Correlative investigations by HAADF‐STEM and Atom Probe Tomography

Abstract

The ultimate capabilities achieved by electron microscopies and their associated techniques inevitably raise the following question: is there room for conceiving new ways of investigating materials at the nano‐scale? Indeed, most recent TEMs and STEMs easily achieve sub‐Angström spatial resolution, while allowing elemental mapping at the same scale. Meanwhile, electron tomography has unambiguously demonstrated the possibility to image atomic positions and defects. In these instruments, some physical properties (e.g. optical, magnetic) are now accessible, again with increased resolution. However, as far as an ultimate machine would allow correlating physical properties with a “perfect” determination of atomic species and atomic positions in 3D, one must recognize that such a tool is not yet available. Aside from electron microscopes, Atom Probe Tomography (APT), which is intrinsically a 3D technique, has received increased attention owing to drastic developments during the last decade. This tool enables reconstructing volumes of matter by determining atom positions in 3D, which nature is determined by time of flight mass spectrometry. Thanks to the improvement of specimen preparation protocols, APT can be applied to much broader areas of materials science (semi‐conductors, bio‐materials, geo‐materials, soft mater and even liquids). Nowadays, intrinsic limitations of this tool reside in its limited detection efficiency (roughly 50% of atoms are detected) and in its anisotropic spatial resolution (though sub‐Angström resolution is currently accessible along the direction of analysis, sub‐nanometer resolution is achieved along transverse directions). Strong advantages of APT rely in its possibility to detect all types of atoms, independently of their atomic number, in its excellent detection limit (few ppm in favorable cases but rarely more than 100 ppm), and in its intrinsic 3D nature. In order to collect a significant amount of information on a same nano‐object, it is relevant to consider a correlative approach combining a TEM/STEM and APT. Motivations for such an approach are numerous. A non‐exhaustive list would evoke: i) the possibility to associate structural defects (in TEM) with segregations (in APT); ii) associating the morphology of a particle (in electron tomography) with a 3D field of composition (in APT); iii) improving the quality of APT reconstructions by accessing additional information about the specimen morphology in TEM/STEM. This presentation will begin with a rapid overview of the efforts made by the APT users community to promote this approach. Then, some illustrations will be given which relate the correlative investigation on alloys and quantum wells. The possibility to image APT specimens in STEM in high resolution mode (cf. Figure), while enabling atom counting will be discussed on the basis of HAADF‐STEM image simulations. The advantages of the correlation of electron tomography and APT will be highlighted in the case of GaN/AlNGaN quantum wells.