bluuebunny/crossref_metadata_2025 huggingface dataset:
Solution synthesis and spectroscopic characterization of high Er3+ content LaF3 for broadband 1.5 μm amplification
Authors: Kumar G. A. , Riman Richard , Snitzer Elias , Ballato John
Published: December 2003

Erbium-doped LaF3 transparent gels have been synthesized by the solution synthesis of metal inorganic precursors. The solid solubility of Er3+ can reach 60 mol %, which is almost double the solubility limit of Er3+ in LaF3 synthesized by the Bridgman technique. A quantitative analysis of the absorption and emission spectral data shows that the quantum efficiency of the 1.55 μm emission band is comparable to that of melt grown single crystal. However, the emission spectral width of the 1.55 μm band is found to be 40% greater than that of the melt grown single crystal, which extends the spectral bandwidth to 11 THz. The high solid solubility of Er3+ supports 36 cm−1 single pass optical gain, which is three times higher than that in zirconium barium lanthanum aluminum sodium glass. The high Er3+ concentration and spectral bandwidth enable LaF3:Er amplifiers to be made by a low-temperature solution-based technique.
Time-resolved carrier tunneling in nanocrystalline silicon/amorphous silicon dioxide superlattices
Authors: Duzhko V. , Tsybeskov L.
Published: December 2003

Eight period nanocrystalline Si/amorphous SiO2 superlattices with Si nanocrystals of ∼5 nm diameter and tunnel-transparent (1.6–1.8-nm-thick) layers of SiO2 reveal a sharp resonance in conductivity at a low (∼0.8 V) applied bias. The performed measurements of time-resolved photocurrent decay show two distinctly different components. A fast, temperature-independent decay dominates at the applied bias close to the resonant conditions. Slower, temperature-dependent photocurrent decay becomes dominant at higher (>1.5 V) voltages. The observed fast photocurrent transient is associated with resonant hole tunneling throughout nanocrystalline Si superlattices.
Magnetic tunnel junctions with ZnSe barriers
Authors: Jiang Xin , Panchula Alex F. , Parkin Stuart S. P.
Published: December 2003

Magnetic tunnel junctions with ZnSe barriers were fabricated with a combination of magnetron sputtering, ion beam sputtering, and effusion cell evaporation. Tunneling magnetoresistance values of ∼10% are observed at room temperature. The temperature and barrier thickness dependences of the junction resistance and tunneling magnetoresistance are consistent with a predominant direct tunneling mechanism when the barrier thickness is less than ∼10 nm thick.
Localized and ultrahigh-rate etching of silicon wafers using atmospheric-pressure microplasma jets
Authors: Ichiki Takanori , Taura Ryo , Horiike Yasuhiro
Published: December 2003

A miniaturized very-high-frequency- driven inductively coupled plasma jet source has been developed for the production of high-temperature and high-density plasmas in a small space, and its application to the localized and ultrahigh-rate etching of silicon wafers has been studied. The developed plasma source consists of a 1-mm-diam discharge tube with a fine nozzle of 0.1 mm in diameter at one end and a three-turn solenoidal antenna wound around it. The electron density of atmospheric-pressure argon plasma jets blowing out from the nozzle was estimated to be 1014–1015 cm−3 by means of optical emission spectroscopy. By the addition of halogen gases into the downstream region of argon plasma jets, high-speed etching of fine holes of several hundreds μm in diameter has been investigated. The highest etch rates of 4000 μm/min and 14 μm/min have been achieved for silicon wafers and fused silica glass wafers, respectively.
Electrical transport in boron nanowires
Authors: Wang Dawei , Lu Jia G. , Otten Carolyn Jones , Buhro William E.
Published: December 2003

Electrical transport is studied in crystalline boron nanowires, using Ni and Ti as the contact electrodes, in which Ni forms ohmic contact and Ti forms Schottky-barrier junction. Three-terminal electrical measurements demonstrate p-type semiconductor behavior with estimated carrier mobility of 10−3 cm2/V s. The conductivities in annealed devices are on the order of 10−2 (Ω cm)−1. At 4.2 K, the I-V shows low conductance at low bias voltage, and increases exponentially beyond a threshold electric field close to 105 V/cm. We attribute this behavior to electric-field-induced impact ionization.
Analytical solution for the electric field in a half space conductor due to alternating current injected at the surface
Authors: Bowler N.
Published: December 2003

An analytical expression for the electric field in a half space conductor, due to alternating current injected at the surface, is derived. Assuming that the injected current flows in wires perpendicular to the surface of the test piece, the problem can be formulated in terms of a single, transverse magnetic, potential. Considering at first one wire, the cylindrical symmetry permits simplification of the calculation by use of the Hankel transform. The final result for a system with two current-carrying wires is obtained by superposition.
X-ray diffraction and photoelectron spectroscopic studies of (001)-oriented Pb(Zr0.52Ti0.48)O3 thin films prepared by laser ablation
Authors: Zhu T. J. , Lu Li
Published: December 2003

Pb(Zr 0.52 Ti 0.48 ) O 3 / LaNiO 3 (PZT/LNO) heterostructural thin films have been fabricated on LaAlO3 (LAO) single crystal substrates in the range of oxygen pressures from 50 to 400 mTorr by pulsed laser deposition. X-ray diffraction analyses reveal that the films grow with the good relationships PZT(001)//LNO(001)//LAO(001) and PZT〈100〉//LNO〈100〉//LAO〈100〉. X-ray photoelectron spectroscopy (XPS) for the surfaces of the as-received PZT films indicates that the O 1s peak can be decomposed into two components, lattice oxygen, and surface adsorbed oxygen. Each high resolution XPS spectrum of Pb 4f, Zr 3d, and Ti 2p consists of only one spin-orbit doublet, which is in agreement with the chemical state of each element in the PZT system. The Ar+-ion sputtering results in the increase in the concentrations of Pb, Zr, and Ti ions and the reduction of partial Pb2+ ions to Pb0 due to preferential removal of the oxygen ions. Oxygen pressures higher than 200 mTorr during the deposition of PZT thin films appear to favor the growth of stoichiometric PZT thin films.
Hole trapping time measurement in low-temperature-grown gallium arsenide
Authors: Adomavičius R. , Krotkus A. , Bertulis K. , Sirutkaitis V. , Butkus R. , Piskarskas A.
Published: December 2003

