bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomic oxygen in solid deuterium
Authors: Danilychev A. V. , Apkarian V. A. , Kajihara H. , Tanaka S. , Koda S.
Published: July 2002

Atomic oxygen is photogenerated in solid D2 by 193 nm irradiation of samples initially doped with molecular oxygen. The atoms are detected by laser-induced fluorescence at the O(1S→1D) transition, which occurs at 559 nm, with a fluorescence lifetime of 230 μs. The absorption leading to this emission is indirect, attributed to O2(X):O(3P) pairs. Complementary studies are carried in solid D2 co-doped with Xe and O2, in which, in addition to ionic XeO centers, the atomic O(1S→1D) transition with a radiative lifetime of 50 μs is observed. The photogeneration of the atomic centers and the stability of the atomic and molecular emissions are sensitive to sample preparation and thermal and irradiation histories. In annealed solids at temperatures below 6.5 K the atomic emission does not bleach, implying that the vertically prepared O(1D) atoms undergo intersystem crossing to form O(3P) rather than react with D2. The barrier to insertion on the O(1D)+D2 potential energy surface in solid D2 is explained as a many-body polarization effect. The recombination of O(3P) atoms can be initiated thermally and can be monitored by their thermoluminescence at the molecular O2(A′→X) transition. The thermal onset of recombination varies between 5.5 K and 9 K, depending on the sample preparation method. In all cases, the thermally induced recombination is catastrophic, accompanied by thermal runaway, pressure burst, and material loss. This is interpreted as an indication that the process is initiated by self-diffusion of the host, consistent with the notion that atomic O centers stabilize the host lattice.
New features of swirling jets
Authors: Shtern Vladimir , Hussain Fazle , Herrada Miguel
Published: July 2002

Important new features are found for a family of swirling jets with velocity v∼z−n, where z is the distance from the jet origin. First, there is a sharp minimum of the pressure coefficient at a certain value of the swirl number Sw which is nearly n independent; this feature can be utilized in technological devices. Second, as Sw increases, a separation zone develops, where the fluid is not at rest in the inviscid limit (contrary to the claims of recent vortex breakdown theories). These results are obtained under the boundary layer approximation for incompressible jets characterized by n and Sw=vφm/vzm, where vφm and vzm are the maximal values of the swirl and longitudinal velocities at z=const. Unlike prior results viewed in terms of parameter L (which is the vφ/vz ratio at the outer edge of the jet), the solution dependence on Sw is found similar for both n<1 and n>1. For any n, (a) the pressure coefficient is minimum at Sw≈0.65; (b) two solutions exist for SwSwf; (c) as Sw decreases, the jets either consolidate near the axis or separate from it, depending on the solution branch; and (d) the flow in the separation zone tends to become swirl-free and potential.
Growth of arc in high-pressure, pulsed glow discharge by gas density depletion
Authors: Imada Go , Yatsui Kiyoshi , Masuda Wataru
Published: July 2002

Effects of gas density depletion on arc formation of high-pressure, pulsed glow discharge have been investigated by eliminating the other factors which may affect the discharge stability, such as shock waves, residual ions, electrode heating, and discharge products. The gas density depletion has been simulated by utilizing a subsonic gas flow between the curved electrodes combined with a convergent nozzle and a divergent diffuser. A comparison has been made on the discharge in the aerodynamically created gas density depletion with the second discharge in the double-pulse discharge within a stable gas. We have found that the large gas density depletion, Δρ/ρ0∼−3.6% corresponding to a pulse repetition rate (PRR) of ∼50 Hz, tends to cause an arc-like filament or an arc without the shocks, ions, electrode heating, and products. However, the second discharge in the double-pulse discharge becomes an arc in much smaller gas density depletion (Δρ/ρ0∼−1.2% corresponding to PRR ∼3 Hz). Therefore, the collapse of high-pressure, pulsed glow discharge is most likely caused by some factor other than the gas density depletion.
Streaming instabilities in a collisional dusty plasma
Authors: Mamun A. A. , Shukla P. K.
Published: July 2002

A pair of low-frequency electrostatic modes, which are very similar to those experimentally observed by Praburam and Goree [Phys. Plasmas 3, 1212 (1996)], are found to exist in a dusty plasma with a significant background neutral pressure and background ion streaming. One of these two modes is the dust-acoustic mode and the other one is a new mode which is due to the combined effects of the ion streaming and ion–neutral collisions. It has been shown that in the absence of the ion streaming, the dust-acoustic mode is damped due to the combined effects of the ion–neutral and dust–neutral collisions and the electron–ion recombination onto the dust grain surface. This result disagrees with Kaw and Singh [Phys. Rev. Lett. 79, 423 (1997)], who reported collisional instability of the dust-acoustic mode in such a dusty plasma. It has also been found that a streaming instability with the growth rate of the order of the dust plasma frequency is triggered when the background ion streaming speed relative to the charged dust particles is comparable or higher than the ion–thermal speed. This point completely agrees with Rosenberg [J. Vac. Soc. Technol. A 14, 631 (1996)].
Lie aspects of the dynamics of self-gravitating ellipsoids in n dimensions
Authors: Carrero J.
Published: July 2002

