bluuebunny/crossref_metadata_2025 huggingface dataset:
In situ, real-time observation of Al chemical-vapor deposition on SiO2 in an environmental transmission electron microscope
Authors: Drucker Jeff , Sharma Renu , Weiss Karl , Kouvetakis John
Published: July 2002

A technique for in situ observation of chemical-vapor deposition (CVD) is demonstrated. An environmental transmission electron microscope with 3.8 Å resolution was used for real-time, in situ observation of Al CVD onto SiO2 from trimethylamine alane (TMAA). Nominal TMAA pressures from 50 to 400 mTorr and substrate temperatures from 50 to 200 °C were investigated for deposition onto both TiCl4 pretreated and untreated surfaces. For deposition onto untreated surfaces, low nucleation densities and polycrystalline, dendritelike growth were observed for all deposition temperatures with deposition commencing at about 150 °C. Deposition on TiCl4-treated surfaces resulted in polycrystalline films with grain sizes of a few nm and lower-temperature (100 °C) growth.
Europium diffusion enhancement in lithium niobate by GeV gadolinium ion irradiations
Authors: Ramos S. M. M. , Brenier R. , Canut B. , Fuchs G. , Thevenard P. , Treilleux M. , Meftah A. , Toulemonde M.
Published: July 2002

Single crystals of lithium niobate (LiNbO3) with y-cut orientation were irradiated at GANIL with 5.17 MeV/amu 155Gd ions at a fluence ranging from 1.5×1011 to 6.0×1011 ions cm−2. After irradiation the samples were implanted with europium ions of 70 keV energy at a fixed fluence of 5×1016 ions cm−2. Both irradiations and implantations were performed at room temperature. Thermal treatments in air, in the range 300–1100 °C, were applied to investigate the influence of the preirradiation damage on the thermal evolution of Eu implanted LiNbO3 crystals. Transmission electron microscopy micrographs exhibit the amorphous track formed by high-energy irradiations. Rutherford backscattering spectrometry results show that these amorphous tracks enhance the europium diffusion in depth of the LiNbO3. A diffusion coefficient and an activation energy were estimated to be about 4.2×10−15 cm2 s−1 and 0.42 eV, respectively.
Sputter-induced grain boundary junctions in YBa2Cu3O7−x thin films on MgO
Authors: Vuchic B. V. , Merkle K. L. , Dean K. A. , Buchholz D. B. , Chang R. P. H. , Marks L. D.
Published: July 2002

A low voltage argon ion sputter technique was used to form grain boundary junctions in YBa2Cu3O7−x thin films on MgO. The YBa2Cu3O7−x thin film grown on a pre-sputtered region of MgO was rotated 45° about the [001] axis relative to the YBa2Cu3O7−x thin film grown on an adjacent unsputtered region of the substrate. YBa2Cu3O7−x thin films were grown using pulsed organometallic beam epitaxy (POMBE). The current-voltage and resistance-temperature characteristics of individual grain boundary junctions demonstrated weak-link-type behavior. Sputter-induced 45° grain boundary junctions are advantageous in device applications because they are planar and simple to form in many configurations.
Study of magnetization relaxation on zone-melted YBa2Cu3O6+x superconductors
Authors: Qin M. J. , Jin X. , Ji H. L. , Shi Z. X. , Yao X. X. , Fan Z. G. , Shan Y. Q.
Published: July 2002

The analytical magnetization versus time expressions for several flux-pinning models for high-Tc superconductors are deduced. Magnetic relaxation and hysteresis studies have been performed on a zone-melted YBa2Cu3O6+x sample by SQUID. Nonlogarithmic magnetization decay is observed at relatively high temperatures; we attribute this to the nonlinear U(J) relationship which may arise from the local pinning potential Up(x), rather than collective pinning. At 70 K, the experimental data fits extremely well to the expression M(t)=M0+a(T)ln ln(t/τ) which is the result of the exponential U(J) relationship. Based on this U(J) law, the local pinning potential is determined to be Up(x)=U0(x/x0)[1−ln (x/x0)]. The voltage–current E(J) resulting from the observed U(J) relationship, as well as the role of the Y2BaCuO5 (211) phase in zone-melted YBCO is discussed.
Diffuse scattering of x rays from nonideal layered structures
Authors: Kopecký M.
Published: July 2002

A new theory of nonspecular x-ray scattering from layered systems with random rough interfaces based on the distorted-wave Born approximation is presented. Calculations of the diffuse scattering from a single gold layer and two W/Si multilayer mirrors has been carried out. The theory explains the existence of maxima and minima in the angular distribution of diffusely scattered intensity resulting from standing-wave-enhanced scattering and other dynamical effects. The influence of the mutual correlation between individual interface profiles on x-ray scattering is discussed.
Modified sheet resistance of ohmic contacts to p-type In0.47Ga0.53As/InP
Authors: Leech Patrick W.
Published: July 2002

