bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct observation of Fermi-level pinning in Cs-doped CuPc film
Authors: Yan Li , Watkins N. J. , Zorba S. , Gao Yongli , Tang C. W.
Published: July 2002

The electronic structures of pristine and Cs-doped CuPc films are investigated using photoemission spectroscopy and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital can be directly observed by IPES and ultraviolet photoemission spectroscopy simultaneously. We found that the Fermi-level position in organic film can be modified by Cs doping. The observed onset of the LUMO of the CuPc film is shifted by Cs doping to less than 0.2 eV above the Fermi level. The result indicates that the energy alignment and charge injection properties of the organic materials can be modified by a simple doping process.
Determination of charge carrier concentration in n- and p-doped SiC based on optical absorption measurements
Authors: Weingärtner R. , Wellmann P. J. , Bickermann M. , Hofmann D. , Straubinger T. L. , Winnacker A.
Published: July 2002

We have investigated the effect of doping on absorption for various SiC polytypes, i.e., n-type (N) 6H–SiC, 4H–SiC, and 15R–SiC, p-type (Al) 6H–SiC, and 4H–SiC, and p-type (B) 6H–SiC. For these polytypes the band-gap narrowing with higher doping concentration is observed. In addition, for n-type doping below band-gap absorption bands at 464 nm for 4H–SiC, at 623 nm for 6H–SiC, and at 422 and 734 nm for 15R–SiC are observed. The peak intensities of these absorption bands show a linear relation to the charge carrier concentration obtained from Hall measurements. The corresponding calibration factors are given. As an application a purely optical wafer mapping of the spatial variation of the charge carrier concentration is demonstrated.
Microwave second-harmonic generation and point-contact spectroscopy of Ni–Nb metallic glasses
Authors: Balkashin O. P. , Yanson I. K. , Halbritter A. , Mihály G.
Published: July 2002

The point-contact spectra (the energy dependence of the amplitude of the second harmonic of the modulation current) of contacts of NixNb1−x metallic glasses are measured at audio (3726 Hz) and microwave (9.5 GHz) frequencies. It is established for the first time in rf dynamic experiments that the low-energy spectral feature due the scattering of electrons on two-level systems (TLSs) in the contact is observed at microwave frequencies as well. This is direct evidence of the existence of a fast relaxation process in the contacts, with a characteristic time τ≪10−10 s. There is also a small contribution from slower scattering processes. It is found that the videodetection signal contains a component due to a steady-state nonequilibrium occupation of the states of the TLSs under microwave irradiation. The coupling efficiency of the contact with the waveguide and rf receiver is estimated to be 5–7%. A slight minimum of the resistance at zero voltage observed for some of the contacts is attributed to superconducting fluctuations of the order parameter at a temperature T∼2.3Tc.
Structural investigation of graphitic foam
Authors: Mukhopadhyay S. M. , Mahadev N. , Joshi P. , Roy A. K. , Kearns K. M. , Anderson D. P.
Published: July 2002

Structural and chemical characteristics of pitch-based graphitic foam have been studied using scanning electron microscopy, transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy. Chemically, the carbon atoms in these materials are found to have identical bonding states as those in pure graphite single crystals. Microstructural studies indicate that they have a cellular morphology with the cell walls made up of graphitic layers. The walls can be smooth or stepped depending upon the orientation of graphitic layers with respect to the cells. Ligaments between neighboring cells and junctions of ligaments (corners of three or more cells) distinctly show layers of graphitic planes, irregular flakes, and beam-like protruding structures made up of folded layers of graphite. The network of interconnecting pores have openings on the cell walls that have ruptured edges and sharp corners. This indicates that they were formed after hardening of the precursor, resulting in brittle fracture of the walls. The sharp cracks at pore openings may enhance mechanical failure of these structures and attempts to minimize them may improve mechanical behavior. Higher magnification studies in the TEM indicate the presence of flaky layers of graphite crystals and also long tubular structures protruding from the cell walls and ligaments. These fibrous structures (possibly rolled stacks of graphitic planes) have lengths in excess of several microns and diameters ranging between several tens of nanometers to several hundred nanometers. The walls are thick: several tens of nanometers. These structures, intermediate in size between micron size graphitic fibers and nanometer size multiwalled “nanotubes,” have been discussed in light of other well investigated graphitic structures.
Inhomogeneous states for small magnetic particles with exchange anisotropy
Authors: Ivanov B. A. , Volk A. Ya. , Merkulov A. Yu.
Published: July 2002

In a ferromagnetic particle with exchange anisotropy a state with a nonuniform distribution of spins can arise in the presence of a magnetic field perpendicular to the easy axis. The presence of inhomogeneous states substantially modifies the standard dependence of the magnetization of the particle on the magnetic field. The form of the boundary conditions on the macroscopic equations for the magnetization is refined on the basis of a comparison of the results with the solutions in the continuum approximation.
Metallic ferromagnetism in a generalized Hubbard model
Authors: Didukh L. , Kramar O.
Published: July 2002

The possibility of realization of a metallic ferromagnetic state in a generalized Hubbard model with correlated hopping and exchange interaction integrals is investigated. The single-electron energy spectrum recently obtained by means of the mean-field approximation is applied for the description of ground-state and finite-temperature properties of the system. An expression for the Curie temperature is found, and the behavior of the temperature dependences of the magnetization and paramagnetic susceptibility is analyzed. Taking the correlated hopping into account allows one to explain certain peculiarities of the ferromagnetic behavior of transition metals and their compounds.
Magnetic phase diagram of the system of manganites Nd0.6Ca0.4(Mn1−xCrx)O3
Authors: Troyanchuk I. O. , Bushinsky M. V. , Eremenko V. V. , Sirenko V. A. , Szymczak H.
Published: July 2002

