In this paper we propose a theory which accounts for the thickness dependence of threading dislocation densities in mismatched heteroepitaxial (001) semiconductors. This theory predicts that, for thick, planar, highly-mismatched heteroepitaxial layers with equilibrium strain, the threading dislocation density should be proportional to f/h, where f is thelattice mismatch and h is the film thickness. These predictions are in good agreement with experimental resultsin the GaAs on Si(001) system.

The role of cation-cation and anion-anion “wrong” bonds at compound semiconductor grain boundaries is investigated. Results are presented for the {110} and {001} inversion domain boundaries in SiC and GaAs. The role of electrostatic effects, compensation of over- and undersaturated bonds, “chemical” reconstructions andbond length relaxations are discussed.

Single crystal niobium films were grown by Molecular Beam Epitaxy (MBE) on (0001)s sapphire substrates. Cross-sectional specimens with thickness of <20 nm were prepared so that the Nb/A1203 interface could be investigated by high resolutionelectron microscopy (HREM). The orientation relationship between the metal film and the ceramic substrate was verified by selected area diffraction: (111)Nb ║(0001)S and [110]Nb║[2110]S. The atomistic structure of the interface was identified by HREM.

The interface structure of MBE grown Nb films on sapphire substrates was studied using grazing incidence x-ray diffraction and x-ray reflectivity measurements. Specifically, the use of these x-ray techniques in probing the buried interfaces was demonstrated. Diffraction effects were observed which are consistent with the presence of misfit dislocations in the interface.

TiO2 thin films grown on (1120) sapphire at 800°C by the MOCVD technique have been characterized by transmission electron microscopy. The TiO2 thin films are single crystalline and have the rutile structure. The epitaxial orientation relationship between the TiO2 thin films (R) and the substrate (S) has been foundto be: (101)[010]R║(1l20)[0001]s. Growth twins in the films are commonly observed with the twin plane{101} and twinning direction <011>. Detailed atomic structures of the twin boundaries and TiO2/α-Al203 interfaces have been investigated by highresolution electron microscopy (HREM).When the interfaces are viewed in the direction of [010]R/[000l]S, the interfaces are found to be structurally coherent in the direction of [1Ol]R/[1100]s,in which the lattice mismatch at the interfaces is about 0.5%.

CuAu-I type ordered structures in InGaAs grown on (110) InP substrates by molecular beam epitaxy, have been studied by transmission electron microscopy. In the electron diffraction pattern from the InGaAs, superstructure spots associated withCuAu-I type ordered structure are found. When the tilting angle of the substrates increases, the ordering becomes stronger. The ordering is also stronger in crystals grown on substrates tilted toward the <001> or the <001> direction than those on substrates tilted toward the <110> direction. From these results, one can conclude that atomic steps on the growth surface play an important role in the formation of ordered structures. The ordering becomes stronger when the growth temperature increases in the range 360-485°C. In high resolution images of the crystal, doubling in periodicity of 220 and 200lattice fringes is found, which is associated with CuAu-I type ordered structure. Moreover, anti-phase boundaries are very often observed in the ordered regions. It is also found that ordering is not perfect, and that ordered regions are plate-like microdomains lying on planes slightly tilted from the (110) plane.

Deep level transient spectroscopy (DLTS) measurements have been performed on Schottky diodes fabricated on MBE grown InGaAs/GaAs heterostructures. The dominant electron trap in this material is found at a depth of 0.30eV below the GaAs conduction band and is believed to be the previously observed M3 defect. Two other defects, at depths of 0.50eV and 0.58eV below the GaAs conduction band, were also observed. Defect depth profiling shows the 0.50eV defect to be spatially locatednear the heterointerface. The 0.58eV defect is not observed near the heterointerface but is observed in large concentrations deep in the GaAs epilayer. Optical DLTS measurements reveal deep defects at 0.54eV and 0.31eV above the InGaAs valence band as well as a large, broad peak, most likely consisting of several energy levels with varying capture cross sections,located at the heterointerface. Two carrier accumulation peaks were also seen in the CV carrier profiling measurements and are suggested to be due to two heterointerface defects located at 0.68eV and 0.87eV below the GaAs conduction band.Thermally stimulated capacitance measurements also indicate minority hole emission in this n-InGaAs/N-GaAs heterostructure.

High quality GaAs/SiO2 MIS( Metal Insulator Semiconductor ) diodes were fabricated using (NH4)2S treatment and photo-assisted CVD( Chemical Vapor Deposition ). The density of states at the GaAs and SiO2 interface is the order of 1011 cm-2eV-1 throughout the forbidden energy range, which is smaller by the order of two than that of the MIS devices made by the conventional CVD process. The mechanism attributable to the interface improvement was investigated through XPS( X-ray Photoelectron Spectroscopy ) analyses.

