Local atomic structure and lattice defect analysis in heavily Co-doped ZnS thin films using X-ray absorption fine structure spectroscopy

X-ray absorption fine structure (XAFS) spectroscopy was used to address both the local atomic structure and the local electronic structure around Zn, Co, and S in Co-doped ZnS thin films. X-ray absorption near-edge spectroscopy (XANES) revealed changes in the strong pre-edge feature of the S K-edge spectra that depend on the Co concentration and are related to atomic bound-state transitions through hybridization between 3d transition metal and sp host semiconductor electronic states. These changes reveal intrinsic effects, such as S vacancies and dopant effects. From the Co K-edge XANES spectra, substitution of Co2+ into ZnS is observed. The local atomic structure around the dopant, obtained by XAFS spectroscopy, indicates a reduction of the Co S bond length. Optical analysis shows deep absorption centers or intermediate states in the bandgap, ascribed to d-d transitions in the Co atoms that modify the electronic band structure and cause a decrease in the bandgap as a function of Co concentration. This work provides direct evidence of dopant effects in heavily Co-doped ZnS, including bond distances and Debye-Waller factors, providing key elements to generate a realistic atomic structural model, which is crucial to understand the observed electronic properties of this material.