Positron annihilation studies of defects in helium-irradiated silicon

Positron annihilation studies have been carried out on Si-n irradiated with He⁺ ions at the V.U.B. cyclotron, to a dose of 4×10¹⁷ He/cm². No temperature dependence on the S-parameter and lifetimes is seen below the irradiation temperature. The positron lifetime associated to the created defects is 290 ps. During the isochronal annealing, this lifetime stays constant up to 700 K. It is attributed to the annihilation of positrons from large vacancy-clusters filled with He atoms. From the isochronal annealing results, only one annealing stage is seen. This annealing stage which extends over a long range of temperature 700–1000 K, is ascribed to the degassing of helium atoms from defects and the growth of vacancy-clusters. The lifetime of positrons in those defects reaches a value of about 530±30 ps at 1000 K, indicating that the vacancy-clusters formed have a mean size of more than 8 vacancies.     

Synthesis and characterization of magnesium oxide nanocrystallites and probing the vacancy-type defects through positron annihilation studies

Magnesium oxide nanocrystallites exhibit certain abnormal characteristics when compared to those of other wide band gap oxide semiconductors in the sense they are most prone to water absorption and formation of a hydroxide layer on the surface. The problem can be rectified by heating and pure nanocrystallites can be synthesized with controllable sizes. Inevitably the defect properties are distinctly divided between two stages, the one with the hydroxide layer (region I) and the other after the removal of the layer by annealing (region II). The lattice parameters, the optical band gap and even the positron annihilation characteristics are conspicuous by their distinct behavior in the two stages of the surface configurations of nanoparticles. While region I was specific with the formation of positronium-hydrogen complexes that drastically altered the defect-specific positron lifetimes, pick-off annihilation of orthopositronium atoms marked region II. The vacancy clusters within the nanocrystallites also trapped positrons. They agglomerated due to the effect of the higher temperatures and resulted in the growth of the nanocrystallites. The coincidence Doppler broadening spectroscopic measurements supported these findings and all the more indicated the trapping of positrons additionally into the neutral divacancies and negatively charged trivacancies. This is apart from the Mg²⁺ monovacancies which acted as the dominant trapping centers for positrons.     

Positron lifetime study of vacancy-type defects in amorphous and polycrystalline nanometer-sized alumina

2 O₃ and nanocrystalline Al₂O₃ specimens. The short-lifetime (170±20 ps), intermediate-lifetime (410±20 ps) and long-lifetime components correspond to three different kinds of defects: monovacancy-like free volumes, microvoids, and larger voids. The appearance of lifetimes in the range 1–5 ns indicates the formation of positronium. The influence of thermal annealing from 873 K to 1373 K on positron lifetime parameters was also analyzed. The components with lifetimes τ₁=170 ps and τ₂=410 ps persisted even after the grains had grown to 100 nm in size, while the long-lifetime component declined significantly when grain sizes exceeded 10 nm. The interface characteristics of polycrystalline nano-Al₂O₃ prepared by the two methods were compared by analyzing the variations of the positron-lifetime parameters with grain growth. Received: 1 April 1997/Accepted: 13 August 1997     

Positron annihilation studies in ZnO nanoparticles

We report results on positron annihilation spectroscopic (PAS) studies using lifetime and coincidence Doppler broadening techniques in zinc oxide (ZnO) nanoparticles (4 to 40 nm) synthesized by solid state pyrolytic reaction followed by annealing in the temperature range of 200 ^°C to 800 ^°C. Positron lifetime in the nanoparticles are observed to be higher than bulk lifetime in all the cases. Theoretical calculation of lifetime indicates the presence of either Zn or (Zn, O) vacancy clusters which migrate and anneal out at high temperature. Comparison of ratio spectra from coincidence Doppler broadening measurement and calculated electron momentum distribution indicates the presence of either Zn or (Zn, O) vacancies. In addition, photoluminescence (PL) measurements have been carried out to examine the role of defects on the intensity of emission in the visible region.     

Slow positron annihilation studies of vacancy-type defects in the near-surface region of Cu and Nb before and after wear

Received: 22 June 1998 / Accepted: 9 November 1998     

Positron annihilation studies of a conjugated polymer

Lineshapes of the Doppler broadened annihilation radiation (DBAR) of a conjugated polymer polydiacetylene, 4-butoxycarbonylmethylurethane (P-4BCMU), in its amorphous state, were measured over the temperature range of 30 to 190°C. A differential-scanning-calorimeter (DSC) was used to correlate the transition temperatures. The DBAR lineshapes show a distinct change during the structural transitions of the polymer. A two states trapping model is used to qualitatively describe the temperature dependence of the lineshape. The nature of formation of the trapped states of positrons above T_c, through the interplay of electronic structure/conformation change, is 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 studies in alpha-irradiated n-type GaAs

The positron annihilation technique has been used to study the recovery of defects in α-irradiated n-type GaAs. The Doppler broadening lineshape parameter, S, showed, for the first time, the recovery in three steps beginning around 120°C, 300°C and 510°C indicating the presence of three types of defects.     

Positron annihilation studies in alpha-irradiated n-type GaP

The positron annihilation technique has been used to study the defect recovery in 22.0 MeV alpha-irradiated n-type GaP. The increase in the lineshape parameter S along with defect specific parameter R between 200 and 350°C indicates the migration of vacancies and the formation of vacancy clusters. The defects start disappearing between 350 and 600°C.     

Positron annihilation studies in Fe doped SiC materials

Positron annihilation lineshape studies have been made in 5%, 10%, 20% and 30% iron doped SiC specimens sintered at 1050°C and 1100°C at hydrogen atmosphere and at 1100°C in vacuum. The annihilation lineshape parameter, S, remains constant for the first three doping concentrations for every sintered condition and increases sharply for the 30% case. The increase in the S value could be a signature of the structural phase change in the system.     

