bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of a traveling wave tube tuned by a cavity
Authors: Schachter Levi , Nation John A.
Published: July 2002

We present a theoretical analysis of a system composed of two periodic structures separated by a uniform waveguide section, taking into consideration the impedance mismatch at both ends of the system. First we examine the effect of the reflections on the output gain for a single stage system, i.e., when the uniform waveguide is not present. It is shown that if the product of the gain and the reflection coefficients is of order of unity, the output gain might be significantly smaller than the one-pass gain as calculated by ignoring reflections. Introducing the uniform waveguide section, additional reflections occur from the two new planes of impedance mismatch. However, a proper location of these planes generates a wave, in the first slow wave structure, that practically may cancel the wave reflected toward the input end. The principle is similar to quarter wavelength tuning in transmission line or optical systems. The main difference is the fact that the electrons may have a significant influence on the waves, affecting the tuning condition.
Structural and electrical properties of ZrSi2 and Zr2CuSi4 formed by rapid thermal processing
Authors: Setton Michael , Van der Spiegel Jan
Published: July 2002

The growth of ZrSi2 and Zr2CuSi4 has been achieved by rapid thermal processing in vacuum. Fast heating rates and temperatures of at least 950 °C are required to form smooth Zr disilicide with a room-temperature resistivity of 34 μΩ cm. In agreement with the bulk phase diagram, the ternary compound can be prepared by reacting Cu with the disilicide. This phase is formed through the diffusion and the insertion of Cu atoms into the twin faults of the pseudolamellar C 49 Zr silicide structure; it belongs to the same Cmcm space group. Electrical measurements indicate that ZrSi2 is an electron conductor whereas a mixed conduction mechanism is found for the ternary samples.
Theoretical studies of high-power Cerenkov amplifiers
Authors: Schächter L. , Nation J. A. , Shiffler D. A.
Published: July 2002

The main theoretical aspects of the experiments performed and reported recently are discussed here. First the one stage amplifier is considered. As a preliminary step the behavior of the electrons is followed in the phase space at different points along the interaction region. This analysis reveals that about 30% of the interaction region is utilized for construction of the electron bunches. It is shown that although the average energy of the electrons remains unchanged along most of the amplifier, their energy spread increases substantially. Since the system consists of two long tapered sections, it is suggested that the effective length of the interaction region might be significantly longer than the physical length of the uniform structure. It is further suggested that the electrostatic periodic potential induced by the beam may also improve the interaction process. The next subject addressed here is the bandwidth of a single stage amplifier. It is shown that the reason for the narrow measured bandwidth is the gain of the system. In fact the output signal from a short amplifier is narrowed by exactly the same amount the amplitude of the electromagnetic wave is increased. This result is general as long as part of the radiation field is reflected from both ends of the amplifier. In the second part of this paper the two stage amplifier is analyzed. As in the case of the single stage amplifier the behavior of the electrons is followed in phase space at various locations along the system. This discussion leads to an analysis of the development of ‘‘sidebands’’ which are not symmetrically located around the initial frequency at power levels that do not correspond to a nonlinear process. It is suggested that these sidebands are amplified noise−produced basically in the first stage. The sharp selection of frequencies is due to the constructive interference of the waves bouncing between the two ends of the second stage. This selection is not symmetric relative to the initial frequency. Some considerations regarding the bandwidth of the system are also presented.
Analysis of the acoustic response of vascular tissue irradiated by an ultraviolet laser pulse
Authors: Crazzolara H. , von Muench W. , Rose C. , Thiemann U. , Haase K. K. , Ritter M. , Karsch K. R.
Published: July 2002

Stress waves were generated in sections of human vascular tissue by transmitting laser pulses from a XeCl laser source through a glass fiber. Needle-type polyvinylidenefluoride hydrophones were used to detect the acoustic response of the tissue samples during ablation. The experimental arrangement allowed the discrimination in vitro between calcified hard tissue and normal arterial wall immersed in normal saline solution. Atheromatous vessels exhibited a shorter rise time and a higher peak stress than normal tissue.
Band offset determination in analog graded parabolic and triangular quantum wells of GaAs/AlGaAs and GaInAs/AlInAs
Authors: Kopf R. F. , Herman M. H. , Schnoes M. Lamont , Perley A. P. , Livescu G. , Ohring M.
Published: July 2002

The band offset parameter Qc = ΔEc/ΔEg for both GaAs/AlGaAs (lattice matched to GaAs), and GaInAs/AlInAs (lattice matched to InP) was extracted from the optical interband transition energies obtained from both triangular and parabolic quantum well shapes of various widths. The wells were grown using continuous analog compositional grading as opposed to the discrete, superlattice (digital) grading used by previous researchers. Electron beam electroreflectance (EBER) was the primary technique used to measure the interband transition energies. By combining the theoretical energies from quantum mechanical potential well calculations with EBER measured energies, it was possible to extract band offset values in a self-consistent manner. Qc values obtained were 0.658±0.009 and 0.650±0.001 for GaAs/AlGaAs and GaInAs/AlInAs, respectively. Measurements also revealed that Qc was both temperature and concentration independent within the range of composition studied.
Tunneling through thin oxide interface layers in a-Si:H Schottky diodes
Authors: Vink T. J. , Nieuwesteeg K. J. B. M. , Oversluizen G.
Published: July 2002

