bluuebunny/crossref_metadata_2025 huggingface dataset:
Dissociative and diffractive scattering of H2 from Pt(111): A four-dimensional quantum dynamics study
Authors: Pijper E. , Kroes G. J. , Olsen R. A. , Baerends E. J.
Published: July 2002

Following earlier three-dimensional (3D) calculations, we present results of four-dimensional (4D) calculations on dissociative and diffractive scattering of H2 from Pt(111) by extending the 3D model with a second degree of freedom parallel to the surface. A 4D potential energy surface (PES) is constructed by interpolating four 2D PESs obtained from density-functional theory calculations using the generalized gradient approximation and a slab representation of the metal surface. The 4D calculations show that out-of-plane diffraction is much more efficient than in-plane diffraction, providing a partial explanation for the paradox that diffraction experiments measure little in-plane diffraction, whereas experiments on reaction suggest the surface to be corrugated. Calculations for off-normal incidence of v=0 H2 show that, in agreement with experiment, initial parallel momentum inhibits dissociation at low normal translational energies, and enhances reaction for higher energies. Our 4D calculations also show that the reaction of initial v=1 H2 is vibrationally enhanced with respect to v=0 H2, as was found in the 3D model, even though H2+Pt(111) is an early barrier system.
Experimental validation of the Stokes law at nonisothermal conditions
Authors: Bar-Ziv Ezra , Zhao Bin , Mograbi Elaad , Katoshevski David , Ziskind Gennady
Published: July 2002

Novel experimental results on the hydrodynamic forces acting on a small heated particle at low Reynolds and Grashof numbers are presented. The temperature difference between the particle and the ambient causes free convection flow around the particle. As a result, the drag force exerted on the particle by forced flow deviates significantly from the Stokes law. This deviation is represented quantitatively for three basic directions of forced flow, namely assisting, opposing, and crossing with respect to free convection flow. Validity of the Stokes formula at mixed-convection creeping flow conditions is discussed.
Electron irradiation induced expansion in amorphous chalcogenide films
Authors: Romero J. S. , Fitzgerald A. G. , Mietzsch K.
Published: July 2002

An expansion of the film surface is observed in amorphous germanium based chalcogenide films when an electron beam is focused in pulsed or fixed mode on the film surface. For thin films of 0.1–1 μm thick the expansion extends laterally over a range of 0.5–1.5 μm and the vertical expansion extends from 0.1–0.6 μm. Increasing the current density of the probe results in an increase in the thickness and a decrease in the width of these features. This phenomenon can be explained in terms of theoretical electrostatics.
Experimental investigation of high temperature superconducting imaging surface magnetometry
Authors: Espy Michelle A. , Matlashov Andrei N. , Kraus Robert H. , Volegov Petr , Maharajh Keeran
Published: July 2002

The behavior of high temperature superconducting quantum interference devices (SQUIDs) in the presence of high temperature superconducting surfaces has been investigated. When current sources are placed close to a superconducting imaging surface (SIS) an image current is produced due to the Meissner effect. When a SQUID magnetometer is placed near such a surface it will perform in a gradiometric fashion provided the SQUID and source distances to the SIS are much less than the size of the SIS. We present the first experimental verification of this effect for a high temperature SIS and SQUID. Results are presented for two SQUID-SIS configurations, using a 100-mm-diam YBa2Cu3O(7−δ) disk as the SIS. These results indicate that when the current source and sensor coil (SQUID) are close to the SIS, the behavior is that of a first-order gradiometer. The results are compared to analytic solutions as well as the theoretical predictions of a finite element model.
L1
ordering at different stages of Fe0.5Pd0.5 epitaxial growth
Authors: Halley D. , Auric P. , Bayle-Guillemaud P. , Gilles B. , Marty A. , Jalabert D.
Published: July 2002

FePd layers deposited by molecular beam epitaxy at 620 K on a (001) Pd buffer are investigated. At this growth temperature, an L10 structure is expected; we compare the crystal ordering at different depths in these thin films. Fe57 is used as a local probe at an atomic scale, located at different places within an equimolar FePd layer. The relationship between the alloy structure and the magnetic anisotropy of the films is investigated by Mössbauer spectroscopy. The possible decrease of the L10 ordering during growth is not observed. On the contrary, the quality of the L10 structure seems to improve, as confirmed by the low number of near-neighbors for iron atoms located on the top level of the alloy layer. Iron-rich clusters shaping into large vertical defects across the whole alloy layer are also observed; these clusters correspond to chemically disordered areas within the film. At the buffer interface, disordered, and dilute iron alloys (mean composition: Fe0.25 Pd0.75) with in-plane momenta are identified and explained assuming iron atom diffusion through about 1 nm thick Pd buffer, due to Pd segregation. Some iron rich aggregates could be the consequence of iron homocoordination or Pd surface segregation. The L10 structure is assumed to occur after about 1.7 nm alloy deposit.
Study of cobalt-disilicide formation from cobalt monosilicide
Authors: Appelbaum A. , Knoell R. V. , Murarka S. P.
Published: July 2002

The reaction CoSi+Si→CoSi2 has been investigated in the temperature range of 500–600 °C, using Rutherford backscattering, x-ray diffraction, transmission electron microscopy, and four-point sheet-resistance measurements. The reaction is very slow at 500 °C and extremely fast at 600 °C, and appears to occur in four stages: (a) nucleation of the CoSi2 phase at the grain-boundary triple points, (b) lateral growth from nucleation sites to form a continuous layer on the silicon surface, (c) growth in thickness by diffusion through this disilicide layer, and (d) the lateral epitaxial growth by eliminating the grain boundaries. The diffusion through the disilicide layer seems to be a rate-limiting process with high activation energy.
Interstitial and vacancy concentrations in the presence of interstitial injection
Authors: Hu S. M.
Published: July 2002

