Optical transitions in alkaline earth sulfide electroluminescent phosphors

The optical transitions in three alkaline earth (AE) sulfide material systems were investigated. In undoped and oxygen doped Ca{dollar}rmsb{lcub}0.55{rcub}Srsb{lcub}0.45{rcub}Gasb2Ssb4{dollar}:Ce the optical transitions occur between Ce energy levels, which are sensitive to the local bonding of the Ce. The oxygen doped films were found to be brighter, more efficient, and emitted higher energy photons than the undoped materials. Analytical data showed that while Ce was bonded to eight sulfur ions in the undoped films, in the oxygen doped films the Ce was bonded to 5 sulfur and three oxygen ions. To model the effects that oxygen has on the Ce energy levels, semiempirical cluster calculations were performed. The calculated Ce energy levels for the undoped and oxygen doped thin films were in good agreement with the experimentally determined energy levels.; The optical transitions of different Ce doped AE sulfide phosphors (MgS:Ce, CaS:Ce, SrS:Ce and BaS:Ce) were also studied. Semiempirical cluster calculations were performed to model the Ce energy levels in MgS, CaS, and SrS and a good agreement with the experimental values was obtained. The crystal field model was also used to calculate the energy levels in these phosphors, and while it accurately described the CaS:Ce and SrS:Ce, it had to be modified for the MgS:Ce. The crystal field model also predicted that the BaS:Ce transition energy would be higher than SrS:Ce. Sr{dollar}sb{lcub}rm x{rcub}{dollar}Ba{dollar}sb{lcub}rm 1-x{rcub}{dollar}S:Ce thin films were grown, and analytical data revealed that films with higher BaS concentrations had lower transition energies than pure SrS:Ce. The discrepancy between the model calculations and the experimentally determined transition energies was explained by a local change in the Ce symmetry.; The properties of SrS:Te powders and thin films were also studied and excitons bound to Te sites were determined to luminesce efficiently. Two emission peaks (360 and 400 nm) were identified, and their intensities were found to be dependent on the Te concentration. From temperature dependent photoluminescent data several important bound exciton properties were determined, including: (1) the binding energy, (2) the thermal quenching activation energy, and (3) the vibrational energy.