bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct-mode interactions in the wake behind a stepped cylinder
Authors: Vallès B. , Andersson H. I. , Jenssen C. B.
Published: July 2002

A computerized flow analysis of wake phenomena caused by a discontinuity in cylinder diameter is presented. The vortex linkage and half-loop formation observed experimentally by Lewis and Gharib [Phys. Fluids A 4, 104 (1992)] have for the first time been reproduced numerically. The instantaneous vorticity and pressure fields provide a distinct picture of the direct-mode interaction, from which it is concluded that a new half-loop is formed every third shedding cycle.
High photovoltage multiple-heterojunction organic solar cells incorporating interfacial metallic nanoclusters
Authors: Yakimov A. , Forrest S. R.
Published: July 2002

We demonstrate high open circuit voltage (Voc) organic photovoltaic (PV) cells that incorporate two, three, or five stacked, thin heterojunctions (HJs) consisting of Cu-phthalocyanine as a donor, and 3,4,9,10 perylenetetracarboxylic bis-benzimidazole as an acceptor. Ultrathin (∼5 Å average thickness) layers of Ag clusters are placed between each HJ to serve as recombination centers for unpaired charges that are photogenerated in the device’s interior. High power conversion efficiencies with high Voc have been demonstrated in cells containing up to five optically active HJs and four interstitial metallic recombination regions. The power conversion efficiencies of the two and three HJ cells under one sun, AM 1.5 illumination are ηp=2.5±0.1% and ηp 2.3±0.1%, with Voc=0.93±0.05 and 1.20±0.05 V, respectively. These values of ηp are more than twice that of a comparable single-junction cell based on the same materials, where ηp=1.1±0.1%.
Miscibility of amorphous ZrO2–Al2O3 binary alloy
Authors: Zhao C. , Richard O. , Bender H. , Caymax M. , De Gendt S. , Heyns M. , Young E. , Roebben G. , Van Der Biest O. , Haukka S.
Published: July 2002

Miscibility is a key factor for maintaining the homogeneity of the amorphous structure in a ZrO2–Al2O3 binary alloy high-k dielectric layer. In the present work, a ZrO2/Al2O3 laminate thin layer has been prepared by atomic layer chemical vapor deposition on a Si (100) wafer. This layer, with artificially induced inhomogeneity (lamination), enables one to study the change in homogeneity of the amorphous phase in the ZrO2/Al2O3 system during annealing. High temperature grazing incidence x-ray diffraction (HT-XRD) was used to investigate the change in intensity of the constructive interference peak of the x-ray beams which are reflected from the interfaces of ZrO2/Al2O3 laminae. The HT-XRD spectra show that the intensity of the peak decreases with an increase in the anneal temperature, and at 800 °C, the peak disappears. The same samples were annealed by a rapid thermal process (RTP) at temperatures between 700 and 1000 °C for 60 s. Room temperature XRD of the RTP annealed samples shows a similar decrease in peak intensity. Transmission electronic microscope images confirm that the laminate structure is destroyed by RTP anneals and, just below the crystallization onset temperature, a homogeneous amorphous ZrAlxOy phase forms. The results demonstrate that the two artificially separated phases, ZrO2 and Al2O3 laminae, tend to mix into a homogeneous amorphous phase before crystallization. This observation indicates that the thermal stability of ZrO2–Al2O3 amorphous phase is suitable for high-k applications.
Double transport barrier experiments on Alcator C-Mod
Authors: Wukitch S. J. , Boivin R. L. , Bonoli P. T. , Fiore C. L. , Granetz R. S. , Greenwald M. J. , Hubbard A. E. , Hutchinson I. H. , In Y. , Irby J. , Lin Y. , Marmar E. S. , Mossessian D. , Porkolab M. , Schilling G. , Rice J. E. , Snipes J. A. , Wolfe S. M. ,   Alcator C-Mod group
Published: July 2002

Double transport barrier modes (simultaneous core and edge transport barrier) have been observed with off-axis ion cyclotron range of frequencies (ICRF) heating in the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. An internal transport barrier (ITB) is routinely produced in enhanced DαH-mode (EDA) discharges where the minority ion cyclotron resonance layer is at r/a∼|0.5| during the current flat top phase of the discharge. The density profile becomes peaked without the presence of a particle source in the plasma core and continues to peak until the increased core impurity radiation arrests the improved energy confinement, ultimately leading to a barrier collapse. With the addition of moderate (0.6 MW) central ICRF heating, the double barrier mode was maintained for as long as the ICRF power was applied. Modeling shows that the internal thermal barrier was maintained throughout the discharge. The presence of sawteeth throughout most of the ITB discharge allows sawtooth-induced heat pulse analysis to be performed. This analysis indicates that there is an abrupt radial discontinuity in the heat pulse time to peak profile when an ITB is present. Furthermore, this discontinuity appears to move into the core plasma from the edge region in about 0.2 s, several confinement times. The deduced thermal diffusivity, χhp, indicates that a barrier exists in the electron thermal transport, the barrier is limited to a narrow radial region, and the transport is unaffected outside this narrow radial extent.
Designing a heterostructure for the quantum receiver
Authors: Kiselev A. A. , Kim K. W. , Yablonovitch E.
Published: July 2002

