bluuebunny/crossref_metadata_2025 huggingface dataset:
Degradation of the dielectric permittivity of a strongly oriented Ba0.25Sr0.75TiO3 layer by replacing a SrRuO3 electrode with an Ag one
Authors: Boikov Yu. A. , Claeson T.
Published: July 2002

Films of (Ba,Sr)TiO3 have been deposited with a quality allowing a ferroelectric Curie–Weiss behavior to be seen and a study of the influence of electrode material and interfaces in a capacitor. The measured value of the real part of the dielectric permittivity, ε′, of a 700 nm thick Ba0.25Sr0.75TiO3 layer decreased three times at temperatures around the Curie point, when one of the two SrRuO3 electrodes (the top one) was replaced with a silver one. This may be interpreted as if there were a series capacitance, Cs, at the ferroelectric-to-metal electrode. From the decreased ε′, Cs=1.2 μF/cm2 was estimated for the Ag/Ba0.25Sr0.75TiO3 interface, corresponding to a characteristic length of about 2.6 Å for the interface layer.
Calculating the Debye–Scherrer diffraction pattern for large clusters
Authors: Hall B. D. , Monot R.
Published: July 2002

An efficient algorithm is presented that calculates the Debye–Scherrer electron diffraction pattern, in the kinematical approximation, for clusters with several thousand atoms. The algorithm creates a histogram of interatomic distances in the cluster and applies a fast Fourier transform to it, to calculate the diffraction pattern. The approximation of binning interatomic distances into discrete, evenly spaced, histogram classes is discussed. When the class width is small, the distortion introduced to the diffraction pattern can be represented by a pseudotemperature factor, analogous to thermal movement of the cluster.
Spin density waves in Cu-Mn
Authors: Gotaas J. A. , Rhyne J. J. , Werner S. A.
Published: July 2002

The incommensurate magnetic peaks occurring in Cu1−x Mnx at (1,0.5±δ,0) and equivalent positions in reciprocal space have been studied in an extensive series of neutron scattering experiments. It is found that δ varies linearly with composition, and that the width of these peaks corresponds to a correlation range of about 10 fcc unit cells. The symmetry of the magnetic scattering indicates that there are 12 spin density wave domains. Experiments performed under high-resolution conditions (61 μ eV) show that the elastic component of the magnetic scattering cross section at (1,0.5±δ,0) approaches zero in the vicinity of the ‘‘freezing’’ temperature Tf, closely resembling the behavior of an order parameter going to zero at a Néel point.
Nb-based A15 compound Josephson tunnel junctions fabricated using a CF4 cleaning process: Fabrication conditions and barrier properties
Authors: Tanabe K. , Michikami O.
Published: July 2002

Josephson tunnel junctions with Pb counterelectrodes have been fabricated on magnetron-sputtered Al5 Nb3X(X=Al,Ge) thin films using a CF4 plasma cleaning process (CFCP) in conjuction with standard photolithographic processing. The fabrication conditions necessary to produce optimized junctions and their tunneling properties have been investigated. The junction quality rapidly decreases with the increase in the discharge cathode self-bias voltage (VCSB) during the plasma oxidation as well as the CF4 cleaning. The optimum voltage for the cleaning necessary to obtain high-quality junctions with a uniform current distribution is common to both compounds and exists in the narrower range of 120–140 V than for Nb/Pb junctions. Nevertheless, high-quality Nb3X/Pb junctions with Vm >20 mV can be highly reproducibly fabricated for a wide range of the critical current density (1–103 A/cm2). Such junctions show high barrier height values of 0.8–1.0 eV and good aging stability, which is also the case for the Nb/Pb junctions fabricated using CFCP. X-ray photoelectron spectroscopy (XPS) and ellipsometric measurements on the CF4-cleaned base electrode surface indicate the presence of an ultrathin fluoride overlayer which is composed of Nb-F and X-fluorides. The former also show that the tunnel barrier is a mixture of Nb2O5, Nb-F-O, and X-F-O. These results, in addition to those for barrier height measurements, suggest that F atoms prevent the defect creation in the barrier and the selective Nb oxidation. They, thus, play an important role in the formation of the highly insulating barriers as well as a nearly ideal barrier-electrode interface without a substantial proximity layer.
Physics of amorphous silicon alloy p-i-n solar cells
Authors: Hack M. , Shur M.
Published: July 2002

We present a comprehensive computer simulation of amorphous silicon alloy p-i-n solar cells which has enabled us to establish the physical mechanisms governing device operation. This has been carried out by analyzing the free-carrier, space-charge, recombination rate, and electric field profiles as well as the corresponding current-voltage characteristics for a variety of device parameters and illumination conditions. We interpret these results in terms of the asymmetrical density of localized states in these alloys and its effect on the basic recombination paths. We also demonstrate that for uniformly absorbed light, cell performance is primarily controlled by the hole transport properties, and not by the sum of the carrier drift lengths, or their mobility-lifetime products. This concept of a ‘‘limiting carrier’’ also applies to the case of nonuniform illumination. Photovoltaic performance is calculated both as a function of device thickness as well as material quality. Finally, we compare the results from our simulation with those obtained from other models for these devices.
Diffusion of ion-implanted phosphorus within thermally grown SiO2 in O2 ambient
Authors: Yamaji Tetsuo , Ichikawa Fumio
Published: July 2002

