bluuebunny/crossref_metadata_2025 huggingface dataset:
Production of D−Amino Acid Using Whole Cells of Recombinant Escherichiacoli with Separately and Coexpressed D−Hydantoinase and N‐Carbamoylase
Authors: Park Joo‐Ho , Kim Geun‐Joong , Kim Hak‐Sung
Published: July 2002

We developed a fully enzymatic process employing D−hydantoinase and N‐carbamoylase for the production of D−amino acid from 5′ ‐monosubstituted hydantoin. For the comparison of the reaction systems using two sequential enzymes, D−hydantoinase of Bacillus stearothermophilus SD1 and N‐carbamoyl‐D−amino acid amidohydrolase (N‐carbamoylase) of Agrobacterium tumefaciens NRRL B11291 were separately expressed in each host cell and coexpressed in the same host cell. A high level and constitutive expression of both enzymes in Escherichia coli in a soluble form was achieved using a promoter derived from B. stearothermophilus SD1. The expression levels of both enzymes ranged from 17% to 23% of the total soluble protein, depending on the expression system. In the case of employing separately expressed enzymes, the product yield of D−hydroxyphenylglycine from D,L−p‐hydroxyphenylhydantoin and productivity were 71% and 2.57 mM/g‐cell/h in 15 h, respectively. The accumulation of N‐carbamoyl‐D−hydroxyphenylglycine was significant over the reaction time. On the other hand, use of coexpressed enzymes resulted in 98% product yield of D−hydroxyphenylglycine in 15 h, minimizing the level of intermediates in the reaction mixture. The productivity of coexpressed whole cell reaction was estimated to be 6.47 mM/g‐cell/h in 15 h. The coexpressed system was tested for an elevated concentration of D,L−p‐hydroxyphenylhydantoin, and efficient production can be achieved.
Biotechnology of the Banana: A Review of Recent Progress
Authors: Rout G. R. , Samantaray S. , Das P.
Published: July 2002

Abstract: A number of biotechnological tools have been developed which could help breeders to evolve new plant types to meet the demand of the food industry in the next century. Available techniques for the transfer of genes could significantly shorten the breeding procedures and overcome some of the agronomic and environmental problems which would otherwise not be possible through conventional methods. In vitro protocols have been standardized to allow commercially viable propagation of desired clones of Musa. An overview of the regeneration of banana by direct and indirect organogenesis, and somatic embryogenesis is presented in this article. In addition, the use of several other biotechnological techniques to enrich the genome of banana, such as selection of somaclonal variants, screening for various useful characteristics, cryopreservation, genetic transformation and molecular genetics are reviewed. In conclusion, the improvement of banana through modern biotechnology should help ensure food security by stabilizing production levels in sustainable cropping systems geared towards meeting domestic and export market demands.
Morphological Evidence for an Extended Tribe ‐ Coccocypseleae (Rubiaceae‐Rubioideae)
Authors: Piesschaert F. , Huysmans S. , Jaimes I. , Robbrecht E. , Smets E.
Published: July 2002

Abstract: Coccocypselum is a small neotropical genus of creeping herbs belonging to the Rubiaceae (Rubioideae). A recent macromolecular study suggested a previously unknown relationship between Coccocypselum and two other neotropical genera, Declieuxia and Hindsia, but the morphological evidence for this relationship was obscure. In the present paper, morphological characters supporting the conclusions from macromolecular data are presented. The most important similarities between Coccocypselum, Declieuxia, and Hindsia are found in their pollen and stipule morphology. Pollen of the three genera share a typical exine ornamentation, called a complex reticulum, which consists of a psilate suprareticulum and spinulate infrareticulum. In some specimens of Hindsia longiflora, a transitional stage between a complex reticulum and a simple, spinulate reticulum is encountered. The interpetiolar stipules of Hindsia, Declieuxia and Coccocypselum have an identical basic structure: they bear a central, colleter‐tipped awn that extends downwards to the next node as a distinct stem ridge. Some less conspicuous similarities of habit, calyx, and corolla are also discussed. The morphological data are, in part, confirmed by distributional data, Declieuxia and Hindsia having a remarkably congruent centre of diversity in the southeast of Brazil. It is concluded that the macromolecular data, combined with this new morphological and distributional evidence, justify an extended tribe Coccocypseleae, comprising the genera Coccocypselum, Hindsia, and Declieuxia.
Carbon‐Isotope Ratios and Photosynthetic Pathways in the Neotropical Family Rapateaceae
Authors: Crayn D. M. , Smith J. A. C. , Winter K.
Published: July 2002

Abstract: The Rapateaceae is a small, mainly Neotropical family of terrestrial or occasionally epiphytic herbs that grow on mesic, nutrient‐poor sites. Some recent studies suggest that the Rapateaceae may be closely related to the Bromeliaceae, one of the major families containing CAM plants. To investigate the photosynthetic pathway in Rapateaceae, the plant carbon‐isotope ratio (δ13C) was determined for samples from dried herbarium specimens for 85 of the approximately 100 species in the family. The δ13C values ranged from ‐ 37.7 to ‐ 19.8 ‰. Most Rapateaceae showed δ13C values typical of C3 plants. However, six species (Kunhardtia rhodantha Maguire, Marahuacaea schomburgkii (Maguire) Maguire, Saxofridericia compressa Maguire, Stegolepis grandis Maguire, St. guianensis Klotzsch ex Körn. and St. squarrosa Maguire) showed δ13C values less negative than ‐ 23 ‰, i.e., at the higher end of the range for C3 plants and at the lower end of the distribution for plants exhibiting CAM. The δ13C values became significantly less negative with increasing altitude (regression analysis indicating a change from about ‐ 30.7 ‰ at sea level to ‐ 22.5 ‰ at 2500 m). Although other environmental factors and the type of tissue analysed may also influence δ13C values, these results suggest that some Rapateaceae may be capable of performing CAM. Further studies, including measurements of diel gas exchange patterns and leaf organic‐acid fluctuations, would be needed to demonstrate CAM in Rapateaceae unequivocally, but living material of many of these enigmatic plants is difficult to obtain.
27.6% efficiency (1 sun, air mass 1.5) monolithic Al0.37Ga0.63As/GaAs two-junction cascade solar cell with prismatic cover glass
Authors: Chung B.-C. , Virshup G. F. , Hikido S. , Kaminar N. R.
Published: July 2002

