bluuebunny/crossref_metadata_2025 huggingface dataset:
Three-dimensional optical circuitry using two-photon-assisted polymerization
Authors: Joshi Mukesh P. , Pudavar Haridas E. , Swiatkiewicz J. , Prasad P. N. , Reianhardt B. A.
Published: July 2002

An approach using an as-formed solid bulk sample containing a blend of photocurable and thermally curable epoxies together with a highly efficient two-photon excitable fluorophore is demonstrated for in situ fabrication of three-dimensional optical circuitry with a precise control of dimension and spatial locations. Confocal volume images of a 1×2 splitter and a grating coupler are shown. End-fire coupling of a He–Ne laser beam into the 1×2 splitter has been achieved.
Wiedemann–Franz law at boundaries
Authors: Mahan G. D. , Bartkowiak M.
Published: July 2002

The full equations are derived for the resistances to the transport of heat and electricity through boundaries of thermoelectrics. We show that the boundary resistances of heat and electricity are proportional. This relationship is a boundary form of the Wiedemann–Franz law. We also show there is a boundary Seebeck coefficient.
Injection-controlled size effect on switching of ferroelectric thin films
Authors: Tagantsev A. K. , Stolichnov I. A.
Published: July 2002

The effect of near-by-electrode charge injection on switching of a thin film ferroelectric capacitor is theoretically analyzed. We develop a model of switching affected by charge injection through a surface dielectric layer to calculate the coercive field of the capacitor as a function of both film thickness and maximal polarization of the switching cycle. The predictions of the model are verified by electrical measurements on sol–gel derived Pb(Zr, Ti)O3 thin films of thickness ranging from 100 to 1000 nm with Pt electrodes. The model gives a good description of the size effect on switching in the Pt/Pb(Zr, Ti)O3/Pt system and enables an explanation for a much smaller magnitude of this effect in Bi-containing and oxide–electrode thin films.
The adsorption of oxygen at GaN surfaces
Authors: Zywietz Tosja K. , Neugebauer Jörg , Scheffler Matthias
Published: July 2002

A critical point in device fabrication based on GaN is the controlled doping and the incorporation of impurities like, e.g., oxygen. We have explored the adsorption of oxygen at the wurtzite (0001) and (0001̄) GaN surfaces employing density-functional theory. Our results show that both surface orientations are very active towards oxygen adsorption, explaining the high oxygen concentrations typically observed in GaN. However, the (0001) and (0001̄) surfaces behave differently and oxygen incorporation is expected to be higher at the (0001̄) surface. The different reactivity is explained in terms of the specific structural configurations.
Effects of embedded crystallites in amorphous silicon on light-induced defect creation
Authors: Kamei Toshihiro , Stradins Paul , Matsuda Akihisa
Published: July 2002

We investigate effects of embedded crystallites in hydrogenated amorphous silicon on light-induced metastable dangling-bond defect creation in a systematic manner. Inclusion of a small volume fraction of crystallites into the amorphous matrix significantly suppresses defect creation against moderate light illumination. Excess carriers generated in the amorphous matrix tend to recombine in the embedded crystallites, which suppresses nonradiative recombination within the amorphous matrix and the subsequent defect creation. The presence of a small volume fraction of crystallites, however, is no longer effective to improve the stability against strong light exposure such as pulsed laser irradiation. In this case, the higher carrier concentration favors bimolecular direct carrier recombination within the amorphous matrix.
Microelectronic cooling using the Nottingham effect and internal field emission in a diamond (wide-band gap material) thin-film device
Authors: Miskovsky N. M. , Cutler P. H.
Published: July 2002

We propose a method for heat dissipation in microelectronic devices which uses internal field emission through the interface of a composite thin-film device {i.e., metal [semiconductor (S)]/ chemical vapor deposition doped diamond [wide-band gap (WBG) material]} in conjunction with a heat sink. These composite thin-film devices are of micron to submicron dimensions and composed of materials which can be integrated with existing semiconductor technology. As distinct from conventional field emission into vacuum and thermionic devices, the relatively high metallic (S) work function (>2 eV) is here circumvented by use of internal field emission through a Schottky barrier at a metal/diamond (WBG) interface. It is the large band gap in these materials which introduces a filtering effect on the injected electrons which allows one to restrict the tunneling of electrons through the Schottky barrier from states below the Fermi energy, εF. For applied fields below a certain value, the average energy of the field-emitted electrons is greater than the average energy of the electrons which replace them, leading to the so-called Nottingham cooling. It has recently been shown that the replacement electrons have an energy up to 100 meV or more lower than εF, enhancing the cooling process by field emission. Using a kinetic field emission formalism, a tip density of 107/cm2, a local electron gas temperature of 500 K, and a tip radius of 50 nm (blunt tip), an average cooling rate per area of 1.6 W/cm2 can be achieved. Higher tip densities lead to average heat dissipation rates which can scale up to 100 W/cm2 or higher, rates competitive with or exceeding other techniques for thermal dissipation (i.e., thermoelectric and thermionic).
Evidence for grain-boundary-assisted diffusion of sulfur in polycrystalline CdS/CdTe heterojunctions
Authors: Herndon M. K. , Gupta A. , Kaydanov V. , Collins R. T.
Published: July 2002

