bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermoreflectance spectroscopy of Te-based II–VI compounds ZnTe and CdTe
Authors: Suzuki Ken-ichi , Adachi Sadao
Published: July 2002

Thermoreflectance (TR) spectra of Te-based II–VI compounds ZnTe and CdTe are presented. Measurements are carried out on the bulk single crystals in the 2.0–5.6 eV photon-energy range at room temperature. The TR spectra obtained reveal distinct structures at energies of the E0, E0+Δ0, E1, E1+Δ1, E0′, and E2 critical points (CPs). Detailed analyses are made using a simplified model of the interband transitions. Results suggest that the E0 and E0+Δ0 structures can be characterized by a three-dimensional M0 CP, the E1 and E1+Δ1 structures by a two-dimensional (2D) M0 CP, and the E0′ and E2 structures by a damped harmonic oscillator (a broadened 2D M1 CP). It is also concluded that the broadening terms cannot be neglected in the analysis of TR spectra.
On the origin of recalescence in amorphous Ge films melted with nanosecond laser pulses
Authors: Vega F. , Afonso C. N. , Szyszko W. , Solis J.
Published: July 2002

Rapid solidification phenomena have been studied in amorphous germanium films on silicon substrates by means of real time reflectivity measurements performed during irradiation with nanosecond laser pulses. The influence of the thermal response of the film/substrate system has been investigated by comparing the behavior of films with thicknesses in the range of 30–180 nm. Two different solidification scenarios are observed depending on the ratio between film thickness (d) and the thermal diffusion length (l) of amorphous germanium (l≈80 nm for 12 ns laser pulses). In the thinner films (d<l), reamorphization occurs upon solidification. Recalescence is observed in the thicker ones (d⩾l) when the melt depth induced is above of ≈80 nm. Above this threshold, crystalline phases are nucleated upon solidification. The origin of this melt depth threshold is discussed in terms of the heat flow into the substrate, the supercooling prior to solidification, and the need of a minimum amount of initially solidified material.
Wet oxidation of AlGaAs: the role of hydrogen
Authors: Ashby Carol I. H. , Sullivan John P. , Choquette Kent D. , Geib K. M. , Hou Hong Q.
Published: July 2002

Wet oxidation of AlGaAs to form Al2O3 by the reduction of H+ from water to H produces intermediate As2O3. Reduction of As2O3 by H to elemental As enables the escape of arsenic from the oxidized film. Further reduction of As to AsH3 can provide another volatile As species. Formation of intermediate As is problematic for the use of wet oxidation in metal-insulator-semiconductor applications. The kinetic balance between As2O3 formation and As escape can explain the transition between the linear and parabolic time dependence of the wet oxidation of buried AlGaAs layers. The near-total suppression of wet oxidation by O2 is attributed to the suppression of volatile product formation through consumption of atomic hydrogen by reaction with O2 to form H2O in preference to the hydrogen reduction of As2O3.
Annealing method for operating quantum-cellular-automaton systems
Authors: Akazawa M. , Amemiya Y. , Shibata N.
Published: July 2002

We propose an annealing method as an effective way of operating quantum-cellular-automaton (QCA) systems, which are devices for computation that utilize the minimum energy state of electrons in a quantum cell system. A QCA system has an energy function with many local minima and therefore cannot be operated as desired if placed under the conditions of a thermodynamically open system. Accordingly, for successful operation of a QCA system (i.e., making the QCA system converge successfully to its minimum-energy state), we propose a method of operation based on the concept of thermodynamic annealing. We simulate the dynamics of various QCA logic-gate systems operated by this annealing method, and show that data processing in QCA systems can be carried out accurately by means of this annealing method. The applicability of QCA systems to non-Neumann parallel-processing computation is also described.
Surface thermal expansion of metal under femtosecond laser irradiation
Authors: Maznev A. A. , Hohlfeld J. , Güdde J.
Published: July 2002

Transient surface displacement of gold under femtosecond laser irradiation is studied using a probe beam deflection technique. A surface thermal expansion rise time of about 100 ps is explained in terms of nonequilibrium diffusion and thermalization of photoexcited electrons. Transient displacement provides direct information on the lattice temperature profile established once the electron-lattice relaxation is completed.
Generalized trapping kinetic model for the oxide degradation after Fowler–Nordheim uniform gate stress
Authors: Pananakakis G. , Ghibaudo G. , Papadas C. , Vincent E. , Kies R.
Published: July 2002

The practicality of modeling the power law degradation observed in thin dielectrics after Fowler–Nordheim stress has been demonstrated on the basis of a generalized trapping approach with appropriate trap cross-section and density profiles. A detailed mathematical analysis of the negative bulk oxide charge kinetics has been established using incomplete Gamma and generalized hypergeometric functions, after assuming exponentially varying trap cross-section and density profiles throughout the oxide. These spatial distributions could be due to the structural nature of the oxide after growth. Moreover, the asymmetry of the charge distribution centroid for negative and positive gate bias stress has been satisfactorily interpreted by neglecting the trapping in the tunneling region near the cathode. Overall this generalized kinetic trapping model provides very good fitting of the variation of the trapped oxide charge with the injection dose for oxide thicknesses between 5.5 and 10 nm. The evolution of the charge centroid is also well predicted but with less accuracy, due to the presence of other concurrent charge generation processes associated with positive and/or negative charge buildup.
Defects in YAG:Yb crystals
Authors: Yin Hongbing , Deng Peizhen , Gan Fuxi
Published: July 2002

