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To achieve high-power performance in β-Ga₂O₃-based lateral power devices, it is indispensable to ensure a high breakdown voltage and suppress surface channel leakage currents. These requirements can be realized by using semi- insulating substrates with high electrical resistivity. Unintentionally doped (UID) β-Ga₂O₃ typically exhibits n-type conduction due to residual impurities in the source material (such as Si, Sn, and Ge) and oxygen vacancies (Vo) formed during crystal growth. Thus, charge compensation using deep acceptor dopants (such as Mg and Fe) is necessary to trap free electron carriers and obtain high resistivity. In this study, UID (001)-oriented and Fe-doped (100)-oriented β-Ga₂O₃ single crystals were grown using the edge-defined film-fed growth (EFG) method. The crystal orientation, crystal quality, defect morphology, EPD (etch pit density), optical properties, and electrical characteristics were evaluated for β-Ga₂O₃ samples grown with different Fe concentrations (0 mol%, 0.005 mol%, 0.01 mol%, and 0.02 mol%). The results showed that as the Fe doping concentration increased, the EPD increased, leading to a decrease in crystal quality and optical transmittance. On the other hand, SIMS analysis confirmed that for the samples with 0.01 mol% and 0.02 mol% Fe, the Fe dopant concentration was higher than the residual Si impurity concentration. This proves that the Si donors were effectively compensated by the Fe dopants. The electron traps formed by Fe doping significantly reduced the free electron concentration, leading to a large increase in resistivity. As a result, a semi-insulating β-Ga₂O₃ substrate with a high resistivity (r) of 5×10¹¹ Ω·cm (n = 1×10¹⁵ cm⁻³) was successfully demonstrated at a doping condition of 0.02 mol% Fe.

In this study, as the expectation ratio Ar increases, the internal convective velocity |U|max decreases sharply due to the expansion of wall frictional resistance. This lowers the heat and mass transfer efficiency (Nu, Sh) at the crystal growth interface to its lowest level, ultimately causing an exponential decrease in the single-crystal growth rate of mercury(I) bromide (Hg₂Br₂). To efficiently manufacture high-speed and high-quality mercury(I) bromide crystals, it is essential to design low expectation ratio Ar ampoules and optimize the process to ensure appropriate convective intensity within a range that does not induce convective instability. The results of this study are expected to serve as important basic computational data for designing constricted diffusion systems under microgravity or terrestrial conditions, such as in future Korean Spaceflight.

The effects of deposition temperature and post-annealing on the structural and optical properties of Ga₂O₃ thin films deposited by RF magnetron sputtering were investigated. As the deposition temperature increased from room temperature to 400°C, the deposition rate slightly increased from 2.29 nm/min to 2.45 nm/min. The Ga₂O₃ thin films deposited at various substrate temperatures primarily exhibited the ε-Ga₂O₃ phase, with a slight improvement in crystallinity as the deposition temperature increased. After annealing at 800 C in air, the ε-Ga₂O₃ and β-Ga₂O₃ phases coexisted due to the progression of the phase transition to the β-Ga₂O₃ phase. After annealing at 1,000°C, the films were completely transformed into the thermodynamically stable β-Ga₂O₃ phase. In addition, the grain size increased with increasing annealing temperature. The as-deposited Ga₂O₃ films exhibited a high visible-light transmittance of approximately 88 % and optical bandgaps of 4.67~4.80 eV. Post-annealing at 1,000°C in air was found to induce stabilization of the crystal structure and optical properties of the Ga₂O₃ films, resulting in the convergence of the optical bandgap to a narrow range of 4.73~4.76 eV. The Ga₂O₃ thin films deposited by RF magnetron sputtering and subsequently annealed at 1,000°C in air exhibited an optical absorption edge of approximately 260~262 nm, indicating their potential suitability for solar-blind UV photodetector applications.