bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport properties of stoichiometric and nonstoichiometric GdAs single crystals
Authors: Li D. X. , Haga Y. , Shida H. , Suzuki T.
Published: July 2002

A systematic investigation of the transport properties of GdAs single crystals is presented. We report on measurements of the electric resistivity, magnetoresistance and Hall effect performed on a stoichiometric and a nonstoichiometric sample at temperatures between 1.6 and 300 K in fields up to 10 T. The stoichiometric sample behaved as a well compensated semimetal that orders antiferromagnetically, while the nonstoichiometric sample showed some anomalies that could be explained qualitatively by the model of trapped magnetic polarons.
Optical properties of ultrarough silver films on silicon
Authors: Neff H. , Henkel S. , Sass J. K. , Steinbeiss E. , Ratz P. , Müller J. , Michalke W.
Published: July 2002

The optical properties of inhomogeneously grown rough silver films have been analyzed on the basis of reflectance measurements. Data have been recorded within the wave number range 50 cm−1<λ−1<50 000 cm−1. The results are compared with compact and fairly smooth films, made from the same metal. Rough films reveal very low reflectance and high absorptivity values of nearly 1, at wave numbers ≳200 cm−1. The reflectance of these films is peaking at the bulk plasma resonance hvp of silver at 3.87 eV. Smooth compact films, in contrast, show a pronounced minimum at the same energy. Based on an effective medium approach and available literature data, the dielectric function (DF) and absorption coefficient have been calculated. For rough films, the real part of the DF remains positive within the whole spectral range, but is negative for compact films below hvp, in agreement with published data. The calculated DF of the inhomogeneously grown films fully resembles the experimental observations.
Infrared-absorption spectra associated to transitions from donor states to conduction subbands in GaAs-(Ga,Al)As quantum wells
Authors: Sabin del Valle J. , de Dios-Leyva M. , Oliveira L. E.
Published: July 2002

The infrared-absorption spectra associated with transitions from donor states to conduction subbands in GaAs–(Ga,Al)As quantum wells are systematically studied. The absorption spectra are calculated within the effective-mass approximation and with a variational procedure for evaluating donor energies and envelope wave functions. Calculations are presented for incident radiation polarized both in the growth direction and in the plane perpendicular to it. Theoretical results are analyzed for different quantum-well widths and n=1,2 conduction subbands, and compared with available experimental measurements.
Effect of large dielectric constant of SrTiO3 substrate on the characteristics of high T
c dc superconducting quantum interference device
Authors: Enpuku K. , Maruo T. , Minotani T.
Published: July 2002

Effects of large dielectric constant of SrTiO3 substrate on the characteristics of high Tc dc superconducting quantum interference device (SQUID) are studied theoretically. It is shown that parasitic capacitance, which is caused by the substrate and distributes along an inductance of the SQUID, cannot be neglected when we use SrTiO3 substrate. Due to the distributed parasitic capacitance, the inductance can no longer be described as lumped element, but behaves like a transmission line when the length of the inductance becomes long. Taking into account the resonant characteristics of the transmission line, we have modified the conventional equivalent circuit of the SQUID, and presented a circuit model for this case. Using numerical simulation, we have clarified quantitatively how the SQUID characteristics are affected by the resonance due to the parasitic capacitance. The obtained results indicate the importance of taking into account the resonance when we use the SrTiO3 substrate, especially for the case of large junction resistance. Numerical simulation of the model cases shows the possibility that the transfer function can be much increased without degrading the flux noise when the SQUID is properly designed. The present theoretical results explain well the experimental results reported recently.
Sharp line injection luminescence from InP quantum dots buried in GaInP
Authors: Anand S. , Lindahl J. , Pistol M-E. , Samuelson L.
Published: July 2002

InP quantum dots embedded in Ga0.5In0.5P are investigated by injection luminescence. By using a masking technique we have improved the spatial resolution. At 77 K, the luminescence peak of the fully formed InP dots occurs at about 1.62 eV. In addition, in the 1.7–1.8 eV energy range, we observe a rich structure in the spectra with several sharp lines typically 3 meV in width. The origin of this luminescence is attributed to the partially formed InP quantum dots. This injection luminescence band also exhibits spatial variations both in the envelope as well as in the fine structure.
Dynamics of laser-induced synthesis of ZnSe
Authors: Kántor Z. , Laude L. D.
Published: July 2002

Results of a study of laser-induced synthesis of ZnSe from a stack of elemental layers are presented. The process is traced by real-time, simultaneous measurement of both reflectivity and transmission of the irradiated thin films. Numerical solution of the thermal conductivity equation reveals that synthesis may develop at and above a well-defined threshold temperature, which is set by the melting of selenium. Further, it is shown that the whole process comprises three independent successive steps: interlayer mixing (or alloying), chemical reaction (i.e., formation of an amorphous compound), and crystallization of that compound. Among other findings, this analysis shows that large atom transport velocities (in mm/s) are necessary to account for the extremely short duration of the mixing step.
Influence of oxygen precipitation along dislocations on the strength of silicon crystals
Authors: Yonenaga Ichiro , Sumino Koji
Published: July 2002