A detailed study of hydrogenated amorphous silicon (a-Si:H) surfaces before and after thermal and plasma oxidation treatments was carried out using x-ray photoelectron spectroscopy. The thickness of the surface oxides is correlated with the electrical properties of corresponding Mo Schottky barrier structures. Oxide layers up to 1.5 nm in thickness cause a decrease of the reverse current of nearly two orders in magnitude, while the forward current is hardly affected. For oxide thicknesses above 2.0 nm a large reduction in the forward current is observed. Surprisingly, the associated tunneling probabilities of the oxide interface layers in the a-Si:H Schottky diodes are the same as those previously reported for c-Si-based tunnel diodes. Tunneling in the a-Si:H devices cannot be simply described by the properties of a rectangular barrier, which is adopted most frequently in these studies. A potential form where the barrier height increases quadratically with thickness fits the observed tunneling characteristics more quantitatively, both in absolute magnitude and oxide-thickness dependence of the tunnel current.
Mechanism and conditions for anomalous strain relaxation in graded thin films and superlattices
Authors: LeGoues F. K. , Meyerson B. S. , Morar J. F. , Kirchner P. D.
Published: July 2002

Compositionally graded films of SiGe/Si(100) and GaInAs/GaAs were grown under different conditions in order to investigate the different modes of strain relaxation associated with the compositional grading. We show that, when the growth conditions are very clean and the gradient is shallow enough (about 1% misfit per half micron), very good, relaxed films are obtained. This coincides with the introduction of large numbers of dislocations in the substrate itself, which is counter-intuitive at first since the substrate is under negligible strain. We show that this introduction of dislocations is the result of the activation of novel Frank–Read-like sources located in the graded region, and is directly correlated to the lack of other low energy nucleation sites for dislocations. We detail the conditions of growth necessary for this phenomenon to occur, and show that it operates both for the SiGe/Si system and the GaInAs/GaAs system. Pure, relaxed Ge films have been grown in this manner on Si(100), with a defect density as low as 106/cm2.
The measurement of deep level states caused by misfit dislocations in InGaAs/GaAs grown on patterned GaAs substrates
Authors: Watson G. Patrick , Ast Dieter G. , Anderson Timothy J. , Pathangey Balu , Hayakawa Yasuhiro
Published: July 2002

Rectangular Schottky diodes were fabricated on In0.06Ga0.94As grown by organometallic vapor phase epitaxy on GaAs substrates patterned with mesas. The density of α and β misfit dislocations at the strained-layer interface changed with the size of the rectangular mesas. Since all mesas (four sizes and two orientations) are processed simultaneously, all other defect concentrations are expected to remain constant in each diode. Scanning cathodoluminescence showed that the misfit dislocation density varied linearly with rectangle size. Deep-level transient spectroscopy showed that an n-type majority-carrier trap is present at 0.58 eV below the conduction band with a concentration that increases with increasing α-type misfit dislocation density. The β misfit dislocation density had no influence the deep level spectra, indicating that this trap is related to the cores of only α-type misfit dislocations. The capture rate trend corroborates the view that the trap is associated with the dislocation cores and not with isolated defects. Calculations indicate that the trap concentration is comparable to the concentration expected if all of the dislocation core atoms are electrically active.
Monte Carlo simulations of sputter atom transport in low-pressure sputtering: The effects of interaction potential, sputter distribution, and system geometry
Authors: Myers A. M. , Doyle J. R. , Ruzic D. N.
Published: July 2002

A comparative study of the various model assumptions in Monte Carlo simulations of low-pressure sputter-atom transport is presented. The few-collision conditions and actual ‘‘racetrack’’ magnetron geometry, typical of low-pressure magnetron sputtering, are emphasized. For the gas phase scattering problem, a comparison is made between hard sphere, Lennard–Jones 6-12, and Abrahamson Thomas–Fermi–Dirac [Phys. Rev. 178, 76 (1969)] interatomic potentials. The hard sphere potential results in both a significantly lower energy distribution and a more diffuse angular distribution for the depositing flux, as compared with the more realistic ‘‘softer’’ potentials. Because energy-dependent cross sections are obtained when using the 6-12 and Abrahamson potentials, an ‘‘energy filtering’’ effect is observed, i.e., high-energy particles arrive at the substrate preferentially to those at low energy. It is concluded that the hard sphere model will lead to serious errors in both the energy and angular distributions of the arrival flux, and that the 6-12 and Abrahamson potentials yield results that are similar to each other. For the nascent sputter distribution, fractal TRIM (transport of ions in matter) simulations are compared to the analytic Thompson distribution. While both distributions give nearly identical results for the angle-integrated fluxes, the fractal TRIM distribution shows a strong angular dependence of the energy distribution. The implications of this effect for finite geometry systems are discussed.
Low-pressure chemical vapor deposition of copper: Dependence of the selectivity on the water vapor added to a hydrogen or helium carrier gas
Authors: Lecohier B. , Calpini B. , Philippoz J.-M. , van den Bergh H.
Published: July 2002

The selectivity of copper deposition from copper (II) bis-hexafluoroacetylacetonate on SiO2 patterned with a platinum seeding layer is studied as a function of the reagent gas mixture. On platinum, the copper film growth rate increases with the amount of water vapor in the gas flow, and is independent of the chemical nature of the carrier gas used (H2 or He). The selectivity of the copper deposition is significantly improved when using He rather than H2 as carrier gas, especially at high water vapor concentrations where rapid film growth can be obtained.
Relationship between the stress and bonding properties of amorphous SiNx:H films
Authors: Hasegawa S. , Amano Y. , Inokuma T. , Kurata Y.
Published: July 2002

