Summary:The microstructure of the free volume and its temperature dependence in two poly(dimethyl siloxane)s (PDMS), one in the pure state and the other filled with 35 wt.‐% of an in situ hydrophobized fumed silica with a specific surface area of 200–300 m2 · g−1, were studied by pressure‐volume‐temperature experiments (PVT, T = 22–156 °C, P = 10–200 MPa) and positron annihilation lifetime spectroscopy (PALS, T = −173–100 °C, P = 10−5 MPa). The Simha‐Somcynsky equation of state was used to estimate the hole free volume fraction, h, and free and occupied volumes, Vf = hV and Vocc = (1 − h)V, from the specific total volume, V. The PALS spectra were analyzed with the routine LT9.0, which allowed for a dispersion, σi, in all three of the lifetimes: the para‐positronium (p‐Ps, τ1), positron (e+, τ2), and ortho‐positronium (o‐Ps) lifetime (τ3). This kind of analysis delivered correct p‐Ps lifetime parameters, τ1, σ1, and I1. It was speculated that e+, like o‐Ps, undergoes Anderson localization at empty sites of the, static or dynamic, disordered structure. The hole size distribution, its mean value, 〈vh〉, and dispersion, σh, were calculated from the o‐Ps lifetimes. A comparison of 〈vh〉 with Vf was used to estimate the specific hole number, $N'_{\rm h}$. During melting of the semicrystalline samples at−38 °C (Tm), 〈vh〉 increased abruptly, and σh suddenly decreased. Both effects are explained by the disappearance of the rigid‐amorphous fraction (RAF) and, thus, a reduction in the dynamic heterogeneity. The following leveling‐off in 〈vh〉 and the low value of σh are attributed to the fast segmental relaxation in the PDMS melts which leads to a smearing of the molecule density distribution around a hole during the o‐Ps lifetime. magnified image

Microstructure and thermal stability of the radiation defects in n-FZ-Si ([P] ≈ 7 × 10(15) cm(-3)) single crystals have been investigated. The radiation defects have been induced by irradiation with 15 MeV protons and studied by means of both the positron lifetime spectroscopy and low-temperature measurements of the Hall effect. At each step of the isochronal annealing over the temperature range ∼60-700 °C the positron lifetime has been measured for the temperature interval ∼30-300 K, and for samples-satellites the temperature dependences of the charge carriers and mobility have been determined over the range ∼4.2-300 K. It is argued that as-grown impurity centers influence the average positron lifetime by forming shallow (E(b) ≈ 0.013 eV) positron states. The radiation-induced defects were also found to trap positrons into weakly bound (E(b) ≤ 0.01 eV) states. These positron states are observed at cryogenic temperatures during the isochronal annealing up to T(anneal.) = 340 °C. The stages of annealing in the temperature intervals ∼60-180 °C and ∼180-260 °C reflect the disappearance of E-centers and divacancies, respectively. Besides these defects the positrons were found to be localized at deep donor centers hidden in the process of annealing up to the temperature T(anneal.) ≈ 300 °C. The annealing of the deep donors occurs over the temperature range ∼300-650 °C. At these centers positrons are estimated to be bound with energies E(b) ≈ 0.096 and 0.021 eV within the temperature intervals ∼200-270 K and ∼166-66 K, respectively. The positron trapping coefficient from these defects increases from ∼1.1 × 10(16) to ∼6.5 × 10(17) s(-1) over the temperature range ∼266-66 K, thus substantiating a cascade phonon-assisted positron trapping mechanism whose efficiency is described by ≈T(-3) law. It is argued that the value of activation energy of the isochronal annealing E(a) ≈ 0.74-0.59 eV is due to dissociation of the positron traps, which is accompanied by restoration of the electrical activity of the phosphorus atoms. The data suggest that the deep donors involve a phosphorus atom and at least two vacancies. Their energy levels are at least at E > E(c) - 0.24 eV in the investigated material.

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For the first time the samples, cut from the same wafer of crystals of float‐zone silicon, n−FZ−Si(P) and n−FZ−Si(Bi), were subjected to irradiation with 0.9‐MeV electrons and 15‐MeV protons at RT for studying them by low‐temperature positron annihilation lifetime spectroscopy. Measurements of Hall effect have been used for the materials characterization. The discussion is focused on the open vacancy volume (Vop) of the thermally stable group‐V‐impurity‐vacancy complexes comprising the phosphorus atoms; the bismuth‐related vacancy complexes are briefly considered. The data of positron probing of PV pairs (E‐centers), divacancies, and the thermally stable defects in the irradiated n−FZ−Si(P) materials are compared. Beyond a reliable detecting of the defect‐related positron annihilation lifetime in the course of isochronal annealing at ∼ 500 °C, the recovery of concentration of phosphorus‐related shallow donor states continues up to ∼650–700 °C. The open vacancy volumes Vop to be characterized by long positron lifetimes Δτ2 ∼271–289 ps in (gr.‐V‐atom)–Vop complexes are compared with theoretical data available for the vacancies, τ(V1), and divacancies, τ(V2). The extended semi‐vacancies, 2Vs‐ext, and relaxed vacancies, 2Vinw, are proposed as the open volume Vop in (gr.‐V‐atom)–Vop complexes. It is argued that at high annealing temperature the defect Ps–Vop–Ps is decomposed.

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A probing of the atomic environment of positron in Cz-Si single crystal heat-treated at T=600C and T=450C has been performed by one-dimensional angular correlation of annihilation radiation (ACAR). It has been established that positrons get trapped by the oxygen-related complexes. The penetration of positrons into the core region of surrounding atoms results in emission of the elementally specific high-momentum annihilation radiation. The processes of expelling of positron from ion cores and its penetration into the core region are regulated by the potential barrier (to be considered as the Coulomb’s one as a first approximation). The characteristic electron-positron ion radius and the probabilities of correlated events of the highmomentum annihilation are due to the chemical nature of the ion cores of atoms involved in the composition of the oxygen-related complexes. The interpretation of the results is based on the notion of the positron localization in the field of negative effective charge resulted from comparatively high electron affinity of the oxygen impurity atom. The presence of a free volume (perhaps, a vacancy) as well as the carbon atom in the microstructure of the oxygen-related positron-sensitive thermal defects is briefly discussed.

Temperature dependency of the average positron lifetime has been investigated for n‐type float‐zone silicon, n‐FZ‐Si(P), subjected to irradiation with 0.9 MeV electrons at RT. In the course of the isochronal annealing a new defect‐related temperature‐dependent pattern of the positron lifetime spectra has been revealed. Beyond the well known intervals of isochronal annealing of acceptor‐like defects such as E‐centers, divacancies and A‐centers, the positron annihilation at the vacancy defects has been observed in the course of the isochronal annealing from ∼ 320 °C up to the limit of reliable detecting of the defect‐related positron annihilation lifetime at ≥ 500 °C. These data correlate with the ones of recovery of the concentration of the charge carriers and their mobility which is found to continue in the course of annealing to ∼ 570 °C; the annealing is accomplished at ∼650 °C. A thermally stable complex consisting of the open vacancy volume and the phosphorus impurity atom, Vop‐P, is suggested as a possible candidate for interpreting the data obtained by the positron annihilation lifetime spectroscopy. An extended couple of semi‐vacancies, 2Vs‐ext, as well as a relaxed inwards a couple of vacancies, 2Vinw, are suggested as the open vacancy volume Vop to be probed with the positron. It is argued that a high thermal stability of the Vs‐ext PVs‐ext (or VinwPVinw.) configuration is contributed by the efficiency of PSi5 bonding. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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