bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron spin resonance evidence for the structure of a switching oxide trap: Long term structural change at silicon dangling bond sites in SiO2
Authors: Conley John F. , Lenahan Patrick M. , Lelis Aivars J. , Oldham Timothy R.
Published: July 2002

We provide direct and unambiguous experimental spectroscopic evidence for the structure of a switching oxide trap in thermally grown SiO2 gate oxides on Si. Switching oxide traps can ‘‘switch’’ charge state in response to changes in the voltage applied to the gate of a metal-oxide-semiconductor field-effect transistor. Electron spin resonance measurements reveal that some Eγ′ centers (a hole trapped at an oxygen vacancy) can behave as switching oxide traps.
High quality CeO2 film grown on Si(111) substrate by using low energy dual ion beam deposition technology
Authors: Huang Dading , Qin Fuguang , Yao Zhengyu , Ren Zhizhang , Lin Lanying , Gao Weibin , Ren Qingyu
Published: July 2002

By using the mass-analyzed low energy dual ion beam deposition technique, a high quality epitaxial, insulating cerium dioxide thin film with a thickness of about 2000 Å has been grown on a silicon (111) substrate. The component species, cerium and oxygen, are homogeneous in depth, and have the correct stoichiometry for CeO2. X-ray double-crystal diffraction shows that the full width at half maximum of the (222) and (111) peaks of the film are less than 23 and 32 s, respectively, confirming that the film is a perfect single crystal.
Recrystallization of high energy As-implanted GaAs studied by transmission electron microscopy
Authors: Jasinski J. , Chen Y. , Washburn J. , Liliental-Weber Z. , Tan H. H. , Jagadish C. , Kaminska M.
Published: July 2002

Ion implantation damage and the regrowth process during thermal annealing of 2 MeV As-ion-implanted GaAs were studied by transmission electron microscopy. With low-temperature annealing, a high denisty of stacking faults was formed during the recrystallization process, but they were rarely observed with high-temperature annealing. At intermediate temperatures, a much lower denisty of stacking faults was generated at the upper interface between the buried amorphous layer and the crystal than that at the lower interface, where a higher concentration of as-implanted arsenic exists. Based on the observed experimental results, an atomic model is proposed to explain the formation of stacking faults induced by As clusters.
ZnCdSe/ZnCdMgSe quantum wells on InP substrates for visible emitters
Authors: Cavus A. , Zeng L. , Tamargo M. C. , Bambha N. , Semendy F. , Gray A.
Published: July 2002

We have grown ZnCdSe/ZnCdMgSe quantum well (QW) structures nearly lattice matched to InP substrates. Emission energies from 2.307 to 2.960 eV were measured by low-temperature photoluminescence at 10 K for samples with QW thicknesses between 5 and 80 Å. By using exactly lattice-matched QWs, the lower limit of the energy range can be lowered to about 2.2 eV (at 10 K). We propose that these structures could be used in entirely lattice-matched semiconductor lasers operating at room temperature in the blue, green, and yellow regions. Because of the absence of strain, these materials are expected to be less prone to degradation than the current blue-green lasers grown on GaAs.
Fabrication of polymer nanodots with single electron patterning technology
Authors: Yoshimura Toshiyuki , Shiraishi Hiroshi , Yamamoto Jiro , Terasawa Tsuneo , Okazaki Shinji
Published: July 2002

Polymer nanodots 3–5 nm in diameter have been fabricated using conventional polymer resists. Negative-type resists of cresol novolak resins combined with crosslinkers are exposed with a uniform 50-kV electron beam. The uniform beam is considered to be a flux of ultrafine beams of individual electrons that causes crosslink reactions. By selecting electron beam doses and wet development conditions, dense patterns of polymer nanodots are successfully obtained. We call this process ‘‘single electron patterning technology’’ (SEPT). Sizes of the nanodots reflect the molecular-weight distribution characteristics of the resin polymers. A resist polymer with a small molecular distribution (polydispersity) leads to uniform dots. The dots are thought to be polymer microgels that consist of a few crosslinked resin molecules, or the molecules themselves. X-ray photoelectron spectroscopy confirms that the dots consist of resist polymers.
Extraction of superthermal electrons in a high current, low emittance, steady state electron gun with a plasma cathode
Authors: Hershcovitch Ady
Published: July 2002

Major limitations of plasma cathodes have been overcome in an electron gun based on extraction of superthermal electrons from a discharge characterized by a large component of high energy electrons with a low thermal spread. A grid is employed to select these electrons for extraction while retaining the bulk electrons in the discharge. Steady state extraction of electron beams corresponding to over 60% of the total arc discharge current has been observed. This extracted electron current far exceeds the thermal electron flux. A perveance of over 280 microperv was reached with the extraction of 9 A at 1 keV from a 6 mm aperture.
Nonlinear optical response of As+-ion implanted GaAs studied using time resolved reflectivity
Authors: Janz S. , Akano Usman G. , Mitchell Ian V.
Published: July 2002

The transient reflectivity response of GaAs with a 2% excess As concentration (GaAs:As), prepared by As+ ion implantation, has been measured at photon energies near the band gap. Results are compared with similar measurements on implanted GaAs with a 0.01% excess As concentration, and unimplanted GaAs. For GaAs:As, the transient refractive index change Δn, is larger than, but of the opposite sign to that of unimplanted GaAs. The measured carrier lifetime of 1±0.1 ps is identical to that of low-temperature GaAs. The wavelength dependence of Δn indicates the presence of an induced absorption peak at photon energies near the band gap, which is attributed to band-gap renormalization.
Improved thin films of pentacene via pulsed laser deposition at elevated substrate temperatures
Authors: Salih A. J. , Lau S. P. , Marshall J. M. , Maud J. M. , Bowen W. R. , Hilal N. , Lovitt R. W. , Williams P. M.
Published: July 2002

