Bruce Steiner

Photodetachment of Electrons from SH

The cross section for photodetechment of electrons from SH− for the first 0.75 eV above threshold (λ = 534.7 to 403.0 nm) has been measured in a crossed‐beam experiment. The cross section rises rapidly in the first 0.15 eV (Δλ = 35 nm) to the value 1.9 × 10−17 cm2 determined with a total uncertainty of ± 0.4 × 10−17 cm2 given by the square root of the sum of the squares of the maximum observed deviation and the allowances for various systematic factors; over the rest of the observed range it is constant. The detailed shape of the cross section vs λ curve has been employed to derive the SH− structural parameters (and their upper limits of uncertainty), which are indistinguishable from those of the neutral free radical: R = 0.135 ± 0.002 nm, ωe = 2700 ± 300 cm−1, and Be = 9.46 ± 0.32 cm−1. The electron affinity is identified with the observed photodetechment threshold, 2.319 ± 0.010 eV. These results are in general agreement with the recent calculations of Cade. Evidence is presented for the importance of i...

Direct x-ray synchrotron imaging of strains at 180° domain walls in congruent LiNbO3 and LiTaO3 crystals

We present direct evidence for wide regions of strain on length scales of many micrometers associated with 180° domain walls in congruent LiNbO3 and LiTaO3 crystals. Strain contrast in symmetric high-resolution diffraction images of these crystals in Bragg geometry arises in part from curvature in the basal planes (0001) across a domain wall as well as from lateral variation in the lattice spacing of the basal planes extending over many micrometers in going across a wall.

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.

Electrically based spectral power measurements through use of a tunable cw laser

A new approach to radiant power measurements is described. A continuously tunable cw dye laser was used to measure the absolute spectral response of a silicon photodiode and narrow band−pass filter by comparison with an electrically calibrated pyroelectric detector. The filtered photodetector was then used to measure the spectral power density from a standard lamp that had been calibrated by the classical technique. The agreement of better than 1% between the two measurements is well within the uncertainties identified with each measurement individually, of the order of 1%. A number of advantages in the new technique are identified.

Enhancement of mercuric iodide detector performance through increases in wafer uniformity by purification and crystal growth in microgravity

Wafers from mercuric iodide crystals grown in microgravity on two occasions have previously been found to be characterized by a higher hole mobility-lifetime product, which enables energy dispersive radiation detectors with superior resolution. In the present work, we have identified the specific structural modifications that are responsible for this enhanced performance. As a result of this study, the performance of terrestrial wafers also has been improved but not yet to the level of wafers grown in microgravity. High resolution synchrotron x-ray diffraction images of a series of wafers, including those grown both in microgravity and on the ground, reveal two principal types of structural changes that are interrelated. One of these, arrays of inclusions, affects performance far more strongly than the other, variation in lattice orientation. Inclusions can be formed either from residual impurities or in response to deviations from ideal stoichiometry. The formation of both types is facilitated by gravity...

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.

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...

On Domain Wall Broadening in Ferroelectric Lithium Niobate and Tantalate

We present evidence for broad regions of domain wall strains extending over micrometers in ferroelectric lithium niobate and tantalate due to the presence of cation nonstoichiometry. The influence of a corresponding polarization broadening on intrinsic coercive fields of ferroelectrics is explored.

Residual disorder in low pressure, low thermal gradient liquid encapsulated Czochralski gallium arsenide observed in high resolution synchrotron diffraction imaging

High resolution synchrotron X-radiation diffraction images of low thermal gradient, low pressure, liquid encapsulated Czochralski (LEC) silicon-doped gallium arsenide display a degree of crystalline order that is far higher than that found in undoped conventional LEC material. No distinct volume irregularity is observed in one crystal where it is unstrained. Only a set of surface-treatment-related dislocations is prominent in a second. A third crystal exhibits two distinct sets of quasi periodic dislocations, which have been analyzed in detail. The results support a model previously proposed for the evolution of disorder in gallium arsenide and suggest the role that silicon plays in its control.

Growth and characteristics of tgs crystals grown aboard first international microgravity laboratory (IML-1)

Abstract The growth and detector characteristics of TGS crystals grown in a low gravity environment of space are presented. High resolution monochromatic synchrotron X-ray diffraction imaging of the space grown crystal indicates an extraordinary crystal quality.