Electro-luminescence and associated two carrier effects in cadmium sulphide

Abstract

The properties of cad sulphide are such that the material has some potential for use in the semiconductor industry. The potential has so far not been realised because of the difficulty of preparing crystals with controlled properties. In order to improve the material available, much work has been carried out to identify and measure the parameters of the defects in the material. The study of double injection phenomena is one tool which enables defects to be investigated. During the work which forms the basis of this thesis, a large number of double injection devices have been fabricated from crystals grown by sublimation of CdS powder in a stream of argon. The devices have been studied at temperatures in the range 90 to 300 ºk and at temperatures below about 150 ºk visible green light is emitted when currents above about 5 mA flow. The light is due to recombination in the bulk and on the surface of the crystals, A comparison of the photo-, electro-, and cathodo- luminescent spectra has been made. The electrical measurements reveal that the current flow is governed by a class IT centre located 0.98 eV above the valence band. The presence of this centre leads to a switching effect which is such that the device can be turned on when a voltage pulse or a pulse of light is applied to the device when biased below threshold. When pulsed voltages are applied, the current pulse has a step in it due to a fraction of the class II centres capturing electrons while the pulse is off. The maximum frequency of sine wave modulation of the light has been found to be less than 1 megacycle so that the CdS device is unlikely to compete with injection lasers in other materials as a source of light for optical telecommunication systems.