bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetotransport studies of AlGaN/GaN heterostructures grown on sapphire substrates: Effective mass and scattering time
Authors: Wang T. , Bai J. , Sakai S. , Ohno Y. , Ohno H.
Published: July 2002

Temperature-dependent magnetotransport measurements have been carried out on high-quality two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures with different Al content grown on sapphire substrates. The GaN electron effective mass and the quantum scattering time are determined by well-resolved Shunbikov–de Hass oscillations. The electron effective mass is determined to be 19m0. The ratio of the classic scattering time to the quantum scattering time increases with increasing 2DEG sheet carrier density, which agrees very well with the previous calculation based on an ideal 2DEG in conventional semiconductor systems. Our result indicates that a low density of deep centers results in the much higher mobility of our structure compared with other reports, which is of critical importance in fabricating a high-quality 2DEG structure in AlGaN/GaN systems.
Topographic measurement of electromigration-induced stress gradients in aluminum conductor lines
Authors: Wang P.-C. , Noyan I. C. , Kaldor S. K. , Jordan-Sweet J. L. , Liniger E. G. , Hu C.-K.
Published: July 2002

We report a set of data on the evolution of stress in thin-film metallization wires during the transient region of electromigration. The excellent strain sensitivity of the x-ray microbeam topography technique allows real-time, spatially resolved measurements at the lowest currents reported to date (1.0×104–1.4×105 A/cm2). While the steady-state results agree qualitatively with the Blech’s stress gradient model [I. A. Blech, J. Appl. Phys. 47, 1203 (1976)], the threshold-length product calculated from our data is about 2–3 times smaller than previously reported values. Stress evolution during the transient state displays local fluctuations which cannot be attributed to experimental errors, indicating possible microstructural effects on local flux divergence even in the case of wide, nonbamboo wires.
Neutron irradiation induced degradation of the collector–emitter offset voltage in InP/InGaAs single heterojunction bipolar transistors
Authors: Shatalov Alexei , Subramanian S. , Dentai A. , Chadrasekhar S. , Goodnick S. M.
Published: July 2002

In this article, we report the results of our investigation of neutron irradiation effects on the collector–emitter offset voltage of InP/InGaAs single heterojunction bipolar transistors. We find that the offset voltage of these devices increases by more than 0.1 V for neutron doses ∼6×1014 cm−2. We present an analysis of the forward and inverse Gummel plots that clearly shows that the increase in offset voltage is caused by the degradation of the base–collector junction due to the neutron-induced displacement damage in the collector.
Stability of viscoelastic shear flows subjected to parallel flow superposition
Authors: Ramanan V. V. , Graham M. D.
Published: July 2002

Steady viscoelastic shear flows in curved geometries are susceptible to instabilities due to the radial force associated with elastic stresses along curved streamlines. Recent work has shown that the addition of steady or oscillatory shear flow in a transverse direction (orthogonal superposition) can suppress these instabilities. The present work instead investigates the effect of oscillatory parallel superposition, for the particular case of circular Couette flow. For flow of an Oldroyd-B fluid, the oscillation has a weak stabilizing effect if the oscillation amplitude is not too large. If on the other hand, the oscillation amplitude is such that the angular velocity of the moving cylinder changes sign over part of the cycle, the flow is destabilized. However, in the limit where the motion is purely oscillatory (i.e., large amplitude oscillatory shear), the flow is stabilized relative to steady circular Couette flow with the same maximum shear rate. Finally, in the limit where curvature goes to zero—plane Couette flow—parallel superposition makes the already stable flow slightly more stable, increasing the decay rate of fluctuations in the range of parameters studied. Furthermore, two-dimensional disturbances are the most slowly decaying; we have extended Squire’s theorem in this case to encompass an arbitrary time dependence.
Bistability and formation of low work function areas in the O2+H2 reaction on a Rh(111) surface
Authors: Schaak A. , Imbihl R.
Published: July 2002

The catalytic O2+H2 reaction on Rh(111) has been investigated in the 10−6–10−5 mbar range using photoelectron emission microscopy (PEEM) as spatially resolving method. Depending on the pretreatment of the sample in which the sample was exposed to pO2=2×10−4 mbar at T=770 K for varying times (tOX) different dynamic behavior was found. For tOX=12 h we found simple bistable behavior with reaction fronts initiating transitions between an unreactive high oxygen coverage state and a reactive almost bare surface. For tOX=36 h low work function (WF) areas developed in the area where two reaction fronts collided. For very long oxygen pretreatments with tOX=48 h the formation of secondary fronts traveling backwards from the area where two primary fronts collided were seen. The properties of the low WF areas which have been tentatively assigned to subsurface oxygen were studied in adsorption and titration experiments.
Trapping in organic field-effect transistors
Authors: Schön J. H. , Batlogg B.
Published: July 2002

Current–voltage characteristics of single- and polycrystalline organic field-effect transistors are analyzed. The effect of bulk, interface, and grain boundary traps is investigated. The frequently observed dependence of the field-effect mobility on the gate voltage is ascribed to trapping processes rather than to an intrinsic charge transport mechanism in these organic semiconductors. Furthermore, the thermally activated mobility in polycrystalline devices, frequently observed, is ascribed to the formation of a potential barrier at the grain boundaries of the polycrystalline semiconductor. The barrier height depends significantly on the trap density and the position of the Fermi energy and therefore on the gate voltage.
Strong, easy-to-manufacture, transition edge x-ray sensor
Authors: Tanaka Keiichi , Morooka Toshimitsu , Chinone Kazuo , Ukibe Masahiro , Hirayama Fuminori , Ohkubo Masataka , Koyanagi Masao
Published: July 2002

