Positron Annihilation Analysis Research Articles

Oxygen vacancy–induced ferromagnetism in un-doped ZnO thin films

ZnO films became ferromagnetic when defects were introduced by thermal-annealing in flowing argon. This ferromagnetism, as shown by the photoluminescence measurement and positron annihilation analysis, was induced by the singly occupied oxygen vacancy with a saturated magnetization dependent positively on the amount of this vacancy. This study clarified the origin of the ferromagnetism of un-doped ZnO thin films and provides possibly an alternative way to prepare ferromagnetic ZnO films.

Study the Properties of Cu-Zn Ferrite Substituted with Rare Earth Ions by Using Positron Annihilation Analysis

Positron annihilation lifetime spectroscopy (PALS) is a direct probe of size and concentration of nano-scale defects in materials, because it is very sensitive to electron density. The lifetime of positron (τ) and its intensity (I) can be used to characterize the defects concentration. In the present work, PALS has been applied to measure the variation of positron lifetime parameters for polycrystalline samples with chemical formula Zn0.5Cu0.5Fe1.98R0.02O4 (R = Gd, Sm, Nd and La). The variation of positron annihilation lifetime parameters (I1, I2, τav and k) with the ionic radius of rare earth ions, homogeneity, grain size and electrical resistivity have been studied. The inter-granular pores and grain boundaries defects are increased with the increasing of the ionic radius of the rare earth ions and decreased for Gd-sample. The defects inside and outside the grains are distributed homogeneously for Sm-sample only. A positive correlation has been found between positron lifetime parameters and ionic radius of rare earth ions as well as the electrical resistivity.

Investigation on the structural origin of n-type conductivity in InN films

This work presents a study of the correlation between the electrical properties and the structural defects in nominally undoped InN films. It is found that the density of edge-type threading dislocations (TDs) considerably affects the electron concentration and mobility in InN films. The Hall-effect measured electron concentration increases, while the Hall mobility decreases with the increase in the edge-type TD density. With the combination of secondary ion mass spectrometry and positron annihilation analysis, we suggest that donor-type point defects at the edge-type TD lines may serve as dominant donors in InN films and affect the carrier mobility.

Effect of lightly Si doping on the minority carrier diffusion length in n-type GaN films

We investigate the effects of lightly Si doping on the minority carrier diffusion length in n-type GaN films by analyzing photovoltaic spectra and positron annihilation measurements. We find that the minority carrier diffusion length in undoped n-type GaN is much larger than in lightly Si-doped GaN. Positron annihilation analysis demonstrates that the concentration of Ga vacancies is much higher in lightly Si-doped GaN and suggests that the Ga vacancies instead of dislocations are responsible for the smaller minority carrier diffusion length in the investigated Si-doped GaN samples due to the effects of deep level defects.

Positron annihilation and ion beam analysis of ion-bombardment-induced hydrogen release and oxidation of ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene was bombarded with He + and Ar + ions to fluences ranging from 10 13 to 2×10 16 ions/cm 2. Rutherford backscattering and nuclear reaction analysis were applied to study mechanism of oxygen uptake and hydrogen release induced by ion beam bombardment. The influence of ion bombardment on positron annihilation lifetime parameters is also discussed. Hydrogen release was observed with increasing ion dose and was correlated to the ion stopping power. An important effect observed, was the rapid oxidation of samples after exposure to air.

Molecular-level free volume as a crucial complementary factor affecting miscibility and nanoscopic homogeneity of polyarylate/poly(vinyl chloride) blends

