Sputter deposited electroluminescent zinc sulfide thin films doped with rare earths

Abstract

The effects of deposition conditions on the physical properties and threshold voltages of ZnS alternating current thin film electroluminescent (ACTFEL) phosphors are reported. The ZnS thin films were deposited by rf magnetron cosputtering of both a pure ZnS and a ZnS target doped with either 1.5mol% ErF3, NdF3, or TmF3, and EL peaks in both the visible and near infrared (NIR) were observed. The substrate temperature during deposition was varied between 140 and 190°C, and the highest NIR EL intensity was measured from as-deposited films at 140°C. Energy dispersive x-ray analysis was used to determine the rare earth concentrations between 1.0 and 2.6at.%, and the maximum emission was at ∼0.8–1at.%. The EL intensity decreased at higher concentrations due to poor crystallinity and decreased at lower concentrations due to lower rates of excitation. The threshold voltage and phosphor thickness for these ACTFEL devices both decreased as the deposition temperature was increased due to re-evaporation of the incongruently sputter transported zinc and sulfur. Electrical data confirmed that the optical threshold voltage for both NIR and visible emission were equal to one another and to the electrical breakdown voltage, indicating that at breakdown the energies of ballistic electrons are sufficient to excite both visible and infrared emissions.