Graphene FETs With Combined Structure and Transparent Top

8/12/2019 Graphene FETs With Combined Structure and Transparent Top

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Physics Procedia 32 ( 2012 ) 869 874

1875-3892 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS).doi: 10.1016/j.phpro.2012.03.649

18th International Vacuum Congress (IVC-18)

Graphene FETs with Combined Structure and TransparentTop

Yuanlin Yuan a, Zhen Chen a, Congxiang Lu a, Hongzhong Liu b*, Yuan Wu a, Xin Li a *aDepartment of Microelectronics, Xian Jiaotong University, Xian 710049, China

bKey Laboratory for Manufacturing Systems Engineering, Xian Jiaotong University, Xian 710049, China

Abstract

Graphene, comprising of monolayer of carbon atoms packed into a two-dimensional honeycomb lattice, has a series of peculiar properties such as the anomalously quantized Hall effects, the large charge carrier mobility and so on. Micromechanical cleavagemethod is used to produce grapheme, which is acquired by peeling graphite foil off from transparent sticky tape repeatedly.Graphenes pattern has been placed between the source and the drain electrodes as the channel by location transplantation method.The results show that the graphene samples of micromechanical cleavage method have better lattice structure. The IDS-VDScurves of FET properties of graphene sheet channel are measured. Graphene channel obvious responses to the gate voltage.

2010 Published by Elsevier B.V. PACS : 81.05.Uw; 81.07.-b; 81.15.Gh; 81.16.-c

Keywords graphene Micromechanical cleavage hydrazine hydrate; Ethanol; Field effect transistor; location transplantation gas sensing

1. Introduction

In last few years as nanotechnology rapidly progress, especially the continuous discovery of novel nanostructuressuch as fullerene, carbon nanotube and graphene, researches of carbon nanomaterials draw extensive concern.Graphene was acquired in 2004 by A.K.Geim's research group in Manchester University[1][2], its particularquantum Hall effect, high electron mobility[3-5] etc. become glaring focus of carbon family, which decrease the sizeof device and power consumption and enhance conduction velocity. Fast velocity and transportation with noscattering ensures its application in high frequency transistor [6]. It is expected to start a new era of carbon materials.

In this paper we use two ways to prepare graphene sample, on one hand through transparent sticky tape method(micromechanical cleavage method), on the other hand through hydrazine reduction process in ethanol. We take thetwo kinds of samples under microanalysis, investigating the influence of process on crystal structure. Then wetransplant graphene films to backgated FET devices, measuring the IV character of graphene channel.

* Corresponding author. Tel.: +86-029-8266-3041-602; fax: +86-029-8266-3426. E-mail address : [email protected], [email protected].

Available online at www.sciencedirect.com

2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS).


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870 Yuanlin Yuan et al. / Physics Procedia 32 ( 2012 ) 869 874

2. Experiments

2.1 Mechanical CleavageGraphite flakes are readily provided, while transparent sticky tape should have apposite stickiness (we choose

3M company's Scotch Magic Tape 810) and no residue. Place a graphite flake on the sticky tape with tweezer, andfold the sticky tape, forming a "sandwich structure"(two layers of sticky tape with a graphite flake between them).

Press the tape to ensure good contact of graphite and tape, and tear the two layers of sticky tape rapidly, then thegraphite flake is divided into two flakes with glossy surfaces. Repeat this process, tightly and appropriatelydistribute the graphite flakes on the sticky tape and avoid overlapping. Then cut the tape (with graphite flakessticked on it) into small pieces, disperse them in deionized water, ethanol and analytical reagent acetone respectively,apply ultrasonic oscillationto obtain "graphene solution".

2.2 Chemical ReductionDisperse graphite oxide in ethanol while graphite oxide should be less than 0.02wt%. Applying ultrasonic

oscillation no less than two hours, the dispersed graphite oxide suspension is not clear liquid but rather turbid. Dilute80% analytical reagent hydrazine with ethanol. Graphene reduced in ethanol environment directly deposit on bottomof the receptacle due to gravity. The sediment is not result from polymeric degeneration, but rather a loose group ofgraphene films which can be scattered by ultrasonic oscillation.

