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“Study of GeSi:H materials deposited

by PECVD at low temperatures

(Td<200 °C) for device applications”

presented by

Ismael Cosme Bolaños

A thesis submitted in partial fulfillment of the

requirements for the degree of

Ph. D. in Electronic Sciences

at the

National Institute for Astrophysics, Optics and

Electronics

January 2013

Tonantzintla, Puebla

Advisors:

Ph. D. Andrey Kosarev

and

Ph.D. Alfonso Torres J.

©INAOE 2013

All rights reserved

The author hereby grants to INAOE premisson to

reproduce and to distribute copies of this

thesis document in whole or in part

iii

“Porqué el amor de mis padres es tan grande que es la sustancia de mis sueños”

Gracias por todo

Dedicada a:

Marielena Bolaños,

Vicente Cosme,

y mis hermanas

Araceli y Claudia

v

AGRADECIMIENTOS

Eternamente agradecido con el Dr. Andrey Kosarev por su confianza incondicional

y los nuevos horizontes proyectados, sin su guía y sus consejos este trabajo jamás hubiera

sido posible.

Agradezco también al Consejo Nacional de Ciencia y Tecnología (CONACyT) por

la beca otorgada durante mi doctorado y a la Secretaria de Energía por el soporte al

proyecto SENER-CONACYT-INAOE No. 152244.

De igual manera quedo agradecido con la comunidad INAOE, profesores, técnicos y

administrativos que me brindaron su ayuda incondicional. En particular a mi co-asesor Dr.

Alfonso Torres y al departamento de formación académica. Gracias especiales a mi amigo

y maestro Adrián Itzmoyotl, el cual compartiera su experiencia y literalmente me mostrara

las entrañas del sistema PECVD. Gracias al Dr. Yuri Kudriavtsev y a su institución,

CINVESTAV, por el apoyo con las mediciones de SIMS. Gracias al Dr. Andrei Sazonov

por el apoyo al facilitar los substratos flexibles usados en este trabajo. También quiero

agradecer a Francisco Temoltzi por las mediciones de fotoconductividad.

Gracias a mi más grande soporte y corazón Rosalba Tecpanecatl Quechol. Y por

último, gracias a la familia que me prestara la vida en esta gran etapa: Francisco Sánchez

Canto, Rodolfo Trejo Guerra, Judith Onchi Vázquez y su hijo hermoso Héctor Rodolfo.

vii

RESUMEN

Recientemente, los materiales basados en aleaciones de silicio amorfo depositadas

por PECVD tienen un gran nicho en aplicaciones que requieren grandes áreas y flexibilidad

en sus procesos de fabricación. Aleaciones basadas en germanio y silicio amorfos tienen

un gran potencial en dichas aplicaciones debido a que el contenido de germanio permite

modificar las propiedades del material como el ancho de banda óptico, aumentando de esta

manera el rango de absorción.

Actualmente las aleaciones de silicio-germanio son la mejor opción para usarse en

estructuras de celdas solares multiuniónes. Películas Si1-xGex:H con bajo contenido de

Germanio (x < 0.5) han permitido reducir el ancho de banda óptico comúnmente a valores

de Eg= 1.5 eV y como consecuencia se han mejorado las eficiencias de dispositivos

fotovoltaicos aumentando el espectro de absorción. Es por esto que aleaciones con menor

ancho de banda óptico (Eg<1.5 eV) son deseables. Este tipo de materiales basados en

germanio-silicio con alto contenido de germanio (GexSi1-x:H) y reducido ancho de banda

óptico (Eg=1.1 eV) depositadas por PECVD han sido estudiadas en el INAOE. El reciente

surgimiento de nuevas aplicaciones ha aumentado los requerimientos en el desarrollo de

materiales amorfos basados en silicio-germanio, siendo una de éstas, la fabricación de

dispositivos sobre substratos plásticos flexibles requiriendo bajar la temperatura de depósito

en los procesos de fabricación.

Este trabajo de investigación tiene como objetivo estudiar la fabricación y

caracterización de materiales basados en aleaciones GexSi1-x:H (x>0.5) depositados por

PECVD a baja temperatura (Td<200°C) para su aplicación en dispositivos compatibles con

substratos de plástico flexible. La investigación tiene tres direcciones principales: 1) El

estudio del efecto de reduccion de temperatura de deposito de Td=300°C a Td=70°C en las

propiedades optoelectronicas de las peliculas GeSi:H. 2) Seleccionar una temperatura

óptima de deposito compatible con substratos flexibles para el estudio de las propiedades

viii

electronicas de peliculas dopadas con diborano y fosfina, y 3) el estudio de la fabricación

de dispositivos basados en las películas GeSi:H compatibles con substratos de plástico

flexible.

El estudio del efecto de la temperatura de depósito sobre las propiedades

electrónicas y ópticas de películas GexSi1-x:H depositadas por PECVD mostraron una

temperatura óptima de depósito a Td=160°C compatible con substratos flexibles. Se estudió

y demostró el control de las propiedades electrónicas de las películas GeSi:H a la

temperatura seleccionada compatible con substratos flexibles. La incorporación en fase

solida de los dopantes (boro y fósforo) en las películas depositadas fue estudiada mediante

SIMS. Finalmente las películas estudiadas fueron empleadas en dispositivos foto-voltaicos

compatibles con substratos flexibles fabricados por PECVD a baja temperatura Td=160°C

obteniendo los mejores resultados sobre substrato flexible.

ix

ABSTRACT

The Silicon-based alloys deposited by Enhanced Plasma Chemical Vapor

Deposition (PECVD) are of much interest to large area applications and comercial mass

production.

Germanium is currently the most interesting low-optical gap material that can

provide many potential advantages to silicon-based technologies. For example, the

hydrogenated silicon-germanium (SiGe:H) alloys are used in multiple-junction devices due

to their potentially higher conversion efficiency than only silicon-based devices. The

narrow optical gap of these alloys and compatibility with silicon films deposited by plasma

are the main advantages. The silicon-germanium alloys with relative low-optical gap

(Eg=1.5 eV) for Ge content less than 40% are commonly used in multiple-junction

amorphous solar cells, although a lower optical gap (Eg~1.2 eV) for high Ge content

(>50%) is highly wishful. Morehover the low optical gap and the high absorption

coefficient in the wavelength 1500nm make the germanium-based alloys an interesting

material for near infrared applications (micro-bolometers). Another potential advantage

with the uses of Ge materials is the higher hole (four times) and electron (two times)

mobility rather than Si materials.

The versatility of PECVD process allows relative low deposition temperatures

Td<300°C in comparison to the thermal deposition techniques. This versatilityis of much

interest for the industry and scienties because of the energy reduction of the deposition

process, and recently, the compatibility with potential applications in flexible devices on

low-cost plastic substrates.

x

The objective of this thesis is to the study the fabrication and the characterization of

a-Gex Si1-x:H (x>0.5) films deposited by low temperature plasma process (Td< 200°C) for

applications in devices on flexible substrates.

In order to reach the objective the following research was performed: 1) the study of

deposition temperature effect on composition, electronic and optical properties for intrinsic

a-GeSi:H with high Germanium content (<90%) deposited by PECVD. 2)The study of the

doping control of n- and p-type GeSi:H films deposited at low deposition temperature by

PECVD and 3) the deposition of different p-i-n//n-i-p photovoltaic structures based on a-

GeSi:H thin films deposited at low deposition temperature (Td=160°C).

In this work the reduction of deposition temperature of intrinsic a-GeSi:H films

were studied. The effect of deposition temperature reduction and its correlation with the

electrical characteristic demonstrated an optimal deposition temperature Td=160°C. The

doping was studied at low deposition temperature Td=160°C and the electronic properties

have been controlled by boron and phosphorus doping. The solid composition of dopants

was studied by SIMS. Finally, the intrinsic and doped films were used for the study of p-i-n

structures at low deposition temperature which is compatible with flexible substrates. Best

results in devices structures were obtained on flexible substrate.

xi

LIST OF ACRONYMS

Al aluminum

AM1.5 air mass coefficient

ANTID anti-diffusion configuration

a-SixGe1-x:H hydrogenated amorphous silicon-germanium (high Si content)

CINVESTAV Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional

CONACyT Consejo Nacional de Ciencia y Tecnología

DOS density of states

FTIR Fourier transform infrared spectroscopy

Ge germanium

HIT hetero-junction intrinsic thin film

HT high temperature

INAOE Instituto Nacional de Astrofísica, Óptica y Electrónica

IR infrared

LF low frequency

LT low temperature

nc- nano-crystalline

NIR near infrared

PECVD plasma enhanced chemical vapor deposition

PEN polyethylene naphthalene

PET polyethylene terephthalate

PV Photovoltaic

rf radio frequency

RIE reactive ion etching

SENER Secretaria Nacional de Energía (México)

Si Silicon

a-GexSi1-x:H Hydrogenated amorphous silicon-germanium (high Ge content)

SIMS secondary ion mass spectroscopy

TFT thin film transistor

Ti titanium

xii

LIST OF SYMBOLS

C concentration atoms/cm3

Ea thermal activation energy eV

EF Fermi energy level eV

EgTAUC

Tauc optical gap eV Eg optical gap eV

EU Urbach energy eV

EU-ph Urbach energy from photo-conductivity spectrum eV

Ev valence energy level eV

f frequency Hz

FF fill factor

IDC polarization current A

IE intensity of secondary beam a.u

IM intensity of matrix a.u

Iph photocurrent A

Isc short circuit current A

Jsc short circuit current density A/cm2

Jsc_max current density maximum A/cm2

K wavenumber cm-1

n refraction index

R dilution ratio

t thickness cm Td deposition temperature °C

Tg glass transition temperature °C

Tm measurement temperature °C

Ts substrate temperature °C

Vd deposition rate A°/s

Voc open circuit voltage V

α absorption coefficient cm-1

λ wavelength μm

σdark dark conductivity Ω-1 cm-1

σph photoconductivity Ω-1 cm-1

χ thermal coefficient eV/K

xiii

CONTENT

RESUMEN V

ABSTRACT VII

LIST OF ACRONYMS IX

LIST OF SYMBOLS X

1. INTRODUCTION 1

1.1. Background and justification 1

1.2 Objectives and tasks 3

1.3 Thesis organization 4

2. ANALYSIS OF LITERATURE 5

2.1 Introduction 5

2.2 The effect of deposition temperature reduction on a-GeSi:H films deposited by PECVD 5

2.2.1The role of deposition temperature on PECVD 6

2.2.2 The characteristics of a-GeSi:H alloys 11

2.2.3 The effect of temperature reduction onGe-Si films deposited by PECVD 12

2.2.4 Literature of a-GeSi:H films deposited by PECVD at low temperatures 16

2.3 Doping of a-GeSi:H films deposited by PECVD at low temperature 18

2.4 Devices based on a-GeSi:H layers deposited at low temperature 20

3. METHODOLOGY AND EXPERIMENTAL TECHNIQUES 22

3.1 Introduction 22

3.2 Experimental PECVD system for deposition of a-GeSi:H films 22

3.2.1 Substrate preparation 24

xiv

3.3 Methodology used for the temperature reduction in a-GeSi:H film deposition 25

3.3.1 Deposition conditions for study of deposition temperature reduction 26

3.3.2 Temperature reduction for deposition process for a-GeSi:H films 27

3.4 Doping of a-GeSi:H films deposited by PECVD at low temperature 29

3.4.1 Doping process by PECVD at low temperature by diborane 29

3.4.2 Doping process by PECVD at low temperature by phosphine 30

3.5 Experimental methods for film characterization 31

3.5.1 Deposition rate characterization. 31

3.5.2 Fourier transform Infrared (FTIR) spectroscopy 32

3.5.3 Secondary ion mass spectroscopy 36

3.5.4 Temperature dependence of dark conductivity 38

3.5.5 Photoconductivity measurements 40

3.6 Fabrication of devices based on a-GeSi:H films deposited by PECVD at low temperature42

3.6.1 Device structures 42

3.6.2 Deposition conditions for device fabrication 43

3.6.3 Flow charts for devices fabrication 44

3.7 Characterization methods for devices 47

3.7.1 Themapping methodology of electronic characteristic for devices 47

3.7.2 Spectral response for devices 48

4. FILMS EXPERIMENTAL RESULTS 49

4.1 Introduction 49

4.2 Results of temperature reduction effect on intrinsic a-GeSi:H films 49

4.2.1 Fabrication results for temperature reduction 49

4.2.2 Deposition rate as a fuction of the deposition temperature 52

4.2.3 Hydrogen bonding as a function of the deposi tion temperature 53

4.2.4 Sub-gap absorption measurements by FTIR 55

4.2.5 Electronic properties as a function of the deposition temperature 60

4.2.6 Photoconductivity measurements results 63

4.3 Doping results for a-GeSi:H films deposited at low temperature 65

4.3.1 Fabrication results for doping at low deposition temperature 66

4.3.2 Deposition rate results for doping at low deposition temperature 66

4.3.3 Composition study by SIMS for doping at low deposition temperature 67

4.3.4 Electronic properties as a function of the dopant concentration 70

xv

5. DEVICES EXPERIMENTAL RESULTS 74

5.1Introduciton 74

5.2 Current density progress for device structures 74

5.3 Stage 1: a-GeSi:H “BASIC” structure 76

5.3.1 Electronic and mapping characterization of BASIC structure 78

5.4 Stage 2: “ANTID” structures, anti-diffusion silicon layers 79

5.4.1 Thickness increase of intrinsic a-GeSi:H layer 82

5.4.2 Third anti-diffusion Si:H layer 84

5.4.3 Optimization of doping in GeSi:H layer 86

5.4.4 Optimization of thickness of anti-diffusion Si:H layers 89

5.5 Stage 3: “HIT” structure based in Si:H and GeSi:H absorption layers 92

5.5.1 Transition from ANTID to HIT structure 95

5.5.2 Optimization of absorption layers thickness 98

5.5.3 Sputtered contacts, n-i-p structure and plastic substrate 100

5.4 SIMS characterization for selected process 1086 103

5.5 Short circuit current spectra for selected devices structures 104

5.6 a-GeSi:H device characteristics on flexible substrate 106

6. DISCUSSION OF THE RESULTS 109

6. 1 introduction 109

6.2 Disscusion about deposition temperature reduction effect on intrinsic a-GeSi:H films 109

6.2.1 Effect of deposition temperature reduction on hydrogen incorporation 109

6.2.2 Effect of deposition temperature on deposition rate 111

6.2.3 Effect of deposition temperature on optical characteristics 111

6.2.4 Effect of deposition temperature on electronic properties. 113

6.2.5 Effect of deposition temperature on photoconductivity 115

6.2.6 General discussion about deposition temperature reduction 116

6.3 Discussion about doping for a-GeSi:H films deposited by PECVD at low temperature 120

6.3.1 Doping at low deposition temperature (160°C) and high deposition temperature (300°C) 120

6.3.2 Doped a-GeSi:H films at low deposition temperatures T d<200°C 123

6.3.3 Special discussion of compensated intrinsic a-GeSi:H film at low deposition temperature 124

xvi

6.4 Disccusion about devices based in a-GeSi:H films fabricated at low temperature 125

6.4.1Electronic characterization and structure evolution 125

6.4.2 SIMS results discussion 131

6.4.3 Discussion of the short circuit current spectra 131

6.5 Future improvements 133

7.CONCLUSIONS 135

Main conclusion 135

Conclusion for each study 135

PUBLICATIONS 139

CONFERENCES 140

PROJECTS 141

REFERENCES 142

LIST OF FIGURES 149

P a g e | 1

1. INTRODUCTION

1.1 Background and justification

Technology based on amorphous semiconductors has become an important part and

multi-billion businesses in electronics in recent years. It has been several years since J.M.

Ziman wrote: "A theory of the physical properties of solids would be practically impossible

if the most stable structure for the solids were not regular crystal lattice" [1] and nowadays

the theory of amorphous materials hasn’t only been developed, but also increases the

interest in technological applications [2-4].

The first amorphous semiconductor films were deposited more than 40 years ago by

Sterling and Swann and these has been reported in ref. [2], using a completely new method

called Plasma Enhanced Chemical Vapor Deposition (PECVD) The systematical study of

electronic characteristics of the films deposited by the new method was taken up and

improved by Chittick in 1969 in ref. [3], silane (SiH4) and doping with phosphine source

(PH3) were used. The limitations of the first PECVD systems were high leak rates and high

pre-deposition pressure, which results in hydrogenated amorphous silicon (a-Si:H) films

with unremarkable electronic characteristics as is shown in ref.[4], and the real potential of

the amorphous semiconductors would not jet be discovered.

The real development of amorphous semiconductors starts in 1970 with Spears and

LeComber investigations [5-6]. They studied the density of states within “bandgap” in a-Si

films using a field effect technique at different deposition temperatures [5]. They

demonstrated that amorphous semiconductors could be doped p-type with diborane (B2H6)

and n-type with phosphine (PH3) gas sources [6]. In 1976 the first functional p-n junction

was reported by Spears and LeComber in ref. [7]. Carlson and Wronski report a p-i-n

structure employing an intrinsic a-Si:H film with a relative high photoconductivity [8]. One

of the first studies of hydrogenated amorphous germanium (a-Ge:H) films (intrinsic and

doped films) was made by Spears and LeComber and it is reported in ref. [9].

P a g e | 2 CHAPTER 1: INTRODUCTION

Justification

Germanium is currently the most interesting low-optical gap material and it provides

potential advantages to silicon-based technologies. For example, hydrogenated amorphous

silicon-germanium alloys (a-SiGe:H) are used in multiple-junction devices due to they have

a higher conversion efficiency than silicon-based devices. The narrow optical gap of a-SiGe

alloys and the compatibility with silicon films deposited by plasma are the main advantages

of this material. Silicon-germanium alloys with relative low-optical gap (1.5 eV) for Ge

content less than 40% are commonly used in multiple-junction amorphous solar cells,

however a lower optical gap (1.2 eV) is highly desirable and this is obtained for high

concentrations of Ge (>>50%) (See section 2.4). Furthermore the low optical gap and a

high absorption coefficient in the wavelength λ=1500 nm are interesting for near infrared

applications (micro-bolometers) [10]. Other potential advantages for Ge materials are the

higher hole (four times) and the higher electron (two times) mobility than for silicon in

crystalline materials. These advantages could be transferred to amorphous materials.

The development of a-Ge:H technology has been slower than a-Si:H and now some

stages of development for silicon are being studied for germanium. One of these stages is

the study of deposition temperature effect on the characteristics of the films deposited by

PECVD process. The main benefit of glow discharge process is the considerable reduction

of deposition temperatures then the first studies were focused on getting the best electronic

properties of the films through the study of the fabrication parameters and optimal

deposition temperatures (Td). The studies found an optimal deposition temperature in the

range from Td=250 to 350 °C. The versatility of PECVD process allows relative low

deposition temperatures in comparison to the thermal deposition techniques. The possibility

of temperature reduction is of high interest for the industry and scientists, because of the

energy reduction in the deposition process and also the compatibility with potential

applications in flexible devices on low-cost plastic substrates. The first flexible

photovoltaic devices were produced on stainless steel due to the high deposition

temperatures in the PECVD processes (> 200 °C). Nowadays these devices had become

commercial sold products (Fig. 1.1) and new substrates as low-cost plastic are what the

industry and scientist are looking for. .


a)

b)

Figure 1.1 Amorphous silicon PIN devices from Energy Conversion Devices Inc. a) Flexible one-

square-foot Ovonic solar cell 1985 [11] and b) PowerBond PVL uni-solar product 2012 [12].

The development of flexible devices requires the study of intrinsic and doped (n- and

p- type) films of a good quality at low deposition temperatures. In this thesis we studied the

deposition of intrinsic and doped films by LF PECVD at low deposition temperatures based

on germanium-silicon alloys with high concentration of Germanium (GexSi1-y:H) and we

applied the films in photovoltaic application at low deposition temperature compatible with

flexible substrates. The solar cells are very attractive as a renewable energy source and

devices based on Ge:H, which provide main advantages as a low optical gap, a large area

scale and flexibility that increase the possibilities of photovoltaic devices.

This thesis is part of SENER-CONACYT-INAOE project No. 152244 and it shows

the interest of these institutions to increase the basic research in renewable technologies

according to the national goals in the national energy strategy [13].

1.2 Objectives and tasks

The objective of this thesis is the study of the fabrication and characterization of

GexSi1-x:H (x>0.5) films deposited by low temperatures in the plasma processes (Td<

200°C) for application in devices on flexible substrates.


In order to reach the objective, the following research will be performed:

1. Study of the deposition temperature effect on composition, electronic and optical

properties for intrinsic aGexSi1-x:H (x>0.5) films deposited by PECVD.

2. Study of the doping control (n- and p-type) in a-GeSi:H films deposited at low

deposition temperature (Td=160°C) by PECVD.

3. The deposition of different p-i-n//n-i-p photovoltaic structures based on a-GeSi:H thin

films deposited at low deposition temperature (T=160°C) in order to demonstrate the

application of the films on flexible substrates.

1.3 Thesis organization

The chapters of this thesis are organized as follow: In Chapter 2 we presented the

analysis of literature on temperature effect, doping control and devices based on a-GeSi:H

films. In Chapter 3 we describe the methodology of deposition, the doping control, and the

characterization techniques of the films and devices. The results obtained on deposition

processes for intrinsic and doped a-GeSi:H films at low deposition temperatures are

discussed in Chapter 4. The Chapter 5 contains the results of structures and fabrication of

devices at low deposition temperature. In Chapter 6 the results obtained are discussed in

comparison to data available in literature. Finally, in Chapter 7 we concluded our findings.

P a g e | 5

2. ANALYSIS OF LITERATURE

2.1 Introduction

Plasma Enhanced Chemical Vapor Deposition (PECVD) is a standard method in the

electronic industry. An accidental discovery was that the preparation of amorphous silicon

using SiH4 results in low Density of State (DOS) films due to the incorporation of the

hydrogen atom in the lattice of amorphous silicon [3]. Another advantage of PECVD is the

use of plasma in order to allow low temperatures in the deposition process. The low

deposition temperatures were studied recently by scientist and the industry with the goal to

explore new applications which are not compatible with high deposition temperatures

(Td>300°C) at the moment

The purpose of this chapter is the analysis of the state-of-art technology for

deposition, doping and photovoltaic (PV) devices based on germanium-silicon thin films

with high concentration of germanium deposited by PECVD at low deposition temperatures

(Td<200°C). The study of the effect of deposition temperature on the composition,

electrical and optical characteristics of the films is discussed referring to previous

investigations. A review of the performance of PV devices based on GeSi:H thin films (p-i-

n structures) fabricated at low temperature are analyzed at the end of this chapter.

2.2 The effect of deposition temperature reduction

on a-GeSi:H films deposited by PECVD

Actually, the glow discharge is used in a wide variety of processes because of the

advantages. Plasma can be created by DC or RF electric field applied to electrodes; the

radicals of molecules are formed taking advantage of the kinetic energy and also the

collisions between electrons, ions and neutral particles. The growing surface of the films is

controlled by microscopic parameters of these particles such as the gained energy,

P a g e | 6 CHAPTER 2: ANALYSIS OF LITERATURE

collisions, particle and radical concentrations. The characteristics of these depend on the

macroscopic parameters such as pressure, frequency, power, gas flows and temperature

deposition. This interrelation of many factors is the origin of the flexibility of the PECVD

technique.

2.2.1 The role of deposition temperature on PECVD

The main factors that influence the growth of the films in PECVD process are:

deposition temperature, type of substrate and surface conditions. The stages of growth are

shown in the Figure 2.1 and it exposes the three main factors that are involved in the

growth process: a) The incorporation of the hydrogen atoms to the lattice, b) the deposition

of the atoms on the growing surface and c) the restructuring of the atoms in the lattice of

the films. In this section we investigated the effects of the deposition temperature on the

main growth mechanisms of growth on the films..

Figure 2.1 The factors of the formation of thin films in PECVD growth process; i) condition and type

of the surface, ii) chemical conditions of the substrate and iii) the deposition temperature.


a) Incorporation of hydrogen

The resulted hydrogen atoms in the plasma from hydride radicals and from molecular

hydrogen (dilution gas) are of principal importance. This has been found important because

of the dangling bound passivation and because of the incorporation of H atoms relieves the

extra strain on the lattice of the material[14-21].

The role of the deposition temperature is a strong effect on the incorporation of

hydrogen during the deposition of the films. In general, the studies of deposition

temperature observed that the total H content decreases with a deposition temperature

increase [22-26] (see Fig.2.2).

In reference to [26] three types of activate processes in relation to deposition

temperature are described for the absorption-elimination of hydrogen in a-Si films: (Fig.

2.3) a) the surface desorption is observed at high temperatures Td > 400 °C (hydrogen

elimination activation energy ~2.2 eV), b) the bulk H diffusion is observed for Td ranges

from 200°C to 300 °C (hydrogen elimination activation energies ~1.6 eV) and c) the H*

rearrangement is shown for Td<200°C (hydrogen elimination activation energies ~0.15 eV.

Figure 2.2 Hydrogen content CH versus deposition temperature for Ge0.5Si0.5:H films deposited by

glow discharge deposition system [13.56 MHz]in the temperature range from 230°C to 372°C (fig.

taken from [23]). Total hydrogen content CH decreases with the deposition temperature Td increase.


Figure 2.3 Deposition temperature Td dependence versus hydrogen content CH, and elimination

hydrogen mechanisms in amorphous silicon deposition (figures taken from [26])

It has been established that deposition temperature has an effect on electronic

properties such as: Urbach energy EU, density of state (DOS) and optical band gap Eg due

to the direct relation with the hydrogen absorption in the films.

b) Deposition of involved atoms on growing surface

Deposition rate Vd in PECVD process is less affected by deposition temperature than

in other techniques not related to plasma. In glow discharge process, the precursors are

generated by the glow discharge and they are relatively independent of the substrate

temperature. Direct effect of deposition temperature on deposition rate has not been

observed for Si:H in ref. [25]. Other studies for Si:H [27-28] observe an increase of

deposition rate with the increase of deposition temperature , in contrast to [46] that reports a

general decrease of deposition rates with the increase of deposition temperature for Ge:H

films in the range of temperature between Ts=30 °C and 310°C. However, generally for

GeSi:H alloys, the deposition rates are controlled by other parameters such as power and

frequency [29-30].


c) Restructuration of the atoms in the lattice of the films.

Many factors had an effect on the formation of the thin film during the deposition and

the deposition temperature is only one among many. The effect of deposition temperature

on the restructuring of the atoms in the lattice depends on the interrelation with other

parameters and conditions.