In processes such as thermal oxidation and phosphorus diffusion, where self-interstitials are generated in excess, what happens to the vacancy concentration may significantly affect many diffusion processes which occur via dual mechanisms. Most authors in diffusion modelling have assumed as self-evident that, for a very long time or in steady state, the product of the vacancy and the interstitial concentrations should be a constant. It is shown here that this assumption is generally invalid. The fallacy in the analogy between this case and the solubility product is pointed out. A correct relationship is first derived on a uniform defect concentration approximation. Then rigorous expressions for both the vacancy and the interstitial concentrations are given from an exact solution of simultaneous vacancy and interstitial continuity equations that include diffusion, recombination, and generation terms, under appropriate boundary conditions. The errors in the results from previous speculations are especially severe in the surface region.
Ba–ferrite thin-film disk for perpendicular magnetic recording
Authors: Matsuoka M. , Naoe M. , Hoshi Y.
Published: July 2002

The authors have prepared Ba–ferrite film disks and evaluated their magnetic read/write characteristics. The c-axis-oriented Ba–ferrite thin films were formed epitaxially on c-axis-oriented ZnO films by means of a targets-facing-type sputtering system. The direction of easy magnetization in the film with perpendicular crystalline anisotropy is almost perpendicular to the film plane. Thus, the perpendicular remanence is more than 60 times as large as the in-plane one. The simple Ba–ferrite films, however, have a coercive force, Hc⊥ above 2 kOe which is too large to write on by means of a conventional head. The substitution of In3+ in the Ba–ferrite films makes the Hc⊥ values decrease to about 1 kOe without increasing the critical temperature, Tcritical, of c-axis orientation. The read/write characteristics of the films were measured with a MnZn–ferrite ring head. The reproduced signal exhibits clear dipulsive waveforms as the dipulse ratio exceeds 0.7. Furthermore, the overwrite ratio decreases below −28 dB for the In3+-substituted Ba-ferrite film disk. The Ba–ferrite film disks have promising potential as a high-density perpendicular magnetic recording medium.
Determination of the electrostriction tensor components in single-crystal CaF2 from the uniaxial stress dependence of the dielectric permittivity
Authors: Meng Z. Y. , Cross L. E.
Published: July 2002

The separated electrostriction tensor components have been measured for single-crystal calcium fluoride by the converse effect, using a specially designed uniaxial elastic stressing jig. Values obtained for the individual components of the electrostrictive voltage (M) coefficients were M11=−0.132, M12=0.117, and M44=0.507 in units of 10−20 m2/V2. These values are smaller than those observed by direct measurement of electric-field-induced elastic strain (K. Rittenmyer, Ph.D. thesis, The Pennsylvania State University, 1984), but are closer to expected values and likely to be more reliable.
Band discontinuity for GaAs/AlGaAs heterojunction determined by C-V profiling technique
Authors: Watanabe Miyoko Oku , Yoshida Jiro , Mashita Masao , Nakanisi Takatosi , Hojo Akimichi
Published: July 2002

The band discontinuity has been determined for a GaAs/AlGaAs heterojunction prepared by molecular beam epitaxy. The conduction band-discontinuity ΔEc and the valence-band discontinuity ΔEv were independently obtained by the C-V profiling technique, taking into account a correction for the interface charge density. The simulation was employed to confirm the reliability of the obtained band discontinuity. The ΔEc dependence on both the Al composition of the AlGaAs layer and the heterojunction structure (AlGaAs on GaAs, or GaAs on AlGaAs) was examined. We found that ΔEc and ΔEv were determined to be 62 and 38% of the band-gap discontinuity ΔEg, being independent of the structure.
Thermal analysis by photoacoustic phase measurements: Effect of sample thickness
Authors: Lachaîne André
Published: July 2002

The phase of the photoacoustic signal is measured as a function of chopping frequency for different thicknesses of Pb resting on a water backing. Analysis of the results is in agreement with the theory of Rosencwaig and Gersho [J. Appl. Phys. 47, 64 (1976)] and leads to a simultaneous determination of thermal conductivity and specific heat of the sample.
Optical absorption and x-ray diffraction in narrow-band-gap InAs/GaSb superlattices
Authors: Arch D. K. , Wicks G. , Tonaue Tom , Staudenmann Jean-Louis
Published: July 2002

We report on the optical transmission properties of narrow-band-gap (Eg<0.1 eV) InAs/GaSb superlattices grown by molecular-beam epitaxy. Energy band gaps of 0.15 and 0.085 eV at 4.8 K are determined for a 102-Å-period and a 124-Å-period superlattice, respectively. The absorption edge is extremely soft due to the spatial mismatch of hole and electron wave functions. In addition we show the first reported x-ray diffraction measurements on this materials system.
Hydrogen depth profiles and optical characterization of annealed, proton-implanted n-type GaAs
Authors: Zavada J. M. , Jenkinson H. A. , Sarkis R. G. , Wilson R. G.
Published: July 2002

