Ronald C. Dobbyn

Growth and characterization of Cd1−xZnxTe and Hg1−yZnyTe

Abstract We describe the modified vertical Bridgman conditions required for bulk growth of Cd 1-x Zn x Te and Hg 0.87 Zn 0.13 Te. Good quality single crystals were obtained, with dislocation densities of approximately 4x10 4 to 1x10 5 cm -2 . No precipitates were observed under IR microscopic examination. The Hg 0.87 Zn 0.13 Te crystal exhibited n-type behavior after a 250°C mercury saturated post-anneal, with a carrier density of 8x10 15 cm -3 and mobility of 1.5x10 5 cm 2 /V·s at 77 K. X-ray lattice constant measurements and atomic absorption analysis were used to establish accurately the zinc mole fraction in the crystals. The structural properties were studied using X-ray diffraction and topography; photoreflectance, a contactless form of the electric field modulated reflectivity technique, was used for optical characterization.

Insight into the genesis of irregularity during crystal growth achieved monochromatic high sensitivity monochromatic synchrotron x-radiation diffraction imaging (topography)

Abstract Lattice irregularities observed in monochromatic synchrotron x-radiation diffraction imaging (topography) are summarized. Important recent advances in sensitivity and resolution are described. The resulting insight into the genesis of variation in high quality crystals is reviewed. Finally, areas in which additional progress can be anticipated in light of these advances are indicated.

A study of the coarsening of liquid-solid mixtures using synchrotron radiation microradiography

The coarsening of liquid-solid mixtures with a high volume fraction of solid is an important process in metallurgy. The development of secondary arm spacings in castings and the sintering of powders using a liquid phase are but two examples. The theory of Ostwald ripening is widely used to model some of the events during these processes, yet microstructural details important at a high volume fraction of solid are usually ignored. The authors report preliminary studies on the coarsening of a liquid-solid mixture observed at temperature by microradiography using synchrotron radiation where local microstructural features can be examined in situ.

Diffraction imaging of high quality bismuth silicon oxide with monochromatic synchrotron radiation: Implications for crystal growth

Abstract Three slices from a high quality boule of bismuth silicon oxide have been examined by X-ray diffraction imaging (topography) with monochromatic synchrotron radiation. The absence of macroscopic inhomogeneous strains, which are usually found in large single crystals, permits us to observe several types of microscopic strain pattern in the form of growth striations and interface boundaries. Each is associated with distinct aspects of the crystal growth. Analysis of these strains leads to a detailed understanding of the formation of a high quality bismuth silicon oxide boule. The model developed suggests ways to realize further improvement in crystal perfection.

NBS materials science beamlines at NSLS

Abstract Synchrotron radiation beamlines for topography, spectroscopy and small angle scattering at energies from 5 to 20 keV, are described.

Real time topography with X-ray image magnification

Abstract An X-ray optical configuration for real time synchrotron ratiation topography is described. Asymmetric diffraction from perfect Si crystals is used to control the beam size, wavelength, and collimation before the sample, and to magnify the X-ray image after the sample. Preliminary results using this system are reported. Video images of moving magnetic domain walls under a varying magnetic field were obtained from Ni single crystals in the anomolous transmission geometry.

Basic Biological Sciences Application of an X-ray Image Magnifier to the Microradiography of Dental Specimens

A highly parallel incident x-ray beam combined with x-ray image magnification was used to obtain high-resolution microradiographs of dental specimens. Preliminary results obtained using a rotating anode x-ray generator show that limitations associated with conventional contact microradiography regarding (1) spatial resolution, (2) sample thickness, and (3) sample orientation, relative to the film, were significantly reduced.

Structural anomalies in undoped gallium arsenide observed in high-resolution diffraction imaging with monochromatic synchrotron radiation

Novel, streaklike disruption features restricted to the plane of diffraction have recently been observed in images obtained by synchrotron radiation diffraction from undoped, semi‐insulating gallium arsenide crystals. These features were identified as ensembles of very thin platelets or interfaces lying in {110} planes, and a structural model consisting of antiphase domain boundaries was proposed. We report here the other principal features observed in high resolution monochromatic synchrotron radiation diffraction images: (quasi)cellular structure; linear, very low‐angle subgrain boundaries in 〈110〉 directions, and surface stripes in a 〈110〉 direction. In addition, we report systematic differences in the acceptance angle for images involving various diffraction vectors. When these observations are considered together, a unifying picture emerges. The postulate of thin {110} antiphase boundaries leads to an understanding not only of the streak‐like diffraction features but of the other principal features a...

Parallel beam microradiography of dental hard tissue using synchrotron radiation and X-ray image magnification

Abstract A novel technique utilizing a highly parallel beam of monochromatic synchrotron radiation combined with X-ray image magnification has been used to obtain microradiographs of caries lesions in relatively thick tooth sections. Preliminary results reveal structural features not previously reported. This technique holds the promise of allowing one to follow the structural changes accompanying the formation, destruction and chemical repair of mineralized tissue in real time.

Microradiography with an x‐ray image magnifier: Application to dental hard tissue

The superior spatial resolution obtained with parallel-beam microradiography over conventional contact microradiography has allowed us to image microstructural features of dental hard tissue not previously reported. Our efforts to extend these techniques to provide a real-time capability for viewing in situ demineralization and remineralization effects, at and below the 1-micron level, have resulted in an instrument with several novel and unique features. Using a synchrotron radiation source of x rays and diffraction image magnification, we are now able to change magnification at will (x-ray zoom lens). In addition, the energy range over which the instrument operates gives one considerable flexibility in optimizing image contrast. The techniques of parallel-beam microradiography, and diffraction image magnification are applicable to problems in many other areas of science. Using examples within dental research, the uniqueness and versatility of these new techniques are discussed.