Showing papers on "Lattice constant published in 1997"

Flux Pinning in a Superconductor by an Array of Submicrometer Magnetic Dots

Abstract: Triangular arrays of submicrometer magnetic dots, with typical spacing of 400{endash}600nm and diameters close to 200nm, have been fabricated by electron beam lithography to study pinning effects on Nb thin films. The resistivity versus magnetic field curves exhibit regular structure. Minima appear at constant field intervals, given by the lattice parameter of the dot array. The angular, current, and temperature dependencies of the resistivity imply synchronized pinning by the magnetic array which is relevant at high vortex velocities, when the order in the vortex lattice increases. {copyright} {ital 1997} {ital The American Physical Society}

Photonic crystal properties of packed submicrometric SiO2 spheres

Abstract: In this letter, we investigate the optical properties of packed monodisperse silica submicron spheres by means of optical transmission measurements. The results are compatible with a three dimensional face centered cubic order in these solid structures. The lattice parameter of these structures, and therefore their optical properties, can be easily tuned through the sphere size (between 200 and 700 nm) thus covering the whole visible and near infrared spectrum.

Use of Rietveld refinement for elastic macrostrain determination and for evaluation of plastic strain history from diffraction spectra

Abstract: Macrostrain variations in engineering components are frequently examined using neutron diffraction, at both reactors and pulsed sources. It is desirable to minimize the sampling volume in order to maximize the spatial resolution, although this increases the required measurement time. At reactors, macrostrain behavior is inferred from a single lattice reflection (deemed to be representative of the bulk response). At a pulsed source, a complete diffraction pattern is recorded and accordingly it is natural to fit the entire diffraction spectra using a Rietveld [J. Appl. Cryst. 2, 65 (1969)] refinement. This means that an idealized crystal structure is fit to the measured distorted crystal structure, which includes deviation of the measured lattice reflections from the ideal due to elastoplastic strain anisotropies, which are dependent on the particular lattice reflection (hkl) considered. We show that elastic macrostrains calculated from lattice parameter changes in Rietveld refinements (without accounting for hkl dependent anisotropies) are almost identical to the bulk elastic response and are comparable to the response obtained from a single lattice reflection typically used by practitioners at a steady state source. Moreover good refinements on the complete pattern are obtained with short measurement times compared to what is required for good statistics for single reflections. By incorporating a description of the elastic strain anisotropy expected in cubic materials into the Rietveld code, an empirical prediction of plastic strain history is possible. The validity of these arguments is demonstrated by analysis of a uniaxial tensile load test and a reanalysis of previously reported data taken on a deformed stainless steel ring. The plastic strain predictions compare favorably with a finite element model.

Determination of the Al mole fraction and the band gap bowing of epitaxial AlxGa1−xN films

Abstract: AlxGa1−xN alloys were grown on c-plane sapphire by plasma-induced molecular beam epitaxy. The Al content x was varied over the whole composition range (0⩽x⩽1). The molar Al fraction was deduced from x-ray diffraction and for comparison by elastic recoil detection analysis. The composition of the alloys calculated from the lattice parameter c underestimates x. This is due to a deformation of the unit cell. The exact Al mole fraction and the biaxial strain of the alloys can be calculated by an additional determination of a, using asymmetric reflections. The results obtained by x-ray diffraction and elastic recoil detection provide evidence for the validity of Vegard’s law in the AlGaN system. In addition, the deviation of the band gap from a linear dependence on x was investigated. We found a downward bowing with a bowing parameter b=1.3 eV.

Laser Rapid Prototyping of Photonic Band-Gap Microstructures

Abstract: Three-dimensional periodic microstructures of aluminum oxide, which are important for creating photonic band-gap structures (PBGs), were fabricated by laser rapid prototyping by means of laser-induced direct-write deposition from the gas phase. The structures consisted of layers of parallel rods forming a face-centered tetragonal lattice with lattice constants of 66 and 133 micrometers. These structures showed transmission minima centered around 4 terahertz (75 micrometers) and 2 terahertz (150 micrometers), respectively. PBGs will allow precise control of the optical properties of materials, including lasers without threshold.

