Positron Trapping Defects Research Articles

Extended Positron–Positronium Trapping Defects in the MgAl2O4 Spinel Ceramics

Extended free‐volume positron trapping defects (positron trapping channel) and nanopores (positronium decaying channel) in the initial and modified MgAl2O4 ceramics sintered at 1200, 1300, and 1400 °C are investigated using positron annihilation lifetime spectroscopy within four‐component fitting procedure. It is established that the number and size of extended defects near grain boundaries (described by the second component) decrease with increasing sintering temperature of ceramics, which correlates with the content of additional phases in the materials. The agglomeration of nanopores occurs in the initial ceramics (described by the third component) and the decrease of number and size of nanopores in the modified ceramics. The corresponding processes of the evolution of nanopores described by the fourth component are reflected mainly in their reduction, except for ceramics sintered at 1400 °C. Such changes are associated with the evolution of porous structure and increased contact area between the grains of ceramics.

Thermal Stability of Rolled Metastable Austenitic Stainless Steel 1.4307 Studied Using Positron Annihilation

Positron annihilation spectroscopy (PAS) was used in the study of cumulative isochronal and isothermal annealing of rolled 1.4307 (EN) stainless steel (SS). Due to different rolling temperatures, the SS samples varied in the α′-martensite volume fraction from 0.09 to 0.91. The measurements of positron annihilation characteristics, i.e., Doppler broadening (DB) of the annihilation line, showed a gradual annealing of vacancies in the temperature range between 200 °C and 400 °C, which indicated the first stage of recovery. This first stage of recovery did not change the microhardness. In the temperature range from 475 °C to 600 °C, a decrease in the microhardness and generation of new open volume defects accompanying the reversion of α′-martensite to austenite was observed. The amount of these defects correlated with the initial α′-martensite volume fraction. Their formation could be related to the volume contraction occurring during bcc/fcc transformation. The different chemical surroundings suggested that the positron trapping defects were associated with the metal carbide precipitates.

Positron annihilation spectroscopic investigation of high energy ball – milling engendered defects in CaCu3Ti4O12

Polycrystalline sample of calcium – copper – titanate (CaCu3Ti4O12) was synthesized by two – steps solid state reaction route and then high energy ball-milled for several hours. Samples collected at different hours of milling were characterized and studied. X-ray diffraction patterns indicated monophase formation in the initial stage of grinding and a few other phases were found being formed with further milling. The elemental analysis indicated the possible presence of defects possibly due to non-stoichiometry, which was subsequently confirmed through positron annihilation studies. The positron trapping defects in the samples were found evolving from monovacancies to larger vacancy clusters and microvoids during increased hours of milling. The concomitant process of reduction of the sizes of the crystallites to nanometre dimensions resulted in the diffusion of thermalized positrons to the nanocrystallite surfaces and subsequent annihilation. The results indicated defects as a strong entity of influence on the structure and properties of CaCu3Ti4O12 and deserving serious consideration when the high dielectric material is sought for use in practical applications.

Defects characterization and study of amorphous phase formation in xV2O5-(1-x)Nd2O3 binary glass nanocomposites using positron annihilation and correlated experimental techniques

