Positron trapping in indium

The positron annihilation lineshape was measured in indium in the temperature range between 74 K and the melting point (430 K). A model based on self-trapping of positrons and trapping by vacancies was fitted through the data. A vacancy formation enthalpy of (0.48±0.03) eV was obtained.     

On the trapping of positrons in cadmium in the temperature range from 80 to 330 K

The positron lifetime was measured in cadmium in the temperature range between 80 K and 500 K. For the first time a plateau was observed by this method in polycrystalline samples. The obtained data are well explained by depletion of shallow traps forT180 K and by self-trapping in the prevacancy region.     

Positron lifetimes and lineshape factors in deformed copper

Measurements of positron annihilation lifetime and lineshape factors in deformed OFHC copper, for 14 different pairs of specimen, ranging up to 66% deformation, are discussed. The influence of annihilation in the source is carefully examined and the data are consequently corrected. The lifetime varies from (124±2) ps for annealed copper to (179±2) ps for dislocation saturated samples. Trapping cross sections and trapping efficiencies are calculated. The possibility of interference of vacancies formed during the deformation is discussed.     

Positron annihilation in the quenched and electron-irradiated NiSb intermetallic compound

Samples of the near equiatomic NiSb compound were irradiated by 3 MeV electrons at 20 K or quenched from 1103 K and 1333 K and subsequently annealed isochronally. The behaviour of defects created by quench or irradiation were studied by the positron annihilation technique. Only one recovery stage was found around 425 K for quenched specimens, but two distinct stages (100 K and 425 K) were observed after irradiation. The 425 K stage is ascribed to the migration of Ni vacancies giving dislocation loops. The recombination of mobile interstitials with vacancies after irradiation is assumed to occur between 100 K and 250 K. Doppler broadening and lifetime variations of positrons as a function of the measuring temperature in these irradiation samples are discussed.     

Positron lifetimes in V3Si

From positron annihilation lifetime measurements on V₃Si single crystals we estimated that about 17.2% of the positrons annihilate with core electrons.     

Effect of doping and thermal vacancies on positron annihilation in semiconductors

The effect of lattice defects on positron annihilation in semiconductors was studied. In silicon, any detectable doping effect could not be found. In germanium, the thermal equilibrium measurements of annihilation lineshapes showed no vacancy effect. From these experimental facts, the interaction of positrons with lattice defects was discussed.     

Positron study of the martensitic transformation in Fe-29.5at.%Ni

Precision measurements of positron lifetime spectra in Fe-29.5at.%Ni alloy specimens were performed to clarify the nature of the martensitic transformation in this alloy. The forward transformation did not introduce any defects which behave as strong trapping sites for positrons. On the other hand, the reverse transformation accompanied the production of defects, to which the positron lifetime is sensitive, maybe dislocations. From the present results, the nature of the transformation was discussed.     

Positron annihilation studies of defects in 3CSiC hot-implanted with nitrogen and aluminum ions

2 ⁺ and Al⁺ at temperatures from room temperature (RT) to 1200 °C at doses of 10¹³ and 10¹⁵/cm². It is found from Doppler broadening spectra of annihilation gamma-rays obtained by varying the incident positron energy that hot-implantation gives rise to clustering of vacancies, whereas it suppresses amorphization and diminishes the thickness of damaged layers. The average size of such clusters increases with increasing implantation dose and temperature. Vacancy clustering by hot-implantation can be interpreted by the combination of vacancies during implantation. Vacancy type defects in the low-dose (10¹³/cm²) implanted samples are found to be removed by annealing at 1400 °C, whereas large vacancy clusters still remain after 1400 °C annealing in the high-dose (1 0¹⁵/cm²) implanted samples. It is also derived from the depth profile of positron diffusion length that positron scattering centers are produced after annealing at 1400 °C in all implanted samples. Received: 7 March 1997/Accepted: 6 May 1997     

Positron annihilation at defects in sintered high-T c perovskite superconductors

We report positron lifetime measurements in sintered superconducting YBa₂Cu₃O_7–x and GdBa₂Cu₃O_7–x oxides. It is shown that the thermal behaviour of the positron lifetime spectra strongly depends on the preparation of the ceramics. A lifetime of 190±3 ps is attributed to oxygen deficient regions. Two lifetimes of 251±7 ps and 225 ±5 ps are attributed to a cation vacancy presenting a temperature dependent atomic arrangement. The lifetime transition (251225ps) occurs during decreases in temperature across the resistivity superconducting transition. This lifetime change indicates that the volume of the cation vacancy decreases in the superconducting state.     

