Optical spectroscopic characterization of silica layers containing silicon nanocrystals (Si-nc) is described in detail. Red-
NIR photoluminescence (PL) is studied and correlations of the PL with optical and structural properties are analyzed.
The surface mechanism of PL involving Si=O bonds is supported by our results. Wavelength-selective optical
waveguiding by Si-nc/SiO2 layers is studied. The found spectral filtering allows optical properties of Si-nc/SiO2 layers to
be measured. Laser-induced thermal effects on structural and optical properties of free-standing Si-nc/SiO2 films are
reported. The obtained results suggest very efficient Si-SiO2 phase separation by intense laser light. Laser-controlled
stress of Si-nc in silica is demonstrated. The laser manipulations with Si-nc stress offer an approach to Si-nc memory
with an extremely long retention time, which can be written, read, and erased by optical means.
The Si/SiO2 superlattices were prepared by a molecular beam deposition method, high temperature furnace annealing (1100 °C), and back-side Si wafer etching in tetramethyl ammonium solution. Transmission electron microscopy and Raman spectroscopy show that the layered structure is not preserved during high temperature treatment. The etching of the substrate increases photoluminescence of the Si/SiO2 material. Optical waveguiding was realized for the free-standing sample demonstrating its reasonable optical quality and providing the optical parameters.
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 SiOx material. In the waveguiding detection geometry, the spectral narrowing of the photoluminescence is observed and used to estimate the refractive index.
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