Amorphous SiNx:H (a-SiNx:H) films were deposited at 300 °C on single-crystal Si and fused quartz substrates using SiH4-NH3 mixtures. The stress and vibrational absorption were investigated as a function of the N content x. Increased tensile stress subsequent to a reduction in the compressive stress with increasing x was observed. From the values of stress determined for films on two different substrates, values of Y/(1−ν) for a-SiNx:H films were estimated, where Y is Young’s modulus and ν the Poisson ratio. The values of Y/(1−ν) rapidly decreased with an increase in x, from 4.2×1012 dyn/cm2 for a-Si:H films to about 2.5×1011 dyn/cm2 for a-SiNx:H films having x above 1.0. It was found that the measured tensile stress in a-SiNx:H films for high x above 1.0 was caused by the intrinsic stress, and that incorporated NH bonds act to relax the intrinsic stress. These results were discussed in terms of the change in the bonding configuration as a function of x, based on a modified random bonding model.
Interdiffusions in Cu/reactive-ion-sputtered TiN, Cu/chemical-vapor-deposited TiN, Cu/TaN, and TaN/Cu/TaN thin-film structures: Low temperature diffusion analyses
Authors: Olowolafe J. O. , Mogab C. J. , Gregory R. B. , Kottke M.
Published: July 2002

A comparative study of reactively-ion-sputtered (RIS) and low pressure chemical-vapor-deposited (LPCVD) TiN, and sputter-deposited TaN thin films as diffusion barriers for Cu has been done using Auger electron spectroscopy, Rutherford backscattering spectrometry, x-ray diffraction, scanning electron microscopy, and sheet resistance measurements. For the RIS TiN and sputtered TaN diffusion barriers, the integrity of the individual layers was preserved and there was no evidence of interdiffusion for samples annealed up to 500 °C in N2–H2 ambient for 1 hour. The LPCVD TiN was stable up to 450 °C for 30 minutes only, after which Cu started to diffuse into the TiN layer. The reasons for the higher thermal stability of RIS TiN compared to LPCVD TiN can be deduced from the microstructural differences in the two films, as observed with x-ray diffraction technique.
Magnetoresistance in (Co10 Å/Cu10 Å)n multilayer films as n increases
Authors: Plaskett T. S. , McGuire T. R.
Published: July 2002

Multilayers of Cu10 Å/(Co10 Å/Cu10 Å)n, where n is the number of bilayers, shows a nonlinear increase of magnetoresistance ratio (MR%) with increasing n. For each value of n the resistivity decreases with applied field. MR values at 4.2 K are less than 2% for n≤16 but increase to 38% at n=128. Conduction electron mean free paths (ℓ), calculated from film resistivity as a function of total film thickness, increase from ℓ=150 to 470 Å when the applied magnetic field changes from zero to 18 kOe. The values obtained for ℓ indicate that for n≤16 MR is reduced by diffuse surface scattering while for n≥16 MR surface scattering is less important and the Co atomic moment ordering is the most important factor. About half the decrease in resistivity with field is associated with the final 2% increase in magnetization for fields above 5 kOe which we interpret in terms of Co interface disorder.
29Si magnetic resonance study of amorphous hydrogenated silicon plasma deposited at 50 °C
Authors: Cheung Man Ken , Petrich Mark A.
Published: July 2002

We deposit compact amorphous hydrogenated silicon (a-Si:H) films at 50 °C, using low silane partial pressures in silane/hydrogen reaction mixtures. The 29Si chemical shift and infrared signature of our films are strongly affected by the silane feed partial pressure, but are insensi- tive to the hydrogen feed partial pressure, indicating that hydrogen ‘‘etching’’ does not play a significant role in the film growth process. Interestingly, the 29Si chemical shift and infrared signature of our compact 50 °C films are similar to those of a-Si:H films deposited at standard ‘‘optimum’’ conditions, but the electronic properties are very different. Upon thermal annealing at 150 °C for 3 h, the spin defect density may be reduced by as much as 2 orders of magnitude, while the hydrogen content, 29Si chemical shift and infrared signature remain unchanged. Therefore, it seems possible to first grow a compact silicon network structure in a-Si:H at 50 °C, and then equilibrate the electronic structure at 150 °C, without significantly altering the silicon network.
Growth and characterization of (111) and (001) oriented MgO films on (001) GaAs
Authors: Tarsa E. J. , De Graef M. , Clarke D. R. , Gossard A. C. , Speck J. S.
Published: July 2002

The effects of substrate preparation on the structure and orientation of MgO films grown on (001) GaAs using pulsed laser deposition has been investigated. Textured MgO films displaying a (111)MgO∥(001)GaAs orientation relation with x-ray rocking curve full width at half maximum (FWHM) values as low as 1.8° were obtained in cases where the native GaAs surface oxide was only partially desorbed prior to growth. Reflection high-energy electron diffraction, transmission electron microscopy (TEM), and x-ray pole figure analysis of these films reveals a preferential orientation within the plane of the substrate: [11̄0]MgO∥[11̄0]GaAs and [112̄]MgO∥[110]GaAs. An interfacial layer (∼5 nm thick) was observed in high resolution TEM analysis, and was attributed to a remnant native GaAs oxide layer. Complete desorption of the native GaAs oxide at ∼600 °C in vacuum prior to MgO growth led to significant surface roughening due to Langmuir evaporation, and resulted in randomly oriented polycrystalline MgO films. Growth of MgO on Sb-passivated GaAs substrates, which provided smooth, reconstructed surfaces when heated to 350 °C in vacuum, resulted in cube-on-cube oriented films [i.e., (001)MgO∥(001)GaAs,[100]MgO∥[100]GaAs] with x-ray rocking curve FWHM values as low as 0.47°. TEM analysis of the cube-on-cube oriented films revealed evidence of localized strain fields at the MgO/GaAs interface, indicating the presence of misfit dislocations in the MgO layer.
Elastic constants and thermal expansion coefficient of metastable C49 TiSi2
Authors: Jongste J. F. , Loopstra O. B. , Janssen G. C. A. M. , Radelaar S.
Published: July 2002

