Effects of amorphous layers on ADF-STEM imaging

Area of Science:

• Materials Science
• Electron Microscopy
• Solid-State Physics

Background:

• High-resolution imaging in scanning transmission electron microscopy (STEM) is crucial for materials characterization.
• Amorphous surface layers can degrade image quality by scattering electrons.
• The impact of amorphous layers on aberration-corrected STEM imaging requires further investigation.

Purpose of the Study:

• To investigate the effect of amorphous surface layers on high-resolution annular dark-field (ADF) imaging in STEM.
• To compare the influence of these layers in both uncorrected and aberration-corrected STEM systems.
• To understand how crystal structure, orientation, and atomic composition affect image contrast in the presence of amorphous layers.

Main Methods:

• Multislice simulation was employed to model high-resolution ADF imaging.
• Simulations were performed for both uncorrected and aberration-corrected STEM probes.
• The study analyzed the propagation of electron beams through amorphous layers and their interaction with crystalline specimens.

Main Results:

• Amorphous layers significantly reduce the visibility of atomic columns in ADF STEM images.
• Aberration-corrected STEM requires only 60 Å of amorphous material to reduce visibility, compared to 200 Å for uncorrected STEM.
• An amorphous layer at the beam entry surface has a greater impact on the ADF image than one at the exit surface.
• Image contrast reduction is approximately linear with increasing amorphous layer thickness.

Conclusions:

• Amorphous layers pose a significant challenge for high-resolution ADF STEM imaging, particularly in uncorrected systems.
• Aberration correction substantially mitigates the detrimental effects of amorphous layers, enabling imaging with thinner surface contamination.
• Understanding the influence of amorphous layers is critical for accurate interpretation of STEM images of real-world crystalline materials.