bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of magnesium/Alq3 interaction in organic light emitting devices
Authors: Zhang R. Q. , Hou X. Y. , Lee S. T.
Published: July 2002

The interaction between the magnesium (Mg) atom and Alq3 molecule in organic light emitting devices has been theoretically studied using the PM3 method of the molecular orbital theory. It has been shown that various interactions can happen when the Mg atom approaches the Alq3 molecule, leading to significant changes in the molecular geometry and electronic structure of Alq3. The resulting configurations with insertion of Mg into the central part of the Alq3 molecule are found to be energetically more favorable than those with Mg forming bond(s) with a benzene ring of Alq3. In agreement with recent experiments, this result shows that the interaction between Mg and the Alq3 molecule depends on the order of deposition. Interestingly, the density of states calculated for these systems reveals that the creation of the new electronic gap states observed in the experiments is mainly relating to the aluminum atomic orbital due to interaction, instead of a direct contribution of Mg.
Coherent array of tungsten ultrafine particles by laser irradiation
Authors: Kawakami Yuji , Ozawa Eiichi , Sasaki Shinya
Published: July 2002

We found a coherent array of tungsten ultrafine particles of about 700 nm around a laser-irradiated mark on a single-crystalline surface of tungsten. The tungsten specimens were irradiated by a short-pulse Nd:YAG laser under low pressure in an inert gas atmosphere. Two types of coherent arrays were observed that included the two-dimensional cubic and hexagonal systems. Such a coherent array of ultrafine particles may have potential applications for emission devices of plasma display and microelectronic devices.
Analysis of the improved contact resistance in metal-p+ silicon Schottky barriers using the BF2/B mixed implantation
Authors: Lee Jung-Ho , Yeo In-Seok , Lee Jeong-Youb , Lee Sahng-Kyoo
Published: July 2002

Remarkable reduction of contact resistance in the metal-p+ silicon Schottky barriers is obtained using the BF2/B mixed implant instead of the BF2 implant. Upon the annealing, higher remaining contents of both boron and fluorine in a shallow p+ surface layer are observed in the mixed implanted silicon. Variation of the dopants loss observed with different F doses is explained by introducing the following two types of surface reaction, i.e., (1) recombination of F–Si to make gaseous fluorosilyl/oxyfluorosilyl products and (2) recombination of F–B to form gaseous boron fluorides. Increasing the amount of the F–B reaction accelerates the recombination reaction of F–Si due to the reduced surface p+ doping level, and then results in the anomalous enhancement of the F removal. For the mixed implant, shortening of the average interdopant separation due to higher B contents within the contact depletion width reduces the potential fluctuations induced by the dopant discreteness, and thus leads to the improved uniformity of contact resistance.
Insulating GaN:Zn layers grown by hydride vapor phase epitaxy on SiC substrates
Authors: Kuznetsov N. I. , Nikolaev A. E. , Zubrilov A. S. , Melnik Yu. V. , Dmitriev V. A.
Published: July 2002

Fabrication of high-performance GaN microwave devices, which are the excellent candidates for new generation of high-power solid-state components, requires insulating GaN substrate materials. Due to lack of bulk GaN crystals and particularly lack of semi-insulating GaN substrates, we propose insulating GaN layers on silicon carbide as substrates for the fabrication of GaN-based microwave devices. In this work, we demonstrate insulating GaN layers on silicon carbide substrates. Insulating GaN layers doped with zinc were grown on silicon carbide substrates by hydride vapor phase epitaxy. High crystal quality of the grown material was proved by x-ray diffraction measurements showing the full width at a half maximum of ω-scan rocking curve of about 100 arcsec. Temperature dependence of specific resistivity of the GaN:Zn layers was measured in the temperature range from 200 to 500 K. The value of the specific resistivity was found to be 1012 Ω cm at 300 K and 109 Ω cm at 500 K.
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omnidirectional reflector and microcavity resonator via the sol-gel method
Authors: Chen Kevin M. , Sparks Andrew W. , Luan Hsin-Chiao , Lim Desmond R. , Wada Kazumi , Kimerling Lionel C.
Published: July 2002

Thin films of SiO2 and TiO2 were used to fabricate one-dimensional photonic crystal devices using the sol-gel method: an omnidirectional reflector and microcavity resonator. The reflector consisted of six SiO2/TiO2 bilayers, designed with a stopband in the near infrared. Reflectivity over an incident angle range of 0°–80° showed an omnidirectional band of 70 nm, which agrees with theoretical predictions for this materials system. The microcavity resonator consisted of a TiO2 Fabry–Perot cavity sandwiched between two SiO2/TiO2 mirrors of three bilayers each. We have fabricated a microcavity with resonance at λ=1500 nm and achieved a quality factor of Q=35. We measured a resonance frequency modulation with a change in incident angle of light and defect layer thickness.
Scanning field emission from patterned carbon nanotube films
Authors: Nilsson L. , Groening O. , Emmenegger C. , Kuettel O. , Schaller E. , Schlapbach L. , Kind H. , Bonard J-M. , Kern K.
Published: July 2002

