bluuebunny/crossref_metadata_2025 huggingface dataset:
Resputtering effects on the stoichiometry of YBa2Cu3Ox thin films
Authors: Selinder T. I. , Larsson G. , Helmersson U. , Rudner S.
Published: July 2002

Energetic particle bombardment of the substrate during deposition will under certain process conditions adversely affect the properties of YBa2Cu3Ox thin films. Films were grown by dc magnetron sputtering from stoichiometric compound targets in a planar geometry. An increased oxygen fraction in the sputtering gas entailed a drastic decrease in the film deposition rate. Etching of the substrates was observed in pure oxygen plasmas for pressures below 2 Pa. For the whole range of sputtering gas mixtures, the film composition deteriorated if too low total pressures were used; that is, even if pure argon was used as working gas. Copper deficiency was observed under these conditions and attributed to resputtering of the growing film by energetic particles. This conclusion was supported by the copper deficiency observed in films grown at negative substrate bias, i.e., during ion bombardment from argon plasmas. The origin of various energetic particles during normal sputtering is discussed. We concluded that oxygen neutrals were responsible for the resputtering. At elevated substrate temperatures the film composition was more sensitive to resputtering than at low temperatures. If the energetic oxygen particles were thermalized, stoichiometric films were grown at temperatures up to at least 760 °C.
Composition dependence of lattice spacing for low dislocation density undoped liquid encapsulated Czochralski GaAs crystals
Authors: Usuda K. , Yasuami S. , Fujii T. , Higashi Y. , Kawata H. , Ando M.
Published: July 2002

Crystals with varied composition were pulled by the As-partial-pressure controlled liquid encapsulated Czochralski method. The dislocation density was found to be as low as 2×103 cm−2 at the cores of the crystals. Lattice spacing (d) was measured with a precision of Δd/d ∼ 5.9 × 10−6 using synchrotron radiation. For a variation of 7×10−5 in the As-atom fraction in the crystals, the lattice spacing varied by less than 1×10−5 Å.
Raman scattering and photoluminescence analysis of silicon on insulator structures obtained by single and multiple oxygen implants
Authors: Pérez-Rodríguez A. , Cornet A. , Morante J. R. , Jiménez J. , Hemment P. L. F. , Homewood K. P.
Published: July 2002

An analysis of silicon on insulator structures obtained by single and multiple implants by means of Raman scattering and photoluminescence spectroscopy is reported. The Raman spectra obtained with different excitation powers and wavelengths indicate the presence of a tensile strain in the top silicon layer of the structures. The comparison between the spectra measured in both kinds of samples points out the existence in the multiple implant material of a lower strain for a penetration depth about 300 nm and a higher strain for higher penetration depths. These results have been correlated with transmission electron microscopy observations, which have allowed to associate the higher strain to the presence of SiO2 precipitates in the top silicon layer, close to the buried oxide. The found lower strain is in agreement with the better quality expected for this material, which is corroborated by the photoluminescence data.
Piezoelectric properties of GaAs for application in stress transducers
Authors: Fricke K.
Published: July 2002

In this paper piezoelectric stress transducers are proposed on the base of GaAs and AlxGa(1−x)As. These materials exhibit reasonable piezoelectric properties and since GaAs based integrated circuits have reached a high maturity they are a promising choice for integrated stress transducers. An additional feature of the high band-gap material GaAs is its superior high-temperature performance in comparison with Si. In this paper the piezoelectric and related properties of GaAs and AlxGa(1−x) are discussed. Therefore the piezoelectric tensors for the commonly used (100) and (111) oriented substrates are derived. These calculations are verified by measurements of some realized sensors using different substrate orientations.
Three-dimensional optical trapping and laser ablation of a single polymer latex particle in water
Authors: Misawa Hiroaki , Koshioka Masanori , Sasaki Keiji , Kitamura Noboru , Masuhara Hiroshi
Published: July 2002

We developed a laser trapping-ablation system comprised of CW and pulsed Nd3+:YAG lasers as well as of an optical microscope. Three-dimensional manipulation of various kinds of particles and laser ablation of a single optically trapped, poly(methyl methacrylate) latex particle in water were demonstrated. A minute hole with its diameter of ∼sub-μm was fabricated on the latex particle (∼6 μm diameter). The hole size produced was much smaller than the effective diameter of the excitation laser beam, suggesting nonlinear optical (self-focusing of the laser beam) and photochemical (multiphoton absorption) mechanisms for the present laser trapping ablation. Characteristic features of the laser trapping-ablation are discussed in detail.
Substrate dependent internal stress in sputtered zinc oxide thin films
Authors: Jou Jwo-Huei , Han Min-Yung , Cheng Duen-Jen
Published: July 2002

Stresses in zinc oxide (ZnO) films prepared by radio frequency (rf) magnetron sputtering have been studied using a bending beam technique. The stresses exhibited at room temperature have been found to be strongly substrate dependent. The films deposited on gallium–arsenide (GaAs) substrates have exhibited much less compressive stress than those on silicon. This was contrary to what was expected. This may be attributed to: as revealed by x-ray diffraction experiments, the films grown on GaAs are less crystalline so that peening effect could occur less intensively during deposition. The stress varies with different processing conditions, especially substrate temperature and total gas pressure.
Vacuum ultraviolet generation by anti-Stokes Raman scattering of KrF laser radiation in H2
Authors: Huo Yunsheng , Shimizu Kouki , Yagi Takashi
Published: July 2002

