bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of N to Ga flux ratio on the GaN surface morphologies grown at high temperature by plasma-assisted molecular-beam epitaxy
Authors: Tsai Jenn-Kai , Lo Ikai , Chuang Keng-Lin , Tu Li-Wei , Huang Ji-Hao , Hsieh Chia-Ho , Hsieh Kung-Yu
Published: December 2003

The surface morphology of GaN epitaxial films grown by plasma-assisted molecular-beam epitaxy has been investigated. We found that the surface morphology was sensitive to the N to Ga flux ratio (N/Ga) when grown at a high temperature (i.e., 788 °C). At that temperature, we did not observe large sized Ga droplets on the surface even at Ga-rich conditions. Furthermore, we found a transition from two-dimensional (2D) to three-dimensional (3D) growth in the intermediate Ga-stable regime. The slope of the growth rate was different: Slope=(0.39±0.06) was observed in the 2D-growth mode and (0.14±0.03) in the 3D-growth mode. In the high N/Ga ratio, the total dislocation concentration was reduced, and the mixed threading dislocation concentration had a minimum value at N/Ga=22.5. By comparing with the Hall carrier concentration results, we found that the mixed threading dislocations influence the number of electronic carriers.
Effect of Mn interstitials on the lattice parameter of Ga1−xMnxAs
Authors: Kuryliszyn-Kudelska I. , Domagała J. Z. , Wojtowicz T. , Liu X. , Łusakowska E. , Dobrowolski W. , Furdyna J. K.
Published: December 2003

Structural investigation of as-grown as well as annealed Ga1−xMnxAs epilayers was carried out using high-resolution x-ray diffraction (XRD) measurements for a wide range of Mn concentrations (0.027⩽x⩽0.083), with special attention on how the interstitial Mn atoms (MnI) influence the lattice parameter of this material. We observe a distinct decrease of the lattice parameter after low-temperature annealing of Ga1−xMnxAs, which is known to reduce the MnI concentration. The reciprocal space maps measured for all of the investigated samples showed that the Ga1−xMnxAs layers are fully strained—i.e., they remain pseudomorphic to the GaAs (001) substrate—for the entire thickness of the samples used (in the present case over 100 nm). In all cases studied, the XRD measurements revealed high crystalline perfection of both as-grown as well as annealed Ga1−xMnxAs epilayers.
Transition from electrostatic-to-electromagnetic mode in a radio-frequency Ar inductively coupled plasma in atmospheric pressure
Authors: Razzak M. Abdur , Kondo Kenji , Uesugi Yoshihiko , Ohno Noriyasu , Takamura Shuichi
Published: December 2003

The dynamics of mode transition from the electrostatic (E) to electromagnetic (H) mode of a radio-frequency argon inductively coupled plasma (ICP) in atmospheric pressure is investigated, both experimentally and theoretically. High-speed (4500 f/s) camera imaging is performed to investigate the dynamics of E–H discharge mode transition. The temporal plasma loading impedance of Ar inductive discharges at the E–H transition stage is also observed to investigate the transition dynamics. The experimental observations reveal that the formation of the multiple streamerlike electrostatic discharge paths followed by the strong ring-shaped azimuthal discharges leads to ignite the high-pressure (around 1 atm or more) ICPs. The time scale of E–H discharge mode transition is estimated by using proposed models and the estimated results are compared with that of the experimental one. It is found that the estimated results agree well with that of the experimental one.
Fatigue-free RuO2/Pb(Zr,Ti)O3/RuO2 capacitor prepared by metalorganic chemical vapor deposition at 395 °C
Authors: Asano Gouji , Morioka Hitoshi , Funakubo Hiroshi , Shibutami Tetsuo , Oshima Noriaki
Published: December 2003

We deposited an RuO2/Pb(Zr0.40Ti0.60)O3/RuO2 capacitor by metalorganic chemical vapor deposition. RuO2 and Pb(Zr0.40Ti0.60)O3 films were prepared at 350, 395, and 445 °C from respective Ru(C7H11)(C7H9)–O2 and Pb(C11H19O2)2–Zr(O⋅t-C4H9)4–Ti(O⋅i-C3H7)4–O2 systems. Good ferroelectricity was observed for PZT films deposited at 445 °C but not at 395 °C. However, we obtained ferroelectricity with a remanent polarization above 30 μC/cm2 by inserting a 10-nm-thick sputtered-Pt layer between the PZT and RuO2 bottom electrodes, which improved the crystallinity of PZT films even those deposited at 395 °C. This capacitor had hardly any fatigue after 1×1010 switching cycles. This demonstrates the possibility of preparing fatigue-free capacitor all deposited below 400 °C for high-density ferroelectric random-access memory applications.
Field-induced domain interpenetration in tetragonal ferroelectric crystal
Authors: Tan X. , Shang J. K.
Published: December 2003

Ferroelectric domain structures of a 〈001〉-oriented lead magnesium niobate–lead titanate tetragonal crystal were examined under cyclic bipolar electric fields. Complex patterns of orthogonal domain strips were found to emerge from a simple structure of parallel strips of 90° domains. Near the boundary between the two orthogonal sets of the domain strips, domains were forced to intersect, creating charged domain walls at the intersections. With continued electric cycling, direct impingement of individual domains resulted in domain interpenetration and fine domain cells in the boundary region. Away from the boundary region, initial domain walls were withdrawn and replaced by the walls along a different orientation, resulting in separate areas that each contained a single set of parallel strips of domains. A model based on 180° domain switching is suggested to explain interpenetration of the domains and the withdrawal of the original domain walls.
Single-lobe, surface-normal beam surface emission from second-order distributed feedback lasers with half-wave grating phase shift
Authors: Witjaksono G. , Li S. , Lee J. J. , Botez D. , Chan W. K.
Published: December 2003

