bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of impurity-free and impurity-induced disordering on the optical properties of GaAs/(Al,Ga)As distributed Bragg reflectors
Authors: Floyd P. D. , Merz J. L.
Published: July 2002

The variation of the reflectivity and stopband width of disordered GaAs/(Al,Ga)As distributed Bragg reflectors (DBRs) is studied by thermal annealing of semiconductor DBRs and modeled by simulating interdiffusion of Al and Ga at the interfaces within the DBR. Undoped mirrors show stability in their reflectivity and stopband width for annealing durations of up to 24 h. Heavily Si-doped mirrors show strong degradation for annealing durations as short as 1 h. Using the model, excellent agreement with experimental results is found and interdiffusion coefficients can be estimated. The results show that important optical parameters are unchanged for short diffusion lengths, but drastically degrade for larger diffusion lengths. The results indicate the limits of thermal processing for device structures containing GaAs/(Al,Ga)As DBRs. The ability to accurately predict the effects of disordering on DBRs permits the proper design of vertical cavity structures containing DBRs, by specifically accounting for the effects of disordering.
Theoretical electronic properties of silicon-containing bismuth
Authors: Ferreira da Silva A.
Published: July 2002

The electrical conductivities of n-doped silicon and, in particular Si:Bi, have been investigated for doping levels greater than the impurity critical concentration Nc for the metal-nonmetal transitions. A general feature of the conductivity for concentration normalized to Nc is presented in the order σ(Bi)≳σ(As)≳σ(P)≳σ(Sb). For Si:Bi, the value of Nc is calculated for different criteria. The mobility of electrons presents a lower value compared to Si:P. The results for Si:P and Si:As are compared to the experimental data available in the literature.
Direct measurement of free-energy barrier to nucleation of crystallites in amorphous silicon thin films
Authors: Shi Frank G.
Published: July 2002

A method is introduced to a measure the free-energy barrier W*, the activation energy, and activation entropy to nucleation of crystallites in amorphous solids, independent of the energy barrier to growth. The method allows one to determine the temperature dependence of W*, and the effect of the preparation conditions of the initial amorphous phase, the dopants, and the crystallization methods on W*. The method is applied to determine the free-energy barrier to nucleation of crystallites in amorphous silicon (a-Si) thin films. For thermally induced nucleation in a-Si thin films with annealing temperatures in the range of from 824 to 983 K, the free-energy barrier W* to nucleation of silicon crystals is about 2.0–2.1 eV regardless of the preparation conditions of the films. The observation supports the idea that a-Si transforms into an intermediate amorphous state through the structural relaxation prior to the onset of nucleation of crystallites in a-Si. The observation also indicates that the activation entropy may be an insignificant part of the free-energy barrier for the nucleation of crystallites in a-Si. Compared with the free-energy barrier to nucleation of crystallites in undoped a-Si films, a significant reduction is observed in the free-energy barrier to nucleation in Cu-doped a-Si films. For a-Si under irradiation of Xe2+ at 105 eV, the free-energy barrier to ion-induced nucleation of crystallites is shown to be about half of the value associated with thermal-induced nucleation of crystallites in a-Si under the otherwise same conditions, which is much more significant than previously expected. The present method has a general kinetic basis; it thus should be equally applicable to nucleation of crystallites in any amorphous elemental semiconductors and semiconductor alloys, metallic and polymeric glasses, and to nucleation of crystallites in melts and solutions.
Diffusive buoyancy force and concentration-dependent diffusivities
Authors: Zimmels Y.
Published: July 2002

The effect of pressure gradients that develop in diffusion systems consisting of particulates dispersed in a continuous fluid is considered. It is shown that the gradient of chemical potential which drives the diffusion flux induces a pressure gradient that opposes this flux. This effect, which exists in addition to the induced bulk flow, is expressed in terms of a diffusive buoyancy force (DBF). For dispersions consisting of monodisperse particulates in a single-component fluid, the net driving force is the negative product of the volume fraction occupied by the fluid and the gradient of the chemical potential of the particulates. For polydisperse particulates, the DBF is the negative product of the total volume fraction occupied by the particulates and the expectation of gradient of their chemical potential. The joint effect of the DBF and the hydrodynamic hindrance is expressed in terms of a concentration-dependent diffusion coefficient. It is shown that the effect of the DBF yields a fundamental diffusion coefficient Dφ, which is the product of the volume fraction occupied by the fluid 1−φ, and the Stokes–Einstein diffusion coefficient D. The intrinsic diffusion coefficient, which is defined as the product of 1−φ and Dφ, thus becomes the product of the square of 1−φ and D. At steady state the concentration profile cannot be analytically linear unless the buoyancy and hydrodynamic effects are offset by changes of size, energy per particulate and the activity coefficient. Finally, implications regarding the diffusion equation and effects of combined fields on the DBF are considered.
Transfer-matrix analysis of waveguide phase modulator using the linear electro-optic effect of asymmetric quantum wells
Authors: Chen Y. W. , Li H. S. , Zhou Z. , Wang K. L.
Published: July 2002