We present a near-field scanning optical microscopy (NSOM) study of S interdiffusion in polycrystalline CdS/CdTe heterojunctions. S diffusion from CdS into CdTe leads to the formation of a CdTe1−xSx ternary phase. Because the band gap of CdTe1−xSx varies with S composition, we were able to combine NSOM with a tunable laser source to microscopically identify S-rich regions in the CdTe layer. S composition was found to be very nonuniform and frequently to be greater along grain boundaries than in the grain centers, identifying grain boundaries as locations of enhanced interdiffusion.
Localized states at InGaN/GaN quantum well interfaces
Authors: Brillson L. J. , Levin T. M. , Jessen G. H. , Ponce F. A.
Published: July 2002

Low-energy electron-excited nanoscale-luminescence (LEEN) spectroscopy of GaN/InGaN/GaN double-heterojunction structures reveal the formation of electronic states localized near the quantum well interfaces under relatively In-rich conditions. These states are due to formation in a cubic GaN region comparable to the quantum well layer in thickness rather than the bulk native defects typically associated with growth quality. The nanoscale depth dependence of the noncontact, nondestructive LEEN technique enables detection of this competitive recombination channel within a few nanometers of the “buried” heterojunction interfaces.
THz emission from coherently controlled photocurrents in GaAs
Authors: Côté D. , Fraser J. M. , DeCamp M. , Bucksbaum P. H. , van Driel H. M.
Published: July 2002

We report broadband terahertz radiation from ballistic photocurrents generated via quantum interference of one- and two-photon absorption in low-temperature-grown and semi-insulating GaAs at 295 K. For 90 fs, 1550 and 775 nm optical pulses, we obtain phase-controllable near-single cycle 4 THz radiation. Higher frequency THz emission should be achievable with shorter pulses. At a 250 kHz repetition rate and average powers of 10 mW (1550 nm) and 400 μW (775 nm), we measure 3 nW of THz power, limited mainly by phase walkoff of the optical beams within the 1.5-μm-thick sample and collection efficiency.
Measuring average tip-sample forces in intermittent-contact (tapping) force microscopy in air
Authors: Fain S. C. , Barry K. A. , Bush M. G. , Pittenger B. , Louie R. N.
Published: July 2002

A method to measure the average normal force on a surface produced by intermittent-contact (tapping) force microscopy is presented. This force is determined by measuring the average deflection of a calibrated piezoresistive cantilever in intermittent contact with an oscillating active cantilever. Results obtained with this method are presented for a two-state cantilever motion where the piezolever force is higher for the state with the lower amplitude of vibration.
Dynamic visualization of stress distribution by mechanoluminescence image
Authors: Xu Chao-Nan , Zheng Xu-Guang , Akiyama Morito , Nonaka Kazuhiro , Watanabe Tadahiko
Published: July 2002

We report the realization of the dynamic image of stress distribution by developing a remarkably strong mechanoluminescence (ML) material of Sr0.975Al2O3.985:Eu0.01, which can emit four orders of magnitude larger intensity than that of the reported strong ML material of quartz crystal. This ML material can be mixed in the target composite or coated on the surface to sense stress by emitting visible light. This method is applicable to the dynamic visualization of stress distribution in a solid not only in the atmosphere but also in an aqueous environment.
Optical gain and collector current characteristics of resonant-cavity phototransistors
Authors: Lin Chien-chung , Martin Wayne , Harris James S. , Sugihwo Fred
Published: July 2002

An analytic model for current gain as a function of optical input power in heterojunction bipolar phototransistors is developed. The model provides excellent agreement with the dc measurements of a resonant-cavity-enhanced heterojunction bipolar phototransistor. The model is extended to explain small-signal results.
Epitaxial Y2O3 films grown on Si(111) by pulsed-laser ablation
Authors: Hunter M. E. , Reed M. J. , El-Masry N. A. , Roberts J. C. , Bedair S. M.
Published: July 2002

Y 2 O 3 has a relatively high dielectric constant (13–17) leading to several potential applications. In this work, pulsed-laser deposition was used to grow epitaxial Y2O3 films on Si(111) substrates. Structural characterization indicated two-dimensional growth without the formation of an amorphous interfacial layer. Annealing in either Ar or O2 was found to induce an O2 diffusion reaction resulting in the formation of two interfacial amorphous layers. Electrical characterization by capacitance–voltage and current–voltage indicated that the as-grown samples were poor insulating films. Annealing the samples improved the electrical performance by lowering leakage currents and exhibiting inversion during capacitance–voltage testing. This epitaxial growth points toward the possibility of the heteroepitaxial growth of silicon on insulator device structures.
Ferroelectric properties of Sr2Bi4Ti5O18 thin films
Authors: Zhang S. T. , Xiao C. S. , Fang A. A. , Yang B. , Sun B. , Chen Y. F. , Liu Z. G. , Ming N. B.
Published: July 2002

Bi-based layered ferroelectric thin films of Sr2Bi4Ti5O18 (SBTi) were prepared by pulsed-laser deposition. The c-axis-oriented SBTi films were grown on SrRuO3 seeded Pt/TiO2/SiO2/Si substrates while polycrystalline SBTi films were grown on Pt/TiO2/SiO2/Si substrates. The measurements of ferroelectricity revealed that the direction of spontaneous polarization was not along the c axis since the polarization of c-axis-oriented films was much less than that of randomly oriented films. There was no significant degradation of switchable charge at least up to 1011 cycles for the randomly oriented films, suggesting that, even with Ti which was widely accepted to contribute to the fatigue of Pb(Zr, Ti)O3, SBTi showed superior fatigue characteristics. The randomly oriented films also showed excellent retention characteristics after 105 s.
Monolithic active mode locking of quantum cascade lasers
Authors: Paiella Roberto , Capasso Federico , Gmachl Claire , Hwang Harold Y. , Sivco Deborah L. , Hutchinson Albert L. , Cho Alfred Y. , Liu H. C.
Published: July 2002

