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Demonstrates that the presence of low-temperature-grown GaAs in LT-GaAs/AlAs/GaAs:Si heterostructures increase the Al/Ga interdiffusion at the heterostructure interfaces. Causes of the interdiffusion enhancement.

This paper describes an approach to retrieve the three-dimensional atomic structure of a nanocrystalline particle from the reconstructed electron exit wave function in a single projection direction. The method employs wave propagation to determine the local exit surface of each atomic column together with its mass. The exit wave in between colums is used as internal calibration so as to remove the background noise and improve the precision to the level of single atom sensitivity. The validity of the approach is tested with exit wave functions of a gold wedge reconstructed from simulated images containing different levels of noise. [ABSTRACT FROM AUTHOR]

The recent development of atomic resolution, low dose-rate electron microscopy allows investigating 2D materials as well as catalytic nano particles without compromising their structural integrity. For graphene and a variety of nanoparticle compositions, it is shown that a critical dose rate exists of <100 e − /Å 2 s at 80 keV of electron acceleration that allows maintaining the genuine object structures including their surfaces and edges even if particles are only 3 nm large or smaller. Moreover, it is demonstrated that electron beam-induced phonon excitation from outside the field of view contributes to a contrast degradation in recorded images. These degradation effects can be eliminated by delivering electrons onto the imaged area, only, by using a Nilsonian illumination scheme in combination with a suitable aperture at the electron gun/monochromator assembly. [ABSTRACT FROM AUTHOR]

This contribution touches on essential requirements for instrument stability and resolution that allows operating advanced electron microscopes at the edge to technological capabilities. They enable the detection of single atoms and their dynamic behavior on a length scale of picometers in real time. It is understood that the observed atom dynamic is intimately linked to the relaxation and thermalization of electron beam-induced sample excitation. Resulting contrast fluctuations are beam current dependent and largely contribute to a contrast mismatch between experiments and theory if not considered. If explored, they open the possibility to study functional behavior of nanocrystals and single molecules at the atomic level in real time. [ABSTRACT FROM AUTHOR]

Abstract: The distribution of indium in a GaN/In x Ga1− x N/Al y Ga1− y N quantum well with x±Δx=0.24±0.07 is quantitatively investigated by extraction of displacement fields from lattice images. Simulations accurately describe the measured strain relaxation across a wedge-shaped sample for a sample thickness up to 150nm. The proportionality between indium concentration and resulting lattice constant c(x) is approximated by c(x)=0.5185+0.111x nm. In general, it is challenging to discriminate the effects of random alloying against clustering. In In x Ga1− x N this is particularly true at low indium concentrations x<0.2. For an accurate quantitative analysis, sample preparation and imaging were developed such that radiation damage can be recognized if present. Further, an analysis of detection limits and knowledge of the sample thickness are crucial for obtaining reproducible results. Data averaging is necessary to reach sufficient precision. Consequently, the size of the indium-rich clusters is poorly known if x is small. Beyond the interest in physical properties of In x Ga1− x N alloys, the analysis of strain and its relaxation exemplifies how quantitative analysis is possible at an atomic level and is in excellent agreement with theoretical predictions. [Copyright &y& Elsevier]

Abstract: The In x Ga1− x N alloy system is used as an example to describe achievements and limitations that are given by noise levels in lattice images from transmission electron microscopes. Unlike many other experimental techniques, noise limits must be determined afresh from image to image since they vary with microscope performance, sample preparation, radiation damage if present, and operator skills. Both, the determination of the indium distribution in In x Ga1− x N alloys by strain mapping and the detection of indium clusters in InN by imaging, are largely affected by noise limitations. As a result, it is challenging to probe for the distribution of indium atoms in alloys with x<0.2 or x>0.8, which is of importance if one aims at understanding the nucleation of indium atom clusters in GaN or InN. We present results that point towards cluster formation in quantum wells with x<0.2 and show that photoluminescence at 0.5–0.7eV relate to the presence of indium clusters in InN. A reliable detection of single indium atoms, however, will require further improvement of detection limits, which one can expect from the next generation of electron microscopes that are developed in DoE''s TEAM Project. [Copyright &y& Elsevier]

