Structural and optical characterization of beta-gallium oxide

Abstract

Demands for cheaper solar cells have led researchers to less complex, low-temperature, vacuum-free thin-film deposition processes, like spray pyrolysis and sol–gel spin coating. Previous studies of β-Ga2O3 thin-film deposition have used aqueous solutions of gallium nitrate which have strong tendencies to form hydroxide precipitates. This leads to the blockage of spray gun nozzles. To avoid precipitation, tetrahydroxogallate (III) ammonium was used as a novelty precursor in this study for the synthesis of β-Ga2O3. In the spray pyrolysis technique, the precursor was deposited on sapphire substrates at 200 ℃, with a carrier gas pressure of 200 kPa. The same precursor, with added monoethanolamine to enhance viscosity, was used in the spin coating method. A polycrystalline β-Ga2O3 structure was obtained by post-annealing films at 750 ℃ in ambient air. The spin-coated films with thicknesses ranging from 165 to 354 nm exhibited an average crystallite size of 17.78 nm and an optical band gap range between 4.80 eV and 4.95 eV. Films produced by spray pyrolysis had thicknesses ranging between 158 and 255 nm, an average crystallite size of 17.55 nm, and a band gap ranging between 4.69 eV and 4.93 eV. From Raman spectroscopy, the molecular vibrational modes Ag and Bg were detected, featuring three blue shifts and two red shifts. Films showed a UV-blue region originating from oxygen and gallium vacancies in the lattice, an important characteristic for good photodetectors and vital for solar cell passivation. When utilizing β-Ga2O3 as dielectric coating, the refractive index between air and solar cells is reduced, enhancing solar energy absorption. Similar results were obtained for both synthesis techniques confirming the reliability of the methods.