Low-temperature Transmission Electron Microscopy Research Articles

Step-Bunching Evidence in Strained InxGa1?xAs/GaAs Quantum Wells Grown on Vicinal (001) Substrates

In this study the influence of the interface morphology upon the photoluminescence of thick InGaAs/GaAs strained quantum wells has been investigated. Samples grown by molecular beam epitaxy, using GaAs (001) substrates with a miscut of 6° towards (111)A, have been studied using low temperature photoluminescence and transmission electron microscopy. The photoluminescence linewidth broadening was correlated with the interface morphology. The blueshift observed in the optical emissions of samples grown on vicinal substrate was well explained in terms of a simple model that includes indium segregation and strain modulation. Corrugation along the [110] direction, similar to tiles, was observed in the 200 and 300 A thick quantum wells grown on vicinal substrate. Such observation was taken as an indication of the presence of step bunches at the upper interface of the quantum wells.

Charge/orbital ordering structure and microstructure of Pr 1− xCa xMnO 3 ( x⩽0.5) studied by low-temperature TEM

The crystal structure of perovskite-type manganese oxides, Pr 1− x Ca x MnO 3 ( x⩽0.5) in the charge/orbital ordering state has been investigated by means of low-temperature transmission electron microscopy (TEM). The superlattice reflection spots with wave vector a * [0,1/2,0] appear for 0.3⩽ x⩽0.5 compounds below near the charge-ordering transition temperature (∼230 K). With further decreasing of temperature, the new structural modulation is generated additionally with the superstructure for 0.36⩽ x⩽0.4 compounds. The superlattice reflections appear as sharpen spots with wave vector a *[1/4,1/4,1/2] below ∼100 K. Such modulation structure can be interpreted in terms of ordering of d 3 z 2− r 2 -type of e g electrons, which are caused by a deviation of x from 1/2, along c-axis. The relations between chemical composition and microstructures in charge-ordering phase were discussed.

Low temperature X-ray diffraction study of La 2−xM xCuO 4, M [dbnd], Sr

The low temperature phase diagrams for La 2−xM xCuO 4, M Ba and Sr, were redetermined using high resolution synchroton x-ray diffractionm by monitoring the low temperature orthorhombic (LTo1, standard space group Cmca indexed Bmab) 020/200 and low temperature tetragonal (LTT, P4 2/ncm) 200 peaks. For M Ba, the LTT phase exists as a minority phase at the 10–15% level in the nominally LTO1 phase well above the bulk LTT transformation temperature. For M Sr there is no bulk LTT transformation, but up to 10% LTT phase is observed at low temperatures. For unsubstituted La 2CuO 4 no LTT contribution to diffraction is observed, down to an estimated sensitivity of 3%. For x>0.12, both for M Sr and Ba, the LTO1 200/020 diffraction peak splitting at low temperatures is inadequate to allow the observation of a possible minority LTT phase. Based on low temperature transmission electron microscopy studies of La 2−xBa xCuO 4, we propose a two step transformation from the LTO1 to the LTT phase. The LTT phase may be nucleated at relatively high temperatures at the LTO1 twin boundaries. For M Ba, near T=60K, the LTT phase also develops within the LTO1 twins, and the sample essentially completely transforms to LTT.

ZnMgSe/ZnCdSe-based distributed bragg mirrors grown by molecular-beam epitaxy on ZnSe substrates

Molecular-beam epitaxy was used to grow distributed Bragg mirrors on ZnSe substrates. These mirrors are composed of 10.5 and 20 pairs of alternating quarter-wave ZnMgSe and ZnCdSe layers with reflectance peaks at the wavelengths of 530 and 560 nm, respectively, which fall in the transparency region of the substrate. These structures were studied by low-temperature cathodoluminescence, atomic-force microscopy, and transmission electron microscopy. The maximum of the reflection coefficient was 78% for a 20-pair mirror and 66% for a 10.5-pair mirror. This result is interpreted in terms of a model that takes into account the roughness of the interlayer boundaries.

3D Reconstruction from STEM Images of Xanthine Dehydrogenase and Insulin Receptor: Refinements and Molecular Modelling.

Abstract We have reconstructed the three-dimensional quaternary structure of the complete 480 kDa insulin receptor (IR), complexed with Nanogold - labeled insulin, and the 290 kDa Xanthine Dehydrogenase (XDH). Both molecules were imaged by low-dose, low-temperature dark field scanning transmission electron microscopy (STEM). XDH and IR were both freeze-plunged in liquid ethane, and transferred to the STEM (VG HB 601) where they were freeze dried at -130°C. Reconstruction was carried out using the method of quaternion-assisted angular reconstruction (IQAD) as described previously. XDH was further refined by an iterative process in which the IQAD produced volume was used as a reference for further refinements. Separate reconstructions of two sets of half the images indicated an inter-reconstruction resolution of 20 Å and 9 Å by phase residual criteria for the IR and XDH reconstructions, respectively.

