bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of electronic temperature and distribution on the second-order surface nonlinear susceptibility of metals
Authors: Guo Chunlei , Rodriguez George , Hoffbauer Mark , Taylor Antoinette J.
Published: July 2002

We performed ultrafast, optical pump and second-harmonic-generation (SHG) probe measurements to study the effects of electronic temperature (Te) and distribution on the second-order nonlinear susceptibility [χ(2)] for a gold surface. These measurements clarify the physical origin of the recently observed power-law deviation of the SHG signal at high illuminating intensities. The study of SHG is extended to the high Te regime, and the results agree with a model calculation presented in this letter. The consistency between the experimental and theoretical results in this letter provides a uniform picture of the effects of the electronic temperature and distribution on χ(2).
Vertical p-type high-mobility heterojunction metal–oxide–semiconductor field-effect transistors
Authors: Chen Xiangdong , Ouyang Qiqing , Jayanarayanan Sankaran Kartik , Prins Freek E. , Banerjee Sanjay
Published: July 2002

We have fabricated a vertical p-channel metal–oxide–semiconductor field-effect transistor called high-mobility heterojunction transistor (HMHJT). Compared with a Si control device, reduced short channel effects, reduced floating body effect, and high drive current have been achieved with this device structure. A SiGe/Si heterojunction barrier at the source/bulk junction suppresses drain induced barrier lowering and bulk punchthrough, which are significant problems for sub-100 nm devices. A SiGe source also helps to reduce the charge built-up in the floating body. The higher mobility in a strained SiGe channel and the absence of a hetero-barrier between the source and channel result in higher drive current. The fabricated HMHJT has a 60% higher drive current at VDS=VGS−VT=−1.6 V, and a 70× lower off-state leakage current at VDS=−1.6 V and VGS=0.0 V, compared with the Si control device.
Spin-polarized scanning tunneling microscopy on half-metallic manganite thin film with half-metallic manganite tip
Authors: Akiyama R. , Tanaka H. , Matsumoto T. , Kawai T.
Published: July 2002

Spin-polarized tunneling was demonstrated on La0.7Sr0.3MnO3 epitaxial thin film with a La0.7Sr0.3MnO3-coated PtIr tip at room temperature. A magnetic-field-dependent gap structure was found in the differential tunneling spectra due to the spin-valve effect. The spatial mapping of the differential tunneling conductance shows strongly contrasting variations, which demonstrates the presence of magnetic domains with different spin alignments.
Hydrostatic pressure effect on photoluminescence from a GaN0.015As0.985/GaAs quantum well
Authors: Tsang M. S. , Wang J. N. , Ge W. K. , Li G. H. , Fang Z. L. , Chen Y. , Han H. X. , Li L. H. , Pan Z.
Published: July 2002

Photoluminescence from a GaN0.015As0.985/GaAs quantum well has been measured at 15 K under hydrostatic pressure up to 9 GPa. Both the emissions from the GaNAs well and GaAs barrier are observed. The GaNAs-related peak shows a much weaker pressure dependence compared to that of the GaAs band gap. A group of new peaks appear in the spectra when the pressure is beyond 2.5 GPa, which is attributed to the emissions from the N isoelectronic traps in GaAs. The pressure dependence of the GaNAs-related peaks was calculated using the two-level model with the measured pressure coefficients of the GaAs band gap and N level as fitting parameters. It is found that the calculated results deviate seriously from the experimental data. An increasing of the emission intensity and the linewidth of the GaNAs-related peaks was also observed and briefly discussed.
Microcavity-enhanced surface-emitted second-harmonic generation from 200 fs pulses at 1.5 μm
Authors: Ulmer Todd G. , Hanna Marc , Washburn Brian R. , Verber Carl M. , Ralph Stephen E. , SpringThorpe Anthony J.
Published: July 2002

We present theoretical results for short-pulse surface-emitted second-harmonic generation and show that significant cavity enhancement is possible in the femtosecond regime. Experimentally, we demonstrate enhanced surface-emitted second-harmonic generation using 200 fs fundamental pulses near 1.5 μm by use of a 5λSH/2n vertical microcavity in [211]-oriented AlGaAs waveguides. The microcavity height is minimized by allowing the distributed Bragg reflectors that define the vertical cavity to also serve as cladding layers for the fundamental waveguide, thereby increasing the resonance width and hence the overlap with the second-harmonic spectrum.
Growth, structure, and optical properties of carbon-reinforced silica fibers
Authors: Zhang Z. J. , Ajayan P. M. , Ramanath G. , Vacik J. , Xu Y. H.
Published: July 2002

We report the synthesis of carbon-reinforced silica fibers by methane exposure of metallocene-treated oxidized-Si(001) substrates at 1100 °C. The SiO2 cap layer transforms into silica fibers reinforced by glassy carbon in the core during methane exposure. High-resolution electron microscopy and spatially resolved spectroscopy measurements of the fibers reveal an amorphous structure without a hollow, and domains of glassy carbon embedded at the fiber core. The carbon-reinforced fibers are optically transparent and have an optical band gap of ≃3.1 eV. These fibers are organized in radial patterns that vary for different metallocene species. On nickelocene-treated substrates, the fibers originate from the circumference of the circular templates and grow outwards, forming radial patterns. On ferrocene-treated substrates, randomly oriented fibers grow within as well as slightly outside the perimeter of the templates, forming wreath-like patterns. Aligned growth of such fibers could be useful for fabricating optoelectronics devices and reinforced composites.
“Inverted hut” structure of Si–Ge nanocrystals studied by extended x-ray absorption fine structure method
Authors: Soo Y. L. , Kioseoglou G. , Huang S. , Kim S. , Kao Y. H. , Peng Y. H. , Cheng H. H.
Published: July 2002

