Studies on excess noise in stabilized amorphous selenium used in X-ray photodetectors

We report investigations of the dependence of 1/f noise on surface condition, alloy composition and the substrate temperature during evaporation in samples of amorphous selenium alloys. The 1/f noise was measured using DC bias currents ranging from 2 to 200 nA; the measurements were complicated by the low current levels and highly non-linear I–V relationship of electroded amorphous selenium. The noise power density spectrum varies approximately as 1/fα from 0.5 Hz to 1 KHz where α ranges from 0.7 to 1.1. A roughened surface produces noise almost two orders of magnitude lower than an as-deposited surface. The dependence of the noise power on Cl concentration was weak but was quite significant for As concentration. The substrate temperature during evaporation of the selenium alloy did not have any effect on 1/f noise for a particular applied voltage, though typically the dark current of a sample deposited at a particular substrate temperature falls sharply when the temperature is increased or decreased. The results stated in this paper have implications for designing the photoconductive layer in the X-ray imaging detectors.