bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical alignment and fixation of liquid crystals using azopolymer networks
Authors: Zhao Yue , Chénard Yanick , Galstian Tigran V.
Published: July 2002

Anisotropic gels of a nematic liquid crystal (NLC) were prepared using optical alignment and fixation in the absence of surface orientation layers. The method consists of first making a homogeneous mixture that contains a reactive diacrylate monomer bearing azobenzene group, a nonreactive NLC and a photoinitiator, then irradiating the mixture by a laser at λ=532 nm for the liquid crystal (LC) orientation induction. Subsequently, the mixture is exposed by a second laser at λ=350 nm for photopolymerization giving rise to anisotropic azobenzene network. Infrared dichroism measurements show that a remnant long-range LC orientation is retained by the network.
Annihilation of nonradiative defects on hydrogenated silicon surfaces under pulsed-laser irradiation
Authors: Timoshenko V. Yu. , Dittrich Th. , Koch F. , Kamenev B. V. , Rappich J.
Published: July 2002

Annihilation of nonradiative recombination defects on hydrogenated Si surfaces has been found after irradiation with XeCl laser pulses of energies below the melting threshold. A decrease of the total numbers of the stretched Si–Hx (x=1,2,3) bonds and an increase of the relative part of the Si–H bonds of the surface coverage have been established simultaneously by Fourier transform infrared spectroscopy. The drop of the number of surface nonradiative defects despite the efficient hydrogen desorption is explained by the mechanism of bonding of Si dangling bonds under the high electronic excitation induced by the XeCl laser pulse.
First-principles study of one-dimensional quantum-confined H-passivated ultrathin Si films
Authors: Agrawal B. K. , Agrawal S.
Published: July 2002

An ab initio study of the electronic structure of the one-dimensional quantum-confined H-passivated crystalline ultrathin Si in large thickness range of 2.7–27.0 Å has been made, employing a self-consistent full potential linear muffin-tin orbital (FPLMTO) method along with the density functional theory in local approximation. Calculations have been performed for a quite big unit cell with sufficient separation between the unit cells to avoid interference effects between the two surfaces of a unit cell as well as between the two successive unit cells. The present results show the absence of overlocalized states in quantum-confined systems and the occurrence of the extended states responsible for the luminescence observed in quantum-confined nanostructures. The presently calculated values of the band gap show an exponential rise with a decrease in the quantum-confined size in the ultrathin film region. The present results have been obtained for a more extended film thickness region as compared to earlier ab initio calculations.
Polarization effects in near-ground-state quantum wire lasers
Authors: Percival Chris , Woodhead J. , Houston P. A. , Cullis A. G. , Hill G. , Roberts J. S.
Published: July 2002

We report measurements of the lasing polarization from GaAs quantum wires grown on v-grooved (100) oriented substrates. Comparison is made with the polarization of emission from other quantum well structures formed on similar nonplanar substrates. The quantum wells lase in classic TE modes. The quantum wire emission is linearly polarized at an angle to the growth plane, but not aligned with any obvious device feature.
Peculiarities of radio frequency YBa2Cu3O7−x superconducting quantum interference device response with a finite amplitude of the second harmonic in the current-phase relationship
Authors: Il’ichev E.
Published: July 2002

The signal properties of radio frequency (rf) (SQUIDs) based on YBa2Cu3O7−x symmetrical 30° and asymmetrical 45° grain-boundary Josephson junctions have been experimentally investigated. Both amplitude as well as phase detection have been used to read out the SQUID output signal. Comparison of the SQUID output signals to each other is given. It is shown that peculiarities of the rf SQUID response of SQUIDs with a 45° grain-boundary Josephson junction are consistent with an unconventional current-phase relation (CPR) of the Josephson junction, namely, the CPR contains a finite amplitude of the second harmonic.
Dispersion and pulse-duration dependence of the nonlinear optical response of Gd2 at C80
Authors: Yaglioglu Gul , Pino Robinson , Dorsinville Roger , Liu J. Z.
Published: July 2002

Single-beam z-scan and transient absorption experiments were carried out between 400 and 1000 nm in Gd2 at C80 thin films and solutions. The third-order nonlinearity was large, negative, and strongly dependent on pulse duration and wavelength. Transient absorption results suggest that the fast nonlinear response is enhanced by two-photon-induced ground-state absorption and excited-state bleaching effects.
Catalyst enhanced high power radio frequency excited CO2 slab laser
Authors: Starostin S. A. , Udalov Y. B. , Peters P. J. M. , Witteman W. J.
Published: July 2002

A substantial improvement in the specific laser output power (up to 30%) and efficiency (up to 50%) of a radio frequency (rf) excited slab CO2 laser with a distributed gold catalyst is observed when a large amount of CO (more than 60% of CO2 concentration) is added to the laser gas mixture. A maximum output power of 160 W with an efficiency of 17% was obtained from a laser with an active volume of 2×15×370 mm3. Gas mixtures with a high CO concentration combined with a distributed gold catalyst also accelerate the start-up time of rf pumped CO2 lasers considerably.
Observation of a 6×6 superstructure on 6H–SiC (0001) by reflection high energy electron diffraction
Authors: Xie Xian Ning , Loh Kian Ping
Published: July 2002

