bluuebunny/crossref_metadata_2025 huggingface dataset:
Second-harmonic generation of poled BaTiO3 thin films
Authors: Lu H. A. , Wills L. A. , Wessels B. W. , Lin W. P. , Zhang T. G. , Wong G. K. , Neumayer D. A. , Marks T. J.
Published: July 2002

Second-harmonic generation of 1.064 μm incident light was measured on BaTiO3 thin films prepared by metalorganic chemical vapor deposition. Upon corona poling of the film, the second-harmonic signal was significantly enhanced. The second-order nonlinear optical susceptibility, d, of the poled film reaches ∼7.5 times that of the quartz. The enhanced second-harmonic generation shows a very slow decay at room temperature.
Threshold characteristics of semiconductor microdisk lasers
Authors: Slusher R. E. , Levi A. F. J. , Mohideen U. , McCall S. L. , Pearton S. J. , Logan R. A.
Published: July 2002

This letter describes the threshold characteristics of InGaAs/InGaAsP microdisk lasers with optical emission near a wavelength λ=1.52 μm. More than 5% of the total spontaneous emission feeds into the lasing mode as the microdisk diameters reach 2 μm.
Paradoxical spatial distribution of the electron temperature in a low pressure rf discharge
Authors: Godyak V. A. , Piejak R. B.
Published: July 2002

Spatially resolved probe measurements of the electron energy distribution function (EEDF) in a capacitive rf discharge in argon reveal that the electron temperature decreases with increasing plasma heating rf field. This paradoxical behavior in the electron temperature distribution in an inhomogeneous rf field is shown to be the combined effect of nonlocal electron kinetics and a non-Maxwellian EEDF typical of a low pressure capacitive rf discharge in a Ramsauer gas.
Magnetoresistance in magnetic manganese oxide with intrinsic antiferromagnetic spin structure
Authors: Chahara Ken-ichi , Ohno Toshiyuki , Kasai Masahiro , Kozono Yuzoo
Published: July 2002

Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin-dependent electron scattering due to spin canting of the manganese oxide.
Site characterization of rare-earth-doped LiNbO3 using total site selective spectroscopy
Authors: Gill D. M. , Wright J. C. , McCaughan L.
Published: July 2002

A technique, total site selective spectroscopy, is presented to optically identify spectroscopic sites in Er:LiNbO3. Six sites are seen; four major nonclustered sites and two minor cluster sites. When pumping the 4F9/2 manifold at low temperatures (∼12 K), only cluster sites are found to upconvert using a nonradiative energy transfer mechanism. The polarization of Er3+ absorption cross sections are shown to be site dependent while the manifold lines from different sites are highly degenerate. These characteristics likely contribute to the observed simultaneous lasing at two wavelengths of devices based in this medium.
Superconductivity and valence state of Tb in Lu1−xTbxBa2Cu3O7−δ (0≤x≤0.7)
Authors: Gnanasekar K. I. , Selvam P. , Keer H. V. , Pinto R. , Purandare S. C. , Tamhane A. S. , Gupta L. C. , Vijayaraghavan R.
Published: July 2002

Although both lutetium and terbium do not form superconducting REBa2Cu3O7−δ, we report here the formation of superconducting phase in thin films. Films are highly c-axis oriented with high current densities (3.0×106 A cm−2 at 77 K) observed in Lu0.5Tb0.5Ba2Cu3O7−δ. X-ray photoelectron spectroscopic analysis of Tb in the above film indicates that Tb is in a trivalent state. Further, as the redox potential of Tb (Tb4++e−→Tb3+:E0=3.1 V) is close to that of Pr (Pr4++e−→Pr3+:E0=3.2 V), we suggest that Pr also would be in a trivalent state and the mechanism by which the latter depresses Tc must be of trivalent origin.
GaN epitaxial layers grown on 6H-SiC by the sublimation sandwich technique
Authors: Wetzel C. , Volm D. , Meyer B. K. , Pressel K. , Nilsson S. , Mokhov E. N. , Baranov P. G.
Published: July 2002

We report on the structural and optical properties of GaN epitaxial layers grown on 6H-SiC. We employed the sublimation sandwich method to grow single crystal layers at high growth rates with free carrier concentrations of 2×1017 cm−3. Very narrow x-ray diffraction peaks of the GaN (0002) plane are obtained indicating the high quality of this system. These findings are directly reflected in the optical properties. The photoluminescence shows a single sharp exciton line with a half width of 4 meV. Impurity related donor acceptor transitions are seen with very weak intensities. However, at lower energies the internal luminescence transitions of the 3d transition metal ions Fe and V are observed.
Electroluminescence from CdSe quantum-dot/polymer composites
Authors: Dabbousi B. O. , Bawendi M. G. , Onitsuka O. , Rubner M. F.
Published: July 2002

Electroluminescence is obtained from nearly monodisperse CdSe nanocrystallites (quantum dots) incorporated into thin films (1000 Å) of polyvinylcarbazole (PVK) and an oxadiazole derivative (t-Bu-PBD) and sandwiched between ITO and Al electrodes. The electroluminescence and photoluminescence spectra (bandwidths ≤40 nm) are nearly identical at room temperature and are tunable from ∼530 to ∼650 nm by varying the size of the dots. Voltage studies at 77 K indicate that while only the dots electroluminesce at the lower voltages, both the dots and the PVK matrix electroluminesce at higher applied voltages. Variable temperature studies indicate that the electroluminescence efficiency increases substantially as the films are cooled down to cryogenic temperatures.
Dynamic piezoelectric translation devices
Authors: Pohl D. W.
Published: July 2002

