bluuebunny/crossref_metadata_2025 huggingface dataset:
Indium–tin–oxide surface treatments: Influence on the performance of CuPc/C60 solar cells
Authors: Djurišić A. B. , Kwong C. Y. , Chui P. C. , Chan W. K.
Published: April 2003

In this work, we investigate the influence of different indium tin oxide (ITO) surface treatments on the performance of organic solar cells. ITO substrates have been characterized by Hall measurements, Seebeck coefficient measurements, surface sheet resistance measurements, and surface probe microscopy. Single layer (ITO/copper phthalocyanine (CuPc)/Al) and double layer (ITO/CuPc/C60/Al) solar cells were fabricated. It was found that the surface treatments changed the parameters of the ITO (work function, carrier concentration, sheet resistance, surface roughness) and significantly influenced the solar cell performance. The AM1 power conversion efficiency of the ITO/CuPc/C60/Al cell with optimal surface treatment (∼0.1%) is 1 order of magnitude larger than the power conversion efficiency of the solar cell fabricated on untreated ITO substrate (∼0.01%). The AM1 power conversion efficiency can be further enhanced with improved device structures. Obtained AM1 power conversion efficiency for a three layer structure ITO/CuPc/CuPc:C60 (1:1)/C60/Al was measured to be 0.16%.
Photoluminescence of iodine-doped ZnTe homoepitaxial layer grown by metalorganic vapor phase epitaxy
Authors: Tanaka Tooru , Hayashida Kazuki , Nishio Mitsuhiro , Guo Qixin , Ogawa Hiroshi
Published: April 2003

Photoluminescence (PL) properties of I-doped ZnTe homoepitaxial layers grown by atmospheric pressure metalorganic vapor phase epitaxy have been investigated as a function of n-butyliodide (n-BuI) transport rate. The PL spectrum changed drastically even when a low dopant transport rate was used. A structured emission band at 2.22 eV was detected predominantly at low n-BuI transport rates, whereas a structureless broad emission band at around 2.17 eV or less dominates the PL spectrum at high n-BuI transport rates. PL spectra of I-doped ZnTe layers were analyzed at varying excitation power and temperature. These luminescence bands are due to donor–acceptor pair recombination, and may be ascribed to complexes consisting of Zn vacancies and I on Te sites close to donors.
A computational method of temperature characteristics of magneto-optical Kerr effect in amorphous TbFeCo films with a multilayered structure
Authors: Zou Z. Q. , Wang H. , Yu C.
Published: April 2003

A computational method for evaluating temperature characteristics of the Kerr rotation angle, Kerr ellipticity, and thus the readout signal-to-noise ratio of an amorphous TbFeCo film with a multilayered structure is presented. In this method, a generalized mean-field theory for an n-component system is applied to analyze the temperature dependence of the sublattice magnetizations and the total saturation magnetization of the ternary TbFeCo alloy, and the characteristic matrix method of film optics is adopted to analyze the magneto-optical Kerr effect in the multilayered film. By using this method, the optimum power of the readout laser for a practical magneto-optical disk structure can be determined. As an example, we present a calculation on the Tb0.21Fe0.66Co0.13 quadrilayer disk structure. The result shows that the Kerr rotation angle, Kerr ellipticity, and the readout figure of merit of the quadrilayer structure decrease monotonically with increasing temperature, similar to the variation of Fe sublattice magnetization with the temperature, MFe(T). The calculated temperature dependences of the saturation magnetization MS(T) and the Kerr rotation angle θK(T) are in good accordance with the previous experimental results.
Polymer nanocomposite containing CdS–ZnS core–shell particles: Optical properties and morphology
Authors: Lu Shih-Yuan , Wu Mei-Ling , Chen Hsin-Lung
Published: April 2003

Composites of nanosized CdS–ZnS core–shell particles well dispersed in polycetyl- p-vinylbenzyldimethylammonium chloride (PCVDAC) were fabricated and their optical properties and morphology were studied. The core-shell particles of less than 10 nm in size were prepared with the reverse micellar solutions formed by a polymerizable anionic surfactant, CVDAC, and a direct thermal radical polymerization of the micellar solution led to the nanocomposite containing well-dispersed core–shell particles. The shell was inferred from small angle x-ray scattering to be around a monolayer thick. This monolayer coating, however, was enough to produce an appreciable enhancement in photoluminescence for the ZnS-coated CdS system (wider band gap material on narrower band gap material), and a degradation in photoluminescence for the CdS-coated ZnS system. However, a complete coverage of the CdS core with ZnS was required to realize the photoluminescence enhancement for the ZnS-coated CdS system. The polymer domains in the composite were found to be composed of lamellar stacks due to the comb-like structure of PCVDAC. The interlamellar distance was 3.9 nm with smectic liquid crystalline texture observed under polarized optical microscope. The core–shell particles were believed to locate at the grain boundary of the liquid crystalline domains.
Asymmetrical leakage currents as a possible origin of the polarization offsets observed in compositionally graded ferroelectric films
Authors: Bouregba R. , Poullain G. , Vilquin B. , Le Rhun G.
Published: April 2003