Figure 2.1 shows the general stages of growth in PECVD process. The materials

move toward the substrate during the re-evaporation stage in different particles such as

molecules, atoms, radicals and ions. The particles move on the surface of the substrate with

same thermal energy of neutrals and ions in the plasma. Some of this particles with low

energy will be diffused in the substrate and other particles with extra energy will be

evaporated from the surface influenced by the substrate temperature, ion bombardment and

the energy obtained from a chemical reaction e.g. H-subtraction. The diffused atoms will

be added to other single atoms to form doublets, triplets and nuclear regions, which create

the quasi islands during the nucleation and island stages. These islands grow in size large

enough to touch with others growing islands in the coagulation stage, this process continues

on to it reaches the continuity of the formed film. The microstructure of the films (scale

~10 nm) is determinates by the hydrogen content in the first stages of growth. In several

works the influence of hydrogen on microstructure of germanium and silicon films has

been studied and some recent examples are mentioned in references [31-33]. They found

that hydrogen bonding configurations, density fluctuations, clustered and dispersed

hydrogen distributions all have an effect on the formation of voids in the films. In the

PECVD these factors are controlled by a complex process related to micro particles in the

plasma, ion and electron bombardments [34], density of radicals, pressure, feed gas,

dilution, doping species and deposition temperature. As it was mentioned earlier, hydrogen

content increases with reduction of Td.

Recently, some other aspects are being considered in which the deposition

temperature has an important role. The PECVD deposition at low deposition temperatures

Td< 200°C is required for deposition on plastic substrates for light-weight flexible

electronic devices. Flexible electronics is not something new; as Ovshinsky presents roll to


roll technology for commercial flexible solar cells in 1988 [35]. But since 1997, when it

was shown that polycrystalline Silicon TFT can be used on flexible plastic substrates [36],

the research of this field have increased in the international scientific and industrial

communities. These new applications also bring forth new developments in substrate

materials. Table 2.1 shows some characteristics for different substrates used in PECVD

process.

Table 2.1Characteristics for substrates used in PECVD process [37 and

http://www.teijindupontfilms.jp/english/product/hi_film.html 23/11/012]

Characteristic Glass

(1737)

Plastic Substrates Stainless steel

PET PEN Polyimide

Weight (g/m2) 250 120 -- -- 800

Visually

transparent

YES YES YES SOME N0

Maximum process

temperature (°C)

600 120 180 300 1000

Plastic is a cheap and flexible material that can be used in mass production and

applications. Polyethylene terephthalate (PET), Polyethylene naphthalene (PEN) and

polyimide are the most common plastic substrates used in electronic applications. The

highest deposition temperature and best performances are in the polyimide substrates with a

highest cost of the three plastic substrates followed by PET and PEN. Table 2.1 highlights

the maximum process temperatures for glass, plastics and stainless substrates. The

maximum process temperature T< 200°C for plastic limits the PECVD deposition and the

electronic properties of the films deposited at low deposition temperatures need to be

compensated by both plasma and surface conditions in growth process. The complex

interrelations of the operator parameters as; pressure, frequency, power, flows, gas source,

substrates have an influence on the films properties. The data on a-GeSi:H alloys deposited

at different deposition temperatures will be analyzed in the section 2.2.2 and 2.2.3

revealing the effect of this parameter on the electronic properties of the films.

http://www.teijindupontfilms.jp/english/product/hi_film.html


2.2.2 The characteristics of a-GeSi:H alloys

Germanium-Silicon alloys deposited by low temperature process (Td< 250°C) are of

much interest because they provide narrow gap materials compatible with Si:H films and

they have potential applications for the devices on plastic substrates. These films can be

used for different device applications i.e., solar cells [38]. The main problem associated

with these materials is the high density of states that is generated with the increase of

germanium content in the alloy.

The Ge atom is slightly larger and remarkably heavier compare to Si atom.

Ovshinsky wrote in 1994 in ref. [39] that this not only affects the mobility of the

germanium atom on the growing surface, but also requires higher temperatures during

deposition and they affect the hydrogen evolution. Because of this, the alloys with more

than 50 % content of germanium with good opto-electronic qualities which are difficult to

obtain even at high deposition temperatures. Several efforts have been made to solve this

negative effect of high Ge content in alloys; ion bombardment [34, 38], thickness

optimization [17] high dilution rate in hydrogen [33]. At INAOE, we have an extensive

background research in high germanium content alloys. One study for different dilutions

and concentrations of germanium revealed that low concentration of silicon (< 5%)

improves the electronic and structural properties of films deposited at high deposition

temperature (300°C) [34]. These improvements were used in thermo-sensing films for

infrared applications [40].

By increasing the germanium content to x >0.5, the lower optical gaps are obtained

(Eg =1.1 eV) however this affects the hydrogen content, increasing the micro-voids

formation and the reduction of opto-electronic properties in contrast to the silicon films.

The reduction of deposition temperature is expected to increase the negative effects of the

high germanium content. The properties referring to the first films a-Ge:H obtained by

PECVD are Eg=1.01 eV, activation energy Ea=0.45-0.49 eV, dark conductivities from

σdark=10-4

-10-5

Ω-1

cm-1

and Urbach energies in the range of EU=55-60 meV [41].


2.2.3 The effect of temperature reduction Ge-Si films deposited by

PECVD

Few studies [9, 22, 23, 42, 43-46] have been conducted on SiGe:H films deposited by

PECVD at low temperature, and many of them were made in the eighties. These studies

were aimed at finding deposition conditions to achieve good electronic properties. The

optimal temperatures were found to be around Td=250-300°C, depending on the

concentration of Ge, installation, and some other parameters. The effect of deposition

temperature in GexSi1-x:H films (x~50%) in the range from Td=230 to 372 °C was studied

by Mackenzie in 1985 using rf PECVD (13.56 MHz)[23]. The optical gap shows a linear

decrease from Eg=1.6 eV to 1.4 eV with the increase of Td. These changes have a

correlation between the reduction of hydrogen content and the increase of Td in the same

range. Also the photoconductivity has been studied as a function of the deposition

temperature (Fig. 2.4). The photoconductivity shows a rise with the increase of

temperature in the range from Td=230 to 300 °C and above this range the photoconductivity

is halved with an optimal deposition temperature around Td=300°C.

In ref. [42] an increase of refractive index from n=4.20 to 4.30 and reduction in IR

absorption peak at 1970 cm-1

as a function of deposition temperature in the temperature

range from Td=150-250°C is reported for Ge:H film. In the total range studied Td=110-

280°C the increase of temperature results in a decrease of optical gap from Eg=1.22 to 1.16

eV. In ref. [43] it is studied Si0.58Ge0.42:H alloys in the range of deposition temperature from

Td=150 to 230°C. The increase of deposition temperature causes a reduction of bonded

hydrogen with the decrease of optical gap from Eg=1.52 to 1.43 eV. A sample deposited at

Td=175°C is reported with photoconductivity of σph=4.5x10-5

Ω-1

cm-1

and photosensitivity

of σph/σdark= 9.4x104

Ω-1

cm-1

. The incorporation of hydrogen in nc-Ge:H films influenced

by deposition temperature is studied in ref. [44] and transitions from amorphous to

crystalline and amorphous phase within the range of deposition temperature are reported.

These results are explained by the influences of deposition temperature on growing surfaces

and changes in diffusion length of absorbed precursors [see section 2.2.1].


Figure 2.4 The effect of deposition temperature for a-Ge0.5Si0.5:H films deposited by PECVD taken

from ref. [23]; a) Energy gap E04 (Eg), b) photoconductivity Δσ, versus deposition temperature Td.

In ref. [45] a-SiGe:H films obtained by low dilution ratio (H2:Si4 2.5:1) and high

dilution ratios (27:1 and 54:1) at temperature range from Td=180°C to 230 °C are reported.

Figure 2.5a shows the Td effect of dark conductivity σdark and photoconductivity σph, at low

dilution ratio, σph drops six orders of magnitude (10-1

to 10-7

Ω-1

cm-1

) in deposition

temperature ranges from Td=230°C to 170 °C. For high dilution rates, the

photoconductivity maintains around σph~1x10-5

Ω-1

cm-1

without drastic descent. Figure

2.5b shows the incorporation and bounding configuration of hydrogen for different

deposition temperatures. It shows that for low dilution ratio (27:1) the configuration CH(Si-


H2) decreases from 5.5 % to 1.6 with the increase of temperature and for the high dilution

ratios, the same configuration has not been affected by the temperature.

Figure 2.5 Effect of deposition temperature Td [45] on a) photoconductivity and dark conductivity, b)

bonding configurations of hydrogen, at different dilution ratios

In ref. [45] the incorporation of hydrogen is kept constant at different deposition

temperatures by high dilution ratios. These results suggest that the very high dilution ratio

conditions could compensate for the low deposition temperature conditions. The extra

radicals and ions of hydrogen provide some additional energy and possibly it compensates

for the low temperatures on the growth surfaces.

More recently was studied the structural transition, optical and electrical transport

properties of germanium films in ref. [46]. Ge:H films were obtained by rf (13.56 MHz)

PECVD in the deposition temperature ranges from Td=30 to 310°C. The deposition rate Vd

decreases with a temperature increase in the range between Vd=0.13 to 0.26 A°/sec with a

rise in deposition temperature to Td=280°C. The electronic characteristic versus deposition

temperature is shown in Figure 2.6a. Two different trends are identified for activation


energy trends; one region, from Td=40°C to 250°C, shows almost constant activation

energies and the second region, from Td~240 to 310°C, shows an increase of the activation

energy between Ea=0.40 eV and 0.1 eV. It is interesting to notes that the activation energy

Ea decreases, and dark conductivity σdark increases five orders of magnitude at Td=310°C.

The highest photosensitivity σph/σd was obtained at Td=120°C.

Figure 2.6 a) Dark conductivity σdark, photosensitivity σph/σdark and thermal activation energy Ea

versus deposition temperature, b) SIMS hydrogen concentration profile of Ge layers deposited at

different deposition temperatures [46]

The Figure 2.6b shows a SIMS profile for Ge:H layers deposited at different

deposition temperatures. The decrease of hydrogen incorporation is continuous in the

temperature range from Td=30 to 190°C with a rise at temperature Td=240°C. The Figure

2.6b also shows transitions from amorphous to nano-crystalline structure and vice versa.

They are only controlled by deposition temperature and this effect is also reported in refs.

[44 and 47].This transition involves the desorption of hydrogen in the range from Td= 200

to 300°C and the transitions at low temperatures from Td=30 to 80°C merits further study

as mentioned by the authors.


2.2.4 Literature of a-GeSi:H films deposited by PECVD at low

temperatures

Silicon-germanium alloys are considered to be the best low optical gap material for

multiple-junction silicon solar cells. The potentially higher conversion efficiency for low-

optical gap material, less than Eg=1.5 eV, is the reason for the interest in these materials.

However, the electronic properties of germanium-silicon alloys are deteriorated by high

concentration of germanium. Only a few studies exist in Ge:H and GexSi1-x:H films with

high concentrations of germanium (x>>0.5) and there are fewer studies with low deposition

temperatures (Td<200°C). Table 2.2 shows the optical and electronic properties for a-Ge:H

films and a-GexSi1-x (x>0.5) alloys deposited at low deposition temperatures (Td< 200°C)

by PECVD.

In ref. [46] Ge:H films with values for dark conductivity σdark~2x10-4

Ω-1

cm-1

and

values of photoconductivity

from σph/σdark~6-1 are reported. These studies at low

deposition temperature are very interesting because they obtained better properties than

with a high deposition temperature. In ref. [42] it is reported an a-Ge:H film deposited at

Td=150°C with optical gap Eg=1.22 eV and Urbach energy values from EU=44-46 meV for

different film thickness. The deep defect density obtained by PSD spectra is reported in the

range from ND=2.5x1017

– 3x10-17

cm-3

for samples deposited at Td=120°C and 225 °C

respectively. The values of refraction index are shown from n=4.20 to 4.30. In ref. [49] a

good quality Ge:H films with deep defect density range from ND=18x1016

-13x1017

cm-3

deposited by PECVD for power variation at deposition temperature Td= 200°C is reported.

The values of optical gap were obtained in the range from Eg=1.19 to 1.25 eV with

refraction index from n=4.23 to 3.96. Other a-Ge:H films are reported in ref. [38] with

application in p-i-n devices with good quality [see section 2.4] deposited at Td=200°C.

Other techniques, such as dual target reactive magnetron sputtering, are promising

techniques as reported in ref. [50] for Ge:H film deposition at low temperatures but they are

still in development.


Table 2.2 Intrinsic GeSi:H films properties based on high concentration of germanium (Eg<1.5 eV) deposited at different deposition temperatures.

Td (°C) CGe

(%)

CH

(%)

Eg

(eV)

σph

(Ω-1

cm-1

)

σd

(Ω-1

cm-1

)

σph/σd Ea

(eV)

EU

(meV)

n

Deposition rate

(A/s)

Ref

150-250

RFPECVD

(13.56;Hz)

100 0 1.22-

1.16 - - - -

46 -60

4.20

-

4.30

--

[42]

30-310

RFPECVD

(13.56MHz)

100 10-1 0.8 -- 10-5

-100

~6-1 0.4-

<0.1 - - 0.24-0.13

[46]

200

RFPECVD

(13.56MHz)

100 4.4-7.7 1.19-

1.25 - - - - -

4.23

-

3.96

10.5

[49]

174

RFPECVD

(13.56MHz)

100 - - - - - 0.3 - - --

[38]

170-230

RF PECVD <50 13-5 1.32 10

-7-10

-5 10

-11-10

-9 - -

- - --

[45]

230-372

RFPECVD

(13.56MHz)

50 13-4 1.6-1.3 10-9

-10-7

- - - - --

[23]


2.3 Doping of a-GeSi:H films deposited by PECVD at

low temperature

As previously mentioned, Si:H films function in many significant applications

thanks to the possibility of controlling of the electronic properties by doping the films in

n- and p-type. For a-GexSi1-x:H films with a high concentration of Ge (x>0.5), the doping

control has not been well studied in reference to the application in devices, for example, a

device based on intrinsic a-Ge:H films is reported in ref. [38] using p- and n-type Si:H

films.

Only a few works [52-56] have reported about doped germanium-silicon films with a

high germanium concentration deposited by PECVD, and only ref. [56] has a focus on low

deposition temperatures (Td<200°C). In ref. [52] the defect density ND of Ge:H doped by

phosphine and diborane have been researched. A strong increase of ND with dopant

concentration for both phosphorous and boron was observed. A control of Fermi energy

was reported in this work practically consisting of the entire optical gap Eg= 1.1 eV from

EF-Ev ~ 0.4 to 0.9 eV. The doped n- and p- GexSi1-x:H films deposited at high temperatures

(Td ~300°C) and the application of the films in micro-bolometer devices was reported in

references [53-54]. In ref. [54-55] GexSi1-x:H films with x>0.90 are reported. The

incorporation of boron from gas phase to solid phase was studied, and the electronic

properties were studied as function of the boron concentration. The activation energy

control from Ea= 0.22 to 0.47 eV by increasing the B content was reported in ref. [54] and it

suggests a compensation of dark n- type conductivity in non-doped films by the B

incorporation of the films that changes from σdark= 3.5x10-3

to 3.5x10-6

Ω-1

cm-1

in the

studied range of boron concentration. On the other hand, the increasing B content above the

compensation level causes the activation energies to decrease again from Ea=0.47 to 0.32

eV while the room temperature conductivity remained almost constant. The solid

incorporation of hydrogen obtained by FTIR and SIMS was studied showing a non-

monotonous change in the H incorporation with the gradual incorporation of boron. The

author in ref. [55] noted that for low concentration of solid boron, the dark conductivity


was almost constant, but changes occur in the structure films, as indicated by the changes

of the refractive index n(∞) and the Urbach energy values.

Figure 2.7 Electrical properties of phosphorus-doped and boro- doped nc-Ge films deposited by

PECVD at deposition temperatures Td=150°C and Td=190°C [56] a) thermal activation energy b) dark

conductivity as a function of dopants concentration

In ref. [56], the authors reported thin phosphorous-doped and boron-doped

germanium films deposited at temperatures Td=150°C and Td=190°C (Fig. 2.7).For

phosphorus doped films (150 and 190°C), the changes of activation energy occur at low

concentration and remain constant (Ea~0.12 eV) in the studied range (Fig. 2.7a). The boron

doped films at low dopant ratio ([B]gas/[Ge]gas=0.04) show a similar activation energy as

the intrinsic film at Td=190°C and it shows a decrease in activation energy at Td=150 °C.


The activation energy decreases with an increase of dopants until the ratio

[B]gas/[Ge]gas=0.20, and then it shows an increase in activation energy (Fig. 2.7a). The dark

conductivity data (Fig 2.7b) shows values within the ranges from σdark=10-4

to 100 Ω

-1cm

-1.

2.4 Devices based on a-GeSi:H layers deposited at

low temperature

Deposition of device based on hydrogenated germanium (Ge:H) of good quality is of

much importance to infrared detectors and tandem devices. However Germanium is still

used as a secondary material in a-SiGe:H alloys, and the real potential of germanium, as

low optical gap and high hole mobility (two times higher than silicon for crystalline

materials), has not yet been revealed. In ref. [56] a dark J-V characteristic of Ge:H device

with n-type and p-type films is presented, but it does not result in a response to light. .

Figure 2.8 shows the J-V characteristic of p-i-n Ge:H device and the rectification effect are

observed [56]. The intrinsic and n-type layers were deposited at deposition temperature

Td=150°C and the p-type layer was deposited at Td=190°C

Figure 2.8 Dark J-V characteristics at room temperature for p-i-n germanium device deposited at

150°C and p-type layer deposited at 190°C, [56]


The intrinsic a-Ge:H and doped a-Si:H films are used in n-i-p photodiodes deposited

at low temperatures Td=150-170°C with an efficiency of 2.1 % in ref. [38]. The J-V

characteristics of the devices are shown in Figure 2.9.

Figure 2.9 The J-V characteristics of n-i-p germanium photodiodes with an absorption layer thickness

of 40, 80 and 120 nm. The graph was taken from ref. [38]

The Intrinsic germanium films used in ref. [38] was fabricated under the conditions of

high hydrogen dilution and an Ar ion bombardment. These conditions were suggested in

order to compensate for the lower deposition temperatures in the process. The thickness of

several intrinsic films was used with the deposited structures (Fig. 2.9). The best sample

was grown at the low deposition temperature Td=150 °C with intrinsic Ge:H film thickness

of t=60 nm (J-V characteristics were not reported for this particular sample), The thickness

of n- and p- type Si:H films were t=20 nm and t=35 nm respectively showing a short

circuit current density value of Jsc=20.6 mA/cm2 (AM1.5), an open circuit voltage of

Voc=253 mV, a fill factor of FF=39.4 and an efficiency of η=2.05%.

P a g e | 22

3. METHODOLOGY AND

EXPERIMENTAL TECHNIQUES

3.1 Introduction

This chapter describes the methodology applied to the study of deposition

temperature effect on the electronic and optical characteristics of films. The Section 3.2

presents the Plasma-Enhanced Chemical Vapor Deposition system used for the deposition

of the samples. The deposition temperature Td was varied from 300°C to 70°C, the

procedure is explained in section 3.3. The doping at low deposition temperature is

presented in section 3.4; both boron and phosphorous doping have been studied. The

techniques for the film characterization are presented in section 3.5, the intrinsic and doped

films for device applications were characterized; a prototype of p-i-n germanium diode was

fabricated at low deposition temperature. The stages of fabrication are shown in section 3.6

and the characterization techniques for the devices are presented in section 3.7.

3.2 Experimental PECVD system for deposition of a-

GeSi:H films

Plasma-Enhanced Chemical Vapor Deposition or glow discharge deposition is the

standard technique for fabrication of device based in amorphous silicon and germanium

films as was discussed in section 2.2. The main advantage of this technique is that the

plasma-assisted process decreases the optimal deposition temperature for the deposition of

the films. The PECVD installation used for fabrication in this thesis is a modernized system

“Model AMP 3300” of APPLIED MATERIAL Inc. The schematic representation of the

system is shown in Figure 3.1.

P a g e | 23 CHAPTER 3: METHODOLOGY

Figure 3.1 Scheme of the plasma deposition system “APPLIED MATERIALS” Model AMP330 used

for deposition process of the films and structures at INAOE.

The system consists of five general elements: reactor, temperature, gas, rf power and

pressure subsystems. The reactor is a planar capacitor electrode with 66 cm (26 inches) of

diameter and 5 cm (2 inches) of inter-electrode distance. The system includes three heater

zones and an independent controller that heat the bottom electrode (substrate electrode)

from room temperature to 400°C. The gas subsystem includes the gas sources for (SiH4,

GeH4, PH3, B2H6, Ar), and the flow meters and controllers for each source. The pressure

subsystem has a controller and a mechanical pump to maintain the pressure in the range

from 0.3 to 4 Torr. The initial vacuum is reached by turbo molecular pumping. A rf power

source provided the power by a matching network connect to the chamber. The power and

frequency in this thesis were fixed to 300 Watt and 110 kHz, respectively.


3.2.1 Substrate preparation

The types of substrates used in this thesis were Glass (Corning 1737) and Si-Wafer

(p-type, resistivity ρ≤ 3 Ω cm-1

and 001 oriented). These substrates were used in the

characterization of the samples while others two types of flexible plastic substrates were

used to demonstrate the compatibility of the the deposition process at low deposition

temperatures. The plastic substrates used for the experiments were polyimide and

polyethylene-naphthalate (TEONEX© PEN) films from DuPont Inc. The glass transition

temperatures of these substrates are Tg >360 [57] and Tg>180°C [58], respectively. The

plastic substrates were tested in the temperature range from Td=70 to 200°C.

The preparation of the substrates is as follows:

The glass substrates with 2.5 cm x 2.5 cm (1 inch2) size were submerged in

Trichloroethylene during 5 minutes and then were submerged 5 minutes in Acetone

solution followed by three rinses in water and they were dried in a centrifugation

system (super Q).

The c-Si substrates were cleaned using RCA1, RCA2 and three water rinses. In

order to remove the native oxide, it was employed a HF:H2O (7:1) solution.

The Plastic substrates were cleaned with three trichloroethylene flashes followed

by three acetone flashes and three water rinses.

For the electronic characterization of films, titanium contacts were deposited on the

substrates (c-Si, glass and plastics) by electron beam evaporation (Blazer system). A

mechanical mask was used in order to from the contacts during the evaporation. A non-

transparent contact of titanium was deposited with thickness of t=0.5 µm and a transparent

contact was deposited with thickness of t=70 A° (depending on the configuration of the

sample). Immediately, after the deposition of titanium the samples were transferred to

PECVD chamber for vacuum.


3.3 Methodology used for temperature reduction in

in a-GeSi:H film deposition

Previously in the INAOE, the deposition of amorphous silicon and germanium films

at high temperature (Td=300°C) had been studied. In the ref. [59] it is shown that the films

based in germanium with low concentration of silicon (Ge0.97Si0.03:H films) deposited at

high dilution ratios of hydrogen (R=75) have a low band tail and a low defect density.(Fig.

3.2). The study conclude that exists a correlation between the morphology and electronic

properties and it was shown that the film with the best morphological characteristics have

superior electronic properties.

Figure 3.2 Spectral dependence of optical absorption coefficient α(hv) at different hydrogen dilution

for Ge0.97Si0.03:H films deposited at Td=300°C [59]

Figure 3.2 shows the hydrogen dilution effect on the spectral absorption in the films

deposited at temperature Td=300°C [59], It is shown that the sample with a dilution ratio of

R=75 has the lowest band tail. The spectral dependence of absorption coefficient α(hv)

shows the characteristics such as the lowest Urbach energy Eu= 30 meV and the defect


absorption αd=2x103 cm

-1 (hv≈1.04 eV). Applications of these films as thermo-sensing

layers deposited at Td=300°C in micro-bolometers are reported in ref. [60]. In this thesis,

the best film obtained in ref [59] was used as reference process (process 468) in order to

start the study of deposition at low temperatures.

3.3.1 Deposition conditions for the study of deposition temperature

reduction

The program starts with the selection of the initial conditions (previous section).

Once the process 468 was selected as the reference, the deposition conditions of this

process were used to deposit the films at different deposition temperatures. The deposition

conditions for process 468 are shown in the Table 3.1.

Table 3.1 Deposition conditions for intrinsic GexSi1-x:H film (process 468 with H-dilution R=75) [59]

Process

#468

QSiH4

99%

(sccm)

QGeH4

99%

(sccm)

QH2

100%

(sccm)

Pressure

(Torrs)

Frequency

(kHz)

Power

(Watts)

Deposition

Temperature

(°C)

GeSi:H 25 25 3750 0.76 110 300 300

The deposition conditions in Table 3.1 (pressure, frequency and power) were fixed

for the study of deposition temperature reduction. Changes on GeH4 flow was made due to

the changes in the gas source dilution during the experiments in this thesis. The GeH4

source was changed from 100% to 10% of GeH4 in dilution of H2 due to maintenance and

modernization procedure of the installation.. The initial conditions used in this study are

shown in Table 3.2

Table 3.2 Final fixed conditions with R=76 to the study of deposition temperature effect in deposited

films by PECVD in this work.

Process

film

QSiH4

99%

(sccm)

QGeH4

10%

(sccm)

QH2

100%

(sccm)

Pressure

(Torrs)

Frequency

(kHz)

Power

(Watts)

GeSi:H 25 250 3700 0.76 110 300


The dilution ratio was maintained closely to R~75 and it is calculated as R=QH2/(SiH4

+ GeH4) where QH2 include the flow of hydrogen in QGeH4.

3.3.2 Temperature reduction for deposition process for a-GeSi:H films

As discussed in section 2.3.1, the study of low temperature reduction in PECVD

process is important and an advantage of the low deposition temperature is the

compatibility with the flexible plastic substrates. However the temperature decrease is

typically accompanied by deterioration of the electronic characteristics of the deposited

films. We have not observed in literature a systematic study of deposition for a-GeSi:H

films at low temperatures, In this thesis the selected range of temperature reduction was of

Td=300°C to 70°C.

The Figure 3.1 shows the temperature subsystem in the PECVD installation. The

temperature system consists of a temperature controller and three heaters (Outer, central

and inner) with thermophiles in order to sense the temperature. The controller turned off or

on each heater and the deposition temperature is set in the individual controller zone to

maintain the temperature in the plate of the chamber. In this thesis, the temperature defined

as deposition temperature (Td) is the temperature set in the temperature controllers of the

installation. Table 3.3 shows the differences between Td and the measured temperature in

each heater. The measurements of temperature were made in the conditions showed in the

Table 3.2 with the power source turned off. For outer and inner heaters, the temperatures

values were above “Td + 5°C” and the central heater values were below of “Td - 10 °C”.

The installation shows reproducibility for temperature measurements in the ranges of

studied temperature.

The Figure 3.3 shows the three main configurations of the samples for deposition

experiments: a) a stripe configuration is used in the electronic characterization on glass

substrates, b) the film configuration is utilized for SIMS and FTIR characterization on glass

and silicon substrates and c) the Step configuration is used for thickness measurements on

glass substrates.


