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

Amorphous 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 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.Comment: 22 pages, 5 figure

Investigating the medium range order in amorphous Ta₂O₅ coatings

Ion-beam sputtered amorphous heavy metal oxides, such as Ta2O5, are widely used as the high refractive index layer of highly reflective dielectric coatings. Such coatings are used in the ground based Laser Interferometer Gravitational-wave Observatory (LIGO), in which mechanical loss, directly related to Brownian thermal noise, from the coatings forms an important limit to the sensitivity of the LIGO detector. It has previously been shown that heat-treatment and TiO2 doping of amorphous Ta2O5 coatings causes significant changes to the levels of mechanical loss measured and is thought to result from changes in the atomic structure. This work aims to find ways to reduce the levels of mechanical loss in the coatings by understanding the atomic structure properties that are responsible for it, and thus helping to increase the LIGO detector sensitivity. Using a combination of Reduced Density Functions (RDFs) from electron diffraction and Fluctuation Electron Microscopy (FEM), we probe the medium range order (in the 2-3 nm range) of these amorphous coatings

Mechanism of dopant distribution: an example of nickel-doped ceria nanoparticles

The microstructure, geometry and accurate distribution of dopants in nickel-doped ceria nanoparticles prepared by spray pyrolysis were characterized. Nickel dopant concentration variation with the distance from the centre of the particle can be approximated as linear with a higher concentration of nickel found closer to the centre. It is suggested that the dopant distribution is mainly controlled by the difference in the diffusion rates of the precursors from the centre to the surface of the particle

Understanding atomic structures of amorphous C-doped Ge2Sb2Te5 phase-change memory materials

The atomic structure of thin films of the carbon-doped Ge2Sb2Te5 (GST) rapid phase-change memory material Ge2Sb2Te5C (10% C-GST) was investigated by reverse Monte Carlo refinement using experimental electron diffraction reduced scattering data accompanied by density functional theory (DFT) molecular dynamics (MD) simulations and energy optimizations. For comparison, the structure of amorphous Ge2Sb2Te5C2 (18% C-GST) was obtained by DFT MD simulation of cooling from the melt. The results suggest that the carbon dopant forms atomic scale carbon clusters coordinated predominantly by Ge atoms. This becomes more evident with increasing carbon concentration. For 10% C-GST the building blocks of the matrix can be identified as squares of Ge(Sb)-Te-Sb(Ge)-Te atoms, related to the elementary building blocks of the corresponding crystalline structure of the metastable cubic phase of pure GST. The increased contribution of homopolar Te-Te bonds and Sb(Te)-Te-Sb(Te)-Te square fragments is suggested with the higher dopant level in 18% C-GST

Photo-induced optical activity in phase-change memory materials

We demonstrate that optical activity in amorphous isotropic thin films of pure Ge2Sb2Te5 and N-doped Ge2Sb2Te5N phase-change memory materials can be induced using rapid photo crystallisation with circularly polarised laser light. The new anisotropic phase transition has been confirmed by circular dichroism measurements. This opens up the possibility of controlled induction of optical activity at the nanosecond time scale for exploitation in a new generation of high-density optical memory, fast chiroptical switches and chiral metamaterials