A glass nanocomposite system with the combination of transition metal oxide and rare earth oxide of the general nomenclature xV2O5-(1-x)Nd2O3 for x = 0, 0.2, 0.4, 0.6 and 0.8 have been synthesized through the rapid quenching process and characterized by X-ray diffraction, transmission electron microscopy and UV–Vis absorption studies. Positron annihilation lifetime and coincidence Doppler broadening measurements are further employed to identify the vacancy or void-type defects existing in the composites of the different V2O5 concentrations (x). The X-ray diffraction patterns indicate distinct diffraction peaks with finite broadening in some of them signifying the co-existence of nanocrystallites and a certain level of amorphous phases in some of them. The crystal morphology, lattice fringes and the selected area electron diffraction patterns identified from the transmission electron microscopy images confirmed the nanocrystallite formation at lower V2O5 concentrations (x) and increasing amorphous character of the composite as x increased. The optical band gap energy has been estimated from the optical absorption spectra and it is found to reduce with increasing x. It is found from positron annihilation studies that a fraction of positrons are getting selectively confined in the interfacial gaps around the nanocrystallites for smaller values of x (i.e., 0, 0.2, 0.4 and 0.6). Porous defects surrounded by an amorphous glassy environment became the dominant trapping centres for the highest value of x (i.e., 0.8). The total free volume fraction decreases with x due to the interfacial gaps gradually disappear and give way to the generation of porous defects within the glassy matrix. The coincidence Doppler broadening spectroscopic measurements also established the transformation of the surroundings of the positron trapping defects.

Positron trapping defects in free-volume investigation of Ge–Ga–S–CsCl glasses

Evolution of free-volume positron trapping defects caused by crystallization process in (80GeS2–20Ga2S3)100−х(СsCl)x, 0 ≤ x ≤ 15 chalcogenide-chalcohalide glasses was studied by positron annihilation lifetime technique. It is established that CsCl additives in Ge–Ga–S glassy matrix transform defect-related component spectra, indicating that the agglomeration of free-volume voids occurs in initial and crystallized (80GeS2–20Ga2S3)100−х(СsCl)x, 0 ≤ x ≤ 10 glasses. Void fragmentation in (80GeS2–20Ga2S3)85(СsCl)15 glass can be associated with loosing of their inner structure. Full crystallization in each of these glasses corresponds to the formation of defect-related voids. These trends are confirmed by positron-positronium decomposition algorithm. It is shown, that CsCl additives result in white shift in the visible regions in transmission spectra. The γ-irradiation of 80GeS2–20Ga2S3 base glass leads to slight long-wavelength shift of the fundamental optical absorption edge and decreasing of transmission speaks in favor of possible formation of additional defects in glasses and their darkening.

Increased p-type conductivity in GaNxSb1−x, experimental and theoretical aspects

The large increase in the p-type conductivity observed when nitrogen is added to GaSb has been studied using positron annihilation spectroscopy and ab initio calculations. Doppler broadening measurements have been conducted on samples of GaNxSb1−x layers grown by molecular beam epitaxy, and the results have been compared with calculated first-principle results corresponding to different defect structures. From the calculated data, binding energies for nitrogen-related defects have also been estimated. Based on the results, the increase in residual hole concentration is explained by an increase in the fraction of negative acceptor-type defects in the material. As the band gap decreases with increasing N concentration, the ionization levels of the defects move closer to the valence band. Ga vacancy-type defects are found to act as positron trapping defects in the material, and the ratio of Ga vacancy-type defects to Ga antisites is found to be higher than that of the p-type bulk GaSb substrate. Beside Ga vacancies, the calculated results imply that complexes of a Ga vacancy and nitrogen could be present in the material.

Defects structure characterization of NiMnGa alloys by PALS

We have studied the behaviour of defects in off-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys by means of positron lifetime spectroscopy. The measurements presented in this work have been performed in six ternary alloys. The studied samples cover a large composition range. Positron experiments have been performed at room temperature after subsequent isochronal annealing at different temperatures up to a maximum temperature of 700°C. Results show a large variation of the average positron lifetime value with the isochronal annealing temperature in three of these samples, with significant differences between them. In the other three, the response is quite different. The results are discussed in terms of different types of positron trapping defects and their evolution with the annealing temperature. The present work shows a high dependence of recovery behaviour with composition in NiMnGa ferromagnetic shape memory alloys.

