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Cite this: RSC Adv., 2019, 9, 9678
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Field-plate engineering for high breakdown voltage
b-Ga2O3 nanolayer field-effect transistors†
Jinho Bae,a Hyoung Woo Kim,
b
In Ho Kangb and Jihyun Kim
*a
The narrow voltage swing of a nanoelectronic device limits its implementations in electronic circuits. Nanolayer
b-Ga2O3 has a superior breakdown field of approximately 8 MV cm1, making it an ideal candidate for a nextgeneration power device nanomaterial. In this study, a field modulating plate was introduced into a b-Ga2O3
nano-field-effect transistor (nanoFET) to engineer the distribution of electric fields, wherein the off-state
three-terminal breakdown voltage was reported to be 314 V. b-Ga2O3 flakes were separated from a singlecrystal bulk substrate using a mechanical exfoliation method. The layout of the field modulating plate was
Received 15th February 2019
Accepted 16th March 2019
optimized through a device simulation to effectively distribute the peak electric fields. The field-plated bGa2O3 nanoFETs exhibited n-type behaviors with a high output current saturation, exhibiting excellent
DOI: 10.1039/c9ra01163c
switching characteristics with a threshold voltage of 3.8 V, a subthreshold swing of 101.3 mV dec1, and
an on/off ratio greater than 107. The b-Ga2O3 nanoFETs with a high breakdown voltage of over 300 V
rsc.li/rsc-advances
could pave a way for downsizing power electronic devices, enabling the economization of power systems.
Introduction
b-Ga2O3 is an attractive material for high-efficiency power
devices owing to its ultra-wide energy bandgap (4.85 eV at room
temperature), large breakdown eld (approximately 8
MV cm1), and excellent chemical and thermal stability.1,2 Its
Baliga's gure of merit is estimated to be 3214.1 times greater
than that of Si, which surpasses that of conventional widebandgap materials such as GaN (846.0) and 4H-SiC (317.1),
suggesting its great potential as a near-future power device
material.3,4 The commercial availability of a large single-crystal
b-Ga2O3 substrate at its early R&D stage makes it more
competitive compared to III-nitride semiconductors, which
have suffered from the absence of a commercial freestanding
substrate. Many techniques of growing an epitaxial b-Ga2O3
layer have been previously reported, including metal organic
chemical vapor deposition and molecular beam epitaxy (MBE),
and pulsed laser deposition.5–9 Thin lm b-Ga2O3 power devices
such as metal-oxide semiconductor eld-effect transistors,
Schottky diodes, and metal-semiconductor eld-effect transistors (MESFETs) have demonstrated their potential as nearfuture, next-generation high-voltage electronic devices.10–15
Yang et al. fabricated edge-dened lm-fed grown (EFG) Sidoped b-Ga2O3 thin-lm vertical Schottky diodes with
a
Department of Chemical and Biological Engineering, Korea University,
Anamdong-5-Ga, Seoul 02841, South Korea. E-mail: hyunhyun7@korea.ac.kr
b
Korea Electrotechnology Research Institute (KERI), Seongsan-gu, Changwon-si,
Gyeongsangnam-do 51543, South Korea
† Electronic supplementary information (ESI) available: DC output characteristics
of the b-Ga2O3 nanoFETs with and without the eld-modulating plate and the
materials parameters for the device simulations. See DOI: 10.1039/c9ra01163c
9678 | RSC Adv., 2019, 9, 9678–9683
a reverse breakdown voltage of 2300 V.16 Wong et al. demonstrated MBE-grown Si-implanted thin-lm b-Ga2O3 MOSFET
with a breakdown voltage of 755 V.17
Although b-Ga2O3 is not a van der Waals material, it can be
easily separated into a single-crystalline nanolayer via
mechanical exfoliation owing to its large anisotropy in the
monoclinic lattice constants (a [100] ¼ 12.225 Å, b [010] ¼
3.039 Å, and c [001] ¼ 5.801 Å). Mechanically exfoliated bGa2O3 nanolayer akes feature a strain-free and at surface
with a high crystallinity maintained. Hwang et al. reported
a quasi-two-dimensional (quasi-2D) b-Ga2O3 layer obtained
through the mechanical exfoliation of a b-Ga2O3 crystal, grown
