Abstract
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.
