Phase Diagram and Growth Mechanism of Rutile-Type GeO2 Epitaxial Film on c-Plane Sapphire Substrate

Abstract

Rutile-type GeO₂ (r-GeO₂) is expected as a next-generation ultra-wide-band-gap oxide semiconductor with controllable p-type and n-type conduction. However, the existence of other polymorphs, such as α-quartz and amorphous phases, makes it challenging to grow a single-phase r-GeO₂ film. Here, we investigate the effect of growth temperature (T_g) and oxygen pressure (P_O2) on the phase stability of GeO₂ films grown on c-plane α-Al₂O₃ substrates by pulsed laser deposition and establish a growth phase diagram. The GeO₂ films deposited at 400 °C are amorphous, while (100)-oriented r-GeO₂ crystalline films are stabilized only under reducing conditions (P_O2 ≤ 0.1 Pa) within a T_g range of 500–600 °C due to severe re-evaporation at higher T_g. However, the deep defect states, which are detected by optical absorption in its band gap and probably related to oxygen vacancies (V_O), form in the r-GeO₂ films, and the amount of V_O increases under lower P_O2 conditions. On the other hand, increasing P_O2 suppresses both re-evaporation and V_O formation, but further higher P_O2 promotes amorphous phase growth simultaneously. The r-GeO₂ films with 6-fold-rotational columnar domains are grown epitaxially on c-plane Al₂O₃ substrates, with the amorphous phase forming in the gaps between the domains. Precise control of T_g and P_O2 is crucial for obtaining high-quality r-GeO₂ films, as there is a competition between re-evaporation and the formation of amorphous phase.