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

• Published in: Physica status solidi. PSS-RRL. Rapid research letters Vol. 14; no. 5
• Main Authors: Raty, Jean-Yves, Noé, Pierre
• Format: Journal Article Web Resource
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01-05-2020; Wiley
• Subjects: ab initio; ab initio simulations; Antimony; chalcogenide; Conduction bands; Electron states; electronic properties; Electrons; Glass; glasses; Memory devices; Molecular dynamics; ovonic threshold switching; Percolation; Physical, chemical, mathematical & earth Sciences; Physics; Physique; Physique, chimie, mathématiques & sciences de la terre; selectors; Switching; Tetrahedra
• ISSN: 1862-6254; 1862-6270; 1862-6270
• DOI: 10.1002/pssr.201900581

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.
The effect of electronic excitation on two glasses exhibiting ovonic threshold switching (OTS) is studied by ab initio molecular dynamics. The glasses are shown to undergo modifications in bond angle distributions upon excitation. The modified bonds are better aligned and exhibit anomalous Born effective charge values. These are at the origin of the conductivity increase during OTS.