Thickness-dependent annular dark-field STEM

Abstract

The demonstration of Z-contrast atom structure images and high spatial resolution using annular dark field STEM has drawn considerable interest. This approach is well suited for quantitative image and diffraction pattern acquisition. Experimental data has been used to validate calculations of convergent beam electron diffraction (CBED) patterns in silicon and ADF STEM images of (100) InP. These procedures can be extended to explore the dependence of images on factors such as thickness.Energy filtered CBED patterns over an angular range of over 150 mrads and covering a range of over four orders of magnitude in intensity from silicon were recorded with a VG HB501A STEM for thicknesses of 217Å, 326Å and 543Å. Simulation using a “frozen phonon” approximation successfully modeled this data provided an accurate phonon amplitude was used. A similar quantitative study for images has been carried out for (100) oriented InP. This sample contains indium columns at the comer of a 2.9Å square and phosphorus atoms at the square center.