bluuebunny/crossref_metadata_2025 huggingface dataset:
Hole transport and quasi-Fermi level splitting at the emitter-base junction in Pnp heterojunction bipolar transistors
Authors: Datta S. , Roenker K. P. , Cahay M. M.
Published: July 2002

Hole transport across the emitter-base heterojunction in Pnp heterojunction bipolar transistors is considered and the amount of hole quasi-Fermi level splitting at the interface is analyzed. Hole injection at the emitter is shown to significantly limit the emitter current to a level more than an order of magnitude less than that predicted by the hole-base diffusion model. Due to the low hole mobility, hole drift diffusion across the emitter-base space charge region is shown to be of comparable importance to thermionic-tunneling emission in controlling hole injection into the base.
Domain structure of epitaxial PbTiO3 thin films on LaAlO3(100) grown by metal organic chemical vapor deposition
Authors: Yen Bi Ming , Chen Haydn
Published: July 2002

Lead titanate (PbTiO3) thin film has been deposited on lathanum aluminate LaAlO3(100) (using pseudocubic index) single crystalline substrate by metal organic chemical vapor deposition at 550–700 °C. Microstructure of the films has been studied by the four-circle x-ray diffraction and transmission electron microscopy methods. Due to the noncubic symmetry of the LaAlO3 substrate at room temperature, the grown films exhibited more complex domain structure than those grown on cubic MgO or SrTiO3 substrates. The x-ray pole figure of LaAlO3 pseudocubic (002) peak has revealed two separate peaks at room temperature, indicating the existence of the two types of domain in the substrate. Consequently, the epitaxially grown PbTiO3 films were found to exhibit two distinguishable c domains, and eight a-domain peaks, as demonstrated by the x-ray pole figures employing PT(002) and PT(200) Bragg peaks. The domain structure and the domain formation mechanism are discussed in this article.
Digital signal propagation in dispersive media
Authors: Jordan P. M. , Puri Ashok
Published: July 2002

In this article, the propagation of digital and analog signals through media which, in general, are both dissipative and dispersive is modeled using the one-dimensional telegraph equation. Input signals are represented using impulsive, Heaviside unit step, Gaussian, rectangular pulse, and both unmodulated and modulated sinusoidal pulse type boundary data. Applications to coaxial transmission lines and freshwater signal propagation, for both digital and analog signals, are included. The analysis presented here supports the finding that digital transmission in dispersive media is generally superior to that of analog. The boundary data (input signals) give rise to solutions of the telegraph equation which contain propagating discontinuities. It is shown that the magnitudes of these discontinuities, as a function of distance, can be found without the need of solving the governing equation. Thus, for digital signals in particular, signal strength at a given distance from the input source can be easily determined. Furthermore, the magnitudes of these discontinuities are found to be independent of both the dispersion coefficient k and the elastic coefficient b. In addition, it is shown that, depending on the algebraic sign of k, one of two distinct forms of dispersion is possible and that for small-time intervals, solutions are approximately independent of k.
Mixing and noise in diffusion and phonon cooled superconducting hot-electron bolometers
Authors: Burke P. J. , Schoelkopf R. J. , Prober D. E. , Skalare A. , Karasik B. S. , Gaidis M. C. , McGrath W. R. , Bumble B. , LeDuc H. G.
Published: July 2002

We report a systematic, comprehensive set of measurements on the dynamics and noise processes in diffusion and phonon-cooled superconducting hot-electron bolometer mixers which will serve as ultralow noise detectors in THz heterodyne receivers. The conversion efficiency and output noise of devices of varying lengths were measured with radio frequency between 8 and 40 GHz. The devices studied consist of 100-Å-thin film Nb bridges connected to thick (1000 Å), high conductivity normal metal (Au) leads. The lengths of the devices studied range from 0.08 to 3 μm. For devices longer than the electron–phonon interaction length Le–ph≡Dτe–ph, with D the diffusion constant and τe–ph−1 the electron–phonon interaction rate, the hot electrons are cooled dominantly by the electron–phonon interaction, which in Nb is too slow for practical applications. If the device length is less than πLe–ph(≈1 μm at 4.2 K), then out diffusion of heat into the high conductivity leads dominates the cooling process. In this limit, the intermediate frequency (IF) bandwidth is found to vary as L−2, with L the bridge length, as expected for diffusion cooling. The shortest device has an IF bandwidth greater than 6 GHz, the largest reported for a low-Tc superconducting bolometric mixer. The dominant component of the output noise decreases with frequency in the same manner as the conversion efficiency, consistent with a model based on thermal fluctuations. The noise bandwidth is larger than the gain bandwidth, and the mixer noise is low, ranging from 100 to 530 K (double sideband). The crossover from phonon dominated to diffusion dominated behavior is also demonstrated using noise thermometry measurements in the normal state. Scalar measurements of the device differential impedance in the intermediate state agree with a theoretical model which takes into account the thermal and electrical dynamics. We also present detailed comparisons with theoretical predictions of the output noise and conversion efficiency.
Surface and exchange anisotropy fields in MnFe2O4 nanoparticles: Size and temperature effects
Authors: Bakuzis A. F. , Morais P. C. , Pelegrini F.
Published: July 2002