The investigation of the field emission (FE) properties of carbon nanotube (CNT) films by a scanning anode FE apparatus, reveals a strong dependence on the density and morphology of the CNT deposit. Large differences between the microscopic and macroscopic current and emission site densities are observed, and explained in terms of a variation of the field enhancement factor β. As a consequence, the emitted current density can be optimized by tuning the density of CNTs. Films with medium densities (on the order of 107 emitters/cm2, according to electrostatic calculations) show the highest emitted current densities.
Picosecond hot-electron energy relaxation in NbN superconducting photodetectors
Authors: Il’in K. S. , Lindgren M. , Currie M. , Semenov A. D. , Gol’tsman G. N. , Sobolewski Roman , Cherednichenko S. I. , Gershenzon E. M.
Published: July 2002

We report time-resolved characterization of superconducting NbN hot-electron photodetectors using an electro-optic sampling method. Our samples were patterned into micron-size microbridges from 3.5-nm-thick NbN films deposited on sapphire substrates. The devices were illuminated with 100 fs optical pulses, and the photoresponse was measured in the ambient temperature range between 2.15 and 10.6 K (superconducting temperature transition TC). The experimental data agreed very well with the nonequilibrium hot-electron, two-temperature model. The quasiparticle thermalization time was ambient temperature independent and was measured to be 6.5 ps. The inelastic electron–phonon scattering time τe–ph tended to decrease with the temperature increase, although its change remained within the experimental error, while the phonon escape time τes decreased almost by a factor of two when the sample was put in direct contact with superfluid helium. Specifically, τe–ph and τes, fitted by the two-temperature model, were equal to 11.6 and 21 ps at 2.15 K, and 10(±2) and 38 ps at 10.5 K, respectively. The obtained value of τe–ph shows that the maximum intermediate frequency bandwidth of NbN hot-electron phonon-cooled mixers operating at TC can reach 16(+4/−3) GHz if one eliminates the bolometric phonon-heating effect.
Organic ligand and solvent kinetics during the assembly of CdSe nanocrystal arrays using infrared attenuated total reflection
Authors: Kim Bosang S. , Avila Luis , Brus Louis E. , Herman Irving P.
Published: July 2002

The self-assembly of amorphous three-dimensional arrays of CdSe nanocrystals is probed in real time using multiple-reflection, infrared attenuated total reflection spectroscopy by following the solvent and the organic ligands that passivate the nanocrystal surface. During the self-assembly of a 250 ML array from pyridine-capped CdSe nanocrystals in pyridine solvent, the solvent molecules evaporate in ∼30–40 min and the pyridine-capping molecules leave the array very slowly, apparently limited by diffusion, with ∼30±8% remaining after three days.
GaN–AlGaN heterostructure field-effect transistors over bulk GaN substrates
Authors: Asif Khan M. , Yang J. W. , Knap W. , Frayssinet E. , Hu X. , Simin G. , Prystawko P. , Leszczynski M. , Grzegory I. , Porowski S. , Gaska R. , Shur M. S. , Beaumont B. , Teisseire M. , Neu G.
Published: July 2002

We report on AlGaN/GaN heterostructures and heterostructure field-effect transistors (HFETs) fabricated on high-pressure-grown bulk GaN substrates. The 2d electron gas channel exhibits excellent electronic properties with room-temperature electron Hall mobility as high as μ=1650 cm2/V s combined with a very large electron sheet density ns≈1.4×1013 cm−2. The HFET devices demonstrated better linearity of transconductance and low gate leakage, especially at elevated temperatures. We also present the comparative study of high-current AlGaN/GaN HFETs (nsμ>2×1016 V−1 s−1) grown on bulk GaN, sapphire, and SiC substrates under the same conditions. We demonstrate that in the high-power regime, the self-heating effects, and not a dislocation density, is the dominant factor determining the device behavior.
Effective surface Debye temperature for NiMnSb(100) epitaxial films
Authors: Borca C. N. , Komesu Takashi , Jeong Hae-kyung , Dowben P. A. , Ristoiu D. , Hordequin Ch. , Pierre J. , Nozières J. P.
Published: July 2002

The surface Debye temperature of the NiMnSb (100) epitaxial films has been obtained using low energy electron diffraction, inverse photoemission, and core-level photoemission. The normal dynamic motion of the (100) surface results in a value for the effective surface Debye temperature of 145±13 K. This is far smaller than the bulk Debye temperature of 312±5 K obtained from wave vector dependent inelastic neutron scattering. The large difference between these measures of surface and bulk dynamic motion indicates a soft and compositionally different (100) surface.
Dipole-induced ordering in nematic liquid crystals. II. The elusive holy grail
Authors: Syvitski Raymond T. , Burnell E. Elliott
Published: July 2002

Similar size and shape molecules with different electric multipoles are used to investigate effects of molecular dipole, quadrupole, and polarizability interactions on the average orientational order of solutes in nematic liquid crystals. Solutes are codissolved in the same sample tube so that the orientational ordering among solutes can be directly compared and analyzed using mean-field models. Permanent dipoles have a negligible influence on solute orientational order. Effects from molecular polarizability interactions could not be separated from short-range interactions. However, order parameters predicted from strong, short-range repulsive forces coupled with interactions between the solute quadrupole and the average electric field gradient felt by the solute are consistent with experimental values. For the nematic mixture 55 wt% ZLI 1132 in EBBA [N-(pethoxybenzylidene)-p′-n-butylaniline] the contribution to solute ordering from long-range electrostatic interactions is negligible.
Spatial and energetic-entropic decomposition of surface tension in lipid bilayers from molecular dynamics simulations
Authors: Lindahl Erik , Edholm Olle
Published: July 2002

