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

•Optical and structural properties of heat-treated Ge-Al2O3 films is studied.•Ge nanoclusters, interface and host oxide defects contribute to luminescence.•Ge phase crystallizes at 550–600 °C depending on Ge content.•The higher Ge content, the shorter annealing time for Ge nanoclusters formation.•Film stoichiometry, free carrier concentration and mobility were estimated. 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 TA ≤ 550 °C were found to be amorphous whatever the Ge content. The formation of amorphous Ge clusters occurs at TA = 550 °C, whereas their crystallization is prominent at TA = 600 °C requiring a shorter annealing time for the higher Ge content. The films annealed at TA = 550 °C showed broad photoluminescence spectrum. Its shape and intensity depend on Ge content and excitation energy. Annealing at TA = 600 °C results in the appearance of additional UV bands originated from the formation of GeOx 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.