bluuebunny/crossref_metadata_2025 huggingface dataset:
Picosecond four-wave-mixing in GaN epilayers at 532 nm
Authors: Taheri Bahman , Hays J. , Song J. J. , Goldenberg B.
Published: July 2002

Pulsed probe degenerate four wave mixing experiments were performed on GaN epilayers using 13 ps pulses at 532 nm. Intensity and time response of the scattering efficiency was studied. Intensity dependence of the observed signal suggests carrier generation by both single and two photon effects. The absolute scattering efficiency was measured and related to pump-induced nonlinear index change. The nonlinear refractive coefficient found was 1×10−3 cm2/GW which is greater than an order of magnitude larger than the expected value. Time response of the signal was found to be dictated by carrier lifetimes. Double-exponential decays to trap levels with lifetimes of 100 ps and 1.1 ns are suggested as the dominant recombination processes.
Fiber-optic-coupled, laser heated thermoluminescence dosimeter for remote radiation sensing
Authors: Huston Alan L. , Justus Brian L. , Johnson Tommy L.
Published: July 2002

We describe an optically transparent, laser heated, thermoluminescent glass dosimeter that is used for fiber-optic-coupled remote radiation monitoring. The glass consists of ZnS nanocrystallites embedded in a Vycor glass host, doped with Cu1+ and Nd3+ ions. An 807 nm, 1 W GaAlAs laser is used to rapidly heat microscopic regions, surrounding the Nd3+ ions, in the composite glass to stimulate blue-green thermoluminescence from metastable traps populated by exposure to ultraviolet or 60Co γ radiation.
Time-resolved photoluminescence studies of InGaN epilayers
Authors: Smith M. , Chen G. D. , Lin J. Y. , Jiang H. X. , Asif Khan M. , Chen Q.
Published: July 2002

Time-resolved photoluminescence (PL) has been employed to study the optical transitions and their dynamic processes and to evaluate materials quality of InGaN epilayers grown by metalorganic chemical vapor deposition. Our results suggest that the PL emissions in InGaN epilayers result primarily from localized exciton recombination. The localization energies of these localized excitons have been obtained. In relatively lower quality epilayers, the localized exciton recombination lifetime τ, decreases monotonically with an increase of temperature. In high quality epilayers, τ increases with temperature at low temperatures, which is a well-known indication of radiative exciton recombination. Our results demonstrate that time-resolved PL measurements uniquely provide opportunities for the understanding of basic optical processes as well as for identifying high quality materials.
High-sensitive superconducting magnetometry on a two-dimensional electron gas up to 10 Tesla
Authors: Meinel Ines , Grundler Dirk , Bargstädt-Franke Silke , Heyn Christian , Heitmann Detlef , David Bernd
Published: July 2002

We report on new magnetization studies on a two-dimensional electron system (2DES) revealing spin splitting of the Landau levels. For this, we have built a high-sensitive susceptometer consisting of a low-noise thin-film dc superconducting quantum interference device (SQUID) with a multiturn input coil and a wire-wound first-order gradiometer. The system noise level is only 40×10−6 Φ0/√(Hz) down to a frequency of a few Hz in unshielded environment. In background fields up to 10 T, the system exhibits significant low-frequency noise. At frequencies above 1 kHz, however, the SQUID sensitivity is barely affected and we have reached a value of about 10−14 J/T at 1 T and better than 10−13 J/T at 10 T. With this, we have studied the de Haas–van Alphen effect for a tunable 2DES starting from zero carrier density.
Phase-matched mid-infrared difference frequency generation in GaAs-based waveguides
Authors: Fiore A. , Berger V. , Rosencher E. , Bravetti P. , Laurent N. , Nagle J.
Published: July 2002

We demonstrate difference frequency generation of 4 μm radiation from 1 to 1.32 μm pumps in GaAs-based waveguides. Phase matching is obtained by using form birefringence in selectively oxidized AlGaAs/AlAs multilayers. A tunability over 50 cm−1 around 4 μm is demonstrated by changing the waveguide temperature and one pump wavelength. A much larger tunability may be achieved by using two tunable sources.
Stable field-induced electron emission from a solidified liquid metal ion source
Authors: Chen L. W. , Wang Y. L.
Published: July 2002

Stable field-induced electron emission is derived from a tungsten tip covered by indium which has been solidified from its liquid state during an ion emission process. By choosing different ion emission currents at the beginning of the solidification process, the final geometry of the solidified emitter can be reshaped and the characteristics of the electron emission changed. The Fowler–Nordheim equation is used to infer the geometry of the electron emitter prepared by various solidification processes. The dual ion/electron point source is expected to have high potential applications in focused ion and electron beam technologies.
Annealing of ion implanted gallium nitride
Authors: Tan H. H. , Williams J. S. , Zou J. , Cockayne D. J. H. , Pearton S. J. , Zolper J. C. , Stall R. A.
Published: July 2002

In this paper, we examine Si and Te ion implant damage removal in GaN as a function of implantation dose, and implantation and annealing temperature. Transmission electron microscopy shows that amorphous layers, which can result from high-dose implantation, recrystallize between 800 and 1100 °C to very defective polycrystalline material. Lower-dose implants (down to 5×1013 cm−2), which are not amorphous but defective after implantation, also anneal poorly up to 1100 °C, leaving a coarse network of extended defects. Despite such disorder, a high fraction of Te is found to be substitutional in GaN both following implantation and after annealing. Furthermore, although elevated-temperature implants result in less disorder after implantation, this damage is also impossible to anneal out completely by 1100 °C. The implications of this study are that considerably higher annealing temperatures will be needed to remove damage for optimum electrical properties.
In situ diagnostics of film thickness and surface roughness of diamond films on a Ti–6Al–4V alloy by optical pyrometry
Authors: Catledge Shane A. , Comer Walton , Vohra Yogesh K.
Published: July 2002

