Laser-induced thermal effects on Si/SiO2 free-standing superlattices

Abstract

The thermal effects produced by continuous-wave laser radiation on free-standing Si/SiO2 superlattices are studied. We compare two samples with different SiO2 layer thicknesses (2 and 6 nm) and the same Si layer thickness (2 nm). The as-prepared free-standing superlattices contain some amount of Si nanocrystals (Si-nc). Intense laser irradiation at 488 nm of the as-prepared samples enhances the Raman scattering of Si-nc by two orders of magnitude. This laser-induced crystallization originates from melting of Si nanostructures in silica, which makes Si-nc better ordered and better isolated from the oxide surrounding. Continuous-wave laser control of Si-nc stress was achieved in these samples. In the proposed model, intense laser radiation melts Si-nc, and Si crystallization upon cooling down from the liquid phase in a silica matrix leads to compressive stress. The Si-nc stress can be tuned in the ∼3 GPa range using laser annealing below the Si melting temperature. The high laser-induced temperatures were verified with Raman spectroscopy. The laser-induced heat leads to a strongly nonlinear rise of light emission. The light emission is also observed in the anti-Stokes region, and its temperature dependence is practically the same for the two studied samples. The laser-induced temperature is essentially controlled by the absorbed laser power.