bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature variation of the magnetoresistance in cobalt-enhanced spin-valve structures
Authors: Szucs J. , O’Brien T. , Lottis D. K. , Gangopadhyay S. , Mao S. , Murdock E.
Published: July 2002

We have conducted a survey of the temperature variation of the magnetoresistance in a series of FeMn exchange-biased spin-valve structures. These permalloy-based samples were prepared in an ion-beam sputtering system and feature Co layers inserted at the interfaces with the Cu spacer layer to enhance the interfacial spin-dependent scattering. Typical values for the MR are 4.0% at 295 K and 8.4% at 8 K (sample with 15 Å Co thickness). A control sample with no cobalt showed MR values of 1.3% and 4.3% for those same temperatures. Both the MR ratio and the un-normalized magnetoresistive change ΔR are plotted vs temperature. The MR ratio for the cobalt-enhanced samples exhibits nearly linear decrease with rising temperature. The sample with no cobalt exhibits a temperature variation deviating substantially from linearity, with an upward curvature. The temperature dependence for the MR in these spin valves is examined in the light of interchannel spin-mixing and intrachannel scattering.
Magnetic properties and structure of a new multilayer [FeTaN/TaN]n for recording heads
Authors: Li Shuxiang , Freitas P. P. , Rogalski M. S. , Azevedo M. , Sousa J. B. , Dai Z. N. , Soares J. C. , Matsakawa N. , Sakakima H.
Published: July 2002

In this article, the structure and soft magnetic properties of [FeTaN/TaN]n multilayers have been studied. Best multilayer films have coercivity of 1.2 Oe, which is further decreased to 0.5 Oe by annealing up to 400 °C. The saturation magnetization is 19.5 kG. The initial permeability of multilayer films is 1200 and is constant up to 100 MHz. A predominant α-Fe(110) texture is observed by x-ray diffraction patterns and Mössbauer spectra (MS), but a small amount of ferromagnetic ε-FexN (x⩾3) and paramagnetic ε-FexN (2<x<3) phases are identified by MS, which may account for the relatively large magnetostriction (∼5×10−6). The temperature dependence of MS and x ray show good thermal stability up to 430 °C.
Extrinsic dislocation loop behavior in silicon with a thermally grown silicon nitride film
Authors: Herner S. B. , Krishnamoorthy V. , Jones K. S. , Mogi T. K. , Thompson M. O. , Gossmann H.-J.
Published: July 2002

The effect of a thermally grown silicon nitride (SiNx) film on end-of-range extrinsic dislocation loops in a silicon substrate was investigated by transmission electron microscopy. A layer of extrinsic dislocation loops was formed by annealing a Si wafer amorphized by a Ge+ ion implant. A nitride film was grown on the Si by further annealing in ammonia (NH3) at 810 and 910 °C for 30–180 min. Wafers with a loop layer were also annealed in argon (Ar) at the same conditions as the NH3-annealed wafers to determine loop behavior in an inert environment. Samples annealed in NH3 had a significant decrease in the net number of interstitials bound by the loops, while those annealed in Ar showed no change. The results are explained by a supersaturation of vacancies caused by the presence of the nitride film, resulting in loop dissolution. By integrating the measured vacancy flux over the distance from the nitride/Si interface to the loop layer, we extract an estimate for the relative supersaturation of vacancies at 910 °C, CV/CV*∼4, where CV is the concentration of vacancies and the asterisk denotes equilibrium. We rule out interstitial undersaturation-induced loop dissolution based on loop stability with temperature and oxidation-enhanced loop growth calculations. A comparison with estimated CV/CV* values from a previous report using the same processing equipment and parameters but monitoring the change in Sb diffusivity with nitridation shows excellent agreement.
Optical properties of Cu-implanted LiNbO3
Authors: Saito Y. , Shang D. Y. , Kitsutaka R. , Kitahara A.
Published: July 2002

Cu ion implantation in LiNbO3 was done at widely different energies. Optical absorption spectra were measured in the visible range. Samples were heated to 500 °C in air for different times. Depth profiles and the electronic state of Cu for the samples at low ion energies were measured by x-ray photoelectron spectroscopy. Identification of the elements and measurement of the concentration of droplets on the surface were done by inductive coupled plasma spectroscopy. A large, sharp absorption peak is observed at 596 nm for the as-implanted sample at 3 MeV and a large, broad peak at 610 nm for the sample at 25 keV. These absorption peaks are explained in terms of Cu colloids formed in LiNbO3, but the electronic state of Cu in the implanted layer is not that of pure copper. Cu atoms segregate from the implanted layer to the surface, where they form small droplets. The droplet precipitation occurs even at room temperature and is accelerated at high temperature.
Oxygen precipitation and denuded zone characterization with the electrolytical metal tracer technique
Authors: Obermeier Guenther , Hage Juergen , Huber Diethard
Published: July 2002

In the investigation of precipitated wafers after two-step anneals good correlation was found between the bulk microdefects counted and the diffusion length measured using the electrolytical metal tracer technique. A spatially resolved image of the precipitated bulk of the wafer is thus obtained thereby providing more information about the regularity of oxygen precipitation. To characterize a wafer after a thermal denuding/precipitation cycle a two-region model (defect-free/precipitated region) was assumed with two different diffusion lengths LD separated in a depth t. The measurements were made by using two different laser wavelengths. By solving the diffusion and continuity equations of this system information on the whole wafer around the denuded zone depth and the diffusion length in the precipitated bulk can be obtained. The results are in good agreement with the cleavage face etching method.
Interaction of coherent collinear acoustic waves in a liquid/solid system
Authors: Duquesne J.-Y. , Parshin D. A.
Published: July 2002