The thickness and surface roughness of diamond films grown on a Ti–6Al–4V alloy in a microwave plasma reactor was measured in situ using optical pyrometry. The growing film results in oscillations of the apparent temperature with time, which can be explained by interference effects caused by reflections from the film/air and film/substrate interfaces. The equation governing the transmittance of the diamond/metal system has been derived by taking into account the complex index of refraction of an absorbing substrate. The apparent temperature was modeled using this relation for the transmittance in order to extract the time dependence of film thickness, surface roughness, and the true temperature of the substrate. The growth rate was observed to exhibit two regimes: an initial period of slowly increasing growth followed by a growth rate that was about 50% higher. The surface roughness increased at a nearly uniform rate but quickly reached a saturation roughness for long deposition times (high surface roughness). The limitation of the model for films in the high surface roughness regime is discussed.
High-precision film thickness determination using a laser-based ultrasonic technique
Authors: Banet Matthew J. , Fuchs Martin , Rogers John A. , Reinold James H. , Knecht Jeffrey M. , Rothschild Mordechai , Logan Randy , Maznev A. A. , Nelson Keith A.
Published: July 2002

A noncontact and nondestructive laser-based acoustic technique called impulsive stimulated thermal scattering (ISTS) is used to measure thicknesses of metal films including Cu, Ta, W, Al, Ti, and others in single-layer and multilayer assemblies on silicon substrates. Other opaque film materials and substrates have also been examined. Thicknesses are determined with a repeatability of a few angstroms with data acquisition times of about 1 s. ISTS and conventional measurements (scanning electron microscopy, profilometry, and four-point electrical sheet resistance) are made on the same samples and the results are found to compare favorably.
Large third-order optical nonlinearity of nanocluster-doped glass formed by ion implantation of copper and nickel in silica
Authors: Falconieri M. , Salvetti G. , Cattaruzza E. , Gonella F. , Mattei G. , Mazzoldi P. , Piovesan M. , Battaglin G. , Polloni R.
Published: July 2002

Composite glasses containing different metal nanoclusters were obtained by implanting copper, nickel, or copper+nickel ions in SiO2 glass. The nonlinear refractive index of the composites was determined by the Z-scan method at a wavelength of 770 nm and with a laser pulse duration of 130 fs. Values of n2 up to 0.68 cm2 gW−1 were measured in the case of the Cu+Ni implanted sample.
Gain bandwidth and noise characteristics of millimeter-wave YBa2Cu3O7 hot-electron bolometer mixers
Authors: Li C.-T. , Deaver B. S. , Lee Mark , Weikle R. M. , Rao R. A. , Eom C. B.
Published: July 2002

Conversion gain bandwidth and noise characteristics are reported for lattice-cooled superconducting YBa2Cu3O7 hot-electron bolometers acting as low-power, wide-bandwidth heterodyne mixers at 75 and 585 GHz. Near 77 K, these mixers show intrinsic conversion gain as high as −16 dB using ∼1 μW of local oscillator power. The measured gain bandwidth is well described by a single Lorentzian with −3 dB frequency of 5–9 GHz. Measurements of output noise combined with conversion gain indicate an optimal input noise temperature of ∼5000 K, significantly lower than calculations predict.
Temperature dependence of deformation-assisted crystallization in amorphous Fe78B13Si9
Authors: Xu J. , Atzmon M.
Published: July 2002

Low-energy ball milling of amorphous Fe78B13Si9 has been performed at temperatures between 25 and 250 °C. Primary crystallization is observed after milling at elevated temperatures, but not after annealing at the same temperatures. Although milling at room temperature creates nucleation sites for primary crystallization, subsequent annealing at elevated temperature does not result in significant crystallization. Heating by ball impact and contamination by the milling tools during milling are ruled out as the causes for crystallization. The observed behavior is interpreted to be caused by diffusivity enhancement due to milling-produced defects.
Mass transport equations unifying descriptions of isothermal diffusion, thermomigration, segregation, and position-dependent diffusivity
Authors: Tan T. Y.
Published: July 2002

Via the combined use of the jump frequency and chemical force formulation methods, a set of generalized mass transport equations has been derived. This set of equations unifies the descriptions of isothermal diffusion, thermomigration induced by a thermal gradient, and segregation and the position-dependent diffusivity arising from the crystal inhomogeneity. The equations reproduce Fick’s laws for the isothermal homogeneous crystal case, and the diffusion-segregation equations for the isothermal inhomogeneous crystal case. Also, a new expression for the heat of transport of thermomigration is obtained.
Conducting atomic force microscopy for nanoscale electrical characterization of thin SiO2
Authors: Olbrich Alexander , Ebersberger Bernd , Boit Christian
Published: July 2002

In this work, we demonstrate the applicability of conducting atomic force microscopy (AFM) for the quantitative electrical characterization of thin (3–40 nm) SiO2 films on a nanometer scale length. Fowler–Nordheim (F–N) tunneling currents on the order of 0.02–1 pA are measured simultaneously with the oxide surface topography by applying a voltage between the AFM tip and the silicon substrate. Current variations in the F–N current images are correlated to local variations of the oxide thickness on the order of several angströms to nanometers. From the microscopic current–voltage characteristics the local oxide thickness can be obtained with an accuracy of ±0.3 nm. Local oxide thinning of up to 3.3 nm was found at the edge between gate oxide and field oxide of a metal-oxide-semiconductor capacitor with a 20-nm-thick gate oxide.
Observation of two atomic configurations for the {12̄10} stacking fault in wurtzite (Ga, Al) nitrides
Authors: Vermaut P. , Nouet G. , Ruterana P.
Published: July 2002