We demonstrate active mode locking of a high-speed 8 μm quantum cascade laser in a monolithic configuration, at a repetition rate of 11.6 GHz. Evidence of mode locking is obtained from the measured optical spectra and corresponding interferograms, as well as from the power spectra of the photocurrent detected with a fast quantum-well infrared photodetector. An estimate for the pulse width of approximately 5 ps is inferred from the experimental results. Mode-locked operation is observed up to a maximum temperature of over 120 K.
Model for discontinuous actuation of ferromagnetic shape memory alloy under stress
Authors: Murray S. J. , O’Handley R. C. , Allen S. M.
Published: July 2002

Ferromagnetic shape memory alloys have demonstrated strains up to 5% resulting from the rearrangement of crystallographic variants by twin boundary motion under an applied field. A model is proposed that describes twin boundaries moving abruptly in order to accommodate either the mechanical energy of an applied stress σε0 or the magnetic interaction energy of the applied field MsH. This model provides predictions of the materials response under both field and load. The principal conclusion of the model is that when one energy, either mechanical or magnetic, becomes larger than the other, the twins will move to abruptly rearrange the entire sample to a variant stable under the new condition. Experiments were performed to verify this model in Ni–Mn–Ga shape memory alloy. The abrupt change of the variant structure was observed experimentally although the twin boundaries only swept through limited potions of the sample. Experimental field-induced strains reached 2.2%. Samples were also tested under cycled field with constant load and showed cyclic strains of 1.5%.
Gas dynamics of laser ablation: Influence of ambient atmosphere
Authors: Gusarov Andrey V. , Gnedovets Alexey G. , Smurov Igor
Published: July 2002

A two-stage two-dimensional (2D) gas-dynamic model of laser ablation in an ambient gas atmosphere is proposed. The initial one-dimensional stage of the process is related to the ablation plume formation under the action of a laser pulse (duration of the order of 10 ns; fluence about several J/cm2; laser spot diameter about 1 mm) and describes heating, melting, and evaporation of the target, the target–vapor interaction in the Knudsen layer, and the vapor dynamics. The final 2D stage is responsible for the formation of the energy and angular distributions of the ablated material. Considerable compression of the ambient gas around the expanding plume of the laser-evaporated material and a shock front propagating through the undisturbed ambient gas are found. The pressure of the compressed ambient gas behind the shock may be much higher than the ambient one. However, at the investigated ambient pressures below 100 Pa, it remains still much lower than the vapor pressure during laser evaporation. Therefore, the initial stage of laser ablation is essentially independent of the ambient atmosphere. Once the laser pulse is over, the vapor pressure eventually drops down to a value comparable to the compressed ambient gas pressure. From this time on, the gas considerably suppresses vapor expansion. There is a noticeable difference between the vapor distribution in vacuum and the one in the ambient atmosphere: the vapor fills the entire plume volume in vacuum while in the presence of ambient atmosphere it is accumulated near the plume boundary and tends to form a thin shell. The angular and energy distributions of the ablated material are especially sensitive to the nature and pressure of the ambient gas. Both the kinetic energy of the ablated atoms and the width of their angular distribution decrease with the ambient pressure.
Magnetic properties of Sm- and Cu-doped oriented SmCo5 ribbons prepared by melt spinning
Authors: Yan A-ru , Zhang Wen-yong , Zhang Hong-wei , Shen Bao-gen
Published: July 2002

Oriented melt-spun ribbons with composition Sm1+Y(Co1−XCux)5 (X=0–0.3 and Y=0–0.2) were fabricated with a wheel speed of 5 m/s, followed by annealing in the temperature range of 600–1000 °C for 30 min. Our results show that a high degree of texture, in which the c axis is parallel to the longitudinal direction of the ribbons, is obtained in all above ribbons. The room temperature intrinsic coercivity can be significantly enhanced by both Sm additions and Cu substitution and coercivities of 21 and 12.4 kOe an achieved in Sm1.15Co5 and Sm(Co0.8Cu0.2)5 ribbons, respectively. Compared with the Sm-doped ribbons, Cu-doped ribbons exhibit a better thermal stability of coercivity and a high intrinsic coercivity of 5.2 kOe can be maintained at 300 °C in Sm(Co0.8Cu0.2)5 ribbons. Further analysis indicates that the coercivities of Sm- and Cu-doped ribbons are determined by different demagnetization mechanisms.
Mechanisms for the reduction of the Schottky barrier heights of high-quality nonalloyed Pt contacts on surface-treated p-GaN
Authors: Jang Ja-Soon , Seong Tae-Yeon
Published: July 2002

