Optical characterization of As-Se-Ag thin films

1. Transmission spectra recorded within the spectral range 400 – 2500 nm. Thickness of (AsSe)100-xAgx films as a function of the Ag content. Typical X-ray diffraction patterns of AsSe and AsSe-Ag films. (αhν)1/2 versus photon energy hν plots, from which the optical band gap Eg of amorphous (AsSe)100-xAgx films is determined. Determination of the Eg values Tauc procedure: plotting a graph of (αhν)1/2 versus hν extrapolation of the straight line part to the energy axis of zero absorptioncoefficient. Influence of the Ag content on the optical band gap of amorphous (AsSe)100-xAgx films . Spectral dispersion of the refractive index n of amorphous (AsSe)100-xAgx films Optical characterization of As-Se-Ag thin films V. Ilcheva1*, T. Petkova1, P. Petkov2 and V. Boev1 1Institute of Electrochemistry and Energy Systems, BAS, 1113 Sofia, Bulgaria 2Departments of Physics, University of Chemical Technology and Metallurgy, Kl.Ohridski Blvd.8, 1756 Sofia, Bulgaria * vania_ilcheva@yahoo.com Abstract Thin arsenic-selenium-silver films (AsSe)100–xAgx (x = 0 - 25 at.%) have been deposited on glass and silicon substrates by either vacuum thermal evaporation (VTE) and pulsed laser deposition (PLD) from the corresponding bulk materials. The amorphous character of the coatings was confirmed by X-ray diffraction investigations. Their transmission was measured within the wavelength range (400 – 2500) nm and the obtained spectra were analyzed by the Swanepoel method to derive the optical band gap Eg and the refractive index n. The possibility to deposit by both methods high-quality films with the desired composition and appropriate optical parameters in view of potential applications in optics has been demonstrated. EXPERIMENTAL RESULTS α =1/d [ln (1-R)2 / T] d - thickness of the film R – reflectivity of the film Absorption coefficient of amorphoussemiconductors: (ahν) = B{hν -Eg}mm=1/2 SUMMARY • Thin amorphous (AsSe)100-xAgx films were grown by vacuum thermal evaporation and pulsed laser deposition from the corresponding bulk glassy materials and characterized with respect to their optical properties. • A red shift of the absorption edge is observed with the addition of Ag to the glassy matrix. The introduction of Ag causes also a decrease in the optical band gap, attributed to structural transformation and bond rearrangement in the films after Ag introduction. The atomic substitution of arsenic by silver probably is related to an increase of disorder and defects present in amorphous structure, resulting in optical band gap decrease. • The refractive index n of the films varies in the range 2.5 – 3.4. A common trend for all films under investigation is the increase of n with the silver content due to the higher polarizability of Ag atoms and corresponding increase of optical density. Acknowledgment: The authors are grateful for the final support of European Social Fund, Human Resources Development Program, under contract BG 051PO001/07/3.3-02/58/ 17.06.2008.