bluuebunny/crossref_metadata_2025 huggingface dataset:
An effective-mass model of hydrogenated amorphous silicon: A tail state analysis
Authors: O’Leary Stephen K. , Zukotynski Stefan , Perz John M.
Published: July 2002

An effective-mass model of hydrogenated amorphous silicon (a-Si:H) is proposed. This model focuses upon the impact of hydrogen on the electronic structure of the tail states. Tail state density-of-states calculations are performed in the high hydrogen concentration limit, and the results are found to be in general agreement with present experimental data. These results are shown to be quite robust to variations in the modeling parameters.
Low-frequency transport in semi-insulating GaAs
Authors: Shulman Dima D.
Published: July 2002

By extending to the frequency domain the analysis of transport in semi-insulating GaAs two-terminal structures, in which one terminal injects only electrons and the other may inject holes, closed forms were obtained for ac charge and potential distribution under conditions of low-level injection. The presence of deep traps results in frequency dependence of both the excess free and trapped carriers. At low frequencies free electrons move so as to neutralize injected holes, but at higher frequencies charge separation of free carriers due to zero local recombination, typical of relaxation semiconductors, occurs. The corresponding admittance can be represented by an equivalent circuit consisting of a frequency-dependent conductance in parallel with a frequency-dependent capacitance. At very low frequencies the conductance decreases with increasing hole injection. At higher frequencies it increases and then saturates with frequency. At low frequencies the capacitance increases with increasing hole injection and is a strongly decreasing function of frequency. At higher frequencies the admittance depends only weakly on the hole injection ratio. Although we focus on semi-insulating GaAs, the equations presented are in a general form, which is applicable to the frequency-dependent transport in a variety of other semiconductors under conditions of low-level injection.
Thermostability and decomposition of the (Bi,Pb)2Sr2Ca2Cu3O10 phase in silver-clad tapes
Authors: Luo J. S. , Merchant N. , Maroni V. A. , Gruen D. M. , Tani B. S. , Carter W. L. , Riley G. N. , Sandhage K. H.
Published: July 2002

The stability of the Bi2−xPbxSr2Ca2Cu3O10 (Pb-2223) phase contained in silver-sheathed oxide-powder-in-tube specimens has been investigated by x-ray diffraction, transmission electron microscopy, and energy dispersive x-ray analysis. Silver tubes loaded with Pb-2223 precursor powders were processed into tapes using established metallurgical techniques. The tapes were heat-treated in a specially designed equilibration apparatus at selected temperatures (800–845 °C) for a range of times (10–5500 min) and quenched in liquid gallium held at ∼40 °C. The results showed that the Pb-2223 phase is stable in a limited temperature interval between 810 and 830 °C in 7.5% oxygen. At 800 °C, this phase decomposes to Bi2Sr2CaCu2O8 (2212), Ca2PbO4, and CuO; while at temperatures ≥840 °C it partially melts with precipitation of Bi2Sr2CuO6 (2201) and Ca2CuO3. The effects of the silver cladding on the Pb-2223 phase stability and microstructure are also discussed.
Temperature dependence of elastic nonlinearities in single-crystal gallium arsenide
Authors: Joharapurkar D. , Gerlich D. , Breazeale M. A.
Published: July 2002

The six third-order elastic moduli (TOEM) of single-crystal gallium arsenide were determined by a combination of measurements of ultrasonic second-harmonic generation, and pressure dependence of the second-order elastic moduli, at room temperature. In the temperature range 77–300 K, the nonlinearity parameter for the propagation directions [100], [110], and [111] was measured. Utilizing the Keating model, these data were used in evaluating all six TOEM as a function of temperature. The TOEM C111, C144, and C456 turn out to be nearly constant in the above temperature range. The Cauchy relations seem to be obeyed somewhat better as 0 K is approached. The measured values of the TOEM have been employed in calculating a Murnaghan equation of state, which predicts a somewhat higher volume change than the measured one. The elastic Grüneisen constants deduced from the TOEM are in reasonable agreement with the thermal ones in the high-temperature limit.
Pressure relaxation in atomic mixing and preferential sputtering
Authors: Collins R. , Sigmund P.
Published: July 2002

It is demonstrated explicitly that the collective-current concept in atomic mixing, as formulated by Collins, is equivalent with the homogeneous-relaxation model proposed by Hofer and Littmark. Both schemes lead to the balance equation of Sigmund, Oliva, and Falcone. The relation to Titov’s model of high-fluence ion implantation is clarified.
Optimum separate confinement structure for midinfrared HgCdTe heterostructure lasers
Authors: Singh Jasprit , Zucca Ricardo
Published: July 2002

A study for the optimization of HgCdTe heterostructure lasers for applications as midinfrared wavelength sources has been carried out. Structures are examined to emit photons at 2.5 and 4.5 μm at 77 K. For the 2.5 μm case, it is found that a quantum-well laser with well width of 200 Å in a separate confinement structure is optimum. For the 4.5 μm case the optimum structure is one with a 1000 Å active region. For the 4.5 μm case the high carrier density at threshold in quantum wells and the consequent high Auger rates do not allow the decrease of threshold current with smaller well sizes. This result is rather general for narrow-gap zinc-blende semiconductors and represents a cautionary warning against the commonly held belief that narrow quantum wells will always improve threshold currents.
Localization in one-dimensional random superlattices
Authors: Su W. P. , Shih H. D.
Published: July 2002

