Strain transition and mosaicity evolution in m-plane non-polar <b> <i>α</i> </b> -Ga2O3 heteroepitaxy

The heteroepitaxy of α-phase gallium oxide (α-Ga2O3) is fundamentally limited by its metastability and the strong coupling between strain relaxation and phase transformation. Here, non-polar α-Ga2O3 films were grown on m-plane sapphire by metal-organic chemical vapor deposition at 550–790 °C to elucidate growth temperature-driven strain evolution, lattice mosaicity, and phase stability. Phase-pure α-Ga2O3 is obtained within a narrow growth window of 550–730 °C, whereas β-phase nucleation above 750 °C disrupts epitaxial coherence. Reciprocal-space mapping reveals a temperature-driven transition from out-of-plane compressive to tensile strain accompanied by increasing in-plane compression, reflecting a crossover from coherent-length limited growth to tilt-dominated strain relaxation. ψ-dependent rocking-curve analysis reveals reduced twist mosaicity and threading-dislocation densities within the α-phase stability window, as confirmed by two-beam transmission electron microscopy, which also identifies α-to-β transformation and domain-boundary strain accumulation. These results establish a temperature-controlled strain-relaxation framework that defines an optimal growth regime for low-defect α-Ga2O3 heteroepitaxy.