bluuebunny/crossref_metadata_2025 huggingface dataset:
Dramatic photoluminescence quenching of phenylene vinylene oligomer thin films upon submonolayer Ca deposition
Authors: Choong V. , Park Y. , Gao Y. , Wehrmeister T. , Müllen K. , Hsieh B. R. , Tang C. W.
Published: July 2002

We report that Ca atoms severely quench the photoluminescence (PL) of an organic thin film 1,4-bis[4-(3,5-di-tert-butylstyryl)stryyl]benzene (4PV). A submonolayer coverage (0.035 monolayer or 0.1 Å) of Ca on the 4PV surface reduced the PL of a 300 Å layer of 4PV by as much as 50%. Three distinct quenching rates were observed throughout the 200 Å Ca deposition. An exciton diffusion length of 200 Å in 4PV is estimated assuming that the PL quenching is diffusion controlled.
Manipulation of growth modes in heteroepitaxy: Ni/Cu(111)
Authors: Wulfhekel Wulf , Beckmann Ingo , Lipkin Nuphar N. , Rosenfeld Georg , Poelsema Bene , Comsa George
Published: July 2002

Growth manipulation methods, which have been successfully used to improve the growth of homoepitaxial films, are applied to molecular beam epitaxy of the heteroepitaxial system Ni/Cu(111). The procedures applied are temperature reduction during nucleation and pulsed ion bombardment during deposition. While the first does not lead to smoother films, the ion beam assisted growth is successful in reducing the film roughness.
Spin-dependent tunneling in HfO2 tunnel junctions
Authors: Platt C. L. , Dieny B. , Berkowitz A. E.
Published: July 2002

We identified reactively sputtered HfO2 as a particularly good material for making thin insulating barriers for spin-dependent tunnel junctions. This material allows one to form pinhole-free tunnel barriers with good transmission of the spin polarization of the tunneling electrons. Magnetic tunnel junctions consisting of a thin layer of HfO2 sandwiched between transition metal electrodes (Co and Fe, for instance) exhibit changes of tunnel resistance up to 30% at low temperature as a function of applied field. This effect can be used in a variety of magnetic field sensing applications or in magnetic random access memory.
Nitrogen transport during rapid thermal growth of silicon oxynitride films in N2O
Authors: Baumvol I. J. R. , Stedile F. C. , Ganem J.-J. , Trimaille I. , Rigo S.
Published: July 2002

We investigated the transport of nitrogenous species during rapid thermal growth of silicon oxynitride films on Si in N2O, using N isotopic tracing and high resolution depth profiling techniques. The results indicate that the diffusion of nitrogenous species (most probably NO) through the growing oxynitride film to react with Si at the oxynitride/Si interface, induces the incorporation of N near this interface. This mechanism acts in parallel with a site-to-site jump mechanism (interstitialcy or vacancy) of diffusion and chemical reaction of nitrogenous species in the volume of the growing oxynitride film. The characteristic N accumulation only near the interface obtained by rapid thermal processing growth in N2O is due to the removal of N from the near surface region of the films, here attributed to atomic exchanges O↔N taking place during growth. Furthermore, N↔N exchange was also observed.
Self-assembling GaN quantum dots on AlxGa1−xN surfaces using a surfactant
Authors: Tanaka Satoru , Iwai Sohachi , Aoyagi Yoshinobu
Published: July 2002

Nanoscale GaN quantum dots were fabricated on AlxGa1−xN layer surfaces via metalorganic chemical vapor deposition. In order to achieve a self-assembling dot structure, a two-dimensional growth mode (step flow) of GaN films on AlxGa1−xN (x=0–0.2) surfaces that is energetically commenced under the conventional growth conditions was intentionally modified into a three-dimensional mode by using a ‘‘surfactant.’’ The surfactant is believed to inhibit the GaN film from wetting the AlGaN surface due to the change in surface free energy. The resulting morphological structures of GaN dots were found to be sensitive to the doping rate of tetraethyl silane used as a surfactant, the Al content (x) of the AlxGa1−xN layer, and the growth temperature. A very intense photoluminescence emission was observed from the GaN dots embedded in the AlGaN layers.
Nonlinear ac susceptibility of high temperature superconducting rings
Authors: Mrowka F. , Wurlitzer M. , Esquinazi P. , Brandt E. H. , Lorenz M. , Zimmer K.
Published: July 2002

We have studied the ac response of structured thin, narrow rings of high-Tc superconducting films at ac and dc fields ⩽1 mT. The ac susceptibility as a function of reduced ac field h/hm (hm being the ac field at the dissipation maximum) shows no temperature dependence and can be well described by a model presented in this work. We show that the measurement of the magnetic moment of samples with ring geometry enables the identification of regions with degraded superconducting properties. The response of the ring shows up to ∼ 1000 times smaller critical current density than the bulk. The magnetic moment shows a crossover from the ring to strip response upon increasing the applied ac field.
Room-temperature holographic grating recording in CdF2:Ga
Authors: Suchocki A. , Koziarska B. , Langer T. , Langer J. M.
Published: July 2002

