Carbon-based nanocrystals, formed in silica by low-energy carbon implantation and subsequent thermal annealing, have been previously reported, but ambiguities as to the equilibrium morphology and its relationship to the source of photoluminescence. In our study, silica samples were implanted with 2.0x 1017 atoms/cm(2) C- ions at energies of 8.5 keV, 40 keV and 70 keV. Samples were annealed for 15, 30, 60, 120, 240 minutes at 1100 degrees C in a forming gas (4%H-2+96%Ar). Rutherford backscattering (RBS), particle induced x-ray emission (PIXE), and photoluminescence (PL) were used to characterize the nanocrystals formed by these conditions. The resulting photoluminescence analysis indicated that the size distribution of the nanocrystals could be tailored to yield a broad spectral output between 2.0-2.8 eV by controlling the formation parameters. Also a few notable factors, such as liberation of oxygen in the silica during implantation, as well as uptake of ambient oxygen in the annealing furnace, will be shown to play a critical role in the formation process. Additional passivation with 96%Ar+4%H-2 at a lower temperature (600 degrees C) had little effect. The results of this work will be presented and compared to previous studies.

Due to their unique structure, which provides a large ratio of surface area to volume, carbon nanotubes (CNTs) may prove to be ideal materials for hydrogen storage. Typically, hydrogen is stored in CNTs by exposure of the material to a high-pressure H-2 atmosphere at various temperatures. The maximum hydrogen concentrations stored following this method and measured using Ion Beam Analysis do not exceed 1 wt.%. Introduction of defects by ion irradiation prior to high-pressure H, treatment, offers an alternative method to activate H adsorption and enhance the chemisorption of hydrogen. In this work, a low-energy H- beam at 13.5 keV was used to irradiate single-wall nanotubes (SWNTs) and to produce defects in SWNTs to enhance hydrogen adsorption. Hydrogen desorption measurements are presented, which compare the behavior in irradiated and as-prepared SWNTs after hydrogen storage treatment. These desorption measurements showed a large enhancement of hydrogen desorption for irradiated samples. (C) 2007 Elsevier B.V. All rights reserved.

An initial study was performed on the feasibility of forming Ru2Si3 nanocrystals using an implantation and oxidation technique. Silicon (100) was implanted with Ru− ions at a fluence of 1.0×1017atoms/cm2 at an energy of 45keV. The implant was then oxidized at 900°C to segregate nanocrystals at the silicon/oxide interface using two methods, a dry and wet thermal oxidation. Oxidation times were chosen so the oxide thickness roughly matched for the two methods. Rutherford backscattering (RBS) with 1.5MeV alphas was used to monitor the concentration of Ru as a function of depth, and indicated both techniques resulted in segregated Ru. However, the Ru did not self-assemble into a thin layer as expected, but instead diffused further in to the sample for both cases. The results of this initial study and suggested techniques to overcome the enhanced diffusion are presented here.

An MeV energy beam line for ion implantation of carbon and silicon to fashion optically-active nanocrystals has been constructed at the Ion Beam Modification and Analysis Laboratory at the University of North Texas (UNT). The implantation line is at 15° and consists of a set of slits to define the beam, a doublet quadrupole focusing lens, a 40kV electrostatic ion beam raster scanner and target chamber for 2cm diameter circular implants. A 1.95m drift section between the scanner and target is sufficient to accommodate 10MeV carbon and silicon ions. It was determined that the separation between the slits and quadrupole should be at least 10cm to achieve point-to-parallel focusing in the beam line. It was also determined that, for a slit opening of 2mm, the distance between the upstream bending magnet and the analyzing slit should be at least 20cm to allow only single isotopes of carbon to pass into the beam line.

Thermal oxidation of Ge+-implanted silicon (100) has been investigated over a wide range of Ge-fluences and temperatures under both wet and dry oxidation conditions. Implanted samples were pre-oxidized and then etched to produce a common sample set with a thin Ge:Si layer at the surface to ensure a common sample morphology for various experimental conditions. Oxidation kinetics as a function of Ge-fluence and annealing temperature are reported and compared to results from virgin silicon (100). Random and ion-channeled Rutherford backscattering (RBS) characterization techniques, accompanied by computer simulations (SIMNRA), were used for oxide and Ge-segregated layer thickness and composition measurements. Ge-rich GexSi1−x segregated layer composition are presented as a function of implantation and oxidation conditions.

Optically active metal silicides offer an interesting direct band gap material that in principle can be integrated into Si-based microelectronics to provide optoelectronic functionality. In particular the isostructual phases of Os2Si3 and Ru2Si3 and their alloys could be tailored to yield a response ranging from the visible blue to the near infrared. The metal-rich phase, Os2Si3 poses some difficulties for the annealing process since it is the low temperature phase, and thus difficult to fabricate in silica due to the high temperature needed for precipitation. Ion implantation of Os and Si ions above the stoichiometric ratio resulted in the formation of Os2Si3 at 1100°C, as indicated by Rutherford backscattering. Faint visible photoluminescence was detected with a peak centered near (520nm) corresponding to the previously reported bandgap of 2.3eV [L. Schellenberg, H.F. Braun, J. Muller, J. Less-Common Met. 144 (1988)]. To the best of our knowledge this is the first reported photoluminescence from this material.

Carbon nanoclusters formed using ion implantation and thermal annealing are shown to photoluminescence in the visible range. Silica samples were implanted with a fluence of 2×1017atoms/cm2, 70keV carbon ions and thermally annealed for 4h at 1100°C. Photoluminescence measurement made at select intervals during the anneal process show continued growth of the nanoclusters within the silica throughout the process. However, Rutherford backscattering showed a rapid loss of carbon during the initial 15min of annealing indicating a competition between the growth of the second-phase nanoparticles and the formation of CO, a volatile form of carbon.