A 27.6% efficiency measured under 1 sun, air mass 1.5 illumination has been achieved in a two-terminal, monolithic two-junction cascade solar cell consisting of an Al0.37Ga0.63As (Eg=1.93 eV) upper cell and a GaAs lower cell. The component cells were electrically connected together using a metal-interconnect contact fabricated during post-growth processing. Also, a prismatic cover glass was bonded to the front surface of the cascade structure to minimize the obscuration effect introduced by the grid lines and metal-interconnect contacts. As the cascade cell was operated under air mass 0 illumination, an efficiency of 23.0% was obtained. Both results represent the highest 1 sun efficiencies ever reported. The implications of achieving this high efficiency in a two-junction cascade solar cell for terrestrial and space applications are also described.
Deposition of diamond films at low pressures and their characterization by positron annihilation, Raman, scanning electron microscopy, and x-ray photoelectron spectroscopy
Authors: Sharma S. C. , Dark C. A. , Hyer R. C. , Green M. , Black T. D. , Chourasia A. R. , Chopra D. R. , Mishra K. K.
Published: July 2002

We have deposited diamond films with micron-size crystals on Si〈111〉 using low-pressure hot-filament-assisted chemical vapor deposition. These films have been characterized by positron annihilation, Raman spectroscopy, scanning electron microscopy, and x-ray photoelectron spectroscopy. In addition to the results for the electronic structure and morphology, we also present new results for the lattice defects present in these films.
Accurate determination of the conduction-band offset of a single quantum well using deep level transient spectroscopy
Authors: Letartre X. , Stievenard D. , Barbier E.
Published: July 2002

Using deep level transient spectroscopy, we have determined the band offset of a GaAs/GaInAs/GaAs single quantum well. To interpret the data, we propose an original model which takes into account the fact that the emission rate of electrons depends on the charge density in the well, and thus varies continuously during the emission process, contrary to previous models. The validity of the analysis is tested with success by using capacitance-voltage measurement to determine the band offset.
Signature of the gallium-oxygen-gallium defect in GaAs by deep level transient spectroscopy measurements
Authors: Neild S. T. , Skowronski M. , Lagowski J.
Published: July 2002

Experimental results of deep level transient spectroscopy (DLTS) and Fourier transform infrared spectroscopy (FTIR) on oxygen-doped GaAs crystals are presented. A positive identification of the DLTS signature of the Ga-O-Ga defect (tentatively identified as an oxygen-arsenic vacancy complex) is made through correlation with FTIR measurements. The energy level for the two-electron state of the defect is found to be located at 0.55 eV below the conduction band, and direct proof is given that this center is actually a negative U center. A calibration factor for local vibrational mode absorption is calculated to be 8×1016 cm−1.
Fabrication of thin-film LiMn2O4 cathodes for rechargeable microbatteries
Authors: Shokoohi F. K. , Tarascon J. M. , Wilkens B. J.
Published: July 2002

We have fabricated thin-film cathodes of lithiated manganese oxide (LixMn2O4, where x≤1), which can reversibly intercalate nearly one Li atom per unit formula, at a steady potential of 4.1 V. This results in a theoretical energy density of about 600 Wh/kg. The polycrystalline films (thickness of 0.5–2 μm) show electrochemical performance exceeding that of bulk materials. This is attributed to the small grain size and the porosity within these LixMn2O4 three-dimensional spinel films, reducing the diffusion path length for Li+ ions. The films are tested as the cathodes in about 70 charging/discharging cycles, in a secondary lithium cell, at current rates of 10–100 μA cm−2. No measurable self-discharge over 12 h periods is observed. These thin-film cathodes with above electrochemical properties are very promising for a new generation thin-film secondary batteries.
Evidence for quantum confinement in the photoluminescence of porous Si and SiGe
Authors: Gardelis S. , Rimmer J. S. , Dawson P. , Hamilton B. , Kubiak R. A. , Whall T. E. , Parker E. H. C.
Published: July 2002

We have used anodization techniques to process porous surface regions in p-type Czochralski Si and in p-type Si0.85Ge0.15 epitaxial layers grown by molecular beam epitaxy. The SiGe layers were unrelaxed before processing. We have observed strong near-infrared and visible light emission from both systems. Analysis of the radiative and nonradiative recombination processes indicate that the emission is consistent with the decay of excitons localized in structures of one or zero dimensions.
a-axis oriented YBa2Cu3O7−x thin films on Si with CeO2 buffer layers
Authors: Luo Li , Wu X. D. , Dye R. C. , Muenchausen R. E. , Foltyn S. R. , Coulter Y. , Maggiore C. J. , Inoue T.
Published: July 2002