Phosphorus, implanted into thermally grown SiO2, was diffused in O2 ambient, and the phosphorus distribution was analyzed by secondary ion-mass spectrometry. A profile of the highly doped phosphorus revealed enhanced diffusion in its high-concentration region (above about 1×1019 cm−3). An empirical diffusion model, in which phosphorus diffusivity depends on local phosphorus concentration, is proposed. Numerical simulations using this model give well-matched profiles to the SIMS data, and parameters for the model are determined. The concentration-dependent diffusivity is ascribed to the formation of P=O bond in SiO2 layer, and to the resulting loosening of SiO2 network or increase of liquid phase with increasing phosphorus concentration. It was also found that the diffusivity becomes less dependent on phosphorus concentration with increasing annealing temperature.
Raman-scattering studies of silicon-implanted gallium arsenide: The role of amorphicity
Authors: Holtz M. , Zallen R. , Geissberger Art E. , Sadler R. A.
Published: July 2002

A series of Raman-scattering experiments were carried out on GaAs implanted with Si+ and with SiF+3 ions, both before and after annealing, for samples subjected to fluences spanning a wide range. The implantation-induced amorphization of the damage layer was cleanly observed via the evolution, with increasing fluence, of the broad three-band continuum of amorphous GaAs which extends from near zero up to 300 cm−1. Annealing recovers the simple line spectrum of crystalline GaAs, but with a changed longitudinal-optical/transverse-optical intensity ratio which indicates a departure from epitaxial regrowth. Three lines observed near 400 cm−1 in heavily implanted samples were identified with silicon vibrational local modes. The effect of annealing on these local-mode lines is not an intensity increase but is instead a line narrowing which reveals an annealing-induced sharpening of the distribution of local settings sampled by the substitutional silicons. In particular, a line (at 381 cm−1) assigned to the Si-at-a-Ga-site donor impurity is clearly seen before annealing, even though annealing is needed to transform the highly resistive implanted material into semiconducting n-type GaAs. We propose that the primary role of annealing in the ‘‘electrical activation’’ of implanted semiconductors is not to shift the impurity atoms into substitutional donor or acceptor sites (they already occupy such sites), but is instead to recover the high carrier mobility of the crystalline form. ‘‘Healing’’ and ‘‘activation’’ thus correspond to the same process, the elimination of amorphicity.
Semiconductor analysis using organic-on-inorganic contact barriers. I. Theory of the effects of surface states on diode potential and ac admittance
Authors: Forrest S. R. , Schmidt P. H.
Published: July 2002

A new method for measuring the density of states at semiconductor surfaces using organic-on-inorganic (OI) semiconductor contact barriers is suggested. This work is an extension of previous models of OI diode behavior which includes the ac admittance characteristics, and which considers the range of validity of approximations to OI diode capacitance used in previous experiments. The theory describes the potential distribution across the device. We consider the case of ideal OI diodes, as well as diodes with significant densities of states at the inorganic semiconductor surface. This analysis leads to a technique whereby the low-frequency conductance and capacitance characteristics can be used to obtain information about the magnitude and the energy distribution of surface states in the inorganic semiconductor bandgap. Also, the carrier concentration profiles of the substrate can be conveniently obtained. Due to the noninvasive nature of the organic/inorganic contact, information about the density of states at relatively undisturbed semiconductor surfaces obtained via this technique may prove useful in determining the processes involved in Schottky barrier formation and metal-insulator-semiconductor diode surface properties. This theory has been applied to the investigation of surfaces of III-V alloy semiconductor-based OI diodes, and the experimental results will be presented in a subsequent paper. In addition, an expression for the OI diode n value obtained from the forward current-voltage characteristics is derived. It is found that the n value depends on surface states which are in equilibrium with the organic as well as the inorganic materials. Estimates of the surface state densities obtained from previously reported n values are consistent with expectations for the semiconductors under test.
Measurement of deep states in undoped amorphous silicon by current transient spectroscopy
Authors: Kida H. , Hattori K. , Okamoto H. , Hamakawa Y.
Published: July 2002

Localized deep states in undoped a-Si:H have been investigated by current transient spectroscopy (CTS). After a trap-filling light pulse, current transients associated with thermal emission of trapped carriers are measured over a wide time range under the isothermal condition. Slow saturation of the CTS signal J×t, with increased filling-pulse duration is well explained by taking account of spatially and energy dependent carrier capture and emission processes, yielding an estimate of attempt-to-escape frequency. The attempt-to-escape frequency has been found to be energy dependent and to range from 1012 to 1013 s−1. Detailed analysis of the CTS signal has revealed that deep-states distribution displays a broad peak locating near the center of the gap and a less pronounced structure at 0.5–0.6 eV below the conduction band edge.
Rapid thermal annealing and regrowth of thermal donors in silicon
Authors: Stein Herman J. , Hahn S. K. , Shatas S. C.
Published: July 2002

Rapid thermal annealing (RTA) of 450° thermal donors in Si at temperatures between 600 and 1000 °C and their subsequent regrowth between 375 and 450 °C have been investigated by infrared absorption and resistivity measurements. Thermal donors for small oxygen clusters identified by their excited state absorption are present in as-grown Czochralski Si, whereas a large range of cluster sizes are observed after extended heating at 450 °C. The temperature required for RTA is lower when only small clusters are present. Removal of thermal donors by RTA restores 5–8 oxygen atoms to interstitial sites per electrically measured donor removed, which supports oxygen cluster models for thermal donors. Restoration of oxygen to interstitial sites upon removal of thermal donors also impacts models treating relationships between thermal donors and oxygen precipitation in Si. An activation energy of 2.5±0.2 eV is obtained for RTA removal of thermal donors, consistent with oxygen diffusion. Initial regrowth of thermal donors after RTA at 1000 °C occurs with an activation energy of 1.7±0.2 eV.
Minimum energy principle in magnetic insulation theory
Authors: Slutz Stephen A.
Published: July 2002

