【Expert Interview】Prof Tao Xutang, The industrialisation of Gallium Oxide should be quality-oriented, blooming, and the future can be expected!

Personal Resume

      Tao Xutang, a native of Xinjian County, Jiangxi Province, is a professor at Shandong University. He received his Ph.D. degree in engineering from Tokyo University of Agriculture and Technology in 1995. In 2002, he was appointed an extraordinary professor of “Chang Jiang Scholars Program” of the Ministry of Education, won the 2003 National Science Fund for Distinguished Young Scholars, and won the 2007 Innovative Research Group Fund of the Foundation Committee as an academic leader. He was the director of the Institute of Crystal Materials, Shandong University, and the State Key Laboratory of Crystal Materials. He is also the director of the Chinese Ceramic Society, the director of the Chinese Crystallographic Society, the deputy director of the Crystal Growth Committee of the Chinese Ceramic Society, the deputy director of the Solid Defect Professional Committee of the Chinese Physical Society, and the member of the sixth and seventh Science and Technology Committee of the Ministry of Education. Academic member of State Key Laboratory of Silicon Materials, Zhejiang University, State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, etc. He has been engaged in the application research of various crystal materials and devices for a long time.

Q：Mr. Tao, would you please introduce your current team?

A：

      Our team has gradually grown up under the guidance of academician Jiang Minhua. Following the discipline proposed by Academician Jiang, “Demand-driven, Single-crystal Oriented, Dedicated and United, Educating Crystals and People,” we carry out the growth and application exploration of various crystal materials. Adhere to the multi-dimensional, full-chain development concept in the bulk crystals, two-dimensional crystals, single crystal fiber, micro nanocrystals and other aspects are involved. Our research group not only has a variety of crystal growth methods, such as Solution Method, CZ, EFG, Descent Method, Micro-pulling-down, Laser-heated Pedestal Growth, etc., and the related crystal growth equipment is designed by us and manufactured in cooperation with relevant domestic manufacturers. At the same time, we have also developed some new growth methods, such as micro-spacing in-air sublimation, compressed flux growth, etc., and the relevant procedures have been adopted by many units domestically and internationally. In addition, we are at the international leading level in the manufacture of single-crystal optical fiber equipment and the research of single-crystal optical fiber. We also research drug crystal control. Now, our team has all kinds of crystal growth equipment and related talents, and we are involved in all aspects of crystal materials research.

      It should also be added that crystals exhibit some unique properties, such as homogeneity and anisotropy, due to their long-range ordered structure. As a medium for transforming various energy forms such as light, electricity, sound, magnet, force and heat, functional crystals are the primary materials in the modern information age. The four significant inventions of the twentieth century, the transistor, the integrated circuit, the laser, and atomic energy, are closely related to crystal. One might think that nuclear power has nothing to do with crystal. Still, one of the critical materials studied in laser inertial confinement fusion is a nonlinear optical crystal called KDP. Therefore, although it takes a long time to grow crystals, the research is still significant, so academician Jiang Minhua often said that “crystals are not only beautiful but also useful.”

Q：What made you start working on Gallium Oxide?

