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

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 (Tg) and oxygen pressure (PO2) 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 (PO2 ≤ 0.1 Pa) within a Tg range of 500–600 °C due to severe re-evaporation at higher Tg. However, the deep defect states, which are detected by optical absorption in its band gap and probably related to oxygen vacancies (VO), form in the r-GeO2 films, and the amount of VO increases under lower PO2 conditions. On the other hand, increasing PO2 suppresses both re-evaporation and VO formation, but further higher PO2 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 Tg and PO2 is crucial for obtaining high-quality r-GeO2 films, as there is a competition between re-evaporation and the formation of amorphous phase.