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Several shallow donor bound exciton photoluminescence (PL) transitions are reported in ZnO nanowires doped with carbon. The emission energies are in the range of 3360.8-3361.9 meV, close to previously reported emission lines due to excitons bound to donor point defects, such as Ga, Al, In, and H. The addition of small amounts of hydrogen during growth results in a strong enhancement of the PL of these carbon related emission lines, yet PL and annealing measurements indicate no appreciable bulk hydrogen. The observation of two electron satellites for these emission lines enables the determination of the donor binding energies. The dependence of exciton localization energy on donor binding energy departs somewhat from the usual linear relationship observed for group III donors, indicating a qualitatively different central cell potential, as one would expect for a complex. Emission lines due to excitons bound to ionized donors associated with these defects are also observed. The dependence of the PL emission intensities on temperature and growth conditions demonstrates that the lines are due to distinct complexes and not merely excited states of each other. [ABSTRACT FROM AUTHOR]

We report on the surface passivation of Au-assisted Te-doped GaAs nanowires (NWs) grown by metalorganic vapor phase epitaxy. The electrical properties of individual free standing NWs were assessed using a tungsten nano-probe inside a scanning electron microscope. The diameter independent apparent resistivity of both strained and relaxed passivated NWs suggests the unpinning of the Fermi level and reduction of sidewalls surface states density. Similar currentvoltage properties were observed for partially axially relaxed GaAs/GaP NWs. This indicates a negligible contribution of misfit dislocations in the charge transport properties of the NWs. Low temperature micro-photoluminescence (μ-PL) measurements were also carried out for both uncapped and passivated GaAs NWs. The improvement of the integrated (μ-PL) intensity for GaAs/GaP NWs further confirms the effect of passivation. [ABSTRACT FROM AUTHOR]

We report on the growth of GaSb shells on Au-catalyzed GaAs or GaP nanowires (NWs) using metalorganic vapor phase epitaxy. The large lattice mismatch between GaSb and GaAs (GaP), 7.8% (11.8%), results in surface roughening and GaSb island formation via the Stranski-Krastanov (S-K) growth mode. Based on transmission electron microscopy (TEM) analysis, coherent GaSb islands on GaAs NWs could be grown up to a thickness of 1.8 nm for a core diameter of 34 ± 5 nm. For greater shell thickness of 9 ± 3 nm, equal axial and radial strain relaxation occurred increasing from 74% ± 3% for GaAs/GaSb NWs and 91% ± 2% for GaP/GaSb NWs to 100% with increasing core diameter from 15 ± 2 nm to 55 ± 3 nm. Axial strain is relieved by periodic misfit dislocations with edge components parallel to the growth direction. Tangential relaxation is presumed to occur partially by roughening via the S-K growth mode but dislocations with edge components perpendicular to the growth direction were not detected. Raman scattering measurements were performed on ensembles of NWs and the absolute residual strain in the core and shell were determined from the shift of the zone-center phonon modes. Raman results were consistent with the TEM analysis. It was found that the residual strain is higher in GaAs/GaSb NWs (7.3%) compared to GaP/GaSb NWs (1.7%). [ABSTRACT FROM AUTHOR]

We demonstrate the use of Fourier transform infrared (IR) spectroscopy for midinfrared emission measurements following x-ray or electron excitation. Spectra from an InAs low band-gap semiconductor film, which emits in the IR from 3000 to 3400 cm-1, are presented. There is good agreement between the present results and previously published laser-excited spectra. Using focused beams, it should be possible to perform sub-diffraction-limited IR imaging. In addition, simultaneous structural and electronic analysis could be performed using the x-ray or electron excitation probes. [ABSTRACT FROM AUTHOR]

InAsSb/InAs type II strain balanced superlattices lattice matched to GaSb have recently been proposed as an alternative to InAs/(In)GaSb short period superlattices for mid- to long infrared photodetectors. Photoluminescence data at 4 K of OMVPE grown InAsSb (multi-) quantum wells in an InAs matrix on InAs and GaSb substrates is presented for Sb compositions between 4% and 27%. The measured transition energies are simulated with a self-consistent Poisson and Schroedinger equation solver that includes strain and band-offsets. The fitted parameters are then used to predict the type II transition energies of InAsSb/InAs strain balanced superlattice absorber stacks at 77 K for different compositions and periods. The optical matrix element was calculated and compared with InAs/(In)GaSb superlattices. The InAsSb/InAs structures can be designed with higher or equal matrix elements for longer periods. Finally, the initial optical response data of an unoptimized strain balanced InAs0.79Sb0.21/InAs detector with a 40 nm period are shown. Its cutoff wavelength is 0.15 eV (8.5 μm), in good agreement with the predicted type II transition energy of 0.17 eV. [ABSTRACT FROM AUTHOR]

