Optical properties of nanocrystalline silicon embedded in SiO₂

Nanocrystalline silicon embedded SiO₂ matrix has been formed by annealing the a-SiO_x films fabricated by plasma enhanced chemical vapor deposition technique. Absorption and photoluminescence spectra of the films have been studied in conjunction with micro-Raman scattering spectra. It is found that absorption presents an exponential dependence of absorption coefficient to photon energy in the range of 1 .5-3 .0 eV, and a sub-band appears in the range of 1.0-1.5 eV. The exponential absorption is due to the indirect band-to-band transition of electrons in silicon nanocrystallites, while the sub-band absorption is ascribed to transitions between surfaces and/or defect states of the silicon nanocrystallites. The existence of Stokes shift between absorption and photoluminescence suggests that the phonon-assisted luminescence would be enhanced due to the quantum confinement effects.