Medium range structural order in amorphous tantala spatially resolved with changes to atomic structure by thermal annealing

Abstract

International audienceAmorphous tantala (a-Ta2O5) is an important technological material that has wide ranging applications in electronics, optics and the biomedical industry. It is used as the high refractive index layers in the multi-layer dielectric mirror coatings in the latest generation of gravitational wave interferometers, as well as other precision interferometers. One of the current limitations in sensitivity of gravitational wave detectors is Brownian thermal noise that arises from the tantala mirror coatings. Measurements have shown differences in mechanical loss of the mirror coatings, which is directly related to Brownian thermal noise, in response to thermal annealing. We utilise scanning electron diffraction to perform a modified version of Fluctuation Electron Microscopy (FEM) on Ion Beam Sputtered (IBS) amorphous tantala coatings, definitively showing an increase in the medium range order (MRO), as determined from the variance between the diffraction patterns in the scan, due to thermal annealing at increasing temperatures. Moreover, we employ Virtual Dark-Field Imaging (VDFi) to spatially resolve the FEM signal, enabling investigation of the persistence of the fragments responsible for the medium range order, as well as the extent of the ordering over nm length scales, and show ordered patches larger than 5 nm in the highest temperature annealed sample. These structural changes directly correlate with the observed changes in mechanical loss. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

Measurement and Modeling of Short and Medium Range Order in Amorphous Ta2O5 Thin Films

“…The first observation is that PDF of all three amorphous films show at least some order up to 1 nm. This is in agreement with recent fluctuation electron microscopy (FEM) and electron-PDF studies where evidence for ordering up to 1–1.5 nm was found in amorphous Ta 2 O 5 and Ti-doped Ta 2 O 5 thin films2743. The second observation is that even though the overall PDF looks similar for all three films, there are significant differences among them, not only in the short-range correlation but also in medium-range order.…”

Towards quantitative treatment of electron pair distribution function

“…As the medium-range order has a significant effect on the physical behavior of the material (Pagon et al, 2010;Bassiri et al, 2011Bassiri et al, , 2013Yan et al, 2012;Hart et al, 2016), studies of fine details of amorphous structures were triggered with the development of a dedicated method -Fluctuation Electron Microscopy (FEM), which demonstrated the paracrystalline nature of amorphous silicon (Borisenko et al, 2012;. Ishimaru (2006) demonstrated the importance of energy filtering for the PDF analysis: the unfiltered data of amorphous SiC showed a significantly different behavior for Q below 10 Å À1 , while the high-Q regions of filtered and unfiltered data were the same (energy slit used was 20 eV).…”

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