bluuebunny/crossref_metadata_2025 huggingface dataset:
Model for InGaP/GaAs/InGaP double heterojunction bipolar transistor
Authors: Lew K. L. , Yoon S. F.
Published: July 2002

The current–voltage characteristics of an InGaP/GaAs/InGaP double heterojunction bipolar transistor (DHBT) are modeled where the tunneling effect at the base–collector (B–C) junction and base–emitter (B–E) junction is taken into account. Therefore, this model can be applied to both conventional DHBT and composite collector heterojunction bipolar transistor. The role of the n−GaAs layer and n+InGaP layer are discussed and the effects of variation in the doping level and thickness of these layers are considered in the model. Good agreement between our predictions of the model and reported experimental results is achieved.
Disorder-activated infrared modes and surface depletion layer in highly Si-doped hexagonal GaN
Authors: Kasic A. , Schubert M. , Kuhn B. , Scholz F. , Einfeldt S. , Hommel D.
Published: July 2002

Three infrared-active low-polar modes are reported for highly Si-doped hexagonal (α-) GaN. The 0.8–1.6 μm thick films, grown by metal organic vapor phase epitaxy or molecular beam epitaxy on (0001) sapphire substrates, were studied by infrared spectroscopic ellipsometry. For GaN epilayers with free-electron concentration N⩾8×1018 cm−3 we observe, besides the usual GaN transverse-optical lattice modes and coupled longitudinal-optical phonon-plasmon modes, a band of additional modes at 567.4±2.5, 752.5±0.9, and 855.0±0.9 cm−1. We tentatively assign the first one to the disorder-activated high E2 GaN mode and the third mode to an acoustic-optical combination band, whereas the origin of the second mode remains unclear. Furthermore, the ellipsometric spectra of highly n-conductive Si-doped GaN reveal thin carrier-depleted regions at the sample surface.
Formation of shallow acceptor states in the surface region of thin film diamond
Authors: Williams Oliver A. , Whitfield Michael D. , Jackman Richard B. , Foord John S. , Butler James E. , Nebel Christoph E.
Published: July 2002

Considerable interest exists in fabrication of electronic devices from thin film polycrystalline diamond. To date, doping this material to achieve good free carrier concentrations and mobilities at room temperature has proved difficult. In this letter we report low temperature Hall effect measurements made on diamond films subjected to a hydrogenation process, such that the near surface region becomes p type without the addition of conventional dopant atoms. High carrier concentrations and mobilities can be achieved. The change in carrier concentration within the temperature range 10–300 K does not change as expected for most films, actually increasing as the temperature falls. This effect could be related to the confinement of carriers at the surface caused by the dipole provoked by adsorbed hydrogen on the diamond. However, polished films display more conventional behavior in that the carrier concentration falls with falling temperature.
Excited-state quenching of a highly luminescent conjugated polymer
Authors: Belton C. , O’Brien D. F. , Blau W. J. , Cadby A. J. , Lane P. A. , Bradley D. D. C. , Byrne H. J. , Stockmann R. , Hörhold H-H.
Published: July 2002

The optical properties of a luminescent polymer, poly[1,4-phenylene-1,2-di(phenoxyphenyl) vinylene], have been investigated. Its photoluminescence yield increases unusually in the solid-state over solution, 52%–6% respectively. Investigations into the stimulated emission properties of this material were carried out but no amplified spontaneous emission was observed. To investigate the presence of excited-state absorption features, the photoinduced absorption spectrum was measured. An observed polaron absorption band, from 1.5 to 2.25 eV, overlaps the emission spectra and therefore quenches stimulated emission. This highlights the need to consider the effects of excited-state absorption on the emission when synthesizing new materials.
Strong luminescence from Tamm states in modulation-doped superlattices
Authors: Henriques A. B.
Published: July 2002

Calculations are presented for the photoluminescence spectrum of InP/In0.53Ga0.47As superlattices doped with Si. When doping is confined to the inner barriers, the photoluminescence is dominated by transitions between Tamm states in the electron and valence bands, which contribute with an optical band at energies higher than the In0.53Ga0.47As band gap. These results are in good agreement with experimental observations.
Kinetics of Si growth from hydride precursors on As-passivated Si(001) surface
Authors: Tok E. S. , Hartell A. D. , Zhang J.
Published: July 2002

The presence of arsenic on a Si(001) surface is known to have a strong effect on the growth rate during epitaxial growth of silicon from hydride precursors. In this letter, the authors describe a study of this effect using the reflection high energy electron diffraction (RHEED) intensity oscillation technique. A number of surface As coverages produced by cycles of arsine adsorption at temperature below 400 °C and hydrogen desorption at 600 °C were considered. The period of the first RHEED intensity oscillation during overgrowth on a partially As-passivated Si(001) surface is found to be proportional to that during homoepitaxy. The constant of proportionality is equal to the inverse of the fraction of the clean Si surface not passivated by As. This experimental finding is explained using a model based on rate equations for surface hydrogen coverage and site exclusion by arsenic.
Wide-bandwidth high-frequency electro-optic modulator based on periodically poled LiNbO3
Authors: Lu Yan-qing , Xiao Min , Salamo Gregory J.
Published: July 2002

We propose a high-frequency traveling-wave integrated electro-optic modulator based on a periodically poled LiNbO3. The traveling velocity of the optical wave and the electrical wave velocity in the waveguide can be quasimatched due to the periodic structure. Using this design, a modulation frequency of several hundred GHz can be realized. Wide-bandwidth modulation is also achievable by employing an aperiodic domain grating.
UV photodissociation dynamics of ethyl radical via the Ã 2A′(3s) state
Authors: Amaral Gabriel , Xu Kesheng , Zhang Jingsong
Published: July 2002

