Phase change materials and phase change memory
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Introduction
Properties of phase change materials
Novel information storage concepts have been continuously developed throughout history, from cave paintings to printing, from phonographs to magnetic tape, dynamic random access memory (DRAM), compact disks (CDs), and flash memory, just to name a few. Over the last four decades, silicon technology has enabled data storage through charge retention on metal-oxide-silicon (MOS) capacitive structures. However, as silicon devices are scaled toward (sub-) 10 nm dimensions, minute capacitors become leaky by simple quantum mechanical considerations, and the memory storage density appears to plateau. Novel information storage concepts are under development that include storing data in the direction of the magnetic orientation (magnetic RAM,1 spin torque transfer RAM,2 racetrack RAM3), in the electric polarization of a ferroelectric material (ferroelectric RAM4), in the resistance of a memory device (resistive RAM,5 memristor,6 conducting bridge RAM,7 carbon nanotube memory8,9), or in the resistance of the storage media itself (phase change RAM10). Phase change materials store information in their amorphous and crystalline phases, which can be reversibly switched by the application of an external voltage. In this article, we describe the properties of phase change materials and their application to phase change memory (PCM).
Phase change materials exist in an amorphous and one or sometimes several crystalline phases, and they can be rapidly and repeatedly switched between these phases. The switching is typically induced by heating through optical pulses or electrical (Joule) heating. The optical and electronic properties can vary significantly between the amorphous and crystalline phases, and this combination of optical and electrical contrast and repeated switching allows data storage. This effect was initially uncovered in 1968,11 but it took the breakthrough discovery12 of fast (i.e., nanosecond time scale) switching materials along the pseudo-binary line between GeTe and Sb2Te3, notably the most studied and utilized Ge2Sb2Te5 (GST), to enable phase change storage technology.13 Many technologically useful phase change materials are chalcogenides, which owe their success in this regard to a unique combination of properties, which include strong optical and electrical contrast, fast crystallization, and high crystallization temperature (typically several hundred degrees Celsius). Figure 1 shows the ternary phase diagram of the Ge-Sb-Te system. As mentioned previously, alloys along the pseudo-binary line between Sb2Te3 and GeTe with compositions (GeTe)m(Sb2Te3)n have been intensely studied14 and are used in state-of-the-art PCM devices.15 In search of
Simone Raoux, Institute Nanospectroscopy for Energy Material Design and Optimization, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany; [email protected] Feng Xiong, Electrical Engineering, Stanford University, USA; [email protected] Matthias Wuttig, Physikalisches Institut and Jülich Aachen
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