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Crystallization kinetics of phase change materials for novel data storage concepts

By Michael Klein


After a general definition of phase change materials and a description of their defining properties, such as the possibility to switch between two room temperature stable phases at high speed and a contrast in optical and electrical properties, the history of phase change research is shortly outlined. In this context also already realized and possible technological applications are presented. These have been a major driving force of phase change research. Until now, phase change materials have been employed in rewritable optical media, but the realization of commercial electronical memories is within reach.For both sorts of applications, the crystallization kinetics is of fundamental importance. In any kind of application the contrast of the material's amorphous and crystalline phase is exploited. The device speed is governed by the speed of crystallization, as this is the slowest of the switching processes. In many experimental techniques, the nucleation and growth part of the crystallization process cannot be separated. This information is, however, crucial for many applications, as due to the presence of crystalline surroundings, the nucleation part of crystallization may only play an inferior role. Crystallization can be described with the classical thermodynamical concept for phase transitions introduced by Gibbs. Two models to describe the crystallization kinetics are presented in this study, the Johnson-Mehl-Avrami-Kolomogorov model and the Kissinger model. The results of both models are then connected, in order to be able to compare the experimental results obtained by both of them later on. Starting with calorimetry measurements and temperature dependent resistivity measurements, the crystallization process of a large variety of materials is reviewed. With a combination of the precise furnace of the calorimeter and imaging techniques like atomic force microscopy and optical microscopy, single crystals are experimentally observed. Thus, nucleation and crystal growth are unraveled. Within this study the data base on crystallization kinetics of germanium-antimony-tellurium alloys is vastly enhanced. Thus, it is possible to derive stoichiometrical trends for the crystallization temperature, the activation barriers for growth and steady state nucleation, as well as the Kissinger activation barrier. This can be used to tailor phase change materials according to the needs of applications. Finally, this work moves its focus to the effect of the generally oxidized surface of the measured samples. It will be shown that for some materials the surface oxidation leads to an earlier crystallization of a layer close to the material's surface. Afterwards, the influence of capping layers preventing the surface oxidation will be shown. The effect of a step-like transition in temperature dependent resistivity measurements is explained with a simple model based on a parallel circuit. The measurements on the crystallization kinetics under a capping layer reveal that there is a strong effect of the capping layer in regards of the crystal growth velocity and the nucleation rate. Hence, many new research opportunities open up

Topics: info:eu-repo/classification/ddc/530, Kinetik, Kristallisation, Kraftmikroskopie, Amorpher Festkörper, Festkörperphysik, Kalorimeter, Phase-Change-Technologie, Physik, crystallization, kinetics, phase change
Publisher: Publikationsserver der RWTH Aachen University
Year: 2009
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Provided by: RWTH Publications
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