The following sequence of phase transformations is observed in the Nd0.6Ca0.4(Mn1−xCrx)O3 system: antiferromagnet (x=0), mixed magnetic state (0.015⩽x⩽0.04), ferrimagnet (0.04⩽x⩽0.8), and spin glass (x=1). At chromium concentrations near x∼0.5 the Néel temperature increases sharply, and the contribution from the Nd sublattice increases substantially. Since the chromium and manganese ions have a tendency to order, these results can be interpreted in a model of superexchange interactions.
Spectroscopic studies of nitrogenated amorphous carbon films prepared by ion beam sputtering
Authors: Das Debajyoti , Chen K. H. , Chattopadhyay S. , Chen L. C.
Published: July 2002

Spectroscopic analysis of the unhydrogenated amorphous carbon nitride (a-CNx) films, prepared by ion beam sputtering, was done by ellipsometry, Raman scattering and x-ray photoelectron spectroscopic (XPS) studies. The optical gap of the films was estimated from the Tauc’s analysis of the (n,k) data obtained from spectroscopic ellipsometry. In addition to the commonly observed D and G bands at ∼1350 cm−1 and 1550 cm−1, respectively, we detected a separate band at ∼1450 cm−1 in the Raman spectrum of a-CNx films. This intermediate peak was unambiguously identified as the N band arising out of the nitrogen–nitrogen bonding, due to the incorporation of nitrogen in the a-C network. High resolution XPS C 1s and N 1s peaks were used to estimate the nitrogen content in the films and various bonding configurations were identified from their Gaussian deconvolution. An increase in the intensity ratio of CN and CC components, I(CN)/I(CC), in the C 1s spectra signified increasing nitrogenation of the carbon network. Deconvolution of the N 1s spectra revealed that the sp3-hybridized C–N component remained unchanged at ∼40%, however, the sp1-hybridized C≡N component increased in intensity when the nitrogen content in the network increased systematically. Increasing nitrogenation led to the graphitization of the network and resulted in a gradual decrease in the optical gap, along with simultaneous increase in the ID/IG and IN/IG intensity ratios in the Raman band.
V/Al/Pt/Au Ohmic contact to n-AlGaN/GaN heterostructures
Authors: Schweitz K. O. , Wang P. K. , Mohney S. E. , Gotthold D.
Published: July 2002

A study of V/Al/Pt/Au and Ti/Al/Pt/Au contacts to n-Al0.3Ga0.7N/GaN heterostructures is presented. Vanadium was chosen as a potential replacement for Ti because V is expected to form a thermally stable nitride with a low work function. Low-resistance Ohmic contacts are achieved with V/Al/Pt/Au layers after annealing at 650 °C, which represents a decrease of 150 °C compared to the Ti/Al/Pt/Au counterpart. This contact exhibits two orders of magnitude lower specific contact resistance compared to the Ti/Al/Pt/Au contacts studied in this work when annealed at temperatures less than 800 °C, although the specific contact resistance of the Ti/Al/Pt/Au contacts is lower when annealed at higher temperatures. A contact resistance and specific contact resistance of 0.8±0.1 Ω mm and (1.4±0.3)×10−5 Ω cm2, respectively, are obtained after annealing at 650 °C for 45 s.
Extension of integrated high performance regimes with impurity and deuterium particle control in Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U)
Authors: Kubo H. ,   JT-60 Team
Published: July 2002

In JT-60U [Horiike et al., Fusion Eng. Des. 16, 285 (1991)] extension of high performance plasmas toward high density, high radiation-loss-power fraction and high fuel purity has been explored by pellet injection, Ar injection and study of impurity production. By pellet injection from the high-field-side, the density range for the high-poloidal-beta high-confinement mode plasmas with edge localized modes (high-βp ELMy H-mode plasmas) has been extended to 70% of the Greenwald density (nGW). In ELMy H-mode plasmas with Ar injection, confinement time given by the International Thermonuclear Experimental Reactor ELMy H-mode scaling IPB98(y,2), electron density of 0.8 nGW and radiation-loss-power fraction of 0.8 have been achieved simultaneously. Type-I ELMs disappeared, and the heat flux to the divertor plates due to ELMs decreased by a factor of 10. At the carbon divertor plates, the chemical sputtering yields due to C2Hx/C2Dx (where x=2, 4, and 6) production in addition to CH4/CD4 production have been measured systematically. It was shown that the C2Hx/C2Dx production was dominant in the low temperature divertor plasmas.
Determination of adsorption-induced variation in the spring constant of a microcantilever
Authors: Cherian Suman , Thundat Thomas
Published: July 2002

Although resonating microcantilevers are demonstrated to be excellent mass sensors, adsorption-induced changes in the spring constant result in errors in the calculation of adsorbed mass from shifts in resonance frequencies. However, simultaneous measurement of resonance frequency and adsorption-induced cantilever bending can be used to determine the variation in spring constant. Plotting the change in surface stress as a function of analyte concentration, the surface excess of adsorbed molecules and, therefore, the mass adsorbed can be determined. Here, we demonstrate this concept for adsorption of Na+ ions on microcantilevers in NaCl solutions where a change in the spring constant was found to increase from 9.5×10−4 to 7.5×10−3 N/m as the NaCl concentration increased from 0.05 to 0.8 M.
Mechanism for the changes with temperature of the EPR spectrum of the Fe3+ ion in polycrystalline materials containing complexes with a multiwell potential
Authors: Vasyukov V. N.
Published: July 2002