We developed a membrane structure with a silicon-on-insulator (SOI) wafer by using a micromachining technique to create a transition edge x-ray sensor. In this membrane structure, the part of the SOI layer between the silicon nitride (SiNx) film and the buried oxide layer was etched from the front side to form the SiNx membrane. Advantages of this membrane are that (a) it is stronger than conventional membranes and is therefore suitable for large format arrays, (b) the Si etching time is reduced from 12 h (for conventional etching) to 4 h, and (c) all the fabrication processes are done from the front of the wafer, thus simplifying the manufacturing process.
Transient and steady state simulations of internal temperature profiles in high-power semi-insulating GaAs photoconductive switches
Authors: Kayasit P. , Joshi R. P. , Islam N. E. , Schamiloglu E. , Gaudet J.
Published: July 2002

Simulations have been performed to determine the internal temperature profiles of high-power GaAs photoconductive switches in the presence of a current filament. No thermal instability is predicted below a power generation density level of about 1.3×1014 W/m3. This prediction is in keeping with recent experimental data on photoconductive semiconductor switch devices. It is shown that this power dissipation density threshold for stability exists under both dc and transient conditions. A simple model provides qualitative support for the power density threshold, and an explanation of the filamentary current radii that have been observed experimentally.
Yellow luminescence and persistent photoconductivity of undoped n-type GaN
Authors: Chung S. J. , Cha O. H. , Kim Y. S. , Hong C.-H. , Lee H. J. , Jeong M. S. , White J. O. , Suh E.-K.
Published: July 2002

Deep-level defect-related optical properties of undoped n-type GaN grown by metalorganic chemical vapor deposition are investigated using photoluminescence (PL), optical absorption (OA), photoconductivity (PC), and persistent photoconductivity (PPC) measurements. From the temperature dependence of the PL and OA, we find that the yellow luminescence (YL) is due to shallow-to-deep donor recombination. PL, PC, and PPC results manifest a strong correlation in properties related to deep levels. Samples which emit YL exhibit a PC peak at 1.9 eV due to the photoionization of deep levels as well as to the persistent photoconductivity effect, whereas samples with no YL have no PC peak in the forbidden gap and no PPC at any photon energy, suggesting a common origin. Furthermore, two types of PPC behavior were observed depending on the sample quality: typical stretched exponential decay in relatively thick samples and photocurrent quenching and a subsequent reduction of the dark current in thin samples. An explanation of the latter phenomenon based on photoinduced metastable electron traps in a highly defective layer near the interface is suggested from the temporal behavior of the PC. These traps seem to disappear slowly after the illuminating light is turned off.
Adhesion of ultrathin ZrO2(111) films on Ni(111) from first principles
Authors: Christensen A. , Carter Emily A.
Published: July 2002

We have studied the ZrO2(111)/Ni(111) interface using the ultrasoft pseudopotential formalism within density functional theory. We find that ZrO2(111) adheres relatively strongly at the monolayer level but thicker ceramic films interact weakly with the Ni-substrate. We argue that the cohesion changes character from dominantly image charge interactions for thick ceramic films to more covalent for monolayer ZrO2(111) films. We provide an analysis of energetic, structural and electronic aspects of the ZrO2/Ni interface as a function of the thickness of the oxide layer. We also address the role of the exchange-correlation density functional parameterization for modeling the oxide and metal/oxide interface and discuss the sensitivity of the supercell approximation for metal/oxide interface properties.
Boolean algebra operations performed on optical bits by the generation of holographic fields through second-order nonlinear interactions
Authors: Andreoni Alessandra , Bondani Maria , Potenza Marco A. C. , Denisyuk Yury N. , Puddu Emiliano
Published: July 2002

Boolean algebra operations such as AND or XOR are performed on optical bits encoded as amplitude modulations in two wave fronts that are made interact in a crystal (β-BaB2O4) endowed with second-order nonlinearity. If the corresponding wave fields are at the same frequency ω and the crystal is tuned for the phase-matched generation of 2ω, we show that the generated wave front reconstructs a holographic image containing the outputs of the desired operations. Since a nearly diffraction-limited optical resolution can be easily achieved in the holographic image at 2ω, a correspondingly high density of data is encodeable in the wave front at ω playing the role of object wave front. The experiments demonstrate the feasibility of the operation of a parallel half-adder performing the sum of multiple data with a one-digit binary number, which is encoded in the reference wave front.
Resolution enhancement in the magnetic bottle photoelectron spectrometer by impulse electron deceleration
Authors: Giniger Rina , Hippler Thomas , Ronen Shai , Cheshnovsky Ori
Published: July 2002

