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At least 1,045 records · Page 58
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Functionalization of cubic boron nitride films with rhodamine B and their fluorescent properties

Fluorophore-functionalized cubic boron nitride (cBN) films grown by chemical vapor deposition were achieved by immobilizing rhodamine B isothiocyanate onto their surfaces. To perform the immobilization, the cBN substrates were modified with amino groups by photochemical reaction between hydrogen-terminated cBN surfaces and allylamine. The surface analysis of hydrogen-terminated cBN films surfaces and after functionalization with x-ray photoelectron spectroscopy verified that rhodamine B was indeed attached to the cBN surfaces with covalent bonding. The rhodamine B-functionalized cBN surfaces showed significant variation in fluorescent spectra and confocal imaging upon the treatment in acidic or basic solutions.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Excitation wavelength dependence of water-window line emissions from boron-nitride laser-produced plasmas

We investigated the effects of laser excitation wavelength on water-window emission lines of laser-produced boron-nitride plasmas. Plasmas are produced by focusing 1064 nm and harmonically generated 532 and 266 nm radiation from a Nd:YAG laser on BN target in vacuum. Soft x-ray emission lines in the water-window region are recorded using a grazing-incidence spectrograph. Filtered photodiodes are used to obtain complementary data for water-window emission intensity and angular dependence. Spectral emission intensity changes in nitrogen Ly-{alpha} and He-{alpha} are used to show how laser wavelength affects emission. Our results show that the relative intensity of spectral lines is laser wavelength dependent, with the ratio of Ly-{alpha} to He-{alpha} emission intensity decreasing as laser wavelength is shortened. Filtered photodiode measurements of angular dependence showed that 266 and 532 nm laser wavelengths produce uniform emission.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Diffusion of co-implanted carbon and boron in silicon and its effect on excess self-interstitials

Diffusion of co-implanted carbon (C) and boron (B) in silicon (Si) and its effect on excess Si self-interstitials (I's) after annealing at 800 and 1000 deg. C were investigated by means of secondary ion mass spectrometry. The experimental results showed that C diffusion was not significant at 800 and 1000 deg. C but later became visible for longer annealing times at 1000 deg. C. B diffusion was reduced by the presence of C when no significant C diffusion was observed, but it was enhanced when C diffusion was observed. These results indicate that all implanted C atoms form immobile CI clusters with excess I in the amount of implanted C and that these CI clusters are stable and trap I to reduce B diffusion. On the contrary, CI clusters are dissolved to emit I for longer annealing times at 1000 deg. C and both B and C diffusion are enhanced. Diffusion simulation based on these models fits the experimental profiles of B and C.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Explicitly correlated Gaussian calculations of the {sup 2}D Rydberg states of the boron atom

Accurate non-relativistic variational calculations are performed for the seven lowest members of the {sup 2}D Rydberg series (1s{sup 2}2s2p{sup 2}, and 1s{sup 2}2s{sup 2}nd, n= 3, Horizontal-Ellipsis , 8) of the boron atom. The wave functions of the states are expanded in terms of all-electron explicitly correlated Gaussian basis functions and the effect of the finite nuclear mass is directly included in the calculations allowing for determining the isotopic shifts of the energy levels. The Gaussian basis is optimized independently for each state with the aid of the analytic energy gradient with respect to the Gaussian parameters. The calculations represent the highest accuracy level currently achievable for the considered states. The computed energies are compared with the available experimental data.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Modeling boron profiles in silicon after pulsed excimer laser annealing

In this work, we investigated four possible mechanisms which were candidates to explain the shape of boron profiles after ion implantation and melting excimer laser annealing in silicon. A laser with a wavelength of 308 nm and a pulse duration of {approx}180 ns was used. To simulate this process, an existing model for the temperature and phase evolution was complemented with equations for the migration of dopants. Outdiffusion, thermodiffusion, segregation, and adsorption were investigated as possible mechanisms. As a result, we found that outdiffusion and segregation can be excluded as major mechanisms. Thermodiffusion as well as adsorption could both reproduce the build-up at low melt depths, but only adsorption the one at deeper melt depths. In both cases, ion beam mixing during SIMS measurement had to be taken into account to reproduce the measured profiles.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Energy fluxes in a radio-frequency magnetron discharge for the deposition of superhard cubic boron nitride coatings

Energy flux measurements by a calorimetric probe in a rf-magnetron plasma used for the deposition of super-hard c-BN coatings are presented and discussed. Argon as working gas is used for sputtering a h-BN target. Adding a certain amount of N{sub 2} is essential for the formation of stoichiometric BN films, since a lack of nitrogen will lead to boron rich films. Subsequently, the contributions of different plasma species, surface reactions, and film growth to the resulting variation of the substrate temperature in dependence on nitrogen admixture are estimated and discussed. In addition, SRIM simulations are performed to estimate the energy influx by sputtered neutral atoms. The influence of magnetron target power and oxygen admixture (for comparison with nitrogen) to the process gas on the total energy flux is determined and discussed qualitatively, too. The results indicate that variation of the energy influx due to additional nitrogen flow, which causes a decrease of electron and ion densities, electron temperature and plasma potential, is negligible, while the admixture of oxygen leads to a drastic increase of the energy influx. The typical hysteresis effect which can be observed during magnetron sputtering in oxygen containing gas mixtures has also been confirmed in the energy influx measurements for the investigated system. However, the underlying mechanism is not understood yet, and will be addressed in further investigations.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Lattice and grain-boundary diffusions of boron atoms in BaSi{sub 2} epitaxial films on Si(111)

