Crystallization kinetics of phase change materials on a ns-timescale at elevated temperatures

摘要

Phase-change materials have gained considerable interest in recent years and decades, respectively. This is not only due to their application in optical data storage, but also in the field of electrical random access memories in personal computers. They are one of the most promising candidates for future memory technology applications and are considered to replace FLASH-Memory or even DRAM. First prototypes have already been developed and some products in niche markets are already working on the basis of phase-change materials. One of the key features is their non-volatility which is enabled by a permanent structural (re)- arrangement. At this time, DRAM requires an electrical refresh of the stored information in certain periods. Thus, if power is turned off, the information is lost and the personal computer needs to boot at restart. Hence, if the random access memory was based on phase-change materials, booting of personal computers would not be necessary any longer. However, their non-volatility is not the only reason for the growing interest in these materials. Their rapid switching within a few nanoseconds is another attractive phenomena. It occurs between an amorphous and crystalline state which is high resistive and low reflective or low resistive and high reflective, respectively. Though, the fundamental mechanism of crystallization in these materials is still not fully understood. At temperatures close to room temperature, a phase change cannot be observed. Hence, phase-change materials are long-term stable. Anyhow, at elevated temperatures, a rapid switching can occur on a nanosecond time-scale. Since, within the temperature regime between the glass transition temperature and the melting point, phase-change materials possess an extremely rapid switching speed, it is challenging to access the regime of fast crystallization. Hence, until now, no data in the regime of fast crystallization were published. Usually, the lack of data is closed with an extrapolation from low to high temperatures, but the relevance of these extrapolations is questionable. Therefore, a comprehensive investigation of the temperature dependence of crystal nucleation and growth is highly desirable. This work has the overall aim to contribute to the research of crystallization kinetics in such a way that extrapolations are more profound or even unnecessary. Anyway, it will start with giving a brief introduction to phase-change materials and relevant aspects of this special class of materials will be presented. Subsequently, a theoretical background of crystallization kinetics will give insights to theoretical models which are the basis of the mentioned extrapolations. In the past decades, there have been many attempts in unraveling the fast crystallization kinetics. A selected review of concepts and tools, developed to investigate crystallization kinetics will be given. However, it will be shown that none of the presented approaches is capable of accessing fast crystallization kinetics completely for different reasons. Thus, there is a need for a different setup and concept. Hence, the Phase-change Optical Tester (POT) and the concept using it will be explained as a tool being capable to access fast crystallization kinetics. It has to be noted that the focus of research in the field of crystallization kinetics within this work is crystal growth. Therefore, this concept in combination with POT is applied to the phase-change material AgInSbTe and will be compared to theoretical models. Unfortunately, POT alone is not able to access the structural rearrangement of phase-change materials which occur during the rapid switching process. Thus, a recheck of the performed experiments is achieved with Transmission Electron Microscopy (TEM), since TEM is a powerful tool to perform structural investigation on local scale. In the summary of this thesis, the results will be condensed and future experiments are elaborated which benefit from the POT setup, the experimental concept and TEM.