The frequencies of adjacent γ′ pairs having in-phase and out-of-phase relationships in quartet configurations, determined by high resolution electron microscopy [Calderon et al., Phil. Mag. Lett. 85 51 (2005)], were examined in detail for γ′ particles belonging to the same translational order domain. Consequently, there were always at least two out-of-phase adjacent pairs in any quartet. It is concluded that the quartet split patterns in γ′ particles, but are produced by the migration of particles due to diffusion, these particles having been precipitated separately and independently prior to migration. An elasticity calculation is provided to show that two γ′ particles migrate to align along 〈001〉 when their initial position deviates from this direction. [ABSTRACT FROM AUTHOR]

During the past 50 years, transmission electron microscopy (TEM) has evolved from an imaging tool to a quantitative method that approaches the ultimate goal of understanding the atomic structure of materials atom by atom in three dimensions both experimentally and theoretically. Today's TEM abilities are tested in the special case of a Ga-terminated 30° partial dislocation in GaAs:Be where it is shown that a combination of high-resolution phase contrast imaging, scanning TEM (STEM), and local electron energy loss spectroscopy (EELS) allows for a complete analysis of dislocation cores and associated stacking faults. We find that it is already possible to locate atom column positions with picometre precision in directly interpretable images of the projected crystal structure and that chemically different elements can already be identified together with their local electronic structure. In terms of theory, the experimental results can be quantitatively compared with ab initio electronic structure total energy calculations. By combining elasticity theory methods with atomic theory, an equivalent crystal volume can be addressed. Therefore, it is already feasible to merge experiments and theory on a picometre length scale. Whereas current experiments require the utilization of different, specialized instruments, it is foreseeable that the rapid improvement of electron optical elements will soon generate a next generation of microscopes with the ability to image and analyze single atoms in one instrument with deep sub-angstrom spatial resolution and an energy resolution better than 100&ThickSpace;meV. [ABSTRACT FROM AUTHOR]

Abstract: A novel sample preparation technique is reported to fabricate electron transparent samples from devices utilizing a FIB process with a successive wet etching step. The high quality of the obtained samples allows for band gap—and chemical composition measurements of In x Ga1− x N quantum wells where electron beam induced damage can be controlled and shown to be negligible. The results reveal indium enrichment in nanoclusters and defects that cause fluctuations of the band gap energy and can be measured by low loss Electron Energy Spectroscopy with nm resolution. Comparing our time, energy, and spatially resolved measurements of band gap energies, chemical composition, and their related fluctuations with literature data, we find quantitative agreement if the band gap energy of InN is 1.5–2eV. [Copyright &y& Elsevier]

New elements of a symmetric [001] 67.4∘ SrTiO3 near Σ 13 tilt grain boundary are identified by a quantitative analysis of lattice images, reconstructed electron exit waves, and HAADF images. The analysis reveals local, geometrical variations of structural grain boundary units that relate to the presence of defects introduced by a tilt deviation of 0.65 + 0.02 degrees from the perfect Σ13 geometry. Sr and TiO columns are discriminated in HAADF images while the reconstructed electron exit wave reveals all oxygen columns in addition. Both methods depict the crystal and boundary structure directly while lattice imaging with a high voltage instrument requires image simulations to link the image intensity to structure. For the first time we observe a Sr column splitting by 90 pm that supports theoretical predictions. An inhomogeneous, preferential etching at the grain boundary is attributed to local charge variations and hampers a quantitative investigation and local stoichiometry. The near Σ13 boundary forms a dense and compact structure with chemically identical columns in close proximity. Therefore, it is different from the relaxed, bulk like configurations described in previous reports. [ABSTRACT FROM AUTHOR]

High-resolution transmission electron microscopy (HRTEM) employing focus-variation phase-reconstruction methods is used to image the atomic structure of grain boundaries in a silicon nitride ceramic at subangstrom resolution. Complementary energy-dispersive X-ray emission spectroscopy experiments revealed the presence of yttrium ions segregated to the 0.5-0.7-nm thin amorphous boundary layers that separate individual grains. Our objective here is probing if yttrium ions attach to the prismatic planes of the Si[sub 3]N[sub 4] at the interface toward the amorphous layer, using Scherzer and phasereconstruction imaging, as well as image simulation. Crystal structure images of grain boundaries in thin sample (<100 Å) areas do not reveal the attachment of yttrium at these positions, although lattice images from thicker areas do suggest the presence of yttrium at these sites. It is concluded that most of the yttrium atoms are located in the amorphous phase and only a few atoms may attach to the terminating prism plane. In this case, the line concentrations of such yttrium in the latter location are estimated to be at most one yttrium atom every 17 Å,. [ABSTRACT FROM AUTHOR]

