Low-energy Positron Annihilation Research Articles

Vacancy complexes in nonequilibrium germanium-tin semiconductors

Depth-profiled pulsed low-energy positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy were combined to identify vacancy-related complexes and probe their evolution as a function of Sn content in GeSn epitaxial layers. Regardless of the Sn content in the 6.5–13.0 at. % range, all GeSn samples showed the same depth-dependent increase in the positron annihilation line broadening parameters, relative to that of epitaxial and bulk Ge references, thus confirming the formation of open volume defects during growth. The measured average positron lifetimes were found to be the highest (380–395 ps) in the region near the surface and monotonically decrease across the analyzed thickness but remain above 350 ps. All GeSn layers exhibit average lifetimes that are 20–160 ps higher than those recorded for the Ge reference. Surprisingly, these lifetimes were found to decrease as the Sn content increases in GeSn layers. These measurements indicate that divacancies are the dominant defect in the as-grown GeSn layers. However, their corresponding lifetime was found to be shorter than in epitaxial Ge, thus suggesting that the presence of Sn may alter the structure of divacancies. Additionally, GeSn layers were also found to contain a small fraction of vacancy clusters, which become less important as the Sn concentration increases. The interaction and possible pairing between Sn and vacancies have been proposed to explain the reduced formation of larger vacancy clusters in GeSn when the Sn content increases.

Subnanopore filling during water vapor adsorption on microporous silica thin films as seen by low-energy positron annihilation

Positron annihilation lifetime spectroscopy (PALS) using a low-energy positron microbeam extracted into air was applied to elucidating molecular-level pore structures formed in silicon-oxide-backboned microporous thin films under controlled humidity conditions; as a result, a direct observation of the interstitial spaces in the micropores filled with water molecules was achieved. It was demonstrated that PALS using a microbeam extracted into air in combination with water vapor adsorption is a powerful tool for the in-situ elucidation of both open and closed subnanoscaled pores of functional thin materials under practical conditions.

Mode coupling and multiquantum vibrational excitations in Feshbach-resonant positron annihilation in molecules

The dominant mechanism of low-energy positron annihilation in polyatomic molecules is through positron capture in vibrational Feshbach resonances (VFR). In this paper we investigate theoretically the effect of anharmonic terms in the vibrational Hamiltonian on the positron annihilation rates. Such interactions enable positron capture in VFRs associated with multiquantum vibrational excitations, leading to enhanced annihilation. Mode coupling can also lead to faster depopulation of VFRs, thereby reducing their contribution to the annihlation rates. To analyze this complex picture, we use coupled-cluster methods to calculate the anharmonic vibrational spectra and dipole transition amplitudes for chloroform, chloroform-$d_1$, 1,1-dichloroethylene, and methanol, and use these data to compute positron resonant annihilation rates for these molecules. Theoretical predictions are compared with the annihilation rates measured as a function of incident positron energy. The results demonstrate the importance of mode coupling in both enhancement and suppression of the VFR. There is also experimental evidence for the direct excitation of multimode VFR. Their contribution is analyzed using a statistical approach, with an outlook towards more accurate treatment of this phenomenon.

Effect of heat treatment on the nanoporosity of silica PECVD films elucidated by low-energy positron annihilation and ellipsometric porosimetry

The nanoporosity of silica thin films, prepared through plasma enhanced chemical vapor deposition with different flow rates of tetraethyl orthosilicate, was investigated by means of vapor-adsorption ellipsometric porosimetry (EP) and pulsed, low-energy positron annihilation lifetime spectroscopy (PALS). The effect of heat treatment on the subnano-scaled pore structure was observed via changes in the porosity and the pore dimension, obtained from EP and PALS, respectively. The open porosity of the as-deposited films, elucidated from the methanol (MeOH) adsorption isotherms at 26 °C, was found to range up to 5%. After annealing, the open porosity was reduced to almost zero, whereas the film total porosity was not significantly changed. This suggests that bottlenecks connecting pores near the film surface were developed by heat treatment, so that those bottlenecks prevented the MeOH molecules from diffusing into the open pores. On the other hand, the PALS results indicated that subnano-scaled pores of the films were enlarged along with the heat treatment; as a result, their sizes, evaluated from the lifetimes of ortho-positronium, were in the range of 0.31 nm to 0.37 nm in radius. These results from EP and PALS signified that the heat treatment enhanced the structural non-uniformity with respect to nanoporosity of the silica films.

