Stochastic random network model in Ge and Si chalcogenide glasses

A stochastic random network model is proposed for the structure of Geₓ{S}_{1{{-}}x}$, ${Ge}ₓSe_1-x, Siₓ{S}_{1{{-}}x}$, and ${Si}ₓSe_1-x (x{≤}0.33) glasses. This model is constructed to explain the existence of two types of microcrystalline states induced by photoirradiation above or below the threshold intensity. This model characterizes the glass structure by one parameter P which is related to the existing probability of the edge-sharing bonds between the tetrahedral MX₄ molecules relative to the corner-sharing bonds. P depends only on the species of atoms forming the glass and not on x. In order to prove the validity of the present model, Raman scattering experiments were made and the x dependence of the intensity ratio of the A₁ᶜ companion peak to the A₁ peak, I(A₁ᶜ)/I(A₁), was obtained. From the viewpoint of phonon localization, the A₁ mode is assigned to the breathing mode of MX₄ molecules and the A₁ᶜ mode to the vibration of chalcogen atoms on the edge-sharing double bonds. The x dependence of the intensity ratio I(A₁ᶜ)/I(A₁) calculated by the present model is in good agreement with the experimentally obtained ratio. The P obtained increases in order from Geₓ{S}_{1{{-}}x}$, ${Ge}ₓSe_1-x, Siₓ{Se}_{1{{-}}x}$ to ${Si}ₓS_1-x with the same order of tendency of getting edge-sharing bonds in the crystals. The value of P is independent of the method of making the amorphous but it can be changed by photoirradiation. P decreases with irradiation below the threshold intensity, but it increases with irradiation above the threshold. The local energy in the glass is lower in the corner-sharing bonds, but the total energy is lowest in the same structure as the crystal. The threshold irradiation intensity for Se glass is less than one-hundredth of that for GeSe₂ glass.