De-Excitation of ZnS and ZnCdS Phosphors by Electric Fields

The change of impedance (photoconductivity) of certain ZnS and ZnCdS powder phosphor samples, brought about by external ac (60 cps) and dc fields (up to 15 000 volts/cm), applied both during and after excitation with ultraviolet light (3660 A), is described and compared with impedance changes due to irradiation with infrared light. Application of a dc field simultaneously with the exciting radiation results first in a sudden drop in both the capacitance (ΔCC₀) and the dissipation factor (D=1/Q) followed by a recovery to a new equilibrium level below the field-free case. Removal of the field is accompanied by another drop and a recovery to the original field-free values. The partial recovery is not seen during ac field application under the same conditions but the complete recovery also occurs upon field removal. Applied during the photoconductivity decay, both ac and dc fields accelerate the decay. The sudden drop and partial recovery under dc fields is also seen during the photoconductivity decay but to a lesser extent.These phenomena are explained by a consideration of the distribution of conduction electrons created by the application of the field. The sudden capacitance drop seen under a dc field is attributed to a polarization effect. The free charge is swept close to the electrodes leaving the bulk of the phosphor sample nonconducting. The partial recovery is due to the buildup of the back field as well as to an increase in the conductivity of the bulk of the phosphor grains due to the creation of additional free charge by the exciting radiation and the emptying of traps into the conduction band. There is no partial recovery during ac field application because of the absence of the polarization field. The finite recovery time after field removal when the phosphor is under excitation as well as the acceleration of the photoconductivity decay due to the field application is attributed to a quenching action brought on by an increase in the recombination rate in those regions of the phosphor powder sample where conduction electrons have been piled up by the action of the applied field.