bluuebunny/crossref_metadata_2025 huggingface dataset:
A model for the Fe-related emission at 3057 cm−1 in GaAs
Authors: Pressel K. , Bohnert G. , Rückert G. , Dörnen A. , Thonke K.
Published: July 2002

We present Fourier transform infrared photoluminescence (PL) and absorption studies on GaAs:Fe to analyze the origin of the Fe-related emission at 3057 cm−1. Temperature-dependent PL spectra show additional hot lines and a characteristic phonon sideband linked to this Fe-related peak. Time-resolved studies, using the 1.06 μm line of a Nd:YAG laser for excitation, reveal a decay time of 1.9±0.3 ms. The long lifetime and the fine-structure splitting fit a model where the transitions take place between the 4T1 excited state and the 6A1 ground state of Fe3+ (3d5) in tetrahedral environment.
Mössbauer–Fresnel zone plate
Authors: Mooney T. M. , Alp E. E. , Yun W. B.
Published: July 2002

We present a novel application of the Fresnel zone plate geometry in which the Mössbauer effect is used to focus nuclear-resonant x-ray photons and thereby to monochromatize synchrotron radiation. The calculated efficiency and energy bandpass of the Mössbauer–Fresnel zone plate monochromator compare well with those of other Mössbauer-effect monochromators currently in use. The unique capability of focusing Mössbauer radiation into a small (few μm) spot may have applications in spatially resolved and high-pressure Mössbauer-effect studies.
Frequency up-conversion of a high-power microwave pulse propagating in a self-generated plasma
Authors: Kuo S. P. , Ren A.
Published: July 2002

In the study of the propagation of a high-power microwave pulse, one of the main concerns is how to minimize the energy loss of the pulse before reaching the destination. In the very high-power region, one has to prevent the cutoff reflection caused by the excessive ionization in the background air. A frequency autoconversion process that can lead to reflectionless propagation of powerful electromagnetic pulses in self-generated plasmas is studied. The theory shows that under the proper condition the carrier frequency ω of the pulse shifts upward during the growth of local plasma frequency ωpe. Thus, the self-generated plasma remains underdense to the pulse. A chamber experiment to demonstrate the frequency autoconversion during the pulse propagation through the self-generated plasma is conducted. The detected frequency shift is compared with the theoretical result calculated by using the measured electron density distribution along the propagation path of the pulse. Good agreement is obtained.
Determination of crack size and shape using relaxation methods
Authors: Hodgson R. J. W.
Published: July 2002

The relaxation method is used together with surface-potential values in order to determine the size and shape of surface cracks in metallic structures. The method is based on the ac potential difference technique for sizing surface-breaking cracks. The original problem is transformed to a form where the X and Y coordinates are expressed as functions of the potential and stream functions. The crack shape is then given by the set of points formed by the computed values of X and Y along the boundary where the potential vanishes. The procedure is tested using synthetic input data, and including the effect of noisy data. An experimental technique is proposed in order to obtain the required input data.
Spin cast thin films of fullerenes and fluorinated fullerenes: Preparation and characterization by x-ray reflectivity and surface diffuse x-ray scattering
Authors: Belaish I. , Entin I. , Goffer R. , Davidov D. , Selig H. , McCauley J. P. , Coustel N. , Fischer J. E. , Smith A. B.
Published: July 2002

We demonstrate that high quality thin films of fullerenes (C60) and fluorinated fullerenes can be prepared from solution by the spin coating technique on float glass or silicon wafers. The films were characterized by x-ray reflectivity and diffuse x-ray scattering. A systematic study of films of different thicknesses allows estimation of the distance between the fullerenes spheres to be 10.9±1 Å in our C60 films. The C60 film-air surface is very smooth. Annealing in air above 90 °C leads to an irreversible increase of the film thickness, which is attributed, at present, to oxidation.
Electric probe measurements of the normal cathode fall in CO2 laser discharges
Authors: Stańco J. , Uhlenbusch J.
Published: July 2002

An electric probe was used for measurements of the normal cathode fall in a glow discharge in flowing CO2 laser mixtures (CO2:N2:He=4.5:13.5:82 and 5:20:75) for uncooled copper, stainless-steel and tungsten cathodes. The results imply that the cathode fall value in the mixture is governed mainly by the presence of carbon dioxide.
Fundamental optical absorption edge of reactively direct current magnetron sputter-deposited AlN thin films
Authors: Zarwasch R. , Rille E. , Pulker H. K.
Published: July 2002

Aita et al. [J. Appl. Phys. 66, 4360 (1989)] studied the ultraviolet optical behavior of rf sputter-deposited AlN thin films. The same optical behavior is found for reactively dc magnetron sputter-deposited AlN thin films at different total gas flows for constant Ar/N2 ratios. The microcrystallites of these films exhibit diameters between 5 and 20 nm.
Ti/GaAs Schottky barriers prepared by ion beam sputtering
Authors: Cola A. , Lupo M. G. , Vasanelli L. , Valentini A.
Published: July 2002

Schottky barrier diodes of Ti on n-GaAs wafers were made by means of an ion beam sputtering (IBS) system under various sputtering conditions. We have investigated Schottky diode parameters by I–V and C–V measurements, and deep level centers by deep level transient spectroscopy (DLTS). The electrical characteristics are typical of good quality Ti/GaAs Schottky diodes for all samples, although a slight deterioration of the electrical performances has been observed for samples prepared at higher beam voltage. DLTS investigation shows the presence of damage defects in concentration lower than 1 × 1014 cm−3 for all samples with the exception of those prepared at higher beam voltage. Such low concentration of defects introduced by the sputtering deposition shows that IBS can be a very useful technique to prepare Schottky barriers on GaAs.
Transport electronic properties and magnetic susceptibility studies of Al90R10 amorphous alloys with R=Y, La, Ce, Nd, Sm, Gd, Tb, Ho, and Er
Authors: Pont M. , Puig T. , Rao K. V. , Inoue A.
Published: July 2002