A 6×6 reconstruction consisting of a silicon-rich phase has been observed on 6H–SiC (0001) after annealing a silicon-enriched 3×3 6H–SiC (0001) surface at 900 °C. Our results show that the 6×6 reconstruction obtained is a long-range reconstruction and not a psuedoperiodic structure due to incommensurately adsorbed graphite honeycombs as suggested previously. We propose that the 6×6 structure arises from the absence of consecutive tetrahedral clusters in a tetrahedral cluster array with 3×3 periodicity. Annealing the 6×6 structure further at 1200 °C results in a carbon-rich 6√3×6√3R30 structure.
Temperature dependence of photoinduced Au particle formation in polyvinyl alcohol films
Authors: Tanahashi Ichiro , Kanno Hiroshi
Published: July 2002

Au particles were embedded in polyvinyl alcohol films (Au/PVA films) by a photoreduction technique. Formation of Au particles in the films was investigated at temperatures between 0 and 50 °C in the dark. The Au/PVA film was colorless at 0 °C for 20 h after the photoreduction. On the other hand, the film became purple with increasing ambient temperatures. In the optical absorption spectra of the film, the absorption band of surface plasmon resonance of Au particles became clear with time. The average diameter of the Au particles in the film was estimated to be 29.1 nm from transmission electron microscopy observation. A thermal-sensing reagent was fabricated using the Au particles in PVA. The color of the reagent irreversibly changed from white to purple when the ambient temperature increased from 0 to 25 °C.
Magnetic behavior of the La1−yCayMn1−xFexO3 perovskites
Authors: Tzavellas A. , Trohidou K. N. , Kechrakos D. , Moutis N.
Published: July 2002

The magnetic ordering of the compounds La1−yCayMn1−xFexO3 where 0.2<y<0.5 and 0<x<0.1 has been studied within the molecular field theory. We introduce a model based on the competition between ferromagnetic coupling between the Mn ions and strong antiferromagnetic coupling induced by the presence of Fe ions. The magnetization as a function of temperature and the critical temperature have been calculated for several values of the parameter x. We show that even for very small x, the magnetic order of the system is reduced. Our results are in good agreement with experimental findings on these systems.
Interaction of a Ti-capped Co thin film with Si3N4
Authors: Li Hua , Bender Hugo , Conard Thierry , Maex Karen , Gutakovskii Anton , Van Landuyt Jozef , Froyen Ludo
Published: July 2002

The reaction of a Ti (8 nm) capped Co film (15 nm) with a Si3N4 layer (150 nm) is studied after rapid thermal annealing at 660 °C for 120 s in a N2 ambient. X-ray photoelectron spectroscopy, transmission electron microscopy, electron energy-loss spectroscopy, and Auger electron spectroscopy are used to study the reaction products. Combining the results of the different analyses yields a layer stack consisting of: TiO2/TiO/unreacted Co/(Ti,Co)2N/Co2Si, followed by amorphous Si3N4. The reaction mechanisms are discussed. Conclusions concerning the risk for degradation of nitride spacers in advanced devices are drawn.
The Biot–Savart operator for application to knot theory, fluid dynamics, and plasma physics
Authors: Cantarella Jason , DeTurck Dennis , Gluck Herman
Published: July 2002

The writhing number of a curve in 3-space is the standard measure of the extent to which the curve wraps and coils around itself; it has proved its importance for molecular biologists in the study of knotted DNA and of the enzymes which affect it. The helicity of a vector field defined on a domain in 3-space is the standard measure of the extent to which the field lines wrap and coil around one another; it plays important roles in fluid dynamics and plasma physics. The Biot–Savart operator associates with each current distribution on a given domain the restriction of its magnetic field to that domain. When the domain is simply connected, the divergence-free fields which are tangent to the boundary and which minimize energy for given helicity provide models for stable force-free magnetic fields in space and laboratory plasmas; these fields appear mathematically as the extreme eigenfields for an appropriate modification of the Biot–Savart operator. Information about these fields can be converted into bounds on the writhing number of a given piece of DNA. The purpose of this paper is to reveal new properties of the Biot–Savart operator which are useful in these applications.
Self-pulsations in vertical-cavity semiconductor lasers
Authors: Willemsen M. B. , van de Nes A. S. , van Exter M. P. , Woerdman J. P. , Brunner M. , Hövel R.
Published: July 2002

We report the observation of strong self-pulsations in oxide-confined vertical-cavity surface-emitting lasers. From optical spectra and intensity noise spectra, we deduce that the pulses are strongly chirped.
Storage and release of self-field-generated vortices in percolative YBa2Cu3O7−δ films
Authors: Kalbeck A. , Terheggen M. , Zepezauer E. , Prettl W. , Puig T.
Published: July 2002

Pulsed-laser heating of current-biased percolative YBa2Cu3O7−δ films can be used to generate and release self-field-induced vortices. A laser pulse yields a voltage signal due to penetration of bias-current-induced magnetic flux in absence of an external magnetic field. Upon recooling, strongly pinned vortices remain in the film. These remanent vortices have been detected after disconnecting the bias source in the currentless film. Applying a subsequent second laser pulse again yields a signal voltage but of opposite sign due to magnetic flux redistribution. This phenomenon could be applied as write-read memory.
High sensitivity measurement of implanted As in the presence of Ge in GexSi1−x/Si layered alloys using trace element accelerator mass spectrometry
Authors: Datar S. A. , Wu Liying , Guo B. N. , Nigam M. , Necsoiu D. , Zhai Y. J. , Smith D. E. , Yang C. , Bouanani M. E. , Lee J. J. , McDaniel F. D.
Published: July 2002

