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Graphenedielectric integration for graphene transistors

Lei Liao a, Xiangfeng Duan a,b,*aDepartment of Chemistry & Biochemisty, University of California, Los Angeles, CA 90095, USAbCalifornia Nanosystems Institute, University of California, Los Angeles, CA 90095, USA

Contents

1. Introduction: the promise and challenges for graphene electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

1.1. The promise of graphene electronics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

1.2. Graphene dielectric integration and its impact on transistor performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

2. Physical vapor deposition of top-gate dielectrics on graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

2.1. Raman characterization of graphenedielectric integration by PVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

2.2. Top-gated graphene transistors using PVD dielectrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357

3. Atomic layer deposition of top-gate dielectrics on graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357

3.1. Challenges in direct ALD of top-gate dielectrics on graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357

3.2. Molecular buffer layer for ALD deposition of oxide dielectrics on graphene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358

3.3. Metal oxide buffer layer for ALD of dielectrics on graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359

3.4. Low-k polymer buffer layer for ALD of dielectrics on graphene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361

4. Physical assembly of high-k oxide nanostructures as top-gate dielectrics for graphene transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361

4.1. Dielectric properties of Al2O3 nanoribbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3624.2. Dielectric nanostructuregraphene integration and interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362

4.3. Al2O3 nanoribbons as the top-gate dielectrics for graphene transistors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363

4.4. ZrO2 nanowire as top-gate dielectric for graphene nanoribbon transistors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365

4.5. Conductor/dielectric coreshell nanowires as the top-gate for graphene nanoribbon transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366

4.6. Integrated device array from nanowire gated graphene nanoribbon transistors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369

Materials Science and Engineering R 70 (2010) 354370

A R T I C L E I N F O

Article history:

Available online 8 August 2010

Keywords:

Semiconductor

Graphene

Transistor

Dielectrics

Nanowires

Nanoribbons

A B S T R A C T

Graphene is emerging as an interesting electronicmaterial for future electronics due to its exceptionally

high carrier mobility and single-atomic thickness. Graphenedielectric integration is of critical

importance for the development of graphene transistors and a new generation of graphene based

electronics. Deposition of dielectric materials onto graphene is of significant challenge due to the

intrinsic material incompatibility between pristine graphene and dielectric oxide materials. Here we

review various strategies being researched for graphenedielectric integration. Physical vapor

deposition (PVD) can be used to directly deposit dielectric materials on graphene, but often introduces

significant defects into the monolayer of carbon lattice; atomic layer deposition (ALD) process has also

been explored to deposit high-k dielectrics on graphene, which however requires functionalization ofgraphene surface with reactive groups, inevitably leading to a significant degradation in carrier

mobilities. Using naturally oxidized thin aluminum or polymer as buffer layer for dielectric deposition

can mitigate the damages to graphene lattice and improve the carrier mobility of the resulted top-gated

transistors. Lastly, a physical assembly approach has recently been explored to integrate dielectric

nanostructures with graphene without introducing any appreciable defects, and enabled top-gated

graphene transistors with the highest carrier mobility reported to date. We will conclude with a brief

summary and perspective on future opportunities.

2010 Elsevier B.V. All rights reserved.

* Corresponding author at: Department of Chemistry & Biochemistry, and California Nanosystems Institute, University of California, Los Angeles, CA 90095, USA.

E-mail address: [email protected] (X. Duan).

Contents lists available at ScienceDirect

Materials Science and Engineering R

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m s e r

0927-796X/$ see front matter 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.mser.2010.07.003
http://dx.doi.org/10.1016/j.mser.2010.07.003mailto:[email protected]://www.sciencedirect.com/science/journal/0927796Xhttp://dx.doi.org/10.1016/j.mser.2010.07.003http://dx.doi.org/10.1016/j.mser.2010.07.003http://www.sciencedirect.com/science/journal/0927796Xmailto:[email protected]://dx.doi.org/10.1016/j.mser.2010.07.003


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1. Introduction: the promise and challenges for graphene

electronics

1.1. The promise of graphene electronics

Graphene is a one-atom-thickplanar sheet of sp2-bondedcarbon

atoms arranged in a honeycomb crystal lattice (Fig. 1a and b) [1,2].

Graphene has attracted considerable interest in the past several

years due to its significant potential for both the fundamental

studies and technological applications [321]. Graphene is charac-

terizedby a linear dispersion relation withthe Dirac point separating

the valence and conduction bands witha zerobandgap, which limits

the achievable onoff current ratios but does not rule out analogue

radio frequency (RF) device applications. In particular, it has been

demonstrated that graphene exhibits the highest carrier mobilities

exceeding 200,000 cm2/V s [11], which is not only $100 timesgreater than that of silicon but also about 10 times better than the

state-of-the-art high mobility semiconductors lattice-matched to

indium phosphide. Along with many other desirable properties,

including large critical current densities ($2 108 A/cm2) [22], ahigh saturation velocity (5.5 107 cm/s) [23], and a micrometer-scale mean free path at room temperature, graphene is currently

regarded as a highly promising materials for high-speed electronics

[2328]. These desirable properties promise graphene to offerexcellent short-circuit current-gain cutoff frequency (fT) for high

frequency applications. In particular, it has been recently demon-

strated that high speed graphene devices were achieved with a

cutoff frequency fT reaching up to 100 GHz, demonstrating the

significant potential of graphene devices for radio frequency (RF)

applications (Fig. 1c and d) [25].

Dueto thezero-band-gap nature, bulk graphene remains highly

conductive even at the charge neutrality point, and therefore

cannot be used for effective field-effect transistors (FETs) at room

temperature for digital applications. The formation of graphene

nanostructures with lateral quantum confinement can open up a

finite band gap to enable room temperature FET operation ( Fig. 2)

[10,2937]. It has been demonstrated that sub-10 nm graphene

nanoribbons (GNRs) can be created, and used to fabricate FETs that

can be effectively switched off at room temperature [31,32]. We

have recently developed a rational approach to fabricate GNRs

with controllable widths from 6 to 20 nm using chemical

synthesized nanowires as the physical etch masks [35]. Impor-

tantly, using such GNR as the semiconducting channel, we have

fabricated room temperature FETs with onoff ratios> 100

(Fig. 2a). It has also been demonstrated that graphene nanomesh,

as a mimick of graphene nanoribbon networks, can effectively

introduce a finite conduction band gap into the two-dimensional

graphene to enable room temperature transistors with comparable

on/off ratios (Fig. 2b) [34].

1.2. Graphene dielectric integration and its impact on transistor

performance

The gate dielectric is an essential component of a transistor.Compared to the semiconductor channel, the gate dielectrics have

received much less attention due to the relative uninteresting

nature of an insulator, despite its significant impact on the critical

device parameters including transconductance, subthreshold

swing and frequency response. Among the various strategies

explored to fabricate graphene or graphene nanostructure based

Fig. 1. Graphene as a potential electronic material. (a) Schematic illustration of graphene as an atomic-scale honeycomb lattice of carbon atoms. (b) TEM image of graphene,

adapted from [2]. (c) Image of radio frequency devices fabricated on a 2-inch graphene wafer and schematic cross-sectional view of a top-gated graphene FET. (d) Measured

small-signal currentgain jh21j as a function of frequency f fora 240-nm-gateand a 550-nm-gate graphene FETat VD = 2.5 V. Cutofffrequencies,fT, were53 and 100 GHzforthe

550 and 240-nm devices, respectively. Adapted from [25].

