Positron Annihilation Measurements Research Articles

Time‐resolved luminescence studies on AlN and high AlN mole fraction AlGaN alloys

AbstractImpacts of point defects and impurities on the recombination dynamics for the near‐band‐edge (NBE) emission in AlN and high AlN mole fraction AlxGa1‐xN epilayers are revealed by means of deep ultraviolet (DUV) time‐resolved luminescence and positron annihilation measurements. Extremely radiative nature of AlN is identified, as the radiative lifetime (τR) for a free excitonic polariton emission is as short as 10 ps at 7 K and 180 ps at 300 K, which are the shortest ever reported for spontaneous emission of bulk semiconductors. However, τR increases with the increase in impurity and Al‐vacancy (VAl) concentrations up to 530 ps at 7 K, irrespective of the threading dislocation (TD) density. Continuous decrease in τR with temperature rise up to 200 K for heavily‐doped samples may reflect the carrier release from band‐tail states. Similar to these AlN, low‐temperature τR for low‐temperature‐grown high‐x AlxGa1‐xN are longer than that for low‐x AlxGa1‐xN, AlN, or GaN due to the contribution of bound and localized tail‐states. However, τR shows little change with temperature rise, and is still a few ns at 300 K. The results essentially indicate an excellent radiative performance, although the luminescence efficiency of AlN and AlxGa1‐xN DUV light‐emitting‐diodes (LEDs) reported so far is limited by short nonradiative lifetimes (τNR). To increase τNR, high temperature growth with appropriate defect management is preferable (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Room-temperature ferromagnetism observed in C-/N-/O-implanted MgO single crystals

MgO single crystals were implanted with 70keV C/N/O ions at room temperature with respective doses of 2×1016 and 2×1017ions/cm2. All samples with high-dose implantation showed room temperature hysteresis in magnetization loops. Magnetization and slow positron annihilation measurements confirmed that room temperature ferromagnetism in O-implanted samples was attributed to the presence of Mg vacancies. Furthermore, the introduction of C or N played more effective role in ferromagnetic performance than Mg vacancies. Moreover, the magnetic moment possibly occurred from the localized wave function of unpaired electrons and the exchange interaction formed a long-range magnetic order.

Positron Annihilation Measurements Performed on Oxide-Dispersion Strengthened Steels

This paper is focused on investigation of oxide-dispersion strengthened steels in a basic state (before an experimental treatment) by two positron annihilation techniques – positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy. Three high chromium commercial steels – MA 956, MA 957 and PM 2000 were measured in order to identify and quantify defects in samples as well as to demonstrate defect depth profile in subsurface layers up to 1.6µm. Results indicate presence of di-vacancies in PM 2000, three-vacancies in MA 956 and six-vacancies together with dislocations in MA 957. Positron techniques were supplemented by scanning electron microscopy, which demonstrated also presence of larger defects as pores and different density of precipitates.

Defect mediated ferromagnetism in Ni-doped ZnO nanocrystals evidenced by positron annihilation spectroscopy

NiO/ZnO nanocomposites with NiO content of 4 at. % and 20 at. % were annealed up to 1200 °C to get Ni doped ZnO nanocrystals. Raman scattering spectra illustrate a broad and strong band at 500-600cm−1 in all nanocomposites after annealing above 700 °C, which suggests incorporation of Ni in the ZnO lattice. However, x-ray diffraction measurements show that NiO phase can be still observed in all nanocomposites after annealing, which indicates that Ni is partially doped into the ZnO structure. Positron annihilation measurements reveal large number of vacancy defects in the interface region of all nanocomposites, and they are gradually recovered with increasing annealing temperature up to 1000 °C. Room temperature ferromagnetism can be observed in the NiO/ZnO nanocomposites, which is stronger in the 20 at. % NiO/ZnO nanocomposites, and the magnetization decreases continuously with increasing annealing temperature. This indicates that the ferromagnetism at low annealing temperatures originates from the NiO nanograins, and they become antiferromanetic after subsequent higher temperature annealing which leads to the weakening of ferromagnetism. After annealing up to 1000 °C, the ferromagnetism in both the two samples becomes nearly invisible. The disappearance of ferromagnetism shows good coincidence with the recovery of vacancy defects in NiO/ZnO nanocomposites. It can be inferred that the ferromagnetism is mediated by vacancy defects which are distributed in the interface region.

