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Semiconductor Science and Technology
Semicond. Sci. Technol. 31 (2016) 034001 (11pp)

doi:10.1088/0268-1242/31/3/034001

Recent progress in Ga2O3 power devices
Masataka Higashiwaki1, Kohei Sasaki1,2, Hisashi Murakami3,
Yoshinao Kumagai3, Akinori Koukitu3, Akito Kuramata2,
Takekazu Masui2 and Shigenobu Yamakoshi2
1

National Institute of Information and Communications Technology, Koganei, Tokyo 184–8795, Japan
Tamura Corporation, Sayama, Saitama 350–1328, Japan
3
Tokyo University of Agriculture and Technology, Koganei, Tokyo 184–8588, Japan2

E-mail: mhigashi@nict.go.jp
Received 29 May 2015, revised 30 October 2015
Accepted for publication 7 December 2015
Published 18 January 2016
Abstract

This is a review article on the current status and future prospects of the research and developmenton gallium oxide (Ga2O3) power devices. Ga2O3 possesses excellent material properties, inparticular for power device applications. It is also attractive from an industrial viewpoint sincelarge-size, high-quality wafers can be manufactured from a single-crystal bulk synthesized bymelt–growth methods. These two features have drawn much attention to Ga2O3 as a new widebandgap semiconductor following SiC and GaN. In this review, we describe the recent progressin the research and development on fundamental technologies of Ga2O3 devices, covering
single-crystal bulk and wafer production, homoepitaxial thin ﬁlm growth by molecular beam
epitaxy and halide vapor phase epitaxy, as well as device processing and characterization of
metal–semiconductor ﬁeld-effect transistors, metal–oxide–semiconductor ﬁeld-effect transistors
and Schottky barrier diodes.
Keywords: gallium oxide, Ga2O3, power devices, edge-deﬁned ﬁlm-fed growth, halide vapor
phase epitaxy, molecular beam epitaxy(Some ﬁgures may appear in colour only in the online journal)
voltage (Vbr ) and efﬁciency than their SiC and GaN
counterparts.
SiC wafers are produced from bulk ingots typicallygrown using a sublimation method at extremely high temperatures above 2000 ◦C. Various growth methods are being
used for GaN bulk and/or wafer production, such as the highnitrogen pressure method, the ammonothermal technique andhydride vapor phase epitaxy; however, it is an open question
which technique will dominate the market. As mentioned
above, SiC and GaN wafers are produced from bulks synthesized by growth methods other than melt growth, because
no atmospheric melt growth methods, which are standard forthe production of semiconductor single-crystal bulks to date,are applicable for both SiC and GaN. However, in the case of
Ga2O3, a single-crystal bulk can be synthesized using several
standard melt growth methods; therefore, this is a strong
advantage for Ga2O3 over SiC and GaN, in particular formass production.
This review article mainly summarizes the recent progress in research and development (R&D) activities on this

1. Introduction
SiC and GaN power devices have attracted much attention as
key components of the high-efﬁciency power conversion
required for energy saving in the near future. Their device
performance can far exceed that of the Si-based devices
mainly used in current power electronics. However, SiC and
GaN do not have to be the only candidates for next-generation
power devices in the near future, because the materials anddevices with the best performance and cost advantage should
be chosen for society. Against this background, we have been
proposing a new oxide compound semiconductor, gallium
oxide (Ga2O3), as another promising candidate because of its
excellent material properties and suitability for mass
production.
The bandgap (Eg ) and the expected Baliga’s ﬁgure of
merit (FOM) of Ga2O3 are much larger than those of SiC and
GaN. These outstanding material properties will in principle
enable Ga2O3 power devices with even higher breakdown
0268-1242/16/034001+11$33.00

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M Higashiwaki et al

the reported Eg values, between 4.5 and 4.9 eV [6–9]. The ntype doping technology is relatively easy and mostly established. Sn and Si atoms are known to be shallow donors withsmall activation energies in Ga2O3. Electron densities (n) can
be controlled in the wide range of 1015–1019 cm−3
[7, 8, 10, 11]. In contrast, there has been no report on
effective hole conduction in Ga2O3. We consider that it is
difﬁcult to ﬁnd acceptor atoms with a small activation energy
since Ga2O3 is a wide bandgap oxide semiconductor. Furthermore, a very low hole mobility (μ) prohibiting effective ptype conductivity in Ga2O3 was expected from the ﬁrstprinciples calculation of the Ga2O3 band structure [12–14].
Even in the worst case, tight localization of holes at speciﬁcsites in a Ga2O3 bulk was also predicted [15]. The thermal
conductivity of Ga2O3 strongly depends on the crystal
orientation due to its asymmetric crystal structure as shown inﬁgure 1. The highest thermal conductivity of 0.27 W (cm K)–1
can be obtained in the [010] direction, which is more than
twice as large as that in the [100] direction [16]. However,even the [010] thermal conductivity of Ga2O3 is still muchsmaller than those of other semiconductors.