Carbon nanotubes (CNTs) are studied as a possible hydrogen storage medium for future energy needs. Typically, hydrogen is stored in the CNTs by exposure of the material to a high-pressure H2 atmosphere at different temperatures. The maximum hydrogen concentrations stored following this method and measured using ion beam analysis do not exceed 1wt.%. Introduction of defects by ion irradiation (i.e. implantation) prior to high-pressure H2 treatment, offers an alternative method to activate H adsorption and enhance the chemisorption of hydrogen. This is a preliminary work where hydrogen was introduced into single-wall nanotubes and carbon films by low-energy (13.6keV) hydrogen ion implantation. Elastic recoil detection was used to measure the quantity and depth distribution of hydrogen retained in the carbonaceous materials. Results show that there are substantial differences in the measured profiles between the CNT samples and the vitreous carbon. On another hand, only ∼43% of the implanted hydrogen in the CNTs is retained in the region where it should be located according to the SRIM simulations for a solid carbon sample.

While Si-based integrated circuits dominate the microelectronics marketplace, they cannot be fabricated with optical functionality since Si is indirect. Alternative materials have been used in such applications but the ability to integrate an optically active material directly onto a silicon substrate to co-opt the advances in Si technology and processing capabilities is the better solution. Many of the transition metals form silicides that are direct band gap semiconductors and, as such, may be integrated with Si to achieve the desired optical properties. Ion implantation of the transition metal into Si was used to form the desired silicide phase by reaction of the metal with the Si substrate. Depending upon the fluence the resulting implanted layer can consist of a two-phase region in which the silicide phase forms as isolated precipitates randomly oriented within a heavily dislocated Si matrix. Rutherford backscattering/ion channeling spectrometry was used to monitor this process as a function of temperature and time. A unique method for orienting the silicide precipitates to align them crystallographically with the Si substrate and eliminate the ion-induced dislocations that form during the initial implant is discussed. This method involves oxidation of the implanted region to segregate the silicide phase at the oxide interface. Initial results of Os- ions implanted into Si(100) are presented. (c) 2005 Published by Elsevier B.V.

The compound semiconductor GaN has become the electronic material for many semiconductor applications because of its high-power, high-temperature and high-frequency characteristics. The introduction of impurities to GaN, either by direct doping or by contamination of unwanted impurities during fabrication leads to changes in its electronic properties [CAARI 2002: 17th International Conference on the Application of Accelerators in Research and Industry, American Institute of Physics, Vol. 680, 2002, p. 369]. There are a number of techniques routinely used to analyze the depth profiles of impurities in semiconductor substrates, but they lack the sensitivity required for today's applications. For example, secondary ion mass spectrometry (SIMS) has molecular interferences which obscure analysis of some elements. Trace element accelerator mass spectrometry (TEAMS), which is a combination of accelerator mass spectrometry (AMS) and SIMS, removes molecular interferences by breaking apart the molecules in an accelerator. Like SIMS, secondary ions are sputtered from a target, the negative ions are then extracted and injected into a 3 MV tandem accelerator. As they pass through a gas at the center of the terminal, molecular breakup is achieved via a coulomb explosion due to electron stripping. The ions are then passed through momentum/charge and energy/charge filters, which are used to separate out many of the unwanted interferences. In most cases, TEAMS is a complimentary technique to SIMS and can eliminate the interferences that may not be resolvable by SIMS. The purpose of this work is to examine the depth profiles and detection limits of various impurities implanted into a GaN substrate.

GaN is one of the most promising electronic materials for applications requiring high-power, high frequencies, or high-temperatures as well as opto-electronics in the blue to ultraviolet spectral region. We have recently measured depth profiles of Mg, Si, and Zn implants in GaN substrates by the TEAMS particle counting method for both matrix and trace elements, using a gas ionization chamber. Trace Element Accelerator Mass Spectrometry (TEAMS) is a combination of Secondary Ion Mass Spectrometry (SIMS) and Accelerator Mass Spectrometry (AMS) to measure trace elements at ppb levels. Negative ions from a SIMS like source are injected into a tandem accelerator. Molecular interferences inherent with the SIMS method are eliminated in the TEAMS method. Negative ion currents are extremely low with GaN as neither gallium nor nitrogen readily forms negative ions making the depth profile measurements more difficult. The energies of the measured ions are in the range of 4-8 MeV. A careful selection of mass/charge ratios of the detected ions combined with energy-loss behavior of the ions in the ionization chamber eliminated molecular interferences.

When an in vitro assay system and radioimmunoassays specific for juvenile hormones (JH) I and III were used to probe the effect of co-incubating pupal brains with last instar larval corpora allata (CA) from the tobacco hornworm, Manduca sexta, a selective activation of JH III synthesis by the CA was observed. This homolog-specific activation suggested the presence of an allatotropic factor for the synthesis of JH III (JH III ATF), and its presence was demonstrated by the ability of a postmicrosomal supernatant of a day 0 pupal brain homogenate to activate the CA in vitro in a dose-dependent manner. This moiety appears to be a protein, based on its heat lability and protease sensitivity, and has an apparent molecular size of 40 kD and an isoelectric point of 5.5. JH III ATF activity is localized in specific neural tissues of the day 0 pupa, the brain and first three abdominal ganglia, with the brain containing 4 times the activity in the ganglia. The existence of this factor suggests that JH III synthesis by the CA of Manduca is regulated by a neuropeptide.