[en] The charge states of divacancies induced by 5 MeV self-implantation of doped silicon were investigated by positron annihilation methods. For low doping concentrations, results were found to be in agreement with the predictions of Fermi statistics. For the case of heavily boron-doped silicon (n_B=1times10¹⁹cm^-3) an anomalous single-negative divacancy charge state was detected. We attribute this to the introduction of new levels in the band gap, due to the capture of boron by divacancies, resulting in a boron-divacancy complex. Detailed analysis of positron annihilation spectra suggests that the boron does not reside on a nearest-neighbor site to the divacancy. Isothermal annealing experiments yield activation energy of 0.9±0.1eV for migration of this defect. [copyright] 2001 American Institute of Physics

[en] Molten flibe (2LiF · BeF₂) is a candidate as a cooling and tritium breeding media for future fusion power plants. Neutron interactions with the salt will produce tritium and release excess free fluorine ions. Beryllium metal has been demonstrated as an effective redox control agent to prevent free fluorine, or HF species, from reacting with structural metal components. The extent and rate of beryllium solubility in a pot design experiments to suppress continuously supplied hydrogen fluoride gas has been measured and modeled. This paper presents evidence of beryllium loss from specimens, a dependence of the loss upon bi-metal coupling, i.e., galvanic effect, and the partitioning of the beryllium to the salt and container materials. Various post test investigative methods, viz., scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) were used to explore this behavior

[en] Silicon nanoclusters/nanocrystals (Si-nc) in an SiO₂ matrix exhibit strong visible luminescence, and so are of interest in the pursuit of a silicon-based light emitter for optoelectronics. We have investigated the formation of Si-nc by implanting excess Si at 90 keV into SiO₂ films and then annealing to form nanoclusters by precipitation and ripening. The use of ion implantation provides control over composition and so allows us to optimize the light output. Positron annihilation provides information on vacancy-type defects produced during implantation. Our results suggest that defects may play a key role in Si-nc formation. The depth and size distributions of Si-nc are obtained by transmission electron microscopy, and are correlated with light emission measured by photoluminescence.

[en] We have investigated the nature of violet-blue emission from (Ge, Er) codoped Si oxides (Ge+Er+SiO₂) using photoluminescence (PL) and positron annihilation spectroscopy (PAS) measurements. The PL spectra and PAS analysis for a control Ge-doped SiO₂ (Ge+SiO₂) indicate that Ge-associated neutral oxygen vacancies (Ge-NOV) are likely responsible for the major emission in the violet-blue band. For Ge+Er+SiO₂, both Ge-NOV and GeO color centers are believed to be responsible for the emission band. The addition of Er has a significant influence on the emission, which is discussed in terms of Er-concentration-related structural change in the Ge+Er+SiO₂

[en] Luminescence of amorphous Si quantum dots (Si QDs) in a hydrogenated silicon nitride (SiN_x:H) matrix was examined over a broad range of stoichiometries from Si_3N_2_._0_8 to Si_3N_4_._1_4, to optimize light emission. Plasma-enhanced chemical vapor deposition was used to deposit hydrogenated SiN_x films with excess Si on Si (001) substrates, with stoichiometry controlled by variation of the gas flow rates of SiH_4 and NH_3 gases. The compositional and optical properties were analyzed by Rutherford backscattering spectroscopy, elastic recoil detection, spectroscopic ellipsometry, photoluminescence (PL), time-resolved PL, and energy-filtered transmission electron microscopy. Ultraviolet-laser-excited PL spectra show multiple emission bands from 400 nm (3.1 eV) to 850 nm (1.45 eV) for different Si_3N_x compositions. There is a red-shift of the measured peaks from ∼2.3 eV to ∼1.45 eV as Si content increases, which provides evidence for quantum confinement. Higher N content samples show additional peaks in their PL spectra at higher energies, which we attribute to defects. We observed three different ranges of composition where Tauc band gaps, PL, and PL lifetimes change systematically. There is an interesting interplay of defect luminescence and, possibly, small Si QD luminescence observed in the intermediate range of compositions (∼Si_3N_3_._1_5) in which the maximum of light emission is observed

