bluuebunny/crossref_metadata_2025 huggingface dataset:
Free-carrier absorption in Be- and C-doped GaAs epilayers and far infrared detector applications
Authors: Korotkov A. L. , Perera A. G. U. , Shen W. Z. , Herfort J. , Ploog K. H. , Schaff W. J. , Liu H. C.
Published: July 2002

Far infrared (FIR) absorption, reflection, and transmission in heavily doped p-GaAs multilayer structures have been measured for wavelengths 20–200 μm and compared with the calculated results. Both Be (in the range 3×1018–2.6×1019 cm−3) and C (1.8×1018–4.7×1019 cm−3)-doped structures were studied. It is found that the observed absorption, reflection, and transmission are explained correctly by the model with a dominant role of free-carrier absorption in highly doped regions. High reflection from heavily doped thick layers is attractive for the resonant cavity enhanced FIR detectors.
Effect of heating ramp rates on transient enhanced diffusion in ion-implanted silicon
Authors: Mannino G. , Stolk P. A. , Cowern N. E. B. , de Boer W. B. , Dirks A. G. , Roozeboom F. , van Berkum J. G. M. , Woerlee P. H. , Toan N. N.
Published: July 2002

Boron marker-layer structures have been used to analyze the heating ramp-rate dependence of transient enhanced dopant diffusion (TED) during rapid thermal annealing of Si implantation damage. The study uses short anneals with heating ramp rates in the range 0.1–350 °C/s, and peak temperatures in the range 900–1100 °C. Increasing the ramp rate is found to reduce the amount of profile broadening caused by TED, as well as reducing the smaller amount of normal “thermal-equilibrium” diffusion which is related to thermal budget. The results show why high ramp rates lead to improved B-implant activation and junction-depth control in Si devices. An Ostwald ripening model of interstitial-cluster evolution describes the detailed trends in the data and predicts further improvements in the case of ultrarapid annealing.
Anomalous ion heating from ambipolar-constrained magnetic fluctuation-induced transport
Authors: Gatto R. , Terry P. W.
Published: July 2002

A kinetic theory for the anomalous heating of ions from energy stored in magnetic turbulence is presented. Imposing self-consistency through the constitutive relations between particle distributions and fields, a turbulent Kirchhoff’s Law is derived that expresses a direct connection between rates of ion heating and electron thermal transport. This connection arises from the kinematics of electron motion along turbulent fields, which results in granular structures in the electron distribution. The drag exerted on these structures through emission into collective modes mediates an effective ambipolar constraint on transport. Resonant damping of the collective modes by ions produces the heating. In collisionless plasmas the rate of ion damping controls the rate of emission, and hence the ambipolar-constrained electron heat flux. The heating rate is calculated for both a resonant and nonresonant magnetic fluctuation spectrum and compared with observations. The theoretical heating rate is sufficient to account for the observed twofold rise in ion temperature during sawtooth events in experimental discharges.
The Jahn–Teller and related effects in the cyclopentadienyl radical. II. Vibrational analysis of the Ã 2A2″–X̃ 2E1″ electronic transition
Authors: Applegate Brian E. , Bezant Andrew J. , Miller Terry A.
Published: July 2002

The laser excited, jet-cooled Ã 2A2″–X̃ 2E1″ electronic spectrum of the cyclopentadienyl radical yields detailed information about the vibronic structure of both its Ã and X̃ states. A straightforward assignment of the Ã vibronic structure is presented. The X̃ state vibronic structure reveals a comprehensive picture of the Jahn–Teller distortion of its potential energy surface. The molecular parameters characterizing the Jahn–Teller interaction provide the stabilization energy and distorted geometry, which are compared to previous experimental and ab initio results.
Quantum dynamics: Path integral approach to time correlation functions in finite temperature
Authors: Sethia Ashok , Sanyal Sudip , Hirata Fumio
Published: July 2002

We propose a method to calculate time correlation functions using path integral formulation of quantum mechanics. The accuracy of the proposed method is examined by comparing the calculated result with exact and centroid molecular dynamics results.
Fast ignitor research at the Institute of Laser Engineering, Osaka University
Authors: Kodama R. , Mima K. , Tanaka K. A. , Kitagawa Y. , Fujita H. , Takahashi K. , Sunahara A. , Fujita K. , Habara H. , Jitsuno T. , Sentoku Y. , Matsushita T. , Miyakoshi T. , Miyanaga N. , Norimatsu T. , Setoguchi H. , Sonomoto T. , Tanpo M. , Toyama Y. , Yamanaka T.
Published: July 2002

The physics element relevant to the fast ignitor in inertial confinement fusion has been extensively studied. Laser-hole boring with enormous photon pressures into overcritical densities was experimentally proved by density measurements with XUV laser probing. Ultra-intense laser interactions at a relativistic parameter regime were studied with a 50-TW glass laser system and a 100-TW glass laser system synchronized with a long pulse laser system. In the study of relativistic laser beam propagation in a 100-μm scale-length plasma, a special propagation mode (super-penetration mode) was observed, where the beam propagated into overdense regions close to the solid target surface. At the super-penetration mode, 20% of the laser energy converted to energetic electrons toward the target inside, while the coupling efficiency was 40% without the long scale-length plasmas. The high-density energetic electron transport and heating of solid material was also studied, indicating beamlike propagation of the energetic electrons in the solid target and effective heating of solid density ions with the electrons. Based on these basic experimental results, the heating of imploded plasma by short-pulse-laser light with three different ways of injecting the heating pulse has been studied.
Direct observation of voids in the vacancy excess region of ion bombarded silicon
Authors: Williams J. S. , Conway M. J. , Williams B. C. , Wong-Leung J.
Published: July 2002

