Effect of Oxygen upon Sintered Cadmium Sulphide Photoconducting Films

Abstract

The dark conductivity and decay time of sintered CdS films prepared by firing in nitrogen gas were higher than those of sintered CdS films prepared by firing in the mixture of nitrogen gas and hydrogen sulphide. Accordingly it was thought that the sintered CdS films prepared by firing in nitrogen gas have many sulphur vacancies acting as donors.
As a result of heat-treatment in oxygen of the sintered CdS films prepared by firing in nitrogen gas, the dark conductivity and decay time decreased. A new peak appeared at 680 mμ in spectral response of the photocurrent. The activation energy obtained by measuring the slope of the curve of logarithm of dark conductivity plotted against 1⁄T(°K) increased, while the trap concentration obtained by measuring the thermally stimulated current decreased with the increase in temperature of heat-treatment in oxygen. The results of experiments supported the interpretation that sulphur vacancies acting as donors had been occupied by oxygen atoms, and then vanished. At the same time there were produced new exciting centers at about 1.8 eV below the bottom of the conduction band.
By electron-microscopic observations it was found that by heat treatment above 400°C in oxygen, CdO grains were produced on the surface or near the grain boundary of the sintered CdS film, and when the whole surface of the CdS film was coated by CdO layers, photoconductivity was not observed.