Offsets of hysteresis loops along the polarization axis have been observed on a step graded Pb(Zr,Ti)O3 (PZT) thin film using a Sawyer–Tower (ST) circuit. However, the same effect may be artificially reproduced by adding adequate resistors and diodes in parallel with a nongraded PZT thin film. The hypothesis that the offsets were mainly due to the asymmetrical charging up of the standard capacitor used in the ST circuit, allows us to establish that the graded ferroelectric sample behaves as a kind of rectifying device. It is concluded that the presence of asymmetrical leakage currents in compositionally graded devices may allow the elucidation of the origin of the offsets often observed in these structures. Correlatively, it is demonstrated that such offsets do not represent an abnormal static polarization but a dc voltage. The Em4 power law dependence of the offsets (where Em is the amplitude of the electric field) was found to be attributable to the nonlinear increase of the net leakage current.
Substrate induced strain effects in epitaxial La0.67−xPrxCa0.33MnO3 thin films
Authors: Wu T. , Ogale S. B. , Shinde S. R. , Biswas Amlan , Polletto T. , Greene R. L. , Venkatesan T. , Millis A. J.
Published: April 2003

We have studied the physical properties of epitaxial La0.67−xPrxCa0.33MnO3(LPCMO) (x=0.13, 0.20, 0.27) thin films under different type and degree of substrate induced biaxial strain. Films with thickness of 300, 600, and 1000 Å grown on crystalline NdGaO3 (negligible strain), LaAlO3 (compressive strain), and SrTiO3 (tensile strain) by pulsed laser deposition are examined. Our data suggest occurrence of metastable phase mixtures in these films, with the volume fractions controlled by strain. In particular, melting of the charge ordered phase is seen to occur in thin films on SrTiO3 at fields as low as 1 T, in the low temperature regime. The deviation of ρ(T) from the effective medium theory further indicates that LPCMO thin films cannot be described as simple metal–insulator mixtures.
Crystallinity, surface morphology, and magnetic properties of La0.7Sr0.3MnO3 thin films: An approach based on the laser ablation plume range models
Authors: Koubaa M. , Haghiri-Gosnet A. M. , Desfeux R. , Lecoeur Ph. , Prellier W. , Mercey B.
Published: April 2003

We report on a systematic investigation of the influence of pulsed laser deposition (PLD) parameters upon the microstructure, surface morphology, and magnetic properties of La0.7Sr0.3MnO3 (LSMO) thin films grown on (100) SrTiO3 substrates. The optimization of the physical properties requires a careful exploration of the main parameters such as the oxygen pressure P and the target-to-substrate distance D. We show that there is a strong correlation between both these parameters and an optimal distance (D=L0), which can be calculated from a PD3 scaling law (in accordance with a shock wave model). This particular value L0 corresponds to the distance for which all species are thermalized in the plume. In the D–P diagram, L0 defines two distinct regions for the morphology and the microstructure: (i) when D1 μm) morphology. These films are perfectly epitaxial on the substrate, single-phase and not relaxed. (ii) When D≫L0, the films are columnar with a high peak-to-peak roughness (Rp-v>10 nm). While a good surface morphology and an good internal microstructure are obtained in the low P and D part of the diagram, on the opposite, the best magnetic and electrical properties are observed in films deposited in a sharp transition window between these two regions.
Limitations of split-gate ballistic electron waveguides
Authors: Raichev O. E. , Debray P.
Published: April 2003

We study the ballistic conductance of electron waveguides created by lateral depletion of the two-dimensional (2D) electron gas caused by negatively biased split surface Schottky gates. The maximal number of resolved steps of the quantized conductance staircase and their temperature stability are examined as functions of slit width w, depth d of the 2D layer from the surface, and 2D electron density n. Conditions to obtain a large number of well-resolved steps at high temperatures are explored and formulated. Limitations of the split-gate devices are also discussed. In particular, we find that while the increase of n and decrease of d are always desirable, the width w has to be optimized to have a maximum number of steps at a given temperature.
Sputtering yield measurements during low energy xenon plasma bombardment
Authors: Doerner R. P. , Whyte D. G. , Goebel D. M.
Published: April 2003

The sputtering yields of molybdenum, titanium, beryllium, and carbon have been measured during xenon ion bombardment from a plasma in the energy range between 10 and 200 eV. The erosion rates of Mo, Be, and C are measured both spectroscopically in the plasma and using the standard weight loss technique. Spectroscopic measurements of Ti sputtering yields, where no atomic physics data is available, are normalized to the weight loss measurements. The erosion rates of the metals decrease with the reduced mass of the metal–xenon combination and decrease with the increasing metal’s binding energy, as expected. The erosion results for bombardment of graphite indicate that the sputtering rate of carbon, as atoms, from the surface is insufficient to explain the total carbon weight loss measured. The multiple mechanisms for carbon erosion during plasma bombardment are discussed and the sputter rates of carbon atoms and carbon dimers are presented.
Influence of the interface layers on the transport properties of InP/InGaAs quantum barrier structures grown by low pressure metalorganic vapor phase epitaxy
Authors: Hanamoto L. K. , Farias C. M. A. , Henriques A. B. , Tribuzy C. V. B. , Souza P. L. , Yavich B.
Published: April 2003

