Table of contents

A new fabrication technique of p-type CdS thin films by He-Ne laser illumination of bilayer Cu-nCdS structures at room temperature was investigated. The n-type CdS films were obtained by vacuum evaporation in a quasi-closed volume. X-ray diffraction was used to provide crystalline structure and composition data of CdS films and Cu-CdS structures. The band gap of CdS films was estimated from the optical transmission spectra. The hot probe and Hall effect studies were used for the determination of conductivity type and concentration of charge carriers in films. The formation of a p-n homojunction in CdS films or conversion of the film all over to the p-type, depending on the duration of laser illumination, was shown by I-V characteristics, the photovoltaic, the hot probe and Hall effect measurements. Analysis of concentration distributions of Cu in CdS films, arising as a result of laser-accelerated diffusion, by energy dispersive x-ray spectroscopy gave the effective diffusion coefficient of copper, D = 8 × 10^-12 cm² s^-1 at T = 25 °C.

We report the fabrication of ordered arrays of silicon nanopillars via reactive ion etching (RIE). Self-assembled polymer spheres are used as masks for the deposition of hexagonal arrays of Ag islands on a silicon substrate. Following the removal of the polymer spheres, the sample is etched with SF₆ and CF₄ at 100 W leading to hexagonal arrays of silicon nanopillars. The aspect ratio of the pillars can be influenced by the etching time; maximum aspect ratios of 15:1 have been produced.

A new method for the removal of harmful organic molecules from water is described. A low power corona discharge is created over the aqueous solution. Chemically active species diffuse into the water and then oxidize the target compound, which in this case is the model compound phenol. The energy consumption per removed phenol molecule is one order of magnitude lower compared to the discharge techniques that create a plasma in the water. The reaction mechanism of the conversion is shown by measuring the ozone concentration over the water, the intermediate/final oxidation products and the release of CO₂ from the water. Indications are found that the discharge is more than merely an ozone generator.

There is growing interest in organic electroluminescence (EL). A great deal of progress has been made recently in improving the performance of various classes of organic EL devices. Some of these are now adequate for many applications. However, specialists focusing on selected aspects of organic EL devices have often lost contact with the general subject of EL. Therefore, a review covering all aspects of EL mechanisms and their experimental manifestation seemed necessary. This article is concerned with the new EL device physics that can be realized using crystals, or films made of organic materials, as electrically and optically active components, in devices ranging from simple single-component light emitting diodes (LEDs), through double- and multi-layer LEDs to light emitting electrochemical cells (LECs) and organic LED-based light transducers. The investigation of the properties of these devices has provided in turn a very effective method for studying the basic EL phenomena in these materials. Since the subject of the present review has generated a huge amount of literature, and it is impossible to mention here all that has been done, we have attempted to provide an outline of the background of the field of organic EL, and discussed in some detail those aspects most relevant to the EL device physics. Because of the diversity of the types of material and EL structure, there is no single, simple description of EL in organics. Therefore, the initial sections of the article are devoted to a discussion of the types of EL and related phenomena, such as carrier injection and recombination or nature of emitting states. Then, the fundamentals of the fabrication of various types of EL devices are discussed along with the most representative examples. In general, the reader will find in the article a brief historical review of the subject as well as a description of the latest trends in organic EL research covering all the new concepts and most important data which have become available before the time of publication.