Table 3.3 Deposition temperature Td and measured temperature in each region of the bottom

electrode in the deposition chamber with vacuum conditions reached for the processes (0.7 Torrs)

Process

Temperature controllers

Outer

Set /measured + 5

(°C)

Central

Set /measured + 10

(°C)

Inner

Set /measured + 5 (°C)

857 70/ 75 70/70 70/75

858 100/105 100/100 100/105

859 130/135 130/125 130/135

860 160/165 160/150 160/165

861 190/195 190/170 190/195

862 220/225 220/200 220/225

863 250/255 250/225 250/255

864 280/285 280/250 280/285

865 300/305 300/270 300/305

Figure 3. 3 Sample configurations for temperature series characterization: a) stripe configuration:

electronic characterization b) film configuration: SIMS and FTIR and c) step configuration:

thickness measurements


3.4 Doping of a-GeSi:H films deposited by PECVD at

low temperature

At INAOE the doping control in a-GeSi:H alloys at Td=300°C has been systematic

studied [55]. The boron incorporation in solid phase and the effects on electronic properties

for a-Ge:H films deposited at Td=300°C is reported in ref [55]. The optimal deposition

temperature for doping experiments in this thesis was selected from the study of non-doped

films deposited at low deposition temperatures discussed in the previous section. The

sources of diborane and phosphine were used at high dilution of H2. The flows of silane

and germane were increased in proportion of 2 in relation to non-doped film and the doping

flow meters were selected for low flows in order to increase the accuracy of the control of

the doping gas. This is important because of the doping with boron shows a compensation

of conductivity in non-doped films from n-type to intrinsic at low doping concentrations

reported in ref [55]. The results of doping at low deposition temperature will be compared

to the doping at high temperature (Td=300°C) reported in ref. [55] to understand the

temperature effect. The sample configurations for doping series are shown in Figure 3.3

(stripe, film and step configurations).

3.4.1 Doping process by PECVD at low temperature by diborane

In this thesis the study of silicon-germanium doping at Td=300°C is considered as

reference for doping at low deposition temperatures. To improve the low concentration

control of doping gas in the chamber in the experiments it was necessary to increase the

main flows in comparison to the intrinsic films. The concentration of doping gas for boron

in the chamber was calculated as:

100*)(2

][44

62

GeSiH

HBB gas

(1)

Where, B2H6, SiH4 and GeH4 are the real concentrations of gas without dilution in

H4. The factor of 2 is used due to the configuration of the diborane molecule. The dilution


of diborane source is of 1% in H2. The deposition conditions for boron doping series are

showed in Table 3.4.

Table 3.4 Deposition doping conditions for study of boron doping

Proccess Fixed

conditions

QSiH4

(sccm)

QGeH4

(sccm)

QH2

(sccm)

QB2H6

(sccm) [B]gas%

0.99SiH4 +

0.01H 0.1GeH4 + 0.9H2 H2

0.01B2H6+0.99H2

B2H6/2(GeH4+SiH

4)

972

Power=

300Watts

Frequency=110

kHz

Pressure=0.6

Torrs

50 500 3700

0

Intrinsic

film

973 8 0.04%

974 12 0.06%

975 16 0.08%

976 20 0.10%

977 24 0.12%

978 28 0.14%

The series is prepared without interruptions of other film process from low to high

concentration of doping to prevent the contamination for the lowest concentration

processes, taking into account that the PECVD is a single chamber system.

3.4.2 Doping process by PECVD at low temperature by phosphine

The study of phosphorus doping was at the same selected temperature that boron

based in the results for the temperature reduction series. The gas concentration of

phosphorus was calculated as:

(2)

Where, PH3, SiH4 and GeH4 are the real concentrations of gas without dilution in H2.

The dilution of Phosphine source is of 1% in H2. In Table 3.5 shows the deposition

conditions used for phosphorous doping.

100)(

][44

3

%

GeHSiH

HPP gas


Table 3.5 Deposition doping conditions for study of phosphorus doping

Process Fixed conditions

QSiH4 (sccm) QGeH4 (sccm) QPH3 (sccm) [P]gas%

0.99SiH4 +

0.01H2 0.1GeH4 + 0.9H2 0.01PH3 +0.99H PH3/GeH4+SiH4

979

Power=300Watts

Frequency=110kHz

Pressure=0.6 Torr

50 500

4 0.04%

980 10 0.10%

981 16 0.16%

982 22 0.22%

3.5 Experimental methods for film characterization

The characterization of the films is principal to obtain the correlation between the

deposition conditions and the application of the films in devices. The characterization

methods were used to characterize the films in the temperature reduction and doping

programs. The deposition rate, the film composition (FTIR and SIMS), the dark

conductivity temperature dependence, the optical transmission and the photoconductivity

were studied to show the effect of deposition temperature on the films and these

characteristics were used to select the best film to demonstrate the potentially applications

in photovoltaic devices.

3.5.1 Deposition rate characterization.

The deposition rate is an important parameter of the deposition process. The effects

of temperature on the growth are discussed in section 2.2.1. In this thesis the deposition

rate was calculated from measurement of thickness of the films and deposition time. The

deposition time for all samples was 40 min and the thickness of the film was measured

with 2 separate techniques. For the films was used Veeco's Dektak 150 stylus, a non-

contact optical profiling system, with Step Height Repeatability < 6A° and the samples was

measured by Dr. Claudia Reyes at INAOE. For measurement of devices a stylus-type surf


meter was used, the measurements were performed with the collaboration of the

CINVESTAV. The sample configuration for the thickness measurements is shown in the

Figure 3c. The step in the films was formed using photolithography on a glass substrate

using a positive photoresist applied by spinning, and it was exposed to UV light. The film

was etching using a reactive ion etcher (RIE) with CF4 gas. The results of thickness were

averaged from 10 measurements in different places of the film. The mean value was used to

calculate the deposition rate as follows:

s

tV m

d2400

[A°/s] (3)

Where tm is the mean thickness and as mentioned the deposition time was fixed at 40

minutes (2400 seconds), the deposition rate is presented in A°/s for comparison to literature

and plotted as function of the deposition temperature to reveal the temperature effect on the

deposition rate.

3.5.2 Fourier transform infrared (FTIR) spectroscopy

Transmittance spectroscopy by Fourier transform infrared is commonly used to get

information on bonding hydrogen for silicon and germanium films. Also, it can be used to

determinate the coefficient absorption spectra in the sub-gap region.

IR absorption spectra for a-GeSi:H films

IR absorption lines for Ge-H bonding modes (Ge:H, Ge:H2 and Ge:H3) and Si-H

bonding modes (Si:H, Si:H2 and Si:H3) are shown in Table 3.6.

The FTIR spectroscopy can be used to detect oxygen in the films. Bonding mode for

Si:O oxygen also is shown in Table 3.6. There are two main molecular modes for each

bond type; the stretching and the deformation modes. In this work, the hydrogen content is

calculated using the region in the spectrum between 565-1

to 750 cm-1

.


Table 3.6 Main bounding modes distributions for Germanium-silicon alloys take it from [61]

Bound type Wavenumber (cm-1

) Mode

GeH 1880 stretching

570 bending

GeH2 1980 stretching

830 bending

GeH3 2050-2060 stretching

770, 830 Bending

SiH 2000 Stretching

630 Bending

SiH2

2080-2090 Stretching

680 Bending

SiH3 2120-2140 Stretching

SiO 1150 Surface

950-110 Stretching

SiHy-Ox 2100, 2185and 2160 --

The IR spectra of the films were measured using a Fourier transform infrared

spectrometer; model Vector-22 of BRUNKER Inc. The light source was a lamp of silicon

carbide and detector used (model DTGS4) has a resolution of 0.5 cm-1

allow a range of

measurements from k=10,000 to 400 cm-1

. For solve the problem of noise it was necessary

turned on the equipment four hours before the measurements in ambient of N2. and lay the

sample into the chamber 20 min before each measure. The time for the measurement

spectrums was of 5 minutes with a resolution of 5 cm-1

in the covered range from K=400

cm-1

to 2500 cm-1

.The primary data was process with a “base-line” function using the

software “OPUS” in order to subtract the background effect of the substrate. An example

of the data is shown in the Figure 3.4a.


500 1000 1500 2000 2500 3000 3500 4000

0.000

0.005

0.010

0.015

0.020

0.025

0.030

Ab

so

rban

ce (

a.u

)

Wavenumber, k, (cm-1)

857

a)

500 1000 1500 2000 2500

0

100

200

300

400

500

600

700

800

900

1000

Streching vibration

mode Ge:H

1880 cm-1

Ab

so

rpti

on

co

eff

icie

nt,

, (c

m-1)


857Bending vibration

mode Ge:H

565 cm-1

2300-2400 cm-1

b)

450 475 500 525 550 575 600 625 650 675 700 725 750

0

100

200

300

400

500

600

700

800

900

1000

Ab

so

rpti

on

co

eff

icie

nt,

, (c

m-1)


857

Equation y=y0 + (A/(w*sqrt(PI/2)))*exp(-2*((x-xc)/w)^2)

Adj. R-Square 0.99058

-- Value Standard Erro

-- y0 -17.01828 8.81336

-- xc 569.31926 0.31097

-- w 77.15416 1.08322

-- A 94844.24731 1862.50974

Bending vibration

mode Ge:H

565 cm-1

c)

1750 1800 1850 1900 1950 2000 2050 2100

0

100

200

300

400

500

Streching vibration

mode Ge:H

1880 cm-1

Ab

so

rpti

on

co

eff

icie

nt,

, (c

m-1)


857

Equation y=y0 + (A/(w*sqrt(PI/2)))*exp(-2*((x-xc)/w)^2

Adj. R-Squar 0.99807

Value Standard Err

-- y0 -10.31314 2.68816

-- xc 1875.91841 0.1291

-- w 69.35968 0.54749

-- A 41497.94823 490.55536

d)

Figure 3. 4 Fourier transform infrared spectroscopy data: a) primary data of absorbance c) absorption coefficient data, c) bonding mode for GeH and

d) stretching mode for GeH


The absorption coefficient was calculated using the following equation:

mtkA /))(1ln( (4)

Where A(k) function is the data primary and tm is the mean thickness of the sample.

The absorption coefficient is shown in Figure 3.4b. The bending and stretching modes for

Ge:H are shown in Figure 4c and Figure 4d. To determine the concentration of hydrogen

bonded, the peaks in the figures are de-convoluted. The information of this process shows a

width (w), a peak position (x) and an area under the curve (A).

Sub-gap absorption measurements by FTIR

The optical transmission spectrum in the near infrared (NIR) range from λ=0.87µm to

5µm can be used to extract the optical gap for a-GeSi:H films with a high concentration of

germanium. A NIR source is used in the FTIR spectrometer, in order to cover a range

between k=10500 to 5000 cm-1 to extract useful information about the sub-gap absorption

of the films based in germanium (Fig 3.5).

Figure 3.5 Example of absorption coefficient spectral for a-GeSi alloy with E04, E03 and Urbach

energy extraction.


As mentioned, the equipment was turned on four hours before the measurements in

ambient of N2 For solve the problem of noise and it was necessary lay the sample into the

chamber 20 min before each measure. The time for the measurement spectral was 5 min

with resolution of 5 cm-1

in the covered range from K=12500 to 2500 cm-1

. The data of

transmittance is processed with PUMA software [62] to calculate the information of

thickness, transmittance spectral T(λ), refraction index (λ) and attenuation coefficient k(λ).

The absorption coefficient is calculated as follows:

k

4 (5)

Finally, the absorption coefficient is plotted as function of the photon energy hv and

an example is shown in Figure 3.5. The Characteristics Energies for the absorption

coefficients corresponding at 103

cm-1

(E03) and 104 cm

-1

(E04) are showed. The Urbach

energy EU can be extracted using the relation )/(

exp)( ug EEhv

gh

where αg is the

absorption coefficient at hν=Eg.

3.5.3 Secondary ion mass spectroscopy

Secondary Ion Mass Spectroscopy (SIMS) is a technique used to monitor the profile

content, doping and impurities in films and structures. The technique applies an ion beam

(primary beam) to remove the atomic layers from the surface and produces particles as

electrons, photons, atoms and molecules. Some of these particles are ionized particles

(secondary beam) and they are analyzed by a mass spectrometer. The advantages of this

technique are 1) the high sensitivity, and b) a depth profile for the film composition (Fig.

3.6).


Figure 3.6 Example for SIMS depth profile for GeSi:H films in regimes Cs2 (B, C, O, Si and Ge

detection)

The SIMS Measurements were provided by Dr. Y. Kudriavtsev (CINVESTAV)

using the installation CAMECA IMS-6f ion microprobe. The intensity, energy and particles

utilized in the primary beam (regimen) and the characteristics of the mass spectrometer

determinate the particles of the secondary ion mass. In total 4 regimens were used: Cs+,

CsM+r (Cs1), Cs+,M- (Cs2) , O+ (used for oxygen ions), Cs+,M- HD (used for

phosphorous ions). The ion beam had energy from 5 to 15 KeV and a scanning area around

of 200 μm.

The composition in the bulk of different elements was estimated using the relation:

xRSFI

IC

M

Ey (5)

Where, IE is the intensity of secondary beam, IM is the intensity of matrix and RSF is

Relative Sensitivity Factor that depends of the regimen, isotope and matrix element. The

absolute Si and Ge concentrations were obtained assuming densities of Si atoms and Ge

atoms of NSi=5x1022

Atoms/cm2 and NGe=4.4x10

22 Atoms/cm

2 respectively.


3.5.4 Temperature dependence of dark conductivity

The temperature dependence of dark conductivity is a technique to extract the

electronic parameters as the dark conductivity σdark, activation energy Ea, and Fermi energy

level EF. These parameters are principal for the study of doping control in the films.

The I-V characteristics were measured using an electrometer Keithley model. 6517A

and a cryostat system of JANIS Inc. The measurements were made in the range of

temperature Tm= 27-140 °C with a maximum ramp of 1 degree/minute, vacuum of

30mTorrs, and a range of voltage from V=-10 to 10 V with step of 2mV. The Figure 3.7

shows the installation configuration for the measurements.

Figure 3.7 Experimental setup for temperature dependence of dark conductivity measurements

(JANIS cryostat system)


The conductivity was calculated by the Ohm´s law R=V/I at 5V.

)/( tLRd (6)

Where L and d are geometrical parameters described in Figure 3.3a and t is the

thickness of the sample. Each sample was measured 3 times: 1) SUB1: the first increase of

temperature to see the effect on the film properties and then the temperature was decreased

without measures. 2) S2: the second temperature increase, the sample was measured again

with the increase of temperature and finally 3) B2: the samples were measure meanwhile

the temperature was decreased. Some samples shows changes in the slope for each

measurement, an example of the primary data is in Figure 3.8.

Figure 3.8 Data primary for extraction of activation energy, a) I-V characteristic at different

temperature of measurement b) conductivity as function of the temperature for different

measurement (see text SUB1, B2 and S2)

For Fermi energy, the experimental curves can be analyzed according to the

Arrhenius expression as follows

)/(

0 exp)( kTEaT (7)


Where the activation energy (Ea) is the Fermi energy (EF) at temperature equal to

zero. Writing the temperature dependence of Fermi energy level as:

TEETETE FcFc )0()0()()( (8)

Where the term )()( TETE Fc is Fermi energy level at temperature T and χ is the

temperature coefficient the theoretical temperature dependence of conductivity can be

rewritten as:

kTEk aT//

min expexp)( (9)

And using the conventionally accepted value of σmin= 200 Ω-1

cm-1

[59], finally, the

temperature coefficient χ can be extracted by the expressionk/

min0 exp .

3.5.5 Photoconductivity measurements

The photoconductivity is an important parameter for photovoltaic applications and it

is related to the photo generation and the transport of charge in the film. The

photoconductivity spectrum in subgap region of the films is employed to determinate the

mid-gap band tail defect states and the Urbach energy EU-ph. In the Figure 3.9 it is shown

the diagram of the installation for this measurements .

Figure 3.9 Experimental installation of photoconductivity sub-gap measurements


The spectrometer used for characterizations is “model TRIAX320” with a resolution

of 0.06nm, a dispersion of 2.64 nm/mm and a focal distance of 32 cm. The lock-in

amplifier model was SR520 and it was used in a current configuration. The samples were

illuminated by halogen lamp and the photoconductivity was calculated as:

Lt

d

I

I

dc

ph

ph

)(

(10)

Where d and L are geometrical factors (see Fig. 3.3c), t is the thickness of the film,

Idc is the polarization current and Iph(λ) is the primary data. The data is normalized to the

intensity of the equipment. An example of spectra is shown in Figure 3.10.

Figure 3.10 Example of photoconductivity spectral in subgap region for a-GeSi film and Urbach

energy EU-ph extracted from photoconductivity [63]

The Figure 3.10 shows the extraction of Urbach energy of photoconductivity spectral

EU-ph, this photoconductivity has implicit the influence of the transport electronic

characteristic of the films (ημτ-quantum efficiency, mobility and recombination time

product). The extraction of this energy is from the relation:


phUg EEhv

phoh/)(

exp)(

(11)

Where Eg is the optical gap, σph is the photoconductivity at hν=Eg and hν is the photon

energy. For details of data processing see reference [63]. The photoconductivity

measurements in this work were performed by Francisco Temoltzi.

3.6 Fabrication of devices based on a-GeSi:H films

deposited by PECVD at low temperature

A-GeSi:H alloys provide a short length absorption for IR applications and These are

compatible with technology of amorphous silicon. As mentioned in Chapter 2, the real

problem of germanium technology is the not sufficient knowledge in the study of

deposition of intrinsic and doped germanium films. Using the results in this work for

germanium films, we propose a device to demonstrate the application of the studied films.

The results for the devices structures fabricated in this thesis are presented in Chapter 5.

3.6.1 Device structures

In the Figure 3.11, three structures for device are proposed.

Figure 3.11 The proposed structures for device application in this work are: a) (BASIC) p-i-n

structure based totally in a-GeSi:H films, b) (ANTID) p-i-n structure with Silicon anti-diffusion

layers and c) p-i-i-n structure with a-SiGe and a-Si absorption layer (HIT structure).


Figure 3.11a shows the BASIC structure; this structure changes during this work to

the other two structures (see Chapter 5). The structure in the Figure 3.11b (ANTID) is a p-

i-n structure with two anti-diffusion silicon layers in order to avoid the diffusion of dopants

through the structure. The final structure is identified as “HIT” structure (Hetero-junction

with Intrinsic Thin Layer). We know that the HIT label is used only for Hetero-junction

between crystalline and amorphous thin films however we used this label for our structure

for simplicity and because the acronym is not contradictory to the structure. The HIT

structure is shown in Figure 3.11c. This structure has two absorption intrinsic layers (a-

GeSi and a-Si) with the configuration: Substrate-(pGeSi)-(iGeSi)-(iSi)-(nGeSi). The a-

GeSi:H films used in the devices were taken from the result of this thesis in chapter 4 and

silicon film was taken from a previous study in the INAOE.

3.6.2 Deposition conditions for the device fabrication

The deposition conditions for the initial structure (process 1004) are shown in Table

3.7. The conditions were selected from the study of deposition temperature reduction

program in chapter 4:

Table 3.7 Selected process conditions from deposition temperature reduction used for the first

structure (BASIC structure)

Process

Fixed conditions QSiH4

99.99% Si

(sccm)

QGeH4

10 % Ge

(sccm)

QH2

100%

(sccm)

QPH3

1%

(sccm)

QBH3

1%

(sccm)

P a-GeSiB:H

(978) Power=300Watts

Temp=160 °C

F=110 KHz

P=0.76 Torrs

50 500 3700 0 28

I a-SiGe:H

(972) 50 500 3700 0 0

N a-SiGeP:H

(981) 50 500 3700 16 0

The power, deposition temperature, frequency and pressure were constant conditions,

while the flows were changing during the study. The deposition conditions for the anti-

diffusion silicon and the absorption silicon layers are shown in the Table 3.8.


Table 3.8 Process condition for the silicon films used as anti-diffusion and absorption layer in the

structures.

Process

1004

Fixed conditions

(see table 3.7)

QSiH4

99.99%

(sccm)

QGeH4

10 %

(sccm)

QH2

100%

(sccm)

i-Si Si:H P=1.4 Torr 50 0 1000

The conditions for silicon deposition were changed during the study for several

reasons (Chapter 5). The deposition conditions are similar for a-GeSi and a-Si films except

for pressure, for a-Si the pressure was 1.4 Torr and for a-GeSi was 0.6 Torrs The objective

the device study was to obtain a photovoltaic response using the a-GeSi:H films obtained in

the study of deposition temperature reduction and the study of doping. The methodology of

this section of the program can be described as follows: 1) it is suggested a structure with a

hypothesis to increase the electronic response of the structures, 2) the suggested structure is

fabricated, 3) a characterization stage is performed in order to identified the problems, and

4) a change in the flow process are proposed as result of the characterization stage

3.6.3 Flow charts for devices fabrication

A main problem in the device fabrication is the contamination through the deposition

processes of each layer (s-, p-. and i-) in a single chamber configuration. In the Figure 3.12

is shown the methodology used for the process flow in the device fabrication. The main

implementations are flashes of H2 to prevent contamination and the interruption of power

during the film growing to prevent void continuity in the intrinsic GeSi:H films. For the

used conditions, the plasma of H has not shown etching of the films.

P-film deposition flow

The device fabrication starts with the deposition of the p-film. The gas flows was set

before turn on the power source. Once the flows were constant in the chamber the power

was turn on and the deposition of the film starts.


Figure 3.12 Processes flow for devices fabrication methodology to prevent cross contamination (n - p-

and intrinsic flow processes); main implementations are Flash stages of H2 during the process and

power interruptions to prevent voids in intrinsic films.


To finalize the process deposition, the flows of silane, germane and diborane are

interrupted and only the H2 flow is maintained in the chamber as flash gas. The power

source is turned off after 5 min of the first flash. The process finalizes with 2 more flashes

stages of H2 to prevent the contamination in the next layers (Fig 3.12).

Intrinsic film deposition process

The intrinsic a-GeSi:H film process starts with set of temperature, pressure and flows

in the chamber. Then the power source is turned on for growing. During the initial time of

the process, the power is interrupted four times to prevent the voids continuity and the short

circuits. After the interruption, the growth process continues. To finalize, the silane and

germane flows are closed and a flash of H2 is maintained in the chamber. After 5 minutes

the power source is turned off.

N film deposition process

N-type film deposition starts with the flows in the chamber. The power source is

turned on and the growth is continuous. To finalize the phosphine, silane and germane

flows are closed and a flash of H2 flow is maintained in the chamber. After 5 minutes the

power source is turned off.

The devices have two transparent contacts of titanium deposited by the same

technique that the contact of the films in the stripe configuration. The thickness of the

Contacts is of 70 A° to get transmittance of 70 %. The configuration of the devices is

shows in the Figure 3.13

Figure 3.13 Sample configuration for devices with transparent titanium contacts


3.7 Characterization methods for devices

Some techniques for characterization of the films were applied in devices

characterization as SIMS and photoconductivity measurements. The J-V characteristics

were used to extract the short circuit current density Jsc and the open circuit voltage Voc.

3.7.1 The mapping methodology of electronic characteristic for devices

The J-V characteristics were made with three illumination types: the “H_3V”

illumination made with a halogen lamp with intensity of 100 mW/cm2. This illumination is

the most simple and easy to make in INAOE lab, the second illumination (SUN) was direct

to the sun with an intensity measured of 100 mW/cm2. The third illumination (H_8V) was a

shift of H_3V to the sun response. The shift was made with a resulted intensity of 500

mW/cm2 with the halogen lamp and it is shown in the Figure 3.14. The J-V characteristics

were measured with the “H_8V” illumination in the lab in order to get the control of the

reproduction in the lab.

Figure 3.14 Three illuminations types for device; H_3V (Halogen lamp 100 mW/cm 2), H_8V(Halogen

lamp 500 mW/cm2) and Sun lighting (100 mW/cm2)


The H_3V illumination was used for mapping. The incident intensity was measured

with thermopile as:

areaActivesponsivity

CurrentThermopileIntensity

_Re

_

(12)

The thermophile sensor used to measure the intensity was model “71938” with a

responsivity R=260 + 40 μA/W in the wavelength range from λ=0.13 to 11 μm and an

active area of A=2.8x10-3

cm2. The results of mapping were used for calculate process

YIELD.

3.7.2 Spectral response for devices

The spectral response of Isc in sub gap region was analyzed for some representative

devices using the installation showed in the Figure 3.15.

Figure 3.15 Experimental installation for spectral Isc response for devices (measurements performed

by Francisco Temotzil)

The spectrometer used is model “TRIAX320” with a resolution for 0.06nm,

dispersion of 2.64 nm/mm and a focal distance of 32 cm. The short circuit current was

measured with the electrometer Keithley model. 6517A. The samples were illuminated by a

halogen lamp with a density power of 1 mWatts/cm2. Finally, the current was plotted

versus photon energy.

P a g e | 49

4. FILMS EXPERIMENTAL RESULTS

4.1 Introduction

The deposition temperature is the main parameter of PECVD process and the

technique allows the study of depositions at low temperatures. In this chapter the low

results of temperature deposition are presented. This chapter is divided in two section: 1)

the effect of the temperature reduction on the characteristics of intrinsic GeSi:H films and

2) the study of doping at low deposition temperature. The films are applied in devices and

this is presented in the Chapter 5.

4.2 Results of temperature reduction effect on

intrinsic a-GeSi:H films

In this section, the results of the temperature reduction are reported. The films were

deposited by PECVD in the temperature range from Td= 300 to 70°C and the others

fabrication parameters were fixed (see section 3.3). All experiments were on glass

substrates (Corning), unless some notification specifies other substrate.

4.2.1 Fabrication results for temperature reduction

In the Figure 4.1 is shown series of experimental samples for the fabrication process

at Td= 70°C. The films do not show visible problems of adhesion on glass substrate,

crystalline silicon wafer or plastic substrates (polyimide or PEN). However, the samples on

polyimide substrates show a considerable deformation at all tested temperatures (70°C,

100°C, 130°C, 160°C and 190°C). The comparison of this deformation for Td= 70°C and

190°C is shown in Figure 4.2. This effect is not observed in the films deposited on PEN

substrate (Fig 4.3).

P a g e | 50 CHAPTER 4: FILMS EXPERIMETAL RESULTS

Figure 4.1 Fabrication results at low deposition temperature; study of effect of deposition

temperature in the range from Td=300°C to 70°C.

a)

b)

Figure 4.2Films deposited on polyimide substrates at deposition temperatures: a) 70°C and b) 190°C,

the deformation increases with the temperature.


Figure 4.3 Sample on PEN substrate at deposition temperature Td=190°C, no visible deformation for

any tested temperature and it shows wide flexibility without adhesion problems.

The Figure 4.2 shows the increase of deformation with the increase of the deposition

temperature in the samples on polyimide substrates. The deformation in the samples

deposited at Td=70°C is located in the center of the sample; this deformation is due to the

temperature, stress and the way that the sample was fixed in the chamber (the ends of the

samples were fixed but not the center - see Fig. 4.1). The sample at deposition temperature

Td=190°C shows an increase of deformation although the transition temperature of the

substrates is above Tg> 360°C and it is twisted by the increase of temperature (the samples

were fixed in the same way for all series). Despite this large deformation, the deposited

films show a good adhesion on the polyimide substrates for all deposition temperatures.

