【Specialist Intro】Ou Xin —— the Member of Technical Expert Committee

Introduction

Ou Xin, secondary researcher in Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences, director of silicon-based materials and integrated device laboratory, doctoral supervisor, National Leading Talent, major project leader of the National Key Research and Development Plan, the Fund Committee, IEEE Senior Member. On the basis of silicon-based SOI material technology for the important demand of 5G acoustic, optical and electric core chips for heterointegrated materials, his research lies in the development of "universal ion knife" heterointegrated XOI materials and device technology, and applied to radio frequency, photoelectric and power chip. He has published more than 140 papers on SCI / EI, among which, with his name as the first / corresponding author, 110 papers were published in famous journals such as Nature Com.,PRL、Adv. Mater.、Nano Lett.、Light、Optica、Appl. Phys. Rev. and so on. More than 170 patents have been applied, more than 70 of which have been authorized, and more than 40 achievements have been transformed. As the first chairman of the conference, he launched the first Domestic RF Filter Innovation Technology Conference, and formed an important innovation forum in this field. In recent years he has won many award and prizes, such as China Youth Science and Technology Award, First Prize of Beijing Science and Technology Progress Award, China Industry-University-Research Cooperation Innovation Award, Ten Optical Advances in China, China Semiconductor Research Top Ten Progress Nomination Award, Science and Technology Progress Award of China Electrotechnical Society, Award of the National Disruptive Innovation Technology Competition, the International IEEE Microwave Award, International Award for Outstanding Contribution to Ion Beam and Material Modification IBMM Prize, The International Young Scientist Award for Ion Injection Technology, Annual Research Award of the HZDR Research Center, Helmholtz, Germany, etc.

Related Information

The bandwidth of β-Ga₂O₃ is 4.8 eV, equivalent to more than 4 times that of Si, and Baliga’s figure-of-merit is as high as 3444, which is about 10 times that of SiC and 4 times that of GaN. Therefore, the devices developed based on Ga₂O₃ will have smaller conduction loss, higher power conversion efficiency and higher breakdown voltage, which can be applied to higher power devices. Meanwhile, as a semiconductor material that can be prepared by melting method, the low cost of large size wafer and controllable n-type doping are also important advantages. However, the thermal conductivity of Ga₂O₃ crystal material is very low, only 10~27 W / mK, less than one-twentieth of SiC, one fifth of Si, which makes it not effective enough to dissipate the heat produced in the working process by the device based on Ga₂O₃ material, producing serious self-heating effect. And the high channel temperature will make the substrate insulation deterioration. These factors will lead to decreased stability of β-Ga₂O₃ devices and reduce their competitiveness in the field of power and RF. So, it is of great significance to integrate β-Ga₂O₃ thin films into other highly thermal conductive substrates to solve the thermal dissipation of β-Ga₂O₃ materials and devices. Our team is committed to providing solutions to the serious thermal dissipation problem of the gallium oxide power device through a heterointegrated approach.

Introduction of the team

The team includes 2 researchers, 3 postdocs, and 15 doctoral students. In cooperation with Han Genquan Team of Xidian University, the team transferred β-Ga₂O₃ to the high thermal conductivity SiC and Si substrate and optimized the quality and characterised the quality of the heterointegrated β-Ga₂O₃ film. The overall inhomogeneity was lower than ±1.8%, and the film surface RMS roughness was 0.2 nm. Given the decrease of the thermal and electrical performance of heterointegrated β-Ga₂O₃ films caused by ion injection, the thermal and electrical performance of hetero-integrated β-Ga₂O₃ films is greatly improved by high-temperature annealing technology, and the improved film quality, thermal and electrical are close to the quality of uninjected β-Ga₂O₃ bulk material. High-performance MOSFETs and SBDs power devices were prepared based on heterointegrated β -Ga₂O₃ films, which proved the excellent heat dissipation characteristics of high thermal conductivity heterointegrated β-Ga₂O₃ wafers. The equivalent thermal resistance of GaOSiC SBD is only less than 1/4 that of GaO Bulk SBD.

