bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication and characterization of sol-gel planar waveguides doped with rare-earth ions
Authors: Orignac X. , Barbier D. , Du X. M. , Almeida R. M.
Published: July 2002

Silica-titania sol-gel planar waveguides were deposited on silica glass disks by spin coating. Fluorescence lifetimes as high as 375 μs were measured in neodymium-doped sol-gel planar waveguides and 1.78 ms for an erbium-doped film. Quenching was found to occur for the former at concentrations above 1 at. % of neodymium.
Infrared quenching of photoinduced persistent conductivity in YBa2Cu3O6+x
Authors: Chew D. C. , Federici J. F. , Gutierrez-Solana J. , Molina G. , Savin W. , Wilber W.
Published: July 2002

Experimental evidence for infrared quenched photoinduced persistent conductivity in oxygen deficient YBa2Cu3O6+x films is presented. Results are interpreted in terms of both a charge transfer model, in which photogenerated electrons are trapped in oxygen vacancy defects, and a photoassisted oxygen ordering model. Infrared illumination of the photoinduced state results in detrapping of electrons, a recombination of photogenerated holes/electrons, and a partial quenching of the photoinduced state. Experimental results suggest the possibility of optically controlled superconducting devices in which the superconducting state is generated and quenched with different colors of light.
Phase identification of boron nitride thin films by polarized infrared reflection spectroscopy
Authors: Plass M. F. , Fukarek W. , Mändl S. , Möller W.
Published: July 2002

Six different types of boron nitride films were investigated by polarized infrared reflection spectroscopy. Films with a highly cubic, mixed cubic and noncubic, and exclusively noncubic phase composition were synthesized using ion beam assisted deposition. Additionally, postdeposition argon ion irradiated cubic and noncubic boron nitride films as well as a nitrogen implanted boron sample were analyzed. Using this technique, besides the cubic phase, two different noncubic modifications, layered anisotropic and amorphous, could be distinguished. A preferential orientation of the normal axis of the sp2-bonded basal planes parallel to the substrate surface was observed.
1.54 μm photoluminescence of Er3+ doped into SiO2 films containing Si nanocrystals: Evidence for energy transfer from Si nanocrystals to Er3+
Authors: Fujii Minoru , Yoshida Masato , Kanzawa Yoshihiko , Hayashi Shinji , Yamamoto Keiichi
Published: July 2002

SiO 2 films containing Si nanocrystals (nc-Si) and Er were prepared and their photoluminescence (PL) properties were studied. The samples exhibited luminescence peaks at 0.81 and 1.54 μm, which could be assigned to the electron-hole recombination in nc-Si and the intra-4f transition in Er3+, respectively. Correlation between the intensities of the two luminescence peaks was studied as functions of Er concentration and excitation power. The present results clearly demonstrate that excitation of Er3+ occurs through the recombination of photogenerated carriers spatially confined in nc-Si and the subsequent energy transfer to Er3+.
How tunneling currents reduce plasma-induced charging
Authors: Hwang Gyeong S. , Giapis Konstantinos P.
Published: July 2002

As semiconductor manufacturing moves towards smaller logic devices and thinner gate oxides, there is serious concern that pattern-dependent charging during plasma etching will impede progress by distorting etch profiles and by causing oxide breakdown. Simulations of the final overetch predict that the use of ultrathin oxides (⩽5 nm), combined with a low substrate potential, will actually eliminate notching by enabling electron tunneling from the substrate to decrease surface charging potentials at the bottom of high aspect ratio trenches. Comparison with published experimental results validates the simulations.
Valence intersubband lasers with inverted light-hole effective mass
Authors: Sun G. , Lu Y. , Khurgin J. B.
Published: July 2002

We propose a novel intersubband laser based on transition between the ground-state heavy-hole subband (HH1) and light-hole subband (LH1) in a k-space region where the light-hole effective mass is inverted. The laser structure can be electrically pumped with a simple quantum cascade scheme. Our calculation shows that with only a small fraction of the carrier population in the upper subband (LH1), it is possible to achieve population inversion between the two subbands locally in K space where the light-hole effective mass is inverted. Optical gain in excess of 150/cm can be achieved with a pumping current density on the order to 100 A/cm2 at the temperature of liquid nitrogen.
X-ray photoelectron spectroscopy study of excimer laser treated alumina films
Authors: Georgiev D. G. , Kolev K. , Laude L. D. , Mednikarov B. , Starbov N.
Published: July 2002

Amorphous alumina layers are deposited on a single crystal Si substrate by a e-gun evaporation technique. These films are then thermally annealed in oxygen to be crystallized and, further, irradiated with an excimer laser beam. At each stage of the film preparation, an x-ray photoelectron spectroscopy analysis is performed at the film surface and in depth, upon ion beam grinding. Results give evidence for the formation of an aluminosilicate upon thermal annealing of the film in oxygen. At the surface itself, this compound is observed to decompose upon excimer laser irradiation at energy densities exceeding 1.75 J/cm2, giving rise to free Si atoms and SiO2, however with complete disappearance of Al atoms. Model photochemical reactions are proposed to explain such transformations.
Mapping of the lateral polar orientational distribution in second-order nonlinear thin films by scanning second-harmonic microscopy
Authors: Vydra Jan , Eich Manfred
Published: July 2002