Depth profiles of 300-keV protons implanted in n-type GaAs at room temperature have been obtained using secondary ion mass spectrometry and correlated with optical effects determined by infrared reflectance measurements. The profiles of the implanted hydrogen (1H) have been measured as a function of annealing for temperatures up to 600 °C. These profiles display a major redistribution of the hydrogen atoms with movement beginning at 200 °C and terminating by 700 °C. The hydrogen diffusion into the substrate can be approximated by an Arrhenius process with an activation energy of 0.62 eV and a diffusion constant of 1.54×10−5 cm2/s. The reflectance spectra indicate that while an optically uniform layer is present in the as-implanted specimen, more complicated optical profiles exist in the annealed samples. Annealing at 300 °C causes the layer to nearly double in thickness but higher temperature annealing produces optical profiles similar to the as-implanted state. Qualitatively, these optical changes follow the behavior of the hydrogen depth profiles.
Heteroepitaxial growth of Ge films on the Si(100)-2×1 surface
Authors: Asai M. , Ueba H. , Tatsuyama C.
Published: July 2002

The initial stage of Ge heteroepitaxy on a Si(100)-2×1 surface has been investigated by low-energy electron diffraction (LEED) and Auger-electron spectroscopy (AES). The growth mode of the Ge films was studied by measuring the decrease in the Si(LVV) AES line at 92 eV with an increase in the Ge overlayer thickness. The Ge films deposited at room temperature exhibit layer-by-layer growth up to at least six monolayers. When the substrate is heated up to 350 °C, the growth mode is characterized by the Stranski–Krastanov type; i.e., the first three monolayers of growth is followed by island formation. Although these characteristics of the growth mechanism are similar to the case of Ge on Si(111)-7×7 surfaces, annealing behavior of the Ge films suggests that the bond strength between Ge and Si is stronger on Si(100) than on Si(111) surfaces. In contrast to the case of Ge on Si(111) surfaces, where the original 7×7 superstructure of the Si surface is replaced by a new 5×5 pattern at about two-monolayer coverage of Ge, the original 2×1 LEED pattern is not strongly disturbed up to about 1–2 monolayers of Ge. In addition to the detailed study on the initial stage of heteroepitaxial growth, we observed that thick Ge films deposited onto Si(100) surfaces held at 350 and 470 °C display a sharp 2×1 LEED pattern and demonstrate a single-crystal growth of a Ge(100) face on the Si(100) surface. This is further supported by a measurement of the x-ray diffraction pattern of the Ge films.
Absolute response and noise equivalent power of cyclotron resonance-assisted InSb detectors at submillimeter wavelengths
Authors: Brown Elliott R. , Wengler M. J. , Phillips T. G.
Published: July 2002

Spectra are presented of the responsivity and noise equivalent power (NEP) of liquid-helium-cooled InSb detectors as a function of magnetic field in the range 20–110 cm−1. The measurements are all made using a Fourier transform spectrometer with thermal sources. The results show a discernable peak in the detector response at the conduction electron cyclotron resonance (CCR) frequency for magnetic fields as low as 3 kG. The magnitude of responsivity at the resonance peaks is roughly constant with magnetic field and is comparable to the low-frequency hot-electron bolometer response. The NEP at the peaks is found to be comparable to the best long wavelength results previously reported. For example, we measure NEP=4.5×10−13 W/Hz1/2 at 4.2 K, 6 kG, and 40 cm−1. The InSb CCR will provide a much improved detector for laboratory spectroscopy, as compared with hot electron bolometers, in the 20–100 cm−1 range.
Study of relaxation effects in the 57Fe Mössbauer spectra of carbon-supported iron carbide particles
Authors: Lin Sho-Chen , Phillips Jonathan
Published: July 2002

A modified model of relaxation for systems containing both a magnetic field and an electric field gradient is presented. This model is used to explain the 57Fe Mössbauer spectra of small iron χ-carbide particles supported on two types of high surface area carbon. On the basis of this model, it is concluded that χ-carbide particles interact weakly with graphitic carbon, but very strongly with nearly amorphous carbon. On the nearly amorphous carbon, the particles may, in fact, have an unusually high surface-to-volume ratio.
Teflon-clad As-S glass infrared fiber with low-absorption loss
Authors: Saito M. , Takizawa M.
Published: July 2002

An As-S glass fiber with Teflon Fluorinated Ethylene Propylene (FEP) cladding has been fabricated. The transmission range of this fiber was 1–7 μm with two pronounced absorption peaks at 2.8 and 4.1 μm. It was found that these absorptions were caused by moisture adsorbed on the surface of the As-S glass as well as the hydrogen impurity due to the silica ampule. By removing these impurities the absorption loss was reduced and a minimum optical loss of 0.15 dB/m was obtained for this infrared fiber.
Hole-scattering mechanisms in modulation-doped heterostructures
Authors: Walukiewicz W.
Published: July 2002

Hole mobilities in the two highest spin-split subbands in a p-type modulation-doped heterostructure are calculated. The light-hole mobility is shown to be very low because of efficient scattering by the holes in the heavy mass subband. This finding significantly simplifies calculation of the charge transport, since only the heavy-hole contribution has to be considered. It is shown that for the state-of-the-art heterostructures the heavy-hole mobility is limited by the deformation potential acoustical and optical phonon scatterings with small contribution from ionized impurities at low temperatures. The valence-band deformation potential constant has been determined comparing the theoretical calculations with available experimental data on temperature dependence of the hole mobility.
Large-scale-length nonuniformities in gas puff implosions
Authors: Hussey T. W. , Matzen M. K. , Roderick N. F.
Published: July 2002

