bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum transport anomalies in semiconductor nanosystems
Authors: Joe Yong S. , Khatun M. , Cosby Ronald M.
Published: July 2002

We present quantum transport anomalies in the theoretical conductance of various semiconductor nanostructures. We first investigate a quantum channel with a chain of quantum boxes connected by slits, called a superlattice structure, and study the miniband and minigap effects associated with resonances and anti-resonances in the conductance. We also report studies of electron transport in a quantum wire containing series or parallel slits and a detector slit. In these systems, strong conductance oscillations due to quantum interference effects are predicted as a detector slit is moved across the wire. In the case of a single and multi-series slits, we attribute these effects to multiple reflections of the phase-coherent electron along the quantum wire. The transmission coefficients and electronic phase shifts are examined, which provide insights into the origins of these conductance oscillations. In the case of multi-parallel slits, peaks with two- (four-) fold splitting in the conductance are exhibited due to the quantum branch interference between the two (four) alternative electron paths. We also study the conductance of a quantum structure containing an artificially produced impurity. It is shown that the conductance modulations are strong when the impurity is scanned across the channel. We explain these oscillatory features of the conductance by a simple optical interference model and a simple adiabatic model.
Photoreflectance and photoluminescence spectroscopy of low-temperature GaAs grown by molecular-beam epitaxy
Authors: Sinha S. , Arora B. M. , Subramanian S.
Published: July 2002

Photoreflectance (PR) and photoluminescence (PL) measurements have been carried out on low-temperature GaAs (LT GaAs) and LT-GaAs/AlGaAs modulation-doped photodetector (MODPD) structures grown by molecular-beam epitaxy. Samples with LT GaAs grown at 350 °C show several PL lines associated with (i) transitions involving two-dimensional electron gas in LT GaAs near the interface, (ii) band-edge transitions in the bulk LT GaAs, and (iii) transitions involving deep level defect complexes in LT GaAs. In addition a PL emission band at 1.65 eV observed in all the MODPD structures is attributed to a crossover transition at the LT-GaAs/AlGaAs interface. PR spectra of these modulation-doped structures show Franz–Keldysh oscillations which are attributed to high electric fields (≳105 V/cm) at the LT-GaAs/AlGaAs interface. PR measurements on bare LT-GaAs layers suggest that the E0 transition in LT GaAs is about 20 meV above that of the normal GaAs E0 gap.
Lifetime in proton irradiated silicon
Authors: Hallén Anders , Keskitalo Niclas , Masszi Ferenc , Nágl Viliam
Published: July 2002

Deep energy levels caused by high-energy low-dose proton irradiation of both n- and p-type silicon have been investigated. Energy positions in the band gap, capture coefficients, and their temperature dependences for majority and minority carrier capture and entropy factors have been measured by deep level transient spectroscopy. Computer simulations have been employed to obtain the correct numbers of injected charge carriers needed for the evaluation of minority carrier capture data. From these measurements, it is possible to deduce the charge carrier lifetime profiles in proton irradiated n-type silicon for different injection concentrations and temperatures. At room temperature and for low injection, it is found that the singly negative divacancy level with a band-gap enthalpy of HC−HT=0.421 eV has the largest influence on the lifetime. At high injection, the vacancy–oxygen center, HC−HT=0.164 eV, is mostly responsible for the lifetime reduction.
Magneto-optical properties of Sc-substituted holmium iron garnet single crystals
Authors: Ostoréro J. , Guillot M.
Published: July 2002

The Faraday rotation (FR) of scandium substituted holmium iron garnets single crystals Ho3Fe5-zSczO12 (z=0.35, 0.52, 0.56, and 0.73) has been investigated at 1152 nm wavelength in the 6 K<T<TNéel temperature range and magnetic field up to 20 kOe applied along the [111] easy axis. At this wavelength, in a narrow concentration range (0.5<z<0.6), in addition to the classic magnetic compensation temperature Tcomp, the spontaneous FR Φs(T, z=Cte) curves present two magneto-optic (MO) compensation points TMOComp(1) and TMOComp(2) where the FR vanishes although the ferrite magnetization is different from 0. When z≳0.6, only TMOComp(1) remains. These results, due to the influence of the diamagnetic dilution of the octahedral [a] Fe3+ sublattice, are interpreted using the one ion FR model of Crossley et al. and the Dionne-refined Néel model of garnets. They give an indirect experimental confirmation of the temperature dependence of the MO electric dipole coefficient Ce(λ=1152 nm, T), and of the preponderance of the (d)−{c} magnetic interactions compared to the [a]−{c} ones.
Random free energy model for the description of hysteresis
Authors: Bertotti G. , Basso V. , Durin G.
Published: July 2002

The connection between hysteresis phenomena and free energy metastable states in magnetic systems is discussed. A random free energy model is introduced, which leads to a stochastic differential equation for the evolution of magnetization in time. We show that the solutions of this equation are equivalent to the Preisach model of hysteresis. The analytical form of the Preisach distribution is calculated.
Unified bulk mobility model for low- and high-field transport in silicon
Authors: Schenk Andreas
Published: July 2002

