【Domestic Papers】Strain-mediated compositional pulling effect in α-(AlₓGa₁₋ₓ)₂O₃ heteroepitaxy（α-(AlₓGa₁₋ₓ)₂O₃

      Researchers from the Nanjing University have published a dissertation titled " Strain-mediated compositional pulling effect in α-(AlₓGa₁₋ₓ)₂O₃ heteroepitaxy（α-(AlₓGa₁₋ₓ)₂O₃" in Applied Physics Letters.

Background

      Gallium oxide (Ga₂O₃), with its ultrawide bandgap and high theoretical breakdown field, has emerged as a compelling semiconductor for high-power and high-frequency electronic devices. Among its various polymorphs, monoclinic β-Ga₂O₃ has received particular attention owing to the availability of large-area, high-quality single-crystal substrates grown by melt-based techniques. Nevertheless, the relatively modest electron mobility of bulk β-Ga₂O₃ remains a key obstacle to realizing high-performance radio frequency (RF) transistors, as it fundamentally limits carrier transport and frequency response. To overcome this limitation, extensive efforts have focused on heterostructure engineering. In particular, high-quality β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructures grown on various β-Ga₂O₃ substrate orientations have enabled the formation of two-dimensional electron gases (2DEGs) through band-offset engineering and modulation doping, providing a viable route toward β-Ga₂O₃-based high-frequency and high-power transistors.

Abstract

      Precise compositional control is essential for tailoring α-(Al_xGa_1−x)₂O₃ heterostructures, yet is often compromised by the compositional pulling effect (CPE) during heteroepitaxy. Here, α-(Al_xGa_1−x)₂O₃ epilayers spanning a broad Al-composition range of x = 0.42–0.92 were grown on nonpolar m-plane sapphire substrates to elucidate the strain-mediated origin of the CPE. Critical-thickness calculations, reciprocal-space mapping, energy-dispersive x-ray spectroscopy, and geometric phase analysis collectively identify a composition-dependent transition from strain-relaxed growth of α-(Al_xGa_1−x)₂O₃ at x = 0.42–0.62 to fully strained pseudomorphic growth at x = 0.71–0.92. The relaxed α-(Al_xGa_1−x)₂O₃ epilayers exhibit pronounced Al enrichment near the heterointerface, with the compositional grading length contracting from approximately 72 to 22 nm as the Al composition increases, whereas the fully strained α-(Al_xGa_1−x)₂O₃ epilayers are compositionally uniform. Comparative growth of α-(Al_0.40Ga_0.60)₂O₃ on sapphire and α-Ga₂O₃ templates further reveals a reversal of the cation-redistribution pathway: compressive strain promotes Al incorporation, while tensile strain drives Ga enrichment. These results establish the CPE as a bidirectional, strain-mediated cation-incorporation response and provide a physical basis for interfacial composition engineering in α-(Al_xGa_1−x)₂O₃ heterostructures.

Highlights

      In α-(AlₓGa₁₋ₓ)₂O₃epitaxial layers with an Al composition ranging from 0.42 to 0.92, strain relaxation and interfacial Al enrichment were observed when the Al composition was between 0.42 and 0.62, whilst fully strain-controlled growth and essentially uniform composition were maintained in the range of 0.71 to 0.92.

      Compressive strain promotes Al incorporation, whilst tensile strain promotes Ga enrichment, indicating that the direction of interfacial strain determines the direction of compositional redistribution.

      By linking the composition pull effect with strain relaxation and elastic strain energy, this study provides both theoretical and experimental grounds for the design and control of interfacial composition in α-(AlₓGa₁₋ₓ)₂O₃heterostructures.

Conclusion

      In summary, we uncover a strain-mediated, bidirectional compositional pulling effect in α-(AlₓGa₁₋ₓ)₂O₃ heteroepitaxy. A composition-dependent transition from relaxed, compositionally graded growth to fully strained pseudomorphic growth is identified and directly correlated with the evolution of local strain fields. We further demonstrate that the polarity of the interfacial strain selectively favors Al or Ga incorporation, establishing elastic strain energy as the governing driving force for alloy redistribution. This mechanism provides a foundation for strain-engineered composition control and the rational design of corundum-structured oxide heterostructures.