Optical properties vacuum deposited and chlorine doped a-Se thin films: aging effects Wee Chong Tan Æ George Belev Æ K. Koughia Æ Robert Johanson Æ Stephen K. O’Leary Æ Safa Kasap Ó Springer Science+Business Media, LLC 2007 Abstract The optical properties of cold deposited films (substrate temperature T s =3 °C, below room temperature and the glass transition temperature T g ), hot deposited (T s = 50 °C, above T g ) and Cl-doped a-Se films have been examined by measuring the transmission spectra and then using the Swanepoel technique to extract the refractive index, dispersion relation, absorption coefficient and film thickness. We report and discuss the dependence of the optical properties on the deposition temperature, Cl-doping, and aging. 1 Introduction While amorphous selenium (a-Se) is no longer used in xerography, it still has a number of useful and interesting applications, for example, as a photoconductive layer in direct conversion X-ray image detectors and in ultra-sen- sitive video HARP (an acronym for ‘‘high-gain avalanche rushing photoconductor’’) video tubes that are based on avalanche multiplication [1, 2]. The optical properties of a-Se films have been widely studied by various authors in the past; there are numerous references in the literature as in the classic Mott and Davis book [3] and in various reviews (e.g. [4]). The purpose of this work was to inves- tigate the optical properties of undoped and chlorine doped a-Se films prepared by conventional vacuum deposition, which is the most convenient method to fabricate a-Se films [4]. Recently there has been an interest in cold deposited a-Se films (deposited below the room tempera- ture in contrast to films deposited above the glass transition temperature) because such films can act as n-like blocking layers in a-Se based x-ray photoconductor structure [5]. Amorphous Se is a typical glass and exhibits changes in its properties with aging and annealing; aging has been asso- ciated with structural relaxations that occur with time [6, 7]. Its glass transition temperature T g is about 40 °C. We have examined the changes in the optical properties with aging and have also obtained the dispersion relation for the refractive index. 2 Experimental procedure Films of undoped and Cl-doped a-Se were prepared by the thermal evaporation of selenium pellets from a molybde- num boat in a conventional stainless steel vacuum deposi- tion system under a base pressure of 10 –6 Torr. Typical film thicknesses ranged from 1 to 4 lm but we report mainly on 2 lm thick films. A shutter protected the substrates until steady evaporation conditions had been established and also during the cool-down period following the deposition. The substrate temperature was either approximately 3 °C (cooled substrate) or 50 °C, above T g (heated substrate). The first will be referred as the ‘‘cold’’ and the latter as the ‘‘hot’’ deposited film, where ‘‘cold’’ and ‘‘hot’’ are relative to room temperature and also T g . The glassy pellets used for the starting material were high-purity (photoreceptor grade) selenium either undoped or doped with 67 ppm chlorine. W. C. Tan (&)  G. Belev  K. Koughia  R. Johanson  S. Kasap Department of Electrical and Computer Engineering, University of Saskatchewan, Saskatoon, SK, CanadaS7N 5A9 e-mail: wet445@mail.usask.ca S. K. O’Leary Department of Electrical and Computer Engineering, University of Windsor, 401 Sunset Avenue, Windsor, ON, CanadaN9B 3P4 123 J Mater Sci: Mater Electron DOI 10.1007/s10854-007-9227-3