Highlights

Interface Structure of ZrO₂/In₂O₃ Multilayer Films

Scientific Achievement

Multilayers comprised of ZrO₂ and In₂O₃ are expected to exhibit enhanced ionic and electronic conductivities, which are important properties for solid oxide fuel cells.  This enhanced conductivity can be explained by the electronic structure of sharp interfaces and/or local intermixing. Knowledge of the chemistry and structure of the interface on an atomic level is therefore essential for understanding changes in conductivity at this interface.  The roughness of the interface ZrO₂/In₂O₃ requires a 3-dimensional analysis of the elemental distribution.

A multilayer of ZrO₂ and In₂O₃ was deposited on yttria-stabilized zirconia (YSZ)(001) by rf-magnetron sputtering.  Energy-filtered transmission electron microscopy (EFTEM) was used to record elemental maps of Zr and In.  An interface roughness of up to 5nm could be measured at the interface using these elemental maps as well as high-resolution TEM (HRTEM) images.  The roughness of the interfaces where the In₂O₃ layer is closer to the substrate is significantly higher than for interfaces with the ZrO₂ closer to the substrate.  The roughness increases with the thickness of the layers.  These results hint to a three dimensional growth process for the In₂O₃ layers as the origin of the observed interface roughness.  Under these circumstances conventional TEM imaging and analysis is hampered by projecting a 3D structure into two dimensions.  Therefore, we used tomographic tilt series of energy-filtered images to reconstruct a 3D elemental map of a complex interface.  Iterative reconstruction methods were compared with respect to their ability to reduce noise and minimizing distortions.  The SIRT algorithm [1] provided the best results for EFTEM tomography and was used to reconstruct a 3D distribution of Zr in a ZrO₂/In₂O₃ multilayer.  The 3D map shows interface roughness of the ZrO₂ layer on the In₂O₃ layer on a nanometer scale.  Resolution is limited due to lens aberrations and delocalization of energy-loss electrons to ~2nm.  The (001) lattice planes adjacent to a smooth segment of a  ZrO₂/In₂O₃ interface have a lattice distance which is about 5% larger than the bulk value as can be measured from phase reconstructed HRTEM images.

Significance

Conventional studies of interface structure and chemistry based on 2D imaging are limited to smooth interfaces which can be imaged “edge-on”.  3D elemental mapping extends the analytical capabilities of TEM to arbitrarily shaped interfaces.  This will allow a better understanding of the correlation of interface properties to the conductivity at the interface ZrO₂/In₂O₃ and other thin film systems.

Performers

X. Zhong, B. Kabius, D. Fong, J. Eastman, D. Schreiber, A. Petford-Long (Argonne-MSD)