bluuebunny/crossref_metadata_2025 huggingface dataset:
Scanning impedance microscopy of an active Schottky barrier diode
Authors: Kalinin Sergei V. , Bonnell Dawn A.
Published: July 2002

Electrostatic force sensitive scanning probe microscopy is used to quantify dc and ac transport properties of an active Schottky barrier diode. Scanning surface potential microscopy (SSPM) of the laterally biased device is used to quantify the potential drop at the metal–semiconductor interface. Ramping the lateral bias allows the local voltage and I–V characteristics of the diode to be reconstructed. Scanning impedance microscopy (SIM) demonstrates the phase and amplitude change of voltage oscillations across the interface. The frequency dependence of voltage phase shifts across the interface defines the appropriate equivalent circuit for the reverse biased junction. Excellent agreement between junction capacitance obtained from SIM measurements and impedance spectroscopy is demonstrated. Variation of the dc component of lateral bias in SIM yields the local capacitance–voltage characteristics of the junction. SIM contrast of grain boundaries in p-doped silicon was interpreted in terms of minority carrier generation in the interface region. The combination of SSPM and SIM provides an approach for the quantitative analysis of local dc and ac transport properties which were demonstrated for a Schottky diode but can be applied to any semiconductor device.
Uniaxial-stress dependence of Hall effect in an AlGaAs/GaAs modulation-doped heterojunction
Authors: Liu Y. , Rang Z. L. , Fung A. K. , Cai C. , Ruden P. P. , Nathan M. I. , Shtrikman H.
Published: July 2002

Uniaxial compressive stress was applied to an AlGaAs/GaAs heterojunction. The uniaxial stress coefficients of sheet resistivity, sheet electron concentration, and mobility were obtained. The hydrostatic pressure coefficient of sheet resistivity was also obtained and was used to explain the different magnitude of the uniaxial stress coefficients of sheet electron concentration in the [110] and [11̄0] directions. We obtain a value for the piezoelectric constant e14 of AlAs to be −0.26 C/m2, compared to the value −0.225 C/m2 calculated by K. Hübner [Phys. Status Solidi B 57, 627 (1973)].
Actively induced transmission via a quadratic nonlinear optical interaction in a potassium titanyl phosphate microcavity
Authors: Cojocaru Crina , Vilaseca R. , Martorell Jordi , Carvajal J. , Massons J. , Diaz F.
Published: July 2002

We report an actively induced change in the transmission of a light pulse passing through a microcavity filled with a 40 μm thick nonlinear potassium titanyl phosphate crystal. This change was induced by an intense pulse at the frequency of the second harmonic of the original pulse. The magnitude of the change was measured to be 20%. The observed changes in transmission are shown to be in very good agreement with the numerical results from a model that considers the intrinsically fast second-order nonlinear interaction between both pulses within the nonlinear microcavity.
Selective processing of individual carbon nanotubes using dual-nanomanipulator installed in transmission electron microscope
Authors: Kuzumaki Toru , Sawada Hidetaka , Ichinose Hideki , Horiike Yasuhiro , Kizuka Tokushi
Published: July 2002

Nanoscale processings such as deformation, cutting off, and bonding of individual carbon nanotubes (NTs) have been selectively performed using a dual-nanomanipulation system installed in a high-resolution transmission electron microscope. These processes are directly observed in situ at a lattice resolution of 0.1 nm. At high applied electric field between the NT tip and Au tip, we have found a carbon monolayer extending out from the carbon cluster which was deposited on the NT tip. The cutting off and bonding of individual NT tips can be performed by contacting the NT tips and the opposite nanometer-sized tips at an applied bias voltage. The threshold voltage of the processing is approximately 2 V.
Nanovoid-related large redshift of photoluminescence peak energy in hydrogenated amorphous silicon
Authors: Han Daxing , Yue Guozhen , Wang Keda , Baugh Jonathan , Wu Yue , Xu Yueqin , Wang Qi
Published: July 2002

A large redshift of the photoluminescence (PL) peak energy is found in hydrogenated amorphous silicon films prepared by hot-wire chemical-vapor deposition with a high-growth rate ⩾50 Å/s. The PL intensity is as high as that in the standard film and its temperature dependence shows thermalization behavior. The origin of the redshift is clarified by employing 1H nuclear magnetic resonance and mass density measurements. A ∼2% volume fraction of tube-like nanoscale voids is identified. The long spin-lattice relaxation time of H2 in the nanovoids implies a negligible density of silicon dangling bonds on the nanovoid surfaces. We suggest that highly strained bonds on these surfaces form broad conduction-band tail states that are responsible for the PL redshift.
Seismo-electrical effects: Experiments and field measurements
Authors: Röder H. , Büttner R. , Zimanowski B.
Published: July 2002

We report on laboratory and field measurements of electrical effects caused by dislocation of different geomaterials at a shear plane. The onset of the electrical signals precedes the macroscopic dislocation. From the results, we propose a phenomenological model in four steps: mechanical loading, charge generation in the creep phase, charge separation during dislocation, and mechanical relaxation and electrical discharge. This technique is aimed to be used in landslide surveillance operation.
Surface magnetic anisotropy of Co(0001) and effects of ultrathin Cu overlayers studied by in situ spin-wave Brillouin light scattering
Authors: Hyomi Kyoko , Murayama Akihiro , Oka Yasuo , Hiller Uli , Falco Charles M.
Published: July 2002

