【Domestic Papers】Development Status and Prospects of Gallium Oxide Materials

      Ultra-wide band gap semiconductor materials, with their advantages of large Bandgap, high breakdown electric field and low energy consumption, have great application prospects in high-precision and cutting-edge fields such as ultra-high voltage power electronic devices, radio frequency electronic transmitters, radiation detectors, quantum communication and extreme environment applications, and can make up for the deficiencies of existing semiconductor materials. In the military field, it is expected to be applicable to power control systems for high-power electromagnetic guns, tanks, fighter jets and ships, as well as radiation-resistant and high-temperature resistant aerospace power supplies, which can significantly reduce the system loss of weapons and equipment, reduce the volume and weight of thermal and cooling systems, and meet the requirements of military equipment for miniaturization, lightweighting, speed, radiation-resistant and high-temperature resistance. In the civilian sector, it is expected to be used in areas such as power grids, electric traction, photovoltaics, electric vehicles, medical equipment and consumer electronics to achieve greater energy conservation and emission reduction effects, and is considered an important foundation for the rapid development of multiple fields including information, energy, transportation, manufacturing and defense in the future. In August 2022, the Bureau of Industry and Security (BIS), U.S. Department of Commerce announced export controls on two types of ultra-wide band gap semiconductor materials, Gallium Oxide (Ga₂O₃) and Diamond, because "equipment using these materials significantly enhances military potential", believing that Ga₂O₃ and diamond "can work faster, more efficiently, for longer periods and under harshness conditions. It could change the rules of the game in both business and military ".

The Current Development Status of Gallium Oxide Materials

      Gallium Oxide power devices can meet the demands of future power systems for high withstand voltage and low power consumption of power electronic devices, and become the most powerful competitors in the next generation of power electronic devices. To advance the research on Ga₂O₃ materials and devices, Japan, Germany, the United States and other countries have adopted a series of research programs on Ga₂O₃materials and devices. In 2014, the National Institute of Advanced Industrial Science and Technology (AIST) launched the "Commercial Development of β-Ga₂O₃ Schottky Barrier Diode" project under its Strategic Energy-saving Technology Innovation Program, aiming to promote the research and commercialization of Gallium Oxide power devices. In order to further enhance the performance of radio frequency devices and power switch devices in military radars, electronic warfare and communication systems, the United States Air Force Research Laboratory has approved a 27-month research program on the preparation of Ga₂O₃ single crystal materials. In January 2016, the Defense Advanced Research Projects Agency of the United States Department of Defense launched a research program related to the epitaxial growth technology of Gallium Oxide materials. The proposal of this program was based on the fact that the electromagnetic railguns, air defense radar systems and DDG-51 destroyer propulsion systems equipped on future naval warships all require high-voltage and efficient power converters to achieve the required power density. According to statistics, other countries around the world have invested more than 10 million US dollars each year in the research and development of Gallium Oxide materials and devices from 2016 to the present. In September 2023, the US Department of Defense announced the implementation of the Microelectronics Sharing Program, listing "high-voltage Gallium Oxide power switches" as key technologies of priority concern. Japan is also accelerating the industrialization of Gallium Oxide, with several leading Japanese enterprises entering the Gallium Oxide field.

      Gallium Oxide single crystal can be grown by the liquid-phase melt technique and has a relatively low hardness. It has advantages in both material growth and processing costs. Ga₂O₃ single crystal can be prepared by various melt methods, including the Floating Zone method, Czochralski method, EFG method, Vertical Bridgerman method, casting method, Cold Crucible method, etc. The 4-inch Ga₂O₃ single crystal substrate produced by the EFG method has been commercialized. The EFG method, Vertical Bridgerman method and casting method have achieved technological breakthroughs in 6-inch Ga₂O₃ single crystal substrates. The Czochralski method and Cold Crucible method have also successfully prepared 2-inch Ga₂O₃ single crystal. The EFG method, casting method and Czochralski method require expensive iridium crucibles. The future large-scale commercial development of Ga₂O₃ may be limited by cost. The Vertical Bridgerman method, Cold Crucible method and Floating Zone method do not require expensive iridium crucibles, and the cost of single crystal growth is expected to drop significantly.