We have grown a-axis oriented YBa2Cu3O7−x (YBCO) thin films on Si(100) substrates with (110) oriented insulating buffer layers of cerium dioxide (CeO2) using the pulsed laser deposition technique. The films are highly oriented and textured as determined by θ–2θ x-ray diffraction, x-ray pole-figure scan, scanning electron microscopy, Rutherford backscattering spectroscopy, and ion channeling. No diffusion at the interface has been found at growth temperatures up to 760 °C, indicating the CeO2 is a chemically stable and structurally compatible intermediate material for the growth of YBCO on Si. A zero resistance superconducting transition temperature of 87 K and a critical-current density (Jc) of 1.5×105 A/cm2 at 75 K have been measured; Jc obtained represents the highest value for the a-axis oriented YBCO films.
Photoluminescence spectra from porous silicon (111) microstructures: Temperature and magnetic-field effects
Authors: Perry Clive H. , Lu Feng , Namavar Fereydoon , Kalkhoran Nader M. , Soref Richard A.
Published: July 2002

Visible and near-infrared (IR) photoluminescence emission spectra (0.9–3.0 eV) from p-type porous Si(111) microstructures are reported as a function of temperature and magnetic field. The visible peak located at 1.84 eV at 4 K shifted to ∼1.56 eV at 575 K where it disappeared; the intensity reached a maximum value at ∼150 K. The photoluminescence spectrum showed no measurable shift in the peak position with magnetic field from 0 to 15 T. Strong IR intrinsic band-to-band emission above and below the bulk silicon band gap at ∼1.09 eV at 300 K was observed. This luminescence was found to be enhanced by two orders of magnitude or more over the IR spectrum from an unanodized wafer.
Fast intensified-CCD photography of YBa2Cu3O7−x laser ablation in vacuum and ambient oxygen
Authors: Geohegan David B.
Published: July 2002

The propagation of laser ablation plumes from 248 nm laser-irradiated YBCO into vacuum and 100 mTorr ambient oxygen has been photographed with a gated, intensified CCD array camera system. The thermalization of the laser plasma and onset of shock structures due to collisions with the background gas are investigated from two-dimensional digitized images of the visible plume emission.
Single-electron quantization of electric field domains in slim semiconductor superlattices
Authors: Korotkov A. N. , Averin D. V. , Likharev K. K.
Published: July 2002

It is shown that dc I-V curves of the semiconductor superlattices of small (practically, submicron) cross section should exhibit oscillations with the dc voltage period e/C, where C is the capacitance between adjacent conducting layers. These oscillations are due to the single-electron quantization of electric charge of the boundaries of static high-electric-field domains.
Modulation of a semicondutor-to-semimetal transition at 7 THz via coherent lattice vibrations
Authors: Cheng T. K. , Acioli L. H. , Vidal J. , Zeiger H. J. , Dresselhaus G. , Dresselhaus M. S. , Ippen E. P.
Published: July 2002

This letter introduces the general notion of significantly modulating the physical characteristics of a solid on a terahertz time scale with coherent lattice vibrations. We show, as an example, experiments in which coherent phonons are optically excited in single-crystal Ti2O3, which is a narrow-gap semiconductor at 300 K and gradually transforms into a semimetal at 600 K. Quantitative comparison of previous equilibrium measurements to our new transient measurements suggests that the induced coherent lattice vibration is significantly modulating the semiconductor-to-semimetal transition at 7 THz.
Role of interfacial nitrogen in improving thin silicon oxides grown in N2O
Authors: Carr E. C. , Buhrman R. A.
Published: July 2002

We have used chemical depth profiling, with a depth resolution of 10 Å, in conjunction with x-ray photoelectron spectroscopy to study the composition and chemical bonding in thin silicon oxides grown in N2O with both a conventional furnace and a rapid thermal annealer (RTA) process. The nitrogen profiles of RTA and furnace oxides differ, with the RTA oxides showing an increase in nitrogen concentration at the interface and the furnace oxides showing a more uniform nitrogen distribution. The percentage of nitrogen at the interface also differs, and correlates with a reduction in interface state generation under current injection for increased nitrogen concentration. The chemical environment for the nitrogen changes with distance from the interface, and this is attributed to an increasing number of nitrogen—silicon bonds near the interface.
Native oxide top- and bottom-confined narrow stripe p-n AlyGa1−yAs-GaAs-InxGa1−xAs quantum well heterostructure laser
Authors: Maranowski S. A. , Sugg A. R. , Chen E. I. , Holonyak N.
Published: July 2002

A new form of AlyGa1−yAs-GaAs-InxGa1−xAs quantum well heterostructure (QWH) laser that is confined above and below the active region by an insulating low refractive index native oxide is demonstrated. The laser diodes are defined from a mesa edge by the selective lateral oxidation and anisotropic oxidation of high Al composition AlyGa1−yAs layers (y=0.85, 0.87) located above and below the QW and waveguide active region. This structure provides excellent current and optical confinement, resulting in continuous wave threshold currents of ∼8 mA and maximum output powers (uncoated laser) of 35 mW/ facet for a∼2.5 μm aperture.
1 000 000 °C/s thin film electrical heater: In situ resistivity measurements of Al and Ti/Si thin films during ultra rapid thermal annealing
Authors: Allen L. H. , Ramanath G. , Lai S. L. , Ma Z. , Lee S. , Allman D. D. J. , Fuchs K. P.
Published: July 2002

