bluuebunny/crossref_metadata_2025 huggingface dataset:
Degradation of dielectric breakdown field of thermal SiO2 films due to structural defects in Czochralski silicon substrates
Authors: Satoh Y. , Shiota T. , Murakami Y. , Shingyouji T. , Furuya H.
Published: July 2002

We used heat treatment to intentionally introduce various structural defects in Czochralski silicon substrates. The type, size, and number density of the induced defects were surveyed with transmission electron microscopy, and the defects were then incorporated into SiO2 films (10–50 nm thick) during thermal oxidation in dry O2. The effect of the defects on dielectric strength of the SiO2 films was examined with a time zero dielectric breakdown method. Larger platelet oxygen precipitates caused greater decreases of the breakdown field, and precipitates smaller than the SiO2 film thickness did not appreciably reduce the breakdown field. Every large platelet oxygen precipitate incorporated in the SiO2 film caused a degradation. Octahedral oxygen precipitates caused little degradation. The breakdown field was higher than 7 MV/cm and did not depend much on the SiO2 film thickness and precipitate size. We discussed possible mechanisms for the degradation due to both kinds of precipitates. Oxidation-induced stacking faults formed by a surface oxidation did not markedly reduce the breakdown field when only segments of dislocations and stacking faults were incorporated in the SiO2 film. Another serious degradation was caused by pits that were formed by dissolving octahedral oxygen precipitates in a HF solution. The breakdown field was lower for thicker oxide films, and it recovered as the pit shape became smoother during chemical etching. We proposed that this degradation was caused by a local thinning of SiO2 film due to stress generated in the oxidation of pits. These results suggest that voids rather than the other reported grown-in defects play the most important role in the degradation observed for as-grown silicon.
Mechanisms behind green photoluminescence in ZnO phosphor powders
Authors: Vanheusden K. , Warren W. L. , Seager C. H. , Tallant D. R. , Voigt J. A. , Gnade B. E.
Published: July 2002

We explore the interrelationships between the green 510 nm emission, the free-carrier concentration, and the paramagnetic oxygen-vacancy density in commercial ZnO phosphors by combining photoluminescence, optical-absorption, and electron-paramagnetic-resonance spectroscopies. We find that the green emission intensity is strongly influenced by free-carrier depletion at the particle surface, particularly for small particles and/or low doping. Our data suggest that the singly ionized oxygen vacancy is responsible for the green emission in ZnO; this emission results from the recombination of a photogenerated hole with the singly ionized charge state of this defect.
Electrical characterization of n-amorphous/p-crystalline silicon heterojunctions
Authors: Marsal L. F. , Pallarès J. , Correig X. , Calderer J. , Alcubilla R.
Published: July 2002

n-type amorphous silicon on p-type crystalline silicon heterojunction diodes were fabricated and electrically characterized. The a-Si:H film was deposited by plasma enhanced chemical vapor deposition. Electrical properties were investigated by capacitance–voltage and current–voltage measurements at different temperatures. The capacitance–voltage results confirm an abrupt heterojunction. Current–voltage characteristics show good rectifying properties (50000:1 at ±0.5 V). A detailed analysis of transport mechanisms was developed in order to establish a unified model of conduction. Two carrier transport mechanisms are believed to be at the origin of the forward current. At low bias voltage (V<0.4 V), the current is determined by recombination at the amorphous side of the space charge region, while at higher voltages (V≳0.6 V), the current becomes space charge limited.
The origin of photoluminescence from thin films of silicon-rich silica
Authors: Kenyon A. J. , Trwoga P. F. , Pitt C. W. , Rehm G.
Published: July 2002

We have carried out a study of the photoluminescence properties of silicon-rich silica. A series of films grown using plasma enhanced chemical vapor deposition over a range of growth conditions were annealed under argon at selected temperatures. Photoluminescence spectra were measured for each film at room temperature and for selected films at cryogenic temperatures. The photoluminescence spectra exhibit two bands. Fourier transform infrared and electron spin resonance spectroscopies were used to investigate bonding and defect states within the films. The data obtained strongly suggest the presence of two luminescence mechanisms which exhibit different dependencies on film growth conditions and postprocessing. We make assignments of the two mechanisms as (1) defect luminescence associated with oxygen vacancies and (2) radiative recombination of electron-hole pairs confined within nanometer-size silicon clusters (‘‘quantum confinement’’).
Emission studies of the gas-phase oxidation of Mn during pulsed laser deposition of manganates in O2 and N2O atmospheres
Authors: Lecoeur P. , Gupta A. , Duncombe P. R. , Gong G. Q. , Xiao Gang
Published: July 2002

The plasma produced during pulsed laser deposition of manganate films has been probed using optical emission spectroscopy. The studies have been carried out using Mn, Mn2O3, and La0.67Sr0.33MnO3 (LSMO) as target materials in the presence of two different oxidizing gases: nitrous oxide (N2O) and oxygen (O2). Emission from excited MnO (MnO*) has been observed in all cases resulting primarily from reaction of the ablated Mn atoms with the background gas. Consistent with the oxidation reaction energetics, the emission intensity from MnO* is found to be about an order of magnitude stronger with N2O than with O2. Magnetization measurements of LSMO films show improved magnetic properties of films prepared in N2O compared to O2 at low pressures. The improvement in film quality can be attributed, at least in part, to the increased oxidation of Mn in the plasma plume.
Comment on ‘‘Temperature limits on infrared detectivities of InAs/InxGa1−xSb superlattices and bulk Hg1−xCdxTe’’ [J. Appl. Phys. 74, 4774 (1993)]
Authors: Piotrowski J. , Rogalski A.
Published: July 2002