The parameter known as the modified sheet resistance Rsk has been used in the modeling of ohmic contacts to define the region of alloying/reaction immediately below the metallization. In this study, the value of Rsk has been measured for Pd/Zn/Pd/Au, Pd/Au, Zn/Pd/Au, Au/Zn/Au, Ni/Zn/Ni/Au, Cr/Au, and Pd/Mn/Sb/Pd/Au contacts to In0.47Ga0.53As/InP as a function of the annealing temperature to 500 °C. In the as-deposited contacts, the measured value of Rsk was lower than the sheet resistance outside of the contact, Rsh, by a factor of ∼2 to 3. This reduction in Rsk from Rsh has been attributed to (a) the formation of low-resistivity phases at the interface and (b) the creation of a doped region by indiffusion of the Zn. In addition, in the Pd/Zn/Pd/Au contacts, the effect of varying the thickness of the layer of Zn between 0 and 400 Å on Rsk was examined. A thickness of Zn≥50 Å was required in order to produce a significant reduction in Rsk from the value for the Pd/Au (Zn=0 Å) contacts. This effect was enhanced by annealing at 500 °C, which produced a value of Rsk lower than Rsh by a factor of ∼10. The reduction in Rsk by annealing treatment was consistent with an enhanced formation of the interfacial phases and an increase in the rate of indiffusion of the Zn. The exceptions to these trends were in the Pd/Mn/Sb/Pd/Au contacts which showed an approximately constant value of Rsk and Pd/Au which showed a minor reduction in Rsk with increase in annealing temperature.
Relationship between hydrogenation and optical properties of dielectric a-SiC:H films prepared by tetrakis(trimethylsilyl)silane in remote H2 plasma
Authors: Wickramanayaka Sunil , Nakanishi Yoichiro , Hatanaka Yoshinori
Published: July 2002

The effect of hydrogen content in dielectric a-SiC:H films, fabricated in remote H2 plasma using tetrakis(trimethylsilyl)silane as the source gas, on the optical energy gap Eopt and the photoluminescence (PL) was investigated. The amount of hydrogen bonded to C in the fabricated a-SiC:H films was three orders of magnitude higher than that bonded to Si. The total hydrogen content of the films decreased with an increase of substrate temperature maintained during the deposition. Maximum Eopt, 3.2 eV, was observed for the sample prepared at room temperature in which the hydrogen content was about 7.2×1022 cm−3. This sample shows blue-white PL with the peak energy at 475 nm. After this sample was annealed at 400 °C, the PL peak energy shifted to 425 nm with an improved chromaticity, even though the hydrogen concentration was reduced by about 14%. Further reduction of hydrogen after annealing at 600 °C resulted in a decrease of Eopt. The variation of Eopt is observed to depend on the density of unoccupied dangling bonds and the nature of Si—C bonding structure with the former appearing to be the prime factor in controlling the Eopt.
Space-charge oxidant diffusion model for rapid thermal oxidation of silicon
Authors: Kazor A.
Published: July 2002

A model based on space-charge assisted diffusion of O− is proposed to describe the rapid thermal oxidation kinetics of silicon in dry oxygen ambient. The space-charge region within the growing oxide layer is assumed to have a constant planar charge density which results in a characteristics length, λc, the extend of the space-charge region. The diffusion of O− and therefore the oxide growth rate will be driven by two components, the concentration gradient and space-charge drift. The discrepancy in the calculated values of diffusion activation energies for O− is explained by comparing the strength of the space-charge regions. The strength of space charge depends on the range of photon energies irradiated by the heating lamps used in different rapid thermal processors.
Thermoelectric figure of merit of n-Hg1−xCdxSe
Authors: Sofo Jorge O.
Published: July 2002

The Seebeck coefficient and conductivity of Hg1−xCdxSe is calculated for 0<x<0.5, at temperatures below 400 K, and donor concentration between 1016 and 1018 cm−3. These transport coefficients are obtained by solving the Boltzman equation including carriers scattering with optical and acoustic phonons, ionized impurities, and alloy potential. The thermoelectric figure of merit is calculated using these coefficients and measured values of the thermal conductivity. In spite of the very low thermal conductivity of this material (≊10 mW/cm K) the thermoelectric figure of merit times temperature is lower than 0.3 at 300 K, and decreases monotonically with decreasing temperature.
Coherent states of two-dimensional Josephson-junction networks
Authors: Krech W. , Platov K. Yu.
Published: July 2002

We have investigated numerically the phase-locking behavior of two-dimensional Josephson-junction arrays, taking into account a finite inductance (l≳1) of the unit cell and external magnetic fields. Within this model we demonstrate the existence of multivalued I-V curves of the network. Different branches of the I-V curve correspond to in-phase and anti-phase coherent states with high and low levels of output power, respectively. The arrays show hysteretic transitions between these states in external magnetic fields.
Biepitaxial Josephson junctions with high critical current density based on YBa2Cu3O7−δ films on silicon on sapphire
Authors: Boikov Yu. A. , Ivanov Z. G. , Vasiliev A. L. , Claeson T.
Published: July 2002

High-current-density Josephson junctions have been produced in high-temperature superconducting YBa2Cu3O7−δ films deposited on silicon on sapphire and using biepitaxial grain boundaries. The technique is estimated to be useful in integrating superconducting and semiconducting components. A multilayer system of epitaxially grown films was used to form the junctions. A double buffer layer CeO2/ZrO2(9.5% Y2O3) prevented interactions between YBa2Cu3O7−δ and Si during the high-temperature deposition and promoted formation microcrack-free films with a critical current density of 2×106 A/cm2 at 77 K. A MgO seed layer, with (001)MgO∥(001)CeO2 orientation, was used to induce a 45° crystallographic grain boundary in YBa2Cu3O7−δ at its edge. An additional epitaxial buffer double layer of YBa2Cu3O7−δ and SrTiO3 on top of the seed layer promoted the formation of a grain boundary of better crystallinity and stoichiometry. It improved the critical current of the junction about tenfold and resulted in characteristic IcRn products of 150 μV at 77 K in microbridges crossing the grain boundary. Microwave-induced steps were detected at 77 K up to voltages corresponding to the characteristic IcRn value. Peak-to-peak responses to superconducting quantum interference devices reached values of 7 μV at 77 K.
Flux pinning and motion of high-J
c Tl(2223) Ag-sheathed tapes
Authors: Sun Yuping , Zhang Faipei , Jiang Jianyi , Hu Anming , Du Jiaju , Zhang Yuheng , Sheng Z. Z.
Published: July 2002