High resolution electron microscopy has been used to determine the atomic structure of the {12̄10} stacking fault which easily folds from prismatic to basal plane in wurtzite (Al, Ga)N epitaxial layers. This letter reports experimental evidence for a simultaneous occurrence of two atomic configurations for the {12̄10} stacking fault. In more than 90% of the cases, it takes the atomic structure of the 1/2〈101̄1〉{12̄10} stacking fault; and in very few nanometric areas, it exhibits a 1/6〈202̄3〉 stacking fault atomic configuration.
Optical properties of SrS:Cu,Ag two-component phosphors for electroluminescent devices
Authors: Park W. , Jones T. C. , Summers C. J.
Published: July 2002

The luminescence characteristics of a two-component electroluminescent (EL) phosphor, SrS:Cu,Ag, were investigated by optical spectroscopy. Motivated by the discovery of efficient EL from SrS:Cu, SrS:Ag was investigated but exhibited poor EL emission, even though it showed a superior blue color. Thus, SrS:Cu,Ag samples were prepared in order to investigate if the Ag luminescence could be sensitized to produce a more efficient and saturated blue EL phosphor. A detailed spectroscopic study on SrS:Cu,Ag revealed that the emission characteristics of SrS:Cu,Ag were identical to SrS:Ag, whereas the excitation spectra resembled SrS:Cu. These results demonstrated the efficient energy transfer from Cu to Ag, confirming the successful sensitization.
Superconducting multiplexer for arrays of transition edge sensors
Authors: Chervenak J. A. , Irwin K. D. , Grossman E. N. , Martinis John M. , Reintsema C. D. , Huber M. E.
Published: July 2002

We report the design and testing of an analog superconducting time-division multiplexer to instrument large format arrays of low-temperature bolometers and microcalorimeters. The circuit is designed to multiplex an array of superconducting quantum interference devices, thereby simplifying wiring and room temperature electronics. We have fabricated a prototype 8×1 multiplexer chip and show a switching rate of 1 MHz. We calculate that a 32×32 array or larger is feasible.
Arrays of distributed-Bragg-reflector waveguide lasers at 1536 nm in Yb/Er codoped phosphate glass
Authors: Veasey David L. , Funk David S. , Sanford Norman A. , Hayden Joseph S.
Published: July 2002

We have demonstrated an array of monolithic, single-frequency-distributed-Bragg-reflector (DBR), waveguide lasers operating near 1536 nm wavelengths. The lasers were fabricated by forming waveguides in Yb/Er-codoped phosphate glass by ion exchange. The slope efficiency for each laser as a function of launched pump power is 26% and the thresholds occur at 50 mW of launched pump power. An output power of 80 mW was achieved with 350 mW of coupled pump power. Each laser exhibits stable operation on a single longitudinal mode and all have linewidths less than 500 kHz. A comb of waveguides with varying effective indices allows the selection of wavelength using a single-period grating.
Guiding radio frequency waves on metallic foils
Authors: Friedman M. , Fernsler R. F.
Published: July 2002

The propagation of electromagnetic surface waves along thin metallic strips was investigated. Efficient guidance of surface waves over distances of many kilometers can now be realized since: (1) low attenuation of the electromagnetic waves can be achieved by increasing the width of the strip, (2) losses by scattering are small because of the large reduction in the radial extension of the electric field, and (3) thin foils are lightweight. Other means (e.g., coaxial line or waveguides) of transporting electromagnetic waves produce orders of magnitude increase in weight, cost, and attenuation.
Electric fatigue properties of sol–gel-derived Pb(Zr, Ti)O3/PbZrO3 multilayered thin films
Authors: Jang Jae Hyuk , Yoon Ki Hyun
Published: July 2002

The effect of various stacking sequences of sol–gel-prepared Pb(Zr, Ti)O3/PbZrO3 (PZT/PZ) multilayered thin films on the electric fatigue properties was investigated. The antiferroelectric layer between the Pt electrode and the ferroelectric layers acted as a barrier to fatigue. The initial P*r-P∧r (about 5 μC/cm2) of the PZ(1 layer)/PZT(5 layers)/PZ(1 layer) film remained nearly unchanged after 109 cycles of a ±10 V square-wave pulse. The leakage current of the multilayered films did not show any change during 109 cycles.
Energetics of segregation in β-C2BN
Authors: Zhang R. Q. , Chan K. S. , Cheung H. F. , Lee S. T.
Published: July 2002

We show that β-phase boron–carbon–nitrogen (C2BN) structures energetically favor segregation into superlattices of alternating diamond and cubic boron nitride (cBN) layers. Local-density functional calculations of total energies using pseudopotential approximations and a plane-wave basis set of the β-C2BN systems with different thicknesses of alternating diamond and cBN layers grown along the (001) direction found a decrease in the formation energy with an increase in the layer thickness. We consider this phenomenon to be a result of the charge redistribution in each layer due to interface formation. Our study indicates that the deviation of charge distributions from those in pure diamond and cBN structures due to the existence of BN–C junctions in β-C2BN may result in difficulties in its synthesis.
Efficient operation of a high-power X-band traveling wave tube amplifier
Authors: Wang Pingshan , Xu Zhou , Ivers James D. , Nation John A. , Naqvi Shahid , Schachter Levi
Published: July 2002

