High-angle Annular Dark-field Scanning Transmission Electron Microscopy Images Research Articles

Determination of Thickness for the Individual Layer of Strained P-Type Al<sub>0.11</sub>Ga<sub>0.89</sub>NGaN Superlattice by High Angle Annular Dark Field Scanning Transmission Electron Microscopy

AlGaN/GaN superlattice are the important structure of optoelectronic devices such as light-emitting diodes and laser diodes. The nanostructure of the superlattice can greatly influences the optical electrical properties of final LD and LED . It is impossible to evaluate their thickness by TEM when it’s aluminum component lower than 14%.We investigated the nanostructure of strained p-type Al0.11Ga0.89N/GaN superlattice grown on sapphire by high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM).By an average filtering technique, the thickness of the AlGaN layers and GaN layers were determined to be 2.409±0.092 nm and 2.371±0.062 nm from the HAADF-STEM images, respectively.

Morphology and optical properties of single- and multi-layer InAs quantum dots

An understanding of the structural and optical properties of quantum dots (QDs) is critical for their use in optical communication devices. In this study, single- and multi-layer self-organized InAs QDs grown on (001) GaAs substrates by molecular beam epitaxy (MBE) were investigated. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images show that the lateral size of multi-layer InAs QDs are larger and flatter than single-layer InAs QDs, which are oval-shaped. The change in shape and size may be attributed to the presence of InGaAs spacer layers in multi-layer InAs QDs. Reciprocal spacer mapping and fast Fourier transformation images clearly show that InGaAs spacer layers present in the multi-layer InAs QDs structures help to release the strain originally existing in the QDs. In addition, the photoluminescence peak of the multi-layer InAs QDs is broader than QD in the single-layer one, which implies that the multi-layer InAs QDs size variation is more random than the single-layer one and this corresponds with the HAADF-STEM images. These results prove that spacer layers release strain influencing the morphology and optical properties of the QDs.

The Formation and Utility of Sub-Angstrom to Nanometer-Sized Electron Probes in the Aberration-Corrected Transmission Electron Microscope at the University of Illinois

We evaluate the probe forming capability of a JEOL 2200FS transmission electron microscope equipped with a spherical aberration (Cs) probe corrector. The achievement of a real space sub-Angstrom (0.1 nm) probe for scanning transmission electron microscopy (STEM) imaging is demonstrated by acquisition and modeling of high-angle annular dark-field STEM images. We show that by optimizing the illumination system, large probe currents and large collection angles for electron energy loss spectroscopy (EELS) can be combined to yield EELS fine structure data spatially resolved to the atomic scale. We demonstrate the probe forming flexibility provided by the additional lenses in the probe corrector in several ways, including the formation of nanometer-sized parallel beams for nanoarea electron diffraction, and the formation of focused probes for convergent beam electron diffraction with a range of convergence angles. The different probes that can be formed using the probe corrected STEM opens up new applications for electron microscopy and diffraction.

Through-focal HAADF-STEM of buried nanostructures

High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) in combination with strain mapping techniques provides a powerful tool for quantitative analysis of crystalline semiconductor materials. Due to the complex interaction of a focused probe and a sample in HAADF, the calculation of each pixel in a simulation process requires a complete multislice iteration, making the overall computing process a rather demanding task in time and memory. SICSTEM is a parallel software code recently developed for running on the University of Cadiz Supercomputer (3.75 Tflops) that allows the simulation of images from large nanostructures containing more than one million atoms. The software has been designed to be able to generate not only one dimensional line scans or two dimensional images, but also to perform optical sectioning in the STEM simulation process, providing an easy way to simulate 3D HAADF-STEM images. In this work we consider GaAs capped GaSb nanostructures epitaxially oriented on a GaAs substrate. A methodology has been developed by combining the through-focal series STEM imaging and image analysis to estimate shape and position of buried GaSb nanostructures.

Effect of convergent beam semiangle on image intensity in HAADF STEM images

In this study, we experimentally and theoretically show that the intensities of bright spots in a spherical aberration (C(s))-uncorrected high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) image of [011]-oriented Co(3)O(4), which has two different numbers of Co atoms in the projected atomic columns, are reversed with increasing sample thickness. However, C(s)-corrected HAADF STEM images produce intensities that correctly depend on the average number of atoms in the projected atomic columns. From an analysis based on the Bloch-wave theorem, it is found that an insufficient semiangle of the incident convergent beam yields intensities that do not depend on the average atomic number in the atomic columns.

