Photoconductivity in the infra-red region of the spectrum Part I. The preparation and properties of photconductive films of lead telluride Part II. The mechanism of photoconductivity in lead telluride

Abstract

PART I The method of preparation of photoconductive films of lead telluride is described. Details are given of the construction of cells suitable for infra-red detection in spectroscopic applications. Lead telluride is not photosensitive at room temperature, but is strongly photoconductive at temperatures in the region of —190° C. The conductivity of lead telluride as a function of the applied voltage, and the wave-length and intensity of illumination has been investigated, for seven films of widely different conductivity, at liquid-air temperature. The application of the photoconductive effect to infra-red detection is discussed, and attention is drawn to the effect of any background of thermal radiation on the performance of a cooled photoconductor. PART II A method is described by which thin semi-conducting films of nearly stoichiometric lead telluride can be prepared by evaporation, in a sequence with increasing impurity of lead. The conductivity and infra-red photoconductivity of the specimens has been investigated as a function of temperature and of lead impurity. It is found that the magnitude of the photo-effect is a function of the dark conductivity, and depends only on the quantity of excess lead. The absorption of oxygen at room temperature is equivalent to the removal of excess lead; the presence of oxygen is not a requirement for the appearance of photoconductivity. These observations are used to draw conclusions concerning the mechanism of the photoconductivity. It is shown that all the phenomena can be explained by a single lattice model, without recourse to internal potential barriers or inhomogeneities in the composition of the samples.