The spatial and groupwise distribution of surface tension in a fully hydrated 256 lipid dipalmitoylphosphatidylcholine (DPPC) bilayer is determined from a 5 ns molecular dynamics simulation by resolving the normal and lateral pressures in space through the introduction of a local virial. The resulting surface tension is separated into contributions from different types of interactions and pairwise terms between lipid headgroups, chains and water. By additionally performing a series of five simulations at constant areas ranging from 0.605 to 0.665 nm2 (each of 6 ns length), it is possible to independently resolve the energetic contributions to surface tension from the area dependence of the interaction energies. This also enables us to calculate the remaining entropic part of the tension and the thermal expansivity. Together with the total lateral pressures this yields a full decomposition of surface tension into energetic and entropic contributions from electrostatics, Lennard-Jones and bonded interactions between lipid chains, headgroups and water molecules. The resulting total surface tension in the bilayer is found to be a sum of very large terms of opposing signs, explaining the sensitivity of simulation surface tension to details in force fields. Headgroup and headgroup–water interactions are identified as attractive on average while the chain region wants to expand the bilayer. Both effects are dominated by entropic contributions but there are also substantial energetic terms in the hydrophobic core. The net lateral pressure is small and relatively smooth compared to the individual components, in agreement with experimental observations of DPPC lipids forming stable bilayers.
Imaging surface plasmon resonance sensor based on multiple wavelengths: Sensitivity considerations
Authors: Johansen Knut , Arwin Hans , Lundström Ingemar , Liedberg Bo
Published: July 2002

A new, multiple wavelength surface plasmon resonance apparatus for imaging applications is presented. It can be used for biosensing, e.g., for monitoring of chemical and biological reactions in real time with label-free molecules. A setup with a fixed incident angle in the Kretschmann configuration with gold as the supporting metal is described, both theoretically and experimentally. Simulations of the sensor response based on independently recorded optical (ellipsometric) data of gold show that the sensitivity for three-dimensional recognition layers (bulk) increases with increasing wavelength. For two-dimensional recognition layers (adlayer) maximum sensitivity is obtained within a limited wavelength range. In this situation, the rejection of bulk disturbances, e.g., emanating from temperature variations, decreases, with increasing wavelength. For imaging surface plasmon resonance the spatial resolution decreases with increasing wavelength. Hence, there is always a compromise between spatial resolution, bulk disturbance rejection, and sensitivity. Most importantly, by simultaneously using multiple wavelengths, it is possible to maintain a high sensitivity and accuracy over a large dynamic range. Furthermore, our simulations show that the sensitivity is independent of the refractive index of the prism.
Reactive excitation functions for F+p-H2/n-H2/D2 and the vibrational branching for F+HD
Authors: Dong Feng , Lee Shih-Huang , Liu Kopin
Published: July 2002

Complementary to our recent report on the F+HD reaction, the reactive excitation functions for the other isotopomers are presented. Through analysis of the differential cross section data, the collisional energy dependencies of product vibrational branchings for F+HD are also reported here. Several important conclusions can be drawn from this work. First, the transition-state properties, in particular the barrier height, of this reaction are well-characterized by the SW PES, despite its neglect of spin–orbit couplings. Second, contrary to the theoretical conclusion in recent literatures, an experimental observation is presented which seems to suggest that a resonance may indeed exist for the F+H2 reaction in support of the original interpretation proposed by Lee and co-workers. Third, the vibrational branching for the F+HD→HF+D reaction elucidates another facet of resonance effects in the integral cross sections. Finally, the nonadiabatic reactivity of the spin–orbit excited F*(2P1/2) atom is found to be small, which is in line with the conclusion inferred from a most recent, full quantum mechanical multisurface calculation.
Magnetic exciton in a two-layer system
Authors: Vol E. D. , Shevchenko S. I.
Published: July 2002

The bound state of a light electron of mass me and a heavy hole of mass mh (mh≫me) is considered for a two-layer system in a magnetic field. The field is assumed strong only for the electron (aBe≫l0, where aBe=ℏ2/(mee2) is the Bohr radius, and l0=cℏ/(eB) is the magnetic length). A new method of calculation is proposed by which one can find the ground-state energy of a magnetic exciton and the spectrum of its excited states without assuming that the Coulomb interaction is small. The effective mass m* is found, and the dependence of the energy of the exciton on its momentum P is obtained. The behavior of the exciton in crossed electric and magnetic fields is investigated. The results can be used for analysis of experiments in real magnetic fields ∼104–105 Oe for such semiconductors as InSb, InAs, GaAs, etc., where the ratio me/mh≲0.1.
Crystallization behavior of as-deposited, melt quenched, and primed amorphous states of Ge2Sb2.3Te5 films
Authors: Khulbe Pramod K. , Wright Ewan M. , Mansuripur Masud
Published: July 2002

