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Quasi-two-dimensional h-BN/#-GaO heterostructure metal–insulator–semiconductor field-effect transistor Janghyuk Kim, Michael A. Mastro, Marko J. Tadjer, and Jihyun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 1, 2017

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ACS Applied Materials & Interfaces

Quasi-two-dimensional h-BN/β-Ga2O3 heterostructure metal–insulator– semiconductor field-effect transistor Janghyuk Kim, † Michael A. Mastro, ‡ Marko J. Tadjer ‡ and Jihyun Kim*,† †

Department of Chemical and Biological Engineering, Korea University, Anam-dong, Sungbuk-gu, Seoul 136-713 Korea ‡

US Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC 20375 USA

ABSTRACT β-gallium oxide (β-Ga2O3) and hexagonal boron nitride (h-BN) heterostructure-based quasitwo-dimensional metal-insulator-semiconductor field-effect transistors (MISFETs) were demonstrated by integrating mechanical exfoliation of (quasi)-two-dimensional (2D) materials with dry transfer process, where nano-thin flakes of β-Ga2O3 and h-BN were utilized as the channel and gate dielectric, respectively, of the MISFET. The h-BN dielectric, which has extraordinary flat and clean surface, provides a minimal density of charged impurities on the interface between β-Ga2O3 and h-BN, resulting in superior device performances (maximum transconductance, on/off ratio, subthreshold swing, and threshold voltage), compared with the conventional back-gated configurations. Also, double-gating of the fabricated device was demonstrated by biasing both top and bottom gates, achieving the modulation of the threshold voltage. This heterostructured wide bandgap nanodevice shows a new route toward stable and high power nano-electronic devices.

KEYWORDS: wide bandgap semiconductor; heterostructure; two-dimensional material; 1

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field-effect transistor; gallium oxide

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INTRODUCTION β-Ga2O3, is a promising material for power electronics because of its wide bandgap of ~4.9 eV and high thermal and chemical stability.1-3 The expected Baliga’s figure of merit (measure of the specific on-resistance in the drift region) of β-Ga2O3 material (at 3214) is far larger than those of other wide-bandgap semiconductors such as 4H-SiC (at 317) and GaN (at 846), owing to the ultrahigh theoretical critical field strength of β-Ga2O3: as high as 8 MV/cm.4-7 Also, the Johnson’s figure of merit (measure of power-frequency capability of the material) is 2844.4, which is much higher than 4H-SiC (277.8) and GaN (1089.0). Thus, βGa2O3 has a superior power-switching capability with a higher efficiency to SiC and GaN. Single-crystalline β-Ga2O3 bulk substrates are already commercially available, which is a distinct advantage for β-Ga2O3 over GaN.1 Also, direct bandgap of β-Ga2O3 can open many possibilities for optoelectroic applications including solar-blind photodetectors compared with indirect bandgap of SiC.8 Owing to their monoclinic structure with large discrepancies in lattice constant among a-, b- and c-axis, three-dimensional β-Ga2O3 crystals can be cleaved into ultrathin flakes in the (100) direction, even though β-Ga2O3 is not a two-dimensional (2D) van der Waals force material.9, 10 This is similar to the conventional van der Waals materials, such as graphene11 and transition metal dichalcogenides (TMDs)12, 13, which can be easily exfoliated due to week force between the layers. Recently, 2D β-Ga2O3 has been investigated as a nanoscale building block for future high-power (opto)electronic devices9, 14. Hexagonal boron nitride (h-BN), which is widely studied 2D materials as an alternative dielectric material, has an atomically flat surface without unnecessary dangling bonds.15, 16 The clean surface of h-BN with a low density of charged impurities can offer the better device performance than conventional insulating materials including SiO2, SiNX and Al2O3 gate dieletrics.17 Moreover, the thermal 3