We present a new type of experiment on the nonlinear interaction between coherent collinear longitudinal acoustic waves: Two waves with different frequencies propagate together successively in a liquid and in a solid. They interact because of the nonlinear elastic properties of both media. We have measured the magnitude of the signal resulting from the interaction, as a function of the liquid thickness. The variation law is sensitive to the phase difference between the waves generated in both media (in phase or out of phase). From this dependence the ratio and relative sign of two parameters characterizing the non-linear properties of both media can be derived without any measurement of the primary and harmonic waves amplitude.
Magnetization curling in films used for perpendicular magnetic recording
Authors: Ishii Y. , Hasegawa S. , Saito M. , Tabayashi Y. , Kasajima Y. , Hashimoto T.
Published: July 2002

The magnetization-curling mode in a columnar-structure film with perpendicular magnetization is studied on the assumption that the film is a two-dimensional array of cylinders. The angular dependence of the nucleation field HN is calculated by the Ritz method and compared with that for the coherent-rotation mode. It is found that the magnetization rotates in the curling mode for a large reduced radius S and in the coherent-rotation mode for a small S. The angular dependence of HN is different from that in an infinite cylinder. If S is large, the magnetization rotates in the curling mode for any value of θ0, where θ0 is the angle between the applied field and the cylinder axis. When S is medium, the magnetization rotates in the coherent-rotation mode for a small θ0 and in the curling mode for a large θ0.
A self-consistent nonlinear theory of a premodulated electron beam propagating through a helix-loaded waveguide
Authors: Uhm Han S.
Published: July 2002

A self-consistent nonlinear theory of a relativistic electron beam propagating through a waveguide loaded with a sheath helix is developed by making use of Maxwell equations. A closed integrodifferential equation is obtained for the normalized beam current described in terms of the normalized time θ and propagation distance ζ. An analytical investigation of the partial integrodifferential equation of the current modulation is carried out by Fourier decomposing the current modulation with harmonic number ℓ. The mode strength cl(ζ) is obtained in terms of the spatial oscillation frequency ηl and growth rate ξl of the mode amplitude, which are, in turn, determined in terms of the electrostatic and helix effects. The spatial oscillation frequency increases as the electrostatic effect (gl) increases. On the other hand, the growth rate increases with the strength of the helix effect. It is found that the mode strength cl is oscillatory and that it grows during propagation when the spatial frequency ηl (the electrostatic effect) is considerably larger than the growth rate ξl (the helix effect). Otherwise, the exponential growth dominates. Investigation of the helix mode must include the electrostatic effect for an intense beam.
Coherent Raman spectroscopy of nanoshocks
Authors: Tas Guray , Hambir Selezion A. , Franken Jens , Hare David E. , Dlott Dana D.
Published: July 2002

A new technique is described, where picosecond laser pulses generate and probe 4.2 GPa nanoshocks in polymeric and polycrystalline solids at a high repetition rate of ∼100/s. The term nanoshock refers to the short duration (a few ns) of the shock pulse and the very small shocked volume (a few ng). The nanoshock wave form is characterized by the shock front risetime, shock falltime, peak pressure, and velocity. Coherent Raman spectroscopy during nanoshock propagation in a 700-nm-thick layer of polycrystalline anthracene, called an optical nanogauge, is used to determine these quantities. A powerful method of analysis, singular value decomposition (SVD), is applied to Raman spectroscopy of shock waves for the first time. Using SVD analysis, the risetime of the nanoshock pulses is found to be less than 25 ps, and the velocity of the shock front in the nanogauge is monitored in real time. Some possible applications of nanoshock technology in the areas of shock-induced material transformation and shock-induced mechanical deformation processes, are discussed briefly.
Stochastic resonance and computation
Authors: Torres José-Leonel , Trainor Lynn
Published: July 2002

Stochastic resonance (SR) occurs in bistable nonlinear systems subject to noise, as the entrainment of their output by a weak periodic modulation added to the input. Electronic computation involves switching of memory elements between two states that correspond to 1 and 0, respectively. The possibility of switching errors due to SR in memory elements is considered, showing that it represents a negligible danger to reliable computation.
Magnetic properties in Laves phase CexDy1−xFe2 intermetallics
Authors: Tang Chengchun , Chen Dongfeng , Li Yangxian , Wu Guangheng , Jia Kechang , Zhan Wenshan
Published: July 2002

We report the measurement of the lattice parameter, Curie temperature, low-temperature magnetization, and room-temperature magnetostriction of the pseudobinary compounds CexDy1−xFe2. The analysis of these results revealed a Ce-valence change from +3.29 toward +3 in the concentration range 0.6⩽x⩽0.8. This is manifested by an anomalous change in the lattice parameter and the Curie temperature in this particular concentration range. In addition, a strong evidence for the mentioned variation of valence was found from magnetostriction measurements (λ∥−λ⊥ increases from 8 to 75 ppm).
Focused ion-beam implanted lateral field-effect transistors on bulk silicon
Authors: Crell C. , Friedrich S. , Schreiber H.-U. , Wieck A. D.
Published: July 2002