Various devices can be realized on strained GeSi/Si substrates by doping the substrate with different impurities such as As. As is an n-type dopant in both Ge and Si. As cross contamination can also arise during germanium preamorphization implantation due to inadequate mass resolution in the implanter. Thus, it is important to be able to accurately measure low-level As concentrations in the presence of Ge. Secondary ion mass spectrometry (SIMS) is the standard technique for these types of measurements but is constrained by mass interferences from molecular ions (74GeH, 29Si30Si16O). The trace element accelerator mass accelerator technique allows the breakup of interfering molecules. As is measured in a GeSi matrix with sensitivity significantly better than SIMS.
Speed limit of frozen pellets (H2, D2, and Ne) through single-loop and multiloop tubes and implications for fusion plasma research
Authors: Combs S. K. , Griffith A. E. , Foust C. R.
Published: July 2002

Frozen pellets (H2, D2, and Ne at 8 K) of nominal 2.7 mm diam were shot through a coiled tube (single loop of ≈0.6 m diam and 8.5 mm bore), and the speed limit for survival was recorded for each pellet type. Intact H2 pellets were observed at speeds approaching 500 m/s; but neon pellets could not survive much more than 100 m/s. The speed limit for D2 pellets fell in the middle at ≈300 m/s. Some D2 pellets were also shot through a 30 m coiled tube consisting of 11 loops (average loop diameter of ≈0.8 m), and a speed limit of ≈100 m/s was observed. Injection of frozen H2 or D2 pellets is commonly used for core fueling of magnetically confined plasmas, and frozen neon pellets are sometimes used for impurity transport studies in similar experiments. The results from these tests add to a pellet database for injection lines with single- and complex multiple-curved guide tubes. All of the information to date suggests that frozen pellets can be delivered reliably from a pellet source to any accessible plasma location on a fusion device via “roller-coaster” tubes as long as the pellet speed is maintained below a threshold limit.
A pulsed particle injection system for shock tube studies of powders
Authors: Parker T. E. , Foutter R. R. , Rawlins W. T.
Published: July 2002

Shock tubes offer a unique method for studying high temperature properties and/or combustion of particulate. The particles are simply introduced into the shock tube prior to firing and are subsequently heated by gases behind the shock waves. For dilute particle systems, the temperature of the particulate rapidly equilibrates with that of the processed gas; the experimenter can then monitor the high temperature behavior of the particles until the event is quenched by the wave processes within the shock tube. However, effective delivery of particles into the shock tube is not a trivial task and a method that has been proven to be effective is the subject of this article. The injection system uses mass loadings much less than 1 g, produces a uniform cloud at the end of a shock tube, and can be tailored to operate with a variety of different particle types. This system has been used to study the high temperature optical properties of Al2O3 (Parker et al., 18th JANNAF Conference, Monterey, CA, 1989; Rawlins et al., AIAA Conference, 1993) as well as the ignition/combustion properties of boron (Rawlins et al., Joint Eastern and Central States Technical Meeting of the Combustion Institute, 1993).
Optically pumped InGaN/GaN lasers with wet-etched facets
Authors: Stocker D. A. , Schubert E. F. , Redwing J. M.
Published: July 2002

Optically pumped laser action is demonstrated in InGaN/GaN double heterostructure lasers with wet-etched facets. The facets are formed by a two-step etching process which creates vertical facets with less than 5 nm roughness. The first step, photoenhanced electrochemical wet etching, is used to define the laser cavities. The second step reduces the facet roughness by crystallographic wet chemical etching. Lasing is demonstrated by an increase in the differential quantum efficiency, linewidth narrowing, and strongly polarized output above threshold. The threshold varies with cavity length from 2.4 MW/cm2 for 500 μm cavities to 23 MW/cm2 for 50 μm cavities. A modal loss of 15 cm−1 is deduced from an analysis of the threshold pumping power as a function of cavity length.
Electron cloud effect on current injection across a Schottky contact
Authors: Nabet B. , Cola A. , Quaranta F. , Cesareo M. , Rossi R. , Fucci R. , Anwar A.
Published: July 2002

The electron cloud that is formed in the narrow gap material in a modulation-doped heterostructure affects the Schottky contact made to the wide gap material. It also influences absorption and collection of the optically generated carriers. Photocurrent spectra, current–voltage, and current–temperature measurements show that the increase in electron cloud density decreases dark current flow while increasing photoresponsivity. We propose that the Coulombic interaction between the confined electron cloud and the emitted electrons from metal to the wide gap material increases the barrier height. The electric field in the direction of growth due to modulation doping accounts for the increase in photoelectron collection efficiency. Implementation of this effect increases efficiency of photodetectors while, simultaneously, reducing the noise due to dark current.
Carbon substrates for backscattering spectrometry analysis
Authors: Giauque Pierre H. , Nicolet Marc-A.
Published: July 2002

Graphite coated with a thin layer of photoresist and annealed in an inert ambient to vitrify the coating makes light, smooth substrates that are as good as polished diamond for the analysis of thin films by MeV He backscattering spectrometry.
Highly efficient unibond silicon-on-insulator blazed grating couplers
Authors: Ang T. W. , Reed G. T. , Vonsovici A. , Evans A. G. R. , Routley P. R. , Josey M. R.
Published: July 2002

In this letter, we report, design and fabrication of good asymmetrical Si-blazed gratings with a pitch of 383 nm, fabricated in unibond silicon-on-insulator using angled ion-beam etching. An output efficiency of 84% was achieved. The devices allow efficient coupling to/from small waveguides, of the order of 1 μm in height.
Optical characterization of relaxation processes in nitrogen-doped ZnSe layers
Authors: Worschech L. , Ossau W. , Nürnberger J. , Faschinger W. , Landwehr G.
Published: July 2002