Quantum barrier structures of lattice matched InGaAs/InP composition, and doped with Si in the middle of the InP layers, were studied by Shubnikov–de Haas measurements. For a given doping level, the density of free carriers shows a rapid decrease when the thickness of the InP barrier is reduced. This is correlated with the amount of Si atoms that reaches the InGaAs/InP interface by thermal diffusion. To interpret these results, a simple model is proposed, in which it is assumed that some of the Si atoms that are located near the interface form nondonor centers, due to local stress and compositional disorder. The model is used to describe quantitatively the loss in the free carrier concentration as a function of the distance between the doped layer and the interfacial one. Calculations of the subband populations and scattering rates at the Fermi level provide support for the interpretation of the experimental data.
Evaluation of intrinsic and extrinsic contributions to the piezoelectric properties of Pb(Zr1−XTX)O3 thin films as a function of composition
Authors: Kim Dong-Joo , Maria Jon-Paul , Kingon Angus I. , Streiffer S. K.
Published: April 2003

The piezoelectric, dielectric, and ferroelectric properties of highly (111)-textured, 200-nm-thick polycrystalline lead zirconate titanate (PZT) films have been investigated as a function of Zr/Ti ratio. The distinct peak in piezoelectric coefficient at the morphotropic phase boundary found in bulk PZT ceramics is not observed in thin film PZTs. Measurements of the temperature dependence of relative permittivity and the nonlinear behavior of relative permittivity and piezoelectric coefficient suggest that non-180° domain wall motion in these films is negligible, indicating that the extrinsic contribution to the room temperature permittivity is dominated by only 180° domain wall motion. The semiempirical phenomenological equation relating the piezoelectric coefficient to measured polarization and permittivity values is demonstrated to give an excellent description of the piezoelectric behavior in these films, assuming bulk electrostrictive and elastic coefficients. The small deviation between calculated and measured piezoelectric coefficients as well as the dependence of piezoelectric and polarization behavior on the external field, i.e., hysteresis loop, are suggested to be primarily due to backswitching of 180° domains.
Effects of crystallographic orientations on the charging time in silicon nanocrystal flash memories
Authors: de Sousa J. S. , Leburton J.-P. , Thean A. V. , Freire V. N. , da Silva E. F.
Published: April 2003

The effect of crystallographic orientations in nanocrystal silicon on the charging time of flash memory devices is investigated by using the Bardeen’s transfer formalism within a Schrodinger–Poisson scheme. Besides the strong dependence on the nanocrystal shape, we found that the crystallographic orientations strongly affect the electronic structure by changing the symmetry of the wave functions and level degeneracy, which can result in variation in the charging time by one order of magnitude.
Evaluation of improved efficiency with a diamond coating for a plasma display panel electrode
Authors: Matsunaga Yasushi , Kato Tomokazu , Hatori Tadatsugu , Hashiguchi Seishiro
Published: April 2003

Application of diamond to electrode coating of a plasma display panel (PDP) is evaluated, since we expect diamond to emit much secondary electron due to the Auger neutralization induced by Xe ions. In a conventional magnesium oxide-xenon (MgO/Xe) system, the most abundant Xe+ produced in the discharge does not effectively cause the secondary electron emission, because the condition of the Auger neutralization is not satisfied. In order to increase the efficiency of ultraviolet (UV) radiation, being especially important for engineering, we should avoid such inefficiency. Under suitable conditions in diamond/Xe system the Auger neutralization can occur. Further, if the electron affinity χ is negative, i.e., negative electron affinity (NEA), the condition of the Auger neutralization in diamond/Xe system is sufficiently satisfied. First, we calculate the coefficients of the secondary electron emission on diamond of clean surface or of hydrogenated surface where the dangling bonds are terminated, on the basis of the Hagstrum model. If the NEA with the electron affinity χ=−0.5 is realized, the high ion-induced secondary electron coefficients such as γXe=0.25 and γNe=0.32 are obtained. Next, we carry out a one-dimensional fluid simulation of the electric discharge in which the two secondary electron emission coefficients of Ne and Xe obtained theoretically are set. Results are presented with changing the sustaining voltage, the fraction of Xe, and the duration of the zero-voltage phase. For 10%Xe mixture, it is shown that the efficiency of UV radiation in the diamond coating increases two times higher than that in the MgO coating. An unnecessarily spontaneous breakdown during the zero-voltage phase, which is peculiar to high secondary electron emission materials, is observed. A mechanism of the breakdown is discussed and means to suppress it are proposed. Taking the zero-voltage phase short and increasing the fraction of Xe up to 50%, in comparison with MgO, a maximum improvement of 3.5 times in the efficiency of UV radiation can be performed. The high secondary electron emission due to Xe ion brings about a decrease in the sustaining voltage, and the sustaining voltage can be further lowered by shortening the duration of the zero-voltage phase. The operation with an increase of Xe fraction becomes attainable. The feasibility of the high performance operation on the ac type PDP by utilizing the materials including diamond with high secondary electron emission for Xe is quantitatively shown.
Electromagnetic wave technique to determine radiation torque on micromachines driven by light
Authors: Collett W. L. , Ventrice C. A. , Mahajan S. M.
Published: April 2003

A method to determine radiation torque on a dielectric microrotor driven by a focused laser beam, while considering the wave nature of the incident light, is introduced. The method involves numerically determining the electric and magnetic field quantities of the scattered light on the surface of the rotor using the finite difference time domain method. Maxwell’s stress tensor is then employed to calculate the force over the rotor surface, enabling the determination of the rotor torque about the axis of rotation. The method is demonstrated using a simple rotor shape, with results that are in good qualitative agreement with observations reported in literature.
Quantum-size effects in n-type bismuth thin films
Authors: Rogacheva E. I. , Grigorov S. N. , Nashchekina O. N. , Lyubchenko S. , Dresselhaus M. S.
Published: April 2003

