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     

Phase transformations and vacancy formation energies of transition metals by positron annihilation

The technique of the coincidence count rate at the peak of the angular correlation curve (CCR) in positron annihilation has been applied to the investigation of vacancy formation energies in thermal equilibrium in nickel, cobalt, and iron. The monovacancy formation energyE ^1v/F has been determined to (1.55±0.05) eV and (1.34±0.07) eV for nickel and cobalt, and (1.60±0.10) eV for α-iron, and (1.40±0.15) eV for γ-iron, respectively. The structural phase transformations in cobalt (693 K) and iron (1183 K, 1663 K) are exhibited by discontinuities of the CCR. In the case of cobalt the CCR follows exactly the change of the thermal expansion at the transition temperature. The temperature dependence of the CCR in the prevacancy region is found to be proportional to the thermal expansion for all metals investigated.     

Irradiation-induced atomic defects in SiC studied by positron annihilation

The present paper reports on positron lifetime measurements on atomic defects in SiC after low-temperature (80 K) electron irradiation of low (0.47 MeV) and high (2.5 MeV) electron energies and doses from 1.8×10¹⁷ to 1.9×10¹⁹ e/cm² as well as after subsequent isochronal annealing up to 1900 K. For these studies the single crystals of nitrogen doped (2–3×10¹⁸ cm^–3) SiC grown by a modified Lely technique with hexagonal structure (6H polytype) were used.According to the positron lifetime measurements, very different types of vacancy-like positron traps are introducted after irradiation with electrons of either low or high energy. The formation of defect agglomerates and their decay at high temperatures is studied during isochronal annealing and related to earlier studies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Temperature characteristics of positron trapping at defects in electron-irradiated silicon

Positron-annihilation lifetime and Doppler-broadening measurements are used to investigate defects in silicon irradiated at 373 K with 6 MeV electrons to a dose of 1×l0¹⁹e/cm². In the unirradiated silicon sample (p type) a temperature-independent behaviour of the bulk-lifetime is observed in the temperature interval 110–500 K with a constant value of 220±1 ps. The slight effect observed on the S-parameter evolution is explained taking into account the thermal expansion of the lattice. The lifetime results obtained at 80 K and at 300 K after isochronal annealing as well as the behaviour of the intensity of the second lifetime componentI ₂ during lifetime measurements below the irradiation temperature in the irradiated silicon sample (n type), clearly indicate the temperature dependent characteristics of the positron trapping cross section _t(T) T ⁿ withn= –1.905±0.016. From isochronal annealing results, an annealing stage is observed in which di-vacancies agglomerate into quadri-vacancies. The mean positron lifetime in those quadri-vacancies is 350 ps.A.B.O.S., on leave from University of Kinshasa, Zaïre     

An investigation of point defects in silicon carbide

Positron lifetime measurement on bulk samples of single crystal SiC wafers have shown that both 6H and 4H polytypes exhibit a bulk lifetime of 150±2ps. All samples contained a second, defect-related lifetime component, ranging in value from 250 to about 300 ps with rather low intensities. The defect structure exhibited by the nanocrystalline samples is, not unexpectedly, much more complex.Positron beam experiments on Electron Cyclotron Resonance Chemical Vapour Deposited (ECR-CVD) SiC thin films showed that the positrons are very sensitive to changes in important film parameters as a function of the deposition conditions. It was found that the film density is lower than expected, probably due to hydrogen incorporation; variations in compositions among different films were detected through variations in theS parameters, and differences were observed in the electric field at the film-substrate interface due to hydrogen passivation of dangling bonds and different substrate resistivities.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Lattice defects of V3Si single crystals studied by positron annihilation

The sensitivity of positrons to point defects created by the irradiation of V₃Si with neutrons is demonstrated. We found no indication of thermal vacancies by thermal equilibrium measurement up to 1273 K which indicates that the monovacancy formation enthalpy for V₃Si isH _1V ^F ≧(1.84±0.14) eV. Investigations within the range of homogeneity for excess vanadium suppot the idea that substitutional defects are the dominating defect type, whereas for excess silicon a direct confirmation of existing structural vacancies as the dominating defect type is given.     