An analytical bulk mobility model for hydrodynamic transport equations is developed from a microscopic level and designed for silicon device simulation. Applying Kohler’s variational method extended to the regime of nonlinear transport yields the general expression for the mobility as function of carrier temperature, lattice temperature, and doping. Assuming a nonparabolic, isotropic band model and a heated Maxwellian allows for the analytical calculation of the collision integrals. A nonelastic approximation for intravalley acoustic-phonon scattering is proposed, which improves the model in the low-temperature range. Intervalley scattering is treated in a one-mode, equipartition approximation. Here, an accurate analytical approach for all carrier temperatures is derived. For impurity scattering the Brooks-Herring theory is used including Fermi statistics and the effect of dispersive screening. The influence of other effects like anisotropic valleys and perturbation of the density of states by heavy doping are discussed quantitatively. Despite the oversimplified band structure, all essential features of the measured mobility in silicon can be reproduced except in the heavy doping range. The adjusted deformation potentials coincide with estimated sums of the corresponding sets used in full-band Monte Carlo simulation. The method has the potential of an extension to the Si−SiO2 system.
Effect of target size on dose uniformity in plasma-based ion implantation
Authors: Sheridan T. E.
Published: July 2002

Plasma-based ion implantation of a square bar is modeled using a particle-in-cell plasma simulation for three different size bars. When the sheath width is significantly greater than the bar width, it is found that the incident ion dose is largest at the center of the bar and decreases precipitously at the corners. When the sheath width is comparable to the bar width, the incident dose is largest near to, but not at, the corners. It may be possible to optimize dose uniformity by straddling these two regimes.
Interface states in In0.5Ga0.5P/AlxGa1−xAs heterostructures grown by liquid phase epitaxy
Authors: Cho Yong-Hoon , Choe Byung-Doo , Kim Y. , Lim H.
Published: July 2002

We report the observation of deep traps localized near the heterointerface of the InGaP/AlGaAs structure grown by liquid-phase epitaxy. In the case of low-quality InGaP/AlGaAs heterojunctions containing interface traps, the shape of capacitance-voltage (C-V) carrier profiles is affected by these deep states. To elucidate the characteristics of the interface traps, the deep-level transient spectroscopy (DLTS) measurements are performed. It is found from the DLTS measurements that the electron traps distributed over the energy range of 0.4–0.8 eV below the conduction-band minimum are presented at the InGaP/AlGaAs heterointerface. These interface traps are found to induce an abnormal dependence of C-V profiles and the conduction-band offset on the temperature. The origin of these states and their influence to the electrical and optical properties are also discussed.
Substrate bias effects on low temperature polycrystalline silicon formation using electron cyclotron resonance SiH4/H2 plasma
Authors: Nozawa Ryoichi , Takeda Hirohisa , Ito Masafumi , Hori Masaru , Goto Toshio
Published: July 2002

Polycrystalline silicon films were deposited at a substrate temperature of 300 °C by electron cyclotron resonance SiH4/H2 plasma-enhanced chemical vapor deposition. The effects of substrate dc bias during deposition on the crystallinity and the surface roughness of the deposited films were investigated in the range from −150 to +50 V by using x-ray diffraction, scanning electron microscopy, and atomic force microscopy. It was found that the positive biases applied to the substrate improved the crystallinity and the surface roughness. To clarify the substrate dc bias effects on the crystallinity and the surface roughness, the sheath potential, ion current, and temperature on the substrate surface were measured. It is determined that improvement of the crystallinity and the surface roughness is due to the decrease of ion flux to the substrate when positive bias is applied to the substrate.
New type of active device based on galvanomagnetic effect
Authors: Kakuno K.
Published: July 2002

A new type of active device composed of metals is fabricated based on the galvanomagnetic electromotive force effect, which is at the origin of the magnetoresistance effect and of the so-called planar Hall effect. A power gain (the ratio of the signal output to a signal input) of 2.5 is experimentally obtained for this active device. This experimental result shows the feasibility of active devices based on quite a different principle from existing ones.
Defect states in strain-relaxed Si0.7Ge0.3 layers grown at low temperature
Authors: Mooney P. M. , Tilly L. , D’Emic C. P. , Chu J. O. , Cardone F. , LeGoues F. K. , Meyerson B. S.
Published: July 2002

Two shallow hole traps dominate the deep level transient spectroscopy (DLTS) data for strain-relaxed Si0.7Ge0.3 layers grown on Si(100) by ultrahigh vacuum chemical vapor deposition at temperatures ⩽560 °C. The trap energy levels are at Ev+0.06 and Ev+0.14 eV and trap concentrations are ⩽5×1014 cm−3 in relaxed layers having threading dislocation densities of 2–4×107 cm−2. A logarithmic dependence of the filling rate indicates that these traps are associated with extended defects and this is confirmed by their absence in a sample having no dislocations. The annealing temperature of the DLTS peaks is consistent with the interpretation of these traps as states of defect complexes at dislocations, rather than intrinsic dislocation states or isolated defect complexes. The trap concentrations are proportional to the oxygen concentration in the film, suggesting that oxygen may be a constituent of the defect complex.
Growth kinetics of furnace silicon oxynitridation in nitrous oxide ambients
Authors: Singhvi Shri , Takoudis Christos G.
Published: July 2002