Oscillatory thickness dependences of the electrical conductivity, Hall coefficient, charge carrier mobility, and Seebeck coefficient were obtained at room temperature for n-type thin Bi films (d=3–300 nm) fabricated by the thermal evaporation of a bismuth crystal in a vacuum and deposition on mica substrates at 380 K. We attribute this oscillatory behavior to quantum-size effects, which are observable when the electron mean-free path and Fermi wave length exceed the film thickness d.
Hole-transport properties of a furan-containing oligoaryl
Authors: Wu Chung-Chih , Hung Wen-Yi , Liu Tsung-Li , Zhang Ling-Zhi , Luh Tien-Yau
Published: April 2003

We report the carrier transport properties of a furan-containing oligoaryl PF6, which contains no arylamine moiety in the molecular structure but exhibits competitive hole-transport capability in comparison with conventional arylamine-based hole-transport materials often used in organic light-emitting devices (OLEDs) and xerography. Thin films of this compound exhibit both morphological stability and appropriate energy levels for OLED applications. OLEDs using PF6 as the hole-transport layer show low turn-on voltage, high efficiency, and high brightness competitive with those using conventional hole-transport materials, strongly indicating superior hole-transport properties of PF6. The carrier mobility of PF6 was directly measured by the time-of-flight transient photocurrent technique under various temperatures and electric fields. Nondispersive hole transport was observed and a room-temperature hole mobility in excess of 10−3 cm2/V s was obtained under high fields. The field and temperature dependence of hole mobility were analyzed and were found consistent with the Bässler (disorder) formalism.
Metastable ordered arrays of size-selected Ag clusters on graphite
Authors: Couillard M. , Pratontep S. , Palmer R. E.
Published: April 2003

We employ high-resolution transmission electron microscopy to explore the deposition of large size-selected AgN clusters (N=1100 to 5000 atoms) on the model graphite surface. We find that Ag5000 clusters (only) form metastable, ordered arrays, nucleated at surface defects, which are stable for at least a day. The results suggest that the cluster coalescence is strongly dependent on cluster size, and that it should be possible to fabricate large-scale, two-dimensional, and metastable arrays of clusters in the size regime 5–10 nm.
Finding interstitial oxygen in an Si substrate during low-temperature plasma oxidation
Authors: Kim Bo Hyun , Ahn Jin Hyung , Ahn Byung Tae
Published: April 2003

An Si substrate (100) was oxidized at a low temperature in inductively coupled oxygen plasma. Interstitial oxygen was found in the Si substrate at the initial stage of oxidation by IR measurements. The penetration depth of the interstitial oxygen was about 4 nm. An x-ray rocking curve of Si substrates showed a lower peak intensity due to lattice distortion by the interstitial oxygen. The refractive index of thin oxides, below which interstitial oxygen existed in the Si substrate, was smaller than the refractive index of thick oxides, below which no interstitial oxygen existed. The interstitial oxygen was found by plasma oxidation using O2 gas and N2O gas. The inductively coupled plasma oxidation using N2O gas was performed by atomic oxygen, not by molecular oxygen, indicating that atomic oxygen in plasma is responsible for the incorporation of interstitial oxygen.
Bias dependence of tunneling magnetoresistance on ferromagnetic electrode thickness
Authors: Zhu T. , Xiang X. , Xiao John Q.
Published: April 2003

We have investigated the bias dependence of the tunneling magnetoresistance (TMR) in Py/CoFe(t)/Al2O3/Py magnetic tunneling junctions with a wedge-shaped CoFe layer. The value of TMR increases slowly with CoFe thickness and saturates at t⩾2.3 nm, implying a significant bulk-like contribution in spin-dependent tunneling. The bias dependence of TMR also shows a similar trend as a function of CoFe thickness, and becomes independent of CoFe thickness at t⩾2.3 nm. The extracted tunneling characteristic length decreases slightly with increasing bias voltage due to the hot electron excitations.
Photoluminescence dependence of ZnO films grown on Si(100) by radio-frequency magnetron sputtering on the growth ambient
Authors: Jeong Sang-Hun , Kim Bong-Soo , Lee Byung-Teak
Published: April 2003

We report the effects of the growth ambient on photoluminescence (PL) emission properties of ZnO films grown on Si (100) by rf magnetron sputtering. Upon increasing the O2/Ar+O2 ratio in the growing ambient, the visible emission in the room-temperature PL spectra was drastically suppressed without sacrificing the band-edge emission intensity in the ultraviolet region. This tendency is estimated to be due to the reduction of the oxygen vacancies and zinc interstitials in the film induced by the improvement of the film stoichiometry with respect to high oxygen content, indicating that the visible emission in ZnO originates from oxygen vacancy or zinc interstitial related defects. The violet emission peaked at about 401 nm (3.09 eV) was observed in the low-temperature PL spectra of the ZnO films grown under oxygen-rich conditions. This emission band was assigned to the electron transition from the bottom of the conduction band to the Zn vacancy level, positioned approximately 3.06 eV below the conduction band edge.
Grazing incidence x-ray scattering investigation of Si surface patterned with buried dislocation networks
Authors: Leroy F. , Eymery J. , Buttard D. , Renaud G. , Lazzari R. , Fournel F.
Published: April 2003

