Phase-change materials and rigidity
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Introduction One has to go back to the 1960s to find the first mention of phase-change materials (PCMs). In 1968, S. Ovshinsky proposed that these materials had the potential for the development of information storage devices. These materials were characterized by a series of interesting nonconventional properties. Unlike most semiconductors, the crystalline and amorphous forms of PCMs possess different optical and electrical properties. They also have the ability to switch between the two states rapidly (with nanosecond switching speeds) and in a reversible way when an energetic stimulus is applied. Most PCMs are telluride materials, with the most wellknown compositions being Ge2Sb2Te5, Ge1Sb2Te4, and GeTe.1,2 PCMs used as nanometric layers were extremely successful as the active materials of several devices for optical data storage, such as CD-RW (compact-disc rewriteable), DVD-RW (digital versatile disc-rewriteable) and BD (Blu-ray disc). Intense investigations into these materials are ongoing with the aim to develop new types of electrical memories such as PC-RAM (phasechange-random access memory) with improved performance compared to current flash memory devices. Expectations for improvements concern the speed, energy efficiency, and toughness of the devices. The principle of operation of a PC-RAM memory, similar to that used for optical memory, is shown in Figure 1.3 Improvements in PC-RAM memory performance
rely on understanding the link between structure, dynamics, and more subtle properties such as drift and aging in PCMs. It will be shown how rigidity theory can help in this respect.
PCMs and rigidity The combined ability of an amorphous phase to crystallize rapidly and to yield a strong improvement in electrical properties upon crystallization raises a number of questions. High among these is the relationship between the amorphous and crystalline structures, which have to be distinct enough to ensure the stability of the amorphous phase and have good data retention at operational temperatures in memory applications, and at the same time, be topologically close enough to ensure crystallization on a nanosecond time scale. Rigidity theory first proposed by Phillips4 provides a useful tool to understand these unique properties of PCMs (see the Introductory article in this issue). It classifies amorphous networks into flexible or rigid glasses. In this concept, strong covalent forces are expected to act as mechanical Lagrangian constraints defining the local atomic structure of the disordered solid. Covalent solids demonstrate two different types of bonding constraints. The bond-stretching constraints, nBS, are defined by the bond between two neighboring atoms, whereas the bond-bending constraints, nΒΒ, correspond to the bonding angle between an atom and two of its neighbors.
Andrea Piarristeguy, Institut Charles Gerhardt, Université de Montpellier, France; [email protected] Annie Pradel, Centre National de la Recherche Scientifique, Institut Charles Gerhardt, Université de Montpellier, France; annie
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