In the Figure 4.3 is shown a sample on PEN substrate deposited at the temperature

Td=190°C (the highest tested temperature for flexible substrates). The samples on PEN do

not show visible deformation at any tested deposition temperature. However a further study

is recommended.

Films on glass and crystalline silicon were included in all deposition processes and

the films deposited on these substrates don’t show visible problems and present good

adhesion within the studied temperature range. The temperatures studied were 857(70°C),

858(100°C), 859(130°C), 860(160°C), 861(190°C), 862(220°C), 863(250°C), 864(280°C)

and 865(300°C). The fixed parameters were Power=300 Watts, Frequency=110KHz,

Pressure=0.6Torr, QSiH4=25 sccm (99%), QSiH4=250 sccm (10%) and QH2=3700 sccm. The


H dilution ratio was calculated as R=QH2/(SiH4 + GeH4)=76. The deposition time for all

processes was fixed in 40 minutes.

4.2.2 Deposition rate as a function of the deposition temperature

In Figure 4.4 is shown the deposition rate as function of the deposition temperature.

The maximum value of the deposition rate was Vd=1.45 A°/s and it was obtained at the

deposition temperature Td=190 °C. The minimum value of the deposition rate was Vd=0.91

A°/s and it was obtained at Td=280°C. Other important result is the value at the highest

temperature Td=300°C and the lowest temperature Td=70°C, with values of Vd=1.08 A°/s

and Vd=1.25 A°/s respectively.

Process

Temperature

deposition

(°C)

Deposit

rate

(A°/s)

857 70 1.25

858 100 1.12

859 130 1.25

860 160 1.33

861 190 1.45(H)

862 220 1.29

863 250 0.95

864 280 0.91(L)

865 300 1.08

OPTICAL PROFILE Td=70°C

Figure 4.4 Deposition rate of a-GeSi films as function of the deposition temperature in the range from

Td= 70°C to 300°C deposited by PECVD. (OPTICAL PROFILE for sample deposited at Td=70°C)


The deposition rate shows discontinuous change in relation to the increases of the

deposition temperature. The highest values are obtained at the deposition temperatures

between the range 160°C >Td >220°C and the lowest deposition rates are obtained at

temperatures above Td>250°C. The deposition rates were within the range from Vd=0.91

A°/s to 1.45 A°/s and it shows indeterminate effect of the deposition temperature. The

optical profile for the lowest deposition temperature Td=70°C is shown in the Figure 4.4,

the a-GeSi:H films on glass substrate show visible uniformity and not adhesion problems.

This result is observed for all deposition temperature studied in this work.

4.2.3 Hydrogen bonding as a function of the deposition temperature

The study of hydrogen bonding by FTIR is used to calculate the relative hydrogen

content in the films . In Figure 4.5 is shown the absorption spectra of each sample. The

spectra show the main peaks for germanium-hydrogen bounding; bending mode (k= 565

cm-1

) and stretching mode (k=1880 cm-1

). Not absorption peaks in relation to silicon-

hydrogen absorption is observed. The stretching mode Si-H k=2000 cm-1

and bending mode

k=630 cm-1

are not observed. This is because of the high concentration of germanium,

which is more than 95% of the composition of the films, and the low concentration of

silicon in solid phase.

A specific absorption peak is observed at deposition temperatures 130°C, 280°C and

300°C, this peak corresponds to the O-H stretching mode (k~1100 cm-1

). The

measurements of absorption show clear spectra with dominant peaks of monohydrate Ge-H

bond. This mode is a characteristic of films of good quality. The Figure 4.6a shows Ge-H

stretching mode peaks at Td=70°C and Td=300°C in the selected region k=1800 to 2000 cm-

1 and Figure 4.6b shows hydrogen content CH as a function of the deposition temperature.

The Figure 4.6a shows the IR spectrum for the film deposited at Td= 70 °C with a higher

concentration of Ge-H bonds (i.e. hydrogen content) in comparison to the sample deposited

at Td= 300 °C, both spectra demonstrate the Ge-H stretching mode at k ≈ 1875 cm-1

. The

shift of the line position of the peak from k=1872 cm-1

at Td=70°C to k=1875 cm-1

at

Td=300°C suggests some change in the lattice structure in vicinity of Ge-H bonding.


Figure 4.5 FTIR absorption spectra for a-GeSi:H films deposited by PECVD at deposition

temperature range from Td=70°C to 300°C


Figure 4.6Hydrogen content calculated by FTIR a) Ge-H stretching mode peaks for a-GeSi:H films at

the deposition temperatures 70°C and 300°C, b) relative hydrogen bonding content versus deposition

temperature.

The Figure 4.6b demonstrates a general decrease of bonded hydrogen content with

an increase of the temperature except for the sample at deposition temperature Td=250 °C

that shows an abrupt increase in hydrogen content, this results are discussed in chapter 6.

The relation of hydrogen content at the highest (300°C) and the lowest (70°C) temperatures

is RHT=CH70/CH300=1.46. The process at Td=160°C is not included in hydrogen content

study due to an accidental problem with the sample on c-Si.

4.2.4 Sub-gap absorption measurements by FTIR

The absorption coefficient for the experiment of temperature reduction is shown in

Figure 4.7. The spectral dependence of absorption coefficient, α(hv), in sub-gap absorption

region for different deposition temperatures, is plotted. The filled symbols in the Fig. 4.7

show the highest and lowest deposition temperature for the series. The α(hv) curves could

be described by three main regions due to the transitions : a) extended to extended states:

band-to-band transitions that correspond to the valence band to conduction band,

b)extended to localized states and vice versa: transitions related to valence band tails, and

conduction bands tails and c) localized to localized: defect related to the transitions.


Figure 4.7 Spectral dependence of optical absorption coefficient for different deposition temperatures

extracted by FTIR transmission measurements.

From the absorption spectra can be extracted the characteristic energies related to the

extended states. This energies are the E04 and E03 energies that correspond to the absorption

coefficients α=10-4

cm-1

and α=10-3

cm-1

, respectively, and the Tauc optical gap EgTAUC

.

The characteristic energies for the localized states are: 1) ΔE= E04- E03, it is the energy

described by the slope of α(hv), and 2) The Urbach energy EU that describe the

exponential decreases of the tails in the states near to the valence band and conduction

band edges. The optical gap in this thesis is characterized by E04 and EgTAUC

energies and

these are plotted in Figure 4.8 versus the deposition temperature.


Figure 4.8 Optical gap characterized by E04 and EgTAUC energies as function of the deposition

temperature for a-GeSi:H films . The solid lines are guide to the eyes.

In Fig. 4.8, the energy E04 shows a minimum value E04=1.09 eV at the deposition

temperature Td=190°C and a maximum value E04=1.2 eV at Td=280°C. In the extremes of

the temperature, the values of E04 energies are E04=1.11 eV (Td=70°C) and E04=1.19 eV

(Td=300°C). An increase of the energy E04 is shown at the deposition temperatures

Td>190°C. For Tauc energies, the minimum value EgTAUC

=0.90 eV is obtained at the

deposition temperature Td=190°C and the maximum value EgTAUC

= 1.01 eV is obtained at

Td=280 °C coinciding with E04 energy. The lowest temperature (Td=70°C) shows a value of

EgTAUC

= 0.98 eV and the highest temperature (Td=300°C) shows a value of EgTAUC

= 1.00

eV. The maximum values are obtained at deposition temperature Td>250°C. Both data

show discontinuous changes with the increases of the deposition temperature and they

show a similar behavior.

The Urbach energy EU and the ΔE characteristic energy are shown in Figure 4.8 as

function of the deposition temperature. The energies are plotted independently to distinct

tendencies.


a)

b)

Figure 4.9 a) Urbach energy EU and b) Characteristic energy ΔE=E04-E03 as function of deposition

temperature for GeSi:H films extracted from absorption coefficient spectra.

The Urbach energy is plotted in Figure 4.9a as function of the deposition temperature

Td. The Lower values of Urbach energy have relation to the good electronic properties of

the films. The lowest value EU=33 meV is obtained at the deposition temperature Td=

160°C. The best values are obtained at the deposition temperature range from Td=160 to

220°C. An interesting result is that the highest values of Urbach energy are not obtained at

the lowest deposition temperature. The tendency of the Urbach energy is discontinuous

with the decrease of the deposition temperature. The ΔE characteristic energy has relation

to the slope in the distribution of states within the optical gap. Low ΔE energy has relation

to low density of states (DOS). The energy ΔE is shown in Fig. 4.9b as function of the

deposition temperature. The minimum value ΔE= 50 mV is obtained at the deposition

temperature Td=190°C. The Lowest values are obtained in the region from Td=160 to

220°C in correlation to the Urbach energy data. The data of ΔE shows a discontinuous

change with the deposition temperature and the maximum value is obtained at the lowest

temperature Td=70°C. The refraction index n∞ is calculated and plotted as a function of

the deposition temperature in Figure 4.10. This parameter reveals information of the

density of the films. The maximum value is obtained at the deposition temperature

Td=280°C and the minimum value of refraction index is obtained at the deposition

temperature Td=160°C.


Figure 4.10Refractive index n∞ as function of the deposition temperature for GeSi:H

Decrease of the refraction index is observed for the deposition temperatures between

Td=300°C to 160°C then it changes of trend from n∞= 3.92 to 4.1 for the low temperatures

from. This changes in the refraction in the range of n∞=3.93 to 4.18 reveals a strong effect

of the deposition temperature on the film structure. The temperature extremes show values

from n∞=4.06 at the deposition temperature Td=70°C to n∞=4.1 at the deposition

temperature Td=300°C. The results of optical characterization are concentrated in Table

4.1.

Table 4.1Energys characteristic and refraction index extracted by sub-gap absorption measurements

by FTIR

Process

Deposition

Temperature

(°C )

EgTAUC

(eV)

E04

(eV)

EU

(meV)

ΔE

(meV) n∞

857 70 1.00 1.19 50 260 4.06

858 100 1.01 1.2 51 130 3.98

859 130 0.95 1.17 72 170 4.03

860 160 1.00 1.14 33 90 3.92

861 190 0.90 1.09 43 50 3.95

862 220 0.95 1.13 43 90 3.99

863 250 0.97 1.12 77 180 4.02

864 280 0.98 1.13 71 210 4.18

865 300 0.98 1.11 76 140 4.1


4.2.5 Electronic properties as a function of the deposition temperature

The temperature dependence of dark conductivity σ(T) was used to characterize the

electronic properties of the films as function the deposition temperature Td. The samples

were measured with a temperature measurement range from Tm=27 °C to 140°C, some

samples (857, 858 and 859) were deposited at lower temperatures Td than the temperature

of measurement due to this annealing effect is observed for the samples. In the Figure

4.11 is shown the temperature (Tm) dependence of dark conductivity. Each sample was

measured tree times (see section 3.5.4); increasing (SUB1 and S2) and decreasing (B2) the

temperature.

Figure 4. 11 Temperature dependence of the dark conductivity, SUB1 (increase of measurement

temperature), S2 (second increase of measurement temperature) and B2 (final decrease of

measurement temperature)


Fig. 4.11 shows the comparison of temperature dependence of the dark conductivity

for SUB1, S2 and B2 slopes. It shows that for low deposition temperatures Td<200°C, the

slope is modified by measurement temperature, this effect is high at deposition

temperatures Td=70°C and Td=130°C.The changes could be due that the temperature in the

measurement is higher than deposition temperature and this produces changes in the

structure of the films. In the Figure 4.12 is shown the dark conductivity at room

temperature as a function of deposition temperature.

Figure 4.12 Dark conductivity as function of the deposition temperature.

In the plot of dark conductivity in Fig. 4.12 is observed a minimum value of

conductivity of σdark=5x10-5

Ω-1

cm-1

at the deposition temperature Td=190°C and a

maximum value of σdark=9.6x10-4

Ω-1

cm-1

at the deposition temperature Td=300 °C. The

ratio of the maximum and minimum values obtained is about one order of magnitude. For

the lowest deposition temperature Td=70°C the dark conductivity value is σdark=3.48x10-4

Ω-1

cm-1

and the highest deposition temperature has a value of σdark=9.6x10-4

Ω-1

cm-1

. The

tendency of the dark conductivity is discontinuous with the increase of the deposition

temperature. At lower temperatures, the increase of deposition temperature from Td=70 to

220°C results in a decrease of the dark conductivity and the increase of deposition


temperature from Td=220°C to 300°C results in a change of tendency with the increase of

dark conductivity.

Thermal activation energy Ea and Fermi Energy level EF is plotted as function of the

deposition temperature in Figure 4.13. The activation energy and Fermi Energy are

calculated from the slope B2. In Table 4.2 is showed values for SUB1, S2 and B2.

Figure 4.13 Activation energy Ea and Fermi Energy EF as function of the deposition temperature Td

Fig. 4.13 shows the minimum values of activation energy Ea=0.23 eV and Fermi

level EF=0.16 eV at the highest temperature Td=300°C. The maximum values, Ea=0.38eV

and EF=0.37 eV, were obtained at the deposition temperature Td=220°C. The lowest

deposition temperature Td=70°C results in values of Ea=0.3 eV and EF=0.25 eV.

Two regions are identified in the tendency of the curves, constant values are obtained

in region 1 for low deposition temperatures Td<160°C and region 2 for high deposition

temperatures Td>160°C. The activation energy and Fermi level shows a discontinuous

change, The energies are more sensitive at deposition temperatures within region 2 (Td>160

°C) .


The temperature coefficient was calculated and the maximum value of χ = 22.9x10-5

eV/K was obtained at the deposition temperature Td=300°C. The minimum temperature

coefficient χ = 0.62x10-5

eV/K was obtained at Td=220° and a clear tendency is not

observed in relation to the deposition temperature.

Table 4.2Electrical parameters of a-GeSi films for temperature reductions.

Process

Deposition

temperature

(°C)

σdark

(10-4

Ω-1

cm-1)

σ0

(Ω-1

cm-1

)

χ (10

-5 eV/K)

Ea

(eV)

B2-SUB1

EF

(eV)

857 70 3.48

29 16.6 0.30-0.29 0.25

858 100 2.63 31 16 0.30 0.25

859 130 2.68 38 14.3 0.30-0.29 0.25

860 160 1.3 37 14.5 0.32-0.30 0.27

861 190 0.57 20 19.84 0.33 0.27

862 220 0.58 186 0.62 0.38 0.37

863 250 4.6 18 20 0.27 0.21

864 280 4.5 30 16.3 0.29 0.24

865 300 9.6 14 22.9 0.23 0.16

4.2.6 Photoconductivity measurements

Spectral photoconductivity response was measured for samples deposit at low

temperature (LT samples). The Urbach energy was extracted from the sub-gap region and

this energy was defined as EU-ph. In Figure 4.14 is shown the spectral dependence of

photoconductivity for different deposition temperatures. The photoconductivity values were

in between the ranges from σph=7.5x10-8

to 4.9x10-7

Ω-1

cm-1

. The lowest value was

obtained at the deposition temperature Td=160°C and the highest value was obtained at the

deposition temperature Td=300°C. An increase of conductivity is observed with the

increase of temperature in the region from Td=160 to 300°C (Fig. 4.14b). The “Photo”

Urbach energy is plotted in Fig. 4.14b, the lowest values were obtained at low deposition

temperatures, and a clear trend is not observed..


a)

b)

Figure 4.14 Results of photo-conductivity characterization: a) photo-conductivity spectra at different

deposition temperature b) “photo” Urbach energy and photoconductivity as function of the deposition

temperature


Table 4.3Photo-electrical parameters for different deposition temperatures.

Deposition

Temperature

(°C)

Eu-ph

(meV)

σph

[hv~1.1eV]

(10-7

Ω-1

cm-1

)

σph/σdark

(10-4

)

857 70 50 2.13 0.61

858 100 50 1.2 0.45

859 130 62 0.77 0.28

860 160 50 0.75 0.76

861 190 50 1.36 2.39

862 220 100 1.27 2.19

863 250 68 2.6 0.56

864 280 66 4.5 1.00

865 300 52 4.9 0.51

Photo-electrical characteristics are summarized in Table 4.3. The photo-sensitivity

σph/σdark was calculated and the values obtained were in the range from σph/σdark= 4.5x10-3

to

2.39x10-4

. The Maximum photosensitivity was obtained at the deposition temperature

Td=190°C.

4.3 Doping results for a-GeSi:H films deposited at

low temperature

Results of the study of low deposition temperature in previous section demonstrated a

low value of Urbach energy EU=33 meV for the sample deposited at Td=160°C. The lowest

dark conductivity (σdark) and photosensitivity (σph/σdark) values are obtained in the range of

temperature from Td =130°C to220°C. For this reason, the deposition temperature

Td=160°C was selected for the study of doping (see discussion Chapter 6).

In this section, the results of doping at low deposition temperature Td=160°C are

reported. The studied range for boron doping was from [B]gas= 0 to 0.14% and phosphorus

doping was studied in the ranges from [P]gas= 0 to 0.22 %. The composition, incorporation

of boron and phosphorus in solid phase was obtained by SIMS.


4.3.1 Fabrication results for doping at low deposition temperature

At deposition temperature Td=160°C the process of deposition demonstrated

compatibility with plastic substrates and the deposited films have not visible problems of

adhesion on glass substrate, crystalline silicon wafer or plastic substrates (polyimide or

PEN). With the deposition temperature selected at Td=160°C others parameters were fixed

as follows: H dilution R= QH2/( QSiH4+QGeH4)=76, QSiH4 =50 sccm, QGeH4= 500 sccm,

discharge power 300 Watts and pressure P= 0.76 Torr. The processes of doping are

summaries in Table 4.4 for both phosphorus and boron doping..

Table 4.4 Doping range in fabrication processes for the study of doping

PROCESS [B]gas% PROCESS [P]gas%

973 0.04% 979 0.04%

974 0.06% 980 0.10%

975 0.08% 981 0.16%

976 0.10% 982 0.22%

977 0.12% 972 Intrinsic film

978 0.14%

4.3.2 Deposition rate results for doping at low temperature

In Fig. 4.15 is shown the deposition rate for doping at low deposition temperature.

Figure 4.15 Deposition rate as function of a) boron and b) phosphorus gas concentrations for GeSi:H

films deposited at Td=160°C


Figure 4.15 shows the deposition rate for boron and phosphorous doping films

deposited at low deposition temperature Td=160°C. For doped a-GeSi:H films with boron,

the deposition rate changed slightly in the range from Vd=1.2 to 1.4 A°/s. For doped a-

GeSi:H films with phosphorus, the deposition rate changes in the range Vd =1.3 to 1.4 A°/s.

The results of deposition rate show a discontinuous trend as function of doping, both

boron or phosphorous in gas phase.

4.3.3 Composition study by SIMS for doping at low deposition

temperature

The composition of the intrinsic films and the incorporation of dopants in solid phase

for doping at low deposition temperature Td=160°C were determined by SIMS. In Figure

4.16 is shown the SIMS profile for intrinsic film deposited at Td=160°C. The results of

SIMS shows that the composition of intrinsic film deposited at low temperature Td=160°C

has a composition as GexSi1-x:H with Ge concentration in the range from x=0.95 to 0.99.

Figure 4.16 SIMS profile for intrinsic a-GeSi film deposited at deposition temperature Td=160°C,

hydrogen, carbon, oxygen, silicon and germanium concentrations


The concentration of hydrogen in solid phase was 5.59x1021

Atoms/cm3 with a

relation as

%11

HSiGe

HH

NNN

NC

The oxygen concentration in solid phase was 2.8x1020

Atoms/cm3 with a relation: as:

%5.00

OSiGe

ONNN

NC

Other important impurity was carbon with a concentration of 4.3x1018

Atoms/cm3.

The profile for the highest doping concentration with diborane and phosphine in this study

is shown in Figure 4.17. The highest dopants concentrations obtained for doping with

boron was 1.8x1020

Atoms/cm3 in the sample 971 and for phosphorus doping was 2x10

20

Atoms/cm3 for sample 982.

Figure 4.17 SIMS profiles; concentration versus sputtering time for doping series a) phosphorus

doping and B)boron doping

In Table 4.5 is shown the concentrations of dopants for doping. The results are

plotted in Fig. 4.17. The factor of incorporation for doping is calculated as the slope of the

relation between the incorporation in solid phase from gas phase.


Table 4.5 Results of solid incorporation for boron doping and phosphorus doping

Boron series Phosphorus series

Sample [B] Gas

%

[B] Solid

% Sample

[P] Gas

%

[P] Solid

%

973 0.04 0.29 979 0.04 0.2471

974 0.06 0.26 980 0.08 0.486

975 0.08 0.37 981 0.12 0.6121

976 0.1 0.65 982 0.16 0.7467

977 0.12 0.51 -- -- --

978 0.14 0.70 -- -- --

Figure 4.18 Incorporation of dopants in solid phase as a function of dopants gas concentration, the

solid concentration was determined by SIMS for boron and phosphorous series.

The Figure 4.18a shows the boron incorporation from gas phase to solid phase. The

incorporation of boron at low deposition temperature Td=160°C for a-GeSi:H films was

described by the linear fit as [B]sol= KBi [B]gas (KBi= 4.8 ± 0.7). The incorporation of

phosphorous from gas phase to solid phase in a-GeSi films is shown in Figure 4.18b. The


fit for n-type a-GeSi:H films is in the range of [P]gas= 0.04 to 0.22 % and it describes the

incorporation of phosphorous in solid phase as [P]sol= KPi[P]gas(KPi= 2.77 ± 0.02)

4.3.4 Electronic properties as a function of dopant concentration

Temperature dependence of dark conductivity σdark(T) were used to characterize the

electronic properties. The samples were characterized with a temperature measurement in

the range from Tm=27 to 140°C. The electronic characterization is important for the design

of device based in p-i-n structures; The dark conductivity σdark, activation energy Ea and

Fermi energy level EF was extracted for boron and phosphorous doped films.

In Figure 4.19 is shown the electronic properties as a function of the gas

concentration for boron doping.

Figure 4.19 Electronic properties as a function of the boron concentration in gas phase a) dark

conductivity σdark and b) Activation energy Ea and Fermi energy level EF for GeSi:H films deposited

at low deposition temperature Td=160°C

The dark conductivity σdark for boron films is shown in Figure 4.19a. The changes in

orders of magnitude of the dark conductivity demonstrate doping. The changes in the

activation energy demonstrates the control of doping in the films deposited at low


deposition temperature Td=160°C. The dark conductivity σdark shows a drastic reduction

from σdark= 10-4

to 10-7

Ω-1

cm-1

with an increase of boron from [B]gas= 0 to 0.04%- This

effect suggests a compensation of electron conductivity in non-doped films and it has

correlation with maximum the activation energy Ea=52 eV and the lowest value for dark

conductivity σdark=10-7

Ω-1

cm-1

with [B]gas=0.04%.. The increase from [B]gas=0.04 to 0.14%

results in an increase of conductivity from σdark= 10-7

to 10-4

Ω-1

cm-1

. The Figure 4.19b

shows a discontinuous change in the activation energy and Fermi level position, both

parameters show an increase of energy from Ea=0.32 to 0.53 eV and EF= 0.29 to 0.52 eV

respectively with the increase of boro gas concentration from [B]gas= 0 to 0.04%. Then a

decrease in the activation energy Ea= 0.33 eV and Fermi energy level EF= 0.27 eV is show

with the increase of boron gas concentration from [B]gas = 0.04 to 0.14%. Figure 4.20

shows the electronic characterization for phosphorus series.

Figure 4.20 Electronic properties as a function of the phosphorus concentration in gas phase a) dark

conductivity σdark and b) Activation energy Ea and Fermi energy level EF for GeSi:H films deposited

at low deposition temperature Td=160°C

Dark conductivity σdark for phosphorus doping is shown in the Figure 4.20a. For

changes of [P]gas from 0% to 0.22%, the dark conductivity changes in more than 2 orders of

magnitude from σdark =10-4

to 10-2

Ω-1

cm-1

and the activation energy decreases from

Ea=0.32 to 0.18 eV. The films demonstrated saturation of the conductivity with change in


one order of magnitude. The changes in orders of magnitude demonstrate doping and

control of the electronic characteristics for phosphorous doping at low deposition


Table 4.6 Summary of the electronic characteristics extracted from n-type films.

phosphorous doping

Process [P]gas% σdark

(Ω-1 cm

-1)

σ0 (Ω-1

cm-1

)

χ (x10-5

eV/K)

Ea (eV) Ef (eV) 300K

979 0.04 1.8x10-3

18.17 20 0.24 -0.18

980 0.08 3.8x10-3

13.4 23 0.21 -0.14

981 0.12 8x10-3

16.4 21 0.19 -0.12

982 0.16 1.8x10-2

18.1 20 0.18 -0.12

Table 4.7 Summary of the electrical characteristic extracted from p-type films

boron doping

Process [B]gas% σdark

(Ω-1 cm

-1)

σ0

(Ω-1

cm-1

)

χ

(10-5

eV/K)

Ea

(eV)

Ef

(eV)

972 intrinsic 2.3x10-4

81.4 7.7 0.32 0.29

973 0.04 1.2x10-7

148.4 2.5 0.53 0.52

974 0.06 3.3x10-6

12.1 23 0.39 0.32

975 0.08 5.1x10-6

49.4 12 0.39 0.35

976 0.1 1.2x10-6

29.9 16 0.44 0.39

977 0.12 1.7x10-5

244.6 1.7 0.41 0.40

978 0.14 1.5x10-4

44.7 12.9 0.31 0.27


In Tables 4.6 and 4.7 is shown the summary of electronic parameters for boron and

phosphorous doping at low deposition temperature.

P a g e | 74

5. DEVICES: EXPERIMENTAL

RESULTS

5.1Introduction

Amorphous germanium-silicon films deposited by low temperature process (Td<

200°C) are of much interest because they provide a narrow gap compatible with a-Si:H

films. These films have potential applications for devices on plastic substrates. The films

of a-GeSi:H can be used for different device applications i.e., p-i-n solar cells. In the

previous chapter, the experimental results of film fabrication were presented. The intrinsic

and doped films in this thesis were obtained at low deposition temperature Td=160°C. In

this chapter the experimental results of p-i-n structures based on the GeSi:H films are

presented. The fabrication process was optimized for low deposition temperature

Td=160°C and we describe the experimental results on fabrication and the study of PV

structures based on a-GeSi:H films deposited at Td=160°C. As mentioned in the section

3.6.1, the fabricated p-i-n structures include three structures: “BASIC structure” based only

on a-GeSi:H films (Fig. 3.11a), ANTID structure based on a-GeSi:H layers with two

silicon anti-diffusion layers (Fig. 3.11b) and finally, the HIT structure based on a-GeSi-Si

absorption layers (Fig. 311c). The fabrication results, electronic mapping and J-V curve

are presented for each structure in the follow sections.

