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Structure and phase characterisation

The structural properties (e.g. lattice parameter, symmetries, crystal orientations, lattice defects) of materials can be determined by electron diffraction or by high resolution imaging procedures.

The advantage of electron microscopy in relation to other techniques is the high spatial resolution.

Electron diffraction pattern of an AlMg-alloy (left image) and a convergent beam electron diffraction pattern of silicon (right image)

Electron diffraction

Electrons behave in the electron microscope like waves with very small wavelengths (a few pico-meters). In a TEM these waves pass through the thin sample and are diffracted by the crystal lattice to produce diffraction patterns. By analysing these patterns structural information can be obtained about the material. The structural data can be used together with chemical analysis (EDX or EELS) in order to perform a phase characterisation.

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EBSD pattern showing a set of kikuchi lines and an OIM map of an aluminum alloy.

EBSD (Electron backscatter diffraction)

Electron diffraction can also be used in an SEM for determining the material structure. If electrons hit a polished surface at an angle of incidence of 20° some of the scattered electrons leave the surface while beeing diffracted at the crystal lattice of the sample.

The diffracted electrons form an EBSD pattern on a camera and are recorded digitally.  An EBSD pattern consists of so-called Kikuchi lines, from which the structure and orientation of the interacting volume can be determined.

By scanning a selected sample range with the electron beam, an OIM map (Orientation Imaging Microscopy) can be produced, which provides information about grain size, phase distributions, grain boundaries angles or texture.


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ZnSe on GaAs substrate showing lattcie defects.

HRTEM

High resolution transmission electron microscopy (HRTEM) enables imaging the crystallographic structure of a sample with atomic resolution. HRTEM is a widespread tool for the characterisation of nanostructures in crystalline materials like semiconductors or metals.

By HRTEM objects with a thickness of a few nanometer are investigated. The image contrast is mainly influenced by differences of the phase instead of the amplitude of the electron wave when passing through the sample. Interpretation of phase contrast images typically requires image simulations. 

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HRSTEM

Silicon dubbells in (110)-orientation: distance between the dumbbells: 0.136 nm.

For HRSTEM High-Angle Annular Darkfield Detectors (HAADF) are used for the electronic treatment of data produced by electrons scattered at large angles. The contrast in the HAADF image is - beside of the inner and outer detector angles - only dependent on the thickness of the sample, the atomic number of the probed atoms, and the density of matter. HAADF images are therefore called Z-contrast images.

The magnification of the image is contrary to HRTEM not depending on the settings of the projector system, but only influenced by the angle of electron beam deflection during scanning across the specimen. The resolution of the image is mainly influenced by the focussing of the small electron probe, which is focussed down to the atomic level for HRSTEM applications. In this situation the atomic positions are imaged and the HRSTEM image is directly interpretable, which is the largest advantage compared to HRTEM.

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