The temperature dependence of the EPR spectrum of the Fe3+ ion in polycrystalline materials is investigated on the basis of a numerical modeling of the spectrum. A comparison is made of the changes in the spectrum with temperature for two different mechanisms. The first mechanism involves the temperature dependence of the parameters of the spin Hamiltonian, and the second the existence of a system with a multiwell potential. A comparison of the outcomes for these two mechanisms with the experimental data shows that the second mechanism is the governing one. In that mechanism a change in the occupation of the excited vibronic states leads to a change in the shape of the EPR spectrum. The possibility of obtaining additional information about the parameters characterizing the distribution of the vibronic states over energy is demonstrated.
Growth mechanism of carbon nanocoils
Authors: Pan Lujun , Zhang Mei , Nakayama Yoshikazu
Published: July 2002

Carbon nanocoils were prepared by catalytic pyrolysis of acetylene using iron-coated indium tin oxide as the catalyst. The effects of the constitution of catalyst, the growth temperature and time, the flow rate of acetylene gas on the growth of carbon nanocoils were investigated. It is found that the coils grow mainly from the interface of iron and indium tin oxide films. The coils generally consist of two or more nanotubes. Each coil has its own external diameter and pitch, which is determined by the structure of the catalyst at its tip. The growth of the carbon nanocoil is considered to be due to the nonuniformity of the carbon extrusion speed at different parts of the catalyst particle containing iron, tin, indium and/or oxygen. It is confirmed that iron is crucial in the formation of a nanotubule and indium tin oxide induces the helical growth of the nanotubule.
Field emission from nonaligned carbon nanotubes embedded in a polystyrene matrix
Authors: Poa C. H. , Silva S. R. P. , Watts P. C. P. , Hsu W. K. , Kroto H. W. , Walton D. R. M.
Published: July 2002

Low threshold fields of 1.6 V/μm and 2.6 V/μm were obtained for field emission from multiwalled carbon nanotubes and boron-doped multiwalled carbon nanotubes embedded in polystyrene, respectively. A Fowler–Nordheim analysis of the results together with sheet resistivity data illustrate that the higher carbon nanotube concentrations in a polystyrene matrix result in larger effective emission areas, at the expense of higher operating threshold fields.
Polymerized carbon nitride nanobells
Authors: Zhang G. Y. , Ma X. C. , Zhong D. Y. , Wang E. G.
Published: July 2002

Well-aligned carbon nitride nanotubes are fabricated by microwave plasma assisted chemical vapor deposition using iron as the catalyst. These nanotubes are linearly polymerized by carbon nitride nanobells with catalyst particles in the root, as revealed by high-resolution transmission electron microscopy. Carbon nitride nanotube film is analyzed by x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and Raman spectroscopy. Both XPS and AES reveal a nitrogen content of about 2% in the film. Raman spectroscopy exhibits a good crystallization of these carbon nitride nanotubes. Electron energy loss spectroscopy is used to study the local distribution of nitrogen in a single nanotube, which indicates that nitrogen prefers to locate at curved graphite sheets, at the top of the nanobells, and that incorporation of nitrogen results in the decrease of the crystallization. Based on these results, a growth model is proposed to explain this periodically stacked nanobell structure. In this model, we propose that graphite sheets only precipitate along the surface of catalyst particles and that lower growth rate at the top curve surface of the bell-like catalyst particle is the key factor influencing formation of this special microstructure. A heterojunction between a tubular carbon nanotube and a carbon nitride nanobell also has been fabricated by a two-step growth technique. And short carbon nitride nanotubes with a few nanobells or even just one nanobell are obtained by both physical and chemical methods. Furthermore, we studied the field emission properties and have obtained a threshold field of as low as about 1 V/μm. A novel side-emission mechanism has been proposed based on the special polymerized nanobell structures.
Equation of state of an equimolar He3–He4 mixture
Authors: Karnatsevich L. V. , Sibileva R. M. , Khazhmuradov M. A. , Shapoval I. N. , Meriuz A. V.
Published: July 2002

Analytical forms of the empirical equations of state of the system are obtained for an equimolar He3–He4 mixture in the homogeneous liquid and dense fluid phases at temperatures 1.5–14 K and pressures 0–10 MPa on the basis of the existing experimental P–V–T data. This is done by choosing approximating expressions, setting up a computer program, and calculating the fitting coefficients of the expressions. The quality of the approximation corresponds to the accuracy of the experimental determinations and is on average 0.5%.
A new surface forces apparatus for nanorheology
Authors: Restagno Frédéric , Crassous Jérôme , Charlaix Élisabeth , Cottin-Bizonne Cécile , Monchanin Michel
Published: July 2002

We present an original surface forces apparatus which enables us to measure the interaction forces between any solid surfaces such as, e.g., metallic surfaces, opaque surfaces, or rough surfaces. The relative displacement of the surfaces is measured with a capacitive sensor. The forces are measured by a stiff and highly sensitive interferometric sensor. The measurements are performed in a dc to 100 Hz bandwidth. This feature allows us to study the mechanical response of a nanometric confined medium to rapid strain variations in the linear regime. An example of nanorheological measurement of dodecane confined in a nanometric gap is given at the end of this article.
Electrical transport properties of individual gallium nitride nanowires synthesized by chemical-vapor-deposition
Authors: Kim Jae-Ryoung , So Hye Mi , Park Jong Wan , Kim Ju-Jin , Kim Jinhee , Lee Cheol Jin , Lyu Seung Chul
Published: July 2002

