Observation of GaAs—AlxGa1 — x as heterostructures and quantum-well-wire structures using backscattered electron image

As a microscale tool for observing GaAs-Al_x Ga_1–xAs heterostructures, backscattered electron (BE) images in the scanning electron microscope (SEM) were compared with conventional secondary electron (SE) images. BE images were found to be more sensitive to compositional differences between GaAs and Al_xGa_1–xAs and less sensitive to surface roughness. BE images have a spatial resolution of 10 nm or better. This method enables the nondestructive observation of ultrafine lateral periodic structures, such as quantum-well-wire (QWW) structures, fabricated by compositional disordering technology using focused Ga ion-beam (Ga-FIB) implantation into GaAs-Al_xGa_1–xAs material.     

Composition mapping in InGaN by scanning transmission electron microscopy

We suggest a method for chemical mapping that is based on scanning transmission electron microscopy (STEM) imaging with a high-angle annular dark field (HAADF) detector. The analysis method uses a comparison of intensity normalized with respect to the incident electron beam with intensity calculated employing the frozen lattice approximation. This procedure is validated with an In_0.07Ga_0.93N layer with homogeneous In concentration, where the STEM results were compared with energy filtered imaging, strain state analysis and energy dispersive X-ray analysis. Good agreement was obtained, if the frozen lattice simulations took into account static atomic displacements, caused by the different covalent radii of In and Ga atoms. Using a sample with higher In concentration and series of 32 images taken within 42 min scan time, we did not find any indication for formation of In rich regions due to electron beam irradiation, which is reported in literature to occur for the parallel illumination mode. Image simulation of an In_0.15Ga_0.85N layer that was elastically relaxed with empirical Stillinger-Weber potentials did not reveal significant impact of lattice plane bending on STEM images as well as on the evaluated In concentration profiles for specimen thicknesses of 5, 15 and 50 nm. Image simulation of an abrupt interface between GaN and In_0.15Ga_0.85N for specimen thicknesses up to 200 nm showed that artificial blurring of interfaces is significantly smaller than expected from a simple geometrical model that is based on the beam convergence only. As an application of the method, we give evidence for the existence of In rich regions in an InGaN layer which shows signatures of quantum dot emission in microphotoluminescence spectroscopy experiments.     

Electron energy-loss near edge structure (ELNES) of InGaN quantum wells

Lasers and light‐emitting diodes (LEDs) that emit in the blue to green region are often based on In_xGa_1–xN quantum well structures. Ionization edges in the electron energy‐loss spectrum contain fine structures (called the energy‐loss near edge structure (ELNES)) and provide information about the electronic structure. In this paper we compare the experimental and calculated ELNES for the N‐K ionization edge of In_xGa_1–xN quantum wells. When the effects of the core‐hole are included in the calculations, agreement between experimental and calculated spectra is very good. Strain has been shown to accentuate the effects of In on the ELNES and moves the ionization edge onset down in energy, relative to the other features. These results suggest that ELNES may provide an alternative method to lattice imaging to determine the presence of strain in this system.     

Heteroepitaxy of AIIIBV films on vicinal Si(001) substrates

GaAs films on Si substrates miscut from the (001) plane by 6° in the [110] direction are grown by molecular beam epitaxy (MBE). GaAs films are grown both on the Si surface terminated by arsenic atoms and on thin pseudomorphic GaP/Si layers. The condition of formation of the As sublayer and the first monolayer of GaP on the Si surface is defined as the GaAs film orientation (001) or \((00\bar 1)\) . The processes of Si surface preparation and formation of the As sublayer and GaAs and GaP epitaxial layers are monitored by means of high-energy electron diffraction reflection (RHEED). The grown structures are investigated by methods of X-ray diffraction, atomic force microscopy (ATM), high-resolution transmission electron microscopy (HRTEM), and low-temperature luminescences. It is shown that the epitaxial film orientation affects both the surface morphology and its crystalline properties. Intense photoluminescence is obtained from the In_0.17Ga_0.83As quantum well structure grown on the GaAs/Si buffer layer.     