The chemical kinetics and process–property relationships of silicon oxynitride films grown in nitrous oxide ambients were investigated. Gas phase compositions in the furnace were experimentally determined with mass spectrometry and were found to be within ±5% of the ones calculated theoretically. Experimental observations in the furnace suggested that the reaction between NO and oxygen to form NO2 inside the furnace was negligible. Silicon oxynitride films were grown at 950 °C and atmospheric pressure for times ranging between 30 min and 24 h. Ellipsometry was used to measure the film thickness. The observed thin-film growth was parabolic with time and appeared to saturate after about 24 h, that is, significantly slower than the oxynitride growth by rapid thermal processing. Secondary ion mass spectrometry (SIMS) used in the depth profiling of oxynitrides showed a sharp accumulation of nitrogen at the oxynitride–silicon interface. Additional experimental data suggested that nitric oxide is the species responsible for nitrogen incorporation at the interface and removal of nitrogen from the bulk film. Mass spectrometric, ellipsometric, and SIMS analyses of furnace silicon oxynitridation in nitrous oxide ambients were used for the development of a model of the process. Model predictions are in agreement with all trends of the experimental data obtained.
Characterization of refractive index change induced by electron irradiation in amorphous thin As2S3 films
Authors: Nordman Nina , Nordman Olli
Published: July 2002

Binary diffraction gratings in As2S3 films were prepared with the aid of an electron beam. The dose of the electrons was varied. The gratings were read with a HeNe laser, and the zero-order and first-order diffraction efficiencies were noted. Rigorous diffraction theory was used to give a homogeneous approximation for the refractive index change. Reactive ion etching was applied to reduce the thickness of the film, and a new value for the refractive index change was evaluated. The refractive index change versus film thickness dependence was found to be linear at low electron doses and Gaussian shaped at higher doses. Through a simple mathematical analysis, the absolute value of the refractive index was determined as a function of the position inside the film. At higher doses, the refractive index change was found to have a maximum value of 3%, approximately 1.2 μm from the film surface.
Spallation of metal targets subjected to intense laser shocks
Authors: de Rességuier T. , Couturier S. , David J. , Niérat G.
Published: July 2002

Piezoelectric polymer gauge measurements were performed on the rear surface of plane copper and aluminum foils irradiated by infrared laser pulses of 1.5 ns duration, with intensities up to about 7 T W/ cm2, using recently developed transducers made of a vinylidene-fluoride and trifluoroethylene copolymer. Some gauge records have indicated that incipient spall damage occurs in the samples subjected to very intense shocks. This was confirmed by post-test examination of the targets. The experiments were simulated using two computer codes: one describes laser–matter interaction and provides input to the second code that handles wave propagation. A spall model for ductile metals is integrated in the wave code. The ability of those computer models to predict the experimental results was thereby tested. Overall agreement was obtained between the gauge measurements and calculations, except for some discrepancies that are discussed.
Neutral product analysis of the microwave C2H2 plasma: Cn, CnH2, CnH3, CnH4, CnH5, and larger species
Authors: Fujii Toshihiro
Published: July 2002

Product analysis of microwave (MW) C2H2 discharge by Li+ ion attachment mass spectrometry demonstrates the unexpected formation of many unfamiliar hydrocarbon neutral products. The experiments were conducted under MW discharge conditions in which the peak intensities for the ionic species leaving the MW C2H2 plasma were barely detectable which permits confirmation of these neutral compound in the gas phase. Various free radicals as well as stable polymer molecules were clearly present. The latter are classified as Cn (n=4, 6, and 8), CnH2 (n=2, 3, 4, 6, and 8), CnH4 (n=2, 3, 4, 6, and 8), and so on, where n is the carbon number. Radicals such as CnH3 (n=2 and 4), CnH5 (n=2, 4, and 6), CnH7 (n=3 and 4), and CnH9 (n=4) were detected. Some of these species, which have been observed in the interstellar space, have been identified mass spectrometrically for the first time. We believe that integrating a Li+ ion attachment reactor with quadrupole mass spectrometry can create a powerful instrument for installation on spacecraft.
Spectroscopic ellipsometry and magneto-optic Kerr effects in Co/Pt multilayers
Authors: Gao Xiang , Glenn Darin W. , Heckens Scott , Thompson Daniel W. , Woollam John A.
Published: July 2002

Control of thickness and microstructure is critical in the preparation of ultrathin metallic multilayers for magneto-optic memory, computer read heads, and improved wear surfaces. This paper describes use of in situ spectroscopic ellipsometry to calibrate sputter deposition rates, control layer thicknesses, determine optical constants, and study oxidation rates and processes. In this work, the information gained in situ is used together with ex situ Kerr measurements to determine the complete magneto-optic response including the spectroscopic figure of merit for Co/Pt multilayers.
A remark on fast decay effect in the electromagnetic missile theory
Authors: Wang Gang , Liang Chang Hong
Published: July 2002

The fast decay effect of energy in electromagnetic (EM) radiation as deduced from the EM missile theory is demonstrated to be unrealistic. The inferences drawn from the EM missile results involving such a fast decay effect must be modified by taking into account the contribution from the band-limited spectrum, which will yield reasonable results.
Real-time detection of laser-induced transient gratings and surface acoustic wave pulses with a Michelson interferometer
Authors: Shen Yao-chun , Hess Peter
Published: July 2002