We compare the crystallization behavior of thin films of Ge2Sb2.3Te5 in various amorphous states, namely, as-deposited, melt-quenched, and primed. These films are embedded in a quadrilayer stack similar in structure to the commercially available phase-change optical disks. This study shows that the melt-quenched amorphous film has a shorter crystallization onset time and a higher crystallization rate in comparison to the as-deposited amorphous film. We also observed that variable priming leads to crystallization behavior falling between that of the as-deposited and melt-quenched states. A qualitative model of the modification in crystallization behavior due to priming is given based on the notion that priming produces crystalline embryos which hastens crystallization process.
Impact of in-plane anisotropic strain on the polarization behavior of vertical-cavity surface-emitting lasers
Authors: Panajotov K. , Nagler B. , Verschaffelt G. , Georgievski A. , Thienpont H. , Danckaert J. , Veretennicoff I.
Published: July 2002

We experimentally demonstrate that the presence of switching between two fundamental modes with orthogonal linear polarization in vertical-cavity surface-emitting lasers and the current at which this phenomenon occurs depend on both the magnitude and the orientation of an externally induced in-plane anisotropic strain. We interpret this behavior by considering the anisotropy in gain and in refractive index, both induced by the in-plane strain, and by accounting for a redshift of the two gain curves as a result of current-induced heating.
New millimeter wave and infrared spectra of the CO dimer: Assignment and precise location of quantum states
Authors: Walker Kaley A. , Xia Changhong , McKellar A. R. W.
Published: July 2002

Millimeter wave and infrared spectra of the weakly bound dimer (CO)2 have been studied in low-temperature pulsed supersonic jet expansions. Twenty-five new millimeter wave transitions have been observed and assigned, mostly in the 78–107 GHz region. Combined with previous data, they enable the relative energies of most of the known rotational levels (28 out of 31) in the ground vibrational state (vCO=0) of the dimer to be determined with “microwave” accuracy (≲0.1 MHz). Four new subbands in the infrared spectrum have been assigned in terms of two new stacks of rotational levels in the excited vibrational state (vCO=1), one stack with K=0 and the other with K=1. Energies for these levels have been determined with “infrared” accuracy (≲10 MHz). These results contribute significantly to the considerable body of precise experimental information available for a system that is ripe for further theoretical investigation.
The effect of particle size on the structural transitions in zinc sulfide
Authors: Qadri S. B. , Skelton E. F. , Dinsmore A. D. , Hu J. Z. , Kim W. J. , Nelson C. , Ratna B. R.
Published: July 2002

Studies of pressure induced phase transformations of ZnS nanoparticles using diamond anvil cells and synchrotron radiation were carried out to 20.0 GPa. Nanoparticles initially in the zinc-blende and wurtzite phases both transformed to the NaCl phase under the application of pressure. The zinc-blende particles, which were of 2.8 nm size, and the wurtzite particles, which were of 25.3 nm size, transformed to the NaCl phase at 19.0 and 15.0 GPa, respectively. Nanoparticles of the wurtzite phase never regained their initial wurtzite structure but returned to the zinc-blende phase upon downloading the pressure. The resultant zinc-blende nanoparticles transformed to the NaCl phase upon the reapplication of a pressure of 15.0 GPa. Nanoparticles initially in the zinc-blende phase returned to their original phase.
Radio-frequency plasma cleaning for mitigation of high-power microwave-pulse shortening in a coaxial gyrotron
Authors: Cohen William E. , Gilgenbach Ronald M. , Jaynes Reginald L. , Peters Christopher W. , Lopez Mike R. , Lau Y. Y. , Anderson Scott A. , Brake Mary L. , Spencer Thomas A.
Published: July 2002

Results are reported demonstrating that radio-frequency (rf) plasma cleaning is an effective technique for mitigating microwave-pulse shortening (i.e., lengthening the pulse) in a multimegawatt, large-orbit, coaxial gyrotron. Cleaning plasmas were generated by 50 W of rf power at 13.56 MHz in nitrogen fill gas in the pressure range 15–25 mTorr. Improvements in the averaged microwave energy output of this high-power-microwave device ranged from 15% to 245% for different initial conditions and cleaning protocols. The mechanism for this improvement is believed to be rf plasma sputtering of excess water vapor from the cavity/waveguide and subsequent removal of the contaminant by cryogenic vacuum pumps.
Dose dependence and dose-rate dependence of the optically stimulated luminescence signal
Authors: Chen R. , Leung P. L.
Published: July 2002

The expected behavior of the dose dependence of the optically stimulated luminescence (OSL) signal has been studied using numerical simulation. A simple model of one trapping state and one kind of recombination center is presented, and the sequence of sets of simultaneous differential equations governing the processes during excitation, relaxation, and light exposure are numerically solved. With the choice of reasonable trapping and recombination parameters, it has been shown that a quadratic dose dependence of this effect results from the model when the irradiation stage starts with empty trapping states. This may explain reports in the literature of an initial supralinear dose dependence of OSL. It is also shown that within the same model, one can get an initial linear dose dependence of OSL if one starts with partly filled traps. Also has been studied the influence of dose rate on the measured OSL signal for a constant total dose, and some effect has been seen for a certain dose-rate range. The similarities and dissimilarities of OSL as compared to thermoluminescence with respect to these phenomena are discussed.
Erbium emission from porous silicon one-dimensional photonic band gap structures
Authors: Lopez Herman A. , Fauchet Philippe M.
Published: July 2002

