bluuebunny/crossref_metadata_2025 huggingface dataset:
Interference of locally excited surface plasmons
Authors: Novotny L. , Hecht B. , Pohl D. W.
Published: July 2002

Surface plasmon interactions on a finite silver layer are theoretically investigated using a coupled dipole formalism. The studied system consists of several protruding particles located on the surface of the layer that are scanned with an optical probe. An optical scan-image of the silver surface is obtained by assigning the recorded far-field radiation to the momentary position of the optical probe. Both, probe and protrusions are considered as single dipolar particles. Interferences of the locally excited surface plasmons can be recorded by detecting the radiation emitted into the lower half-space at angles beyond the critical angle of total internal reflection (forbidden light). The resulting scan images show excellent agreement with recent experimental measurements. The theory of the coupled dipole formalism using Green’s functions of a layered reference system is outlined and electromagnetic properties of surface plasmons are discussed.
Structural and dielectric properties of epitaxial SrTiO3 films grown on Si(100) substrate with TiN buffer layer
Authors: Lee M. B. , Koinuma H.
Published: July 2002

SrTiO 3 thin films have been grown on Si(100) substrates by pulsed laser deposition using an epitaxial TiN film as a buffer layer, and their structural and dielectric properties were investigated as functions of deposition parameters. X-ray diffraction analysis showed that the SrTiO3 films were grown epitaxially in the wide range of substrate temperatures (400–650 °C) and ambient oxygen pressure (10−5 Torr–150 mTorr) with an orientation relationship of SrTiO3(100) // TiN(100) // Si(100) and SrTiO3〈010〉 // TiN〈010〉 // Si〈010〉. The crystallinity of the epitaxial films was improved with the increase of the substrate temperature and decrease of the ambient oxygen pressure, while the film surface morphology was degraded with increasing either of the two parameters. The relative dielectric constant of the films was revealed to depend both on the crystallinity and on the surface roughness of the films, the highest value of which was εr=270 at 1 MHz, comparable to that of bulk SrTiO3.
Schottky barrier formation in a Au/Si nanoscale system: A local density approximation study
Authors: Zavodinsky V. G. , Kuyanov I. A.
Published: July 2002

First-principles local density cluster calculations show that a Schottky junction may be formed in a nanoscale Au–Si(111) systems. The calculated Schottky barrier height value depends on the nanosystem’s geometry and varies from 0.5 to 1.35 eV. The total energy calculations show that the metallic gold/silicon system is unstable if the thickness of the gold is more than 2–3 monolayers.
Influence of aperture diameter on image contrast and resolution in scanning near-field optical microscopy
Authors: Schöfer J. , Gregor M. J. , Blome P. G. , Ulbrich R. G.
Published: July 2002

The influence of aperture diameter on image contrast and resolution in scanning near-field optical microscopy has been investigated. Evanescent standing wave fields at glass–air interfaces were probed with well-characterized aluminum-coated fiber tips. A strong decrease in image contrast was found when the aperture diameter exceeds a critical value. Numerical simulations with the concept of an intensity transfer function using the axial component of the Poynting vector inside the probe demonstrate that this phenomenon is the result of the increasing influence of higher-order waveguide modes on power transmission through the probe. It has been shown that standing wave patterns are convenient and reliable reference devices for individual probe characterization and quantification of their resolving power.
Henkel plots for thermally relaxing systems
Authors: Klik I. , Yao Y. D. , Chang C. R.
Published: July 2002

Henkel and δM plots for an array of thermally relaxing, interacting uniaxial particles are analyzed using master equation formalism. It is found that the peaks of δM depend on coupling strength only indirectly, through the stability of the demagnetized and remanent states and that an unstable initial demagnetized state results in δM≈0 even for a strongly coupled system.
Strain-induced magnetocrystalline anisotropy in Ni on Cu(001)
Authors: Wu Ruqian , Chen Lujun , Freeman A. J.
Published: July 2002

The origin of the observed perpendicular magnetic anisotropy (PMA) in Ni/Cu(001) for thick Ni layers is investigated using the ab initio full potential linearized augmented plane wave method. With the aid of the state tracking and torque approaches, very stable results are obtained for the magnetocrystalline anisotropy (MCA) energies for both the Ni/Cu(001) overlayer systems and the distorted bulk fct Ni. We find that the PMA is due mainly to the strain induced bulk contribution rather than to interfacial hybridization. The calculated value of the bulk MCA energy, 65 μeV/atom, is very close to recent experimental data extrapolated to zero temperature, 70 μeV/atom.
Theory of optical modes in semiconductor microdisk lasers
Authors: Wang Ruo Peng , Dumitrescu Maria-Manuela
Published: July 2002

A method for the calculation of optical modes in disk-like resonant cavities is presented. The eigenvalue equation for resonant wavelengths and threshold optical gain as well as the optical field distribution is established. The characteristics of the optical modes in microdisk lasers are studied using this method. Numerical calculations are carried out for some long wavelength microdisk lasers. The theoretical analyses and numerical results show that the optical modes in a microdisk laser with a very small thickness can be approximately classified into transverse electric and transverse magnetic modes, but the lasing modes and other lower threshold modes are transverse electric polarized, and a spontaneous emission coefficient between 0.2 and 0.3 can be achieved in long wavelength semiconductor microdisk lasers. The possibility of a microdisk laser to lase in a surface emitting mode is also demonstrated.
Phonon dispersion and electron–polar optical phonon interaction in coupled quantum-well structures in the modified image–charge ansatz approach
Authors: Wang Jin , Leburton J.-P. , Pozela J.
Published: July 2002

