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A Bloch wave analysis of optical sectioning in aberration-corrected STEM
1School of Physics, Trinity College Dublin, Dublin 2, Ireland.
Ultramicroscopy
|February 20, 2007
Summary
Optical depth sectioning in aberration-corrected STEM uses reduced focal depth for 3D information. Bloch wave calculations reveal intensity variations in crystals due to electron states and wave interference, complicating depth interpretation.
Area of Science:
- Materials Science
- Solid-State Physics
- Electron Microscopy
Background:
- Aberration-corrected STEM utilizes larger apertures, reducing focal depth of field.
- This reduction enables optical depth sectioning for 3D information retrieval.
- Understanding electron intensity distribution within crystals is crucial for this technique.
Purpose of the Study:
- To explore depth sectioning in zone-axis crystals using Bloch wave calculations.
- To explain the depth-dependent electron intensity within crystals.
- To identify factors causing deviations in intensity maxima from expected defocus values.
Main Methods:
- Utilizing Bloch wave calculations for depth sectioning analysis.
- Decomposing calculations into contributions from individual states and partial plane waves.
- Analyzing electron intensity as a function of depth within crystals.
Main Results:
- Identified two effects causing intensity maximum deviation from defocus values.
- Unbound, high-angle excited states exhibit prefocusing due to atomic column lensing.
- Interference between the 1s state and the rest of the wavefunction causes oscillations.
Conclusions:
- The observed prefocusing and oscillatory effects complicate the interpretation of optical sectioning data.
- These effects can prevent intensity maxima formation at certain crystal depths.
- Further research is needed to accurately interpret 3D information from STEM optical sectioning.