Because they are less susceptible to the hydromagnetic Rayleigh–Taylor instability than other fast Z-pinch imploding liner systems, gas puffs offer the possibility of higher implosion velocity. This higher specific energy appears necessary for optimizing high-energy x rays required in a photoionization-pumped soft x-ray laser. Nevertheless, large-scale-length nonuniformities created as the gas flows from the nozzle across the electrode gap are a potential problem. One- and two-dimensional calculations suggest that gas near the nozzle will implode before that which is further from the nozzle, leading to an effect described as ‘‘zippering.’’ Because the number of such two-dimensional calculations that can be done is limited and because the density distribution of nozzles is uncertain, we have developed a simple quasi-two-dimensional interface code that is able to quickly survey the effect of arbitrary initial gas distributions on the implosion dynamics. Results of this survey suggest that zippering contributes significantly to thermalization time, and we propose two methods to counteract this problem. These techniques, each of which involves tailoring the initial density distribution to offset effects of nonuniformities, appear promising. Nevertheless, we will never completely eliminate these nonuniformities, therefore, they must be accounted for in x-ray laser target design.
Low-temperature deposition of high-quality silicon dioxide by plasma-enhanced chemical vapor deposition
Authors: Batey J. , Tierney E.
Published: July 2002

Thin films of high-quality silicon dioxide have been deposited at low temperatures by plasma-enhanced chemical vapor deposition. A deposition rate much lower than that used in conventional plasma-enhanced processes is found to be crucial in obtaining material with reproducible, good properties. Controlled, slow deposition is achieved by using very low flow rates of reactive gases, together with a much higher flow of inert carrier gas to ensure uniformity. Films deposited at usual high deposition rates (∼500 Å/min) exhibit irreproducible and poor electrical properties and are porous. Those deposited slowly (∼60 Å/min) have very reproducible properties, are relatively dense and exhibit very good electrical integrity. Oxides deposited using a substrate temperature of 350 °C compare favorably with those deposited at 700°C using atmospheric-pressure chemical vapor deposition and can be deposited routinely over a wide range of oxide thickness. Deposition at 275 °C results in similar properties but with increased electron capture associated with deep bulk traps. Thicker layers can be deposited onto polycrystalline metal (sputtered films on glass substrates) without any deterioration in film properties. The as-deposited Si:SiO2 interface state density is quite high (∼1012 eV−1 cm−2), although it is reduced to very respectable levels (in the mid 1010–1011 eV−1 cm−2 range) by conventional postmetallization anneals. The suitability of these low-temperature oxides for thin film transistor applications is discussed.
Puff-gas coaxial-injected electromagnetic coaxial plasma gun
Authors: Degnan J. H. , Baker W. L. , Warren S. W. R. , Price D. W. , Snell M. P. , Richter-Sand R. J. , Turchi P. J.
Published: July 2002

We have designed, built, and tested a pulsed gas-injection coaxial plasma gun driven by a 72-μF capacitor bank, with operating voltages of 60–80 kV (stored energy 130–230 kJ), and 1.6-MA, 1-μs rise-time current discharges. Using deuterium gas, we have obtained reliable delivery of all current to the gas and neutron yields of greater than 109. Current, voltage, inductance, and current position data are discussed and are compared with circuit solver models. Magnetic-probe and filtered scintillator photomultiplier detector array data on high-energy photon spectra were taken. A second puff-gas valve near the anode (inner electrode) axis at the muzzle end substantially improved the neutron yield.
The chemistry of and physics with porous sol-gel glasses
Authors: Shafer M. W. , Awschalom D. D. , Warnock J. , Ruben G.
Published: July 2002

Porous media with well-defined pore diameters can be employed as hosts within which one may study a wealth of physical phenomena in well-characterized confining geometries. In this paper we describe the procedures for forming variable pore size glasses via the sol-gel process. Two general methods were used to form the gels from which the porous glasses were formed; the hydrolysis of the alkoxides and the gelling of colloidal silica, e.g., DuPont LUDOX. By controlling the pH of the gelling solution, the water/silica ratio and the firing temperature, glasses with pore diameters ranging from less than 2–12 nm were formed from the alkoxide hydrolysis. Larger pore diameters were obtained from colloidal silica gels, the largest being in the 45-nm range in glasses made from leached alkali silicate gels. Extensive high resolution transmission electron microscopy analysis showed both ‘‘colloidal’’ and ‘‘polymeric’’ type glasses, in addition to an intraparticle structure in the 1–3 nm range. An example is given where subpicosecond optical birefringence techniques have been used to directly probe the rotational dynamics of different molecular liquids in several glasses having different pore sizes. Thus, a systematic evaluation of the liquid behavior as a function of its geometrical confinement is obtained and the results discussed.
Structure and magnetic properties of sputtered Co-Si alloy films
Authors: Ishiguro T. , Fujii H. , Ichinose Y. , Endo J. , Harada H.
Published: July 2002

An interesting behavior of Si as an additive to Co alloys is expected, since Si is supposed to change the structure and the magnetic properties of Co alloys as a metalloid element. From the above reason, the relationship between the microstructures and the magnetic properties of the sputtered Co-Si alloy films was investigated in detail. As the Si content was increased to 35 at. % Si, such changes of magnetic properties were found that the coercive force of the sputtered Co-Si alloy films reached the peak of about 550 Oe at 6–10 at. % Si, below or above which the coercive force is relatively small. On the other hand, according to the transmission electron microscopic observation at relatively low magnification, the grain size of α-Co crystallites forming the films was found to decrease as the Si content increases. The results of Lorentz electron microscopy showed large magnetic domains in the films with a Si content below 6 at. %, very fine magnetic domains in the films with 6–10 at. % Si content, and the disappearance of magnetic domains in the films with greater than 25 at. % Si. From the above, it can be mentioned that the magnetic properties of the sputtered Co-Si alloy films are very closely related to the microstructures and/or the magnetic domain structures.
The effect of Ag and Cu overlayers on the magnetization of thin cobalt films
Authors: Nair R. , Schröder K.
Published: July 2002

