Academic Paper attributes
Nominally phase-pure gamma-Ga2O3 was deposited on (100) MgAl2O4 within a narrow temperature window centered at sim470 circC using metal-organic chemical vapor deposition (MOCVD). The film deposited at 440 circC exhibited either poor crystallization or an amorphous structure; the film grown at 500 circC contained both beta-Ga2O3 and gamma-Ga2O3. A nominally phase-pure beta-Ga2O3 film was obtained at 530 circC. Atomic-resolution scanning transmission electron microscopy (STEM) investigations of the gamma-Ga2O3 film grown at 470 circC revealed a high density of antiphase boundaries. A planar defect model developed for gamma-Al2O3 was extended to explain the stacking sequences of the Ga sublattice observed in the STEM images of gamma-Ga2O3. The presence of the 180circ rotational domains and 90circ rotational domains of beta-Ga2O3 inclusions within the gamma-Ga2O3 matrix is discussed within the context of a comprehensive investigation of the epitaxial relationship between those two phases in the as-grown film at 470 circC and the same film annealed at 600 circC. The results led to the hypotheses that (i) incorporation of certain dopants including Si, Ge, Sn, Mg, Al, and Sc, into beta-Ga2O3, locally stabilizes the gamma-phase and (ii) the site preference(s) for these dopants promotes the formation of the gamma-phase and/or gamma-Ga2O3 solid solutions. However, in the absence of such dopants, pure gamma-Ga2O3 remains the least stable Ga2O3 polymorph, as indicated by its very narrow growth window, lower growth temperatures relative to other Ga2O3 polymorphs, and the largest calculated difference in Helmholtz free energy per formula unit between gamma-Ga2O3 and beta-Ga2O3 than all other polymorphs.