We report experimental results demonstrating 54% power conversion efficiency (43% energy conversion efficiency), from a two-stage X-band traveling wave tube amplifier designed for high-power operation. The first stage of the amplifier is a 12-cm-long Boron Nitride dielectric section used to modulate the electron beam. The second stage consists of a long high-phase-velocity bunching section followed by a short low-phase-velocity output section. Output powers of up to 78 MW with narrow spectrum width were obtained with ∼700 kV, ∼200 A beam.
Empirical relationship between low-frequency drain current noise and grain-boundary potential barrier height in high-temperature-processed polycrystalline silicon thin-film transistors
Authors: Angelis C. T. , Dimitriadis C. A. , Farmakis F. V. , Brini J. , Kamarinos G. , Gueorguiev V. K. , Ivanov Tz. E.
Published: July 2002

The relationship between low-frequency drain current noise spectral density SI and grain-boundary potential barrier height Vb is investigated in high-temperature-processed polycrystalline silicon thin-film transistors. It is demonstrated that a general empirical relationship exists between SI and Vb indicating that the noise sources are located at the grain boundaries. The implications of the obtained relationship between SI and Vb from the practical viewpoint are discussed.
Epitaxial strain induced metal insulator transition in La0.9Sr0.1MnO3 and La0.88Sr0.1MnO3 thin films
Authors: Razavi F. S. , Gross G. , Habermeier H.-U. , Lebedev O. , Amelinckx S. , Van Tendeloo G. , Vigliante A.
Published: July 2002

We are reporting an unexpected metal insulator transition at the ferromagnetic phase-transition temperature for thin films of La0.9Sr0.1MnO3 (<50 nm), grown on a (100) face of SrTiO3 substrate. For the thicker films (>50 nm), similar to the single crystal, no such transition is observed below TC. Additionally, we observe the suppression of the features associated with charge or orbital ordering in intentionally La-deficient thin films of La0.88Sr0.1MnO3 (<75 nm). In thin films, transmission electron microscopy reveals a compressive strain due to the epitaxial growth, that is, lattice parameters adopt those of the cubic lattice of SrTiO3. As the film thickness increases, coherent microtwinning is observed in the films and the films relax to a orthorhombic structure.
In situ transmission electron microscope studies of the kinetics of abnormal grain growth in electroplated copper films
Authors: Hau-Riege Stefan P. , Thompson Carl V.
Published: July 2002

We have performed in situ transmission electron microscopy studies of abnormal grain growth in electroplated copper films at room temperature and 55 °C, starting minutes after the plating process by electroplating directly onto electron-transparent membranes. A kinetic analysis indicates that the grains that grow abnormally, reject impurities as they grow, so that the impurities accumulate in (and must diffuse through) the untransformed matrix. This suggests that while impurities impair normal grain growth, impurity rejection contributes to the energetics that drive abnormal grain growth. This result also indicates that removal of impurities from the film can control the kinetics of the transformation to large-grained structures.
Field emission from molybdenum carbide
Authors: Rouse Ambrosio A. , Bernhard John B. , Sosa Edward D. , Golden David E.
Published: July 2002

The thermal stability and the resiliency of molybdenum carbide field-emission tips deposited at room temperature by electrophoresis have been studied. The field emission from Mo2C films deposited on Mo tips does not change after being heated to 800 °C while exposed to 360 L of air, although MoO2, MoO3, and possibly MoO, are present in the films. The field-emission thresholds agree with photoelectric work functions determined from photoelectron spectroscopy measurements of similarly grown flat samples. These films are found to exist in three distinct phases as a function of temperature after formation by room-temperature electrophoresis. From room temperature to 500 °C, MoO3 is the dominant oxide, from 500 to 775 °C, MoC2 is the dominant oxide, and above 825 °C both oxides have virtually disappeared.
Electronic structure and energy band gap of poly (9,9-dioctylfluorene) investigated by photoelectron spectroscopy
Authors: Liao L. S. , Fung M. K. , Lee C. S. , Lee S. T. , Inbasekaran M. , Woo E. P. , Wu W. W.
Published: July 2002

The electronic structure of poly (9,9-dioctylfluorene) (PFO) film on a Au-coated Si substrate was investigated by ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). From the UPS measurement, we obtained the ionization potential (Ip) of the PFO film, Ip=5.60±0.05 eV. From the XPS shake-up peaks of the C1s core level, we estimated the electron energy band gap (Eg) of the film, Eg=3.10±0.10 eV. By comparing the Eg with the optical absorption gap, we found that the value of Eg is closer to the optical absorption maximum than to the optical absorption edge. Therefore, we suggest that the optical absorption maximum may be a better approximation than the optical absorption edge in estimating Eg.
Pretreatment effects by aqua-regia solution on field emission of diamond film
Authors: Han Sang Youn , Kim Jong Kyu , Lee Jong-Lam , Baik Young-Joon
Published: July 2002

Field emission of diamond film was enhanced after surface treatment using boiling aqua-regia solution. The current generated by the emission of electrons was distinctly increased and the threshold field was reduced from 18.1 to 13.8 V/μm by the treatment. The amount of C–C bonds was decreased, but the O–C one was increased by the treatment. The Fermi level at the treated surface was increased by 2.2 eV. This provides evidence that the enhancement of electron emission originated from the reduction of the work function, caused by the chemisorption of oxygen atoms at the surface of diamond during the aqua regia treatment. Thus, the effective electron affinity changed from positive to negative, leading to the reduction of the potential barrier height and width for electron emission at the surface of diamond.
Molecular dynamics results on the pressure tensor of polymer films
Authors: Varnik F. , Baschnagel J. , Binder K.
Published: July 2002

