Ben Aziza, Z.

Benz, C.

Danneau, R.

Löhneysen, H. von

Mohrmann, J.

Taniguchi, T.

Thürmer, M.

Watanabe, K.

Wu, F.

English

KITopen

Graphene on boron nitride microwave transistors driven by graphenenanoribbon back-gatesC. Benz,1,2 M. Th€urmer,1 F. Wu,1 Z. Ben Aziza,1 J. Mohrmann,1 H. v. L€ohneysen,1,2,3K. Watanabe,4 T. Taniguchi,4 and R. Danneau1,2,a)1Institute of Nanotechnology, Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany2Institute of Physics, Karlsruhe Institute of Technology, D-76049 Karlsruhe, Germany3Institute for Solid State Physics, Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany4Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba, Japan(Received 7 November 2012; accepted 8 January 2013; published online 24 January 2013)We have designed ultra-thin graphene microwave transistors by using pre-patterned metal orgraphene nanoribbon back-gates and hexagonal boron nitride as a dielectric substrate. Despite theinhomogeneities induced by the graphene transfer process, we show that it is possible to operatethese types of devices across a broad range of microwave frequencies. For the graphene nanoribbongates, we observe a deviation of the current gain from the usual 1/f trend that can be attributed to thelarge gate resistance of these systems as we demonstrate with our small-signal model. The scatteringparameter analysis shows a very limited back-action from the channel onto the graphene nanoribbongates. Our work thus proves that graphene microwave transistors could be driven by graphenenanoribbon gates. VC 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4788818]The remarkable properties of graphene field-effect tran-sistors (GFET) are mirrored in the amount of recent efforts topush the transit frequencies fT to new limits1–5 setting the cur-rent record to fT¼ 427 GHz.6 Aside from transistor optimiza-tion, whole integrated GFET circuits have been designed.7,8Finding appropriate substrates and dielectrics is crucial toimprove GFET characteristics. Common material like SiO2has a certain roughness9 and contains charge traps10 whichboth induce Coulomb scattering centers and residual chargecarrier inhomogeneities into the graphene channel, ultimatelylimiting the GFET performance.11,12 A promising substratecandidate for graphene is hexagonal boron nitride (h-BN).13h-BN offers several benefits: it has the same morphology asgraphene with a lattice mismatch of just 1.7%.14 It is anatomically flat crystal which contains a very limited numberof charge traps and dangling bonds, permitting very highmobilities.13 Moreover, h-BN is an insulator and can thereforealso be used as a gate dielectric. Here, we report on the fabri-cation of GFET on h-BN substrates where we have studiedthe possibility to use graphene nanoribbons as a gate materialthat operates at microwave frequencies.We have proceeded along two GFET device layouts. Inthe first design, we patterned the dual-finger back-gate on asapphire substrate15,16 by electron beam lithography (EBL)followed by the evaporation of a thin Ti/Au bilayer (1/30 nm)and lift-off in acetone. Then, we transfer h-BN flakes onto thepre-patterned back-gate (see Fig. 1(a) for a schematic draw-ing). We used high purity h-BN crystals, grown as describedin Ref. 17. These crystals are superior to CVD-grown h-BNfilms whose quality is degraded by wrinkles and step edges.18The h-BN serves both as the gate dielectric and the substratefor the graphene channel which is transferred in a subsequentstep. The connections to the microwave transmission linewere defined by EBL, metal evaporation (Ti/Al 10/100 nm) inultra high vacuum (109 mbar), and lift-off in acetone. Intotal, four GFET with this layout have been fabricated andcharacterized, all working in a similar fashion. Here, we pres-ent the data of one of these devices.In Fig. 1(d), the ratio of the drain current Ids to the mini-mum current Ids0 at the Dirac point is plotted versus theback-gate voltage Vg. Note that the Dirac point is very closeto zero gate voltage which implies low unintentional dopingof the graphene sheet. With increasing