Electron tomography of (In,Ga)N insertions in GaN nanocolumns grown on semi-polar (11-22) GaN templates
Imperfections of crystalline semiconductor materials (microstructure) affect the physical properties of opto-electronic devices. Nanoanalytics aims at the understanding of these structure-property relations on a nanometer scale. For a long time, the analysis of cross-sections through devices by transmission electron microscopy (TEM) has reliably revealed crystalline defects in projected images of the specimen. Nowadays, devices do not only shrink but they are also changed by morphological design. Originally, 2D grown semiconductor layers have been grown thinner and thinner. Recent challenges concern the fabrication of 3D structures, e.g. nanocolumns (NCs), covered by thin shells. The increasing requirements for a three-dimensional analysis and understanding of such material systems and their microstructure necessitate the application of tomography in TEM. Moreover, the prerequisite of electron transparent specimen for TEM (thickness in the order of 100 nm) demands an advanced preparation method. The dual-beam microscope comprising a focused ion beam (FIB) and a scanning electron microscope (SEM) enables the production of specimen with a needle-like shape. Such specimen allow the acquisition of projected images over a large tilt-range which are needed for the tomographic reconstruction. A versatile sample stage and an in-situ micromanipulator within the FIB-SEM help to overcome former geometric constrains of conventional sample preparation. We paradigmatically demonstrate the capability of transmission electron tomography for a case of GaN nanocolumns (NCs). The NCs have been grown on a prepatterned, semi-polar (11-22) GaN template by molecular beam epitaxy (MBE). Resulting 3D structures exhibit a preferred  growth direction which is inclined towards the surface normal (Figure 1(a)). The inclusion of (In,Ga)N for opto-electronic applications has been one goal of the growth study. The geometrical complex situation due to the inclination of the NCs or the low symmetry of the substrate surface, respectively, impedes the analysis of the (In,Ga)N distribution from conventional cross-section or plan-view samples. Consequently, a lamella has been prepared with the FIB-SEM device. The  direction has been systematically aligned with the tilt axis for the tomographic image acquisition. Afterwards the lamella has been trimmed with the ion beam to obtain an electron transparent specimen. The tilt-series has been recorded with the high-angle annular dark-field (HAADF) detector in the scanning transmission electron microscopy (STEM) mode. This signal is sensitive to the mean atomic number of the probed specimen volume. Results of the 3D reconstruction of a NC from this data set are presented in Figures 1 and 2. The isosurface representation 3D reconstruction reveals the outer morphology of the NC (Figure 1 left column). The non-uniform development of symmetrically equivalent facets is apparent. The rough surface parallel to the (0001) capping plane is well resolved. The right column shows the shape of the (In,Ga)N inclusion (red). The outer GaN shell of the NC appears semitransparent and (blue). The NC morphology is adapted by the inclusion except for the part that is leaned toward the substrate surface. Slices through the reconstructed volume (Figure 2) offer a more detailed insight into the (In,Ga)N occurrence. The amount of indium differs in layers parallel to different lattice planes as does the layer thickness. On the one hand, the incorporation of indium and the growth velocity differs for different crystallographic planes. On the other hand, the anisotropic character of material supply in MBE causes variations of the layer thickness even on symmetrically equivalent planes. Eventually, the unique capability of electron tomography allowed to reveal the core-shell like (In,Ga)N inclusion on inclined GaN NCs with a cap parallel to the (0001) plane and with the limitation that the shell is not closed towards the substrate due to the shadowing of the molecular beam.
Figure 1 - Isosurface visualization: the opaque representation on the left column illustrates the morphology of the nanoobject. The right column reflects the red opaque (In,Ga)N shell that resembles the outer morphology which is presented semitranparent in this montage of two isosurfaces.
Figure 2 - Slices through the reconstructed volume: the position of the slices 1-6 is marked in the isosurface representations on the left.