The phase modulation of asymmetric quantum-well embedded waveguides is studied. In order to accurately analyze the modulation from quantum wells and optimize the performance of waveguide modulators, a transfer-matrix method is developed to calculate the bias-controlled phase modulation of waveguides with an arbitrary refractive index distribution. The refractive index change may include those from free-carrier effects and the nonlinear effects of bulk material and quantum wells. The linear electro-optic effect introduced by asymmetric quantum wells can potentially be used to improve the frequency performance of semiconductor waveguide phase modulators.
Response to ‘‘Comment on ‘Structural properties of CuInxGa1−xSe2 thin films prepared by rf sputtering’ ’’ [Phys. Status Solidi A 124, 427 (1991)]
Authors: Yamaguchi Toshiyuki , Yoshida Akira
Published: July 2002

The structure of the CuInxGa1−xSe2 system can be determined from the viewpoint described in this comment by Tinoco et al. Our argument concerning the x-ray study published by T. Tinoco, C. Rincon, M. Quintero, and G. Sanchez Perez [Phys. Status Solidi A 124, 427 (1991)] is summarized.
Further examinations on the thermodynamic stability of the Mie–Grüneisen equation of state
Authors: Segletes Steven B.
Published: July 2002

The Mie–Grüneisen equation of state (EOS) is further examined for restrictions upon the form of the Grüneisen parameter, which is given as a function of volume and relates the pressure and energy changes at constant volume. Employing basic thermodynamic rules and applying them to the Mie–Grüneisen EOS, a more restrictive condition is offered for the Grüneisen parameter, which extends previous work on the subject.
Acoustic-phonon limited mobility in periodically modulated quantum wires
Authors: Stocker J. E. , Leburton J. P. , Noguchi H. , Sakaki H.
Published: July 2002

We investigate acoustic-phonon scattering in quantum wires subject to a periodic potential along the propagating direction. A technique for modeling the electronic structure of the periodic system is introduced using the imaginary time propagation method. The acoustic-phonon scattering rate is evaluated by taking umklapp processes into account. We found umklapp processes can cause a significant increase in intersubband scattering but is negligible for intrasubband scattering. Overall, the exact treatment of the electron dispersion relation improves the acoustic-phonon limited mobility compared to earlier estimates [H. Noguchi, J. P. Leburton, and H. Sakaki, Phys. Rev. B 47, 15593 (1993)].
Modeling of the rotation of polarization in polymers using an inhomogeneous birefringence model
Authors: Watanabe Yukio
Published: July 2002

Polymers exhibiting a weak birefringence mostly posses optical properties which are predicted well by conventional models for birefringence. However, the rotation of polarization behavior was found to disagree with these models. This deviation has been overlooked in usual measurements characterizing birefringence in polymers. In order to accurately predict the rotation of polarization, we propose a model which includes a birefringence inhomogeneity. The model is shown to be related to the birefringence-induced optical activity. The model predictions are shown to agree well with experimental results using a parallel laser beam. Practical implications with respect to unusually high low-frequency noise of magneto-optical disks are discussed briefly.
Simple model of foam drainage
Authors: Fortes M. A. , Coughlan S.
Published: July 2002

A simple model of foam drainage is introduced in which the Plateau borders and quadruple junctions are identified with pools that discharge through channels to pools underneath. The flow is driven by gravity and there are friction losses in the exhausting channels. The equation of Bernoulli combined with the Hagen–Poiseuille equation is applied to describe the flow. The area of the cross section of the exhausting channels can be taken as a constant or may vary during drainage. The predictions of the model are compared with standard drainage curves and with the results of a recently reported experiment in which additional liquid is supplied at the top of the froth.
Interfacial chemistry at metal/CdTe contacts as probed by differential scanning calorimetry
Authors: Lin W.-Y. , Wei C. , Rajeshwar K.
Published: July 2002

All four possible chemical reactivity patterns, namely, outdiffusion of Te (metal-Cd alloy formation), Cd outdiffusion (metal telluride compound formation), comparable chemical reactivity of the metal towards both Cd and Te (no Cd or Te outdiffusion), and chemical inertness of the metal towards CdTe, were differentiated via the differential scanning calorimetry (DSC) technique from a study of the interaction of nine different metals toward CdTe powder. The fusion signatures of free Cd or Te, exotherms due to compound or alloy formation, along with the thermal transitions of the metal telluride and/or the intermetallic were used for this purpose. These reactivity patterns are discussed within the framework of two different thermodynamic models. Both virgin and chemically etched CdTe surfaces were examined, and found to exhibit rather different reactivity trends towards the metal. The ramifications of these results in terms of the electronic properties of metal/CdTe contacts are discussed. Finally, DSC is shown to be useful for probing alterations in the CdTe surface chemistry as a result of the etch treatment.
Space-charge dipole domains dynamics theory of frequency locking, quasiperiodicity, and chaos in extrinsic Ge
Authors: Jiang Zongfu
Published: July 2002

Space-charge dipole domain dynamics theory has been used to investigate the frequency locking, quasiperiodicity, and chaos in extrinsic Ge. Our numerical results agree with the experimental phenomena.
Anomalous Hall effect in strongly modulated two-dimensional systems
Authors: Gumbs Godfrey , Huang Danhong
Published: July 2002

