Characterization of Grain Boundary Geometry in the TEM, exemplified in Si thin films

It has been long observed that phenomena associated with grain boundaries (GB) in metals (e.g. corrosion, energy, segregation, etc) are influenced by the grain boundary geometry. For many years the coincidence site lattice (CSL) model, which describes the grain boundary in terms of the misorientation between neighbouring grains, has been a cornerstone of grain boundary research and formation of special boundaries with low-Σ (assumed to result in better material properties) was pursued. Further work subsequently indicated that a low-Σ CSL was a necessary but not sufficient criterion for specialness, but the interpretation of similar results is complicated by the effect of impurities on boundary energies. The structure of the GB is more related to the orientation of the GB plane (planar density of coincidence sites), which is difficult to determine experimentally. Bicrystals with known misorientation and BG plane indices were artificially fabricated and studied both by HRTEM and other characterization methods from the 80’s. Structural characterization (including indexing the GB plane) in real-life polycrystalline materials proved to be more challenging. One approach involves following the change in the location of GB traces in a scanning electron microscope (SEM) while thin layers of material are removed mechanically (either by polishing or by focused ion beam, FIB). The other approach reported here, records the projections of the GB at two different tilt values in a transmission electron microscope (TEM) and deduces the orientation of the GB plane from them.