Three papers published recently [P. M. Young, C. H. Grein, H. Ehrenreich, and R. H. Miles, J. Appl. Phys. 74, 4774 (1993); G. M. Williams, J. Appl. Phys. 77, 4153 (1995); C. H. Grein, M. E. Flatte, H. Ehrenreich, and R. H. Miles, J. Appl. Phys. 77, 4156 (1995)] concerning the comparison of theoretically predicted performance of HgCdTe photodiodes with InAs/InGaSb superlattice photovoltaic detectors are obscure with respect to optimal selection of HgCdTe photodiode structures. Both Comments [G. M. Williams, J. Appl. Phys. 77, 4153 (1995); C. H. Grein, M. E. Flatte, H. Ehrenreich, and R. H. Miles, J. Appl. Phys. 77, 4156 (1995)] have not noticed the important results of papers published by Humpreys [Infrared Phys. 23, 171 (1983); Infrared Phys. 26, 337 (1986)], who critically reexamined the role of a radiative mechanism in the detection of infrared radiation. To explain our point of view on competition between InAs/InGaSb SLs and ‘‘bulk’’ HgCdTe detectors, we present a generalized model of an infrared photodetector and derive the figure merit of any material for infrared photodetector as the ratio of absorption coefficient to the thermal generation rate. This determines the detectivity of infrared photodetectors. Using that model for the most common n+-p and p+-n long wavelength HgCdTe photodiodes, it is shown that the highest performance can be obtained with a low doping of the base photodiode regions. This means that the previously assumed (Young et al.) highly doped HgCdTe photodiode structure with a thick base region is far from optimal. Our calculations carried out for optimal device structures indicate that the ultimate detectivities of long wavelength HgCdTe photodiodes operating at 77 K are higher than those for InAs/InGaSb SLs.
Composition of AlGaAs
Authors: Wasilewski Z. R. , Dion M. M. , Lockwood D. J. , Poole P. , Streater R. W. , SpringThorpe A. J.
Published: July 2002

Although the AlxGa1−xAs alloy system has been extensively investigated, there are still considerable uncertainties in measuring the value of x. Here a new AlxGa1−xAs calibration structure, grown by molecular beam epitaxy, has been used to establish unambiguous alloy compositions. Such “standard’’ AlxGa1−xAs layers were measured by high-resolution x-ray diffraction, photoluminescence, and Raman spectroscopy to determine the compositional variations of the measured physical parameters. The phenomenological equations derived from these measurements can now be used to establish the Al content of unknown alloys with confidence. In addition, the results show that Vegard’s law does not hold for the variation of the AlxGa1−xAs lattice constant with x. The small quadratic term has very important implications for a correct analysis of x-ray results.
Single-crystal Pb(ZrxTi1−x)O3 thin films prepared by metal-organic chemical vapor deposition: Systematic compositional variation of electronic and optical properties
Authors: Foster C. M. , Bai G.-R. , Csencsits R. , Vetrone J. , Jammy R. , Wills L. A. , Carr E. , Amano Jun
Published: July 2002

Single-crystal thin films of Pb(ZrxTi1−x)O3 (PZT) covering the full compositional range (0⩽x⩽1) were deposited by metal-organic chemical vapor deposition. Epitaxial SrRuO3(001) thin films grown on SrTiO3(001) substrates by rf-magnetron sputter deposition served as template electrode layers to promote the epitaxial growth of PZT. X-ray diffraction, energy-dispersive x-ray spectroscopy, atomic force microscopy, transmission electron microscopy, and optical waveguiding were used to characterize the crystalline structure, composition, surface morphology, microstructure, refractive index, and film thickness of the deposited films. The PZT films were single crystalline for all compositions exhibiting cube-on-cube growth epitaxy with the substrate and showed very high degrees of crystallinity and orientation. The films exhibited typical root mean square surface roughness of ∼1.0–2.5 nm. For tetragonal films, the surface morphology was dominated by grain tilting resulting from ferroelectric domain formation. We report the systematic compositional variation of the optical, dielectric, polarization, and electronic transport properties of these single-crystalline PZT thin films. We show that the solid-solution phase diagram of the PZT system for thin films differs from the bulk due to epitaxy-induced strains and interfacial defect formation. High values of remanant polarization (30–55 μC/cm2) were observed for ferroelectric compositions in the range of 0.8⩽x⩽0.2. Unlike previous studies, the dielectric constant exhibited a clear dependence on composition with values ranging from 225 to 650. The coercive fields decreased with increasing Zr concentration to a minimum of 20 kV/cm for x=0.8. The undoped films exhibited both high resistivity and dielectric-breakdown strength (1013–1014 Ω cm at 100 kV/cm and 300–700 kV/cm, respectively).
Very thin Fe/Ni modulation multilayer films under ion bombardment
Authors: Amaral L. , Scorzelli R. B. , Brückman M. E. , Paesano A. , Schmidt J. E. , Shinjo T. , Hosoito N.
Published: July 2002