2.3 Preparation of Graphene FETUse graphene as the channel to construct a backgated FET, and its structure is showed in Figure 1. The electrode

material is Pt, and substrate is insulating material. Transfer the readily prepared graphene sample on SiO2/Si backgate substrate, and bond it with insulating substrate which already have source and drain electrodes on it toform graphene FET.

Figure 1 Schematic Picture of graphene FET device's structureMeasure graphene channel's IV character with Keithley 4200, and the test circuit is showed in Figure 2. The two

metal electrodes are respectively source and drain. Silicon dioxide is gate dielectric. Silicon substrate in the back isgate. Source electrode is always grounded during measurement.

Figure 2 Graphene FET's structure and its test circuit

graphene

Highly doped silicon

source drain

substrate

silicon dioxide


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3. Result and Discussion

Use Holland FEI company's Tecnai F30 G2 scanning transmission electron microscope to characterize graphenesample. Point resolution is 0.20 nm and line resolution is 0.10 nm. Figure 3 shows the graphene sample obtainedthrough mechanical cleavage method. Cleavage is a rapid process, leading to uneven detachment between graphitelayers and therefore fragments. Transplantation is a process to dissociate the fragments. When the graphitefragments have a thickness of one atomic layer (10 atoms), we get graphene. Graphene prepared through mechanicalcleavage have fine crystal structure as shown in figure3b.

(b)

Figure 3 TEM scanning photograph of graphene sample obtained from sticky tape cleavage and ultrasonic oscillation indeionized water, (a) with the lower resolution and (b) with the higher resolution

Figure 4 shows a TEM photo of graphene sample obtain through chemical reduction under the resolution of100nm. Through careful observation we found that there were tiny granules adhering to carbon atom film as shownin figure 4a. Observation of the crystal lattice stripes of those graphene films showed that graphene obtained from

hydrazine reduction turned to be amorphous. X-ray energy spectrum analysis told that the elemental component ofthose small granulesis is the same with that of graphene, e.i. carbon atoms. Those small granules manifest differentcrystal morphology as shown in figure 4b and figure 4c. Preliminary analysis suggests that certain factor in

preparation process leads to crystallization of carbon atoms and results in those small granules. Polycrystallinesmaller than 5nm is very rare, its mechanism needs further research. Graphene in Figure 4a have the small granulesadhering to its surface, whose size is around 5nm. Small granules manifest different chirality, because the granule inFigure 4b has the same chirality while in Figure 4c there are several chiralities.


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Figure 4 TEM scanning microphotograph of graphene sample reduced by hydrazine in ethanol. (a)The tiny granules adhering to

carbon atom film, (b) and (c) the small granules manifest different crystal morphologyTEM images also suggest that although graphene obtained from hydrazine reduction is amorphous, hydrazinereduction in ethanol environment can produce high quality graphene whose thickness is less than 5 carbon atomlayers. Meanwhile, this process is easy to operate, has high yield, and its product graphene is large in size. Yet theadhering small granule and its mechanism still need further research.

Figure 5 is the IV characteristic curve of graphene channel obtained from mechanical cleavage. In Figure 5a thegate voltage varied from -1V to 1V with a step of 0.5V, the source-drain voltage varied form 0V to 20V. While thevoltage between the source and the drain was 5V and the gate voltage varied from -1V to 1V, the drain currentvaried from -0.83 A to 2.24 A. In Figure 5b the gate voltage varied from 0V to 20V with a step of 5V, and draincurrent varied from -0.51 A to 0.66 A. Analysis of those pictures suggests that graphene channel is able to becontrolled by the gate. In Figure 5b graphene manifests ambipolar feature, but linearity is not so good, suggestingthat the contact between electrodes and graphene is non-ohmic contact, which accords with the fact.

