bluuebunny/crossref_metadata_2025 huggingface dataset:
Fracture toughness and fatigue-crack propagation in a Zr–Ti–Ni–Cu–Be bulk metallic glass
Authors: Gilbert C. J. , Ritchie R. O. , Johnson W. L.
Published: July 2002

The recent development of metallic alloy systems which can be processed with an amorphous structure over large dimensions, specifically to form metallic glasses at low cooling rates (∼10 K/s), has permitted novel measurements of important mechanical properties. These include, for example, fatigue-crack growth and fracture toughness behavior, representing the conditions governing the subcritical and critical propagation of cracks in these structures. In the present study, bulk plates of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy, machined into 7 mm wide, 38 mm thick compact-tension specimens and fatigue precracked following standard procedures, revealed fracture toughnesses in the fully amorphous structure of KIc∼55 MPa√m, i.e., comparable with that of a high-strength steel or aluminum alloy. However, partial and full crystallization, e.g., following thermal exposure at 633 K or more, was found to result in a drastic reduction in fracture toughness to ∼1 MPa√m, i.e., comparable with silica glass. The fully amorphous alloy was also found to be susceptible to fatigue-crack growth under cyclic loading, with growth-rate properties comparable to that of ductile crystalline metallic alloys, such as high-strength steels or aluminum alloys; no such fatigue was seen in the partially or fully crystallized alloys which behaved like very brittle ceramics. Possible micromechanical mechanisms for such behavior are discussed.
Organic–inorganic dielectric multilayer systems as high reflectivity distributed Bragg reflectors
Authors: Convertino A. , Valentini A. , Ligonzo T. , Cingolani R.
Published: July 2002

Unprecedentedly high reflectivity distributed Bragg reflectors consisting of fluorocarbon polymer (CFx) and high refractive index inorganic oxide multilayers have been fabricated by means of assisted ion-beam sputtering at room temperature. Multilayer stacks consisting of a CFx/TiOx and CFx/HfOx pairs exhibit reflectivities larger than 98% in the infrared and ultraviolet spectral region, respectively. The superior wide tunability and the high efficiency, with respect to inorganic–inorganic multilayers is due to the exceptionally low refractive index of the fluorcarbon polymer (n≃1.35) on a very large spectral region (300–2000 nm).
Pressure-dependent photoluminescence study of InxGa1−xN
Authors: Shan W. , Song J. J. , Feng Z. C. , Schurman M. , Stall R. A.
Published: July 2002

We present the results of pressure-dependent photoluminescence (PL) studies of single-crystal InxGa1−xN (0⩽x<0.15) films grown on top of thick GaN epitaxial layers by metalorganic chemical vapor deposition with sapphire as substrates. PL measurements were performed at 10 K as a function of applied hydrostatic pressure using the diamond-anvil-cell technique. The luminescence emissions from the InxGa1−xN epifilms were found to shift linearly toward higher energy with increasing pressure. By examining the pressure dependence of the PL spectra, the pressure coefficients for the emission structures associated with the direct band gap of InxGa1−xN were determined. The values of the pressure coefficients were found to be 3.9×10−3 eV/kbar for In0.08Ga0.92N and 3.5×10−3 eV/kbar for In0.14Ga0.86N.
The elimination of surface cross-hatch from relaxed, limited-area Si1−xGex buffer layers
Authors: Hammond R. , Phillips P. J. , Whall T. E. , Parker E. H. C. , Graf T. , Von Känel H. , Shields A. J.
Published: July 2002

The influence of lateral dimensions on the relaxation and surface topography of linearly graded Si1−xGex buffer layers has been investigated. A dramatic change in the relaxation mechanism has been observed for depositions on Si mesa pillars of lateral dimensions 10 μm and below. Misfit dislocations are able to extend unhindered and terminate at the edges of the growth zone, yielding a surface free of cross-hatch. For lateral dimensions in excess of 10 μm orthogonal misfit interactions occur and relaxation is dominated by the modified Frank–Read (MFR) mechanism. The stress fields associated with the MFR dislocation pile-ups result in a pronounced cross-hatch topography.
Organic electroluminescence cells based on thin films deposited by ultraviolet laser ablation
Authors: Matsumoto N. , Shima H. , Fujii T. , Kannari F.
Published: July 2002

Organic thin films of copper-phthalocyanine (CuPc) and aluminum tris-8-hydroxyquinline (Alq3) were fabricated via KrF laser ablation. In spite of poor crystallinity exhibited in the reconstructed organic films, the CuPc film still works as a hole-transport layer in electroluminescence cells based on the CuPc and Alq3 layers. Electroluminescence at ∼500 nm was obtained with a low initiation voltage of ∼12 V. There is no significant difference in electroluminescence performance for devices fabricated via laser ablation and thermal evaporation. Emission at ∼600 nm caused by energy transfer from Alq3 to 4-(Dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) is also observed from a DCM-doped Alq3 emission layer.
Remote electron beam induced current imaging of electrically active regions in YBa2Cu3O7−x single crystals
Authors: Dı́az-Guerra C. , Piqueras J.
Published: July 2002

Remote electron beam induced current (REBIC) measurements have been carried out to investigate electrically active regions in YBa2Cu3O7−x single crystals. Enhanced REBIC contrast, found in growth steps and other topographic features of the samples, is discussed in terms of charged oxygen-related defects. The capability of REBIC to image structural inhomogeneities caused by strain or plastic deformation in these crystals is also established. Charge carrier diffusion length has been estimated at different temperatures from REBIC linescan profiles.
Direct measurement of strain effects on magnetic and electrical properties of epitaxial SrRuO3 thin films
Authors: Gan Q. , Rao R. A. , Eom C. B. , Garrett J. L. , Lee Mark
Published: July 2002

