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Phase change materials show a very remarkable combination of properties. On the one hand side they possess a pronounced difference of their optical properties between the amorphous and the crystalline state. This pronounced optical contrast can only be explained if the atomic structure of the amorphous and crystalline state shows significant differences. Recent work has shed light into the structural differences between the amorphous and crystalline state for GeSbTe based phase change materials and the reasons for the existence of the two different atomic arrangements. At the same time the structural transition between the amorphous and crystalline state is very rapid even though severe atomic rearrangements are necessary. To fully exploit the potential of phase change materials it is hence crucial to understand the origin of the fast transformation kinetics observed e.g. in laser induced crystallization.
To understand the kinetics of fast crystallization processes we have combined experiments on different time and length scales that enable surprising insight in the transformation kinetics. Ex-situ atomic force microscopy in combination with a high-precision furnace has been employed for a systematic study of crystallization kinetics of sputtered amorphous Ag0.055In0.065Sb0.59Te0.29, Ge4Sb1Te5 and Ge2Sb2Te5 thin films used for optical data storage. Direct observation of crystals enabled us to establish the temperature dependence of the crystal nucleation rate and crystal growth velocity around 150 C. While these alloys exhibited similar crystal growth characteristics, the crystal nucleation behaviour of Ag0.055In0.065Sb0.59Te0.29 differed significantly from that of Ge4Sb1Te5 and Ge2Sb2Te5. These observations provide an explanation for the different re-crystallization mechanisms observed upon laser-heating of amorphous marks. The combination of experiments described enables an in-depth understanding of fast crystallization phenomena in phase change alloys and should facilitate both the proper simulation of transformation kinetics as well as the design of new compounds.
Coworkers in this project: Michael Klein1, Johannes Kalb1, Dominic Lencer1 and Frans Spaepen2 1. I. Physikalisches Institut der RWTH Aachen, 52056 Aachen, Germany wuttig@physik.rwth-aachen.de 2. Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA |
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