Thin films of pentacene were deposited by thermal evaporation and by pulsed laser deposition (PLD). Surface characterization using atomic force microscopy shows that PLD leads to a reduction in surface roughness, which can be reduced further using heated substrates. Observation of enhanced field-effect carrier mobilities in the PLD films, together with increased electrical conductivity and reduced activation energy for electrical conduction, is consistent with increased molecular ordering in the pentacene films and suggests a direct correlation between molecular ordering in the bulk and surface roughness.
High electron mobility in bipolar composites of organic molecules
Authors: Lin L.-B. , Jenekhe S. A. , Borsenberger P. M.
Published: July 2002

Electron and hole mobilities in bipolar molecular composites of N,N′-bis(1,2-dimethyl- propyl)-1,4,5,8-naphthalenetetracarboxylic diimide (NTDI) and tri-p-tolylamine (TTA) are reported. The electron mobility in NTDI/TTA composites is found to be enhanced by factors of 4 to 6 compared to pure NTDI. These results demonstrate a new approach to improving electron mobility in organic materials.
Reactivation of acceptors and trapping of hydrogen in GaN/InGaN double heterostructures
Authors: Pearton S. J. , Bendi S. , Jones K. S. , Krishnamoorthy V. , Wilson R. G. , Ren F. , Karlicek R. F. , Stall R. A.
Published: July 2002

The apparent thermal stability of hydrogen passivated Mg acceptors in GaN is a function of the annealing ambient employed, with H2 leading to a reactivation temperature approximately 150 °C higher than N2. The dissociation of Mg–H complexes and the loss of hydrogen from GaN are sequential processes, with reactivation occurring at ⩽700 °C for annealing under N2, while significant concentrations of hydrogen remain in the crystal even at 900 °C in implanted samples. The hydrogen is gettered to regions of highest defect density such as the InGaN layer in GaN/InGaN double heterostructure.
On the determination of internal optical mode loss of semiconductor lasers
Authors: Smowton P. M. , Blood P.
Published: July 2002

Use is made of a numerical simulation of the light—current characteristic to examine the errors which may arise in the determination of the optical mode loss (αi) from the cavity length dependence of the external differential efficiency (ηextd). In particular, we focus on the effects of incomplete Fermi level pinning and carrier leakage, and show that αi can only be determined correctly if ηextd is determined under conditions where it is invariant both with current level and temperature.
Investigation of the early stages of ZnSe epitaxy on GaAs(001) via scanning tunneling microscopy
Authors: Ahsan S. , Kahn A. , Pashley M. D.
Published: July 2002

We present a scanning tunneling microscopy (STM) study of the initial stages of ZnSe deposition on the GaAs(001)-(2×4) surface. The deposition of elemental Se and of ZnSe on the bare GaAs surface induces considerable atomic disorder attributed to the Se–As exchange reaction. The deposition of elemental Zn weakens the 2× periodicity of the surface but induces no apparent changes in the STM images of the As dimers. Comparison of STM images of submonolayers of ZnSe on GaAs with and without a Zn pretreatment suggests that Zn reduces the interaction of Se with the GaAs surface.
Effect of roughness on the measurement of nanohardness—a computer simulation study
Authors: Bobji M. S. , Biswas S. K. , Pethica J. B.
Published: July 2002

This letter reports a simulation study of depth sensing nanoindentation on a rough surface. The estimated mean hardness and scatter are both influenced by roughness, irrespective of whether there is a genuine variation in mechanical property with deformation volume/depth. The scatter due to roughness always decreases with penetration depth. The roughness dependency of hardness is the product of two terms. The first term is independent of property variation with deformation volume/depth and arises due to the discrete nature of multiple contacts during indentation. The second term is due to property variation with deformation volume/depth, as changing roughness changes the aggregate strength of each contact island.
Measurement of optical mode loss in visible emitting lasers
Authors: Mogensen P. C. , Smowton P. M. , Blood P.
Published: July 2002

We describe a technique for direct measurement of the passive optical mode loss of a semiconductor laser or similar semiconductor waveguide structure, based upon measurement of the attenuation of optically excited luminescence in the guided mode as a function of distance traveled along the passive guide. A spectrometer is used to select luminescence in the low energy tail of the spectrum which is subject to very little reabsorption. We have applied the method to a series of highly strained GaInP quantum well laser structures and observe an increase in the mode loss from 9.9 cm−1 for 1% strain to 46 cm−1 for 1.7% strain. This correlates with the appearance of clustered regions in the highly strained wells observed by transmission electron microscopy (TEM).
Poly(tetraflouro-p-xylylene), a low dielectric constant chemical vapor polymerized polymer
Authors: Senkevich Jay J. , Desu Seshu B.
Published: July 2002