By lifting an epitaxial thin film off its growth substrate, we directly and quantitatively demonstrate how elastic strain can alter the magnetic and electrical properties of single-domain epitaxial SrRuO3 thin films (1000 Å thick) on vicinal (001) SrTiO3 substrates. Free-standing films were then obtained by selective chemical etching of the SrTiO3. X-ray diffraction analysis shows that the free-standing films are strain free, whereas the original as-grown films on SrTiO3 substrates are strained due to the lattice mismatch at the growth interface. Relaxation of the lattice strain resulted in a 10 K increase in the Curie temperature to 160 K, and a 20% increase in the saturation magnetic moment to 1.45 μB/Ru atom. Both values for the free-standing films are the same as that of the bulk single crystals. Our results provide direct evidence of the crucial role of the strain effect in determining the properties of the technologically important perovskite epitaxial thin films.
6.1 W continuous wave front-facet power from Al-free active-region (λ=805 nm) diode lasers
Authors: Wade J. K. , Mawst L. J. , Botez D. , Nabiev R. F. , Jansen M. , Morris J. A.
Published: July 2002

Al-free active-region diode lasers grown by low-pressure, metal-organic chemical vapor deposition and emitting at λ=805 nm have been optimized for high continuous wave output power. The 1-mm-long devices consisting of an InGaAsP/In0.5Ga0.5P/In0.5(Ga0.5Al0.5)0.5P laser structure have a threshold-current density, Jth, of 310 A/cm2 and relatively high values for the characteristic temperatures of the threshold current, T0 (135 K), and differential quantum efficiency, T1 (900 K). Lasers with 10%/90% coatings and a 100-μm-wide stripe provide a maximum cw output power of 6.1 W at a heatsink temperature of 10 °C. The devices fail due to catastrophic optical mirror damage (COMD), where the internal power density, P̄COMD, is 17.4 MW/cm2; that is, twice that for conventionally facet-coated, 810 nm emitting, AlGaAs active-region diode lasers.
Multiple-junction single-electron transistors for digital applications
Authors: Chen R. H. , Likharev K. K.
Published: July 2002

The concept of the capacitively coupled single-electron transistor (CSET) is generalized to a device based on a linear array of N tunnel junctions. The basic characteristics of such multiple-junction CSETs are calculated for several distributions of tunnel junction and coupling capacitances. The results indicate that for optimized parameters, the operating temperature and parameter tolerances increase appreciably with N, with the most striking gains for N≲5. For example, a five-junction transistor may provide a 2.5-fold increase of the maximum operating temperature, for the same minimum feature size.
Trapping of size-selected Ag clusters at surface steps
Authors: Carroll S. J. , Seeger K. , Palmer R. E.
Published: July 2002

We have investigated the deposition of size-selected metal clusters (Ag400) onto a stepped graphite surface. The clusters were produced with an inert gas condensation cluster source and were imaged on the surface with a high-resolution scanning electron microscope. For modest cluster deposition energies, cluster aggregation is much more limited at the steps than on the flat terrace regions of the surface, suggesting the possibility of the fabrication of structured arrays (e.g., chains) of size-selected particles. A theoretical analysis of the particle-particle gap size distribution along the step probes the 1D mobility of the particles trapped at the step.
Photoconductivity of an inorganic/organic composite containing dye-sensitized nanocrystalline titanium dioxide
Authors: Kocher M. , Däubler T. K. , Harth E. , Scherf U. , Gügel A. , Neher D.
Published: July 2002

The photophysical properties of solid films of an inorganic/organic composite composed of dye-sensitized nanocrystalline titanium dioxide (TiO2) particles, a conjugated polymer, and a [60] fullerene derivative have been investigated. Large charge collection efficiencies of up to 10% at a field of only 10 V/μm were observed. The photoaction spectrum of the composite is interpreted in terms of three major contributions: a weak photocurrent due to the absorption of photons by the polymer, photogeneration of charges involving the fullerene, and a broad region below the onset of the polymer absorption which involves photophysical processes in the dye-loaded TiO2 nanoparticles.
On the optical properties of InAs/InP systems: The role of two-dimensional structures and three-dimensional islands
Authors: Mendonça C. A. C. , Laureto E. , Brasil M. J. S. P. , Cotta M. A. , Carvalho M. M. G. , Meneses E. A.
Published: July 2002

We investigate the effects of the interface morphology on the electronic properties of InAs/In systems using in-air atomic force microscopy and low temperature photoluminescence. Atomic force microscopy results show that the distribution of InAs strained film into three-dimensional islands and the two-dimensional wetting layer—typical of the Stranski–Krastanov growth mode—is strongly affected by the characteristics of the substrate and by the morphology of the InP buffer layer. The differences in the optical data are correlated to the different interface characteristics observed by atomic force microscopy. We discuss the origin of emission peaks taking into account the diffusion process of adsorbed atoms on the different types of surface.
Dose rate effects in focused ion beam synthesis of cobalt disilicide
Authors: Hausmann Stephan , Bischoff Lothar , Teichert Jochen , Voelskow Matthias , Grambole Dieter , Herrmann Folker , Möller Wolfhard
Published: July 2002

The influence of the dwell-time in focused ion beam synthesis has been investigated. Cobalt disilicide layers have been produced by 70 keV Co2+ implantation into silicon and have been investigated by Rutherford backscattering spectroscopy and scanning electron microscopy. At an implantation temperature of about 400 °C it is only possible to form continuous CoSi2 layers using sufficiently short pixel dwell-times. This result is explained by an enhanced damage accumulation for longer dwell-times.
How good is the polarization selection rule for intersubband transitions?
Authors: Liu H. C. , Buchanan M. , Wasilewski Z. R.
Published: July 2002