A perturbed F center associated with rare earth ion was identified in YAG:Yb boules with absorption bands at 370 and 625 nm. The dislocation density in the crystal is determined to be less than 100 units cm2. From the absorption and emission spectras of YAG:Yb the trails of Er3+ and Ho3+ were detected. Hydroxyl groups were also found in the YAG:Yb crystals. To ensure high laser efficiency of diode pumped YAG:Yb system, the practical doping concentration of Yb3+ must not exceed 20 atm%.
Electron transport in wurtzite indium nitride
Authors: O’Leary Stephen K. , Foutz Brian E. , Shur Michael S. , Bhapkar Udayan V. , Eastman Lester F.
Published: July 2002

We present the velocity-field characteristics of wurtzite indium nitride, determined using an ensemble Monte Carlo approach. It is found that indium nitride exhibits an extremely high peak drift velocity at room temperature, 4.3×107 cm/s, at a doping concentration of 1.0×1017 cm−3. We also demonstrate that the saturation drift velocity of indium nitride, 2.5×107 cm/s, is comparable to that of gallium nitride, and much larger than that of gallium arsenide. Our results suggest that the transport characteristics of indium nitride are superior to those of gallium nitride and gallium arsenide, over a wide range of temperatures, from 150 to 500 K, and doping concentrations, up to 1.0×1019 cm−3. Hence, indium nitride has considerable potential for device applications.
Study of frequency dependencies of the complex magnetic ac-permeability aftereffect in amorphous Fe64Co21B15 and Fe8Co77B15 alloys
Authors: Turtelli R. Sato , Grössinger R. , Kussbach C. , Sinnecker J. P.
Published: July 2002

The frequency dependencies of the complex magnetic permeability and its aftereffect phenomenon in as-cast and annealed Fe64Co21B15 and in as-cast Fe8Co77B15 amorphous ribbons were studied. The measurements of these properties were carried out at frequencies between 1 and 120 kHz. The behavior of the frequency dependence of real and imaginary parts of the ac permeability and aftereffect were analyzed using the magnetic domain wall motion model. A change of the restoring force coefficient and the damping parameter with time was considered for the study of frequency dependency of the magnetic relaxation.
Excited-state photoemission with combined laser/synchrotron pulse excitation from C60 chemisorbed on Ni(110)
Authors: Quast T. , Bellmann R. , Winter B. , Gatzke J. , Hertel I. V.
Published: July 2002

Photoemission from C60 chemisorbed on Ni(110) has been studied by one-photon photoelectron spectroscopy (1ppe) and two-photon photoelectron spectroscopy (2ppe) with synchrotron radiation (SR) and with synchronized SR and laser radiation. The 1ppe photoelectron spectra are studied as a function of C60 coverage. Significant peak broadening relative to the thick film features is observed for the three leading valence bands. The large peak width of the valence levels obtained for the monolayer system is consistent with changes of the physical structure of C60 due to the strong chemical interactions with the nickel substrate. The 2ppe experiment demonstrates transient excitation of the (t2u,hg) level of C60.
Deep levels in heavily Zn-doped InP layers implanted with Ti and Ti/P
Authors: Si Sang Kee , Kim Sung June , Moon Youngboo , Yoon Euijoon
Published: July 2002

We have investigated deep level peaks observed in the photoluminescence spectrum of heavily Zn-doped InP layers grown by metalorganic chemical vapor deposition at energies centered at 0.89 and 0.94 eV. These peaks are enhanced when the samples are implanted with Ti. When P is co-implanted, however, the intensity of these peaks decrease, and at an increased dosage, the peaks disappear from the spectrum. The peaks are, therefore, dependent on the phosphorus vacancy produced by the excessive Zn doping or the implant damage. Hall measurement data show that the Ti/P-implanted p-type InP layer is converted to n type with its sheet resistance decreasing and the donor activation of Ti increasing for higher P co-implant dose. In addition, the photoluminescence intensity of the deep level peaks is highly correlated with the sheet resistance.
Red to green rainbow photoluminescence from unoxidized silicon nanocrystallites
Authors: Yamani Zain , Ashhab Sahel , Nayfeh Ammar , Thompson W. Howard , Nayfeh Munir
Published: July 2002

We prepared luminescent silicon by incorporating H2O2 in the anodizing process. Under UV excitation, our samples appear reddish, orangish, yellowish to greenish as the electrolyte-air interface at the meniscus is approached. Under high current anodization, the emission becomes broad but unstable. The time characteristics of all the emission regions are in the range of 1–10 μs. The results are consistent with a significant reduction of crystallite sizes, and may be explained via novel Si–Si dimer surface states that are induced by quantum confinement.
Probing ultrafast carrier and phonon dynamics in semiconductors
Authors: Othonos Andreas
Published: July 2002