A low dielectric constant polymer, poly(tetrafluoro-p-xylylene) (VT-4) was synthesized by a chemical vapor polymerization process using 4,5,7,8,12,13,15,16-octafluoro-[2.2]-paracyclophane as a precursor. The VT-4 polymer has a dielectric constant of 2.42 and a dielectric loss of 0.008 at 1 MHz, perpendicular to the plane of the film. Thermal stability of VT-4 satisfied the SEMATECH criteria, exhibiting an onset of degradation at 460 °C, and 1% weight loss at 480 °C in an argon environment. The x-ray diffraction data suggest a disordered semicrystalline polymer as-deposited (at ∼12 °C). The crystalline phase became more ordered due to a decrease in the d spacing from 4.850 to 4.594 Å and an increase in the percent crystallinity from 39% as-deposited to 66% after successive postdeposition anneals to 300 °C. Optical measurements showed a highly anisotropic thin film with ne @630 nm=1.601 and n0 @630 nm=1.471, progressively becoming more negatively birefringent after postdeposition anneals, reaching a plateau at ∼250 °C, due to the polymer chain becoming more conformationally ordered.
Electronic structure induced by lateral composition modulation in GaInAs alloys
Authors: Mattila T. , Bellaiche L. , Wang L.-W. , Zunger Alex
Published: July 2002

It has been recently shown that growth of [001]-oriented short period (AC)n/(BC)n vertical superlattices (n∼1−2) spontaneously creates a lateral composition modulation in the substrate plane ([110] direction), where wire-like AC-rich and BC-rich domains alternate with a period of ∼100−200 Å. This creates a new type of lattice structure with orthogonal [001] and [110] strain fields and compositional waves. Using a three-dimensional plane-wave pseudopotential approach, we study here the electronic properties of this type of structure in a GaInAs alloy, predicting valence band splittings into distinctly polarized components, a ⩽100 meV band-gap reduction and strong, type I electron and hole confinement in the In-rich lateral channels.
Contactless electroreflectance characterization of GaInP/GaAs heterojunction bipolar transistor structures
Authors: Huang Y. S. , Sun W. D. , Pollak Fred H. , Freeouf J. L. , Calder I. D. , Mallard R. E.
Published: July 2002

We have characterized two GaInP/GaAs (001) heterojunction bipolar transistor structures, fabricated by organometallic chemical vapor deposition and chemical beam epitaxy, using contactless electroreflectance, including the dependence of the signals on the polarization {[110] and [11̄0]} of the incident radiation. The ordering parameters deduced from the polarization dependence of the GaInP emitter signals are consistent with transmission electron microscope measurements. From the observed Franz–Keldysh oscillations we have evaluated the electric fields in the collector/base and emitter/base regions. In general, these fields are in good agreement with a calculation based on a comprehensive, self-consistent model, including the photovoltaic effect.
High birefringence and wide nematic range bis-tolane liquid crystals
Authors: Wu Shin-Tson , Hsu C.-S. , Shyu K.-F.
Published: July 2002

Several dialkyl and fluoro bis-tolane liquid crystals were synthesized and their physical properties evaluated. These highly conjugated liquid crystals exhibit a wide nematic range, small heat fusion enthalpy, high birefringence, and relatively low viscosity. Their excellent compatibility with commercial cyano mixtures makes these liquid crystals attractive for many electro-optic applications.
Passive Q-switching of fiber lasers using a broadband liquefying gallium mirror
Authors: Petropoulos P. , Offerhaus H. L. , Richardson D. J. , Dhanjal S. , Zheludev N. I.
Published: July 2002

Using a nonlinear cavity element, a liquefying gallium mirror, we demonstrate stable, self-starting, passive Q-switching of both erbium and ytterbium fiber laser cavities operating at wavelengths of 1550 and 1030 nm, respectively. The performance at 1550 nm is shown to be equivalent to that achieved with a state of the art semiconductor saturable absorber designed to work at this wavelength. The results highlight the suitability of this tremendously broadband, inexpensive nonlinear medium for a wide range of passive Q-switch applications.
Three-color organic light-emitting diodes patterned by masked dye diffusion
Authors: Pschenitzka Florian , Sturm J. C.
Published: July 2002

A method of masked dye diffusion to locally pattern the emissive color of organic light-emitting devices (OLEDs) over a large area is introduced. By using a large-area diffusion source, which may be a spin-coated doped polymer film, the entire process of masked diffusion of dye into a polymer film of an OLED to create an integrated three-color device has been demonstrated at atmospheric pressure. The materials used to demonstrate this method are the polymer poly(9-vinylcarbazole) combined with electron transport molecules, and the dyes bimane, coumarin 6, and Nile red.
On the formation and nature of nanometer size clusters on the surface of ZnSe epilayers
Authors: Zhang X. B. , Hark S. K.
Published: July 2002

ZnSe epilayers were grown on GaAs (001) substrates by metal organic chemical vapor phase deposition, using different group VI–II precursor flow ratios. Atomic force microscopy (AFM) examinations of their surface show that epilayers grown with a high VI/II ratio are not as stable as those grown with a low ratio. When exposed to air, Se clusters would appear and grow on the surface of the unstable epilayers. The ripening process could take as long as 50 days at room temperature. Secondary electron and cathodoluminescence images indicate that the clusters are more likely to emerge from areas of high defect density. Moreover, AFM topographic images of epilayers at an intermediate stage of ripening suggest that clusters near surface depressions would grow in size at the expense of others. By using a low accelerating voltage and allowing the clusters to grow to a size larger than the electron interaction volume, we used energy dispersive x-ray analysis to show that the clusters are made up entirely of Se.
Direct measurement of strain in a Ge island on Si(001)
Authors: Miller Peter D. , Liu Chuan-Pu , Henstrom William L. , Gibson J. Murray , Huang Y. , Zhang P. , Kamins T. I. , Basile D. P. , Williams R. Stanley
Published: July 2002

