Search

We have used high-resolution triple-axis x-ray diffraction analyses to monitor the defect structure in tetragonal crystals of hen egg-white lysozyme as a function of x-ray irradiation time. At long irradiation times we observed the expected decrease in peak intensity and increase in the angular extent of the peak breadth. In contrast, the initial stages of irradiation showed relatively complex changes in both the peak breadth and the intensity; in fact, during the period from 25 to 45 h of irradiation the angular breadth of the intensity (both the full-width at half-maximum and the full-width at 1% of the maximum intensity) decreased to a minimum value. We have found that the unambiguous analysis of defects at high angular resolution is complicated by the fact that the diffraction characteristics of protein crystals apparently lie at the confluence of the kinematic (ideally imperfect) and dynamic (ideally perfect) treatments of diffraction.

Lithium gallate (LiGaO2) is gaining increasing attention as a potential substrate for the growth of the important semiconductor GaN. In order to better understand this material we have performed high-resolution double- and triple-axis x-ray diffraction analyses of both the starting LiGaO2 and GaN/LiGaO2 following epitaxial growth. A high-resolution triple-axis reciprocal space map of the substrate showed a sharp, well-defined crystal truncation rod and a symmetric streak of intensity perpendicular to q002, suggesting high structural quality with mosaic spread. Triple-axis scans following GaN growth showed (1) the development of isotropic diffuse scatter around the LiGaO2 (002) reflection, (2) the presence of a semi-continuous intensity streak between the LiGaO2 (002) and GaN (0002) reflections, and (3) a compact pattern of diffuse scatter around the GaN (0002) reflection that becomes increasingly anisotropic as the growth temperature is increased. These results suggest that LiGaO2 permits the epitaxial growth of GaN with structural quality that may be superior to that observed when growth is performed on SiC or Al2O3.

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.

Two novel x-ray diffraction techniques with enhanced surface sensitivity, grazing incidence x-ray diffraction (GIXD) and inclined Bragg plane x-ray diffraction (IBXD), have been used to study surface damage in gallium arsenide (GaAs) due to bromine/methanol (Br2/MeOH) chemical mechanical (CM) polishing. A factorial design was implemented to determine the effects of four polishing variables on the surface structure of GaAs. Precise lattice parameter measurements were made in both the surface regions using GIXD and deeper into subsurface regions using IBXD after the various CM polishing treatments. Bromine concentration was found to primarily affect the surface lattice parameter, while the total polish time influenced both the surface and subsurface lattice parameters in GaAs samples that were heavily damaged prior to CM polishing. The combined effect of polishing pad rotation speed and the force exerted on the sample was found to have a much greater effect on the surface lattice parameter than either variable had alone.

A novel pre-resonant conductance structure is observed in single and double well GaAs/Ga/sub x/Al/sub 1-x/As resonant tunneling heterostructures. This structure is attributed to single electron tunneling through donor bound states in the quantum well as they cross the Fermi level. Peaks in conductance are observed in devices as large as (64/spl mu/)/sup 2/. We observe for the first time donor binding energies in quantum wells as large as 35 meV, which is probably due to the formation of a donor complex. An impressive impurity tunneling conductance structure is observed in double quantum well structures with conductance peak heights varying over 3 orders of magnitude. Conductance peaks are observed before and after the main resonant current peak which are attributed to tunneling of electrons through impurities in one quantum well and the quantum state in the other well. >

An X-ray powder diffraction analysis has been performed on several samples from the naturally occuring patina of the Statue of Liberty. This work, which was conducted as a service to the National Park Service as part of the restoration activities for the Statue, was performed to assess the impact of acid deposition on the phase composition of the patina. Samples of the patina that were obtained from various locations on the copper skin of the Statue were found to consist primarily of the basic copper sulfate known as brochantite or CuSO4·3Cu(OH)2. Another less stable form of basic copper sulfate CuSO4·2Cu(OH)2 known as antlerite was also observed in samples taken from areas that are more exposed to the incoming weather in New York harbor. The presence of antlerite supports the contention that acid deposition is promoting undesirable changes in the phase composition of the patina. Analyses were also performed on patina samples that were taken from pieces of the Statue's copper skin that had been removed in the years 1905 and 1980. X-ray powder diffraction of the corrosion product on the 1905 sample showed that it consisted primarily of the stable brochantite phase, while the 1980 sample displayed both copper chlorides as well as the less stable antlerite. Both samples also contained cuprite (Cu2O) which appears to have formed prior to either of the sulfates.

The angular misorientation of GaAs epitaxial layers grown on silicon substrates by molecular beam epitaxy has been measured using x-ray diffraction. A significant misorientation, or tilt, between the epitaxial layer and the substrate has been observed. The magnitude of the tilt depends on the initial substrate orientation, the silicon substrate type (float zone or Czochralski), postgrowth annealing, and epitaxial layer thickness. In almost all cases, the sense of the tilt is such that the GaAs 〈001〉 lies between the surface normal and the silicon 〈001〉. While the presence of interfacial dislocations with Burgers vectors that are approximately parallel to the heterointerface does predict a tilt between the substrate and the epitaxial layer, the sense of the tilt that arises from these dislocations is opposite to that observed experimentally. A model, based on the relief of misfit by dislocations inclined approximately 45 degrees to the interface, is proposed that correctly describes the observed tilt.

Fibers of PET were drawn at 80°C and investigated by small-angle x-ray scattering (SAXS) after annealing with free ends at temperatures up to 240°C. While the as-drawn fiber had no identifiable discrete SAXS pattern, evidence for an alternating two-phase microstructure was seen after annealing at 100°C. Annealing at temperatures up to 200°C resulted in well oriented SAXS patterns which doubled in integrated intensity. During this stage of (low-temperature) annealing, it is envisioned that oriented amorphous regions partially crystallize, leading to transverse growth of oriented microfibrils. Annealing at or above 225°C causes the SAXS pattern to become arced, implying disorientation of the fiber via the presence of a largely molten structure. Quenching from these highest annealing temperatures results in a slight decrease in the SAXS intensity which is attributed to the formation of thicker transition zones at the crystal-amorphous interface during rapid crystallization. By comparison to previous...