Luminescence-center-mediated excitation as the dominant Er sensitization mechanism in Er-doped silicon-rich $\mathrm{Si}{\mathrm{O}}_2$ films

The structural and optical properties of erbium-doped silicon-rich silica samples containing $12\mathrm{at.}\%$ of excess silicon and $0.63\mathrm{at.}\%$ of erbium are studied as a function of annealing temperature in the range $600–1200°\mathrm{C}$. Indirect excitation of ${\mathrm{Er}}^{3+}$ ions is shown to be present for all annealing temperatures, including annealing temperatures well below $1000°\mathrm{C}$ for which no silicon nanocrystals are observed. Two distinct efficient $({\eta }_{\mathrm{tr}}>60\%)$ transfer mechanisms responsible for ${\mathrm{Er}}^{3+}$ excitation are identified: a fast transfer process $({\tau }_{\mathrm{tr}}<80\mathrm{ns})$ involving isolated luminescence centers (LCs), and a slow transfer process $({\tau }_{\mathrm{tr}}\sim 4–100\mu \mathrm{s})$ 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 $600–1000°\mathrm{C}$. The proposed interpretation provides an alternative explanation for several observations existing in the literature.

Figure 1

High-resolution bright field TEM images of Er-doped Si-rich $\mathrm{Si}{\mathrm{O}}_2$ samples (a) as-deposited, (b) annealed at $600°\mathrm{C}$, (d) annealed at $1000°\mathrm{C}$, and (e) annealed at $1200°\mathrm{C}$. Electron diffraction patterns for (c) the sample annealed at $600°\mathrm{C}$ and (f) the sample annealed at $1200°\mathrm{C}$. The insets show a magnified view of the area indicated by the dashed squares.

Figure 2

(Color online) Photoluminescence spectra of passivated Si-rich $\mathrm{Si}{\mathrm{O}}_2$ films with (XSE) and without (XSO) erbium, annealed at temperatures in the range $600–1200°\mathrm{C}$. The inferred spectral dependence of the transfer efficiency is included for annealing temperatures of 600 and $1100°\mathrm{C}$.

Figure 3

(Color online) Near-infrared photoluminescence spectra of passivated Er-doped Si-rich $\mathrm{Si}{\mathrm{O}}_2$ samples annealed at temperatures in the range $600–1200°\mathrm{C}$. Inset: the ${\mathrm{Er}}^{3+}$-related peak emission intensity at 981 and $1535\mathrm{nm}$ as a function of pump power. The dashed line indicates a linear dependence.

Figure 4

(Color online) Photoluminescence excitation spectra of passivated Si-rich $\mathrm{Si}{\mathrm{O}}_2$ films with (XSE) and without (XSO) erbium for an as-deposited sample as well as samples annealed at 600 and at $1100°\mathrm{C}$, measured at emission wavelengths predominantly corresponding to luminescence center emission $\left(650\mathrm{nm}\right)$, Si nanocrystal emission $\left(875\mathrm{nm}\right)$, and ${\mathrm{Er}}^{3+}$ emission $\left(1535\mathrm{nm}\right)$.

Figure 5

Measured and deduced PL parameters of passivated Si-rich $\mathrm{Si}{\mathrm{O}}_2$ films with (XSE) and without (XSO) erbium as a function of annealing temperature, showing (a) the Er PL peak intensity $\left(I_{\mathrm{Er}}\right)$ and the decay time $\left({\tau }_{\mathrm{Er}}\right)$ at $1535\mathrm{nm}$ in XSE samples. (b) The PLintensities at $650\mathrm{nm}$ $\left(I_{\mathit{LC}}^{\mathit{XSE}}\right)$ and $900\mathrm{nm}$ $\left(I_{\mathit{NC}}^{\mathit{XSE}}\right)$ in XSE samples, the derived ${\mathrm{Er}}^{3+}$ excitation density in XSE samples $(I_{\mathrm{Er}}∕{\tau }_{\mathrm{Er}})$, the relative concentration of ${\mathrm{Er}}^{3+}$ ions coupled to sensitizers $\left(N_{\mathrm{Er},c}\right)$, and the transfer efficiency at $650\mathrm{nm}$ derived from the spectral measurements $\left({\eta }_{\mathit{tr},\mathit{PL}}\right)$. (c) The decay rate of the emission at $900\mathrm{nm}$ in XSO and XSE samples ($R_{\mathit{dec},\mathrm{NC}}^{\mathit{XSO}}$ and $R_{\mathit{dec},\mathrm{NC}}^{\mathit{XSE}}$, respectively), the derived transfer rate at $900\mathrm{nm}$ $\left(R_{\mathit{tr}}\right)$, the excitation rate of the nanocrystals measured at $900\mathrm{nm}$ in XSO samples $\left(R_{\mathit{exc},\mathrm{NC}}^{\mathit{XSO}}\right)$, the ${\mathrm{Er}}^{3+}$ excitation rate measured in XSE samples $\left(R_{\mathit{exc},\mathrm{Er}}\right)$, the transfer efficiency at $900\mathrm{nm}$ derived from the lifetime measurements $\left({\eta }_{\mathit{tr},\mathit{LT}}\right)$, and the transfer efficiency at $900\mathrm{nm}$ derived from spectral measurements $\left({\eta }_{\mathit{tr},\mathit{PL},\mathit{NC}}\right)$.

Figure 6

(Color online) Schematic representation of the ${\mathrm{Er}}^{3+}$ excitation processes in Er-doped Si-rich $\mathrm{Si}{\mathrm{O}}_2$ samples (a) at low annealing temperatures, showing a high concentration of excess-Si-related LCs indicated by crosses, as well as ${\mathrm{Er}}^{3+}$ ions, indicated by open circles, and (b) the corresponding schematic band diagram, indicating the $\mathrm{Si}{\mathrm{O}}_2$ valence band and conduction band, LC-related electronic levels in the band gap indicated by the horizontal lines, as well as the ${\mathrm{Er}}^{3+}$ energy levels (LC-mediated excitation is indicated by the vertical arrows); (c) at intermediate annealing temperatures, showing a reduced concentration of LCs; and (d) at high annealing temperatures, showing the formation of Si nanocrystals (dark circles), and (e) the corresponding band diagram, indicating the presence of Si nanocrystals with a quantum confined band gap. A weak exciton-mediated contribution to the Er excitation is indicated by the light vertical arrow.