【Domestic News】Gallium Oxide Epitaxial Growth and Power Devices（Ⅰ）

1、Enhanced gallium oxide field effect transistors and review of progress in performance studies

      Power electronic devices, as the most important electronic components in the realization of electric energy conversion and control circuit of electric power equipment and electronic system, are known as the nuclear "core" of electric energy conversion and transmission. In recent years, the third generation of semiconductor gallium nitride (GaN) and silicon carbide (SiC) have injected new impetus into the development of power electronic devices, and their device performance has exceeded the theoretical limit of traditional silicon (Si) -based devices. The new ultra-wide band gap semiconductor gallium oxide (Ga₂O₃) has a band gap up to 4.9 eV, and has more excellent breakdown field strength and Baliga’s figure of merit, which has a broad development prospect in power electronic devices.

      At present, although the performance of Ga₂O₃ unipolar devices has exceeded the SiC material limit, the problems such as p-type Ga₂O₃ doping restrict the further improvement of the device performance. Unipolar Ga₂O₃ field-effect transistor (FET) devices therefore fail to realize the working mode of enhanced (E-mode), which has become an obstacle for Ga₂O₃ power electronic devices in practical circuit applications. Therefore, how to achieve stable and reliable E-mode Ga₂O₃ FET has become a hot spot and difficulty in the field of Ga₂O₃ power electronic devices.

      Recently, the nanomimining platform of Suzhou Institute of Nanotechnology, Chinese Academy of Sciences, has sorted out and summarized the research progress of E-mode Ga₂O₃ FET material epitaxy and device process through the investigation of relevant literature. First of all, from the process of metal organic compound vapor deposition (MOCVD) and halide vapor phase epitaxy (HVPE), the key technology of Ga₂O₃ material epitaxial process is introduced in detail, and the research situation of Ga₂O₃ power electronic devices is shown. Secondly, the recent progress of E-mode Ga₂O₃ FET are reviewed, including specific details of the relevant preparation process and key parameters such as threshold voltage and on-resistance of the device. By comparing the device performance of different structures, the advantages and disadvantages of the design structure are clearly demonstrated, and the design ideas are clarified, which is of guiding significance for the research and development of E-mode Ga₂O₃ FET. In addition, the review gives a brief introduction of Ga₂O₃ FET breakdown voltage and other parameters such as heat dissipation performance. The essay is published in Journal of Semiconductors, titled “A comprehensive review of recent progress on enhancement-mode β-Ga₂O₃ FETs: growth, devices and properties”. Li Botong, doctoral student of Suzhou Institute of Nanometer, Chinese Academy of Sciences, is the first author of the paper, and Zhang Baoshun of Suzhou Institute of Nanometer, Chinese Academy of Sciences is the corresponding author.

Figure 1. (a) Enhanced gallium oxide field-effect transistor of oxygen annealing treatment process;  (b) Enhanced gallium oxide field-effect transistor with a p-type nickel oxide gate;  (c) enhanced gallium oxide field-effect transistor of multilayer ferroelectric gate medium;  (d) Vertical concave gate strong-enhanced gallium oxide field effect transistor of nitrogen ion injection process

Paper Information：A comprehensive review of recent progress on enhancement-mode β-Ga₂O₃ FETs: Growth, devices and properties

doi: 10.1088/1674-4926/44/6/061801

2、Progress of NiO / β-Ga₂O₃ heterojunction in the field of power devices

      Power devices are often used for switch control and high-power circuit drive, and are widely used in many occasions. With the rapid development of electric vehicles, 5G networks and the Internet of Things (IoT), silicon-based devices have gradually reached their physical limits, and traditional silicon-based power devices have been unable to meet the needs of many ultra-high power applications. The wide band semiconductor material gallium oxide (β-Ga₂O₃) has a band width of about 4.8 eV, a critical electric field up to 8 MV / cm, a Baliga’s figure of merit (BFOM) of about 3000, obtained by melting growth and other excellent properties. It is a potential material for the preparation of next-generation power devices. With silicon or tin doping, β-Ga₂O₃ can achieve a tunable N-type conductivity in the range of 10¹⁵-10¹⁹ cm^-3. However, to achieve β-Ga₂O₃ P-type conduction is very difficult due to the large impurity-dependent activation energy and hole self-trap energy, which is also a major obstacle to the development of β-Ga₂O₃ homogenous bipolar devices. Bipolar structures usually have low leakage current, high thermal stability and good surge handling capability, and are widely used in power device design. In order to realize β-Ga₂O₃ bipolar devices, a feasible method is to construct heterojunctions with P-type semiconductor materials such as tin oxide (SnO), copu oxide (Cu₂O), nickel oxide (NiO) and N-type β-Ga₂O₃. And NiO is the mainstream choice with its large forbidden band width of 3.6-4.0 eV and controllable P-type doping. In 2020, Lu et al. reported a heterojunction prepared by NiO films and β-Ga₂O₃, realizing the first β-Ga₂O₃ bipolar type device with kV breakdown voltage. Subsequently, many researchers have made significant progress in improving the performance of NiO / β-Ga₂O₃ heterogeneous pn junction. In addition, NiO / β-Ga₂O₃ heterojunction structure is also widely used in other device structures, such as junction barrier Schottky diode (JBS), junction field effect transistor (JFET) and edge terminal (ET) structure.

