The configuration of Cu centers in electroluminescent SrS:Cu phosphors. An x-ray absorption fine structure and optical study

X-ray absorption fine structure and photoluminescence measurements are combined to analyze the structure of Cu centers in SrS:Cu thin-film and powder samples. We find that all the data can be understood on the basis of a model involving three kinds of Cu centers: (i) dumbbell-type Cu-Cu complexes (which are the dominant centers), (ii) substitutional centers (CuSr), and (iii) interstitial centers (Cui). The bond lengths in the dumbbell centers are determined from the x-ray absorption data. Two characteristic emission bands peaking at about 460 and 510 nm are attributed to Cui and dumbbell centers, respectively. The dependences of the emission spectrum on temperature and Cu concentration are explained by a dissociation-association reaction of the type Cu-Cu<-->CuSr + Cui, assuming center-dependent excitation efficiencies and thermal quenching rates.