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Ga vacancies and their unusual structure in β-Ga2O3 have been studied in recent years by several experimental techniques as well as theoretical calculations. Theory predicts that the so-called split Ga vacancy configuration is the one with the lowest formation energy. Positron annihilation spectroscopy faces challenges with β-Ga2O3 due to the colossal signal anisotropy and lack of a proper reference sample. Nevertheless, the majority of the data strongly suggests that all studied β-Ga2O3 samples contain high concentrations of split Ga vacancies in a wide distribution of configurations. EPR, IR spectroscopy and scanning transmission electron microscopy experiments all agree on the split Ga vacancy configuration as being the most likely interpretation of the data. However, the exact structure of the split Ga vacancies remains to be solved in terms of complexes with O vacancies and hydrogen.

Positron annihilation signals from VMCO-like samples grown by atomic layer deposition at different temperatures are utilized for the characterization of differences in open volume defects in TiN/TiO[Formula: see text]/a-Si heterostructures. Doppler and coincidence Doppler mode of positron annihilation spectroscopy combined with a monoenergetic positron beam were used for this study. Differences observed in the Doppler parameters indicate differences in the positron trapping states of the TiO[Formula: see text] epilayers grown at different temperatures. Furthermore, the coincidence-Doppler results show that these differences cannot be due to intermixing of the TiO[Formula: see text] and a-Si layers and formation of thin SiO[Formula: see text] layers at the interface during the growth process. The results indicate that the amount of open volume defects in the TiO[Formula: see text] layer of the VMCO-structure seems to increase with an increase in the growth temperature.

Defect evolution under irradiation is investigated in a set of single-phase concentrated solid solution alloys (SP-CSAs) containing Ni with Co, Fe and/or Cr. We show that atomic segregation of Ni takes place already at very early stages of radiation damage in the 2-4 element SP-CSAs containing Fe or Cr, well below 1 dpa. We arrive at this conclusion by following the evolution of positron annihilation signals as a function of irradiation dose in single crystal samples, complemented by molecular dynamics simulations in the same model systems for high entropy alloys (HEAs). This manifestation of short-range order calls attention to composition fluctuations at the atomic level in irradiated HEAs. Ion irradiation may induce short-range order in certain alloys due to chemically biased elemental diffusion. The work highlights the necessity of updating the assumption of a totally random arrangement in the irradiated alloys, even though the alloys before irradiation have random arrangements of different chemical elements. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd.

We present evidence of homogenization of atomic diffusion properties caused by C and N interstitials in an equiatomic single-phase high entropy alloy (FeMnNiCoCr). This phenomenon is manifested by an unexpected interstitial-induced reduction and narrowing of the directly experimentally determined migration barrier distribution of mono-vacancy defects introduced by particle irradiation. Our observation by positron annihilation spectroscopy is explained by state-of-the-art theoretical calculations that predict preferential localization of C/N interstitials in regions rich in Mn and Cr, leading to a narrowing and reduction of the mono-vacancy size distribution in the random alloy. This phenomenon is likely to have a significant impact on the mechanical behavior under irradiation, as the local variations in elemental motion have a profound effect on the solute strengthening in high entropy alloys. (C) 2021 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Publisher Copyright: © 2021 Author(s). Copyright: Copyright 2021 Elsevier B.V., All rights reserved. Positron annihilation spectroscopy, Fourier transform-infrared absorption spectroscopy, and secondary ion mass spectrometry have been used to study the behavior of gallium vacancy-related defects and hydrogen in deuterium (D) implanted and subsequently annealed β-Ga2O3 single crystals. The data suggest the implantation generates a plethora of VGa-related species, including VGa1- and VGa2-type defects. The latter’s contribution to the positron signal was enhanced after an anneal at 300 °C, which is driven by the passivation of VGaib by hydrogen as seen from infrared measurements. Subsequent annealing near 600 °C returns the positron signal to levels similar to those in the as-received samples, which suggests that split VGa-like defects are still present in the sample. The almost complete removal of the VGaib-2D vibrational line, the appearance of new weak O-D lines in the same spectral region, and the lack of D out-diffusion from the samples suggest that the 600 °C anneal promotes the formation of either D-containing, IR-inactive complexes or defect complexes between VGaib-2D and other implantation-induced defects. The degree of electrical compensation is found to be governed by the interactions between the Ga vacancies and hydrogen.

