We report dual-gate modulation of topological insulator field-effect
transistors (TI FETs) made on Bi2Te3 thin flakes with integration of
atomic-layer-deposited (ALD) Al2O3 high-k dielectric. Atomic force microscopy
study shows that ALD Al2O3 is uniformly grown on this layer-structured channel
material. Electrical characterization reveals that the right selection of ALD
precursors and the related surface chemistry play a critical role in device
performance of Bi2Te3 based TI FETs. We realize both top-gate and bottom-gate
control on these devices, and the highest modulation rate of 76.1% is achieved
by using simultaneous dual gate control.Comment: 4 pages, 3 figure

Liu, Han

Ye, Peide D.

English

arXiv

We report dual-gate modulation of topological insulator field-effect transistors (TI FETs) made on Bi2Te3 thin flakes with integration of atomic-layer-deposited (ALD) Al2O3 high-k dielectric. Atomic force microscopy study shows that ALD Al2O3 is uniformly grown on this layer-structured channel material. Electrical characterization reveals that the right selection of ALD precursors and the related surface chemistry play a critical role in device performance of Bi2Te3 based TI FETs. We realize both top-gate and bottom-gate control on these devices, and the highest modulation rate of 76.1% is achieved by using simultaneous dual gate control. (C) 2011 American Institute of Physics. [doi:10.1063/1.3622306

