Uri Laor

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...

Superposition of thermal loads model for measuring thermal diffusivity in solids

Abstract A new method for the measurement of the thermal diffusivity in solids using transient conditions is demonstrated. This method utilizes the principle of superposition of thermal loads. Only time measurements are required, and no accurate temperature and heat flow rate measurements are needed. The method utilizes the Seebeck effect for the measurement of the time dependence of the temperature. A solution of the heat transfer equation for the model pertinent to the described method is derived, and results obtained for several materials are presented.

Keys to the Enhanced Performance of Mercuric Iodide Radiation Detectors Provided by Diffraction Imaging

High resolution monochomatic diffraction imaging is playing a central role in the optimization of novel high energy radiation detectors for superior energy resolution at room temperature. In the early days of the space program, the electronic transport properties of mercuric iodide crystals grown in microgravity provided irrefutable evidence that substantial property improvement was possible. Through diffraction imaging, this superiority has been traced to the absence of inclusions. At the same time, other types of irregularity have been shown to be surprisingly less influential. As a result of the knowledge gained from these observations, the uniformity of terrestrial crystals has been modified, and their electronic properties have been enhanced. Progress toward property optimization through structural control is described.

High resolution monochromatic synchrotron X-radiation diffraction imaging of triglycine sulfate (TGS) crystals

In order to shed light on the principal irregularities found in various optical crystals for infrared detectors, their origins, and their influence on device performance, we have carried out high resolution monochromatic synchrotron X-ray diffraction imaging of triglycine sulfate (TGS) crystals. Images of crystals grown during the First International Microgravity Laboratory (IML-1) flight in 1992 were taken and compared with others of terrestrially-grown crystals. The local acceptance angle for diffraction from the uncut TGS crystal grown on the IML-1 mission was found to be 1-2 arc seconds, indicating extraordinary crystal uniformity. The only inclusions visible in the topographs were polystyrene particles intentionally introduced into the growth solution of the space crystal. No demarcation was seen between the terrestrial seed and the new growth of the crystal grown in space. TGS crystals grown on the ground were found to have edge dislocations near the cleaved surface. The results of the topographic studies of the space- and ground-grown crystals are discussed and their implications identified.

Defects in III-V materials and the accommodation of strain in layered semiconductors

High resolution monochromatic synchrotron-radiation diffraction images of five, high quality epitaxial heterojunctions on silicon, gallium arsenide, and indium phosphide substrates display several forms of accommodation to lattice mismatch. From the images, we deduce a coherent set of factors for the loss of crystalline order in layered semiconducting crystals. Lattice mismatch is demonstrated in each of the systems by warping after layer deposition. Nevertheless, local lattice orientation is maintained across each layer interface. In two of the systems, one severely mismatched while the other is not, no arrays of dislocations appear. Sets of mixed linear lattice mismatch dislocations, consistent with identification as 60° dislocations, are found in two of the other systems with intermediate degrees of mismatch. A set of pure edge dislocations penetrating all layers is found in a system with a grid structure. These observations indicate that the formation of extensive arrays of dislocations during uniform one micrometer layer deposition depends not only on the extent of lattice mismatch and layer thickness but also on the degree of crystalline order of the substrate. Establishment of a nonpseudomorphic layer mismatched with the substrate by several tenths of a percent is an important factor, as previously determined. However, localized absence of crystalline order, e.g. in the form of scratches or dislocations in the substrate, appears also to be required for the formation of arrays of interface mismatch dislocations. Where these criteria are not fulfilled, the formation of dislocations in uniform layered systems is inhibited. Localized residual stress can initiate dislocation formation even where it would not appear in uniform layers. The images show also that crystalline disorder in state-of-the-art indium phosphide differs markedly from that in comparable gallium arsenide. Understanding of crystalline order in both monolithic materials is extended by this work.

A New Method for Thermal Diffusivity Measurements in Metals Utilizing the Thermo-Electric Effect

A new method for the measurement of the thermal diffusivity in metals, using transient conditions, is demonstrated. This method is using the principle of superposition of thermal loads. Only time measurements are required, and no accurate temperature and heat flow rate measurements are needed. The method utilizes the Seebeck effect for the measurement of the time dependence of the temperature. A solution of the heat transfer equation for the model pertinent to the described method is derived, and results obtained for several materials are presented.