We present a nondestructive experimental technique for the determination of the lateral distribution of the polar order in second-order nonlinear optical thin films. The sample, which consists of a poled polymer film is scanned through the focus of an infrared laser beam in a second-harmonic generation setup and the second-harmonic intensity is monitored stepwise. In combination with a conventional electro-optic (EO) characterization it is possible to create an EO-coefficient map of the sample. The resolution of this mapping technique can be significantly increased by using high numerical aperture microscope optics for the illumination of the poled polymer. This method, for instance, allows the evaluation of poling inhomogeneities due to high-field poling and field distortions at the edges of poling electrodes.
Fabrication of quantum devices by Ångström-level manipulation of nanoparticles with an atomic force microscope
Authors: Junno T. , Carlsson S.-B. , Xu Hongqi , Montelius L. , Samuelson L.
Published: July 2002

We describe a technique for the fabrication of lateral nanometer-scale devices, in which individual metallic nanoparticles are imaged, selected and manipulated into a gap between two electrical leads with the tip of an atomic force microscope. In situ, real-time monitoring of the device characteristics is used to control the positions of the particles down to atomic accuracy and to tune the electrical properties of the device during fabrication. Using this technique we demonstrate a nanomechanical switch as well as atomic-scale contacts that are stable at quantized conductance levels on the time scale of hours at room temperature.
Kinetics of charge carrier recombination in organic light-emitting diodes
Authors: Kalinowski J. , Camaioni N. , Di Marco P. , Fattori V. , Martelli A.
Published: July 2002

The temporal behavior of the electroluminescence (EL) from thin-film indium-tin-oxide (ITO)/aromatic diamine (TPD)/Al(III) 8-hydroxyquinoline (Alq3)/Mg/Ag light-emitting diodes (LEDs) upon the application of a rectangular driving voltage is analyzed in terms of the electron-hole recombination on the TPD/Alq3 interface. From the long-time component of the temporal decay of the EL intensity the electron-hole recombination coefficient γ=(1.1±0.5)×10−10 cm3/s is determined in good agreement with the value expected on the basis of the Langevin theory of recombination with the recombination process controlled by the motion of holes in the hole-transporting TPD layer of the LED.
Self-ordering and confinement in strained InGaAs/AlGaAs V-groove quantum wires grown by low-pressure organometallic chemical vapor deposition
Authors: Martinet E. , Reinhardt F. , Gustafsson A. , Biasiol G. , Kapon E.
Published: July 2002

The structure and low temperature luminescence properties of compressively strained InGaAs/AlGaAs quantum wire (QWR) arrays grown by low-pressure organometallic chemical vapor deposition on V-grooved substrates are reported. The strain gives rise to quasi-periodic undulations of the wire facets along the wire axis, resulting in ordered chains of quantum dotlike structures. Low-temperature photoluminescence shows efficient emission from the wires with narrow (as low as 9.8 meV) linewidths and relatively high intensities. At high excitation densities, several quasi-one-dimensional QWR subbands appear as a result of bandfilling, presenting virtually no energy shifts (<2 meV), even when several (⩾3) subbands are filled.
Growth and characterization of thin Si80C20 films based upon Si4C building blocks
Authors: Kouvetakis J. , Chandrasekhar D. , Smith David J.
Published: July 2002

The growth of thin Si80C20 diamond-structured material on (100)Si has been achieved using the novel C–H free, carbon source tetrasilyl methane, C(SiH3)4. The precursor decomposes at temperatures in the range 600–700 °C to give thin amorphous layers with a composition of Si0.80C0.20, which corresponds to the same relative concentrations of Si and C as in the precursor. The amorphous material is crystallized via solid-phase epitaxy by annealing at 825 °C to yield a potentially ordered structure in which Si4C tetrahedra are linked together in a three-dimensional diamond-cubic framework. Measured lattice parameters are larger than expected from Vegards’ Law, a discrepancy which is attributed to steric repulsions causing bond elongation.
On the environment of optically active Er in Si-electroluminescence devices
Authors: Lanzerstorfer S. , Palmetshofer L. , Jantsch W. , Stimmer J.
Published: July 2002

We report sharp, atomlike electroluminescence spectra close to 1.54 μm from a low-dose (3.5×1018 cm−3) erbium-implanted silicon light-emitting diode operating under forward bias. The well-resolved Stark splitting identifies the isolated interstitial Er with cubic site symmetry as the source. The full width at half maximum of the most intense line is 0.5 nm. A comparison with a highly Er (5×1019 cm−3) and O (1×1020 cm−3) doped diode with a high doping gradient grown by molecular beam epitaxy and with Er-implanted silica is given with respect to fine structure and thermal quenching. The room-temperature emission of the highly Er and O doped diode is ascribed to Er containing silica precipitates within the c-Si matrix.
An in situ Raman study of polarization-dependent photocrystallization in amorphous selenium films
Authors: Poborchii Vladimir V. , Kolobov Alexander V. , Tanaka Kazunobu
Published: July 2002

Photocrystallization of amorphous selenium (a-Se) films under illumination by polarized light with 632.8 or 647.1 nm wavelength has been studied by Raman spectroscopy. Preferential orientation of trigonal crystalline, selenium (t-Se) obtained as a result of photocrystallization has been observed, threefold c axis of t-Se being oriented perpendicular to the direction of the polarization of the illuminating light. Although the mechanism of polarization-dependent photocrystallization seems to be optical in origin, an alternative, essentially thermal, mechanism of the polarization-dependent photocrystallization of a-Se is discussed.
High figure-of-merit porous Pb1−xCaxTiO3 thin films for pyroelectric applications
Authors: Seifert Andreas , Muralt Paul , Setter Nava
Published: July 2002