Half-wave phase shifts were fabricated in the center of second-order GaAs gratings, for use in surface-emitting, horizontal-cavity, semiconductor diode lasers (λ=0.98 μm). Incorporating such gratings in diode lasers with distributed-feedback (DFB) active regions and distributed Bragg reflectors (DBRs) is found to provide surface-normal, single-lobe beam emission, as predicted by theory. InGaAs/AlGaAs/InGaP, two-quantum-well structures are employed. A 500-μm-long GaAs/Au second-order grating with half-wave phase shift represents the DFB region, which provides feedback and unidirectional light outcoupling. GaAs/SiO2/Au, 500-μm-long, second-order gratings are the DBR regions, on either side of the DFB region, which provide both frequency-selective feedback as well as unidirectional outcoupling. Lateral-mode control is achieved via a 2.5-μm-wide ridge waveguide. Surface emission is obtained through a 80-μm-wide window stripe in the metallization on the substrate n-side. Single-frequency lasing in an orthonormally emitted, single-lobe, diffraction-limited beam is obtained to 3× threshold under pulsed-drive conditions (200 ns wide pulses, 1 kHz repetition rate).
Nonlinear optical spectroscopy in one-dimensional photonic crystals
Authors: Schneider Garrett J. , Watson George H.
Published: December 2003

We have conducted a spectroscopic investigation of the enhancement of nonlinear optical effects around a defect in a one-dimensional photonic crystal. Degenerate four-wave mixing studies were performed on a dielectric stack that contained a polymer thin-film defect layer doped with a nonlinear organic dye. This sample exhibited a large nonlinear response at a resonant defect frequency. Nonlinear spectroscopy was performed around the defect resonance and at frequencies well away from resonance. We have shown that the four-wave mixing signal exhibits extremely high-quality resonance, consistent with the expected cubic dependence on the calculated intensity within the defect layer.
Exciton spin relaxation time in quantum dots measured by continuous-wave photoluminescence spectroscopy
Authors: Mackowski S. , Nguyen T. A. , Jackson H. E. , Smith L. M. , Kossut J. , Karczewski G.
Published: December 2003

We demonstrate a method of measuring the exciton spin relaxation time in semiconductor nanostructures by continuous-wave photoluminescence. We find that for self-assembled CdTe quantum dots (QDs) the degree of circular polarization of emission is larger when exciting polarized excitons into the lower energy spin state (σ−-polarized) than in the case when the excitons are excited into the higher energy spin state (σ+-polarized). A simple rate equation model gives the exciton spin relaxation time in CdTe QDs equal to τS=4.8±0.3 ns, significantly longer than the QD exciton recombination time τR=300 ps.
Inelastic electron tunneling spectroscopy study of traps in ultrathin high-k gate dielectrics
Authors: He Wei , Ma T. P.
Published: December 2003

We report the use of inelastic electron tunneling spectroscopy (IETS) as an effective tool in studying traps in high-k gate dielectrics, particularly the electrical stress-induced traps, in metal–oxide–semiconductor (MOS) structures. Two kinds of traps may be identified by the IETS technique: (1) those that contribute to trap-assisted conduction mechanisms and (2) those that contribute to trapping in the gate dielectric. These two kinds of traps can be distinguished from each other, because each of them exhibits a distinct feature in the IETS spectra. The trap energies are readily obtained from the voltage locations where these features occur. From voltage polarity dependence of the IETS spectra, one can get information about the spatial distribution of the traps. Examples will be shown to demonstrate the capability of the IETS technique for studying traps in MOS structures with high-k gate dielectrics.
Electron-beam-induced strain within InGaN quantum wells: False indium “cluster” detection in the transmission electron microscope
Authors: Smeeton T. M. , Kappers M. J. , Barnard J. S. , Vickers M. E. , Humphreys C. J.
Published: December 2003

InGaN quantum wells have been found to be extremely sensitive to exposure to the electron beam in the transmission electron microscope (TEM). High-resolution TEM images acquired immediately after first irradiating a region of quantum well indicates no gross fluctuations of indium content in the InGaN alloy. During only a brief period of irradiation, inhomogeneous strain is introduced in the material due to electron beam damage. This strain is very similar to that expected from genuine nanometer-scale indium composition fluctuations which suggests there is the possibility of falsely detecting indium-rich “clusters” in a homogeneous quantum well.
Phototunable lasing in dye-doped cholesteric liquid crystals
Authors: Chanishvili Andro , Chilaya Guram , Petriashvili Gia , Barberi Riccardo , Bartolino Roberto , Cipparrone Gabriella , Mazzulla Alfredo , Oriol Luis
Published: December 2003

In this letter, we report the results of phototunable lasing in dye-doped cholesteric liquid crystals (DD–CLC). Photoexcitation of DD–CLC films gives rise to laser emission in the violet-UV range. Control of the structure of the chiral dopant driven by UV phototransformation is exploited in order to obtain a permanent variation of the cholesteric pitch. Laser emission wavelength tuning, by means of photoinduced shifting of the selective reflection band of the cholesteric liquid crystals is established. A tuning interval of about 35 nm, in the wavelength range of 385–415 nm, is observed.
Diffusion and drift in terahertz emission at GaAs surfaces
Authors: Heyman J. N. , Coates N. , Reinhardt A. , Strasser G.
Published: December 2003