The longitudinal (ρxx) and Hall (ρxy) resistivities are calculated for a two-dimensional square array of scatterers, which are simulated by an array of positive delta functions. We find that when the potential strength is strong ρxy can be negative and quenched and ρxx shows commensurate oscillations simultaneously. Similar features have been observed experimentally for antidots in both the quasiclassical and the quantum regimes. The quenching of ρxy is explained by the collimated states formed by electrons scattering resonantly off the lattice sites in a magnetic field when the magnetic translational period and reciprocal-lattice vector are commensurate. Impurities do not qualitatively change ρxy but they can quantitatively change ρxx. Numerical results show how the lattice, impurity scattering, and the electron density affect the quenching of the Hall effect and the commensurate oscillations
Measurement of microwave dielectric properties using the parallel-plate dielectric resonance technique
Authors: Sheen Jyh , Bhalla A. S. , Cross L. E.
Published: July 2002

A modified field model of the parallel-plate open dielectric resonator is adopted to measure the dielectric constants and loss tangents of low loss materials. The measurement accuracy using this model is justified by comparing the results with those measured by other techniques.
Concentration and temperature dependence of electrical resistivity in Zr1−xHoxCo2 (0≤x≤1) intermetallic compounds and their hydrides
Authors: Ramesh R. , Rama Rao K. V. S.
Published: July 2002

The electrical resistivity studies have been carried out on the C 15-type Laves phase intermetallic compounds Zr1−xHoxCo2 (0≤x≤1) and their hydrides in the temperature range 16–300 K. The first-order magnetic transition occurring for x=1 disappears for lower Ho concentrations. Anomalies in the form of resistivity minima observed for the intermediate x values are attributed to the spin fluctuations in the vicinity of the magnetic ordering temperature. On hydrogen absorption, the first-order transition observed for x=1.0 as well as the anomalies observed for the other x values disappear at higher hydrogen concentrations. A concentration-dependent metal-semiconductor-like transition has been observed in all cases. This may be a consequence of charge transfer taking place between hydrogen and the 3d band, which leads to a strong decrease in the conduction electron density. The activation energy is evaluated from the ln ρ vs 1/T plots.
Couplings between Tamm states and Stark ladders in a finite-length superlattice
Authors: Glessner William , Yu Roger
Published: July 2002

The electronic structure of a finite superlattice exposed by an external electric field along the growth direction is studied using the Fourier expansion in electron wave functions. We have found surface localized [I. Tamm, Phys. Z. Sowjetunion 1, 733 (1933)] states and, in addition, two Stark ladders whose states are localized in the quantum wells and evenly spaced in energy. Strong interactions between Tamm states and Stark ladders are studied in detail.
Effects of high oxygen pressures and temperatures on the stability of the three superconducting Y-Ba-Cu-O compounds
Authors: Williams R. K. , Kroeger D. M. , Martin P. M. , Mayotte J. R. , Specht E. D. , Brynestad J.
Published: July 2002

In the course of investigating synthesis routes, stability information was developed for YBa2Cu3O7−x, Y2Ba4Cu7O15−x, and YBa2Cu4O8. The results were obtained between ∼900 and 1040 °C and involve oxygen pressures between 10 and 145 atm. The observations show that the PO2-T ranges for stability of the three phases overlap and that some stable mixtures of the phases can be synthesized. The results are compared to the results of other experimental studies and to calculations based on calorimetric data.
Characterization of intrinsic a-Si:H in p-i-n devices by capacitance measurements: Theory and experiments
Authors: Caputo D. , de Cesare G. , Irrera F. , Palma F. , Tucci M.
Published: July 2002

A technique has been developed for characterizing the intrinsic layer in a-Si:H p-i-n structures. The method is based on the measurement of the differential capacitance under forward bias, i.e., with injection of free carriers in the intrinsic layer. In these conditions, capacitance is extremely dependent on charges trapped in the band-gap defects. Measurements were performed on several p-i-n samples, in a wide range of frequencies and voltages. All the samples showed the same trend: Capacitance diminished with increasing signal frequency and increased with forward applied voltage. An analytical model explains the obtained behavior. In particular, the model shows that the band-tail contribution to capacitance decreases slowly with frequency, while deeper defects are effective only below 100 Hz. At higher frequencies, trapping phenomena play a lesser and lesser role in the measurement while depletion charge becomes relevant and the measured capacitance tends to its asymptotic junction value (i.e., the ratio dielectric constant/thickness of the p-i-n). The model predicts a high sensitivity to defect distribution in the gap of the intrinsic layer, thus allowing an effective characterization of the semiconductor material as is in the actual device. Up to 1017 cm−3 defects have been detected via the capacitance technique, presumably located in the portion of the intrinsic layer closest to the p/i interface. The origin of such a large amount of defects can be related to the presence of inhomogeneities due to the interfaces in the p-i-n device. These defects behave as free-carrier traps, and do not affect the recombination mechanism. A picture of the electronic defect distribution in the device material is finally proposed which accounts for both midgap neutral dangling bonds and shallower charged defects.
Time-resolved study on the impact excitation and quenching processes of the 1.54 μm electroluminescence emission of Er ions in InP
Authors: Kimura Tadamasa , Isshiki Hideo , Ishida Hiroyuki , Yugo Shigemi , Saito Riichiro , Ikoma Toshiaki
Published: July 2002