Polymeric thin films of various thicknesses, confined between two repulsive walls, have been studied by molecular dynamics simulations. Using the anisotropy of the perpendicular, PN(z), and parallel components, PT(z), of the pressure tensor the surface tension of the system is calculated for a wide range of temperature and for various film thicknesses. Three methods of determining the pressure tensor are compared: the method of Irving and Kirkwood (IK), an approximation thereof (IK1), and the method of Harasima (H). The IK- and the H-methods differ in the expression for PT(z) (z denotes the distance from the wall), but yield the same formula for the normal component PN(z). When evaluated by molecular dynamics (or Monte Carlo)-simulations PN(z) is constant, as required by mechanical stability. Contrary to that, the IK1-method leads to strong oscillations of PN(z). However, all methods give the same expression for the total pressure when integrated over the whole system, and thus the same surface tension, whereas the so-called surface of tension, zs, depends on the applied method. The difference is small for the IK- and H-methods, while the IK1-method leads to values that are in conflict with the interpretation of zs as the effective position of the interface.
Radiation trapping of the Hg 185 nm resonance line
Authors: Menningen K. L. , Lawler J. E.
Published: July 2002

The decay rate of the Hg 61P1 level was measured as a function of cold spot temperature (Hg density) and buffer gas pressure in cylindrical, sealed fused silica cells. The decay rates were obtained using a time-resolved laser-induced 185 nm fluorescence experiment with multi-step excitation. Cold spot temperatures from 25 to 100 °C were studied. The Hg densities for this temperature range and with no buffer gas yield the lowest possible decay rates due to radiation trapping with partial frequency redistribution. Decay rates with argon buffer gas pressures of 3 and 30 Torr were also studied. The results are in agreement with published data from a discharge afterglow experiment. Monte Carlo simulations of radiation transport in the cells, including the effects of hyperfine and isotope structure, the effects of foreign gas broadening, and partial frequency redistribution are compared to the experimental data. Reasonably good agreement is obtained, however there is evidence of quenching of Hg 61P1 atoms in collisions with ground state Hg and Ar atoms. An analytic formula for the fundamental mode trapped decay rate of the 61P1 level, which is applicable over a substantial region of parameter space, was devised from the Monte Carlo results.
Locked modes in two reversed-field pinch devices of different size and shell system
Authors: Malmberg J.-A. , Brunsell P. R. , Yagi Y. , Koguchi H.
Published: July 2002

The behavior of locked modes in two reversed-field pinch devices, the Toroidal Pinch Experiment (TPE-RX) [Y. Yagi et al., Plasma Phys. Control. Fusion 41, 2552 (1999)] and Extrap T2 [J. R. Drake et al., in Plasma Physics and Controlled Nuclear Fusion Research 1996, Montreal (International Atomic Energy Agency, Vienna, 1996), Vol. 2, p. 193] is analyzed and compared. The main characteristics of the locked mode are qualitatively similar. The toroidal distribution of the mode locking shows that field errors play a role in both devices. The probability of phase locking is found to increase with increasing magnetic fluctuation levels in both machines. Furthermore, the probability of phase locking increases with plasma current in TPE-RX despite the fact that the magnetic fluctuation levels decrease. A comparison with computations using a theoretical model estimating the critical mode amplitude for locking [R. Fitzpatrick et al., Phys. Plasmas 6, 3878 (1999)] shows a good correlation with experimental results in TPE-RX. In Extrap T2, the magnetic fluctuations scale weakly with both plasma current and electron densities. This is also reflected in the weak scaling of the magnetic fluctuation levels with the Lundquist number (∼S−0.06). In TPE-RX, the corresponding scaling is ∼S−0.18.
Photoluminescence up-conversion in InAs/GaAs self-assembled quantum dots
Authors: Paskov P. P. , Holtz P. O. , Monemar B. , Garcia J. M. , Schoenfeld W. V. , Petroff P. M.
Published: July 2002

We report up-converted photoluminescence in a structure with InAs quantum dots embedded in GaAs. An efficient emission from the GaAs barrier is observed with resonant excitation of both the dots and the wetting layer. The intensity of the up-converted luminescence is found to increase superlinearly with the excitation density. The results suggest that the observed effect is due to a two-step two-photon absorption process involving quantum dot states.
Modeling the dynamics of Si wafer bonding during annealing
Authors: Han Weihua , Yu Jinzhong , Wang Qiming
Published: July 2002

The bonding behavior of silicon wafers depends on activation energy for the formation of siloxane bonds. In this article we developed a quantitative model on the dynamics of silicon wafer bonding during annealing. Based on this model, a significant difference in the bonding behaviors is compared quantitatively between the native oxide bonding interface and the thermal oxide bonding interface. The results indicate that the bonding strength of the native oxide interface increases with temperature much more rapidly than that of the thermal oxide interface.
An extended Maxwell–Wagner theory for the electric birefringence of charged colloids
Authors: Saville D. A. , Bellini T. , Degiorgio V. , Mantegazza F.
Published: July 2002