In this paper we study the dynamics of self-gravitating ellipsoids in n dimensions, from the point of view of the Poisson structure of the dual of a suitable Lie algebra. When n=3 this was done by Rosenteel. In this setting we describe explicitly the ring of invariant functions. In the two-dimensional case we apply a technique due to Pedersen to find globally defined Darboux coordinates on the coadjoint orbits. Finally we derive a characterization of tensors coming from potentials, and use it to exhibit the dynamical equations of the ellipsoid in the Hamiltonian form.
Dynamics of GaAs/AlGaAs microdisk lasers
Authors: Luo K. J. , Xu J. Y. , Cao H. , Ma Y. , Chang S. H. , Ho S. T. , Solomon G. S.
Published: July 2002

Dynamic response of a GaAs/AlGaAs microdisk laser has been experimentally investigated using femtosecond optical pumping. Below the lasing threshold, the delay time of the emission pulse from the microdisk hardly changes with the pump power. Above the lasing threshold, the delay time is shortened dramatically, and it decreases with increasing the pump power. The theoretical simulation based on the rate equations reproduces the experimental observation after the effect of carrier diffusion is taken into account. The simulation result illustrates that the speed of a microdisk laser is limited mainly by the carrier diffusion in the disk plane.
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interface contamination and its effect on electrical properties
Authors: Im Jaemo , Streiffer S. K. , Auciello Orlando , Krauss Alan R.
Published: July 2002

Surface contamination and cleaning processes of (BaxSr1−x)Ti1+yO3+z(BST) and Pt films were investigated using in situ, real-time mass spectroscopy of recoiled ions (MSRI). MSRI analysis revealed that BST film surfaces exposed to atmospheric ambient are contaminated with carbon and hydrogen containing species, which could be removed by thermal decomposition/desorption in an oxygen ambient. Cleaning of the BST surface was accomplished by annealing at 500 °C in ⩾1 mTorr O2, resulting in complete elimination of these species. Similar contamination on Pt film surfaces could be eliminated with only 200 °C annealing in 5×10−4 Torr of O2. Annealing of the BST film surface in oxygen prior to deposition of the top Pt electrode results in a clean top Pt/BST interface that yields BST capacitors with much lower and more symmetric leakage current characteristics, and lower dielectric losses compared to BST capacitors with contaminated top Pt electrode/BST interfaces.
Enhanced luminescence in InGaN multiple quantum wells with quaternary AlInGaN barriers
Authors: Zhang Jianping , Yang J. , Simin G. , Shatalov M. , Khan M. Asif , Shur M. S. , Gaska R.
Published: July 2002

We report on the comparative photoluminescence studies of AlGaN/GaN, GaN/InGaN, and AlInGaN/InGaN multiple quantum well (MQW) structures. The study clearly shows the improvement in materials quality with the introduction of indium. Our results point out the localized state emission mechanism for GaN/InGaN structures and the quantum well emission mechanism for AlInGaN/InGaN structures. The introduction of indium is the dominant factor responsible for the observed differences in the photoluminescence spectra of these MQW structures.
Resonances between the cavity mode and five excitonic transitions in an InxGa1−xAs/GaAs/AlAs/AlGaAs vertical-cavity surface-emitting laser structure using photomodulated reflectance
Authors: Choulis Stelios A. , Ghosh Sandip , Hosea Thomas J. C.
Published: July 2002

An InxGa1−xAs/GaAs/AlAs/AlGaAs vertical-cavity surface-emitting laser structure has been studied by conventional reflectance and photomodulated reflectance (PR) spectroscopies. Slight fluctuations in molecular beam epitaxy growth conditions led to thickness variations of <∼12% along the wafer radius. While this did not appreciably affect the energy of the quantum well (QW) transitions, it did give rise to a significant but smooth variation in the cavity mode energy. PR spectroscopy was used to study the interaction between the cavity mode and QW excitons, as the overlap between them was varied by probing different wafer regions. The PR signal was strongly enhanced when the cavity mode and a QW transition were in good alignment. We were able to investigate five distinct such resonances between the cavity mode and the ground-state and four other, higher-order, QW transitions. A theory has already been developed for the PR modulation of the coupled cavity and exciton modes, based on energy-dependent Seraphin coefficients. A similar but simplified model was used to fit all the PR spectra, and the resulting QW transition energies then compared with those predicted by a theoretical model which includes excitonic binding energy effects.
Analysis of spatial light modulation characteristics of C60
Authors: Roy Sukhdev , Singh C. P. , Reddy K. P. J.
Published: July 2002

Spatial light modulation based on nonlinear absorption of light by excited-state molecules in C60–toluene solution is demonstrated. Amplitude modulation of probe laser beam transmissions at 747 and 885 nm, corresponding to the peak absorption of T1 and S1 states, by the modulation laser beam intensity induced population changes at 532 nm has been analyzed using the rate equation approach, for various values of the small-signal absorption coefficient. It is shown that for T1 and S1 states, modulation up to 100% and 94.5% can be achieved for a laser beam intensity of 800 W/cm2 and 1 kW/cm2 at 532 nm, respectively.
Analysis of linear and quadratic optical response of mixed Langmuir–Blodgett films of stearic acid and 5-CT
Authors: in het Panhuis M. , Munn R. W.
Published: July 2002

Published experimental optical second-harmonic generation (SHG) results for mixed films formed between stearic acid and the mesogen 5-CT (4″-n-pentyl-4-cyano-p-terphenyl) are reanalyzed. Experimental refractive index results for the pure films are used to deduce molecular polarizabilities in order to calculate local fields. At low and high fractions of 5-CT, a consistent interpretation of the SHG is obtained using a mean molecular response based on a fixed axial hyperpolarizability βLLL for the mesogen, but at high mesogen fractions molecular tilt makes small off-diagonal components βLLM≈−βLLL/40 essential. At intermediate fractions the treatment can interpret the SHG only by requiring molecular tilt not found in the experiments, implying that a more detailed structural model is required.
Optical study of ZnSexTe1−x alloys using spectroscopic ellipsometry
Authors: Lee Hosun , Kim S. M. , Seo B. Y. , Seong E. Z. , Choi S. H. , Lee S. , Furdyna J. K.
Published: July 2002