We have developed an in situ spin-wave Brillouin light scattering system, for the study of surface magnetic anisotropies under an ultrahigh vacuum. The surface uniaxial anisotropy constant is determined as −1.1 mJ/m2 for a bare surface of Co(0001), indicating a strong in-plane anisotropy. With only 0.5 monolayer (ML) of Cu deposited on, the strong perpendicular anisotropy is induced, which is followed by an intensity peak at 1 ML of the Cu. We attribute these results to a steep change in the surface electronic state due to the Cu-atom deposition.
Organic insulator/semiconductor heterostructure monolayer transistors
Authors: Schön J. H. , Bao Z.
Published: July 2002

Field-effect transistors based on self-assembled monolayers of molecular, organic insulator/semiconductor heterostructures are demonstrated. The alkyl chains of the molecule act as the gate insulator and the π-electron moieties as the active semiconductor of the device. Mobilities up 0.05 cm2/V s and on/off ratios exceeding 105 are achieved. In addition, using self-assembled monolayers for patterning transistors with channel lengths as short as 2 nm are demonstrated.
Polarized excimer laser-induced birefringence in silica
Authors: Borrelli N. F. , Smith C. M. , Price J. J. , Allan D. C.
Published: July 2002

We have used linearly polarized 193-nm-excimer laser irradiation to create polarization-induced birefringence in silica. We have observed the polarization-induced birefringence irrespective of whether the sample undergoes net compaction or expansion. The sign of the birefringence is dependent on whether the glass is expanding or compacting. It is suggested that the birefringence derives from an anisotropic density change effected by the linearly polarized exposure. This effect can be expressed formally as an optical frequency-induced dc electrostriction.
Monte Carlo simulation of Fickian diffusion in the critical region
Authors: De S. , Teitel S. , Shapir Y. , Chimowitz E. H.
Published: July 2002

In this article we describe a novel, phenomenologically based computer simulation approach for studying relaxation dynamics in fluid systems. The method utilizes an ensemble consisting of two isothermal chambers initially separated by an impermeable partition. The fluid configurations in each chamber are initially pre-equilibrated at densities ρ̄+ε and ρ̄−ε respectively, where ρ̄ reflects an average density of interest and ε a small perturbation about this value. After the pre-equilibration step the partition is removed and the entire ensemble allowed to relax towards an equilibrium state guided by a kinetic Monte Carlo computer simulation algorithm. Fickian transport coefficients are found from quantities calculated during this relaxation process. We present an analysis of the approach and illustrate its application to transport property calculations in purely diffusive lattice-gas systems. Our results focus upon the critical region for which there are few published results and where simulation results face the most challenges because of finite-size effects and the phenomenon known as critical slowing-down.
High differential efficiency (>16%) quantum dot microcavity light emitting diode
Authors: Chen H. , Zou Z. , Cao C. , Deppe D. G.
Published: July 2002

Data are presented on high differential efficiency quantum dot microcavity light emitting diodes. The data show that differential efficiencies >16% can be achieved by the use of quantum dots to reduce carrier diffusion in small oxide-apertured microcavities. The measured efficiencies are sensitive to both microcavity tuning of the resonance peak to the quantum dot light emitters, and nonradiative recombination effects brought on by temperature, bias current, and edge effects. The peak efficiencies are obtained at a resonance temperature of ∼160 K.
Film thickness dependence of the NiSi-to-NiSi2 transition temperature in the Ni/Pt/Si(100) system
Authors: Liu J. F. , Feng J. Y. , Zhu J.
Published: July 2002

The effect of film thickness on the NiSi-to-NiSi2 transition temperature in the Ni/Pt/Si(100) system has been studied. Three sets of Ni/Pt/Si(100) bilayered samples with the same Ni:Pt ratios but with different film thicknesses were annealed by rapid thermal annealing at 750–900 °C. Both the x-ray diffraction analysis and the sheet resistance measurement show that the thermal stability of Ni(Pt)Si films improves with a decrease in film thickness. This property of Ni(Pt)Si films reveals the good potential for its applications in ultrashallow junctions. The experimental results are explained in terms of classical nucleation theory.
Near-infrared waveguide photodetector with Ge/Si self-assembled quantum dots
Authors: Elkurdi M. , Boucaud P. , Sauvage S. , Kermarrec O. , Campidelli Y. , Bensahel D. , Saint-Girons G. , Sagnes I.
Published: July 2002

We have investigated near-infrared p-i-n photodetectors with Ge/Si self-assembled quantum dots. The self-assembled quantum dots were grown by chemical vapor deposition on Si(001). A vertical stacking of 20 layers of quantum dots was inserted into a near-infrared waveguide obtained with a Si0.98Ge0.02 alloy. The samples were processed into ridge waveguides. The photoresponse of the device covers the near-infrared spectral range up to 1.5 μm. At room temperature, a responsivity of 210 mA/W is measured at 1.3 μm and 3 mA/W at 1.5 μm. The photocurrent is compared to the photoluminescence and to the absorption of the quantum dots measured in the waveguide geometry. At room temperature, the onset of the absorption is around 1.9 μm (0.65 eV). The photocurrent is blueshifted as compared to the absorption.
Superhard conductive carbon nanocrystallite films
Authors: Hirono S. , Umemura S. , Tomita M. , Kaneko R.
Published: July 2002

