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Four nanostructured active semiconducting materials currently used in electronic inks have been structurally characterised using a combination of small angle scattering techniques and scanning electron microscopy. The percolation theory and scaling laws have been used to obtain quantitative correlations of the network topologies and the local micro-structures with the electronic and electrical properties of the printed, electronic devices. The small angle light scattering has been used to expand the lower q-range of the Ultra Small Angle x-ray Scattering curves of the 2503 metallurgical grade silicon (mSi), silicon dioxide (SiO

The authors report on a large area, fully screen-printed transistors, using nanostructured silicon as the active material. The transistors are produced by simple screen printing processes under ambient conditions without the need for post-processing steps. The devices can be operated as two-way switches both with a direct or alternating current (AC). The authors demonstrated the operation of the devices printed on flexible substrates (office paper 80 g/m2) using silver as the conductive electrodes and highly doped p-type nanostructured silicon as the active layer. A method of switching sinusoidal AC using a single printed transistor is demonstrated at a frequency of 50 Hz at ambient condition. The silicon used as active materials, present domains, and blocks at a microscopic level, with an electric behaviour similar to one of the varistor-like components used as surge suppressors. So, unlike conventional transistors which rely on electric field modulation or charge injection, these AC switches operate by activation transport of charge carriers in the active silicon layer. Switching AC is achieved by applying a signal to the base which results in a change of the current's direction from between the collector and base to between the base and emitter.

The presence of native oxide on the surface of silicon nanoparticles is known to inhibit charge transport on the surfaces. Scanning electron microscopy (SEM) studies reveal that the particles in the printed silicon network have a wide range of sizes and shapes. High-resolution transmission electron microscopy reveals that the particle surfaces have mainly the (111)- and (100)-oriented planes which stabilizes against further oxidation of the particles. X-ray absorption spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) measurements at the O 1s-edge have been utilized to study the oxidation and local atomic structure of printed layers of silicon nanoparticles which were milled for different times. XANES results reveal the presence of the +4 (SiO2) oxidation state which tends towards the +2 (SiO) state for higher milling times. Si 2p XPS results indicate that the surfaces of the silicon nanoparticles in the printed layers are only partially oxidized and that all three sub-oxide, +1 (Si2O), +2 (SiO) and +3 (Si2O3), states are present. The analysis of the change in the sub-oxide peaks of the silicon nanoparticles shows the dominance of the +4 state only for lower milling times.

This article reports on the variable temperature performance of a flexible printed transistor which works as a current driven switch. In this work, electronic ink is formulated from nanostructured silicon produced by milling polycrystalline silicon. The study of the silicon active layer shows that its conductivity is based on thermal activation of carriers, and could be used as active layers in active devices. We further report on the transistors switching operation and their electrical performance under variable temperature. The reliability of the transistors at constant current bias was also investigated. Analysis of the electrical transfer characteristics from 340 to 10 K showed that the printed devices’ current ON/OFF ratio increases as temperature decreases making it a better switch at lower temperatures. A constant current bias on a terminal for up to six hours shows extraordinary stability in electrical performance of the device.

This work reports on the successful fabrication and characterization of fully screen printed resistors, inductors, capacitors and LRC resonant circuit. Ink formulation and screen printing techniques were used to demonstrate the feasibility and reproducibility of fabricating components of resonant circuits. The equivalent circuit models of the individual printed components were established and used to model and extract parameters of individual components, from the fits of the impedance measurements data. We showed that the sample thickness and compaction were important factors necessary to reduce the resistance of the inductors. A quality factor of 25.5 at 13.5MHz was achieved on the printed inductor. A fully printed LRC circuit was further produced and characterize. Furthermore, a quality factor of 1.91, was achieved with band width of 2.42MHz. The results demonstrated that printed components had similar electric models with their discrete counterparts. Thus passive components could be screen printed with extraordinary accuracy for a given component's design. Display Omitted Screen printed passive components on flexible substrate are presented.Printed passive components and circuits behave similarly to their conventional counterpart.LRC components and circuit were successfully modeled with impedance spectroscopy.Resonance is achieved with a fully printed LRC circuit.