We study terahertz (THz) emission from GaAs as a function of photon energy and electric field. THz radiation arises from transport of photogenerated charge in an electric field and by hot carrier diffusion (the photo-Dember effect). These mechanisms can be separated by experiments in which either the electric field or the kinetic energy of the carriers is varied. For electric fields E∼4 kV/cm, we find that the electric field controls THz emission for carrier temperatures kBTC⩽0.1 eV, while hot-carrier diffusion dominates for kBTC≈1 eV. Both mechanisms contribute at intermediate fields and carrier temperatures. Our results are consistent with estimates of the relative magnitudes of these two effects.
Increased photo- and electroluminescence by zone annealing of spin-coated and vacuum-sublimed amorphous films producing crystalline thin films
Authors: Liu Chong-Yang , Bard Allen J.
Published: December 2003

Spin-coated and vacuum sublimed amorphous thin films (∼100 nm thick) were converted into single crystalline films with a simple moving-zone-annealing technique, in which a heated metal wire generated a narrow annealed zone in the film as it is moved across the sample. This annealing resulted in both the photoluminescence and electroluminescence to increase dramatically (by 3–4 times), for example with Ru(bpy)3(ClO4)2 and aluminum 8-hydroxyquinoline, (Alq3), as light emitting devices. This technique is of interest in improving the behavior of highly luminescent thin film optoelectronic devices.
Tunable repetition-rate multiplication of a 10 GHz pulse train using linear and nonlinear fiber propagation
Authors: de Matos C. J. S. , Taylor J. R.
Published: December 2003

The temporal Talbot effect and soliton propagation in an optical fiber were exploited to yield a series of pulse train sources with tunable repetition rate simply through variation of the pulse train power in sections of the fiber. In a dual-repetition-rate configuration, 10 and 20 GHz or 10 and 30 GHz repetition rates could be achieved depending on the fiber length used, with pulse durations lower than 21 ps. In a triple-repetition-rate configuration, 10, 20, and 30 GHz repetition rates were obtained, with pulse durations lower than 15 ps.
Capacity-coupled multidischarge for atmospheric plasma production
Authors: Mase Hiroshi , Fujiwara Tamiya , Sato Noriyoshi
Published: December 2003

We propose a method of plasma production by capacity-coupled multidischarge (CCMD) at atmospheric pressure. The discharge gaps in the CCMD consist of a common electrode and a number of compact electrodes (CCE) which are directly coupled with small capacitors for quenching the discharge. A simple CCE structure is provided by a cylindrical capacitor, the inner conductor of which is used as a gap electrode. A short pulse discharge is observed to appear homogeneously at each CCE. A charge transfer for the single-pulsed discharge is 10–100 times as large as that of the conventional dielectric barrier discharge. A high efficiency of ozone production has been confirmed in the CCMD using O2 gas. A device configuration of the CCMD is quite flexible with respect to its geometrical shape and size. The CCMD could be used to produce plasmas for various kinds of industrial applications at atmospheric pressure.
Passivation of Si nanocrystals in SiO2: Atomic versus molecular hydrogen
Authors: Wilkinson A. R. , Elliman R. G.
Published: December 2003

Photoluminescence measurements were used to investigate the effect of atomic and molecular hydrogen on the passivation of Si nanocrystals in SiO2. Significant increases in the luminescence intensity and lifetime were found in samples coated with a thin Al layer prior to a standard anneal in molecular hydrogen. This is shown to be consistent with enhanced passivation of the nanocrystals by atomic hydrogen.
Remote spectral measurement using entangled photons
Authors: Scarcelli Giuliano , Valencia Alejandra , Gompers Samuel , Shih Yanhua
Published: December 2003

By utilizing the frequency correlation of two-photon states produced via spontaneous parametric down conversion, the working principle of a remote spectrometer is demonstrated. With the help of a local scanning monochromator, the spectral transmission function of an optical element (or atmosphere) at remote locations can be characterized for wide range of wavelengths with expected high resolution.
Printing nanoparticle building blocks from the gas phase using nanoxerography
Authors: Barry Chad R. , Lwin Nyein Z. , Zheng Wei , Jacobs Heiko O.
Published: December 2003

This letter reports on the electrostatic driven self-assembly of nanoparticles onto charged surface areas (receptors) from the gas phase for nanoparticle based device fabrication. The charged areas were generated by a parallel technique that uses a flexible, conductive electrode to pattern electrons and holes in a thin film electret. Samples, 1 cm2 in size, were patterned with charge in 10 s with 100 nm scale resolution. Charge based receptors, 100 nm×100 nm in size, contained ∼100 elementary charges. A transparent particle assembly module was designed to direct and monitor the assembly of metallic nanoparticles at a resolution of 100 nm, which is ∼3 orders of magnitude greater than the resolution of existing xerographic printers.
Orbital motion of metallic carbon nanotubes in an axial magnetic field
Authors: Jiang Jie , Dong Jinming , Xing D. Y.
Published: December 2003

It is found that the orbital effect of a single-wall carbon nanotube quantum dot with rather longer length in the axial magnetic field can become the same order as the Zeeman one or even exceed it for the states nearest to the Fermi level. We predict that the spin singlet–triplet transition can be driven almost entirely by the orbital effect in the nanotube quantum dot. Thus, a type of Kondo effect realized by coupling the singlet state to all triplet states may also be observed in the nanotubes.
Strong coupling in high-finesse organic semiconductor microcavities
Authors: Connolly L. G. , Lidzey D. G. , Butté R. , Adawi A. M. , Whittaker D. M. , Skolnick M. S. , Airey R.
Published: December 2003