Carbon films whose hardness is comparable to diamond’s, but whose electrical conductivity is 19 orders of magnitude larger, have been discovered. The films were deposited onto silicon substrates by electron cyclotron resonance plasma sputtering. The bonding structure of the film was mainly sp2. The film consisted of sp2 nanocrystallites having parallel and curved graphene sheets vertically oriented to the film surface. The sp2 nanocrystallites were connected with adjacent crystallites by sp3 bonding, which gives the film both its high hardness and conductivity. These sp2 nanocrystallite films will have many applications.
Dependences of the activation volumes on Ar sputtering pressure in Co/Pt multilayers prepared by dc magnetron sputtering
Authors: Cho Yoon-Chul , Choe Sug-Bong , Shin Sung-Chul
Published: July 2002

We investigated the wall-motion and nucleation activation volumes of Co/Pt multilayer films prepared by dc magnetron sputtering under various Ar sputtering pressures. Delicate analysis of time-resolved domain evolution patterns reveals that the nucleation activation volume is generally smaller than the wall-motion activation volume in all the samples, which is consistent with the nucleation-dominant magnetization reversal behavior observed in this system. Interestingly, the activation volume is found to decrease with increasing Ar pressure, despite a decreasing trend in saturation magnetization. Decreasing grain size with increasing Ar pressure, smaller than the typical size of a Co single domain, is believed to be the origin of the unexpected observation.
Role of elasticity forces in thermodynamics of intercalation compounds: Self-consistent mean-field theory and Monte Carlo simulations
Authors: Kalikmanov V. I. , de Leeuw S. W.
Published: July 2002

We propose a self-consistent mean-field lattice-gas theory of intercalation compounds based on effective interactions between interstitials in the presence of the host atoms. In addition to short-range screened Coulomb repulsions, usually discussed in the lattice gas models, the present theory takes into account long-range effective attractions between intercalants due to elasticity of the host matrix. The mean-field phase diagram in the space of interaction parameters contains the domains of first- and second-order transitions of the order-disorder type, separated by a tricritical line, and the domain of the first-order transition of the gas–liquid-type separated from the homogeneous state by a critical line. Theoretical predictions are shown to be in qualitative agreement with the grand canonical Monte Carlo simulations. The peculiarities of the phase diagram give an insight into different types of behavior of the open circuit voltage observed in rechargeable batteries, in which an intercalation compound is used as an electrode material.
A first-principles study of acetylene and its evolution products on Cu(001)
Authors: Yuan Lan-Feng , Yang Jinlong , Li Qunxiang , Zhu Qing-Shi
Published: July 2002

Acetylene and its evolution products (CCH, C2, and CCH2) on the Cu (001) surface are studied using the density functional method with cluster models. The binding energies, geometries, STM images, and vibrational frequencies for the adsorbates are obtained. The results agree well with the available experimental results. By comparing the calculated frequencies with the experimental ones, we identify the thermal evolution product of acetylene on Cu(001) in the HREELS experiment as CCH2, and establish its orientation relative to the surface. CCH2 is found to be more stable than C2H2 on Cu(001).
Acoustic communications in an enclosure using single-channel time-reversal acoustics
Authors: Heinemann M. G. , Larraza A. , Smith K. B.
Published: July 2002

This letter presents experimental results of a scheme for encrypted acoustic communication in the reverberant environment of an enclosure using single-channel time-reversal acoustics. Because time-reversal acoustics in an enclosure can focus a signal within a half wavelength of sound, it possesses natural encryption for points other than the intended receiver. In this way, distinct multiple messages can be sent simultaneously from one source to different locations in the enclosure. The communication scheme is based on multiple broadband signals with identical bandwidth and windowed source spectra.
Chirped pulse excitation in condensed phases involving intramolecular modes. II. Absorption spectrum
Authors: Fainberg B. D. , Narbaev V.
Published: July 2002

We have calculated the absorption spectrum of an intense chirped pulse exciting a solute molecule in a solvent. The excitation of quantum intramolecular modes has been also taken into account. In general absorption depends on both the real and imaginary part of the susceptibility (a phase-dependent absorption in the nonstationary media). We have shown that for strongly chirped pulses, the absorption spectrum can be expressed by the difference of the convolutions of the “intramolecular” absorption and luminescence spectra with the instantaneous population wave packets in the ground and excited electronic states, respectively. Incorporating of optically active high-frequency intramolecular vibrational modes eliminates the qualitative discrepancies between experimental and calculated absorption spectra which occurred in the model of one vibronic transition.
Low-loss crystal-ion-sliced single-crystal potassium tantalate films
Authors: Izuhara T. , Osgood R. M. , Levy M. , Reeves M. E. , Wang Y. G. , Roy A. N. , Bakhru H.
Published: July 2002

The dielectric response has been studied in 10-μm-thick, single-crystal potassium tantalate films formed by crystal ion slicing. Scanning microwave microscopy shows that the implanted, pre-etched samples exhibit a bulk-like permittivity and low-loss tangent (0.0009) at 1.7 GHz. The separated free-standing films have somewhat higher loss tangents due to residual-ion-induced stress. Selective relaxation of this stress by etching or annealing reduces the dielectric loss.
Transient model for terminal current noise
Authors: Nedjalkov M. , Grasser T. , Kosina H. , Selberherr S.
Published: July 2002

The condition of a stationary transport process is a common origin for the approaches for terminal current noise. Based on a recently-proposed stationary model and the integral form of the Boltzmann equation, we derive a transient stochastic process whose autocovariance function characterizes the terminal current noise. The properties and the domain of application of this transient model and its stationary counterpart are investigated. Numerical experiments are presented which support the conclusions drawn for the two models.
Supercurrent dissipation and strain-induced damage in (Bi, Pb)2Sr2Ca2Cu3O10/Ag tape
Authors: Spal Richard D. , Riley Gilbert N. , Christopherson Craig J.
Published: July 2002