Using GaAs based quantum well infrared photodetectors (QWIPs) with either GaAs or InGaAs wells, we experimentally investigate the accuracy of the polarization selection rule for conduction band intersubband transitions. We employ a device structure and a light coupling geometry where the parasitic light scattering is negligible. The experiments imply that the selection rule is followed to an accuracy of 0.2% for a 8.1 μm QWIP with GaAs wells; this degrades to 3% for a 4.6 μm QWIP with In0.1Ga0.9As wells.
Magnetoelastic properties of epitaxial holmium and erbium thin films
Authors: Ciria M. , Arnaudas J. I. , del Moral A. , Wells M. R. , Ward R. C. C.
Published: July 2002

The magnetoelastic (MEL) stresses of single crystal Ho and Er thin films are measured at low temperatures and in applied magnetic fields up to 12 T by means of a capacitive cantilever technique, to determine the irreducible second-order basal plane MEL stress Bγ,2. For Ho, the data are well fit by the Callen and Callen law, giving Bγ,2=0.29 GPa at 0 K and 12 T, slightly larger than the bulk value, which suggest a negligible effect of the clamping on Bγ,2. For Er, the lack of saturation prevents the determination of the single-ion contribution to Bγ,2 at 0 K. Nevertheless, its sign, which is negative, agrees with the theoretical crystal field prediction.
Strategies for the synthesis of highly concentrated Si1−yCy diamond-structured systems
Authors: Chandrasekhar D. , McMurran J. , Smith David J. , Kouvetakis J. , Lorentzen J. D. , Menéndez J.
Published: July 2002

Precursor chemistry and ultrahigh-vacuum chemical vapor deposition have been used to deposit Si1−yCy thin films on (001) Si substrates. Films with carbon compositions ranging up to 20 at. % were deposited at substrate temperatures of 600–740 °C using gas mixtures of SiH4 with C(SiH3)4 or C(SiH2Cl)4, which are (C–H)-free precursors incorporating Si4C tetrahedra. The composition of the resulting materials was obtained by Rutherford backscattering spectrometry, including carbon resonance analysis. Cross-sectional transmission electron microscopy and infrared spectroscopy were used to provide microstructural and bonding information. Raman spectroscopy suggested that the substitutional C concentration obtained using this protocol was higher than that obtained by other methods. The addition of small amounts of GeH4 to the gas mixture had a remarkable effect on growth rates and film crystallinity.
Vertical cavity surface emitting lasers utilizing native oxide mirrors and buried tunnel contact junctions
Authors: Wierer J. J. , Evans P. W. , Holonyak N. , Kellogg D. A.
Published: July 2002

Vertical cavity surface emitting lasers (VCSELs) are demonstrated with high-index-contrast native-oxide-based (AlxOy) distributed Bragg reflectors (DBRs) on both sides of a “2λ” cavity, thus creating a compact (thin, ∼2.8 μm) laser structure. Selective oxidation of high Al composition AlxGa1−xAs layers yields a structure with a four period upper AlxOy/GaAs DBR, a 5.5 period lower AlxOy/GaAs DBR, and a buried oxide current aperture. A reverse-biased tunnel contact junction provides hole injection via lateral electron current between the upper DBR and the oxide aperture layer. These VCSELs operate with submilliampere thresholds, high spontaneous efficiencies, and excellent polarization control.
Optical characterization of lateral epitaxial overgrown GaN layers
Authors: Freitas Jaime A. , Nam Ok-Hyun , Davis Robert F. , Saparin Gennady V. , Obyden Sergey K.
Published: July 2002

The optical properties of homoepitaxial GaN layers deposited by organometallic vapor phase epitaxy on stripe-patterned GaN films on 6H-SiC substrates have been investigated. Analysis of the spatially-resolved Raman scattering spectra indicate an improvement in material quality of the overgrown region. Room-temperature color cathodoluminescence imaging and low-temperature photoluminescence measurements indicate that a donor and an acceptor, different from those detected in the underlying GaN/AlN/SiC substrate, have been incorporated in the epitaxial layer. Detailed photoluminescence studies of the near-band-edge emission strongly suggest that Si is the additional donor detected in the homoepitaxial GaN layer. Its occurrence, along with that of an acceptor-related defect which is primarily found in the laterally overgrown region, is discussed.
Structure and composition of the c(4×4) reconstruction formed during gallium arsenide metalorganic vapor-phase epitaxy
Authors: Han B.-K. , Li L. , Fu Q. , Hicks R. F.
Published: July 2002

Surfaces of GaAs (001) were prepared by metalorganic vapor-phase epitaxy and characterized by scanning tunneling microscopy, x-ray photoelectron spectroscopy, infrared spectroscopy, and low-energy electron diffraction. Upon removal from the reactor, the gallium arsenide surface exhibits a (1×2) reconstruction, which is a disordered variant of the c(4×4). The disorder arises from the presence of adsorbed alkyl groups. Heating the sample to 350 °C desorbs the hydrocarbons and produces a well-ordered c(4×4) structure. A model is proposed for the alkyl-terminated (1×2) reconstruction.
Understanding scanned probe oxidation of silicon
Authors: Dagata J. A. , Inoue T. , Itoh J. , Yokoyama H.
Published: July 2002