Over the past 2 decades there has been tremendous advancements in the field of ultrafast carrier dynamics in semiconductors. The driving force behind this movement other than the basic fundamental interest is the direct application of semiconductor devices and the endless need for faster response and faster processing of information. To improve and develop microelectronics devices and address these needs, there must be a basic understanding of the various dynamical processes in the semiconductors which have to be studied in detail. Therefore, the excitation of semiconductors out of their equilibrium and the subsequent relaxation processes with various rates has become a key area of semiconductor research. With the development of lasers that can generate pulses as short as a few femtoseconds the excitation and subsequent probing of semiconductors on an ultrashort timescale have become routine. Processes such as carrier momentum randomization, carrier thermalization, and energy relaxation have been studied in detail using excite-and-probe novel techniques. This article reviews the status of ultrafast carrier and phonon dynamics in semiconductors. Experimental techniques such as excite-and-probe transmission, time-resolved up-conversion luminescence, and pump-probe Raman scattering along with some of the significant experimental findings from probing semiconductors are discussed. Finally, a selfconsistent theoretical model, which correlates the carrier and phonon dynamics in germanium on an ultrashort time scale, is described in detail.
Field-tuned quantum tunneling of the magnetization
Authors: García-Pablos D. , García N. , De Raedt H.
Published: July 2002

The response of the magnetization to a time-dependent applied magnetic field in single-spin models for uniaxial magnets is studied. We present staircase magnetization curves obtained from the numerically exact solution of the time-dependent Schrödinger equation. Steps are shown to correspond to field-tuned quantum tunneling between different pairs of nearly degenerate energy levels. We investigate the role played by different terms that allow for tunneling processes: transverse fields and second-order and fourth-order transverse anisotropies. Magnetization curves for nonsaturated initial states and for excited initial states showing steps when the field decreases in absolute value are also presented. These results are discussed in relation to recent experiments on high-spin compounds.
Analysis of electron paramagnetic resonance experiments in colossal magnetoresistance materials
Authors: Huber D. L.
Published: July 2002

An analysis is given of recent electron paramagnetic resonance (EPR) experiments in manganite materials displaying colossal magnetoresistance behavior. These experiments have established that in highly homogeneous samples characterized by a single resonance line, the EPR linewidths above the critical region associated with the paramagnetic–ferromagnetic transition are inversely proportional to Tχ(T), where χ denotes the susceptibility. This behavior indicates that the linewidth does not reflect a coupling to the lattice vibrations but rather is caused by a spin-only mechanism. The possibility that the linewidth arises from the dipolar interaction is assessed using an effective interaction model. On the basis of the model calculation, it is concluded that the dipolar interaction is probably too weak to account for the observed broadening and that the linewidth most likely arises from tetragonal and orthorhombic distortions of the octahedral crystal field at the sites of the Mn ions. The EPR linewidth in samples with very low concentrations of Mn4+ ions is expected to vary with temperature as [Tχsp(T)]−1, where χsp(T) is the susceptibility of the ferromagnetically coupled spin polaron. By measuring the temperature dependence of the linewidth, one may be able to determine the temperature dependence of the polaron susceptibility.
Magnetization processes in hybrid magnets
Authors: Emura M. , Neiva A. C. , Missell F. P. , Babcock K. L. , Ormerod J. , Constantinides S.
Published: July 2002

Injection-molded, oriented hybrid magnets consisting of mixtures of BaO⋅6Fe2O3 and MQP-Q (exchange-coupled Nd2Fe14B+α-Fe) are compared to bonded magnets made only from ferrite or MQP-Q. The magnetic fractions of the hybrid magnets consist of 80, 60, or 40 wt % ferrite blended with 20, 40, or 60 wt % MQP-Q. The microstructure was investigated by x-ray diffraction, scanning electron microscopy, atomic force microscopy, and magnetic force microscopy. Atomic and magnetic force microscopy images indicated differences between the physical and magnetic microstructures. Magnetic interactions were studied via isothermal remanence (IRM) and dc-demagnetization (DCD) remanence curves and Henkel plots. In contrast to what is observed in exchange spring magnets, the IRM and DCD susceptibilities of all magnets present peaks of nearly the same width, centered at roughly the same value of the magnetic field, consistent with weak dipolar interactions between magnetic particles. The IRM susceptibilities show structure associated with magnetic inhomogenieties in the samples.
Domain models for aging in spin glasses
Authors: Walton Derek
Published: July 2002

A simple domain growth model is shown to account for aging in spin glasses. It accounts quantitatively for the change in glass temperature with frequency. It is shown that percolation clusters present in disordered systems are responsible for the peak in ∂M/∂ ln t, where M is the magnetization, and t is the time.
A Mössbauer study of plasma nitrided iron
Authors: Cabral F. A. O. , de Araujo J. H. , Araujo R. C. , Gama S.
Published: July 2002

Iron powder has been nitrided in a glow discharge plasma of a mixture of nitrogen and hydrogen, in order to verify the existence of a metastable iron nitrogen solid solution, previously observed from the Sm2Fe17N3 phase decomposition process. Our results show that, for short nitridation times, we obtain a mixture of Fe4N, a small amount of α-Fe and an iron nitrogen solid solution the amount of which increases after heat-treatment at 610 °C. This iron nitrogen solid solution is characterized by Mössbauer spectroscopy which shows a typical α-Fe spectrum, but with lower hyperfine field and broader linewidth. TMA measurements show that the Curie temperature is similar to that of α-Fe.
Spin reorientation in (Pr,RE)–(Co,TM)–B magnets (RE=Tb,TM=Fe,Cr,Mn)
Authors: Kim D. H. , Hadjipanayis G.
Published: July 2002