We introduce a new technique for rapidly heating (106 °C/s) thin films using an electrical thermal annealing (ETA) pulse technique. By applying a high-current dc electrical pulse to a conductive substrate-heater material (Si), joule heating occurs thus heating the thin film. This method was demonstrated by heating thin films of aluminum at rates ranging from 103 to 106 °C/s. The temperature of the system is measured by using the substrate heater as a thermistor and is found to be within ≊±10 °C during anneals at ≊105 °C/s. Phase transformations in the Ti-Si system were also observed using in situ resistivity measurements during ETA at ≊104 °C.
Band-gap shift in CdS semiconductor by photoacoustic spectroscopy: Evidence of a cubic to hexagonal lattice transition
Authors: Zelaya-Angel O. , Alvarado-Gil J. J. , Lozada-Morales R. , Vargas H. , Ferreira da Silva A.
Published: July 2002

The band-gap energies of the CdS semiconductor are obtained by a photoacoustic spectroscopy (PAS) technique over a range of temperature of thermal annealing (TTA), in which the evolution of the sample structure is characterized by x-ray diffraction patterns. The PAS experiment gives a set of data for the band-gap shift in the region of the fundamental absorption edge. With increasing TTA the band-gap shift increases up to a critical TTA when its slope decreases in a roughly symmetrical way. It is suggested that at this temperature a cubic to hexagonal-lattice transition occurs.
Thermodynamic properties of Pd40Ni40P20 in the glassy, liquid, and crystalline states
Authors: Wilde G. , Görler G. P. , Willnecker R. , Dietz G.
Published: July 2002

Bulk specimens of the easy glass-forming alloy Pd40Ni40P20 have been undercooled consistently into the glassy state at cooling rates as low as 10 K/min applying the melt-fluxing technique in boron trioxide. Due to this low cooling rate, heat capacity measurements could be performed in a commercial heat-flow differential calorimeter, covering for the first time the entire undercooling regime of a liquid metal from the melting temperature down to the glass transition temperature. Based on the measured specific heat data of the undercooled liquid and the crystalline state, the differences in the thermodynamic functions enthalpy, entropy, and Gibbs free energy are determined in dependence on temperature. The entropy balance yields a value of T0=500±5 K for the ideal glass transition temperature of this metallic system. The experimental values are compared to the corresponding thermodynamic functions, derived from commonly applied Gibbs free energy approximations for the undercooled liquid.
Electronic properties of InGaP grown by solid-source molecular-beam epitaxy with a GaP decomposition source
Authors: Shitara Tomoya , Eberl Karl
Published: July 2002

We have grown high-quality In0.48Ga0.52P layers on GaAs(001) substrates using solid-source molecular-beam epitaxy. A dimer phosphorous (P2) molecular beam was produced from a GaP decomposition source. The full width at half-maximum of the (004) double-crystal x-ray diffraction peak from a 1.7-μm-thick InGaP layer on GaAs(001) substrate was 16 arcsec, which is comparable with the smallest values ever reported. The photoluminescence linewidth at 10 K is 16 meV. Hall measurements are carried out on undoped, Si-doped, and Be-doped InGaP layers grown with a 2×1 surface reconstruction. The electron mobilities are comparable to similar InGaP/GaAs layers grown by gas-source molecular-beam epitaxy. Undoped InGaP layers grown with a 2×2 reconstruction, however, showed a high resistivity of ≥102 Ω cm at 300 K.
Coupled semiconductor microcavities
Authors: Stanley R. P. , Houdré R. , Oesterle U. , Ilegems M. , Weisbuch C.
Published: July 2002

The coupled semiconductor microcavity is a system in which there are three oscillators, two photonic and one electronic (quantum well excitons). It develops three strongly coupled modes which allow a wide design range for a variety of optoelectronic applications. The MBE grown structure is comprised of two λ sized GaAs cavities containing InxGa1−xAs quantum wells, separated by a common mirror. Reflectivity measurements show both two coupled photon mode behavior and three coupled mode behavior, i.e., two photon and one exciton, depending on the relative position of the exciton resonance.
Epitaxial orientation and magnetic properties of MnAs thin films grown on (001) GaAs: Template effects
Authors: Tanaka M. , Harbison J. P. , Park M. C. , Park Y. S. , Shin T. , Rothberg G. M.
Published: July 2002

We have studied template effects in molecular beam epitaxy (MBE) of ferromagnetic MnAs thin films on (001) GaAs substrates. When As2 flux was first supplied without Mn flux on the (001) GaAs prior to the MnAs growth, the surface reconstruction was disordered c(4×4), a more As-rich surface than c(4×4). The growth direction of the MnAs thin film grown on this surface is [1̄100] and the easy magnetization axis was found to be along the [1̄1̄20] of MnAs and the [110] of GaAs. In contrast, when one monolayer of Mn was first deposited on the c(4×4) GaAs surface and then As2 flux was supplied to grow MnAs, the growth direction of the MnAs thin film was found to be mainly [1̄101], and the easy magnetization axis was along the [1̄1̄20] of MnAs and the [1̄10] of GaAs, 90° different with respect to the substrate. These results indicate the importance of the very first monolayer in controlling the epitaxial orientation and magnetic properties of epitaxial ferromagnetic MnAs thin films.
Improved adhesion of thin conformal organic films to metal surfaces
Authors: Stewart Kevin R. , Whitesides George M. , Godfried Herman P. , Silvera Isaac F.
Published: July 2002

A technique is described for attaching thin, conformal, pin-hole-free electrically insulating polyethylene films to flat gold surfaces (previously modified by adsorption of a monolayer of an organic disulfide) by plasma polymerization. These polyethylene films are tough enough to support the attachment of gold electrodes.
Frequency modulated (FM) time delay photoacoustic and photothermal wave spectroscopies. Technique, instrumentation, and detection. Part I: Theoretical
Authors: Mandelis Andreas
Published: July 2002