The pyroelectric properties of Pb1−xCaxTiO3 (x=0–0.3) thin films have been greatly improved by including porosity in the microstructure. Control of nucleation and growth by variation of the heating rate was used to process porous, low dielectric constant films on platinized silicon wafers. Electrical characterization of poled films showed pyroelectric coefficients of up to ∼200 μC/m2 K and permitivities as low as εr≈55. The resulting high figure-of-merit p/εr in combination with small dielectric losses of 0.5%–2.0% at 1 kHz make the films good candidates for pyroelectric thin-film devices. Relationships between microstructure, composition, and properties were established.
Investigation of bulk and interfacial properties of Ba0.5Sr0.5TiO3 thin film capacitors
Authors: Zafar Sufi , Jones Robert E. , Chu Peir , White Bruce , Jiang Bo , Taylor Deborah , Zurcher Peter , Gillepsie Sherry
Published: July 2002

In this letter, we report the results for capacitance versus frequency measurements on a set of Ba0.5Sr0.5TiO3 (BST) capacitors with platinum electrodes (Pt) and varying BST film thicknesses. The study shows that Pt/BST interfacial capacitance is independent of frequency whereas the bulk dielectric constant has a power law dependence on frequency. Also, the bulk dielectric constant is observed to decrease whereas the interfacial capacitance increases with increasing temperature. In addition, we report the dependence of dielectric dispersion on BST film thickness and temperature. Calculations are performed which provide insights into the observed dispersion effects.
CdS photoluminescence inhibition by a photonic structure
Authors: Blanco A. , López C. , Mayoral R. , Mı́guez H. , Meseguer F. , Mifsud A. , Herrero J.
Published: July 2002

Here we present experimental evidence of the strong modification of the CdS photoluminescence when it is embedded in a SiO2 colloidal photonic crystal. When the emitted light matches a forbidden photonic band in the matrix, inhibition of the semiconductor photoluminescence is achieved. In this work we prove the effective control of this effect by means of the photonic lattice parameter of the host. CdS was grown by chemical bath deposition and its quality has been checked employing Raman spectroscopy and x-ray diffraction. Scanning electron microscopy is used to study the morphology of the composite.
Dual-color polymer light-emitting pixels processed by hybrid inkjet printing
Authors: Chang Shun-Chi , Bharathan Jayesh , Yang Yang , Helgeson Roger , Wudl Fred , Ramey Michael B. , Reynolds John R.
Published: July 2002

A hybrid inkjet printing (HIJP) technology, which combines a pin-hole free polymer buffer layer and an inkjet printed polymer layer, allows the patterning of high quality polymer light-emitting devices. In this letter, we present a successful demonstration of controllable patterning of dual-color polymer light-emitting pixels using this HIJP technique. In this demonstration, the polymer buffer layer is a wide band gap, blue emitting semiconducting polymer prepared by the spin-casting technique. The inkjet printed layer is a red-orange semiconducting polymer which was printed onto the buffer layer. When a proper solvent was selected, the printed polymer diffused into the buffer layer and efficient energy transfer took place generating a red-orange photoluminescence and electroluminescence from the inkjet printed sites. Based on this principle, blue and orange-red dual-color polymer light-emitting pixels were fabricated on the same substrate. The use of this concept represents an entirely new technology for fabricating polymer multicolor displays with high-resolution, lateral patterning capability.
Electronic states at aluminum nitride (0001)-1×1 surfaces
Authors: Wu C. I. , Kahn A.
Published: July 2002

We investigate the electronic structure of aluminum nitride (0001)-1×1 surfaces via direct and inverse photoemission spectroscopy. Bulk and surface sensitive measurements on clean surfaces and surfaces exposed to oxygen or cesium demonstrate the existence of filled and empty surface states which extend more than 1 eV beyond the valence- and conduction-band edges. The filled states are tentatively associated with Al dangling or back bonds. The measurement of the top of the valence band upon removal of the filled states leads to a determination of an electron affinity equal to 1.9±0.2 eV. The empty surface states are presumed to play a role in the pinning of the Fermi level in the upper part of the gap and are consistent with the anticipated metallicity of the surface.
Third-order optical nonlinearity of the carbon nanotubes
Authors: Liu Xuchun , Si Jinhai , Chang Baohe , Xu Gang , Yang Qiguang , Pan Zhengwei , Xie Sishen , Ye Peixian , Fan Junhua , Wan Meixiang
Published: July 2002

Third-order optical nonlinearity of the carbon nanotubes was investigated with both 30-ps-wide and 8-ns-wide Nd:YAG laser beams. Both 1064 and 532 nm were used in each case. The nonlinear optical temporal response was obtained both on the picosecond scale and on the nanosecond scale, and the corresponding mechanism is discussed. Finally, we have compared the average contribution of one carbon atom to the third-order optical nonlinearity in the carbon nanotubes with that in C60, finding some enhancement of the third-order optical nonlinearity in the carbon nanotubes.
Fabrication of high-quality one- and two-dimensional electron gases in undoped GaAs/AlGaAs heterostructures
Authors: Harrell R. H. , Pyshkin K. S. , Simmons M. Y. , Ritchie D. A. , Ford C. J. B. , Jones G. A. C. , Pepper M.
Published: July 2002