Supercurrent dissipation and strain-induced damage in (Bi, Pb)2Sr2Ca2Cu3O10/Ag high-temperature superconductor composite tape, at 77 K in self-field, are studied by I–V curve analysis. Five dissipation models are considered, and differently processed tapes are compared. After making self-field corrections, which are normally ignored but we find essential to do, collective flux creep stands out as the only dissipation mechanism which explains the data. Confidence in the analysis comes not only from the goodness of fit, but also from experimental confirmation of two fitted parameters. The identification of the main dissipation mechanism has great practical value, because it enables definitive tape characterization, based on intrinsic physical properties.
An x-ray rocking curve technique for the absolute characterization of epitaxial layers and single-crystal solids
Authors: Fatemi M.
Published: July 2002

A rapid double-crystal x-ray diffractometer technique for the absolute measurement of lattice parameters in single crystals and epitaxial layers is introduced. The method is based on the principle that in a diffractometer aligned for rocking curve measurements within a given zone, the zero setting correction is the same for all crystal angles within the zone. The peak positions can thus be measured and corrected quickly, and the lattice parameters evaluated within each layer independently of others. The proof-of-principle tests are given for Si and InP, and the method is applied to a GaN film grown on an a-plane sapphire wafer. The method has unlimited accuracy depending only on the instrumental alignment and material quality.
Thermal stability and recording properties of sub-100 nm patterned CoCrPt perpendicular media
Authors: Albrecht M. , Anders S. , Thomson T. , Rettner C. T. , Best M. E. , Moser A. , Terris B. D.
Published: July 2002

Patterned magnetic media, where the bit cells are predefined, offer a potential path to ultrahigh density data storage. Here we report results on the magnetic and recording properties of islands with lateral dimensions on the order of 80 nm. Prototype nanometer-scale magnetic structures were made by patterning single-layer Co70Cr18Pt12 perpendicular media using a focused ion beam of Ga+. The effects of patterning were investigated by comparing patterned and unpatterned regions of the same medium. The magnetic reversal properties of the patterned media were obtained from magnetic force microscopy images, while those for the unpatterned media were obtained from vibrating sample magnetometer measurements. At remanence the unpatterned region decays at 1.4% per decade, whereas the patterned region shows no measurable change over the course of the experiment (5×105 s). We have also studied aspects of the recording physics of patterned media using a quasistatic write/read tester with a giant magnetoresistance head. We have written a square wave bit pattern that matches the respective island periodicity and investigated the readback signal for out-of-phase and in-phase writing.
Tunnel magnetoresistance oscillations in current perpendicular to plane geometry of CoAlO granular thin films
Authors: Yakushiji K. , Mitani S. , Takanashi K. , Fujimori H.
Published: July 2002

We have fabricated current-perpendicular-to-plane geometry samples consisting of a thin CoAlO granular film sandwiched by top and bottom electrodes with a submicron-sized contact area, and measured magnetotransport properties at 4.2 K. Clear Coulomb staircases due to the charging effect of Co granules are observed by the addition of an Al–O bottleneck layer which leads to an asymmetric configuration of tunnel resistances. Furthermore, tunnel magnetoresistance (TMR) shows oscillatory behavior associated with the Coulomb staircase. It is considered that the TMR oscillation is based on the interplay of spin-dependent tunneling and single electron tunneling.
Common-emitter current–voltage characteristics of a pnp GaN bipolar junction transistor
Authors: Kumakura Kazuhide , Makimoto Toshiki , Kobayashi Naoki
Published: July 2002

We fabricated a pnp GaN bipolar junction transistor and investigated its common-emitter current–voltage characteristics. The device structures were grown by metalorganic vapor phase epitaxy on a sapphire substrate. The base thickness was 0.12 μm and its doping concentration was estimated to be lower than mid-1017 cm−3. We have obtained a maximum common-emitter current gain of 50 at room temperature for collector current ranging from −10−5 to −10−4 A.
Negative differential resistance in scanning-tunneling spectroscopy of diamond films
Authors: Ma D. D. D. , Wang Y. G. , Lu Li , Lee C. S. , Lee S. T.
Published: July 2002

Heavily boron-doped diamond film surfaces were investigated using ultra high-vacuum scanning tunneling microscopy. Negative differential resistance (NDR) was observed in the I–V curves measured from the diamond facets using scanning tunneling spectroscopy. NDR peaks with a peak-to-valley ratio as high as 90 were observed in about half of the I–V curves measured at room temperature. The NDR phenomenon is attributed to the existence of localized electron states on the diamond film surfaces. The I–V curves obtained at the diamond grain boundaries showed the characteristics of the graphite phase, while those acquired from the grain facets manifested obvious rectification in accordance with the wide band gap of diamond.
Photocurrent studies of an active polymer layer in a resonant microcavity
Authors: Singh Th. B. , Waghmare U. V. , Narayan K. S.
Published: July 2002

A microcavity enhanced photodiode, consisting of an active semiconducting-polymer layer is studied in detail. The photodiode spectral response indicates features which specifically arise from the cavity geometry factors, as expected from the simulation of the optical-field pattern within the cavity. The results indicate the tuning of photodetection energy range far below the band gap of semiconducting polymer along with sizable gain and speed.
Hydrogen-mediated quenching of strain-induced surface roughening during gas-source molecular beam epitaxy of fully-coherent Si0.7Ge0.3 layers on Si(001)
Authors: Spila T. , Desjardins P. , Vailionis A. , Kim H. , Taylor N. , Cahill D. G. , Greene J. E. , Guillon S. , Masut R. A.
Published: July 2002

