CORRELATION BETWEEN THE LUMINESCENCE AND RAMAN PEAKS IN QUANTUM-CONFINED SYSTEMS

Two of the main models for the explanation of strong visible-light emi ssion in silicon, namely physical quantum confinement of electrons in nanometer-size ''wires'' or spherical crystallites, and chemical quant um confinement to subnanometer-size silicon particles due to the isola ting effect of oxygen atoms, are considered with regard to a possible correlation between Raman shift and photoluminescence (PL) peak positi on. The physical confinement model predicts opposite shifts with chang ing size of the confinement. In the chemical confinement model, the sh ift of the gap is not a size effect; it occurs owing to chemical subst itution of the bond terminators of the silicon atoms. This is accompan ied by a parallel shift in the Raman frequency. The calculation of the vibration spectra of small size wires and spheres allows the correct assignment of experimentally observed Raman peaks. With the help of th is assignment, the analysis of the observed spectra shows a parallel s hift of Raman and PL peaks. The calculated frequencies for siloxene de rivatives (a known manifestation of chemical quantum confinement) are lower than those observed in most porous silicon samples; still the pa rallel shift favors the idea of chemical as against physical quantum c onfinement.