Figure 1. Atomic unit cell of β-Ga2O3.

emerging semiconductor Ga2O3 power device technology.
Details about the above two attractive features of Ga2O3 will
be introduced in the following sections. At present, the device
technology is still at a primitive stage; however, the developed transistors and diodes with simple structures have veriﬁed its great potential for power devices.

4. Advantages of Ga2O3 power devices from thematerial point of view
Here, we discuss why there is a real possibility that Ga2O3based transistors and diodes can have excellent power device
characteristics such as high Vbr , high power, and low loss due
to its material properties. Table 1 summarizes the physicalproperties of β-Ga2O3 and other major semiconductors for
power devices. The Ga2O3 bulk electron μ of 300 cm2 Vs–1
was estimated on the basis of the experimental data for the
typical n range of 1015–1016 cm−3 for the drift layers ofvertical power devices [11, 17]. From the interpolation of therelationships among the Eg and breakdown electric ﬁelds (Ebr )of the other semiconductors, the Ebr of Ga2O3 is expected tohave a very large value of about 8 MV cm–1 [17]. The highEbr is the most attractive property of Ga2O3, because theBaliga’s FOM, which is the basic parameter to show how
suitable a material is for power devices, is proportional to the
cube of Ebr , but is only linearly proportional to μ. The fundamental limits of on-resistances (Ron ) as a function of Vbr for
Ga2O3 and representative semiconductors are plotted inﬁgure 2. It suggests that the conduction loss of Ga2O3 devices
can be one order of magnitude lower than those of SiC and
GaN devices at the same Vbr . Therefore, we consider that
Ga2O3 will best show its potential in unipolar devices.

2. Crystal structures of Ga2O3
The Ga2O3 single crystal is known to show ﬁve polymorphs,
labeled α, β, γ, δ and ò [1]. Among the crystal structures,
Ga2O3 mainly crystallizes in the monoclinic β-gallia structurewith lattice constants of a = 12.2 Å, b = 3.0 Å and c = 5.8 Å
as shown in ﬁgure 1. The β phase is the most stable structure,and the other phases are meta-stable ones. Note that β-Ga2O3
is the only crystal structure that can be grown from the melt.
Most of the reported scientiﬁc studies have been on the crystal
growth and material properties of β-Ga2O3. The second metastable polymorph is α-Ga2O3 with a corundum structure,
which is often stabilized by low-temperature heteroepitaxial
growth on sapphire substrates [2, 3]. The heteroepitaxial
growth of γ-Ga2O3 thin ﬁlms, which has a defective-spinel
structure, has also been reported [4, 5]. Formation of themeta-stable crystalline phases is strongly dependent on thesubstrate lattice and growth temperature. Typically, highgrowth temperatures lead to the formation of β-Ga2O3 even in
the case of heteroepitaxial growth on foreign substrates. In
this review, we focus on β-Ga2O3.

5. Single-crystal Ga2O3 bulks and wafers
In general, large-diameter single-crystal wafers are required tomass produce vertical devices that are favorable for highvoltage and high-current power devices. Single-crystal Ga2O3
bulks have been synthesized by several melt growth methods
such as the Czochralski [18, 19], the ﬂoating-zone (FZ)

3. Material properties of β-Ga2O3
Some of the basic material parameters of β-Ga2O3 have notbeen fully studied and understood. There is great variability in2

Semicond. Sci. Technol. 31 (2016) 034001

M Higashiwaki et al

Table 1. Material properties of major semiconductors and β-Ga2O3.

BandgapEg (eV)Electron mobilityμ (cm2 Vs–1)
Breakdown ﬁeldEbr (MV cm–1)
Relative dielectric constantò
Baliga’s FOM μEbr 3
Thermal conductivity (W (cm K)–1)

Si

GaAs

4H-SiCGaN

Diamondβ-Ga2O3

1.1
1,400
0.3
11.8
1
1.5

1.4
8,000
0.4
12.9
15
0.55

3.3
1,000
2.5
9.7
340
2.7

3.4
1,200
3.3
9.0
870
2.1

5.5
2,000
10
5.5
24,66410

4.5–4.9
300
8
10
3,444
0.27 [010]0.11 [100]Figure 2. Theoretical unipolar performance limits of Ron as afunction of Vbr for β-Ga2O3 and other major semiconductors powerdevices. Reproduced with permission from [17]. Copyright 2012,
AIP Publishing LLC.

Figure 3. Photograph of 4 inch diameter single-crystal Ga2O3 wafer.