Recent frequency resolved electric birefringence experiments on dilute suspensions of charged, ellipsoidal particles reveal new relaxation features in the MHz frequency range. Such behavior can be explained in terms of surface transport processes. Here we develop a model for the electric polarizability of spheroidal particles by combining features of electrokinetic and Maxwell–Wagner theories. The model accurately depicts the high frequency features and relates them to the colloidal properties of the polyelectrolyte. Using the model, particle charge can be extracted from electric birefringence measurements.
Localized quantum well excitons in InGaN single-quantum-well amber light-emitting diodes
Authors: Chichibu S. F. , Azuhata T. , Sota T. , Mukai T. , Nakamura S.
Published: July 2002

Optical properties of the InGaN single-quantum-well amber light-emitting-diodes were investigated to verify the importance of localized quantum well (QW) excitons in their spontaneous emission mechanisms. The internal piezoelectric field (FPZ) across the QW of the order of 1.4 MV/cm was confirmed to point from the surface to the substrate. Transmittance and photovoltaic spectra exhibited a broad band tail state, and the emission lifetime increased with decreasing detection photon energy. The electroluminescence spectra did not show remarkable energy shift between 10 and 300 K. The spectra exhibited an exponential tail and the higher energy portion increased more rapidly than that of the lower energy one, reflecting a thermal distribution of the localized carriers within the tail states. Since the well thickness is only 2.5 nm and is atomically flat, the device is considered to exhibit reasonably efficient emission with the external quantum efficiency of 5% at 20 mA in spite of the presence of FPZ and large number of threading dislocations due to radiative decay of the localized QW excitons.
Application of an ultrathin LiF/Al bilayer in organic surface-emitting diodes
Authors: Hung L. S. , Tang C. W. , Mason M. G. , Raychaudhuri P. , Madathil J.
Published: July 2002

Organic surface-emitting diodes have been constructed with a multilayer stacked cathode consisting of (1) an ultrathin LiF/Al bilayer acting as an effective electron injector, (2) an optically low-loss and electrically conducting silver intermediate layer for sheet resistance reduction, and (3) a transparent and nonconducting capping layer for refractive index matching to optimize optical transmission. The entire cathode structure is prepared by conventional thermal evaporation without incurring radiation damage, and the resulting organic surface-emitting diodes exhibit superior electrical and optical characteristics.
Low optical loss planar waveguides prepared in an organic–inorganic hybrid system
Authors: Ribeiro S. J. L. , Messaddeq Y. , Gonçalves R. R. , Ferrari M. , Montagna M. , Aegerter M. A.
Published: July 2002

HfO 2 -(3-glycidoxipropil)trimethoxisilane (GPTS) planar waveguides were prepared by a sol–gel route. A stable sol of Hafnia nanocrystals was prepared and characterized by photon correlation spectroscopy and high resolution transmission electron microscopy. The suspension was incorporated in GPTS host and the resulting sol was deposited on borosilicate substrates by the spin coating technique. Optical properties such as refractive index, thickness, number of propagating modes, and attenuation coefficient were measured at 632.8, 543.5, and 1550 nm by the prism coupling technique as a function of the HfO2 content.
Improved algorithm for corner-cutting tunneling calculations
Authors: Fernandez-Ramos Antonio , Truhlar Donald G.
Published: July 2002

We present an improved version of the large-curvature tunneling method that more accurately treats the anharmonic potentials encountered along low-energy corner-cutting tunneling paths. The method is illustrated by applications to the reaction CH3+H2→CH4+H and to the double proton transfer in formamidine hydrate.
Stark effect and single-electron charging in silicon nanocrystal quantum dots
Authors: Thean A. , Leburton J. P.
Published: July 2002

In this article, we investigate numerically the electronic structure of semispherical nanocrystals in quantum-dot based flash memory devices. We model three different sizes of nanocrystals with diameters of 125, 70, and 50 Å by solving self-consistently a system of three-dimensional (3D) Kohn–Sham and Poisson equations. We show that the unique combination of symmetries in the bandstructure and the 3D confinement geometry produce Stark effects with energy-level crossings in the nanocrystals. We also show that the quantum states in large nanocrystals (125 Å diameter) respond markedly different to the applied control-gate electric field compared to the smaller nanocrystals (diameter <70 Å). Finally, we simulate single-electron charging and obtain the addition energy spectrum of the 125-Å-diam nanocrystal.
Improved performance of amorphous silicon thin film transistors by cyanide treatment
Authors: Aiyer Hemantkumar N. , Nishioka Daikichi , Matsuki Nobuyuki , Shinno Hiroyuki , Perera V. P. S. , Chikyow Toyohiro , Kobayashi Hikaru , Koinuma Hideomi
Published: July 2002

We have examined the impact of a simple, wet chemical postgrowth treatment of “immersing in KCN solution” on the performance of inverted staggered amorphous silicon n-channel thin film transistors. Results show that the cyanide treatment significantly improves the overall transistor performance by the elimination of defect states.
Comparison of ultrathin CoTiO3 and NiTiO3 high-k gate dielectrics
Authors: Pan Tung Ming , Lei Tan Fu , Chao Tien Sheng
Published: July 2002

High-k cobalt–titanium oxide (CoTiO3) and nickel–titanium oxide (NiTiO3) were formed by directly oxidizing sputtered Co/Ti and Ni/Ti film. Al/CoTiO3/Si3N4/Si and Al/NiTiO3/Si3N4/Si capacitor structures were fabricated and measured. The effective dielectric constant (k value≅45) with buffer layer for CoTiO3 is larger than that of NiTiO3. In addition, CoTiO3 depicts excellent electrical properties at the same time. This metal oxide thus appears to be a very promising high-k gate dielectric for future ultralarge scale integrated devices.
Relativistic density functional study of gold coated magnetic nickel clusters
Authors: Krüger Sven , Stener Mauro , Rösch Notker
Published: July 2002