Purdue E-Pubs

Atomic-layer-deposited Al2O3 on Bi2Te3 for topological insulator field-effect transistorsHan Liu and Peide D. Ye  Citation: Appl. Phys. Lett. 99, 052108 (2011); doi: 10.1063/1.3622306 View online: http://dx.doi.org/10.1063/1.3622306 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i5 Published by the AIP Publishing LLC.  Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 23 Jul 2013 to 128.46.221.64. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsAtomic-layer-deposited Al2O3 on Bi2Te3 for topological insulatorfield-effect transistorsHan Liu and Peide D. Yea)School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University,West Lafayette, Indiana 47907, USA(Received 22 April 2011; accepted 5 July 2011; published online 4 August 2011)We report dual-gate modulation of topological insulator field-effect transistors (TI FETs) made onBi2Te3 thin flakes with integration of atomic-layer-deposited (ALD) Al2O3 high-k dielectric.Atomic force microscopy study shows that ALD Al2O3 is uniformly grown on this layer-structuredchannel material. Electrical characterization reveals that the right selection of ALD precursors andthe related surface chemistry play a critical role in device performance of Bi2Te3 based TI FETs.We realize both top-gate and bottom-gate control on these devices, and the highest modulation rateof 76.1% is achieved by using simultaneous dual gate control. VC 2011 American Institute ofPhysics. [doi:10.1063/1.3622306]Three dimensional topological insulator (TI) materials,such as Bi2Te3, Bi2Se3, and Sb2Te3, have recently attractedmuch attention due to their unique physical properties.1–5These structures are layered like graphene but appear tobehave like insulators having a band gap in the materialbulk. However, these materials show metallic behavior ontheir surfaces. The surface states of TIs consist of an oddnumber of massless Dirac cones, around which the linear dis-persion of the electron spectrum is such that the carriersreach extremely high surface carrier mobilities up to 9000-10 000 cm2/Vs.6 Moreover, these surfaces, protected by timereversal symmetry,7 result in a non-scattering carrier trans-port regime, making TIs rather promising for future nanoe-lectronics applications with ultralow power dissipation.In order to realize practical TI field-effect transistors(FETs), it is important to study the integration of gate dielec-trics on these materials so that the channel current can becontrolled by the top-gate. In some early studies of Bi2Te3based TI devices, where a heavily doped silicon back gatewas used, no modulation was observed within a back-gatevoltage sweep from 50 V to 50 V at room temperature.8The highest modulation obtained is around 20% measured atless than 10 K.9 For Bi2Se3, which has a larger bandgap of0.3 eV than 0.14 eV of Bi2Te3, minuscule gate modulationwas reported using top-gate control at room temperature,10while much larger modulation was achieved by the back-gate sweeps.11 Although these are inspiring results toobserve the field effect by using a global back-gate, this can-not satisfy the requirement for real device applications,which requires individual device top-gate control and roomtemperature operation. Therefore, the realization of highlyefficient top-gate modulation for Bi2Te3 and other TI materi-als at room temperature is urgently needed.In this letter, we demonstrate Al2O3 growth by atomic-layer deposition (ALD) with two different precursors onBi2Te3 top surfaces. The uniformity of the surface morphol-ogy after the ALD Al2O3 deposition is also studied. Electricalcharacterization has shown a strong modulation of Bi2Te3based TI FETs with both top-gate and back-gate controls.Bi2Te3 thin flakes were peeled off from bulk ingots by stand-ard 3M scotch tape techniques12 and were then transferred toa highly doped silicon wafer with 300 nm thick SiO2. Thethickness of these ultrathin flakes is less than 50 nm. Tennanometer Al2O3 high-k dielectric layers were deposited byan ASM F-120 ALD system at 200 C by using tri-methyl-alu-minum (TMA) and H2O or TMA and O3 as precursors. Thepulse time is 0.8 s TMA and 1.2 s H2O, or 1s TMA and 1 sO3, and the purge time is 6 s N2 for each precursor. Source/drain regions were defined by optical lithography. Al2O3 wasetched away using buffered oxide etch (BOE) at these source/drain regions, followed by e-beam evaporation of a 20 nm/40nm Cr/Au metal and lift-off process for ohmic contacts. A10 nm/50 nm Cr/Au layer was finally deposited as the top-gate. Final device structure is shown in Figure 1(a), which issimilar to a traditional metal-oxide-semiconductor FET, butwith two conducting surfaces and one conducting bulk chan-nel in the channel region.The crystal structure of Bi2Te3 has been shown to besimilar to graphene, with stacking layers bonded by the Vander Waals force. The lattice constant for hexagonal Bi2Te3 is0.4384 nm for the a-axis and 3.045 nm for the c-axis.13–16Each layer of Bi2Te3, a quintuple layer with the thickness of1 nm, consists of the five layer sequence Te-Bi-Te-Bi-Te.Three quintuple layers form one unit cell.8 On the top andbottom Te layers, there are no dangling bonds at the surface.This may lead one to suspect that it is not possible to depositALD Al2O3 directly on these flakes