Investigation of a surface patterned by buried dislocation networks is performed with grazing incidence x-ray scattering (GIXS). It is shown that surface long-range undulations lead to a azimuth-dependent diffusion spot, the scattering vector of which is mainly parallel to the x-ray propagation direction. This unusual scattering direction with GIXS is explained by the scales of the scattering objects. A geometrical model is proposed to extract the periodicity of the surface from GIXS.
Oriented single-crystal diamond cones and their arrays
Authors: Zhang W. J. , Meng X. M. , Chan C. Y. , Wu Y. , Bello I. , Lee S. T.
Published: April 2003

One of the major problems in material science has been the difficulty in modification of the most endurable material, diamond, due to its extreme hardness and chemical inertness. Here, we report the development of a conical structure of diamond by performing bias-assisted reactive ion etching in hydrogen plasma. The diamond cones produced by this method are uniformly distributed over large areas on silicon substrates. Each cone was identified to be a single crystal with an apical angle as small as 28° and a very sharp tip (tip radii ∼2 nm). Their [001] axes are perpendicular to the substrate surface and parallel to each other. Such striking structures of individual single-crystal diamond cones and their arrays, in addition to their scientific value, may lead to a breakthrough in the design of high-performance mechanical and electronic devices.
Determination of the triplet state lifetime of vibrationally excited C60
Authors: Hedén M. , Bulgakov A. V. , Mehlig K. , Campbell E. E. B.
Published: April 2003

The lifetime of the ground triplet state of vibrationally excited C60 has been determined in a two-color pump–probe experiment. An intense pump pulse (337 nm, 4 ns) was used to excite the neutral molecules. The triplet state was probed via single photon ionization with a weak probe pulse (193 nm). The lifetime was found to be on the order of 1 μs. This is rather low to explain recent delayed ionization results but higher than expected from an extrapolation of earlier measurements at lower temperatures.
Noise performance of polycrystalline silicon thin-film transistors made by sequential lateral solidification
Authors: Bonfiglietti A. , Valletta A. , Mariucci L. , Pecora A. , Fortunato G.
Published: April 2003

Low-frequency noise in polycrystalline silicon (polysilicon) thin-film transistors (TFTs), made using the sequential lateral solidification (SLS) technique, has been investigated. Two types of devices have been analyzed: SLS polysilicon TFTs, where the noise is dominated by the carrier number fluctuations, and extended grain polysilicon TFTs, where an excess noise was present for low drain currents. The noise induced by carrier number fluctuations was seen to scale, in both types of devices, with the inverse of the channel area, as expected from the theory. The excess noise was explained considering the mobility fluctuations induced by the scattering with low angle grain boundaries present in the extended grain polysilicon.
Ion extraction from carbon shunting arc plasma
Authors: Takaki K. , Mukaigawa S. , Fujiwara T. , Kumagai M. , Yukimura K.
Published: April 2003

The ion current characteristics of the shunting arc discharge are described in this letter. A carbon rod with 2 mm diameter and 40 mm length was employed for arc generation. The combination of the shunting arc and the negative pulse voltage applied to target is promising for plasma-based ion implantation and deposition for metallic or semimetallic three-dimensional materials. The delay time, which is defined as the time between the start of the arc current and applying the pulse voltage, was varied. The ions are extracted from the shunting arc plasma by applying a pulsed bias voltage to a target set nearby the arc source. The arc current lasts 40 μs, of which peak is 1.7 kA. The extracted target current has a sharp peak at the initial stage, followed by a stationary state. The stationary current decreases with increasing the delay time and increases with increasing the bias voltage. Under the assumption of a collisionless ion sheath, the plasma density was estimated. At the boundary between the ion sheath and the shunting arc plasma, the density was approximately 5×108 cm−3 and 3×107 cm−3 for the delay times of 100 and 1000 μs, respectively. The stationary ion current linearly increases with increasing the bias voltage and can be expressed in a simple equation.
Streaming distribution of two-dimensional electrons in III-N heterostructures for electrically pumped terahertz generation
Authors: Korotyeyev V. V. , Kochelap V. A. , Kim K. W. , Woolard D. L.
Published: April 2003

We studied anisotropic electron distributions in group-III-nitride quantum wells and analyzed formation of the streaming regime under moderate electric fields. We found that the streaming effect can occur in high-mobility heterostructures (μ⩾30 000 cm2/V s) with low electron concentrations (n⩽1012 cm−2) at temperatures above 77 K. The effect holds in the electric fields of a few kV/cm. These conditions can be met in contemporary high-quality nitride heterostructures that have advantages over the bulk materials due to the specifics of electron–phonon and electron–electron interactions in two-dimensional structures. Subsequently, the streaming effect may enable the development of electrically-pumped THz sources operating above the nitrogen temperature based on nitride heterostructures.
Improved performance of 325-nm emission AlGaN ultraviolet light-emitting diodes
Authors: Chitnis A. , Zhang J. P. , Adivarahan V. , Shatalov M. , Wu S. , Pachipulusu R. , Mandavilli V. , Khan M. Asif
Published: April 2003