Small substitutions of Pr with Tb and Co with Fe, Cr, Mn were used to increase the spin reorientation temperature of the Pr2Co14B compound which is around 395 °C. Spin reorientation temperatures were obtained by measuring magnetization vs temperature curves at temperatures up to 750 °C. Several ribbons with high spin reorientation temperature were fabricated by melt-spinning and they were annealed in the temperature at 600–700 °C to obtain the optimum coercivity. Our results show that spin reorientation temperature is sharply increased with Fe substitution and reaches a maximum (575 °C) in the sample with 25% Fe substituted for Co. Small amounts of Tb, Cr, and Mn are effective in increasing the spin reorientation temperature. The optimum coercivities of ribbons are in the range of 7–10.5 kOe and ribbons with high spin reorientation temperature show an improved temperature dependence of coercivity.
Orbital magnetic moment enhancement at surfaces and interfaces within the framework of the local density approximation+U method
Authors: Shick A. B. , Freeman A. J. , Liechtenstein A. I.
Published: July 2002

Stimulated by MCXD measurements, the determination of orbital magnetic moments, ML, at surfaces and interfaces has become a subject of strong interest. We combine the rotationally invariant local density approximation (LDA)+U total energy functional with a relativistic self-consistent spin-polarized version of the total energy full-potential linearized augmented plane wave method in order to determine ML at surfaces and interfaces. As illustration, the method is applied to Co overlayers on Cu(001). We find an enhancement of the Co orbital magnetic moment in LDA+U calculations in comparison with local spin density functional approximation (LSDA) results. ML as a function of magnetic Co-film thickness is analyzed. The ML/MS ratio is calculated to be different for different Co layers (i.e., interface, surface, or remaining Co layers) in Co/Cu(001), and is shown to be dependent on the Co overlayer thickness. Finally, the use of LDA+U is shown to give ML/MS in better agreement with MCXD data than does LSDA.
Magnetomechanical instability in FeTb/Fe multilayers
Authors: Wuttig Manfred , Su Quanmin , Masson Fabrice , Quandt Eckhard , Ludwig Alfred
Published: July 2002

Multilayers composed of alternate layers of iron–rare earth and Fe or FeCo alloys feature a high magnetostriction at low external fields and a coercive force as low as 5 mT. The coercive fields of as sputtered multilayers are significantly higher than that and the low values are attained after an anneal at 473 K. Since the hysteresis remaining after the heat treatment might be of magnetomechanical origin it is of interest to investigate the dynamic magnetomechanical characteristics of this class of multilayers. Experiments on multilayers of the composition (7 nm FeTb)/(8 nm/Fe) show a pronounced magnetomechanical damping maximum and an attendant decrease of Young’s modulus upon annealing. Both occur in the vicinity of the coercive field. Their evolution as a function of the annealing and magnetic field will be described and it will be shown that they reflect an instability principally caused by the magnetomechanical interaction of the component layers of the multilayer.
Magnetic and structural properties of PrCo3−xSix compounds
Authors: Matsui T. , Stevenson R. D. , Kirby R. D. , Sellmyer D. J.
Published: July 2002

We have systematically investigated the magnetic properties of the rare-earth compounds PrCo8Si and PrCo3−xSix, the crystal structures of which are hexagonal CeNi3-type and rhombohedral Be3Nb-type, respectively. Sharp cusps were observed in the real and imaginary parts of the ac susceptibility at temperatures between 282 (x=0.08) and 209 K (x=0.23). Also observed were bifurcations of the zero-field-cooled and field-cooled magnetization curves at the same temperatures, showing that the magnetization was irreversible below these temperatures. The effect of Si substitution for Co in this system is to decrease the average ferromagnetic exchange interaction, thus decreasing the Curie temperature, without introducing significant random magnetism effects due to exchange fluctuations or random magnetic anisotropy.
Artificial FeCu(100) epitaxial ordered alloy films: Element-selective magnetic properties
Authors: Kuch W. , Salvietti M. , Gao Xingyu , Klaua M. , Barthel J. , Mohan Ch. V. , Kirschner J.
Published: July 2002

Epitaxial artificial FeCu alloy thin films in the fcc L10 ordered phase were prepared by pulsed laser deposition on Cu(100). Magnetic circular dichroism in x-ray absorption at the Fe and Cu L2,3 edges was used to study the electronic and magnetic properties of the normally immiscible FeCu(100) alloy films. The Fe exhibits magnetic moments comparable to that of fcc Fe films, ruling out the occurrence of nonmagnetic fcc Fe. The ratio between orbital and spin contribution to the Fe d moments is significantly enhanced in FeCu with respect to Fe/Cu(100), and amounts to ≈0.12. An induced magnetic moment of the Cu atoms is observed, which carries about 7% of the total d-band moment of the FeCu film.
Overpotential driven perpendicular magnetization of electrodeposited ultrathin cobalt films
Authors: Bubendorff J. L. , Beaurepaire E. , Mény C. , Bucher J. P.
Published: July 2002

Depending on the overpotential applied during electrodeposition of ultrathin cobalt films (η=0.1–2.2 V), either in-plane or out-of-plane magnetization can be stabilized. The regime of high supersaturation in particular allows to engineer electrodeposited cobalt films on Au(111) that show perpendicular magnetization (in the thickness range from 2 to 8 atomic layers) exactly like their ultrahigh vacuum grown counterparts. The film topography depends on the overpotential as shown by atomic force microscopy: continuous films are obtained at low overpotential whereas high overpotential leads to the formation of islands. The perpendicular magnetization results from a subtle thickness dependent competition between the shape and the interface anisotropy.
Effect of buffer layer on antiferromagnetic grain size and exchange-coupling field of Cr70Al30/Fe19Ni81 bilayers
Authors: Ikarashi K. , Otani Y. , Fukamichi K. , Kitakami O. , Shimada Y. , Echigoya J. , Uyama H. , Makino A.
Published: July 2002