The principle of inertial sliding of a platform on a periodically accelerated support is exploited for the design of a piezoelectric fine-positioning device. The device provides step sizes of 0.04–0.2 μ, speeds of up to 0.2 mm/s, and practically unlimited translation range. It is powered by a sawtooth electric waveform of 60–300-V amplitude and useable for loads of up to 1 kg and probably even more. Mechanical parts and driver electronics are extremely simple, reliable, and easy to operate.
Luminescent thermometry for aerodynamic measurements
Authors: Gallery Jean , Gouterman Martin , Callis James , Khalil Gamal , McLachlan Blair , Bell James
Published: July 2002

A luminescent temperature sensitive paint containing the molecule rhodamine B base in a silicone polymer paint has been developed that utilizes video imaging detection to produce temperature maps of an entire surface from luminescence intensity measurements. The luminescent thermometry system has been calibrated in the laboratory to have an intensity loss of 2.30% per °C, in the range of 10 to 46 °C, with a root mean square prediction error (RMSPE) of 0.26 °C. To evaluate the technique, transient temperature field measurements have been made on a copper plate with a heater at one end. In spite of a loss in sensitivity of the paint due to aging effects that reduced sensitivity to 1.39% intensity loss per °C, the calculated RMSPE was 0.25 °C and the time series of temperature profiles predicted from luminescence agree with those predicted by a numerical solution to the heat flow equation. Data was also collected from the surface of an airfoil in a wind tunnel experiment. In spite of systematic temperature differences between the thermocouples used for calibration and the paint, RMSPE’s of less than 1 °C were obtained.
Use of a quadrupole mass filter for high energy resolution ion beam production
Authors: Smolanoff Jason , L/apicki Adam , Anderson Scott L.
Published: July 2002

We have found focusing/operating conditions that allow use of a commercial quadrupole mass filter for producing ion beams that simultaneously have high mass and kinetic energy resolution, and good transmission efficiency. Under proper conditions, our design perturbs the ion beam energy distribution by only ∼50 meV, while allowing mass resolution (M/ΔM)≥100, and net transmission of ∼50%. The critical factors are injection/collection lens design, an exit aperture which rejects ions that have been strongly perturbed, and stable dc balance in the quadrupole electronics.
Numerical removal of ring artifacts in microtomography
Authors: Raven Carsten
Published: July 2002

Defective and insufficient calibrated detector elements introduce ring or half-circle artifacts in microtomographic image reconstructions. A computationally efficient numerical filter is presented which suppresses these defects by taking advantage of the particular appearance of ring artifacts in the Fourier transforms of the recorded sinograms. The performance of the filter is demonstrated on experimental data taken with high-energy synchrotron radiation in phase-contrast (outline) mode.
The superconducting gravimeter
Authors: Goodkind John M.
Published: July 2002

The superconducting gravimeter is a spring type gravimeter in which the mechanical spring is replaced by a magnetic levitation of a superconducting sphere in the field of superconducting, persistent current coils. The object is to utilize the perfect stability of supercurrents to create a perfectly stable spring. The magnetic levitation is designed to provide independent adjustment of the total levitating force and the force gradient so that it can support the full weight of the sphere and still yield a large displacement for a small change in gravity. The gravimeters provide unequaled long term stability so that instrumental noise can be either below geophysical and cultural noise or indistinguishable from it over periods ranging from years to minutes. This article reviews the construction and operating characteristics of the instruments, and the range of research problems to which it has been and can be applied. Support for operation of the instruments in the United States has been limited so that operation of multiple instruments for periods much longer than a year has not been possible. However, some of the most appropriate applications of the instrument will require records of several years from arrays of instruments. Commercial versions of the instruments have now been purchased in sufficient numbers elsewhere in the world so that a world-wide array has been organized to maintain instruments and share data over a period of six years.
Strain and relaxation in InAs and InGaAs films grown on GaAs(001)
Authors: Woicik J. C. , Miyano K. E. , Pellegrino J. G. , Shaw P. S. , Southworth S. H. , Karlin B. A.
Published: July 2002

The strain and relaxation of InAs and InGaAs films grown on GaAs(001) have been examined by the x-ray standing wave and extended x-ray absorption fine-structure techniques. While 1 monolayer (ML) films of both InAs and InGaAs are found to be tetragonally distorted in accordance with the prediction of macroscopic-elastic theory, thicker InAs films are found to collapse to their natural-lattice constant past a critical thickness Tc, of ∼2 ML’s. By 8 ML’s, bond-length strain is no longer evident, and a large degree of structural disorder is observed.
Organic electroluminescent devices with improved stability
Authors: Van Slyke S. A. , Chen C. H. , Tang C. W.
Published: July 2002

Highly stable organic electroluminescent devices based on vapor-deposited Alq thin films have been achieved. The improvement in stability is derived from several factors including: (1) a multilayer thin-film structure with a CuPc stabilized hole-injection contact, (2) a hole-transport diamine layer using a naphthyl-substituted benzidine derivative, and (3) an ac drive wave form. These emissive devices have shown an operational half-lifetime of about 4000 h from an initial luminance of 510 cd/m2.
Contactless determination of current–voltage characteristics and minority-carrier lifetimes in semiconductors from quasi-steady-state photoconductance data
Authors: Sinton Ronald A. , Cuevas Andres
Published: July 2002

A simple method for implementing the steady-state photoconductance technique for determining the minority-carrier lifetime of semiconductor materials is presented. Using a contactless instrument, the photoconductance is measured in a quasi-steady-state mode during a long, slow varying light pulse. This permits the use of simple electronics and light sources. Despite its simplicity, the technique is capable of determining very low minority carrier lifetimes and is applicable to a wide range of semiconductor materials. In addition, by analyzing this quasi-steady-state photoconductance as a function of incident light intensity, implicit current–voltage characteristic curves can be obtained for noncontacted silicon wafers and solar cell precursors in an expedient manner.
Gallium vacancies and the yellow luminescence in GaN
Authors: Neugebauer Jörg , Van de Walle Chris G.
Published: July 2002