Near-surface defects in hydrogen-plasma-treated boron-doped silicon studied by positron beam spectroscopy

Near-surface vacancy-type defects have been studied by positron beam spectroscopy in three boron-doped Si wafers; a control sample, second sample exposed to atomic hydrogen in electron cyclotron resonance (ECR) plasma, and a third sample annealed at 500 °C following plasma treatment. From the analysis of the Doppler broadening spectra, measured as a function of positron implantation depth, we obtain positron diffusion lengths of about 100 and 250 nm for the damaged layer and bulk of the wafer, respectively. For the plasma-treated wafer, our measurements provide a defect density of about 5×10¹⁷ cm^-3. Received: 4 September 1998 / Accepted: 5 January 1999 / Published online: 28 April 1999     

A positron lifetime study in quenched Pb(Ag) alloys

Positron lifetime measurements have been made in quenched or irradiated pure Pb and in quenched Pb(Ag) alloys. From positron investigation of annealing behaviour, the precipitation of silver atoms in dilute alloys should be understood in terms of (Ag-Pb) interstitially migrating pairs. The presence of di-interstitials (Ag-Ag) or complexes [Ag(S)-V] as mobile defects responsible for the Ag transport process in concentrated alloys is discussed.     

Positron-lifetime study of vacancy annealing in neutron-irradiated GaAs

Positron-lifetime measurements have been used to study the annealing of vacancies in neutron-irradiated GaAs. The vacancies which are interpreted as defects in the Ga sublattice disappear in a single annealing stage (at 500°C in GaAs doped with Si or Zn, and at 600°C in Cr-doped GaAs).     

Nucleation process of stacking fault tetrahedra in gold studied by positron lifetime spectroscopy

By component analyses of positron lifetime spectra in quenched gold, it was found that positrons are strongly trapped by stacking fault tetrahedra (SFT) at which the lifetime is about 160 ps remarkably smaller than that for monovacancies (200 ps) or divacancies (220 ps). Positron lifetimes at small vacancy clusters were also estimated in relation to the nucleation process of SFT, with the aid of computer simulation of kinetics of vacancy clustering during quenching and on subsequent isochronal annealing. The results show that large atomic relaxation occurs in small vacancy clusters among which pentavacancies have the largest open-space, thereby having the lifetime of about 230 ps larger than that of trior tetra-vacancies (160ps). It is also suggested that tetra- and penta-vacancies act as prenuclei for stable nuclei of SFT consisting of six or more vacancies.Present situation: Professor Emeritus     

Positron lifetime as a function of grain size

Published data on positron annihilation lifetime in copper as a function of grain size have been analyzed to show that there is a linear relationship between the internal grain boundary surface area, per unit volume,S _v, and the positron lifetime, τ. The analysis indicates that grain boundaries are important in the trapping of positrons. It is suggested that the slope of the resulting straight line,dS _v/dτ, can be used to determine the annihilation rate of the grain boundaries.     

Review of defect investigations by means of positron annihilation in II-VI compound semiconductors

1-x Cd_xTe after post-growth annealing. The experiments on defect generation and annihilation after low-temperature electron irradiation of II-VI compounds are also reviewed. The characteristic positron lifetimes are given for cation and anion vacancies. Received: 19 November 1997/Accepted: 20 November 1997     

The recovery of electron irradiated zinc and cadmium by positron annihilation spectroscopy

Positron lifetime and Doppler-broadening measurements have been performed in Zn and Cd specimens irradiated with 3 MeV electrons at 20 K. Isochronal annealing of the irradiation induced defects between 20 and 280 K has been studied. No evidence of three-dimensional vacancy agglomerates was found.     

