【Domestic Papers】Strain transition and mosaicity evolution in m-plane nonpolar α-Ga₂O₃ heteroepitaxy

      Researchers from the Nanjing University have published a dissertation titled "Strain transition and mosaicity evolution in m-plane nonpolar α-Ga₂O₃ heteroepitaxy" in Applied Physics Letters.

Background

      Gallium oxide (Ga₂O₃), an ultra-wide-bandgap semiconductor with a critical breakdown field exceeding 8 MV/cm, has emerged as a promising platform for high-power electronics and deep-ultraviolet optoelectronics. While research has predominantly focused on the thermodynamically stable β-Ga₂O₃ due to the availability of large-scale melt-grown substrates, the corundum α-phase gallium oxide (α-Ga₂O₃) offers unique opportunities for heteroepitaxy on sapphire owing to their shared crystal symmetry. This compatibility enables oriented growth despite lattice mismatch and facilitates full-range Al_xGa_1-x)₂O₃ and (In_xGa_1-x)₂O₃ alloying for bandgap engineering, as well as the integration with p-type corundum oxides for all-oxide bipolar device architectures.

Abstract

      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.

Highlights

      A pure-phase growth temperature window of 550–730 °C is determined for m-plane nonpolar α-Ga₂O₃, and α→β phase transformation occurs above 750 °C.

      Temperature drives out-of-plane strain from compressive to tensile, and the growth mechanism transitions from coherence limitation to dislocation-dominated mosaic relaxation.

      Twist mosaicity and threading dislocation density are significantly reduced within the α-phase stable temperature window, establishing a temperature-controlled mechanism for low-defect epitaxy.

Conclusion

      In summary, the heteroepitaxial growth of non-polar α-Ga₂O₃ on m-plane sapphire is constrained by the strong coupling between strain relaxation and phase stability. A narrow temperature window of 550–730 °C enables phase-pure α-Ga₂O₃ growth, while β-phase nucleation above 750 °C disrupts epitaxial coherence. Within the α-phase stability regime, growth temperature drives a transition from coherence length-limited broadening to dislocation-mediated mosaic relaxation, characterized by reduced twist mosaicity and lower threading-dislocation densities. These findings establish a temperature-controlled strain-relaxation mechanism that defines an optimal growth regime for low-defect α-Ga₂O₃ heteroepitaxy.

Project Support

      This work was supported by the National Key R&D Program of China (No. 2024YFE0205200), the Jiangsu Provincial Science and Technology Major Project (Nos. BG2024030 and BK20253003), and the National Natural Science Foundation of China (Nos. 62425403, 62234007, and 62293522).