In this letter, we develop optimal parameters for a structure which is suitable for the realization of a coherent quantum receiver. Conditions including predefined photon wavelength, strain, small Zeeman splitting of the electron levels, and large Zeeman effect for quantum-confined light holes are satisfied simultaneously for the structure based on the InGaAsP solid solutions. We are able to achieve designs with wavelengths of 1.3 and 1.55 μm that are desirable for optoelectronic applications.
TTC: Symbolic tensor and exterior calculus
Authors: Castellví P. , Jaén X. , Llanta E.
Published: July 2002

The package TTC (Tools of Tensor Calculus) implements the majority of the basic tools of tensor and exterior calculus in a differentiable manifold using the point-of-view of the modern differential geometry (DG). It can be of interest to any physicist using DG, for research or teaching purposes, in relativity theory, mechanics, electromagnetism, thermodynamics, Hamiltonian theory, fluid dynamics, elementary particle physics, etc. It is written in the computer algebra language Mathematica, which gives the user a powerful environment. Emphasis has been put on using the full expression of tensors, on defining objects with no restrictions at all, and on using a close-to-textbook notation.
Radiative properties of puffed-gas mixtures
Authors: Duston D. , Davis J. , Agritellis C.
Published: July 2002

The radiative yield of dense, high-temperature plasma mixtures has been studied theoretically for conditions typically attained in puffed-gas implosions from high-voltage pulse power generators. The mixtures are characterized using an equilibrium ionization/radiation model which calculates the self-consistent state density populations of detailed level configurations by solving the complete set of atomic rate equations. In order to accurately model the quantitative line emission as well as the radiation spectrum from the plasma mixtures, detailed atomic models of L- and K-shell ion level structure and collisional processes in neon, argon, and krypton are employed; these models are described in detail. Specific mixtures reported on include neon/argon, argon/krypton, helium/argon, and helium/krypton. They are considered as compressed stationary plasmas, radiating at constant temperature and density (the time-history of the implosion dynamics is neglected in this report). While no power amplification is detected in the optically thin approximation via the various mixtures, broadband spectral enhancement is demonstrated using various constituents.
Current-voltage characteristics of ideal silicon diodes in the range 300–400 K
Authors: Cappelletti P. , Cerofolini G. F. , Polignano M. L.
Published: July 2002

Current-voltage characteristics of silicon diodes which were found to be ideal at room temperature have been investigated in the range 300–400 K. The diodes remain ideal at higher temperatures, too.
The classical theory of the transverse optical klystron for a nonmonoenergetic beam
Authors: Boscolo I. , Leo M. , Leo R. A. , Soliani G. , Stagno V. , Pellegrini C.
Published: July 2002

In this paper the classical theory of the transverse optical klystron is presented. The gain is obtained by solving analytically the Vlasov equation which governs the evolution of the electron beam along the undulator magnet. The initial energy distribution of the electron beam is assumed gaussian. Both the small signal and the saturation regime are investigated.
Interface-trap generation modeling of Fowler–Nordheim tunnel injection into ultrathin gate oxide
Authors: Horiguchi Seiji , Kobayashi Toshio , Saito Kazuyuki
Published: July 2002

Two mechanisms for interface-trap generation during Fowler–Nordheim tunnel injection into gate oxide are confirmed by the results of experiments with changes in the interface-trap density for positive and negative gate bias injections. One mechanism is independent of gate bias polarity during injection, the other mechanism is present only in negative gate bias injection. Agreements between the measured and calculated generation cross sections for both mechanisms indicate that: (i) The first mechanism is quantitatively explained using a broken-bond model by taking account of electron heating due to an oxide field during passage through the oxide conduction band. (ii) The second mechanism is quantitatively explained using a heated electron impact model where electrons heated by the oxide field generate interface traps by directly breaking the interface weak bonds when electrons cross the interface between the SiO2 and the Si substrate.
High responsivity HgCdTe heterojunction photoconductor
Authors: Arch D. K. , Wood R. A. , Smith D. L.
Published: July 2002

We present an experimental and theoretical study of n-type Hg1−xCdxTe photoconductors in which a large band-gap alloy was grown on top of a smaller band-gap active region and contacts were made to the larger gap material. The larger band-gap material causes an energy barrier to holes which decreases the rate at which they reach the high recombination region of the metal-semiconductor interface. As a result, this heterojunction contact greatly reduces the effects of carrier sweepout on device performance and leads to much higher detector responsivities. Experimental results in a symmetric device with a cutoff wavelength of 7.8 μm at 77 K show responsivities in excess of 106 V/W and detectivities close to the background limited value and nonsaturation of responsivity with bias voltage. In an asymmetric device, in which only one heterojunction contact was used, an order of magnitude increase in responsivity was observed when the heterojunction contact was biased to attract minority carriers, compared with the opposite bias polarity. A theoretical model of the heterojunction contact photoconductor is presented. Calculated results are in good agreement with experimental results. The results of the calculation suggest that the optimum compositional difference Δx of the two layers should be Δx∼0.04, and that the thickness of the large band-gap region should be 2–3 μm.
Direct measurement of the energy distribution of hot electrons in silicon dioxide
Authors: Brorson S. D. , DiMaria D. J. , Fischetti M. V. , Pesavento F. L. , Solomon P. M. , Dong D. W.
Published: July 2002

