Thermal vibrations in simulated ADF STEM

Abstract

Thermal vibrations reduce Bragg scattering, create a thermal diffuse background, and disrupt channeling, all of which may affect the ADF STEM signal. There are claims that thermal vibrations can change the relative contrast of different elements in the ADF STEM image, which is different from the simple Z- contrast interpretation. Evaluating the effects of thermal vibrations may be necessary for correct interpretation of experimental results. We present an investigation of ADF STEM imaging including thermal vibrations based on the frozen phonon model.Thermal vibrations can be included in the multislice calculation with a Monte Carlo technique. Since, the interaction time for 100 keV electrons is ≤ 10-4 atomic vibration periods, the atoms may be considered stationary. The time between successive electrons in the STEM is ≥ 102 vibration periods, so the atomic positions seen by different electrons are uncorrelated. The technique is to offset every atom in the specimen (many slices) by a small random amount perpendicular to the beam before performing a standard multislice calculation. A set of random offsets freezes one phonon configuration into the specimen. The calculation is then repeated and averaged over an ensemble of different configurations.