We have investigated native defects and native defect-impurity complexes as candidate sources for the yellow luminescence in GaN. Using state-of-the-art first-principles calculations, we find strong evidence that the Ga vacancy (VGa) is responsible. The dependence of the VGa formation energy on Fermi level explains why the yellow luminescence is observed only in n-type GaN. The VGa defect level is a deep acceptor state, consistent with recent pressure experiments. Finally we show that the formation of VGa is enhanced by the creation of complexes between VGa and donor impurities.
Investigation of charge carrier injection in silicon nitride/silicon junctions
Authors: Elmiger J. R. , Kunst M.
Published: July 2002

Contactless measurements of the conductance of silicon wafers with silicon nitride top layers are presented. The deposition of silicon nitride layers leads to an increase of the conductance, both for p-Si and n-Si. This is attributed to the increase of the number of electrons in the Si wafer, in an inversion layer in p-Si, and in an accumulation layer in n-Si. Comparison of the increase of the conductance with the fixed charge density in the nitride layer points to a strongly reduced electron mobility in these layers. It is shown that the change of the conductance increases with growing thickness of the nitride layer up to a thickness of 20 nm, whereafter a constant value is attained.
MoS2 as microtubes
Authors: Remskar M. , Skraba Z. , Cléton F. , Sanjinés R. , Lévy F.
Published: July 2002

We report on the existence of MoS2 hollowed microtubes, several mm in lengths and less than 0.1 μm wall thickness, grown from the vapor phase. Scanning electron microscopy studies reveal that instability of thin weakly bonded sheets against folding causes the tube growth directly or indirectly beyond the formation of turbulent gas flow. Electron diffraction of a single tube wall proves a parallel growth mode of successive layers while the crystal lattices of both tube walls are mutually rotated. A spiral growth mode is proposed.
Large magnetic entropy change in La0.75Ca0.25MnO3
Authors: Guo Z. B. , Zhang J. R. , Huang H. , Ding W. P. , Du Y. W.
Published: July 2002

Magnetic entropy change of La0.75Ca0.25MnO3 with different particle size has been investigated. Magnetic entropy change ∼4.7 J/kg K larger than that of Gd has been observed at 224 K under a magnetic field of 1.5 T in comparatively big particles. This phenomenon of large magnetic entropy change indicates that the perovskite-type manganese oxides have potential applications for magnetic refrigerants in high temperature.
Damage-free separation of GaN thin films from sapphire substrates
Authors: Wong W. S. , Sands T. , Cheung N. W.
Published: July 2002

Gallium nitride thin films grown on sapphire substrates were successfully separated and transferred onto Si substrates using single 38 ns KrF excimer laser pulses directed through the transparent substrate at fluences in the range of 400–600 mJ/cm2. The absorption of the 248 nm radiation by the GaN at the interface induces rapid thermal decomposition of the interfacial layer, yielding metallic Ga and N2 gas. The substrate is easily removed by heating above the Ga melting point of 30 °C. Scanning electron microscopy and x-ray diffraction of the GaN films before and after lift-off demonstrate that the structural quality of the GaN films is not altered by the separation and transfer process.
X-ray photoelectron spectroscopy study of Bi4Ti3O12 ferroelectric ceramics
Authors: Jovalekić Č , Pavlović M. , Osmokrović P. , Atanasoska Lj.
Published: July 2002

Ferroelectric Bi4Ti3O12 ceramics have been prepared by the method of reactive sintering. The ceramics exhibit good ferroelectric properties with a remanent polarization Pr=5.5×10−2 C/m2 and a coercive field Ec=3MV/m. High resolution x-ray photoelectron spectra of the atomic constituents of Bi4Ti3O12 were recorded. The obtained results indicate that oxygen vacancies are preferentially sited in the vicinity of Bi ions, in the Bi2O2 layers.
Novel microwave plasma reactor for diamond synthesis
Authors: Füner M. , Wild C. , Koidl P.
Published: July 2002

Numerical simulations were performed to predict the performance of microwave plasma reactors with various reactor geometries. The simulations include the calculation of the electric field distribution using the finite integration theory and the determination of the plasma density distribution based on a breakdown field algorithm. One reactor geometry with a cavity having the shape of a rotational ellipsoid turned out to be very promising. The electric field within this cavity exhibits two pronounced maxima at the two focal points of the ellipsoid. By coupling microwave energy into one maximum via an antenna, large electric field strengths can be generated in the counter maximum. This effect has been used to excite intense discharges that are very stable, spatially extended, homogeneous, and free from wall contact. These discharges were employed for the chemical vapor deposition of large area diamond wafers.
Synthesis of boron nitride nanotubes by means of excimer laser ablation at high temperature
Authors: Yu D. P. , Sun X. S. , Lee C. S. , Bello I. , Lee S. T. , Gu H. D. , Leung K. M. , Zhou G. W. , Dong Z. F. , Zhang Z.
Published: July 2002