Reply to “Comment on ‘Gamma-ray spectra from low-energy positron annihilation processes in molecules’ ”

Reply to “Comment on ‘Gamma-ray spectra from low-energy positron annihilation processes in molecules’ ”

Comment on “Gamma-ray spectra from low-energy positron annihilation processes in molecules”

In the article by Ma~\emph{et al.}~[Phys.~Rev.~A {\bf 94}, 052709 (2016)], $\gamma$-ray spectra for positron annihilation on molecules were calculated in the independent-particle approximation with the positron wavefunction set to unity. Based on comparisons with experimental data they concluded that inner valence electrons play a dominant role in positron annihilation. These conclusions are incorrect and resulted from fallacious analysis that ignored the known effect of the positron wavefunction on the spectra.

Enhanced Magnetization of Cobalt Defect Clusters Embedded in TiO2−δ Films

High magnetizations are desirable for spintronic devices that operate by manipulating electronic states using built-in magnetic fields. However, the magnetic moment in promising dilute magnetic oxide nanocomposites is very low, typically corresponding to only fractions of a Bohr magneton for each dopant atom. In this study, we report a large magnetization formed by ion implantation of Co into amorphous TiO2-δ films, producing an inhomogeneous magnetic moment, with certain regions producing over 2.5 μB per Co, depending on the local dopant concentration. Polarized neutron reflectometry was used to depth-profile the magnetization in the Co:TiO2-δ nanocomposites, thus confirming the pivotal role of the cobalt dopant profile inside the titania layer. X-ray photoemission spectra demonstrate the dominant electronic state of the implanted species is Co0, with a minor fraction of Co2+. The detected magnetizations have seldom been reported before and lie near the upper limit set by Hund's rules for Co0, which is unusual because the transition metal's magnetic moment is usually reduced in a symmetric 3D crystal-field environment. Low-energy positron annihilation lifetime spectroscopy indicates that defect structures within the titania layer are strongly modified by the implanted Co. We propose that a clustering motif is promoted by the affinity of the positively charged implanted species to occupy microvoids native to the amorphous host. This provides a seed for subsequent doping and nucleation of nanoclusters within an unusual local environment.

Investigation of graphene using low energy positron annihilation induced Doppler broadening spectroscopy

We report the first measurements on 6-8 layers graphene using a low energy positron beam employing a high efficiency rare gas moderator. Doppler broadening of the annihilation gamma was measured from graphene over layers at positron kinetic energies as low as 2 eV. The efficient trapping of positrons on the surface state of graphene at these low energies has been utilised to get Doppler broadened gamma spectra predominantly from graphene over layer on Cu. The ratio of the annihilation gammas measured at low positron energies (a few eV) to that measured at 20 keV show features corresponding to carbon. W-parameter calculated from gamma spectra at different positron energies point towards trapping of positrons in defected graphene as well as at the interface of graphene over layer and Cu. This is the first ever measurements of graphene film of 2-3 nm thickness on a substrate with a depth resolved Doppler broadening spectroscopy at positron kinetic energies below 10 eV.

Nanopore structure relevant to the D2O permeation into silica thin films as studied by secondary ion mass spectrometry, ellipsometric porosimetory and positron annihilation

Subnanoscopic pore structure relevant to the water permeation for two types of silica thin films, fabricated by plasma-enhanced chemical vapor deposition (PECVD) and thermal oxidization (TO), were examined by means of dynamic secondary ion mass spectrometry (D-SIMS), vapor-adsorption ellipsometric porosimetry (EP), and low-energy positron annihilation spectroscopy (PALS). The D− secondary ion intensity for the PECVD film, observed using D-SIMS, was much higher than for the TO film, indicative of the higher permeance of D2O molecules in the matrix of the PECVD film. The results from PALS and EP suggest that this is ascribed to the larger pores and the higher open porosity of the PECVD film as compared to the TO film.

Gamma-ray spectra from low-energy positron annihilation processes in molecules

The theoretical $\ensuremath{\gamma}$-ray spectra from positron-electron annihilation process in a wide variety of atoms and molecules are studied. The theoretical Doppler broadened $\ensuremath{\gamma}$-ray spectra for core, inner valence, and outer valence electrons of these molecules are studied systematically. The present results show that the experimental $\ensuremath{\gamma}$-ray spectra agree well with inner valence electrons, especially the lowest occupied valence orbital electrons rather than the outer valence electrons. These inner valence electrons show a strong correlation with the corresponding available experimental measurements. These findings are interesting because the outer valence electrons have been supposed to have larger probability annihilating with positrons rather than the inner valence electrons. The present work also suggests that the accurate positron wave functions must be involved to identify the dominance of the bound electrons clearly in positron-electron annihilation process.

