Studies on excess noise in stabilized amorphous selenium used in X-ray photodetectors Shaikh Hasibul Majid, Robert E. Johanson * Department of Electrical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9 Available online 4 March 2008 Abstract 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/f a from 0.5 Hz to 1 KHz where a 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 concen- tration 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. Ó 2008 Elsevier B.V. All rights reserved. PACS: 05.40.Ca; 74.40.+k; 87.57.Às; 71.23.Cq; 81.05.Gc Keywords: Alloys; Amorphous; Selenium; Photodetector; 1/f noise 1. Introduction With a steady dc voltage applied, the current through most conductive materials fluctuates which results from conductance fluctuations inside the material. The power density of the fluctuations is greater at lower frequencies and typically approximates a 1/f power law [1–3]. The physical origin for most of materials is unknown. We reported some of the first measurements of 1/f noise in sta- bilized amorphous selenium (a-Se) alloys [4] that is Se with 0.3%–0.5% As (to retard crystallization) and tens of ppm Cl (to passivate defects). Stabilized a-Se is used in direct-conversion, X-ray digital radiography as the photo- conductive layer. A high dc voltage across the a-Se layer is necessary to collect the charges produced by the X-rays [5,6]. The large electrical field also produces a dark current which has no relationship to the X-ray exposure. Although the average signal from the dark current can be subtracted, the fluctuating part of the dark current degrades the image quality. To better understand the performance of the X-ray detectors, we need to measure and minimize the 1/f noise. In this paper, we examine the effect of alloy composi- tions, surface condition and substrate temperature at depo- sition on the 1/f noise. The concentrations of As and Cl as well as the substrate temperature are known to affect charge transport. Investigating a roughened versus as- deposited surface is an attempt to separate bulk noise from contact noise. Due to the high resistance of a-Se, it is quite difficult to do noise measurements on this material. Furthermore, the non-linear I–V relationship of the metal to a-Se junction has made the interpretation of the noise more complicated, hence making the research work more challenging. 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.093 * Corresponding author. Tel.: +1 306 966 5392; fax: +1 306 966 5407. E-mail address: robert.johanson@usask.ca (R.E. Johanson). www.elsevier.com/locate/jnoncrysol Available online at www.sciencedirect.com Journal of Non-Crystalline Solids 354 (2008) 2763–2766