Heat Treatment of CdSe Thin Films and Application to Photopotentiometers

Effects of heat treatment have been studied in a nitrogen atmosphere for improving the characteristics of poor photoconductive CdSe evaporated films. The following results were obtained. At first light resistance and dark resistance greatly decrease after the heat treatment of evaporated films in a pure nitrogen atmosphere for an hour. The ratio of dark resistance to light resistance under irradiation of 1,000/ux remained as low as an order of magnitude of 10. Secondary increase of an order of magnitude of 10 6 or 10 7 in dark resistance is obtained, adding to the post treatment in N 2 + O 2 atmosphere for thirty second after the main heat treatment in N 2 for an hour. The ratio of dark to light resistance becomes an order of 10 8 and as for photoresponse, both rise and decay time of photocurrent become 0.5 ms. The results indicates that heat treated films have characteristics of good photosensitivity and quick response in photocurrent. The photopotentiometer has been analysed by using the transmission line theory and device input–output characteristics have been related to the properties of the resistive film and photoconductive film of the device. The photopotentiometer using the improved photoconductive CdSe thin film has been fabricated and examined. A good independent linearity of less than 2% at a speed of 2000 rpm is obtained.