bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlating the location of structural defects with the electrical failure of multiwalled carbon nanotubes
Authors: de Pablo P. J. , Howell S. , Crittenden S. , Walsh B. , Graugnard E. , Reifenberger R.
Published: July 2002

Electrical failure of carbon nanotubes was investigated by obtaining I(V) data with a voltage ramp from a rope of multiwalled carbon nanotubes. Noncontact scanning force microscope images were obtained before and after each I(V) curve until electrical failure of the tube resulted. Following this procedure, it was possible to correlate a defect on the surface of a nanotube with the exact location of the tube failure.
Electron drift mobility and electroluminescent efficiency of tris(8-hydroxyquinolinolato) aluminum
Authors: Chen B. J. , Lai W. Y. , Gao Z. Q. , Lee C. S. , Lee S. T. , Gambling W. A.
Published: July 2002

The electron drift mobility in films of tris(8-hydroxyquinolinolato) aluminum (Alq) deposited at different rates (0.2, 0.4, and 0.7 nm/s) on silicon has been determined by the time-of-flight technique. It has been found that the drift mobility of electrons in Alq increased by about two orders of magnitude as the deposition rate decreased from 0.7 to 0.2 nm/s. Further, the electron drift mobility in all Alq samples increased linearly with the square root of the applied electric field. Electroluminescent devices with a structure of indium tin oxide/α-naphthylphenylbiphenyl amine (NPB, 90 nm)/Alq (90 nm)/Mg:Ag were fabricated at different Alq deposition rates. The device efficiency was found to increase with increasing electron mobility in Alq. As the electron is the minority carrier in the present device, an increase in electron mobility in Alq would thus lead to an increase in device efficiency.
Saturation of THz-radiation power from femtosecond-laser-irradiated InAs in a high magnetic field
Authors: Ohtake Hideyuki , Ono Shingo , Sakai Masahiro , Liu Zhenlin , Tsukamoto Takeyo , Sarukura Nobuhiko
Published: July 2002

THz-radiation power from femtosecond-pulse-irradiated InAs is found to be saturated at the magnetic field around 3 T. Additionally, we find that this saturation magnetic field strongly depends on geometrical layout. Interesting magnetic-field dependence of the center frequency for THz radiation is also observed.
Low angle grain boundary transport in YBa2Cu3O7−δ coated conductors
Authors: Verebelyi D. T. , Christen D. K. , Feenstra R. , Cantoni C. , Goyal A. , Lee D. F. , Paranthaman M. , Arendt P. N. , DePaula R. F. , Groves J. R. , Prouteau C.
Published: July 2002

Second generation, high-temperature superconducting wires are based on buffered, metallic tape substrates of near single crystal texture. Strong alignment of adjacent grains was found to be necessary from previous work that suggested large angle, YBa2Cu3O7−δ [001]-tilt boundaries reduce Jc exponentially with increasing misorientation angle (θ). We pursue the low-θ regime by evaluating single grain boundaries (GB) and biaxially aligned polycrystalline films utilizing both the rolling-assisted biaxially textured substrates and ion-beam assisted deposition coated conductor architectures. Analysis concludes that an exponential dependence on Jc is applicable for θ≳4°, where the spacing between the periodic disordered regions along the GB become smaller than a coherence length.
Field-effect transistor made of individual V2O5 nanofibers
Authors: Kim G. T. , Muster J. , Krstic V. , Park J. G. , Park Y. W. , Roth S. , Burghard M.
Published: July 2002

A field-effect transistor (FET) with a channel length of ∼100 nm was constructed from a small number of individual V2O5 fibers of the cross section 1.5 nm×10 nm. At low temperature, the conductance increases as the gate voltage is changed from negative to positive values, characteristic of a FET with n-type enhancement mode. The carrier mobility, estimated from the low-field regime, is found to increase from 7.7×10−5 cm2/V s at T=131 K to 9.6×10−3 cm2/V s at T=192 K with an activation energy of Ea=0.18 eV. The nonohmic current/voltage dependence at high electric fields was analyzed in the frame of small polaron hopping conduction, yielding a nearest-neighbor hopping distance of ∼4 nm.
Silicon nanowire devices
Authors: Chung Sung-Wook , Yu Jae-Young , Heath James R.
Published: July 2002

Transport measurements were carried out on 15–35 nm diameter silicon nanowires grown using SiH4 chemical vapor deposition via Au or Zn particle-nucleated vapor-liquid-solid growth at 440 °C. Both Al and Ti/Au contacts to the wires were investigated. The wires, as produced, were essentially intrinsic, although Au nucleated wires exhibited a slightly higher conductance. Thermal treatment of the fabricated devices resulted in better electrical contacts, as well as diffusion of dopant atoms into the nanowires, and increased the nanowire conductance by as much as 104. Three terminal devices indicate that the doping of the wires is p type.
Synthesis and field-emission behavior of highly oriented boron carbonitride nanofibers
Authors: Bai X. D. , Guo J. D. , Yu Jie , Wang E. G. , Yuan Jun , Zhou Wuzong
Published: July 2002

Large-area highly oriented boron carbonitride (BCN) nanofibers with various compositions were synthesized directly on polished polycrystalline nickel substrates from a gas mixture of N2, H2, CH4, and B2H6 by bias-assisted hot-filament chemical-vapor deposition. The morphology of BCN nanofibers was examined by scanning electron microscopy, the nanofiber structure was studied by high-resolution transmission electron microscopy, and the chemical composition of individual nanofibers was determined by electron energy-loss spectroscopy. Field-emission behavior of the BCN nanofibers was characterized and a high emission current density of about 20–80 mA/cm2 at a low electric field of 5–6 V/μm implies a promising application as field-emission sources.
Defect structure in selective area growth GaN pyramid on (111)Si substrate
Authors: Tanaka Shigeyasu , Kawaguchi Yasutoshi , Sawaki Nobuhiko , Hibino Michio , Hiramatsu Kazumasa
Published: July 2002

