bluuebunny/crossref_metadata_2025 huggingface dataset:
Fast recombination processes in lead chalcogenide semiconductors studied via transient optical nonlinearities
Authors: Klann Robert , Höfer Thomas , Buhleier Rainer , Elsaesser Thomas , Tomm Jens W.
Published: July 2002

Third-order optical nonlinearities and recombination processes of photoexcited electron-hole plasma in PbSe and PbTe are directly monitored by picosecond pump-probe experiments in the wavelength range from 3 to 10 μm. After excitation, a strong blue shift of the absorption edge and large changes of the refractive index are found. Time-resolved studies of the absorption changes at lattice temperatures between 70 and 160 K reveal a fast partial decrease of the carrier density within 100 ps at excitation densities exceeding 7×1017 cm−3. Comparative emission spectra demonstrate that this behavior is due to recombination by stimulated emission. At room temperature the nonradiative Auger recombination dominates on a 0.5–2 ns time scale below the threshold of stimulated emission. The overall Auger coefficients for carrier densities of about 1018 cm−3 are derived from the data and have values of cAPbSe=1.1±0.3×10−28 cm6/s and cAPbTe=2.5±1.3×10−28 cm6/s.
Characterization of reflective cholesteric liquid-crystal displays
Authors: St. John W. D. , Lu Z.-J. , Doane J. W.
Published: July 2002

We report on the optical reflective properties of the planar texture of cholesteric liquid crystal reflective displays. The reflecting liquid-crystal medium is partitioned into many distinguishable domains, each of which is characterized by its local dielectric anisotropy. Such a multidomain cholesteric liquid crystal is made bistable by either dispersing a low concentration of polymer or by treating the cell substrate surfaces, both of which affect the domain alignment. Both types of cells result in a bistable colorful reflective display. We determine the role that the polymer network and surface alignment has on the reflective properties in regard to their photometric and colorimetric properties as a function of viewing angle using both collimated and diffuse illumination. Both the polymer network and surface anchoring have the effect of distributing the orientation of the cholesteric helix axes about the cell normal. Theoretically we characterize these cells by this distribution. Our goal is to properly model both types of display cells so as to quantitatively elucidate their distinguishing features.
Raman scattering from anisotropic LO-phonon–plasmon–coupled mode in n-type 4H– and 6H–SiC
Authors: Harima Hiroshi , Nakashima Shin-ichi , Uemura Tomoki
Published: July 2002

LO-phonon–plasmon–coupled modes in n-type 4H– and 6H–SiC single crystals with free-carrier concentrations of 1016–1018 cm−3 have been measured by Raman scattering at room temperature. The axial-type mode for which plasma oscillation and atomic displacement are parallel to the c axis, and the planar-type mode for which these oscillations lie in the c plane, have been individually observed. From a line-shape analysis of the observed spectra, the plasmon frequency, carrier damping, and phonon damping have been deduced. These quantities have large differences between the axial- and planar-type mode in 6H–SiC, indicating its large crystal anisotropy. On the contrary, 4H–SiC shows small anisotropy. The longitudinal and transverse effective mass components of the electron have been determined from the plasmon frequency using carrier densities derived from Hall measurements. The deduced values are m∥=1.4m0 and m⊥=0.35m0 for 6H–SiC, and m∥=0.48m0 and m⊥=0.30m0 for 4H–SiC. The carrier mobility obtained from the analysis is also anisotropic. This is consistent with reported electrical measurements.
Carbon incorporation into Si at high concentrations by ion implantation and solid phase epitaxy
Authors: Strane J. W. , Lee S. R. , Stein H. J. , Picraux S. T. , Watanabe J. K. , Mayer J. W.
Published: July 2002