6. Epitaxial growth of Ga2O3 thin ﬁlms

[7, 20, 21] and the edge-deﬁned ﬁlm-fed growth (EFG) [22]methods. In contrast, SiC and GaN bulk crystals are grownusing methods such as sublimation, vapor phase epitaxy andhigh-pressure synthesis other than melt growth methods [23–25]. We propose that EFG has an advantage against the othermelt growth methods regarding the bulk size, which is a key
factor for high-volume production of large-size single-crystal
Ga2O3 wafers. Figure 3 shows a photograph of a 4 inch
diameter single-crystal Ga2O3 wafer produced from an EFGgrown Ga2O3 bulk. The crystal quality of Ga2O3 wafers
prepared from EFG bulks has already been very good, with afull-width at half-maximum of the x-ray diffraction rocking
curve (XRC) as narrow as 17 arcsec and a dislocation density
on the order of 103–104 cm−2 as characterized by surface etchpits. The wafer surface was atomically ﬂat and smooth after
chemical–mechanical polishing (CMP) with a small rootmean-square (rms) surface roughness of 0.11 nm. The goodmaterial workability of Ga2O3 is another important feature.
Undoped Ga2O3 bulks show n-type conductivity due tounintentional Si incorporation from the Ga2O3 powder source.
Thus, at present, compensation doping with deep acceptors
such as Mg and Fe is required for the production of semiinsulating bulk crystals and wafers.

For the epitaxial growth of Ga2O3 thin ﬁlms, there have been
many reports on molecular beam epitaxy (MBE) [26–31],
halide vapor phase epitaxy (HVPE) [32–35], metal–organic
chemical vapor deposition [36–39] and mist chemical vapor
deposition [2, 40]. In this section, the current status of our
MBE and HVPE technologies for Ga2O3 thin ﬁlm growth will
be introduced.

6.1. MBE

At the time of writing this article, scientiﬁc studies on MBE
growth have been the most advanced among the epitaxial
growth techniques of Ga2O3 thin ﬁlms. In general, normal
gas-source MBE machines equipped with a turbo molecularpump are used for Ga2O3 growth. There are two types ofoxygen sources used for the MBE growth of oxide semiconductors; one is oxygen radicals, generated by an radiofrequency (RF)-plasma cell [26–29], and the other is ozone[30, 31]. Our work on the ozone MBE growth of undoped andSn-doped Ga2O3 ﬁlms on native Ga2O3 (010) substrates will
be described in the following [31].
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M Higashiwaki et al

Figure 4. (a) Surface AFM images and (b) their rms roughnesses of Ga2O3 epitaxial ﬁlms grown at various temperatures by ozone MBE.
Reproduced with permission from [31]. Copyright 2014, Elsevier.

The speciﬁcation of our ozone MBE machine used for
Ga2O3 thin ﬁlm growth is as follows. Ga and Sn (n-type
dopant) are, respectively, supplied from Ga metal with sixnines purity and four nines SnO2 powder heated in conventional Knudsen cells. The oxygen source is an ozone (5%)–
oxygen (95%) gas mixture generated by an ozone generator.
One of the noteworthy advantages of ozone over RF-plasma
is its high growth rate, reaching a few μm h–1.

Figure 4(a) shows the surface morphologies of Ga2O3
epitaxial ﬁlms grown at various temperatures, which wereobserved by atomic force microscopy (AFM). The rms surface roughnesses of the Ga2O3 epitaxial ﬁlms are plotted as afunction of growth temperature (Tg ) in ﬁgure 4(b). As shownin ﬁgure 4(a), step bunching along the [100] direction isobserved for Tg of higher than 700◦C , and the surface
becomes more rough with increasing Tg . A different type of4

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M Higashiwaki et al

Figure 6. Schematic diagram of the in-house-built atmospheric-pressure HVPE system for Ga2O3 growth.
Figure 5. Nd –Na depth proﬁles in Sn-doped Ga2O3 epitaxial ﬁlms
grown at 540, 570, and 600°C. Reproduced with permission from[31]. Copyright 2014, Elsevier.

substrates for the HVPE growth. Based on thermodynamic
analysis, GaCl and O2 were determined to be appropriate
precursors for Ga2O3 growth [33]. In the HVPE system, GaClis generated by the reaction between high-purity Ga metal andCl2 gas at 850◦C in an upstream region of the reactor. The
GaCl and O2 are separately introduced into a growth zone
using N2 as a carrier gas, and the Ga2O3 substrate is placed ona quartz glass susceptor. The SiCl4 dopant gas is simultaneously supplied in case of n-type Ga2O3 growth. The growthrate has been controlled up to as high as about 20 μm h–1
without degradation of the material quality of the Ga2O3
epitaxial ﬁlm [33]. Unintentionally doped (UID) Ga2O3 thinﬁlms grown by HVPE possess excellent structural and electrical properties, evidenced by an XRC peak as narrow as theone from the EFG bulk single crystal and an extremely low
residual carrier concentration of less than 1×1013 cm−3
[34]. The n-type conductivity of Ga2O3 ﬁlms can be controlled by the Si doping density in the wide range on the orderof 1015 ∼ 1018 cm−3. Details of the material and electricalproperties of HVPE-grown n-Ga2O3 thin ﬁlms will bereported elsewhere.

