Optical properties of thermally evaporated SnS thin films M.M. El-Nahass a , H.M. Zeyada b, * , M.S. Aziz b , N.A. El-Ghamaz b a Department of Physics, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt b Department of Physics, Faculty of Science at New Damietta, 34517 New Damietta, Egypt Received 27 April 2001; received in revised form 28 November 2001; accepted 7 January 2002 Abstract Thermally evaporated SnS amorphous chalcogenide films undergo structural transformation upon annealing in the temperature range between 432–573 K. The optical properties of amorphous and annealed films were investigated using spectrophotometric measurements of the transmittance and reflectance at normal incidence in the wavelength range 250–2500 nm. The films are transparent for a wavelength >1250 nm. The refractive index (n) and the absorption index (k) are independent of film thickness in the measured film thickness range (55–365 nm). The dispersion energy, E d , of amorphous films increased from 20.2 to 23.85 eV for crystalline films. The types of optical transition responsible for optical absorption are indirect allowed and direct forbidden transitions with energy gaps of 1.4 and 2.18 eV for the amorphous films and of 1.38 and 2.33 eV for the crystalline films, respectively. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal evaporation; Optical constants; SnS 1. Introduction SnS is one of the tin chalcogenide layered semiconductors in group IV–VI. It may exhibit the p-type conductivity [4–7,9,10,16], n-type conduc- tivity [1,4] depending on the concentration of tin and it may also changes its type of conduction from p to n-type conduction in accordance with treatment temperature [11,12]. Different values of energy gap have been ob- tained [1,2,4–7] for SnS ranging from 1 to 2.33 eV depending on the resulting structure obtained by different techniques and the occurring type of electron transitions. The requirements imposed on films used as a light absorber are (i) they must have an energy gap of about 1.5 eV with indirect al- lowed transition and (ii) a high absorption coeffi- cient >10 4 cm 1 . Because SnS crystallizes in orthorhombic structure, it can be used in n–p ho- mojunction [11] and n–p heterojunction [11,12]. The nature is abundant in elements Sn and S which moreover are non-polluting during SnS growing process. SnS thin films can be prepared by a va- riety of methods [1,2,4,5,7,9] with the purpose of manufacturing thin films suitable for use as a solar absorber in optoelectronic devices and photovol- taic applications [12]. Among these methods, the thermal evaporation technique received little in- terest in the literature [8,13]. The most recently studies on thermally evaporated SnS films were probably made by Deraman et al. [13] who Optical Materials 20 (2002) 159–170 www.elsevier.com/locate/optmat * Corresponding author. Fax: +20-2-057-403868. E-mail address: hamdyzeyada@masrawy.com (H.M. Zey- ada). 0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0925-3467(02)00030-7