We have developed a technique for the fabrication of high-mobility electron gases formed in undoped GaAs/AlGaAs heterostructures. The use of an insulated gate allows independent control over the carrier density in the Hall bar and ohmic contact regions of the device. This unique design eliminates difficulties in obtaining reliable ohmic contacts, particularly in the low carrier density regime. In the absence of remote ionized impurity scattering, extremely high transport mobilities are obtained at low carrier densities (1×106 cm2 V−1 s−1 at 1×1010 cm−2). This design has been adapted to the formation of undoped one-dimensional electron gases that show clean and reproducible conductance plateau at 1.5 K.
Room-temperature InAsSb strained-layer superlattice light-emitting diodes at λ=4.2 μm with AlSb barriers for improved carrier confinement
Authors: Pullin M. J. , Hardaway H. R. , Heber J. D. , Phillips C. C. , Yuen W. T. , Stradling R. A. , Moeck P.
Published: July 2002

Room-temperature InAs/InAs1−xSbx strained-layer superlattice light-emitting diodes (x∼8%) are reported that emit at λ∼4.2 μm with an internal efficiency of 2.8%. The structures are grown by molecular beam epitaxy on slightly mismatched InAs substrates and include a strained AlSb barrier layer to prevent electron migration to the dislocated substrate–epilayer interface region. Comparison with a near identical structure grown without the barrier layer indicates a factor of four improvement in device efficiency at room temperature.
Temperature dependence of Raman scattering in single crystal GaN films
Authors: Liu Ming S. , Bursill Les A. , Prawer S. , Nugent K. W. , Tong Y. Z. , Zhang G. Y.
Published: July 2002

Micro-Raman scattering from single crystal GaN films, both free-standing and attached to Al2O3 substrates, was performed over the temperature range from 78 to 800 K. These measurements reveal that the Raman phonon frequency decreases and the linewidth broadens with increasing temperature. This temperature dependence is well described by an empirical relationship which has proved to be effective for other semiconductors. The experiments also demonstrate that the strain from Al2O3 substrates compresses the epitaxial GaN in the c-axis direction.
Anisotropic holographic scattering in centrosymmetric sodium nitroprusside
Authors: Imlau M. , Schieder R. , Rupp R. A. , Woike Th.
Published: July 2002

Anisotropic holographic light scattering has been discovered in centrosymmetric single crystals of sodiumnitroprusside, Na2[Fe(CN)5NO]⋅2H2O, in which the photorefractive response is based on the excitation of metastable electronic states. Besides a bright homogeneous scattering corona generated by isotropic scattering, a sharp scattering ring builds up with light polarized orthogonal to that of the pump beam and the corona. This anisotropic scattering resulting from interference of scattered waves with the pump beam occurs although there is no linear electro-optic effect in centrosymmetric crystals.
Observation of a large third-order nonlinear response of exciton polaritons in GaAs thin film
Authors: Akiyama Koich , Tomita Nobuyuki , Nomura Yoshinori , Isu Toshiro
Published: July 2002

We investigated the third-order nonlinear response of excitons in GaAs thin films grown by molecular beam exitaxy. Degenerate four-wave mixing (DFWM) measurements were made using picosecond pulses of 3.2 ps auto correlation width at T=5 K. The DFWM signal of a 110-nm-thick GaAs film at exciton resonance energy is 25 times larger than that of a 1-μm-thick GaAs film. This enhancement is explained by the confinement effect of exciton polaritons, which is due to the nonlocal response of the center-of-mass quantized states of excitons in a radiation field.
Activation and deactivation of implanted B in Si
Authors: Pelaz Lourdes , Venezia V. C. , Gossmann H.-J. , Gilmer G. H. , Fiory A. T. , Rafferty C. S. , Jaraiz M. , Barbolla J.
Published: July 2002

The temporal evolution of the electrically active B fraction has been measured experimentally on B implanted Si, and calculated using atomistic simulation. An implant of 40 keV, 2×1014 cm−2 B was examined during a postimplant anneal at 800 °C. The results show a low B activation (∼25%) for short anneal times (⩽10 s) that slowly increases with time (up to 40% at 1000 s), in agreement with the model proposed by Pelaz et al. [Appl. Phys. Lett. 74, 3657 (1999)]. Based on the results, we conclude that B clustering occurs in the presence of a high interstitial concentration, in the very early stages of the anneal. For this reason, B clustering is not avoided by a short or low-temperature anneal. The total dissolution of B clusters involves thermally generated Si interstitials, and therefore, requires long- or high-temperature anneals.
Polarization-insensitive wavelength selection in an axially symmetric liquid-crystal Fabry-Perot filter
Authors: Lee Ju-Hyun , Kim Hak-Rin , Lee Sin-Doo
Published: July 2002

A polarization-insensitive, electrically tunable Fabry-Perot filter (FP) with a nematic liquid crystal (NLC) is demonstrated in an axially symmetrical configuration. Due to the axial symmetry, the wavelength-selection property of the FP filter is completely independent of the polarization state of the input beam in the whole range of tuning. For normal incidence, the in-plane symmetry of the input polarization state is preserved in such configuration that the alignment of the NLC molecules are homeotropic on one mirror surface and axially homogeneous on the other surface.
Stretching quantum wells: A method for trapping free carriers in GaAs heterostructures
Authors: Negoita V. , Snoke D. W. , Eberl K.
Published: July 2002

We have demonstrated a method of using inhomogeneous stress to create an in-plane harmonic potential in GaAs quantum wells which works equally well for excitons and for free conduction electrons. The depth of the well can be continuously varied via an external control. This essentially provides a type of gate for controlling the motion of carriers, e.g., a two-dimensional electron gas, without using electric field.
Atomic structure and composition of the barrier in the modified interface high-Tc Josephson junction studied by transmission electron microscopy
Authors: Wen J. G. , Koshizuka N. , Tanaka S. , Satoh T. , Hidaka M. , Tahara S.
Published: July 2002