We present ac susceptibility, electrical resistivity, and Hall effect measurements for a new series of Al-rich amorphous alloys Al90R10 with R=Y, La, Ce, Nd, Sm, Gd, Tb, Ho, and Er. The electrical resistivity at room temperature is approximately constant and around 70 μΩ cm, and increases with increasing temperature. The ac susceptibility, measured between 4 and 150 K shows that most of the alloys follow a Curie–Weiss behavior with an effective magnetic moment close to that of the pure rare-earth elements. The ordinary Hall coefficient has a negative value around half of the expected using a free electron approach. Due to the magnetic moment carried by the rare-earth elements in these alloys a magnetic contribution to the Hall effect cannot be ruled out although a detailed comparison with the theory is not available at present.
WSe2-based Schottky junctions: The effect of polyiodide treatment on junction behavior
Authors: Hodes G. , Watkins E. , Mantell D. , Brillson L. J. , Peisach M. , Wold A.
Published: July 2002

The effect of polyiodide solution treatment on WSe2−both n and p type−prior to Schottky junction formation, has been studied. The junctions have been characterized mainly by (photo)current/voltage and spectral response measurements. Barrier heights of ≳1 V have been found [Eg(WSe2)∼1.2 eV]. X-ray photoelectron spectroscopy and Rutherford backscattering analyses of polyiodide-treated WSe2 show that some iodine species (the nature of which has not been conclusively identified) exists to considerable depths (10−6–10−5 cm) in the WSe2, and that the I uptake is much faster and more extensive on defects (∥c faces) than on the van der Waals (⊥ c) face, if indeed it is adsorbed on the latter at all. Also, the surface charge differs between the two faces for both the nontreated and I-treated cases. The results are consistent with a passivation mechanism whereby some I species at the defects chemically attack the deposited metal (Au,Al), effectively removing the metal/defect–semiconductor junctions which short the good diode behavior of the greater part of the surface. The main effect of the defects is to decrease the photovoltage by increasing the dark forward current. The assignment of these defects as recombination centers, as is usually believed to be their main effect on these photovoltaic cells, while often (though not always) important, is secondary to their effect on the dark forward current.
Critical currents in high-T
c ceramics by force-induced fluxon movement
Authors: Weyers A. , Kliem H. , Lützner J. , Arlt G.
Published: July 2002

The effects of external magnetic fields on the critical currents jc in YBaCuO ceramic due to Lorentz force-induced fluxon movement are discussed. Two cases for applied magnetic fields are investigated: (i) The external field and the self-field are circular in the superconductor; (ii) the external field is homogeneous and perpendicular to the current in the superconductor. The first case is a one-dimensional problem that allows a simple analytical calculation. It is assumed that the critical state in the cross section of the sample is governed by the condition jc B=const with B as local magnetic induction. The critical currents calculated under these conditions agree qualitatively with measurements.
Epitaxial growth of superconducting YBa2Cu3O7−x on Si(100) with CaF2 as intermediate buffer
Authors: Tiwari A. N. , Blunier S. , Zogg H. , Lerch Ph. , Marcenat F. , Martinoli P.
Published: July 2002

Epitaxial superconducting YBa2Cu3O7−x (YBCO) thin films have been grown on Si(100) using CaF2 as an intermediate buffer layer. The CaF2(100) layers were grown by molecular beam epitaxy while for the YBCO layers a laser ablation process was used. Electron channeling patterns of the YBCO surface show the characteristic four-fold symmetry for c-axis orientation. The epitaxial relationship is (001)YBCO//(001)Si and [010]YBCO//[110]Si. This corresponds to a lattice mismatch of only 1% in the a-b-YBCO interface plane. The 60-nm-thick layers are crack free. Magnetic shielding properties of the epitaxial YBCO layers have been studied by measuring the zero temperature penetration depth λab(0) and the power law dependence of the transition temperature versus magnetic field.
Thermal-wave study of the microstructure of lead titanate zirconate and PbTiO3-based ceramics
Authors: Dimitrov S. , Velinov Ts. , Yanchev R. , Vasileva D. , Bransalov K.
Published: July 2002

The microstructure of lead titanate zirconate and PbTiO3-based ceramics is investigated with thermal waves. The photoacoustic data obtained by a gas-microphone method show the presence of thermal-wave diffraction within the samples. The comparison with scanning electron micrographs allows the influence of the grain size and the volume of the glassy phase on the photoacoustic signal to be determined. The influence of the thermal parameters is also discussed. Analysis of the photoacoustic behavior of the investigated ceramic samples is made using the Opsal–Rosencwaig theory.
Gas-source molecular-beam epitaxy using Si2H6 and GeH4 and x-ray characterization of Si1−xGex (0≤x≤0.33) alloys
Authors: Li S. H. , Chung S. W. , Rhee J. K. , Bhattacharya P. K.
Published: July 2002

