JOURNAL OF MATERIALS SCIENCE LETTERS 11 (1992) 380-381 Photoconductivity of SnSe thin films V. P. BHATT, K. GIREESAN and C. F. DESAI Physics Department, Faculty of Science, M.S. University of Baroda, Baroda 390 002, India Tin monoselenide (SnSe) is one of the most impor- tant photovoltaic materials of the type AIVB vI. There has been an ample study on growth and electrical properties of SnSe thin films with a few reports on their photoelectronic properties [1-7]. We report here the results of our study on the dark current, photocurrent, response time and effect of temperature on the photocurrent. The films used were of thickness 75nm, prepared by resistive evaporation at 373 K on glass substrates. The deposition and temperature variation studies were done at a pressure of 1.3 mPa. A 100 W, 220 V tungsten-filament lamp and a foot-candle meter were used for illumination and intensity measure- ment, respectively. Aluminium films were used as ohmic electrodes. The variation of photocurrent with light intensity is shown in Fig. 1. It is seen that the variation is linear, as expected for this type of photoconductors. The change in slope at 20 mW cm-2 is indicative of a trap centre changing over to behave as a recombina- tion centre, decreasing the photosensitivity at higher intensities. These data were obtained at room temperature (about 300 K). The variation of photocurrent with temperature under a constant intensity of 25 mW cm -2 is shown in Fig. 2. The heating rate was about 1 °Cmin -1. There is a peak at 350 K above which the photocur- rent decreases with temperature. This is due to the fact that if the traps are filled by excitons at low temperatures, they may be emptied by raising the temperature. This sharp maximum in the plot (at 350 K) is a clear indication of the single trap depth. The photocurrent of SnSe thin films with various 7 ~e % x 5 E ~4 L 0 0 10 20 30 40 50 60 70 80 90 100 Intensity (roW crn -2) Figure1 Variation of photocurrent with light intensity of SnSe thin film. 380 times and intensities of illumination is shown in Fig. 3a and b. The sample was exposed to light for 60 s, then the light was extinguished and readings were taken continuously. The slow photocurrent and its decay are of considerable interest as this phe- nomenon is sensitive to the trapping effect due to impurity levels. It can be seen from Fig. 3a that when low-intensity light was incident on the sample, the photocurrent increased suddenly from A to B, then it slowly increased from B to C and almost levelled off from C to D. When the light was extinguished, the photocurrent suddenly decreased from E to F. This type of behaviour was observed up to the intensity of 20 mWcm -2. Under strong illumination (above 20 mW cm -2) there was a rapid rise in photocurrent with time of illumination (A'B') and the current slowly increased and reached the maximum value. After switching off the light the current rapidly decreased initially (B'C') and then slowly decreased and became constant. The photoconductivity rise and decay results obtained agree with those observed in p-type silicon [8], in which the rise and decay of photoconductivity was found to be due to the presence of the shallow 3 2 ~0 o X 8 O .E2 13_ G 270 2cj0 3i0 3:30 350 370 390 410 Temperature (K) Figure2 Variation of photocurrent with temperature of SnSe thin films (intensity 25 mW cm-2). 0261-8028 © 1992 Chapman & Hall