Martin Hÿtch

Quantitative measurement of displacement and strain fields from HREM micrographs

A method for measuring and mapping displacement fields and strain fields from high-resolution electron microscope (HREM) images has been developed. The method is based upon centring a small aperture around a strong reflection in the Fourier transform of an HREM lattice image and performing an inverse Fourier transform. The phase component of the resulting complex image is shown to give information about local displacements of atomic planes and the two-dimensional displacement field can be derived by applying the method to two non-colinear Fourier components. Local strain components can be found by analysing the derivative of the displacement field. The details of the technique are outlined and applied to an experimental HREM image of a domain wall in ferroelectric–ferroelastic PbTiO3.

Nanoscale holographic interferometry for strain measurements in electronic devices

Strained silicon is now an integral feature of the latest generation of transistors and electronic devices because of the associated enhancement in carrier mobility. Strain is also expected to have an important role in future devices based on nanowires and in optoelectronic components. Different strategies have been used to engineer strain in devices, leading to complex strain distributions in two and three dimensions. Developing methods of strain measurement at the nanoscale has therefore been an important objective in recent years but has proved elusive in practice: none of the existing techniques combines the necessary spatial resolution, precision and field of view. For example, Raman spectroscopy or X-ray diffraction techniques can map strain at the micrometre scale, whereas transmission electron microscopy allows strain measurement at the nanometre scale but only over small sample areas. Here we present a technique capable of bridging this gap and measuring strain to high precision, with nanometre spatial resolution and for micrometre fields of view. Our method combines the advantages of moiré techniques with the flexibility of off-axis electron holography and is also applicable to relatively thick samples, thus reducing the influence of thin-film relaxation effects.

Quantitative comparison of high resolution TEM images with image simulations

Abstract The quantitative comparison of a simulation with an experimental image can be divided into three parts: the comparison of their mean intensity, contrast and pattern. The theoretical basis for such an approach has been discussed elsewhere and here we apply it to the evaluation of a typical through-focal series of images of a representative III–V specimen. Particular attention will be payed to the independent determination of the experimental parameters. A quantitative assessment of the effect on the matching process of noise and errors in the simulation parameters will also be made. The application of the approach is then used to demonstrate the inadequacy of current image simulations and the reasons for the mismatch which is typically found between the experimental and simulated image intensities are discussed.

Strain mapping in nanowires

A method for obtaining detailed two-dimensional strain maps in nanowires and related nanoscale structures has been developed. The approach relies on a combination of lattice imaging by high-resolution transmission electron microscopy and geometric phase analysis of the resulting micrographs using Fourier transform routines. We demonstrate the method for a germanium nanowire grown epitaxially on Si(111) by obtaining the strain components epsilon(xx), epsilon(yy), epsilon(xy), the mean dilatation, and the rotation of the lattice planes. The resulting strain maps are demonstrated to allow detailed evaluation of the strains and loading on nanowires.

Electron microscopy nanoscale characterization of ball-milled Cu–Ag powders. Part I: Solid solution synthesized by cryo-milling

Abstract We present a study of nanostructured Cu–Ag material obtained by low temperature ball milling. The microstructural characterization is carried out using a wide variety of transmission electron microscopy (TEM) techniques from conventional dark-field imaging, selected area diffraction and energy dispersive X-ray spectrometry (EDS) to more modern techniques of scanning TEM high-angle dark-field imaging (HAADF), nanodiffraction and high resolution imaging (HREM). A novel method of HREM image analysis is also presented which consists of calculating geometric phase images by Fourier filtering. Real-space maps of lattice spacings and lattice rotations are thus obtained. The analysis addresses the following essential points of nanostructural characterization: dislocation densities, grain sizes and morphologies, grain boundaries and local lattice rotations, local textures, local variations in lattice parameters. A new description of the microstructure emerges from the observations, quite different to that expected. Analytical modeling suggests that large lattice rotations can be expected in nanomaterials produced by intense deformation.

Quantitative criteria for the detection and characterization of nanocrystals from high-resolution electron microscopy images

Abstract A new method of high-resolution electron microscopy image analysis is presented which improves considerably the visibility of nanocrystals in amorphous materials. The technique is based on individual Bragg filtering of the lattice fringes in the image. The significant feature is that the amplitude and phase of the fringes as functions of position are calculated separately. This is similar to a holographic reconstruction with the Bragg interference representing the carrier wave. The phase information is shown to be particularly robust with respect to noise due to amorphous material. The images so produced are in a form suitable for statistical analysis, and criteria for judging quantitatively the extent of the crystal are proposed. The application of the technique to a wide range of similar problems concerning partially ordered materials is discussed.

Strain mapping of tensiley strained silicon transistors with embedded Si1−yCy source and drain by dark-field holography

Dark-field holography, a new transmission electron microscopy technique for mapping strain distributions at the nanoscale, is used to characterize strained-silicon n-type transistors with a channel width of 65 nm. The strain in the channel region, which enhances electron mobilities, is engineered by recessed Si0.99C0.01 source and drain stressors. The strain distribution is measured across an array of five transistors over a total area of 1.6 μm wide. The longitudinal tensile strain reaches a maximum of 0.58%±0.02% under the gate oxide. Theoretical strain maps obtained by finite element method agree well with the experimental results.

HRTEM study of defects in twin boundaries of ultra-fine grained copper

Twin boundaries (TBs) in ultra-fine grained (UFG) copper prepared by powder metallurgy were investigated using high-resolution transmission electron microscopy (HRTEM) and geometric phase analysis (GPA). Specimens were analyzed both before and after mechanical deformation (compression of 40%) and emphasis placed on the study of TB defects. Twin boundaries in the as-processed specimens are mainly disoriented from the perfect Σ3 coincidence. They present a faceted structure with coherent {111} and incoherent {112} facets. The latter have a 9R structure and the {111}/{112} junctions are associated with sessile dislocations of Frank type . Shockley glissile dislocations with Burgers vector of type are also present. This microstructure is interpreted in terms of the absorption and decomposition at room temperature of lattice dislocations (60° type). After mechanical deformation, an enrichment of twins at dislocations and a decrease of step density and height is observed and quantified by statistical analysis. Deformation mechanisms of UFG copper are discussed in light of these observations.

Interface characterization in electrodeposited Cu–Co multilayers

Multilayers of Cu–Co made by electrodeposition are characterized. The interface width and layer roughness are measured by the Fresnel technique in electron microscopy. It is shown that the quality of the interfaces is comparable to that of layers made by physical deposition techniques. By depositing the layers on a glass substrate, we also show that it is possible to measure the magnetoresistance of the multilayers without removing the substrate. The values obtained are discussed in relation to the roughness of the layers and the electrodeposition conditions.

End of range defects in Ge

We show that the solid-phase epitaxial regrowth of amorphous layers created by ion implantation in Ge results in the formation of extended defects of interstitial-type. During annealing, these defects evolve in size and density following, as in Si, an Ostwald ripening mechanism. However, this process becomes nonconservative as the annealing temperature increases to 600 °C. This suggests that the recombination/annihilation of Ge interstitial atoms becomes important at these temperatures. These results have important implications for the modeling of diffusion of implanted dopants in Ge.