A GaN pyramid grown selectively on a (111)Si substrate with a patterned dot structure of a SiO2 mask, by metalorganic vapor phase epitaxy using AlGaN as an intermediate layer, was characterized by transmission electron microscopy. The dot pattern has an array of 5.0-μm-diameter window openings with a 10 μm period. The density of threading dislocations observed in the window region decreased gradually with increasing distance from the interface. This was mainly due to the dislocation reaction and bending of threading dislocations for the first 2 μm region from the interface and for the upper region, respectively. Dominantly observed defects in the lateral-growth part were dislocations parallel to the interface. An amorphous layer was formed at the interface in the window region. Nitride particles were observed at the interface in the mask region.
Time-resolved optical characterization of InAs/InGaAs quantum dots emitting at 1.3 μm
Authors: Fiore A. , Borri P. , Langbein W. , Hvam J. M. , Oesterle U. , Houdré R. , Stanley R. P. , Ilegems M.
Published: July 2002

We present the time-resolved optical characterization of InAs/InGaAs self-assembled quantum dots emitting at 1.3 μm at room temperature. The photoluminescence decay time varies from 1.2 (5 K) to 1.8 ns (293 K). Evidence of thermalization among dots is seen in both continuous-wave and time-resolved spectra around 150 K. A short rise time of 10±2 ps is measured, indicating a fast capture and relaxation of carriers inside the dots.
Oxidized Ni/Pt and Ni/Au ohmic contacts to p-type GaN
Authors: Chen Li-Chien , Ho Jin-Kuo , Jong Charng-Shyang , Chiu Chien C. , Shih Kwang-Kuo , Chen Fu-Rong , Kai Ji-Jung , Chang Li
Published: July 2002

The Ni/Pt and Ni/Au contacts to p-GaN heat treated in air are investigated to study the effect of the metallic phase of high work function on specific contact resistance (ρc). The current–voltage measurement suggests that there is an optimal microstructure for the oxidized Ni/Au contact to obtain a minimum ρc of 4×10−6 Ω cm2. Such low ohmic contact resistance could be related to the formation of the NiO and the specific microstructure. However, high ρc of about 2–5×10−2 Ω cm2 are obtained for the oxidized Ni/Pt contacts, which is attributed to the different microstructure from that of the oxidized Ni/Pt contact. The NiO phase of the oxidized Ni/Pt contact is separated from p-GaN by a continuous face-centered-cubic Pt–Ni–Ga film. Therefore, the oxidized Ni/Pt contact behaves just like a common metal contact, but not a metal/NiO composite contact. Comparison of the two oxidized contacts implies that the previously proposed model based on metal/p-NiO/p-GaN heterojunction is reliable to explain the low ρc to p-GaN.
Measurement of nanohardness and nanoelasticity of thin gold films with scanning force microscope
Authors: Kracke B. , Damaschke B.
Published: July 2002

With a scanning force microscope operating in ultrahigh vacuum, we measured local mechanical properties of single crystalline gold islands. The elastic moduli of the gold islands and the substrates sapphire, mica, and quartz were evaluated from force-distance curves. The value of Young’s elastic modulus in [111] direction of the gold grains is (50±16) GPa and was found to be significantly smaller than the bulk modulus of gold. With an indentation technique, the nanohardness of gold islands was investigated. The nanohardness is smaller than the bulk value and depends on the indentation area. This can be interpreted as nanohardening by dislocation interaction in very pure single crystals.
Initial etching of GaAs (001) during H2 plasma cleaning
Authors: Robey S. W. , Sinniah K.
Published: July 2002

The initial etching of GaAs (001) during remote H2 plasma etching for oxide removal was examined using atomic force microscopy, coupled with in situ reflection high energy diffraction and Auger spectroscopy. Localized etching of the GaAs surface is particularly evident for etch temperatures below ∼650 K and leads to the formation of etch pits 5–10 nm deep and 20–50 nm in diameter. Etch pit formation was found to occur well before complete oxide removal. The development of etch pits is influenced by surface mis-cut steps, possibly due to increased hydrogen incorporation into the near surface at surface defects such as steps.
Structure optimization via free energy gradient method: Application to glycine zwitterion in aqueous solution
Authors: Okuyama-Yoshida Naoto , Kataoka Ken , Nagaoka Masataka , Yamabe Tokio
Published: July 2002

The free energy gradient method was applied to the multidimensional geometry optimization of glycine zwitterion (ZW) in aqueous solution in order not only to demonstrate its applicability, but also to examine its efficiency. The method utilizes force on the free energy surface that can be directly calculated by the molecular dynamics method and the free energy perturbation theory. Then, the most stable ZW structure in aqueous solution was obtained within the tolerance assumed, and it was found that the free energy (FE) and enthalpy changes of stabilization from the initial geometry optimized in the gas phase are −0.9 and −3.5 kcal/mol, respectively, and the amino and carboxyl groups are spatially separated by each other due to their solvating with water molecules. Comparing the contributions of enthalpy and entropy to FE, the former is attributed to the main origin of FE stabilization during the optimization procedure, and it was found that solvation entropy prevents water molecules from solvating the ZW more strongly.
Photovoltaic and spectral photoresponse characteristics of n-C/p-C solar cell on a p-silicon substrate
Authors: Krishna K. M. , Umeno M. , Nukaya Y. , Soga T. , Jimbo T.
Published: July 2002