H-atom channels in the photodissociation of jet-cooled ethyl radical (C2H5) via the Ã 2A′(3s) state are studied near 245 nm by using the high-n Rydberg-atom time-of-flight technique. Bimodal product translational energy release and energy-dependent angular distribution suggest two dissociation pathways. A slow (〈fT〉∼0.35) and isotropic channel corresponds to unimolecular dissociation of the radical, presumably after internal conversion. A previously unobserved fast (〈fT〉∼0.78) and anisotropic (β=0.5±0.1) channel is consistent with direct H-atom scission via a nonclassical H-bridged transition state from the 3s state to yield H+C2H4(X̃ 1Ag). The fast/slow branching ratio is ∼0.2. Site-selective loss of the β hydrogen atom is confirmed by using the partially-deuterated CH3CD2 radical.
Nonvolatile grating in an azobenzene polymer with optimized molecular reorientation
Authors: Wu Pengfei , Rao D. V. G. L. N. , Kimball B. R. , Nakashima M. , DeCristofano B. S.
Published: July 2002

We demonstrated a nonvolatile grating using an azobenzene polymer film with polarized two color beams. The reorientation of azobenzene molecules can be optimized when the two color light beams are polarized perpendicularly. The stored information can be read repeatedly without volatility with the same wavelength as the writing beam.
Adaptation to low temperatures of a differential interference contrast microscope with subnanometer sensitivity
Authors: Müller X. , Kinoshita T. , Dupont-Roc J.
Published: July 2002

We show how a differential interference contrast microscope can be adapted for low temperature operation by having only the microscope objective at low temperatures. The microscope’s body, with delicate birefringent optics, is kept at room temperature. A key part is the use of two Wollaston prisms with opposite anisotropic media to ensure a flat field. The device can detect optical path differences of the order of 10 pm/μm with a few seconds of integration time. Images taken at several hour intervals can be subtracted, indicating good mechanical stability.
Spontaneous formation of horizontal chevrons in smectic-C* liquid crystals
Authors: Strangi G. , Coleman D. A. , Maclennan J. E. , Copic M. , Clark N. A.
Published: July 2002

We report an unusual structural formation observed in surface-stabilized ferroelectric liquid crystal cells with asymmetrically treated surfaces of 3-Glycidoxypropyl trimethoxysilane and nylon. On cooling the material W415 from the smectic-A* to the smectic-C* phase, the bookshelf smectic layers spontaneously rearrange to form a horizontal chevron structure, manifested optically as a periodic texture of uniform stripes. The polarization is perpendicular to the boundary plates and reverses direction from stripe to stripe, while the director is oriented in the plane of the cell and alternates symmetrically about the stripe direction. In an applied field the cells show a V-shaped (analog) electro-optic response.
Epitaxial crystallization of keV-ion-bombarded α quartz
Authors: Roccaforte F. , Bolse W. , Lieb K.-P.
Published: July 2002

In this article, our results on the epitaxial crystallization of ion-bombarded crystalline silicon dioxide (α quartz) are reviewed. The epitaxial recrystallization of amorphized layers was achieved after alkali irradiation and annealing in air in the temperature range 650–875 °C. The systematic behavior of alkali ions in enhancing the regrowth rate both with decreasing ion size and increasing concentration is shown. The role of oxygen in the recrystallization was investigated by means of nuclear reaction analysis, by performing thermal treatments of the samples in O18. A large amount of O18 diffuses inside the amorphous layer in the alkali-ion implanted samples at 600–800 °C. From the strong correlation between the migration of O18 and implanted alkali, it was possible to gain further insights into the recrystallization mechanism.
Temperature dependence of the Faraday rotation for CdMnCoTe films
Authors: Ahn J. Y. , Tanaka M. , Imamura M.
Published: July 2002

The temperature dependence of magneto-optical property in the visible wavelength region has been studied on four-element semimagnetic semiconductor CdMnCoTe films deposited on quartz glass substrates by using MBE equipment. A large dispersion of Faraday rotation was observed, and the peak of the Faraday rotation was shifted to the higher photon energies with increasing Mn concentration at low temperatures. At 180 K, the value of the Faraday rotation observed for the Cd0.647Mn0.34Co0.013Te film on quartz glass was −0.36 deg/cmG at 630 nm. It is equivalent to the value of −0.36 deg/cmG observed at 77 K for the Cd0.52Mn0.48Te film on quartz glass. At 77 K, the Faraday rotation observed for the Cd0.647Mn0.34Co0.013Te film on quartz glass was −0.49 deg/cmG at 610 nm. The value is approximately two times larger than that of the Cd0.52Mn0.48Te film deposited on the same quartz glass substrate. The origin of the enhancement of Faraday rotation in CdMnCoTe films has been discussed in terms of the magnetic susceptibility χ.
Size and shape effects on exchange field of patterned NiO/NiFe films
Authors: Wang Yu-Jen , Lai Chih-Huang
Published: July 2002

The exchange field and domain configurations were investigated in patterned polycrystalline and (100) NiO/NiFe films. The exchange field was enhanced in the polycrystalline patterns with the aspect ratio larger than one (the long edge parallel to the original easy axis). Exchange field of patterned (100) NiO/NiFe on MgO, in contrast, showed little dependence on the aspect ratio, but strong dependence on the pattern size. The exchange field increased from 8.6 Oe in the sheet film to 48.4 Oe in the 2 μm patterns. The different dependence of the exchange field of polycrystalline and (100) NiO/NiFe on the pattern size can be plausibly explained by the difference of the domain size in NiO, and the variation of the blocking temperature distribution. Magnetic force microscopy images of patterned polycrystalline NiO/NiFe showed two kinds of domain configurations (single-domain state and multidomain state) in the sample with the pattern size of 2 μm, which may be the direct observation of various exchange paths at the interface of NiO/NiFe.
Electron heating by photon-assisted tunneling in niobium terahertz mixers with integrated niobium titanium nitride striplines
Authors: Leone B. , Gao J. R. , Klapwijk T. M. , Jackson B. D. , Laauwen W. M. , de Lange G.
Published: July 2002