We report the fabrication of high-finesse strongly coupled microcavities composed of a polystyrene film doped with the dye tetraphenyl–porphyrin zinc positioned between two high reflectivity dielectric mirrors. The bottom mirror was deposited by plasma enhanced chemical vapor deposition, and was composed of 11 λ/4 thick (silicon oxide/silicon nitride) pairs. The organic layer was deposited on to this by spin coating. Finally, the top mirror was deposited by thermal evaporation and consisted of 12 λ/4 thick (tellurium oxide/lithium fluoride) pairs. Such cavities are characterized by Q factors of between 440 and 620. Strong coupling was evidenced via white light reflectivity measurements. Due to the high cavity Q factor, a Rabi splitting of 135 meV at resonance was very clearly resolved.
Breakdown characteristics of InP/InGaAs composite-collector double heterojunction bipolar transistor
Authors: Lin Y. S.
Published: December 2003

This investigation proposes an InP/InGaAs composite-collector double heterojunction bipolar transistor (CC-DHBT) grown by metalorganic chemical vapor deposition. The improved structure exhibits the advantages of no knee-shaped characteristics, no switching effect, low output conductance, a high two-terminal base–collector breakdown voltage (BV) that exceeds 20 V, and high three-terminal breakdown voltages (BVCEO>15 V, BVCBO>20 V). The current gain is over unity at ultralow collector current density of 10−4 A/cm2. These characteristics are attributed to the optimized collector design. Furthermore, this study elucidates complex breakdown mechanisms in the CC-DHBTs.
Effects of hydrogen incorporation in GaMnN
Authors: Baik K. H. , Frazier R. M. , Thaler G. T. , Abernathy C. R. , Pearton S. J. , Kelly J. , Rairigh R. , Hebard A. F. , Tang W. , Stavola M. , Zavada J. M.
Published: December 2003

Single-phase (Ga,Mn)N layers grown by molecular beam epitaxy with ∼3 at. % Mn were exposed to H2 or H1 plasmas under conditions designed to permeate layers with hydrogen. No evidence of the formation of Mn–H complexes could be detected from either infrared spectroscopy or the changes in magnetic properties. However, the residual magnetization increased ∼20% after hydrogenation, consistent with passivation of crystal defects in (Ga,Mn)N. Hydrogenation may be an attractive method for improving the magnetic properties of these heteroepitaxial materials.
Broadband locally resonant sonic shields
Authors: Ho Kin Ming , Cheng Chun Kwong , Yang Z. , Zhang X. X. , Sheng Ping
Published: December 2003

We demonstrate a class of sonic shield materials based on the principle of locally resonant (LR) microstructures. Each local resonator is found to vibrate almost like an independent unit, and two layers of such resonators can even be regarded as a sonic crystal. By combining several LR layers of different resonant frequencies, a broadband (200–500 Hz) sound shield, with an average transmission intensity 11 dB lower than that dictated by mass density law, has been achieved.
Conductivity and Hall effect measurements on intentionally undoped and doped AlGaN/GaN heterostructures before and after passivation
Authors: Bernát J. , Javorka P. , Marso M. , Kordoš P.
Published: December 2003

Conductivity and Hall effect measurements were performed before and after Si3N4 passivation of intentionally undoped and doped AlGaN/GaN heterostructures on Si and SiC substrates. An increase of the sheet carrier density (up to ∼30%) and a slight decrease of the electron mobility (less than 10%) are found in all samples after passivation. The passivation induced sheet carrier density is 1.5–2×1012 cm−2 in undoped samples and only 0.7×1012 cm−2 in 5–10×1018 cm−3 doped samples. The decrease of the electron mobility after passivation is slightly lower in highly doped samples. The channel conductivity in both types of unpassivated samples on Si and SiC substrates increases with an increase in doping density. After passivation, a well-resolved increase of channel conductivity is observed in the undoped or lightly doped samples and nearly the same channel conductivity results in the highly doped samples.
Thermal diffusivity and thermoelectric figure of merit of Al1−xInxN prepared by reactive radio-frequency sputtering
Authors: Yamaguchi Shigeo , Izaki Ryohei , Yamagiwa Ken-ichi , Taki Kyoichiro , Iwamura Yasuo , Yamamoto Atsushi
Published: December 2003

We studied the thermal properties of AlN, InN, and Al1−xInxN films, prepared by reactive sputtering, as a function of the temperature. The results indicated minimum thermal diffusivities of 3.14×10−6 m2/s for AlN, 7.65×10−7 m2/s for InN, 7.53×10−7 m2/s for Al0.57In0.43N, and 7.03×10−7 m2/s for Al0.28In0.72N. We estimated the dimensionless thermoelectric figure of merit to be 0.1 for Al0.28In0.72N at 873 K.
InGaAs/InP double heterostructures on InP/Si templates fabricated by wafer bonding and hydrogen-induced exfoliation
Authors: Fontcuberta i Morral A. , Zahler J. M. , Atwater Harry A. , Ahrenkiel S. P. , Wanlass M. W.
Published: December 2003

