Bandgap nonlinearity and composition-dependent bowing in <i>α</i>-(AlxGa1−x)2O3 epilayers

Designing functional materials with tailored properties often involves alloying different semiconductors, yet the nonlinear bandgap bowing effect complicates precise bandgap engineering, particularly in ultrawide-bandgap systems, such as Ga2O3 ternary alloys. In this work, we examined the bandgap nonlinearity and composition-dependent bowing in pure-phase α-(AlxGa1-x)2O3 epilayers (0 < x < 0.62) grown via laser molecular beam epitaxy on m-plane sapphire substrates. The variations in the x-ray rocking curve full width at half maximum and surface roughness of epilayers with increasing Al composition x follow the trend predicted by the theoretical formation enthalpy of α-(AlxGa1-x)2O3 alloys. The high crystalline quality of the α-(AlxGa1-x)2O3 epilayers was further confirmed by x-ray diffraction and transmission electron microscopy characterizations. While lattice constants adhered to Vegard's law, the optical bandgap (5.28-7.22 eV) exhibited nonlinearity, with a bowing factor of 1.33 eV, aligning closely with theoretical predictions. Our findings suggest that the observed optical bandgap nonlinear effect in the α-(AlxGa1-x)2O3 alloy primarily stems from charge exchange, rather than volume deformation or strain relaxation effects, providing a pathway for precise bandgap tuning in Ga2O3 for high-performance power electronics.