X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical bonding and electronic properties of the interfaces between Pt and p-GaN layers that were two-step surface treated using a buffered-oxide etch solution, and hence, to understand the surface-treatment time dependence of the Schottky barrier height (SBH). Current–voltage (I–V) measurements show that the effective SBH decreases with increasing surface-treatment time. The XPS results show that as the treatment time increases, the Ga 2p and Pt 4f core levels for the 20-min-treated samples shift toward the lower-binding-energy side by 0.6 and 1.5 eV, respectively, compared to the 0.5-min-treated one. It is further shown that the intensity of the oxygen core-level peak decreases with increasing treatment time. Based on the I–V and XPS results, the observed reduction of the effective SBHs is attributed to the combined effects of the effective removal of the native oxide and the shift of the surface Fermi level toward the valence-band edge.
Reaction class transition state theory: Hydrogen abstraction reactions by hydrogen atoms as test cases
Authors: Truong Thanh N.
Published: July 2002

We present a new method called Reaction Class Transition State Theory (RC-TST) for estimating thermal rate constants of a large number of reactions in a class. This method is based on the transition state theory framework within the reaction class approach. Thermal rate constants of a given reaction in a class relative to those of its principal reaction can be efficiently predicted from only its differential barrier height and reaction energy. Such requirements are much less than what is needed by the conventional TST method. Furthermore, we have shown that the differential energetic information can be calculated at a relatively low level of theory. No frequency calculation beyond those of the principal reaction is required for this theory. The new theory was applied to a number of hydrogen abstraction reactions. Excellent agreement with experimental data shows that the RC-TST method can be very useful in design of fundamental kinetic models of complex reactions.
Template-grown high-density nanocapacitor arrays
Authors: Shelimov Konstantin B. , Davydov Dmitri N. , Moskovits Martin
Published: July 2002

The fabrication and electrical properties of high-density arrays of cylindrical nanoscale capacitors grown in anodic aluminum oxide templates is described. Using chemical vapor deposition, alternating metallic (carbon) and insulating (boron nitride) layers are created within the template pores, thereby forming composite metal/insulator/metal nanotubules. With the metal electrodes evaporated on the two sides of the template, the structure is converted to an array of nanocapacitors connected in parallel. For 50-μm-thick templates, specific capacitances as high as 2.5 μF/cm2 were measured and capacitances as high as 13 μF/cm2 should be attainable by optimizing the insulating layer properties. The fabrication process can be made compatible with the silicon technology and might, therefore, be used to fabricate high-capacitance elements on tightly packed chips. At the same time, the leakage resistance of the arrays fabricated in the preliminary studies reported here is rather low, presumably due to the contamination of the insulating layer.
Concentration-dependent sedimentation of colloidal rods
Authors: Dogic Z. , Philipse A. P. , Fraden S. , Dhont J. K. G.
Published: July 2002

In this paper we first develop an approximate theory for the leading order concentration dependence of the sedimentation coefficient for rodlike colloids/polymers/macromolecules. To first order in volume fraction φ of rods, the sedimentation coefficient is written as 1+αφ. For large aspect ratios L/D (L is the rod length, D its thickness) α is found to vary like ∝(L/D)2/ln(L/D). This theoretical prediction is compared to experimental results. Then, experiments on fd virus are described, both in the isotropic and nematic phase. First-order in concentration results for this very long and thin (semiflexible) rod are in agreement with the above-mentioned theoretical prediction. Sedimentation profiles for the nematic phase show two sedimentation fronts. This result indicates that the nematic phase becomes unstable with the respect to isotropic phase during sedimentation.
The fluid mechanics of fire whirls: An inviscid model
Authors: Battaglia Francine , Rehm Ronald G. , Baum Howard R.
Published: July 2002

Whirling fire plumes are known to increase the danger of naturally occurring or post-disaster fires. In order for a fire whirl to exist, there must be an organized source of angular momentum to produce the large swirl velocities as air is entrained into the fire plume. These vorticity-driven fires occur over a large range of length and velocity scales, and significantly alter the entrainment and combustion dynamics. A new model is derived for a buoyant plume that incorporates angular rotation and neglects dissipation; the result is a form of the steady state Euler equations. Included is a general solution for large density and temperature variations. Results are presented that identify the mechanisms and their effects toward creating a fire whirl.
Microscopic lock-in thermography investigation of leakage sites in integrated circuits
Authors: Breitenstein O. , Langenkamp M. , Altmann F. , Katzer D. , Lindner A. , Eggers H.
Published: July 2002

The detection limit of infrared thermographic investigations can be improved down to 10 μK by using a highly sensitive high-speed infrared camera in an on-line averaging lock-in thermography system. Together with a microscope objective, this allows lock-in thermography to be used as a simple and sensitive technique to localize the sites of leakage currents and other heat sources in electronic components. The practical realization of a novel lock-in thermography system is described and both test measurements and practical applications are introduced. The detection limit for surface-near local heat sources in silicon is a few microwatts with a spatial resolution down to 5 μm. Leakage sites in several microelectronic structures are imaged and assigned to the layout of the integrated circuit by comparing direct images with lock-in ones. The direct comparison of an averaged and background-subtracted stationary thermogram with a lock-in one, both measured under similar conditions at the same sample, clearly demonstrates the gain in information obtained by using lock-in thermography.
Study on electron trapping and interface states of various gate dielectric materials in 4H–SiC metal-oxide-semiconductor capacitors
Authors: Cho Won-ju , Kosugi Ryoji , Senzaki Junji , Fukuda Kenji , Arai Kazuo , Suzuki Seiji
Published: July 2002