The elastic constants and thermal expansion coefficient of C49 TiSi2 thin films have been investigated by in situ curvature measurement during heat treatment and ex situ x-ray diffraction measurements. The C49 TiSi2 compound was formed from Ti-Si multilayers deposited on monocrystalline silicon and sapphire substrates. The films were polycrystalline without any evident texture. Young’s modulus (142 GPa), Poisson’s ratio (0.27), and the thermal expansion coefficient (10.9×10−6 K−1) have been determined. Note that these values are averages over random crystal orientations. Directly after formation C49 TiSi2 films exhibit tensile stress. This stress relaxes considerably above 375 °C. Below this temperature the thermal expansion is found to be independent of the state of relaxation of the C49 TiSi2 film.
Chemical stresses in composite circular cylinders
Authors: Chu J. L. , Lee Sanboh
Published: July 2002

The chemical stresses in a composite circular cylinder have been investigated. The mass transfer follows the diffusion equation which was solved using the Laplace transformation technique. Both diffusion processes of constant average concentration and constant surface concentration are considered. The stresses arising from the mass transfer are derived. Two situations are considered: one is plane strain and the other is zero axial force. The largest maximum shear stress occurs at the position near the surface in the outer medium. The positions having the largest maximum shear stress are most probable to have plastic deformation. The outer medium cannot avoid plastic deformation during the mass transfer at constant average concentration.
Characterization of deep-level defects in GaAs irradiated by 1 MeV electrons
Authors: Lai S. T. , Nener B. D. , Faraone L. , Nassibian A. G. , Hotchkis M. A. C.
Published: July 2002

Deep level transient spectroscopy has been employed to determine the defect energy levels, capture cross sections, and trap densities in Si-doped vapor phase epitaxy GaAs both before and after irradiation by 1 MeV electrons at room temperature for electron fluence ranging from 1.1×1014 to 5.0×1015 e cm−2. The results indicate that the irradiated samples have an electron trap at Ec-0.334 eV(EL6) in addition to the two electron traps at Ec-0.815 eV(EL2) and Ec-0.420 eV(EL3) which are present in the nonirradiated sample. The density of the EL6 trap increases monotonically with irradiation fluence from 6.7×1013 to 24.4×1013 cm−3 as electron fluence is increased from 1.1×1014 to 3.1×1014 e cm−2. In contrast, both the EL2 and EL3 trap densities were found to be only moderately affected by electron irradiation with trap densities slightly greater than the nonirradiated sample. These results, along with the fact that the EL6 trap was not observed in the nonirradiated sample, strongly suggest that this trap is created by the electron irradiation. In addition to creating the EL6 trap, electron irradiation results in a nonexponential transient for the EL2 deep level at Ec-0.815 eV which can be resolved into the sum of two exponential transients arising from two closely spaced deep levels at Ec-0.815 eV and Ec-0.843 eV.
Electron and metastable densities in parallel-plate radio-frequency discharges
Authors: Greenberg K. E. , Hebner G. A.
Published: July 2002

Electron densities were measured in continuous and pulse-modulated, 13.56-MHz, helium and argon discharges. These measurements were made in a symmetrically driven Gaseous Electronics Conference Reference Cell. Pulse modulation of the applied potential allowed observation of the time required for the electron density to achieve steady state. In general, helium discharges reached steady state in approximately 1.5 ms, taking three to ten times longer than argon discharges under similar operating conditions. As much as a threefold increase of the electron density was observed in the afterglow of a pulse-modulated helium discharge, indicative of large metastable densities. Absorption spectroscopy was used to measure the helium singlet and triplet metastable densities throughout the volume of the discharge. For a 1.0-Torr discharge, helium triplet metastable densities at the center of the discharge were as large as 2.5×1011 cm−3 while the peak singlet density was 3.0×1010 cm−3. The steady-state electron density varied from 3.8×1010 cm−3 at 50 V to 2.3×1011 cm−3 at 200 V for the 1.0-Torr helium discharge.
Electron cyclotron resonance microwave discharge for oxide deposition using tetramethylcyclotetrasiloxane
Authors: Pai C. S. , Miner J. F. , Foo P. D.
Published: July 2002

Results of the dielectric oxide films deposited at 300 °C using tetramethylcyclotetrasiloxane/oxygen chemistry in a reactor with electron cyclotron resonance microwave discharge are presented. We have found that quality oxide is deposited with an O2/tetramethylcyclotetrasiloxane flow-rate ratio of greater than 3. The properties of the deposited films are characterized by prism coupler, infrared spectroscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, and triangular voltage sweep measurements. The deposition rate using tetramethylcyclotetrasiloxane is found to be about four times higher than tetraethylorthosilicate under similar processing conditions. We have obtained oxide films with superior quality (both material and electrical properties) at a deposition rate of 5000 Å/min. The step coverage of oxide is found to be excellent when rf bias is applied on the substrate during the deposition. We have demonstrated that trenches with aspect ratios ≥1.50 can be filled without voids. Details of reaction chemistries for oxide deposition in the electron cyclotron resonance reactor and the effect of ion bombardment on the oxide profile are discussed.
Epitaxial growth of YBa2Cu3O7−δ films on oxidized silicon with yttria- and zirconia-based buffer layers
Authors: Pechen E. V. , Schoenberger R. , Brunner B. , Ritzinger S. , Renk K. F. , Sidorov M. V. , Oktyabrsky S. R.
Published: July 2002

