No description available.

No description available.

Sputter-deposited heteroepitaxial layers of (100) CdTe $\Vert$ (100) KBr prepared at growth temperatures of 300-325$\sp\circ$C are found to be p-type with high dark resistivities on the order of 2 $\times$ 10$\sp5$ ohm-cm. The epilayers are highly photoconducting. The photoconductivity $\sigma\sb{ph}$ (expressed as $\sigma\sb{ph} \propto f\sp{\gamma}$) is observed to exhibit a sublinear ($\gamma$ $\prec$ 1) dependence on illumination intensity f. Photo-Hall effect measurements show that the photoconductivity is due to change in carrier concentration and not due to any enhancement in the carrier mobility. Moreover, the photocurrent is carried by holes. Hall measurements on sputter-deposited epitaxial PbTe films have confirmed that samples prepared at high growth temperatures are n-type and those grown at low substrate temperatures are p-type, irrespective of the substrate bias applied during growth. The low temperature mobility values in the sputter-deposited samples are found to be limited by a temperature-independent neutral impurity scattering mechanism. One of the features of heteroepitaxial growth is the existence of a large density of structural defects at the interface due to lattice mismatch. (Abstract shortened by UMI.)

An ultrahigh vacuum surface physics system was designed and built to study ion-surface interactions at various incident angles by detection of the forward scattered particles. The proposed initial measurement is the scattering of keV ions by a clean Si(100) surface.

Double barrier heterostructures of Al$\sb{\rm x}$Ga$\sb{\rm 1-x}$As/GaAs are studied. The presence of a quantized level in the well is confirmed by the observation of a region of negative differential resistance (NDR) in the current-voltage (I-V) characteristics of the heterostructures. A spectrum analyser and a network analyser are thus used to measure the oscillator properties of the samples and to investigate the effects of the external circuit on these properties and on the I-V characteristics. For the sample with AlAs barriers, electron oscillations, of frequency greater than 1 GHz, are measured at room temperature; the observation of these frequencies depends on the matching of the impedances of the external circuit to the device under test. Photoconductivity spectra and photocurrent-voltage (PC-V) characteristics at fixed wavelengths are measured in order to study the effects of light incident on the sample. I-V measurements are carried out in a quantizing magnetic field parallel to the tunneling current. With increasing magnetic field, the voltage periodic structures are shifted to higher bias positions at a similar rate of change. (Abstract shortened by UMI.)

The elastic properties of a material have long been a subject of interest in materials science and physics. Especially, a complete determination of the single crystal compliance and stiffness tensor is of great importance, as the single crystal elastic tensor provides a complete description of the elastic properties of a material. There are numerous materials that are only available in polycrystalline form. Many of these polycrystalline materials are of great interest, such as the polycrystalline materials synthesized under high pressure conditions, for which the elastic properties under high pressure conditions are particularly important. However, traditional methods to measure the elastic constants apply only to single crystals. Recently, Singh and co-workers have developed a method, using energy dispersive X-ray diffraction to measure the single crystal elastic constants of a material at elevated pressures, which, for the first time, enabled the single crystal elastic tensors of numerous polycrystalline samples to be determined. Inspired by the energy dispersive X-ray diffraction method, we have undertaken to develop a novel method, using angle dispersive X-ray diffraction techniques combined with a two-dimensional X-ray recording area detector, to measure the single crystal elastic constants of powder samples. We have obtained important results that will enable the single crystal elastic constants of concerned material to be determined from Debye rings recorded on the X-ray recording image plate. In comparison to the energy dispersive X-ray diffraction method, the angle dispersive X-ray diffraction method offers advantages, as we will demonstrate.

The model of Clem and Hao and others is extended to account for the enhancement of the Meissner effect observed in single crystals of hysteretic type II superconductors upon thermal cycling below Tc in static applied magnetic fields. Predictions are made about the features of the final closed thermal hysteretis loop achievable by extensive cycling and their dependence on the temperature limits T1 and T2 < Tc chosen for the cycles. A large variety of observations, by several workers, of a narrow peak of enhanced Meissner effect near Tc in polycrystalline type II superconductors upon slow warming in static applied fields after fast field cooling, are qualitatively and quantitatively accounted for by a model where we introduced the scheme developed above for single crystals into a weak-linked intergranular network (matrix). This "two tier" framework is then extended to describe the enhancement of the Meissner effect observed by Hyun by thermal cycling of weak-linked Nb3Sn below Tc in a static field. A simple framework is presented which quantitatively develops the proposal of Evetts and Glowacki that the superposition of the applied field H a and the return field, Hr of the magnetized grains, is the cause of the hysteretic behaviour of Ic in weak-linked high T c superconductors and the occurrence of a peak in Ic versus Ha descending and reascending, after an excursion to various values, denoted Hcycle, or after field cooling in different H cool. Observations by several workers on the dependence, of the position of four categories of peaks of Ic, on Hcycle and Hcool are reproduced by this model and yield estimates of the "compression" factor C in the linear dipole approximation, H r = C Mg. We also show that, ratios of the measured plateau values for the position of these peaks, lead to an estimate for C which is independent of H·g, the penetration field into the grains, and of the model chosen to calculate the dependence of the magnetization of the grains, Mg, on Ha. Instead of the artificial pseudo-Josephson - junction expression generally used by other workers in the analysis of Ic hysteresis phenomena, we develop a family of formulae based on the critical state concept applied to idealized planar geometry. Exploiting an especially simple case from this family of formulae we reproduce a panoply of experimental curves of Ic versus Ha displayed in the literature and exhibiting a variety, of features. Analysis of the extensive data of List et al reveals that our approach leads to results in accord with observations whereas the Josephson junction format does not. (Abstract shortened by UMI.)