We demonstrate that semiconducting CdF2 crystals doped with gallium provide an efficient medium for optical storage of information in static and dynamic regimes in a temperature range close to 300 K. Ga is a bistable center in CdF2 crystals. Illumination by visible and UV light below 500 nm causes phototransformation of these centers from a deep-localized to a shallow-hydrogenic state. They are separated by a vibronic barrier that causes metastability below 250 K. The phototransformation changes the local polarizability, and thus, the local refractive coefficient. This, in turn, allows writing a phase hologram with a diffraction efficiency and decay time being temperature dependent with the activation energy Eact=0.65±0.1 eV. A spontaneous decay of the grating is caused by a thermal recovery of the Ga impurity from the metastable hydrogenic state to the localized ground state. The writing is a local process.
Growth patterns of self-assembled InAs quantum dots near the two-dimensional to three-dimensional transition
Authors: Colocci M. , Bogani F. , Carraresi L. , Mattolini R. , Bosacchi A. , Franchi S. , Frigeri P. , Rosa-Clot M. , Taddei S.
Published: July 2002

Self-assembled InAs quantum dots have been grown by molecular beam epitaxy in such a way as to obtain a continuous variation of InAs coverages across the wafer. Structured photoluminescence spectra are observed after excitation of a large number of dots; deconvolution into Gaussian components yields narrow emission bands (full width at half-maximum 20–30 meV) separated in energy by an average spacing of 30–40 meV. We ascribe the individual bands of the photoluminescence spectra after low excitation to families of dots with similar shapes and with heights differing by one monolayer, as strongly supported by numerical calculations of the fundamental electronic transitions in quantum dot structures.
A surface-emitting vacuum-deposited organic light emitting device
Authors: Bulović V. , Tian P. , Burrows P. E. , Gokhale M. R. , Forrest S. R. , Thompson M. E.
Published: July 2002

We demonstrate a vacuum-deposited organic light emitting device which emits from its top surface through a transparent indium-tin-oxide anode. This device employs a novel protective cap layer which prevents damage to the organic layers during sputter deposition of the anode, while also improving hole injection. Mechanisms of current transport and carrier injection from the contacts are investigated. This device configuration allows for integration of organic light emitting devices with n-channel field effect transistors used in display active matrix backplanes.
Deactivation of Be acceptors by atomic hydrogen in AlGaAs/GaAs quantum well structures
Authors: Zhao Q. X. , Fimland B. O. , Södervall U. , Willander M. , Selvig E.
Published: July 2002

We present a systematic study of atomic hydrogen passivation of Be doped AlGaAs/GaAs quantum well (QW) structures by photoluminescence spectroscopy. High quality AlGaAs/GaAs QWs with centrally doped Be acceptors have been subjected to dc H plasma at different sample temperatures and gas pressures. It is found that the percentage of deactivated Be acceptors strongly depends on sample temperature during H-plasma treatments and on the cooling down procedure after dc H-plasma treatment, while the duration of H-plasma treatments is less critical. Without causing degradation of the AlGaAs/GaAs interfaces, we have obtained a deactivation of up to 82% of the Be acceptors.
Selective Stark-ladder transitions in an asymmetric double-well GaAs/AlAs superlattice
Authors: Fujiwara Kenzo , Hinooda Shin-ichi , Kawashima Kenji
Published: July 2002

Optical transitions in a novel biperiodic GaAs/AlAs superlattice consisting of asymmetric double quantum wells have been investigated as a function of an applied electric field by photocurrent spectroscopy. Due to the existence of two different heavy-hole localized states under the applied field, two types of Stark-ladder transitions are possible in the superlattice. It is found that, among four possible indirect ladder transitions between the nearest neighbor wells, two of them have larger oscillator strengths. This selectivity of the indirect ladder transitions is rigorously explained by selective resonance coupling between the inequivalent neighboring wells because of the introduced structural asymmetry.
A model for the current–voltage characteristics and the quantum efficiency of single-layer organic light emitting diodes
Authors: Kawabe Y. , Jabbour G. E. , Shaheen S. E. , Kippelen B. , Peyghambarian N.
Published: July 2002

A model for the current–voltage characteristics and the quantum efficiency of single-layer organic light emitting diodes is presented. With variables such as the electric field, the hole, and electron carrier densities, a set of coupled nonlinear differential equations is derived by using classical electrostatics and by assuming Fowler–Nordheim injection. Numerical calculations for different carrier mobility and barrier height conditions show that low barrier height at both electrodes leads to higher efficiency and higher carrier mobility leads to higher brightness. We find that for applications that require high current injection such as lasers, materials with high mobility are desired to reduce space charge effects.
Field emission from tetrahedral amorphous carbon
Authors: Satyanarayana B. S. , Hart A. , Milne W. I. , Robertson J.
Published: July 2002

Field emission has been measured from a series of tetrahedrally bonded amorphous carbon (ta-C) films produced by the filtered cathodic vacuum arc. The threshold field and current densities achievable have been studied as a function of their sp3 content and of nitrogen incorporation. Typical undoped ta-C films are found to have a threshold field of 10–20 V/μm, decreasing with increasing sp3 content, and optimally nitrogen doped films exhibit threshold fields as low as 3–5 V/μm.
Layer-by-layer electrostatic self-assembly of nanoscale Fe3O4 particles and polyimide precursor on silicon and silica surfaces
Authors: Liu Yanjing , Wang Anbo , Claus Richard O.
Published: July 2002

Monolayer and multilayer ultrathin films comprised of nanosized iron oxide (Fe3O4) particles and polyimide molecules have been fabricated on single crystal silicon and quartz substrates by a novel layer-by-layer electrostatic self-assembly process. This process involves the alternate dipping of a substrate into an aqueous solution of anionic polyimide precursor (polyamic acid salt, PAATEA), followed by dipping into an aqueous solution of polycation polydiallyldimethylammonium chloride (PDDA) which coats on nanoscale Fe3O4 particles as a stabilizer. The growth process and the structure have been characterized using UV-vis spectroscopy, contact angle, and ellipsometry measurements. The results suggest that well-ordered uniform monolayer and multilayer magnetic films have been formed on silicon and silica surfaces. A recently developed highly sensitive fiber optic magnetic field sensor was used to probe the small magnetic field intensity produced by the multilayer films.
Structure of GaN films grown by hydride vapor phase epitaxy
Authors: Romano L. T. , Krusor B. S. , Molnar R. J.
Published: July 2002

