【Expert Interview】Xiamen University of Zhang Hongliang：Need patience and perseverance to treat Gallium Oxide, make it becomes a "material" into a "device"

      In 2023, the Asian Gallium Oxide Alliance (hereinafter referred to as"AGOA") expanded a new channel, —— Expert Interviews. It is provided by the governing units, member units and expert committee within the Alliance to explain their different opinions on the field of gallium oxide, leading us to know, understand, and explore the world of gallium oxide.

      This time, we have the honor to interview Professor Zhang Hongliang of Xiamen University, one of the governing units of the Alliance, to share his unique views on gallium oxide as a material.

Expert Profile

      Zhang Hongliang, a professor at Xiamen University, is a national high-level "young" talent. He received his PhD in inorganic chemistry from Oxford University in 2012, his master’s degree in Physics of Condensed Matter from the National University of Singapore in 2008, and his bachelor's degree from Shandong University in 2003. From 2012 to 2017, he worked as a postdoctoral fellow at the Pacific Northwest National Laboratory in the United States and University of Cambridge. His research interests include wide band gap oxide semiconductor thin film epitaxy, electronic structure regulation and optoelectronic devices. So far, he has published more than 150 papers in Phys. Rev. Lett., Nat. Commun., Adv. Mater., J. Am. Chem. Soc., Phys. Rev. B., etc., and applied for 15 patents. He has presided over 10 projects of National Key Research and Development Plan, National Natural Science Foundation of China, and enterprise cooperative research and development. He has won the Herchel Smith Research Fellowship of Cambridge University and the Best International postgraduate Research Award of Taiwan Semiconductor Manufacturing Company (TSMC).

AGOA：Could you briefly introduce the research work of yourself or your research group?

      The research work of our team mainly focuses on oxide semiconductor materials and their device applications. These materials and devices have important applications in TV, mobile phone touch screen, new energy vehicles, 5G communications and other fields used in our daily life. Specifically speaking, our research direction is mainly in the following two aspects: (1) wide band gap Gallium Oxide semiconductor and photoelectric devices; (2) Development of P-type Oxide semiconductor materials.

(1) Our team is now focusing on wide band gap Gallium Oxide semiconductor materials and photodetectors.

       Gallium Oxide has an ultra-wide band gap width of 4.9 eV, which is higher than the third-generation semiconductor Silicon Carbide (SiC) and the Gallium Nitride (GaN). The wider band gap means that electrons need more energy to transition from valence band to conduction band, so Gallium Oxide has the characteristics of high withstand voltage, high efficiency, high power, anti-irradiation and so on. Power and photodetector components based on Gallium Oxide have important applications in new energy vehicles, charging piles, rail transit, missile warning, smart grid detection and other fields. As a rising star in semiconductor field, Gallium Oxide has received strong attention from governments, academics and industry. The United States and Japan successively issued policies in 2022 to ban the export of Gallium Oxide single crystal and thin films to China. Based on this background, our work mainly focuses on the key scientific and technical issues related to the growth of high-quality Gallium Oxide epitaxy films, the regulation of photoelectric properties, defect and doping mechanisms for the development of Gallium Oxide based optoelectronic devices.

(2) Our other work is the development of P-type oxide-semiconductor materials.

       The wide band gap oxide semiconductor has the advantages of transparency, stability, large area preparation, etc., and is widely used in today's booming flat panel display, touch screen, photovoltaic and other industries. However, most of the oxide-semiconductors are N-type, and the lack of matching P-type oxides greatly restricts the development of new devices with higher performance and more functions. The development of P-type oxide-semiconductors will greatly promote the innovation of optoelectronics technology. Our work is to solve the problem of low mobility of P-type oxide-semiconductors, and to develop novel P-type oxide-semiconductor materials through the strategy of "band structure regulation with top of valence band".

AGOA：Research on the current difficulties encountered by the team, or the difficulties recently solved

      For the research and development of Gallium Oxide semiconductor materials, its industrial chain generally includes single crystal substrate, epitaxial film, device production and other links. Among them, the preparation of single crystal and epitaxial film are the key basic materials for the preparation of devices, whose quality directly determines the performance of devices. Therefore, in order to realize high-performance Gallium Oxide power electronic devices, it is necessary to prepare Gallium Oxide epitaxial thin films with low defect density, high crystal quality and high mobility (including the Gallium Oxide substrate). This is also an urgent problem that colleagues in the field of Gallium Oxide materials are currently facing. Gallium Oxide material itself is prone to forming various defect, and its crystal structure also has great anisotropy, which brings many challenges to the growth and processing of single crystal substrate and subsequent epitaxy. It is necessary to integrate the industrial chain and work together with substrate - processing - epitaxy - device to tackle key problems. On the one hand, our team cooperated with Gallium Oxide enterprises to carry out process research and development; On the other hand, we also make full use of the research and development advantages of the university team to carry out basic research on defect and doping mechanism, especially systematic research on key scientific issues such as the defect chemistry, doping physical mechanism and microscopic electronic structure of Gallium Oxide epitaxy films, hoping to make breakthroughs from the basic source.