Four nanostructured active semiconducting materials currently used in electronic inks have been structurally characterised using a combination of small angle scattering techniques and scanning electron microscopy. The percolation theory and scaling laws have been used to obtain quantitative correlations of the network topologies and the local micro-structures with the electronic and electrical properties of the printed, electronic devices. The small angle light scattering has been used to expand the lower q-range of the Ultra Small Angle x-ray Scattering curves of the 2503 metallurgical grade silicon (mSi), silicon dioxide (SiO2), aluminium dioxide (Al2O3) and titanium dioxide (TiO2) materials by close to an order of magnitude, thereby providing valuable clustering properties for each material. Each scattering curve presented a series of multiple structural levels, which are then quantified using the Unified power-law approach to provide valuable clustering characteristics such as the degree of aggregation, polydispersity and geometry standard deviation. Subsequently, a fully screen-printed field effect transistor that uses mSi as the active material is demonstrated. The transistor had an ON/OFF current-ratio of 104; an electron mobility of 0.7 cm2/V s; a leakage current in the order of 5 × 10-9 A, and no current saturation.

The 16 O(α,α) 16 O resonance scattering was applied to study the effects of ion implantation on the oxygen distribution in the near surface region of polycrystalline titanium implanted with 180 keV krypton ions at fluences, ranging between 1 × 10 14 and 5 × 10 15 Kr + /cm 2 . Two sample sets were chosen: as-received polycrystalline titanium discs rolled and annealed in half-hard condition which had a thick oxygen layer and similar samples in which this surface layer was removed by polishing. An increase of the mean oxygen concentration observed in both unpolished and polished samples at low fluence suggests a knock-on implantation of surface oxygen atoms. At high fluence, an overall decrease in the mean oxygen concentration and mean oxygen depth suggests an out-diffusion of near-surface oxygen atoms.

Ab initio electronic structure calculations have been performed to study the formation and migration of Kr impurities, and the stability of small Kr–vacancy clusters for clusters with up to four vacancies and four Kr atoms, in hcp Ti. Both the substitutional and the interstitial configurations of Kr are found to be stable. The octahedral configuration is however found to be more stable than the tetrahedral. Interstitial Kr atoms are shown to have attractive interactions and a low migration barrier, suggesting that, at low temperature, Kr bubble formation is possible, even in the absence of vacancies. We also find vacancy clusters to be stable. The binding energies of an interstitial Kr atom and a vacancy to a Kr–vacancy cluster are obtained from the calculated formation energies of the clusters. The stability of small-vacancy clusters is found to be dependent on Kr–vacancy ratio. The trends of the calculated binding energies are discussed in terms of providing further insights on the behaviour of Kr in implanted Ti.

The stress profile in polycrystalline titanium implanted with krypton ions at different fluences has been determined using synchrotron radiation diffraction. For each fluence, the krypton profile has been measured using Rutherford backscattering geometry. The results were compared to model calculations obtained from the SRIM 2008 computer code. A strong stress relaxation was found for high fluence implantation, whereas for low fluence implantation an additional source of tensile stress was introduced in the near surface region. The projected range of the implanted krypton was significantly reduced compared to the expected range. A possible cause of this discrepancy is the drift of implanted ions under the influence of the pre-existing stress gradient.

By means of an ab initio plane-wave pseudopotential method, monovacancy, divacancy and self-interstitials in hcp titanium are investigated. The calculated monovacancy formation energy is 1.97 eV, which is in excellent agreement with other theoretical calculations, and agrees qualitatively with published experimental results. The relaxation of the atoms around a single vacancy is observed to be small. Two divacancy configurations, the in-plane and the off-plane, have also been shown to be equally stable. With regards to the interstitials, of the eight configurations studied, two (octahedral and basal octahedral) have relatively lower formation energies and are, thus, the most likely stable configurations. We find small energy differences between them, suggesting their possible co-existence. It is also observed that the tetrahedral configuration decays to a split dumbbell configuration, whereas both the basal tetrahedral and the basal pseudocrowdion interstitials decay to the basal octahedral configuration....

