Sputter deposition of rare earth doped zinc sulfide for near infrared electroluminescence

Near infrared emitting alternating current thin film electroluminescent (ACTFEL) phosphors were fabricated by simultaneous R.F. magnetron sputtering from both a target of doped ZnS and an undoped ZnS target. The intensities of both near infrared (NIR) and visible emission from ZnS doped with thulium (Tm), neodymium (Nd), or erbium (Er) fluorides were dependent on deposition parameters such as target duty cycle (varied from 25 to 100% independently for the two targets) and substrate temperature (140--180°C), with lower temperatures giving 400% better NIR brightness. By optimizing the rare earth concentration between 0.8 and 1.1 at%, the near infrared irradiance was improved by 400% for each dopant. The increase in brightness and optimal concentrations are attributed to decreased crystallinity and increased dopant interaction at higher rare earth concentrations. The brightness increase with decreasing deposition temperature was attributed to a reduction of thermal desorption of the ZnS during deposition, and consequently thicker films and optimized rare earth concentration. Luminescent decay lifetimes were short (20--40 musec) because of a high concentration of non-radiative pathways due to defects from the strain of the large rare earth ions on the ZnS lattice. The threshold voltage for visible and near infrared emission was identical despite emission of NIR and visible light resulting from electrons relaxing from low and high energy excited levels, respectively. The optical threshold voltages were identical to the electrical threshold voltages, and it was concluded that at the voltages necessary for electrical breakdown, the accelerated electrons had enough energy to excite either the visible or NIR emitting levels. Phosphors doped with Nd exhibited increased internal charge at higher dopant concentrations despite a reduction in phosphor field (i.e. reduced applied voltage) In contrast; the charge did not change appreciably for Er and decreased for Tm doped films at reduced fields. The charge differences were attributed to dopant effects on the distribution of states near the interfaces. It was postulated that Nd doped devices have a shallower state distribution, while the majority of states in Tm doped devices are deeper and require higher fields for tunnel injection. The electrical behavior of all of the devices also demonstrated that field clamping occurred despite non-ideal phosphor breakdown during device operation. It is postulated that a high breakdown strength, low dielectric constant, interface layer is formed during deposition, and reduces capacitance before and after phosphor breakdown and results in field clamping. The thickness calculated for the interface layer decreases with increasing deposition temperature implying that the layer is formed during deposition, and this decreasing thickness results from increased atomic mobility at higher temperatures.