Local structure around Ge in Si/Ge superlattices containing the “inverted hut” nanocrystals has been investigated by using the extended x-ray absorption fine structure (EXAFS) technique. In contrast to the usual nanometer-sized Ge “hut clusters” commonly grown on top of Si layers using the conventional Stranski–Krastanow self-organized growth mode, SiGe-alloy nanocrystals can be formed beneath the Ge wetting layer and grown into the Si layer in Si/Ge superlattices prepared in a low-temperature molecular beam epitaxy growth mode, and exhibit inverted hut nanocrystal structures regularly spaced along the Si/Ge interface. The EXAFS results obtained with varying Ge wetting layer thickness provide a direct evidence that intermixing of Ge and Si atoms takes place in a zone of about 1–3 monolayers on each side of the Si/Ge interface. The intermixing of constituent atoms allows a mechanism other than the usual formation of misfit dislocations to release the strain energy resulted from lattice mismatch between Si and Ge at the interface.
Etching trenches to effectively create electron quantum wires for single-electron-transistor applications
Authors: Fu Y. , Willander M. , Wang T. H.
Published: July 2002

By self-consistently solving two-dimensional Schrödinger and Poisson equations, we have studied electron quantum wires formed by etching two shallow trenches in GaAs/InGaAs/AlGaAs and GaAs/AlGaAs heterostructures for single-electron-transistor applications. Two-dimensional electron gases (2DEGs) form in the InGaAs layer in GaAs/InGaAs/AlGaAs and at the GaAs/AlGaAs interface in GaAs/AlGaAs heterostructure, the two etched trenches deplete the carriers of the 2DEGs underneath and thus effectively define an electron quantum wire. It has been determined that two etched trenches with a depth of 50 nm and a spatial separation of 250 nm are optimal to create an electron quantum wire in a GaAs/InGaAs/AlGaAs heterostructure with a cross section of 80×10 nm2 and a peak electron concentration of 1.5×1018 cm−3 at 4.2 K.
Current-driven insulator–conductor transition and nonvolatile memory in chromium-doped SrTiO3 single crystals
Authors: Watanabe Y. , Bednorz J. G. , Bietsch A. , Gerber Ch. , Widmer D. , Beck A. , Wind S. J.
Published: July 2002

Materials showing reversible resistive switching are attractive for today’s semiconductor technology with its wide interest in nonvolatile random-access memories. In doped SrTiO3 single crystals, we found a dc-current-induced reversible insulator–conductor transition with resistance changes of up to five orders of magnitude. This conducting state allows extremely reproducible switching between different impedance states by current pulses with a performance required for nonvolatile memories. The results indicate a type of charge-induced bulk electronic change as a prerequisite for the memory effect, scaling down to nanometer-range electrode sizes in thin films.
Structure, interface roughness, and growth mechanism of reactive deposition epitaxy of CoSi2 on Si(100) substrates
Authors: Sarkar D. K. , Rau I. , Falke M. , Giesler H. , Teichert S. , Beddies G. , Hinneberg H.-J.
Published: July 2002

Thin CoSi2 films have been grown on Si(100) substrates using the relative deposition epitaxy method. The structure of the silicide films have been analyzed using x-ray diffraction, transmission electron microscopy (TEM), and Rutherford backscattering spectrometry and channeling, and the interface roughness of the CoSi2/Si(100) is analyzed using specular x-ray reflectivity and cross-sectional TEM. The structure and interface roughness of CoSi2/Si(100) is found to be dependent on the substrate temperature. Highly epitaxial CoSi2 with minimum interface roughness is obtained when the film is grown at substrate temperatures around 900 K. The observed interface roughness is a parabolic function of temperature. The achievement of the best silicide at a substrate temperature around 900 K is explained on the basis of the instantaneous diffusion of Co through growing CoSi2.
Fast modulation scheme for a two laterally coupled laser diode array
Authors: Carpintero G. , Lamela H. , Leonés M. , Simmendinger C. , Hess O.
Published: July 2002

The present letter reports a modulation scheme that takes advantage of the unique characteristics of a two laterally coupled laser diode (also known as twin stripe array) to overcome the limit on the modulation imposed by the laser’s relaxation oscillation frequency. Through the use of the rate equation description of the device we uncover the device dynamics behind the modulation scheme generating 35 ps (full width at half maximum) laser pulses at 8 Gb/s modulation rate. Our scheme relies on the fast dynamics of the phase difference, controlled by means of the current injection on each stripe.
Anomalously high thermoelectric figure of merit in Bi1−xSbx nanowires by carrier pocket alignment
Authors: Rabina Oded , Lin Yu-Ming , Dresselhaus Mildred S.
Published: July 2002

Electronic transport calculations were carried out for Bi1−xSbx nanowires (0⩽x⩽0.30) of diameters 10 nm⩽dW⩽100 nm at 77 K. A band structure phase diagram was generated, showing the dependence of the relative band edge positions on diameter and composition. Calculations of the thermoelectric figure-of-merit (ZT) predict that the performance of Bi1−xSbx nanowires is superior to that of Bi nanowires and to that of the bulk alloy. An exceptionally high value of ZT for p-type nanowires at 77 K was found for dW∼40 nm and x∼0.13, which is explained by the coalescence in energy of up to ten valence subband edges to maximize the density-of-states at the Fermi energy.
Scaling behavior in adsorption on bivariate surfaces and the determination of energetic topography
Authors: Bulnes F. , Ramirez-Pastor A. J. , Zgrablich G.
Published: July 2002

