Evolution of structural disorder using Raman spectra and Urbach energy in GeSe0.5S1.5 thin films

• Urbach energy reduction is attributed to increase of order with film thickness.
• Homopolar to heteropolar structure transition evident from Raman spectra
• Phonon–phonon scattering is responsible for the variation in thermal diffusivity.
• Crystal system of GeSe0.5S1.5 thin films has been solved as orthorhombic.

We report thickness dependent structural and optical properties of GeSe0.5S1.5 thin films prepared by vacuum evaporation. From Urbach energy the structural disorder is addressed on the basis of band tailing due to creation of localized energy states. The obtained decrease in band gap and Urbach energy with increase in film thickness is linked to the increase in crystallinity and structural order. Raman spectra also supports the observed change in crystallinity with thickness. The lower thickness samples possess homopolar bonds such as GeGe, SeSe and SS while in higher thickness film heteropolar GeSe2S2 structural units are formed. These findings suggest that the prepared GeSe0.5S1.5 thin films show an increase in the proportion of edge-sharing tetrahedral units with thickness. The effect of thermo elastic bending on GeSe0.5S1.5 thin films is studied from photo-acoustics and phonon–phonon scattering is found to be the dominating scattering mechanism.