Use of positron annihilation measurements to detect the defect beneath worn surface of stainless steel 1.4301 (EN) under dry sliding condition

Positron measurements were performed for detection of the subsurface zone in the stainless steel 1.4301 (EN) exposed to dry sliding. They revealed the defect profile induced by dry sliding which is extended inward the worn surface from 85μm to 400μm depending on the applied load during the tribo-test. This result was obtained using the positron techniques, while the microhardness profile exhibited shorter range of the microhardness depth profile, i.e., from 70μm to 150μm. However, at the depth of about 5μm from the worn surface the sudden increase in the microhardness was observed. The complementary measurements of X-ray diffraction and SEM micrographs of the worn surface were also performed.

Time-resolved photoluminescence, positron annihilation, and Al0.23Ga0.77N/GaN heterostructure growth studies on low defect density polar and nonpolar freestanding GaN substrates grown by hydride vapor phase epitaxy

Time-resolved photoluminescence (TRPL) and positron annihilation measurements, as well as Al0.23Ga0.77N/GaN heterostructure growth by metalorganic vapor phase epitaxy were carried out on very low defect density, polar c-plane and nonpolar m-plane freestanding GaN (FS-GaN) substrates grown by hydride vapor phase epitaxy. Room-temperature photoluminescence (PL) lifetime for the near-band-edge (NBE) excitonic emission of the FS-GaN substrates increases with increasing positron diffusion length (L+); i.e., decreasing gross concentration of charged and neutral point defects and complexes. The best undoped c-plane FS-GaN exhibits record-long L+ being 116 nm. The fast component of the PL lifetime for its NBE emission increases with temperature rise up to 100 K and levels off at approximately 1.1 ns. The result implies a saturation in thermal activation of nonradiative recombination centers. The surface and interface roughnesses for a Si-doped Al0.23Ga0.77N/GaN/Al0.18Ga0.82N/GaN heterostructure are improved by the use of FS-GaN substrates, in comparison with the structure fabricated on a standard GaN template. The emission signals related to the recombination of a two-dimensional electron gas and excited holes are recognized for an Al0.23Ga0.77N/GaN single heterostructure grown on the c-plane FS-GaN substrate.

Study of Coincidence Doppler Broadening in Carbon Nanotubes

Coincidence Doppler broadening measurements of positron annihilation for multi-walled carbon nanotubes, double-walled carbon nanotubes, single-walled carbon nanotubes and graphite were performed. The ratio curves of the Doppler broadening for these samples to silicon were obtained. It is shown that there are distinct peaks at the position of 10×10−3m0c for both carbon nanotubes and graphite, however the amplitudes of the peaks are not the same. We have the opinion that these peaks arise from the annihilation of positron with the 2s and 2p electron of carbon element.

Precipitation Behavior of Al-Cu-Mg Alloy During Electric Field-Assisted Aging

In this article, we report the effect of electric field (EF) on the precipitation behavior of Al-Cu-Mg alloy. It is found that the applied field increases the hardness of Al-Cu-Mg alloy at initial aging stage. The peak of the S phase formation determined by differential scanning calorimetry also shifts to lower temperature with the applied EF. These are attributed to a lower decreasing rate of density of vacancies under field which is verified by positron annihilation measurements. The built-in field caused by the rearrangement of vacancy-solute atom complexes might lower the barrier for vacancy to escape from a complex. It leads to a longer survival time of vacancies, resulting in more possible nucleation sites for clusters.

Correlation between ferromagnetism and defects in MgO nanocrystals studied by positron annihilation

High purity MgO nanopowders were pressed into pellets and annealed in air from 100 to 1400°C. Variation of the microstructures was investigated by X-ray diffraction and positron annihilation spectroscopy. Annealing induces an increase in the MgO grain size from 27 to 60nm with temperature increasing up to 1400°C. Positron annihilation measurements reveal vacancy defects including Mg vacancies, vacancy clusters, microvoids and large pores in the grain boundary region. Rapid recovery of Mg monovacancies and vacancy clusters was observed after annealing above 1200°C. Room temperature ferromagnetism was observed for MgO nanocrystals annealed at 100, 700, and 1000°C. However, after 1400°C annealing, MgO nanocrystals turn into diamagnetic. Our results suggest that the room temperature ferromagnetism in MgO nanocrystals might originate from the interfacial defects.