We report on AlGaN multiple-quantum-well light-emitting diodes over sapphire with peak emission at 325 nm. A pulsed-atomic-layer-epitaxy growth process was used to improve the material quality of the AlN buffer and the AlN/AlGaN strain-relief layers for reducing the nonradiative recombination. In addition, a modified device epilayer structure was used to improve the carrier confinement and the hole injection. A 40% improvement of external quantum efficiency is obtained, resulting in record high optical powers of 10.2 mW at a pulsed pump current of 1 A.
Effect of ambient on the thermal parameters of a micromachined bolometer
Authors: Nagapriya K. S. , Raychaudhuri A. K. , Jain V. K. , Jalwania C. R. , Kumar Vikram
Published: April 2003

The thermal characterization of bolometers is needed for optimal design as well as applications. In this letter, we present results of the effect of environment on the thermal properties of micromachined bolometers. We find that while in vacuum, the thermal response can be represented by a single time constant, in presence of an ambient gas, the thermal response can no longer be described by a single time constant. This will have a direct implication on frequency dependence of responsivity. We present a model to explain our data which involves the finite diffusion time in the ambient gas and the associated extra thermal mass.
Suppression of interdiffusion in InGaAs/GaAs quantum dots using dielectric layer of titanium dioxide
Authors: Fu L. , Lever P. , Tan H. H. , Jagadish C. , Reece P. , Gal M.
Published: April 2003

In this work, titanium dioxide (TiO2) film was deposited onto the In0.5Ga0.5As/GaAs quantum-dot structure by electron-beam evaporation to investigate its effect on interdiffusion. A large redshifted and broadened spectrum from the dot emission was observed compared with that from the uncapped (but annealed) reference sample, indicating the suppression of thermal interdiffusion due to TiO2 deposition. The structure was also capped with a silicon dioxide (SiO2) single layer or SiO2/TiO2 bilayer with the thickness of SiO2 varied from ∼6 to ∼145 nm. In the former case, an increased amount of impurity-free vacancy disordering (IFVD) was introduced with the increase of SiO2 thickness due to the enhanced Ga outdiffusion into the film. With TiO2 deposited on top, IFVD and thermal interdiffusion were suppressed to different extents with the variation of SiO2 thickness. To explain the suppression of interdiffusion, thermal stress introduced by the large thermal expansion coefficient of TiO2 (when compared with GaAs) as well as the metallurgical reactions between the TiO2 and GaAs were proposed as possible mechanisms.
Femtogram mass detection using photothermally actuated nanomechanical resonators
Authors: Lavrik Nickolay V. , Datskos Panos G.
Published: April 2003

Nanomechanical devices with very small mass and size have the potential for mass sensing at the level of individual molecules. In the present study, we designed nanomechanical mass sensors, demonstrated their operation under ambient pressure and temperature, and achieved femtogram-level mass sensitivity. Our nanomechanical resonators were gold-coated silicon cantilevers with resonance frequencies in the range of 1 to 10 MHz, characteristic thicknesses of 50–100 nm, and force constants of about 0.1 N/m. Using a cantilever with a resonant frequency of 2.2 MHz that was excited photothermally, we measured a mass change of 5.5 fg upon chemisorption of 11-mercaptoundecanoic acid. Our analysis indicates that, by decreasing the mass of the cantilever and increasing the excitation amplitude, even higher mass sensitivity can be realized in an easily accessible frequency range (<100 MHz).
450 meV hole localization in GaSb/GaAs quantum dots
Authors: Geller M. , Kapteyn C. , Müller-Kirsch L. , Heitz R. , Bimberg D.
Published: April 2003

The electronic properties of self-organized GaSb quantum dots (QDs) embedded in GaAs n+p diodes were investigated by capacitance–voltage and deep level transient spectroscopy. The localization energy of the hole ground state is 450 meV. State filling lowers the activation energy to 150 meV for completely charged QDs containing 15 holes. The hole retention time at room temperature for a single hole per QD is extrapolated to be in the microsecond range, about five orders of magnitude longer than in In(Ga)As/GaAs QDs. Hence, we consider GaSb/GaAs to be a suitable material system for future QD memory applications which require long storage times.
Simulation of Schottky barrier tunnel transistor using simple boundary condition
Authors: Jang Moongyu , Kang Kicheon , Lee Seongjae , Park Kyoungwan
Published: April 2003

The current–voltage characteristics of a Schottky barrier tunnel transistor (SBTT) are simulated by considering the internal voltage drop at the Schottky barrier and using the current continuity condition between the tunneling and channel current. The numerical results show typical behaviors as can be found in many experimental results. From these results, a significantly higher threshold voltage is expected for the SBTT compared to the conventional metal–oxide–semiconductor field-effect transistors, because of the suppression of the tunneling current at low gate voltage. For the nanometer-size device application, a metal gate should be used to decrease the threshold voltage.
Nanoscale dopant-induced dots and potential fluctuations in GaAs
Authors: Jäger N. D. , Urban K. , Weber E. R. , Ebert Ph.
Published: April 2003

We identified p-type nanoscale dopant-induced dots that are formed by fluctuations of the dopant atom distribution in sufficiently thin GaAs p–n multilayers. Their electronic structure and the resulting potential variations were investigated by cross-sectional scanning tunneling microscopy and spectroscopy as a function of the number of dopant atoms within the dot. We find significant changes in the current–voltage characteristics of the dots compared to spatially nonconfined material, due to a reduced ability to screen the tip’s electric field. This indicates a limited ability to deplete the dots of free holes arising from the presence of confining potentials surrounding the dopant-induced dots.
Transport through an isolated artificial molecule formed from stacked self-assembled quantum dots
Authors: Bryllert T. , Borgstrom M. , Wernersson L-E. , Seifert W. , Samuelson L.
Published: April 2003