We report pseudodielectric function data 〈ε〉=〈ε1〉+i〈ε2〉 of ZnSexTe1−x samples grown on GaAs substrates. The data were obtained from 1.5 to 6.5 eV using spectroscopic ellipsometry. Critical-point parameters were obtained by fitting model line shapes to numerically calculated second-energy derivatives of 〈ε〉. The bowing parameters of E0, E1, and E1+Δ1 were determined and were comparable to that of E0 quoted from the literature. We observed a monotonic increase of the linewidth of the E1 gap up to x=0.85, whereas that of E1+Δ1 showed a maximum value near x=0.5. We attribute this anomalous broadening of the E1 gap to sample microstructures developed in the low-Te composition alloys.
Admittance of metal–insulator–semiconductor tunnel contacts in the presence of donor–acceptor mixed interface states and interface reaction
Authors: Chattopadhyay P.
Published: July 2002

The admittance of a metal–insulator–semiconductor tunnel contact is evaluated considering the presence of donor–acceptor mixed interface states and chemical reaction in the interfacial oxide layer. Both the voltage and frequency behavior of the device has been studied. It has been found that, due to interface reaction, the current, conductance, and capacitance of the device drift considerably with time yielding an aging effect. Further, it is revealed that the dependence of the conductance and capacitance on the aging time stem rapidly from changing time constants of the interface states with aging time. The results are discussed with special reference to well known admittance spectroscopy used for the characterization of interface states.
Refractory metals in molten salts: Theory and simulation of geometry, electronic structure, and electron transport
Authors: Koslowski Thorsten
Published: July 2002

In this work, we present a theoretical and numerical study of the microscopic and electronic structure of solutions of refractory metal halides in alkali halide melts, [NbCl5]x[KCl]1−x and [TaCl5]x[KCl]1−x with 0⩽x⩽0.5. The geometry of the melts is described by ensembles of charged hard spheres, the electronic structure is modeled by a tight-binding Hamiltonian, which is extended by a reaction field to describe the diabatic energy profile of the electronic self-exchange in many-orbital mixed-valence systems. Despite its simplicity, the model leads to the formation of distorted octahedral [NbCl6]− and [TaCl6]− clusters, as evident both from the inspection of the simulation geometries and from the analysis of the partial pair distribution functions. Even in the presence of the strong potential energy fluctuations characteristic of ionic liquids, the octahedral structure is manifest in the density of states in a t2g–eg splitting of the conduction band. The Hamiltonian that describes mixed-valence systems is solved self-consistently. Using an attractive Hubbard parameter of 1.5 eV, we show that the numerical results can be interpreted by Marcus’ theory of outer-sphere electron transfer reactions with a reorganization energy of 2.2 eV, an electronic coupling parameter of 0.12 eV, and an activation energy of 0.42 eV. Both anion-d metal cation and intervalence charge transfer excitations contribute to the optical absorption spectrum, the latter leads to a pronounced polaron absorption peak. These findings are compared to recent experimental results.
Dislocations induced by bubble formation in high energy He implantation in silicon
Authors: Oliviero E. , Beaufort M. F. , Barbot J. F.
Published: July 2002

He + ions were implanted into a (111) epitaxial n-type silicon wafer at 1.6 MeV and at a dose of 2×1016 cm−2. After implantation the samples were subjected to thermal annealing at 800 °C for 30 min. Cross section transmission electron microscopy (TEM) was used to characterize microstructural features of the induced defects. Even in the as-implanted samples the TEM observations revealed the formation of a buried layer containing a dense array of bubbles. After annealing, a large band of defects (bubbles, Frank dislocation loops) was observed as well as rows of prismatic punching related dislocations which can extend over several micrometers away from the buried layer. Planar clusters of helium bubbles, lying on the {001} planes, were also observed and were supposed to be involved in the generation of dislocation loops in the matrix. Their nucleation is discussed in terms of the trap-mutation process.
Sample mounting and transfer for coupling an ultrahigh vacuum variable temperature beetle scanning tunneling microscope with conventional surface probes
Authors: Nafisi Kourosh , Ranau Werner , Hemminger John C.
Published: July 2002

We present a new ultrahigh vacuum (UHV) chamber for surface analysis and microscopy at controlled, variable temperatures. The new instrument allows surface analysis with Auger electron spectroscopy, low energy electron diffraction, quadrupole mass spectrometer, argon ion sputtering gun, and a variable temperature scanning tunneling microscope (VT-STM). In this system, we introduce a novel procedure for transferring a sample off a conventional UHV manipulator and onto a scanning tunneling microscope in the conventional “beetle” geometry, without disconnecting the heating or thermocouple wires. The microscope, a modified version of the Besocke beetle microscope, is mounted on a 2.75 in. outer diameter UHV flange and is directly attached to the base of the chamber. The sample is attached to a tripod sample holder that is held by the main manipulator. Under UHV conditions the tripod sample holder can be removed from the main manipulator and placed onto the STM. The VT-STM has the capability of acquiring images between the temperature range of 180–500 K. The performance of the chamber is demonstrated here by producing an ordered array of island vacancy defects on a Pt(111) surface and obtaining STM images of these defects.
Amorphization and anisotropic fracture dynamics during nanoindentation of silicon nitride: A multimillion atom molecular dynamics study
Authors: Walsh Phillip , Kalia Rajiv K. , Nakano Aiichiro , Vashishta Priya , Saini Subhash
Published: July 2002