In in-plane-gate transistors, gate, drain, and source are lying in one plane, leading to a very efficient lateral electric field effect. This concept proved to be reliable and such transistors have been realized on a number of different semiconductor material systems. Here, we present focused ion-beam implanted in-plane-gate transistors on standard bulk silicon and some attempts to understand their physics, possibly opening the way to significantly reduce their dimensions from what is now, typically, 1–3 μm into the quantum regime. The transistors are written using a 100 keV gallium ion beam with a focus of 100 nm into a thin arsenic doped n-type layer on a prestructured standard 3 in. p-type (100) silicon wafer and subsequent rapid thermal annealing. The devices are characterized electrically by direct current measurements at room temperature. They show very promising features, for example, a drain saturation current of 35 μA combined with a relatively high differential output resistance of 900 kΩ and a transconductance of up to 29 μS, which is much higher than the value recently obtained on separation-by-implantation-of-oxygen substrates. The voltage dependent spreading of the depletion zones shows a square-root law. The drain current of the devices depends strongly on the substrate bias. It is shown that this is due to a biasing of the lateral gate lines via the substrate, caused by the vertical geometry of the transistors.
Magnetism of perovskite manganese oxide La1/3Nd1/3Ca1/3MnO3
Authors: Rao G. H. , Sun J. R. , Liang J. K.
Published: July 2002

Magnetism of bulk La1/3Nd1/3Ca1/3MnO3 with a magnetoresistance ratio as high as −96% in a field of 0.67 T is investigated. Two shoulders are observed on both thermal magnetization curves and magnetization isotherms. A magnetic inhomogeneity model, i.e., simultaneous occurrence of ferromagnetic La-rich domains and nonferromagnetic Nd-rich domains in the compound, is proposed. The model gives a satisfactory explanation for the experimental results. It is argued that a field-induced magnetic transition occurs in the Nd-rich phase and the coexisting La-rich phase can promote the magnetic transition.
Ni/Si based multilayer for the reflection of soft x rays in the “water window”
Authors: Cilia M. , Verhoeven J.
Published: July 2002

As nickel and silicon have no absorption edges in the “water window” (2.4–4.4 nm), these materials form suitable components for multilayers to be applied as optical components in this wavelength region. The practical feasibility of using these components is limited by their chemical reactivity, resulting in intermixing at the interfaces. A procedure, based on the application of ion implantation and ion beam mixing, has been developed to produce silicon nitride and nickel silicide layers. As these processes also cause ion etching, an additional reduction of the surface roughness of the layers has been observed.
Electrical properties of high energy 120Sn implantation in GaAs
Authors: Narsale Arun , Ali Yousuf Pyar , Bhambhani Uma , Salvi V. P. , Arora B. M. , Kanjilal D. , Mehta G. K.
Published: July 2002

Single crystal n-GaAs substrates have been implanted at room temperature with 120Sn ions at an energy of 70 MeV. The optical microscope and scanning electron microscope studies of the cross section of the implanted samples have shown the formation of two sharp layers at a depth of 8.7 and 11 μm from the surface of the substrate. The electrical characteristics of the Schottky diodes fabricated on the implanted substrates at room temperature indicate the presence of high series resistance due to radiation defects. The electrical properties of the implanted samples were investigated after implantation and annealing to 850 °C. Low temperature resistance measurements of these samples indicate that the samples annealed to 450 °C are dominated by a variable range hopping conduction mechanism, whereas for the samples annealed at 550 and 650 °C the electrical conduction is due to hopping between the neighboring defect sites. At annealing temperatures higher than 650 °C, the electrical transport below room temperature seems to be dominated by carriers in the extended states, which are also responsible for the electrical conduction at room temperature and above, for the samples annealed at temperatures higher than 450 °C.
Effect of long term heat treatment on the electrical properties of RSe1.47 (R=La and Nd) and LaSe1.48
Authors: Michiels J. , Gschneidner K. A.
Published: July 2002

The phase stability of the γ-cubic LaSe1.47, LaSe1.48 and NdSe1.47 compounds with the Th3P4 type structure, along with their electrical resistivity, Seebeck coefficient, and Hall effect were measured periodically as the materials were annealed at 800 °C or 1000 °C to determine their potential as thermoelectric materials for long term applications at high temperatures. The LaSex samples showed the presence of the ternary oxide La10Se14O (β-phase). The content of this phase increased with increasing anneal time, whereas the NdSe1.47 samples showed no sign of a ternary oxide second phase. The room temperature electrical resistivity and Seebeck coefficient of LaSex increased with increased annealing time, while those of NdSe1.47 decreased. The power factors remained relatively stable with annealing time. The high temperature electrical resistivity of LaSe1.47 and NdSe1.47 increased with increasing temperature up to 1000 °C, while the Seebeck coefficient became more negative. The power factor, however, peaks at 800 °C for LaSe1.47 and at 500 °C for NdSe1.47. Based on this study we find that LaSex is not a promising high temperature thermoelectric material due to the formation of the β-phase, whereas the NdSe1.47 which does not form the β-phase oxide is a potential candidate material.
Spatial, temporal, and spectral effects and conversion efficiencies in second-harmonic generation from mode-locked lasers in surface-emitting geometry
Authors: Ding Yujie J. , Khurgin Jacob B.
Published: July 2002

