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.

Project Report

" (07/01/09-12/31/11; $240,000). The phase change chalcogenides primarily belong to the Germanium (Ge)- Antimony (Sb)- Tellurium (Te) system. Large differences in optical reflectivity or electrical resistivity between the crystalline and the amorphous phases of these materials provide a means to store binary information. These materials have recently received remarkable attention due to their extraordinary technological importance in rewritable optical data storage [e.g. CD, DVD, Blu-ray] and non-volatile memory applications. The primary focus of this NSF grant has been to develop direct atomic-scale understanding of the structure-dynamics-property relationships associated with the phase change phenomena in tellurium-based chalcogenide glasses. The major scientific findings obtained under the support of this grant and the broader impacts in terms of education and outreach are summarized below. 1. Bulk phase change materials have been invented in the system Ge-As-Te. This opens up the possibility of fabricating large scale devices for optical memory applications in future and we are no longer limited to thin films of Ge-Sb-Te materials for such purposes. 2. Diffraction and spectroscopic studies have indicated that the chemical order in Te-based chalcogenide glasses are structurally fundamentally different from their sulfur and selenium –based analogs. This may hold the key in understanding the uniqueness of tellurides for phase change applications. 3. 125Te NMR spectroscopy has been established as an experimental technique for studying the structural environment of Te atoms in amorphous tellurides. This breakthrough has allowed us to develop a tellurium-centric structural model of the phase change process in Ge-Sb-Te alloys at the atomic scale. The results thus obtained can explain the observed ultrafast kinetics of phase change in these materials. 4.The thermodynamic nature of phase stabilities and transformations have been investigated in crystalline and amorphous Ge1Sb2Te4 as a function of pressure and temperature using synchrotron x-ray diffraction in a diamond anvil cell. The phase equilibria relations are consistent with the existence of multiple amorphous phases of these materials that are thermodynamically distinct and can be accessed via different processing routes that include thin film deposition, melt quenching and application of high pressure. These findings have far-reaching implications in optimizing materials and processing for the next generation optical storage media. Scientifically this work impacts materials science, physical chemistry and solid state physics. The materials studied have actual applications in diverse areas including optical memory storage, telecommunication, photovoltaics and environmental remote sensing. This project has supported four Ph.D. students, two of them are women (Sezen Soyer and Erica Gjersing) who obtained their PhD in 2009 and 2010 and are currently employed as scientists at Conoco-Phillips and at the National Renewable Energy Lab. Graduate students Trent Edwards and Alvin Mao are in their third and second years, respectively, and have been supported by this grant until the end of 2011. Undergraduate students Zheng Xue and Joel Lammatao have worked in the group in the Summer of 2010 and 2011 to gain research experience. The PI has lectured at the Winter School on the application of synchrotron techniques in glass research that was held at Zhejiang University in China . These extensive set of lecture notes have been made available to the glass research community at the website: www.lehigh.edu/imi/WinterSchool/10china2R.html. The PI has also been personally active in local outreach efforts at K-12 and community college levels. Some of his recent outreach efforts include: lecturing and experimental demonstration on materials science at the Einstein Education Center (www.ycoe.org/eec), an alternative high school for youth who have left school or are not succeeding in traditional high schools (see photograph to the left) and coorganizing a visit of senior undergraduate students from the Society for the Advancement of Chicanos and Native Americans in Science (SACNAS; https://sacnas.org/) to the college of engineering at UC Davis. This project has fostered a number of fruitful collaborations (see Table 1 below). They provide important opportunities for our group to interact with other universities, national labs and industry. Table 1: Collaborations Person Institution Collaboration Dr. Chris Benmore Argonne National Lab High-energy Diffraction RMC simulation Dr. Bruce Aitken Corning Incorporated Molecular chalcogenides Prof. Hideki Maekawa Tohuku University, Japan Ultra high field NMR Dr. Zhehong Gan National High Magnetic Field Lab Ultra high field NMR Prof. George Papatheodorou ICEHT, Greece Aerodynamic levitation melting The PI co-organized a symposium on Glass structure-property relationships at the American Ceramic Society Glass & Optical Materials Division meeting at Corning, NY in 2010. During this grant period, PI presented 5 invited talks and graduate students Soyer, Gjersing and Edwards presented contributed talks at various national and international conferences and workshops. PI also co-organized the 7th international conference on borate glasses, crystals and melts that was held in Halifax, Nova Scotia in Canada in August 2011.

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Division of Materials Research (DMR)
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Lynnette D. Madsen
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University of California Davis
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