rough surface due to three-dimensional growth is shown for
Tg = 500◦C . These experimental data indicate that
550–650 ◦Cis the optimum window of Tg for obtaining asmooth surface.
Next, the electrical properties of the ozone–MBE Ga2O3
ﬁlms were characterized. The dependence of effective carrier
concentration (Nd –Na ) on Tg was characterized by using
electrochemical capacitance–voltage (C–V) measurements.
Figure 5 plots the Nd –Na depth proﬁles in three Sn-doped
Ga2O3 epitaxial ﬁlms grown at Tg = 540, 570 and 600°C.
Uniform distributions of the doped Sn atoms along the growth
direction were observed for the samples grown at Tg = 540°C
and 570 ◦C. On the other hand, a delay in the Sn doping at theinitial stage was conﬁrmed for the sample grown at
Tg = 600 ◦C, which was likely to be due to Sn segregation.
Therefore, the optimum Tg for Sn-doped n-Ga2O3 ﬁlms wasdetermined to be 550–570 ◦C from the overlap of the growth
conditions for the best structural and electrical properties.

7. Ga2O3 transistors

6.2. HVPE

MBE is a very useful tool for scientiﬁc research, because we
can obtain high-quality, high-purity epitaxial ﬁlms by omitting the possibility of the incorporation of unintentional
impurities. However, the productivity of MBE, which
requires an ultra-high-vacuum atmosphere, is open to question. We chose HVPE from the several epitaxial growthmethods as the ﬁrst candidate for Ga2O3 thin ﬁlm growth for
future mass production of Ga2O3 epitaxial wafers. Ga2O3
HVPE R&D is still at a primitive stage; however, we havealready established several important technologies [33, 34].
An atmospheric-pressure horizontal hot-wall HVPE
system as schematically shown in ﬁgure 6 is used for Ga2O3
growth [33]. Currently, we use EFG-grown Ga2O3 (001)7.1. Ga2O3 MESFETs

Transistor action for single-crystal Ga2O3 devices was ﬁrst
demonstrated using simple metal–semiconductor ﬁeld-effecttransistor (MESFET) structures [17]. By ozone MBE, a Sndoped n-type Ga2O3 layer with a thickness of 300 nm wasgrown on a Mg-doped semi-insulating Ga2O3 (010) substrate.
The Sn doping concentration was set at 7×1017 cm−3. The
activation energy of Sn in Ga2O3 was estimated to be about
60–80 meV; therefore, about half of the Sn dopants wereactivated in the MBE-grown Ga2O3 layer at room temperature(RT). Figure 7(a) shows a cross-sectional schematic of the nGa2O3 MESFET. We employed a circular FET pattern as
5

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Figure 7. (a) Cross-sectional schematic illustration and (b) optical microscope micrograph of Ga2O3 MESFET. Reproduced with permission
from [17]. Copyright 2012, AIP Publishing LLC.

shown in ﬁgure 7(b). In the ﬁrst process of ohmic contact
formation, a reactive ion etching (RIE) treatment was performed using a gas mixture of BCl3 and Ar followed byevaporation of Ti(20 nm)/Au(230 nm) and lift off. We foundthat the RIE process is effective in improving the ohmic
contacts. Finally, Schottky gates were fabricated by Pt(15 nm)/Ti(5 nm)/Au(250 nm) deposition and lift off. Surface dielectric passivation was not performed for the device.
The gate length, source–drain spacing and diameter of theinner circular drain electrode were 4, 20 and 200 μm,
respectively.
Figures 8(a) and (b) show the dc output and transfer
characteristics of the Ga2O3 MESFET, respectively. The draincurrent density (Id ) was effectively modulated by a gate
voltage (Vg ), and the device exhibited a perfect pinch off. The
maximum Id was 26 mA mm–1 at a drain voltage (Vd ) of 40 V
and a Vg of +2 V. The three-terminal destructive breakdown
in the off-state, which resulted in permanent degradation of
the gate electrode, happened at a Vd of over 250 V. The
maximum transconductance (gm ) was 2.3 mS mm–1 for
Vd = 40 V. The off-state drain leakage current was as small as
5 μA mm–1, and the on/off Id ratio reached about four ordersof magnitude.
The Ga2O3 MESFET showed excellent device characteristics as the ﬁrst FET demonstration. However, thedevices also exhibited two clear issues. The most serious
problem in the device characteristics was the high contact
resistance of source/drain (S/D) electrodes. In addition, the Id
on–off ratio was limited by the small leakage current throughthe unpassivated Ga2O3 surface.