We describe the gap voltage depression and current–voltage (I–V) characteristics in pumped niobium superconductor–insulator–superconductor junction with niobium titanium nitride tuning stripline by introducing an electron heating power contribution resulting from the photon-assisted tunneling process. Theoretical fits using the extended Tien–Gordon theory are obtained that reproduce the most salient features of the pumped I–V characteristics.
Multiple-phase behavior and its microscopic implication for 4-acrylamidosalicylic acid gel
Authors: Annaka Masahiko , Shibayama Mitsuhiro , Ikkai Fumiyoshi , Sugiyama Masaaki , Hara Kazuhiro , Nakahira Takayuki , Tanaka Toyoichi
Published: July 2002

Poly(4-acrylamidosalicylic acid) gels exhibited multiple-phase behavior depending on their histories in the parameter space of pH and temperature. Four different phases, denoted as phase091 (as-prepared), phase244 (swollen at high pH), phase064 (heat-treated), and phase233 (swollen at high pH after heat treatment) were clearly resolved, where the three digits denote their linear swelling ratios in percentage with respect to their sizes at preparation. Each phase was stable and did not change its swelling ratio with pH or temperature as long as the values of pH or temperature were within limited ranges. Transitions among different phases were discrete with hysteresis loops. The structure factors corresponding to these four phases were obtained by small-angle neutron scattering, which indicated the presence of characteristic structures depending on pH and temperature, particularly in the shrunken state (i.e., phase064).
Infrared spectra of the CO–H2O 1–1 cluster trapped in an argon matrix
Authors: Abe Hisashi , Yamada Koichi M. T.
Published: July 2002

The carbon monoxide–water (c–w) cluster produced in an argon matrix at cryogenic temperatures has been reinvestigated by FTIR spectroscopy. In the 4.7 μm region, a line at 2149.4 cm−1 was assigned to the CO stretching mode of the CO–H2O 1–1 cluster. In the 2.7 μm region, two lines, at 3627.8 and 3723.5 cm−1, were assigned to the OH stretching modes of the CO–H2O 1–1 cluster. By the deuterium enriched experiment, the lines of the CO–D2O 1–1 cluster and the CO–HDO 1–1 cluster were identified for the CO stretching vibration and the OD stretching vibration. The tunnel splitting observed in gas phase has not been detected in the present study. Combining with the gas phase data the matrix shifts in the line positions were obtained precisely, with which we have predicted some band centers of the free complexes so far not reported in literature. Migration of trapped molecules, CO and H2O, in the argon matrix has been observed both by annealing and by high temperature deposition. The spectral behavior shows that CO can move at a lower temperature than H2O in the argon matrix.
Magnetoluminescence of Zn(Mn)Se/Be(Mn)Te semimagnetic heterostructures with a type-II band alignment
Authors: Yakovlev D. R. , Sas C. , König B. , Hansen L. , Ossau W. , Landwehr G. , Molenkamp L. W. , Waag A.
Published: July 2002

Zn(Mn)Se/Be(Mn)Te semiconductor heterostructures with a type-II band alignment have been fabricated by molecular-beam epitaxy. Giant Zeeman splitting of the band states, caused by their interaction with the localized magnetic moments of Mn ions, has been observed for the spatially direct and indirect optical transitions. Strong p–d exchange interaction for the valence band states in (Be,Mn)Te has been demonstrated experimentally.
Hot-electron transport through Ni80Fe20 in a spin-valve transistor
Authors: Vlutters R. , Jansen R. , van ’t Erve O. M. J. , Kim S. D. , Lodder J. C.
Published: July 2002

Hot-electron transport in Ni80Fe20 thin films was studied using a spin-valve transistor. By varying the NiFe thickness from 10 to 100 Å we obtain an attenuation length of 43 Å for majority-spin hot electrons at 0.9 eV above the Fermi level. Based on such relatively long bulk attentuation lengths, one would expect a current transfer ratio that is much larger than the measured value. We propose that the discrepancy can be accounted for by considering interfacial scattering. Increasing the growth quality should thus provide a means to improve the current transfer ratio.
Investigation of inversion domains in GaN by electric-force microscopy
Authors: Jones K. M. , Visconti P. , Yun F. , Baski A. A. , Morkoç H.
Published: July 2002

Inversion domains in III-nitride semiconductors degrade the performance of devices fabricated in them. Consequently, it is imperative that we understand their electrostatic manifestation, the growth conditions under which such domains form, and an effective means of their identification. In what is nominally referred to as Ga-polarity samples, N-polarity domains have a polarization that is reversed with respect to the remainder of the surface, and therefore, have a different potential under strain. We have used surface-potential electric-force microscopy (SP-EFM) to image the electrostatic surface potential of GaN grown on sapphire, which is strained due to the thermal mismatch between the substrate and GaN. Employing a control sample with side-by-side Ga- and N-polarity regions, we have established the EFM mode necessary to identify inversion domains on GaN samples grown by molecular-beam epitaxy. This method is not sensitive to topology and has a spatial resolution of under 100 nm. The measured surface potentials for Ga-face and N-face regions are +25±10 and −30±10 mV, respectively, with respect to the sapphire substrate, where the sign is consistent with Ga- and N-polarity GaN under compressive strain due to thermal mismatch with the sapphire substrate.
Degradation and time dependent breakdown of stressed ferromagnetic tunnel junctions
Authors: Das J. , Degraeve R. , Boeve H. , Duchamps P. , Lagae L. , Groeseneken G. , Borghs G. , De Boeck J.
Published: July 2002