We have theoretically investigated ultrafast surface-emitting second-harmonic (SH) generation from mode-locked laser pulses with arbitrary pulse shapes. The spatial dependence of the SH energy density per pulse is almost the same as the temporal profile of the SH power emitted from the entire surface per waveguide width for a long waveguide. For a sinc fundamental pulse, we have obtained, rich and unique, temporal, spatial, and spectral behavior of the second harmonic. We have found that both the temporal and spectral profiles of the emitted second harmonic depend on the location at the surface. These effects can be observed with lasers currently available, and used for auto correlation, in optical signal processing, optical communications, and practical frequency doubling of ultrafast optical pulses with an advantage of no broadening of the SH pulse. We have compared these results with those for a Gaussian fundamental pulse. After introducing saturation intensity to quantify the saturation regime, we have obtained quantitative expression on the second-harmonic intensity in terms of the pump and saturation intensities.
Wave propagation in an epoxy-graphite laminate
Authors: Clements B. E. , Johnson J. N. , Addessio F. L.
Published: July 2002

The third-order, nonhomogenized, dynamic method of cells is used to calculate the particle velocity for a shock wave experiment involving an epoxy–graphite laminate. Constitutive relations suitable for the various materials are used. This includes linear and nonlinear elasticity and, when appropriate, viscoelasticity. It is found to be beneficial to incorporate artificial viscosity into the analysis. Artificial viscosity successfully removes the unphysical high-frequency ringing in the numerical solutions of the theory, while leaving the physical oscillations, characteristic of wave propagation in a periodic laminate, largely undiminished. It also allows the viscoelastic relaxed moduli to be closer to their unrelaxed counterparts than in a previous calculation, thus making them more acceptable. The results agree well with the corresponding plate-impact experiment, and are compared to the second-order theory of Clements, Johnson, and Hixson [Phys. Rev. E, 54, 6876 (1996)].
Nanostructured Ta-Si-N diffusion barriers for Cu metallization
Authors: Kim Dong Joon , Kim Yong Tae , Park Jong-Wan
Published: July 2002

Diffusion barrier properties of nanostructured amorphous Ta-Si-N thin films with different N concentrations have been investigated in metallurgical aspects of Cu metallization. When the N content exceeds 40 at. %, the Ta-Si-N film remains in the nanostructure phase even after annealing at 1100 °C for 1 h and effectively prevents Cu diffusion after annealing at 900 °C for 30 min. The reason for the excellent thermal stability is that excess N atoms disturb the grain growth of TaSi2 phase and keep the Ta-Si-N film in the nanostructure phase during the high temperature annealing. The Ta-Si-N film with N content less than 40 at. % fails to prevent the Cu diffusion after annealing at 700 °C for 30 min.
Experimental estimation of the hot spot size in Nb-based Josephson tunnel junctions using Abrikosov vortices
Authors: Cristiano R. , Frunzio L. , Pagano S. , Palmieri V. G. , Lisitskii M. P.
Published: July 2002

We report on a new experimental approach to the size estimation of the hot spot induced by ionizing particles in a Josephson tunnel junction. Here, in contrast to the case of a superconducting strip, it is possible to investigate the hot spot dynamics in absence of effects due to the heating induced by the bias current. The reported experiment is based on the motion of Abrikosov vortices, trapped in the thin films constituting the junction electrodes, under 5.6 MeV α-particle irradiation. The fast time evolution of a hot spot, combined with the presence of Abrikosov vortices, produces a change of the static magnetic field in the junction area and thus a change of the critical current value, Ic. Measurements of Ic during the α-particle irradiation and in presence of trapped Abrikosov vortices allow to determine the rate of appearance of those Ic changes. The behavior of the average appearance rate as function of the Abrikosov vortices density provides a direct determination of the maximum hot spot area. The experiment is performed on a high quality Nb/Al–AlOx/Nb junction of circular geometry and with “small” dimensions with respect to the Josephson penetration depth. A value of 4.7±1.2 μm2 is found for the maximum hot spot area.
Current–voltage characterization of the vortex motion in YBa2Cu3O7−δ microbridges and the implications on the development of superconducting flux flow transistors
Authors: Bernstein P. , Picard C. , Pannetier M. , Lecoeur Ph. , Hamet J. F. , Doan T. D. , Contour J. P. , Drouet M. , Regi F. X.
Published: July 2002

In order to develop superconducting thin films or multilayer structures suitable for Abrikosov vortex flux flow transistors, we propose a model to describe the current–voltage characteristics measured on microbridges operating in the flux creep regime and we show how the effective superconducting thickness of the samples, as well as the pinning potential range, the maximum velocity, the pinning energy, and the depinning current of the vortices can be determined. This model is applied to microbridges with structures containing one or several YBa2Cu3O7−δ layers which were deposited either on LaAlO3 or on SrTiO3 substrates. The results demonstrate that most YBa2Cu3O7−δ microbridges exhibit very similar properties and are not suitable for flux flow devices. However, bilayer structures comprised of a YBa2Cu3O7−δ film and a conductive capping layer display interesting properties.
Space-charge limited conduction with traps in poly(phenylene vinylene) light emitting diodes
Authors: Campbell A. J. , Bradley D. D. C. , Lidzey D. G.
Published: July 2002

