Understanding Nucleation in Glassy Phase Change Materials

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

Technical: The proposed work will experimentally test many theoretical predictions in nucleation science, and provide data on the evolution of nanoscale nuclei in glassy phase change materials under various conditions. The nucleation process is ubiquitous in nature, and the knowledge obtained from this work will be applicable to essentially any material systems involving phase transformation. One example is phase change memory technology, since nucleation and crystallization are the key processes that govern fast data writing and long-term retention. A pump-probe laser technique, atomic force microscopy, and membrane electrical thermometry will be combined for understanding nucleation in glassy phase change materials such as AgInSbTe, Ge2Sb2Te5, and GeSb. The experimental results will be correlated with theoretical model calculations. First, some predictions of the general nucleation theory will be tested experimentally. The temperature-dependence of steady-state subcritical nuclei distribution and the dependence of nucleation rate on pre-existing subcritical nuclei will be explored. Second, the effects of interfaces, compositional variation, and thermal history on the evolution of nuclei in phase change materials will be investigated. Since the distribution of nuclei is one of the most important factors that influence the speed and stability of a phase change memory, FTEM will be used to determine the concentration of nuclei in actual memory devices. Third, as a relatively exploratory work, electrical thermometry will be employed to characterize the glass transition and nucleation at very high heating rates. Important thermal parameters are dependent on temperature and heating rate, but conventional techniques can only provide heating rates many orders of magnitude lower than the rates involved in actual memory operations. The data from this study will provide crucial information for technology, and serve as fundamentally illustrative examples of transient nucleation. The principal task of the proposed work is the investigation of the fundamental science of nucleation. Non-technical: The project addresses basic research issues in a topical area of materials science with technological relevance, and is expected to provide unique opportunities for graduate and undergraduate training in an interdisciplinary field. This research project is also expected to have broader impacts through the training of women and men leaders in this research field, through the wide dissemination of the findings of this research through publications. One or two undergraduates will carry out portions of this work as their senior theses. Notably, the experimental results can be used in textbooks and lectures as excellent examples to illustrate the nucleation theory, which is an important subject of materials science education. Qualified students will be identified and advised, in particular through the SURGE (Support of Under-represented Groups in Engineering) program at the University of Illinois.