Time-resolved impact excitation and de-excitation processes of the 1.54 μm electroluminescence (EL) emission of Er3+-doped InP are investigated. Samples are impact excited by applying electrical pulses and the time response of the EL emission is measured in the temperature range from 77 K to 330 K. The decay of the emission proves almost exponential in all the temperature ranges and shows little thermal quenching with the decay time decreasing from 2 ms at 77 K to only 1 ms at 330 K. This result contrasts with the large thermal quenching and nonexponential characteristics of the photoluminescence (PL) time decay at high temperatures in the same sample, suggesting different excited Er3+ centers between EL and PL. Also measured is the emission rise time as a function of excitation pulse current, giving us the impact cross section of 9×10−16 cm2 for Er3+ ions in InP. The excitation and quenching processes as well as the efficiency of Er emission are analyzed. A model taking into consideration the presence of different Er centers explains the different behaviors in the time responses between EL and PL.
Pulsed and continuous-wave thermal characteristics of external-cavity surface-emitting laser diodes
Authors: Chen G. , Hadley M. A. , Smith J. S.
Published: July 2002

The thermal characteristics of p-type substrate external-cavity surface-emitting laser diodes (SELDs) under pulsed and continuous-wave operation are studied both theoretically and experimentally. The theoretical work establishes models for the current flow, spontaneous emission transfer, heat generation, and temperature rise in the laser. The models incorporate the reduced and anisotropic electrical and thermophysical properties of the Bragg reflectors. The temperature rise at the active region of an external-cavity SELD is determined by measuring the output wavelength as a function of the injection current, modulation frequency, and substrate temperature. Simulation results agree reasonably well with experimental data on the external-cavity SELD. It is shown that the thermal resistance of the studied laser is smaller than previously reported values for the n-type substrate SELDs because of the current spreading in the p-type mirror and its close proximity to the substrate.
Velocity spread measurements on a magnetron injection gun beam
Authors: Guss W. C. , Basten M. A. , Kreischer K. E. , Temkin R. J.
Published: July 2002

The parallel electron velocity distribution function has been measured on an electron beam from a magnetron injection gun (MIG) using an E∥B energy analyzer. The magnetic field, beam voltage, and beam current were scaled down from normal operating parameters. The perpendicular velocity spread, inferred under the assumption that the electrons are monoenergetic, is relatively constant for electron velocity ratios β⊥/β∥≳0.7 and increases approximately linearly with the beam current. The current scaling of the perpendicular velocity spread is also consistent with electron-loss currents measured at the control anode of the MIG. Observed perpendicular velocity spreads for the gun design parameters are substantially larger than computational values.
Role of implantation-induced defects in surface-oriented diffusion of fluorine in silicon
Authors: Szeles Cs. , Nielsen B. , Asoka-Kumar P. , Lynn K. G. , Anderle M. , Ma T. P. , Rubloff G. W.
Published: July 2002

Open-volume defects introduced in Si(100) crystals during fluorine implantation were investigated by variable-energy positron beam depth profiling. The behavior of the implantation-induced lattice defects upon high temperature annealing and their role in the surface-oriented diffusion of F impurities were examined. The defects become mobile and undergo recovery at temperatures below 550 °C, i.e., well before the onset of fluorine diffusion as seen by secondary ion mass spectroscopy (SIMS) profiling. This behavior suggests that after irradiation and annealing the fluorine occupies substitutional sites to which positrons are insensitive. The anomalous F diffusion seen in SIMS has been explained through a two-step diffusion mechanism, in which the diffusion kinetics is determined by dissociation of the substitutional F into an interstitial F and a vacancy, followed by a rapid diffusion of the interstitial F and the vacancy through the crystal to the surface.
Chemical vapor deposition of Al from dimethylethylamine alane on GaAs(100)c(4×4) surfaces
Authors: Karpov I. , Bratina G. , Sorba L. , Franciosi A. , Simmonds M. G. , Gladfelter W. L.
Published: July 2002

Al films were grown by chemical vapor deposition at 400, 550, and 700 °C on GaAs(100) substrates using the molecular precursor dimethylethylamine alane. The film morphology and composition were studied in situ by reflection high-energy electron diffraction and Auger electron spectroscopy, and ex situ by atomic force microscopy and scanning electron microscopy. Chlorine (at 400 °C) and C and N (at 550 and 700 °C) at or below the percent level were found to be the major contaminants of the deposited films. Systematic studies for deposition at 400 °C established that the film microstructure evolves via the growth and coalescence of three-dimensional faceted islands with (100)Al∥(100)GaAs or (110)Al∥(100)GaAs preferential orientation. Coalescence of such crystallites was observed only for equivalent coverages of Al above 150 nm. Comparison with the microstructure of Al films obtained by evaporation suggests that in the temperature range examined the evolution of film morphology during chemical vapor deposition from dimethylethylamine alane was mainly determined by surface diffusion of isolated adsorbed Al atoms.
Quench in superconducting magnets. II. Analytic solution
Authors: Shajii A. , Freidberg J. P.
Published: July 2002

A set of analytic solutions for the Quencher model as described in Part I, is presented in this paper. These analytic solutions represent the first such results that remain valid for the long time scales of interest during a quench process. The assumptions and the resulting simplifications that lead to the analytic solutions are discussed, and the regimes of validity of the various approximations are specified. The predictions of the analytic results are shown to be in very good agreement with numerical as well as experimental results. Important analytic scaling relations are verified by such comparisons, and the consequences of some of these scalings on currently designed superconducting magnets are discussed.
Effect of in-plane magnetic fields on the interband optical absorption spectra of GaAs-(Ga,Al)As superlattices
Authors: de Dios-Leyva M. , da Silva E. Z. , Oliveira L. E.
Published: July 2002