Angular measurements of magnetic resonance are used to investigate the surface anisotropy field as well as the exchange anisotropy field in spherical MnFe2O4 magnetic nanoparticles as a function of temperature and particle diameter (D). The resonance field is a combination of angular dependent and angular independent fields, both affected by the surface anisotropy field, which in turn follows a D−α power law, with α very close to unity. The angular dependent component probes the surface anisotropy field while the angular independent component probes the exchange anisotropy field. In the temperature range from 100 to 250 K a negative surface anisotropy field is found, which increases as the particle size is reduced, indicating a radial orientation of the spins at the MnFe2O4 nanoparticle surface.
Theory of ferromagnetism and colossal magnetoresistance in doped manganites
Authors: Alexandrov A. S. , Bratkovsky A. M.
Published: July 2002

An exchange interaction of polaronic carriers with localized spins leads to a new ferromagnetic transition in doped charge-transfer insulators with strong electron–phonon coupling. The relative strength of the exchange and electron–phonon interactions determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of bipolaron formation in the paramagnetic phase, is extremely sensitive to an external magnetic field. We show that the carrier density collapse describes the colossal magnetoresistance, anomalous specific heat, and tunneling gap of doped manganites close to the transition. The dependence of the binding energy on ion mass explains the giant isotope effect in magnetization and resistivity upon substitution of 16O by 18O.
Conditions for the representation of vector hysteresis by the vector Preisach model
Authors: Friedman G.
Published: July 2002

Three important properties of the two-dimensional (2D)-vector Preisach model are investigated. It is shown that these properties are necessary and sufficient conditions for the representation of a 2D-vector hysteresis relation by this model.
Power losses of soft magnetic composite materials under two-dimensional excitation
Authors: Zhu J. G. , Zhong J. J. , Ramsden V. S. , Guo Y. G.
Published: July 2002

Soft magnetic composite materials produced by powder metallurgy techniques can be very useful for construction of low cost small motors. However, the rotational core losses and the corresponding B–H relationships of soft magnetic composite materials with two-dimensional rotating fluxes have neither been supplied by the manufacturers nor reported in the literature. This article reports the core loss measurement of a soft magnetic composite material, SOMALOY™ 500, Höganäs AB, Sweden, under two-dimensional excitations. The principle of measurement, testing system, and power loss calculation are presented. The results are analyzed and discussed.
Geometrically induced giant magnetoresistance behavior in quarter-micrometer strip pattern
Authors: Matsuyama K. , Matsuo K. , Nozaki Y.
Published: July 2002

The present article reports a geometrically induced magnetoresistance (MR) behavior observed in magnetic strips consisting of a symmetric layer structure with lateral sizes down to 0.2 μm. The MR ratio was markedly improved from 2.8% for an as-grown film of Co(6 nm)/Cu(6 nm)/Co(6 nm) to 4.6% by structuring it into a strip with 0.3 μm width. The parabolic MR behavior observed in the hard-axis direction suggests that the magnetization in the two magnetic layers rotates toward the opposite directions, which can be explained by the magnetostatic interaction between the surface magnetic charges at the pattern edges. The dependence of MR saturation field on the pattern width agrees well with a theoretical prediction based on a coherent rotation model. The MR ratio was markedly increased by the increase of the layer number and the thickness of the magnetic layer, the highest value of 7.8% was obtained for a strip made of [Co(6 nm)/Cu(4 nm)]2Co(6 nm). A linear- and hysteresis-free MR response was performed with an alternative field modulation under a transverse bias field.
Structure and magnetic properties of SmCo thin films on Cr/Ag/Si templates
Authors: Zangari G. , Lu B. , Laughlin D. E. , Lambeth D. N.
Published: July 2002

SmCo thin films were grown onto highly oriented Cr/Ag/Si templates of various orientations and their properties compared to those of SmCo/Cr/glass thin film structures. No conclusive evidence of local crystalline order in SmCo films was observed by microscopy techniques. However, in-plane uniaxial magnetic anisotropy was observed for SmCo films grown onto Si(111) and (110) templates, indicating interfacial effects, while no anisotropy was observed for SmCo on Si(100). It is hypothesized that epitaxial growth of SmCo takes place on Cr(112) surfaces, but local crystalline order disappears with increasing SmCo thickness. Uniaxial SmCo films on Si(110) exhibit magnetic properties superior to those of SmCo on other Si templates. Process optimization could lead to SmCo media largely improved with respect to current SmCo films on glass.
Measurement of elastic modulus, Poisson ratio, and coefficient of thermal expansion of on-wafer submicron films
Authors: Zhao Jie-Hua , Ryan Todd , Ho Paul S. , McKerrow Andrew J. , Shih Wei-Yan
Published: July 2002

A bending beam method has been developed to measure the elastic modulus E, the coefficient of thermal expansion (CTE) and the Poisson ratio ν for on-wafer dielectric films with thicknesses in the submicron range. The method was demonstrated for 0.5 μm thick silicon dioxide films made from tetraethylorthosilane (TEOS). First, the biaxial elastic modulus E/(1-ν) and CTE were measured on blanket TEOS on Si and GaAs substrates and found to be 77 GPa and 1.0 ppm/°C, respectively. The Poisson ratio ν was determined by combining the finite element calculation and the experimental result of the thermal stresses of TEOS fine lines on the Si substrate. The Poisson ratio of TEOS was determined to be 0.24 and, as a consequence, the Young’s modulus was 59 GPa. Fourier transform infrared spectra were obtained for TEOS films on the Si and GaAs substrates to ensure that the chemical structure of the film is independent of the substrate.
High speed conversion of infrared images with a planar gas discharge system
Authors: Portsel L. M. , Astrov Yu. A. , Reimann I. , Ammelt E. , Purwins H.-G.
Published: July 2002

