A further experiment which belies the fischer model for the electroluminescence process

Abstract

Experiments have already been reported which provide evidence against the Fischer double-injection model for the a.e. electroluminescence (EL) in ZnS (1). They concern respectively the behaviour of low temperature of the EL yield (2) and the effect of a sharp UV light spot on the EL comets (a). We relate here another experiment which, as will be seen, also conflicts with the Fischer model. I t consists in studying the enhancement of the E1 brightness waves emit ted when the ZnS crystals are irradiated with UV light. This effect, which can be referred to as photoelectroluminescenee (PEL), was previously investigated by some authors (4), mainly by PATEK (5), but only in a rather quali tat ive manner and without succeeding in obtaining definite information about the mechanism of the EL process. To perform our measurements we used condenser shaped EL cells, 0.08 mm thick, filled with ZnS microcrystals embedded in araldite. The crystals were doped with 8.10 -4 Cu, C1 atoms per Zn atom. Both the cell electrodes were built with conducting quartz, to allow the UV light to reach the phosphor crystals. The cell was excited with a 1500 Hz sinusoidal voltage, while it was irradiated with light from a 250 W low-pressure UV lamp supplied with a 50 Hz stabilized voltage. The distance from the lamp to the cell was 30 cm. Light from the cell was detected with a 6810 A RCA phototube placed 20 em away. Suitable collimators and filters were used, to prevent visible light from the lampe reaching the cell and UV light scattered from the cell striking the phototube. In these conditions the EL signal, even though of relevant amplitude, was hidden by a predominant photoluminescence (PL) signal due, besides the ZnS crystals, to araldite. For this reason and taking into account that PL emission was pulsed with a frequency (100 Hz, say twice that of the voltage applied to the UV lamp) very low with respect to that of EL, we connected, after the photomultiplier, a high-pass filter to improve the EL to PL signal ratio.