5.2 Current density progress for device structures

Figure 5.1 shows the current density from the initial stage to the final stage for

devices structures in this thesis. It shows the progress of the mapped current density for

device structures deposited at low deposition temperature Td=160°C and it shows the

progress of the rectification effect through this work.

P a g e | 75 CHAPTER 5: DEVICES EXPERIMENTAL RESULTS

Figure 5.1 Current density progress for device structures deposited at low deposition temperature T d=160°C. It is shows progress in mapping result for illumination

H_3V and I-V curves. (Blue processes are maintain process for the installation)


This chapter is organized in three main stages:

Section 5.3: it presents the results of the study on BASIC structure based only in a-

GeSi:H layers, (process 1004).

Section 5.4: it presents the results of anti-diffusion layer (ANTID) structures. This

section consists in four programs: a) increase of thickness of intrinsic a-GeSi:H

layer (process 1021, 1034 and 1037), b) the use of three a-Si:H anti-diffusion layers

(process 1035), c) the optimization of n- and p-type a-GeSi:H layers (process 1066

and 1065), and d) the optimization of a-Si:H anti-diffusion thickness (processes

1058, 1061)

Section 5.5: it presents the results on Hetero-junction Intrinsic Thin Film (HIT*)

structures, this section consists in three programs: a) transition from ANTID to HIT

structure (Processes 1075, 1078 and 1079), b) optimization of thickness of the

absorption layer (Processes 1009, 1114 and 1115), and c) fabrication of n-i-p

structure on flexible substrate (Process 1146).

Results presented in Figure 5.1 are the mapping current densities obtained under

illumination H_3V. The results shows a current density increase of three orders of

magnitude from not response samples to a density of Jsc=10-4

mA/cm2. In the next section

are presented details of each stage in this thesis.

5.3 STAGE 1: a-GeSi:H “BASIC” structure

The basic structure based only on a-GeSi:H layers (p-i-n configuration) is shown in

Figure 5.2. The structure is based on the results obtained for the study of deposition

temperature reduction and doping at low deposition temperature. The electronic

characteristics for selected processes are presented in Table 5.1.


Figure 5.2BASIC structure based on a-GeSi:H films deposited at low deposition temperature selected

processes; 972(intrinsic), 981(n-type) and 978 (p-type).

Table 5.1Electronic properties for selected a-GeSi:H films to use in BASIC structure

Reference process Type-Film Ea (eV) EF(eV) 300k

978 p-type -0.31 -0.27

972 intrinsic -0.32 -0.29

981 n-type -0.18 -0.12

The films have an optical gap of Eg~1.14 eV for the a-GeSi:H films and were

deposited at low deposition temperature Td=160°C. The device structures were fabricated

on plastic substrates simultaneously to glass substrates to identify general problems of

adhesion. In the Fig. 5.3 is shown an example of the fabricated structure on different

substrates. The structures deposited on plastic substrates show not visible problems of

adhesion.

Figure 5.3p-i-n structure on plastic substrates based on a-GeSi:H layers deposited at low deposition



The fabrication parameters for BASIC structure are shown in Table 5.2.

Table 5.2 Fabrication parameters for process 1004 (BASIC structure)

Process

1004

General

parameters

QSiH4

99.99%

Si

(sccm)

QGeH4

10 % Ge

(sccm)

QH2

100%

(sccm)

QPH3

1%

(sccm)

QBH3

1%

(sccm)

Time/

Thickness

(min/A°)

P GeSiB:H

(978) Power=

300Watts

Td=160 °C

F=110 KHz

P=0.76

Torrs

50 500 3750 0 28 3 min / ~200

I

SiGe:H

(972) 50 500 3750 0 0

48min /

~3500

+ 10 min

interruptions

N SiGeP:H

(981) 50 500 3750 16 0

3 min / ~200

The power, deposition temperature, pressure and frequency were kept as constant

parameters in the structure.

5.3.1 Electronic and mapping characterization of BASIC structure

The fabricated samples were mapping under “H_3V” illumination (halogen lamp

with intensity of 100 m W/cm2) on the samples. For the initial process was not found a

functional sample. The open circuit voltage Voc and short circuit current Isc were not

detectable. In the Fig. 5.4 is shown the I-V curve for the process 1004. The curve shows a

lineal behavior and it is not possible see a rectification effect.


a)

b)

Figure 5.4 a) current-voltage curve for BASIC structure process 1004 deposited at Td=160°C b) pin-

holes in the sandwich structure as a cause of short circuit and a linear behavior in the curve.

The main hypothesis of the results in BASIC structure process 1004 is the short

circuits caused by “pin-holes” in the structures as is show in Fig.5.4b. The diffusion of

metal or dopants through the structure could generate the short circuits. For this reason, it

was proposed a structure with anti-diffusion layers that is described in next section.

5.4 STAGE 2: “ANTID” structures, anti-diffusion

silicon layers

In order to solve the possible short circuits in the BASIC structure, a new structure

was proposed using two anti-diffusion silicon layers between the intrinsic and the doped

films. These anti-diffusion layers are used to prevent the diffusion of dopants or metal

through the structure. The initial ANTID structure is shown in the Fig. 5.5, the anti-

diffusion silicon layer was used with a thickness of 500°A to make the films transparent to

the charge transport.


Figure 5.5 Initial ANTID structure based on anti-diffusion silicon layers (process number 1005)

fabricated at low deposition temperature Td=160°C

The anti-diffusion a-Si:H layer was taken from ref. [64], a previous study in INAOE.

The electronic characteristics for the a-Si:H film are shown in Table 5.3 and the fabrication

parameters for the complete ANTID structure are presented in Table 5.4.

Table 5.3 Electronic energies for silicon anti -diffusion layer [64]

Eg (eV) Ea (eV) EF(eV)

a-Si:H 1.97 0.42 0.47

Table 5.4 Fabrication parameters for process 1005 (ANTID structure)

Process

General

parameters

Power=300Watts

T=160 °C

F=110 KHz

QSiH4

99.99%

(sccm)

QGeH4

10 % Ge

(sccm)

QH2

100%

(sccm)

QP

1%

(sccm)

QB

1%

(sccm)

Time/

Thickness

(min/A°)

P a-

GeSiB:H

(978)

P=0.76 Torr

50 500 3750 0 28

3 / ~200

I-

Si Si:H (18) P=1.4 Torr 50 0 1000 0 0

7/500

I a-GeSi:H

(972)

P=0.76 Torr

50 500 3750 0 0

48 / ~3500

+ 10 min

interruptions

I-

Si Si:H P=1.4 Torr 50 0 1000 0 0

7/500

N a-

GeSiP:H

(981)

P=0.76 Torr

50 500 3750 16 0

3 / ~200


The mapping was made under “H_3V” illumination. For the first time, signals of

open circuit voltage were obtained and it was used as reference of YIELD of the process.

However, no signal of short circuit current was detectable. The results of electronic

characterization are shown in Table 5.5.

Table 5.5 Mapping results for the sample 1005 (anti-diffusion layers) at different illumination

conditions

Process

Jsc_max (A/cm2) Voc (mV)

YIELD %

(Voltage) (H_3V) (H_8V) (H_3V) (H_8V)

1005 -- -- 28 -- 59

The total mapping consisted in 96 samples, with a detectable open circuit voltage in

59% of the structures. The maximum open circuit voltage was Voc=28 mV and not response

of short circuit current was detectable. The current-voltage characteristic is shown in Fig.

5.6. The current-voltage curve shows not visible change under illumination. However, it is

observed a change in the linear behavior in comparison with sample 1004. This change and

the open circuit voltage response demonstrate that the anti-diffusion silicon layers prevent

short circuit in the samples. In order to get short circuit current Isc we proposed to increase

the thickness of the intrinsic a-GeSi:H film in the structure.

Figure 5.6 Current-voltage characteristic for ANTID structure process 1005 deposited at

Td=160°Cunder illumination H_3V


5.4.1 Thickness increase of intrinsic a-GeSi:H layer

The program consists in the increase of thickness of the intrinsic a-GeSi:H film. This

program includes the process numbers 1021, 1034 and 1037. The fabrication parameters of

the processes are shown in the Table 5.7 After the process 1021 due to maintenance of

installation the silane source at 99% was change to a new gas source at 10%. The flows of

silane and germane were adjusted for this new dilution in the gas source.

The started structure (ANTID 1021) had two modifications in comparison to the

sample 1005: 1) The increase of deposition time from 58 min to 80 min that suppose a

increase of thickness in the intrinsic a-GeSi:H layer from t= ~4200 A° to ~6800 A° and 2) a

reduction of the deposition time for the doped films from 3 min to 2 min. For the sample

1034 the deposition time of the intrinsic film was 120 min. with an expected thickness

~9000 A° and for the doped films the deposition time was 2 min. For the process 1037 the

deposition time of intrinsic GeSi:H film was 180 min. and the deposition time of doped

films was 2 min. The structures have the same ANTID structure configuration showed in

Fig. 5.5.

The mapping and electronic characterization results for the process 1021, 1034 and

1037 are shown in Table 5.6. The mapping results show that process 1021 and 1034 The

mapping shows that thickness increase of i-GeSi:H film results in an increase of the short

circuit current and it demonstrates current contribution of the i-GeSi:H film in the structure.

Table 5.6 mapping results with H_3V lighting of process 1021, 1034 and 1037

Process


VOLTAGE

YIELD % (H_3V) (H_8V) (H_3V) (H_8V)

1021 3.49x10-7

-- 75 -- 92.2

1034 1.14x10-6

1.6x10-5

420 450 93.5

1037 -- -- -- -- DEAD


Table 5.7 Fabrication parameters for processes 1021, 1034 and 1037 (ANTID structures)

Process

General

parameters

Power=300Watts

T=160 °C

F=110 KHz

QSiH4

99.99%/10%

(sccm)

QGeH4

10 % Ge

(sccm)

QH2

100%

(sccm)

QP

1%

(sccm)

QB

1%

(sccm)

Time/

thickness

(min/A°)

PROCESS 1021

P a-GeSiB:H

(978) P=0.76 Torr 50 500 3750 0 28

2:30/-

I-

Si Si:H P=1.4 Torr 50 0 1000 0 0 7/500

I a-SiGe:H

(972) P=0.76 Torr 50 500 3750 0 0

80 /~

5840

I-

Si Si:H P=1.4 Torr 50 0 1000 0 0 7/500

N a-SiGeP:H

(981)

P=0.76 Torr

50 500 3750 16 0 2:30 / -

PROCESS 1034

P a-GeSiB:H

(978)

P=0.76 Torr

500 500 3300 0 28

2 / --

I-

Si Si:H P=1.4 Torr 500 0 550 0 0

7/~500

I a-SiGe:H

(972)

P=0.76 Torr

500 500 3330 0 0

120 /

~8748

I-

Si Si:H P=1.4 Torr 500 0 550 0 0

7/~500

N a-SiGeP:H

(981)

P=0.76 Torr

500 500 3300 16 0

2 / --

PROCESS 1037

P a-GeSiB:H

(978)

P=0.76 Torr

500 500 3300 0 28

2 / --

I-

Si Si:H P=1.4 Torr 500 0 550 0 0

7/~500

I a-SiGe:H

(972) P=0.76 Torr 500 500 330 0 0

180

/~13122

I-

Si Si:H P=1.4 Torr 500 0 550 0 0

7/ ~500

N a-SiGeP:H

(981) P=0.76 Torr 500 500 3300 16 0

2 / -


Process 1037 shows not response in open circuit voltage or short circuit. The

maximum current densities for the process 1021 and 1034 were Jsc=3.49x10-7

A/cm2 and

Jsc=1.14x10-6

A/cm2 respectively. The increase of thickness of the intrinsic Ge film resulted

in an increase of current in one order of magnitude from the process 1021 to the process

1034. For voltage response, the open circuit voltage increases from Voc= 74 to 400 mV for

the processes 1021 to 1034. The yield increases to 90% in contrast to ~60% for the process

1005. The current-voltage curve of the processes 1021 and 1034 are shown in Fig.5.7.

Figure 5.7Current-voltage characteristics for ANTID structures a) process 1021 and b) process 1034

dark and under illumination “H_3V”

The curves show an increase of the rectification effect for the processes 1021 and

1034 in comparison to the process 1004. Figure 5.7b shows response to the lighting in low

voltage regimen.

5.4.2 Third anti-diffusion a-Si:H layer

Previous results show that the anti-diffusion silicon layers prevent the short circuits in

the structures. In order to continue with this idea it was proposed a third anti-diffusion

silicon layer as it is shown in Fig. 5.8 (process 1035).


Figure 5.8 Proposal structure based in three anti-diffusion silicon layers fabricated at low deposition

temperature Td=160°C (process 1035)

The parameters of process 1035 are identical to the process 1034 and only it was

inserted a third anti-diffusion layer in the half of the intrinsic a-GeSi:H layer. The mapping

and electronic characterization results are shown in Table 5.8.

Table 5.8 Mapping results under the illuminations H_3V and H_8V for third anti-diffusion layer

structure (process 1035)

Process


VOLTAGE

YIELD % (H_3V) (H_8V) (H_3V) (H_8V)

1035 3.8x10-7

-- 400 -- 92.2

The mapping results for the process 1034 show a maximum short circuit current of

Jsc=3.8x10-7

A/cm2 and an open circuit voltage of Voc=400 mV. In comparison to the

process 1034 ( Jsc=1.14x10-6

A/cm2, Voc=420 mV) the current density decreases one order

of magnitude for the process 1035. The results demonstrated the best performances in the

structure 1034 with two anti-diffusion layers.

Si:H 500°A

5100 °A i-SiGe

5100 °A i-SiGe

1035


5.4.3 Optimization of doping in a-GeSi:H layers

This program consists in optimization of the dopant flows for doped (p-type and n-

type) a-GeSi:H films. The dopant flows were increased in relation to the best process

obtained in the previous program (1034) and the others parameters of fabrication were kept

constant to the reference process. The flows used are presented in Table 5.9 in comparison

to the process reference 1034.

Table 5.9 Dopant flows for optimization program of doped GeSi:H (process 1066 and 1075)

Process Diborane flow

(sccm)

Phosphine flow

(sccm)

1034 (Reference) 28 16

1066 32 20

1075 34 22

Mapping and electronic characterization results for the process 1066 and 1075 are

shown in Table 5.10 under the H_3V and H_8V illuminations (see section 3.7.1).

Table 5.10 Mapping results with H_3V lighting of process 1066 and process 1075

Process


VOLTAGE

YIELD % (H_3V) (H_8V) (H_3V) (H_8V)

1066 7.4x10-6

8.4x10-5

180 330 93.8

1075 7.6x10-6

1.0x10-4

442 600 79.5

Voltage YIELDs were of 93.8 % and 79.5% for process 1066 and 1075 respectively,

in contrast to the reference process 1034 with a voltage YIELD of 93.5%. In this section of

the program the voltage YIELD decreases for the process 1075 in comparison to the

reference process 1034. The maximum current density Jsc for the process 1075 shows an


increase from Jsc =1.14x10-6

mA/cm2 (process 1034) to 7.6x10

-6 mA/cm

2 (process 1075).

The open circuit voltage for process 1066 was Voc= 180 mV and for process 1075 was Voc=

442mV. The value of Voc decreases for the process 1066 and increase for the process 1075

in relation to the process 1034 with value of Voc=420mV. The dark and light current-

voltage characteristics for process 1066 and 10375 are shown in Fig. 5.9- Fig. 5.12 under

illumination H_8V conditions.

Figure 5.9 Current-voltage curve for the process 1066, dark and light current density illuminated by

Halogen lamp at 8V

Figure 5.10 Current-voltage curve for process 1075, dark and light current density illuminated by

Halogen lamp at 8V


Figure 5.11 Current-voltage characteristic for process 1066, dark and light current density

illuminated by Halogen lamp at 8V.(low voltage regimen)

Figure 5.12 Current-voltage characteristic for process 1075, dark and light current density by

Halogen lamp at 8V (low voltage regimen)


The curves in the Fig. 5.11 and 5.12 show the light response of the samples at low

voltages regimen. The rectification effect increases for this fabrication program. The

current density under adjusted H_8V illumination shows an increase of value in one order

of magnitude in comparison to H_3V illumination. The values of current density are Jsc=

8.4x10-5

mA/cm2 and Jsc= 1.0x10

-4 mA/cm

2 for processes 1066 and 1075 respectively. A

result is the increases of open circuit voltage for the sample 1075 with value Voc= 600 mV.

However, this sample shows a not regular behavior for the low voltage regimen. This effect

is discussed in section 5.5. In the next section was performed an increase of dopants flows

and Si:H thickness in order to study the increase of doping and possible diffusion of

contamination.

5.4.4 Optimization of thickness of anti-diffusion a-Si:H layers

This program consists in an increase of doping flows for n-type and p-type GeSi:H

layers and the increase of thickness of the anti-diffusion silicon layer to prevent the dopants

diffusion The thickness and dopant flows are presented in Table 5.11, others deposition

parameters are fixed as reference process 1034.

Table 5.11 Dopant flows and thickness of a-Si:H layers for process 1034, 1058 and1061

Process

Thickness

(anti-diffusion)

(A°)

Diborane flow

(sccm)

Phosphine flow

(sccm)

1034 (Reference) 500 28 16

1058 640 29 18

1061 710 32 20

The electronic characteristics and mapping results for process 1058 and 1061 are

shown in Table 5.12. The open circuit voltage for process 1058 shows a maximum value of

Voc=294 mV and process 1061 shows maximum value of Voc=210 mV under H_3V


illumination. This values decrease in comparison to the process reference 1034 with value

of Voc=420mV. The maximum short circuit current increases to values of Jsc_max=2.5x10-6

A/cm2 and 4.7x10

-6 A/cm

2 for process 1058 and 1061 respectively in comparison to the

reference process 1034 with value of Jsc=1.14x10-6

A/cm2.

Table 5.12 Mapping results with H_3V lighting of process 1058 and process 1061

Process


VOLTAGE

YIELD % (H_3V) (H_8V) (H_3V) (H_8V)

1058 2.5x10-6

--

294 -- 93.8

1061 4.7x10-6

1.6x10-5

210 330 79.5

The value for current density under H_8V illumination for process 1061 was Jsc=

1.6x10-5

A/cm2 and the open circuit voltage value of Voc=330 mV. The J-V characteristics

for the best process (1061) are shown in the Fig. 5.13.

Figure 5.13 Current-voltage characteristic for process 1061, dark and lighting by Halogen l amp at 8V

adjusted response to un intensity.

A similar behavior between process 1075 and process 1061 is observed at low voltage

regimen.. The Figure 5.14 shows a result summary for the section 5.4.3 and section 5.4.5.

It shows that an increase of dopant flows and the no increase of the thickness of anti-

diffusion layers, improve the short circuit current from the process 1034 Jsc=1.4x10-6

A/cm2

to the process 1075 Jsc=7.6x10-6

A/cm2 (lighting H_3V).


Figure 5.14 ANTID structures: a) Changes of the structure configuration, b) current-voltage curves

and c) Low voltage regimen curves


Short circuit current density increases from process 1034 Jsc=1.6x10-6

A/cm2 to

process 1075 Jsc=7.6x10-6

A/cm2

( Fig.5.14b). The open circuit voltage decreases from

process 1035 Voc=420 mV to process 1075 Voc=442 mV under H_3V illumination. Fig

5.14b shows the current voltage characteristics for the discussed structures, the structure

1075 shows the best rectification effect and this result reveals that the increase of dopant

flows improves the electronic characteristic of the structures.

5.5 STAGE 3: “HIT” structure based in Si:H and

GeSi:H absorption layers

The results obtained in sections 5.4.3 and 5.4.4 for the process 1075 show an increase

in the open circuit voltage and short circuit current. The behavior of current at low voltage

regimes are a probe of a potential barrier in the flow of current as is shown in Figure 5.15.

Figure 5.15 The experimental curve for low voltage regiment for process 1075(black), ideal trend for

current voltage characteristics of shut and serial resistances (blue)


In Figure 5.16 is presented a simplified analysis of the bands diagram of the ANTID

structure. It shows potential barriers due to the difference in the optical gaps of the anti-

diffusion a-Si:H layers and intrinsic a-GeSi:H layer.

Figure 5.16 Analysis of band diagram for ANTID structure (p-GeSi//i-Si//i-GeSi//i-Si:H//n-GeSi:H)

In order to confirm the hypothesis of the potential barriers is proposed the

asymmetrical reduction of thickness for anti-diffusion silicon layer (process 1075). The

reduction of the anti-diffusion Si:H layer in p-type region was performed. The layer was

reduced from 500°A to 350 A° while the anti-diffusion Si:H layer from n-type region was

kept constant as it is shown in Fig. 5.17.

Figure 5.17 Structure design to probe the potential barriers between layers (reduction of anti-

diffusion thickness between p-type and intrinsic a-GeSi:H layers)


The structure 1076 pretends remove one of the two potential barriers and see the

effect on the current-voltage characteristics. The electronic characteristics and mapping

results are shown in Table 5.13. The short circuit current and open circuit voltage

drastically decrease with the reduction of thickness in the anti-diffusion layer in p- region.

The short circuit current density decreases from Jsc=7.6x10-6

A/cm2

(1075) to Jsc=1x10-7

A/cm2 (1076) and the open circuit voltage decreases from Voc=442 mV to 64 mV,

respectively.

Table 5.13 Mapping results with H_3V lighting of process 1076 and process

Process


VOLTAGE

YIELD % (H_3V) (H_8V) (H_3V) (H_8V)

1075 (REF.) 7.6x10-6

1.0x10-4

442 600 79.5

1076 4.7x10-7

--

64 -- 94.3

The analysis of current-voltage characteristic in Figure 5.18 shows a change of

polarization for the structure.

Figure 5.18 Current-voltage characteristic for process 1076 (decreases of thickness for anti-diffusion

layer) change bias in relation previous process


The change on polarization reveals the potential barrier in the n-region and this

barrier is the cause of the trend in previous structures at low voltage regimens (Figure

5.15). The results suggest that the anti-diffusion layer in p-region contributes one order of

magnitude more than anti-diffusion layer in n-region (the current density decreases one

order of magnitude when the p-region is reduced). The previous results in the process 1076

and process 1075 show the contribution of anti-diffusion layers in the ANTID-structure. A

transition from ANTID-structure to Hetero-junction with Intrinsic Thin Layer Films “HIT”

structure is proposed to solve the problem of the potential barrier in n- region.

5.5.1 Transition from ANTID to HIT structure

The proposal of this program is to eliminate one of the two anti-diffusion silicon

layers and study the contribution of the remaining silicon layer as second absorption layer.

The program is shown in the Figure 5.19. The reference process is the process 1075 and

the modifications are shown in the Table 5.14.

Table 5.14 Modification to reference process 1075 for program HIT structure (other parameter was

fixed like process 1075)

Process Anti-diffusion

thickness A°

(p-type region)

Anti-Diffusion

thickness A°

(n-type region )

Others modifications

1075 500 500 --

1078 630 500 --

1079 715 100 --

1086 1071 0 37sccm (Diborane)

As is shown in Fig. 5.19 and Table 5.16 the elimination of the ant-diffusion silicon

layer from n-type region was done gradually as follow: a) Process 1078, the anti-diffusion

layer from p-region was increased in thickness b) Process 1079 consisted in the reduction

of anti-diffusion Si:H layer in the n-type region from t=500 to100A° and an increase of


anti-diffusion Si:H layer in p-type region from 500 to 715 A°, c) Process 1086: the anti-

diffusion layer in n-type region was eliminated and the anti-diffusion layer from p-type

region was converted to an absorption layer with a thickness of t=~1000 A°. The increase

of this layer allows the increase in diborane flow from 35 to 37 sccm.

Figure 5.19 Transition from ANTID to HIT structure, process 1078, 1079 and 1086

The mapping and electronic characterization results for process 1078, 1079 and 1086

are shown in Table 5.15. The results show an increase of the short circuit current density

under H_3V illumination from reference process 1075 with Jsc=7.6x10-6 A/cm2 to process


1086 with Jsc=17.2 x10-6 A/cm2 This increase in current is consistent with the increase of

the thickness of the intrinsic silicon layer. The maximum open circuit voltage for process

1086 is Voc=499 mV.

Table 5.15 Mapping results with H_3V lighting of process 1078,1079 and 1086.

Process


VOLTAGE

YIELD % (H_3V) (H_8V) (H_3V) (H_8V)

1078 1.08x10-5

1.70x10-4

358 400 98.3

1079 1.17x10-5

4.20x10-4

357 500 92.5

1086 1.72x10-5

8.24x10-4

500 550 95.5

YIELD for current and voltage was above 90 %. The current-voltage characteristics

for process 1078, 1079 and 186 are shown in the Figure 5.20.

Figure 5.20 J-V characteristics for transition program from ANTID to HIT structure: a) under

illumination b) low voltage regimen under illumination c) log curve under illumination and d) dark

log characteristic (process 1078, 179, and 1086)


Figure 5.20 shows current-voltage characteristics for comparison. The Figures 5.20

a), b) and c), show the best rectification effect for the process 1086 and the Figure.5.20b)

shows the reduction of potential barrier in low voltage regimen. It is demonstrated the

hypothesis that the barrier was caused by anti-diffusion layer in n-type region. The short

circuit current density increases for processes 1078, 1079 and 1086 from Jsc=1.7x10-5

to

8.4x10 -5

A/cm2. The maximum voltage was in the process 1086 with value of Voc=550

mV. These parameters were obtained with adjusted illumination “H_8V”. This results show

a functional p-i-n structure deposited at low temperature based on a-GeSi:H films.

5.5.2 Optimization of absorption layers thickness

This program consist in the optimization of thickness for the intrinsic a-Si:H and

intrinsic a-GeSi:H films used as absorption layers. The best structure obtained in section

5.5.1 (process 1086) is used as reference structure for this study. The deposition

parameters for the reference process are showed in Table 5.16.

Table 5.16Deposition parameter for reference process 1086

Process

General

parameters

Power=300Watts

T=160 °C

F=110 KHz

QSiH4

99.99%

(sccm)

QGeH4

10 % Ge

(sccm)

QH2

100%

(sccm)

QP

1%

(sccm)

QB

1%

(sccm)

Time/

thickness

(min/A°)

P a-GeSiB:H

(978) P=0.76 Torr 500 500 3300 0 37

2

I-

Si Si:H P=1.4 Torr 500 0 550 0 0 15/1093

I a-SiGe:H

(972) P=0.76 Torr 500 500 3300 0 0

100 /7980

N a-SiGeP:H

(981)

P=0.76 Torr 500 500 3300 23 0 2/


The study of film thickness includes the processes number 114, 115 and 1109. All

fabrication parameters were fixed as process 1086. The thickness for each absorption layer

is shown in Table 5.17.