Ion implantation has been performed on polycrystalline titanium samples with 180 keV Kr+ ions at various doses from 1 × 1015 to 5 × 1016 ions cm− 2 at room temperature. The samples where characterised by Rutherford backscattering spectrometry, positron annihilation lifetime spectroscopy and X-ray diffraction. By means of the sin2Ψ technique the near surface stress has been determined for both unimplanted and implanted samples. The initial stress state has been shown to be strongly tensile in the first 75 nm below the surface, and weakly compressive deeper inside. The main effect of the implantation process was to relax the pre-existing tensile stress in the track region. An additional compressive stress was introduced deeper in the sample and could be attributed to the presence of larger defect clusters.

The stress states encountered in ion implanted materials are often complicated, and, so analysis, and hence presentation thereof, is incomplete due to unavailability of computational and visual tools. Stress states determined by X-ray and neutron diffraction are usually calculated from the gradient of sin 2 Ψ curves. This process, inter alia, contracts the stress to a scalar, losing the directionality and multivariate nature of the real stress state. In this paper we present a novel approach to the stress profile analysis and presentation obtained by X-ray diffraction. Stress is considered as a tensorial quantity, from the initial measurement phase to the final presentation. This discussion inherently brings in the depth resolution of stresses, which in turn are explained by means of clear visual explanations. Visual tools, which are not common use in the field, are introduced and the benefits as applied to ion implantation are discussed.

The effect of ion implantation on surface modification was focused on the residual stress aspect in the model system krypton/titanium. The krypton ions have been implanted in the polycrystalline titanium samples with different doses, 1 × 10 16 and 5 × 10 16 Kr + /cm 2 . The stress results obtained by X-ray diffraction show that krypton implantation has a beneficial effect in terms of reducing the initial stress developed during titanium manufacturing. However, it was found that the significance of stress relaxation depends on the (hkl)-reflection. Considering the concentration profiling, the RBS shows that the Kr ions penetrate deeper into the material when implanted with lower doses, 1 × 10 16 Kr + /cm 2 .

A phosphosilicate polymer spin-on glass dopant has been adapted to produce a screen printable N-type diffusion pastes using different types of nanoparticles as functional additives to quantitatively change the doping strength of the paste. Strong qualitative and quantitative differences in the resulting phosphorous concentration profiles after diffusion have been found between different compositions. Not only is an intermediate doping level obtainable if silicon nanoparticles are used instead of silica but also a shallower dopant depth is also achieved. The electrical quality of the layer formed by diffusing phosphorus into the surface of a P-type silicon wafer has been investigated by the fabrication and testing of P-N junction solar cells. The devices exhibit diodelike current–voltage (IV) characteristics with open-circuit voltages of 0.437 V and 0.523 V and short-circuit current densities of 1.88 mA/cm2 and 4.78 mA/cm2 indicating a low doping level of the cell emitter and a relatively high series resistance of the junction.

The microstructure and optical properties of a series of hydrogenated amorphous silicon layers deposited on glass substrates at different temperature have been characterized by means of X-ray diffraction techniques and optical spectroscopy. The radial distribution function of the as-deposited samples showed an increase in the bond angle and a decrease in the radial distance indicating a relaxation of the amorphous network with increasing the deposition temperature. Light induced degradation was studied using a simulated daylight spectrum. The changes in hydrogen bonding configuration, associated with the light soaking at different stages of illumination, was monitored via the transmission bands of the vibrational wag and stretch modes of the IR spectrum.

Hydrogenated amorphous silicon has been deposited on 80 g m− 2 wood-free paper, with and without an intermediate metallic interlayer, using low temperature hot wire chemical vapor deposition (HW-CVD). Electrical measurements show these layers to be of good quality. In this paper we compare the differences in microstructural properties of the two types of layer, concentrating on the influence of the substrates, including their effect on the deposition rate of the material and substrate temperature. During the deposition process, the metallized substrates reach a higher temperature than plain paper. Both X-diffraction and positron annihilation lifetime studies indicate that the growth rate on the uncoated substrate is slightly higher than with prior metallization. There is no evidence of a crystalline phase or voids in the a-Si:H layers, and the internal defect structure is similar, with a dominant dangling-bond complex of similar size.