Ferromagnetic tunnel junctions are very sensitive to degradation and breakdown, due to the ultrathin (∼1 nm) tunnel barrier. When the junction is stressed with a constant current or voltage, a conductance change of the tunnel junction is observed. Sufficiently high stress will lead to breakdown of the junction. As in SiO2 gate oxide reliability studies, the Weibull distribution plot can be obtained from the time to breakdown data. The dependence of the Weibull function on the area and the stress conditions is studied for the Al2O3 barrier of the tunnel junctions. This is the first step of a systematic study of reliability, which is an important issue for the use of tunnel junctions in, e.g., magnetic random access memory applications.
The action of pressure-radiation forces on pulsating vapor bubbles
Authors: Hao Y. , Og̃uz H. N. , Prosperetti A.
Published: July 2002

The action of pressure-radiation (or Bjerknes) forces on gas bubbles is well understood. This paper studies the analogous phenomenon for vapor bubbles, about which much less is known. A possible practical application is the removal of boiling bubbles from the neighborhood of a heated surface in the case of a downward facing surface or in the absence of gravity. For this reason, the case of a bubble near a plane rigid surface is considered in detail. It is shown that, when the acoustic wave fronts are parallel to the surface, the bubble remains trapped due to secondary Bjerknes force caused by an “image bubble.” When the wave fronts are perpendicular to the surface, on the other hand, the bubble can be made to slide laterally.
Monolithic-integrated two-wavelength laser diodes for digital-versatile-disk/compact-disk playback
Authors: Nemoto Kazuhiko , Kamei Takafumi , Abe Hiroaki , Imanishi Daisuke , Narui Hironobu , Hirata Shoji
Published: July 2002

We have developed a monolithic two-wavelength laser diode, which emits 650 and 780 nm wavelengths. This device, which has a separated-confinement-heterostructure multi-quantum-well active region and a gain-guiding tapered-stripe structure, is fabricated using only two steps of metal organic chemical vapor deposition. The operating currents at 5 mW are 57.0 and 61.5 mA for the 650 and 780 nm elements, respectively. The relative intensity noise of the 650 and 780 nm elements was below −130 dB/Hz up to 70 °C without high-frequency superposition circuits.
Deposition of [Ni–Fe/Al–O/Co–Fe] films with tunneling magnetoresistance effect using the interfacial modulation technique
Authors: Miyamoto Yasuyoshi , Machida Kenji , Hayashi Naoto , Tamaki Takahiko , Okuda Haruo
Published: July 2002

[Ni–Fe/Al–O/Co–Fe] films with tunneling magnetoresistance (TMR) were deposited by the dual ion beam sputtering method. An interfacial modulation technique using Kr ion bombardment on the initial growth region of each layer in TMR devices changed the crystallographic structures and interfacial conditions of the films. Kr was used as the sputtering gas in order to reduce the damage to growing films by recoiled particles. The effect of Kr ion bombardment on the initial growth region seemed to be effective in decreasing the film resistivity of Ni–Fe and Co–Fe films, especially at the optimized bombarding energy of 100 eV. In addition, stoichiometric Al2O3 films could be attained by reactive ion beam sputtering with O2 concurrently flow to the substrate. These optimized films were applicable as the ferromagnetic and insulation layers in TMR devices.
Observations of bi-Maxwellian and single Maxwellian electron energy distribution functions in a capacitively coupled radio-frequency plasmas by laser Thomson scattering
Authors: Mansour ElSabbagh M. A. , Bowden M.D. , Uchino K. , Muraoka K.
Published: July 2002

Electron energy distribution functions in low-pressure capacitively coupled radio-frequency argon discharges were measured using the technique of laser Thomson scattering. It was found that the distribution functions changed from a bi-Maxwellian at lower pressures to a single Maxwellian at higher pressures. These measurements provide independent confirmation of probe measurements made in similar discharges. The electron temperature and density of the cold group of electrons were measured with an accuracy of better than 10%.
Formation of a SiO2/SiC structure at 203 °C by use of perchloric acid
Authors: Kobayashi Hikaru , Sakurai Takeaki , Nishiyama Masayoshi , Nishioka Yasushiro
Published: July 2002

A chemical method for the low-temperature formation of a SiO2/SiC structure is developed using perchloric acid (HClO4). The thickness of the SiO2 layer increases linearly with the time of the immersion of 6H-SiC(0001) in the HClO4 solution at 203 °C, indicating the reaction-limited process. The immersion in 20 h results in the formation of an 80-nm-thick SiO2 layer. The SiO2 layer contains little carbon and suboxide species, but includes chlorine (Cl) species with the atomic concentration of 0.2%, leading to a high leakage current density. Heat treatment at 900 °C removes the Cl species, and the leakage current density becomes negligibly low.
Structural and electrical characteristics of Pb0.90La0.15TiO3 thin films on different bottom electrodes
Authors: Bhaskar S. , Majumder S. B. , Dobal P. S. , Katiyar R. S. , Krupanidhi S. B.
Published: July 2002