We study hole dynamics in GaAs layers grown by molecular-beam epitaxy at 270 °C by two-color pump-and-probe experiments employing femtosecond 800-nm-wavelength pulses for sample’s excitation and 9-μm-wavelength pulses for probing the induced intervalence band absorption. Hole trapping time in as-grown, undoped layer is equal to 2 ps; it increases after thermal annealing or Be doping, and decreases in Si-doped layer. The mechanism of the hole trapping is discussed; it is shown that experimental observations are consistent with the hole trapping at neutral arsenic antisites model.
Large optical nonlinearities in BiMnO3 thin films
Authors: Sharan Alok , An Ilsin , Chen Chi , Collins Robert W. , Lettieri James , Jia Yunfa , Schlom Darrell G. , Gopalan Venkatraman
Published: December 2003

Large third-order optical nonlinearities were observed in epitaxial thin films of BiMnO3 grown on SrTiO3 substrates. Using 140 fs laser pulses at 900 nm, a negative nonlinear refractive index (nI∼−0.53 cm2/GW) and a nonlinear absorption coefficient (αI∼−0.08 cm/kW) for BiMnO3 are measured at room temperature. Large electric-field-induced enhancement of three to four orders of magnitude in the second-harmonic response at 450 nm is observed with effective nonlinear coefficients of deff∼40(115) pm/V at 300 (473) K under applied fields of ∼707 (177) V/mm, respectively, from a 110 nm multivariant thin film.
Monocrystalline YBa2Cu3O7−x thin films on vicinal SrTiO3 (001) substrates
Authors: Dekkers J. Matthijn , Rijnders Guus , Harkema Sybolt , Smilde Henk Jan H. , Hilgenkamp Hans , Rogalla Horst , Blank Dave H. A.
Published: December 2003

The correlation between the vicinal properties of SrTiO3 (001) substrates and the twinning in YBa2Cu3O7−x thin films grown by pulsed-laser deposition is studied using x-ray diffraction with reciprocal space mapping. The vicinal properties, i.e., angle and in-plane orientation, play a significant role in the anisotropic strain starting at the interface between substrate surface and film, and affect the twin behavior of YBa2Cu3O7−x. On substrates having an [110] in-plane orientation of the step edges, a completely preferred twin pair is observed if the vicinal angle is increased to 0.60°. Whereas on substrates having their step edges oriented along one of the crystallographic axis, the films exhibit a detwinning as the vicinal angle increases. For α=1.10° a maximum detwinned, i.e., monocrystalline film is obtained. At this angle, the diffusion length of YBa2Cu3O7−x during the growth matches the terrace length of SrTiO3. Up to this specific angle, the films are grown perpendicular to the optical instead of the crystallographic surface.
Slip systems and misfit dislocations in InGaN epilayers
Authors: Srinivasan S. , Geng L. , Liu R. , Ponce F. A. , Narukawa Y. , Tanaka S.
Published: December 2003

We have studied the microstructure of InGaN layers grown on two different GaN substrates: a standard GaN film on sapphire and an epitaxial lateral overgrown GaN (ELOG) structure. These two materials exhibit two distinct mechanisms of strain relaxation. InGaN epilayers on GaN are typically pseudomorphic and undergo elastic relaxation by the opening of threading dislocations into pyramidal pits. A different behavior occurs in the case of epitaxy on ELOG where, in the absence of threading dislocations, slip occurs with the formation of periodic arrays of misfit dislocations. Potential slip systems responsible for this behavior have been analyzed using the Matthews-Blakeslee model and taking into account the Peierls forces. This letter presents a comprehensive analysis of slip systems in the wurtzite structure and considers the role of threading dislocations in strain relaxation in InGaN alloys.
Charge detrapping in HfO2 high-κ gate dielectric stacks
Authors: Gusev E. P. , D’Emic C. P.
Published: December 2003

We investigated the kinetics of charge detrapping in high-κ gate stacks fabricated with ultrathin HfO2 dielectric films grown by atomic layer deposition and a polycrystalline silicon gate electrode. It was observed that charge trapped after electron injection in the high-κ stack was unstable and slowly decayed over time. The decay does not follow a simple first-order exponential law suggesting complex detrapping mechanism(s), possibly involving more than one type of trap present in the stack. The detrapping rate was found to depend strongly on gate voltage, temperature, and light illumination.
Effects of self-affine surface roughness on the friction coefficient of rubbers in the presence of a liquid interlayer
Authors: Palasantzas G. , De Hosson J. Th. M.
Published: December 2003

In this article, we investigate how the friction coefficient is affected by the presence of a liquid layer in between a self-affine rough surface and a sliding rubber surface. The liquid layer will reduce energy dissipation from the small surface asperities and cavities of lateral sizes smaller than the healing length ζ and, therefore, will reduce the coefficient of friction. Significant coefficients of friction are attained for small healing lengths ζ (≪ξ with ξ the in-plane roughness correlation length). Finally, it is shown that the effect of the substrate roughness exponent H becomes less important especially for large healing lengths (ζ⩾ξ).
Near-equilibrium strain relaxation and misfit dislocation interactions in PbTe on PbSe (001) heteroepitaxy
Authors: Wiesauer Karin , Springholz Gunther
Published: December 2003