The results reported in this letter indicate that the spatial separation of the vacancy and interstitial excesses which result from ion bombardment gives rise to stable voids upon annealing at 850 °C even for implants where the projected ion range is only of the order of a few thousand Ångstrom. Such voids have been observed directly by transmission electron microscopy. Furthermore, in cases where both voids and interstitial-based defects are present at different depths, it is found that Au has a strong preference for decorating void surfaces and hence Au can, indeed, be used as a selective detector of open volume defects in Si.
High-frequency asymptotics for Maxwell’s equations in anisotropic media Part II: Nonlinear propagation and frequency conversion
Authors: Garnier Josselin
Published: July 2002

This paper is devoted to the derivation of the equations that govern the propagation and frequency conversion of pulses in noncentrosymmetric crystals. The method is based upon high-frequency expansions techniques for hyperbolic quasi-linear and semilinear equations. In the so-called geometric regime we recover the standard results on the frequency conversion of pulses in nonlinear crystals. In the diffractive regime we show that the anisotropy of the diffraction operator involves remarkable phenomena. In particular the phase matching angle of a divergent pulse depends on the distance between the waist and the crystal plate. Finally we detect a configuration where the beam propagation in a biaxial crystal involves the generation of spatial solitons thanks to an anomalous one-dimensional diffraction.
Enhanced metal–insulator transition and magnetoresistance in melt-processed La0.67Ca0.33MnO3 and Ho-doped manganites
Authors: Pradhan A. K. , Roul B. K. , Feng Y. , Wu Y. , Mohanty S. , Sahu D. R. , Dutta P.
Published: July 2002

We report significant enhancement of magnetoresistance (MR) in melt-processed La0.67Ca0.33MnO3 (LCM) and in Ho-doped LCM samples. The LCM system exhibits surprising enhancement of metal–insulator transition (TIM) that coincides with the Curie transition temperature (Tc), TIM≈Tc, illustrating the enhanced percolative transport and spin-polarization through grain boundaries. Surprisingly, 1 mol % of addition of Ho enhances Tc of ∼70 K with remarkable colossal MR. The nanoscale Ho distribution is consistent with the magnetic inhomogeneity-induced MR due to phase segregation in Ho-doped sample whereas charge-segregation picture is valid for undoped LCM.
Luminescence from Si nanocrystals in silica deposited by helicon activated reactive evaporation
Authors: Cheylan S. , Elliman R. G. , Gaff K. , Durandet A.
Published: July 2002

An alternative method is investigated for the preparation of Si-rich SiO2 films used for the fabrication of light-emitting Si nanocrystal structures. The technique, helicon-activated reactive evaporation (HARE), combines e-beam evaporation of silicon with plasma activation of a reactive argon–oxygen atmosphere, and has the advantage of being able to produce thick, H-free films suitable for planar photonic device applications. The nanocrystal-rich films were formed by annealing as-deposited films at 1100 °C for 1 h. Room temperature photoluminescence was then measured and compared with that from ion-implanted samples annealed under similar conditions. The HARE-deposited films exhibited strong visible luminescence for a range of excess Si concentrations, demonstrating their potential for the manufacture of such materials. The films also exhibited a concentration dependence comparable to that of ion-implanted samples: the luminescence intensity initially increased with excess Si concentration up to a maximum before decreasing with increasing concentration thereafter. The cause of the decrease at higher concentrations is briefly discussed.
Microwave propagation characteristics of a high-temperature superconducting variable spacing parallel plate transmission line
Authors: Wu Chien-Jang
Published: July 2002

A sinusoidal field solution for a high-temperature superconducting variable spacing parallel plate transmission line is derived. This solution demonstrates how to relate the propagation characteristics to the thicknesses and material parameters of the variable spacer and superconducting films in addition to the dielectric substrates. Then, a numerical analysis of the attenuation constant and phase velocity as a function of the dielectric spacer thickness is made. In addition, some further numerical results from this solution show that the roles played by the thicknesses of the two dielectric substrates are insignificant and can be neglected in practical microwave applications.
Growth and characterization of a novel In2Se3 structure
Authors: Groot C. H. de , Moodera J. S.
Published: July 2002

Thin films of In2Se3 deposited by thermal co-evaporation crystallize upon vacuum annealing almost single phase into an, up to now, unknown structure. Only when the films are capped with a thin oxide layer before annealing, the reportedly stable γ-In2Se3 structure, single phase and aligned along the c axis forms. Rutherford backscattering confirms an In to Se ratio of 2 to 3 for both structures. Nevertheless, the new structure has distinct x-ray diffraction peaks and Raman spectra. The new structure has a much lower resistivity than the γ-In2Se3 structure, consistent with its smaller electrical and optical energy gap. Both structures show large photoconductivity.
Inherent temperature effects in magnetic tunnel junctions
Authors: Davis A. H. , MacLaren J. M. , LeClair P.
Published: July 2002