We have studied the incorporation of heavily supersaturated C into Si using solid-phase epitaxy (SPE) of implanted amorphous layers. The strain in the Si1−xCx/Si heterostructures was measured using rocking curve x-ray diffraction. The microstructure and defect introduction were examined using ion channeling and transmission electron microscopy (TEM). The fraction of C located on substitutional lattice sites in the Si was monitored using Fourier transform infrared absorption spectroscopy and ion channeling at resonance energies. Carbon-depth profiles were monitored by secondary ion mass spectroscopy. The metastable solubility limit for the incorporation of C into Si by SPE was found to be 3.0–7.0×1020 atoms/cm3, which is over three orders of magnitude above the equilibrium solubility at the Si melting point. This limit was determined by the ability to regrow without the introduction of microtwins and stacking faults along {111} planes. We postulate the local bond deformation resulting from the atomic size difference between C and Si leads to the faceting of the amorphous–crystalline interface and allows defect introduction, thus limiting the C supersaturations achieved in Si by SPE. It was also found that the defect density in the regrown alloys could be reduced by higher SPE regrowth temperatures in rapid thermal anneal processing.
Growth and characterization of anodic oxides on Si(100) formed in 0.1 M hydrochloric acid
Authors: Bardwell J. A. , Draper N. , Schmuki P.
Published: July 2002

Anodic oxides of thickness 1–10 nm have been grown on Si(100) using anodic oxidation at room temperature. The electrolyte was 0.1 M HCl. The effects of the anodic potential, growth time, external visible illumination intensity, substrate doping type, and density on the oxide thickness were determined. Coulometric and etching experiments suggest that no silicon is lost to the 0.1 M HCl solution. Under sufficient visible illumination, the oxide thickness was independent of dopant level for n-type substrates and only weakly dependent for p-type substrates. These results suggest that this technique can be used for accurate dopant profiling, when combined with cyclical etch back and four-point probe electrical measurements. In the dark, the oxide thickness was substantially reduced for n-type substrates. This suggests that this technique can be used for lateral delineation of n- and p-type regions on patterned wafers. The as grown oxides were characterized by x-ray photoelectron spectroscopy, ellipsometry, and x-ray reflectometry. The substrates were characterized by Mott–Schottky analysis which, together with electrochemical polarization curves, assisted in a qualitative description of the oxide growth as a function of doping density and type. Flatband potentials obtained from Mott–Schottky analysis were obtained at pH 1.1 and 11.7, and show a pH dependence of approximately 60 mV/pH unit, which in contrast to previously reported data indicates an ideal behavior.
Colossal magnetoresistance in the antiferromagnetic La0.5Ca0.5MnO3 system
Authors: Gong G. Q. , Canedy C. L. , Xiao Gang , Sun J. Z. , Gupta A. , Gallagher W. J.
Published: July 2002

We have explored the colossal magnetoresistance (CMR) effect in the antiferromagnetic La0.5Ca0.5MnO3 compound. In the absence of a magnetic field (H), the solid is a canted antiferromagnetic (AFM) insulator. An applied H in the Tesla scale induces a first order AFM to FM phase transition, and correspondingly, an insulator to metal transition. The observed CMR is attributed to the H-induced charge localization-delocalization behavior associated with the AFM–FM transition. At low temperatures (T<100 K), the solid remains in the AFM phase, where we have observed a phenomenal one millionfold change in resistivity between 0 and 8 Tesla. The origin of CMR in low T-region is a thermal activation energy gap which is strongly dependent on H.
Effect of boron diffusion on the high-voltage behavior of 6H-SiC p+nn+ structures
Authors: Ortolland S. , Raynaud C. , Chante J. P. , Locatelli M. L. , Lebedev A. A. , Andreev A. N. , Savkina N. S. , Chelnokov V. E. , Rastegaeva M. G. , Syrkin A. L.
Published: July 2002

Boron diffusion can be used to compensate the n-type layer of a p+nn+ 6H-silicon carbide structure in order to increase its high-voltage capabilities. Measurements under reverse biases for a current range from 10 to 500 μA show that this process is very efficient for working temperatures about 300 K. Indeed we obtained a voltage of 670 V for a reverse current of 10 μA instead of the 120 V calculated for a structure without boron diffusion. Nevertheless, the breakdown voltage decreases rapidly when the temperature increases. Capacitance measurements show that the measured doping level in the n-type layer evolves in the same way as the temperature (it ranges from 1013 cm−3 at 300 K to 1017 cm−3 at 500 K). A great concentration of boron seems to be responsible for this doping variation with temperature. Admittance spectroscopy reveals the presence of D centers at 0.62 eV above the valence band associated to boron at concentration similar or superior to nitrogen concentration in the n-type layer. The increase of the doping level with the temperature is responsible for this decrease of the breakdown voltage.
Nitrogen modification of hydrogenated amorphous carbon films
Authors: Silva S. R. P. , Robertson J. , Amaratunga G. A. J. , Rafferty B. , Brown L. M. , Schwan J. , Franceschini D. F. , Mariotto G.
Published: July 2002