We report the electrical properties of n-InAsSb/n-GaSb heterojunctions as a function of the GaSb doping concentration. Because of the staggered type II band alignment, strong electron accumulation occurs on the InAsSb side. For low GaSb doping, depletion occurs on the GaSb side resulting in a Schottky-like junction as previously reported. As the GaSb doping increases, the built-in voltage as well as depletion width decreases as shown using self-consistent simulations. For GaSb doping levels above 5×1017 cm-3, the junction loses its rectifying properties due to tunneling. Under zero and reverse bias voltage, the photoresponse of these diodes is solely due to the photovoltaic effect in the GaSb depletion region. For forward bias voltages >400 mV, we also observed a photoconductive response from the InAsSb layer. The proposed physical mechanism is quite different from the one suggested in a recent paper. [ABSTRACT FROM AUTHOR]

Annealing measurements performed on heavily carbon-doped InAs samples grown by organometallic vapor phase epitaxy confirm that the n-type behavior observed in this material is due to the formation of dicarbon donor defects. The structural, electrical, and optical properties of n-type carbon-doped InAs annealed at temperatures of 400 °C and higher were studied. Raman and infrared spectroscopy showed a reduction in the intensity of carbon acceptor and carbon-hydrogen lines along with an increase in the strength of the dicarbon local vibrational mode after annealing. X-ray diffraction revealed that the lattice constant of annealed samples increased toward that of the undoped samples. These changes were accompanied by increases in the electron concentration and mobility. Based on the expected energy level of dicarbon donors in GaAs, we predict that the dicarbon defects in InAs should introduce a resonant level close to or slightly above the conduction band minimum. [ABSTRACT FROM AUTHOR]

Provides guidelines for interpretation of electron beam computed tomography (EBCT) calcium scores from the Dallas Heart Disease Prevention Project, a cross-sectional epidemiologic study designed to determine the gene-environment interactions causing cardiovascular disease in the ethnic populations of Dallas County, Texas. Accepted modalities for detection of coronary calcium, EBCT and helical CT; Correlation of the degree of calcium of the coronary arteries with the coronary atherosclerosis.

We report on the electrical properties of Te-doped GaAs nanowires (NW) grown via the vapor-liquid-solid mechanism. Gold nanoparticles were used as growth catalysts and contacts for electrical measurements using a nanoprobe technique. Semi-log I-V curves show 6-8 decades of forward-voltage linearity giving an ideality factor of 1.25 ± 0.06 and barrier height of 0.78 ± 0.04 eV. When normalized to NW cross-sectional area, all curves overlapped consistent with an n-type carrier concentration (9 ± 1) × 1017 cm-3 and a constant NW resistivity (2.5 ± 0.5) × 10-3 Ω cm. Raman spectroscopy indicated a small surface depletion width of less than 5 nm. [ABSTRACT FROM AUTHOR]

We report the growth and optical characterization of InAsSb/InAs strain balanced superlattice structures on GaSb substrates for potential application in midinfrared photodetectors. Photoluminescence (PL) emission was observed in the range 5 μm≤λ≤10 μm at 4 K for Sb compositions 0.14≤xSb≤0.27. The PL energy was found to depend approximately linearly on antimony, consistent with a type II band lineup. The dependence of the emission energies on the Sb mole fraction is in agreement with trends predicted by various theoretical works. The data suggest that this transition reaches zero energy for a composition of xSb=0.37. [ABSTRACT FROM AUTHOR]

Carbon-doped InAs samples grown by organometallic vapor phase epitaxy were studied by Raman spectroscopy. A local vibrational mode (LVM) identified at 527 cm-1 correlated with carbon concentration from secondary ion mass spectroscopy measurements, and based on LVM trends in other III-Vs, appears to be consistent with substitutional acceptors. On the other hand, an increase in n-type doping is observed with increasing carbon concentration, which suggests that carbon also generates donors, either substitutional point defects, or complexes. An additional LVM at 386 cm-1 is present in as-grown samples, and can be eliminated by annealing under nitrogen. This mode is attributed to a carbon-hydrogen complex. [ABSTRACT FROM AUTHOR]

The addition of small quantities of a Bi surfactant is shown to have dramatic effects on the microstructure of GaAsSb alloys grown by organometallic vapor phase epitaxy (OMVPE). Epilayers grown without Bi show weak atomic ordering in plan-view selective area electron diffraction measurements, with periodicity of three or six times the [110] lattice spacing as previously reported. The addition of Bi at a ratio of 1% Bi/Ga to the gas flow results in the appearance of strong CuAu and chalcopyrite ordering, as determined from electron diffraction measurements in both undoped, and heavily carbon doped layers. High-resolution, transmission electron microscopy lattice images clearly show the coexistence of {100}, {210} ordered and disordered structures with domain sizes of ∼10–20 nm. Photoluminescence shows no band gap changes in GaAsSb samples with and without {100} and {210} ordering. [ABSTRACT FROM AUTHOR]