The eleventh anti-Stokes order at 116 nm of the KrF excimer laser radiation was obtained in H2 gas at room temperature. The effects of the depletion of the first Stokes order by the S1-S2 Raman conversion on the efficient vacuum ultraviolet anti-Stokes generation was studied, and it determined the optimal Raman gain coefficient in this experiment. Ne gas was added into hydrogen gas to suppress the S1-S2 conversion, and the significant increase in the conversion efficiency for the lower anti-Stokes orders was observed in certain circumstances.
A study of minority carrier lifetime versus doping concentration in n-type GaAs grown by metalorganic chemical vapor deposition
Authors: Lush G. B. , MacMillan H. F. , Keyes B. M. , Levi D. H. , Melloch M. R. , Ahrenkiel R. K. , Lundstrom M. S.
Published: July 2002

Time-resolved photoluminescence decay measurements are used to explore minority carrier recombination in n-type GaAs grown by metalorganic chemical vapor deposition, and doped with selenium to produce electron concentrations from 1.3×1017 cm−3 to 3.8×1018 cm−3. For electron densities n0<1018 cm−3, the lifetime is found to be controlled by radiative recombination and photon recycling with no evidence of Shockley–Read–Hall recombination. For higher electron densities, samples show evidence of Shockley–Read–Hall recombination as reflected in the intensity dependence of the photoluminescence decay. Still, we find that radiative recombination and photon recycling are important for all electron concentrations studied, and no evidence for Auger recombination was observed.
On the dielectric response of complex layered oxides: Mica-type silicates and layered double hydroxides
Authors: Mehrotra Vivek , Giannelis Emmanuel P.
Published: July 2002

The dielectric properties of mica-type silicates and layered double hydroxides have been studied in the pristine and various intercalated forms in the frequency range 101–107 Hz. A relaxation peak has been observed for the pristine silicate, whereas the pristine layered double hydroxide exhibits an anomalous low-frequency dispersion. The dielectric response is rationalized in terms of structural ordering and fluctuation of charge carriers as well as models invoking fractal time processes and fractal structure. The response is also related to the structure and mobility of the intercalated water molecules. In both pristine hosts, the predominant conduction mechanism is proton hopping between sites generated by a network of intercalated water molecules. Silicate intercalated with the insulating form of polyaniline exhibits an almost frequency-independent response. In the case of conducting polyaniline intercalated silicate, where polarons are the majority charge carriers, an anomalous low-frequency dispersion is observed and the response is typical of a metal-insulator composite. Finally, impedance measurements have been used to calculate the spatial disorder and/or surface irregularity of the host layers, expressed by the fractal dimension ds. The changes observed in ds upon intercalation of high-charge ions are correlated to the stacking disorder of the host layers.
Variation of the refractive index and polarizability of sapphire under high pressures
Authors: Balzaretti N. M. , Denis J. P. , da Jornada J. A. H.
Published: July 2002

In this article we present the experimental results for the pressure variation of the refractive index of sapphire up to 16 GPa, obtained with an interferometric method, using the diamond anvil cell. In the range of hydrostatic pressures, up to about 12 GPa, the analysis of the results with the classic Lorentz–Lorenz approach provides a nearly linear relation between polarizability (α) and volume, corresponding to a constant strain polarizability parameter. For pressures above 12 GPa, there is a substantial nonlinear deviation, associated with nonhydrostatic effects inside the diamond anvil cell.
Fractal formation in a-Si:H/Ag/a-Si:H films after annealing
Authors: Bian Bo , Yie Jian , Li Boquan , Wu Ziqin
Published: July 2002

The crystallization behavior of a-Si:H/Ag/a-Si:H sandwich films has been studied in detail. Fractals of Si caused by metal enhanced crystallization appear after annealing at 350–600 °C. The fractal dimension decreases (the Si fractals become more open) with the increasing annealing temperature. The number density of fractals increases at 350–450 °C and turns to decrease at 500–600 °C. The average fractal size increases from 350 to 550 °C and shows a decreasing tendency at 600 °C. The formation of fractals can be explained by a random successive nucleation and growth model. The x-ray energy dispersive spectroscopy (EDS) microanalysis indicates that although there is lateral interdiffusion of Ag and Si atoms, the thicknesses of the fractal region and the matrix remain nearly the same. At the same time, EDS shows that there are also Ag aggregates extending out of the films. It is suggested that besides the preferred nucleation at the Ag/Si interface the break of Si—H bonds may also stimulate the crystallization of a-Si:H so that the crystallization temperature of an a-Si:H/Ag system is much lower than that of an a-Si/Ag system.
Fe-Al2O3 nanocomposites prepared by high-energy ball milling
Authors: Linderoth So/ren , Pedersen Michael S.
Published: July 2002

Nanocomposites of α-Fe and α-Al2O3, prepared by high-energy ball milling, exhibit coercivities which are enhanced by about two orders of magnitude with respect to the bulk value. The degree of enhancement depends on the volume fraction (xv) of Fe, with a maximum for xv≊0.25. The effect is ascribed to the production of single-domain magnetic grains. Mössbauer spectroscopy reveals the presence of iron oxide phases which could not be seen by x-ray and electron diffraction measurements.
Thermal effects in end-pumped Nd:phosphate glasses
Authors: Yan Li , Lee Chi H.
Published: July 2002

