The hall effect and the associated phenomena in cadmium sulphide

Abstract

Hall effect and photo Hall measurements have been made on a number of crystals. These crystals were grown (in this laboratory) using a modified method of the technique of growth by vacuum sublimation. The object of the photo Hall measurements was to determine some of the parameters of the imperfection centres with energy levels in the forbidden gap in photosensitive cadmium sulphide crystals. Two crystals (crystals 78 and 79) with dark conductivities less than 10(^-7) mho cm(^-1)were chosen for the investigation. Their conductivities could be increased to 10(^-2) mho cm(^-1) by optical excitation with an intensity of 3200 ft-c. The photo Hall effect was investigated at different temperatures. Measurements were made to determine the variation in the value of Hall mobility as a function of the location of the electron Fermi level which was achieved by changing the intensity of photoexcitation. From the plots of 1/µ versus E(_fn) (at different temperatures) four electron trapping levels with energy depths of 0.12, 0.l6, 0-22 and 0.33 eV and six electron trapping levels with energy depths of O.O98, 0.13, 0'19, 0.23, 0.33 and 0.42 eV below the conduction band were obtained for crystals 78 and 79 respectively. The photoHall data were also used to determine the charge state and the scattering cross-sections of the imperfection centres. The experimental values of the scattering cross-sections were of the -11 2order of 10cm. From a study of the temperature dependence of concentration of photoexcited carriers, the height of the energy levels of the sensitizing centres above the valence band was found to be 1.04 eV. The Hall coefficient was also measured for a number of semiconducting samples between 14 K and 300 K. The donor ionization energies and the donor and acceptor concentrations were determined from the variation of carrier concentration with temperature above 30 K. Below 30 K, the variation of carrier concentration with temperature was found to be dominated by an impurity band conduction mechanism (non-metallic type). Polar optical mode scattering was the dominant intrinsic scattering mechanism at the higher temperatures. The experimental mobility data could be fitted to theoretically computed values of µ assuming that polar optical mode, piezoelectric and ionized impurity scattering processes were operative. The effective mass was used as an adjustable parameter. The best fit was obtained with m*(_e) = 0.19 m.