The influence of surface relaxation on the imaging of dislocations in thin single crystal films, using white beam synchrotron radiation topography in grazing Bragg-Laue geometry, has been assessed. The predicted visibility of dislocation images on grazing Bragg- Laue topographs, for the particular case of interfacial edge dislocations in a GaAs/Si heterostructure, is shown to be strongly influenced. Agreement between predicted and observed visibility could only be obtained by incorporating the surface relaxation effects, which thus strongly influence the depth sensitivity of the technique, i.e. the ability to pinpoint the depth of a dislocation. Dislocation image widths are also influenced by these effects.

MOCVD grown GaAs and InP epitaxial layers have been studied using photoreflectance spectroscopy. Homogeneous and heterogeneous structures were employed to investigate the influence of the mismatch induced strain and dislocations. All the tested spectra contained a sharp peak related to the fundamental absorption edge and a pronounced Franz-Keldysh oscillation. The data analysis revealed a consistent difference in bandgap, temperature coefficients of the bandgap, and surface electric field, in the order of the degree of mismatch. For GaAs/GaAs and GaAs/Si samples, the bandgaps derived from the three point method were 1.436 and 1.324eV, respectively. Values of 1.334, 1.325, and 1.294 eV for the bandgap were found for InP/InP, InP/GaAs, and InP/GaAs/Si, respectively. For GaAs epitaxial layers, the intensity of the surface field bore a ratio of 1.18:1 between GaAs and Si substrates. For InP epitaxial layers, the ratio was 1.23:1.12:1 in the sequence of InP, GaAs, and GaAs/Si substrates. Such a measure must be related to the mismatch in the heteroepitaxy structures. A shoulderlike peak, 18-23 meV below the gap-energy peak (Eo), was found for GaAs samples, which could be impurity related. A broad shoulder-like peak 30 meV below the fundamental absorption edge was also observed only for InP/GaAs/Si and hence attributed to some shallow defect levels induced by antiphase disorder.

Extended regions of hexagonal zinc sulfide (wurtzite) structure are found to exist in AlGaAs grown by low-pressure OMCVD at 750°C. The specimen was prepared on a (001) GaAs substrate patterned with [110]-orientedV-grooves, intended for a quantum wire laser structure. A high density of planar faults was observed to originate in theAl0.66Ga0.34As cladding layers near the inner corners of the V-grooves and propagate towards the surface along the {111} planes. Many of these faults are stacking faults and microtwins. However, there also exhibit extended regions of hexagonal structure, revealed in electron diffraction patterns and high resolution lattice images. The hexagonal phase shares the same close-packed layers with the cubic phase, i.e., (0001)hexagonal // {111}cubic. The mechanism for the formation of hexagonal structure in this specimen is not yet fully understood.

Ferroelastic domain switching is one of the possible toughening mechanisms in ceramic materials. Microstructural evidence of domain reorientation (switching) in polydomain tetragonal zirconia single crystals is observed upon the application of a unidirectional compressive stress. Dark field imaging of the three {112} tetragonal twin variants in a {111} zone indicates that two sets of twin variants grow at the expense of the third set upon application of uniaxial compression. The diminishing variant is the one with its c axis parallel to the compression axis. Indentation experiments on uniaxially compressed samples show an anisotropy in crack length. Crack propogates more easily along the loading direction. A construction for the orientation relationship of domains and their twin boundaries is presented.

Geometric criteria for low interface energy and interface structure were tested for the cubic-rhombohedral system YAG/alumina. Orientation relationships near (111){110}a // (112){110}y and facets on (111)a(112)y were observed in both sol-gel derived composites and directionally solidified eutectic composites. The ΣYAG=12 near-CSL of 2{111}, {110}, {112} was inferred to be the preferred structural unit. Dislocations with b=1/3{111}a and b=1/2{110}a were observed and inferred to accomodate deviation from the structural unit, respectively. The {110}a,y direction met some of the criteria for an invariant line. Although the OR was explained by geometric criteria it would have been difficult to predict it with such criteria

A new method for measuring interface stress is presented. The curvature of substrates coated with artificial multilayers is measured as a function of bilayer repeat length. The interface stress is determined from the slope of the curvature plotted versus inverse repeat length. As an example, the (111) Au/amorphous Al2O3 interface stress was determined to be 1.13±0.06 J/m2 (compressive) . X-ray and TEM observations allowed the possibility of interfacial phases to be ruled out as possible alternative sources of the stress.

We review our work on irradiation effects in single crystal YBa2Cu3O7-x. Transmission electron microscopy has been employed to study the defect microstructures produced by irradiations with fast neutrons, MeV ions (Kr, Ne and p), and electrons. The atomic structure within defect cascades was investigated using 50 keV Kr and Xe ion irradiations to low doses. Evidence is shown for an amorphous structure with some incoherent recrystallization within individual cascades. Correlation with enhancements in critical current density produced by neutron irradiations suggest that this cascade structure effectively pins magnetic flux lines.

