Table of contents

Co/Au(001) superlattices were prepared using an ultra-high-vacuum deposition technique. When the Co layer thickness is above 7 AA or when the Au layer thickness is below 12 AA, a drastic increase is found in the interface roughness parameter, indicating the change from a coherent to an incoherent structure of the interface. From magnetization measurements at 5 K, the magnetic moment for the Co layers in the Co/Au(100) superlattices is found to be 1.7 mu _B per atom. The easy magnetization direction lies in the film plane for all the samples. A strong in-plane magnetic anisotropy is observed for the samples with small Co layer thickness and highly ordered interface structure.

A thermocouple which consists of two parts of a Fe_77.5Si_7.5B₁₅ wire-an amorphous part and a crystalline part-which is crystallized by passing a DC current through it, supplies a thermoelectromotive force that depends linearly, up to 470 K in the first approximation, on the junction temperature. The amorphous part is electropositive compared to the crystalline part. The value of the thermoelectric power depends on the evolution of the crystallization process which is simultaneously studied by measurements of the magnetic induction and of the electrical resistance. The quantitative determination of the thermo emf is a sensitive method for the study of the crystallization process in ribbon- and wire-shaped amorphous materials.

An algorithm is described which uses the Monte Carlo method to evaluate the fraction of the light produced by charged particles in the bulk which reaches the scintillator's output face, by taking into account surface and optical absorption. Local light production is estimated using the model of Michaelian and Menchaca-Rocha (1994 Phys. Rev. B 49 15550-62). These absorption mechanisms are shown to introduce range-dependent effects which are most important when the penetration of the particles is comparable with the depth of the scintillator.

We report on the performance of a hollow-cathode discharge continuous wave He-Cd⁺ laser module, which is capable of simultaneously delivering stable, milliwatt output power at the three primary spectral lines: blue ( lambda =441.6 nm), green ( lambda =533.7 and 537.8 nm), and red ( lambda =635.5 and 636.0 nm). In particular, the laser output characteristics for single- and multi-line operation and the power interaction between the laser lines are presented. The results show that the most efficient simultaneous laser oscillations at the red, green and blue lines occur for the He pressure (8 mbar at an inner diameter of the hollow cathode of 4 mm) at which the hollow-cathode effect is strongest. We find strong power interaction between the green and red lines, showing experimentally that, apart from the already known increase in the He-Cd⁺ laser output power in the green owing to the radiative cascade transitions in the red, also the red lines gain in their output power under simultaneous operation with the green lines, which de-populate the lower laser levels of the red transitions. Under some conditions the de-population of the lower laser levels of the red transitions by the green lines' oscillations can be the dominant mechanism for the build-up of the population inversion between the upper and lower levels of the red transitions.

A simple formula giving parameter limits for stable operation with respect to thermal instabilities of high-frequency excitation of CO₂ laser discharges is derived from a discharge model including the continuity equation for the electron density and the neutral gas density, the energy equation for the neutral gas and Ampere's law for the electric field. For stable discharge operation, a limiting value of the product of the neutral gas density and the discharge width must not be exceeded. This limit is also determined by the excitation frequency. The discharge is stabilized mainly by the frequency-dependent reduction in the electric field accompanying a corresponding increase in the electrical conductivity and by those processes that lead to a smoothing of particle densities and the temperature. These processes are diffusion and heat conduction and especially turbulence-induced transport processes. A quantitative description of the turbulent transport processes is beyond the scope of the present paper. The effect of the turbulent gas flow is roughly approximated by an effective heat conductivity.

Detailed experimental and theoretical studies of a phototriggered XeCl excimer laser have been performed through the development and the experimental validation of a zero-dimensional model, which self-consistently coupled the solution of the Boltzmann equation, the kinetic equations of heavy particles, the electrical circuit and the photon equations. A detailed description of the XeCl molecule and of the associated kinetics has been taken into account, together with the effect of the gas temperature growth on the stimulated emission cross section. The model predictions have been compared with the experimental results obtained from a complete set of time-resolved electrical and optical diagnostics including emission and laser absorption spectroscopy on various xenon and neon states. A specific output energy of 7.6 J l^-1 and an efficiency of 3.5% are reported, which indicates the effectiveness of the phototriggered discharges to produce high-performance excimer lasers. Simple rules allowing the calculation of the required voltage for best efficiency or maximum output laser energy are given.

