Structural Evolution in Thin Films of Ag2Te1-xSex (x∼0.3) Alloy Prepared by Thermal Evaporation Technique

Evolution of structure and properties in ternary Ag2Te1-xSex (x similar to 0.3) alloy is examined in the present paper. Stoichiometric quantity of pure powders of Ag2Te and Ag2Se was mixed in a ball mill and then melted at high temperature, 1025 degrees C in a vacuum sealed quartz tube, followed by annealing at 700 degrees C for 12 hours. Thin films of Ag2Te.7Se.3 of different thickness viz.25, 50, 100, and 150 nm are produced by thermal evaporation method. Thermal evaporation method is carried out by resistive heating of the solid of Ag2Te.7Se.3 sample at 1000 degrees C under a high vacuum (pressure in the chamber is less than 10(-5) mbar). The thin films were subjected to structural analysis by XRD, microstructural study by scanning electron microscopy and optical property study by UV-Vis. spectroscopy. It is found that Ag2Te1-xSex (x similar to 0.3) is formed by alloying of two solid solutions, of which one is based on Ag2Te with Se as solute and the other is Ag2Se-based solid solution in which Te is the solute. The peak position of Ag2Te-based solid solution is shifted toward higher angle whereas a reverse trend is observed in Ag2Se-based solution. The microstructure is composed of a solid solution of Ag2Te and Ag2Se along with the fine dispersion of elemental silver, tellurium and selenium. The optical band gap varies within 1.4 to 2.2 eV over the thickness range from 25 to 150 nm. The band gap initially increases with the film thickness, and beyond 50 nm, the band gap continuously decreases to 1.7 eV. The transmittance is lower for higher film thickness; while transmittance increases with wavelength, the rate of increase is enhanced in the infrared region. The absorption coefficient of thin films of Ag2Te.7Se.3 is as high as of the order of 10(5) cm(-1).