A：

      I engaged in crystal research relatively early, influenced by Mr. Jiang. In 1983, after my undergraduate graduation, I entered the master’s program and began to conduct related research as Mr. Jiang’s graduate student. Later, I went to Japan to study for about 11 years. After returning to China, I reorganized my team with the support of Mr. Jiang. The research on Gallium Oxide can be said to have started in 2004. At that time, our country had a significant project: developing heat-capacity lasers. One of the critical materials is doped “GGG,” that is, doped “Gadolinium Gallium Garnet,” that is, Gallium Oxide and Gadolinium Oxide are combined to form a garnet structure, that is, a cubic crystal structure. At that time, the domestic development level was less than 3 inches, while the United States had reached 6 inches, and the project team required it to reach or exceed the level of the United States. Two years were given to develop the project. At that time, I just returned to China, and I was relatively young and dared to work, so I boldly accepted the project. After taking over the project, I found the equipment was not up to the requirements. At first, I wanted to spend a lot of money to buy French equipment, but later, because of the laser weapons, the other side did not sell us. From then on, we decided to build our equipment. And I think making our equipment is the way we have to go. Some of the equipment purchased abroad is not so good and expensive. We independently developed the CZ single-crystal furnace through this project and grew “GGG” crystals close to 8 inches. Why do we start with this crystal? That is because it contains Gallium Oxide components, and when doing “GGG” research, we have encountered a series of problems with Gallium Oxide, such as easy to volatilize and decompose at high temperatures and more trouble when seed crystal. In particular, the issues of high-temperature decomposition of Gallium Oxide and floating matter on the melt surface are encountered when the crystal is grown using the CZ method. Therefore, it also makes us the first team in China to use EFG to do Gallium Oxide, which we have also learned from the experience and lessons of growing “GGG” by the CZ method. When the EFG grows the crystal, it needs a mold, equivalent to having a lid, so that the seed crystal does not need to be put into the melt. The melt passes through the small hole of the mold due to the capillary effect on the top of the mold, and the growth starts from the mold, which avoids many problems. At first, some domestic units, such as the Shanghai Institute of Optics Fine Mechanics, were the first to research Gallium Oxide in China, using the optical floating zone method, and the long crystal is relatively tiny. However, various approaches are being tried as wide bandgap semiconductors become more popular. We took the lead in adopting EFG growth in China, and later, many units also turned to guide mode. I have been calling on everyone to “hundred blossoms” and not stick to one method because it is still difficult to say which method of growing Gallium Oxide will win out in the end, that is, be recognized by industrialization, so more attempts must be made.

Q：There are many ways to grow crystals of Gallium Oxide; what do you think are the advantages and disadvantages of each?

A：

      Each method has its advantages and disadvantages. However, we have a lot of crystal growth methods, such as CZ, EFG, Descent Method, Skull Melting Method, optical Floating Zone Method and so on. The current domestic and international maximum size is 2 inches, and no reports of larger crystals exist. The CZ technique is relatively mature, and columnar crystals can be obtained, which is conducive to cutting wafers with different orientations and provides more options for device research. However, it is difficult for Gallium Oxide to obtain larger crystals due to high temperature volatilization, decomposition and floating materials on the surface. In the growth of crystals by descent method, the temperature gradient is relatively small, and the growth in a sealed container can avoid the problem of volatilization of Gallium Oxide. The crystals grown using the descent method reported in Japan have relatively few twins, and the crystallinity is better. However, no one has used the descent method for Gallium Oxide crystal growth in China, and it is worth trying. However, the metal rhodium will enter the crystal; this problem still needs to be solved. There are domestic and international methods of growing Gallium Oxide using the skull melting method, which does not require Iridium and is theoretically cheaper. However, the size and quality of the growing crystals still need to be improved. Before the pandemic, Iridium was less expensive, being hundreds of thousands of a kilo. After the 2019 pandemic, the price of Iridium went up a lot, which significantly increased the cost. This is a very negative thing for Gallium Oxide. Initially, when Gallium Oxide and Silicon Carbide were compared, the price advantage was obvious; of course, this situation would change with the availability of Iridium. Gallium Oxide is the only wide bandgap semiconductor that can be grown by melt method. The characteristic of growing crystals by the melt method is that they are fast. Silicon carbide is mainly grown by the gas method, which is relatively slow and relatively challenging to expand in diameter, unlike the melt method, which allows for a shoulder growth process. The gas method can only grow a little wider than the seed crystal, and the diameter expansion can only be carried out once and for a long time. The casting method proposed by Academician Yang Deren does not use the EFG mold, and the amount of Iridium is relatively tiny. The advantage of the casting method is that it is relatively simple, and it is a casting process that slowly cools down after melting. Problems remain, such as the inability to use seed crystal directional growth. Gallium Oxide's growth can also be considered a hydrothermal method, but there is no report on this.