Hydrogen-induced exfoliation combined with wafer bonding has been used to transfer ∼600-nm-thick films of (100) InP to Si substrates. Cross-section transmission electron microscopy (TEM) shows a transferred crystalline InP layer with no observable defects in the region near the bonded interface and an intimately bonded interface. InP and Si are covalently bonded as inferred by the fact that InP/Si pairs survived both TEM preparation and thermal cycles up to 620 °C necessary for metalorganic chemical vapor deposition growth. The InP transferred layers were used as epitaxial templates for the growth of InP/In0.53Ga0.47As/InP double heterostructures. Photoluminescence measurements of the In0.53Ga0.47As layer show that it is optically active and under tensile strain, due to differences in the thermal expansion between InP and Si. These are promising results in terms of a future integration of Si electronics with optical devices based on InP-lattice-matched materials.
Electronic and magnetic properties of MnAs nanoclusters studied by x-ray absorption spectroscopy and x-ray magnetic circular dichroism
Authors: Okabayashi J. , Mizuguchi M. , Oshima M. , Shimizu H. , Tanaka M. , Yuri M. , Chen C. T.
Published: December 2003

We have investigated the electronic and magnetic properties of a MnAs:GaAs granular film with MnAs clusters embedded in the GaAs matrix fabricated by high-temperature annealing of Ga1−xMnxAs using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The XAS line shapes in the Mn 2p core level changed from a localized structure to an itinerant NiAs-type one. Magnetic-field dependence of the XMCD revealed no hysteresis curves at the fixed photon energy where the large XMCD signals were observed, suggesting the superparamagnetic behavior at 100 K.
Clusters formation in ultralow-energy high-dose boron-implanted silicon
Authors: Cristiano F. , Hebras X. , Cherkashin N. , Claverie A. , Lerch W. , Paul S.
Published: December 2003

The formation and evolution of small cluster defects in 500 eV, 1×1015 cm−2 boron-implanted silicon is investigated. These clusters are identified by high-resolution transmission electron microscopy (TEM) as small dislocation loops lying on {100} planes with an interstitial character. Weak-beam dark-field TEM analysis shows that, during annealing at 650 °C, they evolve following an Ostwald ripening mechanism. Spike anneals at high temperatures make them dissolve but an immobile boron peak is still detected in the secondary ion mass spectroscopy profiles. Upon oxidation, the average size of the clusters increases, while boron electrical deactivation occurs. These results strongly indicate that the observed clusters contain both boron interstitials and silicon self-interstitials atoms.
Heating and structural disordering effects of the nonlinear viscous flow in a Zr55Al10Ni5Cu30 bulk metallic glass
Authors: Kato Hidemi , Inoue Akihisa , Chen H. S.
Published: December 2003

The heat evolution of stress-induced structural disorder, ΔHs(ε), of a Zr55Al10Ni5Cu30 bulk metallic glass (BMG) during compressive constant ram-velocity deformation at the glass transition region (Tg=680 K) was deduced from in situ measurements of temperature change of the deforming sample. At the transition from the linear to nonlinear viscoelasticity, the behavior of viscosity change with strain, η(ε), is qualitatively consistent with the enthalpy evolution of the structural disordering, ΔHs(ε), but not with the temperature change, ΔT(ε). It is concluded that the initial softening deformation is due to the stress-induced structural disordering. The change in the nonlinearity, −log η̃ ≡−log η /ηN, is found to be proportional to the ΔHs and the slope of ΔHs(−log η̃) can be estimated to 400 J/mol, where ηN is the Newtonian viscosity. On the other hand, the temperature raise, ΔT(ε), is pronouncedly delayed as compared with the η (ε) and ΔHs(ε) at the transition, but is determined by a product of stress and plastic strain-rate, σ⋅ε̇p, and is nearly proportional to it at the steady state. The slope of ΔT(σ⋅ε̇p) can be estimated to 5.2×10−2 K mol/W.
Direct demonstration for changes in surface plasmon resonance induced by surface-enhanced Raman scattering quenching of dye molecules adsorbed on single Ag nanoparticles
Authors: Itoh Tamitake , Hashimoto Kazuhiro , Ikehata Akifumi , Ozaki Yukihiro
Published: December 2003

Changes in surface plasmon resonance (SPR) bands induced by surface-enhanced Raman scattering (SERS) quenching of rhodamine 6G (R6G) molecules adsorbed on single Ag nanoparticles were investigated by light scattering microspectroscopy. It was found that SPR bands show a peak shift to a higher-energy side and that their intensities increase after SERS quenching. It was also revealed that these SPR bands are accompanied by an enhanced absorption band of R6G and that it has the same anisotropy as SERS and SPR bands. Assuming that the changes in the SPR bands are caused by the desorption of R6G molecules, we compared our experimental findings with calculation results obtained based on Rayleigh scattering theory.
Surface potential profiling and contact resistance measurements on operating pentacene thin-film transistors by Kelvin probe force microscopy
Authors: Puntambekar Kanan P. , Pesavento Paul V. , Frisbie C. Daniel
Published: December 2003

Surface potentials of operating pentacene thin-film transistors (TFTs) with two different contact geometries (bottom or top) were mapped by Kelvin probe force microscopy (KFM). The surface potential distribution was used to isolate the potential drops at the source and drain contacts. These potential drops were converted to resistances by dividing by the appropriate drain current values. The bottom contact TFTs were contact limited at large gate voltages, while the top contact TFTs were not contact limited. In both geometries, the contact and the channel resistances decreased strongly with increasing (negative) gate bias but did not depend strongly on the drain bias. This study demonstrates the utility of KFM for visualizing charge transport bottlenecks in operating pentacene devices and for correlating electrical behavior with device structure by comparison of surface potential and topographic maps.
Coulomb blockade of electron transport in a ZnO quantum-dot solid
Authors: Roest Aarnoud L. , Kelly John J. , Vanmaekelbergh Daniël
Published: December 2003

