Results of the comprehensive study of deuterium-implanted hexagonal SiC (4H and 6H) using optical absorption and infrared measurements, elastic recoil detection analysis, thermal desorption and positron annihilation spectroscopies are reported. It is shown that implanted deuterium mainly forms bonds with lattice atoms. The amount of deuterium in the form of interstitial molecules and in vacancies is considerably smaller. Ion implantations with fluences exceeding 10(15) D+/ cm(-2) create point defects in concentrations sufficiently high for complete positron trapping. Recrystallisation of the amorphised SiC does not remove the positron traps.

This paper describes a study of the effect of an external electric field on the behavior of positrons in metal-oxide-silicon (MOS) systems. Doppler broadening measurements of the annihilation radiation were performed on capacitors with identical thermally grown SiO2 layers and with Al, W and Au layers as a gate. The data were analyzed by the combined use of the shape- and wing-parameters of the photo peak. The observed effects of the electric field are due to the field-driven transport of positrons through the SiO2, silicon and the interfaces. By applying a field of the order of 1 MV/cm the positrons can be efficiently transported through the approximately 100 nm thick SiO2 layer. From the transport behavior of the positrons it is concluded that the positron affinity is higher for SiO2 than for silicon and for the gate metal. By properly choosing the direction of the field, the positrons implanted into the SiO2 layer are collected either at the Si/SiO2 interface or at the SiO2/gate interface. For negative gate bias the positrons implanted into the substrate, that diffuse back to the SiO2, are transported through the oxide layer and injected into the gate metal. This is the first time that field-assisted transport of positrons across an insulating layer has been demonstrated.

OsRu and other alloys are used as a top-layer on impregnated W thermionic cathodes to obtain a high emission performance. We apply a cathode with a sputtered OsRu layer in cathode ray tubes (crt). Due to the sputtering process Ar is incorporated in this layer. Part of this Ar desorbs into the crt on first heating. The Ar desorption mechanism has been studied by thermal desorption spectrometry and the metal layer structure has been studied with a variable energy positron beam. It has been found that no Ar bubbles are formed during deposition. Ar is partially released on heating by thermal-vacancy assisted diffusion at ca. 1300 K from the W pellet, and at ca. 1100 K from the OsRu layer. At the same time Ar bubbles are formed in the OsRu layer that desorb at ca. 1800 K.

Doppler-broadening measurements can be improved by using a second Ge-detector for the coincidence detection of the second annihilation photon. The coincidence condition in combination with an energy relation results in a reduction of the background by a factor of 100 compared to a single detector system. This background reduction opens up the possibility of performing accurate measurements of the high momentum part of Doppler-broadened annihilation spectra. We have used this technique for the analysis of a metal oxide semiconductor system and electron-irradiated. As- and Sb-doped silicon.

Bulk copper and nickel samples containing 2.5 and 5 at.% krypton present in overpressurized bubbles have been studied by EXAFS spectroscopy. A fit to the experimental data yields krypton nearest-neighbour distances of 3.62(4) and 3.69(4) Å in a copper and a nickel matrix respectively. These values confirm that krypton bubbles are overpressurized. Calculated krypton packing densities are in good agreement with results from other techniques. In addition, average krypton - metal nearest-neighbour distances of 3.0(1) Å were determined. Atomistic calculations show a wide variation of krypton - metal distances, in agreement with the experiment.

In this paper we discuss the potential of positrons for the study of defects in the MOS system. It is shown that Doppler broadening measurements of the positron annihilation radiation reveals properties not detected by conventional techniques such as electron spin resonance measurements. Guiding the positrons towards the Si/SiO2 interface constitutes an advantageous approach for studying the properties of this region. Examples of the correlation between technologically affected electrically active defect centers and positron annihilation data are discussed. We also briefly deal with the extension of the technique to provide more detailed information using lifetime, correlation and two-dimensional angular correlation annihilation radiation measurements.

This paper describes the testing of a positron beam which is primarily based on copper activation near the core of a nuclear reactor and extraction of the positrons through a beam guide tube. An out-of-core test with a 22Na source and an in-core test with the reactor at reduced power have been performed. Both tests indicated a high reflectivity of moderated positrons at the tungsten surfaces of the moderation discs which enhanced the expected yield. Secondary electrons generated in the source materials during the in-core test caused electrical field distortions in the electrode system of the system by charging of the insulators. At 100 kW reactor power during one hour, positrons were observed with an intensity of 4.4 × 104 e+ s−1 of which 90% was due to positrons created by pair formation and 10% by copper activation.