-2 0 2 4 6 8 10 12 14 16 18 20 22-0.000002

0.000000

0.000002

0.000004

0.000006

0.000008

0.000010

0.000012

0.000014

0.000016

I D S

( A )

VDS (V)

VGS = -1V

VGS = -0.5V

VGS = 0V

VGS = 0.5V

VGS = 1V

a


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-1.0 -0.5 0.0 0.5 1.0-0.0000006

-0.0000004

-0.0000002

0.0000000

0.0000002

0.0000004

0.0000006

0.0000008

I D S

( A

)

VDS (V)

VGS =0V

VGS =10V

VGS =20V

b

Figure 5 The V DS~IDS characteristic curve of Graphene FET's under room temperature(25 ), atmosphere and different gatevoltage. (a) V DS~IDS character when Gate voltage V GS=-1V -0.5V 0V 0.5V 1V. (b) V DS~IDS character when Gate voltageVGS=0V 10V 20V

4. Conclusion

We use micromechanical cleavage HOPG and reduction of graphite oxide respectively to obtain graphene. In thefirst method, we use transparent sticky tape to cleave graphite, and disperse it in solvent through ultrasonicoscillation. In the second method, with ethanol as dispersant, we use hydrazine reduction technic to obtain evenlydispersed graphene. SEM, TEM and laser scanning confocal microscope are used to characterize graphene'sstructure and morphology. Result suggests that micromechanical cleavage method can produce graphene which has

better crystal lattice structure, while chemical reduction produce graphene with crystal lattice defects. IV charactertest of backgated FET which use the above two kinds of graphene as channel suggest that both of them have goodgate control characteristic.

Acknowledgements

This paper was supported by National Natural Science Foundation of China (No. 729092923040, 6080102260476037, 50975226, 60976058HZ), National Basic Research Program of China (No. 2009CB724202), NewCentury Excellent Talents (NCET-08-0447), and the Fundamental Research Funds for the Central Universities.

References[1] Novoselov, KS, et al. Electric Field Effect in Atomically Thin Carbon Films[J], Science, 2004,Vol 306

(5696): 666-669.

[2] Novoselov, KS et al. Two-dimensional atomic crystals[R], PNAS, 2005-01-26, Vol 102 (30): 10451-10453.[3] KS Novoselov, AK Geim, SV Morozov, et al. Two-dimensional gas of massless Dirac fermions in

graphene[J]. Nature, 2005, 438: 197200.[4] SV Morozov, KS Novoselov, F Schedin, et al. Two-dimensional electron and hole gases at the surface of

graphite[J]. Physical Review B, 2005, 72: 201401.[5] MS Purewal, Y Zhang, P Kim. Unusual transport properties in carbon based nanosclaed materials:

nanotubes and grapheme[J]. Phys. Stat. Sol. (b), 2006, 243(13): 34183422.[6] Editorial. Natural Material[J], 2007, 6:169.


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Conference Title 18th International Vacuum Congress (IVC-18)

The title of paper: Graphene FETs with Combined Structure andTransparent Top

Author: Yuanlin Yuan, Zhen Chen, Congxiang Lu, Hongzhong Liu*, Yuan Wu, Xin Li *Email: [email protected], [email protected] affiliation: Department of Microelectronics, Xian Jiaotong University, Xian 710049, ChinaSecond affiliation: Key Laboratory for Manufacturing Systems Engineering, Xian Jiaotong University, Xian710049, China

AbstractGraphene, comprising of monolayer of carbon atoms packed into a two-dimensional honeycomb lattice, has a seriesof peculiar properties such as the anomalously quantized Hall effects, the large charge carrier mobility and so on.Micromechanical cleavage method is used to produce grapheme, which is acquired by peeling graphite foil off fromtransparent sticky tape repeatedly. Graphenes pattern has been placed between the source and the drain electrodes asthe channel by location transplantation method. The results show that the graphene samples of micromechanicalcleavage method have better lattice structure. The IDS-VDS curves of FET properties of graphene sheet channel aremeasured. Graphene channel obvious responses to the gate voltage.

2010 Published by Elsevier B.V.PACS: 81.05.Uw; 81.07.-b; 81.15.Gh; 81.16.-cKeywords graphene Micromechanical cleavage hydrazine hydrate; Ethanol; Field effect transistor; locationtransplantation gas sensing

2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS).

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Graphene FETs With Combined Structure and Transparent Top