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Si nanocrystals embedded in a SiO2 matrix were studied with positron annihilation and photoluminescence spectroscopies. Analysis of the S- and W-parameters for the sample annealed at 800 °C reveals a positron trap at the interface between the amorphous nanodots and the surrounding matrix. Another trap state is observed in the 1150 °C heat treated samples where nanodots are in a crystalline form. Positrons are most likely trapped to defects related to dangling bonds at the surface of the nanocrystals. Passivation of the samples results on one hand in the decrease of the S-parameter implying a decrease in the open volume of the interface state and, on the other hand, in the strengthening of the positron annihilation signal from the interface. The intensity of the photoluminescence signal increases with the formation of the nanocrystals. Passivation of samples strengthens the photoluminescence signal, further indicating a successful deactivation of luminescence quenching at the nanocrystal surface. Strengthening of the positron annihilation signal and an increase in the photoluminescence intensity in passivated silicon nanocrystals suggests that the positron trap at the interface does not contribute to a significant extent to the exciton recombination in the nanocrystals. [ABSTRACT FROM AUTHOR]

Positron annihilation spectroscopy has been used to investigate the role of vacancies in the interdiffusion of Al and Ga in AlSb/GaSb superlattices. The samples were grown by metalorganic vapor-phase epitaxy on undoped and Te doped GaSb and consisted of ten periods of GaSb quantum wells (thickness 13 nm) and AlSb barriers (thickness 2–3 nm) and an approximately 50 nm thick capping layer of GaSb. The superlattices were annealed at 908 K for up to 250 s, resulting in interdiffusion of Al and Ga between well and barrier. A secondary ion mass spectrometry study showed that the Te dopant diffused from the substrate through the superlattice structure in the annealing process. In the positron annihilation study we observe that the vacancy concentration clearly decreases with annealing for the samples grown on undoped substrates, whereas the samples grown on Te doped substrates show a different annealing behavior. [ABSTRACT FROM AUTHOR]

Vacancy-type defect production in Al- and Si-implanted 4H–SiC has been studied as a function of ion fluence, ion flux, and implantation temperature in the projected ion range region by positron annihilation spectroscopy and Rutherford backscattering techniques. Ion channeling measurements show that the concentration of displaced silicon atoms increases rapidly with increasing ion fluence. In the ion fluence interval of 1013–1014 cm-2 the positron annihilation parameters are roughly constant at a defect level tentatively associated with the divacancy VCVSi. Above the ion fluence of 1014 cm-2 larger vacancy clusters are formed. For implantations as a function of ion flux (cm-2 s-1), ion channeling and positron annihilation measurements behave similarly, i.e., indicating increasing damage in the projected range region with increasing ion flux. However, for samples implanted at different temperatures the positron annihilation parameter S shows a clear minimum at approximately 100 °C, whereas the normalized backscattering yield decrease continuously with increasing implantation temperature. This is explained by the formation of larger vacancy clusters when the implantation temperature is increased. [ABSTRACT FROM AUTHOR]

The defect distributions in [sup 11]B-, [sup 14]N-, and [sup 27]Al-implanted epitaxial 4H–SiC are studied using monoenergetic positron beams. At least three types of defects are needed to account for the Doppler broadening annihilation spectra and two of the defects are tentatively identified as V[sub Si], and V[sub Si]V[sub C]. By comparing the defect profiles extracted from the annihilation spectra to the chemical profiles determined by secondary ion mass spectrometry, and to the primary defect profiles obtained from binary collision approximation simulations, it is concluded that the defects found at depths considerably deeper than the projected range of the implanted ions mainly originate from deeply channeled ions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Positron annihilation spectroscopy in both conventional and coincidence Doppler broadening mode is used for studying the effect of growth conditions on the point defect balance in GaSb:Bi epitaxial layers grown by molecular beam epitaxy. Positron annihilation characteristics in GaSb are also calculated using density functional theory and compared to experimental results. We conclude that while the main positron trapping defect in bulk samples is the Ga antisite, the Ga vacancy is the most prominent trap in the samples grown by molecular beam epitaxy. The results suggest that the p-type conductivity is caused by different defects in GaSb grown with different methods. [ABSTRACT FROM AUTHOR]

Weakly n-type doped germanium has been irradiated with protons up to a fluence of 3×1014 cm-2 at 35 K and 100 K in a unique experimental setup. Positron annihilation measurements show a defect lifetime component of 272±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. [ABSTRACT FROM AUTHOR]

