Localization and ALCHEMI for zone axis orientations

A standards-based method for compositional analysis by energy dispersive X-ray spectrometry using multivariate statistical analysis: application to multicomponent alloys

Given an unknown multicomponent alloy, and a set of standard compounds or alloys of known composition, can one improve upon popular standards-based methods for energy dispersive X-ray (EDX) spectrometry to quantify the elemental composition of the unknown specimen? A method is presented here for determining elemental composition of alloys using transmission electron microscopy-based EDX with appropriate standards. The method begins with a discrete set of related reference standards of known composition, applies multivariate statistical analysis to those spectra, and evaluates the compositions with a linear matrix algebra method to relate the spectra to elemental composition. By using associated standards, only limited assumptions about the physical origins of the EDX spectra are needed. Spectral absorption corrections can be performed by providing an estimate of the foil thickness of one or more reference standards. The technique was applied to III-V multicomponent alloy thin films: composition and foil thickness were determined for various III-V alloys. The results were then validated by comparing with X-ray diffraction and photoluminescence analysis, demonstrating accuracy of approximately 1% in atomic fraction.

Theoretical and experimental limits of the analysis of III/V semiconductors using EELS

The optimisation of acquisition conditions for EELS spectroscopy of Al(x)Ga(1-x)As heterostructures permits one to find the absolute concentration with a precision of better than delta = +/-0.02 and to detect changes in concentration of +/-0.01 for x = 0-0.5. In order to achieve this concentration precision, we investigated ways to reduce the influence of three major sources on the inaccuracies of the measurement: the effect of electron channelling which biases the ionisation probabilities on the different atomic sites, the contribution of the sample surface layers and the accuracy in the spectral analysis. An optimal specimen orientation that maximises the stability of the electron densities on Al and As sites without introducing an unacceptable loss of spatial resolution due to sample tilt is found by computing the electron channelling intensity as a function of the specimen tilt angle. The influence of a Ga enriched surface layer on the analysis is demonstrated. Two methods for the extraction of the edge intensity from the spectra are compared. These methods are shown to give the upper and the lower limit of the Ga concentration.

Theory of zone axis electron diffraction

The theory of high-energy electron diffraction is reviewed, with an emphasis on methods that help most in understanding the form of zone axis convergent-beam patterns. A Bloch wave description is used throughout the paper, and the theory is formulated in real space, leading to general expressions for both zero-layer and higher-order Laue zone (HOLZ) diffraction amplitudes. HOLZ diffraction is discussed in detail, and it is shown that first-order perturbation theory is usually applicable and provides a clear picture of the diffraction. The computation of diffracted intensities is discussed with reference to the many-beam equations. Diffraction at the [0001] axis in 1T-VSe2 is analysed both to illustrate the theory and to provide an example of the special effects that occur at well defined string zone axes.