A study of epitaxial growth of YBa2Cu3O7−δ films on oxidized Si with yttria- and zirconia-based buffer layers is reported. Using substrates with either SiO2 free or naturally oxidized (100) surfaces of Si it was found that a thin SiO2 layer on top of the Si favors high-quality superconducting film formation. Compared to yttria-stabilized ZrO2 (YSZ) single layers, YSZ\Y2O3 double and YSZ/Y2O3\YSZ triple layers allows the deposition of thin YBa2Cu3O7−δ films with improved properties including reduced aging effects. In epitaxial YBa2Cu3O7−δ films grown on the double buffer layers a critical temperature Tc(R=0)=89.5 K and critical current densities of 3.5×106 A/cm2 at 77 K and 1×107 A/cm2 at 66 K were reached.
Blue-light electroluminescence from p-phenylene vinylene-based copolymers
Authors: Sokolik Igor , Yang Zhou , Karasz Frank E. , Morton David C.
Published: July 2002

Electroluminescence (EL) in the blue region of the spectrum with a maximum at 465 nm was observed from polymer films sandwiched between indium-tin-oxide and Al electrodes. The specially designed soluble multiblock copolymers used consisted of chromophoric units that have uniform conjugation length and are spaced with polymethylene units of various lengths. EL originates from the radiative decay of the chromophore singlet excitation created as a result of the recombination of injected charge carriers.
Amorphization of transition metal-Si alloy powders by mechanical alloying
Authors: Lee Pee-Yew , Lin Chung-Kwei , Lin Hong-Ming
Published: July 2002

Amorphization of TM-Si (TM: trasition metal) alloy powders by mechanical alloying has been systematically investigated. Pure element powder mixtures were mechanically alloyed for ten hours with a high energy ball mill. The results show that amorphous metallic silicide powders were successfully synthesized in Ti-Si, Zr-Si, Nb-Si, and Ta-Si systems. Prediction of the amorphization range of mechanically alloyed TM-Si powders by the Miedema model is inappropriate. The possible criteria for the amorphization of TM3Si powders after mechanical alloying 10 h in a shaker ball mill are (a) the free energy of amorphous state of TM3Si powders is less than −31.95 kJ/mole, and (b) the silicon to trasition metal atomic radius ratio is less than 0.985.
Photo-induced antiferromagnetic interlayer coupling in Fe superlattices with iron silicide spacers (invited)
Authors: Mattson J. E. , Fullerton Eric E. , Kumar Sudha , Lee S. R. , Sowers C. H. , Grimsditch M. , Bader S. D. , Parker F. T.
Published: July 2002

Sputtered Fe/FeSi films possessing antiferromagnetic (AF) interlayer coupling at room temperature develop ferromagnetic remanence when cooled below 100 K, but the AF coupling can be restored at low temperature by exposure to visible light of sufficient intensity (≳10 mW/mm2). We attribute these effects to charge carriers in the FeSi spacer layer, which, when thermally or photogenerated, are capable of communicating spin information between the Fe layers.
On finite element implementation of impedance boundary conditions
Authors: Mayergoyz I. D. , Bedrosian G.
Published: July 2002

In the paper, impedance boundary conditions are represented in terms of scalar magnetic potential. This leads to a scalar magnetic potential formulation of three-dimensional (3-D) eddy current problems with small skin depths. The scalar potential formulation is then reduced to a weak Galerkin form. The finite element discretization of this form results in two (volume and surface) ‘‘stiffness’’ matrices. This approach completely avoids vectorial calculations in 3-D eddy current analysis.
Low field giant magnetoresistance in discontinuous magnetic multilayers
Authors: Hylton T. L. , Coffey K. R. , Parker M. A. , Howard J. K.
Published: July 2002

Giant magnetoresistance of order 4%–6% has been observed in fields of 5–10 Oe at room temperature in annealed multilayers of Ni80Fe20/Ag prepared by magnetron sputtering. For a wide range of NiFe and Ag thicknesses, no giant magnetoresistance was observed in the unannealed films. We attribute the appearance of giant magnetoresistance to a magnetostatic interlayer interaction that promotes antiparallel order of the moments in adjacent layers fostered by a breakup of the NiFe layers. We discuss the effects of variations in the underlayers, spacer thickness, and the sputtering process on the magnetoresistance. Our results suggest that maximizing magnetoresistance and minimizing hysteresis require samples with continuous Ag layers that prevent contact between the NiFe layers and NiFe layers that are discontinuous but not too severly disrupted.
Resonant holographic interferometry measurements of laser ablation plumes in vacuum, gas, and plasma environments
Authors: Lindley R. A. , Gilgenbach R. M. , Ching C. H. , Lash J. S. , Doll G. L.
Published: July 2002