MgO/Cr 70 Al 30 /Fe 19 Ni 81 and ZnO/Cr70Al30/Fe19Ni81 layered films with different buffer layer thicknesses were prepared on (100)Si substrates in order to investigate the relationship among the exchange-coupling field, the grain size, and the lattice constant of the antiferromagnetic layer. The exchange-coupling field was found to consist of the intragrain exchange coupling and the intergrain coupling fields. The former field is inversely proportional to the product of the lattice constant and the grain size. The latter field is constant and changes its sign in association with the increase of the buffer layer thickness.
Finite size scaling in the thin film limit
Authors: Waldfried C. , Welipitiya D. , McAvoy T. , Dowben P. A. , Vescovo E.
Published: July 2002

The thickness dependent spin-polarized electronic structure of strained thin and ultrathin films of Gd is characterized by a distinct change in the critical exponent λ in the formalism of finite size scaling. The reduced critical exponent in the ultrathin films can be correlated to the increased dominance of the surface magnetic structure and the increasing paramagnetic-like behavior of the bulk.
Properties of the titanium related level in Cd0.96Zn0.04Te crystals
Authors: Zerrai A. , Bremond G.
Published: July 2002

Semi-insulating and n-type conductor titanium (Ti) doped CdZnTe has been investigated by deep level transient spectroscopy, deep level optical spectroscopy and photoinduced current transient spectroscopy. A main electron trap at 0.82 eV is detected and its electrical and optical characteristics are given. The σno optical cross section spectrum of this level exhibits a resonance band attributed to the internal transition of the Ti2+ ion in CdZnTe. Starting from this identification, this deep trap is formally proposed to be the Ti2+/Ti3+ single donor level. The semi-insulating properties of the CdZnTe:Ti are interpreted in relation to this deep donor. Finally, the impact of the optical cross sections σno and σpo of this level on the photorefractive behavior of CdZnTe crystal is also discussed.
Frequency response of cantilever beams immersed in viscous fluids with applications to the atomic force microscope
Authors: Sader John Elie
Published: July 2002

The vibrational characteristics of a cantilever beam are well known to strongly depend on the fluid in which the beam is immersed. In this paper, we present a detailed theoretical analysis of the frequency response of a cantilever beam, that is immersed in a viscous fluid and excited by an arbitrary driving force. Due to its practical importance in application to the atomic force microscope (AFM), we consider in detail the special case of a cantilever beam that is excited by a thermal driving force. This will incorporate the presentation of explicit analytical formulae and numerical results, which will be of value to the users and designers of AFM cantilever beams.
Quantum transport in high mobility modulation doped Ga0.25In0.75As/InP quantum wells
Authors: Ramvall P. , Carlsson N. , Omling P. , Samuelson L. , Seifert W. , Wang Q. , Ishibashi K. , Aoyagi Y.
Published: July 2002

We report on edge state transport in a ternary material, a modulation-doped InP/Ga0.25In0.75As/InP quantum well, where the electron transport takes place in the highly strained Ga0.25In0.75As layer. The electron mobility is, even though fundamentally limited by alloy-disorder scattering, determined by the two-dimensional electron concentration. By varying the distance between the modulation-doped layer and the two-dimensional electron gas we influence the single particle relaxation time but not the electron mobility. Special attention is paid to the effect of dislocation formation in the conducting Ga0.25In0.75As layer. In addition to the quantum Hall and the Shubnikov–de Haas effect a strong, nonlocal transport behavior, which is maintained after illumination, is observed. This effect is explained by the low defect density and the Fermi level pinning on the etched Ga0.25In0.75As surface, at an energy close to the same as the Fermi energy of the two-dimensional electron gas. Furthermore, overshoot effects of the quantum Hall plateaus introduced by the high and varying effective (many-body) g value are investigated. The g value is further addressed in an experiment on a wet etched quantum wire in which values enhanced up to around 45 were found.
Electron transport in metallic dot arrays: Effect of a broad dispersion in the tunnel junction dimensions
Authors: Cordan A. S. , Goltzené A. , Hervé Y. , Mejias M. , Vieu C. , Launois H.
Published: July 2002

We have shown that for a one-dimensional multijunction array with a broad junction length distribution, we can increase the mean threshold voltage Vth for a given maximal tunnel resistance without increasing the scatter of Vth. For two-dimensional arrays we can increase strongly the output of devices which do not behave as open circuits, without any loss on the scatter on Vth, and still increase the latter with respect to a single-island device. The experimental background which justified the calculation will be described, as our model needs the experimental distribution of the island dimensions and junction lengths. Such a result may either show a way to increase the threshold voltages for a given technology, or allow for larger and therefore more controllable dimensions, or even extend the choice of materials.
Spectroscopic evidence for different laser gain mechanisms in optically pumped ZnCdS/ZnS quantum well structures
Authors: Chen P. , Nicholls J. E. , O’Neill M. , Stirner T. , Hogg J. H. C. , Lunn B. , Ashenford D. E.
Published: July 2002