Gas-source molecular-beam epitaxy (MBE) has been used to grow SiGe alloys with Si2H6 and GeH4 as sources on (100) Si substrates. Single-crystalline epilayers with Ge composition as high as 33% have been produced at 610 °C, the lowest temperature hitherto used for gas-source SiGe MBE. Growth parameters, growth modes, and the structural characteristics have been studied by a variety of in situ and ex situ techniques. Double-crystal x-ray diffraction data for the alloys have been obtained for the first time in thin mismatched layers.
Surface and step reconstructions on {100} and {111} planes of diamonds prepared by combustion-flame deposition
Authors: Okada Katsuyuki , Komatsu Shojiro , Ishigaki Takamasa , Matsumoto Seiichiro , Moriyoshi Yusuke
Published: July 2002

The growth steps were observed on both {100} and {111} planes of diamonds prepared by a combustion-flame deposition method using C2H2-O2. The etch pits on {100} planes of diamonds were also observed with a scanning electron microscope. The orientations of both growth steps and etch pits on {100} planes were parallel to 〈110〉, whereas the step directions on {111} planes were [1̄1̄2]-oriented and the lateral growth rate was the slowest in the [1̄1̄2] direction. The former is described with the hypothesis of so-called (2×1) surface reconstruction, and the latter is successfully explained by a step reconstruction model. The occurrence of the surface reconstruction would indicate that not only atomic hydrogen but also OH radicals and atomic oxygen, which adequately exist in the diamond growing region in a flame, prevent the surface from being passivated or adsorbed by impinging atoms and/or radicals under the growing conditions.
Design considerations for z-pinch driven photoresonant x-ray lasing in neonlike krypton
Authors: Thornhill J. W. , Apruzese J. P. , Davis J. , Clark R. W.
Published: July 2002

A neonlike x-ray laser photoresonant pumping scheme is explored. An attractive design is a coaxial z-pinch consisting of an inner krypton lasant plasma surrounded by a carbon shell that itself is surrounded by a stagnated krypton plasma. The photoresonant radiation emitted from the outer plasma passes inwardly through the carbon shell and photoexcites lasant electrons to the 3s, 3d, and 4d levels. It is calculated that monochromatic pump powers of 30 GW/cm eV can be achieved for the 4d neonlike resonance line while powers of 100–200 GW/cm eV are attainable for the 3s and 3d resonance lines. The gain in several neonlike 3s-3p transitions is calculated as a function of temperature and density of the lasant plasma. Reasonable gain in only the 3p-3s (J=0–1) transition is obtained for high-density, high-temperature lasant conditions. This gain, which is driven by monopole excitation from the ground state to the 3p (J=0) level, is nearly independent of the presence of pump radiation. For low-density, low-temperature lasants, optimal gain is obtained in the 3p-3s (J=2–1) transitions. Under these conditions, the pump radiation is necessary to photoionize and photoexcite the lasant plasma to the neonlike stage while radiative decay from the resonantly pumped 4d level to the 3p (J=2) states provides the major upper lasing level pump mechanism.
Suppression of repetitively pulsed electrical discharges by spurious voltage pulses
Authors: Curzon F. L. , Cobb R. W. , Mikoshiba S.
Published: July 2002

Repetitive discharges, powered by a train of negative, flat-topped maintenance pulses of amplitude V1, can be suppressed by applying a similarly shaped ‘‘spurious’’ voltage pulse of amplitude V2 at a time interval of tq after each maintenance pulse. Suppression occurs, even with quite small values of V2, a phenomenon that can be explained by taking account of how the spurious pulses affect the electric charges left behind in the discharge tube by each of the driven discharges. The results are relevant to elucidating ‘‘cross talk’’ between discharge tubes in display panels.
InP tunnel metal-insulator-semiconductor devices irradiated with 1 MeV electrons
Authors: Singh A. , Reinhardt K. C. , Anderson W. A.
Published: July 2002

Yb/p-InP and Au/n-InP tunnel metal-insulator-semiconductor (MIS) diodes were irradiated with 1 MeV electrons to a fluence of 1×1015 e−/cm2. The forward current-voltage (I-V) and high-frequency capacitance-voltage (C-V) characteristics of electron-irradiated Yb/p-InP MIS diodes were measured over the temperature range 80–450 K. From the I-V/T data, the temperature dependence of the ideality factor n and the reverse saturation current I0 was determined. The temperature dependence of I0 fit very well to the thermionic emission model over the temperature range 250–450 K. The zero bias, zero temperature barrier height, φ0=0.93±0.04 V, obtained from the above fitting procedure, agreed within experimental error with the value of φ0=0.99±0.05 V, obtained from the C-V/T measurements. The interface-state density at the insulator-semiconductor interface of the irradiated Yb/p-InP tunnel diodes, deduced from the forward bias dependence of n at 309.1 K, was three times smaller than the one for the nonirradiated diodes. The deep-level transient spectroscopy studies in irradiated Yb/p-InP diodes indicated that the electron irradiation annealed an electron trap, introduced a new hole trap, shifted the other two original hole traps to slightly higher energies, increased their capture cross section by an order of magnitude, and reduced their concentration by about 50%. In Au/n-InP MIS diodes, electron irradiation annealed one electron trap and created a new electron trap, shifted the other two original electron traps to slightly lower energies, lowered the capture cross sections by an order of magnitude, and increased their concentrations by nearly 50%.
Influence of growth conditions and annealing parameters on the near-infrared optical absorption of epitaxial magnetic garnet films
Authors: Jovanovic C. , Sure S. , Clausing E. , Scharfschwerdt C. , Neumann M. , Alwes H. , Lorenz K. , Dötsch H. , Tolksdorf W. , Willich P.
Published: July 2002

