【Domestic News】Collection of Gallium Oxide Reports by Major Academies and Universities at the "2023 Power and Optoelectronic Semiconductor Device Design and Integration Application Forum" (2)

Professor Zhang Hongliang， Xiamen University：Study on epitaxy and electronic structure of Gallium Oxide films

      Gallium oxide (Ga₂O₃) semiconductor has the advantages of ultra-wide band gap of 4.85 eV, high breakdown field strength, and large size substrates at low cost. It is expected to achieve higher voltage withstanding, lower loss and higher efficiency. Low-cost, large-size single-crystal substrates can be prepared by melting method. Gallium oxide semiconductor has attracted great attention in high-power devices and solar blind ultraviolet photovoltaic detection applications.

      Recently, in the "2023 Power and Photoelectric Semiconductor Device Design and Integration Application Forum" held in Xi'an, Professor Zhang Hongliang of Xiamen University brought the theme report of "Study on epitaxy and electronic structure of wide-band Gallium Oxide films”. The report introduced the development status of gallium oxide materials, and focuses on the research progress in gallium oxide film epitaxy, doping and defect mechanism, and electronic structure of surface interface.

      At present, in view of the broad development prospects, gallium oxide materials and devices have become the world's leading science and technology frontier and national strategic competition. For example, Japan's Ministry of Economy, Trade and Industry (METI) has invested more than $100 million in the past five years to support the development of gallium oxide semiconductor materials. The Defense Department / Air Force Laboratory joint Cornell / UCSB and Enterprises established the Gallium Oxide Materials and Devices Research Division in 2012. The Leibniz Institute for Crystal Growth (IKZ) in Germany began developing Ga₂O₃ crystals in 2009, realizing 4-inch crystals and supplying epitaxial substrates for US Air Force laboratories.

      From the perspective of gallium oxide semiconductor industry chain, single crystal substrate and epitaxial thin film are the key material basis of power and photoelectric devices, and their quality directly determines the device performance. High quality gallium oxide epitaxial film and defect and doping regulation are the key to device applications. At present, foreign countries have realized the preliminary mass production of 4-inch single crystal via EFG method and the breakthrough of 6-inch crystal billet. China has gradually achieved 4-inch crystals.

      In terms of epitaxial progress of gallium oxide film, MOCVD, MBE, HVPE and other technologies are used to grow gallium oxide film. Regulation of the carrier concentration in the range from 10¹⁵ to 10²⁰ cm^-3 was achieved by doping like Si and Sn. For the problem of gallium oxide film epitaxy, the report believes that most of the epitaxial research work focuses on (010) plane as substrate; however, (100), (001) plane are more likely to obtain large-size substrate，but there are defects, high dislocation density, slow growth rate and other problems. The epitaxial films with low background carrier concentration and low defect density need to be further developed.

      The report introduces the equipment and characterization technology, as well as the research results of Si and Sn doped gallium oxide electronic structure. The Si and Sn doping of gallium oxide epitaxial film carriers from3×10¹⁷ cm^-3 to  2.6×10²⁰ cm^-3 by pulsed laser deposition and molecular beam epitaxy technology respectively. Among them, the 1% Si-doped Ga₂O₃ film achieves the highest conductivity of 2520 S/cm and the highest carrier concentration of 2.6×10²⁰ cm^-3 reported in current literature reports. Combined with synchrotron radiation hard X-ray photoelectron spectroscopy (HAXPES) and density functional theory (DFT) calculations, the report illustrates that Si is the most preferred n-type dopant of gallium oxide, and proposes a theoretical model of Si resonance doping mechanism. The report clarifies defects and doping mechanisms and proposes a suppression strategy with a very small amount of impurities / defects.

      On the other hand, the research group also conducted a systematic study on the surface interface electronic structure of gallium oxide film. It is found that the upward band bends up to 0.8 eV on the gallium oxide surface. The reason is that the bottom energy of the conduction band formed by the Ga 4s orbit is relatively high, and low gallium oxide work function (3.2 eV only). The upward band bending challenges the Ga₂O₃ Ohm contact. Si heavy-doped gallium oxide films can effectively alleviate the Schottky barrier formation.

      The results on the electronic structure of epitaxy, defect and doping mechanism and surface interface have some guiding significance for the development of gallium oxide materials and optoelectronic devices.

Wang Yifei, Xidian University: Research based on the performance regulation of gallium oxide material and high performance solar blind ultraviolet photodetector

      During the Forum, on “Optoelectronic devices and their applications”, Wang Yifei, a doctoral student from the School of Microelectronics of Xidian University, delivered a keynote report on "Research based on the performance regulation of gallium oxide material and high performance solar blind ultraviolet photodetector”. Ultra-wide band gap semiconductor gallium oxide has attracted wide attention in recent years because of its high breakdown field strength, good stability and natural solar blind band gap. However, there are still some problems in practical applications, such as the effective acquisition p-type gallium oxide semiconductor and the insufficient response of gallium oxide-based solar-blind ultraviolet optical electric detectors to ultraviolet light.