Resonant holographic interferometry and dye-laser-resonance-absorption photography have been utilized to investigate the expansion of the laser ablation plumes produced by a KrF excimer laser beam (248 nm) focused onto an aluminum target (≊0.1 cm2, 2–6 J/cm2). Plume expansion was studied in vacuum and in background argon gas pressures of 14 mTorr, 52 mTorr, 210 mTorr, 1 Torr, and 35 Torr. The existing theory for the interpretation of resonant interferograms has been extended to account for Doppler shift effects, the diagnostic laser bandwidth, and the selective absorption of the laser beam. Absolute line densities in the range 4.3×1013–1.0×1015 cm−2 have been measured in the ablation plumes, which imply measured Al neutral densities of up to 1×1015 cm−3. The total number of Al neutral atoms in a plume has been measured to be ≊3×1014, which corresponds to a surface etch rate of ≊1 nm/pulse. Expansion velocities in the range 1.1–1.4 cm/μs were measured for the pressures ≤210 mTorr, while ≊0.3 cm/μs was measured for 1 Torr and ≊0.08 cm/μs was measured for 35 Torr. Ablation plume expansion into a 1 Torr rf argon plasma environment was compared with the expansion into a 1 Torr argon gas. The ablation plume appeared to expand and dissipate slightly faster in the plasma.
Nonlinear effects in high-temperature superconducting bolometers
Authors: Epifani M.
Published: July 2002

In this paper deviations from ideal behavior of high-temperature superconducting bolometers are analyzed. Formulas include corrections to the bolometric parameters for electrical nonlinearity. A simple empirical model for the resistive transition together with the standard bolometer theory are used to calculate the electrical and thermal parameters. It is shown that the standard method used to derive the detector responsivity would lead to wrong results. A new criterion for deducing the responsivity from electrical measurements in the presence of nonlinear effects is illustrated.
Erbium in oxygen-doped silicon: Electroluminescence
Authors: Lombardo S. , Campisano S. U. , van den Hoven G. N. , Polman A.
Published: July 2002

Room-temperature electroluminescence at 1.54 μm is demonstrated in erbium-implanted oxygen-doped silicon (27 at. % O), due to intra-4f transitions of the Er3+. The luminescence is electrically stimulated by biasing metal-(Si:O, Er)-p+ silicon diodes. The 30-nm-thick Si:O, Er films are amorphous layers deposited onto silicon substrates by chemical-vapor deposition of SiH4 and N2O, doped by ion implantation with Er to a concentration up to ≊1.5 at. %, and annealed in a rapid thermal annealing furnace. The most intense electroluminescence is obtained in samples annealed at 400 °C in reverse bias under breakdown conditions and it is attributed to impact excitation of erbium by hot carriers injected from the Si into the Si:O, Er layer. The electrical characteristics of the diode are studied in detail and related to the electroluminescence characteristics. A lower limit for the impact excitation cross section of ≊6×10−16 cm2 is obtained.
Magnetic and structural properties of iron nitride thin films obtained by argon-nitrogen reactive radio-frequency sputtering
Authors: Bobo J.-F. , Chatbi H. , Vergnat M. , Hennet L. , Lenoble O. , Bauer Ph. , Piecuch M.
Published: July 2002

We have prepared iron nitride thin films by reactive rf sputtering of an iron target in an argon-nitrogen plasma. The films present a wide variety of structures and compositions: α expanded bcc iron, amorphous Fe1−x-Nx, γ′-Fe4N, ε-Fe2–3N, and ζ-Fe2N. Evolutions of the crystallographic structures and of the magnetic properties of the obtained phases are discussed versus the nitrogen concentration in the gaseous flow and the substrate temperature during deposition.
Growth of Ge, Si, and SiGe nanocrystals in SiO2 matrices
Authors: Zhu Jane G. , White C.W. , Budai J. D. , Withrow S. P. , Chen Y.
Published: July 2002

Nanocrystals of group-IV semiconductor materials (Si, Ge, and SiGe) have been fabricated in SiO2 by ion implantation and subsequent thermal annealing. The microstructure of these nanocrystals has been studied by transmission electron microscopy. Critical influences of the annealing temperatures and implantation doses on the nanocrystal size distributions are demonstrated with the Ge-implanted systems. Significant roughening of the nanocrystals occurs when the annealing temperature is raised above the melting temperature of the implanted semiconductor material.
Initial interface formation study of the Mg/Si(111) system
Authors: An K. S. , Park R. J. , Kim J. S. , Park C. Y. , Lee S. B. , Abukawa T. , Kono S. , Kinoshita T. , Kakizaki A. , Ishii T.
Published: July 2002

The initial interface and silicide formation induced by Mg adsorption on the Si(111)7×7 surface have been studied using low-energy electron diffraction, x-ray photoelectron spectroscopy, and synchrotron radiation photoelectron spectroscopy. At room temperature, it is found that Mg atoms are preferably adsorbed on top sites of Si adatoms and rest atoms on the Si(111)7×7 surface and with increasing of Mg deposition, a Mg2Si epitaxial layer is formed and the surface structure transforms from the diffuse (1×1) phase into the (2/3√3×2/3√3)R30°. After growing up to a critical thickness, the Mg film grew in a disordered phase on the epitaxial layer. The Fermi level of the Mg2Si film is positioned at 0.51±0.05 eV above the valence band maximum. On the other hand, at 300 °C the Mg2Si epitaxial layer was formed in the (1×1) phase on the Si(111)7×7 and grew up to a critical thickness in the initial stage. For the successive evaporation, the Mg film grew in a disordered phase on the Mg2Si(111)1×1 surface.
Excitons in silicon diodes and solar cells: A three-particle theory
Authors: Corkish Richard , Chan Daniel S.-P. , Green Martin A.
Published: July 2002