Laser-induced transient gratings (LITGs) at surfaces of absorbing materials were utilized to generate narrowband surface acoustic waves (SAWs). In these experiments, SAWs were excited thermoelastically by two crossed picosecond laser pulses and detected with an actively stabilized Michelson interferometer by measuring transient surface displacements in the sub-angstrom range in real time. In addition, coherent broadband SAW pulses with frequencies up to 350 MHz were excited by sharply focusing the laser beam with a cylindrical lens system onto the sample surface. The LITG experiments provide an extension of the frequency range achieved with the broadband SAW pulse technique. From the measurements of the dispersive SAW phase velocity for a 650 nm aluminum film on fused silica in the frequency range 10 MHz–1 GHz the density and elastic constants were determined by fitting the experimental data to the exact solution of the wave equations taking into account the boundary conditions.
Magnetic anisotropy and spin diffusion through spin disordered interfaces in magnetoresistive manganites
Authors: Martı́nez B. , Balcells Ll. , Fontcuberta J. , Obradors X. , Cohenca C. H. , Jardim R. F.
Published: July 2002

The magnetoresistance and the magnetization of ceramic La2/3A1/3MnO3 (A=Sr, Ca) oxides have been studied as a function of the grain size. It is found that these ceramics become magnetically harder when reducing the particle size exhibiting a large magnetic anisotropy that increases when reducing the grain size. In concomitance with this enhancement of the magnetic hardness, a gradual increase of the low-field magnetoresistance is also detected, signaling the relevance of the surface and interfaces contribution to the magnetic and transport properties. We suggest that both phenomena could be closely related and associated to the existence of some degree of spin disorder at the grain boundaries. We propose that these features are fingerprints of quenched spin disorder at interfaces that can be tuned through bandwidth modulation of the magnetic frustration.
Influence of microstructure on thermal stability of spin-valve multilayers
Authors: Maesaka Akihiro , Sugawara Nobuhiro , Okabe Akihiko , Itabashi Masao
Published: July 2002

We investigated the influence of microstructure on the thermal degradation of giant magnetoresistance ΔR on spin-valve multilayers constructed by IrMn/CoFe/Cu/CoFe/NiFe, using transmission electron microscopy and energy dispersive x-ray spectroscopy. We found that the main cause of thermal degradation at about 250 °C is the interlayer diffusion along the grain boundary, which had more influence on ΔR than the compositional mixing at the multilayer interface. The polycrystal spin-valve multilayers deposited on Ta and CoZrTa underlayers had rapid degradation of ΔR above the annealing temperature of 225 °C, resulting in 16% and 20% degradation at 275 °C, respectively. A Ta underlayer with amorphous structure has the effect of reducing the density of the grain boundary by improving the surface smoothness, as compared with a crystalline CoZrTa underlayer, leading to slightly less degradation. By contrast, a spin-valve epitaxial layer deposited on a MgO(111) substrate brought about a marked reduction of the thermal degradation of ΔR to within 3%, even at 275 °C, due to a remarkable reduction of crystallographic defects like the grain boundary.
Transmission electron microscopy analysis of the oriented diamond growth on nickel substrates
Authors: Liu W. , Yang P. C. , Wolden C. A. , Davis R. F. , Prater J. T. , Sitar Z.
Published: July 2002

Transmission electron microscopy (TEM) was used to investigate the interfacial microstructure and the phases that developed during the nucleation and growth of oriented diamond on Ni by a hot filament process. Oriented Ni4C nuclei were identified by plan-view TEM in a sample quenched during the nucleation stage. Likewise, the presence of the Ni4C phase between the diamond and the Ni substrate was observed by cross-section TEM in samples grown for several hours. The orientational relationship among the diamond, Ni4C, and Ni substrate was examined by selected area diffraction. Diamond and Ni4C interfacial phase had a good epitaxial relationship, while the interfacial Ni4C phase and the Ni substrate developed with a small misfit and rotation. Based on these experimental results, the nucleation mechanism of oriented diamond growth on Ni is proposed.
Pulsed Fowler–Nordheim current stress resistance of Si oxynitride grown with helicon-wave excited nitrogen–argon plasma
Authors: Okamoto Yoshinaga , Kimura Shinjiro , Oka Fumihito , Tsuchiya Shigehiro , Ikoma Hideaki
Published: July 2002

Pulsed Fowler–Nordheim (FN) current stress resistance was investigated for the Si oxynitride grown in the helicon-wave excited N2–Ar plasma. The shift of the gate threshold voltage Vth increased with an increase in the pulse frequency for both polarities of the applied stress voltage. At low frequencies (<1 kHz), the Vth shift was larger for the negative gate-voltage stress than for the positive one. However, as the frequency exceeds about 1 kHz, the Vth shift become much higher for the positive stress than for the negative one. The Vth shift was smaller as the pulse duty ratio was larger. These findings could be explained with the surface–plasmon and avalanche breakdown models combined with the effect of the total amount of the injected carriers to the oxynitride from the Si substrate or the gate electrode. The effect of Ar ion etching during plasma processing on the FN stress resistance was also investigated. The Ar ion etching effect was found to be substantially reduced as the plasma-sheath width was large and Si oxynitride samples were grown under this condition. The mean time to failure was highly improved by the Si oxynitride samples grown under the condition of reduced Ar ion etching effect.
Effects of viscoelasticity on rubber vibration isolator design
Authors: Lin Chen-Ron , Lee Yu-Der
Published: July 2002

