Gallium oxide (Ga2O3) is a semiconductor with a wide bandgap of ~5.0 eV and large breakdown voltages (>8 MV·cm−1). Among the crystal phases of Ga2O3, the monoclinic β-Ga2O3 is well known to be suitable for many device applications because of its chemical and thermal stability. The crystalline quality of polycrystalline β-Ga2O3 films on c-plane sapphire substrates was studied by rapid thermal annealing (RTA) following magnetron sputtering deposition at room temperature. Polycrystalline β-Ga2O3 films are relatively simple to prepare; however, their crystalline quality needs enhancement. The β-phase was achieved at 900 °C with a crystallite size and d-spacing of 26.02 and 0.2350 nm, respectively, when a mixture of ε- and β-phases was observed at temperatures up to 800 °C. The strain was released in the annealed Ga2O3 films at 900 °C; however, the clear and uniform orientation was not perfect because of the increased oxygen vacancy in the film at that temperature. The improved polycrystalline β-Ga2O3 films with dominant (−402)-oriented crystals were obtained at 900 °C for 45 min under a N2 gas atmosphere.
Improvement in crystallinity was investigated by compensating for stoichiometric deviations of non-selenization processed Cu0.9In0.7Ga0.3Se2 (CIGS) thin films due to highly volatile Se by co-sputtering them with Te followed by rapid thermal annealing. The prepared CIGS:Te thin films did not show any linear correlation between the compositional ratio and the co-sputtering time of Te; however, the deviation parameter (Δs) from the stoichiometry and normalized stoichiometric deviations of Se + Te and In + Ga were largely consistent with the behavior of thin-film properties. The proposed method provides better crystallinity with a large grain size, clear grain boundaries, and low microstrain and dislocation density, resulting in a large volume of the unit cell. The CIGS:Te thin films used as absorbers show improved optical properties compared to the conventional CIGS thin films, with Eg = 1.548 eV. These results can advance the low-cost commercialization of the enhanced-efficiency CIGS:Te thin films without the selenization process.
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