Effects of energy filtering in LACBED patterns

Abstract

Electron energy loss spectra (EELS) from 250 keV large-angle convergent beam electron diffraction (LACBED) patterns of InP, using either 2 or 5 μm selected-area diffraction apertures, are recorded as a function of vertical displacement Δ z of the specimen from the eucentric height. These parameters define the effective collection semi-angle α for the EELS spectrum, varied in this set of experiments from 0.05 to 3 mrad. Mean free paths λ p from a plasmon model as a function of α correlate well with measurements of zero-loss to plasmon-loss intensities I 0/ I p, as does thickness t from EELS data with diffraction measurements. It is also shown that a plot of measured α versus a theoretical value α th, calculated from I 0/ I p and t, yields a linear relationship. An Einstein model for thermal diffuse scattering (TDS) is used to calculate the effective mean free path λ TDS that contributes to the LACBED pattern. This contribution is greatly reduced by small collection angles, and we provide estimates for idealized kinematic and “worst case” conditions that may be expected to operate. The background TDS contribution to elastic scattering contrast when using LACBED mode may be reduced by several orders of magnitude in comparison with standard CBED techniques, where no angular filtering of inelastically scattered electrons occurs.