We have applied positron annihilation spectroscopy to study a wide range of β-Ga2O3bulk crystals and thin films with various doping levels. The Doppler broadening of the 511 keV positron-electron annihilation line exhibits colossal anisotropy compared to other three-dimensional crystalline semiconductors. State-of-the-art theoretical calculations of the positron characteristics in the β-Ga2O3lattice reveal that the positron state is effectively 1-dimensional, giving rise to strong anisotropy. Strongly relaxed split Ga vacancies are found to exhibit even stronger anisotropy and to dominate the positron annihilation signals in almost all experiments. The evidence leads to the conclusion that split Ga vacancies are abundant, with concentration of 1018 cm-3 or more, in β-Ga2O3samples irrespective of conductivity.

A unique experimental setup at the Accelerator Laboratory of the University of Helsinki enables in situ positron annihilation spectroscopy (PAS) analysis on ion irradiated samples. In addition, the system enables temperature control (10-300 K) of the sample both during irradiation and during subsequent positron annihilation measurements. Using such a system for defect identification and annealing studies comes with a plethora of possibilities for elaborate studies. However, the system also poses some restrictions and challenges to these possibilities, both related to irradiation and to the PAS analysis. This review tries to address these issues.

The influence of shear strain on the microstructural, physical, and mechanical properties was studied on large bulk samples (diameter: 30 mm, thickness: 1 or 8 mm), which were consolidated by high-pressure torsion (HPT) from a commercial powder DD0.7Fe3CoSb12. Particularly, the thick sample (mass similar to 53 g) allowed measuring the thermoelectric (TE) properties with respect to various orientations of the specimen in the sample. All data were compared with those of a hot-pressed (HP) reference sample, prepared with the same powder. Transmission electron microscopy, as well as X-ray powder diffraction profile analyses, Hall measurements, and positron annihilation spectroscopy, supported these investigations. Furthermore, synchrotron data for the temperature range from 300 to 825 K were used to evaluate the changes in the grain size and dislocation density as well as the thermal expansion coefficient via the change in the lattice parameter during heating. In addition, hardness and direct thermal expansion measurements of the HPT samples were performed and compared with the HP reference sample's values. With the increase of the shear strain from the center to the rim of the sample, the electrical resistivity becomes higher, whereas the thermal conductivity becomes lower, but the Seebeck coefficient remained almost unchanged. For the thin as well as thick samples, the enhanced electrical resistivity was balanced out by a decreased thermal conductivity such that the maximum ZT values (ZT = 1.3-1.35 at 856 K) do not vary much as a function of the shear strain throughout the sample, however, all ZTs are higher than that of the HP sample. The thermal-electric conversion efficiencies are in the range of 14-15% (for 423-823 K). With similar high ZT values for the n-type skutterudites, fabricated in the same fast and sustainable way, these p- and n-type skutterudites may serve as legs for TE generators, directly cut from the big HPT bulks.

We report a systematic first-principles study on positron annihilation parameters in the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ lattice and Ga monovacancy defects complemented with orientation-dependent experiments of the Doppler broadening of the positron-electron annihilation. We find that both the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ lattice and the considered defects exhibit unusually strong anisotropy in their Doppler broadening signals. This anisotropy is associated with low symmetry of the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ crystal structure that leads to unusual kind of one-dimensional confinement of positrons even in the delocalized state in the lattice. In particular, the split Ga vacancies recently observed by scanning transmission electron microscopy produce unusually anisotropic positron annihilation signals. We show that in experiments, the positron annihilation signals in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ samples seem to be often dominated by split Ga vacancies.