Coating of magnetic clusters by gold atoms is becoming an experimental technique of increasing interest for passivation and stabilization of these small metal particles. To computationally investigate the effect of gold coating, we have studied the magnetic clusters Ni6 and Ni13 employing an all-electron scalar-relativistic density functional method. We examine two series of octahedral clusters with increasing gold coverage of up to a monolayer: Ni6Aun (n=0,8,32) and Ni13Aun (n=0,6,8,14,24,30,42). Structural features, binding energies, and gold adsorption energies are determined and discussed. The different atomic radii of Au and Ni lead to rather short Au–Au and relatively long Ni–Ni distances in these clusters. The Au–Ni contacts are found to be the longest nearest-neighbor distances; a detailed analysis indicates these bonds to be the strongest in these Au-covered Ni clusters. The atomization energies change only slightly with increasing Au coverage. Also, the effect of gold adsorption on the magnetic properties of the Ni cores is analyzed. For the Ni6Aun series the magnetism decreases with n, while for Ni13Aun a maximum cluster magnetization is calculated for incomplete gold coverage. This different behavior of the two cluster series can be traced to differing numbers of unpaired electrons per atom in the pure Ni clusters and to an increased magnetic moment due to the adsorption of isolated Au atoms. Both series exhibit a residual magnetism at full monolayer coverage of the Ni cores.
Particle transport in the scrape-off layer and its relationship to discharge density limit in Alcator C-Mod
Authors: LaBombard B. , Boivin R. L. , Greenwald M. , Hughes J. , Lipschultz B. , Mossessian D. , Pitcher C. S. , Terry J. L. , Zweben S. J. ,   Alcator Group
Published: July 2002

Cross-field particle transport in the scrape-off layer (SOL) of Alcator C-Mod [Phys. Plasmas 1, 1511 (1994)] can be characterized by an effective particle diffusivity (Deff) that increases markedly with distance from the separatrix. As a consequence, recycling onto the main-chamber walls is large compared to plasma flows into the divertor volume. The SOL exhibits a two-layer structure: Steep gradients and moderate fluctuation levels are typically found in a ∼5 mm region near the separatrix (near SOL) where parallel electron conduction typically dominates energy losses. Small gradients and larger fluctuation levels with longer correlation times are found outside this region (far SOL). Deff in the near SOL increases strongly with local plasma collisionality normalized to the magnetic connection length. As the discharge density limit is approached, Deff and associated fluctuation levels become large across the entire SOL and cross-field heat convection everywhere exceeds parallel conduction losses, impacting the power balance of the discharge.
Large magnetostrictive susceptibility in Tb–FeCo/FeCo multilayers
Authors: Duc N. H. , Danh T. M. , Tuan N. A. , Teillet J.
Published: July 2002

[Tb(Fe 0.55 Co 0.45 ) 1.5 /Fe] n and [Tb(Fe0.55Co0.45)1.5/(Fe0.5Co0.5]n multilayers were fabricated by rf-magnetron sputtering from composite targets. Magnetostriction was measured using an optical deflectometer. The as-deposited films showed a soft magnetic and magnetostrictive character, with a coercive field μ0Hc=5 mT and a parallel magnetostrictive susceptibility χλ∥=dλ∥/d(μ0H)=1.4×10−2 T−1. This magnetostrictive softness was strongly improved by heat treatments: the magnetostrictive susceptibility attains the huge value of 5×10−2 T−1 in applied field of 6 mT. The low-field dependence of the magnetostrictive susceptibility and their technical characters are described in detail.
Local structure analysis of an optically active center in Er-doped ZnO thin film
Authors: Ishii Masashi , Komuro Shuji , Morikawa Takitaro , Aoyagi Yoshinobu
Published: July 2002

The local structure of an optically active center in erbium-doped zinc oxide (ZnO:Er) thin film produced by a laser ablation technique and its optical activation process are investigated by Er LIII-edge x-ray absorption fine structure analysis using a synchrotron radiation as an x-ray source. In as-ablated ZnO:Er thin film, Er has an approximately five-fold coordination of O surrounded by eight other O atoms as second-nearest neighbors. The high-order coordination of O decreases the Er-related photoluminescence (PL) intensity due to an undesirable crystal field for 4f radiation transition. After annealing in O2 ambient, the local structure of Er changes to a pseudo-octahedral structure with C4v symmetry, similar to the optically activated Er-doped Si (Si:Er), resulting in strong PL. The bond lengths of Er–O are evaluated, and differences in the optical activation processes between ZnO:Er and Si:Er thin films are discussed.
RADAR upper hybrid resonance scattering diagnostics of small-scale fluctuations and waves in tokamak plasmas
Authors: Bulyiginskiy D. G. , Gurchenko A. D. , Gusakov E. Z. , Korkin V. V. , Larionov M. M. , Novik K. M. , Petrov Yu. V. , Popov A. Yu. , Saveliev A. N. , Selenin V. L. , Stepanov A. Yu.
Published: July 2002

