Optical emission from $\mathrm{Si}{\mathrm{O}}_2$-embedded silicon nanocrystals: A high-pressure Raman and photoluminescence study

We investigate the optical properties of high-quality Si nanocrystals $\left(\mathrm{NCs}\right)/\mathrm{Si}{\mathrm{O}}_2$ multilayers under high hydrostatic pressure with Raman scattering and photoluminescence (PL) measurements. The aim of our study is to shed light on the origin of the optical emission of the Si $\mathrm{NCs}/\mathrm{Si}{\mathrm{O}}_2$. The Si NCs were produced by chemical-vapor deposition of Si-rich oxynitride $\left(\mathrm{SRON}\right)/\mathrm{Si}{\mathrm{O}}_2$ multilayers with 5- and 4-nm SRON layer thicknesses on fused silica substrates and subsequent annealing at 1150 °C, which resulted in the precipitation of Si NCs with an average size of 4.1 and 3.3 nm, respectively. From the pressure dependence of the Raman spectra we extract a phonon pressure coefficient of $8.5\pm 0.3\mathrm{c}{\mathrm{m}}^{-1}/\mathrm{GPa}$ in both samples, notably higher than that of bulk $\mathrm{Si}(5.1\mathrm{c}{\mathrm{m}}^{-1}/\mathrm{GPa})$. This result is ascribed to a strong pressure amplification effect due to the larger compressibility of the $\mathrm{Si}{\mathrm{O}}_2$ matrix. In turn, the PL spectra exhibit two markedly different contributions: a higher-energy band that redshifts with pressure, and a lower-energy band which barely depends on pressure and which can be attributed to defect-related emission. The pressure coefficients of the higher-energy contribution are $(-27\pm 6)$ and $(-35\pm 8)\mathrm{meV}/\mathrm{GPa}$ for the Si NCs with a size of 4.1 and 3.3 nm, respectively. These values are sizably higher than those of bulk $\mathrm{Si}(-14\mathrm{meV}/\mathrm{GPa})$. When the pressure amplification effect observed by Raman scattering is incorporated into the analysis of the PL spectra, it can be concluded that the pressure behavior of the high-energy PL band is consistent with that of the indirect transition of Si and, therefore, with the quantum-confined model for the emission of the Si NCs.

Figure 1

Raman spectra up to $\sim 5\mathrm{GPa}$ of multilayered Si NCs embedded in a $\mathrm{Si}{\mathrm{O}}_2$ matrix with a NC size $L=4.1\mathrm{nm}$ (sample A, upper panel) and $L=3.3\mathrm{nm}$ (sample B, bottom panel). The top spectra (in blue) correspond to the as-grown samples, before the mechanical polishing for the high-pressure experiments.

Figure 2

Pressure dependence of the frequency of the first-order optical phonon of Si in the two Si $\mathrm{NCs}/\mathrm{Si}{\mathrm{O}}_2$ samples studied in this work. The dashed lines are linear fits to the experimental data.

Figure 3

Photoluminescence (PL) spectra of two Si $\mathrm{NC}/\mathrm{Si}{\mathrm{O}}_2$ samples with different NC sizes ($L$). (a) Sample $\mathrm{A}(L=4.1\mathrm{nm})$; (b) the PL spectra of sample $\mathrm{B}(L=3.3\mathrm{nm})$. The spectra on top (in blue) correspond to the as-grown samples before mechanical polishing for the high-pressure experiments.

Figure 4

Selected photoluminescence (PL) spectra of sample $\mathrm{A}(L=4.1\mathrm{nm})$ obtained during the upstroke and downstroke cycles. The results of the line-shape fitting analysis of the PL spectra, carried out with two Gaussian functions (as a function of photon energy), are shown.

Figure 5

PL peak energies for the high-energy (peak P1) and low-energy (peak P2) optical emission of the two Si $\mathrm{NC}/\mathrm{Si}{\mathrm{O}}_2$ samples studied in this work. The dots correspond to sample A (with a NC size $L=4.1$ nm) and the squares to sample B ($L=3.3$ nm).