drain voltage Vds, weobserve a shift towards positive gate voltages as expectedwith the additional bias-inducing doping of the channel. Inaddition, the on/off ratio decreases. Both are due to short-channel effects.19,20 Fig. 1(e) shows I-V curves of the samedevice, reaching current densities of up to 0:6 mA=lm.High frequency measurements were carried out on a probestation using a Rohde & Schwarz ZVA40 vector network ana-lyzer (VNA) under ambient conditions. De-embedding wasperformed by measuring “open” and “short” structures on-chip and extracted from the measured scattering (S) parame-ters using conventional two-port network analysis.21 Thesestructures are located right next to the actual device. Greatcare was taken to achieve an identical layout. We extracted thesmall-signal current gain jh21j from both the bare measuredand de-embedded S-parameters to determine the correspond-ing transit frequencies as described in Ref. 22. Fig. 1(f) showsjh21j of the device (h-BN thickness d¼ 12 nm, gate lengthlg¼ 100 nm, source-drain distance lsd¼ 300 nm, total channelwidth wch¼ 16 lm) at Vg¼ 0.48 V and Vds¼ 0.8 V. Maximumvalues fT¼ 6.4 GHz (extrinsic) and fT  60 GHz (intrinsic)have been found. The current gain follows the ideal 1/f slopeover the whole measurement range from 0.01 to 40 GHz.The extracted rf transconductance grfm is 90 lS/lm at 1 GHzfor Ids¼ 9.2 mA and stays almost constant in the measuredbandwidth.The intrinsic transit frequencies of our h-BN supportedGFETs exceed those of the devices presented so far.18,23,24However, the performance of our devices is most probablya)Author to whom correspondence should be addressed. Electronic mail:romain.danneau@kit.edu.0003-6951/2013/102(3)/033505/4/$30.00 VC 2013 American Institute of Physics102, 033505-1APPLIED PHYSICS LETTERS 102, 033505 (2013)Downloaded 24 Jan 2013 to 141.52.94.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionslimited by ripples in the graphene due to the (wet) transferprocess.25 These inhomogeneities can be clearly seen in theSEM and AFM images in Figs. 1(b) and 1(c). Additionally,the finite thickness of the metallic back-gate leads to a bend-ing of both the h-BN crystal and graphene, ultimately intro-ducing even further inhomogeneities in the channel.Motivated by this last issue, our second device layout uses afew-layer graphene nanoribbon back-gate (Fig. 2(a)) with athickness of less than 2 nm and a gate length of 100 nm.Together with the very thin h-BN flakes of about d¼ 4 nmand a monolayer graphene with a channel length of 500 nmand a total channel width of 30 lm, the cumulative deviceheight is 7nm (excluding metallic electrodes).Figs. 2(b)–2(g) depict the various steps of the devicefabrication process. We begin by exfoliating natural graph-ite onto our sapphire substrates. Using optical microscopy,we singled out suitable flakes for the back-gate. The dual-finger gate layout is patterned by EBL using a negativeresist (ma-N 2401). We use oxygen plasma to pattern theflakes. Then, the resist is removed by acetone and the sam-ple is cleaned in a vacuum furnace. The h-BN and gra-phene flakes are transferred using an optimized wedgingtechnique.13 Accurate alignment of the order of 1 lm isachieved with the high-precision piezo stage of our home-made transfer set-up. In the last step, the source, drain,and gate electrodes are defined by EBL, and a Ti/Albilayer (10/100 nm) is evaporated in UHV. Two deviceswere fabricated and measured, both showing similarperformances.We find that the Dirac point is slightly shifted withrespect to zero gate voltage indicating low doping level (seeFig. 3(a)). I-V curves show clear gate dependence (seeFig. 