Solid phase epitaxy of rutile-type GeO2 on TiO2 (001)

Rutile-type GeO2 (r-GeO2) is an ultra-wide bandgap oxide semiconductor, which attracts growing interest because of its ambipolar dopability and high carrier mobility predicted by recent first-principles calculations. While epitaxial thin films of r-GeO2 have been synthesized using various vapor phase epitaxy techniques, desorption of germanium suboxide limits the growth conditions in a narrow range. Here, we demonstrated solid phase epitaxy (SPE) of r-GeO2 thin films on TiO2 (001) substrates. Amorphous GeO2 thin films fabricated using pulsed laser deposition at ≤250 °C were crystallized by post-deposition annealing at 700 °C. X-ray diffraction and cross-sectional scanning transmission electron microscopy analysis revealed that the amorphous GeO2 thin films epitaxially crystallized to r-GeO2 phase without any impurity phases. The epitaxial crystallization of r-GeO2 was significantly promoted by introducing a seed layer of a rutile GexSn1−xO2 epitaxial thin film, which was coherently grown on the TiO2 substrate, probably due to chemical interactions between the amorphous film and the seed layer. The crystallinity of the SPE-grown r-GeO2 was comparable to that synthesized via vapor phase epitaxy, indicating that the SPE is an alternative route for synthesizing r-GeO2 epitaxial thin films.