We investigated the effect of noble gas irradiation (He, Ne, and Xe) on Fe–Ni multilayers with a very thin modulation and nominal composition in the Invar region Fe0.63Ni0.37. The evaluation of the formation/stability of the Fe–Ni phases formed under irradiation with different ions and doses was followed by conversion electron Mössbauer spectroscopy. The magnetic hysteresis curves were also obtained in order to correlate the hyperfine pattern with magnetic properties. The as-deposited sample reveals mainly the characteristic α-Fe while He- and Ne-irradiated samples clearly show a phase transformation with segregation of γ-FeNi phases with different Ni concentrations, a magnetic atomically ordered phase (∼50% Ni), and a nonmagnetic phase (⩽30% Ni). However, mixing with Ne is more effective than with He for similar doses. The results obtained with Xe showed a large distribution of hyperfine fields similarly to previous results reported for Kr [C. Tosello, F. Ferrari, R. Brand, W. Keune, G. Marest, M. A. El Khakani, J. Parellada, G. Principi, S. Lo Russo, V. Rigato, and S. Enzo, Nucl. Instrum. Methods B 80/81, 417 (1993)].
A noise detection scheme with 10 mK noise temperature resolution for semiconductor single electron tunneling devices
Authors: Glattli D. C. , Jacques P. , Kumar A. , Pari P. , Saminadayar L.
Published: July 2002

The experimental requirements for low frequency electrical noise measurements of single electron tunneling (SET) semiconductor samples are discussed and a two-channel cross-correlation spectrum analysis method combined with ultralow noise detection is proposed which gives 10 mK noise temperature resolution. We emphasize the effect of the high frequency photon radiation originating from the external circuit which may strongly affect sub-kelvin low frequency noise measurements if not filtered. We quantitatively show that hot photons are important as a mechanism of activation of electron traps for semiconductor SET devices, increasing low frequency 1/f or telegraphic noise, and as a mechanism to increase the electronic temperature. We describe a new type of cryogenic coaxes whose attenuation drastically thermalizes the photons. As a check, we show for the first time resonant tunneling peak linewidths in the Coulomb blockade regime decreasing with temperature down to ≃60 mK.
Current suppression and harmonic generation by intense terahertz fields in semiconductor superlattices
Authors: Lei X. L.
Published: July 2002

The time-dependent current response of superlattice miniband conduction to a strong submillimeter wave electric field of varying frequency and strength is investigated at lattice temperature T=77 and 300 K by means of the three-dimensional balance-equation method with accurate microscopic treatment of impurity and phonon scatterings. The terahertz-radiation-induced current suppression and the generated third harmonic power as functions of the input power of the high-frequency field predicted theoretically are in reasonable agreement with recent experimental findings in heavily doped GaAs superlattices.
Optical, structural, and electrical properties of indium oxide thin films prepared by the sol-gel method
Authors: Bel Hadj Tahar Radhouane , Ban Takayuki , Ohya Yutaka , Takahashi Yasutaka
Published: July 2002

This article reports the fabrication of thin films of In2O3 from an aqueous sol and an organic solution. The stability of the aqueous sol with respect to the concentration of indium and pH are discussed. Thermo-gravimetric and differential thermal analysis were performed on the dried sol particles as well as the gel for better understanding of thermal events occurring during the sintering process. Microstructure, phase purity, optical, and electrical properties of the thin films deposited on glass substrates by dip-coating process from aqueous sol and organic solution were studied by transmission electron microscopy, x-ray diffractometry, visible light spectrometry, and the four-probe method, respectively. The electrical properties of the films derived from the two systems are discussed based on the differences in their morphologies.
Identification of the symmetry of phonon modes in CsPbCl3 in phase IV by Raman and resonance-Raman scattering
Authors: Calistru Dana M. , Mihut L. , Lefrant S. , Baltog I.
Published: July 2002

The first- and second-order Raman phonon spectra of CsPbCl3 were measured in phase IV using Raman and resonance-Raman scattering. Twelve Raman active phonons were resolved at 32, 35.5, 42, 52, 72, 90, 110, 115, 121, 189, 200.5, and 375 cm−1 and correlated to the phonon spectrum of CsPbCl3 in phase I. The highest frequency longitudinal optical mode is 375 cm−1.
Low temperature metal induced crystallization of amorphous silicon using a Ni solution
Authors: Yoon Soo Young , Kim Ki Hyung , Kim Chae Ok , Oh Jae Young , Jang Jin
Published: July 2002

Amorphous silicon (a-Si) was crystallized by metal-induced crystallization (MIC) using a Ni standard absorption solution. The a-Si films spin-coated with a 5000 ppm Ni solution were crystallized at as low as 500 °C. Needlelike morphology, developed as a result of the migration of NiSi2 precipitates, appears in the MIC poly-Si. The growth of the needlelike crystallites proceeds to a direction parallel to 〈111〉. The a-Si can be fully crystallized at 500 °C for 20 h.
Cation disorder and size effects upon magnetic transitions in Ln0.5A0.5MnO3 manganites
Authors: Damay F. , Martin C. , Maignan A. , Raveau B.
Published: July 2002