Metal Nanoparticle−Block Copolymer Composite Assembly and Disassembly

Ligand-stabilized platinum nanoparticles (Pt NPs) were self-assembled with poly(isoprene-block-dimethylaminoethyl methacrylate) (PI-b-PDMAEMA) block copolymers to generate organic-inorganic hybrid materials. High loadings of NPs in hybrids were achieved through usage of N,N-di-(2-(allyloxy)ethyl)-N-3-mercaptopropyl-N-3-methylammonium chloride as the ligand, which provided high solubility of NPs in various solvents as well as high affinity to PDMAEMA. From NP synthesis, existence of sub-1 nm Pt NPs was confirmed by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images. Estimations of the Pt NP ligand head group density based on HAADF-STEM images and thermogravimetric analysis (TGA) data yielded results comparable to what has been found for alkanethiol self-assembled monolayers (SAMs) on flat Pt {111} surfaces. Changing the volume fraction of Pt NPs in block copolymer-NP composites yielded hybrids with spherical micellar, wormlike micellar, lamellar and inverse hexagonal morphologies. Disassembly of hybrids with spherical, wormlike micellar, and lamellar morphologies generated isolated metal-NP based nano-spheres, cylinders and sheets, respectively. Results suggest the existence of powerful design criteria for the formation of metal-based nanostructures from designer blocked macromolecules.

HAADF‐STEM analysis of layered double perovskite La2CuSnO6 grown epitaxially

Structural observation of layered double perovskite oxide La(2)CuSnO(6) thin films grown epitaxially on SrTiO(3) is reported by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Particularly the transition layer at the interface was observed, and the first B site layer at the interface was found to be almost formed by the Cu atomic layer as the random structure, followed by formation of the layered structure. In addition, HAADF-STEM images indicate that the thin film is not single crystalline, but some irregular structures were observed to grow around the interface near atomic steps of the substrate of SrTiO(3). Therefore, the steps largely affect the growth process of the thin film.

Atomic scale high-angle annular dark field STEM analysis of the N configuration in dilute nitrides of GaAs

While high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) has been successfully used for the analysis of heavy atoms in a lighter matrix, the detection of light atoms in a heavy matrix remains challenging. In this paper, we show that the combination of first-principles total-energy calculations with aberration-corrected HAADF-STEM experimental and simulated images can be used to overcome this problem. The application of this methodology to the analysis of dilute nitrides of GaAs points to the existence of a major proportion of (2 NAs) nn in the alloy, which is a relatively stable configuration in GaAsN as revealed by our energetic calculations. Our study has allowed us to shed light in the effect of the local distortion of the lattice due to different configuration of atoms in HAADF-STEM imaging. © 2009 The American Physical Society.

Age-hardening response of Mg–0.3 at.%Ca alloys with different Zn contents

We have investigated the age-hardening responses and corresponding microstructures of Mg–0.3Ca–xZn (x=0.0, 0.1, 0.3, 0.6, 1.0, 1.6at.%) alloys by hardness test, transmission electron microscopy (TEM), and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). Zn additions up to x=0.6 lead to enhanced age-hardening responses with the highest peak hardness of HV=69 for x=0.6. Further addition of Zn degraded the age-hardening responses. HAADF-STEM images revealed that the finely dispersed monolayer G.P. zones with internal ordered structure are the major contributor to the age-hardening. Excess addition of Zn resulted in the formation of Ca2Mg6Zn3 precipitates suppressing the formation of the ordered G.P. zones.