We present a modified time of flight magnetic bottle photoelectron spectrometer (MBPES), designed to operate with mass-selected ion beams. It is based on a configuration in which the ion beam is coaxial with the electron flight tube. High resolution combined with high efficiency is achieved by applying impulse deceleration at the electron detachment zone immediately after detachment. The main effect of the impulse is to improve the resolution of the spectrometer by subtracting the ion velocity from the electron velocity. The impulse can also counteract the spread in the flight times caused by the alignment of the electrons in the magnetic bottle. Unlike previous designs of the MBPES, ultimate resolution is achieved without any loss in signal intensity. Resolutions of 8, 25, and 48 meV were obtained for 0.28, 1.22, and 2.11 eV electrons, respectively, with 60% collection efficiency. This design also allows easy switching between high-resolution high-yield and polarization-sensitive photoelectron spectroscopy modes. Our simulations indicate that, with careful attention to details, sub-meV resolution can be obtained using this approach.
Complex permittivity measurements and mixing laws of porous alumina
Authors: Gershon D. , Calame J. P. , Birnboim A.
Published: July 2002

The dependence of the permittivity of porous alumina on the microstructure was studied. Three different algebraic mixing laws inaccurately predicted the measured effective permittivity of the three-phase material, which was alumina, air, and water. Finite-difference electrostatic simulations of physically realistic microstructures accurately predicted the experimental results. The electrostatic simulations also provided physical insight into the arrangement of water in the material and its significant effect on the effective permittivity.
Hard x-ray spectrometers for the National Ignition Facility
Authors: Seely John , Back Christina , Deslattes Richard , Hudson Lawrence , Holland Glenn , Bell Perry , Miller Michael
Published: July 2002

A National Ignition Facility (NIF) core diagnostic instrument has been designed and will be fabricated to record x-ray spectra in the 1.1–20.1 keV energy range. The High-Energy Electronic X-Ray (HENEX) instrument has four reflection crystals with overlapping coverage of 1.1–10.9 keV and one transmission crystal covering 8.6–20.1 keV. The spectral resolving power varies from approximately 2000 at low energies to 300 at 20 keV. The spectrum produced by each crystal is recorded by a modified commercial dental x-ray charge coupled device detector with a dynamic range of at least 2500.
A variable-temperature ultrahigh vacuum scanning tunneling microscope
Authors: Zhang H. , Memmert U. , Houbertz R. , Hartmann U.
Published: July 2002

A variable-temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM) was designed and tested. Design details and initial results are presented. The STM is directly attached to the cold face of a continuous flow cryostat which is mounted into a two-chamber UHV system. A significant advantage of this system in comparison to many others is, that samples can be cooled down to base temperature of 6.5 K within very short times of below 2 h. This feature not only increases the potential sample throughput, it also allows to cycle the sample temperature within the regime below 20 K without losing track of given sample locations. The instrument was tested by imaging Au layers on graphite. The vertical stability at low temperature was found to be below 3 pm. Images recorded at 6.5 K show crystalline Au islands and the Au(111)22×√3 reconstruction with atomic resolution. Using a resistive heater, the sample temperature was adjusted between 6.5 and 20 K. After an equilibration time of 15 min, the displacement due to the temperature change remained below 150 nm. Scanning tunneling spectroscopy on Au(111) grains resolves the Au(111) surface state.
Near-infrared Raman instrument for rapid and quantitative measurements of clinically important analytes
Authors: Qu Jianan Y. , Shao Lan
Published: July 2002

We present the use of a near-infrared (IR) laser Raman spectroscopy instrument to measure the concentrations of many important analytes at their clinically relevant levels in the simulated human serum. The Raman signal is generated by a 745 nm diode laser in a disposable waveguide capillary cell that contains a submicroliter sample. The Raman spectrum is acquired from the sample in 10 s. The major error in quantitative Raman spectroscopy caused by the variation in laser power, optical alignment, and capillary cell size from measurement to measurement is eliminated by normalizing the spectrum to the dominant water peak at 3350 cm−1. Concentrations of glucose, acetaminophen, albumin, and other analytes are predicted using partial least squares (PLS) calibration. An effective multiple bandpass-filtering method was developed to enhance the signal of the desired analytes to interfering background ratio for improvement of PLS calibration accuracy. It is demonstrated that the accuracy of predicted concentrations for all analytes in the simulated human serum samples are highly acceptable for clinical diagnosis. The results promise the potential applications of the near-IR Raman instrument in medical practice.
Characteristics of an electronically tunable surface-coil-type resonator for L-band electron paramagnetic resonance spectroscopy
Authors: Hirata Hiroshi , Walczak Tadeusz , Swartz Harold M.
Published: July 2002

This note describes considerable additional information on the key parameters of an electronically tunable surface-coil-type resonator for L-band continuous-wave electron paramagnetic resonance (EPR) spectroscopy [Hirata et al., J. Magn. Reson. 142, 159 (2000)]. For the 1.1 GHz prototype tunable surface coil (10.6 mm in diameter), the measured efficiency for generating the rf magnetic field is 77 μT/W1/2. The unloaded quality factor of the tunable surface coil is in the range of 260–310. In absence of an EPR sample, 57% of the magnetic energy in the system is stored in the parallel coaxial line, 16% in the surface coil, and a half of the energy is dissipated in the parallel coaxial line. The energy dissipation for the surface coil occurs mainly through radiation (25% of the total energy dissipation). The level of the second harmonic in the reflected microwaves generated by the tunable surface coil was measured. The varactor diodes used in the matching and tuning circuits are the dominant source of the second harmonic.
An averaging method for optical triangulation displacement sensors using diffraction grating
Authors: Oh Se Baek , Kim Kyung-Chan , Kim Soo Hyun
Published: July 2002

