Electrons and positrons in metal vacancies

The electron density profiles at monovacancies of simple metals are calculated by the self-consistent Kohn-Sham method and by a number of statistical methods. The metal is described by a uniform positive background charge together with an interacting electron gas, and the vacancy is approximated as a spherical hole in the background. The Kohn-Sham electron density inside the vacancy is found to be in average 15 of the density in the bulk material. Of the various statistical methods, the simple Thomas-Fermi approximation is found to describe best the average electron density over the whole metallic density range when compared to the Kohn-Sham results. The energies of vacancy formation are calculated by using the Kohn-Sham electron densities together with three lattice models, and reasonable numerical success is achieved for alkali metals. In the case of polyvalent metals the results are not satisfactory even if the uniform background were replaced by point ions or if the electron-ion interactions were described by Ashcroft empty-core pseudopotentials. The lifetimes of a positron trapped at the vacancies of several metals are calculated by using both the Kohn-Sham and the Thomas-Fermi electron densities. The results for most metals are in agreement with experimental values. The angular-correlation curve of the positron in aluminum vacancy is calculated directly from the Kohn-Sham one-electron wave functions. The result agrees with the curve calculated from the so-called mixed-density approximation and also with the experimental result.Peer reviewe

Density-functional theory of positronium and electron bubbles in helium fluids

The density-functional method is applied to excess electrons and positronium atoms in helium fluids. The self-trapping is investigated in a fully self-consistent fashion, and formulas are given for the particle energy and Ps pick-off annihilation rate in quasifree as well as localized states. The numerical results compare well with experimental data. However, the need for a more sophisticated treatment of threshold effects near the onset of bubble formation is indicated.Peer reviewe

Temperature dependence of positron trapping at voids in metals

We report positron-lifetime measurements in void-containing aluminum samples, which show strong temperature dependence for the positron trapping probability. A theory is presented for the positron motion and trapping in a three-dimensional array of large voids, which compares favorably with the experimental data. It is shown that at low temperatures the trapping is transition limited and strongly temperature dependent with a crossover to diffusion-limited and weakly-temperature-dependent behavior at high temperatures.Peer reviewe

Identification of vacancy defects in compound semiconductors by core-electron annihilation: Application to InP

We show that the Doppler broadening of positron annihilation radiation can be used in the identification of vacancy defects in compound semiconductors. Annihilation of trapped positrons with surrounding core electrons reveals chemical information that becomes visible when the experimental backgorund is reduced by the coincidence technique. We also present a simple calculational scheme to predict the high-momentum part of the annihilation line. The utility of the method is demonstrated by providing results for vacancies in InP. In electron irradiated InP the isolated In and P vacancies are distinguished from each other by the magnitude of the core-electron annihilation. In heavily Zn-doped InP we detect a native vacancy defect and identify it to a P vacancy decorated by Zn atoms.Peer reviewe

Identification of Vacancy-Impurity Complexes in Highly n-Type Si

We show that the detailed atomic structure of vacancy-impurity complexes in Si can be experimentally determined by combining positron lifetime and electron momentum distribution measurements. The vacancies complexed with a single impurity, V−P and V−As, are identified in electron irradiated Si. The formation of native vacancy defects is observed in highly As-doped Si at the doping level of 1020cm−3. The defects are identified as monovacancies surrounded by three As atoms. The formation of a V−As3 complex is consistent with the theoretical descriptions of As diffusion and electrical deactivation in highly As-doped Si.Peer reviewe

Theoretical and experimental study of positron annihilation with core electrons in solids

A theory for calculating the momentum distribution of annihilating positron-electron pairs in solids is presented. To test the theory, momentum distributions are measured by the Doppler broadening of the annihilation radiation for several bulk metals and semiconductors, as well as for semiconductor alloys and for positrons trapped at vacancies in semiconductors. The theory is based on a two-particle description of the annihilating electron-positron pair. Then, the electron-positron correlation effects, i.e., the enhancement of the electron density at the positron, depend on the electronic state in question. The theory is suited for calculating the high-momentum part of the annihilation spectrum that arises from the core electrons and which can be measured by the Doppler broadening using coincidence techniques. The ideas of the theory are justified by a good agreement between theory and experiment in the case of positron annihilation in undefected bulk lattices. Moreover, the comparison of the theoretical and experimental spectra for alloys and vacancy defects tests the theoretical description for the positron distribution in delocalized and localized states, respectively.Peer reviewe

Target chamber for a slow positron beam: optimization of count rate and minimization of backscattering effects

Abstract Positrons, which scatter back from the target and annihilate in chamber walls near the detectors, may cause a significant error in annihilation parameters. We have constructed a new UHV target chamber for slow positron beam studies. In our design special care has been taken to reduce the effect of backscattered positrons. Detector wells are designed for two-detector coincidence measurements and they are situated on both sides of the target. The distance of the wells from the target can be adjusted by simple manipulators. This enables optimization regarding the count rate and the rate of backscattered positrons hitting the detector wells. The magnetic field in front of the target is increased by permanent magnets situated behind the target. The increased magnetic field guides the backscattered positrons effectively away from the detectors. The increased magnetic field also focuses the beam spot strongly.

Clustering of vacancy defects in high-purity semi-insulating SiC

Positron lifetime spectroscopy was used to study native vacancy defects in semi-insulating silicon carbide. The material is shown to contain (i) vacancy clusters consisting of 4--5 missing atoms and (ii) Si vacancy related negatively charged defects. The total open volume bound to the clusters anticorrelates with the electrical resistivity both in as-grown and annealed material. Our results suggest that Si vacancy related complexes compensate electrically the as-grown material, but migrate to increase the size of the clusters during annealing, leading to loss of resistivity.Comment: 8 pages, 5 figure