Depth sectioning of aligned crystals with the aberration-corrected scanning transmission electron microscope

The implementation of aberration correction for the scanning transmission electron microscope (STEM) enables the use of larger probe-forming apertures, improving the transverse resolution significantly and also bringing depth resolution at the nanometer scale. This opens up the possibility of three-dimensional imaging by optical sectioning, and nanometer-scale depth resolution has been demonstrated for amorphous and off-axis samples. For crystalline materials it is usual to image in a zone axis orientation to achieve atomic resolution. In this case, the tendency for the beam to channel along the columns complicates the simple optical sectioning technique. Here we conduct a series of simulations which demonstrate that higher beam convergence angles available in next generation aberration correctors can overcome this limitation. Detailed simulations with realistic values for residual aberrations predict nanometer-scale depth resolution for Bi dopant atoms in Si (110) for an instrument corrected up to fifth order. Use of a monochromator appears to significantly improve the depth resolution.