      The report shares progress in gallium oxide material performance regulation and gallium oxide daily blind detectors. The report points out that the co-doping method of electron-deficient element and N element will effectively reduce the position of the main energy level, realizing the p-type gallium oxide; Transition metal doping regulated gallium oxide. While revealing the formation energy of transition metal element doped gallium oxide and the law of introducing defect energy level, it is also found for the first time that I_B and II_B group metals will be introduced into the AX center, which is a new reason to hinder the acquisition of p-type gallium oxide; The properties of low-dimensional gallium oxide materials, including electrical properties, optical properties, mechanical properties and thermal properties, and uniaxial strain, external electric field and impurity atomic passivation, to provide theoretical guidance for the future preparation of high-performance low-dimensional gallium oxide based devices; The report for the first time proposed the use of organic polymers to regulate gallium oxide photodetector, whose response increased by 36%, improving the response characteristics to solar blind ultraviolet light; The report explores the effects of doping of different In components, realizes gallium oxide thin film transistors with high mobility (μ_sat= 3.63 cm²/V·s) and low subthreshold amplitude (SS= 1.38 V / decade), and further obtains a photodetector with high response characteristics to solar blind ultraviolet light; The report builds a In₂O₃/IGO heterojunction transistor to obtain a heterojunction transistor with higher carrier mobility (μ_sat= 10.22 cm²/V·s), providing a device basis for future photoelectric detection applications; The report expounds the interface characteristics of gallium oxide and different two-dimensional material systems, which enriches the gallium oxide interface theory and provides theoretical guidance for the preparation of high-performance gallium oxide / two-dimensional material-based photodetectors.

Dun Shaobo, CETC 13^th Institute: Development progress and thinking of gallium oxide power devices which withstand high voltage

      During the Forum, on “Power Semiconductor Device Design and Integrated Applications”, Dun Shaobo, senior engineer of the key laboratory of the 13th Institute of CETC, brought a keynote report entitled "Development progress and thinking of gallium oxide power devices which withstand high voltage".

      The 13^th Research Institute of China Electronics Technology Group Corporation is the comprehensive semiconductor research institute with the earliest, the largest scale, strong technical force and supporting professional structure in China. It mainly involves microelectronics, optoelectronics and microechanoelectronic systems (MEMS). The report points out that the lack of P-type doping technology is still a major bottleneck problem facing the development of gallium oxide power devices, and the development of new terminal structures and seeking alternative solutions is one of the international research hotspots. Gallium oxide diode is relatively more mature, small size diode progress rapidly, some of whose performance has exceeded the theoretical limit of SiC materials, showing certain advantages in high pressure and low conduction characteristics.

Huo Xiaoqing, CETC 46^th Institute: Large-size Gallium Oxide Single Crystal Material Technology for Gallium Oxide Power Devices

      During the Forum, Huo Xiaoqing, senior engineer of the 46th Research Institute of China Electronics Technology Group, made a keynote report on "Large-size gallium oxide single crystal material technology for gallium oxide power devices".

      Ga₂O₃ has five kinds of crystalline phases: α, β, γ, δ, ε, which will have a phase transition between each other under specific conditions, among which β-Ga₂O₃ is the most stable phase. β-Ga₂O₃ can be grown by melt method, with the advantages of large size, low defects, low cost, and it is the most likely to realize the industrialization of ultra-wide band gap semiconductor materials. According to Japanese Fuji Economy predicts by 2025, the market size of Ga₂O₃ power devices will reach about 70 billion yen, with substrate and extension accounting for 50% of the total scale, and it is estimated to be 35 billion Japanese yen, or about 25 million US dollars. China's "14th Five-Year" overall plan and each special plan also put Ga₂O₃ semiconductor materials into the development focus.

      From the development of gallium oxide at home and abroad, in 2014, a 2-inch Mg-doped gallium oxide single crystal was successfully grown by CZ method. This method was not suitable for high-doped N-type crystals, and the cost of iridium crucible was high. EFG method can achieve large size gallium oxide single crystal growth, and can achieve low resistance, high resistance doping, and fast growth rate, but iridium crucible costs high. The VB method uses the intermediate frequency induction heating / resistance heating method, using platinum and rhodium crucible, to achieve 2~3 inches (100) / (010) / (001) plane gallium oxide single crystal growth. It is worth looking forward to achieving high resistance, low resistance doping, with relatively low cost. In 2022, C & A Japan and Tohoku University jointly developed a 2-inch gallium oxide substrate by cold crucible method. It was the first gallium oxide single crystal prepared without precious metal method in the world though the technology is not yet mature.

      CETC 46^th Institute has successfully prepared China's first 6-inch gallium oxide single crystal, reaching the highest level in the world. The report introduces the research progress of CETC 46^th Institute, including the achievement of preparation on (100), (001), (0101), (-201) plane substrates. Gallium oxide crystal growth is very reproducible, is expected to achieve low resistance, high resistance doping, and maintain high crystallization quality. The quality of gallium oxide substrate processing is good, with clear atomic steps and high flatness, which can be supplied in small batches. At the end of 2021, the 46^th Institute took the lead in breaking through the HVPE gallium oxide homogeneous epitaxial technology, and became the only unit that prepared the 2-inch HVPE β-Ga₂O₃ homogeneous epitaxial wafer in China.

      The design simulation of HVPE thermal field flow field and process regulation leads to the high precision control of gallium oxide epitaxial surface flattening, obtaining the HVPE gallium oxide epitaxial with high crystallization quality and high surface quality and good uniformity. The current system can realize effective and controllable Si microdoping, and the uniformity of Si doping with the thickness is good, and the carrier concentration of the epitaxial layer can be regulated at 5E15cm^-3～1E18cm^-3. According to the report, in recent years, gallium oxide has made rapid progress in substrate and epitaxial growth, presenting a situation where multiple methods are advancing side by side.

      The application direction of gallium oxide single crystal materials has been continuously expanded, and the performance of gallium oxide-based devices has been greatly improved. The breakdown field strength of SBD and MOSFET far exceeds the theoretical limit of SiC, GaN and other materials, and better performance can be expected in the future. However, due to its structural particularity, there are still many challenges in the substrate and epitaxial growth process.