High-resolution X-ray Diffraction Methods Research Articles

Structural characterization of GaAs grown at low temperatures by molecular beam epitaxy

High-resolution X-ray diffraction methods have been used to characterize the structural characteristics of GaAs grown by molecular beam epitaxy at low substrate temperatures. Triple-crystal X-ray diffraction scans demonstrate that, as the As4/Ga incorporation ratio was increased, there occurred increases in the magnitude of diffuse scattering as well as an anisotropy in the scattering. A degradation of the crystal truncation rod is also observed. Differences have also been observed in the diffuse scattering from n- and p-doped GaAs samples grown in this way; we attribute this to changes in the kinetics of arsenic incorporation due to the electrical characteristics of the material. Analysis of the annealed GaAs layers has shown the development of long-range strain fields from distortions generated by the growth of the arsenic precipitates. The diffuse scattering has been used to develop estimates of the average size of the arsenic precipitates.

Structure and Oxygen Affinity of Crystalline of DesArg141α Human Hemoglobin A in the T State

The correlation of a protein structure determined crystallographically to its functional properties determined in solution can be an extremely complex problem due to potential differences of protein conformational flexibility in the two physical states. A more direct approach to the correlation of structure with function is to examine both the structure and the function of a protein in the same crystalline environment. In this paper, the structural and functional properties of T state desArg hemoglobin (human hemoglobin modified by removal of the α-chain COOH-terminal residue, Arg141α) have been studied in the same crystal form by high resolution X-ray diffraction methods and by polarized absorption microspectrophotometry. Specifically, the crystal structure of deoxygenated desArg human hemoglobin has been refined at a 2.1 Å resolution using crystals grown at low salt concentration from solutions of polyethylene glycol. The loss of Arg141α and all of the salt bridges in which it participates is associated with subtle structural perturbations of the α-chains which include an increase in the conformational flexibility of both the NH 2and COOH-terminal peptides. Although the heme pockets appear unchanged and even the side-chain of Tyr140 is oriented nearly as in HbA, the functional characterization by microspectrophotometric measurements indicates that crystals of desArg hemoglobin bind oxygen with an affinity which is roughly 15-fold greater than that of crystals of human hemoglobin A. There is no alkaline Bohr effect or effect of chloride ions, but an acid Bohr effect is observed. The oxygen affinities measured along two principal axes of the crystals differ by 25% indicating heterogeneity in the affinities of the oxygen binding sites. This finding and the measured Hill coefficient of unity suggest significant cooperativity in the binding of oxygen in these crystals. The origins of the observed heterogeneity and the implied cooperativity are unknown.

Depth Dependence of Residual Strains in Textured Mo Thin Films Using High-Resolution X-Ray Diffraction

AbstractThe magnitude of the average stress in a thin film can be obtained by measuring the curvature of the film-substrate couple. However, the details of the strain distribution, as a function of depth through the thickness of the film, can have important consequences in governing film quality and ultimate morphology. A high-resolution x-ray diffraction method was used to determine the depth dependence of strain in a textured Mo film, with a nominal thickness of 260 nm, which was deposited by planar magnetron sputtering onto Si (100) substrates. The principal strains, resolved onto a laboratory reference frame, displayed a negligible gradient in the azimuthal directions (x and y), but displayed a large gradient in the direction normal to the film (z). A similar trend was previously observed for a 100 nm polycrystalline film, but the magnitude of the normal strain very near the free surface was about a factor of 2 less. The increase in the normal strain may be due to the development of a preferred growth direction and grain facetting. A linear elastic model was also used to determine the strains in successive slabs of the film, where strain variations between slabs were indicated.

Investigation of lattice distortions in InP crystals implanted with Fe+ ions by means of high-resolution x-ray diffraction and x-ray standing-wave methods

Lattice distortions due the implantation of Fe+ ions in InP semi-insulating crystals have been investigated by means of high-resolution x-ray-diffraction and x-ray standing-wave methods. The effects of both the implantation dose and the annealing time were studied. It is shown that the x-ray standing-wave method provides valuable complementary information on strain and damage in the subsurface layer and permits one to distinguish between different distortion profiles that give practically the same kinematical diffraction curve.

High Resolution X-Ray Study on Anomalous Diffraction Peak Shift in Dimerized Langmuir-Blodgett Superlattice Films

Structures of Langmuir-Blodgett (LB) superlattice films formed with alternately deposited fatty acid Cd salts with different alkyl-chain length are studied by high resolution X-ray diffraction method using a synchrotron radiation source. The remarkable diffraction peak shift is observed in the superlattice film. The shift is interpreted in terms of the Guinier effect in partially-disordered superlattice system. Simultaneous modulation of molecular position and of molecular scattering factor results in the peak shift. The effect of alkyl-chain species substitution and of cooling on the peak shift are also described. The partial disorder in the superlattice is caused by dissolution of LB film.