Sn-doped Ga2O3 layer with a thickness of 300 nm was grown
at 560◦C on an Fe-doped semi-insulating Ga2O3 (010) substrate by ozone MBE. The device process started with multiple Si-ion (Si +) implantations to the regions under the S/Delectrodes to form a 150 nm deep box proﬁle with
Si = 5×1019 cm−3, followed by activation annealing at
925 ◦C for 30 min in an N2 gas atmosphere. Then, a Ti/Au
metal stack was deposited on the implanted regions andannealed again at 470◦C for 1 min in an N2 gas atmosphere.
This metal annealing process is effective to further decrease
contact resistance, probably due to the reaction between themetal and Ga2O3 at the interface. We obtained a speciﬁc
contact resistance as low as 8.1×10−6 Ω cm2 for theannealed contacts. Further details of the Si + implantation
doping in Ga2O3 and its application to the fabrication of lowresistance ohmic contacts are reported elsewhere [42]. A 20
nm thick Al2O3 gate dielectric and passivation ﬁlm wasformed on the Ga2O3 layer at 250◦C by plasma atomic layer
deposition. Note that the conduction band offset at the Al2O3–
Ga2O3 interface was estimated to be about 1.5 eV [43]. The
gate metal was formed with a Pt–Ti–Au stack on top of the
Al2O3 ﬁlm. The gate length and width were 2 μm and500 μm, respectively. The spacing between the S/D Si +implanted regions was 20 μm.
Figure 10(a) shows the dc output characteristics of the
Ga2O3 MOSFET at RT. As in the case of the MESFET, the Id
was effectively modulated by Vg with good saturation andsharp pinch-off characteristics. Device self-heating caused by
the poor thermal conductivity of Ga2O3 gave rise to thenegative output conductance with increasing Vd . The maximum Id was 39 mA mm–1 at Vg = +4 V. The three-terminal
off-state Vbr was as high as 370 V at Vg = −20 V. The Id –Vgcharacteristic of the MOSFET at Vd = 25 V is shown inﬁgure 10(b). The Id on–off ratio was extremely high,exceeding ten orders of magnitude with the measured offstate leakage reaching the lower limit of the measurement

7.2. Depletion-mode Ga2O3 MOSFET version 1: MBE-grown
Sn-doped Ga2O3 channel

To overcome the drawbacks of the MESFETs, we next fabricated depletion-mode Ga2O3 MOSFETs [41]. Figure 9shows a schematic cross section of the Ga2O3 MOSFET. The
6

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Figure 9. Schematic cross section of Ga2O3 MOSFET structure withan MBE-grown Sn-doped channel layer. Reproduced with permission from [41]. Copyright 2013, AIP Publishing LLC.

7.3. Depletion-mode Ga2O3 MOSFET version 2: Si + implanted Ga2O3 channel

The Ga2O3 MOSFETs with a Sn-doped n-Ga2O3 channel
suffered from reproducibility of the doping proﬁle, because
the suppression of the delay of Sn incorporation into the
Ga2O3 at the initial stage of the MBE growth requires theprecise control of Tg in the narrow temperature window of
550–570◦C . This issue led to both poor channel thickness
control and non-uniform in-plane n. We addressed the problem in[44] by applying Si +implantation doping to not only
the S/D electrodes but also the channel layer for reliable
doping.
The device structure of the Ga2O3 MOSFET presented inthis subsection was mostly the same as that of the Sn-dopedchannel MOSFET shown in ﬁgure 9, except that the channel
was formed by Si + implantation doping to a UID Ga2O3 layergrown by MBE. The thickness of the MBE-grown UID
Ga2O3 layer was 300 nm, which was the same as the Sndoped channel. The implanted Si atom densities in thechannel layer and the S/D regions were 3×1017 and5×1019 cm−3, respectively. Multiple implantations were
performed to obtain box proﬁles with depths of 300 nm forthe channel and 150 nm for the S/D regions. Implant activation annealing was performed once at 925 ◦C for 30 min in
N2 atmosphere for both the channel and S/D regions. Afterthe implantation and activation process, all fabrication procedures were identical to those employed for the Sn-dopedchannel Ga2O3 MOSFETs.
All the device characteristics of the Si +-implantedchannel MOSFETs were almost the same as or slightly
improved over those of the Sn-doped-channel Ga2O3 MOSFETs. Figures 11(a) and (b) show the dc output and breakdown characteristics of the Ga2O3 MOSFET with a gate
length of 4 μm at RT. The maximum Id was 65 mA mm–1 atFigure 8. (a) Dc I–V and (b) transfer characteristics of Ga2O3MESFET. Reproduced with permission from [17]. Copyright 2012,
AIP Publishing LLC.

instrument. These device characteristics were much superior
to those of the Ga2O3 MESFETs.
We also investigated the performance of the Ga2O3
MOSFET at high-temperature operation. The device characteristics evolved smoothly with increasing device operating
temperature. No kink or abrupt change that might be indicative of breakdown events and/or permanent degradation wasobserved, suggesting stable device operation in the wholetemperature range from 25 to 250 ◦C. The MOSFET maintained a high Id on–off ratio of about four orders of magnitude
even at 250 ◦C. Furthermore, the device characteristics were
almost entirely recovered after operation at 250 ◦C and cool
down to RT.
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M Higashiwaki et al

Figure 10. (a) Dc output and (b) Id –Vg characteristics of Ga2O3

MOSFETs with an MBE-grown Sn-doped channel layer. Reproduced with permission from [41]. Copyright 2013, AIP Publishing LLC.