Resolving the electron-positron mass annihilation mystery

<p class="abstract"><span lang="EN-US">Mutual annihilation of low energy electrons and positrons results in two photons of equal energy. The annihilation is consistent with charge conservation but both particles have positive mass, so how do two positive masses annihilate? The issue is resolved by considering particles electromagnetic (EM) energy localized by curvature of the space-time metric. The curvature extends into the surrounding metric forming the particle’s gravitational field, usually attributed as due to mass by the observer, but only the curved space-time metric exists. In principle both positive and negative metric curvatures could exist and display positive and negative masses respectively, but both would possess positive energy. For the electron EM energy circulates in the observer domain and outside but close to an event horizon (EH), the positive metric curvature results in the impression of positive mass. Symmetry suggests positron energy circulates inside an EH and should have negative curvature. It is posited metric field curvature reverses on passage through an event horizon, thus the positron positive mass apparent to the observer arises from negative metric curvature inside the particle EH. The opposite metric curvatures of the electron and positron cancel on annihilation, eliminating their gravitational effects and thereby their apparent masses.</span></p>

The improvement of Si0.5Ge0.5/Si interface quality by using low energy hydrogen plasma cleaning process and positron annihilation spectroscopy

Positron Annihilation Spectra (PAS), Raman, and photoluminescence spectroscopy reveal that Si 0.5 Ge 0.5 /Si interface quality can be dramatically improved through a low energy plasma cleaning process using hydrogen. In the PAS, the particularly small values of lifetime and intensity near the Si 0.5 Ge 0.5 /Si interface in the treated sample indicate a 2.25 times reduction in defect concentration. Fewer defects were found in the interface, resulting in a high compressive strain of about 0.36 % in the top layer, which can be observed in Raman spectra, and a 1.39 times increase to the radiative recombination rate for the infrared emission, which can be observed in the photoluminescence spectra. Improved Si 0.5 Ge 0.5 /Si interface quality leads to improved optical and electrical characteristics in SiGe-based devices in a broader range of photovoltaic applications. The PAS is also shown to be a useful metrology tool for quantifying defects in SiGe-based devices.

Transport theory for low-energy positron thermalization and annihilation in helium

A transport theory that explicitly incorporates loss of flux due to annihilating collisions is developed and applied to low-energy positron diffusion and annihilation. The use of more complete momentum transfer and annihilation cross sections for helium has resulted in improved descriptions of the time dependence of $\ensuremath{\langle}{Z}_{\mathrm{eff}}\ensuremath{\rangle}$ for positrons injected into gaseous helium. Similarly, the variation of $\ensuremath{\langle}{Z}_{\mathrm{eff}}\ensuremath{\rangle}$ versus $E/{n}_{0}$ for experiments where the annihilation region is immersed in an electric field is in closer agreement with experimental data. Inclusion of loss of flux due to annihilation was found to have a very small effect on the derived $\ensuremath{\langle}{Z}_{\mathrm{eff}}(t)\ensuremath{\rangle}$ for helium.

Calculation of transport cross sections for positrons in solid targets via improved expressions

Transport cross sections (TCS) for positron impinging on a number of selected atomic targets have been calculated in the energy range 1–4 keV. The calculations were performed by using a simplified correction of the approximate analytical expression of transport cross sections reported by Jablonski [A. Jablonski, Phys. Rev. B 58 (1998) 16470]. An argued parameter–obtained from both an exponential and polynomial fits–was then introduced. The agreement generally observed between our results and those previously reported in the literature is quite good, especially for positron energies greater than 2 keV. Furthermore, an attempt has been made for scaling the TCS with the atomic number of solid targets of interest. Such a scaling was found to be possible for both exponential and third order polynomial fits. The information gathered by the present study may be useful for the evaluation of parameters needed for quantitative low-energy positron annihilation spectroscopy.

Influence of cracks generation on the structural and optical properties of GaN/Al 0.55Ga 0.45N multiple quantum wells

Both cracked and crack-free GaN/Al 0.55Ga 0.45N multiple quantum wells (MQWs) grown on GaN template by metalorganic chemical vapor deposition have been studied by triple-axis X-ray diffraction, grazing-incidence X-ray reflectivity, atomic force microscope, photoluminescence spectroscopy and low-energy positron annihilation spectroscopy. The experimental results show that cracks generation not only deteriorates the surface morphology, but also leads to a period dispersion and roughens the interfaces of MQWs. The mean density of dislocations in MQWs, determined from the average full-width at half-maximum of ω-scan of each satellite peak, has been significantly enhanced by the cracks generation. Furthermore, the measurement of annihilation-line Doppler broadening reveals a higher concentration of negatively charged vacancies in the cracked MQWs. The combination of these vacancies and the high density of edge dislocations are assumed to contribute to the highly enhanced yellow luminescence in the cracked sample.