We have synthesized high-quality gallium nitride (GaN) nanowires by a chemical-vapor-deposition method and studied the electrical transport properties. The electrical measurements on individual GaN nanowires show a pronounced n-type field effect due to nitrogen vacancies in the whole measured temperature ranges. The n-type gate response and the temperature dependence of the current–voltage characteristics could be understood by the band bending at the interface of the metal electrode and GaN wire. The estimated electron mobility from the gate modulation characteristics is about 2.15 cm2/V s at room temperature, suggesting the diffusive nature of electron transport in the nanowires.
Origin of strong G′ band in Raman spectra of carbon whiskers
Authors: Dong Jian , Shen Wanci , Tatarchuk Bruce
Published: July 2002

Raman scattering has been studied on one kind of carbon whiskers. The G′ band, overtone of D band, is the most intense of all bands—its intensity is up to ten times stronger than that of the G band. Such phenomena can be observed with different excitation wavelength. The strong intensity of the G′ band may be related to the disclination existing in the whiskers. Polarized Raman spectra are obtained from the edge plane of the whiskers. The intensity of G′ bands obtained with polarized light parallel to carbon layers (E⊥c) is ten times as strong as that with polarized light normal to carbon layers (E∥c).
Unusual properties of the fundamental band gap of InN
Authors: Wu J. , Walukiewicz W. , Yu K. M. , Ager J. W. , Haller E. E. , Lu Hai , Schaff William J. , Saito Yoshiki , Nanishi Yasushi
Published: July 2002

The optical properties of wurtzite-structured InN grown on sapphire substrates by molecular-beam epitaxy have been characterized by optical absorption, photoluminescence, and photomodulated reflectance techniques. These three characterization techniques show an energy gap for InN between 0.7 and 0.8 eV, much lower than the commonly accepted value of 1.9 eV. The photoluminescence peak energy is found to be sensitive to the free-electron concentration of the sample. The peak energy exhibits very weak hydrostatic pressure dependence, and a small, anomalous blueshift with increasing temperature.
Tremor classification and tremor time series analysis
Authors: Deuschl Günther , Lauk Michael , Timmer Jens
Published: July 2002

The separation between physiologic tremor (PT) in normal subjects and the pathological tremors of essential tremor (ET) or Parkinson’s disease (PD) was investigated on the basis of monoaxial accelerometric recordings of 35 s hand tremor epochs. Frequency and amplitude were insufficient to separate between these conditions, except for the trivial distinction between normal and pathologic tremors that is already defined on the basis of amplitude. We found that waveform analysis revealed highly significant differences between normal and pathologic tremors, and, more importantly, among different forms of pathologic tremors. We found in our group of 25 patients with PT and 15 with ET a reasonable distinction with the third momentum and the time reversal invariance. A nearly complete distinction between these two conditions on the basis of the asymmetric decay of the autocorrelation function. We conclude that time series analysis can probably be developed into a powerful tool for the objective analysis of tremors.
Intrinsically irreversible light-driven engine
Authors: Watowich Stanley J. , Hoffmann Karl Heinz , Berry R. Stephen
Published: July 2002

We examine a reciprocating heat engine which necessarily operates far from equilibrium and about an unstable steady state. The piston of the engine is driven by the nonlinear coupling of the working fluid to an external light source which provides high quality heat and to the environment into which waste heat is dumped. We determine the piston trajectories that optimize two different criteria of process performance, the maximization of work output, and the minimization of entropy production. The trajectories optimizing different performance goals are qualitatively different. In engines not dominated by friction losses, the cycle optimizing work output requires that the expansion stroke begins with a slight compression and the temperature of the working fluid increases briefly.
Atomic layer epitaxy
Authors: Goodman Colin H. L. , Pessa Markus V.
Published: July 2002

Atomic layer epitaxy (ALE) is not so much a new technique for the preparation of thin films as a novel modification to existing methods of vapor-phase epitaxy, whether physical [e.g., evaporation, at one limit molecular-beam epitaxy (MBE)] or chemical [e.g., chloride epitaxy or metalorganic chemical vapor deposition (MOCVD)]. It is a self-regulatory process which, in its simplest form, produces one complete molecular layer of a compound per operational cycle, with a greater thickness being obtained by repeated cycling. There is no growth rate in ALE as in other crystal growth processes. So far ALE has been applied to rather few materials, but, in principle, it could have a quite general application. It has been used to prepare single-crystal overlayers of CdTe, (Cd,Mn)Te, GaAs and AlAs, a number of polycrystalline films and highly efficient electroluminescent thin-film displays based on ZnS:Mn. It could also offer particular advantages for the preparation of ultrathin films of precisely controlled thickness in the nanometer range and thus may have a special value for growing low-dimensional structures.
Variable angle of incidence spectroscopic ellipsometry: Application to GaAs-AlxGa1−xAs multiple heterostructures
Authors: Snyder Paul G. , Rost Martin C. , Bu-Abbud George H. , Woollam John A. , Alterovitz Samuel A.
Published: July 2002