Figure 11. (a) Dc output and (b) breakdown characteristics of Ga2O3

MOSFET with a Si-doped channel layer by ion implantation.
Reproduced with permission from [44]. Copyright 2013, IEEE.

leakage current of less than 5 μA mm–1 and a high Id on–off
ratio of about four orders of magnitude at 250 ◦C.

Vg = +6 V. The three-terminal off-state Vbr was as high as
415 V at Vg = −30 V. The off-state Id remained below thedetection limit prior to device breakdown. At Vd = 30 V, ahigh Id on–off ratio of about ten orders of magnitude wasachieved, and the maximum gm was 3.6 mS mm–1. Figure 12
plots the temperature dependence of the transfer characteristics at Vd = 30 V of the MOSFET. The presented devices
with the Si +-implanted channel also revealed stable operation
up to 250 ◦C as did the devices with the MBE-grown Sndoped channel. The MOSFETs maintained a low drain

8. Ga2O3 Schottky barrier diodes (SBDs)
8.1. Ga2O3 SBDs fabricated on UID Ga2O3 native substrates

For the purpose of evaluating the basic device performance ofGa2O3 SBDs, we ﬁrst fabricated simple SBD structures on a8

Semicond. Sci. Technol. 31 (2016) 034001

M Higashiwaki et al

Figure 12. Temperature dependence of the transfer characteristics at
Vd = 30 V of Ga2O3 MOSFET with a Si-implanted channel.
Reproduced with permission from [44]. Copyright 2013, IEEE.

UID Ga2O3 (010) substrate with a thickness of 600 μm, whichwere produced from an FZ-grown bulk crystal [45]. The
substrates showed n-type conductivity due to the unintentionally incorporated Si in the Ga2O3 powder source. The
100 μm diameter circular Schottky anode electrodes were
fabricated on the front side of the substrate with a Pt–Ti–Au
stack. Next, BCl3/Ar RIE treatment was performed on thewhole back area of the substrate, and the cathode electrode of
Ti/Au was evaporated onto it. These were the same procedures employed for the MESFET to fabricate S/D ohmic
contacts [17].
Note that n was uniform along the thickness of thesubstrate but showed in-plane variation from 3×1016–
1×1017 cm−3 as evaluated by C–V measurements. The inplane distribution of n was caused by a non-uniform Si atom
density, which was attributed to the characteristic of the
Ga2O3 FZ growth. Here, we introduce device characteristicsof the two selected SBDs having different n of 3×1016 cm−3
and 5×1016 cm−3, which were fabricated at different locations on the same substrate. The forward current density–
voltage (J–V) characteristics of the SBDs are shown inﬁgures 13(a) and (b). The near-unity ideality factors of
1.04–1.06 were estimated for both devices, indicating thehigh crystal quality of the FZ Ga2O3 substrate and good
Schottky interface property. A Schottky barrier height of
1.3–1.5 eV was extracted for the Pt–Ga2O3 interface from thesemi logarithmic-plotted J–V and 1/C2–V characteristics. The
Ron of the Ga2O3 SBDs, which were determined from theslopes of the linear regions in ﬁgure 13(a), were relatively
high at 7.85 and 4.30 mΩ·cm2 because of the low conductivity of the bulk substrate. Figure 13(b) shows the reverse
J–V characteristics of the Ga2O3 SBDs. The reverse Vbr values
were 150 and 115 V for n = 3×1016 and 5×1016 cm−3,

Figure 13. (a) Forward and (b) reverse J–V characteristics of two

different Ga2O3 SBDs. They were fabricated at different locations on
the same substrate with n = 3×1016 and 5×1016 cm−3.
Reproduced with permission from [45]. Copyright 2013, IEEE.

respectively, which were reasonably high for the n and thesimple device structure.
8.2. Ga2O3 SBDs fabricated on HVPE-grown n − -Ga2O3 drift
layersRecently, we started development on full-scale Ga2O3 SBDs
with the advancement of the HVPE technology [33, 34]. The
Ga2O3 SBDs were fabricated on epitaxial wafers with HVPE9

Semicond. Sci. Technol. 31 (2016) 034001

M Higashiwaki et al

Figure 14. Schematic cross section of Ga2O3 SBD structure with an
HVPE-grown n--Ga2O3 drift layer.