L. Liao, X. Duan/ Materials Science and Engineering R 70 (2010) 354370 355


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FETs, most efforts employ a silicon substrate as a global back gate

and a 300 nm thick silicon dioxide (SiO2) as the gate dielectrics.

This substrate is used primarily because the graphene can be

readily visualized using optical microscope on this particular

substrate due to optical interference [3842]. The readily viewable

graphene on the 300 nm SiO2 substrate has allowed easy devicefabrication and led to many interesting scientific discoveries. The

requirement of 300 nm SiO2, however, can intrinsically limit the

resulted device performance in several perspectives. First, optical

detection technique to visualize graphene has been demonstrated

and widely used for a SiO2 thickness of 300 nm, but a 5% variation

in thickness (e.g. to 315 nm) can significantly lower the contrast

[40]. Second, the devices made on 300 nm SiO2 substrate have

small gate capacitance due to the relatively large dielectric

thickness and the small dielectric constant, and thus require high

switching voltage [43].

Exploring high-k dielectric substrate may mitigate these

problems [43,44]. For example, we have demonstrated that a

72 nm Al2O3/Si substrate is superior to the 300 nm SiO2/Si

substrate for both the visualization of graphene and the transistorperformance [43]. Compared to the 300 nm SiO2/Si substrate, the

72 nm Al2O3/Si substrate can enhance the optical contrast of single

layer graphene by at least 3 times. Furthermore, using the Al2O3film as the gate dielectrics, the back-gated graphene FETs have

been fabricated to exhibit more than 7-fold increase in transcon-

ductance.

It is important to note that the carrier mobility of the resulting

graphene transistors can be significantlyimpactedby the nature of

the dielectric layer and graphenedielectric interface. Theoretical

studies suggest that the intrinsic mobility of graphene, set by

longitudinal acoustic (LA) phonon scattering, can reach

105 cm2 V1 s1 at room temperature [45]. However, the achiev-

able carrier mobility in an actual device can often be limited by

extrinsic scattering sources, many of which arise from the surfacemorphology, chemistry, structural, and electronic properties of the

substrate. For example, recent calculation suggests that a mobility

as high as 44,000 cm2 V1 s1 can be achievedin graphene on SiO2/

Si substrate at room temperature, which is limited by the phonon

scattering [4548]. Experimental studies have demonstrated that

mobility values in the range of 220 103 cm2 V1 s1 for thegraphene transistor made on SiO2/Si substrate [49]. The variability

in mobility is attributed to various degrees of local disorder in

graphene and in the dielectric substrate.

Increasing the mobility beyond the extrinsic limits is one of the

central challenges of the graphene community. Recently, two

groups have reported a significant improvement in the mobility of

suspended graphene after current-heating annealing [12,50].

However, a more device friendly solution may rely on placing

graphene on a different substrate. Several alternatives have

been explored,however, the mobility obtained so far is comparable

to that on SiO2 [51]. Utilization of high-k dielectric material as

the gate insulator is expected to partially screen charged

impurities and enhance the carrier mobility [48]. A recent report

demonstrates significant carrier mobility improvement to7 104 cm2 V1 s1 in graphene transistors using single-crystalepitaxial PbZr0:2Ti0:8O3 (PZT) films as the gate oxide [44]. This

remarkable improvement was attributed to the strong screening of

PZT film and possible ordered adsorbate molecular layer at

interface that may reduce the scattering [52].

On the other hand, most studies of graphene transistors

discussed above employ a silicon substrate as a global back gate,

which, although useful for fundamental investigations, will be of

limited use for practical applications due to the inability to

independently address multiple devices on the same chip. Top-

gated devices can readily allow independently addressable device

arrays and functional circuits, and therefore are of significant

interest [5355]. The formation of top-gated graphene transistors

requires direct deposition of dielectric oxide material on top ofpristine graphene, which is of significant challenge due to the

intrinsic incompatibility between these two types of materials.

Deposition of oxide dielectrics onto graphene for top-gated

transistors can often introduce substantial defects into graphene

lattice and lead to significant degradation in carrier mobilities

[23,27,5661]. Here we review various strategies, including

physical vapour deposition (PVD), atomic layer deposition (ALD)

and physical assembly approach, for graphenedielectric integra-

tion for top-gated graphene transistors, discussing the potential

merits and challenges of each.

2. Physical vapor deposition of top-gate dielectrics on

graphene

Physical vapour deposition (PVD) such as electron-beam

evaporation or sputtering process is a common approach to

deposit a variety of oxide dielectric thin films on many different

substrates including graphene. However, the PVD process usually

yields lower quality dielectrics and can cause significant damages

to graphene.

2.1. Raman characterization of graphenedielectric integration by

PVD

Raman spectroscopy has been used to study the interaction

between single layer graphene and PVD dielectrics ( Fig. 3) [62].

Various PVD approaches, including electron beam evaporation,

pulsed laser deposition (PLD) and radio frequency (RF) sputter,

Fig. 2. Graphene nanostructures for the opening of a conduction gap in graphene. (a) Schematic view of a graphene nanoribbon (inset) and transfer characteristics of a

grapnene nanoribbon transistor with on/off ratio > 100. (c) Schematic view of a graphene nanomesh (inset) and transfer characteristics of a grapnene nanomesh transistor

with on/off ratio > 100. Adapted from [34] and [35].

L. Liao, X. Duan/ Materials Science and Engineering R 70 (2010) 354370356


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were explored to deposit SiO2 layer on graphene. Raman spectra of

the graphene covered with these PVD SiO2 layers all show largedefect band (D-band), indicating substantial defects introduced

into the graphene lattice. Strong defect-induced D band was also

observed in Raman spectra of graphene covered with PLD

deposited HfO2. In contrast, simple spin coating of polymer

dielectric such as polymethyl methacrylate does not introduce D-

band. These results clearly demonstrate that PVD process can

result in significant damage to carbon lattice, and simple spin

coating would not introduce obvious defects.

2.2. Top-gated graphene transistors using PVD dielectrics

The PVD dielectric film can be used as top-gate dielectrics for

graphene transistors. However, due to the significant damage the

process can cause, the mobility values observed in the top-gateddevices fabricated from this process are typically nearly one order

of magnitude smaller than those achieved in the back-gated

devices. For example, Lemme et al. reported the top-gated

graphene transistors, in which electron-beam evaporated SiO2was used as the top-gate dielectric [58]. Before the deposition of

the top-gate dielectric, the charge carrier mobilities in the

uncovered graphene are estimated to be mh = 4790 cm2 V1 s1

and me = 4780 cm2 V1 s1. In stark contrast, the room tempera-

ture mobility valuesare severely degraded tomh = 710 cm2 V1 s1

and me = 530 cm2 V1 s1 after the deposition of the SiO2 (Fig. 4).