The energy distribution of hot electrons in high-field stressed amorphous silicon dioxide (SiO2) films have been measured using a vacuum emission technique. Electrons having average energies ≳2 eV and an energy relaxation length of λ≊32 Å are observed at all fields studied (≳ 2 MV/cm). However, contrary to previous theoretical expectations, the majority of carriers in the distribution remains stable at all fields. The results are in agreement with other recent experiments (electroluminescence and carrier separation) which only measure the average energy of hot electrons in SiO2 and with recent Monte Carlo transport calculations which include scattering by both optical and acoustic phonon modes. Results for varying SiO2 thickness, metal gate thickness, oxide composition, and metal gate composition will be discussed.
Self-consistent calculation of electron and hole inversion charges at silicon–silicon dioxide interfaces
Authors: Moglestue C.
Published: July 2002

The charge distribution at a semiconductor-insulator interface has been calculated, both for holes and electrons, by solving Schrödinger’s and Poisson’s equations self-consistently for particles obeying Fermi–Dirac statistics. The results have been applied to carriers in the channel of a crystalline MOSFET (metal-oxide-semiconductor field-effect transistor) with the (100) axis perpendicular to the gate oxide. For weak inversion, the self-consistent results do not deviate significantly from those obtained assuming a triangular potential well, but for strong inversion the carriers tend to move closer to the oxide. The energy and occupation levels of the subbands are only affected by a few percent on passing from weak to strong inversion, keeping the transversal interface electric field fixed. Finally the gate capacitance has been calculated and found to agree with the experimental data published elsewhere.
Kinetics of ultrathin SiO2 growth
Authors: Murali V. , Murarka S. P.
Published: July 2002

A new model for explaining the initial rapid oxidation of silicon in dry oxygen has been proposed. Creation of an oxygen-diffused zone near the Si-SiO2 interface is postulated. Our model takes into account the heretofore ignored phenomenon of diffusion of oxidizing species into the silicon substrate during initial exposure to such species. This occurs because of the low diffusion resistance offered by the ultrathin oxide films to the oxidizing species. The oxidizing species diffuses in such large amounts that not all of it is consumed at the Si-SiO2 interface; instead, a part of it diffuses into the substrate creating an oxygen-diffused zone in silicon. The reaction occurs over this zone instead of occurring just at the interface. As the oxide thickness builds up, the concentration of the oxidizing species at the Si-SiO2 interface decreases due to the increase in the diffusion resistance offered by the growing oxide film. Once the oxide becomes sufficiently thick all of the oxidizing species reaching the Si-SiO2 interface is consumed by the reaction at the interface, and the zone disappears. The occurrence of the reaction over a zone instead of occurring at the interface alone leads to an enhancement in the oxidation rate in the ultrathin-film regime. This enhancement has been formulated, and new oxide thickness-time relationships have been derived. This model explains all phenomena observed in silicon oxidation, and the Deal-Grove model is found to be a special case at larger thicknesses.
The hot-electron problem in small semiconductor devices
Authors: Hänsch W. , Miura-Mattausch M.
Published: July 2002

Decreasing device dimensions will cause an increasing internal field strength in a semiconductor device. The average carrier energy is different from the thermal equilibrium value 3/2 kT. Modification of current transport is considered on different levels of approximation. In a local approximation we derive a field-dependent carrier mobility and temperature from a more general self-consistent formulation. Numerical estimation of hot-electron effects are given for a realistic n-channel metal-oxide-semiconductor field-effect transistor of various channel lengths. It is shown that both high-field and field-gradient effects will contribute.
Molecular hydrogen in electroless copper deposits
Authors: Graebner J. E. , Okinaka Y.
Published: July 2002

Clusters of molecular hydrogen have been detected at a concentration of 930 ppm H2 in electroless Cu deposits using a new, nondestructive, calorimetric technique. The H2 is located in microvoids at a pressure of 700 atm at room temperature. The observation of H2 supports previous suggestions that it is responsible for the embrittlement of electroless Cu. Gentle annealing at 150 °C restores the ductility and also removes the H2.
Structure and magnetism of Co-Cr thin films with an artificial superlattice structure
Authors: Sato Noboru
Published: July 2002

Cobalt-chromium (Co-Cr) thin films deposited in an alternating multilayer structure by the magnetron sputtering method have an artificial superlattice hexagonal closed pack (hcp) structure with the c-axis normal to the film plane, where the c-plane of hcp-Co grows epitaxially on the (110) plane of the bcc-Cr. The film, composed of monatomic layers of Cr and then several atomic layers of Co in an alternating fashion, has an especially excellent hcp structure and shows a clearly separated spot pattern in reflection high-energy electron diffraction analysis. The artificial superlattice structure of the film is stable thermally to temperatures up to 500 °C. Small-angle x-ray diffraction analysis confirmed the layering of monatomic layers of Co and Cr, where the periodicity of layers was found to be about 5 Å. Transmission electron microscopic observation proved the artificially layered structure in these films. The films showed ferromagnetism over a wide Cr composition range, where the thickness ratio of Co and Cr layers was varied from 4/1 to 1/4. The perpendicular magnetization hysteresis loop was obtained in films with a Co layer thinner than about 12.5 Å for various thickness ratios of Co and Cr layers. It is concluded that the origins of the perpendicular magnetization in the Co-Cr films with artificial superlattice structures are the magnetocrystalline anisotropy and the shape anisotropy related to the columnar structure.
Thermoelectric power of two-dimensional electron gas in heterojunctions at low temperatures
Authors: Kundu S. , Sarkar C. K. , Basu P. K.
Published: July 2002

