Undoped Ga2O3 thin films were synthesized from an aqueous solution by using mist chemical vapor deposition. The defective-spinel-type γ-phase and corundum-type α-phase were epitaxially stabilized when deposited on (100) MgAl2O4 and (0001) sapphire substrates, respectively. The epitaxial relationship for the γ-phase film was identified to be cube on cube. We found that these metastable phases were obtained at lower growth temperatures, which gradually transformed to the stable β-phase with increasing temperatures. The temperature dependence of growth rate on the MgAl2O4 exhibited a clear transition from surface-reaction-limited to diffusion-limited regime and a growth window for the γ-phase was found in the vicinity of the transition. For a pure γ-phase film, refractive index in a visible region, direct and indirect band-gaps were estimated to be 2.0∼2.1, 5.0 and 4.4 eV, respectively, at room temperature.
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