A conceptual basis for the technique of time delay photothermal wave spectroscopy is presented. The signal generation and appropriate system functions in the time delay and frequency domains are introduced and discussed from the point of view of correlation and spectral analysis and processing.
Novel Langmuir probe technique for the real-time measurement of the electron temperature
Authors: Van Nieuwenhove R. , Van Oost G.
Published: July 2002

A new method for the evaluation of the electron temperature from the voltage–current characteristic of a single Langmuir probe under the application of an ac signal was developed and tested. This method exploits harmonics in the current spectrum which arise from the nonlinearity of the characteristic; from the amplitude of these harmonics the electron temperature can be deduced. Langmuir probe measurements on the TEXTOR edge plasma show a good agreement of the new method with the conventional one. The advantages of this method over other methods are discussed. Proposals for further improvements are given.
Schottky barrier contacts of titanium nitride on n-type silicon
Authors: Dimitriadis C. A. , Logothetidis S. , Alexandrou I.
Published: July 2002

Schottky contacts of TiNx thin films on n-type Si(100) were fabricated by reactive magnetron sputtering at room temperature. In situ spectroscopic ellipsometry was used to determine the stoichiometry of the TiNx films. Dark forward bias current–voltage as well as dark reverse bias capacitance–voltage techniques was used to characterize the diodes in the temperature range from 77 to 300 K. The electrical characteristics of the contacts (i.e., barrier height, ideality factor, and leakage current) as well as the inhomogeneity of the spatial distribution of the barrier heights at the interface are improved drastically for overstoichiometric TiNx films obtained by using low negative bias voltage and/or high N2 flow rates during the TiNx deposition.
A fluorescence-based fiber-optic flow sensor—Design considerations
Authors: Basarab I. , Ghassemlooy Z. , Shaw A. J. , Grattan K. T. V.
Published: July 2002

A design study of a proposed fiber-optic flow sensor is presented. The concept used is that the temperature-dependent fluorescent decay time of a ruby crystal may be measured to determine the temperature of the crystal. The heat induced by light from a laser shone on the crystal is dissipated through it and may be conducted away by a flowing fluid. The measurement of crystal temperature can therefore be used as a means of measurement of fluid flow. In this work, the transient heat distribution in the crystal is modeled; as the basis of the design study, the results produced by the model indicate that this is a method that has potential as a practical system
Ultralow frequency oscillator using a pendulum with crossed suspension wires
Authors: Barton Mark A. , Kuroda Kazuaki
Published: July 2002

A pendulum using four suspension wires in an X formation is described which can provide an arbitrarily long period in one dimension. Design equations are derived, and tested experimentally. A number of applications are outlined, including a low-frequency vibration isolation system for a laser-interferometric gravity wave observatory.
On the use of the open photoacoustic cell technique for studying photosynthetic O2 evolution of undetached leaves: Comparison with Clark-type O2 electrode
Authors: Pereira Antonio C. , Neto Graciliano de O. , Vargas Helion , Cella Norberto , Miranda Luiz C. M.
Published: July 2002

The leaf photosynthetic O2 evolution detected by the open photoacoustic cell (OPC) technique is compared to the measurements using a Clark-type O2 electrode detector. In vivo and in situ measurements were carried out with undetached maize leaves. From the correlation between these two methods the possibility of a calibration is proposed for the oxygen evolution OPC signal.
Excited state spectroscopy of excitons in single quantum dots
Authors: Gammon D. , Snow E. S. , Katzer D. S.
Published: July 2002

We describe the results of a photoluminescence study of quantum dots which are formed by interface fluctuations in narrow GaAs/AlAs single quantum wells. The photoluminescence measurements were made with lateral spatial resolution ranging from the macroscopic down to the optical near-field regime. For spatial resolution below a few square microns the photoluminescence from individual quantum dots is resolved. Photoluminescence excitation spectroscopy is used to study the excited state spectrum of an exciton bound in a single quantum dot.
Effects of rf sidebands on spectral reproducibility for infrared synchrotron radiation
Authors: Biscardi R. , Ramirez G. , Williams G. P. , Zimba C.
Published: July 2002

The effects of audio sidebands from the main rf of the NSLS VUV storage ring on beam motion and specifically on spectral reproducibility in the infrared region were investigated. For this spectral region it is advantageous to use Michelson interferometers because of their high throughput (Jacquinot advantage). A second advantage is that interferometers inherently give a multiplex or Felgett advantage, since one is always looking at all the wavelengths for all of the measuring time, even though there is only one detector. In such instruments it is beneficial to scan the moving mirror at a fast rate so that any mechanical or other low-frequency noise shows up as a slow modulation in the interferogram and disappears altogether in the Fourier transform from the spectral region of interest. However, audio-frequency sidebands appear from the rf energy restoring cavity as noise on the beam. These always occur at multiples of 60 Hz and can thus be readily identified. They can also be confirmed by changing the mirror velocity which changes where they appear in the spectra in a predictable way. In the present work, spectra were measured while simultaneously reducing and shifting the sidebands in the rf and thus the effects were correlated and ultimately eliminated as a source of noise. Ultimately reproducibilities of <1% in 15 s of scanning time across the entire spectral region from 800 to 4000 cm−1 with a sample throughput of only 10−10 m2 sr were achieved.
Low magnetic field dynamic nuclear polarization using a single-coil two-channel probe
Authors: TonThat Dinh M. , Augustine Matthew P. , Pines Alexander , Clarke John
Published: July 2002