We report on the electrical properties of Au-catalyzed C-doped GaAs nanowires (NWs) grown by metal organic vapor phase epitaxy. Transport measurements were carried out using a tungsten nanoprobe inside a scanning electron microscope by contacting to the Au catalyst particle of individual nanowires. The doping level could be varied from approximately (4 ± 1) × 1016 cm-3 to (1.0 ± 0.3) × 1019 cm-3 by varying the molar flow of the gas phase carbon precursor, as well as the group V to group III precursor ratio. It was found that the current transport mechanism switches from generation-recombination to tunnelling field emission by increasing the doping level to 1 × 1019 cm-3. Based on a diameter-dependent analysis of the apparent resistivity of the C-doped NWs, we propose that C incorporates into GaAs NWs through the triple boundary at the Au/NW interface. The p-type conductivity of the C-doped NWs was inferred by observing a rectification at negative bias (applied to the Au electrode) and confirmed by back-gating measurements performed on field effect transistor devices. [ABSTRACT FROM AUTHOR]

We have measured the Raman spectra of heavily carbon doped (p > 10[sup 19] cm[sup -3]) GaSb and GaAsSb. A local vibrational mode (LVM) due to carbon residing on group-V lattice sites was observed at 540 cm[sup -1] for GaSb and 568 cm[sup -1] for GaAs[sub 0.44]Sb[sub 0.56]. A gap mode at 164 cm[sup -1] was observed for GaSb. The frequency of the LVM as well as the gap mode is in quantitative agreement with recent theoretical predictions. [ABSTRACT FROM AUTHOR]

We have detected three- and six-fold lateral ordering in undoped and carbon-doped GaAs[sub 1-x]Sb[sub x] films (0.4

We measure the donor-bound electron longitudinal spin-relaxation time (T1) as a function of magnetic field (B) in three high-purity direct band-gap semiconductors: GaAs, InP, and CdTe, observing a maximum T1 of 1.4, 0.4, and 1.2 ms, respectively. In GaAs and InP at low magnetic field, up to ∼2 T, the spin-relaxation mechanism is strongly density and temperature dependent and is attributed to the random precession of the electron spin in hyperfine fields caused by the lattice nuclear spins. In all three semiconductors at high magnetic field, we observe a power-law dependence T1∝B-ν with 3≲ν≲4. Our theory predicts that the direct spin-phonon interaction is important in all three materials in this regime in contrast to quantum dot structures. In addition, the "admixture" mechanism caused by Dresselhaus spin-orbit coupling combined with single-phonon processes has a comparable contribution in GaAs. We find excellent agreement between high-field theory and experiment for GaAs and CdTe with no free parameters, however a significant discrepancy exists for InP. [ABSTRACT FROM AUTHOR]

GaAs core-shell p-n junction tunnel diodes were demonstrated by combining vapor-liquid-solid growth with gallium oxide deposition by atomic layer deposition for electrical isolation. The characterization of an ensemble of core-shell structures was enabled by the use of a tungsten probe in a scanning electron microscope without the need for lithographic processing. Radial tunneling transport was observed, exhibiting negative differential resistance behavior with peak-to-valley current ratios of up to 3.1. Peak current densities of up to 2.1 kA/cm(2) point the way to applications in core-shell photovoltaics and tunnel field effect transistors.

Carbon is a commonly used p-type dopant in planar III-V semiconductors, however its use in nanowire (NW) growth has been much less reported. In this work we show that the morphology of gold assisted GaAs NWs can be strongly modified by the presence of CBr(4) vapor during growth by metalorganic vapor phase epitaxy. GaAs NWs were grown under conditions which result in strong tapering and lateral growth at low growth temperatures by the use of triethylgallium (TEGa) instead of the more usual precursor, trimethylgallium (TMGa). Under these conditions, NWs grown in the presence of CBr(4) exhibit higher axial and lower radial growth rates, and negligible tapering compared with NWs grown in the absence of CBr(4) under the same conditions. We attribute this primarily to the suppression of the 2d growth rate by CBr(4), which enhances the axial growth rate of the nanowires. NWs grown with CBr(4) show stacking-fault-free zincblende structure, while the NWs grown without CBr(4) show a high density of stacking faults. This work underlines the striking effects which precursor chemistry can have on nanowire morphology.

1. The polymerase chain reaction has been used to amplify specifically the cDNA coding for the secreted form of ovine trophoblast protein-one from a preparation of total cellular RNA extracted from sheep embryos removed from ewes 16 days after mating. 2. Cloning and sequencing of the amplified cDNA revealed two new sequence variants: SPW49 having 93% similarity with deduced amino acid sequences from published cDNA data, and SPW27 a variant coding for a deleted form of ovine trophoblast protein-one. 3. The gene for ovine trophoblast protein-one is intronless. 4. This study provides further evidence for the existence of an ovine trophoblast protein-one gene family. 5. Both variants contain a potential N-glycosylation site not apparent in published sequences for ovine trophoblast protein-one.