Absorption spectra of nanocrystalline silicon embedded in SiO₂ matrix

Nanocrystalline silicon (nc-Si) embedded SiO₂ matrix has been formed by annealing the SiO_x films fabricated by plasma-enhanced chemical vapor deposition (PECVD) technique. Absorption coefficient and photoluminescence of the films have been measured at room temperature. The experimental results show that there is an 'aUrbach-like'b exponential absorption in the spectral range of 2.0-3.0 eV. The relationship of (αhv) ^1/2 ∝ (hv - E_g) demonstrates that the luminescent nc-Si have an indirect band structure. The existence of Stokes shift between photoluminescence and absorption edge indicates that radiative combination can take place not only between electron states and hole states but also between shallow trap states of electrons and holes.