Table of contents

A critical review of the different theoretical models proposed for explaining the metal-semiconductor and semiconductor-semiconductor interfaces is presented. Although no attempt to present a thorough review of the experimental evidence is made, the authors discuss the most relevant information concerning those interfaces. Their discussion concentrates mainly on the models that have received more widespread acceptance: the induced density of interface states model and the defect model. A few comments on other cases are also made, and a critical evaluation of the different models is presented.

The temperature dependence of the dielectric permittivity ( epsilon ') of polymer single crystals of bis-(p-toluene sulphonate) or 2,4-hexadiyne-1,6-diol (PTS) has been determined for the range 140-300K. In contrast to earlier published results, the measurements along the b axis ( epsilon '/sub ///) show anomalous behaviour with a maximum of epsilon '/sub /// at T_c approximately=195K. Various models are discussed in an effort to provide an explanation for this result. A comparison is made with previous crystallographic data on the temperature evolution of the crystal structure. It is concluded that the anomalous behaviour of epsilon '/sub /// is due to a two-dimensional order-disorder-type phase transition, which occurs at about 195K. In the low-temperature phase (T<T_c) the Frohlich model accounts for the increase with temperature of epsilon '/sub ///, and for T>T_c a slightly modified Kirkwood approach explains its decrease with temperature satisfactorily.

Uniaxial-stress and magnetic field perturbations confirm that the 'W' zero-phonon line, produced in crystalline silicon by radiation damage, occurs between spin-singlet, nondegenerate orbital states at a trigonal centre. The authors show that the excited state of the W line undergoes strong stress-induced interactions with both non-degenerate and doubly degenerate orbital states lying about 30 meV to higher energy. Although this is the major stress perturbation, there is negligible vibronic interaction of the excited states, and the vibronic properties of the W band are explained in terms of simple relaxation involving perturbed lattice modes. The one observed local vibrational mode of the centre has Si isotope effects consistent with its involving only Si atoms. The stress data are used to show that the series of W-like bands produced by bombarding Si with inert-gas ions may be W centres perturbed by the strain of a nearby inert-gas atom.

Detailed study of the cathodoluminescence emission spectra of ¹⁶⁶Er⁺-implanted zinc selenide cubic single crystals as functions of the post-implantation isochronal annealing temperature has allowed three luminescence centres to be identified. Centre A consists of an Er³⁺ ion at a zinc interstitial site surrounded by four nearest-neighbour (NN) Zn²⁺ ions and six next-nearest-neighbour (NNN) Se²⁺ ions. The other two centres each have a rare-earth ion at a substitutional site, centre B when an Er³⁺ ion occupies a selenium substitutional site surrounded by four NN Zn²⁺ ions and twelve NNN Se^2- ions and centre C when an Er³⁺ ion occupies a zinc substitutional site surrounded by four NN Se^2- ions and twelve NNN Zn²⁺ ions.

It is shown how the quasi-harmonic approximation can be applied to crystals with several varying internal strains. The quasi-harmonic theory is linear in the anharmonic parametric of lattice dynamical models, which can therefore easily be fitted to a large body of experimental data: macroscopic and internal expansion coefficients, third-order elastic constants and spectroscopically measured mode Gruneisen parameters. The method is applied to a short-range central-force model for alpha -quartz, which is fitted to experimental thermal expansion up to room temperature. For T>or approximately=90K a fair fit is obtained to both the macroscopic and the internal expansion, thus permitting a detailed examination of the roles played by tilt and by distortion of the SiO₄ tetrahedra. At lower temperatures the model is in qualitative agreement with the observed behaviour, including the negative expansion along the trigonal axis. With a different set of anharmonic parameters, the model is fitted simultaneously both to the thermodynamic Gruneisen functions and to spectroscopically measured mode Gruneisen parameters. It is not possible to fit simultaneously all the available experimental data, confirming that a more sophisticated model is needed to give a complete representation of the inter-atomic forces.

The author presents a detailed analysis of conduction in discontinuous metals based on the traditional model but taking into account both the statistical aspects of relevant structural features and the presence of potential disorder. The analysis predicts conductivity with a temperature dependence close to simple activation at all temperatures. It is argued that an essential feature of the physics of transport in these systems has yet to be identified and incorporated into a satisfactory detailed model.

Second-order phase transitions are studied for anisotropic disordered systems composed of randomly distributed metal and non-metal regions, the former ones having tensor-type conductivity along the principal axes. Equations are derived for the effective Hall and Seebeck coefficients. It is shown that near the percolation threshold the decrease of the anisotropy of both the effective conductivity and Hall coefficient can be described by two new critical exponents that characterise the transition to the quasi-two-dimensional and the quasi-one-dimensional states. The anisotropy of the thermopower at close thermal conductivity values of the components is characterised by the same exponents that describe the electric conductivity, while at larger ratios of these values the thermopower anisotropy shows no decay at the non-metal side of the transition. The transition from the quasi-two-dimensional state to the two-dimensional one takes place at the two-dimensional threshold, while that from the quasi-one-dimensional state to the one-dimensional one occurs at the puncture threshold that depends on the size of the sample.