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conductivity of h-BN (50 Wm-1K-1)18 is higher than those of other dielectric materials such as SiO2 (1.4 Wm-1K-1)19 and Al2O3 (35 Wm-1K-1)20, which improve the thermal profile of the fabricated nano-layer power electronic devices. β-Ga2O3 has been fabricated into metal-oxide semiconductor field-effect transistors (MOSFETs)10, 21, metal-semiconductor field effect transistors (MESFETs)22, and Schottky barrier diodes23 for the potential implementation in high power electronics, including heavyduty electric motor drives for automotive applications and microscale on-chip powermanagement controllers. Recently, Ahn et al. demonstrated the front and back gate operations of a depletion-mode β-Ga2O3 FET using SiO2 and Al2O3 as the gate oxides for the back and front sides, respectively.24 Zhou et al. reported β-Ga2O3 ultrathin flakes based FET with high Sn doping, resulting in higher source-drain current density of 600 mA/mm.25 In this study, we fabricated a quasi-2D heterostructure-based MISFET by integrating mechanically exfoliated β-Ga2O3 nanobelts with the exfoliated h-BN flakes. Nano-thin flakes of β-Ga2O3 and h-BN were exfoliated from bulk crystals via the typical adhesive-tape method and used as the channel and gate dielectric, respectively, of the MISFET. The electrical properties of the topgated MISFET fabricated with the h-BN dielectric were analyzed and compared with those of a bottom-gated FET with a SiO2 dielectric. Also, the electrical performance of a β-Ga2O3 FET with a single gate was compared with the electrical performance of a β-Ga2O3 FET with double gate.

EXPERIMENTAL SECTION Device Fabrication and Characterization. A bulk β-Ga2O3 crystal (-201) grown by the edge-defined film-fed method that was unintentionally n-doped and had an effective carrier 4


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density (Ndonor-Nacceptor) of ~3 × 1017 cm−3 was employed in this study (Tamura Corp.). Nanoscale-thickness β-Ga2O3 flakes were cleaved from the bulk β-Ga2O3 crystal by the mechanical exfoliation method and subsequently transferred onto a 1×1 cm2 SiO2 (300 nm)/p++ Si (500 μm) substrate by using commercial adhesive tape, followed by the deposition of a full back-gate electrode (Ti (20 nm)/Au (80 nm)) onto the opposite face of the substrate using an electron-beam evaporator. The source and drain electrodes were defined by Ti/Au (20 nm/80 nm) metallization, with a source-to-drain electrode spacing of 42 µm, via a conventional photolithography and a lift-off processes. h-BN flakes were mechanically exfoliated from a freestanding h-BN sample (HQ graphene company) and transferred onto the β-Ga2O3 flakes as a gate dielectric via a polydimethylsiloxane (PDMS) transfer method. The top-gate electrode metal, Ni/Au (20 nm/ 80nm) was deposited using an electron-beam evaporator following patterning via a conventional photolithography and lift-off process. Characterization. The surface morphology of the β-Ga2O3 nanobelts was examined via atomic force microscopy (AFM) (XE100, PSIA). To analyze the structural properties of the β-Ga2O3 and h-BN flakes, micro-Raman spectroscopy was performed at room temperature with a backscattering geometry using the 532-nm line of a diode-pumped solid-state laser (Omicron). The atomic structure and crystal orientations were investigated via transmission electron microscopy (TEM, G2 F30ST, Tecnai) after the specimen was prepared using focused ion beam (FIB) technique (Quanta2003D, FEI). To protect the surface of the samples from Ga-ion bombardment during the FIB fabrication, a Pt layer was deposited above the βGa2O3 and h-BN flakes. The current–voltage (I–V) and transport characteristics of the FET were obtained using an Agilent 4155C semiconductor parameter analyzer that was connected to the probe station.

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RESULTS AND DISCUSSION Schematic of the β-Ga2O3 MISFET with h-BN as the top-gate dielectric and SiO2 as the bottom-gate dielectric is shown in Fig. 1(a), which corresponds to an optical image in Fig. 1(b). The morphology and thickness of the transferred β-Ga2O3 and h-BN flakes were determined using AFM, as shown Fig. 1(c). The thickness of the β-Ga2O3 and h-BN flakes were approximately 400 and 60 nm, respectively. The quality of the β-Ga2O3 nanoflakes covered with h-BN was evaluated using micro-Raman spectroscopy, as shown in Figs. 2(a) and (b). The phonon peaks corresponding to β-Ga2O3 are observable in the range of 60 to 800 cm-1.26 The stability of these peaks indicates that the high crystal quality of the β-Ga2O3 was not perturbed through the fabrication process. The green circle symbol around 1,350 cm-1 in Fig. 2(b) is the Raman features of h-BN.27 When compared with the pristine β-Ga2O3 flakes, it confirms there was no significant disruption to either material following the formation of the van der Waals heterostructure. The heterostructure between the exfoliated β-Ga2O3 flake and the exfoliated h-BN layer was investigated by the cross-sectional TEM images of the hBN and β-Ga2O3 flakes on a SiO2/Si substrate, as shown Fig. 3(a). No deformation or fault in each layer and the interface between the β-Ga2O3 and h-BN flakes was observed, implying the formation of van der Walls heterostructure. The selected area electron diffraction (SAED) pattern of this sample is shown in inset of Figure 3(b), indicating the lattice symmetry and the lattice parameters of the exfoliated β-Ga2O3 thin flakes. The exfoliated β-Ga2O3 flakes appeared high quality and strain-free. The d-spacing of ~0.61 nm and the diffraction pattern of the β-Ga2O3 thin flakes, which correspond to the (200) lattice plane, indicates that the βGa2O3 ultrathin flakes were exfoliated along the [100] a-direction. Monoclinic β-Ga2O3 crystal can easily be cleaved into ultrathin flakes in the (100) plane direction because it has a lattice constant that is larger (~1.22 nm) than other directions of (010) ~ 0.303 nm and (001) 6