In thin film electronic applications, the limiting factor, in terms of cost and usability, is generally the substrate material. As a consequence, different materials are being investigated as potential lightweight, inexpensive and flexible substrates. In this respect, we have been the first research collaboration to produce silicon-based electronics on paper substrates. Here we present structural characterisation of hydrogenated amorphous silicon (a-Si:H) layers deposited on 80 g m −2 wood-free paper, with and without an intermediate metallic interlayer, using low temperature hot wire chemical vapour deposition (HW-CVD). Both pulsed positron beam profiling and X-ray diffraction studies indicate that the growth rate on the uncoated substrate is slightly higher than with prior metallization. There is no evidence of a crystalline phase or voids in the a-Si:H layers. The internal defect structure is similar, with a dominant dangling bond complex of similar size, which has a slightly longer lifetime than in layers grown at higher temperatures on conventional substrates.

The production of active semiconductor thick-film components typically involves the deposition of precursor materials and subsequent thermal processing to produce a massive semiconductor layer. In this paper, we present electronic materials, based on nanoparticulate silicon, to produce the active semiconducting layer, which can simply be printed onto low-temperature substrates such as paper. Particular emphasis will be given to the structure, morphology, and composition of the nanoparticles, which are produced by either gas-phase decomposition of silane or mechanical attrition of bulk silicon. Of further importance are the electrical characteristics of the composite materials, in which the active semiconductor is formed from an interconnecting backbone of silicon particles. These will be discussed for example structures, including junction field effect transistors (FETs), insulated-gate FETs, and photodiodes.

Nanocrystalline materials are quite promising in the fabrication of photovoltaic devices because of their advantages over conventional polycrystalline and amorphous materials. Also, nanocrystalline materials have great potential for technological applications. This paper discusses how nanocrystalline semiconductors were characterized structurally using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Further investigations of the deposited layers included the elemental analysis using Energy-Dispersive X-Ray (EDXR). The results obtained revealed that the nc-Si is polycrystalline in nature with the grain size of ≤100 nm. The elemental analysis reveal that the indium tin oxide consists of Sn 35 wt%, Si 15.54 wt%, In 11.84 wt% and Al 11.74 wt%. The XRD results are found to be consistent with EDXR results. The photovoltaic application of the deposited layer is discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we study the effect of blending the conducting polymers, P3HT with the fullerene complex PCBM on its structural and defect characteristics. The films were deposited on glass substrates by spin casting, and were characterized with positron annihilation, microscopy and other techniques with regard to thickness, and structural homogeneity. The unblended polymers have positron annihilation characteristics similar to most non-polar polymers, exhibiting a relatively broad electron momentum distribution, a long-lived (>1 ns) positron state corresponding to the formation of orthopositronium, and strong-localization of the positron. Blending with PCBM causes the sample electron momentum distribution to narrow, and results in a single state with a lifetime of around 370 ps in both polymer mixtures. We postulate that this state corresponds to annihilation with low-momentum electrons in the fullerene cage.

The effect of light-soaking, using a simulated daylight spectrum (colour temperature 5800 K) on the crystallinity, defect structure, and total hydrogen concentration of a-Si:H grown by HW-CVD is being investigated in an ongoing project. In this article, positron beam based electron momentum spectroscopy is applied to monitor the evolution of the open-volume defect structure after each illumination stage. The results indicate an initial increase in free volume at dangling-bond complexes on illumination, with no indication of a change in defect concentration or generation of larger microvoids. On further illumination, there is a reduction in the low electron momentum fraction, which is not accompanied by a similar change in the free-volume. This can be interpreted as a reconfiguration of the hydrogen in the dangling-bond complex, followed by its release into a mobile state.

A study of the residual stress and bubble pressure in bulk samples of copper, containing 3 at.-% krypton, using X-ray diffraction techniques is presented here. The authors have confirmed that the Kr forms solid precipitates with an fcc structure, which is consistent with an estimated pressure of 2.4 GPa. Stress measurements in the surrounding Cu matrix indicate a zero normal stress, confirming that the matrix experiences only a shear strain. The magnitude of the shear stress is estimated from the bubble pressure to be below the yield stress of the matrix, thus explaining the long term stability of the bubbles.

