【Member News】Research progress in Gallium oxide materials and power devices by Mr. He Yunlong and his team from Xidian University

Progress of Gallium Oxide Materials and Power Devices

HE Yunlong, HONG Yuehua, WANG Xichen, ZHANG Zhouning, ZHANG Fang, LI Yuan, LU Xiaoli, ZHENG Xuefeng, MA Xiaohua

School of Microelectronics, Xidian University, Xi’an 710071, China

Abstract: Gallium oxide (Ga₂O₃) is expected to become the main force in the future semiconductor power electronics field with its advantages of large forbidden band width, high breakdown field strength and strong radiation resistance. Compared with the common wide band gap semiconductor SiC and GaN, Ga₂O₃ has advantages of superior Baliga’s figure of merit and lower cost in growth, and more potential for high voltage, high power, high efficiency, and small size electronic devices. The latest domestic and international research on Ga₂O₃ epitaxial materials, power diodes and power transistors are summarized, and the future applications and development prospects of Ga₂O₃ are anticipated.

Keywords: Ga₂O₃; epitaxial material; power diode; power transistor

1     Introduction

      With the implementation of the national strategy of "carbon peak, carbon neutrality", energy conservation, emission reduction and improvement of conversion efficiency are important development directions in the energy field at present. Power semiconductor devices are the core of power conversion. In order to achieve this strategic goal, power devices with super-high power, high conversion efficiency, and small size have become a research hotspot. At present, after decades of continuous optimization of chip manufacturing, device structure, packaging and application technology, the first generation of semiconductor devices represented by Si and Ge have approached or reached the physical limit of materials, and the space for future development is limited. Therefore, relying on the existing Si-based power devices has been unable to meet the working needs of power semiconductor devices for miniaturization and high efficiency, finding and developing semiconductor materials with excellent performance has gradually become the main direction of researchers at home and abroad. The ultra-wide band gap semiconductor material represented by gallium oxide (Ga₂O₃) has ultra-wide band gap (about 4.8 eV) and ultra-high critical breakdown field strength (about 8 MV/cm), so it has the advantages of high breakdown voltage and high output power, and has become one of the hotspots of current research.

      Compared with the common wide band gap semiconductors GaN and SiC, the mobility of Ga2O3 materials is not high, but its ultra-high breakdown field strength and fast electronic saturation speed, make it has great advantages in the field of power electronics with low on resistence under high voltage. At present, there are 6 known crystal phases of Ga₂O₃ materials, among which β-Ga₂O₃ has the highest thermal stability and the best single crystal growth quality, so it is the best choice for device preparation. As to the power electronic devices prepared based on β-Ga₂O₃, the Baliga’s figure of merit is 4 times that of GaN devices, 10 times that of SiC devices, and 3444 times that of Si devices. Therefore, under the same operating voltage, the β-Ga₂O₃ device has lower on resistance and less power consumption, which can greatly reduce the power loss of the device. In addition, the preparation process of β-Ga₂O₃ single crystal substrate is similar to sapphire substrate, which can be directly obtained by metal melting method. At present, the 4-inch Ga₂O₃ single crystal substrate process is relatively mature, and the cost is expected to be reduced to one-third of the SiC substrate of the same size, so it has great potential in aspect of low cost. Compared with GaN power devices, β-Ga₂O₃ power devices have higher reliability without current collapse; and they have higher breakdown field strength than GaN and theoretically have higher output power. Compared with SiC power devices, β-Ga₂O₃ power devices have a simple fabrication method and a planar structure can be used, which is easier to integrate. On the other hand, the thermal conductivity of β-Ga₂O₃ is low, which also becomes a bottleneck restricting the development of β-Ga₂O₃ devices. Therefore, how to improve the heat dissipation effect of the device has become a key problem to be solved urgently.

      Ga₂O₃ is considered to be a promising material for the next generation of high power, high efficiency and low power consumption power supply system. Developed countries such as Europe countries and the United States have listed it as the next generation of strategic semiconductor materials, and carried out a series of research. This paper summarizes the latest research progress in Ga₂O₃ materials, power diodes and power transistors at home and abroad, hoping to provide a reference for the research of Ga₂O₃ material and its devices.