by the Czochralski method.18 Kim et al. demonstrated quasi2D b-Ga2O3 MOSFETs with a stable operation at a high
temperature of 250 C, where the Ga2O3 was grown by the EFG
method.19 Zhou et al. fabricated FET devices with an on/off
current ratio of 1010 using Sn-doped b-Ga2O3 akes.20 The
exfoliated quasi-2D b-Ga2O3 nanolayers can be easily integrated with other 2D materials such as graphene, hexagonal
boron nitride (h-BN), or transition metal dichalcogenides. The
formation of a heterostructure by combining diverse nanolayers is intrinsically strain-free, i.e., the high crystal quality of
each layer is maintained. The integration of n-Ga2O3 with a ptype 2D material, which is absent in b-Ga2O3, can enable
a versatile device conguration that facilitates p–n heterojunctions and bandgap engineering.21–23 Kim et al. fabricated
a 2D material-integrated b-Ga2O3 MOSFET by stacking h-BN
on top of an exfoliated b-Ga2O3 nanolayer and analyzed the
single- and dual-gate operations.24 A b-Ga2O3 quasi-2D MESFET with a stepped-gate structure was demonstrated by using
2D h-BN as a dielectric layer.25
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The premature electrical breakdown induced by the
concentrated electric elds limits the device operation under
a high bias voltage and threatens the device reliability. Various
techniques, such as a eld-modulating plate, a stepped gate,
and a recess gate structure, have been introduced to ease the
peak electric eld and enhance the breakdown voltage for highpower electronics.26–30 Among these, the eld-modulating plate
technique has been widely used owing to its efficacy and ease of
fabrication. Studies have been conducted to increase the offstate electrical breakdown voltage using a gate or source eldplate, or multiple eld-plates.31–33 However, such methods are
rarely studied in nanodevices despite their potential in power
nanoelectronics. The integration of a eld-modulating plate
with a b-Ga2O3 nanolayer can miniaturize the power circuit
system and simplify the layout. In this study, we optimized the
structure of a eld-modulating plate on a b-Ga2O3 nanoFET
through electric eld simulations. Based on them, we fabricated
high breakdown voltage quasi-2D b-Ga2O3 nanoFETs. The
structural and electrical properties of the fabricated b-Ga2O3
nanoFETs with a eld-modulating plate were systematically
investigated.
Experimental details
A single crystalline b-Ga2O3 substrate (Tamura Corp.) with an
effective carrier density of approximately 3.5 1017 cm3, grown
by the EFG method, was mechanically exfoliated into quasi-2D
nanolayers using a commercial adhesive tape. The exfoliated bGa2O3 nanolayer akes were transferred onto a thermally grown
SiO2 (300 nm)/Si (500 mm) substrate via a standard dry transfer
method. Both the source and drain electrodes were dened using
an electron beam lithography (EBL) technique, followed by Ti/Au
(50 nm/100 nm) metal deposition using an electron-beam evaporator. The optimal design of a eld-modulating plate, in order to
distribute the localized peak electric eld of a b-Ga2O3 nanoFET
device, was simulated using the device simulation soware SILVACO Atlas. Rapid thermal annealing (Mila-5050, Ulvac Technologies, Inc.) under a low-vacuum condition (<10 mTorr) was
performed at 500 C for 1 min to improve the ohmic contact. The
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top-gate electrode of the b-Ga2O3 nanoFETs was deposited with Ni/
Au (50 nm/100 nm), which was dened by the EBL and electron
beam evaporation procedures. A SiO2 dielectric layer with a thickness of 200 nm was deposited using plasma-enhanced chemical
vapor deposition (PECVD, VL-LA-PECVD, Unaxis), followed by the
patterning of the eld-modulating plate (Ti/Au (50 nm/100 nm)).
The overall device fabrication process is shown in Fig. 1.
The surface morphology and thickness of the fabricated
eld-plated FET devices were characterized using atomic force
microscopy (AFM; Innova, Bruker). Micro-Raman spectroscopy
was used to analyze the structural properties of the exfoliated bGa2O3 akes under a back-scattering geometry using a 532 nm
wavelength line of a diode-pumped solid-state laser (Omicron).