Insulin Receptor: 3D Reconstruction from Darkfield Stem Images, Structural Interpretation and Functional Model

We have reconstructed the three-dimensional quaternary structure of the complete 480 kDa insulin receptor (IR), complexed with NanoGold-labelled insulin, via sets of electron micrographs obtained by low-dose low-temperature dark field scanning transmission electron microscopy (STEM).Insulin binding to IR in mammalian cell membranes is essential for its manifold effects such as glucose homeostasis, increased protein synthesis, growth, and development. IR belongs to the superfamily of transmembrane receptor tyrosine kinases that include the monomeric epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor (PDGFR). In contrast, IR and its homologues IGF-1R (insulin-like growth factor 1 receptor) and IRR (insulin receptorrelated receptor) are sub-types of this family that are intrinsic disulfide-linked dimers of two αβ heterodimers. Monomeric receptor TKs are inactive, but are activated by ligand-induced dimerization that results in autophosphorylation. IR-like TKs are also inactive even though they are already dimeric, and are activated by ligand binding without further oligomerization. Insulin binding to the extracellular domain of IR results in autophosphorylation of specific tyrosines to initiate an intracellular signal transduction cascade. However, because the quaternary structure of IR is not known, the structural basis for the mechanism of IR activation by extracellular insulin binding has not been elucidated.The insulin receptor was purified from human placenta. Bovine insulin was derivatized with NanoGold at the B chain Phel, a location not directly involved in receptor binding. Binding of derivatized insulin to the purified receptor was reduced only slightly compared to binding of the native insulin.

Phonon signatures of spontaneous CuPt ordering in Ga0.47In0.53As/InP

Polarized Raman scattering spectra of spontaneously ordered Ga0.47In0.53As alloys, combined with low-temperature photoluminescence and transmission electron microscopy, are used to find a unique phonon signature of CuPtB-type ordering. In the Raman spectra of ordered alloys, a new phonon peak that is absent in the spectrum of the random alloy appears at 181 cm−1. The intensity of this peak correlates with the degree of ordering inferred from electron diffraction measurements. We propose that Raman scattering measurements can be used as a nondestructive probe to detect spontaneous ordering in this alloy.

Anisotropic Growth and Screw Dislocation of {100} Tabular Grains

Silver chloride tabular grains with {100} faces on the surface (hereinafter referred as {100} tabular grain) were prepared by introducing a small amount of silver bromide at their nucleation stage. Their dislocation structure was observed by low-temperature transmission electron microscopy (LTEM) in order to study the mechanism of their anisotropic growth. The results of the observations were as follows: First, a nucleus was observed at one of the corners of almost every {100} tabular grain. Second, two dislocations extended from a nucleus trace in the <310> directions. Third, a {100} tabular grain grew anisotropically from the nucleus only in the two directions to which these two dislocations extended. Fourth, the dislocations thus observed in {100} tabular grains were identified as screw dislocations. We conclude that anisotropic growth of {100} tabular grains is caused by screw dislocations.

Self-organized nanosize InP and InAsP clusters obtained by metalorganic compound hydride epitaxy

Results are presented of investigations of the growth of self-organizing nanosize InP and InAsP clusters in an In0.5Ga0.5P matrix. The structure was characterized by low-temperature photoluminescence and transmission electron microscopy. The photoluminescence measurements revealed highly efficient radiative recombination from the quantum dots and indicated that the structures were of good optical quality. The average density and size of the InP clusters, determined from the results of the transmission electron microscope measurements, are 3×109 cm−2 and 80 nm, respectively.

Strained SiGe/Si quantum well dots and wires selectively grown by LPCVD and their optical properties

To realize quantum dots and quantum wires a variety of approaches are currently being explored. Among the growth approaches there is the selective epitaxial growth using a mask to define the growth region. In this case the feature size and geometry are mainly limited by the lithographic process. With optical lithography we achieved a lateral dimension of ⩾0.4 μm. Therefore, to further reduce the lateral dimension, but still using optical lithography, the tendency of facet formation during selective epitaxial growth was investigated. Multiple quantum well Si/Si 0.70Ge 0.30 structures with Si 0.935Ge 0.065 spacers and buffers were deposited and characterized by low temperature photoluminescence and transmission electron microscopy. The buffer thickness was varied so as to achieve facet junction. While on large areas the Si 0.935Ge 0.065 buffer was relaxed, for dots ⩽300 μm or narrower the structures remained strained even for buffers exceeding by a factor of two to three the critical thickness of large area. In dots and wires where facet junction has taken place a rounded (001) region between facets (approximately 50 nm broad) in the quantum well layers was observed. In wires oriented parallel to 〈100〉 sidewalls growth induced wire formation was observed and in addition vertical correlation of these self-organized wires. The photoluminescence of all dots and wires down to the lowest achieved dimension and including the self-organized wires is strong, the integral intensity normalized to the surface coverage by far exceeding that of the substrate related peaks.