The electronic structure of random sequences of superlattice layers is studied within the one-dimensional Anderson model. We find significant variation of the degree of localization of the band-tail states with the random stacking sequence. Comparison of the calculated results with existing data on the photoluminescence of disordered superlattices is made.
An improved layered ferrite slab waveguide with a corrugated interface for mm-wave radiation
Authors: Sidick Erkin , Seki Ryoichi
Published: July 2002

An improved model of a layered ferrite waveguide is proposed as an extension of a previously studied structure, and the radiation characteristics of millimeter waves in the waveguide are investigated theoretically. The waveguide has a corrugation located on the interface between a ferrite slab and a grounded dielectric slab. The radiation efficiency is shown to be increased in the present waveguide in comparison to that of the previously studied structure with corrugation on the upper surface of a ferrite slab.
A new GaAs sawtooth-doping-superlatticed switching device
Authors: Sun Chung-Yih , Liu Wen-Chau
Published: July 2002

A GaAs switching device with a double sawtooth-doping-superlattice (SDS) structure has been fabricated and demonstrated. By using the Kronig–Penny model, the dispersion relations of the electrons and holes in the zigzag quantum well were investigated. Two different delta-doping sheets, 1 × 1012 and 1 × 1013 cm−2, were used in the double SDS structure. The period length of the SDS was 600 Å. Thus, due to the sufficient barrier height and width, the calculated transmission coefficient of electrons was negligible when the studied structure was at thermal equilibrium or under a small-biased condition. However, when a larger bias was applied, the studied structure exhibited an interesting negative-differential-resistance (NDR) performance in the experimental current-voltage characteristics. This was caused by an avalanche multiplication process and the potential redistribution in SDS periods. Particularly, an interesting intermediate state between the initial off state and final on state was observed at 77 K. Consequently, with a suitable adjustment of structural parameters, e.g., the delta-doping density and the period number of the SDS, the proposed device provides good potential for switching circuit applications.
Nonlinear optical response of amorphous Si:H
Authors: Henari F. , Blau W.
Published: July 2002

We have carried out laser-induced grating and nonlinear transmission measurements near the band gap of amorphous Si:H. The nonlinear four-wave mixing response deviated substantially from traditional third-order effects. An intensity dependence of less than cubic for the diffracted intensity was observed. The nonlinearity does not follow the absorption profile. An effective nonlinear susceptibility χ(3)=5×10−9 esu was found at 572 nm. The nonlinear transmission showed saturable absorption at low intensities, and induced absorption at high intensities. This behavior can be attributed to light-induced defects.
Mechanisms of high-current pulses in lightning and long-spark stepped leaders
Authors: Larigaldie S. , Roussaud A. , Jecko B.
Published: July 2002

The mechanisms of high-current transients in lightning stepped leaders and in long laboratory sparks at negative polarity are analyzed both from improved time resolution measurements and from systematic identifications of the various elements of the discharge during a pulse. A qualitative model was first constructed when the observations made were correlated with some previously known basic phenomena. Then the model was made quantitative by means of a computer simulation of spark formation, carried out from a modified program in electromagnetics. The relevance of the proposed model was checked for long sparks at negative polarity when computed and recorded current pulse wave forms were compared. Finally, the model of stepped-leader development was applied to a full scale event: an in-flight lightning strike on an instrumented aircraft. Slight discrepancies between computed and recorded current pulse wave forms may indicate possible underestimation of the electromagnetic high-frequency threat to sensitive airborne equipment due to the frequency limitations of the transient recorders used for lightning characterization on aircraft.
A study of the kink-related excess low-frequency noise in silicon- on-insulator n-metal-oxide-semiconductor transistors operated at liquid helium temperatures
Authors: Simoen E. , Dierickx B. , Claeys C.
Published: July 2002

In this paper, the drain voltage dependence of the low-frequency (lf) noise of partially depleted silicon-on-insulator n-metal-oxide-semiconductor-transistors (n-MOSTs) is investigated in detail at liquid helium temperature. As will be shown, an increase in the noise spectral density is observed at the kink position, similar to bulk n-MOSTs operated at 4.2 K. This excess noise introduces a Lorentzian generation-recombination (GR) component in the lf noise spectrum. The physical mechanism underlying the GR noise is thought to be the same as for bulk transistors: ionization and capture in the depletion region of carriers, which are created at the drain. This generates a fluctuation in the depletion charge, which is translated into a fluctuation of the drain current via a change in the threshold voltage. A model will be proposed that is derived from the analysis previously established for the bulk case. Extension of the model to fully depleted, thin-film transistors and to higher temperatures (77 K, 300 K) will be briefly outlined. As will be demonstrated, a good overall agreement between theory and experiment is obtained whereby the key features of the noise overshoot, i.e., its position and amplitude, are consistently reproduced.
X-valley tunneling in single AlAs barriers
Authors: Boykin Timothy B. , Harris James S.
Published: July 2002

We examine tunneling through GaAs/AlAs/GaAs single-barrier heterostructures of varying widths using a tight-binding model, including in the calculation for the first time the effects of space-charge regions. Our calculation concentrates on quantities more representative of what one can measure in an experiment than previous efforts and we compare our results to both those earlier tight-binding and pseudopotential-based scattering matrix calculations, none of which include space-charge regions, and results obtained with the more usual envelope-function approach. We find that including the accumulation and depletion layers in the calculation leads to results which are somewhat unexpected in light of the older efforts. Finally, we briefly examine the role of nonzero k∥ (wave vector parallel to the interface) on tunneling and present current-voltage characteristics for a 10 monolayer barrier structure.
Performance of Bi-Sr-Ca-Cu-O thick film rf SQUIDs at liquid nitrogen temperatures
Authors: Chaudhry Sangeeta , Khare Neeraj , Gupta A. K. , Tomar V. S.
Published: July 2002

