This study measures the x-ray-absorption spectra of a crystalline ͑c͒-Si-C-N thin film at the C and Si K edge using the sample drain current mode and at the N K edge using the fluorescence mode. A resonance peak resembling the C 1s core exciton in the chemical-vapor-deposition-diamond/Si is observed. In addition, a broad feature is found in the energy range between ϳ290 and 305 eV, which can be assigned to the antibonding C 2p-Si 3sp hybridized states and the C 2p-N 2sp hybridized states as well. The fact that the resonance peak is located ϳ1.5 eV below the C 1s ionization energy suggests that the Frenkel-type exciton model can appropriately describe the core exciton of carbon atoms in c-Si-C-N. Closely examining the N K edge near edge absorption spectra reveals similar features in both c-Si-C-N and ␣-Si 3 N 4 , indicating that nitrogen atoms generally have a similar local environment in these two materials. Moreover, results obtained from Si K-edge absorption spectra of c-Si-C-N demonstrate a proportional combination of local Si-N and Si-C bonds associated with the local tetrahedral C-Si-N 3 arrangement as well as the long-range ordered atomic structure around Si atoms. Theoretical calculations using the first-principles pseudofunction method are also presented and compared with experimental data. ͓S0163-1829͑98͒08037-0͔

The structural parameters, band structures and density of states of anatase TiO 2 co-doped with Cu and S were calculated by first-principles based on the density functional theory. The results indicate that the volumes of the co-doped TiO 2 increase due to the lattice distortion. The calculated X-ray diffraction pattern shows that the crystal phase of TiO 2 is still kept as anatase after Cu and S co-doping. The band gap of TiO 2 broadened when S substitutes for Ti or O along with Cu substitutes for Ti. The calculated partial density of states shows that the impurity energy levels mainly come from the Cu 3d and S 3p orbital. The calculated results may provide some theoretical foundations for the photocatalytic activity enhancement of TiO 2 co-doped with Cu and S.

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