Laser grooving is a very complicated process to analyse, due to the molten material's behaviour. Based on the assumption of complete removal of molten material, a theoretical analysis is performed to derive groove depth as a function of process parameters. The heat balance at the cutting front is used as the governing equation to determine groove depth. Conduction heat direction and area change are approximated to obtain a closed form of the groove depth. The theoretical results show good agreement with experimental data. This approach provides a simple way of predicting groove depths and is applied to the prediction of depths of cuts in laser cutting.

A TEA CO₂ laser (500 mJ, 100 ns) has been focused on a Zn plate in a surrounding gas at 0.5 Torr. The characteristics of the laser plasma were analysed using a unique time-resolved spectroscopic method. Time-resolved spatial distributions of Zn I 481.0 nm and Zn II 492.4 nm emission lines were observed. It was clearly shown that the resulting plasma has a thin shell structure and expands with time, and the ionization of Zn atoms precedes at slower rate than the excitation of neutral Zn atoms. The displacement of the neutral Zn emission was proportional to the two-fifths power of time. These experimental results proved that the plasma was excited by a blast wave induced by the laser bombardment.

We have measured CF₃, CF₂ and CF radical densities in a RF discharge CHF₃ plasma using infrared diode laser absorption spectroscopy and have observed the variation of radical densities with passage of time after discharge initiation. The results show that the CF_x radical densities increased with discharge time and then became stable. The thickness of fluorocarbon polymer films grown on the electrodes was also measured to investigate the influence of fluorocarbon polymer formation on CF_x radical densities in the plasma. It was considered from these results that the radical density continues to increase until the electrodes are covered with a certain thickness of fluorocarbon polymer film. Moreover, the CF₂ radical showed a different dependence on discharge duration to CF₃ and CF radicals because of its longer lifetime.

In order to have a better understanding of the thermal processes occurring in an electrode submitted to a moving arc root, we have studied the displacement mode of cathode and anode arc roots. We have also elaborated a three-dimensional model able to solve the diffusion problem of the heat brought by the arc root to a hollow cathode of a plasma torch. This model takes into account the temperature dependence of the material properties (specific heat and thermal conductivity), the various phase changes occurring during the arc and the arc root motion.

The electrical properties of Langmuir-Blodgett films of polypyrrole fabricated on a copper stearate template have been studied. The films exhibit domains of a highly organized two-dimensional linear array of polypyrrole granules sandwiched between insulating fatty acid layers together with some disorganized domains. The measured in-plane conductivity does not show any appreciable anisotropy and is as high as 1.2 S cm^-1 with a thermal activation energy of 0.15 eV. The transverse conductivity of the film in a metal-insulator-metal structure is shown to be Poole-Frenkel emission-limited and has a value of the order of 10^-8 S cm^-1 in the low-field region. The film then shows a high degree of conductivity anisotropy.

The influence of rapid thermal annealing in vacuum on the properties of thermally grown SiO₂ has been investigated by means of high-field Fowler-Nordheim tunnelling injection and breakdown field techniques. The results indicate that the rapid thermal annealing process anneals the original electron traps in the oxide but at the same time introduces a positive charge. These two mechanisms are a strong function of the technological history of the samples and of the oxide quality. Rapid thermal annealing in vacuum improves the properties of the as-grown oxides without conventional post-oxidation annealing in N₂ but degrades the oxides that are preliminarily annealed in N₂. The extent of the improvement (degradation) depends on the temperature of rapid annealing (1073-1473 K).

