Growth and optical properties of polar-, nonpolar, and semi-polar wurtzite ScGaN films

Wurtzite ScGaN offers a promising platform for anisotropic optoelectronic and piezoelectric functionalities, provided that its crystallographic orientation can be precisely controlled. Here, we demonstrate systematic orientation engineering of ScGaN thin films grown by reactive magnetron sputter epitaxy on c-, r-, and m-plane Al2O3 substrates, enabled by the strategic use of an ultra-thin GaN buffer layer. X-ray diffraction (XRD) and scanning transmission electron microscopy confirm the growth of polar, non-polar, and semi-polar ScGaN films on sapphire. A comparison of growth conditions reveals that the GaN buffer layer is essential for stabilizing these orientations and ensuring high epitaxial quality. Consequently, the c-plane ScGaN (0002), a-plane ScGaN (112¯0), and semi-polar-plane ScGaN (112¯2) were indexed through combined XRD and selected area electron diffraction. Spectroscopic ellipsometry combined with Mueller-matrix analysis reveals pronounced orientation-dependent optical anisotropy across all films. Cody-plot analysis further reveals that ScGaN has a wider bandgap (3.504 eV) than GaN (3.385 eV). This confirms polarization-resolved absorption onsets reflecting the wurtzite valence band anisotropy. The extracted optical-axis tilt angles quantitatively confirm polar, non-polar, and semi-polar configurations of ScGaN on sapphire substrates. Together, these results highlight buffer-assisted orientation control as an effective strategy to tailor the anisotropic properties of ScGaN.