Preparation of48V source for measurements of lineshapes of annihilation radiations and positron lifetimes

The preparation of the⁴⁸V positron source induced in a 1 μm thick Ti foil by (³He, pxn) reaction with a cyclotron is described. This source is very convenient for measurements of lineshapes of annihilation radiations and positron lifetimes at low or high temperatures, in a vacuum or for liquid metals. The absence of the mixing of long lifetime components is also convenient for the study of defects.     

Positron diffusion in solids and the reconstruction of inhomogeneous defect distributions from lifetime measurements

The time dependent diffusion trapping equations for positrons implanted into inhomogeneous solids are analyzed. This problem is of central importance in the study of polycrystalline materials and for the application of pulsed positron beams to defect studies in materials research. The main problem in previous investigations was the necessity to solve the time-dependent diffusion equation. It prevented analytical treatment in all but the simplest applications. For the first time this difficulty is eliminated by invoking a new concept, the observable local annihilation characteristics for local implantation of positrons into the thermalized ensemble. It will be shown that the local annihilation characteristics are governed by field equations which reduce to the well known quantities of the standard trapping model in the case of homogeneous defect distributions. Furthermore, inhomogeneous defect distributions are uniquely determined from the field equations provided the local annihilation characteristics are known. Analytical solutions are derived and applied successfully to recent experimental results for a selection of simple, but realistic problems. The formal procedure includes internal drift fields and could be extended to cover also the epithermal period of positron thermalization, if necessary.     

Positron lifetime and 2D-ACAR studies of divacancies in Si

We have measured positron lifetime and Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) distributions of Floating-Zone grown (FZ) Si specimens containing divacancies (V₂) with the definite charge states, V ₂ ⁰ , V ₂ ^–1 or V ₂ ^–2 from room temperature to about 10 K. These charge states are accomplished by an appropriate combination of dopant species, their concentration and irradiation doses of 15 MeV electrons. with reference to the currently accepted ionization level of divacancies. The positron lifetime of the negatively charged divacancy increases with temperature, while that of the neutral divacancy shows little change with temperature. The positron trapping rate, obtained from lifetime and 2D-ACAR measurements, increases markedly with decreasing temperature. This is found not only for the negative divacancies but also for the neutral divacancy. We need a model which explains this temperature dependence. The 2D-ACAR distribution from positrons trapped at divacancies shows nearly the same distribution for the different charge states, which differs considerably from the case of As vacancies in GaAs studied by Ambigapathy et al. We have observed a small but definite anisotropy in the distribution of trapped positrons in V ₂ ^– using a specimen containing oriented divacancies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Positron lifetime and Doppler-broadening measurements in noble metals and their alloys

Positron annihilation measurements in pure copper and silver as well as in the alloysCuGe,CuIn,CuSb, andAgSb have been performed. Most of the measurements have been carried out in thermal equilibrium. In this way it is possible to deduce vacancy formation — and vacancy — solute atom binding energies. ForCuIn alloys with 3.3 at.% and 7.1 at.% indium precipitation effects have been studied.This paper is based upon a talk given by W.L.-T. at the Int. Symp. Production of Low-Energy Positrons with Accelerators and Applications Justus-Liebig-Universität Giessen (1986)     

Pressure dependence of the electric field gradient in Sb and Sb1−x M x (M=ln,Zn, Ge,Pb, Cd,Sn) at the site of111Cd