Zinc Vacancy‐Induced Room‐Temperature Ferromagnetism in Undoped ZnO Thin Films

Undoped ZnO thin films are prepared by polymer‐assisted deposition (PAD) and treated by postannealing at different temperatures in oxygen or forming gases (95% Ar + 5% H2). All the samples exhibit ferromagnetism at room temperature (RT). SQUID and positron annihilation measurements show that post‐annealing treatments greatly enhance the magnetizations in undoped ZnO samples, and there is a positive correlation between the magnetization and zinc vacancies in the ZnO thin films. XPS measurements indicate that annealing also induces oxygen vacancies that have no direct relationship with ferromagnetism. Further analysis of the results suggests that the ferromagnetism in undoped ZnO is induced by Zn vacancies.

Tuning the Relative Concentration Ratio of Bulk Defects to Surface Defects in TiO2 Nanocrystals Leads to High Photocatalytic Efficiency

TiO(2) nanocrystals with tunable bulk/surface defects were synthesized and characterized with TEM, XRD, BET, positron annihilation, and photocurrent measurements. The effect of defects on photocatalytic activity was studied. It was found for the first time that decreasing the relative concentration ratio of bulk defects to surface defects in TiO(2) nanocrystals could significantly improve the separation efficiency of photogenerated electrons and holes, thus significantly enhancing the photocatalytic efficiency.

Local structure and hardness change of Zr50Cu40Al10 bulk glassy alloy after heavy ion irradiation

Zr50Cu40Al10 bulk glassy alloys before and after 200MeV Xe ion irradiation with a maximum dose of about 1×1014ions/cm2 and 5MeV Al ion (“self-ion”) irradiation with a dose of about 3×1014ions/cm2 have been investigated by positron annihilation and micro-Vickers hardness measurements. No crystallization took place in the samples after the irradiation according to X-ray diffraction measurement. We found that the hardness decreases by irradiation with increasing the irradiation dose. A decreasing trend of positron lifetime, which reflects the size of free volume, was observed. These facts imply that heavy ion irradiation can be used for the local modification of hardness, which is not thermal effect, in Zr50Cu40Al10 bulk glassy alloy.

Positron annihilation spectroscopy on binary Fe–Cr alloys and ferritic/martensitic steels after neutron irradiation

The irradiation-induced behaviour of iron–chromium model alloys and high chromium ferritic/martensitic steels have been studied using positron annihilation measurements. Both the positron lifetime and coincidence Doppler broadening techniques were used in a complementary way. It is shown that the presence of chromium in iron-based alloys significantly reduces the concentration of vacancies to the extent that the formation of clusters is hindered. The observed vacancy behaviour as a function of the chromium content is fully consistent with the findings of void swelling suppression in chromium-rich alloys and can be rationalized based on existing models. Finally, the information obtained for the ferritic/martensitic steels shows good agreement with the results for binary model alloys. Compared with the vacancy behaviour in pure iron, this confirms the major influence of chromium on the nanostructural evolution of these steels under irradiation.

Correlation between interfacial defects and ferromagnetism of BaTiO 3 nanocrystals studied by positron annihilation

Abstract High purity BaTiO 3 nanopowders were pressed into pellets and annealed between 100 and 1200 °C. The crystal quality and grain size of the BaTiO 3 nanocrystals were characterized by X-ray diffraction measurements. Annealing induces an increase in the grain size from 44 to 82 nm with temperature increasing up to 1200 °C. XRD and Raman spectroscopy studies confirm that all the samples were single phase with a tetragonal structure after annealing at different temperatures. Positron annihilation measurements reveal large number of vacancy defects in the grain boundary region. These interfacial defects remain stable after annealing at temperatures below 400 ° C and begin to disappear rapidly above 700 °C. After annealing at 1200 °C, most of the interfacial defects have been removed. Hysteresis loops are observed for the 100 ° C annealed samples, which indicate ferromagnetism in BaTiO 3 nanocrystals. The ferromagnetism becomes a little weaker after annealing at 700 °C, and it disappears after 1200 ° C annealing. This change of ferromagnetism coincides with the defect recovery process after annealing, suggesting that ferromagnetism might originate from the interfacial defects.