The mechanical strength of dislocated crystals of Czochralski-grown silicon as influenced by the precipitation of oxygen impurities on dislocations was investigated. The yield strength increases during the early stage of precipitation of silicon oxide, but decreases remarkably during the later stage of precipitation. The enhancement of the yield strength is brought about by the immobilization of dislocations due to locking by closely aligned precipitates along the dislocations during the early stage of precipitation. The locking effect diminishes during the late stage when precipitates on dislocations coalesce with the generation of free portions of dislocations with large separation.
Initial phase formation in Nb/Si multilayers deposited at different temperatures
Authors: Zhang Ming , Yu Wen , Wang W. H. , Wang W. K.
Published: July 2002

The initial phase formation in Nb/Si multilayers deposited at 25 and 200 °C was studied by high-resolution transmission electron microscopy and x-ray diffraction. When Nb/Si multilayers were deposited at 25 °C, the multilayers with a modulation period L of 4 nm were in the amorphous state, with Nb-rich amorphous silicide layers and Si-rich amorphous silicide layers. The multilayers with a modulation period L of 100 nm also had an amorphous structure, consisting of intermixed layers of amorphous Nb silicide between the amorphous Si (a-Si) and amorphous Nb (a-Nb) layers. The initial amorphization reaction in Nb/Si multilayers is thermodynamically and kinetically favored. When the multilayers were deposited at 200 °C, a crystalline cubic Nb3Si phase with AuCu3 structure was formed in the multilayer samples. The interfacial energy and modified heat of formation are used to explain why the crystalline phase is formed and Nb3Si is the first phase formed during deposition at so low a temperature.
Al composition dependence of Schottky barrier heights and conduction band offsets of AlxIn1−xP/Ga0.47In0.53As on InP
Authors: Zhu Yu , Ishimaru Yoshiteru , Takahashi Naoki , Shimizu Masafumi
Published: July 2002

The Al composition dependence of the Schottky barrier heights (Φb) and the conduction band offsets (ΔEc) in the AlxIn1−xP/Ga0.47In0.53As heterostructure (0<x<0.5) were investigated for the first time. The thickness and Al composition of AlxIn1−xP layers were evaluated using infrared ellipsometry and x-ray diffraction, and Φb and ΔEc were extracted from the current–voltage and capacitance–voltage characteristics of Schottky barrier diodes. Both Φb and ΔEc show linear dependencies on Al composition, and the composition dependence of Φb follows closely that of ΔEc. Φb=0.90 eV and ΔEc=0.62 eV, which are much larger than those of the widely used Al0.48In0.52As/Ga0.47In0.53As heterostructure, were inferred for the Al0.28In0.72P/Ga0.47In0.53As heterostructure.
Carrier dynamics in oxidized porous silicon
Authors: Komuro Shuji , Kato Takashi , Morikawa Takitaro , O’Keeffe Patrick , Aoyagi Yoshinobu
Published: July 2002

A systematic study of carrier dynamics in oxidized porous silicon (PS) has been carried out using both steady-state and time-resolved photoluminescence (TRPL) measurements probed by tunable excitation over the photon energy range 3.68–1.94 eV for a temperature range 20 K to room temperature. The TRPL results show two emission bands, a high energy band (HEB) and a low energy band (LEB) with distinctively different temperature dependence that is attributed to different and competing carrier recombination mechanisms. The origin of the HEB PL is identified as oxygen-induced defect states with a broad distribution of 2.5±0.3 eV. These oxygen-induced defect states act as radiative recombination centers into which selective excitation of carriers results in the pronounced enhancement of the PL with the decay time of 5–6 ns. The LEB with the decay time of microsecond order is located around 2.1–2.0 eV. The time decay behavior and thermal quenching of the LEB PL intensity at less than 100 K are very similar to that of amorphous materials characterized by carrier recombination via band-tail states. Taking into account the results of the tunable-excitation steady-state PL and TRPL, a plausible model is proposed to explain carrier recombination in oxidized PS based on an analogy with amorphous materials.
Photoemission electron microscopy studies of Pt/GaP(001) buried interfaces
Authors: Almeida J. , Coluzza C. , dell’Orto T. , Barbo F. , Bertolo M. , Bianco A. , Cerasari S. , Fontana S. , Margaritondo G.
Published: July 2002

Using (secondary) photoelectron emission microscopy, we studied the fully formed 80 Å Pt/n–GaP(001) interface with a lateral resolution better than 2 μm. We probed the chemically etched and sulfur passivated GaP(001) surface by ultraviolet and soft x rays. The radiation source was either a deuterium lamp or the radiation from ELETTRA’s U12.5 undulator. Due to their escape depth, the photoemitted secondary electrons carry chemical information of buried interfaces. The use of tunable synchrotron radiation enabled us to obtain chemical contrast by digital subtraction of the microimages taken at photon energies above and below each core-level absorption edge. The microimages revealed lateral changes in photoyield efficiency and chemical composition. The results were confirmed by x-ray photoemission spectromicroscopy measurements performed using Al Kα radiation.
Monte Carlo particle simulation of a quantum well heterojunction field-effect transistor: Comparison with experimental data
Authors: Moglestue C.
Published: July 2002

A heterojunction field-effect transistor with a quantum well channel has been studied using the Monte Carlo particle model. The transport in the channel has been calculated both considering and neglecting the effect of energy quantization of the conduction band. The results of the two approaches were found to agree well with each other and with experimental data. The significant differences between classical and quantum mechanical calculations are the better confinement of the carrier in the channel and the better agreement of the transfer characteristics with experiment for the latter.
Elastoplastic stress analysis of Nb3Sn superconducting magnet
Authors: Vaghar Mohammad Reza , Garmestani Hamid , Markiewicz W. Denis
Published: July 2002

