bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal noise in high-temperature superconducting-normal- superconducting step-edge Josephson junctions
Authors: Berkowitz S. J. , Skocpol W. J. , Mankiewich P. M. , Ono R. H. , Missert N. , Rosenthal P. A. , Vale L. R.
Published: July 2002

We have fabricated YBa2Cu3O7−δ-normal metal-YBa2Cu3O7−δ step-edge Josephson junctions that fit the resistively shunted junction model with Johnson–Nyquist thermal noise. The I-V curves are well fit over a large temperature range for junctions of varying critical current values. There is good agreement between the fitted thermal noise temperature and the measured ambient temperature. This is strong evidence that these junctions are not dominated by superconducting shorts longer than the superconducting coherence length.
Enhanced Fe moment in nitrogen martensite and Fe16N2 (invited)
Authors: Wallace W. E. , Huang M. Q.
Published: July 2002

Nitrogen martensite was prepared by treating fine Fe powder with NH3/H2 gas mixtures at temperatures around 665 °C. Upon quenching to a temperature Tq, the γ phase which had formed at the elevated temperature undergoes a martensitic transformation to form nitrogen martinsite (α′ Fe-N alloy), a tetragonal material. Heat treating this material for 1–2 h at 140±10 °C produced the α″ phase Fe16N2. The α′ phase occurred along with γ Fe-N. From x-ray line intensities, the amount of α′ phase was ascertained. The α′ phase exhibits a room-temperature moment of 250±10 emu/g. Fe16N2 is formed along with α Fe and also there is retained N-austenite. Using XRD and conventional magnetic measurement procedures, one obtained 280±10 emu/g for the saturation moment of Fe16N2. The experimental Fe moment, 2.88 μB, is in excellent agreement with the most recent band-structure calculations, 2.85 μB. The ternary systems (Fe,M)16N2 were studied with M=Mn or Ni. γ Fe-Mn nitride readily forms, but it does not undergo the γ→α′ transformation. The α′ phase and possibly the α″ phase form in the Fe-Ni nitride.
Giant magnetoresistance in Co/Cu multilayers after annealing
Authors: McGuire T. R. , Harper J. M. , Cabral C. , Plaskett T. S.
Published: July 2002

Multilayer films of [Co 10 Å/Cu(t)]64 with copper thicknesses from t=10 to 29 Å annealed for 1 h at temperatures about 350 °C showed a decrease in sample resistivity at 4.2 K. The giant magnetoresistance (GMR) maximums for as-deposited films at t=10 Å and t=23 Å shifted with annealing. The GMR decreased for t=10 Å and t=23 Å but increased for t=19 Å and t=29 Å indicating a complex behavior with annealing. Similarities with granular films are discussed.
Magnetic properties of nanophase cobalt particles synthesized in inversed micelles
Authors: Chen J. P. , Sorensen C. M. , Klabunde K. J. , Hadjipanayis G. C.
Published: July 2002

Cobalt particles were prepared with the microemulsion method in the binary system of DDAB (didodecyldimethylammonium bromide)/toluene by reduction of CoCl2 with NaBH4. The average particle size of the as-prepared samples could be varied from 1.8 to 4.4 nm by controlling the concentration of CoCl2 in the solution of DDAB in toluene. TEM studies showed that the particles were quite uniform and well isolated. The particle sizes determined from magnetic data were consistent with those measured by TEM. The coercivity of the particles at 10 K increased from 640 to 1250 Oe as particle size increased from 1.8 to 4.4 nm. The blocking temperature of the particles increased from 19 to 50 K for the same size range. The saturation magnetization σs at 2 K increased with decreasing particle size. The value of σs of the particles with average size of 1.8 nm was about 200 emu/g, which is 20% higher than the bulk value. This implies that the magnetic moment per atom is enhanced in the nanoparticle system.
Laser ablation of polymethylmethacrylate and polystyrene at 308 nm: Demonstration of thermal and photothermal mechanisms by a time-of-flight mass spectroscopic study
Authors: Tsunekawa Makoto , Nishio Satoru , Sato Hiroyasu
Published: July 2002

Laser ablation of polymethylmethacrylate and polystyrene films by an excimer laser at 308 nm was studied by detecting the fragments by time-of-flight mass spectroscopy after multiphoton ionization. Neutral fragments ablating from the polymer surface were predominantly monomer and dimer methylmethacrylate and monomer styrene, as deduced from the mass spectra. Thus, ‘‘unzipping’’ of these polymers occurs. The dependence of fragment yields on ablating laser fluence and kinetic energy distributions of ablating species demonstrates thermal and photothermal dissociation of these polymers under the experimental conditions.
Considerations on the precursor decay anomaly in lithium fluoride
Authors: Sano Yukio
Published: July 2002

The purpose of this study is to investigate the cause of the precursor decay anomaly in LiF single crystals, based on the stress-time profiles measured by Asay et al. [J. Appl. Phys. 43, 2132 (1972)] which suggest that the decay is caused by an evolving follower wave overtaking the precursor. It is initially inferred using the profiles that this follower wave consists in turn of a contraction wave C, two degenerate contraction waves I, II, a subrarefaction wave R′, and a rarefaction wave Rb, which appear consecutively. An exact equation for the relaxation function, which can be applied to all five followers, is then derived. Next, inequalities for the dislocation density at the front of each follower are derived by incorporating their various properties into the equations; these are then evaluated using the experimental data of Gupta, Duvall, and Fowles [J. Appl. Phys. 46, 532 (1975)]. The validity of the approach is confirmed by showing that their predicted densities satisfy the inequalities, although both the evaluations and the predictions are still too large to be true in the early stage of the decay process. Finally, this anomaly is traced to the common use of an illusive precursor decay curve of unrealistically large slope.
Sheath structure in a plane probe inmersed in an electronegative plasma
Authors: Fernández Palop J. I. , Ballesteros J. , Colomer V. , Hernández M. A. , Dengra A.
Published: July 2002