Adsorption of particles with repulsive nearest-neighbor interactions is studied through Monte Carlo simulation on bivariate surfaces characterized by patches of weak and strong adsorbing sites of size l. Patches can be either arranged in a deterministic chessboard structure or in a random way. Quantities are identified which scale obeying power laws as a function of the scale length l. Consequences of this finding are discussed for the determination of the energetic topography of the surface from adsorption measurements.
Image capture array with an organic light sensor
Authors: Street R. A. , Mulato M. , Lau R. , Ho J. , Graham J. , Popovic Z. , Hor J.
Published: July 2002

A large-area image capture device using an organic sensor is reported. The 512×512 pixel array, with 100×100 micrometer pixel size, combines amorphous silicon matrix addressing with a continuous organic sensor. The bilayer sensor comprises a tetraphenyldiamine hole transport layer on top of a benzimidazole perylene generator layer. This combination provides high sensitivity across the visible with low dark current. We present imaging properties and in particular show that the lateral charge transport between pixels is small, and that the effective fill factor is ∼90%. X-ray imaging with a phosphor converter is demonstrated.
A new Monte Carlo simulation approach for the prediction of sorption equilibria of oligomers in polymer melts: Solubility of long alkanes in linear polyethylene
Authors: Zervopoulou E. , Mavrantzas V. G. , Theodorou D. N.
Published: July 2002

This work is concerned with the study of the solubility of small alkanes and ethylene oligomers in polyethylene melts of prescribed molecular weight distribution. The infinite dilution case is examined first, using Widom test particle insertion method. Simulation estimates of Henry’s constant for alkanes from C4 to C8 in a melt with average chain length C78 at T=450 K and P=1 atm are compared to older simulation results and to experimental data and are found to be adequately accurate. Cases where the oligomer is dissolved in the melt at finite concentrations are of greater practical interest. To address these cases, two different methods are implemented. The first one employs Monte Carlo (MC) simulations in the [f1NpnPTμ*] statistical ensemble, in which conventional insertion and deletion moves are used to equilibrate the concentration of the small molecules in the PE melt. It is observed that this technique leads to satisfactory results only for short alkanes, of length up to C10. Configurationally-biased insertions and deletions of alkane molecules in dense polymer phases become extremely difficult and time-consuming as the length of the inserted molecule is increased, rendering equilibration within reasonable CPU time practically impossible. To study the solubility of longer alkanes and oligomers in a polyethylene melt two novel MC moves are introduced: scission of a polymer chain to create an alkane molecule and a shorter chain, and fusion of a polymer chain and an alkane to create a longer polymer. For implementing these two new moves, a new statistical ensemble, the [f 1′Npn0PTμ*], is formulated. The new moves lead to extremely fast equilibration of the concentration of alkane molecules in the polymer melt and allow studying the solubility of long oligomers in the polymer matrix at high pressures. Their advantage lies exactly in the fact that they generate alkane molecules from already existing polymer chains, thereby eliminating the need for insertion and deletion moves. Results are presented from both MC methods for the sorption isotherms of C5, C10, and C20 in molten linear polyethylene and their dependence on the average chain length of the polymer matrix. The structure of the polymer–alkane mixtures and the volume changes (swelling) of the polyethylene matrix upon sorption are also examined. Predictions obtained from the atomistic simulations are compared extensively to available experimental data and to calculations based on the Flory–Huggins theory and the SAFT equation of state (EoS). The agreement between simulation and experiment is seen to be very good, demonstrating the value of the new simulation scheme designed and implemented in the course of this work.
High p-type doping of ZnBeSe using a modified delta-doping technique with N and Te
Authors: Guo S. P. , Lin W. , Zhou X. , Tamargo M. C. , Tian C. , Kuskovsky I. , Neumark G. F.
Published: July 2002

High crystalline quality ZnBeSe epilayers were grown nearly lattice matched to GaAs (001) substrates by molecular beam epitaxy with a Be–Zn co-irradiation. A (1×2) reflection high energy electron diffraction pattern was observed after the Be–Zn co-irradiation of the GaAs (2×4) surface. A high p-type doping level of 1.5×1018 cm−3 was achieved for (N+Te) triple-delta doping (δ3 doping) of ZnBeSe epilayers, whereby three adjacent δ layers of N and Te were deposited in each doping cycle. X-ray diffraction measurements reveal that (N+Te) δ3-doped ZnBeSe samples with a Te concentration of about 0.5% remain of very high crystalline quality with an X-ray rocking curve linewidth of 51 arcsec. Low temperature photoluminescence measurements show some emission peaks related to Te2 clusters and/or Ten⩾3 clusters.
Lateral heterogeneity in the surface composition after laser processing of Ti/Si interface contaminated with oxygen
Authors: Larciprete R. , Danailov A. , Barinov A. , Casalis L. , Gregoratti L. , Kiskinova M.
Published: July 2002

The lateral variations in the surface composition of an oxygen-contaminated Ti/Si(001) interface processed by pulsed laser annealing were investigated by synchrotron radiation x-ray photoemission spectromicroscopy. It has been found that SiO2, which segregates on top of the Ti silicide film, appears only in a circular edge region of the laser spots and is completely absent in the hotter internal area, where SiO evaporation has occurred. The results demonstrate that the temperature gradient within the laser-irradiated area can affect substantially the lateral homogeneity of the fabricated interfaces, an important issue for microdevice technology.
Localized excitons in an In0.06Ga0.94N multiple-quantum-well laser diode lased at 400 nm
Authors: Chichibu Shigefusa F. , Azuhata Takashi , Sota Takayuki , Mukai Takashi
Published: July 2002