Co films, several nm thick, were prepared in a high-vacuum magnetometer and their dipole moments were measured. Then an Ag or a Cu overlayer was deposited on the Co film and the dipole moment was again measured. The magnetization change was less than 0.02 μB /surface atom. This agrees with calculations on the Co-Cu system.
Flash x radiography of laser-accelerated foils
Authors: Whitlock R. R. , Obenschain S. P. , Grun J. , Stamper J. A. , Sprague J. A. , Sweeney B. V. , Ripin B. H. , Craxton R. S.
Published: July 2002

The successful compression of laser-driven pellets to thermonuclear ignition depends on the stability and uniformity of the motion with which dense shells can be imploded. The motion of planar foils accelerated by the Pharos II laser has been studied by two-dimensional, flash x radiography employing pinhole imaging and slitted crystal imaging. The acceleration was driven by a 3–5-ns duration, 1.05-μm laser focused to 3–6×1012 W/cm2 in a millimeter diameter spot, while a second laser beam of shorter duration produced the x-ray flash for imaging purposes. The x-ray images obtained clearly show that the planar foil targets are ablatively accelerated to velocities of 3×106 cm/s while maintaining a density above 3% of solid. The axial extent of the accelerated, high-density material has been observed to be as small as 25% of the distance traveled. The sides of accelerated portions of the foil connect smoothly to the stationary regions removed from the laser illumination. This connection apparently isolates the rear surface from the hot ablation plasma and helps explain the low rear surface temperatures which have been observed. The overall appearance of the accelerated foil is localized and nearly planar at early times when it has moved distances small compared to its diameter. The x-radiographic results are in general agreement with two-dimensional hydrodynamic simulations and with earlier Naval Research Laboratory studies of target motion, ablation pressures, and symmetrization employing other diagnostics.
Multiphonon relaxation and infrared-to-visible conversion of Er3+ and Yb3+ ions in barium-thorium fluoride glass
Authors: Yeh D. C. , Sibley W. A. , Suscavage M. , Drexhage M. G.
Published: July 2002

The upconversion of infrared radiation into green and red fluorescence has been studied for Er3+ and Yb3+ ions in BaF2/ThF4 fluoride glass over a wide temperature range and several dopant concentrations. At room temperature an upconversion efficiency of 3.3×10−5 has been obtained for the green emission from the glass with 1 mol % ErF3 and 19 mol % YbF3 pumped by 973-nm radiation with intensity of 16.5 mW/cm2. For an absorbed intensity of 10 W/cm2 the efficiency for green upconversion emission is predicted to be 2%. Radiative transition rates for the excited states of Er3+ were calculated using Judd–Ofelt theory and intensity parameters obtained from measured integrated absorption coefficients. The calculated values of the multiphonon relaxation and the upconversion fluorescence intensity, obtained from the rate equation model of Wright, are discussed in light of the experimental results.
An IR, optical, and electron-spin-resonance study of as-deposited and annealed a-Ge1−xCx:H prepared by rf sputtering in Ar/H2/C3H8
Authors: Shinar J. , Wu H. S. , Shinar R. , Shanks H. R.
Published: July 2002

Hydrogenated amorphous germanium carbide (a-Ge1−xCx:H ) films were prepared by rf sputtering, at various rf power levels and mixtures of Ar, H2, and propane. As in the case of a-Si:H rf sputtered under similar conditions, the concentration of Ge–H bond, as determined by the IR absorption spectra, and the Tauc determined optical gap, generally increase as the rf power is decreased. The optical gap of the a-Ge1−xCx:H films range from 0.85–2.3 eV, and the electron-spin-resonance defect spin densities from 6.5×1017 to 3×1018 spins/cm3. Auger spectroscopy was used to determine the C/Ge ratio and indicated that in most of the samples, this ratio was ≲0.15. Isochronal annealing up to 400 °C indicated that (i) Ge-C segregation effects already initiated at 100 °C are greatly enhanced above 300 °C, (ii) at 300 °C C–H bonds are formed at the expense of Ge–H ones, and (iii) all of the hydrogen bonded to Ge and most of that bonded to carbon evolve out of the sample at or slightly below 400 °C.
Shear strength of shock-loaded alumina as determined with longitudinal and transverse manganin gauges
Authors: Rosenberg Z. , Yaziv D. , Yeshurun Y. , Bless S. J.
Published: July 2002

The shear strength of shock-loaded commercial alumina (AD-85 manufactured by Coors) is determined in the 0–140-kbar range of shock stresses. Longitudinal and transverse manganin gauges were used to determine the principal stresses in the shocked specimens. Shear strengths were determined from the difference between the longitudinal and lateral stresses. It was found that the shear strength remains essentially constant at about 27 kbar for shock stresses between 60 kbar (the Hugoniot elastic limit) and the maximum shock amplitude tested in this series (142 kbar). The source for the high shear strength is attributed to the confining pressures that strengthen the comminuted ceramic. Evidence for this interpretation is obtained by considering the release profiles as recorded by the longitudinal gauges when the free-surface rarefactions reach gauge location.
Spectral hole-burning and gain saturation in semiconductor lasers: Strong-signal theory
Authors: Agrawal Govind P.
Published: July 2002