We report on a direct measurement of the strain in a single Ge “quantum dot” island grown on Si by chemical vapor deposition. This transmission electron microscopy method is reliable: without the need for detailed modeling of the strain field, it measures the maximum in-plane displacement. Good agreement is found between the experimental value of 0.86±0.17% average strain and finite element simulations assuming pure Ge. Thus no evidence of significant alloying with Si is observed.
Selective area metalorganic molecular-beam epitaxy of GaN and the growth of luminescent microcolumns on Si/SiO2
Authors: Guha S. , Bojarczuk N. A. , Johnson M. A. L. , Schetzina J. F.
Published: July 2002

We demonstrate the selective area growth of gallium nitride on patterned Si(111)/GaN/SiO2 wafers by metalorganic molecular beam epitaxy using triethyl gallium as a Ga source. We show that such selective area deposition may be used to grow isolated microcolumns of GaN with lateral dimensions of tens of nanometers on Si/SiO2 wafers. Via high resolution cathodoluminescence imaging we show that such microcolumn structures are highly luminescent inspite of a large surface to volume ratio, indicating that nonradiative recombination at free surfaces is not a significant issue in this system.
Cross-sectional atomic force microscopy of ZnMgSSe- and BeMgZnSe-based laser diodes
Authors: Ankudinov A. V. , Titkov A. N. , Shubina T. V. , Ivanov S. V. , Kop’ev P. S. , Lugauer H.-J. , Reuscher G. , Keim M. , Waag A. , Landwehr G.
Published: July 2002

Atomic force microscopy (AFM) of cleaved facets of ZnSe-based lasers with various active region designs is reported. Different AFM probe friction on the materials forming the laser structures are exploited for imaging their basic layers. Unlike ZnMgSSe-based lasers, the cleaved surface of cladding layers in BeMgZnSe-based structures is atomically flat, which is attributed to hardening of the II–VI materials by Be incorporation. Nanometer-high steps and undulations are observed at the laser heterointerfaces on cleaved facets. The shape and height of such topographic singularities located in the vicinity of a (Zn,Cd)Se quantum well active region depend on the strain distribution in the laser waveguide.
GaInNAs resonant-cavity-enhanced photodetector operating at 1.3 μm
Authors: Héroux J. B. , Yang X. , Wang W. I.
Published: July 2002

We demonstrate a GaAs-based p-i-n resonant-cavity-enhanced (RCE) GaInNAs photodetector operating near 1.3 μm. The device design was optimized using a transfer matrix method and experimental absorption spectra obtained from p-i-n structures grown without a resonant cavity. The RCE photodetector was fabricated in a single growth step by using GaAs/AlAs distributed Bragg reflectors for the top and bottom mirrors. A 72% quantum efficiency was obtained with a full width at half maximum of 11 nm.
Synthesis and characterization of amorphous carbon nanowires
Authors: Tang Y. H. , Wang N. , Zhang Y. F. , Lee C. S. , Bello I. , Lee S. T.
Published: July 2002

By heating a pressed tablet of graphite powder mixed with nickel in a quartz tube mounted inside a high-temperature tube furnace at 1200 °C, amorphous carbon nanowires were formed on the inner wall of the quartz tube near a copper cooling finger. Bright-field images of transmission electron microscopy show that the diameters of the nanowires are around 40 nm. Selected-area electron diffraction and Raman study reveal that the nanowires have an amorphous structure.
Magnitude of the threshold energy for hot electron damage in metal–oxide–semiconductor field effect transistors by hydrogen desorption
Authors: Hess K. , Tuttle B. , Register F. , Ferry D. K.
Published: July 2002

Based on the energetics for hydrogen desorption from the interface between silicon and silicon-dioxide, we argue that the hard threshold for this effect may be considerably lower than the previously assumed value (∼3.6 eV). We support these findings further by recent experimental results related to the giant isotope effect in hydrogen related transistor degradation and the fact that degradation occurs also with relatively low supply voltages. We also show that the high threshold energy model is difficult to defend at these low voltages, even though electron–electron interactions provide a mechanism to create hot electrons with energies of ∼3.6 eV.
A characterization technique for quantum well infrared photodetectors
Authors: Chen C. J. , Choi K. K. , Rokhinson L. , Chang W. H. , Tsui D. C.
Published: July 2002

Despite the rapid development of the quantum well (QW) infrared technology, the intrinsic properties of the QW infrared photodetectors (QWIPs) have not been directly measured under the operating conditions of the detector. In this work, we introduce a characterization technique, which utilizes the surface corrugation to probe the absorption coefficient and the photoconductive gain of a QWIP under different operating conditions. This technique enables the intrinsic properties of the detector to be more accurately characterized and its performance better assessed. A mid-wavelength QWIP is used for the demonstration of this technique. The results are compared to those deduced from the conventional measurements.
Patterning a surface on the nanometric scale by ion sputtering
Authors: Rusponi S. , Costantini G. , de Mongeot F. Buatier , Boragno C. , Valbusa U.
Published: July 2002

