ATOMIC AND ELECTRONIC STRUCTURES OF GLASSY GeXSe1-X AROUND THE STIFFNESS THRESHOLD COMPOSITION

Anomalous x-ray scattering experiments on glassy GexSe1-x have been carried out at energies close to the Ge and Se K edges at concentrations close to the onset and completion of the rigidity percolation threshold (x = 0.195 and 0.23). The total structure factors S(Q) show rapid changes in both the position and intensity of the prepeak around 10 nm-1, while remaining almost unchanged in the other Q ranges. The differential structure factors ∆iS(Q) obtained have characteristic features of their own. A detailed comparison among them suggests that the prepeak originates from only the Ge-Ge correlation. On the basis of the concentration dependence of the spectra and the existing partial structure factors of glassy GeSe2 obtained by Petri et al., the origin of the prepeak is discussed. Valence- and conduction-band electronic density of states of glassy GexSe1-x (0 x 0.33) were also investigated by measuring the ultraviolet photoemission and inverse-photoemission spectra. They exhibit a remarkable change in their spectral features near x = 0.20. These observations in both the atomic and electronic structures are consistent with the occurrence of a percolation threshold in non-crystalline covalent network systems as predicted by Phillips and Thorpe. The threshold is characterized by the percolation of a specific Ge(Se1/2)4 molecular unit spread over the network