The self-limited impedances of magnetically insulated transmission lines obtained by minimizing energy are compared to previous results obtained by minimizing the total line current. Although the relationship between the line voltage and the fraction of the current carried by free electrons is significantly different, the impedance obtained by these two approaches is nearly identical. We discuss why the two solutions are so similar.
Strain in GaAs by low-dose ion implantation
Authors: Paine B. M. , Hurvitz N. N. , Speriosu V. S.
Published: July 2002

The production of strain in (100) GaAs by low-dose ion implantation has been investigated. Implantations were conducted at room temperature with ions of He, B, C, Ne, Si, P, and Te. Energies were between 100 and 500 keV, and each species was implanted over a range of doses sufficient to create perpendicular strain below 0.3%. The perpendicular strains ε ⊥ were measured by x-ray double-crystal diffractometry about the (400) Bragg condition. Detailed depth profiles of ε⊥ were obtained by fitting the resulting rocking curves with a kinematic model for the diffraction. For all implantations the maximum in the ε⊥ distribution was found approximately from the separation of the lowest-angle prominent oscillation from the substrate peak. The depth profiles of perpendicular strain had the same shape as the calculated profiles of energy deposited per ion by nuclear collisions, FD. The maximum perpendicular strains scaled linearly with the dose φ of the implanted ions for all ion species. Also the ratio of maximum strain to dose was found to vary linearly with FD over more than 2 orders of magnitude in FD. We therefore conclude that ε⊥=KφFD at all depths, where K is a constant. The value of K was found to be (5±1)×10−2 Å3/eV. Our results suggest that this holds for any ion species in the mass range 4–128 amu, with energy in the hundreds of keV, implanted into (100) GaAs at room temperature, provided the maximum strain is less than 0.3%.
Preparation and characteristics of pyroelectric infrared sensors made of c-axis oriented La-modified PbTi03 thin films
Authors: Takayama Ryoichi , Tomita Yoshihiro , Iijima Kenji , Ueda Ichiro
Published: July 2002

Highly c-axis oriented La-modified PbTiO3 (PLT) thin films were deposited on (100) oriented Pt thin film or (100) cleaved MgO single crystals by rf-magnetron sputtering. Figures of merit for infrared detector were studied on the PLT thin films. It was found that a composition Pb1−xLaxTi1−x/4O3 where x=0.10 (PL10) was the most suitable for the infrared detector. The films have the advantage of having pyroelectric effect and polarization with the same direction even without a poling treatment. The films have high figures of merit Fv(=γ/cvεr) of 0.83×10−10 (C cm/J) and Fm(=γ/cv(εr tan δ)1/2) of 1.2×10−8 (C cm/J) because of relative low-dielectric constant εr of 200 and high-pyroelectric coefficient γ of 5.3×10−8 C/cm2 K. Both the Fv and the Fm of the films are 3 times as large as those of PbTiO3 ceramics. High sensitive pyroelectric infrared sensors were fabricated by using the PL10 films. The thickness of the films is 2 μm, and the electrodes were vacuum evaporated Ni-Cr of approximate area 7×10−4 cm2. The MgO substrate under the sensing area is etched off by phosphoric acid. The sensors have a large voltage responsivity of 2400 (V/W) at 25 Hz. A high specific detectivity D*(500,25,1) of 5×108 (cm(Hz)1/2 /W) has been attained.
1/f  noise in a quarter-micron GaAs Hall device made by focused ion-beam implantation
Authors: Tacano Munecazu , Kanayama Toshihiko , Hiroshima Hiroshi , Komuro Masanori , Sugiyama Yoshinobu
Published: July 2002

A quarter-micron-width Greek-cross Hall device was fabricated by focused ion-beam implantation with a practical product sensitivity of 85 V/AT. The Hooge’s noise parameter αH [Phys. Lett. A 29, 139 (1969)] was evaluated from the exact carrier number and the 1/f  noise level to be 4.5×10−6, in good agreement with the quantum theory for 1/f  noise.
Nonlinear theory of the dislocation-enhanced Snoek effect and its connection with the geometric and/or thermal kink oscillations on nonscrew dislocations in body-centered-cubic metals
Authors: Oğurtani Tarik Ö. , Seeger Alfred K.
Published: July 2002

The dislocation-enhanced Snoek effect, which is induced by the forced vibrations of the interacting kinks (geometric and/or thermal) on nonscrew dislocations in the atmosphere of mobile interstitials, is formulated by taking into account the nonlinear character of the atomic hopping motion of interstitials, as well as the existence of the strong and inhomogeneous static interaction field associated with the kinked dislocation lines. The desired explicit and transparent expression for the internal friction behavior of defects is also worked out, which reveals clearly the effect of the site saturation on the relaxation strength as well as on the activation enthalpies, especially for the case of highly localized binding interaction (hydrostatic) between interstitial foreign atoms and the dislocation lines.
Second-harmonic generation in sodium lanthanium fluoride
Authors: Velsko Stephan P. , Eimerl D.
Published: July 2002