The cross-sectional device structure and crystal orientation of
the exfoliated b-Ga2O3 were investigated using scanning transmission electron microscopy (JEM-2100F, JEOL) aer the specimen was prepared using the focused ion beam (FIB) technique
(Quanta 3D FEG, FEI). The surface of the specimen was protected from FIB damage by a carbon layer. The electrical properties of the eld-plated b-Ga2O3 MESFETs were monitored
using an Agilent 4155C semiconductor parameter analyzer and
41501B single measurement unit expander connected to
a probe station. The three-terminal off-state breakdown voltages
of the fabricated b-Ga2O3 MESFETs were measured using
a Keithley 6485 picoammeter connected with a Keithley 248
high-voltage supply. The fabricated b-Ga2O3 nanoFETs were
immersed in a Fluorinert solution (FC-40, 3M) to prevent an
unintended dielectric breakdown during the measurement of
the three-terminal off-state breakdown voltage.
Results and discussion
Prior to the device fabrication, the optimal eld-modulating
structure that can effectively distribute the concentrated electric elds was investigated in order to prevent a premature
breakdown and maximize the off-state breakdown voltage of the
fabricated nanoFET devices. A schematic of the simulated
device structure and LFP are presented in Fig. 2a. The simulations of electric eld distribution were performed while varying
Fig. 1 Fabrication process of a top-gated b-Ga2O3 nanoFET with a field-modulating plate. (a) Patterning of the source and drain ohmic contacts
to the exfoliated b-Ga2O3 flake. (b) Deposition of the Ni/Au top gate electrode. (c) Deposition of the PECVD-SiO2 dielectric layer. (d) Patterning of
the field-modulating plate (Ti/Au).
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Fig. 2 (a) Schematic of the b-Ga2O3 nanoFET with the field-modulating plate. (b) Simulation results of the electric field distribution between the
gate and drain electrodes. (c) Maximum electric fields with varying field-plate lengths (LFP). Simulation results of the electric field distribution in bGa2O3 channel (d) without and (e) with the field-modulating plate of LFP y 0.8 mm. (f) Depth profile of the electric fields calculated along the
dotted line in the inset figure at varying LFP.
the length of the eld-modulating plate from the edge of the
gate electrode to the source-grounded eld-plate electrode (LFP).
Using the numerical device analysis, the electric eld distributions, which varied with the LFP under the conditions of VDS ¼
+400 V and VGS ¼ 50 V, are shown in Fig. 2b. The electric elds
at the middle of the gate and drain electrodes were less than 3
MV cm1, which is much lower than the breakdown eld of bGa2O3 (8 MV cm1). However, the electric elds were much
higher at the drain edge of the gate (x ¼ 0.5 mm) and the edge of
the drain electrode (x ¼ 2.5 mm). Generally, the peak electric
eld was observed at the drain-side edge of the gate electrode
due to the bias condition of the FET. Once defective sites are
created under the intense electric elds, they will grow and
damage the device. These high-intensity, highly localized electric elds can eventually destroy it.34,35 Fig. 2b indicates the
redistribution of the concentrated electric elds due to the
presence of the eld-modulating plate. In particular, at the
drain-side edge of the gate electrode (x ¼ 0.5 mm), the electric
eld was greatly alleviated by introducing the source-grounded
eld-plate structure, which is consistent with the previous
reports of AlGaAs/GaAs, AlGaN/GaN, and SiC devices. In AlGaAs/
GaAs HEMTs, the peak electric eld was lowered by employing
the eld-modulating plate.36 Fig. 2c shows the maximum electric eld values varying with LFP and proposes that the layout
optimized for the dispersion of the concentrated electric elds
is LFP y 0.8 mm, which decreases the peak electric eld from
11.4 MV cm1 to 6.6 MV cm1. The peak electric eld (6.6
MV cm1) redistributed by the source-grounded eldmodulating plate was lower than the intrinsic breakdown eld
(8 MV cm1) of b-Ga2O3, which can prevent a premature
9680 | RSC Adv., 2019, 9, 9678–9683
electrical breakdown during device operation and help to
improve the device reliability. Fig. 2d and e compare the electric
eld distribution in b-Ga2O3 channel layer without and with
a eld-modulating plate, respectively. Fig. 2f shows the depth
prole of the electric eld in the b-Ga2O3 channel that varies
with LFP. The electric eld at the hot gate edge which is located
in the drain-side gate edge was greatly mitigated by the introduction of eld-modulating plate, enhancing the off-state
breakdown voltage of the FET device.