2   Ga₂O₃ Denitaxial material

      The Ga₂O₃ material epitaxial technology mainly includes Hydride Vapour Phase Epitaxial (HVPE), Metal-Organic Chemical Vapor Deposition (MOCVD) and Molecular Beam Epitaxial (MBE), etc, among which, HVPE is the mainstream way of material growth at present, with large size, low defect density, low growth cost. However, due to a monoclinic crystal system of Ga₂O₃, its surface roughness is relatively higher and requires Planarization treatment. Meanwhile, it is not easy to control the epitaxial thickness due to its high growth rate. At present, the mainstream research institutions are mostly in Japan, South Korea and other countries. In 2014, Tokyo University of Agriculture and Technology in Japan adopted GaCl₂ and O₂ as the precursor source, and epitaxial on the (001) crystal orientation of Ga₂O₃ substrate by HVPE growth method, the FWHM of the rocking curve of the (002) crystal orientation is 90", and the background impurity carrier concentration is as low as 10¹³cm^-3[1]. In 2019, Chonnam National University of South Korea introduced Pd nanoparticles and used the HVPE method to grow α phase Ga₂O₃ films on sapphire, with a FWHM of its rocking curve 879" and a roughness 4.98 nm^[2]. In 2022, Soongsil University of South Korea grew α phase Ga₂O₃ film with HVPE on sapphire. The XRD rocking curve of the epitaxial material is shown in Figure 1, and the FWHM of the rocking curve of the epitaxial Ga₂O₃ film is 73". And a MOSFET device was made on this material, with its breakdown voltage up to 2300 V, indicating that the quality of Ga₂O₃ grown by HVPE was significantly improved ^[3].

Figure 1 The XRD rocking curve of the epitaxial material

      Compared with HVPE growth method, the thickness of epitaxial material grown by MBE is accurate and controllable, but the method has low growth efficiency and cannot meet the growth requirements of large size epitaxy, which is not the mainstream method in the industry. Even so, it is still the main method of growing high-quality epitaxial materials. In 2020, the Leibniz Institute in Germany tried to improve the growth rate using metal indium assisted metal exchange catalysis through a homogeneous epitaxial Ga₂O₃ film layer on the (100), with a growth rate up to 1.5 nm/min and a roughness (RMS) 0.3 nm^[4]. In 2020, Cornell University used the novel S-MBE method to epitaxy Ga₂O₃ films on sapphire and Ga₂O₃, respectively, and the epitaxial growth rate on sapphire was 1.6 µm/h, and the epitaxial growth rate on the Ga₂O₃ substrate is 1.5 µm/h. The rocking curve of the film has a FWHM of 71" and a roughness of 0.7 nm ^[5]. In 2022, the research team introduced In₂O and SnO, which improved the growth rate of Ga₂O₃ film, and the FWHM of its rocking curve was 10.3" and the roughness was 0.25 nm ^[6], as shown in Figure 2, which is the best level of Ga₂O₃ growth through MBE.

Figure 2 Exaxial material test results by Cornell University

      The MOCVD epitaxial growth method combines the advantages of both HVPE and MBE. It can not only obtain a large film size, but also effectively control the growth rate, which is an effective way to develop Ga₂O₃ epitaxial growth into industrialization. Due to the late start of this method, the quality of grown materials needs to be improved. In 2018, the University of California reported Ga₂O₃ films with ultra-high electron mobility using the MOCVD growth method with 176 cm²/(V·s) at room temperature, and 3481 cm²/(V·s)^[7] at 54 K. In 2021, Jilin University carried out the MOCVD homoepitaxial Si-doped Ga₂O₃ thin film on the substrate of (100), and realized controllable doped^[8] with doping concentration from 6.5×10¹⁶cm^-3 to 2.6×10¹⁹ cm^-3 ; In 2022, By optimizing the growth process, Ga₂O₃ film material was achieved, with the carrier doping concentration as low as 3.6×10¹⁶ cm^-3, an electron mobility 137 cm²/ (V·s). Its FWHM of the (002) diffraction surface is 26.3", and the roughness of the film is 0.323 nm^[9]. As shown in Figure 3, this is currently the best quality Ga₂O₃ material grown by the MOCVD method.