Quantitative atomic resolution mapping using high-angle annular dark field scanning transmission electron microscopy

A model-based method is proposed to relatively quantify the chemical composition of atomic columns using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images. The method is based on a quantification of the total intensity of the scattered electrons for the individual atomic columns using statistical parameter estimation theory. In order to apply this theory, a model is required describing the image contrast of the HAADF STEM images. Therefore, a simple, effective incoherent model has been assumed which takes the probe intensity profile into account. The scattered intensities can then be estimated by fitting this model to an experimental HAADF STEM image. These estimates are used as a performance measure to distinguish between different atomic column types and to identify the nature of unknown columns with good accuracy and precision using statistical hypothesis testing. The reliability of the method is supported by means of simulated HAADF STEM images as well as a combination of experimental images and electron energy-loss spectra. It is experimentally shown that statistically meaningful information on the composition of individual columns can be obtained even if the difference in averaged atomic number Z is only 3. Using this method, quantitative mapping at atomic resolution using HAADF STEM images only has become possible without the need of simultaneously recorded electron energy loss spectra.

High-Angle Annular Dark Field Scanning Transmission Electron Microscopy on Carbon-Based Functional Polymer Systems

Two purely carbon-based functional polymer systems were investigated by bright-field conventional transmission electron microscopy (CTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). For a carbon black (CB) filled polymer system, HAADF-STEM provides high contrast between the CB agglomerates and the polymer matrix so that details of the interface organization easily can be revealed and assignment of the CB phase is straightforward. For a second system, the functional polymer blend representing the photoactive layer of a polymer solar cell, details of its nanoscale organization could be observed that were not accessible with CTEM. By varying the camera length in HAADF-STEM imaging, the contrast can be enhanced between crystalline and amorphous compounds due to diffraction contrast so that nanoscale interconnections between domains are identified. In general, due to its incoherent imaging characteristics HAADF-STEM allows for reliable interpretation of the data obtained.

Effects of electron channeling in HAADF-STEM intensity in La 2CuSnO 6

Atomic resolution imaging using the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) can be applied to analyze the atomic structures of materials directly. This technique provides incoherent Z-contrast with the atomic number of the constituent elements. In the present work, unique contrasts that make intuitively interpreting the HAADF-STEM image in double perovskite oxide La 2CuSnO 6 difficult were observed. Multislice simulation confirmed that this occurred as an effect of the channeling process of electrons in combination with the effect of Debye–Waller factors. This was confirmed because in the La 2CuSnO 6 crystal, two independent Sn atoms and four independent La atoms in the unit cell had different Debye–Waller factors, and the La columns consisted of pairs of columns with a small separation, whereas the Sn atoms were arranged straight. Furthermore, the image contrast was examined by mutislice simulation, and two atomic La columns were separated in a projected plane and appeared as one column contrast using multislice simulation. As a result, the HAADF intensity did not decrease constantly with the increase in column separation, with the exception of a very thin sample, which could be interpreted by the specific change in the electron-channeling process.

Direct observations of Ca ordering in Ca0.33CoO2 thin films with different superstructures

Ca-ion superstructures such as 3a×3a (hexagonal) and 2a×3a (orthorhombic) in layered Ca0.33CoO2 crystalline film, which were prepared by the reactive solid-phase epitaxy and the subsequent ion-exchange treatment, were directly distinguished by using spherical-aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The fluctuation of Ca intensities in the HAADF-STEM image indicates the existence of many Ca vacancies in the 2a×3a orthorhombic superstructure, which is consistent with the partial occupation of Ca (x=0.33) in the cation sites of x=0.5.

Simulation of high angle annular dark field scanning transmission electron microscopy images of large nanostructures

High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) is a powerful tool to quantify size, shape, position, and composition of nano-objects with the assessment of image simulation. Due to the high computational requirements needed, nowadays it can only be applied to a few unit cells in standard computers. To overpass this limitation, a parallel software (SICSTEM) has been developed. This software can afford HAADF-STEM image simulations of nanostructures composed of several hundred thousand atoms in manageable time. The usefulness of this tool is exemplified by simulating a HAADF-STEM image of an InAs nanowire.