Recent studies have shown the application of amorphous carbon as a semiconductor in C/Si heterojunction photovoltaic solar cells. In this letter, we report the rectifying current–voltage characteristics of the phosphorus-doped carbon/undoped-carbon (n-C/p-C) junction. The p- and n-carbon films were deposited by pyrolysis and ion-beam sputtering, respectively, on a p-Si substrate, using camphor as a natural carbon precursor. The preliminary photovoltaic characteristics of the cell reveals a short-circuit current density of 17.1 mA/cm2, open-circuit voltage of 0.339 V, and photoelectrical conversion efficiency of 1.82%, a reproducible result, under air mass zero and 1 sun illumination conditions. The spectral photoresponse characteristics of the cell of the above configuration was explained in terms of transmission/absorption characteristics of the two individual carbon layers.
Aligned CNx nanotubes by pyrolysis of ferrocene/C60 under NH3 atmosphere
Authors: Han Wei-Qiang , Kohler-Redlich Philipp , Seeger Torsten , Ernst Frank , Rühle Manfred , Grobert Nicole , Hsu Wen-Kuang , Chang Bao-He , Zhu Yan-Qiu , Kroto Harold W. , Walton David R. M. , Terrones Mauricio , Terrones Humberto
Published: July 2002

Aligned CNx (x<0.1) nanotubes have been generated by pyrolyzing ferrocene/C60 mixtures at 1050 °C in an ammonia atmosphere. The structure and composition of the product were determined by high-resolution transmission electron microscopy and high spatial resolution electron energy-loss spectroscopy. The CNx tubes (15–70 nm diameter, <50 μm length) grown in large flakes (<3 mm2) consist of a reduced number of “graphitic” layers (<15 on either side) arranged in a bamboo-like structure. Areas of high nitrogen concentration were found within curved or corrugated “graphite-like” domains. The observation of a well-developed double peak in the σ* feature of the N K-edge suggests that the material has not undergone the transition to the fullerene-like phase known for nitrogenated carbons. Incorporation of nitrogen from the gas phase (NH3) into CNx nanotubes therefore leads to improved and more efficient N substitution into the network as compared to the synthesis with solid nitrogen-containing precursors reported earlier.
Electrical characterization of Mg-related energy levels and the compensation mechanism in GaN:Mg
Authors: Seghier D. , Gislason H. P.
Published: July 2002

We investigated GaN:Mg samples grown by metal-organic chemical vapor deposition using various electrical measurement techniques. Annealing of highly resistive as-grown samples for different duration of time gives gradual activation of acceptors to concentrations up to 1×1017 cm−3. Conductance measurements of the annealed samples show the presence of two hole traps H1 and H2 with activation energies 130 and 170 meV from the valence band, respectively. The concentration of the H2 trap is always found to be low in our samples. The H1 trap is the shallowest one in our samples. Its concentration is directly proportional to the electrically active acceptor concentration in the samples, increasing with annealing. Hence, we attribute it to a Mg-related acceptor. This assignment is confirmed by dark current measurements. Two electron traps at 280 and 580 meV from the conduction band are observed in optical deep-level transient measurements. They have too weak concentrations to influence the free carrier concentration. We conclude that the hole conductivity observed in the annealed GaN samples is produced through thermal dissociation of passivating H donors from Mg acceptors. Most of the Mg concentration in the samples remains electrically inactive, however.
Synthesis of nanocrystalline nickel oxide by controlled oxidation of nickel nanoparticles and their humidity sensing properties
Authors: Das D. , Pal M. , Di Bartolomeo E. , Traversa E. , Chakravorty D.
Published: July 2002

Nickel–silica nanocomposites were prepared by the sol-gel route. By subjecting these to an oxidation treatment in the temperature range 723–1023 K, nickel oxide films of estimated thicknesses in the range 0.5–1.5 nm were grown on the nickel nanoparticles. dc electrical resistivity was measured in the temperature range 300–570 K. The data indicate that an amorphous phase in the interfacial region of the oxide-coated nickel nanoparticles determines the electrical conduction. A small polaron hopping conduction is found to be operative. The humidity sensing properties of these nanocomposites were measured at 300 K in the range of relative humidity (RH) from 3% to 87%. Electrical conductance in all specimens showed an increase of about three orders of magnitude as the RH was raised. Specimens subjected to higher oxidation treatment showed a higher conductance for a specific value of RH. This is ascribed to the presence of a larger number of Ni3+ ions in such specimens.
Scanning second-harmonic/third-harmonic generation microscopy of gallium nitride
Authors: Sun Chi-Kuang , Chu Shih-Wei , Tai Shi-Peng , Keller Stacia , Mishra Umesh K. , DenBaars Steven P.
Published: July 2002

Scanning second-harmonic generation and third-harmonic generation microscopy of a gallium nitride (GaN) sample was demonstrated using a femtosecond Cr:forsterite laser. Taking advantage of the electric-field enhanced second-harmonic generation effect and bandtail state resonance effect, the obtained second-harmonic and third-harmonic generation microscopic images revealed the piezoelectric field and bandtail state distributions in a GaN sample.
Deposition of aligned bamboo-like carbon nanotubes via microwave plasma enhanced chemical vapor deposition
Authors: Cui H. , Zhou O. , Stoner B. R.
Published: July 2002