Figure 3 Surface roughness of MOCVD epitaxial material by Jilin University

      With its excellent material performance, Ga₂O₃ attracts attention from researchers at home and abroad, and it is considered as the core of the next generation of power electronic devices. Therefore, power devices based on Ga₂O₃ materials have potential in the fields of high power, high temperature and strong radiation. Key research in Ga₂O₃ power devices focus on Ga₂O₃ power diodes and Ga₂O₃ power transistors.

3     Ga₂O₃ Power diode

      At present, Ga₂O₃-based power diode can be divided into Schottky structure, field terminal structure, fin groove structure and PN junction structure, etc., and the parameters to evaluate the main performance of the device include breakdown voltage, on-resistance, Baliga’s figure of merit, etc.

3.1   Schottky structure

      The conventional Ga₂O₃-based power diode is mainly composed of a Schottky contact anode and an ohmic contact cathode. In 2013, the Japanese Tamura company successfully prepared the first β-Ga₂O₃-based diode, as shown in Figure 4, with a breakdown voltage of 150 V^[10]. In 2015, National Institute of Information and Communication Technology (NICT) of Japan used HVPE method, and achieved successfully growth of low 7 µm of epitaxial layer (doping concentration of 1×10¹⁶ cm^-3), and prepared the diode based on this material. Through characteristic study of variable temperature C-V, I-V , they found that the forward turn-on current of the device conforms to the thermal electron emission mechanism, the reverse leakage current transmission conforms to the thermal field emission mechanism^[11].

Figure 4 Conventional power diodes made by Tamura, Japan

3.2   Field terminal structure

      It can be seen from section 3.1, there is still a big gap between the theoretical limit and the diode breakdown field strength using only conventional Schottky structure. Therefore, in order to obtain a higher breakdown voltage and Baliga’s figure of merit, the device needs to be developed and improved on the new field terminal technology. The field terminal technology mainly makes the electric field distribution of the device more uniform through the ion injection or metal electrode at the edge of the anode, so as to weaken the peak electric field at the edge of the anode electrode. In 2019, the National Institute of Information and Communication Technology in Japan injected nitrogen ions in the anode edge region of Ga₂O₃, and used the composite terminal structure of the field ring superimposed field plate to achieve a breakdown voltage of 1.43 kV and a on resistance ^[12] of 4.7 mΩ·cm². In 2021, the University of Utah prepared the field plate structure with double-layer high-k dielectric BTO / STO, whose on resistance is as low as 0.32 mΩ·cm², breakdown voltage is 687 V, and Baliga’s figure of merit exceeds 1 GW/cm². The on resistance is the lowest value reported^[13]. In 2022, Xidian University made the field terminal power diode by etching to form a groove structure and filling with silica dielectric. The breakdown voltage of the device is up to 6000 V, the on resistance is 3.4 mΩ·cm², and the Baliga’s figure of merit of the device is as high as 10.6 GW/cm^2[14].

      Although great progress has made on the Ga₂O₃ field terminal structure, the lack of P-type doping of Ga₂O₃ material seriously restricts the development of Ga₂O₃ power diode towards higher performance. The researchers tried to use P-type NiO_x instead of Ga₂O₃ to form a heterogeneous PN junction, which filled the regret of the loss of P-type Ga₂O₃ and has become the focus of current research. In 2020, CETC 13 took advantage of NiO_x natural P type and combined it with Ga₂O₃ to produce a Junction Barrier Schottky diode (JBS), whose structure is shown in Figure 5. The on resistance of the device is 3.45 mΩ·cm², the breakdown voltage is 1715 V, and the Baliga’s figure of merit exceeds 0.85 GW/cm^2[15]. In 2021, Xidian University made a groove-type JBS structural power diode, the breakdown voltage of the device reaches 1340 V, and the on resistance is as low as 1.94 mΩ·cm^2[16].