Strain relaxation of PbTe layers on PbSe (001) by misfit dislocation formation is shown to take place near equilibrium without kinetic barriers. A comparison of the experimental data with different strain relaxation models shows that mutual dislocation interactions are of crucial importance for the strain relaxation process. This results in faster relaxation than predicted by the models for noninteracting dislocation arrays.
Photodetecting properties of ZnO-based thin-film transistors
Authors: Bae H. S. , Yoon M. H. , Kim J. H. , Im Seongil
Published: December 2003

We report on the photodetecting properties of a ZnO-based thin-film transistor (TFT) that has been fabricated on a SiO2/p-Si substrate by rf magnetron sputtering at room temperature. Our ZnO-based TFT exhibited a saturation current level of about 6.5 μA under a gate bias of 40 V, decent electron mobility of 0.1 cm2/V s, and on/off current ratio of ∼106 in the dark. Illuminated by ultraviolet (λ=340 nm), blue (λ=450 nm), and green (λ=540 nm) light with intensity of 0.7 mW/cm2, our TFT displays high photocurrent gain of 50, 32, and 15 μA, respectively, under a gate bias of 40 V. In the channel depletion state with gate bias of −30 V, the photodetecting sensitivity becomes much higher than in the accumulation state. It is thus concluded that our ZnO-based TFT can be a good UV photodetecting device as well as an electronic device.
Quantum conductance of carbon nanotube peapods
Authors: Yoon Young-Gui , Mazzoni Mario S. C. , Louie Steven G.
Published: December 2003

We present a first-principles study of the quantum conductance of hybrid nanotube systems consisting of single-walled carbon nanotubes (SWCNTs) encapsulating either an isolated single C60 molecule or a chain of C60 molecules (nanotube peapods). The calculations show a rather weak bonding interaction between the fullerenes and the SWCNTs. The conductance of a (10,10) SWCNT with a single C60 molecule is virtually unaffected at the Fermi level, but exhibits quantized resonant reductions at the molecular levels. The nanotube peapod arrangement gives rise to high density of states for the fullerene highest occupied molecular orbital and lowest unoccupied molecular orbital bands.
Parallel writing by local oxidation nanolithography with submicrometer resolution
Authors: Cavallini Massimiliano , Mei Paolo , Biscarini Fabio , García Ricardo
Published: December 2003

We demonstrate that the process of local oxidation of surfaces by atomic force microscopy (AFM) can be upscaled in a straightforward way by using a solid support with multiple protrusions as the cathode electrode. A metallized digital video disk DVD polymeric support has been used as a stamp to generate an array of features of variable length and 100 nm in width on a silicon surface covering a 5×6 mm2 region. The parallel patterning process involves the formation of as many liquid bridges as there are protrusions in the stamp. The growth rate of the parallel local oxides is slightly smaller than the one obtained by AFM experiments. Nonetheless, results from AFM local oxidation experiments can be readily extended to parallel oxidation which in turn opens the possibility of patterning centimeter-square regions with 10 nm motives.
Band structure and fundamental optical transitions in wurtzite AlN
Authors: Li J. , Nam K. B. , Nakarmi M. L. , Lin J. Y. , Jiang H. X. , Carrier Pierre , Wei Su-Huai
Published: December 2003

With a recently developed unique deep ultraviolet picoseconds time-resolved photoluminescence (PL) spectroscopy system and improved growth technique, we are able to determine the detailed band structure near the Γ point of wurtzite (WZ) AlN with a direct band gap of 6.12 eV. Combined with first-principles band structure calculations we show that the fundamental optical properties of AlN differ drastically from that of GaN and other WZ semiconductors. The discrepancy in energy band gap values of AlN obtained previously by different methods is explained in terms of the optical selection rules in AlN and is confirmed by measurement of the polarization dependence of the excitonic PL spectra.
Effect of hydrogen and deuterium on the microstructure of amorphous Si solar cell material
Authors: Sutter E. , Williamson D. L.
Published: December 2003

We investigate hydrogenated and deuterated amorphous Si (a-Si:H and a-Si:D) solar cell material, prepared under conditions of high hydrogen and deuterium dilution, respectively. Using cross-sectional transmission electron microscopy, we provide direct observation of the microstructure of these materials. We observe that the high hydrogen dilution results in homogeneous a-Si:H material with no evidence of microvoids. In contrast, the replacement of hydrogen with deuterium produces a heterogeneous mixture of amorphous and microcrystalline material. The amorphous phase contains significant numbers of microvoids or low-density deuterium-rich domains. The observed heterogeneity is related to the effect of the substitution of hydrogen with deuterium on the microstructure of the Si film.
Photonic crystal optical filter based on contra-directional waveguide coupling
Authors: Qiu Min , Mulot Mikaël , Swillo Marcin , Anand Srinivasan , Jaskorzynska Bozena , Karlsson Anders , Kamp Martin , Forchel Alfred
Published: December 2003

Wavelength-selective operation of an optical filter (add/drop) based on a contra-directional photonic crystal waveguide coupler is demonstrated. The waveguides are defined as line defects in a two-dimensional triangular photonic crystal fabricated in an InP/GaInAsP heterostructure. The device is characterized using the end-fire method for the drop functionality. The experimental data are in good agreement with the theoretical results predicted by finite-difference time-domain simulations.
Cavity-length effects of high-temperature high-power continuous-wave characteristics in quantum-cascade lasers
Authors: Yu J. S. , Evans A. , David J. , Doris L. , Slivken S. , Razeghi M.
Published: December 2003