The plastic behavior of a superconducting material is investigated and the corresponding elastoplastic formulation for the distribution of stress and strain in a superconducting solenoid magnet is presented. The analysis calculates stress and strain at the midsection, where tangential stress exhibits its maximum value and shear stress is negligible. The prediction of stress and strain is essential for both the mechanical and electrical design of high-field superconducting magnets containing Nb3Sn superconductor. The concept of plasticity is introduced for the first time in the context of magnet design for Nb3Sn conductor and compared to alternative approaches using conventional elasticity theory. Individual coil sections of a superconducting magnet can be reinforced by an outer section of structural material, the effect of which is included in this formulation. The results show that the elasticity approach using the ‘‘secant modulus method’’ does not fully predict the strain distribution; however, it can be used to approximate the stresses. It is shown that for an accurate strain prediction the true nonlinear elastoplastic nature of the superconducting materials should be considered and proper yield criteria should be used. The inaccurate prediction of strains (tangential or radial) can affect critical current density and the evaluation of the reinforcements.
Monte Carlo simulation of electron behavior in an electron cyclotron resonance microwave discharge sustained by circular TM11 mode fields
Authors: Kuo S. C. , Kuo S. P.
Published: July 2002

Electron behavior in an electron cyclotron resonance microwave discharge sustained by TM11 mode fields of a cylindrical waveguide has been investigated via a Monte Carlo simulation. The time averaged, spatially dependent electron energy distribution is computed self-consistently. At low pressures (∼0.5 mTorr), the temperature of the tail portion of the electron energy distribution exceeds 40 eV, and the sheath potential is about −250 V. These results, which are about twice as high as the previous results for TM01 mode fields [S. C. Kuo, E. E. Kunhardt, and S. P. Kuo, J. Appl. Phys. 73, 4197 (1993)], suggest that TM11 mode fields have a stronger electron cyclotron resonance effect than TM01 mode fields in a cylindrical waveguide.
Effects of external fields on the far-infrared 1s→2p+ intradonor absorption spectra in quantum wells
Authors: Ribeiro F. J. , Latgé A. , Oliveira L. E.
Published: July 2002

The effects of both electric and magnetic fields (applied perpendicular to the interfaces) on the donor transition energies on a GaAs-Ga1−xAlxAs quantum well are studied by following a variational calculation within the effective-mass approximation with two-parameter variational envelope hydrogenic wave functions for the 1s- and 2p+-like donor states. A detailed analysis of the far-infrared intradonor absorption spectra, taking into account a proper consideration of the impurity-doping profile, is performed and results are compared with recent experimental data. The agreement between the reported magnetospectroscopic data and the average photon energy at the width at half-maximum of the calculated infrared-absorption spectra is quite apparent. It is unambiguously shown that the absorption spectra must be evaluated with an adequate choice of the variational envelope wave function for a correct quantitative understanding of the experimental measurements.
Wannier–Stark localization in strained barrier GaAs/InXAl1−XAs superlattices
Authors: Tominaga K. , Hosoda M. , Watanabe T. , Fujiwara K.
Published: July 2002

We report the observation of the Wannier–Stark localization effect in short period GaAs/ InXAl1−XAs superlattices with strained barriers. The superlattices, each of which is contained in the intrinsic region of a p-i-n diode structure, consist of GaAs (3.0 nm) wells and strained shallow InXAl1−XAs (0.9 nm) barriers (X=0, 0.1, 0.2, and 0.3) grown on GaAs by molecular beam epitaxy. In spite of the use of strained barriers, the Wannier–Stark localization effect is clearly observed for all samples at room temperature. Even the superlattice sample with the highest In content of X=0.3 exhibits distinct photocurrent spectra showing several peaks associated with Wannier–Stark ladder transitions as well as Franz–Keldysh oscillations. It is found that the transition intensities are consistent with theoretically calculated oscillator strengths based on the simplified tight-binding model. By increasing the In content X, the miniband width increases and the absorption peak energy due to the zeroth order ladder (e1-hh1 and e1-lh1) transitions decreases because of the reduced barrier height. The transition energies are consistently explained by taking modulation effects into account on the valence subbands due to the compressively strained barriers. The above results show that the use of a large strain effect on the barriers is possible in Wannier–Stark localization effect type devices.
Dielectric charging by an electron beam
Authors: Upatov V. Y.
Published: July 2002

Experimental discovery of a charge spot field effect (CSFE) has altered considerably our understanding of dielectric charging by an electron beam, under conditions typical for the operation of a large class of cathode ray tubes (CRT). Dielectric charging by an electron beam was studied using a specific pulse method for the measurement of the potential. The accuracy of this method is discussed. Measurements were made of the potential relief of a positively charged spot on muscovite mica (quartz, aluminum oxide). The potential at the spot center, under conditions described in the paper and at a relatively long charging time, was shown to be considerably lower than that of the collector. Potential dependence on charging time, determined under the same conditions, is shown for the charged spot center and a number of adjacent points. During creation of the charged spot charging current was measured. The results of the measurements are discussed. A new mechanism of dielectric charging by electron beam is proposed. A CSFE is formulated, and its significance for the operation of CRT is stated. Criticism is given of in-plane grid effect. The paper presents calculations of fields for grid target models determining the mechanism of dielectric charging by electron beam.
Enhanced x-ray optical contrast of Mo/Si multilayers by H implantation of Si
Authors: Schlatmann R. , Keppel A. , Xue Y. , Verhoeven J. , Marée C. H. M. , Habraken F. H. P. M.
Published: July 2002

