Analysis of silicon nanocrystals in silicon-rich SiO2 synthesized by CO2 laser annealing

摘要

The localized synthesis of 4.2-5.6 nm-Si nanocrystals (nc-Si) in Si-rich SiO2 (SRSO) by CO2 laser annealing at laser intensity of below ablation-threshold (6 kW/cm2) is demonstrated. Since the SRSO exhibits a high absorption coefficient of up to 0.102 cm-1 at wavelength of 10.6 μm, a direct-writing CO2 laser annealing system with focusing spot size of 0.2 mm2 is used to locally anneal the SRSO and precipitate the nc-Si. A thermophysical model reveals that the surface temperature of SRSO ranging from 130oC to 3350oC is achieved by varying the laser power densities from 1.5 to 13.5 kW/cm2. The CO2 laser-ablation-threshold power density is about 6 kW/cm2, corresponding to the optimized annealing temperature 1285oC at the ablation threshold. The CO2laser annealing is capable of the precise control on power density and spot size, which benefits from the in-situ and localized annealing temperature control of SRSO film, and also prevents from the eternal damage of the other electronic devices nearby the annealing site. The nc-Si dependent photoluminescence (PL) were observed at 806 nm or longer, whereas the laser-ablation damaged SRSO film exhibits significant blue PL at 410 nm due to the oxygen-related structural defects. The refractive index of the lasertreated SRSO film is increasing from 1.57 to 2.31 as the laser intensity increases from 1.5 to 6.0 kW/cm2 which is mainly attributed to the increasing density of nc-Si embedded in SRSO. High resolution transmission electron microscopy (HRTEM) analysis reveals that the average size of nc-Si embedded in SRSO film is about 5.3 nm, which correlates well with the theoretical prediction of a corresponding PL at 806 nm. The HRTEM estimated square density of the nc-Si in SRSO film under the laser intensity of 6 kW/cm2 is about 1018 cm-3.