Optical properties of amorphous (As0.33S0.67)100-xTex (x=0, 1, 5 and 10) chalcogenide thin films, photodoped step-by-step with silver

We have analysed in detail the effect of silver-content on the optical properties of Ag-photodoped amorphous (As0.33S0.67)(100-x)Te-x (with x = 0, 1, 5 and 10 at.%) chalcogenide thin films; the chalcogenide host layers were prepared by vacuum thermal evaporation. Films of composition Ag-y[(As0.33S0.67)(100-x)Te-x](100-y), with y less than or similar to 18 at.%, were successfully obtained by successively photodissolving about 20- or 40-nm-thick layers of silver. The optical constants (n, k) have been accurately determined by an improved envelope method [J.M. Gonzilez-Leal, R. Prieto-Alcon, J.A. Angel, D.A. Minkov, E. Marquez, Appl. Opt. 41 (2002) 7300], based on the two envelope curves of the optical-transmission spectrum, obtained at normal incidence. The dispersion of the refractive index of the Ag-photodoped chalcogenide films is analysed in terms of the Wemple-DiDomenico single-effective-oscillator model: n(2)(h omega) = 1 - EoEd/(E-o(2) - (h omega)(2)), where E-o is the single-oscillator energy, and E-d the dispersion energy. We found that the refractive index of the Ag-doped samples strongly increases with the Ag-content, whereas the optical band gap, E-g(opt), decreases also notably. For instance, in the particular case of x = 10 at.%, the largest Te-content, E-g(opt) decreases from 2.17 down to 1.67 eV. It should also be mentioned that, in the case of the undoped samples, when the Te-concentration increases from zero up to 10 at.%, the value of E-g(opt) decreases from 2.49 down to 2.17 eV. (C) 2007 Elsevier B.V. All rights reserved.