A 180-nm-thick boron (B) layer was deposited on a 300-nm-thick a-axis-oriented BaSi{sub 2} epitaxial film grown by molecular beam epitaxy on Si(111) and was annealed at different temperatures in ultrahigh vacuum. The depth profiles of B were investigated using secondary ion mass spectrometry (SIMS) with O{sup 2+}, and the diffusion coefficients of B were evaluated. The B profiles were reproduced well by taking both the lattice and the grain boundary (GB) diffusions into consideration. The cross-sectional transmission electron microscopy (TEM) image revealed that the GBs of the BaSi{sub 2} film were very sharp and normal to the sample surface. The plan-view TEM image exhibited that the grain size of the BaSi{sub 2} film was approximately 0.6 {mu}m. The temperature dependence of lattice and GB diffusion coefficients was derived from the SIMS profiles, and their activation energies were found to be 4.6 eV and 4.4 eV, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

The location and doping effect of boron in Si nanocrystals embedded silicon oxide film

Electrically activated doping of boron (B) atoms into the Si-nanocrystals (Si-NCs) embedded silicon oxide film is achieved by co-sputtering technique following with the annealing treatment. The evolution of the size, the shape, and the density of Si-NCs with the doping of B atoms is investigated. The observation of x-ray photoelectron spectroscopy of Si 2p and B 1s and the decrease in lattice spacing of Si (111) plane suggest that B atoms are doped into Si-NCs. The activated doping is confirmed by the Fano effect of the micro-Raman spectra for Si-NCs and the drastic decrease of the sheet resistance.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Two-dimensional excitons in three-dimensional hexagonal boron nitride

The recombination processes of excitons in hexagonal boron nitride (hBN) have been probed using time-resolved photoluminescence. It was found that the theory for two-dimensional (2D) exciton recombination describes well the exciton dynamics in three-dimensional hBN. The exciton Bohr radius and binding energy deduced from the temperature dependent exciton recombination lifetime is around 8 Å and 740 meV, respectively. The effective masses of electrons and holes in 2D hBN deduced from the generalized relativistic dispersion relation of 2D systems are 0.54m{sub o}, which are remarkably consistent with the exciton reduced mass deduced from the experimental data. Our results illustrate that hBN represents an ideal platform to study the 2D optical properties as well as the relativistic properties of particles in a condensed matter system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Layered insulator hexagonal boron nitride for surface passivation in quantum dot solar cell

Single crystalline, two dimensional (2D) layered insulator hexagonal boron nitride (h-BN), is demonstrated as an emerging material candidate for surface passivation on mesoporous TiO{sub 2}. Cadmium selenide (CdSe) quantum dot based bulk heterojunction (BHJ) solar cell employed h-BN passivated TiO{sub 2} as an electron acceptor exhibits photoconversion efficiency ∼46% more than BHJ employed unpassivated TiO{sub 2}. Dominant interfacial recombination pathways such as electron capture by TiO{sub 2} surface states and recombination with hole at valence band of CdSe are efficiently controlled by h-BN enabled surface passivation, leading to improved photovoltaic performance. Highly crystalline, confirmed by transmission electron microscopy, dangling bond-free 2D layered h-BN with self-terminated atomic planes, achieved by chemical exfoliation, enables efficient passivation on TiO{sub 2}, allowing electronic transport at TiO{sub 2}/h-BN/CdSe interface with much lower recombination rate compared to an unpassivated TiO{sub 2}/CdSe interface.

14 SOLAR ENERGY↗
DOE OSTI2013

Ab initio study of phase transition of boron nitride between zinc-blende and rhombohedral structures

Boron nitride has polymorphs such as zinc-blende (c-BN), wurtzite (w-BN), rhombohedral (r-BN), and graphite-like (h-BN) forms. We simulate the direct conversion of r-BN to c-BN through electronic excitation. In our calculation, the conversion is made possible by increasing the hole concentration to over 0.06/atom. This conversion should be experimentally possible by hole-doping via an electric double layer transistor (EDLT) or capacitor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

A first-principles core-level XPS study on the boron impurities in germanium crystal

We systematically investigated the x-ray photoelectron spectroscopy (XPS) core-level shifts and formation energies of boron defects in germanium crystals and compared the results to those in silicon crystals. Both for XPS core-level shifts and formation energies, relationship between defects in Si and Ge is roughly linear. From the similarity in the formation energy, it is expected that the exotic clusters like icosahedral B12 exist in Ge as well as in Si.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Ab initio modeling of graphene layer functionalized with boron and nitrogen