The sheath zone near a plane probe inmersed in an electronegative plasma is analyzed. A theoretical model for the sheath zone was used to derive a new expression for the Bohm criterion. The new expression holds at any negative probe potential. Solving the model equations allows one to obtain a wealth of information including the spatial distribution of various quantities (electric field, potential, and particle densities), the sheath thickness, and the floating potential.
Kinetic study of atomic hydrogen etching of GaAs (100)
Authors: Elzey John W. , Meharg Paul F. A. , Ogryzlo Elmer A.
Published: July 2002

Semi-insulating GaAs (100) was exposed to hydrogen atoms that were produced in a remote microwave plasma. The flux of hydrogen atoms was determined by isothermal calorimetry. Hydrogen atom partial pressures between 3 and 51 mTorr were found to etch GaAs continuously at rates that ranged from 3 to 40 nm/min at temperatures above 200 °C. The rate coefficients could be fitted to the Arrhenius equation, k=105.2±0.6 nm min−1 Torr−1 e−0.25±0.05 eV/kT, between 200 and 360 °C. The surfaces etched crystallographically and were found to become arsenic deficient. The possible influence of hydrogen atom absorption by GaAs is considered.
Performance of a plasma opening switch in positive polarity on Gamble I using flashboard plasma sources
Authors: Renk T. J.
Published: July 2002

The successful development of the Plasma Opening Switch (POS) for inductive storage applications has been largely confined to negative polarity operation. Some models of POS behavior suggest that this is because in a positive polarity coaxial configuration, the weaker magnetic field at the cathode position retards the switch opening process. This article describes experiments in which both conductor radii in the POS region were significantly reduced. Anode- and cathode-side current monitors indicate that voltages greater than open-circuit are generated at the POS position, but there is a significant amount of electron flow out of the POS, depending upon load impedance. Flow impedance analysis indicates that a relatively small gap appears in the POS plasma after switch opening. Switch performance is also compared between flashboard and carbon gun plasma sources, with the latter operated both in positive and negative polarity.
Giant magnetoresistance in Fe/Mo multilayers formed by magnetron sputtering
Authors: Yan M. L. , Lai W. Y. , Wang Y. Z. , Li S. X. , Yu C. T.
Published: July 2002

Fe/Mo multilayers have been prepared by magnetron sputtering. Giant magnetoresistance (GMR) has been found in the samples with an antiferromagnetic interlayer coupling. The magnetoresistance (MR) ratio exceeds 12% at 4.2 K, and it oscillates as a function of Mo spacer thickness. The oscillation period is about 10–12 Å, which is consistent with the case of the reported Kerr effect. The results suggest that the presence of antiferromagnetic coupling and the absence of GMR are the result of the sharp interfaces, and that relatively rough interfaces and moderately thin Fe layer thickness are the key factors for enhancing MR in the sputtered films.
High-level injection in quasi-neutral region of n/p junction devices: Numerical results and empirical model
Authors: Yue Y. , Liou J. J. , Ortiz-Conde A.
Published: July 2002

The ambipolar transport equation is solved numerically to provide the physical insight of high-level free-carrier injection in the quasi-neutral region of n/p junction devices. It is shown that the diffusion current-only approximation used conventionally to model the minority current fails when high-level injection prevails. Our results further suggest that in high-level injection, the drift current is comparable to the diffusion current and that the minority current in the quasi-neutral region is position dependent even if the region is thin. Based on the numerical results, an empirical model is also developed which, while retaining the form of the conventional diffusion current-only model, can accurately describe the current transport in an n/p junction including the effect of high injection.
Collisional sheath dynamics
Authors: Sheridan T. E. , Goeckner M. J.
Published: July 2002

Using a collisional fluid model, we investigate the dynamics of the collisional sheath following the application of a large, negative, finite-rise-time, voltage pulse to a planar target. For a moderate amount of collisionallity, we find that the ion impact energy decreases significantly, while the sheath width and ion flux (i.e., the ion current) are not greatly reduced. The theory shows fair agreement with experimental measurements.
Multiple internal reflection infrared spectroscopy of hydrogen adsorbed on diamond(110)
Authors: McGonigal M. , Russell J. N. , Pehrsson P. E. , Maguire H. G. , Butler J. E.
Published: July 2002

Multiple internal reflection infrared spectroscopy is used to investigate the reaction of atomic hydrogen with a polished, natural type IIa diamond (110) dehydrogenated surface held at 673 K. A single infrared absorption band at 2880 cm−1 is observed and is attributed to the C-H stretching mode on an sp3 hybridized surface carbon. The band is stable up to surface temperatures between 1073 and 1173 K, and is absent when the dehydrogenated surface is exposed to atomic deuterium.
Compositional influence on photoluminescence linewidth and Stokes shift in InxAlzGa1−x−zAs/InyAluGa1−u−yAs/InP heterostructures grown by molecular beam epitaxy
Authors: Hillmer H. , Lösch R. , Schlapp W.
Published: July 2002