We describe the design and construction of a single-coil, two-channel probe for the detection of low-field magnetic resonance using dynamic nuclear polarization (DNP). The high-frequency channel of the probe, which is used to saturate the electron spins, is tuned to the electron Larmor frequency, 75 MHz at 2.7 mT, and matched to 50 Ω. Low-field, 1H nuclear magnetic resonance (NMR) is detected through the second, low-frequency channel at frequencies <1 MHz. The performance of the probe was tested by measuring the DNP of protons in a manganese (II) chloride solution at 2.7 mT. At the proton NMR frequency of 120 kHz, the signal amplitude was enhanced over the value without DNP by a factor of about 200.
A simplified power transmission method used for measuring the complex conductivity of superconducting thin films
Authors: Wu Ruixin , Qian Min
Published: July 2002

A simplified power transmission method, used for determination of complex conductivity of superconducting thin film, is proposed. The method has the advantage of avoiding direct measurements, which are difficult and complicated, of substrate parameters. By means of measuring the total transmission coefficient of superconducting samples, the substrate coefficients of transmission and reflection, the conductivity σ can be calculated. In the paper, a transmission equation T(σ) is given and its approximate solution is discussed. Under the condition that the thickness d of superconductive film is much smaller than the penetration depth λ, an analytical expression for σ is derived. A numerical calculation shows that this expression could be used until the d/λL<0.5 with small computing errors.
Diagnostic strategy of the W7-X stellarator
Authors: Hartfuss H. J. , Brakel R. , Endler M. , Geist T. , Grigull P. , Hofmann J. V. , Junker J. , Kick M. , Kühner G. , Niedermeyer H. , Ringler H. , Teubel A. , Wagner F. , Weller A.
Published: July 2002

The Wendelstein 7-X experiment is a concept test for properties of reactor relevant plasmas in advanced stellarators. Prominent features include a modular superconducting coil assembly, a fivefold toroidal symmetry, and a helical magnetic axis. Due to the optimization process, W7-X is characterized by a vacuum magnetic field configuration with smooth magnetic surfaces, improved equilibrium properties with a weak dependence of rotational transform and shear on the plasma pressure β, good magneto-hydrodynamic stability properties due to magnetic well stabilization, reduced neoclassical transport losses and negligible bootstrap current in the long mean-free-path regime, good collisionless α-particle confinement in an equivalent reactor, and, as a technical aspect, good feasibility of the superconducting modular coils. W7-X will be heated by continuous electron cyclotron resonance heating and pulsed neutral beam injection and ion cyclotron resonance heating. The envisaged parameters are Te⩽10 keV, Ti⩽6 keV central densities ⩽3×1020 m−3 with an averaged 〈β〉⩽5%. Despite the complicated geometrical structure, all basic diagnostics are compatible with W7-X. Generally, diagnostic methods and applications in a stellarator are not different from those in tokamaks. However, special efforts are being made to equip the experiment with those diagnostics necessary to measure the quantities directly related with the optimization of the machine: the verification of the predicted magnetic topology and characterization of the configuration throughout the entire parameter range, the identification of equilibrium and stability, and the determination of the confinement properties. The article describes the strategy developed which assures that the detailed measurement needs of the W7-X experimental program can be met.
Impedance analysis of thickness-shear mode quartz crystal resonators in contact with linear viscoelastic media
Authors: Nwankwo E. , Durning C. J.
Published: July 2002

An electromechanical description of the impedance response of thickness-shear mode quartz crystal resonators in contact with linear viscoelastic media is presented. Explicit assignments for equivalent circuit parameters (resistances, inductances, and capacitances), are derived in terms of the properties of the resonant piezoid and damping fluid medium. The calculation shows that the effect of fluid elasticity is a reduction in both the equivalent resistance and inductance relative to a corresponding viscous bulk. The lower inductance is due to a reduced decay envelope for shear wave propagation into the liquid; in effect, a smaller mass of liquid is “dragged” by the crystal. The lower equivalent resistance is due to the combined effect of lower energy dissipation and enhanced recoverable energy storage within the mechanically active fluid layer near the crystal surface. Comparison of model predictions with published results on a variety of undamped, viscously damped, and viscoelastically damped crystals shows very good agreement.
Optical characterization of continuous compositional gradients in thin films by real time spectroscopic ellipsometry
Authors: Kim Sangbo , Collins R. W.
Published: July 2002

An analysis procedure for real time spectroscopic ellipsometry data is described that allows one to characterize compositional gradients in thin films prepared by continuously varying the deposition parameters. This approach provides the time evolution of (i) the instantaneous deposition rate, (ii) the surface roughness layer thickness, and (iii) the near-surface dielectric function and composition of the film with a depth resolution as high as 7 Å. We apply the analysis to obtain the depth profile of the relative void volume fraction for an amorphous silicon-carbon alloy film prepared by plasma-enhanced chemical vapor deposition.
Advances in experimental technique for quantitative two-dimensional dopant profiling by scanning capacitance microscopy
Authors: Zavyalov V. V. , McMurray J. S. , Williams C. C.
Published: July 2002