We produced self-assembled quantum dot (QD) samples of InAs on GaAs by molecular beam epitaxy (MBE). With these, we explored growth effects as a function of InAs coverage for three arsenic pressures, and as a function of arsenic pressure at a specific InAs coverage. During growth, the samples were studied using reflection high energy electron diffraction (RHEED). These RHEED measurements were compared to low energy electron diffraction (LEED) measurements. To perform this ex-situ LEED characterisation, some samples were covered with an amorphous arsenic cap. This cap was thermally evaporated producing a clean, non-oxidised surface that was studied using LEED. We obtained non-ambiguous identification of the GaAs (001) surface reconstructions as well as timing information for the 2D to 3D transition during the growth of InAs on GaAs. Post growth characterisation of two sets of self-assembled QD samples, twelve samples in all, revealed the following: As a function of increasing the arsenic pressure used in QD growth, the photoluminescence (PL) of capped QDs is first redshifted at low arsenic pressures, and then blueshifted at high arsenic pressures. Scanning electron microscopy and atomic force microscopy of uncapped QDs show that as the arsenic pressure increases, the QD density increases while the average QD width and height decrease monotonically; these trends are consistent with the shift in PL for the high arsenic pressure samples, but are inconsistent with the shift in PL for the low pressure samples. This leads us to proposing a mechanism by which QDs may be modified as they are overgrown with capping material. We discuss the effects of adjusting the arsenic pressure on the formation of QDs and the mechanism by which QDs may be modified during capping.

The structures of two electron defect systems in alkali halide crystals are studied. The systems which have been studied include: two electrons localized at an anion vacancy (F$\sp\prime$-centre); positronium self-trapped at an anion vacancy (Fe$\sp+$-centre); positron self-trapped at a cation vacancy (F$\sb{\rm anti}$-centre); positronium self-trapped at an interstice; and positronium in a Bloch state. An improved version of the extended-ion method which is based on the one electron Hartree-Fock approximation is used to perform these calculations. Its main feature is the exclusive use of floating 1s Gaussian functions as basis. For the multi-electron defect systems, the calculation of matrix elements of two electron interaction terms is a most difficult problem. We developed an effective approach to treat this interaction approximately. The correlation effect of defect electrons is partly accounted for by properly arranged Gaussian basis. The binding energy, thermal dissociation energy, and transition energy between ground state and excited state are calculated for F$\sp\prime$-centres. A defect model with negative-U properties was introduced to interpret the deeply bound F$\sp\prime$-centre. Calculations of positron binding energies are made for Fe$\sp+$-centres and F$\sb{\rm anti}$-centres. In addition, we evaluate the angular correlation and lifetime of an annihilated electron-positron pair for Fe$\sp+$-centres, localized positronium and Bloch state positronium. The observed phenomena such as the transition of positronium from Bloch state to localized state, and the crystallographic effect are examined theoretically. The calculated results regarding various properties of crystals are in reasonably good agreement with experiment.

A systematic study was performed on the structural, magnetic and optical properties of polycrystalline I$\sb2$.Mn.IV.VI$\sb4$ compounds with I = Ag or Cu, IV = Si, Ge, Sn, VI = S, Se and Te. In X-ray diffractometry, three different crystal structures were observed, two corresponding to the tetragonal stannite and orthorhombic wurtz-stannite structures reported previously, and thirdly a cubic rock salt-like structure. Magnetic susceptibility ($\chi$) measurements showed that most of the selenide phases were ferrimagnetic. Optical absorption measurements were made to determine values of optical energy gap E$\sb{\rm o}$ for a number of the single phase samples. When the values of these various parameters ($\rm a\sb{e}, T\sb{N}, \theta\sb{a}$ and E$\sb{\rm o}$) were compared for the various phases, it was found that the phases could be classified into five different sets. In each case investigated, the variation of E$\sb{\rm o}$ with temperature was found to agree well with the theoretical predictions of Alexander et al. (76A1). (Abstract shortened by UMI.)