Some peculiarities of the mechanism of irreversible photostructural transformations in thin As-S-Se layers

Abstract

The present paper is concerned with the peculiarities of the mechanism of irreversible photostructural transformations in As-S-Se layers. The starting point of the irreversible photostructural transformations is the state which is determined by the conditions of the fabrication of the As-S-Se layers. It is thermodynamically unstable and differs from the structure of the glasses or annealed films. Their state is the final one for the structure of as-prepared layers changing under the influence of external factors. For the considered As-S-Se compositions the correlation of the composition dependencies for dispersion energy and optical dielectric constant of the exposed or annealed layers and structurally-dependent parameters of As-S-Se glasses: glass transition temperature Tg, relaxation enthalpy DH, heat capacity Cp, mole volume, compactness, is characteristic. Irreversible photostructural transformations are characterized by the absence of the essential influence of the diffusion processes. This is connected with the close proximity and high concentration of the non-stoichiometric molecular fragments which contain homopolar (As-As, S-S (Se-Se)) bonds. The swit­ching of the bonds with the decrease of the homopolar ones and various defects is energetically favorable. The consideration of the evolution of the number of such fragments (number of homopolar bonds) as a result of polymeric processes that take place due to the exposure influence and change the local structure of the amorphous layers towards that inherent for the glass, gives the exponential decay of their number with the increasing exposure. This is supported by the exponential decay of Raman spectra bands intensity with the exposure which correspond to the presence of molecular fragments containing homopolar bonds.