Optical triangulation displacement sensors are widely used in many applications due to the advantages of noncontact measurement, simple structure, good resolution, and long operating range. However, they commonly have several errors, such as speckle effects and electronic noises, etc. Among them, reducing electronic noises is very tedious and hard. To reduce the effect of electronic noises, conventional systems use sequential averaging techniques. Since electronic noises are random in nature, their variances can be reduced with an averaging operation. However, the averaging is inherently a time-consuming process which requires many measurements. To reduce the time for averaging and the number of measurements, several sensor modules or faster signal processing hardware are required. Therefore, these alternatives are not cost effective and increase the size of the measurement system. In this article, we propose a simple and cost-effective system structure for optical triangulation displacement sensors, which reduces time for averaging and the number of measurements by using a transmission-type diffraction grating. The diffraction grating helps us to obtain several signals simultaneously. Therefore, it is achievable to reduce time for averaging and the number of measurements. The feasibility of the proposed system was verified through experiments. When only two diffracted orders are used for obtaining signals, the rms of noise is reduced by 1/√.
A yield stress scaling function for electrorheological fluids
Authors: Choi Hyoung J. , Cho Min S. , Kim Ji W. , Kim Chul A. , Jhon Myung S.
Published: July 2002

The yield stress dependence on electric field strength for electrorheological (ER) fluids is examined. A proposed scaling function incorporates both the polarization and conductivity models. Proper scaling allows yield stress data for ER fluids to collapse onto a single curve for a broad range of electric field strengths.
Some topological invariants for three-dimensional flows
Authors: Dufraine Emmanuel
Published: July 2002

We deal here with vector fields on three manifolds. For a system with a homoclinic orbit to a saddle-focus point, we show that the imaginary part of the complex eigenvalues is a conjugacy invariant. We show also that the ratio of the real part of the complex eigenvalue over the real one is invariant under topological equivalence. For a system with two saddle-focus points and an orbit connecting the one-dimensional invariant manifold of those points, we compute a conjugacy invariant related to the eigenvalues of the vector field at the singularities.
Essential self-adjointness of the elliptic Ruijsenaars models
Authors: Komori Yasushi
Published: July 2002

We study the elliptic Ruijsenaars models associated with arbitrary root systems, which are difference analogs of the Calogero–Moser model. We give a dense subspace in the space of square integrable functions invariant under the action of the Weyl group on a torus as a domain of its Hamiltonian and prove its essential self-adjointness by using perturbation theory. It is also clarified that these models consist of pure point spectrum.
Large-N theory from the axiomatic point of view
Authors: Shvedov O. Yu.
Published: July 2002

The state space and observables for the leading order of the large-N theory are constructed. The obtained model (“theory of infinite number of fields”) is shown to obey Wightman-type axioms (including invariance under boost transformations). The considered class of exactly solvable relativistic quantum models involves good examples of theories containing such difficulties as volume divergences associated with the Haag theorem, Stueckelberg divergences, and infinite renormalization of the wave function.
Pressure-induced amorphization of ZrTiCuNiBe bulk glass-forming alloy
Authors: Wang Wei Hua , Wang R. J. , Dai D. Y. , Zhao D. Q. , Pan M. X. , Yao Y. S.
Published: July 2002

Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 alloy can be cooled under high pressure (HP) to a bulky glassy state at very low cooling rates. The structure and properties of the bulk metallic glass (BMG) are investigated by differential scanning calorimetry, x-ray diffraction, ultrasonic study, and density measurements. The effects of pressure on the glass formation are discussed from the points of view of nucleation kinetics and thermodynamics. The BMG obtained under HP is in a high-density amorphous state that is different in structure and property from low-density amorphous phase quenched in water. Solidification under HP is a promising way not only for synthesizing BMGs with more densely packed structures and different properties, but also for understanding the glass formation mechanism.
Versatile gas-loading system for diamond-anvil cells
Authors: Kenichi Takemura , Sahu P. Ch. , Yoshiyasu Kunii , Yasuo Toma
Published: July 2002

A gas-loading system has been constructed to load diamond-anvil cells with various kinds of high-pressure gases at room temperature. The gases either serve as hydrostatic pressure media or samples by themselves. The present high-pressure vessel can accommodate various types of diamond-anvil cells simply by changing the holder. The system has been used successfully for high-pressure powder x-ray diffraction experiments with a helium-pressure medium.
Electronic and bonding structures of amorphous Si–C–N thin films by x-ray absorption spectroscopy
Authors: Tsai H. M. , Jan J. C. , Chiou J. W. , Pong W. F. , Tsai M.-H. , Chang Y. K. , Chen Y. Y. , Yang Y. W. , Lai L. J. , Wu J. J. , Wu C. T. , Chen K. H. , Chen L. C.
Published: July 2002

