Nanosecond pulsed laser melting can be used to rapidly recrystallize ion-implanted Si through liquid phase epitaxy. The rapid resolidification that follows the melting results in a supersaturation of impurities and hyperdopes the Si, inducing novel optoelectronic properties with a wide range of applications. In this work, structural changes in the Si lattice in Au-hyperdoped Si are studied in detail. Specifically, we show that the local skewing of the lattice observed previously in regions of extremely high Au concentrations (>1.4 at. %) can be related to the displacement of Au from perfect lattice positions. Surprisingly, although the incorporation of the larger Au atoms into Si is expected to cause swelling of the lattice, reciprocal space mapping shows that a small amount (0.3 at. %) of lattice contraction (decrease in lattice parameter) is present in the hyperdoped layer. Furthermore, positron annihilation spectroscopy shows an elevated concentration of vacancies in the hyperdoped layer. Based on these observations and with the aid of density functional theory, we propose a phenomenological model in which vacancies are kinetically trapped into lattice sites around substitutional Au atoms during resolidification. This vacancy trapping process is hypothesized to occur as a means to minimize lattice strain and may be universal in pulsed laser melted Si systems.

Luminescence of amorphous Si quantum dots (Si QDs) in a hydrogenated silicon nitride (SiNx: H) matrix was examined over a broad range of stoichiometries from Si3N2.08 to Si3N4.14, to optimize light emission. Plasma-enhanced chemical vapor deposition was used to deposit hydrogenated SiNx films with excess Si on Si (001) substrates, with stoichiometry controlled by variation of the gas flow rates of SiH4 and NH3 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 Si3Nx compositions. There is a red-shift of the measured peaks from similar to 2.3 eV to similar 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 (similar to Si3N3.15) in which the maximum of light emission is observed. (C) 2015 AIP Publishing LLC.

Luminescence of amorphous Si quantum dots (Si QDs) in a hydrogenated silicon nitride (SiNx:H) matrix was examined over a broad range of stoichiometries from Si3N2.08 to Si3N4.14, to optimize light emission. Plasma-enhanced chemical vapor deposition was used to deposit hydrogenated SiNx films with excess Si on Si (001) substrates, with stoichiometry controlled by variation of the gas flow rates of SiH4 and NH3 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 Si3Nx 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 (∼Si3N3.15) in which the maximum of light emission is observed.

Using variable-energy positron annihilation spectroscopy, we demonstrate that a different near-surface vacancy concentration accompanies drastic differences in surface resistance of superconducting niobium cavities for particle acceleration. Our data suggest that vacuum baking at 120 °C leads to the doping of a near-surface layer with vacancy-hydrogen complexes, and that higher vacancy-type defect concentration distinguishes electropolished from chemically etched cavities. Our findings may help to explain a strong dependence of cavity performance on heat and chemical treatments, and may be of interest to other physics fields including cavity quantum electrodynamics (QED), microresonators, and single photon detectors.

Rutherford backscattering spectroscopy (RBS) has been used in combination with X-ray photoemission spectroscopy (XPS) to investigate Ge diffusion in Al2O3 (0001) samples. Ge was implanted in c-plane α-Al2O3 (0001) at 80keV to a fluence of 1×1016cm-2 at room temperature followed by thermal annealing in the 30–180min range at 1200°C in a N2 environment. RBS results indicate that implantation-induced damage does not fully amorphize the substrate, while incurred defects are partially annealed after 1h accompanied by Ge phase crystallization. XPS data confirms the existence of GeO2 and GeO. There is a decrease in the Ge content compared to the as-implanted sample, attributed to GeO desorption, which is evident after 30min and by 180min 15% of the original Ge concentration remains. Integrated intensity of the Ge peak in aligned geometry is much lower compared to random geometry with a bimodal distribution of Ge evident in both spectra indicating Ge substitutional incorporation and the formation of a distinct Ge layer. XPS data shows a peak thought to be associated with the distinct Ge layer at ≈1216.5eV, which is a lower binding energy than a Ge reference peak. The lower binding energy is thought to result from a net positive electron density in the substrate due to excess Al atoms in the peak defect-region.

Time-resolved photoluminescence measurements of silicon nanocrystals formed by ion implantation of silicon into silicon dioxide reveal multi-exponential luminescence decays. Three discrete time components are apparent in the rise and decay data, which we associate with different classes of nanocrystals. The values of decay time are remarkably constant with emission energy, but the relative contributions of the three components vary strongly across the luminescence band. In keeping with the quantum confinement model for luminescence, we assign emission at high energies to small nanocrystals and that at low energies to large nanocrystals. By deconvolving the decay data over the full emission band, it is possible to study the migration of excitation from smaller (luminescence donor) to larger (luminescence acceptor) nanocrystals. We propose a model of diffusion of excitation between neighboring nanocrystals, with long lifetime emission being from the largest nanocrystal in the local neighborhood. Our data also allow us to study the saturation of acceptor nanocrystals, effectively switching off excitation transfer, and Auger recombination in non-interacting nanocrystals. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3622151]

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.

The electronic states and optical properties of Si quantum dots (QDs) with variable size prepared by ion implantation in a SiO2 matrix are studied by x-ray photoemission spectroscopy (XPS), photoluminescence (PL), and Raman spectroscopy. The results are compared with several theories of quantum confinement. Our Si 2p binding energies and the valence band energies do not change as a function of QD diameter nor compared to the bulk Si values. Raman spectra show no signs of stress on the Si-QDs. XPS data indicates the presence of a Si2O3 interfacial layer between the Si-QDs and the surrounding SiO2 matrix, which is understood to relieve stress in the QDs and to cause pinning of the valence level. Our XPS results for ion-beam implanted QDs are compared with other group's studies for Si-QDs prepared by alternative methods, and discrepancies in the interfacial compositions are discussed. These results call into question the fundamental predictions and assumptions of many quantum confinement models. It is concluded that the lack of a shift in the valence band is due to a symmetry-breaking process in the hole states, which is not currently accounted for by theory, demonstrating the importance of the hole states during radiative events. This work is intended as a first step in highlighting the features that should be present in a theoretical formalism for embedded Si-QDs, and cause is given to abandon particular formalisms.