Abstract: Photoluminescence (PL), high-precision X-ray diffraction, and positron annihilation studies were performed on compressively strained GaIn(N)As/GaAs quantum-well heterostructures in attempts to elucidate optical and structural effects originating from incorporation of nitrogen into the crystal lattice and from post-growth thermal annealing. It was found that annealing left the properties of the nitrogen-free samples rather unchanged, but strongly enhanced PL and blue-shifted the spectrum for GaInNAs/GaAs. None of the as-grown samples contained negatively charged or neutral vacancies within the sensitivity limits of the positron experiments. However, positron traps (likely Ga vacancies) appeared in the annealed GaInNAs/GaAs sample. These defects did not deteriorate strong light emission from annealed GaInNAs/GaAs. [Copyright &y& Elsevier]

Abstract: The aim of this contribution is to present a review of the results related to vacancy complexes in SiGe obtained in the positron group at Helsinki University of Technology. We have studied proton irradiated undoped and P doped SiGe layers with Ge concentration from 4% to 30%. The dominating defect after irradiation in the n-type layers is found to be the E-center, i.e. the vacancy phosphorus complex, no preferred association with either Ge or Si is found for the vacancy after irradiation. The E-center is observed to be mobile in the temperature interval 150–200°C and to migrate until it encounters a Ge atom and forms the V–P–Ge complex. This complex anneals out in temperatures above 200°C. We estimate that the binding energy increases by approximately 0.1–0.2eV when a Ge atom neighbours an E-center. For the undoped irradiated samples, we find no indication of vacancies surrounded by one or several Ge atoms, i.e. the presence of Ge around a vacancy is not enough to make the defect stable at room temperature. The dominating defect in undoped irradiated samples is most likely the divacancy. [Copyright &y& Elsevier]

Abstract: We have used positrons annihilation spectroscopy to study vacancy type defects in GaAs based superlattices used for quantum well lasers. The samples were molecular beam epitaxy grown on p-type GaAs substrates. Three sets of samples were used for the experiment. For each set the superlattice structure consisted of 10 periods of the quantum well material separated by a GaAs buffer. As quantum well material we used GaInAs, GaNAs and GaInNAs. The growth temperature was for each sample set and some of the samples were annealed. For annealed samples, it could be shown that vacancies were formed in the superlattice structure or at interface between the GaAs cap and the superlattice structure in the annealing process and that the formation was related to the nitrogen content. Annealed GaNAs and GaInNAs superlattice samples were found to contain vacancies, most likely in the Ga sublattice. [Copyright &y& Elsevier]

Abstract: Positron annihilation spectroscopy was applied to study relaxed P-doped n-type layers with Ge concentrations up to 30%. As-grown SiGe layers were defect-free and annihilations are superpositions from bulk Si and Ge. Proton irradiation at 2MeV energy with a fluence was used to produce saturated positron trapping in monovacancy related defects. The defects were identified as V–P pairs, the E-center. The distribution of Si and Ge atoms surrounding the E-center is the same as in the host lattice. The vacancy migration process leading to the formation of V–P pairs therefore does not seem to have a preference for either Si or Ge atoms. [Copyright &y& Elsevier]

We have used positron annihilation spectroscopy to study vacancy type defects in strained P doped Si1−xGex layers grown on Si substrates and irradiated with 2 MeV protons. The results show that the dominant defect in the SiGe layer after irradiation is the E-center, the V–P pair. When the sample is annealed at 150–175°C the dominant defect species in the SiGe layer changes into a complex consisting of a vacancy, a P dopant and a Ge atom (V–P–Ge complex). The V–P–Ge complex anneals out at 200°C. The 50°C higher annealing temperature of the V–P–Ge complex corresponds to about 0.1–0.2 eV larger binding energy than that of the V–P pair. [Copyright &y& Elsevier]