The low-temperature thermopower of two-dimensional electron gas has been calculated by using the expression obtained from the Boltzmann equation. The relevant scattering mechanisms are taken into account. It is found that the calculated thermopower increases almost linearly with temperature in the GaAs-AlGaAs and InGaAs-InP structures, but in all cases the results differ widely from the values given by the Mott formula and also from the experimental values. The results are not influenced by phonon scattering and are slightly affected if the impurity density is changed by the order of magnitude. For high-mobility samples, the experimental values are higher than the theoretical ones and it seems that phonon drag thermopower may account for the difference. For low-mobility samples, however, the experimental values are lower and in some cases decrease with temperature. The present model is then inadequate. Possible refinements of the theory are suggested.
A small-signal equivalent-circuit model for GaAs-AlxGa1−xAs resonant tunneling heterostructures at microwave frequencies
Authors: Gering J. M. , Crim D. A. , Morgan D. G. , Coleman P. D. , Kopp W. , Morkoç H.
Published: July 2002

This paper presents a study of the microwave impedance of GaAs-AlxGa1−xAs resonant tunneling heterostructures. An equivalent-circuit model is proposed that accounts for the frequency variation of the measured impedance and whose elements correspond to physical phenomena believed to be present in the device. Empirical formulas are obtained which can be used to calculate the values of the equivalent-circuit elements from the structural parameters and the dc current-voltage characteristics of the device.
Kinetics of laser-induced chemical vapor deposition of gold
Authors: Kodas T. T. , Baum T. H. , Comita P. B.
Published: July 2002

Rates of photothermal gold deposition onto alumina substrates heated by a focused argon-ion laser beam were measured by determining the time required for deposits to grow through the focal spot of a HeNe laser probe beam directed parallel to the substrate. Deposition rates from 0.25 to 6 μm/s were measured for deposits with heights ranging from 5 to 100 μm. Rates of gold deposition using dimethyl gold hexafluoroacetylacetonate as a precursor depend linearly on the gold precursor partial pressure and for a wide variety of conditions do not depend on the laser power or focal spot diameter. A theory is presented to describe reactant mass transfer-controlled deposition with and without buffer gas. Comparison of measured with calculated growth rates showed that gold deposition rates using dimethyl gold hexafluoroacetylacetonate as a precursor were transport limited for the conditions studied. Theory and experiment also showed that the deposition rate decreases inversely with increasing buffer gas pressure above a critical pressure.
Characteristics of Cl-doped ZnSe layers grown by molecular-beam epitaxy
Authors: Ohkawa K. , Mitsuyu T. , Yamazaki O.
Published: July 2002

High-quality n-type ZnSe layers have been grown by molecular-beam epitaxy using chlorine (Cl) as a dopant. The Cl-doped ZnSe layers showed mirrorlike morphology and good crystallinity, although some degrade in crystallinity is observed at a heavy doping. The carrier concentration of the layer could be widely controlled by a ZnCl2 Knudsen cell temperature. The carrier concentration attained 1×1019 cm−3, where the resistivity was as low as 3×10−3 Ω cm, indicating a remarkable improvement compared to the previous work using group-III elements as a dopant. Hall mobilities at room temperature were in the range of 200–400 cm2/(V s), depending on the doping level. The Cl-doped ZnSe layer exhibited strong blue near-band-gap photoluminescence (PL) with suppressed deep-level emission at room temperature. The 4.2-K PL of the layer was dominated by strong emission of excitons bound to neutral donors originating from substitutional Cl atoms. It was found by a secondary ion-mass-spectroscopy analysis that diffusion of Cl atoms in the ZnSe layer during growth was negligible (<370 Å).
Some aspects of damage annealing in ion-implanted silicon: Discussion in terms of dopant anomalous diffusion
Authors: Servidori M. , Šourek Z. , Solmi S.
Published: July 2002

Lattice damage has been introduced in silicon wafers by silicon ion implantation at doses below and above amorphization threshold. In the first case, the annealing behavior of damage is consistent with the presence of amorphous islands near the surface and interstitial clusters in deeper regions. Unlike the former kind of defects, the temperature evolution of the latter strongly affects the diffusion of dopant atoms. Enhanced diffusion for dopant diffusing via interstitialcy mechanism accompanies in fact the interstitial cluster dissolution. The extent of this effect depends on the relative depth position of dopant and defects. When the local concentration of the interstitials increases (e.g., as a consequence of an implant made with the same dose, but with a lower energy) defect clustering begins to compete with their recovery, which makes the fraction of interstitials available for dopant diffusion smaller. Interstitial clustering is the dominant process occurring during annealing at the original amorphous-crystal interface in a high-dose implanted sample. Dislocation loops are then formed and evolve (increase in size, decrease of density) via dissolution of the small clusters in the damage tail. The process is accompanied by a net reduction of the interstitials collapsed in the residual dislocation loops. The consequence is a supersaturation of interstitials, which assists the movement of the dopant, if initially localized at depths greater than loop position. Conversely, when the dopant junction is shallower than the original amorphous layer, negligible enhanced diffusion occurs, which indicates an effective sink action of the loops for the interstitials coming from cluster dissolution.
Hole trapping in SiO2 films annealed in low-pressure oxygen atmosphere
Authors: Hofmann K. , Young D. R. , Rubloff G. W.
Published: July 2002