We have grown InxAlzGa1−x−zAs/InP heterostructure layers of different geometry and composition by molecular beam epitaxy showing very narrow photoluminescence linewidth, small Stokes shift, and high epitaxial quality. For rising z we observe experimental linewidths strongly increasing from 1.45 meV (InGaAs) to 13 meV (InAlAs). The Stokes shift is found to grow with rising z. Several contributions are discussed to explain the measured compositional variation of linewidth and Stokes shift. The possibility of arbitrarily combining different material compositions is demonstrated by the growth and characterization of multiple quantum well structures, including possible combinations of ternary and quaternary materials in the wells and barriers.
The growth dynamics of thick sputtered copper coatings under the influence of surface diffusion: A quantitative atomic force microscopy study
Authors: Eisenmenger-Sittner C. , Bergauer A. , Bangert H. , Bauer W.
Published: July 2002

The evolution of surface features during the growth of a coating is of vital importance for the resulting overall film structure and morphology. In the low substrate temperature regime (T/TM<0.3) the growth front of a single component material roughens due to fluctuations in the incoming particle beam (kinetic roughening) and relaxes via surface diffusion. The continuous deposition process thus leads to the evolution of surface features of large lateral extensions compared to the extension of the film-forming particles. A detailed experimental study of the surface evolution of sputtered copper coatings with thicknesses ranging from 300 to 105 Å for a working gas pressure of 0.4 Pa is presented. The roughening as well as the relaxation of the film growth front is studied by means of atomic force microscopy (AFM). The increasing lateral extension of the surface features during growth (coarsening) can well be described by a deterministic continuum model of surface diffusion. The model is capable of predicting the evolution of the surface profile’s Fourier transform for increasing deposition times (i.e., increasing film thickness). A retransformation from k space to real space allows for a direct comparison of surface profiles obtained from AFM scans with those resulting from the continuum model and gives good qualitative agreement of the profile shapes.
Alternating current losses in mono- and multicored silver sheathed (Bi,Pb)2Sr2Ca2Cu3O10 tapes at T=27 K in direct current magnetic fields
Authors: Fukumoto Y. , Wiesmann H. J. , Garber M. , Suenaga M. , Haldar P.
Published: July 2002

Alternating current (ac) losses in silver sheathed mono- and multicored (Bi,Pb)2Sr2Ca2Cu3O10 superconducting tapes were investigated at liquid neon temperature (27 K) under various direct current (dc) background magnetic fields. The loss behavior was well accounted for by the Bean critical state model with the inclusion of eddy current loss in the silver sheath. The interfilamentary coupling loss was found to be negligibly small for the present orientation of the tape. In addition, a severe distortion of the loss signal and the B-H hysteresis curve was observed at 27 K when a high dc magnetic field was applied simultaneously with ac fields. The losses of the tapes were also measured at 77 K and compared with the 27 K values.
Effects of process parameters on low-temperature silicon homoepitaxy by ultrahigh-vacuum electron-cyclotron-resonance chemical-vapor deposition
Authors: Tae Heung-Sik , Hwang Seok-Hee , Park Sang-June , Yoon Euijoon , Whang Ki-Woong
Published: July 2002

The effects of the process parameters on the low-temperature Si homoepitaxial growth in an ultrahigh-vacuum electron-cyclotron-resonance chemical-vapor-deposition (UHV-ECRCVD) system are examined by reflection high-energy electron diffraction and transmission electron microscopy (TEM). The substrate dc bias during plasma deposition drastically changes the crystal structure from polycrystalline silicon at negative bias to single crystalline at positive bias. The defect production during plasma deposition is mainly caused by the energetic ions impinging on the Si substrate, and it can be effectively suppressed by the proper control of the process parameters in the direction of minimizing the ion energy. The positive substrate dc bias is a prerequisite for better crystallinity of low-temperature Si, but additionally the other process parameters such as microwave power, distance of the ECR layer from the substrate, SiH4 partial pressure, and total pressure should be definitely optimized to obtain dislocation-free Si epilayers. Dislocation-free Si epilayers are successfully grown at 560 °C at the positive dc bias greater than +10 V with the optimal control of the other process parameters. At temperatures below 470 °C, a high density of defects in the Si epilayers is observed by plan-view TEM, and the growth of the single-crystalline silicon is possible even without substrate heating but with a high density of defects. It is concluded that the substrate dc bias is a critical process parameter and the other process parameters do play a small but significant role as well in determining the crystallinity of the Si epilayers grown by UHV-ECRCVD.
Repetition rate effect on the laser ablation of polymer structures
Authors: Reyna L. G. , Soběhart J. R.
Published: July 2002

Multilayer polyimide structures may be used as an etch stop mechanism in laser ablation processing. By changing the optical properties of the polymer it is possible to better control the depth of the patterns produced by the lasers. We present the effect of the repetition rate in the ablation of multilayer structures. The results are obtained from a photothermal model that includes the light absorption by the decomposed fragments, which shield the polymer from the incoming laser beam. The model also includes an intermediate zone in which the polymer suffers a phase transition. The evolution of the temperature profiles is simulated during each pulse taking into account the pulse shape; however, a simple diffusion model is used between pulses. The results of the simulations indicate the range of values for which the multilayer structure may be used as an effective etch stop mechanism. We found that the effectiveness of the multilayer structure deteriorates for increasing repetition rates.
Photoluminescence of three phonon replicas of the bound exciton in undoped InGaP prepared by liquid phase epitaxy
Authors: Jiang G. C. , Chang Y. , Chang L. B. , Juang Y. D. , Lu W. L. , Lu Luke S. , Chang K. H.
Published: July 2002