Aligned multiwall carbon nanotubes have been grown on silicon substrates by microwave plasma enhanced chemical vapor deposition using methane/ammonia mixtures. Scanning electron microscopy shows that the nanotubes are well aligned with high aspect ratio and growth direction normal to the substrate. Transmission electron microscopy reveals that the majority phase has a bamboo-like structure. Data are also presented showing process variable effects on the size and microstructure of the aligned nanotubes, giving insight into possible nucleation and growth mechanisms for the process.
Carrier transfer in self-assembled coupled InAs/GaAs quantum dots
Authors: Tarasov G. G. , Mazur Yu. I. , Zhuchenko Z. Ya. , Maaßdorf A. , Nickel D. , Tomm J. W. , Kissel H. , Walther C. , Masselink W. T.
Published: July 2002

Photoluminescence (PL) spectra and time-resolved PL data from AlGaAs/GaAs superlattice structures containing thin InAs layers of about 1–3 monolayer grown on semi-insulating (001)-oriented GaAs substrates at lowered temperatures are studied. The size distribution of InAs quantum dots (QDs) among different families (modes) is controlled by variation of growth temperature and/or growth interruption. We demonstrate the stabilization of the PL magnitude caused by strong coupling between different modes and the full width at half maximum of “large size” QD modes within a certain temperature interval (50–150 K) due to feeding of the radiative transitions from nonradiative decay and carrier transfer arising from decaying excitonic states of the small size QD modes. Strong competition between different channels of ground state relaxation leads to an oscillating dependence of the PL transient for the small size QD mode. Efficient inter- and intramode tunneling rules out “bottleneck restrictions” for the PL. The parameters of intra- and intermode tunneling are determined from time-resolved PL.
Long time self-diffusion in suspensions of highly charged colloids: A comparison between pulsed field gradient NMR and Brownian dynamics
Authors: Wagner Joachim , Härtl Wolfram , Walderhaug Harald
Published: July 2002

The long time self-diffusion of highly charged colloidal particles consisting of polybutylacrylate was studied employing pulsed field gradient NMR (PFGNMR) and Brownian dynamics (BD) simulations. The potential parameters for the Yukawa interaction potential used in the BD simulations were derived from the static structure factor measured by means of light scattering. The long time self-diffusion coefficients resulting from NMR and BD were compared for different ionic strengths, which cause a screening of the interaction potential due to stray ions. Increasing the ionic strength of the colloidal suspensions, we observed a phase transition from a crystalline to a liquidlike structure. Whereas at high salt concentrations both long time diffusion coefficients agree within a few percent, at intermediate ionic strengths the values measured via PFGNMR are by a factor of 2 higher than those predicted by the BD simulation. This deviation is explained by hydrodynamic interactions which are expected to enhance long time self-diffusion and which are not taken into account in our BD algorithm.
Diffusion constant of the TIP5P model of liquid water
Authors: Mahoney Michael W. , Jorgensen William L.
Published: July 2002

The diffusion constant of TIP5P [J. Chem. Phys. 112, 8910 (2000)], the recently developed five-site rigid nonpolarizable model of liquid water that significantly improves the description of water’s density anomaly, has been calculated at a range of temperatures between −25 °C and 75 °C and pressures between 1 atm and 3000 atm. The diffusion constant, in units of 10−5 cm2/s, for TIP5P water at 25 °C and 1 atm is 2.62±0.04 as compared with the experimental value of 2.30. This is a significant improvement over most commonly used water models, e.g., for TIP4P and TIP3P [J. Chem. Phys. 79, 926 (1983)] the diffusion constants are 3.29±0.05 and 5.06±0.09, respectively, and for SPC it is 3.85±0.09. The diffusion constant of TIP5P decreases dramatically with decreasing temperature, as is observed experimentally, and the change in the diffusion constant as pressure is increased is also consistent with experimental results.
Ga-doped ZnO films grown on GaN templates by plasma-assisted molecular-beam epitaxy
Authors: Ko H. J. , Chen Y. F. , Hong S. K. , Wenisch H. , Yao T. , Look D. C.
Published: July 2002

We have investigated the structural and optical properties of Ga-doped ZnO films grown on GaN templates by plasma-assisted molecular-beam epitaxy. The carrier concentration in Ga-doped ZnO films can be controlled from 1.33×1018/cm3 to 1.13×1020/cm3. Despite high Ga incorporation, the linewidth of (0002) ω-rocking curves of Ga-doped ZnO films still lies in the range from 5 to 15 arc min. Photoluminescence (PL) spectra of Ga-doped ZnO films show dominant near-bandedge emission with negligibly weak deep-level emission, independent of carrier concentration. The PL spectrum exhibits a new emission line at 3.358 eV, which corresponds to exciton emission bound to a Ga donor. To avoid degradation of the PL intensity, the maximum dopability of Ga in ZnO is determined to be around 2.6×1019/cm3.
Measurements of discrete electronic states in a gold nanoparticle using tunnel junctions formed from self-assembled monolayers
Authors: Petta Jason R. , Salinas D. G. , Ralph D. C.
Published: July 2002

We present results from nanometer-scale tunnel junctions fabricated using organic self-assembled monolayers (SAMs) as tunnel barriers. Single junctions have resistances consistent with tunneling through a single layer of molecules. In several devices, a gold nanoparticle nucleated within the SAM barrier. Such samples allow a sensitive test of mechanical stability—some are sufficiently stable to permit the observation of electron tunneling through individual electron-in-a-box states at low voltages and mK temperatures. We also observe anomalous transport characteristics at larger voltages, which may be due to the motion of the gold nanoparticle within the monolayer.
BACH, the beamline for advanced dichroic and scattering experiments at ELETTRA
Authors: Zangrando M. , Finazzi M. , Paolucci G. , Comelli G. , Diviacco B. , Walker R. P. , Cocco D. , Parmigiani F.
Published: July 2002

