Intrinsic Thermal Instability of δ-Ga2O3 Thin Films Grown via Mist Chemical Vapor Deposition

Abstract This study demonstrates that δ-Ga2O3 thin films grown by mist chemical vapor deposition (CVD) are intrinsically thermally unstable, and that rapid postgrowth cooling is essential to stabilize the δ phase. δ-Ga2O3 thin films were deposited on YSZ (111) substrates using a β-Fe2O3 buffer layer, and their thermal stability was systematically investigated. X-ray diffraction (XRD) showed that δ-Ga2O3 is stabilized only within a narrow growth-temperature window of 550–625 °C. Postgrowth annealing caused the disappearance of δ-Ga2O3 diffraction peaks at temperatures as low as 500 °C, indicating thermal instability below the growth temperature. Photoluminescence excitation (PLE) spectroscopy probed structural changes beyond the XRD detection limit. The PLE peak shifted from 4.94 eV in as-grown films to 5.12 eV after annealing at 700 °C, consistent with a δ → γ phase transformation, and further to 5.07 eV after annealing at 900 °C, suggesting mixed γ/β phases. The absence of additional diffraction peaks is attributed to nanocrystalline γ or γ/β phases below XRD detectability. δ-Ga2O3 also loses crystallinity when held at the growth temperature for more than 10 min, emphasizing the critical role of immediate cooling. These findings provide fundamental insight into δ-Ga2O3 metastability and define key processing constraints for its device applications.