Thermal effects in continuous-wave end-pumped Nd:phosphate glass were studied. Thermally induced fracture was due to the tangential stress at the peripheries of the samples near the front entrance. For pump beam size much smaller than the diameter of a cylinder rod or the thickness of the cross section of a rectangular slab, the critical pump power causing thermal fracture depends only on the heat absorbed per unit length. The focal length of thermal lensing was measured to be ∼1 cm at an incident pump power of 2 W. Thermally induced birefringence significantly affected the laser oscillation.
Oxygen diffusion in cuprate superconductors
Authors: Routbort J. L. , Rothman S. J.
Published: July 2002

Superconducting properties of the cuprate superconductors depend on the oxygen content of the material; the diffusion of oxygen is thus an important process in the fabrication and application of these materials. In the present article, we review studies of the diffusion of oxygen in La2−xSrxCuO4, YBa2Cu3O7−δ, YBa2Cu4O8, and the Bi2Sr2Can−1CunO2n+4 (n=1 or 2) superconductors, and attempt to elucidate the atomic mechanisms responsible.
Growth of gallium nitride by electron-cyclotron resonance plasma-assisted molecular-beam epitaxy: The role of charged species
Authors: Molnar R. J. , Moustakas T. D.
Published: July 2002

The role of ionic and nonionic excited species of nitrogen in the growth of GaN thin films by electron-cyclotron resonance (ECR) plasma-assisted molecular-beam epitaxy has been investigated. It was found that the kinetics of film growth is significantly affected by the microwave power in the ECR discharge. Specifically, a transition from the island to a layer-by-layer and, finally, to a three-dimensional growth has been observed as a function of power. These morphological changes are accompanied by degradation of the electrical and luminescence properties, a result attributable to increased native defects and impurities. Secondary-ion-mass spectroscopic (SIMS) analysis indicates that impurity levels in the films increase with the plasma power levels used during the growth. To study the relative role of ion-induced native defects in these films, strategies for charged species extraction were developed by using an off-axis solenoid to modify the magnetic environment during growth. Films grown under a reduced ionic/excited neutral ratio environment show marked improvement in the electrical and luminescence properties. These data, together with SIMS analysis, indicate that observed improvements in these films are due to a reduction of native defects and not impurities.
Reaction of Ta thin film with single crystalline (001) β-SiC
Authors: Chen J. S. , Kolawa E. , Nicolet M.-A. , Ruiz R. P. , Baud L. , Jaussaud C. , Madar R.
Published: July 2002

The reaction between a sputter-deposited Ta film (320 nm thick) and a single crystalline (001) β-SiC substrate induced by vacuum annealing at temperatures of 600–1200 °C for 1 h (30 min at 1100 °C) is investigated by 3 MeV He++ backscattering spectrometry, x-ray diffraction, secondary ion mass spectrometry, and transmission and scanning electron microscopies. No significant reaction is observed at 800 °C or at lower temperatures. At 900 °C, the main product phases are Ta2C and carbon-stabilized Ta5Si3. A minor amount of unreacted Ta is also present. After annealing at 1000 °C, all the tantalum has reacted; the reaction zone possesses a multilayered structure of β-SiC/TaC/carbon-stabilized Ta5Si3/α-Ta5Si3/Ta2C. The diffusion path at 1000 °C is plotted on the isothermal section of the Ta-Si-C phase diagram. At 1100 °C, the reacted layer has an interface with the SiC substrate that is still quite flat but has a rough surface due to the formation of macroscopic voids within the reacted layer. The equilibrium products predicted by the phase diagram are TaC and TaSi2. This final state is reached by annealing at 1200 °C for 1 h. At that point, the reacted layer has a laterally very uneven structure and morphology.
Band states and shallow hole traps in Pb(Zr,Ti)O3 ferroelectrics
Authors: Robertson J. , Warren W. L. , Tuttle B. A.
Published: July 2002

Band structure calculations and electron paramagnetic resonance measurements are used to show that Pb states determine many of the electronic properties of Pb(Zr,Ti)O3 ferroelectric materials. The valence-band edge consists of hybridized Pb s and O p states at all compositions. The conduction-band minimum changes from a Ti d-like Γ25′ state to a Pb p-like X1 state with increasing Zr content. The Pb p character accounts for the relatively small 0.2 eV increase in band gap in the Pb(Ti,Zr)O3 alloys with Zr content compared to the large 2 eV increase in band gap in Ba(Ti,Zr)O3 alloys. The paramagnetic Pb3+ hole center is found to become deeper and acquire some p character as the Zr content is raised. This is attributed to the change in conduction-band character combined with a local off-center displacement of the Pb3+ ion.
Incoherent electron holography
Authors: Ru Q.
Published: July 2002