We report positron annihilation results on vacancy defects in Si-doped Al0.90Ga0.10N alloys grown by metalorganic vapor phase epitaxy. By combining room temperature and temperature-dependent Doppler broadening measurements, we identify negatively charged in-grown cation vacancies in the concentration range from below 1 x 10 16 cm(-3) to 2 x 10 18 cm(-3) in samples with a high C content, strongly correlated with the Si doping level in the samples ranging from 1 x 10 17 cm(-3) to 7 x 10 18 cm(-3). On the other hand, we find predominantly neutral cation vacancies with concentrations above 5 x 10 18 cm(-3) in samples with a low C content. The cation vacancies are important as compensating centers only in material with a high C content at high Si doping levels.

We have applied positron annihilation spectroscopy to study the formation of Ga vacancy related defects in Mg and Mn doped bulk GaN crystals grown by the ammonothermal method. We show that Mn doping has little or no effect on the formation of Ga vacancies, while Mg doping strongly suppresses their formation, in spite of both dopants leading to highly resistive material. We suggest the differences are primarily due to the hydrogen-dopant interactions. Further investigations are called for to draw a detailed picture of the atomic scale phe-nomena in the synthesis of ammonothermal GaN.

Using a combination of optical and electrical measurements, we develop a model for metastable defects in Ag-alloyed Cu(In,Ga)Se-2, one of the leading thin film photovoltaic materials. By controlling the pre-selenization conditions of the back contact prior to the growth of polycrystalline (Ag,Cu)(In,Ga)Se-2 absorbers and subsequently exposing them to various stresses (light soaking and dark-heat), we explore the nature and role of metastable defects on the electro-optical and photovoltaic performance of high-efficiency solar cell materials and devices. Positron annihilation spectroscopy indicates that dark-heat exposure results in an increase in the concentration of the selenium-copper divacancy complex (V-Se-V-Cu), attributed to depassivation of donor defects. Deep-level optical spectroscopy finds a corresponding increase of a defect at E-v+0.98eV, and deep-level transient spectroscopy suggests that this increase is accompanied by a decrease in the concentration of mid-bandgap recombination centers. Time-resolved photoluminescence excitation spectroscopy data are consistent with the presence of the V-Se-V-Cu divacancy complex, which may act as a shallow trap for the minority carriers. Light-soaking experiments are consistent with the V-Se-V-Cu optical cycle proposed by Lany and Zunger, resulting in the conversion of shallow traps into recombination states that limit the effective minority carrier recombination time (and the associated carrier diffusion length) and an increase in the doping density that limits carrier extraction in photovoltaic devices.

We show that $\mathrm{N}$-polar $\mathrm{Ga}\mathrm{N}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ heterostructures exhibit significant $\mathrm{N}$ deficiency at the bottom $(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ interface, and that these $\mathrm{N}$ vacancies are responsible for the trapping of holes observed in unoptimized $\mathrm{N}$-polar $\mathrm{Ga}\mathrm{N}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ high electron mobility transistors. We arrive at this conclusion by performing positron annihilation experiments on $\mathrm{Ga}\mathrm{N}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ heterostructures of both $\mathrm{N}$ and $\mathrm{Ga}$ polarity, as well as state-of-the-art theoretical calculations of the positron states and positron-electron annihilation signals. We suggest that the occurrence of high interfacial $\mathrm{N}$ vacancy concentrations is a universal property of nitride semiconductor heterostructures at net negative polarization interfaces.

Positron annihilation spectroscopy has been applied to study vacancy defects in Ga2O3. Both positron lifetime and Doppler broadening experiments have been performed, both in bulk single crystals and in thin films. The results show that Ga vacancy defects are efficiently formed in thin film growth and account for the electrical compensation in semi-insulating and highly resistive Ga2O3. Their concentrations are very low in n-type material. In (InxGa1−x)2O3 alloys, the nature and behavior of the cation vacancy defects change from Ga2O3-like to In2O3-like along with the crystalline phase. Further work is important to elucidate the details of the vacancy formation mechanisms and the origins of the exceptionally strongly anisotropic positron annihilation signals.