Surface etching by ion sputtering, besides producing equilibrium-oriented patterns similar to those obtained by molecular beam epitaxy (MBE), can also be used to pattern the surface along nonequilibrium orientations, thus extending the possibilities of MBE. By tuning the competition between ion erosion at grazing angles and diffusion-induced surface reorganization, it is, for example, possible to pattern a substrate characterized by a square symmetry with a well-ordered ripple structure running along any desired direction.
Critical issues for the growth of high-quality (Al,Ga)N/GaN and GaN/(In,Ga)N heterostructures on SiC(0001) by molecular-beam epitaxy
Authors: Brandt O. , Muralidharan R. , Waltereit P. , Thamm A. , Trampert A. , von Kiedrowski H. , Ploog K. H.
Published: July 2002

We identify and discuss the essential strategies for the growth of (Al,Ga)N/GaN and GaN/(In,Ga)N heterostructures on SiC(0001) by both plasma-assisted and reactive molecular-beam epitaxy. Substrate preparation, nucleation, and growth conditions are optimized for simultaneously satisfying the requirements of high structural, morphological, optical, and electrical quality. The results demonstrate that molecular-beam epitaxy is a competitive technique for the growth of group-III nitrides.
Dynamics of an air breakdown plasma on a solid surface during picosecond laser ablation
Authors: Mao Samuel S. , Mao Xianglei , Greif Ralph , Russo Richard E.
Published: July 2002

Using picosecond time-resolved shadowgrams and interferograms, we measured the lateral expansion of an early stage ablation plasma induced by a 1064 nm, 35 ps laser pulse on a copper target. The plasma was found to have an electron density on the order of 1020 cm−3 near the target surface. Prior to the expanding material vapor plume, this high density plasma originates from the breakdown of air, assisted by laser-induced electron emission from the target surface. The longitudinal expansion of the plasma was suppressed due to the development of a strong space-charge region. At postpulse times, the relation r∼t1/2 was found for the temporal lateral expansion of the radius of the plasma. Measurements of energy absorption by the plasma provide an interpretation for the experimentally measured reduction in ablation efficiency as the laser fluence increases beyond approximately 100 J/cm2.
Room-temperature thousandfold magnetoresistance change in MnSb granular films: Magnetoresistive switch effect
Authors: Akinaga H. , Mizuguchi M. , Ono K. , Oshima M.
Published: July 2002

A huge positive magnetoresistance effect has been discovered in MnSb granular films. Granular film consisting of nanoscale MnSb dots that are grown on a sulfur-passivated GaAs (001) substrate by molecular-beam epitaxy, then covered with an Sb thin layer, exhibits magnetic-field-sensitive current–voltage characteristics. When a constant voltage, above the threshold value, is applied to the film, more than 1000% change in the current, which we term magnetoresistive switch, is driven by the magnetoresistance effect under a relatively low magnetic field (less than 0.5 T) at room temperature.
Crystallographic and magneto-optical studies of nanoscaled MnSb dots grown on GaAs
Authors: Mizuguchi M. , Akinaga H. , Ono K. , Oshima M.
Published: July 2002

MnSb ultrathin films with the nominal thickness of 0–1.40 nm were grown on sulfur passivated GaAs substrates by molecular beam epitaxy. Atomic force microscopy analysis showed that MnSb formed nanosize clusters on the substrate, and the coalescence of the clusters occurred at the nominal thickness between 0.70 and 1.05 nm. The intensity of the polar magnetic circular dichroism of MnSb clusters suddenly increased when the nominal thickness reached the critical value of 1.05 nm. The coalescence among the dots can be correlated with the sharp increase of the magnetic circular dichroism intensity.
Distributed feedback laser action from polymeric waveguides doped with oligo phenylene vinylene model compounds
Authors: Kretsch Kevin P. , Blau Werner J. , Dumarcher Vincent , Rocha Licinio , Fiorini Celine , Nunzi Jean-Michel , Pfeiffer Steffen , Tillmann Hartwig , Hörhold Hans-Heinrich
Published: July 2002

We report lasing studies of poly(styrene) waveguides doped with amino- and cyano-substituted oligo phenylene vinylene (distyryl benzene) model compounds under picosecond excitation. Optical feedback is provided by distributed Bragg gratings formed in the film by interference patterns in the pump beam. We demonstrate broad tunability of laser emission in these materials and simultaneous lasing at two wavelengths separated by 23 nm. Tuning ranges of the model compounds are compared with previous experiments.
Observation of Li-atomic array in spinel-LiMn2O4 films spin-coated on Si substrates using an atomic force microscopy
Authors: Kushida K. , Kuriyama K.
Published: July 2002

The thin films of spinel-LiMn2O4 were synthesized on (100) Si substrates by a sol-gel method and subsequent furnace annealing at various temperatures (300–800 °C) for 30 min. Li and Mn acetates (the atomic ratio Li/Mn of 1:2) were used as a coating solution, and dissolved in methanol with a small amount of a chelating agent. The films were confirmed to be a single phase of LiMn2O4 (space group: Fd3m) by x-ray diffraction studies. The surface morphologies of the films were examined by an atomic force microscopy, showing that the films consisted of bar-shaped crystalline parts. The area of these parts extended with increasing the annealing temperature. In the films annealed at 300 °C, the Li atomic array with slight distortions was observed in an atomically flat surface, which was associated with an unit cell with a lattice spacing of 8.195±0.005 Å on the (100) surface in spinel-LiMn2O4.
Photoluminescence of ZnS:Mn embedded in three-dimensional photonic crystals of submicron polymer spheres
Authors: Zhou Ji , Zhou Y. , Buddhudu S. , Ng S. L. , Lam Y. L. , Kam C. H.
Published: July 2002

