Defects in Si-Rich SiO2 Films Prepared by Radio-Frequency Magnetron Co-sputtering Using Variable Energy Positron Annihilation Spectroscopy

Si-rich SiO2 films prepared by rf magnetron co-sputtering method are studied by slow positron beams. The nega- tively charge point defects （probably Pb centres or peroxy radicals） at the silicon nanocluster （nc-Si）/SiO2 interface are observed by Doppler broadening spectra. Coincidence Doppler-broadening spectra show that positrons have a higher annihilation probability with core electrons nearby oxygen atoms than silicon atoms. The formation of N-related bonds may be the reason for the prevention of the migration reaction of Si and 0 atoms, hence nc-Si formation is inhibited by annealing in nitrogen compared to in vacuum.     

Annealing Behaviour of Helium Bubbles in Titanium Films by Thermal Desorption Spectroscopy and Positron Beam Analysis

Helium-containing Ti Glms are prepared using magnetron sputtering in the helium-argon atmosphere. Isochronal annealing at different temperatures for an hour is employed to reveal the behaviour of helium bubble growth. Ion beam analysis is used to measure the retained helium content. Helium can release largely when annealing above 970K. A thermal helium desorption spectroscopy system is constructed for assessment of the evolution of helium bubbles in the annealed samples by linear heating （OAK/s） from room temperature to 1500K. Also, Doppler broadening measurements of positron annihilation radiation spectrum are performed by using changeable energy positron beam. Bubble coarsening evolves gradually below 680K, migration and coalescence of small bubbles dominates in the range of 68-970 K, and the Ostwald ripening mechanism enlarges the bubbles with a massive release above 970K.     

Study of the surface contamination of copper with the improved positron annihilation-induced Auger electron spectrometer at NEPOMUC

The high intensity positron source NEPOMUC at the FRM-II in Munich enables measurement times for positron annihilation-induced Auger electron spectroscopy (PAES) of only 2.4 h/spectrum, in contrast to usual lab beams with measurement times up to several days. The high electron background due to surrounding experiments in the experimental hall of the FRM-II has been eliminated and hence background free experiments have become possible. Due to this, the signal to noise ratio has been enhanced to 4.5:1, compared to 1:3 with EAES. In addition, a long-term measurement has been performed in order to observe the contamination of a polycrystalline copper foil at 150 °C.     

Study of the ionic transport in polymer electrolyte using positron lifetime distribution method

Positron annihilation spectroscopy has been applied to measure the free-volume hole distribution in poly(ether urethane) as a function of temperature. The hole radius distribution determined from orthopositronium lifetime distribution is found to shift to a larger values with increasing temperature. This result, combined with the variation of ionic conductivity, suggests that carrier ions do not migrate naked but are bound to polymer segments through ion-dipole interaction forces, and the ion migration is controlled primarily by segmental motion of the polymer.     

Influence of Dopants in ZnO Films on Defects

The influence of dopants in ZnO films on defects is investigated by slow positron annihilation technique. The results show S that parameters meet S_Al>S_un>S_Ag for Al-doped ZnO films, undoped and Ag-doped ZnO films. Zinc vacancies are found in all ZnO films with different dopants. According to S parameter and the same defect type, it can be induced that the zinc vacancy concentration is the highest in the Al-doped ZnO film, and it is the least in the Ag-doped ZnO film. When Al atoms are doped in the ZnO films grown on silicon substrates, Zn vacancies increase as compared to the undoped and Ag-doped ZnO films. The dopant concentration could determine the position of Fermi level in materials, while defect formation energy of zinc vacancy strongly depends on the position of Fermi level, so its concentration varies with dopant element and dopant concentration.     

Effect of pseudogap on the positronium in void

In ceramic high-Tc superconductor (Bi, Pb)₂Sr₂Ca₂Cu₃O₇ the anomalies in temperature dependence τ₂(T)-lifetime of positrons trapped by microvoids are observed in Tc-region [V.L. Sedov, S.N. Kuznetsov, O.A. Tsigelnik, N.N. Oleynikov, D.I. Grigorashev, Phys. Lett. A 222 (1996) 455; D. Sanyal, D. Banerjee, U. De, Phys. Rev. B 58 (1998) 15226]. This effect is considered on the basis of the model in which the positrons trapped in microvoids form positronium atoms, interacting with surrounding medium by the processes Ps↔e⁺+e⁻ [V.L. Sedov, O.A. Tsigelnik, Phys. Lett. A 332 (2004) 423]. It is shown that the local density of states of the interacting positronium forms the δ-peak near the lower boundary of the gap 2Δ at ω>−Δ. The conclusion was made that the experimentally observed discontinuous increase τ₂ at the transition of the superconducting phase into pseudogap state is the result of the discontinuous increased weight of the peak at this transition.     