X-ray absorption near edge structure (XANES) spectra of hard amorphous a-Si–C–N thin films with various compositions were measured at the C and N K-edge using sample drain current and fluorescent modes. The C K-edge XANES spectra of a-Si–C–N contain a relatively large 1s→π* peak, indicating that a substantial percentage of carbon atoms in the a-Si–C–N films have sp2 or graphite-like bonding. Both the observed sp2 intensity and the Young’s modulus decrease with an increase in the carbon content. For N K-edge XANES spectra of the a-Si–C–N films we find the emergence of a sharp peak near the threshold when the carbon content is larger than between 9% and 36%, which indicates that carbon and nitrogen atoms tend to form local graphitic carbon nitride.
Mechanical aging behavior of oriented Pb(Mg1/3Nb2/3)O3–PbTiO3 and Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals
Authors: Priya Shashank , Ryu Jungho , Uchino K. , Viehland D.
Published: July 2002

The aging characteristics of 〈001〉-oriented Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) single crystals have been studied under electrical and mechanical drive. A significant increase in hysteretic loss and decrease in piezoelectric constant were observed under continuous excitation. Systematic investigations of piezoelectric aging were then performed using a novel mechanical fatigue method. Piezocrystals were placed between two magnetostrictive plates and clamped. Periodic longitudinal compressive stresses were generated by applying magnetic fields. At both short and long times, the piezoelectric constant was found to decrease slowly. However, in an intermediate range, a pronounced decrease was observed over a narrow time range. The dynamics of aging have been shown to scale to a hierarchical relaxation process, typical of disordered systems such as random field and glassy states.
The Fermat principle in general relativity and applications
Authors: Giannoni F. , Masiello A. , Piccione P.
Published: July 2002

In this paper we use a general version of Fermat’s principle for light rays in general relativity and a curve shortening method to write the Morse relations for light rays joining an event with a smooth timelike curve in a Lorentzian manifold with boundary. The Morse relations are obtained under the most general assumptions and one can apply them to have a mathematical description of the gravitational lens effect in a very general context. Moreover, Morse relations can be used to check if existing models are corrected.
The absence of a rheological effect on the spreading of small drops
Authors: Neogi P. , Ybarra R. M.
Published: July 2002

The fluid mechanics of the movement of the contact line region for non-Newtonian fluids has been analyzed using the method of Joanny and de Gennes [C. R. Acad. Sci., Ser. II: Mec., Phys., Chim., Sci. Univers Terre 299, 279 (1984)] where under lubrication theory approximation the rate of viscous dissipation is equated to the rate of surface work to get the expression for the rate of spreading. Two kinds of fluids are considered, one where the viscosity is dependent on shear rate but there are no normal stresses, and another where there are normal stresses but the coefficients are not dependent on the shear rate. Both show the spreading rates to be independent of the non-Newtonian nature of the fluids to a good approximation, with the zero shear viscosity replacing the Newtonian viscosity in the result of Joanny and de Gennes. This appears to be in keeping with the available experimental data.
Elastostatic fields in an anisotropic substrate due to a buried quantum dot
Authors: Pan E. , Yang B.
Published: July 2002

We present an efficient and accurate continuum-mechanics approach for the numerical prediction of displacement, stress, strain, and strain energy density fields in an anisotropic substrate (modeled as a half-space) due to a buried quantum dot (QD). Our approach is based on Green’s function solution in anisotropic and linearly elastic half-space combined with the Betti’s reciprocal theorem. Numerical examples clearly show that the crystalline anisotropy of the III–V semiconductor group has great influence on the elastic fields, as compared to the isotropic solution. In particular, it is found that the hydrostatic strain and strain energy density on the surface of anisotropic half-space made of different crystalline materials due to a cubic QD can be substantially different, and therefore, the isotropy approximation neglecting their differences should not be used in general. Furthermore, the hydrostatic strains on the surface of an anisotropic half-space due to a finite-size (cubic) QD and an equal-intensity point QD at relatively large depth (about twice the side length of the cubic QD) can still be quite different, in contrast to the corresponding isotropic result. These observations indicate that in modeling and analyzing the mechanical and electronic behaviors of QD semiconductor structures, the effect of crystalline anisotropy should be considered in general.
Mobility of CF3+ in CF4, CHF2+ in CHF3, and C+ in Ar
Authors: Basurto E. , Urquijo J. de
Published: July 2002

A drift tube-double mass spectrometer technique has been used to study the transport of CF3+ in CF4 and of CHF2+ in CHF3. The reduced mobilities of these two ions were measured over the density-normalized electric field strength E/N, between 30 and 750 Td (1 Td=10−17 V cm2). It was found that substantial amounts of C+ were produced thorugh the reaction of CF3+ with Ar. Thus, the mobility of C+ could be measured over the E/N range from 30 to 260 Td. Further measurements of the abundance of secondary ions formed by the reactions of CF3+ and CHF2+ with Ar revealed that for E/N>250 Td and E/N>120 Td, respectively, Ar+ predominates as a secondary species over F+, CF+, CF2+, and ArH+. The injection of Ar+ into CF4 was also found to be a very efficient dissociative charge transfer process, leading predominantly to CF3+ formation, with conversion efficiencies of practically 100% for E/N>100 Td. The above finding is consistent with a previous beam study.
Electrical field dependence of hopping conduction in self-organized carbon networks
Authors: Govor L. V. , Bashmakov I. A. , Boehme K. , Parisi J.
Published: July 2002