The sensitivity of spectroscopic ellipsometry data to multilayer model parameters is shown to be a strong function of the angle of incidence. A quantitative study of sensitivity versus angle of incidence is performed for a GaAs-AlxGa1−xAs-GaAs substrate structure, showing that maximum sensitivity to layer thicknesses and AlGaAs composition occurs near the wavelength-dependent principal angle. These results are verified by experimental measurements on two molecular-beam epitaxy grown samples. New spectral features, not found in previous ellipsometry studies of similar structures, are also reported.
Scaling law for contoured length of engineering surfaces
Authors: Ling Frederick F.
Published: July 2002

An experimentally based scaling law for the contoured length of engineering surfaces is presented. The contoured length between two points on the surface, normalized by the straight-line distance, is expressed as a function of the length of the measuring unit with two parameters. One parameter is quite universal while the other is surface specific. The embodiment of the stochastic nature of surfaces in inferred through the reference of fractals.
Size-distribution in sudden breakage by the use of entropy maximization
Authors: Englman R. , Rivier N. , Jaeger Z.
Published: July 2002

We derive the number and weight distributions of fragment pieces (of linear size a) that are obtained in a sudden fragmentation process. The information-entropic derivation is based on a fragment energy e(a) which incorporates the physical mechanisms and material properties responsible for breakage. For not too large fragments the number distribution follows a power law a−Θ, where Θ is between 2 and 5, depending on the size range and circumstances of the breakage. Our results are compared with other theories of size distributions.
Optimal performance of an endoreversible-combined refrigeration cycle
Authors: Chen Jincan , Yan Zijun
Published: July 2002

We investigate the performance of a class of endoreversible-combined refrigeration cycles operating between a low-temperature heat reservoir at TL and a high-temperature heat reservoir at TH , and derive the optimal configuration and the relation between the optimal coefficient of performance and the rate of refrigeration of these cycles. It is shown that the relation is similar to that of an optimal endoreversible refrigeration cycle. However, it is worth noticing that the overall heat resistance of a combined endoreversible refrigeration cycle is not identical with that of a single one, so that they are somewhat different from each other in performance. Simultaneously, it is pointed out that some performances of an endoreversible refrigeration cycle are quite different from those of an endoreversible power cycle.
Fourier transform infrared spectroscopy studies at low temperatures of MPX3 layered compounds
Authors: Jouanne M. , Julien C.
Published: July 2002

In this work experimental results in infrared absorption at low temperatures on the layered compounds of the MPX3 (M=Fe,Ni and X=S,Se) family are reported. The absorption spectra in the far infrared (between 50 and 500 cm−1) recorded on thin slabs of these compounds (10 μm thick) are compared with the corresponding Raman spectra and this comparison confirms the hypothesis of the phonon branches folding in the antiferromagnetic phase of FePS3 below the Néel transition at 118 K. In the midinfrared (between 700 and 4000 cm−1), broad bands have been observed at low temperatures for the iron phosphorus trichalcogenide compounds. These absorption bands are attributed to electronic transitions for both FePS3 and FePSe3 lamellars, which are mainly due to the splitting of the 3d-metal levels by the pseudooctahedral field created by the nearest-neighbor sulfur atoms.
Crystallized Si films by low-temperature rapid thermal annealing of amorphous silicon
Authors: Kakkad R. , Smith J. , Lau W. S. , Fonash S. J. , Kerns R.
Published: July 2002

Low-temperature rapid thermal annealing has been used to crystallize both undoped and doped amorphous silicon (a-Si) films deposited at low temperatures. The polycrystalline films produced are completely crystallized with time temperature budgets such as 4 min at 700 °C. Unlike deposited polycrystalline Si films, the grain size in these crystallized films is not limited by film thickness. In the case of undoped a-Si films crystallized by this approach, the resulting conductivity is comparable to that achieved in undoped polycrystalline Si films produced by much higher processing temperatures. In the case of doped a-Si films, the resulting crystallized film yields a conductivity of 160 S/cm, a value which is comparable to the highest reported for doped polycrystalline and microcrystalline silicon. These doped films are found to have mobility values of ∼13 cm2/V s.
Charge trapping and mechanism of stimulated luminescence in CaS:Ce,Sm
Authors: Chakrabarti K. , Mathur V. K. , Thomas L. A. , Abbundi R. J.
Published: July 2002

Optically and thermally stimulated luminescence in CaS:Ce,Sm have been studied. Subsequent to exposure to ionizing radiation, this material can be optically stimulated by infrared laser sources such as an InGaAsP (1.20 μm) laser and a Nd:YAG (1.06 μm) laser. The resulting optically stimulated luminescence is the characteristic Ce3+ doublet emission. This material has a deep trap with a ∼330 °C thermoluminescence peak. However, the thermal stimulation results in both Sm3+ and Ce3+ characteristic emissions. It appears that charge compensation in the form of additional sulfur ions in the interstitials is possible in CaS:Ce,Sm and the charge motion between the Ce3+ ions at different symmetry sites results in Ce3+ emissions from the 330 °C peak.
Phase locking of high-power microwave oscillators
Authors: Woo W. , Benford J. , Fittinghoff D. , Harteneck B. , Price D. , Smith R. , Sze H.
Published: July 2002

