Fragility crossover in chalcogenide glass-forming liquids: Importance for phase-change memory application
Jiri Orava* & A. Lindsay Greer
Chalcogenide phase-change materials have long been of interest for optical data storage, in which writing is by local melting with a laser pulse to create a glassy mark. Erasure of the mark, heated by a less intense pulse, occurs by nucleation and growth of new crystals within it (for nucleation-dominated
materials, e.g. Ge-Sb-Te alloys) or by crystal growth inwards from the mark perimeter (for growth-dominated
materials, e.g. Ag,In-doped Sb2Te alloys). Transformation mode affects performance: in the growth-dominated case, erasure time depends on mark diameter only, and not on incubation effects. Current interest in phase-change materials focuses on random-access memory, in which switching is by Joule heating, with recording time limited by crystal growth rate. Device scaling and smaller memory cells give extreme transformation conditions: after melting, the cell cools into the glassy state at ~1010 K s–1, and crystallization takes < 10 ns. Operation of such devices is technologically challenging and useful, while opening up interesting fundamental questions.
Phase-change materials are poor glass-formers and conventional kinetic measurements (near the glass-transition temperature T
g) are far from the regime of fast crystallization relevant for device performance. Recently, it has been found experimentally and confirmed by molecular-dynamic simulations that the nucleation-dominated Ge-Sb-Te glasses can crystallize, around T
g, up to 105 times faster than would be predicted from temperature dependence of viscosity, h
. Such decoupling, between crystal growth and h
, means that the memory has poor data retention as the glassy mark can crystallize spontaneously at slightly elevated temperatures.
Very recent studies (on the growth-dominated material Ag-In-Sb-Te) give clear evidence for a fragile-to-strong crossover
, similar to what has been claimed in several metallic glass-forming melts. This crossover behaviour may be important in understanding and optimizing memory performance.