The speed of conversion of infrared (IR) images by a planar semiconductor gas discharge system into the visible range has been investigated. Argon or nitrogen are used in the discharge gap having an electrode distance of 100 μm. Using pulse radiation from an IR laser to excite the system, we have shown that the characteristic response time of the device with the cryogenic discharge in the gap can lie in the submicrosecond range. This characteristic of the system can be applied for a fast IR imaging at a rate higher than 106 frame/s.
Transport property in narrow barrier GaAs/AlAs superlattice under hydrostatic pressure
Authors: Wu J. Q. , Jiang D. S. , Sun B. Q. , Liu Z H. X.
Published: July 2002

The current–voltage (I–V) characteristics of a doped weakly coupled GaAs/AlAs superlattice (SL) with narrow barriers are measured under hydrostatic pressure from 1 bar to 13.5 kbar at both 77 and 300 K. The experimental results show that, contrary to the results in SL with wide barriers, the plateau in the I–V curve at 77 K does not shrink with increasing pressure, and becomes wider after 10.5 kbar. It is explained by the fact that the EΓ1–EΓ1 resonance peak is higher than the EΓ1–EX1 resonance peak. At 300 K, however, because of the more important contribution of the nonresonant component to the current, the plateau shrinks with increasing pressure.
Effects of layer thickness on orientation distribution and magnetic properties of CoCrTa/Cr films
Authors: Shan Z. S. , Zeng H. , Zhu C. X. , Azarisooreh M. , Honardoost K. , Liu Y. , Sellmyer D. J.
Published: July 2002

Evolution of orientation distribution of Co(110) crystal planes was determined by x-ray rocking curves. It has been found that: (i) The full-width at half maximum W of the Co(110) rocking curve decreases with both increasing Cr underlayer thickness dCr, and increasing CoCrTa magnetic layer thickness dCo, especially in the thin layer regime. (ii) For the thin dCo regime, the interlayer diffusion between the Cr underlayer and the magnetic layer affects the rocking curves and magnetic properties significantly. (iii) Film magnetic properties, e.g., a significant jump in coercivity with increasing magnetic layer thickness in the thin dCo regime may be related in part to the evolution of the Co(110) orientation distribution.
Effects of Na on the electrical and structural properties of CuInSe2
Authors: Wei Su-Huai , Zhang S. B. , Zunger Alex
Published: July 2002

We found theoretically that Na has three effects on CuInSe2: (1) If available in stoichiometric quantities, Na will replace Cu, forming a more stable NaInSe2 compound having a larger band gap (higher open-circuit voltage) and a (112)tetra morphology. The ensuing alloy NaxCu1−xInSe2 has, however, a positive mixing enthalpy, so NaInSe2 will phase separate, forming precipitates. (2) When available in small quantities, Na will form defect on Cu site and In site. Na on Cu site does not create electric levels in the band gap, while Na on In site creates acceptor levels that are shallower than CuIn. The formation energy of Na(InCu) is very exothermic, therefore, the major effect of Na is the elimination of the InCu defects and the resulting increase of the effective hole densities. The quenching of InCu as well as VCu by Na reduces the stability of the (2VCu−+InCu2+), thus suppressing the formation of the “Ordered Defect Compounds.” (3) Na on the surface of CuInSe2 is known to catalyze the dissociation of O2 into atomic oxygen that substitutes Se vacancy (shallow donors), converting them into OSe. We find, however, that OSe is an (isovalent) deep rather than shallow acceptor. We also find that having removed the donors, O atoms in CuInSe2 form Cu2O and In2O3 compounds, and phase separate, forming precipitates at the surfaces and grain boundaries. Our results are compared with previous models and provide new insights into the defect physics of Na in CIS.
Chemical and structural transformation of sapphire (Al2O3) surface by plasma source nitridation
Authors: Cho Y. , Kim Y. , Weber E. R. , Ruvimov S. , Liliental-Weber Z.
Published: July 2002

The chemical, morphological, and structural characteristics of nitrogen plasma treated c-plane sapphire substrate surfaces were studied by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The plasma treatment was carried out by exposing sapphire substrates (Ts=700 °C) to a flow of 35 sccm of activated nitrogen species generated by a constricted-plasma source for 5–60 min. The emergence of the N 1s peak in XPS indicates nitrogen incorporation in sapphire as soon as after 5 min of nitridation. AFM images show that the sapphire contained a high density of islands after 1 h of nitridation. A thin polycrystalline AlN layer was observed on the nitridated sapphire surface by TEM. Both the thickness of the AlN layer and the N 1s photoelectron peak intensity increase nonlinearly with respect to nitridation time. The nonlinear relationship between the thickness of the nitridated layer and the reaction time suggests the growth of the AlN layer follows a diffusion limited growth mechanism.
Strain characterization of polycrystalline diamond and silicon systems
Authors: Anastassakis E.
Published: July 2002

The phonon deformation potentials of polycrystalline diamond and silicon, derived through the Voigt–Reuss–Hill averaging technique, are used to establish the systematics of strain characterization of polycrystalline films, considering all possible stress configurations. The results are compared to Raman data from the literature.
Effects of the epitaxial layer thickness on the noise properties of Schottky barrier diodes
Authors: Gomila G. , Bulashenko O. M.
Published: July 2002