Stress migration of point and open-volume defects is an important problem in a wide variety of applications, ranging from semiconductor technology and basic metallurgical problems to ion implantation and surface modification. Often the driving force of the drift is the local stress field of other defects in the material, although other sources of residual stress as well as external loads play an important role. This paper concentrates on the behaviour of non-equilibrium vacancies in a non-uniformly strained material. Illustrative models of one-dimensional problems are developed and compared to available experimental results in different ion implanted materials.

We have used slow positron beam-based positron lifetime spectroscopy to study positron diffusion in thick epitaxial n-type 6H-SiC layers. The layers are considerably thicker than the maximum positron penetration depth, and can therefore, be treated as homogeneous semi-infinite bulk material in an analysis including the time-dependent diffusion of a single group of probe particles. Temperature-dependent measurements show a reduction in positron diffusion at low temperatures, which has been interpreted by an increase in trapping to negatively charged defect states.

Low-hydrogen-concentration a-Si:H grown by HW-CVD forms a continuous random network with no detectable free volume in the form of microvoids, and no evidence of a microcrystalline phase. Over a wide temperature range we observe a single positron state in the amorphous network with a temperature-independent lifetime of 322 ps. From the temperature dependence of the positron diffusion we show that this is a localized state and present direct observation of hopping diffusion of positrons. On annealing up to 400 °C the amorphous network is seen to relax and the first stages of crystallization occur. There is also evidence of vacancy clustering to form a low concentration of microvoids. The structural relaxation has a very low-activation energy, around 0.1 eV, and is interpreted in terms of a reconfiguration of the fundamental defect identifed by positron annihilation.

The microstructure of hydrogenated amorphous silicon grown by hot-wire chemical vapour deposition (HW-CVD) on glass substrates, at different substrate temperatures ranging from 300 to 500 °C, has been studied using X-ray diffraction and positron annihilation techniques. In previous studies it has been shown that recrystallization is accompanied by a relaxation of the defect structure with an increase in the free volume at the positron annihilation site. The object of this work is to relate the initial defect configuration to the degree of order in the structure, which has been characterized through its radial density function giving accurate estimates of the nearest-neighbour separation and bond angles.

We present the results of a combined study of the defect structure and residual stress in a diamond layer, grown by PECVD on a polycrystalline copper substrate with a titanium interlayer. For the defect studies, electron spectroscopies based on electron-positron annihilation were applied. X-ray diffraction techniques were used for both the stress determination in the diamond layer and for a phase analysis of the complete composite structure. The layer was found to contain a significant fraction of vacancy clusters and single vacancy type defects, which are probably situated within the individual grains. The presence of the larger defects may be related to a compressive stress in the layer.

Hydrogenated amorphous silicon (a-Si:H) grown by hot wire chemical vapour deposition is a promising candidate for robust inexpensive solar cells. However, prolonged exposure to light is known to lead to a reduction in efficiency of a-Si:H devices. The causes for this ageing effect are still unclear, but may be related to a structural relaxation or change in hydrogen content. In this work, results are presented for positron beam studies of the defect structure, using both lifetime and Doppler-broadening spectroscopy, of a-Si:H grown under different conditions.

This paper applies a recent semi-analytical treatment of the slowing-down and penetration of positrons in condensed matter to the slowing down of slow positrons in aluminium. The computations are based on the age equation (formally identical with the equation of heat conduction), of which a simplified derivation is given. The dominant energy loss mechanisms, scattering off conduction electrons and long wavelength acoustic phonons, have been treated using the Lindhard dielectric function and the deformation potential approximation, respectively.

