Heterostructure WSe2âGa2O3 Junction Field-Effect Transistor for Low

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Heterostructure WSe–GaO junction field-effect transistor for low-dimensional high-power electronics Janghyuk Kim, Michael A. Mastro, Marko J. Tadjer, and Jihyun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07030 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Applied Materials & Interfaces

Heterostructure WSe2-Ga2O3 junction field-effect transistor for lowdimensional high-power electronics

Janghyuk Kim,a Michael A. Mastro,b Marko J. Tadjer b and Jihyun Kima,* a

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

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

Abstract Layered materials separated from each bulk crystal can be assembled to form a strain-free heterostructure by using the van der Waals interaction. We demonstrated a heterostructure n-channel depletion-mode β-Ga2O3 junction field-effect transistor (JFET) through van der Walls bonding with an exfoliated p-WSe2 flake. Typical diode characteristics with a high rectifying ratio of ~105 were observed in a p-WSe2/nGa2O3 heterostructure diode, where WSe2 and β-Ga2O3 were obtained by mechanically exfoliating each crystal. Layered JFETs exhibited an excellent IDS-VDS output as well as IDS-VGS transfer characteristics with a high on-off ratio (~108) and low subthreshold swing (133 mV/dec). Saturated output currents were observed with a threshold voltage of −5.1 V and a three-terminal breakdown voltage of 144 V. Electrical performances of the fabricated heterostructure JFET were maintained at elevated temperatures with outstanding air stability. Our WSe2-Ga2O3 heterostructure JFET paves a route to the low-dimensional high-power devices, enabling a miniaturization of the integrated power electronic systems

Keywords: wide bandgap semiconductor; heterostructure; two-dimensional material; field-effect transistor; gallium oxide

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Introduction β-type gallium oxide (β-Ga2O3), which has an energy bandgap of approximately 4.8 eV (direct) at room temperature,1-3 is one of the promising wide bandgap semiconductor materials for next-generation high-power electronics. Owing to its ultra-high (theoretical) critical field strength of approximately 8 MV/cm, Baliga’s figure of merit (FOM), which is the metric of the conduction loss in power FETs, of βGa2O3 (at 3214) outperforms other wide bandgap semiconductors including SiC (at 317) and GaN (at 846), indicating that β-Ga2O3 offers a higher power switching capability with higher efficiency than the competitors do.3-8 Single-crystalline β-Ga2O3 wafers up to 4 inch in diameter have been produced using a melt-growth technique, which uses edge-defined film-fed growth.1 Epitaxially grown β-Ga2O3 thin films have been successfully fabricated into various metal-oxide semiconductor field-effect transistors (MOSFETs) for their use in ultra-high power electronics. Ahn et al. demonstrated the front- and back-gate modulations of a depletion-mode β-Ga2O3 MOSFET using SiO2 and Al2O3 as the gate oxides for the back and front sides, respectively.9 Zhou et al. reported that β-Ga2O3 MOSFETs with high Sn doping resulted in a higher source-drain current density of 600 mA/mm.10 Wong et al. demonstrated enhancement-mode β-Ga2O3 MOSFETs with a low series resistance using Si-ion implantation doping.11 Although MOSFETs have an advantage of high operating efficiency due to their high impedance, the quality of oxide dielectrics has severely limited their performances and stability.12,13 Heteroepitaxial growth of the high-quality gate dielectric layer with high breakdown field, low surface state density, and high dielectric constant is very challenging in compound semiconductors. By contrast, junction field effect transistors (JFETs) do not require a dielectric layer, resulting in a simple device structure. As well, JFETs are less susceptible to damage from electrostatic discharge and high temperature.14,15 JFETs based on β-Ga2O3 still have not been demonstrated due to the difficulty of p-type doping with effective hole conduction.3,16 A β-Ga2O3 single crystal is intrinsically n-type because the oxygen vacancy acts a donor.3,17 Interestingly, a single crystalline β-Ga2O3 with a monoclinic structure can be 2 ACS Paragon Plus Environment

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cleaved into ultra-thin flakes along the (100) plane direction, even though it is not a layered two-dimensional material held together by the van der Waals force. Its strong in-plane force and weak out-of-plane force are attributed to the large anisotropy in the lattice constants of a single crystalline β-Ga2O3.18 Recently, the β-Ga2O3 nanostructures9,19 and novel 2D/3D6,20,21 and mixed-dimensional heterojunctions formed by integrating 2D/3D materials have been fabricated,22-24 presenting novel (opto)electronic device applications. Transition metal dichalcogenides (TMDs) which are 2D materials are strong candidate materials for next-generation electronic and photonic devices due to their moderate bandgap and strong light-matter coupling.2527