The dependence of resistance on temperature has been measured for a Tl-system 2223-phase Ag-sheathed tape with Jc(Jc=1.5×104 A/cm2, 77 K, 0 T) in an applied magnetic-field range from 0 to 0.8 T. The experimental results can be explained in terms of the thermally activated flux creep model. The relationship between pinning potential U0 and the applied magnetic field H oriented parallel to and perpendicular to the tape surface is, respectively, U0∥=0.12H−0.22 eV and U0⊥=0.065H−0.35 eV. The relationship between pinning potential U and T follows: U(T,H)=U0(H)(1−t) (t=T/T0,T0=141 K). When H and the measuring current I are laid in the same tape surface, the broadened resistive transition is independent of the configuration between H and I, namely, an energy dissipation is independent of a macroscopic Lorentz force. The phenomenon is accounted for according to the two-dimensional pancakelike vortex-antivortex model based on the characteristic of a stronger intrinsic plane pinning of layer superconductors.
Effect of quantum carrier and escape time on frequency chirp in multiple-quantum-well lasers
Authors: Wartak M. S. , Makino T. , Chen Y.
Published: July 2002

The ‘‘reservoir’’ model of Kan, Vassilovski, Wu, and Lau [Appl. Phys. Lett. 61, 752 (1992)] is extended to analyze frequency chirp in separate confinement heterostructure (SCH) multiple-quantum-well lasers. Small-signal analysis is performed and compared between linear and logarithmic material gain. It is shown that there is an optimum value of SCH width at which frequency chirp becomes minimum.
Crystal plasticity simulations of thermal stresses in thin-film aluminum interconnects
Authors: Povirk G. L. , Mohan R. , Brown S. B.
Published: July 2002

Thermal stresses that develop in aluminum interconnects upon cooling from an annealing temperature are investigated using crystal plasticity theory. The crystal plasticity model is implemented assuming either isotropic hardening or a physically based hardening model developed for single crystals. The dependence of the predicted thermal stresses on crystal orientation and on interconnect aspect ratio is examined. The results are compared with recent observations of electromigration-induced transgranular voids to ascertain the contribution of the thermal stresses in the failure of aluminum interconnects. The analysis clearly demonstrates the necessity of accounting for all slip systems when modeling the effects of texture in single-crystal thin films. In addition, the calculations suggest that proper consideration of the hardening description is necessary for accurate predictions of thermal stresses.
A model hyperfrequency differential-mobility for nonlinear transport in semiconductors
Authors: Varani L. , Vaissiere J. C. , Nougier J. P. , Houlet P. , Reggiani L. , Starikov E. , Shiktorov P. , Gruzhinskis V. , Hlou L.
Published: July 2002

We present analytical expressions for the differential-mobility spectra which are obtained from a linear analysis of the balance equations under stationary and homogeneous conditions. The expressions are rigorously related to an eigenvalue expansion of the response matrix and are applicable to ohmic as well as to non-ohmic conditions. The coefficients appearing in the formula can be calculated from the knowledge of three parameters as functions of the electric field, namely, the reciprocal effective mass, the drift velocity, and the average energy of the carriers. The theory is applied to the case of holes in Si at T=300 K and validated by comparison with the results obtained by a direct numerical resolution of the perturbed Boltzmann equation.
Population mechanisms of the green Er3+:LiYF4 laser
Authors: Pollnau M. , Lüthy W. , Weber H. P.
Published: July 2002

In computer simulations the mechanisms that lead to room-temperature continuous-wave green upconversion lasing in Er3+:LiYF4 are investigated. The rate-equation system considers the full erbium level scheme up to 2H9/2, ground-state depletion, excited-state absorption on the pump and laser wavelengths, three interionic processes, stimulated emission, and the crystal and resonator data of the experiments. Experimental results performed at the University of Hamburg, Germany, are reproduced in the simulation. The influence of different parameters as pump wavelength, absorption cross sections, interionic parameters, dopant concentration, and temperature is investigated. An avalanche effect which exploits the strong cross relaxation from the upper laser level and the upconversion from 4I13/2 leads to an efficient population of the upper laser level. At higher dopant concentrations the cross relaxation becomes detrimental to stimulated emission due to the depletion of the upper laser level. This concentration dependence can be considered as a general behavior of rare-earth-doped avalanche lasers.
Formation of thin AlN films on NiAl(001) upon thermal decomposition of ammonia
Authors: Gassmann P. , Bartolucci F. , Franchy R.
Published: July 2002

The formation of thin AlN films on NiAl(001) has been studied by means of high resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and Auger electron spectroscopy. The AlN films were grown by the adsorption of NH3 on NiAl(001) at T=80 K and subsequent thermal decomposition at elevated temperatures. After annealing to T=1250 K, a distinct LEED pattern appears which exhibits pseudo-twelvefold symmetry. This indicates the formation of two hexagonal domains of AlN which are rotated by 90° with respect to each other. HREEL spectra of the ordered AlN film show a Fuchs–Kliewer phonon mode at 865 cm−1 in good agreement with theoretical spectra calculated on the base of the dielectric theory. The electronic energy gap of the thin AlN films is determined to be Eg≂6.1 eV.
Polycrystalline interlayer formed by deposition of thin-film iridium on silicon
Authors: Ballesteros C. , Rodriguez T. , Jimenez-Leube J. , Clement M.
Published: July 2002