A beamline for advanced dichroism (BACH), to perform light polarization dependent experiments in the 35–1600 eV photon energy range is under construction at the ELETTRA Synchrotron Radiation Source in Trieste, Italy. The radiation source, based on two APPLE-II helical undulators, is designed for high photon flux and high resolving powers. The photons dispersion system is based on a Padmore variable angle spherical grating monochromator with a typical resolving power of 20 000–6000, 20 000–6000, and 15 000–5000 in the energy ranges 35–200 eV, 200–500 eV, and 500–1600 eV, respectively. Two separate branches after the monochromator allow setting two independent experimental chambers. The photon flux in the experimental chamber(s), calculated at the best resolutions achievable and with the aperture of the slits set at 10 μm, is expected to be above 1011 photons’s with linearly or circularly polarized light. In addition, a fourth grating operates in the 400–1600 eV range to provide a higher flux, 1012 photons’s with smaller resolving power (10 000–2000), allowing fluorescence and x-ray scattering experiments. The refocusing section(s), based on plane elliptical mirrors in a Kirkpatrick–Baez scheme, will provide on the sample, a nearly free-aberration spot(s), whose dimensions are expected to be 200×10 μm2 (horizontal×vertical). In the following, the general layout of the beamline is reported and the characteristics of the optical elements, as well as the optical performances (resolving powers and efficiencies of the monochromator, flux, and spot dimensions) are described in detail.
Competition between deep impurity and dopant behavior of Mg in GaN Schottky diodes
Authors: Schmeits M. , Nguyen N. D. , Germain M.
Published: July 2002

The effect of the deep acceptor Mg on the electrical characteristics of p-doped GaN Schottky diodes is analyzed. The theoretical study is based on the numerical resolution of the basic semiconductor equations, including the continuity equation for the Mg-related acceptor level. It gives the steady-state and small-signal analysis of p-doped GaN:Mg Schottky diodes, yielding as final result the frequency dependent capacitance and conductance of the structure. It is shown that the low-frequency characteristics are determined by the carrier exchange between the Mg related impurity level and the valence band, whereas above the impurity transition frequency, the hole modulation of the depletion layer edge governs the electrical response. Detailed results are shown on the effect of temperature, applied steady-state voltage and series resistance. The study of two back-to-back connected GaN Schottky diodes reveals the appearance of typical features in the electrical characteristics, depending on the respective Schottky barrier height of the two junctions.
Quasi-one- and two-dimensional transitions of gases adsorbed on nanotube bundles
Authors: Gatica S. M. , Bojan M. J. , Stan G. , Cole M. W.
Published: July 2002

Grand canonical Monte Carlo simulations have been performed to determine the adsorption behavior of Ar and Kr atoms on the exterior surface of a rope (bundle) consisting of many carbon nanotubes. The computed adsorption isotherms reveal phase transitions associated with the successive creation of quasi-one-dimensional lines of atoms near and parallel to the intersection of two adjacent nanotubes.
On combined standard–nonstandard or hybrid (q,h)-deformations
Authors: Aneva B. L. , Arnaudon D. , Chakrabarti A. , Dobrev V. K. , Mihov S. G.
Published: July 2002

Combined (q,h)-deformations proposed by Kupershmidt and Ballesteros–Herranz–Parashar are studied. In each case a transformation is shown to lead to an equivalent, standard q-deformation. We briefly indicate that appropriate singular limits of the same type of transformations can however lead from standard biparametric (p,q)-deformations to nonhybrid but biparametric nonstandard (g,h) ones. Finally a case of hybrid (q,h)-deformation is recalled, related to the superalgebra GL(1|1).
Generalized hydrodynamics, bulk viscosity, and sound wave absorption and dispersion in dilute rigid molecular gases
Authors: Eu Byung Chan , Ohr Young Gie
Published: July 2002

Generalized hydrodynamic equations are derived from an irreversible kinetic equation for rigid diatomic molecular gases and applied to extract the bulk viscosity by making use of sound wave absorption and dispersion. As a first application of the generalized hydrodynamic equations for molecular gases, the linearized versions of the generalized hydrodynamic equations are applied to study sound wave absorption and dispersion in molecular gases. The results obtained for sound wave absorption and dispersion with the linearized set yield good agreement with experimental data on nitrogen, hydrogen, deuterium, and HD reported in the literature.
Secondary electron emission yields from MgO deposited on carbon nanotubes
Authors: Yi Whikun , Yu SeGi , Lee Wontae , Han In Taek , Jeong Taewon , Woo Yoonseong , Lee Jeonghee , Jin Sunghwan , Choi Wonbong , Heo Jungna , Jeon Dongryul , Kim J. M.
Published: July 2002

Enormously high secondary electron emission yields under electric field are observed from MgO deposited on carbon nanotubes. The yields reach a value as high as 15 000 and are strongly dependent upon the bias voltage applied to the sample. The creation of the electric field across the MgO film after bombardment of primary electrons is considered as one of key features, since positive charges are generated at the surface by departure of secondary electrons. Subsequent bombarding electrons produce other secondary electrons inside the MgO film, then the liberated secondaries are accelerated towards the surface under the strong field. Under this condition, the secondary electrons gain sufficient energy to create further electrons by impact ionization. The process continues until an equilibrium avalanche is established. To elucidate the earlier explanations, the kinetic energy spectra of secondary electrons are measured by an energy analyzer at various bias voltages in MgO/carbon nanotube samples. The analysis of spectral results with the energy band diagram gives us strong evidence for the suggested mechanism.
Very-low-specific-resistance Pd/Ag/Au/Ti/Au alloyed ohmic contact to p GaN for high-current devices
Authors: Adivarahan V. , Lunev A. , Khan M. Asif , Yang J. , Simin G. , Shur M. S. , Gaska R.
Published: July 2002