Especially, tungsten diselenide (WSe2) shows p-type conduction with excellent air

stability.28,29 Because of them, WSe2 has been used in various p-n heterojunctions such as WSe2/MoS230 and WSe2/MoSe2.31 Nevertheless, TMDs, e.g., as the current carrying channel in a transistor, cannot inherently handle high power densities. In this work, we demonstrated β-Ga2O3 heterojunction JFETs by integrating exfoliated β-Ga2O3 layer with the exfoliated WSe2 layer. Our WSe2-Ga2O3 heterojunction p-n diode shows good static rectification characteristics with a high on/off ratio, promising a high performance and high power multi-functional nano-devices based on β-Ga2O3 and TMDs.

Experimental details Figures 1(a–c) show the optical microscope images, which present the fabrication sequence of a WSe2-Ga2O3 heterojunction JFET. The β-Ga2O3 flakes were separated from a (-201) β-Ga2O3 single crystalline using a mechanical exfoliation process. The β-Ga2O3 single crystal, which is unintentionally n-type with an effective carrier density (Nd-Na) of approximately 3 x 1017 cm−3, was grown by the edge-defined film-fed method (Tamura Corp.). Then, the exfoliated β-Ga2O3 flakes were dry-transferred onto a SiO2 (300 nm)/p++−Si (500 μm) substrate, which has a back-gate Ti/Au (20 nm/80 nm) electrode. The source and drain electrodes were defined by Ti/Au (50 nm/100 nm) metallization with a source-to-drain electrode spacing of 16 µm using a conventional photolithography and lift-off process. The channel width was 2 μm. Rapid thermal annealing (RTA) in an argon ambient was 3 ACS Paragon Plus Environment


performed for 60 sec at 480 °C to form ohmic contact to the exfoliated β-Ga2O3. The multilayer WSe2 was mechanically exfoliated from a bulk substrate (an effective carrier density of ~1017 cm−3) grown by a chemical vapor deposition method (HQ graphene), followed by a dry transfer onto the pre-deposited β-Ga2O3 flake using a polydimethylsiloxane (PDMS) transfer technique. The contact metallization to WSe2 was obtained by depositing Pt/Au (20/80 nm) after patterning using electron beam lithography. A schematic of the final WSe2-Ga2O3 heterojunction JFET is shown in Figure 1(e). The thickness and surface morphology of β-Ga2O3 and WSe2 were characterized by using atomic force microscopy (AFM) (XE100, PSIA). Micro-Raman spectroscopy at room temperature was conducted under a back-scattering geometry with a 532-nm line of a single-mode diode-pumped solid-state laser (Omicron) to analyze the structural properties of the β-Ga2O3 and WSe2 layers. The atomic structure and crystal orientations were investigated using transmission electron microscopy (TEM) (G2 F30ST, Tecnai), where the specimen was prepared by using focused ion beam (FIB) equipment (Quanta2003D, FEI). The Pd layer was deposited above the β-Ga2O3 and WSe2 flakes to protect the surface of the specimen from Gaion bombardment during the FIB fabrication. The electrical and transport characteristics of the fabricated JFET and diode devices were obtained using an Agilent 4155C semiconductor parameter analyzer connected to a probe station.

Results and discussion The morphology and thickness of the transferred β-Ga2O3 and WSe2 were determined using AFM, as shown in Figures 2(a–c). The β-Ga2O3 and WSe2 nanosheets were approximately 320- and 55-nm thick, respectively. AFM and optical microscope images confirm that the transferred WSe2 layer directly covered the βGa2O3 flake, forming an out-of-plane van der Walls junction. The cross-sectional TEM image of WSe2 and β-Ga2O3 flakes on the SiO2/Si substrate (inset of Figure 2(c)) indicates no noticeable deformations or faults in the interface between β-Ga2O3 and WSe2 flakes, suggesting the formation of the van der Waals heterostructure. The quality of the β-Ga2O3 layer covered with a WSe2 nanosheet was evaluated using 4 ACS Paragon Plus Environment