Positron Annihilation Spectroscopy Study of Ni-Mn-Ga Ferromagnetic Shape Memory Alloys

Abstract We have studied the role that vacancy type defects play in the martensitic transformation of Ni-Mn-Ga ferromagnetic shape memory alloys by means of positron lifetime spectroscopy. The measurements presented in this work have been performed in five ternary alloys. Three of them transform to modulated and two to non-modulated martensitic phases. With these five samples we cover a large range in composition. Positron experiments have been performed at room temperature after subsequent isochronal annealing at different temperatures and up to a maximum temperature of 600°C. Results show a large variation of the average positron lifetime value with the isochronal annealing temperature in non-modulated samples. However, the response in the modulated samples is quite different. The results are discussed in term of different type of positron trapping defects and their evolution with the annealing temperature. The present work shows a correlation between vacancy concentration and martensitic transformation temperature of ferromagnetic shape memory alloys.

Extended defects in insulating MgAl2O4 ceramic materials studied by PALS methods

Extended positron-trapping defects in technological modified insulating nanoporous MgAl2O4 ceramics are characterized by positron annihilation lifetime spectroscopy. The results are achieved using three-component fitting procedure with arbitrary lifetimes applied to treatment of measured spectra. Within this approach, the first component in the lifetime spectra reflects microstructure specificity of the spinel structure, the second component responsible to extended defects near intergranual boundaries and the third component correspond to ortho-positronium "pick-off" decaying in nanopores of ceramics. It is shown that in ceramics of different technological modifications the same type of positron traps prevails.

Positron annihilation in (Ga, Mn)N: A study of vacancy-type defects

The vacancy-type defects in (Ga,Mn)N films grown by metal organic chemical vapor deposition were studied by positron annihilation technique. Doppler broadening spectra were measured for the films. Compared to the undoped GaN film, the positron trapping defects in the (Ga,Mn)N films have been changed to a new type defects and its concentration increases with the increasing Mn concentration. By analyzing the S-W correlation plots and our previous results, we identify this type defects in the (Ga,Mn)N as VN-MnGa complex. This type of defects should be considered when understand the magnetic properties in a real (Ga,Mn)N system.

Defect dynamics in P + implanted 6 H ‐SiC studied by positron annihilation spectroscopy

We study P+-implanted 6H-SiC samples by means of a variable energy slow positron beam. In as-grown samples we observe a slow fall of the annihilation S-parameter from the surface to the bulk value, indicating a long diffusion length, i.e. absence of positron-trapping defects. This result is also confirmed by positron-lifetime measurements yielding essentially a single-component lifetime of 148 ps. In the implanted samples we detect a wide flat region of slightly risen defectiveness. However, we note that, in order to reproduce reasonably well the depth of the damaged layer, one has to assume the presence of strong electric fields in the implanted region. With annealing, S-curves show a rising maximum, moving towards surface – probably, in as-implanted samples the defects are decorated by P+ ions and do not trap positrons in effective way. After the highest temperature annealing a highly defected region extends for the first 50 nm depth. We perform also atomic force microscopy to monitor the evolution of the surface defects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of annealing ambient on the formation of compensation defects in InP

Positron annihilation lifetime (PAL) and photoinduced current transient spectroscopies (PICTS) have been employed to study the formation of compensation defects in undoped InP under different annealing processes with pure phosphorus (PP) ambience and iron phosphide (IP) ambience, respectively. The different annealing ambiences convert the as-grown n-type undoped InP into two types of semi-insulating (SI) states. The positron average lifetimes of as-grown InP, PP SI-InP, and IP SI-InP are found to be 246, 251, and 243 ps, respectively, which are all longer than the bulk lifetime of 240 ps, indicating the existence of vacancy-type positron-trapping defects. For as-grown InP, VInH4 complexes are the dominant defects. They dissociate into VInHn(0⩽n⩽3) acceptor vacancies under PP ambience annealing, compensating the residual shallow donors and turning the material semi-insulating. In forming IP SI-InP, diffusion of iron into VIn complexes under IP ambience annealing produces the substitutional compensation defect FeIn, causing a shorter positron average lifetime. The PICTS measurements show that a group of vacancy-type defects has been suppressed by iron diffusion during the annealing process, which is in good agreement with the PAL results.