The structure of GaN films grown by hydride vapor phase epitaxy on sapphire substrates has been studied by x-ray diffraction, transmission electron microscopy (TEM), and atomic force microscopy. Films, 15–80 μm thick, were grown on c-plane sapphire that were either pretreated with GaCl or contained a ZnO sputter deposited layer. The defect density, for both types of films, was found by plan view TEM to range between mid-107 to mid-108 dislocations/cm2 despite very different structural defects at the film/substrate interface. Nanovoids were found; however, no cracks were observed in the films that were investigated by TEM.
Annealing stability and device application of nonalloyed ohmic contacts using a low temperature grown GaAs cap on thin n+ GaAs layers
Authors: Ueng H. J. , Kolagunta V. R. , Janes D. B. , Webb K. J. , McInturff D. T. , Melloch M. R.
Published: July 2002

This letter summarizes a study of nonalloyed ohmic contact structures consisting of Au/Ti metallization deposited on a thin (3.5 nm) layer of low-temperature-grown GaAs (LTG:GaAs) on a thin (10 nm) layer of heavily doped n-type GaAs. We demonstrate that this Au/Ti:LTG:GaAs/n+GaAs contact structure can be used to make effective contacts to thin n+ layers, that the resulting contact survives annealing at temperatures between 300 °C and 400 °C, and that the contact resistivity, ρc, is reasonably stable for these anneals. This is contrasted with conventional Au/Ge/Ni alloyed contacts. The contact structure has also been applied to a resonant tunneling diode (RTD). The characteristic current-voltage curves of RTDs show that the performance of the intrinsic barrier/well/barrier region of the device is not degraded after anneal.
Fast bistable nematic display with grey scale
Authors: Barberi R. , Giocondo M. , Li J. , Bartolino R. , Dozov I. , Durand G.
Published: July 2002

We present a novel principle for a fast bistable nematic display with intrinsic grey scale. The geometry of a single pixel is the usual hybrid texture in between two conductive flat plates. The device is written by creating surface walls when an electric field is applied, above the thresholds to achieve both the planar anchoring breaking and an electro-hydrodynamic flow. Erasing is obtained by simply breaking the anchoring in absence of vortices. The distorted regions around surface walls depolarize the incident light. As the surface defect density can be modulated, a grey scale is achieved.
The interstitial fraction of diffusivity of common dopants in Si
Authors: Gossmann H.-J. , Haynes T. E. , Stolk P. A. , Jacobson D. C. , Gilmer G. H. , Poate J. M. , Luftman H. S. , Mogi T. K. , Thompson M. O.
Published: July 2002

The relative contributions of interstitials and vacancies to diffusion of a dopant A in silicon are specified by the interstitial fraction of diffusivity, fA. Accurate knowledge of fA is required for predictive simulations of Si processing during which the point defect population is perturbed, such as transient enhanced diffusion. While experimental determination of fA is traditionally based on an underdetermined system of equations, we show here that it is actually possible to derive expressions that give meaningful bounds on fA without any further assumptions but that of local equilibrium. By employing a pair of dopants under the same point-defect perturbance, and by utilizing perturbances very far from equilibrium, we obtain experimentally fSb⩽0.012 and fB⩾0.98 at temperatures of ∼800 °C, which are the strictest bounds reported to date. Our results are in agreement with a theoretical expectation that a substitutional dopant in Si should either be a pure vacancy, or a pure interstitial(cy) diffuser.
Evidence for atomic H insertion into strained Si–Si bonds in the amorphous hydrogenated silicon subsurface from in situ infrared spectroscopy
Authors: von Keudell A. , Abelson J. R.
Published: July 2002

The reaction of atomic hydrogen with a growing amorphous hydrogenated silicon film plays a crucial role in determining the final material properties: hydrogen saturates dangling bonds in the film and thereby lowers the defect density, and it is assumed that hydrogen is inserted into strained bonds of the silicon network thereby reducing the local disorder. The latter reaction can be inferred indirectly since the total hydrogen uptake exceeds the decrease in the defect density. This letter presents the first direct experimental evidence from in situ infrared spectroscopy for the insertion of hydrogen into strained Si–Si bonds. This reaction becomes visible during the addition of atomic hydrogen to an as-grown film by the appearance of a characteristic SiH vibrational mode at 2033 cm−1, indicating a different embedding Si matrix compared to the standard SiH bulk vibration at 2000 cm−1.
High energy products in rapidly annealed nanoscale Fe/Pt multilayers
Authors: Liu J. P. , Luo C. P. , Liu Y. , Sellmyer D. J.
Published: July 2002

Magnetic properties of nanocomposite Fe–Pt films with Fe concentration higher than 50 at % have been investigated in this study. Fe/Pt multilayers were produced by sputtering and magnetic hardening was observed after heat treatment including rapid annealing. The final nanocomposite films consisted of the hard face-centered tetragonal FePt phase and a soft face-centered-cubic phase. The maximum energy products of the optimally processed samples exceeded 40 MGOe. Evidence for exchange coupling of the hard and soft phases was found.
Analytic expression for triple-point electron emission from an ideal edge
Authors: Schächter Levi
Published: July 2002

