Using molecular dynamics for multislice TEM simulation of thermal diffuse scattering in AlGaN.

Area of Science:

• Materials Science
• Computational Physics
• Solid-State Chemistry

Background:

• Accurate simulation of transmission electron microscopy (TEM) images requires precise modeling of thermal diffuse scattering.
• The frozen phonon multislice algorithm necessitates realistic quantum mechanical distributions for thermal displacements.
• For alloys, the Einstein approximation is often used but neglects crucial atomic correlations.

Purpose of the Study:

• To introduce and validate molecular dynamics (MD) simulations as an alternative to the Einstein approximation for calculating atomic displacements in alloys.
• To develop and apply an empirical potential for AlGaN alloys suitable for MD simulations.
• To assess the accuracy of MD-based TEM simulations compared to density functional theory (DFT) and Einstein approximation methods.

Main Methods:

• Development of an empirical Stillinger-Weber type potential for AlGaN by fitting to DFT-derived force constants and elastic constants.
• Classical molecular dynamics simulations to approximate quantum mechanical dynamics and capture atomic correlations.
• Application of MD-based displacements in the multislice algorithm for conventional and scanning TEM image simulations.

Main Results:

• The devised empirical potential accurately reproduces phonon dispersions and atomic displacement expectations from DFT calculations for AlGaN.
• MD simulations incorporating atomic correlations show excellent agreement with DFT-based TEM image calculations.
• A significant deviation was observed between MD and Einstein approximation results, highlighting the importance of included correlations.

Conclusions:

• Molecular dynamics simulations with the developed empirical potential provide a more accurate method for TEM image simulation of AlGaN alloys.
• The MD approach inherently includes atomic correlations and static displacements, leading to improved simulation fidelity over the Einstein approximation.
• This validated potential serves as a valuable tool for precise TEM analysis of complex alloy systems.