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When measuring heterogeneous samples with atom probe tomography (APT), the different evaporation behaviour of the materials can lead to non-hemispherical apex topographies [1]. Because the local curvature is one of the main factors determining the ion trajectories, any deviation from a hemispherical shape can lead to locally varying magnifications and therefore, spatial inaccuracies [2]. In our work, we investigate the dependence of the local magnification on the applied voltage and laser power in APT.
Pre-sharpened silicon microtip coupons were coated with vanadium oxide by atomic layer deposition [3], yielding silicon/vanadium oxide core/shell tips. Identical behaviour was observed for coupons coated with vanadium dioxide (VO2) and vanadium pentoxide (V2O5). We will show that the applied voltage (electric field) is driving a change of the (local) magnification of the silicon core. Furthermore, atomic force microscopy on APT tips [4] reveals a clear link to the apex topography, where a protruding centre corresponds to a high magnification of the core and a concaving centre to a low magnification. Together with 2D evaporation simulations, where the experimental ion maps were reproduced qualitatively by varying the ratio of the evaporation fields of the materials, we conclude that silicon and vanadium oxide exhibit a field driven switch in preferential evaporation. We suggest a evaporation barrier reduction dependent on the hole density in each material as underlying reason. A redistribution of holes dependent on the electric field leads to a change in the relative evaporation barriers und thus the switch in preferential evaporation of silicon and vanadium oxide.
[1] Miller, M.K., and Hetherington, M.G., Surface Science 246.1-3 (1991): 442-449.
[2] Grenier, A., et al. Ultramicroscopy 136 (2014): 185-192.
[3] Mattelaer, F., et al. RSC advances 6.115 (2016): 114658-114665.
[4] Op de Beeck, J., et al. J. Phys. Chem. C 124.11 (2019): 6371-6378.