Phase locking is considered both for a case in which an oscillator is driven by an external signal without feedback, and for a case in which two coupled oscillators drive each other. A comprehensive sustained oscillator model is used for the driven microwave cavity. The new locking conditions for two coupled oscillators show that phase locking can occur only when the connector contributes the zero or π phase delay. Temporal behavior is solved numerically. Calculations with large priming power agree with the experiments on a high-power magnetron driven vircator in which there is no feedback to the magnetron. The mutual drive calculations also agree with the experiments on high-power coupled magnetrons.
A revised model for the metallurgical behavior of 2:17-type permanent magnet alloys
Authors: Ray A. E.
Published: July 2002

The 2:17 magnet alloys form a complex metallurgical system with a large number of interactive compositional and heat-treating variables. A model for the metallurgical behavior of these alloys was proposed by the author in 1983. Subsequent experiments designed to test that model have verified some aspects, significantly modified others, and added much detail. The current version of the model is reported.
Reaction between SiC and W, Mo, and Ta at elevated temperatures
Authors: Geib K. M. , Wilson C. , Long R. G. , Wilmsen C. W.
Published: July 2002

The stability of W, Mo, and Ta in contact with single-crystal β-SiC at elevated temperatures has been investigated using Auger sputter profiling. All three metals were found to form a thin-mixed layer of metal carbide and silicide upon metal deposition at room temperature. This layer is thought to be the result of surface defects which weaken the Si—C bonds and allow a low-temperature reaction to occur. Upon heating, the Ta readily reacts with the SiC substrate and forms a mixed layer of Ta carbide and silicide at annealing temperatures as low as 400 °C, however, the W/SiC and Mo/SiC systems are stable and change very little after annealing at 850 and 800 °C, respectively.
Study of the optical transitions in electrodeposited CuInSe2 thin films
Authors: Herrero J. , Guillén C.
Published: July 2002

Optical absorption coefficients of electrodeposited CuInSe2 thin films were determined from the measured transmittance and reflectance at normal incidence of light in the wavelength range 400 to 2000 nm. As-deposited CuInSe2 is found to be a direct-gap semiconductor with an allowed direct transition whose gap energy is Ega= 0.96 eV and a forbidden direct transition whose gap energy is Egf= 1.38 eV. After heat treatment of the films at 400 °C, a decrease in the optical absorption was observed at energies below 1.1 eV. Heat-treated films showed an allowed direct transition with E′ga=1.00 eV, close to the value for as-deposited samples, but in this last case the forbidden direct transition was not observed.
Computation of electron-diffraction patterns in Lorentz electron microscopy of thin magnetic films (abstract)
Authors: Mansuripur M.
Published: July 2002

Lorentz electron microscopy is a powerful tool for high-resolution studies of magnetic structure in thin films.1–3 The physical mechanism that underlies all known modes of Lorentz microscopy is the interaction between the propagating electron wave and the magnetic vector potential field. For a given electron trajectory the interaction, commonly known as the Aharonov–Bohm effect, results in a phase delay directly proportional to the path integral of the vector potential.4 Lorentz microscopy is therefore a branch of phase-contrast microscopy whose various modes (e.g., Fresnel, Foucault, differential phase contrast, small-angle diffraction, electron interference, and holography) simply represent different designs for capturing the information contained in the phase of the beam after passage through the sample. This paper introduces a general technique for computing the phase imparted to the electron beam by a two-dimensional pattern of magnetization. The vector potential field for a thin film with arbitrary magnetization is calculated using fast Fourier transforms. This field is then used to compute the phase modulation of the electron beam. Calculated phase patterns and the corresponding Fresnel/Foucault intensity distributions for several magnetic configurations of practical interest (e.g., ripples, vortices, sinks, and sources) are presented.
Effect of CaCl2 and NdCl3 on the manufacturing of Nd-Fe-B by the reduction-diffusion process
Authors: Chen Chi-Jen , Liu Tin-Yu , Hung Ying-Chang , Lin Cheng-Hao , Chen Shou-Hsiung , Wu Cherng-Dean
Published: July 2002

The effect of an additional amount of CaCl2 and the amount of NdCl3 to replace Nd2O3 on the characteristics of the washed Nd-Fe-B powder alloys, and the role of chlorides on the disintegration process have been investigated. CaCl2, CaO, and unreacted Ca surrounded Nd2Fe14B grains uniformly as the chloride amount exceeded 10 wt. %. The residual Ca and oxygen content of the washed powder alloys from high chloride amount samples were relatively low; residual Ca ∼400–800 ppm, oxygen content ∼2600–4000 ppm. After sintering and heat treatment, the magnet made using CaCl2 has remarkable magnetic properties; Br ≂ 11.5 kG, iHc ≂ 10.3 kOe, and (BH)max ≂ 29 mGOe. However, the magnet made using NdCl3 has much lower coercivity, Br = 11 kG, iHc = 6.4 kOe, (BH)max = 25 mGOe.
Coercivity and Mössbauer spectroscopy studies of cobalt-adsorbed γ-Fe2O3
Authors: Tsuji Toshihide , Ando Kazuhide , Naito Keiji , Matsui Yasushi
Published: July 2002