This study highlights that the PVD deposited dielectric layer can

significantly disrupt the charge transport characteristics in

graphene.

3. Atomic layer deposition of top-gate dielectrics on graphene

Atomic layer deposition (ALD) is a well developed approach for

growing high-k gate dielectric layers, owing to its precise control

over the film thickness and uniformity [63]. However, the direct

deposition of high-k dielectric materials, such as Al2O3 and HfO2,

on graphene using H2O-based ALD is not possible because of the

hydrophobic nature of graphene basal plane [64] and the lack of

functional groups for molecular absorption which is necessary for

ALD.

3.1. Challenges in direct ALD of top-gate dielectrics on graphene

The attempt to directly deposit oxide dielectrics on pristine

graphene using ALDfailedto produce continuous layer of dielectrics.

Because a perfect graphite surface is chemically inert [65], attempts

togrowALDAl2O3 layer on a clean highly orientedpyrolyticgraphite

surface leadto onlya selective growth at the steps between graphite

layers, where the broken carbon bonds along the terraces serve as

one-dimensional nucleation center for the initial ALD process [66].

Fig. 5a and b shows AFM images of the same area before and afterALD of $2 nm Al2O3. Before ALD, the height of the triangulargraphene pieceat thebottom and thelargepiece onthe leftwas$1.7and$2.0 nm, respectively.Near the edgeof the graphene, there wasalso a narrow $1.0 nm high stripe. After ALD, $2.0 nm Al2O3 wascoated on SiO2. The apparent topography height of the three

graphene sheets was obviously reduced to a level similar to that of

the ALD-coated SiO2. The height difference before and after ALD

suggests that no Al2O3 was effectively coated on pristine graphene

sheets. This is because ALD relies on chemisorption and rapid

reaction of precursor molecules with surface functional groups

[67,68]. As a result, no ALD canoccur on thepristine grapheneplane

since it does not have any dangling bonds or surface functional

groups to react with the precursors. Interestingly, quasicontinuous

bright lines of Al2O3 preferentially grown on the edges of thegraphene sheets, suggesting dangling bonds or possible termination

by OH or other reactive species on the edges. Some bright dots in

the middle of graphene sheets may correspond to ALD Al2O3 island

around the defects such as pentagonhexagon pairs or vacancies

known to exist in graphite [69].

The unsuccessful ALD attempts to deposit oxide dielectrics on

graphene call for additional surface functionalization to create

functional groups for oxide nucleation. The key idea enabling the

high-k dielectric layer growth on graphene by ALD is to provide

intentional nucleation sites on the inert surface of graphene. To

this end, Lin et al. has used a functionalization layer consisting of

Fig. 3. Raman spectra of the graphene show a clear defect band emerging at

1350 cm1 after PVD dielectric deposition using various approaches, suggesting

significant defects are introduced into graphene lattice during the dielectric

deposition process. Adapted from [62].

Fig. 4. (a)SEM image of a graphene transistor with PVDtop-gate dielectrics. (b)Back-gatetransfer characteristics of graphene-FETwith andwithout a topgate. Adapted from

[58].



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50cyclesof NO2-TMA (trimethylaluminum)in ALD process prior to

the growth of gate oxide. This NO2-TMA functionalization layer

was essential for the ALD process and allowed us to achieve a

uniform thin (


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$2 nmAl2O3. Before ALD,the height of the graphene under the linecut was $1.6 nm, which increased to $3.0 nm after ALD. Therelative height increase of Al2O3-coated graphene was partly

attributed to the thickness of PTCA layer, which was usually $0.50.8 nm observed by AFM after PTCA coating step. The actual Al2O3on graphene was $2.8 0.2 nm thick in Fig. 7c. AFM images showcontinuous coverage of dielectric layer on the entire graphene area

with a root of mean square roughness of the Al2O3 film on graphene

about 0.33 nm, suggesting uniform packing of the underlayer PTCA

molecules. Previous high vacuum STM studies have confirmed two

modes of epitaxial packing of the PTCA precursor, perylene

tetracarboxic dianhydride, along the lattice lines of highly oriented

pyrolytic graphite [75,76]. As thepartitioning of PTCA from methanol

to the graphene interface is highly favorable, it is likely that similar

epitaxial packing is occurring in the solution phase, yielding dense,

uniform coating on graphene. This self-assembled PTCA layer on

graphene is responsible for uniform Al2O3 film coating. The non-

covalent functionalization method is not destructive to graphene

lattices andmay be used fordepositing ultrathinhigh-k dielectrics for

future graphene electronics [27].

3.3. Metal oxide buffer layer for ALD of dielectrics on graphene

An alternative buffer layer is oxidized metal thin film. In this

approach, a very thin layer of metal film is first deposited on

graphene and allowed to oxidize to form a thin oxide layer, which

can function as the nucleation layer for subsequent oxide

deposition by ALD. For example, Kim et al. introduced a thin

nucleation layer of oxidized aluminum between the graphene

layer and the Al2O3 dielectric [77]. Prior to the Al2O3 layer growth

by ALD, a 12 nm thick aluminum layer was deposited on thegraphene surface by e-beam evaporation. This aluminum nucle-

ation layer was completely oxidized as soon as the sample was

exposed in air before transferring into ALD chamber [78,79]. The

samples were transferred to the ALDchamber for the deposition of

Al2O3 using trimethylaluminum as the aluminum source and H2O

as the oxidizer. Initial stage of ALD growth starts with an H2O

oxidizing cycle at elevated temperatures to ensure the complete

oxidation of aluminum.

Top-gated graphene transistors have been fabricated to test the

dielectric quality and its impact on the graphene devices. To

fabricate the device, nickel thin film as the source and drain

electrodes were first defined using electron beam lithography on

mechanically peeled graphene, followed by an annealing step in a

hydrogen atmosphere at 200 8C to remove possible contaminantssuch as resist residues [80]. Al nucleation layer was then deposited

by high vacuum e-beam evaporator. The sample was then exposed

in air and transferred into the ALD chamber and went through 167

cycles of Al2O3 deposition, resulting in a 15 nm thick Al2O3 film. A

50 nm thick nickel top-gate electrode was subsequently fabricated

to obtain the final device (Fig. 8a and b).

Fig. 7. ALD ofAl2O3 on PTCA-coated graphene. (a) Schematic illustration of perylene

tetracarboxylic acid (PTCA)-coated graphene. PTCA selectively adheres to graphene

on SiO2 surfaces, providing binding sites for TMA deposition. Inset is a top view of

PTCA structure. (b) AFM image of graphene on SiO2 before ALD. The height of the

triangular shaped graphene is $1.6 nm as shown in the height profile along thedashed line cut. Scale bar is 500 nm. (c) AFM image of the same area as (a) after

$2 nm Al2O3 ALD deposition. The height of the triangular shaped graphenebecomes $3.0 nmas shownin the heightprofilealong the dashed linecut.Scale baris500 nm. (dand e)Schematicsof graphene onSiO2 beforeand after ALD.The Al2O3grows uniformly on non-covalently PTCA-coated graphene. Adapted from [73].

