Bandgap engineering of α-(Al_xGa_1-x)₂O₃ by a mist chemical vapor deposition two-chamber system and verification of Vegard's Law

This work reports growth of α-(Al_xGa_1-x)₂O₃ single crystals with high incorporation of Al by a Mist Chemical Vapor Deposition two-chamber system, which was rationally designed to avoid side-reactions between different precursors during solution preparation for multi-component thin film growth. Multiple acceleration voltages were used in Energy Dispersive X-ray measurements to reliably obtain the Al composition x of the films. As a result, Vegard's law for lattice constants was verified and found to be valid in the α-(Al_xGa_1-x)₂O₃ system. However, Vegard's law for optical bandgaps, derived from different models, required an additional term to account for the bowing effect. At x = 0.71, the gaps were 7.74, 7.03, 7.26, and 7.34 eV as derived from the Tauc plots for the direct bandgap, indirect bandgap, Tauc-Lorentz model, and O'Leary-Johnson-Lim model, respectively. The two-chamber system provides reliable and effective control of the Al content in α-(Al_xGa_1-x)₂O₃ alloys and heterostructures.