As-deposited iridium-silicon interface structures have been characterized by cross-section high-resolution transmission electron microscopy. Iridium films were deposited by an e-beam gun on (100) Si wafers heated at 200 °C during the deposition. A planar polycrystalline interfacial layer, 2–3 nm thick, was formed between the iridium and the silicon substrate. Thermodynamic calculations are presented which describe the development of the interfacial layer. Correlation between the observed structure and that proposed by thermodynamic calculations is discussed.
Space charges of different mobilities: A novel data processing method
Authors: Zhang Yewen , Lewiner Jacques , Alquié Claude , Hampton Nigel
Published: July 2002

A new way to process the information obtained in space-charge distribution measurements is presented. It is applied to data given by the pressure wave propagation method. This method can be carried out with or without an applied voltage. It is shown however that it does not always allow, under dc stress, for quantitative evaluation of the charge density, if the material contains space charges of very different mobilities, and particularly of high mobility. The new processing technique described here overcomes this difficulty. It is illustrated by application to cross-linked polyethylene samples. This permits the discrimination of two different kinds of space charge. One is the normal space charge which has been reported many times, the second is a type that displays a high mobility. This charge is not directly visible even at very short times after the suppression of the applied voltage. In these measurements, this very mobile space-charge is always seen as heterocharge near the anode.
Fabrication of inherently stable and adjustable contacts of atomic size
Authors: Muller C. J. , de Bruyn Ouboter R.
Published: July 2002

As dimensions of devices shrink new phenomena are revealed. We present a technique which allows the dimension of a point contact to ‘‘shrink’’ down to a single atom. Furthermore, this technique enables us to adjust a tunnel barrier between two inherently stable electrodes. Utilizing this technique we show that it is possible to apply a pressure or a tensile force on a single atom contact and study the resulting effects on the transport properties of this microscopic structure.
Temperature dependence of intrasubband relaxation time and its influence on high-temperature characteristics of InP-based quantum-well lasers
Authors: Seki Shunji , Yokoyama Kiyoyuki , Sotirelis Paul
Published: July 2002

By taking into account the temperature dependence of the intrasubband relaxation times for electrons and holes, we study the temperature sensitivity of the optical gain and of the gain saturation coefficient for 1.3-μm InP-based strained-layer (SL) quantum-well (QW) lasers. The band structures are obtained by the self-consistent numerical solution of the Poisson equation, the scalar effective-mass equation for the conduction band, and the multiband effective-mass equation for the valence band. The intrasubband relaxation times are then calculated within the fully dynamic random phase approximation including carrier-carrier and carrier-phonon interactions on an equal basis. We demonstrate that the temperature dependence of the intrasubband relaxation times plays a key role in determining the temperature sensitivity of the optical gain and of the gain saturation coefficient of InP-based SL-QW lasers.
Study of band-gap narrowing effect and nonradiative recombination centers for heavily C-doped GaAs by photoluminescence spectroscopy
Authors: Yan B. P. , Luo J. S. , Zhang Q. L.
Published: July 2002

The band-gap narrowing effect of GaAs as a function of carbon doping concentration has been measured using photoluminescence (PL) spectroscopy on samples grown by metalorganic chemical-vapor deposition. The range of carbon concentration varies from 3.4×1018 to 1.1×1020 cm−3. The experimental results obtained from PL spectra taking tailing effects into account are in good agreement with recent theoretical calculations. The intensity of PL spectra decreases rapidly when the free-carrier concentration is higher than about 4×1019 cm−3. This phenomenon cannot be explained with only the varying tendency of the minority-electron lifetime of the radiative recombination process, indicating the introduction of additional nonradiative recombination centers in heavily C-doped GaAs.
Photoluminescence study of Sr+ ion-implanted GaAs
Authors: Shen Hong-Lie , Makita Yunosuke , Dittrich Werner , Kimura Shinji , Tanoue Hisao , Yamada Akimasa , Iida Tsutomu , Obara Akira
Published: July 2002

Low-temperature photoluminescence experiments were carried out for Sr+ ion-implanted GaAs substrates as a function of Sr concentration and excitation intensity. It was found that the Sr impurity produces four emissions denoted by (Sr°, X), ‘‘G1’’, ‘‘G2’’, and ‘‘H’’ in the near band emission region in GaAs grown by molecular beam epitaxy. ‘‘G1’’ emission presents a red shift with increasing Sr concentration, while ‘‘G2’’ does not shift. In a Sr+ ion-implanted impure GaAs substrate grown by the liquid-encapsulated Czochralski method, all four of the above emissions were missing. Instead there appeared two other emissions, SM1 and SM2, which are considered to be related to two deep acceptor levels of Sr in GaAs. All results indicate that the Sr impurity produces both shallow and deep energy levels in GaAs and a small amount of the residual impurity will quench the emissions related to shallow energy levels.
Transmission electron microscopy and atomic force microscopy analysis of Nb-Al-AlOx-Nb superconducting tunnel junction detectors
Authors: Rando N. , Videler P. , Peacock A. , van Dordrecht A. , Verhoeve P. , Venn R. , Wright A. C. , Lumley J.
Published: July 2002