A theoretical study of the effects of in-plane magnetic fields on the interband optical absorption spectra of GaAs-(Ga,Al)As superlattices is presented. The envelope wave functions were obtained by an expansion in terms of sine functions. Whereas in the bulk case the selection rule Δn=0 for transitions between Landau levels is satisfied, the presence of the superlattice potential makes possible the occurrence of interband transitions with Δn≠0. The 19 T calculated interband peaks in the absorption spectrum agrees quantitatively well with the main experimental peaks by Belle, Maan, and Weimann.
Novel solid-phase epitaxial growth of YBa2Cu3O7−δ films from precursor oxides
Authors: Hussey B. W. , Gupta A. , Olsson E.
Published: July 2002

The pulsed laser deposition technique has been used to study the process of layering and intermixing of precursor oxide phases of YBa2Cu3O7−δ (YBCO) during deposition from separate component targets. The films are grown on (100)-oriented SrTiO3 substrates by ablation from the component targets YBa2Ox/CuO or Y2Cu2O5/BaCuO2 under process conditions similar to those needed for the growth of YBCO films from a composite target. For precursor oxide layers below a critical thickness, which can be as large as 1000 Å, facile formation of high-quality epitaxial YBCO films is observed resulting from interdiffusion and reaction of the constituent phases. On the other hand, while some degree of intermixing occurs above the critical thickness, there is no evidence of formation of the YBCO phase. The novel solid-phase epitaxy observed below the critical thickness is believed to result from diffusion of species through grain boundaries of the initial precursor oxide layer, followed by reaction at the substrate interface to epitaxially nucleate the YBCO layers. The process continues until all of the initial oxide layer is consumed and can be repeated with deposition of subsequent alternate layers. The dynamics of the diffusion process has been investigated using in situ resistance measurements to monitor the growth of the YBCO phase. The results show that the time scale for interdiffusion and reaction is very rapid (∼10–100 ms), and occurs soon after arrival of the ablated species from the second component target on the surface of the initial precursor oxide layer. These results have important implications for the choice of component oxide blocks for layer-by-layer growth of artificially structured films.
Nanoscale structural evolution and associated changes in magnetoresistance in the granular FexAg100−x thin films
Authors: Xu Yingfan , Sumiyama Kenji , Wakoh Kimio , Makhlouf Salah A. , Suzuki Kenji
Published: July 2002

The microstructure of sputter-deposited granular FexAg100−x thin films (0<x<60) has been investigated by using a high resolution transmission electron microscope. For x≤14 at. % Fe, two kinds of morphological regions exist, i.e., the crystallized and the highly disordered. As the x changes within 20–36 at. % Fe, small clusters with sizes around 1 nm are formed in the films. Further increase of x leads to the crystal grain fining and crystal frustration. The observed microstructural evolution in the films with the increase of Fe content can be correlated with the evolution of giant magnetoresistance at 4.2 K in the Fe-Ag films: a linear relationship of the magnetoresistance ratio, Δρ/ρ, with H for x≤20 at. %, the saturation trend and the decrease of Δρ/ρ for x≥36 at. % Fe.
Long-pulse, electron beam pumped, atomic xenon laser
Authors: Patterson E. L. , Samlin G. E.
Published: July 2002

Characteristics of a long-pulse, low pump rate, atomic xenon (XeI) laser are described. Energy loading up to 170 J/L at pulse widths between 5 and 55 ms is achieved with an electron beam in transverse geometry. The fraction of energy in each wavelength obtained with electron beam pumping is in good agreement with results from fission fragment pumping in a reactor pumped laser. Values for the small-signal gain coefficient, loss coefficient, and saturation intensity as a function of pump rate are presented. Laser energy as a function of pulse width and the effects of air and CO2 impurities are described. An investigation of the dominant laser wavelength in a high-Q cavity indicates that the 2.6 μm radiation dominates.
Surface pinning and Lorentz force dependence of transport critical current in NbTa thin films and heterostructures
Authors: Kumar D. , Blamire M. G. , Doyle R. , Campbell A. M. , Evetts J. E.
Published: July 2002

The critical current densities of NbTa and Ta/NbTa/Ta thin films have been measured as a function of the orientation of an applied magnetic field with respect to both the current and the film surface. It is shown that appropriate control of the deposition conditions and the film composition allow the surface pinning and Lorentz force dependence to be reliably and reproducibly altered. Comparison of the results with those reported elsewhere for high Tc films shows broadly similar behavior in the two systems. In particular, we show that surface pinning results in a (sin φ)−1/2 dependence of Jc(φ) for B perpendicular to current, similar to the dependence attributed to intrinsic pinning in high Tc systems. The results also confirm that a lack of Lorentz force dependence of transport critical current is a characteristic of a two dimensional pinning system.
Fourier spectroscopy of elastic modes and anharmonic effects in LiNbO3
Authors: de Souza Ricardo E. , Engelsberg M. , do Nascimento George C.
Published: July 2002

Elastic vibrational thickness modes in LiNbO3 crystal were studied in the frequency range 20–100 MHz using a modified Fourier transform pulsed nuclear-magnetic-resonance spectrometer. The phase discrimination capability, absence of baseline distortions and drifts, and the excellent spectral resolution permit a very accurate determination of the elastic constants and the observation of subtle anharmonic coherent effects in the fine structure of the overtones of several modes.
A back-side passivation film on a-Si:H thin film transistor
Authors: Uchikoga Shuichi , Kakinoki Masami , Nakajima Mitsuo , Suzuki Kouji
Published: July 2002