ZnS:Mn has been in-filled in photonic crystals of submicron polymer spheres. The effect of the photonic band gap on the photoluminescence (PL) properties of ZnS:Mn has been investigated. Because of the overlap of the transmission dip of the photonic crystal and the photoluminescence band of ZnS:Mn, both suppression and enhancement in the PL of the phosphor have been observed. A strong dependence of the fluorescence lifetime on the emission wavelength in the range of the stop band has been found. This strong dependence is believed to arise from the very low photon density of state within the stop band of the ZnS:Mn in-filled photonic crystal as result of a high dielectric contrast between ZnS:Mn and the polystyrene spheres.
Study of the crystalline quality of exfoliated surfaces in hydrogen-implanted silicon
Authors: Breese M. B. H. , Alves L. C. , Hoechbauer T. , Nastasi M.
Published: July 2002

Hydrogen-implanted “Smart-Cut®” silicon is attracting considerable interest for silicon on insulator applications in high-speed, low-power microelectronics. To optimize the fabrication process the mechanisms of bubble growth, defect formation and the origin of the residual surface roughness and damage must be studied under a variety of implantation and annealing conditions. This letter describes the analysis of the crystalline quality of the surfaces of exfoliated bubbles and delaminated layers in hydrogen-implanted silicon. Transmission and backscattering ion channeling are used to observe the separate contributions of lattice damage and internal gas pressure to the exfoliation process. Significant additional damage occurs during the exfoliation/delamination process, which is attributed to mechanically induced plastic deformation of the surface.
Discrete energy level separation and the threshold temperature dependence of quantum dot lasers
Authors: Shchekin Oleg B. , Park Gyoungwon , Huffaker Diana L. , Deppe Dennis G.
Published: July 2002

Data are presented on one- and two-stack InAs quantum dot lasers that have reduced temperature sensitivity of their lasing threshold. Adjustment of dot size and composition is used to increase the energy separation between the ground and first excited radiative transition energies to 104 meV, with a dot density of ∼3.1×1010 cm−2. The one- and two-stack lasers show broad area as-cleaved room temperature threshold current densities as low as 43 and 35 A/cm2, respectively. The wide energy separation between the ground and first excited radiative transitions leads to significant improvements in the temperature sensitivity of threshold.
Reduction of stress-induced leakage currents in thin oxides by application of a low post-stress gate bias
Authors: Ang C. H. , Ling C. H. , Cheng Z. Y. , Cho B. J. , Kim S. J.
Published: July 2002

We have observed that stress-induced leakage currents (SILC) in thin gate oxides (4.5 nm) could be reduced by applying a low gate bias to the oxides after stress, regardless of the polarity of the applied gate bias. The reduction of SILC increased with the applied gate bias and began to saturate after 105 s. In addition, the reduction of SILC was significantly enhanced in a hydrogen ambient, suggesting a strong link between the reduction of SILC and trapped-hole annealing.
Thermodynamics of heterogeneous multicomponent condensation on mixed nuclei
Authors: Djikaev Y. S. , Donaldson D. J.
Published: July 2002

We consider a nucleating center consisting of both an insoluble core and soluble species and develop the thermodynamics of isothermal formation of a droplet on such a nucleus in a multicomponent vapor mixture. Two different approaches to the derivation of the free energy of droplet formation within the framework of the capillarity approximation are considered. If condensation is not barrierless, the free energy of formation describes a multidimensional free-energy surface having a “well” point and a “saddle” point. It is shown that in a strict theory, taking account of surface enrichment effects, the compositions of droplets corresponding to these two points are equal and can be found without knowing the surface tension of the droplet. For the case of no surfactants in the droplet, we extend the Kuni method of investigating the behavior of the free energy of droplet formation to the case of heterogeneous multicomponent condensation on mixed nuclei, which makes it possible to find out all the main features of the free-energy surface without explicitly knowing the free-energy itself. The theoretical results are illustrated by numerical calculations for the water–methanol condensation on mixed nuclei.
Microstructure related influence on the electrical properties of pulsed laser ablated (Ba, Sr)TiO3 thin films
Authors: Saha S. , Krupanidhi S. B.
Published: July 2002

The microstructural dependence of electrical properties of (Ba, Sr)TiO3(BST) thin films were studied from the viewpoint of dc and ac electrical properties. The films were grown using a pulsed laser deposition technique in a temperature range of 300 to 600 °C, inducing changes in grain size, structure, and morphology. Consequently, two different types of films were realized, of which type I, was polycrystalline, multigrained, while type II was [100] oriented possessing a densely packed fibrous microstructure. Leakage current measurements were done at elevated temperatures to provide evidence of the conduction mechanism present in these films. The results revealed a contribution from both electronic and ionic conduction. In the case of type I films, two trapping levels were identified with energies around 0.5 and 2.73 eV, which possibly originate from oxygen vacancies VÖ and Ti3+ centers, respectively. These levels act as shallow and deep traps and are reflected in the current–voltage characteristics of the BST thin films. The activation energy associated with oxygen vacancy motion in this case was obtained as 1.28 eV. On the contrary, type II films showed no evidence of deep trap energy levels, while the identified activation energy associated with shallow traps was obtained as 0.38 eV. The activation energy obtained for oxygen vacancy motion in type II films was around 1.02 eV. The dc measurement results were further elucidated through ac impedance analysis, which revealed a grain boundary dominated response in type I in comparison to type II films where grain response is highlighted. A comparison of the mean relaxation time of the two films revealed three orders of magnitude higher relaxation time in the case of type I films. Due to smaller grain size in type I films the grains were considered to be completely depleted giving rise to only grain boundary response for the bulk of the film. The activation energy obtained from conductivity plots agree very well with that of dc measurements giving values 1.3 and 1.07 eV for type I and type II films, respectively. Since oxygen vacancy transport have been identified as the origin of resistance degradation in BST thin films, type I films with their higher value of activation energy for oxygen ion mobility explains the improvement in breakdown characteristics under constant high dc field stress. The role of microstructure in controlling the rate of degradation is found useful in this instance to enhance the film properties under high electric field stresses.
Molecular engineering of push–pull dipolar and quadrupolar molecules for two-photon absorption: A multivalence-bond states approach
Authors: Barzoukas M. , Blanchard-Desce M.
Published: July 2002

