Scientific Advisory Board - Erich Schmid Institute

ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE
Revealing the relationship of the lattice constant and nitrogen vacancy
in CrN using atomic resolution electron microscopy (Advanced
imaging and analytics)
transition metal nitrides like crn have, besides applications as wear-resistant coatings, recently
gained considerable interest for its unique antiferromagnetic configurations, and have been used
as a prototype material for strong magneto-structural interactions. the study of the atomic and
electronic structure of crn films is of vital importance for understanding the antiferromagnetic
property. ordered n vacancies distributed at {111} planes were detected, which lead to a
distorted lattice. Such ordered n vacancies can form at the interface crn–cr during the film
synthesis. Advanced tem techniques, such as electron energy-loss spectroscopy (eeLS)
and energy-loss near-edge structure (eLneS) analysis, spherical aberration (c S)-corrected
high-resolution transmission electron microscopy (HRtem) and scanning tem (Stem) and
geometrical phase analysis (gpA), are applied to characterize the atomic and electronic
structures of the ordered n vacancies, furthermore, exploring the relationship between the
lattice constant and nitrogen vacancy concentration and a comparison will be made with the
theoretical calculations using the density functional theory (dft) (cooperation with prof. c.
mitterer, department of materials physics, Univ Leoben, cooperation with prof. draxl claudia,
Humboldt-Universität zu Berlin)
independent of the substrates used, the crn films grow frequently in a fine grained randomly
oriented microstructure during the initial nucleation stage and then continuously grow in a
columnar manner. However, when a cr interlayer is introduced between the crn and the
substrate, crn can grow directly on the cr layer. the columnar grain size is about 60 -100
nm in width. to examine the atomic and electronic structure of the interface in detail, a large
columnar grain is selected for the atomic resolved imaging. A c S-corrected HRtem image (fig.
1) acquired in [110] direction shows some “stripe” features along the {111} plane adjacent to the
cr-crn interface, forming a “defective layer” (crn x) in the film. the thickness of the defect layer
is in the range of a few to tens of nanometers. the diffractograms illustrate that the defective
layer possesses the identical pattern as the bulk crn except a slight expansion, which means
that it retains the same crystal structure as the crn, of fcc lattice, (also corroborated by the
HRtem image from the [010] direction), but possesses a slightly different lattice constant. A
cs-corrected high resolution Stem images is also shown in fig.1, in which the stripe features
are still visible (cooperation with dr. Herbert Schmid, Leibniz-institut für neue materialien,
Saarbrücken, germany).
the strong distortions at the region containing numerous ordered n vacancies can be visualized
using the geometrical phase analysis. the distortion mapping performed at the same region
(where the eeLS analysis was carried out) clearly shows three distinguishable regions (fig.
2a). the corresponding integrated line profile crossing the three layers shows a different
magnitude of distortion. from the map, relative to the perfect crn layer, the defective layer is
slightly compressed while the cr layer is somewhat expanded. the distortion distributions in
the defective layer appears inhomogeneous, which could imply the presence of anisotropic
distortion distributions attributed to the nitrogen vacancies located at the {111} planes.
the displacement map (d 200) performed on the same region was acquired using the crn
(200) reflection as shown in fig. 2b. As the defective layer retains the cubic structure and
assuming a direct relationship between the structure and composition, the displacement map
actually demonstrates the lattice constant (2×d 200) variations with the n vacancy, which gives
displacement information at each individual position as comparing with the analysis of the
page 16 Scientific RepoRt 2012