The thermal evolution of vacancies and vacancy clusters in tungsten (W) has been studied. W (100) single crystals were irradiated with 200 keV hydrogen (H) ions to a low damage level (5.8 x 10(-3) dpa) at 290 K and then annealed at temperatures in the range of 500-1800 K. The resulting defects were characterized by positron annihilation lifetime spectroscopy (PALS) and positron annihilation Doppler broadening spectroscopy (DBS). Annealing at 700 K resulted in the formation of clusters containing 10-15 vacancies, while at 800 K and higher temperatures clusters containing about 20 vacancies or more were formed. Reduction of the defect concentration likely accompanied by further coarsening of the clusters started at 1300 K and ended at 1800 K with the complete defect recovery. The determined cluster sizes at 700 K and 800 K were larger than the estimated minimum cluster sizes that are thermally stable at these temperatures, indicating that the migration and ensuing coalescence of small clusters plays an important role in cluster growth. (C) 2020 Max-Planck-Institut fur Plasmaphysik. Published by Elsevier B.V. All rights reserved.

Single crystals of oxygen-free copper oriented to easy glide of dislocations were tensile tested in order to study the hydrogen effects on the strain localisation in the form of slip bands appearin...

Positron annihilation spectroscopy and secondary ion mass spectrometry have been applied to study the evolution of polishing-induced defects in hydrothermally grown ZnO samples depending on the annealing temperature. Annealing of the as-grown ZnO wafer at 1200–1500 °C is found to lead to Li accumulation in the sub-surface layer, and significant reduction of Li content in the bulk. Polishing is shown to introduce vacancy complexes involving both V Zn and V O . Post-polishing annealing of hydrothermally grown ZnO with removed Li layer at 800 ° C reduces the concentration of polishing-induced defects below the detection limit.

Funding Information: This work was funded by the KACST-KAUST-UCSB Technology transfer program and the Solid State Lighting and Energy Electronics Center (SSLEEC) at UC Santa Barbara; a part of this work was carried out in the California NanoSystems Institute at UCSB. The research reported here made use of shared facilities of the UCSB MRSEC (Grant No. NSF DMR 1720256). A portion of this research was conducted in the UCSB nanofabrication facility, NSF NNIN network (Grant No. ECS-0335765). This work was partially funded by the Academy of Finland, Project No. 315082. The authors would like to gratefully thank Dr. Tom Mates for SIMS measurements and Dr. Stacia Keller and Dr. Mohammed Abo Alreesh for their insightful inputs. In addition, the authors would like to thank Dr. Youli Li from MRL and the cleanroom staff at the UCSB nanofabrication facility for the technical support provided. Publisher Copyright: © 2021 Author(s). The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been investigated using metalorganic chemical vapor deposition. Growth parameters including growth temperature, growth rate, and trimethylindium (TMI) flow have been systematically studied to minimize the resistivity of AlGaN:Si. We observed a strong anticorrelation between AlGaN:Si conductivity and growth temperature, suggesting increased silicon donor compensation at elevated temperatures. Secondary ion mass spectrometry and positron annihilation spectroscopy ruled out compensation by common impurities or group-III monovacancies as a reason for the observed phenomenon, in contrast to theoretical predictions. The underlying reason for AlGaN:Si resistivity dependence on growth temperature is discussed based on the possibility of silicon acting as a DX center in Al0.65Ga0.35N at high growth temperatures. We also show remarkable enhancement of AlGaN:Si conductivity by introducing TMI flow during growth. A minimum resistivity of 7.5 mΩ cm was obtained for n-type Al0.65Ga0.35N, which is among the lowest reported resistivity for this composition.

Positron annihilation Doppler broadening spectroscopy characterizes lattice point defects and is sensitive to very small vacancy densities. High-purity germanium detectors are generally used for recording the Doppler broadening spectrum because they provide good energy resolution and stability. However, the energy resolution of a germanium detector is somewhat dependent on the photon absorption geometry in the detector crystal. This change in the energy resolution changes also the Doppler broadening parameters. To observe the dependency of the resolution function and the Doppler broadening parameters, we performed experiments on Si samples in standard sandwich configuration with a Na-22 source. We changed the radiation geometry of the incident gamma photons via altering the distance of the sample-source package from the detector and by adding steel between the source and the detector. We observed the change of the absorption geometry in the germanium detector crystal by doing Monte Carlo simulations. The aim of this study is to help understand and decide what is the best way to compare the Doppler broadening parameters obtained with different measurement setups and even with the same setup when the geometry in the measurements has changed.