A model for scanned probe microscope (SPM) silicon oxidation is presented. The model was derived from a consideration of the space-charge dependence of this solid-state reaction as a function of substrate doping type/level and has been verified experimentally by integrating an in situ electrical force characterization technique, scanning Maxwell stress microscopy (SMM), into the SPM fabrication process. This system enables us to obtain SPM topographic, SMM capacitance, and SMM surface potential information of nanometer-scale oxide features as a function of ionic concentrations within the growing oxide film. SPM oxide properties are compared to those of anodic and thermal oxides. The predictive power of the resulting model is demonstrated by showing how the growth rate and electrical character of the SPM-oxide features can be altered dramatically by modulating the applied oxidation voltage.
Photon-spin controlled lasing oscillation in surface-emitting lasers
Authors: Ando H. , Sogawa T. , Gotoh H.
Published: July 2002

We report on photon-spin controlled lasing oscillation in GaAs surface-emitting lasers at room temperature. We demonstrate experimentally that the partial electron-spin alignment, created by optically pumping the GaAs laser active media with circularly polarized pulses, drastically changes the polarization state of the lasing output, causing circularly polarized lasing emission. We discuss the laser polarization characteristics in relation to the measured electron-spin relaxation time.
Pathways for initial water-induced oxidation of Si(100)
Authors: Stefanov Boris B. , Raghavachari Krishnan
Published: July 2002

First-principles molecular orbital methods and gradient-corrected density functional calculations on silicon clusters are used to study possible pathways for the initial oxidation of Si (100)-2×1. In these reactions, the adsorbed hydroxyl oxygen inserts into the dimer Si–Si bond to form a suboxide (≡Si–O–Si≡) surface structure. The reaction typically follows a two-step pathway involving an intermediate energy minimum. In the case of an ideal surface with full water coverage, the reaction is exothermic by 1.3 eV and the overall reaction barrier is estimated at 2.4 eV. However, an alternative pathway involving a dangling bond site lowers the activation barrier to 2.1 eV. The implications for the oxidation reaction rates are discussed as well as possible alternative pathways.
Wavelength dependence of 4-dimethylamino-4′-nitrostilbene polymer thin film photodegradation
Authors: Zhang Qiang , Canva Michael , Stegeman George
Published: July 2002

The polymeric electro-optical stilbene material, 4-dimethylamino-4′-nitrostilbene (DANS), was illuminated at wavelengths ranging from the visible to near-infrared in order to quantify its photodegradation processes. Photodegradation due to one-photon and two-photon absorption was studied. The quantum efficiency of the chromophore degradation is found to be strongly wavelength and absorption mechanism dependent. This suggests that, in different regions of the spectrum, different excited states or different mechanisms are involved in the degradation process. In the near-infrared spectral region, operation lifetimes of electro-optic devices made from this chromophore are expected to be around only few hundreds hours, dramatically shorter than it was previously believed.
The large magnetoresistance property of La0.5Sr0.5CoO3−x thin films prepared by pulsed laser deposition
Authors: Liu J.-M. , Ong C. K.
Published: July 2002

High-quality (001) thin film La0.5Sr0.5CoO3−x (LSCO) has been prepared on (001) SrTiO3 substrates by pulsed laser deposition under different oxygen pressures with and without postannealing. C-axis expansion of the LSCO with reducing oxygen pressure was revealed. The electrical resistivity increased over five orders of magnitude when oxygen pressure varied from 1.0 mbar (plus postannealing) to 10−3 mbar. The negative magnetoresistance (n-MR) property as a function of temperature and oxygen pressure was investigated. Linear dependence of the n-MR ratio on magnetic field and temperature was found and significant effect of oxygen stoichiometry on the n-MR was demonstrated. The film prepared at 650 °C and 0.1 mbar oxygen shows a n-MR ratio of −16% at 81 K under a field of only 0.2 T.
Optical implementation of the wavelet transform by using a bacteriorhodopsin film as an optically addressed spatial light modulator
Authors: Joseph Joby , Aranda F. J. , Rao D. V. G. L. N. , DeCristofano B. S. , Kimball B. R. , Nakashima M.
Published: July 2002

An optical system utilizing the photoinduced dichroism in a bacteriorhodopsin film has been demonstrated for the optical implementation of wavelet transforms. The dichroism, induced by the image of a wavelet filter on a bacteriorhodopsin film leads to polarization rotation of the Fourier components of an image. The polarization-rotated Fourier components of an input scene are analyzed with a polarizer to give the wavelet transform components. The dichroism is induced with beams whose profiles are determined by wavelet filters in order to perform the optical wavelet transform.
Arsenic flux dependence of incorporation of excess arsenic in molecular beam epitaxy of GaAs at low temperature
Authors: Suda A. , Otsuka N.
Published: July 2002

Incorporation of excess As in GaAs layers grown by molecular beam epitaxy was studied by varying As fluxes for four different substrate temperatures, 210, 240, 270, and 290 °C. Concentrations of excess As in GaAs layers were estimated by measuring increases of lattice spacings with x-ray diffraction, and the substrate surface temperature was monitored by using a quartz rod connected to an infrared pyrometer with its end placed in the vicinity of the substrate surface. Nearly stoichiometric GaAs layers without any detectable increase of the lattice spacing are grown at all substrate temperatures under the As atom flux equal to the Ga atom flux. With a slight increase of the As flux from the above stoichiometric condition, the concentration of excess As sharply increases for all substrate temperatures. For the substrate temperature of 210 °C, the concentration of excess As is saturated in the range of As atom fluxes more than three times the Ga atom flux, while similar tendencies are observed for other substrate temperatures. The incorporation process of excess As is discussed on the basis of these results.
Numerical modeling of superconducting coplanar resonators for radio frequency superconducting quantum interference devices
Authors: Yi H. R. , Zhang Y. , Braginski A. I.
Published: July 2002

