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°C. Indirect excitation of Er 3+ ions is shown to be present for all annealing temperatures, including annealing temperatures well below 1000°C for which no silicon nanocrystals are observed. Two distinct efficient ͑ tr Ͼ 60% ͒ transfer mechanisms responsible for Er 3+ excitation are identified: a fast transfer process ͑ tr Ͻ 80 ns͒ involving isolated luminescence centers ͑LCs͒, and a slow transfer process ͑ tr ϳ 4 -100 s͒ 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 Er 3+ excitation rate, a strong similarity between the LC and Er 3+ excitation spectra, as well as an excellent correspondence between the observed LC-related emission intensity and the derived Er 3+ excitation density for annealing temperatures in the range of 600-1000°C. The proposed interpretation provides an alternative explanation for several observations existing in the literature.

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