We present a computational study of the phenomenon of opening the band gap in graphene by means of functionalization with boron and nitrogen atoms. For most of the considered structures, we observe a nonzero energy gap with the width slightly dependent on the concentration of the substituent atoms. Additionally, elastic properties for graphene functionalized with B/N atoms for concentrations of 2% and 4% have been predicted. N-substitution almost does not influence the elastic moduli of graphene, while changes caused by B-substitution are more remarkable.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Note: An ion source for alkali metal implantation beneath graphene and hexagonal boron nitride monolayers on transition metals

The construction of an alkali-metal ion source is presented. It allows the acceleration of rubidium ions to an energy that enables the penetration through monolayers of graphene and hexagonal boron nitride. Rb atoms are sublimated from an alkali-metal dispenser. The ionization is obtained by surface ionization and desorption from a hot high work function surface. The ion current is easily controlled by the temperature of ionizer. Scanning Tunneling Microscopy measurements confirm ion implantation.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Local environment of silicon in cubic boron nitride

Si-doped cubic boron nitride (c-BN) is synthesized at high pressure and high temperature, and the local environment of Si is investigated using X-ray absorption near edge structure (XANES) and first-principles calculations. Si-K XANES indicates that Si in c-BN is surrounded by four nitrogen atoms. According to first-principles calculations, the model for substitutional Si at the B site well reproduces experimental Si-K XANES, and it is energetically more favorable than substitutional Si at the N site. Both the present experimental and theoretical results indicate that Si in c-BN prefers the B site to the N site.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Properties of boron-doped thin films of polycrystalline silicon

The properties of polycrystalline-silicon films deposited by low pressure chemical vapor deposition and doped heavily in situ boron-doped with concentration level of around 2×10{sup 20}cm{sup −3} has been studied. Their properties are analyzed using electrical and structural characterization means by four points probe resistivity measurements and X-ray diffraction spectra. The thermal-oxidation process are performed on sub-micron layers of 200nm/c-Si and 200nm/SiO{sub 2} deposited at temperatures T{sub d} ranged between 520°C and 605°C and thermally-oxidized in dry oxygen ambient at 945°C. Compared to the as-grown resistivity with silicon wafers is known to be in the following sequence <ρ{sub 200nm/c−Si}> < <ρ{sub 200nm/SiO2}> and <ρ{sub 520}> < <ρ{sub 605}>. The measure X-ray spectra is shown, that the Bragg peaks are marked according to the crystal orientation in the film deposited on bare substrates (poly/c-Si), for the second series of films deposited on bare oxidized substrates (poly/SiO{sub 2}) are clearly different.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Effective dopant activation by susceptor-assisted microwave annealing of low energy boron implanted and phosphorus implanted silicon

Rapid processing and reduced end-of-range diffusion result from susceptor-assisted microwave (MW) annealing, making this technique an efficient processing alternative for electrically activating dopants within ion-implanted semiconductors. Sheet resistance and Hall measurements provide evidence of electrical activation. Susceptor-assisted MW annealing, of ion-implanted Si, enables more effective dopant activation and at lower temperatures than required for rapid thermal annealing (RTA). Raman spectroscopy and ion channeling analyses are used to monitor the extent of ion implantation damage and recrystallization. The presence and behavior of extended defects are monitored by cross-section transmission electron microscopy. Phosphorus implanted Si samples experience effective electrical activation upon MW annealing. On the other hand, when boron implanted Si is MW annealed, the growth of extended defects results in reduced crystalline quality that hinders the electrical activation process. Further comparison of dopant diffusion resulting from MW annealing and rapid thermal annealing is performed using secondary ion mass spectroscopy. MW annealed ion implanted samples show less end-of-range diffusion when compared to RTA samples. In particular, MW annealed P{sup +} implanted samples achieve no visible diffusion and equivalent electrical activation at a lower temperature and with a shorter time-duration of annealing compared to RTA. In this study, the peak temperature attained during annealing does not depend on the dopant species or dose, for susceptor-assisted MW annealing of ion-implanted Si.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Performance analysis of boron nitride embedded armchair graphene nanoribbon metal–oxide–semiconductor field effect transistor with Stone Wales defects

We study the performance of a hybrid Graphene-Boron Nitride armchair nanoribbon (a-GNR-BN) n-MOSFET at its ballistic transport limit. We consider three geometric configurations 3p, 3p + 1, and 3p + 2 of a-GNR-BN with BN atoms embedded on either side (2, 4, and 6 BN) on the GNR. Material properties like band gap, effective mass, and density of states of these H-passivated structures are evaluated using the Density Functional Theory. Using these material parameters, self-consistent Poisson-Schrodinger simulations are carried out under the Non Equilibrium Green's Function formalism to calculate the ballistic n-MOSFET device characteristics. For a hybrid nanoribbon of width ∼5 nm, the simulated ON current is found to be in the range of 265 μA–280 μA with an ON/OFF ratio 7.1 × 10{sup 6}–7.4 × 10{sup 6} for a V{sub DD} = 0.68 V corresponding to 10 nm technology node. We further study the impact of randomly distributed Stone Wales (SW) defects in these hybrid structures and only 2.5% degradation of ON current is observed for SW defect density of 3.18%.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