Amplitude-division electron holography achieved in a nonbiprism transmission electron microscope is presented. A single-crystal thin film is used to divide the amplitude of the incident electron beam. The lattice fringes of the crystal film are used to form off-axis holograms. The relation between the spatial coherence of the illuminating beam and the resolution of the reconstructed image is derived and compared with that of the conventional wave-front-division holography achieved with a biprism. It is shown that using a low-coherence illuminating beam will reduce the resolution of the reconstructed images in the amplitude-division holography, whereas it will reduce sensitivity in conventional wave-front-division holography. Experimental results obtained using a gold crystal thin film as the beam splitter show that the amplitude-division electron holography permits any coherent or incoherent illumination when high resolution is not strongly required. Advantages over the wave-front-division electron holography achieved with a biprism system have been pointed out.
LiTaO3 crystal periodically poled by applying an external pulsed field
Authors: Zhu Shi-ning , Zhu Yong-yuan , Zhang Zhi-yong , Shu Hong , Wang Hai-feng , Hong Jing-fen , Ge Chuan-zhen , Ming Nai-ben
Published: July 2002

A method of periodically poling LiTaO3 single crystal at room temperature by applying an external pulsed field is proposed. The relationship between the growth of inverted domains and switching current as well as switching time has been studied. The growth of inverted domains can be controlled by the duration of the pulsed field. The domain structure with period Λ≳8 μm has been fabricated in a 0.3-mm-thick plate of LiTaO3 by partial switching.
Inverse analysis for estimating the electron-phonon coupling factor in thin metal films
Authors: Orlande H. R. B. , Özisik M. N. , Tzou D. Y.
Published: July 2002

The electron-phonon coupling factor controls the heat transfer between electrons and lattice when thin metal films are heated by laser pulses with duration of order of femtoseconds. An inverse analysis is used to estimate this factor from transient measurements of the normalized variation of reflectivity and transmissivity of the metal film. The sensitivity coefficients are examined in order to determine the optimum time interval for the measurements. Results obtained with simulated measurements containing random errors show that the inverse analysis provides accurate estimates for the electron-phonon coupling factor. A statistical analysis is performed for the evaluation of the confidence interval for the estimates. Actual experimental data of reflectivity and transmissivity available in the literature are used to estimate the electron-phonon coupling factor for films of gold and copper.
Correlation of magnetic Barkhausen noise with core loss in oriented 3% Si–Fe steel laminates
Authors: Krause Thomas W. , Szpunar J. A. , Birsan M. , Atherton David L.
Published: July 2002

The angular dependence of surface magnetic Barkhausen noise (MBN) with flux densities up to 1.7 T on eight different samples of oriented 3% Si–Fe steel laminate was investigated. On two of the samples encircle MBN measurements were performed. MBN energy values were obtained by integrating the square of the MBN voltage signal with respect to time. The angular variation of the MBN energy signal was modeled by considering anisotropic internal fields that moderate 180° domain wall motion and the local eddy current field interactions between spatially correlated Barkhausen events. The eddy current field interactions arising between 180° domain walls that lie along the sample’s primary easy axis direction were represented by a parameter, α′. α′ had a cos θ angular dependence where θ is the angle of the applied sweep field with respect to the sample rolling direction. The angular averaged MBN energy signal, 〈Energy〉, was evaluated and compared with core losses measured using the standard Epstein technique at 1.5 T. Surface MBN measurements on samples with higher 〈Energy〉 demonstrated lower core loss. This was associated with a larger proportion of microscopic eddy currents generated parallel to the sample plane which were, therefore, uncoupled from macroscopic eddy currents generated by the bulk magnetization of the sample. Larger surface 〈Energy〉 signals were identified with a finer domain wall structure. For samples with a nonzero α′, the core loss was observed to decrease with increasing α′ magnitude. This was interpreted as a result of a reduction of microscopic eddy currents arising from an increase in interactions between simultaneously moving 180° domain walls.
Negative surface ionization of hydrogen atoms and molecules
Authors: Seidl M. , Cui H. L. , Isenberg J. D. , Kwon H. J. , Lee B. S. , Melnychuk S. T.
Published: July 2002

The total yield of H− ions, Y(Ein), produced in backscattering of low-energy H+ and H+2 ions from polycrystalline gold, tungsten, and molybdenum converter surfaces was measured at normal incidence in the energy range Ein=2–30 eV per nucleus. The yield per nucleus is independent of the ion mass. This indicates that the molecular ions are dissociated before colliding with the converter surface. A universal expression for Y(Ein) was developed by combining the electron tunneling theory with atomic scattering theory. This expression agrees well with measurements. The yield is completely characterized by two parameters, Eth/RE and RNη0, which can be determined experimentally: Y=0 for Ein=Eth/RE, and Y approaches the maximum yield Rη0 as Ein increases. These parameters were determined from measured H− yields in ion beam backscattering experiments, as well as for backscattering of thermal distributions of hydrogen atoms. For beam experiments, the maximum yield of 0.3 per nucleus was obtained for Mo/Cs converters with 1.5 eV work function. A higher maximum yield of 0.42 was obtained from experiments on backscattering thermal distributions of H atoms. This is attributed to high extraction fields. The universal yield formula made it possible to compare the results of the two different types of experiments.
Size dependence of the magnetoresistance in submicron FeNi wires
Authors: Adeyeye A. O. , Bland J. A. C. , Daboo C. , Lee Jaeyong , Ebels U. , Ahmed H.
Published: July 2002

