Grand Séminaire d'Institut

Vendredi 22 Novembre 2019 à 15h00.

Amorphous materials: Ideality, Losses, and Two Level Systems (TLS) in Amorphous Silicon Films

''ATTENTION horaire spécial: 15h.
Abstract. 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.''