A systematic study of the magnetic properties of the manganites Ln0.5(A,A′)0.5MnO3 (A,A′=Ba, Ca, Sr) has been carried out. The variations of TC and TN vs two parameters, the average size of the interpolated cations 〈rA〉 and the mismatch effect represented by the variance σ2, have been studied for the series of manganites La0.5(A,A′)0.5MnO3, Pr0.5(A,A′)0.5MnO3, Nd0.5(A,A′)0.5MnO3 and Sm0.5(A,A′)0.5MnO3 (A,A′=Ba, Ca, Sr), involving at most three different cations on the perovskite A site. The results obtained for other (Ln, Ln′)0.5Sr0.5MnO3(Ln, Ln′=Gd, Sm Y, Pr) have been taken into consideration. A σ2−〈rA〉 diagram has been established, which displays the different types of magnetic behaviors that can be exhibited by Ln0.5A0.5MnO3 manganites, i.e., when the temperature is decreasing, either a paramagnetic (PM) to ferromagnetic (FM) transition, or PM to FM followed by FM to antiferromagnetic transitions, or a PM to weak ferromagnetic transition.
Electronic transport in tin-doped indium oxide thin films prepared by sol-gel technique
Authors: Bel Hadj Tahar Radhouane , Ban Takayuki , Ohya Yutaka , Takahashi Yasutaka
Published: July 2002

Hall effect measurements have been performed on tin-doped indium oxide thin films prepared by the dipping method. The apparent variation of electron mobility with carrier concentration in these films has been quantitatively interpreted in terms of scattering at charged and neutral impurities. An electrical resistivity as low as 3.2×10−4 Ω cm was obtained for the films doped with about 6 at. % Sn. A further increase in the doping content lead to the formation of neutral defects without contributing carriers to the material.
H
−
laser photodetachment at 1064, 532, and 355 nm in plasma
Authors: Nishiura M. , Sasao M. , Bacal M.
Published: July 2002

The fundamental frequency (1064 nm), the second harmonic (532 nm), and the third harmonic (355 nm) of a Nd yttrium aluminum garnet laser have been used to investigate the influence of the energies of photodetached electrons upon the determination of the H− density and the H− drift velocity in a hydrogen plasma, where the electron density and temperature range from 1010 cm−3 to 1011 cm−3 and from 0.5 to 3 eV, respectively, with the n−/ne ratio less than 2%. From the δIp−Vp characteristics, where δIp is the photodetachment current and Vp is the probe voltage, it is found that there is no dependence of the energies of photodetached electrons on the photon energies. The effective temperature of photodetached electrons Teff is close, within 40%, to that of background electrons. As a result the H− density and the H− drift velocity are not affected by photon energies.
The effect of grain and particle size on the microwave properties of barium titanate (BaTiO3)
Authors: McNeal Mark P. , Jang Sei-Joo , Newnham Robert E.
Published: July 2002

The use of ferroelectric ceramics and thin films in microwave devices requires that they possess frequency-stable, low-loss dielectric properties. At microwave frequencies, ferroelectric polycrystalline ceramic materials typically exhibit a large dielectric relaxation, characterized by a decrease in the relative permittivity (εr) and a peak in the dielectric loss (tan δ). Mechanisms attributed to the relaxation phenomenon include piezoelectric resonance of grains and domains, inertia to domain wall movement, and the emission of gigahertz shear waves from ferroelastic domain walls. As a result, the relaxation phenomenon appears to be intimately linked to the domain state of the ferroelectric. The domain state of a ferroelectric is, in part, dependent upon its microstructure. In this study, the microwave dielectric properties of ferroelectric barium titanate were measured as a function of grain and particle size. Polycrystalline ceramic ferroelectric BaTiO3 (having average grain sizes of 14.4, 2.14, and 0.26 μm) and BaTiO3 powder-polymer matrix composites (possessing average particle sizes of 1.33 μm, 0.19 μm, and ∼66 nm) were employed. The composite samples were used to decouple resonances between adjacent grains as well as reduce the three dimensional clamping experienced by grains in ceramic. Characterization studies were performed to determine the effects of grain size and particle size on the crystal structure and degree of tetragonality. Microwave dielectric measurements through 6 GHz were carried out using lumped impedance, cavity perturbation, and post resonance experiments. All samples exhibited evidence of relaxation or resonance phenomena in their dielectric spectra. Except for the 0.26 μm grain size ceramics and the 66 nm particle size composites, all other samples exhibited relaxation in their dielectric spectra. The 0.26 μm ceramic and 66 nm composite showed evidence of resonance in their dielectric spectra. This work clearly shows the potential to tune the microwave properties of ferroelectrics through control of grain/particle size and the domain state. In general, relaxation frequencies increased and loss tangents decreased with decreasing grain/particle size. The relaxation mechanisms were identified and correlated with the material characterization results and theoretical models. Relaxation frequencies were generally governed by the smallest resonant width, i.e., the domain width.
Modeling of an axially inhomogeneous microwave argon plasma column at a moderate pressure
Authors: Benova E. , Petrova Ts. , Blagoev A. , Zhelyazkov I.
Published: July 2002

In this article, we present theoretical results for axial distributions of charged particles and excited atoms in an axially inhomogeneous microwave sustained argon plasma column at a pressure of 1.8 Torr. The model includes two self-consistently linked parts—electrodynamic and kinetic ones. It is found that the electron temperature is not a constant along the column length. The dependence of the mean power required for sustaining an electron–ion pair in the discharge on the electron number density is obtained taking into account different elementary processes. The theoretical results are in agreement with the available experimental data.
Electro-optic properties in pure LiNbO3 crystals from the congruent to the stoichiometric composition
Authors: Abdi F. , Aillerie M. , Bourson P. , Fontana M. D. , Polgar K.
Published: July 2002