Molecular-level miscibility and scale of mixing were characterized and evaluated for three model blends of polyarylates having structural variants with PVC through cross polarization/magic-angle spinning (CP/MAS) 13C nuclear magnetic resonance (NMR) spectroscopy. From this study, we found that the polyarylate with sulfonyl central connector and tetramethyl substitutions on bisphenol rings (shortly, TMBPS-PAr) was nanoscopically miscible with PVC as homogeneously mixed down to a scale of ∼2.3 nm. In contrast, the structurally similar polyarylate, just in absence of tetramethyl substitutions from TMBPS-PAr, (BPS-PAr) was partially mixed with PVC at a homogenization scale of at least ∼27–32 nm, regardless of almost identical specific intermolecular interaction relative to TMBPS-PAr. In the case of polyarylate with isopropyl central connector other than the sulfonyl (TMBPA-PAr) was found to be completely immiscible with PVC, despite the presence of tetramethyl substitutions, due to the lack of polar sulfonyl groups that is known to promote mixing. These observations, in conjunction with the structural combinations of polyarylates, suggest the need to seek another complementary key parameter to promote miscibility and nanoscopic homogeneity in addition to the specific interaction. From the results of the positron annihilation analysis (PALS) on the intermolecular free volume through the entire composition ratio, the TMBPS-PAr/PVC blend resulted in a larger negative deviation of the ortho-positronium ( o-Ps) pick-off lifetimes ( τ 3) and the free cavity sizes ( R) from linear additivity of those parameters of component polymers compared to the BPS-PAr/PVC blend. The larger negative deviation was interpreted to be a higher/greater contraction of free space and more tightly packing of molecular chains in TMBPS-PAr/PVC upon mixing irrespective of similar specific intermolecular interaction between two blend systems, which shows good agreement with the results of densities with blend compositions. Recognizing that the only structural variation between TMBPS-PAr and BPS-PAr was tetramethyl substitutions and that TMBPA-PAr/PVC was completely immiscible despite the tetramethyl substitutions, the combined results obtained from NMR and PALS lead to rationalize additional effects of the molecular free volume space created by the tetramethyl groups on the good miscibility of the blends under the circumstance when enough specific intermolecular interaction existed upon mixing.

Coincidence Doppler broadening and 3DAP study of the pre-precipitation stage of an Al–Li–Cu–Mg–Ag alloy

Pre-precipitation solute clustering in Al–Li–Cu–Mg–Ag and Al–Cu–Mg–Ag alloys has been investigated by coincidence Doppler broadening (CDB) spectroscopy of positron annihilation and three-dimensional atom probe (3DAP) analysis. Although Ag–Mg co-clusters form in the Al–Cu–Mg–Ag alloy in the early stage of aging, no evidence for the co-cluster formation was obtained from the Li containing alloy using 3DAP. While CDB spectra indicated that vacancies are associated with Ag after aging for 15 s in the Al–Cu–Mg–Ag alloy, vacancy–Ag association is suppressed in the Li containing alloy. Based on the 3DAP and CDB results, the reasons for the completely different clustering behaviors observed in these two similar alloys are discussed.

Yield and internal stresses in aluminum filled epoxy resin. A compression test and positron annihilation analysis

The influence of the filler content on the mechanical properties of an epoxy resin composite filled with aluminum powder was investigated. Compressive tests were performed at room temperature and at different strain rates. The response of the composites was also studied by positron annihilation lifetime spectroscopy. The dependence of the yield stress on the filler content is shown. The results are discussed in terms of a proposed model that takes into account the contribution of the filler powder. To this purpose information from positron spectroscopy is important since it allows to correctly evaluate the internal stresses introduced in the composite epoxy lattice by the metal filler.

Evolution of a defect structure of Pd–Ag alloys during tritium exposure

Pd–Ag alloys, a material for palladium diffuser of the ITER fuel clean-up system, were investigated after long-term usage exposition in tritium. Nucleation and evolution of the alloy structure defects as a result of a radiogenic helium-3 accumulation have been examined using electron microscopy, positron annihilation and X-ray analysis. The types of helium containing defects and their characteristics were determined. The early stage of helium bubbles forming was observed. It was shown that the simple defect concentration decreased slowly and helium-3 bubble sizes and concentration increased during the tritium exposure.