Electron-beam-induced current (EBIC) measurements can be used in order to investigate microscopic electrical properties of semiconductor materials and devices. However, for the estimation of material parameters and the unequivocal interpretation of EBIC micrographs, very often additional simulations are necessary. Up to now, these simulations have been carried out mostly using analytical models based on some restrictive simplifications. These models can only be used for simple specimen structures of high symmetry or for specimens that can be definitely separated into such simple structures. These models are not well suited to the simulation of electron beam currents in small devices such as MESFETs (metal-semiconductor field-effect transistors). Thus, we have used the more adaptable finite difference method for two-dimensional numerical simulations of electron-beam-induced gate currents in these devices. In this first part of the paper, the model and tools used are presented.

Metal-semiconductor field-effect transistors (MESFETs) are important semiconductor devices, which can be produced on many semiconductor materials. Owing to their simple structure, MESFETs are often used as pioneering devices when new material systems are developed. Electron-beam-induced gate current (gate EBIC) measurements in a scanning electron microscope (SEM) can be used as an important tool for investigating the electrical homogeneity of semiconductor materials, electrical contacts and process-induced defects in MESFETs. Furthermore, in combination with suitable simulations, a profound understanding of the importance of different material and device parameters to the electron-beam-induced currents is possible. With this aim, results of two-dimensional numerical gate EBIC simulations obtained for a GaAs MESFET are presented in this paper and partially compared with measured line scans.

Thermally stimulated current (TSC) spectra and photocurrent (I_pc) measurements are used for determination and evaluation of deep levels, as well as for the comparison of their distributional uniformity in undoped semi-insulating (SI) GaAs crystals. Liquid-encapsulated Czochralski-grown (LEC) GaAs wafers from various sources were analysed. It was found that crystal growth and post-growth treatment of samples from each producer give characteristic and distinctive TSC spectra. The samples from different positions at wafers show good uniformity of deep-level distribution for some producers, whereas the uniformity is quite poor for the others. Some characteristic peaks appear in spectra of all crystals, suggesting either the presence of native defects or some of the most common contaminants. The results obtained are compared with those in previous reports. Furthermore, a considerable difference in photocurrent values after 100 s illumination has been found, indicating large free carrier lifetime scattering for samples of different origins. This study shows how, by using quick and simple TSC and I_pc measurements, the quality of the material and uniformity of the distribution of defects with deep levels in the forbidden energy gap can be assessed.

Electrostriction is the strain response of a dielectric material proportional to the square of an applied electric field. This effect is related to the strain-dependence of the dielectric function. The strain signal observed using optical interferometry is a superposition of true electrostriction and other effects like electrostatic forces and thermal stresses. Therefore, it is necessary to have a formula that allows the measured values to be corrected for these spurious effects. In this paper, a thermodynamic approach and the force law are used, combining continuum mechanics and Maxwell's electromagnetic equations, to derive a correction formula. The result can be applied to measurements of both the longitudinal and the transversal effect in single crystals of any symmetry.

An iterative automatic method is described for the characterization of lossy piezoelectric materials in the radial resonance mode based on the use of the more general expression for the complex admittance. From the experimental data of Y at four adequately selected frequencies, the constants of the material are determined with the necessary accuracy to reproduce the piezoelectric behaviour of the sample around resonance. The IEEE-176 Standard procedure has been automatized for the initial estimation of the real parts of the elastic constants. The method is applicable even to those materials in which said standard does not allow one to determine the piezoelectric constants.

A theoretical study on the thermoelastic generation mechanisms of the characteristic elastic waveforms excited axisymmetrically by a pulsed laser in plates is presented by employing the method of eigenfunction expansion. The method gives a systematic treatment and allows one to investigate not only the thermoelastic generation of longitudinal, transverse and surface acoustic waves in thick samples, but also the excitation of the Rayleigh-Lamb wave modes in thin materials. The three-dimensional theoretical modelling takes account of both the sub-surface thermoelastic source (due to thermal diffusion and optical penetration into the solid) and the surface source resulting from the thermoelastic mode conversion at the illuminated surface. The excitation of the transient Lamb waves is also studied thoroughly in thin isotropic materials, and the results indicate that the Lamb waves characterized by a spike (termed 'sheet waves') and the frequency-modulated oscillation signal in a very thin sample will approach the Rayleigh wave in nature as the thickness of sample increases. The numerical technique provides a quantitative tool for experimental determination of material properties.

