Graphene FETs with Combined Structure and Transparent Top

Available online at www.sciencedirect.com Physics Procedia 32 (2012 ) 869 874 18th International Vacuum Congress (IVC-18) Graphene FETs with Combined Structure and Transparent Top Yuanlin Yuan a, Zhen Chen a, Congxiang Lu a, Hongzhong Liu b *, Yuan Wu a, Xin Li a * adepartment of Microelectronics, Xi an Jiaotong University, Xi an 710049, China bkey Laboratory for Manufacturing Systems Engineering, Xi an Jiaotong University, Xi an 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 cleavage method 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-VDS curves of FET properties of graphene sheet channel are measured. Graphene channel obvious responses to the gate voltage. 2012 2010 Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). Open access under CC BY-NC-ND license. 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 nanostructures such 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 particular quantum Hall effect, high electron mobility[3-5] etc. become glaring focus of carbon family, which decrease the size of device and power consumption and enhance conduction velocity. Fast velocity and transportation with no scattering 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 the two kinds of samples under microanalysis, investigating the influence of process on crystal structure. Then we transplant 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: lx@mail.xjtu.edu.cn, hzliu@mail.xjtu.edu.cn. 1875-3892 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). Open access under CC BY-NC-ND license. doi:10.1016/j.phpro.2012.03.649

870 Yuanlin Yuan et al. / Physics Procedia 32 ( 2012 ) 869 874 2. Experiments 2.1 Mechanical Cleavage Graphite 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, and fold 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 the graphite flake is divided into two flakes with glossy surfaces. Repeat this process, tightly and appropriately distribute the graphite flakes on the sticky tape and avoid overlapping. Then cut the tape (with graphite flakes sticked 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 Reduction Disperse 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. Dilute 80% analytical reagent hydrazine with ethanol. Graphene reduced in ethanol environment directly deposit on bottom of the receptacle due to gravity. The sediment is not result from polymeric degeneration, but rather a loose group of graphene films which can be scattered by ultrasonic oscillation. 2.3 Preparation of Graphene FET Use 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 to form graphene FET. Highly doped silicon silicon dioxide source drain graphene substrate Figure 1 Schematic Picture of graphene FET device's structure Measure 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 is gate. Source electrode is always grounded during measurement. Figure 2 Graphene FET's structure and its test circuit

Yuanlin Yuan et al. / Physics Procedia 32 ( 2012 ) 869 874 871 3. Result and Discussion Use Holland FEI company's Tecnai F30 G2 scanning transmission electron microscope to characterize graphene sample. Point resolution is 0.20 nm and line resolution is 0.10 nm. Figure 3 shows the graphene sample obtained through mechanical cleavage method. Cleavage is a rapid process, leading to uneven detachment between graphite layers and therefore fragments. Transplantation is a process to dissociate the fragments. When the graphite fragments have a thickness of one atomic layer (10 atoms), we get graphene. Graphene prepared through mechanical cleavage 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 in deionized 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 of 100nm. Through careful observation we found that there were tiny granules adhering to carbon atom film as shown in 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 of those small granulesis is the same with that of graphene, e.i. carbon atoms. Those small granules manifest different crystal 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. Polycrystalline smaller than 5nm is very rare, its mechanism needs further research. Graphene in Figure 4a have the small granules adhering to its surface, whose size is around 5nm. Small granules manifest different chirality, because the granule in Figure 4b has the same chirality while in Figure 4c there are several chiralities.

872 Yuanlin Yuan et al. / Physics Procedia 32 ( 2012 ) 869 874 g g g g y ( ) 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 morphology TEM images also suggest that although graphene obtained from hydrazine reduction is amorphous, hydrazine reduction in ethanol environment can produce high quality graphene whose thickness is less than 5 carbon atom layers. Meanwhile, this process is easy to operate, has high yield, and its product graphene is large in size. Yet the adhering 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 the gate voltage varied from -1V to 1V with a step of 0.5V, the source-drain voltage varied form 0V to 20V. While the voltage between the source and the drain was 5V and the gate voltage varied from -1V to 1V, the drain current varied 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 drain current varied from -0.51 A to 0.66 A. Analysis of those pictures suggests that graphene channel is able to be controlled by the gate. In Figure 5b graphene manifests ambipolar feature, but linearity is not so good, suggesting that the contact between electrodes and graphene is non-ohmic contact, which accords with the fact. I DS (A) 0.000016 0.000014 0.000012 0.000010 0.000008 0.000006 0.000004 0.000002 0.000000 V GS = -1V V GS = -0.5V V GS = 0V V GS = 0.5V V GS = 1V -0.000002-2 0 2 4 6 8 10 12 14 16 18 20 22 V DS (V) a

Yuanlin Yuan et al. / Physics Procedia 32 ( 2012 ) 869 874 873 0.0000008 0.0000006 0.0000004 V GS =0V V GS =10V V GS =20V I DS (A) 0.0000002 0.0000000-0.0000002-0.0000004-0.0000006-1.0-0.5 0.0 0.5 1.0 V DS (V) b Figure 5 The V DS ~I DS characteristic curve of Graphene FET's under room temperature(25 ), atmosphere and different gate voltage. (a) V DS ~I DS character when Gate voltage V GS =-1V -0.5V 0V 0.5V 1V. (b) V DS ~I DS character when Gate voltage V GS =0V 10V 20V 4. Conclusion We use micromechanical cleavage HOPG and reduction of graphite oxide respectively to obtain graphene. In the first method, we use transparent sticky tape to cleave graphite, and disperse it in solvent through ultrasonic oscillation. In the second method, with ethanol as dispersant, we use hydrazine reduction technic to obtain evenly dispersed graphene. SEM, TEM and laser scanning confocal microscope are used to characterize graphene's structure 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 character test of backgated FET which use the above two kinds of graphene as channel suggest that both of them have good gate control characteristic. Acknowledgements This paper was supported by National Natural Science Foundation of China (No. 729092923040, 60801022 60476037, 50975226, 60976058HZ), National Basic Research Program of China (No. 2009CB724202), New Century 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.

874 Yuanlin Yuan et al. / Physics Procedia 32 ( 2012 ) 869 874 Conference Title 18th International Vacuum Congress (IVC-18) The title of paper: Graphene FETs with Combined Structure and Transparent Top Author: Yuanlin Yuan, Zhen Chen, Congxiang Lu, Hongzhong Liu*, Yuan Wu, Xin Li * Email: lx@mail.xjtu.edu.cn, hzliu@mail.xjtu.edu.cn First affiliation: Department of Microelectronics, Xi an Jiaotong University, Xi an 710049, China Second affiliation: Key Laboratory for Manufacturing Systems Engineering, Xi an Jiaotong University, Xi an 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 cleavage method 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-VDS curves of FET properties of graphene sheet channel are measured. Graphene channel obvious responses to the gate voltage. 2012 2010 Published by by Elsevier Elsevier B.V. B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). 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