The charge compensation mechanisms in U 1-x Gd x O 2 and Th 1-x Gd x O 2-x/2 have been systematically studied using X-ray Absorption Spectroscopy (XAS) upon gradually increasing the Gd content. Gd doped nuclear fuels are widely used for optimizing the fresh core neutronics, yet when Gd 3+ is substituted into U 4+ or Th 4+ lattice position in UO 2 or ThO 2 , respectively, charge must be compensated for charge neutrality. In U 1-x Gd x O 2 the general hypothesis has been that the U 4+ will oxidise to U 5+ /U 6+ while in Th 1-x Gd x O 2-x/2 the fixed Th 4+ valence requires generation of O vacancies. Our XAS results for a series of technologically relevant Gd contents (x = 0.04 to 0.14) in U 1-x Gd x O 2 clearly demonstrate that upon increasing the Gd content U 5+ is formed inducing slight increase in the U coordination number and contraction for the U-O distances while the Gd local environment remains virtually unchanged. For the Th 1-x Gd x O 2-x/2 larger Gd fractions were applied (x = 0.10 to 0.34). Nonetheless, both Gd and Th local environments show changes upon increasing the Gd content; the average Gd-O and Th-O distances decrease gradually and the Gd and Th coordination numbers follow the expected trend considering the O vacancy formation to obtain charge neutrality. Finally, comparison to Gd 2 O 3 allowed us to propose that one of the Gd L 3 -edge XANES resonance features is directly connected to the generation of O vacancies.

Positron annihilation spectroscopy and thermal desorption spectroscopy experiments were combined to ascertain the role of hydrogen on generation of vacancies and vacancy clusters in Ni alloys. The effects of grain size and deformation temperature are emphasized for pure Ni single crystals and polycrystalline Ni-201 alloy samples with two grain sizes that were thermally pre-charged with 3000 appm hydrogen. Variation in positron lifetime and intensity suggests that hydrogen enhances and stabilizes vacancies and vacancy clusters. Additionally, grain boundaries and the regions adjacent to them are preferential sites for vacancy and cluster formation. Hydrogen-altered vacancies and vacancy clusters are manifest in yield behavior differences: uniform vacancy distributions augment strength increases after hydrogen charging; enhanced yield strength during cryogenic deformation is ascribed to an ‘Orowan type’ strengthening mechanism while cross-slip restriction dominates hardening behavior at room temperature.

The effect of radiation damage on the defect and alloy structure in InxGa1− xN thin films grown on Si substrates was studied using positron annihilation spectroscopy. Prior to the measurements, the samples were subjected to double He+ implantation at 40 and 100 keV. The results show the presence of cation vacancy-like defects in high concentrations (>1018 cm−3) already in the as-grown samples. The evolution of the annihilation characteristics after the implantation suggests strong alloy disorder rearrangement under irradiation. © 2017 Author(s)., V.P. and I.M. acknowledge financial support from the Academy of Finland (Project Nos. 285809 and 293932). This work has been partially supported by the Spanish MINECO under Contract No. MAT2015-71035-R.

In summary, PAS gives microscopic information about vacancy defects in semiconductors in the concentration range 10 15 -10 19 cm -3 . The positron lifetime is the fingerprint of the open volume associated with a defect, and it can be used to identify mono- and divacancies and larger vacancy clusters. Doppler broadening of the annihilation radiation, on the other hand, can be used to identify the nature of the atoms surrounding the vacancy. Consequently, vacancies on different sub lattices of a compound semiconductor can be distinguished, and impurities associated with the vacancies can be identified. The charge state of a vacancy defect can be determined by the temperature dependence of the positron -trapping coefficient, and positron localization into Rydberg states around negative centers yields information about ionic acceptors that have no open volume. Importantly, as shown in this chapter, the methods based on positron annihilation are not restricted by the nature or physical dimensions of the semiconductor. Defects can be studied in narrow- and wide-bandgap semiconductor materials in samples of any conductivity. Bulk crystals as well as thin films can be subjected to the experiments and defects identified.