Self-aggregation of quantum dots for very thin InAs layers grown on GaAs.

Low-temperature photoluminescence and transmission electron microscopy of InAs/GaAs quantum wells grown by molecular-beam epitaxy show that InAs self-aggregation of InAs quantum dots is a continuous phenomenon and that quantum dots nucleate at the well interfaces for nominal InAs layer thicknesses much smaller than commonly reported in the literature (i.e., 1.6\ifmmode\pm\else\textpm\fi{}0.1 ML). A good agreement is also found between the self-aggregated dot sizes estimated from the photoluminescence emission energies and those directly obtained from transmission electron microscopy measurements.

Visible luminescence from semiconductor quantum dots in large ensembles

Visible luminescence has been obtained from ensembles of defect-free, InxAl(1−x)As islands of ultrasmall dimensions embedded in AlyGa(1−y)As cladding layers. These structures were grown by molecular beam epitaxy and studied with low-temperature photoluminescence (PL) and transmission electron microscopy. Visible luminescence is produced using various compositions of InxAl(1−x)As/AlyGa(1−y)As. Quantum dot size distributions, planar densities, dot heights, and wetting layer thicknesses are presented and correlated with the PL spectra.

Molecular beam epitaxial growth of high quality Zn1−xCdxSe on InP substrates

High quality, lattice matched Zn1−xCdxSe has been grown on InP substrates by molecular beam epitaxy. The quality of the epilayers was monitored by reflection high energy electron diffraction, and by low temperature photoluminescence and transmission electron microscopy (TEM). The use of an As flux during the pregrowth substrate treatment followed by a low initial growth temperature were needed to optimize the growth. TEM images of samples grown under these conditions show abrupt interfaces and good crystalline quality. Epilayers exhibit excellent optical properties, indicated by a very narrow, high intensity near-band-edge excitonic emission peak (full width at half maximum of 10 meV), and almost negligible deep level emission.

Rare gas bubbles in muscovite mica implanted with xenon and krypton

Xenon and krypton have been implanted into muscovite mica at room temperature and at liquid nitrogen temperature. The behavior of the implanted Xe and Kr was followed by low-temperature transmission electron microscopy and energy dispersive x-ray analysis. An electron diffraction pattern of diffuse bands is observed at room temperature due to the presence of fluid rare gas and to noncrystalline mica. Visible cavities with diameters 10–300 nm formed in the Xe-implanted mica. Visible cavities in room-temperature Kr-implanted mica ranged from 5–50 nm in diameter. The gas pressures at room temperature in the cavities are estimated, assuming all of the implanted gas precipitated in cavities to be ∼10 MPa for Xe and ∼20 MPa for Kr. These pressures are considerably lower than found for rare gases implanted in metals and ceramics, but sufficient to liquefy the rare gases at room temperature. The Xe and Kr were observed by dark-field microscopy to form fcc crystalline solids within the cavities at temperatures below their triple points, with lattice parameters of a(xe) = 0.630 ± 0.0015 nm and a(Kr) = 0.565 ± 0.005 nm. The solid Xe within bubbles was unstable under the electron beam of the transmission electron microscope at temperatures above 80 K, while the solid Kr within bubbles was unstable at temperatures as low as 35 K. The crystalline mica matrix undergoes a transformation from a crystalline structure to an amorphous structure as a result of implantation.

The study of misfit dislocations in InxGa1-xAs/GaAs strained quantum well structures

Abstract InxGa1-xAs/GaAs strained-layer structures containing four single quantum wells (SSQWs) were grown by molecular beam epitaxy on a (001) semi-insulating GaAs substrate with In contents of x = 0.09 and x = 0.16. The structures contained four InxGa1-xAs quantum wells with well width and growth sequence given by 20, 40, 80 and 160 A and were separated by 500 A GaAs layers. The optical and structural properties of the SSQW samples have been analysed by low-temperature cathodoluminescence (CL) and transmission electron microscopy. Misfit dislocations were observed in the sample with higher In content. Large-angle convergentbeam electron diffraction has been newly applied to this system as a reliable method for determining dislocation character. All misfit dislocations studied in plan-view specimens revealed dark line contrast in appropriately chosen monochromatic CL images irrespective of the dislocation character or its α or β form. A simple one-to-one correlation between dislocations and dark lines wa...

