294 Andreas Stoffers et al. The HAADF STEM image can already provide highly
interesting structural details without further extensive simulations. The present Σ3 tilt GB shows nearly perfect structure of a coherent and symmetric twin boundary. The term “coherent” in this context refers to the fact that one atomic layer belongs consistently to both adjacent grains. The Si dumbbells are rotated by 70° and the GB plane is (111) with respect to both crystals. Following the trace of the GB, no change in this nearly perfect symmetry was observed. The atomic structure observed along the Σ9 GB is much
more complex. Its structural motifs are repeated periodically, with a length of its underlying structural units of 1.5 nm, over a distance of >20 nm. A single structural unit is highlighted by the yellow frame. The complex atomic structure makes it difficult to assign a distinct GB plane direction, but in agreement with the expectations from trace analysis performed on the EBSD measurements (not shown here), the GB plane is close to (114). The misorientation of both crystals measured in the HAADF micrograph is 36±1°, whereas a misorientation angle of 39.0±0.5° was deter- mined by means of EBSD. The small deviation from the ideal Σ9 misorientation (38.94°) in the HAADF micrograph falls well within the Brandon criterion (Brandon, 1966), and may in addition be altered by the presence of scan distortions and sample drift. The atomic structure of the present Σ27 GB reveals a
feature to compensate for non-ideal GB properties. Here, the misorientation of the two crystals is 33±1° in the HAADF micrograph and 31.7±0.5° in the EBSD measure- ment, which is close to the ideal misorientation of 31.58°. The global GB plane could not be identified by the trace analysis in this case, indicating that the global GB plane is asymmetric. The HAADF image in Figure 2c confirms the asymmetry and reveals the faceted atomic structure of this interface. A set of nanograins with a diameter of 3 nm and a third orientation is introduced (orange ellipses). This results in dissociation of the Σ27 interface into numerous triangular shaped grains bordered by symmetrical Σ3, Σ3, and Σ9 interfaces: Σ27→Σ3(111) +Σ3(112) +Σ9(122). The corresponding EBIC map in Figure 2a shows a
strong recombination activity at the Σ27 interface, whereas the recombination activity at the Σ3 and Σ9 interfaces are negligible. This difference may be caused by the observed GB structure of the Σ27 interface or by additional impurity segregation. APT analysis of that interface is shown in Figure 2d and reveals a slight C segregation at the position of the GB in the 3D ion map. The presented APT volume was reconstructed on the basis of the tip profile observed in SEM, using the corresponding reconstruction routine in the IVAS® software package. APT analysis of the Σ9 interface was not successful, but previous work on recombination inactive interfaces revealed no impurity segregation (Ohno et al., 2013; Stoffers et al., 2015a). The GB dissociation at the Σ27 interface most probably promotes the observed segregation of C and thus increases the recombination activity. This implies a significant influence of C segregation
on the GB recombination activity, in agreement with our own work (Stoffers et al., 2015a) and other researchers (Pizzini et al., 2005). A second example of a GB analyzed by the correlative
approach is shown in Figures 3–6. In this case the correlative approach was used to study a single Σ9 interface with a higher recombination activity than the Σ27 interface discussed above. Figure 3 shows the EBIC map (a), GB CSL character extracted from the EBSD measurements (b) toge- ther with an SEM/TKD overlay image of the corresponding TEM lamella (c), and APT specimen (d). The grains appear nearly in the same color in the TKD map, as both grains are oriented close to the <110> zone axis. The GB position (white line) is highlighted by the white arrows. The mis- orientation of both crystals measured in the HAADF micrograph in Figure 4 is 38±1°, which is very close to the ideal misorientation of 38.94°. The observed atomic struc- ture in this case is again very complex and shows a strong faceting of numerous Σ9 segments with different GB habit planes. Several straight (114) and (122) facets of varying lengths in the range of 10nm were observed. The interface structure observable in Figure 4 contains a straight (122) segment and a slightly bent segment close to (114), which consists of the same structural atomic motifs as observed in the first example in Figure 2c. Due to the strong curvature of the GB plane, the atomic structure is heavily distorted and no periodicity similar to the previous example (Fig. 2) was observed, although the same structural units can be found along the whole GB. The corresponding APT measurement is presented in Figures 5 and 6. The observed impurity decoration follows the GB interface, which consists of two segments within this APT specimen. Figure 5a represents the complete APT measurement viewed along the common GB
plane direction, where only C and Si were plotted. The wide field of view and high detection efficiency of the LEAP5000 XS system enables the clear observation of multiple crystal- lographic poles on the detector, as can be seen on the detector density maps in Figures 5b and 5c for the blue and red plane shown in (a). The (113) pole in the upper grain and the (111) pole in the lower grain were selected for optimization of the reconstruction parameters. Small volumes corresponding to the dashed circles were recon- structed to confirm correct lattice plane distances, as can be seen in Figures 5d and 5e. In Figure 6, the 3D ion maps of the volume around the
GB are plotted for the detected impurity species. These 3D maps nicely demonstrate the preferential segregation of different impurity species at the facets: Fe andNare detected in form of the complex FeN2+ at m/z = 35Da and decorate both interface segments. The detection as a complex ion does not necessarily imply that these species are segregating in a molecular form. Rather, it is likely that they are segregating individually. However, the atomic species Fe and N were not observed in the mass spectra, most probably due to peak overlap of Fe2+ with Si+ (28 Da) and N+ with Si2+ (14 Da). Therefore, the given (Fe+N) concentration should be taken as a lower limit of the effectively segregated amount.
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