Silicon Nanocrystals on the Surface of Standard Si Wafers: A Micro-Raman Investigation

Abstract

The presence of silicon nanocrystals on the surface of standard wafer samples of Si, conserved under “usual” laboratory conditions, has been investigated by micro-Raman analysis, performed for increasing intensity of laser irradiation. The poor thermal connection of such small crystals to the Si wafer bulk allows for the appearance of two well distinct Raman bands in the spectra, with a different evolution for increasing irradiance levels: the first, expected, due to bulk silicon response, the other one assignable to the silicon nanocrystals. A careful analysis of peak position and linewidth has been carried out, both for the Raman contribution from the nanocrystals, reaching high temperatures under irradiation (up to 1400 K), and for the one from the “bulk” Si, which remains practically at room temperature. The analysis of the spectra and the comparison with previous studies on nc-Si suggest that such nanocrystals do not have a very small size, so that the observed changes of spectral parameters are mainly due to laser heating, rather than quantum confinement effects. In any case, we performed also an independent size deter-mination by AFM mapping, confirming a size distribution well peaked be-tween 50 and 100 nm. As a corollary from this analysis, we get the indication that apparent linewidths and positions, at low laser irradiation levels, can be slightly changed in the presence of nc-Si on the surface. It is due to the differ-ent thermal responses of bulk and nanocrystalline components, inducing un-resolved separate components; this hypothesis suggests reanalysing some previous experimental data, in particular for many Raman spectra of Si col-lected at “room temperature”.