Why are chalcogenide glasses the materials of choice for Ovonic switching devices?

Non-destructive switching from a high resistance OFF state to a highly conducting ON state occurs at a critical field between 0.5 and 0.7 MV/cm in thin films of chalcogenide glasses placed between two non-reacting contacts as discovered by S.R. Ovshinsky in the 1960s. The switching device returns to its original high resistance state when the ON state current falls below a holding current value. We present a theory of this electronic switching and of the conducting ON state, which allows many billion reproducible switching cycles. In particular the question is addressed why the unique properties and the defect chemistry of chalcogenicle glasses lead to this non-destructive and reproducible switching effect. It is proposed that the switching at the critical field is initiated by a modified Zener tunneling breakdown. In the ON state most of the holding voltage drop occurs near the electrodes and the current is confined to a current filament. The high current density of the ON state is supplied by Fowler-Nordheim field emission from the contacts. This is made possible by the narrowness of the potential barriers at the contacts which in turn is a consequence of the defect chemistry of chalcogenicle glasses. Electronic switching is also observed in Ovonic memory devices having a preferred composition of Ge2Sb2Te5. The large atomic sizes of Sb and Te result in a partial 3-fold covalent coordination of Te which might affect the defect chemistry of this and related memory glasses. (C) 2007 Elsevier Ltd. All rights reserved.