Linear polarization of the nitrogen acceptor bound exciton I1N luminescence, which reflects the anisotropic strain relief of the ZnSe lattice, is exploited to monitor two independent relaxation processes in nitrogen-doped ZnSe films. An inhomogeneous vertical strain profile is identified by a maximum in-plane polarization when 800 nm of the sample surface are removed. After exposure of nitrogen-doped ZnSe films to intense light, an isotropic strain relief is observed. The I1N peak position is shifted to lower energies, whereas the anisotropic polarization remains constant.
Evidence for an isotope effect in the electrical transport of thermally generated mobile charges in amorphous SiO2
Authors: Devine R. A. B.
Published: July 2002

The thermal generation of mobile, positively charged species in the gate oxide of metal–oxide–semiconductor field effect transistors has been studied. Static hysteresis in the source-drain current versus gate voltage curves indicates the presence of ∼3.2×1012 cm−2 mobile charges in devices annealed in D2 and H2 containing atmospheres at 700 °C for 30 min. Pulsed switched gate voltage experiments give clear evidence of a (mass)1/2 effect in the charge transport consistent with identification of the charges as deuterons and protons.
A multiple pin, flip-chip system for microwave and gigabit per second cryogenic device testing at variable temperatures
Authors: Wire M. S. , Durand D. J. , Silver A. H. , Wagner M. K. , Hamilton C. A.
Published: July 2002

We have developed a cryogenic probe and cryostat system to test both active and passive superconducting devices and circuits up to microwave frequencies at variable temperatures. Our system consists of two basic parts: the cryostat and the matching probe. The cryostat is a unique, very efficient, variable temperature, flow-type cryostat, where we control temperature without electrical heaters. It is magnetically shielded and allows rapid testing between 4.2 K and room temperature. Probes developed for this cryostat have a larger number of shorter cables than standard dip-type probes. They are designed to test chips or chip assemblies without additional packaging. Chips or chip assemblies can be quickly mounted and dismounted from a fixed spring-contact assembly. Each probe has 56 wide-bandwidth signal cables. We have repeatedly used these probes for testing both active and passive superconducting integrated circuits up to 20 GHz. The probe and cryostat combination provides a testing capability that is simultaneously high frequency, dc magnetic shielded, has variable cryogenic temperature, and quick turnaround.
Trench-type narrow InGaAs quantum wires fabricated on a (311)A InP substrate
Authors: Sugaya Takeyoshi , Ogura Mutsuo , Sugiyama Yoshinobu , Matsumoto Kazuyuki , Yonei Kenji , Sekiguchi Takashi
Published: July 2002

InGaAs quantum wires (QWRs) with cross sections as narrow as 10 nm×20 nm have been fabricated on a (311)A InP V-grooved substrate under an As2 source. Trench-type InGaAs QWRs consist of (111)A and (331)B facets with an angle of about 22°. Cathode-luminescence and photoluminescence measurements confirmed the luminescence peak arising from the QWRs.
Digital thin-film color optical memory
Authors: Chi C. J. , Steckl A. J.
Published: July 2002

A promising optical memory device called digital thin-film (DTF) color optical memory is presented. The DTF optical memory utilizes localized regions of varying thickness to adjust the spectral characteristic of reflected light from a broad band source. The DTF structure has been fabricated by Ga+ focused ion beam milling on thermally grown silicon dioxide on Si to prove the concept. A charge-coupled device array is used as the optical detector for the readout of the stored data. The reflected light image of the DTF memory reveals easily discriminated color levels and proves the suitability of using optical means to extract the stored data. DTF optical memory structures with 16 physical levels or 4 bits/pixel have been fabricated providing an equivalent storage density in excess of 5 Gb/in.2
Enhanced oxygen precipitation in silicon due to grown-in spatial inhomogeneities in the oxygen distribution
Authors: Wang Zhihong , Sinno Talid , Brown Robert A.
Published: July 2002

Fine control of oxygen precipitation during annealing of silicon wafers is required for modern microelectronics processing. We show that the presence of microscopic spatial inhomogeneities in the oxygen concentration caused by fluctuations during crystal growth, can lead to significantly enhanced effective nucleation rates. This effect is demonstrated for a simple nonlinear reaction-diffusion model and then confirmed by simulations of oxide precipitation.
Hole-trapping-related transients in shallow n+–p junctions fabricated in a high-energy boron-implanted p well
Authors: Poyai A. , Simoen E. , Claeys C.
Published: July 2002

This letter describes a transient phenomenon in the reverse hole current of large-area shallow n+–p-well junctions, giving rise to a hump at a specific reverse bias. This corresponds to a certain depletion depth in the retrograde p well, which has been fabricated by a deep (200 keV) and a shallow (55 keV) boron ion implantation. No such a reverse hole current hump occurs for reference diodes, processed in p-type Czochralski substrates. The effect is also absent in large-perimeter p-well junctions, suggesting a correlation with defects in the p-well region. The occurrence at a specific depletion depth indicates a nonuniform defect distribution, for example related to the displacement damage created by the 200 keV B implantation. This idea is further supported by deep level transient spectroscopy results, which reveal the presence of a nonuniform density of hole traps, corresponding to a broad range of energy levels from about 0.3 to 0.5 eV above the valence band. A discussion of the possible nature of the underlying defects is given.
Growth and optical properties of type-II InP/GaAs self-organized quantum dots
Authors: Wang Benzhong , Chua Soo-Jin
Published: July 2002