A high degree of in-depth compositional uniformity is an important prerequisite for obtaining device-quality CuInSe₂ absorber films. In general, it is reported that two-stage growth processes lack reproducibility due to material losses during the high-temperature selenization stages. In this study, absorber films were prepared by a typical two-stage process in which selenium-free (In/Cu/In) and selenium-containing (InSe/Cu/InSe) precursors were reacted with H₂Se/Ar. Scanning electron microscopy (SEM) studies revealed a significant improvement in the morphological properties of the absorber films in the latter case. X-ray fluorescence (XRF) K_1,2 line intensity measurements of the samples indicated no loss of In or any other element during the selenization stages, irrespective of the precursor alloy considered. The in-depth compositional uniformity of the samples was determined by measuring the XRF K_1,2 line intensities of successively etched samples. For samples obtained from the selenium-free precursors, this in-depth analysis revealed a pronounced separation of the elements: non-uniform Se interdiffusion and a sharp increase in the In concentration towards the Mo back contact. In samples prepared from selenium-containing precursors, the concentration of all three elements remained virtually unchanged as a function of the sample thickness. X-ray diffraction (XRD) studies also revealed a single-phase material with a very strong preferred [112] orientation.

We have fabricated and characterized Si-implanted thermistors to be used as phonon sensors in very-low-temperature, single-quantum detectors. After a short review of the required thermistor properties for this type of application, a detailed description of the production process and of the characterization experimental techniques is reported. The data show that the resistivity-temperature behaviour of all of the devices follows the prediction of the variable range hopping conduction model in the investigated temperature range (4.2-0.03 K). Phonon-electron decoupling and excess low-frequency noise show up at low temperatures, reducing the thermistor sensitivity. These phenomena are discussed and conveniently parametrized in view of a complete detector optimization.

A variational procedure in the effective-mass approximation was used in the study of both the density of donor states and the associated optical absorption spectra of GaAs-(Ga,Al)As semiconductor heterostructures. Calculations were performed for a finite, cylindrical-shaped quantum box of GaAs within the infinite-barrier approximation and considering an homogeneous distribution of impurities within the low-dimensional heterostructure. Theoretical results for the density of impurity states and donor-related optical absorption spectra were found to be in good agreement with previous studies in the limiting cases of quantum wells and quantum-well wires.

We report on a powerful 4.6 µm light emitting diode (LED), operating at room temperature, suitable for carbon monoxide gas detection. The source is based on a InAs_0.55Sb_0.15P_0.30 /InAs_0.89Sb_0.11/InAs_0.55Sb_0.15P_0.30 symmetrical double heterostructure with large band offsets. To improve performance, the InAs_0.89Sb_0.11 ternary material in the active region was purified by using rare-earth ion gettering during liquid phase epitaxial growth. A pulsed optical output power in excess of 1 mW at room temperature has been measured, making these emitters suitable for use in cost-effective instruments for the environmental monitoring of carbon monoxide.

The Auger coefficient of the InAs_0.89Sb_0.11 active region material was evaluated by analysis of the temperature quenching of the LED output power measured at constant current. It was found to be C₀ = 1.5 × 10^-26 cm⁶ s^-1 with an activation energy of E = 31 meV, which is in good agreement with previous findings in the literature.

We investigated the spatial distributions regarding the pathlength, the penetration depth, and the lateral displacement of 200-2000 eV electrons elastically backscattered from copper and silver. We calculated these distributions by the Monte Carlo method using elastic scattering cross sections and inelastic inverse mean free paths for volume and surface excitations. In our approach, we applied the partial wave expansion method and the finite difference technique to calculate electron elastic cross sections by the Hartree-Fock-Winger-Seitz scattering potential for solid atoms. We employed the extended Drude dielectric function to estimate electron inelastic mean free paths inside the solid and electron surface excitation parameters outside the solid. Our study was focused on the energy dependence of the pathlength distribution, the maximum depth distribution and the radial distribution. We found that both the radial displacement and the maximum depth of backscattered electrons were on the order of a few angstroms. The maximum depth and the pathlength distributions obeyed the exponential attenuation form. The ratio of the attenuation lengths for the pathlength and the maximum depth distributions saturated to a value somewhat greater than two. Considering the back and forth trajectories of backscattered electrons, it revealed that most electrons were backscattered from the solid through a single elastic scattering or a few elastic scatterings. As electron energy decreases, this ratio became larger, indicating that the probability for smaller scattering angles increases.