The generation of hole traps in thermal SiO2 films on Si(100) has been characterized as a function of O2 partial pressure in the annealing ambient in order to address the chemistry associated with the hole traps. The annealing treatments were carried out in ultrahigh vacuum (base pressure∼5×10−9 Torr) without and with the presence of an intentional (controlled) partial pressure of O2 in the range 10−6–5×10−2 Torr. Hole trapping was characterized using the avalanche injection technique. Annealing in vacuum results in an increased hole trapping rate similar to that observed for high-temperature (T>900 °C) furnace annealing in N2. The hole trapping is reduced upon annealing in O2 containing ambients if the O2 partial pressure exceeds the SiO vapor pressure by at least one order of magnitude. Thus, the presence of sufficient O2 in the postoxidation annealing process suppresses hole trapping. These results appear analogous to the reduction in low-field breakdown when O2 is present, as recently reported. In both cases, it is likely that the O2 serves to reoxidize a defect related SiO product which is generated by Si-SiO2 reaction at the interface.
Antimony-doped ZnSe grown by molecular-beam epitaxy
Authors: Park R. M. , Kleiman J. , Mar H. A. , Smith T. L.
Published: July 2002

Antimony-doped ZnSe layers were grown by molecular-beam epitaxy (MBE) on (100) GaAs substrates using a variety of antimony oven temperatures in the range of 550–800 °C and substrate temperatures in the range of 250–330 °C. Sb concentrations as high as mid-1019 cm−3 were measured in the ZnSe layers by the secondary ion mass spectrometry technique, although an Sb concentration limit of ≤1019 cm−3 was established in this work for the growth of single-crystal Sb-doped material. The Sb incorporation coefficient in ZnSe was found to depend strongly on substrate temperature, increasing from ∼10−2 at 330 °C to ∼1 at 250 °C. In addition, a strong dependence was found of the ZnSe lattice parameters measured normal to the heterointerface on Sb concentration in the layers, the lattice parameter increasing monotonically with increasing Sb concentration. Acceptor bound excitonic emission was not detected from the Sb-doped ZnSe layers by 4.2-K photoluminescence analysis, indicating that Sb incorporation under the MBE growth conditions did not produce a shallow acceptor level in the ZnSe band gap.
Wigner function modeling of resonant tunneling diodes with high peak-to-valley ratios
Authors: Mains R. K. , Haddad G. I.
Published: July 2002

Wigner function simulations of structures with experimentally observed high peak-to-valley ratios are carried out. It is shown that if care is taken with the numerical method used, the simulations reproduce these sharp resonances. When scattering is ignored, peak-to-valley ratios of 33.7 are obtained for a pseudomorphic InGaAs-AlAs structure. The effects of phonon scattering are included to first order. Also, a small-signal analysis is carried out and the results are used to predict the rf power generation capability of these devices.
Photoluminescence study of carrier collection and recombination in thin GaInAs/InP single quantum wells
Authors: Reihlen E. H. , Persson A. , Wang T. Y. , Fry K. L. , Stringfellow G. B.
Published: July 2002

The photoluminescence (PL) from thin GaInAs/InP single quantum wells (SQWs) grown by atmospheric pressure organometallic vapor phase epitaxy is investigated. The 10-K PL intensity from the SQWs is as much as 25 times stronger than that from approximately 1.5-μm-thick epitaxial GaInAs layers. The underlying PL processes, namely photogeneration of carriers, carrier collection by the well, and recombination in the well, are studied. The photogeneration of carriers in the well is calculated to be negligible compared to that occurring in the InP barriers. In contrast, the quantum-well PL is approximately a factor of 4500 stronger than the barrier PL for all samples at temperatures ranging from 10–300 K. This necessitates rapid and efficient transfer of photogenerated carriers from the barriers into the well. The transfer is investigated by applying a rate equation model relating the barrier and quantum well PL intensities to the lifetimes governing the recombination dynamics in the barriers and in the well. The transfer is calculated to occur within a few picoseconds at 10 K with nearly 100% transfer efficiency. The temperature dependence of the barrier PL spectra shows that the carrier-collection efficiency of the well remains high up to room temperature. The integrated quantum-well PL intensity decreases by approximately two orders of magnitude as the temperature is raised from 10 to 300 K, which is attributed to a decrease of the radiative quantum efficiency of the well. Results of a PL-excitation study suggest that the PL is due to interface- or cluster-localized exciton recombination at 10 K at low excitation intensities. At high temperatures and excitation intensities, the PL spectra show evidence for delocalization and/or dissociation of the excitons.
Dynamic magnetic forces in superconducting ceramics
Authors: Moon F. C. , Weng K-C. , Chang P-Z.
Published: July 2002

Dynamic forces between small permanent magnets and bulk high-Tc superconducting ceramic materials have been measured, including magnetic stiffness and damping effects. The vibration frequencies and the related magnetic stiffness show a large dependence on the magnet-superconductor distance. The dynamic magnetic stiffness is shown to be related to small static reversible magnetic forces, but is not correlated with the static hysteretic forces. The magnetic damping is inversely related to the magnet-superconductor distance and can critically damp the oscillations at small gaps. These data also show that the flux pinning forces depend on the prior flux history in the superconductor.
Long-wavelength lattice dynamics of GaxIn1−xAsySb1−y quaternary alloys
Authors: Jaw D. H. , Cherng Y. T. , Stringfellow G. B.
Published: July 2002