Theoretical studies of the temperature dependence of the tunneling magnetoresistance ratio (TMR) are presented. A successful elastic tunneling model has been extended to handle temperature dependence. It treats Fermi smearing and applies Stoner-like behavior to the exchange split band structure in the electrodes to calculate TMR(T). As expected, the effects of Fermi smearing are small, but small changes in the magnetic band structure produce large changes in TMR. For a Co/I/Co junction produced by LeClair et al. [Phys. Rev. Lett. 84, 2933 (2000)], calculations using bulk magnetization predicted 33% of the experimental loss of TMR from 0 to 300 K with only a 1.5% change in magnetization. A mere 3.2% change in magnetization produced 100% of the observed drop in TMR. These results imply larger than imagined intrinsic temperature dependence for TMR.
Real time dynamics in epitaxial Fe(100) disks
Authors: Gardiner S. , Rothman J. , Xu Y. B. , Tselepi M. , Bland J. A. C. , Cheng Y. , Rousseaux F.
Published: July 2002

Real time resolved scanning Kerr microscopy has been used to study the switching dynamics of 50 μm diameter epitaxial Fe(100) disks. The measurements were performed using a sinusoidal sweeping field with a sweep rate of dH/dt=10 kOe/s. By performing repetitive one-shot measurements, we have mapped the statistical fluctuations and the probability distribution of characteristic switching parameters as the switching instant t0, and the switching speed, V. We observe a substantial difference in the parameters estimated from the average of several measurements compared to the parameters extracted from the probability distributions. This illustrates the potential risks of using averaging techniques in dynamic measurements, in addition to the loss of the statistical information. The disks were found to display an inhomogeneous switching, which is believed to be caused by defect damped motion of the domain walls and a inhomogeneous distribution of defects.
Ab initio nonequilibrium calculation of magnetic tunnel junctions
Authors: Uiberacker C. , Wang K. , Heide C. , Levy P. M.
Published: July 2002

We present a nonequilibrium band-structure calculation of a magnetic tunnel junction using the ab initio screened Korringa–Kohn–Rostoker multiple scattering formalism together with the local spin density approximation. We find that the current is determined by the surface density of states (DOS) and the bias dependence of the barrier in our Fe(100)/vacuum(30 Å)/Fe(100) model junction. The existence of quasilocalized states at the interfaces leads to the effect of negative differential resistance at low bias. For high bias the surface DOS yields an enhanced spin polarization of the current. Therefore, the magnetoresistance ratio becomes strongly quenched since the current is dominated by the minority channel for both the parallel and the antiparallel configuration of the magnetic electrodes.
Analysis of multiphase clocked electron pumps consisting of single-electron transistors
Authors: Amakawa Shuhei , Mizuta Hiroshi , Nakazato Kazuo
Published: July 2002

Pump circuits consisting of single-electron transistors are analyzed in which electrons are pumped by multiphase clock pulses. An optimal low-temperature operation condition is presented where pumped current is maximized, yet the power consumption is not. Approximate formulas for the number of electrons transferred per clock cycle and the power consumption are derived for that condition, which clearly show the advantage of the pump circuits for low-power applications. The power consumption becomes even less at higher temperatures. However, the relatively large island capacitance between transistors limits the operation temperature.
Intermolecular energy transfer after vibrational excitation of a perylene dye in solution, in polymer binder, and in a side-chain copolymer
Authors: Baier Johannes , Pösch Peter , Jungmann Gert , Schmidt Hans-Werner , Seilmeier Alois
Published: July 2002

Skeletal modes between 1550 cm−1 and 1750 cm−1 of the perylene chromophor are vibrationally excited in different surroundings via resonant absorption of ultrashort IR pulses. The vibrational energy transfer is monitored via electronic transitions to the S1 state with the help of a second, delayed visible pulse. In liquid chloroform solution of the dye an intermolecular energy transfer time of τe=(15±1) ps was determined. In the solid dye/PMMA blend the intermolecular energy transfer time was with τe=(10,5±1) ps substantially smaller. In the copolymer a similar time constant of τe=(9±1) ps is found. Obviously the spacer decouples in the copolymer not only the electronic systems but also the vibrational manifolds.
Study of dynamics and mechanism of metal-induced silicon growth
Authors: Guliants Elena A. , Anderson Wayne A.
Published: July 2002

The present study addresses the mechanism of metal-induced growth of device-quality silicon thin films. Si deposition was performed by magnetron sputtering on a 25-nm-thick Ni prelayer at 525–625 °C and yielded a continuous, highly crystalline film with a columnar structure. A Ni disilicide intermediate layer formed as a result of the Ni reaction with Si deposit provides a sufficient site for the Si epitaxial growth because lattice mismatch is small between the two materials. The reaction between Ni and Si was observed to progress in several stages. The NixSiy phase evolution in a Ni:Si layer was studied by x-ray photoelectron spectroscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, transmission electron microscopy, and x-ray diffraction and found to be controlled by the Ni-to-Si concentration ratio at the growing front. After Ni is completely consumed in the silicide, continued Si deposition leads to the nucleation and growth of Si crystals on the surface of the NiSi2 grains. The issues related to the nature of NixSiy phase transformations and Si heteroepitaxy are discussed.
Temperature dependence of tunnel magnetoresistance
Authors: Hagler T. , Kinder R. , Bayreuther G.
Published: July 2002

