Spectroscopic characterization of phase transformation in Ge-rich Al₂O₃ films grown by magnetron co-sputtering

Thermally-stimulated evolution of optical and structural properties of Ge-rich-Al₂O₃ films with different Ge contents was investigated. As-deposited films and films annealed at T_A ≤ 550 °C were found to be amorphous whatever the Ge content. The formation of amorphous Ge clusters occurs at T_A = 550 °C, whereas their crystallization is prominent at T_A = 600 °C requiring a shorter annealing time for the higher Ge content. The films annealed at T_A = 550 °C showed broad photoluminescence spectrum. Its shape and intensity depend on Ge content and excitation energy. Annealing at T_A = 600 °C results in the appearance of additional UV bands originated from the formation of GeO_x phase covered Ge clusters. An analysis of excitation spectra was performed to distinguish the mechanism of light emission in these films as well as to discriminate contribution of carrier recombination in the Ge phase (amorphous clusters and/or nanocrystals) as well as via interface or host defects. The concentration and mobility of free carriers was also estimated.