The influence of viscoelasticity of rubber materials on the performance of rubber isolators is presented. Dynamic properties of both neat rubber and filled rubber with filler content φ<φc are linear viscoelastic while those of filled rubbers with filler content φ⩾φc are strongly strain dependent and nonlinear viscoelastic. The Kramers & Kronig relationship and its approximation is used to simulate the performance of a linear viscoelastic rubber isolator. Two specific conditions for viscoelastic rubber systems, (a) systems with constant loss tangent and (b) systems in the region of ω0.5 frequency dependence, are studied. The limitation on the transmissibility of a linear viscoelastic rubber isolator is also illustrated through the Kramers & Kronig relationship. The performance of a vibration isolator made from filled rubbers with filler content φ⩾φc, however, can be simulated by the L-N-B model. When the strain amplitude at natural frequency is greater than the apparent strain amplitude εapp, the stability problems such as Jump effect are likely to happen.
Hybrid superconductor/semiconductor step junctions with three terminals
Authors: Lachenmann S. G. , Kastalsky A. , Förster A. , Uhlisch D. , Neurohr K. , Schäpers Th.
Published: July 2002

We report on experimental results with three-terminal superconductor/semiconductor hybrid junctions, which are based on the two-dimensional electron gas at the surface of p-type InAs. A short distance (≈150 nm) between superconducting Nb contacts is obtained using a step geometry. The step geometry allows the realization of different heterostructure potential profiles along the two-dimensional channel. The critical current of the step junctions can be controlled by applying a voltage to highly doped (δ-doped) layers embedded in the heterostructure. With p-δ-doped layers, a p-n junction is introduced in the two-dimensional channel and an asymmetric change of the critical current with respect to the gate voltage or gate current is observed. With n-δ-doped layers, the change is symmetrical.
Structure and magnetic properties of a Fe–Zr–N thin film
Authors: Baek Jong-Sung , Yu Seong-Cho , Lim Woo-Young , Kim Chul-Sung , Kim Taek-Soo , Kim Chong-Oh
Published: July 2002

Fe–Zr–N thin films were deposited in an argon and nitrogen mixed atmosphere using a rf magnetron reactive sputtering apparatus at room temperature. The x-ray diffraction pattern of the Fe76Zr8N16 thin film obtained at room temperature indicates that the as-sputtered film consists of amorphous and crystalline phases. The mixed structure of the sample was also confirmed by the temperature dependence of the saturation magnetic flux density. The as-sputtered film exhibited soft magnetic properties with a saturation magnetic flux density of 1.1 T and effective permeability of 1400 or more at 1 MHz. The film also showed good thermal stability, sustaining the effective permeability of 3000 at temperatures up to 550 °C. The film annealed in a magnetic field of 500 Oe at 450 °C showed a permeability of 2200 or more up to 40 MHz. The conventional Mössbauer spectrum of the kapton substrate sample obtained at room temperature could be fitted by a mixture of three kinds of six-line hyperfine splittings.
Magnetically textured γ-Fe2O3 nanoparticles in a silica gel matrix: Structural and magnetic properties
Authors: Bentivegna F. , Ferré J. , Nývlt M. , Jamet J. P. , Imhoff D. , Canva M. , Brun A. , Veillet P. , Višňovský Š. , Chaput F. , Boilot J. P.
Published: July 2002

This paper is devoted to magnetic and structural properties of anisotropic γ-Fe2O3 superparamagnetic particles dispersed in a transparent xerogel matrix. The effect of frozen anisotropy axes and magnetic texture, induced by a magnetic field applied during the solidification of the matrix on the in-field magnetization process, is studied by alternating gradient force magnetometry and first and second order magneto-optical effects. The changes of magnetization curves with respect to the ferrofluid solution at the same particle concentration are interpreted on the basis of an existing statistical approach extended to systems with particle size distribution, which has to be taken into account for real samples. A very good agreement between the experiment and theory was achieved for a log-normal distribution of diameters which well resembles that deduced from electron microscopy observations in different imaging modes. This structural analysis states the parameter values used in calculations and confirms the relevance of basic assumptions of the model for the specimens studied. The experimental results and the related theoretical discussion should be of use to understand magnetic properties of other magnetically textured superparamagnetic systems.
Temperature-domain analysis of primary and secondary dielectric relaxation phenomena in a nonlinear optical side-chain polymer
Authors: Cheng Zhong-Yang , Yilmaz Şükrü , Wirges Werner , Bauer-Gogonea Simona , Bauer Siegfried
Published: July 2002

The investigation of dipole relaxation processes in nonlinear optical (NLO) polymers containing chromophore dipoles with large hyperpolarizabilities is important for optimizing the poling process and for predicting the long-term stability of the poling-induced order. The primary or α relaxation is difficult to assess by dielectric spectroscopy in polymers with high glass transition temperature due to thermally induced chromophore degradation. A fast experimental procedure is developed for the investigation of dielectric relaxation processes in NLO polymers, without severely inducing chromophore degradation. The procedure is based on the measurement of the dielectric function ε̃(T)=ε′(T)−iε″(T) at a few frequencies from 30 Hz to 30 kHz, while heating the polymer at a constant rate. The complex plane representation of the temperature-dependent dielectric function is used to determine the distribution of relaxation times, while the temperature-dependent mean relaxation time τα(t) is numerically determined from the dielectric loss ε″(T). With only three decades in frequency, information on five decades in time or 90 K in temperature is gained above the glass transition temperature. The strong α and the weak β relaxation below the glass transition are separately investigated by thermally stimulated depolarization after a suitable two-step poling procedure. The method has been applied to a typical polyimidelike side-chain nonlinear optical polymer with modified Disperse Red 1 chromophores. Even below room temperature, a γ-relaxation process is observed, demonstrating significant mobility of the chromophores much below the glass transition. From the results of thermally stimulated depolarization it is concluded that the initial fast decay of the electro-optical response to a temporally stable value is related to the partial depolarization caused by the β relaxation.
Luminescence study of thermal treated and laser irradiated Bi12GeO20 and Bi12SiO20 crystals
Authors: Cremades A. , Piqueras J. , Remón A. , Garcı́a J. A. , Santos M. T. , Diéguez E.
Published: July 2002