The performance of photon detectors based on superconducting tunnel junctions are related to their current - voltage (I-V) curve characteristics and, ultimately, to the quality of the thin tunnel barriers (of order 1 nm) which separate the two superconducting thin films. Both the optimization of the spectroscopic performance of these detectors and the development of a reproducible and high yield fabrication route, require a better understanding of barrier quality and growth techniques. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) provide valuable tools for the investigation of the barrier region and for the control of the quality of the different thin films and related interfaces. In this paper, the results of a TEM and AFM evaluation of Nb-Al-AlOx-Nb tunnel junctions are reported, together with their interpretation on the basis of the I-V curve performance at low temperature (T≥0.3 K). Thickness disuniformities of the Al plus AlOx overlayer and evidence of barrier defects have been found, which may place constraints on the spectroscopic performance of such devices. Through the use of TEM it has also been possible to confirm the epitaxial nature of the Nb base electrode. The junction counter electrode however appears to be polycrystalline, with a columnar morphology and an average grain width of 40 nm. The overall structure of the various layers may well place constraints on the tunneling characteristics of the device.
Nonparabolicity effects and cyclotron mass in GaAs-(Ga,Al)As superlattices in an in-plane magnetic field
Authors: Bruno-Alfonso A. , Diago-Cisneros L. , de Dios-Leyva M.
Published: July 2002

A quantum-mechanical calculation of the conduction cyclotron mass in GaAs-(Ga,Al)As superlattices in an in-plane magnetic field is performed. Calculation is carried out in the envelope function approximation using the parabolic and nonparabolic models for the conduction band. The cyclotron mass is studied as a function of the Al concentration x in the barrier for different magnetic fields. A discussion of the influence of nonparabolicity on the x dependence of the cyclotron mass and a comparison with previously reported experimental data are presented.
Evolution of microcracks in brittle solids under intense dynamic loading
Authors: Wang Ze-Ping , Lam K. Y.
Published: July 2002

An evolution equation of microcracks for brittle cracked materials subjected to an applied dynamic tensile pressure is presented. The influence of inertial effects (or kinetics) on the dynamic growth of microcracks, which is a major characteristic of intense dynamic loading, is taken into account. Numerical analysis of the evolution equation clearly indicates that microcrack growth at high strain rate (ε̇≥103 s−1) is unstable. The critical condition for microcrack dynamic growth is obtained. It is predicted that the threshold stress for dynamic growth of microcracks is dependent on the average strain rate at super-high strain rate (ε̇≥106 s−1); however, numerical result is also shown that when the average strain rate ≤105 s−1, the threshold stress can be, approximately, taken to be a constant.
Thermal hole emission from Si/Si1−xGex/Si quantum wells by deep level transient spectroscopy
Authors: Chretien O. , Apetz R. , Vescan L. , Souifi A. , Lüth H. , Schmalz K. , Koulmann J. J.
Published: July 2002

We report on the determination of the valence band offset between strained Si1−xGex and unstrained Si layers by deep level transient spectroscopy (DLTS) on Si/Si1−xGex/Si quantum well (QW) structures. A problem of this technique is to store the holes long enough (≥1 ms) in the QW so that the thermal emission of holes is the dominating process. We achieved sufficiently long hole storage times by using two different structures. In the first ones, this is obtained by selective growth which leads to a lateral limitation of the smooth QW layer, and with good Schottky contacts. For the second ones, the localization of holes is due to the presence of Si1−xGex islands. For a sample containing a smooth QW with XGe=0.17 a valence band offset of 140±20 meV was obtained and for the island layer with XGe=0.3 a value of 258±20 meV was found. These results are in good agreement with theory. The DLTS measurements are compared to admittance spectroscopy results and photoluminescence measurements.
Concentration-dependent oxygen diffusion in Tl1Ba2Ca2Cu3 and Ag1Tl2Ba2Ca2Cu3 superconductor precursor alloys
Authors: Chou H. , Chow T. C. , Chen H. S.
Published: July 2002

To study the growth mechanism in growing superconducting thin films by the liquid-gas-solidification process, oxygen diffusivities in liquid Tl1Ba2Ca2Cu3 and Ag1Tl1Ba2Ca2Cu3 precursor alloys were measured by a modified coulometric titration method. The diffusivities of oxygen are very high of the order of 10−4 to 10−3 cm2/s. The logarithm of oxygen diffusivities is found to be reciprocally proportional to the logarithm of oxygen concentrations. This concentration-dependent oxygen diffusivity phenomenon has not been observed in many metallic liquids investigated previously. An interstitial diffusion mechanism may contribute to the fast oxygen diffusion. The strong bonds between Ba and O atoms are proposed to enhance the configurational structural order and hence to quicken the oxygen diffusion that varies the oxygen concentration in Tl1Ba2Ca2Cu3 and Ag1Tl1Ba2Ca2Cu3 superconductor precursor liquids.
ZnMgSeS/ZnSSe/CdZnSe strained quantum well lasers grown on (511)A orientation
Authors: Tao I. W. , Wang Y. , Jurkovic M. , Wang W. I.
Published: July 2002

Although low threshold II-VI blue/green lasers have been demonstrated, the device lifetime for these lasers remains an important issue. It has been observed that the dark line defects, which are caused by stacking faults or point defects, are related to the degradation of the lasers. In order to improve the reliability of the laser, we initiated studies of molecular beam epitaxial growth and device characteristics of II-VI quantum well lasers grown in the (511)A orientation. The photoluminescence spectra of (511)A epilayers always exhibited stronger and sharper emission peaks than their (100) counterparts, which indicated that less defects are incorporated in the (511)A epilayers. Room-temperature threshold current densities of 1 kA/cm2 have been obtained. The performance of these lasers merits further studies on the use of (511)A as a possible solution to the reliability problem of II-VI lasers.
Ensemble Monte Carlo particle investigation of hot electron induced source-drain burnout characteristics of GaAs field-effect transistors
Authors: Moglestue C. , Buot F. A. , Anderson W. T.
Published: July 2002

