Crystallinity degradation and defect development in (Al_xGa_1−x)₂O₃ thin films with increased Al composition

We systematically analyzed the Al composition dependences of the structural properties of (Al $x$Ga $1 − x$⁠)₂O₃ thin films grown on $β$-Ga₂O₃ (010) substrates. The crystal structure was characterized by x-ray diffraction, and the surface morphology was observed by reflection high-energy electron diffraction and atomic force microscopy. In the 100-nm-thick thin films, the crystallinity began to degrade and defects appeared on the surface when the Al composition $x$ exceeded about 0.16. The defects developed mainly along the $[201]$ direction and slightly along the $[001]$ direction as $x$ increased. The boundary where the thin film quality changed was close to a critical thickness curve calculated using the Matthews–Blakeslee model assuming the slip system of $⟨201⟩{10 2 ¯}$⁠.