We report the cavity-length dependent high-temperature high-power cw characteristics in λ=6 μm quantum-cascade lasers with a thick electroplated Au top contact layer. For a high-reflectivity (HR) coated 15 μm wide and 3 mm long laser, the cw operation is achieved up to 313 K (40 °C) with an output power of 17 mW. At 298 K, a very high cw output power of 213 mW is obtained for a HR coated 15 μm wide and 4 mm long laser. Thermal resistance is analyzed at temperatures above 283 K for HR coated lasers with different cavities.
Role of Ga2O3 template thickness and gadolinium mole fraction in GdxGa0.4−xO0.6/Ga2O3 gate dielectric stacks on GaAs
Authors: Passlack M. , Medendorp N. , Gregory R. , Braddock D.
Published: December 2003

Amorphous GdxGa0.4−xO0.6/Ga2O3 gate dielectric stacks have been grown onto the GaAs(001) surface by molecular beam epitaxy using high temperature effusion cells. The Ga2O3 template thickness and the Gd mole percent have been systematically varied from 73 to 0 Å and from 8.8 to 22 at. % (0.088⩽x⩽0.22), respectively. Oxide/n-type GaAs samples have been characterized by high-frequency capacitance–voltage measurements. Optimum gate oxide stack and oxide/GaAs interface properties are obtained with a Ga2O3 template thickness of 9–11 Å and a minimum Gd mole percent of 15–17 at. %. While gate oxide films with thicker Ga2O3 templates and/or lower Gd mole fraction show kinks in capacitance–voltage measurements attributed to charge trapping in the oxide, thinner Ga2O3 templates lead to strong stretch-out of capacitance–voltage curves indicating severely degraded oxide/GaAs interface properties.
Nonlinear behavior of piezoelectric lead zinc niobate–lead titanate single crystals under ac electric fields and dc bias
Authors: Ren Wei , Liu Shi-Fang , Mukherjee Binu K.
Published: December 2003

The nonlinear behaviors of the dielectric and piezoelectric responses of 〈001〉 oriented Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) single crystals for x=4.5% and 8% have been investigated as a function of ac electric fields and dc bias fields. At relatively low applied fields, the polarization and strain of PZN-PT single crystals poled along the 〈001〉 direction show little hysteresis and have a linear dependence on the applied field, which is a consequence of engineered domain stability. Hence the dielectric and piezoelectric coefficients of the material do not exhibit any field dependence. However, when the applied electric field exceeds a threshold value, the strain and dielectric responses become nonlinear. The dielectric and piezoelectric constants are a function of the applied field, and hysteresis loops are observed. The results suggest that the observed nonlinear behavior in the PZN-PT single crystals is caused by domain motion/switching in response to the large ac fields. A positive dc bias effectively stabilizes the domain configuration in the crystals and enhances the linear response. The threshold field, at the onset of nonlinearity, is a linear function of the dc bias field in the field range investigated.
dc characteristics of a nanoscale water-based transistor
Authors: Chiragwandi Z. G. , Nur O. , Willander M. , Calander N.
Published: December 2003

We demonstrate a nanoscale water-based transistor. The presented nanoscale water-based transistor relies on the controlled modification of the pH in deionized water through the base applied electric field. The dc characteristics are presented and studied with a focus on the influence of the base applied electric field, the base electrode design, and their proximity to the sensing emitter and collector nanoelectrodes. The demonstrated water-based nanoscale device is of interest for many bioelectrical applications due to the biocompatibility and the wide usage and presence of water in biological systems.
Low-frequency noise and radiation response of metal-oxide-semiconductor transistors with Al2O3/SiOxNy/Si(100) gate stacks
Authors: Xiong H. D. , Fleetwood D. M. , Felix J. A. , Gusev E. P. , D’Emic C.
Published: December 2003

Low-frequency noise and irradiation response are studied for n-channel metal-oxide-semiconductor field-effect transistors with Al2O3/SiOxNy/Si(100) gate stacks. Radiation-induced threshold-voltage shifts and low-frequency noise increase with dose and decrease with postirradiation annealing. The border trap density in the gate dielectric inferred from the noise measurements is significantly higher than that typically observed in thermal SiO2. Similarly, the effective radiation-induced-hole trapping efficiency in Al2O3 gate dielectrics is significantly higher than for SiO2 gate dielectrics of comparable thickness. The low-frequency noise in these high-κ devices can be described well by a number fluctuation model.
Limit of the Rayleigh scattering loss in silica fiber
Authors: Saito K. , Yamaguchi M. , Kakiuchida H. , Ikushima A. J. , Ohsono K. , Kurosawa Y.
Published: December 2003

The limit of the Rayleigh scattering and total losses in silica fiber was determined from the results of the Rayleigh scattering and structural relaxation measurements. Annealing condition during fiber drawing was optimized, and much potential to reduce the Rayleigh scattering loss has been verified. Total loss can be reduced below 0.15 dB/km at 1.55 μm, for example, even when annealing time is 0.1 s. Total loss below 0.14 dB/km can be realized by increasing the annealing time.
Self-assembled nanorings in Si-capped Ge quantum dots on (001)Si
Authors: Lee S. W. , Chen L. J. , Chen P. S. , Tsai M.-J. , Liu C. W. , Chien T. Y. , Chia C. T.
Published: December 2003