In situ positron annihilation analysis produced by high energy photon-induced neutron deficient positron emitting nuclei

Thick samples of materials have been investigated by the in situ production of positrons arising from β + decay of neutron deficient positron emitting nuclei photo-induced by high energy γ rays. The Idaho State University–Idaho Accelerator Center (ISU–IAC) possesses an electron LINAC that produces 10–20 MeV bremsstrahlung photons when a tungsten target is bombarded with the electron beam. These γ photons induce positron active radio-nuclei in the sample by means of photonuclear reactions. The radio-nuclei subsequently β + decay via a positron within the sample which can be up to several inches thick. Materials that undergo such positron emitting excitation include most of the metals and semi-conducting elements. Once the positrons are produced within the material they diffuse and thermalize in the normal manner seeking out defects and vacancies deep within the material and further produce the characteristic Doppler broadened annihilation spectra dependent on defect type and environment. As examples of this technique, data are presented for a set of fatigued stainless steels and an aluminum alloy under heat treated aging. Possible applications as a field deployable diagnostic technique will be discussed.

Investigation of physical ageing in polymethylmethacrylate using positron annihilation, dielectric relaxation and dynamic mechanical thermal analysis

The physical ageing of polymethylmethacrylate is investigated using a combination of positron annihilation, dielectric relaxation and dynamic mechanical thermal analysis. Correlations are observed between the three types of observation; however, no simple theory can be seen to fit all of the data. The dielectric data indicates that ‘thermorheologically simplicity’ is probably not a valid assumption for this system. The data also suggest that the free volume distribution probably changes during ageing; however, it was not probably to quantify these changes from the current positron annihilation study. Comparison of the experimental data from these various techniques clearly indicates that processes occurring at a molecular level are complex and not simply the result of one single type of conformational rearrangement and have an apparent activation energy comparable to that of the glass-rubber transition. © 1997 Elsevier Science Ltd.

PAT study on stress-field pinning in high-T c superconductors

The simultaneous measurements of Doppler-broadened radiation spectra and lifetime spectra were performed by positron annihilation analysis on Eu-doped Y-123 superconductors. When the doping concentration reaches 30% and 70%, the density of defeat and its stress-field reach the maximum for (Y1-x Eu x )Ba2CU3O7-y The pinning mechanism of such superconductors is discussed, and the results support the stress pinning mechanism.

Secondary defect formation in self-ion irradiated silicon

A detailed experimental study has been made of the evolution of extended secondary decondary defects which form during rapid thermal anneals of 0.5 MeV energy self-ion irradiated silicon. The implant fluence (2 × 10 15 ions/cm 2), flux and substrate temperature (91°C) were chosen so that primary damage levels were well below saturation. Cross-sectional transmission electron microscopy (X-TEM), Rutherford backscattering-channeling spectroscopy (RBS-C) and variable-energy positron annihilation techniques (VEP) have been used to allow partial discrimination between vacancy- and interstitial-type defects. The growth and development of the defect band and of specific types of extended defects within the band has been followed up to anneal temperatures of 1000°C, where the majority are shown to have dissipated. X-TEM has revealed the formation of a previously unreported tubular defect which is found in a narrow temperature range of 700–765°C. The occurrence of this defect correlates with the positron annihilation analysis which shows that a small concentration of defects with vacancy character is present after annealing in the same temperature range. In addition, positron annihilation analysis has allowed an assessment of the role played by defects lying in regions appearing defect-free by the other techniques (RBS-C and TEM). The implications of these findings to existing models involving secondary defect production are discussed.

Morphologies and Growth Modes of Fesi and β-Fesi2 Layers Prepared by Rapid Thermal Annealing

AbstractThis paper reports on the formation of FeSi and β-FeSi2by rapid thermal anneal (RTA). Sputtered Fe layers with a thickness of 25 nm and 40 nm on (100) Si wafers were subjected to RTA-treatment at temperatures ranging between 500 and 700 °C. This leads to three different types of growth. Two types of lateral growth and a hillock like growth. All the growth modes were nucleation controlled. The lateral growth rates of β-FeSi2 were measured at temperatures ranging from 625 to 700 °C, this provided a value for the activation energy of this process of about 1.35 eV. Positron annihilation analysis and scanning electron microscopy were used to study the onset of the transformation.It is suggested that the lateral growth appears when the Fe layer is completely transformed to FeSi before FeSi2 nucleates and that the hillock-like growth emerges when FeSi2 nucleates before the complete transformation of the Fe layer to FeSi.The RTA formed P3-FeSi2 layers have a roughness of the order of the thickness of the layer and a high defect density as observed by cross-section transmission electron microscopy. In order to decrease the surface roughening a-Si and Si3N4 capping layers were deposited with plasma enhanced vapour deposition before RTA.

