Crystallinity and optical properties of post-annealed Ga2O3 thin films deposited under varying oxygen partial pressures

Abstract

Amorphous Ga₂O₃ thin films were deposited on c-plane sapphire substrates by RF magnetron sputtering under varying oxygen partial pressures and crystallized into the monoclinic β-phase via post-annealing at 1000 °C in an oxygen atmosphere. Atomic force microscopy revealed that films grown under low oxygen conditions exhibited surface cracks, while those deposited at higher oxygen pressures showed smooth, crack-free surfaces. Raman spectroscopy showed a progressive increase in the intensities of Ga–O vibrational modes around 200 and 415 cm⁻¹ with increasing oxygen partial pressure, indicating enhanced structural ordering. Optical transmittance measurements revealed a non-linear change in bandgap energy from 4.8 to 4.95 eV, likely due to variations in defect concentration and stoichiometry. The refractive index remained nearly constant at ~ 1.76, suggesting low film density or structural inhomogeneity. These results suggest that β-Ga₂O₃ thin films fabricated under different oxygen partial pressures may still exhibit variations in microstructure and optical properties even after post-annealing.