Periodic oscillations in the V-Φ characteristics have been observed up to 97 K in radio frequency SQUIDs fabricated from screen printed Bi-Sr-Ca-Cu-O thick films. The SQUID has been operated successfully in the flux-locked-loop mode at 77 K. Stability of the SQUID output and its noise performance have been found to improve considerably when the SQUID is operated in the flux-locked-loop mode as compared to that in the open-loop mode. The spectral noise density, in the white noise region, is 5×10−4 Φ0/√Hz at 77 K for the flux-locked-loop mode.
A silicon diverging beam modulator fabricated by part of metal-oxide-silicon process modules
Authors: Huang H. C. , Yee S. , Darling R. B. , Chan C. H.
Published: July 2002

We hereby report a silicon diverging beam modulator fabricated by part of metal-oxide-silicon process modules. The particular feature of the fabrication is that a mode modulation technique is utilized. This technique changes intensity confinement of the propagation modes of a silicon partial waveguide such that an intensity modulation is achieved. Modulation indices of greater than 15% have been observed with a forward-bias current of 24 mA and a modulation length of 3 mm. By reducing the core width of the modulator, the modulation index can be further increased.
Formation of titanium silicide during rapid thermal annealing: Influence of oxygen
Authors: Richter F. , Bugiel E. , Erzgräber H. B. , Panknin D.
Published: July 2002

Rapid thermal annealing of Ti/Si couples has been investigated by transmission electron microscopy using cross-section preparation, four-point probe measurement of sheet resistance, and Auger electron spectroscopy. The development of crystalline phases was determined in this way with high spatial and temporal resolution. Our results indicate the decisive influence of the solubility of oxygen on the phase sequence.
Infrared fluorescence spectra of Nd3+ sites in gadolinium gallium garnet:Nd and gadolinium gallium garnet:Nd,Cr
Authors: Guyot Y. , Bausá L. E. , Camarillo E. , García Solé J. , Vergara I. , Monteil A. , Moncorgé R.
Published: July 2002

The fluorescence (excitation and emission) spectra of Nd3+ ions in gadolinium gallium garnet (GGG):Nd and GGG:Nd,Cr have been investigated in the spectral region of laser interest, 4F3/2→4I11/2 transitions. Four sites, denoted as α, β, γ, and δ are present in GGG:Nd, while two additional sites, ε and ζ, appear in GGG:Nd,Cr crystals. Site selective excitation (by using a Ti-sapphire laser) has permitted to obtain the energy level values for each Nd3+ site. This information is necessary for making lasers with these materials. Thus, it has been determined that the γ site is the responsible for laser action in the GGG:Nd minilasers.
Soft x-ray photoemission investigation on the effect of In doping in CdS/CuInSe2 heterojunction formation
Authors: Nelson Art J. , Niles David W. , Rioux D. , Patel R. , Höchst Hartmut
Published: July 2002

Synchrotron radiation soft x-ray photoemission spectroscopy was used to investigate the development of the electronic structure at the CdS(In)/CuInSe2 heterojunction interface. In-doped CdS overlayers were deposited in steps on single-crystal n-type CuInSe2 at 250 °C. Results indicate that the CdS(In) grows in registry with the substrate, initially in a two-dimensional growth mode followed by three-dimensional island growth as is corroborated by reflection high-energy electron diffraction analysis. Photoemission measurements were acquired after each growth in order to observe changes in the valence-band electronic structure. The results were used to correlate the interface chemistry with the electronic structure at these interfaces and to directly determine the CdS(In)/CuInSe2 heterojunction valence-band discontinuity and the consequent heterojunction band diagram as a function of In dopant concentration. We measured a valence-band offset ΔEv=0.3 eV, independent of In doping.
SnO2 grazing-incidence antireflection films for monochromatization of synchrotron radiation: Design, preparation, and characterization
Authors: Homma H. , Kentjana M. , Alp E. E. , Mooney T. M. , Witthoff E. , Toellner T.
Published: July 2002

Grazing-incidence antireflection (GIAR) films are used to suppress electronic reflectivity, while maintaining good nuclear resonant reflectivity to help monochromatize the synchrotron radiation down to the μeV level of energy resolution. Such monochromators will have superior throughput by a factor of 100–500 compared to natural linewidth of individual nuclei. 119SnO2 GIAR films of various thicknesses were designed and fabricated on Pd-coated quartz substrates. The performance of these films was evaluated based on the measurement of off-resonance x-ray reflectivity using MoKα radiation and conversion electron Mössbauer spectroscopy. A Pd film was deposited on a quartz substrate at room temperature and was subsequently annealed at 500 °C in an Ar atmosphere at a substrate temperature of 200 °C, SnO2 was sputtered onto the Pd by reactive sputtering in an Ar and O2 atmosphere. Employing quantitative analysis of surface and interface roughness, a series of experiments was also conducted to determine optimal sputtering conditions.
Compaction, distribution, and chemical bonding of tungsten-implanted glassy carbon
Authors: Hoffman A. , Evans P. J. , Cohen D. D. , Paterson P. J. K.
Published: July 2002