Phonon dispersion and Fröhlich interaction in coupled quantum-well structures are derived by using a modified image–charge ansatz that shows that the mode symmetry in the surface phonon dispersion of single quantum-well (QW) structures is modified by the addition of a thin inner barrier. The electron–phonon scattering rates are evaluated for phonons in the entire coupled QW configuration and compared with the corresponding rates calculated for phonons in the separated-single QW approximation. Our calculations show that the separated-single QW phonon model provides reasonable estimates of the electron–phonon scattering rates except for intersubband processes when the quantized energy separation between the initial and final electron states is in resonance with the polar optical phonon energy.
Charge carrier recombination in organic bilayer electroluminescent diodes. II. Experiment
Authors: Tak Y.-H. , Bässler H.
Published: July 2002

Diodes consisting of a hole transporting layer containing tri-stilbeneamine as active medium and a hole blocking layer (an oxadiazole blend) sandwiched between indium-tinoxide and aluminium electrodes have been characterized via both their j(F) characteristics and the dependence of intensity, quantum efficiency, and spectrum of the electroluminescence on the thickness Le of the hole blocking layer. The results are discussed within the framework of the analytic model advanced in the preceding paper. Both the decrease of the effect of electric field screening at the anode due to the space charge accumulation at the internal interface as well as the recombination efficiency with decreasing Le is in good agreement with theory.
Activation volumes and interparticle interaction effects in nanostructured Fe networks
Authors: Butera A. , Weston J. L. , Barnard J. A.
Published: July 2002

We have studied the film thickness dependence of the remanent magnetization (delta M plots), the magnetic viscosity, and the activation volumes of Fe thin films sputtered on commercial nanochannel alumina of 20 nm average pore diameter. The films have the form of a contiguous interconnected network. A critical film thickness of φ∼20 nm defines two regions with very different magnetic behavior. For this critical value of φ we have found a maximum in the remanent coercivity, a minimum in the interparticle interaction parameter α, and a rapid increase in the magnetic viscosity, the irreversible susceptibility and the activation volume. Our results are interpreted in terms of the pinning effects caused by nonmagnetic inclusions. The functional relationship between the remanent coercivity and the activation volume is similar to the one found in long cylinders that reverse magnetization by a curling mechanism.
The binding energies of shallow donor impurities in GaAs quantum-well wires under applied electric fields
Authors: Montes A. , Duque C. A. , Porras-Montenegro N.
Published: July 2002

Using a variational procedure, within the effective mass-approximation, we calculate the binding energy of a shallow-donor impurity in a rectangular cross-sectional area of a GaAs quantum-well wire, under the action of an electric field applied perpendicular to one of the interfaces, assuming an infinite-confinement potential. We study the binding energy of the donor impurity as a function of the system geometry, the applied electric field, and the donor-impurity position. It was found that the presence of the electric field breaks down the degeneracy of the states for impurities symmetrically positioned within the structure. Our results for a large length of one side of the cross-sectional area coincide quite well with previous results in quantum wells. We show, unambiguously, that the impurity binding energy depends strongly not only on quantum confinement, but also on the applied electric field and on the distribution of impurities inside the quantum-well wires; these aspects must be taken into account in the quantitative understanding of optical phenomena related to shallow impurities when an electric field is applied.
Hot-electron-induced quasibreakdown of thin gate oxides
Authors: Umeda Kazunori , Taniguchi Kenji
Published: July 2002

An oxide-breakdown mode induced by hot-electron injection into an oxide is reported for n-channel metal–oxide–semiconductor field-effect transistors with 6 nm thick gate oxides. The diameter of the current path formed after breakdown was estimated to be less than 10 nm based on spreading-resistance measurements. The measured charge to breakdown was 102–104 C/cm2 in an oxide field of 5–7 MV/cm. This value is much smaller than that extrapolated from the results of Fowler–Nordheim (FN) tunneling experiments. The charge to breakdown in the hot-electron injection method decreased as the electric field increased in a silicon substrate. This was quite different from the case with FN tunneling injection, where the oxide field plays a significant role in oxide breakdown. Good correlation between the calculated electron energy in the oxide and the electric field in the silicon substrate suggests that the difference between hot-electron injection and FN tunneling can be explained in terms of the average electron energy in the oxide.
Reduction of time-averaged irradiation speckle nonuniformity in laser-driven plasmas due to target ablation
Authors: Epstein R.
Published: July 2002