Boron nitride nanotubes (BN-NTs) were synthesized by using excimer laser ablation at 1200 °C in different carrier gases. The main characteristic of the BN-NTs produced by this method is that nanotubes are of only one to three atomic layers thick, which could be attributed to the dominance of the axial growth rate over the radial growth rate. The diameter of the BN-NTs ranged from 1.5 to 8 nm. The tips of the BN-NTs are either a flat cap or of polygonal termination, in contrast to the conical ends of carbon nanotubes. The atomic ratio of boron to nitrogen as measured by means of parallel electron energy loss spectroscopy is 0.8, which is within the experimental error of the stoichiometry of hexagonal BN structure.
Ordered InAs quantum dots in InAlAs matrix on (001) InP substrates grown by molecular beam epitaxy
Authors: Li Hanxuan , Wu Ju , Xu Bo , Liang Jiben , Wang Zhanguo
Published: July 2002

InAs self-organized quantum dots in InAlAs matrix lattice-matched to exactly oriented (001) InP substrates were grown by solid source molecular beam epitaxy (MBE) using the Stranski-Krastanow mode. Preliminary characterizations have been performed using photoluminescence and transmission electron microscopy. The geometrical arrangement of the quantum dots is found to be strongly dependent on the amount of coverage. At low deposition thickness. InAs QDs are arranged in chains along [11̄0] directions. Luminescence from the quantum dots and the wetting layer consisting of quantum wells with well widths of 1, 2, and 3 monolayers is observed.
Crystal growth and low coercive field 180° domain switching characteristics of stoichiometric LiTaO3
Authors: Kitamura Kenji , Furukawa Yasunori , Niwa Kazuo , Gopalan Venkatraman , Mitchell Terence E.
Published: July 2002

We grew LiTaO3 single crystals with a composition close to stoichiometry by using a double crucible Czochralski method. The switching field required for 180° ferroelectric domain reversal and the internal fields originating from nonstoichiometric point defects were compared for the stoichiometric and conventional commercially available crystals. The switching fields for the domain reversal in the stoichiometric crystal with a Curie temperature of 685 °C was 1.7 kV/mm. This is about one thirteenth of the switching field required for the conventional LiTaO3  crystals with a Curie temperature near 600 °C. The internal field in the stoichiometric crystal drastically decreased to 0.1 kV/mm.
Spin-dependent band structure, Fermi surface, and carrier lifetime of permalloy
Authors: Petrovykh D. Y. , Altmann K. N. , Höchst H. , Laubscher M. , Maat S. , Mankey G. J. , Himpsel F. J.
Published: July 2002

Angle-resolved photoemission is used to determine the energy bands of permalloy (Ni0.8Fe0.2) and compare them to Ni, Co, and Cu. The energy and momentum resolution (≈0.01 eV and ≈0.01 Å−1) is high enough to resolve the magnetically split bands at the Fermi level that are responsible for spin-dependent conductivity and tunneling. For the Σ1 band we find the magnetic exchange splittings δEex=0.27 eV (0.23 eV for Ni), δkex=0.16±0.02 Å−1 (0.12±0.01 Å−1 for Ni), the Fermi velocity vF↑=(0.22±0.02)106 m/s (0.28×106 m/s for Ni, 0.33×106 m/s for fcc Co), and the widths δk↑⩽0.11 Å−1 and δk↓=0.24 Å−1. Compared to Ni, permalloy features a 27% larger magnetic splitting of the Fermi surface and an extremely short mean free path of 4–8 Å for minority spins.
Electron mobility in modulation-doped AlGaN–GaN heterostructures
Authors: Gaska R. , Shur M. S. , Bykhovski A. D. , Orlov A. O. , Snider G. L.
Published: July 2002

We report on the measurements of the electron mobility in modulation-doped Al0.2Ga0.8N–GaN heterostructures grown on sapphire, conducting 6H–SiC, and insulating 4H–SiC substrates as a function of the sheet electron density, ns, at the heterointerface in a wide temperature range. The mobility increases with an increase in ns up to approximately 1×1013 cm−2 and decreases with a further increase in ns. This is explained by the electron spillover at high values of ns from the two-dimensional states at the AlGaN/GaN heterointerface into the delocalized states in the doped GaN channel. The maximum electron Hall mobility in excess of 2000 cm2/V s at room temperature and 11 000 cm2/V s at 4.2 K was measured in the heterostructures grown on 6H–SiC at the values of ns close to 1013 cm−2 and 7×1012 cm−2, respectively.
Thermomechanical properties and moisture uptake characteristics of hydrogen silsesquioxane submicron films
Authors: Zhao Jie-Hua , Malik Irfan , Ryan Todd , Ogawa Ennis T. , Ho Paul S. , Shih Wei-Yan , McKerrow Andrew J. , Taylor Kelly J.
Published: July 2002

This letter describes measurement of the biaxial modulus, coefficient of thermal expansion (CTE), and moisture uptake characteristics of hydrogen silsesquioxane (HSQ) thin films. The biaxial modulus and CTE were determined using a bending beam method, and moisture uptake was studied using a quartz crystal microbalance method. The biaxial modulus and CTE of a 0.5 μm HSQ film were measured on Si and Ge substrates and found to be 7.07 GPa and 20.5 ppm/°C, respectively. The value determined for the diffusion constant of water in a 0.7-μm-thick HSQ films is 3.61×10−10 cm2/s at room temperature.
Quantum-well width dependence of threshold current density in InGaN lasers
Authors: Chow W. W. , Amano H. , Takeuchi T. , Han J.
Published: July 2002

The quantum-confined Stark effect was found to result in a strong quantum-well width dependence of threshold current density in strained group-III nitride quantum well lasers. For an In0.2Ga0.8N/GaN structure with quantum-well width in the neighborhood of 3.5 nm, our analysis shows that the reduction in spontaneous emission loss by the electron–hole spatial separation outweighs the corresponding reduction in gain to produce a threshold current-density minimum.
Surface smoothing of floating gates in flash memory devices via surface nitrogen and carbon incorporation
Authors: Cha Cher-Liang , Chor Eng-Fong , Gong Hao , Bourdillon Anthony J. , Jia Yu-Min , Pan Jin-Sheng , Zhang An-Qing , Chan Lap
Published: July 2002