The electric field in the vicinity of a metallic edge attached to a dielectric half-space is calculated analytically. The resulting electric field is used to evaluate the current emitted from the edge using the Fowler-Nordheim formula. It is shown analytically that the emitted current is proportional to the dielectric coefficient of the material.
Study of Cu gettering to cavities in separation by implantation of oxygen substrates
Authors: Zhang Miao , Lin Chenglu , Hemment P. L. F. , Gutjahr K. , Gösele U.
Published: July 2002

Gettering of Cu impurities to cavities induced in separation by implantation of oxygen (SIMOX) substrates has been investigated. The cavities were introduced beneath the buried oxide layer (BOX) of SIMOX by H+ implantation and subsequently annealing. 5×1013/cm2 or 5×1015/cm2 of Cu impurities were implanted in the top Si layer. The results indicate that the BOX layer does not appear to prevent the movement of Cu at temperatures higher than 700 °C. Profiles of Cu indicate that 92% of the initial 5×1013/cm2 Cu has diffused through the buried oxide layer and been captured by the cavities, with 1% of Cu left in the top Si layer after a 1000 °C annealing, and 73.6% of the 5×1015/cm2 Cu is gettered to the cavities with 13% of Cu in the top Si layer. The gettering effect of cavities is stronger than the damage around the BOX. H+ implantation-induced cavities have been demonstrated to be an effective method to getter Cu impurities away from the top Si layer in SIMOX substrates.
Spin relaxation of excitons in zero-dimensional InGaAs quantum disks
Authors: Gotoh H. , Ando H. , Kamada H. , Chavez-Pirson A. , Temmyo J.
Published: July 2002

We report the observation of spin relaxation of excitons in zero-dimensional semiconductor nanostructures. The spin relaxation is measured in InGaAs quantum disks by using a polarization dependent time-resolved photoluminescence method. The spin relaxation time in a zero-dimensional quantum disk is as long as 0.9 ns at 4 K, which is almost twice as long as the radiative recombination lifetime and is considerably longer than that in quantum wells. The temperature dependence of the spin relaxation time suggests the importance of exciton–acoustic phonon interaction.
Enhanced nonlinear optical response of an endohedral metallofullerene through metal-to-cage charge transfer
Authors: Heflin J. R. , Marciu D. , Figura C. , Wang S. , Burbank P. , Stevenson S. , Dorn H. C.
Published: July 2002

A new mechanism for increasing the third-order nonlinear optical susceptibility, χ(3), is described for endohedral metallofullerenes. A two to three orders of magnitude increase in the nonlinear response is reported for degenerate four-wave mixing experiments conducted with solutions of Er2@C82 (isomer III) relative to empty-cage fullerenes. A value of −8.7×10−32 esu is found for the molecular susceptibility, γxyyx, of Er2@C82 compared to previously reported values of γxxxx=3×10−34 esu and γxyyx=4×10−35 esu for C60. The results confirm the importance of the metal-to-cage charge-transfer mechanism for enhancing the nonlinear optical response in endohedral metallofullerenes.
Extension of Tsu-Esaki model for effective mass effects in resonant tunneling
Authors: Schulman J. N.
Published: July 2002

The Tsu-Esaki model for resonant tunneling is extended to include the effects of differing in-plane masses in the emitter and well. The improvement produces features in the current versus voltage characteristic previously thought to require a much higher level of theoretical sophistication. The interband tunneling case can for the first time also be included in this simple and convenient framework. Calculations demonstrate that the first order contribution to the magnitude of the negative resistance is due to the mass difference, even when the same semiconductor is used in the emitter and well. A simple analytical SPICE-compatible I(V) formula is also derived.
Eyesafe pulsed microchip laser using semiconductor saturable absorber mirrors
Authors: Fluck R. , Häring R. , Paschotta R. , Gini E. , Melchior H. , Keller U.
Published: July 2002

We passively Q-switched a diode-pumped Er/Yb:glass microchip laser at a 1.535 μm wavelength using semiconductor saturable absorber mirrors and demonstrated pulses as short as 1.2 ns. By varying the design parameters of the saturable absorber, the pump power, and the pump spot size, we achieved repetition rates from 300 Hz to 100 kHz with pulse energies up to 4 μJ.
Recombination lifetime of In0.53Ga0.47As as a function of doping density
Authors: Ahrenkiel R. K. , Ellingson R. , Johnston S. , Wanlass M.
Published: July 2002

We have fabricated devices with the structure InP/In0.53Ga0.47As/InP, with a InGaAs doping range varying from 2×1014 to 2×1019 cm−3. These isotype double heterostructures were doped both n and p type and were used to measure the minority-carrier lifetime of InGaAs over this doping range. At the low doping end of the series, recombination is dominated by the Shockley–Read–Hall effect. At the intermediate doping levels, radiative recombination is dominant. At the highest doping levels, Auger recombination dominates as the lifetime varies with the inverse square of the doping concentration. From fitting these data, the radiative- and Auger-recombination coefficients are deduced.
Second harmonic generation in laser ablated zinc oxide thin films
Authors: Cao H. , Wu J. Y. , Ong H. C. , Dai J. Y. , Chang R. P. H.
Published: July 2002

We have observed large second-order nonlinear optical response from zinc oxide (ZnO) thin films deposited on sapphire substrates by pulsed laser ablation. By comparing the second harmonic signal generated in a series of ZnO films with different crystallinity and thickness, we conclude that a significant part of the second harmonic signal is generated at the grain boundaries and interfaces.
Combined millimeter-wave near-field microscope and capacitance distance control for the quantitative mapping of sheet resistance of conducting layers
Authors: Lann A. F. , Golosovsky M. , Davidov D. , Frenkel A.
Published: July 2002