The atomic structure and composition of modified interface junctions which showed reproducible critical current Ic (Ic1σ<8% for 100 junctions) are investigated by transmission electron microscopy. Transmission electron microscopic observations show the existence of a thin barrier (1–2 nm) homogeneously covering the ion milled edge of the base YBa2Cu3Oy film although there is no barrier deposition and annealing process. High-resolution electron microscopy images and energy dispersive x-ray analysis with a spot size of 1 nm indicates that the barrier is a Ba-based perovskite-like structure, (Y1−xCux)BaOy with x<0.5. A thin amorphous layer whose composition deviates from YBa2Cu3Oy is formed due to the preferential sputtering of Cu. The amorphous layer recrystallizes into the nonequilibrium phase (Y1−xCux)BaOy after heating up to the deposition temperature.
Spontaneous formation of complex and ordered structures on oxygen-plasma-treated elastomeric polydimethylsiloxane
Authors: Chua Diana B. H. , Ng H. T. , Li Sam F. Y.
Published: July 2002

In this letter, we describe the spontaneous formation of complex and ordered structures on polydimethylsiloxane when subjected to oxygen plasma treatment. Periodicity of wavy structures from submicrometer to micrometer length scales could be generated reproducibly and quantitatively accounted for. The origin of these patterns could be related to the relief of compressive stress by buckling of the silica-like thin film that was formed as a result of the plasma exposures. Atomic force microscope has been used to characterize the varied trends of the modifications. The present approach could be extended to the fabrication of intricate ordered patterns on polymeric surfaces with integrated relief structures obtained by soft lithography and micromolding.
Ductile quasicrystalline alloys
Authors: Inoue A. , Zhang T. , Chen M. W. , Sakurai T. , Saida J. , Matsushita M.
Published: July 2002

An icosahedral (I) quasicrystalline phase with a grain size below 40 nm was formed as a metastable phase in crystallization of the bulk glassy Zr65Al7.5Cu17.5−xNi10Mx (M=Ag, Pd, Au, or Pt; x=5 and 10 at %) alloys. The volume fraction (Vf) of the I phase is about 85% for the 5% M alloy and nearly 100% for the 10% M alloy. The I phase changes to Zr2Cu+Zr2Ni+Zr2Al3 in a fully annealed state. Compressive fracture strength (σc,f) and fracture elongation (εc,f) of the 10% Pd cylinder with a diameter of 2 mm are respectively 1640 MPa and 2.2% for the glassy phase and increase to 1830 MPa and 3.1% for the I phase. The increase in σc,f is due to the suppression effect of the I particles against the shear deformation of the intergranular glassy phase, and the increase in εc,f results from the localization effect of deformation into the glassy layer. The precipitation of the I phase implies that the glassy alloys include randomly oriented I configurations. The present work shows promise for the new class of high-strength nanoquasicrystalline materials.

At the request of the authors, this article is being retracted effective 24 June 2011.
Nanostructure fabrication using pulsed lasers in combination with a scanning tunneling microscope: Mechanism investigation
Authors: Lu Y. F. , Mai Z. H. , Zheng Y. W. , Song W. D.
Published: July 2002

Nanostructure fabrication using lasers in combination with a scanning tunneling microscope has been reported in the past several years. Different mechanisms have been discussed for the formation of these nanostructures. However, they are controversial. In this study, we investigated the mechanism of nanostructure fabrication on both gold films and hydrogen-passivated Ge surfaces. Current-distance curves for a gold film and for an H-passivated Ge surface under an electrochemically etched tungsten tip were measured to determine the tip-sample distance. An analytical model was proposed to explain different mechanisms for nanostructure fabrication on gold films and on H-passivated Ge surfaces. Thermal expansion of the tip under laser irradiation was calculated. With comparison between the tip-sample distance and the thermal expansion of the tip, we can determine whether the mechanism is based on optical enhancement or on thermal mechanical indentation.
Strong exciton-erbium coupling in Si nanocrystal-doped SiO2
Authors: Kik P. G. , Brongersma M. L. , Polman A.
Published: July 2002

Silicon nanocrystals were formed in SiO2 using Si ion implantation followed by thermal annealing. The nanocrystal-doped SiO2 layer was implanted with Er to a peak concentration of 1.8 at. %. Upon 458 nm excitation the sample shows a broad nanocrystal-related luminescence spectrum centered around 750 nm and two sharp Er luminescence lines at 982 and 1536 nm. By measuring the excitation spectra of these features as well as the temperature-dependent intensities and luminescence dynamics we conclude that (a) the Er is excited by excitons recombining within Si nanocrystals through a strong coupling mechanism, (b) the Er excitation process at room temperature occurs at a submicrosecond time scale, (c) excitons excite Er with an efficiency >55%, and (d) each nanocrystal can have at most ∼1 excited Er ion in its vicinity.
Physical origin of nonlinearity in the Fowler–Nordheim plot of field-induced emission from amorphous diamond films: Thermionic emission to field emission
Authors: Xu N. S. , Chen Jun , Deng S. Z.
Published: July 2002

Nonlinearity is observed in Fowler–Nordheim (FN) plots of field emission from nondoped and nitrogen-doped amorphous diamond films. Based on a unified electron emission equation a detailed analysis is carried out. The results from numerical calculation of the unified equation are consistent with the experimental data. It is shown that the nonlinearity in the FN plot originates from a transition from thermionic emission to field emission as the applied field increases. The electrical field ranges are derived in which the field emission and thermionic emission approximation applies.
Local structure and bonding of Er in GaN: A contrast with Er in Si
Authors: Citrin P. H. , Northrup P. A. , Birkhahn R. , Steckl A. J.
Published: July 2002