Formation of defect structures in Au-polysilane interfaces probed by low energy positron beams

Formation and accumulation of defect structures at interfaces between polysilanes and vacuum-evaporated gold (Au) electrodes are discussed quantitatively by low energy positron annihilation spectroscopy. The size distribution of the defects at the interface is analyzed based on the values of ortho-positronium ( o-Ps) lifetime ( τ 3), and no effect of the evaporation process is observed in the polymer films. The intensity of o-Ps ( I 3) indicates no considerable change before and after Au evaporation on dialkyl-substituted polysilanes, however, the values of I 3 is increased ∼20% in phenyl-substituted polysilane (PMPS) by the evaporation. The I 3 dependence on incident positron energy suggests the formation of the defects not only at an Au-PMPS interface but also in PMPS bulk phase as deep as 400 nm from the interface. Phenyl ring dissociation from the polymer backbone will play a significant role in the selective formation of the defects in PMPS. This is the first report on the direct measurement of defect structures at conjugated polymer–metal interface with non-destructive way, implying that electrode fabrication by vacuum evaporation affects the solid state structure of polymers.

Vibrational-resonance enhancement of positron annihilation in molecules.

The rate of annihilation of low-energy positrons in many molecular gases is orders of magnitude larger than can be explained on the basis of simple collisions. Developments in positron beam technology have enabled the first energy-resolved measurements of this annihilation process. The results of these experiments provide direct evidence that the large observed values of annihilation rate are due to the excitation of long-lived vibrational resonances of the positron-molecule complex. These results are generally consistent with a recent theoretical model of resonant annihilation.

Mechanisms of positron annihilation on molecules

The aim of this work is to identify the mechanisms responsible for very large rates and other peculiarities observed in low-energy positron annihilation on molecules. The two mechanisms considered are the following: (i) Direct annihilation of the incoming positron with one of the molecular electrons. This mechanism dominates for atoms and small molecules. I show that its contribution to the annihilation rate can be related to the positron elastic scattering cross section. This mechanism is characterized by a strong energy dependence of ${Z}_{\mathrm{eff}}$ at small positron energies and high ${Z}_{\mathrm{eff}}$ values (up to ${10}^{3})$ for room-temperature positrons, if a low-lying virtual level or a weakly bound state exists for the positron. (ii) Resonant annihilation, which takes place when the positron undergoes resonant capture into a vibrationally excited quasibound state of the positron-molecule complex. This mechanism dominates for larger molecules capable of forming bound states with the positron. For this mechanism ${Z}_{\mathrm{eff}}$ averaged over some energy interval, e.g., due to thermal positron energy distribution, is proportional to the level density of the positron-molecule complex, which is basically determined by the spectrum of molecular vibrational states populated in the positron capture. The resonant mechanism can produce very large annihilation rates corresponding to ${Z}_{\mathrm{eff}}\ensuremath{\sim}{10}^{8}.$ It is highly sensitive to molecular structure, and shows a characteristic ${\ensuremath{\varepsilon}}^{\ensuremath{-}1/2}$ behavior of ${Z}_{\mathrm{eff}}$ at small positron energies $\ensuremath{\varepsilon}.$ The theory is used to analyze calculated and measured ${Z}_{\mathrm{eff}}$ for a number of atoms and molecules.

Evolution of vacancy defects in the surface layers of a metal irradiated with a pulsed electron beam

The distribution of vacancy defects in the surface layers of α-Fe after irradiation with a high-current pulsed electron beam is studied experimentally by unique nuclear-physical methods — low-energy positron annihilation, Rutherford backscattering (RBS), and proton-induced x-ray emission (PIXE). Regions with low local density, which are sources of crater formation on the surface of the irradiated sample, are observed by scanning a proton microbeam. Positron lifetime measurements reveal that as the electron beam power increases, nonequilibrium vacancies tend to be captured by carbon impurity atoms.

Time-dependent diffusion models of the annihilation of low energy positrons implanted in solids

Two different models for positron transport in solids after implantation are presented. In the simplest model positrons are assumed to be implanted thermally and diffuse to the surface. In the second, two group model, the epithermal distribution acts as time-dependent source for the thermal distribution. Using experimental results from single crystal aluminium, silver and gold, it is shown that for deeply implanted positrons a single group diffusion model can be applied. At lower incident energies, neither model is applicable. In particular, a single epithermal state does not satisfactorily describe the intermediate stages of thermalization.