Silicon nitride (SiNx) films of various quality were deposited on the back-side of a-Si:H thin film transistors (TFTs) as a passivation layer. Although the films were not deposited directly on the channel region, the N/Si ratio of the film affected the TFT performance. It changed the threshold voltage and the subthreshold slope. In the linear region, the effect of the passivation layer was explained by assuming fixed charges in the nitride layer. Furthermore, the current-voltage characteristics in the saturation region were explained by a parasitic TFT model. It was determined that a nitrogen rich passivation film is preferrable for liquid crystal displays.
Valley current of three-dimensional resonant tunneling diode studied by the improved optical model
Authors: Zohta Yasuhito , Tanamoto Tetsufumi
Published: July 2002

Using the energy-dependent optical potential determined by experimental data on hot electrons, the optical model is improved to incorporate the effect of longitudinal optical-phonon scattering that is important in GaAs/AlGaAs systems. On the basis of this model, simulations have been made of double-barrier resonant tunneling diodes with emitters of high and low donor concentrations. The simulation results are in good agreement with experimental results, i.e., the improved optical model explains the experimental peak-to-valley ratio well in the case with the high donor concentration emitter, and reproduces a phonon satellite observed in the experiment in the case with the low donor concentration emitter. It is shown that this simulator is easy to use and simple to interpret, and can be easily obtained by a little modification of the widely used one-dimensional simulator.
Range profiles of 600–1200 keV Xe+ implanted in KTiOPO4
Authors: Wang Ke-Ming , Ding Pei-Jun , Wang Wei , Lanford W. A. , Shi Bo-Rong , Yu Zheng-Gang , Lu Qing-Ming
Published: July 2002

600–1200 keV Xe+ were implanted into KTiOPO4 (potassium titanyl phosphate) in increments of 100 keV. The depth distributions of implanted Xe+ in KTiOPO4 were measured by Rutherford backscattering. The effects of channeling, temperature, and dose on the depth distribution were briefly investigated. All range distributions were nearly Gaussian. The mean projected range and range straggling obtained were compared with the TRIM’91 code, projected range algorithm (PRAL) and Wang and Shi’s (WS) calculation procedure based on Biersack’s angular diffusion model. The results show that TRIM’91 systematically underestimates the projected range by approximately 25%, PRAL underestimates the range by approximately 28%, and WS underestimates the range by approximately 7%. All calculate the underestimated range straggling by approximately a factor of two.
Polarization dependence of spectral transmission and photoconductive response of a p-doped multiple quantum well structure
Authors: Fenigstein A. , Finkman E. , Bahir G. , Schacham S. E.
Published: July 2002

A comparison between the optical transmission and the photoconductive response of p-type quantum well infrared photodetectors is demonstrated. The dependence on polarization was found to be different for these two processes. Light polarized perpendicular to the growth layers was more efficiently absorbed by the quantum wells, while the photoconductive response was stronger for the parallel polarized light. The difference is attributed to the different vertical transport behavior of light and heavy holes and the mixed nature of hole subbands.
Optical and electrical properties of evaporated 2,5-bis-methylthio-7,7′,8,8′-tetracyanoquinodimethane
Authors: Zambounis J. S. , Mizuguchi J. , Rihs G. , Chauvet O. , Zuppiroli L.
Published: July 2002

2, 5 -bis-methylthio - 7 , 7′, 8, 8′ - tetracyanoquinodimethane (BMT-TCNQ), a strong π-electron acceptor, has been newly synthesized and its optical and electrical properties have been investigated in evaporated films. A large bathochromic shift of the visible absorption band (474–590 nm) has been observed in going from solution to the solid state. This is attributed to enhanced electron delocalization in the solid state through close intermolecular S-N contacts (3.2 Å) between the S atom of the -SCH3 group of one BMT-TCNQ molecule and the N atom of the -C≡N group of the neighboring molecule. Evaporated BMT-TCNQ exhibits a high electrical conductivity of 2×10−5 S cm−1 at room temperature as well as a high spin concentration of 2×1017 spins/cm3. The origin of the charge carriers is presumably ascribed to electron-donating impurities which cause the ESR signals, whereas the close intermolecular S-N contacts considerably facilitate the carrier hopping throughout the crystal.
Lattice and continuum predictions of crack-tip stability
Authors: Anderson Peter M. , Thomson Robb
Published: July 2002