Figure 5 The JBS diode structure produced by CETC 13

3.3   Fin-type groove structure

      In 2018, Ga₂O₃ diode with fin groove structure was first prepared by Cornell University in the United States. The structure is shown in Figure 6. Al₂O₃ dielectric was deposited above the deep groove, and the breakdown voltage of the device is up to 2440 V and the on resistance is 11.3 mΩ·cm^2[17]. The advantage of the device is that the multiple fin grooves are used to separate the active region. The structure not only has a flat field terminal structure, but also adds the longitudinal field terminal, and the multiple fin grooves increase the heat dissipation area. In 2020, the research group used two-stage field plates to optimize the Ga₂O₃ diode with fin groove structure. The breakdown voltage of the device reaches 2890 V, the on resistance is 10.5 mΩ·cm², and the Baliga’s figure of merit of the device is as high as 0.80 GW/cm^2[18].

Figure 6 The Ga₂O₃ diode of the first fin-type groove structure

3.4   PN junction structure

      With the fully-fledged technology of NiO_x deposition, the researchers tried to combine high-quality P-type nickel oxide with N-type Ga₂O₃ to prepare a Ga₂O₃ diode with heterogenous PN junction structure, and replace the Schottky junction with PN junction to improve its pressure resistance characteristics. In 2020, Nanjing University deposited two layers of NiO_x with different concentrations by changing the oxygen atmosphere and chamber pressure, and made Ga₂O₃ PN junction diodes on this basis. The breakdown voltage is 1860 V, the on resistance is 10.6 mΩ·cm², and its Baliga’s figure of merit is 0.33 GW/cm^2[19]. In 2021, Xidian University made a PN junction diode through magnetron sputtering. The breakdown voltage of the device is 1220 V, its on resistance is reduced to 1.08 mΩ·cm², and its Baliga’s figure of merit reaches 1.38 GW/cm^2[20]. In the same year, USTC optimized the interface between NiO_x and Ga₂O₃ through post-annealing technology to reduce the interface state defect density, thus improving the performance of the device. After annealing, the breakdown voltage is increased from 900 V to 1630 V, the on resistance is reduced to 4.1 mΩ·cm², and the Baliga’s figure of merit is 0.65 GW/cm^2[21].

      Due to the excellent characteristics of the heterogeneous PN junction formed by NiO_x and Ga₂O₃, the diode performance of the PN junction structure has been greatly improved. Therefore, the researchers constantly combine the PN junction structure with the field terminal structure to improve the characteristics of Ga₂O₃ power diodes. In 2022, the NiO_x of the trapezoidal countertop was made by Nanjing University and combined with Ga₂O₃ to prepare heterogenous PN junction diodes. The breakdown voltage reaches 2230 V and the on resistance is 1.9 mΩ·cm^2[22]. In the same year, CETC the 13th Institute made the device combined the heterogeneous PN junction structure with the trapezoidal field plate, the breakdown voltage reaches 2410 V and the on resistance is as low as 1.12 mΩ·cm², and the Baliga’s figure of merit reaches 5.18 GW/cm^2[23]. In Xidian University, the heterogenous PN junction structure diode was fabricated by using the bilayer concentration of NiO_x, combined with various terminal technologies such as Mg ion injection field ring and SiO₂ dielectric field plate. The device structure is shown in Figure 7, N_A and N_D are the acceptor and the donor doping concentration respectively, and T stands for the thickness. The breakdown voltage of the device reaches 8320 V, the on resistance is 5.24 mΩ·cm², and the Baliga’s figure of merit reaches 13.2 GW/cm^2[24], which is the highest reported in the world.

Figure 7 Composite terminal structure of gallium oxide PN diode made by Xidian University

      Through the above research, it can be noted that using the heterogeneous PN junction structure is still one of the most effective methods to improve the Baliga’s figure of merit. Meanwhile, in order to improve the breakdown field strength of the device and make it closer to the theoretical limit, the field terminal technology is still indispensable. Therefore, the major direction of Ga2O3-based power diodes is still aiming at increasing its breakdown voltage and reducing its on resistance. Absolutely, with the continuous improvement of the device performance, the breakdown field strength of the device is constantly approaching its theoretical limit, and the thermal stability and reliability of the device will become a more important standard for consideration.