Electric transport through magnetic tunnel junctions (MTJs) has been studied at various temperatures to gain understanding of the transport mechanisms in such devices. Between 15 and 400 K, MTJs with Al2O3 barriers have been tested at low voltage (barrier height: 2.0–2.1 eV, barrier width: 1.5 nm). For the soft-magnetic electrode a sputtered 1 nm Co/6 nm Fe double layer was used. The hard-magnetic electrode is realized with a 1.5 nm Co/1.0 nm Cu/1.0 nm Co system. Antiferromagnetic coupling between the two Co layers leads to a high saturation field. The 1.5 nm Co layer is used as the second electrode of the MTJ. The conductance increases with growing temperature while the tunnel magnetoresistance (TMR) shows a slight decrease. For interpretation of the results, the temperature dependence of direct tunneling, of the hopping conductance via trapped states, and of the interface magnetization have to be taken into consideration. The dominant factor for the TMR is proportional to 1−BT3/2 and follows the temperature dependence of the interface magnetization. The experimental data allow us to separate transport mechanisms and characterize the junction quality. At room temperature the spin-independent hopping conductance of our junctions is calculated to be less than 10% of the total conductance. Concerning the magnetic properties, a ferromagnetic orange-peel coupling corresponding to a field of about 4 Oe (0.3 kA/m) was found at 15 K, which decays exponentially with increasing temperature to less than 0.6 Oe (0.05 kA/m) at 300 K. The coercive field of the soft layer also shows an exponential decay.
Computational investigation of dielectric absorption at microwave frequencies in binary oxides
Authors: Wu Eric J. , Ceder Gerbrand
Published: July 2002

In this work, a computational study of the dielectric absorption and Q-factor at microwave frequencies in rocksalt oxides is presented. This work is performed within a theory for the two-phonon anharmonic absorption process in terms of third-order force constant matrices, phonon eigendata, and Born effective charges. The complex dielectric permittivity is expressed by means of Green’s functions. This theory is used on model systems that are described using empirical Buckingham potentials with shells. The Q-factor and its temperature dependence in AO (A=Mg, Ca, Ba, Sr) oxides with the rocksalt structure is calculated. For MgO, the calculated Q-factor at room temperature agrees relatively well with experiments, and the temperature dependence is in qualitative agreement. The Q-factor in a model “AO” system is also calculated to determine the effects of cation mass and size on the Q-factor. Increased cation size is found to lower the Q-factor, whereas increasing cation mass is found to increase the Q-factor when the cation mass is higher than that of Ca.
Evolution of nanoscale texture in ultrathin TiN films
Authors: Williamson M. J. , Dunn D. N. , Hull R. , Kodambaka S. , Petrov I. , Greene J. E.
Published: July 2002

We investigate the evolution of texture and grain morphology in fine-grained TiN thin films using cross correlation of dark-field images obtained using annular objective apertures with radii that correspond to different low index TiN reflections. This technique enables parallel analyses of the orientations of thousands of grains, with a spatial resolution of order 10 nm. Preferred grain orientations were determined for 40 and 100 nm thick TiN layers grown on SiO2 by magnetically unbalanced reactive magnetron sputter deposition. We find that no single orientation is dominant in the 40 nm films but that a 〈100〉 texture has developed by the time these films reach 100 nm in thickness.
Kinetics of the glass-transition and crystallization process of Fe72−xNbxAl5Ga2P11C6B4 (x=0, 2) metallic glasses
Authors: Mitrovic N. , Roth S. , Eckert J.
Published: July 2002

The kinetics of the glass-transition and the crystallization process of multicomponent ferromagnetic Fe72−xNbxAl5Ga2P11C6B4 (x=0, 2) metallic glasses have been investigated by constant-rate heating differential scanning calorimetry at different heating rates. The glass-transition temperature Tg and the crystallization peak temperature Tp shift to higher temperatures with increasing heating rate. The apparent activation energy and the frequency factor are evaluated by the Kissinger method, and the rate constant of the crystallization process is estimated by an Arrhenius law. The kinetics of the glass-transition process have been analyzed in terms of a Vogel–Fulcher–Tammann equation describing the heating rate dependence of the glass-transition temperature. The effect of the niobium addition on the glass-forming ability has been investigated with respect to the glass-transition and crystallization kinetics, revealing that substitution of Nb for Fe does not improve the glass-forming ability and it lowers the thermal stability of the material.
Electrical and materials properties of ZrO2 gate dielectrics grown by atomic layer chemical vapor deposition
Authors: Perkins Charles M. , Triplett Baylor B. , McIntyre Paul C. , Saraswat Krishna C. , Haukka Suvi , Tuominen Marko
Published: July 2002

Structural and electrical properties of gate stack structures containing ZrO2 dielectrics were investigated. The ZrO2 films were deposited by atomic layer chemical vapor deposition (ALCVD) after different substrate preparations. The structure, composition, and interfacial characteristics of these gate stacks were examined using cross-sectional transmission electron microscopy and x-ray photoelectron spectroscopy. The ZrO2 films were polycrystalline with either a cubic or tetragonal crystal structure. An amorphous interfacial layer with a moderate dielectric constant formed between the ZrO2 layer and the substrate during ALCVD growth on chemical oxide-terminated silicon. Gate stacks with a measured equivalent oxide thickness (EOT) of 1.3 nm showed leakage values of 10−5 A/cm2 at a bias of −1 V from flatband, which is significantly less than that seen with SiO2 dielectrics of similar EOT. A hysteresis of 8–10 mV was seen for ±2 V sweeps while a midgap interface state density (Dit) of ∼3×1011 states/cm eV was determined from comparisons of measured and ideal capacitance curves.
Low field intergranular tunneling effect in CrO2 nanoparticles and characterization of the barriers
Authors: Dai Jianbiao , Tang Jinke
Published: July 2002