Pb 0.90 La 0.15 TiO 3 (PLT15) thin films were deposited by the sol-gel method on Pt, Pt/Si, and RuO2 on Si and Pt/Si bottom electrodes. X-ray diffraction, micro-Raman spectroscopy, and atomic force microscopy techniques were used for structural characterization of these films. PLT15 films on different electrodes showed good surface morphology with dense and uniform microstructure. PLT15 films on solution derived RuO2 bottom electrodes show (100) preferred orientation of growth and result in larger crystallites. Films deposited on a Pt bottom electrode show sharp and intense Raman features indicating better crystallinity and insignificant film–electrode interactions. PLT15 film on a Pt bottom electrode exhibited higher dielectric constant (1300 at 100 kHz) and high values of Pm and Pr, 68 and 46 μC/cm2, respectively, compared to other electrodes. Films on a RuO2 bottom electrode showed relatively inferior dielectric and ferroelectric properties. The ac field dependence of dielectric permittivity at subswitching fields was fitted using the Rayleigh law. It was found that ∼22% of the total measured permittivity was due to irreversible domain wall displacement for the films on a Pt electrode. The reversible polarization components estimated from the capacitance–voltage (C–V) and quasistatic hysteresis measurements showed that Prev/Psat at Vmax for the case of Pt/Si (24%), was larger than that of Pt (11%) bottom electrodes. The observed results were correlated with the domain wall pinning at the disturbed film–electrode interface.
Self-organized (553)BIn0.15Ga0.85As/GaAs quantum-wire field-effect transistors
Authors: Yan Fa-Wang , Li Xian-Jie , Zhang Wen-Jun , Zhang Rong-Gui , Liu Wei-Ji , Ao Jin-Ping , Liang Chun-Guang , Liu Shi-Young
Published: July 2002

Extremely uniform and high-density In0.15Ga0.85As/GaAs quantum wires (QWRs) were naturally formed on a (553)B-oriented GaAs substrate by molecular-beam epitaxy. The density of the QWRs is as high as 4.0×105 cm−1. The strong photoluminescence peak at λ=868 nm from the (553)B QWRs shows a large polarization anisotropy [p=(I∥−I⊥)/(I∥+I⊥)=0.22] and a very small full width at half maximum of 9.2 meV at 12 K. Based on the modulation-doped (553)B QWR structure, self-organized QWR field-effect transistors were fabricated (the channel along the QWRs’ direction). The devices demonstrate very good saturation characteristics and pinch-off behavior at room temperature. A maximum transconductance (gm) of 135 mS/mm is measured for 2 μm gate-length devices.
Solvent-enhanced dye diffusion in polymer thin films for color tuning of organic light-emitting diodes
Authors: Pschenitzka F. , Sturm J. C.
Published: July 2002

A method of solvent-enhanced dye diffusion in polymer films for organic light-emitting diode (OLED) application is introduced. After an initial dye transfer from a dye source substrate into the top of the device polymer film, the device substrate is then exposed to solvent vapor. Due to solvent absorption by the polymer film, the glass transition temperature of the polymer is significantly decreased which leads to enhanced diffusion of the dye in the polymer film. Secondary ion mass spectroscopy and depth dependent photoluminescence show that the temperature for dye diffusion can be decreased by at least 100 °C, to room temperature. OLEDs with 0.6% external quantum efficiency have been demonstrated with this method using the polymer poly(9-vinylcarbazole) combined with electron transport molecules and the dyes coumarin 47 and coumarin 6.
Nanoscale fracture studies using the scanning force microscope
Authors: Baumeister Bettina , Jung Thomas A. , Meyer Ernst
Published: July 2002

We present a variety of experiments concerning friction and fracture mechanisms using two-dimensional arrays of microfabricated nanotowers as templates. The scanning force microscope tip is used as a tool to apply well-defined forces to the surface of the patterned substrate. Force statistic measurements reveal information about the forces involved in the fracture process and the probability of fracture of selected towers. These methods are discussed in the context of nanometer-scale mechanisms. Using optimized parameters, a controlled removal of individual nanotowers and the ability to “write” predefined patterns on a nanometer scale can be achieved.
Sum rules for generalized electron-pair moments of Hartree–Fock orbitals
Authors: Koga Toshikatsu
Published: July 2002

For many-electron systems, the generalized electron-pair density function g(q;a,b) represents the probability density function for the magnitude |ari+brj| of two-electron vector ari+brj to be q, where a and b are real-valued parameters. For Hartree–Fock wave functions, decomposition of g(q;a,b) into spin–orbital-pair components gij(q;a,b) shows that the second moments 〈q2〉(a,b)ij, associated with gij(q;a,b), satisfy several rigorous sum rules which connect one- and two-electron properties of spin–orbitals i and j. The same is also true in momentum space. As an illustrative application, the orbital kinetic energies of the Rn atom are separated into the relative motion and center-of-mass motion contributions of electrons in two relevant orbitals.
Direct patterning of nanometer-scale silicide structures on silicon by ion-beam implantation through a thin barrier layer
Authors: Mitan M. M. , Pivin D. P. , Alford T. L. , Mayer J. W.
Published: July 2002

CoSi 2 structures were formed by focused ion-beam implantation. Patterned silicide lines with dimensions down to 150 nm were produced on (100) silicon. The process involved the ion implantation of 200 keV As++ through a cobalt (34 nm)/oxide (∼2 nm) thin film structure. The thin oxide at the Si/Co interface acted as a selective reaction barrier. Ion-beam mixing disrupted the oxide layer to allow silicidation to proceed during subsequent rapid thermal anneal treatments. Reactions were inhibited in nonimplanted areas. A threshold dose of 3×1015 cm−2 was required for process initiation. Electrical measurements resulted in resistivities ranging from 15 to 30 μΩ cm.
Characteristics of n+–p junction leakage induced by tantalum pentoxide gate insulator and gate reoxidation
Authors: Kang Chang-Yong , Kim Young-Gwan , Kang Dae-Gwan
Published: July 2002