Low temperature transmission electron microscopy and differential scanning calorimetry characterization of latexes stabilized with surface active block oligomers

The spatial distribution of a short-chain amphiphilic diblock copolymer (i.e. block oligomer), used as an emulsifier during the preparation of a latex, is determined by low temperature transmission electron microscopy and differential scanning calorimetry (d.s.c.). The main question is whether intermixing of the amphiphilic block oligomer and latex core materials actually takes place to a significant extent. Electron microscopy gives a clear indication that a surfactant-rich layer exists in all latex samples examined. At very high surfactant concentration (>6%, not typically used in commercial latexes), electron beam radiation damage reveals the presence of a surfactant gradient from the particle surface to its core. These results are in good agreement with our d.s.c. data.

Broad Band vs Phonon Resolved Luminescence in Si1−xGex/Si Heterostructures Grown by Molecular Beam Epitaxy

ABSTRACTSi1−xGex alloys and multilayers synthesized by solid source MBE on Si(100) substrates have been characterized by low temperature photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM). Phonon resolved transitions originating from excitons bound to shallow impurities were observed in addition to a broad band of intense luminescence. PL spectroscopy over the temperature range 2K to 100K has been used to characterize Si1−xGex/Si heterostructures exhibiting both types of PL spectra. Thin alloy layers exhibited phonon-resolved PL spectra, similar to bulk material, but shifted in energy due to strain and hole quantum confinement. In single quantum wells confinement shifts up to ∼200 meV were observed (1.2 nm wells with x = 0.38) and NP linewidths down to 1.37 meV were obtained. However, the broad PL band (peak energy ∼120meV below the strained bandgap) was predominant when the alloy layer thickness was greater than 2 – 10nm, depending on x, growth temperature, and substrate surface preparation. The strength of the broad PL band was correlated with the areal density of strain perturbations (∼109cm−2 per quantum well; local lattice dilation ∼1.5 nm in diameter) observed in plan-view TEM. The role of MBE growth parameters in determining optical properties was investigated by changing growth temperature, substrate preparation procedures and exploring the effect of surface passivation in a hydrogen ambient. In addition, post growth anneals at temperatures in the range 700°C to 1 100°C were carried out, where interdiffusion removes interfacial asperities and the broad luminescence band decays to zero intensity.

Low substrate temperature molecular beam epitaxial growth and the critical layer thickness of InGaAs grown on GaAs

We report on the critical layer thickness of InxGa1−xAs on GaAs grown at low substrate temperatures in a wide range of indium compositions. Compared with ordinary growth conditions, the transition between pseudomorphic and relaxed regions (in the epilayer thickness versus x plane) occurred at higher indium compositions when the growth temperature was lowered. An increase in critical thicknesses for pseudomorphic growth by at least a factor of seven for alloy compositions with less than 45% indium was observed. This was determined by low temperature photoluminescence spectroscopy and transmission electron microscopy measurements on single quantum wells.

Defect-free growth of AlxGa1−xAs by liquid-phase epitaxy on V-grooved (001) GaAs substrates

The crystalline quality of AlxGa1−xAs grown on a V-grooved (001) GaAs substrate is characterized by low-temperature cathodoluminescence and transmission electron microscopy. High-crystalline perfection visualized by narrow neutral donor bound exciton luminescence is attained above the groove. Growth of (Al, Ga) As in the groove takes place without significant change of the aluminium distribution. An order of magnitude increase of silicon acceptor concentration induced by As-vacancy creation is detected at the interface n-GaAs substrate/V groove. Transmission electron microscopy pictures taken from sample cross sections do not show any kind of defects in the groove.

Structure of cubic mesomorphic phases determined by low-temperature transmission electron microscopy and small-angle x-ray scattering

Cubic phases exist in many amphiphile/water systems at high solute concentrations between the lamellar and hexagonal phases, and at lower amphiphile concentrations between micellar and hexagonal phases. The structure of the latter is still controversial. Structural models that have been suggested to satisfy the experimental data all lacked direct evidence to prove their validity. The structural model we report here is based on a combination of direct and indirect experimental techniques. Direct imaging by low-temperature (cryo) transmission electron microscopy (TEM) of vitrified specimens provides the geometry and size of the «building blocks» of the phase, in this case globular micelles, and shows directly the existence of a cubic phase