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SLAC-R-598 UC-404 (SSRL-M) THE CHEMICALLY-SPECIFIC STRUCTURE OF AN AMORPHOUS MOLYBDENUM GERMANIUM ALLOY BY ANOMALOUS X-RAY SCATTERING∗ Hope Ishii Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Stanford University, Stanford, California 94309 SLAC-Report-598 May 2002 Prepared for the Department of Energy under contract number DE-AC03-76SF00515 Printed in the United States of America. Available from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161 ∗Ph.D. thesis, Stanford University, Stanford, CA 94309. THE CHEMICALLY-SPECIFIC STRUCTURE OF AN AMORPHOUS MOLYBDENUM GERMANIUM ALLOY BY ANOMALOUS X-RAY SCATTERING a dissertation submitted to the department of materials science and engineering and the committee on graduate studies of stanford university in partial fulfillment of the requirements for the degree of doctor of philosophy Hope Ishii May 2002 Abstract Since its inception in the late 1970s, anomalous x-ray scattering (AXS) has been employed for chemically-specific structure determination in a wide variety of non- crystalline materials. These studies have successfully produced differential distribu- tion functions (DDFs) which provide information about the compositionally-averaged environment of a specific atomic species in the sample. Despite the wide success in obtaining DDFs, there are very few examples of successful extraction of the fully- chemically-specific partial pair distribution functions (PPDFs), the most detailed description of an amorphous sample possible by x-ray scattering. Extracting the PPDFs is notoriously difficult since the matrix equation involved is ill-conditioned and thus extremely sensitive to errors present in the experimental quantities that enter the equation. Instead of addressing this sensitivity by modifying the data through mathematical methods, sources of error have been removed experimentally: A focussing analyzer crystal was combined with a position-sensitive linear detector to experimentally eliminate unwanted inelastic scattering intensity over most of the reciprocal space range probed. This instrumentation has been used in data collection for the extraction of PPDFs from amorphous (a)-MoGe3. This composition arises as a phase separation endpoint in the Ge-rich region of the vapor-deposited Mo-Ge amorphous alloy system but is not ii present at equilibrium. Since the first Ge-rich compound in the Mo-Ge equilibrium system is MoGe2, previous workers have speculated that perhaps a unique MoGe3 compound exists in the amorphous system. Rather than indicating a distinct MoGe3 compound with definitive local structure, however, the coordination results are more consistent with a densely-packed alloy having a wide range of solid solubility. Significant improvement in the quality and reliability of experimental PPDFs from a-MoGe3 by AXS has been achieved solely through the experimental modifications to eliminate inelastic scattering. The coordination uncertainties are estimated at 5% for the Mo-Ge and Ge-Ge coordinations and 15% for the Mo-Mo coordination. These PPDFs from data collected at a second generation synchrotron source demonstrate the promise of the technique for routine PPDF extraction from binary alloys when applied in the future on dedicated beamlines at third generation synchrotron sources. iii Acknowledgements There are many people who have contributed to this work through scientific, as well as non-scientific, contributions and support. Although the Bienenstock research group has a tradition of brief and concise acknowledgments, the Clemens research group does not. Since I have been involved with both groups, I trust that the reader will forgive my deference to the latter. Prof. Arthur Bienenstock has been my principle advisor in this work, and he has given me the invaluable gift – and occasional curse – of a tremendous amount of freedom. In spite of his busy schedule, first as Director of the Stanford Synchrotron Radiation Laboratory (SSRL) and then as Director of Science for the Office of Science and Technology Policy (OSTP) in Washington, D.C., he has always found the time to interact with me in person, by phone or by e-mail to share his experience and insights in science, diplomacy and life. I especially thank him for his personal encouragement and strong support throughout my graduate career. Dr. Sean Brennan has been a second advisor to me. He is a highly talented exper- imentalist with uncanny trouble-shooting abilities and an internal GPS unit that can locate any piece of useful beamline hardware. It was Sean who first patiently showed me the ropes in synchrotron experiments and the flies and nymphs in fly-fishing. Without his presence during our month-long beamtimes at SSRL, this research would iv simply not have been possible, and I am very grateful for his involvement and his friendship. Prof. Bruce Clemens has been my adoptive advisor. Bruce took me in during Artie’s tenure at OSTP, and through my involvement with him and his research group, I’ve