The field-dependent magnetoresistance (MR) characteristic and magnetic hysteresis behavior has been studied in 300–500 Å thickness Ni80Fe20 wires of variable width (w) in the range from 0.2 to 10 μm. As the width of the wire decreases, a marked increase in the easy axis coercive field is seen for fields applied along the wire axis and the form of the MR characteristic is markedly modified for the in-plane perpendicular hard axis direction with a large field-dependent MR response observed for applied field strengths exceeding the edge demagnetizing field. The low field hard axis results are discussed in terms of an inhomogeneous spin configuration across the width of the wires arising from the spatial variation of the demagnetizing field.
Electrical conduction in platinum–gallium nitride Schottky diodes
Authors: Suzue K. , Mohammad S. N. , Fan Z. F. , Kim W. , Aktas O. , Botchkarev A. E. , Morkoç H.
Published: July 2002

Gallium nitride is a highly promising wide band gap semiconductor with applications in high power electronic and optoelectronic devices. Among the devices considered for high power generation is the ubiquitous field-effect transistors which require Schottky barriers for modulating the channel mobile charge. It is in this context that we have undertaken an investigation of likely metal-GaN contacts. Here we report on the electrical conduction and other properties of Pt–GaN Schottky diodes. These Schottky diodes were fabricated using n-GaN grown by the molecular beam epitaxy method. Both capacitance–voltage and current–voltage measurements have been carried out as a function of temperature to gain insight into the processes involved in current conduction. Based on these measurements, physical mechanisms responsible for electrical conduction at low and high voltages and temperatures have been suggested. Schottky barrier height determined from the current–voltage and capacitance–voltage measurements is close to 1.10 eV.
Competition between band gap and yellow luminescence in GaN and its relevance for optoelectronic devices
Authors: Grieshaber W. , Schubert E. F. , Goepfert I. D. , Karlicek R. F. , Schurman M. J. , Tran C.
Published: July 2002

The competition between band gap and the 2.2 eV (yellow) luminescence of epitaxial GaN is studied for excitation densities ranging from 5×10−6 to 50 W/cm2. The ratio of the peak intensities of the band gap-to-yellow luminescence changes from 4:1 to 3000:1 as the excitation density is increased by 7 orders of magnitude. At room temperature, the band gap luminescence linewidth is 2.3kT, close to the theoretical minimum of 1.8kT. A model is developed describing the intensity of the two radiative transitions as a function of the excitation density. This model is based on bimolecular rate equations and takes into account shallow impurities, deep levels, and continuum states. The theoretically predicted dependences of the two different luminescence channels follow power laws with exponents of 1/2, 1 and 3/2. Thus the intensity of the yellow luminescence does not saturate at high excitation densities. These dependences are in excellent agreement with experimental results. The relevance of the results for optoelectronic GaN devices is discussed. It is shown that the peak intensity of the yellow luminescence line is negligibly small at typical injection currents of light-emitting diodes and lasers.
Correlation between dielectric constant and chemical structure of sodium silicate glasses
Authors: Hsieh C. H. , Jain H. , Kamitsos E. I.
Published: July 2002

The chemical structure of sodium aluminosilicate glasses is determined by high resolution x-ray photoelectron spectroscopy (XPS) as silicon is gradually replaced by aluminum. A well-defined chemical state is found for silicon, aluminum, and sodium atoms, while three different environments are identified for oxygen atoms corresponding to Si–O–Si, Si–O–Al, and Si–O–Na bonds. The binding energy of Na 1s photoelectrons increases significantly with increasing aluminum substitution while that of Al 2p and components of O 1s photoelectrons remains approximately constant. Thus, the ionicity of sodium increases with aluminum amount, but the over all electron density around silicon, aluminum, and different types of oxygen atoms remains unchanged. The dielectric constant of the glasses increases with increasing aluminum substitution. It is analyzed in terms of the polarizabilities of constituent structural units, viz., silicon tetrahedra, nonbridging oxygen–sodium ion pairs, and aluminum tetrahedron–sodium ion pairs. The electronic polarizability of oxygen ions depends linearly on their negative charge and can be correlated to the O 1s XPS binding energy. The ionic polarizability of sodium ions increases with an increasing aluminum amount, and correlates directly with the Na 1s XPS binding energy.
Preferred orientation of TiN films studied by a real time synchrotron x-ray scattering
Authors: Je J. H. , Noh D. Y. , Kim H. K. , Liang K. S.
Published: July 2002

The orientational crossover phenomena in a radio frequency (rf) sputtering growth of TiN films were studied in in situ, real time synchrotron x-ray scattering experiments. For the films grown with pure Ar sputtering gas, the crossover from the (002)-oriented grains to the (111)-oriented grains occurred as the film thickness was increased. As the sputtering power was increased, the crossover thickness, at which the growth orientation changes from the 〈002〉 to the 〈111〉 direction, seemed to decrease. The addition of N2 besides Ar as sputtering gas suppressed the crossover, and consequently resulted in the (002) preferred orientation without exhibiting the crossover. We attribute the observed crossover phenomena to the competition between the surface and the strain energy. The x-ray powder diffraction, the x-ray reflectivity, and the ex situ atomic force microscopy surface topology studies consistently suggest that the microscopic growth front was in fact always the (002) planes. In the initial stage of growth, the (002) planes were aligned to the substrate surface to minimize the surface energy. At later stages, however, the (002) growth front tilted away from the surface by about 60° to relax the strain, which caused the crossover of the preferred growth direction to the 〈111〉 direction.
Evidence of partial disorder in a frustrated Heisenberg spin system
Authors: Santamaria C. , Diep H. T.
Published: July 2002