      Gallium Oxide semiconductors have inherent material advantages in Solar-blind deep ultraviolet photodetection applications, and our team is also carrying out research on Gallium Oxide based Solar-blind ultraviolet photodetectors. Solar-blind UV photodetection has important applications in the military and civil fields such as corona detection of power grid, deep ultraviolet sterilization and disinfection, missile warning and so on. At present, the response performance of Gallium Oxide detectors has been very high. However, the response time of the detector is still relatively slow, which may be caused by factors such as defect inside the material, insufficient optimization of the interface or device structure. Our team also developed fast response detector device by preparing high-quality materials, optimizing device interface, building heterojunction and other strategies. In addition, the development of detectors needs to be integrated with specific applications. According to the specific application scenario to develop relative to the performance of the indicator detector.

      On the other hand, for most oxide semiconductors (such as Gallium Oxide, Zinc Oxide, Tin Oxide, etc.), the lack of corresponding P-type doping materials greatly limits the development of devices. Although there are many reports about P-type oxide materials in the literature, there are still many problems in their reliability, stability and mobility. The internal reason limiting P-type oxide semiconductor materials is that the top electron state of oxidation valence band is mainly composed of strongly localized O 2p electron states, resulting in low hole mobility and high activation energy. The electronic structure of oxide itself is the internal source of P-type doping, and it is also a difficulty in the field. Our research group has been engaged in this research for a long time. A strategy we have recently proposed is to modulate the top of the oxidation valence band by introducing elements containing the electronic state of Sn 5s² or Bi 6s² to obtain high P-type mobility.

AGOA：Optimistic about the development potential of Gallium Oxide and future application fields

      The ultra-wide band gap of Gallium Oxide materials makes it have very good potential applications in power electronic devices and Solar-blind photodetectors, especially in high voltage and high-power electronic devices. On the other hand, compared with Gallium Nitride and Silicon Carbide, Gallium Oxide can be prepared by melt method with large substrate size, which may greatly reduce the material preparation cost. There have been many reports about the potential and applications of Gallium Oxide materials in many media and professional journals, and I will not go into details. However, the Gallium Oxide semiconductor industry is still in the early stage of the industry, and there are still many key scientific and technical problems to be overcome, and it is very necessary for scientific research and industry to continue to work together to overcome them. It requires a certain amount of patience and perseverance to treat Gallium Oxide, so that it becomes a "material" into a "device". A semiconductor material from the appearance to the real application is a long iterative process. For example, the current hot SiC, from the concept of SiC to the commercialization has also experienced more than 30 years of development. Finally, I would like to add here that Gallium Oxide is relatively easy to obtain highly conductive, deep ultraviolet transparent films or substrates by N-type doping. I think that Gallium Oxide may also have some potential applications in this regard.

AGOA：How to view the role of university-enterprise cooperation and industry-university-research combination in the development of Gallium Oxide industrialization

      After nearly ten years of scientific research and joint efforts of the industry, Gallium Oxide semiconductor materials and devices have been made great progress, and breakthroughs have been made in many aspects, such as the realization of 6-inch substrates, and the successful development of high withstand voltage power electronic devices. However, compared with mature compound semiconductors such as GaAs, GaN and SiC, the current Gallium Oxide industry is still in its initial stage, and there are still many basic scientific problems to be solved in the whole industry chain of Gallium Oxide from single crystal substrate, thin film epitaxy and devices. School-enterprise cooperation and industry-university-research collaboration are certainly the most suitable mode for the current development stage of Gallium Oxide. In terms of scientific research projects, universities can better play the advantages of talent team and equipment platform, as well as the analysis of basic scientific problems and the output of original technologies. Universities and enterprises can jointly tackle the key problems encountered in the process of industrialization, which can reduce the economic and time costs of enterprises to a great extent, and also improve the competitiveness of core technologies. On the other hand, I think university-enterprise-industry-university-research cooperation is also more conducive to the cultivation of talents. Especially in the semiconductor industry, there are many basic scientific problems from materials to devices that need to be solved. That's how you train people to solve real problems in the industry. Of course, how to ensure the efficient cooperation between enterprises, universities and research institutes really requires the both sides to explore the most suitable cooperation mode.

AGOA：Whether there is a research and development project that needs financial support

      There are many R&D projects that require funding. For example, the epitaxy of Gallium Oxide films I mentioned just now, the research on defect and doping mechanism, and the solar blind detector need financial support to speed up the advancement from the material basis to the device and then to the application.

AGOA：Comments or suggestions on the work of the alliance

      Our alliance has gathered experts, scholars and industry leaders in the field of Gallium Oxide domestic and international and has played a huge role in promoting basic research of Gallium Oxide, industry-university-research cooperation and exchanges, and industrialization process. I would like to express my heartfelt thanks to the Alliance and hope that the Alliance will continue to do better and better and be in the ascendant. Our team will also continue work together with the alliance, maintain communication with colleagues and jointly promote the development of Gallium Oxide.