The presence of large polysilicon grains and facetal growth in floating gate (polysilicon) of memory devices causes its surface to be rough, and degrades the quality of its interface with the adjacent interpoly dielectric. A simple and efficient solution has been proposed in this work. Via low-energy N2+ or Ar+ ion implantation into deposited polysilicon film, surface silicon nitride (SiNy) or silicon carbide (SiC) are formed and they act as nucleation centers to promote the growth of small surface grains during thermal processing. Smaller surface grains, a restriction of film facetal growth, a smoother polysilicon surface, and a better floating gate/interpoly dielectric interface are all achieved via such implantation.
Photoemission study of the interface between phenyl diamine and treated indium–tin–oxide
Authors: Le Quoc Toan , Nüesch F. , Rothberg L. J. , Forsythe E. W. , Gao Yongli
Published: July 2002

The interface formation between indium–tin–oxide (ITO) having various work functions and N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) was investigated using x-ray and ultraviolet photoelectron spectroscopy. Phosphoric acid and tetrabutylammonium hydroxide were used to modify the ITO work function. The energy difference between the highest occupied molecular orbital (HOMO) position of NPB and the ITO Fermi level can be varied substantially by surface treatment. The work function of acid-treated ITO dramatically decreases after deposition of a thin NPB layer. We attribute this to the reaction between the NPB nitrogen and the proton of the adsorbed dipole layer. No significant reaction was observed for NPB deposited on standard ITO.
Exposure of 38 nm period grating patterns with extreme ultraviolet interferometric lithography
Authors: Solak H. H. , He D. , Li W. , Singh-Gasson S. , Cerrina F. , Sohn B. H. , Yang X. M. , Nealey P.
Published: July 2002

Extreme ultraviolet (EUV, λ=13 nm) lithography is considered to be the most likely technology to follow ultraviolet (optical) lithography. One of the challenging aspects is the development of suitable resist materials and processes. This development requires the ability to produce high-resolution patterns. Until now, this ability has been severely limited by the lack of sources and imaging systems. We report printing of 38 nm period grating patterns by interferometric lithography technique with EUV light. A Lloyd’s Mirror interferometer was used, reflecting part of an incident beam with a mirror at grazing incidence and letting it interfere with the direct beam at the wafer plane. High-density fringes (38 nm pitch) were easily produced. Monochromatized light of 13 nm wavelength from an undulator in an electron storage ring provided the necessary temporal and spatial coherence along with sufficient intensity flux. This simple technique can be extended to sub-10 nm resolution.
Field emission from well-aligned, patterned, carbon nanotube emitters
Authors: Murakami Hirohiko , Hirakawa Masaaki , Tanaka Chiaki , Yamakawa Hiroyuki
Published: July 2002

We have developed a process to fabricate well-aligned, patterned, carbon nanotube field emitters on glass substrates. The process consists of depositing and patterning a nickel-based metal line on the glass substrate followed by a bias-enhanced microwave plasma chemical vapor deposition to grow carbon nanotube emitters. A turn-on field of 1.2 V/μm, and emission currents of 1 mA/cm2 at 3 V/μm were achieved on well-aligned carbon nanotube emitters. A test of cathode-ray tube lighting elements now underway suggests a lifetime of exceeding 10 000 h.
Temperature dependence of the Raman spectra of single-wall carbon nanotubes
Authors: Li H. D. , Yue K. T. , Lian Z. L. , Zhan Y. , Zhou L. X. , Zhang S. L. , Shi Z. J. , Gu Z. N. , Liu B. B. , Yang R. S. , Yang H. B. , Zou G. T. , Zhang Y. , Iijima S.
Published: July 2002

Raman spectra of single-wall carbon nanotubes (SWCNTs) were measured at different temperatures by varying the incident laser power. The elevated temperature of the SWCNTs and multiwall carbon nanotubes (MWCNTs) is confirmed to be due to the presence of impurities, defects, and disorder. The temperature coefficient of the frequency of the C–C stretching mode E2g (GM) and that of the radial breathing mode in the SWCNT were determined to be ∼−0.038 and ∼−0.013 cm−1/K, respectively. It is found that the temperature coefficient of the GM in the SWCNT is larger than that of the MWCNT, highly oriented pyrolytic graphite, and the graphite. This is attributed to the structural characteristic of the SWCNT—a single tubular carbon sheet with smaller diameter.
Oxidation resistance of protective coatings studied by spectroscopic ellipsometry
Authors: Hones P. , Zakri C. , Schmid P. E. , Lévy F. , Shojaei O. R.
Published: July 2002

Spectroscopic ellipsometry is applied to determine the oxidation resistance of TiN and CrN coatings. This technique proves to provide accurate measurements of the oxide layer thickness in a convenient, fast, and nondestructive manner. The magnetron-sputtered thin films were heated in air in the temperature range between 673 and 925 K. The verification with scanning electron microscopy and x-ray reflectometry shows an excellent agreement between these complementary techniques. The rate constant k of the oxidation is directly related to the film morphology in TiN thin films. While understoichiometric films with a dense fine-grained structure exhibit a moderate k of 4 nm2/s at 773 K, overstoichiometric films with a pronounced columnar structure oxidized over seven times faster. The nonstoichiometry in TiNy and CrNy leads to a reduced activation energy for oxidation compared to stoichiometric compounds.
Solution of the dark state problem in antiferroelectric liquid crystal displays
Authors: D’havé Koen , Rudquist Per , Lagerwall Sven T. , Pauwels Herman , Drzewinski Witold , Dabrowski Roman
Published: July 2002