The stability of a crack which has a nonlinear core emerging from the tip onto the plane ahead of the crack is investigated in terms of KI−KII loading. The principal focus is to address a paradox that the Griffith relation, 𝒢=2γs, permits equilibrium cracks to exist in pure mode-II loading, yet under such conditions there would be insufficient tensile force to pull atoms apart to maintain free surfaces. Using a Peierls-type framework developed by Rice [J. Mech. Phys. Solids 40, 239 (1992)], a continuum analysis of a crack with a nonlinear core ahead of the crack tip is presented to demonstrate that, indeed, the crack-core structure is stable to self-similar translation when the Griffith condition is met. However, the portions of the Griffith curve which have a sufficient fraction of mode-II loading—approximately ‖KII‖/KI≥0.4 for the particular bonding properties considered here, are unattainable because dislocation emission intervenes. Corresponding studies of an equilibrium crack in a 2D hexagonal lattice demonstrate that there is a band of KI−KII values within which the crack is stable. The band is approximately centered on the Griffith curve and extends to critical KII values comparable to those at which the continuum model predicts dislocation emission to intervene. The finite width of the band occurs due to lattice trapping, and that width is observed to broaden as KII is increased. In this context, the continuum model represents the case of zero trapping. Consequently, a satisfactory explanation of the Griffith crack paradox is that pure mode-II equilibrium cracks are unattainable because dislocation emission from the crack tip intervenes before the pure mode-II Griffith value can be reached.
Monte Carlo simulation of cation diffusion via vacancies in quasibinary spinel solid solutions involving multiple types of cation-vacancy exchanges
Authors: Lu F.-H. , Franke P. , Nichols C. S. , Dieckmann R.
Published: July 2002

Previously [F.-H. Lu, P. Franke, C. S. Nichols, and R. Dieckmann, J. Appl. Phys. 75, 819 (1994)], the correlation factors of cations and vacancies diffusing by a vacancy mechanism in quasibinary spinel solid solutions were reported for cases in which only one type of cation-vacancy exchange prevails. Cases involving significant contributions from more than one type of cation-vacancy exchange are considered for quasibinary spinel solid solutions of the type (A1−xBx)3−δO4 and their binary end members, for very small values of δ. Correlation factors for the cations A and B and for cation vacancies have been determined for different frequency combinations and different vacancy and cation distributions as a function of the cationic composition x. In cases where no cation-vacancy exchanges occur between the octahedral and tetrahedral sublattices, the simulation results can be related to correlation factors for different diffusion processes involving only one type of cation-vacancy exchange.
Diffusion of both Ga and Bi into InSb seeds during growth of InGaSbBi
Authors: Hayakawa Y. , Ando M. , Matsuyama T. , Hamakawa E. , Koyama T. , Adachi S. , Takahashi K. , Lifshits V. G. , Kumagawa M.
Published: July 2002

In1−xGaxSb1−yBiy (0<x≤0.21, 0<y≤0.005) quaternary bulk single crystals were grown on InSb seed crystals using a rotary Bridgman method. In order to investigate the quality of these crystals, various kinds of measurements were carried out, such as optical microscope, x-ray topograph, four-crystal x-ray diffractometry, electron-probe microanalysis, energy-dispersive spectroscopy, and secondary-ion-mass spectroscopy. All grown crystals were 10 mm in diameter and more than 10 mm in length. This indicates that the rotary Bridgman method was useful to grow quaternary bulk single crystals. Owing to segregation, the compositional ratio of Bi (y) increased and that of Ga (x) decreased as crystals grew. During growth of InGaSbBi, both Ga and Bi diffused into the InSb seed and there appeared domains of InBi. For comparison, InSb1−yBiy (0<y≤0.05) and In1−xGaxSb (0<x≤0.16) were grown on InSb. It turned out that Bi did not diffuse into InSb without Ga, but Ga diffused without Bi. The incorporation of Ga produced the excess In and as a result InBi domains were formed.
Crystal structure of proton-exchanged LiNbO3 waveguides
Authors: Fei Y. , Wong H. K.
Published: July 2002

It is generally agreed that the crystal structure of proton-exchanged LiNbO3 waveguides is not consistent with the Li1−xHxNbO3 phase diagram established using powder samples. We propose a simple strain model to remove such anomaly and show that the proton-exchanged layer on a LiNbO3 substrate is in a strained β ‴ phase.
Spatial distribution of recombination centers in GaAs:Te: Effects of the doping level
Authors: Castaldini A. , Cavallini A. , Fraboni B. , Mendez B. , Piqueras J.
Published: July 2002

The distribution in liquid-encapsulated-Czochralski (LEC) GaAs:Te wafers of point and complex defects has been investigated together with their influence on the minority-carrier diffusion length L. Three wafers with different Te-doping concentration (2.2×1017, 4.5×1017, and 1.5×1018 cm−3) have been studied by means of the electron-beam-induced-current (EBIC) mode of scanning electron microscopy and of the surface photovoltage (SPV) method. The morphology and electrical activity of the defects observed across each wafer have been correlated to the formation and distribution of deep electronic levels, which are significantly affected by the tellurium concentration. The diffusion length has been found to be mainly controlled by deep levels associated with dislocations. EBIC localized measurements of L and of the net ionized free-carrier concentration provide evidence for the influence of Te concentration on impurity segregation at complex defects.
Variable-area resonant tunneling diodes using implanted in-plane gates
Authors: Goodings C. J. , Mizuta H. , Cleaver J. R. A. , Ahmed H.
Published: July 2002

Variable-area resonant tunneling diodes have been fabricated using a process in which the lateral confinement is produced by an in-plane implanted gate. The basic operation of such devices is discussed, and the lateral confinement shown by both measurements and numerical modeling to be very nearly symmetrical about the resonant tunneling diode (RTD) barriers. Fine structure has been observed near threshold for relatively large area devices and this has been attributed to single-electron or few-electron tunneling through donor states in the quantum well. Additional fine structure seen in the valley current of small-area devices has been shown to be consistent with the effects of lateral quantization in the system. Finally, results are presented for strip devices in which the resonant tunneling peak breaks up into numerous subpeaks on application of the gate voltage, and this is attributed to pinning of the confining potential by donors close to the RTD barriers.
Experimental proof of saturation in nonresonant multiple-photon ionization
Authors: Schütze M. , Trappe C. , Kurz H.
Published: July 2002

