Characterization of Atomic Defects and Their Aggregates Using Positron Annihilation Spectroscopy

Defects present in ZnO nanocrystals prepared by a wet chemical method have been characterized by photoluminescence (PL) and positron annihilation spectroscopy (PAS) techniques. The as-prepared sample was heat treated at different temperatures to obtain nanocrystals in the size range of 19–39nm. X ray diffractograms confirmed the single-phase wurtzite structure formation. Photoluminescence measurements showed a strong violet band at 434nm, which has been identified as due to electronic transitions from the zinc interstitial defect level to the top of the valence band. A marked decrease in the intensity of the violet emission with increasing heat-treatment temperature has been observed, which is attributed to recombination of zinc interstitials with zinc vacancies. Positron annihilation spectroscopy has been employed to understand the dynamics of the vacancy-type defects and their annealing behavior. The observed variation of the defect related lifetime components with heat-treatment temperature has been successfully explained by using a three-state trapping model. The results of PL and PAS studies in the present case are found to be complementary to each other.

In recent years, short, pulsed laser ablation has been gaining popularity for machining small-scale test geometries from bulk samples and for efficient serial sectioning. These laser-based techniques are being added to the toolbox in material science, which makes it necessary to understand the changes in the material that occur from the laser–material interaction. Positron annihilation spectroscopy is a unique, nondestructive technique to investigate small defects in materials difficult to investigate by other tools. In this work, Doppler broadening and positron lifetime annihilation spectroscopy are utilized to help quantify the damage in materials treated with short, pulsed lasers. Using a femtosecond laser on single crystal silicon, this manuscript shows that clusters of vacancy-like defects and small voids increase systematically with laser power. The damage induced by the laser can also reach to micrometer depths.

The photovoltaic performance of Cu(In1-x,Gax)Se2 (CIGS) materials is commonly assumed to be degraded by the presence of vacancy-related defects. However, experimental identification of specific vacancy defects remains challenging. In this work we report positron lifetime measurements on CIGS crystals with x = 0, and x = 0.05, saturation trapping to two dominant vacancy defect types, in both types of crystal, is observed and found to be independent of temperature between 15–300 K. Atomic superposition method calculations of the positron lifetimes for a range of vacancy defects in CIS and CGS are reported. The calculated lifetimes support the assignment of the first experimental lifetime component to monovacancy or divacancy defects, and the second to trivacancies, or possibly the large In-Se divacancy. Further, the calculated positron parameters obtained here provide evidence that positron annihilation spectroscopy has the capability to identify specific vacancy-related defects in the Cu(In1-x,Gax)Se2 chalcogenides.

Self-irradiation effects in plutonium alloys

Study of defect complexes and their evolution with temperature in hydrogen and helium irradiated RAFM steel using positron annihilation spectroscopy

The Japan Atomic Energy Agency (JAEA) has started to study and develop zirconium carbide (ZrC)‐coated fuel particles for advanced high‐temperature gas‐cooled reactors. The ZrC‐coated particles have been fabricated at JAEA and then heat‐treated to investigate the effects of the fuel compact sintering process. Remarkable ZrC crystal grain growth occurred in one batch, but not in another, in spite of both having the same C/Zr ratio and ZrC density. TEM/STEM observation of the specimens before heat treatment clarified that considerably more free carbons or voids, which would be expected to hinder the grain growth of ZrC with heat treatment, were distributed in the ZrC layer in the batch in which the remarkable ZrC crystal grain growth did not occur than in the other batch. Differences between these batches, which could not be detected by estimating the C/Zr ratio or the ZrC density, could be detected with positron annihilation spectroscopy (PAS). This result indicates that there is a possibility that PAS can be applied for quality control for ZrC‐coated fuel particles in the future, as preparing specimens for PAS is much easier than for TEM/STEM observation. Moreover, PAS is much more suitable than microstructural observation for quantitative estimation. Characterization of defects detected with PAS would be the next step in this process for the investigation of the feasibility of the application of PAS to the quality control of ZrC‐coated fuel particles.

Besides its conventional applications in defect characterization, variable‐energy positron annihilation spectroscopy can be employed to monitor internal electric fields in the depletion regions in semiconductor structures. In this work, electric fields were studied in pre‐amorphized Cz Si wafers (background dopant level ∼ 1015 cm–3) implanted with 0.5 keV B ions at a dose of 1015 cm–2, and then annealed isothermally at 800oC for times ranging from 1 to 2700 s. Differences in the S parameter with annealing time were observed in samples implanted (a) with B ions only and (b) with B followed by F ions at 10keV; these were attributed to different electric fields, which drift positrons back (a) to the surface, or (b) to a vacancy‐like defected layer. Fitting of the data revealed depletion regions of widths between 150‐350 nm centered at depths between 250‐350 nm, with electric field values in the range ‐9 x 106 to ‐3 x 106 Vm–1. The depth and width of the depletion regions increase significantly for annealing times greater than 100 s, attributed to B diffusion. The results are consistent with simple theoretical estimates, but the uncertainties on the latter are large. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Defect evolution in single crystalline tungsten following low temperature and low dose neutron irradiation

Defects introduced in epitaxially grown cubic silicon carbide (3C-SiC) by implantation of nitrogen (N2+) and aluminum ions (Al+) at a wide temperature range from room temperature to 1200 °C were studied using electron spin resonance (ESR), photoluminescence (PL), and positron annihilation spectroscopy (PAS). It is found that while hot-implantation reduces paramagnetic defects and improves the crystallinity of implanted layers, it causes the simultaneous formation of vacancy clusters. These results can be explained in terms of the migration and combination of point defects during hot-implantation. The formation and reduction of defects by hot-implantation are discussed in connection with implantation temperature, dose, and ion species. Postimplantation annealing of the defects in hot-implanted 3C-SiC was also examined by the ESR, PL, and PAS technique. The influence of residual defects on the electrical properties of implanted 3C-SiC layers is also discussed.

