Electrical conductivity and luminescence in Cadmium Sulphide

Abstract

The work is conveniently divided into two sections:- (i) the electrical properties of cadmium sulphide and (ii) the optical properties of cadmium sulphide.(i) Measurements have been made on thin pure platelets of cadmium sulphide from which it is clear that electrical conduction in these samples is governed by two complex defect centres with associated levels within the forbidden gap (0.41 eV - 0.61 eV; 0.25 eV - 0.83). This has been deduced from measurements of the static current- voltage curves made over a wide range of temperatures coupled with an investigation of thermally stimulated current curves obtained after electron injection and optical excitation. The measurements also demonstrate that Lamport theory of Space Charge Limited Current Flow in an insulator with traps is applicable at low current densities but that trap emptying occurs when high current densities are passed through a crystal.(ii) Measurements of the spectral distribution of ultra-violet stimulated fluorescence at 77 K and 300 K, demonstrate the existence of electronic energy levels within the forbidden gap which can be correlated with those determined from the electrical experiments. The spectral distribution of luminescent emission has been measured in pure and doped samples from which it is concluded that edge emission is associated with both sulphur vacancies and interstitials (0.13 eV below the conduction band and 0.16 eV above the valence band respectively.). The two emission series can be separate in suitable crystals. Further work, with better resolution of the emission bands should clarify the position further. Electro-luminescence, due to the recombination of injected electrons and holes, is attributes to bulk processes which produce the edge emission series, surface and impurity recombination.