Shpotyuk M. Physical peculiarities of radiation-structural modification of As-S covalent-network glass

Thesis is devoted to the studying of radiation-structural modification (RSM) of chalcogenide semiconductor glasses (ChSG), particularly to the establishing of physical peculiarities, adequate mechanisms and criterial parameters of RSM being induced by high-energy γ-irradiation in covalent-network As-S glass. It is detected that γ-irradiation stimulate essential structural-relaxation processes in AsxS1-x CgSG with x < 0.40, which appear in increasing of glass transition temperature and area of endothermic peak of DSC curve and testify to γ-induced approaching of their network to more equilibrium thermodynamic state. On the other hand, AsxS1-x ChSG with 0.30  x  0.42 are resistant to the natural physical ageing (PhA). Effect of γ-induced PhA in As-S system increases with increase in deviation from stoichiometry to S-enriched composition. Observed changes are explained by considering the radiation-induced breaking of covalent chemical bonds in ChSG network. It is argued the methodological reasonability of experimental investigation of intrinsic radiation-induced optical effects in ChSG in the backward chronology of continuous in-situ measuring. Restrictions of the radiation-induced metastability investigation methods applied in direct chronology of cycled ex-situ measurements are exposed. It is demonstrated the possibility of delimitation of the radiation-optical response from intrinsic destruction-polymerization transformations and accompanying natural and radiation-induced physical ageing effects. Compositional trends in RSM of the spectral dependences of optical transmission in the network ChSG of As-S system are investigated by the in-situ optical spectroscopy within the methodology of backward chronology. Effects of short- and long-wave radiation-induced shift of the fundamental optical absorption edge are observed in dependence on the chemical composition and thermal prehistory of ChSG (thermally aged or rejuvenated glasses). It is shown that γ-induced darkening owing to the intrinsic destruction-polymerization transformations dominates in near-stoichiometric As-S ChSG, as well as radiation-induced PhA processes cause the γ-induced bleaching of chalcogen-enriched ChSG. It is defined the defect-related origin of the radiation-induced metastability in ChSG in the frameworks of free volume concept. By the methods of positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy, it is identified the processes of the free volume voids evolution in As-S ChSG (agglomeration-fragmentation, expansion-contraction and charging-discharging) being connected with transformations of abnormally-coordinated charged defects. The effects of thermal and γ-induced PhA are shown to be caused by the processes of voids fragmentation, as well as formation of the γ-induced defects accompanies by the additional voids charging. The methodology of criterial parameterization of RSM in glassy chalcogenide network systems is worked on considering the transient (in-situ) and post-irradiation (ex-situ) relaxation inputs. Metastable radiation-optical response is described as intrinsic destruction-polymerization process in chemically-ordered or random covalent network depending on the radiation-induced misbalance in the distributions of covalent bonds, available for relaxation accompanying free volume and excess of the uncompensated electrical charge on the abnormally-coordinated atoms in ChSG. Proposed numerical criterion describes adequately the compositional trends in reversible induced changes of the optical parameters of As-S(Se) ChSG in the full networking range. Efficiency of the structural modification of glassy arsenic sulfides is found to be much higher in respect to arsenic selenides. This parameter is maximal for stiochiometric compositions in the case of chemically-ordered covalent network, but maximal efficiency of RSM shifts to the sulfur-enriched As0.30S0.70 composition with the increase in randomness of covalent bonds distribution. It is proposed the configurational-enthalpic diagram for the describing of the induced effects in ChSG. In the frameworks of this diagram, it is foreseen the occurrence of ChSG in the different metastable structural states (corresponding to the as-prepared, aged and γ-induced glasses) and short-lived excited state. Transitions between the different states agree with the changes on the corresponding ChSG enthalpic diagram. On the example of As-S glasses, configurational-enthalpic diagram is shown to sufficiently describe effects of natural, thermal and γ-induced PhA, intrinsic processes of γ-induced defects formation and thermal renovation of ChSG.