The magnetoresistance (MR) and microstructures of half-metallic CrO2 nanoparticle systems were studied. Using field alignment, the needle-shaped CrO2 single domain particles were aligned in the same orientation. The MR of this structure showed a magnetic junction-like behavior with two well-separated peaks in the MR at coercivity fields and the MR of the aligned CrO2 particles reached >41% at a relatively low field of about 1000 Oe. The magnetotransport mechanism was analyzed in terms of spin dependent tunneling between CrO2 nanoparticles. Using transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy techniques, the intergranular tunneling barrier was characterized to be a very thin Cr2O3 interface layer between the CrO2 particles. Temperature dependence of MR and conductivity in cold-pressed CrO2 nanopowders were studied. The MR significantly decreased with increasing temperature and the spin independent hopping conduction is suggested to be responsible for the suppression of MR at high temperature.
Effect of thermal annealing on high indium content InGaN/GaN single quantum well structures
Authors: Chen Chii-Chang , Hsieh Kun-Long , Chi Gou-Chung , Chuo Chang-Cheng , Chyi Jen-Inn , Chang Chin-An
Published: July 2002

Photoluminescence measurement in surface-emitting geometry, optical pumping in edge-emitting geometry and spatially resolved spectra measurement were performed to study high-indium-content InxGa1−xN/GaN single quantum well (SQW) structures (x⩾32%) grown by low-pressure metalorganic chemical vapor deposition. Stimulated-emission (SE) was observed in the optical pumping spectra. Thermal annealing was performed to investigate the influence of high temperature treatment to the optical properties of the samples. Redshift of the SE peaks took place after the samples were annealed at 700 °C for 30 and 60 min. Blue shift of these SE peaks was observed after the samples were annealed for 120 min. Red shift of the SE peaks may be attributed to the reduction of the compositional fluctuation in quantum well (QW) leading to the decrease of the quantized energies. The interdiffusion of In and Ga atoms across the interface of barrier and QW can result in the increase or decrease of the quantized energies leading to the redshift or blueshift of the SE peaks. The relaxation of the strain in the QW by annealing can induce the blueshift of the SE peaks.
Self-diffusion in Ga69Sb121/Ga71Sb123 isotope heterostructures
Authors: Bracht H. , Nicols S. P. , Haller E. E. , Silveira J. P. , Briones F.
Published: July 2002

Gallium and antimony self-diffusion experiments have been performed in undoped Ga69Sb121/Ga71Sb123 isotope heterostructures at temperatures between 571 and 708 °C under Sb- and Ga-rich ambients. Ga and Sb profiles measured with secondary ion mass spectrometry reveal that Ga diffuses faster than Sb by several orders of magnitude. This strongly suggests that the two self-atom species diffuse independently on their own sublattices. Experimental results lead us to conclude that Ga and Sb diffusion are mediated by Ga vacancies and Sb interstitials, respectively, and not by the formation of a triple defect proposed earlier by Weiler and Mehrer [Philos. Mag. A 49, 309 (1984)]. The extremely slow diffusion of Sb up to the melting temperature of GaSb is proposed to be a consequence of amphoteric transformations between native point defects which suppress the formation of those native defects which control Sb diffusion. Preliminary experiments exploring the effect of Zn indiffusion at 550 °C on Ga and Sb diffusion reveal an enhanced intermixing of the Ga isotope layers compared to undoped GaSb. However, under the same conditions the diffusion of Sb was not significantly affected.
Proton irradiation of InAs/AlSb/GaSb resonant interband tunneling diodes
Authors: Magno R. , Weaver B. D. , Bracker A. S. , Bennett B. R.
Published: July 2002

Room temperature current–voltage measurements have been made on InAs/AlSb/GaSb resonant interband tunnel diodes irradiated with 2 MeV protons to determine the effect of displacement damage on the negative resistance peak current Ip and the peak-to-valley current ratio P/V. Diodes with 5 and 13 ML AlSb barrier thickness were irradiated and measured several times until the total fluences reached 1×1015 and 2×1014 H+/cm2, respectively. The current due to radiation-induced defects has a nonlinear voltage dependence, with a large increase occurring in the voltage range between the negative resistance peak and the valley. Ip increased <50% while a large increase in the valley current decreased the P/V ratios to about 2.
Inelastic effects in dissociative sticking of diatomic molecules on metals
Authors: Brivio G. P. , Grimley T. B. , Trioni M. I.
Published: July 2002

A quantum mechanical model for the dissociative sticking, and scattering of a diatomic molecule on a potential surface containing both elastic and inelastic couplings is solved exactly. The inelastic potential can be due to the excitation of either electron–hole pairs, or phonons. The model is parametrized for vibrationally cold H2 on copper assuming that electron–hole pairs are the source of the inelastic potential. The main effect is a dynamical polarization of the adiabatic electron system, which increases the sticking coefficient over that calculated with the elastic potential alone, for most couplings. It is a no-loss contribution. Within this model the real energy loss contributes little to the sticking coefficient because a negligible fraction of reaction exothermicity is dissipated before the two atoms are separated on the surface. If the electron–hole pair coupling is very strong and highly localized within the molecule bond, the effect of inelasticity is to act as a reflection barrier reducing the sticking coefficient compared to the elastic calculation. The mass dependence of these effects is also discussed.
Biphotonic holographic recording in a liquid crystalline cyanoazobenzene side-chain polymethacrylate. Polarization, intensity, and relief gratings
Authors: Sánchez C. , Cases R. , Alcalá R. , López A. , Quintanilla M. , Oriol L. , Millaruelo M.
Published: July 2002