Stabilization of Cubic AlN in Epitaxial AlN/TiN Superlattices

Abstract: The high-pressure rocksalt structure of AlN was stabilized in epitaxial AlN/TiN(001) superlattices with AlN layer thickness $\ensuremath{\le}2.0\mathrm{nm}$. The AlN layers were shown to be pure rocksalt-structure AlN, with a stress-free lattice parameter of $0.408\ifmmode\pm\else\textpm\fi{}0.002\mathrm{nm}$, using x-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy. The stable hexagonal phase was observed for AlN layer thickness $g2\mathrm{nm}$. The rocksalt structure formed at small layer thicknesses since it provided lower AlN/TiN interfacial energy than the hexagonal or zinc-blende structures.

H 2 O hydrogen bonding in density-functional theory

Abstract: The sublimation energy, lattice constants, and bulk modulus of a periodic model of the common phase of ice, Ih, vary dramatically when calculated with several commonly employed approximations to the electron exchange-correlation energy functional. Several gradient-corrected functionals give accurate descriptions of this hydrogen-bonded material.

Ab initio lattice dynamics of BN and AlN: Covalent versus ionic forces

Abstract: We report first-principles calculations of the structural, lattice-dynamical, and dielectric properties for zinc-blende and wurtzite BN and AlN. The ground-state properties, i.e., the lattice constants, the bulk moduli, the ionicity factors of the chemical bonds, and the elastic constants, are calculated using a plane-wave-pseudopotential method within the density-functional theory. A linear-response approach to the density-functional theory is used to derive the Born effective charges, the high-frequency dielectric constants, and the phonon frequencies and eigenvectors. The different behavior of the structural and lattice-dynamical properties of BN and AlN is discussed in terms of the different ionicities, strengths of the covalent bonds, and the atomic masses. Our results are in excellent agreement with the experimental data available. {copyright} {ital 1997} {ital The American Physical Society}

Solar battery module

Abstract: PROBLEM TO BE SOLVED: To decrease the lattice defect density of the III-V compound semiconductor film on an amorphous film by a method wherein an amorphous film is formed on the III-V compound semiconductor film formed in a heterogeneous substrate, a window is perforated, a III-VB compound semiconductor film is formed on the amorphous film using the window as a nucleus, and a solar battery is arranged therein. SOLUTION: The III-V compound semiconductor film 2 formed on a heterogeneous substrate is composed of a III-V compound semiconductor film, having the lattice constant approximate to that of GaAs, formed on the substrate 1. An amorphous film 3 is formed by oxidizing the III-V compound semiconductor film. Windows 4 are formed by photolithography and etching. A III-V compound semiconductor film 5 can be formed by the growth method generating no polycrystalline in a dielectric film such as a MOCVD method and the like. As the lower layers of the III-V compound semiconductor film 2 and the III-V group semiconductor film 5 are separated by the amorphous film 3, only the lattice defect generated in the windows 4 affects on the III-V compound semiconductor film 5, defect density does not affect the deterioration of efficiency of a solar battery 6, and lattice defect density can be lowered.

Strain dependence of surface diffusion: Ag on Ag(111) and Pt(111)

Abstract: Using density-functional theory with the local-density approximation and the generalized gradient approximation we compute the energy barriers for surface diffusion for Ag on Pt(111), Ag on one monolayer of Ag on Pt(111), and Ag on Ag(111). The diffusion barrier for Ag on Ag(111) is found to increase linearly with increasing lattice constant. We also discuss the reconstruction that has been found experimentally when two Ag layers are deposited on Pt(111). Our calculations explain why this strain driven reconstruction occurs only after two Ag layers have been deposited.

Lattice engineered compliant substrate for defect-free heteroepitaxial growth

Abstract: Presented here is proof-of-principle that a thin single crystal semiconductor film—when twist-wafer bonded to a bulk single crystal substrate (of the same material)—will comply to the lattice constant of a different single crystal semiconductor thick film grown on its surface. In our experiment, a 100 A film of GaAs was wafer bonded to a GaAs bulk substrate, with a large twist angle between their 〈110〉 directions. The resultant twist boundary ensures high flexibility in the thin film. Dislocation-free films of In0.35Ga0.65P(∼1% strain) were grown with thicknesses of 3000 A, thirty times the Matthews–Blakeslee critical thickness, on twist-wafer-bonded films of GaAs.