Photoluminescence and Raman spectroscopies are employed to study undoped InGaP layers grown on GaAs (100) substrates at 750 °C by liquid phase epitaxy. There are four peaks in the photoluminescence spectrum in the energy range between 1.55 and 2.25 eV. Besides a bound exciton recombination, three longitudinal optical phonon replicas with one superimposed donor-acceptor emission are identified based upon their dependences of emission energies on temperature and excitation intensity.
Visible electroluminescence from semitransparent Au film/extra thin Si-rich silicon oxide film/p-Si structure
Authors: Qin G. G. , Li A. P. , Zhang B. R. , Li Bing-Chen
Published: July 2002

Visible electroluminescence (EL) has been reported from semitransparent Au film/extra thin Si-rich silicon oxide film/p-Si diodes at room temperature. The Si-rich silicon oxide films, with thickness of about 40 Å, were grown using the magnetron sputtering technique. At forward bias of 4 V, EL spectra with peak energy of 1.9 eV and full width at half maximum of 0.5 eV can be observed from diodes with such extra thin Si-rich oxide films having not been annealed. EL peak energy shows a small red shift under low forward bias but does not shift again when increasing the bias further. Annealing at 800 °C, EL spectra widen and show several shoulders at about 1.5, 2.2, and 2.4 eV, and the EL peak energy shows blue shift with increasing forward bias. These results are shown to be consistent with light emission at several types of luminescence centers in the Si-rich silicon oxide films.
Characteristics of thermal stresses in Al(Cu) fine lines. I. Unpassivated line structures
Authors: Yeo I.-S. , Ho P. S. , Anderson S. G. H.
Published: July 2002

The stress evolution of unpassivated Al(Cu) fine lines under thermal cycling has been investigated as a function of linewidth using a bending beam technique. The results show that the stresses parallel (σ∥) and perpendicular (σ⊥) to the line direction are lower than those measured for thin films as a consequence of stress reduction at the line edges. The edge effect is particularly effective in reducing σ⊥, although σ∥ is also affected. With decreasing linewidth, the effect became more pronounced, leading to an increasingly unequal biaxial stress state and a reduction in line deformation. The observed linewidth dependence can be satisfactorily accounted for by analytic or finite element calculations of the average line stresses as a function of the line aspect ratio. Results of this study validates the use of the bending beam technique for measuring the average stresses in fine line structures.
X-ray photoelectron spectroscopy and optoelectrical properties of low-concentration erbium-doped GaSb layers grown from Sb-rich solutions by liquid-phase epitaxy
Authors: Sun Yuh-Maoh , Wu Meng-Chyi
Published: July 2002

We have attempted to grow low hole-concentration GaSb layers by introducing the rare-earth element Er into Sb-rich solutions by liquid-phase epitaxy. The x-ray photoelectron spectra suggest that the strong affinity of Er will lead to interactions between the Er and residual group-VI impurities (e.g., O, S, and Te) in the growth melt for efficient Er gettering and between the Er and the low electronegativity of group-V element Sb for the formation of stable chalcogenides. The carrier concentration of GaSb layers can be lowered due to the Er gettering and the suppression of complex acceptor defects. Intense sharp luminescence lines of free-exciton and excitons bound to neutral acceptors dominate the low-temperature photoluminescence spectra. The higher breakdown voltage exhibited in the Er-doped GaSb mesa diodes is due to the reduction of carrier concentration in the Er-doped GaSb layers.
Planarization of epitaxial GaAs overgrowth over tungsten wires
Authors: Wernersson Lars-Erik , Georgsson Kristina , Litwin Andrej , Samuelson Lars , Seifert Werner
Published: July 2002

We report on thin GaAs epitaxial overgrowth over tungsten wires. The aim of the study is to overgrow the grating without the formation of voids above the tungsten wires and to investigate the planarization of the growth front over the grating. It is established that the most important factors for rapid planarization of the overgrowth for given epitaxial conditions are the crystallographic orientation of the grating, the grating period, and the ratio of the growth rates for the different facets formed in the growth front. For a 300 nm period grating and a metal width of 100 nm, a planarized growth front is demonstrated after 200 nm of growth.
The optical properties of a-C:H films between 1.5 and 10 eV and the effect of thermal annealing on the film character
Authors: Logothetidis S. , Petalas J. , Ves S.
Published: July 2002

The optical properties of amorphous hydrogenated carbon films prepared with various techniques are studied with conventional and synchrotron-radiation spectroscopic ellipsometry (SE) and the pseudodielectric function 〈ε(ω)〉 of diamondlike and graphitelike films is presented in the energy region 1.5–10 eV. Characteristic features of the measured 〈ε(ω)〉 and the calculated electron-energy-loss (EEL) function are found to serve as useful criteria for the classification of such materials. The results and information obtained by SE are compared to those obtained by EEL and Raman spectroscopy techniques, which are the most widely used for this purpose. Thermal annealing experiments up to 675 °C with in situ monitoring of the 〈ε(ω)〉 reveal the undergoing structural changes in the material character from diamondlike into graphitelike during the annealing. The major modifications which turn the material into sp2-like are found to take place around and above 550 °C. The fundamental gap along with other optical parameters of the materials are compared to those of diamond and graphite and their shift with temperature is discussed and used to illustrate further the prevalence of the graphitic character during and after the annealing. Finally, the optimum growth parameters for the production of diamondlike material are discussed in the case of the glow-discharge and ion-beam deposited films.
Hydrogenated amorphous silicon-nitrogen alloys, a-SiNx:Hy: a wide band gap material for optoelectronic devices
Authors: Demichelis F. , Crovini G. , Giorgis F. , Pirri C. F. , Tresso E.
Published: July 2002