Fig. 8. Dual gate graphene transistors with Al2O3 top-gate dielectric deposited using thin aluminum as buffer layer. (a) Schematic of dual-gated graphene FET structure. (b)

Opticalmicroscope image of a graphene FET. (c) Rtot vs. VTG data measured at different VBG values.The inset shows theposition ofVDirac,TG at different VBG. Optical microscope

image of a graphene FET. Adapted from [77].



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The electrical transport characterizations were conducted at

room temperature in a vacuum probe station. The top-gate

electrode and the silicon substrate were used as a local gate and

global back-gate, respectively, to control the carrier concentration

and polarity in the graphene layer. Fig. 8c shows the total device

resistance (Rtot) as a function of top-gate voltage measured at

different back-gate biases from 40 to 40 V and a drain bias ofVD = 0.1 V. At VBG = 0 V, the sample resistance reaches a maximum

(Dirac point) at VDirac,TG = 0.08 V. This observation indicates that

there was little unintentional doping of the graphene sample after

the top-gate stack deposition. As jVTGVDirac,TGj increases, theelectron or hole concentration in the graphene channel increases

and Rtot decreases, resulting in V-shaped traces. Importantly, the

top-gated device shows a very small hysteresis less than 0.05 V,

and low leakage current less than 0.75 pA/mm2 through the Al2O3dielectric, highlighting a high dielectric quality and a low interface

state density.

The Rtot vs. VTG measured at different VBG values shows an

applied VBG bias changes the position of the Dirac point and also

shifts vertically the measured resistance values (Fig. 8c). The

change in the Dirac point position can be explained as follows: a

positive (negative) VBG bias induces a finite concentration of

electrons (holes) in the active area, which is proportional to the

back-gate capacitance (CBG). In order to restore the device to theDirac point, where the carrier concentration is minimum, a

negative (positive) applied VTG is required. The vertical shift is

caused by the resistance change in the un-top-gated regions of the

graphene flake. The shift of the top-gate Dirac points as a function

of VBG can be used to determine top-gate and back-gate

capacitances, CTG/CBG$ 28 (inset of Fig. 8c). Using the back-gatecapacitance value ofCBG = 11 nF/cm

2, the top-gate capacitance can

be estimated to be CTG = 306 nF/cm2, corresponding to a relative

dielectric constant of 6.0 for the Al2O3 film.

To determine the impact of the dielectric deposition, it is

important to determine the carrier mobility. To accurately derive

the mobility value, the contact resistance should be carefully

excluded as it is comparable to the graphene transistor channel

resistance. The authors have adopted a model to exclude theimpact of the contact resistance as detailed below. The carrier

concentrations (electrons or holes) in the graphene channel

regions ntot can be approximated by

ntot ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

n20 nVTG2q

(1)

where n0 represents residual carrier concentration at Dirac point,

which should be zero for an ideal, disorder-free graphene layer and

none zero in presence of charged impurities located either in the

dielectric or at the graphene/dielectric interface [81]. nVTGrepresents the carrier concentration induced by the top-gate bias

away from the Dirac point, VTG VTG VTG;Dirac. The expression fornVTG is obtained from thefollowing equation relating VTG, Cox, andthe quantum capacitance of the two-dimensional electrons in the

graphene channel:

VTG VTG;Dirac e

Cn hvF

ffiffiffiffiffiffiffipn

pe

(2)

The total device resistance Rtot is given by

Rtot Rcontact Rchannel Rcontact L=W

nem

Rcontact L=Wffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffin20 nVTG2em

q (3)

where Rchannel is the resistance of the graphene channel coveredby top-gate electrode, the contact resistance Rcontact consists of

the uncovered graphene section resistance and the metal/

graphene contact resistance, and L and W represent the channel

length and width the top-gated area [23]. By fitting this model

to the measured data, the authors extracted a phenomenal

mobility m of 8600 cm2/Vs, representing the highest value

achieved in top-gated transistors at the time of their report. This

approach to use aluminum nucleation layer for ALD was also

adopted by IBM scientists to fabricate top-gated graphene

devices to achieve field effect mobility about 2700 cm2/V s [24],

a respectable value, although not as high as the first report

(Fig. 9).

The metal oxide as the buffer/nucleation layer has also been

explored for the deposition of oxide on graphene using otherapproach such as molecular beam epitaxy (MBE). Wang et al.

demonstrated that the growth of atomically smooth magnesium

Fig. 9. (a) Device schematic of the dual-gate graphene transistor. (b) SEM image of a double-channel graphene transistor. The channel width is 27 mm, and the gate length is350 nm for each channel. (c) Measured channel conductance as a function of the back-gate voltage of a graphene device before and after the deposition of 12-nm-thick ALD

Al2

O3

. Prior to the ALD process, a layer of 2-nm aluminum is deposited and oxidized as the nucleation layer. Adapted from [24].



8/17

oxide (MgO)films on graphene surfaces by MBE [82]. By examining

AFM images of MgO films with different growth rates on highly

oriented pyrolytic graphite (HOPG) substrates, they found that the

high surface diffusion led to non-uniform MgO films with root

mean square roughness> 0.8 nm. To reducethe mobilityof surface

atoms, they deposit Ti atoms to dress the graphene surface prior to

the MgO deposition. Remarkably, with as little as a 0.5 ML

(monolayer) coverage of Ti, the root mean square roughness of a

1 nm MgO film is dramatically reduced to be near the atomic

spacing in MgO (0.211 nm). Because the metallic Ti islands on

graphene maybe undesirable for lateral transport, they oxidize the

Ti prior to MgO growth and find that the MgO layer is atomically

smooth under this condition as well. However, the mobility of

these devices is still limited, as low as 8001700 cm2/V s.

The use of a thin metal (Al or Ti) film as a nucleation layer has

enabled the ALD of continuous oxide dielectric (e.g. Al2O3) on

graphene. The device results show mobility values of several

thousand at room temperature, an encouraging finding that

indicates that the top-gate stack does not severely increase the

carrier scattering and consequently degrade the device character-

istics.

3.4. Low-k polymer buffer layer for ALD of dielectrics on graphene

Farmer et al. reported that the introduction of a low-k polymer

buffer layer for ALD of high-k dielectrics on graphene, and

demonstrated top-gated devices without significant mobility

degradation [14]. The low-k bufferlayer consists of a commercially

available polymer NFC 1400-3CP, which is a derivative of

polyhydroxystyrene that is conventionally used as a planarizing

underlayer in lithographic processes. It can be diluted in propylene

glycol monomethyl ether acetate (PGMEA), and spin-coated onto

the graphene surface. The dilution and spin speed can be adjusted

to control the desired thickness and uniformity of the buffer layer.

Methyl and hydroxyl groups contained within the polymer

structure serve as functional groups for ALD of HfO2 using

tetrakis(dimethylamido)-hafnium and water as the precursors.

In a typical process, approximately 10 nm thick polymer bufferlayer was first spin coated and cured at 175 8C for 5 min to remove

residual solvent, and 10 nm thick HfO2 is deposited onto the buffer

layer to complete the dielectric stack. The ALD was conducted at a

relatively low temperature of 125 8C to produce HfO2 films with a

dielectric constant ofk = 13. Capacitance analysis of the complete

gate stack yields a dielectric constant ofk = 2.4 for the buffer layer.