Direct observations of the vacancy and its annealing in germanium

Weakly $n$-type doped germanium has been irradiated with protons up to a fluence of $3\ifmmode\times\else\texttimes\fi{}{10}^{14}$ cm${}^{\ensuremath{-}2}$ at 35 K and 100 K in a unique experimental setup. Positron annihilation measurements show a defect lifetime component of 272$\ifmmode\pm\else\textpm\fi{}$4 ps at 35 K in in situ positron lifetime measurements after irradiation at 100 K. This is identified as the positron lifetime in a germanium monovacancy. Annealing experiments in the temperature interval 35--300 K reveal two annealing stages. The first at 100 K is tentatively associated with the annealing of the Frenkel pair, the second at 200 K with the annealing of the monovacancy. Above 200 K it is observed that mobile neutral monovacancies form divacancies, with a positron lifetime of 315 ps.

Depth profiling of defects in ion-implanted Ni and Fe by positron annihilation measurements

Pure Ni and Fe samples were irradiated with 150 keV Ar ions at room temperature, 300 and 500 °C to introduce defects near the surface. The irradiated samples were characterized by energy-variable slow-positron beams with Doppler broadening and positron lifetime measurements to investigate defect profiles. The irradiation-induced defects were detected at regions much deeper than ion projected ranges for all the temperatures. The origin of such defects beyond the projected ranges was discussed in terms of vacancy migration, ion channeling, surface contamination and depth-scale error. Among these effects, the ion channeling may account for the obtained results.

Spin-polarized positron annihilation measurements of polycrystalline Fe, Co, Ni, and Gd based on Doppler broadening of annihilation radiation

Spin-polarized positron annihilation measurements of polycrystalline Fe, Co, Ni, and Gd based on Doppler broadening of annihilation radiation

Free-volume defects and microstructure in ion-conducting Ag/AgI-As 2S 3 glasses as revealed from positron annihilation and microhardness measurements

The positron annihilation lifetime spectroscopy (PALS) study and load dependent Vickers microhardness data were applied together to find correlation between free-volume defects and microstructure in the pure As 2S 3 and metal-modified (As 2S 3) 85Ag 15 and (As 2S 3) 85(AgI) 15 glasses. The PALS data were analyzed by the well-developed formalism of the two-state positron trapping model while microhardness data were analyzed according to the strain-gradient plasticity theory presented in the form: H = H 0(1 + d 0/ d), where H 0 is the load independent hardness and d 0 is a constant. Analysis of the experimental results showed that: (1) the hole-like defects with volume larger than or comparable with the size of the Ag + ions but smaller than the size of the AgI clusters are present in the structure of the host glass matrix As 2S 3 and are effective positron traps, and (2) these free-volume defects determine the load-independent indentation microhardness H 0 of a glass in the region of applied load where the H V decreases with an increase in the applied load P.

Major impacts of point defects and impurities on the carrier recombination dynamics in AlN

Impacts of point defects and impurities on the carrier recombination dynamics in AlN are revealed by time-resolved spectroscopy and positron annihilation measurements. Intrinsically short low-temperature excitonic radiative lifetime (τR∼10 ps) was elongated with the increase in Al-vacancy concentration up to 530 ps, irrespective of threading dislocation density. A continuous decrease in τR with temperature rise up to 200 K for heavily doped samples revealed the carrier release from the band-tail formed due to impurities and point defects. Because room-temperature nonradiative lifetime was equally short for all samples, high temperature growth with appropriate defect management is necessary in extracting radiative nature of AlN.

Experimental determination of the state-dependent enhancement of the electron-positron momentum density in solids

The state-dependence of the enhancement of the electron-positron momentum density is investigated for some transition and simple metals (Cr, V, Ag and Al). Quantitative comparison with linearized muffin-tin orbital calculations of the corresponding quantity in the first Brillouin zone is shown to yield a measurement of the enhancement of the s, p and d states, independent of any parameterizations in terms of the electron density local to the positron. An empirical correction that can be applied to a first-principles state-dependent model is proposed that reproduces the measured state-dependence very well, yielding a general, predictive model for the enhancement of the momentum distribution of positron annihilation measurements, including those of angular correlation and coincidence Doppler broadening techniques.