ELECTRICAL STUDIES ON TRAP LEVELS PRESENT IN N-TYPE AND P-TYPE SPRAY PYROLYZED CDS THIN-FILMS

Trap levels in n- and p-type spray pyrolysed CdS thin films were analy sed using thermally stimulated current (TSC) and dark-conductivity mea surements. TSC measurements of the n-type sample revealed the presence of only one peak under light excitation for a short period due to mob ility of sulphur vacancies, but longer excitation could give evidence of one more level which was due to a complex of cadmium and sulphur va cancies. TSC spectra of the p-type sample indicated the presence of tw o levels of comparable cross section irrespective of light excitation time, which correspond to mobility of the sulphur vacancy and copper i mpurity. TSC measurements on air annealed n-type samples indicated the presence of a level due to the sulphur vacancies and another level ma y be due to chemisorbed oxygen. However vacuum annealed samples (10(-2 ) Torr) showed results similar to those of as-prepared n-type samples. The dark-conductivity measurements of these samples were found to be in good agreement with the TSC measurement results.