An analytical theory to describe the combined effects of the epitaxial layer thickness and the ohmic contact on the noise properties of Schottky barrier diodes is presented. The theory, which provides information on both the local and the global noise properties, takes into account the finite size of the epitaxial layer and the effects of the back ohmic contact, and applies to the whole range of applied bias. It is shown that by scaling down the epitaxial layer thickness, the current regime in which the noise temperature displays a shot-noise-like behavior increases at the cost of reducing the current range in which the thermal-noise-like behavior dominates. This improvement in noise temperature is limited by the effects of the ohmic contact, which appear for large currents. The theory is formulated on general trends, allowing its application to the noise analysis of other semiconductor devices operating under strongly inhomogeneous distributions of the electric field and charge concentrations.
Tight binding simulation of the thermodynamic behavior of amorphous silicon
Authors: Rosato V. , Celino M.
Published: July 2002

Structures of amorphous silicon have been generated by using a suitable implementation of the reverse Monte Carlo technique, based on the reproduction of different experimental data. The structures have been subsequently relaxed at fixed temperature and pressure via tight binding molecular dynamics. The amorphous structures have been further characterized by evaluating structural, dynamic and electronic structure properties, as a function of temperature, up to and above the melting point. The model of amorphous silicon undergoes a melting transition at Tal≃0.55Tm (where Tm is the homogeneous melting temperature of the bulk crystal). In the temperature range between Tal and Tm, the system exhibits thermodynamic and structural properties typical of an undercooled liquid.
Electrical properties of iron-silica nanocomposites synthesized by electrodeposition
Authors: Banerjee S. , Ghosh A. K. , Chakravorty D.
Published: July 2002

Nanometer-sized iron particles with diameters in the range 5.5–11.1 nm were grown within a silica gel by an electrodeposition method. Electron diffraction measurements show that some of the iron particles were oxidized to Fe3O4. dc resistivity measurements over the temperature range 110–300 K show a T−1/4 variation indicating a variable range hopping transport. ac conductivity over the frequency range 100 Hz–2 MHz show an overlapping large polaron tunneling mechanism to be operative. The dielectric modulus spectra as a function of frequency were analyzed on the basis of a stretched exponential relaxation function. The values of the exponent β as extracted from this analysis were in the range 0.38–0.46. The activation energies corresponding to the maximum of the imaginary part of the dielectric modulus were in the range 0.13–0.20 eV. These are ascribed to an electron tunneling mechanism.
Augmented Fokker–Planck equation for electron transport in arbitrary electric fields
Authors: Bringuier E.
Published: July 2002

Electron transport in a solid subjected to an arbitrary electric field is analyzed in the energy-position manifold instead of phase space. In the absence of deeply inelastic electron-lattice scattering, the spectral carrier density satisfies a differential equation of the Fokker–Planck type. Subsumption of deeply inelastic events results in an integrodifferential equation which agrees exactly with Monte Carlo simulations in the limit of a vanishing drift-to-instantaneous velocity ratio. Reasonable agreement is found for ratios as high as 0.4, enabling the augmented Fokker–Planck equation to tackle a number of transport issues at a much lower cost than Monte Carlo simulations.
Asymmetrical heating behavior of doped Si channels in bulk silicon and in silicon-on-insulator under high current stress
Authors: Liao C. N. , Chen C. , Huang J. S. , Tu K. N.
Published: July 2002

With continuing scaling down in microelectronic devices, the current density and the power consumption in the devices must increase. Hence, device reliability under high current density is an issue for ultralarge-scale integration technology. This study investigates the heating behavior of the heavily doped Si channels under high current stress. Thermal and electrical characterization of the channels in bulk Si and in silicon-on-insulator were conducted. An abnormal asymmetrical heating along the channels in bulk Si has been observed. We propose a junction leakage mechanism to explain the phenomenon observed. Other asymmetrical thermal effects, such as electron–hole recombination and Peltier effect, have also been discussed.
Measurement of Tc suppression in tungsten using magnetic impurities
Authors: Young B. A. , Saab T. , Cabrera B. , Cross J. J. , Clarke R. M. , Abusaidi R. A.
Published: July 2002

We have measured the effects of dilute magnetic-atom doping on the superconducting transition temperature of tungsten thin films. Our “Tc tuning” technique is accurate, precise, and simple. Experiments were performed using dc-magnetron-sputtered tungsten films with undoped values of Tc in the range of 70–150 mK. The magnetic-atom doping was achieved using ion implantation. Specific Tc suppressions of between 5% and 65% were targeted and observed in this study. The transition width of each undoped sample was ≈1 mK and the transition widths remained sharp after implantation with Fe+56 ions. Our data are in good agreement with predictions of a linear dependence of Tc suppression with increasing magnetic-atom concentration, in the small concentration limit. At higher concentrations, antiferromagnetic coupling between the magnetic dopant atoms becomes important and the Tc-suppression effect is diminished. We use our Tc data to calculate the Abrikosov–Gor’kov (AG) and Ruderman–Kittel–Kasuya–Yosida (RKKY) spin–flip relaxation parameters τAG and τRKKY. We conclude with a brief discussion of applications of the Tc-tuning technique, and present our plans for future studies in this area.
Analysis of neutral fragments from ultraviolet laser irradiation of a photolabile triazeno polymer
Authors: Lippert T. , Langford S. C. , Wokaun A. , Savas Georgiou , Dickinson J. T.
Published: July 2002