The characteristics of electron trapping and interface states for various gate dielectrics formed on the 4H–SiC metal-oxide-semiconductor capacitors have been studied. Thermal gate oxides prepared by dry oxidation (in pure O2) or wet oxidation (in H2:O2=1:2), oxide-nitride-oxide (ONO) stacked films and low-temperature oxide (LTO) films formed by low-pressure chemical vapor deposition were used. Post-oxidation anneal was performed at 1200 °C for 1 h in Ar ambient. It is found that the LTO film has lower interface state density and effective oxide charge density than the other gate dielectrics. Also, the LTO film showed excellent electron trapping characteristics against the electron injection. On the contrary, the electron trapping of ONO stacked film was larger than that of the thermal oxide and LTO film.
The accuracy of the pseudopotential approximation. III. A comparison between pseudopotential and all-electron methods for Au and AuH
Authors: Schwerdtfeger Peter , Brown J. Reuben , Laerdahl Jon K. , Stoll Hermann
Published: July 2002

The quality of the pseudopotential approximation has been tested thoroughly by calculating spectroscopic properties of the gold atom and ground state AuH for eight different effective core potentials using Hartree–Fock, second-order Mo/ller–Plesset and coupled cluster methods. The pseudopotential valence basis set {φ}v for Au was chosen to be identical for all pseudopotentials, a subset of the all-electron basis set {φ}v⊂{φ}AE, and the condition was applied that all sets are of near basis set limit quality. The pseudopotential results are compared with data obtained from nonrelativistic, scalar relativistic Douglas–Kroll and fully relativistic four-component all-electron calculations. The variation between the results obtained for all valence electron small-core pseudopotentials and all electron Douglas–Kroll calculations is found to be small (for the Stuttgart pseudopotential Δre=0.001 Å, ΔDe=0.03 eV, Δωe=9 cm−1, Δμe=0.04 D). Sizable differences to all electron results are only found for the 11 valence electron large-core pseudopotentials. The effects of the basis set superposition error on spectroscopic constants were investigated. Calculated coupled cluster electron affinities and ionization potentials for gold and spectroscopic properties for AuH were found to be in excellent agreement with available experimental data. The variation between the different small-core pseudopotentials for one particular spectroscopic property is shown to be less than the error due to the incompleteness of electron correlation procedure or the basis set and approximately of the same size as the basis set superposition error. The results show that scalar relativistic effects for valence properties are perfectly described by the pseudopotential approximation.
Charge-transfer correction for improved time-dependent local density approximation excited-state potential energy curves: Analysis within the two-level model with illustration for H2 and LiH
Authors: Casida Mark E. , Gutierrez Fabien , Guan Jingang , Gadea Florent-Xavier , Salahub Dennis , Daudey Jean-Pierre
Published: July 2002

Time-dependent density-functional theory (TDDFT) is an increasingly popular approach for calculating molecular excitation energies. However, the TDDFT lowest triplet excitation energy, ωT, of a closed-shell molecule often falls rapidly to zero and then becomes imaginary at large internuclear distances. We show that this unphysical behavior occurs because ωT2 must become negative wherever symmetry breaking lowers the energy of the ground state solution below that of the symmetry unbroken solution. We use the fact that the ΔSCF method gives a qualitatively correct first triplet excited state to derive a “charge-transfer correction” (CTC) for the time-dependent local density approximation (TDLDA) within the two-level model and the Tamm-Dancoff approximation (TDA). Although this correction would not be needed for the exact exchange–correlation functional, it is evidently important for a correct description of molecular excited state potential energy surfaces in the TDLDA. As a byproduct of our analysis, we show why TDLDA and LDA ΔSCF excitation energies are often very similar near the equilibrium geometries. The reasoning given here is fairly general and it is expected that similar corrections will be needed in the case of generalized gradient approximations and hybrid functionals.
Thermal conductivity of amorphous carbon thin films
Authors: Bullen Andrew J. , O’Hara Keith E. , Cahill David G. , Monteiro Othon , von Keudell Achim
Published: July 2002

Thermal conductivities Λ of amorphous carbon thin films are measured in the temperatures range 80–400 K using the 3ω method. Sample films range from soft a-C:H prepared by remote-plasma deposition (Λ=0.20 W m−1 K−1 at room temperature) to amorphous diamond with a large fraction of sp3 bonded carbon deposited from a filtered-arc source (Λ=2.2 W m−1 K−1). Effective-medium theory provides a phenomenological description of the variation of conductivity with mass density. The thermal conductivities are in good agreement with the minimum thermal conductivity calculated from the measured atomic density and longitudinal speed of sound.
Electron magnetohydrodynamic turbulence in a high-beta plasma. I. Plasma parameters and instability conditions
Authors: Stenzel R. L. , Urrutia J. M.
Published: July 2002

The interaction of a dense discharge plasma with a weak external magnetic field has been studied experimentally. The electron pressure exceeds the field pressure and forms a magnetic hole in the plasma interior. The ions are unmagnetized, while the electrons are in a transition regime from none to full magnetization. The electron confinement changes from Boltzmann equilibrium to magnetic confinement. The pressure balance equation does not describe the diamagnetism because ambipolar E×B drifts oppose the diamagnetic drift. The net drift exceeds the sound speed by an order of magnitude and produces a strong two-stream cross-field instability. Although its spectrum is close to the lower hybrid instability, there are significant differences from the classical lower hybrid instability, e.g., the presence of strong magnetic fluctuations. These fall into the regime of electron magnetohydrodynamics (EMHD) with unmagnetized but mobile ions. While the EMHD turbulence is the main focus of the two following companion papers, this first paper describes the plasma diamagnetism and basic parameters that lead to the instability.
Statistical rate theory for the HO+O⇔HO2⇔H+O2 reaction system: SACM/CT calculations between 0 and 5000 K
Authors: Harding L. B. , Maergoiz A. I. , Troe J. , Ushakov V. G.
Published: July 2002