Annealing (in vacuum) at a temperature near the glass transition temperature and exposure (in air) with bandgap light of thermally-evaporated As₄₀S₄₀Se₂₀ amorphous chalcogenide thin films, were found to be accompanied by structural changes, which lead to changes in the refractive index and shifts in the optical absorption edge. Indications of photo-oxidation were found after light exposure in air. An optical characterization method, based on the transmission spectra at normal incidence of uniform thickness thin films, has been used to obtain the optical constants and the film thicknesses corresponding to the virgin, annealed and exposed As₄₀S₄₀Se₂₀ samples. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model. The optical absorption edges are described using the `non-direct transition' model proposed by Tauc.

Short-range order in Fe/Al multilayers has been determined using Fe K fluorescence x-ray absorption fine structure (XAFS) experiments. The two multilayers studied (prepared by pulsed laser deposition) exhibited different Fe single layer thickness (4.3 nm and 30 nm, respectively) but comparable total amount of Fe (six thin and one thick layers, respectively). From XAFS signals arising at different regions of the sample it was concluded that a solid solution of -Fe had been formed in the vicinity of the Fe/Al interface, the thickness of which was estimated to be 3.8 nm. The average Fe neighbourhood in these interface regions contained about 50% Al substituting for Fe.

The magneto-impedance effect Z/Z_H = [Z(H)-Z(H_max)]/Z(H_max) has been measured in (Fe_0.95Co_0.05)_72.5B₁₅Si_12.5 wire under torsion stress, (torsion angle per unit length) with axial magnetic field (H) as parameter. Without stress (Z/Z)_H(H) dependence has a non-monotonous shape with first an increase of total impedance Z and then a decrease, i.e. shows a maximum at certain axial magnetic field H_m. It was found that the torsion stress dependence of electrical impedance (torsion impedance), (Z/Z) = [Z()-Z(_max)]/Z(_max), has asymmetric character with a clear maximum at torsion angle, around 7 rad m^-1 in as-cast wire, while (Z/Z) reaches a maximum around 170%. Thermal treatments under torsion stress (without and with a previous annealing stage) develop a helical anisotropy on the amorphous wire, which drastically modifies the (Z/Z) response. Such treatments were carried out by using current annealing which resulted in a drastic increase of the maximum (Z/Z) up to 225%, and a change of torsion dependence of Z/Z with a tendency to a finally symmetric dependence of (Z/Z)(). The maximum (Z/Z) ratio, (Z/Z)_m, was obtained under torsion stress of = 20 rad m^-1 (in a torsion annealed sample) and = 11 rad m^-1 (with pre-annealing and torsion annealing). Observed dependences were explained taking into account the frozen-in magneto-elastic anisotropy owing to the internal stress distribution during the fabrication process, the helical anisotropies induced by the torsion strain and that developed by thermal treatment under torsion stress (torsion annealing). The differences in the shape and intensity of the maximum (Z/Z)_m between the torsion annealed and pre-annealed and torsion annealed samples should be ascribed to the visco-elastic character of the helical anisotropy induced by torsion stress.

The stochastic model of the accumulation of particles in a filter matrix composed of magnetized ferromagnetic spheres is considered. The distribution function, which gives the time dependence of the saturation radius that is characterized by both the capturing and detachment events occurring in the capture region, is determined. Taking into account the motional properties of the particles in the capture region, the distribution is evaluated from the solution of the Fokker-Planck equation. The results obtained are compared with, and shown to be similar to, the results of the other methods and some experiments reported in the literature.

Iron-based amorphous ribbons with a positive magnetostriction constant have been investigated by means of the vibrating sample magnetometer technique. When the samples are subjected to torsion and tensile stress, their hysteresis loops exhibit distinct steps. The correlation between the shape of the loops and the stresses applied was explored both experimentally and by evoking a simple realistic model. In addition to the fundamental interest in the hysteresis phenomena in these materials, the observed good agreement has an immediate practical implication, in that it is more reliable to use direct current magnetization curves in the interpretation of neutron depolarization studies.