Molecular dynamics simulations of nanoindentation of 10 million atom α-Si3N4 films using a rigid indenter are reported. Local pressure distributions and configuration images of the plastically deformed region are presented. Residual tensile pressures correspond to voids and cracks that separate regions of compacted, plastically deformed material and elastically recovered crystalline material. Structural analysis shows that pile-up material on the surface and deformed material under the indenter is amorphous. With this indenter geometry, Si3N4 deforms primarily by amorphization, which is arrested by cracking at the indenter corners and piling-up of material along the indenter sides. Indentation fracture exhibits anisotropic behavior consistent with the orientation-dependent fracture toughness values.
Elemental B distributions and clustering in low-energy B+ ion-implanted Si
Authors: Wang T.-S. , Cullis A. G. , Collart E. J. H. , Murrell A. J. , Foad M. A.
Published: July 2002

A detailed study is presented of characteristic elemental B distributions in Si produced by low-energy B+ ion implantation and annealing. Implant concentration profiles have been determined with approximately nanometer spatial resolution using energy-filtered imaging in the transmission electron microscope, for a B+ ion dose close to those relevant to electronic device processing. It is demonstrated that, for as-implanted Si, the near-surface B distribution shows a smooth concentration peak which correlates well with theoretical simulation and shows no anomalous surface buildup of the type generally indicated by secondary ion mass spectrometry measurements. After annealing of the layers, the present direct observations reveal that the final B distribution is characterized by residual nanometer-scale elemental clusters which comprise disordered zones within the restructured Si lattice.
Thermal stability of copper precipitates in silicon
Authors: McHugo Scott A. , Flink C.
Published: July 2002

The dissolution of copper precipitates in Czochralski silicon has been studied with synchrotron-based x-ray fluorescence. Copper has been introduced and allowed to precipitate at oxygen precipitates and growth-related stacking faults. The dissolution of copper precipitates is monitored after low-temperature anneals. This study is designed to determine whether copper can be released from these precipitation sites at low temperatures such that contamination and subsequent device degradation of an integrated circuit device could occur. Our results demonstrate copper dissolution back into the silicon matrix at temperatures as low as 360 °C.
Three-domain architecture of stress-free epitaxial ferroelectric films
Authors: Roytburd A. L. , Alpay S. P. , Bendersky L. A. , Nagarajan V. , Ramesh R.
Published: July 2002

Epitaxial ferroelectric films undergoing a cubic-tetragonal phase transformation relax internal stresses due to the structural phase transformation and the difference in the thermal expansion coefficients of the film and the substrate by forming polydomain structures. The most commonly observed polydomain structure is the c/a/c/a polytwin which only partially relieves the internal stresses. Relatively thicker films may completely reduce internal stresses if all three variants of the ferroelectric phase are brought together such that the film has the same in-plane size as the substrate. In this article, we provide experimental evidence on the formation of the three-domain structure based on transmission electron microscopy in 450 nm thick (001) PbZr0.2Ti0.8O3 films on (001) SrTiO3 grown by pulsed laser deposition. X-ray diffraction studies show that the film is fully relaxed. Experimental data is analyzed in terms of a domain stability map. It is shown that the observed structure in epitaxial ferroelectric films is due to the interplay between relaxation by misfit dislocations at the deposition temperature and relaxation by polydomain formation below the phase transformation temperature.
Resonant Raman scattering in self-organized InAs/GaAs quantum dots
Authors: Heitz R. , Born H. , Hoffmann A. , Bimberg D. , Mukhametzhanov I. , Madhukar A.
Published: July 2002

The exciton-phonon coupling in self-organized InAs/GaAs quantum dots (QDs) is investigated under resonant excitation of the ground-state transition. First- and second-order phonon sidebands of the TO (30.3 meV) and LO (33.2 meV) modes of the strained InAs QDs as well as an interface (35.9 meV) mode are resolved. Huang–Rhys factors of 0.012, 0.026, and 0.006, respectively, indicate enhanced polar exciton-phonon coupling in such strained low-symmetry QDs. Time-resolved measurements support the local character of the phonon modes.
Guiding effects of electric and magnetic fields on the plasma output of a cathodic arc magnetic filter
Authors: Zhang T. , Kwok D. T. K. , Chu P. K. , Brown I. G.
Published: July 2002

A magnetic filter was inserted between the cathodic arc plasma source and chamber to reduce the amount of macroparticles transmitted from the plasma to the sample. The plasma output of the magnetic filter was determined as a function of magnetic field and bias voltage, for the cases when the bias was applied to the entire duct wall or to a Bilek bias plate alone. The factors affecting plasma diffusion in the duct were investigated. As well as collisional and inhomogeneous magnetic field effects, our computer simulation and experimental results indicate that the E×B drift results in an additional diffusion flux for the case when a Bilek bias plate is used. Hence the Bilek biasing mode results in a lower plasma output than for the case in which the entire duct is biased.
Phase diagram and the spectra of coupled magnetoelastic waves of a biaxial ferromagnet with a biquadratic interaction in an external magnetic field
Authors: Fridman Yu. A. , Kosmachev O. A. , Baı̆ramalieva G. É.
Published: July 2002