We present a dual-frequency electromagnetic scanning probe and apply it for quantitative mapping of the sheet resistance of conducting films. The high-frequency (82 GHz) mode is used for image acquisition, while the low-frequency (5 MHz) mode is used for distance control. We measure magnitude and phase of the near-field microwave reflectivity from conducting films of varying thickness and develop a model which accounts fairly well for our results. This brings us to a quantitative understanding of the contrast in the microwave near-field imaging using an aperture probe, and allows us to achieve quantitative contactless characterization of conducting layers with sheet resistance even below 2 Ω.
Local dynamics in polymer-dispersed liquid crystals studied by near-field scanning optical microscopy
Authors: Mei Erwen , Higgins Daniel A.
Published: July 2002

Electric-field-induced molecular reorientation dynamics are studied in localized regions within polymer-dispersed liquid-crystal (PDLC) films, using time-resolved near-field scanning optical microscopy (NSOM) methods. A concentrated electric field is applied across the PDLC film, between the metallized NSOM probe and the conductive, transparent substrate supporting the sample. The electric field is modulated as a square wave form at (sub)-kilohertz frequencies. Subsequent liquid-crystal (LC) reorientation is observed by crossed-polarized, transmitted-light NSOM methods. The results show dramatic spatial variations in the time scale over which the molecules reorient. The variations are interpreted based on proximity of the region probed to the polymer–LC interface.
Ultimate quantum efficiency of a superconducting hot-electron photodetector
Authors: Il’in K. S. , Milostnaya I. I. , Verevkin A. A. , Gol’tsman G. N. , Gershenzon E. M. , Sobolewski Roman
Published: July 2002

The quantum efficiency and current and voltage responsivities of fast hot-electron photodetectors, fabricated from superconducting NbN thin films and biased in the resistive state, have been shown to reach values of 340, 220 A/W, and 4×104 V/W, respectively, for infrared radiation with a wavelength of 0.79 μm. The characteristics of the photodetectors are presented within the general model, based on relaxation processes in the nonequilibrium electron heating of a superconducting thin film. The observed, very high efficiency and sensitivity of the superconductor absorbing the photon are explained by the high multiplication rate of quasiparticles during the avalanche breaking of Cooper pairs.
Electron emission spectra from lead zirconate titanate ferroelectric films on stainless-steel substrates
Authors: Sviridov E. , Le Bihan R. , Liateni S. F. , Désécures A.
Published: July 2002

Emission of electrons from lead zirconate titanate films was measured using excitation voltage pulses of different waveforms. Time-resolved measurements of the emission, voltage, and circuit current were conducted simultaneously. Electron emission spectra excited by a sine-wave voltage exhibited two distinguishable peaks arising every second half period of the applied voltage. Electron emission was also induced by film excitation with either square-wave voltage below coercive field, or unipolar pulses of appropriate polarity. The peculiarities of the emission are considered in terms of reversal of both induced (dielectric) and spontaneous (ferroelectric) polarization. Excitation of the electron emission in the “inversion of induced charge” mode seems to eliminate problems associated with reversal of spontaneous polarization as fatigue and necessity of resetting.
Determination of built-in field by applying fast Fourier transform to the photoreflectance of surface-intrinsic n+-type doped GaAs
Authors: Wang D. P. , Wang K. R. , Huang K. F. , Huang T. C. , Chu A. K.
Published: July 2002

Photoreflectance spectroscopy of surface-intrinsic n+-doped (s-i-n+) GaAs has been measured at various power densities (Ppu) of a pump beam. Many Franz–Keldysh oscillations (FKOs) were observed above the band-gap energy, which will enable the electric-field strength (F) to be determined from the periods of the FKOs. Field F thus obtained is subject to photovoltaic effects. In order to reduce the photovoltaic effects from the pump beam, Ppu was kept below 10 μW/cm2 in the previous experiments. Here, we demonstrate that the built-in field can be determined at a larger Ppu by using fast Fourier transform techniques.
Permanent photoreduction of Sm3+ to Sm2+ inside a sodium aluminoborate glass by an infrared femtosecond pulsed laser
Authors: Qiu Jianrong , Miura K. , Suzuki T. , Mitsuyu T. , Hirao K.
Published: July 2002

We report the observation of permanent photoreduction of Sm3+ to Sm2+ inside a transparent and colorless Sm3+-doped sodium aluminoborate glass. After irradiation by an 800 nm focused femtosecond pulsed laser, the focused part of the laser in the glass became orange. Absorption and photoluminescence spectra showed that a part of Sm3+ was reduced to Sm2+ after the laser irradiation. Electron spin resonance spectra of the glass before and after the laser irradiation were also measured. The observed phenomenon is inferred to be useful for the fabrication of optical memory devices with an ultrahigh storage density.
Distributed feedback ridge waveguide lasers fabricated by nanoscale printing and molding on nonplanar substrates
Authors: Rogers John A. , Meier Martin , Dodabalapur Ananth , Laskowski Edward J. , Cappuzzo Mark A.
Published: July 2002

