F. Plazaola

Positron lifetime calculation for the elements of the periodic table

Theoretical positron lifetime values have been calculated systematically for most of the elements of the periodic table. Self-consistent and non-self-consistent schemes have been used for the calculation of the electronic structure in the solid, as well as different parametrizations for the positron enhancement factor and correlation energy. The results obtained have been studied and compared with experimental data, confirming the theoretical trends. As is known, positron lifetimes in bulk show a periodic behaviour with atomic number. These calculations also confirm that monovacancy lifetimes follow the same behaviour. The effects of enhancement factors used in calculations have been commented upon. Finally, we have analysed the effects that f and d electrons have on positron lifetimes.

Detection of Ga vacancies in electron irradiated GaAs by positrons

Positron lifetime measurements have been used to study the recovery of electron irradiated GaAs between 77 and 800 K. Below room temperature positrons are trapped by vacancies in Ga sublattices. The Ga vacancies recover between 200 and 350 K.

Positron annihilation in II-VI compound semiconductors: theory

Positron states and annihilation rates are calculated for nine different II-VI compound semiconductors. Positron annihilation from the delocalized states in the perfect lattice as well as from the localized states at vacancies and divacancies is considered. The calculations are based on the local-density approximation (LDA) for the electron-positron correlation effects. The calculations are performed using non-self-consistent electron densities and electrostatic potentials obtained by atomic superposition and solving for the three-dimensional positron wavefunctions by a relaxation method. For the perfect lattices, also self-consistent electron densities and positron states (linear muffin-tin orbital method within atomic-spheres approximation, LMTO-ASA) are calculated. The results show that positron annihilation with the outer d electrons of the group II metal atoms plays an important role. The positron lifetimes calculated for perfect lattices are a few per cent shorter than the experimental ones, indicating an LDA overestimation of the d-electron enhancement around the positron. These bulk lifetimes can be corrected efficiently by a semiempirical model introduced in this work. In the case of positrons trapped by defects, the present theoretical description is less satisfactory, because atomic relaxations due neither to rearrangements in the electronic structure nor to the localized positron are taken into account. However, the calculated annihilation probabilities with core and valence electrons will serve as an important database for methods in which defects can be identified using the positron angular correlation or Doppler broadening measurements.

Post-implantation annealing of SiC studied by slow-positron spectroscopies

The effects of post-implantation annealing of damage in 6H-SiC caused by ion implantation at two different fluences have been studied by monoenergetic positron Doppler broadening and lifetime techniques. The measurements are supported by new calculations of positron lifetimes in vacancy clusters in SiC. At both fluences two defected layers are identified and characterized by depth and defect type as a function of annealing temperature. The results indicate that it is impossible to remove the radiation damage by annealing at temperatures up to .

A positron annihilation study of the formation and dissolution of L12 precipitates in Al—Li alloys

Abstract The formation and dissolution of δ′ precipitates has been followed by positron annihilation spectroscopy in two Al-Li alloys (Al–9·9 at.% Li and Al-8·6 at.% Li-0·04 at.% Zr). Lifetime measurements have been performed by annealing the samples from the as-quenched state to 750 K, which is well above the dissolution temperature for the δ′-phase Al3Li. The results suggest that small δ′-like regions are present in the as-quenched state, and that they dissolve at around 430 K, in agreement with previous results in the literature. The technique has proved to be very sensitive to the growth and dissolution of δ′ precipitates owing to the high positron affinity to Li in an Al matrix.

Calculation of positron characteristics for elements of the periodic table

Positron characteristics have been calculated in bulk and monovacancies for most of the elements of the periodic table. Self-consistent and non-self-consistent schemes have been used for the calculation of the electronic structure in the solid, and different parametrizations for the positron enhancement factor and correlation energy. As it is known, positron lifetimes in bulk show a periodic behaviour with atomic number. These calculations also confirm that monovacancy lifetimes follow the same behaviour. The results obtained have been compared with selected experimental lifetime data, which confirms the calculated theoretical trends. Positron binding energies to a monovacancy have been calculated also for most of the elements of the periodic table. The binding energy shows a periodic behaviour with atomic number too.

Positron lifetime calculations for defects in Zn

The effect of the lattice relaxation at vacancy clusters and interstitial-type dislocation loops on the lifetime of positrons in Zn has been studied. Defective relaxed structures have been generated for the lifetime calculations by using a many-body potential for Zn. From the results, it is inferred that the effect of the atomic relaxation is mainly significant for small vacancy clusters. The lifetime associated with interstitial-type loops is very sensitive to the loop structure and its surroundings. Previous experimental results are compared with the theoretical calculations.

Positron Lifetime Calculations of Hexagonal Metals with the True Geometry

The approaches to get lifetimes of the hexagonal metals have been performed making use of the face centred cubic (f.c.c.) structure, instead of the real hexagonal one. Although some structural reasons allow this approximation, it is expected to fail when free volume is created in the solid. In this work, we have used the real hexagonal geometry to study these metals. For the cases of bulk, mono-, di- and trivacancies no appreciable differences have been observed in the calculated lifetimes. However, when studying clusters of vacancies in the real hexagonal structure and in the f.c.c. one, differences between 3.2% and 7.4% appear in lifetimes between these structures. This fact shows that the approximation of using the f.c.c. structure is not good for studies of extended defects. Positron lifetimes in these metals have also been obtained by means of the local density and general gradient approximations, using for enhancement factors Arponen-Pajanne and Boronski-Nieminen approximations. In alkaline earth and early transition metals the calculations performed with the general gradient approximation do not improve the results obtained using Boronski-Nieminens formula for the enhancement factor.

The influence of Li on the nucleation of defects of quenched AlLi alloys

Abstract The nucleation and kinetics of defects formed by quenching in two AlLi alloys having Li content of 1.7 and 3.74 at.%Li have been studied by positron annihilation spectroscopy. It has been found that the defect formation is sensitive to the aging time at the temperature from which the samples are quenched. This fact has been related to the Li loss experienced by the alloys aged at high temperature. The quenched-in defects have been identified as vacancy-Li clusters and dislocation loops. The latter are formed by the collapse of the Li-rich vacancy complexes and are very sensitive to the Li content; as a consequence, the loops are decorated by Li-rich zones and are revealed as very effective positron traps in comparison to the vacancy-Li complexes, giving rise to an enhanced trapping.

The nature of defects in electron irradiated and deformed indium

Abstract Positron annihilation spectroscopy is used to study the recovery behaviour in electron irradiated In. A long component of 270 ps in the as-irradiated samples has been attributed to monovacancies produced during the irradiation, which anneal out at 100 K to give rise to dislocation loops. The results obtained are compared with previous works performed in low temperature deformed In, concluding that the plastic deformation in In is mainly due to twinning.