High-resolution transmission electron microscopy is utilized to examine the crystal structure of a silicon nitride ceramic using focus variation methods to achieve sub-a˚ngstro¨m resolution at the absolute theoretical information limit of the transmission electron microscope. Specifically, crucial requirements of high instrumental stability, a coherent electron source and optimum imaging conditions have been met by the one-A˚ngstrom microscope (OA˚M) at the National Center for Electron Microscopy in order to obtain a resolution of 0.8 A˚. The resulting high-resolution images reveal the individual atom positions of the in-plane projected crystal structure of silicon nitride and permit detailed structural information. The images correspond closely to computed and simulated images of this crystal structure. [ABSTRACT FROM AUTHOR]

Hexagonal InN layers were grown by molecular beam epitaxy and studied by high resolution electron microscopy and by photoluminescence spectroscopy. Inclusions of a few nanometers in diameter were found, which are among the smallest reported. Image simulation, beam sensitivity, and photoluminescence of the samples corroborate that these inclusions are indeed metallic indium. This letter provides evidence that nanoscopic metallic indium inclusions can be present in InN crystals and have a strong influence on its optical properties. [ABSTRACT FROM AUTHOR]

A letter is presented in response to the article "Three-dimensional atom probe studies of an In(sub x)Ga(sub 1-x)N/GaN multiple quantum well structure: Assessment of possible indium clustering," which appeared in issue no. 90 of 2007.

Abstract: Annular dark field TEM images exhibit a dominant mass-thickness contrast that can be quantified to extract single atom scattering cross sections. On top of this incoherent background, additional lattice fringes appear with a nonlinear information limit of 1.2Å at 150kV. The formation of these fringes is described by coherent nonlinear imaging theory and good agreement is found between experimental and simulated images. Calculations furthermore predict that the use of aberration corrected microscopes will improve the image quality dramatically. [Copyright &y& Elsevier]

Decomposition of the solid solution of an Al–3Mg–0.4Cu–0.12Si (wt.%) alloy has been studied by examining samples aged for periods of 1–16 h at a temperature of 180 °C. High resolution transmission electron microscopy (HRTEM) was the primary method of investigation. Within the 4–16 h aging interval, nanometer scale, homogeneously dispersed, coherent, lath-like particles were detected in the matrix. The orientation of the lath particles in the matrix is such that the edges are aligned along the 〈1 0 0〉Al directions. From the Fourier transformation of the HRTEM images obtained on 〈1 0 0〉Al zones, it was possible to confirm that two variants give rise to diffraction spots that were previously associated with the presence of S″-phase in “Cu lean” Al–Mg–Cu alloys. Two other variants give rise to streaked diffraction spots positioned around the (1 1 0)Al. The detected particles are termed GPBII in this study, instead of previously suggested S″-phase notation. The findings of this study are also compared with those of the frequently studied “Cu rich” Al–Cu–Mg alloying system. [Copyright &y& Elsevier]

Microstructure evolution of an age hardenable Al–3Mg–0.4Cu–0.12Si (wt%) alloy has been studied during artificial aging at 180 °C prior to the formation of the stable S-phase. The primary investigation method used in this study was high-resolution transmission electron microscopy (HRTEM), coupled with image processing and image simulation. After 1 h of aging, the presence of super-lattice reflections was detected in the Fourier spectra of the HRTEM images, suggesting an L10 type ordering of Mg and Cu atoms in the Al matrix. After 4 and 8 h of aging, coherent particles were observed in the microstructure. These particles give rise to diffraction spots that in previous literature have been considered to be characteristic of the S″-phase in the “Cu-lean” Al–Mg–Cu alloys. It is shown that these diffraction spots can be indexed in terms of a crystal structure that is closely related to the L10 ordering formed at the shorter aging times. The crystal structure is orthorhombic with lattice parameters a=1.2 nm, b=0.4 nm, c=0.4 nm and space group Cmmm. We propose to identify these coherent particles as GPB-II zones, and the ordering that precedes them as GPB zones. [Copyright &y& Elsevier]

