Ga2O3 Schottky barrier diodes fabricated on single-crystal -Ga2O3 substrates

Kohei Sasaki1,2, Masataka Higashiwaki2,3, Akito Kuramata1, Takekazu Masui4, and Shigenobu Yamakoshi1 1Tamura Corporation, Sayama, Saitama 350-1328, Japan

2National Institute of Information and Communications Technology, Koganei, Tokyo 184-8795, Japan 3PRESTO, Japan Science and Technology Agency, Chiyoda, Tokyo 102-0075, Japan

4Koha Co., Ltd., Nerima, Tokyo 176-0022, Japan Phone : +81-4-2900-0045 / Fax : +81-4-2900-0059 / Email : kohei.sasaki@tamura-ss.co.jp

-Ga2O3 has an extremely large bandgap of 4.8-4.9 eV and excellent material properties for power device

applications, as summarized in Tab. 1. The breakdown field of -Ga2O3 is estimated to be 8 MV/cm, from the relation between the band gaps and breakdown fields of other major semiconductors. Another important feature is that large single-crystal -Ga2O3 substrates can be fabricated with melt-growth methods such as floating-zone (FZ) and edge-defined film-fed growth (EFG). Figure 1 shows a photograph of a 2-inch-diameter single-crystal -Ga2O3 wafer fabricated by the EFG method. The wafer size depends on the production machine size. Therefore, we consider that Ga2O3 is a very attractive new semiconductor for power device applications because of its material properties and ease of mass production. However, it has not been studied to any extent up to now.

We have been working on a new project to develop Ga2O3 power devices and recently succeeded in fabricating Ga2O3 transistors on single-crystal Ga2O3 substrates for the first time [1,2]. The devices exhibited good characteristics such as high breakdown voltage (VBR) (~250 V), low off-state drain leakage current (5 μA/mm), and high on/off ratio (~104), even though they had a simple circular MESFET structure. These characteristics clearly indicate the potential of Ga2O3 power devices. In this study, we fabricated and characterized Ga2O3 Schottky barrier diodes (SBDs) on single-crystal -Ga2O3 substrates made from melt; these devices will be key components in power electronics.

We used unintentionally-doped -Ga2O3 substrates prepared from bulk grown by the FZ method. The bulk crystal was cut along the (010) plane. The substrate thickness was 600 μm. The surface of the substrate was atomically flat after chemical mechanical polishing with a root-mean-square surface roughness of 0.11 nm. The crystal quality of the substrate was excellent, as represented by a narrow XRD rocking curve peak with an FWHM of 30 arcsec and low dislocation density of less than 1×104 cm-2. The substrate showed n-type conductivity due to unintentionally-doped Si, which was incorporated in the Ga2O3 powder source (5N). The electron density (n) was uniform in the depth direction, but a little distributed in a range of 3 1016-1 1017 cm-3 in the plane, as evaluated by the capacitance-voltage measurement.

Figure 2 shows a cross-sectional schematic illustration of the -Ga2O3 SBD structure fabricated on the single-crystal -Ga2O3 substrate. First, circular Schottky contacts with a diameter of 100 m were fabricated on the front side of the substrate as anode electrodes by standard photolithography patterning, Pt/Ti/Au evaporation, and lift off. Next, a reactive ion etching (RIE) treatment using a mixture gas of BCl3 and Ar was performed on the back side, followed by cathode metal evaporation (Ti/Au). We found that the RIE treatment changes the electrode property from Schottky to ohmic and significantly decreases the contact resistance [1].

Figures 3(a) and (b) show the forward current density-voltage (J-V) characteristics of two different Ga2O3 SBDs with n=3×1016 and 5×1016 cm-3, which were at different places on the same substrate. They showed excellent ideal factors of 1.04-1.06, indicating the high crystal quality of the Ga2O3 substrate and good Schottky interface property. On-resistances (Ron) were high, 7.85 and 4.30 m cm2, and they were larger than those of the state-of-the-art devices made from other semiconductors. However, these high values were simply because of the small conductivity of the substrate due to the low n. Therefore, they can be much improved simply by using the usual SBD structure consisting of an n--Ga2O3 epitaxial layer on an n+-Ga2O3 substrate. Figure 4 shows the reverse J-V characteristics. The reverse VBR values were about 150 and 115 V for n=3×1016 and 5×1016 cm-3, respectively, which were reasonably high for the n. Note that all the densities were simply derived by dividing the measured values by the area of the anode metal.

In conclusion, we fabricated Ga2O3 SBDs on a single-crystal -Ga2O3 (010) substrate. The devices showed good device characteristics such as an ideal factor close to 1.0 and reasonably high reverse VBR. These results indicate that Ga2O3 SBDs have comparable or even more potential than Si and typical widegap semiconductors SiC and GaN have for power device applications. This work was partially supported by NEDO and JST PRESTO programs, Japan. [1] M. Higashiwaki et al., Appl. Phys. Lett. 100, 013504 (2012), [2] K. Sasaki et al., Appl. Phys. Exp. 5, 035502 (2012).

978-1-4673-1164-9/12/$31.00 ©2012 IEEE 159

Fig. 1: Single-crystal 2-inch-diameter -Ga2O3 (010) substrate fabricated by the EFG method.

Table 1: Material properties of major semiconductors and Ga2O3.

Fig. 2: Cross-sectional schematic illustration of Ga2O3 SBD.

100 mm Pt/Ti/Au (Schottky)

Ti/Au (Ohmic)

Unintentionally doped -Ga O

(010) substrate 600 μm2 3

Si 4H-SiC GaN Diamond -Ga2O3

Band gap (eV) 1.1 3.3 3.4 5.5 4.8-4.9Electron mobility (cm2/(Vs)) 1,500 1,000 1,200 1,800 300Breakdown field (MV/cm) 0.3 3.0 3.3 10 8Dielectric constant 11.8 10 9.5 5.5 10Baliga's FOM (low freq.) 1 570 860 21,000 3,200

(a) (b) Fig. 3: Forward J-V characteristics of Ga2O3 SBDs in (a) single logarithmic and (b) linear plots.

0.0 0.5 1.0 1.5 2.0 2.5 3.00

50

100

150

200

Nd-Na = 5×1016 cm-3

Cur

rent

den

sity

(A/c

m2 )

Voltage (V)

Nd-Na = 3×1016 cm-3

0.0 0.5 1.0 1.5 2.0 2.5 3.010-9

10-7

10-5

10-3

10-1

101

103

Nd-Na = 3×1016 cm-3

Nd-Na =

5×1016 cm-3

Cur

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Voltage (V)

Lower limit ofmeasuring instrument

Fig. 4: Reverse J-V characteristics of Ga2O3 SBDs.

-200 -150 -100 -50 010-12

10-10

10-8

10-6

10-4

10-2

100

102

Nd-Na = 3×1016 cm-3

VBR = 150 V

Nd-Na = 5×1016 cm-3

VBR = 115 V

Cur

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(A/c

m2 )

Voltage (V)

Lower limit ofmeasuring instrument

978-1-4673-1164-9/12/$31.00 ©2012 IEEE 160