grown Si-doped n--Ga2O3 drift layers on n+-Ga2O3 (001)substrates. SiCl4 was simultaneously supplied during the
growth as an n-type dopant gas. The growth rate of the Ga2O3
epitaxial layers was set at 10 μm h–1. After the HVPE growth,
the surfaces of the epitaxial layers were rough with many pits;
therefore, the CMP process was performed to ﬂatten thesurfaces. The thickness of the post-CMP drift layer was
about 7 μm.
A schematic cross section of the SBD structure is shownin ﬁgure 14. The depth proﬁles of Nd –Na extracted by thed (1 C 2 )/dV method in two n--Ga2O3 epitaxial layers grownwith different SiCl4 ﬂow rates were almost constant at
1.4×1016 cm−3 and 2.0×1016 cm−3, respectively.
Figure 15(a) shows the forward J–V characteristics of the
SBDs. From linear ﬁts to the slopes within the range of J =100–200 A cm–2, the speciﬁc Ron were estimated to be
3.0 mΩ·cm2 for the device with Nd –Na = 1.4×1016 cm−3 inthe drift layer and 2.4 mΩ·cm2 for the one with
Nd –Na = 2.0×1016 cm−3. Note that the Ron values included
a substrate resistance as large as 1.0 mΩ·cm2. The ideality
factors of the SBDs were 1.03–1.07, indicating that the Pt/
HVPE-grown Ga2O3 interface followed the ideal thermionicemission theory. The reverse J–V characteristics shown inﬁgure 15(b) reveal a high Vbr of around −500 V for both
SBDs. Note that hard breakdown events happened with catastrophic damage in both devices due to electric-ﬁeld concentration at the edge of the anode electrodes. Therefore,
implementation of edge termination designs such as a ﬁeld

Figure 15. (a) Forward and (b) reverse J–V characteristics of Ga2O3

SBDs with an HVPE-grown n--Ga2O3 drift layer.

plate and/or a guard ring to the presented device structure
should lead to further improvement in Vbr .

9. Conclusion
This paper has attempted to give a review of the present status
of R&D on new wide bandgap semiconductor Ga2O3 powerdevices. Ga2O3 bulk single crystals can be synthesized byseveral melt growth methods, and large-size, high-quality
Ga2O3 wafers produced from the EFG bulk have already been
available up to the 4 inch diameter size. Among several
10

Semicond. Sci. Technol. 31 (2016) 034001

M Higashiwaki et al[16] Guo Z, Verma A, Wu X, Sun F, Hickman A, Masui T,
Kuramata A, Higashiwaki M, Jena D and Luo T 2015 Appl.
Phys. Lett. 106 111909[17] Higashiwaki M, Sasaki K, Kuramata A, Masui T and
Yamakoshi S 2012 Appl. Phys. Lett. 100 013504[18] Tomm Y, Reiche P, Klimm D and Fukuda T 2000 J. Cryst.
Growth 220 510[19] Galazka Z et al 2014 J. Cryst. Growth 404 184[20] Víllora E G, Shimamura K, Yoshikawa Y, Aoki K and
Ichinose N 2004 J. Cryst. Growth 270 420[21] Ohira S, Yoshioka M, Sugawara T, Nakajima K and
Shishido T 2006 Thin Solid Films 496 53[22] Aida H, Nishiguchi K, Takeda H, Aota N, Sunakawa K and
Yaguchi Y 2008 Japan. J. Appl. Phys. 47 8506
[23] Müller S G, Glass R C, Hobgood H M, Tsvetkov V F,
Brady M, Henshall D, Jenny J R, Malta D and Carter C H Jr
2000 J. Cryst. Growth 211 325[24] Fujito K, Kubo S, Nagaoka H, Mochizuki T, Namita H andNagao S 2009 J. Cryst. Growth 311 3011
[25] Dwiliński R, Doradziński R, Garczyński J, Sierzputowski L P,
Puchalski A, Kanbara Y, Yagi K, Minakuchi H and
Hayashi H 2009 J. Cryst. Growth 311 3015
[26] Víllora E G, Shimamura K, Kitamura K and Aoki K 2006
Appl. Phys. Lett. 88 031105[27] Oshima T, Okuno T and Fujita S 2007 Japan. J. Appl. Phys.
46 7217[28] Okumura H, Kita M, Sasaki K, Kuramata A,
Higashiwaki M and Speck J S 2014 Appl. Phys. Express 7095501[29] Vogt P and Bierwagen O 2015 Appl. Phys. Lett. 106 081910[30] Sasaki K, Kuramata A, Masui T, Víllora E G,
Shimamura K and Yamakoshi S 2012 Appl. Phys. Express 5035502[31] Sasaki K, Higashiwaki M, Kuramata A, Masui T and
Yamakoshi S 2014 J. Cryst. Growth 392 30[32] Matsumoto T, Aoki M, Kinoshita A and Aono T 1974 Japan.
J. Appl. Phys. 13 1578
[33] Nomura K, Goto K, Togashi R, Murakami H, Kumagai Y,
Kuramata A, Yamakoshi S and Koukitu A 2014 J. Cryst.
Growth 405 19[34] Murakami H et al 2015 Appl. Phys. Express 8 015503
́[35] Oshima Y, Villora
E G and Shimamura K 2015 Appl. Phys.
Express 8 055501[36] Kim H W and Kim N H 2005 Appl. Phys. A 81 763[37] Huang C Y, Horng R H, Wuu D S, Tu L W and Kao H S 2013
Appl. Phys. Lett. 102 011119[38] Gogova D, Wagner G, Baldini M, Schmidbauer M,
Irmscher K, Schewski R, Galazka Z, Albrecht M and
Fornari R 2014 J. Cryst. Growth 401 665[39] Baldini M, Albrecht M, Gogova D, Schewski R and Wagner G2015 Semicond. Sci. Technol. 30 024013[40] Kawaharamura T, Dang G T and Furuta M 2012 Japan. J.
Appl. Phys. 51 040207[41] Higashiwaki M, Sasaki K, Kamimura T, Wong M H,
Krishnamurthy D, Kuramata A, Masui T and Yamakoshi S
2013 Appl. Phys. Lett. 103 123511[42] Sasaki K, Higashiwaki M, Kuramata A, Masui T and
Yamakoshi S 2013 Appl. Phys. Express 6 086502[43] Kamimura T, Sasaki K, Wong M H, Krishnamurthy D,
Kuramata A, Masui T, Yamakoshi S and Higashiwaki M
2014 Appl. Phys. Lett. 104 192104[44] Higashiwaki M, Sasaki K, Wong M H, Kamimura T,
Krishnamurthy D, Kuramata A, Masui T and Yamakoshi S
2013 IEEE Int. Electron Devices Mtg 28.7.1[45] Sasaki K, Higashiwaki M, Kuramata A, Masui T and
Yamakoshi S 2013 IEEE Electron Device Lett. 34 493