Achievement

In 2019, in cooperation with Professor Han Genquan, Academician of Hao Yue's team at Xidian University, we used ion beam stripping and transfer technology to realize the first heterogeneous integration of wafer grade β-Ga₂O₃ single crystal film and high thermal conductive silicon and silicon carbide substrate in the world, and prepared 2 inch β-Ga₂O₃ / Si and β-Ga₂O₃ / SiC heterointegrated materials. Compared with the devices based on the homogeneous β-Ga₂O₃ substrate, the thermal stability of the heterointegrated β-Ga₂O₃ device was significantly improved, and the relevant results were published in IEDM, the top conference in the field of microelectronics. After this, we have been systematically investigating the properties of heterointegrated β-Ga₂O₃ materials and devices. In terms of materials, starting from the stripping mechanism, we realized β-Ga₂O₃ stripping and transfer by single H injection, calculated the activation energy of β-Ga₂O₃ stripping transfer (2.28 eV) using the thermal kinetic model, accurately predicted the relationship between β-Ga₂O₃ foaming time and temperature, calculated the utilization rate of stripping H⁺ is only 9%, and most of H is captured by defects caused by injection and remain in β-Ga₂O₃, and could not be dissociated at low temperature. In addition, we also found that He and H co-injection in β-Ga₂O₃ caused a large number of empty volume defects that were difficult to migrate in vivo, and it was difficult to realize the stripping of the film. For bonding, we have achieved stripping of β-Ga₂O₃ single crystal films by SAB bonding and hydrophilic bonding. After optimizing the ion injection procedure and bonding conditions, we have achieved the stripping and transfer of the β-Ga₂O₃ single crystal film with 4 inch, see Figure 3. At present, we can prepare heterogeneous β-Ga₂O₃ films with different thicknesses in the range of 200-1000 nm, while the remaining single crystal β-Ga₂O₃ substrate can be reused to achieve multiple transfer. This greatly reduces the cost of our process in the situation where β-Ga₂O₃ single crystal substrate price is expensive in the current.

Figure 1  Preparation method of heterogenous β-Ga₂O₃ thin films

Ion injection will inevitably produce defects in the heterogeneous β-Ga₂O₃ film, which reduces the electrical and thermal properties. We restore the thermoelectric performance of β-Ga₂O₃ by repairing the defects caused by the ion injection and dispersing the H⁺ in the film. At 900℃, The defects in the film were repaired. The swing curve of the annealed β-Ga₂O₃ film was 80 arcsec. The stress peaks caused by ion injection in the film disappear. By The high resolution XRD, the tensile stress in the film decreased from 0.382% to 0.043%; We extracted the thermal dissipation ability of heterogeneous β-Ga₂O₃ films before and after annealing and compared it with SiC and β-Ga₂O₃ bulk materials; Moving β-Ga₂O₃ to SiC substrates by heterointegration methods greatly improves the thermal dissipation capacity of heterogeneous β-Ga₂O₃-based materials. After annealing, the thermal conductivity of the heterogenous β-Ga₂O₃ single crystal film is doubled to 0.093 W / cmK, which is close to the thermal conductivity of the single crystal material (0.13 W / cmK), and the thermal conductivity of the annealed film with temperature is consistent with that of the single crystal block; In addition, the thermal resistance of the interface decreases to 1/3 after the annealing, which results from the recrystallization of the interface. In conclusion, after annealing, the quality and thermal performance of the film are greatly improved.Figure 2  Comparison of material quality and thermal properties before and after annealing of heterogeneous β-Ga₂O₃ films

We prepared Schottky diodes (SBDs) on Si and SiC substrates respectively, with a switch ratio of 10¹¹ and an open-state resistance of 6.7 mΩ∙cm². As the temperature increases, the device characteristics stabilize, with no significant change in the open-state current when the temperature rises to 150℃; while the open-state current of homoepitaxy-based SBDs decreases by 40%. With the help of infrared thermal imaging technology, we intuitively observed that the surface temperature of SBDs based on β-Ga₂O₃ / SiC heterointegrated materials at the same power is significantly lower than that of β-Ga₂O₃ body material devices, and the equivalent thermal resistance of β-Ga₂O₃ / SiC heterogeneous SBDs is 43.55 K / W, which is only 1 / 4 of β-Ga₂O₃ body material (188.24 K / W), indicating that the thermal dissipation of β-Ga₂O₃ devices can be effectively improved by integrating with high thermal conductivity substrate.

Figure 3 Excellent thermal stabilization of heteroSi, SiC, β-Ga₂O₃ SBDs

Message from he Expert

As a new generation of semiconductor materials, gallium oxide has broad prospects in the field of power electronic devices with the significant advantages such as a wide bandwidth, high breakdown electric field, high quality wafer availability and controllable n-type doping and so on. The preparation of power electronic devices based on gallium oxide not only needs to improve its Baliga figure of merit through various terminal structures, so that it constantly approaches its theoretical limit, but also pay special attention to the thermal dissipation problem caused by high power, which is particularly important for gallium oxide with low thermal conductivity. At present, our team has now improved the heat dissipation performance of the device through heterogeneous integration. The future research direction will mainly focus on two aspects: on the one hand, optimize the structure of β-Ga₂O₃ devices to further improve the thermal dissipation capacity of the devices. Combined with the packaging method,  realize the efficient cooperative thermal dissipation from inside to outside of the β-Ga₂O₃ device. On the other hand, optimize the process parameters to achieve the quality of the heterogeneous gallium oxide films, so as to realize the collaborative design of electrical and thermal properties of gallium oxide devices. Finally, we would like to thank Professor Han Genquan of Xidian University, Professor Tadatomo Suga of Star University of Japan, Professor Sun Huarui of Harbin Institute of Technology and Professor An Zhenghua of Fudan University for their help! May the Alliance grow stronger and all colleagues work together to realize the industrialization of gallium oxide as soon as possible.