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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-driven convection during growth. As the level of inclusions is reduced, through growth from material of higher purity, through the achievement of balanced stoichiometry, or by suppression of convection mixing during crystal growth, the hole mobility-lifetime product is enhanced in spite of an accompanying decreased uniformity in lattice orientation. Sixfold enhancement in the performance of x- and γ-ray detectors has been accomplished to date. Further augmentation in performance appears likely.

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.

High resolution monochromatic synchrotron X-radiation diffraction images of several high quality multilayer systems suggest several factors in the establishment of irregularities in layerd semiconducting crystals. The nucleation of extensive arrays of dislocations during uniform one micro meter layer deposition appears to depend not only on the extent of lattice mismatch and layer thickness but also on the regularity of the substrate. Propagation of arrays of mismatch dislocations appears to depend on the character of the unit cell.

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.

Gravity can affect the physical vapor growth of mercuric iodide in two distinct ways. First, gravity will induce convection during growth, which strongly mixes residual impurities and any elementary gases resulting from imperfect stoichiometry, either of which can then form precipitates in the growing crystal. Second, gravity loads the resulting crystal, which is particularly soft while still hot, especially in the absence of precipitates. We have investigated the effects of these processes on the resulting crystalline regularity and the effects of various types of irregularity, in turn, on performance.High resolution synchrotron x-radiation diffraction imaging of three generations of crystals, grown both in microgravity and in full gravity, provide graphic evidence of the influence of gravity on mercuric iodide crystal growth. These images tie together the results of other characterization studies, identifying the crystallographic sources of the observed property enhancement in microgravity. The first process, convection, is found to be particularly important, both in its influence on observed crystalline regularity and in the resulting electronic performance of detectors made from these crystals.As a result of these investigations, the crystalline regularity and performance of terrestrial crystals has been substantially improved, although the resulting crystals have not yet achieved parity with the performance of crystals grown in microgravity. We propose new experiments in microgravity for property optimization.

The hole-mobility•carrier-lifetime product of α mercuric iodide high energy radiation detectors has been enhanced through vapor crystal growth in microgravity. This improvement is closely correlated with specific characteristics of the crystal lattice, which have been identified by high resolution synchrotron x-ray diffraction imaging. These structural features and the associated performance are now being approached in terrestrial growth of α mercuric iodide.Gravity may affect the uniformity of this crystal lattice in two distinct ways: 1) directly through deformation that it imposes on the lattice during growth and 2) indirectly through convection, which mixes any extraneous material. Inclusions associated with these processes harden the lattice and facilitate lattice folding. These changes affect the electronic parameters of detectors made from the crystals. As purification procedures are optimized, the incorporation of extraneous material is curtailed, enhancing electronic properties in spite of lattice flexing through loss of precipitation hardening.These studies provide insight into the contribution of various aspects of crystalline order in α-mercuric iodide crystals to property improvement. This knowledge has led to modification of requirements for starting materials, adjustment of physical vapor growth procedures, and change in crystal handling procedures. As a result, the electronic performance of terrestrially grown radiation detectors has been improved, and we provide evidence that further enhancement is still possible.

The overall crystalline lattice uniformity in recently available, highly purified mercuric iodide single crystals has been shown to be impacted by crystal handling techniques that were previously satisfactory. High resolution diffraction imaging of the surface regularity of crystals of various levels of purity and growth orientation shows: 1) that the newer materials have a generally lower level of precipitates, 2) that the incidence of these precipitates is now closely correlated with growth direction, and 3) that the deformation resistance and resulting sensitivity to crystal handling procedures are also closely correlated with these factors in this soft material.As a result, gentler cutting and polishing procedures have been developed and are shown to be effective in preserving overall lattice regularity in the new material. The polishing required to remove residual surface scratches affects the lattice orientation of the softer, precipitate-free regions, while not affecting those regions with detectible levels of precipitates. These results correlate closely with the electrical properties of devices made from these crystals.

The formation of mismatch dislocations in layered semiconductor structures was found recently in high resolution monochromatic synchrotron x-radiation diffraction images to be correlated with characteristics of the substrate as well as with the layer thickness and degree of lattice mismatch of non pseudomorphic layers.1,2We have now extended these studies to examine the accommodation to strain as a function of lattice mismatch in a series of high electron mobility transistor (HEMT) structures grown by molecular beam epitaxy (MBE) on indium phosphide substrates.Five distinct types of irregularity are observed: 1) lattice warping, 2) the formation of a nonpseudomorphic layer, 3) the formation of extended arrays of linear mismatch dislocations at the interface between the substrate and a nonpseudomorphic layer, 4) the formation of oval regions of tweed-like local lattice variation imbedded among these arrays, and 5) extended tweed-like local lattice variation over large peripheral areas in which the formation of straight mismatch dislocation arrays is not observed.Warping of the lattice is found in nearly all layered structures. A distinct layer with a different lattice parameter but without visible misfit dislocations is formed with a mismatch of 0.27 %. With increase of the mismatch to 0.5 %, the other three forms of accommodation appear in distinct regions of the structure: arrays of mismatch dislocations; oval regions of tweed-like irregularity, oriented in the [011] direction; and peripheral regions of extended tweed-like local lattice variation.

