Optical and structural properties of Si nanocrystals in SiO2 film

Silicon nanocrystals (Si-nc) embedded in a SiO2 matrix is a promising system for siliconbased photonics. We studied optical and structural properties of Si-rich silicon oxide SiOx (x < 2) films annealed in a furnace at temperatures up to 1200 °C and containing Si-nc. The measured optical properties of SiOx films are compared with the values estimated by using the effective medium approximation and X-ray photoelectron spectroscopy (XPS) results. A good agreement is found between the measured and calculated refractive index. The results for absorption suggest high transparency of nanoscale suboxide. The extinction coefficient for elemental Si is found to be between the values for crystalline and amorphous Si. Thermal annealing increases the degree of Si crystallization; however, the Si–SiO2 phase separation is not complete after annealing at 1200 °C. The 1.5-eV photoluminescence probably originates from small (~1 nm) oxidized Si grains or oxygen-related defects, but not from Si-nc with sizes of about 4 nm. The SiOx films prepared by molecular beam deposition and ion implantation are structurally and optically very different after preparation but become similar after annealing at ~1100 °C. The laser-induced thermal effects found for SiOx films on silica substrates illuminated by focused laser light should be taken into account in optical measurements. Continuous-wave laser irradiation can produce very high temperatures in free-standing SiOx and Si/SiO2 superlattice films, which changes their structure and optical properties. The center of a laser-annealed area is very transparent and consists of amorphous SiO2. Large Si-nc (up to 300 nm) are observed in the ring around the central region. These Si-nc produce high absorption and they are typically under compressive stress, which is connected with the crystallization from the melt phase. Some of the large Si-nc exhibit surface features, which is interpreted in terms of eruption of pressurized Si from the film. A part of large Sinc is removed from the film forming holes of similar sizes. The presence of oxygen in the laser-annealing atmosphere decreases the amount of removed Si-nc. The structure of laserannealed areas is explained by thermodiffusion, which leads to the macroscopic Si–SiO2 phase separation. Comparison of the structure of central regions for laser annealing in oxygen, air, and inert atmospheres excludes the dominating effect of Si oxidation in the formation of laser-annealed area. By using a strongly focused laser beam, the structural changes in the free-standing films can be obtained in submicron areas, which suggests a concept of nonvolatile optical memory with high information density and superior thermal stability.