Photo-Resistance Effect for Metals at Low Temperatures

Photo-resistance effect for sputtered metallic films at low temperatures.—Waterman's equilibrium theory of resistance predicts an effect for metals, increasing as the temperature is decreased. To test this, thin sputtered films, after aging, were held at liquid air temperature and illuminated with light from a quartz mercury vapor arc, with or without screens. Bismuth showed the largest decrease in resistance, 16×${10}^{-6\mathrm{}}$, palladium 14×${10}^{-6\mathrm{}}$, copper 1.6×${10}^{-6\mathrm{}}$, platinum even less and gold and silver no detectable change. The order of these metals is in agreement with the theory. The effect was not in general instantaneous but increased with time along a saturation curve, and in the dark returned only slowly toward the original value. The effective wave-length was found to be beyond 3000A for fresh Bi films, but the threshold seemed to move to longer wave-lengths with age. Some older Bi films showed a reverse positive effect of even larger magnitude which apparently is due to longer wave-lengths, but the exact circumstances were not definitely fixed. The magnitude of the negative effect for Bi decreased rapidly with increasing temperature, to 3×${10}^{-6\mathrm{}}$ at -110°. Tellurium, sensitive at room temperature, showed a 70 percent greater effect at -185°C.