Strain, composition and disorder in ADF imaging of semiconductors

Element specific atom counting for heterogeneous nanostructures: Combining multiple ADF STEM images for simultaneous thickness and composition determination

In this paper, a methodology is presented to count the number of atoms in heterogeneous nanoparticles based on the combination of multiple annular dark field scanning transmission electron microscopy (ADF STEM) images. The different non-overlapping annular detector collection regions are selected based on the principles of optimal statistical experiment design for the atom-counting problem. To count the number of atoms, the total intensities of scattered electrons for each atomic column, the so-called scattering cross-sections, are simultaneously compared with simulated library values for the different detector regions by minimising the squared differences. The performance of the method is evaluated for simulated Ni@Pt and Au@Ag core-shell nanoparticles. Our approach turns out to be a dose efficient alternative for the investigation of beam-sensitive heterogeneous materials as compared to the combination of ADF STEM and energy dispersive X-ray spectroscopy.

Influence of plasmon excitations on atomic-resolution quantitative 4D scanning transmission electron microscopy

Scanning transmission electron microscopy (STEM) allows to gain quantitative information on the atomic-scale structure and composition of materials, satisfying one of todays major needs in the development of novel nanoscale devices. The aim of this study is to quantify the impact of inelastic, i.e. plasmon excitations (PE), on the angular dependence of STEM intensities and answer the question whether these excitations are responsible for a drastic mismatch between experiments and contemporary image simulations observed at scattering angles below \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim $$\end{document}∼ 40 mrad. For the two materials silicon and platinum, the angular dependencies of elastic and inelastic scattering are investigated. We utilize energy filtering in two complementary microscopes, which are representative for the systems used for quantitative STEM, to form position-averaged diffraction patterns as well as atomically resolved 4D STEM data sets for different energy ranges. The resulting five-dimensional data are used to elucidate the distinct features in real and momentum space for different energy losses. We find different angular distributions for the elastic and inelastic scattering, resulting in an increased low-angle intensity (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim $$\end{document}∼ 10–40 mrad). The ratio of inelastic/elastic scattering increases with rising sample thickness, while the general shape of the angular dependency is maintained. Moreover, the ratio increases with the distance to an atomic column in the low-angle regime. Since PE are usually neglected in image simulations, consequently the experimental intensity is underestimated at these angles, which especially affects bright field or low-angle annular dark field imaging. The high-angle regime, however, is unaffected. In addition, we find negligible impact of inelastic scattering on first-moment imaging in momentum-resolved STEM, which is important for STEM techniques to measure internal electric fields in functional nanostructures. To resolve the discrepancies between experiment and simulation, we present an adopted simulation scheme including PE. This study highlights the necessity to take into account PE to achieve quantitative agreement between simulation and experiment. Besides solving the fundamental question of missing physics in established simulations, this finally allows for the quantitative evaluation of low-angle scattering, which contains valuable information about the material investigated.

Insights into image contrast from dislocations in ADF-STEM

Competitive mechanisms contribute to image contrast from dislocations in annular dark-field scanning transmission electron microscopy (ADF-STEM). A clear theoretical understanding of the mechanisms underlying the ADF-STEM contrast is therefore essential for correct interpretation of dislocation images. This paper reports on a systematic study of the ADF-STEM contrast from dislocations in a GaN specimen, both experimentally and computationally. Systematic experimental ADF-STEM images of the edge-character dislocations reveal a number of characteristic contrast features that are shown to depend on both the angular detection range and specific position of the dislocation in the sample. A theoretical model based on electron channelling and Bloch-wave scattering theories, supported by numerical simulations based on Grillo's strain-channelling equation, is proposed to elucidate the physical origin of such complex contrast phenomena.