We study the classical Heisenberg spin model on a bcc lattice with nearest neighbor interaction. Next-nearest neighbor interaction on one sublattice is included to create a frustration. We show, for the first time, evidence of a partial disorder in this frustrated vector spin system at equilibrium at finite temperature. Such an effect found commonly in frustrated Ising spin systems has never been encountered before in a vector spin system. A phase diagram is shown and discussed.
Magnetization switching in nanoscale ferromagnetic grains: Simulations with heterogeneous nucleation
Authors: Kolesik M. , Richards Howard L. , Novotny M. A. , Rikvold Per Arne , Lindgård Per-Anker
Published: July 2002

We present results obtained with various types of heterogeneous nucleation in a kinetic Ising model of magnetization switching in single-domain ferromagnetic nanoparticles. We investigate the effect of the presence of the system boundary and make comparison with simulations on periodic lattices. We also study systems with bulk disorder and compare how two different types of disorder influence the switching behavior.
n-Type conduction in Pb doped Se–In chalcogenide glasses
Authors: Mehra R. M. , Kohli Sandeep , Pundir Amit , Sachdev V. K. , Mathur P. C.
Published: July 2002

This paper reports on the p to n transition in Pb doped Se–In chalcogenide glasses. Measurements of thermoelectric power in the temperature range 300 K⩽T⩽315 K, dc conductivity in the temperature range 100⩽T⩽300 K, and optical band gap (Egopt) have been carried out for Se75In25−xPbx (x=0,5,10,15) samples. The p-n transition occurs with very low addition of Pb impurity (5 at. %). The conductivity and pre-exponential factor also change by five to six orders of magnitude with Pb doping. Results are explained on the basis of the formation of ionic Pb–Se bonds, instead of covalent bonds. Formation of ionic bonds disturbs the equilibrium between the charged defect states of Se–In glass and unpins the Fermi level and thus leads to n-type conduction in these glasses.
Domain switching and spatial dependence of permittivity in ferroelectric thin films
Authors: Chai Francis K. , Brews J. R. , Schrimpf R. D. , Birnie D. P.
Published: July 2002

A domain model consistent with the measured capacitance–voltage (CV) characteristics of lead zirconate titanate (PZT) capacitors is proposed. Two variants of this model are presented and compared with experimentally measured CV data. The basic model is developed adopting a macroscopic electric field that is spatially uniform through the depth of the film. Then, this model is generalized to allow a variation of the electric field with depth and to include a physically reasonable, position-dependent domain structure. Specifically, the spatial variation of the electric field is related to dopant–ion charges. As a result of the interaction between the domain properties and the electrical doping, a position dependent permittivity is induced, and the electrical properties of the capacitors are affected. Finally, computer simulations to fit the measured CV characteristics are performed to help understand the extent of the coupling between the domain properties and the electrical doping. It is found that there is a minimum doping level below which the doping does not affect the CV characteristic. A method for determining this minimum doping level from the CV curve is presented. The analysis of observed CV data demonstrates that niobium doping is responsible for partially compensating the p-type nature of PZT thin films. For the films measured here, the minimum noticeable doping level is about 1018 cm−3. It is also found that niobium doping slows the growth rate of polarization as the electric field increases, and has a tendency to increase the coercive field.
Angular dependence of nucleation by curling in a prolate spheroid
Authors: Aharoni Amikam
Published: July 2002

The nucleation field is calculated for a small ferromagnetic prolate spheroid as a function of its size, its elongation, and the angle between the applied field and the spheroid axis. If there is no other switching mode except for magnetization curling and coherent rotation, these results probably represent the coercivity. The calculation is rigorous only for an isotropic particle, but a first order approximation is also given for a uniaxial or cubic anisotropy whose easy axis is parallel to the long axis of the spheroid.
Stress and frequency dependence of the direct piezoelectric effect in ferroelectric ceramics
Authors: Damjanovic Dragan
Published: July 2002

It is shown that at weak alternating stress the relationship between the piezoelectrically induced charge and applied stress in ferroelectric ceramics has the same form as the Rayleigh law for magnetization versus magnetic field in ferromagnetic materials. Applicability of the Rayleigh law to the piezoelectric effect is demonstrated in detail for lead zirconate titanate (PZT) ceramics. Experimental results indicate that the dominant mechanism responsible for piezoelectric hysteresis and the dependence of the piezoelectric coefficient on the applied ac stress is the pinning of non-180° domain walls. The dependence of the piezoelectric coefficient on the frequency of the driving stress is examined and is shown to be due to the frequency dispersion of both reversible and irreversible components of domain-wall displacement. Analysis of the stress dependence of the piezoelectric phase angle reveals piezoelectric hysteresis contributions that are not necessarily due to Rayleigh-type displacement of domain walls. Piezoelectric properties of a modified lead titanate composition that exhibits non-Rayleigh type behavior are examined and compared with the properties of PZT ceramics.
Calculation of the electron initiated impact ionization transition rate in cubic and hexagonal phase ZnS
Authors: Bellotti E. , Brennan K. F. , Wang R. , Ruden P. P.
Published: July 2002