The potential energy surface of the HO+O⇔HO2⇔H+O2 reaction system is characterized by ab initio calculations. The complex-forming bimolecular reaction is then treated by statistical rate theory, using statistical adiabatic channel and classical trajectory calculations for the HO+O⇔HO2 and HO2⇔H+O2 association/dissociation processes. Specific rate constants k(E,J) of both reactions as well as thermal rate constants are calculated over wide ranges of conditions. Open shell quantum effects are important up to room temperature. The good agreement with experimental results suggests that the ab initio potential is of sufficient accuracy. There is no evidence for non-statistical effects or for a significant contribution from electronically excited states. The comparison with rate data for the H+O2→HO+O reaction, because of the remaining uncertainty in the heat of formation of HO, is somewhat inconclusive. Apart from this problem, the calculated rate constants appear reliable between 0 and 5000 K.
Simultaneous measurement of liquid density and viscosity using remote query magnetoelastic sensors
Authors: Grimes Craig A. , Kouzoudis Dimitris , Mungle Casey
Published: July 2002

Earlier work [C. A. Grimes et al., Smart Mater. Struct. 8, 639, (1999)] has shown that upon immersion in liquid the resonant frequency of a magnetoelastic sensor shifts linearly in response to the square root of the liquid density and viscosity product. It is shown that comparison between a pair of magnetoelastic sensors with different degrees of surface roughness can be used to simultaneously determine the liquid density and viscosity.
Photorefraction in LiNbO3 as a function of [Li]/[Nb] and MgO concentrations
Authors: Furukawa Y. , Kitamura K. , Takekawa S. , Miyamoto A. , Terao M. , Suda N.
Published: July 2002

Photorefractive damage, photoconductivities, and photogalvanic currents of LiNbO3 single crystals have been investigated as a function of [Li]/[Nb] and MgO concentrations. Near stoichiometric LiNbO3 single crystal, having large [Li]/[Nb] ratios of 0.993, shows lower photorefractive damage resistance than congruent LiNbO3. However, crystals doped with a small amount of MgO (>0.78 mol %) exhibit no measurable photorefractive damage at 532 nm to intensities of as much as 2 MW/cm2. This remarkable decrease of photorefraction that was found can be attributed to the decrease of saturated space charge field results from a combination of the increased photoconductivity and the decreased photogalvanic current of the crystal.
Ionic Wiedemann–Franz law
Authors: Scott J. F. , Bohn H. G. , Schenk W.
Published: July 2002

The Wiedemann–Franz law for electron transport in metals was historically a linch pin for the free-electron theory of metals; it states that, at moderately low temperatures, the ratio of thermal conductivity K to electrical conductivity σ is proportional to absolute temperature, with a proportionality constant called “the Lorenz number” of L=2.45×10−8 W Ω/K2. Here we describe work on yttrium-stabilized zirconia in which we have found that a similar relationship is valid above 1350 K, where nearly all the electrical conduction and thermal conduction arise from ionic currents.
Gate leakage current mechanisms in AlGaN/GaN heterostructure field-effect transistors
Authors: Miller E. J. , Dang X. Z. , Yu E. T.
Published: July 2002

Gate leakage currents in AlGaN/GaN heterostructure field-effect transistor (HFET) structures with conventional and polarization-enhanced barriers have been studied. Comparisons of extensive gate leakage current measurements with two-dimensional simulations show that vertical tunneling is the dominant mechanism for gate leakage current in the standard-barrier HFET and that the enhanced-barrier structure suppresses this mechanism in order to achieve a reduced leakage current. An analytical model of vertical tunneling in a reverse-biased HFET gate-drain diode is developed to evaluate the plausibility of this conclusion. The model can be fit to the measured data, but suggests that additional leakage mechanisms such as lateral tunneling from the edge of the gate to the drain or defect-assisted tunneling also contribute to the total leakage current. The vertical tunneling current mechanism is shown to be more significant to the gate leakage current in III–V nitride HFETs than in HFETs fabricated in other III–V material systems, in which the lateral tunneling current component generally dominates the gate leakage current.
Improved refractive index formulas for the AlxGa1−xN and InyGa1−yN alloys
Authors: Laws G. M. , Larkins E. C. , Harrison I. , Molloy C. , Somerford D.
Published: July 2002

A detailed understanding of the nitride refractive indices is essential for the modeling and design of III–N laser structures. In this article, we report on the assessment of the refractive index data available for the nitride alloys and present formulas for evaluating the refractive indices for variations in both composition and photon energy. For AlxGa1−xN, an expression is given which fits well to experimental data below x<0.38, sufficient for the molefractions found in the cladding layers of III–N lasers. Due to the almost complete lack of experimental refractive index data for InyGa1−yN, we propose an expression to give a first-order approximation for the refractive index.
Deceleration mechanism of spheromak-like compact toroid penetrating into magnetized plasmas
Authors: Suzuki Yoshio , Hayashi Takaya , Kishimoto Yasuaki
Published: July 2002

