Compositional dependence of the optical properties of vacuum evaporated thin GeSe2-GeTe-PbTe films
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2013-01-01
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Chalcogenide glasses from the GeSe2-GeTe-PbTe system were synthesized by taking preliminary prepared GeSe2, GeTe and PbTe in their molecular percentages and melting them in an evacuated quartz ampoule. Thin films were deposited on optical glass substrates BK-7 and Si-wafer substrates by thermal evaporation. Using X-ray microanalysis it was found that the film composition differs to a certain degree from the bulk composition. The changes in the optical properties of thin GeSe2-GeTe-PbTe films were studied as a function of their composition, exposure to light and annealing. It is shown by XRD analysis that the thin films are amorphous up to layers with composition of Ge27Se51Te14Pb8. The optical constants (refractive index, n and absorption coefficient, ), the thickness, d as well as the optical band gap, Eg, depending on the film composition were determined by spectrophotometric measurements in the region 450-2500 nm applying the Swanepoel's envelop method and Tauc's procedure. With the increase of the Te content in the layers, the absorption edge is shifted to the higher wavelengths, the refractive index increases from 2.54 for Ge 31Se66Te3 up to 3.37 for Ge32Se 55Te13 while the optical band gap decreases from 1.86 eV for to 1.30 eV for the same compositions. Similar dependence was observed with the increase of the PbTe content in the layers. After annealing of thin films, the values of n decrease, the optical band gap increases and a shift of the absorption edge to the shorter wavelengths is observed. © 2013 The Authors. Published by Elsevier B.V.