This letter will present the n+–p junction characteristics in tantalum pentoxide gate dielectric (Ta2O5) and gate reoxidation ambient. The n+–p junctions in n-type metal–oxide–silicon field effect transistor fabricated with different gate dielectrics and post-thermal conditions were characterized by current–voltage measurements. The current–voltage measurements of junction leakage of Ta2O5 gate dielectric without gate reoxidation (MT1) show the hump characteristics due to the precipitates of oxide near the junction depletion regime. And their leakage mechanism is phonon-assisted tunneling, which facilitates the hopping of electrons from the valence band into the shallow attractive Coulomb center via the interface states in the precipitate, leaving holes in the valence band. However, the junction leakage currents of Ta2O5 dielectric with gate reoxidation in hydrogen rich ambient (MT2) are higher than those of thermal oxide samples and their leakage mechanism is Schottky barrier lowering due to the enhanced diffusion of oxygen into the junction depletion region.
Charging of single Si nanocrystals by atomic force microscopy
Authors: Boer E. A. , Bell L. D. , Brongersma M. L. , Atwater H. A. , Ostraat M. L. , Flagan R. C.
Published: July 2002

Conducting-tip atomic force microscopy (AFM) has been used to electronically probe silicon nanocrystals on an insulating substrate. The nanocrystal samples were produced by aerosol techniques and size classified; nanocrystal size can be controlled in the size range of 2–50 nm with a size variation of less than 10%. Using a conducting tip, the charge was injected directly into the nanocrystals, and the subsequent dissipation of the charge was monitored. Estimates of the injected charge can be made by comparison of the data with an intermittent contact mode model of the AFM response to the electrostatic force produced by the stored charge.
Self-consistent theory and simulation of quasiuniform states in thin rectangular magnetic nanoparticles
Authors: Tartakovskaya E. V. , Tucker J. W. , Ivanov B. A.
Published: July 2002

A self-consistent theory of the ground-state nonuniform magnetization distribution in small magnetic nanoelements is proposed, valid for thicknesses much less than the exchange length, and with natural fulfillment of boundary conditions allowing application to a variety of element shapes. The theory is applied to rectangular 2p1l×2p2l×2l permalloy elements. In contrast to that of square elements, there exists a range of particle sizes having an “intermediate” ground state (mixed flower and leaf symmetries) with average magnetization inclined at φ to the longer edge. With increasing p1/p2 (p2 fixed), φ gradually decreases to zero (flower state). This intermediate→flower transition is of the second type, unlike the leaf→flower transition (first type) observed in square elements with reduction in p1(=p2). Simulation results support the analytic theory.
Delayed pulses from high-transparency Josephson junctions
Authors: Xu Songtao , Mancini C. A. , Feldman M. J.
Published: July 2002

Calculations of the shape and the timing of single-magnetic-flux quanta (SFQs) generated by high-current-density “self-shunted” Josephson junctions are presented. The junction current is assumed to be due in part to multiple Andreev reflections within the high-transparency barrier. The SFQ pulses from these junctions show several differences when compared to those from lower-current-density resistively shunted Josephson junctions, the most important being that the pulses are significantly delayed in the self-shunted junctions, by as much as many times the pulse width.
Tunable narrow-band terahertz generation from periodically poled lithium niobate
Authors: Lee Y. S. , Meade T. , Norris T. B. , Galvanauskas A.
Published: July 2002

We describe a technique for generating tunable narrow-band terahertz radiation via optical rectification in periodically-poled lithium niobate. Frequency tuning is accomplished by spatially chirping the domain width laterally to the beam propagation direction, and adjusting the temperature of the sample. We demonstrate tuning over a continuous range from 0.8 to 2.5 THz. The bandwidth of the terahertz waveforms is as narrow as 0.02 THz.
In–out intermittency in partial differential equation and ordinary differential equation models
Authors: Covas Eurico , Tavakol Reza , Ashwin Peter , Tworkowski Andrew , Brooke John M.
Published: July 2002

We find concrete evidence for a recently discovered form of intermittency, referred to as in–out intermittency, in both partial differential equation (PDE) and ordinary differential equation (ODE) models of mean field dynamos. This type of intermittency [introduced in P. Ashwin, E. Covas, and R. Tavakol, Nonlinearity 9, 563 (1999)] occurs in systems with invariant submanifolds and, as opposed to on–off intermittency which can also occur in skew product systems, it requires an absence of skew product structure. By this we mean that the dynamics on the attractor intermittent to the invariant manifold cannot be expressed simply as the dynamics on the invariant subspace forcing the transverse dynamics; the transverse dynamics will alter that tangential to the invariant subspace when one is far enough away from the invariant manifold. Since general systems with invariant submanifolds are not likely to have skew product structure, this type of behavior may be of physical relevance in a variety of dynamical settings. The models employed here to demonstrate in–out intermittency are axisymmetric mean-field dynamo models which are often used to study the observed large-scale magnetic variability in the Sun and solar-type stars. The occurrence of this type of intermittency in such models may be of interest in understanding some aspects of such variabilities.
Representations of superconformal algebras in the AdS7/4/CFT6/3 correspondence
Authors: Ferrara Sergio , Sokatchev Emery
Published: July 2002

