2.

AFM scans (2 × 2 μm² area) of Ga₂O₃ films grown on (a–d) a-plane (films A₁–A₄), (e–h) c-plane (films C₁–C₄), (i–l) r-plane (films R₁–R₄), and (m–p) m-plane (films M₁–M₄) sapphire substrates, with thicknesses between 13 and 100 nm (T _gr = 500 °C). Note that film names have the format X _i , where X indicates the substrate orientation (A: a-plane, M: m-plane, C: c-plane, and R: r-plane), and the subscript i corresponds to the film thickness (1:13 nm, 2:20 nm, 3:50 nm, and 4:100 nm). The surface roughness of all 16 samples is plotted in Figure .