The investigation and identification of point defects in GaN is crucial for improving the reliability of light-emitting and high-power electronic devices. The RY3 defect with a characteristic emission band at about 1.8 eV is often observed in photoluminescence (PL) spectra of $n$-type GaN grown by hydride vapor phase epitaxy, and it exhibits unusual properties. Its emission band consists of two components: a fast (10-ns lifetime) RL3 with a maximum at 1.8 eV and a slow (100--300 \ensuremath{\mu}s lifetime) YL3 with a maximum at 2.1 eV and zero-phonon line at 2.36 eV. In steady-state PL measurements, the YL3 component emerges with increasing temperature from 90 to 180 K, concurrently with a decrease in the RL3 intensity. The activation energy of both processes is about 0.06 eV. In time-resolved PL, the YL3 intensity abruptly rises when the RL3 intensity begins to saturate. These and other phenomena can be explained using a model of an acceptor with two excited states. A delocalized, effective-mass state at about 0.2 eV above the valence band captures photogenerated holes. These holes transition to the ground state, which produces the RL3 component with a lifetime of \ensuremath{\sim}10 ns. Alternatively, they may nonradiatively transition over a 0.06 eV-high barrier to a localized excited state with a level at 1.13 eV above the valence band. Recombination of free electrons or electrons at shallow donors with the holes at this localized excited state is responsible for the YL3 component. The relative intensities of the RL3 and YL3 components are dictated by the probabilities of holes at the shallow excited state to transition to the ground or to the localized excited states. Transition metals and complex defects are considered as the main candidates for the RY3 center.

We report positron annihilation spectroscopy measurements revealing the prevalence of VSe-VCu divacancy defects in reactively sputtered ACIGS solar cells. Together with compositional and structural analysis as well as capacitance-voltage measurements, an intricate interplay is observed between divacancies, (Cu+Ag)/III ratio, grain size, and carrier concentration. These properties can be tuned during absorber growth by varying growth temperature and alkali content. Exploiting this interplay may be the key to achieving high efficiency in ACIGS solar cells.

In this work, pulsed photoionization as well as photoluminescence and positron annihilation spectroscopy were combined to detect different species of defects. The GaN crystals, grown by the ammono-thermal method, doped with Mn as well as Mg impurities and irradiated with different fluences of reactor neutrons, were examined to clarify the role of the technological and radiation defects. The evolution of the prevailing photoactive centres was examined by pulsed photoionization spectroscopy. Positron annihilation spectroscopy was applied to reveal vacancy-type defects.

We have applied positron annihilation spectroscopy to study vacancy-type defects in unintentionally doped and Si and Sn doped β-Ga 2 O 3 homoepitaxial thin films grown by metal-organic chemical vapor deposition (MOCVD). We detect Ga vacancy related defects at high concentrations in semi-insulating and highly resistive material, while conductive (ntype) material exhibits very low Ga vacancy concentrations. These findings show that Ga vacancies can act as efficient electrical compensators for n-type conductivity, but their concentrations can be suppressed by controlling the growth environment, leading to efficient n-type doping. We also note the strong anisotropy of the positron annihilation signals and give recommendation for presenting positron data obtained in β-Ga 2 O 3 .