In inertial confinement fusion (ICF) experiments, irradiation uniformity is improved by passing laser beams through distributed phase plates (DPPs), which produce focused intensity profiles with well-controlled, reproducible envelopes modulated by fine random speckle. [C. B. Burckhardt, Appl. Opt. 9, 695 (1970); Y. Kato and K. Mima, Appl. Phys. B 29, 186 (1982); Y. Kato et al., Phys. Rev. Lett. 53, 1057 (1984); Laboratory for Laser Energetics LLE Review 33, NTIS Document No. DOE/DP/40200-65, 1987 (unpublished), p. 1; Laboratory for Laser Energetics LLE Review 63, NTIS Document No. DOE/SF/19460-91, 1995 (unpublished), p. 1.] A uniformly ablating plasma atmosphere acts to reduce the contribution of the speckle to the time-averaged irradiation nonuniformity by causing the intensity distribution to move relative to the absorption layer of the plasma. This occurs most directly as the absorption layer in the plasma moves with the ablation-driven flow, but it is shown that the effect of the accumulating ablated plasma on the phase of the laser light also makes a quantitatively significant contribution. Analytical results are obtained using the paraxial approximation applied to the beam propagation, and a simple statistical model is assumed for the properties of DPPs. The reduction in the time-averaged spatial spectrum of the speckle due to these effects is shown to be quantitatively significant within time intervals characteristic of atmospheric hydrodynamics under typical ICF irradiation intensities.
Evidence of continuous trap distribution in the glow curve of a Brown microcline
Authors: Sakurai Takao , Gartia Raj Kishor
Published: July 2002

The formidable task of determining a mathematical form of the trap distribution in a solid has been solved for the case of an unusually broad thermoluminescence (TL) glow peak of a Brown microcline (feldspar, triclinic form of KAlSi3O8). An exponential distribution of traps gives the best fit to the experimental glow peak. Five TL glow peaks of the Brown microcline which was previously irradiated with γ-rays and then subjected to fading at room temperature are numerically analyzed using the exponential distribution model of traps. The dependence of the best-fit trap parameters are found: (1) the width and the characteristic depth of the trap distribution; (2) the rates of recombination and retrapping of the released electrons from the traps; and (3) the concentration of thermally disconnected traps on the fading time. These trap parameters show a clear dependence on the time of fading. The procedure of the TL analysis using a continuous trap distribution is also reported in detail.
Improved understanding of thermally activated structural changes in Al/SiOx/p-Si tunnel diodes by means of infrared spectroscopy
Authors: Bierhals Andreas , Aberle Armin G. , Hezel Rudolf
Published: July 2002

Thermal treatment at temperatures well above 100 °C is known to lead to a degradation of the current–voltage (I–V) characteristics of Al/SiOx/p-Si metal–insulator–semiconductor (MIS) tunnel diodes. In the present work, the structural changes caused in these devices by annealing at temperatures around 300 °C are investigated by means of grazing internal reflection (GIR) infrared spectroscopy, I–V measurements, and scanning electron microscopy. While all previous studies attributed the structural changes to a single chemical process, we show that at least three different processes occur: the reduction of SiOx by Al, the diffusion of aluminum or oxygen through the tunnel insulator, and the formation of Al spikes through the tunnel insulator. The first two processes lead to significant changes in the Al–O and Si–O bond concentrations in the tunnel insulator, but the impact on the I–V characteristics of the MIS tunnel diode is negligible. In contrast, the third process leads to a drop of the baseline of the GIR spectra and to a significant degradation or even a complete destruction of the diode characteristics.
Photoacoustic determination of non-radiative carrier lifetimes
Authors: Marín E. , Riech I. , Díaz P. , Alvarado-Gil J. J. , Baquero R. , Mendoza-Alvarez J. G. , Vargas H. , Cruz-Orea A. , Vargas M.
Published: July 2002

From photoacoustic (PA) experiments we determine the nonradiative carrier lifetime in direct band-gap semiconductors. We use the Rosencwaig and Gerscho model to calculate the PA signal in semiconductors taking into account the distinction between non-radiative and radiative carrier lifetimes. We have assumed that for our high quality crystalline samples, the main contribution to the non-radiative processes comes from CHCC and CHSH Auger recombination for n and p-type materials, respectively. For GaAs, InSb and GaSb samples, the experimental data obtained by means of an open photoacoustic cell were fitted to the theoretical model and we show that the values we determined for the non-radiative recombination lifetime agree well with those reported in the literature.
Thickness effect on ferro/antiferromagnetic coupling of Co/CrMnPt systems
Authors: Nishioka Koichi , Shigematsu Satoshi , Imagawa Takao , Narishige Shinji
Published: July 2002

To investigate ferro/antiferromagnetic coupling in Co/CrMnPt layers of spin valve films, spin valve films of glass/Ta/NiFe/CoFe/Cu/Co 30 Å/CrMnPt D Å/Ta 30 Å were made with various CrMnPt thicknesses indicated D. Shift fields of the hysteresis loop indicated Hp and coercivities indicated Hc were evaluated. With the increase of D from 150 to 1000 Å, the blocking temperature increases from RT to 340 °C. By increasing the temperature, the coercivity Hc increases, peaks at around the blocking temperature and then decreases. The temperature at which Hc peaks increases with an increase of D. The maximum value of Hc decreases with the increase of D. The grain size distribution of CrMnPt was measured and temperature dependencies of Hp and Hc were calculated based on a thermal fluctuation model using the grain size distribution. Quantitatively, the calculated results agree with the experimental results. The change of temperature dependence of Hp and Hc are explained by the change of the local blocking temperature distribution. With the increase of D, the mean blocking temperature becomes higher and the local blocking temperature shows narrower distributions. The anisotropy constant of the antiferromagnet Ka0, which is a fitting parameter, increases with the increase of D and saturates at above 400 Å.
Urbach’s tails in the absorption spectra of CuInTe2 single crystals with various deviations from stoichiometry
Authors: Wasim S. M. , Marı́n G. , Rincón C. , Sánchez Pérez G. , Mora A. E.
Published: July 2002