Amorphous silicon-nitrogen, a-SiNx:Hy, thin films with optical gap in the range 2.0–5.2 eV have been deposited by 13.56 MHz ultra-high-vacuum plasma enhanced chemical vapor deposition system in SiH4+NH3 gas mixtures. Compositional, optical, dark and photoelectrical and defect characterizations have been performed in order to show that a-SiNx:Hy films can be applied in optoelectronic technology as wide band-gap semiconductor. A comparison between electronic properties of a-SiNx:Hy samples and device quality a-Si1−xCx:H films, already applied in electronic devices, has been carried out. Amorphous silicon-nitrogen films show high deposition rates, good controllability of optical gap, and electronic properties similar to high-quality silicon-carbon films. No doping effect of nitrogen atoms in tetrahedral configuration has been evidenced and spin density below 7×1017 cm−3 have been measured in a-SiNx:Hy films with optical gap as high as 5.2 eV.
Site-selective excitation and polarized absorption spectra of Nd3+ in Sr5(PO4)3F and Ca5(PO4)3F
Authors: Gruber John B. , Morrison Clyde A. , Seltzer Michael D. , Wright Andrew O. , Nadler Melvin P. , Allik Toomas H. , Hutchinson J. Andrew , Chai Bruce H. T.
Published: July 2002

Polarized absorption and fluorescence spectra were analyzed to establish individual energy (Stark) levels of Nd3+ ions in host crystals of Sr5(PO4)3F (SFAP) and Ca5(PO4)3F (FAP). Site-selective excitation and fluorescence facilitated differentiation between Nd3+ ions in emitting sites associated with 1.06 μm stimulated emission, and nonemitting Nd3+ ions in other sites. Measurements were made on samples containing different concentrations of Nd3+ at 4 K and higher temperatures. Substitution of Nd3+ for Sr2+ or Ca2+ was accompanied by passive charge compensation during crystal growth. Crystal-field splitting calculations were performed according to site for Stark levels of Nd3+ ions identified spectroscopically. We obtained a final set of crystal-field parameters Bnm for Nd3+ ions in fluorescing sites with a rms. deviation of 7 cm−1 (52 levels in Nd:SFAP) and 8 cm−1 (59 levels in Nd:FAP). For one of the nonemitting sites in Nd:FAP we obtained a final set of Bnm parameters which gave a rms deviation of 6 cm−1 between 46 experimental and calculated levels.
Effects of dislocations on transport properties of two dimensional electron gas. I. Transport at zero magnetic field
Authors: Bougrioua Z. , Farvacque J. L. , Ferré D.
Published: July 2002

This paper presents a theoretical analysis of the scattering potentials associated with dislocations in the case of two dimensional electron gas. It is shown that dislocations act mainly on the free carrier mobility through the potential associated with their linear electric charge and not with their strain field potentials. Two different experimental cases, both supported by GaAlAs-GaInAs-GaAs samples, have been considered: (i) samples containing long misfit dislocations issuing from a partial relaxation, during the crystal epitaxial growth and (ii) samples containing dislocation segments introduced by plastic deformation. It is shown that in the case of long misfit dislocations, the free carrier mobility parallel to the dislocations remains unaffected by dislocations while it is probably controlled by some tunneling effect in the direction perpendicular to the lines. In the case of dislocation segments, the free carrier mobility is controlled in both directions by a diffusion process which may be described with the help of a phenomenological potential.
Optical and electrical properties of selectively delta-doped strained InxGa1−xAs/GaAs quantum wells
Authors: Ke Mao-long , Chen X. , Zervos M. , Nawaz R. , Elliott M. , Westwood D. I. , Blood P. , Godfrey M. J. , Williams R. H.
Published: July 2002

We report here an investigation of selectively delta-doped strained InGaAs/GaAs quantum wells. Electronic structures of the systems were calculated by self-consistently solving the Schrödinger and Poisson equations and the calculations revealed a systematic variation of the band structure as the delta sheet moved away from the center of the well to the edge and finally to the barrier. The results were found to be in agreement with our photoluminescence (PL) measurements. For center-doped samples, band-gap renormalization was found to be strong from the PL data, and our realistic random-phase approximation calculation for the heavily doped sample is in excellent agreement with the PL data. The radiative lifetimes were measured to be around 450 ps for all the samples, and surprisingly they vary very little from sample to sample although the wave-function overlap was considerably different for some samples. We also report Shubnikov–de Haas (SdH) measurements on the two barrier doped cases. For the heavily doped sample (A12132), two oscillation signals were detected and they were identified as two upper subbands. The measured electron densities were in very good agreement with the self-consistent calculation. Illumination did not make any difference to the measured densities. For the low-doped sample (A12025), however, the measured electron density before illumination is much smaller than the calculated, and illumination was found to make a large difference.
Generation-recombination noise analysis in heavily doped p-type GaAs transmission line models
Authors: Pascal F. , Jarrix S. , Delseny C. , Lecoy G. , Kleinpenning T.
Published: July 2002