Exfoliated b-Ga2O3 nanolayer MESFETs with a sourcegrounded eld-plate were fabricated based on the optimized
length of the eld-modulating plate electrode (LFP ¼ 0.8 mm).
The optical microscopic image (Fig. 3a) and AFM image (Fig. 3b)
conrm that the b-Ga2O3 nanoFETs were fabricated using the
same layout as that suggested by the above electric eld simulation. The b-Ga2O3 akes used in this study had a thickness
ranging from 200 to 350 nm with a root-mean-square roughness
of approximately 1.3 nm (Fig. 3b), which is consistent with the
result of the cross-sectional high-resolution TEM image
(Fig. 3c). PECVD SiO2 conformally covered the Ni/Au top gate
electrode. On top of the PECVD SiO2 layer, the Ti/Au eld-plate
electrodes were seamlessly dened with the optimized LFP (0.8
mm). A clear boundary was maintained between each layer
without interdiffusion aer the device fabrication process, as in
Fig. 3c, which indicates the robustness of the exfoliated b-Ga2O3
nanolayer. The Raman spectrum of the fabricated nanoFETs is
shown in Fig. 4a. No change in the Raman mode was observed
aer the device fabrication process, which also indicates the
chemical and mechanical stability of the b-Ga2O3 nanolayer.37
The high crystallinity of the exfoliated b-Ga2O3 ake is also
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Fig. 3 (a) Optical microscope image, (b) AFM image, and (c) cross-sectional high-resolution TEM image of the fabricated b-Ga2O3 nanoFET with
the field-modulating plate. Note that a carbon layer was deposited to protect the specimen from FIB damage.
Fig. 4 (a) Raman spectrum of the fabricated b-Ga2O3 nanoFET with the field-modulating plate. (b) Cross-sectional high-resolution TEM image
and (c) SAED pattern of the mechanically exfoliated b-Ga2O3 flake.
conrmed by the TEM image (Fig. 4b) and the selected area
electron diffraction (SAED) pattern (Fig. 4c). The d-spacing in
the SAED pattern was 0.609 nm, which matches the (200) lattice
plane. This indicates that the mechanically exfoliated b-Ga2O3
ake was separated along the (100) direction due to the large
anisotropy of the monoclinic b-Ga2O3 unit cell, even though bGa2O3 is not a van der Waals material.
Eight b-Ga2O3 nanoFETs, each with a source-grounded eldplate, were fabricated. The electrical properties of the representative device are shown in Fig. 5. For comparison, the bGa2O3 nanoFET without the eld plate was characterized, where
the current density of the eld-plated FETs was lower than that
of the non-eld-plated FETs because the voltage on the eld
plate competed with that on the gate electrode (Fig. S1†). They
exhibited excellent DC output characteristics at varying VGS
(Fig. 5a). The fabricated device showed n-type characteristics
and was completely pinched off at a VGS of approximately 5 V.
They showed a linear increase in the IDS under low-voltage
operation below the knee voltage, and output currents were
saturated above the knee voltage. Considering that conventional 2D material-based electronic devices suffer from the
absence of output current saturation, the saturated output
Fig. 5 (a) DC output and (b) transfer/transconductance characteristics of the representative b-Ga2O3 nanoFET with the field-modulating plate at
varying VGS.
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Fig. 6 Off-state three-terminal hard-breakdown results of the fabricated b-Ga2O3 nanoFET (a) without and (b) with the source-connected fieldmodulating plate. The insets show the schematics of each device.