In the search of organic molecules with large two-photon absorption cross-sections, the push–pull dipolar and quadrupolar chromophores have both attracted major attention. In order to provide the basis of molecular engineering and comparison of these two families of molecules, we implement multivalence-bond states models based on measurable parameters. The analytical expressions of the resonant two-photon absorption cross-sections are derived for both families of molecules. Difference and likeness in a number of features are outlined by comparing the results obtained for these molecular systems. The multivalence-bond states models provide useful guidelines for the design of push–pull dipolar and quadrupolar chromophores with enhanced two-photon absorption cross-sections.
Invariant measures on polarized submanifolds in group quantization
Authors: Guerrero J. , Aldaya V.
Published: July 2002

We provide an explicit construction of quasi-invariant measures on polarized coadjoint orbits of a Lie group G. The use of specific (trivial) central extensions of G by the multiplicative group R+ allows us to restore the strict invariance of the measures and, accordingly, the unitarity of the quantization of coadjoint orbits. As an example, the representations of SL(2,R) are recovered.
Quantitative analysis on correlation between local coercivity and reversal time in ferromagnetic thin films
Authors: Choe Sug-Bong , Shin Sung-Chul
Published: July 2002

We report a method to quantitatively analyze the correlation between the local coercivity variation and the local reversal-time distribution in ferromagnetic thin films. The spatial distribution of the local coercivity on a film plane was directly measured from the hysteresis loops of each local area of 320×320 nm2 and then, the local coercivity distribution was quantitatively correlated with the local reversal-time distribution obtained from time-resolved domain evolution patterns grabbed at precisely the same position of the film. We demonstrate a clear experimental evidence of the direct correlation between the real coercivity distribution and the magnetization reversal dynamics, which could be explained within a context of a thermally activated relaxation process.
Compactness, aggregation, and prionlike behavior of protein: A lattice model study
Authors: Giugliarelli Gilberto , Micheletti Cristian , Banavar Jayanth R. , Maritan Amos
Published: July 2002

The solubility and compactness of proteins is investigated within the framework of models amenable to an exact numerical study through exhaustive enumeration. We study how the average inter-amino acid interaction potential affects the properties of both isolated and interacting proteins. In a concentrated solution, depending on the value of the average potential, individual proteins may remain stable in the isolated native structure (soluble case), may aggregate preserving their geometrical shape (nonsoluble case) or aggregate changing their geometrical shape (prionlike behavior). The number of sequences that have compact native states and are soluble is maximal at a fine-tuned average interaction potential and of the same order of the corresponding number of nonsoluble prionlike proteins. The viable protein sequences selected by such a fine-tuned potential are found to have an amino acid composition similar to naturally occurring proteins.
Multi-isotope study of fractionation effects in the ozone formation process
Authors: Wolf S. , Bitter M. , Krankowsky D. , Mauersberger K.
Published: July 2002

The isotope fractionation of all ozone isotopologs of mass 48–54 u has been measured with high accuracy. The data represent a standard for the magnitude of the ozone isotope effect since pressure, temperature, and composition of the gas in which ozone is produced are well known. While 17O17O17O and 18O18O18O are depleted by −1.5% and −3.9%, respectively, all others are enriched with 16O17O18O showing the highest value of 19.8%. Enrichments or depletions are independent of the isotope composition of the oxygen gas.
State-to-state studies of ground state NH(X 3Σ−,v=0,J,N)+Ne
Authors: Rinnenthal Jan Leo , Gericke Karl-Heinz
Published: July 2002

State-to-state rotational energy transfer of ground state NH(X 3Σ,v=0,J,N) in collisions with Ne is examined. NH is exclusively generated in the metastable NH(a 1Δ) state via photodissociation of hydrazoic acid at a wavelength of 266 nm. The strongly forbidden NH(a 1Δ→X 3Σ−) intercombination transition around 794 nm is used to generate single state NH(X 3Σ−,v=0,J,N) applying the stimulated emission pumping technique. The ground state radicals are detected after a certain delay time with laser induced fluorescence (LIF) using the intense NH(A 3Π←X 3Σ−) transition around 336 nm with respect to all quantum states. The collision induced energy flux between the different rotation and spin levels is studied in detail and a comprehensive set of state-to-state rate constants for inelastic collisions of NH(X 3Σ−,v=0,J,N) with Ne up to N=7 which include the effect of multiple collisions is given. The state-to-state rate constants are obtained by the use of an iterative integrated profiles method. We find a propensity for (ΔN=0, Δi=±1) and (ΔN=±1, Δi=0) transitions where N represents the quantum state for nuclear rotation and i represents the index of the spin component Fi. In most cases the energy transfer which changes the spin component and conserves the nuclear rotation quantum number N (ΔN=0, Δi=±1), is the most effective energy transfer in collisions with Ne. The energy dependence of the transition efficiency concerning only the nuclear rotation quantum number N obeys an energy-gap law (EGL).
Reynolds number effects on the fine structure of uniformly sheared turbulence
Authors: Ferchichi M. , Tavoularis S.
Published: July 2002

