Depletion-mode Ga2O3 metal-oxide-semiconductor field-effect transistors on -Ga2O3(010) substrates and temperature dependence of their device characteristicsMasataka Higashiwaki, Kohei Sasaki, Takafumi Kamimura, Man Hoi Wong, Daivasigamani Krishnamurthy, Akito

Kuramata, Takekazu Masui, and Shigenobu Yamakoshi

Citation: Applied Physics Letters 103, 123511 (2013); doi: 10.1063/1.4821858 View online: http://dx.doi.org/10.1063/1.4821858 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effective passivation of In0.2Ga0.8As by HfO2 surpassing Al2O3 via in-situ atomic layer deposition Appl. Phys. Lett. 101, 172104 (2012); 10.1063/1.4762833 InAs MOS devices passivated with molecular beam epitaxy-grown Gd2O3 dielectrics J. Vac. Sci. Technol. B 30, 02B118 (2012); 10.1116/1.3678206 Improved electrical characteristics of TaN / Al 2 O 3 / In 0.53 Ga 0.47 As metal-oxide-semiconductor field-effecttransistors by fluorine incorporation Appl. Phys. Lett. 95, 013501 (2009); 10.1063/1.3173820 Gate-first inversion-type InP metal-oxide-semiconductor field-effect transistors with atomic-layer-deposited Al 2 O3 gate dielectric Appl. Phys. Lett. 92, 233508 (2008); 10.1063/1.2937117 GaN metal-oxide-semiconductor high-electron-mobility-transistor with atomic layer deposited Al 2 O 3 as gatedielectric Appl. Phys. Lett. 86, 063501 (2005); 10.1063/1.1861122

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

194.117.10.254 On: Thu, 27 Mar 2014 10:53:58

Depletion-mode Ga2O3 metal-oxide-semiconductor field-effect transistorson b-Ga2O3 (010) substrates and temperature dependence of their devicecharacteristics

Masataka Higashiwaki,1,a) Kohei Sasaki,1,2 Takafumi Kamimura,1 Man Hoi Wong,1

Daivasigamani Krishnamurthy,1 Akito Kuramata,2 Takekazu Masui,3

and Shigenobu Yamakoshi21National Institute of Information and Communications Technology, 4–2–1 Nukui-Kitamachi, Koganei,Tokyo 184–8795, Japan2Tamura Corporation, 2–3–1 Hirosedai, Sayama, Saitama 350–1328, Japan3Koha Co., Ltd., 2–6–8 Kouyama, Nerima, Tokyo 176–0022, Japan

(Received 31 July 2013; accepted 5 September 2013; published online 20 September 2013)

Single-crystal gallium oxide (Ga2O3) metal-oxide-semiconductor field-effect transistors were

fabricated on a semi-insulating b-Ga2O3 (010) substrate. A Sn-doped n-Ga2O3 channel layer was

grown by molecular-beam epitaxy. Si-ion implantation doping was performed to source and drain

electrode regions for obtaining low-resistance ohmic contacts. An Al2O3 gate dielectric film

formed by atomic layer deposition passivated the device surface and significantly reduced gate

leakage. The device with a gate length of 2 lm showed effective gate modulation of the drain

current with an extremely low off-state drain leakage of less than a few pA/mm, leading to a high

drain current on/off ratio of over ten orders of magnitude. A three-terminal off-state breakdown

voltage of 370 V was achieved. Stable transistor operation was sustained at temperatures up to

250 �C. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4821858]

We have been proposing gallium oxide (Ga2O3) as a

promising candidate for power device applications because

of its excellent material properties and suitability for mass

production.1,2 With a wide bandgap of 4.7–4.9 eV,3–5 the

estimated breakdown electric field of 8 MV/cm for Ga2O3

is about three times larger than those of SiC and GaN.

Ga2O3 can be controllably doped to obtain electron den-

sities (n) from semi-insulating to as high as 1019 cm�3.6–9

The extrapolated experimental bulk electron mobility of

Ga2O3 reaches a relatively high value of about 300 cm2/V�sfor n¼ 1015–1016 cm�3, which is typical for the drift layers

of vertical power transistors and diodes.10 This is supported

by theoretical calculations showing that the electron effec-

tive mass of 0.23–0.34m0 (m0: free electron mass) for

Ga2O3 is comparable to those for conventional widegap

(3–4 eV) semiconductors.5,8,11 The Baliga’s power device

figure of merit of Ga2O3 is estimated to be several times

larger than those of SiC and GaN based on their material

properties,1 projecting higher breakdown voltages (Vbr) and

efficiencies for Ga2O3 power devices than their SiC and

GaN counterparts. The other important attribute of Ga2O3 is

that single-crystal bulk materials can be synthesized by

using conventional melt growth techniques such as the

floating-zone (FZ), the edge-defined film-fed growth, and

Czochralski methods.12–15 These technologies, which are

currently employed for manufacturing sapphire substrates,

will offer Ga2O3 a big advantage over other widegap semi-

conductors by enabling simple and low-cost mass produc-

tion of single-crystal native Ga2O3 substrates. Therefore,

Ga2O3 power devices have the obvious potential to surpass

SiC and GaN in not only device performance but also cost

effectiveness.