Several series of epitaxial films of nominal pure and bismuth-, aluminum-substituted yttrium iron garnet are grown on [111]-oriented paramagnetic substrates from PbO-fluxed melts. The growth rate is varied systematically by changing the supercooling. The optical absorption at a wavelength of 1330 nm and the concentrations of the impurities Pb2+, Pb4+, Pt4+, Si4+, and Ca2+ are measured. The absorption shows a minimum at a certain growth rate vmin, which increases linearly with the saturation temperature of the melt. The minimum of the absorption constant reaches values below 0.3 cm−1. The as-grown films contain a surplus of four-valent impurities, the minimum concentration of which correlates with the minimum of the optical absorption. The absorption of samples with growth rates higher than vmin can be reduced considerably by annealing in hydrogen atmosphere at 430 °C for a few minutes.
Two-step rapid thermal annealing of B- and As-implanted polycrystalline silicon films
Authors: Sicart J. , Jeanjean P. , Sellitto P. , Robert J. L. , Chaussemy G. , Laugier A.
Published: July 2002

We investigate the electrical properties of polycrystalline silicon films implanted with boron or arsenic at doses 4×1014, 8×1014, and 5×1015 cm−2, respectively. Two post-implantation rapid thermal annealings (RTAs) have been performed: a standard one-step RTA (1150 °C, 20 s) and a two-step RTA (1150 °C, 4 s+680 °C, 4 min). We find that the electrical behavior depends on the implanted species. Boron-implanted films exhibit the highest electrical properties (high dopant activation and mobility). The trap density Nt at grain boundaries is found to be independent of the heat treatment.
Effect of design variations on the threshold current density of AlxGa1−xAs separate confinement heterostructure single quantum well lasers
Authors: Givens M. E. , Miller L. M. , Coleman J. J.
Published: July 2002

Data from a series of separate confinement heterostructure quantum well AlxGa1−xAs lasers of various structural design are analyzed so as to define material and device parameters suitable for subsequent modeling and threshold current optimization of arbitrary separate confinement laser structures. By modeling variations in inner barrier and cladding layer compositions, inner barrier width, and quantum well size, we show that, through proper design, low threshold current densities can be realized for structures having both indirect and direct barriers, further demonstrating that electron confinement due to a separate confinement heterostructure has no effect on carrier collection in the quantum well active layer. We also demonstrate the insensitivity of threshold current density to deviations in barrier width from an optimum value due to simultaneously induced variations in the optical confinement factor and overall optical loss coefficient.
Theoretical study of the influence of doping concentration on the performance of polycrystalline silicon solar cells
Authors: Bhatt D. P. , Joshi D. P.
Published: July 2002

The influence of the base doping concentration (NA) on the effective diffusion length of minority carriers (Ln*) and the parameters of polycrystalline silicon solar cells with fibrous grain is investigated theoretically utilizing our recently proposed grain-boundary recombination model together with the heavy doping effects. The effects of the grain size, the grain-boundary states density, and the Auger recombination processes on the above-said parameters are also studied. It has been shown that, for very small grain sizes (d≪100 μm), the grain-boundary recombination process is always more effective than the bulk and the Auger recombination processes, provided the base doping level is less than 1019 cm−3. It is found that if the grain size is much larger than the bulk diffusion length of minority carriers, Ln* always decreases with increasing NA, whatever the base doping level be. It is also found that at small values of the grain size and for NA≳3×1015 cm−3, the contribution of the space-charge recombination component of the dark current is greater than that of the diffusion component. It is predicted that the effect of the emitter side on the performance of the solar cell cannot be neglected if NA≳2×1017 cm−3. It is demonstrated that the optimum base doping level (for the maximum conversion efficiency) increases with increasing grain size and attains its corresponding value of monocrystalline silicon. It is also demonstrated that the experimentally observed values of maximum power output cannot be matched with the computed values, unless we consider the grain boundary shunting effects for NA≥1016 cm−3.
Ge-rich Co-Ge contacts to n-type GaAs
Authors: Koltin E. , Eizenberg M.
Published: July 2002

Interfacial reactions for the Co/Ge and Ge/Co contact systems on (001)-oriented n-type GaAs substrates were studied with emphasis on Ge-rich stoichiometries−Co:Ge=1:1 and 2:3. Following annealing at temperatures up to 350 °C, intermetallic compounds of cobalt and germanium were formed, depending on the Co:Ge atomic ratio, while the inner interface with the GaAs substrate remained intact. At higher temperatures (up to 600 °C, the highest temperature used) a limited reaction with the GaAs substrate was detected. This reaction for both configurations was contained near the interface with the substrate, and did not develop with temperature. The extent of reaction decreased with the decrease in the Co:Ge atomic ratio. No reaction could be detected at the GaAs interface when the Co:Ge atomic ratio was 2:3 even at temperatures as high as 600 °C (for 30 min). Contacts produced in these systems were rectifying with a nearly ideal thermionic emission behavior. This is in contrast to a case of a more Co-rich composition previously studied in our laboratory where contacts with low barriers and extensive interfacial reactions were observed after annealing at 400 °C or higher temperatures.
Simple calculations of energy levels in quantum wells of lattice-matched semiconductors with nonparabolic bands
Authors: Hrivnák Ľ.
Published: July 2002