To increase the x-ray optical contrast of Mo/Si multilayers, we study low energy hydrogen ion implantation of amorphous Si layers. Using elastic recoil detection and Rutherford backscattering spectrometry, we measure the result of hydrogen implantation on Si atomic density. We find a lowering of Si atomic density, and, thus, an enhancement of x-ray optical contrast, as a result of H implantation. We find that the Si atomic density saturates at a minimum of 64±5% of the crystalline value. We have also observed a minor smoothing effect of H+ ion bombardment. Combined with Kr+ ion bombardment, causing a very much larger smoothing of the Si surface, the atomic density is found to saturate at a minimum of 77±5% of the crystalline value.
Electrical evaluation of Al–p+-GexSi1−x ohmic contacts
Authors: Jiang H. , Elliman R. G.
Published: July 2002

The electrical properties of Al–p+-GexSi1−x contacts were studied using transmission line measurements. For this study the GeSi alloy layers were selectively formed with 30 or 90 keV Ge implantation into Si, a technique which offers a simple, self-aligned process for the fabrication of such layers. Measurements of the current–voltage characteristics showed that the metal–alloy contacts were ohmic over the voltage range examined. The specific contact resistivity was found to be a function of Ge concentration, decreasing with increasing Ge concentration for concentrations below a critical value and increasing with increasing Ge concentration above this value. The initial decrease in specific contact resistivity is attributed to the effect of Ge on the contact barrier height and width, an effect which is caused by the reduction in the band gap of the alloy. The subsequent increase in specific contact resistivity at higher Ge concentrations is believed to be due to the presence of a high concentration of dislocations in the alloy layer. The thermal stability of contacts is also reported.
Magnetic domain-specific microspectroscopy with a scanning x-ray microscope using circularly polarized undulator radiation
Authors: Kagoshima Y. , Miyahara T. , Ando M. , Wang J. , Aoki S.
Published: July 2002

Absorption spectra as microspectroscopy taken inside single magnetic domains of nickel have been achieved in the 2p–3d excitation region with a 1 μm spot of circularly polarized undulator radiation focused by a scanning x-ray microscope. A sample used as demonstration was a deposited nickel layer. It was confirmed that the spectra exhibited the features corresponding to the opposite magnetization of each domain using the fixed circular polarization. This technique can offer element- and domain-specific x-ray magnetic microspectroscopy in a submicrometer scale.
Photoinduced structures in the exciton luminescence spectrum of InGaAs/GaAs quantum well heterostructures
Authors: Borri P. , Gurioli M. , Colocci M. , Martelli F. , Capizzi M. , Patané A. , Polimeni A.
Published: July 2002

A large set of InGaAs/GaAs quantum well structures was investigated by means of continuous wave photoluminescence (PL) and photoluminescence excitation spectroscopy. Strong photomodulation effects are observed in PL, namely, a strong sensitivity to the excitation energy and strong changes in the line shape when resonant and nonresonant excitations are used together. Correspondingly, the exciton emission exhibits a doublet structure and the excitation spectra, as detected by monitoring the emission at the two peak energies of the PL doublet, show quite different profiles, with peaks and/or dips not directly related to absorption resonances. On the grounds of time-resolved experiments it is shown that band-bending modifications, due to trapping of free carriers at interface defects, account for the observed photomodulation.
Laser-generated pseudo-Rayleigh acoustic wave propagating along the edge of a thin plate
Authors: Jia X. , Auribault D.
Published: July 2002

A pseudo-Rayleigh wave traveling along the edge of a thin plate is excited by a pulsed laser and detected by optical interferometry. The field confinement feature at the top of the plate edge is investigated by means of acoustic dilatation measurement. A nearly dispersionless behavior and little diffraction loss are observed during the guided mode propagation which agree with theoretical prediction. The possibility of using this wave for viscosity sensing is also discussed.
Electron interaction with confined acoustic phonons in cylindrical quantum wires via deformation potential
Authors: Yu SeGi , Kim K. W. , Stroscio Michael A. , Iafrate G. J. , Ballato Arthur
Published: July 2002

The effects of phonon confinement on electron–acoustic-phonon scattering is studied in cylindrical semiconductor quantum wires. In the macroscopic elastic continuum model, the confined-phonon dispersion relations are obtained for several crystallographic directions with the two cardinal boundary conditions: free-surface and clamped-surface boundary conditions. The scattering rates due to the deformation potential interaction are obtained for these confined phonons and are compared with those of bulk-like phonons for a number of quantum wire materials. The results show that the inclusion of acoustic phonon confinement effects may be crucial for calculating accurate low-energy electron scattering rates in nanostructures. It is also demonstrated that the scattering rates may be significantly influenced by the direction of phonon propagation, especially for low-energy electrons. Furthermore, it has been found that there is a scaling rule governing the directional dependence of the scattering rates: the directions characterized by small Poisson ratios exhibit large scattering rates.
Microstructural effects of thermal annealing on CdS films
Authors: Mendoza-Galván A. , Martínez G. , Lozada-Morales R.
Published: July 2002