Quantitative atom column position analysis at the incommensurate interfaces of a (PbS)(1.14)NbS(2) misfit layered compound with aberration-corrected HRTEM

Aberration-corrected HRTEM is applied to explore the potential of NCSI contrast imaging to quantitatively analyse the complex atomic structure of misfit layered compounds and their incommensurate interfaces. Using the (PbS)_1.14NbS₂ misfit layered compound as a model system it is shown that atom column position analyses at the incommensurate interfaces can be performed with precisions reaching a statistical accuracy of ±6 pm. The procedure adopted for these studies compares experimental images taken from compound regions free of defects and interface modulations with a structure model derived from XRD experiments and with multi-slice image simulations for the corresponding NCSI contrast conditions used. The high precision achievable in such experiments is confirmed by a detailed quantitative analysis of the atom column positions at the incommensurate interfaces, proving a tetragonal distortion of the monochalcogenide sublattice.     

A method of normalizing cathodoluminescence images of electron transparent foils for thickness contrast applied to InGaN quantum wells

The uniformity of panchromatic cathodoluminescence (CL) from In_0.09Ga_0.91N/GaN quantum wells at 100 K was investigated using a combined transmission electron microscope–cathodoluminescence instrument. A technique for correcting CL images of electron‐transparent wedge specimens for thickness contrast artefacts is presented. The foil thickness was estimated using the dynamical formulation of the relationship between the thickness and the (experimentally observed) transmitted electron intensity. For a given thickness the CL intensity was calculated using the Everhart–Hoff depth‐dose function and also taking into account surface recombination losses. Experimental CL images were normalized by dividing by the calculated CL value at each point. The procedure was successful in calculating the underlying materials contrast in CL images of thin specimens of InGaN single quantum wells. Non‐uniformities in the CL emission on the scale of ～0.7 µm were observed.     

Synthesis of CuInGaSe2 nanoparticles by low temperature colloidal route

CIGS nanoparticles were synthesized by a low temperature colloidal route for the absorber layer of photovoltaic devices. The CIGS nanoparticles were prepared by reacting CuI, InI₃, GaI₃ in pyridine with Na₂Se in methanol at 0°C under inert atmosphere. The reaction products of dark red and yellow colors were turned out to be NaI and CIGS nanoparticles, respectively, by ICP-AES and SEM-EDS analyses. Chalcopyrite structure of the CIGS nanoparticles was confirmed by XRD and TEM diffraction patterns. As compared to the particles from Cu_0.9In_0.8 Ga_0.3Se₂ ratio, more uniform and smaller nanoparticles were obtained from Cu_1.1In_0.68Ga_0.23Se_1.91 stoichiometric ratio. The CIGS nanoparticles were measured to be in the ranges of 5-20 nm. However, tube like CIGS particles with length of several γn and width in the range of 100-300 nm were obtained from Cu_0.9In_0.8Ga_0.3Se₂, and Cu_0.9In_0.7Ga_0.4Se₂. The morphological change of the CIGS particles seems to be closely related to the ratio of Cu/(In+Ga).     

SiCf–SiBC composites: microstructural investigations of the as-received material and creep tested composites under an oxidative environment

SiC_f–SiBC composites fabricated by Snecma Propulsion Solide (St Médard en Jalles, France) were investigated by SEM and HRTEM in the as‐received state and after creep tests performed in air, in a temperature range 1423–1573 K, under 170 and 200 MPa. These composites are reinforced by Hi‐Nicalon fibres (Nippon Carbon). A pyrocarbon interphase was first deposited on the fibres. The matrix was then deposited on the fibrous preform by several chemical vapour infiltrations (CVI). As a result the matrix is multilayered and based on the Si–B–C ternary system. This matrix is self‐sealing: this is due to the presence of boron inducing the formation of a sealant glass if the material is heated in an oxidative environment. This glass will protect fibres and fibre/matrix interphases against oxidation. Hi‐Nicalon fibres as well as the different matrix layers were studied by HRTEM and EDX. Some investigations were carried out on the creep‐tested specimens in order to characterize modifications observed in the different constituents of the composites, particularly at the interfaces between the matrix layers and at the fibre/matrix interface. It was shown that several matrix layers crystallized during the creep tests. Moreover, a thin silica layer was observed at the pyrocarbon/matrix interfaces. Differences between the behaviour of the same type of material creep tested under neutral atmosphere are discussed.     