The impact of the buffer dielectric processing on the transport

properties of the back-gated graphene devices was examined by

comparing two-point transfer characteristics (Fig. 10b) and

transconductances (inset of Fig. 10b) at different stages of the

processing. The two-point transconductance is defined asgm = dID/

dVG, where ID is the drain current and VG is the gate voltage. The

Dirac point voltage (VDirac) of the device before processing is

VDirac = 3.5 V, in close proximity to 0 V, which signifies that thegraphene is not significantly doped by the supporting 300 nm SiO2substrate. The doping level became highly p-doped after applica-

tion of the buffer layer (VDirac = 42.5 V), and then moderately p-

doped after HfO2 ALD (VDirac = 13.25 V). After ALD, the device was

subjected to an O2 plasma treatment which further shift the device

towards neutral doping (VDirac = 5.75 V), but did not damage the

graphene as indicated by relatively little change in resistance or

transconductance. This also demonstrates that the dielectric stack

caneffectivelyprotect graphene from damaging by theplasma that

is known to etch graphene. Beyond changing the doping level,

buffered dielectric processing has a minimal effect on the transfer

characteristics. Both the minimum current value at VDirac and the

maximum hole transconductance remain within 15% of theirinitial

values. There is a 40% decrease in the maximum electron

transconductance, which is likely associated with doping-induced

conductance asymmetry caused by the electrodes [83,84]. The

mobility values achieved here is 8500 cm2/V s for back gated

configuration (with top-gate dielectrics) and 7600 cm2/V s for top-gated configuration. Overall, the results obtained here show a

significant improvement compared to other methods of dielectric

coating (Fig. 10a). Indeed, the authors have demonstrated in

parallel experiments that devices made with the polymer buffer

layer are significantly better than those made with other

approaches, such as NO2 functionalization and oxidized Al

deposition discussed before.

The addition of a low-k polymer buffer layer between

graphene and conventional gate dielectrics helps minimize

mobility degradation in top-gated graphene transistors. Possible

reasons include the suppression of extrinsic surface phonons by

the buffer layer and the reduction of the impurity concentration

due to the inherent properties of the polymer. This new coating

procedure represents a significant improvement over previousefforts, and will hopefully further the advancement of graphene

transistors.

4. Physical assembly of high-k oxide nanostructures as top-

gate dielectrics for graphene transistors

Despite significant efforts devoted to the functionalization of

graphene surface or introduction of buffer layer for ALD of oxide

Fig. 10. Two-point back-gated measurements of graphene flakes. (a) Transfer characteristics and corresponding transconductances (inset) after the different stages of

buffered dielectric processing: beforeprocessing (gray), afterNFC polymer deposition (green),after HfO2 deposition(blue), andafter50 W O2 plasmatreatments for30 s (red)

. Theschematicshowsthe completeddevice configuration. (b)Transfer characteristics of twodevices before (solidlines)and after (dashedlines) alternative coating processes

areemployed.Two nanometers oxidized Al deposition (red) and NO2 functionalization (blue) is usedinstead of polymer coating. Both processes result in significant mobility

degradation. VD = 10 mV forall measurements,and VBG is swept forwardand backwardto showcurrent hysteresis(For interpretation of thereferences to colour in this figure

legend, the reader is referred to the web version of the article.). Adapted from [14].



9/17

dielectrics on graphene, and significant advancements in this

direction, these processes usually break the chemical bonds or

introduce undesired impurities in the graphene lattice, inevitably

leading to a significant degradation in deviceperformance. Many of

these processes can result in mobility degradation by nearly one

order of magnitude. The use of aluminum buffer layer for high-k

deposition was demonstrated with improved device mobility,

which, however, is still lower than that of normal back-gated

graphene devices [4,45,49,85,86]. Introduction of a polymer buffer

layer prior to high-k deposition can mitigate the potential damage

to graphene lattice, which however can limit the effective gate

coupling due to the low-k polymer layer. To eventually realize

high-performance graphene-based electronics, continued effort is

necessary to develop alternative approaches that can enable

graphenedielectric integration without damaging the pristine

graphene lattice. To this end, we have recently reported an entirely

new strategy to integrate graphene with high-k dielectrics by

physical assembling free-standing oxide nanowires or nanorib-

bons on top of graphene [8789]. Here we review the recent

advancements in this regard.

Nanowires and nanoribbons can be synthesized at high

temperature with nearly perfect crystalline structure, but manip-

ulated and assembled at room temperature. This flexibility allows

the integration of normally incompatible materials and processesand can enable unique new functions in electronics or photonics

[53,9093]. Physical assembly of freestanding dielectric nanos-

tructures on graphene represents the mildest approach for

graphenedielectric integration. Specifically, high quality dielec-

tric nanostructures were first synthesized, and then transferred

onto graphene as the gate dielectrics for top-gated graphene

transistors. This integration approach preserves the pristine nature

of the graphene and allows us to achieve the highest room

temperature carrier mobility in top-gated graphene transistors to

date. Fig. 11 illustrates our approach to fabricate top-gated

graphene transistors using oxide nanostructures as the gate

dielectrics. Oxide dielectric nanostructures (e.g. nanoribbons)

were first aligned on top of the graphene through a physical dry

transfer process, followed by lithography and metallizationprocess to define the source and drain electrodes (Fig. 11a).

Oxygen plasma etch was then used to remove the exposed

graphene, leaving only the graphene protected underneath the

dielectric nanostructure and the source drain electrodes (Fig. 11b).

The top gate electrode was then fabricated to obtain a functional

transistor (Fig. 11c).

4.1. Dielectric properties of Al2O3 nanoribbons

Aluminum oxide (Al2O3) nanoribbons were used as an initial

example to demonstrate the concept of using free-standing

chemical nanostructures as the top-gate dielectrics for graphene

transistors [89], due to their excellent dielectric properties,

thermal and chemical stability [94]. Al2O3 nanoribbons were

synthesized through a physical vapour transport approach at

1200 8C [95]. Transmission electron microscope (TEM) studies

showthattheAl2O3 nanoribbonstypically have a width of 13mm,and a length on the order of 10 mm (Fig. 12a). Selected areaelectron diffraction (SAED) shows the nanoribbon has a single

crystallinea-Al2O3 structure, oriented along h1 1 0i direction in itslong axis, and along h0 0 1i direction (c-plane) in its thickness(inset of Fig. 12a). The high resolution TEM image (HRTEM)

confirms that the nanoribbon is a single crystal with nearly perfect

crystalline structure free of any obvious defects (Fig. 12b). Atomic

force microscopy (AFM) studies show the nanoribbons typically

have a thickness around 15150 nm (Fig. 12c), and nearly

atomically smooth surface with root mean square roughness less

than 0.2 nm (Fig. 13d).

