Atomic-Scale Understanding of Phase-Change Phenomena in Amorphous Chalcogenides

University Of California-Davis, Davis CA

Investigators

Abstract

NON-TECHNICAL DESCRIPTION: We live in the Information Age where storage of large amount of information in a small space is of paramount importance. Glassy materials consisting primarily of Ge, Sb, As and Te (chalcogenides) have recently received significant attention due to their extraordinary technological importance in rewritable optical data storage applications in the forms of compact disk (CD), digital versatile disk (DVD) and Blu-ray disk and in electronic memory applications. These chalcogenides have been aptly termed phase-change materials as they show rapid and repeatable switching between crystalline and glassy phases under suitable conditions that forms the basis of writing, reading and storage of data. However, the details of the structure-property relationships in phase-change chalcogenides are not well understood at the atomic level and they often remain controversial and conjectural, especially in the technologically relevant multi-component systems. This lack of knowledge regarding the connection between the microscopic and the macroscopic results in extensive trial and error tests in composition and processing related optimization in this fast-paced industry. We hope to address the fundamental issues associated with the atomic-scale understanding of the key properties of phase-change chalcogenides based on systematic structural and dynamical studies using state-of-the-art experimental and simulation techniques. Such studies will allow the development of physically more accurate models of structure-property relationships and will better guide future technological development of these materials with improved functionality. Scientifically, the proposed work impacts materials science, solid-state chemistry and solid-state physics. The materials studied have actual or potential applications in a wide range of technologies including optical memory devices, telecommunication, remote-sensing and photovoltaics. The interdisciplinary nature of our research transfers knowledge between fields and provides a unique intellectual environment. This project will continue to foster ongoing collaborations with scientists in industry, universities and Argonne National Laboratory (ANL) and to enrich the graduate education and training experience for participating students through scientific dialogue and interactions between the collaborating scientists and students. Students in this research program will learn to investigate problems in the realm of ?basic science? that underlies industrial applications. This program will coordinate with the underrepresented minority-serving and K-12 outreach programs at UC Davis to attract and recruit underrepresented graduate students and to increase the awareness of students in the science and technology of phase-change chalcogenides. TECHNICAL DETAILS: Chalcogenides that primarily belong to the Ge-Sb/As-Te system constitute an important class of materials known as the ?phase-change? materials that display thermally or electrically induced rapid and reversible transformation between crystalline and amorphous phases under suitable conditions. These chalcogenides have recently received remarkable attention due to their extraordinary technological importance in rewritable optical data storage and non-volatile electronic memory applications. From the point of view of direct atomic-scale understanding of the phase-change phenomena, a number of fundamental questions remain unresolved in these systems: What are the structural similarities and differences between the amorphous and crystalline phases and how do they affect the relevant physical properties such as density, optical absorption and electrical conductivity? What are the possible effects of pressure and temperature on the structure of the amorphous phase, i.e. do these external variables select a particular structure from a ?landscape? of possible structures? What are the nature, timescales and length scales of the atomic/molecular dynamics in the glassy and supercooled liquid state and how are they related to entropy generation, macroscopic relaxation and transport processes and crystallization kinetics? Some of these issues are understood only at the macroscopic level within the framework of phenomenological models. The primary focus of the work proposed here is to address these questions at the microscopic/atomic level using a uniquely powerful combination of neutron/X-ray diffraction, Raman spectroscopy, inelastic neutron scattering, 125Te NMR spectroscopy and Reverse Monte Carlo modeling. Specifically, phase-change chalcogenides in Ge-Sb-Te and Ge-As-Te systems will be investigated. Models linking the atomic-scale structure and dynamics with macroscopic physical and thermodynamic properties will be formulated and tested. Such studies will allow the development of physically more accurate models of structure-property relationships and although they are in the realm of basic science, they should have long-term significance in guiding future technological development of phase-change materials with improved functionality. This work includes significant training of graduate students in state-of-the-art spectroscopic, diffraction and simulation techniques. The equipment and expertise at UC Davis and ANL will provide students with a variety of modern research tools and a supportive structure for learning to use them.