Extraction of structural and chemical information from high angle annular dark‐field image by an improved peaks finding method

With the development of spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM), high angle annular dark filed (HAADF) imaging technique has been widely applied in the microstructure characterization of various advanced materials with atomic resolution. However, current qualitative interpretation of the HAADF image is not enough to extract all the useful information. Here a modified peaks finding method was proposed to quantify the HAADF‐STEM image to extract structural and chemical information. Firstly, an automatic segmentation technique including numerical filters and watershed algorithm was used to define the sub‐areas for each atomic column. Then a 2D Gaussian fitting was carried out to determine the atomic column positions precisely, which provides the geometric information at the unit‐cell scale. Furthermore, a self‐adaptive integration based on the column position and the covariance of statistical Gaussian distribution were performed. The integrated intensities show very high sensitivity on the mean atomic number with improved signal‐to‐noise (S/N) ratio. Consequently, the polarization map and strain distributions were rebuilt from a HAADF‐STEM image of the rhombohedral and tetragonal BiFeO₃ interface and a MnO₂ monolayer in LaAlO₃/SrMnO₃/SrTiO₃ heterostructure was discerned from its neighbor TiO₂ layers. Microsc. Res. Tech. 79:820–826, 2016. © 2016 Wiley Periodicals, Inc.     

Transmission electron microscopy analysis of phase separation in GaInAsSb films grown on GaSb substrate

The GaSb‐based quaternary alloys are a good choice for thermophotovoltaic applications. The thermophotovoltaic cell converts infrared radiation to electricity, using the same principles as photovoltaic devices. The aim of the present work was the microstructural study of such an alloy, namely Ga_0.84In_0.16As_0.12Sb_0.88. A thin film of the material was grown by metal organic vapour phase epitaxy on a (100)α→[111]B (α = 2°, 4°, 6°) GaSb substrate. The GaInAsSb alloy has an appropriate band gap, but suffers from a phase separation consisting of GaAs‐rich and InSb‐rich regions that is disadvantageous for cell efficiency. In this work, we employed a morphological approach to phase separation, with the use of conventional transmission electron microscopy and atomic force microscopy. The phase separation occurs in two different orientations: parallel to the growth direction (vertical) and inclined (lateral). After application of fast Fourier transformation filtering, the vertical periodicity was found to be λ = 5 nm for the pair (black and white) of layers independently of the cut‐off angle, whereas the lateral periodicity was related to it.     

Characteristics of MSM photodetector fabricated on porous In0.08Ga0.92N

In this study, nanoporous structures of In_0.08Ga_0.92N thin films were synthesized using anodic etching at various etching durations. The metal–semiconductor–metal photodetectors subsequentaly have been fabricated by depositing a high work function metal (Pt) on the thin films. Results show that the responsivity of the detector increased with increasing the etching duration to reach to the maximum value at 15 min. Moreover, the rise and recovery times of the device were investigated at 390 nm chopped light.     

Nonradiative recombination at dislocations in III–V compound semiconductors

Carrier recombination at individual dislocations is investigated for the case of misfit dislocations at a heterojunction between Ga_1–xAl_xAs_1–yP_y epitaxial layers. Through the combined use of scanning transmission electron microscopy, electron beam induced current and cathodoluminescence analysis, it is shown that the non-radiative and radiative recombination properties of dislocations are associated with their fine crystallographic configuration. The edge Lomer-Cottrell dislocation is found to be electrically neutral. The absence of carrier recombination strongly suggests that core reconstruction may be important in eliminating dangling bonds and kink sites along the core of these dislocations. An undissociated dislocation with a Burgers vector b =1/2a〈110〉 is proposed as the more likely configuration for the edge Lomer-Cottrell dislocation. An asymmetry in the nonradiative recombination properties and crystallographic structure of the 60° dislocation is also reported and discussed.     

Assessment of green cleaning effectiveness on polychrome surfaces by MALDI‐TOF mass spectrometry and microscopic imaging

