Composition and strain contrast of Si1−xGex (x=0.20) and Si1−yCy (y≤0.015) epitaxial strained films on (100) Si in annular dark field images

Abstract

The annular dark field (ADF) image contrast of Si1−xGex (x=0.20) and Si1−yCy (y≤0.015) strained epitaxial films on (100) Si is investigated in a 200 kV scanning transmission electron microscope (STEM) with ADF detector inner semiangles ranging from 26 to 92 mrad. For the Si1−xGex/Si system, the contrast increases with increasing ADF detector semiangle and decreases with increasing thickness to remain almost constant after the TEM sample thickness reaches 120 nm. The opposite trend is seen in the Si1−yCy/Si system, where the contrast increases with increasing sample thickness and decreases with increasing ADF detector semiangle, and remains almost constant after the sample thickness exceeds 200 nm. In the dilute Si1−yCy/Si system the lower average atomic number strained Si1−yCy layers are brighter than the higher average atomic number Si for an ADF detector semiangle of up to 92 mrad when the sample thickness is greater than 200 nm. This anomalous contrast dependence is also observed for an ADF detector semiangle of up to 50 mrad in thinner TEM samples. The observed ADF-STEM image contrast is explained in relation to the atomic scattering and multislice simulations. The normalized intensity line scan profiles of ADF-STEM images coincide with the composition profiles determined from analytical TEM techniques: energy dispersive x-ray spectroscopy and electron energy loss spectroscopy, as well as secondary ion mass spectrometry.