Continuous-wave laser annealing of free-standing Si∕SiO2 superlattice: Modification of optical, structural, and light-emitting properties

Abstract

Raman, optical, and photoluminescence studies of a free-standing Si∕SiO2 superlattice (SL) are described with emphasis on laser-induced thermal effects (laser annealing). The Si∕SiO2 SL (500 repeats of 2-nm-thick Si and SiO2 layers) on a Si substrate was grown by a molecular beam deposition method and annealed at 1100°C for 1h in an oven, which promoted Si nanocrystals. Then the Si substrate was partially chemically etched producing free-standing areas. Continuous-wave laser annealing (∼104W∕cm2) of the as-prepared free-standing SL strongly increases the Raman band of Si nanocrystals at ∼515cm−1, which features laser-induced crystallization and presumably originates from melting of Si nanostructures. The obtained results show that thermal annealing at 1100°C does not finish structural reorganization of the Si∕SiO2 SL material and a large proportion of Si excess can be undetectable by Raman spectroscopy in related thermally annealed materials. For the laser-crystallized samples, various material characteristics (Raman spectra, light emission, and absorption) have been measured as a function of laser-induced temperature and period of laser annealing. The light emission is found to be a straightforward function of the temperature whereas the absorption coefficient depends on the laser-annealing period suggesting additional laser-induced structural reorganization.