Sub-bandgap optical spectroscopy of epitaxial β -Ga₂O₃ thin films

Room temperature sub-gap optical absorption spectra measured by photothermal deflection spectroscopy were investigated for hetero- and homo-epitaxial β-Ga₂O₃ layers grown by plasma-assisted molecular beam epitaxy as well as for a bulk crystal. The absorption spectra show a pronounced exponential Urbach tail with slope parameters of 120-150 meV in the spectral region between 4.5 and 5 eV, indicating an unusually large self-trapping energy of excitons. In addition, an absorption band related to deep defects is observed in the spectral region from 2.5 to 4.5 eV. The steepness of the Urbach tail as well as the strength of the defect-related absorption can be influenced and optimized by annealing at 900-1000 °C in an oxygen atmosphere. Similar features were also observed for bulk β-Ga₂O₃ crystals and for homoepitaxial β-Ga₂O₃ layers. The present results for β-Ga₂O₃ are compared and discussed in the context of similar measurements for other wide-bandgap semiconductors of current interest in electronics and photocatalysis: GaN, ZnO, TiO₂, and BiVO₄.