Effects of small specimen tilt and probe convergence angle on ADF-STEM image contrast of Si0.8Ge0.2 epitaxial strained layers on (100) Si

The effects of specimen tilt and probe convergence angle on annular dark field (ADF) image contrast of Si0.8Ge0.2 heteroepitaxial strained layers on (100) Si were investigated in a 200 kV scanning transmission electron microscope (STEM) for a TEM specimen thickness of 195 nm. With 0.5 degrees of specimen tilt away from the exact 〈011〉 zone-axis orientation, the signal-to-noise level of atomic columns was significantly reduced for both Si0.8Ge0.2 and Si in high resolution ADF-STEM lattice images. When the specimen was tilted 0.5 degrees around the 〈011〉 axis, or the STEM probe convergence semiangle was reduced from 14.3 to 3.6 mrad, the ADF-STEM image intensity profiles across the Si0.8Ge0.2 and Si layers changed significantly as compared to those obtained at the exact 〈011〉 zone axis orientation, and no longer reflected the composition changes occurring across the layer structure. Multislice image simulation results revealed that the misfit strain between the Si0.8Ge0.2 and Si layers, and strain relaxation near the surface of the TEM specimen, were responsible for the observed changes in image intensity.