This article proposes an innovative methodology which employs nondestructive techniques to assess the effectiveness of new formulations based on ionic liquids, as alternative solvents for enzymes (proteases), for the removal of proteinaceous materials from painted surfaces during restoration treatments. Ionic liquids (ILs), also known as “designer” solvents, because of their peculiar properties which can be adjusted by selecting different cation‐anion combinations, are potentially green solvents due totheir low vapour pressure. In this study, two ionic liquids were selected: IL1 (1‐butyl‐3‐methylimidazolium tetrafluoroborate ([BMIM][BF₄])) and IL2 (1‐ethyl‐3‐methylimidazolium ethylsulphate ([EMIM][EtSO₄])). New formulations were prepared with these ILs and two different proteases (E): one acid (E1—pepsin) and one alkaline (E2—obtained from Aspergillus sojae). These formulations were tested on tempera and oil mock‐up samples, prepared in accordance with historically documented recipes, and covered with two different types of protein‐based varnishes (egg white and isinglass—fish glue). A noninvasive multiscale imaging methodology was applied before and after the treatment to evaluate the cleaning's effectiveness. Different microscopic techniques—optical microscopy (OM) with visible and fluorescent light, scanning electron microscopy (SEM) and atomic force microscopy (AFM)—together with Matrix‐Assisted Laser Desorption/Ionization—Time of Flight Mass Spectrometry (MALDI‐TOF MS) were applied on areas cleaned with the new formulations (IL + E) and reference areas cleaned only with the commercial enzyme formulations (gels). MALDI‐TOF proved particularly very useful for comparing the diversity and abundance of peptides released by using different enzymatic systems. Microsc. Res. Tech. 77:574–585, 2014. © 2014 Wiley Periodicals, Inc.     

Electron Microscope Studies Of Vpe Gainas Layer Structures Suitable For Use As Infrared Leds

Electron microscope investigations have been carried out on vapour grown (100) GaAs/GaInAs structures designed for use as infrared emitters of wavelength 1·06 μm. The structures consist of a GaAs substrate, a graded layer in which the indium concentration is increased from zero to 17 atomic %, and a constant composition Ga_0·83In_0·17As layer which contains a p-n junction. X-ray microprobe analysis of cross-sections of the slices established the uniformity of the grading. TEM analysis showed a dense and extensive asymmetric network of misfit dislocations (1 × 10¹² m^?2 (10⁸ cm^?2)) in the graded layer, threading dislocations and other anomalous contrast features extending from the graded layer through the p-n junction to the surface (local densities of 1 × 10¹¹–1 × 10¹² m^?2 (10⁷–10⁸ cm^?2)), and a planar network of dislocations just below the surface (spacing 0·2–2 μm). SEM EBIC and CL studies of the layer above the junction revealed dark spots, and a cross-grid of dark lines, which could be correlated with the threading defects, and the dislocation network just below the surface, respectively. The SEM results showed that these defects had a deleterious effect on the luminescent and electrical properties of the material in the vicinity of the p-n junction, and would therefore impair the performance of devices made from these layer structures.     

Spectroscopy of Single AlInAs Quantum Dots

A system of quantum dots based on Al _x In_1?xAs/Al _y Ga_1?yAs solid solutions is investigated. The use of Al _x In_1?xAs wide-gap solid solutions as the basis of quantum dots substantially extends the spectral emission range to the short-wavelength region, including the wavelength region near 770 nm, which is of interest for the development of aerospace systems of quantum cryptography. The optical characteristics of Al _x In_1?xAs single quantum dots grown by the Stranski–Krastanov mechanism were studied by cryogenic microphotoluminescence. The statistics of the emission of single quantum dot excitons was studied using a Hanbury Brown–Twiss interferometer. The pair photon correlation function indicates the sub-Poissonian nature of the emission statistics, which directly confirms the possibility of developing single-photon emitters based on Al _x In_1?xAs quantum dots. The fine structure of quantum dot exciton states was investigated at wavelengths near 770 nm. The splitting of the exciton states is found to be similar to the natural width of exciton lines, which is of great interest for the development of entangled photon pair emitters based on Al _x In_1?xAs quantum dots.     

Interfacial reactions and silicate formation in highly dispersed Lu2O3– SiO2 system

Solid state interface reactions in highly dispersed Lu₂O₃– SiO₂ binary oxide system were studied at 600–1100 °C with X‐ray powder diffraction (XRD), high‐resolution transmission electron microscopy (HRTEM) and Fourier Transform Infrared spectroscopy (FTIR). The results show that at 600–900 °C an amorphous, nanometer thick Lu‐O‐Si layer covering SiO₂ particles exists in the system. At higher temperatures the breakage of the layer into amorphous islands occurs and crystalline silicates with various structures are formed. In particular, Lu₄[Si₃O₁₀][SiO₄] silicate, analogue of B‐type Dy – Tm disilicates, forms at 1000 °C.     

