Abstract
Single α-phase (AlxGa1−x)2O3 thin films are grown on m-plane sapphire (α-Al2O3) substrates via metalorganic chemical vapor deposition. By systematically tuning the growth parameters including the precursor molar flow rates, chamber pressure, and growth temperature, the epitaxial growth of high-quality phase pure α-(AlxGa1−x)2O3 films (0 ≤ x ≤ 1) is demonstrated with smooth surface morphologies and alloy homogeneities by comprehensive material characterization. The asymmetrical reciprocal space mapping reveals fully relaxed films for α-(AlxGa1−x)2O3 films with x ≤ 0.5. The coherent growth of α-(AlxGa1−x)2O3/α-Al2O3 superlattice structures is demonstrated with abrupt interfaces and uniform Al distribution for higher Al compositions at x = 0.78 in the α-(AlxGa1−x)2O3 layer. The influence of growth parameters, such as growth temperature and chamber pressure, on the phase stabilization and Al incorporation in the α-(AlxGa1−x)2O3 films is investigated. While lower growth temperatures facilitate the phase stabilization of α-Ga2O3 thin films, lower chamber pressure leads to higher Al incorporation in α-(AlxGa1−x)2O3 films. High resolution x-ray photoelectron spectroscopy was utilized for determining the Al compositions and bandgaps of α-(AlxGa1−x)2O3. Furthermore, the evolution of the valance and conduction band offsets at α-Al2O3/α-(AlxGa1−x)2O3 heterojunctions is evaluated with the variation of Al compositions, which reveals the formation of type-I (straddling) band alignment between α-Al2O3 and α-(AlxGa1−x)2O3.
Highlights
Gallium oxide (Ga2O3), having five different polymorphs (α, β, γ, δ, and ε)[1] and an ultrawide bandgap energy (∼4.5 to 5.3 eV),[2,3] is a transparent semiconducting material promising for high-power electronics due to its high predicted breakdown field strength (∼8 MV/cm).[3]
For the first time, we have investigated the metalorganic chemical vapor deposition (MOCVD) growth of α-Ga2O3 and α-(AlxGa1−x)2O3 alloys on mplane sapphire substrates for the entire Al composition range
The crystalline structure and quality of the epi-films, strain characteristics, compositional homogeneity, surface morphology, and the bandgaps of α-(AlxGa1−x)2O3 are evaluated by comprehensive characterization via x-ray diffraction (XRD), high resolution scanning transmission electron microscopy (HR STEM), energy dispersive x-ray spectroscopy (EDX), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), field emission scanning electron microscopy, and Raman spectroscopy, which reveal the high quality epitaxial growth of α-Ga2O3 and α-(AlxGa1−x)2O3 alloys on m-plane sapphire substrates via MOCVD
Summary
Gallium oxide (Ga2O3), having five different polymorphs (α, β, γ, δ, and ε)[1] and an ultrawide bandgap energy (∼4.5 to 5.3 eV),[2,3] is a transparent semiconducting material promising for high-power electronics due to its high predicted breakdown field strength (∼8 MV/cm).[3]. While achievable film thicknesses of phase pure α-Ga2O3 are limited due to the phase transition that occurs by the formation of c-plane facets on the α-Ga2O3 surface during the growth on c- and r-plane sapphire, a crystal plane that is perpendicular to the c-plane, such as a- or m-planes, can potentially promote the growth of phase pure α-Ga2O3 and α-(AlxGa1−x)2O3 alloys by avoiding the formation of c-plane facets Based on this principle, by using the m-plane sapphire substrate, a recent MBE growth study has revealed a pathway to achieve phase pure (56–84.3 nm thick) α-(AlxGa1−x)2O3 alloys for the entire Al composition range with bandgap energies of 5.4–8.6 eV.[32]. The crystalline structure and quality of the epi-films, strain characteristics, compositional homogeneity, surface morphology, and the bandgaps of α-(AlxGa1−x)2O3 are evaluated by comprehensive characterization via x-ray diffraction (XRD), high resolution scanning transmission electron microscopy (HR STEM), energy dispersive x-ray spectroscopy (EDX), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), field emission scanning electron microscopy, and Raman spectroscopy, which reveal the high quality epitaxial growth of α-Ga2O3 and α-(AlxGa1−x)2O3 alloys on m-plane sapphire substrates via MOCVD
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