      Research on the growth of Gallium Oxide single crystal in Japan and Germany started relatively early. From 2000 to 2006, scholars from Japan and Germany were the first to successfully prepare β-Ga₂O₃ single crystal by using the Czochralski method and the EFG method. Novel Crystal Technology, Inc. of Japan, a leader in the production of β-Ga₂O₃ substrates, has dominated the market for large-sized, high-quality Ga₂O₃ wafers and now holds more than 90 percent of the global substrate market share. Novel Crystal Technology, Inc. achieved its first breakthrough in 2-inch wafer substrate technology in 2012. In 2018, it mass-produced 4-inch β-Ga₂O₃ wafers through the EFG method. In 2022, it broke through the preparation technology of 6-inch β- Ga₂O₃ wafers and announced in 2024 that it could prepare 6-inch β-Ga₂O₃ single crystal substrates using the lower-cost Vertical Bridgman method. The Leibniz Institute for Crystal Growth (IKZ) in Germany has achieved the fabrication of 2-inch Gallium Oxide wafers using the Czochralski method.

      Compared with countries such as the United States, Japan and Germany, China started its research on Gallium Oxide substrates, materials and devices relatively late and is generally in the stage of catching up. In recent years, with the support of relevant ministries and commissions, some device indicators have reached the international leading level, but there is still a gap in Gallium Oxide materials. Although the Gallium Oxide field in China is developing rapidly at present and many enterprises have emerged, the overall industrial chain is still not complete enough, and the engineering and industrialization maturity of "single crystal - epitaxial - device - equipment" is still not high enough. In addition to the continuous support and guidance from the state, it is also necessary for all fields including academia, investment and industry to cooperate sincerely, work together towards the industrialization goal of Gallium Oxide, and carry out iterative optimization.

      A number of domestic research institutions have carried out a series of studies on Gallium Oxide materials and devices, including Shandong University, Tongji University, Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, the 46^th and 13^th Research Institute of China Electronics Technology Group Corporation, Peking University, Jilin University, Xidian University, Nanjing University, Beijing University of Posts and Telecommunications, Sun Yat-sen University, Fudan University, etc. CETC's 46th Research Institute produced 2-inch β-Ga₂O₃ wafers in 2016, developed 4-inch β-Ga₂O₃ wafers in 2018, and successfully produced 6-inch β-Ga₂O₃ wafers in 2023. Shandong University successfully prepared a 6-inch Gallium Oxide substrate by using the EFG method. In 2022, Zhejiang University successfully grew 2-inch β-Ga₂O₃ single crystal by casting method. A 4-inch β-Ga₂O₃ single crystal was fabricated in 2023. The growth of 6-inch β-Ga₂O₃ single crystal will be broken through in 2024. In July 2024, Hangzhou GAREN SEMI Co., LTD., an incubated enterprise of Zhejiang University, made a breakthrough in the technology of Gallium Oxide crystal growth and substrate processing, successfully preparing a 3-inch wafer-level (010) Ga₂O₃ single crystal substrate, which is the largest size reported internationally to date. In October 2024, Hangzhou GAREN SEMI Co., Ltd. successfully grew a 2-inch Gallium Oxide single crystal based on its independently developed Gallium Oxide dedicated crystal growth equipment and by using the Vertical Bridgerman method. This was the first technological breakthrough achieved in China. Hangzhou Fujia Gallium Technology Co., Ltd. and Beijing MIGSEMI Co., Ltd. possess 4-inch single crystal growth technology and have commercialized 2-inch single crystal substrates. A team from the University of Science and Technology of China, in response to the challenge of the lack of effective P-type doping in Gallium Oxide, which makes it difficult to achieve enhanced vertical structure transistors, has developed a KV-level Gallium Oxide vertical slot gate transistor by optimizing the post-annealing process to achieve nitrogen substitutional activation and repair lattice damage. This achievement was presented at the 36th International Sympoisum on Power Semiconductor Devices and Integrated Circuits, providing new ideas for realizing high-performance Gallium Oxide transistors for applications.