Changes on the defect structure of Bi12GeO20 (BGO) and Bi12SiO20 (BSO) crystals induced by thermal treatments and laser irradiation have been studied by means of cathodoluminescence in the scanning electron microscope. The results have been compared to those previously reported for untreated and electron irradiated samples and recombination mechanisms responsible for some of the observed luminescence bands are discussed. Annealing of BGO samples causes the appearance of a new luminescence band at about 390 nm. The centers responsible for this band decorate the deformation slip bands in quenched BGO as observed in the cathodoluminescence images. The emission observed in BSO in the same spectral range is quenched during the annealing treatment. The annealing induced reduction of Bi ions to metallic Bi appears to be related to the quenching of a band at 640 nm observed in untreated samples.
Optical properties of α-ZnAl2S4:Cr single crystals
Authors: Broussell I. , Fortin E. , Kulyuk L. , Popov S. , Anedda A. , Corpino R.
Published: July 2002

The optical properties of the chromium doped spinel type semiconductor α-ZnAl2S4 have been examined over the temperature range 2–540 K. The intrinsic photoluminescence (PL) showed an intense ultraviolet peak at 381 nm at 296 K. From extrinsic absorption and emission spectra the transitions between the ground state A2g4 and the excited levels Eg2, T1g2, T2g2, T2g4, and T1g4 of Cr3+ ions were observed. A fit of the measured temperature dependence of the PL decay, using a four level model, yielded the lifetimes of the Eg2, T1g2, and T2g4 levels and the energy gaps between the Eg2–T1g2 and Eg2–T2g4 states which were found to be very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr3+ ions in a α-ZnAl2S4:Cr spinel host has been constructed. Optical gain measurements at 198, 298, and 380 K displayed two separate spectral regions associated to the Cr3+ ions and possibly Cr4+ impurity centers. Gain values of up to 25 cm−1 at 198 K were obtained.
Spectral decomposition of excitons confined in a quantum dot by a parabolic potential and under the influence of an external field
Authors: Qiao Bi , Ruda Harry E.
Published: July 2002

A general formulation of subdynamics is presented for constructing the spectral decomposition of the Hamiltonian of N excitons confined within a quantum dot and influenced by an external electromagnetic field. The formulation offers a simple means for calculating the spectrum of the N-exciton Hamiltonian using recurrence relations for the creation and destruction operators. The spectral decomposition of the Hamiltonian may therefore be constructed using the intertwining relation. This formulation also can be extended to construct the spectral decomposition of the N-exciton Hamiltonian in more complex or nonintegrable quantum box systems, even when such systems are subject to a strong external field. As a demonstration of the application of this formulation, we consider the calculation of the eigenvalues and eigenvectors of Hamiltonian of an exciton in the system.
Photon emission excited in paraffin-passivated GaAs surfaces by scanning tunneling microscopy
Authors: Carladous A. , Coratger R. , Seine G. , Ajustron F. , Beauvillain J.
Published: July 2002

Photon emission from GaAs surfaces induced by scanning tunneling microscopy has been detected in air. Surfaces have been protected from oxidation by a thin film of paraffin oil. Tunneling topography, related photon mapping, and the corresponding emission spectrum have been simultaneously acquired. Also, the influence of applied bias voltage on the emission yield has been studied. The results allow this photon emission to be ascribed to radiative recombinations in the semiconductor bulk. An irreversible decay of emission yield at high negative sample voltages (−3 V) is also observed.
Electrical resistivity of copper-silica nanocomposites synthesized by electrodeposition
Authors: Banerjee S. , Chakravorty D.
Published: July 2002

Composites containing copper particles with nanometer dimensions in a silica gel medium have been synthesized by an electrodeposition technique. The precursor composition of the gel was in the system Cu(NO3)2–SiO2 and the copper particle diameters were in the range of 3.2–11.4 nm. The dc electrical resistivity of pellets obtained from the nanocomposite powders was measured in the temperature range of 110–300 K. A temperature dependence with a fractional exponent of 0.25 was observed. This behavior has been explained on the basis of a variable range hopping mechanism.
Interface quality and electron transfer at the GaInP on GaAs heterojunction
Authors: Schuler O. , Dehaese O. , Wallart X. , Mollot F.
Published: July 2002

Hall measurements performed on Ga0.50In0.50P/In0.20Ga0.80As structures show abnormally low mobility both at room temperature and at 77 K, and too high electron densities which cannot be attributed to a normal two-dimensional electron gas in the channel. On the other hand, low temperature photoluminescence on asymmetrical AlGaAs/GaAs/GaInP quantum wells and x-ray photoemission spectroscopy measurements reveal the presence of arsenic atoms in the GaInP barrier. Using a one-dimensional Schrödinger–Poisson simulation with a nonabrupt interface model, we show that the presence of arsenic in GaInP leads to the formation of a parasitic GaInAsP well between the δ-doped layer and the channel, trapping the main part of transferred electrons. We experimentally show that the electron transfer can be drastically improved by inserting a thin AlInP layer at the interface. Insertion of at least six monolayers of AlInP is needed to recover a normal electron transfer as high as 2.1×1012 cm−2.
Transport properties of HCl doped polyaniline and polyaniline–methyl cellulose dispersion
Authors: Ghosh M. , Barman A. , De S. K. , Chatterjee S.
Published: July 2002

