Effects of oxygen on electroluminescent characteristics of ZnS:TbOF and ZnS:TmOF devices

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Abstract
ZnS:TbOF and ZnS:TmOF active layers for thin-film electroluminescent devices were deposited in oxygen atmosphere by an electron-beam evaporation method and the effects of oxygen in active layers on electroluminescent (EL) characteristics were studied. With increasing oxygen deposition pressures, the luminance and the luminescent decay time increased and the threshold voltage shifted to lower values in ZnS:TbOF devices, while they showed no notable variations in ZnS:TmOF devices. X-ray-diffraction studies for ZnS thin-films showed that by the deposition in oxygen atmosphere the film orientation of (111) plane, the film uniformity, and the grain size are improved. By an analysis based on the Auger electron spectroscopy spectra for ZnS thin films and EL emission spectra, it was confirmed that a small amount of oxygen is incorporated into films and suppresses the recombination related to zinc and sulfur vacancies. It was found that contrary to ZnS:TmOF devices, ZnS:TbOF devices show a change in the peak intensity of EL spectra by oxygen doping. From the experimental results, it is suggested that in ZnS:TbOF devices oxygen doping could give rise to the increase of the luminance due to a suppression of the nonradiative energy transfer via grain boundaries and/or vacancies, whereas in ZnS:TmOF devices it plays no role in such a mechanism. The effect of oxygen doping on interfaces is also discussed.