Self-organized InP quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix by metalorganic chemical vapor deposition. Strong photoluminescence centered at 986 nm is observed for the sample of InP grown at 490 °C, which can be attributed to radiative recombination of zero-dimensional (0D) electrons located in the InP dots and holes located in the surrounding regions. The indirect recombination of photogenerated carriers has been confirmed by the measurement of luminescence at different excitation densities and temperatures. If the InP is grown at 600 °C, experimental results show that a thicker and much smoother wetting layer is formed which results in much stronger and narrower luminescence located at 875 nm. In addition, state filling of the 0D electrons is also observed for the type-II quantum dots.
Magnetic block array for patterned magnetic media
Authors: Koike Kazuyuki , Matsuyama Hideo , Hirayama Yoshiyuki , Tanahashi Kiwamu , Kanemura Takashi , Kitakami Osamu , Shimada Yutaka
Published: July 2002

We have fabricated a magnetic rectangular block array of 130 Gblock/in.2 with a block-size-to-period ratio of 0.85 by using focused ion beam lithography. Each block has perpendicular crystal magnetic anisotropy and a single domain structure. Its advantage as patterned media is discussed in conjunction with thermal stability in recorded magnetization, reading signal, and area density.
Characteristics of monolithically integrated two-wavelength laser diodes with aluminum-free active layers
Authors: Lu Tien-chang , Fu Richard , Shieh H. M. , Huang K. J. , Wang S. C.
Published: July 2002

A two-wavelength integrated laser diode (TWINLD) with aluminum-free active areas (AAA) has been realized by monolithically combining two different laser material structures in a single chip utilizing the metalorganic chemical vapor deposition growth and regrowth techniques. The single TWINLD chip comprises two ridge wave-guide lasers, one is an InGaP/InGaAlP material structure for a 650 nm red laser, and another is an InGaAsP/AlGaAs material structure with AAA for a 780 nm infrared laser. The chip geometry is 300 μm long and 300 μm wide with a separation of 150 μm between two laser emission spots. The ridge widths are 3.5 and 2 μm for the red and IR laser, respectively, and both lasers emit a single transverse mode with a threshold current of 12 mA for a 650 nm laser and 14 mA for a 780 nm laser under continuous wave operation condition.
X-ray determination of vertical ordering of InAs quantum dots in InAs/GaAs multilayers
Authors: González J. C. , Magalhães-Paniago R. , Rodrigues W. N. , Malachias A. , Moreira M. V. B. , de Oliveira A. G. , Mazzaro I. , Cusatis C. , Metzger T. H. , Peisl J.
Published: July 2002

The degree of vertical alignment of InAs quantum dots in InAs/GaAs(001) multilayers was studied using grazing incidence x-ray scattering. We show that it is necessary to access one of the weak (200) x-ray reflections to observe the modulation of the GaAs lattice periodicity produced by the stacking of the InAs dots. The degree of alignment of the dots was assessed by fitting the x-ray diffuse scattering profiles near a GaAs (200) reciprocal lattice point. By using a model of gaussian lateral displacement of the dots, we show that we can determine the average value of the mistake in stacking positions of the islands from one bilayer to the next.
Resonant Raman scattering on self-assembled GaN quantum dots
Authors: Kuball M. , Gleize J. , Tanaka Satoru , Aoyagi Yoshinobu
Published: July 2002

Self-assembled GaN quantum dots grown on Al0.15Ga0.85N using Si as antisurfactant have been investigated by resonant Raman scattering. Phonons of GaN quantum dots of different sizes and the Al0.15Ga0.85N barrier layer were probed selectively by varying the laser excitation energy from 3.53 to 5.08 eV. Phonon confinement effects were studied on GaN quantum dots of 2–3 nm height. We show that although grown using Si (a common donor for GaN) as an antisurfactant, only a small electron concentration is present in the GaN quantum dots. Implications on the role of Si for the formation of the GaN quantum dots will be discussed.
Nonlinear refractive beam shaping by an organic nonlinear absorber
Authors: Robertson John , Smith Andrew , Duignan Jason , Milsom Philip , Bourhill Grant
Published: July 2002

The response of organic dyes to laser pulses is typically described solely by nonlinear absorption. Recently, spatial profiles of a nanosecond pulse exiting an organic nonlinear absorber have shown energy redistribution from a Gaussian input profile to a central spike and outer ring. It was suggested that the spike and ring resulted from both nonlinear absorption and nonlinear refraction. In this letter, the role of a thermal nonlinear refraction in beam shaping is demonstrated using single and time-delayed double picosecond pulses. It is concluded that the dynamics of nonlinear absorption and nonlinear refraction must be included to correctly describe the laser-material interaction.
Doping by metal-mediated epitaxy: Growth of As delta-doped Si through a Pb monolayer
Authors: Dubon O. D. , Evans P. G. , Chervinsky J. F. , Aziz M. J. , Spaepen F. , Golovchenko J. A. , Chisholm M. F. , Muller D. A.
Published: July 2002

In molecular-beam epitaxy a monolayer of Pb on the Si(111) surface induces single-crystal growth at temperatures well below those required for similar growth on a bare surface. We demonstrate that the suppression of dopant segregation at the lower temperatures attainable by Pb-mediated growth allows the incorporation of As donors at concentrations reaching a few atomic percent. When Pb and Si are deposited on an As-terminated Si(111) substrate at 350 °C, the Pb segregates to the surface without doping the Si film while the As is buried within nanometers of the substrate–film interface. The resulting concentration of electrically active As, 1.8×1021 cm−3, represents the highest concentration of As donors achieved by any delta-doping or thin-film deposition method.
Photoluminescence modulation by high-frequency lateral electric fields in quantum wells
Authors: Zhang S. K. , Santos P. V. , Hey R.
Published: July 2002

