Optical properties of silicon nanocrystals in silica: Results from spectral filtering effect, m-line technique, and x-ray photoelectron spectroscopy

The optical properties of silica layers containing silicon nanocrystals are analyzed in terms of spectral filtering in absorbing planar waveguides (cutoff spectra), m-line measurements, and x-ray photoelectron spectroscopy (XPS). The effects of optical dispersion, approximation of weak guiding, and depth dependence of refractive index in a planar waveguide are studied. We compare the measured optical properties of silicon-rich silicon oxide samples with the values estimated by the Bruggeman theory using the XPS structural components. A good agreement between the measured and calculated refractive indices is found. The results for absorption suggest high transparency of the nanoscale-suboxide component in contrast to the corresponding bulk material. The Raman intensity of silicon nanocrystals is proportional to the XPS amount of bulk silicon. The extinction coefficient extracted for the Si component is between the values for crystalline and amorphous silicon. Annealing at higher temperatures decreases the Si component extinction coefficient, which is interpreted as a decrease in the amorphous Si fraction. The XPS method surprisingly suggests a large proportion of silicon suboxide even after annealing at 1200 degrees C.