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We have applied positron annihilation spectroscopy to study the vacancy-type defects in β-Ga2O3 single crystals. The three different types of crystals were prepared by Czochralski and edge-defined film-fed growth and doped with Fe, Mg, and Sn for semi-insulating and n-type characteristics. The crystals were also subjected to 6-MeV proton irradiation for controlled introduction of mono-vacancy defects. Positron lifetime and the details of the anisotropy of the Doppler broadening signals were measured as a function of temperature, and the results were compared with the annihilation signals predicted by theoretical calculations. We find Ga vacancies in all three basic split Ga vacancy configurations to dominate the positron data in the as-grown crystals. In contrast, unrelaxed Ga vacancies are found as the main defect introduced by the irradiation. [ABSTRACT FROM AUTHOR]

We have applied positron annihilation spectroscopy to study the vacancy-type defects in β-Ga2O3 single crystals. The three different types of crystals were prepared by Czochralski and edge-defined film-fed growth and doped with Fe, Mg, and Sn for semi-insulating and n-type characteristics. The crystals were also subjected to 6-MeV proton irradiation for controlled introduction of mono-vacancy defects. Positron lifetime and the details of the anisotropy of the Doppler broadening signals were measured as a function of temperature, and the results were compared with the annihilation signals predicted by theoretical calculations. We find Ga vacancies in all three basic split Ga vacancy configurations to dominate the positron data in the as-grown crystals. In contrast, unrelaxed Ga vacancies are found as the main defect introduced by the irradiation. [ABSTRACT FROM AUTHOR]

This Perspective focuses on experimental and theoretical aspects of positron annihilation spectroscopy. This set of methods is highly suitable for identifying and quantifying vacancy-type defects in semiconductors and also allows for analyzing their physics characteristics. We present selected examples from the past decade, where the methods have been used for obtaining timely and useful insights into the defect-controlled phenomenon in narrow-gap (Ge, GaSb) and wide-gap (III-nitride, oxide) semiconductors. We also discuss possible future developments that may allow more detailed studies in novel semiconductor materials and devices with ever more complex lattice structures. [ABSTRACT FROM AUTHOR]

We report positron annihilation results on in-grown and proton-irradiation-induced vacancy defects in AlN single crystals grown by physical vapor transport. The samples were irradiated with 100 keV H+ ions to varying fluences in the range of 5 × 1014 − 2 × 1018 ions cm–2. Doppler broadening of annihilation radiation was recorded in as-grown and irradiated samples with a slow positron beam with varying implantation energy. Doppler results combined with first principles theoretical calculations show that the 100 keV H+ irradiation introduces isolated VAl on the ion track and VN-rich vacancy clusters at the end of the ion range. The results suggest that the excess amount of detected VN originates from a high concentration of in-grown VN. So far, these defects have been considered to be unidentified negative ion-like defects in AlN. [ABSTRACT FROM AUTHOR]

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 180–190 ps 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. [ABSTRACT FROM AUTHOR]

Positron annihilation spectroscopy in the Doppler and coincidence Doppler mode was applied on Ge 1 − x Sn x epitaxial layers, grown by chemical vapor deposition with different total As concentrations (∼ 10 19 – 10 21 cm − 3 ), high active As concentrations (∼ 10 19 cm − 3 ), and similar Sn concentrations (5.9%–6.4%). Positron traps are identified as mono-vacancy complexes. Vacancy-As complexes, V-As i , formed during the growth were studied to deepen the understanding of the electrical passivation of the Ge 1 − x Sn x :As epilayers. Larger mono-vacancy complexes, V-As i (i ≥ 2), are formed as the As doping increases. The total As concentration shows a significant impact on the saturation of the number of As atoms (i = 4) around the vacancies in the sample epilayers. The presence of V-As i complexes decreases the dopant activation in the Ge 1 − x Sn x :As epilayers. Furthermore, the presence of Sn failed to hinder the formation of larger V-As i complexes and thus failed to reduce the donor-deactivation. [ABSTRACT FROM AUTHOR]

Owing to their excellent radiation tolerance, some of the high-entropy alloys (HEAs) are considered as potential candidates for structural materials in extreme conditions. In order to shed light on the early-stage irradiation damage in HEAs, we performed positron annihilation spectroscopy on hydrogen implanted equiatomic FeMnNiCoCr and interstitial carbon-containing FeMnNiCoCr HEAs. We reveal primary damage as monovacancies in low dose irradiated HEAs. The enhancement of Frenkel pair recombination by C addition is observed in C-containing HEAs. In addition, the C interstitials suppress the vacancy cluster formation in high dose irradiated HEAs. [ABSTRACT FROM AUTHOR]