We investigate the modulation of the excitonic photoluminescence (PL) of GaAs quantum wells by high-frequency (frf) lateral electric fields. Under these fields, the PL becomes modulated in the form of pulses with repetition frequency of 2 frf. The periodic PL modulation is attributed to the time-dependent ionization of photogenerated excitons under the lateral electric field. The exciton ionization mechanism is proposed to be the impact ionization with electrons accelerated by the electric fields with a threshold field for ionization of about 15 V/cm. The different transport properties of electrons and holes are found to play a role in the exciton ionization process.
Degenerate four-wave mixing and Z-scan measurements of stilbazolium derivatives
Authors: Sun Wenfang , Lawson Chris M. , Gray Gary M. , Zhan Chuanlang , Wang Duoyuan
Published: July 2002

Stilbazolium derivatives are very attractive optical materials for photonics and biophotonics applications because a large number of these derivatives are available and they can exhibit large third-order nonlinear coefficients and rapid responses. To quantitatively characterize their third-order nonlinear coefficients and the temporal responses of their third-order nonlinear process, we have performed degenerate four-wave mixing and Z-scan experiments on four stilbazolium derivatives with different electron donors using 40 ps laser pulses at 532 nm. The γ values for three of the compounds are on the order of 10−31 esu. These high values arise from electronic processes and are associated with both nonlinear absorption and nonlinear refraction. The relative contributions from nonlinear absorption and nonlinear refraction are dependent on the chemical structure and linear absorption of the stilbazolium derivative.
Structural and electronic properties of C36
Authors: Gueorguiev G. K. , Pacheco J. M.
Published: July 2002

We carry out a first-principles and comparative study of structural and electronic properties of the two most stable—and equally stable—isomers of fullerene C36. We find that the remarkable equivalence exhibited by the two isomers in what concerns their stability extends to many other electronic properties. However, their different geometric structure is responsible for sizeable differences in their vibrational spectra; as well we find that the quantization of electronic states around the Fermi level leads to a different distribution of strength for the low-energy electronic excitations. These features may contribute to a rich and distinct interplay between electrons and phonons in the bulk phase.
Nuclear magnetic resonance spin echoes for restricted diffusion in an inhomogeneous field: Methods and asymptotic regimes
Authors: Axelrod Scott , Sen Pabitra N.
Published: July 2002

We develop systematic formulations for calculating the magnetization of spins diffusing in a bounded region in the presence of the surface relaxation and magnetic field inhomogeneity and compute explicitly the relaxation exponent for the Carr–Purcell–Meiboom–Gill spin echoes. The results depend on the echo number n, and three dimensionless parameters: Lρ/LS, D̃0=(LD/LS)2, the dimensionless diffusion constant, and γ̃=LD2LS/LG3=Δωτ, the dimensionless gyromagnetic ratio, where the restriction is characterized by a size LS, the magnetic field inhomogeneity by a dephasing length, LG, the diffusion length during half-echo time by LD, and a length Lρ characterizes the surface relaxation. Here Δω is the line broadening and 2τ is the echo period. Depending on the length scales, three main regimes of decay have been identified: short-time, localization, and motionally averaging regimes (MAv). The short-time and the MAv regimes are described well by the cumulant expansion in terms of powers of the “small” parameter γ̃. We give simple formulas for decay rates in these two asymptotic regimes. We show that the Gaussian phase approximation (GPA), i.e., the exponent up to the second order in γ̃2 in terms of a full eigenmode expansion interpolates well between these two regimes. In the localization regime, the decay exponent depends on a fractional power, γ̃2/3, denoting a breakdown of the GPA and a breakdown of the cumulant expansion in terms of γ̃.
Fine structure on the green band in ZnO
Authors: Reynolds D. C. , Look D. C. , Jogai B.
Published: July 2002

An emission band at 2.4 eV, called the green band, is observed in most ZnO samples, no matter what growth technique is used. Sometimes this band includes fine structure, which consists mainly of doublets, repeated with a longitudinal-optical-phonon-energy spacing (72 meV). We have developed a vibronic model for the green band, based on transitions from two separate shallow donors to a deep acceptor. The donors, at energies 30 and 60 meV from the conduction-band edge, respectively, are also found from Hall-effect measurements.
Ultrafast dynamic holography in nanocrystal solids
Authors: Kraabel B. , Malko A. , Hollingsworth J. , Klimov V. I.
Published: July 2002

We report efficient dynamic gratings in close-packed solids of CdSe nanocrystals. These gratings are formed on the subpicosecond time scale and have diffraction efficiencies up to 0.5% for film thicknesses of ∼0.5 μm. Nanocrystal solids combine the best features of inorganic semiconductors (large resonant nonlinearities and high photostability) and organic semiconducting polymers (chemical flexibility and tunability of optical properties by simple synthetic means). Additionally, nanocrystal solids allow precise control over the spectral position of the nonlinear optical response by simply varying the size of the nanocrystals used in fabricating the solid (quantum confinement effect).
Ultraviolet-extended real-time spectroscopic ellipsometry for characterization of phase evolution in BN thin films
Authors: Zapien J. A. , Messier R. , Collins R. W.
Published: July 2002