Nanorings with an average height and diameter of 1.2 and 65 nm, respectively, were observed to form in Si-capped Ge quantum dots grown at 600 °C by ultrahigh-vacuum chemical vapor deposition. The nanorings were captured with the rapid cooling of the samples with appropriate amount of Si capping. Based on the results of transmission electron microscopy and Raman spectroscopy, the formation of nanorings is attributed to alloying and strain relief in the Si/Ge/(001)Si system. The self-assembly of nanorings provides a useful scheme to form ultrasmall ring-like structure and facilitates the characterization of the physical properties of unconventional quantum structures.
Direct measurements of strain depth profiles in Ge/Si(001) nanostructures
Authors: Moon D. W. , Lee H. I. , Cho B. , Foo Y. L. , Spila T. , Hong S. , Greene J. E.
Published: December 2003

Direct measurements of strain depth profiles in Ge layers consisting of either pyramidal or dome-shaped nanostructures grown on Si(001) by gas-source molecular-beam epitaxy were obtained using medium-energy ion scattering spectroscopy. Layers consisting solely of pyramidal Ge structures (corresponding to total Ge coverages θGe=5.5 ML) exhibit a compressive strain of 2.1% which is uniform with depth. In contrast, Ge layers with a dome-shaped surface morphology (θGe=8.9 ML) undergo significant relaxation giving rise to a strain gradient which varies from 0.6% at the surface to 2.1% at the Ge/Si(001) interface.
Realization of waveguiding epitaxial GaN layers on Si by low-pressure metalorganic vapor phase epitaxy
Authors: Schenk H. P. D. , Feltin E. , Laügt M. , Tottereau O. , Vennéguès P. , Doghèche E.
Published: December 2003

Waveguiding GaN epitaxial layers have been grown by low-pressure metalorganic vapor phase epitaxy on Si(111) substrates using AlN/GaN short period-superlattice (SPS) buffer layer systems. The AlN/GaN SPS has been studied by x-ray diffraction where it appears as pseudoternary AlxGa1−xN alloy. Using elastic theory an effective Al content of 44% is calculated. This value is confirmed by the average Al content calculated from the AlN:GaN layer thickness ratio measured in cross-section transmission electron microscopy. The GaN waveguiding properties have been assessed using the prism coupling method. They sensibly improve with the total thickness of the underlying AlN/GaN superlattice as well as if an additional AlN/GaN SPS is grown atop the GaN waveguiding layer.
Top-emitting organic light-emitting devices using surface-modified Ag anode
Authors: Chen Chieh-Wei , Hsieh Ping-Yuan , Chiang Huo-Hsien , Lin Chun-Liang , Wu Han-Ming , Wu Chung-Chih
Published: December 2003

A high-reflectivity bottom anode is essential for high-performance top-emitting organic light-emitting devices (OLEDs). Ag has the highest reflectivity for visible light among all metals, yet its electronic properties are not ideal for anodes of OLEDs. In this letter, we report that by inducing a thin silver oxide at the surface of Ag, hole injection from Ag anodes into OLEDs is largely enhanced yet with rather high reflectivity retained. Top-emitting devices using such surface-modified Ag anode show device characteristics competitive with those of a bottom-emitting device using the indium tin oxide anode.
InN island shape and its dependence on growth condition of molecular-beam epitaxy
Authors: Cao Y. G. , Xie M. H. , Liu Y. , Ng Y. F. , Wu H. S. , Tong S. Y.
Published: December 2003

During molecular-beam epitaxy of InN films on GaN(0001) surface, three-dimensional (3D) islands are observed following an initial wetting layer formation. Depending on deposition condition, the 3D islands take different shapes. Pyramidal islands form when excess nitrogen fluxes are used, whereas pillar-shaped islands are obtained when excess indium fluxes are employed. The pillar-shaped islands are identified to represent the equilibrium shape, whereas the pyramidal ones are limited by kinetics. As the size of islands increases, their aspect ratio shows a decreasing trend, which is attributed to a gradual relaxation of strain in the layer by defects.
Two types of structures for the GaAs(001)-c(4×4) surface
Authors: Ohtake Akihiro , Koguchi Nobuyuki
Published: December 2003

Scanning tunneling microscopy, reflectance difference spectroscopy, and x-ray photoelectron spectroscopy have been used to study the atomic structures of the As-stabilized GaAs(001)-c(4×4) surface. We found that the c(4×4) surfaces are classified into two phases of α (Ga–As dimer structure) and β (As–As dimer structure). While the α phase is obtained by heating the β phase under As fluxes, we found that the structural change from β to α is kinetically-limited.
Observation of three-dimensional optical stacking of microparticles using a single Laguerre–Gaussian beam
Authors: Lee W. M. , Yuan X.-C.
Published: December 2003

Optical stacking of microparticles has been demonstrated earlier using a single focused Gaussian beam, multiple beams from Laguerre–Gaussian (LG) interference patterns, and Bessel beams. In this letter, we demonstrate that a single focused LG beam has the ability to optically stack multiple high-index microparticles around the intensity annular rings of the LG beam, and thus form a three-dimensional structure. Due to the symmetrically circular shape of the LG, we have been able to stack particles in a circular manner. Hence we propose that this technique of stacking can be extended to optical fields of designed shaped such that the stacking microparticles will be organized according to the shaped of the beam intensity. This is an alternative method to obtain a desired three-dimensional crystalline structure, where shaping the optical vortices beam is used instead of using multiple beams.
Bandgap states in transition-metal (Sc, Y, Zr, and Nb)-doped Al2O3
Authors: Jung Ranju , Lee Jae-Cheol , So Ye-Won , Noh Tae-Won , Oh S.-J. , Lee Jong-Cheol , Shin Hyun-Joon
Published: December 2003

