X-ray absorption spectroscopy study of diluted magnetic semiconductors: Zn1-xMxSe (M = Mn, Fe, Co) and Zn1-xMnxY (Y = Se, Te)

We have investigated 3d electronic states of doped transition metals in II-VI diluted magnetic semiconductors, Zn1-xMxSe (M = Mn, Fe, Co) and Zn1-xMnxY (Y = Se, Te), using the transition-metal L2,3-edge X-ray absorption spectroscopy (XAS) measurements. In order to explain the XAS spectra, we employed a tetragonal cluster model calculation, which includes not only the full ionic multiplet structure but also configuration interaction (CI). The results show that CI is essential to describe the experimental spectra adequately, indicating the strong hybridization between the transition metal 3d and the ligand p orbitals. In the study of Zn1-xMnxY (Y = Se, Te), we also found considerable spectral change in the Mn L2,3-edge XAS spectra for different ligands, confirming the importance of the hybridization effects in these materials.Comment: This paper consists of 22 pages including 4 figures. This paper is submitted to Physical Review

Radiative Recombination in Cadmium Sulfide

Luminescent centers involving Ag impurities were introduced into CdS single crystals through doping with 109Cd radioisotopes. Thus, the Ag concentration increases with time as more 109Cd decays. This enables a study of photoluminescence versus Ag concentration in a given crystal without changing the concentrations of other impurities.A new emission band at 5600 Å results in addition to the 6100 Å band present in Ag-doped crystals using conventional techniques. This new emission is quenched with increasing Ag concentration at high concentrations. Also concentration quenching by the Ag impurities occurs for the green edge, and the bound-exciton emissions I1, and I2. The quenching is explained by assuming a donor–acceptor recombination process.The new emission probably arises from the recombination of a bound electron with a bound hole at a distant donor–acceptor pair, with Ag as the acceptor. The acceptor role of Ag is supported by electrical conductivity measurements on 109Cd-doped crystals. Estimates are obtained for the acceptor binding energy, the donor concentrations, and the separations of pairs responsible for the new 5600 Å emission and the green-edge emission. The 6100 Å emission is attributed to Ag closely associated with other impurities. These conclusions are verified by our temperature annealing and quenching experiments. </jats:p

PHOTOLUMINESCENCE IN CdS ASSOCIATED WITH NITROGEN IMPURITIES

In the photoluminescence spectrum of cadmium sulfide, new emission bands have been produced which can be attributed to nitrogen impurities giving rise to acceptor levels 131 meV above the top of the valence band. </jats:p

PHOTOLUMINESCENCE EFFICIENCY AND PHOTOCONDUCTIVITY IN CADMIUM SULFIDE

The photoluminescence efficiency and photoconductivity response of high-purity cadmium sulfide crystals were measured and interpreted in terms of a simple model involving only the donor and acceptor levels previously established, an effective surface recombination velocity, and a bulk nonradiative recombination rate. The measurements at 64 and 78 °K included the effects on the photoluminescence efficiency of varying the excitation intensity and the wavelength of the excitation light, applying electric fields, removing chemically adsorbed oxygen ions from the surface, and of different acceptor concentrations. It was established that the surface does play an important role in reducing the photoluminescence efficiency and photoconductive response, and that the photogenerated carriers do not diffuse significantly into the interior of the crystal. </jats:p