Electroluminescence of ZnS Single Crystals with Cathode Barriers

Certain ZnS crystals show electroluminescence predominantly near the cathode. The major emission from Cu-activated crystals occurs as a burst of light when the exciting voltage is suddenly removed, as electrons flowing back toward the cathode to neutralize the barrier charge recombine with ionized luminescent centers. This burst may be quenched by a voltage in the initial direction or enhanced by a voltage in the opposite direction. There is also a small steady component of emission due either to capture of electrons that entered the crystal from the cathode or to excitation of centers to discrete levels. In Mn-activated crystals, only the latter has been observed.These results, as well as the emission peaks obtained from copper-activated ZnS under sinusoidal voltage excitation, are explained in terms of ionization of luminescent centers in a barrier region. The "primary" peak, which predominates in powder samples and under low voltages in single crystals with metal contacts, is due to recombination of centers left over from the previous half-cycle. A capture cross section of about 10^-12 cm² is deduced. The peak at about 150 to 170 phase degrees is due to recombination of ionized centers left in the neutral part of the crystal as the barrier width diminishes. A barrier space-charge density of (very roughly) 10¹⁶ e/cm³ is deduced. The in-phase peak is analogous to dc electroluminescence.