The effects of high-temperature thermal annealing on cathodoluminescence (CL) spectra in SiOx (0:9
x
1:87) films prepared by radiofrequency sputtering are investigated. The CL intensities for the as-deposited films are weak but they increase after thermal annealing at 900 and 1100 C. One of features in the CL spectra for the films annealed at 1100 C is a peak at a photon energy of 2:7 eV with an asymmetric tail on the lower energy side. In order to analyze the spectral features, optical transition energies are calculated for Sin clusters with n ¼ 2{5, embedded in a SiOx matrix, by ab initio molecular orbital calculation. In addition, the probabilities of formation are statistically estimated for those Si clusters under the assumption of a chemically ordered random network for the SiOx network. The comparison of the experimental results with the calculated transition energies and the statistics of the Si clusters suggests that a contribution of the Si2 clusters to the CL spectra are dominant, whereas those of the Sin clusters with n > 3 are considerably small. # 2011 The Japan Society of Applied Physics