4   Ga₂O₃ Power Transistor

      Compared with Ga₂O₃ power diodes, the research on Ga₂O₃ power transistor is still relatively lagging behind, mainly because it is difficult to obtain the high quality epitaxial materials on the insulating substrate. At the same time, the size of high quality epitaxial materials is small, and there is less physical space for the improvement of device characteristics. Therefore, there are many problems unsolved in Ga₂O₃ power transistors, including low breakdown voltage, low current density, and it is difficult to prepare the enhanced mode device. The main research subjects covers the field terminal technology, vertical device technology and enhanced technology, etc. The main parameters to evaluate the device performance include breakdown voltage, on resistance, the Baliga’s figure of merit, etc.

4.1   Field Terminal Technology

      Similar to the Ga₂O₃ power diodes, the breakdown field strength of the Ga₂O₃-based power transistors is far below its theoretical limit. It is imperative to develop and optimize the field terminal technology to obtain a high breakdown field strength. In 2020, CETC the 13th Institute achieved Ga₂O₃-based MOSFET using optimized source field plate technology, with a structure as shown in Figure 8, the power figure of merit of the device is 277 W/cm², reaching the world's leading level^[25]. In the same year, the Institute studied the influence of oxygen atmosphere annealing on the device power performance, which is also the first report in China on power performance of Ga₂O₃-based MOSFET. The device output power P_out is 0.4 W/mm at 1 GHz CW, the power added efficiency (PAE) is 10%,  the gain is 3.2 dB^[26].

Figure 8  Ga₂O₃-based MOSFET made by CLP 13 Institute

      In 2019, the Japanese Institute of Electronics and Communications produced MOSFET using double-layer dielectric combined with source field plate technology, and its breakdown voltage reaches 2321 V. This technology provides a stable, low-cost and effective method to promote breakdown voltage^[27] for Ga₂O₃-based MOSFET. In 2022, the University of Buffalo used the polymer passivation layer to significantly increase the breakdown voltage of MOSFET, with the device breakdown voltage up to 8.03 kV^[28]. Xidian University optimized the NiO_x/Ga₂O₃ inerface and combined the groove technology, the on resistance of the device reached 6.24 mΩ·cm² , the breakdown voltage was 2145 V, and the power figure of merit (PFOM) reached 0.74 GW/cm². The value is the highest reported on lateral Ga₂O₃-based MOSFET at present, showing the great application prospect of Ga₂O₃-based MOSFET in high power, high efficiency and power electronics^[29].

4.2   Vertical device technology

      For lateral devices, in addition to adopting effective terminal technology to increase the breakdown voltage of the device, the gate-to-drain spacing can also be increased, which undoubtedly increases the active region area of the device. Therefore, many researchers have begun to focus on vertical devices, which can increase the breakdown voltage of devices without changing the area of their active region area, so it is one of the important directions of current research. In 2019, the National Institute of Information and Communication Technology of Japan produced vertical Ga₂O₃-based MOSFET through N- and Si-ion implantation doping technology, which provides an important reference^[30] for the study of Ga₂O₃-based vertical-type devices. In 2022, the first vertical field effect transistor (VDBFET) of Mg diffusion current blocking layer (CBL) was produced by Stanford University. The switching ratio of the device reaches 10⁹ and the breakdown voltage is 72 V. Although the breakdown voltage is very low, the report of the device provides a theoretical basis for the innovation of Ga₂O₃-based MOSFET in CBL structure^[31].

      In 2019, vertical fin Ga₂O₃-based MOSFET was produced at Cornell University, USA, as shown in Figure 9. The device modulates the threshold voltage of the device by controlling each fin width, and modulates the breakdown voltage through the doped concentration of the drift region. The breakdown voltage of the device is up to 2.66 kV, and the specific on-resistance is 25.2 mΩ·cm². The device uses many key technologies, such as low-damage etching technology, fine-line lithography technology, dielectric conformal coverage technology, so it is difficult to achieve. However, it has the advantages of controllable threshold voltage, good breakdown characteristics, and good heat dissipation, etc., and provides an important support for the application of Ga₂O₃-based MOSFET in the field of power electronics^[32].