Characterisation of open volume defects in Fe–Cr and ODS Fe–Cr alloys after He+ and Fe+ ion irradiations

In the present work we studied microstructure of ultra fine grained (UFG) pure Mg and UFG Mg-based alloys. The initial coarse grained samples were deformed by high pressure torsion (HPT) using pressure of 6 GPa. Such deformation leads to formation of UFG structure in the samples. The severe plastic deformation results in creation of high number of lattice defects. Therefore, we used positron annihilation spectroscopy (PAS) for defect characterizations. PAS represents a well developed non-destructive technique with high sensitivity to open volume defects like vacancies, vacancy clusters, dislocations etc. In the present work we combined PAS with TEM and XRD to obtain complete information about microstructure of the UFG samples studied. We have found that microstructure of HPT-deformed Mg contains two kinds of regions: (a) ”deformed” regions with UFG structure (grain size 100-200 nm) and high number of randomly distributed dislocations, and (b) ”recrystallized” regions with low dislocation density and grain size of few microns. It indicates some kind of dynamic recovery of microstructure already during HPT processing. On the other hand, homogenous UFG structure with grain size around 100 nm and high density of homogeneously distributed dislocations was formed in HPT-deformed Mg-9.33 wt.%Gd alloy. After characterization of the as-deformed microstructure the samples were subsequently isochronally annealed and the development of microstructure with increasing temperature and recovery of defects were investigated.

The sensitivity of positron annihilation characteristics to the structural changes occurring in supersaturated metallic systems has been demonstrated and explained since the early 1980s. This knowledge is currently applied for positron annihilation spectroscopy (PAS) investigations regarding the main phenomena that govern the structural evolution of a supersaturated alloy when various thermal treatments are applied: (a) kinetics of vacancy-type defects retained after quenching; (b) solute migration; (c) formation of solute clusters and/or GP zones; (d) precipitation. The analysis of PAS data gives information on the chemical composition, the defect structure and the density of decomposition products of nanometric size. The present article, that includes an exhaustive repertory of works published since 1987, discusses the value of the contribution that PAS studies can give to physical metallurgy in the field of age-hardenable systems. Most examples are taken from the direct experience of the authors.

Tungsten has demonstrated a competitive figure of merit in its application to plasma-facing components (PFCs) of fusion reactors. During service, the material is exposed to high temperatures and high-level displacement damage. A common interest is fostered in the nuclear materials community to address the issue of defect evolution at operating temperatures, and how they recover throughout service. During maintenance, the application of in situ thermal repair technologies is tempting, featuring attractive efficiency in defect removal via an optimal selection of post-irradiation annealing (PIA) parameters. In previous studies, we examined the role of PIA temperature, PIA duration, and initial defect concentration on defect evolution, and redefined the damage recovery stages for tungsten, but this was done from a room-temperature heavy-ion irradiation perspective; see Wang et al (2023 J. Nucl. Mater. 581 154454), Wang et al (2024 Acta Mater. 273 119942). In this study, the scope is expanded to displacement damage saturation induced by heavy-ions at high temperatures, relevant to the service conditions of tungsten-based PFCs. The damage microstructure evolution in response to varied irradiation temperatures ( T Irr ) and PIA temperatures ( T PIA ) was assessed via transmission electron microscopy and Doppler broadening positron annihilation spectroscopy. Irradiation hardening was evaluated via nano-indentation. A scientific framework is proposed to guide thermal healing of displacement damage in tungsten via PIA treatment. It was ineffective when T PIA ⩽ T Irr . An adverse effect of PIA-induced secondary hardening occurred when T PIA (stage III) > T Irr (stage III). The optimal PIA scheme was confirmed when T PIA (stage IV) > T Irr (stages III–IV), eluding PIA-induced secondary hardening and minimizing PIA-enhanced recrystallization.

Physical properties of MnFeAl-based alloys affected by Mn content and annealing

Cobalt films of thickness nm have been deposited on Si(100) substrates and subsequently annealed to temperatures of 400, 500, 600 and for 2 h. These anneals resulted in the formation of overlayers consisting of Co , CoSi and (600 and ). The various phases are easily distinguishable using both Rutherford backscattering (RBS) and positron annihilation spectroscopy (PAS). The values of the Doppler broadening S parameter for CoSi and (formed at ) are equal to the stoichiometrically weighted averages of the S parameters of the constituents. Consequently, it is proposed that positron trapping sites previously detected in using re-emitted positron spectroscopy are most likely grain boundaries and dislocations. Open volume defects (possibly voids) were detected by PAS in the sample formed at . Using in situ PAS measurements during annealing of an initially unreacted 60 nm Co film, transitions between the various phases were observed.