Table of contents

Interfaces play a key role in the performance of electronic, optoelectronic and photovoltaic devices. Within epitaxial semiconductor heterostructures, interfaces are commonly characterized using various methods including transmission electron microscopy, secondary ion mass spectroscopy, x-ray diffraction, photoluminescence spectroscopy and capacitance–voltage profiling. The averaging present in these conventional techniques limits their ability to resolve critical atomic-scale features. In addition, many of these methods require detailed modelling in order to determine quantitative interface profiles. Thus, these techniques are often not suitable for obtaining the localized information needed to elucidate the structure and properties of heterointerfaces. Alternatively, nanoprobing of heterointerfaces using cross-sectional scanning tunnelling microscopy (XSTM) has emerged as a powerful method for resolving atomic features at interfaces within heterostructures. In this paper, we describe XSTM and discuss its application to several important issues in semiconductor heterostructure materials, including the formation and ordering of quantum dot arrays, direct measurements of interdiffusion and segregation lengths and investigations of the mechanisms of alloy phase separation.

A large magnetoresistive (MR) effect was observed in Bi2212 in which USr₂CaO₆ was added in a proportion of 6 wt%. The resistivity of melt-textured (MT) Bi2212 + 6 wt% USr₂CaO₆ possess a high sensitivity to applied dc fields, as compared to pure Bi2212, in particular at low fields, below 3 T, and in a temperature range between 45 and 85 K. In this temperature range, the MR effects of MT Bi2212 + 6 wt% USr₂CaO₆ display a maximum that may be tuned to a particular temperature within the above range, by changing the amount of added non-superconducting compound. X-ray results show that during the melt-texturing process of this mixture, the USr₂CaO₆ phase is stable and the reaction with Bi2212 matrix is minimal, in spite of a small decrease in the superconducting transition temperature. A cryogenic sensor was build and tested at 77 K in low fields. It shows high sensitivity and no hysteresis of resistivity when the applied field was cycled between 0 and 1 T.

Hybrid substrates comprising both silicon-on-insulator (SOI) and bulk Si regions have been fabricated using the technique of patterned separation by implantation of oxygen (SIMOX) with high-dose (1.5 × 10¹⁸ cm⁻²) and low-dose ((1.5–3.5) × 10¹⁷ cm⁻²) oxygen ions, respectively. Cross-sectional transmission electron microscopy (XTEM) was employed to examine the microstructures of the resulting materials. Experimental results indicate that the SOI/Si hybrid substrate fabricated using high-dose SIMOX is of inferior quality with very large surface height step and heavily damaged transitions between the SOI and bulk regions. However, the quality of the SOI/Si hybrid substrate is enhanced dramatically by reducing the implant dose. The defect density in transitions is reduced considerably. Moreover, the expected surface height difference does not exist and the surface is exceptionally flat. The possible mechanisms responsible for the improvements in quality are discussed.

Undoped microcrystalline silicon thin films have been deposited by radio frequency powered plasma enhanced chemical vapour deposition technique. The structural order of silicon network in the films has been studied by Raman spectroscopy. The role of bonded hydrogen in the films and their consequences in microstructural defects have been obtained from the results of Fourier transform infrared spectroscopy. The presence of microcrystals and growth morphology of the microcrystalline films have been investigated by transmission and scanning electron microscopy, respectively. Optimized deposition conditions are obtained where properties of the microcrystalline films are suitable as absorber layer of solar cells. A highly conducting undoped microcrystalline film with 87% crystalline fraction (f_c) is obtained, which exhibits columnar structure.

Degenerate four-wave mixing (DFWM) laser spectroscopy was used in local studies of atmospheric pressure argon plasma generated in a free-burning arc. The results of plasma diagnostics using the DFWM method were compared to the results obtained with optical emission measurements. In the cathode region of the arc the maxima of both the DFWM signal and the emission coefficient for the 696.5 nm Ar I line depend on the distance from the cathode tip. This effect proves the departure of the plasma state from local thermal equilibrium (LTE) as it has been reported by many authors. On the other hand the Stark shifts of the 696.5 nm Ar I line determined by the DFWM method in relation to plasma diagnostic results show no deviations from LTE on the arc axis down to 1.0 mm from the cathode tip.

