Quantitative composition evaluation from HAADF-STEM in GeSi/Si heterostructures

Abstract

High-angle annular dark field scanning transmission electron microscopy has been successfully used for composition evaluation in various material systems. In this work, the quantitative applicability of this method to GeSi/Si heterostructures was studied. Reference images were simulated by frozen lattice multislice simulations for different Ge concentrations accounting for static atomic displacements and biaxial strain due to pseudomorphic growth. Specimen thickness and composition are obtained by comparison of simulated and normalised experimental intensities. The measured thickness of a pure Si wedge specimen is compared to thickness determined from Pendellösung fringes in dark field micrographs. The deviation is below 10 nm coinciding with the accuracy of prior works. The composition of a GeSi-layer structure was measured in a calibration sample of known concentration and good agreement is found. Two-dimensional concentration maps of a GeSi/Si transistor structure were created. Measured concentrations agree with nominal values. However, strain fields in the Si lead to a variation of the image intensity causing an artificial fluctuation of the measured concentrations of ±4%.