Positron annihilation spectroscopy in tomorrow's material defect studies

Positron Annihilation Spectroscopy (PAS), sometimes refered to as Positron Annihilation Lifetime Spectroscopy (PALS), are non-destructive techniques commonly used for defects and voids study in materials. It provides a handy tool for in-depth profiling down to few microns, and in determining open volume defects like dislocation, agglomerates, and vacancies at ppm concentration. PAS with the aid of Doppler broadening has certainly opened a new gateway in the spectroscopy of solid state physics, metals, RO systems, and semiconductors. Since it is an antiparticle of the electron it gives better information on the electron density in the material of interest. The lifetime of a positron is a complex function of electron density present at the annihilation site. To add beauty, in principle PAS gives the type and concentration of the defect independently by a single measurement. This article presents PAS based on the most relevant, most iconic, and most recent references.     

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.     

Positron annihilation in germanium

High precision long-slit and cross-slit geometry angular distributions of annihilation radiation from oriented germanium are presented. The momentum distributions from the long-slit measurements are compared with recent Compton profile data to test the importance of the positron wavefunction and positron-electron correlations. Evidence for Umklapp annihilation is discussed. No observable differences are found between distributions from heavily doped and intrinsic germanium samples. Differences in the angular distributions between 5 K and 300 K are attributed to positron thermal motion, and the results are analyzed by a new method to obtain an estimate of the positron effective mass. Paper A21 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973). Work supported by the National Science Foundation and the U.S. Army Research Office, Durham, North Carolina USA.     

Positron annihilation in copper

Electron momentum distribution in copper is studied through the angular correlation of positron annihilation radiation, by employing self-consistent linear combination of Gaussian orbitals band structure method. The two-photon momentum density is presented and analysed through three dimensional plots and angular momentum decomposed contributions. Calculation of two-dimensional angular correlation has been carried out in the independent particle approximation. The results for the analysis using Lock, Crisp and West theorem and the reciprocal form factors are also presented. The present theoretical results are compared with the experimental ones wherever possible, and the agreement in general, is good.     

Positron annihilation study of the annealing behaviour of alpha induced defects in tungsten

Abstract

Positron lifetime and Doppler broadening measurements have been performed on alpha-irradiated tungsten to study the evolution of defects during isochronal annealing from 100°C to 1050°C. Vacancy-impurity complexes dissociate at the earlier stages and release the vacancies. Monovacancies start migrating at 400°C. The vacancy clusters coarsen to form microvoids in the region from 350°C to 650°C and remain stable upto 750°C. Some of the clusters collapse into vacancy loops. Dislocation/vacancy loops and microvoids start annealing around 750°C. The role of impurities and grain boundaries on the annealing behaviour of the defects is also discussed.     

Positron annihilation studies in the mixed crystals of KCl:KBr

Positron annihilation line shape studies have been made in five mixed crystals of KCl:KBr with varying concentrations. The annihilation line shape parameter S shows a minimum for the 50 mole% KCl/50 mole% KBr specimen indicating an orderly behaviour of the system. At other concentrations, the mixed crystals appear to have defects.     

Positron annihilation studies of heavy-ion induced radiation damages in stainless steels

Variable-energy (0–30 keV) positrons were used to study the depth distribution of heavy-ion induced vacancy type defects in the following specimens, SUS304 and SUS316 austenitic stainless steels, a SUS444 ferritic stainless steel and nickel metal, which were irradiated by 0.5 MeV He⁺, 2.0 MeV C⁺, 3.5 MeV Si²⁺ and 4.0 MeV Ni²⁺ ions up to 0.01 dpa at peak. Vacancy type effective residual defects (ERD) were evaluated from the line shape parameterS of Doppler broadened positron annihilation photon spectra. With increasing primary knockon atom (PKA) energy, a decrease of the vacancy type ERD was observed. The ERD differences among the specimens are discussed in comparison with theoretical predictions.     

Role of vacancy-type defects in magnetism of GaMnN

Role of vacancy-type(N vacancy(VN) and Ga vacancy(VGa)) defects in magnetism of GaMnN is investigated by first-principle calculation.Theoretical results show that both the VNand VGainfluence the ferromagnetic state of a system.The VNcan induce antiferromagnetic state and the VGaindirectly modify the stability of the ferromagnetic state by depopulating the Mn levels in GaMnN.The transfer of electrons between the vacancy defects and Mn ions results in converting Mn~(3+)(d~4) into Mn~(2+)(d5).The introduced VNand the ferromagnetism become stronger and then gradually weaker with Mn concentration increasing,as well as the coexistence of Mn~(3+)(d~4) and Mn~(2+)(d~5) are found in GaMnN films grown by metal–organic chemical vapor deposition.The analysis suggests that a big proportion of Mn~(3+)changing into Mn~(2+)will reduce the exchange interaction and magnetic correlation of Mn atoms and lead to the reduction of ferromagnetism of material.     

Positron annihilation in polycrystalline metals

The empirical relation θ _p ⁶ /I _p=aK (where θ _p is the limiting angle of the parabolic component in the angular distributions of annihilation photons in metals, I _p is the integrated contribution of this component, K=1, 2, 3, ... is an integer, and a is a constant independent of the type of metal) observed earlier has been tested on magnesium, aluminum, copper, zinc, lead, and bismuth samples. The validity of this relation has been substantiated. The value of the dimensionless constant a has been determined and was found to coincide within experimental error with the result obtained in previous measurements. It is shown that the value of K for the same metal but for different samples may be different. It is conjectured that this may be due to different defect concentrations in samples. Fiz. Tverd. Tela (St. Petersburg) 40, 600–602 (April 1998)