Electrical resistance and magnetoresistance of the HCl doped polyaniline (PANI) in aqueous ethanol medium and polyaniline–methyl cellulose dispersion (PANI–MC) has been investigated at low temperature down to 1.8 K and in magnetic field up to 8 T. The weaker temperature dependence of resistivity characterized by the resistivity ratio ρr=ρ(1.8 K)/ρ(300 K) indicates that a better homogeneity and less disorder can be obtained by protonation with HCl in ethanol media. The temperature dependence of resistivity for both PANI and PANI–MC has been analyzed by three-dimensional variable range hopping (VRH) type of conduction. The samples with the resistivity ratio lying in the range of 102⩽ρr⩽103 exhibit a crossover from Mott to Efros–Shklovskii VRH conduction below 10 K. A large positive magnetoresistance at low temperature has been obtained.
Magnetic flux distribution around Bi2212 superconducting single crystal: The paramagnetic resonance technique
Authors: Drulis H. , Folcik L. , Drulis M.
Published: July 2002

The profile of the magnetic field distribution over the surface of Bi2212 superconducting samples is studied by scanning with an electron paramagnetic resonance (EPR) microprobe. The magnetic field distribution is mapped from the measurements of the spin probe EPR-signal shift. The investigations were carried out for both zero field cooling and field cooling procedures and showed different profile shapes in reversible and irreversible magnetization ranges of the superconducting material. It has been found that this method is particularly suitable to study the field profiles above the sample surface where other techniques are not so successful.
Quantitative interpretation of electron-beam-induced current grain boundary contrast profiles with application to silicon
Authors: Corkish Richard , Puzzer Tom , Sproul A. B. , Luke Keung L.
Published: July 2002

An improved method is described for extracting material parameters from an experimental electron-beam-induced current (EBIC) contrast profile across a vertical grain boundary by directly fitting an analytical expression. This allows the least-squares values of the grain boundary recombination velocity and the diffusion length in each grain to be determined without the need for the reduction of the experimental profile to a few integral parameters, as is required in a previously reported method. Greater accuracy of the extracted values is expected since none of the information contained in the experimental contrast data is discarded and a less extensive spatial range of measured data is required than in the commonly used method. Different models of the carrier generation volume are used in the fitting and the effect of the choice of generation model on extracted values is investigated. In common with other EBIC approaches, this method is insensitive to changes in the diffusion length when the collection efficiency is high and diffusion lengths may not be reliably established in those cases.
Properties of molecular-beam epitaxy-grown GaNAs from optical spectroscopy
Authors: Pozina G. , Ivanov I. , Monemar B. , Thordson J. V. , Andersson T. G.
Published: July 2002

GaN x As 1−x layers with different nitrogen concentrations x grown on (001)GaAs substrates by molecular-beam epitaxy have been studied by photoluminescence, optical absorption, and Raman spectroscopy. The content of nitrogen in the layers was determined by x-ray diffraction and secondary-ion-mass spectrometry. The samples can be classified in three categories with respect to the concentration of N: with doping nitrogen concentration, with average content of N less than 0.3, and with x close to 1. From optical measurements and from analysis of x-ray diffraction spectra, different phases are observed in the GaNxAs1−x layers: GaAs, GaN, and the solid ternary solution GaNxAs1−x. In Raman spectra both GaAs-like and GaN-like optical phonons are observed. We have estimated the fundamental band-gap energy in the GaNxAs1−x alloy with low nitrogen concentration up to x=0.04 from absorption measurements, and in GaNxAs1−x with high nitrogen concentration x>0.96 from photoluminescence spectra. Fitting of the experimental data for low x values gives a constant bowing parameter as big as b=−18 eV. This value predicts the band-gap energy for the high nitrogen concentration in agreement with experimental data. Consequently, GaNxAs1−x is predicted to be semimetallic in the range 0.12<x<0.75.
Accurate thickness/density measurements of organic light-emitting diodes
Authors: Marée C. H. M. , Weller R. A. , Feldman L. C. , Pakbaz K. , Lee H. W. H.
Published: July 2002

We report on the use of Rutherford backscattering spectroscopy for thickness analysis of organic light-emitting diode structures (OLEDs) with subnanometer resolution and a spatial resolution <1 mm. A careful study of ion beam induced effects revealed some organic film degradation, but not so severe as to inhibit meaningful measurements. The method is independent of the substrate and is still applicable if the organic film is capped with a metal cathode. Common OLED materials have been the subject of this study: poly(2-methoxy,5-(2′-ethylhexoxy)-1,4-phenylene-vinylene (MEH-PPV), N′,N′-diphenyl-N, N′-bis(3-methylphenyl)-1,1′ biphenyl-4,4′-diamine (TPD), and tris-(8-hydroxyquinoline) aluminum (Alq3). The densities of thin films of evaporated TPD (ρ=1.22±0.05 g/cm3) and Alq3 (ρ=1.51±0.03 g/cm3) have been established.
Stress-induced leakage current reduction by a low field of opposite polarity to the stress field
Authors: Meinertzhagen A. , Petit C. , Jourdain M. , Mondon F.
Published: July 2002