We report large and nonmonotonic dependences of the electro-optic (EO) coefficient r22 and the corresponding dielectric permittivity in pure LiNbO3 on the concentration of intrinsic defects related to the deviation from the stoichiometry of the crystal. This variation is caused mainly by the electro-mechanical contribution. A stoichiometric crystal possesses both large EO properties and resistance to optical damage which renders it much more suitable for use in laser Q-switching than the congruent crystal.
Effective thermal diffusivity of layered materials measured by modulated photothermal techniques
Authors: Salazar A. , Sánchez-Lavega A. , Terrón J. M.
Published: July 2002

Modulated photothermal techniques provide useful methods based on linear relations to measure the thermal diffusivity of homogeneous materials. We have analyzed theoretically the applicability of such linear relations to two particular cases of layered composites, i.e., two-layer materials and superlattices. In order to measure the through-thickness and in-plane thermal diffusivities of these anisotropic materials, planar and pointlike excitations have been studied. The main result of this article is that the linear relations encountered for homogeneous materials still hold for layered composites, although their slopes do not always give the effective thermal diffusivities of the sample parallel and perpendicular to the layers, as derived from the in-parallel and in-series thermal resistors models, respectively. However, an “apparent” thermal diffusivity is obtained from which the thermal parameters of each layer can be retrieved.
Effects of thermal processes on photoluminescence of silicon nanocrystallites prepared by pulsed laser ablation
Authors: Umezu Ikurou , Shibata Kosaku , Yamaguchi Shoji , Sugimura Akira , Yamada Yuka , Yoshida Takehito
Published: July 2002

The temperature dependence of the photoluminescence (PL) spectra of Si nanocrystallites prepared using an inert-gas-ambient pulsed laser ablation technique was characterized. Although the PL intensity of as-deposited Si nanocrystallites was very weak at room temperature, it increased with annealing in N2 or O2 gas. The PL intensity of the sample annealed in O2 gas was the largest at room temperature since the reduction in intensity with increased temperature was small. The nonradiative recombination process is discussed in terms of the temperature dependence of the PL intensity. Our results suggest that annealing of Si nanocrystallites in O2 gas reduces the nonradiative recombination of electron–hole pairs.
The effect of surface roughness on direct wafer bonding
Authors: Gui C. , Elwenspoek M. , Tas N. , Gardeniers J. G. E.
Published: July 2002

A theory is presented which describes the initial direct wafer bonding process. The effect of surface microroughness on the bondability is studied on the basis of the theory of contact and adhesion of elastic solids. An effective bonding energy, the maximum of which is the specific surface energy of adhesion, is proposed to describe the real binding energy of the bonding interface, including the influence of the wafer surface microroughness. Both the effective bonding energy and the real area of contact between rough surfaces depend on a dimensionless surface adhesion parameter, θ. Using the adhesion parameter as a measure, three kinds of wafer contact interfaces can be identified with respect to their bondability; viz. the nonbonding regime (θ >12), the bonding regime (θ <1), and the adherence regime (1<θ <12). Experimental data are in reasonable agreement with this theory.
Structural and tribological characterization of protective amorphous diamond-like carbon and amorphous CNx overcoats for next generation hard disks
Authors: Scharf T. W. , Ott R. D. , Yang D. , Barnard J. A.
Published: July 2002

Further insight into processing-structure-property relationships have been carried out for existing and candidate carbon-based protective overcoats used in the magnetic recording industry. Specifically, 5 nm thick amorphous diamond-like carbon (a:C) and nitrogenated diamond-like carbon (a:CNx) overcoats were deposited by low deposition rate sputtering onto a thin film disk consisting of either CoCrPt/CrV/NiP/AlMg or CoCrPt/CrV/glass. The wear durability and frictional behavior of these hard disks were ascertained using a recently developed depth sensing reciprocating nanoscratch test. It was determined that the CN0.14/CoCrPt/CrV/glass disk exhibited the most wear resistance, least amount of plastic deformation, and lowest kinetic friction coefficient after the last wear event. Core level x-ray photoelectron spectroscopy (XPS) results of sputter cleaned overcoats indicated that nitrogen up to 14 at. % incorporated into the amorphous network resulted in these improvements near the overcoat/magnetic layer interface, since there was an increase in the number of N-sp3 C bonded sites in a predominantly N-sp2 C bonded matrix. However, nonsputter cleaned overcoats exhibited a more graphitic pyridine-like (nondoping configuration) structure near the surface as evidenced by the increase in C=N versus C–N bonds and the valence band XPS determined appearance of the 2p-π band near the Fermi level (EF). Therefore, XPS sputter cleaning revealed a gradient in the chemical nature of the overcoats through the thickness. In addition, micro-Raman spectroscopy established that a further increase of nitrogen (⩾18 at. %) weakened the overcoat structure due to the formation of terminated sites in the amorphous carbon network, since nitrogen failed to connect the sp2 domains within the network. This, in conjunction with an increase in the intensity of the 2p-π band from the valence band XPS spectra and the increase in the G-band position and ID/IG ratio from the Raman spectra, confirmed the increase in the size and number of sp2 bonds in the CN0.18 overcoat.
Reduction of laser imprinting using polarization smoothing on a solid-state fusion laser
Authors: Boehly T. R. , Smalyuk V. A. , Meyerhofer D. D. , Knauer J. P. , Bradley D. K. , Craxton R. S. , Guardalben M. J. , Skupsky S. , Kessler T. J.
Published: July 2002