The room-temperature coercivity of acicular γ-Fe2O3 particles having cobalt ions adsorbed in the pH range from 5 to 13.5 and in the temperature range from 283 to 373 K was measured. Emission Mössbauer spectroscopy at room temperature was used to study cobalt adsorbed on the surface of acicular γ-Fe2O3 particles. The coercivity of cobalt-adsorbed γ-Fe2O3 particles increased with increasing cobalt content up to about 2 wt %, above which it leveled off. The saturated values of coercivity increased with increasing adsorption temperature from 283 to 303 K, decreased with further increasing temperature up to 343 K, and again increased remarkably at 373 K. It was found from Mössbauer spectroscopy that the sample with high coercivity shows a strong magnetic interaction between the absorbed cobalt ions and the ordered magnetic ions of the substrate. Nonmagnetic components that do not contribute to the coercivity are probably due to the oxides and hydroxides of divalent and trivalent cobalts. The effect of pretreatment with Mg2+, Ca2+, and Sr2+ ions on the adsorption of cobalt on the surface of γ-Fe2O3 at 373 K was also studied by emission Mössbauer spectroscopy. It was found that the average internal magnetic field of γ-Fe2O3 with both Mg2+ and Co2+ ions adsorbed was larger than that for Co2+ ions alone, whereas in the case of Ca2+ and Sr2+ ions, the average internal magnetic field did not change. These phenomena are discussed on the basis of the site preference of cations in ferrites.
Extended long-wavelength λ=11–15-μm GaAs/AlxGa1−xAs quantum-well infrared photodetectors
Authors: Zussman A. , Levine B. F. , Kuo J. M. , de Jong J.
Published: July 2002

Success has been achieved in extending the spectral wavelength of GaAs/AlxGa1−xAs quantum-well infrared photodetectors to significantly longer wavelengths, λ=11–15 μm. High responsivity Rp = 0.5 A/W, high quantum efficiency η=12%, and high detectivity D* = 3 × 1010 cm √Hz/W, as well as an excellent noise equivalent temperature difference NEΔT=4 mK have been achieved at T=50 K. High performance NEΔT=19 mK has also been achieved at an even higher temperature of T=60 K.
Pulsed laser deposition of diamond-like carbon films
Authors: Pappas David L. , Saenger Katherine L. , Bruley John , Krakow William , Cuomo Jerome J. , Gu Tieer , Collins Robert W.
Published: July 2002

Carbon thin films have been prepared by 248 nm excimer laser vaporization of graphite targets. The effect of a variety of process parameters on the film properties is investigated. Deposition at or below room temperature yields diamond-like films with low hydrogen content, high optical transmission, and high resistivity. Electron energy loss spectra indicate sp3 bond fractions of 70–85%. Detailed analyses of the pseudodielectric functions, measured using spectroscopic ellipsometry, show the films to have normal dispersion and an index of refraction of 2.5 in the visible wavelength region. The effects of a low pressure hydrogen background and the use of auxiliary pulsed and dc plasma enhancements are also examined.
Laser generation and detection of surface acoustic waves: Elastic properties of surface layers
Authors: Neubrand A. , Hess P.
Published: July 2002

A noncontact all-optical method for surface photoacoustics is described. The surface acoustic waves (SAWs) were excited employing a KrF laser and detected with a Michelson interferometer using a 633-nm HeNe laser. Due to an active stabilization scheme developed for the interferometer a surface displacement of 0.2 Å could be detected. The materials investigated included pure materials such as polycrystalline aluminum, and crystalline silicon; films of gold, silver, aluminum, iron, and nickel on fused silica; and a-Si:H on Si(100). In the case of pure materials the shape of the acoustic pulse and the phase velocity were determined. The dispersion of the SAW phase velocity observed for the film systems was used to extract information on the film thickness, density, and transverse and longitudinal sound velocity. Models for the theoretical treatment of film systems and the calculation of dispersion curves are presented.
Second-order polarizabilities of nitropyridine derivatives determined with electric-field-induced second-harmonic generation and a solvatochromic method: A comparative study
Authors: Bosshard Ch. , Knöpfle G. , Prêtre P. , Günter P.
Published: July 2002

A comparison is made between the second-order polarizabilities of nitropyridine derivatives determined with two different methods: electric-field-induced second-harmonic generation (EFISH) and a solvatochromic method. Both techniques yield the same values within the experimental errors, showing that the main contribution of the microscopic nonlinearity arises from a single intramolecular charge-transfer transition. Based on the quantum-mechanical two-level model, the solvatochromic method also yields important molecular parameters, such as transition and excited-state dipole moments. The second-order polarizabilities determined with the EFISH method are shown to follow the theoretical two-level dispersion.
Investigation of the transitional pore structure of activated carbon fibers by small-angle neutron scattering
Authors: Kieffer John
Published: July 2002

The small-angle neutron scattering technique was used to determine the geometry of transitional pores in activated carbon fibers, as a function of the degree of activation. A refined numerical method for the data reduction is introduced. This procedure allows one to determine the characteristic size of structural features from density-density correlation functions. Based on this analysis, the activation of the fibers can be described as an erosion process, producing a microporous granular structure. The largest structural features have a radius of gyration of approximately 260 Å. These define the pore structure. The size of these granules is reduced as the activation treatment progresses and the specific surface area increases. A smaller feature, with a radius of gyration of close to 80 Å, was also observed. The size of this structural element is not affected by the activation process. The description of the pore structure within the bulk of the fibers, as deduced from the scattering data, is consistent with scanning electron microscope pictures of the surface of the fibers.
Properties of indium tin oxide films prepared by the electron beam evaporation method in relation to characteristics of indium tin oxide/silicon oxide/silicon junction solar cells
Authors: Kobayashi H. , Ishida T. , Nakamura K. , Nakato Y. , Tsubomura H.
Published: July 2002