Several advances have been made toward the achievement of quantitative two-dimensional dopant and carrier profiling. To improve the dielectric and charge properties of the oxide–silicon interface, a method of low temperature heat treatment has been developed which produces an insulating layer with consistent quality and reproducibility. After a standard polishing procedure is applied to cross-sectional samples, the samples are heated to 300 °C for 30 min under ultraviolet illumination. This additional surface treatment dramatically improves dielectric layer uniformity, scanning capacitance microscopy (SCM) signal to noise ratio, and C–V curve flat band offset. Examples of the improvement in the surface quality and comparisons of converted SCM data with secondary ion mass spectrometry (SIMS) data are shown. A SCM tip study has also been performed that indicates significant tip depletion problems can occur. It is shown that doped silicon tips are often depleted by the applied SCM bias voltage causing errors in the SCM measured profile. Worn metal coated and silicided silicon tips also can cause similar problems. When these effects are tested for and eliminated, excellent agreement can be achieved between quantitative SCM profiles and SIMS data over a five-decade range of dopant density using a proper physical model. The impact of the tip size and shape on SCM spatial accuracy is simulated. A flat tip model gives a good agreement with experimental data. It is found that the dc offset used to compensate the C–V curve flat band shift has a consistently opposite sign on p- and n-type substrates. This corresponds to a positive surface on p-type silicon and to a negative surface on n-type silicon. Rectification of the large capacitance probing voltage is considered as a mechanism responsible for the apparent flat band shift of (0.4–1) V measured on the samples after heating under UV irradiation. To explain the larger flat band shift of (1–5) V, tip induced charging of water-related traps is proposed and discussed.
Deep levels caused by misfit dislocations in GaAsSb/GaAs heterostructures
Authors: Wosiński T. , Ma̧kosa A. , Figielski T. , Raczyńska J.
Published: July 2002

Two deep electron traps induced by lattice mismatch in relaxed GaAs1−xSbx layers (x=0% to 3%) grown by liquid phase epitaxy (LPE) on GaAs substrates have been revealed by means of deep-level transient spectroscopy. One of the traps, that shows nonstandard, logarithmic capture kinetics and whose energy level is tied to the valence-band edge, has been related to electron states associated with α dislocations. The other trap has been attributed to the EL2 defect and possible reasons of its unexpected formation in the LPE-grown layers are briefly discussed.
Microstructure of germanium quenched from the undercooled melt at high pressures
Authors: Zhang F. X. , Wang W. K.
Published: July 2002

The solidification behavior of germanium quenched from the melt at pressure of 2–7 GPa is presented. The solidification parameters of specimen were directly measured under high pressure. The microstructure of specimen was observed by electron microscopy. From the measured undercooling and average grain size, the interfacial free energy and activation energy of crystal growth under high pressures were estimated according to classical nucleation theory. Using high resolution transmission electron microscopy (HRTEM), crystals of sizes ranging from 5 to 100 nm surrounded by an amorphous matrix of germanium were found in those samples which quenched from deeply undercooled melt.
Huge birefringence in selectively oxidized GaAs/AlAs optical waveguides
Authors: Fiore A. , Berger V. , Rosencher E. , Laurent N. , Theilmann S. , Vodjdani N. , Nagle J.
Published: July 2002

Selective wet oxidation of AlAs was used to obtain huge birefringence in GaAs/AlAs optical waveguides. A single polarization waveguide was obtained by oxidizing an AlAs layer buried in a GaAs guiding layer. The TM mode is below cutoff due to the high index contrast between the layers. Applications to phase matching in nonlinear optical conversion are envisaged.
Characteristics of InGaN multi-quantum-well-structure laser diodes
Authors: Nakamura Shuji , Senoh Masayuki , Nagahama Shin-ichi , Iwasa Naruhito , Yamada Takao , Matsushita Toshio , Kiyoku Hiroyuki , Sugimoto Yasunobu
Published: July 2002

InGaN multi-quantum-well (MQW)-structure laser diodes fabricated from III–V nitride materials were grown by metalorganic chemical vapor deposition on (111) MgAl2O4 substrates. The laser consisted of a InGaN MQW, GaN optical guiding layers and AlGaN cladding layers. The observed stimulated emission was at a wavelength around 419 nm, with a threshold current of 320 mA (13 kA/cm2) and a threshold voltage of 28 V under pulsed current injection at room temperature. The stimulated emission also showed a strong transverse electric polarization. The beam full widths at half power for the parallel and perpendicular far-field radiation patterns were 5° and 17°, respectively.
Controlled growth of bulk bicrystals and the investigation of microstructure-property relations of YBa2Cu3Ox grain boundaries
Authors: Todt V. R. , Zhang X. F. , Miller D. J. , St. Louis-Weber M. , Dravid V. P.
Published: July 2002

A new method to prepare bicrystals with well defined planar interfaces in YBa2Cu3Ox (Y123) has been developed. The bicrystal misorientation is controlled by dual seeding using Nd1+xBa2−xCu3Oy single crystals. The grain boundary plane orientation is influenced by the positioning of the seeds and by control of the temperature gradient. The macro-, meso-, and microscopic planarity of the grain boundaries has been established by optical and electron microscopy. In addition, a difference in critical current density between the low (≤10°) and the high (≥20°) misorientation angle regime of nearly two orders of magnitude has been established in a series of [001]-tilt grain boundaries. Thus, this type of grain boundary may allow a less ambiguous interpretation of the relationship between microstructure and transport properties than is possible from bicrystal thin film boundaries by eliminating the potential variations in properties associated with a varying grain boundary plane.
Effects of interdiffusion on the luminescence of InGaAs/GaAs quantum dots
Authors: Leon R. , Kim Yong , Jagadish C. , Gal M. , Zou J. , Cockayne D. J. H.
Published: July 2002