The spectra of coupled magnetoelastic waves of a biaxial ferromagnet with a biquadratic interaction are investigated as a function of the external magnetic field. It is shown that the dynamic properties of magnetoelastic waves and the phase states of the system are determined by the relation between the Heisenberg and biquadratic exchange constants. A three-dimensional phase diagram is constructed for the system.
Rovibrationally inelastic scattering of (v=1, j=1) H2 from Cu(100): Experiment and theory
Authors: Watts E. , Sitz G. O. , McCormack D. A. , Kroes G. J. , Olsen R. A. , Groeneveld J. A. , Van Stralen J. N. P. , Baerends E. J. , Mowrey R. C.
Published: July 2002

A comparison between experiment and theory is performed for the scattering of (v=1, j=1) H2 from Cu(100) at normal incidence. Experimentally, this system was studied using molecular beam techniques, with stimulated Raman pumping employed to overpopulate (v=1, j=1) in the incident beam, and resonance enhanced multi-photon ionization used to detect the H2 scattered in two (v=1, j) states, and two (v=0, j) states. Theoretically, six-dimensional wave packet calculations were performed, employing a new, extended potential energy surface that was computed with density functional theory, using the generalized gradient approximation and a slab representation of the metal surface. Theory and experiment are in good agreement for the survival probability, i.e., the probability for rovibrationally elastic scattering. However, the theory overestimates the probabilities for rotationally inelastic scattering (to v=1, j=3) and for rovibrationally inelastic scattering (to v=0, j=5 and 7) for channels that could be determined experimentally. The cause of these discrepancies is discussed, as are possibilities for future improvements in the theory as well as the experiment.
Characterization of plasma-gas interactions in a copper cathodic arc operated in oxygen
Authors: Kelly H. , Lepone A. , Márquez A.
Published: July 2002

Several results on the plasma-neutral gas structure generated in a dc cathodic arc are presented. The arc is operated at a current level of 100 A, with a copper cathode, and with oxygen gas at a pressure of 0.5 Pa. The employed diagnostics include spherical Langmuir probes and a calorimetric technique. The plasma potential, electron temperature, ion density, and ion kinetic energy are inferred at different axial positions in the discharge chamber. In order to explain the obtained experimental results, a simplified one-dimensional stationary model to describe the plasma-neutral gas structure is developed. A good agreement is found between the experimental data and the theoretical predictions when charge-exchange between metallic ions and neutral gas molecules and lateral diffusion loses of energetic particles are included in the theoretical treatment.
Impurity gettering by vacancy-type defects in high-energy ion-implanted silicon at Rp /2
Authors: Krause-Rehberg R. , Börner F. , Redmann F.
Published: July 2002

Vacancy-type defects were studied after high-energy self-implantation of Si and subsequent rapid thermal annealing by means of a depth-resolution enhanced positron beam technique. Two different types of open-volume defects were found at a depth of Rp/2 and Rp, respectively. The defect type at Rp/2 is an agglomeration of point defects containing vacancies. This defect getters diffusing copper atoms. The vacancy-type defect observed in a depth of Rp could be connected to the interstitial loops formed there. The positron annihilation parameters suggest that this detected defect is not decorated by diffusing copper atoms.
Current transport mechanism of p-GaN Schottky contacts
Authors: Shiojima Kenji , Sugahara Tomoya , Sakai Shiro
Published: July 2002

Transient measurements of I–V and depletion layer capacitance were conducted to clarify the leaky current flow mechanism in Ni Schottky contacts formed on Mg-doped p-GaN. We found that carrier capture and emission from acceptor-like deep level defects cause depletion layer width (Wdep) to vary significantly. Upon ionization of the defects by white light, which results in small Wdep, current can go through the Schottky barrier and a leaky I–V curve is observed. Upon filling by current injection, Wdep becomes larger and the large original Schottky barrier height is seen. The time constant of carrier emission is as long as 8.3×103 min.
Infrared spectra and density functional calculations of platinum hydrides
Authors: Andrews Lester , Wang Xeufeng , Manceron Laurent
Published: July 2002

Laser-ablated and thermally evaporated Pt atoms react with H2 to form PtH2, which is characterized by infrared spectra of PtH2, PtHD, and PtD2 in solid argon and neon, and density functional theory calculations. The insertion reaction of Pt into dihydrogen is spontaneous on annealing solid neon to 6–8 K and solid argon to 20–30 K. This observation requires spin–orbit coupling of the ground D33 and excited S01 states of Pt in the curve-crossing region to give the stable PtH2 (1A1) molecule. Laser-ablated Pt atoms with excess energy also react with H2 to produce PtH. Further reactions of PtH with H2 or PtH2 with H atoms give evidence for a new asymmetric H2PtH molecule, which is also characterized by isotopic substitution and frequency calculations.
Signals from molecular beams using a surface ionization detector
Authors: Wahlbeck P. G.
Published: July 2002