Positron and positronium studies of irradiation-induced defects and microvoids in vitreous metamict silica

To study irradiation-induced defects and structural microvoids in vitreous silica (v-SiO2), positron lifetime, angular correlation of positron annihilation radiation (ACAR), and electron spin resonance (ESR) were measured on v-SiO2 and quartz (c-SiO2) samples irradiated with fast neutrons up to a dose of 4.1×1020 n/cm2. Two kinds of positron-trapping defects have been found to form in v-SiO2 by fast neutron irradiation: type-I and type-II defects. Similar defects also appear in the irradiated c-SiO2, indicating that both the defects are common in v-SiO2 and c-SiO2. The detailed annealing and photo-illumination studies of positron annihilation and ESR for these two defects suggest that the type-I defects are non-bridging oxygen hole centers (NBOHC), while the type-II defects are oxygen molecules which cannot be detected by ESR. Higher dose irradiation than 1.0×1020 n/cm2 causes c-SiO2 to change to metamict (amorphous) phase (m-SiO2). Positronium (Ps) atoms are found to form in microvoids with an average radius of about 0.3 nm in the v-SiO2 and m-SiO2. This suggests that microvoids proved by Ps are structurally intrinsic open spaces and reflect the topologically disordered structure of these phases in the subnanometer scale.

Enhanced depth resolution in positron analysis of ion irradiated SiO2 films

We report the observation of defects following 1.7 MeV Si+ through-implantation of SiO2 films thermally grown on Si(100) substrates. Films were irradiated to fluences of 1012, 1013, and 1014 ions/cm2, and analyzed using a variable energy positron beam. Enhanced depth resolution was achieved via iterative chemical etching and measurement, for the sample irradiated to 1012 Si+/cm2, and for an unirradiated control sample. The positron annihilation “S parameter” is reduced in the implanted SiO2, due to a reduction in the formation of positronium (the atom-like positron–electron bound pair), and is increased in the Si substrate, due to the creation of vacancy-type defects. The concentration of implant-induced defects was found to be very nearly constant as a function of depth in the SiO2 film, indicating a significant contribution of both the electronic and nuclear stopping of the implanted Si+ ions to the production of positron-trapping defects. We find that the maximum overlayer thickness which allows simultaneous detection of defects in the substrate (i.e., through-oxide probing of substrate defects) is ∼600 nm.

Study of argon-irradiation-induced defects and amorphization in silicon using a positron beam, Raman spectroscopy and ion channelling

Studies of irradiation-induced defects have been carried out on silicon single-crystal samples, irradiated with 140 keV argon ions to doses in the range 5 × 1014 to 5 × 1016 cm-2, using a variable-low-energy positron beam, Raman spectroscopy and ion channelling. The Doppler broadening lineshape S-parameter has been found to exhibit a peak as a function of the positron beam energy Ep and a subsequent saturation behaviour for all irradiated samples. The peak damage occurs at a depth of 100 nm, consistent with TRIM code calculations. The measured S-Ep curves are analysed using a positron diffusion model to obtain the depth profile of positron-trapping defects. The behaviour of the S-W correlation plots and the variation of the R-parameter indicate that the nature of the open-volume defects is independent of the dose and the dominant defects are vacancy clusters larger than divacancies. Raman spectroscopic studies indicate that all of the irradiated samples are amorphized, and the degree of residual crystallinity across the irradiated zone of the sample is obtained. Ion-channelling studies carried out on the samples have yielded thicknesses of the amorphized layer which are consistent with positron beam results. In contrast to the TRIM code calculations, the present results show the presence of defects at depths far beyond the Ar-ion range. The results of the positron beam, Raman and ion-channelling studies are discussed in the context of ion-induced defects and amorphization.