Atomistic studies of O2 dissociation on Pt(111) induced by photons, electrons, and by heating

Abstract: The adsorption and subsequent dissociation of O2 on Pt(111) was studied by variable temperature scanning tunneling microscopy in the temperature range of 40 to 215 K. Tight clustering of bridge site molecules is observed on terraces between 40 and 70 K, indicating a highly mobile precursor to chemisorption. Coexistence of bridge and fcc hollow site molecules in fractal-shaped islands is observed after dosing between 70 and 95 K. Dissociation of these species was induced by uv radiation, inelastic tunneling electrons, and heating. In all three cases, two O atoms are found within two lattice constants of the original molecule and one to three lattice constants apart.

Local lattice distortions and thermal transport in perovskite manganites

Abstract: Measurements of thermal conductivity versus temperature and magnetic field are reported for perovskite manganites that exhibit ferromagnetic (FM), charge-ordering (CO), antiferromagnetic, and/or structural phase transitions. The data reveal a dominant lattice contribution to the heat conductivity with $\ensuremath{\kappa}\ensuremath{\sim}1\ensuremath{-}2$ W/mK near room temperature. The rather low values, implying a phonon mean free path on the order of a lattice spacing, are shown to correlate with static local distortions of the ${\mathrm{MnO}}_{6}$ octahedra. Modifications of the local structure are responsible for abrupt anomalies in the zero-field \ensuremath{\kappa} at the FM, CO, and structural transitions, and for colossal magnetothermal resistance near the FM transition.

Laser diodes based on beryllium-chalcogenides

Abstract: Beryllium chalcogenides have a much higher degree of covalency than other II–VI compounds. Be containing ZnSe based mixed crystals show a significant lattice hardening effect. In addition, they introduce substantial additional degrees of freedom for the design of wide gap II–VI heterostructures due to their band gaps, lattice constants, and doping behavior. Therefore, these compounds seem to be very interesting materials for short wavelength laser diodes. Here, we report on the first fabrication of laser diodes based on the wide band gap II–VI semiconductor compound BeMgZnSe. The laser diodes emit at a wavelength of 507 nm under pulsed current injection at 77 K, with a threshold current of 80 mA, corresponding to 240 A/cm2.

Electronic structure and bonding of ultralight LiMg

Abstract: Electronic structure calculation using the Tight Binding Linear Muffin Tin Orbital (TB-LMTO) method has been performed for ultralight LiMg alloys. The band structure, density of states, valence electron charge density and Fermi surface have been computed. The lattice constant, cohesive energy, heat of formation at the equilibrium lattice constant, and bulk modulus are in agreement with available experimental and theoretical results. The calculation shows charge transfer from Mg to Li site in agreement with positron annihilation experiment of Y. Tsuchiya et al. (J. Phys. F8, L29 (1978)).

High-resolution x-ray diffraction from multilayered self-assembled Ge dots

Abstract: We have studied multilayers of self-assembled Ge-rich dots embedded in silicon grown by molecular-beam epitaxy using high resolution x-ray reciprocal space mapping and reflectivity. The Si spacer thicknesses between the dot arrays were in the range of 10‐40 nm, the typical dot size was about 150 nm for the diameter and 7 nm for the height. The measured reciprocal space maps were simulated using statistical kinematical x-ray-diffraction theory, and a good agreement between experimental and simulated data has been achieved. From the measurements, the in-plane strain in the dot lattice was determined. We derived the degree of the vertical correlation of the dot positions ~‘‘stacking’’ ! and a lateral ordering of the dots in a square array with main axes parallel to the ^100& directions, with an array lattice constant of about 500 nm. @S0163-1829~97!05623-3#

Optical properties of GaN grown on ZnO by reactive molecular beam epitaxy

Abstract: High quality wurtzite GaN epilayers have been grown on ZnO(0001) substrates by reactive molecular beam epitaxy. Photoluminescence and reflectivity measurements point to high quality presumably due to the near match of both the crystal lattice parameter and the stacking order between GaN and ZnO. In addition, the good films lack the characteristic yellow photoluminescence band. Any misorientation of the GaN epilayer planes with respect to the ZnO substrate is not detectable with polarized reflectivity. The x-ray double crystal diffraction measurements indicate this misorientation is much smaller than those for GaN epilayers on SiC and Al2O3 .