The role of biexcitons has been studied in two optically pumped quantum well (QW) structures of Zn1−xCdxS/ZnS, one with 18% Cd concentration and the other with 3% in the wells. For the x=18% QW structure, high excitation photoluminescence and stimulated emission indicate that the laser gain mechanism involves biexcitons. For the x=3% QWs, even though biexcitons are clearly observed in the spontaneous emission, they are not responsible for laser gain in this structure. Instead exciton–exciton scattering may be the more likely mechanism responsible for laser gain close to threshold, while at higher densities an estimate of the carrier density indicates an electron–hole plasma as the likely source of optical gain. The different mechanisms in the two cases can, very likely, be attributed to one of differing degrees of localization both within the QW and at alloy fluctuations.
Modeling the kinetics of neutral particles in low-pressure chemically active planar microwave plasmas
Authors: Strobel H. , Ohl A.
Published: July 2002

Low-pressure planar microwave plasmas exhibit a pronounced spatial structure. If the discharge vessels are wide enough, they have a thin, plane homogeneous excitation region adjacent to a microwave coupling window, and they strongly decay in the direction normal to the window. The decay length of plasma parameters and activated neutrals differ by about one order of magnitude. This suggests a two-region approach for modeling of neutral particle kinetics assuming spatial homogeneity inside these two regions. The gaseous reactor volume is subdivided by a virtual interface on which mass exchange by directed gas flow, local microconvective mixing or diffusion, and exchange of enthalpy between the two different regions can be balanced. The main differences between the two regions are that electron-induced generation of activated neutrals is restricted to the excitation region (1) and that basic loss processes for activated neutrals due to the substrate surface and volume reactions are considered to occur in the second, chemical reaction region (2). Processes of loss of active neutral atoms by recombination at inert surfaces of the vessel are considered in both regions. Effectively, the model represents a system of two-coupled continuous flow stirred tank reactors given by a set of differential equations including unsteady-state Bernoulli equations. For the case of an example process concerning an oxygen plasma surface treatment, numerical solutions for initial values at the startup of the reactor are computed. The results for the dynamical behavior and the steady states of the concentrations, the temperatures, and the mass loss are in qualitative agreement with the experimental behavior of a real reactor arrangement.
Formation of In– and Au–In0.52Al0.48As(100) interfaces: A soft x-ray photoemission spectroscopy study
Authors: Cammack D. S. , Clark S. A. , Dunstan P. R. , Pan Min , Wilks S. P. , Elliott M.
Published: July 2002

We have examined the physical and chemical reactions occurring during the formation of intimate In– and Au–InAlAs(100) at room temperature. In deposition results in a two-dimensional mode of growth, followed by clustering. Au deposition however, perturbs the interface with As diffusing into the Au overlayer. A Fermi shift is evident following the deposition of In and Au, but in both cases this is lower than barrier heights measured by transport techniques. Possible mechanisms for the observed adaptation of pinning position are discussed in the light of current models of Schottky barrier formation.
Optical functions from 0.02 to 6 eV of AlxGa1−xSb/GaSb epitaxial layers
Authors: Ferrini R. , Patrini M. , Franchi S.
Published: July 2002

The complex refractive index ñ=n+ik and the dielectric function ε̃=ε1+iε2 at room temperature of AlxGa1−xSb films with 0⩽x⩽0.5, grown by molecular beam epitaxy on a GaSb substrate, were determined from 0.02 to 6 eV by using the complementary data from fast Fourier transform far-infrared, dispersive, and ellipsometric spectrometry. The effect of the native oxide was accounted for and the self-consistency of the optical functions was checked in the framework of the Kramers–Kronig causality relations. In the restrahlen region the dielectric function was well fitted by classical Lorentz oscillators; in the transparent region below the fundamental gap E0, the refractive index was modeled by a Sellmeier dispersion relation, and in the interband region the dielectric function near the critical points was analyzed through standard line shapes. Interpolating the fitting parameters or the interband dielectric spectra, it was possible to obtain the optical functions for any concentration x between 0.0 and 0.5.
Alignment of a nematic liquid crystal induced by anisotropic photo-oxidation of photosensitive polyimide films
Authors: Wang Yinghan , Xu Chunying , Kanazawa Akihiko , Shiono Takeshi , Ikeda Tomiki , Matsuki Yasuo , Takeuchi Yasumasa
Published: July 2002

The alignment of a nematic liquid crystal (LC) could be induced by three polyimide (PI) films exposed to linearly polarized ultraviolet (UV) light of long wavelength (366 nm). The alignment could also be modified by changing the direction of polarization of the linearly polarized light (LPL). Infrared (IR) and UV-visible spectroscopy indicated that among the three PI films the PI with a benzophenone moiety and the PI with a diphenyl methane moiety were sensitive to UV radiation and susceptible to remarkable ablation after irradiation with LPL in air. Polarized UV-visible spectroscopy indicated that dichroism of the PI films was caused by LPL and altered by changing the polarization of the LPL. Furthermore, the PI films irradiated under vacuum showed little reduction in intensity of IR bands, which is in sharp contrast to the marked decreases in samples irradiated in atmospheres containing oxygen. This proves that oxygen is necessary to the ablative process of the PI films. Our results indicate that the mechanism of LC alignment on the photosensitive PI films is photoinduced anisotropic oxidative ablation of the PI films. In contrast, the PI film with a diphenyl ether moiety was quite stable under similar irradiation conditions even though the irradiated PI film could also induce alignment of the LC molecules.
In situ ellipsometric characterization of SiNx films grown by laser ablation
Authors: Samano E. C. , Machorro R. , Soto G. , Cota-Araiza L.
Published: July 2002