We report tunable, narrow, directional, and enhanced erbium emission from one-dimensional photonic band gap structures. The structures are prepared by anodic etching of crystalline silicon and consist of two highly reflecting porous silicon Bragg reflectors sandwiching an active layer. The cavities are doped by cathodic electromigration of the erbium ions into the porous silicon matrix, followed by high temperature oxidation. By controlling the oxidation temperature of the structure, the position of the erbium emission near 1.5 μm is tuned to regions where the natural erbium spectrum is very weak. The erbium emission from the cavity is narrowed to a full width at half maximum of 12 nm with a quality factor Q of 130, highly directional with a 20° emission cone around the normal axis, and enhanced by more than one order of magnitude.
Borel summable solutions to one-dimensional Schrödinger equation
Authors: Giller Stefan , Milczarski Piotr
Published: July 2002

It is shown that so called fundamental solutions the semiclassical expansions of which have been established earlier to be Borel summable to the solutions themselves appear also to be the unique solutions to the one-dimensional (1D) Schrödinger equation having this property. Namely, it is shown in this paper that for the polynomial potentials the Borel function defined by the fundamental solutions can be considered as the canonical one. The latter means that any Borel summable solution can be obtained by the Borel transformation of this unique canonical Borel function multiplied by some ℏ-dependent and Borel summable constant. This justifies the exceptional role the fundamental solutions play in 1D quantum mechanics and completes the relevant semiclassical theory relied on the Borel resummation technique and developed in our other papers.
Principle of ferroelectric domain imaging using atomic force microscope
Authors: Hong Seungbum , Woo Jungwon , Shin Hyunjung , Jeon Jong Up , Pak Y. Eugene , Colla Enrico L. , Setter Nava , Kim Eunah , No Kwangsoo
Published: July 2002

The contrast mechanisms of domain imaging experiments assisted by atomic force microscope (AFM) have been investigated by model experiments on nonpiezoelectric (silicon oxide) and piezoelectric [Pb(Zr,Ti)O3] thin films. The first step was to identify the electrostatic charge effects between the tip, the cantilever, and the sample surface. The second step was to explore the tip–sample piezoelectric force interaction. The static deflection of the cantilever was measured as a function of dc bias voltage (Vdc) applied to the bottom electrode (n-type Si wafers) for noncontact and contact modes. In addition, a small ac voltage (Vac sin ωt) was applied to the tip to measure the amplitude (Aω) and phase (Φω) of the first harmonic (ω) signal as a function of Vdc. By changing from the noncontact to the contact mode, a repulsive contribution to the static deflection was found in addition to the attractive one and a 180° phase shift in Φω was observed. These results imply that in the contact mode the cantilever buckling is induced by the capacitive force between the cantilever and the sample surface. This interaction adds to the tip–sample piezoelectric interaction thereby overlapping the obtained tip vibration signal. Therefore, the antiparallel ferroelectric domain images obtained at zero dc bias voltage will show a variation in Aω but a negligible one in Φω. The capacitive force contribution to the tip vibration signal was further verified in piezoelectric hysteresis loop measurement assisted by the AFM. The observed vertical offset of the loops was explained by the contact potential difference between the cantilever and the bottom electrode. The shape of the curve could be explained by the capacitive force interaction combined with the tip–sample piezoelectric interaction. The experimental results obtained in this study support the interpretation of the cantilever–sample capacitive force contribution to the tip vibration signal in ferroelectric domain imaging experiments using AFM as a probing tool. The use of a large area top electrode between the tip and the sample resulted in the elimination of the electrostatic cantilever–sample interaction with negligible degradation of the domain contrast. This method proved to be successful because the cantilever–sample interaction was hardly detected and only the tip–sample interaction was observed.
Size-controlled short nanobells: Growth and formation mechanism
Authors: Ma X. , Wang E. G. , Tilley R. D. , Jefferson D. A. , Zhou W.
Published: July 2002

Very short nanofibers containing only a few nanobells have been produced from nitrogen-doped carbon nanotubes by two methods, H2 plasma treatment, followed by grinding and oxidation with acidified potassium permanganate. The length of these short nanofibers can be controlled to give a wide range of sizes. The shortest nanofibers are composed of only a single nanobell with dimensions of ∼15 nm in diameter and length. High-resolution transmission electron microscope images reveal that their structures are closed at one end and open at the other. The formation mechanism of these short nanofibers and individual nanobells is discussed along with their potential for interesting nanotube-end physics and chemistry.
Two-dimensional electron-gas AlN/GaN heterostructures with extremely thin AlN barriers
Authors: Smorchkova I. P. , Keller S. , Heikman S. , Elsass C. R. , Heying B. , Fini P. , Speck J. S. , Mishra U. K.
Published: July 2002

Plasma-assisted molecular-beam epitaxy is used to grow a set of two-dimensional electron-gas AlN/GaN structures with AlN barrier thicknesses varied between 24 and 50 Å. The density of the two-dimensional electron gas formed at the GaN/AlN interface increases from 1.51×1013 cm−2 for the AlN barrier width of 24 Å to 3.65×1013 cm−2 for the AlN barrier width of 49 Å. The increase in the electron sheet density is accompanied by a decrease in electron mobility related to tensile strain relaxation and enhanced interface roughness scattering. It is shown that room-temperature sheet resistances below 200 Ω/□ can be achieved in AlN/GaN high electron mobility transistor structures with 35–45 Å AlN barriers.
Microscopic theory of protein folding rates. I. Fine structure of the free energy profile and folding routes from a variational approach
Authors: Portman John J. , Takada Shoji , Wolynes Peter G.
Published: July 2002