An InGaN multiple-quantum-well laser diode wafer that lased at 400 nm was shown to have InN mole fraction x of only 6% and 2% in the wells and barriers, respectively. The Stokes-like shift (SS) and localization depth E0 were estimated to be 49 and 35 meV at 300 K although the compositional fluctuation in the well was as small as 1% or less. Since the well thickness fluctuation is insufficient to reproduce the SS or E0, the quantum-well exciton localization is considered to be an intrinsic phenomenon in InGaN, which is due to the large band-gap bowing and In clustering. The spontaneous emission was thus assigned as being due to the recombination of excitons localized at the exponential tail-type potential minima in the density of states. However, shallow and low-density localized states are leveled by injecting high-density carriers under the lasing conditions.
On the mechanism of the cubic phase formation in the boron nitride thin-film systems
Authors: Khizroev Sakhrat , Litvinov Dmitri
Published: July 2002

This work addresses the issue of cubic phase formation in boron nitride thin-film systems. The presented data suggest strain-induced formation of cubic phase. The compliant nature of the turbostratic boron nitride buffer layer is shown to be an essential factor for cubic phase nucleation. Two distinct regimes of cubic phase formation are observed. First, a cubic boron nitride seed layer is formed on top of the turbostratic boron nitride buffer layer under nitrogen ion irradiation of the growth surface. This is followed by the growth mode of the cubic phase, which requires a different set of growth conditions. The role of nitrogen ion irradiation in two deposition regimes is discussed.
Detailed atomistic molecular-dynamics simulation of the orthorhombic phase of crystalline polyethylene and alkane crystals
Authors: Mavrantza Ioanna-Elisavet , Prentzas Dimitris , Mavrantzas Vlasis G. , Galiotis Costas
Published: July 2002

Results are presented for the structural and thermodynamic properties of the orthorhombic phase of crystalline polyethylene (PE) from detailed atomistic molecular-dynamics (MD) simulations in the NPT statistical ensemble. Two different PE crystal systems have been investigated: (a) paraffins consisting of an odd number of carbon atoms such as C23H48 and (b) infinite length PE monocrystals, at temperatures ranging from T=150 to 298 K. The results support the experimentally verified chain herringbone arrangement in the crystal for both systems. The infinite-chain systems, however, are characterized systematically by a higher density and a more regular structure, particularly at low temperatures, where all chains are found in their all-trans configuration. At temperatures close to 273 K, the finite-chain length crystals start presenting gauche defects whose number increases rapidly upon further increasing the temperature. No such gauche defects are observed for the infinite-chain length crystals at the range of temperatures (T<300 K) investigated. In a second step, the atomistic, crystalline PE configurations accumulated during the NPT MD runs are analyzed within the quasi-harmonic approximation to obtain their infrared (IR) vibrational spectra. IR spectra of the infinite-chain length crystals obtained at different temperatures show a shift of the peaks of the CH2 symmetric and asymmetric stretching bands to lower wave numbers with increasing temperature. No shift is observed for the vibrational wave numbers of the other bands. The wave number shift of the CH2 symmetric and asymmetric stretching bands is found to be related linearly to temperature with a slope equal to −0.033 cm−1/K for both bands, in approximate agreement with experiment.
Electronic properties of hard and soft ions in solution: Aqueous Na+ and Ag+ compared
Authors: Vuilleumier Rodolphe , Sprik Michiel
Published: July 2002

The electronic structure of model aqueous solutions of Na+ and Ag+ is investigated using ab initio molecular-dynamics methods. We compute a number of electronic response coefficients in solution, such as global hardness and nuclear Fukui functions. The nuclear Fukui functions are found to be particularly sensitive to the chemical nature of the component species giving for Ag+ a susceptibility 3.5 times the value for a H2O molecule while the result for Na+ is more than a factor of 4 smaller compared to a solvent molecule. The electronic structure of the solution is further characterized by construction of effective molecular orbitals and energies. This analysis reveals that the effective highest occupied molecular orbital (HOMO) of the hard cation, Na+, remains buried in the valence bands of the solvent, whereas the HOMO of Ag+ is found to mix with the lone pair electrons of its four ligand H2O molecules to form the (global) HOMO of the solution. This observation, highlighting the importance of the electronic structure of the solvent, is used to rationalize the results for the electronic response.
Liquid crystal phase formation for the linear tangent hard sphere model from Monte Carlo simulations
Authors: Vega Carlos , McBride Carl , MacDowell Luis G.
Published: July 2002

Monte Carlo simulations have been performed for the linear tangent hard sphere model. The models considered in this work consisted of m=3, 4, 5, 6, and 7 monomer units. For the models m=3 and m=4 we find an isotropic fluid and an ordered solid. For the m=5 model we find the sequence of phases isotropic–nematic–smectic A on compression, and the sequence solid–smectic A–isotropic on expansion. We suggest that the nematic phase for this model is meta stable. For the model m=6 we observe the phase sequence isotropic–nematic–smectic A on compression, and the sequence ordered solid–smectic A–nematic–isotropic on expansion. We observe a similar sequence on expansion of the m=7 model. The results for the m=7 model are in good agreement with those of Williamson and Jackson [J. Chem. Phys. 108, 10294 (1998)]. It was suggested by Flory [Proc. R. Soc. London, Ser. A 234, 73 (1956)] that liquid crystal phases could exist for length to breadth ratios ⩾5.437, i.e., m⩾6. In this work we place the lower bound at m⩾5.
Large eddy simulation wall-modeling based on suboptimal control theory and linear stochastic estimation
Authors: Nicoud F. , Baggett J. S. , Moin P. , Cabot W.
Published: July 2002

