GaN-based nitride semiconductors have been put to practical use as blue light emitting devices, and their application to electronic devices is also being attempted. While research and development using heteroepitaxial layers on dissimilar substrates such as Si, SiC, and sapphire is the mainstream, the need for GaN substrates is also increasing. However, since commercially available GaN substrates are expensive and their quality is low, GaN on GaN devices have not become widespread, and high quality and thick GaN crystals are required. We have succeeded in obtaining low dislocation and large diameter wafers by using the point-seed technique in the Na-flux method. Thick growth is also possible with homoepitaxial growth by the Halide vapor phase epitaxy (HVPE) method. This paper describes the barriers we have overcome to establish these technologies, the history of research on improving the quality of GaN crystals by the Na flux method, and future prospects.

  In this paper we discuss the qualities of GaN substrates manufactured using the oxide vapor-phase epitaxy (OVPE) method and assess the characteristics of PN diodes examined as examples of their device applications. The OVPE-GaN substrates had ultralow resistivity on the order of 10^-4 Ω cm and a low threading dislocation density on the order of 10⁴ cm^-2. Our evaluation of the PN diodes confirmed that the use of the OVPE-GaN substrates resulted in a significant reduction in specific on-resistance by highly efficient photon recycling. The on-resistance was reduced by 1/8 compared to commercial conductive GaN substrates (resistivity: 1 × 10^-2 Ω cm, threading dislocation density: 1~4 × 10⁶ cm^-2). We also introduce recent progress in OVPE-GaN growth, including a high-speed growth rate of 200 μm/h, high-quality growth of 0.7-mm-thick film, and large-diameter growth of 6 inches.

  Tri-halide vapor phase epitaxy (THVPE) of thick GaN using GaCl₃ was investigated for the fabrication of low cost, high crystalline quality GaN substrates instead of the conventional hydride vapor phase epitaxy (HVPE) using GaCl. It was found that the growth rate and the upper growth temperature limit of GaN using THVPE were much higher than that using conventional HVPE under the same growth conditions. The upper growth rate limit without the degradation of crystalline quality also increased as the growth temperature increased due to the enhancement of precursor migration on the growing surface. It was found that the incorporation of impurities such as O, C and Cl could be suppressed even on N-polarity GaN by increasing the growth temperature and eliminating the origin of oxygen. The possibility of enlargement of the crystal diameter by growing N-polarity GaN layer using THVPE is also proposed.

  We have been developing a SiC crystal growth technique using the solution method. As a result, we have achieved the growth of ultra-high quality crystals with extremely low dislocation density. The key to this is the reduction of dislocation density by utilizing the macro-step dislocation conversion phenomenon and the suppression of surface morphology roughness by controlling the flow in the solution. In order to put these technologies to practical use, we have developed a new machine learning technique for optimizing crystal growth conditions for large-diameter crystals. In this method, a model is constructed in the computer that reproduces the actual experiment quickly and accurately, and then hundreds of thousands or millions of trials are performed using the model to derive the experimental conditions with high efficiency. This means that optimization by surrogate models, which is one of the methods of process informatics, has been realized in crystal growth. By using these techniques, we were able to achieve 6-inch crystal growth in a very short time.

  The growth of β-Ga₂O₃ films by metalorganic vapor phase epitaxy (MOVPE) has been investigated by thermodynamic analysis and growth experiments. Triethylgallium (TEGa) and oxygen (O₂) were selected as the source gases, and the inert gas was selected as the carrier gas. Thermodynamic analysis revealed that O₂ is consumed in the combustion of TEGa-derived hydrogen and hydrocarbons, and as a result, a high input VI/III ratio is essential for the growth of β-Ga₂O₃. Based on the thermodynamic analysis, a smooth β-Ga₂O₃ film oriented to (201) and showing an optical bandgap of 4.84 eV was successfully grown at 900℃ at about 1.4 μm/h on a c-plane sapphire substrate. By growing at higher temperatures, the concentrations of hydrogen and carbon impurities in the grown films became lower than the background levels of the measurement system. These results indicate that the MOVPE is promising as one of the growth methods for β-Ga₂O₃.

  Formation and control of electron spin in diamond is attracting much attention for next-generation quantum information and quantum sensing devices. With this fact in mind, we provide a guideline for the growth of homoepitaxial diamond films that possess higher crystalline quality, higher chemical purity, and a higher carbon isotopic ratio and nitrogen doping. To improve both the purity and crystalline quality of homoepitaxial diamond films, an advanced growth condition was applied: higher oxygen concentration in the growth ambient. Under this growth condition for high-quality diamond, a thick diamond film of >30 µm was deposited reproducibly while maintaining high purity and a flat surface. Then, combining this advanced growth condition for non-doped diamond (100) film with a nitrogen doping technique that provides parts-per-billion order doping, single nitrogen-vacancy centers that show excellent properties were formed. These advanced growth techniques are expected to accelerate the research fields of quantum information and quantum sensing devices using diamond.

β-Ga₂O₃ single crystal growth by the vertical Bridgman method using a platinum-rhodium alloy crucible in ambient air was studied. 2-inch diameter β-Ga₂O₃ single crystals with the growth direction perpendicular to (100), (010) and (001) planes were grown and machined wafers with each plane were produced. Large sized crystals with 3-inch and 4-inch diameter have been successfully grown by the same technique.