Optical Properties of Thin Ge-Se-In Chalcogenide Films for Sensor Applications

Abstract

(GeSe₅)_1−xIn_x calcogenide glasses with 5,10,15,20 mol% In have been investigated. Some physico-chemical characteristics have been obtained: the density ρ, compactness δ, and molar volume V_m. The free volume percentage FVP has been attained; correlations with the mean coordination number Z are discussed. The number of constraints per atom N_co as a function of the average coordination number has been calculated. The correlations between composition and properties of the glasses are discussed in terms of structural transformations in the glassy matrix. The processes of vacuum evaporation and condensation in the Ge-Se-In system were investigated. The films deposited under the conditions used are amorphous and without 3D defects as proven by TEM analysis. The optical spectra of the thin films have been studied with a view to understand the role of indium on the film behavior. An optical characterization method based on transmission and reflection spectra at normal incidence of uniform thin films has been used to obtain the thicknesses and optical constants of as-deposited and annealed samples. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model. The absorption edges are described using both the Urbach rule and the ‘nondirect transition’ model proposed by Tauc. The variations in the refractive index, the band-gap, and the oscillation energy of the films after annealing are discussed in terms of rearrangements of the main structure units.