Electrical Structure of PbS Films

Abstract

The electrical properties of chemically deposited lead sulfide films have been investigated as a function of frequency from dc into the microwave range. Conductivity and Hall mobility measurements have been made on both the dark and the photoexcited carriers. The dark conductivity is independent of frequency below 100 Mc/sec. It increases by almost an order of magnitude over a decade in frequency and then becomes constant again at higher frequencies. This behavior is typical of a material whose low-frequency conductivity is limited by barriers. Surprisingly the photoconductivity behaves quite differently. The photoconductivity has a hump around 100 Mc/sec but otherwise is relatively independent of frequency. The Hall mobilities of the dark and photocarriers are equal at dc in agreement with earlier studies. Microwave Hall measurements indicate that the dark carriers have about the same apparent mobility at 10 kMc/sec as at dc. It is shown that the conventional barrier model with a photoinduced change in carrier concentration cannot account for the behavior of these films. Nor can a model in which the barrier height is modified by illumination. The present studies suggest that there are connecting channels through the films and that the photoresponse of the films is determined by the behavior of these channels. A simple model is constructed and its parameters are adjusted to fit the conductivity data and the dc Hall mobility. As a check on the model the measured microwave Hall mobility is compared with the predicted value. Good agreement is found for high-quality commercial films. The hump observed in the photoconductivity follows naturally from this model, being associated with a modulation of the barrier capacitance. Studies of the photoconductive decay yield the same time constant at dc and microwave frequencies, supporting the picture that the same photocarriers are responsible for the dc and microwave photoconductivity.