Gap states in a-SiC from optical measurements and band structure models

Undoped and boron-doped a-Si1-xCx:H, for x∼0.5, films have been prepared by means of plasma-enhanced chemical-vapour deposition using methyltrichlorosilane. The optical absorption spectra of these films demonstrate three characteristic peaks at about 1.6, 2.0 and 2.5 eV consistent with other experimental measurements. To explain the observed peculiarities of the spectra, the atomic and electronic structures of a-SiC have been investigated using both molecular dynamics simulations based on an empirical potential and the recursion method. The results of the calculations show that five-coordinated (T5) atoms (floating-bond atoms), anomalous four-coordinated (T4a) sites (weak-bond atoms), three-coordinated (T3) defects (dangling-bond atoms) and normal four-coordinated (T4n) atoms which are nearest neighbours of T3, T4a or T5 atoms give rise to three gap peaks. It was established that three peaks in the low-energy region of the optical absorption spectra are due to the electronic transitions: the valence band → the empty gap peak and two occupied gap peaks → the conduction band. Boron doping effects upon the optical spectra was not revealed. (author)