The composition, distribution, and chemical bonding of tungsten-implanted glassy carbon have been studied for ion doses between 3.5×1014 and 1.5×1017 ions/cm2. The implantations were performed using a metal-vapor vacuum arc ion source which for an extraction potential of 20 kV yields a mean implant energy of 60 keV. The implanted layer was examined by Rutherford backscattering spectrometry and Auger profiling spectroscopy. In addition, volume effects were assessed by measuring the step height between implanted and unimplanted regions. It has been established that the implanted tungsten is bonded in a carbidic configuration. This is in agreement with thermodynamic data which favors carbide formation rather than separate phases of metallic tungsten and graphitic carbon. For the highest dose studied, 1.5×1017 ions/cm2, the tungsten distribution was found to be nearly constant near the surface followed by a decrease at larger depths. For ion doses up to 3.6×1016 ions/cm2, the maximum tungsten distribution was found to occur at a depth 300±100 Å and the implanted and retained doses were found to be equal. For tungsten ion doses between 7.35×1014 and 1×1016 ions/cm2, the implanted region was found to be compacted by 300±100 Å. The latter is attributed to an irradiation-induced densification of glassy carbon from 1.5 to 2.1 g/cm3. This compaction effect is also found to occur upon C+ (30 keV) implantation for ion doses between 3×1016 and 1.3×1017 ions/cm2. In this case the implanted layer was compacted by 400±100 Å. For higher tungsten doses, a steep increase in compaction of the tungsten implanted layer was observed. This is attributed to chemical and sputtering effects.
Voltage-dependent scanning tunneling microscopy image of the GaInAs/InP multiquantum well structure
Authors: Kato Takashi , Osaka Fukunobu
Published: July 2002

The cross-sectional (110) surface of the GaInAs/InP multiquantum well (MQW) structure was observed by scanning tunneling microscopy (STM). In this STM observation using the constant-current mode, it was found that the corrugation amplitude of the MQW image sharply increased upon decreasing the applied positive bias voltage of the tip. Using a simple free-electron model, this phenomenon is explained: Due to the difference of the valence-band energy levels in the well and the barrier layers, the number of valence electrons above the Fermi level of the tip is different between these layers; the ratio of the tunneling current (Iwell/Ibarrier) becomes larger when the Fermi level of the tip is closer to the valence-band energy level in the barrier layers, resulting in a larger corrugation amplitude of the MQW image.
Corrections to the constant photoconductivity method for determining defect densities, with application to amorphous silicon
Authors: Bube Richard H. , Benatar Lisa E. , Grimbergen Michael N. , Redfield David
Published: July 2002

The constant photoconductivity measurement (CPM) technique has been widely used to determine the density of defects in thin films of materials where only a small absorption coefficient exists for these defects, particularly in determining the dangling-bond density in hydrogenated amorphous silicon. Interpretation and modeling in the literature assume that CPM gives an accurate value for the density of defects present. The limitations of this method are examined as a function of several defect parameters, and particularly as a function of the Fermi energy, in models consisting of one level, a Gaussian distribution of levels, two independent levels, and the levels typical of a multivalent defect, applying the results of the latter to actual data on optical degradation kinetics in amorphous silicon. It is concluded that a prerequisite for reliably relating CPM-determined densities to actual defect densities is a knowledge of the relative location of the equilibrium Fermi level and the energy level of the defect.
Microstructure and magnetic anisotropy of ultrathin Co/Pt multilayers grown on GaAs (1̄1̄1̄) by molecular-beam epitaxy
Authors: Cho N.-H. , Krishnan Kannan M. , Lucas C. A. , Farrow R. F. C.
Published: July 2002

Multilayers of (Co3 Å, Pt15 Å)x, x=15 or 30 repeats, with or without a 200 Å silver buffer layer, were grown on GaAs (111) substrates by molecular-beam epitaxy. Vibrating sample magnetometry measurements confirmed that the samples with the Ag buffer layer show strong uniaxial magnetic anisotropy perpendicular to the surface. The perpendicular anisotropy exhibited by these metallic superlattices is discussed in terms of the microstructure of the overall multilayer stack, as well as the structural characteristics of the Co interface layer. Samples grown on the Ag buffer layer show strong (111) texture with 30–40-nm-size twin-related grains. These grains, correspond to the two possible (111) stacking sequence for an fcc lattice, i.e., double positioning. However, direct growth on GaAs (111) results in randomly oriented 10–20 nm grains. All samples exhibit a repeat period of 1.83 nm in both low-angle reflectivity and high-angle Θ–2Θ x-ray scattering measurements. In addition, transverse scans through the low-angle multilayer Bragg peaks show the interfaces to be diffuse in nature indicative of considerable in-plane inhomogeneity and/or compound formation. High-resolution electron microscopy measurements of cross sections compared with image simulations confirm that the interface layer is diffuse and its stoichiometry is such that the Co occupation is less than 40%. Redistribution of Co should then extend over at least four monolayers. The nanostructure of the samples grown with the Ag buffer layer comprises an eight atomic layer repeat with the Co interface layer diffuse over four monolayers. The microstructure is strongly (111) textured with columns of twin related 30-nm-sized grains. It is suggested that the combination of interdiffusion, highly oriented but twin-related columnar growth, small grain size with a possible nanometer-scale second phase may be the key to the understanding of the perpendicular anisotropy observed in these (111) superlattices.
Theoretical study on silicon-nitride film growth: Ab initio molecular orbital calculations
Authors: Koseki Shiro , Ishitani Akihiko
Published: July 2002

