A New Bridge Photo-Cell Employing a Photo-Conductive Effect in Silicon. Some Properties of High Purity Silicon

Abstract

A pure photo-conductive effect was found in pyrolytically deposited and vaporized silicon films. An apparatus is described for making bridge type photo-cells by reaction of silicon tetrachloride and hydrogen gases at ceramic or quartz surfaces at high temperatures. The maximum photo-sensitivity occurs at 8400–8600A with considerable response in the visible region of the spectrum. The sensitivity of the cell appears about equivalent to that of the selenium bridge and its stability and speed of response are far better. For pyrolytic films on porcelain there are three distinct regions in the conductivity as a function of temperature. At low temperatures the electronic conductivity is given by the expression σ=Af(T) exp−(E/2kT). At temperatures between 227°C and a higher temperature of 400–500°C σ=A exp−(E/2kT), where E lies between 0.3 and 0.8 ev; and at high temperatures σ=A exp−(E/2kT), where E=1.12 ev. The value 1.12 ev represents the separation of the conducting and non-conducting bands in silicon. The long wave limit of the optical absorption of silicon was found to lie at approximately 10,500A (1.18 ev). The data lead to the conclusion that the same electron bands are concerned in the photoelectric, optical, and thermal processes and that the low values of specific conductances found (1.8×10−5 ohm−1 cm−1) are caused by the high purity of the silicon rather than by its polycrystalline structure.