Low-frequency noise measurements are performed on heavily doped p-type GaAs transmission line models. Excess noise exhibits 1/f noise and generation-recombination (GR) noise components. A study of the GR components vs device geometry shows the spectral densities due to contact resistances to be negligible. Thus the noise sources due to the volume resistances are predominant, and have to be located in the bulk layer or in the space-charge region of the devices. These two possibilities concerning the location of the GR noise sources are investigated. For both cases, expressions for the variance and the relaxation time associated to fluctuations in the charge carriers are given. The comparison between the experimental data with the theoretical results shows that the GR noise sources are located in all probability in the space-charge region.
In situ laser reflectance interferometry measurement of diamond film growth
Authors: Zuiker Christopher D. , Gruen Dieter M. , Krauss Alan R.
Published: July 2002

A model is presented to interpret in situ laser reflectance interferometry (LRI) measurements of film growth. The model uses no adjustable parameters and measures film thickness, growth rate, attenuation, and root-mean-square (rms) surface roughness in situ and in real time. For the particular case of diamond film growth on silicon substrates, it is shown that the assumptions of the model are satisfied, and that film properties determined in situ by LRI and by various means ex situ are in good agreement. With this model, the evolution of diamond film properties are followed under typical diamond growth conditions using mixtures of CH4 and H2. In agreement with texture formation models, the growth rate and surface roughness were observed to increase as film growth progressed. The growth rate was found to be proportional to CH4 fraction for CH4 fractions less than 8.5%. At higher CH4 fractions, the growth rate decreased and significant attenuation was observed.
A computer analysis of the effect of a wide-band-gap emitter layer on the performance of a-Si:H-based heterojunction solar cells
Authors: Chatterjee Parsathi
Published: July 2002

A first principles computer model for simulating the performance of amorphous-silicon-based solar cells has been applied to study the effect on solar cell performance of using an emitter layer with band gap larger than that of the intrinsic absorber. We surprisingly find that if the transparent conducting oxide/p-layer contact barrier height φb0 is held constant, the effect of increasing the band gap of the emitter layer Eμ(p) is to depress the open-circuit voltage Voc, fill factor (FF), and cell efficiency η, although the short-circuit current Jsc increases as expected. The main reason for the decrease of Voc, FF, and η at constant φb0 is that as Eμ(p) expands, the field corresponding to the gradient in the electron affinity at the p/i interface increases also, leading more and more to a collapse of the field over the intrinsic absorber layer. Considering the effect of φb0, we find that Jsc in these structures is practically independent of this parameter. However, φb0 exerts considerable influence on Voc, FF, and η. In fact, the observed improvement in a-Si:H-based single junction solar cell performance when a wider-band-gap p-a-SiC:H layer is introduced can only be explained by assuming that φb0 in this case is larger than that for a p-i-n homojunction structure a-Si:H solar cell. A study of the p/i interface states induced by the heterojunction reveals that these mainly limit Jsc, FF, and the efficiency of such cells. A reduction in the number of these interface states by using a graded band-gap buffer layer at this junction has been found to enhance the conversion efficiency by more than 25%. We further find that cells with p-layer thickness ∼80 Å have the highest conversion efficiency when this layer has a wider band gap than the intrinsic absorber. Finally, we show that when a buffer layer is introduced at the p/i heterojunction, the energy location of the p-layer/i-layer band-gap discontinuity does not have a crucial impact on heterojunction device performance.
Modeling the post-burn-in abnormal base current in AlGaAs/GaAs heterojunction bipolar transistors
Authors: Sheu S. , Liou J. J. , Huang C. I. , Williamson D. C.
Published: July 2002

The base current of AlGaAs/GaAs heterojunction bipolar transistor subjected to a long burn-in test often exhibits an abnormal characteristic with an ideality factor of about 3, rather than a normal ideality factor between 1 and 2, in the midvoltage range. We develope an analytical model to investigate the physical mechanisms underlying such a characteristic. Consistent with the finding of an experimental work reported recently, our model calculations show that the recombination current in the base has an ideality factor of about 3 in the midvoltage range and that such a current is responsible for the observed abnormal base current in heterojunction bipolar transistor after a long burn-in test. Post-burn-in data measured from two different heterojunction bipolar transistors are also included in support of the model.
Interaction between chromium and oxidized polyphenylquinoxaline surfaces
Authors: Bellard L. , Ammar C. Fauquet-Ben , Themlin J.-M. , Cros A.
Published: July 2002

In this work, x-ray photoemission spectroscopy is performed on standard polyphenylquinoxaline surfaces treated by ultraviolet irradiation under oxygen atmosphere and by plasma. Spectra reveal the appearance of oxidized carbon species and of amine, NH+3 or NO groups, indicating that polymer bonds are broken during the treatments. Interaction of chromium with these surfaces is further reported. A solid-state reaction occurs, where the new species produced by the treatments are reduced, and chromium oxide, as well as nitrogen and carbon electron-rich species are formed.
Fabricating nanoscale magnetic mounds using a scanning probe microscope
Authors: Bessho K. , Iwasaki Y. , Hashimoto S.
Published: July 2002

Nanometer-scale mounds were fabricated by applying voltage pulses between a substrate and an atomic force microscope cantilever coated with magnetic material. Mounds were formed on both insulator and conducting substrates. Magnetic force microscopy (MFM) observations of the fabricated mounds were performed, and the contrast was turned over by reversing the magnetization of the tip, which is convincing proof that the mounds are magnetic. The MFM images also suggest that the mounds are perpendicularly magnetized. These results demonstrate that scanning probe microscope based nanofabrication is a promising method to fabricate nanoscale magnetic dots on any kind of substrates.
Free-carrier absorption of Hg1−xCdxTe epitaxial films
Authors: Li Biao , Chu J. H. , Chen Z. H. , Chang Y. , Ji H. M. , Tang D. Y.
Published: July 2002