This letter describes the use of printing and molding techniques to fabricate plastic photopumped laser that use distributed feedback resonators on ridge waveguides. The results indicate (i) potential optoelectronic applications of a set of emerging low cost lithographic techniques that can pattern a range of materials with submicron resolution on nonplanar substrates, (ii) a new type of plastic laser based on molded organic gain materials, and (iii) that thresholds of plastic distributed feedback ridge waveguide lasers are similar to their planar counterparts.
The effect of annealing on the microwave properties of Ba0.5Sr0.5TiO3 thin films
Authors: Chang Wontae , Horwitz James S. , Carter Adriaan C. , Pond Jeffrey M. , Kirchoefer Steven W. , Gilmore Charles M. , Chrisey Douglas B.
Published: July 2002

Oriented, single phase thin films (∼5000 Å thick) of Ba0.5Sr0.5TiO3 (BST) have been deposited onto (100) MgO and (100) LaAlO3 (LAO) substrates using pulsed laser deposition. The capacitance and dielectric Q (1/tan δ) of as-deposited and annealed films have been measured from 1 to 20 GHz as a function of electric field (0–80 kV/cm) at room temperature using interdigitated Ag electrodes deposited on top of the film. For films deposited onto MgO, it is observed that, after a postdeposition anneal (1000–1200 °C), the dielectric constant decreases and the dielectric Q increases. For films deposited onto LAO, a postdeposition anneal (⩽ 1000 °C) resulted in a significant increase in the dielectric constant and a decrease in Q. The observed dielectric properties of the BST films are attributed to the changes in film stress, which affects the extent of ionic polarization.
Direct observation of traps responsible for positive space charge in alternating-current thin-film electroluminescent devices
Authors: Krasnov Alex N.
Published: July 2002

Space charge has significant influence on the operation of alternating-current thin-film electroluminescent displays. This letter introduces a relatively simple method for direct observation of the distribution of traps responsible for the space charge in the active layer of the devices. Contribution of native and dopant defects in the space charge formation is discussed.
Step-edge magnetoresistance in La0.7Ca0.3MnO3 films
Authors: Ziese M. , Heydon G. , Höhne R. , Esquinazi P. , Dienelt J.
Published: July 2002

The magnetoresistance of step-edge structures in La0.7Ca0.3MnO3 films was investigated. Step-edge arrays with 200 steps of height 140–200 nm and step separation 20 μm along [110] were fabricated on LaAlO3 substrates by chemically assisted ion-beam etching. Thin La0.7Ca0.3MnO3 films were deposited on the structured substrates by pulsed-laser deposition. Measurements of the large low-field magnetoresistance, the dynamic conductance, and the anisotropic magnetoresistance lead to the proposal of a model of spin-polarized tunneling in a ferromagnet/spin-glass/ferromagnet geometry.
Direct view of stress distribution in solid by mechanoluminescence
Authors: Xu Chao-Nan , Watanabe Tadahiko , Akiyama Morito , Zheng Xu-Guang
Published: July 2002

Visualization of stress distribution has been realized by a nondestructive mechanoluminescence (ML) from SrAl2O4:Eu, which can emit three magnitudes higher visible light than that of well-known ML substance of quartz. A simulation result confirms that such a ML image successfully reflects the stress distribution. A kinetic model for ML of SrAl2O4:Eu is proposed.
Single-mode, single-lobe operation of surface-emitting, second-order distributed feedback lasers
Authors: Lopez J. , Witjaksono G. , Botez D.
Published: July 2002

A thin p-cladding InGaAs/InGaP/GaAs laser structure (λ=0.977 μm) with a second-order Au/air grating (70% duty cycle) and asymmetrically coated (30%, 5% reflectivity) cleaved facets emits in a diffraction-limited (0.11°) single lobe in a direction virtually normal to the chip surface. The near-field pattern corresponds to (grating) phase shifts of 10° and 40° at the low- and high-reflectivity cleaved-mirror facets. An analysis of 30%/0% coated, 500-μm-long devices shows that single-lobe surface emission occurs for a wide variation in grating phase with respect to the high reflectivity mirror, ΔφHR: 10°–80°. For 99%/0% coated devices, single-lobe emission occurs with relatively uniform near-field intensity profile and external differential quantum efficiency ηd around 20% for ΔφHR values close to π/4 (i.e., in the 45°–65° range). Single-lobe emission normal to the chip surface (i.e., symmetric-mode lasing) can then be obtained from devices without cleaved mirrors by introducing a phase shift close to π/2 in the center of the second-order grating. For the structure used, a phase shift of 90°–130° in the center of 1-mm-long gratings is found to provide single-lobe surface emission with substantially uniform near-field profile, and ηd values as high as 22%. Increasing the grating-section length increases ηd (e.g., 35% for 1.5-mm-long gratings) at some price in near-field uniformity.
Electro-optic recovery of the photoquenching effect
Authors: Rubinger R. M. , de Oliveira A.G. , Ribeiro G. M. , Bezzera J. C. , Moreira M. V. B. , Chacham H.
Published: July 2002

We have carried out dynamic low-temperature measurements of the current density for a semi-insulating GaAs sample grown at 300 °C. The standard photoquenching (PQ) effect was observed at low temperatures. We found that high electric fields inhibit the PQ effect and that the recovery of the PQ effect was made possible by simultaneous application of a high electric field and illuminating the sample with infrared light.
Size effect on the superconducting transition of embedded lead particles in an Al–Cu–V amorphous matrix
Authors: Tsai A. P. , Chandrasekhar N. , Chattopadhyay K.
Published: July 2002

We have synthesized specimens of nanometric lead dispersion in a glassy Al–Cu–V matrix by rapid solidification of the corresponding melt. The microstructure has been designed to avoid superconducting percolation due to coupling of the neighboring particles by the proximity effect. Using these specimens, we have determined quantitatively the effect of size of the ultrafine lead particles on the superconducting transition.
Attachment-induced ionization instability of a radio frequency excited discharge in oxygen
Authors: Katsch H.-M. , Goehlich A. , Kawetzki T. , Quandt E. , Döbele H.-F.
Published: July 2002

