Bismuth-assisted stabilization of rutile GeO2 thin films grown by Mist-CVD

• Bi incorporation acts as an efficient surfactant during Mist-CVD growth of rutile GeO 2 . • Bi promotes Ge 4+ diffusion into TiO 2 , forming a graded Ti 1-x Ge x O 2 interfacial region. • The transition layer reduces lattice mismatch and induces controlled strain in GeO 2 . • Low Bi concentrations enhance crystallinity, while high Bi content disrupts surfactant action. • Results enable an effective interface strategy for the controlled synthesis of metastable UWBG oxides. In this work, we investigated the effect of bismuth incorporation on the growth and stabilisation of r-GeO 2 films deposited on rutile TiO 2 (001) substrates by mist chemical vapour deposition (Mist-CVD). The addition of only 1% Bi acted as an effective surfactant, facilitating the growth of the rutile GeO 2 phase. The grown films were homogeneous, flat and resistant to water degradation. High-Resolution XRD (HRXRD) measurements suggested the formation of a diffusion zone in the upper side of the r-TiO 2 substrate, which reduced the mismatch and enhanced the crystallinity of the r-GeO 2 . Further examinations using Secondary Ion Mass Spectrometry (SIMS) and High-Resolution TEM (HRTEM) confirmed the presence of this diffusion zone. Cross-sectional TEM revealed a single-crystal rutile structure in the main body of the film. This study demonstrates the effectiveness of Bi-assisted growth to obtain r-GeO 2 thin films while providing insight into the effects occurring at the interphase that govern structural formation and chemical stability.