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~ 0.580 nm, which is consistent with previous reports.28, 29 Figures 4(a) and 4(c) show the source-drain (IDS-VGS) current–voltage characteristics of the h-BN-covered β-Ga2O3-flake FET with bottom and top gates operational, respectively, at room temperature under an air ambient condition. The h-BN and SiO2 were utilized as the top- and bottom-gate dielectrics, respectively, of the β-Ga2O3-flake MISFET. For the bottomand top-gate operation, the IDS decreased by lowering the gate-source bias (VGS), indicating that the channel layer (β-Ga2O3) was of n-type conductivity. The source-drain current density was ~0.6 μA/μm for both the bottom- and top-gate operation, which is explained by the low carrier concentrations of the β-Ga2O3 crystal and the non-optimized Ohmic contacts between the β-Ga2O3 and the metal electrodes, as indicated by the relatively high knee voltage.25 Also, the output curves with superlinear behaviors indicates the high ohmic contact resistance. High specific contact resistance of 0.0145 Ω·cm2 was obtained by transfer length measurement (Figure S1). Current density can be controlled by using the highly doped βGa2O3 flake as a channel layer. Figures 4(b) and 4(d) compare the transfer characteristics of the h-BN-covered β-Ga2O3-flake MISFET under bottom-gated (SiO2) or top-gate (h-BN) operation. A threshold voltage (Vth) of −48 V and IDS on/off ratio of 106 and subthreshold swing (SS) of 502 mV/dec were extracted for bottom-gate operation. It has been reported that the transfer characteristics of the bottom gate MISFET using SiO2/doped Si substrate suffer from a large negative threshold voltage (Vth) shift, which can be attributed to the existence of positive fixed charges in the SiO2/Si interface.30 The room-temperature field-effect mobility (μFE) was calculated as follows: μFE = (Lg×gm)/(LW×COX×VDS), where Lg and LW are the channel length and width, respectively, and COX is the capacitance of the dielectric material. The μFE of the back-gated β-Ga2O3-flake FET was ~4.87 cm2/V·s. Low carrier mobility can be attributed to the dangling bonds at its surface, which can act as carrier scattering centers. 7



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The randomly distributed Coulomb/charged impurities and adsorbed gas molecules introduced during the device fabrication can lower the carrier molecules.31- 33 In contrast, a Vth and gmax of -24 V and ~0.05 μS/μm, respectively, were extracted for top-gate operation with high IDS on/off ratio of 107 and low SS of 348 mV/dec, showing the interface is high quality as shown in Figure 4(d). These inequalities in the electrical characteristics between the bottom- and top-gated β-Ga2O3-flake FETs are attributed to the difference in the surface states between h-BN and SiO2 because the dielectric constant of h-BN (~3.9) is close to that of SiO2.34 We believe that the interface state between the channel and the dielectric significantly affected the performance of the FETs. The h-BN had a flat and clean surface with a low density of charged impurities compared with SiO2, which in turn contributed to its high device performance.15, 17, 31 The μFE of the β-Ga2O3-flake FET was approximately 1.77 cm2/V·s for top-gate operation, which is lower than that of bottom-gate one because of the fabrication damages and chemical residue during additional h-BN transfer and photolithography processes. The transfer characteristics of the β-Ga2O3-flake FET with backand top-side gate are summarized in Table 1. Figure 5(a) shows the IDS-VGS of the β-Ga2O3-flake MISFET with operating both bottom- and top-side gates. The drain-source current was more effectively modulated under the operation of both gates than under solely bottom- or top-gate operation. The double-gate devices with independent gate control are capable of connecting the bottom- and top- gates together or to control them separately while designing a circuit resulting in new circuit technology. For example, the utilization of double-gate structures helps to control and enhance of carrier mobility for 2D TMDS such as MoS235, 36 or WSe237 based FETs. Figure 5(b) shows the transfer curves of a double-gated β-Ga2O3-flake FET with a sweeping top-gate voltage and a fixed bottom-gate voltage. In the independent gate operation, top-gate is used 8