The quaternary semiconductor alloy GaxIn1−xAsySb1−y has been investigated using Raman scattering spectroscopy in the spectral region from 150 to 300 cm−1 at room temperature. The Raman spectra exhibit two- or three-mode behavior depending on the composition. A modified random-element isodisplacement model is used to describe the behavior of the optical phonons. The calculations of the dependence of the long-wavelength optical phonon frequencies on the composition are in good agreement with the experimental results.
A tuned Langmuir probe for measurements in rf glow discharges
Authors: Paranjpe Ajit P. , McVittie James P. , Self Sidney A.
Published: July 2002

Measurements of charged-particle concentrations and the electron energy distribution function (EEDF) have been made in Ar and SF6 glow discharges using a tuned Langmuir probe technique. A simple passive circuit connected to the probe when properly tuned increases the impedance between the probe and ground, thereby forcing the probe to follow the instantaneous plasma potential. In this manner, rf-induced distortion of the probe characteristic is mitigated. At 13.56 MHz the electron collection characteristic of a detuned probe is distorted by rf interference; the ion collection characteristic is unaffected. The EEDF is highly non-Maxwellian in argon discharges, but quite Maxwellian in SF6 discharges. The mean electron energy increases with decreasing pressure and increasing power in argon discharges, but is independent of pressure and power in SF6 discharges. The measured distribution functions and charged particle concentrations are in good agreement with calculations.
The self-biased heterojunction effect of ferroelectric thin film on silicon substrate
Authors: Xu Yuhuan , Chen Ching Jih , Xu Ren , Mackenzie John D.
Published: July 2002

Several ferroelectric thin films with n-type or p-type conductivity, including undoped and doped lead zirconate titanate, barium titanate, strontium barium niobate, and potassium niobate, as well as lead barium niobate, were made on silicon single-crystal substrates by the sol-gel process. Self-biased heterojunction effects were observed in both p-ferroelectric thin film on n-silicon and n-ferroelectric thin film on p-silicon by the measurement of current-voltage characteristics. It has been observed that either p-n junction or n-p junction in the ferroelectric-semiconductor systems behave like a rectifying diode. However, the junction effect is weak in the p-ferroelectric thin film on p-silicon system. A physical model based on the consideration of energy-band theory has been constructed in explaining this effect. Possible applications of this effect are discussed.
Study of interface impurity sputtering in partially ionized beam deposition of Cu on Si
Authors: Bai P. , Yang G.-R. , Lu T.-M.
Published: July 2002

Cu thin films have been deposited on Si(111) substrate using partially ionized deposition technique in which the self-ions, i.e., the ions derived from the depositing materials themselves, are being used to concurrently bombard the substrate surface during thin film growth. The ion percentage in the deposition beam is in the range from 0% to 5% with the ion energy varying between a few eV and several keV. The Si(111) substrates are chemically cleaned by a RCA cleaning process before loading into the vacuum chamber of deposition. No in situ cleaning of the substrate surface is performed prior to deposition. A reduction of impurities such as oxygen, carbon, and hydrogen at the Cu/Si interface is observed as a function of the ion bombardment parameters using secondary-ion mass spectrometry. The estimates of the sputtering cross section of oxygen, carbon and hydrogen at the Cu/Si interface by Cu ions and their energy dependence are discussed in the framework of mechanisms of the impurity sputtering by low-energy ions.
Alloy-disorder-induced intrasubband scattering in a quantum well under an electric field
Authors: Basu P. K. , Raychaudhury D.
Published: July 2002

The scattering rates in the lowest subband in a quantum well are calculated for alloy-disorder scattering when an electric field is applied perpendicular to the layer plane. Calculations for the InGaAs quantum well indicate that the scattering rate increases with increase in the electric field.
Resonant interband coupling in single-barrier heterostructures of InAs/GaSb/InAs and GaSb/InAs/GaSb
Authors: Luo L. F. , Beresford R. , Longenbach K. F. , Wang W. I.
Published: July 2002

A new mechanism for negative differential resistance due to electron/light hole coupling has been observed using broken gap heterostructures of InAs/GaSb/InAs and GaSb/InAs/GaSb. The best peak-to-valley ratio is about 2:1 (3.7:1 at 77 K) for a GaSb layer width of 10 nm. The peak current density of 4.2 kA cm−2 and the peak voltage of 300 mV are consistent with the interpretation of these experiments as interband coupling between the InAs conduction band and the GaSb valence (light hole) band.
Force between a superconductor and a permanent magnet due to trapped flux
Authors: Adler Ronald J. , Anderson W. W.
Published: July 2002

Type-II and multiply connected type-I superconductors may experience magnetic forces that are significantly different from the well-known repulsive Meissner effect. In particular flux trapped by a superconductor may result in a force which has a stable position. We have calculated the magnetic force acting on a superconductor, based on a trapped flux model; the results have been compared with experiments at 4.2 K on (1) a block of Nb3Sn, (2) a disk of YBa2Cu3Ox, and (3) a hollow cylinder of Pb. Most of the experimental results are in reasonably good agreement with the calculations. However, an interesting discrepancy exists in the frequency of small oscillations about the stable position.
Numerical modeling of the filament-assisted diamond growth environment
Authors: Goodwin D. G. , Gavillet G. G.
Published: July 2002