In this study we have estimated the density of SiO molecules produced in atmospheric dielectric barrier discharges by means of spectroscopic investigations based on the self-absorption diagnostic. The (2, 0), (1, 0) and (0, 0) ro-vibrational bands of the SiO(A ¹Π–X ¹Σ⁺) electronic transition were investigated. The effective values of Einstein coefficients and absorption cross sections for the considered rotational transitions were calculated. The SiO concentration during the plasma phase was estimated by comparisons between experimental and computed spectra for the three explored bands. For example, the estimated value was (5 ± 1) × 10¹³ cm⁻³, for (Ar/SiH₄/N₂O) = (98.43%/0.38%/1.19%). The conversion rate from silane to silicon monoxide was deduced as a function of silane fraction. The influence of the molecular fraction on the rotational temperature was investigated. A simple technique based on the ratio of band heads was presented as a useful tool, in order to roughly estimate the density of SiO.

This paper reports on the first experimental investigations into the physics of a self-sustained volume discharge (SSVD) in strongly electronegative gases on their vibrational excitation by irradiation of TEA CO₂ laser. Examined are pure SF₆, mixtures of SF₆ with He, Ne and C₂H₆, and pure C₂HCl₃. In these gases, the discharge voltage and current oscillograms have been obtained for different delays between the laser and voltage applications, and on varying the laser fluences over a wide range. The observed effects of increasing the burning voltage and decreasing the discharge current in a CO₂ laser-irradiated SSVD are treated in terms of enhanced electron attachment due to electron capture by vibrationally excited molecules of SF₆ and C₂HCl₃. For the first time the effect of radically affecting a spatial electric current distribution over the discharge gap in strongly electronegative gases by irradiation of a specially profiled CO₂ laser beam is found. Starting from this finding, the possibility of performing laser-controlled SSVD spatial structures in SF₆ and SF₆-based mixtures has been experimentally demonstrated. The problem of stability of a laser-irradiated SSVD in strongly electronegative gases is also touched upon.

We present the results of a study of a capacitively coupled hydrogen discharge by means of a one-dimensional numerical fluid model and experiments. The model includes a detailed description of the gas-phase chemistry taking into account the production of H⁻ ions by dissociative attachment of H₂ vibrational levels. The population of these levels is described by a Boltzmann vibrational distribution function characterized by a vibrational temperature T_V. The effect of the dissociative-attachment reaction on the discharge dynamics was investigated by varying the vibrational temperature, which was used as a model input parameter. Increasing the vibrational temperature from 1000 to 6000 K affects both the chemistry and the dynamics of the electrical discharge. Because of dissociative attachment, the H⁻ ion density increases by seven orders of magnitude and the H⁻ ion density to electron density ratio varies from 10⁻⁷ to 6, while the positive ion density increases slightly. As a consequence, the atomic hydrogen density increases by a factor of three, and the sheath voltage drops from 95 to 75 V. Therefore, clear evidence of a strong coupling between chemistry and electrical dynamics through the production of H⁻ ions is demonstrated. Moreover, satisfactory agreement between computed and measured values of atomic hydrogen and H⁻ ion densities gives further support to the requirement of a detailed description of the hydrogen vibrational kinetics for capacitively coupled radio frequency discharge models in the Torr regime.