The growth mechanism of SiN films in the chemical vapor deposition process was theoretically investigated using ab initio molecular orbital methods. A reactive intermediate SiH2 (or SiCl2) is produced by the thermal decomposition of SiH4 (or SiH2Cl2). The intermediate inserts into the N—H bond of NH3 to form H3Si—NH2 (or HCl2Si—NH2). From the Si atom of the insertion product, H2 (or HCl) is eliminated and a silylene-like reaction site (—Si—H) is created again. Such processes generate various kinds of SiN particles. On the surface of SiN film, SiH2 (or SiCl2) insertion and H2 (or HCl) elimination seem to occur repeatedly, which leads to the SiN film growth. Thus, the chemical features of SiH2 and SiCl2 may explain the difference of the film-growth mechanisms between the mixtures of SiH4+NH3 and SiH2Cl2+NH3.
Fidelity of optical phase conjugation by photorefractive degenerate four-wave mixing in barium titanate
Authors: Naylor D. L. , Tam P. W. , Hellwarth R. W.
Published: July 2002

The fidelity of optical phase conjugation resulting from photorefractive degenerate four-wave mixing in barium titanate was investigated using an interferometric technique. The effect on the fidelity of varying the relative intensities of the beams entering the crystal was studied. It was found that the intensity of the image to be conjugated as a fraction of the total intensity in the crystal is the dominant factor. The effects of angular misalignment of the incident laser beams were investigated and a strict angular criterion established. The angular tolerance appears to be determined by the Bragg diffraction criterion for infinite plane waves, which may be much less than the divergence of the image bearing beam to be phase conjugated.
Carrier removal and defect behavior in p-type InP
Authors: Weinberg I. , Swartz C. K. , Drevinsky P. J.
Published: July 2002

A simple expression, obtained from the rate equation for defect production, was used to relate carrier removal to defect production and hole trapping rates in p-type InP after irradiation by 1-MeV electrons. Specific contributions to carrier removal from defect levels H3, H4, and H5 were determined from combined deep-level transient spectroscopy (DLTS) and measured carrier concentrations. An additional contribution was attributed to one or more defects not observed by the present DLTS measurements. The high trapping rate observed for H5 suggests that this defect, if present in relatively high concentration, could be dominant in p-type InP.
Thermoelastic behavior of grain-boundary superlattices
Authors: Phillpot S. R.
Published: July 2002

The thermal-expansion coefficients of superlattices of twist grain boundaries on the (100) plane in fcc Cu and Au are calculated using a generalization of the Grüneisen relation. A strong anisotropy in the in-plane and out-of-plane thermal-expansion coefficients is found. The thermal expansion of these superlattices is found to be determined from a complex interplay of the structural disorder arising from the interfaces with the associated zero-temperature lattice-parameter changes. The relationship between the thermal expansion and the elastic properties is explored.
Structural properties of CuInxGa1−xSe2 thin films prepared by rf sputtering
Authors: Yamaguchi Toshiyuki , Matsufusa Jiro , Yoshida Akira
Published: July 2002

Cu-In-Ga-Se alloy thin films were prepared by rf sputtering from powder targets previously synthesized by reacting Cu, In, Ga, and Se in the various ratios. From x-ray diffraction analyses, these thin films have chalcopyrite structure of CuInxGa1−xSe2, and the lattice parameters (a and c) are varied linearly with indium content x in the thin films from CuInSe2 to CuGaSe2. The preferred orientation factor f(112) in these thin films was estimated. Its value was always more than 0.79, specifically 0.96 in a CuInSe2 thin films. From scanning electron microscope images, the grain sizes in these thin films decrease with the decrease of indium content. From x-ray photoelectron spectroscopy analyses, the binding energies of the Se 3d, In 3d5/2, Cu 2p3/2 and Ga 2p3/2 in these thin films shifts to higher energies with decreasing indium content, where a slight bowing behavior is observed.
Diffraction characterization of totally relaxed, partially relaxed, and unrelaxed semiconductor strained-layer superlattices
Authors: Duan X. F. , Fung K. K.
Published: July 2002

Diffraction from totally relaxed, partially relaxed, and unrelaxed strained-layer superlattices is considered in the light of the similar rocking curves obtained by convergent-beam electron diffraction and x-ray diffraction. Diffraction of the superlattice can be described by an intensity expression given in terms of the strains and thicknesses of the superlattice bilayer. It is shown that the intensity profile of a reflection of a given superlattice depends on the difference of the tensile and compressive strains, i.e., the oscillating strain, of the superlattice bilayer. The oscillating strain of the superlattice bilayer, to a very good approximation, is independent of the state of strain in the step model of the superlattice. The insight gained leads to the understanding and prediction of the general form of the rocking curve of a superlattice from its growth parameters, although simulation of the rocking curve is still needed in order to obtain accurate structural parameters.
Theory of the electron-plasmon interaction in Monte Carlo calculations through the direct solution of the Poisson equation
Authors: Mansour Nabil S. , Janzou Steven H. , Brennan Kevin F.
Published: July 2002