Biphotonic holographic recording has been performed on thin films of a side-chain liquid crystalline p-cyanoazobenzene polymethacrylate. Polarization and intensity holographic gratings have been recorded by irradiation of the films with blue incoherent light and simultaneous or subsequent illumination with the red light pattern generated by the interference of two properly polarized beams from a He–Ne laser. The influence of temperature and illumination conditions on the diffraction efficiency of those gratings has been studied. Surface topography of the films has been explored using a profilometer. The appearance of relief gratings using biphotonic processes has been observed only in gratings recorded with simultaneous illumination of blue light and an intensity modulated light pattern of the red light.
Characterization of electrical conduction in silicon nanowire by scanning Maxwell-stress microscopy
Authors: Fujii Hideo , Matsukawa Takashi , Kanemaru Seigo , Yokoyama Hiroshi , Itoh Junji
Published: July 2002

An experimental approach to investigate anomalous electrical conduction in a 100 nm width silicon wire by scanning Maxwell-stress microscopy (SMM) has been reported. The silicon wire exhibited negative resistance and hysteresis characteristics by sweeping the applied voltage up to 50 V. Potential profile along the wire obtained from the SMM images exhibited that lateral electric field significantly increased at the ground-side end of the wire after the hysteresis characteristics emerged. The field increase is interpreted as the conductivity decrease. The origin of the conductance decrease is considered to be generation and trapping of hot holes at the ground-side end.
Kinetics of the photorefractive response of bismuth silicon oxide
Authors: McCullough J. S. , Georgalas A. M. , Hunt C. A. , Hoefler-Coster Susan P. , Peakheart D. W. , Dixon G. S. , Martin J. J.
Published: July 2002

The kinetics of production and dark decay of index gratings in Bismuth Silicon Oxide was investigated as a function of write-beam intensity at 300 K and of temperature over the 20–300 K temperature range. At 300 K and an intensity of about 200 mW/cm2 the gratings show a fast leading-edge peak which drops to a steady saturated value. At lower intensities the peak slows and broaden, but the grating strength at saturation remains the same. As the temperature is lowered the gratings grow more slowly, and the peak disappears at about 200 K. The slowing and disappearance of the peak are probably related to the large decrease in electron mobility in this temperature range. Below 225 K the grating strength at the end of the 500 ms write time grows significantly and reaches a maximum in the 125–150 K temperature region. The dark decays disappear quickly for temperatures down to about 180 K. At lower temperatures the decays become much slower and become persistent below 60 K. The largest change appears between 150 and 123 K. A persistent grating was written at 20 K, and its strength was measured as the sample warmed. The persistent grating annealed out between 110 and 150 K. The slowing of the dark decays and the anneal of the persistent grating correlate with the reported recovery of Fe3+.
Resonant interband tunnel diodes with AlGaSb barriers
Authors: Magno R. , Bracker A. S. , Bennett B. R.
Published: July 2002

The peak current density of InAs/AlSb/GaSb/AlSb/InAs resonant interband tunneling diodes has been enhanced by replacing the AlSb barriers with Al1−xGaxSb that has a narrower band gap. The devices were grown by molecular beam epitaxy and tested at room temperature. Diodes with nominally identical 7-ML-thick ternary alloy barriers with x=0.35 are found to have peak current densities three times larger than those with AlSb barriers. The peak-to-valley current ratio decreases by only one third from 18 for the AlSb diodes to 12 for diodes with the ternary alloy barriers.
Time-domain analysis of reflections influence on gyrotron operation
Authors: Grudiev A. , Jelonnek J. , Schünemann K.
Published: July 2002

A time-domain modal analysis of gyrotron operation in the presence of reflections is presented. An eigenfunction expansion method, along with a convolution technique, are used to accurately represent the time-dependent electromagnetic field in the gyrotron cavity, taking into account a mismatched termination of the output port. The relativistic equations of motion for the particles associated with the electron beam and the equations for the field expansion coefficients are solved self-consistently by means of a multi-step integration scheme. The influence of reflections of different kind on gyrotron operation has been investigated numerically for a wide range of parameters. Numerical results for a 1 MW, 140 GHz, TE22,6-mode gyrotron developed at Forschungszentrum Karlsruhe, Germany, are compared with experimental results. Generally, the comparison shows quite good agreement. Moreover, for a specific parameter range, the simulations show unstable operation of the gyrotron. In that case, the output signal has a broad spectrum up to about 12% of relative width. Finally, different mechanisms of output power reduction and frequency pulling are discussed.
Dissociative photodetachment studies of O−(H2O)2, OH−(H2O)2, and the deuterated isotopomers: Energetics and three-body dissociation dynamics
Authors: Clements Todd G. , Luong A. Khai , Deyerl Hans-Jürgen , Continetti Robert E.
Published: July 2002

