Photoexcitation Mechanisms of the Green Defect Emission from Annealed and Sulfur Doped ZnO Phosphor Powders Through Direct Comparison of Optical Properties and Characterization

The mechanism for defect related green emission from annealed ZnO and sulfur doped ZnO are determined through optical characterization of the green and UV emission bands. Annealed ZnO is prepared by heating ZnO in a slightly reducing atmosphere for 1 hour, while sulfur doped ZnO is fabricated in the same manner with the addition of sulfur. Photoluminescence, photoluminescence excitation spectra, and quantum efficiency measurements are analyzed to determine the mechanism of the green defect emission. Low temperature PL and PLE measurements are used to assign activation energies to the emission processes and connect them with donor bound excitons in ZnO. It was determined that both annealed ZnO and sulfur doped ZnO have a similar green emitting mechanism. Localized donor bound excitons I3a and I9 are determined to be the mediators between photoexcitations and green emission. I9 can be excited directly or indirectly by first ionizing I3a and then creating an I9 bound exciton. The addition of sulfur creates ZnS domains within the lattice leading to a type II band alignment between the ZnO and ZnS domains. The concentration of electrons is increased in sulfur doped ZnO due to the type II band alignment, which also increases the recreation of the donor bound exciton I3a, which can then be ionized to transfer an exciton to I9 for green emission. These results can lead to informed optimization of ZnO:S as a potential white light emitting phosphor.