A photolabile triazeno polymer was irradiated with pulsed excimer laser light at 248 nm and 30 ns pulse width. The ablation fragments were analyzed using time-of-flight (TOF) mass spectrometry. At fluences below 1.3 J/cm2, only neutral products were found. At these fluences, N2 is by far the most intense neutral signal along with measurable phenyl radical (mass 76) production. The N2 TOF shows a fast shoulder corresponding to kinetic energies of about 1.1 eV and a long slow tail persisting for hundreds of microseconds. The tail is attributed to delayed emission of reaction products from the polymer. The kinetic energy of the fast peak is attributed to direct ejection of products from surface sites undergoing exothermic decomposition. A weaker signal due to the phenyl radical is also observed. The observed fluence dependence of the two major products is highly nonlinear and is shown to fit an Arrhenius equation. We discuss the implications of these measurements regarding photochemical versus photothermal processes.
Precipitation and dissolution of Co granules in CuCo alloys: Reverse effects of Joule heating
Authors: da Silva F. C. S. , Ferrari E. F. , Knobel M.
Published: July 2002

Measurements of resistance R versus electrical current I were performed during annealing of melt-spun pure Cu and Cu90Co10 ribbons using linearly varying current Joule heating. Typical results of Cu90Co10 samples show three characteristic stages. For low applied currents (I<4.0 A), a metallic behavior is observed and compared with pure Cu samples. Precipitation is the dominant process for intermediate currents (5.0 A<I<9.0 A). Re-solution of precipitated Co back to Cu matrix appears for high current values (I>9.0 A). Competition between precipitation and dissolution of Co granules depends also on the cooling rates, and we observed that it is possible to freeze high temperature off-equilibrium configurations down to room temperature after an appropriate quenching. Experimental annealing conditions were simulated using the Monte Carlo–Metropolis method, with Kawasaki dynamics of diffusing atoms, to study the kinetics of transformations in the Cu–Co system. Simulations show that precipitation and re-solution competition occurs as functions of both temperature and time. A relationship between simulated Co atoms configuration and resistance measurements is made
Experimental observation of band inversion in the PbSnTe system
Authors: Ferreira S. O. , Abramof E. , Motisuke P. , Rappl P. H. O. , Closs H. , Ueta A. Y. , Boschetti C. , Bandeira I. N.
Published: July 2002

The band inversion in the PbSnTe compound has been observed by optical and electrical measurements. The samples, high quality Pb1−xSnxTe epitaxial layers, have been grown by molecular-beam epitaxy on (111)BaF2 substrates. Optical transmission measurements have revealed a change in signal of the energy gap temperature derivative for samples with 0.35<x<0.70. In the same samples and at the same temperature, a minimum in the resistivity has been observed, showing a interrelation between the optical and electrical measurements. However, the temperature, for which the inversion occurs, is not that predicted by the band inversion model. This discrepancy is supposed to be due to the relatively high hole concentration of these samples.
Dynamic self-enhanced diffraction from written photorefractive gratings
Authors: Zhang Liangmin , Prasad Paras N. , Burzynski Ryszard , Chen Huanchu , Han Jianru
Published: July 2002

The self-enhanced diffraction with oscillatory behavior for unfixed holograms is observed and investigated in new photorefractive (K1−xNax)2m(Sr1−yBay)1−mNb2O6 crystals. The recorded diffraction efficiency can be increased 10 times during the readout process. The maximum peak diffraction efficiency obtained is close to unity. A closed solution of dynamic two-wave mixing equations and a coupling coefficient expression obtained from the standard band transport model are used to fit the observed results. The observed self-enhancement is caused by the interference between a new grating, formed by the reading beam and the diffracted signal, and the existing grating. The fitted results show that the internal space-charge field originating from the photovoltaic effect and the pyroelectric effect can reduce the phase shift.
Theoretical analysis of thin film epitaxial growth monitored by differential reflectance
Authors: Yang Guo-Zhen , Li Zhi-Yuan , Gu Ben-Yuan , Lee S. T.
Published: July 2002

The thin film growth at the level of a single molecular layer can be achieved via laser molecular beam epitaxy approach. Such a growth can be monitored not only via the reflection high-energy electron diffraction technique, but also with the use of relative reflectivity change between s- and p-polarized light. The epitaxy growth of thin film is modeled as a microscopic two-dimensional (2D) gratings on the surface of the substrate with a period over two orders of magnitude less than the wavelength of probe light. The solution of Maxwell’s equations for such a 2D gratings leads to an effective multilayer medium model, from which the reflectance-difference (RD) signal in the process of epitaxy growth can be calculated in the framework of general optics. The simulation results demonstrate that the growth of thin film at the atomic level can be detected by such an optical RD technique. The theoretical calculations of RD signal are in agreement with the experimental measurements both in the order of magnitude and variation character of RD signal curve.
Magnetic and magnetostrictive properties in amorphous (Tb0.27Dy0.73)(Fe1−xCox)2 films
Authors: Duc N. H. , Mackay K. , Betz J. , Givord D.
Published: July 2002