Photoelectron–photofragment coincidence spectroscopy was used to study dissociative photodetachment of the doubly hydrated clusters of oxide and hydroxide, M−(H2O)2→M+2H2O+e− (M=O, OH). These experiments yield information on the energetics of the parent anion and the dissociation dynamics of the photodetached neutral species. Photoelectron spectra and photoelectron–photofragment coincidence spectra are presented and compared to data for O−(H2O) and OH−(H2O). Unlike the singly hydrated species, no evidence of vibrationally resolved product translational energy distributions is observed. The second hydration energy of O− with both H2O and D2O was also measured to be 0.80±0.08 and 0.81±0.08 eV, respectively. The three-body dissociation dynamics of the neutral clusters produced by photodetachment were studied by measuring the velocities and recoil angles of all the particles in coincidence. The observed partitioning of momentum is consistent with a two-step mechanism or dissociation from a wide range of starting geometries.
Brownian dynamics study of dynamic scaling and related freezing criteria in quasi-two-dimensional dispersions
Authors: Pesché Raphaël , Kollmann Markus , Nägele Gerhard
Published: July 2002

We present a Brownian dynamics simulation study of static and dynamic properties of quasi-two-dimensional dispersions of colloidal spheres interacting by long-range screened electrostatic and by dipolar magnetic forces, respectively. The calculated van Hove real-space dynamic correlation functions, mean squared displacements, and hydrodynamic functions are shown to obey a dynamic scaling behavior in terms of a characteristic relaxation time related to the geometrical mean particle distance. Hydrodynamic interactions introduce a second characteristic length scale, and they lead to a more restricted scaling behavior with an enhancement of self-diffusion. As a consequence of dynamic scaling, the dynamical criterion of Löwen [Phys. Rev. E 53, R29 (1996)] for the onset of colloidal freezing is shown to be equivalent to a two-dimensional freezing criterion related to the static structure factor.
Biochemical reaction equilibria from the point of view of a semigrand partition function
Authors: Alberty Robert A.
Published: July 2002

When the pH is specified in studying equilibria in a system of biochemical reactions, the criterion for spontaneous change and equilibrium is provided by a transformed Gibbs energy G′ that is the thermodynamic potential corresponding to a semigrand ensemble. The semigrand partition function for a system at a specified pH provides a global view of the thermodynamics of the reaction system because all the other thermodynamic properties can be calculated from this function. These operations are illustrated by a system consisting of two species with different numbers of hydrogen atoms, by a system involving two isomeric monoprotic acids, and by a series of metabolic reactions that reduce to a pseudoisomer group when the steady-state concentrations of coenzymes are specified.
New and simple mass calibration procedure for time-of-flight studies of electron stimulated desorption of ions from solid samples
Authors: Rocco Maria Luiza Miranda , de Souza Gerardo Gerson Bezerra , Weibel Daniel Eduardo
Published: July 2002

A technique that provides a simple mass scale calibration for linear time-of-flight (TOF) measurements is presented. This procedure has been used in the calibration of the mass scale in experiments of electron stimulated desorption of ions from solid surfaces. It is based on the admission of one or more calibration rare gases inside the ultrahigh vacuum chamber while a TOF spectrum of desorbing ions from the surface is simultaneously recorded. Peaks related to desorbing ions as well as to the atomic gases can consequently be observed in the same mass spectrum, thus allowing an exact mass scale calibration. The present calibration procedure is especially suitable in the low mass range (<200 amu) and can be easily adapted for photon stimulated desorption experiments.
Properties of GaN films deposited on Si(111) by radio-frequency-magnetron sputtering
Authors: Miyazaki Takayuki , Fujimaki Tamotsu , Adachi Sadao , Ohtsuka Kohji
Published: July 2002

GaN films have been deposited on Si(111) substrates by reactive rf-magnetron sputtering at nitrogen pressures from 0.08 to 2.70 Pa without intentionally heating the substrates. X-ray diffraction (XRD), spectroscopic ellipsometry (SE), and ex situ atomic-force microscopy (AFM) observations have been carried out. The XRD patterns indicate that the GaN films deposited at pressures lower than 1.10 Pa are polycrystalline films highly oriented with the (0001) plane preferred, while those deposited at ⩾1.10 Pa display mixed orientations or amorphous form. The pseudodielectric function ε(E)=ε1(E)+iε2(E) of the sputter-deposited GaN films has been measured by SE in the range between 1.50 and 5.00 eV at room temperature. The measured ε(E) spectra are analyzed by taking into account the effects of surface roughness based on an effective medium model. The roughness thickness for the film deposited at 0.27 Pa is determined to be ∼17 Å, which is comparable to the AFM rms value (∼11 Å).
The effects of interdiffusion on the subbands in GaxIn1−xN0.04As0.96/GaAs quantum well for 1.3 and 1.55 μm operation wavelengths
Authors: Chan Michael C. Y. , Surya Charles , Wai P. K. A.
Published: July 2002

The interdiffusion of GaxIn1−xN0.04As0.96/GaAs single quantum well (QW) structure with well width of 6 nm is studied theoretically. The as-grown Ga concentration in the QW is chosen to be 0.7 and 0.6 for the operation wavelengths of 1.3 and 1.55 μm, respectively. We studied the effects of interdiffusion on the in-plane strain, confinement potential, and subband energy levels of the QW using Fick’s law. The diffusion coefficients of both the well and barrier layers are assumed to be constant. The effects of valence band mixing and strains are included in the calculation of the electron and hole subband structures. We find that the group-III interdiffusion effects can result in blueshifts of 123 and 211 nm in the GaxIn1−xN0.04As0.96/GaAs QW at operation wavelength of 1.3 μm (x=0.7) and 1.55 μm (x=0.6), respectively. Our results show that interdiffusion technique can be used to tune the operating wavelengths of GaInAsN/GaAs lasers for multiwavelength applications such as in the sources of dense wavelength division multiplexed optical communication systems.
Mirror modes: Non-Maxwellian distributions
Authors: Gedalin M. , Lyubarsky Yu. E. , Balikhin M. , Russell C. T.
Published: July 2002

