Raman scattering in cluster-deposited nanogranular silicon films

Abstract

We study nanograin size confinement effects, and the effect of the increase of local temperature on the first-order Raman spectrum in silicon nanogranular films obtained by cluster deposition. The local temperature was monitored by measuring the Stokes/antiStokes peak ratio with the laser power up to similar to20 kW/cm(2). We find large energy shifts, up to 30 cm(-1), and broadenings, up to 20 cm(-1), of the Raman-active mode, which we attribute to both laser heating and confinement effects. The phonon softening and phonon linewidth are calculated using a phenomenological model which takes into account disorder effects through the breakdown of the k = 0 Raman-scattering selection rule, and also anharmonicity, which is incorporated through the three- and four- phonon decay processes. Very good agreement with experimental data is obtained for calculated spectra with nanograin sizes of about 10 nm, and with an increase in the anisotropy constants with respect to those of bulk silicon.