A microscopic theory of the free energy barriers and folding routes for minimally frustrated proteins is presented, greatly expanding on the presentation of the variational approach outlined previously [J. J. Portman, S. Takada, and P. G. Wolynes, Phys. Rev. Lett. 81, 5237 (1998)]. We choose the λ-repressor protein as an illustrative example and focus on how the polymer chain statistics influence free energy profiles and partially ordered ensembles of structures. In particular, we investigate the role of chain stiffness on the free energy profile and folding routes. We evaluate the applicability of simpler approximations in which the conformations of the protein molecule along the folding route are restricted to have residues that are either entirely folded or unfolded in contiguous stretches. We find that the folding routes obtained from only one contiguous folded region corresponds to a chain with a much greater persistence length than appropriate for natural protein chains, while the folding route obtained from two contiguous folded regions is able to capture the relatively folded regions calculated within the variational approach. The free energy profiles obtained from the contiguous sequence approximations have larger barriers than the more microscopic variational theory which is understood as a consequence of partial ordering.
High-temperature operation of distributed feedback quantum-cascade lasers at 5.3 μm
Authors: Hofstetter Daniel , Beck Mattias , Aellen Thierry , Faist Jérôme
Published: July 2002

High-temperature operation of a low-threshold 5.3 μm quantum-cascade distributed feedback laser is presented. The emission spectrum was single mode with more than 20 dB side mode suppression ratio for all investigated temperatures and up to thermal rollover. For 1.5% duty cycle and at 0 °C, the laser emitted 1.15 W of single mode peak power; at 120 °C, a value of 92 mW was seen. For a 3 mm long device, we observed a room-temperature threshold current density of 3.6 kA/cm2. This remarkable performance is mainly due to a 4 quantum-well active region using a double phonon resonance for the lower laser level.
Dynamics of fine particles in magnetized plasmas
Authors: Sato Noriyoshi , Uchida Giichiro , Kaneko Toshiro , Shimizu Shinya , Iizuka Satoru
Published: July 2002

Here are presented experiments on fine particles levitating in low-pressure weakly ionized plasmas under a vertical magnetic field. The magnetic field is useful to provide a vertically long cylindrical column of fine-particle clouds, yielding even string-shaped vertically aligned fine particles, under the double-plasma configuration. Measurements show that fine-particle clouds rotate in the azimuthal direction on the horizontal plane even in such a weak magnetic field that positive ions are slightly magnetized. With an increase of the magnetic field, the rotation speed increases, being followed by subsequent saturation. The rotation speed and direction can be controlled by varying radial plasma potential and/or density profiles. The rotation is induced under the condition that the interparticle distance is small enough for the strong Coulomb coupling among fine particles. A mechanism of the rotation could be explained by effects of ion motions on fine particles, which are modified in the presence of the vertical magnetic field.
Monte Carlo study of heat conductivity in stochastic boundaries: Application to the TEXTOR ergodic divertor
Authors: Runov A. M. , Reiter D. , Kasilov S. V. , Heyn M. F. , Kernbichler W.
Published: July 2002

The heat balance equation is derived and solved for fusion edge plasma conditions with (partially developed) ergodic magnetic-field structures. For this purpose, a three-dimensional (3D) Monte Carlo code, “E3D,” based upon the “multiple local magnetic coordinate system approach” has been developed. Parameters typical for the Dynamic Ergodic Divertor (DED) of TEXTOR-94 (Torus Experiment for the Technology Oriented Research) [K. H. Finken et al., Fusion Eng. Des. 37, 1 (1997)] are chosen in the applications. The plasma temperature fields and the profiles of the radial component of heat flux due to the classical parallel and anomalous perpendicular diffusion are calculated. Because of magnetic-field ergodization and diversion of field lines, parallel conduction also can contribute to this radial flux. The results are compared with theoretical predictions for two limiting cases: With the Rechester–Rosenbluth model of ergodization-induced transport and with a “laminar flow model” proposed in the present paper. This latter model describes the effects of field line diversion. The diversion effect is shown to be dominant for TEXTOR-DED conditions.
A quantum and semiclassical study of dynamical resonances in the C+NO→CN+O reaction
Authors: Abrol Ravinder , Wiesenfeld Laurent , Lambert Bruce , Kuppermann Aron
Published: July 2002

Accurate quantum mechanical reactive scattering calculations were performed for the collinear C+NO→CN+O reaction using a polynomial-modified London Eyring Polanyi Sato (PQLEPS) potential energy surface (PES), which has a 4.26 eV deep well in the strong interaction region, and a reference LEPS PES, which has no well in that region. The reaction probabilities obtained for both PESs show signatures for resonances. These resonances were characterized by calculating the eigenvalues and eigenvectors of the collision lifetime matrix as a function of energy. Many resonances were found for scattering on both PESs, indicating that the potential well in the PQLEPS PES does not play the sole role in producing resonances in this relatively heavy atom system and that Feshbach processes occur for both PESs. However, the well in the PQLEPS PES is responsible for the differences in the energies, lifetimes, and compositions of the corresponding resonance states. These resonances are also interpreted in terms of simple periodic orbits supported by both PESs (using the WKB formalism), to further illustrate the role played by that potential well on the dynamics of this reaction. The existence of the resonances is associated with the dynamics of the long-lived CNO complex, which is much different than that of systems having an activation barrier. Although these results were obtained for a collinear model of the reaction, its collinearly-dominated nature suggests that related resonant behavior may occur in the real world.
Solvation induced morphological effects on the polymer/metal contacts
Authors: Liu Jie , Guo Tzung-fang , Shi Yijian , Yang Yang
Published: July 2002