Direct analysis of mirror mode instabilities from the general dielectric tensor for several model distributions in the long wavelength limit is performed. The growth rate at the instability threshold depends on the derivative of the distribution for zero parallel energy. The maximum growth rate is ∼k∥vT∥ well above the instability threshold. The instability threshold and growth rate and their dependence on the propagation angle depend on the shapes of the ion and electron distribution functions.
Environmental aging of Schottky contacts to n-AlGaN
Authors: Readinger E. D. , Luther B. P. , Mohney S. E. , Piner E. L.
Published: July 2002

Gold Schottky contacts to n-AlGaN were fabricated, and the influence of the semiconductor surface preparation on the electrical performance of the diodes was examined. More significantly, the electrical characteristics of the diodes were found to be sensitive to the environment in which they were exposed. Diodes stored in vacuum had stable but poor electrical characteristics, exhibiting the same high reverse leakage currents, low barrier heights, and high ideality factors as the freshly prepared diodes. On the other hand, didoes exposed to air changed over the course of days, in some cases with decreases in the reverse leakage currents by four or more orders of magnitude and increases in the barrier height by 0.3–0.5 eV. Further study of this change in electrical properties showed that the effect was reversible with exposure to N2 gas or vacuum and adequate temperature. In addition, the effect was more pronounced when the metal contact was thin, indicating that diffusion of gases through the metal was significant. This study suggests that nitride semiconductor devices with Schottky barriers to n-AlGaN as components may exhibit improved performance if they are initially stored in air for a few days prior to encapsulation and will also exhibit a pronounced sensitivity to their storage and operational environment if not adequately protected from it.
Implementation of a high-resolution two-color spectrometer for rotational coherence spectroscopy in the picosecond time domain
Authors: Weichert Andreas , Riehn Christoph , Barth Hans-Dieter , Lembach Gerhard , Zimmermann Marcus , Brutschy Bernhard , Podėnas Donatas
Published: July 2002

A high-resolution two-color spectrometer for rotational coherence spectroscopy (RCS) in the picosecond time domain has been developed and applied to several molecular systems. This time-resolved method enables the structural characterization of large molecules and clusters in the gas phase by determination of their rotational constants with high precision. Our spectrometer is based on a well stabilized solid-state chirped pulse amplified laser setup with an adjustable pulse duration in the picosecond range. Two optical parametric generators provide tunable radiation in the UV spectral range, with a pulse duration of 2 ps. An interferometric setup is employed in order to perform pump–probe experiments with up to 8 ns delay between pump and probe pulse. Two different pulse to pulse switching modes and the online monitoring of laser parameters have been integrated to enable the measurement of RCS traces with high resolution and good signal/noise ratio. These modes also permit the identification of weak features that would otherwise not have been observed. The molecular samples are prepared in a supersonic expansion and either the laser induced fluorescence or the photoion current are detected as a function of the time delay between pump and probe pulse. The performance of the spectrometer is demonstrated by recording a high-resolution RCS spectrum of para-difluorobenzene⋅argon and benzene. From their analysis the rotational constants of the ground state and electronically excited state of both species were obtained with an estimated uncertainty on the order of 10−4 and 10−5, respectively, in excellent agreement with high-resolution frequency-resolved data.
All-perovskite-oxide ferroelectric memory transistor composed of Bi2Sr2CuOx and PbZr0.5Ti0.5O3 films
Authors: Ota H. , Fujino H. , Migita S. , Xiong S.-B. , Sakai S.
Published: July 2002

An all-perovskite-oxide transistor composed of a tetragonal perovskite PbZr0.5Ti0.5O3 (ferroelectric) and a layered perovskite Bi2Sr2CuO6 (conducting channel) is fabricated on a cubic perovskite Nb-doped SrTiO3 (gate electrode). We demonstrate a considerably large conductance modulation of Bi2Sr2CuO6 by varying the direction and magnitude of the polarization. The ratio of the ON- and OFF-state drain currents reaches 196. A memory retention time as long as about 8 h is also observed. The drain current versus the drain voltage curves analyzed by adopting a semiconductor model give fairly good agreement with the measurement data. We also discuss the advantages and future prospects of all-perovskite-oxide transistors.
Neutral uniformity and transport mechanisms for plasma etching
Authors: Yun Seokmin , Kolobov Vladimir , Tynan George R.
Published: July 2002

Dissociated neutral (radical) uniformity on the wafer has been studied in a high-density large area plasma reactor. Radial profiles of radicals on the wafer are measured by scanning optical probe and by spatially resolved actinometry and are also estimated by a simple analytic model and two-dimensional (2D) commercial fluid simulation code. Center-peaked radial profiles of radical species are observed experimentally and are also predicted by simple calculation and by simulation code. The radial radical density profiles are compared with the radial profiles of etching rate of blanket photoresist films on 200 mm wafers etched by oxygen plasmas. Radial profiles of etch rate and atomic oxygen radical densities are compared and discussed along with other parameters such as the profiles of ion density, ion energy, and wafer temperature with various chuck bias voltages. At low chuck bias voltage the etch rate uniformity is correlated with radical uniformity. As the chuck bias voltage increases, the etch rate profile begins to follow the ion density profile.