Strain relaxation in α-(Al_xGa_1-x)₂O₃ thin films grown on M-plane and R-plane sapphire via metalorganic chemical vapor deposition

The growing interest in gallium oxide (Ga₂O₃) as an ultrawide bandgap semiconductor stems from its potential for high-efficiency power electronics and solar-blind ultraviolet photodetectors. However, realization of single-crystalline α-(Al_xGa_1−x)₂O₃ with multilayer structures suitable for device applications remains challenging. This study establishes a quantitative framework for strain relaxation in corundum α-(Al_xGa_1−x)₂O₃ thin films grown on m-plane and r-plane sapphire via metalorganic chemical vapor deposition (MOCVD), integrating experimental and first-principles approaches. High-quality α-(Al_xGa_1−x)₂O₃ films with aluminum content ranging from 0.46 to 0.99 were synthesized through optimized growth parameters, achieving an exceptionally low full width at half maximum (FWHM) of ∼0.1° for compositions up to Al content x around 0.88. Reciprocal space mapping (RSM) and density functional theory (DFT) reveal a composition-dependent critical thickness variation: films with x < ∼0.7 exhibit complete or partial relaxation at 100 nm, whereas those with x ≥ ∼0.7 maintain pseudomorphic coherence. Despite observed similarities in relaxation behavior, substrate orientation governs strain evolution. Specifically, m-plane films relax via shear strain and exhibit reduced overall strain magnitude, whereas r-plane films retain slightly higher strain through prismatic and/or basal slip. First-principles calculated elastic constants and lattice parameters align with experimental RSM-derived data, validating a predictive mechanical model for critical thickness and providing a foundational reference for device structure design.