We have investigated the electronic structure of transition-metal (TM=Sc, Y, Zr, and Nb)-doped Al2O3 by x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS). In valence bands of these TM-doped Al2O3 measured by XPS, the highest occupied levels and the shapes of valence bands are almost unchanged from the pure alumina. On the other hand, XAS spectra obtained at the oxygen K-edge show that Nb- and Zr-doped Al2O3 show localized d states in the bandgap below the conduction band minimum, while Y- and Sc-doped Al2O3 have d states inside the conduction band of the original Al2O3. This implies that Y- and Sc-doped Al2O3 will show little bandgap degradation and maintain the same band offset as silica at the Si interface, and can serve as promising candidates for an alternative gate oxide.
Terahertz quantum-cascade laser operating up to 137 K
Authors: Williams Benjamin S. , Kumar Sushil , Callebaut Hans , Hu Qing , Reno John L.
Published: December 2003

We report operation of a terahertz quantum-cascade laser at 3.8 THz (λ≈79 μm) up to a heat-sink temperature of 137 K. A resonant phonon depopulation design was used with a low-loss metal–metal waveguide, which provided a confinement factor of nearly unity. A threshold current density of 625 A/cm2 was obtained in pulsed mode at 5 K. Devices fabricated using a conventional semi-insulating surface-plasmon waveguide lased up to 92 K with a threshold current density of 670 A/cm2 at 5 K.
Suppression of D’yakonov–Perel spin relaxation in InAs and InSb by n-type doping at 300 K
Authors: Murzyn P. , Pidgeon C. R. , Phillips P. J. , Merrick M. , Litvinenko K. L. , Allam J. , Murdin B. N. , Ashley T. , Jefferson J. H. , Miller A. , Cohen L. F.
Published: December 2003

We have made direct pump-probe measurements of spin lifetimes in intrinsic and degenerate n-InAs at 300 K. In particular, we measure remarkably long spin lifetimes (τs∼1.6 ns) for near-degenerate epilayers of n-InAs. For intrinsic material, we determine τs∼20 ps, in agreement with other workers. There are two main models that have been invoked for describing spin relaxation in narrow-gap semiconductors: the D’yakonov–Perel (DP) model and the Elliott–Yafet (EY) model. For intrinsic material, the DP model is believed to dominate in III–V materials above 77 K, in agreement with our results. We show that in the presence of strong n-type doping, the DP relaxation is suppressed both by the degeneracy condition and by electron–electron scattering, and that the EY model then dominates for the n-type material. We show that this same process is also responsible for a hitherto unexplained lengthening of τs with n-type doping in our earlier measurements of n-InSb.
Influence of interface alloying on the magnetic properties of Co/Pd multilayers
Authors: Carrey J. , Berkowitz A. E. , Egelhoff W. F. , Smith David J.
Published: December 2003

The origin of perpendicular anisotropy in Co/Pd multilayers has been investigated. We examined this question by studying multilayers in which the total equivalent thicknesses of Co and Pd are kept constant, but in which the Co/Pd interface is progressively alloyed using codeposition of the two elements. The multilayers have the structure: [(0.3 nm Co codeposited with y nm Pd)/(2.0−y) nm Pd]15, with y=0, 0.1, 0.3, 0.5, 1, 1.5, 2.0. X-ray diffraction confirms that the total thicknesses of the samples and their overall Co/Pd ratios stay constant. Magnetic properties are almost unaffected by alloying up to y=0.5 nm, suggesting that the interface of the usual Co/Pd multilayer could be modeled by a mix of 0.3 nm of Co with 0.5 nm of Pd embedded between 2 monolayers of Pd. Using this model and reasonable assumptions about the strain and magnetostriction of a Co-Pd alloy, we show that the magnetoelastic effect yields a perpendicular anisotropy between 2.3×105 and 3.5×105 J/m3. The perpendicular anisotropy estimated in our samples (1.5×105 J/m3) can thus be satisfactorily explained.
In surface segregation in M-plane (In,Ga)N/GaN multiple quantum well structures
Authors: Jun Sun Yue , Brandt Oliver , Jenichen Bernd , Ploog Klaus H.
Published: December 2003

We investigate the composition profile of M-plane (In,Ga)N/GaN(11̄00) multiple quantum well structures by both high-resolution x-ray diffractometry and secondary ion mass spectrometry. The results reveal significant In surface segregation, resulting in a reduced In content and much wider wells than intended. The resulting In content of ∼7% is lower than that obtained (∼15%) for corresponding C-plane structures grown under identical conditions, suggesting a lower In incorporation efficiency on the (11̄00) plane compared to the (0001) plane. Finally, the dependence of the transition energies on the well thickness of these M-plane quantum wells evidences the absence of internal electrostatic fields along the growth direction.
The role of the cooperative Jahn–Teller effect in the charge-ordered La1−xCaxMnO3 (0.5⩽x⩽0.87) manganites
Authors: Zheng R. K. , Li G. , Tang A. N. , Yang Y. , Wang W. , Li X. G. , Wang Z. D. , Ku H. C.
Published: December 2003

Based on the magnetoresistance, magnetization, ultrasound, and crystallographic data, we studied the role of the cooperative Jahn–Teller (JT) effect in the charge-ordered (CO) state for La1−xCaxMnO3. We found that, with increasing the fraction of Q3 mode of JT distortion and decreasing that of Q2 mode in the CO state, the magnetic structure evolves from charge-exchange-type to C-type and the orbital ordering changes from 3dx2−r2/3dy2−r2-type to 3dz2−r2-type, with the strength of ferromagnetism and the phase separation tendency being suppressed. At the same time, the stability of the CO state and the cooperative JT lattice distortion increase. These effects imply that the cooperative JT effect with different vibration modes is the key ingredient in understanding the essential physics of the CO state.
Nanotubes of rare-earth manganese oxide
Authors: Levy P. , Leyva A. G. , Troiani H. E. , Sánchez R. D.
Published: December 2003