Magnetic and magnetostrictive properties have been investigated for amorphous (Tb0.27Dy0.73)(Fe1−xCox)2 thin films. An increase in the 3d magnetic moment due to the enhancement of T–T interactions in substituted (Fe, Co) alloys was found. This leads to stronger R–(Fe, Co) exchange energies and then to enhancements of R–sublattice magnetization as well as magnetostriction in these amorphous R(Fe, Co) thin films. In addition, a well-defined in-plane anisotropy is created by magnetic-field annealing for the Co-rich films. A large magnetostriction of 480×10−6 developed in low fields of 0.3 T was observed for films with x=0.47 after magnetic-field annealing. The differing roles of Fe and Co atoms on the magnetization process have also been discussed.
Deformation mechanism in the forcefill process
Authors: Janssen G. C. A. M. , Jongste J. F. , Verbruggen A. H.
Published: July 2002

Forcefill is a method used in metallization of ICs to fill via holes with metal. At elevated temperature and under applied pressure the metal flows into the via hole. The method is applicable to aluminum as well as to copper. In this article the mechanism of the process is discussed based on measurements of the kinetics and on finite element calculations of the shear and hydrostatic stress in the film covering the via hole. It is demonstrated that naı̈ve use of deformation maps constructed by Frost and Ashby for the description of the forcefill process easily leads to highly inconsistent results. The calculated stress distribution shows that the relevant shear stress is more than one order of magnitude lower that the applied pressure. Since no deformation map was available for the grain size appropriate for the forcefill experiment a map for 1 μm grains was constructed. This map indicates that for the forcefill process diffusional flow is the dominant deformation mechanism. It is shown that under these conditions the diffusional flow process described by Frost and Ashby is in essence identical to our previously reported stress-induced diffusion model.
Improvement in photoluminescence efficiency of SiO2 films containing Si nanocrystals by P doping: An electron spin resonance study
Authors: Fujii Minoru , Mimura Atsushi , Hayashi Shinji , Yamamoto Keiichi , Urakawa Chika , Ohta Hitoshi
Published: July 2002

SiO 2 and phosphosilicate glass (PSG) films containing Si nanocrystals (nc-Si) as small as a few nanometers were studied by electron spin resonance (ESR) and photoluminescence (PL), and the correlation between the two measurements was examined. It is shown that the incorporation of nc-Si in SiO2 results in the drastic increase in the ESR signal; the signal is assigned to the Si dangling bonds at the interfaces between nc-Si and matrices (Pb centers). The ESR signal becomes weaker by doping P into SiO2 matrices, i.e., by using PSG as matrices. By increasing the P concentration, the ESR signal decreases further. By decreasing the ESR signal, the low-energy PL peak at 0.9 eV decreases, while the band-edge PL at 1.4 eV increases. These results suggest that the 0.9 eV peak is related to Pb centers, and that the decrease in the density of the Pb centers by P doping brings about an improvement in the band-edge PL efficiency of nc-Si.
Transitions from nanoscale to microscale dynamic friction mechanisms on polyethylene and silicon surfaces
Authors: Niederberger S. , Gracias D. H. , Komvopoulos K. , Somorjai G. A.
Published: July 2002

The dynamic friction mechanisms of polyethylene and silicon were investigated for apparent contact pressures and contact areas in the ranges of 8 MPa–18 GPa and 17 nm2–9500 μm2, respectively. Friction force measurements were obtained with a friction force microscope, scanning force microscope, and pin-on-disk tribometer. Silicon and diamond tips with a nominal radius of curvature between 100 nm and 1.2 mm were slid against low- and high-density polyethylene and Si(100) substrates under contact loads in the range of 5 nN–0.27 N. The low friction coefficients obtained with all material systems at low contact pressures indicated that deformation at the sliding interface was primarily elastic. Alternatively, the significantly higher friction coefficients at higher contact pressures suggested that plastic deformation was the principal mode of deformation. The high friction coefficients of polyethylene observed with large apparent contact areas are interpreted in terms of the microstructure evolution involving the rearrangement of crystalline regions (lamellae) nearly parallel to the sliding direction, which reduces the surface resistance to plastic shearing. Such differences in the friction behavior of polyethylene resulting from stress-induced microstructural changes were found to occur over a relatively large range of the apparent contact area. The friction behavior of silicon was strongly affected by the presence of a native oxide film. Results are presented to demonstrate the effect of the scale of deformation at the contact interface on the dynamic friction behavior and the significance of contact parameters on the friction measurements obtained with different instruments.
Quantification of the low temperature infrared vibrational modes from interstitial oxygen in silicon
Authors: De Gryse O. , Clauws P.
Published: July 2002

Accurate conversion factors are obtained to determine the concentration of interstitial oxygen in silicon from the low temperature local vibrational mode absorption at 1136, 1128 and 1205 cm−1 for different resolutions and apodization functions. The absorption spectra at 6 K were fitted with fit functions in order to extract the amplitudes of interest in an accurate and reproducible manner. The ratio of the amplitude at room temperature to the low temperature amplitude then gives the conversion factors for 6 K. Based on a phonon model [H. Yamada-Kaneta Materials Science Forum, edited by G. Davies and M. H. Hazare (Trans Tech, Aveiro, 1997), 258–263, p. 355] and on occupation statistics the use of the conversion factors is extended to temperatures as high as 100 K, taking into account the broadening of the absorption peaks and the variation in the occupation of the different excited states.
Thermally stimulated current investigations on diamond x-ray detectors
Authors: Tromson D. , Bergonzo P. , Brambilla A. , Mer C. , Foulon F. , Amosov V. N.
Published: July 2002