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micro-Raman spectroscopy, as shown in Figure 2(d). Three phonon modes were observed in the range of 175 to 280 cm-1 from the overlapped areas of the WSe2Ga2O3 heterostructure. The peak near 200 cm-1 corresponds to the A3g vibration mode of β-Ga2O3,32 and the other two peaks of approximately 247 and 254 cm-1 correspond to the E12g and A1g vibration modes of WSe2,33 respectively. There were no significant changes in the position and the ratio of the phonon peaks of β-Ga2O3 and WSe2 as compared to each pristine β-Ga2O3 flake, which confirms that no significant disruption to either material occurred following the formation of the van der Waals (vdW) heterostructure. The IDS-VDS output characteristics of a bottom-gated β-Ga2O3 FET with thermally grown SiO2 dielectric, before and after the multilayer WSe2 was transferred onto the β-Ga2O3 flake, are shown in Figure S1. After the multilayer WSe2 was deposited on top of the β-Ga2O3 channel layer, not only IDS but also the knee voltage decreased to 1.6 μA/μm and 5 V, respectively. The n-type transfer characteristics (Figure 1(d)) under a bottom-gate operation of the FET at VDS = +20 V are compared before and after multilayer WSe2 was transferred. A threshold voltage (Vth) of −30 V, an IDS on/off ratio of 108, and a subthreshold swing (SS) of 374.5 mV/dec were extracted before multilayer WSe2 was transferred onto β-Ga2O3 channel layer. The positive shift of Vth from −30 V to -7 V and the decrease of IDS were observed after multilayer WSe2 was transferred onto the β-Ga2O3 channel layer, as shown in Figure 1(d), indicate the formation of a depletion region originating from the hetero p-n junction between the p-type WSe2 and n-type β-Ga2O3 although there was no electrical contact to WSe2. The current-voltage (I-V) characteristics of the WSe2-Ga2O3 heterojunction pn diode is shown in Figure 3(a) after the WSe2 is electrically contacted by Pt/Au electrode, resulting in a diode-like rectifying behavior with on/off ratio of approximately 105. The microscope image of the heterojunction p-n diode is shown in the inset of Figure 3(a). Another parameter indicating the quality of the pn junction is the ideality factor (n), which can be extracted using the following equation:34 I = I exp

-1

(1)

where I0 is the saturation current, q is the elementary charge, kB is Boltzmann’s 5 ACS Paragon Plus Environment


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constant, T is the absolute e temperatture, and V is the applied volttage. The ideality factor o of our device was 4.3, which iss much hig gher than the t typical value. This large idealityy factor va alue which originatess from the e interfacia al trap stattes is a common characcteristic of van v der Waals W heterrostructure e diodes be ecause thee transfer process p inevitab bly introdu uces the re esidue and d the adsorrption of th he environm mental mo olecules such as H2O, O2 and CO2 at a the interrface.24,35,36 Since the e free carriier concentrations of β-Ga a2O3 and WSe W 2 are comparab ble, the spa ace charge e region w would form in both materia als, formin ng the typ pical p-n jjunction. Figure F 3(b) shows tthe energy band diagram m of the WSe W 2-Ga2O3 heteroju nction, which presen nts a type- II band alignment at the iinterface between b the e WSe2/β- Ga2O3 hetterojunction n. Based oon the And derson’s rule (tyype-II inte erface), the conducction band offset (Δ ΔEC=χWSe2− −χGa2O3) and a the valence e band offsset (ΔEV=△ △EC+△Eg) are 0 and 3.6 eV, res spectively.337,38 Figure 4(a a) shows the t circuit configuration of the WSe2-Ga2 O3 heterojjunction JFET. T The β-Ga2O3 and WS Se2 were u utilized as the t channe el and gatee of the n-c channel JFET, respectivvely. The IDS-VDS output characterist c tics of tthe WSe2-Ga2O3 heterojjunction JF FET showe ed good sa aturation and sharp pinch-off p chharacteristics with a knee e voltage of o 5 V, as shown s in F Figure 4(b b). Its trans sfer characcteristics at a VDS = +10 V shows a threshold t voltage v of −5.1 V an nd SS of 133 mV/de c with high h on/off ratio of ~108, ass shown in n Figure 4 4(c). The room-temp r erature fieeld-effect electron e mobilityy (μch) wass extracted d via39

(2) where L and W are a the cha annel leng th and wid dth, respec ctively, gmaxx is the ma aximum onductance e, q is the elementarry charge, d is the ch hannel thicckness, an nd μch is transco the carrier conce entration of o the n-ch hannel. Th he estimated μch of the WSe2-Ga2O3 heterojjunction JF FET was approximate ely 4.3 cm2/V⋅s. The gate leakaage curren nt of the heterojjunction JFET main ntained a significan ntly low le evel of appproximate ely 10-8 mA/mm m at VGS < 0 V, but in ncreased e exponentially at VGS > 2 V becaause of the e diodelike recctifying behavior of the t WSe2--Ga2O3 hetterojunction. When nnegative VGS was applied d, the deplletion regio on of the g gate-chann nel junction n widenedd and the channel c became simultan neously na arrower, ressulting in an a increase in the chhannel res sistance 6 ACS Paragon Plus Environment

