High-quality β-(Al_xGa_1−x)₂O₃ thin films on sapphire substrates by face-to-face annealing

High-quality β-(Al_xGa_1−x)₂O₃ thin films are fabricated through face-to-face annealing on sapphire substrates covered with epitaxial Ga₂O₃. Al atoms during high-temperature annealing are uniformly diffused from the sapphire substrate into the overlying Ga₂O₃ for (Al_xGa_1−x)₂O₃ formation. The transformation of the (−201) crystal facet into the (300) crystal facet is observed, which is likely due to the lowest E^(hkl)_surf of the (100) surface. The crystallinity can be further improved with increasing the annealing temperatures and the duration of the thermal treatment under a tolerance limit of 1400 °C, which also increases the Al content to 0.68 in β-(Al_xGa_1−x)₂O₃ with a bandgap of up to 6.0 eV. As a result, highly-crystalline (300)-plane β-(Al_xGa_1−x)₂O₃ is obtained with a low X-ray diffraction (XRD) full width at half maximum (FWHM) at around 0.08° (288 arcsec). These findings contribute to the understanding of the preparation of high-quality β-(Al_xGa_1−x)₂O₃ films on sapphire substrates.