SulfurL2,3sand zincM2,3ssoft-x-ray fluorescence spectra in CdS and ZnS

The sulfur ${\mathrm{L}}_{2,3}$ soft-x-ray emission spectra of wurtzite CdS and cubic ZnS and the zinc ${\mathrm{M}}_{2,3}$ spectra of ZnS have been measured using both photon and electron-beam excitation. The photon-excited sulfur L spectra are found to be much sharper than electron-beam-excited spectra. Using photon excitation between the ${\mathrm{L}}_2$ and ${\mathrm{L}}_3$ thresholds, a pure ${\mathrm{L}}_3$ spectrum is obtained for both materials. Broad features of the S L spectra, consisting of a lower valence band (LVB) derived from the sulfur 3s orbital and an upper valence band (UVB) derived from S 3p and Cd(Zn) s and p orbitals, are typical of materials with mixed ionic-covalent character. A narrow band between the UVB and LVB is associated with overlap of Zn 3d and Cd 4d orbitals onto the S site. The s+d local partial density of states derived from these spectra provide a rigorous test of band-structure calculations; they are in good agreement for the position, width, and structure of the LVB and UVB, but place the d band deeper in the subband gap than theory predicts. The Zn ${\mathrm{M}}_{2,3}$ spectrum has large lifetime broadening and thus provides limited information. However, near-threshold excitation of this spectrum shows dramatic effects resulting from an electronic resonance Raman process.