Sectional voltage contrast topography of quantum well diodes

Detailed scanning electron microscopy (SEM) of an operational quantum well laser was performed using the differential voltage contrast (DVC) method. Quantitative measurements of Fermi energies in an operational device at high magnifications were possible due to a new calibration procedure described in this study. Precise tailoring of digitized DVC images of an operational laser taken at high and low magnifications is described. The ability to record and to analyze very detailed quasi‐Fermi energy (QFE) profiles across an operational laser or transistor with 30‐Angstrom features improved the DVC technique. A layered GaAs/Ga_xAl_1?xAs/In_yGa_1?y As quantum well light emitter was tested through various operational steps, that is, inverse population, threshold, and full emission. The detailed QFE profile allowed us to analyze collective electron behavior at various steps of operation of the laser.     

Three‐dimensional morphometric comparison of normal and apoptotic endothelial cells based on laser scanning confocal microscopy observation

Three‐dimensional (3D) morphometric analysis of cellular and subcellular structures provides an effective method for spatial cell biology. Here, 3D cellular and nuclear morphologies are reconstructed to quantify and compare morphometric differences between normal and apoptotic endothelial cells. Human umbilical vein endothelial cells (HUVECs) are treated with 60 μM H₂O₂ to get apoptotic cell model and then a series of sectional images are acquired from laser scanning confocal microscopy. The 3D cell model containing plasma membrane and cell nucleus is reconstructed and fused utilizing three sequential softwares or packages (Mimics, Geomagic, and VTK). The results reveal that H₂O₂ can induce apoptosis effectively by regulating the activity of apoptosis‐related biomolecules, including pro‐apoptotic factors p53 and Bax, and anti‐apoptotic factor Bcl‐2. Compared with the normal HUVECs, the apoptotic cells exhibit significant 3D morphometric parameters (height, volume and nucleus‐to‐cytoplasm ratio) variation. The present research provides a new perspective on comparative quantitative analysis associated with cell apoptosis and points to the value of LSCM as an objective tool for 3D cell reconstruction. Microsc. Res. Tech. 76:1154–1162, 2013. © 2013 Wiley Periodicals, Inc.     

Microstructure and interfacial chemistry of pure and La‐doped BiFeO3 thin films

We report on the microstructure and interfacial chemistry of thin films of pure and La‐doped multiferroic bismuth ferrite (Bi_1‐xLa_xFeO₃ or BLFO), synthesized on Indium Tin Oxide‐coated glass substrates by solution‐deposition technique and studied using scanning transmission electron microscopy. Our results show that undoped and La‐doped thin films are polycrystalline with distorted rhombohedral structure without any presence of any line or planar defect in the films. In addition, the films with La doping did not show any structural change and maintain the equilibrium structure. Cross section compositional analysis using X‐ray energy dispersive spectrometry did not reveal either any interdiffusion of chemical species or formation of reaction product at the film‐substrate interface. However, a closer examination of the microstructure of the films shows tiny pores along with the presence of ～2–3 nm thin amorphous layers, which may have significant influence on the functional properties of such films. Microsc. Res. Tech. 76:1304–1309, 2013. © 2013 Wiley Periodicals, Inc.     

Surface micromorphology of dental composites [CE‐TZP]‐[Al2O3] with Ca+2 modifier

The objective of this study was to characterize the three‐dimensional (3D) surface micromorphology of the ceramics produced from nanoparticles of alumina and tetragonal zirconia (t‐ZrO₂) with addition of Ca⁺² for sintering improvement. The 3D surface roughness of samples was studied by atomic force microscopy (AFM), fractal analysis of the 3D AFM‐images, and statistical analysis of surface roughness parameters. Cube counting method, based on the linear interpolation type, applied for AFM data was used for fractal analysis. The morphology of non‐modified ceramic sample was characterized by the rather big (1–2 μm) grains of α‐Al₂O₃ phase with a habit close to hexagonal drowned in solid solution of t‐ZrO₂ with smooth surface. The pattern surfaces of modified composite content a little amount of elongated prismatic grains with composition close to the phase of СаСеAl₃О₇ as well as hexahedral α‐Al₂O₃‐grains. Fractal dimension, D, as well as height values distribution have been determined for the surfaces of the samples with and without modifying. It can be concluded that the smoothest surface is of the modified samples with Ca⁺² modifier but the most regular one is of the non‐modified samples. A connection was observed between the surface morphology and the physical properties as assessed in previous works. Microsc. Res. Tech. 78:840–846, 2015. © 2015 Wiley Periodicals, Inc.