epitaxial growth techniques, MBE and HVPE succeeded in
producing device-quality Ga2O3 epitaxial wafers. Signiﬁcantprogress has been made in the development of Ga2O3 transistors and diodes in the last few years, and the device
characteristics demonstrated the great potential of Ga2O3
power devices for future high-power and high-voltage
applications. We consider that vertical transistors and diodes
operating at the voltage range of higher than 3 kV will be themain target for Ga2O3 power devices in future R&D. However, Ga2O3 devices are not limited to the power device
applications and thus should also be developed for several
different application ﬁelds such as high-frequency andextreme environment electronics. Further efforts and progress
in R&D on Ga2O3 devices should pave the way for nextgeneration semiconductor electronics.

Acknowledgments
This work was partially supported by ’The research anddevelopment project for innovation technique of energy
conservation’ of the New Energy and Industrial Technology
Development Organization (NEDO), and Council for Science, Technology and Innovation (CSTI), Cross-ministerial
Strategic Innovation Promotion Program (SIP), Next-generation power electronics (funding agency: NEDO).

References[1] Roy R, Hill V G and Osborn E F 1952 J. Am. Chem. Soc.
74 719[2] Shinohara D and Fujita S 2008 Japan. J. Appl. Phys. 47 7311
[3] Kumaran R, Tiedje T, Webster S E, Penson S and Li W 2012
Opt. Lett. 35 3793
[4] Hayashi H, Huang R, Ikeno H, Oba F, Yoshioka S,
Tanaka I and Sonoda S 2006 Appl. Phys. Lett. 89 181903[5] Oshima T, Nakazono T, Mukai A and Ohtomo A 2012
J. Cryst. Growth 359 60[6] Tippins H H 1965 Phys. Rev. 140 A316
[7] Ueda N, Hosono H, Waseda R and Kawazoe H 1997 Appl.
Phys. Lett. 70 3561
[8] Orita M, Ohta H, Hirano M and Hosono H 2000 Appl. Phys.
Lett. 77 4166
[9] Onuma T, Fujioka S, Yamaguchi T, Higashiwaki M, Sasaki K,
Masui T and Honda T 2013 Appl. Phys. Lett. 103 041910[10] Víllora E G, Shimamura K, Yoshikawa Y, Ujiie T and Aoki K
2008 Appl. Phys. Lett. 92 202120[11] Sasaki K, Kuramata A, Masui T, Víllora E G,
Shimamura K and Yamakoshi S 2012 Appl. Phys. Exp. 5035502[12] He H, Orlando R, Blanco M A, Pandey R, Amzallag E,
Baraille I and Rérat M 2006 Phys. Rev. B 74 195123[13] Varley J B, Weber J R, Janotti A and Van de Walle C G 2010
Appl. Phys. Lett. 97 142106[14] Yamaguchi K 2004 Solid State Commun. 131 739[15] Varley J B, Janotti A, Franchini C and Van de Walle C G 2012
Phys. Rev. B 85 081109(R)11