Structure characteristics of nanocrystalline element selenium with different grain sizes

Abstract: Porosity-free nanocrystalline (nc) element Se samples with mean grain sizes ranging from 13 to 70 nm were synthesized by crystallizing a melt-quenched amorphous Se solid. Microstructures of the nc-Se (with a hcp structure) samples were characterized by means of quantitative x-ray-diffraction measurements. The Bragg reflection-and the background intensities, as well as the reflection shape of the x-ray-diffraction patterns for the ne Se were analyzed according to data fitting of Me measurement results. The grain-size dependencies of the microstrain, lattice parameters, unit-cell volume, and the mean Debye-Waller parameter were determined. With a reduction of grain size, the microstrain increases significantly along [100] direction but decreases along [104] direction, and exhibits an increasing anisotropic microstrain behavior. The lattice parameter a was found to increase evidently while c decreased slightly with a decreasing gain size, resulting in a significant lattice distortion vith a dilated unit-cell volume. it agrees with the observation that the mean Debye-Waller parameter increases with a reduction of grain size, suggesting larger displacements of atoms from their ideal lattice sites in the nc-Se samples with smaller grains. The similarity of the grain-size dependencies of these structural parameters as that of the grain-boundary volume fraction implies that the intrinsic microstructure feature of ne materials is closely related to the crystallite dimension and the amount of grain boundaries.

Infrared and X-ray studies of hydrogen intercalation in different tungsten trioxides and tungsten trioxide hydrates

Abstract: Hydrogen intercalation via spillover reaction in various tungsten trioxides leads to the formation of blue hydrogen bronzes. These reversible reactions induce changes in the W-O bond system while maintaining the W-O skeleton. The effect of the intercalation process on the host crystalline structure has been studied with respect to the ν(O-W-O) stretching vibration changes and lattice parameter variations by means of infrared and X-ray diffraction measurements. Among the main results, the intercalation process is shown to be strongly influenced by the structural type of the host compound as well as its amorphous versus crystalline nature. For instance, for the ReO3 type oxides (monoclinic and cubic WO3) and hexagonal WO3, ν(O-W-O) shifts to higher frequency are assigned to a shortening effect of W-O bonds. A W-O bond system arrangement is also measured for the crystallized and amorphous hydrates WO3 · H2O, but no detectable changes could be found in the pyrochlore WO3 and in the hydrate WO3·1/3 H2O.

Quenched-in vacancies in B2-structured intermetallic compound FeAl

Abstract: The concentrations of quenched-in vacancies retained in B2 intermetallic compound Fe1-cAlc were obtained as functions of composition (0.39 < c < 0.51) and quenching temperature (773–1273 K) from lattice constant and density measurements. Obtained vacancy concentration indicates rather gradual increase with composition in lower Al content region, while it increased rapidly as the composition approaches to stoichiometric composition. Further, the data for slow-cooled (1 K min−1) samples showed that retained vacancy concentration is higher than that for samples annealed at and quenched from 773 K. Observed lattice constant for each composition decreases linearly with vacancy concentration, which is interpreted in terms of atomic size effect. From changes in vacancy concentration with quenching temperature, apparent vacancy formation energies were estimated. The relation between vacancy concentration and microhardness was also examined. Present results of dependency of lattice constant on heat treatment condition confirm the hardening effect due to retained vacancy. Further, the proposed linear relation of hardness to the square root of vacancy concentration is supported by the present study.

Composition of AlGaAs

Abstract: Although the AlxGa1−xAs alloy system has been extensively investigated, there are still considerable uncertainties in measuring the value of x. Here a new AlxGa1−xAs calibration structure, grown by molecular beam epitaxy, has been used to establish unambiguous alloy compositions. Such “standard’’ AlxGa1−xAs layers were measured by high-resolution x-ray diffraction, photoluminescence, and Raman spectroscopy to determine the compositional variations of the measured physical parameters. The phenomenological equations derived from these measurements can now be used to establish the Al content of unknown alloys with confidence. In addition, the results show that Vegard’s law does not hold for the variation of the AlxGa1−xAs lattice constant with x. The small quadratic term has very important implications for a correct analysis of x-ray results.