Figure 9. Vertical fin-type Ga₂O₃-based MOSFET made by Cornell University, USA

4.3   Enhanced technology

      The mobility of Ga₂O₃ material is low, usually lower than 200 cm²/(V·s). Besides, in order to obtain a higher current density, the channel layer of the material is usually thick, so the threshold voltage of Ga₂O₃-based MOSFET is negative (less than -20 V), which severely limits the application of the device in power conversion. Therefore, the production of high-performance enhanced power devices is the future development trend. In 2019, the U. S. Air Force Laboratory produced the enhanced Ga₂O₃-based MOSFET with groove gate structure with a threshold voltage of +3V at 0.1 mA/mm current density, the density of drain current greater than 20 mA/mm, current switch ratio greater than 10⁷, and the breakdown voltage of the device higher than 198 V ^[33]. The gate oxygen annealing technology is adopted by CETC 13th Institute to form the gate oxide layer (OA zone), which effectively reduces the electronics in groove. The threshold voltage of the enhanced device is 4.1 V, and the breakdown voltage is above 3000 V ^[34]. The device structure is shown in Figure 10. In 2022, Xidian University made Ga₂O₃ enhanced power device by optimizing NiO_x/Ga₂O₃ interface and combining Ga₂O₃ groove technology. The on resistance is 13.75 mΩ·cm², the breakdown voltage is 1977 V, and the PFOM value is 0.28 GW/cm². This parameter is the best indicator  of Ga₂O₃ enhanced power device at present^[29].

Figure 10 Enhanced device structure made by CETC 13th Institution

      Due to the low thermal conductivity of Ga₂O₃ material and the defects of oxygen vacancy and gallium vacancy, the thermal performance and the carrier transport of Ga₂O₃ are affected. Too low thermal conductivity leads to severe self-heating phenomenon of the device, which will lead to severe decline of the device performance. Therefore, the heat dissipation efficiency becomes the bottleneck restricting the application of Ga₂O₃ power devices. In 2019, Xidian University adopted ion knife lift-off technology for the first time to realize the substrate transfer of Ga₂O₃-based MOSFET, as shown in Figure 11, the "I" in GaO@ISiC (GaO@ISi) represents the Al₂O₃ dielectric layer. It provides an important solution to the heat dissipation problem of Ga₂O₃ power devices^[35]. In addition, substrate thinning is also one of the effective method to solve the heat dissipation problem of Ga₂O₃ power devices^[36].

Figure 11 Ion knife stripping and bonding technology of Xidian University

 

      From the above research progress, it can be seen that the optimization of field terminal technology is still one of the effective ways to raise the breakdown voltage of Ga₂O₃-based MOSFET. At present, the highest breakdown voltage of Ga₂O₃ power transistors can reach 8.03 kV. By optimizing the interface characteristics and combining the groove technology, the highest PFOM value (0.74 GW/cm²) can be obtained. Although some progress has been made in the research of Ga₂O₃-based MOSFET, compared with the diode, the development of MOSFET is in the initial stage, and there are still many problems to be solved.

5   Conclusion

      This paper summarizes the latest research on Ga₂O₃ epitaxial material, power diodes and power transistors at home and abroad. Ga₂O₃ epitaxial materials are mainly obtained through three growth methods: HVPE, MBE and MOCVD. Among them, the growth method of MOCVD takes both the large epitaxial size and controllable growth rate into account, which is an effective way to push Ga₂O₃ materials to industrialization. Optimizing its growth process, preparing epitaxial materials with the large size and high quality will become the main research direction in the future. Ga₂O₃ power devices are mainly divided into power diodes and power transistors. At present, Ga₂O₃ power diode has approached the theoretical limit through the combination of heterogeneous PN junction and a variety of field terminal technologies, and is one of the closest to commercial devices. While Ga₂O₃ power transistor has achieved some breakthroughs with various technologies, the performance of the device still lags its theoretical limit due to the material properties and size, and the preparation technology of enhanced devices is not perfect, so technical development and theoretical research need to be continued. In addition, limited by the inherent low thermal conductivity characteristics of Ga₂O₃ materials, the thermal management technology of Ga₂O₃ power devices will also become one of the key research directions in the future.

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