Ovonic Threshold Switching in Se-Rich GexSe1−x Glasses from an Atomistic Point of View: The Crucial Role of the Metavalent Bonding Mechanism

The ovonic threshold switching (OTS) phenomenon, a unique discontinuity of conductivity upon electric-field application, has been observed in many chalcogenide glasses, some of which are presently used as selector elements in latest ultimate phase-change memory devices. Herein, ab initio molecular dynamics is used to simulate the structure of two prototypical glasses that are shown to exhibit significantly different OTS properties and switching performance in OTS devices. The first glass, Ge30Se70 (GS), has a typical structure of connected Ge tetrahedra, whereas in the second GS-based glass that contains antimony and nitrogen, the structure around Ge atoms is quite more complex. By the simulation of the excitation of electrons in the conduction band, slight modifications of the local order are shown to be sufficient to delocalize electronic states. The electron delocalization involving both Ge and Se (as well as Sb atoms in the case of Sb-containing glass) ensures the percolation of conductive paths for electrons, giving, therefore, to the excited material a metallic behavior. These conductive channels result from the local formation of “metavalent” bonds in the amorphous structure as characterized geometrically and with associated Born effective charges.