We demonstrate a laser beam-smoothing technique known as polarization smoothing. A birefringent optical wedge splits the individual laser beams into two orthogonally polarized beams that, when coupled with a distributed phase plate, produce two speckle patterns shifted with respect to one another. This instantaneously reduces the on-target nonuniformity by a factor of √. We measured this reduction optically and its effect is demonstrated in laser-driven targets.
The crucial role of particle morphology in the magnetic properties of haematite
Authors: Sorescu M. , Brand R. A. , Mihaila-Tarabasanu D. , Diamandescu L.
Published: July 2002

Haematite particles of four different morphologies (polyhedral, platelike, needlelike and disk shaped) were synthesized by the hydrothermal method. The morphology and average particle diameter (1.4, 7.4, 0.2, and 0.12 μm, respectively) were determined by transmission electron microscopy combined with electron diffraction. The haematite samples were studied by transmission Mössbauer spectroscopy in the temperature range 4.2–300 K. In all cases, a weak ferromagnetic (WF) phase was present above the Morin temperature of 230 K and found to coexist with an antiferromagnetic (AF) phase below this temperature. However, the populations of the two phases at 230 K were demonstrated to depend on the morphology of the particles. Moreover, the WF and AF phases exhibit a different dependence of the magnetic texture on temperature and particle morphology.
Synthesis and magnetic properties of a novel ferrite organogel
Authors: Li Sichu , John Vijay T. , Irvin Glen C. , Rachakonda Suguna H. , McPherson Gary L. , O’Connor Charles J.
Published: July 2002

A novel magnetic organogel that can be considered a precursor example of a magnetoresponsive gel is reported. The gel is formed by the bridging of ferrite containing anionic bis(2-ethlhexyl) sodium sulfosuccinate reverse micelles with 2,6-dihydroxynaphthalene (2,6-DHN). The addition of 2,6-DHN leads to a room temperature quotes “freezing in” of the liquid solution to a clear organogel. Ferrite particles in the size range 10–15 nm are doped into the gel network and are thus suspended in the optically clear gel media. The magnetic properties of the gel were measured using a superconducting quantum interference device magnetometer. The results reveal that the gel exhibits superparamagnetic behavior with a blocking temperature of 6 K (at an applied field of 1000 G), and a coercivity of 850 G at 2 K. The ferrites introduced into the gel serve the function of magnetic “seeds” via which magnetic properties are acquired by the gel.
Domain configurations of nanostructured Permalloy elements
Authors: Gomez R. D. , Luu T. V. , Pak A. O. , Kirk K. J. , Chapman J. N.
Published: July 2002

The magnetization distributions of an array of small NiFe elements were studied using Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). The dependence of the domain configurations at zero field as a function of the aspect ratio was observed using MFM, and confirms the earlier observations using LTEM. Comparison of the images of similar islands using both techniques elucidate the complementarity between the LTEM and MFM measurements which individually show different facets of the magnetization distributions on soft magnetic thin films.
Investigation of DX center in silicon doped GaAs–AlAs short period superlattices
Authors: Bosc F. , Sicart J. , Robert J. L.
Published: July 2002

Electrical properties of various GaAs–AlAs short period superlattices uniformly doped with silicon are investigated. Hydrostatic pressure is used to capture electrons from the conduction miniband onto the deep donor states associated with the large lattice relaxation of the Si donor in GaAs, AlAs, and at the GaAs–AlAs interface. The ionization energies and densities of different deep donors are determined. We find that a two-band model of conduction, assuming the existence of three donor states (discrete structure of the Si-DX center), accounts for the Hall data in all samples. Application of hydrostatic pressure allows us to vary the miniband energy in some samples and leads to improved determination of the donor energies and concentrations. When the AlAs growth rate is half of the GaAs growth rate, the fraction of DX states decreases in AlAs, whereas the fraction of DX states increases at the interface indicating a strong Ga/Al cation exchange across the interface.
Ultrashort-pulse laser machining of dielectric materials
Authors: Perry M. D. , Stuart B. C. , Banks P. S. , Feit M. D. , Yanovsky V. , Rubenchik A. M.
Published: July 2002

There is a strong deviation from the usual τ1/2 scaling of laser damage fluence for pulses below 10 ps in dielectric materials. This behavior is a result of the transition from a thermally dominated damage mechanism to one dominated by plasma formation on a time scale too short for significant energy transfer to the lattice. This new mechanism of damage (material removal) is accompanied by a qualitative change in the morphology of the interaction site and essentially no collateral damage. High precision machining of all dielectrics (oxides, fluorides, explosives, teeth, glasses, ceramics, SiC, etc.) with no thermal shock or distortion of the remaining material by this mechanism is described.
Andreev reflection: A new means to determine the spin polarization of ferromagnetic materials
Authors: Soulen R. J. , Osofsky M. S. , Nadgorny B. , Ambrose T. , Broussard P. , Cheng S. F. , Byers J. , Tanaka C. T. , Nowack J. , Moodera J. S. , Laprade G. , Barry A. , Coey M. D.
Published: July 2002

