Epitaxial Growth of β-Ga ₂ O ₃ Thin Films on α-Al ₂ O ₃ Substrates via Two-Step Metal–Organic Chemical Vapor Deposition: Influence of Growth Temperature on Crystallinity and Interface Formation

In this study, β-Ga₂O₃ thin films were epitaxially grown on Al₂O₃(0001) substrates via a two-step metal-organic chemical vapor deposition (MOCVD) process. The effects of growth temperatures (820 °C-920 °C) on the crystallinity, microstructure, and interfacial properties of thin films were investigated via X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XRD θ-2θ scans confirmed that the as-grown thin films comprised a monoclinic β-phase Ga₂O₃ layer with a pronounced out-of-plane orientation along the (-201) plane. As the growth temperature increased, the primary β-Ga₂O₃ diffraction peaks became sharper, and the <i>d</i>-spacing of the (-201) plane approached that of the theoretical bulk value, indicating the progressive relaxation of tensile strain in the thin films. Rocking curves (ω scans) revealed that the full-width at half-maximum (FWHM) of the β-Ga₂O₃(-201) reflection narrowed from ∼2.50° (8680 arcsec) at 820 °C to ∼1.93° (6960 arcsec) at 920 °C, reflecting improved crystallinity and reduced defect density at high growth temperatures. Cross-sectional TEM revealed the epitaxial relationship of Ga₂O₃(-201)//Al₂O₃(0006) and Ga₂O₃[0-20]//Al₂O₃[-3300]. High-angle annular dark-field (HAADF) images showed an ultrathin (∼1 nm) intermediate layer with a bright contrast, which was identified as a coherently strained β-Ga₂O₃ layer with an epitaxial orientation of β-Ga₂O₃(3-10)// α-Al₂O₃(-114). This layer served as a structural template, enabling subsequent β-phase nucleation and orientation transition along the <-201> direction. SEM results further confirmed that high growth temperatures enhanced vertical column alignment and surface smoothness. These findings highlight the critical role of interfacial phase engineering and thermal conditions in optimizing the β-Ga₂O₃ film quality for advanced electronic and optoelectronic applications.