There are different types of optical capillary cells in use for the modern on-line UV-VIS analysis of liquids using fibre spectrometers. These cells have the advantages of low sample volume, long optical pathlengths and compatibility to fibre optics. In this paper, we will show that the liquid core waveguide (LCW) is a particularly useful device because the light intensity is confined in the liquid core. This is shown in two ways. Firstly, the measured total optical losses of the LCW capillary device indicate typical guiding losses less than 3 dB m^-1 without a significant dependence on either fibre diameter or on the diameter of the capillary. Secondly, measurements of the radial intensity distribution exhibit a clear maximum in the core in the case of the LCW device.

The efficiency of a liquid core waveguiding cell is investigated for different core diameters (0.3-4 mm) in combination with different coupling fibre diameters (200-600 µm). A thin cell with a 360 µm diameter with a sample volume in the µl range and low coupling losses will be discussed as well.

The memory-mode operation of a polymer dispersed liquid crystal (PDLC), which depends on the stiffness of the layer structure in the smectic-A phase, has been analysed by comparing ordinary PDLC and holographic PDLC (HPDLC). Although both PDLC films were fabricated from the same material, the temperature range of the memory-mode operation was lower in the HPDLC than in the ordinary PDLC. In calorimetric analysis, the differential scanning calorimeter curves of the ordinary and holographic PDLC films were the same. Therefore, the liquid-crystal phase does not depend on the structural difference. This suggests that the force exerted against the layer structure of the smectic phase is greater in HPDLC because the droplet size is smaller and the ratio of the surface to volume is higher.

The effect of small amounts of electronegative NF₃ molecules on the electron swarm characteristics in Ar:NF₃ mixtures is studied numerically under steady-state Townsend conditions. Two different methods of calculation are used: the solution of the two-term Boltzmann equation (BE) and the Monte Carlo (MC) simulation. The BE analysis is performed for an established electron energy distribution function. The MC calculations give the distance at which the shape of the distribution function approaches that resulting from the BE analysis.

It is shown that the losses of electrons in the attachment processes play a significant role in the formation of the electron energy distribution function. In a wide range of the reduced electric field strength E/N and NF₃ concentrations the distribution function with a maximum at finite energy is established. It is also shown that the diffusion-modified drift velocity V_d is a non-monotonous function of E/N and that the so-called field-determined part of the V_d is negative in a wide range of E/N values. These effects are explained in terms of the specific energy dependence of the cross section for electron attachment to the NF₃ molecules and the momentum transfer cross section of Ar.

Gas amplification in a proportional counter filled with propane, propane-based tissue-equivalent gas mixture and dimethyl-ether is measured. By using low pressures (2-50 kPa) relatively strong reduced electric fields are produced, which enabled a study of non-equilibrium effects. The gas gain data are analysed according to the field-gradient model and the reduced ionization coefficient /P is thus obtained. The assumption of electrons being in equilibrium with the electric field fails to be valid in low-pressure gases, where the electric field strengths at the anode surface, S_a, above about 5 × 10⁵ V m^-1 kPa^-1 are obtained. Under non-equilibrium conditions /P is not a unique function of the reduced electric field strength, but depends on the gas pressure as well. Good agreement with other experimental or theoretical /P obtained mostly under equilibrium conditions is obtained at relatively weak electric field strengths. The presented /P in strong fields may be used for a more accurate modelling of low-pressure counters.

Self-consistent Monte Carlo simulations of the positive column of gas discharges with a range of gas densities and currents are described. These simulations are performed using highly efficient Monte Carlo codes for electron transport in cylindrical geometry and using a realistic particle model of ion transport. No internal boundary conditions or assumptions about the plasma-sheath boundary are used in the simulations. Techniques to speed up the convergence are discussed. The self-consistent radial potential for each plasma is presented as a Taylor series expansion in radius squared terms. The 16 converged simulations presented here were checked using two independent codes, written by different authors, in different languages and executed on different computers. These positive column results are useful in comparisons with other types of fluid and kinetic theory simulations.

