On red-shift of UV photoluminescence with decreasing size of silicon nanoparticles embedded in SiO2 matrix grown by pulsed laser deposition

Ensembles of silicon nanoparticles (Si-nps) embedded in SiO2 matrix were grown by alternate ablation of Si and SiO2 targets using KrF excimer laser based pulsed laser deposition (PLD). The sizes of Si-nps (mean size ranging from 1-5 nm) were controlled by varying the ablation time of silicon target. Transmission electron microscopy (TEM) along with selected area electron diffraction (SAED) and Raman spectroscopy were used to confirm the growth of silicon nanoparticles, its size variation with growth time and the crystalline quality of the grown nanoparticles. TEM analysis showed that mean size and size distribution of Si-nps increased with increase in the ablation time of Si target. Intense peaks ~521 cm−1 in Raman analysis showed reasonably good crystalline quality of grown Si-nps. We observed asymmetric broadening of phonon line shapes which also redshift with decreasing size of Si-nps. Photoluminescence (PL) from these samples, obtained at room temperature, was broad band and consisted of three bands in UV and visible range. The intensity of PL band in UV spectral range (peak ~3.2 eV) was strong compared to visible range bands (peaks ~2.95 eV and ~2.55 eV). We observed a small red-shift (~0.07 eV) of peak position of UV range PL with the decrease in the mean sizes of Si-nps, while there was no appreciable size dependent shift of PL peak positions for other bands in the visible range. The width of UV PL band was also found to increase with decrease of Si-nps mean sizes. Based on the above observations of size dependent redshift of UV range PL band together with the PL lifetimes and PL excitation spectroscopy, the origin of UV PL band is attributed to the direct band transition at the Г point of Si band structure. Visible range bands were ascribed as defect related transitions. The weak intensities of PL bands ~2.95 eV and ~2.55 eV suggested that Si nanoparticles grown by PLD were efficiently capped or passivated by SiO2 with low density of surface/interface related defects.