The intrinsic dielectric properties (current tunnelling, break-

down and dielectric constant) of the Al2O3 nanoribbons were

characterized using a metalinsulatormetal (MIM) device

(Fig. 12e). Currentvoltage (IV) measurements of the MIM device

show typical FowlerNordheim (FN) tunnelling behaviour with a

breakdown field of$8.5 MV/cm (Fig. 12f and inset), comparable tothe best quality ALD Al2O3 film [94]. This type of field assisted

tunnelling can be described by charge carrier tunnelling through a

triangular barrier with:

J AE2OXexpBEOX

(4)

where

A 1:54 106 1mFB

(5)

and

B 6:83 107m1=2FB3=2 (6)

Jis current density, EOX is the oxide electric field, m* is the effective

mass of the charge carrier, which is about 0.23me, and FB is the

barrier height [94]. Fitting the IV characteristics with FN

tunnelling model gives a tunnel barrier of about 2.0 eV betweenAl2O3 and Ti, comparable to previous reports of the barrier height

between ALD Al2O3 and metals of similar work function [94,96].

The relative dielectric constant is also determined as 8.5 from

capacitancevoltage measurement, which is larger than typical

values observed in ALD Al2O3 films. These studies clearly

demonstrate that the Al2O3 nanoribbons have dielectric properties

comparable to or better than the best quality ALD Al2O3 film, and

can function as an excellent dielectric material for top-gated

graphene transistors.

4.2. Dielectric nanostructuregraphene integration and interface

The Al2O3 nanoribbons can be aligned onto the top of the

graphene through a solution assembly or physical transfer process.

Fig.11. Schematic illustration of thefabrication processto obtain top-gated graphenetransistors using dielectricoxidenanostructures(e.g.nanoribbons) as theetching mask

and top-gate dielectric. (a) A dielectric nanostructure is aligned on top of graphene using a dry-transfer process without any additional chemical functionalization to

minimize the possibility to introduce defects/impurities into the graphenedielectric interface, and the sourcedrain electrodes are fabricated by electron-beamlithography.

(b) Oxygen plasma etch is used to remove the unprotected graphene, leaving only the graphene underneath the dielectric nanostructure connected to two large graphene

blocks underneath the source and drain electrodes. (c) The top gate electrode is defined through lithography and metallization process. Adapted from [89].



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Previous studies have shown that the deposition of oxide on top of

graphene often introduced significant defects into the graphene

structure with an obvious defect band (D-band) emerging around

1350 cm1 in Raman spectra [62]. To this end, micro-Raman

spectroscopy was employed to investigate the interaction between

an Al2O3 nanoribbon and the underlying graphene (Fig. 13a).

Micro-Raman spectra were collected from bare graphene (point a)

and Al2O3 nanoribbon covered graphene (point b). Significantly,

there is no clear difference between two Raman spectra and there

is no obvious D-band (Fig. 13b), in contrast to previous study in

which an obvious D-band was observed[109]. Cross-sectionalTEM

was used to study the graphenedielectric interface (Fig. 13c). The

image of the device shows that the graphene layers are intimately

integrated with the crystalline Al2O3 nanoribbon without any

obvious gap or impurities between them (Fig. 13d). Together, these

studies clearly demonstrate that the physical assembly approach

can effectively integrate Al2O3 nanoribbon with graphene without

introducing any appreciable defects into the graphene lattice, and

thus can effectively preserve the high carrier mobility in the

resulting devices.

4.3. Al2O3 nanoribbons as the top-gate dielectrics for graphene

transistors

The electrical transport studies of the top-gated graphene

transistors using Al2O3 nanoribbon gate dielectrics were carried

Fig. 12. Evaluation of the Al2O3 nanoribbons as dielectric material. (a) TEM image (inset, SAED pattern) and (b) HRTEM image of an Al2O3 nanoribbon show nearly perfect

crystalline structure with a-Al2O3 structure. (c) AFM image of an Al2O3 nanoribbon with thickness $50 nm. The image area is 5 mm 5 mm. (d) AFM image of the surface ofthe Al2O3 nanoribbon, highlighting the smooth surface with a root mean square roughness < 0.2 nm. The image area is 250 nm 250 nm. (e)The schematic device diagram(inset) and SEM image of an Al2O3 nanoribbon metalinsulatormetal (MIM) device. (f) Current densityelectric field (JE) curve of an MIM device made from an Al2O3nanoribbon, and the inset shows the corresponding FowlerNordheim (FN) curve. Adapted from [89].



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out at room temperature. Fig. 14a shows the drainsource current

(Ids) vs. drainsource voltage (Vds) output characteristics of the

transistor at various top-gate voltages (VTG). The device delivers an

on-current of 675mA at Vds = 1 V and Vg =

1.5 V. Importantly, the

transfer characteristics (Ids vs. top-gated voltage (VTG) and back-gated voltage (VBG)) show the required gate voltage swing to

achieve similar current modulation in top-gate configuration is

more than one order of magnitude smaller than that in back-gate

configuration (Fig. 14b and inset). The transconductance gm = dIds/

dVTG can be extractedfrom the IdsVTG curve (Fig. 14c). At Vds = 1 V,

the top-gated device exhibits a max gm of about 290mS, which isabout 15 times larger than that of the back-gated configuration

(gm $ 19.5mS).Fig. 15a further shows two-dimensional plot of the device

conductance as a function of varying VBG and VTG bias, from which

we can determine the top-gate Dirac point (VTG_Dirac) shift as a

function of VBG (Fig. 15b). It gives the ratio between top-gate and

back-gate capacitances, CTG/CBG% 14.3. This gate capacitance ratio

is consistent with the improvement factor ($15) in transconduc-tance of top- vs. back-gated configurations. Using the back-gate

capacitance value ofCBG = 11.5 nF/cm2, the top-gate capacitance is

estimated to be CTG = 164.5 nF/cm2, corresponding to a relative

dielectric constant of 8.4 for Al2O3 nanoribbon, which is alsoconsistent with the value obtained from MIM devices.

The same model described above [Section 3.3] was used to fit

the measured data and extract the relevant parameters, n0, Rcontactand m. Fig. 15c shows the measured Rtot vs. VTG (black line), along

with thefitted curve derived from Eq. (3) (red line).The fitted curve

agrees well with the experimental data, with a single value of the

residual concentration n0 = 4.1 1011 cm2, Rcontact = 1240V, andthe mobility m = 22,400 cm2/V s, which represents the highest

carrier mobility value observed in top-gated graphene devices to

date. Multiple devices were fabricated and tested with the same

approach, all of which exhibited carrier mobilities well exceeding

10,000 cm2/V s with highest mobility of reach 23,600 cm2/V s,

comparable to the best reported values in back-gated devices and

Fig.13. Characterization of the graphene/Al2O3 nanoribbon interface.(a) An optical image of an Al2O3 nanoribbon on graphene, thescalebar is 2mm. (b)Ramanspectra of thegraphene with (b) and without (a) Al2O3 nanoribbon covering. There is no D-band in either spectrum, indicating that Al2O3 nanoribbon does not introduce any appreciable

defects into graphene lattice. (c) A cross-section TEM image of the top gate stack, the scale bar is 100 nm. The inset shows an SEM image of a typical device, the scale bar

indicates 5mm. The dotted line in the inset shows the cross-section cutting direction. (d) A cross-section HRTEM image of the interface between Al2O3 nanoribbon and a tri-layer graphene. The partially incomplete graphene layers in the image are caused by electron beam damage during the TEM imaging process. Adapted from [89].