To understand the fueling process in a fusion device by a spheromak-like compact toroid (SCT) injection method, magnetohydrodynamic numerical simulations, where a SCT is injected into magnetized target plasmas, have been carried out so far. As a result, it has been found that the SCT penetration into magnetized target plasmas is accompanied by complex physical dynamics, which is not adequately described by the conventional simple theoretical model. In this study, based on the previous simulation results, a new theoretical model to determine the penetration depth of the SCT is represented. Here, the SCT is considered to be decelerated not only by the magnetic pressure force but also by the magnetic tension force, which is generated by the bending of the target magnetic field as a result of the SCT penetration. Furthermore, by comparing the penetration depth of the SCT estimated from the theoretical model with that in the simulation, the accuracy of the model is examined. Finally, the effect of magnetic reconnection on the SCT penetration is discussed.
Spectroscopic identification of light emitted from defects in silicon devices
Authors: Rasras Mahmoud S. , De Wolf Ingrid , Groeseneken Guido , Maes Herman E.
Published: July 2002

A comprehensive study of different fundamental aspects of light emission from defects in Si semiconductor devices is presented. Based on an experimental analysis, using a new highly sensitive spectroscopic photon emission microscope (SPEM) for continuous wavelength analysis (2.5 eV–1.2 eV), a unique assignment of the spectrum of the emitted light and the corresponding failure mechanism is established. Three distinguishable basic spectral categories were identified. They were attributed to gate oxide breakdown, metal shorts, and electro-static discharge caused junction spiking. The focused ion beam technique was used to look at the damage sites for confirmation of the SPEM results.
Experiments measuring the initial energy deposition, expansion rates and morphology of exploding wires with about 1 kA/wire
Authors: Sinars D. B. , Hu Min , Chandler K. M. , Shelkovenko T. A. , Pikuz S. A. , Greenly J. B. , Hammer D. A. , Kusse B. R.
Published: July 2002

Wire-array Z-pinch implosion experiments begin with wire heating, explosion, and plasma formation phases that are driven by an initial 50–100 ns, 0–1 kA/wire portion of the current pulse. This paper presents expansion rates for the dense, exploding wire cores for several wire materials under these conditions, with and without insulating coatings, and shows that these rates are related to the energy deposition prior to plasma formation around the wire. The most rapid and uniform expansion occurs for wires in which the initial energy deposition is a substantial fraction of the energy required to completely vaporize the wire. Conversely, wire materials with less energy deposition relative to the vaporization energy show complex internal structure and the slowest, most nonuniform expansion. This paper also presents calibrated radial density profiles for some Ag wire explosions, and structural details present in some wire explosions, such as foam-like appearance, stratified layers and gaps.
Arsenic incorporation during Si(001):As gas-source molecular-beam epitaxy from Si2H6 and AsH3: Effects on film-growth kinetics
Authors: Kim H. , Glass G. , Soares J. A. N. T. , Desjardins P. , Greene J. E.
Published: July 2002

The effects of As doping, at concentrations CAs⩽4.8×1018 cm−3, on the growth kinetics of Si(001):As layers deposited at temperatures Ts=575–900 °C by gas-source molecular-beam epitaxy from Si2H6 and AsH3 have been investigated. With constant AsH3 and Si2H6 fluxes, film deposition rates RSi increase while CAs decreases with increasing Ts. All incorporated As resides at substitutional electrically active sites for CAs up to 3.8×1018 cm−3 (Ts=800 °C), the highest value yet reported for Si(001):As growth from hydride source gases. Immediately following film growth or partial-monolayer As adsorption on clean Si(001), the samples were quenched to 300 °C and exposed to atomic deuterium (D) until saturation coverage. In situ D2 temperature-programmed desorption (TPD) spectra from both as-deposited Si(001):As and As-adsorbed Si(001) layers are composed of β1 and β2 peaks, due to D2 desorption from Si monodeuteride and dideuteride surface phases, together with a new peak β3 which we attribute to desorption from Si–As mixed dimers. Analyses of the TPD spectra show that, because of the lone-pair electrons associated with each As surface atom, the total dangling-bond coverage, and hence RSi, decreases with increasing incoming flux JAsH3 at constant Ts. From measurements of the steady-state As surface coverage θAs vs CAs and Ts, we obtain an As surface segregation enthalpy ΔHs=−0.92 eV. Dissociative AsH3 adsorption on Si(001) was found to follow second-order kinetics with a relatively Ts-independent reactive sticking probability of 0.3. Associative As2 desorption is also second order with a rate constant kd,As2=1×1013 exp(−3.0 eV/kTs). From the combined set of results, we develop a predictive model with no fitting parameters for CAs vs JAsH3, JSi2H6, and Ts.
Hall mobility and free electron density at the SiC/SiO2 interface in 4H–SiC
Authors: Saks N. S. , Agarwal A. K.
Published: July 2002

The electron mobility and free electron density have been measured in 4H– and 6H–SiC metal-oxide-semiconductor inversion layers using the Hall effect. The 4H–SiC inversion layers are found to have very poor conductance which is caused by severe trapping of electrons at the Si/SiO2 interface. The trapping causes reduced conductance through a reduction in the number of free electrons in the inversion layer, and also because of a drop in the mobility due to increased Coulombic scattering.
Epitaxial diamond encapsulation of metal microprobes for high pressure experiments
Authors: Weir Samuel T. , Akella Jagannadham , Aracne-Ruddle Chantel , Vohra Yogesh K. , Catledge Shane A.
Published: July 2002