Investigations of epoxy-based adhesives with PLEPS

Contamination-tolerant adhesives are of ever increasing importance in industrial applications. The possible failure mechanisms of adhesive bonds in these adhesives are however still poorly understood. Results of a series of investigations with our pulsed low energy positron beam system (PLEPS) in an epoxy-based contamination-tolerant adhesive are presented.     

Studies on the microstructure of phase transitions in two kinds of liquid crystals by PAT and DSC

The positron lifetime and DSC measerments for EBBA and DOBAMBC have been made with heating and cooling clycles. The experimental results show that a shorter lifetime (₁) is essentially independent of temperature while the longer lifetime (₂) and the intensity (I ₂) change abruptly double or triple with temperature. Consequently, the EBBA has only nematic phases while the DOBAMBC has two liquid-crystalline phases (smectic C* and smectic A) with transition temperatures as follows: for EBBA, solid nematic (304.5 K), nematic isotropic (356.5 K), isotropic nematic (356.5 K), nematic solid (301 K); and for DOBAMBC, solid smectic C* (346 K), smectic C* smectic A (357.5 K), smectic A isotropic (389 K). These critical temperatures are in accordance with the transition temperatures measured by DSC. In addition, the difference in the solid-nematic transition temperature in the heating and cooling cycles was also observed. A discussion about the correlation of the observed changes in lifetime (₂) with the changes in molecular orientational order (S) and dielectric anisotropy () is presented.     

Anisotropy of the electron momentum distribution in Bi2Sr2CaCu2O8+δ superconductor studied by positron annihilation

The temperature dependent (30-300 K) Doppler broadening of the positron annihilated γ-radiation measurement has been investigated on single crystalline Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) high T_c superconducting samples along two different crystallographic orientations. It has been observed that throughout the temperature range the electron momentum distribution has a larger value along the crystallographic c-axis than in the a-b plane. The temperature dependent Doppler broadened positron annihilation γ-radiation lineshape analysis shows a step like increase of S-parameter at the temperature region 92-116 K.     

Evolution of Structural Defects in SiOx Films Fabricated by Electron Cyclotron Resonance Plasma Chemical Vapour Deposition upon Annealing Treatment

We study the structural defects in the SiO, film prepared by electron cyclotron resonance plasma chemical vapour deposition and annealing recovery evolution. The photoluminescence property is observed in the as-deposited and annealed samples. [-SiO3]^2- defects are the luminescence centres of the ultraviolet photoluminescence （PL） from the Fourier transform infrared spectroscopy and PL measurements. [-SiO3]^2- is observed by positron annihilation spectroscopy, and this defect can make the S parameters increase. After 1000℃ annealing, [-SiO3]^2- defects still exist in the films.     

On the sensitivity limit of positron annihilation: detection of vacancies in as-grown silicon

14 cm^-3 in FZ-Si was obtained. Received: 2 June 1998 / Accepted: 20 November 1998 / Published online: 24 February 1999     

Slow positron annihilation studies of vacancy-type defects in the near-surface region of Cu and Nb before and after wear

Received: 22 June 1998 / Accepted: 9 November 1998     

Study of the effect of annealing on defects in Fe-Mn-Si-Cr-Ni-C alloy by slow positron beam

FeMnSi shape memory alloys (SMAs) have received much attention as one-way SMAs due to their cost-effectiveness. Variable-energy (0-30 keV) positron beam studies have been carried out on a Fe-Mn-Si-Cr-Ni-C alloy with different degrees of deformation. Doppler broadening profiles of the positron annihilation as a function of incident positron energy were shown to be quite sensitive to defects introduced by deformation. The variation of the nature and the concentration of defects are studied as a function of isochronal annealing temperature. These results are correlated with the data measured with the positron annihilation lifetime spectroscopy (PALS). The positron annihilation results are compared to XRD and optical microscopy (OM).     