Sodium lanthanum fluoride (NaLaF4) is a new nonlinear fluoride crystal that can be phase matched at 1.06 μm. It has an unusually small angular sensitivity for a critically phase-matchable material (800 cm−1/rad). Its nonlinear coefficients are at least 5 times larger than those of BaZnF4 and twice as large as those of Na2SbF6, the only other fluorides known to be phase matchable at 1.06 μm. This paper describes measurements of phase-matched second-harmonic generation in small (submillimeter) crystals and discusses the relationship of the observed nonlinear properties of the unusual structure of this crystal.
Reduction of stored energy in a heavily irradiated ceramic
Authors: Klemens P. G. , Clinard F. W. , Livak R. J.
Published: July 2002

The decrease of stored energy that accompanies redamage of a metamict ceramic is compared with the calculated elastic strain energy resulting from the volume misfit between the disordered inclusions and the crystalline matrix. In the case of self-damaged CaPuTi2O7 it is found that the elastic energy contributes no more than 20% to the observed energy decrease. The major part of this decrease must be attributed to rearrangement of atoms by the redamage process into configurations of lower energy.
Rutherford backscattering and transmission electron microscopy study on phase transformation of As heavily doped Si during post-rapid-thermal annealing
Authors: Lu Zhiheng
Published: July 2002

Development of cluster models of As in Si is briefly reviewed. The random and aligned backscattering and the channeling angle scanning were performed on As heavily doped Si samples. Not only the displacement of As but also that of Si atoms during post-rapid-thermal annealing were measured as 0.15 and 0.12 Å, respectively. The potential variations of the deviated atoms of As and Si were estimated as 0.10 and 0.06 eV, respectively, according to Morse potential. The transmission electron microscope picture, with computer image processing, exhibited a microstructure with a mean period of about 20 Å. These data demonstrated that a certain coherent phase transformation occurs. The cluster models were used as the basis of some questions.
Magnetic, Mössbauer, and neutron diffraction investigations of W-type hexaferrite BaZn2−xCoxFe16O27 single crystals
Authors: Paoluzi A. , Licci F. , Moze O. , Turilli G. , Deriu A. , Albanese G. , Calabrese E.
Published: July 2002

Macroscopic magnetic measurements, Mössbauer, and neutron diffraction studies have been carried out on BaCoxZn2−xFe16O27 [(Zn−Co)2-W] single-crystal hexaferrites. The extrapolated saturation magnetization values σs(0) are slightly lower than those found in the literature. Such differences have been justified as due to the different Co and Zn distributions among the tetrahedral sites.The Co ions induce a strong planar contribution to the magnetic anisotropy. At low temperature, the spin order, found from the macroscopic magnetic measurements and preliminary neutron diffraction studies, is planar, while the Mössbauer data indicate a conical order with an angle of 70° with respect to the c axis. By increasing the temperature, the cobalt anisotropy decreases and a spin reorientation to easy axis is observed.
The relation of dark and illuminated diode parameters to the open-circuit voltage of amorphous silicon p-i-n solar cells
Authors: Hegedus Steven S. , Salzman Neil , Fagen Edward
Published: July 2002

We have measured the current-voltage characteristics of a-Si:H p-i-n solar cells having open-circuit voltages (Voc) between 0.70 and 0.90 V as a function of temperature and light intensity. The diode parameters (diode factor, saturation current density, and built-in voltage) were calculated in the light and in the dark. We find that parameters obtained in the dark have no relevance to the analysis of Voc, but they do indicate that tunneling may be a significant effect in the dark. The Voc is dominated by recombination at the p/i interface or in the i layer near this interface. Reducing interface recombination with a carbon buffer layer and reducing bulk recombination with less i layer impurities improves Voc by reducing the forward recombination current density.
Theoretical and experimental study of ultrasonic surface modes at a solid-fluid periodic interface
Authors: Jungman A. , Leroy O. , Quentin G. , Mampaert K.
Published: July 2002

The generation of ultrasonic surface waves on a solid-liquid periodic interface is demonstrated. Characterization of leaky Stoneley modes and leaky Rayleigh modes is obtained by looking at the reflected and the transmitted broadband spectra. It is shown both experimentally and theoretically that the frequency and amplitude of such surface resonances are significantly affected, not only by the spatial periodicity of the profile, but also by the peak-to-valley height of the roughness. As a result, the practical problem of measurement of the geometrical parameters of the periodic surface, knowing the ultrasonic velocities in the coupling medium, is addressed. Conversely, we discuss a method to estimate the surface wave velocities on a given known profile.
The effects of deposition conditions on microstructure and magnetic properties of TbFeCo
Authors: Lee J.-W. , Shieh H-P. D. , Kryder M. H. , Laughlin D. E.
Published: July 2002

Transmission electron microscopy (TEM) has been employed to characterize micro/magnetic structural details of magneto-optical (MO) recording TbFeCo thin films as a function of dc-magnetron sputtering parameters using bright/dark field imaging, selected area diffraction, convergent beam electron diffraction, and Lorentz electron microscopy. It is found that the preparation conditions have a strong impact on both microstructure and magnetic properties of the films. The microstructures of the films deposited at low argon bleeding pressures (<10 mTorr) are featureless, and their magnetic domains are of the ‘‘stripe’’ type with a perpendicular anisotropy. In contrast, high argon pressures (>20 mTorr) give rise to microvoids surrounding ‘‘honeycomblike’’ networks and in-plane domains with ‘‘ripple type’’ contrast. The microstructure and magnetic domain structures are related to the films’ magnetic properties as characterized by a MO loop tracer.
Harmonic multiplication using resonant tunneling
Authors: Sollner T. C. L. G. , Brown E. R. , Goodhue W. D. , Correa C. A.
Published: July 2002