Combining Theory and Experiment to Characterize the Atomic Structures of Surface-Deposited Au309Clusters

Gold clusters with icosahedral, decahedral, and cuboctahedral shell structures, have been studied using the Gupta many-body potential, to aid in the structural characterization of surface-deposited Au309 clusters using high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). In this size regime, the calculations indicate that the icosahedral geometries are lower in energy than the decahedral and cuboctahedral structures but that the energy differences are small. This is consistent with the spread of different geometries observed by HAADF-STEM. Analysis of the different outlines and intensity profies of the HAADF-STEM images indicate that there are roughly equal numbers of decahedral and cuboctahedral clusters on the surface. The unambiguous assignment of icosahedral geometries is more difficult because of the more-spherical nature of these nanoparticles. Because the experimental uncertainty in the deposited cluster size is ±5%, a genetic algorithm has been used to search for the lowest energy isomers for AuN, clusters with N = 309 ± 15 atoms. A variety of highly faceted structures have been found, many corresponding to incomplete or distorted icosahedra, including a puckered icosahedral geometry for N = 309.

Defect-modulated structures in Al–Ni–Rh crystalline approximants

Defect-modulated Al–Ni–Rh crystalline approximants, which are structurally related to a decagonal quasicrystal with 1.6 nm periodicity, have been found to form in an as-solidified Al75Ni15Rh10 alloy. Their structural features were studied by a combination of high-resolution electron microscopy (HREM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The results show that the prefect structures of Al–Ni–Rh crystalline approximants are characterized by periodic arrangements of hexogen units (H) constructed from atom columnar clusters. Defect-modulated structures were formed due to ordered arrangement of one- or two-dimensional structural defects of high density. The basic structural defect is associated with incorrect arrangements of atom columnar clusters which appear as polygons different from the H units. These polygons of atom columnar clusters represent a type of linear defect extending along the columnar axis direction, and they can array to form a planar defect. By means of the HAADF-STEM imaging technique, the structural nature of the core area for the linear defect, which usually exhibits dark contrast in HREM observations, has been clearly revealed.

Imaging of high‐angle annular dark‐field scanning transmission electron microscopy and observations of GaN‐based violet laser diodes

The first part of this paper is devoted to physics, to explain high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging and to interpret why HAADF-STEM imaging is incoherent, instructing a strict definition of interference and coherence of electron waves. Next, we present our recent investigations of InGaN/GaN multiple quantum wells and AlGaN/GaN strained-layer superlattice claddings in GaN-based violet laser diodes, which have been performed by HAADF-STEM and high-resolution field-emission gun scanning electron microscopy.

Quantitative Strain Mapping Applied to Aberration-Corrected HAADF Images

A systematic distortion in high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images, which may be caused by residual electrical interference, has been evaluated. Strain mapping, using the geometric phase methodology, has been applied to images acquired in an aberration-corrected STEM. This allows this distortion to be removed and so quantitative analysis of HAADF-STEM images was enabled. The distortion is quantified by applying this technique to structurally perfect and strain-free material. As an example, the correction is used to analyse an InAs/GaAs dot-in-quantum well heterostructure grown by molecular beam epitaxy. The result is a quantitative measure of internal strain on an atomic scale. The measured internal strain field of the heterostructure can be interpreted as being due to variations of indium concentration in the quantum dot.

An expanded approach to noise reduction from high-resolution STEM images based on the maximum entropy method

An expanded use of the maximum entropy method (MEM) is suggested to reduce noise from an experimental high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) image. The MEM is combined with an estimate of the standard deviation of noise from an experimental HAADF STEM image and low-pass filtering using the information limit for an incoherent STEM image. Consequently, the present method has just one parameter of a Lagrange multiplier. It is demonstrated that the present method can reduce noise efficiently in high-resolution HAADF STEM images.

Determination of thickness and lattice distortion for the individual layer of strained Al0.14Ga0.86N∕GaN superlattice by high-angle annular dark-field scanning transmission electron microscopy

Al 0.14 Ga 0.86 N ∕ Ga N and GaN layers in the strained-layer superlattice (SLS) in GaN-based laser diodes were distinguished as dark and bright bands, respectively, in a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image. From the HAADF-STEM images the thickness of the AlGaN layers was determined to be 2.24±0.09nm and that of GaN layer 2.34±0.15nm, which corresponds to nine atom planes in the [0001] direction. The parameters of the distorted AlGaN and GaN lattices were evaluated to be a=0.32, c=0.50nm and a=0.32, c=0.52nm, respectively. This shows that the resultant good lattice matching on the (0001) AlGaN∕GaN interfaces suppressed the generation of misfit dislocation in the SLS cladding.