A novel graphene transistor architecture is reported. The transistor has a U-shaped geometry and was fabricated using a gallium focused ion beam (FIB). The channel conductance was tuned with a back gate. The Ion/Ioff ratio exceeded 10E5

Boden, Stuart

Ghiass, Mohammad Adel

Mizuta, Hiroshi

Moktadir, Zakaria

Rutt, Harvey

English

Southampton (e-Prints Soton)

U-shaped bilayer graphene channeltransistor with very high Ion/Ioff ratioZ. Moktadir, S.A. Boden, A. Ghiass, H. Rutt and H. MizutaA novel graphene transistor architecture is reported. The transistor has aU-shaped geometry and was fabricated using a gallium focused ionbeam (FIB). The channel conductance was tuned with a back gate.The Ion/Ioff ratio exceeded 105.Introduction: Graphene has ignited tremendous interest since its discov-ery in 2004 [1]. This material is made of a single layer of carbon atomsassembled in a hexagonal structure, providing extraordinary electricaland mechanical properties. The observation of ﬁeld effect in graphenehas particularly attracted the attention of the electron devices communityresulting in a large volume of work on graphene ﬁeld effect transistors(GFETs). Indeed graphene is one of the possible materials for the neosilicon era, according to the 2009 International Technology Roadmapfor Semiconductors. One outstanding property of graphene is its highcarrier mobility at room temperature. In exfoliated supported graphene,mobility as high as 70000 cm2 V21 s21 has been reported [2].Suspended graphene shows even higher values, reaching 106 cm2 V21s21 as the interaction with the substrate is suppressed [3].Graphene grown on metals and transferred to a substrate shows lowervalues. However, large area monolayer or bilayer graphene suffers fromthe absence of a band gap, which results in poor switching capability ofGFETs. Despite intense work to improve the Ion/off ratio, only valuesless than 100 were achieved. There are two ways used to open a bandgap in graphene. The ﬁrst is to cut it into narrow ribbons (graphenenanoribbons (GNRs)). If the width of the ribbon is small enough (fewnanometres), carriers are conﬁned in a one-dimensional quantum wire,resulting in a nonzero band gap. The band gap value is inversely pro-portional to the width of the GNR. Band gaps larger than 0.5 eV(needed for device operation at room temperature) require the width ofthe GNR to be less than 5 nm [4]. Unfortunately existing lithographictechniques cannot provide such accuracy. The second way is to applya strong electric ﬁeld perpendicular to the bilayer graphene plane, indu-cing a strong coupling between the two graphene atomic layers, whichresults in the detachment of the valence and the conduction band atthe K point in the Brillouin zone. The largest band gap values wereachieved with dual gated GFETs which are controlled using a top gateand a back gate. Values of Ion/Ioff ratios of around 100 have beendemonstrated [5].We report here a new GFET architecture. The GFET is U-shaped andfabricated using state-of-the-art gallium FIB technology. The obtainedIon/Ioff exceeds 105.Device fabrication: All fabrication was carried out using a dual focusedion beam/scanning electron microscope system (Zeiss NVision40 FIB/FEGSEM) equipped with a gas injection system (GIS). A bilayergraphene ﬂake on a 300 nm layer of SiO2 on Si (sample purchased fromGraphene Industries Ltd) was imaged (Fig. 1a) and areas were identiﬁedfor the formation of a graphene U-shaped transistor. Electron beam-induced tungsten deposition was then used to deposit thin protectivelayers across the edges of the graphene ﬂake in areas where contactformation would take place. This acted as a barrier layer to minimisethe damage caused to the graphene by the gallium ions in the subsequention beam-induced metal deposition step. This process involved using thereduced raster scan to scan the electron beam over these edge areas. Thevalve to the GIS was then opened to allow tungsten precursor (W(CO)6)to ﬂow into the chamber and deposit on the surface, leading to the con-trolled deposition of a few nanometres of tungsten across the edges ofthe graphene ﬂake. Square contact pads, 50 × 50 mm, and smaller rec-tangular strips to connect the pads to the graphene were deposited usinggallium ion beam-induced tungsten deposition in the same system. Theion beam current was adjusted in the range 0.08–13 nA for maximumdeposition rate depending on the areas over which the deposition wasoccurring. Care was taken not to expose the unprotected parts of the gra-phene ﬂake to the gallium ion beam during deposition so as to minimisedamage to the graphene. After each deposition, the gas injection systemvalvewas closed and the chamber vacuum was allowed to recover so thatany remaining precursor could desorb from the surface before imagingthe sample. The gallium ion beam was then used to mill the contactwires and graphene ﬂake to form and isolate the device. A beamcurrent of 150 pA was used when milling the tungsten and 80 pAwhen milling the graphene; the Ga+ beam voltage was 30 kV. Thesamples were annealed at 400 8C for 10 min prior to theircharacterisation.Fig. 1 Scanning electron