Luminescence-center-mediated excitation as the dominant Er sensitization mechanism in Er-doped silicon-richSiO2films

Abstract

The structural and optical properties of erbium-doped silicon-rich silica samples containing $12\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ of excess silicon and $0.63\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ of erbium are studied as a function of annealing temperature in the range $600--1200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Indirect excitation of ${\mathrm{Er}}^{3+}$ ions is shown to be present for all annealing temperatures, including annealing temperatures well below $1000\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ for which no silicon nanocrystals are observed. Two distinct efficient $({\ensuremath{\eta}}_{\mathrm{tr}}g60%)$ transfer mechanisms responsible for ${\mathrm{Er}}^{3+}$ excitation are identified: a fast transfer process $({\ensuremath{\tau}}_{\mathrm{tr}}l80\phantom{\rule{0.3em}{0ex}}\mathrm{ns})$ involving isolated luminescence centers (LCs), and a slow transfer process $({\ensuremath{\tau}}_{\mathrm{tr}}\ensuremath{\sim}4--100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s})$ 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 ${\mathrm{Er}}^{3+}$ excitation rate, a strong similarity between the LC and ${\mathrm{Er}}^{3+}$ excitation spectra, as well as an excellent correspondence between the observed LC-related emission intensity and the derived ${\mathrm{Er}}^{3+}$ excitation density for annealing temperatures in the range of $600--1000\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The proposed interpretation provides an alternative explanation for several observations existing in the literature.