3(b)). In Fig. 3(c), the high frequency measurementresults are plotted. At Vds¼ 0.53 V, the cut-off frequenciesare fT¼ 4.8 GHz before and fT  30 GHz after de-embeddingthe parasitic capacitances of the contact pads. A full open/short de-embedding process however was impossible to per-form since the graphene back-gate would have to be replacedby normal metal conductors in the dummy structures. A clearindication of this issue is the departure of the de-embeddedcurrent gain from the usual 1/f dependence at the highestfrequencies.Although the performance does not fully reach that ofour metal-gate devices, it is still surprisingly high given theextreme reduction of the gate thickness (30 nm versus 4 nm)and the larger gate resistance Rg of the graphene nanoribbonback-gates. Gate resistance should be as small as possible inhigh frequency transistors.26,27 We observe that the extrinsiccurrent gain curve starts to deviate from the ideal 1/f charac-teristic before reaching the optimum cutoff frequency. Thisis strong evidence of the very high gate access resistance Rg.The jh21j behavior can be understood by a simplified small-signal model,28 which accounts for the access resistance Rg,FIG. 2. (a) Schematic of the graphene nanoribbon back-gate device layout.(b)-(g) Optical micrographs of the sample fabrication steps: (b) Exfoliationof graphene (few layer of 2 nm thickness) for back-gate defined by EBLand etched using reactive ion etching (c). (d) Transfer of a thin layer ofh-BN covering the gate fingers. (e) Transfer of a monolayer graphene flake(channel). (f) Patterning of the contacts and gate electrode by EBL.(g) Metal evaporation Ti/Al.FIG. 1. (a) Schematic of the metal back-gate device layout. (b) SEM and (c)AFM images of the active area. Note the wrinkles in the graphene due totransfer. (d) Ratio of Ids to Ids0 vs. Vg. The drain voltages Vds0 at the mini-mum drain currents Ids0 are indicated by the legend. (e) Ids vs. Vds for variousVg. (f) Current gain jh21j vs. frequency extracted from as-measured (greensquares) and de-embedded (blue circles) S-parameters. The achieved maxi-mum transit frequencies are fT¼ 6.4 GHz and fT  60 GHz, respectively.The ideal 1/f dependence is indicated by black lines. The inset shows the rftransconductance grfm vs. Vg.033505-2 Benz et al. Appl. Phys. Lett. 102, 033505 (2013)Downloaded 24 Jan 2013 to 141.52.94.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsthe total gate capacitance Cg ¼ Cgs þ Cgd of the GFET29(Cgs and Cgd corresponding to gate to source and gate todrain capacitances, respectively) and the contact pad capaci-tance Cpg. The circuit used to model our system is depictedin Fig. 3(e). We found that the resulting frequency depend-ence can be described byh21ðxÞ ¼ðgm  ixCgdÞ  ðxRgðCgd þ CgsÞ  iÞxð1þ ixRgðCgd þ CgsÞÞ  ðixRgðCgd þ CgsÞCpg þ Cpg þ ðCgd þ CgsÞÞ;where its module is used to fit our raw data. We keep Cpgand gm as static parameters while only leaving Rg and Cg asfitting parameters, we show that our model fits well thebehavior of jh21j. For a measured transconductance ofgm¼ 2.6 mS, Cpg¼ 31.2 fF (extracted from the open struc-ture), we obtain a Cgs  2.9 fF, Cgd  13.8 fF, and Rg  3.3kX. Despite this large gate resistance Rg (approximately twoorders of magnitude larger than Rg of the metal back-gatetransistor), Fig. 3(d) shows a high rf transconductance of thedevice. At 10 MHz, gm takes on comparable values as themetal back-gate device. At higher frequencies, however, weobserve a notable decrease, mainly affected by the large gateresistance of our graphene nanoribbon gate devices.The analysis of the S-parameters offers a deeper insightinto the quality of the GFET. Figs. 4(a) and 4(b) show polarplots of the recorded S-parameters for the metal and the gra-phene gate, respectively. S-parameters relate incoming andreflected voltages of the two-port transistor devices, one portbeing defined by gate and source and the other by drain andsource.21 S11 gives the impedance mismatch to the character-istic impedance Z0 which in almost all rf applications isgiven by 50 X. While intuitively, one would expect strongerreflection for higher frequencies in the graphene gated de-vice,30 we find indeed the opposite. We may attribute thisresults to the effect of the quantum capacitance that willbecome increasingly dominant for decreasing gate dielectricthickness,31,32 as well as to the very thin graphene nanorib-bon back-gate and its reduced fringing fields compared tothicker metal-gates.Another important impact of the graphene