On the possible identification of defects using the autocorrelation function approach in double Doppler broadening of annihilation radiation spectroscopy

The recent revived interest in the use of double-Doppler broadening of annihilation radiation (D-DBAR) spectroscopy, which employs two Ge detectors in back-to-back geometry, has stemmed mainly from its potential in defect identification as a result of its elemental sensitivity through core annihilations in atoms at the defect site. Emphasis has thus largely concentrated on the high momentum spectral range. In contrast the present work emphasizes the need to also focus attention on the low momentum region of the D-DBAR spectra. It is argued that the improved resolving power of D-DBAR, in conjunction with spectral deconvolution, should give future 1D (one dimensional) momentum data approaching in quality those obtainable using 1D-ACAR (angular correlation of annihilation radiation), thus forming an alternative technique for observing the structure containing diffraction patterns that originate from annihilations with localized electron states at positron trapping defects. Rotation of the sample about a specified crystal axis, and the binning of events by angle, is suggested as a means of extending the technique to form a 2D- (two dimensional) DBAR counterpart to 2D-ACAR. The advantages of considering the real space positron electron wavefunction product AF (autocorrelation function), obtained by simple manipulation of the D-DBAR data in Fourier space, are outlined. In particular the possible visualization offered in real space of a defect's physical geometry, with the prospect of building up a library of contour patterns for future defect identification, is discussed, taking the silicon monovacancy in Si and the negative As vacancy in GaAs as examples.

Annealing of defects induced by Ge irradiation of silica probed with variable energy positrons

Fused silica (Suprasil 2) has been implanted with 2 MeV Ge ions. Defects produced by the implantation have been studied using a variable energy positron beam. The measurements allow depth resolved characterisation of the implantation induced defects, which are found to be produced by both the electronic and nuclear stopping of the ions. It is suggested that the previously unidentified positron-trapping defect associated with the nuclear stopping is an oxygen-decorated vacancy. Heating the sample to a temperature of 600°C demonstrates an annealing stage for this defect between 350–400°C. A region with a low concentration of positron-trapping defects is found to be present close to the sample surface, even in the as-irradiated sample prior to annealing. The depth of this region increases with increasing annealing temperature.

Structural and electrical defects in amorphous silicon probed by positrons and electrons

The structure of pure amorphous Si, prepared by ion implantation, has been investigated by variable-energy positron annihilation spectroscopy (PAS) and lifetime measurements of optically generated free carriers. In general, PAS measurements are thought to be sensitive to vacancy-type defects while the carrier lifetime depends on the density of band-gap states (e.g., dangling bonds). The PAS measurements indicate that the density of positron-trapping defects can be reduced by thermal annealing at 500 °C. Concurrent with the removal of structural defects the density of band gap states is reduced as indicated by an increased photocarrier lifetime by a factor of 10. Some material has been implanted with H+ and annealed at a low temperature (150 °C). The hydrogen is expected to passivate electrical defects associated with strained and dangling bonds and indeed the photocarrier lifetime is increased in this material. Moreover, the PAS measurements cannot distinguish this material from 500 °C annealed amorphous Si, indicating that (some of) the electrical defects are associated with positron-trapping, and therefore possibly vacancy-type, structural defects. Finally, both methods have been used to detect small amounts of ion irradiation damage in annealed amorphous Si.

Vacancy-type and electrical defects in amorphous silicon probed by positrons and electrons

Amorphous Si has been investigated by variable-energy positron annihilation spectroscopy (PAS) and lifetime measurements of optically generated free carriers. The density of positron-trapping defects can be reduced by thermal annealing at 500°C. Simultaneously, the density of bandgap states is reduced as indicated by an increased photocarrier lifetime. Hydrogen, implanted and annealed at 150°C, leads to an increased photocarrier lifetime and reduced positron trapping. It appears that (some of) the electrical defects are associated with positron-trapping, and therefore possibly vacancy-type, structural defects. Finally, both methods have been used to detect small amounts of ion irradiation damage in amorphous Si.