The optical absorption coefficient α near the band edge is found to vary exponentially with photon energy in several samples of CuInTe2 single crystals. This behavior, where the extrapolation of ln α vs hυ at different temperatures converges to a single point, is consistent with Urbach’s rule. The temperature dependence of Urbach energy KT/σ, calculated from the steepness parameter σ, agrees with the theoretical model that takes into account the contribution of both electron-phonon interaction and structural disorder. It is established that the observed phonon energy higher than the average value calculated from the reported optical modes is due to structural disorder which is caused by the compositional deviation from the ideal stoichiometry.
The threshold effect of incident light intensity for the photorefractive light-induced scattering in LiNbO3:Fe,M (M=Mg2+, Zn2+, In3+) crystals
Authors: Zhang Guoquan , Zhang Guangyin , Liu Simin , Xu Jingjun , Sun Qian , Zhang Xinzheng
Published: July 2002

Based on the two-center model and the multi-three-wave mixing model, we theoretically study the possible origins of the threshold effect of the incident light intensity for the photorefractive light-induced scattering in LiNbO3:Fe,M, (M=Mg2+, Zn2+, In3+) crystals. Our results show that the value of the threshold intensity of incident light is strongly dependent on the dark conductivity of the crystal, and the maximum total fanning noise is dependent on the concentration of Fe ion on the Li site. These two factors together determine the threshold behavior of incident light intensity for the photorefractive light-induced scattering in LiNbO3:Fe,M crystals when the incident light intensity is in moderate range.
Thermodynamics of polydomain heterostructures. III. Domain stability map
Authors: Alpay S. Pamir , Roytburd Alexander L.
Published: July 2002

A map showing regions of stability of possible domain structures and relative fractions of domains in a polytwin structure is developed for epitaxial heterostructures with active layers which undergo a cubic–tetragonal or a tetragonal–orthorhombic transformation. This map, which is also applicable to epitaxial film–substrate systems, shows the dependence of the polytwin structure on misfit strain, lattice parameters of the product phase, and mechanical stress. A uniaxial stress field applied during cooling down from the growth temperature strongly affects the domain structure selection and perfect single-domain structures may be obtained with such fields. For polytwin layers with thicknesses close to the critical thickness for domain formation, microstresses must be taken into consideration and the domain stability map is modified accordingly. Misfit dislocation generation at the deposition temperature is taken into account through a temperature dependent effective substrate lattice parameter. As examples, PbTiO3(001) films grown on MgO(001) and SrTiO3(001) substrates are analyzed. Theoretical predictions are in good agreement with experimental results from the literature. The maps for epitaxial BaTiO3(001) films grown on various substrates are also included. Experimental observations for YBa2Cu3O7−x films strongly support the ideas developed in this paper.
Quantification of magnetic force microscopy images using combined electrostatic and magnetostatic imaging
Authors: Gomez R. D. , Pak A. O. , Anderson A. J. , Burke E. R. , Leyendecker A. J. , Mayergoyz I. D.
Published: July 2002

A method for calibrating the force gradients and probe magnetic moment in phase-contrast magnetic force microscopy (MFM) is introduced. It is based upon the combined electrostatic force microscopy EFM and MFM images of a conducting non magnetic metal strip. The behavior of the phase contrast in EFM is analyzed and modeled as a finite area capacitor. This model is used in conjunction with the imaging data to derive the proportionality constant between the phase and the force gradient. This calibration is further used to relate the measured MFM images with the field gradient from the same conducting strip to derive the effective magnetic moment of the probe. The knowledge of the phase-force gradient proportionality constant and the probe’s effective moment is essential to directly quantify field derivatives in MFM images.
Structure and magnetic properties of SmCo7−xZrx alloys (x=0–0.8)
Authors: Huang M. Q. , Wallace W. E. , McHenry M. , Chen Q. , Ma B. M.
Published: July 2002

The alloys with composition of SmCo7−xZrx(x=0–0.8) were synthesized and characterized in the temperature range of 10–1273 K and at fields up to 5 T. The experimental results show that a small amount of Zr substitution can contribute to a stabilization of the TbCu7 structure, and improve magneto-anisotropy Ha from 90 kOe for x=0–180 kOe for x=0.5 at room temperature, and from 140 kOe for x=0–300 kOe for x=0.5 at 10 K. It is probable that Zr may partly replace a dumbbell of Co atom pair in these alloys. The phase transition between CaCu5, TbCu7, Th2Zn17, and Ce2Ni7 at different heat treatment conditions was also discussed.
Effect of surface roughness on magnetization reversal of Co films on plasma-etched Si(100) substrates
Authors: Li M. , Zhao Y.-P. , Wang G.-C. , Min H.-G.
Published: July 2002

Co films ∼970 Å thick were deposited, simultaneously, on ten plasma-etched Si(100) substrates with various etch times t. The surface morphologies and magnetic properties of the Co films were measured by atomic force microscopy (AFM) and magneto-optic Kerr effect (MOKE) technique. The analysis of the AFM images shows that as the etch time t increased from 0 to 100 min, the vertical interface width w increased from ∼5 to ∼1400 Å; the lateral correlation length ξ increased from ∼300 to ∼10 500 Å. The MOKE measurements provided the in-plane azimuthal angular dependence of the hysteresis loops and the change of loop shapes with the surface roughness. It was found that the magnetization reversal process changed with the surface roughness. Magnetization rotation dominated the magnetization reversal for the smoothest films. As the films roughened, the domain-wall pinning set in, eventually dominating the magnetization reversal for the roughest films. Additionally, the magnetic uniaxial anisotropy in the Co films disappeared as the roughness parameters increased. It was also found from MOKE that the surface roughness strongly affected the coercivity.
Electrically active point defects in n-type 4H–SiC
Authors: Doyle J. P. , Linnarsson M. K. , Pellegrino P. , Keskitalo N. , Svensson B. G. , Schöner A. , Nordell N. , Lindström J. L.
Published: July 2002

