Optical Characterization of Gallium Oxide α and β Polymorph Thin-Films Grown on c-Plane Sapphire

In this work, the optical properties of thin-film α-Ga₂O₃ are examined and contrasted with those of β-Ga₂O₃ thin films grown on similar substrates. Thin films of α-Ga₂O₃ were synthesized employing mist chemical vapor deposition (mist CVD) on double-side polished c-plane sapphire. Optical properties were studied through absorbance, photoluminescence (PL), and photoluminescence excitation (PLE). Absorbance measurements showed spectra consistent with that previously reported in the literature, with E_g ~ 5.19 eV for α-Ga₂O₃. Despite an absorption edge laying at higher energies, the PL spectra in α-Ga₂O₃ films yield similar features to those of metal-organic chemical vapor deposition (MOCVD) grown β-Ga₂O₃ thin-films. In both cases, when excited above the band gap, the PL spectra consist of UV, UV′, blue and green broad bands. As the excitation photon energy was varied, the relative intensity of these PL bands changed. This is attributed to a non-uniform defect concentration distribution across the sample depth. With smaller concentration of defects associated with blue and green emission in the bulk compared to the surface in the α-phase films.