Thin solid films of CdS chemical bath deposited on glass substrates were analyzed by spectroscopic ellipsometry in the spectral range of 1.5–5.0 eV, before and after thermal annealing in Ar atmosphere, at different temperatures. Analyses of the data reveal that the films possess a multilayer structure of two or three layers. The samples which were as-grown and thermal annealed at 208 °C are described by cubic phase layers of CdS. For temperatures of thermal annealing ≥260 °C, the films are better described by layers in the hexagonal phase of CdS. The as-grown sample shows features of three graded density-deficient layers. As the annealing temperatures increases, the density and size of the layer closest to the substrate increases, while the other two remain as density-deficient layers. The sample annealed at the maximum temperature (447 °C) is described by just two layers. All the spectra of the thermally treated samples show a shift in the optical band gap with respect to the as-grown sample. These results are compared with the analyses of previous works.
Interfacial reaction studies of Cr, Ni, Ti, and Pt metallization on InP
Authors: Persson Leif , El Bouanani Mohamed , Hult Mikael , Whitlow Harry J. , Andersson Margaretha , Bubb Ian F. , Johnston Peter N. , Walker Scott R. , Cohen David D. , Dytlewski Nick , Zaring Carina , Östling Mikael
Published: July 2002

Interfacial reactions between (100) InP and thin films of the transition metals Cr, Ni, Pt, and Ti have been studied. A thin layer of metal was deposited onto the InP substrates using e-beam evaporation and parts of the samples were then subjected to heat treatment in vacuum for 30 min at several temperatures up to 500 °C. Separate characterizations of the metal, In, and P depth distributions were carried out using mass and energy dispersive recoil spectrometry. The different crystalline phases observed were determined using x-ray diffraction. The near-noble metals (Ni, Pt) formed ternary phases, while Ti and Cr formed phosphides. The phases formed were generally stable up to 500 °C with the major exception being Pt where the ternary phase decomposed to form PtIn2, PtP2, and Pt3In7.
Substitutional, interstitial, and neutral zinc incorporation into InP grown by atmospheric pressure metalorganic vapor phase epitaxy
Authors: Chu S. N. G. , Logan R. A. , Geva M. , Ha N. T. , Karlicek R. F.
Published: July 2002

Substitutional zinc incorporates linearly into device quality InP at low Zn source flow rate grown by atmospheric pressure metalorganic vapor phase epitaxy (AP-MOVPE) at 625 °C, and saturates around 4×1018 cm−3 at high Zn source flow rate. Increase in the Zn source flow rate beyond saturation significantly enhances interstitial incorporation. The excess interstitial diffuses into the undoped region via an interstitial-substitutional diffusion mechanism and reveals itself as an enhanced diffusion. Previously we have proposed a surface adsorption-desorption-trapping model for substitutional Zn incorporation during AP-MOVPE, where the saturation level is determined by the surface incorporation sites for substitutional Zn. This model is applied to interstitial Zn incorporation at Zn source flow rate above the saturation level for substitutional Zn to explain the enhanced Zn diffusion. The analysis is further extended to the incorporation of neutral Zn in the presence of excess phosphorus vacancies, therefore, this model can be used for simultaneous incorporation of Zn of all three types during epitaxy as long as the incorporation processes are independent.
Electron and hole escape times in single quantum wells
Authors: Lefebvre Kevin R. , Anwar A. F. M.
Published: July 2002

The calculation of the carrier escape time is important for quantum well devices. In this article a model for calculating the escape time of both electrons and holes is presented. The escape time is found by solving the Schrödinger equation by the method of the logarithmic derivative of the wave function which yields: the continuum density of states within a biased quantum well, the proper group velocity, and the partitioning between the thermionic emission and tunneling currents. Excellent agreement between the theoretical and previously reported experimental results for electron and hole escape times is achieved.
Stopping powers of 200–3200 keV 4He and 550–1550 keV 1H ions in polyimide
Authors: Plompen A. J. M. , Munnik F. , Räisänen J. , Wätjen U.
Published: July 2002

Accurate stopping powers of polyimide have been determined for mean energies (E) in the range of 200 keV≤E≤3200 keV for 4He ions and 550 keV≤E≤1550 keV for 1H ions, using the transmission method for foils with areal densities of 30–225 μg/cm2. The overall uncertainties of ∼2% are mainly due to the foil thickness determination. The present data have been parametrized and compared to results obtained by previous authors in the upper energy range and to various parametrizations found in the literature. The best description of the data is given by Bragg’s rule calculations using parametrized elemental stopping powers that were derived from large data sets (deviations <2%). No support is found for the cores-and-bonds model showing deviations of 3%–5%, that may be due to the neglect of phase state effects.
Self-consistent kinetic modeling of low-pressure inductively coupled radio-frequency discharges
Authors: Yang Yun , Wu Hanming
Published: July 2002

An efficient method for solving the spatially inhomogeneous Boltzmann equation in a two-term approximation for low-pressure inductively coupled plasmas has been developed. The electron distribution function (EDF), a function of total electron energy and two spatial coordinates, is found self-consistently with the static space-charge potential which is computed from a 2D fluid model, and the rf electric field profile which is calculated from the Maxwell equations. The EDF and the spatial distributions of the electron density, potential, temperature, ionization rate, and the inductive electric field are calculated and discussed.
Electrical behavior of implanted carbon impurities in fluorine co-implanted GaAs
Authors: Peroni Marco , D’Eustacchio Patrizio , Di Virginio Nazzareno , Graffitti Roberto , Gasparotto Andrea
Published: July 2002