The lattice heating rate has been calculated for GaAs field-effect transistors of different source-drain channel design by means of the ensemble Monte Carlo particle model. Transport of carriers in the substrate and the presence of free surface charges are also included in our simulation. The actual heat generation was obtained by accounting for the energy exchanged with the lattice of the semiconductor during phonon scattering. It was found that the maximum heating rate takes place below the surface near the drain end of the gate. The results correlate well with a previous hydrodynamic energy transport estimate of the electronic energy density, but shifted slightly more towards the drain. These results further emphasize the adverse effects of hot electrons on the Ohmic contacts.
Epitaxial Sr2(AlTa)O6 films as buffer layers on MgO for YBa2Cu3O7−x thin film growth
Authors: Chen K. Y. , Afonso S. , Wang R. C. , Tang Y. Q. , Salamo G. , Chan F. T. , Guo R. , Bhalla A. S.
Published: July 2002

Sr2(AlTa)O6 thin films (2000–3000 Å) have been deposited on MgO (001) substrates using pulsed laser deposition (PLD). X-ray-diffraction analysis shows that the Sr2(AlTa)O6 grows with the c axis highly oriented normal to the substrate plane and very good in-plane epitaxy. The subsequently deposited YBa2Cu3O7−x films using PLD on Sr2(AlTa)O6 buffered MgO substrates exhibit excellent epitaxial growth with a narrow rocking curve width and a small φ scan peak width. The critical temperature Tc0 of 90–92 K has been achieved reproducibly and the critical current density is over 2.7×106 A/cm2 at 77 K.
Estimation of strain arising from the assembling process and influence of assembling materials on performance of laser diodes
Authors: Shigihara Kimio , Nagai Yutaka , Karakida Shoichi , Aiga Masao , Otsubo Mutsuyuki , Ikeda Kenji
Published: July 2002

A method for estimating strain in a semiconductor laser chip arising from bonding to foreign materials. The method is based on measurements of dependence of lasing wavelengths on duty ratios of pulsed operation with a constant peak current. The wavelength at zero duty ratio (λ0) is extrapolated from this dependence, from which the strain is estimated. Estimated strain from this method is in good agreement with result obtained from the change of polarization ratio, transverse-magnetic to transverse-electric light outputs. The strain arising from packaging affects not only lasing wavelength but also operating characteristics such as threshold current. Moreover, thermal resistance of laser diodes can be obtained from this dependence of lasing wavelengths on duty ratios.
Low-level photomodulation of exciton absorption in CdTe single quantum wells
Authors: Naumov A. , Mi Donglin , Sturge M. D. , Ge Weikun , Dang Le Si , Mariette H. , Magnea N.
Published: July 2002

An optical nonlinearity at very low light levels in a CdTe/CdZnTe single quantum well at 4 K is reported. Excitation above the band gap produces a significant decrease in the heavy-hole exciton absorption at intensities down to μW/cm2. The dependence of the change in absorption on the pump intensity is sample dependent and is strongly sublinear. At low pump intensity the recovery of the absorption after switching off the pump is exponential with a time constant of about 150 ms, which is nearly independent of pump intensity up to ∼3 mW/cm2. Above 30 μW/cm2 an additional process on a time scale ∼100 μs is observed. The rise time after turning on the pump varies inversely as pump intensity. It is shown that the nonlinearity arises from the presence of excess electrons in the CdTe quantum well, which reduce the excitonic absorption by phase-space filling. These electrons are charge compensated by holes trapped in the barrier. The time and intensity dependence of the optical nonlinearity can be fitted by a kinetic model of the trapping, in which a range of traps with different recovery times participates.
Influence of quantum-mechanical reflection at the emitter-base spike on the base transit time through abrupt heterojunction bipolar transistors
Authors: Kumar T. , Cahay M. , Shi S. , Roenker K.
Published: July 2002

A model to simulate electron transport through the base region of abrupt heterojunction bipolar transistors has been developed taking into account the finite probability for electrons in the base to tunnel through the emitter-base spike back into the emitter. The average base transit time is calculated as a function of the emitter-base voltage using the impulse response technique. For all biases, the average base transit time is found to be smaller than its value computed while neglecting the finite probability for electrons with energy below the emitter-base spike to tunnel back into the emitter. For the case of an AlGaAs/GaAs structure, the average base transit time is found to increase with the forward emitter-base voltage.
Correlation of effective recombination velocity of a backsurface field with a boron concentration profile
Authors: Leguijt C. , Lölgen P. , Burgers A. R. , Eikelboom J. A. , Steeman R. A. , Sinke W. C. , Alkemade P. F. A. , Sarro P. M.
Published: July 2002

The efficiency of silicon solar cells can be increased by reducing the recombination of minority carriers at the backsurface of a cell. This can be achieved by application of a low-high junction, commonly called a backsurface field (BSF). The dependence of the effective backsurface recombination velocity Seff of a BSF on the BSF doping profile and the base doping concentration was studied experimentally. The doping profiles were analyzed using sheet resistance measurements, stripping Hall measurements, and secondary ion mass spectrometry. The effective recombination velocity was obtained from photoconductivity decay probed with microwaves on symmetrical p+pp+ structures, and from photocurrent decay measurements on n+pp+ solar cells. The measured values of Seff were compared with calculated values, based on the measured doping profiles. Both from photoconductivity and photocurrent decay measurements, Seff was found to decrease with an increasing difference in acceptor concentration between the BSF and the base of the solar cell. The measured values of Seff, obtained with the two experimental techniques, are in agreement with each other.
On acoustic propagation in exponential ocean surface waveguides
Authors: Ye Zhen
Published: July 2002