The plasma of a 13.56 MHz capacitively coupled oxygen discharge of the Gaseous Electronics Conference Reference Cell type exhibits fluctuations of the plasma potential and of light emission in the kilohertz range. This behavior can be explained in terms of an attachment-induced ionization instability in connection with a sufficiently high fraction of negative ions. A necessary condition for the appearance of this type of instability is a strong positive dependence on electron temperature of the electron attachment coefficient describing the formation of negative oxygen ions. A strong volume loss of negative ions by neutral particles—metastable molecules in the present case—is an additional condition. Calculations of the temperature dependent coefficients for ionization and attachment by solving the time-independent Boltzmann equation in the two-term approximation in combination with the fluid equations of the charge carriers show that the criteria for the development of an attachment-induced ionization instability are fulfilled in the pressure domain around 30 Pa and for medium radio frequency voltages around 300 V.
Ultralow-dark-current wafer-bonded Si/InGaAs photodetectors
Authors: Levine B. F. , Pinzone C. J. , Hui S. , King C. A. , Leibenguth R. E. , Zolnowski D. R. , Lang D. V. , Krautter H. W. , Geva M.
Published: July 2002

A method of wafer bonding is demonstrated which significantly reduces the thermal expansion mismatch stress, by removing the substrate before the high temperature anneal, thereby allowing elastic accommodation of the thin device layers. Record low-dark-current Si/InGaAs pin detectors have been realized.
High rate etching of SiC using inductively coupled plasma reactive ion etching in SF6-based gas mixtures
Authors: Khan F. A. , Adesida I.
Published: July 2002

Inductively coupled plasma (ICP) reactive ion etching of SiC was investigated using SF6 plasmas. Etch rates were studied as a function of substrate bias voltage (−3 to −500 V), ICP coil power (500–900 W), and chamber pressure (1–6 mT). The highest etch rate (970 nm/min) for SiC yet reported was achieved. Anisotropic etch profiles with highly smooth surfaces free of micromasking effects were obtained. The addition of O2 to the SF6 plasma was found to slightly increase the etch rate.
Ion acceleration in the radially expanding plasma of the hot refractory anode vacuum arc
Authors: Beilis I. I. , Boxman R. L. , Goldsmith S. , Paperny V. L.
Published: July 2002

The ion energy and plasma potential as a function of radial distance were measured in a vacuum arc mode—the hot refractory anode vacuum arc. A 175 A arc was sustained between a 30-mm-diam water-cooled Cu cathode and a thermally isolated graphite anode. The plasma potential and electron temperature were determined using a triple probe, and the ion energy distribution was measured with a retarding field analyzer. It was found that the electron temperature decreased from 1.2 to 0.6 eV with increasing radial distance from 3 to 18 cm. In the same distance range, the directed ion energy per unit charge increased from 8 to 20 eV. The 12 eV ion energy enhancement significantly exceeded the measured drop of the plasma potential of about 2 V, indicating that the ions were not accelerated electrostatically.
Structural and magnetic properties of FePt:SiO2 granular thin films
Authors: Luo C. P. , Sellmyer D. J.
Published: July 2002

Nanocomposite FePt:SiO2 films have been fabricated by annealing the as-deposited FePt/SiO2 multilayers at temperatures from 450 to 650 °C. These films consist of high-anisotropy tetragonal L10 FePt particles embedded in a SiO2 matrix. The structural and magnetic properties of these films were investigated. We have found that coercivity and grain size are highly dependent on the annealing temperature and SiO2 concentration. Films with coercivities in the range from 2 to 8 kOe and grain sizes of 10 nm or less were obtained. These films have considerable potential as high-density magnetic recording media.
1540 nm light emission from Er-doped amorphous GaAsN films
Authors: Zanatta A. R.
Published: July 2002

Erbium-doped amorphous gallium–arsenic–nitrogen (a-GaAsN) films have been prepared by cosputtering from a crystalline GaAs wafer partially covered with metallic Er pieces. The films were deposited at room temperature under different partial pressures of Ar and N2. After deposition, the films were characterized by optical transmission in the visible-ultraviolet energy range, photoluminescence (PL) in the infrared region, and Raman scattering spectroscopy. Compositional measurements were also performed indicating an Er content of ∼0.5 at. % and a N concentration that scales with the N2 partial pressure during deposition. According to the experimental results, to higher N contents correspond larger optical band gaps and more intense Er3+-related PL signals. This dependence is analyzed in terms of the compositional, electronic, and structural characteristics of each film.
Analysis of the stress-induced leakage current and related trap distribution
Authors: Riess P. , Ghibaudo G. , Pananakakis G.
Published: July 2002

The amplitudes of stress-induced leakage currents (SILC) generated for a constant injected dose are measured for oxide thicknesses between 3.5 and 9 nm. Then, the doses necessary to generate the same amplitude of SILC for all oxide thicknesses at 6 MV/cm are measured. The analysis of these results, considering that the SILC is due to a trap-assisted tunneling mechanism, demonstrates that a uniform trap distribution throughout the oxide cannot explain the thickness dependence of the SILC, indicating that the trap distribution is an important feature in the understanding of the oxide thickness dependence of the SILC.
First-principles study of stability, band structure, and optical properties of the ordered Ge0.50Sn0.50 alloy
Authors: Pandey Ravindra , Rérat Michel , Causà Mauro
Published: July 2002