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for switching while bottom gate is used for threshold voltage (Vth) control. The double-gated β-Ga2O3-flake FET shows a high IDS on/off ratio of 107 and a small SS of 175 mV/dec due to the reduced surface scatterings38, 39, which means these devices can work at a small gate bias. The linear shift of the top-gate threshold voltage with respect to the back-gate voltage is shown in Figure 5(c), where we effectively controlled the threshold voltage of the device. As carriers are not confined to either interface in double gate operation, the influences of scattering related to surface trapped charges and surface roughness can be reduced, which can benefit field-effect mobility and subthreshold swing. Figure 5(d) shows the gate leakage current with bottom- and top-gate operation. The gate leakage current with top-gate operation was far lower than that with bottom-gate operation owing to the excellent surface properties, as previously mentioned. The lower gate leakage current for the top-gate operation yielded a large IDS on/off ratio of ~107. Overall, the results indicate that the heterostructure of the β-Ga2O3 and h-BN ultrathin-flake device is very promising for the fabrication of high-power nanodevices, as well as suggest that wide bandgap 2D materials offer a general route for achieving high-power nanodevices.40, 41

CONCLUSIONS We fabricated a quasi-2D h-BN/β-Ga2O3 heterostructure MISFET on a SiO2/Si substrate using h-BN as a top gate dielectric and SiO2 as a bottom gate dielectric material. The heterostructure of the h-BN and β-Ga2O3 flake-based top-gated MISFET exhibited excellent transistor characteristics with higher Vth due to the superior interface between the exfoliated β-Ga2O3 and the exfoliated h-BN, which showed a low gate leakage current and a large on/off ratio of ~107—compared with a bottom-gated FET with a SiO2 dielectric. The 9



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application of a dual-side gate enabled us to tune the subthreshold swing and threshold voltage of the fabricated MISFET. This synergetic fusion of 2D h-BN and quasi-2D β-Ga2O3 introduces β-Ga2O3 as a nanoscale building block for future high-power devices.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J.K.). Notes The authors declare no competing financial interest.

Supporting information. The measured total resistance as a function of the electrode spacing

Acknowledgements The research at Korea University was supported by National Research Foundation of Korea (2016M3D1A1952967) funded by the Ministry of Science, ICT and Future Planning of Korea and the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Korea (No. 20163010012140).

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(27) Reich, S.; Ferrari, A. C.; Arenal, R.; Loiseau, A.; Bello, I.; Robertson, J. Resonant Raman Scattering in Cubic and Hexagonal Boron Nitride. Phys. Rev. B 2005, 71, 205201. (28) Bermudez, V. M. The Structure of Low-Index Surfaces of β-Ga2O3. Chem. Phys. 2006, 323, 193-203. (29) Ortiz, A.; Alonso, J. C.; Andrade, E.; Urbiola, C. Structural and Optical Characteristics of Gallium Oxide Thin Films Deposited by Ultrasonic Spray Pyrolysis. J. Electrochem. Soc. 2001, 148, F26-F29. (30) Liu, H.; Ye, P. D. D. MoS2 Dual-Gate MOSFET with Atomic-Layer-Deposited Al2O3 as Top-Gate Dielectric. IEEE Electron Device Lett. 2012, 33, 546-548. (31) Cheng, R.; Jiang, S.; Chen, Y.; Liu, Y.; Weiss, N.; Cheng, H. C.; Wu, H.; Huang, Y.; Duan, X. Few-layer Molybdenum Disulfide Transistors and Circuits for High-speed Flexible Electronics. Nat. Commun. 2014, 5, 5143 (32) Li, S.; Tsukagoshi, K.; Orgiu, E.; Samori, P. Charge Transport and Mobility Engineering in Two-dimensional Transition Metal Chalcogenide Semiconductors, Chem. Soc. Rev. 2016, 45, 118-145 (33) Li, S.; Tsukagoshi, K.; Xu, Y.; Nakaharai, S.; Komatsu, K.; Li, W.; Lin, T.; AparecidoFerreira, A.; Tsukagoshi, K. Thickness-Dependent Interfacial Coulomb Scattering in Atomically Thin Field-Effect Transistors, Nano Lett. 2013, 13, 3546-3552 (34) Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722-726. (35) Lee, G. H.; Cui, X.; Kim, Y. D.; Arefe, G.; Zhang, X.; Lee, C. H.; Ye, F.; Watanabe, K.; Taniguchi, T.; Kim, P.; Hone, J. Highly Stable, Dual-Gated MoS2 Transistors Encapsulated by Hexagonal Boron Nitride with Gate-Controllable Contact, Resistance, and Threshold Voltage. Acs Nano 2015, 9, 7019-7026. 14