Fully-coherent Si0.7Ge0.3 layers were deposited on Si(001) by gas-source molecular beam epitaxy (GS-MBE) from Ge2H6/Si2H6 mixtures in order to probe the effect of steady-state hydrogen coverages θH on surface morphological evolution during the growth of compressively strained films. The layers are grown as a function of thickness t at temperatures, Ts=450–550 °C, for which strain-induced roughening is observed during solid-source MBE (SS-MBE) and deposition from hyperthermal beams. With GS-MBE, we obtain three-dimensional (3D) strain-induced growth mounds in samples deposited at Ts=550 °C for which θH is small, 0.11 monolayer (ML). However, mound formation is dramatically suppressed at 500 °C (θH=0.26 ML) and completely eliminated at 450 °C (θH=0.52 ML). We attribute these large differences in surface morphological evolution primarily to θH(Ts)-induced effects on film growth rates R, adatom diffusion rates Ds, and ascending step-crossing probabilities. GS-MBE Si0.7Ge0.3(001) growth at 450 °C remains two dimensional, with a surface width 〈w〉<0.15 nm, at all film thicknesses t=11–80 nm, since both R and the rate of mass transport across ascending steps are low. Raising Ts to 500 °C increases R faster than Ds leading to shorter mean surface diffusion lengths and the formation of extremely shallow, rounded growth mounds for which 〈w〉 remains essentially constant at ≃0.2 nm while the in-plane coherence length 〈d〉 increases from ≃70 nm at t=14 nm to 162 nm with t=75 nm. The low ascending step crossing probability at 500 °C results in mounds that spread laterally, rather than vertically, due to preferential attachment at the mound edges. At Ts=550 °C, the ascending step crossing probability increases due to both higher thermal activation and lower hydrogen coverages. 〈w〉(t) increases by more than a factor of 10, from 0.13 nm at t=15 nm to 1.9 nm at t=105 nm, while the in-plane coherence length 〈d〉 remains constant at ≃85 nm. This leads, under the strain driving force, to the formation of self-organized 3D {105}-faceted pyramids at 550 °C which are very similar to those observed during SS-MBE.
Simple expression for vacancy concentrations at half ion range following MeV ion implantation of silicon
Authors: Coleman P. G. , Burrows C. P. , Knights A. P.
Published: July 2002

Mean concentrations CD of aggregated vacancy-type point-defect structures in float-zone Si implanted with H+, B+, Si+, O+, and Ge2+ ions at energies between 0.45 and 4.0 MeV have been measured as a function of ion dose φ at depths ∼RP/2 (half projected ion range) by beam-based positron spectroscopy. By adjusting φ to φA using factors given by the code TRIM, one arrives at the universal expression CD=(2.79×1010) φA0.63; CD (cm−3) can be estimated to ±50% for MeV ions implanted for φA from 109×1013 cm−2, which corresponds to an upper limit dose approaching 1014 cm−2 for 2 MeV Si+ implantation.
Strain-induced crystal structure change in ultrathin films of Pr0.7Sr0.3MnO3
Authors: MacLaren Ian , Wang Zhong Lin , Wang H. S. , Li Qi
Published: July 2002

Ultrathin films (<30 nm) of Pr0.7Sr0.3MnO3 on LaAlO3 have been studied using transmission electron microscopy (TEM). It was shown that the films are highly uniform and defect-free, and that they are coherently strained to the smaller lattice parameter of the substrate, resulting in a tetragonal expansion perpendicular to the film plane and a change of crystal structure from the ordered orthorhombic of bulk materials to a simple tetragonal perovskite. The variation of the tetragonality with distance from the interface was also determined from high-resolution TEM images.
Channel formation in organic field-effect transistors
Authors: Li T. , Balk J. W. , Ruden P. P. , Campbell I. H. , Smith D. L.
Published: July 2002

Results of two-dimensional electrostatic modeling of organic field-effect transistors, focusing on the formation of the conductive channel, are reported. The effect on channel formation of the choice of the source and drain contact metal is investigated for both top- and bottom-contact device structures. High-work-function metal (e.g., gold) source and drain contacts produce a conducting p-type region near these contacts. In contrast, low-work-function metal source and drain contacts (e.g., magnesium) lead to depleted regions. In the center of the device, between the source and drain contacts, the channel carrier density at a fixed gate bias is determined by the work function of the gate contact material, and is essentially independent of the metal used to form the source and drain contacts. The principal difference between top- and bottom-contact structures is the spatial variation of the charge density in the vicinity of the source and drain contacts. The channel carrier density for a fixed gate bias (and gate contact material) between the source and drain electrodes is essentially the same for the two structures. Finally, the dependence of the transistor threshold voltage on the gate contact metal work function and the device implications of the spatial variation of the induced charge density are discussed.
Microstructure of laser-crystallized silicon thin films on glass substrate
Authors: Nerding M. , Dassow R. , Christiansen S. , Köhler J. R. , Krinke J. , Werner J. H. , Strunk H.-P.
Published: July 2002

