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High order phase contrast and source divergence in low energy electron microscopy.
Lei Yu1, Weishi Wan2, Ka Man Yu3
1College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China.
Ultramicroscopy
|April 19, 2021
Summary
High order phase contrast in aberration corrected low energy electron microscopy (AC-LEEM) reveals new interference fringes. This study quantifies AC-LEEM source divergence, a key parameter for microscopy imaging.
Area of Science:
- Materials Science
- Surface Science
- Electron Microscopy
Background:
- Aberration corrected low energy electron microscopy (AC-LEEM) is a powerful surface imaging technique.
- Phase contrast in LEEM typically involves first-order interference fringes.
Purpose of the Study:
- To report the observation of high order phase contrast in AC-LEEM.
- To investigate the dependence of these fringes on experimental parameters.
- To quantitatively determine the source divergence of AC-LEEM.
Main Methods:
- Experimental observation of phase contrast using AC-LEEM on a Ag (111) surface.
- Comparison of experimental results with Fourier optics (FO) model predictions.
- Exploitation of source divergence effects on fringe amplitudes for quantitative measurement.
Main Results:
- Observation of previously unreported high order interference fringes in AC-LEEM phase contrast.
- Demonstration of the dependence of these fringes on defocus and incident electron energy.
- Quantitative determination of field emission source divergence (0.055 ± 0.005 mrad) in AC-LEEM.
Conclusions:
- High order phase contrast provides new insights into AC-LEEM imaging.
- Source divergence significantly impacts fringe amplitudes at large defocus.
- This work establishes a method for characterizing a critical AC-LEEM instrumental parameter.
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