nanoribbonback-gate can be seen when comparing the frequency devel-opment of S12 which indicates the back-coupling of the elec-tromagnetic fields from the channel into the gate. Thisprocess is strongly suppressed in the graphene-gated deviceat high frequencies. For cases in which good source/loadback-action isolation is needed, this might be of benefit. S21and S22 are mainly governed by the chosen operating pointof the transistor. In both devices, the transconductance gm atthe operating point is 2 mS and the differential output con-ductance gds ¼ dIdsdVds is of the order of 100 mS. S21 being thevoltage amplification factor is therefore rather low in alldevices but comparable to those reported on GFET in the lit-erature, and deviates at higher frequencies mainly due to thestrong difference in gate access resistance as discussedabove. We aim to improve device performance, especiallywith respect to voltage amplification, by increasing currentsaturation33 (i.e., increasing the wch) and therefore gds usingvery thin high-j gate dielectrics as suggested in Ref. 8.FIG. 3. Graphene back-gate device: (a) Current ratio Ids=Ids0 vs. back-gatevoltage Vg. (b) Ids vs. Vds for varying back-gate voltages Vg. (c) At Vg¼ 0.48V and Ids¼ 8.5 mA, maximum transit frequencies of fT¼ 4.8 GHz (extrinsic)and fT  30 GHz (intrinsic) are found (green squares and blue circles,respectively). The red curve corresponds to the fit with our small-signalmodel. (d) The rf transconductance at 10 MHz is comparable to the firstmetal-gate device but decreases with increasing frequency. (e) small-signalmodel used to fit our GFET current gain jh21j.FIG. 4. Polar plot of the S-parameters before de-embedding of (a) the metalback-gate and (b) graphene back-gate device. The frequency range from 10MHz to 40 GHz has to be read clockwise.033505-3 Benz et al. Appl. Phys. Lett. 102, 033505 (2013)Downloaded 24 Jan 2013 to 141.52.94.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsIn conclusion, we have presented graphene field-effecttransistors on h-BN substrates with metal as well as graphenenanoribbon back-gates of the same length. Despite their largegate resistance, GFET with few-layer graphene nanoribbongates can be driven at microwave frequencies. These ultra-thin devices represent a further step towards all-sp2 carbonelectronics.We acknowledge E. Pallecchi and L. Petzold for fruitfuldiscussions. R.D.’s Shared Research Group SRG 1-33received financial support by the Karlsruhe Institute of Tech-nology within the framework of the German Excellence Ini-tiative. This work was supported by the EU Project No.MMM@HPC FP7-261594.1I. Meric, N. Baklitskaya, P. Kim, and K. L. Shepard, Tech. Dig. - Int.Electron Devices Meet. 2008, 4796738.2Y.-M. Lin, K. A. Jenkins, A. Valdes-Garcia, J. P. Small, D. B. Farmer, andP. Avouris, Nano Lett. 9, 422 (2009).3Y.-M. Lin, C. 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Micro-wave Theory Tech. 36, 1151 (1988).29In this small-signal model, we neglect all inductances (including kineticinductance) and series resistances in drain and source since they mainlyscale jh21j up or down, and do not affect dramatically the value of Rg.30This is due to the wider channel and thinner h-BN thickness of the gra-phene gate devices as to compare to the metal gate devices (and thereforeincreases the gate coupling capacity).31T. Fang, A. Konar, H. Xing, and D. Jena, Appl. Phys. Lett. 91, 092109(2007).32E. Pallecchi, A. C. Betz, J. Chaste, G. Fève, B. Huard, T. Kontos, J.-M.Berroir, and B. Plaçais, Phys. Rev. B 83, 125408 (2011).33I. Meric, M. Y. Han, A. F. Young, B. €Ozyilmaz, P. Kim, and K. L.Shepard, Nat. Nanotechnol. 3, 654 (2008).033505-4 Benz et al. Appl. Phys. Lett. 102, 033505 (2013)Downloaded 24 Jan 2013 to 141.52.94.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Graphene on boron nitride microwave transistors driven by graphene nanoribbon back-gates

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Graphene on boron nitride microwave transistors driven by graphene nanoribbon back-gates

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