The electron drift velocity, the longitudinal diffusion coefficient and the effective ionization coefficients in C₂F₆ and its mixtures with Ar and N₂ have been studied in a pulsed Townsend experiment. The density-reduced electric field strength E/N, was varied widely in the range 0.1–500 Td (1 Td = 10⁻¹⁷ V cm²). In all cases, negative differential conductivity (NDC) regions were found in the plots of the electron drift velocity as a function of E/N. Our drift velocity values were found to be in good agreement with previous work on C₂F₆ and its mixtures. In some cases, the E/N range was increased. The longitudinal diffusion coefficient also displays the NDC effects; these are more pronounced as the C₂F₆ content in the mixture decreases. The effective ionization coefficient for C₂F₆ was measured over a wider range of E/N than that covered by previous studies. Our effective ionization coefficients for C₂F₆ and those derived previously disagree strongly. We are not aware of any previously published data on effective ionization coefficients for the mixtures herein studied. The limiting field strength for C₂F₆–N₂ turned out to be nearly a factor of two higher than that for C₂F₆–Ar, for C₂F₆ concentrations being less than 10% in both mixtures.

Radiation trapping and transport are important to the power balance of low pressure non-equilibrium plasma lighting sources. This is particularly the case for radio frequency inductively coupled lamps having complex geometries and where control of radiation trapping is an important design consideration. To investigate these issues, a Monte Carlo radiation transport simulation was developed and integrated into a two-dimensional plasma dynamics model. Investigations were performed on the 254 nm (6 ³P₁–6 ¹S₀) and 185 nm (6 ¹P₁–6 ¹S₀) resonance radiation transitions from Hg in Ar/Hg electrodeless discharges. We found that analytically computed radiation trapping factors are less accurate when there is a non-uniform density of absorbers and emitters, as may occur in low pressure lamps, in our case due primarily to cataphoresis. For typical lamp conditions (hundreds of mTorr fill pressure of argon with the vapour pressure of Hg, a few megahertz driving frequency), the electromagnetic skin depth is much larger than the size of the vessel. Therefore, the frequency of excitation does not appreciably affect the distribution of absorbers and emitters, and so has little effect on radiation trapping. Studies were performed on industrially available lamp geometries. We found that the shape of the plasma cavity influences trapping factors, primarily due to the consequences of transport of Hg ions on the distribution of radiators.

Momentary emission spectra of iron and argon lines were measured in a plasma plume induced during welding with a continuous wave CO₂ laser. Time-dependent spectra were registered using a fast gate, lens coupled microchannel plate image intensifier placed between a spectrograph and a 1254 silicon intensified target detector connected to an optical multichannel analyser. The results, together with the analysis of the colour images from a fast camera, show that in the case when argon is the shielding gas, two plasmas exist: the argon plasma and the iron plasma. It has been found that during strong bursts the plasma plume over the keyhole consists mainly of metal vapour, not being diluted by the shielding gas. No apparent mixing of the metal vapour and the shielding gas has been observed. The space-averaged electron densities determined from the Stark broadening of the 7503.87, 7514.65 Å Ar I lines amounts to (0.75–1.05) × 10²³ m⁻³ depending on the distance from the surface. Assuming that argon is not mixed with the metal vapour and is in local thermodynamic equilibrium these electron densities correspond to the temperatures of 12–13 kK. At the peaks of strong vapour bursts the space-averaged electron densities determined from the Stark broadening of the 5383.37 Å Fe I line are (0.6–1) × 10²³ m⁻³. Numerical simulations showed that the maximum densities in the plasma centre are considerably higher and amount to ∼1.8 × 10²³ m⁻³ and ∼2.45 × 10²³ m⁻³ in the case of the argon and metal plasma, respectively. Consequently the absorption of the laser beam in the plasma plume amounts to ∼5% of the beam power in the case of argon and 10% in the case of metal plasma.

Multipolar plasma confinement is known to result in enhanced plasma density, homogeneous plasma of a large volume, and quiescent plasmas. The effects of a multipolar magnetic field on the dc discharge are confirmed using the particle-in-cell/Monte Carlo (PIC/MC) method. The gas is argon. The present PIC/MC simulation made it possible to consider three roles: (1) magnetic mirror effect near the cusp of two magnetic poles, (2) suppressed electron diffusion in the direction normal to magnetic field line, and (3) electron repelling in the sheath on the chamber wall. Electron number density for the magnetic field |B_θ|_max = 0.1 T is up to nine times higher than that for no field for our plasma dimensions. The multipolar confinement is effective when the mean free path of fast electrons is of the order or larger than the discharge size. In order to give design criteria of the magnet system, the effects of magnetic field, number of magnetic poles, and gas pressure on the plasma density are examined systematically.