The response of a surface ionization detector with a Re-heated filament was examined as a function of arrival rate of CsCl molecules. The source of CsCl was an effusion cell. The surface ionization detector was moved kinematically on a track toward the CsCl source. The intensity was dependent on the vapor pressure of CsCl (dependent on effusion cell temperature), on the inverse square root of absolute temperature of the effusion cell, and on the inverse square of the distance between the source and the detector. For small arrival rates of CsCl with high-Re-filament temperatures (CsCl dissociated on the Re surface), the ion current of Cs+ from the detector was found to be linearly dependent on the arrival rate over a range of magnitude of 160. For larger arrival rates, deviations from linearity were observed which may be explained by desorption of neutral CsCl(ad) molecules [incomplete dissociation of CsCl(ad)] due to the presence of large amounts of Cl(ad). The equilibrium between Cl(ad) and Cl2(ad) also is significant since the formation of Cl2(ad) provides a sink for Cl(ad). In these experiments negative changes in the work function of Re should be insignificant.
Electrical and microstructural properties of highly boron-implantation doped 6H–SiC
Authors: Panknin D. , Wirth H. , Mücklich A. , Skorupa W.
Published: July 2002

Boron was implanted with four energies and doses at 400 °C into 6H–SiC epilayers to form a 500 nm thick doped layer with a mean concentration between 1×1018 and 1.5×1021 cm−3. Two annealing techniques were used: furnace and flash lamp annealing. The electrical and microstructural effects were investigated using temperature dependent Hall measurements, cross sectional electron microscopy, and secondary ion mass spectrometry. During the annealing two competing processes occurred: boron outdiffusion and growth of boron containing precipitates. The efficiency of these individual processes is different for varying dopant concentrations as well as annealing techniques. After furnace annealing at temperatures between 1550 and 1750 °C and for a mean boron concentration of 5×1019 cm−3 boron containing clusters are found mainly around the region of the three deeper implantation peaks. In the surface region boron outdiffusion is observed adjusting a concentration of 1.5×1019 cm−3. Using flash lamp annealing, the outdiffusion is negligible. For high dopant concentrations (1.5×1021 cm−3) the growth of random distributed boron precipitates is the dominating effect independent of the used annealing techniques. The electrical activation is limited due to the solubility of boron in SiC. After furnace annealing Hall effect measurements show a maximum hole concentration of about 2×1016 cm−3 for the boron concentration of about 5×1018 cm−3. Alternative to the furnace annealing, the electrical properties after flash lamp annealing at about 2000 °C, 20 ms show a slight enhancement of the maximum hole concentration for boron concentrations <3×1020 cm−3.
Magnetoluminescence studies in InGaP alloys
Authors: Zeman J. , Martinez G. , Bajaj K. K. , Krivorotov I. , Uchida K.
Published: July 2002

We have measured both the diamagnetic shift and the linewidth of an excitonic transition in In0.48Ga0.52P as a function of magnetic field up to 22 T at 4.2 K using photoluminescence spectroscopy. The sample was grown on a GaAs substrate using low-pressure metalorganic vapor phase epitaxy at 700 °C. The substrate was misoriented by 15° from [001] towards [011] direction. We find that the variations of both the diamagnetic shift and the linewidth with magnetic field are about one half of those reported earlier [E. D. Jones, R. P. Schneider, Jr., S. M. Lee, and K. K. Bajaj, Phys. Rev. B 46, 7225 (1992)] in a In0.48Ga0.52P sample grown with only 2° misorientation and also those calculated using a free exciton model. We suggest that this behavior may be due to the fact that our sample was grown with much larger misorietation. We have calculated both of these variations using a model in which we assume that in this sample the hole is completely localized and find an excellent agreement with the observed data.
Acoustics of early universe—Lifshitz versus gauge-invariant theories
Authors: Golda Zdzisław A. , Woszczyna Andrzej
Published: July 2002

Appealing to classical methods of order reduction, we reduce the Lifshitz system to a second order differential equation. We demonstrate its equivalence to well known gauge-invariant results. For a radiation dominated universe we express the metric and density corrections in their exact forms and discuss their acoustic character.
Effect of nanometer-scale corrugation on densities of gap states and fixed charges at the thermally grown SiO2/Si interface
Authors: Ishikawa Yasuhiko , Kosugi Masaaki , Tabe Michiharu
Published: July 2002

Effect of nanometer-scale corrugation on gap state and fixed charge densities at the SiO2/Si interface in Si (001) metal–oxide–semiconductor (MOS) capacitors has been studied by capacitance–voltage (C–V) and conductance–voltage (G/ω–V) measurements. As the interface corrugation, a high density of Si protrusions, composed of a microscopic (001) plane at the top and high-index planes [typically (115) planes] at the sides, were formed by a microscopic selective oxidation technique developed by the authors. By changing the selective oxidation condition, the area ratio of the top and side planes was varied. As a result, it was found that each of C–V and G/ω–V curves showed a mixture of two curves with different flatband voltages. This reflects that the interface fixed charge density at the side high-index planes is larger than that at the top (001) plane, similar to previous reports for classical MOS capacitors with a macroscopic flat interface. Furthermore, it was found that the side high-index interface has a higher gap state density than the top (001), also similar to the previous macroscopic results. Such orientation dependence of gap state and fixed charge densities in the nanometer regime will play a significant role in electrical properties of ultrasmall Si devices, which accompany a relatively large area of curved interfaces.
Sequential quenching of square-well particles
Authors: Danwanichakul Panu , Glandt Eduardo D.
Published: July 2002

In previous work we discussed the integral-equation formalism for the computation of the structure of systems built through sequential addition, equilibration and irreversible quenching in place of individual particles. This sequential quenching model, appropriate for slow irreversible deposition, can be investigated by the techniques of equilibrium liquid theory. In the case of hard particles the problem is identical to that of random sequential addition. Our earlier calculations showed that the integral equation results for hard disks are in good agreement with simulation. In this paper we explore the structures arising from sequential quenching of square-well disks, which are found to be very different from those for the corresponding equilibrium case. The most interesting result is the much higher degree of clustering observed when particles are quenched one by one, as opposed to what is observed from the instantaneous quenching of an entire equilibrium system.
Optical absorption of Zn1−xMnxGa2Se4 diluted magnetic semiconductors: Variation of the energy gap with composition and temperature
Authors: Millán A. , Morón M. C.
Published: July 2002