Table 5.17Thickness for absorption layers in processes 1114, 1109, 1115 and reference process 1086

Thickness

absorption layers (A°)

Process

1114

Process

1086

(Reference)

Process

1115

Process

1109

i-Si 1093 1093 1093 1263

i-GeSi 6384 7980 9576 7980

For process 1114 and 1115 the intrinsic a-GeSi:H thickness was varied from 7980 A°

to 6384 A° and 9576 A° respectively with an a-Si:H film of thickness fixed at 1093°A,

while for the process 1109 the a-Si:H thickness was increased from 1093 A°(reference) to

1263 A° with an a-GeSi:H thickness fixed as the reference structure. The results for

electronic characteristics and mapping are shown in Table 5.18.

Table 5.18 Mapping results of process 1114,1115 and 1086.

Process


VOLTAGE

YIELD % (H_3V) (H_8V) (H_3V) (H_8V)

1086 1.72x10-5

8.24x10-4

500 580 95.2

1114 1.60x10-5

6.10x10-4

550 430 95

1115 1.50x10-5

5.51x10-4

450 575 93.5

1109 2.5x10-5

7.90x10-4

428 430 95.3

Results in Table 5.18 show the maximum currents under illuminations H_3V and

H_8V. The maximum current reached Jsc=2.15x10-5

A/cm2

under the illumination H_3V for

the process 1109 and Jsc=8.24x10-4

A/cm2 under illumination H_8V for process 1086. The

open circuit voltage have maximum values obtained for process 1114 with a maximum


value of Voc=580 mV. The current-voltage characteristics under H_8V illumination are

shown in Figure 5.21

Figure 5.21J-V characteristics for HIT program a) rectification curve b) low voltage regimen

comparison under H_8V illumination.

The results show a maximum value of short circuit current density for process

reference 1086 with value fo Jsc= 8.24x10-5

A/cm2. The reduction or increase of thickness

for the absorption a-GeSi:H film results in a decrease of current densities for the process

1114 Jsc=6.10x10-4

A/cm2 and the process 1115 Jsc=5.51x10

-4 A/cm

2. The increase of

thickness for a-Si:H absorption film decreases the open circuit voltage from Voc= 580mV

(reference process 1086) to 430 mV (process 1109). The best results for Jsc and Voc were

obtained for the process 1086.

5.5.3 Sputtered contacts, n-i-p structure and plastic substrate

The process 1084 demonstrated a functional p-i-n structure deposited at low

deposition temperature Td=160°C. A change in the technique for deposit the contacts and a

change in the configuration of the structures (from p-i-n to n-i-p) were proposed. The

deposit structures on different substrates are shown in Figure 5.22. All fabrication

parameters were fixed as process 1086.


Figure 5.22 Structures deposited on glass substrate and flexible plastic substrates . Reference process

1086, sputtering contacts 1145 and n-i-p configuration process 1146.

The Figure 5.22 shows different configurations for the structures using the deposition

parameters of the reference process 1086 with contacts deposited by electron beam

evaporation (Blazer system). The process 1146 was a reproduction of the process 1086 but

with a change to a n-i-p configuration with contacts deposited by ATC Orion Sputtering

System of AJA International Inc. The structures were deposited on glass substrate

(1146_A), on PEN substrate with Ti contacts (1146_B) and on PEN substrate with a no

transparent aluminum bottom contact (1146_C). The structures on polyimide substrate were

not functional due to the large deformation in the sample (Figure 5.22). The current-

voltage characteristics under H_8V lighting are compared for the samples in Figure 5.23.


Figure 5.23 Current voltages characteristics for process 1146: b) under illumination (linear) b) low

voltage regimen under illumination c) log curve (dark )c) under illumination (log), n-i-p process

1146_A (glass substrate), 1146_B (PEN substrate) and 1146_C (PEN substrate and Al contacts)

The Figure 5.23 shows the current-voltage characteristics for the process 1146

(substrate//n-i-p structure). All samples show a rectification effect in the studied range. The

values of short circuit current density for each samples were Jsc=8.54x10-4

A/cm2

on glass

(1146_A), Jsc=6.10x10-4

A/cm2 on PEN substrate (1146_B) and Jsc=1.12x10

-3 A/cm

2 Al

bottom contact on flexible substrate PEN. The comparison shows an increase of current

density (Fig. 5.23 b) and a better rectification effect (Fig. 5.23 a) for the sample 1146_C.


5.4 SIMS Characterization for selected process 1086

The SIMS profile for process 1086 is shown in Figure 5.24. The content of

germanium, silicon, hydrogen, phosphorus and boron were determined by SIMS

measurements.

Figure 5.24 SIMS profile for HIT STRUCTURE (process 1086): a) B- and P- dopant profile and b)

Carbon and Oxygen impurities concentrations through structure.

SIMS profile shows an intrinsic a-GexSi1-x:H material with a composition of x=0.95

in the bulk. The Fig. 5.24a shows a dopants profile for boron and phosphorus

concentrations through the structure 1086. The main structure profile is defined by the

concentration of germanium and silicon . The thickness of the completed structure was

calculated with the ALPHA STEP equipment. The diffusion of boron and phosphorus was

indicated are indicated in Fig.5.2. For boron the maximum peak of concentration decreases

four orders of magnitude in a depth of 170 A° while phosphorus decreases in a depth of

300°A from the doped films to the intrinsic film. The maximum concentration of dopants


in the doped regions are for boron CB=2.5x1021

atoms/cm2 and phosphorus CP=2.2x10

20

atoms/cm3. The concentrations of hydrogen in the bulk for a-GeSi:H and a-Si:H layers are

CH(GeSi)=5.5x1020

atoms/cm3 and CH(Si)=1.9x10

21 atoms/cm

3 respectively. The impurity

concentrations for GeSi:H layer in the bulk are for Carbon CC(GeSi)=3.9x1017

atoms/cm2,

and for Oxygen CO(GeSi) =2.6x1020

atoms/cm3, and for the Si:H layer the impurity

concentrations are CC(Si)=2.3x1018

atoms/cm3 and CO(Si)=1.9x10

20 atoms/cm

3.

5.5 Short circuit current spectra for selected devices

structures

Figure 5.25 and Figures 5.26 show the short circuit current spectra for selected

devices structures. The Figure 5.25 shows the processes fabricated for the transition from

ANTID structure to HIT structure..

1.0 1.5 2.0 2.5 3.0 3.5 4.0

10-2

10-1

100

101

102

103

1075

1076

1078

1079

1086

Sh

ort

Cir

cu

it C

urr

en

t (I

SC,

nA

)

Photon Energy (hv, eV)

Figure 5.25 Short circuit current spectra for structures 1075, 1076, 1078 1079 and 1086


The spectra in Fig. 5.25, show the sub-gab absorption region in Eg<1.97 eV that

corresponds to the optical gap of silicon layer. As discussed in the introduction for section

5.3, the process 1075 has an ANTID structure configuration with symmetrical silicon anti-

diffusion layers. The short circuit current decreases for the asymmetrical ANTID structure

process 1076. The current decreases from Isc(hv= 2.5 eV)= 90 nA to 9 nA as result of the

thickness reduction of intrinsic anti-diffusion layer in p- region. The reduction of anti-

diffusion layer thickness in of p-interface decreases the absorption in the region Eg>1.97

eV. In contrast, the anti-diffusion layer reduction in n-interface and the increases of

thickness of anti-diffusion layer in p- interface (section 5.3.1) result in an increase of short

circuit current from Isc(hv= 2.5 eV)= 90 nA to 3000 nA. For region Eg<1.97 eV, the short

circuit current has no significant change with a value of Isc(hv= 1.1 eV)~ 0.01 nA from the

process 1075 to the process 1076. For the processes 1076 and 1078 the short circuit current

increases from Isc(hv= 1.1 eV)= 0.01 nA to 0.3 nA, respectively. For the process 1078 and

1079, the short circuit current decreases from Isc(hv= 1.1 eV)= 0.3 nA to 0.1 nA,

respectively. The maximum short circuit current in the region Eg<1.97 eV is obtained for

the process 1086 (HIT structure) with value of Isc (hv= 1.1 eV)=1 nA. Figure 5.26 shows

the short circuit current spectrum for the HIT structure 1086 under different illumination

conditions: 1) Halogen lamp and 2) Halogen lamp + IR (800 nm laser) in order to reveal the

IR absorption of the intrinsic a-GeSi:H layer.

Figure 5.26 Spectral responses for HIT structure, Halogen lamp (red) and Halogen lamp+IR (800nm)


The results show an increase of current Isc(hv=1.1ev) from 1 nA to 45nA under IR

illumination. The spectral response shows an increase of absorption for the energies E<

1.97 eV. This demonstrates that the intrinsic a-SiGe layer contributes to the absorption in

the device.

Finally, the superficial recombination has an interesting behavior for the studied

structures. The transition from ANTID configuration to HIT configuration results in a

change of the absorption for high energies (E<3.0 eV) that suggest an increase of the

superficial recombination from ANTID to HIT structure.

5.6 a-GeSi:H device characteristics on flexible

substrate

The Figure 5.27 shows the current-voltage characteristics for the sample n-i-p 1046

on flexible substrate.

Figure 5.27 Current-voltage characteristic for flexible substrate n-i-p structure, for process 1046.

*Energy conversion efficiency calculated using adjusted intensity for 100mW/cm2

The sample shows values of short circuit current density of Jsc=1.2x10-3

A/cm2 and an

open circuit voltage of Voc=480 mV. The energy conversion efficiency η, of solar cell was

calculated as:


in

ocsc

P

FFVJ

And

)(

)(

ocsc

mm

VI

VIFF

Where Im and Vm are values at maximum power and Pin was 100 mWatts/cm2

adjusted to the responses of the sun (real 500 mWatts/cm2). The final structure is shown in

Figure 5.28.

Figure 5.28 Structure for flexible substrate n-i-p structure process 1046, approximate thicknesses.

The best electronic characteristic structure is shown in Figure 5.28. The

approximated thickness of the layers are: for p layer 150 A°, for i-Si:H 1100A°, for i-

SiGe:H 7500 A° and for n layer 150 A°.


In next chapter is presented a discussion for the structure fabricated in this work, the

results of fabrication, characterization and comparison with literature has been analyzed

P a g e | 109

6. DISCUSSION OF THE RESULTS

6. 1Introduction

In this chapter, the results are discussed and contrasted with the literature. The first

part of the chapter is dedicated to the intrinsic film results and the deposition temperature

reduction effect on intrinsic a-SiGe:H films. The characteristics at low deposition

temperature of the intrinsic films are discussed. In the second part the results of doping are

discussed for low deposition temperatures Td<200°C. The results of the electronic

characteristics and solid incorporation for doped films are contrasted at low deposition

temperature Td=160°C and high deposition temperature Td=300°C, to observe the influence

of the reduction of deposition temperature. Finally, the results of the structures are

compared with others p-i-n structures based in high Ge content fabricated at low deposition

temperatures Td<200°C.

6.2 Disscusion about deposition temperature

reduction effect on intrinsic a-GeSi:H films

In this section the deposition temperature reduction effect on intrinsic films is

discussed for the results showed in chapter 4 [Section 4.2]. The deposition temperature

reduction effect is discussed through hydrogen bounding, deposition rate, optical and

electronic characteristics. The data is contrasted with literature in Chapter 2 [Section 2.2.4].

6.2.1 Effect of deposition temperature reduction on hydrogen incorporation

The results in this thesis demonstrate that hydrogen incorporation decreases with the

increase of deposition temperature (Fig. 4.6b studied by FTIR). This reduction is

continuous, except for the sample at deposition temperature Td=250°C that shows an abrupt

P a g e | 110 CHAPTER 6: DISCUSSION OF RESULTS

increase of hydrogen incorporation. A similar study using SIMS for Ge:H is reported in ref.

[46] and an rise of hydrogen is observed in the range of deposition temperature from

Td=190 °C to 240 °C and it can be explained with the hydrogen desorption in the

deposition process. However, the high content of hydrogen cannot be fully explained by

this mechanism in our samples. A more detailed study in temperature range around

Td=250°C should be done to understand abrupt incorporation of hydrogen at this deposition

temperature. The continuous hydrogen decrease with the increase of deposition temperature

observed in ref. [22-26], i.e. Mackenzie [23] observes the reduction of hydrogen in the

temperature range from Td=220°C to 380°C for a-Ge0.5Si0.5:H films (Fig 2.4a). In ref. [45]

it is shown that the effect of deposition temperature reduction on H incorporation is

compensated by high H dilution ratios (54:1) for Si0.6Ge0.4:H alloys (Fig. 2.5b) , however,

the results in this thesis and the data reported in ref. [46] show no compensation of this

effect in GexSi1-x:H (x>>0.5) deposited at high H dilution ratios (75:1 and 450:1

respectively).

The relation between the hydrogen content at the highest (300°C) and the lowest

(70°C) temperatures in this work was RHT=CH70/CH300=1.46. In this thesis the shift of the

line position is reported from k=1872 cm-1

at Td=70°C to k=1875 cm-1

at Td=300°C for Ge-

H stretching mode. This means that the extra energy comes from the increase of deposition

temperature and these results in changes in the lattice structure on vicinity of Ge-H

bonding. In the literature that we have investigated has not been reported any data in

relation to these results.

The composition of intrinsic film deposited at low temperature (Td=160°C) was

studied by SIMS (section 4.3.3). The total hydrogen content was CH=5.59x1021

atoms/cm-3

meaning a concentration of 11% in the i-Ge0.95Si0.05:H film. The H content in this work is

one order of magnitude more than the reported one in the work [46] for Ge:H films

deposited at Td=150°C (Fig. 2.6b). In ref. [45] it is reported the H content of 13 % for

Si0.6Ge0.4:H alloys deposited at Td=170°C. The differences of concentrations are explained

by the conditions of the experiments of each work, however, the results show that


deposition temperature affects the hydrogen elimination and the reconstruction of the atoms

during the growing process in a-GeSi:H films.

6.2.2 Effect of deposition temperature on deposition rate

As discussed in Chapter 2, the deposition rate of films based in silicon-germanium

deposited by PECVD process is poorly affected by temperature deposition [27-28] and can

be controlled by other parameters such as high frequency [17-18].

The results in this work show deposition rates in the range from Vd=0.91 A°/s to 1.45

A°/s [Fig. 4.4] obtained at power density W=20 mWatts/cm2 and H dilution ratio of R=75

and it shows a discontinuous change as function of deposition temperature. Generally,

higher power means higher deposition rates. However, in ref. [46] it is reported the

deposition rates from Vd=0.24-0.13 A°/s for Ge films obtained in the range of deposition

temperature Td=30°C-310°C with power density at 43mWatts/cm2 and H dilution ratio of

R=450. These values are one order of magnitude lower than values obtained in this work at

lower power density. The best deposition rate value reported [49] for Ge:H is Vd= 10 A°/s

obtained at low deposition temperature Td=200°C with power density at 11mWatts/cm2 and

H dilution ratio of 40. The comparison of the three works shows the lowest deposition rate

value with the lowest dilution ratio [46] and the highest deposition rate for the highest

dilution ratio [49]. As discussed in section 6.2.1, the low deposition temperature (Td <

200°C) increases the hydrogen incorporation and maybe lower H dilution ratios can be used

to obtain higher deposition rate in this temperature range. The results in this work show

reasonable values in the deposition rate for a-GeSi:H deposited at low deposition

temperatures i.e. Vd= 1.33 A°/s at Td=160°C.

6.2.3 Effect of deposition temperature on optical characteristics

The results presented in the section 4.2.4, Fig 4.8 show minimum optical gap energy

E04=1.09 eV at deposition temperature Td=190°C and maximum value E04=1.2 eV at

Td=280°C with a slight increase at deposition temperatures Td>190°C. In ref. [23], E04


shows a linear increase from Eg=1.4 eV to 1.6 eV with a decrease of Td in GexSi1-x:H films

(x~50%) deposited in the deposition temperature range from Td=230 to 372 °C. A Similar

effect is shown in ref. [42] for Ge:H films, it shows increase of optical gap from Eg= 1.16 to

1.22 eV with the decrease of deposition temperature in the range from Td=110-280°C. The

increase of optical gap with an increase of deposition temperature is related to the high H

concentration at a low deposition temperature. In contrast with references [23] and [42], in

this work the increase of optical gap is not present with decrease of Td (reduction of H

content Fig. 4.6b). The trend observed in our results is interesting due to the lower optical

gaps that are obtained at low deposition temperature. The results for E04 are according to

the minimum value of EgTAUC

=0.90 eV at deposition temperature Td=190°C and to the

maximum value EgTAUC

= 1.01 eV at Td=280.

For optical characterization results, a decrease of refraction index from n∞=4.06 to

3.92 at deposition temperatures from Td=70°C to 160°C and a change of trend with

refraction index increase from n∞=3.92 to 4.1 at deposition temperature from Td=160°C to

300°C have been observed. The variations in refraction index from n∞=3.93 to 4.18 reveal

a strong effect of deposition temperature on the structure of the films. The minimum value

of refraction index n∞=3.92 obtained was at the deposition temperature Td= 160°C. The

changes in refraction index without a significant change in optical gap can be explained by

changes in the material density. The increase of refraction index can be related to density

increase in the films. As mentioned in section 2.2.2, the reduction of deposition

temperature in these films emphasizes the effects of high germanium content and the

increase of micro-voids formation. In ref. [42] it is reported an increase of refractive index

from n=4.20 to 4.30 at deposition temperature range from Td=150-250°C for Ge:H films,

this data is according to the observations of our results. The values of refraction index for

Ge:H films are reported in ref. [49]. Refraction index values were obtained in the range

from n=4.23 to 3.96 at deposition temperature Td=200°C. The comparison of ref. [49] and

our results show that our films are a less dense material at deposition temperature

Td=200°C. In relation with the Urbach energy characteristic, values are reported in the

range from Eu=44-46 meV for a-Ge:H film deposited at Td=150°C [42]. We know that


Urbach energy values is larger for Ge:H films than Si:H films with the values between the

range EU =50-60meV [52]. In INAOE low values of Urbach energy (EU=30meV ) for

nanostructured SiGe:H films deposited at high deposition temperature Td=300°C has been

reported [45]. For this thesis the lowest value of Urbach energy was EU=33meV and it was

obtained at low deposition temperature Td=160°C. The low Urbach energy was reflected in

the spectral dependence of absorption coefficient, α(hv). It showed a relative low tail defect

density and mid-gap defect states (Fig. 4.7) for the sample deposited at Td=160°C. These

characteristics of the sample deposited at low deposition temperature shows improvement

of the transport properties of the material in comparison to other deposition temperatures.

A lower value of EU in material means lower density of tail defects and good electronic

characteristics. The Urbach energy value obtained in this work at Td=160°C was the best

value in comparison with the literature dealing with GexSi1-x:H (x>0.5)films deposited at

low temperature (Td<200°C).

6.2.4 Effect of deposition temperature on electronic properties.

The results of electronic characteristics in this thesis are presented in section 4.2.5. It

shows that for low deposition temperatures Td<200°C the slope for the dark conductivity

temperature curve is modified by measurement of temperature, this effect is higher at

deposition temperatures Td=70°C and Td=130°C (Fig. 4.11).The changes could be because

of the temperature of measurement Tm is higher than the deposition temperature Td and this

produces changes in the transport properties of the films during the measurement

(annealing effect by the temperature of measurement).

The results of this work on dark conductivity are presented in Figure 4.12. The

effect of deposition temperature reduction shows a trend of discontinuous change of dark

conductivity in the range of deposition temperature from Td=70°C to 300°C. The minimum

and maximum values obtained with dark conductivity were σdark=1.3x10-5

Ω-1

cm-1

(Td=190°C) and σdark=9.6x10-4

Ω-1

cm-1

(Td=300°C). These results demonstrated a strong

effect of deposition temperature on the transport characteristics of the films. The changes in

the conductivity shows an optimal region of values in the temperature range from


Td=160°C to 220°C with lower values than other temperature regions as is shown in Figure

6.1b. Lower dark conductivity is related to the passive dangling bond defects and low

impurity contaminations. The representative values for dark conductivity in Ge:H films are

reported in ref. [46] from σdark= 10-5

to 100 Ω

-1cm

-1 at deposition temperature ranges from

Td= 30 to 310 °C in Ge:H films. In ref. [23] the study of dark conductivity as a function of

deposition temperature for a-SiGe:H (Eg=1.32 eV) alloys (Figure 2.5a) is presented. The

study shows discontinuous change of dark conductivity (σdark=10-11

to 10-7

Ω-1

cm-1

) in the

deposition temperature range from Td=230 to 370°C with the maximum of dark

conductivity at deposition temperature around 320°C. The low values of dark conductivity

was achieved at low deposition temperature Td<300°C. In ref. [45] it is studied the effect of

deposition temperature on dark conductivity in a-SiGe:H films deposited at different

dilution ratios and shows increase of dark conductivity from σdark=10-11

to 10-9

Ω-1

cm-1

with

an increase of deposition temperature from Td=170 to 230°C with a low dilution ratio (5:1).

The dark conductivity seems unaffected by the deposition temperature with high dilution

ratios (27:1 and 54:1). According to this discussion, our results and the data reported in the

literature show an increase of dark conductivity with the increase of deposition temperature

in the range from Td=120 to 300°C and it shows similar values of dark conductivity to our

results. The good quality films are reported in literature for low optical gap materials

(Table 6.1). These characteristic are important for the application of the intrinsic a-GeSi:H

films as absorption device layer..

For activation energy, our results show a constant activation energy with the increase

of deposition temperature from Td= 70°C to 190°C and then a discontinuous change in

activation energies from deposition temperature Td= 190 °C to 300°C is achieved with a

final decrease in activation energy from Ea=030 eV to 0.23 eV. An interesting observation

is that more an intrinsic characteristics in the films is not obtained at high deposition

temperature with maximum values, Ea=0.38eV and EF=0.37 eV at deposition temperature

Td=220°C (Fig. 4.13). It is generally found that un-doped amorphous germanium-silicon

materials has n-type characteristics due to uncompensated dangling bonds and then un-

doped material, where EF is near mid-gap, has a relation with low defect states density and


low impurity contaminations. According to this, our results show an optimal deposition

temperature in the range from Td=160 °C to 220°C.

Activation energies reported in literature for Ge:H films as a function of deposition

temperature are published in ref. [46], two parts are identified for activation energy trend:

a) In part one activation energy stays constant Ea~0.35 with the increase of deposition

temperature from Td=40°C to 250°C and b) Part two shows a decrease of activation energy

from Ea= 0.40 eV to0.1 eV with the increase of deposition temperature in the range from

Td~240 to 310°C (Fig.2.6a). Both works, [46] and this thesis, have similar values of

activation energy at optimal deposition temperature Td=160°C. As a comparison, in ref.

[41] activation energies for un-doped Ge:H films deposited at high temperature are reported

in the range from Ea=0.45-049 eV . For device applications, in ref [38] it is reported a

device based in i-Ge:H film deposited at low temperature Td=174 C° with activation energy

Ea=0.3 eV. In comparison to our results, we have similar values of activation energy at

lower deposition temperature Td=160°C and more intrinsic activation energy (Ea=0.38eV)

for deposition temperature Td=190°C. The Fermi level follows the same activation energy

trend discussed previously. Finally, the temperature coefficient was extracted; the results

show no effect of temperature reduction on the temperature coefficient. In literature, there

are no reported studies of deposition temperature effect on temperature coefficient for Ge:H

films.

6.2.5 Effect of deposition temperature on photoconductivity

The photoconductivity results in this work are shown in section 4.2.6. The Figure

4.14 shows photoconductivity as a function of the deposition temperature. The

photoconductivity shows discontinuous change with the decrease of deposition

temperature. The photoconductivity values were obtained in the range σph=7.5x10-8

to

4.9x10-7

Ω-1

cm-1

at deposition temperature range from Td=70 to 300°C. The

photosensitivity was calculated and it is shown in Table 4.3. The best values of

photosensitivity were σph/ σd==2.39x10-4

, 2.19x10-5

Ω-1

cm-1

and 7.7x10-5

Ω-1

cm-1

. It was

achieved at low deposition temperatures: Td=190°C, Td= 220°C and Td=160°C


respectively. As a reference, the values of photoconductivity, that are reported in ref [23]

from σph=10-9

to 10-7

Ω-1

cm-1

for GexSia1-x:H (x=50) deposited at temperature range from

Td=230°C to 372°C (Fig. 2.4). The comparison of [23] with this thesis shows a similar

trend, an increase of photoconductivity with the increase of deposition temperature in the

range from Td=220 °C to 300 °C. In ref. [45] the effect of deposition temperature on

photoconductivity for SixGe1-x:H films (x=0.5) deposited with different dilution ratios is

reported. The study shows an increase of photoconductivity from σph=10-7

to 10-1

Ω-1

cm-1

with the increase of deposition temperature in the range from Td=170 to 230 °C with the

low dilution ratio of 2.5:1. The photoconductivity σph=1x10-5

Ω-1

cm-1

seems unaffected to

the deposition temperature for films deposited with high dilution ratios (27:1 and 54:1)

being similar to the dark conductivity data discussed previously. In relation to the

photosensitivity, in ref [46] the photosensitivity for Ge:H films as a function of deposition

temperature is reported. Values are reported in the range from σph/σd= 6 to 1 and the

deposition temperatures range are from Td=30°C to 310°C. The maximum value of

photoconductivity can be extracted from the graphic and it shows a value of σph~5x101 Ω

-

1cm

-1 at deposition temperature Td=120°C. The photoconductivity is related to the

generation, transport and recombination of holes and electrons. The discussion in this

section shows a similar trend of the photoconductivity as a function of temperature showing

increase of the photoconductivity with increase of deposition temperature. Our results show

standard values of photoconductivity for GexSi1-x (x>>0.5) films deposited at low

deposition temperature, except for ref. [46] which shows exceptional values. The Urbach

energy from the photoconductivity spectra EU-ph was extracted; values of EU-ph are higher

than the EU values extracted in section 4.2.4 due to EU-ph includes the influences of the

charge transport.. The low values of EU-ph were obtained at low deposition temperature but

EU-ph as a function of deposition temperature shows unclear trend Data for EU-ph is not

reported in any literature

6.2.6 General discussion about deposition temperature reduction

In this work, we studied the effect of deposition temperature reduction on the

interrelation properties of GexSi1-x:H (x=0.95) films. In our results, we found a general


decrease of hydrogen content CH, with a non-corresponded change on optical gap Eg (with

the increase of deposition temperature).. These results contrast with the data reported in

literature [23] and [42] as we discussed previously. For optoelectronic characteristics, in

Figure 6.1 the summary of optical and electronic properties of the films as a function of

deposition temperature is shown. Our results in this work for Ge0.95Si0.05:H films show a

discontinuous change in electronic and optical characteristic with increases of deposition

temperature. The Figure 6.1 shows that for continuous increases of deposition temperature

we can be identified three parts in the trend of optical and electrical characteristics. The

three regions identified for minimums and maximums are: Region 1 from Td=70°C to 130

°C, Region 2 from Td=160 to 200°C and Region 3 from Td= 250 to 300°C.