We perform a general analysis of representations of the superconformal algebras OSp(8/4, ↛) and OSp(8*/2N) in harmonic superspace. We present a construction of their highest-weight UIR’s by multiplication of the different types of massless conformal superfields (“supersingletons”). In particular, all “short multiplets” are classified. Representations undergoing shortening have “protected dimension” and may correspond to BPS states in the dual supergravity theory in anti-de Sitter space. These results are relevant for the classification of multitrace operators in boundary conformally invariant theories as well as for the classification of AdS black holes preserving different fractions of supersymmetry.
Tracking of conduction phenomena and degradation in organic light emitting diodes by current noise measurements
Authors: Sampietro M. , Ferrari G. , Natali D. , Scherf U. , Annan K. O. , Wenzl F. P. , Leising G.
Published: July 2002

Noise current analysis, both in time and frequency, is proposed as a means to sense variations of the microscopic conduction in organic light emitting diodes and to track their time evolution. The sensitivity of the technique would allow to correlate the carriers conduction properties with the corresponding changes in the microscopic morphology of the organic layers as obtained with structural or spectroscopic investigations. The method is shown to be very effective also in sensing the initial state and the growth of organic diodes catastrophic degradation in large advance to current monitoring or other techniques.
Effect of A-site cation size mismatch on charge ordering behavior in (La1−xYx)0.5(Ca1−ySry)0.5MnO3
Authors: Wang Y. Q. , Maclaren Ian , Duan X. F. , Wang Z. H. , Shen B. G.
Published: July 2002

Electron energy-loss spectroscopy (EELS) has been used to determine the valence state of manganese for all the samples at low temperature (93 K). EELS results indicate that the valence state of manganese keeps constant (∼3.5) for all the samples. The effects of the A-site cation size mismatch on the charge ordering (CO) behaviors in the manganites (La1−xYx)0.5(Ca1−ySry)0.5MnO3 are studied by transmission electron microscopy (TEM). TEM images show that the size mismatch and disorder of A-site cations have a suppression effect on the CO transition. The schematic models are proposed for the incommensurate CO modulation in the samples with size mismatch σ2⩽0.003. The disappearance of the CO transition in the sample with the largest mismatch (σ2=0.005) is explained by the random arrangements of Mn3+ and Mn4+ ions.
Microstructure and its effect on the conductivity of magnetron sputtered carbon thin films
Authors: Dimitriadis C. A. , Hastas N. A. , Vouroutzis N. , Logothetidis S. , Panayiotatos Y.
Published: July 2002

Carbon thin films were grown by magnetron sputtering at room temperature on silicon substrates, with the substrate bias voltage varying from +10 to −200 V. Transmission electron microscopy analysis has shown that films deposited at Vb=+10 and −40 V are amorphous (α-C), while films deposited at Vb=−200 V are nanocrystalline (nc-C). Temperature dependent conductivity measurements were carried out in the temperature range 300–77 K. With respect to conductivity, the results indicate that the investigated carbon films are classified in three groups: (i) In α-C films deposited at Vb=+10 V (sp2 rich bonds), the variable range hopping (VRH) conduction dominates below 300 K. (ii) In α-C films deposited at negative Vb up to −100 V (sp3 rich bonds), VRH conduction dominates at low temperatures (T<150 K) and a thermally activated process satisfying the Meyer–Neldel rule at higher temperatures (T>150 K). (iii) In nc-C film deposited at Vb=−200 V, the conductivity is explained by a heteroquantum-dots model based on a thermal-assisted tunneling process. The earlier differentiation in the conductivity mechanisms may play a significant role in the field electron emission properties of the films.
Superparticle and superstring in AdS3×S3 Ramond–Ramond background in the light-cone gauge
Authors: Metsaev R. R. , Tseytlin A. A.
Published: July 2002

We discuss superparticle and superstring dynamics in AdS3×S3 supported by R–R 3-form background using light-cone gauge approach. Starting with the superalgebra psu(1,1|2)⊕psu∼(1,1|2) representing the basic symmetry of this background we find the light-cone superparticle Hamiltonian. We determine the harmonic decomposition of light-cone superfield describing fluctuations of type IIB supergravity fields expanded near AdS3×S3 background and thus the corresponding Kaluza–Klein spectrum. We fix the fermionic and bosonic light-cone gauges in the covariant Green–Schwarz AdS3×S3 superstring action and find the corresponding light-cone string Hamiltonian. We also obtain a realization of the generators of psu(1,1|2)⊕psu∼(1,1|2) in terms of the superstring 2-d fields in the light-cone gauge.
Non-Abelian D-branes and noncommutative geometry
Authors: Myers Robert
Published: July 2002

We discuss the non-Abelian world-volume action which governs the dynamics of N coincident Dp-branes. In this theory, the branes’ transverse displacements are described by matrix-valued scalar fields, and so this is a natural physical framework for the appearance of noncommutative geometry. One example is the dielectric effect by which Dp-branes may be polarized into a noncommutative geometry by external fields. Another example is the appearance of noncommutative geometries in the description of intersecting D-branes of differing dimensions, such as D-strings ending on a D3- or D5-brane. We also describe the related physics of giant gravitons.
Mismatch and chemical composition analysis of vertical InxGa1−xAs quantum-dot arrays by transmission electron microscopy
Authors: Zhang Qi , Zhu Jing , Ren Xiaowei , Li Hongwei , Wang Taihong
Published: July 2002

Vertically stacked InxGa1−xAs/GaAs quantum dots have been fabricated by molecular beam epitaxy. Cross-sectional high resolution electron microscopy analyses demonstrate that the quantum dot size increases, and the interplanar spacings of {111} for quantum dots as well as the mismatch between the quantum dots and the spacer layers, is increased with the layer number. Chemical analysis shows that, as fabrication proceeds, indium content is increased and gallium content is decreased, leading to the changes of mismatch and interplanar spacing for the quantum dots.
Transport properties of La0.8Ca0.2MnO3 epitaxial films prepared by rf magnetron sputtering using soft targets
Authors: Prokhorov V. G. , Kaminsky G. G. , Komashko V. A. , Park J. S. , Lee Y. P.
Published: July 2002