Ensemble Monte Carlo calculations of the steady-state electron drift velocity in degenerate bulk GaAs using a self-consistent algorithm specially tailored to directly include the electron-plasmon interaction are presented. The critical issues implicit in the direct approach are the mesh size, charge assignment to the mesh nodes, interpolation of the field at the particle location, and the frequency with which the solution of the Poisson equation is updated. All of these factors determine the stability of the system, the accuracy, and the computational time required in the calculation. Comparison is made to quantum mechanically based techniques in which the electron-plasmon interaction is treated as an additional scattering mechanism. It is found that the steady-state electron drift velocity in bulk degenerate GaAs calculated using the semi-classical approach for the electron-plasmon interaction is significantly less than that calculated assuming no electron-plasmon interaction is present. The steady-state drift velocity calculated using the quantum mechanical approach, wherein the electron-plasmon interaction is treated as a separate scattering mechanism, is affected in the opposite way, i.e., the velocity is greater than in the absence of the electron-plasmon interaction. Neither model can at present be considered complete and caution should be exercised in adopting either approach in device simulation until a more satisfactory model has been developed.
Assessment of density of states extraction from the space charge limited current measurement: A numerical simulation
Authors: Smith J. H. , Fonash S. J.
Published: July 2002

The den Boer analysis of single carrier space charge limited currents is commonly used to determine the density of states function within the gap of amorphous silicon hydride and other newly emerging materials. By using a device simulator, we assess the validity and accuracy of this analysis. In particular, den Boer’s assumptions of 0 K occupation statistics, a spatially uniform distribution of injected charge carriers, spatially uniform material, no generation/recombination, and no contact effects are investigated. We find that in the absence of contact effects and with spatially uniform states the den Boer analysis is accurate to within a factor of 2. However, current limiting due to contact effects at higher current values and diffusion dominated current at lower applied biases should not be mistaken for features of the density of states. In addition, if the gap states are not spatially uniform, gross overestimates (by as much as an order of magnitude for the device studied here) of the density of states can result. In general, only a limited range of the gap of the semiconductor can be mapped with this technique for a single device length, although multiple device lengths can be used to increase this range.
Electrochemical capacitance-voltage profiling of n-type ZnSe
Authors: Wang S. Y. , Simpson J. , Prior K. A. , Cavenett B. C.
Published: July 2002

In this article we report the first detailed study of electrochemical capacitance-voltage profiling of ZnSe. An electrolyte consisting of 1 M sodium hydroxide and 1 M sodium sulphite has been developed that does not deposit selenium while etching the surface of n-type ZnSe during C-V profiling. The dissolution number of the electrolyte/ZnSe system is a function of the strength of electrolyte and the etching current and in order to obtain an accurate etching depth a constant etching current mode was used. A wide range of doping concentrations including both uniformly doped and staircase structures have been measured demonstrating that the electrochemical C-V profiler can now be a routine tool for assessing the growth parameters of ZnSe.
Wide band-gap hydrogenated amorphous silicon carbide prepared from a liquid aromatic carbon source
Authors: Nevin W. A. , Yamagishi H. , Tawada Y.
Published: July 2002

By utilizing the aromatic molecule xylene, we have prepared hydrogenated amorphous silicon carbide (a-SiC:H) for the first time from an aromatic carbon source. Good-quality films are obtained, over a wide range of optical band gaps from 2.2 to 3.5 eV and carbon content from 0.4 to 0.9 atomic fraction. Infrared measurements indicate that the films contain an underlying inorganic network of bonded Si, C, and H atoms, as found in a-SiC:H prepared from conventional carbon sources, but in addition, have substantial organic character, evidenced by a large amount of both aromatic and olefinic sp2 carbon bonding.
Tunneling spectroscopy of a quantum resonator
Authors: Takagaki Y. , Ferry D. K.
Published: July 2002

A tunneling current across a thin wall of a narrow-wide-narrow (NWN) cavity region in an electron waveguide is calculated. A lead is attached to the cavity region at right angle through a thin tunneling barrier to drain the current. The transmission coefficient through the NWN waveguide shows successive dips due to resonances through quasibound states in the cavity region as the length of the cavity region or the Fermi energy of an incident electron is varied. The transmission coefficient leaking out the tunneling barrier shows peaks that line up with the dips in the forward transmission probability, demonstrating that a tunneling spectroscopy of the quasibound state levels is provided through sweeping the Fermi energy.
Elasto-optical characterization of strained heterostructures
Authors: Cai Jiguang , Anastassakis E.
Published: July 2002

It is shown through a detailed calculation that in strained layers grown along arbitrary crystallographic directions, the strain components can be determined from oblique transmission experiments. The analysis is based on the elasto-optical effect. Electro-optical contributions due to piezoelectric fields are also considered. Specific examples are treated in detail.
Laser-induced temperature rise in anisotropic substrates
Authors: Lu Yong-Feng
Published: July 2002

Laser-induced temperature rise in anisotropic substrates has been investigated by solving the heat equation in anisotropic media. A model was derived to calculate the laser-induced temperature in anisotropic substrates. It is found that the heat equation in anisotropic media can be simplified to a heat equation in isotropic media by coordinate transformation. The anisotropy of the media can then be reflected by heat source transformation. The results reveal that the laser-induced temperature rise can be significantly affected by the anisotropy of the substrate. The thermal conductivity change along the Z direction (downward into the substrate) will affect the temperature rise more significantly than that along the X and Y directions (parallel to the substrate surface). The anisotropy in thermal conductivity not only affects the peak temperature rise, but also affects the shape of the temperature profile.
High-field electron transport for ellipsoidal multivalley band structure of silicon
Authors: Samudra Ganesh , Chua Soo Jin , Ghatak Ajoy K. , Arora Vijay K.
Published: July 2002

