Table of contents

Surface accumulation of carriers not only affects the conductance of a semiconductor sample, but is traditionally invoked as a means of reducing the effects of surface recombination velocity on minority carrier lifetime. It is shown that circumstances can easily arise where the latter effect is actually reversed, to the detriment of device performance. Simple expressions are derived for the effective surface recombination velocity, and the relationship between minority carrier lifetime and sheet conductance in truly surface dominated samples.

The K-absorption spectrum has been studied in vermiculite mica with a view to studying its Fe²⁺/Fe³⁺ ratio and Fe-O bond length before and after subjecting it to heat treatment at 800 degrees C. The heat treatment is found to lead to near total conversion of Fe²⁺ to Fe³⁺, but the value of the bond length does not seem to change significantly. The 'white line' observed in its spectrum, ascribed to exciton levels, is found to broaden on heat treatment. The results are compared with those obtained on other trioctahedral micas in an earlier study.

Threshold and electro-optical characteristics are studied at domain instability in the nematic phase, with small positive anisotropy or the dielectric constant ( epsilon _a>0) and smectic order, for the seventh, eighth and ninth homologues of p-n-alkyloxybenzoic acid. The Orsay parameter zeta ² is found to be negative for the seventh and eighth homologues and approximately zero for the ninth homologue. The dependence of the domain formation threshold, the Fredericksz threshold the elastic modulus and the Orsay parameter zeta ² on temperature show a characteristic change in the middle of the nematic phase, due to a change from the classic nematic structure to the planar short range order structure. The molecular director reaction and relaxation and the break-up and formation of domains are also studied.

Experimental values have been given for both the drift velocity and the ratio of longitudinal diffusion coefficient to mobility for electrons in helium (1.4<or=E/N<or=226 Td) and neon (1.4<or=E/N<or approximately=339 Td). Comparison has been made with a Monte Carlo computer simulation and a good fit has been obtained with the present and previously measured data. The simulation results for ionisation coefficient and ratio of radial diffusion coefficient to mobility have been compared with both experimental and other theoretical values over a wide range of E/N. A comparison between the mean swarm energy and the breakdown voltage has clearly shown that equilibrium conditions exist at the highest E/N (2260 Td for helium and 5600 Td for neon) for which the computations were carried out.

The technique for studying swarms of electrons in a gas discharge by observing the photon flux from the discharge, has been adapted to investigate the role of the secondary mechanisms in hydrogen and nitrogen. The results show that, contrary to previous indications, ion bombardment of the cathode plays only a negligible, if any, part in the low pressure discharge in hydrogen and nitrogen at low E/N, while at high E/N only the contribution of the atomic ion is significant.

The temporal and spatial variations of the population of excited-state He atoms (2¹S, 2¹P, and 2³S and 2³P) in a pulsed discharge of short duration ( approximately 1 mu s) have been studied using the hook method. Though, in general, the populations of these states show a rapid increase immediately after the current ceases, the temporal variation for 2³S shows two maxima which are likely to arise from two different processes. The profile of the radial distribution of these states varies with time more rapidly than could be expected from the decay process, due to diffusion to the wall.

The authors' aim is to find the voltage-current characteristic of the graphite DC arc in argon flow at normal pressure and with currents between 10 and 40 A. The anode and cathode voltage drops, across the anode and the cathode regions of space charge, were determined. The existence of quasi-neutral transition regions between the space charge regions and the positive column where the ionisation processes are thought to be dominant were assumed. The sum of voltage drops across the transition regions was found, as well as the ion fraction of the total cathode current.

The authors demonstrate that the essentially constant width of high angle Kikuchi bands does not necessarily imply that they are formed by almost monoenergetic electrons. They show that formation of sets of Kikuchi bands from the wide range of energies present in a back-scattered electron energy distribution can give rise to an apparently sharp, well defined Kikuchi band edge. Furthermore, the dependence of the back-scattered electron spectrum on atomic number can account for why high angle Kikuchi bands are better defined for high-Z elements and compounds.

The third-order elastic moduli of polycrystalline Al-rich Al-Mg and Al-Cu alloys are determined by measuring the changes in ultrasonic wave velocities under applied uniaxial stress. A solid-state pulse interferometer sensitive to a few parts per million for the relative velocity measurement was used. Pressure derivatives of elastic moduli and Gruneisen gamma are evaluated from the measured second- and third-order moduli.

Careful specific heat measurements on trissarcosine calcium chloride (TSCC) have been performed in the range 50-330K, using an adiabatic calorimeter whose calibrated accuracy in the range of measurements reaches 0.1%. The characteristic thermodynamic parameters for the ferro-paraelectric phase transition are: T_c=130.27+or-0.03K, Delta H=39.5+or-2 cal mol^-1 and Delta S=0.32+or-0.02 cal mol^- K^-1. A comparison is made with the Landau and dipolar theories. The critical behaviour agrees with the classical predictions for mod T_c-T mod *0.05, but discrepancies appear closer to the transition temperature.

The dependence of current density on applied voltage has been studied in the composite system ZnO-Sn, where the proportion of Sn was 5% or 10% by weight. All samples showed a power-law dependence of current density on voltage, with the exponent being either equal to 2 or in the range 2.6-2.9. The conductivity was ascribed to traps of density 1*10¹⁶-2*10¹⁷ cm^-3 which are situated below the conduction band edge of the ZnO. In some cases the traps were distributed exponentially according to the relation N(E)=N₀exp(-E/kT_t), where the temperature parameter T_t was typically about 500-600K; in other cases the traps were situated at a single level of 0.255-0.366 eV below the conduction band.

Optical absorption and thermoluminescence spectra of nominally pure X-irradiated KDP have been obtained between 13K and room temperature. Absorption spectra and their associated thermal bleaching curves exhibit a strong dichroism. Two glow peaks are observed at 73K and 123K which correlate well with main decay steps on the bleaching curve. From these data the existence of two different centres with activation energies of approximately 0.042 eV and approximately 0.22 eV is concluded. Different possibilities for the structure of these centres are discussed.