Relations are presented for calculating electron and light-hole energy levels in quantum wells on the basis of knowledge of the electron and light-hole effective masses, the lattice constant, and the width of the well. The electron and light-hole band nonparabolicity of semiconductors forming the well is accounted for. The nonparabolicity of the heavy-hole band is neglected. The calculated values of En − LHn and En − HHn transition energies are in good agreement with recently published experimental data for various AlxGa1−xAs-GaAs, Ga0.51In0.49P-GaAs, and In0.53Ga0.47As-InP quantum wells.
Regrowth of a GaAs layer for n-GaAs ohmic contacts
Authors: Li Baoqi , Holloway Paul H.
Published: July 2002

Reaction of Au with GaAs in conventional NiAuGe/GaAs ohmic contacts results in rough interfaces, poor surface morphology, and deterioration at the edges of films. To improve these properties, ohmic contacts to GaAs were formed by heating initially discrete layers of volatile arsenic and nonvolatile gallium under a Ge–Au alloy thin film. The layer sequence beginning at the air interface was AuGe(1500 Å)/As(500 Å)/Ga(400 Å), all evaporated onto chemically cleaned (100) GaAs surfaces, followed by annealing at 400 to 650 °C in forming gas. A regrown layer of highly doped GaAs was formed between the AuGe layer and the original GaAs substrate. This layer was characterized by scanning Auger electron spectroscopy, scanning electron microscopy, and electrical measurements. The carrier concentration of the regrown GaAs layer was estimated from four-point probe data to be in the range of 1019 to 1020 cm−3. The specific contact resistivity for the AuGe/As/Ga/GaAs regrown at 550 °C for 10 min was ≊1.5×10−5 Ω cm2, which is similar to values measured for AuGe/GaAs annealed at 450 °C for 2 min. The mechanism of GaAs regrowth and ohmic contact formation are discussed.
Superconducting properties of (Bi,Pb)2Sr2Ca2Cu3Ox tapes
Authors: Ullrich M. , Heinemann K. , Freyhardt H. C.
Published: July 2002

(Bi,Pb)2Sr2Ca2Cu3Ox Ag-sheathed tapes were fabricated by the standard ‘‘powder in tube’’ technique. Because of the platelike growth of the Bi HTc-phase, it is possible to texture the wires by a rolling process, which improves the mechanical density. This implies an increase of the critical current density not only in zero magnetic field, but also in all external magnetic fields. By measuring ρ(T,B) curves, we have estimated the activation energy for thermically activated flux creep, U0, to vary between 200 and 20 meV in magnetic fields between 0.005 and 10 T. These low U0 values are approximately ten times smaller than in Y1Ba2Cu3O7−δ. For the Bi 2223 tapes, a power law U0∼B−α with α=0.277±0.030 was obtained.
Heteroepitaxial InP layers grown by metalorganic chemical vapor deposition on novel GaAs on Si buffers obtained by molecular beam epitaxy
Authors: Olego D. J. , Tamura M. , Okuno Y. , Kawano T. , Hashimoto A.
Published: July 2002

Heteroepitaxial InP layers were grown by metalorganic chemical vapor deposition on novel GaAs buffer layers on Si substrates. The GaAs buffers were prepared by molecular beam epitaxy and show a reduced threading dislocation density achieved by inserting one nanometer thick Si interlayers. The structural and optoelectronic properties of the heteroepitaxial InP layers, which were investigated by transmission electron microscopy, x-ray diffraction, and photoluminescence spectroscopy, show improvements attributed to the optimized GaAs buffer layers.
Charge storage in the quasineutral regions of Si homojunction, AlGaAs/GaAs heterojunction, and Si/SiGe heterojunction bipolar transistors
Authors: Liou J. J.
Published: July 2002

Charge storage in the quasineutral regions of a bipolar transistor often limits the switching speed of the device, particularly when the transistor is operated in saturation mode (both emitter-base and base-collector junctions are forward biased), a bias condition which occurs frequently in logic circuits. This paper studies and compares the free-carrier charges stored in the quasineutral regions of the conventional Si homojunction bipolar transistor (Si BJT) and the increasingly important AlGaAs/GaAs and Si/SiGe heterojunction bipolar transistors (HBTs). The diffusion capacitances associated with this stored charge, which are used in the Gummel–Poon bipolar transistor model, are also calculated and discussed. It is shown that the AlGaAs/GaAs HBT has the least carrier accumulation in the quasineutral regions among the three devices due primarily to the relatively large energy band gap of GaAs-related materials. Also, the Si/SiGe HBT is expected to operate at a higher switching speed compared to the Si BJT, because the valence band discontinuity at the Si/SiGe base-collector heteroface impedes the minority-carrier injection from the base into the collector.
Al/Au and Cu/Au bilayer-metal contacts to YBa2Cu3O7−x thin films
Authors: Ma Q. Y. , Schmidt M. T. , Yang E. S. , Chan Siu-Wai , Bhattachayra D. , Zheng J. P. , Kwok H. S.
Published: July 2002

Electrical contact resistivities of both Al and Cu single layer contacts as well as Cu/Au and Al/Au bilayer metal contacts to YBa2Cu3O7−x (YBCO) thin film have been studied. It was found that aluminum and copper make poor electrical contacts to YBCO due to interface reactions. These contacts have large contact resistivity (10−1–1 Ω cm2), orders of magnitude higher than that of a gold/YBCO contact (10−6 Ω cm2). When an ultrathin Au interlayer (10–30 Å) was inserted between an Al or Cu overlayer and a YBCO film, interface reaction was greatly reduced. The contact resistivities of Al/Au and Cu/Au bilayer-metal contacts dropped by five orders of magnitude when Au interlayer thickness was increased from 0 to 10 Å. With the gold interlayer thickness of 15 Å, the Al/Au and Cu/Au bilayer-metal contacts reached a minimum contact resistivity, approaching that of a thick single-layer Au metal contact.
New methods for using the Q-V technique to evaluate Si-SiO2 interface states
Authors: Lin Ta Cheng , Young Donald R.
Published: July 2002

