Preparation, characterization, and bistable photoconduction properties of thin CdS layers

In recent years the interest in hybrid bistable devices has increased rapidly because of the possibility of using these properties for optical computing systems. Therefore we investigate optoelectronic features of various CdS layers, pointing out especially the bistability in photoconduction presented by thermo-optical hysteresis. We grow our thin (2–10-μm) CdS layers on various substrates (glass, Pyrex, and CaF2) by three methods: evaporation, close-spaced vapor transport, and spray deposition. By means of x-ray diffraction, transmission, and secondary-ion mass-spectroscopy measurements, we analyze the crystal structure, the slope of the absorption edge (which is important in attaining bistable behavior), and the impurities of our samples. From photocurrent measurements we determine the free-carrier lifetime, the photoconductivity gain, and the sensitivity of the layers. In this context a new theory is presented that gives the ratio of the surface recombination time to the free-carrier lifetime. We are most successful in exploring thermo-optical hysteresis in the case of sprayed layers. To our knowledge, this is the first case in which a room-temperature sprayed-film self-electro-optic-effect device presents the best-resolved thermo-optical hysteresis, especially in photoconductivity, when compared with recent measurements.