Phase change materials for non-volatile electronic memories

Starting from a brief introduction into the physics of phase change materials their applications in the field of data storage are reviewed. Without the latter there would certainly be less attention paid to the topic today, even in basic research. For years now, optical data storage media based on phase change materials have been successfully produced for the mass market. Despite its maturity there are still new ideas on how to further improve this technology in order to stay competible with other storage media in the future. In contrast, for the application of phase change materials in electronic memories the situation is completely different. Although proposed decades ago, today it is still a rather young and promising technology. In this work both applications are analyzed with respect to their requirements to the incorporated phase change material, with an emphasis on the second one. For optical storage and especially in the case of electronic memories the author arrives at the conclusion, that the crystallization kinetics of phase change materials is the most urgent and fundamental problem that needs to be solved. The emphasis on a deep and quantitative understanding of this phenomenon implicitely criticizes the popular approach of studying the switching of electronic cells for phase change based memories without an in-depth research of the crystallization kinetics. After this identification of the research objective, the result of a thorough review of the literature on the theory of crystallization is presented. The understanding of glass formation turns out to be extremely important, since it deals with the stability of an amorphous solid or undercooled liquid against structural reconfiguration. Consequently, a separate chapter is dedicated to the theory of glass formation. Based on the knowledge of the theoretical connections between glass transition and crystallization it is investigated how meaningful a calculation of the enthalpy of atomization is for a prediction of stoichiometric trends not only of glass transition, but also of crystallization. The sparce experimental evidence on the glass transition temperature of phase change materials is compared to the calculated enthalpies. The same is done for a series of measurements of crystallization. These comparisons show that the proposed strategy works well for predicting the influence of a stoichiometric variation of a phase change material on its stability against crystallization. The author's critical review of the literature on crystallization kinetics reveals that the widely used classical crystallization theory still lacks a rigorous experimental prove of its validity. The latter is difficult, because the multitude of quasi-free parameters generally ensures a good mathematical agreement between theory and the often very limited experimental data. Such a check of validity of the theory is especially challenging for phase change materials: In a wide range of high temperatures crystallization proceeds so fast, that until now an experimental quantification of nucleation rate and crystal growth velocity was only possible in a rather limited regime of low temperatures. The extrapolation of such data over the whole temperature range up to the melting temperature by application of the equations provided by the classical theory is assessed to be too uncertain to be trusted. To close that gap with experimental evidence and to advance therewith towards a disentanglement of electronic and thermal effects involved in the switching of an electrical cell, a new experimental setup has been designed and realized. It combines laser induced annealing experiments with the capability to apply and measure fast electrical pulses. The implementation of a control of the sample's base temperature is an additional, valuable component. Each of the sections of the new setup on its own is already a sophisticated tool, that enables its user to investigate phase change materials on very short time scales. Examples for this are unprecedented laser experiments that have been performed by the author. Some of those innovatively separate crystal nucleation and growth. Others demonstrate a path towards a quantitative measurement of crystallization kinetics in the melt-quenched amorphous state. The latter is highly relevant for technology, since it is that amorphous phase, that is realized in applications. But the new setup is more than just the sum of its parts. The combination of optical, electrical and thermal experiments opens up a wide field of new possibilities. This is indicated by the demonstration of an electrical experiment for the investigation of the threshold-switching effect, the phenomenon describing an abrupt breakdown of the resistivity of the amorphous phase upon application of a critical electric field. The initialization of the sample by laser annealing allows for a clean'' experiment in which the starting conditions are not dependent on the property of interest itself, in this case the electrical behaviour. Such a successive approach from purely thermal experiments towards the testing of the realistic but complex cells of a phase change based electronic memory is necessary to stepwise synchronize the numerical simulations. This strategy is essential for achieving a deep understanding of the physical processes involved in the switching of such cells. Beyond these technologically important measurements the new setup is pioneering for a multitude of further optical and electrical experiments that are likely to make valuable contributions to the research of the physical properties of phase change materials. Examples for such experiments are proposed at the end of this work.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2008

Author dc:creator

• Salinga, Martin

Contributors dc:contributor

• Wuttig, Matthias

• info:eu-repo/classification/ddc/530
• Datenspeicherung
• Festkörper
• Festkörperphysik
• Amorpher Festkörper
• Amorpher Zustand
• Kristallisation
• Kristallin
• Kinetik
• Rekristallisationskinetik
• Tellurlegierung
• Antimon
• Antimonlegierung
• Germanium
• Germaniumlegierung
• Amorphisierung
• Physik
• Phasenwechsel
• Phasenwechselmaterialien
• elektronisch
• PCRAM
• PRAM
• Phase change
• non-volatile
• memory
• crystallization
• kinetics

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-112663%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A50106