Investigations of As-antisite-related defects in GaAs

Optically Detected Magnetic Resonance (ODMR) is a powerful tool to study paramagnetic defects in semiconductors. As an example for the successful application of various methods based on ODMR, investigations of As antisite-related defects in GaAs are presented. Information about defect properties such as their microscopic structure, their metastability and thermal stability can be obtained by ODMR which was measured via the Magnetic Circular Dichroism of the optical Absorption (MCDA).Low temperature irradiation of GaAs (4.2 K) with 2 MeV electrons produces the Ga vacancy and some new As antisite-related defects. Annealing steps were investigated at 77 K and above. At 77 K the isolated As antisite defect is detected if semi-insulating (si) GaAs was irradiated. Between 200 K and 300 K the Ga vacancy decays. Its decay is correlated to the formation of an anti-structure pair. A second annealing step was found at about 520 K. There the anti-structure pair decays. EL2 is formed at that temperature if si material was irradiated, but this EL2 formation is not correlated with the anti-structure pair decay. We performed also magneto-optical measurements to investigate the metastability properties of the three As antisite-related defects. They all show metastability.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Defect study on electron irradiated GaAs by means of positron annihilation

Measurements of the positron lifetime and Doppler-broadened annihilation-radiation have been performed in electron-irradiated GaAs. The positron lifetime at the irradiation induced defects was 0.250 ns at 300 K. The defect clustering stage was found to occur at around 520–620 K, and the coarsening and annealing stage is believed to be above 620 K. Similar annealing stages were also observed in GaAs lightly doped with Si (0.2×10¹⁸ cm^–3). Both the lifetime and the S-parameter in the irradiated GaAs were found to decrease with temperature from 300 K to 100 K, suggesting the coexistence of shallow traps in electron irradiated GaAs.     

Semi-insulating GaAs: A possible substrate for a field-assisted positron moderator

Positron lifetime spectroscopy measurements have been carried out for semi-insulating GaAs with applied electric fields in the samples directed towards, and away from the positron injecting contact. The lifetime spectra have been decomposed into two components, the longer of which (400 ps) is characteristic of open volume defects at the metal-semiconductor interface through which positrons are injected. The interesting feature of these experiments is the large increase in the intensity of this interface component as the field is directed towards the contact. We show that this increase is caused by a significant fraction of implanted positrons drifting under the influence of a strong electric field produced by a layer of space-charge formed adjacent to the positron injecting contact. The general trend of the intensity variation is well explained by the proposed model. Experiments involving the application of an ac bias to the samples strengthen the suggestion that the space charge region is largely formed from ionized EL2 donors. The results of the present work indicate that semi-insulating GaAs possesses properties that make it a suitable material for the fabrication of a high efficiency (10%) room-temperature field-assisted positron moderator. The extraction of positrons from the GaAs substrate into the vacuum through a thin metalization is discussed based upon available positron affinities for the GaAs and various elemental metals. These data suggest that a few monolayers of a strongly electronegative metal such as Au or Pd may allow vacuum emission through quantum tunneling.     

The study of native defects in as-grown GaAs by positron annihilation

A mono-energetic positron beam was applied for a study of native defects in GaAs. From measurements of Doppler broadening profiles and lifetime spectra of positrons, it was found that Ga-vacancies were introduced by Si-doping. On the other hand, for p-type GaAs, interstitial clusters were found to be introduced by Zn-doping. The observed species of defects are in agreement with those expected from the Fermi level.     

A study of defects in deformed FeSi alloys using positron annihilation techniques

Steels with high amounts of silicon are used in electrical applications due to their low magnetostriction, high electrical resistivity and reduced energy losses, but they exhibit poor formability. The slow positron beam of Gent is used to investigate defects in different deformed FeSi alloys. It was found that the concentration of defects for the alloys deformed at high temperatures are different from the ones related to the alloys deformed at room temperature. These results are correlated to the results of positron annihilation lifetime spectroscopy (PALS).