Structural analysis of as-deposited and annealed low-temperature gallium arsenide

The structure of GaAs grown at low substrate temperatures (LT-GaAs) by molecular beam epitaxy has been studied using high resolution X-ray diffraction methods. Double crystal rocking curves from the as-deposited LT-GaAs show well defined interference fringes, indicating a high level of structural perfection. Triple crystal diffraction analysis of the as-deposited sample showed significantly less diffuse scattering near the LT-GaAs 004 reciprocal lattice point compared with the substrate 004 reciprocal lattice point, suggesting that despite the incorporation of approximately 1% excess arsenic, the epitaxial layer had superior crystalline perfection than did the GaAs substrate. Triple crystal scans of annealed LT-GaAs showed an increase in the integrated diffuse intensity by approximately a factor of three as the anneal temperature was increased from 700 to 900°C. Analogous to the effects of SiO2 precipitates in annealed Czochralski silicon, the diffuse intensity is attributed to distortions in the epitaxial LT-GaAs lattice by arsenic precipitates.

High resolution x-ray diffraction analysis of annealed low-temperature gallium arsenide

High resolution x-ray diffraction methods have been used to characterize GaAs grown at low substrate temperatures by molecular beam epitaxy and to examine the effects of post-growth annealing on the structure of the layers. Double crystal rocking curves from the as-deposited epitaxial layer show well-defined interference fringes, indicating a high level of structural perfection despite the presence of excess arsenic. Annealing at temperatures from 700 to 900 °C resulted in a decrease in the perpendicular lattice mismatch between the GaAs grown at low temperature and the substrate from 0.133% to 0.016% and a decrease (but not total elimination) of the visibility of the interference fringes. Triple-crystal diffraction scans around the 004 point in reciprocal space exhibited an increase in the apparent mosaic spread of the epitaxial layer with increasing anneal temperature. The observations are explained in terms of the growth of arsenic precipitates in the epitaxial layer.

Structural studies of phase transformations in ultrafine zirconia powders

The factors governing the existence of metastable cubic and tetragonal phases in zirconia powders are still controversial. In order to elucidate this question, the effects of calcination temperature on ultrafine powders prepared from different precursors by different low-temperature chemical routes were studied. The morphological and structural characteristics of the powders depending on the calcination temperature were determined by means of conventional and high-resolution transmission electron microscopy (CTEM and HREM) and X-ray diffraction methods (whole and peak profile fitting procedures). Important structural differences between the powders depending on the precursor were revealed by HREM. The presence of impurities and/or microstrains seems to play a major role in the stabilization of the cubic and the tetragonal phases.

Unusual X-Ray Diffraction Vector Shift Observed in Heterogeneous Langmuir-Blodgett Multilayer Films

ABSTRACTStacking of heterogeneous Langmuir-Blodgett films was studied by high resolution X-ray diffraction method using synchrotron radiation (SR) source. The diffraction patterns of the heterogeneous films formed with alternately deposited Cd salts of fatty acids with different length of alkyl chains exhibit anomalous oscillatory structure in diffraction vector.

Water structure in vitamin B12 coenzyme crystals. I. Analysis of the neutron and x-ray solvent densities

The disordered solvent distribution in crystals of vitamin B12 coenzyme was examined using the methods of high-resolution neutron and x-ray diffraction. One set of neutron (0.95 A) and two sets of x-ray (0.94 and 1.1 A) data were collected and the resulting models were extensively refined using least-squares and Fourier syntheses. The solvent regions were analyzed in two stages: first, main sites were assigned to the well defined regions of solvent density and refined using least squares; second, continuous sites were assigned representing the more disordered diffuse and elongated regions of solvent density. During the analysis an acetone molecule was also located. Water networks were formulated from the assigned sites in the above models and also from those assigned in the original structure determination (Lenhert, 1968), using criteria that included hydrogen bonding (derived from small crystal hydrates), van der Waals contact distances, side-chain disorder, water molecule orientations, and the presence or absence of foreign solvent. The well established networks extend throughout all the solvent regions of the crystal with interesting orientational arrangements of the individual waters around both polar and apolar groups of the coenzyme molecule. The networks were seen to be consistent among each of the four models in terms of occupying relatively similar positions. However, the occupancy values of the individual networks varied between the models; some networks were clearly visible in one but attenuated in another. The specific details of the water structure (bonding geometries, short-range nonbonded contacts, orientations of the waters, polar and apolar interactions, etc.) are described in the following paper.

Study of microstructural defects induced by microwave electric fields in silicon single crystals by a high-resolution X-ray diffraction method

Study of microstructural defects induced by microwave electric fields in silicon single crystals by a high-resolution X-ray diffraction method

Study of microstructural changes in silicon single crystals subject to high electric fields by using a high resolution X-ray diffraction method

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