The temperature dependence of electron transport was studied in an assembly of ZnO quantum dots. The number of electrons per quantum dot was controlled by the electrochemical potential. For assemblies permeated with organic electrolyte solutions, the electron conductance measured in the linear response regime shows an activation energy of 80–120 meV. Our analysis indicates that this is due to Coulomb blockade of electron transport.
Lateral epitaxial growth of (Ba,Sr)TiO3 thin films
Authors: Lee Jang-Sik , Wang H. , Lee Sang Yeol , Foltyn S. R. , Jia Q. X.
Published: December 2003

We report a technique for epitaxial thin-film growth by combination of selective heteroepitaxial and lateral homoepitaxial growth. (Ba,Sr)TiO3 (BST) thin films were deposited on LaAlO3 having amorphous SiOx masking layers with stripe patterns at 450 °C by pulsed-laser deposition. Postannealing was carried out thereafter for lateral epitaxial growth. The difference in BST nucleation temperatures from the amorphous masking regions and lattice-matched single-crystalline substrates enables selective nucleation. Heteroepitaxial growth takes place in the regions of single-crystalline substrates, whereas lateral homoepitaxial growth evolves from the crystallized BST towards the SiOx masked region during postannealing process.
Characterization of InP electron-initiated impact ionization in low electric fields using abrupt junction NpN InP/GaAsSb/InP double heterojunction bipolar transistors
Authors: Lam S. , Bolognesi C. R. , Watkins S. P.
Published: December 2003

The electron impact ionization coefficient αn(E) in InP is determined from electron multiplication measurements in NpN InP/GaAsSb/InP double heterojunction bipolar transistors (DHBTs). The staggered (“type-II”) band lineup between the p+ GaAs0.51Sb0.49 base and the InP collector layer allows the injection of a pure electron initiating current directly into the InP collector across an abrupt base/collector heterojunction, without the ambiguities associated with the compositional grading otherwise required to overcome the blocking band discontinuity between Ga0.47In0.53As and InP. InP/GaAsSb/InP DHBTs thus enable us to characterize αn(E) in InP in a low-field regime. Whereas the αn(E) values we extract at higher fields are in good agreement with previously reported photodiode based measurements, our data reveal a clear low-field “tail” for αn(E)<102 cm−1, indicating that InP has a soft impact ionization threshold similar to that observed for electrons in Ga0.47In0.53As. This finding can be understood in terms of the similarities between the conduction band structures of InP and Ga0.47In0.53As.
Roughening kinetics of thin films in the presence of both stress and Ehrlich–Schwobel barrier
Authors: Liu Z.-J. , Shen Y. G.
Published: December 2003

The growth of thin films under the interplay of shot noise, Ehrlich–Schwobel barrier (ESB), stress, and surface diffusion has been studied within a linear continuum model. It is shown that the roughening kinetics of such a system is sensitive to the growth temperature. At low temperatures, the surface growth is dominated by the ESB, while the stress controls the morphology instability at high temperatures. In the intermediate temperature regime, both the ESB effect and the stress instability become weak, thus the surface roughening is primarily driven by noise. An analysis of the film growth in this relatively stable regime reveals that at the early stages of growth the surface roughness shows an exponential dependence on the growth time, with a time-dependent growth exponent β being larger than the value of 0.25 generated by the competition between pure noise and surface diffusion. At the late stages of growth, however, the scaling law is broken and the growth becomes unstable due to the enhanced stress and ESB effects induced by growth time.
Omnidirectional gap and defect mode of one-dimensional photonic crystals containing negative-index materials
Authors: Jiang Haitao , Chen Hong , Li Hongqiang , Zhang Yewen , Zhu Shiyao
Published: December 2003

We show theoretically that a one-dimensional photonic crystal containing a negative-index material has an omnidirectional gap, owing to the mechanism of zero (volume) averaged refractive index. In contrast to the Bragg gap, the edge of such a zero-n̄ gap is insensitive to incident angle and polarization. When an impurity is introduced, a defect mode appears inside the zero-n̄ gap with a very weak dependence on incident angle and invariant with scaling.
Temperature dependence of the wavelength and threshold of fiber-taper-coupled L-band Er3+-doped tellurite glass microsphere laser
Authors: Peng Xiang , Song Feng , Kuwata-Gonokami Makoto , Jiang Shibin , Peyghambarian N.
Published: December 2003

We report on the temperature dependence of L-band laser emission of fiber-taper-coupled Er3+-doped tellurite glass microsphere. The microsphere laser emission threshold increased and the emitted laser wavelength shifted with temperature. The experimental results are explained with a quasi-four-level model, showing that a significant reduction of laser threshold can be achieved at lower temperatures and higher Q values of this microsphere laser.
Microspherical surfaces with predefined focal lengths fabricated using microfluidic capillaries
Authors: Lien Victor , Berdichevsky Yevgeny , Lo Yu-Hwa
Published: December 2003

We demonstrate a technique to fabricate spherical micromirrors with a programmable focal length by microfluidic capillary. Micromirrors with focal lengths of 13 μm, 19 μm, 28 μm, 50 μm, and a flat mirror were fabricated and characterized. These micromirrors show a great ability to converge light into a focal point. This technique also introduces the concept of fabricating and integrating optical components with microfluidic channels.
Epitaxial silicon and germanium on buried insulator heterostructures and devices
Authors: Bojarczuk N. A. , Copel M. , Guha S. , Narayanan V. , Preisler E. J. , Ross F. M. , Shang H.
Published: December 2003