Abstract: We have studied the annealing of vacancy defects in neutron and proton irradiated germanium. After neutron irradiation the Sb-doped samples were annealed at 473, 673 and 773K for 30min. The positron lifetime was measured as a function of temperature (30 – 295K). A lifetime component of 330ps with no temperature dependence is observed in as irradiated samples, identified as the positron lifetime in a neutral divacancy. The average positron lifetime in the samples annealed at 473K has a definite temperature dependence, suggesting that the divacancies become negative as the crystal recovers and the Fermi level moves upward in the band gap. Proton irradiation of germanium at 37K with subsequent room temperature annealing also resulted in a similar lifetime component 315ps, in good agreement with the neutron irradiation experiment. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Evolution of the chemical surroundings of vacancy complexes in Sb-doped ([Sb]=2×1018 and 2×1019 cm-3) Si0.8Ge0.2 was studied with positron annihilation spectroscopy in Doppler broadening mode. The study was performed by annealing the samples both isochronally and isothermally. Defect evolution was observed at the temperature range 450-650 K. Both treatments were shown to induce changes in the chemical surroundings of the E-centers via introduction of Ge near the defects. Moreover, Sb was found to hinder these changes by stabilizing the E-centers and thus preventing them from finding Ge. The stable state reached after the anneals was found to differ from that measured from an as-grown sample. This difference was deemed to be the result of Ge gathering in small clusters during the annealing thus breaking the initially random Ge distribution. [ABSTRACT FROM AUTHOR]

Abstract: Positron annihilation spectroscopy (PAS) in Doppler broadening mode was used to study the vacancy profile of crystalline Si after He and B implantation and subsequent annealing. In the He-implanted samples two different void layers were observed, one consisting of large voids at the projected range of He and another containing “nanovoids” slightly larger than divacancies at roughly halfway between R p of He and the surface. The nanovoid layer was shown to be absent from samples co-implanted with B, implying that interstitials created during B implantation get trapped in the nanovoids and fill them, thus hindering interstitial-mediated B diffusion. [Copyright &y& Elsevier]

We have applied positron annihilation spectroscopy to study native point defects in Te-doped n-type and nominally undoped p-type GaSb single crystals. The results show that the dominant vacancy defect trapping positrons in bulk GaSb is the gallium monovacancy. The temperature dependence of the average positron lifetime in both p- and n-type GaSb indicates that negative ion type defects with no associated open volume compete with the Ga vacancies. Based on comparison with theoretical predictions, these negative ions are identified as Ga antisites. The concentrations of these negatively charged defects exceed the Ga vacancy concentrations nearly by an order of magnitude. We conclude that the Ga antisite is the native defect responsible for p-type conductivity in GaSb single crystals. [ABSTRACT FROM AUTHOR]

Abstract: We have studied proton irradiated and annealed n-type SiGe with a P dopant concentration in the 1×1018 cm−3 range by positron annihilation spectroscopy. The Ge contents in the samples were 10, 20 and 30%. The results show that by annealing the irradiated samples at temperatures 250–350°C, the Ge content around the E center increases. Based on temperature-dependent and coincidence Doppler measurements, we conclude that the increase in Ge content around the E center pulls down the localized second acceptor state, found in Ge, into the SiGe band gap. This charge transition is observed even in samples annealed at 350°C, with a considerably higher thermal budget than expected for the complex to anneal completely. [Copyright &y& Elsevier]

In this paper we highlight the existence of tensile stress in heavily arsenic-doped epitaxial silicon (Si:As) prepared by low pressure chemical vapor deposition. Despite the large size of As atoms compared to Si ones, we demonstrate with x-ray diffraction and convergent electron beam diffraction that the heavily doped epitaxial layers show a tetragonal lattice with a reduced out of plane parameter. Using positron annihilation spectroscopy, we highlight the formation of arsenic-vacancies defects during the growth. We show that the tensile strain is related to this type of defects involving inactive As atoms and not to the As active concentration. [ABSTRACT FROM AUTHOR]

In this work, we address the charge trapping properties of Ge dangling bond (DB) defects - GePb1 centers as typified by electron spin resonance spectroscopy (ESR) - found at the interfaces between condensation-grown Si1− xGe x (0.28 < x < 0.8) alloys and insulating SiO2. The ESR observation of singly-occupied paramagnetic GePb1 centers, carried out at 4.3 K, is complemented by temperature-dependent positron annihilation spectroscopy (PAS) in the Doppler broadening mode, which enables observation of the neutral-to-negative defect transitions as the temperature increases from 50 to 300 K. Through correlation of this re-charging behavior with the temperature-induced shift of the Fermi energy, the energy of the Ge DB −/0 transition in the Si0.27Ge0.73 alloy is inferred to be energetically distributed in a ∼0.1 eV interval above the top of the semiconductor valence band. This result refines previous estimates from capacitance-voltage measurements, thus providing independent affirmation that the energy levels of Ge DBs lie inside the band gap of the Si1− xGe x alloys. [ABSTRACT FROM AUTHOR]