Optical absorption and magnetic susceptibility data of the Zn1−xMnxGa2Se4 family of compounds have been collected in the temperature range 10–300 K. One of the most interesting aspects of this series is the availability of samples in the whole composition range, in contrast with most diluted magnetic semiconductors that are only miscible in a limited range of compositions. Temperature-independent absorption bands with maxima at 2.33, 2.49, and 2.66 eV have been found that are assigned to electron transitions from the A16 ground state to the T14, T24, and A14, or E4 excited states of the Mn++ ion in a tetrahedral crystalline field. The optical spectra exhibit a temperature dependent absorption edge in the 350–600 nm region that corresponds to a direct band gap. The band gap energy has been determined as a function of atomic concentration and temperature. At room temperature, the variation of the energy gap Eg with the Mn content shows a rather anomalous behavior that consists of an initial decrease for x<0.1, followed by a roughly flat variation for 0.1<x<0.5, and a nearly linear increase for x>0.5. An analogous evolution of Eg with x is found when decreasing temperature. Such behavior has been compared with that reported for analogous systems. The variation of Eg with temperature follows Varshni’s relation for all compositions. A monotonic increase of the magnetic susceptibility with x is found within the whole range of Mn content.
Chemical vapor deposition of 4H–SiC epitaxial layers on porous SiC substrates
Authors: Mynbaeva M. , Saddow S. E. , Melnychuk G. , Nikitina I. , Scheglov M. , Sitnikova A. , Kuznetsov N. , Mynbaev K. , Dmitriev V.
Published: July 2002

Epitaxial 4H–SiC layers were grown by chemical vapor deposition (CVD) on porous silicon carbide. Porous SiC substrates were fabricated by the formation of a 2 to 15 μm thick porous SiC layer on commercial off-axis 4H–SiC substrates. The thickness of CVD grown layers was about 2.5 μm. The concentration Nd–Na in the layers was about 7×1015 cm−3. The layers were investigated for their surface roughness, crystal structure, deep level concentration, and minority carrier diffusion length. It was found that the characteristics of SiC epitaxial layers grown on porous SiC substrates were significantly improved compared to those of SiC layers grown on standard SiC substrates.
X-ray analysis of spontaneous lateral modulation in (InAs)n/(AlAs)m short-period superlattices
Authors: Li J. H. , Holý V. , Zhong Z. , Kulik J. , Moss S. C. , Norman A. G. , Mascarenhas A. , Reno J. L. , Follstaedt D. M.
Published: July 2002

The lateral composition modulation in (InAs)n/(AlAs)m short-period superlattices was studied by means of synchrotron x-ray diffraction. By choosing specific diffraction vectors having a large component closely parallel to the modulation direction, we are able to observe a number of lateral satellite peaks around the zero-order short-period superlattice peak. A model, incorporating both composition and strain, is used to simulate the intensities of these satellites. Our results provide a quantitative fit and permit the evaluation of the composition amplitude.
Vibrational dynamics of the C–O stretch vibration in alcohols
Authors: van den Broek M. A. F. H. , Nienhuys H. -K. , Bakker H. J.
Published: July 2002

We present a study on the vibrational dynamics of the C–O stretch vibration of methanol and ethanol in carbon tetrachloride (CCl4) solution. The relaxation of the excited C–O stretch vibration was observed to occur in two steps. In the first step energy is transferred from the v=1 state of the C–O stretch vibration to an intermediate state with a time constant of 3.2±0.2 ps for methanol and 3.2±0.7 ps for ethanol. The intermediate state is most likely formed by the n=3 or 4 state of the C–O–H bending mode, which is also known as the torsional mode. In the second step energy is transferred from this intermediate state to low-energy modes, leading to a full equilibration of the energy. In methanol this thermalization occurs with a time constant of 28±1 ps. In ethanol the second step is faster, with a time constant of 12±2 ps.
Quantitative estimation of the threefold and fourfold carbon coordination in amorphous CNx films
Authors: Mezzasalma A. M. , Mondio G. , Neri F. , Trusso S.
Published: July 2002

Reflection electron energy loss spectroscopy and x-ray photoemission spectroscopy results obtained from laser-ablated amorphous CNx thin films have been analyzed in order to quantitatively deduce the change of the threefold/fourfold carbon coordination ratio induced by the nitrogenation. It has been found that nitrogenation induces an increase of the threefold-coordinated carbon atoms with respect to the fourfold-coordinated ones, evidencing the graphitization of the material. All the bond concentrations of the threefold and fourfold coordinations have been evaluated.
A five-dimensional quantum scattering model for the type AB+XCD3⇌ABX+CD3 reaction in hyperspherical coordinates: Application to OH+CH4⇌H2O+CH3
Authors: Yu Hua-Gen
Published: July 2002