Free-carrier absorption (FCA) of Hg1−xCdxTe epitaxial films is analyzed by considering the composition-in-depth nonuniformity of epilayers. The results show that epilayers exhibit different FCA behavior from bulk materials. Based on the analyses, the carrier concentration, the density and size distribution of Te precipitates, as well as the inclusion in Hg1−xCdxTe epilayers are derived from fitting the measured FCA spectra.
Adjustment of temperature coefficient of resistance in NiCr/CuNi(Mn)/NiCr films
Authors: Brückner W. , Baunack St. , Elefant D. , Reiss G.
Published: July 2002

The thin-film system Ni0.37Cr0.63/Cu0.57Ni0.42Mn0.01/Ni0.37Cr0. 63 with a typical thickness of 1 μm is used for low-ohmic precision resistors. The necessary adjustment of the temperature coefficient of resistance (TCR) by annealing has been studied by investigating the irreversible changes of the resistance during various annealing steps of NiCr/CuNi(Mn)/NiCr multilayers in comparison with single layers of CuNi(Mn) and NiCr. Auger depth profiles showed that the interdiffusion of CuNi(Mn) and NiCr results in an impoverishment of Ni in CuNi(Mn), explaining the TCR shift by comparison with data of Cu1−xNix bulk material. The decrease of the resistivity and the reduction of the width of the copper-nickel conductive layer by formation of a Ni0.6Cr0.2Cu0.2 interdiffusion zone phase (in accordance with the Cu-Ni-Cr phase diagram) cause a significant curvature of the resistance-temperature curve. As main result, it is shown that the NiCr base and cover layers and their interdiffusion with CuNi(Mn) play the decisive role in adjusting the TCR. It was checked that oxidation and topography effects have no remarkable influences.
The influence of chemisorption on the defect equilibrium of metal oxide thin films
Authors: Geistlinger H.
Published: July 2002

Using the exact charge density of intrinsic point defects of metal oxides (MOs), the phenomenological electron theory of chemisorption, developed by Volkenstein, is applied to acceptorlike and donorlike chemisorption on MO thin films for the whole ξ region (where ξ=D/LD, D is the film thickness, and LD is Debye length). The experimental temperature and oxygen partial pressure dependence of the averaged electron concentration 〈n〉(pO2,T)∝e−EA/kTpO2−m(T) for polycrystalline ZnO films are discussed on the bases of three different models: the Schottky-defect model, the Volkenstein model for electronic equilibrium, and the comprehensive model for complete equilibrium. It turns out that a Schottky-defect model that uses single-crystal-mass action constants will not yield the experimental high temperature limit (T=1000 K: EA=1.6 eV, m=0.26). This limit is obtained using a higher averaged concentration of oxygen vacancies for polycrystalline films (due to the presence of grain boundaries). The comparison between the electronic and complete equilibrium shows that the screening of the surface charge through mobile positively charged oxygen vacancies has a tremendous reducing effect of about 30% of the surface potential in the temperature range considered.
Early stages of diamond growth by chemical-vapor deposition monitored both by electron spectroscopies and microstructural probes
Authors: Le Normand F. , Arnault J. C. , Parasote V. , Fayette L. , Marcus B. , Mermoux M.
Published: July 2002

We performed a complete study of the nucleation and growth kinetics of chemical-vapor-deposition (CVD) diamond on Si(100). The diamond film was grown using the microwave-assisted MWACVD method and the substrate was preliminary pretreated by ultrasonic agitation with 300 μm diamond grains, which provides a high nucleation density at saturation (≳108 cm−2). The evolution of the diamond particles coverage was investigated by two independent ways. The size distribution, mean size of the individual diamond particles, the surface coverage, and the nucleation density were monitored by scanning electron microscopy, including analysis of the pictures, and the overall carbon coverage was recorded by x-ray photoemission spectroscopy. Results agree to predict a break point in the growth law: Initially the kinetics obey a law in t1/3, whereas after about 30–45 min the behavior becomes linear. This is interpreted as a change of the rate limiting step of the growth which is governed by the surface coverage of diamond particles. At low diamond islands surface coverage (S≪0.04), the growth process is limited by an indirect route including the impingement onto the bare surface, surface diffusion of the reactive carbon species, and interface reactivity. At larger surface coverage, the route through reactivity of the diamond surface is preferred.
Diffusion and alloy formation of Co ultrathin films on Pt(111)
Authors: Tsay J. S. , Shern C. S.
Published: July 2002

The kinetics of the interdiffusion of Co adatoms on a Pt(111) surface was studied by Auger electron spectroscopy at various temperatures with Co coverage near 1 ML. After annealing, the Co concentration decreases dramatically before it approaches a constant value. The Co–Pt alloy is confined in the few topmost layers. Activation energies and pre-exponential diffusion coefficients are obtained from an Arrhenius plot. The values are 0.90 eV and 6.6×10−11 cm2/s, respectively. They are independent on the coverage in this ultrathin system. After annealing the 1 ML Co ultrathin film on a Pt(111) surface at 600 K, Co atoms diffuse into the first two monolayers only. Both the topmost and the second layers contain 50% of Co atoms. In addition, ultraviolet photoelectron spectroscopy was used to monitor the alloy formation of Co/Pt(111) in the annealing process.
Morphology of wetting reaction of eutectic SnPb solder on Au foils
Authors: Kim P. G. , Tu K. N.
Published: July 2002