Current–voltage, impedance, and transient conductance measurements have been carried out on indium-tin-oxide/poly(phenylene vinylene)/Al light emitting diodes. In these devices injection and transport is expected to be dominated by positive carriers. Fowler–Nordheim tunneling theory cannot account for the temperature dependence, the thickness dependence, or the current magnitude of the current–voltage characteristics. Space-charge limited current theory with an exponential distribution of traps is however in extremely good agreement with all of the recorded current–voltage results in the higher applied bias regime (approximately 0.7⩽V/d⩽1.6×106 V cm−1). This gives a trap density Ht of 5(±2)×1017 cm−3 and the product of μNHOMO of between 1014 and 5×1012 cm−1 V−1 s−1. Assuming NHOMO is 1020 cm−3 gives an effective positive carrier mobility between 10−6 and 5×10−8 cm2 V−1 s−1. The characteristic energy Et of the exponential trap distribution is 0.15 eV at higher temperatures (190⩽T⩽290 K), but this decreases as the devices are cooled, indicating that the distribution is in fact a much steeper function of energy closer to the highest occupied molecular orbital (HOMO) levels. The current–voltage characteristics in the lower applied bias regime (approximately V/d⩽0.7×106 V cm−1) can be fitted to pure space-charge limited current flow with a temperature and field dependent mobility of Arrhnenius form with a mobility at 290 K close to the above values. If NHOMO lies between 1021 and 1019 cm−3, then the trap filled limit bias gives a mobility independent value of Ht of 3(±1)×1017 cm−3. Capacitance–voltage measurements show that at zero bias the devices are fully depleted, and that the acceptor dopant density NA must be less than about 1016 cm−3. The impedance results show that the devices can be modeled on a single, frequency independent, parallel resistor-capacitor circuit with a small series resistor. The variation of the resistor and capacitor in the parallel circuit with applied bias and temperature are consistent with the space-charge limited current theory with the same exponential trap distribution used to model the current–voltage characteristics. Initial results for transient conductance measurements are reported. The transients have decay times greater than 300 s and exhibit a power-law dependence with time. This is shown to be exactly the behavior expected for the decay of an exponential trap distribution. Measurements at higher temperatures (290⩾T⩾150 K) give an Et of 0.15 eV, in excellent agreement with that found from the current–voltage measurements. This value of Et is exactly that found by similar analysis of the current–voltage characteristics in negative carrier dominated dialkoxy poly(phenylene vinylene) and Mq3 devices. It is proposed that this bulk transport dominated behavior is purely a consequence of hopping conduction through an approximately Gaussian density of states in which the deep sites act as traps.
The probe dynamics under shear force in near-field scanning optical microscopy
Authors: Wei P. K. , Fann W. S.
Published: July 2002

We present here the results of theoretical as well as experimental studies of shear force interactions in near-field scanning optical microscopy. A continuum mechanical model that considers the realistic tapered fiber shape was used to calculate the shear force during the interactions. The frequency spectra at different probe/sample separations show that the shear force is a combination of elastic and frictional force. The relative strengths of these two forces depend on the tilt angle between the probe and the sample surface.
O
2
18
isotope labeling studies of stress effect on oxidation kinetics
Authors: Imai Keitaro , Yamabe Kikuo
Published: July 2002

It is observed that in sequential two-step thermal oxidation in dry O2 at 800 °C with Ar annealing at 1100 °C following the first oxidation, the oxidation rate increases by nearly four times at the early stage in the second oxidation. Because an intrinsic stress of about 1.4×109 dyne/cm2 induced during the thermal oxidation in the as-grown SiO2 films was sufficiently relaxed following the annealing, the increase in the oxidation rate is supposed to be due to relaxation of the intrinsic oxide stress. For the purpose of investigating this stress effect in terms of the oxygen transport mechanism during thermal oxidation of silicon, O218 isotope oxidation is performed and the isotope profiles in oxides are measured by secondary ion mass spectroscopy. It is found that the bulk O18 concentration in the annealed oxide is much higher than that in the unannealed oxide and increases with the annealing temperature. This strongly suggests that the increase in oxygen concentration is due to the stress relaxation. Since the bulk O18 concentration in the oxide annealed at 1100 °C is over three times higher than that in the unannealed oxide, the increase in oxygen concentration is most likely the cause for the enhanced oxidation. The mechanism of the stress effect is discussed. Furthermore, it is found that the oxygen concentration in oxide on the (111) Si substrate is higher than that on the (100) Si substrate. This could also be explained by the stress effect.
Dependence of high-temperature superconductivity on rare-earth ions
Authors: Blackstead Howard A. , Dow John D.
Published: July 2002

The main facts concerning the dependence of superconductivity on rare-earth ion R in the homologues of RBa2Cu2NbO8, RBa2Cu3O7, RBa2Cu4O8, R2−zCezCuO4, and R2−zCezSr2Cu2NbO10 are organized using the principles that the supercurrent is where the holes are and the superconducting condensate is beyond the pair-breaking range of active rare-earth magnetic moments. Experiments to test this organization are proposed.
Cd self-doping of CdTe polycrystalline films by co-sputtering of CdTe–Cd targets
Authors: Picos-Vega A. , Becerril M. , Zelaya-Angel O. , Ramı́rez-Bon R. , Espinoza-Beltrán F. J. , González-Hernández J. , Jiménez-Sandoval S. , Chao B.
Published: July 2002