We have previously demonstrated Ga2O3 metal-

semiconductor field-effect transistors (MESFETs) on single-

crystal b-Ga2O3 substrates.1 The MESFET showed excellent

device characteristics such as a three-terminal off-state Vbr

of 250 V and a drain current (Id) on/off ratio of about four

orders of magnitude. However, the device suffered from

high contact resistance of source and drain electrodes. In

addition, its Id on/off ratio was limited by a small leakage

current through the unpassivated Ga2O3 surface. To address

these shortcomings, we fabricated Ga2O3 metal-oxide-semi-

conductor FETs (MOSFETs) in the present work by adopting

Si-ion (Siþ) implantation doping for improved ohmic con-

tacts and atomic layer deposition (ALD) of a dielectric film

for surface passivation and gate insulation. Temperature

dependences of current–voltage (I-V) characteristics of the

Ga2O3 MOSFETs were closely examined to estimate their

capability for high-temperature application.

The depletion-mode Ga2O3 MOSFETs were fabricated

on Fe-doped semi-insulating single-crystal b-Ga2O3 (010)

FZ substrates. A 300-nm-thick n-Ga2O3 channel layer with a

nominal Sn doping concentration of 7� 1017 cm�3 was

grown by molecular-beam epitaxy at a substrate temperature

of 560 �C and a growth rate of 0.6 lm/h. Ga and Sn fluxes

were, respectively, supplied by Ga metal and SnO2 powder

from conventional Knudsen cells. A gas mixture of ozone

and oxygen was used as the oxygen source. The activation

energy of Sn in Ga2O3 was estimated to be about 60 meV

from the temperature dependence of n; therefore, a little less

than half of the Sn dopants were activated in the Ga2O3

epitaxial layer at room temperature (RT). In fact, an effec-

tive carrier density (Nd-Na) of about 3� 1017 cm�3 wasa)Electronic mail: mhigashi@nict.go.jp

0003-6951/2013/103(12)/123511/4/$30.00 VC 2013 AIP Publishing LLC103, 123511-1

APPLIED PHYSICS LETTERS 103, 123511 (2013)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

194.117.10.254 On: Thu, 27 Mar 2014 10:53:58

experimentally confirmed for the channel layer at RT by

capacitance–voltage measurements on MOS diodes simulta-

neously fabricated on the same substrate.

Figures 1(a) and 1(b) show a cross-sectional schematic

and an optical micrograph of the Ga2O3 MOSFET, respec-

tively. We employed a circular FET pattern as shown in Fig.

1(b). Multiple Siþ implantations were performed to the

regions for source and drain electrodes to form a 150-

nm-deep box profile with Si¼ 5� 1019 cm�3, followed by

30-min activation annealing at 925 �C in a N2 gas atmosphere

by using an infrared-lamp annealing system. The near-

surface region of the implanted areas had a lower Si density

due to the nature of the ion implantation process. To expose

the highly doped subsurface layer for direct contact with

the ohmic metal, a 13-nm-deep recess was performed to the

implanted regions by BCl3 reactive ion etching (RIE) at

chamber pressure of 5.0 Pa and plasma power of 100 W prior

to evaporation of Ti(20 nm)/Au(230 nm) and liftoff. To fur-

ther reduce the contact resistance, the Ti/Au metal stack was

annealed at 470 �C for 1 min in N2 by using a rapid thermal

annealing system. As shown in Fig. 2(a), the annealed Ti/Au

contacts on an implanted Ga2O3 film with Si¼ 5� 1019 cm�3

were ohmic with linear I–V behavior whereas the non-

annealed ones showed weak Schottky behavior. The specific

contact resistance of the annealed electrodes as measured by

the circular transmission-line method was as low as

8.1� 10�6 X � cm2, which is comparable to typical values for

conventional Ti/Al-based alloyed contacts on n-GaN and/or

AlGaN/GaN heterostructures. Cross-sectional transmission

electron microscopy (TEM) revealed a defective Ga2O3

region and a reacted Ti–Ga2O3 region at the annealed

Ti/Ga2O3 interface [Fig. 2(b)]. We speculate that these inter-

facial reactions contributed to the improved contact ohmicity.

