Widespread adoption of β-Ga₂O₃ power devices is expected to lead to energy savings for inverters used in industrial machinery, residential photovoltaic power generation systems, and next-generation EVs. Our recent achievement will overcome the issue of the high cost of epitaxial deposition of β-Ga₂O₃ and lead to epitaxial deposition equipment capable of mass-producing large-diameter and epitaxial wafers.

1. Overview
Gallium oxide (β-Ga₂O₃)¹⁾ has a larger bandgap energy²⁾ than silicon carbide (SiC)³⁾ or gallium nitride (GaN) ⁴⁾, and transistors and diodes made from this material are expected to have excellent power-device ⁵⁾ characteristics, such as high breakdown voltage, high output, and high efficiency (low loss). Japan leads the world in the development of β-Ga₂O₃ power devices, and Novel Crystal Technology, Inc. has already developed small-diameter 4-inch β-Ga₂O₃ epitaxial wafers⁶⁾ using the halide vapor phase epitaxy (HVPE) method in 2021.⁷⁾ We currently manufacture and sell these wafers.⁸⁾

Unlike SiC and GaN wafers, β-Ga₂O₃ wafers are advantageous for lowering the price of power devices because they can be manufactured in the melt growth method in which bulk crystals grow quickly and it is easy to obtain a large-diameter wafer. However, while the HVPE method has a low raw-material cost and provides a high-purity deposition, the current equipment can make only small diameter (2 inches or 4 inches) single wafers. This problem means there is great demand for equipment capable of mass-producing large-diameter (6 inch or 8 inch) wafers in batches in order to reduce the epitaxial deposition cost involved in the HVPE method.

We have been working with Taiyo Nippon Sanso and Tokyo University of Agriculture and Technology to develop epitaxial deposition equipment for mass-producing large-diameter β-Ga₂O₃ wafers in NEDO’s “Strategic Energy Conservation Technology Innovation Program⁹⁾ / Large-diameter mass-produced gallium oxide for next-generation power devices” project. In particular, we developed external supply technology¹⁰⁾ for the metal chloride¹¹⁾ that is the raw material of the HVPE method during the “incubation” research and development phase of this program in FY2019. Moreover, we set up 6-inch single-wafer HVPE equipment and evaluated it to establish the basic technology of mass production in the practical development phase in FY2020-2021. This year, we succeeded in depositing β-Ga₂O₃ on a 6-inch wafer for the first time in the world.

2. Current achievements
Taiyo Nippon Sanso, Tokyo University of Agriculture and Technology and Novel Crystal Technology have developed a 6-inch single-wafer HVPE equipment (Fig. 2) and succeeded in depositing β-Ga₂O₃ on a 6-inch test wafer (using a sapphire substrate) for the first time in the world (Figure 1).
In addition, we demonstrated β-Ga₂O₃ epitaxial deposition on a 6-inch test wafer and achieved a film thickness distribution of ± 10% or less and furthermore confirmed that uniform epitaxial deposition is possible (Fig. 3) by optimizing the epitaxial deposition conditions and using a unique raw-material nozzle structure. These achievements will greatly advance the development of the large-diameter-wafer batch-type mass-production equipment. It is estimated that the energy saving effect of the β-Ga₂O₃ epitaxial deposition process and mass adoption of devices will be about 210,000 kL/year by 2030.

Fig. 2 Photograph of the developed 6-inch single-wafer HVPE equipment
for β-Ga₂O₃ epitaxial deposition

Fig. 3 Film thickness distribution of β-Ga₂O₃ on a 6-inch test wafer

3. Future plans
Taiyo Nippon Sanso, Tokyo University of Agriculture and Technology and Novel Crystal Technology will continue to develop mass-production equipment for β-Ga₂O₃ epitaxial deposition in the NEDO project and perform epitaxial film deposition using 6-inch β-Ga₂O₃ wafers and β-Ga₂O₃ thin film. We will develop high-quality β-Ga₂O₃ epitaxial deposition technology by evaluating the electrical characteristics and defects existing in the film and aim to commercialize the equipment in 2024. β-Ga₂O₃ epitaxial wafers manufactured by the HVPE method are mainly used for SBDs¹²⁾ and FETs¹³⁾, and the market is expected to grow to about 59 billion yen by 2030.¹⁴⁾ By entering the market with our mass-production equipment and popularizing β-Ga₂O₃ power devices, we will provide energy savings for next-generation EVs and other power applications.

[Notes]

1 Gallium oxide (β-Ga₂O₃)
Gallium oxide is a compound semiconductor that is attracting attention as a third wide bandgap semiconductor for power devices following the adoption of silicon carbide and gallium nitride. Its theoretical performance as a power device is overwhelmingly higher than that of silicon and surpasses that of silicon carbide and gallium nitride.

2 Gallium nitride (GaN)
GaN is a compound of gallium and nitrogen and is a wide-bandgap semiconductor with a more stable bond structure and higher dielectric breakdown strength than SiC. It is mainly used for small and high-frequency applications such as switching power supplies.

3 Silicon carbide (SiC)
SiC is a compound of carbon and silicon and is a wide-bandgap semiconductor material mainly used for high-breakdown-voltage and high-current applications.

4 Bandgap energy
The energy required for electrons and holes to transition from the valence band to the conduction band. A semiconductor with a large value is called a wide-bandgap semiconductor, and the larger the bandgap, the higher the dielectric breakdown strength. The bandgap of β-Ga₂O₃ is about 4.5 eV, which is larger than that of Si (1.1 eV), 4H-SiC (3.3 eV) or GaN (3.4 eV).

5 Power device
A semiconductor element used for power conversion in power inverters and converters.

6 β-Ga₂O₃ epitaxial wafer
A wafer with a thin film of β-Ga₂O₃ deposited on the wafer substrate. Crystal growth is performed on seed crystals that will become the wafer. Growth in which the atoms of the thin film are aligned to the crystal plane of the underlying wafer is called epitaxial growth.

7 Halide vapor phase epitaxy (HVPE) method
A crystal growth method that utilizes metal chloride gas as a raw material. Its advantages are that high-speed growth and high-purity epitaxial deposition are possible with it.

8 4-inch β-Ga₂O₃ epitaxial wafers now available; opportunities for manufacturing and sales
Novel Crystal Technology, Inc. News release dated June 16, 2021
High-quality 100-mm β-type gallium oxide epitaxial wafers have been developed.

9 Strategic energy-saving technology innovation program
Summary at https://www.nedo.go.jp/english/activities/activities_ZZJP_100039.html

10 External supply technology
In the HVPE method, this technology separates the metal chloride generation part and the epitaxial deposition part and supplies metal chloride from the outside of the reactor through piping.

11 Metal chloride
A compound that is a raw material in the HVPE method: typical metal chlorides include GaCl, GaCl₃, AlCl, AlCl₃, InCl, and InCl₃.

12 SBD
Schottky Barrier Diode. A diode that utilizes a junction made of a certain metal and an n-type semiconductor instead of a PN junction. Its advantages are higher efficiency and faster switching than other diodes.

13 FET
Field Effect Transistor (FET). A transistor that controls the current between source and drain electrodes by applying a voltage to a gate electrode to vary the density of electrons or holes in the channel region.

14 Quoted from Fuji Keizai
Quoted from Fuji Keizai “2020 Current Status and Future Prospects of the Next Generation Power Devices & Power Electronics Related Equipment Market”