Cadmium self-doped CdTe polycrystalline films were grown on Corning glass substrates at room temperature by cosputtering from a CdTe–Cd target. The electrical, structural, and optical properties of the films were analyzed as a function of the Cd concentration. Films with a stoichiometric composition, and slightly below and above it, were prepared. In films where the Te exceeds 50 at. %, it is found segregation of Te and its electrical resistivity is about 107 Ω cm. In those with an excess of Cd, the electrical resistivity drops several orders of magnitude, the carrier concentration increases, and the resistivity activation energy drops. From these results, we concluded that using this deposition method, n-type Cd self-doped CdTe polycrystalline films can be produced.
Microstructure evolution of boron doped homoepitaxial diamond films
Authors: Brunet F. , Germi P. , Pernet M. , Deneuville A. , Gheeraert E. , Laugier F. , Burdin M. , Rolland G.
Published: July 2002

We study the (004) diffraction peaks of (001) homoepitaxial diamond films (deposited by microwave plasma chemical vapor deposition) vs their boron content from 2×1017 to 8×1020 cm−3. The lattice parameter calculated for the relaxed films slightly increases according to the linear Vegard’s law up to 2.7×1020 [B] cm−3, then more rapidly with another linear law. We ascribed this second law to an additional electronic contribution from the hole on the boron impurity band with a positive deformation potential around 16 eV. There is overall agreement between the deduced in-plane stresses and the Raman peak positions. The full width at half maximum of the diffraction peak is dominated by the concentration of residual defects which are lower in the film than in the substrate up to 1020 [B] cm−3.
Effect of high-temperature annealing on electrical and optical properties of undoped semi-insulating GaAs
Authors: Fang Z.-Q. , Reynolds D. C. , Look D. C. , Paraskevopoulos N. G. , Anderson T. E. , Jones R. L.
Published: July 2002

A comprehensive characterization, including room temperature Hall effect, near infrared absorption, temperature dependent dark current and photocurrent (using 1.13 eV light), normalized thermally stimulated current (NTSC), photoluminescence at 4.2 K in both near band edge and deep level regions, and selective pair photoluminescence (SPL) at 2 K, has been carried out on undoped semi-insulating GaAs samples, cut from four wafers which were grown by the low pressure liquid encapsulated Czochralski technique and annealed by three different schedules: a 1100 °C anneal with either fast or slow cooling, or a 1000 °C standard anneal. The 1100 °C anneal clearly introduces higher concentrations of NTSC traps near 0.3 and 0.5 eV, a PL center at 0.8 eV, and acceptor centers, which are mainly due to the point defects and increase the resistivity. Slow cooling to some extent reduces all of these additional centers. The SPL measurements show changes in the relative intensities of C, Zn, and Si related emissions with changes in annealing conditions.
The variation of microstructure in Czochralski silicon induced by low-high two step anneal
Authors: Kung C. Y. , Liu C. M. , Hsu W. , Horng R. H.
Published: July 2002

Systematic low-high two-step and high-low-high three-step annealing studies were conducted to investigate the microdefects generated in Czochralski silicon wafers. It was found that cluster precipitates entangled with dislocations are annihilated during extended low temperature anneal. A model involving the recombination of vacancies with interstitials is proposed to explain this phenomenon.
Photoionization cross-section of the DX center in Te-doped AlxGa1−xSb
Authors: Baraldi A. , Ghezzi C. , Magnanini R. , Parisini A. , Tarricone L. , Zerbini S.
Published: July 2002

The photoionization of DX centers in Te-doped AlxGa1−xSb layers grown by molecular beam epitaxy is investigated by measuring the increase of the Hall free electron density after illumination by monochromatic light in the temperature range typical of the persistent photoconductivity (PPC) effect. The investigated samples have AlSb molar fractions in the 0.3⩽x⩽0.5 range and n-type doping in the 1017−1018 cm−3 range. An accurate investigation of the isothermal photoionization transients is performed to evidence features in the curve not directly related to the phenomenology of the DX center, the free electron density being influenced by the possible occupancy of other impurity levels. The transients show, in particular, an initial nonexponential behavior which is demonstrated as due to localization of a fraction of the photoexcited electrons into a nonmetastable impurity state which is responsible for the semiconductor-to-metal transition observed under the PPC regime. When this effect is accounted for, the dependence of the photoionization cross section of the DX center on the photon energy was obtained from the analysis of the linear part of the transients and analyzed through a model given in the literature. The analysis gives values of the optical ionization energy and of the Frank-Condon shift varying in the ranges of 0.84–0.95 and 0.70–0.74 eV, respectively, depending on the alloy composition. This confirms a large lattice relaxation for the DX center related to the Te-impurity in AlxGa1−xSb.
Thickness monitoring of GaAs growth by surface photoabsorption in metalorganic chemical vapor deposition
Authors: Nakamura F. , Kim Y. D. , Yoon E. , Forbes D. V. , Coleman J. J.
Published: July 2002

We have monitored monolayer (ML) growth of GaAs at 650 °C by metalorganic chemical vapor deposition using surface photoabsorption (SPA). AlGaAs quantum well (QW) structures were grown by monitoring SPA oscillations. Photoluminescence measurements of these QWs show excellent agreement between the observed spectra and model calculations in both peak energies and line width, confirming that the growth rate was 1 ML per cycle and that the roughness at the interface was <1 ML.
Charge accumulation in GaAs/AlGaAs triple barrier resonant tunneling structures
Authors: Buckle P. D. , Dawson P. , Kuo C. Y. , Roberts A. H. , Truscott W. S. , Lynch M. , Missous M.
Published: July 2002