An electrically active defect has been observed at a level position of ∼0.70 eV below the conduction band edge (Ec) with an extrapolated capture cross section of ∼5×10−14 cm2 in epitaxial layers of 4H–SiC grown by vapor phase epitaxy with a concentration of approximately 1×1013 cm−3. Secondary ion mass spectrometry revealed no evidence of the transition metals Ti, V, and Cr. Furthermore, after electron irradiation with 2 MeV electrons, the 0.70 eV level is not observed to increase in concentration although three new levels are observed at approximately 0.32, 0.62, and 0.68 eV below Ec with extrapolated capture cross sections of 4×10−14, 4×10−14, and 5×10−15 cm2, respectively. However, the defects causing these levels are unstable and decay after a period of time at room temperature, resulting in the formation of the 0.70 eV level. Our results suggest strongly that the 0.70 eV level originates from a defect of intrinsic nature. The unstable behavior of the electron irradiation-induced defects at room temperature has not been observed in the 6H–SiC polytype.
The Raman spectrum of Ti3SiC2
Authors: Amer M. , Barsoum M. W. , El-Raghy T. , Weiss I. , Leclair S. , Liptak D.
Published: July 2002

In this paper, the Raman spectrum of Ti3SiC2 is reported and compared with that of TiC0.67. All the TiC0.67 first order Raman disorder-induced modes are active, but shifted, in Ti3SiC2. Two additional peaks at 150 and 372 cm−1 are observed in Ti3SiC2. The former is ascribed to a shear mode between the Si and Ti planes; the origin of the latter is unknown. No second order Raman bands are detected. Micro-Raman spectroscopy also reveal the presence of ≈50 Å graphite crystallites in samples hot pressed in graphite dies—these crystallites are not detected in samples processed by hot isostatic pressing in molten glass containers.
Polarized cathodoluminescence study of selectively grown self-assembled InAs/GaAs quantum dots
Authors: Rich D. H. , Tang Y. , Konkar A. , Chen P. , Madhukar A.
Published: July 2002

We have examined the optical properties of self-assembled InAs quantum dots (QDs) grown on prepatterned GaAs(001) substrates with polarization sensitive and time-resolved cathodoluminescence (CL) imaging and spectroscopy techniques. The InAs QDs were formed using a novel application in self-assembled molecular beam epitaxial growth, which entailed the growth of InAs on preformed [11̄0]-oriented stripe mesas. Interfacet In adatom migration occurred along the stripe side-walls during growth, enabling the selective formation of linear arrays of InAs QDs on the stripe mesas. The total InAs deposition needed to induce the two-dimensional to three-dimensional morphology change on the stripes is less than that required to initiate QD formation on the unpatterned substrates. The QDs formed on the mesa top were found with a luminescence distribution redshifted relative to QDs in the valley region, indicating that QDs with a larger average size were formed on the mesa top. The lower density of QDs in the valley region led to a weaker emission and sharper δ-like transitions at lower beam currents, relative to emission from QDs on the mesa. CL imaging was employed to study the spatial distribution of luminescence and identified the presence of relatively small QDs situated near the edges of the valley region along the lower surface of the stripe edges. An excitation- and energy-dependent polarization anisotropy relative to the 〈110〉 directions was observed in CL emission. A polarization anisotropy reversal was found between CL from QDs on the mesa and in the valley regions, revealing the importance of the stress anisotropy in both the formation of QDs and their subsequent optical properties. The CL was examined as a function of temperature to evaluate the thermal re-emission of carriers and the associated activation energies. The carrier relaxation kinetics were studied with time-resolved CL to measure differences in the CL onset and decay rates for QDs in the valley and mesa regions.
Electron stimulated decomposition of adsorbed hexafluoroacetylacetonate Cu(I) vinyltrimethylsilane, Cu(I)(hfac)(vtms)
Authors: Mezhenny S. , Lyubinetsky I. , Choyke W. J. , Yates J. T.
Published: July 2002

The effect of a low energy electron beam on the decomposition of hexafluoroacetylacetonate Cu(I) vinyltrimethylsilane (Cu(I) precursor) on the Si(111)-(7×7) surface at room temperature has been studied using x-ray photoelectron spectroscopy and Auger electron spectroscopy. It was found that electron bombardment causes the decomposition of the Cu(I) precursor and an electron energy threshold for decomposition was found to be 4±0.5 eV. This result demonstrates that the decomposition of the Cu(I) precursor in the threshold region occurs through a dissociative electron attachment mechanism. Elemental composition studies of the grown films show that they are contaminated with carbon, and that the copper fraction is increased in the film compared to the precursor composition.
Experimental study on magnetic and heating characteristics of magnetic wood
Authors: Oka Hideo , Fujita Hiromitu
Published: July 2002