It has been observed experimentally that bubbly layers generated by breaking waves form nearly exponential waveguides near ocean surfaces and that noise propagation in such waveguides occurs in distinct frequency bands [D. M. Farmer and S. Vagle, J. Acoust. Soc. Am. 86, 1897 (1989)]. It was conjectured that the well-defined frequency bands are due to the normal mode propagation of sound. Motivated by the experimental observations, a theory is presented to investigate the normal mode propagation in exponential waveguides, by analogy with the optical waveguides formed by diffusion [Z. Ye, Appl. Phys. Lett. 65, 3173 (1994)]. The theoretical results compare favorably with the experimental data. In addition, we show that a waveguide with an exponential sound speed profile has great mathematical convenience in that it has simple characteristic equations which can be solved in a closed form. The number of normal modes, which is shown to correspond to the number of discrete acoustic bands, can be determined by waveguide parameters through a simple equation. The results are useful for solving inverse problems associated with exponential waveguides. The present exponential sound speed profile is also compared to the inverse square profile that has been studied extensively in the past.
Three-dimensional Monte Carlo calculation of Ga+ ion penetration in an a-Se75Ge25 thin film
Authors: Lee Hyun-Yong , Chung Hong-Bay
Published: July 2002

Three-dimensional (3D) Monte Carlo (MC) simulation for Ga+ ion penetration in an amorphous Se75Ge25 thin film which acts as a positive resist in focused ion beam lithography is presented. For 3D MC simulation, both the depth and the lateral distance in the resist are divided into discrete cubic pixels with 25×25×25 Å. Scattering and stop positions as well as deposited energy density by ∼104 ion trajectories are summed up in these pixels. Also, these MC data are expanded for a focused ion beam with a proper ion dose. Therefore, the 3D stop-ion distributions and the 3D deposited energy profiles by ions implanted into the resist are obtained. The peak concentration of MC simulation associated with a dose of 1015/cm2 and an incident energy of 80 keV amounts to 2×1020 ions/cm3 and is located at a distance of about 400 Å from the surface which is shifted about 40 Å to the surface in comparison with Lindhard, Scharff, and Schiott-based calculation.
Two-band structure in photoluminescence spectra from porous silicon and its dependence on excitation wavelength
Authors: Duan J. Q. , Yao G. Q. , Song H. Z. , Zhang B. R. , Zhang L. Z. , Qin G. G.
Published: July 2002

We have systematically measured the room-temperature photoluminescence spectra with a two-band structure from porous silicon (PS), as a function of excitation wavelengths in a wavelength range from 260 to 460 nm. Each spectrum can be fitted by two Gaussian functions centered at about 610 and 700 nm, the intensities of the two bands change with excitation wavelength and the intensity maxima occur when the excitation wavelength is about 340 and 400 nm, respectively. When the excitation wavelength exceeds 420 nm, the band at 610 nm is very weak. The above phenomena can be accounted for in the quantum confinement/luminescence centers model [G. G. Qin and Y. Q. Jia, Solid State Commun. 86, 559 (1993)], where it is supposed that there are two kinds of luminescence centers in SiOx layers covering the nanoscale silicon units in PS.
Pattern generation on silicon surfaces and YBa2Cu3Ox thin films by a scanning tunneling microscope
Authors: Shekhawat G. S. , Gupta Ram P. , Agarwal A. , Garg K. B. , Vyas P. D.
Published: July 2002

In this article, recent results employing scanning tunneling microscopy-based techniques for the generation of nanometer-scale patterns on hydrofluoric acid treated silicon(100) and YBa2Cu3Ox superconducting thin films are presented. Furthermore, we were able to extract silicon (Si) atoms from Si(100)−1×1 surfaces, thereby producing silicon vacancies in the surface. These results thus demonstrate a possible approach for the construction of an atomic scale data memory as well as fabrication of artificial nucleation sites. The emission mechanism is believed to be field assisted evaporation due to the close proximity of the surface and the probe of the microscope.
Influence of metal sheet resistance on capacitance measurements of Schottky barriers
Authors: Myrtveit T. Æ.
Published: July 2002

The effect of metal sheet resistance on capacitance measurements of Schottky barriers is investigated through modelling. The calculations show that when the capacitance is determined by conventional ac impedance measurements, the value obtained is less than the true capacitance when the metal sheet resistance is high. The 1/C2 versus V plot will be nonlinear for extreme cases, but remains linear even for metal films with quite high resistivities. In the linear regime, the curves for different resistances will have a near identical slope but will be shifted along the 1/C2 axis. This causes the barrier height to be overestimated. The error increases with increasing metal resistances. The series resistance determined from I-V plots may not necessarily indicate whether the metal sheet resistance is detrimental for capacitance measurements.
Mean-square displacements of Ta and C atoms in TaC encapsulated in carbon nanotubes
Authors: Yosida Yositaka
Published: July 2002