Measurement of oxide thickness using a variable-energy positron beam

Thermally grown, wet silicon oxides were studied using a variable-energy positron beam. Positron data were modelled without prior knowledge of the film thicknesses, as a “blind” test, to evaluate our ability to interpret the positron data. Good agreement was found between film thicknesses obtained by positron annihilation, time-resolved reflectivity, and nuclear reaction analysis.

Synthesis, Gas Permeability, and Gas Sorption Properties of Fluorine-Containing Norbornene Polymers

Two fluorine-containing glassy polynorbornenes were synthesized, and their properties (permeability (P), diffusion (D), and solubility (S) coefficients, sorption isotherms, and lifetime positron annihilation spectra) were studied. Both polymers, poly(5,5-difluoro-6,6-bis(trifluoromethyl)norbornene) (PFMNB) and poly(5,5,6-trifluoro-6-(heptafluoropropoxy)norbornenel (POFPNB), were demonstrated to be highly permeable materials having Pvalues much larger than the Pvalue of nonsubstituted polynorbornene. Positron annihilation analysis and density measurements showed that these polymers have high free volumes. However, the reasons for the high permeabilities of these two materials were different. The permeability of PFMNB (Tg = 169 C) is related to its very high solubility coefficients while that of POFPNB (Tg = 77 C) appears to be caused by both its diffusion and solubility coefficients. Sorption isotherms have the form expected for materials behaving according to the dual-mode sorption model. The parameters for this model were determined by a least squares analysis of the isotherms. It was shown that the Henry's law solubility coefficient ko is enhanced in these fluorine-containing polymers compared to other polynorbornenes. On the other hand, the Langmuir capacity parameter C'H, which characterizes the nonequilibrium state of a polymer, also correlates with the Tgvalues of these norbornene polymers. Hence, both equilibrium and nonequilibrium factors appear to be responsible for the enhanced solubility and permeability seen for these fluorine-containing norbornene polymers.

Reply to "Comment on 'Atomic model for the EL2 defect in GaAs"

After correcting misimpressions regarding our atomic model for EL2 and that of Zou et al., we show that our model is in good agreement with thermochemical and positron annihilation analysis and with the compensation mechanisms which render GaAs semi-insulating, while the alternative model of von Bardeleben et al. is not. We give a plausible account of other electronic, optical, and electron-nuclear double resonance data regarding EL2.

Point Defects, Dislocations in GaAs and Material Homogeneity

ABSTRACTCombined positron annihilation and differential thermal analysis have shown that, in bulk GaAs materials, dislocations are decorated with vacancy related defects which recombine with As interstitials originating from As clusters. The role of these dislocations, the role of As source of these clusters are described and the consequences on the material homogeneity discussed.

Positron-annihilation study of silver irradiated by energetic protons or neutrons

To study the relationship of recoil energy to damage production we have irradiated well-annealed samples of silver at room temperature with energetic protons from 4.5 to 22.5 MeV or D-T fusion neutrons up to maximum fluences of $6\ifmmode\times\else\texttimes\fi{}{10}^{17}p{\mathrm{cm}}^{\ensuremath{-}2} \mathrm{and} \ifmmode\times\else\texttimes\fi{}{10}^{17}n{\mathrm{cm}}^{2}$. Positron-annihilation analysis including both lifetime and Doppler-broadening profile data was used to determine the characteristics of the surviving defects. The trapping-rate data exhibit a strong nonlinear fluence dependence, which may be due to radiation annealing of the damage. There is also clear evidence of a threshold recoil energy below which the defect survival efficiency is very low. This threshold is determined to be between 50- and 100-keV recoil energy.