Magnetic wood, wood which demonstrates magnetic characteristics, was introduced by the Oka group in 1991. Based on previous studies, we have proposed that magnetic wood can be used as a heating board by induction heating, applying a high frequency magnetic field. This article covers experimental magnetic and heating characteristics for three types of magnetic wood: an impregnated type (the wood is impregnated with a water-based magnetic fluid), a powder type (Mn–Zn ferrite powder and wood powder are mixed and pressed into boards), and a coating type (Mn–Zn ferrite powder is coated onto a fiber board). Our results showed that the coating-type magnetic wood had the highest surface temperature when using a solenoid excitation coil. In this case the surface temperature rose 50 °C. The heating board we propose can be used in furniture and medical equipment that require a warm and smooth surface.
Maximum entropy principle for hydrodynamic transport in semiconductor devices
Authors: Trovato M. , Reggiani L.
Published: July 2002

A hydrodynamic (HD) transport approach based on a closed system of balance equations is developed from the maximum entropy principle. By considering a nonlinear expansion with respect to a local thermodynamic equilibrium, we determine an analytic expression for the distribution function as a function of macroscopic quantities such as density, velocity, energy, deviatoric stress, heat flux associated with charge carriers. From the determined distribution function and considering the collision interactions of carriers with phonons, all the constitutive functions appearing in the fluxes and collisional productions of the balance equations are explicitly calculated. The analytical closure so obtained is applied to the case of some n+nn+ submicron Si structures. Numerical HD calculations are found to compare well with those obtained by an ensemble Monte Carlo simulator thus validating the approach developed here.
Low-frequency noise in a thin active layer α-Si:H thin-film transistors
Authors: Chen X. Y. , Deen M. J. , van Rheenen A. D. , Peng C. X. , Nathan A.
Published: July 2002

Low-frequency noise of hydrogenated-amorphous-silicon (α-Si:H) thin-film transistors (TFTs) with a thin active layer and an inverted staggered device structure operating in the conducting mode has been investigated. Pure 1/f-noise spectra were observed. The results show that the physical location of the noise in α-Si:H TFTs is different from that in crystalline metal–oxide–semiconductor field-effect transistors. The noise contributions from the channel and interface have been determined for the device operating in different modes. The 1/f noise of α-Si:H TFTs stems from the channel when the device is operated in the linear region at high gate voltages. However, the 1/f noise of α-Si:H TFTs generated at the interface becomes significant when the device is operated in the saturation region. The interface noise can be explained by the number fluctuation model (ΔN model). The channel noise can be explained by either the ΔN model or the mobility fluctuation model (Δμ model).
A diffusion model for picosecond electron bunches from negative electron affinity GaAs photocathodes
Authors: Hartmann P. , Bermuth J. , v. Harrach D. , Hoffmann J. , Köbis S. , Reichert E. , Aulenbacher K. , Schuler J. , Steigerwald M.
Published: July 2002

Even though theoretical estimates predict response times for the photoemission process of electrons from a negative electron affinity GaAs photoemitter in excess of hundreds of picoseconds, recent measurements found electron bunch durations of 40 ps or less. This work presents precise measurements of picosecond electron bunches from a negative affinity bulk GaAs photocathode and develops a model which explains the measured bunch durations as well as the observed bunch shapes. The bunch shape turns out to be independent from the quantum efficiency of the photoemitter.
CdZnTe pixel array detectors and implications for producing large volume gamma-ray spectrometers
Authors: Brunett B. A. , Van Scyoc J. M. , James R. B. , Schlesinger T. E.
Published: July 2002

In this article we show that the behavior of new devices fabricated from horizontal Bridgman CdZnTe reported recently can be explained by the same conventional electrostatics arguments used to analyze detectors made from high pressure Bridgman material, and no qualitative differences in the material or contacts are necessary to explain the behavior of these new devices. Our work is an extension of the results obtained with a similar device geometry fabricated on high pressure Bridgman material. In addition we discuss the possibility of extending the design concepts learned here to fabricate large volume spectrometers based on a pixel array design.
Strain splitting of the Γ5 and Γ6 free excitons in ZnO
Authors: Reynolds D. C. , Look D. C. , Jogai B. , Litton C. W. , Collins T. C. , Harris M. T. , Callahan M. J. , Bailey J. S.
Published: July 2002

High-quality ZnO crystals have been grown by vapor-phase techniques and by the hydrothermal method. Depending on the surface preparation technique, some hydrothermally grown crystals contain strain. These strains result in energy shifts of the free excitons as well as relaxation of the selection rules. The Γ6 unallowed exciton is observed in these samples without the application of a magnetic field. The Γ6 exciton is also observed to split in a strain field, consistent with the Γ9 symmetry for the top valence band in ZnO. The Γ5 and Γ6 excitons have been observed to split in the strain field. The splitting is believed due to combined strain and electron–hole spin exchange.
Thermal diffusivity measurement of polymeric thin films using the photothermal displacement technique. II. On-wafer measurement
Authors: Hu Chuan , Ogawa Ennis T. , Ho Paul S.
Published: July 2002