The influence of the electrical field on the variable range hopping process of porous carbon networks is examined in the range of validity of the law ln σ(T)∝T−1/2, where σ and T mean electrical conductivity and temperature, respectively. We show that the field dependence of the samples investigated in the vicinity of the metal–insulator transition clearly distinguishes four characteristic regions. At low values of the applied electrical field, we have ohmic conductivity. Upon increasing the electrical field E, the electrical conductivity σ rises, first following the law ln σ(E)∝En, where n changes from 1.4 to 2.6 with increasing distance from the metal–insulator transition on the insulating side. Then, at higher electrical field, the conductivity turns to the relation ln σ(E)∝E1.0. The temperature dependence of the hopping length of the charge carriers, determined within the above field regime, develops as l(T)∝T−0.9. At temperatures where the ohmic behavior in the Coulomb gap occurs and obeys the law ln σ(T)∝T−1/2, the electrical conductivity caused by thermally nonactivated charge carriers at high fields complies with ln σ(E)∝E−1/3. The current density j changes as ln j(E)∝E−1/6. The temperature dependence of the threshold electrical field, which characterizes the transition from the low-field to the high-field range, follows Eth∝T1.5.
Linear response coupled cluster calculation of Raman scattering cross sections
Authors: Pecul Magdalena , Rizzo Antonio
Published: July 2002

The linear response coupled cluster singles and doubles (CCSD) method has been used to evaluate electric dipole polarizabilities, their geometric derivatives and Q-branch Raman scattering cross sections for five diatomic molecules: N2, CO, HF, HCl, and Cl2. A systematic study of basis set convergence has been carried out. Geometric derivatives of the frequency dispersion coefficients have been computed. This allowed to evaluate Raman intensities for arbitrary laser frequencies outside the resonance region. The geometric derivatives of polarizabilities, and therefore Raman intensities, exhibit stronger basis set dependence and dispersion effects than the polarizabilities themselves. Whereas the calculated polarizabilities are very close to experiment, the agreement of their geometric derivatives and of the Raman scattering cross sections with experiment is less excellent. We attribute this mainly to the limitations of Placzek’s theory (for Raman intensities) and to the low precision of the reference experimental data.
Improvement of the discharge efficiency in plasma displays
Authors: Oversluizen G. , Klein M. , de Zwart S. , van Heusden S. , Dekker T.
Published: July 2002

The dependence of the efficacy of an alternating current surface-discharge plasma display panel on the gas pressure is investigated for several Xe–Ne gas mixtures. In monochrome green 4 in. test panels the efficacy trends and emission spectra are examined for increasing gas pressure and/or Xe concentration. The measured panel efficacy and emission characteristics are compared with the results of a numerical discharge model. It is found that the discharge efficiency for the cell geometry used in present-day commercial products can be increased significantly by using a larger Xe partial pressure. An increase of the electron heating efficiency and of the Xe excitation efficiency contribute about equally to the efficacy increase. The contribution of the increasing Xe dimer radiation fraction to the efficacy improvement is relatively small. These findings are applied in a 4 in. color test display with a design that resembles the one used in present-day commercial products and contains a gas mixture of 13.5% Xe in Ne at 800 hPa. For realistic operating conditions an efficacy of 3.8 lm/W at a white luminance of 2010 cd/m2 is obtained. Furthermore, the panel chromaticity improves for increasing Xe partial pressure due to decreasing Ne emission.
Two mechanisms of blueshift of edge emission in InGaN-based epilayers and multiple quantum wells
Authors: Kuokstis E. , Yang J. W. , Simin G. , Khan M. Asif , Gaska R. , Shur M. S.
Published: July 2002

We present the results of a comparative photoluminescence (PL) study of GaN and InGaN-based epilayers, and InGaN/GaN multiple quantum wells (MQWs). Room-temperature PL spectra were measured for a very broad range of optical excitation from 10 mW/cm2 up to 1 MW/cm2. In contrast to GaN epilayers, all In-containing samples exhibited an excitation-induced blueshift of the peak emission. In addition, the blueshift of the emission in the InGaN epilayers with the same composition as the quantum well was significantly smaller. The comparison of the blueshift in the “bulk” InGaN and in the MQWs allowed us to separate two different mechanisms responsible for this effect: (i) filling of the localized states in In-rich areas and (ii) screening of the polarization electric field in strained MQW structures.
Interface reaction of Ta/Ni81Fe19 or Ni81Fe19/Ta and its suppression
Authors: Yu G. H. , Zhao H. C. , Li M. H. , Zhu F. W. , Lai W. Y.
Published: July 2002

Ta / Ni 81 Fe 19 and Ni81Fe19/Ta structures are commonly used in the magnetic multilayers with giant magnetoresistance. For a Ta/Ni81Fe19/Ta fundamental structure, Ta seed and Ta cap layers resulted in a loss of moment equivalent to a magnetically dead layer of thickness 1.6±0.2 nm. In order to find out the reason, the composition and chemical states at the interface regions of Ta/Ni81Fe19 and Ni81Fe19/Ta were studied using the x-ray photoelectron spectroscopy and peak decomposition technique. The results show that there are thermodynamically favorable reactions at the Ta/Ni81Fe19 and Ni81Fe19/Ta interfaces: 2Ta+Ni=NiTa2. However, the thickness of a magnetically dead layer was significantly reduced by the insertion of a small amount of Bi in the Ta/Ni81Fe19/Ta structure. This result indicates that a surfactant Bi can suppress the interface reaction in multilayers.
Color imaging with a low temperature scanning tunneling microscope
Authors: Hoffmann Germar , Kröger Jörg , Berndt Richard
Published: July 2002

