Photoconductivity in Vacuum Coated Selenium Films*

Abstract

Photoconductive layers which have a dark resistivity higher than 1012 ohm×cm, are of particular interest for electrophotography and television pick-up tubes. Selenium coatings prepared by vacuum evaporation are suited for these purposes. At base plate temperatures lower than 50°C, vitreous selenium is deposited which shows photoconductivity for wavelengths shorter than 600 millimicrons. At higher coating temperatures, many small hexagonal selenium crystals embedded in the vitreous matrix are introduced. Both the nucleation and velocity of crystal growth are investigated. A small percentage of crystalline selenium produces an additional red sensitivity, while high dark insulation qualities are preserved. The incident light causes positive holes from the hexagonal crystals to be injected into the vitreous selenium. A small addition of tellurium induces at low coating temperatures a shift of the photosensitivity towards longer wavelengths. At higher temperatures coatings with fairly panchromatic response can be produced.