So far, it has proven impossible to achieve an acceptable dark state between crossed polarizers for antiferroelectric liquid crystals (AFLCs), which otherwise would have an enormous potential for electro-optic applications, in particular for high-resolution full color displays. The reason lies in static and dynamic spatial fluctuations of the optic axis. As both have intrinsic causes it is not likely that the problem is ever going to be solved by improvement in alignment and addressing methods. We show that if the directors in alternating layers are orthogonal to each other, the AFLC acquires new optical properties such that the problem is eliminated, and a dark state extinction is achieved which is only limited by the quality of the polarizers. After having synthesized such a material, we have been able to demonstrate the predicted unique electro-optical properties of this new class of AFLC materials.
Electric-field dependence of mobility in conjugated polymer films
Authors: Rakhmanova S. V. , Conwell E. M.
Published: July 2002

It has been suggested that Poole–Frenkel behavior, i.e., the linear dependence of the log of the mobility μ on the square root of the electric-field intensity, found for many conjugated polymer films is due to the interaction between the charge carriers and randomly distributed permanent dipoles. However, the dipole concentration in the polymers is much too small to achieve this. We show that introduction of short-range correlation between the energies of the hopping sites, on the scale of the coherence length measured by x rays, can account for the field and temperature dependence of films of poly(2-methoxy,5-(2′-ethyl-hexoxy)–p-phenylene vinylene), a representative conjugated polymer. We investigate also the effects of inhomogeneity on the field and temperature dependence of μ.
Photoelectrochemical etching of InxGa1−xN
Authors: Hwang J. M. , Hsieh J. T. , Ko C. Y. , Hwang H. L. , Hung W. H.
Published: July 2002

Photoelectrochemical (PEC) etching of InxGa1−xN in the KOH solution under illumination of a Hg-arc lamp is studied. An indium oxide surface layer is formed during PEC etching, which slows down the etching rate. The PEC etch rate of InxGa1−xN is determined by dissolution of indium oxides into the solution. Increase of the solution temperature results in an increase of solubility of indium oxides and thus enhances the PEC etch rate. It is found that stirring the solution can accelerate indium oxides to dissolve into the solution and increase the etch rate. The thick indium oxide layer on the PEC-etched InxGa1−xN surface can be effectively removed by the treatment of using a hot 6N HCl solution. A post-treatment by using a 3.2 M KOH solution can provide a smooth sidewall on the PEC-etched surface for the potential application to laser cavity.
Self-induced light polarization rotation in azobenzene-containing polymers
Authors: Nikolova L. , Nedelchev L. , Todorov T. , Petrova Tz. , Tomova N. , Dragostinova V. , Ramanujam P. S. , Hvilsted S.
Published: July 2002

We report here a light-induced phenomenon—a self-induced rotation of the azimuth of elliptically polarized light passing through photobirefringent azopolymers. The experiments are carried out with films of amorphous and liquid-crystalline polymers. It has been shown that the induced rotation angle depends on the ellipticity of the input light. A theoretical analysis of the phenomenon has been done and it has been shown that light induces chiral structure in the polymer films.
Neoclassical theory of rotation and electric field in high collisionality plasmas with steep gradients
Authors: Claassen H. A. , Gerhauser H. , Rogister A. , Yarim C.
Published: July 2002

The equation describing the radial transport of toroidal momentum in a collisional subsonic plasma with steep gradients has been obtained via a systematic expansion of the two-fluid equations. The diffusion rate is classical; the poloidal rotation, driven by the temperature gradient, generates, in turn, a toroidal flow gradient, also in Ohmic discharges. Moreover, important modifications of the parallel momentum equation are found to arise if Λ1≡(νi/Ωi)(q2R2/rLT) is ⩾O(1/3); the poloidal rotation velocity is then no longer unique but obeys a cubic equation which may allow for bifurcated equilibria under certain conditions. The toroidal velocities predicted for Ohmic discharges compare well with those measured in PLT [Princeton Large Torus; S. Suckewer et al., Nucl. Fusion 21, 1301 (1981)]; the relevance of the extended equation providing the poloidal rotation velocity to selected experimental edge plasmas is discussed.
Electrical characterization of TiC ohmic contacts to aluminum ion implanted 4H–silicon carbide
Authors: Lee S.-K. , Zetterling C.-M. , Danielsson E. , Östling M. , Palmquist J.-P. , Högberg H. , Jansson U.
Published: July 2002

We report on the investigation of epitaxial TiC ohmic contacts to Al ion implanted 4H–SiC. TiC ohmic contacts were formed by coevaporation of Ti and C60 at low temperature (<500 °C). A sacrificial silicon nitride (Si3N4) layer was deposited on the silicon carbide substrate prior to Al implantation in order to reach a high Al dopant concentration at the surface while maintaining a low dose. The combination of epitaxially grown TiC and the silicon nitride layer resulted in a promising scheme to make low resistivity ohmic contacts. The lowest contact resistivity (ρC) and sheet resistance (Rs) of the implanted layer at 25 °C were as low as 2×10−5 Ω cm2 and 0.6 kΩ/□, respectively.
Dynamics of ammonia decomposition on Ru(0001)
Authors: Mortensen H. , Diekhöner L. , Baurichter A. , Jensen E. , Luntz A. C.
Published: July 2002

