Orientation-Dependent Heteroepitaxy and Growth-Mediated Stabilization of (111) Rutile GeO2 on Rutile TiO2 by MOCVD

Abstract

Rutile GeO₂ (r-GeO₂) is a promising ultrawide bandgap semiconductor; however, achieving controlled epitaxial films remains challenging due to competing phases and growth kinetics. In this work, we demonstrate orientation-dependent growth of GeO₂ films on rutile TiO₂ substrates with (100), (110), (001), and (111) orientations using metal–organic chemical vapor deposition (MOCVD). Among the investigated orientations, the (111) surface supports the formation of continuous, predominantly r-GeO₂ films with comparatively reduced surface roughness, whereas the (001) orientation also enables rutile phase formation but with higher roughness, and the other orientations showed mixed phases and non-uniform morphologies. Reducing growth pressure improves crystalline quality, with the rocking curve full width at half maximum (FWHM) decreasing from 1.86° to 0.396° at 300 and 80 Torr, respectively. A direct comparison between continuous and seed-driven stepwise crystallization (SDSC) growth indicates that the segmented growth pathway assists rutile-phase stabilization under the present MOCVD conditions, whereas continuous growth yields amorphous films. The resulting films exhibit continuous morphology with comparatively reduced roughness (∼23 nm) and remain stable after water treatment. Under the present high-temperature X-ray diffraction conditions, amorphous films show no detectable crystallization up to 950 °C, suggesting that crystallization is strongly influenced by growth-mediated conditions. These results offer guidelines for the controlled growth of (111)-oriented r-GeO₂ films for ultrawide-bandgap semiconductor applications.