Gas desorption spectrometry has been used to investigate the composition of the gas and the quantities that are present in blowholes in interstitial free cold rolled steel. Besides monitoring of the gas release during ramp heating of selected parts containing blowholes, measurements have also been performed for in vacuo cutting of the blowholes at room temperature. The main component (> 99%) of the gas is argon. Gas pressure in cavities with a volume of similar to 0.1 mm(3) is a few bar. Some methane gas is also present, but the amount of hydrogen gas is negligible. (C) 1997 The Institute of Materials.

We report the use of a layered foil as a positron rectifying device for transmission geometry positron moderation and remoderation, thereby increasing the efficiency of such devices. Positron transmission through foils consisting of a 10 nm W layer on top of a 100 nm Mo foil has been measured. The measurements are compared with similar results for a 100 nm W foil. A strong rectifying effect with a forward to reverse current ratio of more than 10 is observed making this type of well matched layered foils an obvious candidate for positron remoderation. Retarding energy spectra of the transmitted positrons were obtained and measurements of Doppler broadening and surface branching ratios were performed to further characterize the foils. The obtained transmission results again reveal the discrepancy in positron implantation profiles for thin foils between experiments and existing theory.

The thermal annealing of 1.5 μm thick SiO2 layers deposited on Si with a High Density Plasma (HDP) has been monitored by Thermal Desorption Spectrometry (TDS), Fourier Transform Infra-Red absorption (FTIR) and Positron Annihilation Doppler Broadening (PADB). Two samples were prepared with different production conditions (substrate temperature, Ar flow, ratio) in order to get oxide layers rich in SiH bonds or rich in SiOH bonds. The deposition temperatures of these layers were 545 K and 575 K, respectively. The formation of SiH rich and SiOH rich oxides was confirmed by FTIR absorption measurements. Desorption measurements showed that at a heating rate of 5 K/s release of hydrogen takes place around 1100 K. For the layer deposited at 575 K additional release of hydrogen is observed at 900 K. In both samples argon release is seen around 1200 K. The release of hydrogen at 1100 K is consistent with FTIR measurements at annealed samples. The absorption peaks associated with the SiH and SiOH bonds only disappear after heating above 1000 K. For the PADB measurements thermal anneals were carried out from 300 to 1300 K in steps of 50 and 100 K of 16 min duration. The positron measurements show different annealing behavior of the two oxides. It is observed that up to 1200 K the 545 K oxide shows a significant higher value for the defect parameter. The argon and hydrogen release is observed by an increase of the defect parameter at 1000 K.

This paper investigates the trapping of helium in pure vanadium and V5Ti, V3Ti1Si alloys using thermal helium desorption spectroscopy (THDS). The implantation of helium has been carried out with energies varying from 50 eV subthreshold implantation up to 3 keV with irradiation temperatures between 300 and 1000 K. On a separate set of samples of the same alloys, tensile measurements were performed at 773, 873, and 973 K. The samples used for the tensile measurements were pre-irradiated with neutrons up to 6.4 dpa. Helium was injected by cyclotron irradiation. The differences observed in the desorption spectra for pure vanadium and the alloys after different irradiation conditions are discussed with respect to their mechanical properties. The role of interstitially dissolved impurities in helium-vacancy clustering and helium trapping was investigated both experimentally and by Monte-Carlo simulations of clustering processes. By matching the calculation results to the THDS results the interstitially dissolved fraction of impurities is estimated.

Thermal helium desorption spectrometry (THDS) is applied to the study of helium trapping and clustering in vanadium and a number of selected vanadium alloys. A comparative analysis of the desorption spectra reveals the presence of at least two different trap sites responsible for the helium desorption observed in the investigated materials. These are (1) helium-vacancy clusters nucleated at interstitial impurities (C, N, O) and developed during the irradiation, and (2) pre-existing traps, e.g. precipitates and undersized alloying elements. The contribution of these two types of traps in pure vanadium and the alloys appeared to vary in magnitude depending on the material. In addition to the experiments, a Monte Carlo computer program has been developed for the simulation of both the irradiation and the annealing of the materials. The results obtained for nucleation, growth and dissociation of helium-vacancy clusters are compared to the experimental results. The model takes into account interstitially dissolved impurities, i.e. C, N and O interacting with helium-vacancy clusters.

Positron beam experiments have been performed to establish the diffusion behavior and surface branching of positrons implanted with energies varying from 0–25 keV into an epitaxially grown layer of β-SiC on a silicon substrate. The diffusion length of the positrons amounted to 42 nm. The surface branching was as follows: 30% positron emission, 30% positronium formation and 40% trapping at the surface. The energy distribution of re-emitted positrons consisted of a narrow contribution centered around 0.5 eV with a wider contribution up to 3 eV. The use of this wide band gap material for field assisted positron moderation is discussed.