We report the synthesis of rare-earth manganese-oxide-based nanotubes. The pore wetting technique was used to obtain structures of nominal composition La0.325Pr0.300Ca0.375MnO3 with 800 nm external diameter, 4000 nm length, and wall thickness below 100 nm exhibiting magnetic and magnetoresistive behavior below 200 K, including nonvolatile memory. Walls are found to be formed by small crystals of approximately 20 nm. Structures obtained using different diameter of pores, as small as 100 nm, have a similar aspect ratio. Results show the realization of nanotubes of manganites exhibiting intrinsic phase separation.
Three-dimensional dielectric network structures with large photonic band gaps
Authors: Maldovan Martin , Carter W. Craig , Thomas Edwin L.
Published: December 2003

Several three-dimensional dielectric network structures possessing large photonic band gaps are presented. Two of these photonic crystals show gaps as large as 26% of the midgap frequency for a refractive index contrast 3.6:1. Four of the proposed structures consist of three-connected networks and a fifth structure has a mixture of three- and four-connected nodes. The realization of these structures using block polymer self-assembly, interference lithography and layer-by-layer lithography is briefly discussed.
Robust self-assembled monolayer as diffusion barrier for copper metallization
Authors: Mikami Noboru , Hata Nobuhiro , Kikkawa Takamaro , Machida Hideaki
Published: December 2003

Excellent results on copper (Cu) diffusion barrier characteristics of a self-assembled monolayer (SAM) of 2-(diphenylphosphino)ethyltriethoxy-silane are reported. The thickness and roughness of the SAM were determined by grazing incidence x-ray reflectometry as 1.7 and 0.3 nm, respectively. To evaluate Cu diffusion barrier performance of the SAM, Cu/SiO2/Si and Cu/SAM/SiO2/Si metal-oxide-semiconductor capacitors were prepared to measure their lifetimes under the 2 MV/cm electric bias at 498–548 K. The mean times to failure obtained from the Weibull plots of time to failures were 33.6, 9.24, 4.57, and 2.03 h at 498, 523, 533 and 548 K, respectively. These values show that the barrier characteristic of the SAM of 1.7 nm in thickness is comparable to that of physical-vapor-deposited Ta film of 20 nm in thickness. The estimated lifetime of the SAM barrier at the device operation temperature of 392 K is longer than 10 yr.
Elliptic flux-line-cutting critical-state model
Authors: Romero-Salazar C. , Pérez-Rodrı́guez F.
Published: December 2003

An anisotropic model for describing the critical state of hard superconductors subjected to crossed and rotating magnetic fields is proposed.
High-speed switching of nanoscale ferroelectric domains in congruent single-crystal LiTaO3
Authors: Fujimoto Kenjiro , Cho Yasuo
Published: December 2003

The nanodomain reversal characteristics of congruent LiTaO3 (CLT) single crystal are investigated. It is found that fast nanosecond domain switching can be achieved by reducing the sample thickness, even for CLT, which contains many Li vacancy defects that pin domain-wall movement. As an example, the authors obtain a polarization inverted domain dot with a radius of 7.9 nm by application of a 4 ns 10 V pulse. These results demonstrate that the speed of polarization reversal is closely related to the thickness of the medium.
Evolution of crystallographic ordering in Hf1−xAlxOy high-κ dielectric deposited by atomic layer deposition
Authors: Wiemer C. , Fanciulli M. , Crivelli B. , Pavia G. , Alessandri M.
Published: December 2003

The evolution of the morphology and of the crystallographic ordering of hafnium aluminates deposited by atomic layer deposition has been investigated. Annealing at temperatures as high as 900 °C in N2 or O2 atmosphere is found to promote crystallization of the high-κ layer, together with the growth of an interfacial low-κ oxide. The crystallographic phase has been identified by indexation of transmission electron microscopy selected area diffraction patterns and by Rietveld refinement of grazing incidence x-ray diffractograms. The high κ is found to crystallize in an orthorhombic ternary Hf1−xAlxO2 phase even for an Al content as high as x=0.74. The temperature of crystallization is higher for the Al-richer alloy. The thickness and the electronic density of the interfacial layer are evaluated by combining cross-sectional transmission electron microscopy and x-ray reflectivity analysis.
Low-temperature, in situ tunable, uniaxial stress measurements in semiconductors using a piezoelectric actuator
Authors: Shayegan M. , Karrai K. , Shkolnikov Y. P. , Vakili K. , De Poortere E. P. , Manus S.
Published: December 2003

We demonstrate the use of a piezoelectric actuator to apply, at low temperatures, uniaxial stress in the plane of a two-dimensional electron system confined to a modulation-doped AlAs quantum well. Via the application of stress, which can be tuned in situ and continuously, we control the energies and occupations of the conduction-band minima and the electronic properties of the electron system. We also report measurements of the longitudinal and transverse strain versus bias for the actuator at 300, 77, and 4.2 K. A pronounced hysteresis is observed at 300 and 77 K, while at 4.2 K, strain is nearly linear and shows very little hysteresis with the applied bias.
Particle size dependent x-ray linewidth of rhombohedral phase in Pb(Zn1/3Nb2/3)O3–(6,7)%PbTiO3
Authors: Rajan K. K. , Lim L. C.
Published: December 2003

The full-width-at-half maximum (FWHM) of rhombohedral (002)R x-ray diffraction peak as a function of particle size in Pb(Zn1/3Nb2/3)O3–(6,7)%PbTiO3 [PZN–(6,7)%PT] solid solutions has been investigated. The sharpest (002)R peak, which has a FWHM of 0.10°–0.12°, was obtained from powder samples of intermediate particle sizes of about 120 μm. The broad (002)R peak typically observed in PZN–PT single crystal plates and coarse powder samples is the result of trapped metastable phases and, to a lesser extent, the transformation stresses/strains in the material. The increased FWHM at particle sizes <60 μm, on the other hand, can be attributed to crystal defects and, possibly, freshly formed metastable phases induced by the sample pulverization process.
Interface formation and defect structures in epitaxial La2Zr2O7 thin films on (111) Si
Authors: Seo J. W. , Fompeyrine J. , Guiller A. , Norga G. , Marchiori C. , Siegwart H. , Locquet J.-P.
Published: December 2003