We have simulated the superconducting coplanar resonators of different designs that we have fabricated and tested as the tank circuit of radio frequency superconducting quantum interference devices. The coplanar resonator is formed by two microstrip lines surrounding a flux concentrator with each line having a slit. The simulated resonant frequencies agreed well with the experimental values, where frequencies decreased with an increasing mutual angular position between the two microstrip slits, and with a short circuit between the two microstrip lines. The simulation also showed that the loss in the system is mainly contributed by the dielectric loss of the substrate materials.
Type II photoluminescence and conduction band offsets of GaAsSb/InGaAs and GaAsSb/InP heterostructures grown by metalorganic vapor phase epitaxy
Authors: Hu J. , Xu X. G. , Stotz J. A. H. , Watkins S. P. , Curzon A. E. , Thewalt M. L. W. , Matine N. , Bolognesi C. R.
Published: July 2002

The optical properties of lattice-matched GaAsSb/InGaAs/InP heterostructures with a varying InGaAs layer thickness (0–900 Å) were investigated. These structures display strong low temperature type II luminescence, the energy of which varies with the InGaAs layer thickness and ranges from 0.453 to 0.63 eV. The type II luminescence was used to determine directly and accurately the conduction band offset of these structures. The values obtained herein are 0.36 and 0.18 eV at 4.2 K for the GaAsSb/InGaAs and GaAsSb/InP heterojunctions, respectively, with the GaAsSb conduction band higher in energy.
Synthesis of boron nitride nanotubes from carbon nanotubes by a substitution reaction
Authors: Han Weiqiang , Bando Yoshio , Kurashima Keiji , Sato Tadao
Published: July 2002

A method involving carbon nanotubes substituted reaction was developed for the synthesis of mass quantities of boron nitride nanotubes. Boron oxide vapor was reacted with nitrogen gas in the presence of carbon nanotubes to form boron nitride nanotubes, whose diameters and lengths are similar to those of the starting carbon nanotubes. It is proposed that carbon atoms of carbon nanotubes can be fully substituted by boron and nitrogen atoms through a general chemical reaction. The results suggest that the synthesis methodology developed here may also be extended to form nanotubes from other novel materials.
Photoluminescence anisotropy from laterally anodized porous silicon
Authors: Givant A. , Shappir J. , Sa’ar A.
Published: July 2002

We report on a technique to fabricate planar structures of porous silicon using standard photolithography and processing steps. Uniform stripes of porous silicon with smooth interfaces with the surrounding silicon have been observed. Using now the freedom to define the crystallographic direction of the anodization process we studied the correlation between the crystallographic orientation of the porous silicon medium and the polarization selection rules of the emitted light. Our main conclusion is that the polarization selection rules are independent of the crystallographic direction of the porous silicon medium and determined solely by the anodization direction.
Gallium vacancies and the growth stoichiometry of GaN studied by positron annihilation spectroscopy
Authors: Saarinen K. , Seppälä P. , Oila J. , Hautojärvi P. , Corbel C. , Briot O. , Aulombard R. L.
Published: July 2002

We have applied positron spectroscopy to study the formation of vacancy defects in undoped n-type metal organic chemical vapor deposition grown GaN, where the stoichiometry was varied. Ga vacancies are found in all samples. Their concentration increases from 1016 to 1019 cm−3 when the V/III molar ratio increases from 1000 to 10 000. In nitrogen rich conditions Ga lattice sites are thus left empty and Ga vacancies are abundantly formed. The creation of Ga vacancies is accompanied by the decrease of free electron concentration from 1020 to 1016 cm−3, demonstrating their role as compensating centers.
Energy-dispersive x-ray imaging of an InGaN/GaN bilayer on sapphire
Authors: O’Donnell K. P. , Middleton P. G. , Trager-Cowan C. , Young C. , Bayliss S. C. , Fletcher I. , Van der Stricht W. , Moerman I. , Demeester P.
Published: July 2002

In this letter we discuss the potential and the limitations of quantitative characterization of the distribution of In and Ga atoms in III–N mixed alloys using energy dispersive x-ray (EDX) analysis. Spatial fluctuations of the indium content in an InGaN epilayer are found to correspond to changes in luminescence efficiency. Large hexagonal pyramids, which appear sparsely in such layers, appear to be relatively deficient in indium. Monte Carlo simulations, used to profile the Ga Ka x-ray fluorescence, highlight several limitations of the EDX technique in this context, but confirm our interpretation of the data. Finally, we identify differential growth rates as a possible explanation for the concentration/efficiency variations in InGaN layers.
Scanning localized viscoelastic image using a quartz crystal resonator combined with an atomic force microscopy
Authors: Kim Jong Min , Chang Sang Mok , Muramatsu Hiroshi
Published: July 2002

A quartz crystal resonator and an atomic force microscopy were applied for the measurement of local viscoelasticity and surface morphology. For the reduction of signal noise from the quartz crystal resonator, we designed an oscillation circuit based on a referential quartz crystal method. A polystyrene bead-coated quartz crystal was used as a signal quartz crystal, and a bare Au quartz crystal was used as a reference. By approaching the cantilever of atomic force microscopy to the surface of working quartz crystal, the differential resonant frequency in the two quartz crystals showed changes by the interaction between the tip and the quartz crystal. The changes of differential resonant frequency in the two quartz crystals were influenced by the local viscoelasticity. The image resolution of differential resonant frequency was observed under 80 nm for the polystyrene bead-coated quartz crystal.
Band gaps and band offsets in strained GaAs1−ySby on InP grown by metalorganic chemical vapor deposition
Authors: Peter M. , Herres N. , Fuchs F. , Winkler K. , Bachem K.-H. , Wagner J.
Published: July 2002

