Effect of Composition and Annealing on Some of the Optical Parameters of Gex Te100-x Thin Films

Abstract

The dependence of the absorption coefficient α on photon energy hν was determined in the spectral range from 1.1 to 3.9 eV at room temperature, for evaporated Ge x Te 100-x films. The variation of the optical gap E g opt , dielectric constant e ,and absorption coefficient,α with composition and annealing are reported. The optical gap increases with x followed by a sharpe decrease beyond x =20. This can be explained in terms of increase in the number of Te-Te bonds and formation of GeTe 4 tetrahedral with an increase in the chalcogen content. The observed behaviour is explained on the basis of chemical bond approach and on Mott and Davis model for the density of states in amorphous solids. The absorption coefficient for Ge -Te films exhibits exponential dependence on photon energy obeying Urbach’s rule in the absorption edge . The optical absorption measurements on the as-deposited and annealed films indicate that the absorption mechanism is due to direct transition. The optical gap E g as well as the high frequency dielectric constant, e ,depends on the film composition and the annealing temperature. The shift in the absorption edge due to the heat treatment is explained by a change in the defect structure of the films. The decrease in the optical gap after annealing is due to the effect of oxidation . The optical gap, E g opt , as well as the high