Future microelectronics will be based upon silicon or germanium-on-insulator technologies and will require an ultrathin (<10 nm), flat silicon or germanium device layer to reside upon an insulating oxide grown on a silicon wafer. The most convenient means of accomplishing this is by epitaxially growing the entire structure on a silicon substrate. This requires a high quality crystalline oxide and the ability to epitaxially grow two dimensional, single crystal films of silicon or germanium on top of this oxide. We describe a method based upon molecular beam epitaxy and solid-phase epitaxy to make such structures and demonstrate working field-effect transistors on germanium-on-insulator layers.
Influence of alloy composition and interlayer thickness on twist and tilt mosaic in AlxGa1−xN/AlN/GaN heterostructures
Authors: Lafford T. A. , Parbrook P. J. , Tanner B. K.
Published: December 2003

High-resolution x-ray diffraction, in surface symmetric, skew symmetric, and grazing incidence in-plane diffraction geometries, has been used to investigate the effect of an AlN interlayer between micron thick GaN and AlxGa1−xN layers grown by metalorganic vapor phase epitaxy on basal plane sapphire. No change is found in the tilt mosaic (threading screw dislocation density) with thickness or Al fraction x of the upper layer. A linear increase in the twist mosaic (threading edge dislocation density) was observed as a function of interlayer thickness and x. For all samples the twist mosaic of the AlGaN was significantly greater, by at least a factor of two, than that of the GaN layer. With increasing interlayer thickness the in-plane lattice parameter of the AlGaN decreased. The results are explained in terms of extra threading edge dislocations resulting from relaxation at the GaN/AlN interface.
Stimulated emission in nanocrystalline silicon superlattices
Authors: Ruan J. , Fauchet P. M. , Dal Negro L. , Cazzanelli M. , Pavesi L.
Published: December 2003

We studied the conditions under which optical gain is measured in nanocrystalline silicon (nc-Si) using the variable stripe length method. Waveguide samples have been produced by magnetron sputtering of alternating layers of Si and SiO2, followed by high temperature annealing. No optical gain was observed under continuous wave pumping conditions. Under high intensity pulsed excitation, a superlinear fast (10 ns) recombination component yielding an optical gain up to 50 cm−1 has been independently measured in two different laboratories. A control experiment confirmed that the presence of nc-Si is necessary to achieve gain in our structures.
Effect of thermal heat treatment on oxygen-deficiency-associated defect centers: Relation to 1.8 eV photoluminescence bands in silica glass
Authors: Sakurai Yuryo
Published: December 2003

Our previous studies have reported the existence of a 1.8 eV photoluminescence (PL) band [full width at half-maximum (FWHM) of 0.2–0.4 eV, decay time τ of ∼200 ns, PL excitation peak of 2.1 eV] in silica glass exposed to visible light. Oxygen-deficiency-associated defect centers [Si clusters or SiOx (x<2) structure] were thought to be the origin of this PL band. In this article, we describe the thermal heat treatment characteristics of the 1.8 eV PL band in silica glass (excited by visible light). In the low-temperature region (∼600 °C), we observed a decrease in the intensity of this PL band. On the other hand, the 2.1 eV PL band (FWHM of ∼0.32 eV, τ unknown) was observed for the case of annealing at a temperature around 1050 °C. We propose that the high probability SiOx (x<2) structure rather than the Si clusters plays a predominant role in the generation of the 1.8 eV PL band, and that crystalline Si nanoparticles are probably the origin of the 2.1 eV PL band.
Electronic structures and the estimated Curie temperatures of (Ga1−yIny)1−xMnxAs
Authors: Miura K. , Iwasawa M. , Imanaga S. , Ami T.
Published: December 2003

The electronic structures of (Ga1−yIny)1−xMnxAs have been investigated using the Korringa, Kohn and Rostoker (KKR) method with the coherent potential approximation (CPA). The estimated Curie temperature (TC) of Ga1−xMnxAs is higher than that of (Ga0.5In0.5)1−xMnxAs and In1−xMnxAs when x≲0.10. On the other hand, the estimated TC of Ga1−xMnxAs saturates with an increase of x when x≳0.05, but that of (Ga0.5In0.5)1−xMnxAs and In1−xMnxAs does not saturate even when x>0.10. These results are in good agreement with the previous experimental results. Our calculated results predict that the TC of (Ga0.5In0.5)1−xMnxAs and In1−xMnxAs will be higher than that of Ga1−xMnxAs when x≳0.10.
Growth and luminescence of zinc-blende-structured ZnSe nanowires by metal-organic chemical vapor deposition
Authors: Zhang X. T. , Liu Z. , Leung Y. P. , Li Quan , Hark S. K.
Published: December 2003

Zinc-blende-structured single-crystalline ZnSe nanowires and nanoribbons were grown on (001) silicon substrates by metal-organic chemical vapor deposition. The as-synthesized nanowires were characterized by x-ray powder diffraction and scanning electron microscopy. The diameters of the nanowires range from a few tens to 100 nm and the typical length is in the tens of micrometers. Individual strands of the nanowires were examined by transmission electron microscopy and cathodoluminescence spectroscopy. They were found to be single crystals elongated along the 〈112̄〉 crystallographic direction. Strong and narrow room-temperature band-gap light emissions indicate that their optical and electronic properties rival those of the epitaxial layers employed in diode lasers. A possible growth mechanism of the ZnSe nanowires is also discussed.
Subthalamic Nucleus Stimulation Restores Glucose Metabolism in Associative and Limbic Cortices and in Cerebellum: Evidence from a FDG-PET Study in Advanced Parkinson's Disease
Authors: Hilker Ruediger , Voges Juergen , Weisenbach Simon , Kalbe Elke , Burghaus Lothar , Ghaemi Mehran , Lehrke Ralph , Koulousakis Athanasios , Herholz Karl , Sturm Volker , Heiss Wolf-Dieter
Published: December 2003