      At present, the breakthrough in the industrialization technology for the preparation of 6-inch and above Gallium Oxide single crystal substrate materials without iridium requires the mutual coordination of equipment and processes. The equipment (including the thermal field and flow field) should be optimized according to the process, and then the process should be optimized based on the equipment. Only through multiple iterations can the technical maturity level of the material be elevated to a higher stage. In addition, ultra-wide band gap semiconductors such as Gallium Oxide and Diamond are confronted with the problem of difficult bipolar doping, making it hard to manufacture homogeneous bipolar devices to simultaneously meet the requirements of high current and high voltage carrying. Adopting the heterojunction strategy and constructing p-Diamond/n-Ga₂O₃ diodes with good interface and band matching is an ideal combination for achieving high-performance ultra-wide band gap bipolar diodes, which is conducive to giving full play to the application advantages of ultra-wide band gap semiconductors in advanced power electronic devices. The research teams from Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Zhengzhou University, Nanjing University and Harbin Institute of Technology worked closely together and successfully fabricated p-Diamond/n-Ga₂O₃ heterojunction diodes with breakdown voltages exceeding 3,000 V through the regulation of heteroepitaxial interfaces and the optimized design of device structures. This work provides a manufacturing solution for ultra-wide band gap semiconductor heteroPN junction diodes that feature high withstand voltage characteristics, low on-resistance and efficient thermal management strategies, which will further promote the development of ultra-wide band gap semiconductors in the field of power devices.

Development Directions and Challenges

      Gallium Oxide materials possess outstanding properties such as an extremely wide Bandgap, ultra-high critical breakdown field strength, and radiation resistance, making them a key semiconductor for promoting the miniaturization and lightweighting of power devices. They will first emerge in the medium and high-voltage markets with low entry barriers and high cost sensitivity, such as consumer electronics, home appliances, and industrial power supplies. Although the Gallium Oxide single crystal material is relatively easy to prepare and process, it is still in the initial research stage in the field of electronic devices. There are still many key technical and scientific problems that need to be solved urgently, such as the easy cracking of large-sized single crystal, the strong self-heating effect caused by the low thermal conductivity of the material, the difficulty in P-type doping, the low carrier mobility and the numerous heterogeneous interface defect, etc. The performance of the device is still far from the theoretical value.

      The key research directions for future development mainly include: growth of large-sized and high-quality Gallium Oxide and Diamond single crystal substrate materials, epitaxial growth and performance regulation of semiconductor films such as Gallium Oxide and Diamond, N-type and P-type doping and efficient activation of ultra-wide band gap semiconductors, mechanisms and methods for improving carrier mobility, and fabrication and heterogeneous integration of high-performance ultra-wide band gap semiconductor devices, etc.

Policy Recommendations

      1) We will balance development and security, launch special support programs, increase investment intensity, promote systematic basic research guided by national strategies, and comprehensively and systematically plan ultra-wide band gap semiconductors

      2) Explore the use of credible third-party institutions to drive, connect and organize innovation forces from all sides, and adopt flexible mechanisms that can constantly adjust the form of organization according to the situation and tasks

      3) Through innovation policies and science and technology finance, develop systematic innovation capabilities, realize the pull of national credit on the R&D chain, industrial chain and capital chain, and guide and integrate social resources to be invested in applied basic research

Conclusion

      At present, the world is experiencing a new round of electrification. The rapid deployment of emerging applications such as transportation and big computing power facilities has created an urgent need for more efficient and powerful power processing capabilities, which has become a new driving force for the development of the next generation of power electronics systems. Therefore, it is necessary to promptly lay out forward-looking, systematic and application-oriented research on the next-generation strategic electronic materials represented by ultra-wide band gap semiconductor materials, as well as their devices, circuits and systems. The material growth and device development of ultra-wide band gap semiconductors are still in their initial stages. Developed countries are all actively investing their efforts in this new direction of development in the semiconductor field.