Variation in Conductivity of Cadmium Sulfo-Selenide Sintered Films with Vacuum Heat Treatment

Abstract

The conductivity of CdS-Se sintered film increases irreversibly about 109-fold with vacuum heat treatment from 100° C to 200° C. The mechanism of such a large variation has been studied using highly sensitive Hall effect and conductivity measurements. The obtained results can be analyzed by the grain-boundary potential-barrier model with the aid of the charge neutrality condition for the grains, the maximum allowable values of surface potential and potential barrier height, and the density of interface states. Increase in conductivity of 8 orders of magnitude is brought about by increase in the free electron density, which occurs in two steps: in the low-temperature region, the release of trapped electrons from the surface state occurs due to desorption of chemisorbed oxygen; in the high-temperature region, the increase in effective doping density through the decrease in acceptor density occurs due to the release of occluded oxygen from the bulk crystallites near the surface. The mobility of free electrons contributes to an increase of one order of magnitude in conductivity, which decreases once to a minimum and then increases. The variation in mobility is attributed to the barrier height at the grain boundary which is produced by trapped electrons at the intrinsic interface state.