We investigate the microstructure of polycrystalline silicon films (grain size, orientation distribution, and grain boundary population). These films are produced by laser crystallization of amorphous silicon on glass substrates by a frequency doubled Nd:YVO4 laser operating at a wavelength of 532 nm. Transmission electron microscopy reveals that the grains have an average width between 0.25 and 5 μm depending on the crystallization parameters and a length of several 10 μm. Electron backscattering diffraction experiments show that the grain orientation of the poly-Si films is textured. Type and extent of texture depend in a complex way on the thickness of the crystallized amorphous silicon layer, on the repetition rate of the laser pulses, and on whether or not an additional buffer layer is present on the glass substrate. In any case, the grain boundary population is dominated by first and second order twin boundaries.
Interface controlled electrical and magnetic properties in Fe–Fe3O4–silica gel nanocomposites
Authors: Das D. , Roy S. , Chen J. W. , Chakravorty D.
Published: July 2002

Iron nanoparticles with a shell of Fe3O4 phase with a total diameter of 5.3 nm have been grown within a silica gel matrix in the percolative configuration by suitable reduction followed by oxidation treatments. dc electrical resistivity measurements were carried out in the temperature range 80–300 K. The resistivity of the nanocomposites was found to be about 7 orders of magnitude lower than that of the reference gel. The electrical conduction has been explained on the basis of a small polaron hopping mechanism. The activation energy in the case of the composites was calculated from experimental data to be about one-fifth that for the reference sample. An interfacial amorphous phase is believed to cause such reduction in resistivity. The effective dielectric constant of this phase was estimated to be about four times that of the reference glass. Magnetization measurements on these specimens were carried out in the temperature range 5–300 K both in zero field cooled and field cooled states. A peak in the magnetization at ∼120 K was ascribed to an order–disorder (Verwey) transition. Another peak at ∼55 K was explained as arising due to a spin glass like disorder at the interface between the ferromagnetic iron ores and the ferrimagnetic Fe3O4 shell. A loop shift was observed as a result of the spin freezing below this temperature.
Tribocharging of the magnetic hard disk drive head–disk interface
Authors: Kiely J. D. , Hsia Y.-T.
Published: July 2002

As head–disk separation diminishes and the sensitivity of magnetic recording transducers increases, the effects of spurious charge buildup caused by friction become an increasing concern. We have investigated some of the characteristics of tribocharging during frictional contact between head and media typically found in magnetic hard disk drives. From constant velocity tests with an electrically isolated head contacting the laser texture zone, we find that current and voltage are closely related, both undergoing a rapid initial rise followed by a long-term rise. We have found that the long-term rise is controlled by the rate of discharge and that the overall magnitude of both current and voltage is dependent upon the rate of rotation. Additionally, we find that the magnitude of current and voltage increases sequentially from test to test, suggesting that wear is occurring at the interface. We develop models for wear-induced tribocharging and surface potential difference-induced tribocharging and contrast their predictions for charge generations as functions of velocity. These models suggest that wear is the dominant tribocharging mechanism.
Ballistic spin transport through electronic stub tuners: Spin precession, selection, and square-wave transmission
Authors: Wang X. F. , Vasilopoulos P. , Peeters F. M.
Published: July 2002

Ballistic spin transport is studied through electronic tuners with double stubs attached to them. The spins precess due to the spin–orbit interaction. Injected polarized spins can exit the structure polarized in the opposite direction. A nearly square-wave spin transmission, with values 1 and 0, can be obtained using a periodic system of symmetric stubs and changing their length or width. The gaps in the transmission can be widened using asymmetric stubs. An additional modulation is obtained upon combining stub structures with different values of the spin–orbit strength.
Real-time x-ray scattering study on the thermal evolution of interface roughness in CoSi2 formation
Authors: Kang Tae Soo , Je Jung Ho
Published: July 2002

The thermal evolution of interface roughness during cobalt silicide formation in the Co/Ti/Si(001) and Co/Si(001) systems was investigated using real-time synchrotron x-ray scattering measurement. We find that the enhancement of the CoSi2/Si(001) interface roughness is caused by the retardation of silicide phase formation almost up to the CoSi2 nucleation temperature. In the Co(120 Å)/Ti(50 Å)/Si(001), the interface roughness increases only to 6 Å during silicidation, by suppressing the reaction between the Co overlayer and Si substrate with a Ti diffusion barrier nearly up to the CoSi2 nucleation temperature of 660 °C. In the Co(120 Å)/Si(001), however, the reaction already starts at a low temperature of 300 °C, resulting in a significant rise of the interface roughness up to 13 Å, which is mainly attributed to the formation of Si{111} facets that act as the nucleation sites of misoriented CoSi2 grains.
Influence of MgO and Sc2O3 passivation on AlGaN/GaN high-electron-mobility transistors
Authors: Luo B. , Johnson J. W. , Kim J. , Mehandru R. M. , Ren F. , Gila B. P. , Onstine A. H. , Abernathy C. R. , Pearton S. J. , Baca A. G. , Briggs R. D. , Shul R. J. , Monier C. , Han J.
Published: July 2002

Unpassivated AlGaN/GaN high-electron-mobility transistors show significant gate lag effects due to the presence of surface states in the region between the gate and drain contact. Low-temperature (100 °C) layers of MgO or Sc2O3 deposited by plasma-assisted molecular-beam epitaxy are shown to effectively mitigate the collapse in drain current through passivation of the surface traps. These dielectrics may have advantages over the more conventional SiNX passivation in terms of long-term device stability.
Nanoscale imaging of domains and domain walls in periodically poled ferroelectrics using atomic force microscopy
Authors: Wittborn J. , Canalias C. , Rao K. V. , Clemens R. , Karlsson H. , Laurell F.
Published: July 2002