      As a semiconductor material, doping is also a significant problem. N-doped EFG can be solved, but p-doped is not a real solution. I used to tell some units in China that we still need to explore different methods when we don’t know which method will win—for example, the sapphire growth method--Kyropoulos Method. Sapphires can now grow very large in tons. Logically speaking, Gallium and Aluminum belong to the same family of elements, and the oxide difference is not so big. Is it possible to grow Gallium Oxide using the Kyropoulos method? Of course, the growth method can not be copied entirely, but it is still worth exploring. The domestic trend is that people tend to rush in the same direction, which is a problem. Now, there are many startups in China after introducing capital; they will pay special attention to the size issue. I believe that it is necessary first to improve the quality. If the quality is poor, it is useless to grow bigger. To achieve the industrialization of Gallium Oxide, everyone needs to calm down and seriously improve the quality of the crystal. Based on the improvement of crystal quality, then increase the size and reduce costs. This is the development path for the industrialization of Gallium Oxide. If we only focus on improving the size without considering quality, we deviate from the right path.

      Gallium Oxide has the advantages of a wide bandgap, great value for BFOM, etc., which will benefit energy saving. Still, we must admit that Gallium Oxide thermal conductivity is relatively low, but I think this is not an unsolvable problem. Using thinning, flip device structure and other methods can reduce the impact of heat. In addition, the on-resistance of Gallium Oxide is relatively low, and its heat will not be as high as that of silicon-based devices. A data-processing center based on a silicon-based device currently consumes a lot of power to dissipate heat. If Gallium Oxide is used in the future, it will generate much less heat. In my opinion, the development of Gallium Oxide will still face various problems in the future, and we need to make multi-directional efforts to overcome these difficulties down-to-earth. The Japanese descent method has grown to 6 inches, but the problem of Rhodium entering the crystal in the platinum-rhodium alloy also needs to be solved. It is essential for everyone to first focus on growing high-quality crystals and then, based on various high-quality substrates, proceed with the development of epitaxy, devices, and applications research. This is the necessary path for the industrialization of Gallium Oxide.

Q：Future industrialization needs large-size, high-quality, and low-cost crystals; which method is more likely to be suitable for the road of industrialization?

A：

      Many times at academic conferences, it has been said that the EFG is appropriate if the quality, size, and doping of the crystal are considered comprehensively.

      One reason is its maturity; the other is its superior crystal size and quality. Moreover, it is currently the most popular. If the descent method can solve the problem of Rhodium entering the crystal, it is also a good choice because its temperature gradient will be relatively lower. We must find a way to solve the problem. People now use platinum-rhodium alloy, and Rhodium will grow into a crystal lattice. How do we solve this problem? Is there a more suitable crucible material? This also needs to be considered. You can also try to do this.

       In addition, the casting method proposed by Academician Yang Deren can also continue to be explored. Casting has developed rapidly because of its relative simplicity. Academician Yang’s research group quickly grew 2-inch and 4-inch crystals by casting method and recently reported 6-inch crystals, which quickly developed. As a semiconductor material, it also needs to be uniformly doped, and as a low-symmetry Gallium Oxide, it also needs to choose the best crystal direction. More research is required to solve these problems.

      I think that from the perspective of crystal quality, size, doped and so on, the EFG is better. At present, the biggest problem is that Iridium is too expensive and the cost is too high, and a new method can be tried for the crucible. Is it possible to try to make a pot with other metals and coat it with a thin layer of Iridium to reduce the amount of Iridium and cost? Another problem is that many crystal growth units in our country store a large amount of Iridium in tons; if the existing Iridium is used, it is enough for Gallium Oxide. As far as the industry is concerned, everyone believes that the direction of power devices is better, whether, in electric vehicles, high-voltage power transmission and transformation, electromagnetic ejection, etc., some scenarios requiring large voltage and large current are the direction of Gallium Oxide application. If the Gallium Oxide crystal can be grown to 10 kg or more extensively, it can play a more outstanding market value. With Gallium Oxide, many medium- and low-voltage devices can be made very small, so the consumption is more petite and compact, which aligns with the development trend. When we do Gallium Oxide, we must first consider the quality and then think about the market. We should not engage in internal conflicts domestically but discover issues from an international perspective to find solutions. Currently, in the growth of Gallium Oxide crystals, it is worth considering the low symmetry of the β-phase, which leads to specific growth challenges. While we focus on Gallium Oxide research, we should not restrict ourselves to β Gallium Oxide, as other crystal forms of Gallium Oxide are equally worth exploring.

