Evolution of structural disorder in amorphous GeSeS thin films by thickness variation

• Urbach energy reduction is attributed to increase of order with film thickness.
• At higher thickness heteropolar bonds become dominant.
• Phonon–phonon scattering is responsible for the variation in thermal diffusivity.

Amorphous GeSeS thin films are deposited for five different thicknesses using thermal evaporation technique and their microstructures are analyzed. The oscillations obtained in the optical transmittance spectra are resulted from the interference between the air-film and film-substrate interfaces. The observed decrease in the optical band gap with increase in the thickness of the film is associated with the change in the bonds formed in the structure of the network. Structural disorder like unsaturated bonds (dangling bonds) are studied by means of Tauc parameter (B1/2), Urbach energy (EU), and thermal diffusivity. The formation of heteropolar bonds become enormous for higher thickness of GeSeS thin films. From Raman spectra the bonds associated with symmetric stretching and tetrahedral units are discussed. Phonon–phonon scattering is the dominating mechanism which determines the phonon mean free path and hence the measured variation in thermal diffusivity.