Further details of the Siþ implantation doping in b-Ga2O3

and its application to fabrication of low-resistance ohmic con-

tacts have been reported elsewhere.16 Next, a 20-nm-thick

Al2O3 gate dielectric and passivation film was formed on the

Ga2O3 layer by plasma-assisted ALD at 250 �C using trime-

thylaluminum and oxygen plasma. The inductively coupled

plasma (ICP) power was 300 W, the O2 gas flow rate was 20

sccm, and the Al2O3 deposition rate was 0.9 A/cycle. The

gate metal was formed with Ti(3 nm)/Pt(12 nm)/Au(280 nm)

on top of the Al2O3 film. Probing pads of the source and drain

electrodes were subsequently opened by BCl3 ICP-RIE. We

fabricated and characterized two types of devices with a gate

length (Lg) of 2 or 4 lm. The gate width, source-drain implant

spacing, and diameter of the inner circular drain electrode

were 500, 20, and 200 lm, respectively.

Figure 3(a) shows the DC I-V curves of the Ga2O3

MOSFET with Lg ¼ 2 lm measured at RT. The Id was effec-

tively modulated by the gate voltage (Vg) with good satura-

tion and sharp pinch-off characteristics. The maximum Id

was 39 mA/mm for Vg ¼ þ4 V. Device self-heating caused

by the poor thermal conductivity of the Ga2O3 substrate gave

rise to the negative output conductance with increasing drain

voltage (Vd). The three-terminal off-state Vbr, limited by cata-

strophic dielectric breakdown at the drain side of the gate,

was as large as 370 V at Vg ¼ �20 V. Figure 3(b) shows the

RT transfer characteristics of the MOSFET for Vd ¼ 25 V.

The Id on/off ratio was extremely high, exceeding ten orders

of magnitude with the measured off-state leakage of a few

pA/mm reaching the lower limit of the measurement instru-

ment. These Ga2O3 MOSFET characteristics were superior to

those of the Ga2O3 MESFETs we had reported previously,1

which we attribute to not only the large decrease in the con-

tact resistance with Siþ-implanted source and drain electro-

des but also the low leakage current owing to the Al2O3 gate

dielectric.

FIG. 1. (a) Schematic cross-section and (b) optical micrograph of depletion-

mode Ga2O3 MOSFET.

FIG. 2. (a) I-V characteristics of implanted Ga2O3 film before and after

Ti/Au electrode annealing. The electrode separation was 24 lm. (b) Cross-

sectional TEM micrograph of the Ti/Ga2O3 interface after annealing at

470 �C.

123511-2 Higashiwaki et al. Appl. Phys. Lett. 103, 123511 (2013)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

194.117.10.254 On: Thu, 27 Mar 2014 10:53:58

Next, we investigated the performance of the Ga2O3

MOSFET at elevated temperatures. The temperature-

dependent measurements were performed in atmosphere by

using an on-chip probing setup equipped with a heated sam-

ple stage. Figure 4(a) illustrates the temperature-dependent

transfer characteristics of the MOSFET with Lg ¼ 2 lm at

Vd ¼ 25 V. The corresponding Id on/off ratios, defined as the

Id at Vg ¼ 0 V divided by the Id at Vg ¼ �25 V, are plotted

in Fig. 4(b). These device characteristics evolved smoothly

with increasing device operating temperature. No kinks or ab-

rupt changes that might be indicative of breakdown events

and/or permanent degradation were observed, suggesting sta-

ble device operation in the whole temperature range from 25

to 250 �C, despite a modest reduction in the maximum Id of

approximately 30%. The MOSFET maintained a high Id

on/off ratio of approximately 104 even at 250 �C as shown in

Fig. 4(b). Furthermore, after high-temperature operation at

250 �C and cooling down to RT, the MOSFET exhibited fully

recovered device characteristics. Figure 5 plots drain and gate

currents (Id and Ig) under the pinch-off condition of Ga2O3

MOSFETs with Lg ¼ 4 lm measured at RT and 250 �C.

Different-type source-measure units, which had lower detec-

tion limits of about 200 and 2 pA/mm, were used for the

measurements of Id and Ig, respectively. In case of the RT

operation, both the off-state Id and Ig remained below the re-

spective detection limits within the applied Vd range up to the

device breakdown. Even at 250 �C, the Id and Ig still kept

small values of less than about 20 and 5 lA/mm, respectively.