The effect of secondary sintering additives and/or a post-sintering heat treatment on the semicrystalline atomic structure of the intergranular phase in silicon nitride ceramics is investigated. Three different Yb-doped Si3N4 ceramic compositions are examined using a scanning transmission electron microscope, whereby the intergranular atomic structure is directly imaged with Ångstrom resolution. The resulting high-resolution images show that the atomic arrangement of the Yb takes very periodic positions along the interface between the intergranular phase and the matrix grains, and that a postsintering 1250 °C heat treatment, as well as a change of the secondary sintering additives (Al2O3 vs SiO2), does not alter the atomic positions of Yb. This result has implications for the understanding of how the mechanical properties of ceramics are influenced by the presence of the nanoscale intergranular phase, and for associated computational modeling of its precise role and atomic structure. [ABSTRACT FROM AUTHOR]

Presents a study that investigated homoepitaxial deposition of a GaN overlayer onto a bulk single-crystal GaN substrate using molecular beam epitaxy. Comparison of structural quality of deposited GaN overlayer with heteroepitaxially grown layers; Photoluminescence of bulk GaN and the deposited homoepitaxial GaN layer; Benefits for the development of benchmark values for the optoelectronic properties of GaN thin films.

The crystal structure of FePt nanoparticles of mean size of 6 nm produced by gas-phase condensation is characterized using a combination of high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM). These FePt nanoparticles are found to be chemically ordered, decahedral shaped, and Pt enriched at the surfaces. The experimentally determined crystallographic lattice constants and distribution of Fe and Pt atoms are compared with first-principles calculations of ordered decahedral FePt nanoparticles to confirm the discovery of a unique decahedral structure with Fe/Pt ordedng and Pt surface segregation. [ABSTRACT FROM AUTHOR]

Atomically precise tailoring of graphene can enable unusual transport pathways and new nanometer-scale functional devices. Here we describe a recipe for the controlled production of highly regular "5-5-8" line defects in graphene by means of simultaneous electron irradiation and Joule heating by applied electric current. High-resolution transmission electron microscopy reveals individual steps of the growth process. Extending earlier theoretical work suggesting valley-discriminating capabilities of a graphene 5-5-8 line defect, we perform first-principles calculations of transport and find a strong energy dependence of valley polarization of the charge carriers across the defect. These findings inspire us to propose a compact electrostatically gated "valley valve" device, a critical component for valleytronics. [ABSTRACT FROM AUTHOR]

Abstract: Aberration corrected Transmission Electron Microscope (TEM) images can currently resolve information at significantly better than 0.1nm. Aberration corrected imaging conditions seek to optimize the transfer of high-resolution information but in doing so they prevent the transfer of low spatial frequency information. To recover low spatial frequency information, aberration corrected images must be acquired at a large defocus which compromises high spatial frequency information transfer. In this paper we present two a posteriori solutions to this problem in which the information bandwidth in an exit wave reconstruction is increased. In the first we reconstruct the electron exit wavefunction from two focal series datasets, with different, uniform focal steps, experimentally demonstrating that the width of the transfer interval can be extended from 0.2nm−1 (∼5nm) to better than 10nm−1 (0.1nm). In the second we outline the use of a focal series recorded with a non-uniform focal step to recover a wider range of spatial frequencies without the need for a large number of images. Using simulated data we show that using this non-uniform focal step the spatial frequency interval for a five image data set may be increased to between 0.25nm−1 (4nm) and 8.3nm−1 (0.12nm) compared to between 0.74nm−1 (1.4nm) and 8.3nm−1 (0.12nm) for the standard focal series geometry. [Copyright &y& Elsevier]

Abstract: We apply monochromated illumination to improve the information transfer in focal series reconstruction to 0.5Å at 300kV. Contrary to single images, which can be taken arbitrarily close to Gaussian focus in a C S-corrected microscope, images in a focal series are taken at a certain defocus. This defocus poses limits on the spatial coherence of the illumination, and through this, limits on the brightness of the monochromated illumination. We derive an estimate for the minimum spatial coherence and the minimal brightness needed for a certain resolution at a certain defocus and apply this estimate to our focal series experiments. We find that the 0.5Å information transfer would have been difficult and probably impossible to obtain without the exceptionally high brightness of the monochromated illumination. [Copyright &y& Elsevier]