The nonlinear susceptibility of semiconductor gain media is obtained by including the effect of intraband carrier relaxation within the density-matrix formalism. The result is used to obtain the approximate analytic expressions showing how the optical gain and the refractive index change with an increase in the laser power. The index change is generally small but can be positive or negative depending on which side of the gain peak the lasing mode is located. By contrast, the gain is always reduced because of spectral hole-burning. The implications of gain saturation on the dynamic response of semiconductor lasers are discussed together with the possibility of experimental verification.
Thermal conduction at a contact interface measured by pulsed photothermal radiometry
Authors: Leung W. P. , Tam A. C.
Published: July 2002

We have used pulsed photothermal radiometry to measure the thermal contact resistance at the interface of a smooth polymer film and a polished metal substrate. This method relies on the heating of the film surface by a short light pulse and detecting the subsequent infrared thermal radiation from the surface. An analytical solution to the heat diffusion equation shows that in a suitable delayed time interval, the infrared signal decays exponentially in time with a time constant related to the thermal contact resistance of the interface. By changing gases in the interface at constant pressures, we are able to separate the thermal conductance into two components: that due to solid contacts and that due to gas conduction. The thermal conductance due to gas conduction in the interface is proportional to the thermal conductivity of the gas found in continuum fluid theory, except for He which is more than 30% lower. We believe that the discrepancy in He is partly due to the fact that the mean free path of He gas molecules is not much smaller than the mean width of the gaps in the interface and so the classical continuum fluid theory for heat conduction would no longer hold.
Band-gap narrowing in heavily doped silicon: A comparison of optical and electrical data
Authors: Wagner Joachim , del Alamo Jesús A.
Published: July 2002

The band-gap narrowing in heavily doped silicon has been studied by optical techniques—namely, photoluminescence and photoluminescence excitation spectroscopy—and by electrical measurements on bipolar transistors. The optical experiments give a consistent set of data for the band-gap narrowing in n- and p-type material at low temperatures as well as at room temperature. A good agreement is found between the optical and electrical data removing the discrepancies existing so far in the literature.
Chemistry of Si-SiO2 interface trap annealing
Authors: Reed Michael L. , Plummer James D.
Published: July 2002

The kinetics and chemistry of Si-SiO2 interface trap annealing are examined in detail. Measurements of interface trap density Dit as a function of anneal time were performed with several process variables as parameters: oxide thickness, anneal ambient, temperature, bulk carrier type, metallization damage, and orientation. Experiments were carried out using rapid thermal processing and capacitance-voltage measurements of aluminum gate metal-oxide-semiconductor capacitors. Anneal temperature and crystal orientation have the strongest effect on the kinetics. 〈100〉 interfaces can be described by a power-law temporal variation; 〈111〉 kinetics are slightly more complicated. In both cases the experimentally observed anneal behavior is in conflict with the commonly used second-order surface recombination model. We propose a two-reaction model involving atomic hydrogen dimerization and hydrogen/interface trap reactions. This model sucessfully predicts anneal kinetics over a temperature range of 170–500 °C, representing a 106 dynamic range in anneal rates. The difference in anneal behavior between 〈111〉 and 〈100〉 interfaces is explained by postulating different trap anneal mechanisms for the Pb0 and Pb1 defect centers. This hypothesis is supported by trap production kinetics induced by extended anneals.
Laminated CoZr amorphous thin-film recording heads
Authors: Su James L. , Chen Mao-Min , Lo Jerry , Lee Rod E.
Published: July 2002

Recording heads with four-layer laminated CoZr/Ta films as pole pieces have been fabricated using rf sputter deposition and ion milling techniques. Typical 4πMs of 14 000 G were obtained for CoZr amorphous films compared with 10 000 G for Permalloy films. Recording tests performed with this laminated CoZr/Ta film head indicate 8–12 dB better overwrite than with plated Permalloy film heads having an identical structure. However, the former exhibits a 10%–25% lower amplitude than the latter. This may be attributed to the thermal degradation of the initial permeability μi of the CoZr film during head-fabrication processes. This CoZr/Ta head also shows poorer amplitude stability than the plated Permalloy head.
Monte Carlo simulation of ion transport through rf glow-discharge sheaths
Authors: Thompson Brian E. , Sawin Herbert H. , Fisher Donald A.
Published: July 2002

The transport of ions through rf glow-discharge sheaths was simulated with a Monte Carlo method to determine the distributions of ion-bombardment energy and angle of impact. Several sheath parameters were varied and their effects examined: (1) the type of ion-molecule scattering (hard sphere, potential field interaction, charge exchange), (2) the ratio of ion and neutral masses, (3) the ratio of the sheath width to collision mean free path, (4) spatially uniform, spatially linear, and time-dependent (rf) electric fields in the sheath, and (5) the frequency of an rf component in the sheath. The results of the Monte Carlo simulations indicate that the type of elastic scattering (hard sphere or soft sphere) does not significantly change either the impact angle distributions or the scaled ion-bombardment energy distributions. Charge-exchange scattering produces a much greater ion-bombardment directionality and a different shape of the ion-bombardment energy distribution. The fully developed distributions depend only on the ion-to-neutral mass ratio, type of ion-neutral scattering, and the dc value of the electric field-to-pressure ratio at the electrode. Fully developed distributions are reached in approximately three mean free paths in spatially uniform sheath fields and in about six mean free paths for spatially linear sheath fields. The minimum ion directionality was observed when the ion-to-molecule mass ratio was approximately unity. Time-dependent (rf) variation of the sheath field produces features in the ion energy distributions which are similar for both the collisional and collisionless sheaths.
Energetics and deep levels of interstitial defects in the compound semiconductors GaAs, AlAs, ZnSe, and ZnTe
Authors: Jansen Robert W. , Wolde-Kidane Dawit S. , Sankey Otto F.
Published: July 2002