High-field electron-transport properties of bulk silicon are studied taking into account the multivalley band structure with ellipsoidal energy surface. A distribution function that takes into account the anisotropy introduced in electron motion by the high electric field is described. This puts some order in the otherwise completely random motion. The transition from linear to nonlinear behavior is obtained when the energy gained by an electron in traversing a mean free path is comparable to the thermal energy. The drift velocity is shown to be limited to the random thermal velocity for nondegenerate electrons and to the Fermi velocity for degenerate electrons. This indicates independence of the saturation velocity on momentum-randomizing scattering events which control the mobility. The emission of an optical phonon is significant when inelastic scattering length is comparable to the momentum randomizing mean free path. This further lowers the saturation velocity. When an electric field is applied along the <100≳ direction in silicon, an intervalley transfer from valleys with lower mean free path to the ones with higher mean free path is obtained. When compared with the experimental data on silicon, an excellent agreement is obtained.
Low-frequency measurements of cyclotron resonance in a high-mobility 2D electron gas in GaAs/AlGaAs heterostructures
Authors: Polisskii A. V. , Talyanskii V. I. , Cole J. , Smith C. G. , Pepper M. , Ritchie D. A. , Frost J. E. F. , Jones G. A. C.
Published: July 2002

Cyclotron resonance in GaAs/AlGaAs heterostructures, containing a high-mobility two-dimensional electron gas (2DEG), has been investigated with the help of a resonator technique over the frequency range 26–34 GHz. Heterostructures with mobilities from 2×105 to 1.6×106 cm2/V s have been measured. The microwave conductivity of the 2DEG is well described by the Drude model and momentum relaxation time values are found to be close to dc values. The cyclotron mass was found to be 0.068±0.002m0.
The microstructure and electrical properties of nonalloyed epitaxial Au-Ge ohmic contacts to n-GaAs
Authors: Lee H. S. , Cole M. W. , Lareau R. T. , Schauer S. N. , Fox D. C. , Eckart D. W. , Moerkirk R. P. , Chang W. H. , Jones K. A. , Elagoz S. , Vavra W. , Clarke R.
Published: July 2002

The microstructure and electrical properties of nonalloyed epitaxial Au-Ge contacts were studied. Ohmic behavior was obtained after a 3 h anneal at 320 °C with the lowest average contact resistance and specific contact resistivity found to be ∼0.28 Ω mm and ∼7×10−6 Ω cm2, respectively. Localized reactions in the form of islands were observed across the surface of the contact after annealing and were composed of Au, Ge, and As, as determined by secondary ion mass spectroscopy (SIMS) imaging and Auger depth profiling. Back side SIMS profiles indicate deep Ge and Au diffusion into the GaAs substrate in the island regions. Ohmic contact behavior was found to depend upon both the kinetics of the reactions (localized reactions and island growth) and the thermodynamics (substantial diffusion of both Au and Ge) of the system. A model describing the coupled Au and Ge in-diffusion with respect to the GaAs substrate is presented.
Magnetic suspension, critical current, and morphology in YBa2Cu3O7−x-silver composites
Authors: Reich S. , Veretnik D. , Felner I. , Yaron U.
Published: July 2002

In this article we try to elucidate the link between the morphological structure of YBa2Cu3O7−x (YBCO)-silver composite sintered at 980 °C and the high flux pinning observed in this material. This YBCO/Ag system exhibits the suspension phenomenon in magnetic field gradient. The surface of the large YBCO grains is doped with silver and exhibits a high Tc=95 K. This doped phase shows a longer c axis than regular YBCO ceramic material. We believe that this high Tc phase forms a network of superconducting ‘‘rings’’ localized on the YBCO large grains. This network is probably the source of the high flux trapping phenomenon in the composite material.
Viscosity of molten Pd82Si18 and the scaling of viscosities of glass forming systems
Authors: Tsang K. H. , Kui H. W.
Published: July 2002

The viscosity of molten Pd82Si18 was measured from 1045 to 1210 K by capillary flow method. When the molten specimen was not purified in advance and during the experiment, its viscosity could not be determined for temperatures below the liquidus. On the other hand, in the presence of boron oxide flux, the viscosity could be measured up to an undercooling of 58 K below its liquidus and the entire viscosity can be described very well by the following Vogel–Fulcher equation: ln η=−3.779+(1102.135/T−684), where T is in K. No discontinuity is found at the liquidus. The present data were compared with those of other glass forming systems to test the validity of the principle of corresponding states for atomic transport properties.
Formation of cobalt-silicided p+n junctions using implant through silicide technology
Authors: Chen Bin-Shing , Chen Mao-Chieh
Published: July 2002