Inductively coupled Cl₂/Ar plasma etching of 4H–SiC has been studied. The SiC etch rate has been investigated as a function of average ion energy, Ar concentration in the gas mixtures, inductively coupled plasma power, work pressure and substrate temperature. The etch mechanism has been investigated by correlating the ion current density and relative atomic chlorine content to the etch rate under various etch conditions. For the first time, it has been found that the etch rate of SiC increases by about 50% at lower substrate temperatures (−80°C) than at high substrate temperatures (150°C) with the highest SiC etch rate of 230 nm min⁻¹ being achieved at a substrate temperature of −80°C.

Experimental ac partial discharge (PD) patterns are presented for a disc-void (area greater than length) with metal surfaces, one metal and one insulating surface, and with both surfaces insulating. These patterns indicate that the portion of the surface over which the charge is deposited (discharge area) plays an important role in producing the fluctuations in discharge magnitude commonly observed for such voids. A simulation model for PD patterns in voids is presented, which expressly includes the effects of the charge distribution left on the void surface by consecutive PDs. This model defines two factors that control the PD propagation. These are: a minimum field required to maintain the discharge within an existing surface path E_in and a minimum peripheral field required to extend the discharge path E_p. There is also one other factor, the occurrence probability that controls the incidence of the PD. It is shown that the model gives a variation in PD magnitude throughout the active region of phase without any stochastic factors, i.e. when only variations in the discharge areas are allowed for, but that in order to reproduce the typical (turtle-like) PD patterns observed for the disc-void with insulating interfaces the occurrence probability and surface conductivity have to be included. The current model is compared to previous models and the relationship of the model factors to the change of PD patterns with ageing is briefly discussed.

Y123 thin films were grown by pulsed laser deposition (PLD) on YSZ (001) substrate. Prior to the film deposition, a discontinuous layer of Ag was deposited on the substrate, also using PLD, in the form of separate islands. Atomic force microscopy (AFM) investigation of the Ag layer showed that its morphology consisted of self-assembled islands of nanometre size, randomly distributed on the surface of the substrate, called nano-dots. The Y123 superconducting films grown on such a surface were characterized using AFM, x-ray diffraction, secondary electron microscopy, ac susceptibility and dc magnetization. The results show that there is no significant difference in surface morphology, crystallographic orientation, phase composition or superconducting transition temperature between the Y123 films grown on YSZ (001) with an Ag nano-dots layer and a control Y123 film grown on a virgin YSZ (001) surface. On the other hand, at 77 K, the magnetic critical current density ( ) was three times higher for the Y123 film grown on YSZ with the modified (001) surface than for the film grown on YSZ with a virgin (001) surface. At 5 K the enhancement of was approximately seven times, at both low and high fields. This suggests an increase in pinning, caused presumably by point defects formed in the Y123 film above the Ag islands.

The formation and decay of high temperature phases in H_xLi_1−xNbO₃ waveguide layers have been studied by means of waveguide mode spectroscopy and x-ray rocking curve measuring. The hydrogen doped layers with x ∼ 0.47 in (001) LiNbO₃ were created by proton exchange reaction in melt benzoic acid. The high temperature phases were frozen by quenching from T = 200°C. It has been shown that these phases possess a 9% higher refractive index and 16% lower deformation in reference to the characteristic of H_xLi_1−xNbO₃ phase equilibrium at room conditions. The relaxation of the high temperature phase to the equilibrium one has been traced at T = 100°C.

Light propagation through polymer films with fine randomly distributed liquid crystal droplets is considered. A simple expression for the field-induced optical phase shift is obtained. It is shown that the value of the optical phase shift is proportional to the volume concentration of the droplets, to the difference between the principal refractive indices of the liquid crystal and to the film order parameter. The possibility of retrieving the morphological parameters of the films from measurements of the optical phase shift is also considered.

