Some Electrical Properties of Amorphous Selenium Films

Abstract

The dark current and photocurrentcharacteristics of amorphous selenium and arsenic‐selenium alloy films have been investigated. One millisecond after the application of an electric field, the dark current (neglecting capacitancecharging) is several orders of magnitude higher than the steady‐state dark current. A hysteresis effect is observed in the dark volt‐ampere curves with the descending voltage branch exhibiting an exponential volt‐ampere relationship. The voltage of a charged Se plate in the dark discharges logarithmically with time as anticipated from the exponential volt‐ampere relationship. Upon admitting air to a freshly evaporated Se plate, a negative surface potential of 150 mv is observed. The phenomenon of ``fatigue'' (i.e., the temporary increase in dark current following an exposure to illumination) obtained with highly absorbed radiation indicates surface barrier effects exert a large measure of control over dark currents. Photogenerated holes and electrons in Se films are trapped with trapping parameters such that the hole range/unit field is on the order of 10−8 cm2/v, and the electron range is less than 10−9 cm3/v. The addition of As to the Se greatly increases the electron effective lifetime. Trapped holes are eliminated in the dark exponentially with time through a process having an activation energy of 0.77 ev. A 40‐mv negative surface photovoltage is observed in Se films upon illumination. This voltage may be accounted for by hole diffusion away from the surface. The addition of As to Se extends the spectral response into the red end of the spectrum, results in the initiation of secondary photocurrent effects, intensifies fatigue effects, and increases the softening temperature of the film.