We have developed a simple method to measure the transport spin polarization of ferromagnetic materials. This technique relies on the fact that the Andreev reflection process at the interface between a superconductive and normal is influenced by the spin polarization P of the normal metal. In a very short time we have been able to measure the spin polarization of several metals: NixFe1−x, Ni, Co, Fe, NiMnSb, La0.7Sr0.3MnO3, and CrO2, whose spin polarization ranges from 35% to 90%. Our results compare well with other methods for measuring P.
Analysis of failure mechanisms in electrically stressed Au nanowires
Authors: Durkan C. , Schneider M. A. , Welland M. E.
Published: July 2002

An analysis of polycrystalline Au thin film interconnects of widths ranging from 850 to 25 nm, and lengths ranging from 1.0 μm to 20 nm which have been electrically stressed to the point of failure is presented. For the longer wires (widths 60–850 nm), the failure current density is typically found to be 1012 A m−2, essentially independent of the wire width, and then rapidly approaching zero for thinner wires. For the wider wires, failure occurs at the end towards the negative electrode; for narrow wires, failure tends to occur towards the center of the wire, as observed using scanning electron microscopy and atomic force microscopy. The mean time to failure for fixed current density is seen to decrease with decreasing wire width. The failure current density for a given wire width increases as the length decreases. An analysis of the temperature profile based on calculations of a simple model is presented which shows that this width-dependent behavior of narrow lines is not anticipated from the assumption of a homogeneous line subject to thermally-assisted electromigration alone.
Nonlinear piezoelectric behavior of ceramic bending mode actuators under strong electric fields
Authors: Wang Qing-Ming , Zhang Qiming , Xu Baomin , Liu Ruibin , Cross L. Eric
Published: July 2002

The nonlinear electromechanical behavior of cantilevered piezoelectric ceramic bimorph, unimorph, and reduced and internally biased oxide wafer actuators is studied in a wide electric field and frequency range. It is found that under quasistatic condition, linear relationships between actuator tip displacement-electric field, and blocking force-electric field are only valid under weak field driving. With increasing the driving field, electromechanical nonlinearity begins to contribute significantly to the actuator performance because of ferroelectric hysteresis behavior associated with piezoelectric lead zirconate titanate (PZT)-type ceramic materials. The bending resonance frequencies of all these actuators vary with the magnitude of the electric field. The decrease of resonance frequency with electric field is explained by the increase of elastic compliance of PZT ceramic due to elastic nonlinearity. Mechanical quality factors of the actuators also depend on the magnitude of electric field strength. No significant temperature increase is observed when actuators are driven near resonance frequency under high electric field.
Long lasting phosphorescence and photostimulated luminescence in Tb-ion-activated reduced calcium aluminate glasses
Authors: Kinoshita Takeru , Yamazaki Masaaki , Kawazoe Hiroshi , Hosono Hideo
Published: July 2002

Long lasting phosphorescence (LLP) and photostimulated luminescence (PSL) were found in reduced calcium aluminate glasses activated with Tb3+ ions. The LLP from Tb3+ was observed by illuminating the Tb3+ 4f→5d charge transfer band with ultraviolet (UV) 254 nm light, while the PSL was seen by stimulating the UV-illuminated glasses with 633 nm light. The decay curve of the LLP was fitted with a second-order kinetic for the initial period (0<t<20 min) and a first-order kinetic for the later (t>30 min). An electron paramagnetic resonance (EPR) signal, which is attributed to an F+-like center associated with Ca2+ ions, was induced by illumination with UV light and its intensity decay was fitted with a first-order kinetic similarly to the later stage of the LLP. The appearance of the PSL by illumination is accompanied by a distinct intensity reduction of the EPR signal due to the F+-like center. The thermoluminescence spectra of the specimen illuminated with UV light at 77 K consist of two components peaking at ∼240 and ∼390 K. The low temperature component and the high temperature component were attributed to an F-like center and an F+-like center, respectively. These results lead to a conclusion that electrons of the F-like center and the F+-like center contribute predominantly to the emergence of the LLP and the PSL, respectively.
Thermal decay of ferro/antiferromagnetic exchange coupling in Co/CrMnPt systems
Authors: Nishioka Koichi
Published: July 2002

To investigate thermal decay of ferro/antiferromagnetic coupling, spin valve films with Co/(Cr0.4Mn0.6)90Pt10 exchange coupling layers were heat treated at various temperatures under a magnetic field whose direction was opposite to the pinning direction, and the pinning fields were evaluated at 35 °C. The pinning field decayed as the treatment time increased. During the first 10 min of the heat treatment, the pinning field decreased rapidly and then decreased gradually. The initial decrease in the pinning field became larger and the rate of the change of the pinning field after 10 min became larger at the higher treatment temperatures. A thermal fluctuation model, which assumes coherent rotations and grain size distributions in the antiferromagnets, was used to obtain the activation energy and relaxation times for reversal of the antiferromagnetic moment. Smaller antiferromagnetic grains had lower activation energies and shorter relaxation times. As a result, the smaller grains reversed their moments at an earlier time stage, which gave rise to the rapid decay of the pinning field during the first 10 min of the heat treatment. The model explains the thermal decay of the pinning field when the temperature is lower than 130 °C. However, a narrower distribution of the activation energy than can be expected from the model is required to explain the thermal decay at 150 °C, which suggests that a incoherent magnetic rotation occurs in antiferromagnets above 150 °C.
Temperature effect on a rubbed polyimide alignment layer
Authors: Kim Jong-Hyun , Rosenblatt Charles
Published: July 2002