Krypton atoms incorporated in sputtered a-silicon films are investigated by means of Mössbauer spectroscopy. The hyperfine parameters of the source were determined by taking a spectrum against solid krypton. Mössbauer spectra were taken for films containing krypton concentrations up to 7 at.%. A Debye temperature of 116(4) K has been measured for a sample containing 2.83 at.% Kr. The isomer shifts found for all spectra and the high Debye temperature indicate that krypton resides in small highly pressurized precipitates.

Although many aspects of helium behaviour in metals relevant to fusion reactor technology have been studied, relatively little information is available on the kinetics of high temperature release of helium precipitated into bubbles. Recently, based on the many observations showing that bubble coarsening during annealing is induced initially in regions nearest thermal vacancy sources (e.g. grain boundaries or surfaces), a qualitative model has been suggested in which the vacancy gradient between the vacancy source and initially overpressurised bubble concentrations must lead to directed bubble migration up the gradient towards the source. It is clear that this gives a potential for gas release, either directly for surfaces, or indirectly for grain boundaries. The present paper extends work on this mechanism by discussing quantitative aspects of bubble movement in vacancy gradients and using computer simulations to provide information on the parameters of importance in the gas release mechanism. Among these are the helium content and the associated local swelling, while release kinetics and temperature are primarily controlled by self-diffusion parameters.

An improved approach is presented for the analysis of positron beam Doppler broadening data. Instead of analyzing the energy-dependent shape parameter, the so-called S(E) data, we combined the shape S(E) and wing W(E) data by plotting them as a trajectory in the S–W plane, using the implantation energy as a running parameter. It is shown that this plot is of particular interest for the qualitative interpretation of the data. Furthermore, it allows the independent determination of the characteristic shape and wing parameters of the different positron trapping layers without the use of a numerical simulation and fitting program. The method and its advantages and limitations are illustrated for three cases: a silicon sample implanted with helium, a metal–oxide–silicon system subjected to a bias voltage and a bare oxide layer on silicon.

The modelling and fitting program VEPFIT has been employed in recent years for resolving defect depth profiles and depth structures of deposited layers. Recent activities concerning program development include the testing of a new model of MOS systems for implementation into VEPFIT and a study into decomposition of Doppler-broadened photo-peaks. Further methods are proposed using VEPFIT for analysis of lifetime measurements and for modelling of positron transport with multi-energy groups.

The effect of a post oxidation anneal at 1000 degrees C in a N-2 ambient of the thermally grown Si/SiO2 system was investigated using vacuum ultraviolet irradiation for determining the generation of interface traps of the Al metallized system in combination with positron annihilation spectroscopy to characterize the structure of the oxide network. A correlation was found between the generation of interface traps and the S parameter of the positron trapping sites in the oxide close to the Si. It appears likely that the positrons are trapped in the larger near-interfacial oxide network interstices. These interstices could act as scavengers for the metastable intermediate (atomic hydrogen or excitons) involved in the generation of the interface traps.

Positron beam and helium desorption techniques have been applied to different materials, in particular semiconductor materials, to determine the presence of defects. The positron technique yields values of the positron diffusion length and values of the Doppler broadening parameters. In principle, defect concentrations can be derived and an indication can be obtained about the nature of the defect. Results are presented which show that cavities can be easily detected. It is also demonstrated that gas accumulated in the cavities reduces the observed differences between the defected and the defect-free material. Large cavities were detected in solar cell hydrogenated amorphous silicon and low temperature deposited amorphous silicon. It was found that layers deposited under irradiation with low energy ions (ion assisted deposition) did not show evidence of microcavities. Desorption techniques were successfully employed to detect cavities in silicon

Currently the trend in the Si IC industry is to produce epitaxial material layers by advanced growth and deposition techniques. Examples of these are Si and SiGe low temperature epitaxy, Si selective epitaxy and metallic silicide epitaxy. In order to obtain good electrical properties it is important that the epitaxial material shows no extended lattice defects and has a minimal concentration of lattice point defects. Given the concentrations of these phenomena a sensitive experimental technique is required to characterise the materials. The application of the positron annihilation technique in this IC research area is demonstrated by two examples, namely, characterization of Si Molecular Beam Epitaxy (MBE) and Atmospheric Pressure Chemical Vapour Deposition (APCVD) epi-layers and the assessment of the quality of CoSi2 epi-layers produced by the solid-state reaction with the Si substrate and an amorphous Co75W25 sputtered layer. Results will be presented in terms of defect concentrations derived from positron diffusion lengths.