Annealing effects on the structure and optical properties of GeSe2 and GeSe4 films prepared by PLD

Amorphous thin films of GeSe2 and GeSe4 were prepared by the pulsed laser deposition (PLD) technique. Refractive index, thickness and optical band gap (E-g(opt)) of the films were abtained from the optical transmission spectra and absorption spectra, respectively. Based on the 'non-direct transition' model proposed by Tauc, the absorption edges can be well fitted. As an indicator of the degree of structural randomness of amorphous semiconductors, the Tauc slopes of the prepared thin films were discussed. The dispersion of refractive index was analyzed in terms of the single-oscillator Wemple-Di Domenico model. For as-deposited films, after annealing below the glass transition temperature (T-g), the drop of refractive index and the increase of E-g(opt) can be clearly observed. The thermal-bleaching effect was proposed to the reduction in the density of homopolar bonds confirmed by the Raman spectra analysis. The thermal-contraction effect was ascribed to the elinimination of porous structure of the as-deposited films. (C) 2009 Elsevier B.V. All rights reserved.