Metastable GaAs1−ySby with 0.22<y<0.70 has been grown pseudomorphically strained on (001) InP substrates using metalorganic chemical vapor deposition. The Sb concentration and layer thicknesses, ranging from 24 to 136 nm, were determined by high resolution x-ray diffraction (HRXRD) measurements. Low-temperature photoluminescence (PL) spectroscopy revealed spatially indirect band-to-band emission of electrons localized in the InP and holes in the GaAs1−ySby. At increased excitation power densities samples with layer thicknesses above 65 nm showed, also, spatially direct PL across the band gap of the strained GaAs1−ySby. From the PL data the band gap energy and the band offsets of GaAs1–ySby relative to InP were derived and compared with the predictions of the Model Solid Theory.
Continuous wave laser radiation at 669 nm from a self-frequency-doubled laser of YAl3(BO3)4:Nd3+
Authors: Jaque D. , Capmany J. , Garcı́a Solé J.
Published: July 2002

We report on continuous-wave red laser radiation at 669 nm from a self-frequency-doubled laser of YAl3(BO3)4:Nd3+ operating at a fundamental wavelength of 1338 nm in the F3/24→I13/24 laser channel of Nd3+ ion. Main laser characteristics at the fundamental wavelength are investigated as well as second harmonic generation properties. In nonoptimal preliminary experiments, 1 mW of red radiation is obtained for 900 mW pump power from a Ti:sapphire laser.
Resonant two-photon excitation of silicon nanocrystals
Authors: Diener J. , Kovalev D. I. , Polisski G. , Koch F.
Published: July 2002

We have studied generation of visible photoluminescence (PL) in porous silicon (PSi) under resonant two-photon excitation. While the temporal decays of the resonant photoluminescence excited by one- and two-photon absorption are identical and show the same temperature dependence, the PL response differs significantly near the excitation energy. Contrary to one-photon excitation, no spectral gap between the excitation energy and the onset of the two-photon excited PL is observed. This is explained in the framework of selection rules for dipole allowed and forbidden optical exciton transitions in silicon nanocrystals.
Detectors and sources for ultrabroadband electro-optic sampling: Experiment and theory
Authors: Leitenstorfer A. , Hunsche S. , Shah J. , Nuss M. C. , Knox W. H.
Published: July 2002

A detailed calculation of the amplitude and phase response of ultrathin ZnTe and GaP electro-optic sensors is presented. We demonstrate that the inclusion of the dispersion of the second-order nonlinearity is essential. Significant structures in experimental data can be explained by the theoretical response function. Correcting for the detector characteristics, we determine the precise shape of electromagnetic transients with a time resolution of 20 fs. In addition, we show that ultrafast transport of photocarriers in semiconductors can act as an efficient source for coherent electromagnetic radiation covering the entire far-to-mid-infrared regime.
1.54 μm Er3+ photoluminescent properties of erbium-doped Si/SiO2 superlattices
Authors: Shin Jung H. , Lee Won-hee , Han Hak-seung
Published: July 2002

The 1.54 μm Er3+ photoluminescent properties of erbium-doped Si/SiO2 superlattices are investigated. Two superlattice films, one with erbium in Si layers and the other with erbium in SiO2 layers, were prepared by electron cyclotron resonance plasma-enhanced chemical vapor deposition of SiH4 and O2 with cosputtering of erbium and subsequent rapid thermal anneal. Both display Er3+ luminescence, but it is stronger with longer luminescent lifetime and less temperature quenching when erbium is in the SiO2 layer. The results demonstrate that by using quantum structures, nonradiative deexcitation of Er3+ may be suppressed, and that carrier recombination events, which excite Er3+ ions, may be physically separated from Er atoms and still lead to an efficient Er3+ luminescence.
Third harmonic generation by Bloch-oscillating electrons in a quasioptical array
Authors: Ghosh Avik W. , Wanke Michael C. , Allen S. James , Wilkins John W.
Published: July 2002

We compute the third harmonic field generated by Bloch-oscillating electrons in a quasioptical array of superlattices under THz irradiation. The third harmonic power transmitted oscillates with the internal electric field, with nodes associated with Bessel functions in eEd/ℏω. The nonlinear response of the array causes the output power to be a multivalued function of the incident laser power. The output can be optimized by adjusting the frequency of the incident pulse to match one of the Fabry-Pérot resonances in the substrate. Within the transmission-line model of the array, the maximum conversion efficiency is 0.1%.
Suppressed diffusion of implanted boron in 4H–SiC
Authors: Laube Michael , Pensl Gerhard , Itoh Hisayoshi
Published: July 2002

Transient-enhanced diffusion of boron (B) during anneals at 1700 °C is experimentally observed in B-implanted 4H–SiC samples. This enhanced diffusion can strongly be suppressed by coimplantation of carbon or by a preanneal at 900 °C. It is proposed that B in 4H–SiC diffuses via the kick-out mechanism with the assistance of silicon interstitials in analogy to the B diffusion in Si. From the Fickian diffusion tail into the undamaged bulk, the preexponential factor D0 and the activation energy EA of the B diffusion coefficient D(B,T) are determined.
Precipitate formation in carbon-doped base of InGaP/GaAs heterojunction bipolar transistors grown by low-pressure metal organic chemical vapor deposition
Authors: Yang Q. , Scott D. , Miller J. , Meyer P. , Kuo H. C. , Baker J. E. , Stillman G. E.
Published: July 2002