Abstract: Chromatic aberration limits the resolution in spherical-aberration corrected Transmission Electron Microscopy to approximately 0.7Å at 300kV. The energy spread in the beam is the main contribution to the chromatic aberration. This spread can be reduced with a monochromator. Another limitation to the resolution in TEM can be the finite brightness of the source and the consequent partial spatial coherence of the illumination. This limitation becomes important when spherical aberration and/or defocus are present such as in uncorrected TEM or in focal-series reconstruction in TEM. We used a monochromator optimized for minimum brightness loss and a prototype ‘high-brightness’ gun, and obtained brightness after monochromation comparable to that of the standard Schottky FEG before monochromation. The images were acquired on the prototype TEAM 0.5 microscope, which was developed on a Titan platform by increasing its electrical and mechanical stability. [Copyright &y& Elsevier]

Any tip functionalization of carbon nanotubes, for which the relative orientation between their (metallic) catalyst particle and the nanotube axis is essential, requires a detailed knowledge of the nature of the internal interface between the particle and the outgrown tube. In the present work, this interface is characterized with atomic precision using state-of-the-art low-voltage aberration-corrected transmission electron microscopy in combination with molecular dynamics simulations for the case of hard-magnetically terminated carbon nanotubes. Our results indicate that the physical principle based upon which the interfacial metal facet is chosen is a reduction of the desorption energy for carbon. [ABSTRACT FROM AUTHOR]

Abstract: Using two levels of electron beam lithography, vapor phase deposition techniques, and FIB etching, we have fabricated an electrostatic Boersch phase plate for contrast enhancement of weak phase objects in a transmission electron microscope. The phase plate has suitable dimensions for the imaging of small biological samples without compromising the high-resolution capabilities of the microscope. A micro-structured electrode allows for phase tuning of the unscattered electron beam, which enables the recording of contrast enhanced in-focus images and in-line holograms. We have demonstrated experimentally that our phase plate improves the contrast of carbon nanotubes while maintaining high-resolution imaging performance, which is demonstrated for the case of an AlGaAs heterostructure. The development opens a new way to study interfaces between soft and hard materials. [Copyright &y& Elsevier]

Although the physics of materials at surfaces and edges has been extensively studied, the movement of individual atoms at an isolated edge has not been directly observed in real time. With a transmission electron aberration-corrected microscope capable of simultaneous atomic spatial resolution and 1-second temporal resolution, we produced movies of the dynamics of carbon atoms at the edge of a hole in a suspended, single atomic layer of graphene. The rearrangement of bonds and beam-induced ejection of carbon atoms are recorded as the hole grows. We investigated the mechanism of edge reconstruction and demonstrated the stability of the "zigzag" edge configuration. This study of an ideal low-dimensional interface, a hole in graphene, exhibits the complex behavior of atoms at a boundary. [ABSTRACT FROM AUTHOR]

Employing exit-plane wave function (EPWF) reconstruction in high-resolution transmission electron microscopy (HRTEM), we have developed an approach to atomic scale compositional analysis of III-V semiconductor interfaces, especially suitable for analyzing quaternary heterostructures with intermixing in both cation and anion sub-lattices. Specifically, we use the focal-series reconstruction technique, which retrieves the complex-valued EPWF from a thru-focus series of HRTEM images. A study of interfaces in Al0.4Ga0.6As–GaAs and In0.25Ga0.75Sb–InAs heterostructures using focal-series reconstruction shows that change in chemical composition along individual atomic columns across an interface is discernible in the phase image of the reconstructed EPWF. To extract the interface composition profiles along the cation and anion sub-lattices, quantitative analysis of the phase image is performed using factorial analysis of correspondence. This enabled independent quantification of changes in the In–Ga and As–Sb contents across ultra-thin interfacial regions (approximately 0.6 nm wide) with true atomic resolution, in the In0.25Ga0.75Sb–InAs heterostructure. The validity of the method is demonstrated by analyzing simulated HRTEM images of an InAs–GaSb–InAs model structure with abrupt and graded interfaces. Our approach is general, permitting atomic-level compositional analysis of heterostructures with two species per sub-lattice, hitherto unfeasible with existing HRTEM methods. [ABSTRACT FROM AUTHOR]