The upper hybrid resonance (UHR) scattering technique possessing such merits as one-dimensional probing geometry, enhancement of cross section, and fine localization of scattering region is modified in the new diagnostics under development to achieve wave number resolution. The fluctuation wave number is estimated in the new technique from the scattering signal time delay measurements. The feasibility of the scheme is checked in the proof of principal experiment in a tokamak. The time delay of the UHR scattering signal exceeding 10 ns is observed. The small scale low frequency density fluctuations are investigated in the UHR RADAR backscattering experiment. The UHR cross-polarization scattering signal related to small scale magnetic fluctuations is observed. The lower hybrid (LH) wave propagation and both linear and nonlinear wave conversion are investigated. The small wavelength (λ⩽0.02 cm) high number ion Bernstein harmonics, resulting from the linear wave conversion of the LH wave are observed in a tokamak plasma for the first time.
Classical-limit analysis of the normal-to-local transition by the algebraic effective Hamiltonian approach
Authors: Sako Tokuei , Yamanouchi Kaoru , Iachello Francesco
Published: July 2002

A classical phase space analysis is performed for the stretch vibrations of H2O and SO2 by taking the classical limit of the algebraically expanded effective Hamiltonian. It is demonstrated that the Hamiltonian surface specified by a multiplet quantum number, represented by two intrinsic phase-space variables, is useful in characterizing the transition from a normal mode to a local mode. From the classical trajectories on the Hamiltonian surfaces, the onset of the normal-to-local transition in the highly excited SO2 is clearly identified, which could not appear directly in the nodal patters of the vibrational wave functions.
“Zeptofarad” (10−21 F) resolution capacitance sensor for scanning capacitance microscopy
Authors: Tran T. , Oliver D. R. , Thomson D. J. , Bridges G. E.
Published: July 2002

We describe a sensor for use in a scanning capacitance microscope (SCM) that is capable of “zeptofarad” (10−21 F) capacitance measurement resolution in a 1 Hz bandwidth with a peak-to-peak sense voltage on the probe tip of no more than 300 mV. This sensitivity is based on experimental data and simulation results that are in excellent agreement. The complete sensor incorporates an oscillator (phase locked to a 10 MHz crystal oscillator), a coupled transmission line resonator, an amplifier, and a peak detector. The resonator is fabricated from copper-clad, low-loss dielectric material and its size is such that it is easily incorporated with a scanning probe microscope. The sensor’s use in the SCM enables capacitance resolution that has not previously been possible while retaining the instrumental advantages of imaging at low sense voltages. The performance of this sensor is discussed and compared to alternative scanning capacitance microscopy methodologies.
Electron Interactions With c-C4F8
Authors: Christophorou L. G. , Olthoff J. K.
Published: July 2002

The limited electron collision cross-section and transport-coefficient data for the plasma processing gas perfluorocyclobutane (c-C4F8) are synthesized, assessed, and discussed. These include cross sections for total electron scattering, differential elastic electron scattering, partial and total ionization, dissociation into neutral fragments, and electron attachment, as well as data on electron transport, ionization, and attachment coefficients. The available data on both the electron collision cross sections and the electron transport coefficients require confirmation. Also, measurements are needed of the momentum transfer and elastic integral cross sections, and of the cross sections for other significant low-energy electron collision processes such as vibrational and electronic excitation. In addition, electron transport data over a wider range of values of the density-reduced electric field are needed. The present assessment of data on electron affinity, attachment, and scattering suggests the existence of negative ion states near −0.6, 4.9, 6.9, 9.0, and 10.5 eV.
Fe ion implantation in GaN: Damage, annealing, and lattice site location
Authors: Liu C. , Alves E. , Sequeira A. D. , Franco N. , da Silva M. F. , Soares J. C.
Published: July 2002

In this article we report the damage and annealing behavior as well as lattice site location of Fe atoms in GaN. The Fe ions were homogeneously implanted in GaN films with an energy of 150 keV at room temperature. A two-step annealing (650 °C 15 min and then 1000 °C 2 min) was performed to remove the implantation-induced damage and to drive the dopants into the lattice site. The structure of GaN films before and after the implantation as well as at each stage of the annealing was characterized by Rutherford backscattering/channeling combined with particle induced x-ray emission and high resolution x-ray diffraction. The Fe+ implanted GaN films exhibits an expanded lattice. After the two-step annealing, the lattice distortion does not fully recover. Angular scans along both [0001] and [101̄1] directions show that the Fe atoms occupy the lattice site of Ga atoms in the case of low dose implantation after annealing. However, for the high dose implanted GaN, about 75% of the implanted Fe atoms substitutes Ga atoms, showing that the damage is not annealed out and the dopants are not completely activated.
One-dimensional fluid model for an rf methane plasma of interest in deposition of diamond-like carbon layers
Authors: Herrebout D. , Bogaerts A. , Yan M. , Gijbels R. , Goedheer W. , Dekempeneer E.
Published: July 2002

A one-dimensional (1D) model for a methane rf plasma consisting of 20 species (neutrals, radicals, ions, and electrons) is presented. The equations solved are the particle balances, assuming a drift-diffusion approximation for the fluxes, and the electron energy balance equation. The self-consistent electric field is obtained from the simultaneous solution of Poisson’s equation. The electron–neutral collision rates are expressed as a function of the average electron energy. These expressions are obtained from the solution of the Boltzmann equation using the Lorentz approximation. The results presented in this article are limited to the alpha regime, hence no secondary electrons are considered. In total, 27 electron reactions (vibrational excitation, dissociation, and ionization) have been included in the model, as well as seven ion–neutral reactions and 12 neutral–neutral reactions. The 1D fluid model yields, among others, information about the densities of the different species in the plasma. It is found that in a methane plasma C2H6, C3H8, C2H4, and C2H2 are also present at high densities, together with CH4 and H2 (inlet gases). The main radical in the plasma is CH3. At low pressure (e.g., 0.14 Torr) the most important ion is found to be CH5+, at higher pressure (e.g., 0.5 Torr) C2H5+ becomes the dominant ion.