X-ray absorption measurements from relatively high concentrations of Er (>0.1 at. %) doped in GaN films show that Er occupies the Ga site with an unprecedentedly short Er–N bond length. Electroluminescence intensities from these GaN:Er films correlate with the concentration of Er atoms that replace Ga, not with the abundantly present O impurities in the host. Simple chemical concepts are used to explain each of these results and their striking difference from those obtained for Er-doped Si.
Thermal boundary resistance at Ge2Sb2Te5/ZnS:SiO2 interface
Authors: Kim E.-K. , Kwun S.-I. , Lee S.-M. , Seo H. , Yoon J.-G.
Published: July 2002

The thermal conductivity of sputtered amorphous-Ge2Sb2Te5 (a-GST)/ZnS:SiO2 and crystalline-Ge2Sb2Te5 (c-GST)/ZnS:SiO2 multilayer films has been measured in the temperature range between 50 and 300 K using the 3ω method. The conductivity data in the direction of the cross plane of the films showed lower values than the series conductance of the constituent layers, which was calculated from the thermal conductivity of thick a-GST, c-GST, and ZnS:SiO2 films measured independently. From the reduction in the multilayer thermal conductivity, the thermal boundary resistance at the interface between GST and ZnS:SiO2 films was calculated. The boundary resistance in the c-GST multilayer was lower than that for the a-GST case in the whole measured temperature region.
Asymmetrical magnetoimpedance in as-cast CoFeSiB amorphous wires due to ac bias
Authors: Makhnovskiy D. P. , Panina L. V. , Mapps D. J.
Published: July 2002

Asymmetrical giant magnetoimpedance (AGMI), which utilizes a high frequency bias field hb, is realized in a Co-based amorphous wire having a circumferential anisotropy in the outer region. No asymmetry in the dc magnetic configuration is needed in this case. AGMI is discussed in terms of the surface impedance tensor, demonstrating that the effect of hb is related to the role of the off-diagonal component of the impedance in the voltage response measured across the wire. This effect is important for developing autobiased linear magnetic sensors. Using two oppositely biased wires, a near-linear voltage output (±4 mV) is obtained in the range of ±5 Oe for the sensed dc field at a frequency of 8 MHz.
Electrical properties of in situ phosphorus- and boron-doped polycrystalline SiGeC films
Authors: Anteney I. M. , Parker G. J. , Ashburn P. , Kemhadjian H. A.
Published: July 2002

The sheet resistance, effective carrier concentration, and Hall mobility of in situ boron- and phosphorus-doped polycrystalline Si0.82−yGe0.18Cy films are presented for carbon contents between 0% and 4%. Phosphorus and boron doping levels of 4×1019 and 2×1020 cm−3 were achieved for the n- and p-type layers, respectively, and remained largely unaffected by carbon content. The phosphorus-doped films showed a dramatic increase in sheet resistivity and a corresponding drop in effective carrier concentration and Hall mobility. In contrast, the boron-doped films showed only a minor increase in resistivity. This is attributed to interstitial carbon increasing the defect density and also shifting the defect energy levels at the grain boundaries towards the valence band. This causes an increase in the grain-boundary energy barrier in n-type layers, but leaves the p-type layers largely unaffected.
Product state resolved stereodynamics: Rotational polarization of OH(2Π;υ′,N′,Ω,f ) scattered from the reaction, H+CO2→OH+CO
Authors: Brouard M. , Burak I. , Hughes D. W. , Kalogerakis K. S. , Simons J. P. , Stavros V.
Published: July 2002

The quantum state resolved rotational angular momentum alignments of the OH products of the H+CO2 reaction have been determined for a range of states spanning those most populated by reaction at a collision energy of 2.5 eV. Surprisingly, for all quantum states studied, the angular momentum is shown to be aligned preferentially in the scattering plane, containing the reagent and product relative velocity vectors. The data suggest that out-of-plane HO–CO torsional forces play a significant role in dissociation of the HOCO intermediate. The polarization behavior mirrors observed in the isoelectronic H+N2O reaction [see the accompanying paper, J. Chem. Phys. 113, 3162 (2000)], and the data are compared with those obtained for that system, and with previous theoretical and experimental work on this important reaction.
Splashing impact of a single drop onto very thin liquid films
Authors: Wang An-Bang , Chen Chi-Chang
Published: July 2002

Liquid films with thicknesses on the order of 1 mm were commonly used for the study of drop impingement onto a wetted surface. This is because films thinner than 1 mm are difficult to generate and measure due to capillary meniscus. In this work a novel method to produce thin films of well-defined thickness has been developed. Also a reliable process with minimum uncertainty to determine film thickness was proposed. New splashing phenomena were observed for drop impact onto thin films. It is found that the critical splash level (the threshold Weber number) is insensitive to film thickness for a given solid surface if the film is sufficiently thin. It is also shown that the critical splash level increases with liquid viscosity.
Low harmonic distortion in a Josephson arbitrary waveform synthesizer
Authors: Benz S. P. , Burroughs C. J. , Dresselhaus P. D.
Published: July 2002