We have studied the growth of epitaxial La2Zr2O7 thin films on (111) Si. Although the interface structure can be strongly affected by the Si oxidation during the deposition process, epitaxial growth of La2Zr2O7 was obtained. A detailed study by means of transmission electron microscopy reveals two types of structures (pyrochlore and fluorite) with the same average chemical composition but strong differences in reactivity and interface formation. The structural complexity of the ordered pyrochlore structure seems to prevent excess oxygen diffusion and interfacial SiO2 formation.
Relative dielectric constant of epitaxial BaTiO3 thin films in the GHz frequency range
Authors: Hamano T. , Towner D. J. , Wessels B. W.
Published: December 2003

The relative dielectric constant of epitaxial BaTiO3 thin films deposited on MgO was determined over the frequency range from 0.05 to 40.05 GHz. Coplanar stripline waveguides were formed on the BaTiO3 films and the dielectric constant was determined by matching the reflection (S11-) parameter by measurement with that by simulation. The dielectric constants were ∼2200 and ∼500 at <1 and 40 GHz, respectively. The frequency response of the dielectric constant is well described by a Curie–von Schweidler power law with an exponent of 0.29. The effective index of the coplanar stripline structure on the BaTiO3 film and MgO substrate was 3.6.
Disk substrate deposition techniques for monodisperse chemically synthesized FePt nanoparticle media
Authors: Kodama Hiroyoshi , Momose Satoru , Ihara Nobutaka , Uzumaki Takuya , Tanaka Atsushi
Published: December 2003

Self-assembled FePt nanoparticles hold promise for future ultrahigh-density magnetic recording media because of their high anisotropy and capability to be formed into small and uniform grains. By using a special spin coater, we were able to form a dense array of FePt particles across the entire surface of a 2.5 in. disk substrate. Our method can control the number of layers of FePt nanoparticles. The media deposited with FePt nanoparticles by the spin coater was annealed in a vacuum. We measured read–write properties of the FePt nanoparticle media on a spin stand, and succeeded in detecting a signal of 290 kfci.
Voltage tunable two-color infrared detection using semiconductor superlattices
Authors: Majumdar Amlan , Choi K. K. , Reno J. L. , Tsui D. C.
Published: December 2003

We demonstrate a voltage tunable two-color quantum-well infrared photodetector (QWIP) that consists of multiple periods of two distinct AlGaAs/GaAs superlattices separated by AlGaAs blocking barriers on one side and heavily doped GaAs layers on the other side. The detection peak switches from 9.5 μm under large positive bias to 6 μm under negative bias. The background-limited temperature is 55 K for 9.5 μm detection and 80 K for 6 μm detection. We also demonstrate that the corrugated-QWIP geometry is suitable for coupling normally incident light into the detector.
Fast response wide viewing angle liquid crystal cell with double-side fringe-field switching
Authors: Xiang C. Y. , Sun X. W. , Yin X. J.
Published: December 2003

A four-electrode liquid crystal cell, featuring a double-side fringe-field switching, has been designed with two-layer electrodes on both glass substrates. The two-layer electrode is the same as that used in fringe-field switching liquid crystal displays. Using a negative dielectric liquid crystal, a homogeneous cell has been fabricated with top and bottom stripe electrodes perpendicular to each other. The fringe fields near top and bottom surfaces drive the homogeneous liquid crystal layer to a twisted structure. The distributions of electric field and molecular director are obtained by theoretical analysis. The double-side fringe-field switching cell shows a much faster response and wider viewing cone compared to traditional fringe-field switching one. The improved electro-optical properties are due to the wider and more symmetric director distribution with respect to the midplane of the liquid crystal cell.
Peculiar surface deformation of sapphire: Numerical simulation of nanoindentation
Authors: Nowak R. , Manninen T. , Heiskanen K. , Sekino T. , Hikasa A. , Niihara K. , Takagi T.
Published: December 2003

This report addresses the origin of peculiar anisotropic deformation of sapphire. The three-dimensional finite element simulation of the contact between spherical indenter and elastically anisotropic solid allowed us to analyze stress under the tip that penetrates in the (101̄0) and (0001) planes, and consequently, to localize those regions in which particular deformation mechanisms are likely to be activated. This approach contrasts the available models of “hardness anisotropy,” which routinely apply a modified uniaxial-stress approach to this essentially three-dimensional, nonisotropic contact problem. The calculated results are in agreement with the microscopic inspection of impressions; that is, the surface features reflect the distribution of stress. The computations made it also possible to evaluate the actual radius of the tip (nominally 5 μm ball).
Refractive-index-driven separation of colloidal polymer particles using optical chromatography
Authors: Hart Sean J. , Terray Alex V.
Published: December 2003

Separation of equivalently sized polystyrene, n=1.59, poly(methylmethacrylate), n=1.49, and silica, n=1.43, beads has been accomplished using optical chromatography. The optical separations were performed using a glass flowcell that permits the optical trapping laser to be lightly focused into the fluid pathway against the fluid flow. Separation occurs due to the balance of fluid and optical forces; particles come to rest when the force due to the fluid flow equals the radiation pressure force. The ability to optically separate particles based upon their refractive index opens avenues for the characterization of colloidal samples based upon chemical characteristics, in addition to size.