Uniform and smooth silicon nitride films have been synthesized by laser ablating a Si3N4 sintered target in an ultrahigh vacuum system in vacuum and different gas environments, N2, Ar, and He. The evolution of the composition and thickness control of SiNx overlayers grown on single-crystal Si and quartz substrates kept at room temperature have been in situ monitored by real-time ellipsometry at a fixed photon energy, 2.5 eV, and a fixed incidence angle, 71°. The different stages of the deposition process were momentarily interrupted to analyze the optical properties of the film in the photon energy range of 1.5<hν<5.0 eV. The effective medium approximation is used to analyze the ellipsometric data, finding a film composed by a mixture of noncrystalline Si3N4, polycrystalline Si, and amorphous Si. The ellipsometric results are complemented by in situ characterization of the film by x-ray photoelectron spectroscopy. The films grown on quartz are also used to determine their optical band gap from transmittance measurements. The film deposition in vacuum results in nitrogen deficient films, x≈0.9. The presence of a background gas during deposition produces a strong increase of nitrogen in the SiNx films compared to those grown in vacuum. A nonporous film with a nearly ideal stoichiometry, x≈4/3, has been grown at a critical pressure. The critical pressure value depends on the type of gas, pN2=10 mTorr, pAr=22 mTorr, and pHe=160 mTorr. As a consequence of the control in the film stoichiometry, the film properties can also be adjusted to the desired values by just controlling the gas pressure. The films grown in vacuum are semiconducting with a band gap of 2.8 eV, but the films grown at pN2=10 mTorr are insulating with a band gap of 4.6 eV.
Efficient directional spontaneous emission from an InGaAs/InP heterostructure with an integral parabolic reflector
Authors: Gfroerer T. H. , Cornell E. A. , Wanlass M. W.
Published: July 2002

In order to increase the radiative efficiency and directivity of spontaneous emission from a lattice-matched InGaAs/InP heterostructure, we have polished the substrate into a parabolic reflector. We combine optical and thermal measurements to obtain the absolute external efficiency over a wide range of carrier densities. Using a simple model, the measurement is used to determine interface, radiative, and Auger recombination rates in the active material. At the optimal density, the quantum efficiency exceeds 60% at room temperature. The divergence of the emitted light is less than 20°. In fact, the beam profile is dominated by a 6° wide lobe that can be swept across the field of emission by changing the excitation position. This suggests a way to create an all-electronic scanned light beam.
Intrinsic limits on the Q and intermodulation of low power high temperature superconducting microstrip resonators
Authors: Hammond R. B. , Soares E. R. , Willemsen Balam A. , Dahm T. , Scalapino D. J. , Schrieffer J. R.
Published: July 2002

Cuprate superconducting thin films are being used to make compact low power microwave devices. In recent years, with improved materials and designs, there has been a steady improvement in device performance, notably an increase in resonator Q and a decrease in the nonlinear intermodulation. It is important to understand how much improvement can be expected. Here we discuss the intrinsic limiting behavior that one might achieve with a perfect film, review the present status, and discuss what the limiting behavior implies for high temperature superconducting filters. Our analysis indicates that increases in unloaded Q to ∼106 and decreases in intermodulation by a factor of ∼104, compared with today’s values, might be achieved.
The voltage dependence of switching in a polymer current limiter device
Authors: Sun F. G. , Duggal Anil R.
Published: July 2002

The electrical switching properties of a polymer current limiter device are studied as a function of applied voltage. For voltages ranging from 1 to 400 V, the switching ratio of the device is observed to vary by more than two orders of magnitude and exhibits a maximum around 200 V. Three different regimes of operation are identified based on the accompanying phenomena. A qualitative model which includes the positive temperature coefficient of resistance effect, an ablation-induced contact separation mechanism, and the effect of dielectric breakdown is developed to explain the observations in the three regimes.
Analysis of stability in a solute field under liquid phase epitaxy
Authors: Saitou M. , Motoyama S.
Published: July 2002

We reported a critical gap length between two silicon substrates at which no epitaxial layer was grown on the upper silicon substrate [M. Saitou and S. Itomura, J. Mater. Sci.: Mater. Electron. 8, 321 (1997)]. In this study, using perturbation theory and a numerical solution, the abrupt change in the growth rate is investigated. We find that a nondimensional parameter GrSc∂c/∂y determines the instability in a solute field, and that under a critical value of this parameter no epitaxial growth on the substrate sets in because diffusion governs the mass transport of the silicon solute.
Josephson junction oscillators as probes of electronic nanostructures
Authors: Adourian A. S. , Yang Scott , Westervelt R. M. , Campman K. L. , Gossard A. C.
Published: July 2002

We have fabricated high-quality planar Nb/AlOx/Nb Josephson junctions on-chip adjacent to quantum dots in a near surface two-dimensional electron gas in a GaAs/AlGaAs heterostructure. When used as a voltage-tunable oscillator coupled capacitively to a quantum dot, the Josephson junction can produce a localized time-dependent potential of 200 μV across the dot at frequencies in excess of 300 GHz. The fabrication process involves five separate patterning and processing steps to define the multilayer integrated device.
Effect of annealing on the electrical and optical properties of GaAs/InGaP/n+InGaAs/GaAs epitaxial layers
Authors: Watanabe Kazuo , Hyuga Fumiaki , Nittono Takumi
Published: July 2002