Atomic-scale 3D reconstruction of antiphase boundaries in GaP on (001) silicon by STEM

In order to overcome the limitations of silicon-based electronics, the integration of optically active III-V compounds is a promising approach. Nonetheless, their integration is far from trivial and control as well as understanding of corresponding growth kinetics, and in particular the occurrence and termination of antiphase defects, is of great relevance. In this work, we focus on the three-dimensional reconstruction of such boundaries in gallium phosphide from single scanning transmission electron microscopy images. In the high angle annular dark-field imaging mode, the appearance of these antiphase boundaries is strongly determined by the chemical composition of each atomic column and reflects the ratio of transmitted anti- to mainphase. Therefore it is possible to translate measured intensities to the depth location of these boundaries by utilizing simulation data. The necessary spatial resolution for these column-by-column mappings is achieved via electron optical aberration correction within the microscope. Hence, the complete 3D orientation of these defects can be measured at atomic resolution and correlated to growth parameters. Finally, we present a method to reconstruct large areas from well sampled images and retrieve information about complex embedded nanoscale structures at the atomic scale.

Composition determination of multinary III/V semiconductors via STEM HAADF multislice simulations

Knowledge of the microscopic elemental composition of multinary III/V semiconductor materials is crucial to the development of functionalized opto-electronic devices. Well-proven composition analysis methods, such as high resolution X-ray diffraction (HRXRD), fail to determine the elemental composition when more than three atomic species are involved. In this work we propose a procedure for the composition analysis of multinary III/V semiconductors at atomic resolution using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image simulations. Our method exploits the dependence of HAADF-STEM image intensities on the atomic number and static atomic displacements (SAD) at different detector inner angles. Here, we describe the proposed method in detail using Ga(NAsP) as an example multinary material.

Influence of surface relaxation of strained layers on atomic resolution ADF imaging

Surface relaxation of thin transmission electron microscopy (TEM) specimens of strained layers results in a severe bending of lattice planes. This bending significantly displaces atoms from their ideal channeling positions which has a strong impact on the measured annular dark field (ADF) intensity. With the example of GaAs quantum wells (QW) embedded in a GaP barrier, we model the resulting displacements by elastic theory using the finite element (FE) formalism. Relaxed and unrelaxed super cells served as input for state of the art frozen phonon simulation of atomic resolution ADF images. We systematically investigate the dependencies on the sample´s geometric parameters, i.e. QW width and TEM sample thickness, by evaluating the simulated intensities at the atomic column´s positions as well as at the background positions in between. Depending on the geometry the ADF intensity can be affected in a range several nm from the actual interface. Moreover, we investigate the influence of the surface relaxation on the angular distribution of the scattered intensity. At high scattering angles we observe an intensity reduction at the interface as well as in the GaP barrier due to de-channeling. The amount of intensity reduction at an atomic column is directly proportional to its mean square displacement. On the contrary we find a clearly increased intensity at low angles caused by additional diffuse scattering. We discuss the implications for quantitative evaluations as well as strategies to compensate for the reduced intensities.

Materials characterisation by angle-resolved scanning transmission electron microscopy

Solid-state properties such as strain or chemical composition often leave characteristic fingerprints in the angular dependence of electron scattering. Scanning transmission electron microscopy (STEM) is dedicated to probe scattered intensity with atomic resolution, but it drastically lacks angular resolution. Here we report both a setup to exploit the explicit angular dependence of scattered intensity and applications of angle-resolved STEM to semiconductor nanostructures. Our method is applied to measure nitrogen content and specimen thickness in a GaN_xAs_1-x layer independently at atomic resolution by evaluating two dedicated angular intervals. We demonstrate contrast formation due to strain and composition in a Si- based metal-oxide semiconductor field effect transistor (MOSFET) with Ge_xSi_1-x stressors as a function of the angles used for imaging. To shed light on the validity of current theoretical approaches this data is compared with theory, namely the Rutherford approach and contemporary multislice simulations. Inconsistency is found for the Rutherford model in the whole angular range of 16-255 mrad. Contrary, the multislice simulations are applicable for angles larger than 35 mrad whereas a significant mismatch is observed at lower angles. This limitation of established simulations is discussed particularly on the basis of inelastic scattering.

STEM_CELL: a software tool for electron microscopy: part 1--simulations

The software STEM_CELL, here presented, is a useful tool for (S) TEM simulation. In particular innovative solutions are presented in (1) the supercell manipulation and parameters setting (2) simulation execution through the modified Kirkland routines (3) simulation post-processing with extended output and comprehensive graphic tools (4) image contrast interpretation through a strain channeling equation accounting for strain effects in STEM-ADF.