Natural II-a type diamond is increasingly used for the fabrication of radiation detectors. However, the presence of inherent defects has a strong impact on the detector characteristics such as the time-dependent stability of the detection signal. In order to draw better insight into this effect, comparative investigations of the x-ray responses with thermally stimulated current (TSC) measurements were carried out on natural diamond detectors. TSC revealed the presence of four peaks or shoulders on natural samples in the 200–500 K domain. Three energy levels were identified at about 0.7, 0.71, and 0.95 eV. Time-dependent x-ray detector sensitivity was investigated for various initial conditions. The results give evidence of the improvement of the detection properties after having filled traps in the material by x-ray irradiation. The comparison between the x-ray response and the TSC spectra indicates that trapping levels emptied at room temperature appear to significantly affect the performance of radiation detectors.
High temperature growth of ZnS and ZnMgS by molecular beam epitaxy under high sulfur beam pressure
Authors: Ichino K. , Ueyama K. , Yamamoto M. , Kariya H. , Miyata H. , Misasa H. , Kitagawa M. , Kobayashi H.
Published: July 2002

ZnS and ZnMgS layers have been grown onto GaP substrates by molecular beam epitaxy (MBE). The key parameters of the growth are a high substrate temperature and a high sulfur (S) beam pressure. The S beam pressure was typically 1×10−2 Pa, which was more than one order of magnitude larger than in conventional MBE of ZnS. Using the high S beam pressure, large ZnS growth rate of 0.3–1.0 μm/h could be obtained even at 490 °C. The growth rate was limited by the Zn supply. Optimization of the S beam pressure reduces the full width at half maximum (FWHM) of the (400) double-crystal x-ray rocking curve (DCXRC). For a 2.1-μm-thick ZnS layer the width can be reduced to 400 arcsec. The low temperature photoluminescence (PL) spectra show sharp excitonic emissions including the free exciton emission. ZnMgS layers were grown onto ZnS buffer layers. The ZnMgS layers as well show good crystal and optical qualities. The FWHM of DCXRC of the 1.5-μm-thick Zn0.83Mg0.17S layer is 650 arcsec, which is comparable to the FWHM of a ZnS layer of similar thickness. The low temperature PL of the ZnMgS layer is dominated by a strong excitonic emission. The band gap of Zn1−xMgxS is estimated from reflection spectra. For x=0.20, the band gap is 3.974 eV.
Theoretical calculations of magnetic order and anisotropy energies in molecular magnets
Authors: Pederson M. R. , Porezag D. V. , Kortus J. , Khanna S. N.
Published: July 2002

We present theoretical electronic structure calculations on the nature of electronic states and the magnetic coupling in the Mn12O12 free cluster and the Mn12O12(RCOO)16(H2O)4 molecular magnetic crystal. The calculations have been performed with the all-electron full-potential NRLMOL code. We find that the free Mn12O12 cluster relaxes to an antiferromagnetic cluster with no net moment. However, when coordinated by sixteen HCOO ligands and four H2O groups, as it is in the molecular crystal, we find that the ferrimagnetic ordering and geometrical and magnetic structure observed in the experiments is restored. Local Mn moments for the free and ligandated molecular magnets are presented and compared to experiment. We identify the occupied and unoccupied electronic states that are most responsible for the formation of the large anisotropy barrier and use a recently developed full-space and full-potential method for calculating the spin–orbit coupling interaction and anisotropy energies. Our calculated second-order anisotropy energy is in excellent agreement with experiment.
Micromagnetic simulation of 360° domain walls in thin Co films
Authors: Schrefl T. , Fidler J. , Zehetmayer M.
Published: July 2002

A moving mesh finite element technique is applied to simulate the formation and annihilation of 360° wall structures in thin Co films. Adaptive refinement and coarsening of the finite element mesh controls the discretization error during the simulation of domain wall movement. Elements are refined in regions with high variation of the magnetization, whereas elements are taken out where the magnetization is uniform. The calculated Neel walls have very wide tails and have an extension of about 15–20 nm. The motion of domain walls through the sample gives rise to the formation of 360° domain walls. They are formed when a Bloch line in a domain wall is caught at a nonmagnetic defect in the sample. Pinholes with an extension greater than 6 nm are sufficient to trap a Bloch line. The width of the 360° walls is found to be in the range of 40 to 50 nm. The stability of the 360° walls depends on the strength of the pinning of the Bloch lines. The field required to annihilate the 360° walls increases linearly with the size of the nonmagnetic defect.
Electron emission from clean gold bombarded by slow Auq+ (q=1–3) ions
Authors: Eder H. , Messerschmidt W. , Winter HP. , Aumayr F.
Published: July 2002

Total electron yields for impact of slow singly, doubly, and triply charged gold ions on a clean polycrystalline gold surface have been precisely determined via current measurements for incident ions and emitted electrons. Results of this study are of relevance for a proposed method to establish a new mass standard by determining atomic mass via accumulation of ions to amounts that can be weighed with high accuracy.
Photoluminescence and cathodoluminescence of GaN doped with Pr
Authors: Lozykowski H. J. , Jadwisienczak W. M. , Brown I.
Published: July 2002