The cost of large eddy simulation (LES) in the near-wall region of attached turbulent boundary layers scales as the square of the friction Reynolds number, thus limiting LES to moderate Reynolds numbers. Wall stress boundary conditions are frequently used to alleviate this resolution requirement, but commonly used models are shown to perform poorly at high Reynolds numbers even in turbulent channel flow. Techniques from optimal control theory are used to find wall stresses that yield much better results in turbulent channel flow at high Reynolds numbers than existing models even on extremely coarse grids. In this approach, a suboptimal control strategy is used in which the objective is to force the outer LES towards a desired solution by using the wall stress boundary conditions as control. The suboptimal wall stresses are not necessarily physical, rather they are whatever is necessary to overcome the numerical and modeling errors present in the near-wall region to yield the correct mean velocity profile. Furthermore, the suboptimal control strategy generates reference data for comparing and deriving new wall models. Using linear stochastic estimation it is shown that the dynamically relevant part of the suboptimal wall stresses can be predicted from the local velocity field. A wall model derived from linear stochastic estimation yields good mean flow predictions in LES of turbulent channel flow on a 323 uniform grid for friction velocity Reynolds numbers from 640 to 20 000.
Enhanced As–Sb intermixing of GaSb monolayer superlattices in low-temperature grown GaAs
Authors: Chaldyshev V. V. , Bert N. A. , Musikhin Yu. G. , Suvorova A. A. , Preobrazhenskii V. V. , Putyato M. A. , Semyagin B. R. , Werner P. , Gösele U.
Published: July 2002

As–Sb compositional intermixing was studied by transmission electron microscopy (TEM) in GaAs films grown by molecular-beam epitaxy at low temperature (LT) and δ doped with antimony. The TEM technique was calibrated by imaging the as-grown films with δ layers containing various amounts of Sb. The calibration allowed us to deduce the effective As–Sb interdiffusion coefficient from apparent thickness of the Sb δ layers in the films subjected to isochronal anneals at 400–600 °C. The As–Sb intermixing in LT GaAs was found to be much enhanced when compared to conventional material. Its temperature dependence yields a diffusion coefficient of DAs–Sb=2×10−14 exp(−0.62±0.15 eV/kt) cm2 s−1. Since the kick-out mechanism operating under equilibrium conditions is valid for As–Sb interdiffusion in GaAs, the enhanced intermixing was attributed to an oversaturation of arsenic self-interstitials in the LT GaAs films. The effective activation energy for As–Sb interdiffusion in LT GaAs seems to be reasonably close to the migration enthalpy of As interstitials, whereas their concentration was roughly estimated as 1018 cm−3.
Time-dependent density functional theory for molecules in liquid solutions
Authors: Cossi Maurizio , Barone Vincenzo
Published: July 2002

A procedure based on the polarizable continuum model (PCM) has been applied to reproduce solvent effects on electronic spectra in connection with the time-dependent density functional theory (TD-DFT). To account for solute-solvent interactions, a suitable operator has been defined, which depends on the solute electronic density and can be used to modify the TD-DFT equations for the calculation of molecular polarizabilities and of electronic transition energies. The solute-solvent operator has been derived from a PCM approach depending on solute electrostatic potential: Recently, it has been shown that such an approach also provides an excellent treatment of the solute electronic charge lying far from the nuclei, being particularly reliable for this kind of applications. The method has been tested for formaldehyde in water and in diethyl-ether, and then applied to the calculation of solvent effects on the n→π* transition of diazabenzenes in different solvents. The computed transition energies are in fairly good agreement with experimental values.
Near-field infrared microscopy with a transient photoinduced aperture
Authors: Simanovskii D. , Palanker D. , Cohn K. , Smith T.
Published: July 2002

We report a method of near-field infrared microscopy with a transient optically induced probe. Photoinduced reflectivity in semiconductors is used to generate a relatively large transient mirror with a small aperture (infrared probe) in its center. Properties of this probe have been studied and first images obtained using the technique are presented. Resolution better than λ/5 at 6.25 μm is demonstrated. Among the advantages of this technique are high optical throughput of the probe, ease in simultaneous visible imaging, and a high scanning rate limited primarily by the pulse repetition rate of the laser system.
GaN
metal–semiconductor–metal photodetectors grown on lithium gallate substrates by molecular-beam epitaxy
Authors: Seo S. W. , Lee K. K. , Kang Sangbeom , Huang S. , Doolittle William A. , Jokerst N. M. , Brown A. S.
Published: July 2002

The growth, fabrication, and characterization of ultraviolet metal–semiconductor–metal (MSM) GaN photodetectors grown on LiGaO2 by molecular-beam epitaxy are reported herein. GaN/LiGaO2 material with dislocation densities of approximately 108 cm−2 and x-ray diffraction (00.4) full width at half maximum of 75 arcsec results in MSMs showing high responsivity, 0.105 A/W, at a reverse bias voltage of 20 V at 308 nm, and low dark currents of 7.88 pA at a 60 V reverse bias. Given the etch selectivity of the GaN/LiGaO2 system and the excellent performance of these devices, GaN device integration onto alternative substrates appears promising.
Continuum quantum systems as limits of discrete quantum systems. III. Operators
Authors: Barker Laurence
Published: July 2002