The effect of fluorine co-implantation in carbon-implanted GaAs has been investigated by secondary-ion-mass spectrometry (SIMS), Hall, C–V and deep level transient spectroscopy (DLTS) techniques. The variation of carbon activation with different co-implanted doses of fluorine has been analyzed on the basis of the concentration and mobility versus depth profiles of the samples, obtained via the differential Hall technique. The acceptor activation is found to increase with the co-implanted F dose only in the surface region, whereas in the deeper part of the carbon-implanted layer the co-implantation process is less effective, if not counterproductive. The presence of a midgap hole trap level, with an emission energy ET=0.48 eV, has been observed by means of the DLTS technique. The trap concentration is found to be higher in the samples with the lower C activation level (corresponding to lower co-implanted F dose); furthermore, by comparing C–V measurements (at 77 K and room temperature) and SIMS profiles, the trap concentration profile is found to be almost coincident with the unactivated C concentration: In other words, the difference between the implanted C and the activated acceptor concentration corresponds to the hole trap concentration. This result enables a better understanding of the anomalous behavior observed when implanted carbon is used to compensate donor impurities, with almost 100% efficiency, while as an acceptor its activation is much lower. All these effects cannot be ascribed to any chemical bond of the co-implanted F since from SIMS analysis it results that it completely outdiffuses after the rapid thermal anneal process.
Superconductor–insulator–superconductor tunnel strip line: Features and applications
Authors: Belitsky V. Yu. , Kollberg E. L.
Published: July 2002

The superconductor–insulator–superconductor (SIS) tunnel junction based on Nb material has been investigated theoretically at frequencies of 200–1400 GHz by modeling it as a segment of a superconducting microstrip transmission line and using the Mattis–Bardeen theory of anomalous skin effect. The modeling was used to predict the performance of a flux-flow oscillator (FFO) and a SIS frequency down-converter (mixer). It is shown that in a FFO the internal loss (quasiparticle and surface) limits flux flow to the frequency band of 260–770 GHz. A SIS junction of micron size employed as a mixer for frequency down-conversion demonstrates distributed behavior at frequencies above 550 GHz. We suggest that electrically long SIS junctions of resonant size can be used as self-compensating mixing elements with attractive performance in Nb as well as NbN mixers.
Boltzmann equation analysis of electron-molecule collision cross sections in water vapor and ammonia
Authors: Yousfi M. , Benabdessadok M. D.
Published: July 2002

Sets of electron-molecule collision cross sections for H2O and NH3 have been determined from a classical technique of electron swarm parameter unfolding. This deconvolution method is based on a simplex algorithm using a powerful multiterm Boltzmann equation analysis established in the framework of the classical hydrodynamic approximation. It is well adapted for the simulation of the different classes of swarm experiments (i.e., time resolved, time of flight, and steady state experiments). The sets of collision cross sections that exist in the literature are reviewed and analyzed. Fitted sets of cross sections are determined for H2O and NH3 which exhibit features characteristic of polar molecules such as high rotational excitation collision cross sections. The hydrodynamic swarm parameters (i.e., drift velocity, longitudinal and transverse diffusion coefficients, ionization and attachment coefficients) calculated from the fitted sets are in excellent agreement with the measured ones. These sets are finally used to calculate the transport and reaction coefficients needed for discharge modeling in two cases of typical gas mixtures for which experimental swarm data are very sparse or nonexistent (i.e., flue gas mixtures and gas mixtures for rf plasma surface treatment).
Porous silicon photoluminescence: Implications from in situ studies
Authors: Tallant D. R. , Kelly M. J. , Guilinger T. R. , Simpson R. L.
Published: July 2002

Photoluminescence and Raman measurements have been performed on an anodized silicon surface in an HF/ethanol anodization solution and after replacement of this solution with water. Immediately after anodization and while resident in HF/ethanol, the porous silicon produced does not exhibit intense photoluminescence. Intense photoluminescence develops spontaneously in HF/ethanol after 18–24 h or with replacement of the HF/ethanol with water. The results are consistent with a quantum confinement mechanism in which electron-hole pair migration to traps and nonradiative recombination dominates the de-excitation pathways until silicon nanocrystals are physically separated and energetically decoupled by hydrofluoric acid etching or surface oxidation. Raman spectra show the development of nanometer-size silicon crystals concurrent with intense photoluminescence. Illumination of the silicon surface during anodization tends to inhibit the formation of nanocrystalline silicon, but even very thin layers (tens of nanometers or less) exhibit photoluminescence. As produced by anodization, the porous silicon surface is apparently inhomogeneous on the millimeter scale, showing large variations in photoluminescence intensity and peak wavelength.
In-plane and out-of-plane dissipation in c-axis-oriented (Bi,Pb)2Sr2Ca2Cu3O x silver-sheathed tapes
Authors: Xu Guoyang , Sun Yuping , Du Jiaju , Zhou Yiru , Zeng Rong , Fu Xuekui , Hua Peiwen
Published: July 2002

