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Superlattice Structures (superlattice + structure)
Selected AbstractsThermal Conductivity of the Rare-Earth Strontium AluminatesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2010Chunlei Wan The thermal conductivity of a series of complex aluminates, RE2SrAl2O7, with different rare-earth (RE) ions, has been measured up to 1000°C. There is a strong dependence on the atomic number of the RE ion, ranging from an approximately 1/T dependence for the lanthanum strontium aluminate to an almost temperature-independent behavior of the dysprosium strontium aluminate. The latter conductivity is comparable with that of yttria-stabilized zirconia, the current material of choice for thermal barrier coatings. The temperature dependence of the thermal conductivities of all the aluminates studied can be fit to a standard phonon,phonon scattering model, modified to account for a minimum phonon mean free path, in which the difference in behavior is attributed to increased phonon,phonon scattering with the atomic mass of the RE ion. Although a satisfactory parametric fit is obtained, the model does not take into account either the detailed layer structure of the aluminates, consisting of alternating rock-salt and perovskite layers in a natural superlattice structure, or the site preferences of the RE ion. This suggests that further model development is warranted. [source] The effects of GaAs interval layer on GaNAs/GaAs superlattice structure grown by RF-MBE using modulated N radical beam sequencePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2008Kensuke Fujii Abstract We have investigated the effects of GaAs interval layers on the structure of GaNAs/GaAs superlattices (SLs) grown by radio-frequency molecular beam epitaxy. As the results of x-ray diffraction, it is found that periodic structure of GaNAs/GaAs SL is better in the case of thicker GaAs layer than that of thin GaAs layer, whereas the GaNAs layer thickness is constant of 10 ML. Transmission electron microscope (TEM) studies of SL samples clearly demonstrate the drastic effects of GaAs layer thickness on the GaNAs/GaAs SL structures. High resolution TEM study of the sample with a thin AlAs layer inserted between GaNAs and GaAs, for improved contrast, reveals existence of the roughening process during GaNAs layer growth and the smoothing process during GaAs layer growth. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Transport properties in n-type AlGaN/AlN/GaN-superlatticesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008J. Hertkorn Abstract In order to improve the lateral conductivity in optoelectronic devices, we have investigated Si-doped AlGaN/AlN/GaN-superlattices. As a first step we performed calculations of the band structure of Al-GaN/AlN/GaN modulation doped multi heterostructures. Based on these results we worked on optimizing the growth of low Al content (xAl, 20%) superlattices by MOVPE. Several tens of abrupt and graded AlGaN/AlN/GaN-layer pairs could be grown crack-free on 2 ,m thick n-GaN layers deposited on sapphire substrates with AlN nucleation. By Van-der-Pauw Hall measurements, we determined that the lateral conductivity of a 1.5 ,m thick superlattice structure is a factor of four higher than in highly n-doped bulk material with comparable thickness without compromising too much the vertical conductivity as confirmed by two step TLM-measurements. At 4K we could demonstrate an extremely high effective mobility of 18760 cm2/Vs at n=2×1014 cm,2 (R=1.6,/®), a clear verification of our excellent crystal quality. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Dispersion relation of the optical phonon frequencies in AlN/GaN superlatticesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2005S.K. Medeiros Abstract In this work we study the dispersion relation of the phonon frequencies in heterojunctions composed by III-V nitride materials (GaN and AlN). We are concerned with the superlattice structure, namely /substrate /AlN /AlxGa1,xN/GaN/AlxGa1,xN/,, where the substrate is here considered to be a transparent dielectric medium like sapphire. We make use of a model based on the Fröhlich Hamiltonian, taking into account the macroscopic theory known as the continuum dielectric model. The optical phonon modes are modelled considering only the electromagnetic boundary conditions, in the absence of charge transfer between ions. Numerical results of the confined optical phonon dispersion are presented, characterizing three distinct optical phonon classes designated as interface (IF), half-space (HS) and propagating (PR) modes. Furthermore, due to the dielectric anisotropy presented in the nitride, some additional peculiarities will be presented, like dispersive confined modes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Type II dehydroquinase: molecular replacement with many copiesACTA CRYSTALLOGRAPHICA SECTION D, Issue 1 2008Kirsty Anne Stewart Type II dehydroquinase is a small (150-amino-acid) protein which in solution packs together to form a dodecamer with 23 cubic symmetry. In crystals of this protein the symmetry of the biological unit can be coincident with the crystallographic symmetry, giving rise to cubic crystal forms with a single monomer in the asymmetric unit. In crystals where this is not the case, multiple copies of the monomer are present, giving rise to significant and often confusing noncrystallographic symmetry in low-symmetry crystal systems. These different crystal forms pose a variety of challenges for solution by molecular replacement. Three examples of structure solutions, including a highly unusual triclinic crystal form with 16 dodecamers (192 monomers) in the unit cell, are described. Four commonly used molecular-replacement packages are assessed against two of these examples, one of high symmetry and the other of low symmetry; this study highlights how