The effect of intermediate temperature annealing on the carbon-doped base region of InGaP/GaAs heterojunction bipolar transistors (HBTs) was studied. This work shows that after annealing at only 600 °C a sample doped at 5.5×1019 cm−3 displays carbon precipitation. InGaP/GaAs HBT structures were grown for the annealing study. Hall measurements were used to measure hole concentration. Atomic force microscopy was employed to identify carbon precipitation. The annealing process not only removes hydrogen from the base but also creates carbon precipitates. The dc current gain measurements imply that the carbon precipitates increase base recombination. These results are very important in the growth and postgrowth annealing of high gain HBTs.
Atomic-order layer-by-layer role-share etching of silicon nitride using an electron cyclotron resonance plasma
Authors: Matsuura Takashi , Honda Yasuhiko , Murota Junichi
Published: July 2002

Atomic-order layer-by-layer etching of silicon nitride has been investigated using an ultraclean electron-cyclotron-resonance plasma. The surface nitrogen atoms in silicon nitride within only about one atomic layer from the surface were found to be removed selectively by excited hydrogen gas under well-controlled conditions. The remaining outermost silicon atoms were removed selectively by irradiation of a low-energy Ar+ and H+ ion mixture. By repeating these role-share cycles alternately, the resultant etching amount per cycle corresponded to one mean atomic layer of silicon nitride.
The impact ofin situphotoexcitation on the formation of vacancy-type complexes in silicon implanted at 85 and 295 K
Authors: Yarykin N. , Cho C. R. , Rozgonyi G. A. , Zuhr R. A.
Published: July 2002

Photoexcitation of silicon during low-fluence implantation with MeV Si and Ge ions is observed to suppress vacancy-type point-defect formation, as determined by in situ deep-level transient spectroscopy. The A-center formation after low-temperature implantation is extended over a wide temperature interval indicating that electrically inactive clusters, which emit vacancies during annealing, are formed in the end-of-range region during implantation at 85 K. The number of vacancies stored in these clusters is influenced by low-temperature in situ photoexcitation.
Broadening of submonolayer CdSe sheets in CdSe/ZnSe superlattices studied by x-ray diffraction
Authors: Kyutt R. N. , Toropov A. A. , Sorokin S. V. , Shubina T. V. , Ivanov S. V. , Karlsteen M. , Willander M.
Published: July 2002

We present x-ray diffraction studies of a CdSe distribution profile along the growth direction in CdSe/ZnSe submonolayer superlattices (SLs) grown by molecular beam epitaxy. The performed theoretical simulations show that the shape of both (004)- and (002)-reflection rocking curves is very sensitive to the vertical CdSe distribution around the intended deposition yplanes. In particular, broadening of the CdSe submonolayer insertions results in a decrease in SL (±1) and (±2) satellite intensities. Comparison of the simulations and experimental data allows us to conclude that CdSe sheets in the as-grown SL samples are asymmetrically broaden up to 5 monolayers.
Effect of substrate-induced strain on the charge-ordering transition in Nd0.5Sr0.5MnO3 thin films
Authors: Prellier W. , Biswas Amlan , Rajeswari M. , Venkatesan T. , Greene R. L.
Published: July 2002

We report the synthesis and characterization of Nd0.5Sr0.5MnO3 thin films grown by the pulsed laser deposition technique on [100]-oriented LaAlO3 substrates. X-ray diffraction (XRD) studies show that the films are [101] oriented, with a strained and quasi-relaxed component, the latter increasing with film thickness. We observe that transport properties are strongly dependent on the thickness of the films. Variable temperature XRD down to 100 K suggests that this is caused by substrate-induced strain on the films.
Nitridation of GaP(001) surface by electron cyclotron resonance assisted N2 plasma
Authors: Hashizume Tamotsu
Published: July 2002

Nitridation of GaP(001) surface was achieved at 400 °C by exposure to electron cyclotron resonance assisted N2 plasma. Detailed x-ray photoelectron spectroscopy analysis showed that after the initial stage of nitridation, the GaP surface included both of the Ga–N and P–N bonding components, while a near-stoichiometric GaN layer was formed after a 60 min nitridation process. A weak cathodoluminescence peak was detected at around 3.3 eV for the nitrided surface after 60 min of nitridation.
Atomic steps with tuning-fork-based noncontact atomic force microscopy
Authors: Rensen W. H. J. , van Hulst N. F. , Ruiter A. G. T. , West P. E.
Published: July 2002

Tuning forks as tip–sample distance detectors are a promising and versatile alternative to conventional cantilevers with optical beam deflection in noncontact atomic force microscopy (AFM). Both theory and experiments are presented to make a comparison between conventional and tuning-fork-based AFM. Measurements made on a Si(111) sample show that both techniques are capable of detecting monatomic steps. The measured step height of 0.33 nm is in agreement with the accepted value of 0.314 nm. According to a simple model, interaction forces of 30 pN are obtained for the tuning-fork-based setup, indicating that, at the proper experimental conditions, the sensitivity of such an instrument is competitive to conventional lever-based AFM.
Synthesizing boron nitride nanotubes filled with SiC nanowires by using carbon nanotubes as templates
Authors: Han Weiqiang , Redlich Philipp , Ernst Frank , Rühle Manfred
Published: July 2002

A method is described to synthesize silicon carbide (SiC)-filled boron nitride (BN) nanotubes (NT) simultaneously in high yield by using carbon nanotubes (CNTs) as templates. This method combines both carbon nanotube-substitution reaction and confined reaction. Through the CNT-substitution reaction, CNTs react with boron oxide vapor in the presence of nitrogen gas to form BN NTs, whose diameters and lengths are similar to those of the starting CNTs. The formation of the SiC filling is proceeded by the penetration of SiO vapor into the cavity of the nanotubes and subsequent reaction of SiO vapor with the inner carbon layers or volatile carbon mono-oxide in the interior to form SiC nanowires. The filled length can be up to the entire length of the nanotubes. SiC-filled (BN)xCy nanotubes also form in the product.
Dislocation reduction in GaN thin films via lateral overgrowth from trenches
Authors: Chen Y. , Schneider R. , Wang S. Y. , Kern R. S. , Chen C. H. , Kuo C. P.
Published: July 2002