A numerical model of the filament-assisted diamond growth environment has been developed and used to calculate temperature, velocity, and species concentration profiles, accounting both for transport and detailed chemical kinetics. The computed hydrocarbon concentrations agree well with previously measured values, when allowance is made for 3D effects not included in our model. Upper-bound, diffusion-limited film growth rates for various assumed growth species have been computed, and it has been found no hydrocarbon species other than CH3, C2H2, or CH4 can account for measured diamond growth rates. The effect of thermal diffusion on H-atom profiles has been examined, and found to be only 10%. Although the environment is far from thermodynamic equilibrium, several reactions are close to partial equilibrium throughout the region from the filament to the substrate. It is also shown that homogeneous H-atom recombination is too slow to explain the experimentally observed decrease in the concentration of H with increasing initial methane concentration.
Optical properties of InGaAs lattice-matched to InP
Authors: Nee T. W. , Green A. K.
Published: July 2002

The optical spectra of a molecular-beam-epitaxially grown In0.53Ga0.47As epilayer lattice-matched on a (100) InP substrate are measured in the visible and infrared regions. The transmittance, reflectance, and their wavelength derivatives (wavelength-modulation spectra) are measured at room temperature. The spectra were analyzed by a theoretical spectral line-shape calculation based upon the band structure near the fundamental band gap. The quasidielectric functions and optical constants in the 0.6–3.1 eV range and the critical energies E0, E0+Δ0, E1, and E1+Δ1 are determined by fitting the calculated spectral line shapes to the measured spectra.
Temperature dependence of trap creation in silicon dioxide
Authors: DiMaria D. J.
Published: July 2002

The controversy over whether the rate of trap creation in silicon dioxide or at its interfaces with contacting electrodes increases or decreases at low temperature in metal-oxide-semiconductor structures is resolved. This is done experimentally by separating the strongly temperature-dependent background trapping in the interfacial regions from trap creation by hot electrons. As will be demonstrated here, the trap-generation rate is reduced at low temperatures regardless of the injection mode into the oxide conduction band, but the buildup of the created sites shows no saturation. Tunnel injection through or hot electron emission over the interfacial energy barrier are compared and discussed in detail. Also, our data are compared to radiation damage studies of others and their models using the motion of a hydrogen-related species to the cathodic interface where it interacts with defect sites to produce the observed interface states.
Spatial profiling of electron traps in silicon nitride thin films
Authors: Park Y. C. , Jackson W. B. , Johnson N. M. , Hagstrom S. B.
Published: July 2002

A method is described for determining the spatial profiles of electron traps in dielectric thin films. The method is an extension of avalanche injection and charge centroid measurements. By determining the change in the charge centroid and the injected charge after a sequential series of pulses, the densities of electron traps as a function of depth in both low-pressure-chemical vapor deposited (LPCVD) and plasma-enhanced-chemical vapor deposited (PECVD) silicon nitrides were determined. Contrary to previous assumptions of a uniform trap density in the nitride, both nitrides exhibit interface trap densities extending 10–15 nm into the film that is between 6 and 15 times larger than the bulk trap density of 0.5–2×1018 cm−3. The trap capture cross section was determined to be 6–10×10−13 cm2. The interface trap density of commercial LPCVD nitride deposited at higher temperature was higher than that found for PECVD nitride. The spatial resolution and limitations of the profiling technique, avalanche injection and charge trapping were modeled by numerically solving equations describing charge trapping, current continuity, and electric field. Important issues such as the effects of field, trap density and detrapping on the profiling analysis are examined both experimentally and through the use of simulation.
Carbon doping and lattice contraction of GaAs films grown by conventional molecular beam epitaxy
Authors: Hoke W. E. , Lemonias P. J. , Weir D. G. , Hendriks H. T. , Jackson G. S.
Published: July 2002

Carbon-doped GaAs films have been grown by solid-source molecular beam epitaxy using a graphite filament. The films were doped from 1×1015 cm−3 to 5×1019 cm−3 and the resulting mobilities are equivalent to beryllium-doped films. A slight dependence of As4/Ga flux ratio on carbon doping was observed. The use of either As2 or As4 did not significantly affect the carbon doping concentrations. Lattice contractions were observed for films doped heavily with carbon or beryllium. For a given doping concentration the contraction is more significant for carbon doping which is consistent with the smaller tetrahedral covalent radius of carbon compared to beryllium. Good agreement between observed and calculated lattice contractions with carbon doping is obtained. Annealing studies on a film doped with carbon at 5×1019 cm−3 indicate that the electrical properties and lattice contraction are quite stable.
Excitonic and Raman properties of ZnSe/Zn1−xCdxSe strained-layer quantum wells
Authors: Lozykowski H. J. , Shastri V. K.
Published: July 2002