Stress-induced leakage currents in 7 and 12 nm thick gate oxides of metal-oxide-semiconductor capacitors, created by negative or positive high field stress, were investigated in details. It is known that stress-induced leakage currents have several components. One of these components, which is observed for both stress and measurement polarities, increases drastically when the oxide thickness decreases. We have observed that this component magnitude is reduced when a low field of opposite polarity to the stress field is applied to the oxide after stress. This effect does not seem to be due to electron trapping in the oxide bulk, during the low field application. We propose therefore, that this current decrease is due to a defect relaxation phenomena induced by the low field. This proposition is compatible with any defect creation process which involves a stress-field-induced motion of atoms.
Study of the Kerr effect of CoxAg100−x granular films
Authors: Wang Song-You , Zheng Wei-Min , Qian Dong-Liang , Zhang Rong-Jun , Zheng Yu-Xiang , Zhou Shi-Ming , Yang Yue-Mei , Li Bai-Yun , Chen Liang-Yao
Published: July 2002

A series of CoxAg100−x granular films was annealed at 100, 250, 400, and 500 °C, respectively. Both the dielectric function and polar Kerr effect were measured using a spectroscopic ellipsometer and a magneto-optical Kerr spectrometer in the 1.5–4.5 eV photon energy range. It was found that the Kerr effect was greatly enhanced with increasing annealing temperature, especially for those samples having low Co compositions. After annealing, the strong resonance peaks appear in the 3.7–4.1 eV range. For the typical sample with x=6, the maximum Kerr rotation angle is about 0.26° near 4.0 eV after annealing at 500 °C. Through data analysis, the large negative peak at about 3.85 eV is attributed to the plasma edge of Ag, and the positive peak near 4.0 eV is attributed to the new magneto-optical structure and the properties of the granular film. The larger short-wavelength Kerr effect will be useful for increasing magneto-optical data-storage density.
Characterization and modeling of the ablation plumes formed by intense-pulsed ion beam impact on solid targets
Authors: Davis H. A. , Johnston G. P. , Olson J. C. , Rej D. J. , Waganaar W. J. , Ruiz C. L. , Schmidlapp F. A. , Thompson M. O.
Published: July 2002

An investigation of the properties of the ablation products from intense-pulsed ion beam impact on solid targets is described. Measurements and calculations of the properties of the ablation plume are presented and correlated with incident beam parameters. Experimental techniques include Thomson parabola particle spectroscopy to measure the incident ion beam atomic composition and the energy spectrum of each beam component, thermal imaging to measure the incident-beam energy density, time-resolved photography to measure the plume expansion time history and geometry, and time-resolved energy-density measurements of the plume. The results of a thermal transport model of the beam-target interaction are presented, and a detailed comparison with measurements is made.
Unified model for ordering and staging of intercalated M ions in MxTiS2
Authors: Gu Bing-lin , Song Qing-gong , Ni Jun
Published: July 2002

In this article we propose a unified model for ordering and staging of intercalated M ions in the 1T-type intercalation compounds MxTiS2. The ordered structures and the stage structures of M ions occupying octahedrally coordinated interstitial sites are studied using the concentration-wave theory. Ten three-dimensional completely ordered structures have been derived by minimizing the eigenvalue of ionic interaction Hamiltonian at high-symmetry points in the first Brillouin zone. An explanation is proposed for several experimental results.
Measurement of the conduction band offsets in Si/Si1−x−yGexCy and Si/Si1−yCy heterostructures using metal-oxide-semiconductor capacitors
Authors: Singh D. V. , Rim K. , Mitchell T. O. , Hoyt J. L. , Gibbons J. F.
Published: July 2002

Metal-oxide-semiconductor (MOS) capacitors fabricated on in situ doped n-type Si/Si1−x−yGexCy and Si/Si1−yCy epitaxial layers were used to study the conduction band offsets in these heterojunctions. The heterostructures were grown epitaxially in a rapid thermal chemical vapor deposition reactor. Si/Si1−x−yGexCy samples with a nominal Ge concentration of 20 at. % and carbon fractions up to 1.3 at. % were studied. Carbon fractions up to 1.6 at. % were studied for the Si/Si1−yCy samples. Gate oxides were formed by thermal oxidation of the Si cap at 750 °C. X-ray diffraction measurements confirm that the processing did not affect the strain in the layers. Devices exhibit well-behaved high frequency and quasistatic capacitance–voltage (C–V) characteristics indicating the high electronic quality of the material. Capacitance–voltage measurements performed over a range of temperatures were used to extract the band offsets. Confinement of electrons at the heterointerface is apparent in the C–V curves of the Si/Si1−yCy MOS capacitors. Comparison of the measured C–V data to one-dimensional device simulations yields a conduction band edge lowering of ∼65 meV per at. % C in the Si1−yCy samples. The Si1−x−yGexCy samples, on the other hand show no evidence of electron confinement. Based on a sensitivity analysis of this technique, it is estimated that the conduction band offset in these samples is less than 30 meV. The smaller offsets in Si/Si1−x−yGexCy compared to Si/Si1−yCy can be explained by the competition between strain compensation and the intrinsic chemical effect of carbon in Si1−x−yGexCy.