Energy transfer in Er-doped ${\text{SiO}}_2$ sensitized with Si nanocrystals

We present a high-resolution photoluminescence study of Er-doped ${\text{SiO}}_2$ sensitized with Si nanocrystals (Si NCs). Emission bands originating from recombination of excitons confined in Si NCs, internal transitions within the $4f$-electron core of ${\text{Er}}^{3+}$ ions, and a band centered at $\lambda \approx 1200\text{nm}$ have been identified. Their kinetics were investigated in detail. Based on these measurements, we present a comprehensive model for energy-transfer mechanisms responsible for light generation in this system. A unique picture of energy flow between the two subsystems was developed, yielding truly microscopic information on the sensitization effect and its limitations. In particular, we show that most of the ${\text{Er}}^{3+}$ ions available in the system are participating in the energy exchange. The long-standing problem of apparent loss of optical activity in the majority of Er dopants upon sensitization with Si NCs is clarified and assigned to the appearance of a very efficient energy exchange mechanism between Si NCs and ${\text{Er}}^{3+}$ ions. Application potential of ${\text{SiO}}_2:\text{Er}$, sensitized by Si NCs, was discussed in view of the newly acquired microscopic insight.

Figure 1

(Color online) Time-integrated PL spectra at $T=10$ and 300 K, and excitation wavelength ${\lambda }_{\text{exc}}=450\text{nm}$. Si-NC-related, Er-related, and a third (at low T) PL bands can be observed. The Si-NC-related band is blown up for clarity. The arrows show PL peaks related to emission from higher excited states of the ${\text{Er}}^{3+}$ ion, superimposed to the NC excitonic PL.

Figure 2

(Color online) Time-resolved contour plot of the PL spectrum of the Si-NC excitonic-related band in the $\left(\mathrm{a}\right)$ microseconds and in the $\left(\mathrm{b}\right)$ submicrosecond time ranges. The PL intensity (colored contour plots) is represented as a function of time ($X$ axis) and detection wavelength ($Y$ axis). Panel (a) also shows the spectrum recorded at $\text{t}=100\mu \text{s}$ and the PL decay at $\lambda =850\text{nm}$ (sections from the contour plot). Drift of the band maximum toward longer wavelength is observed for longer time delay in panel (a). Note the difference between the center of the bands in both figures $\left(\approx 200\text{meV}\right)$.

Figure 3

(Color online) Room temperature PL decay kinetics recorded at $\lambda =1200\text{nm}$ (maximum of the PL broad band shown in Fig. 1) and $\lambda =1535\text{nm}$. In the inset, the spectrum recorded at $\text{t}=200\text{ns}$ after the laser excitation pulse, at $T=10\text{K}$, is shown.

Figure 4

(Color online) In panel $\mathrm{a}$: Si-NC PL kinetics, recorded at $\lambda =860\text{nm}$ and room temperature; time resolution of the system was 2 ns. Excitation was provided with a 5 ns pulse at ${\lambda }_{\text{exc}}=450\text{nm}$. Initial nanoseconds decay is stretched and followed by a final decay with characteristic lifetime of thick microseconds. In panel $\mathrm{b}$: Er-related $1.5\mu \text{m}$ PL kinetics for the first microsecond after the excitation pulse is shown at $T=10\text{K}$ and RT.

Figure 5

Er-related PL kinetics in double-logarithmic scale for different excitation conditions at RT. Intensity has been normalized to the maximum of the slow component in order to compare the fast-to-slow intensity ratio. In panels $\left(\mathrm{a}\right)$ and $\left(\mathrm{b}\right)$, the excitation wavelength (450 nm) has been kept constant and flux has been decreased about one order of magnitude. In panels $\left(\mathrm{b}\right)$ and $\left(\mathrm{c}\right)$, flux has been kept in the same order of magnitude and wavelength excitation has been increased to 650 nm.

Figure 6

(Color online) Flux dependence of ${\text{Er}}^{3+}$ and Si-NC PL for several excitation wavelengths is shown, recorded at RT.

Figure 7

(Color online) PL excitation cross section of ${\text{Er}}^{3+}$ and Si-NC PL as a function of excitation wavelength, which have been determined by using formula (1) to fit the curves of Fig. 6.

Figure 8

(Color online) Er- and Si-NC-related effective excitation cross section ${\sigma }_{\text{PL}}$ as a function of the optical-absorption coefficient $\alpha$ for each given excitation wavelength.

Figure 9

(Color online) Electron and hole energy levels in Si NCs in ${\text{SiO}}_2$, as a function of NC diameter. On the right-hand side, the ${\text{Er}}^{3+}$ energy levels are presented. The most effective erbium excitation and deexcitation processes due to intraband carrier transitions are shown.

Figure 10

Factors $I_{i{i}^{\prime }}$ for erbium excitation/deexcitation by electron intraband transitions as a function of $a/R$, where $a$ is the distance of the ${\text{Er}}^{3+}$ ion from the center of the NC and $R$ the radius of the NC.

Figure 11

(Color online) The probability of the most effective excitation and deexcitation processes as a function of $a/R$ ($a$ is the erbium distance from the center of a NC and $R$ the radius of the NC) for a NC with diameter 3.1 nm.

Figure 12

A simulation of the distance distribution of ${\text{Er}}^{3+}$ ions to the center of their nearest neighboring NC, assuming a random distribution of both: 6.6% of the ${\text{Er}}^{3+}$ ions are contained inside NCs.