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.

A model is presented for determining the enhancement of local electromagnetic fields which occurs close to rough metal surfaces through surface plasmon excitation. The model considers a random distribution of metal hemispheroids on a perfectly conducting flat plane. The electrodynamics of this system is described using the long wavelength approximation and with only the dipole coupling between different hemispheroids included. The square of the local field is averaged over all locations on the surface to provide an estimate of the enhancement in Raman intensity pertinent to surface enhanced Raman spectroscopy (SERS). Application is made to rough Ag, Cu, Au, Hg, and Pt surfaces, simulating the roughness produced by electrochemical anodization. For Ag, the roughness induced SERS enhancement is found to be about 102, with a weak dependence on wavelength down to 350 nm, where a sharp drop is observed. The variation of this result on the parameters which characterize the hemispheroid distributions is studied, including the variation with the distribution of hemispheroid heights and widths, with the number and arrangements of hemispheroids on the surface, with hemispheroid dielectric constant, and with hemispheroid coverage. Most of these parameters do not change the overall enhancement factors significantly, although the wavlength dependence of the enhancement does change significantly with hemispheroid coverage. The polarization of the local electric field close to the surface is studied, and found to be primarily perpendicular to the local surface for globally perpendicular applied fields. The SERS enhancement on Cu is found to be comparable to Ag for λ≳600 nm, with a drop in intensity by a factor of 5 at 600 nm. Au is similar to Cu, except that the drop occurs at 500 nm. Hg is similar to Ag in both the magnitude and frequency dependence of the enhancement, while Pt is uniformly smaller throughout the visible. All of these enhancements are ∼103 smaller than is seen experimentally using SERS, although the observed change in SERS intensity due to anodization does closely match the predictions of our model. Overall, we conclude that surface roughness probably contributes 102 to 103 to the overall enhancement with a frequency dependence which is close to that seen experimentally. This conclusion indicates that some other enhancement mechanism must be responsible for the remaining factor of 103 needed to explain the observed overall enhancement of 106.A model is presented for determining the enhancement of local electromagnetic fields which occurs close to rough metal surfaces through surface plasmon excitation. The model considers a random distribution of metal hemispheroids on a perfectly conducting flat plane. The electrodynamics of this system is described using the long wavelength approximation and with only the dipole coupling between different hemispheroids included. The square of the local field is averaged over all locations on the surface to provide an estimate of the enhancement in Raman intensity pertinent to surface enhanced Raman spectroscopy (SERS). Application is made to rough Ag, Cu, Au, Hg, and Pt surfaces, simulating the roughness produced by electrochemical anodization. For Ag, the roughness induced SERS enhancement is found to be about 102, with a weak dependence on wavelength down to 350 nm, where a sharp drop is observed. The variation of this result on the parameters which characterize the hemispheroid distributions is studied, ...

Novel, streak-like 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 presence of ensembles of thin (110) antiphase platelet regions or boundaries is generally consistent not only with the streak-like diffraction features but with the other features reported here as well. For the formation of such regions we propose two mechanisms, operating in parallel, that appear to be consistent with the various defect features observed by a variety of techniques.

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.

The interaction of light with clusters and random distributions of metal hemispheroids on a perfect conducting flat surface is studied. Significant SERS enhancements are found to arise from multiple plasmon contributions to the surface electromagnetic fields.

Streaking images restricted to the direction of the diffraction (scattering) vector have been observed on transmission through undoped GaAs. These disruption images (caused by the reduction of diffraction in the direction of observation) appear both in the forward and in Bragg diffracted directions in monochromatic synchrontron radiation diffraction imaging. This previously unobserved phenomenon can be explained in terms of planar defects (interfaces) or platelets which affects the absorption coefficient in anomalous transmission. Such regions of the crystal look perfect despite the presence of imperfections when the scattering vector is not perpendicular to the normal of the platelets. The observed crystallographic orientation of these interfaces strongly indicates that they are antiphase boundaries.

Structural information of special interest to crystal growers and device physicists is now available from high resolution monochromatic synchrotron diffraction imaging (topography). In this review, the importance of superior resolution in momentum transfer and in space is described, and illustrations are taken from a variety of crystals: gallium arsenide, cadmium telluride, mercuric iodide, bismuth silicon oxide, and lithium niobate. The identification and detailed understanding of local variations in crystal growth processes are shown. Finally, new experimental opportunities now available for exploitation are indicated.