micrographsa Graphene ﬂakeDashed outline marks areas suitable for formation of devicesb Device and contact padsDashed outline marks same area as in Fig. 1ac Graphene U-shaped device with tungsten contact wires and padsd Detail of graphene U-shaped deviceResults: Fig. 1a shows the unpatterned graphene ﬂake where the dashedoutline marks the pristine areawhere the device is fabricated. Fig. 1b is alow magniﬁcation image of thewhole device with tungsten contact pads.Only the bottom two contact pads connect to the U-shaped transistorwhich is the subject of this Letter. The upper contact pad, which con-nects to a second device, can be ignored. The dashed outline marksthe same area as in Fig. 1a. Figs. 1c and d show higher magniﬁcationimages of the fabricated GFET. The width of the graphene wire wasaround 300 nm while the total length was 20 mm. The GFET was con-trolled by a Si/SiO2 substrate back gate.Fig. 2 Id-Vg characteristic at Vd ¼ 1 V, showing off-point of U-shaped tran-sistor (Fig. 2a), and Id-Vd characteristic at Vg ¼ 24V( Fig. 2b)Fig. 2a shows the drain current Id against the back gate voltage, Vg,for a drain voltage of 1V. Fig. 2b shows the drain current againstdrain voltage Vd for Vg ¼ 24 V. The Id-Vg characteristic shows amarked dip around Vg ¼ Voff ¼ 3.3 V, where the GFET channel isswitched off. This characteristic is dissimilar to the usual large areagraphene Id-Vg characteristic which shows a smooth evolution of thedrain current towards the Dirac point. The Ion/Ioff derived here isaround 4.8 × 105 (taking Id ¼ 3.5 × 1024 Aa tV g ¼ 25 V, and Id ¼7.25 × 10210 Aa tV g ¼ 3.3 V), larger by three orders of magnitudethan the highest Ion/Ioff obtained in dual gated GFETs. The U-shapedGFET shows similar bipolar behaviour as observed in usual GFETs,but it also shows an asymmetry in the two conduction regimes (holesand electrons conduction). Our results can be interpreted by the effectof geometrical corners in the graphene channel. Theoretical calculationspredict that the formation of a band gap of an L-shaped channel isassociated with quantum-mechanical quasi-bound states at the corners.These quasi-bound states are strongly dependent on the atomicELECTRONICS LETTERS 3rd February 2011 Vol. 47 No. 3conﬁgurations on both the inner and the outer corners [6]. One shouldnot exclude the formation of a mobility gap [7], however it is unlikelyto be the dominant effect as our fabricated channel shows very lowoff-current ( 1 nA). The result presented here will open up possibilitiesfor the development of efﬁcient graphene logic circuits. For example,two GFETs with two different Voff points can be fabricated side byside to form a complementary GFET (CGFET), with little overlapbetween the Id-Vg characteristics of the two GFETs, resulting in an efﬁ-cient inverter.Conclusion: We have fabricated a graphene ﬁeld effect transistor havinga U-shaped geometry using focused ion beam technology. The achievedIon/Ioff exceeds 105.# The Institution of Engineering and Technology 201126 October 2010doi: 10.1049/el.2010.3029Z. Moktadir, S.A. Boden, A. Ghiass, H. Rutt and H. Mizuta (School ofElectronics and Computer Science, University of Southampton,Highﬁeld, Southampton, SO17 1BJ, United Kingdom)E-mail: zm@ecs.soton.ac.ukReferences1 Novoselov, K.S., Geim, A.K., Morozov, S.V., Jiang, D., Zhang, Y.,Dubonos, S.V., Grigorieva, I.V., and Firsov, A.A.: ‘Electric ﬁeld effectin atomically thin carbon ﬁlms’, Science, 2004, 36, pp. 666–6692 Chen, F., Xia, J., Ferry, D.K., and Tao, N.: ‘Dielectric screeningenhanced performance in graphene FET’, Nano Lett., 2009, 9, (7),pp. 2571–25743 Geim, A.: ‘Graphene update’, Bull. Am. Phys. Soc., 2010, 55, abstr.J21.0004, http://meetings.aps.org/link/BAPS.2010.MAR.J21.44 Li, X.L., Wang, X.R., Zhang, L., Lee, S.W., and Dai, H.J.: ‘Chemicallyderived, ultrasmooth graphene nanoribbon semiconductors’, Science,2008, 319, pp. 1229–12325 Xia, F., Farmer, D.B., Lin, Y., and Avouris, Ph.: ‘Intrinsic limits ofsubthreshold slope in biased bilayer graphene’, Nano Lett, 2010, 10,pp. 715–7186 Xie, Y.E., Chen, Y.P., Sun, L.Z., Zhang, K.W., and Zhong, J.-X.: ‘Theeffect of corner form on electron transport of L-shaped graphenenanoribbons’, Physica B, 404, (12–13), pp. 1771–17757 Miyazaki, H., Tsukagoshi, K., Kanda, A., Otani, M., and Okada, S.:‘Inﬂuence of disorder on conductance in bilayer graphene underperpendicular electric ﬁeld’, Nano Lett., 2010, 10, (10), pp. 3888–3892ELECTRONICS LETTERS 3rd February 2011 Vol. 47 No. 3

Chemically derived, ultrasmooth graphene nanoribbon semiconductors’,

Dielectric screening enhanced performance in graphene FET’, Nano Lett.,

Electric ﬁeld effect in atomically thin carbon ﬁlms’, Science,

Graphene update’,

Inﬂuence of disorder on conductance in bilayer graphene under perpendicular electric ﬁeld’,

Intrinsic limits of subthreshold slope in biased bilayer graphene’,

The effect of corner form on electron transport of L-shaped graphene nanoribbons’,

U-shaped bilayer graphene channel transistor with very high Ion/Ioff ratio

U-shaped bilayer graphene channel transistor with very high Ion/Ioff ratio

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