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(36) Min, S. W.; Lee, H. S.; Choi, H. J.; Park, M. K.; Nam, T.; Kim, H.; Ryu, S.; Im, S. Nanosheet Thickness-Modulated MoS2 Dielectric Property Evidenced by Field-Effect Transistor Performance. Nanoscale 2013, 5, 548-551. (37) Groenendijk, D. J.; Buscema, M.; Steele, G. A.; Michaelis de Vasconcellos, S.; Bratschitsch, R.; van der Zant, H. S.; Castellanos-Gomez, A. Photovoltaic and Photothermoelectric Effect in a Double-Gated WSe2 Device. Nano Lett. 2014, 14, 5846-5852. (38) Francis, P.; Terao, A.; Flandre D.; Van de Wiele, F. Modeling of Ultrathin Double-Gate nMOS/SOI Transistors. IEEE Trans. Electron Devices 1994, 41, 715-720. (39) Hamid, H. A. E.; Guitart. J. R.; Iniguez, B. Two-Dimensional Analytical Threshold Voltage and Subthreshold Swing Models of Undoped Symmetric Double-Gate MOSFETs. IEEE Trans. Electron Devices 2007, 54, 1402-1408. (40) Balestra, F.; Cristoloveanu, S.; Benachir, M.; Brini J.; Elewa, T. Double-Gate Silicon-onInsulator Transistor with Volume Inversion a New Device with Greatly Enhanced Performance. IEEE Electron Device Lett. 1987, 8, 410-412. (41) Suzuki, K.; Tanaka, T.; Tosaka, Y.; Horie, H.; Arimoto, Y. Scaling Theory for DoubleGate SOI MOSFET’s. IEEE Trans. Electron Devices 1993, 40, 2326-2329.

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Figure 1. (a) Scheematic of a β-Ga2O3-fllake FET covered with h an h-BN flake on a SiO2/Si Ga2O3-flake FET with an h-BN fllake. (c) substratte. (b) Optiical image of the fabrricated β-G AFM im mage of the surface of β-Ga2O3 coovered with h-BN.

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Figure 2. Raman spectra of (a) a mechhanically ex xfoliated β-G Ga2O3 flakee and (b) an a h-BN flake affter being trransferred onto o β-Ga2O 3. The inseets show op ptical imagees taken beffore and after thee h-BN trannsfer.

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Figure 33. (a) Crosss-sectional TEM T imagee of the stack ked h-BN and a β-Ga2O3 heterostru ucture on SiO2/Sii. (b) Crooss-sectionaal HR-TEM M image of the stacked h-B BN and β-Ga β 2O3 heterosttructure. Insset shows th he SAED paatterns of β--Ga2O3 flak ke.

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Figure 4. IDS–VDS output chaaracteristics of the β-G Ga2O3-flake FET with ((a) bottom- and (c) VGS transfer characterisstics of the same devicce at VDS = +20 V top-gatee operation. IDS–gm–V with (b)) bottom- annd (d) top-g gate operatioon.

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Figure 55. (a) IDS–V VDS output characterist c tics of the β-Ga β 2O3-flak ke FET with th both bottom- and top-gatee operationn. (b) Transsfer curves of the top--gated β-Gaa2O3-flake FFET with different d bottom--gate voltagges. (c) Plo ots of the ttop-gate thrreshold volttage (Vth) w with respecct to the bottom--gate voltagge (Vbg). (d d) Gate leakkage curren nts under to op-gate opeeration and bottomgate opeeration.

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bottom gate

top gate

Threshold voltage (Vth) [V]

-48

-24

Maximum transconductance (gm) [μS/μm]

0.03

0.05

Subthreshold swing (SS) [mV/dec]

502

348

IDS on/off ratio

106

107

Field effect mobility (μEF) [cm2/V·S]

4.87

1.77

Table 1. Summary of transfer characteristics of the β-Ga2O3-flake FET with bottom (back)and top- gate operations.

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Table O Of Contents (TOC) grraphic

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Î²-Ga2O3 ... - ACS Publications

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