The optical properties of strained-layer ZnSe/Zn0.86Cd0.14Se single quantum wells have been studied. The photoluminescence under direct and indirect excitation is investigated in detail. The temperature dependence of photoluminescence and resonant Raman scattering are investigated. Very strong 2LO-phonon Raman scattering has been observed with Zn0.86Cd0.14Se quantum wells, where the scattered photon energy is in resonance with an exciton transition. Experimental exciton energies are compared with a finite-square-potential quantum-well model including band nonparabolicity and the strain effect. Based on Hill’s theory [J. Phys. C 7, 521 (1974)] we have computed the band gap of Zn1−xCdxSe as a function of composition x.
Low hydrogen content stoichiometric silicon nitride films deposited by plasma-enhanced chemical vapor deposition
Authors: Parsons G. N. , Souk J. H. , Batey J.
Published: July 2002

We have deposited silicon nitride films by plasma-enhanced chemical vapor deposition (PECVD) at 250 °C with properties similar to films prepared at 700 °C by low-pressure chemical vapor deposition (LPCVD). Films are prepared using silane and nitrogen source gases with helium dilution. The film properties, including N/Si ratio, hydrogen content and electrical quality are most sensitive to changes in the silane flow rate during deposition. For films deposited under optimized conditions at a substrate temperature of 250 °C, current versus voltage measurements in metal-insulator-semiconductor structures show the onset of carrier injection at 3–4 MV/cm, slightly lower than LPCVD films. When bias-stressed to 2 MV/cm, capacitance versus voltage measurements show some hysteretic behavior and evidence for positive fixed charge, similar to LPCVD films. For the optimized films: N/Si=1.33±.02; refractive index (λ=6328 Å)=1.980±0.01; dielectric constant (1 MHz) ∼7.5; density=2.7±0.1; and the etch rate in 10% buffered HF ranges from 32 to 70 Å/min. In addition, the hydrogen is distributed equally in Si-H and N-H groups, with a total hydrogen content <10 at.%. These films have a significantly lower hydrogen content than observed in other PECVD silicon nitride films deposited at this temperature. When the substrate temperature is increased to 350 °C, the films have the same Si/N ratio, and similar electrical properties; the hydrogen content is reduced to <6×1021 cm−3, and the etch rate is 17 Å/s in 10% buffered HF solution.
Brittle crack propagation in silicon single crystals
Authors: Brede M. , Hsia K. J. , Argon A. S.
Published: July 2002

Viewing the brittle-to-ductile transition of fracture in intrinsically brittle solids as a crack tip initiated critical event of either nucleation of dislocation loops from the crack tip or the motion away of such dislocations from the crack tip, experiments have been devised to measure the critical activation energy of such events by measuring the arrest temperature of cleavage cracks with different velocities in experiments that were conducted on large Si single crystals subjected to a steep temperature gradient. While such experiments can provide precise information that can be related directly to mechanisms of crack tip bifurcation behavior, they are hampered by nontrivial perturbations that must be controlled. Here in the first of a series of communications we discuss the nature of these perturbations in Si single crystals, cleaving either on the {111} or the {110} planes.
Self-consistent solution of the Poisson and Schrödinger equations in accumulated semiconductor-insulator interfaces
Authors: Suñé J. , Olivo P. , Riccó B.
Published: July 2002

A general method for the study of quantum effects in accumulation layers is presented. The Schrödinger and Poisson equations are self-consistently solved in a finite quantum box which includes the whole metal-insulator-semiconductor structure. An appropriate choice of the boundary conditions allows the achievement of box-independent results. For the first time, the electrostatical potential and quantum energy levels of an accumulated n-type semiconductor are fully self-consistently calculated without considering the electric-quantum limit approximation. Hence, being able to treat the problem even at room temperature, we report results in the whole range from liquid-helium temperature to room temperature and beyond. This has been possible because our method allows the calculation of both bound and mobile electron states and their introduction into the Poisson equation on equal footing. The effect of the penetration of the wave functions into the oxide has been determined, and it has been demonstrated that the consideration of an infinite semiconductor-insulator interface barrier leads to more serious errors than previously estimated by other authors. Having included the oxide-metal interface into the quantum box, we also propose a simple method to calculate the tunnel current which flows through the insulator. Although the contribution of many subbands has to be added up to obtain the total current, oscillations in the Fowler–Nordheim current-voltage characteristic, which are due to reflection resonances at the insulator-anode interface, are clearly observed. Initially conceived for the accumulation layer problem, the presented method is obviously valid for treating inversion layers as well.
Bonding properties of glow-discharge polycrystalline and amorphous Si-C films studied by x-ray diffraction and x-ray photoelectron spectroscopy
Authors: Takeshita T. , Kurata Y. , Hasegawa S.
Published: July 2002

Polycrystalline and amorphous Si-C films were prepared by rf glow-discharge decomposition of silane-methane mixtures at 700 °C. We have demonstrated that polycrystalline SiC films with large grains grow under heavy hydrogen dilution. The bonding properties as a function of film composition and hydrogen dilution were characterized by means of x-ray diffraction and x-ray photoelectron spectroscopy. Crystallization takes place at around C content x=0.5 in Si1−xCx, accompanying some segregation of carbon atoms in grain boundaries, as a result of a preference for heteronuclear bonds. It was shown that C-C(C3−nSin) (n=0–3) bonds appear in the carbidic phase of C-rich films, leading to occurrence of compressive strain in the crystalline SiC grains. In addition, effects of hydrogen dilution were discussed in correlation with the strain.