because there would beno chemical absorption due to the absence of dangling bondsat surface, as generally seen in the case of graphene. Earlystudies also revealed that ALD Al2O3 fails to be depositedon graphene surfaces but would only cluster and grow ongraphene edges and ultimately form nanoribbons.17,18 How-ever, our results show that this problem does not occur forBi2Te3. A Veeco Dimension 3100 AFM was used to studythe flake surface and the layer edges after Al2O3 deposition.The pristine Bi2Te3 surface was studied after peeling andbeing transferred to SiO2/Si substrates. We notice the surfaceis not as smooth as the graphene surface. This might beattributed to the fact that Bi2Te3 is not as highly air-stable asa)Author to whom correspondence should be addressed. Electronic mail:yep@purdue.edu.0003-6951/2011/99(5)/052108/3/$30.00 VC 2011 American Institute of Physics99, 052108-1APPLIED PHYSICS LETTERS 99, 052108 (2011)Downloaded 23 Jul 2013 to 128.46.221.64. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsgraphene; oxygen and water molecules in air can slightlyoxidize the Te-terminated surface. This oxidation facilitatesthe following ALD process by creating nucleation spots forprecursor absorption. The smallest step observed is 1 nmcorresponding to the thickness of one quintuple layer. Mostof terraces have the heights of single or multiple unit cells.Figure 1(b) shows the surface morphology of Bi2Te3 surfaceafter 20 cycles of ALD growth using TMA and H2O, corre-sponding to 1.8 nm Al2O3 deposition. In the graphenecase, the surface of graphene remained intact while Al2O3nanoribbons are clearly formed at the graphene edges.17 NoAl2O3 nanoribbons or clusters can be observed at the Bi2Te3layer edges. Figure 1(c) shows that ALD Al2O3 is conformaland uniformly coated on a series of steps of the peeledBi2Te3 surface with the relative step height remaining simi-lar before ALD. Figure 1(d) shows the measured heights of 3 nm, 9 nm, and 6 nm corresponding to 1–3 unit cells.These observations show that the p-bond in pristine Bi2Te3is not chemically inert in atmosphere as graphene. This insta-bility makes direct ALD growth of dielectrics on such lay-ered structures possible; however, the surface reactionsdefinitely induce potential defects at the high-k/TI interfaceand hence deteriorate device performance as well.Electrical characterization was performed after devicefabrication using a Keithley 4200. Two devices, one usingTMA/H2O (Device 1) and another using TMA/O3 (Device 2)as precursors for Al2O3 are studied here. The typical geomet-ric features are 2 lm in gate length and 1 lm in gate width.The channel resistance, including the contact resistance, forDevices 1 and 2 are 32 kX and 25 kX, respectively, indicatingthat the flake thicknesses are similar. Linear I-V characteris-tics indicate a good ohmic contact to the Bi2Te3 flakes.Figures 2 shows independent top-gate and back-gate modula-tion for both devices with another gate floating. The top-gateand back-gate leakage currents are less than 1010 A. We donot observe an electron-hole or ambipolar transition becausethe strong stoichiometric doping of Bi2Te3 makes it an n-typematerial with an estimated doping concentration of 1019/cm3. For Device 1, a maximum of 45.6% modulation isreached through back-gate control, where the back-gate volt-age is swept from 50 V to 50 V. We calculate the transcon-ductance (gm) to be 9.8 nS. Comparatively, the top gate gm ismeasured 60 nS at its maximum, six times larger than theback-gate gm due to the thinner top dielectric thickness andhigher k value. Though we get an improved value for trans-conductance by top-gate control, the improvement is not ashigh as we expect considering the oxide thickness and dielec-tric constant for both SiO2 as back-gate oxide and Al2O3 fortop-gate oxide. The top-gate gm should be around 60 timeslarger than back-gate gm, if we don’t consider top-gate partialcoverage of the channel and roughly estimate the practicaldielectric constant of Al2O3 to be 7.8, twice as that of SiO2.The stark contrast between predicted and measured valuesindicates non-ideal Al2O3/Bi2Te3 interfaces, which is consist-ent with the discussions above. Also, from the extrinsic trans-conductances, we can estimate the low limit of the effectiveelectron mobility with top-gate control to be 1.69 cm2/Vsand with back-gate control to be 17.0 cm2/Vs. This effectivemobility contains two parts, the surface state mobility and thebulk mobility. The results indicate that the low mobility bulkchannel is the dominant conducting channel and the high mo-bility surface channels are degraded by the poor interfaces, inFIG. 1. (Color online) (a) Schematic device structureof a Bi2Te3 based TI FET. (b) AFM image of Bi2Te3surface with multiple quintuple layer terraces coatedwith an ultrathin ALD Al2O3 layer. (c) Three-dimen-sional profile of the same surface showing differentthicknesses of the broken layers with 1-3 unit cells. (d)AFM height measurement of the smooth terraces alongthe white line illustrated in Figure 1(b).FIG. 2. (Color online) (a) Drain current vs top-gate bias of two Bi2Te3 TIFETs with a 10 nm Al2O3 as top-gate dielectric; (b) drain current vs backgate bias of the same devices with a 300 nm SiO2 as back-gate dielectric.Drain-source voltage is 0.1 V in both measurements. The observed minordrain current change at the same bias condition is ascribed to the change ofcontact resistance and interface traps after gate bias stress.052108-2 H. Liu and P. D. Ye Appl. Phys. Lett. 99, 052108 (2011)Downloaded 23 Jul 2013 to 128.46.221.64. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsparticular, the top interface. Poor interface conditions easilyresult in a strong degradation of field-effect modulation effi-ciency as demonstrated in ALD high-k/III-V MOSFETs.19Compared to III-V MOSFETs, such interface degradation inBi2Te3 could impose more serious problems in device per-formance due to its unique carrier transport properties. It hasbeen stated in previous studies that the conductance of thosetopological insulators is composed of two parts: the bulk con-ductance and the surface conductance, including the top sur-face conductance and bottom surface conductance.10 The twosurface conducting channels are of interests due to its highmobility and non-scattering carrier transport. Considering thelarge intrinsic carrier density in its bulk, the top surface ismore sensitive to the top gate control than the bottom surface,so the two surfaces do not play symmetric roles under gatebias. The bottom surface has a better interface condition withthe back SiO2 dielectric as it has been left intact during thefabrication process, resulting in better back-gate control at thesame electrical field. However, the top surface condition ofthe flakes had been changed to some extent due to Al2O3 dep-osition, reacting with H2O at 200C.20 This reaction mightnot be severe enough to completely damage the material, asthere was only a tiny trace of water vapor as one precursor inone of the alternating ALD pulses. In addition, this water cor-rosion could only take place in the first several cycles ofAl2O3 deposition and would naturally cease when the flake iscovered with Al2O3. But then again, the water corrosion couldconsiderably impact on the interface quality, resulting in a sig-nificant decrease in surface modulation under field-effect.In the same Figures 2(a) and 2(b), the counterparts oftop-gate and back-gate control of Device 1 are also shown.Compared to Device 1, Device 2 shows similar back-gatecontrol, with a maximum modulation of 55.8% for a gatesweep of 50 V to 50 V. However, the two devices differ intop-gate control. Device 1 has a maximum modulation of16.7% with a smoother curve, whereas Device 2 has a modu-lation of only 5.9% with a noisier curve, with the top-gatevoltage ranging from 5 V to 5 V. The weaker top gatemodulation in Device 2 further supports our conclusion thatALD precursors are chemically absorbed to the top surfaceof Bi2Te3. In Device 2, the use of ozone as an oxidant ALDprecursor leads to greater top surface damage during the firstseveral cycles of ALD growth because ozone is a strongeroxidant than H2O. As a consequence, the top interface of De-vice 2 is more defective and the top-gate shows weaker chan-nel modulation and much noisy curves.Finally, the simultaneous dual gate modulations of Bi2Te3are shown in Figure 3. The top gate voltage is swept from 5V to 5 V while the back gate ranges from 50 V to 50 V witha 20 V step. We achieve the highest modulation rate of 76.1%for Device 1 and 61.8% for Device 2. All these indicate a sig-nificant enhancement in modulation for Bi2Te3 thin flakes atroom temperature, using Al2O3 high-k as a top-gate dielectricand dual gate control. Development of a perfect high-k/TIinterface is a must to realize real device applications based onTI FETs. In particular, the truly attractive property of TI as achannel material for device applications is its surface channelwith high carrier mobility and velocity. Any formation of top-gate dielectric on semiconductors cannot be as important as onTI since the conducting channel is on the surface.In conclusion, we have investigated ALD high-k oxideformation on Bi2Te3 as a top-gate dielectric. AFM studiesreveal the feasibility of direct ALD of high-k dielectrics onthis layered material. Electrical characterization shows a pro-nounced modulation by both top-gate and back-gate with thehighest modulation of 76.1% achieved with simultaneous dualgate control. However, at this point, the top-gate modulationis not as effective as the back-gate modulation at the sameelectrical field due to the degraded interface between the ALDdielectric and the TI. Further studies on protecting the TI sur-face during ALD dielectric formation are on-going.21–23The authors would like to thank X. Xu, C. Liu, G. Q. Xu,Y.P. Chen, A.T. Neal, and N. Conrad for valuable discussionsand E. Milligan, W.J. Qian, J.F. Tian, and M. Xu for technicalassistance. 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Atomic-layer-deposited Al2O3 on Bi2Te3 for topological insulator field-effect transistors

Han Liu

Peide D. Ye

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Applied Physics Letters

Atomic-layer-deposited Al2O3on Bi2Te3for topological insulator field-effect transistors

http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1973&amp;context=nanopub

Atomic-Layer-Deposited Al2O3 on Bi2Te3 for Topological Insulator Field-Effect Transistors

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