Theoretical Characterization of Rutile GeₓSn₁₋ₓO₂ Alloys as Ultra-wide Bandgap Semiconductors

GeₓSn₁₋ₓO₂ alloys have recently attracted attention as candidate ultra-wide bandgap (UWBG) materials for power electronics due to their predicted ambipolar dopability, high carrier mobilities, and high thermal conductivity. Experiments show that these alloys can be grown as thin films over a wide composition range, have carrier mobilities that are insensitive to alloy disorder at low Ge content, and potentially exhibit record breakdown fields. In this study, we perform a comprehensive first-principles investigation of the structural, thermodynamic, electronic, and optical properties of GeₓSn₁₋ₓO₂ alloys using DFT and GW calculations. Our results show that these alloys form random solid solutions across all compositions, with weak short-range order and lattice parameters that follow Vegard's Law. The quasiparticle band structures show a direct bandgap with strong compositional bowing and light carrier effective masses. Optical absorption calculations for the 50% alloy reveal that local alloy disorder relaxes the dipole-forbidden character of the fundamental gaps in the binaries to varying strength depending on light polarization. Our results reveal key properties of GeₓSn₁₋ₓO₂ that highlight its potential for applications as an UWBG semiconductor.