X-ray diffraction measurements for stoichiometric TaC single crystals encapsulated in carbon nanotubes in the temperature range from 7 to 273 K are reported. Mean-square displacements of Ta and C atoms, 〈u2〉, in the encapsulated crystals are derived from the integrated intensities of the (hkl) Bragg reflections by assuming 〈uTa2〉=〈uC2〉. The experimental value of √〈u2〉 at 273 K is in agreement with the rippled displacements of the bulk TaC (100) surface atoms, which have been explored in a low energy electron diffraction measurement. The effective Debye temperature Θ′ derived from its temperature dependence is 340±10 K, which is about 70% of that for bulk TaC. The predicted softening of transverse acoustic surface phonon mode (the Rayleigh mode) along the M̄ symmetry axis in the surface Brillouin zone, by Ishida and Terakura, is in good accord with the decrease of Θ′. Linear thermal expansivities, Δa/a273, for the encapsulated crystals and for bulky crystalline particles show a marked difference below 150 K, where a273 is the lattice constant of TaC at 273 K. The predicted surface phonon mode (the Rayleigh mode), with a small anomaly, is adequate for explaining the starting temperature of the observed difference.
Characterization of hydrogen passivation and carbon self-compensation of highly C-doped GaAs by means of x-ray diffraction
Authors: Liu Q. , Brennemann A. , Hardtdegen H. , Lindner A. , Prost W. , Tegude F.-J.
Published: July 2002

Highly C-doped GaAs epilayers grown by low-pressure metalorganic vapor phase epitaxy were studied using high-resolution x-ray diffraction. An x-ray diffraction simulation program based on the dynamical theory has been developed, such that the carbon self-compensation effect and the passivation due to the interstitially incorporated hydrogen in the C-doped GaAs can be taken into account. While the (004) Bragg reflection can only be used to measure the lattice contraction caused by carbon doping, the (002) Bragg reflection is highly sensitive to the carbon occupation of the gallium or arsenic sublattice, respectively. Fitting of the simulated diffraction curves of both the (004) and the (002) reflections to the experimental ones enables the evaluation of the interstitially incorporated hydrogen concentration, and hence allows the calculation of the total carbon concentration and the net hole concentration. This technique was successfully applied to various C-doped GaAs samples and the determined data were proven by Hall measurements and secondary-ion mass spectrometry results.
High power switching with a V/n oil switch
Authors: Pereira N. R. , Gondarenko N. A.
Published: July 2002

Accurate switching is necessary to get reproducible pulses with minimal jitter from pulse power equipment. On a 10 MV pulse power machine, Aurora, we have improved the triggering of the oil closing switch in various ways. These include suppressing fast current oscillations during capacitive charging of the switch electrodes, using a faster trigger pulse that acts only during a fraction of the switch closure time, and localization of the arc. With additional use of the machine’s symmetry the output pulses are now reproducible to better than 2% over the entire pulse shape, and the jitter is reduced about twofold, to 5 ns.
Thermal magnetic noise in a strip wound crystalline ferromagnetic core at 4.2 K
Authors: Snigirev O. V. , Maslennikov Yu. V. , Vitale S. , Cerdonio M. , Prodi G. A.
Published: July 2002

A dc SQUID magnetometer-based system has been developed and used to measure, in the frequency range 50–2300 Hz, the complex magnetic permeability μr(ν) and the magnetization noise at 4.2 K in a strip wound toroid. This toroidal core has been made of the 3-μm-thick ribbon fabricated from a crystalline magnetically soft alloy, Ultraperm. Below 1 kHz a constant value of −arg[μr(ν)]≊2×10−3 and 1/ν shaped noise spectral density have been measured. For frequencies higher than 1 kHz a linear growth of the imaginary part μr and a white noise have been found. The noise due to the sample is found in quantitative agreement with the standard fluctuation-dissipation formula for the thermal noise, while a comparison of the permeability imaginary part magnitude with the theoretical value has indicated a partially shorted windings in the toroid, which have decreased the toroid roll-off frequency down to 1 MHz.
Optical study of heterointerface configuration in narrow GaAs/AlGaAs single quantum wells prepared with growth interruption
Authors: Yuan Z. L. , Xu Z. Y. , Zheng B. Z. , Luo C. P. , Xu J. Z. , Ge Weikun , Zhang P. H. , Yang X. P.
Published: July 2002

Photoluminescence and time-resolved photoluminescence were used to study the heterointerface configuration in GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy with growth interruption. Photoluminescence spectra of the growth-interrupted sample are characterized by multiplet structures, with energy separation corresponding to a 0.8 monolayer difference in well width, rather than 1 monolayer as expected from the ‘‘atomically smooth island’’ picture. By analyzing the thermal transfer process of the photogenerated carriers and luminescence decay process, we further exploit the exciton localization at the interface microroughness superimposed on the extended growth islands. The lateral size of the microroughness in our sample was estimated to be 5 nm, less than the exciton diameter of 15 nm. Our results strongly support the bimodal roughness model proposed by Warwick et al. [Appl. Phys. Lett. 56, 2666 (1990)].
Structural imperfections in amorphous e-beam deposited and ion-beam-mixed Cu–Zr films tested by exoelectron emission
Authors: Dekhtyar Yu. , Katashev A. , Kovalevsky A. , Karpe N.
Published: July 2002

Photothermostimulated exoelectron emission (PTSE) is a relatively new method for studying structural imperfections in solids. The number of emitted electrons depends on the concentration of structural imperfections in a given material, like, for example, differently prepared amorphous metals of similar composition. Thin films of amorphous Cu–Zr have been prepared by e-beam codeposition and ion-beam mixing. A significantly higher electron emission intensity was found for ion-beam-mixed Cu–Zr compared to e-beam deposited films having similar composition. Calculated parameters from the PTSE spectra for the annealing reactions were also found to differ for the ion irradiated specimen. These observations suggest that both the number and type of long-lived nonequilibrium structural imperfections in amorphous Cu–Zr are affected by heavy-ion irradiation.