We have studied the wetting behaviors of eutectic SnPb solder caps on pure Au foils at 200 °C. Surface morphology, wetting angle, and wetting tip stability were examined by scanning electron microscopy and energy dispersion x-ray analysis. In addition, interfacial reaction in the bulk diffusion couple of the solder and Au was studied. Intermetallic compound (AuSn4) was observed on the surface of the solder cap, often called the cold joint, as early as 2 s of reflow at 200 °C. The wetting angle decreased with reflow time but remained constant after 180 s of reflow. However, the side-band width and the diameter of the solder cap continuously increase with the reflow time to 5 min when the entire eutectic SnPb solder cap is fully consumed by intermetallic compound formation. In the bulk diffusion couple of eutectic SnPb and Au, the growth and morphology of the intermetallic compound (AuSn4) was influenced by Au dissolution. Since Au, showing excellent wetting behavior, forms the platelet-type intermetallic compound, we postulate that the wetting rate may be improved with a system containing platelet-type intermetallic compound formation for the eutectic SnPb solder.
Numerical simulation of ZnSe/GaAs interface reflectance difference spectroscopy
Authors: Kwok Tat-Kun , Yang Z.
Published: July 2002

A numerical method based on the matrices established by Yeh [Optical Waves in Layered Media (Wiley, New York, 1988)] is developed to simulate the non-normal incidence reflectance difference spectroscopy (RDS) spectra of biaxial anisotropic (εx≠εy≠εz) multilayer systems. The main features of the RDS spectra obtained from the biaxial anisotropic ZnSe/GaAs interface are reproduced by the numerical method. It has demonstrated that in the cases of near-normal incidence and when the anisotropy within the layer plane (in-plane anisotropy) is small (εx−εy≪εx) a RDS spectrum can be separated into two spectra, namely, the in-plane anisotropic spectrum and the off-plane anisotropic spectrum. The reflectance of the s wave and the p wave can be calculated separately when the in-plane principal axes are at certain orientations, making it possible to obtain the anisotropic dielectric tensor directly from the measured spectra.
Photoacoustic spectroscopic estimation of optical absorption and thermal diffusivity of amorphous Ge15AsxSe85−x
Authors: Ghosh A. K. , Chaudhuri B. K.
Published: July 2002

Photoacoustic spectroscopy (PAS) has been applied to measure the composition dependent optical energy gap (Eo), optical absorption coefficient (α), and thermal diffusivity (σs) of glassy Ge15AsxSe85−x (0≤x≤25) alloys. The energy gap is found to decrease with increase of As concentration and shows a threshold behavior around x∼15, which corresponds to the average coordination number 〈r〉∼2.45. This behavior is found to be consistent with the Phillips-Thorpe theory. The variation of the optical gap (Eo) with composition (x) is analyzed on the basis of the Kastner’s model of bond energies. The observed exponential edge (Urbach edge) can be considered as an internal Franz-Keldysh effect arising due to the charged impurity generated, as well as ‘‘frozen-in’’ optical phonon-generated, electric microfields. It could be described in the framework of Tauc and Dow-Redfield model which ascribes the Urbach rule to the ionization of the exciton as an extension of the stark shift. The concentration (x) dependent thermal diffusivity (σs) estimated from the PAS studies also showed a similar critical behavior at the same concentration x (≊15) which arises due to the threshold percolation of rigidity of the system. The measurements of glass transition temperatures (Tg) and the magnetic susceptibilities (χ) of the samples also support this critical behavior around x=15.
Thermal diffusivity of conducting polypyrrole
Authors: da Costa A. C. R. , Siqueira A. F.
Published: July 2002

We have used the photoacoustic technique in an open photoacoustic cell configuration to measure the thermal diffusivity of conducting polypyrrole films. The samples were obtained by electrochemical deposition from solutions of pyrrole dissolved in 0.08 M p-toluene sulfonic acid and their thicknesses were in the 50–125 μm range. The thermal diffusivity α was found to be very dependent on the synthesis conditions.
Marangoni convection and enhanced morphological stability in float-zone traveling solvent crystal growth of LaB6
Authors: Louchev Oleg A. , Otani Shigeki
Published: July 2002

We analyze the problem of onset of morphological instability and the increasing supersaturation gradient in the particular case of float-zone traveling solvent growth of LaB6 crystals in excess of La, taking into account the actual liquidus line on the relevant phase diagram of Storms and Mueller [J. Phys. Chem. 82, 51 (1978)]. We show that the action of Marangoni convection decreases the concentration gradient at the solidification interface and inhibits the morphological instability and the increasing supersaturation gradient. We suggest that this effect allows the implementation of float-zone traveling solvent growth of LaB6 crystals with pulling rates of ≂1 cm/h without the appearance of defects structures associated with the morphological instability and increasing supersaturation gradient.
Evidence for the formation of ordered layers on SeS2 treated GaAs(110) using atomic force microscopy
Authors: Kuruvilla Beena Annie , Datta A. , Shekhawat G. S. , Sharma A. K. , Vyas P. D. , Gupta R. P. , Kulkarni S. K.
Published: July 2002

The wet chemical treatment using SeS2 is an inexpensive and simple method of depositing selenium on GaAs surfaces. This treatment improves the electronic properties of the surface as seen from the increase in photoluminescence intensity. We present our results on surface structural investigations of GaAs(110) surface passivated by SeS2 treatment using atomic force microscopy. Our results show that SeS2 treatment can passivate the GaAs(110) surface forming ordered overlayers on it.