We investigate resonant tunneling through two coupled self-assembled quantum dots. The strong confinement and the high crystalline quality provided by the self-assembled dots, in combination with the tunneling coupling between the dots, create a system that may be thought of as an artificial molecule. We are able to isolate one single artificial molecule and detailed investigations of the electrical properties are performed. Peak-to-valley ratios above 1000 and full width half maximum of a few millivolts are measured at 4 K. By changing the temperature we also observe Coulomb blockade effects in a different way.
Formation of hexagonal Gd disilicide nanowires on Si(100)
Authors: Lee Dohyun , Kim Sehun
Published: April 2003

The growth of hexagonal Gd disilicide nanowires on Si(100) is studied by scanning tunneling microscopy. Gd disilicide nanowires are grown on Si(100) by submonolayer Gd deposition on the substrate at 600 °C. The formation of nanowires is shown to be due to anisotropic lattice mismatches between hexagonal Gd disilicide and Si. The nanowires have widths of several nanometers and lengths up to micrometer length scales. The top of the nanowires has a c(2×2) structure, indicating that the crystalline structure is Si-deficient Gd disilicide. The nanowires were shown to have metallic properties using scanning tunneling spectroscopy.
Effect of catalyst composition on carbon nanotube growth
Authors: Liao X. Z. , Serquis A. , Jia Q. X. , Peterson D. E. , Zhu Y. T. , Xu H. F.
Published: April 2003

Transmission electron microscopy was used to probe the compositions of individual Co–Mo bimetal catalyst particles and the morphologies of carbon nanotubes (CNTs) catalyzed by these particles under flowing carbon monoxide at 700 °C. It was found that the composition of the catalyst particle at a CNT tip and the distribution of Co within the particle largely determine the morphology of the CNT. A particle with low Co content (<15 at. %) tends to produce a long CNT, while a particle with very high Co content (>85 at. %) tends to produce onion-like structures. These observations provide insight into the CNT growth mechanisms.
Dual frequency 2D-IR of peptide amide-A and amide-I modes
Authors: Rubtsov I. V. , Wang J. , Hochstrasser R. M.
Published: April 2003

Dual frequency two-dimensional infrared (2D-IR) spectra were acquired with three IR fields, two at 3 and one at 6 μm from two pulses, creating a joint nonlinear response from both the amide-A (N–H) and the amide-I transitions of two model peptides AcAlaOMe and caprolactam. The 2D-IR spectra yield anharmonicities and correlations of inhomogeneous distributions. A positive correlation between the fundamental N–H mode frequency and its anharmonicity is found for the dipeptides. The amide-I frequencies are correlated with N–H mode frequencies and anticorrelated with the amide-I/N–H mode coupling. Structural implications of these results are discussed in relation to the anharmonic potential surface.
Specific relonic patterns from non‐specific or useless laboratory data
Authors: Kvitash Vadim I.
Published: April 2003

Discusses how in 20 different hepato‐biliary diseases, relonics for the first time, identifies previously unknown systemic relational networks of biochemical imbalances/dysbalances which can be used as prototype patterns for early diagnosis, instant monitoring of treatment response, and individualized treatment adjustments.
Normal sum decomposition of general systems
Authors: Liu Guoyang
Published: April 2003

This paper introduces normal systems and the normal sum of general systems. A system S=(M,R) is normal if and only if any two relations in R are not contained in the same Cartesian product Mn for any ordinary number n. Normal sum is a new kind of decomposition (composition) of general systems. Given a normal system S=(M,R), and two subsets A1⊆M and A2⊆M. One of the main results is that the normal sum of the A1‐related subsystem and the A2‐related subsystem of S equals the (A1∪A2)‐related subsystem of S. This implies that every normal system is a normal sum of its subsystems which are non‐trivial and non‐discrete.
Effective state estimation of stochastic systems
Authors: Nechval Nicholas A. , Nechval Konstantin N. , Vasermanis Edgars K.
Published: April 2003

In the present paper, for constructing the minimum risk estimators of state of stochastic systems, a new technique of invariant embedding of sample statistics in a loss function is proposed. This technique represents a simple and computationally attractive statistical method based on the constructive use of the invariance principle in mathematical statistics. Unlike the Bayesian approach, an invariant embedding technique is independent of the choice of priors. It allows one to eliminate unknown parameters from the problem and to find the best invariant estimator, which has smaller risk than any of the well‐known estimators. There exists a class of control systems where observations are not available at every time due to either physical impossibility and/or the costs involved in taking a measurement. In this paper, the problem of how to select the total number of the observations optimally when a constant cost is incurred for each observation taken is discussed. To illustrate the proposed technique, an example is given and comparison between the maximum likelihood estimator (MLE), minimum variance unbiased estimator (MVUE), minimum mean square error estimator (MMSEE), median unbiased estimator (MUE), and the best invariant estimator (BIE) is discussed.
A learning model for the dual evolution of human social behaviors
Authors: Nemiche M. , Pla‐Lopez Rafael
Published: April 2003