Using supersonic molecular beam techniques we have investigated the dissociative adsorption of NH3 on a Ru(0001) surface. At high incident energies, the dissociation increases substantially due to a direct breaking of the N–H bond on impact with the surface. For low incident translational energies, the dissociation depends on surface temperature Ts in an unusual manner, peaking sharply around 400 K. Increasing the surface defect density by low-fluence Ar+ sputtering strongly enhances the dissociation probability while preserving the overall Ts-dependence. We interpret the low incident energy behavior as due to a mechanism in which a molecular precursor must undergo diffusion to defects before dissociating. At the lowest surface temperatures, dissociation is limited by the diffusion of the reaction products away from the defects in order to reactivate them. A kinetic model based on this mechanism is developed which is in good agreement with all experimental observations.
Effect of Li substitution on the crystal structure and magnetoresistance of LaMnO3
Authors: Ye S. L. , Song W. H. , Dai J. M. , Wang S. G. , Wang K. Y. , Yuan C. L. , Sun Y. P.
Published: July 2002

The crystal structure and magnetoresistance of the polycrystalline La1−xLixMnO3 (x=0.10, 0.15, 0.20, 0.30) are investigated. The result of the Rietveld refinement of x-ray powder diffraction shows that the room temperature structural transition from rhombohedral (R3̄C) to orthorhombic (Pbnm) symmetry occurs at the Li-doped level x⩾0.2. Accompanying the occurrence of the structural transition, the lattice distortion and the bending of the Mn–O–Mn bond increase and the ferromagnetic transition temperature TC decreases. For x=0.10 and 0.15 samples, double metal–insulator (M–I) transitions accompanying a single ferromagnetic transition and a negative magnetoresistance as high as 26% in a magnetic field of 0.8 T are observed. For x=0.20 and 0.30, the samples manifest nonmetallic behavior throughout the measured temperature range. We suggest that the double M–I transitions phenomena of low Li-doped samples originate from the magnetic inhomogeneity due to the formations of the Mn3+ and Mn4+-rich regions induced by partial substitution of the monovalent Li1+ ions for the trivalent La3+ ions. The transport property of high Li-doped samples (x=0.20 and 0.30) can be explained according to the additional localization of eg electrons induced by a static coherent Jahn–Teller distortion of the MnO6 octahedra stemming from the structural transition from rhombohedral (R3̄C) to orthorhombic (Pbnm) and the reduced bandwidth of eg electrons due to the increased bending of the Mn–O–Mn bond.
Magnetic anisotropy and strain states of (001) and (110) colossal magnetoresistance thin films
Authors: Berndt L. M. , Balbarin Vincent , Suzuki Y.
Published: July 2002

The magnetic anisotropy of epitaxial colossal magnetoresistance films on SrTiO3 substrates is dominated by the strain or magnetocrystalline anisotropy depending on the orientation of the film. While the magnetic anisotropy of (001) films is determined by magnetocrystalline anisotropy, that of (110) films is determined by magnetoelastic effects. Furthermore while the microstructure in the thin films has a significant effect on the coercive field, it has a correspondingly negligible effect on the magnetic anisotropy. We find a roughening surface morphology with increasing thickness that is more pronounced in (110) oriented films and can be attributed to the growth kinetics.
A structure-based coarse-grained model for polymer melts
Authors: Akkermans Reinier L. C. , Briels W. J.
Published: July 2002

In this study we explore a systematic procedure to coarsen a microscopic model towards a mesoscopic model. The procedure is applied to a system of chains of ten beads, representing a low molecular weight polymer melt. Our method consists of defining coarse-grained sites in the microscopic system, and calculating their spatial distribution on the pair level. The effective interaction between the coarse-grained sites is then obtained by bringing the pair interaction in consistence with the pair density. We investigate both a dynamic and a stochastic method for this step. The so obtained mesoscopic interaction is used in a molecular dynamics simulation to investigate the pressure of the coarse-grained system. We found that the pair interaction that reproduces the pair density predicts a pressure that is significantly lower than the microscopic value, even if we take the state-dependency of the coarse-grained interactions into account. We therefore conclude that coarse-grained models lack thermodynamic consistency.
A new determination method of absolute adsorption isotherm of supercritical gases under high pressure with a special relevance to density-functional theory study
Authors: Murata Katsuyuki , El-Merraoui Mustapha , Kaneko Katsumi
Published: July 2002

A new determination method of an absolute adsorbed amount nab from the surface excess mass nex for high-pressure adsorption isotherms of a supercritical gas was proposed. The effectiveness of new method was examined by using the density-functional theory (DFT). The DFT study showed that this analysis can provide reasonable results; both of the absolute adsorbed amount determined from the proposed method and the DFT agreed within 5% at 90 MPa and only 1% below 5 MPa. Furthermore, we applied this new method to the experimental surface excess isotherm in the literature, which has a maximum. The analyzed absolute adsorption isotherm from the surface excess adsorption isotherm having a maximum were of IUPAC type I or type II. This method gave the thickness of the interfacial layer of the adsorbed phase.
Electrospinning: A whipping fluid jet generates submicron polymer fibers
Authors: Shin Y. M. , Hohman M. M. , Brenner M. P. , Rutledge G. C.
Published: July 2002

Polymeric fibers with diameters in the range from 50 nm to 5 μm are produced by accelerating a fluid jet in an electric field, in a process known as “electrospinning.” Here we show that an essential element of the process is a fluid instability, the rapidly whipping jet. The phenomena responsible for the onset of whipping are revealed by a linear instability analysis that describes the jet behavior in terms of known fluid properties and operating conditions. The behavior of two competing instabilities, the Rayleigh mode and the axisymmetric conducting mode, is also described. The results are summarized using operating diagrams, delineating regimes of operation in electrospinning, which are in good agreement with experimental observations.
Electron affinity of AlxGa1−xN(0001) surfaces
Authors: Grabowski S. P. , Schneider M. , Nienhaus H. , Mönch W. , Dimitrov R. , Ambacher O. , Stutzmann M.
Published: July 2002