The equations of motion of a Josephson junction shunted by an LCR circuit and driven by a time-independent current have been solved numerically. This autonomous system displays the familiar sequence of bifurcations, leading to chaos. In addition the nonlinearity of the system leads to discontinuities and hysteresis in the voltage-current curve.

The authors report measurements of resistivity, Hall coefficient and magnetic susceptibility for the intercalation complex Ag_xTiS₂ with x approximately=0.33 obtained by electrochemical intercalation of TiS₂ with silver ions. The intercalation complex shows metallic behaviour as a result of electron occupation of the Ti d band through charge transfer from the silver ions to the host layers. The room-temperature conductivity, n-type Hall coefficient and Pauli magnetic susceptibility are consistent with increased electron occupation in an otherwise unmodified d band. At low temperatures the magnitude of the Hall coefficient falls, and this may be accounted for by the modification of the Fermi surface with the Brillouin zone reconstruction that accompanies the ordering of the silver ions onto a 3^1/2a*3^1/2a superlattice below room temperature.

The authors derive a theory for the Knight shift in narrow-gap semiconductors from a general expression for the Knight shift in solids, which includes spin-orbit and many-body effects from first principles. As an example, they have applied the theory to evaluate the temperature variation of the Knight shift of ²⁰⁷Pb in PbTe. The effects of temperature have been incorporated in their calculation through a change in the energy gap and through the Fermi distribution function and its derivative, which appear in the different contributions to the Knight shift. They formulate a k. pi band calculation in which the interaction of the band-edge states is considered exactly and the resulting states are treated with the far bands by using second-order perturbation theory. The results for two typical values of hole densities in p-PbTe show fairly good agreement with experiment. They emphasis that the general nature of the calculations can be used to calculate the quantity in other narrow-gap semiconductors such as PbS and PbSe.

The hyperfine interaction of the Van Vleck paramagnet, trivalent europium (4f)⁶, dilute in YAlO₃ host crystal is studied using Raman heterodyne spectroscopy in magnetic fields of 80 G in order to determine the directions of the quadrupole interaction in both the ground ⁷F₀ state and the lowest ⁵D₀ excited state. The measured magnetic resonance frequencies as a function of angle for the ground state agree with those frequencies calculated using previously published values of the nuclear Zeeman splitting factors. The excited-state magnetic field splitting factors are nearly isotropic as would be expected from the model of Elliott. Two magnetically inequivalent sites were observed. The NQR frequency against angle patterns demonstrate that the axes for the total quadrupole interaction for both states are identical within the precision of the experiment, with the three axes described by the normal to the mirror plane, and the axes of the pseudo-cell of the YAlO₃ host. It is determined that the principal axes of the nuclear g_N tensor coincide with those of the quadrupole tensor within ten degrees.

The Raman spectrum of paramagnetic CoI₂ has been measured for frequencies in the range 0-1200 cm^-1 at temperatures from 125K down to the Neel temperature. Spectral features due to first-order scattering from Co²⁺ electronic excitations and phonons are analysed and assigned to symmetry species. An effective Hamiltonian model in second order is used to describe the Co²⁺ ion levels within the ⁴T_1g (⁴F) ground term. A comparison is made between the energy matrix parameters obtained for CoI₂ and those of the related trigonal compounds CoCl₂ and CoBr₂.

A new absorption band was found in compensated GaP:Ni:S crystals near 0.8 eV. Optical investigations of the nickel diffusion profile in GaP:Ni proved that this band is associated with the isolated Ni²⁺(d⁸) centre. The interpretation of the 0.8 eV band as being due to the ³T₁(F)-³T₂(F) crystal-field transition is suggested.

Ion-beam-induced luminescence is reported for samples of pure LiNbO₃ obtained from a wide range of sources. Numerous narrow emission bands are observed from 290 to 390 nm. At least four of these lines are ascribed to excitonic transitions. The light is primarily generated by the ion beams in the region of energy loss by nuclear collisions. It appears that the high rate of energy deposition favours luminescence decay paths for the exciton states so formed.

An apparatus is described to measure the optical excitation spectrum of a material whilst it is under X-ray bombardment. Results are presented in the form of the excitation spectra from two distinct optical emissions from a mixed-powder specimen of ZnS and ZnSe. From these spectra site selection in the EXAFS structure above the zinc K edge is demonstrated.