Q：Which way of school-enterprise cooperation is more conducive to the implementation and helpful to the Gallium Oxide industry?

A：

      Building an industry, especially the semiconductor industry, takes a long time. For school-enterprise cooperation, it is hoped there can be some visionary capital attention, investing in businesses from their growth stage. In terms of the starting point, it still requires like-minded individuals. Alliances should also focus on long-term development because Gallium Oxide has its advantages but requires time to grow. There are few Gallium Oxide startups, which is a good thing. I hope everyone can focus on improving technology. If we were to rate the difficulty level of Gallium Oxide substrates, epitaxy, devices, and so on from 1 to 100, the difficulty of substrates should be above 60. To truly excel in substrates, it is necessary to do well in various substrates first, then move on to epitaxy and devices, making the foundation more solid.

      It is hoped that everyone will earnestly tackle problems and overcome various technical difficulties to achieve the industrialization of Gallium Oxide. It is necessary to make plans for at least ten years. For capital, the future market advantage of Gallium Oxide is still undeniable. When using different materials to make the same device, everyone is targeting high-voltage or high-power devices. Once the quality of Gallium Oxide is improved and the price comes down, there will be good prospects for devices in the medium and low voltage range. Because for the same output power, it can be tiny, with low losses, which also aligns well with the energy-saving and emission reduction direction advocated in our current society. The key still lies in those growing the crystal; they must not forget their original intentions and persist in tackling tasks diligently.

Q：Expectations and Suggestions for Alliance.

A：

      Sharing domestic and international information, especially about scientific research achievements, trends, and automotive-grade devices, will boost the confidence of domestic researchers and investors. The messages sent externally should be more realistic and focus on addressing the issue of Gallium Oxide.

      If you want to develop the industry, it is hoped that the alliance will mainly serve as a bridge.

1、Sharing domestic and foreign information significantly advances scientific research, research dynamics, trends, industry, capital, etc.

2、Exchange at international Gallium Oxide conferences, dissemination of information, etc. We cannot just work hard in isolation; we must also examine the global situation and consider the road ahead.

      Organizing more exchange meetings between organizations to facilitate communication is a positive direction. After accumulating knowledge through a study period and gradually gaining experience, it is necessary to unite everyone to organize and convene meetings, as communication is essential. Moreover, it serves as good publicity for the alliance. I will offer full support for the future meeting of the Gallium Oxide, aiming to develop Gallium Oxide, which is crucial. In our industry, there is a need for passionate and committed individuals to undertake this task.

Q：Suggestions for standardization of Gallium Oxide in China? When do you think is a better time to start this work?

A：

      Standardization is something that needs to be done, and this is a critical task. Please organize and collaborate with universities and research institutes to promote this work, such as developing Gallium Oxide testing standards, evaluation standards, equipment standards, etc. As far as I know, there are currently no relevant standards for Gallium Oxide in China. I believe we can take the initiative and research some foreign standards first. We should check if more advanced countries in Gallium Oxide have already standardized. We can use these references to improve our industry standards. As a member of the Crystal Standards Committee, I will strongly support the initiation of standardization development in the future. I suggest that everyone take action together, communicate more, create an outline first, and then delve deeper into it.

Q：What are your suggestions and advice on the talent pool launched by the Alliance?

A：

      The establishment of a talent pool is necessary for future industrial implementation. Currently, not many talents in the country specialize in Gallium Oxide, and few units are dedicated to talent cultivation, which needs to be strengthened. Talent needs to flow and exchange. Establishing a talent pool is of great help to enterprises and research institutions interested in the field of Gallium Oxide. Many visionary companies have already started making notable investments and targeting training in Gallium Oxide. In the future, we also hope to collaborate with such companies.

      We are all in the same boat in Gallium Oxide, and we should treat Gallium Oxide as a career. We need to establish and strengthen the industry’s foundation rather than making superficial efforts to seek rapid economic growth. I often tell young people that as researchers, it is very fortunate to encounter a good topic from research to application in a lifetime. Gallium Oxide may be a topic worth everyone’s efforts.