Real-time spectroscopic ellipsometry with an ultraviolet-extended spectral range (1.5–6.5 eV) has been applied to investigate the sputter deposition of boron nitride (BN) thin films with high cubic content in terms of a two-layer optical model. In this model, the inner and outer layers represent sp2- and sp3-bonded BN (hBN and cBN), respectively. The thickness evolution of the two layers as well as their dielectric functions over the extended spectral range have been determined.
Low-loss YBa2Cu3O7 films on flexible, polycrystalline-yttria-stabilized zirconia tapes for cryoelectronic applications
Authors: Harshavardhan K. S. , Christen H. M. , Silliman S. D. , Talanov V. V. , Anlage S. M. , Rajeswari M. , Claassen J.
Published: July 2002

High-temperature superconducting films on flexible, low-thermal conductivity, low-loss substrates offer a unique base for the development of cryoelectronic digital interconnects. Using an ion-beam-assisted pulsed-laser-deposition technique, we developed biaxially textured YBa2Cu3O7 (YBCO) films on flexible polycrystalline-yttria-stabilized zirconia (YSZ) substrates with the following materials properties: (i) in-plane x-ray Φ-scan full width at half maximum of ∼7°; (ii) transition temperatures (Tc) in the range of 88–89 K with transition widths (ΔTc) of ∼0.5 K; (iii) critical current densities (Jc) in the range 1.5–2×106 A/cm2 at 77 K, zero field; (iv) magnetic penetration depth (λ) of 284 nm at 77 K; and (v) surface resistance (Rs) of 700 μΩ at 77 K, 10 GHz. The low-microwave loss, biaxilly textured YBCO films combined with the low-thermal conductivity YSZ substrate could facilitate a variety of RF cryoelectronic applications.
Stress and piezoelectric properties of aluminum nitride thin films deposited onto metal electrodes by pulsed direct current reactive sputtering
Authors: Dubois Marc-Alexandre , Muralt Paul
Published: July 2002

Polycrystalline aluminum nitride thin films were deposited onto platinum, aluminum, and titanium electrodes by reactive magnetron sputtering in the pulsed direct current mode. The films exhibited all a columnar microstructure and a c-axis texture. The built-in stress and the piezoelectric properties of these films were studied as a function of both the processing conditions and the electrode material. Stress was found to be very much dependent on the growth conditions, and values ranging from strong compression to high tension were observed. The piezoelectric d33,f coefficient was shown to rely on substrate quality and ionic bombardment: The nucleation surface must be stable with regard to the nitrogen plasma and present a hexagonal symmetry and, on the other hand, enough energy must be delivered to the growing film through ionic bombardment.
Thermally poled silica samples are structurally heterogeneous: Electron diffraction evidence of partial crystallization
Authors: Cabrillo C. , Bermejo F. J. , Gibson J. M. , Johnson J. A. , Faccio D. , Pruneri V. , Kazansky P. G.
Published: July 2002

Large structural modifications in v-SiO2 are found to be induced by “thermal poling,” a treatment which makes the glass act as a frequency doubler of an impinging infrared light. The electron diffraction patterns of poled silica plates reveal the presence of a large amount (of order 10%) of crystallites showing patterns consistent with partial crystallization of the glassy matrix into the cristobalite polymorph of silica.
Which nanowire couples better electrically to a metal contact: Armchair or zigzag nanotube?
Authors: Anantram M. P.
Published: July 2002

The fundamental question of how chirality affects the electronic coupling of a nanotube to metal contacts is important for the application of nanotubes as nanowires. We show that metallic-zigzag nanotubes are superior to armchair nanotubes as nanowires, by modeling the metal–nanotube interface. More specifically, we show that as a function of coupling strength, the total electron transmission of armchair nanotubes increases and tends to be pinned close to unity for a metal with Fermi wave vector close to that of gold. In contrast, the total transmission of zigzag nanotubes increases to the maximum possible value of two. The origin of these effects lies in the details of the wave function, which is explained.
Direct patterning of photosensitive low-dielectric-constant films using electron-beam lithography
Authors: Kikkawa Takamaro , Nagahara Tatsuro , Matsuo Hideki
Published: July 2002

A direct patterning technique of interlayer dielectric films was developed for multilevel interconnections. A photosensitive methylsilazane film with a dielectric constant of 2.7 was synthesized. A methylsilazane precursor consists of a photoacid generator, a sensitizer, and a base polymer. The photosensitive methylsilazane film could be patterned by use of electron-beam lithography or ultraviolet lithography. It was demonstrated that the smallest feature size of 50 nm for damascene lines and via holes could be directly patterned in these films by electron-beam lithography.
Imaging the temporal evolution of nuclear resonant x-ray scattering
Authors: Röhlsberger R. , Quast K. W. , Toellner T. S. , Lee P. L. , Sturhahn W. , Alp E. E. , Burkel E.
Published: July 2002

The time response of resonant nuclei in a rotating sample coherently excited by synchrotron radiation pulses maps to an angular scale. At sufficiently high rotational speeds, the temporal evolution of the nuclear decay can be recorded with a position sensitive detector. This has been demonstrated by nuclear resonant scattering of synchrotron radiation from a Fe57 metal foil rotating at 10 kHz. The method has the potential to yield time resolutions of a few 10 ps, thus overcoming principal limits of existing techniques.
Influence of the injection current dependence of gain suppression on the nonlinear dynamics of semiconductor lasers
Authors: Lim C. G. , Iezekiel S. , Snowden C. M.
Published: July 2002

