Optical Absorption and Photoconductivity of Amorphous and Hexagonal Selenium

Abstract

In amorphous and hexagonal selenium optical absorption and photoconductivity were studied in their dependence on temperature. The absorption edge shifts for amorphous selenium toward shorter wavelengths with decreasing temperature. For a temperature change from 300° to 90°K and a layer thickness of 98μ, for example, a shift from 6600 to 6100A is observed at an optical density of 3; for higher densities (shorter wavelengths) the shift is smaller. It can be shown that this shift is related to the thermal excitation of vibrational levels. At higher temperatures the population of the higher vibration levels increases and the separation of these from the excited state diminishes. At low temperature (ca 90°K) photoconductivity is observed only after the light quanta exceed 2.5 ev and the absorption coefficient has reached 10⁵ cm⁻¹. Increase of temperature brings about a shift of the photoconductive threshold toward longer wavelengths, paralleling the absorption‐edge shift. The absorption coefficient has been followed over the wavelength range 6900 to 2100A; its values at the end points of this range are 1.6×10² and 5.2×10⁵ cm⁻¹, respectively; there is a slight maximum at 2600A where the absorption coefficient reaches 5.6×10⁵ cm⁻¹. The transformation of amorphous into hexagonal selenium by heat brings about an increase of the optical absorption in the 5000 to 7200A region and the appearance of a distinct hump in the absorption edge at 5200A. Decrease of temperature sharpens the hump and causes a decrease of absorption at all wavelengths in the 4000 to 7200A range except in the vicinity of the hump. In hexagonal selenium photoconductivity is observed at wavelengths as long as 7500A for films 0.5μ thick. A peak in the photoresponse is found where the optical density of these films reaches ca 1 (10 percent transmission). It can be shown that this behavior is to be expected for a bimolecular law of recombination of electrons and holes when their inhomogeneous distribution is taken into account. With decreasing temperature the peak response shifts consequently toward shorter wavelengths with the absorption edge.