We propose and demonstrate an odd-harmonic resistive multiplier that exploits the unique current-voltage (I-V) relation of resonant-tunneling diodes. It is shown that efficient odd-harmonic conversion is possible and that even harmonics do not appear because of the antisymmetry of the I-V curve. In addition, with the proper choice of resonant-tunneling structure and pump amplitude, most of the harmonic output power can be confined to a single, odd-harmonic frequency. Fifth-harmonic multiplication has been demonstrated with output at 21.75 GHz and a power conversion efficiency of 0.5% in a coaxial circuit. Circuit simulation using an improved I-V curve yields a fifth-harmonic efficiency of 2.7%, suggesting that useful harmonic output in the millimeter and submillimeter spectrum should be achievable.
Characterization of ion damage on p-type cadmium telluride surfaces
Authors: Chien Kuo-Fu , Fahrenbruch Alan L. , Bube Richard H.
Published: July 2002

The effects of low-energy Ar ions (0–800 eV), from an ion beam and in an rf sputtering unit on the surface of single-crystal p-type CdTe have been investigated as a function of the ion energy using measurements of current versus voltage, spectral response, and capacitance versus voltage for indium junctions on CdTe. A threshold for the effects has been identified at an accelerating voltage of slightly less than 100 V for a fluence of 5×10−3C cm−2. Bombardment by Ar ions at voltages larger than this results in formation of an n-type surface layer on the p-type CdTe crystals.
A novel low-temperature preparation of several ferrimagnetic spinels and their magnetic and Mössbauer characterization
Authors: Pannaparayil T. , Marande R. , Komarneni S. , Sankar S. G.
Published: July 2002

Ultrafine subdomain particles of NiFe2O4, Ni0.5Zn0.5Fe2O4, and CoFe2O4 were prepared under mild hydrothermal conditions by precipitating from metal nitrates. Magnetic and Mössbauer effect measurements revealed the superparamagnetic nature of these ferrite particles. The crystallite sizes were in the range 5.0–20.0 nm with a very narrow particle size distribution. The magnetic properties and Mössbauer hyperfine parameters of these subdomain ferrite particles are somewhat different from those reported for multidomain bulk materials. The hydrothermal ferrite powders were found to sinter to almost theoretical density with little or no intergranular porosity.
A tandem radio-frequency plasma torch
Authors: Uesugi Takashi , Nakamura Osamu , Yoshida Toyonobu , Akashi Kazuo
Published: July 2002

The flow and temperature fields in a tandem radio-frequency (rf) plasma torch were calculated. The method employed here is based on our previous rf plasma model. The flow and temperature fields of this tandem plasma show similar features to those of a dc-rf hybrid plasma, and the carrier gas can be injected into the high-temperature region (∼9000 °C) of the second plasma by regulating the linear velocity of the sheath gas. As might be expected, the derived results suggest that the tandem plasma has a possibility to give higher efficiencies for practical processing than conventional rf plasmas. A new type of tandem rf plasma torch was designed based on the results of the calculations. The stability of this plasma was examined under various conditions. Thus, a stable plasma could be maintained even when carrier gas of high flow rate (∼5 ℓ/min of H2) was injected into the top of the second plasma. These theoretical and experimental investigations showed that the tandem plasma torch is an effective reactor for the preparation of refractory compounds by plasma chemical vapor deposition processes.
Y, Ba, Cu, and Ti interface reactions with SrTiO3 (100) surfaces
Authors: Hill D. M. , Meyer H. M. , Weaver J. H.
Published: July 2002

Interface reactions of adatoms of Y, Ba, Cu, and Ti with ordered and disordered SrTiO3 (100) surfaces were examined with x-ray photoemission spectroscopy, Auger spectroscopy, and low-energy electron diffraction. Atomic distributions for these interfaces before and after annealing at 500 °C were determined using Ar ion sputter depth profiling. We observed strong reactions of Y and Ba with O extracted from the substrate for both ordered and disordered SrTiO3, with the strongest interactions occurring for Y on the disordered surface. These reactions were diffusion limited at 300 K, and the growth of a metal overlayer was observed with increasing coverage. Comparison of these results to those for Ti/SrTiO3 made it possible to determine the extent of the reactions and the reaction products. At elevated temperatures, more extended out-diffusion of O from the substrate into the overlayer was observed, completing the conversion to the respective metal oxides. In contrast, Cu deposited on SrTiO3 formed clusters on the surface and did not react with the substrate. O out-diffusion was not observed for the Cu overlayer at elevated temperatures, but clustering was enhanced. The implications of these results are discussed in terms of Y-Ba-Cu-O superconductor synthesis on SrTiO3.
The effect of composition and magnetic heat treatment on the magnetostriction of TbxDy1−xFey twinned single crystals
Authors: Verhoeven J. D. , Ostenson J. E. , Gibson E. D. , McMasters O. D.
Published: July 2002

