Correlation between arsenic precipitates and vacancy-type defects in low-temperature-grown GaAs
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We have utilized x-ray photoelectron and variable energy positron beam spectroscopies for depth profiling excess arsenic, arsenic precipitates, and vacancy-type defects in GaAs grown by molecular beam epitaxy at low temperatures (LT-GaAs). XPS results show about 1.3% excess arsenic in as-grown LT-GaAs and a non-uniform depth profile of arsenic concentration in annealed LT-GaAs. Doppler broadening of the positron-electron annihilation radiation (S parameter) reveals a non-uniform depth profile of defects in annealed LT-GaAs. We observe a clear correlation between the depth profile of the S parameter and As in annealed LT-GaAs.Keywords:
Annihilation radiation
Correlated measurements of the lifetime and of the Doppler broadening of the 511 keV annihilation radiation of positrons (Age‐Momentum Correlation, AMOC) using an MeV positron beam have become a powerful tool for investigating reactions of positrons or positronium as a function of time. The room‐temperature reaction rate of the spin conversion of positronium in methanol induced by the presence of a paramagnetic solute (HTEMPO) has been found to be proportional to the HTEMPO concentration with a reaction‐rate constant of (22.5±0.5)⋅109 l mol−1 s−1 up to the highest concentration investigated (C≤0.1 mol/l). By means of the AMOC technique it was shown that a large Doppler broadening in natural and synthetic diamonds is correlated with a very short lifetime component. At higher positron ages a second positron state with a narrower momentum distribution of the positron‐electron pair indicates positron trapping at defects with concentrations ranging from 10−7 to 10−6 in different types of diamonds.
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We report the measurements of the positron annihilation characteristics, i.e., positron lifetime and coincidence Doppler broadening spectra performed on five Bi–Sb alloys having a Bi content between 12.5 and 81 at.%. The strong effect of the preferential annihilation of positrons with the electrons of the Bi atoms has been revealed in these alloys.
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Positron annihilation lifetime spectroscopy combined with Doppler broadening of annihilation radiation was applied to study free-volume entities in Ge-Ga-S glasses having different amount of CsCl additives. It is shown that the structural changes caused by CsCl additives can be adequately described by positron trapping modes determined within two-state model. The results testify in a favor of rather unchanged nature of corresponding free-volume voids responsible for positron trapping in the studied glasses, when mainly concentration of these traps is a subject to most significant changes with composition.
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Annihilation characteristics of positrons in Ge in thermal equilibrium at high temperature were studied using a monoenergetic positron beam. Precise measurements of Doppler broadening profiles of annihilation radiation were performed in the temperature range between 300 K and 1211 K. The line shape parameters of Doppler broadening profiles were found to be almost constant at 300–600 K. The changes in these parameters were observed to start above 600 K. This was attributed to both the decrease in the fraction of positrons annihilating with core electrons and the lowering of the crystal symmetry around the region detected by positron-electron pairs. This suggests that behaviors of positrons are dominated by some form of positron-lattice coupling in Ge at high temperatures. The temperature dependence of the diffusion length of positrons was also discussed.
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High-precision measurements of the temperature dependence of positron annihilation in Al, over the range 290-930K, have been made via the Doppler-broadening technique and using an internal 68Ge positron source. The results are considered mainly in terms of two lineshape parameters. These are denoted L and W and describe, respectively, the intensities of well defined central and wing portions of a normalised annihilation photopeak area. Both L and W exhibit the characteristic, overall S-shaped temperature-dependence profile associated with the change from free to vacancy-trapped annihilation modes. In detail, however, the temperature dependences of L and W are quite distinct.
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Abstract A new positron beam system, which has high moderation efficiency and can measure positron lifetime spectra, has been constructed at Wuhan university. A solid neon moderator with about 1% moderation effeciency is used. Secondary electron time‐tagging can be used to measure the positron annihilation lifetime. A target chamber has been designed for positron annihilation lifetime spectroscopy (PALS), Doppler broadening (DB) and coincidence Doppler broadening (CDB). The preliminary beam parameters are: a slow positron flux of 5 million per second for 40 mCi 22Na radioactive isotope, a positron energy spread as narrow as 2.0eV, and a moderator decay rate of 4% per day. Variable energy Doppler broadening annihilation radiation data for stainless steel samples are shown, which demonstrate the capabilities of the system. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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