Fig. 14. Room temperature electrical properties of thetop-gatedgraphene deviceusing Al2O3 nanoribbon as thegate dielectric. (a) IdsVds outputcharacteristics, the channel

width and length ofthe deviceis 2.1 and 4.1 mm (b)Transfer characteristics at Vds = 1 V forthe deviceusing topand backgate(inset). (c)Transconductancegm asa functionoftop-gate voltage V

TG, the inset shows the g

mvs. V

BG. The plots indicate the top-gate g

mis about 15 times higher than the back-gate g

m. Adapted from [89].



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significantly better than typical values previously reported for top-

gated devices (Fig. 16). Together, these studies clearly demonstrate

that the presence of Al2O3 nanoribbon on top of graphene does not

lead to any mobility degradation, in contrast to previous efforts in

using ALD or PVD to deposit dielectrics on graphene.

4.4. ZrO2 nanowire as top-gate dielectric for graphene nanoribbon

transistors

Taking a step further, ZrO2 nanowires with even higher

dielectric constant (up to 23) [97,98] were also explored as the

gate dielectric for top-gated graphene devices [88]. An additional

attribute of using ZrO2 nanowires is that narrower graphene

channels (e.g. graphene nanoribbons, GNR) can be readily

fabricated to facilitate gap-opening and improve the transistor

onoff ratio [3033,35,99]. ZrO2 nanowires were synthesized

through a chemical vapour deposition (CVD) process using ZrCl4 as

the precursor. Scanning electron microscope (SEM) image shows

that ZrO2 nanowires are about several tens of microns in length

and 20100 nm in diameter (Fig. 17a). TEM and SAED studies

reveal that ZrO2 nanowires are amorphous (Fig. 17b and inset). The

top-gated graphene transistors using ZrO2 nanowires as the gate

dielectric can be fabricated using the same approach describedabove (Fig. 17c). Here the ZrO2 nanowire also functions as a

nanoscale etch mask to define a narrow graphene nanoribbon with

width in the 1020 nm regime through aggressive over etch (inset,

Fig. 17c) [35]. The IdsVds plots at various top-gate voltages (VTG)

show clearly that the device conductance decreases as the gate

potential increases towards positive direction (Fig. 17d), demon-

strating that the graphene nanoribbon is p-type doped, which can

be attributed to edge oxidation or the physisorbed O2 from

ambient or during the device fabrication process. This device

delivers an on-current of 28mA at Vds = 1 V and Vg = 1.0 V.Transfer characteristics show a top-gated GNR transistor using

ZrO2 nanowire dielectric can be switched on andoff with only $1 Vof gate swing (red curve in Fig. 17e), in contrast to 1040 V

required for back-gated devices (black curve in Fig. 17e). The

device shows a room temperature on/off ratio of$12at Vds = 0.1 V,consistent with a graphene nanoribbon with estimated width of

$1015 nm [30,35]. The maximum transconductance gm atVds = 1 V is about 29 mS, more than 12 times larger than that ofthe back-gated configuration ($2.3mS) (Fig. 17f).

It is interesting to compare the top-gated graphene nanoribbon

devices with state-of-the-art silicon MOSFETs. The effective on

current Ion for an FET device is usually characterized at Vds = Vg(on

off) = Vdd, where Vg(onoff) is the gate voltage swing from off- to on-

state and Vdd is the power supply voltage. Considering Vds = Vg(on

off) = Vdd = 1 V, the Ion of our device at Vds = 1 and 1 V gate swing

from the off state is $25mA. Taking the channel width of thegraphene nanoribbons $ 15 nm, we obtain the scaled values of Ionand gm of our device to be

$1.7 mAmm1 and

$2.0 mSmm1,

already exceeding the values of 0.7 mA mm

1 and 0.8 mS mm

1 insub-100-nm silicon p-MOSFETs and comparable to those of n-

MOSFET devices employing high-k dielectrics [100]. This is

significant considering the relatively large channel length

($1mm) of the current device. It is reasonable to expect thatthe on-current and transconductance can be further improved by

shrinking the channel length. These studies demonstrate that ZrO2

Fig.15. Mobility determinationin thetop-gated graphene deviceusingAl2O3 nanoribbon as thegate dielectric.(a) Two-dimensional plot of thedevice conductanceat varying

VBG and VTG bias.Theunitin thecolor scale ismS. (b)The top-gate Dirac point VTG_Dirac at different VBG. (c)Experimentalplot (blackline) andmodellingfitting (redline)ofRtotvs. VTGVTG_Dirac relation to derive the contact resistance and carrier mobility (For interpretation of the references to colour in this figure legend, the reader is referred to the

web version of the article.). Adapted from [89].

Fig. 16. Table summarizing the highest mobility values obtained in top-gated graphene transistors using various dielectric integration approaches.



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nanowires can also function as effective gate dielectrics for high

performance top-gated graphene nanoribbon devices.

4.5. Conductor/dielectric coreshell nanowires as the top-gate for

graphene nanoribbon transistors

In the above discussions, pure dielectric nanostructures wereexplored as the top-gate dielectric for graphene transistors, in

which the dielectric thickness was controlled by the overall

dimensionof thenanostructuresand may be difficult to scale down

to very small thickness (e.g. 12 nm), due to the challenges in

synthesizing and assembling the oxide nanostructures at such

small dimensions, and the potential difficulties in fabricating

devices with such small structures. Alternatively, one can also

explore conductor/dielectric coreshell nanostructures (nanowires

or nanoribbons), in which the dielectrics are deposited on a

conducting (e.g. metal or highly doped semiconductor) nanos-

tructure using ALD, and can be precisely controlled down to 1 nm

regime. Such coreshell nanowires can be used to fabricate top-

gated graphene transistors using a similar approach described

above, in which the oxide shell functions as the gate dielectricswith controlled thickness, andthe conducting core functionsas the

self-integrated gate. Using such coreshell nanostructures, it is

possible to fabricate top-gated graphene transistors with the

dielectric thickness down to 1 nm or so ( Fig. 18ah). To establish

the electrical contact between the silicon nanowire gate and the

external gate electrode, an additional step of argon plasma

treatment is needed to physically remove the HfO2 film on the

top half of the nanowires prior to the deposition of the top gate

electrode.

Various type of conducting nanowires (highly doped silicon or

metal silicide, NiSi, PtSi) can be explored for this purpose. For

example, Si/HfO2 coreshell nanowires were recently synthesized

by ALD deposition of 2 nm thick HfO2 on highly doped silicon

nanowires (Fig. 19b and c) [87]. Such coreshell nanowires can be

readily explored for top-gated graphene nanoribbon transistors.

Here theHfO2 shell functionsas thegate dielectrics with controlled

thickness down to 1 nm, and the silicon core functions as the self-

integrated gate. To contact the silicon core gate to external

electrode, the top-half of the HfO2 shell was etched away using

argon plasma. The cross section TEM image clearly shows the

integrated silicon gate, 2-nm thick HfO2 gate dielectric, and thegraphene nanoribbon (Fig. 20b and c).