The off-state Vbr were 404 and 378 V for RT and 250 �C,

FIG. 3. (a) DC I-V curves and (b) transfer characteristics at Vd ¼ 25 V of

Ga2O3 MOSFET (Lg ¼ 2 lm) measured at RT.

FIG. 4. (a) Transfer characteristics at Vd ¼ 25 V and (b) Id on/off ratios of

Ga2O3 MOSFET (Lg ¼ 2 lm) as a function of operating temperature.

123511-3 Higashiwaki et al. Appl. Phys. Lett. 103, 123511 (2013)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

194.117.10.254 On: Thu, 27 Mar 2014 10:53:58

respectively. The device breakdowns occurred at both tem-

peratures were permanent ones caused by destructive degra-

dation of the Al2O3 dielectrics and gate electrodes. This is

consistent with the Id and Ig behaviors that increased signifi-

cantly at the point of Vbr due to a short-circuiting between the

gate and drain electrodes. All these results indicate that the

Ga2O3 MOSFET can perform at elevated temperatures up to

at least 250 �C without noticeable irreversible degradation in

electrical characteristics.

In summary, we succeeded in demonstrating depletion-

mode Ga2O3 MOSFETs on single-crystal b-Ga2O3 (010)

substrates. Despite a simple device structure, the MOSFETs

with Lg ¼ 2 lm demonstrated excellent device characteris-

tics represented by a maximum Id of 39 mA/mm for

Vg ¼ þ4 V, an Id on/off ratio of over ten orders of

magnitude, and a three-terminal Vbr of 370 V. The devices

also showed good performance at 250 �C with no significant

permanent degradation. These results demonstrate the great

potential of Ga2O3 electron devices for power electronics

applications in the near future.

This work was partially supported by “The research and

development project for innovation technique of energy con-

servation” of the New Energy and Industrial Technology

Development Organization (NEDO).

1M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, and S. Yamakoshi,

Appl. Phys. Lett. 100, 013504 (2012).2K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, and S. Yamakoshi,

IEEE Electron Device Lett. 34, 493 (2013).3H. H. Tippins, Phys. Rev. 140, A316 (1965).4M. Orita, H. Ohta, M. Hirano, and H. Hosono, Appl. Phys. Lett. 77, 4166

(2000).5H. He, R. Orlando, M. A. Blanco, R. Pandey, E. Amzallag, I. Baraille, and

M. R�erat, Phys. Rev. B 74, 195123 (2006).6N. Ueda, H. Hosono, R. Waseda, and H. Kawazoe, Appl. Phys. Lett. 70,

3561 (1997).7E. G. V�ıllora, K. Shimamura, Y. Yoshikawa, T. Ujiie, and K. Aoki, Appl.

Phys. Lett. 92, 202120 (2008).8J. B. Varley, J. R. Weber, A. Janotti, and C. G. Van de Walle, Appl. Phys.

Lett. 97, 142106 (2010).9K. Irmscher, Z. Galazka, M. Pietsch, R. Uecker, and R. Fornari, J. Appl.

Phys. 110, 063720 (2011).10K. Sasaki, A. Kuramata, T. Masui, E. G. V�ıllora, K. Shimamura, and S.

Yamakoshi, Appl. Phys. Express 5, 035502 (2012).11K. Yamaguchi, Solid State Commun. 131, 739 (2004).12E. G. V�ıllora, K. Shimamura, Y. Yoshikawa, K. Aoki, and N. Ichinose,

J. Cryst. Growth 270, 420 (2004).13H. Aida, K. Nishiguchi, H. Takeda, N. Aota, K. Sunakawa, and Y.

Yaguchi, Jpn. J. Appl. Phys., Part 1 47, 8506 (2008).14Y. Tomm, P. Reiche, D. Klimm, and T. Fukuda, J. Cryst. Growth 220, 510

(2000).15Z. Galazka, R. Uecker, K. Irmscher, M. Albrecht, D. Klimm, M. Pietsch,

M. Br€utzam, R. Bertram, S. Ganschow, and R. Fornari, Cryst. Res.

Technol. 45, 1229 (2010).16K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, and S. Yamakoshi,

Appl. Phys. Express 6, 086502 (2013).

FIG. 5. Off-state breakdown characteristics of Ga2O3 MOSFETs (Lg ¼ 4 lm)

operating at RT and 250 �C; Vg ¼ �20 V.

123511-4 Higashiwaki et al. Appl. Phys. Lett. 103, 123511 (2013)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

194.117.10.254 On: Thu, 27 Mar 2014 10:53:58