program performance can vary significantly depending on the given problem. In addition, the final refined structure of the 16-dodecamer triclinic crystal form is analysed and shown not to be a superlattice structure, but rather an F -centred cubic crystal with frustrated crystallographic symmetry. [source] Interdiffusion phenomena in InGaAs/GaAs superlattice structuresCRYSTAL RESEARCH AND TECHNOLOGY, Issue 5 2010B. Sar, kavak Abstract We have studied structural properties of InGaAs/GaAs superlattice sample prepared by Molecular Beam Epitaxy (MBE) using high resolution X-ray diffractometer (HRXRD). Increasing strain relaxation and defect generations are observed with the increasing Rapid Thermal Annealing (RTA) temperature up to 775 °C. The higher temperatures bring out relaxation mechanisms; interdiffusion and favored migration. The defect structure and the defects which are observed with the increasing annealing temperature were analyzed. Firstly, the in-plane and out-of-plane strains after the annealing of sample were found. Secondly, the structural defect properties such as the parallel X-ray strain, perpendicular X-ray strain, misfit, degree of relaxation, x composition, tilt angles and dislocation that are obtained from X-ray diffraction (XRD) analysis were carried out at every temperature. As a result, we observed that the asymmetric peaks especially in asymmetric (224) plane was affected more than symmetric and asymmetric planes with lower polar or inclination angles due to c-direction at low temperature. These structural properties exhibit different unfavorable behaviors for every reflection direction at the increasing temperatures. The reason is the relaxation which is caused by spatially inhomogeneous strain distribution with the increasing annealing temperature. In the InGaAs superlattice samples, this process enhances preferential migration of In atoms along the growth direction. Further increase in the annealing temperature leads to the deterioration of the abrupt interfaces in the superlattice and degradation in its structural properties. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Semiconductor Nanorod Liquid Crystals and Their Assembly on a Substrate,ADVANCED MATERIALS, Issue 5 2003L.-S. Li The macroscopic alignment and superlattice structures of CdSe nanorods in a nematic liquid-crystalline (LC) phase are determined by the phases that form prior to complete solvent evaporation (e.g., vortex structures in linear arrays, see Figure). By controlling the phase of the LC solution and its orientation using pretreated surfaces or external fields, it may be possible to achieve fine control of order in deposited nanorod films. [source] Optical and magnetic properties of the DyN/GaN superlatticePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003Y. K. Zhou Abstract Ferromagnet/semiconductor DyN/GaN superlattice structures were grown on SiC substrate by radio frequency molecular beam epitaxy. Formation of DyN was confirmed by X-ray diffraction measurements. The temperature dependence of the photoluminescence (PL) spectra for the DyN/GaN superlattice and GaDyN samples was studied. Sharp PL peaks from two samples showed no shift with temperature, indicating the intra-atomic f,f transition at Dy ions. Comparison of the PL spectra for the DyN/GaN superlattice and GaDyN suggests that GaDyN is formed at the GaN/DyN interface. The DyN/GaN superlattice exhibited two ferromagnetic components; one is major at low temperatures (<50 K) and the other is major at high temperatures (>350 K). The former shows large coercivity Hc of about 1800 Oe at 7 K, which was originated from DyN layers. The latter may come from GaDyN, which was formed at the GaN/DyN interface. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Characterization of InGaN/GaN and AlGaN/GaN superlattices by X-ray diffraction and X-ray reflectivity measurementsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010S. Sintonen Abstract The commercial significance of superlattice structures is increasing due to greater demand of optoelectronic devices, such as the light emitting diode (LED). In order to optimize these devices, an accurate and reliable characterization method is needed. This paper describes in detail the characterization of superlattices with X-ray scattering techniques. The thicknesses of the individual layers are determined by X-ray reflectivity (XRR) measurements and the state of strain, the lattice constants and the compositions of ternary compounds by X-ray diffraction (XRD) measurements. The method is non-destructive, and yields unique results, unlike characterizations based on simulation of symmetric XRD scans. These simulations were used for verification of results. The method was tested on InGaN and AlGaN superlattice structures. The measured and simulated parameter values agreed very well. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Photoinduced structural dynamics of polar solids studied by femtosecond X-ray diffractionACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2010Thomas Elsaesser Femtosecond X-ray diffraction allows for real-time mapping of structural changes in condensed matter on atomic length and timescales. Sequences of diffraction patterns provide both transient geometries and charge-density maps of crystalline materials. This article reviews recent progress in this field, the main emphasis being on experimental work done with laser-driven hard X-ray sources. Both Bragg diffraction techniques for bulk and nanostructured single crystals as well as the recently implemented powder diffraction from polycrystalline samples are discussed. In ferroelectric superlattice structures, coherent phonon motions and the driving stress mechanisms are observed in real time. In molecular crystals charge-transfer processes and the concomitant changes of the lattice geometry are analyzed. [source] | ||||||||||||||||