The influence of Fermi level position and annealing ambient on the zinc vacancy ${V}_{\mathrm{Zn}}$ generation and Al diffusion is studied in monocrystalline zinc oxide (ZnO). From secondary-ion mass spectrometry and positron annihilation spectroscopy results, a quadratic dependence between the concentrations of ${V}_{\mathrm{Zn}}$ and Al is established, demonstrating the Fermi level dependence of the formation of the electrically compensating $\ensuremath{-}2$ charge state of ${V}_{\mathrm{Zn}}$ in conductive $n$-type ZnO crystals. In contrast, thermal treatment in the zinc-rich ambient is shown to efficiently reduce the ${V}_{\mathrm{Zn}}$ concentration and related complexes. Using a reaction-diffusion model, the diffusion characteristics of Al at different donor background concentrations are fully accounted for by mobile (${\mathrm{Al}}_{\mathrm{Zn}}{V}_{\mathrm{Zn}}{)}^{\ensuremath{-}}$ pairs. These pairs form via the migration and reaction of isolated ${V}_{\mathrm{Zn}}^{2\ensuremath{-}}$ with the essentially immobile ${\mathrm{Al}}_{\mathrm{Zn}}^{+}$. We obtain a migration barrier for the ${({\mathrm{Al}}_{\mathrm{Zn}}{V}_{\mathrm{Zn}})}^{\ensuremath{-}}$ pair of $2.4\ifmmode\pm\else\textpm\fi{}0.2$ eV, in good agreement with theoretical predictions. In addition to strongly alter the shape of the Al diffusion profiles, increasing the donor background concentration also results in an enhanced effective Al diffusivity, attributed to a reduction in the ${\mathrm{V}}_{\mathrm{Zn}}^{2\ensuremath{-}}$ formation energy as the Fermi level position increases.

We report a positron annihilation study using state-of-the-art experimental and theoretical methods in n-type and semi-insulating β - Ga 2 O 3. We utilize the recently discovered unusually strong Doppler broadening signal anisotropy of β - Ga 2 O 3 in orientation-dependent Doppler broadening measurements, complemented by temperature-dependent positron lifetime experiments and first principles calculations of positron-electron annihilation signals. We find that split Ga vacancies dominate the positron trapping in β - Ga 2 O 3 single crystals irrespective of the type of dopant or conductivity, implying concentrations of at least 1 × 1 0 18 c m - 3.

We have determined positron annihilation characteristics (lifetime and Doppler broadening) in six basic vacancy-type defects of $6H$-SiC and two nitrogen-vacancy complexes using ab initio calculations. The positron characteristics obtained allow us to point out which positron technique in the most adapted to identify a particular defect. They show that the coincidence Doppler broadening technique is the most relevant for observing the silicon vacancy--nitrogen complexes, ${\mathrm{V}}_{\mathrm{Si}}{\mathrm{N}}_{\mathrm{C}}$, and carbon vacancy--carbon antisite ones, ${\mathrm{V}}_{\mathrm{C}}{\mathrm{C}}_{\mathrm{Si}}$. For the other studied defects, the calculated positron characteristics are found to be too close for the defects to be easily distinguished using a single positron annihilation technique. Then it is required to use complementary techniques, positron annihilation based or other.

The behavior of a high density radical source for the plasma assisted molecular beam epitaxy of GaN and AlGaN compounds is studied and compared with the one of a conventional plasma source. Plasma light emission correlates with the GaN growth rate. Both attest to the better efficiency of the new source for producing active nitrogen species with resulting growth rates well beyond 1 µm/h. The present study shows that GaN films with equivalent structural and optical quality can be grown even with a growth rate enhancement by a factor of 5. The purity of the grown films is investigated as well as point defects. Positron annihilation shows that plasma conditions can be tuned in order to limit the increase of the gallium-vacancy related complexes density by about 2x10 16 cm −3 while reaching growth rates as high as 2.1 µm/h.

We present a systematic study of the positron lifetime as a function of measurement temperature in strontium titanate ( SrTiO 3) single crystals grown in different conditions and by different synthesis methods. We combine our experimental results with state-of-the-art theoretical calculations of positron annihilation parameters. We find that the essentially omnipresent 180-190ps lifetime component is most likely the Ti Sr antisite defect, possibly coupled with one or more oxygen vacancies, supporting the importance of the Ti Sr antisite related defects in SrTiO 3.

The ENEN+ project proposes cost-effective actions to attract, develop and retain new talents in nuclear professions. This is a contribution of the ENEN Association, supported by the European Commission, to the common strategic goal of all nuclear stakeholders: to preserve, maintain and further develop the valuable nuclear knowledge for todays and future generations. The ENEN+ project focuses on learners and careers in nuclear reactor engineering and safety, waste management and geological disposal, radiation protection and medical applications. This paper outlines the most important expected contributions of the project and the approaches designed to deliver them.