The electronic properties and the electron affinities of AlxGa1−xN(0001) surfaces were investigated by ultraviolet photoemission spectroscopy (UPS) over the whole composition range. The samples were prepared by N-ion sputtering and annealing. Surface cleanliness and stoichiometry were monitored with x-ray photoemission spectroscopy. Samples with high aluminum content showed traces of oxygen which could not be removed by further cleaning cycles. However, we have evidence that the oxygen is located in the bulk and not at the surface. From the UP spectra the ionization energies and electron affinities as a function of composition x were determined. A decrease in electron affinity with increasing aluminum content was found, but the electron affinity remains positive for all x. Thus, earlier predictions of negative electron affinity for high aluminum content were not confirmed.
The ONIOM-PCM method: Combining the hybrid molecular orbital method and the polarizable continuum model for solvation. Application to the geometry and properties of a merocyanine in solution
Authors: Vreven Thom , Mennucci Benedetta , da Silva Clarissa O. , Morokuma Keiji , Tomasi Jacopo
Published: July 2002

We present the ONIOM-PCM method, which combines the ONIOM (our own n-layered integrated molecular orbital+molecular mechanics) method with the polarizable continuum model (PCM). Four versions of the method have been developed. These schemes differ mainly with respect to the level of coupling between the solute charge distribution and the continuum, which has important consequences for the computational efficiency. Any property that can be calculated by both ONIOM and PCM can also be calculated by the ONIOM-PCM method. In the current paper we use this aspect for the calculation of the derivatives of the energy with respect to the nuclear coordinates to perform geometry optimizations, and the calculation of the nuclear magnetic resonance shielding for solvated molecules. To assess the various versions of the method, we performed ONIOM(B3LYP:Hartree–Fock)-PCM calculations on a merocyanine, H2N(C2H2)3CHO. All four schemes yield results close to the target B3LYP (three-parameter Becke–Lee–Yang–Parr density functional)-PCM, and the method appears to be a promising tool for accurate calculations on large molecules in solution.
Ab initio study of the CH3+O2 reaction: Kinetics, mechanism and product branching probabilities
Authors: Zhu Rongshun , Hsu C.-C. , Lin M. C.
Published: July 2002

The reaction of CH3 radical with molecular O2 has been investigated by ab initio molecular orbital theory and variational transition state theory calculations. The detailed potential energy surfaces, including the crossing seams between the PES, located by means of the intrinsic reaction coordinate approach are presented. The rate constants for the association and product formation channels have been calculated and compared with the experimental data. Under the atmospheric pressure condition, the association reaction (a) producing CH3O2 dominates reaction below 1500 K. The branching probabilities for channels (b) and (c) producing CH2O+OH and CH3O+O, respectively, have been calculated and compared; channel (b) is predicted to be dominant below 2000 K with the rate constant kb=1.14×10−22T2.86exp(−5120/T) cm3 molecule−1 s−1. Over 2000 K, channel (c) becomes competitive; its rate constant could be represented by kc=1.01×10−16T1.54 exp(−13 280/T) cm3 molecule−1 s−1 in the temperature range of 1000–3000 K. In addition, the most exothermic products, CHO+H2O, were found to be kinetically inaccessible because of the large barrier, 47.4 kcal/mol above the reactants.
Materials characterization of ZrO2–SiO2 and HfO2–SiO2 binary oxides deposited by chemical solution deposition
Authors: Neumayer D. A. , Cartier E.
Published: July 2002

The thermal stability, microstructure, and electrical properties of xZrO2⋅(100−x)SiO2 (ZSO) and xHfO2⋅(100−x)SiO2 (HSO) (x=15%, 25%, 50%, and 75%) binary oxides were evaluated to help assess their suitability as a replacement for silicon dioxide gate dielectrics in complementary metal–oxide–semiconductor transistors. The films were prepared by chemical solution deposition using a solution prepared from a mixture of zirconium, hafnium, and silicon butoxyethoxides dissolved in butoxyethanol. The films were spun onto SiOxNy coated Si wafers and furnace annealed at temperatures from 500 to 1200 °C in oxygen for 30–60 min. The microstructure and electrical properties of ZSO and HSO films were examined as a function of the Zr/Si and Hf/Si ratio and annealing temperature. The films were characterized by x-ray diffraction, mid- and far-Fourier transform infrared (FTIR), Rutherford backscattering spectroscopy, and Auger electron spectroscopy. At ZrO2 or HfO2 concentrations ⩾50%, phase separation and crystallization of tetragonal ZrO2 or HfO2 were observed at 800 °C. At ZrO2 or HfO2 concentrations ⩽ 25%, phase separation and crystallization of tetragonal ZrO2 or HfO2 were observed at 1000 °C. As the annealing temperature increased, a progressive change in microstructure was observed in the FTIR spectra. Additionally, the FTIR spectra suggest that HfO2 is far more disruptive of the silica network than ZrO2 even at HfO2 concentrations ⩽25%. The dielectric constants of the 25%, 50%, and 75% ZSO films were measured and were observed to be less than the linear combination of ZrO2 and SiO2 dielectric constants. The dielectric constant was also observed to increase with increasing ZrO2 content. The dielectric constant was also observed to be annealing temperature dependent with larger dielectric constants observed in nonphase separated films. The Clausius–Mossoti equation and a simple capacitor model for a phase separated system were observed to fit the data with the prediction that to achieve a dielectric constant larger than 10 doping concentrations of ZrO2 would have to be greater than 70%.