The temperature dependence of the out-of-plane and in-plane resistance of a c-axis-oriented (Bi,Pb)2Sr2Ca2Cu3O x silver-sheathed tape with high critical current density (Jc>2×104 A/cm2, 77 K, 0 T) has been investigated under applied fields up to 0.9 T. It is found that the in-plane and the out-of-plane resistance transitions are quite different: (1) The out-of-plane zero resistance temperature is much higher than the in-plane one, i.e., Tcab(H)≪Tcc(H), and the difference increases with magnetic field; (2) the out-of-plane zero resistance temperature corresponds to the c-axis decoupling temperature. These phenomena are attributed to different dissipation mechanisms. The in-plane dissipation at low temperature results from the thermally activated flux–flow, while the out-of-plane dissipation originates from the Josephson junction dissipation of weakly coupled c-axis grain boundaries and/or intrinsic Josephson junctions.
Characterization study of GaAs(001) surfaces using ion scattering spectroscopy and x-ray photoelectron spectroscopy
Authors: Wolan John T. , Epling William S. , Hoflund Gar B.
Published: July 2002

A surface characterization study using ion scattering spectroscopy (ISS) and x-ray photoelectron spectroscopy (XPS) has been performed on solvent cleaned, n-type GaAs(001) substrates before and after cleaning by ion sputtering and annealing. The native oxide layer on this surface contains large amounts of As2O5,As2O3, and Ga2O3 according to XPS with Ga2O3 being the predominant species. Before cleaning C is present as hydrocarbons, carbonates, and carbide with hydrocarbons as the predominant chemical state. Ion sputtering converts the hydrocarbons into carbide, which is difficult to remove by further sputtering/annealing cleaning cycles, but O is removed by these cycles. According to ISS data, the outermost atomic layer is enriched in Ga before cleaning, but after cleaning the ISS Ga-to-As atom ratio is about 1:2. The results obtained in this study are consistent with the presence of a layered oxide structure with Ga2O3 just above the interface. A sputter-cleaned surface initially exhibits an increase in the Ga-to-As surface atom ratio which returns to the initial state with time (∼2 h) as observed by ISS. The Ga-to-As surface atom ratio also increases by annealing at temperatures as low as 180 °C.
Analysis of superconducting microstrip resonator at various microwave power levels
Authors: Srivastava G. P. , Jacob Mohan V. , Jayakumar M. , Bhatnagar P. K. , Kataria N. D.
Published: July 2002

The real and imaginary parts of the surface impedance of YBCO superconductors have been studied at different microwave power levels. Using the relations for the critical current density and the grain boundary resistance, a relation for calculating the power dependence of the surface resistance has been obtained. Also, a relation to find the resonant frequency of a superconducting microstrip resonator at various input power levels has been derived. Measurements have been carried out on various microstrip resonators to study the variation of surface resistance and resonant frequency at different rf power levels. The experimental results are in good agreement with theoretical results.
Effect of interfacial roughness on exchange coupling (invited) (abstract)
Authors: Unguris J. , Celotta R. J. , Davies A. , Pierce D. T. , Stroscio J. A.
Published: July 2002

The magnitude and the direction of the oscillatory exchange coupling in multilayer structures depend sensitively on the atomic scale ordering of the layers. Disorder in the form of substrate roughness, interlayer thickness fluctuations, or interfacial interdiffusion can result in reduced bilinear exchange coupling, the disappearance of short period coupling oscillations, and the increase or decrease of the relative strength of the biquadratic coupling. We have investigated the effect of roughness by growing nearly perfect multilayer Fe/Cr and Fe/Au structures on single crystal Fe whisker substrates. The disorder in these structures was measured using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED), while the coupling directions and magnitudes were measured using scanning electron microscopy with polarization analysis and magneto-optic Kerr microscopy. For the nearly perfect trilayer structures, the periodicity and the magnitude of the oscillatory coupling are in agreement with theoretical predictions based on spacer layer Fermi surface properties. For rougher multilayer systems the exchange coupling can be modeled using structural information from STM and RHEED measurements.
Intrinsic localized spin wave modes in easy-axis antiferromagnetic chains
Authors: Lai R. , Sievers A. J.
Published: July 2002

Two types of intrinsic localized spin wave modes are found in perfect antiferromagnetic chains of classical spins with on-site easy-axis anisotropy: the amplitude is either double or single peaked. In the small spin deviation limit, both types become identical envelope solitons. The degree of localization increases as either the maximum spin deviation or the ratio of the anisotropy constant to the exchange coupling constant increase. However, only the single peaked intrinsic localized mode is stable with regard to a noise perturbation.
Structural and magnetic properties of La0.5Ca0.5MnO3 (abstract)
Authors: Dai P. , Fernandez-Baca J. A. , Chakoumakos B. C. , Cable J. W. , Nagler S. E. , Schiffer P. , Kalechofsky N. , Roy M. , Tsui Yee-Kin , McGinn Paul J. , Einloth Sharon L. , Ramirez A. P.
Published: July 2002

The nuclear and magnetic structures of the perovskite La0.5Ca0.5MnO3 were studied as a function of temperature. At low temperatures this system exhibits charge ordering and a magnetic structure of a charge exchange-type in the a–c plane with a ferromagnetic component along the b axis. At intermediate temperatures there is a purely ferromagnetic phase. In this intermediate ferromagnetic phase we find charge ordering associated with antiferromagnetic short-range order. We find no evidence that would support the existence of a charge ordered purely ferromagnetic structure.
Magnetic phase diagram of the spin-density-wave CrFe alloy system from susceptibility measurements
Authors: Galkin V. Yu. , de Camargo P. C. , Ali N. , Fawcett E.
Published: July 2002

