Unraveling the Growth Dynamics of Rutile Sn_{1- x}Ge_xO₂ Using Theory and Experiment

Rutile GeO₂ and related materials are attracting interest due to their ultrawide band gaps and potential for ambipolar doping in high-power electronic applications. This study examines the growth of rutile Sn_1-xGe_xO₂ films through oxygen-plasma-assisted hybrid molecular beam epitaxy (hMBE). The film composition and thickness are evaluated across a range of growth conditions, with the outcomes rationalized by using density functional theory calculations. We find that up to 34% Ge can be successfully incorporated into Sn_1-xGe_xO₂/r-Al₂O₃ (x ≤ 0.34) at 600 °C. Our phase diagram calculations suggest that spinodal decomposition occurs at Ge concentrations exceeding 34%. However, the formation of a Ge-rich rutile phase is inhibited by amorphization of the Ge-rich film and volatility of GeO. We therefore speculate that maximizing the Ge content requires higher Ge flux and more oxidizing environments, providing insights into the growth mechanism of Sn_1-xGe_xO₂ and paving the way toward the synthesis of pure rutile GeO₂ films.