Laser-induced thermal effects on optical and light-emitting properties of free-standing silica films containing Si nanocrystals

Abstract

A Raman and photoluminescence study of a thermally annealed free-standing film of silica containing Si nanocrystals is reported with emphasis on laser-induced thermal effects. The Si-rich silica film on a Si substrate was prepared by a molecular beam deposition method and annealed at 1150 °C for 1 hour in an oven, which promoted Si nanocrystals. Then the Si substrate was partially chemically etched producing free-standing film areas with typical dimensions of 2 mm x 2 mm and thickness of 1.4 μm. For the free-standing film, we observed laser-induced (Ar + laser at 488 nm) thermal effects on the light-emitting and optical properties. In fact, the light emission dramatically increases with the laser intensity, up to 4 orders of magnitude at 840 nm when the laser power increases from ∼100 to 200 mW, and the absorption coefficient rises considerably as well. The anti-Stokes to Stokes Raman intensity ratio suggests a very high temperature of the free-standing silica film containing Si nanocrystals (∼1200 K) upon exposure to a laser power of 100 mW focused to a ∼40 μm spot, and the temperature probably rises up to ∼2000 K for exposure to a laser power of 200 mW. The light emission measured at the high excitation powers is similar to blackbody radiation although some quantitative deviations occur for the temperature dependence. The Ar + laser annealing strongly increases the crystalline Raman peak showing that thermal annealing at 1150 °C does not finish structural reorganization of the SiO x material. In the waveguiding detection geometry, the spectral narrowing of the photoluminescence is observed and used to estimate the refractive index.