Electrical transportstudies of the top-gated device were carried

out in ambient conditions at room temperature. The gate

tunnelling leakage current (Igs) from the Si/HfO2 core/shell

nanowire to the underlying GNR is negligible within the gate

voltage range of 1 V range (Fig. 21a). This measurement

demonstrates that the 2 nm thick HfO2 dielectrics can function as

an effective gate insulator for top-gated GNR transistors and afford

high gate capacitance critical to the high transconductance. The

drainsource current (Ids) vs. drainsource voltage (Vds) plots at

various top-gate voltages (VTG) show clearly that the device

conductance decreases as the gate potential increases towards

positive direction (Fig. 21b), demonstrating that the GNR is p-type

doped, which can be attributed to edge oxidation or the physisorbedO2 from ambient or during the device fabrication process. Fig. 21c

shows the transfer characteristics drainsource current (Ids) vs. top-

gated voltage (VTG) curves for the same device at Vds = 0.1 and 1.0 V.

The transfer characteristics show the device can be switched on and

off with


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The transconductancegm = dIds/dVg can be extracted from the IdsVTGcurve. The maximum gm of the top-gated device at Vds = 1 V is about

32mS (Fig. 21e), nearly 20 times largerthan the value obtained in the

back-gated configuration ($1.7mS) (inset, Fig. 21e). Considering the

nanoribbon width $10 nm, the scaled transconductance gm for GNR

transistor reaches$3.2 mS mm1, representing the highest value everreported for GNR transistors.

4.6. Integrated device array from nanowire gated graphene

nanoribbon transistors

The ability to fabricate the top-gated graphene nanoribbon

devices readily allows integration of multiple GNR FETs into

functional device arrays. (Fig. 22a and b), and therefore allowing

for diverse electronic functions. For example, a logic OR gate was

Fig.19. TEM characterization of Si/HfO2 coreshell nanowires. (a)Schematicillustration of thesynthesis of Si/HfO2 coreshell nanowires. Highlydoped p-typesilicon nanowire

arrays were synthesized using catalytic chemical vapour deposition. Atomic layer deposition was used to grow HfO 2 shell with controlled thickness. (b) TEM and (c) HRTEM

images of Si/HfO2 coreshell nanowires. Adapted from [87].

Fig. 20. Characterization of the graphene/HfO2 interface. (a) A SEM image of a typical device. (b) A cross-section TEM image of the top gate stack. (c) A cross-section HRTEM

image of theinterfacebetween nanowires anda multi-layers graphene, which indicate thatthe graphenelayersare intimately integratedwith theSi/HfO2 nanowire without

any obvious gap or impurities between them. A TEM image of multi-layer graphene device is shown here because it is very difficult to visualize the mono- or few-layer of

graphene nanoribbon under the nanowire due to significant electron-beam damage while conducting TEM studies. Adapted from [87].

Fig. 18. Schematic illustration of the fabrication process to obtain top-gated graphene transistors using Si/HfO2 coreshell nanowires as the etchingmask and top-gate. (a and

e) An Si/HfO2 coreshell nanowire is aligned on top of graphene using a dry-transfer process, and the sourcedrain electrodes are fabricated by electron-beam lithography. (b

and f) Oxygen plasma etch is used to remove theunprotected graphene, leaving only the GNR underneath the nanowire connected to two large graphene blocks underneath

thesourceand drain electrodes. (cand g) Thetop-halfof theHfO2 shell wasetchedawayusingargon plasmato exposethe siliconcoregate forcontact toexternalelectrode. (d

and h) The top gate electrode is defined through lithography and metallization process. Adapted from [87].



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Fig. 21. Room temperature electrical properties of the top-gated GNR device by using Si/HfO 2 coreshell as the top-gate. (a) Gate leakage current vs. top-gate voltage. The

leakage current is negligible (atthe nA level) within1 V range.(b) IdsVds output characteristics at variable top-gate voltage starting from 0.6 V at bottom to1.0 Vat top inthe

step of0.2 V. (c) The transfer characteristics IdsVTG at Vds = 0.10 and 1.0 V. (d) IdsVTG and IdsVBG transfer characteristics at Vds = 1 V. (e) Transconductance as a function of top-

gatevoltage VTG andback gatevoltageVBG (inset). (f) Two-dimensional plotof the device conductance at varyingVBG and VTG bias,the unitin thecolor scaleismS. Adapted from [87].

Fig. 22. Independently addressable GNR device array. (a) An SEM image of two independently addressable top-gated GNR FETs. (b) Transfer characteristics of two top-gated

GNR FETsat Vds = 0.1 V. (c)The SEMimageof a logic OR gates built from GNRtransistors.The inset shows theschematic circuitdiagram.(d) TheOR gate outputcharacteristics

with double top-gates. The operating voltage isVdd

= 1 V. The inputs for the two gates, A and B, are 1 V for state 1 and 0 for state 0. Adapted from [88].



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demonstrated with two independent gate electrodes fabricated on

a GNR in conjunction with a loading resistor (Fig. 22c). The OR

function occurs because the output voltage is low only when the

input of both gates is at low voltages (Fig. 22d). When one or both

gates are at high voltages, the GNR channel is electrically shut off,

resulting in a high output voltage.

5. Summary

Graphene is emerging as an interesting electronic material for

future electronics due to their exceptionally high carrier mobility,

tunable band gap and atomically thin structure. The gate dielectric

is an essential component of a transistor and can significantly

impact the critical device parameters including transconductance,

subthreshold swing and frequency response. Exploring graphene

for future electronics requires its effective integration with high

quality gate dielectrics, in particular the high-kdielectrics, which is

of significant challenge due to the intrinsic incompatibility

between pristine graphene and oxide dielectrics. Physical vapour

deposition (PVD) such as electron-beam evaporation or sputtering

process has been used to deposit dielectrics, although the PVD

process usually yields lower quality of dielectrics and can cause

significant damages to graphene. The deposition of high-k

dielectrics using ALD requires reactive surface groups. Functiona-

lization of graphene surface for ALD either introduces undesired

impurities or breaks the chemical bonds in the graphene lattice,

inevitably leading to a significant degradation in carrier mobilities.

As a result, the mobility values observed in the top-gated devices

are typically nearly one order of magnitude smaller than those

achieved in the back-gated devices. Recently, the introduction of

polymer or aluminum buffer layer for high-k deposition has been

demonstrated with improved device mobility, which, however, is

still lower than best values observed in back-gated graphene

devices. A newly developed physical assembly approach to

integrate graphene with free-standing dielectric nanostructures

can minimize potential damage to graphene lattice, and have

enabled the highest mobility top-gated graphene transistors ever.

This methodopens a new avenue to integrate high-k dielectrics ongraphene with the preservation of high carrier mobility. With

further optimization of dielectric nanostructure growth and

assembly process to precisely control their physical dimension

and spatial location, large arrays of top-gated graphene transistors

or circuits can be envisioned. This physical assembly and

integration approach can thus open a new avenue to high

performance graphene electronics, which can impact significantly

high speed high frequency circuits and enable an entirely new

generation of flexible, wearable or disposable electronics for

computing, storage and wireless communication.

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