Infrared (IR) electroluminescent (EL) thin film phosphors were radio frequency magnetron sputter deposited by cosputtering of an undoped ZnS target together with ZnS: 1.5 mole % ErF3 or ZnS: 1.5 mole % NdF3 targets. The ZnS:ErF3 and ZnS:NdF3 thin film phosphors were annealed in a N2 ambient at temperatures ranging from 350 to 475 °C for 1 h to increase radiance. The maximum EL radiance observed was 28 μW/cm2 at 1550 nm for ZnS:ErF3, and 26 μW/cm2 at 910 nm and 15 μW/cm2 at 1060 nm for ZnS:NdF3 (at 40 V above the threshold voltage) after a 425 °C, 1 h anneal in nitrogen. For anneals above 425 °C visible emission increased, while near infrared (NIR) emission from both ZnS:ErF3 and ZnS:NdF3 was either constant or decreased. For ZnS:ErF3, the 1550 nm NIR peak decreased, but the 990 nm peak remained constant in intensity. The crystallinity of ZnS was improved by annealing, and these results are consistent with the postulate that residual defects limit the acceleration of “hot” electrons for anneals at ⩽425 °C. Under these conditions, hot electrons only have sufficient energy to excite Er+3 into the lower lying I413/2 excited state which leads to 1550 nm NIR emission. With increasing annealing temperatures, hot electrons can excite from the I415/2 ground state into higher energy excited states (e.g., the F47/2 state for 990 nm emission). The NIR emissions from ZnS:NdF3 at 910 and 1060 nm originate from the same excited state and both peaks exhibited maximum NIR intensities after annealing at 425 °C. While the emission spectra from Er were independent of annealing temperature, peak shifts were observed for Nd. These shifts were discussed in terms of the nephelauxetic effect and hybridization of the 5d–4f orbitals.

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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.

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