Abstract
Grain boundary segregation is an important phenomenon in metallurgy and semiconductor technology. Some recent studies by tomographic atom probe field ion microscopy (APFIM) claim to have measured the interfacial excess of atoms segregated to grain boundaries with ultra-high precision, down to 0.01-0.02 atoms/nm2. This study critically evaluates these claims by simulations. It is shown that atom probe tomography is no ‘magic bullet’ and suffers similar physical constraints as analytical scanning transmission electron microscopy (STEM). Data analyses from both methods have much in common in terms of geometry, performance, systematic and statistical errors. It is shown that an analysis method previously developed for (S)TEM called conceptEM can also successfully be applied to APFIM data.
Highlights
Grain boundary segregation describes the accumulation of foreign atoms at internal boundaries between different grains of polycrystalline material
While qualitative evidence of grain boundary segregation can be readily obtained by breaking the material under ultra-high vacuum conditions to expose these grain boundaries without crosscontamination prior to surface analysis by, e.g. Auger electron spectroscopy [1,2], a quantitative measurement of the interfacial excess of atoms at internal boundaries in the bulk is only possible by two techniques: tomographic atom probe field ion microscopy (APFIM) which combines pulsed field evaporation and time-of-flight mass spectroscopy with a position-sensitive detector, and analytical scanning transmission electron microscopy (STEM)
While one would expect that an increase of number of atoms detected by one order of magnitude would improve the signal-to-noise ratio by 10≈3, it can be observed that the slopes of the linear sections on either side of the interface plane reduce with apparent atom number, indicating that if the number of Si atoms per atomic plane evaporated becomes smaller than a critical value below unity, relatively more and more of those few Si atoms dissolved in the gallium arsenide (GaAs) matrix or diffused away from the grain boundary plane can no longer be detected at all
Summary
Measuring grain boundary segregation: tomographic atom probe field ion microscopy (APFIM) vs analytical scanning transmission electron microscopy (STEM)
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