The Q-V technique applied to metaloxide silicon capacitors makes it possible to measure the silicon-oxide interface potential directly with only a few simple assumptions. From these data of interface potential versus gate voltage it is possible to calculate the interface state distribution. The Q-V technique can be used over the entire band gap range and separate n- and p-type samples are not required. New simplified techniques for doing this are presented. The measurements can also be used to determine the band gap of the semiconductor which should have application for the study of silicon-germanium alloys.
Calculated electronic and magnetic structures of the new ternary rare-earth–iron nitride Nd2Fe17N3
Authors: Gu Zongquan , Lai Wuyan
Published: July 2002

The electronic and magnetic structures of Nd2Fe17 and Nd2Fe17N3 have been calculated using the first-principle, spin-polarized orthogonalized linear combination of atomic orbitals method. Comparative studies of the two materials reveal important effects of the nitrogen atoms (at 9e site) on the electronic and magnetic structures. Results are presented for the total density of states, site-projected partial density of states and the spin magnetic moments on four nonequivalent Fe sites. The highest magnetic moments are found to be located on the 6c site for Nd2Fe17 and on the 9d site for Nd2Fe17N3, in agreement with the neutron and Mössbauer experiments. The variation trends of the magnetic moments on different Fe sites are discussed in terms of the separation between Fe and N atoms. Compared with Nd2Fe17, an increase in the exchange splitting of the Fe d band is found in Nd2Fe17N3, which accounts for its higher Curie temperature as observed in experiments. The calculated results show that the nitrogen atoms are charge acceptors in these compounds.
Modeling radio-frequency discharges: Effects of collisions upon ion and neutral particle energy distributions
Authors: May P. W. , Field D. , Klemperer D. F.
Published: July 2002

Using a recently published description of the potentials in a radio-frequency discharge [Y. P. Song, D. Field, and D. F. Klemperer, J. Phys. D 23, 673 (1990)] Monte Carlo trajectories of ions and neutral particles for an Ar plasma have been computed, including collisions at the realistic pressures of tens of mTorr used for reactive ion etching. The model for collisions involves both charge exchange and momentum-transfer scattering. Ion and neutral energy and angular distributions have been calculated for particles impacting on the powered electrode and the calculations agree with an experimental ion energy distribution for Ar+. The presence of a large proportion of energetic neutrals is predicted, and it is suggested that these may have an important role to play in etch mechanisms.
In situ observation of the initial growth stages in GaAs/InAs by coaxial impact collision ion scattering spectroscopy: Transition from two-dimensional-like to three-dimensional island growth
Authors: Saitoh T. , Hashimoto A. , Tamura M.
Published: July 2002

The initial growth stages of the highly lattice-mismatched GaAs/InAs system was studied by coaxial impact collision ion scattering spectroscopy (CAICISS). The GaAs coverage on the InAs substrate during GaAs molecular beam epitaxial growth was monitored in situ by low incident angle CAICISS. The scattering intensity as a function of the deposited amount of GaAs can be divided into three characteristic regions. First, the scattering intensity from In decreases proportionally with the amount of deposited GaAs molecules. However, the intensity decrease stops abruptly before the surface is completely covered with a GaAs layer, and remains constant. Then, the intensity gradually decreases. This result shows that there exist three kinds of growth stages in the process of GaAs deposition on an InAs substrate. The mechanism of the growth mode transition, corresponding to the three kinds of growth stages is discussed from the viewpoint of a strain energy change on the surface.
Carbon-doped long wavelength GaAs/AlxGa1−xAs quantum well infrared photodetectors grown by organometallic vapor phase epitaxy
Authors: Hobson W. S. , Zussman A. , Levine B. F. , deJong J. , Geva M. , Luther L. C.
Published: July 2002

We report p-doped long wavelength GaAs/AlxGa1−xAs quantum well infrared photodetectors (QWIP) grown by organometallic vapor phase epitaxy. The operation of these devices is based on the photocurrent induced through valence-band intersubband absorption by holes and, unlike n-doped QWIPs, can utilize normal incidence illumination. Carbon was used as the p-type dopant in a low-pressure (30 Torr) vertical-geometry reactor. The C-doped QWIPs consisted of fifty periods of 54-nm-thick undoped AlxGa1−xAs (x=0.36 or 0.30) and C-doped GaAs wells (Lz=4 or 5 nm). Using normal incidence illumination, the C-doped QWIP with shorter wavelength response (x=0.36, Lz=4 nm) exhibited a quantum efficiency of η=21.4% and a detectivity at the peak wavelength of Dλ=5.4×109 cm √Hz/W at 77 K. The peak and cutoff wavelengths were λp=8.1 μm and λco=8.9 μm, respectively. The C-doped QWIP with longer wavelength response (x=0.30, Lz=5 nm) exhibited a normal incidence η=22.1% and Dλ*=3.5×108 cm √Hz/W for λp=10.5 μm (λco=11.7 μm). The detectivity of the C-doped QWIPs is about four times less than n-doped QWIPs for the same λp but have the advantage of utilizing normal incidence illumination.
Lowering of the growth temperature by the iodine incorporation for vapor-phase epitaxy of ZnS
Authors: Fuke Shunro , Maezawa Chikako , Nakamura Kazuo , Kuwahara Kazuhiro , Imai Tetsuji
Published: July 2002

