Rutile-Structured GeO ₂ Thin Films Grown on Various Planes of α-Al ₂ O ₃ Substrates with Graded Ge _<i>x</i> Sn _{1– <i>x</i>} O ₂ Buffer Layers

Rutile-structured germanium dioxide (r-GeO2) has gained considerable attention owing to its ultrawide bandgap and ambipolar doping ability. In this study, we investigated the mist chemical vapor deposition of r-GeO2 on various sapphire (α-Al2O3) substrate orientations. X-ray diffraction revealed that graded GexSn1–xO2 buffer layers facilitated the epitaxial growth of r-GeO2 on c-, a-, m-, and r-plane α-Al2O3 substrates. Conversely, GeO2 films grown directly on bare α-Al2O3 substrates exhibited an amorphous or fine crystal state. Scanning electron microscopy demonstrated that the surface morphology of r-GeO2 varied based on the crystallographic orientation of the underlying α-Al2O3. The optical transmittance measurements of the (001)-oriented r-GeO2 grown on m-plane α-Al2O3 demonstrated a forbidden direct transition at 4.60 eV, which is close to the fundamental bandgap of r-GeO2. Furthermore, optical measurements suggested the existence of a defect level with an optical absorption at approximately 4 eV in r-GeO2. These findings present crucial insights into the heteroepitaxial growth behavior of r-GeO2 on α-Al2O3 substrates.