The Effect of Strain on the Band Offsets in Monoclinic β-(Al_xGa_1-x)₂O₃ Alloys

Alloys of gallium oxide and aluminum oxide offer a tunable ultrawide bandgap reaching far into the ultraviolet-C spectral region and permit device architectures with potentially very large breakdown fields, thus are promising for applications in high power electronic devices. The volume of the unit cell of Al₂O₃ is smaller than that of Ga₂O₃. As a result, the incorporation of aluminum into Ga₂O₃ leads to a shrinking of the lattice. In the case of pseudomorphic heteroepitaxial growth, when the epitaxial layer adopts the interfacial lattice spacing of the template, the epitaxial layer is under strain. The alignment of energy bands between the substrate and the strained epitaxial film remains an open question. We propose a model for such band offsets in the specific case of β-(Al_xGa_1-x)₂O₃ alloys grown on beta Ga₂O₃ substrates. Density functional theory (DFT) calculations of the band structure of both, β-Ga₂O₃ and θ-Al₂O₃, under various strain scenarios, in combination with Vegard’s rule allow us to construct a linear model on how the branch-point energy depends on the four independent components of the monoclinic strain tensor across the entire composition range. We apply this model to predict the alignment of the energy bands for specific strain patterns associated with the pseudomorphic growth of β-(Al_xGa_1-x)₂O₃ alloys on, for example, (010) or (-201) faces of Ga₂O₃.