We use the inverse piezoelectric effect for high-resolution imaging of artificially produced ferroelectric domains in periodically poled potassium titanium phosphate, KTiOPO4, and lithium niobate, LiNbO3, crystals using a modified atomic force microscope (AFM). By monitoring the vertical as well as the lateral deflection of the AFM tip, details of the domains and the domain walls were obtained at 1 nm resolution. With this approach we determine the domain wall width to be 20–80 nm for KTiOPO4 and about 150 nm for LiNbO3. The above technique is of importance in tailoring ferroelectric crystals for frequency conversion of lasers and studies of domain walls in ferroelectric and its magnetic analogs.
Characteristics of surface acoustic wave convolver in the monolithic metal–zinc oxide–silicon nitride–silicon dioxide–silicon structure
Authors: Panwar B. S.
Published: July 2002

The nonlinear analysis of the metal–insulator semiconductor shows that the ac currents charging the interface traps lead to large dc operating voltage and an inefficient operation of the monolithic convolvers. These interface traps are annihilated during a low temperature anneal, which utilizes hydrogen atoms implanted underneath the SiO2–Si interface. The overlay piezoelectric ZnO film in the metal–ZnO–Si3N4–SiO2–Sistructure is protected from the influx of hydrogen atoms by an interposed silicon nitride layer. Hydrogen implantation and rapid thermal annealing steps are integrated in the process sequence of realizing an efficient metal–ZnO–Si3N4–SiO2–Si monolithic surface acoustic wave convolver.
Superconductivity of Mg(B1−xCx)2 ternary compounds
Authors: Cheng Zhao-hua , Shen Bao-gen , Zhang Jian , Zhang Shao-ying , Zhao Tong-yun , Zhao Hong-wu
Published: July 2002

The structural properties and superconductivity of Mg(B1−xCx)2 compounds were investigated by means of powder x-ray diffraction (XRD) and magnetization measurements. Powder XRD Rietveld analysis indicates that the samples crystallize in a hexagonal AlB2-type structure. The lattice parameter a decreases slightly with increasing carbon content, while c remains unchanged. The addition of carbon results in a decrease of TC and an increase in the superconducting transition width, while the critical current density Jc data indicate that MgB1.8C0.2 manifests comparable superconducting properties with MgB2.
Band-gap profiling in amorphous silicon–germanium solar cells
Authors: Lundszien Dietmar , Finger Friedhelm , Wagner Heribert
Published: July 2002

Profiled buffer layers at the interfaces of amorphous silicon–germanium (a-SiGe:H) solar cells are routinely used to avoid band-gap discontinuities and high-defect densities at the p/i and i/n interfaces. It is shown that such profiled a-SiGe:H buffer layers can be replaced by a constant band-gap a-Si:H buffer, an inverse profiled a-SiGe:H buffer, or even a 3-nm-thin (δ) buffer at some distance away from the interface without losses in the open-circuit voltage VOC and fill factor while maintaining the same short current density jSC. In view of these results, common model assumptions for a-SiGe:H solar cells have to be revised.
Strain relaxation effect in microdisk lasers with compressively strained quantum wells
Authors: Fujita Masayuki , Ushigome Reona , Baba Toshihiko
Published: July 2002

From lasing wavelengths observed for many samples of microdisk lasers with GaInAsP compressively strained quantum wells (CS-QWs), we estimate the gain peak redshifted by ∼10 meV. We explain this phenomenon as the strain relaxation in the CS-QWs at the disk wing exposed to the air. Band and rate equation analyses show that the built-in potential by strain relaxation accelerates the carrier diffusion toward the disk edge and reduces the threshold to 30%–60% of that without strain relaxation. This result indicates the advantage of CS-QWs not only for the microdisk laser but also for various microlasers with a membrane structure, e.g., photonic crystal slab lasers.
Formation of the GaN conversion layer in GaP (111)B wafers using ammonia gas
Authors: Kuriyama K. , Nagasawa K. , Suzuki Y. , Kushida K.
Published: July 2002

The surface layer of GaP(111)B wafers is converted into α-GaN phase (wurtzite) by annealing at 900 °C for 6 h under NH3 gas flow. The converted layer extends to ∼10 μm from the surface. X-ray diffraction analysis shows that the conversion layer is a highly c-axis oriented GaN, supported by the appearance of an E2 phonon mode originated from the c face of α-GaN observed by a Raman scattering method. The photoluminescence spectra at 20 K consist of the zero-phonon-line transition at 3.34 eV followed by two phonon replicas observed 100 and 180 meV lower in energy. The possible origin of the emission is discussed.
Factors influencing glass formation in rapidly solidified Si,Ge–Ni and Si,Ge–Ni–Nd alloys
Authors: Louzguine Dmitri V. , Louzguina Larissa V. , Inoue Akihisa
Published: July 2002

In this letter we compare the glass-forming capabilities of binary Si,Ge–Ni and ternary Si,Ge–Ni–Nd alloys. The addition of Nd caused the formation of an amorphous single phase in the Ge–Ni–Nd alloy. The structure and crystallization behavior of the amorphous alloy were studied by differential scanning calorimetry and x-ray diffractometry. No amorphous phase formed in any of the Nd-free alloys or in the Si–Ni–Nd alloy. The higher glass-forming capability of the Ge–Ni–Nd alloy compared to Si–Ni–Nd cannot be explained on the basis of the widely used geometrical and chemical factors, viscosity, or diffusion data. The importance of the electronic structure characteristics is suggested to be a reason.
Fokker–Planck approach to impact ionization distributions in space and time
Authors: Jacob Biju , Robson P. N. , David J. P. R. , Rees G. J.
Published: July 2002

