Understanding the effects of annealing induced structural transformations on the UVC absorbance and other optical properties of RF sputter deposited Ga2O3 thin films

Gallium oxide (Ga₂O₃) is a transparent material with high absorption in the UVC region of the electromagnetic spectrum and hence is a very important candidate in the field of short wavelength optical device fabrication. A proper understanding of the different optical parameters is necessary for developing more efficient coatings and devices. In this work, changes in the optical behavior of Ga₂O₃ thin films due to post-deposition annealing (at temperatures 300 °C–900 °C) are discussed in detail. Structural, surface morphological and compositional modifications of the films are identified using the x-ray diffractometer, scanning electron microscopy and x-ray photoelectron spectrometer techniques, respectively. At 900 °C, a highly stable monoclinic β phase of Ga₂O₃ is obtained. The optical transmittance spectra acquired using UV–Vis spectroscopy indicate an improved UVC absorbance of the β-Ga₂O₃ films with an excellent visible transmittance (>80%). The structural transformation from amorphous to crystalline β-Ga₂O₃ phase and the associated reduction in defect density is found to modify other optical attributes, like the bandgap energy, Urbach energy, dispersion parameters, etc.