This paper investigates electrical and material properties of cobalt silicided p+n junctions fabricated using implant through silicide (ITS) technology. The annealing procedure was carried out in an open-tube furnace with flowing nitrogen. To prevent residual oxygen in the furnace from reacting with the cobalt, a passivating film of molybdenum was used during the initial stage of annealing. BF2+ ion implantation was employed for the p+n junction formation. The ITS scheme and the subsequent annealing conditions were evaluated by analysis of the material properties and investigation of the electrical characteristics of the silicided junctions. During high-temperature annealing (≥900 °C), Co silicide releases its high surface energy via silicon precipitation and film agglomeration. High-temperature stability of the Co silicide can be improved by BF2+ ion implantation, as indicated by the retardation of film agglomeration and decreased degradation of sheet resistance. Cobalt-silicided p+n junction diodes with 0.1 μm junction depth measured from the silicide/silicon interface were fabricated at a 700 °C annealing and shown to possess excellent electrical properties. The leakage current density measured at −5 V was 0.5 nA/cm2 and a forward ideality factor of 1.006 was obtained. It was found that Co silicide is suitable to serve as an energy barrier and dopant diffusion source for the ITS scheme at low-temperature annealing. For elevated temperatures (≥900 °C), the use of cobalt silicide as a dopant diffusion source becomes impractical because the dopant evaporation and silicide agglomeration severely degrade junction performance. Nevertheless, the silicide layer can be used as an energy barrier and implantation damage basin for high-temperature processes. No evidence of the electrically activated trapping centers related to the cobalt atoms was observed with BF2+ implantation at energies up to 100 keV. Thus, there is no fundamental limitation prohibiting the formation of cobalt-silicided shallow junctions being fabricated using ITS technology.
Effect of scanning speed on the stability of the solidification interface during zone-melting recrystallization of thin silicon films
Authors: Yoon Sharon M. , Miaoulis Ioannis N.
Published: July 2002

The stability of the moving solidification interface of thin silicon films undergoing zone-melting recrystallization with a graphite strip heater was examined numerically. Unstable growth of the interface occurred for scanning speeds ≥250 μm/s for typical processing conditions. The growth rate of the perturbed interface varied with scanning speed, showing a distinct increase at a velocity of 400 μm/s.
Characterization of thin, doped silicon single crystals by x-ray diffraction
Authors: Joksch Stefan , Graeff Walter , Zaumseil Peter , Winter Ulrich , Csepregi Laszlo , Iberl Franz , Freund Andreas K.
Published: July 2002

A free-standing, extremely thin silicon membrane, prepared by chemical vapor deposition of an epitaxial layer and subsequent anisotropic, selective etching, has been studied. The epitaxial layer consisted of a sandwich structure of undoped and highly boron/germanium-doped films, serving as an etch stop. The results of double- and triple-crystal x-ray diffractometry in the Bragg and Laue case, i.e., in reflection and transmission, respectively, are reported using x radiation between 8.05 (Cu Kα1) and 17.5 keV (Mo Kα1) photon energy. The thickness of the crystal was determined to z0=(30±1) μm and its curvature to <7×10−4 m−1. The influence of the doped etch stop layer on the x-ray reflectivity is discussed in detail.
High-field transport properties of InAsxP1−x/InP (0.3≤x≤1.0) modulation- doped heterostructures at 300 and 77 K
Authors: Yang D. , Bhattacharya P. K. , Hong W. P. , Bhat R. , Hayes J. R.
Published: July 2002

We have measured the high-field transport characteristics of pseudomorphic InAsxP1−x/InP (0.3≤x≤1.0) modulation doped heterostructures at 300 and 77 K. The field dependent steady state average velocities increase steadily with increase in x. The maximum velocities that have been measured in InAs/InP are 1.7×107 cm/s (2.5 kV/cm) and 3.2×107 cm/s (2.2 kV/cm) at 300 and 77 K, respectively. These are the highest velocities measured in any modulation doped heterostructure. The field dependent channel carrier concentration and mobility data indicate that there is very little real space transfer of carriers at high fields and this is confirmed by results from steady state Monte Carlo calculations.
Effects of reducing and oxidizing thermal treatments on epitaxial Bi2.1−xPbxSr2.9−yCayCu2O8+z films
Authors: Balestrino G. , Marinelli M. , Milani E. , Montuori M. , Paoletti A. , Paroli P.
Published: July 2002

We have studied the effects of reducing and oxidizing thermal treatments on truly epitaxial Bi2.1−xPbxSr2.9−yCayCu2O8+z films. Films were grown on NdGaO3 substrates by liquid phase epitaxy with a very narrow mosaic spread (less than 0.1°). Transport and structural properties were investigated for a number of films after various annealing treatments. Tc is shifted to higher values by reducing treatments and to lower values by oxidizing treatments for all lead concentrations. For all films the maximum range of variation Tmaxc − Tminc is about 15 K. However, the Tc for the as-grown lead doped films is lower relative to the undoped ones. Furthermore, in the case of doped films, the optimum hole concentration (corresponding to the highest Tc of 94 K) cannot be reached by reducing treatments. For both doped and undoped films the c-lattice parameter was found to increase slightly after reducing treatments.
Magneto-optical Faraday effect of polystyrene foils containing fine cobalt particles
Authors: Ohmori Kenji , Imamura Takayuki , Yamaguchi Kazuhiro , Fujii Toshitaka
Published: July 2002

The magneto-optical Faraday effect of plastic foils composed of cobalt fine particles fixed by polystyrene was investigated. Colloidal suspensions of the cobalt particles from 5 to 10 nm in diameter were prepared by thermal decomposition of dicobalt octacarbonyl dissolved by toluene. After that, polystyrene was added to the solution. The solution was then dried on glass receptacles, and foils 0.1 to 0.2 mm in thickness were obtained by peeling it off from the receptacles. The Faraday rotation spectra θF(λ) of the foils were quite different from those of a cobalt film and polystyrene. θF(λ) varied with the particle size and agglomerate state of particles during solidification.