Phase change materials: From structures to kinetics
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Review
Phase change materials: From structures to kinetics Wojciech Wełnic,a) Johannes A. Kalb,b) Daniel Wamwangi, Christoph Steimer, and Matthias Wuttig I. Physikalisches Institut (IA), Rheinisch-Westfaˆlisch Technische Hochschule (RWTH) Aachen University of Technology, 52056 Aachen, Germany (Received 19 January 2007; accepted 2 May 2007)
Phase change materials possess a unique combination of properties, which includes a pronounced property contrast between the amorphous and crystalline state, i.e., high electrical and optical contrast. In particular, the latter observation is indicative of a considerable structural difference between the amorphous and crystalline state, which furthermore is characterized by a very high vacancy concentration unknown from common semiconductors. Through the use of ab initio calculations, this work shows how the electric and optical contrast is correlated with structural differences between the crystalline and the amorphous state and how the vacancy concentration controls the optical properties. Furthermore, crystal nucleation rates and crystal growth velocities of various phase change materials have been determined by atomic force microscopy and differential thermal analysis. In particular, the observation of different recrystallization mechanisms upon laser heating of amorphous marks is explained by the relative difference of just three basic parameters among these alloys, namely, the melt-crystalline interfacial energy, the entropy of fusion, and the glass transition temperature.
I. INTRODUCTION
Phase change materials are very promising materials for information technology. They are already used 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 for replacement flash memories.1-6 This raises the question of which material properties are crucial for these storage applications and which compounds posses the required properties. Successful phase change alloys a)
Address all correspondence to this author. Present address: Laboratoire des Solides Irradiés, CNRS-CEA, École Polytechnique, Palaiseau, France, European Theoretical Spectroscopy Facility (ETSF). e-mail: [email protected] b) Present address: Massachusetts Institute of Technology, Department of Materials Science and Engineering, Cambridge, MA 02139. This paper was selected as the Outstanding Meeting Paper for the 2006 MRS Spring Meeting Symposium M Procecedings, Vol. 918. DOI: 10.1557/JMR.2007.0301 2368 J. Mater. Res., Vol. 22, No. 9, Sep 2007 http://journals.cambridge.org Downloaded: 25 Mar 2015
are characterized by a unique property combination. On the one hand, they possess a pronounced contrast of optical properties (reflectivity, transmission) between the amorphous and crystalline state. Figure 1 shows a comparison of (b) the imaginary part of the dielectric function for GeTe, a prototype phase change alloy, in comparison
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