Abstract: A novel reconstruction procedure is proposed to achieve atomic resolution in electron tomography. The method exploits the fact that crystals are discrete assemblies of atoms (atomicity). This constraint enables us to obtain a three-dimensional (3-D) reconstruction of test structures from less than 10 projections even in the presence of noise and defects. Phase contrast transmission electron microscopy (TEM) images of a gold nanocrystal were simulated in six different zone axes. The discrete number of atoms in every column is determined by application of the channelling theory to reconstructed electron exit waves. The procedure is experimentally validated by experiments with gold samples. Our results show that discrete tomography recovers the shape of the particle as well as the position of its 309 atoms from only three projections. Experiments on a nanocrystal that contains several missing atoms, both on the surface and in the core of the nanocrystal, while considering a high noise level in each simulated image were performed to prove the stability of the approach to reconstruct defects. The algorithm is well capable of handling structural defects in a highly noisy environment, even if this causes atom count “errors” in the projection data. [Copyright &y& Elsevier]

Abstract: Electron channelling occurs when the incident electron beam is parallel to the atom columns of an object, such as a crystal or a particular crystal defect. Then, the electrons are trapped in the electrostatic potential of an atom column in which they scatter dynamically. This picture provides physical insight and explains why a one-to-one correspondence is maintained between the exit wave and the projected structure, even in case of strong dynamical scattering. Moreover, the theory is very useful to invert the dynamical scattering, that is, to derive the projected structure from the exit wave. Finally, it can be used to determine the composition of an atom column with single atom sensitivity or to explain dynamical electron diffraction effects. In this paper, an overview of the channelling theory will be given together with some recent applications. [Copyright &y& Elsevier]

Quantitative high-resolution transmission electron microscopy was used to study the distribution of indium atoms in InxGa1-xN alloys by strain mapping. In GaN/InxGa1-xN/GaN quantum wells with x < 0.1 we find that the sample thickness and the precision to which displacement fields can be extracted from a lattice image determine whether or not it is possible to discriminate between random alloy fluctuations and cluster formation. In miscible alloys such as SiGe or AlGaN a precision of better than 1 pm is required to reveal random alloy fluctuations, which presently exceeds experimental capabilities. In InxGa1-xN with x > 0.1, a precision of about 3 pm suffices to distinguish random alloy fluctuations from indium clusters that are present. Thick InxGa1-xN layers with x = 0.6 and x = 0.7 show phase separation with a wavelength between 2 and 4 nm and a fluctuation amplitude of Δx = 0.10 and 0.15, respectively. This produces striped composition fluctuations, which are modulated by dot-like structures. The similarity of the fluctuation magnitudes in quantum wells and thick layers suggests that spinodal decomposition occurs in both materials and our results place the centre of the miscibility gap around x = 0.5-0.6. [ABSTRACT FROM AUTHOR]

Abstract: The interface of an annealed GaN/InN heterostructure was investigated by valence electron energy loss spectroscopy (VEELS). A monochromated electron beam provides an energy resolution of 180meV at a beam size of 1nm in scanning transmission electron microscopy mode. Although simulteanously performed energy dispersive spectra (EDS) scans reveal an 80nm wide InGaN interdiffusion area, alloying is locally minimal and phase separation is commonly found. Various defect transitions in the range between 1 and less than 0.7eV were observed together with room temperature band transitions of 1.7 and 3.3eV for InN and GaN, respectively. An enhanced concentration of vacancies accumulated at the interface may explain the pronounced interdiffusion and some of the observed defect transitions. [Copyright &y& Elsevier]

Abstract: Zincblende and wurtzite phases of InN are found in InN epilayers deposited by molecular beam epitaxy on GaN buffers which were grown by metal organic chemical vapor deposition. Valence electron energy loss spectroscopy (VEELS) was applied to determine band transitions in both phases of InN. GaN buffer layers were used as VEELS reference. The chemistry and crystalline structure of the observed areas was recorded simultaneously to exclude a contribution from oxides and/or metal clusters or extended defects such as grain boundaries. At room temperature a band transition for wurtzite InN was found at and for zincblende InN at (1.4±0.2)eV that are ascribed to the fundamental bandgaps of the respective polytypes. Those values correlate well with recent results of various research groups measuring the bandgap in InGaN alloys with VEELS. [Copyright &y& Elsevier]

