Applications of focused ion beam for preparation of specimens of ancient ceramic for electron microscopy and synchrotron X-ray studies.

In this paper the capabilities of FIB systems as a tool for TEM studies of ancient pottery are explored, especially when the amount of available material is very limited and when, for instance, there is stringent demand for very accurate location of the electron-transparent area as is the case for investigation of outer surface layers, such as slips and patinas. The advantages of the two main FIB milling techniques (H-bar and Lift-out) are discussed in detail and illustrated through the study of metallic lustre decorations and a particular type of Roman Terra Sigillata coating. The H-bar technique is ideal for investigations where the features of interest are near the edges of a ceramic fragment. A significantly large area of surface decoration can be studied without any restriction on the size and the shape of fragment. On the other hand, the Lift-out technique is very powerful for extracting TEM membranes far from the edges. An added advantage of this technique is that the thickness of the foil is very uniform and that allows large tilts and makes it possible to obtain electron diffraction patterns of several zones axes from the same crystal, making crystallographic phase identification easier and precise, and identification of complex structures possible. We also show that the FIB system can be used to deposit very precise registration marks, allowing an experimenter to correlate results from TEM measurements with other complementary techniques, such as synchrotron based microdiffraction and microXANES. Combination of these complementary techniques is becoming a very powerful approach to probe the chemical and morphological microstructure of heterogeneous and complex material from the nanometre to millimetre scale.

New approach for the dynamical simulation of CBED patterns in heavily strained specimens.

A new method for the dynamical simulation of convergent beam electron diffraction (CBED) patterns is proposed. In this method, the three-dimensional stationary Schrödinger equation is replaced by a two-dimensional time-dependent equation, in which the direction of propagation of the electron beam, variable z, stands as a time. We demonstrate that this approach is particularly well-suited for the calculation of the diffracted intensities in the case of a z-dependent crystal potential. The corresponding software has been developed and implemented for simulating CBED patterns of various specimens, from perfect crystals to heavily strained cross-sectional specimens. Evidence is given for the remarkable agreement between simulated and experimental patterns.

Effect of sample bending on diffracted intensities observed in CBED patterns of plan view strained samples.

Convergent beam electron diffraction is used to study the effect of the sample bending on diffracted intensities as observed in transmission electron microscopy (TEM). Studied samples are made of thin strained semiconductor Ga(1-)(x)In(x)As epitaxial layers grown on a GaAs substrate and observed in plan view. Strong variations of the diffracted intensities are observed depending on the thinning process used for TEM foil preparation. For chemically thinned samples, strong bending of the substrate occurs, inducing modifications of both kinematical and dynamical Bragg lines. For mechanically thinned samples, bending of the substrate is negligible. Kinematical lines are unaffected whereas dynamical lines have slightly asymmetric intensities. We analyse these effects using finite element modelling to calculate the sample strain coupled with dynamical multibeam simulations for calculating the diffracted intensities. Our results correctly reproduce the qualitative features of experimental patterns, clearly demonstrating that inhomogeneous displacement fields along the electron beam within the substrate are responsible for the observed intensity modifications.

LACDIF, a new electron diffraction technique obtained with the LACBED configuration and a C(s) corrector: comparison with electron precession.

By combining the large-angle convergent-beam electron diffraction (LACBED) configuration together with a microscope equipped with a C(s) corrector it is possible to obtain good quality spot patterns in image mode and not in diffraction mode as it is usually the case. These patterns have two main advantages with respect to the conventional selected-area electron diffraction (SAED) or microdiffraction patterns. They display a much larger number of reflections and the diffracted intensity is the integrated intensity. These patterns have strong similarities with the electron precession patterns and they can be used for various applications like the identification of the possible space groups of a crystal from observations of the Laue zones or the ab-initio structure identifications. Since this is a defocused method, another important application concerns the analysis of electron beam-sensitive materials. Successful applications to polymers are given in the present paper to prove the validity of this method with regards to these materials.

Quantitative analysis of HOLZ line splitting in CBED patterns of epitaxially strained layers.

A SiGe layer epitaxially grown on a silicon substrate is experimentally studied by convergent beam electron diffraction (CBED) experiments and used as a test sample to analyse the higher-order Laue zones (HOLZ) line splitting. The influence of surface strain relaxation on the broadening of HOLZ lines is confirmed. The quantitative fit of the observed HOLZ line profiles is successfully achieved using a formalism particularly well-adapted to the case of a z-dependent crystal potential (z being the zone axis). This formalism, based on a time-dependent perturbation theory approach, proves to be much more efficient than a classical Howie-Whelan approach, to reproduce the complex HOLZ lines profile in this heavily strained test sample.