Undoped epitaxial films of α-Ga₂O₃ were grown on basal plane sapphire substrates by halide vapor phase epitaxy (HVPE) in three different modes: standard HVPE, HVPE with constant flow of Ga and pulsed supply of O₂ (O₂-control growth regime), and with constant flow of O₂ and pulsed delivery of Ga (Ga-control growth fashion). The best crystalline quality as judged by x-ray symmetric and asymmetric reflection half-widths and by atomic force microscopy morphology profiling was obtained with the O₂-control deposition, and these results appear to be the best so far reported for α-Ga₂O₃ films. All grown α-Ga₂O₃ epilayers were high-resistivity n-type, with the Fermi level pinned near E_c − 1 eV deep traps. Photoinduced current transient spectra also showed the existence in standard HVPE samples and samples grown under the O₂-control pulsed growth conditions of deep hole traps with levels near E_v + 1.4 eV whose density was suppressed in the Ga-control pulsed HVPE samples. The levels of the dominant deep traps in these α-Ga₂O₃ samples are close to the position of dominant electron and hole traps in well documented β-Ga₂O₃ crystals and films.

Effect of annealing to realize high-quality α-Ga₂O₃ epilayers grown by halide vapor phase epitaxy has been studied. The structural and optical properties of α-Ga₂O₃ changed with changing annealing duration. In particular, for an annealing duration of 3 min, the crystal quality improved from 51 and 2757 arcsec to 28 and 1539 arcsec for (0006) and (10–14) FWHM, respectively. X-ray photoelectron spectroscopy results confirmed that the oxygen gas during annealing reacted to compensate the oxygen vacancies in the α-Ga₂O₃ epilayers. These results show that the optimized thermal annealing improved the crystal quality of α-Ga₂O₃ epilayers.