We report on our observation of visible photoluminescence and cathodoluminescence of Pr-implanted GaN. The implanted samples were subjected to isochronal thermal annealing treatments at a temperature of 1100 °C in N2 at atmospheric pressure to recover from implantation damage and activate the rare earth ions. The sharp characteristic emission lines corresponding to Pr3+ intra-4fn-shell transitions are resolved in the spectral range from 400 to 1000 nm and observed over the temperature range from 12 to 335 K. We have developed an energy level diagram for Pr3+ ions in GaN using recorded spectra. The photoluminescence decay kinetics measurements of P13, P03, and D21 levels and quenching mechanism analysis allow us to conclude that the dominant de-excitation process is of electric dipole–electric quadrupole type. We found also that the D21 level separated from the upper P03 level by 3755 cm−1 energy gap can be populated in several ways, by direct energy transfer processes, cross relaxation |3P0,3H4〉→|1D2,3H6〉 and cascade processes from the P03 or higher levels. The full width at half maximum of the strongest photoluminescence line at 653 nm, the P03 level, is 2.9 meV at 13 K, with a 1.86 meV blue peak shift in going from 13 to 330 K temperature, while the line at 670 nm, P13 level, has a full width at half maximum of 4 meV at 13 K and a 1.6 meV red peak shift. The thermal stability of GaN:Pr3+ epilayers indicates the suitability of this material for visible optoelectronic devices.
Accurate prediction of static polarizabilities and hyperpolarizabilities. A study on FH (X 1Σ+)
Authors: Bishop David M. , Maroulis George
Published: July 2002

All the independent components of the α, β, γ, A, C, E, and B tensors were computed near the Hartree–Fock limit for the ground state of hydrogen fluoride, FH (1σ22σ23σ21πx21πy2, X 1Σ+). Higher polarizabilities were found to be strongly basis-set dependent, even when quite close to the Hartree–Fock limit. The inconsistency of the previous theroetical predictions was examined and solutions to existing problems proposed. The following values were found for the isotropy and the anisotropy of the α, β, and γ tensors (in a.u.): ᾱ=4.905, Δα=1.272, β̄=−5.9, Δβ=−7.9, γ̄=334, Δ1γ=−126, and Δ2γ=−70.
Total scattering cross sections and interatomic potentials for neutral hydrogen and helium on some noble gases
Authors: Ruzic David N. , Cohen Samuel A.
Published: July 2002

Measurements of energy-dependent scattering cross sections for 30 to 1800 eV D incident on He, Ne, Ar, and Kr, and for 40 to 850 eV He incident on He, Ar, and Kr are presented. They are determined by using the charge-exchange efflux from the Princeton large torus tokamak as a source of D or He. These neutrals are passed through a gas-filled scattering cell and detected by a time-of-flight spectrometer. The cross section for scattering greater than the effective angle of the apparatus (≈20 mrad) is found by measuring the energy-dependent attenuation of D or He as a function of pressure in the scattering cell. The interatomic potential is extracted from the data.
Collisional deactivation of two-photon laser excited xenon 5p5 6p. II. Lifetimes and total quench rates
Authors: Böwering N. , Bruce M. R. , Keto J. W.
Published: July 2002

Lifetimes and total binary quench rate constants of Xe 5p5 6p were measured using two-photon laser excitation. For Xe 2p5, state-changing three-body quenching is observed, as well. The deactivation rates are discussed on the basis of the molecular potentials involved. Strong coupling of Xe 2p10 with states of Xe 6s′ is observed and analyzed using a kinetic model which gives consistent results with data of other workers. The dominant path for deactivation of the Xe 6s′ levels is found to involve molecular states of Xe 2p10. The total cross sections obtained are compared to measurements of the quenching of Xe 6p by other rare gases.
Dynamics of a spin-1 model with the pair correlation
Authors: Keskin Mustafa , Meijer Paul H. E.
Published: July 2002

A spin-1 or three state system will undergo a first or second order phase transition depending on the ratio of coupling parameters α. Using the pair correlation approximation, the transition temperature is determined in order to establish the unstable, the metastable, as well as the stable regions of this cooperative system. The dynamics of the system is studied by means of the most probable path method and the flow lines and fixed points of the system are given for zero field. The choice of possible initial conditions is discussed. The role of the unstable points in the phase diagram, as separators between the stable and metastable points, is described, and they are computed for a number of cases.
Structures of small carbon clusters: Cyclic ground state of C6
Authors: Raghavachari Krishnan , Whiteside R. A. , Pople J. A.
Published: July 2002

Accurate ab initio calculations have been performed to investigate the nature of the structures and energies of different C6 isomers. The effects of polarization functions and electron correlation have been included in these calculations. A cyclic singlet structure with six π electrons (planar D3h,1A′) is found to be the ground state. The linear 3Σ−g structure lies about 10 kcal/mol higher in energy. The heat of formation, ΔH°0, of C6 is estimated to be ∼281 kcal/mol.
Monte Carlo simulations on concentration self-quenching by statistical traps
Authors: Knoester J. , Van Himbergen J. E.
Published: July 2002

We present high accuracy Monte Carlo simulations on the steady state fluorescence quantum yield and anisotropy for systems in which concentration self-quenching due to incoherent energy transfer between randomly distributed molecules occurs. A simple model of self-quenching is considered, in which luminescent molecules within a critical distance of each other act as perfect traps. The simulations are based on the complete solution of the coupled rate equations for a randomly chosen distribution of molecules in a finite box. Finite size effects are systematically investigated. The results are used to assess the validity of simple analytic theories, such as Burshtein’s theory of hopping transfer, which is found to give a good semiquantitative description of the fluorescence quantities. Furthermore, we show that truncating the transfer rate in such a way that, on the average, 10–20 molecules are within reach of an excitation residing on a given molecule, induces large deviations in the yield. We point out that this is a serious source of error in previous simulations.