Polymer/metal interfaces play an important role in determining the performance of polymer based electronic devices. For most polymer diodes, one of the polymer/metal contacts is usually formed by spin casting a polymer solution onto a metal electrode; we call it polymer-on-metal (POM) contact. The other polymer/metal contact is usually formed by vacuum deposition of metal vapor onto the polymer surface; we call it metal-on-polymer (MOP) contact. In this article, we present evidence that the formation of the POM contact is strongly influenced by the processing of polymer thin films. The POM contact has a greater energy barrier for carrier injection than the MOP contact. The morphology of the polymer thin films, i.e., the relative arrangements of the side groups and conjugated backbone, significantly affects the interfacial characteristics. We also present a simple method of evaluating the morphological effects at the contact.
Ultrafast two-dimensional ultrasonic speckle velocimetry: A tool in flow imaging
Authors: Sandrin Laurent , Manneville Sébastien , Fink Mathias
Published: July 2002

A technique for imaging fluid flows is proposed, based on the ultrafast analysis of the ultrasonic speckle signal backscattered by particles following the flow. Such an “ultrasonic speckle velocimetry” (USV) provides two-dimensional measurements of one component of the fluid velocity field at about 5000 frames per second. USV is applied to three different flows and future improvements of the technique are described.
Reliability theory for receptor–ligand bond dissociation
Authors: Tees David F. J. , Woodward John T. , Hammer David A.
Published: July 2002

Cell adhesion in the presence of hydrodynamic forces is a critical factor in inflammation, cancer metastasis, and blood clotting. A number of assays have recently been developed to apply forces to small numbers of the receptor–ligand bonds responsible for adhesion. Examples include assays using hydrodynamic shear in flow chambers or elastic probe deflection assays such as the atomic force microscope or the biomembrane force probe. One wishes to use the data on the time distribution of dissociation from these assays to derive information on the force dependence of reaction rates, an important determinant of cell adhesive behavior. The dissociation process can be described using the theory developed for reliability engineering of electronic components and networks. We use this framework along with the Bell model for the reverse reaction rate (kr=kr0 exp[r0 f/kT], where f is the applied force and kr0 and r0 are Bell model parameters) to write closed form expressions for the probability distribution of break-up with multiple independent or interacting bonds. These expressions show that the average lifetime of n bonds scales with the nth harmonic number multiplied by the lifetime of a single bond. Results from calculation and simulations are used to describe the effect of experimental procedures in forced unbinding assays on the estimation of parameters for the force dependence of reverse reaction rates.
Reduction of stimulated scattering losses from hohlraum plasmas with laser beam smoothing
Authors: Glenzer S. H. , Berger R. L. , Divol L. M. , Kirkwood R. K. , MacGowan B. J. , Moody J. D. , Langdon A. B. , Suter L. J. , Williams E. A.
Published: July 2002

Laser beam smoothing by spectral dispersion and by polarization smoothing has been observed to significantly reduce the scattering losses by stimulated Brillouin and stimulated Raman scattering from inertial confinement fusion hohlraums. For these measurements, the laser beam smoothing and the high-Z hohlraum wall plasma parameters approach the conditions of future inertial confinement fusion experiments. The simultaneous application of the smoothing techniques has reduced the scattering losses by almost one order of magnitude down to the 1% level. The experimental scaling of the stimulated Brillouin reflectivity compares well to modeling assuming nonlinear damping on the ion acoustic waves in three-dimensional nonlinear wave simulations and calculated hohlraum plasma conditions from radiation-hydrodynamic modeling.
Needlelike motion of prolate ellipsoids in the sea of spheres
Authors: Vasanthi R. , Ravichandran S. , Bagchi Biman
Published: July 2002

Molecular dynamics simulations of translational motion of isolated prolate ellipsoids in the sea of spheres have been carried out for several different values of the aspect ratio (κ), obtained by changing either the length or the diameter of the ellipsoids, at several different solvent densities. The interaction among the spheres is given by the Lennard-Jones pair potential while that between spheres and ellipsoids is given by a modified Gay–Berne potential. Both the mean-square displacements of the center of mass of the ellipsoids and their orientational time correlation function have been calculated. It is found that at short to intermediate times, the motion of ellipsoids is anisotropic and primarily needlelike—the molecules prefer to move parallel to their long axis. The ratio of these two diffusion constants (D∥ and D⊥) approaches κ, suggesting a decoupling of D∥ from the length of the ellipsoid. The diffusion becomes isotropic in the long time with the total diffusion coefficient given by D∥+2D⊥. The crossover from the anisotropic to the isotropic diffusion is surprisingly sharp and clear in most cases.
Red organic light emitting device made from triphenylene hexaester and perylene tetraester
Authors: Seguy I. , Jolinat P. , Destruel P. , Farenc J. , Mamy R. , Bock H. , Ip J. , Nguyen T. P.
Published: July 2002

