Enhanced photoluminescence decay processes of doped CaS phosphors at low temperature.

CaS phosphor samples singly doped with Mn impurity and doubly doped with Mn and X (X=  Fe, Co and Ni) have been synthesized using a flux method. Crystal structure and phase identification of doped CaS phosphors have been carried out by X-ray diffraction (XRD) studies. A pulse excitation method has been employed to study the optical parameters of the doped phosphors. Excited state lifetime measurements at liquid nitrogen temperature were carried out with a pulsed UV laser (nitrogen laser) as the excitation source has a short pulse width (10  ns) and high peak power (200  kW per pulse). The results have been compared with room temperature lifetime studies. Enhancement in photoluminescence intensity and lifetime shortening in these synthesized doped phosphors is reported at low temperature. The lifetime values have been found to be in the microsecond time domain for CaS:Mn phosphors while on addition of a quencher impurity Ni to CaS:Mn phosphors, the lifetime values reduce to the nanosecond time domain at 77  K temperature. A thermally activated carrier transfer model has been proposed to explain the observed abnormal temperature behaviour of emission from CaS:Mn, X-doped phosphors. [ABSTRACT FROM AUTHOR]