Recent work has indicated that a significant number of electrons and holes remain in the free-exciton form in silicon at room temperature, a finding which, if supportable by experimental evidence, requires the inclusion of excitons in diode and solar cell theory. Excitons, although neutral, may contribute to device currents by diffusing to the junction region where they may be dissociated by the field. A generalized three-particle theory of transport in semiconductors is presented. The results of application of the theory to silicon devices indicate a decrease in the dark saturation current as well as an increase in light-generated current when excitons are incorporated in the theory so long as exciton diffusion length exceeds that of the minority carriers. The work includes suggestions for experimental methods to confirm exciton involvement and to estimate the value of the exciton-binding parameter from spectral response measurements on solar cells.
Temperature dependence of the band gap in C60 crystals
Authors: Chiu Kuan-Cheng , Wang Jyh-Shyang , Lin Chung-You
Published: July 2002

The temperature dependence (13–300 K) of the optical-allowed direct band gap Eg(T) deduced from the transmission and photoconductivity spectra in high-quality C60 crystals is presented. A strong temperature-dependent Eg(T) is observed and attributed to the electron–phonon interaction via the localized C60 phonon modes. In addition, a discontinuous change for dEg/dT around 249 K is reported due to the orientational order/disorder phase transition. Finally, three discrete localized bands are observed and the photoconduction for these localized bands is discussed in terms of photothermal ionization.
Deflagration-to-detonation transition in granular pentaerythritol tetranitrate
Authors: Luebcke P. E. , Dickson P. M. , Field J. E.
Published: July 2002

The deflagration-to-detonation transition process has been observed in pressed granular columns of the explosive pentaerythritol tetranitrate. Charges were confined within a steel housing which had been fitted with a polycarbonate window to allow direct recording by high-speed streak photography. The explosive was thermally ignited by a gasless pyrotechnic mixture to minimize pre-pressurization of the charge. The results indicate that upon ignition, early choked flow of the combustion gases prevents the continued propagation of the combustion via a convective heat transfer mechanism and that the propagation of reaction becomes governed by a leading compaction wave which causes ignition through the mechanical formation of hot spots. Detonation finally occurs when the leading front of an accelerating plug of highly compacted material (density close to the theoretical maximum) formed between the reaction front and the leading compaction wave, attains the critical pressure necessary for shock-to-detonation transition.
A time-resolved current method for the investigation of charging ability of insulators under electron beam irradiation
Authors: Song Z. G. , Ong C. K. , Gong H.
Published: July 2002

In the present article, a convenient method for the direct measurement of the displacement current caused by electron trapping is developed to measure the trapped charge in insulators under electron beam irradiation from a scanning electron microscope. The trapping process during electron beam irradiation can be directly observed by this method. By using the conservation of current, a macroscopic formula is derived to describe our observation. The derived formula relates the measured current to the radiated beam energy, current, radiation-induced conductivity, and electron penetration depth of a sample. Experiments have been performed on polymethymethacrylate samples in which the samples are irradiated with electron beams of fixed beam energy as well as fixed beam current. The results are as predicted in theory. An agate sample is found to be unable to trap charge due to the small electron penetration depth and large radiation-induced conductivity of the sample.
A magneto-optical study of interdiffusion in InGaAs/InP quantum wells: Effects of heat treatment, substrates, and dopants
Authors: Wong S. L. , Nicholas R. J. , Martin R. W. , Thompson J. , Wood A. , Moseley A. , Carr N.
Published: July 2002

Interdiffusion in InGaAs/InP quantum wells has been studied using magneto-optical transmission spectroscopy. The effects of dopants in the substrates and quantum wells, overgrowth and annealing processes have been investigated, providing useful information on the interdiffusion effects in typical growth and fabrication processes. The blue shift effect which is often observed in multiple quantum well (MQW) structures subjected to heat treatment, is attributed to a dominant group V interdiffusion which can be suppressed by high defect densities in the substrate. The presence of Zn-dopants in an overgrown layer on top of the MQW structures causes a counteractive redshift effect after long annealing times due to group III diffusion, but in situ Zn- or S-dopants produce no observable shift in transition energies due to interdiffusion. This is attributed to enhanced group III interdiffusion induced by Zn diffusion into the MQWs. We conclude that there are very different interdiffusion mechanisms for group III and group V elements and support the recent suggestion of vacancy-related group V interdiffusion in contrast to the interstitialcy mechanism for the group III interdiffusion.
Surface Debye temperature measurement with reflection high-energy electron diffraction
Authors: Elsayed-Ali H. E.
Published: July 2002

Measurement of the surface mean-square atomic vibrational amplitude, or equivalently the surface Debye temperature, with reflection high-energy electron diffraction is discussed. Low-index surfaces of lead are used as examples. Particular details are given about the temperature-dependent diffraction pattern of Pb(100) in the Debye–Waller region. The use of reflection high-energy electron diffraction for measurement of the substrate surface temperature in thin-film deposition chambers is suggested.
Superparamagnetic magnetic force microscopy tips
Authors: Hopkins P. F. , Moreland John , Malhotra S. S. , Liou S. H.
Published: July 2002