We examined the vacancy trapping proficiency of Sn and P atoms in germanium using positron annihilation spectroscopy measurements, sensitive to the open-volume defects. Epitaxial Ge 1 − x Sn x films were grown by chemical vapor deposition with different P concentrations in the ∼ 3.0 × 10 19 – 1.5 × 10 20 cm − 3 range. We corroborate our findings with first principles simulations. Codoping of Ge with a Sn concentration of up to 9% is not an efficient method to suppress the free vacancy concentration and the formation of larger phosphorus–vacancy complexes. Experimental results confirm an increase in the number of P atoms around the monovacancy with P-doping, leading to dopant deactivation in epitaxial germanium–tin layers with similar Sn content. Vice versa, no impact on the improvement of maximum achieved P activation in Ge with increasing Sn-doping has been observed. Theoretical calculations also confirm that P n -V (vacancy) complexes are energetically more stable than the corresponding Sn m P n -V and Sn m -V defect structures with the same number of alien atoms (Sn or P) around the monovacancy. The strong attraction of vacancies to the phosphorus atoms remains the dominant dopant deactivation mechanism in Ge as well as in Ge 1 − x Sn x. [ABSTRACT FROM AUTHOR]

The E centers (dopant-vacancy pairs) play a significant role in dopant deactivation in semiconductors. In order to gain insight into dopant-defect interactions during epitaxial growth of in situ phosphorus doped Ge, positron annihilation spectroscopy, which is sensitive to open-volume defects, was performed on Ge layers grown by chemical vapor deposition with different concentrations of phosphorus (∼ 1 × 10 18 – 1 × 10 20 cm − 3 ). Experimental results supported by first-principles calculations based on the two component density-functional theory gave evidence for the existence of mono-vacancies decorated by several phosphorus atoms as the dominant defect type in the epitaxial Ge. The concentration of vacancies increases with the amount of P-doping. The number of P atoms around the vacancy also increases, depending on the P concentration. The evolution of P n –V clusters in Ge contributes significantly to the dopant deactivation. [ABSTRACT FROM AUTHOR]

The correlated motion of a positron surrounded by electrons is a fundamental many-body problem. We approach this by modeling the momentum density of annihilating electron-positron pairs using the framework of reduced density matrices, natural orbitals, and natural geminals (electron-positron pair wave functions) of the quantum theory of many-particle systems. We find that an expression based on the natural geminals provides an exact, unique, and compact expression for the momentum density. The natural geminals can be used to define and to determine enhancement factors for enhancement models going beyond the independent-particle model for a better understanding of the results of positron annihilation experiments. [ABSTRACT FROM AUTHOR]

Tungsten is considered to be used in the future fusion reactors as plasma-facing material. In such extreme environments, defects are induced in materials that modify their macroscopic properties such as the mechanical ones. It is of paramount importance to be able to determine concentration and size of the vacancy defects, from the mono vacancy to the large cavities, to validate the models developed to predict the evolution of the microstructure of irradiated materials. Positrons are very useful non-destructive probes that can characterize vacancy-type defects in materials. We present a combined experimental and theoretical study on detecting and estimating the sizes of vacancy clusters that are invisible with electron microscopy in tungsten, using positron annihilation spectroscopy. We here model the positron annihilation in the tungsten lattice and in vacancy-type defects using state-of-the-art first principles methodology. The Doppler broadening spectra and positron lifetimes in tungsten are calculated with two-component density functional theory with local density approximation and weighted density approximation. Our calculations are in excellent agreement with our experimental results. We show that the sizes of vacancy clusters in tungsten can be well estimated by combining both positron lifetimes and Doppler broadening spectra. We also determine the limit of validity of the canonical calculation method, which here is shown to fail when the vacancy clusters grow beyond their nucleation stage. This work is a first step needed to better interpret the measured positron annihilation characteristics (Doppler and lifetime) in tungsten and then extract quantitative data on small vacancy defects required to improve the understanding of early-stage vacancy defect evolution in tungsten. The method used in this paper could be used to study other metallic materials. [ABSTRACT FROM AUTHOR]

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. [Display omitted] [ABSTRACT FROM AUTHOR]