Thermally and photo-induced irreversible changes in the optical properties of amorphous GexSe100-x films

Films of amorphous GexSe100-x were prepared by plasma-enhanced chemical vapour deposition (PECVD) using the hydrides GeH4 and H2Se as precursor gases and by standard thermal evaporation. The optical bandgap obtained from a Tauc plot showed a maximum (E(g) = 2.27 eV) at the stoichiometric composition (x = 33) for the PECVD films. The bandgap values of the virgin evaporated films were much lower in the range x = 40 to 75, the difference being due to the incorporation of hydrogen in the PECVD films, detected in IR and Raman spectra. Thermal annealing and illumination by bandgap light of the evaporated films induced irreversible bleaching. The PECVD films showed a more complex behaviour of the optical shift upon annealing due to the loss of hydrogen. An IR and Raman investigation demonstrated that the changes in the optical transparency are accompanied by an increase of ordering in the local structure.