In order to lower the growth temperature, ZnS layers are grown on GaP(100) substrates by hydrogen-transport vapor-phase epitaxy under conditions of iodine incorporation into the growth ambient. With the optimum iodine transport level, the growth temperature is lowered by more than 200 °C and a growth rate of 1 μm/h is obtained at a substrate temperature of 420 °C. X-ray diffraction measurements show that the crystallinity is improved by the iodine incorporation but has a tendency to become worse at higher iodine transport rates and/or in the lower growth temperature region.
Importance of sample preheating in oxidation of GexSi1−x
Authors: Liu W. S. , Lee E. W. , Nicolet M-A. , Arbet-Engels V. , Wang K. L.
Published: July 2002

Wet thermal oxidation at 1000 °C of a 470-nm-thick epitaxial Ge0.36Si0.64 layer on (100)Si produces oxides of different composition depending on the details of the oxidation procedure. When a cold sample is directly exposed to the hot steam, the surface layer of the oxide contains both Ge and Si. Only SiO2 forms if a preheated sample is exposed to the hot steam. The effect is not present for dry oxidation and is attributed to the known enhancement of the wet oxidation rate by Ge, coupled with the transient warm up of a sample when it is immersed cold in hot steam.
Low-temperature avalanche breakdown in p-Ge: Influence of the acceptor concentration
Authors: Morhard K. D. , Huebener R. P. , Clauss W. , Peinke J.
Published: July 2002

Measurements of the avalanche breakdown electric field εB are reported for p-Ge at 4.2 K for acceptor (In, Ga) concentrations in the range 1013–5×1015 cm−3. The distance between the ohmic contacts has also been varied. The field εB was found to increase linearly with increasing concentration of the doping acceptors. The experimental results on εB are discussed in terms of the important scattering processes using simple hot electron model considerations.
Temporal behavior of population densities of V atoms in an optically pumped V vapor laser
Authors: Ninomiya H. , Yoshida H. , Takashima N.
Published: July 2002

The time variation of population densities of V atoms in absorption levels pertinent to an optically pumped atomic V vapor laser is studied experimentally. V vapor is generated from a V metal by irradiation of a pulsed Nd:YAG (λ=1.06 μm) laser of 2 J/pulse energy and 1.4 ms pulse duration. The population densities of the ground-state and excited-state (E∼2400 cm−1) V atoms have been measured by the interferometric method, and the temperature in the vapor is estimated from these populations. It has been experimentally observed that the populations of the ground state and the excited state are about 1×1014/cm3 and 4×1013/cm3, respectively. The temperature in the vapor increased from about 1800 to 4000 K with time during the YAG laser irradiation.
Measurement of azimuthal anchoring energy in nematic liquid crystals by transmitted and reflected light methods
Authors: Faetti S. , Lazzari C.
Published: July 2002

In a recent paper Yokoyama [Mol. Cryst. Liq. Cryst. 165, 265 (1988)] showed that a correct thermodynamic definition of the anchoring energy at the interface between a nematic liquid crystal (NLC) and another medium requires the preliminary choice of a ‘‘Gibbs dividing surface.’’ As a consequence of this, the measured value of the anchoring energy coefficient can depend on which method is used to perform the experiment. This dependence probably explains the large discrepancies that are often reported in the literature between anchoring energy coefficients measured on the same substrate by different authors. To test this important point the anchoring energy coefficient at a SiOx nematic interface has been measured by using two different methods at the same time: a reflection light method and a transmitted light method. Both these methods have been already used in the literature. In this paper the latter method is analyzed in detail and it is shown that its accuracy is greatly reduced by the presence of spurious bulk contributions. To increase accuracy, a new experimental procedure is proposed that exploits the differing dependence of surface and bulk contributions on the intensity of the magnetic field. This new method allows one to separate bulk and surface contributions without using fitting procedures. Therefore both the anchoring energy and information on the bulk director distortion can be directly obtained from the experiment. The transmitted light and the reflected light methods are used simultaneously to measure the azimuthal anchoring energy coefficient at the SiOx nematic interface on the same NLC sample at the same temperatures. A satisfactory agreement between the anchoring energy coefficients measured by the two methods is found.
Production of compositionally gradient Al-AlN films by reactive sputtering and their mechanical and electrical properties
Authors: Inoue A. , Yamaguchi T. , Kim B. G. , Nosaki K. , Masumoto T.
Published: July 2002

The application of reactive sputtering in a mixed gas of argon (Ar) and nitrogen (N2) to Al metal resulted in the formation of compositionally gradient film consisting of Al and AlN phases. By changing the mixing gas ratio of N2 to N2+Ar so as to be 0, 0.05, 0.1, and 0.2 in a total gas pressure of 1 Pa, the resulting phase changes in the order of Al, Al+AlN, AlN+Al, and AlN and the thickness of each phase is 14, 23, 18, and 8 μm, respectively, after sputtering for 3.6, 5.4, 7.2, and 10.8 ks, respectively. The Al concentration along the sample thickness changes from 100% to about 50% with an increase of mixing ratio from 0 to 0.2. The lattice parameters of Al and AlN phases remain constant along the sample thickness. The Knoop hardness and the electrical resistivity increase steeply from 100 to 3900 and from 3×10−8 Ω m to an infinite value, respectively, with the phase transition from Al to AlN through Al+AlN and AlN+Al. The load leading to an exfoliation among the layers was measured to be 16 N. Thus, the sputtered Al-AlN film is concluded to possess functionally gradient characteristics resulting from the compositional and structural gradients.
Application of the time-resolved optical-beam-induced current method to the investigation of n-metal-oxide-semiconductor inverters
Authors: Bergner H. , Hempel K. , Krause A. , Stamm U.
Published: July 2002