A reduced dimensionality (RD) reactive quantum scattering model has been presented for the general type AB+XCD3⇌ABX+CD3 reaction, where the CD3 fragment has C3V symmetry. The model Hamiltonian and quantum scattering formulas are derived in hyperspherical coordinates, together with a single hyperradius hyperspherical projection method for boundary conditions. The time-independent coupled channel equations are propagated using a log-derivative method. The surface functions are produced by a guided spectral transform (GST) Lanczos algorithm. We also describe a GST Lanczos method to solve the generalized eigenvalue problem in a nonorthogonal basis. The RD model has been applied to the OH+CH4⇌H2O+CH3 reaction. A comparison of calculated results with previous theoretical and experimental ones is made.
Microwave surface resistance of YBa2Cu3Oy films covered by overdoped Y1−xCaxBa2Cu3Oy layers
Authors: Obara H. , Sawa A. , Yamasaki H. , Kosaka S.
Published: July 2002

We have observed the reduction of the microwave surface resistance Rs of YBa2Cu3Oy films deposited on MgO substrates by covering the films with an overdoped Y1−xCaxBa2Cu3Oy layer. At low temperature, Rs of the YBa2Cu3Oy film covered by an Y1−xCaxBa2Cu3Oy (x=0.2) layer was half that of the single YBa2Cu3Oy film. On the other hand, the single Y1−xCaxBa2Cu3Oy (x=0.2) films did not exhibit low loss characteristics. The diffusion of Ca ions into the YBa2Cu3Oy layer was observed and the selective diffusion of Ca ions to the grain boundaries of the YBa2Cu3Oy layer is considered to be the origin of the low Rs in the covered YBa2Cu3Oy films.
Magnetic fields and factored two-spheres
Authors: Dowker J. S.
Published: July 2002

A magnetic monopole is placed at the center of a ball whose surface S2 is tiled by the symmetry group, Γ, of a regular solid. The quantum mechanics on the two-dimensional quotient S2/Γ is developed and the monopole charge is found to be quantized in an expected manner. The heat-kernel and ζ-functions are evaluated and the Casimir energy is computed. Numerical approaches to the calculation of the derivative of the Barnes ζ-function are presented.
On one class of exact Poisson structures
Authors: Przybysz Radosl/aw
Published: July 2002

We discuss some properties of a natural class of Poisson structures on Euclidean spaces and abstract manifolds. In particular it is proved that such structures are always exact and may be reconstructed from their Casimir functions. It is shown that in low dimensions they give the whole class of exact Poisson structures. The dimension of Poisson homology of these structures is computed in terms of the Milnor number of their Casimir functions. We also analyze some concrete examples of such structures in low dimensions and show that their centers are generated by Casimir functions.
Mid-infrared InAs/GaInSb separate confinement heterostructure laser diode structures
Authors: Olesberg J. T. , Flatté Michael E. , Hasenberg T. C. , Grein C. H.
Published: July 2002

Despite recent progress in electronic structure engineering of type-II materials for mid-infrared lasers, suppression of Auger recombination at room temperature has been limited. We present an active region design, consisting of AlAsSb/InAs/GaInSb/InAs/AlAsSb wells separated by an InAs/AlGaSb superlattice, that overcomes this limitation. The 300 K calculated Auger recombination rate in this structure at the optimal lasing density is five times smaller than typical Shockley–Read–Hall (defect-assisted) recombination rates. An integrated separate confinement heterostructure design suitable for this active region is also described. The separate confinement region, which is a lightly doped InAs/AlGaSb superlattice, provides efficient hole transport and injection into the active region. For an estimated nonactive region modal cavity loss of 20 cm−1 and an optical mode width of 1.3 μm, the calculated internal threshold current density is 100 A/cm2 at 300 K for a single quantum well device.
Light emission from tungsten nanoparticles during laser-assisted chemical vapor deposition of tungsten
Authors: Heszler P. , Landström L. , Lindstam M. , Carlsson J. O.
Published: July 2002

Recently we reported that phosphorescent light emission was observed upon irradiation of WF6/H2/noble gas (Ar, Kr, Ne, Xe) mixtures by ArF excimer laser [P. Heszler, P. Mogyorósi, and J.-O. Carlsson, J. Appl. Phys. 78, 5277 1995], originating from tungsten nanoparticles. In this work results of additional measurements on the same system are presented, and we conclude that the reported phosphorescence is a blackbody-like radiation. Taking into account the emissivity of the particles calculated on the base of the Mie theory, it was possible to determine the dependence of the temperature of the tungsten nanoparticles on time after the laser pulse by means of time-resolved spectroscopy. It is shown that surface reactions on the hot nanoparticles contribute significantly to the growth. The size distribution was found lognormal by transmission electron microscopy measurements.
Determination of Ir consumption during thermal oxidation and PbZrxTi1−xO3 processing using Bragg-peak fringe analysis
Authors: Saenger K. L. , Neumayer D. A.
Published: July 2002

Thickness interference fringes can be seen around the Bragg peaks of a variety of polycrystalline thin (10–20 nm) films using standard x-ray diffraction techniques in a conventional Bragg–Brentano geometry. In this article, thickness fringe analysis is used to investigate oxidation and roughening in thin Ir films with and without overlayers of the ferroelectric PbZrxTi1−xO3 (PZT). Changes in fringe spacing were used to determine the Ir thickness consumed by oxidation. Fringe contrast degradation (indicating roughening) was observed both after oxidation anneals (which formed a surface layer of IrO2) and after subsequent reduction anneals (which converted the IrO2 back to Ir). Film overlayers were found to have a protective effect against oxidation and roughening, as evidenced by comparison of postoxidation fringe patterns for bare and PZT-coated Ir films. Overall, our results demonstrate that thickness fringe analysis can be used as a simple, quantitative probe of processing-induced thin film thickness and morphology changes.