Using the¹¹¹Cd-TDPAC (time differential perturbated angular correlation) method, the pressure dependence of the electric field gradient (EFG) in Sb and Sb_1–x M _x (M=ln, Zn, Ge, Pb, Cd, Sn) was investigated. The application of a phenomenological ansatz for the parametrisation of the pressure and temperature dependence of the EFG made it possible to combine temperature data gained in former studies [1], [2] with the pressure dependent data presented in this paper. The resulting pressure dependence of –2±0.2 MHz/kbar is shown to be independent of concentration and element of admixture. Results for the volume and explicit temperature dependence agree with existing information on the mixed system Sb_1–x M _x (M=ln, Zn, Ge, Pb, Cd, Sn); the investigation of the EFG in Sb_1–x–y M _x Pb _y showed that the resulting EFG may be interpreted as the weighted sum of the individual contributions of the two metals.This paper is dedicated to Prof. Dr. W. Kreische on the occasion of his 60th aniversary on 02.02.1995     

Positron annihilation and Hall effect in electron irradiatedn-InP crystals

Positron annihilation and Hall effect inn-InP crystals as a function of electron irradiation up to 1 · 10¹⁹ cm^–2 and post-irradiated isochronal annealing up to 550 °C have been studied. It is concluded that in irradiatedn-InP samples positrons interact with negatively charged acceptor-type defect with level atE _c –0.33 eV, probablyV _In (primary defect). In post-irradiated isochronal annealed (up to 330 °C) samples ofn-InP positron trapping occurs preferably in secondary defects-vacancy clusters, which are formed in the temperature range (150–300 °C). Inn-InP crystals containing radiation induced defects the trapping rate was found to decrease with temperature in the range (300–77) K.     

Positron annihilation in diamond,silicon and silicon carbide

Recent positron lifetime and doppler broadening results on silicon, diamond and silicon carbide are presented in this contribution. In as-grown Czochralski Si ingols vacancies are found to be retained after growth at concentrations typically around 3×10¹⁶/cm³. 10 MeV eleciron irradiation of variously doped Si wafers shows that only high doping concentrations well in excess of the interstitial oxygen concentration causes an increase in the amount of monovacancies retained.In porous silicon very long-lived positronium lifetimes in the range 40–90 ns are found. Polycrystalline diamond films contain various types of vacancy agglomerates but these are found to be inhomogeneously distributed from crystallite to crystallite. Electron irradiation of silicon carbide results in two vacancy-related lifetimes which are interpreted as resulting from carbon and silicon vacancies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

The electron mobility and the static dielectric constant of Cd3As2 at 4.2 K

The ionized-impurity mobility of Cd₃As₂ at 4.2 K with carrier concentration lying in the range 10¹⁸–10¹⁹ cm^?3 has been calculated taking into account the inverted HgTe-type energy band structure of this material. A comparison of the results with experimental mobility data leads to a static dielectric constant of ?₀ = 36. This value represents a lower limit since any compensation effects would increase the fitted estimate of ?₀.     

Pyroelectric properties of lithium triborate in the temperature range from 4.2 to 300 K

Temperature dependences of pyroelectric coefficients γ^T and γ^S corresponding to the mechanically free (T) and clamped (S) states of a lithium triborate (LBO) sample are studied and found to be nonmonotonic. It is proposed that the anomalies are associated with an increase in anharmonism of the lithium sublattice of LBO long before its transition to the superionic state. The spontaneous polarization at T=200 K is estimated to be 0.25 C/m². In the structural motif, the mesotetrahedra responsible for the emergence of spontaneous polarization of LBO are singled out.     

Positron lifetimes and distributions in the infinite-layer compound SrCuO2 and related materials

We have calculated distributions and lifetimes of positrons in the infinite-layer compound SrCuO₂ and those trapped at possible point defects therein. In the delocalized state, positrons show their density maxima at interstitial sites in the Sr planes and have a significant overlap also with Cu and O atoms. The corresponding positron lifetime is 149 ps. It has been revealed that the Sr vacancy strongly localizes positrons with the binding energy of 2.8 eV and the lifetime of 238 ps, while the O vacancy does not trap positrons. Calculations are also performed on related materials Sr₂Cu₄O₆ and Sr₄Cu₆O₁₀, which are characterized by one-dimensional networks of edge-sharing CuO₄ squares. Positrons are predominantly distributed between these networks in these materials and their corresponding lifetimes are 170–171 ps.