We report on Pd/Ag/Au/Ti/Au alloyed metallic contact to p GaN. An 800 °C anneal for 1 min in flowing nitrogen ambient produces an excellent ohmic contact with a specific contact resistivity close to 1×10−6 Ω cm2 and with good stability under high current operation conditions. This high-temperature anneal forms an alloy between Ag, Au, and p GaN resulting in a highly p-doped region at the interface. Using x-ray photoelectron spectroscopy and x-ray diffraction analysis, we confirm that the contact formation mechanism is the metal intermixing and alloying with the semiconductor.
Self-orientation of short single-walled carbon nanotubes deposited on graphite
Authors: Yanagi Hisao , Sawada Eriko , Manivannan Ayyakkannu , Nagahara Larry A.
Published: July 2002

In view of practical handling of single-walled carbon nanotubes (SWNTs), here we present cutting procedures and spontaneous ordering of SWNTs deposited on highly oriented pyrolytic graphite (HOPG). Purified SWNTs were cut by ultrasonication in H2SO4/HNO3 mixture and further in de-ionized water. Thus, prepared short SWNTs were resuspended in methanol and dropped on a freshly cleaved HOPG surface. Scanning tunneling microscopy under a vacuum revealed that the short tubes of 20–100 nm in length were aligned along the basal axis of the HOPG lattice. Tunneling spectroscopy obtained from the oriented tubes indicated a semiconducting behavior, which was characterized in relation to their helical structure. The observed orientation of SWNTs was attributed to their one-dimensional electronic nature along the tube axis with the HOPG lattice.
Electronic structure and conduction in a metal–semiconductor digital composite: ErAs:InGaAs
Authors: Driscoll D. C. , Hanson M. , Kadow C. , Gossard A. C.
Published: July 2002

We have grown epitaxial superlattice structures of layers of semimetallic ErAs particles embedded in an InGaAs matrix on (001) Fe-doped InP substrates. Temperature-dependent Hall measurements, x-ray diffraction, and transmission electron microscopy were performed on the materials. The carrier mobility and the temperature dependence of the charge density imply conduction in the InGaAs matrix. We calculate an offset between the conduction-band minimum of the InGaAs matrix and the Fermi level of the ErAs particles that is strongly dependent on the amount of ErAs deposited. As the size of the ErAs particles increases, the Fermi level decreases from ∼0.01 eV above the InGaAs conduction-band edge to ∼0.2 eV below the InGaAs conduction-band edge and the electrical conduction properties change from metallic to semiconducting.
Percolation transition of thermoelectric properties in PbTe thin films
Authors: Rogacheva E. I. , Krivulkin I. M. , Nashchekina O. N. , Sipatov A. Yu. , Volobuev V. A. , Dresselhaus M. S.
Published: July 2002

Extrema were observed in the film thickness d dependence of various thermoelectric parameters (Seebeck coefficient S, electrical conductivity σ, Hall coefficient RH, charge carrier mobility μ, and power factor P) of epitaxial PbTe/(001) KCl thin films prepared by thermal evaporation in vacuum and protected from oxidation by an EuS layer. We attribute the observed extrema in properties and the high values of μ and P at d≈50 nm to the percolation transition from an island-like to a continuous film and to the self-organization of the islands, which can occur not only in quantum dot superlattices but also in an individual layer.
Derivation of the shear compliance of thin films on quartz resonators from comparison of the frequency shifts on different harmonics: A perturbation analysis
Authors: Johannsmann Diethelm
Published: July 2002

Viscoelastic effects contribute to the shift in resonance frequency of quartz crystal resonators induced by deposition of thin films on the resonator surface. In turn, the mechanical stiffness of the film can be experimentally determined from a comparison of the resonance shifts on different harmonics. When the film is much thinner than the wavelength of shear sound, a series expansion of the viscoelastic effects to third order in film thickness leads to rather simple equations. When plotting the normalized frequency shift δf/f versus the square of the overtone order n2 one finds a linear relationship, where the slope is determined by the film’s elastic compliance. When the same analysis is carried out on the resonance bandwidths rather than the frequency shifts the viscous compliance is obtained. The effects of asymmetric coatings, electrodes, and liquid media are discussed.
Effect of alcohol-based sulfur treatment on Pt Ohmic contacts to p-type GaN
Authors: Huh Chul , Kim Sang-Woo , Kim Hyun-Min , Kim Dong-Joon , Park Seong-Ju
Published: July 2002

The effects of an alcohol-based (NH4)2S solution [t-C4H9OH+(NH4)2S] treatment on Pt Ohmic contacts to p-type GaN are presented. The specific contact resistance decreased by three orders of magnitude from 2.56×10−2 to 4.71×10−5 Ω cm2 as a result of surface treatment using an alcohol-based (NH4)2S solution compared to that of the untreated sample. The O 1s and Pt 4f core-level peaks in the x-ray photoemission spectra showed that the alcohol-based (NH4)2S treatment was effective in removing of the surface oxide layer. Compared to the untreated sample, the alcohol-based (NH4)2S-treated sample showed a Ga 2p core-level peak which was shifted toward the valence-band edge by 0.25 eV, indicating that the surface Fermi level was shifted toward the valence-band edge. These results suggest that the surface barrier height for hole injection from Pt metal to p-type GaN can be lowered by the surface treatment, thus resulting in a drastic reduction in specific contact resistance.
Reflected shock experiments on the equation-of-state properties of liquid deuterium at 100–600 GPa (1–6 Mbar)
Authors: Mostovych A. N. , Chan Y. , Lehecha T. , Phillips L. , Schmitt A. , Sethian J. D.
Published: July 2002