Indium tin oxide (ITO)/silicon oxide/silicon (Si) junction solar cells were produced by depositing ITO on a thin silicon oxide-covered single-crystal Si substrate using the electron-beam evaporation method. The current-voltage (I-V) characteristics strongly depended on the incident angle (θi) of the evaporated ITO vapor to the Si substrate during the ITO deposition, as well as the post-deposition heating temperature (Th) and the kind of the ambient gases during post-deposition heat treatment. The ITO films deposited at θi=0° and treated at Th=380 °C in air formed a high-energy barrier with p-Si, and formed ohmic contact with n-Si. X-ray diffraction analysis showed that the ITO films deposited at θi=0° contained metal indium. The amount of the metal indium decreased either by reducing the deposition rate of the ITO film or by raising the substrate temperature during the ITO deposition. The ITO films deposited at θi=45° and treated at Th=350∼450 °C in hydrogen, on the other hand, formed a high-energy barrier with n Si. In this case, no metal indium was observed in the ITO films. It is concluded that the formation of the metal indium in the ITO films changes their work functions, and thus its presence strongly affects the I-V characteristics of the ITO/silicon oxide/Si solar cells. Darkening observed for the ITO films deposited at θi=0° is also attributed to the presence of the metal indium.
Nucleation of reversed domains at grain boundaries
Authors: Schrefl T. , Schmidts H. F. , Fidler J. , Kronmüller H.
Published: July 2002

Micromagnetic analysis using a finite-element technique confirms that intergrain magnetostatic and exchange interactions drastically affect the magnetization reversal mechanism in nucleation controlled permanent magnets. The investigation of the nucleation fields of magnetically coupled grains emphasizes the important role of nonmagnetic boundary phases and well-aligned grains for the enhancement of coercivity. The long-range magnetostatic interactions between the grains reduce the coercive field of ideally oriented particles. The exchange coupling of misoriented neighboring grains causes a strongly inhomogeneous magnetic state near the interface of the particles which favors the nucleation of reversed domains. Intergrain exchange interactions considerably reduce the coercive field, especially if the angle between the easy axes of adjacent grains is high. An algorithm for adaptive mesh refinement controls the discretization error throughout the calculation of the demagnetization curves.
Stability and confinement of nonrelativistic sheet electron beams with periodic cusped magnetic focusing
Authors: Booske John H. , McVey Brian D. , Antonsen Thomas M.
Published: July 2002

Sheet electron beams focused by periodically cusped magnetic (PCM) fields are stable against low-frequency velocity-shear instabilities (such as diocotron mode). This is in contrast to more familiar unstable behavior in uniform solenoidal magnetic fields. Two rectangular-cross-section magnetic configurations capable of focusing in both transverse dimensions are investigated: (i) a closed-side two-plane PCM configuration that is topologically equivalent to conventional round-cross-section PPM focusing; and (ii) an open-side configuration that uses ponderomotive PCM focusing in the vertical plane and simple vzBy Lorentz force focusing in the horizontal plane. Both configurations are capable of stable sheet beam confinement. The open-side configuration appears more practical both for focusing and for realizing matched (cold) beam conditions in which the beam envelope is free from oscillations. For realistic beams with finite emittance, the existence of a matched cold beam solution implies less emittance growth at beam injection.
Microphotoluminescence and Raman scattering study of defect formation in diamond films
Authors: Bergman L. , Stoner B. R. , Turner K. F. , Glass J. T. , Nemanich R. J.
Published: July 2002

Photoluminescence and Raman spectroscopy are employed to explore the time evolution of defect formation in chemical vapor deposition diamond films for stages of growth spanning nucleation to continuous film formation. Our research is concentrated on three types of defects which give rise to the 1.68 eV optical band, the sp2 phase which centers at 1500 cm−1, and the broadband luminescence at 565–800 nm. The investigation of these types of defects suggests the following conclusions. Si atoms are most likely responsible for the creation of the 1.68 eV optical centers which takes place at the initial stages of growth. Plasma interactions with the Si substrate contribute to the 1.68 eV defect formation. The broad luminescence and sp2 bonding defects were not present in the isolated nuclei but were significantly present when a continuous film was formed. Two rates of diamond growth were obtained and the changes of the rates were attributed to the lowering degree of freedom available for the growth of the nuclei as well as to the formation of the sp2 phase.
Scanning tunneling microscopy studies on n-alkane molecules adsorbed on graphite
Authors: Couto M. S. , Liu X. Y. , Meekes H. , Bennema P.
Published: July 2002

The adsorption of n-alkanes on graphite has been studied by scanning tunneling microscopy. These molecules can form very well ordered layers on the graphite surface. The results suggest that a transition from an ordered to a disordered state of the molecules occurs as a function of temperature. A model for the structure of the adsorbed layer and a possible explanation for the temperature effects are given.
Nucleation and growth of platinum and nickel films on amorphous carbon substrates
Authors: Evans B. L. , Maaroof A. I. , Xu S.
Published: July 2002

The nucleation and growth of Pt and Ni films deposited on amorphous carbon (a-C) substrates by ion-beam sputtering (IBS) have been followed from an early stage through to complete substrate coverage. The influence of IBS parameters and substrate temperature Ts on deposition rate and metal particle (island) size distributions have been measured. Metal island growth is shown to be predominantly two dimensional in character. Soft-x-ray reflectivity measurements on Ni/a-C multilayers detect the single atom layer increments in Ni island thickness tNi′ during growth from the initial (critical) value tNi′ = 0.87 nm which is consistent with a theoretical 22 atom cluster model. The influence of the metal island growth stages on the soft-x-ray reflectivity of multilayer mirrors is considered.