A simple ab initio pseudoatomic orbital method is used to study chemical trends in the near band-gap deep levels and the total energies of interstitial impurities in GaAs, AlAs, ZnSe, and ZnTe. We investigate the impurities K, Zn, Ga, Si, As, Se, and Te at the two nonequivalent interstitial tetrahedral sites: The TA site (surrounded by anions) and the TC site (surrounded by cations). The interstitial site of lowest energy is determined primarily through a competition between two terms in the total energy—the band energy and the deep level energy. The band energy favors the TA site, while the deep level energy favors the TC site. Switching from one site to the other is predicted to occur in some cases due to changes in the Fermi level. Due to this mechanism, we predict a novel bistability of the native Zn interstitial in ZnSe. The derived deep levels associated with the interstitial defects exhibit chemical trends which closely parallel those found through empirical tight-binding methods.
Stoichiometry related deep levels in undoped, semi-insulating GaAs
Authors: Dobrilla P.
Published: July 2002

The influence of stoichiometry on the deep levels found in undoped, semi-insulating GaAs has been investigated by photoinduced transient spectroscopy. Traps shallower than the main midgap donor EL2 have been found to be unambiguously related to the melt stoichiometry.
The importance of thermal stresses and strains induced in laser processing with focused Gaussian beams
Authors: Welsh Lisa P. , Tuchman Judah A. , Herman Irving P.
Published: July 2002

The thermoelastic equations are solved for laser heating of a semi-infinite elastic medium by a focused TEM00 Gaussian beam. Single integral expressions are derived for stresses and strains, which are analytically evaluated on axis at the surface and far from the laser-heated region (for a laser spot size≫absorption depth), and numerically evaluated for the example of laser heating of a silicon substrate. This analysis is extended to the case of laser heating of thin films on substrates. Use of these stress and strain profiles suggests that dislocations may form at the surface during high-temperature laser processing of silicon at scan speeds typically used in direct laser writing. Also the elastic strains induced during laser heating shift phonon frequencies from their thermal equilibrium values, thereby complicating the use of Raman scattering as an optical probe of temperature. This is shown to be particularly significant for laser heating of silicon thin films on fused silica substrates and not very important for laser processing of silicon substrates or silicon thin films on sapphire.
AEM studies of Sm:Co 2:17 permanent magnet alloys
Authors: Zhang Bing , Blachere J. R. , Soffa W. A. , Ray A. E.
Published: July 2002

A cast 2:17 alloy with composition Sm 10.5, Co 60.0, Fe 20.6, Cu 7.1, Zr 1.8 at. % was quantitatively analyzed with an analytical electron microscope (AEM). After a 24-h solution heat treatment at 1160 °C and rapid cooling, the microstructure was highly twinned, mostly 2:17R. Isothermal aging at 800 °C produced the microstructure typical of 2:17 permanent magnets. After 72 h, the 2:17R matrix phase composition was Sm 10.5, Co 61.7, Fe 22.4, Cu 4.8, Zr 0.5 at. %. The coherent boundary and platelet phases were too small to analyze independently, so two phase regions, including the matrix phase, were measured. The procedure yielded excellent results for the boundary phase: Sm 17, Co 33.6, Fe 4.8, Cu 43, Zr 1–2 at. %. Semiquantitative results were obtained for platelet-matrix phase mixtures: Sm 6(5–8), Co 60, Fe 14(13–15), Cu 5, Zr 15(14–17). Selected area diffraction patterns of the platelet could be indexed as with a=0.5 nm and c=0.8 nm. These results for the platelet phase differ significantly from those of previous investigations.
Ultralong optical-pulse corona preionized XeCl laser
Authors: Taylor Rod S. , Leopold Kurt E.
Published: July 2002

A simple corona preionization scheme together with magnetic spiker and pulse forming line technology has resulted in the production of 100-mJ, 1-μs duration as well as 500-mJ, 0.5-μs duration XeCl laser pulses.
Glow-discharge optical spectroscopy as a diagnostic of sputtered permalloy film composition and saturation magnetostriction
Authors: Page Ralph H. , Gudeman Christopher S. , Novotny Vlad J.
Published: July 2002

We show that the composition and saturation magnetostriction of sputtered permalloy films can be correlated with the relative intensities of iron and nickel atomic emissions from a 13.56-MHz rf glow discharge. Wavelength-resolved emission spectra as a function of discharge pressure were recorded while sputtering iron and permalloy targets. Intensities of some emission lines were highly correlated, so that their ratios were nearly independent of argon pressure. This occurred in spite of a ∼50-fold change in the brightness of the glow over the pressure range studied, and suggested that the stoichiometry of the sputtering target and the sputtered permalloy film can each be assessed during the sputtering process. The selection of appropriate emission lines is discussed, along with considerations of the type of equilibrium distribution prevalent in a sputtering discharge. We used five sputtering targets with different Ni/Fe ratios to grow films. The films were analyzed in terms of composition, stress, and saturation magnetostriction. Film stress was not observed to vary significantly with sputtering target composition. A high degree of correlation was observed between Fe/Ni emission intensity ratio, target composition, film composition, and saturation magnetostriction.
Thermally induced structural change of a-Ge:H/a-GeNx multilayer structures
Authors: Honma I. , Komiyama H. , Tanaka K.
Published: July 2002