In this article we present photoluminescence and photoluminescence excitation spectroscopy data from three triple barrier resonant tunneling structures. The spectroscopic techniques are used to estimate the charge accumulation in both tunneling quantum wells of the devices as a function of bias. The charging behavior is extremely asymmetrical, with significant charge accumulation only in the quantum well adjacent to the emitter region of the device and not in the quantum well adjacent to the collector region, irrespective of the direction of bias. This asymmetry in the charging behavior is analogous to highly asymmetrical double barrier resonant tunneling structures. However, due to the two quantum wells present in the triple barrier design it provides a more flexible system to study charge density dependent effects. We also present evidence for negatively charged exciton formation in the first quantum well for both directions of applied bias.
Approximate model of diffuse reflectance from rough polymer surfaces
Authors: Bendler J. T. , Feldman S. F. , Hatti H. , Hobbs S. Y.
Published: July 2002

A simplified version of the Kirchhoff model for the overall reflectance from a rough conducting surface is presented and tested against data obtained on roughened polymer surfaces using a goniospectrophotometer. Samples were injection molded using textured molds having a range of roughness scales and then coated with 600 Å of aluminum. The surfaces were characterized by mechanical profilometry and relevant mathematical descriptors of the surface topographies were extracted by digital image analysis. The calculated and measured reflectance values agree well for incidence angles of 20° and 60° over a wide range of reflected intensities. At higher angles (82°), the measured reflectance values are more specular. Possible reasons for these discrepancies are discussed.
Far infrared induced multi-mode pumping reveals deviations from parabolicity in a ballistic quantum channel
Authors: Tageman Ola , Gorelik L. Y.
Published: July 2002

We consider, theoretically, resonant multi-mode pumping induced by a coherent far infrared field, in a ballistic quantum channel, created in a gated GaAs/AlGaAs heterostructure. For frequencies that enable momentum conservation in a transition between the lowest and a higher mode, we predict complete depopulation of the lowest mode, due to Rabi oscillations. These frequencies are characteristic of the mode spectrum and are therefore closely related to the confining potential. If all but the lowest modes are filtered out by a quantum point contact, we expect the depopulation effect to completely block the transport of current. In addition to providing a spectroscopic tool we believe that the resonance effect may find use as a THz frequency demodulator or as a transistor device.
Laser power and temperature dependence of the transient photoimpedance response of epitaxial YBa2Cu3O7−δ thin films
Authors: Zhao Y. G. , Ogale S. B. , Shreekala R. , Dong Z. W. , Pai S. P. , Rajeswari M. , Venkatesan T. , Cao W. L. , Lu W. , Lee Chi
Published: July 2002

The effect of laser power on the transient response of photoimpedance and the in situ sample temperature is studied for epitaxial YBa2Cu3O7−δ (YBCO) films at a wavelength of 810 nm using 100 fs laser pulses. The temperature dependences of the dc resistance and the amplitudes of the fast and slow photoresponse signals were measured simultaneously. For laser energy density of 20 μJ/cm2 per pulse (average power 22 mW), the average sample temperature is found to increase by about 1 K for 300 nm thick YBCO film with 0.5 mm thick LaAlO3 substrate as shown by the shift of resistance versus temperature curves. Calculations of time constant show that heat diffusion in LaAlO3 is the bottleneck for heat escape which causes the observed increase of the sample temperature. The amplitudes of both slow and fast signals show a peak in the temperature dependence curves near the superconducting transition temperature; and the peak temperature decreases, while the peak amplitude increases with laser power. At 82 K, the laser power dependence of the fast signal amplitude showed nonlinear behavior above 22 mW. These data were analyzed in terms of the kinetic inductance model and bolometric mechanism for the fast and slow signals, respectively. The temperature variation owing to laser pulse has been taken into consideration in this analysis.
Electronically enhanced kink motion on 30° partial dislocations in Ge directly observed by plan-view high resolution electron microscopy
Authors: Inoue Masafumi , Suzuki Kunio , Amasuga Hirotaka , Mera Yutaka , Maeda Koji
Published: July 2002

In the high resolution electron microscopic (HREM) images of dislocations in Ge obtained with the electron beam incident normal to the slip plane, 30° partial dislocation lines were extracted by image processing, and kinks in them were identified at atomic resolution. Analysis of sequential images taken from the same dislocation lines revealed that (1) geometrical kinks of different signs, 30° (left) and 90° (right) kinks, are both mobile even at room temperature (2) generation of kink pairs occurs, albeit less frequently, in the course of HREM observations. These results are reasonably understood when considering that the migration of kinks, at least one of the two types, and the formation of the smallest double kinks are enhanced by the electronic excitation that is caused by the electron beam used for the HREM observations.
Surface modification of n-GaAs by 50 MeV silicon ions
Authors: Hullavarad Shiva S. , Railkar T. A. , Bhoraskar S. V. , Madukumar P. , Gokarna A. S. , Bhoraskar V. N. , Badrinarayanan S. , Pawaskar N. R.
Published: July 2002

