Phase Change Materials - From Structures to Kinetics
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0918-H04-03
Phase Change Materials - From Structures to Kinetics Matthias Wuttig, Wojciech Welnic, Ralf Detemple, Henning Dieker, Johannes Kalb, Daniel Wamwangi, and Christoph Steimer I. Physikalisches Institut IA, Aachen, D52056, Germany ABSTRACT Phase change materials possess a unique combination of properties which include a pronounced property contrast between the amorphous and crystalline state, i.e. a high electrical and optical contrast. In particular the latter observation is indicative for a considerable structural difference between the amorphous and crystalline state. At the same time the crystallization of the amorphous state proceeds on a fast time scale. This raises the question how structure, properties and kinetics are related in phase change alloys. It will be demonstrated that only a small group of covalent semiconductors with octahedral-like coordination has the required property combination. This is related to their thermodynamic properties which govern the kinetics of crystallization.
INTRODUCTION Phase change materials are very promising materials for information technology. They are already employed in rewriteable optical data storage, where the pronounced difference of optical properties between the amorphous and crystalline state is used for data storage. This unconventional class of materials is also the basis of storage concepts to replace flash memory [1-3, 4, 5]
Structure Considerable structural difference between amorphous and crystalline state
Fast crystallization of amorphous state
Pronounced optical contrast between amorphous and crystalline state
Figure 1. Phase change materials show a unique combination of properties. On the one hand side they display a pronounced contrast of optical and electrical properties between the amorphous and crystalline state as well as threshold switching. The former observation is indicative for a pronounced structural difference between the amorphous and crystalline state. At the same time the crystallization of the amorphous state proceeds on a very short time scale. To exploit the full potential of phase change materials it is important to unravel the origin of this unique property combination and to identify the materials which possess these properties.
This raises the question which material properties are crucial for these storage applications, and which compounds posses the required properties. Successful phase change alloys are characterized by a unique property combination which is depicted in fig. 1. On the one hand side they possess a pronounced contrast of optical properties (reflectivity, transmission) between the amorphous and crystalline state. The corresponding dielectric functions, i.e. ε1 and ε2 are displayed in fig. 2 for Ge2Sb2Te5, a prototype phase change alloy. The pronounced optical contrast displayed is a unique and rather rare property. Conventional semiconductors such as GaAs or Si are characterized by a much smaller difference in optical properties. Hence we need to understand what causes the property contrast in phase
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