Convergence of a “discrete” operator to a “continuum” operator is defined. As examples, the circular rotor, the one-dimensional box, the harmonic oscillator, and the fractional Fourier transform are realized as limits of finite-dimensional quantum systems. Limits, thus defined, preserve algebraic structure. The results prepare for a sequel in which some affine canonical transforms will be “discretized.”
Grapevine-like growth of single walled carbon nanotubes among vertically aligned multiwalled nanotube arrays
Authors: Cao Anyuan , Zhang Xianfeng , Xu Cailu , Liang Ji , Wu Dehai , Chen Xihua , Wei Bingqing , Ajayan P. M.
Published: July 2002

Large arrays of self-oriented, multiwalled carbon nanotubes (MWNTs) have been obtained by chemical vapor deposition. It has so far been impossible to translate this idea for growing single walled nanotubes (SWNTs) but we show here that oriented growth of SWNTs among vertically aligned MWNT arrays can be realized by the catalytic pyrolysis of ferrocene and xylene. The MWNTs act as the support structures for SWNTs, forcing them to grow upward steadily, like grapevines. The growth of vertically aligned SWNT over large areas brings about the possibility of exploring their properties in select configurations.
Interface control of Bi4Ti3O12 film growth on Si(100) by use of an ultrathin silicon oxynitride buffer layer
Authors: Rokuta E. , Choi J.-H. , Hotta Y. , Tabata H. , Kobayashi H. , Kawai T.
Published: July 2002

Electrical properties of Bi4Ti3O12 (BiT) films on Si(100) were improved due to insertion of silicon oxynitride (SiON) buffer layers with thicknesses of 1–2 nm. Capacitance–voltage measurements indicated that the improvement was largely attributable to better Si interface properties rather than to the difference of the BiT film quality. By means of x-ray photoelectron spectroscopy and high-resolution transmission microscopy, the Si interfaces of the specimens with and without the SiON buffer layers were investigated. Consequently, we found that a postannealing treatment at 680 °C inevitably resulted in nonuniform growth of Si oxide layers at the Si interface of the specimen without the SiON buffer layer, and that the layer thickness mounted to approximately 10 nm. In contrast, 1–2-nm-thick SiON buffer layers terminated the growth of the additional oxide layer of less than about 3 nm, and the resulting Si oxide layers were quite uniform.
Blue InGaN/GaN multiple-quantum-well optically pumped lasers with emission wavelength in the spectral range of 450–470 nm
Authors: Yablonskii G. P. , Lutsenko E. V. , Pavlovskii V. N. , Marko I. P. , Gurskii A. L. , Zubialevich V. Z. , Mudryi A. V. , Schön O. , Protzmann H. , Lünenbürger M. , Schineller B. , Heuken M. , Kalisch H. , Heime K.
Published: July 2002

Optically pumped lasing in the wavelength range of 450–470 nm in InGaN/GaN multiple-quantum-well heterostructures grown by metalorganic vapor phase epitaxy was achieved and investigated. The energy and power per pulse of the laser were 80 nJ and 10 W correspondingly for one facet at room temperature. The far-field patterns of the laser emission consisted of three light spots near the angles of +30°, −15°, and −45°. The highest operating temperature was 450 K. The photoluminescence and photoluminescence excitation spectrum structures suggest that the quantum dots inside the quantum wells are involved in the recombination mechanism.
Independently addressable subarrays of silicon microdischarge devices: Electrical characteristics of large (30×30) arrays and excitation of a phosphor
Authors: Park S.-J. , Eden J. G. , Chen J. , Liu C.
Published: July 2002

Large arrays (up to 30×30) of microdischarge devices having separately addressable subarrays have been fabricated in Si and operated continuously in Ne, Ne/Ar, and Ne/Xe gas mixtures at pressures up to 800 Torr. Eight 3×3 arrays fabricated on the same substrate operate simultaneously at voltages as low as 210 V in 400 Torr of Ne and exhibit lifetimes beyond 19 h, or approximately 1 order of magnitude larger than those for earlier arrays in which all devices have a common anode. Four 15×15 arrays have also been tested and, when operated in Ne/Ar or Ne/10% Xe gas mixtures, generate intense fluorescence in the green from a phosphor over an area of 16 mm2.
Stimulated-emission phenomena from InGaN/GaN multiple-quantum wells grown by plasma-assisted molecular-beam epitaxy
Authors: Shen X. Q. , Shimizu M. , Okumura H. , Sasaki F.
Published: July 2002

Time-resolved photoluminescence measurements at 5 K were performed to study the optical properties of InGaN/GaN multiple-quantum wells (MQWs) grown by plasma-assisted molecular-beam epitaxy (rf-MBE). Stimulated-emission (SE) phenomena by optical pumping were observed under the high-excitation density from the InGaN/GaN MQW samples with the In composition varying from 0.04 to 0.16. It was found that the threshold density for SE phenomena strongly depended on the In composition, where the lowest threshold density was 69 μJ/cm2 from our samples. Our results show a potential of rf-MBE technique for the future optical device applications.
Pulse-induced switches in a Josephson tunnel stacked device
Authors: Pepe G. P. , Peluso G. , Valentino M. , Barone A. , Parlato L. , Esposito E. , Granata C. , Russo M. , De Leo C. , Rotoli G.
Published: July 2002

