Abstract:A major challenge for atom probe tomography (APT) quantification is the inability to decouple ions that possess the same mass–charge (m/n) ratio but a different mass. For example, 75As+ and 75As22+ at ∼75 Da or 14N+ and 28Si2+ at ∼14 Da cannot be differentiated without the additional knowledge of their kinetic energy or a significant improvement of the mass resolving power. Such mass peak overlaps lead to ambiguities in peak assignment, resulting in compositional uncertainty and an incorrect labeling of the at… Show more
Kingham [(1982). The post-ionization of field evaporated ions: A theoretical explanation of multiple charge states. Surf Sci 116(2), 273–301] provided equations for the probability of observing higher charge states in atom probe tomography (APT) experiments. These “Kingham curves” have wide application in APT, but cannot be analytically transformed to provide the electric field in terms of the easily measured charge state ratio (CSR). Here we provide a numerical scheme for the calculation of Kingham curves and the variation in electric field with CSR. We find the variation in electric field with CSR is well described by a simple two- or three-parameter equation, and the model is accurate to most elements and charge states. The model is applied to experimental APT data of pure aluminium and a microalloyed steel, demonstrating that the methods described in this work can be easily applied to a variety of APT problems to understand electric field variations.
Kingham [(1982). The post-ionization of field evaporated ions: A theoretical explanation of multiple charge states. Surf Sci 116(2), 273–301] provided equations for the probability of observing higher charge states in atom probe tomography (APT) experiments. These “Kingham curves” have wide application in APT, but cannot be analytically transformed to provide the electric field in terms of the easily measured charge state ratio (CSR). Here we provide a numerical scheme for the calculation of Kingham curves and the variation in electric field with CSR. We find the variation in electric field with CSR is well described by a simple two- or three-parameter equation, and the model is accurate to most elements and charge states. The model is applied to experimental APT data of pure aluminium and a microalloyed steel, demonstrating that the methods described in this work can be easily applied to a variety of APT problems to understand electric field variations.
In this paper, the capability for quantifying the composition of Ba-doped SrTiO layers from an atom probe measurement was explored. Rutherford backscattering spectrometry and time-of-flight/energy elastic recoil detection were used to benchmark the composition where the amount of titanium was intentionally varied between samples. The atom probe results showed a significant divergence from the benchmarked composition. The cause was shown to be a significant oxygen underestimation (≳14 at%). The ratio between oxygen and titanium for the samples varied between 2.6 and 12.7, while those measured by atom probe tomography were lower and covered a narrower range between 1.4 and 1.7. This difference was found to be associated with the oxygen and titanium predominantly field evaporating together as a molecular ion. The evaporation fields and bonding chemistries determined showed inconsistencies for explaining the oxygen underestimation and ion species measured. The measured ion charge state was in excellent agreement with that predicted by the Kingham postionization theory. Only by considering the measured ion species, their evaporation fields, the coordination chemistry, the analysis conditions, and some recently reported density functional theory modeling for oxide field emission were we able to postulate a field emission and oxygen neutral desorption process that may explain our results.
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