Quantitative treatment of the penetration of positrons into a sample and of their slowing-down is often the key to the reliable interpretation of positron keV-beam experiments. The close relationship to problems arising in neutron physics and in X-ray and γ-ray penetration is pointed out, and the correspondence between the physical quantities appearing in the different fields is established. Mathematical techniques for solving or simplifying Boltzmann's transport equation originally developed for neutrons are shown to be applicable to positrons as well. The present paper derives in detail the positron version of Fermi's age theory, which, in its simplest form, enables us to reduce the solution of the steady-state Boltzmann equation to that of the equation of heat conduction. In age theory, the time-like variable is a mechanism-dependent quantity that is proportional to the mean square displacement of the positrons during the slowing-down from their original momentum to a given momentum. Time-dependent problems are handled by the Laplace-transformation technique. This approach and its generalisations permit analytical or semi-analytical solutions to problems that had hitherto been thought to be accessible by simulation techniques only. Additional mathematical techniques allowing us to tackle problems beyond the applicability of age theory are outlined.

Amorphous diamond-like carbon coatings, produced by ion beam assisted physical vapour deposition (IBAD) and plasma enhanced chemical vapour deposition (PECVD) on Si(100) substrates have been studied by positron beam and other techniques. Both layers have similar positron annihilation characteristics and a density of 2.4±0.3 g cm−3, and are probably significantly hydrogenated. In the IBAD sample there is significant damage to the substrate, including a deep armorphised region and evidence of defect states at the interface to the carbon.

Platinum films, used in thin film technology, produced by radio-frequency sputter deposition on aluminium oxide substrates under different conditions, have been studied by positron beam and other techniques, before and after production annealing. The defect structure in the layers has been characterised using both positron lifetime and Doppler-broadening spectroscopy, and compared with X-ray studies of crystallinity and texture.

A composite structure consisting of a thick chemical vapour deposition (CVD) polycrystalline diamond layer grown on an aluminium substrate, covered with a thin gold cap layer has been studied using positron beam and other techniques. The diamond layer has a strong 〈110〉 texture and also contains small fractions of aluminium carbide and graphitic phases. Positron beam analysis reveals two states, corresponding to a highly defective thin gold layer and an homogeneous diamond layer with large, essentially defect free, crystallites.

The viability of producing silicon nanoparticles using the HWCVD process is investigated. A system is assembled and particles are produced from silane at pressures between 0.2 and 48 mbar, with hydrogen dilutions of 0–80%, at a total flow rate of 50 sccm and with a tungsten filament maintained at 1650 °C. The as-prepared powder varies in colour from yellowish to dark brown and is deposited on all surfaces inside the reaction chamber. The material is a highly porous agglomeration of nanoparticles of primary size in the order of 40 nm, with a narrow size distribution. The nanoparticles produced are mostly amorphous, hydrogenated and have a partially oxidised surface.

The material properties of hydrogenated amorphous silicon (a-Si:H) have been known to change when exposed to elevated temperatures. In this work we report on the thermal stability of the structural disorder, hydrogen content and defect structure of hot-wire deposited a-Si:H when exposed to temperatures in excess of 100 °C. Prior to thermal hydrogen diffusion a change in the defect structure is observed, caused by the creation of low concentrations of microvoids. There is evidence of vacancy clustering at 400 °C, caused by the alignment of unterminated Si dangling-bonds that consequently results in an increase in the defect size, concentration or both. Raman scattering shows evidence that no crystallization is induced at 400 °C and that the structural disorder increases upon annealing.

The first stage of a state of the art positron beam, being constructed at the University of Cape Town, is currently being brought into operation. This is the first positron beam on the African continent, as well as being the first positron beam dedicated to solid and surface studies in the southern hemisphere. The project also contains a high proportion of local development, including the encapsulated 22 Na positron source developed by our collaboration. Novel features in the design include a purely magnetic in-line deflector, working in the solenoidal guiding field, to eliminate unmoderated positrons and block the direct line of sight to the source. A combined magnetic projector and single pole probe forming lens is being implemented in the second phase of construction to achieve a spot size of ∼ 10 μm without remoderation.