Laser-driven shock experiments are used to study the equation-of-state (EOS) properties of liquid deuterium. Reflected shocks are utilized to increase the shock pressure, to expand the area of EOS phase space probed by the experiment, and to enhance the sensitivity to differences in compressibility. The results of these experiments differ substantially from the predictions of the Sesame EOS. EOS models showing large dissociation effects and large compressibility (up to a factor of 2) agree with the data. By use of independent techniques, this experiment offers the first confirmation of an earlier observation of enhanced compressibility in liquid deuterium.
M
+
/
Rg
bonding: The effects of M+ permanent quadrupole moments (M+= atomic metal ion; Rg=rare gas atom)
Authors: Burns Katherine L. , Bellert D. , Leung Allen W.-K. , Breckenridge W. H.
Published: July 2002

It has been shown, using a model-potential analysis, that the large permanent quadrupole moment of the excited Mg+(3p) ion can play a significant role in the strong physical M+/Rg bonding observed for Mg+(3pπ)⋅Rg[2Π] ionic states. The four permanent quadrupole terms included in the model potential (two proportional to 1/R6, two to 1/R8) contribute substantially to Mg+(3pπ)/Rg attraction near the bond distances Re. In fact, our analysis indicates that the leading charge/induced-dipole 1/R4 attractive term contributes only ∼25–30 % to the physical bonding in the Mg+(3pπ)⋅Ar excited state, in stark contrast to the conventional wisdom that this term is usually dominant in M+/Rg bonding. Empirically derived Ae−bR repulsive terms also show that electron/electron repulsion for a given Mg+(3pπ)⋅Rg excited state is less than for the analogous Mg+(3sσ)⋅Rg ground state, consistent with the fact that the Rg atoms approach the excited 3pπ orbital of Mg+ along its nodal axis. For the Mg+(3pσ)⋅Rg[2Σ+] excited states, however, three of the permanent quadrupole terms are repulsive (with twice the magnitude) and thus contribute significantly to the extremely weak bonds and very large bond distances for the 3pσ ionic states. In contrast, the much smaller quadrupole moments of open-shell d-orbital states of transition metal M+ ions appear to have very little effect on their physical bonding with the Ar atom, at least for the few states which have been well-characterized spectroscopically. For all the M+/Rg states discussed above, our model-potential analysis indicates that no substantial chemical or charge-transfer interactions are needed to rationalize the bond strengths, the bond lengths, and the vibrational frequencies (the “shapes” of the potential curves near their minima).
Growth of aligned carbon nanotubes by biasing during growth
Authors: Avigal Y. , Kalish R.
Published: July 2002

Well aligned multiwalled carbon nanotube (CNT) growth was achieved by positively biasing the substrate during growth. Growth was performed in a flowing mixture of 7% CH4 in Ar onto Co covered Si held at 800 °C with and without the presence of an electric field. High resolution scanning electron microscopy shows that the tube alignment occurs only when a positive bias is applied to the substrate whereas no aligned growth occurs under a negative bias and no tube growth is observed, under the presently applied conditions, with no field. This finding may open up the possibility of realizing cold electron emitting devices based on CNTs with a large electric field enhancement. In particular, it may be possible to utilize the same gate which is needed to turn the device on also to obtain field assisted aligned carbon nanotube growth into the desired regions. Alternatively, due to the fact that no CNTs grow under the conditions of this experiment without bias, selected area biasing may permit selected area growth of vertically aligned carbon nanotubes, a process that may find many applications.
Essential self-adjointness of n-dimensional Dirac operators with a variable mass term
Authors: Kalf Hubert , Yamada Osanobu
Published: July 2002

We give some results about the essential self-adjointness of the Dirac operator H=∑j=1nαj pj+m(x) αn+1+V(x) IN (N=2 [(n+1)/2]), on [C0∞(Rn\{0})]N, where the αj (j=1,2,…,n) are Dirac matrices and m(x) and V(x) are real-valued functions. We are mainly interested in a singularity of V(x) and m(x) near the origin which preserves the essential self-adjointness of H. As a result, if m=m(r) is spherically symmetric or m(x)≡V(x), then we can permit a singularity of m and V which is stronger than that of the Coulomb potential.
Geometry, electronic structure, and energetics of copper-doped aluminum clusters
Authors: Khanna S. N. , Ashman C. , Rao B. K. , Jena P.
Published: July 2002

Using density functional theory and generalized gradient approximation for exchange-correlation potential, we have calculated the equilibrium geometries and energetics of neutral and negatively charged AlnCu (n=11,12,13,14) clusters. Unlike the alkali atom-doped aluminum clusters in the same size range, the copper atom resides inside the aluminum cluster cage. Furthermore, the 3d and 4s energy levels of Cu hybridize with the valence electrons of Al causing a redistribution of the molecular orbital energy levels of the Aln clusters. However, this redistribution does not affect the magic numbers of AlnCu clusters that could be derived by assuming that Cu donates one electron to the valence levels of Aln clusters. This behavior, brought about by the smaller size and large ionization potential of the copper atom, contributes to the anomalous properties of AlnCu− anions: Unlike AlnX− (X=alkali atom), the mass ion intensities of AlnCu− are similar to those of Aln−. The calculated adiabatic electron affinities are also in very good agreement with experiment.
Electromigration threshold in copper interconnects
Authors: Wang P.-C. , Filippi R. G.
Published: July 2002