Twinned single crystals of the giant magnetostrictive intermetallic compound TbxDy1−xFey with a 〈112〉 axial alignment have been prepared by float-zone solidification with Y varying from 1.90 to 1.98 and X being 0.32 or 0.28. The chemical compositions of the crystals were carefully analyzed. A very sharp jump in the magnetostrictive strain λ sometimes occurs in these alloys as the applied field H is increased to values of around 280 Oe under a compressive stress of 6.9 MPa. This large jump in λ is shown to depend strongly on the values of X and Y, and possible reasons for this result are discussed. A magnetic heat treatment has been developed which involves cooling the samples through the Curie temperature under a field applied transverse to the crystal axis. It is shown that this treatment is very effective at increasing the magnetostrictive properties at low applied compressive stresses for certain values of X and Y.
Dielectric properties of portland cement paste as a function of time since mixing
Authors: Camp Paul R. , Bilotta Stephen
Published: July 2002

The dielectric properties of portland cement paste and mortar have been measured in the frequency range 100 Hz–7 MHz as a function of time since mixing. Over much of the spectrum, the ac conductance of the samples appears directly related to the amount of unbound water remaining in the sample and ionic conduction predominates. In addition, interesting structure was found in both the conductance and capacitance data at high frequencies as the free water content was reduced. We conclude that relatively simple measurements of this kind can be a useful tool in concrete research and may provide the basis for simple, in situ, nondestructive measurement of the degree of curing of concrete or for monitoring water migration in concrete structures. Measurements on sealed samples of partially or fully cured concrete reveal also the water–cement ratio of the original mix.
Optical excitation and recombination mechanisms of Tb3+-doped zinc sulphide thin films
Authors: Swiatek Krzysztof , Suchocki Andrzej , Stapor Andrzej , Niinisto Lauri , Leskela Markku
Published: July 2002

Time-resolved spectroscopy measurements show that the excitation of terbium photoluminescence in thin film ZnS:Tb electroluminescent devices, due to energy transfer from donor-acceptor pairs, is independent of temperature. The differences in the decay kinetics of terbium ions excited in various ways demonstrate that terbium ions in different crystal sites have different excitation mechanisms.
Grain-boundary characterization of ZnO varistors by positron annihilation spectroscopy
Authors: Gupta T. K. , Straub W. D. , Ramanachalam M. S. , Schaffer J. P. , Rohatgi A.
Published: July 2002

Positron annihilation spectroscopy (PAS) was performed in a Doppler mode to characterize the negatively charged grain-boundary defect, V′Zn, in ZnO varistor. The PAS study was conducted as a function of annealing treatment. As the annealing temperature increases from 400 to 600 °C the concentration of VZn increases, at intermediate temperatures from 600 to 800 °C the V′Zn concentration decreases, and finally, beyond 800 °C it increases again. These results are explained in terms of a grain-boundary defect model presented earlier [T. K. Gupta and W. G. Carlson, J. Mater. Sci. 20, 3487 (1987)]. The effect of quenching on PAS response was also explained in terms of a defect model.
Photodarkening effects in semiconductor-doped glasses
Authors: Van Wonterghem B. , Saltiel S. M. , Dutton T. E. , Rentzepis P. M.
Published: July 2002

The fading of the phase conjugation efficiency and luminescence quantum yield in CdSxSe1−x-doped glasses was investigated as a function of laser exposure using pulsed and cw mode-locked lasers. An analysis of the kinetics of the photodarkening effect is presented. The rate constant of the fading process depends nonlinearly upon the laser fluence and is sensitive to the origin of the glass.
Effect of sintering aids on the electrical properties of positive temperature coefficient of resistivity BaTiO3 ceramics
Authors: Cheng Hsiu-Fung
Published: July 2002

The effect of sintering aids SiO2, Al2O3, and TiO2 on the microstructure and electrical properties of Sb3+-doped BaTiO3 materials is examined. The SiO2 acts as a liquid-phase former, which not only enhances the atomic diffusion, but also purifies the grain interior and increases the grain conductivity. The room-temperature resistivity of the materials is lowered. An abundance of the liquid phase due to excess TiO2 will, however, result in a clustering of particles and the formation of a nonuniform microstructure. The Al3+ ions inhibit the grain growth and lead to a small and uniform grain size distribution. These ions also act as electron traps which increase the Schottky barrier height and in turn, raise the high-temperature resistivity of the samples.
The effect of Cr underlayer thickness on magnetic and structural properties of CoPtCr thin films
Authors: Johnson K. E. , Ivett P. R. , Timmons D. R. , Mirzamaani M. , Lambert S. E. , Yogi T.
Published: July 2002

Thin Co-based alloy films have shown important recording, magnetic, and structural changes when grown on Cr underlayers of different thicknesses. We have investigated these properties using several different CoPtCr compositions on Cr underlayers ranging from 0 to 200 nm in thickness. We report epitaxial growth of the hcp Co (11.0) planes on the (100) planes of bcc Cr for the first time on a disk appropriate for magnetic recording. The 〈11.0〉 Co preferred orientation occurs only when the Cr underlayer has a 〈100〉 preferred orientation. The 〈100〉 preferred orientation in the Cr layer results from the use of sputtering rates above 150 nm/min and only persists to thicknesses of about 50 nm. The thin Cr underlayers (<50 nm) with a 〈100〉 preferred orientation have fine well-packed grains. Thicker Cr underlayers have larger grains which are uncoupled and have a more random crystal orientation. The CoPtCr film morphologies follow these trends. The noise performance of these films improves with increasing Cr underlayer thickness and decreasing S* values. The CoPtCr films on thicker Cr (>50 nm) show no evidence of epitaxy and develop uncoupled grain structures like the Cr. The uncoupling of grains or the lack of in-plane c-axis orientation could both explain the drop in S* that has been correlated to S0/N improvement. Our results suggest that for the CoPtCr/Cr system, both grain morphology and crystallography must be considered in interpreting the changes in magnetic and recording properties.
Acousto-optic control and modulation of optical coherence by electronically synthesized holographic gratings
Authors: Turunen Jari , Tervonen Eero , Friberg Ari T.
Published: July 2002