Abstract: High efficient green light emitting diodes (LED) on the basis of GaN/InGaN exhibit indium-rich nanoclusters inside the quantum wells (QW) due to InN–GaN phase decomposition. By direct measurements of the variations in the electronic structure, we show for the first time a correlation between indium-rich nanoclusters and local energy band gap minima. Our investigations reveal the presence of 1–3nm wide indium rich clusters in these devices with indium concentrations x as large as x∼0.30–0.40 that narrow the band gap locally to energies as small as 2.65eV. These clusters are able to act as local traps for migrating photon-emitting carriers and seem to boost the overall device performance. [Copyright &y& Elsevier]

Abstract: Valence electron energy loss spectroscopy (VEELS) was applied to determine band transitions in wurtzite InN, deposited by molecular beam epitaxy on (0001) sapphire substrates or GaN buffer layers. The GaN buffer layer was used as VEELS reference. At room temperature a band transition for wurtzite InN was found at (1.7±0.2eV) and for wurtzite GaN at (3.3±0.2eV) that are ascribed to the fundamental bandgap. Additional band transitions could be identified at higher and lower energy losses. The latter may be related to transitions involving defect bands. In InN, neither oxygen related crystal phases nor indium metal clusters were observed in the areas of the epilayers investigated by VEELS. Consequently, the obtained results mainly describe the properties of the InN host crystal. [Copyright &y& Elsevier]

The late stages of coarsening of coherent solid particles is strongly influenced by the reduction of elastic strain energy. This produces migration and alignment of particles as well as some other effects. In this investigation, the origin of the so-called splitting pattern arrangement of ?' precipitate particles, an arrangement which has often been interpreted as being due to splitting of a larger particle into smaller ones, has been studied. The two-particle relationship as to whether they are in-phase or out-of-phase is examined by means of a translation order domain analysis of high-resolution electron-microscopy images along a zone axis parallel to [001]. Ni alloys have been used for the investigation including a binary Ni–Al alloy (producing different volume fractions) and two commercial multicomponent alloys with high volume fraction. About 72% of two-particle pairs forming the splitting configuration are in the out-of-phase relationship, indicating that adjacent pairs are randomly formed and that they are not formed by the splitting of a large particle. In addition, an elasticity analysis shows that the elastic interaction energy of two ?' particles exhibits a minimum at a certain separation distance along? . [ABSTRACT FROM AUTHOR]

Within the last few years indium nitride (InN) gained substantial interest due to its controversially discussed apparent band gap and its predicted highest maximum electron mobility among several III–V compounds, including Al(Ga)N and GaAs. The band gap of epitaxial InN has been recently reported to be around 0.7 eV rather than the previously accepted value of 1.9 eV obtained from polycrystalline films. Thus, InN could be a promising material for applications in infrared opto-electronics or high-speed electronics. However, the structural quality of the InN epilayers is still inferior to GaN and needs to be improved. Also, the role of many contaminants in InN and their effect on the epilayer''s conductivity and/or luminescence properties is still under investigation.This work describes recent studies of InN growth by molecular beam epitaxy on sapphire (0 0 0 1) substrates. The effect of buffer layer variations including a prior substrate nitridation step is discussed. Structural properties (X-ray diffraction, AFM and TEM images) and chemical profiles (SIMS) will be correlated to Hall data and Photo-luminescence spectra. The role of oxygen and hydrogen as possible donors in InN will be discussed. [Copyright &y& Elsevier]

Annular objective apertures are fabricated for a CM300 transmission electron microscope using a focused ion beam system. A central beam stop in the back focal plane of the objective lens of the microscope blocks all electrons scattered up to a semi-angle of approximately 20 mrad. In this manner, contributions to the image from Bragg scattering are largely reduced and the image contrast is sensitive to the atomic number Z. Experimentally, we find that single atom scattering cross sections measured with this technique are close to Rutherford scattering values. A comparison between this new method and STEM-HAADF shows that both techniques result in qualitatively similar images although the resolution of ADF-TEM is limited by contrast delocalization caused by the spherical aberration of the objective lens. This problem can be overcome by using an aberration corrected microscope. [Copyright &y& Elsevier]