Large energy shifts in the luminescence emission from strained InGaAs quantum dots are observed as a result of postgrowth annealing and also when raising the upper cladding layer growth temperatures. These blueshifts occur concurrently with narrowing (from 61 to 24 meV) of the full width at half-maxima for the emission from the quantum dot ensemble. These energy shifts can be explained by interdiffusion or intermixing of the interfaces rather than strain effects due to variations in capping layer thickness. Temperature behavior of the luminescence in annealed and nonannealed samples indicates a change in the shape and depth of the quantum dot confining potential. Quenching of the wetting layer luminescence after interdiffusion is also observed.
Ultraviolet-sensitive, visible-blind GaN photodiodes fabricated by molecular beam epitaxy
Authors: Van Hove J. M. , Hickman R. , Klaassen J. J. , Chow P. P. , Ruden P. P.
Published: July 2002

GaN p–i–n photovoltaic diode arrays were fabricated from epitaxial films deposited on sapphire by molecular beam epitaxy. Peak UV responsivity was 0.11 A/W at 360 nm, corresponding to 48% internal quantum efficiency. Visible rejection over 400–800 nm was 3–4 orders of magnitude. Typical pulsed time response was measured at 8.2 μs. Spectral response modeling was performed to analyze the photocurrent contributions from photogenerated carrier drift in the depletion region and from minority carrier diffusion in the p and n layers. With the model, a maximum internal quantum efficiency of 55% at 360 nm was calculated for the photovoltaic diode structure.
Observation of nanocrystalline diamond in diamondlike carbon films deposited at room temperature in electron cyclotron resonance plasma
Authors: Zarrabian M. , Fourches-Coulon N. , Turban G. , Marhic C. , Lancin M.
Published: July 2002

Diamondlike carbon (DLC) thin films have been prepared by chemical vapor deposition assisted by electron cyclotron resonance plasma at low pressure with radio frequency (rf) power applied to the substrate. The microstructure studies by transmission electron microscopy and electron energy loss spectroscopy show the existence of nanocrystalline diamond grains in DLC films prepared at 0.35 Pa. The cluster’s size varies from 4 to 30 nm with bias voltage (Vb) varying from −200 to −600 V. A phase transition from hexagonal to cubic diamond was also observed with increasing Vb.
Room temperature crystal structure and relaxor ferroelectric behavior of Pb0.5Ca0.5TiO3
Authors: Ranjan Rajeev , Singh Neelam , Pandey Dhananjai , Siruguri V. , Krishna P. S. R. , Paranjpe S. K. , Banerjee Alok
Published: July 2002

Superlattice reflections observed for the first time in the room temperature powder neutron diffraction patterns of (Pb0.5Ca0.5)TiO3 (PCT50) are shown to arise due to an orthorhombic distortion (space group Pbnm) resulting from tilted TiO6 octahedra and off-center location of Pb2+/Ca2+. From a study of the temperature dependence of the real and imaginary parts of the dielectric constant at various frequencies, PCT50 is found to be a relaxor ferroelectric. It is proposed that the structural frustration introduced by the tilted octahedra may be responsible for the relaxor behavior.
Surface melting on small particles
Authors: Peters Kevin F. , Chung Yip-Wah , Cohen Jerome B.
Published: July 2002

Surface melting on small particles was detected using x-ray diffraction from Pb crystallites of 50 nm diam in ultrahigh vacuum. Since the melting temperature depends on particle size, it can be difficult to distinguish between the melting and surface melting. A sharp crystallite size distribution is obtained by melting the smaller particles while a liquid skin forms on the remaining crystallites. When the sample is cooled slightly, the liquid particles remain liquid while the liquid skin on the crystallites solidifies. In situ x-ray diffraction reveals that the liquid skin thickness changes reversibly.
Effect of phonon bottleneck on quantum-dot laser performance
Authors: Sugawara Mitsuru , Mukai Kohki , Shoji Hajime
Published: July 2002

The effect of phonon bottleneck on quantum-dot laser performance is examined by solving the carrier-photon rate equations including the carrier relaxation process into the quantum-dot ground state. We show that the retarded carrier relaxation due to phonon bottleneck degrades the threshold current and the external quantum efficiency. We also show that quantum-dot lasers are quite sensitive to the crystal quality outside as well as inside quantum dots. Our results clarified that the relaxation lifetime should be less than about 10 ps to fully utilize the laser potential originating from the quantum-dot discrete energy states.
Quantum transport in metallic nanowires fabricated by electrochemical deposition/dissolution
Authors: Li C. Z. , Tao N. J.
Published: July 2002

A nonmechanical method for fabricating a metallic narrow constriction between two electrodes using electrochemical deposition is described. The width of the constriction can be adjusted by slowly dissolving metal atoms away or redepositing atoms onto the constriction which can be controlled flexibly by the electrodes’ potentials. Well-defined plateaus near the integer numbers of the conductance quantum have been observed in these constrictions at room temperature. Since no mechanical movements are involved, this method has the potential of fabricating nanoconstrictions with long term stability.
Transmission electron microscopy investigation of structural properties of self-assembled CdSe/ZnSe quantum dots
Authors: Kirmse H. , Schneider R. , Rabe M. , Neumann W. , Henneberger F.
Published: July 2002

CdSe quantum dots on ZnSe, grown by molecular beam epitaxy and formed during reorganization of an initially uniform film by thermal activation, are microstructurally elucidated in cross section and plan view, using transmission electron microscopy. In diffraction contrast, an almost uniform wetting layer is clearly visible. Dark contrast features with a distinctly larger extension into growth direction mark the location of quantum dots. Individual quantum dots can be identified in high-resolution imaging both by lattice expansion and contrasts arising from their strain fields. Plan-view images show the coexistence of two classes of quantum dots with an average lateral size of ⩽10 nm (area density 100 μm−2) and 10–50 nm (20 μm−2), respectively. The shape of the larger entities is pyramidlike.