This study is an experimental investigation of the fine structure of uniformly sheared turbulence within the framework of Kolmogorov’s local isotropy theory. The fine structure was invoked from statistics of derivatives and differences of the streamwise velocity, in both the streamwise and transverse directions, for turbulence Reynolds numbers Reλ between 140 and 660. The streamwise derivative had a pdf with highly asymmetrical, stretched tails, which became more pronounced as Reλ increased. Its skewness grew slowly with Reλ, while its flatness increased at a higher rate, in conformity with previous studies. The transverse derivative had a pdf with a tail asymmetry that decreased as Reλ increased. Its skewness was positive and decreased from about 0.70 at Reλ=140 to about 0.34 at Reλ=660, while its flatness had values near 8.0 and increased only slightly with Reλ. The pdf and flatnesses of velocity differences suggested that intermittency in the inertial spectral range increased with a decreasing separation distance and increasing Reλ. Structure functions up to fourth order were compatible with Kolmogorov’s hypotheses, while the sixth-order structure functions required some correction.
Hole-injection enhancement by copper phthalocyanine (CuPc) in blue polymer light-emitting diodes
Authors: Yu Wang-Lin , Pei Jian , Cao Yong , Huang Wei
Published: July 2002

This article reports the study on enhancement of copper phthalocyanine (CuPc) for hole injection in blue polymer light-emitting diodes based on three blue emissive polymeric materials with different electronic structures under the circumstances of including or excluding an additional poly(N-vinylcarbazole) (PVK) layer. A thin CuPc layer may effectively enhance the hole injection from the anode to the emissive polymer layer and results in a dramatic decrease of operating voltage of the device. The thickness of the CuPc layer is optimized. The addition of a PVK layer between CuPc and emissive polymer may prevent the potential problem of the undesired electron flow from the emissive layer to the CuPc layer. The efficiency of CuPc for enhancing hole injection is almost independent of the energy levels of the highest occupied molecular orbitals of the polymers which are contacted with it. It is suggested that the higher affinity between the two organic materials in comparison with that between indium–tin–oxide and an organic material and the resulted rough interface play a dominated role in the enhancement for hole injection.
Role of CsF on electron injection into a conjugated polymer
Authors: Piromreun Pongpun , Oh HwanSool , Shen Yulong , Malliaras George G. , Scott J. Campbell , Brock Phil J.
Published: July 2002

We studied electron injection from Al and Au cathodes into the conjugated polymer poly[2-methoxy,5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH–PPV). When a thin CsF layer is inserted between MEH–PPV and Al, a substantial enhancement in electron injection is observed. Insertion of the same layer between MEH–PPV and Au does not have a similar effect, indicating that the enhancement mechanism is specific to CsF and Al. Thin Cs layers enhance electron injection regardless of the cathode metal. A mechanism that explains these observations is proposed.
Spectral assignment of dual conformers of 1,2,3,6,7,8-hexahydropyrene in the supersonic jet
Authors: Chakraborty Abhijit , Nath Debnarayan , Halder Mintu , Guchhait Nikhil , Chowdhury Mihir
Published: July 2002

The laser-induced fluorescence excitation (FE) spectra of 1,2,3,6,7,8-hexahydropyrene has been observed in jet-cooled condition. The FE spectrum show the existence of interconvertible conformers, chair and boat, with 0–0 bands 12 cm−1 apart. On the basis of semiempirical calculations and single vibronic level luminescence, the longest wavelength band at 30 653 cm−1 has been ascribed to chair conformer and the one at 30 665 cm−1 to the boat conformer. The low-frequency vibronic bands have been tentatively assigned. The lifetimes of low-frequency vibronic levels follow the trend reported by Jacobson et al. [J. Chem. Phys. 87, 269 (1987)] for naphthalene monoderivatives, namely, they fluctuate at low excess energies, but merge and decrease steadily at high energies of excitation.
The role of conical intersections in the nonadiabatic quenching of OH(A 2Σ+) by molecular hydrogen
Authors: Hoffman Brian C. , Yarkony David R.
Published: July 2002

The role of the C2v(1 2B2–1 2A1), Cs(1 2A′–2 2A′), and C∞v(1 2Π–1 2Σ+) seams of conical intersection in the dynamics of the nonadiabatic quenching of OH(A 2Σ+) by molecular hydrogen is investigated. The locus of the C2v and C∞v conical intersections and the Cs seam bridging them is determined. The accessibility of these seams following optical excitation from the ground 1 2B2 state of the OH–H2 van der Waals complex is examined. Using linear interpolation pathways on the 2 2A′ potential-energy surface connecting these seams and the Franck–Condon region for vertical excitation as a guide, a transition state separating the 2 2A′ van der Waals complex from the seam of conical intersection was located. The barrier of 8.44 kcal mol−1 is due primarily to the required rotation of the OH molecule relative to H2. Finally, gradient descent paths from the conical intersection were determined to identify the possible products of the nonadiabatic quenching. For each seam, pathways leading to both OH+H2 and H2O+H products were found.