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

Rutile-type GeO 2 (r-GeO 2 ) 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-GeO 2 film. Here, we investigate the effect of growth temperature ( T g ) and oxygen pressure ( P O 2 ) on the phase stability of GeO 2 films grown on c -plane α-Al 2 O 3 substrates by pulsed laser deposition and establish a growth phase diagram. The GeO 2 films deposited at 400 °C are amorphous, while (100)-oriented r-GeO 2 crystalline films are stabilized only under reducing conditions ( P O 2 ≤ 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-GeO 2 films, and the amount of V O increases under lower P O 2 conditions. On the other hand, increasing P O 2 suppresses both re-evaporation and V O formation, but further higher P O 2 promotes amorphous phase growth simultaneously. The r-GeO 2 films with 6-fold-rotational columnar domains are grown epitaxially on c -plane Al 2 O 3 substrates, with the amorphous phase forming in the gaps between the domains. Precise control of T g and P O 2 is crucial for obtaining high-quality r-GeO 2 films, as there is a competition between re-evaporation and the formation of amorphous phase.