Laser heating effect on Raman spectra of Si nanocrystals embedded into SiOx matrix

Abstract

Influence of combined size confinement effect and effect of local laser heating on the first-order Raman spectrum of silicon nanocrystals embedded into SiOx matrix has been studied. Increase of the local temperature of Si nanocrystals caused by laser illumination with the power density up to 10 mW/μm² was estimated from the ratio of the Stokes/anti-Stokes peak intensities. Almost linear dependence of nanocrystals local temperature on the power density of exciting radiation with a rate of 63.6 Km² /mW has been found. The phonon line shape at power densities, when no laser heating effect is registered, was shown to be described well within the correlation length model of phonon confinement of Si nanocrystals with the size L = 9.2 nm. Observed phonon softening and broadening with increase of the exciting power density is considered as temperatureinduced vibration anharmonicity with the decay of optical phonons through three and four-phonon processes and corresponding anharmonic constants have been determined.