We introduce a new technique to modify the coherence properties and the far-field intensity distributions of optical fields. Use is made of a combination of Bragg diffraction and Raman–Nath diffraction in an acousto-optic deflector. The high-frequency Bragg grating in the deflector is phase modulated by a synthetic low-frequency Raman–Nath regime grating that has a structure of an optimized phase-only computer-generated hologram. We demonstrate, e.g., electronic control of the state of spatial coherence of light, Gaussian to flat-top conversion, and the generation of arrays of equally intense but mutually incoherent light spots.
Shock induced polymorphic transition in quartz, carbon, and boron nitride
Authors: Tan Hua , Ahrens Thomas J.
Published: July 2002

A theory describing the polymorphism induced by shock waves in silicates, oxides, sulfides, and many inorganic solids is presented. Shock wave experiments conducted on these and other materials indicate that many transformations to high-pressure phases are triggered via the production of shear bands and, in some cases, formation of high-density amorphous phases. Shock states in the mixed phase regimes, of quartz, carbon, and boron nitride, are quantitatively described in terms of the properties of both their low- and high-pressure phases. Good agreement between the calculated results and measured Hugoniot data in the mixed phase regime is obtained. By fitting the pressures of the onset of the phase transition from graphite to diamond, and associating its triggering with crossing the extension of the metastable melting line of graphite, we obtain a similar shaped curve to the metastable melting line obtained by Bundy [J. Geophys. Res. 85, 6930 (1980)]. Similarly, the transition from quartz to stishovite is associated with the metastable melting line of coesite. The present theory, when fit to the onset of the mixed phase regime of graphitelike boron nitride transforming to cubic boron nitride Hugoniot, predicts the standard entropy for cubic BN to be 0.4–0.5 J/g K.
Optimal determination of the elastic constants of composite materials from ultrasonic wave-speed measurements
Authors: Castagnède Bernard , Jenkins James T. , Sachse Wolfgang , Baste Stéphane
Published: July 2002

A method is described to optimally determine the elastic constants of anisotropic solids from wave-speeds measurements in arbitrary nonprincipal planes. For such a problem, the characteristic equation is a degree-three polynomial which generally does not factorize. By developing and rearranging this polynomial, a nonlinear system of equations is obtained. The elastic constants are then recovered by minimizing a functional derived from this overdetermined system of equations. Calculations of the functional are given for two specific cases, i.e., the orthorhombic and the hexagonal symmetries. Some numerical results showing the efficiency of the algorithm are presented. A numerical method is also described for the recovery of the orientation of the principal acoustical axes. This problem is solved through a double-iterative numerical scheme. Numerical as well as experimental results are presented for a unidirectional composite material.
Defect formation in implantation of crystalline Si by MeV Si ions
Authors: Mäkinen J. , Punkka E. , Vehanen A. , Hautojärvi P. , Keinonen J. , Hautala M. , Rauhala E.
Published: July 2002

The distributions of vacancy-type defects and displaced Si atoms in Si(100) produced by the room-temperature implantation of 1014–1016 12-MeV 28Si+ ions/cm2 are measured with low-energy positron- and ion-beam techniques. The observed damage regions are reproduced by computer simulations. The distribution of displaced Si atoms coincides with the deposited energy distribution in elastic collisions. At the fluence of 1×1016 Si+/cm2, no crystalline structure was found in the peak region of the deposited energy at the depth of z=6 μm. Saturation of the divacancy concentration was observed at the ion fluences 3×1015 Si+/cm2 close to the surface (z<0.7 μm) and 3×1014 Si+/cm2 deeper in the sample (z>1 μm). In the region z<0.7 μm, the divacancy concentration is a factor of 4 higher than that in the region z>1 μm. This is also found in the simulated spatial structure of collision cascades.
Laser-induced spall in metals: Experiment and simulation
Authors: Eliezer Shalom , Gilath Irith , Bar-Noy Tuvia
Published: July 2002

Spall at ultra high strain rate (107 s−1) was investigated using short pulsed laser-induced shock waves in copper and aluminum foils. The intensities of the 3.9-ns Nd:Glass laser were in the range of 1010–1012 W/cm2, and the foil thickness was in the 100–600 μm range. The laser-generated shock wave pressure was in the range of a few hundred kilobars (kb). The shock wave traversed the foils in a few tens of nanoseconds. The controlled stepwise increase in laser energies allowed the stages of damage evolution from incipient to complete perforation of the target foils to be found. The energy threshold for spall and the spall width at that energy was measured as a function of the foil thickness for both materials. At threshold energy conditions, spall width of 25–65 μm for Al and 15–45 μm for Cu were obtained for foil thicknesses of 100–600 μm. Computer simulations of the laser-induced spall were performed, including the laser absorption, shock wave travel through the foil, and the spall phenomena. The simulations were based on the one-dimensional medusa code, which was expanded to include the spall phenomena, using simple spall criteria. An estimate of the strain rate was derived from the simulations and it was shown that the strain rates in the present experiments are about an order of magnitude larger than those obtained in spall experiments using other methods. The experimental results of energy threshold for spall and spall width at this energy were compared with the numerical simulations. The experimental results are in good agreement with the simulation results, indicating that spall strength for both materials, Al and Cu, are in the −50 to −60 kb regime.
Gas-phase kinetics in the atmospheric pressure chemical vapor deposition of silicon from silane and disilane
Authors: Giunta Carmen J. , McCurdy Richard J. , Chapple-Sokol Jonathan D. , Gordon Roy G.
Published: July 2002

