Materials Science Research Lecture
Amorphous Materials: Ideality, Losses, and Two Level Systems (TLS) in Amorphous Silicon Films
Webinar ID: 957 0877 2987
Amorphous materials lack structural order, making them difficult to describe and making it difficult to calculate and predict their properties compared to crystalline materials which consist of spatially repeated atoms. This difficulty, however, does not preclude the applicability or scientific impact of amorphous materials. The properties of an amorphous material depend strongly on how it was produced, and there are some well-defined known defects, but it is not clear how to describe the different amorphous structures produced by different methods, even for a single element material, nor what the nature of a defect is in a fully disordered material. Disorder exists on different length and energy scales, ranging from local, atomic-sized disorder to larger scales. Intriguingly, there exists the notion of an "ideal glass", which while remaining thoroughly disordered, lacks imperfections in that disorder and thus approaches the uniqueness of a crystal, including reproducibility and predictability of its properties. LIGO (laser interferometric gravitational observatory) relies on amorphous oxides for the mirror coatings which are critical to its performance; mechanical losses in these coatings currently however are the limiting noise factor in their performance, and are associated with universal yet poorly understand atomic motions in the amorphous structure which are described best as two level systems (TLS) which lead to losses. Amorphous silicon is the single material where these losses can be reduced; TLS can be tuned over nearly 3 decades, from below detectable limits to high in the range commonly seen in glassy systems. This tuning is accomplished by growth temperature, thickness, growth rate, light soaking or annealing. We see a strong correlation with atomic density, as well as with dangling bond density, sound velocity, and bond angle distribution as measured by Raman spectroscopy, but TLS vary by orders of magnitude while these other measures of disorder vary by less than a factor of two. The lowest loss films are grown at temperatures near 0.8 of the theoretical glass transition temperature Tg of Si, similar to results on polymer films and suggestive that high surface mobility even at 0.8Tg produces materials close to an ideal glass, with higher density, lower energy, and low losses due to few nearby configurations with similarly low energy. Comparisons to results on other vapor deposited amorphous materials will be made.
More about the Speaker:
Frances Hellman received her BA in Physics from Dartmouth College and her PhD in Applied Physics from Stanford University, studying what were then considered the high Tc superconductors (the A15's). After a 2 year post-doc in thin film magnetism at AT&T Bell Labs, she went to UCSD as an assistant professor in 1987, where she received tenure in 1994 and became a full professor in 2000. She joined the Physics Dept at UC Berkeley in Jan 2005, and became Chair of the Department in 2007. She stepped down as Chair on July 1, 2013, after serving 6 years. In addition to her work in the Physics Department, she has an appointment in the UCB Materials Science and Engineering Dept. as well as at LBNL in the Materials Sciences Division. She has been on a large number of national and local science boards, including the NSF Advisory Board on Math and Physical Sciences, the NRC Board on Physics and Astronomy, the NRC Solid State Sciences Committee, the DOE Division of Materials Science and Engineering Council, ICAM (Institute for Complex Adaptive Matter), the APS Committee on the Status of Women in Physics, the APS Panel on Public Policy, the editorial board for the Review of Scientific Instruments, the Elementary Institute of Science (in San Diego), COSMOS, a statewide math and science summer program for high school students, and the SF Exploratorium. For many years, she ran the NSF-REU site program at the UCSD Physics Dept. and has supervised many student theses, both undergraduate and graduate. She won the APS Keithley Award in 2006, "In recognition of using emerging micromachining techniques to significantly extend the range of calorimetry into the realm of nanoscale science by construction of Si based microcalorimeters capable of operating in extreme environments with unprecedented sensitivity and accuracy", is a Fellow of the APS, and has been Chair of both the APS Division of Materials Physics and the APS Topical Group on Magnetism and its Applications.
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