Annealing effects on the electrical and optical characteristics of GaAs/InGaP/n+InGaAs:Si/GaAs epilayers grown on GaAs semi-insulating substrates by metal organic chemical vapor deposition have been investigated. This epilayer structure has been used for heterostructure metal–semiconductor field effect transistors. The carrier concentration is decreased and the mobility is increased by annealing at 700 °C or higher, suggesting the deactivation of Si donors in the n+InGaAs layers. With such annealing, the band-edge photoluminescence (PL) peak from the n+InGaAs layers becomes dramatically weak or disappears and deep level PL broad peaks around 1.2 and 1.5 μm appear instead. It is confirmed by the selectively excited PL that these peaks are from the n+InGaAs layer. The deep levels probably act as nonradiative recombination centers for the near band-edge PL and are attributed to Si complex levels created during the Si donor deactivation process. These levels have no relationship with the strain relaxation or dislocation generation. At 600 °C or lower, however, the carrier concentration, mobility, and PL spectra are hardly changed at all by annealing.
Intervalence band absorption in InP
Authors: Kakimoto Syoichi , Watanabe Hitoshi
Published: July 2002

We have calculated intervalence band absorption loss coefficients for InP with various hole densities at 297 and 400 K. The calculated values agree well with the reported measured values at 297 K. However, the calculated values are much smaller than the reported measured values at 400 K. We discuss the difference between our calculated values and the reported calculated values at 297 K and the discrepancy between our calculated values and the reported measured values at 400 K.
Anion control in molecular beam epitaxy of mixed As/Sb III-V heterostructures
Authors: Bennett Brian R. , Shanabrook B. V. , Twigg M. E.
Published: July 2002

Superlattices consisting of As monolayers (MLs) in (In,Ga,Al)Sb and Sb MLs in (In,Ga,Al)As were grown by molecular beam epitaxy and characterized by x-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy. In all cases, well-defined superlattices were formed when the growth temperature was sufficiently low. As temperature increases for the As MLs in antimonides, substantial intermixing occurs. For Sb MLs in arsenides, Sb evaporation from the surface increases with increasing growth temperature. These results are discussed in the context of device heterostructures containing InAs/GaSb and InAs/AlSb heterojunctions.
A frequency response and transient current study of β-Ta2O5: Methods of estimating the dielectric constant, direct current conductivity, and ion mobility
Authors: Mattsson M. Stro/mme , Niklasson G. A. , Forsgren K. , Hårsta A.
Published: July 2002

Dielectric ac measurements in the frequency range of ∼1 mHz–1 MHz were performed on β-Ta2O5 samples made by chemical vapor deposition. A method of estimating the dielectric constant and the dc conductivity from the dielectric response was developed. The high-frequency dielectric constant was found to be 25.84 with no detectable temperature dependence and the dc conductivity due to protons had an activation energy of about 0.4 eV in the studied temperature range (from 24 to 90 °C). Evidence for the conduction mechanism being protonic rather then electronic was found from isothermal transient ionic current (ITIC) measurements. The ITIC recordings could also determine conduction parameters, such as proton mobility, number of charge carriers, and dc conductivity.
High resolution x-ray analysis of pseudomorphic InGaN/GaN multiple quantum wells: Influence of Si doping concentration
Authors: Cho Yong-Hoon , Fedler F. , Hauenstein R. J. , Park G. H. , Song J. J. , Keller S. , Mishra U. K. , DenBaars S. P.
Published: July 2002

We report the structural properties of InGaN/GaN/AlGaN multiple quantum wells (MQWs) by means of two-dimensional reciprocal space mapping (RSM) of high resolution x-ray diffraction. The influence of Si doping in GaN barriers on the characteristics has been studied for 12-period MQWs grown by metalorganic chemical vapor deposition, which have different Si doping concentrations in the GaN barriers ranging from 1×1017 to 3×1019 cm−3. Information on the structural quality of these MQWs was extracted from the linewidth broadening of the higher-order superlattice satellite peaks, as well as from the presence of Pendellösung oscillations. The measured diffraction curves were modeled using kinematic diffraction theory. From the symmetric and asymmetric RSMs around (0002), (0004) and (112̄4) reflections, we found that the InGaN/GaN/AlGaN MQWs are grown coherently on the GaN base layer. Better interface properties are achieved with Si doping. Our results indicate that Si doping in the GaN barriers affects the interface quality of the InGaN/GaN MQW systems, and thus, also influences the optical properties.
Thermally stimulated-current observation of hole traps in undoped semi-insulating GaAs and their photoquenching behavior
Authors: Suemitsu Maki , Sagae Yoshitomo , Miyamoto Nobuo
Published: July 2002

Detailed thermally stimulated-current measurements have been conducted on an undoped, semi-insulating GaAs crystal under 1.06 μm illumination at 15 K. By combining with Hall voltage measurements, we confirmed the presence of hole traps that show similar activation energies with the one proposed for the actuator level in the deep acceptor-mediated photoquenching model of EL2 [Suemitsu et al., Phys. Rev. B 52, 1666 (1995)]. The observed quenching of these hole traps suggests that the actuator level be, most likely, a part of the metastable complex.