Light-induced changes in the optical properties of thin films of Ge–S–Bi(Tl, In) chalcogenides R. Todorov a , Tz. Iliev b , K. Petkov a, * a Central Laboratory of Photoprocesses, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 109, 1113 Sofia, Bulgaria b Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 24, 1113 Sofia, Bulgaria Abstract In this paper, we report some results from the study of changes in transmittance, reflectance and optical band gap of thin Ge–S–Bi(Tl, In) thin layers depending on the composition and the conditions of evaporation and illumination. Using two triple, TR f R m and TR b R m methods, the optical constants (n and k), and the thickness (d ) of very thin layers from the Ge–S system have been determined to an accuracy of ±2 nm. R f , R b and R m is the reflection from the film side, from the glass substrate and that of 100 nm thick film deposited on Si substrate, respectively. The calculated values of the optical constants of the ternary chalcogenide films were compared with those obtained from the ellipsometric measurements and some conclusions for their practical applications were done. Ó 2003 Published by Elsevier B.V. PACS: 78.20.Ci; 78.40.Pg; 78.66.Jg; 81.15.Ef 1. Introduction Photoinduced changes in amorphous chalcoge- nides are an object of systematic investigations with a view to better understanding the mecha- nisms of the phenomena taking place in them as well as their practical applications [1]. The physical properties of chalcogenide glasses from the system Ge x S(Se) 1x and their changes under exposure to light depend considerably on the composition and the conditions of deposition and illumination of the layers [2–4]. When a third element is incorpo- rated in GeS 2 it leads to considerable changes in its structure and properties. Small quantities of Bi or Tl cause some transformations in the glassy net- work of GeS 2 or GeSe 2 , which leads to changes in the band gap as well as their electrical and optical [5–7] properties. The results from X-ray diffrac- tion, IR and Raman spectroscopy [8] show that new bonds and structural units BiS 3=2 are created. The aim of the present paper is to give some new results using different methods for determi- nation of the optical constants of very thin layers and to discuss the influence of the composition and exposure to light on the optical properties of thin Ge–S–Bi(In, Tl) films. 2. Experimental The synthesis of glasses with compositions Ge 20 S 80 ,Ge 33 S 67 ,Ge 40 S 60 and (GeS 2 ) 1x Me x , where * Corresponding author. Tel.: +359-2 718 400; fax: +359-2 722 465. E-mail address: kpetkov@clf.bas.bg (K. Petkov). 0022-3093/$ - see front matter Ó 2003 Published by Elsevier B.V. doi:10.1016/S0022-3093(03)00405-8 Journal of Non-Crystalline Solids 326&327 (2003) 263–267 www.elsevier.com/locate/jnoncrysol