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.
The influence of hydrogen passivation on luminescence-center-mediated excitation of Er 3+ in Er-doped Si-rich SiO 2 films with significantly different microstructures is studied. Photoluminescence measurements are presented for samples containing no detectable silicon nanocrystals ͑annealed at 600°C͒ and for samples containing silicon nanocrystals ͑annealed at 1100°C͒ as a function of hydrogen passivation temperature. Passivation is found to have little effect on the Er 3+ photoluminescence intensity at 1535 nm in the samples that do not contain nanocrystals. In contrast, a pronounced increase in the Er 3+ photoluminescence intensity is observed in the samples containing Si nanocrystals, which is accompanied by a similar increase in the nanocrystal photoluminescence intensity and a gradual increase in the Si nanocrystal emission lifetime. This observation is attributed to two interrelated effects, namely, ͑a͒ an increase in the density of fully passivated optically active nanocrystals due to the passivation-induced removal of silicon dangling bonds and ͑b͒ a concurrent reduction in nonradiative Er 3+ relaxation from levels above the 4 I 13/2 level due to a direct interaction of excited Er 3+ ions with silicon dangling bonds. In addition, the observed counterintuitive gradual increase in the nanocrystal photoluminescence decay time upon passivation is successfully explained taking into account a passivation-induced change in the concentration of optically active nanocrystals with different sizes and the inhomogeneous nature of the nanocrystal-related emission band. It is shown that the combination of luminescence-center-mediated Er 3+ excitation and silicon-dangling-bond-induced Er 3+ de-excitation can explain at least 14 experimental observations reported by independent authors.
The dynamics of Er3+ excitation in low-temperature-annealed Si-rich SiO2 are studied. It is demonstrated that Si-excess-related indirect excitation is fast (transfer time τtr<27 ns) and occurs into higher lying Er3+ levels as well as directly into the first excited state (I413/2). By monitoring the time-dependent Er3+ emission at 1535 nm, the multilevel nature of the Er3+ sensitization is shown to result in two types of excitation of the I413/2 state: a fast excitation process (τtr<27 ns) directly into the I413/2 level and a slow excitation process due to fast excitation into Er3+ levels above the I413/2 level, followed by internal Er3+ relaxation with a time constant τ32>2.3 μs. The fast and slow excitations of the I413/2 level account for an approximately equal fraction of the excitation events: 45%–50% and 50%–55%, respectively.
Erbium sensitization is observed in as-deposited Er 3+ doped Si-rich SiO 2 , ruling out the involvement of Si nanocrystals in the Er 3+ excitation in these samples. The Er 3+ excitation cross section in this material is similar within a factor 3 to that of samples annealed at 600°C under 355 and 532 nm excitation. The density of sensitized Er 3+ ions is shown to be excitation wavelength independent, while the shape of the Er 3+ excitation spectra is governed by a wavelength dependent Er 3+ excitation cross section. These findings enable the use of a broad range of wavelengths for the efficient excitation of this gain medium.
Excitation wavelength independent sensitized Er 3 + concentration in as-deposited and low temperature annealed Si-rich SiO 2 films
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