Saturated red light emission from organic light emitting diodes is less common than emission in the green or the blue. Most organic red light emitting devices are based on rare earth complexes, mainly europium, which are known to exhibit stability problems. The present article describes new diodes made of indium tin oxide-coated glass/triphenylene hexaether/perylene tetraester/aluminum. The band diagram was determined by ultraviolet photoemission spectroscopy, cyclic voltammetry, scanning tunneling microscopy, and absorbance measurements. The interfaces between electrodes and organic layers were investigated by x-ray photoelectron spectroscopy. The current–voltage and luminance–voltage characteristics are very reproducible from device to device, with an emission peak at 620 nm and a full width at half maximum of 80 nm, a current rectification ratio of about 30, I∼V2 at low voltages and I∼Lum∼V6 at higher voltages.
Dynamic effects on force measurements. I. Viscous drag on the atomic force microscope cantilever
Authors: Vinogradova Olga I. , Butt Hans-Jürgen , Yakubov Gleb E. , Feuillebois François
Published: July 2002

When the atomic force microscope (AFM) is used for force measurements, the driving speed typically does not exceed a few microns per second. However, it is possible to perform the AFM force experiment at much higher speed. In this article, theoretical calculations and experimental measurements are used to show that in such a dynamic regime the AFM cantilever can be significantly deflected due to viscous drag force. This suggests that in general the force balance used in a surface force apparatus does not apply to the dynamic force measurements with an AFM. We develop a number of models that can be used to estimate the deflection caused by viscous drag on a cantilever in various experimental situations. As a result, the conditions when this effect can be minimized or even suppressed are specified. This opens up a number of new possibilities to apply the standard AFM technique for studying dynamic phenomena in a thin gap.
Pressure effects on bimolecular recombination and unimolecular dissociation reactions
Authors: Marcus R. A. , Gao Yi Qin
Published: July 2002

The treatment of pressure effects on bimolecular recombinations and unimolecular dissociations is discussed. The analysis of recombination and dissociation reactions is made by showing how the nonequilibrium energy (E) and angular momentum (J)-dependent steady-state population distribution functions for the two reactions are related to each other and to the equilibrium population distribution function at the given E and J. As a special case a strong collision model is then used for the collisional rotational angular momentum transfer, and a ladder model for the collisional energy transfer. An analytical result is obtained for states below the dissociation threshold. The extension to recombinations with two exit channels is described, for application to ozone formation and isotopic effects.
Cross-beam pulsed laser deposition: General characteristic
Authors: Tselev A. , Gorbunov A. , Pompe W.
Published: July 2002

Physical background of the operation of the cross-beam pulsed laser deposition (CBPLD) is considered. In this modification of PLD, gas-dynamic interaction of two plasma plumes from separate targets is used for particulate elimination from the plasma flux to the substrate. The interaction of the plasma plumes by CBPLD in a vacuum was investigated with the help of ion collectors and optical spectroscopy. A minimal plasma number density in the intersection region necessary for an efficient plume interaction amounts to ≈1.4×1015 cm−3 in the geometry of the experiments. For the materials with relatively high evaporation rates, the plasma density in the plume intersection region is larger than this value during almost the entire plume existing time, and the ablated material can be effectively utilized for the deposition of films. CBPLD demonstrates a very good effectiveness of the particulate removal from the plasma flux, and it is reasonable to suggest that CBPLD is one of the most effective working methods for particulate suppression in PLD. The interaction of two plumes results in filtering out the most energetic ions and the slowest neutrals of the original plumes from the plasma flux to the substrate, in decrease of the average plasma particle energy, and in increase of the degree of ionization of plasma up to more than 100%. Mechanisms responsible for these effects are analyzed. An asymmetric CBPLD geometry is proposed that makes it possible to mix materials of different targets directly in the laser plasma with good control over film composition.
High-sensitivity absorption measurement in water and glass samples using a mode-mismatched pump-probe thermal lens method
Authors: Marcano O. A. , Loper C. , Melikechi N.
Published: July 2002

We report on a mode-mismatched pump-probe thermal lens experiment performed to measure absorption coefficients as small as 10−8 cm−1 in liquid and solid samples. We take advantage of the large axial dimension of the thermal lens by focusing the pump beam in the presence of a collimated probe beam. We measure an absorption coefficient at 533 nm of (6.8±0.1)×10−4 cm−1 and of (3±0.3)×10−6 cm−1 for distilled water and BK7 optical glass, respectively.
Fabrication of an all-oxide thermoelectric power generator
Authors: Matsubara Ichiro , Funahashi Ryoji , Takeuchi Tomonari , Sodeoka Satoshi , Shimizu Tadaaki , Ueno Kazuo
Published: July 2002

An oxide thermoelectric device was fabricated using Gd-doped Ca3Co4O9 p-type legs and La-doped CaMnO3 n-type legs on a fin. The power factors of p legs and n legs were 4.8×10−4 Wm−1 K−2 and 2.2×10−4 Wm−1 K−2 at 700 °C in air, respectively. With eight p–n couples the device generated an output power of 63.5 mW under the thermal condition of hot side temperature Th=773 °C and a temperature difference ΔT=390 °C. This device proved to be operable for more than two weeks in air showing high durability.