Photoexcitation Mechanisms of the Green Defect Emission from Zinc and Sulfur Doped ZnO Phosphor Powders Through Measurement and Analysis of Optical Properties and Characterization
The mechanism for defect related green emission from zinc (ZnO:Zn) and sulfur doped ZnO (ZnO:S) are determined through optical characterization of the green and UV emission bands. ZnO:Zn is prepared by heating ZnO in a slightly reducing atmosphere for 1 hour and sulfur doped ZnO is similarly obtained with a small amount of sulfur added. Photoluminescence (PL), photoluminescence excitation spectra (PLE), 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 ZnO:Zn and ZnO:S have a similar green emission mechanism. This common mechanism involves the formation of donor bound excitons <italic>I3a</italic> and <italic>I9</italic>, which were determined to be the mediators between photoexcitation of excitons and the transfer of energy to the defect responsible for green emission. The most efficient excitation processes for both the green and band edge emissions at low temperatures is through direct excitation of the neutral donor bound exciton <italic>I9</italic> or by ionizing the neutral donor bound exciton <italic>I3a</italic>. The ionization of <italic>I3a</italic> eliminates this exciton localization site and simultaneously creates a bound exciton at <italic>I9</italic>. The <italic>I9</italic> bound exciton can then either transfer energy to the defect responsible for the green emission or contribute to the free exciton population through a phonon assisted transition. At room temperature a resonant absorption peak associated with <italic>I9</italic> is still present in the absorption band for ZnO:Zn suggesting partial localization at <italic>I3a</italic> and <italic>I9</italic> of free excitons with low kinetic energy (excitations below the band gap) continues to be the intermediate between excitons and the energy transfer to the green emitting defect.
In ZnO:S, the addition of sulfur creates ZnS domains within the lattice leading to a type II band alignment at the interface of ZnO and ZnS domains. This band alignment at the interface increases the density of free electrons in ZnO, which may then encounter an ionized <italic>I3a</italic> site converting it to its neutral form. Increasing the density of free electrons, a result of the type II band alignment, increases the chances of returning an ionized <italic>I3a</italic> to its neutral form and thus increases the green emission. These results can lead to informed optimization of ZnO:S as a potential white light emitting phosphor.
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