Vacancy defects in epitaxial La_0.7Sr_0.3MnO₃ (LSMO) thin films on LaAlO₃ substrates were detected using a variable energy positron beam. The line-shape S parameter of the epitaxial thin films deposited at different oxygen pressures was measured as a function of the implanting positron energy E. Our results show that the S parameter of the films changes non-monotonically with their deposition oxygen pressures. For the films deposited at lower oxygen pressures, the increase in S value in the films is attributed to the increase in oxygen vacancies and/or related defect–V_O complexes, and for those deposited at higher oxygen pressures, the larger S parameter of the films is caused by the grain boundaries and/or metallic ion vacancies. The surface morphology of the films was also characterized to analyse the open volume defects in the LSMO films.

The thermal conductivity, thermal diffusivity and volumetric heat capacity of monocrystalline AgCl were measured simultaneously, using the Gustafsson probe in the temperature range 77–300 K. The thermal conductivity, as well as the thermal diffusivity, of these crystals follows a T⁻¹ behaviour in the temperature region mentioned here. The heat capacity at constant pressure (C_P), determined from the volumetric heat capacity, agrees with the calculated one at room temperature. The values of C_P obtained from this experiment along with the reported data as a function of temperature are also reported.

In this paper, we apply Monte Carlo simulation to investigate the conductive percolation threshold of granular composite of conductive and insulating powders with amorphous structure. We focus on the effect of insulating to conductive particle size ratio λ = d_i/d_c on the conducting percolation threshold p_c (the volume fraction of the conductive powder). Simulation results show that, for λ = 1, the percolation threshold p_c lies between simple cubic and body centred cubic site percolation thresholds, and that as λ increases the percolation threshold decreases. We also use the structural information obtained by the simulation to study the nonlinear current–voltage characteristics of composite with solid volume fraction of conductive powder below p_c in terms of electron tunnelling for nanoscale powders, dielectric breakdown for microscale or larger powders, and pressing induced conduction for non-rigid insulating powders.

Investigation on phase transformation and electrical transport are carried out for PbO₂–xAg₂O–0.75C composite alloys. When the Ag₂O content decreases, a percolation effect takes place near x = 0.275 for the distribution of the metallic particles embedded in the insulator matrix. With decreasing Ag₂O content, the temperature dependence of resistivity of the composites changes from a typical metallic behaviour to an insulating one. A metal–insulator transition appears in the PbO₂–0.275Ag₂O–0.75C composite, with decreasing temperature. The electronics in the PbO₂–0.23Ag₂O–0.75C composite are described by hopping conductivity in terms of the variable-range-hopping model.

The change in mutual impedance ΔZ₁₂ due to eddy-current induction is considered for an arbitrary pair of air-cored coils located above a conducting plate. A general expression is derived for ΔZ₁₂ and applied to the specific case of cylindrical coils with rectangular cross-section. A series of closed-form expressions for ΔZ₁₂ is presented for the geometries that cover all of the possible coil configurations for which the orientation of the individual coils is either normal or tangential to the plate. Expressions are also obtained for ΔZ₁₂ in the dipole limit and the results are compared with earlier work for dipole loops. The validity of the theory is tested against experimental measurements for representative high-symmetry coil systems above an aluminium-alloy plate. The theoretical calculations and experimental results are in excellent agreement over the experimental frequency range 100 Hz–50 kHz. The significance of the results for eddy-current non-destructive inspection using driver–pickup coils is discussed.

The effects of filler particles on the signal measured using the pressure-wave-propagation method to determine the space charge distribution are analysed. A special test sample geometry is used to classify the influence of particles, and in particular, the role of piezoelectricity is discussed. Applications with Epoxy resin samples loaded either by silica or alumina particles are presented. Silica particles are shown to exhibit piezoelectricity which strongly modifies the measured signals.