Pulse-activated transitions from the metastable to the running state and vice versa have been observed in a stacked double tunnel Nb-based Josephson system. Experimental results are compared with numerical simulations based on the Sine–Gordon model of the stacked junctions by injecting pulses with variable amplitude in one of the junctions of the stack, and observing the voltage response of the other junction. Both experimental and numerical results show the possibility to induce both direct and back-switching transitions from the metastable to the running state simply by changing the amplitude of the electronic pulses injected across the stack device.
Evidence for the precursors of nitrided silicon in the early stages of silicon oxynitridation in N2:N2O atmosphere
Authors: Cerofolini G. F. , Camalleri M. , Galati C. , Lorenti S. , Renna L. , Viscuso O. , Condorelli G. G. , Fragalà I. L.
Published: July 2002

Nitridation of hydrogen-terminated silicon with N2:N2O has been studied by x-ray photoemission spectroscopy. Our analysis has given evidence that the broad N(1s) peak at 398–399 eV, usually reported in the literature, is preceded by the formation of a narrow peak at 397.5 eV, attributed to the moiety Si3N in which silicon is only marginally oxidized, and two other peaks at 400.0 eV and 401.5 eV, attributed to the moieties Si2NOSi and SiNO, respectively.
Irradiation effects in InGaAs/InAlAs high electron mobility transistors
Authors: Jackson E. M. , Weaver B. D. , Shojah-Ardalan S. , Wilkins R. , Seabaugh A. C. , Brar B.
Published: July 2002

The radiation tolerance of high electron mobility transistors (HEMTs) based on InGaAs/InAlAs lattice matched to InP has been studied. At low fluences of 3 MeV He+ ions, the only effect is a reduction in the leakage currents. At higher fluences, the drain current decreases, the threshold voltage increases toward zero, and the transconductance decreases. These results are consistent with increased trapping in the donor layer and increased scattering in the channel layer. Radiation-induced increases in the threshold voltage occur three to nine times more slowly here than in GaAs/AlGaAs HEMTs, indicating high radiation tolerance.
Scanning tunneling microscopic study of boron-doped silicon nanowires
Authors: Ma D. D. D. , Lee C. S. , Lee S. T.
Published: July 2002

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements have been performed on boron-doped and undoped silicon nanowires (SiNWs). STM images clearly showed the presence of nanoparticle chains and nanowires in the B-doped SiNWs sample. Clear and regular nanoscale domains were observed on the SiNW surface, which were attributed to boron-induced surface reconstruction. STS measurements have provided current–voltage curves for SiNWs, which showed clearly enhancement in electrical conductivity by boron doping.
5-ps, 10-GHz pulse generation from an all-optical semiconductor switch embedded in a ring cavity
Authors: Ueno Yoshiyasu , Nakamura Shigeru , Tajima Kazuhito
Published: July 2002

We observed spontaneous generation of nearly transform-limited pulses (5 ps, 10 GHz, 1558 nm) from a semiconductor ring oscillator. This oscillator is completely different from conventional mode-locked lasers in that the pulse width is passively determined by a split delay time inside its ring cavity. This is because the ring cavity includes an all-optical semiconductor switch whose window width is determined by the split delay time.
Detection of C60 using surface-enhanced Raman scattering from metal-coated periodic structures
Authors: Grebel H. , Iqbal Z. , Lan A.
Published: July 2002

Surface-enhanced Raman scattering from C60 deposited on ordered arrays of silver- and gold-coated silica spheres have been measured. Selective enhancement of the Raman-active modes was exhibited in backscattering at surface plasmon resonance conditions. The advantage of signal enhancement at excited (scattered) wavelengths for subwavelength surface plasmons was demonstrated for this photosensitive molecular solid.
Asymmetry in negative differential resistance driven by electron–electron interactions in two-site molecular devices
Authors: Han J. E. , Crespi Vincent H.
Published: July 2002

We demonstrate that Coulomb interactions within the Hartree–Fock approximation can induce a strong asymmetry in the negative differential resistance of a two-site resonant molecular device. This sharp cutoff at high voltage is generic and independent of the detailed single-particle electronic structure of the system. The intersite coupling between distinct molecular subunits controls a transition from staircase to resonant current–voltage characteristics. These studies clarify the relationship between weak screening and strong nonlinearity.
Influence of tensile and compressive strain on the band gap energy of ordered InGaP
Authors: Novák J. , Hasenöhrl S. , Alonso M. I. , Garriga M.
Published: July 2002

The band gap energy of ordered and strained InxGa1−xP as a function of ternary composition was studied. Epitaxial growth using a metalorganic vapor phase epitaxy technique at a reactor pressure of 20 mbar and Tg=580 °C allowed us to prepare a set of samples with nearly constant ordering parameter η. Optical measurements were performed at room temperature using a rotating polarizer ellipsometer with a spectral energy range 1.4–5.1 eV. Comparing the experimental data with the theory, we have shown that the band gap energy Eg dependence on composition closely follows the prediction of Wei and Zunger [S. Wei and A. Zunger, Phys. Rev. B 49, 14337 (1994)]. This prediction is more valid as the commonly used parabolic interpolation of Eg between InP and GaP values.
Programmable organic light-emitting devices
Authors: Wu C.-C. , Chen C.-W. , Lin Y.-T. , Yu H.-L. , Hsu J.-H. , Luh T.-Y.
Published: July 2002

