Studies of element-specific local structures in compound materials using x-ray absorption spectroscopy

The x-ray absorption spectroscopy techniques have been used to study a variety of semiconductor and superconductor materials. In such experiments, synchrotron radiation harnessed by a delicate beamline electronic and control system are used to obtain data with analyzable quality in a reasonable time scale. The element-selectivity is achieved by selecting an energy-scan range close to a characteristic "absorption edge" of the selected element. Peak structures below the absorption edge (pre-edge structures) reflect the local unoccupied states of the selected atomic species. The position of absorption edge (part of the near-edge x-ray absorption fine structure, NEXAFS) provides some qualitative information of the effective valency of the selected element. And, most importantly, the modulation in the spectrum some 40 eV above the absorption edge (extended x-ray absorption fine structure, EXAFS) gives quantitative information of the local structure around the selected atomic species.; The selected atomic species such as magnetic Mn ions in III-V diluted magnetic semiconductors (DMS) {dollar}rm Insb{lcub}1-x{rcub}Mnsb{lcub}x{rcub}As,{dollar} Mn as the luminescent centers in nanocrystals of ZnS, O in the CuO{dollar}sb2{dollar} planes which host the carriers in high-T{dollar}sb{lcub}rm c{rcub}{dollar} superconductors, and F as the electron reservoir in the n-type high-T{dollar}sb{lcub}rm c{rcub}{dollar} superconductors {dollar}rm Ndsb2CuOsb{lcub}4-x{rcub}Fsb{lcub}x{rcub}{dollar} all play an important role in the novel mechanism of these new materials. Along with other detailed information, our EXAFS results have revealed (i) III-V DMS can indeed be prepared by substitutional doping of magnetic impurities under proper processing conditions. (ii) Mn ions substitute for the Zn sites in the nanocrystals of ZnS with significant size-dependent local structural changes. (iii) Only {dollar}sim{dollar}6% of O in the CuO{dollar}sb2{dollar} planes in {dollar}rm Ndsb2CuOsb{lcub}4-x{rcub}Fsb{lcub}x{rcub}{dollar} are substituted by F. The rest of F atoms substitute for O atoms in the NdO layers and serve as electron reservoirs. The NEXAFS results have shown that the effective valency of Mn in ZnS nanocrystals is close to +2 with a weak size dependence. And, our pre-edge structure analysis has confirmed the importance of O 2p holes in the high-T{dollar}sb{lcub}rm c{rcub}{dollar} superconductor mechanism.