Memory switching in a type I amorphous chalcogenide

Abstract

Amorphous chalcogenides may be divided among three classes based upon the changes in local order and bond type on transition from the amorphous to the crystalline phases. These types include: (I) Stoichiometric compounds which do not undergo significant changes in local order on crystallization (e.g., As₂Se₃). (II) Stoichiometric compounds which undergo significant changes in local order on crystallization (e.g., GeTe). (III) Non-stoichiometric alloys which phase separate on crystallization.

Previous work on memory switching has been performed primarily on type II and type III chalcogenides. The memory switching process in thin film specimens of a type I chalcogenide, amorphous As₂Se₃, is examined in this paper. Measurements of the threshold voltage distribution for devices of a given thickness display a sharp peak at some value V_O . A plot of V_O as a function of film thickness shows the initiation of switching to be field controlled, suggesting a non-thermal initiation mechanism. Microscopic examination of switched devices yields evidence for Joule heating, which is enchanced by self-capacitive energy released during switching subsequent to formation of the low resistance path.

From the thickness dependence of V_O and measurements of the failure time of devices biased below threshold, a switching mechanism for this material is deduced which incorporates spinodal decomposition of metallic As clusters in an insulating matrix. These clusters then chain under the influence of an applied field and form a low resistance filamentary path.