We report on magnetic force microscopy (MFM) images of a thin-film magnetic recording head taken using batch micromachined silicon tips coated with nanocomposite Fe60(SiO2)40 and Fe70(SiO2)30 films. The small Fe grain size (<10 nm) and dilute Fe volume fraction (0.29–0.4) of these granular films produce tip coatings of low remanence and essentially zero coercivity, reduced by the superparamagnetic properties of these films. We have used these tips to obtain MFM images of the write field of the head with high spatial and magnetic-field resolution. In comparison to images taken using commercial Co85Cr15-coated tips, these MFM images show reduced tip memory effects and clearly delineate the gap field from the pole pieces.
Henkel plots and the Preisach model of hysteresis
Authors: Buehler C. J. , Mayergoyz I. D.
Published: July 2002

Interpretation of Henkel plots in terms of the Preisach model is revisited. The Preisach model is treated as a purely phenomenological one and the Henkel plots are directly related to experimental data, namely, to the first-order transition curves.
Thickness dependence of giant magnetoresistance of AgNiFe heterogeneous alloys films
Authors: Wiggins J. , Watson M. L. , Gago-Sandoval P. A. , O’Grady K.
Published: July 2002

We have investigated the thickness dependence of the giant magnetoresistance (GMR) observed in AgNiFe heterogeneous alloy films. The films were sputtered from a mosaic target at the system ambient temperature onto glass substrates. In order to maintain interfacial uniformity, and to control the spin-dependent transmission of electrons at the AgNiFe film interfaces, they were covered by 200-Å-thick NiFe under- and overlayers. The samples have total thicknesses in the range 400–3000 Å. The film resistivity, magnetoresistivity, and magnetization were measured in the temperature range 4–300 K and in fields of up to 1.1 T and the structure examined using both large- and small-angle x-ray diffraction. The results indicate that the observed thickness dependence of the GMR of the AgNiFe films is within the experimental error It is postulated that this is due to coherent spin transmission of conduction electrons across the sandwich interfaces which does not degrade the GMR by mixing of the spin currents.
Infrared vibrational bands related to the thermal donors in silicon
Authors: Hallberg T. , Lindström J. L.
Published: July 2002

Two groups of infrared (IR) localized vibrational bands in the regions 975–1015 and 724–748 cm−1 have been correlated with the well-known IR electronic bands due to the thermal double donors (TDs) and with the TD concentration from resistivity measurements. The two groups are suggested to be due to two different vibrational modes of oxygen atoms in a TD core. The vibrational bands at 975, 988, 999, and 1006 cm−1 are correlated to TD1, TD2, TD3, and TDs≥TD4, respectively, while the band at 1012 cm−1 correlates to the NL10 center. A calibration coefficient for the TD-related vibrational bands was determined. This calibration coefficient can be used to estimate the sizes of the TD-related centers, assuming that the calibration coefficient for interstitial oxygen is applicable on the oxygen atoms of these centers. This results in that all of the TD-related bands originates from centers of 1–2 oxygen atoms, suggesting these bands to be due to the vibrations of oxygen atoms in a TD core. The different positions of the TD-related bands could be explained by differently strained environments caused by different oxygen clusters. It is suggested that these clusters will develop into larger oxygen precipitates, which at the end of TD formation appear in the spectrum with a broad IR band at about 1060 cm−1. The early stages of the TD formation at temperatures below about 450 °C are closely related to a transformation process of preexisting clusters related to the 1012 cm−1 band. This explains the formation of the early TDs at low temperatures, when the interstitial oxygen concentration is nearly constant.
Role of Si–H bonding in a-Si:H metastability
Authors: Godet Christian , Roca i Cabarrocas Pere
Published: July 2002

In plasma-deposited a-Si:H films, the time dependence and saturation value of the metastable defect density produced by high-intensity illumination are found to depend on the material properties. From a wide range of deposition techniques, we have observed a strong correlation between the steady-state defect density Nss and the monohydride Si–H bond concentration observed in infrared transmission, with [SiH]2000 values between 2% and 16%. We propose a microscopic description of the light-induced defect creation and light-induced defect annealing phenomena in a-Si:H, based on the trapping of mobile H at two specific sites: the Si–H bond and the Si dangling bond. This model predicts a steady-state defect density proportional to the monohydride [SiH]. For the production of mobile H in three-center (Si–H–Si) configurations, a low-energy mechanism with no necessity for Si–H bond breaking is suggested from the observed changes in the infrared absorption of the various H-bonding configurations during light soaking. With the assistance of an electron-hole pair recombination, it consists of the excitation of two adjacent H atoms from a (Si–H H–Si) configuration along with a Si–Si bond reconstruction.
Disordered oxygen octahedral rotations and glasslike polarization characteristics in rhombohedral lead zirconate titanate
Authors: Dai Xunhu , Xu Z. , Viehland Dwight
Published: July 2002

Disordered oxygen octahedral rotations have been observed in the low-temperature rhombohedral ferroelectric state in the Pb(Zr1−xTix)O3 crystalline solution. The development of disordered oxygen octahedral rotations within the ferroelectric state was found to result in the interruption of long-range polar order and the development of fluctuation effects. Dielectric relaxation was observed below ∼160 °C for Pb(Zr0.65Ti0.35)O3. In addition, double-looplike characteristics were observed in the polarization electric-field (P–E) response in the same temperature range. The results clearly reveal the nature of the coupling between the polarization and oxygen octahedral rotations in a perovskite ferroelectric structure. It was also shown that a small amount of La doping can enhance the degree of ordering of the octahedral tilted state. The enhancement of the ordering was verified by an intensity increase of the 1/2〈111〉 superlattice reflections in the selected area electron-diffraction pattern.