Corundum (InxGa1-x)2O3 alloys: First-principles study, epitaxial growth and thermal annealing

Abstract

Corundum-type $\alpha$‑(In_xGa_1-x)₂O₃ offers a promising platform for deep-ultraviolet optoelectronics and power devices due to their tunable wide bandgap and compatibility with common oxide substrates. This work presents a comprehensive study combining first-principles calculations and mist chemical vapor deposition (mist-CVD) to investigate the structural stability, electronic properties, and epitaxial growth of these alloys. Density functional theory (DFT) and cluster expansion analysis reveal a wide miscibility gap and positive formation enthalpies, indicating a strong tendency for phase separation under equilibrium conditions. However, a metastable composition window was identified for In content up to x ≈ 0.21 at a typical growth temperature of 773 K. Theoretical calculations predict a bandgap reduction from 5.22 to 4.46 eV with increasing In content within this metastable range. In good agreement with these predictions, thin films grown on c-plane sapphire via mist-CVD exhibit systematic XRD peak shifts and a corresponding optical bandgap narrowing from 5.16 to 4.40 eV. Furthermore, post-deposition annealing above 700 °C was found to induce phase separation in films with higher In content, confirming the predicted thermodynamic instability. These results provide key insights into the metastable phase formation and bandgap engineering of $\alpha$‑(In_xGa_1-x)₂O₃, highlighting its potential for high-performance UV photodetectors and power electronics.