      Recently, Associate Professor Lu Xing and Professor Wang Gang of Sun Yat-sen University summarized the research progress of NiO / β-Ga₂O₃ heterojunction in the field of power devices, and made prospects for its future development. The review unfolds from three aspects of the construction, characterization and device performance of NiO / β-Ga₂O₃ heterojunction, discusses the crystallization properties, band structure and carrier transport characteristics of NiO / β-Ga₂O₃ heterojunction prepared by sputtering method. The latest progress of various device structures including NiO / β-Ga₂O₃ heteropn pn-junction diode (HJD), junction barrier Schottky diode, junction field effect transistors and edge terminal structure and superjunction (SJ) structure based on NiO / β-Ga₂O₃ heterojunction are introduced. In addition, the summary and outlook are conducted on the key problems of NiO / β-Ga₂O₃ include material quality, device structure optimization, interfacial state, and device reliability.

      This review summarizes the development status of NiO / β-Ga₂O₃ heterojunction in the field of power devices, which provides a reference for the design of high-performance NiO / β-Ga₂O₃ heterojunction devices, and plays a positive role in the future development of β-Ga₂O₃ bipolar devices.

      The review is published in Journal of Semiconductors, titled“Recent advances in NiO/Ga₂O₃ heterojunctions for power electronics”.

Figure 1. The development milestone of NiO / β-Ga₂O₃ heterojunction power devices.

Paper Information：Recent advances in NiO/Ga₂O₃ heterojunctions for power electronics

doi: 10.1088/1674-4926/44/6/061802

3、Study of doping MOCVD homoepitaxial (100) plane β -Ga₂O₃ film and Si

      β-Ga₂O₃ with its breakdown field strength 8 MV / cm, Baliga’s figure of merit 3444, and the character of single crystal melt growth and other advantages, is one of the most promising ultra-wide band gap semiconductor materials. At present, β-Ga₂O₃ power electronic devices have shown excellent device performance, which is essential to further discover its potential to achieve high-quality epitaxial growth and controllable doping of β-Ga₂O₃ films. β-Ga₂O₃ mainly has four orientation substrates: (100), (010),(001) and (201). Homogenous epitaxial β-Ga₂O₃ films still face various challenges such as twinning and stacking layer errors. Among them, the (100) oriented substrate is relatively easy to prepare and has the advantage of growing large size film, but the surface energy is low, resulting in a significant island growth pattern during the film growth, which makes the epitaxy of the (100) plane very challenging. In addition, the current n-type doping of β-Ga₂O₃ films is limited, and the realization of high concentration doping such as Ohmic Contact usually uses high dose of Si ion injection, producing large damage. Therefore, it is of great significance to achieve homogeneous epitaxial growth of high-quality (100) plane β-Ga₂O₃ films and controllable Si doping.

      Recently, the nanomimining platform of Suzhou Institute of Nanotechnology, Chinese Academy of Sciences and Qi Hongji team of Shanghai Institute of Optics and Machinery have achieved high-quality epitaxial growth and Si doping of β-Ga₂O₃ film on the semi-insulated (100) plane β-Ga₂O₃ substrate through metal-organic chemical vapor deposition (MOCVD). The island growth pattern in β-Ga₂O₃ films was suppressed by optimizing the growth conditions (e. g., VI / III ratio, temperature, and chamber pressure), and it is found that Si as a nucleation point optimized the growth kinetics promoted the lateral merging of the films and improved the surface morphology of the films. The surface roughness of the (100) plane of β-Ga₂O₃ film is 1.3 nm, achieving an effective net doping concentration of 5.41 × 10¹⁵ cm^-3 to 1.74 × 10²⁰ cm^-3. For samples with a carrier concentration of 7.19 × 10¹⁸ cm^-3, the activation efficiency was approximately 61.5% and the room temperature mobility was approximately 51 cm²/(V·s). Moreover, for epitaxial films with Si doping concentration of 1.64 × 10²⁰cm^-3, they showed good Ohmic contact characteristics with contact resistance of 3.73 Ω·mm and square resistance of 1084 Ω/□, as low as 1.29 × 10^-4 Ω·cm², which is comparable to the electrical characteristics of Si injected samples, indicating that controllable and effective n-type doping is realized in the MOCVD growth of β-Ga₂O₃ films, which provides a feasible scheme to reduce the Ohmic contact resistance of β-Ga₂O₃ devices and improve the device performance.