A gas-phase reaction mechanism is proposed for the chemical vapor deposition (CVD) of amorphous silicon from silane or disilane at atmospheric pressure. The gas stream in the CVD reactor is populated by silanes, silylenes, and disilenes in a variety of sizes. Silylenes form by the decomposition of silanes, and they rapidly insert into other silanes to form larger silanes. Although silylenes are expected to stick to growth surfaces to which they diffuse, they are too reactive in the gas phase to deliver a large flux onto the growth surface. Larger silylenes (SiH3SiH and larger) also isomerize to form less reactive disilenes, which we propose to be principally responsible for film growth. Film profiles observed in depositions from silane and disilane are presented, and computed film profiles are compared to these observations. Deposition from silane is explained quite well by the mechanism, as are some qualitative features of deposition from disilane.
KrF/H2 Raman conversion at high repetition rate using a hydrogen gas circulating system
Authors: Lou Qihong , Yagi Takashi , Saito Hideaki
Published: July 2002

A decrease of Raman conversion efficiency pumped by KrF excimer laser at high repetition rate was observed in a sealed hydrogen Raman cell. A hydrogen gas circulating system was designed with gas pressure up to 50 atm; it can be operated at a high repetition rate without decrease of Raman conversion efficiency.
Transport mean free path tabulated for the multiple elastic scattering of electrons and positrons at energies ≤20 MeV
Authors: Liljequist D. , Ismail M. , Salvat F. , Mayol R. , Martinez J. D.
Published: July 2002

The transport mean free path, or transport cross section, is tabulated for the elastic scattering of electrons and positrons in solid matter by means of a correction factor tc applied to the result of a simple screened Rutherford cross section. The correction factor table is based on differential cross-section calculations using a combination of partial-wave analysis (PWA), the Wentzel–Kramers–Brillouin approximation, and the Born approximation, and covers kinetic energies from 100 eV to 20 MeV. Results at low energies are compared with PWA calculations of higher accuracy. Nuclear size effects are discussed but not explicitly included. Applications are discussed.
Plasma sheath thickness in radio-frequency discharges
Authors: Mutsukura Nobuki , Kobayashi Kenji , Machi Yoshio
Published: July 2002

The radio-frequency glow discharges of several kinds of gases were examined to measure the ion sheath thickness at the cathode electrode. At intermediate gas pressures around 0.05– 0.5 Torr, the sheath thickness d depends on the pressure P in the expression P1/2d=K0 for almost all of the discharges examined. It was also pointed out that the constant K0 value decreased linearly against a mass of the predominant ion in the plasma. The discrepancy between the sheath thickness measured in this work and by theoretical solution was discussed for the argon discharge.
Photoreflectance characterization of AlGaAs/GaAs modulation-doped heterostructures
Authors: Pan N. , Zheng X. L. , Hendriks H. , Carter J.
Published: July 2002

Photoreflectance (PR) and reflectance have been applied to characterize undoped and modulation-doped heterostructures of AlGaAs/GaAs grown by metal-organic chemical vapor deposition. The PR spectra were taken on these samples after sequential etching steps in a phosphoric acid etch to study the effects of the surface electric field, the heterointerface, and the two-dimensional electron gas. PR spectra were also taken with an external electric field applied through a transparent gate electrode. The results show that the oscillations appearing near the bandgap energy of GaAs are Franz–Keldysh oscillations originating from the large surface electric field. The surface electric field of the heterostructures can be modified through the application of an external electric field or by etching. The oscillation period is observed to increase with increasing reverse bias or with etching of the GaAs cap layer and the PR features disappear at a forward bias of 0.45 V. The very sharp features associated with the GaAs bandgap energy after etching have also been verified to be Franz–Keldysh oscillations and the presence of a two-dimensional electron gas cannot be confirmed with PR.
Low-energy elastic scattering of electrons by bound silicon and germanium atoms
Authors: Meredith R. J. , Williamson W. , Montemayor V. J. , Öztürk N. , Antolak A. J.
Published: July 2002

Elastic differential and total cross sections were computed for 10 eV to 1 keV electrons scattered from bound silicon and germanium atoms using the method of partial waves. The effects of including combinations of static, polarization, and exchange potentials in the optical potential for partial wave analysis were investigated. A parameterization of our total cross section results is also provided in terms of the screened Rutherford cross section.
Optical study on domain switching of ferroelectric triglycine sulfuric acid and Rochelle salt
Authors: Kobayashi J. , Uchino K. , Matsuyama H. , Saito K.
Published: July 2002

The domain switching of ferroelectric triglycine sulfuric acid and Rochelle salt was studied by observing optical gyration along the ferroelectric axes and measuring rotation of the indicatrices around the same axes. It was established experimentally that the antipolar domains of triglycine sulfuric acid were united by a symmetric twin mechanism and those of Rochelle salt by a hemitrope (rotation twin). It was shown that the high-accuracy universal polarimeter method can become a new general method for distinguishing the twinning mechanism of the ferroelectric domains.