Investigation of absorption of nanocrystalline silicon

Nanocrystalline silicon embedded SiO₂ matrix is formed by annealing the SiO_x films fabricated by plasma enhanced chemical vapor deposition technique. In conjunction with the micro-Raman spectra, the absorption spectra of the films have been investigated. The blue-shift of absorption edge with decreasing size of silicon crystallites is due to quantum confinement effect. It is found that nanocrystalline silicon is of an indirect band structure, and that the absorption presents an exponential dependence of absorption coefficient on photon energy in the range of 2.0-3.0 eV, and a sub-band appears in the range of 1.0-1.5 eV. We believe that 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 the amorphous silicon states existing in the films.