In this letter, we report a promising type of electrically programmable, i.e., reconfigurable, organic light-emitting devices (OLEDs) incorporating a thin carrier-blocking layer as the sacrificial fusing layer. In such devices, the carrier-blocking layer has a lower glass transition temperature than neighboring layers. By raising the internal temperature of the device above the transition temperature of the carrier-blocking layer with a large enough current, interdiffusion between organic layers could occur through such a layer. As a consequence, neighboring layers are fused and a new path for carrier transport is formed, bypassing the carrier-blocking property and altering the device characteristics. A device that emits blue light as fabricated but can be transformed into a green-emitting one is demonstrated. Such a type of device may be used for color pixels in OLED displays, user-programmable OLED applications, and nonvolatile memory devices.
Temporal measurement on and using pulses from spectrally narrowed emission in styrylpyridinium cyanine dye
Authors: Dharmadhikari Aditya K. , Bhowmik Achintya K. , Ahyi Ayayi C. , Thakur Mrinal
Published: July 2002

Highly efficient spectrally narrowed emission (SNE) was observed in the solution of strylpyridinium cyanine dye (SPCD) pumped by fundamental and second harmonic of a picosecond Nd:YAG laser in two separate arrangements. A highly directional emission was observed in both the pumping arrangements without incorporating any mirrors. The pulse duration of the SNE was measured by background free SHG intensity autocorrelation technique. The measured duration of the pulses was 40 ps. These pulses, having a spectral linewidth of 10 nm (full width at half maximum), were used as a probe to measure the transient changes in the transmission in SPCD solution using a pump–probe setup. The transient optical transmission indicated a gain at the overlap and no gain was observed beyond a delay of 40 ps.
Enabling electron diffraction as a tool for determining substrate temperature and surface morphology
Authors: LaBella V. P. , Bullock D. W. , Emery C. , Ding Z. , Thibado P. M.
Published: July 2002

The reconstruction transitions for the GaAs(001) surface have been identified as a function of the band gap-derived substrate temperature and As4 beam equivalent pressure. Surface morphology measurements using in situ scanning tunneling microscopy reveal that the surface spontaneously forms a random distribution of two-dimensional islands. The onset of island formation is coincident with the reflected high-energy electron diffraction pattern changing from the β to α subphase of the (2×4) reconstruction. An electron diffraction-based method for determining the substrate temperature and engineering the surface morphology with a desired amount of roughness is presented.
H–Si doping profile in GaAs by scanning tunneling microscopy
Authors: Grandidier B. , Silvestre S. , Nys J. P. , Mélin T. , Bernard D. , Stiévenard D. , Constant E. , Chevallier J.
Published: July 2002

Hydrogen incorporation in n-type Si-doped GaAs layers results in the neutralization of the active dopants and a change of the conductivity along the growth direction. To characterize the active dopant concentration of doped GaAs layers containing hydrogen, we have used secondary ion mass spectroscopy and cross-sectional scanning tunneling microscopy. Spectroscopic measurements are performed as well as conductance images to visualize the variation of the conduction band-edge position. Such a variation, which is related to the concentration of Si–H complexes, allows the determination of the doping profile.
Degradation in metal-oxide-semiconductor structure with ultrathin gate oxide due to external compressive stress
Authors: Hong Chao-Chi , Hwu Jenn-Gwo
Published: July 2002

The effect of external stress on metal-oxide-semiconductor (MOS) structure with ultrathin gate oxide (∼1.5 nm) was studied. J–V characteristics of fresh and stressed samples revealed that the tensile stress had little effect on J–V curves, whereas the compressive stress obviously increased the leakage current by about several hundred in percentages with respect to the fresh sample, in both positive and negative gate biases. This increase in leakage current was suggested to be attributed to the increase of interface states and silicon bulk traps under external compressive stress in the MOS device with an inherent tensile stressed silicon. In addition, we also found that once the device was damaged by the previously applied compressive stress, the second applied compressive stress of the same magnitude would not create more damage unless the device was breakdown.
Thermal carbonization of porous silicon surface by acetylene
Authors: Salonen J. , Laine E. , Niinistö L.
Published: July 2002

Three different thermal carbonization processes of porous silicon (PS) surface by acetylene occurring between 400 and 950 °C were observed by in situ measurements. The peak temperature of oxidation shifts from 300 to 600 °C and the activation energy from 130 to 500 kJ/mole in the samples treated at 900 °C compared to untreated PS. Using Fourier transform infrared spectroscopy (FTIR), the bonding configurations associated with the temperature dependent processes were found. In the samples treated at or below 600 °C, traces of C–Hx bonds were clearly seen, but above 600 °C the FTIR spectra started resemble those of the SiC. The graphitization process was found to take place on the surface of PS samples treated above 800 °C. By changing the time between the C2H2 flush and thermal treatment, the graphitization can be controlled and even avoided. The weight increase was observed to be nearly proportional to the reduction of the surface area while the decrease in the surface area due to the thermal carbonization was found to be small. The origin of this phenomenon and the effects of other preparation parameters have been discussed.
Low-noise magnetometers with asymmetric superconducting quantum interference devices
Authors: Testa G. , Granata C. , Di Russo C. , Pagano S. , Russo M. , Sarnelli E.
Published: July 2002

Fully integrated low-critical-temperature magnetometers based on asymmetric superconducting quantum interference devices have been designed, fabricated, and characterized. The large value of the flux to voltage transfer coefficient, typical of such configurations, allow a direct coupling to an external preamplifier without the need of an impedance matching flux transformer or additional positive feedback circuitry. This simplifies the read-out electronics, as required in multichannel systems for low-noise measurements. The values of the magnetic flux noise and field sensitivity, obtained so far, are 3–5 μΦ0/Hz1/2 and 6–7 fT/Hz1/2, respectively. These results are already suitable for biomagnetic applications and can be further improved by properly optimizing the device parameters.