The use of broadband integrated filters has enabled practical operating margins for ac waveforms synthesized from the perfectly quantized voltage pulses of Josephson junction arrays. This improvement enabled the digital synthesis of arbitrary waveforms with low harmonic distortion; the second and higher harmonics are all at least 94 dB below the fundamental [−94 dBc (carrier)]. This is a 47 dB lower distortion compared to that of the same sine wave synthesized by the semiconductor-based code generator which drives the array. We present operating margins for synthesized dc voltages and demonstrate synthesized waveforms with multiple tones at kilohertz and megahertz frequencies with harmonic distortion and intermodulation products below −80 dBc.
Wide nematic range alkenyl diphenyldiacetylene liquid crystals
Authors: Wu Shin-Tson , Neubert M. E. , Keast S. S. , Abdallah D. G. , Lee S. N. , Walsh M. E. , Dorschner T. A.
Published: July 2002

Several alkenyl diphenyldiacetylene liquid crystals were synthesized and their physical properties evaluated. The introduction of a double bond in the side chain leads to an unexpectedly wide nematic range and an extraordinarily large figure of merit at elevated temperatures. These liquid crystals are particularly attractive for infrared laser-beam steering.
On the quantum magnetic size oscillation effects in organic conductors
Authors: Azbel’ M. Ya. , Kirichenko O. V. , Peschanskiı̆ V. G.
Published: July 2002

The quantum magnetic size oscillations (QMSOs) of the thermodynamic quantities in layered organic conductors with a quasi-two-dimensional electron energy spectrum of arbitrary form are investigated theoretically. It is shown that the modulation of the QMSOs contains detailed information about the dispersion relation of the charge carriers.
Bi surfactant control of ordering and surface structure in GaInP grown by organometallic vapor phase epitaxy
Authors: Jun S. W. , Lee R. T. , Fetzer C. M. , Shurtleff J. K. , Stringfellow G. B. , Choi C. J. , Seong T.-Y.
Published: July 2002

The surfactant Bi has been added during organometallic vapor phase epitaxial growth (OMVPE) of GaInP using the precursor trimethylbismuth. The addition of a small amount of Bi during growth results in disordered material using conditions that would otherwise produce highly ordered GaInP. Significant changes in the surface structure are observed to accompany the disordering. Atomic force microscopy measurements show that Bi causes an order of magnitude increase in step velocity, leading to the complete elimination of three-dimensional islands for growth on singular (001) GaAs substrates, and a significant reduction in surface roughness. Surface photoabsorption measurements indicate that Bi reduces the number of [1̄10] P dimers on the surface. Secondary ion mass spectroscopy measurements reveal that the Bi is rejected from the bulk, even though it changes the surface reconstruction. Clearly, Bi acts as a surfactant during OMVPE growth of GaInP. The difference in band gap energy caused by the reduction in order parameter during growth is measured using photoluminescence to be about 110 meV for layers grown on singular substrates. Disorder/order/disorder heterostructures were successfully produced in GaInP with a constant solid composition by modulating the TMBi flow rate during growth.
Contribution of ions to the growth of amorphous, polymorphous, and microcrystalline silicon thin films
Authors: Hamers E. A. G. , Fontcuberta i Morral A. , Niikura C. , Brenot R. , Roca i Cabarrocas P.
Published: July 2002

The growth of amorphous, microcrystalline, and polymorphous silicon has been investigated by studying the species contributing to the growth and resulting film structure. The surface reaction probability of the radicals and the contribution of ions to the growth have been determined. In a-Si:H deposition by hot wire chemical vapor deposition, the surface reaction probability (β=0.29) of the depositing radical is compatible with SiH3, whereas the surface reaction probability in microcrystalline silicon growth is higher (0.36⩽β⩽0.54). On the contrary, the deposition of amorphous silicon by plasma enhanced chemical vapor deposition indicates the contribution of more reactive radicals than SiH3. The deposition of polymorphous and microcrystalline silicon by plasma is dominated by ions, which can contribute up to 70% of the deposited film. This is attributed to efficient ionization of silane in charge exchange reactions with hydrogen ions. The surface reaction probability in the case of polymorphous silicon deposition (β≈0.30) is intermediate between that of a-Si:H deposition (β≈0.40) and that of microcrystalline silicon deposition (β≈0.20). Etching of amorphous silicon by means of a hydrogen plasma shows that ions may hinder the process.
Microcalorimetry of dust particles in a radio-frequency plasma
Authors: Swinkels G. H. P. M. , Kersten H. , Deutsch H. , Kroesen G. M. W.
Published: July 2002

The internal temperature of rhodamine B-dyed dust particles (2rp=1.2 μm) immersed in radio-frequency (rf) plasmas has been measured for various plasma conditions. For this purpose, the dye has been excited with an argon-ion laser and the fluorescent emission of the particles has been recorded with an optical multichannel analyzer system. The temperature has been determined after comparison with calibration curves. In argon, the particle temperature increases with rf power and is independent of pressure. In oxygen, an increase with rf power is observed, too. However, the energy flux towards the particles includes also heating by atom recombination (association) and exothermic combustion reactions. These temperature measurements have been compared with calculations based on the thermal balance, where measurements of gas temperature, electron density, and electron temperature have been used. A good agreement between theory and experiment has been found.
Thermoelectric properties of semimetallic (Zr, Hf)CoSb half-Heusler phases
Authors: Xia Y. , Bhattacharya S. , Ponnambalam V. , Pope A. L. , Poon S. J. , Tritt T. M.
Published: July 2002