All electron density functional theory calculations were performed on the ordered Ge0.50Sn0.50 alloy in the zinc-blende phase to study its structural, electronic, and optical properties along with its stability with respect to the elemental components. We employed a linear combination of atomic orbitals approach for calculations in which the Bloch functions were constructed as linear combinations of atom-centered Gaussian orbitals. The calculated results show a relative stability of the GeSn alloy in the zinc-blende phase for which the lattice constant and bulk modulus are predicted to be 6.20 Å and 53 GPa, respectively. Analysis of band structure and density of states shows the cubic-ordered alloy to be a direct-gap semiconductor with a small band gap. The optical properties, such as dielectric constant and plasmon energy of the cubic alloy, appear to be about the average of the respective values in its elemental components.
The generation and detection of high flux atomic oxygen for physical vapor deposition thin film growth
Authors: Ingle N. J. C. , Hammond R. H. , Beasley M. R. , Blank D. H. A.
Published: July 2002

The growth of many epitaxial thin-film oxides is significantly enhanced with the use of an oxidizing agent such as atomic oxygen, ozone, or NO2. We developed a flow-through microwave plasma source to generate large atomic oxygen fluxes while maintaining vacuum pressures of less that 1×10−4 Torr. Continuous and real-time detection of the atomic oxygen was achieved by atomic absorption of the 130 nm atomic oxygen lines. Atomic oxygen fluxes of at least 1.4×1018 atoms/cm2 s and dissociation efficiencies of around 100% were obtained.
Calculated trends of the magnetostriction coefficient of 3d alloys from first principles
Authors: James Peter , Eriksson Olle , Hjortstam Olof , Johansson Börje , Nordström Lars
Published: July 2002

The general behavior of the magnetostriction coefficients for Fe, Co, and Ni and their alloys is reproduced by first-principles theory. It is shown that the low magnetostriction coefficient of soft magnetic materials, observed for specific alloy concentrations, such as permalloy, can be explained theoretically by employing relativistic first-principles calculations in combination with the rigid-band approximation. The connection between magnetostriction and magnetocrystalline anisotropy is discussed as well as the physical mechanisms behind soft magnetic materials. Theory is hence demonstrated to have the ability of predicting values of the magnetostriction coefficient of any alloy, and a microscopic understanding of this entity is starting to emerge.
Magnetic-field-enhanced quantum-cascade emission
Authors: Ulrich J. , Zobl R. , Unterrainer K. , Strasser G. , Gornik E.
Published: July 2002

We have observed an enhancement of terahertz intersubband electroluminescence in a quantum cascade structure in the presence of a magnetic field applied normal to the epitaxial layers. At a field of B=7.2 T the emission efficiency doubles. This effect is attributed to the suppression of nonradiative Auger–intersubband transitions caused by Landau-quantization of the in-plane electron motion. The magnetic field dependence of the luminescence intensity shows strong oscillations. These magnetointersubband oscillations are caused by the modulation of the transition rate via resonant inter-Landau-level transfer.
Ferromagnetic resonance detection with a torsion-mode atomic-force microscope
Authors: Löhndorf Markus , Moreland John , Kabos Pavel
Published: July 2002

We have developed a ferromagnetic resonance (FMR) instrument based on a torsion-mode atomic-force microscope (AFM). The instrument measures the torque on a magnetized thin film in a static out-of-plane field perpendicular to the film surface. The magnetic film is deposited onto an AFM microcantilever. FMR measurements are performed at a fixed microwave frequency of 9.15 GHz with a sweeping in-plane field. At the FMR condition, the change in the average in-plane magnetization of the film is at a maximum corresponding to a maximum change in the torque on the AFM cantilever. Our instrument is capable of measuring fluctuations of in-plane magnetization of 63.3 A/m of NiFe film samples with a total volume of 1.1×10−10 cm3. Given a signal-to-noise ratio of 40, we estimate a magnetic moment sensitivity of 1.7×10−16 A/m2.
Spin-dependent capacitance of silicon field-effect transistors
Authors: Brandt M. S. , Neuberger R. T. , Stutzmann M.
Published: July 2002

Under electron spin resonance conditions, changes of the capacitance of vertical field-effect transistors are observed, due to spin-dependent trapping of charge carriers by defects at the interface between the substrate and the channel region. The spectra obtained by capacitively detected magnetic resonance show the presence of two different defects, tentatively assigned to defects introduced by processing and complexes involving transition-metal impurities. Using a quantitative model, the number of defects resonantly charged by this trapping is estimated. It is shown that the possible cross talk of spin-dependent changes of the conductivity in the substrate is, in fact, suppressed by the large impedance of the space-charge layer.
Hole and electron emission from InAs quantum dots
Authors: Kapteyn C. M. A. , Lion M. , Heitz R. , Bimberg D. , Brunkov P. N. , Volovik B. V. , Konnikov S. G. , Kovsh A. R. , Ustinov V. M.
Published: July 2002

Carrier escape processes from self-organized InAs quantum dots QDs embedded in GaAs are investigated by time-resolved capacitance spectroscopy. Electron emission is found to be dominated by tunneling processes. In addition to tunneling from the ground state, we find thermally activated tunneling involving excited QD states with an activation energy of 82 meV. For holes, the tunnel contribution is negligible and thermal activation from the QD ground state to the GaAs valence band with an activation energy of 164 meV dominates. Extrapolation to room temperature yields an emission time constant of 5 ps for holes, which is an order of magnitude larger than for electrons. The measured activation energies agree well with theoretically predicted QD levels.