An effective passivation of the surface states in n-GaAs has been achieved by high energy (50 MeV) silicon ion irradiation at a fluence of 1×1013 ions cm−2. The effect of passivation on the nature of defects has been studied by the technique of thermally stimulated exoelectron emission (TSEE) measurements. Consequently an enhancement in the intensity of the band edge, photoluminescence (PL) has also been noticed. The results of TSEE and PL measurements are augmented by surface analysis using x-ray photoelectron spectroscopy. An effective migration of silicon was observed from the buried layer towards the surface by radiation enhanced outdiffusion process. A stable passivating layer of silicon dioxide was found on GaAs surface which has been accounted for the enhancement in the PL intensity.
Effects of gallium doping on the properties of amorphous-SiC:H films prepared by magnetron cosputtering
Authors: Saito N. , Inui Y. , Yamaguchi T. , Nakaaki I.
Published: July 2002

Amorphous SiC:H films have been doped with gallium. The films were prepared by magnetron cosputtering of silicon and gallium in an atmosphere of Ar and CH4. The effects of gallium concentration (z) on the structural, optical, electrical, and optoelectronic properties of the films were investigated. Doping at even very low Ga concentrations below Z=10−3 results in a reduction of both the dark and the photoconductivity, which were found to be decreased by more than two orders of magnitude, and the activation energy of the dark conductivity is increased by about 30%. An increase of z up to 10−2 leads to further reduction of the photoconductivity without any significant change of its activation energy, while the dark conductivity slightly increases and the corresponding activation energy drops to about 50% of its value observed for z<10−3. The optical band gap remains unchanged at such low Ga concentrations. These results indicate that the n-type conduction of the undoped film confirmed by a thermoelectric power experiment is compensated by the gallium doping; the type of conduction shifts from n to weak p with increasing z. At higher Ga concentrations z>10−2, infrared absorption measurements indicate a strong reduction of the H concentration in the films, which was taken as evidence for a distortion of the amorphous network by excessively incorporated Ga atoms, which suppress the attachment of H to dangling Si bonds. In this fluence regime the optical band gap shrinks by about a factor of 2 and the photoconductivity becomes quenched, while the dark conductivity increases by about six orders of magnitude.
Unified model for the luminescence and transport data in self-supporting porous silicon
Authors: Mehra R. M. , Agarwal Vivechana , Singh Vijay A. , Mathur P. C.
Published: July 2002

The results of a combined study of the temperature variation of photoluminescence (PL) and dc conductivity from high optical-quality free-standing porous silicon films are presented. PL spectra of the samples have been studied in the range 10–300 K. Dc conductivity (σ) on these samples have been studied from 90 to 300 K in the voltage range 0.25–1.5 V. We attempt to understand the luminescence and transport data in terms of a recently proposed unified model. The conductivity (σ) exhibits a Berthlot-type temperature variation [≅exp(T/TB)]. The luminescence data can be explained in terms of a competition between an activated radiative process [≅ exp−(Tr/T)] and a Berthlot-type hopping process. The Berthlot temperature “TB” obtained independently from the transport and luminescence data are in approximate agreement.
Large second-order optical nonlinearity in a molecular crystal formed from a chiral smectic liquid crystal
Authors: Sprunt S. , Geer R. E. , Crawford G. P. , Naciri J. , Ratna B. R. , Shashidhar R.
Published: July 2002

A new approach to achieving a large second-order nonlinear optical susceptibility in a chiral liquid crystal is described. This approach uses an aligned smectic-C* liquid crystal as a template for forming a molecular crystal, which freezes out the rotation of the molecular dipoles and thereby greatly enhances the noncentrosymmetric order of the structure. Analysis of the measured second harmonic generation gives maximum values of individual susceptibility components of ∼25 pm/V, which are stable when the field is removed.
Ultrasonic beam reflection from fluid-loaded cylindrical shells
Authors: Cloutier T. J. , Safaeinili A. , Chimenti D. E. , Zeroug Smaine , Felsen L. B.
Published: July 2002

The interaction of ultrasonic acoustic transducers with cylindrical shells in pitch-catch mode has been measured and predicted using a two-dimensional theoretical model. The measurements of the received voltage amplitude at selected frequencies and incidence angles are presented as a function of scanned angular position at fixed radius and beam launch angle. In the forward model, the received voltage is expressed in terms of a spectral integral whose integrand contains the spectral amplitudes of the transmitter and the receiver, and a spectral global reflection coefficient that accounts for the pressure wave interaction with the fluid- or air-loaded shell. The transmitter and receiver are modeled using the complex-transducer-point technique to simulate emitting and receiving electro-acoustic transducers with Gaussian profiles. The receiver voltage integral is reduced by uniform high-frequency saddle point asymptotics to closed form solutions for the dominant contributions from the specular reflected and (excited) leaky wave fields. We find generally good agreement between the model prediction and the experimental results, particularly near the voltage maximum. Experimental data are also shown for a solid cylinder, demonstrating in a time-angle plot the existence of whispering gallery modes, which is verified analytically.
Diffusion with multiple kinds of trapping sites
Authors: Kasuya Tadashi , Fuji Masao
Published: July 2002

Since diffusion in solid is influenced by trapping sites which have different binding energies, analytic solutions to describe diffusion with multiple kinds of trapping sites have been derived for dilute systems without using the assumption of local equilibrium. Numerical comparisons with the local equilibrium treatment show that the transport of free atoms evaluated by the present solutions is faster at short times than that of the local equilibrium treatment. As time passes, the difference becomes small. The present solutions also show that the transport of free atoms is influenced by the initial concentrations at trapping sites, and that the apparent transport is accelerated in some cases. Using the present solutions, systematic investigations of diffusion phenomena with many kinds of trapping sites become possible.