We report on an improved optical design for detecting light emitted from a scanning tunneling microscope (STM). Using a charge coupled device camera and a grating spectrometer a photon detection efficiency of ≈2.5% at 550 nm is achieved and count rates of up to 5×104 counts/nA/s are observed on a noble metal surface and a W tip. Statistically significant spectra from noble metal surfaces are detected in tens of milliseconds. Thus, new modes of measurement become available, which encompass spectroscopic imaging (acquisition of fluorescence spectra at each point of a STM image), and excitation spectroscopy (acquisition of fluorescence spectra while varying the tip–sample bias). Spectroscopic imaging is used to observe gradual changes of the emission spectra as the STM tip approaches a monoatomic step of Ag(111) on a nanometer scale. Excitation spectroscopy with high resolution in both wavelength and bias voltage is demonstrated for a Ag(111) surface.
A multichannel superconducting soft x-ray spectrometer for high-resolution spectroscopy of dilute samples
Authors: Friedrich S. , Funk T. , Drury O. , Labov S. E. , Cramer S. P.
Published: July 2002

We have built a high-resolution high-efficiency superconducting soft x-ray spectrometer for synchrotron-based fluorescence-detected absorption spectroscopy. The sensor is a 3×3 array of 200×200 μm2 superconducting Nb–Al–Al2O3–Al–Nb tunnel junctions with an energy resolution around 15 eV below 1 keV and a total count rate capability of ≈100 000 counts/s. This sensor array is cooled to ≈0.1 K by a two-stage adiabatic demagnetization refrigerator while held at the end of a 40-cm-long cold finger that can be inserted into an UHV sample chamber for x-ray fluorescence measurements. We present L-edge absorption spectra of dilute transition metals (≈few 100 ppm) and discuss spectrometer performance with respect to the analysis of metalloproteins.
Coexistence of metallic and insulating phases in epitaxial CaRuO3 thin films observed by scanning microwave microscopy
Authors: Hyun S. , Cho J. H. , Kim A. , Kim J. , Kim T. , Char K.
Published: July 2002

Using a scanning microwave microscope, we investigated the local electrical properties of epitaxial CaRuO3 thin films. The films showed a metal–insulator transition depending on the growth temperature and their thickness. We observed spatially separated highly conducting and poorly conducting regions in the films grown at a high temperature of 800 °C, which showed insulating behavior. The conduction in the CaRuO3 thin films with insulating behavior is percolative through the highly conducting regions and is closely related to this two-phase behavior.
Attitude determination from single-antenna carrier-phase measurements
Authors: Bahder Thomas B.
Published: July 2002

A model of carrier phase measurement (as carried out by a satellite navigation receiver) is formulated based on electromagnetic theory. The model shows that the phase of the open-circuit voltage induced in the receiver antenna with respect to a local oscillator (in the receiver) depends on the relative orientation of the receiving and transmitting antennas. The model shows that using a single receiving antenna, and making carrier phase measurements to seven satellites, the three-axis attitude of a user platform (in addition to its position and time) can be computed relative to an initial point. This measurement model can also be used to create high-fidelity satellite signal simulators that take into account the effect of platform rotation as well as translation.
Effect of nonmagnetic impurities on the spontaneous magnetostriction in β-O2 crystals
Authors: Prokhvatilov A. I. , Freiman Yu. A. , Galtsov N. N. , Stetsenko Yu. E.
Published: July 2002

X-ray investigations of weak solid solutions of oxygen with argon, krypton, and nitrogen are done at temperatures in the existence region of the rhombohedral β-O2 phase. It is found that the spontaneous inhomogeneous magnetostriction effect previously studied in pure oxygen is enhanced by the introduction of the atomic impurities Ar and Kr and weakened by the dissolution of nitrogen molecules. The possible reasons for the different effect of atomic and molecular impurities on the inhomogeneous spontaneous magnetostriction in β-O2 crystals are discussed.
Calculation of damping rates in thin inhomogeneous ferromagnetic films due to coupling to lattice vibrations
Authors: McMichael R. D. , Kunz Andrew
Published: July 2002

This article describes calculations of ferromagnetic resonance damping rates due to coupling between the magnetization and lattice vibrations through inhomogeneities. The mechanisms we have explored include generation of shear phonons through inhomogeneous anisotropy and generation of both longitudinal and shear phonons through inhomogeneous magnetostriction. In both cases, inhomogeneities couple the uniform precession to finite wave vector phonons. For both coupling mechanisms, the predicted damping rate is on the order of 106 s−1 in transition metals. The damping rate by these mechanisms is inversely proportional to the fifth power of the shear phonon velocity, and may play a significant role in mechanically softer materials such as magnet/polymer nanocomposites.
Polarization consistent basis sets. II. Estimating the Kohn–Sham basis set limit
Authors: Jensen Frank
Published: July 2002