current in b-Ga2O3 nanoFETs show a potential for a nanoelectronic power amplier. They also exhibited reproducible
electrical characteristics without a signicant change under the
repeated driving conditions of VDS ¼ +50 V. By contrast, 2Dmaterial based electronic devices using graphene, black phosphorus, and transition metal dichalcogenides cannot withstand
the high bias conditions used in our measurements of the
fabricated b-Ga2O3 devices. Fig. 5b shows the transfer and
transconductance characteristics of the fabricated b-Ga2O3
device. The eld-effect mobility (mFE) was estimated to be 3.1
cm2 V1 s1, which was calculated by the following equation:
mFE ¼
gmax L
qðNd Na ÞdW
where gmax is the maximum transconductance, L is the length,
W is the width, d is the thickness of the b-Ga2O3 channel,
respectively, q is the elementary charge, and (Nd Na) is the
effective carrier concentration of the b-Ga2O3 channel. The bGa2O3 nanoFETs showed a threshold voltage (Vth) of 3.8 V and
a subthreshold swing (SS) value of 101.3 mV dec1, where the
PECVD SiO2 served as both the surface passivation layer for the
exfoliated b-Ga2O3 and the dielectric layer for the eldmodulating plate. Considering that the previous SS of the bGa2O3-based device was 140 mV dec1, the lower SS combined
with a high on/off ratio (>107) can promise to minimize the
power switching loss.
The three-terminal off-state hard-breakdown voltages of the bGa2O3 nanoFETs with and without a eld-modulating plate are
compared in Fig. 6a and b. The three-terminal off-state breakdown voltages were measured under the pinched-off condition.
The devices under the test were immersed in Fluorinert solution
to prevent unintentional dielectric breakdown due to ambient
molecules, which is a standard test condition in power electronics. A high electric eld can initiate carrier multiplication
through impact ionization, where the accelerated carriers collide
with the lattice and release their kinetic energy. The cascade
creations of electron–hole pairs will result in high off-state
currents, which will catastrophically damage the device. The
impact ionization coefficients of b-Ga2O3 were estimated to be
9682 | RSC Adv., 2019, 9, 9678–9683
approximately 6.1 104 cm1 and 9.5 103 cm1 at the peak
electric elds of 11.4 MV cm1 (without eld-modulating plate)
and 6.6 MV cm1 (with eld-modulating plate), which are much
lower than those of 4H-SiC (1.7 106) and GaN (1.8 104) at
electric elds of 6.6 MV cm1.38 The b-Ga2O3 shows a lower impact
ionization coefficient under the same electric eld due to its high
bond strength of the binding energy of Ga–O (531 eV (O1s)),
much larger than that of Ga–N (397 eV (N1s)) and Si–C (283 eV
(C1s)). The breakdown eld is generally proportional to (energy
bandgap)2–2.5.2 This can reduce the off-state leakage currents and
ensure a high hard-breakdown voltage. The hard-breakdown
voltage of the eld-plated b-Ga2O3 nanoFET was 314 V (Fig. 5b),
while the hard-breakdown voltage of the b-Ga2O3 nanoFET
without the eld-modulating plate (Fig. 5a) was observed at VDS ¼
145 V. The two-fold increase of the hard-breakdown voltage is
attributed to the existence of the eld-modulating plate, which is
consistent with the simulation results. The high off-state breakdown voltage of 314 V is much higher than those of conventional
2D devices (MoS2, 120 V) and wide-bandgap GaN nanowire device
(140 V), opening a new route for next-generation high-power
nanoelectronics with wide voltage swing.
Conclusion
A b-Ga2O3 nanoFET with an off-state hard-breakdown voltage of
314 V was fabricated by introducing a source-grounded eldmodulating plate. The numerical device simulation was
employed to analyze the effects of the eld-modulating plate
and determine the optimal structure to effectively distribute the
electric elds concentrated on the hot gate edge. The b-Ga2O3
akes, which were mechanically exfoliated from a single-crystal
bulk substrate, was used as a n-channel layer with their crystallinity maintained. The fabricated nanoFET device showed
excellent device characteristics including low SS and high on/off
ratio with a high off-state hard-breakdown voltage (314 V).
Engineering of the peak electric elds in a nanodevice by using
a eld-modulating plate improved the device stability under
a high-voltage operation, paving the way for high-efficiency
integrated power nanoelectronic systems.
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Conflicts of interest
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There are no conicts to declare.
Acknowledgements
The research at Korea University was supported by the New &
Renewable Energy Core Technology Program of Korea Institute
of Energy Technology Evaluation and Planning (KETEP), which
was granted nancial resources from the Ministry of Trade,
Industry & Energy, Korea (No. 20172010104830) and the Technology Development Program to Solve Climate Changes of the
National Research Foundation (NRF) funded by the Ministry of
Science and ICT (NRF-2017M1A2A2087351).
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