Figure 6.1Main results as a function of deposition temperature with three identified regions for a)

Activation energies and Fermi levels (Fig. 4.13), b) Dark conductivity (Fig. 4.12) c) Photoconductivity

(Fig. 4.14) and d) Urbach Energies (Fig.4.9)


The most interesting results are obtained in region 2 showing a) activation energy

maximum, b) dark conductivity minimum, c) low photoconductivity values but maximum

in photosensitivity (Table 4.3) and d) the lowest value of Urbach energy. The lowest σdark

and more intrinsic characteristics in undoped films are attributed to the passivation of

dangling-bond defects and this effect can be verified by the spectral dependence of the

optical absorption coefficient (Figure 4.7). The density of tail states is proportional to the

Urbach energy, lower Urbach energies means a reduction in the tail defect density and mid-

gap defects. The reduction on tail defects improves the transport properties and reduces the

recombination centers. The Figure 4.7 shows this correlation where the low tail states are

obtained with the temperature Td=300°C and temperature in region 2 from Td=160°C to

220°C.

In Table 6.1 is shown a comparison of the results in our work and studied literature.

The data is organized from the lowest Ge concentration to the highest Ge concentration. It

shows that the studies in literature of the deposition temperature effect on Germanium-

based films are focus only on one type of characteristics (optical or electrical) in contrast to

our work, that is a complete study in the main characteristics (electrical and optical). The

Table 6.1 shows that the best value of Urbach energy obtained in our work is EU=33 meV

and the dark conductivity values are similar for Ge:H films in ref. [46]. The low values of

dark conductivity are obtained by increasing Ge content, but an adjustment with the values

of low optical gap needs to be taking into account [45 and 23].

With regard to photoconductivity and photosensitivity, the higher values are obtained

in ref [46]. This shows the potential of Ge:H films in PV application and these

characteristics needs to be improved in the future. One proposal could be a compensated

intrinsic film by B doping, which is discussed in a special section of the doping results..

Other interesting finding is that the deposition rate is one order of magnitude higher than

RF frequency in ref. [46] under the conditions of high H dilution ratio and LF plasma.


Table 6.1 Comparison of intrinsic a-GeSi:H properties deposited by PECVD at low deposition temperatures.

Technique CGe%

Temperature

range CH% Eg (eV)

σph

(Ω-1

cm-1

)

σdak

(Ω-1

cm-1

)

σph/σdark Ea (eV) EU

(meV)

n

Vd

A°/s Ref

RFPECVD

13.56MHz 100

150-250

0

1.22-

1.16 - - - -

46 -60

4.20-

4.30 -- [42]

RFPECVD

13.56MHz 100

30-310

10-1 0.8 -- 10

-5-10 ~6-1 - - -

0.24-

0.13 [46]

RFPECVD

13.56MHz 100

200

4.4-

7.7

1.19-

1.25 - - - - -

4.23-

3.96

10.5

[49]

RFPECVD

13.56MHz 100

175°C

- - - - - 0.3 - - [38]

LF PECVD

100 kHz 95

75-300 11 @

160°C

1.09-

1.20

7.5x10-8

-

4.9x10-7

5.7x10-5

-

9.6x10-4

2.8x10-

5-

2.3x10-4

0.23-

0.38 33-76

3.92-

4.18

0.91-

1.45

THIS

WORK

RF PECVD <50 170-230

13-5 1.32 10

-7-10

-5 10

-11-10

-9 - -

- - - [45]

RFPECVD

13.56MHz 50

230-372

13-4 1.6-1.3

10-9

-10-7

(Δσ) - - - -

[23]


To summaries, this discussion demonstrated optimal deposition temperatures from

Td=160°C to 220°C for Ge0.95Si0.05:H films deposited in the fabrication ranges of density

power=0.02 Watts/cm2, low frequency=110 kHz, pressure=0.6 Torr and H dilution ratio

R=75. One of more interesting samples was obtained with the low deposition temperature

Td=160°C, Which showed electronic and optical characteristics of a good quality likewise

the lowest Urbach energy EU=33 meV, low dark conductivity σdark= 1.3x10-4

Ω-1

cm-1

and

near activation energy Ea=0.32 eV to the mid gap energy of the material. Moreover, the

temperature Td=160°C was compatible with flexible plastic substrates and for this reason

this temperature was selected for the study of doping at low deposition temperature which

we discussed in the next section.

6.3 Discussion about doping for a-GeSi:H films

deposited by PECVD at low temperature

In this section, the discussion focuses on the doping results. The first part is a special

discussion about the comparison of results obtained at low deposition temperature

Td=160°C versus high deposition temperature Td=300°C deposited with the same

installation and methodology. In the second section the results of doping at low deposition

temperatures are discussed and contrasted with the analyzed literature.

6.3.1 Doping at low deposition temperature (160°C) and high deposition

temperature (300°C)

In [54] it is published a study of doping at deposition temperature Td=300°C obtained

in INAOE as part of the background in Ge:H films.

The study was made with the same doping range and the same installation as this

thesis. The Ge:H films were grown and characterized by N. Delgadillo in capacitive low-

frequency (f=110 KHz) discharge with power W=300 Watts and pressure P=0.76 Torr from

GeH4, PH3 and B2H6 feed gases diluted with H2 as R= QH4/QGeH4 with R=75 at high

deposition temperature (Td=300°C). Boron and phosphorus concentrations in gas phase


were calculated in the deposition process as [B]gas=B2H6/2GeH4 and [P]gas=PH3/GeH4. The

studied range for boron doping was from [B]gas= 0 to 0.15% and the phosphorus doping

was studied in the ranges from [P]gas= 0 to 0.22 %. The compositions of boron and

phosphorus in gas phase to solid phase were extracted from SIMS measurements. In the

Figure 6.2 we show the comparison of doping incorporation in solid phase for boron and

phosphorous.

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

kHTB

=4.3 ±0.2

kLTB

=4.8+ 0.7

Ge (Td=300°C)

Ge0.96

Si0.04

(Td=160°C)

B content in gas phase [B]gas

%

B c

on

ten

t in

so

lid

ph

ase

[B

] so

l, %

0.04 0.08 0.12 0.16 0.200.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Ge (Td=300°C)

Ge0.96

Si0.04

(Td=160°C)

KLTP

=2.77 + 0.02P

co

nte

nt

in s

oli

d p

ha

se

[P

] so

l, %

P content in gas phase [P]gas

,%

KHTP

=3.5 + 0.3

Figure 6.2 Boron and phosphorus solid content dependence as a fuction of the gas content in

deposition process for low deposition temperature (LT) Ge0.96Si0.04:H films and high deposition

temperature (HT) Ge:H films.

The incorporation of boron in LT Ge0.95Si0.05:H films was described by the linear fit

[B]sol= kLTB [B]gas (kLTB= 4.8 ± 0.7) and the incorporation of boron in HT Ge films was

described as [B]sol= kHTB [B]gas (kHTB= 4.3 ± 0.2). It is interesting to notes that the

incorporation of boron is similar for HT Ge:H and LT Ge0.95Si0.05:H films, which suggest

that the doping incorporation has a low influence of Td in glow discharge process. The

electrical characteristics for boron doped films are shown in Figure 6.3. The graph for LT

dark conductivity σdark shows a drastic reduction from σdark= 10-4

to 10-7

Ω-1

cm-1

with the

increase of boron from [B]gas= 0 to 0.04% and an increases of conductivity from σdark= 10-

7 to 10

-4 Ω

-1 cm

-1 results of the increase of the boron concentration in gas phase from

[B]gas=0.04 to 0.14% in . For HT films the conductivity σdark decreases from σdark= 10-3

to

10-6

Ω-1

cm-1

in the range of [B]gas = 0 to 0.05% and then the conductivity shows an


increase from σdark= 10-6

to 10-4

Ω-1

cm-1

in the range of [B]gas= 0.05 to 0.15%. Both cases,

HT as LT, suggest a compensation of electron conductivity in the undoped films. The

compensation effect is relate to the maximum activation energy at HT with Ea=0.46 eV for

[B]gas=0.05% and LT with Ea=52 eV for [B]gas=0.04%. The electronic characteristics for

phosphorus doping are shown in the Figure 6.4.

0.00 0.04 0.08 0.12 0.16

1E-7

1E-6

1E-5

1E-4

1E-3

Ge (Td=300°C)

Ge0.96

Si0.04

(Td=160°C)

B gas concentration [B]GAS

%

Co

nd

uc

tiv

ity, ,

(-1cm

-1)

0.00 0.04 0.08 0.12 0.16

0.28

0.32

0.36

0.40

0.44

0.48

0.52 Ge (T

d=300°C)

Ge0.96

Si0.04

(Td=160°C)

B gas concentration [B]GAS

%

Ac

tiv

ati

on

En

erg

y, e

V

Figure 6.3 Electronic characteristics for boron doped films as a function of the gas content in

deposition process for low deposition temperature Ge0.96Si0.04:H films and high deposition

temperature Ge:H films.

0.04 0.08 0.12 0.16 0.20

10-3

10-2

10-1

100

P gas concentration [P]GAS

%

Ge (Td=300°C)

Ge0.96

Si0.04

(Td=160°C)

Co

nd

uc

tiv

ity, ,

(-1cm

-1)

0.04 0.08 0.12 0.16 0.200.12

0.16

0.20

0.24 Ge (Td=300°C)

Ge0.96

Si0.04

(Td=160°C)

P gas concentration [P]GAS

%

Ac

tiv

ati

on

En

erg

y, e

V

Figure 6.4 Electric characteristics for phosphorus doped films as a function of the gas content in

deposition process for low deposition temperature Ge0.96Si0.04:H films and high deposition

temperature Ge:H films.


For LT films, changes of [P]gas from 0.04% to 0.22% resulted in an increase in more

than 2 orders of magnitude in conductivity and also in a decrease in activation energy from

Ea=0.24 to 0.18 eV. For HT films, the same range demonstrates saturation of conductivity.

Both, HT as LT films, showed a continuous reduction of the Ea with the increase of [P]gas.

To summarize, in HT Ge:H and LT Ge0.96Si0.04 films, no effect of deposition temperature

was observed on boron incorporation from gas phase into solid phase in the samples

deposited at the temperature Td=300 and 160°C. The electrical characteristics of Ge:H and

Ge0.96Si0.04 films are similar, and both (HT as LT samples) demonstrated compensation

effect around [B]gas=0.05%. The lower phosphorus incorporation was observed with LT

Ge:H films in comparison to HT films and which is also reflected in the electrical

characteristics

6.3.2 Doped a-GeSi:H films at low deposition temperatures Td<200°C

Doped GexSi1-x:H (x>>0.5) films deposited at low temperatures Td< 200°C is

insufficiently research in literature. We can see this reflected in ref. [38], where the best

reported device of intrinsic Ge:H film in literature is based on doped Si:H films. However,

it is possible to contrast our results with data reported in ref. [56], in which the

phosphorous-doped and boron-doped germanium thin films deposited at low deposition

temperatures (Td=150°C and Td=190°C) are reported and this is depicted in Fig.2.7. The

phosphorus samples show both a reduction and a saturation of activation energy from

Ea=0.30 to 0.10 eV in the range of the gas concentration from [P]gas/[Ge]gas=0 to 0.30

(¨[P]gas%=0 to 30%). A similar effect is observed on dark conductivity from σdark=10-4

to

100 Ω

-1cm

-1. The comparison of ref. [56] with this thesis shows a similar effect according to

our results shown in Figure 4.20, with the values of activation energies from Ea=0.30 to

0.10 eV and dark conductivity values in the range from σdark=10-4

to 10-2

Ω-1

cm-1

for the

gas concentration from [P]gas%=0 to 0.25%. It is important to notes that our methodology

permits a very low gas concentration of dopants (boron and phosphorus) and consequently

it permits a very accurate control of the electronic characteristics.


For boron doping the comparison is more complex, in ref. [56] it is demonstrated a

discontinuous changes of activation energy and dark conductivity. A “compensation”

effect is observed with the gas concentration [B]gas/[Ge]gas=0.20 ([B]gas%=20%). This effect

is due to the changes in the structure of the films, from nanocrystalline to amorphous. In

our results a compensation of intrinsic film is observed with the gas concentration around

[B]gas%=0.04 %, and this effect could be caused by the compensation of dangling bonds in

undoped films. As a conclusion, the effect of compensation in ref. [56] and in [this thesis]

has different causes and occurs in different gas concentration ranges, [B]gas%=20% and

[B]gas%=0.04%, respectively.

6.3.3 Special discussion of compensated intrinsic a-GeSi:H film at low

deposition temperature

In the previous sections we discussed the results of the study of doping at low

deposition temperatures and one of the most interesting finding is a compensated intrinsic

a-GeSi:H film using low gas B concentration ([B]gas%=0.04%) at low deposition

temperature Td=160° C. In literature, no similar results at low temperature have been

documented. The activation energy changes from undoped material to compensated

material from Ea=0.32 to 0.53 eV with an increase of the gas concentration from [B]gas%=0

to 0.04% and this activation energy is very close to the mid-gap of the material. Another

important results is the change of dark conductivity by 3 orders of magnitude from undoped

material to compensated material from σdark=10-4

Ω-1

cm-1

to 10-7

Ω-1

cm-1

, respectively.

This value of dark conductivity in compensated material is the lowest value obtained for

high concentration a-GeSi:H alloys and it is even lower than the values obtained for

Ge0.50Si0.50 alloys reported in [45]. These characteristics of the compensated material are

very interesting for applications in devices. Moreover, the activation energy near to the

midgap of the material makes the compensated film ideal for application in photovoltaic

devices.


6.4 Disccusion about devices based on a-GeSi:H

films fabricated at low temperature

The results obtained from the fabrication study of p-i-n//n-i-p structures based on

Ge0.95Si0.05:H films deposited by PECVD at low deposition temperature Td=160°C are

discussed in this section. The discussion has been divided in to: a) a general discussion

about electronic characterization and structure evolution, b) a discussion of SIMS study, c)

a discussion of short circuit current spectra and d) a final discussion, a comparison of the

best results obtained and future works.

6.4.1Electronic characterization and structure evolution

In Figure 5.1 it is shown the general progress of current density for the fabrication

study of p-i-n//n-i-p structures at low deposition temperature Td=160°C. As presented in

section 5.2, the evolution of the studied structure was from a single p-i-n structure based in

Ge0.95Si0.95:H films to anti-diffusion Si-Ge-Si structure and finally a hetero-junction

intrinsic thin films (HIT) structure based in Germanium-Silicon (Fig. 3.11). The results of

each structure are discussed below:

a) a-GeSi:H (BASIC) structure:

The first structure was obtained based on the study of deposition temperature

reduction and doping study, which is presented in section 3.6.2 and was discussed in

the previous section. A predictable problem in the initial structure was the increase of

voids due to the amount of clustered hydrogen in the films deposited at low deposition

temperatures as mentioned in section 2.2.1.

In ref. [65] it is shown that the additional hydrogen incorporation increases void

fraction concentration in a-Si:H films without an increase of optical gap. As discussed

in section 6.2.3, our results on a-GeSi:H film are according to this trend. In order to

interrupt the continuous void formation through the structures in the sandwich


configuration, the flow methodology presented in section 3.6.3 was considered. The

methodology consists in power interruptions in the fabrication flow in BASIC structure.

However, the results for basic structure in section 5.3 show no functional samples for

this structure. The linear behavior of the current-voltage characteristics can be a proof

of the presence of short circuits due to the void formation through the sandwich contact

configuration of the sample and the diffusion of dopants in the structure.

In order to continue with the fabrication program, a structure based on anti-

diffusion silicon layers to avoid the diffusion of dopants and metals through the

structure was suggested. In the future work, we consider changes in the fabrication

parameters of intrinsic a-GeSi:H film deposited at low temperatures in order to reduce

the void formation. This idea can be found at the end of this chapter.

b) ANTID structure; anti-diffusion silicon layers

The first ANTID structure results are shown in section 5.4. The use of anti-

diffusion silicon layer was done in order to prevent short circuits through the structures.

The thickness of the anti-diffusion layer was consider in order to get transparency to the

charge transport around t=500 A°. The result of mapping shows first open circuit

voltage in the structures with the value Voc= 28 mV. However, the current response was

not observable in the first structure. The I-V characteristics show that the anti-diffusion

layers increase the shunt resistance of the structure and consequently prevents the short

circuit discussed for the BASIC structure.

In order to continue with this idea, the increase of thickness of the absorption a-

GeSi:H layer was suggested as fabrication program. The mapping results for the

increase of the absorption a-GeSi:H layer are presented in Table 3.7. It shows that the

increase of thickness of intrinsic a-GeSi:H layer results in the first short circuit current

response Jsc=1.6x10-5

A/cm2 and an increase of the open circuit voltage from Voc=75

mV to 420 mV with YIELD process of 93 %. These improvements are related to the a-

GeSi:H film and the increase of the shunt resistance in the structure. These results in


this stage show optimal intrinsic a-GeSi:H thickness at t= 9 000A° and the increase of

the thickness above this value results in dead structures (process 1037). A third anti-

diffusion layer was implemented, but a lower current density was obtained in

comparison with two anti-diffusion layer configurations.

Two simultaneous programs were done in order to reveal the performance of anti-diffusion

layers: a) the optimization of doping program in a-GeSi:H layers (section 5.4.3) that

consists in an increase of doping flows (diborane and phosphine) and keeping a constant

anti-diffusion layer thickness, and b) the optimization of anti-diffusion thickness program

(section 5.4.4) that consists in the increase of doping flows and the thickness of anti-

diffusion layers simultaneously. The results of both programs are summarized in Fig. 5.14

and it shows the improvement of current-voltage characteristics. The best result, Jsc=1x10-

4A/cm

2 (H_8V), is obtained with the maximum of doping flows and the lowest Si film

thickness (process 1075, Table 5.11). The increase of doping in a-GeSi:H layers increases

the short circuit current density in one order of magnitude more than the initial structure in

these programs. The most important findings at this stage are shown in the Figure 5.14c.

The increase of short circuit current reveals a potential barrier in the low voltage range in

current-voltage characteristic (Figure 5.15). This effect is observed with the anti-diffusion

layer thickness value of 500°A. This refutes our initial hypothesis that silicon layers with

the thickness of t=500A° are transparent to charge transport.

To summaries, the anti-diffusion layer program results in the improvement of short circuit

density with the best value of Jsc=1x10-4

(H_8V). The anti-diffusion layers were used to

prevent short circuits through sandwich structures and it allowed the increase of doping

flows and the increase of the potential barrier. We observer, that the anti-diffusion layer

thickness of 500A° is not transparent to the charge transport. A potential barrier in the

interface of GeSi/Si needs to be reduced or eliminated in order to increase the short circuit

current and to improve the current-voltage characteristics.


c) HIT structure based in Silicon-Germanium absorption layers

As we discussed in previous section, the anti-diffusion silicon layers are not

transparent to the charge transport. The simplified band diagram of ANTID structure is

shown in the Figure 5.16.

The diagram shows a potential barrier due to the difference between optical gap

for a-GeSi:H and Si:H film with one barrier for electrons and two potential barriers for

holes. In order to eliminate the highest barrier for holes we suggested the gradual

reduction of the silicon layer in the p-region, and these results are presented in Table

5.13 and Figure 5.18. As a result of the reduction of the anti-diffusion layer in p-region,

we obtained a change of polarity in the structure and a drastic reduction of Jsc and Voc.

As mentioned in section 5.5, the reduction reveals a contribution in current of the anti-

diffusion layers to the structure and it was found that the major contribution of current

is from the anti-diffusion Si layer in p-region which was eliminated at this stage.

In order to eliminate the counter barrier for electrons in n- region and keep the

major current contribution of the anti-diffusion layer in p-region we intended the HIT

structure.

The transition from ANTID to HIT structure is shown in the Figure 5.19. The

results presented in Table 5.15 and Figure 5.20 show the progress of the current-

voltage characteristic at this stage. The transition to HIT structure shows an

improvement of two orders of magnitude in short circuit current density (H_8V) from

the best ANTID structure Jsc= 1x10-6

A/cm2 to the HIT structure process 1086

Jsc=8.24x10-8

A/cm2. We improve the open circuit voltage from Voc=445 to 550 mV

respectively. The Figure 5.20 shows the progress in short circuit current and how the

potential barrier for electrons was eliminated from the structure. This effect is reflected

in the current voltage-characteristic at low voltage ranges..


Once the transition from HIT structure was done, a program for optimization of

absorption layers was considered and the results are shown in Table 5.18 and Figure

5.21. The best result was maintained for the process 1086 with a silicon layer thickness

of t~ 1100 A° and an intrinsic a-GeSi layer thickness of t ~8000 A°.

Finally, the best structure produced on glass substrate was obtained with n-i-p

structure on different substrates including flexible substrates. In order to improve the

serial resistance of the films, we intended to use contacts deposited by sputtering

technique. The different structures are shown in Figure 5.22. In this figure, the increase

of short circuit current density was achieved from glass substrate Jsc=8.24x10-8

A/cm2

to PEN substrate with non-transparent Ti bottom contact Jsc=1.12x10-3

A/cm2. The

current-voltage characteristic achieves better quality as is shown in Figure 5.23. The

exceptional result was that the higher current density was obtained on flexible substrate

rather than glass substrate.

The comparison in Figure 5.23b of the structures 1146_Ti and 1146_Al shows

that the non-transparent Al contact increases the short circuit current from Jsc=6.10x10-4

to 1.12x10-3

A/cm2, but it decreases the open circuit voltage from Voc=520 mV to 438

mV respectively. This suggests an increase of shunt resistance and a reduction of the

serial resistance in the sample 1146_Al. The comparison of glass substrate and plastic

substrate shows higher open voltage values in plastic PEN substrates than glass

substrate, which suggests low serial resistance in PEN substrates.

To summaries, the studied films from deposition temperature reduction program and

doping study were applied in different p-i-n structures. The short circuit structures were

obtained from BASIC structures and we indirectly identified a problem with high

concentration of voids. The problem with short circuit structure was solved with ANTID

structure. The barriers discussed for ANTID structure demonstrate that the doping films

studied in this thesis can be used as functional p-i and i-n junction. The potential barriers in

p- region show a higher contribution of current voltage characteristic than potential barriers

in n- junction. The HIT structure was considered in order to use the contribution of anti-


diffusion Si layer. The best result was obtained in flexible substrate PEN with non-

transparent Al contact, our comparison with literature is shown in Table 6.2. The short

circuit current and the open circuit voltage responses for n-, p- and i- GexSi1-x:H films with

high concentration (x>>50) are not reported in any literature.

Table 6.2 the Comparison of the result in this thesis and literature for p-i-n structures based on a-

GeSi:H layers with a high concentration of germanium (>> 50%) deposited at low deposition

temperatures.

Reference Td (°C) Doped Ge

films

Jsc

(mA/cm2 )

Voc

(mV)

Flexible

substrate

[56] 150 NO 20.6 250 NO

[38] 190 YES -- -- NO

[this work] 160 YES 1.12 530 YES

The comparison shows the better open circuit voltage in our samples, which is two

times more than the reported in ref [56]. Another improvement in this thesis is the

deposition on flexible substrate that is not reported in literature for germanium structures.

In ref [44] it shows a complete p-i-n structure based on Ge:H films with a maximum

deposition temperature Td= 190°C and only the current-voltage curve is shown and this is

presented in Figure 2.8. A note to ref [44] is that neither the values of short circuit current

nor the open circuit voltage are studied. The highest value for Jsc in p-i-n structure is

reported in ref. [56], maximum value of Jsc =20.6 mA/cm2 was reached using intrinsic

Ge:H film and Si:H doped films for n- and p- type layers deposited at deposition

temperature Td=150°C on Corning glass. In order to reach that value of current density, the

i-Ge:H // p-Si:H interface was optimized with a region rich in defects and an elevated

tramping and recombination center to allow the charge transport through the potential

barrier. This idea is consider in the future work for our structures at the end of this chapter.

The comparison with literature of our results shows the highest open circuit voltage, a

deposition on flexible substrates with a future improvement on the short circuit current and

it demonstrates the potential of our final structure obtained in this thesis..


6.4.2 SIMS results discussion

Figure 5.24 shows the results of SIMS profile for HIT structure (p-GeSi//i-GeSi//i-

Si//n-Si) deposited at low deposition temperature Td=160°C. Note that for both interfaces in

doped regions there is an abrupt transition of the dopants concentrations from n-type to i-

layer and p-type to i-layer, with a slight inadvertent doping contamination within intrinsic

layer. For comparison in ref. [65] shows a depth profile of dopants concentration in p-i-n

structures fabricated with single reaction chamber and multi reaction chamber methods.

The values of dopant concentration within of intrinsic layer are reported for boron

contamination around 10-16

in single chamber installation and 1017

atoms/cm3 in multi-

chambers installation. Our results in the single chamber installation show values around

10-16

atoms/cm3 for B contamination, similar to multi-chambers installation results reported

in [65]. In relation to this, it is shown in Figure 5.25, how silicon absorption layer acts as

anti-diffusion layer and how it reduces B concentration from 1018

to 1016

atoms/cm3 in the

intrinsic GeSi-Si interface. For B doping, the maximum concentration peak decreases four

orders of magnitude to a depth of 170 A° while phosphorus decreases to a depth of 300 A°

(no anti-diffusion layer in this region). In this work the oxygen and carbon contaminations

are less than 0.5 %. The levels of contamination ware monitored and controlled in the

maintenance programs indicated in Figure 5.1. For good quality a-Si:H films are reported

with concentration less than 5x1018

Atom/cm3 in oxygen and carbon, with leakage rates

less than 1x10-5

torr-L/s [65]. In this work, our results show in comparison to [65], a low

contamination of carbon and high contamination of oxygen with a leakage rate at ~10-3

torr-L/s. This value needs to be improving in order to increase the quality of the films.

6.4.3 Discussion of the short circuit current spectra

The short circuit current spectra results are presented in section 5.5. The study of

these spectra is based on the transition from symmetrical ANTID structure to HIT structure

and it shows the contribution of the layers to the structures. The Figure 5.25 shows spectral

response for short circuit current.


If the symmetrical ANTID structure (Figure 5.25 process 1075) is used as reference

for the analysis, the results can be discussed as follows:

Process 1076; we observe the reduction of silicon thickness in p-region: it is

observed the reduction of Isc (1.97 eV) in the spectra as a consequence of the

reduction of contribution of silicon layer in p-region. It is important to notes that for

energies ~1.1 eV the a-GeSi layer contribution is not affected.

Process 1078; Increases of silicon thickness in p-region: The current contribution

of silicon layer in p-region increases to Eop>1.97 eV and the potential barrier for

GeSi holes increases but the GeSi contribution is compensated by the next effect.

The decrease of thickness in n-region decreases the thickness of the potential

barrier for electrons in GeSi-Si interface therefore the Isc contribution increases for

GeSi (~1.1 eV) and Si (>1.97) layers, respectively

Process 1079; the increase of silicon thickness in p-region more than in the process

1078: It increases the potential barrier for holes in i-GeSi layer and the current

contribution decreases for energies ~1.1 eV and finally it increases bulk

contribution of silicon layer in p-region (~2.5 eV)

The silicon thickness decreases in n-region more than the process 1078. It

decreases the potential barrier in n-region for GeSi and Si electrons and the current

contribution increases in general except for the value energy at ~1.1 eV. The reason

of this may be because of the potential barrier for holes in GeSi-Si interfaces in p-

region.

