Dominant Effects of Epitaxial Strain on the Phase Control of Heterostructural (In_xGa_1–x)₂O₃ Alloys

While (InxGa1–x)2O3 alloy is a crucial system for the Ga2O3-based ultrawide bandgap semiconductor application, its successful phase control has been struggling because of its heterostructural nature and rich polymorphs. Here, we identified the thermodynamic phase diagrams for both the bulk state and epitaxial state of (InxGa1–x)2O3 alloy by using comprehensive density afunctional theory (DFT) calculations and regular solution models, which is consistent with previous experimental reports. By comparing the phase diagrams under a strain-free condition and an epitaxial strain condition, we demonstrate that the epitaxial strain is a significant factor in the successful growth of alloys in heteroepitaxy processes. While the alloying of (InxGa1–x)2O3 is limited by a miscibility gap under the strain-free condition, the Al2O3 heteroepitaxy substrate opens more metastable regions for various polymorphs. With the choice of a suitable substrate, we also suggest the phase control strategy for (InxGa1–x)2O3 alloys in orthorhombic polymorphs.