Stabilization of α-Ga2O3 on m-plane α-Al2O3 by Plasma-Assisted MBE

*E-mails: jinno.riena.68a@kyoto-u.ac.jp, djena@cornell.edu Metastable α-Ga2O3, which has the rhombohedral corundum structure, has attracted a lot of interest in recent years, because its alloys with α-In2O3 and α-Al2O3 (sapphire) allow wide bandgap engineering from 3.7 to 8.7 eV [1]. Very recently, it has been demonstrated that binary single phase α-Al2O3/Ga2O3 superlattices can be coherently grown on r-plane sapphire by MBE [2]. However, the growth of thick isomorphic α-Ga2O3 is challenging as the stabilization of α-Ga2O3 is limited by the formation of the thermodynamically most stable phase β-Ga2O3, which was found to depend on the orientation of sapphire surface [3]. In this research, by comparatively studying Ga2O3 grown on rand m-plane sapphire, we show that α-Ga2O3 is considerably better stabilized on m-plane sapphire. Ga2O3 was grown on rand m-plane sapphire substrates by oxygen plasma-assisted MBE, at a substrate thermocouple temperature of 650°C. The r-plane sapphire substrate was annealed at 1050°C in air in order to obtain atomic steps in the surface morphology. Single phase epitaxial α-Ga2O3 is confirmed by the XRD (Fig.1). The (αhν)-hν Uv-vis transmission spectra of the Ga2O3 films have two step-like onsets (Fig.2), which has been described earlier as two allowed direct transitions with excitonic effects in the α-Ga2O3 (5.61 and 6.44 eV) [4]. The XRD and in situ RHEED patterns [Fig. 1 (a) and (c)] confirm that isomorphic α-Ga2O3 was stabilized on m-plane sapphire up to a film thickness of ca. 200 nm. On the other hand, RHEED patterns of Ga2O3 grown on r-plane sapphire revealed the inclusion of β-Ga2O3 phases [Fig.1 (d)]. These results suggest that m-plane sapphire is more suitable for the growth of thick α-Ga2O3 films. The explanation of the enhanced stability of α-Ga2O3 can be attributed to geometrical effect: the c-plane facet is believed to facilitate the formation of the β-Ga2O3 [5] and when Ga2O3 is grown on m-plane sapphire, unlike cand r-plane sapphire, the c-plane is exactly perpendicular to the growth direction and therefore the c-plane facet can be greatly shadowed from growth.