Luminescence in Tellurium-Doped Cadmium Sulfide

Abstract

The room-temperature emission in CdS:Te is a single broad band whose half-width and peak position provide a simple means of estimating the tellurium concentration in the range 2×1018−5×1019 atoms cm−3 with a precision of 15%. Of the two discrete bands (at 2.1 and 1.7 eV) seen at low temperatures, only the higher-energy emission can be described by a simple configuration coordinate model. The nophonon line energy, not observed experimentally, is 2.36±0.05 eV, the exciton being bound to the tellurium atom by 0.22 eV. At high tellurium concentrations the exciton can be trapped at pairs or larger groupings of tellurium atoms sited on nearest-neighbor sulfur sites, the binding energy increasing correspondingly. The emission intensity from the 2.1-eV band is reduced by a factor of about 100 as the temperature is increased from near 0° to 300°K. The quenching behavior cannot be described in detail by a single decay process but the effective activation energy is about 0.20±0.03 eV. The drastic temperature quenching above 200°K restricts the usefulness of CdS:Te as a room-temperature scintillation device but can be reduced by the addition of donors. The relative efficiency at 300°K to that at low temperature increases from less than 1% in undoped material to 15%–20% in CdS:Te containing about 1018 donors cm−3. Chlorine, bromine, or iodine appear to serve equally well as donors. Donor-doped crystals exhibit two components in decay lifetime measurements at 300°K but only one (τ∼250±50 nsec) at low temperature. The extra radiative decay time (500±100 nsec) may arise from free-electron to bound-hole transitions at high donor concentrations. Energetically one expects such a process to be near degenerate with the trapped exciton decay and no obvious change occurs in the broad emission band. Hall data show a reduction in the donor binding energy roughly proportional to ND1/3, where ND is the donor concentration. Pulse heights detected after nuclear excitation of CdS:Te are increased about fivefold when 5×1017 donors cm−3 are added, chlorine giving the cleanest pulse shapes.