Process 1086; the increase of silicon layer thickness in p-region. It increases the

contribution of bulk silicon contribution in energies around ~2.5 eV. The

elimination of silicon layer in n-region, it eliminates the potential barrier for GeSi

and Si electrons and the Isc contribution increases in all regions of the spectrum.


In general the previous discussion demonstrates the functionality of i-, p- and n- films

deposited at low temperature Td=160°C. The Figure 5.26 shows a special experiment for

subgap region in HIT structure. The results show increases of current in one order of

magnitude for the region of sub-gap under IR illumination in wavelength of 800nm. The

increase of lower energies suggests that this contribution is mainly attributed to i-GeSi

layer. The analysis of the transition from ANTID to HIT structure showed the control of

current contributions and the formation of functional potential barriers for each interface.

6.5 Future improvements

This thesis is focus on the study of films deposition, doping and device fabrication at

low deposition temperature. The study of the applications of the films in device structures

allows feedback between the film stage and device stage. Thanks to this study, new lines of

research have been identified as future works and in order to continue with the

improvements for films and structures in the INAOE laboratory, the following proposals

will be discussed:

Improvement on films

Study of voids formation and optimization of H-dilution: The discussion of

hydrogen incorporation, optical gap, photoconductivity and short circuits in GeSi

structures reveals a possible high void formation at low deposition temperatures.

Our hypothesis is that the high hydrogen incorporation without the corresponding

change in optical gap suggests that this extra incorporation is the cause of the void

formation by clustered and no-bonded hydrogen. As a future work we propose a

study of the optimization of hydrogen dilution and void formation for the low

deposition temperature Td=160 °C.

Study of compensated intrinsic films by boron doping: The doping control study

in this work demonstrates that a compensated intrinsic film is obtained at a low

concentration of [B]gas=0.04%. . The compensated film showed interesting values of


electronic characteristics and it can be used as absorption layer in photo-voltaic

structures. We suggest a study of these compensated films and opto-electronic

characterization.

Study of the flexible substrate effect on the characteristic of the films: This

work demonstrates the deposition of a-GeSi:H film on flexible plastic substrates

without visible problems of adhesion in polyimide and PEN substrates. However a

more precisely study needs to be done in order to understand the effects on the

electronic and optical characteristics.

Improvement on device structure

Improvement of BASIC GeSi structure: The study of photoconductivity spectra

and current voltage characteristic demonstrate functional p- and n-type a-GeSi:H

films for device applications. The improvement for reduction of voids and a

possible use of compensated intrinsic film are main ideas to r continue work with

the BASIC structure.

Optimization of GeSi-Si interface: In ref [56] an optimization of Ge-Si interface

rich in defect in order to improvement transport charge in the potential barrier is

presented. We advise a similar study for the GeSi-Si interface for HIT structure

studied in this work in order to recollect the concentration of holes in this interface.

The study of photoconductivity spectra for sub gap absorption demonstrated the

potential of this improvement in the structure.

Proposed new HIT structure using doped Si:H layer: A re-order of the

HIT structure is proposed in order to eliminate potential barrier of Ge-Si hetero-

junction and we suggest an optimization of Si:H films deposited at low deposition

temperature.. It is important to note that the Si:H film used in this work was not

optimize for the low deposition temperature.

P a g e | 135

7. CONCLUSIONS

Main conclusion

In this thesis the study of fabrication and characterization of Ge0.95Si0.05:H films were

noticing a non-continuous change in the optoelectronic characteristics. The optimal

deposition temperature for good quality films was found to be Td=160°C. The doping was

systematically studied and the electronic characteristics were controller by boron and

phosphorous doping. The precise study allowed the deposition of intrinsic film

compensated by B doping with good electronic characteristics. The new knowledge of the

films deposited at low temperature was applied in the fabrication of three different devices

structures and the functionality of the films in devices was demonstrated at the low

deposition temperature Td=160°C on flexible substrates.

Conclusion of each study

Some particular conclusions of the programs in this work are:

1. Study of the deposition temperature effect on composition, electronic and optical

properties for i-GexSi1-x:H (x>0.5) films deposited by PECVD.

The study of deposition temperature reduction in intrinsic aGe0.95Si0.05:H (hydrogen

content: 11%) films from Td=70°C to 300 °C revealed a discontinuous change of

electrical and optical properties with the decrease of deposition temperature.

The best values of dark conductivity were obtained with the deposition temperature

range between Td=160°C and 220°C and the best values of photosensitive were σph/

σdark==2.39x10-4

and 7.7x10-5

Ω-1

cm-1

at the low deposition temperatures Td<200

°C for deposition temperatures Td=190 °C and 160°C, respectively.

P a g e | 136 CHAPTER 7: CONCLUSIONS

Concluding on the results and discussion show that the film deposited at T=160°C

has the best electronic and optical properties with the temperature deposition; which

is compatible with flexible substrates. The Urbach energy value EU=33 meV at this

temperature is even better than the films obtained at low or high deposition

temperature Td<200°C reported in literature. The film deposited at this temperature

shows no visible problems of adhesion on flexible plastic substrates.

The increase of hydrogen incorporation at low deposition temperature without a

corresponding change in Eg and the change in refractive index and effect of photo-

conductivity reduction suggest a high formation of voids in the films deposited at

low deposition temperature. This is a problem in the fabrication of sandwich

structure configurations. We suggested a study of optimization of hydrogen dilution

at low deposition temperature and void formation.

2. Study of doping control (n- and p-type) in a-GeSi:H films deposited at low

deposition temperature (Td=160°C) by PECVD.

For p-type films the electrical parameters show discontinuous change with the

increase of B in gas phase. The increase of B concentration from [B]gas%= 0 to

0.04% results in increase of activation energy and Fermi energy, suggesting a

compensation for the electronic conductivity. Dark conductivity shows a drastic

reduction from σdark= 10-4

to 10-7

Ω-1

cm-1

with the increase of boron gas

concentration from [B]gas= 0 to 0.04% and when we increase from [B]gas=0.04 to

0.14% it results in an increase of dark conductivity from σdark= 10-7

to 10-4

Ω-1

cm-1

.

The characteristics of the compensated intrinsic film could be applied for

photovoltaic device applications.


Compensated intrinsic a-GeSi:H film with concentration [B]gas%=0.04% at low

deposition temperature Td=160° C was obtained as a part of the results in this work

and similar results are not reported in any literature in this range of temperature.

The activation energy changes from un-doped material to compensated material

from Ea=0.32 to 0.53 eV (mid-gap) and a change in dark conductivity by 3 orders of

magnitude from un-doped material σdark=10-4

Ω-1

cm-1

to compensated material 10-7

Ω-1

cm-1

. This value of dark conductivity is lower than reported in literature for

GexSi1-x:H (x>>0.5) films.

In p-doped films, gas P concentration were varied in the range from [P]gas% = 0 to

0.22%. In these films room temperature conductivity shows a continuous increase

of dark conductivity from σdark

=2.3x10-4

to 1.8 x10-2

Ω-1

cm-1

and a decrease of

activation energy from Ea=0.32 to 0.18 eV.

3. Fabrication of different p-i-n//n-i-p photovoltaic structures based in a-GeSi:H thin

films deposited at low deposition temperature (T=160°C).

Three different structures were fabricated (BASIC, ANTID and HIT structures) at

low deposition temperature Td=160°C. For the first time we obtained a structure

with photovoltaic response using n- and p-type layer with high Ge content films

deposited at low deposition temperature (Td<200°C) on flexible substrates.

The total progress of mapping of current density shows a current density increase of

three orders of magnitude from BASIC to HIT structure with YIELD over 90%. The

best result was obtained for n-i-p HIT structure deposited on flexible substrates

(PEN) with the values of Jsc= 1.12x10-3

A/cm2 and Voc=438 mV. The open circuit

voltage is the highest value (twice) obtained in comparison to similar structures

reported in literature. The fabrication of structures based on GexSi1-x:H (x<<0.50)

on flexible substrates is not reported in any literature.


For the B contamination we obtained a concentration around 10-16

atoms/cm3

deposited with an single chamber PECVD, Furthermore this level of contamination

is comparable with the obtained results for the multi-chamber installations in the

PECVD systems accomplish with silicon technology. The silicon absorption layer

acts as an anti-diffusion layer and it reduces B concentration from 1018

to 1016

atoms/cm3 in the intrinsic GeSi-Si interface.

For BASIC structure, the short circuit structures might result in setbacks with the

increase of voids due to clustered hydrogen in the films deposited at low deposition

temperatures, so therefore we suggest a study of hydrogen dilution and void

formation.

The discussion of the ANTID structure and the short circuit current spectra

demonstrate the functionality of i-, p- and n- GeSi:H films deposited at low

temperature Td=160°C. We demonstrated that in the short circuit current spectrum,

the potential contribution of Ge0.95Si0.05:H film was within the fabricated structures.

This work was committed to obtaining new technological knowledge of

Ge0.95Si0.095:H films and photovoltaic structures on flexible substrates according to the

interest of SENER-CONACYT-INAOE (Project No. 152244), and to increase the basic

research in renewable technologies in relation to goals of the national energy strategy for

the years 2012 to2016.

139 | P á g i n a

PUBLICATIONS

1. Cosme, I., Kosarev, A., Avila, F. T., & Itzmoyotl, A. (2012). Comparison of

Doping of Gey Si1-y:H (y>0.95) Films Deposited by Low Frequency PECVD at High

(300°C) and Low (160°C) Temperatures. MRS Proceedings, 1426(-1). Retrieved from

http://journals.cambridge.org/abstract_S1946427412008652

2. Cosme, I.; Kosarev, A.; Temoltzi, F.; Itzmoyotl, A.; "Electronic properties

of GeySi1−y:H films deposited by LF PECVD at low temperatures," Electrical

Engineering Computing Science and Automatic Control (CCE), 2011 8th International

Conference on , vol., no., pp.1-4, 26-28 Oct. doi: 10.1109/ICEEE.2011.6106609

(ISB:978-1-4577-1012-4)

3. Cosme, I.; Kosarev, A.; Temoltzi, F.; Itzmoyotl, A.; , "Study of doping of

Ge0.96 Si0.04:H films with B, and P during low frequency plasma deposition at low

temperature," Electrical Engineering Computing Science and Automatic Control (CCE),

2011 8th International Conference on , pp.1-3, 26-28 Oct. 2011 doi:

10.1109/ICEEE.2011.6106619 (ISB:978-1-4577-1012-4)

4.Francisco A., Andrey K., Ismael C, Sub-gap Photoconductivity in GeXSi1-

X:H Films Deposited by Low FrequencyPlasma at Different Substrate Temperatures,

International Congress on Intrumentation and applied Sciences 2nd

(ICIAS) 2011, Oct.

2011 ISB:978-607-02-2298-6

140 | P á g i n a

CONFERENCES

1. Electronic Properties of Gey Si1-y :H Films Deposited by LF PECVD at Low

Temperatures Ismael Cosme, Andrey Kosarev and Carlos Zuniga; MRS 2011

Spring Meeting; Symposium A: Amorphous and Polycrystalline Thin-Film Silicon

and Technology, San Francisco, California, USA (2011)

2. Cosme, I., Kosarev, A., Avila, F. T., & Itzmoyotl, A. (2012). Comparison of

Doping of Gey Si1-y:H (y>0.95) Films Deposited by Low Frequency PECVD at

High (300°C) and Low (160°C) Temperatures. MRS 2012 Spring Meeting;

Symposium A: Amorphous and Polycrystalline Thin-Film Silicon and Technology,

San Francisco, California, USA (2012)

3. Cosme, I.; Kosarev, A.; Temoltzi, F.; Itzmoyotl, A.; "Electronic properties of

GeySi1−y:H films deposited by LF PECVD at low temperatures," Electrical

Engineering Computing Science and Automatic Control (CCE), 2011 8th

International Conference on, Merida, Yucatan, México

4. Cosme, I.; Kosarev, A.; Temoltzi, F.; Itzmoyotl, A.; , "Study of doping of Ge0.96

Si0.04:H films with B, and P during low frequency plasma deposition at low

temperature," Electrical Engineering Computing Science and Automatic Control

(CCE), 2012 8th International Conference on, Merida, Yucatan, México.

5. Francisco A., Andrey K., Ismael C, Sub-gap Photoconductivity in GeXSi1-X:H

Films Deposited by Low FrequencyPlasma at Different Substrate Temperatures,

International Congress on Intrumentation and applied Sciences 2nd

(ICIAS 2011) ,

Puebla, México

6. I. Cosme, A. Kosarev, C.Zuñiga Study of electrical characteristic of Gey Si1-y

films deposited by low temperature PECVD process, Internationa workshop

Advanced Materials for Optoelectronics and Related Physics 2010, October 11-15,

INOAE, Puebla, México.

141 | P á g i n a

PROJECTS

1. “Research in Silicon-Germanium semiconductor alloys obtained by plasma and

new structures for uncooled micro-bolometer, with implementation and development of

analytical methods based in SIMS” No. D48454F (2007-2009) Mexican Council for Science

and Technology-CONACyT

2. “Photovoltaic solar cells based in silicon-germanium deposited by plasma on

flexible substrates” No. 152244 (2011-2013) Mexican Council for Science and Technology-

CONACyT

142 | P á g i n a

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149 | P á g i n a

LIST OF FIGURES

CHAPTER 1

Figure 1.1 Amorphous silicon PIN devices from Energy Conversion Devices Inc.

a) Flexible one-quare-foot Ovonic solar cell 1985 [11] b) PowerBond PVL uni-solar

product 2012 [12] ................................................................................................................... 3

CHAPTER 2

Figure 2.1 Factors in the formation of thin films in PECVD growth process; i)

condition and nature of the surface, ii) chemical nature of the substrate and iii)

deposition temperature. .......................................................................................................... 6

Figure 2.2 Hydrogen content CH versus deposition temperature for Ge0.5Si0.5:H

films deposited by glow discharge deposition system [13.56 MHz]in the temperature

range from 230°C to 372°C (fig. taken from [23]). Total hydrogen content CH decreases

with the deposition temperature Td increase .......................................................................... 7

Figure 2.3 Deposition temperature Td dependence versus hydrogen content CH,

and elimination hydrogen mechanisms in amorphous silicon deposition (figures taken

from [26]) ............................................................................................................................... 8

Figure 2.4 Effect of deposition temperature for a-Ge0.5Si0.5:H films deposited by

PECVD taken from ref. [23]; a) Energy gap E04 (Eg), b) photoconductivity Δσ, versus

deposition temperature Td. .................................................................................................... 13

Figure 2.5 Effect of deposition temperature Td [45] on a) photoconductivity and

dark conductivity, b) bonding configurations of hydrogen, at different dilution ratios ....... 14

Figure 2.6 a) Dark conductivity σdark, photosensitivity σph/σdark and thermal

activation energy Ea versus deposition temperature, b) SIMS hydrogen concentration

profile of Ge layers deposited at different deposition temperatures [46] ............................. 15

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Figure 2.7 Electrical properties of phosphorus-doped and boron doped nc-Ge

films deposited by PECVD at deposition temperatures Td=150°C and Td=190°C [56] a)

thermal activation energy b) dark conductivity as a function of dopants concentration ..... 19.

Figure 2.8 Dark J-V characteristic at room temperature for p-i-n germanium

device deposited at 150°C and p-type layer deposited at 190°C, [56] ................................. 20

Figure 2.9 J-V characteristics of n-i-p germanium photodiodes with absorber layer

thickness of 40, 80 and 120 nm figure taken from ref. [38] ................................................. 21

CHAPTER 3

Figure 3.1 Scheme of plasma deposition system “APPLIED MATERIALS” Mod.

AMP330 used for fabrication process of films and structures ............................................ 23

Figure 3.2 Spectral dependence of optical absorption coefficient α(hv) at different

hydrogen dilution for Ge0.97Si0.03:H films deposited at Td=300°C [59]............................... 25

Figure 3. 3 Sample configurations for temperature series characterization: a) stripe

configuration: electronic characterization b) film configuration: SIMS and FTIR and c)

step configuration: thickness measurements ....................................................................... 28

Figure 3. 4 Fourier transform infrared spectroscopy data: a) primary data of

absorbance c) absorption coefficient data, c) bonding mode for GeH and d) stretching

mode for GeH ...................................................................................................................... 34

Figure 3.5 Example of absorption coefficient spectral for GeSi alloy with E04, E03

and Urbach energy extraction.. ............................................................................................. 35

Figure 3.6 Example for SIMS depth profile for GeSi films in regimes Cs2 (B, C,

O, Si and Ge detection)......................................................................................................... 37

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Figure 3.7 Experimental setup for temperature dependence of dark conductivity

measurements in the JANIS cryostat system. ....................................................................... 38

Figure 3.8 Data primary for extraction of activation energy, a) I-V characteristic at

different temperature of measurement b) conductivity as function of temperature for

different measurement series (see text SUB1, B2 and S2) ................................................... 39

Figure 3.9 Experimental installation for photoconductivity sub-gap measurments ... 40

Figure 3.10 Example of photoconductivity spectral in subgap region for GeSi film

and Urbach energy EU-ph extracted from photoconductivity [63] ......................................... 41

Figure 3.11 Proposal structures for device application a) (BASIC) p-i-n structure

based totally in GeSi:H alloys, b) (ANTID) p-i-n structure with silicon anti-diffusion

layers and c) p-i-i-n structure with SiGe and Si absorption layer (HIT structure). .............. 42

Figure 3.12 porcess flows for devices fabrication methodology to prevent cross

contamination (n- p- and intrinsic flow processes); main implementations are Flash

stages of H2 during the process and power interruptions to prevent voids in intrinsic

films. ..................................................................................................................................... 45

Figure 3.13 Sample configuration for devices with transparent titanium contacts ..... 46

Figure 3.14 Three illuminations types for device; H_3V (Halogen lamp 100

mW/cm2), H_8V(Halogen lamp 500 mW/cm2) and Sun lighting (100 mW/cm2) ............. 47

Figure 3.15 Experimental installation for spectral Isc response for devices

(measurments performed by Francisco Temotzil) ................................................................ 48

CHAPTER 4

Figure 4.1 Fabrication results at low deposition temperature; study of effect of

deposition temperature in the range Td=300°C to 70°C. ...................................................... 50

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Figure 4.2 Samples on polyimide substrates at deposition temperatures: a) 70°C

and b) 190°C, deformation increases with temperature. ...................................................... 50

Figure 4.3 Sample on PEN substrate at deposition temperature Td=190°C, no

visible deformation at tested temperatures and it show wide flexibility without adhesion

problems. .............................................................................................................................. 51

Figure 4.4 Deposition rate of GeSi films as function of deposition temperature in

the range from Td= 70°C to 300°C deposited by PECVD. (OPTICAL PROFILE for

sample deposited at Td=70°C) .............................................................................................. 52

Figure 4.5 FTIRabsorption spectra for GeSi:H films deposited by PECVD at

deposition temperature range from Td=70°C to 300°C ........................................................ 54

Figure 4.6 Hydrogen content calculated by FTIR a) Ge-H stretching mode peaks

for GeSi:H films at deposition temperatures 70°C and 300°C, b) Relative hydrogen

bounding content versus deposition temperature. ................................................................ 55

Figure 4.7 Spectral dependence of optical absorption coefficient for different

deposition temperatures extracted by FTIR transmission measurements. ........................... 56

Figure 4.8 Optical gap characterized by E04 and EgTAUC

energies as function of

deposition temperature for GeSi:H films . Solid lines are guide to the eyes. ...................... 57.

Figure 4.9 a) Urbach energy and b) Characteristic energy ΔE=E04-E03 as function

of deposition temperature for GeSi:H films extracted from absorption coefficient

spectra. .................................................................................................................................. 58

Figure 4.10 Refractive index n∞ as function of deposition temperature for GeSi:H .. 59

Figure 4. 11 Temperature dependence of dark conductivity for temperature series,

SUB1 (increases of measurement temperature), S2 (second increases of measurement

temperature) and B2 (final decreases of measurement temperature)…………………..60

Figure 4.12 Dark conductivity as function of deposition temperature........................ 61

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Figure 4.13 Activation energy Ea and Fermi Energy EF as function of deposition

temperature Td ...................................................................................................................... 62

Figure 4.14 Results of photo-conductivity characterization a) photo-conductivity

spectra for temperature series b) “photo” Urbach energy and photoconductivity as

function of deposition temperature ....................................................................................... 64

Figure 4.15 Deposition rate as a function of a) boron and b) phosphorus gas

concentrations for GeSi:H films deposited at Td=160°C ..................................................... 66

Figure 4.16 SIMS profile for intrinsic GeSi film deposited at deposition

temperature Td=160°C, Hydrogen, Carbon, Oxygen ........................................................... 67

Figure 4.17 SIMS profiles; concentration versus sputtering time for doping series

a) phosphorus doping and B)boron doping .......................................................................... 68

Figure 4.18 SIMS profiles; concentration versus sputtering time for doping series

a) phosphorus doping and B)boron doping .......................................................................... 69

Figure 4.19 Electronic properties as a function of boron concentration in gas

phase a) dark conductivity σdark and b) Activation energy Ea and Fermi energy level EF

for GeSi:H films deposited at low deposition temperature Td=160°C ................................ 70

Figure 4.20 Electronic properties as a function of phosphorus concentration in gas

phase a) dark conductivity σdark and b) Activation Energy and Fermi Energy level for

GeSi:H films deposited at low deposition temperature Td=160°C ...................................... 71

CHAPTER 5

Figure 5.1 Current density progress for device structures deposited at low

deposition temperature Td=160°C. It is shows progress in mapping result for

illumination H_3V and I-V curves. (Blue processes are maintain process for the

installation)……………………………………………………………………………..75

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Figure 5.2BASIC structure based in GeSi:H film results of temperature series and

doping series. Processes selected; 972(intrinsic), 981(n-type) and 978 (p-type)………77

Figure 5.3p-i-n structure on plastic substrates based in GeSi:H layer deposited at

low deposition temperature Td=160°C…………………………………………………77

Figure 5.4 a) current-voltage characteristic for BASIC structure process 1004

deposited at Td=160°C b) pin-holes in the sandwich structure as a cause of short circuit

and linear behavior in the curves……………………………………………………….79

Figure 5.5 Initial ANTID structure based on anti-diffusion silicon layers (process

number 1005) fabricated at low deposition temperature Td=160°C…………………...80

Figure 5.6 Current-voltage characteristic for ANTID structure process 1005

deposited at Td=160°Cunder illumination H_3V …………………………………...81

Figure 5.7Current-voltage characteristics for ANTID structures a) process 1021

and b) process 1034 dark and under illumination “H_3V”…………………………….84

Figure 5.8 Proposal structure based in three anti-diffusion silicon layers fabricated

at low deposition temperature Td=160°C (process 1035)……………………………...85

Figure 5.9 Current-voltage characteristic for process 1066, dark and lighting by

Halogen lamp at 8V adjusted response to Sun intensity……………………………….87


Halogen lamp at 8V adjusted response to Sun intensity……………………………….87


Halogen lamp at 8V adjusted response to Sun intensity. (low voltage regimen)…….88


Halogen lamp at 8V adjusted response to Sun intensity (low voltage regimen)………88

155 | P á g i n a


Halogen lamp at 8V adjusted response to un intensity…………………………………90

Figure 5.14 ANTID structures program a) Comparison of branches 3 and 4

structures, b) current-voltage for branches 3 and 4 and c) Low voltage regimen for

branches 3 and 4………………………………………………………………………..91

Figure 5.15 Experimental curve for low voltage regiment for process 1075(black),

ideal trend for current voltage characteristics of shut and serial resistances (blue) …..92

Figure 5.16 Analysis of band diagram for ANTID structure (p-GeSi//i-Si//i-

GeSi//i-Si:H//n-GeSi:H)………………………………………………………………..93

Figure 5.17 Proposal structure to probe potential barriers (reduction of anti-

diffusion thickness between p-type and i-GeSi:H layers)……………………………...93

Figure 5.18 Current-voltage characteristic for process 1076 (decreases of

thickness for anti-difussion layer) change bias in relation previous process…………..94

Figure 5.19 Transition from ANTID to HIT structure, process 1078, 1079 and

1086…………………………………………………………………………………….96

Figure 5.20 J-V characteristics for transition program from ANTID to HIT

structure: a) under illumination b) low voltage regimen under illumination c) log curve

under illumination and d) dark log characteristic (process 1078, 179, and 1086)……..97

Figure 5.21J-V characteristics for HIT program a) rectification curve b) low

voltage regimen comparison under H_8V illumination………………………………100

Figure 5.22 Fabricated structures for section 5.5.3; on glass substrate and flexible

plastic substrates. Reference process 1086, sputtering contacts process 1145 and n-i-p

configuration process 1146……………………………………………………………101

Figure 5.23 Current voltages characteristics for process 1146: b) under

illumination (linear) b) low voltage regimen under illumination c) log curve (dark )c)

156 | P á g i n a

under illumination (log), n-i-p process 1146_A (glass substrate), 1146_B (PEN

substrate) and 1146_C (PEN substrate and Al contacts)……………………………...102

Figure 5.24 SIMS profile for HIT STRUCTURE (process 1086): a) B- and P-

dopant profile and b) Carbon and Oxygen impurities concentrations through

structure……………………………………………………………………………….103

Figure 5.25 Short circuit current spectra for structures 1075, 1076, 1078 1079 and

1086…………………………………………………………………………………...104

Figure 5.26 Spectral responses for HIT structure, Halogen lamp (red) and Halogen

lamp+IR (800nm)……………………………………………………………………..105

Figure 5.27 Current-voltage characteristic for flexible substrate n-i-p structure

process 1046. *Energy conversion efficiency calculated using adjusted intensity for

100mW/cm2…………………………………………………………………………..106

Figure 5.28 Structure for flexible substrate n-i-p structure process 1046,

aproxímate thicknesses………………………………………………………………..107

CHAPTER 6

Figure 6.1Main results as fuction of deposition temperature with three identified

regions for a) Activation energies and Fermi levels (Fig. 4.13), b) Dark conductivity

(Fig. 4.12) c) Photoconductivity (Fig. 4.14) and d) Urbach Energies (Fig.4.9) ................ 117

Figure 6.2 Boron and phosphorus solid content dependence on gas content in

deposition process for low deposition temperature (LT) Ge0.96Si0.04:H films with and

high deposition temperature (HT) Ge:H films. .................................................................. 121

Figure 6.3 Electric characteristics for boron doped films as dependence on gas

content in deposition process for low deposition temperature Ge0.96Si0.04:H films and

high deposition temperature Ge:H films. ........................................................................... 122

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Figure 6.4 Electric characteristics for phosphorus doped films as dependence on

gas content in deposition process for low deposition temperature Ge0.96Si0.04:H films

and high deposition temperature Ge:H films. .................................................................... 122

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