Microsc. Microanal. 23, 291–299, 2017 doi:10.1017/S1431927617000034
© MICROSCOPY SOCIETY OF AMERICA 2017
Correlating Atom Probe Tomography with Atomic-Resolved Scanning Transmission Electron Microscopy: Example of Segregation at Silicon Grain Boundaries
Andreas Stoffers,1,2,* Juri Barthel,3,4 Christian H. Liebscher,2 Baptiste Gault,2 Oana Cojocaru-Mirédin,1,2 Christina Scheu,2 and Dierk Raabe2
1Institute of Physics (IA), RWTH Aachen University, Otto-Blumenthal-Straße, 52074 Aachen, Germany 2Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany 3Central Facility for Electron Microscopy, RWTH Aachen University, Ahornstraße 55, 52074 Aachen, Germany 4Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
Abstract: In the course of a thorough investigation of the performance-structure-chemistry interdependency at silicon grain boundaries, we successfully developed a method to systematically correlate aberration-corrected scanning transmission electron microscopy and atom probe tomography. The correlative approach is conducted on individual APT and TEM specimens, with the option to perform both investigations on the same specimen in the future. In the present case of a Σ9 grain boundary, joint mapping of the atomistic details of the grain boundary topology, in conjunction with chemical decoration, enables a deeper understanding of the segregation of impurities observed at such grain boundaries.
Key words: correlative microscopy, atom probe tomography, scanning transmission electron microscopy, grain boundary, silicon
INTRODUCTION
Macroscopic properties of materials depend on structure and chemical composition at the atomic scale. Thus, knowledge of the exact atomistic structure, and the corresponding compositional state of interfaces, is the ambitious goal of many researchers in materials science who seek deeper understanding of corresponding structure- chemistry-performance interdependencies (Kuzmina et al., 2015). In the course of investigating these relations in multicrystalline silicon (mc-Si), we have developed a method to correlate high-resolution scanning transmission electron microscopy (HRSTEM) with atom probe tomography (APT). Both techniques have already been used individually to analyze the atomic structure of Si grain boundaries (GBs) (Couillard et al., 2013) and the distribution of impurities and dopants in Si (Thompson et al., 2005, 2007a; Cojocaru- Mirédin et al., 2009; Philippe et al., 2009; Blavette et al., 2014). Several early and also advanced approaches already exist to correlate results from APT and transmission electron microscopic (TEM) measurements (Thuvander et al., 1996; Herbig et al., 2014; Rigutti et al., 2014; Kuzmina et al., 2015; Lefebvre et al., 2015; Weber et al., 2016), however, the direct and joint mapping of atomically resolved STEM information and APT data of an interface has not been yet demonstrated. Mc-Si is still the dominant absorber material used for solar cell fabrication. Compared with monocrystalline Si, it
*Corresponding author.
stoffers@mpie.de Received June 29, 2016; accepted December 26, 2016
contains a high amount of impurities and defects, such as dislocations and GBs, increasing the recombination activity and thus reducing cell efficiency (Kveder et al., 2001; Di Sabatino & Stokkan, 2013). We recently published chemical details of different recombination active GBs using joint electron beam induced current (EBIC) and electron backscatter diffraction (EBSD) experiments and APT (Stoffers et al., 2015a). In that case, we were only able to estimate the details of the GB structure, such as coherency at the micron scale, based on the EBSD data alone, neglecting details at the nano- or atomic scale. However, what the atomic arrangements actually look like for the different GB types and how these structures influence the impurity segregation are still unanswered questions. In a subsequent study, we have already resolved the high complexity of the atomic structure of one of the currently studied GBs by HRSTEM (Stoffers et al., 2015b). We will discuss in this manuscript the combination of APT and HRSTEMand how this approach promotes the interpretation of the APT data.
MATERIALS
Mc-Si ingots were grown by directional solidification in a 200mm crucible. In order to study the influence of the most detrimental transition metals, the ingots were intentionally contaminated with 20ppm Fe and 20ppm Cu during the solidification process to increase the driving force for GB segregation (Riepe et al., 2011; Stoffers et al., 2015a). Samples analyzed in the present study were taken from a thin
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