The electromigration threshold in copper interconnects is reported in this study. The length-dependent electromigration degradation rate is observed and quantified in the temperature range of 295–400 °C. Based on the Blech electromigration model [I. A. Blech, J. Appl. Phys. 47, 1203 (1976)], a simplified equation is proposed to analyze the experimental data from various combinations of current density and interconnect length, as well as to estimate the electromigration threshold product of current density and line length, (jL)th, at a certain temperature. The resulting (jL)th value appears to be temperature dependent, decreasing with increasing temperature in the tested range between 295 and 400 °C.
Integrated porous-silicon light-emitting diodes: A fabrication process using graded doping profiles
Authors: Barillaro G. , Diligenti A. , Pieri F. , Fuso F. , Allegrini M.
Published: July 2002

A fabrication process, compatible with an industrial bipolar+complementary metal–oxide–semiconductor (MOS)+diffusion MOS technology, has been developed for the fabrication of efficient porous-silicon-based light-emitting diodes. The electrical contact is fabricated with a double n+/p doping, achieving a high current injection efficiency and thus lower biasing voltages. The anodization is performed as the last step of the process, thus reducing potential incompatibilities with industrial processes. The fabricated devices show yellow-orange electroluminescence, visible with the naked eye in room lighting. A spectral characterization of light emission is presented and briefly discussed.
Ferroelectric switching mechanism in SrBi2Ta2O9
Authors: Ding Y. , Liu J. S. , MacLaren I. , Wang Y. N.
Published: July 2002

The ferroelectric switching mechanism in strontium bismuth tantalate [SrBi2Ta2O9 (SBT)] has been studied using in situ transmission electron microscopy observations of the nucleation and growth of polarization domains, such as 180° and 90° domains. Thank to this high density of the antiphase boundary (APB), a switching mechanism in SBT based on the nucleation of new polarization domains at APBs as well as the electrode interfaces put forward and the fatigue-free behavior of SBT with a platinum electrode is explained.
Enhanced ultraviolet photoconductivity in semiconducting ZnGa2O4 thin films
Authors: Lee Yong Eui , Norton David P. , Budai John D. , Wei Yayi
Published: July 2002

We have investigated the conductivity and photoconductivity response of undoped and Li-doped ZnGa2O4 epitaxial films grown using pulsed-laser deposition. A significant enhancement of the ultraviolet (UV) photoresponse is observed with Li doping that also correlates with an enhanced luminescent intensity. The wavelength dependence observed for creation of free carriers under UV excitation suggests that the transition is either band-to-band or involves a defect level near the band edge. Moderate n-type dark conductivity is observed for undoped films processed under reducing conditions. With Li doping, dark conductivity is reduced, suggesting that lithium ions in the zinc gallate lattice serve as deep acceptors. In addition, Li doping effectively eliminates persistent photoconductivity that is commonly observed in undoped films, suggesting the possible use of Li-doped ZnGa2O4 as a visible wavelength blind UV photodetector.
Spectroscopy of Nd3+ and Yb3+ codoped fluoroindogallate glasses
Authors: de Sousa D. F. , Batalioto F. , Bell M. J. V. , Oliveira S. L. , Nunes L. A. O.
Published: July 2002

The Nd–Nd and Nd–Yb energy transfer processes are studied in lead fluoroindogallate glasses with the following molar composition: 30PbF2–20GaF3–15InF3–15ZnF2–(20–X)CaF2–XNdF3 (with X=0.1, 0.5, 1, 2, 4, and 5), 30PbF2–20GaF3–15InF3–15ZnF2–(20–X)CaF2–XYbF3 (with X=0.1, 0.5, 1, 2, 3, and 5), and 30PbF2–20GaF3–15InF3–15ZnF2–(19–X)CaF2–XYbF3–1NdF3 (with X=0.1, 0.5, 1, 2, 3, and 5.5). The Dexter, Yokota–Tanimoto, and Holstein formalisms were used to treat the experimental data. The following microparameters of energy transfer were obtained: CDD(Nd–Nd)≈7×10−40, CDA(Nd–Nd)=2.5×10−40, and CDA(Nd–Yb)≈3×10−40 cm6/s. It was also shown that the energy migration between Nd ions depends on the third power of temperature (T3) up to a saturation value of about 80 K. This behavior was attributed to the site to site energy migration. The Yb doped samples presented no nonradiative losses for the Yb3+ emission at 969 nm.
Photophysical and theoretical studies of naphthalene-substituted oligothiophenes
Authors: Seixas de Melo J. , Silva L. M. , Kuroda Masami
Published: July 2002

A comprehensive photophysical characterization of a new class of naphthalene derivatives of the important α-oligothiophenes (αn’s) has been undertaken in solution at room (293 K) and low (77 K) temperature. This includes absorption and fluorescence spectra, fluorescence quantum yields (φF), and lifetimes (τF), as a function of temperature and solvent. Triplet–triplet absorption spectra and triplet formation quantum yields (φT) were also determined. From the above, all the rate constants for the radiative (kF) and radiationless (kIC and kISC) have been calculated. It is shown that the lowest singlet excited state is an allowed π,π* state in all solvents. The results show that although the behavior of the oligomers is similar to their parent compounds (αn’s), significant differences are observed. By comparison of the extinction coefficients of the naph(αn’s) and the (αn’s), a good correlation was found between naph(αn) and α(n+1). On the basis of this proposed pairing, a consistent blue-shift was observed in the absorption maxima between the compounds here considered and the reference α-oligothiophenes. This indicates that there is some significant twist between the naphthalene and the αn chromophores. The most favorable inter-ring angle between the two chromophores, naphthalene and αn, was predicted on the basis of comparison with theory.