A three-dimensional analytical solution as well as experimental verification of the thermoelastically induced deformation in a substrate-constrained thin polymeric film have been developed. In this model, the elastic deformations of the two layers are treated separately since the typical polymer has a much smaller Young’s modulus but a much larger out-of-plane coefficient of thermal expansion than a silicon wafer. Results from measurements of biphenyl tetracarboxylic dianhydride-p-phenyl diamine thin films on silicon wafers agree very well with calculations based on this analysis, especially by using the sensitive frequency modulation method as outlined in part I of this two-part series. Thus, this development of a two-layer thermoelastic deformation model and measurement has advanced this technique to a thinner thickness scale with improved precision. This method is also amenable to studies of the interfacial thermal resistance Rt, and numerical analysis shows that typical interfacial thermal resistances in thin films should be detectable at high chopping frequencies. Additional results on the temperature field and thermoelastic deformation, both within the film and substrate as well as at the film surface, are also discussed.
Ultraviolet emission and microstructural evolution in pulsed-laser-deposited ZnO films
Authors: Im S. , Jin B. J. , Yi S.
Published: July 2002

ZnO thin film has been deposited on a sapphire (001) at a temperature of 400 °C using a pulsed laser deposition with oxygen pressures of 1, 20, 50, 200, 300, and 500 mTorr. As the oxygen pressure for the thin film deposition increases, the grain size of the films increases up to a certain point, but then the size decreases over 50 mTorr as measured by x-ray diffractometry and scanning electron microscopy. In contrast, the intensity of UV photoluminescence continuously increases with the oxygen pressure. This is probably because the stoichiometry of oxygen-deficient ZnO film is improved by increasing oxygen pressure. It is concluded that there exist two different mechanisms for ZnO formation according to applied oxygen pressures, and that the UV luminescence intensity strongly depends on the stoichiometry in the ZnO film rather than the grain size of the thin film.
Influence of base pressure on FeMn exchange biased spin-valve films
Authors: Mao M. , Cerjan C. , Law B. , Grabner F. , Vaidya S.
Published: July 2002

The influence of base pressure, Pb, on physical properties of FeMn exchange biased spin-valve films was studied by varying Pb over two decades from 3×10−8 to 7×10−6 Torr. Giant magnetoresistive value shows a slight increase with increasing Pb until a large decrease occurs at Pb>3.3×10−6 Torr. Exchange bias field and blocking temperature remain constant in the base pressure range between 3×10−8 and 5×10−7 Torr before a large reduction appears. Our results indicate an upper limit for base pressure, Pbu≈5×10−7 Torr, above which significant spin-valve performance modification and deterioration in the crystallographic texture begin as a result of the contamination both at the ferromagnetic/antiferromagnetic interface and in the bulk of FeMn layer.
Influence of nitrogen and temperature on the deposition of tetrahedrally bonded amorphous carbon
Authors: Kleinsorge B. , Ferrari A. C. , Robertson J. , Milne W. I.
Published: July 2002

The effect of nitrogen addition on the properties of tetrahedral amorphous carbon (ta-C) has been studied. The ta-C is deposited by a filtered cathodic vacuum arc. The effect of introducing nitrogen on its plasma was measured by a retarding field analyzer and optical emission spectroscopy. The ta-C:N films were studied as a function of nitrogen content, ion energy, and deposition temperature. The incorporation of nitrogen was measured over the range of 10−2–10 at. % by secondary ion mass spectrometry and elastic recoil detection analysis. The N content was found to vary slightly sublinearly with the N2 partial pressure during deposition. A doping regime was found for N contents of up to 0.4 at. %, in which the conductivity changes while the sp3 content and the optical band gap remain constant. For 0.4%–8% N, the sp3 fraction remains above 80% but the optical gap closes due to a clustering of sp2 sites. Only above about 10% N, the sp3 fraction falls. The influence of nitrogen on the a-C was found to be independent of ion energies between 20 and 220 eV. Deposition above 200 °C causes a sudden loss of sp3 bonding. Raman and optical gap data show however that existing sp2 sites begin to cluster below this temperature.
A cluster variational theory of the hard sphere crystal
Authors: Firey B. , Ashcroft N. W.
Published: July 2002

The crystalline phase of a system of classical rigid spheres is treated using an adaptation of the Kikuchi cluster-variation approximation in the variational formulation of Morita. The theory is developed for an arbitrary choice of basic clusters, but the numerical calculations that are presented are carried out for two sets of basic clusters, namely nearest neighbor pairs and a combination of isosceles and equilateral triangles. The integrations required in the evaluation of the clusters are performed using Monte Carlo techniques, which permit the use of moderately large clusters, and in consequence fairly general parametrizations of the density functions. No symmetry conditions need to be imposed beyond those already inherent in an assumed fcc crystal. Numerical results are given for the entropy and density functions, the latter being presented in a parametrized form suitable for use in a variational calculation which takes the hard sphere crystal as a reference system.
Quenching of CN(A 2Π, v=0,1) by O2, H2, N2, NO, and CO2
Authors: Taherian M. R. , Slanger T. G.
Published: July 2002

Quenching of CN(A) v=0,1 is measured for several molecules (O2, N2, H2, NO, and CO2). The possible pathways are discussed. C2N2 is photodissociated by radiation from an excimer laser, and the radiation from the CN(A−x) bands is then observed. (AIP)
Brownian motion near a plane absorber: Nonanalyticity of the boundary layer solutions to the Fokker–Planck equation
Authors: Mayya Y. S.
Published: July 2002