We demonstrate the deposition of CuInS2 films on single-crystalline (0 0 0 1)-sapphire by radio frequency reactive sputtering with a Cu–In alloy target and H2S gas. X-ray diffraction (XRD) revealed that the as-sputtered films are of mainly (1 1 2)-oriented CuInS2 incorporating a minor CuIn2 phase. XRD rocking curve of CuInS2 (1 1 2) showed a full width at half maximum of 0.1°, indicating an epitaxial-like growth of (1 1 2)-CuInS2 films on (0 0 0 1)-sapphire. Six peaks dominantly show up in the XRD φ–2θ map, between which there are additional regular modulations present, suggesting a multi-domain structure of the thick double-layered films. Furthermore, the morphology and internal microstructure of the quasi-epitaxially sputtered CuInS2 films were characterized by scanning electron microscopy and transmission electron microscopy, respectively. [Copyright &y& Elsevier]

The structure and the interface chemistry of epitaxial BiFeO3–NiFe2O4 nanocomposite thin films on SrTiO3(001) substrates were investigated using the Z-contrast imaging and the electron exit-wave reconstruction methods at the atomic scale. The results show that the NiFe2O4 pillars are nonwetting with respect to the substrate and exhibit {111} facets at the surface. The interface between BiFeO3 and NiFe2O4 lies in the {110} planes and is semicoherent. The atomic configuration of the interface, with the BiFeO layer bonding to the [Ni,Fe]O2 layer, was shown to have the maximized structure continuity and minimized interface charging. [ABSTRACT FROM AUTHOR]

Employing the focal-series reconstruction technique in high-resolution transmission electron microscopy, we obtained true atomic resolution images of interfacial disorder within cation and anion sublattices across interfaces in an InGaSb/InAs heterostructure. This enabled independent quantitative mapping of changes in the In–Ga and As–Sb contents across interfacial regions ∼0.6 nm in width. A comparison of the cation and anion sublattice images revealed that intermixing at the InGaSb-on-InAs interface is confined to the In–Ga sublattice. Also, atomic scale roughness within the As–Sb sublattice of the InAs-on-InGaSb interface was discerned. This approach is general, permitting atomic-scale compositional analysis of heterointerfaces with two species per sublattice. [ABSTRACT FROM AUTHOR]

Nanoscale intergranular films in doped silicon-nitride ceramics are known to markedly affect toughness and creep resistance. They are regarded as being fully amorphous, but are shown here to have a semicrystalline structure in a Ce-doped Si3N4. Using two different but complementary high-resolution electron-microscopy methods, the intergranular atomic structure, imaged with sub-angstrom resolution, reveals that segregated cerium ions take very periodic positions, along the intergranular-phase/matrix-grain interface and as a semicrystalline structure spanning the width of the intergranular phase. This result has broad implications for the understanding of the structure and role of the intergranular phase in enhancing the mechanical properties of ceramics. [ABSTRACT FROM AUTHOR]

InN thin films have been grown epitaxially on GaN-buffered sapphire substrates by molecular-beam epitaxy at 500 °C. A high level of oxygen contamination in the growth chamber led to formation of In2O3 precipitates in the films. These precipitates were characterized in detail by transmission electron microscopy (TEM). The concentration of In2O3 was estimated to be less than 0.07 vol % in the present samples of oxygen content ∼0.5 at. %. Cross-sectional TEM investigations revealed that the precipitates adopt a preferred crystallographic orientation within the InN matrix, and show a characteristic diameter of ∼5 nm with average distance of ∼500 nm. These observations suggest the effective solubility of O in InN could be below 1 at. % at 500 °C. [ABSTRACT FROM AUTHOR]

Single-crystal GaN thin films have been deposited epitaxially on a HfN-buffered Si(111) substrates by molecular-beam epitaxy. The microstructural and compositional characteristics of the films were studied in detail by transmission electron microscopy (TEMs). Cross-sectional TEM investigations have revealed the crystallographic orientation relationship in different GaN/HfN/Si layers. GaN film polarity is studied by conventional TEM and convergent beam electron diffraction simulations, and the results show that the GaN film has a Ga polarity with relatively high density of inversion domains. Based on our observations, growth mechanisms related to the structural properties are discussed. [ABSTRACT FROM AUTHOR]