We show that Be exhibits amphoteric behavior in GaN, involving switching between substitutional and interstitial positions in the lattice. This behavior is observed through the dominance of Be_{Ga} in the positron annihilation signals in Be-doped GaN, while the emergence of V_{Ga} at high temperatures is a consequence of the Be impurities being driven to interstitial positions. The similarity of this behavior to that found for Na and Li in ZnO suggests that this could be a universal property of light dopants substituting for heavy cations in compound semiconductors.

The process of oxidation in zirconium alloys is not straightforward. It includes cyclic periods, where the oxidation rate is decelerating as a function of increasing oxide thickness. Transition to the higher oxidation rate happens approximately after every 2 μm oxide growth and each cycle is associated with a layer of oxide with little or no cracking and a similar thickness layer with a large number of micro-scale lateral cracks. This work uses positron annihilation Doppler broadening spectroscopy to investigate the development of atomic-scale defect features in the oxides on Zircaloy-4 samples exposed to alkaline water at 350 °C. Results from the Doppler broadening measurements supported by Density Functional Theory calculations indicate that complexes involving both zirconium-site and oxygen-site vacancies exist in the oxide. The implications of the vacancies in the oxide layer are considered for oxygen diffusion and the oxidation rate.

The role of the excessive metal vacancy generation in transgranular stress corrosion cracking (TGSCC) of pure copper was studied in relation to crack initiation and growth mechanisms. Electrochemically polarized specimens were strained at room temperature in 0.3 m NaNO2 solution. Cation vacancy redistribution under applied anodic polarization took place at the p-n junction of the duplex Cu2O oxide film, resulting in cation vacancy transport in the p-type layer of the oxide to the metal substrate interface and in excessive metal vacancy accumulation in the metal substrate. The rapid initiation of TGSCC at room temperature in the cold-worked oxygen-free high-conductivity copper samples occurred during slow strain rate tests under anodic polarization in 0.3 m NaNO2 solution. The duplex Cu2O oxide film structure formed by cathodic deposition, before applying the anodic polarization, was characterized with X-ray diffraction and field emission scanning electron microscopy/electron backscatter diffraction/energy-dispersive spectroscopy. The redistribution of cation vacancies in the p-type oxide phase due to the anodic polarization and the metal vacancy accumulation in the copper substrate interface was studied by using positron annihilation spectroscopy.

Understanding the mechanisms of damage formation in materials irradiated with energetic ions is essential for the field of ion-beam materials modification and engineering. Utilizing incident ions, electrons, photons, and positrons, various analysis techniques, including Rutherford backscattering spectrometry (RBS), electron RBS, Raman spectroscopy, high-resolution X-ray diffraction, small-angle X-ray scattering, and positron annihilation spectroscopy, are routinely used or gaining increasing attention in characterizing ion beam modified materials. The distinctive information, recent developments, and some perspectives in these techniques are reviewed. Applications of these techniques are discussed to demonstrate their unique ability for studying ion-solid interactions and the corresponding radiation effects in modified depths ranging from a few nm to a few tens of μm, and to provide information on electronic and atomic structure of the materials, defect configuration and concentration, as well as phase stability, amorphization and recrystallization processes. Such knowledge contributes to our fundamental understanding over a wide range of extreme conditions essential for enhancing material performance and also for design and synthesis of new materials to address a broad variety of future energy applications.

Photoluminescence (PL) was used to estimate the concentration of point defects in GaN. The results are compared with data from positron annihilation spectroscopy (PAS), secondary ion mass spectrometry (SIMS), and deep level transient spectroscopy (DLTS). Defect-related PL intensity in undoped GaN grown by hydride vapor phase epitaxy increases linearly with the concentration of related defects only up to 1016 cm−3. At higher concentrations, the PL intensity associated with individual defects tends to saturate, and accordingly, does not directly correlate with the concentration of defects. For this reason, SIMS analysis, with relatively high detection limits, may not be helpful for classifying unidentified point defects in GaN. Additionally, we highlight challenges in correlating defects identified by PL with those by PAS and DLTS methods.