Unlike semiconducting TiCoSb, ZrCoSb and HfCoSb half-Heusler phases are semimetallic below room temperature and exhibit small Seebeck coefficients of ∼−10 μV/K at 300 K. However, upon substituting (doping) the Co and Sb sites with Pt and Sn, respectively, much larger thermopowers (S) are obtained. For ZrCoSb, S reaches −110 and +130 μV/K while resistivity ρ decreases from ∼5×104 μΩ cm in the undoped phase to 1–2×103 μΩ cm in the substituted phases at 300 K. The lowest thermal conductivity obtained in the substituted alloys is ∼3.0 W/m K at 300 K, which is among the lowest reported for this class of structural phases. There are indications that the thermoelectric properties have not been optimized in these multinary alloys.
A quasiclassical trajectory study of the H+HCN→H2+CN reaction dynamics
Authors: Troya Diego , Baños Irene , González Miguel , Wu Guosheng , ter Horst Marc A. , Schatz George C.
Published: July 2002

We present a quasiclassical trajectory study of the title reaction using a potential energy surface that is derived from ab initio calculations, and which has previously been shown to yield accurate dynamical results for the H2+CN reaction. Results presented include integral and differential cross sections, and product vibrational and rotational distributions for ground and vibrational excited HCN. Vector correlations are also discussed. Detailed comparisons with all available experiments are presented, and most of the theoretical results are in excellent agreement with experiment.
Investigation of electrical breakdown properties in curved electrodes
Authors: Uhm Han S.
Published: July 2002

Properties of electrical breakdown in curved electrodes are investigated by making use of Paschen’s law. Simple analytical expressions for minimum breakdown voltage and its corresponding values of physical parameters are obtained for cylindrical and spherical geometries. The sparking criterion indicates that discharge properties are insensitive to the electrical field polarity inside a diode for both cylindrical and spherical systems, if the second ionization coefficient γ is constant. Due to the curvature effects, the minimum breakdown voltage in cylindrical and spherical geometries increases significantly from its planar value for a large aspect ratio of b/a, where a and b are radii of the inner and outer conductors, respectively. However, the optimum pressure parameter for minimum breakdown voltage in cylindrical geometry is identical to that in planar electrodes. The minimum breakdown voltage for spherical system increases continuously to a finite value, as the curvature aspect ratio b/a increases to infinity. Therefore, pressure for the minimum breakdown voltage may not be zero, even for the infinite aspect ratio. This means that an electrical breakdown with a minimum voltage may occur at surface of a spherical conductor immersed in a gas with an appropriate pressure.
Influence of crystal polarity on the properties of Pt/GaN Schottky diodes
Authors: Karrer Uwe , Ambacher Oliver , Stutzmann Martin
Published: July 2002

Si-doped epitaxial GaN layers with Ga- and N-face polarity were grown by plasma-induced molecular-beam epitaxy (PIMBE) in order to characterize the influence of polarity on the electrical properties of Pt Schottky diodes. Different barrier heights for Pt onto these two materials are obtained from the dependence of the effective barrier height versus ideality factor, determined by I–V measurements to be 1.1 and 0.9 eV for Ga- and N-face GaN, respectively. C–V measurements confirm the greater barrier heights for Ga-face material. A possible explanation for this behavior can be a different band bending of the conduction and valence band, inferred from the self-consistent solution of the Schrödinger–Poisson equation, including polarization-induced surface and interface charges, which result from the different spontaneous polarization in epitaxial layers with different polarity.
Electron energy-loss spectroscopy characterization of pyramidal defects in metalorganic vapor-phase epitaxy Mg-doped GaN thin films
Authors: Benaissa M. , Vennéguès P. , Beaumont B. , Gibart P. , Saikaly W. , Charai A.
Published: July 2002

In the present letter, Mg-doped GaN thin films grown by metalorganic vapor-phase epitaxy were studied using parallel electron energy-loss spectroscopy in a transmission electron microscope. A microstructural characterization of such thin films showed the presence of pyramidal defects (PDs) with a density of about 1018 cm−3. Comparison of energy-loss spectra recorded outside a PD and from the PD showed a significant change in the energy-loss near-edge structure strongly reflecting the presence of inclusions (Mg-based), the electronic properties of which differ from those of GaN. Considering, however, their relatively high density (∼1018 cm−3), one can expect that the optical properties of such inclusions may interfere with those of GaN and, therefore, be at the origin of the frequently obtained blue emission at 2.8–2.9 eV in heavily doped samples.
Single-particle dynamics in dimethyl–sulfoxide/water eutectic mixture by neutron scattering
Authors: Cabral João T. , Luzar Alenka , Teixeira José , Bellissent-Funel Marie-Claire
Published: July 2002

We study water and dimethyl–sulfoxide (DMSO) dynamics in the eutectic DMSO/water mixture, 2H2O:1DMSO, by incoherent quasielastic and inelastic neutron scattering. A temperature range from room down to −27 °C is investigated. Both water and DMSO translational dynamics are significantly slowed in the mixture compared to pure liquids. They exhibit different diffusion dynamics, pointing to the absence of stable hydrogen bonded complexes. Further, the presence of a solute suppresses the dynamic anomalies of water: self-diffusion coefficients for water in the mixture, as well as residence times for jump diffusion reveal an Arrhenius temperature dependence, in contrast to the strong non-Arrhenius behavior in pure water. The density of vibrational states shows a shift of the characteristic librational peak in pure water towards higher energies in the mixture, reflecting hindrance of large amplitude proton motions. These results are compatible with computer simulations which predict longer hydrogen-bond lifetimes in the mixture, compared to pure water. However, rotational relaxation times for water protons in the mixture are estimated to be ∼1 ps, as for pure water. This experimental finding suggests that unlike in pure, low temperature water, proton librations should no longer be the main mechanism responsible for hydrogen-bond breaking in the mixture. Thus, the correspondence between the proton hindered rotational relaxation time and the “hydrogen-bond lifetime” should not apply in DMSO–water mixtures.