      The related result is published in Journal of Semiconductors, titled“Homoepitaxial growth of (100) Si-doped β-Ga₂O₃ films via MOCVD”. Tang Wenbo, PhD student of Suzhou Institute of Nanotechnology, Chinese Academy of Sciences, is the first author of the paper, Zhang Baoshun, researcher of Suzhou Institute of Nanotechnology, Chinese Academy of Sciences, and Qi Hongji, Shanghai Institute of Chinese Academy of Sciences, are the corresponding authors.

Figure 1. Atomic force microscope test morphology of (100) β-Ga₂O₃ films grown at different SiH 4 flow rates, and the sample number and corresponding flow are (a) B2,0.5 sccm; (b) B3,1.2 sccm; (c) B4,3 sccm; (d) B5,6 sccm; (e) B6,10 sccm and (f) B7,20 sccm.

Figure 2. The IV properties of the β-Ga₂O₃ thin film based on the TLM.(a) linear coordinate; (b) semi-logarithmic coordinate; (c) spacing between total resistance and metal contact point.

Paper Information：Homoepitaxial growth of (100) Si-doped β-Ga₂O₃ films via MOCVD

doi: 10.1088/1674-4926/44/6/062801

4、Study on the epitaxial growth and electrical characteristics of α-Ga₂O₃ thin films with different crystal surface orientations

      Gallium oxide (Ga₂O₃) as an emerging third generation semiconductor materials, with large band width, high breakdown field strength, strong radiation resistance superior performance, compared with Si, SiC, GaN, is more suitable for the preparation of power electronic devices, such as Schottky Barrier Diode (SBD), heterojunction diode, metal oxide semiconductor field effect transistor (MOSFET), and it has wide application prospects in the field of advanced information technology and new energy technology. Ga₂O₃ can form five crystal structures, including the thermal steady-state crystal phase (β-Ga₂O₃) and the metastable crystalline phase (α -, ε -, γ-Ga₂O₃, etc.). Compared with the heat-stable monoclinic phase β-Ga₂O₃, the metastable crystalline phase α-Ga₂O₃ has a larger band width and higher breakdown field strength, which is more suitable for the fabrication of power electronic devices.

      However, crystal surface orientation of the substrate has a large effect on the crystallization quality and electrical properties of α-Ga₂O₃ films, which in turn affects the performance of α-Ga₂O₃-based electronic devices. On the one hand, the anisotropy of α-Ga₂O₃ may be related to the anisotropy of the conduction band, and on the other hand, the difficulty of defect formation under different lattice plane also has an important influence on the electrical performance. Therefore, it is very necessary to study the influence of the crystal orientation of the sapphire substrate on the heteroepitaxial and electrical properties of α-Ga₂O₃ films.

      Recently, researchers, from the Silicon-based Solar Cell and Wide Band Gap Semiconductor Team, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences. used PLD technology to systematically study the epitaxial relationship and electrical properties of α-Ga₂O₃ films on a-plane, m-plane and r-plane sapphire substrates. The experimental results show that the epitaxial grown α-Ga₂O₃ film has a consistent out-and-in-plane growth orientation with the sapphire substrate with the same corundum structure. Under the same process conditions, α-Ga₂O₃ films grown on m-plane and r-plane sapphire substrate have better conductive performance than α-Ga₂O₃ films grown on a-plane sapphire substrate. Interestingly, the oxygen vacancy concentration of epitaxial grown α-Ga₂O₃ films on m-and r-plane sapphire substrates was also significantly higher than that of α-Ga₂O₃ films epitaxial grown on a-plane sapphire substrate. Therefore, it is speculated that the conductive behavior of α-Ga₂O₃ films grown epitationally on sapphire substrate with different crystal surface orientations may be related to the concentration of oxygen vacancies, while the formation energy may be strongly dependent on the lattice orientation, leading to significant differences in the activation energy of doped atoms and the carrier transport behavior. However, the mechanism of the influence of anisotropy and crystallinity on the electrical properties needs to be further investigated. This study provides a new idea for substrate selection when growing α-Ga₂O₃ films, and also provides a basis for exploring the mechanism of electrical performance difference of α-Ga₂O₃ films on different crystal planes, and contributes to the development of gallium oxide semiconductor devices. The essay is published in Journal of Semiconductors, titled “Exploring heteroepitaxial growth and electrical properties of α-Ga₂O₃ films on differently oriented sapphire substrates”.

Figure 1. Schematic diagram of PLD epitaxial growth relationship of α-Ga₂O₃ film grown on sapphire substrate on (a) a plane; (b) m plane; and (c) r plane

Figure 2., XPS spectrum of the O 1s peak of α -Ga₂O₃ thin film epitaxial grown on sapphire substrate on (a) a plane; (b) m plane; and (c) r plane

Table 1. Film thickness (d), conductivity (σ), mobility (μ), and carrier concentration (n) of α-Ga₂O₃ films epitaxial grown on sapphire substrate on (a) a plane; (b) m plane; and (c) r plane

Paper Information：Exploring heteroepitaxial growth and electrical properties of α-Ga₂O₃ films on differently oriented sapphire substrates

 doi: 10.1088/1674-4926/44/6/062802