The temperature dependence of the magnetic susceptibility χ(T) of seven Cr1−xFex alloys measured around the Neel transition shows a strong first-order transition at the Neel temperature TN for the Fe content, x=2.7 and 3.7 at. % Fe, greater than the triple-point concentration, xL=2.4 at. % Fe, where the paramagnetic, incommensurate-, and commensurate-spin-density-wave (ISDW and CSDW) phases meet. Alloys with x≲xL (x=2.0 and 2.15 at. % Fe) show a weak transition at TN and a strong first-order transition at TIC (the ISDW–CSDW transition). The magnetic phase diagram of the Cr1−xFex alloy system below xL is re-interpreted in light of these results and the neutron diffraction work. The coexistence of the CSDW and ISDW phases in the mixed state below the branch TIC(x) of the first-order incommensurate-commensurate (IC) phase transition, for values of x well below xL, may result from anisotropy of the large strain at TIC.
Magnetic and structural characteristics of RCo9+δSi4 alloys (R=Pr or Gd, δ=0–4)
Authors: Huang M. Q. , Wallace W. E. , Ma B. M. , Bounds C. O.
Published: July 2002

Studies of the magnetic and structural characteristics of RCo9+δSi4 (δ=0–4), nonstoichiometric alloys, have been extended from R=La in our earlier work to R=Pr and Gd. In the present work, as the extra Co content increases, the magnetic properties M, Tc, and Hc are significantly enhanced compared to that of the stoichiometric alloys (δ=0). Using x-ray diffraction measurements, the sublattice occupancies are calculated. The extra Co atoms are found to occupy the 4a and 16L2 sites in the Ce2Ni17Si9 unit cell. They partially replace R and Si in these sites.
Field emission from surface states of silicon
Authors: Huang Qing-An , Qin Ming , Zhang Bin , Sin Johnny K. O.
Published: July 2002

Field emission from a single level surface state is modelled and numerically estimated. Field emission current from the surface state is dominant at low fields while field emission current from the conduction band is dominant at high fields due to not enough electrons being supplied for the surface state. The field emission Fowler–Nordheim plot shows no linearity.
Comment on “Electron spin resonance studies in β-FeSi2 crystals” [J. Appl. Phys. 80, 1678 (1996)]
Authors: Irmscher K. , Gehlhoff W. , Lange H.
Published: July 2002

In a recent article [J. Appl. Phys. 80, 1678 (1996)] Aksenov et al. reported on electron paramagnetic resonance (EPR) studies in β-FeSi2 crystals grown by chemical vapor transport. They did not perform a rigorous measurement of the angular variation of the EPR line positions. Consequently, there has been a drastic loss of information and most of their conclusions turn out to be erroneous. It is shown that the anisotropic signals (Ai,Bi) do not arise from spin triplet states but from centers with S=1/2 and their origins are not Ni2+ ions but Ni+ (Ai) and Cr− (Bi) ions substituting for Fe on one of its two inequivalent lattice sites. The analysis of the line structure of the isotropic signal (C) is incorrect and hence, the structure cannot be attributed to a ligand hyperfine interaction with four iron atoms. Finally, the determination of an acceptor activation energy from the temperature dependence of the C signal is not justified since no correction for the EPR intensity dependence due to the thermal population difference of the Zeeman levels was included.
The influence of mask thickness on charging damage during overetching
Authors: Hwang Gyeong S. , Giapis Konstantinos P.
Published: July 2002

Feature-scale charging simulations during gate electrode overetching in high-density plasmas reveal that the thickness of the insulating mask plays a critical role in charging damage. When thinner masks are used, the electron irradiance of the conductive part of the sidewalls increases, causing the charging potentials of the polysilicon lines to decrease, thus reducing the probability for catastrophic tunneling currents through the underlying oxide. Simultaneously, changes in the charging potential distribution at the bottom SiO2 surface cause a significant perturbation in the local ion dynamics which, in turn, adversely affects notching. Notches are predicted to form everywhere in a line-and-space structure, even when the lines are electrically isolated. The results suggest that the trend toward thinner (hard) masks—to keep the aspect ratio low as device dimensions shrink—should reduce oxide failure but at the cost of more severe notching.
Electronic states in ZnSe/ZnTe type-II superlattice studied by capacitance transient spectroscopy
Authors: Wang S. Q. , Lu F. , Jung H. D. , Song C. D. , Zhu Z. Q. , Okushi H. , Cavenett B. C. , Yao T.
Published: July 2002

We have studied the electronic states in N-doped ZnSe/ZnTe type-II superlattice (SL) by deep level transient spectroscopy (DLTS) and isothermal capacitance transient spectroscopy (ICTS). The capture and emission processes of holes between a miniband of the SL and the valence band of ZnSe barrier were investigated. From the analysis of DLTS and ICTS spectra, the activation energy for the hole emission from the miniband energy level was determined to be 0.28±0.03 eV, which is consistent with a theoretical value (0.25 eV) of the band offset between the ZnSe/ZnTe SL calculated based on the Kronig–Penney model. A deep level with an activation energy of 0.48±0.03 eV was observed and has been assumed to originate from an interface defect in the SL region. A deep level located at 0.54±0.03 eV above the valence band of ZnSe was also observed in the ZnSe capping layer.