In this work we modelize, with an abstract mathematical model by computer simulation, the processes that have made to appear in the world a strong duality between orient and occident, by combining changes in conditions of initialization, natural system and the opposition gregarious/individualism of the social behaviors. Finally we present a statistical study of the influence of the repression adaptability, resignation and recycling on the ecological destruction and social evolution. This model can help us to analyze if the current capitalist globalization can be stopped, changed or regulated, and if it is possible to overcome it towards a Free Scientific Society.
Axiomatic combinatorial world theory with emergent intelligence: simplifying understanding and professionalizing general education
Authors: Rudin Donald O.
Published: April 2003

A theory of knowledge shows that all four systems of nature are recursive combinatorial‐hamiltonian self‐programmed flow‐wave systems that can be deduced from the usual Conservation Law promoted to the Axiom of Science.
Data self‐create in data storage system
Authors: ke Zhou , Jiangling Zhang , Dan Feng
Published: April 2003

When the controller of a storage system becomes more and more powerful, it sometimes creates new data and stores this data in the system, just like parity information in RAID level 5. We call these phenomena data self‐create. This paper provides a theory about data self‐create which separates data self‐create phenomena into 16 kinds. Three applications are introduced. From a pansystems view, this paper also gives an explanation of data self‐create.
Extreme Response of Floating Structures in Combined Wind and Waves
Authors: Teigen P. , Naess A.
Published: April 2003

The paper highlights the problem of evaluating the extreme surge response of a floating, deep water structure subjected to stochastic loading from concurrent wind and waves. Additional load effects associated with ocean currents are also briefly discussed. Both long-crested and short-crested waves are considered, whereas the wind field is assumed to be unidirectional. The probability density function (PDF) of the combined wave frequency and low frequency response of the structure, due to waves, is calculated by an eigenvalue analysis and convoluted with the corresponding PDF from the wind loads, to obtain the PDF of the global response. The necessity of employing full, biquadratic transfer functions to evaluate the low frequency part of the wave loads is amply documented. The effect of short-crested versus unidirectional seas on the TLP motion response is discussed at some length, along with various numerical aspects related to the mathematical modelling and to the convergence and accuracy of the obtained solutions. Numerical solutions are presented for a wide range of harsh weather type, environmental parameters.
Unified Approach to Risk-Based Inspection Planning for Offshore Production Facilities
Authors: Faber Michael Havbro , Straub Daniel , Goyet Jean
Published: April 2003

Based on new methodological developments in the area of risk-based inspection planning (RBI) for structural components subject to fatigue degradation, the present paper presents a unified approach to risk-based inspection planning for offshore facilities comprising components and systems of both the structural and process type. Suggestions are given for the formulation of acceptance criteria on component level based on overall facility acceptance criteria in terms of risk to personnel, environmental risks and economical risks. The methodology facilitates a generic modeling of fatigue and corrosion degradation for both structural and process type components. Furthermore, special considerations are given to the important aspect of updating of inspection plans when degradation has been observed. The different aspects of RBI for steel components subject to fatigue and corrosion degradation are illustrated on an example considering RBI for the components of a tripod well-head platform.
On Multistage Deep Drawing of Axisymmetric Components
Authors: Sonis Prakash , Reddy N. Venkata , Lal G. K.
Published: April 2003

A process analysis model to determine the limiting drawing ratio for the first draw as well as for redraws is presented considering the effects of normal anisotropy, co-efficient of friction, strain hardening and die arc radius. Predictions of the model presented are in good agreement with the available experimental results. The model can be used by the process planners to determine the minimum number of passes required to achieve the final component geometry. Using the present model, feasible operation sequences which have the minimum number of stages through which the desired cup specification can be achieved is presented for two different cup geometries.
Intelligent Search-Based Selection of Sample Points for Straightness and Flatness Estimation
Authors: Affan Badar M. , Raman Shivakumar , Pulat Pakize S.
Published: April 2003

Form error estimation with a CMM requires prudent sampling and accurate zone fitting. This paper proposes use of optimization search methods for reducing sample size, while maintaining high accuracy. The approach is demonstrated with examples of straightness and flatness. For straightness, region-elimination search is used. For flatness, two pattern search methods: Tabu search and a hybrid search, are employed and their performance is compared. Sampling begins with a necessary number of initial points and a zone is determined. Next points are sampled based on the search methods, with suitable application of intensification/diversification strategies, looking for improvement in the zone fit. Search is conducted in both the +ve and −ve directions from the fit feature and is stopped when a solution for the maximum deviation is realized. The two solution points are added to the initial set and the corresponding tolerance is computed. The tolerance is compared with that obtained for the population of a large sample, to verify the accuracy. It is found that the number of points sampled is potentially less than that typically used to achieve the same accuracy.
Machining Simulation of Ductile Iron and Its Constituents, Part 2: Numerical Simulation and Experimental Validation of Machining
Authors: Chuzhoy L. , DeVor R. E. , Kapoor S. G.
Published: April 2003

To understand the influence of cast iron microstructure on its machinability, a numerical model that simulates machined material on a microstructure scale was developed. This microstructure-level model assembles individual constituents into a composite material based on microstructural composition, grain size, and distribution. Extensive experimentation was performed to determine strain, strain rate, temperature, and load history dependent material properties. The purpose of this work is to validate the microstructure-level model on machining of ductile iron and two of its constituents: pearlite and ferrite. Orthogonal cutting experiments were conducted of the three materials. The measured chip morphology and machining forces were compared with the model predictions, and a good correlation between them was found.