Simulation of Crystallization Processes in Phase Change Optical Discs
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Simulation of Crystallization Processes in Phase Change Optical Discs Bérangère Hyot, Ludovic Poupinet and Pierre Desré1 Léti-CEA/Grenoble, 17 rue des martyrs, Grenoble cedex 9 F-38054, France. 1 Laboratoire de Thermodynamique et Physico-Chimie Métallurgiques, Institut National Polytechnique de Grenoble, Genoble 38402, France. ABSTRACT Future development of erasable phase change recording requires a thorough understanding of the fundamental mechanisms involved in crystallization processes to be able to forecast the behavior of a new material or to guide its optimization when submitted to specific conditions. In this paper, we emphasize that a predictive simulation is valuable only if a good modeling and accurate values of the involved parameters are used. We will present the main characteristics of our phase change model (effect of the short-pulse laser excitation, chemical influence of the materials in contact with the PC layer, stabilization of the amorphous state for very thin film layers) and we will discuss experimental and simulated crystallization behaviors. INTRODUCTION One year ago we built a fast dynamical simulation of the writing and erasing processes in phase change materials. The software includes both optical and thermal models which give access to the spatial and temporal temperature distribution inside the multilayer structure even with a moving laser beam and a specific writing strategy. The key feature of the model is its ability to simulate microstructure during crystallization. Our program allows to simulate the behavior of the two main classes of Phase Change (PC) materials, namely nucleation-dominated materials like GeSbTe (GST) and growth-dominated materials like AgInSbTe. However computer simulation is a powerful tool under two conditions: first of all the use a physicallyrealistic phase change model and then the input within the simulation of the right values of the physical parameters. The first part of this paper presents the more interesting points of our physico-chemical model of crystallization. We will see that it takes into account the effects of the applied temperature variations due to the short-pulse laser excitation, the chemical nature of the material in contact with the PC layer and we will give thermodynamical arguments to explain the observed difficulties in the crystallization process of very thin films. Finally, in the last part we will see that a predictive simulation is based on rigorous and varied experimental works to extract right values of the parameters. We consider in this paper only the case of nucleation-dominated materials, which are more difficult to understand and simulate.
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PHASE TRANSFORMATION MODEL
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Nucleation process
First of all, the temperature variations generated by short-pulse laser heating are so steep, of the order of 10K/ns, that we have to take into consideration the validity of classical nucleation theories. It should be reminded that the steady state treatment is suitable only if the statistical equilibrium which corre
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