Formation and escaping of positronium in porous SiO2 films at low temperature

Positronium formation and escaping has been studied in porous silica thin films at temperature ranging from 13 to 300 K by 2-3 gamma ratio of positronium (3γ-PAS) measurements. Nanoporous silica thin films were deposited by spin coating on p-type (1 0 0) Si substrates and thermal treated in air at temperatures of 600 °C. Two different molar ratios of porogen (polyvinylpyrrolidone) were used in the TEOS-ethanol mixture to obtain samples with close porosity and connected porosity with the surfaces. In both types of sample a reduction of the 2-3 gamma ratio of positronium was observed by decreasing the temperature. This finding, in disagreement with the theoretical expectation, is discussed on the basis of the possible quenching mechanisms.     

Positron annihilation in solid,liquid and undercooled melts of Co80Pd20

Positron annihilation measurements have been performed in solid, liquid and undercooled Co₈₀Pd₂₀ alloy using electromagnetic levitation as containerless processing method. The formation enthalpy for a single vacancy is H_1v = (1.7 ± 0.1) eV. In the melt, the thermal expansion continues linear in the undercooled phase and is larger than that of the solid alloy. The mean free volume in the liquid phase is slightly larger than the volume of a single vacancy. At the Curie temperatures of both solid and liquid phase, the S-parameter indicates no effect on the atomic structure. Different H₂ concentrations in the processing gas atmosphere have no detectable influence on the data.     

Defects-driven appearance of half-metallic ferrimagnetism in Co-Mn-based Heusler alloys

Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula Co₂MnZ where Z is a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co_1−xMn_x]₂MnSi is of particular interest since Mn₃Si is known to crystallize in the Heusler L2₁ lattice structure of Co₂MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts.     

A study of defects in deformed FeSi alloys using positron annihilation techniques

Steels with high amounts of silicon are used in electrical applications due to their low magnetostriction, high electrical resistivity and reduced energy losses, but they exhibit poor formability. The slow positron beam of Gent is used to investigate defects in different deformed FeSi alloys. It was found that the concentration of defects for the alloys deformed at high temperatures are different from the ones related to the alloys deformed at room temperature. These results are correlated to the results of positron annihilation lifetime spectroscopy (PALS).     

New perovskite related-compounds of Ru(V) in K2NiF4 type structure

The preparation of the new K₂NiF₄-type compounds, Sr₂Co_0.5Ru_0.5O₄, Sr₂Ni_0.5Ru_0.5O₄ and Sr₂Ga_0.5Ru_0.5O₄ by direct solid state synthesis and the subsequent characterization by powder X-ray diffraction measurements are described. From Rietveld refinements, we found that the compounds have a K₂NiF₄-type structure with the symmetry of space group I4/mmm. It was found that in these compounds, the substituted trivalent metal ions (M=Co, Ni, and Ga) and the Ru ions are disordered over the octahedral B sites of the K₂NiF₄ structure. The octahedral Ru-O environment is less elongated in the apical direction in these compounds than in the Sr₂RuO₄ compound. We also found that the octahedron in Sr₂Ga_0.5Ru_0.5O₄ is more symmetric than that of Sr₂Co_0.5Ru_0.5O₄ and Sr₂Ni_0.5Ru_0.5O₄. The Ru K-edge X-ray absorption measurements showed that Ru ions were in a pentavalent oxidation state.     

Study of silica aerogel grain surfaces by using a positron age–momentum correlation technique

The effect of heat treatment on silica aerogel has been studied by a positron age–momentum correlation technique and infrared measurement. A difference was observed between the momentum distributions of the electrons on the first layers of the silica aerogel grain surfaces and the electrons in the grains in an as-supplied sample, but not in the sample heat treated at 800 °C. A large change in the S parameter for the momentum distribution of the electrons on the first layer occurs around 200 °C. This change correlates well with that of the infrared spectra, which show oxidation of the methoxyl groups at temperatures above around 200 °C. This correlation reveals that those groups are mainly located on the first layer of the silica grains. Received: 13 November 2000 / Accepted: 23 July 2001 / Published online: 30 October 2001     

Positron diffusion and restoration of local symmetry breaking induced by positrons in liquid metals

We present the effective Lagrangian with spontaneously broken density (the hedgehog-like fluctuation) and the massive internal gauge fields, and propose one explanation for the increase of the positron diffusion length in the liquid metal phase with temperature. From the view point of the restoration of the spontaneously broken density around the positron, the positron diffusion length data in the liquid phase of Bi metal are analyzed.