Quantitative Chemical Mapping of InGaN Quantum Wells from Calibrated High-Angle Annular Dark Field Micrographs

We present a simple and robust method to acquire quantitative maps of compositional fluctuations in nanostructures from low magnification high-angle annular dark field (HAADF) micrographs calibrated by energy-dispersive X-ray (EDX) spectroscopy in scanning transmission electron microscopy (STEM) mode. We show that a nonuniform background in HAADF-STEM micrographs can be eliminated, to a first approximation, by use of a suitable analytic function. The uncertainty in probe position when collecting an EDX spectrum renders the calibration of HAADF-STEM micrographs indirect, and a statistical approach has been developed to determine the position with confidence. Our analysis procedure, presented in a flowchart to facilitate the successful implementation of the method by users, was applied to discontinuous InGaN/GaN quantum wells in order to obtain quantitative determinations of compositional fluctuations on the nanoscale.

Extension of Orientation Mapping to the Transmission Electron Microscope

This paper describes progress in improving the spatial resolution of the well-established Orientation Imaging Microscopy technique, OIM, by developing an analogous procedure for the transmission electron microscope. The transmission orientation micrographs are obtained by recording a large series of dark field micrographs taken from the chosen area in the specimen. This area is selected so that it contains all of the grains of interest and is imaged at sufficiently high magnification to yield the spatial resolution required. The changing intensity of each pixel in different dark field micrographs permits the equivalent of a diffraction pattern for that pixel to be constructed. This enables determination of the lattice orientation of small volumes in the sample corresponding to that imaged in each individual pixel. Experimentation has shown that problems arise however, that decrease the fraction of correctly measured points due to ambiguities in determining the index of higher order reflections, especially when the total number of reflections observed is small. The solution has been to both modify the indexing procedure and to sum the diffraction vectors observed within a single grain. The paper concentrates on a detailed analysis of a heavily deformed aluminium sample, chosen because of the fragmentation of the structure.

Structural changes in Pd electrodeposits during TEM sample preparation

The small grain size and high dislocation density of electrodeposits make it necessary to employ TEM analysis for microstructural evaluation. It had been generally accepted that twinned microstructures of electrodeposits observed by TEM were annealing or growth twins, and subsequently inherent to the properties of the electrodeposit. However, in a recent study, deformation cross-twinning in Pd electrodeposits was unambiguously identified. The purpose of this paper is to report on the influence of TEM sample preparation on the structure of Pd electrodeposits.Electrodeposited Pd (courtesty J. Vanhumbeeck, Siemens NV, Belgium) TEM foils were thinned by the standard techniques of twin jet-electropolishing and ion beam milling. The Pd foils were examined in a Philips EM420T TEM with analytical capabilities operating at 120 kV.FIG. 1 is a series of three dark field micrographs from the same region showing deformation crosstwinning. The dislocation density increases with increasing foil thickness and decreasing deformation cross-twinning.

Observations of Surface Diffusion at the Atomic Level by Means of Microcinematography in the Stem

We have previously indicated the potential application of the STEM for studying surface phenomena on an atomic scale and noted that direct measurements of atom motion in the STEM could be of use in obtaining an improved understanding of a variety of surface processes. In this paper we would like to present some further observations concerning the motion of heavy atom containing molecules on thin carbon substrates due to thermal diffusion.A typical example of heavy atom containing molecules migrating on a carbon substrate is shown in Fig. 1. The specimen is uranylchloride molecules on an ∼ 20 Å thick carbon film. The dark field micrographs have been taken with 17 second exposures. The beam is not irradiating the field of view between exposures. In this set of micrographs it appears that the uranium atom “spot” is migrating towards the large cluster in the lower portion of the field.

Lattice Image Studies of Ordered Alloys

Lattice imaging is becoming a viable technique for investigating the structure, defects and early stages of phase transformations in ceramics and metals. Our interest lies in its application to the study of ordering in alloys. Conventional images yield little information on the local degree of order in the specimen, the image width of defects may be as great as 50Å, and the interpretation of diffaction patterns and superlattice dark field micrographs is sometimes ambiguous. The advantage of the lattice image is its ability to provide data about atomic environment (from the nature of the lattice fringe profile) and to reveal atomic arrangements near defects.

Utilizing Dark Field Electron Microscopy to Produce Lantern Slides

The electron microscope is being utilized more and more in clinical laboratories for pathologic diagnosis. One of the major problems in the utilization of the electron microscope for diagnostic purposes is the time element involved. Recent experimentation with rapid embedding has shown that this long phase of the process can be greatly shortened. In rush cases the making of projection slides can be eliminated by taking dark field electron micrographs which show up as a positive ready for use. The major limiting factor for use of dark field micrographs is resolution. However, for conference purposes electron micrographs are usually taken at 2.500X to 8.000X. At these low magnifications the resolution obtained is quite acceptable.

Fission fragment damage to crystal lattices: track contrast

In previous papers (Bowden & Chadderton 1962; Chadderton & Montagu-Pollock 1963) some account has been given of the type of damage sustained by both stable and unstable crystals, and its relation to the principle mechanisms of energy loss. In particular it has been demonstrated that fission fragments can create so much damage that ‘tracks’ may be observed in the electron microscope, and that these may be black or white, depending upon the thickness of the bombarded crystal. This paper considers the nature of black tracks and describes their formation as images in the electron microscope in terms of ‘diffraction contrast’. A model is established for the damage produced by a fission fragment in an initially perfect crystal. This essentially consists of a central cylinder of disturbed material sur­rounded by a radial strain field, and is mathematically very similar to the model invoked by Ashby & Brown (1963 a ) to describe the spherically symmetrical coherency strain. The model is used in conjunction with the two-beam dynamical theory of electron diffraction (Howie & Whelan 1961) and machine calculations are made of contrast profiles for tracks at different positions in the crystal and for various values of the parameters involved. A good measure of agreement between experiment and theory is demonstrated, and, in particular, it is shown how anomalous images and oscillatory contrast effects may be explained. Asymmetric images on dark-field micrographs can be used in a determination of the direction of radial strain, and anomalous images in both bright and dark field yield information about the direction of particle travel. A method is suggested for obtaining information concerning the pattern of damage in continuous and discontinuous tracks and for testing the validity of the depleted zone concept.