Luminescence studies in cadmium sulphide.

This thesis describes some properties of the luminescence from single crystals and evaporated thin films of CdS under optical and electron beam excitation. A comparison . of optical and electron beam excitation mechan-ismSj as applied to the study of luminescence from solids,, is made. It is concluded that the variable penetration depth of high energy electrons with accelerating voltage can be readily used to distinguish between surface and bulk effects,, and that electron beam excitation can be used to advantage when the efficiency of a radiative process is low and high excitation intensity is required. The main advantage of optical excitation is in the vertical excitation of electrons from lower to higher energy levels which arises from the negligible momentum of the incident photons. Optical excitation spectra of various bound exciton emission lines in single crystals of Cds are , presented . It is shown that -the complex responsible for the so-called I, emission ^ due to the annihilation of an exciton bound to a neutral acceptor,, is created by the direct formation of bound excitons on the impurity site and by the formation of free or intrinsic excitons which are subsequently trapped on the impurity. The complexes responsible for the so-called 1 and I,- emission are also created by the formation of intrinsic excitons but more importantly they are created by direct phonon assisted formation of excitons bound to neutral and ionized impurities. It is concluded that the impurity involved is a donor. Electron beam excited luminescence from evaporated thin films of CdS is obtained. The transitions giving rise to this luminescence are identified as resulting from the recombination of free electrons with bound holes (free-bound emission) and from the recombination of bound electrons with bound holes (bound-bound emission) with the simultaneous emission of n LO phonons (n = 0,, 1, 2, ...). It is shown that if the films are coated with a 2000A thick layer of SiO , causing the CdS energy bands to be bent down at the X SiO - CdS interface,, the peak position of the free-bound emission x is shifted toward higher energies when the incident electrons have a small penetration depth (low energy). This is shown to result from an accumulation layer of electrons at the SiO - CdS inter- X. face .