Deep brain stimulation of the subthalamic nucleus (STN-DBS) is a highly effective surgical treatment in patients with advanced Parkinson's disease (PD). Because the STN has been shown to represent an important relay station not only in motor basal ganglia circuits, the modification of brain areas also involved in nonmotor functioning can be expected by this intervention. To determine the impact of STN-DBS upon the regional cerebral metabolic rate of glucose (rCMRGlc), we performed positron emission tomography (PET) with 18-fluorodeoxyglucose (FDG) in eight patients with advanced PD before surgery as well as in the DBS on- and off-conditions 4 months after electrode implantation and in ten age-matched healthy controls. Before surgery, PD patients showed widespread bilateral reductions of cortical rCMRGlc versus controls but a hypermetabolic state in the left rostral cerebellum. In the STN-DBS on-condition, clusters of significantly increased rCMRGlc were found in both lower thalami reaching down to the midbrain area and remote from the stimulation site in the right frontal cortex, temporal cortex, and parietal cortex, whereas rCMRGlc significantly decreased in the left rostral cerebellum. Therefore, STN-DBS was found to suppress cerebellar hypermetabolism and to partly restore physiologic glucose consumption in limbic and associative projection territories of the basal ganglia. These data suggest an activating effect of DBS upon its target structures and confirm a central role of the STN in motor as well as associative, limbic, and cerebellar basal ganglia circuits.
Regulation of the Profile of Iron-Management Proteins in Brain Microvasculature
Authors: Burdo Joseph R. , Simpson Ian A. , Menzies Sharon , Beard John , Connor James R.
Published: December 2003

The distribution of brain iron is heterogeneous, but the mechanism by which these regional differences are achieved and maintained is unknown. In this study, the authors test two hypotheses related to brain iron transport. The first is that there is regional variability in the profile of proteins associated with iron transport and storage in the brain microvasculature. The second hypothesis is that the iron status of the brain will dictate the response of the protein profile in the microvasculature to changes in systemic iron status. The profile analysis consists of transferrin (iron transport), ferritin (iron storage), transferrin receptor (iron uptake), and divalent metal transporter 1 (release of iron from endosomes). An additional protein involved in cellular iron efflux, ferroportin, was not detected in brain microvasculature. The results show that there are significantly higher levels of these proteins in the microvasculature from each area of the brain compared to a whole brain homogenate, but no regional differences within the microvasculature. The levels of ferritin observed in the microvasculature indicate that the microvascular endothelial cells have significant iron storage capacity. There are no significant changes in the regional protein profiles in response to systemic iron manipulation when brain iron status was normal. In contrast, in Belgrade rats, whose brain is iron deficient, the expression of both divalent metal transporter 1 and transferrin receptor was increased compared with control in almost all brain regions examined, but not transferrin or ferritin. These findings indicate that regional brain iron heterogeneity is not maintained by differences in microvascular iron-management protein levels. The results also indicate that brain iron status dictates the response of the microvascular protein profile to systemic iron manipulation.
A New Primate Model of Focal Stroke: Endothelin-1—Induced Middle Cerebral Artery Occlusion and Reperfusion in the Common Marmoset
Authors: Virley David , Hadingham Sarah J. , Roberts Jenny C. , Farnfield Belinda , Elliott Heather , Whelan Greg , Golder Jackie , David Chris , Parsons Andrew A. , Hunter A. Jackie
Published: December 2003

The purpose of the present set of studies was to develop a new primate model of focal ischemia with reperfusion for long-term functional assessment in the common marmoset. Initially, the cerebral vascular anatomy of the marmoset was interrogated by Araldite-cast and ink-perfusion methods to determine the feasibility of an intravascular surgical approach. The methods showed that the internal carotid artery was highly tortuous in its passage, precluding the development of an extracranial method of inducing temporary middle cerebral artery occlusion in the marmoset. A pilot dose-response study investigated an intracranial approach of topically applying endothelin-1 (ET-1) to the M2 portion of the middle cerebral artery in a small sample of marmosets for up to 6 hours (n = 2 or 3 per group). Dose-dependent reductions in middle cerebral artery vessel caliber followed by gradual reperfusion were inversely related to increases in corrected lesion volume after ET-1 treatment, relative to vehicle control application. Finally, the functional consequences of ET-1–induced lesions to the M2 vascular territory were assessed up to 24 hours after surgery using the optimal dose established in the pilot study (2.5 nmol/25 μL). ET-1–treated marmosets (n = 4) showed marked contralateral motor deficits in grip strength and retrieval of food rewards and contralateral sensory/motor neglect towards tactile stimulation, relative to their ipsilateral side and vehicle-treated marmosets (n = 4). Strong correlations were shown between contralateral impairments and histopathologic parameters, which revealed unilateral putamen and cortical damage to the middle cerebral artery territory. No deficits were shown on general mobility, and self-care was promptly resumed in ET-1 marmosets after surgery. These results show that this novel model of ischemia with reperfusion in the marmoset has the potential to assess long-term function and to gauge the efficacy of novel therapeutic strategies targeted for clinical stroke.