Few non-destructive techniques are available which provide information regarding defect type, concentration and depth distribution in semiconductors. The variable-energy positron beam technique has recently demonstrated a sensitivity to near surface defects and impurities at low defect concentrations. In the present study, intrinsic silicon (100) epilayers of ~3000 Å thickness grown by MBE at different temperatures were examined by this method for evidence of changing defect concentration and type.

The microstructural evolution of polymers induced by ion beam irradiation was investigated using gas permeation measurements with different molecule size gases and positron annihilation spectroscopy (PAS) using variable-energy positron. Simultaneous large increases in gas permeability and permselectivity of polymer-ceramic composite membranes modified by 180 keV H+ ion irradiation indicated that ion irradiation of polymers can modify the microstructure of polymer at sub-nanometer level in a controlled way. PAS results were consistent with the gas permeation results. The results of this work demonstrated ion beam irradiation has a promising application potential in the separation industry.

Changes in the defect structure in carbon rich a-SiC:H films deposited on various substrates using ditertiary butyl silane were investigated as a function of thermal treatment. Films grown at high deposition rates exhibit hydrogen trapped in voids. The incorporation of CHn, groups is thought to be the origin for these microvoids. With increasing annealing temperature the effusion of CHn, groups as determined by thermal desorption experiments promotes void growth which was studied using a variable energy positron beam. At annealing temperatures above 600 °C the films densify due to the breaking of C-H bonds and the formation of additional C-C bonds when the voids anneal out.

There are currently several approaches to the development of combined PET/MRI systems, all of which need to address adverse interactions between the two systems. Of particular relevance to the majority of proposed PET/MRI systems is the effect that static and dynamic magnetic fields have on the performance of PET detection systems based on photomultiplier tubes (PMTs). In the work reported in this paper, performance of two conventional PMTs has been systematically investigated and characterized as a function of magnetic field exposure conditions. Detector gain, energy resolution, time resolution, and efficiency were measured for static field exposures between 0 and 6.3mT. Additionally, the short-term recovery and long-term stability of gain and energy resolution were measured in the presence of repeatedly applied dynamic magnetic fields changing at 4T/s. It was found that the detectors recovered normal operation within several milliseconds following the end of large pulsed magnetic fields. In addition, the repeated applications of large pulsed magnetic fields did not significantly affect detector stability. Based on these results, we implemented a proof-of-principle PET/field-cycled MRI (FCMRI) system for small animal imaging using commercial PMT-based PET detectors. The first PET images acquired within the PET/FCMRI system are presented. The image quality, in terms of spatial resolution, was compared between standalone PET and the PET/FCMRI system. Finally, the relevance of these results to various aspects of PET/MRI system design is discussed.

We studied photoluminescent properties and luminescent decay dynamics in Si quantum dots (QDs) produced by Si implantation in SiO2, and their modification by the application of an implantation mask. Silicon quantum dots were prepared by ion implantation, followed by high temperature annealing leading to nanocrystal nucleation and growth. The mask was prepared by spin-coating silica microspheres to achieve laterally-selective implantation, to control QD size and separation. Transmission electron microscopy (TEM) images were obtained to verify the diameter of the quantum dots. We observe a noticeable peak shift and narrowing in the photoluminescence spectra with the application of the implantation mask. Observed maxima in the photoluminescence spectra are compared with a quantum field theoretical model using an infinite confining 1D potential for Si quantum dots. We comment on the role of excitation transfer by observing a change in the dispersion exponent of the luminescent decay dynamics due to the mask.

The location and size distribution of silicon nanocrystals (Si-nc) formed in SiO2 by Si ion implantation and annealing were investigated. Transmission electron microscopy images revealed that the location and size distribution of the Si-nc are strongly correlated with the location of the vacancies produced in the SiO2 network by the implantation, as simulated by stopping and range of ions in matter and measured by positron annihilation spectroscopy (PAS). Simultaneous consideration of PAS and photoluminescence measurements suggest that the diffusion necessary for Ostwald ripening of Si-nc depends on the concentration and location of vacancy-type defects, and when the majority of these defects have been removed by annealing, the growth of the Si-nc slows dramatically or stops entirely. We demonstrate the capability for modification and enhancement of emission from Si-nc through the controlled introduction of defects into the oxide prior to annealing.

Amorphous erbium-doped silicon oxide (SiyO1−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 ∼4×1013 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.

Most indentation studies to date on crystalline germanium (c-Ge) and related covalent semiconductors have been carried out on pristine defect-free material. This paper addresses the paucity of studies on imperfect crystalline materials by exploring the impact of defects generated by ion implantation, prior to contact damage, upon the mechanical properties of c-Ge. Implantation with Ge ions is carried out to generate a layer of highly defective but still-crystalline Ge. Under nanoindentation with a sharp diamond tip, enhanced plasticity is observed relative to pristine material. Characterization by cross-sectional transmission electron microscopy, atomic force microscopy, and load curve analysis shows softening, quasiductile extrusion, and cracking suppression taking place. These changes can be explained by the high density of defects, and dangling bonds in particular, created by ion implantation and revealed by positron-annihilation spectroscopy, and are proportional to the fraction of "missing bonds" or vacancies in the material. A thermal annealing step at 200 degrees C is sufficient to restore the mechanical response of pristine material, despite incomplete recovery of the original pristine crystal structure.

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.