A pump and probe beam technique is demonstrated in conjunction with the optical-beam-induced current method of laser scanning microscopy which allows one to measure the dynamics of internal signals in integrated circuits. The time resolution of the measurement is limited only by the duration of the applied light pulses as well as the integrated circuit (IC) itself. The method is based on the generation of electrical pulses inside the IC switching a turned-off metal-oxide-semiconductor (MOS) transistor optically with short light pulses. The propagation of the electrical pulse in a chain of n-MOS inverters is studied on a subnanosecond time scale. A simple approach to the mechanisms of the generation of the optical-beam-induced current as well as the optical switching of MOS transistors is given. Optimum conditions for the laser beam power for both the excitation and the probe beam are discussed. The generation of the optical-beam-induced current in dependence on the position of the illuminating laser beam is considered.
Negative electron affinity based vacuum collector transistor
Authors: Williams M. D. , Feuer M. D. , Shunk S. C. , Sauer N. J. , Chang T. Y.
Published: July 2002

We demonstrate the operation of a vacuum collector–semiconductor transistor based on negative electron affinity cold cathode technology. The unique aspect of this transistor is that the collector is separated from the emitter-base junction by a vacuum drift region, yielding an intrinsic collector capacitance which is an order of magnitude lower than that for conventional bipolar transistors. The collector charging time is thus proportionally smaller. Transport in the vacuum drift region is truly ballistic and depends only on the collector-base bias, enabling a wide range of device concepts which are impossible or impractical in conventional transistors.
Sputter deposition of highly 〈100+001〉-textured tetragonal barium titanate on unheated silica using a neon discharge
Authors: Abuhadba N. M. , Aita C. R.
Published: July 2002

The sputter deposition of highly 〈100+001〉-textured tetragonal BaTiO3 on unheated substrates using a pressed powder BaTiO3 target and a radio frequency-excited Ne discharge is reported. For comparison, amorphous BaTiO3 was also produced, using an Ar discharge operated at the same value of all other independent process parameters. In situ discharge diagnostics using optical emission spectrometry was used to study the plasma volume. Data show that there was atomic Ti but no atomic Ba in the Ne discharge used to produce t-BaTiO3. However, both atomic Ba and Ti were identified in the Ar discharge used to produce a-BaTiO3. A probable source of free Ba atoms in the plasma is the reduction of BaTiO3 at the target surface due to the sputtering action of Ar+ ions. By comparison with other sputter-deposited metal oxide systems, the effect on film crystallinity of atomic metal versus molecular metal oxide flux incident on the substrate is discussed.
The effect of V/III ratio on the initial layer of GaAs on Si
Authors: Itoh Yoshio , Sugou Mitsuru , Mori Hidefumi
Published: July 2002

We studied the initial layer of GaAs films grown on Si substrates by a two-step metalorganic chemical vapor deposition method, in which an initial GaAs layer is grown on a Si substrate at low temperature. It was found that the V/III ratio during the growth of the initial layer plays a key role in improving the quality of GaAs film on a Si substrate. The initial GaAs layer grows two dimensionally as a continuous film when it is grown at a low V/III ratio.
Comment on ‘‘Accelerated particle radiation in chiral media’’ [J. Appl. Phys. 69, 34 (1991)]
Authors: Lakhtakia Akhlesh
Published: July 2002

The premise for the basic approximation made by the authors of the subject paper is scrutinized.
Reply to ‘‘Comment on ‘Accelerated particle radiation in chiral media’ ’’ [J. Appl. Phys. 69, 34 (1991)]
Authors: Sun Xiaoguang , Jaggard Dwight L.
Published: July 2002

We acknowledge this most recent comment and several previous comments by the same author on our work during the past six months. This comment concerns a different problem than the one considered in our paper and is characterized by scientific shortcomings. We stand by all statements in the original paper.
A model for correlating 4.2-K performance with room-temperature mechanical characteristics in superconducting test dipole magnets for the Superconducting Super Collider (SSC)
Authors: Ige O. O. , Lyon R. H. , Iwasa Y.
Published: July 2002

The longitudinal attenuation of impact-generated pulses in ten superconducting dipole magnets was measured at room temperature. A lumped-parameter model was constructed for the collared dipole. Using the method of nonlinear least-squares, the model was used to estimate the internal damping in the main components of the dipoles and the coupling resistances between the components: collars, inner, and outer coils. A positive correlation was found between the collar-inner coil coupling resistance and the 4.2-K performance of the magnets: the higher the coupling resistance, the fewer the number of quenches required to reach design operating current. There was virtually no correlation between any of the other internal or coupling resistances and 4.2-K performance. These observations are explained in terms of frictional slip of the inner coil against the collars causing premature quenches. The magnets are more susceptible to quenches at the collar-inner coil interface than at the collar-outer coil interface because the inner coil is subject to higher fields and forces. The experiment is potentially useful as a technique for screening high-performance superconducting magnets such as Superconducting Super Collider (SSC) dipoles at room temperature.