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and decrease in IDS. Transconductance exhibit the maximum value of 0.9 μS at a 10 V gate voltage. The excellent transistor characteristics of the WSe2-Ga2O3 heterojunction JFET are attributed to the formation of van der Waals p-n heterojuction and appropriate band alignment between the exfoliated β-Ga2O3 and exfoliated WSe2. The temperature-dependent transfer characteristics of the WSe2-Ga2O3 heterojunction JFET show that the IDS decreased with increasing operating temperature although the transistor characteristics were well maintained (Figure 5(a). Figure 5(b) displays IDS-VDS output characteristics of the WSe2-Ga2O3 heterojunction JFET as-fabricated (solid line) and after storage for a month (dotted line), where the air-stability of WSe2-Ga2O3 heterojunction JFET is observed. This long-term reliability is outstanding because some low-dimensional devices suffer from air-instability due to their active reaction with the ambient molecules. Three terminal off-state breakdown voltage, measured at a gate bias of −10 V, presents a high breakdown voltage of 144 V as shown in Figure 5(c), which is a critical metric for power amplifiers. The currents were drastically increased upon breakdown in our heterojunction JFET. Low three-terminal breakdown of the low-dimensional devices has limited their applications in nanoelectroncis because the voltage swing is proportional to power. Our results put forward heterostructure devices composed of β-Ga2O3 and WSe2 as a promising low-dimensional design for stable high-power nanodevices.

Conclusion We demonstrated a low-dimensional heterojunction p-n diode and JFET by integrating a mechanically exfoliated n-type β-Ga2O3 flake with p-type multilayer WSe2. The fabricated WSe2-Ga2O3 heterojunction p-n diode showed proper rectifying behavior with a high rectifying ratio of approximately 105. The fabricated JFET displayed excellent transistor characteristics with a high IDS on/off ratio of ~108, a low subthreshold swing of 133 mV/dec, and three-terminal breakdown voltage of +144 V. This result was enabled by the formation of a high-quality interface at the WSe2-Ga2O3 heterojunction, which presents an appropriate band alignment for this 7 ACS Paragon Plus Environment


JFET. The JFET device exhibited excellent long-term air-stability, which further motivates β-Ga2O3-based high-power nano-electronics.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Jihyun Kim).

Notes The authors declare no competing financial interest.

Supporting information. Output characteristics of the β-Ga2O3 flake bottom-gated FET before and after the multilayer WSe2 was transferred

Acknowledgements Research at the Naval Research Lab is supported by the Office of Naval Research. The research at Korea University was supported by National Research Foundation of Korea (2017M1A2A2087351) funded by the Ministry of Science and ICT and the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Korea (No. 20172010104830).

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Figure captions Figure 1 (a)–(c) Optical images of the fabrication sequence of the WSe2-Ga2O3 heterojunction JFET. Scale bars are 10 μm. (d) IDS-VGS transfer characteristics of the device at VDS = +20 V before (Figure 1(a)) and after (Figure 1(b)) the multilayer WSe2 was transferred. (e) Schematic of a WSe2-Ga2O3 heterojunction JFET on a SiO2/Si substrate. Figure 2 (a) AFM image of the fabricated WSe2-Ga2O3 heterojunction JFET. (b) Height profile of the exfoliated β-Ga2O3 flake. (c) Height profile of the exfoliated multilayer WSe2. (Inset: Cross-sectional TEM image of the stacked WSe2 and βGa2O3 heterostructure. Scale bar is 2 nm.) (d) Raman spectra of the multilayer WSe2 before and after it was transferred onto the β-Ga2O3 flake. Figure 3 (a) Typical rectifying curve for a WSe2-Ga2O3 heterojunction p-n diode (inset: circuit of the fabricated WSe2-Ga2O3 diode). (b) Energy-band diagram of a ptype multilayer WSe2/n-type β-Ga2O3 flake heterojunction with vdW gap. Figure 4 (a) Optical image and circuit of the fabricated WSe2-Ga2O3 heterojunction JFET. (b) IDS-VDS output characteristics of the WSe2-Ga2O3 heterojunction JFET. (c) Transfer and transconductance curves of the WSe2-Ga2O3 heterojunction JFET. Figure 5 (a) Temperature-dependent measurement of the transfer characteristics of the WSe2-Ga2O3 heterojunction JFET. (b) IDS-VDS output characteristics of the WSe2Ga2O3 heterojunction JFET as fabricated (solid line) and after a month (dotted line). (c) Three-terminal breakdown measurement of WSe2-Ga2O3 heterojunction JFET at VGS= -10 V.



Figure e1


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Figure e2



Figure e3


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Figure e4



Figure e5


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Heterostructure WSe2âGa2O3 Junction Field-Effect Transistor for Low

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