[en] Amorphous erbium-doped silicon oxide (Si_yO_1-y:Er, y≥1/3) thin films are currently under investigation as a luminescent material system for complementary metal-oxide semiconductor compatible light emitters. We have grown films with y≅1/3 and investigated their properties using both positron annihilation and photoluminescence (PL) spectroscopies. Films were characterized ''as deposited,'' following irradiation with 1 MeV Si⁺ ions and after isochronal annealing. The PL yield from both Er³⁺ ions and sensitizing defects is reduced by irradiation, depending strongly on the irradiation fluence and reaching saturation at ∼4x10¹³ Si⁺/cm². Higher implantation fluences result in an open-volume defect structure in the film that persists after annealing. This annealing behavior is similar to that of an unrecoverable quenching effect on Er³⁺-related PL near 1540 nm, and we suggest that these open-volume defects may cause a decoupling of the Er³⁺ ions from sensitizing oxide point defects that form as a result of the film deposition process

[en] We show the strong dependence on annealing ramp rate of residual open-volume defects in silicon following helium ion implantation and annealing. Helium was implanted at 60 keV energy, 1 x 10¹⁶ cm^-2 fluence into silicon and subsequently annealed to 800 deg. C for 30 min, with ramp rates ranging from 1 to 100 deg. C s^-1. The residual defect distribution was probed by means of positron annihilation spectroscopy and ion channeling, with results demonstrating a strong dependence on the ramp rate. For these conditions, open-volume defects to which the positron technique is sensitive are present in significant concentrations only for annealing ramp rates greater than 5 deg. C s^-1

[en] We present a detailed study of the comparative thermal evolutions of H- and D-related defects in silicon implanted with 2x10¹⁶ H or D/cm², or coimplanted with 0.25x10¹⁶ He/cm² and 0.7x10¹⁶ H/cm², in both orders. By using ion channeling, positron annihilation spectroscopy, and Raman scattering spectroscopy, we found that hydrogen and deuterium interact remarkably differently with primary point defects. He post implantation is found to destroy vacancies highly passivated by hydrogen, whereas He preimplantation accelerates their evolution into atomically smooth internal surfaces. By comparing different systems, subtle points in the interactions between the implanted atoms and point defects are evidenced, and critical defect complexes involved in silicon blistering are identified. Finally, the origins of the isotopic and synergistic effects observed in low energy ion-induced silicon blistering are discussed

[en] During the synthesis of ZnSe nanowires various point and extended defects can form, leading to observed stacking faults and twinning defects, and strong defect related emission in photoluminescence spectra. In this paper, we report on the development of a simple thermodynamic model for estimating the defect concentration in ZnSe nanowires grown under varying Se vapour pressure and for explaining the results of our experimental findings. Positron annihilation spectroscopy was used successfully for the first time for nanowires and the results support predictions from the defect model as well as agreeing well with our structural and optical characterization results. Under very high Se vapour pressure, Se nodules were observed to form on the sidewalls of the nanowire, indicating that beyond a limit, excess Se will begin to precipitate out of the liquid alloy droplet in the vapour-liquid-solid growth of nanowires

[en] Current-voltage measurements have been made at room temperature on a Si-rich silicon oxide film deposited via electron-cyclotron resonance plasma enhanced chemical vapour deposition (ECR-PECVD) and annealed at 750-1000 ⁰C. The thickness of the oxide between Si quantum dots embedded in the film increases with increasing annealing temperature. This leads to a decreasing current density as the annealing temperature is increased. Assuming the Fowler-Nordheim tunnelling mechanism in large electric fields, we obtain an effective barrier height φ_eff of ∼0.7 ± 0.1 eV for an electron tunnelling through an oxide layer between Si quantum dots. The Frenkel-Poole effect can also be used to adequately explain the electrical conduction of the film under the influence of large electric fields. We suggest that at room temperature Si quantum dots can be regarded as traps that capture and emit electrons by means of tunnelling