Using the method of asymptotics, a systematic solution is developed for the velocity distribution in the boundary layer, governed by the one dimensional Fokker–Planck equation for the steady-state Brownian motion near a plane absorber. From the formal solution to the Fokker–Planck equation, a set of integral equations are constructed and solved for the unknown part of the boundary layer distribution, denoted by f (0,u). It is found that f (0,u) can be uniquely expanded as (−u)1/2 exp(−u2/4) times an analytic power series in the interval u∈ (0,−∞), while it is constrained to be zero for positive values of u. From this it is shown that the number density function, N(ξ)∼N0+1.023 a0ξ1/2+O(ξ), for small values of the dimensionless distance ξ from the boundary where N0, a0 are nonzero positive constants. This gives rise to a singularity in the density gradient at the boundary, leading to the vanishing of the corresponding diffusion coefficient defined in a Fickian sense. These results thus provide a quantitative basis for similar conclusions [Y. S. Mayya and D. C. Shani, J. Chem. Phys. 79, 2302 (1983)] drawn earlier.
The determination of hydrogen coordinates in lanthanum nicotinate dihydrate crystals by Gd+3–proton double resonance
Authors: Fields Renny A. , Hutchison Clyde A.
Published: July 2002

We have given a description of the EPR spectrum of Gd+3 located at a crystal site with no spatial symmetry by means of a spin Hamiltonian that fits our 833 measurements of ‖B0‖ required for resonance, with B0 in various directions in three mutually perpendicular crystal planes, with a root mean square deviation 11.4 G. We have shown that by use of the EPR absorption by Gd+3 at this site ENDOR spectra that provide values of proton coordinates for the Gd+3 water ligands can be obtained. By adjustment of coordinate values in a simplified model of the physics of the Gd+3–proton interaction, to give a fit to a simple six-parameter spin Hamiltonian that we used to describe our ENDOR data, we obtained the proton coordinates in a crystal fixed axis system with origin at the Gd+3 site. These coordinates had standard deviations in the range 0.051–0.099 Å and gave values of the Gd+3–proton separation distances with standard deviations in the range from 0.044 to 0.060 Å. We have made comparisons of values of Ln+3–proton and proton–proton separation distances obtained by this method for the present Gd+3 case with corresponding values for the previously studied Nd+3 case, and with x-ray data for the rare earth nicotinates. The comparisons show agreements within the uncertainties of the methods and demonstrate the reliability of the values that have been obtained in the present study. We have also employed a very simple experimental procedure for measuring values of coordinates of protons in the vicinity of Gd+3. In this procedure the ENDOR frequency is measured for only one direction of B0, namely that direction for which the shift of the frequency from the free proton frequency is maximum. This measured frequency, together with the values of the direction cosines of B0 in a crystal fixed axis system with Gd+3 at its origin, gives the values of: (a) the three coordinates of the proton; and of (b) its distance from Gd+3 with estimated standard deviations, 0.10 and 0.01 Å, respectively. The values obtained by this very simple technique for proton coordinates, Gd+3–proton separation distances, and interproton distances in the H2O ligands, are in agreement with their values obtained by the first described method, within the somewhat larger uncertainties associated with the second method. The peculiar merits of the Gd+3 ion for use in magnetic resonance crystallography have been reviewed in this article. The demonstrations that reliable structural information can be obtained in spite of the very considerable complications introduced in the spectra by (a) the fine structure, and by (b) the occurrence of the Gd+3 at a site in the molecule with no spatial symmetry that would simplify interpretation of the spectra, i.e., the type of site at which in general Gd+3 would be bound to a protein, are thus of special significance.
The libron–phonon coupling and the IR absorption spectra of diatomic molecules in a simple liquid
Authors: Garzón I. L. , Blaisten-Barojas Estela , Fujita S.
Published: July 2002

The IR absorption coefficient for diatomic impurities dissolved in liquid solutions is calculated from a 3D model Hamiltonian. The liquid atoms surrounding the impurity form a cage that hinders strongly the molecular rotation reducing it to a librational motion. The coupling between the collective motion of the liquid and the impurity-librational motion generates the libron–phonon interaction. The shifts and broadenings of the IR spectral lines are calculated on the basis of linear response theory with a proper connected diagram analysis. Recent experimental IR measurements for CO and ClF in liquid argon are analyzed in terms of the present theory and the strength of the libron–phonon coupling for such systems is reported.
Resonance Raman scattering from molecules with a nonequilibrium vibronic distribution
Authors: Fujimura Y. , Ohtsuki Y. , Arai M. , Lin S. H.
Published: July 2002

Expressions for the differential cross section of the resonance Raman scattering from molecules in which initially prepared nonequilibrium vibronic levels undergo vibrational and/or electronic relaxations are derived by using the generating function method. Two kinds of the initial nonequilibrium vibronic distributions, single vibronic level and Poisson ones are taken into account. A displaced harmonic oscillator model for the initial and resonant vibrational states and a linear vibrational quantum number dependence of the relaxation constants are adopted in deriving the cross sections. Model calculations of the time-resolved resonance Raman scattering cross sections and of the excitation profiles of the nth order Stokes and anti-Stokes Raman bands are performed by using the derived expressions. The results of the calculation indicate that the anti-Stokes Raman bands make a significant contribution to the resonance Raman scattering from the nonequilibrium vibronic distributions. It is suggested that measurements of the excitation profiles are useful for analyzing the nonequilibrium distribution initially prepared and the subsequent relaxation mechanism.