Direct Imaging on Strain Relaxation of MBE-grown Single Phase alpha-(Al,Ga)₂O₃ on m-sapphire Substrate in Atomic Resolution Using Scanning Transmission Electron Microscopy

MBE-grown corundum structured alpha-Ga₂O₃ on c-plane sapphire substrates often contains considerable amount of beta-Ga₂O₃ due to c-plane facets. Recently, a successful MBE-growth of pure alpha-(Al, Ga)₂O₃ on m-plane sapphire was reported. Here, we show the relaxation mechanism of such films by performing strain analysis with scanning transmission electron microscopy (STEM). The films are partially relaxed through dislocation cores at the interface, some of which tend to climb up into the film. High angle annular dark field (HAADF)-STEM images in plan-view shows the formation of very thin gamma-Ga₂O₃ at the interface occupying less than 1% area density. Owing to its defect spinel-type structure that requires two cation vacancies, gamma-Ga₂O₃ is believed to form naturally at the interface to accommodate the strain arising from lattice mismatch.