Luminescence-center-mediated excitation as the dominant Er sensitization mechanism in Er-doped silicon-rich SiO2 films

The structural and optical properties of erbium-doped silicon-rich silica samples containing 12 at. % of excess silicon and 0.63 at. % of erbium are studied as a function of annealing temperature in the range 600-1200 degrees C. Indirect excitation of Er3+ ions is shown to be present for all annealing temperatures, including annealing temperatures well below 1000 degrees C for which no silicon nanocrystals are observed. Two distinct efficient (eta(tr)>60%) transfer mechanisms responsible for Er3+ excitation are identified: a fast transfer process (tau(tr)< 80 ns) involving isolated luminescence centers (LCs), and a slow transfer process (tau(tr)similar to 4-100 mu s) involving excitation by quantum confined excitons inside Si nanocrystals. The LC-mediated excitation is shown to be the dominant excitation mechanism for all annealing temperatures. The presence of a LC-mediated excitation process is deduced from the observation of an annealing-temperature-independent Er3+ excitation rate, a strong similarity between the LC and Er3+ excitation spectra, as well as an excellent correspondence between the observed LC-related emission intensity and the derived Er3+ excitation density for annealing temperatures in the range of 600-1000 degrees C. The proposed interpretation provides an alternative explanation for several observations existing in the literature.