Multilayer structures of a-Ge:H/a-GeNx and a-Ge/a-GeNx were prepared by a reactive-sputtering technique and their structural stability was studied through thermal anneals. High-resolution transmission-electron-microscope analyses show that crystallization takes place only in a-Ge:H(a-Ge) layers without disturbing atomically smooth and uniform a-Ge:H(a-Ge)/ a-GeNx interfaces. The crystallization temperature of a-Ge:H(a-Ge) layers increases with either decreasing thickness of those layers or increasing thickness of a-GeNx layers. The rise in crystallization temperature is most remarkable when the layer thickness becomes smaller than a couple of hundred angstroms. On the basis of the experimental results, the structural stability of multilayer films is discussed in the light of classical nucleation theory using the free-energy change of the system. It is demonstrated that the crystallization temperature is strongly affected both by layer thickness and by the nature of a heterointerface which phenomenologically explains well the experimental observations.
Thermal reaction between Pt thin films and SixGe1−x alloys
Authors: Hong Q. Z. , Mayer J. W.
Published: July 2002

Thermal reaction of Pt with amorphous SixGe1−x alloys (x=0.25–0.75) was investigated in the temperature range of 200–650 °C. At low temperatures (200–400 °C), a uniform reacted layer containing a mixture of Pt2Si and Pt2Ge was produced. The reaction continued with the formation of monosilicide and monogermanide. The formation of the first phases was diffusion controlled with a measured activation energy of 1.2 eV for all the SixGe1−x alloys. At high temperatures (400–650 °C), phase separation occurred in depth with a surface layer enriched with Si and a bottom layer enriched with Ge. At the same time, the formation of PtGe2 and the crystallization of unreacted amorphous SixGe1−x were observed.
White-light emitting thin film electroluminescent devices with stacked SrS:Ce/CaS:Eu active layers
Authors: Ono Yoshimasa A. , Fuyama Moriaki , Onisawa Ken-ichi , Tamura Katsumi , Ando Masahiko
Published: July 2002

By stacking blue-green emitting SrS:Ce and red-emitting CaS:Eu active layers, white-light emitting electroluminescent (EL) devices were fabricated. Luminance improvement and EL characteristics of SrS:Ce and CaS:Eu EL devices were discussed. The electrooptical characteristics of white-light emitting EL devices with stacked SrS:Ce/CaS:Eu active layers were presented. Color changed from blue-green to white by changing the voltage or frequency. Finally, feasibility of multicolor EL devices by using the fabricated white-light emitting EL devices with color filters were discussed. UFpg5523,5527 UFid992922JAP UFttIn situ ac magnetic susceptibility of gadolinium thin films UFauF. H. Salas$SUPa),b)$ and M. Mirabal-Garciaa$SUPc)$ UFloInstitut fuur Atom- und Festkourperphysik, Freie Universitaut Berlin, D-1000 Berlin 33, ufquadFederal Republic of Germany UFsd(Received 27 April 1989; accepted for publication 3 August 1989) UFabWe report measurements of the ac magnetic susceptibility on Gd(0001)/W(110) thin films grown in ultrahigh vacuum. The measurements were made by using a pickup coil, in which the geometry and the number of turns were optimized. We applied an alternating magnetic field of about 2 Grms at frequencies of 180 and 340 Hz. The growth mode and the deposition rate of the Gd films were determined by performing conventional Auger electron spectroscopy during film growth. In films with thickness larger than 10 nm our technique is sensitive up to 1016 atoms/Grms , which allowed us to study the critical behavior of the magnetic susceptibility as the Curie temperature is approached from above, TT+C . A sharp maximum, which may be related to the Hopkinson effect, is observed at a temperature TH 289 K.
Stochastic simulation of the flow of a thin liquid film over a rough rotating disk
Authors: Ma F. , Hwang J. H.
Published: July 2002

This article addresses the depletion of thin liquid films due to centrifugation. A stochastic model of surface roughness is used, and the equation describing the flow of a thin viscous film over a rough rotating disk is solved by Monte Carlo techniques. Depletion histories of the thin film are given for several cases involving random surface asperities. It is found that surface roughness markedly enhances the retention of a thin liquid film on a rotating disk. The thinner the liquid layer, the more dominant is the effect of surface roughness. Moreover, different degrees of randomness of the surface asperities lead to different asymptotic limits of liquid retention.
Photoreflectance of AlxGa1−xAs and AlxGa1−xAs/GaAs interfaces and high-electron-mobility transistors
Authors: Sydor Michael , Jahren Neal , Mitchel W. C. , Lampert W. V. , Haas T. W. , Yen M. Y. , Mudare S. M. , Tomich D. H.
Published: July 2002

Photoreflectance is used to measure AlxGa1−xAs composition, and to determine carrier concentrations in Si-doped AlGaAs epilayers capped with GaAs. Undoped caps are generally depleted, and do not show a usual GaAs photoreflectance. However, photoreflectance from the cap/(doped AlGaAs) interface produces a broad signal which distorts the entire spectrum, making it hard to locate the GaAs and AlGaAs band edges precisely. A similar broad signal from modulation-doped heterostructures is apparently associated with samples that show the presence of two-dimensional electron gas.