A time-dependent, one-dimensional model was developed for simulating the behaviour of an impulse arc discharge in N₂ gas at atmospheric pressure on the basis of an electromagnetic hydrodynamics. In the model, simultaneous equations including mass, momentum and energy conservation equations were solved considering Ohm's and Ampere's laws and the equation of state, where 18 chemical reactions were taken into account. Time variations in the radial distributions of radial flow velocity, temperature and pressure in the discharge were derived for three different impulse currents with peak values of 74, 124 and 164 A and a time to peak current of 2.4 µs. Spectroscopic measurements of the N spectral line and continuous spectrum were made to experimentally obtain transient temperature variations in the impulse arcs. The arc temperatures were estimated to be about 7500 K up to 80 µs after arc ignition for all the three currents. These arc temperatures were found to agree fairly well with that calculated by the developed model.

The validity of actinometry to measure N and H atoms in nitrogen-hydrogen direct current (dc) glow discharges was investigated. The experiments were conducted in positive columns of dc glow discharges, in mixtures of N₂-xH₂, where x varies from zero to one, pressures between 133.33 and 533.32 Pa and discharge currents from 1 to 50 mA. The electric fields were measured by electric probes (17 V cm^-1<E<32 V cm^-1), and the gas temperatures (490 K<T_g<910 K) were deduced from the rotational transitions of molecular-band systems. The actinometry was performed using argon as the actinometer gas and compared with laser induced fluorescence measurements of ground-state atoms, in order to establish the limits of the validity of actinometry. A theoretical approach was used in order to interpret the behaviour of the emission lines. In nitrogen-hydrogen positive columns, the actinometry method indicated the correct behaviour of the N atoms density in the range of zero to one for x and for H atoms in the region from x = 0 to x = 0.2.

The dynamics of organic compounds 2-cyclo-octylamino-5-nitropyridine (COANP), (S)-2-N--(methylbenzylamino)-5-nitropyridine (MBANP), 2-(N-prolinol)-5-nitropyridine (PNP), and N-(4-nitrophenyl)-(L)-prolinol (NPP) were studied by dielectric relaxation spectroscopy in the frequency range of 10 Hz-2 MHz and differential scanning calorimetry (DSC). The dielectric and DSC studies showed that COANP, MBANP and PNP underwent glass transition. However, NPP crystallized so rapidly upon cooling that the glass state could not be observed. It was found that the crystalline process of COANP did not slow the structure relaxation of COANP glass. The relaxation times fitted well to the empirical Vogel-Fulcher equation = exp[E_a/k_b(T-T_VF)]. The activation energies E_a and the Vogel-Fulcher temperature T_VF were 54.5 meV and 239 K for COANP, 86.2 meV and 249 K for MBANP and 84.9 meV and 245 K for PNP, respectively. The crystalline temperatures of COANP and MBANP were given as 300 K and 330 K, respectively. An anomalous behaviour of the dielectric permittivity of PNP glass was observed.

In this paper the possibility of designing a structure whose microwave properties are similar to the optical properties of cholesteric liquid crystals is demonstrated. Unlike cholesteric liquid crystals, the considered multilayer structure can have considerable local chiral properties. The exact analytical solution for the reflection and transmission from a slab sample is given and illustrated by numerical examples. The modified de Vries formula for the rotation of the polarization plane of electromagnetic waves is found.

Stationary solitary signals with eigenfrequencies in a nonlinearity-induced gap of the linear spectrum are studied analytically on a nonlinear mono-inductance line. The stability of these nonlinear waves, which are kink or antikink solitons, are established theoretically as well as numerically. Computer experiments show how the electrical transmission line would behave in practice.