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2 by Positron Lifetime and Electrical Measurements
Published in R D Tomlinson, A E Hill, R D Pilkington, Ternary and Multinary Compounds, 2020
J. Klais, H. J. Müller, R. Krause-Rehberg, D. Cahen, V. Lyakhovitskaya
Because of the complexity of the defect chemistry experimental methods are required that are particularly sensitive to single defects. The positron annihilation spectroscopy is such a method which allows one to study vacancy-like defects in solids. Positrons can be trapped in the open volume of the defects which increases their lifetime. Since the trapping rate is sensitive to the charge states of a vacancy in semiconductors the lifetime can yield valuable information on the type of vacancies. In this paper positron lifetime measurements in CuInSe2, single crystals of various compositions are presented and compared with the results of IV, CV and admittance measurements.
Atom/Ion Sources
Published in Peter E. J. Flewitt, Robert K. Wild, Physical Methods for Materials Characterisation, 2017
Peter E. J. Flewitt, Robert K. Wild
A positron annihilates by reacting with an electron with the release of a gamma ray. The time between the emission of positrons from a radioactive source and the detection of gamma rays is the lifetime of the positron. The electron and positron may exist in a metastable hydrogen-like bound state known as a positronium. Positron annihilation spectroscopy (PAS) or positron annihilation lifetime spectroscopy (PALS) has potential for quantifying the types and densities of defects in solids. Following the injection of positrons into a solid material, the positron pairs annihilate at a rate depending on the density of electrons near the injection site. If there are lattice vacancies or dislocation-induced defects (voids) near the injection site, the positrons are attracted to these regions, which have a lower electron density and therefore a longer positron lifetime. In PAS, the detected photons provide information regarding defects in crystalline materials, such as dislocations, grain boundaries, single/cluster vacancies and voids. For conductor-type materials (metals) with many free electrons, the positron lifetimes can be quite short in defect-free regions (e.g. ~100 ps) and considerably longer in defect-containing regions (e.g. ~200 ps). When a positron hits an electron, both particles annihilate into electromagnetic radiation which is emitted as two or three photons depending on the relative spin orientations of the positron and the electron. In the two-phonon case, both phonons are emitted with an energy of ~511 keV. Isotope sources used to produce positrons are 22Na, 64Cu, 58Co and 68Ge. A schematic of the experimental arrangement for PALS is shown in Figure 6.90, where the radioactive source is placed between the specimen and the gamma rays impinge on scintillator crystals where the pulses of light are converted to electrical pulses by using a photomultiplier which are then analysed by using a computer.
High concentration of vacancies induced by β″ phase formation in Al–Mg–Si alloys
Published in Philosophical Magazine Letters, 2020
Koji Inoue, Ken Takata, Kenji Kazumi, Yasuharu Shirai
Positron annihilation spectroscopy is sensitive technique for detecting vacancy-type defects. So far, in order to clarify the behaviour of vacancy in Al–Mg–Si alloys, several studies by employing this technique have been performed [8–14]. However, almost samples in their studies were aged at below 70°C, where solute clusters were formed. The clusters are coherent with Al lattice and have FCC structure. The β″ and β′ phases are incoherent with them and have the other structures. Therefore, vacancy behaviour in the samples aged at above 180 °C are quite different from the one in samples aged at below 70°C. In the process of the investigation of the vacancy behaviour aging at 180°C, we found that high concentration of vacancies were generated during β″ phase formation. The present paper reports the evidence for this finding.