The performance of the previously proposed polarization consistent basis sets is analyzed at the Hartree–Fock and density functional levels of theory, and it is shown that each step up in basis set quality decreases the error relative to the infinite basis set limit by approximately an order of magnitude. For the largest pc-4 basis set the relative energy error is approximately 10−7, and extrapolation further improves the results by approximately a factor of 2. This provides total atomization energies for molecules with an accuracy of better than 0.01 kJ/mol per atom. The performance of many popular basis sets is evaluated based on 95 atomization energies, 42 ionization potentials and 10 molecular relative energies, and it is shown that the pc-n basis sets in all cases provides better accuracy for a similar or a smaller number of basis functions.
Generating chaos with a switching piecewise-linear controller
Authors: Lü Jinhu , Zhou Tianshou , Chen Guanrong , Yang Xiaosong
Published: July 2002

This paper introduces a new chaos generator, a switching piecewise-linear controller, which can create chaos from a three-dimensional linear system within a wide range of parameter values. Basic dynamical behaviors of the chaotic controlled system are investigated in some detail.
Transparent p-type conducting BaCu2S2 films
Authors: Park Sangmoon , Keszler Douglas A. , Valencia Melinda M. , Hoffman Randy L. , Bender Jeffrey P. , Wager John F.
Published: July 2002

p -type conducting films of α-BuCu2S2 have been deposited onto glass and KBr substrates, yielding a conductivity of 17 S/cm and a Hall mobility of 3.5 cm2/V s. For a 430-nm-thick film, the optical transparency approaches 90% in the visible portion of the spectrum at 650 nm, and a transparency of 40% extends throughout the infrared to the long-wavelength cutoff of the KBr substrate at 23 μm.
Least-squares fits when there are errors in X
Authors: Jolivette Peter L.
Published: July 2002

The equations for a least-squares fit when the data have errors in both X and Y are developed using a Taylor expansion of both the fitting function y and the ‘‘independent’’ variable x. The method is an extension of the usual treatment for fitting data with a nonlinear function. It is not especially complex and can be implemented in a small program. Examples show strengths and weaknesses of the approach.
Diffusion and solubility of copper in germanium
Authors: Stolwijk N. A. , Frank W. , Hölzl J. , Pearton S. J. , Haller E. E.
Published: July 2002

Diffusion profiles and the solubility of Cu in Ge were measured in the temperature interval 850–1200 K by means of the spreading-resistance technique. From these data it is concluded that the diffusion of Cu in Ge involves the interchange between a highly mobile interstitial configuration, Cui, and a practically immobile substitutional configuration, Cus, with the aid of vacancies, V, via the so-called dissociative mechanism, Cui+V⇄Cus. The excellent agreement of the values of the vacancy contribution to the tracer self-diffusion coefficient in Ge, as calculated from our diffusivity and solubility data on Cu in Ge, with directly measured values of the 71Ge tracer self-diffusion coefficient from the literature demonstrates that self-diffusion in Ge occurs via vacancies. A comparison with the mechanisms of Au and self-diffusion in Si is presented.
New method for complete electrical characterization of recombination properties of traps in semiconductors
Authors: Wang Alex C. , Sah C. T.
Published: July 2002

In the experimental characterization of deep-level trapping centers in semiconductors, the popular capacitance and current transient techniques have reached a degree of maturity. However, two major limitations remain: (i) capture rates are often too fast to be measured by majority-carrier methods, and (ii) optical excitation and specially designed samples are necessary for certain traps. In this work we give detailed analysis and demonstration of our new minority-carrier capture techniques to overcome these limitations. Combined with existing methods, which are also illustrated, this allows purely electrical measurement of all capture and emission rates of deep levels. We explain the physics underlying our suggestion that in most cases even for a multilevel deep trap all one needs is a single p-n junction to obtain the complete set of data.
Optimal paths for thermodynamic systems: The ideal diesel cycle
Authors: Hoffmann Karl Heinz , Watowich Stanley J. , Berry R. Stephen
Published: July 2002

Optimal control theory is used to determine the piston trajectory which yields maximum power output for a model which incorporates the diesel engine’s major irreversibilities. Optimal trajectories were obtained for the cases of unconstrained and constrained piston acceleration. Optimizing the path for our standard engine increased both the net work output per cycle and the net efficiency by about 10%.
Photoluminescence optimization and characteristics of the rare-earth element erbium implanted in GaAs, InP, and GaP
Authors: Pomrenke Gernot S. , Ennen H. , Haydl W.
Published: July 2002

The characteristic 1.54-μm emission from the rare-earth element erbium implanted in GaAs, InP, and GaP was investigated through 10-K photoluminescence essentially as a function of anneal temperature, time, and method. The strip-heater, forming-gas, and quartz-ampoule anneal methods were utilized in the range of 400 to 1000 °C. Erbium-related emissions were observed from 1.48 to 1.64 μm and were observable at emission temperatures of up to 260 K for InP:Er and 296 K for GaP:Er and GaAs:Er. Out of the three semiconductors, GaAs:Er was observed to exhibit the highest optical activation using a square-profile implantation technique. Dependent on the anneal method, optimum Er emissions occurred between 650 and 800 °C for GaAs, for InP between 575 and 625 °C, and for GaP between 800 and 950 °C. In general, the forming-gas anneal method proved most successful; however, maximum luminescence including sharper emission lines was achieved through the strip-heater method. This method, with an anneal time of 10 s, showed also the importance of short-time anneals in GaAs:Er, results which were also paralleled by isothermal anneals of InP:Er. The difference in emissions at different anneal temperatures and times gives preliminary evidence of different Er3+ centers.