Epitaxial La0.8Ca0.2MnO3 films have been prepared by the rf magnetron sputtering method using so-called ”soft” (or powder) targets instead of solid-state ones. The as-grown films have a highly oriented perovskite-like structure, a sharp metal–insulator transition in zero magnetic field at a temperature (Tp) near 220 K, and a temperature coefficient of resistance (TCR) of 900%. We found that a reduction in the film thickness leads to a shift of Tp in the low-temperature range and a significant suppression in the TCR value. The obtained films also exhibited an anisotropic temperature behavior of the magnetoresistance in a magnetic field of 1.5 T.
Determination of the dispersion of the index of refraction and the elastic moduli for molecular-beam-epitaxy-grown Zn1−xBexSe alloys
Authors: Peiris F. C. , Bindley U. , Furdyna J. K. , Kim Hyunjung , Ramdas A. K. , Grimsditch M.
Published: July 2002

The prism coupler technique, together with reflectivity channeled spectra, were used to determine the dispersion of the indices of refraction for a series of ternary alloys of Zn1−xBexSe grown by molecular-beam epitaxy on GaAs substrates. The measurements covered the wavelength range of 400–1300 nm, and the entire Be concentration range, from x=0 to 1.00. The availability of accurate values of the index-of-refraction then enabled us to determine the elastic moduli c11 for the Zn1−xBexSe and its dependence on the Be concentration x from frequency shifts in Brillouin scattering spectra observed on the epilayers of these ternary alloys. The c11 results clearly indicate that the bonding in Zn1−xBexSe becomes more robust as the Be concentration increases.
Anomalous degradation of solar cells induced by carrier trapping
Authors: Karazhanov S. Zh.
Published: July 2002

A mechanism for the effect of anomalous increase of carrier lifetime and the phenomenon of anomalous degradation of solar cells has been proposed. Distinct from previously known mechanisms, based on carrier recombination, the mechanism is based on carrier trapping. It is shown that the increase of carrier lifetime and short-circuit current with increasing trap concentration is caused by the abrupt decrease of total free carrier density due to carrier trapping. Then the Fermi level will be shifted towards the middle of the band gap and the open-circuit voltage decreases. It is shown that the anomalous increase of the short-circuit current can overcome the decrease of the open-circuit voltage, resulting in the increase of the cell efficiency. However, the efficiency improvement is not the result of carrier photoemission from the defect level, but is caused by the trap-induced increase of carrier lifetime.
Application of optical emission microscopy for reliability studies in 4H–SiC p+/n−/n+ diodes
Authors: Galeckas A. , Linnros J. , Breitholtz B. , Bleichner H.
Published: July 2002

An optical emission microscopy technique with spatial and spectral resolution capabilities is applied for stability studies of 4H–SiC material properties. From the example of a 4H–SiC p+/n−/n+ diode imaged at different stages of electrical overstress the mechanism of degrading performance is directly unveiled. We correlate this phenomenon with irreversible structural changes within the active region created by a nonuniform heating related stress. The stress-generated features are interpreted as multiple stacking faults spreading throughout the whole base region and nucleated in the vicinity of built-in defects and process-induced structural deficiencies.
Positron annihilation study of Pd contacts on impurity-doped GaN
Authors: Lee Jong-Lam , Kim Jong Kyu , Weber Marc H. , Lynn Kelvin G.
Published: July 2002

Pd contacts on both n-type and p-type GaN were studied using positron annihilation spectroscopy, and the results were used to interpret the role of Ga vacancies on the band bending below the contacts. The concentration of Ga vacancy in Si-doped GaN was higher than that in the Mg-doped one. In Si-doped GaN, implanted positrons were annihilated at the nearer surface region and the interface of Pd/n-type GaN was detected by positrons clearly shifted toward the surface of Pd. This suggests that Ga vacancies could act as an interface state, pinning the Fermi level at the interface of Pd with GaN, leading to the production of a negative electric field below the interface.
Destructive interference of freely propagating terahertz pulses and its potential for high-resolution spectroscopy and optical computing
Authors: Grebenev V. , Knoesel E. , Bartels L.
Published: July 2002

Freely propagating terahertz pulses have been superimposed on a detection crystal leading to their mutual annihilation. An extinction ratio as good as 10:1 is found. The interference pattern represents the sum of the pulses, as measured individually, with high fidelity. Its application for high-resolution spectroscopy and optical computing are discussed.
Mass transport growth and optical emission properties of hydride vapor phase epitaxy GaN
Authors: Paskova T. , Goldys E. M. , Paskov P. P. , Wahab Q. , Wilzen L. , de Jong M. P. , Monemar B.
Published: July 2002

The optical emission properties of mass-transport regions of GaN grown by hydride vapor phase epitaxy are studied by cathodoluminescence imaging and spectroscopy. A strong donor–acceptor pair emission is observed from the mass-transport regions. Spatially resolved cathodoluminescence reveals a strong intensity contrast between the exciton and donor–acceptor bands from mass-transport and nontransport regions. Focused Auger electron and x-ray photoelectron spectroscopies were employed to investigate the impurity incorporation in the different regions. A preferential moderate increase of residual impurity incorporation or redistribution in mass-transport regions is suggested to be responsible for the observed change of the dominant radiative mechanism.