At sufficiently high fluences of fast neutrons or MeV Kr and Ne ions, a cellular microstructure is found. This structure consists of cells or microcrystallites of good crystalline and superconducting material (in the case of neutron irradiation), with cell walls of amorphous material. Full amorphization proceeds with the growth of cell wall volume. The formation of this microstructure coincides with a decrease in critical transport current, but is not observed by magnetization measurements.

Increases in critical current density under proton irradiation, comparable to those produced by neutron irradiation, have been reported. The defect structure produced by proton irradiations is examined here and found to differ from that of neutron irradiations. The structure is suggested to be consistent with the clustering of mobile defects (at 300 K) produced by the lower energy recoils which dominate in proton irradiations. In both the proton and fast neutron irradiations, to fluences producing the maximum enhancements in critical current densities, the degradations in critical temperature are not severe, <10 K.

Our most recent measurements of changes in critical temperature and current density, and defect microstructure following electron irradiations will be described

This paper investigates both transport properties and nature of superconductivity breakdown or, conversely, enhancement in oxide ceramics, due to radiation-induced defects.

Low-fluence neutrons (≈107 n cm-2 at 3 MeV) can sensitively damage the samples, giving experimental evidence that the breakdown of coherent percolating paths produces decoupled domains. A set of preliminary measurements shows that high-fluence proton implantation can either damage or enhance critical current density in a currently non controllable way. In both cases strongly damaged or enhanced superconducting paths short-circuit the unaffected bulk network.

Local structure around the constituent atoms in the compound system Y1-xPrxBa2CU3O7-y has been investigated by means of x-ray absorption fine structure (XAFS) techniques. By comparing the local structure in the compound x=1 with its counterpart x=0, the XAFS results provide a direct evidence that Pr has replaced Y in the material. The nearest-neighbor structure in the CuO2 planes seems to remain intact as × varies from 0 to 1, indicating that substitution of Pr for Y does not disturb the local distribution of holes in the CuO2 planes. We suggest that local disorder and distortions in the second-neighbor bonding configuration could be responsible for suppression of superconductivity with increasing Pr content in the system.

First principles calculations of the positron lifetimes are used to interpret experimental lifetime data for Ba1-xKxBiO3. The observed lifetimes are found to be very sensitive to the presence of defects in the sample. The temperature dependence of the lifetime and the change in lifetime with sample aging are both attributed to defects. The qualitative differences between metal vacancy defects and oxygen vacancy defects in metal oxides are also discussed.

Variations in superstructure modulations and transition tempeartures (Tc) in undoped and yttrium-doped Bi2Sr2CaCu2O8 (2212) superconductors annealed in different environments have been examined by microstructural analysis. The hole concentration is changed either by annealing the samples in reducing or oxidizing environments, or by partial substitution of Y3+ cations on the Ca-sites. Systematic investigations of the 2212 samples annealed in both the reducing (nitrogen) and oxidizing (oxygen) environments show commensurate and incommensurate modulations, respectively, and a decrease of Tc in oxygen. Commensurate modulations have also been found for samples prepared entirely in nitrogen. High spatial resolution microanalysis reveals that the modulations are insensitive to cation point defects. These results indicate that modulations are intrinsic to the 2212 materials and that they are caused by the lattice mismatch between the perovskite and Bi-O layers. Such periodic variations in interplanar spacings are expected to lead to satellite spots as observed in electron diffraction. The results are consistent with single crystal X-ray diffraction studies reported in the literature. In the Y- doped samples Tc values are similar in Bi2Sr2Ca1-xYxCu2O8 for x=0.2, in both the environments, but decrease with increasing value of x.

X-ray diffraction and transmission electron microscopy have been used to study the morphology and defect structure of YBa2Cu3O7-δ films on [001] MgO substrates polished away from the [001] crystallographic axis towards either [010] or [110] by angles ranging from 1° to 20°. On the [010] off-axis substrates it was found that growth of the YBCO film was highly influenced by the surface normal of the substrate with only very weak alignment to the substrate [001] direction. All [010]-angled samples maintained Tc's of 86K. Angles of 1°, 2.5°, and 5° toward [010] produced films with large mosaic spread parallel to the direction of the angle, as high as 4° FWHM for the 5° sample. Critical current densities of these films remained as high as those of films grown on on-axis substrates, e.g. 4.4×107 A/cm2 at 4.2 K and zero applied field. Larger off-axis angles led to samples with reduced mosaic spread but still alignment to the surface normal, and reduced Jc's of 2×107 A/cm2 and 1×l05 A/cm2 for 10° and 20° angles, respectively. TEM investigation and in-plane x-ray examination of the 5°, 10°, and 20° samples show good in-plane texture except for the 20° sample, and increased granularity with angle. Films on substrates polished off-axis towards [110] have degraded Tc's and Jc's for all angles.