Measurements are reported for the optical phase retardation of a polyimide-coated liquid crystal alignment layer that was rubbed at different temperatures. It was found that rubbing at higher temperatures results in an enhancement of the retardation, sometimes quite significantly. Two mechanisms that may explain the anisotropy of the alignment layer, one by local heating and the other by mechanical pressure, are discussed.
Structure-enhanced yield stress of magnetorheological fluids
Authors: Tang X. , Zhang X. , Tao R. , Rong Yiming
Published: July 2002

The yield stress of magnetorheological (MR) fluids depends on the induced solid structure. Since thick columns have a yield stress much higher than a single-chain structure, we improve the yield stress of MR fluids by changing the fluid microstructure. Immediately after a magnetic field is applied, we compress the MR fluid along the field direction. Scanning electron microscopy images show that particle chains are pushed together to form thick columns. The shear force measured after the compression shows that the structure-enhanced static yield stress can reach as high as 800 kPa under a moderate magnetic field, while the same MR fluid has a yield stress of 80 kPa without compression. This improved yield stress increases with the magnetic field and compression pressure and has an upper limit well above 800 kPa. The method may also be useful for electrorheological fluids.
ac demagnetization of particulate and thin film recording media
Authors: Bissell Philip R. , Cerchez Mihai , Stancu Alexandru
Published: July 2002

ac demagnetization processes have been investigated for different starting remanence states and compared to dc demagnetization processes. Measurements have been made in a range of magnetic recording media including acicular particulate flexible media, Ba–ferrite coatings and sputtered thin films. In all cases an offset of the ac demagnetization relative to that expected from dc measurements was observed, which could be to higher or lower fields. This could not be explained in terms of expected trends from interaction effects or time dependence and additional processes had to be invoked.
Electrical and structural properties of polycrystalline silicon
Authors: Tringe Joseph W. , Plummer James D.
Published: July 2002

This work links the structural and electrical properties of thin polycrystalline silicon films at the level of individual grains. A four point “transparent probe” technique was developed which allowed a small number of grains to be isolated and measured electrically in test structures which can be nondestructively prepared for direct imaging in a transmission electron microscope (TEM). By measuring the temperature dependence of electrical resistance in many test structures, the distribution of individual grain boundary activation energies for resistance was determined. After electrical measurements, grains in the test regions were imaged by TEM. Grain size, shape and crystallographic grain boundary character were measured and the distribution of relative grain misorientations was found. Measured boundaries were compared to coincident-site lattice-related boundaries whose character has been the subject of single boundary studies. Single grain boundaries were characterized both electrically and structurally. Electrical results from the transparent probe structures were compared with standard energy band theory developed for blanket film properties and with results from a Monte Carlo model of a network of thermally activated resistors. These results demonstrate how a distribution of grain structures and orientations fundamentally determines the electrical properties of the blanket film.
Highly sensitive photocurrent spectroscopy of resonant couplings between GaAs/AlAs superlattice Stark ladders and GaAs single quantum well states
Authors: Nogami T. , Kawasaki K. , Kawashima K. , Fujiwara K.
Published: July 2002

We have investigated interactions between the localized states in a single quantum well (SQW) embedded in the center of a superlattice (SL) and the Wannier–Stark ladder states in the SL by low temperature photocurrent (PC) spectroscopy using numerical differentiation for an analysis of the measured PC spectra. It is found that the PC spectra show drastic changes of transition energies and oscillator strengths due to resonances between the SQW localized states and the extended SL Stark ladder states. We demonstrate high sensitivity of PC spectroscopy by introducing the differentiated spectra and gray-scale representation. Based on transfer matrix calculations of the eigen energies and wave function overlap integrals, the observed anticrossings and delocalization phenomena are rigorously explained.
Low temperature electron paramagnetic resonance anomalies in Fe-based nanoparticles
Authors: Koksharov Yu. A. , Gubin S. P. , Kosobudsky I. D. , Beltran M. , Khodorkovsky Y. , Tishin A. M.
Published: July 2002

A study of the electron paramagnetic resonance of Fe-based nanoparticles embedded in polyethylene matrix was performed as a function of temperature ranging from 3.5 to 500 K. Nanoparticles with a narrow size distribution were prepared by the high-velocity thermodestruction of iron-containing compounds. A temperature-driven transition from superparamagnetic to ferromagnetic resonance was observed for samples with different Fe content. The unusual behavior of the spectra at about 25 K is considered evidence of a spin-glass state in iron oxide nanoparticles.
Doppler-limited dye laser excitation spectroscopy of the Ã 1A′′(000)–X̃ 1A′(000) band of HSiF
Authors: Suzuki Tetsuo , Hakuta Kohzo , Saito Shuji , Hirota Eizi
Published: July 2002

The Ã 1A′′(000)–X̃ 1A′(000) band of HSiF was observed by Doppler-limited dye laser excitation spectroscopy. The HSiF molecule was produced by the reaction of SiH3F with microwave discharge products of CF4. The observed spectrum was found to be almost free of perturbations and was readily assigned to about 1300 transitions of K′a−K′′a=5–6, 4–5, 3–4, 2–3, 1–2, 0–1, 1–0, 2–1, 3–2, 4–3, 5–4, 0–0, 1–1, and 2–0. A least-squares analysis of the observed spectrum yielded the rotational constants and the centrifugal distortion constants for both the Ã and X̃ states. The molecular structure was discussed using the observed rotational constants.