The wave-vector dependent, electron initiated, impact ionization transition rates for the cubic and hexagonal phases of ZnS have been numerically determined using a pseudopotential calculated band-structure and wave-vector dependent dielectric function. The rates for both phases show a strong dependence on the relative band structures. A detailed band-by-band analysis of the transition rate in the first Brillouin zone has been performed to determine the wave-vector dependence of the rate. The energy dependent rates have also been computed by averaging the transition rate over energy. It is found that the hexagonal phase of ZnS exhibits a higher ionization transition rate than the cubic phase. A preliminary estimation of the nature of the threshold has been made using a previously published phonon scattering rate.
High power density laser etching of silicon
Authors: Ngan M. L. , Lee K. C. , Cheah K. W.
Published: July 2002

Silicon that was immersed in hydrofluoric acid can be etched photochemically by laser, and it is found to produce long and regular columnar structure if the laser power density is greater than 10 mW/mm2. Another criterion is that the laser wavelength should be at the blue end of the visible spectrum. Fine wires with diameter 300–200 nm were also observed at the top of these columns. The dimension of these fine wires is near to quantum confinement dimension and thus can be taken as supporting evidence for quantum confinement. The photoluminescence spectra full width half maximum was narrower than that from porous silicon fabricated from the conventional anodization method. The narrower full width was attributed to the uniformity of the porous silicon structure. A physical model is proposed to explain the observed strong directional etching. The model showed that once the etch sites have been randomly initiated, the etching rate becomes directional under the influence of the laser. The intensity of the laser controls the etching direction such that silicon columns are formed if the intensity of the laser is strong enough.
Electroluminescence from heterostructures of poly(phenylene vinylene) and inorganic CdSe nanocrystals
Authors: Mattoussi Hedi , Radzilowski Leonard H. , Dabbousi Bashir O. , Thomas Edwin L. , Bawendi Moungi G. , Rubner Michael F.
Published: July 2002

Electroluminescence (EL) and photoluminescence (PL) from heterostructure thin films made of organic poly (phenylene vinylene), PPV, and inorganic semiconductor CdSe nanocrystals are investigated. In these devices, the organic PPV structure is built next to an indium tin oxide anode, using the technique of molecular layer-by-layer sequential adsorption, and serves primarily as the hole transport layer. The inorganic layer, adjacent to an Al electrode, is made of spin cast CdSe nanocrystals, passivated with either organic groups or with a wider band gap semiconductor, e.g., ZnS in the present case. We find that the electroluminescence signal is almost exclusively generated within the inorganic layer, with a very weak contribution from the PPV layer at higher applied voltage. The performance of these heterostructure devices is influenced by the thickness of the dot layer. Lifetime tests reveal promising stability, with devices operating continuously over 50–100 h. Values of the external quantum efficiency, ηext, as high as 0.1% are reached. The quantum efficiency is not enhanced by the presence of ZnS overcoating, as opposed to the observed increase in the PL quantum yield. This reflects a difference in the efficiency of charge injection into the nanocrystals due to the ZnS overlayer.
Comparison of a rate-equation model with experiment for the resonant tunneling diode in the scattering-dominated regime
Authors: Mattia J. P. , McWhorter A. L. , Aggarwal R. J. , Rana F. , Brown E. R. , Maki P.
Published: July 2002

A quasistatic rate-equation model for the resonant tunneling diode (RTD) is presented. In this model, the RTD is divided into three regions, each assumed in quasithermal equilibrium. The electron transfer between states in adjacent regions, assumed to be by elastic tunneling, is then proportional to the probability that the initial state is occupied and the final state is empty. Using this approach, we derive a small-signal equivalent circuit. The model parameters may be either calculated or measured in a fairly straightforward manner, as is demonstrated for an In0.53Ga0.47As/AlAs RTD. We find that the dc characteristic and ac behavior up to 4 GHz are modeled well; we also find that the measured and calculated parameters agree to approximately a factor of 2 except in the vicinity of the current valley. We show that this approach is accurate enough to provide insight into the operation of the device, yields results that are simple enough to be suitable for ac circuit simulation, and should be valid as long as the frequency of the small-signal excitation is much less than the scattering rate.
Amorphous B–C–N semiconductor
Authors: Yao B. , Chen W. J. , Liu L. , Ding B. Z. , Su W. H.
Published: July 2002

Amorphous BC2N powders were prepared by mechanical milling with hexagonal boron nitride and graphite as starting materials. A bulk amorphous BC2N compound was produced by sintering the as-milled amorphous BC2N powders in a vacuum of 10−5 Torr at a temperature of 1470 K. The conductivity measurement for the bulk amorphous BC2N compound showed that it behaves as a semiconductor with band gap energy of 0.11 eV for temperatures ranging from room temperature to 560 K and a semimetal for temperatures between 560 and 740 K. The mechanism of the formation of the amorphous BC2N powders is discussed.
The molten state of graphite: An experimental study
Authors: Musella M. , Ronchi C. , Brykin M. , Sheindlin M.
Published: July 2002