A technology to reduce the dislocation density in GaN thin films by lateral overgrowth from trenches (LOFT) is reported. In LOFT, a GaN thin film was grown on sapphire substrate first, then trenches were formed into the thin film by etching. GaN material was regrown laterally from the trench sidewalls to form a continuous thin film. The average surface density of threading dislocations is reduced from 8×109/cm2 in the first GaN thin film to 6×107/cm2 in the regrown GaN thin film.
Elastic and plastic properties of GaN determined by nano-indentation of bulk crystal
Authors: Nowak R. , Pessa M. , Suganuma M. , Leszczynski M. , Grzegory I. , Porowski S. , Yoshida F.
Published: July 2002

The major obstacle to the production of a blue laser is posed by difficulties with the preparation of defect-free GaN layers. A considerable amount of empirical work is presently being undertaken to achieve this goal. However, there is a lack of basic research on the reduction of residual stress and defects in these epilayers since the mechanical characteristics of GaN have not been measured yet. This is due to difficulties with experimental examination of thin films. This work addresses the mechanical properties of bulk GaN obtained by a high-pressure method. Young’s modulus (295 GPa), hardness (20 GPa), yield strength (15 GPa), and the stress–strain curve of GaN have been evaluated using nano-indentation. The cause of the sudden depth excursions during indentation of GaN epilayers has been clarified.
(
Pb
1−x
La
x
)(
Zr
1−y
Ti
y
)
O
3
patterns on Pt-coated silicon prepared by pulsed laser deposition process
Authors: Liu Kuo-Shung , Chen Yu-Jen , Jamn Gwo , Lin I-Nan
Published: July 2002

Ferroelectric (PbxLa1−x)(ZryTi1−y)O3 (PLZT) thin films on Pt(Si) substrates have been synthesized using a two-step process, which includes low temperature pulsed laser deposition (PLD) and high temperature rapid thermal annealing (RTA). Pure perovskite PLZT phase can be achieved by RTA at 550 °C (1–60 s), provided that the crystalline SrRuO3 layer is used as buffer layer on top of Pt(Si) substrates. Interdiffusion between layers is efficiently suppressed. Increasing the RTA duration insignificantly modifies the crystalline structure and the microstructure of the PLZT/SRO/Pt(Si) films, but significantly improves the ferroelectric properties and leakage characteristics of the films. The electrical properties, which are optimized for the films RTA at 550 °C (30 s), are Pr=19 μC/cm2, Ec=70 kV/cm, and JL<10−5 A/cm2 for Ea<450 kV/cm applied field. A self-aligned pattern consisting of crystalline PLZT dots with good ferroelectric properties (Pr=14 μC/cm2, Ec=80 kV/cm) has been demonstrated.
Effect of indium segregation on optical properties of V-groove InGaAs/GaAs strained quantum wires
Authors: Lelarge F. , Constantin C. , Leifer K. , Condo A. , Iakovlev V. , Martinet E. , Rudra A. , Kapon E.
Published: July 2002

Self-ordered, strained InGaAs/GaAs quantum structures are grown on V-grooved GaAs substrates. The lateral patterning of these nonplanar heterostructures allows the growth of defect-free strained structures with thickness exceeding that achieved with planar epitaxy. Indium segregation at the bottom of the groove results in the formation of a vertical InGaAs quantum-well structure with In-enriched composition. We studied in detail the influence of nominal thickness and In content on the photoluminescence peak energy of these quantum wires. Room-temperature emission at 1.16 μm with a relatively narrow linewidth (30–35 meV) is achieved as a demonstration of the potential of this approach for fabricating long-wavelength semiconductor light sources on GaAs substrates.
Temperature- and oxygen partial pressure-dependent electrical conductivity in nanoporous rutile and anatase
Authors: Dittrich Th. , Weidmann J. , Koch F. , Uhlendorf I. , Lauermann I.
Published: July 2002

The dependence of the electrical conductivity (σ) of sintered nanoporous TiO2 (rutile and anatase) layers on temperature (T) and partial pressure of oxygen (pO2) has been studied by impedance spectroscopy for T up to 450 °C and pO2 up to 10−5 mbar. The diameter of the TiO2 nanoparticles was changed between 5 and 60 nm. σ is thermally activated with EA=0.85 eV independent of the absolute value of σ and depends on pO2 by power law for pO2<1–10 mbar. The electrical properties of reduced nanoporous TiO2 are determined by surface chemical reactions which lead to the formation of shallow donor and deep trap states.
Strain relief and its effect on the properties of GaN using isoelectronic In doping grown by metalorganic vapor phase epitaxy
Authors: Yamaguchi Shigeo , Kariya Michihiko , Nitta Shugo , Amano Hiroshi , Akasaki Isamu
Published: July 2002

We studied the effect of isoelectronic In doping on the crystalline and optical properties of GaN grown on sapphire with H2 or N2 carrier gas by metalorganic vapor phase epitaxy. The relationship between lattice constants c and a obtained by x-ray diffraction analysis showed that with increasing trimethylindium (TMI) flow during growth, the strain in GaN decreased, and accordingly, the tilting and the twisting components of crystalline mosaicity also decreased. In addition, the Raman shift, the excitonic photoluminescence peak energy, and the its linewidth shifted in accordance with the magnitude and the sign of the strain in GaN, regardless of the carrier gas used. These results revealed that for a smaller TMI flow region, In was incorporated so that the crystallinity of GaN improved, and for a larger TMI flow region, In substituted for Ga so that alloying formation might have occurred.