had a much broader education than I otherwise would have. (I have been exposed, for example, to the fundamentals of both thin film stress development and tele-hot-doggin’.) It has truly been a privilege to have had three advisors during my grad student career – and a pleasure to have skied with all of them! In addition to Artie, Sean and Bruce, I’d like to thank Prof. Paul McIntyre and Prof. Piero Pianetta, the other two members of my defense committee. Piero, together with Artie and Sean, helped fund a new position-sensitive detector, optimized for efficiency at high energies, which we used during our final beamtime. I’ve benefited from generous help with equipment, code and samples from many people. At SSRL, Andreas Freund built the variable focus graphite analyzer crys- tal necessary for the experiments, and Jim Patel repeatedly lent us his low energy position-sensitive detectors. Bart Johnson did some beautiful design and building of the optimized analyzer instrumentation, and Henry Bellamy helped me with the DISCO code to carry out the Kramers-Kr¨onig transform in the early stages of this work. I would like to acknowledge the entire x-ray staff at SSRL for their efficient and genuinely friendly support. These experiments are technically challenging, and they have helped us tackle these challenges gracefully. At the former Center for Materi- als Research, Bill Holmes kept the sputtering chamber in which I grew my samples running long past its “expiration date”, and Bob Jones helped out with preliminary sample characterization. My special thanks go to Alice Fischer-Colbrie and Steve Laderman who first v revealed the world of synchrotron radiation to me when I was a young undergraduate intern at HP Labs. Without their enthusiastic introduction to synchrotron work, I would never have considered keeping the odd hours required during beamtimes or received the rewards of this challenging, complex thesis project. Along with the advantage of multiple advisors comes the advantage of working with multiple groups of people, and they’ve all been fun and enriching. In the Bienen- stock group, I’d particularly like to thank Mike Regan for finding data for me long after he’d begun work elsewhere and for offering advice and comments via e-mail. I’d also like to thank Anneli Munkholm who taught me a lot about x-ray scattering although I never did learn to drink Coke for breakfast. I’ve been fortunate to also be part of the dynamic group of students in the Clemens group: Vidya Ramaswamy, Gaurav (Goo) Khanna, Fred Mancoff, James Freitag, Brennan Peterson, Paul Cic- colella, Christine Esber, Mark Phillips, Stephen Bluestein, Anant Chintamananei, David Barbero, Lisa Kinder, Scott Andrews, Guleid Hussen, Aditi Chandra and Raj Kelekar. Todd Hufnagel, a Clemens group alumnus when I joined, is my anomalous x-ray scattering predecessor, and he made many improvements to the data analysis code. In particular, he translated the comments of one of Artie’s former postdocs from Finnish to English for which I thank him profusely. Vidya and Goo have become two of my closest friends, and I’m grateful for their cameraderie and friendship over the years. I’d also like to thank the faculty, staff and students of SSRL, the Department of Materials Science and Engineering, and the Laborabory for Advanced Materials. In particular, I have always been able to turn to Phillippa MacFarlane-Thorne, Lisa Buckley, Billie Kader and Lisa Dunn to straighten out paperwork snafus. It’s truly the people who make these such great places to work. Finally, there’s the “SSRL vi Gang” of Apurva Mehta, Matthew Latimer and Srinka Ghosh with whom I’ve proba- bly had more philosophical and political discussions over SLACburgers and fries than scientific ones. I come now to my family and friends who have always offered sympathetic ears and encouraging words even as they wondered what on earth I was doing. Johan Erikson, Chris Thomas, Michelle Pierce, Nora Flemming de Sandoval, Kathy Wilder Guarini, Steve Guarini, Emmanuel Limal and Paul Braun have provided support from near and far as well as glimpses of life outside. I am grateful to Johan, in particular, for being continually supportive of my educational goals. Friends at Stanford who’ve managed to keep me more-or-less sane throughout my grad years include Vidya, Assad Oberai, Goo, Mark, Marc Glazer, Ankur Mohan, Nicole Meier, Gonzalo Feijoo, Monica Colaiacovo, Ranjana Saha, Jeffrey Snodgrass, Julie Ngau, Sylvia Sprytte and Kathy Flores. Thank you all! I especially thank my parents for instilling in me a love of learning and a love of living and my sisters, parents and grandparents for all their love and encouragement during the many years of my formal education. Finally, I thank my best friend, Paul Ciccolella, whom I cannot thank enough, for his unwavering support and confidence in me. I’d like to acknowledge the U.S. Department of Energy Basic Energy Sciences division, grant DE-AC03-76SF00515, for funding this work.

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