Saturation in nonresonant multiple-photon ionization is directly proved by a novel two-pulse technique. Ultrashort laser pulses with a pulse duration of 17.5 ps are used for multiple-photon ionization. They provide experimental conditions which allow double-pulse experiments on sub-ns time-scales. Data of saturation intensities and cross sections are derived from the experiments.
Interference effects in x-ray double-crystal rocking curves of GexSi1−x/Si superlattices
Authors: Li J. H. , Mai Z. H. , Cui S. F.
Published: July 2002

Interference effects in the x-ray double-crystal rocking curves of GexSi1−x/Si strained-layer superlattices have been simulated based on the x-ray dynamical scattering theory. We found that the A1BA2 type of structure due to unstable growth conditions causes a series of strong oscillation fringes beside the superlattice reflection satellite peaks. The positions and intensities of these oscillation fringes with respect to the superlattice satellite peaks are strongly correlated to the number of periods in the subsuperlattice layer B and the relative mean lattice mismatch between A and B with respect to the substrate.
Positive temperature coefficient of resistivity effect in undoped barium titanate ceramics
Authors: Kuwabara M.
Published: July 2002

A distinct positive temperature coefficient of resistivity (PTCR) effect has been observed in undoped barium titanate ceramics sintered in the temperature range 1300–1400 °C in air. Although undoped barium titanate is normally insulating and ferroelectric at room temperature, it has been found that semiconducting barium titanate ceramics with a distinct PTCR effect could be produced by sintering high-purity, highly sinterable barium titanate powders in air. The resulting semiconducting barium titanate ceramics, with a relative sintered density of about 98%, had a core-shell structure with a semiconducting phase inside and an insulating phase outside. The outer insulating part had a microstructure consisting of rather small grains with an average grain size of less than 10 μm, while the inner semiconducting part had a large-grained microstructure with an average grain size of around 50 μm.
Magnetic hardening mechanisms in Nd-Fe-B nanocrystalline material
Authors: Clavaguera-Mora M. T. , Diego J. A. , Clavaguera N. , Hernando A. , Vázquez M.
Published: July 2002

Microstructural and magnetic measurements of the evolution by heat treatment of initially amorphous Nd16Fe76B8 alloys prepared by melt spinning are presented. Evidence of magnetic hardening above a threshold temperature induced by magnetic isolation of the Nd2Fe14B grains is provided. A thermodynamic and kinetic explanation of local melting of the intergranular nanostructured Nd-rich eutectic phase at temperatures below 900 K based on capillary effects is presented. A subsequent Ostwald ripening process moves Nd to wet intimately the hard magnetic grains, becoming, on cooling, a real paramagnetic isolating thin film (∼2.5 nm). By using a simple analogy, it is shown that the switching magnetization field in a single-domain crystal can be drastically affected through the exchange coupling to neighboring grains with different orientation of the easy axis. This effect should be important enough to reinforce the coercive field of polycrystalline hard magnetic materials and explains the observed enhancement from 0.9 to 1.9 T.
Multidomain and incomplete alignment effects in giant magnetoresistance trilayers
Authors: Idzerda Y. U. , Chen C.-T. , Cheng S. F. , Vavra W. , Prinz G. A. , Meigs G. , Lin H.-J. , Ho G. H.
Published: July 2002

The element specific magnetic hysteresis curves of Fe/Cu/Co trilayer structures can be used in conjunction with the measured magnetoresistance curves to extract the coefficient of the giant magnetoresistance (GMR) independent of magnetic domain effects and incomplete alignment effects, allowing for a measure of the maximum attainable GMR for that trilayer. Information concerning the details of the trilayer switching can be extracted showing that sputtered polycrystalline films of Fe/Cu/Co deposited on Si switch their magnetization directions like multidomain Ising magnets.
Microwave testing of high-T
c based direct current to a single flux quantum converter
Authors: Kaplunenko V. K. , Fischer G. M. , Ivanov Z. G. , Pedersen N. F. , Claeson T. , Mygind J. , Wikborg E.
Published: July 2002

Design, simulation, and experimental investigations of a direct current to a single flux quantum converter loaded with a Josephson transmission line and driven by an external 70 GHz microwave oscillator are reported. The test circuit includes nine YBaCuO Josephson junctions aligned on the grain boundary of a 0°–32° asymmetric Y-ZrO2 bicrystal substrate. The performance of such converters is important for the development of the fast Josephson samplers required for testing of high-Tc rapid single flux quantum circuits in high-speed digital superconducting electronics.
Mica as a waveguiding support for investigating ultrathin layers
Authors: Lawall R. , Knoll W.
Published: July 2002

The attenuated total reflection Kretschmann configuration is used for exciting guided optical modes in thin mica sheets. It is shown that this allows for detecting an additional mica monolayer (1 nm thick) on top of a few micron thick mica waveguide and therefore will allow for the determination of the optical properties of ultrathin layers (Langmuir–Blodgett–Kuhn-self-assembly monolayers) on top of the mica surface. Optical waveguide microscopy was performed with this nearly ideal slab configuration, allowing for the detection of plateaus on the mica surface.