A subsecond laser heating technique has been successfully applied for graphite melting under controlled isobaric conditions. During the applied pulses, relatively large amounts of graphite were melted and subsequently solidified under good stability conditions of the liquid mass. The solid–liquid-vapor triple point was determined. From metallographic analysis of the quenched liquid, the expansion upon melting could be estimated. A mathematical model was then applied to analyze the measured thermograms and the thermal conductivity of liquid carbon was deduced. Both experimental observations and calculation results indicate a nonmetallic nature of liquid carbon in the pressure range of 110–2500 bar. Finally, an analysis of the melting line Tm(p) based on Simon’s empirical equation of state confirms the self-consistency of all results obtained.
Electromigration damage and failure distributions in Al-4 wt % Cu interconnects
Authors: Shih W. C. , Greer A. L.
Published: July 2002

The spatial distributions of damage and failure are characterized in entire test lines under electromigration. The distribution of hillocks is random but that of voids is not, showing clear evidence for the roles of temperature gradients (at the ends of the lines only) and of the near-bamboo microstructure. The evolution of void distributions is consistent with local microstructural observations on void splitting and coalescence. Mass flow from voids to hillocks is not detected in the damage distributions, but damage spacings do correlate with grain structure. Statistically valid characterization of damage and failure distributions can assist in identifying the controlling factors in the development of electromigration damage and failure.
Picosecond thermal pulses in thin metal films
Authors: Al-Nimr M. A. , Arpaci V. S.
Published: July 2002

A simplified approach is introduced to describe the thermal behavior of a thin metal film exposed to picoseconds thermal pulses. The approach is based on the assumption that the metal film thermal behavior occurs in two separate stages. In the first stage, electron gas transmits its energy to the solid lattice through electron-phonon coupling and other mechanisms of energy transport are negligible. In the second stage, electron gas and solid lattice are in thermal equilibrium, the energy transfer through electron-phonon coupling is negligible, and thermal diffusion dominates. The proposed approach eliminates the coupling between the energy equations and the reduced differential equations are easier to handle. The proposed approach applies to metal films whenever GL2/Ke≫1.
Hall-array gradiometer for measurement of the magnetic induction vector in superconductors
Authors: Abulafia Y. , Wolfus Y. , McElfresh M. , Shaulov A. , Yeshurun Y. , Paltiel Y. , Shtrikman H. , Zeldov E.
Published: July 2002

An experimental technique for measuring the distribution of the normal and planar components of the magnetic induction near the surface of a superconducting sample is described. This technique utilizes a design of a double-layered Hall sensor array fabricated from a GaAs/AlGaAs heterostructure containing two parallel layers of a two-dimensional electron gas. Applications of this technique are demonstrated in measuring the current density distribution and in characterizing the flux creep process in a thin YBa2Cu3O7 crystal.
Charge carrier trapping effect by luminescent dopant molecules in single-layer organic light emitting diodes
Authors: Uchida Masahiro , Adachi Chihaya , Koyama Toshiki , Taniguchi Yoshio
Published: July 2002

We investigated electroluminescent (EL) characteristics of single-layer organic light emitting diodes (SOLEDs). Our SOLED devices are composed of an inert polymer as a binder, in which hole transport molecules, emissive electron transport molecules (ETMs), and highly fluorescent dopants as luminescent centers are dispersed. We examined two typical dopants: rubrene and coumarin 6. These exhibited different charge carrier recombination and emission mechanisms. The dopant concentration dependence of the current density–voltage–luminance relationships clearly showed the importance of carrier trapping by dopant molecules for obtaining high luminance. When the dopant was rubrene, we observed that charge carriers were well trapped by the dopant molecule. This means that direct recombination of holes and electrons occurred on the dopant molecules and trapping significantly enhanced the external EL quantum efficiency ΦEL. For coumarin 6, on the other hand, we observed that charge carriers primarily recombined at the emissive ETMs and that the energy transfer from the host to the guest coumarin 6 molecule dominated the EL process. A comparison of these distinct processes revealed that carrier trapping by dopant molecules was necessary to enhance ΦEL in SOLED devices. In our best SOLED device with rubrene as a dopant, we measured luminance of 2800 cd/m2 at J=100 mA/cm2, which corresponds to ΦEL=0.85%.
Equivalent retarder approach to reflective liquid crystal displays
Authors: Stallinga S.
Published: July 2002

Reflective liquid crystal displays (LCDs) are studied using the Jones 2×2 matrix method. The reflective LCD effectively behaves as a single retardation layer. Conditions on the retardation and optical axis orientation of this equivalent retarder in order to obtain high brightness and high contrast are derived and applied to twisted nematic layers without and with a compensating waveplate. The optimization of the display performance by numerical calculations is greatly simplified by analytical results relating the parameters of the liquid crystal, incident polarization, and compensator.
Depletion region effects in Mg-doped GaN
Authors: Kozodoy Peter , DenBaars Steven P. , Mishra Umesh K.
Published: July 2002

The deep nature of the Mg acceptor will have important implications for the performance of high-speed GaN-based bipolar devices. In this work, the effect of the deep acceptor on the band bending within the depletion region is examined in detail. The width of the transition region, which separates the mobile holes from the space-charge edge, is carefully investigated. High-frequency modulation of the depletion region is discussed for both the large- and small-signal cases. For the small-signal case, calculated results are compared to experimental measurements of frequency-dependent capacitance which have been performed on Mg-doped GaN samples.