【International Papers】Atomic-scale investigation of γ-Ga₂O₃ deposited on MgAl₂O₄ and its relationship with β-Ga₂O₃

      Researchers from the Carnegie Mellon University have published a dissertation titled " Atomic-scale investigation of γ-Ga₂O₃ deposited on MgAl₂O₄ and its relationship with β-Ga₂O₃ " in APL Materials.

ABSTRACT

      Nominally phase-pure γ-Ga₂O₃ was deposited on (100) MgAl₂O₄ within a narrow temperature window centered at ∼470 °C using metal-organic chemical vapor deposition. The film deposited at 440 °C exhibited either poor crystallization or an amorphous structure; the film grown at 500 °C contained both β-Ga₂O₃ and γ-Ga₂O₃. A nominally phase-pure β-Ga₂O₃ film was obtained at 530 °C. Atomic-resolution scanning transmission electron microscopy (STEM) investigations of the γ-Ga₂O₃ film grown at 470 °C revealed a high density of antiphase boundaries. A planar defect model developed for γ-Al₂O₃ was extended to explain the stacking sequences of the Ga sublattice observed in the STEM images of γ-Ga₂O₃. The presence of the 180° rotational domains and 90° rotational domains of β-Ga₂O₃ inclusions within the γ-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 °C and the same film annealed at 600 °C. The results led to the hypotheses that (i) incorporation of certain dopants, including Si, Ge, Sn, Mg, Al, and Sc, into β-Ga₂O₃ locally stabilizes the “γ-phase” and (ii) the site preference(s) for these dopants promotes the formation of “γ-phase” and/or γ-Ga₂O₃ solid solutions. However, in the absence of such dopants, pure γ-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 γ-Ga₂O₃ and β-Ga₂O₃ than all other polymorphs.

FIG. 1. (a) XRD patterns (log scale) for Ga₂O₃ films grown on (100) MgAl₂O₄ substrates for 1 h. The black stars indicate the (400) reflection from the substrate. (b) XRD ϕ-scans of {440} γ-Ga₂O₃ (grown at 470 °C) and MgAl₂O₄. (c) Pole figure of diffraction from {444} γ-Ga₂O₃ grown at 470 °C. (d) XRD rocking curves of (800) symmetric and (440) asymmetric diffraction peaks of γ-Ga₂O₃ grown at 470 °C. (e) RSM around 804 diffraction spot of film/substrate combination (film grown at 470 °C).

FIG. 2. Low-magnification cross-sectional bright-field STEM images of the film grown for 2 h at 470 °C, acquired along the (a) [010] zone and (d) [011] zone. SAED patterns of the same γ-Ga₂O₃ layer acquired along the (b) [010] zone and (e) [011] zone. Simulated SAED patterns of γ-Ga₂O₃ acquired along the (c) [010] zone and (f) [011] zone.

FIG. 3. HAADF-STEM image acquired along the [010] zone axis for the γ-Ga₂O₃ film grown at 470 °C (left) and corresponding integrated EDX composition profiles (right). The region and direction of the corresponding EDX profiles are highlighted in the yellow dashed box.

Paper Link：https://doi.org/10.1063/5.0180922