A detailed numerical analysis of the nonlinear dynamics of a directly modulated 1.55 μm InGaAsP distributed-feedback laser diode (LD) is carried out. Results show that when simulating the nonlinear dynamics of LDs, it is important to account for the gain suppression being dependent on the injection current. Only when this dependency of gain suppression is included in the conventional single-mode rate equations, can excellent agreement between simulated and measured results be achieved.
All-fiber phase modulator based on lead zirconate titanate thin-film coating
Authors: Costantini D. M. , Limberger H. G. , Salathé R. P. , Muller C. A. P. , Muralt P. , Setter N.
Published: July 2002

An all-fiber phase modulator based on lead titanate zirconate coating deposited by dc reactive magnetron sputtering was fabricated and optically characterized. The frequency response showed a flat broadband structure up to 600 kHz, with an efficiency of 0.73 rad/(V m). This is an order of magnitude higher than that for sputtered zinc oxide based phase modulators. Radial mode resonance peaks were observed between 10 and 700 MHz, with a maximum phase modulation amplitude of 0.3 rad at the fundamental radial resonance frequency of 25.6 MHz.
Spectrum dynamics of negative-effective-mass oscillators under terahertz radiation
Authors: Cao J. C. , Lei X. L. , Li A. Z. , Liu H. C.
Published: July 2002

We report on a theoretical investigation of power spectrum dynamics in negative-effective-mass (NEM) p+pp+ oscillators under the influence of terahertz (THz) electromagnetic radiation. Possible types of transport states (periodic or chaotic) and transitions between them are examined with the intensity and frequency of the radiation as controlling parameters. When the driving frequency is fixed to the self-oscillating frequency times the inverse Golden ratio, the resulting power spectrum pattern displays a very complex mosaic scenario with a self-similar emergence of high-order mixing frequencies.
Shallow-deep transitions of impurities in semiconductor nanostructures
Authors: Ranjan V. , Singh Vijay A.
Published: July 2002

We study the hydrogenic impurity in a quantum dot (QD). We employ the effective mass theory with realistic barrier and variable effective mass. The model is simple, but it predicts features not previously observed. We observe that the shallow hydrogenic impurity becomes deeper as the dot size (R) is reduced and with further reduction of the dot size it becomes shallow and at times resonant with the conduction band. Such a shallow-deep (SHADE) transition is investigated and a critical size in terms of the impurity Bohr radius (aI*) is identified. A relevant aspect of a QD is reduction in the dielectric constant, ε, as its size decreases. Employing a size dependent ε(R), we demonstrate that the impurity level gets exceptionally deep in systems for which aI* is small. Thus, carrier “freeze out” is a distinct possibility in a wide class of materials such as ZnS, CdS, etc. The behavior of the impurity level with dot size is understood on the basis of simple scaling arguments. Calculations are presented for III–V (AlGaAs) and II–VI (ZnS, CdS) QDs. We speculate that the deepening of the impurity level is related to the high luminescence efficiency of QDs. It is suggested that quantum dots offer an opportunity for defect engineering.
Charge-ordering modulation observed in the (La0.5Mn0.5)MnO3 phase of the multiphased manganite La0.9Sn0.1MnO3 at room temperature
Authors: Wang Y. Q. , Duan X. F. , Wang Z. H. , Sun J. R. , Shen B. G.
Published: July 2002

At room temperature, the lattice image of a modulated structure associated with charge ordering has been observed in the (La0.5Mn0.5)MnO3 phase of La0.9Sn0.1MnO3, which is composed of two-type phases: ABO3 and A2B2O7. Results of electron energy loss spectroscopy and energy dispersive x-ray spectroscopy show that the need of the chemical balance and small A-site radius for the (La0.5Mn0.5)MnO3 phase are the main reasons for the appearance of charge ordering stripes.
Spinodal decomposition in the BxGayIn1−x−yAs alloys
Authors: Asomoza R. , Elyukhin V. A. , Peña-Sierra R.
Published: July 2002

Spinodal decomposition of the BxGayIn1−x−yAs quaternary alloys lattice-matched to the GaAs as the result of the internal deformation and coherency strain energies is described. The alloys are represented in the strictly regular approximation. The internal deformation energy is considered by the valence-force field model. The estimated bond-stretching and bond-bending elastic constants of BAs are αBAs=6.69×10−2 N/m and βBAs=1.81×10−2 N/m, respectively. Ranges of spinodal decomposition of the BxGayIn1−x−yAs quaternary alloys up to x⩽0.1 with and without coherency strain energy are demonstrated.
Transmission electron energy-loss spectroscopy study of carbon nanotubes upon high temperature treatment
Authors: Reed B. W. , Sarikaya M. , Dalton L. R. , Bertsch G. F.
Published: July 2002

Two batches of carbon nanotube materials, grown with a pulsed-laser deposition technique but purified and heat treated under different conditions, are investigated with a combination of high- resolution transmission electron microscopy techniques, including electron nanodiffraction and low-loss and carbon K-edge electron energy-loss spectroscopy. These techniques were used to achieve a detailed profile of each material. Heat treating one batch at 1100 °C is shown to increase the sp2/sp3 hybridization ratio, while a 2150 °C treatment of the other batch fundamentally restructured the material from single walled to a mixture of amorphous and multiwalled material.
Potential applications of dark resonance to subpicosecond optical switches in hyper-terahertz repetition rates
Authors: Ham Byoung S.
Published: July 2002

Subpicosecond optical switching in hyper-terahertz repetition rates is numerically demonstrated using dark resonance based two-photon coherence swapping. The switching time and repetition rate demonstrated are one tenth the population relaxation time T1, and it is a breakthrough in the critical limitation of current technologies of optical switches. This demonstration implies potential applications of ultrafast optical switches to ultrahigh-capacity fiber-optic communications.