Diamond anvils with diamond encapsulated thin-film microcircuits have been fabricated for ultrahigh pressure electrical conductivity experiments. The diamond films were homoepitaxially deposited onto the diamond anvil substrates with microwave plasma chemical vapor deposition using a 2% methane in hydrogen gas mixture and a diamond substrate temperature of 1300 °C. The diamond embedded thin-film microprobes remain functional to megabar pressures. We have applied this technology to the study of the pressure-induced metallization of KI under pressures up to 1.8 Mbar. This technology has the potential of greatly advancing the pressure range of a number of existing high-pressure diagnostic techniques, and for expanding the capabilities of diamond anvil cells into new directions.
Spin–lattice interaction in colossal magnetoresistance manganites
Authors: Lobad Ahmed I. , Averitt Richard D. , Kwon Chuhee , Taylor Antoinette J.
Published: July 2002

The metal–insulator transition and underlying spin dynamics in La0.7D0.3MnO3 (D=Ca, Sr) are investigated using optical pump–probe spectroscopy at 1.5 eV. Our measurements, which span the ferromagnetic–paramagnetic transition temperature, reveal that the dynamics of the optically induced spectral weight transfer follow the temperature dependence of the magnetic specific heat. This dependence reflects the intrinsic interdependence between the optical conductivity and magnetism in the manganites allowing for the determination of the spin-lattice coupling magnitude.
Effect of sheath evolution on metal ion implantation in a vacuum arc plasma source
Authors: Bilek M. M. M.
Published: July 2002

Plasma immersion ion implantation is a surface modification technique in which ions to be implanted are drawn directly from surrounding plasma by a pulse biased substrate. Ion acceleration occurs in the electric sheath that forms around the substrate. The technique is most often used to implant ions, such as nitrogen, from plasmas formed by excitation of a gas. More recently it has been applied with cathodic vacuum arc plasmas in order to implant metal ions. These plasmas have a directed ion drift velocity and generally a higher density than gas based plasmas. Both of these differences influence the development of the electric sheath, which determines the implantation profile and the overall stability of the process. If the sheath expands too much during the pulse, the plasma is depleted and implantation ceases. If, however, the sheath is too thin at any point in space breakdown occurs also stopping the implantation process. The major parameters affecting the sheath formation are plasma density, ion drift velocity, and substrate curvature. The influence of each of these factors is discussed and strategies for forming optimum sheath structures are identified.
An analytic model of electroluminescence in bilayer organic light emitting diodes with Ohmic injection of charge carriers
Authors: Nikitenko V. , Bässler H.
Published: July 2002

A simple analytic model for the electroluminescence (EL) intensity and the total cell current in bilayer light-emitting diodes (LEDs) with Ohmic contacts is presented. Results are in good agreement with both existing experimental data and numerical simulations. This model permits the semiquantitative analyses of the influence of the most important parameters on the EL and cell currents. In particular, dependence of EL current on the ratio of electron to hole mobilities and on the LED geometry is analyzed. The predictions of the model are compared with analogous results for LEDs with barrier-limited injection of electrons. It is shown that the values of EL currents can be comparable.
Electron beam generation from semiconductor photocathodes
Authors: Arneodo F. , Cavanna F. , De Mitri I. , Mazza D. , Nassisi V.
Published: July 2002

Several measurements on a variety of semiconductor photocathodes were performed in order to determine their photoelectric quantum efficiency. Two different excimer lasers (XeCl and KrCl) and a pulsed Xe lamp were used as light sources for electron photoextraction from doped and undoped samples of cadmiun telluride, indium antimonide, and indium phosphide. Large current densities were obtained up to the limit of the Child–Langmuir law. This suggests the use of these materials for the production of intense electron sources, which could also be used for purity measurements of noble liquids.
Thermal lens scanning of the glass transition in polymers
Authors: Rohling J. H. , Caldeira A. M. F. , Pereira J. R. D. , Medina A. N. , Bento A. C. , Baesso M. L. , Miranda L. C. M. , Rubira A. F.
Published: July 2002

In this article we discuss the use of the thermal lens technique for investigating the thermal properties of polymers as a function of temperature. It is also discussed how the experimentally determined thermal lens parameters can be used to locate the glass transition in polymers. The methodology is tested using a solution casted films of poly(vinyl chloride) as a testing sample. A comparison with conventional differential scanning calorimetry data is made. It is proposed that the current transient thermal lens methodology, with minor changes in its experimental configuration, could be adapted to develop a new methodology called differential thermal lens scanning especially designed for the investigation of the phase transitions in polymers. It is shown that this new methodology could be equally used for the measurement of the thermal expansion coefficient, above and below the glass transition.
Electrical properties of p-type GaN:Mg codoped with oxygen
Authors: Korotkov R. Y. , Gregie J. M. , Wessels B. W.
Published: July 2002

Codoping of p-type GaN with Mg and oxygen was investigated. By codoping with oxygen the hole concentrations increased to 2×1018 cm−3 at 295 K, an order of magnitude greater than in Mg-doped epilayers. The resistivity of codoped layers decreased from 8 to 0.2 Ω cm upon oxygen codoping. Variable temperature Hall effect measurements indicated that the acceptor activation energy decreases from 170±5 meV in Mg-doped films to 135±5 meV upon oxygen doping. The higher hole concentration results in part from a decrease in the ionization energy of the acceptor.