A new small-angle scattering technique in reflection geometry is described which enables a topological study of rough surfaces. This is achieved by using long-wavelength soft X-rays which are scattered at wide angles but in the low-Qrange normally associated with small-angle scattering. The use of nanometre-wavelength radiation restricts the penetration to a thin surface layer which follows the topology of the surface, while moving the scattered beam to wider angles preventing shadowing by the surface features. The technique is, however, only applicable to rough surfaces for which there is no specular reflection, so that only the scattered beam was detected by the detector. As an example, a study of the surfaces of rough layers of silicon produced by the deposition of nanoparticles by blade-coating is presented. The surfaces of the blade-coated layers have rough features of the order of several micrometers. Using 2 nm and 13 nm X-rays scattered at angular ranges of 5° ≤ θ ≤ 51° and 5° ≤ θ ≤ 45°, respectively, a combined range of scattering vector of 0.00842 Å−1≤Q≤ 0.4883 Å−1was obtained. Comparison with previous transmission SAXS and USAXS studies of the same materials indicates that the new method does probe the surface topology rather than the internal microstructure.

We present the K-edge X-ray absorption near edge spectra of hexagonal-closed packed iron at pressure and temperature conditions relevant to Earth's mantle conditions. The calculated spectra have been obtained using the first-principles scheme based on the continued-fraction approach and norm-conserving pseudopotentials. The atomic configurations used for the X-ray absorption near edge spectroscopy calculations were obtained from classical molecular dynamics simulations, using an optimized embedded-atom potential. We compare our calculated spectra to recently available experiment results (R. Boehler, H.G. Musshoff, R. Ditz, G. Aquilanti, and A. Trapananti, Rev. Sci. Instrum. 80 (2009), pp. 045103–045108) and identify the main features of the spectra that may indicate onset of melting in iron.

An optical column for a scanning positron microprobe is described. A beam of variable energy from 1 to 30 keV and a spot diameter of 1 mu m, that can be scanned over an area of 0.6*0.6 mm2 is formed using electron optical elements that are already well known but rarely used in an optical column. The idea allows the construction of a sample chamber free of material and electric fields in a hemisphere above the sample with a radius of some 100 mm or more. The absence of diaphragms and the comfortable inner diameter of the lenses used makes it possible to work with a beam of comparatively large diameter and aperture. This is necessary because of the low intensity of the positron sources available, which makes a system with a very high transmission essential. Although the idea was created for a positron microprobe, other applications, such as with a retarding field analyser introduced into the sample chamber, are conceivable.

A pulsed low-energy positron system for positron-lifetime spectroscopy has been up-graded and modified with regard to its three main components. A new moderator preparation chamber has been added, the radiofrequency pulsing concept has been modified using a pre-buncher and a pre-chopper in front of the existing chopper-buncher section. Furthermore, a new target station allowing measurements at variable temperatures has been incorporated. The lifetime spectra now reveal a strongly increased ratio of peak to background and of peak to satellite peak.

The two complementary techniques of positron lifetime spectroscopy and x-ray diffraction have been applied to the structural characterization of commercial titanium nitride hard coatings grown by hollow cathode arc evaporation. Use of a pulsed positron beam allows the layers to be analysed more or less independently of the substrate. The layers are found to consist of a single delta -phase TiN, with pronounced texture and residual stress. The dominant defects appear to be grain boundaries with a positron lifetime of 225 ps. Values for the bulk positron lifetime and diffusion coefficient are also obtained.

We report on the migration of monovacancies, divacancies, and substitutional krypton atoms in hcp titanium using the nudged elastic band method, in the framework of ab initio density functional theory. The divacancy migration energy barrier is found to be lower than that of a monovacancy. The migration of substitutional krypton in titanium is governed by the vacancy mechanism, if there is an excess of nonequilibrium vacancies after the implantation. However, if thermal vacancies dominate, krypton atom migration is expected to proceed via the dissociation mechanism. We also present ab initio calculations on the formation of clusters of multiple substitutional impurity krypton atoms and the interactions between krypton impurities and vacancies in titanium. We have found that, analogous to multiple interstitial krypton clusters, clusters of substitutional krypton atoms are energetically unstable, but are stabilized by vacancies. However, the efficiency of stabilization by vacancies depends strongly on the spatial distribution of the vacancies within the clusters. This study indicates the possibility of inert gases to nucleate in voids created by vacancies in ion implantation processes.