The spatial distribution of the birefringence of a gamma -Fe₂O₃ ferrofluid sample, submitted to a constant magnetic field, has been analysed by means of different polarimetric criteria, evaluated from the m_ij coefficients of the Mueller matrix. These criteria introduced by different authors are used to control the validity and the quality of our calculus method for the determination of the birefringence spatial distribution. Results are compared with the phase-delay spatial distribution of a nematic liquid crystal cell obtained by the same method.

A flexible numerical procedure for the calculation of thin-film optical constants from specular transmittance and reflectance data is presented. The method is based on the minimization of a quadratic error function, which may be adapted to the specifics of the optical behaviour of the given sample (or set of samples), and the given wavenumber region. The flexibility in choosing an appropriate form of the minimized error function, in combination with the powerful minimization method of conjugated gradients, allowed us to investigate the optical constants of very different types of novel thin-film material with a complicated optical loss behaviour. In particular, the results concerning the investigation of single- and two-layer systems based on the following technologically interesting optical thin film materials are presented: (1) amorphous silicon as an example of-an anorganic solar cell material; (2) as-deposited (rough) CVD diamond layers as an example of a polycrystalline protective material; (3) hydrogenated amorphous carbon, applicable as a protective long-wavelength in antireflection coating as well as a spectrally selective solar absorber; (4) copper phthalocyanine layers as an example of a molecular solid, potentially applicable as an organic solar cell material; (5) rare-earth diphthalocyanine layers, interesting because of their electrochromic behaviour.

An attempt is made to give a physical picture of the dynamic thermal deformation in thin films under transient laser pulse heating, which has been observed by one of the authors previously. To simplify the real problem, the transient bulk heating model is used, then using Lord-Shulman theory, the dynamic thermal strain as a function of time and relaxation time is derived. For a thin film, the theoretical calculation shows the same result as the experiment. The present analysis is a simple method which is particularly suitable for systems with a random metallic structure and thin samples.

Mass spectra of negative secondary ions emitted from the surface of a stoichiometrical superconducting YBa₂Cu₃O_7-x ceramic bombarded by 250 eV positive argon and oxygen ions were measured. The results show that the reduction of the primary ion energy reduces the yield of heavy secondary ions. It was observed that the supply of oxygen to the sample bombarded by Ar⁺ ions enhances the yield of secondary negative ions, although this yield is further enhanced by switching to positive oxygen primary ions. Moreover, the bombardment of a YBCO ceramic sample by oxygen ions is characterized by a particular enhancement of the yield of barium and barium-containing molecular ions.

The nature of the surface plays a critical role in the charging ability and the insulating character of insulating materials. By submitting the polymer (for instance polypropylene) to corona discharges with streamers, the corona-activated species change the chemical and morphological properties of the surface. The physico-chemical properties had previously been investigated by electron-micrography and infrared absorption spectroscopy, and both techniques have clearly shown that nodules and crystallites of oxidized constituents of the polymer appear on the surface. By using both surface potential decay and return voltage analysis, the present work will be shown to reveal a significant correlation between the physico-chemical properties of the treated polymer and the transparency of the surface to charges deposited on it by a direct current corona. Whilst the voltage of charged untreated samples decays rapidly and substantially, it decays less, and less quickly, for treated samples. Conversely, whilst the return voltage for untreated samples is hardly measurable, it is quite appreciable for treated samples. These observations cannot be explained in terms of one-dimensional charge transfer models but their interpretation may be based on the defects mentioned above, which modify the local field on the sample during charge deposition.

It is pointed out that the approximately 12 Hz fluctuations measured in a dusty plasma in the paper by Chu et al. (1994) are very likely the first laboratory observation of the type of wave known as dust-acoustic.