A Fokker–Planck equation for carrier transport in semiconductors is derived from the Boltzmann transport equation by expanding in Legendre polynomials and assuming the phonon energy exchanged at momentum randomizing collisions is small compared with the mean carrier energy. The method is used to compute impact ionization probability distributions in space and time and the results agree well with those generated by an equivalent Monte Carlo model over a wide range of electric fields from 300 kV/cm to 1 MV/cm.
Pulse broadening in a Ne–CuBr–Zn laser system by optical resonance pumping
Authors: Fujii K. , Uno K. , Tawada F. , Hishida T. , Nishizawa M. , Suzuki M. , Oouchi K. , Tsuji R. , Saito H.
Published: July 2002

A Zn atom is employed as the light source for optical resonance pumping to a high lying energy level of the Cu atom, thereby depopulating both the Cu lower laser level and enhancing the upper laser level via cascading decays that is carried out simultaneously. The width of the lasing pulse broadened by 220% and the peak value increased by 180%. The cascading decay route was clarified by observing the fluorescence lines, λ=675.1, 793.5, and 809.0 nm, thereby specifying five levels of excitation loop.
Correlation of surface molecular composition to nanoscale elastic behavior and topography of stretched polyurethane films
Authors: Amitay-Sadovsky E. , Komvopoulos K. , Tian Y. , Somorjai G. A.
Published: July 2002

Correlation of the surface molecular composition with the nanoscale elastic behavior and topography of stretched polyurethane films was examined by surface-specific techniques, such as sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM). Identification of the SFG spectra of the soft and hard copolymer segments elucidated changes in the molecular composition due to stretching. The surface molecular composition is correlated to AFM results for the roughness and elastic modulus. It is shown that, in addition to molecular reorientation, stretching promotes exposure of hard segments at the surface, and that this is a continuous time-dependent process at constant elongation.
Hysteresis curves and self-field alternating-current losses in superconducting tapes with transport current: Edge barrier effect
Authors: Maksimov I. L. , Elistratov A. A.
Published: July 2002

The combined influence of both bulk pinning and edge barrier on zero-field transport properties and alternating-current (ac) losses in superconducting tapes and films is considered. The equilibrium distributions of the magnetic flux density n(y) and of the sheet current density i(y) are found analytically for the tapes with an oscillating transport current. The critical current is calculated taking into account both bulk and edge pinning of the magnetic flux, and the dependence of the dissipation loss power upon the ac amplitude I0 is found. The crossover from barrier- to pinning-controlled ac losses is predicted with the increase of current oscillation amplitude I0.
Electrical properties of thin film zirconia grown by ultraviolet ozone oxidation
Authors: Ramanathan Shriram , Park Chang-Man , McIntyre Paul C.
Published: July 2002

Zirconia films of varying thickness (ranging from 20–55 Å) have been grown by the method of UV ozone oxidation at room temperature. The electrical properties of these films have been studied in detail by capacitance–voltage (C–V) and I–V measurements. Capacitors were subjected to various anneals in differing ambient to study their effects on C–V hysteresis, dispersion and charge trapping. It was found that annealing in nitrogen followed by forming gas resulted in C–V curves with negligible hysteresis. The effects of different underlayers on the electrical properties of zirconia films have also been studied and are briefly discussed. It was found that zirconia films grown on UV-ozone grown SiO2 had lower hysteresis and lower interface trap density compared to zirconia films grown on chemical oxide. The effect of oxidation time and oxygen pressure have been investigated; in particular, detailed electrical studies have been performed on partially oxidized zirconia. Defective oxides are shown to have significant frequency dispersion in both the accumulation and depletion regions of the C–V curves along with very high loss tangent factor compared to stoichiometric oxides. A physical mechanism based on Maxwell–Wagner interfacial polarization is presented here for a model system of ZrO2–Zr to explain the experimental data qualitatively. Finally, we have attempted to correlate the C–V hysteresis to the presence of traps in the zirconia film using temperature-dependent current–voltage measurements. The leakage current was found to be nearly independent of temperature at low voltages, suggesting a tunneling mechanism, while at higher voltages the data can be modeled using the Poole–Frenkel conduction mechanism. It is suggested that C–V hysteresis in zirconia films possibly arises from electrical traps in the film and can be identified by modeling their I–V characteristics.
Magnetic force microscopy study of perpendicular media: Signal-to-noise determination and transition noise analysis
Authors: Svedberg Erik B. , Khizroev Sakhrat , Litvinov Dmitri
Published: July 2002

Signal-to-noise ratio (SNR) from a limited bit sequence has been extracted from magnetic force microscopy (MFM) images by two different methods and compared. The autocorrelation SNR properties extracted from the MFM image data are in better agreement with spinstand data than “normal” Fourier extracted SNR values. Transition noise data were also extracted and analyzed, “Squeezing” of the noise was measured as the periodicity of the recorded bit pattern became smaller than 250 nm and subsequently a relaxation at ∼200 nm occurred. MFM analysis based SNR data for two media types (CoCrPtTa alloy and CoB/Pd multilayer based) are compared. Ni45Fe55 alloy was used for the soft underlayer material.