The effect of nitrogen addition on the structural and electronic properties of hydrogenated amorphous carbon (a-C:H) films has been characterized in terms of its composition, sp3 bonding fraction, infrared and Raman spectra, optical band gap, conductivity, and paramagnetic defect. The variation of conductivity with nitrogen content suggests that N acts as a weak donor, with the conductivity first decreasing and then increasing as the Fermi level moves up in the band gap. Compensated behavior is found at about 7 at. % N, for the deposition conditions used here, where a number of properties show extreme behavior. The paramagnetic defect density and the Urbach tailwidth are each found to decrease with increasing N content. It is unusual to find alloy additions decreasing disorder in this manner.
Effects of the Au/CdTe back contact on IV and CV characteristics of Au/CdTe/CdS/TCO solar cells
Authors: Niemegeers Alex , Burgelman Marc
Published: July 2002

A simple analytical theory is presented to explain the measured roll over and cross over behaviour of the IV characteristics of thin film CdTe solar cells. It involves a classical description of the CdS/CdTe junction and the CdTe/back contact structure and is extended with a new description of minority carrier current in the CdTe contact region. This extension is crucial in describing the light dependence of the forward IV curves, and hence cross over. The same model also explains the measured CV curves. It is shown that analysis of the capacitance measurement can yield additional information about the doping density of CdTe in the vicinity of the contact. A relationship between the fill factor of the solar cell and the barrier height of the back contact is derived; this relation is useful as a new, practical criterion for the quality of the back contact. The results of this simple analytical model are confirmed by full numerical calculations of the dc and ac characteristics.
Emissivity of liquid silicon in visible and infrared regions
Authors: Takasuka Eiryo , Tokizaki Eiji , Terashima Kazutaka , Kimura Shigeyuki
Published: July 2002

Normal spectral emissivity of Si melt in visible and infrared regions was determined by the direct measurement of thermal radiations from the melt and a blackbody cavity which was located close to the melt. The spectral emissivity slightly decreases with wavelength. The emissivity slightly changes with temperature. The spectral emissivity values in visible and in infrared region are 0.27 and 0.21, respectively. The wavelength dependence of the emissivity can be interpreted by a dielectric response of free electrons in the melt.
Domain-wall curvature and coercivity in pinning type Sm–Co magnets
Authors: Skomski Ralph
Published: July 2002

The origin of coercivity in precipitation hardened Sm–Co magnets is investigated. By considering domain-wall pinning at boundary phase and Z phase inhomogenities, it is found that the hexagonal Z phase has no direct influence on the coercivity. The intuitive assumption of plane domain walls made in a recent calculation is unrealistic but has no direct effect on the coercivity. However, due to the comparatively large size of the 12:17 cells, the calculated three dimensional domain-wall curvature is sufficiently large to assure a nearly ideal nestling of the domain walls to the 1:5 cell boundary. This nestling yields a coercivity increase from about 0.1 to about 1 T and is therefore responsible for the high coercivity of 2:17 based Sm–Co magnets.
Influence of raw materials and milling technological process on magnetic properties of sintered anisotropic Sr-ferrite magnet
Authors: Teng Yangming , Lu Baosun , Yao Jizhong
Published: July 2002

It is necessary to reduce the Sr-ferrite grain size below 1 μm for high coercivity, besides a high degree of density and orientation for high residual induction. However, submicron Sr-ferrite particles are difficult to orient in a magnetic field at pressing stage because of their cohesion. For preventing this cohesion, treatments with dispersants and reducing coercive force of S-ferrite particles are very effective. In this research, a study was done correlating purity and aggregation state of raw materials with intrinsic coercive force and microstructure of presintered Sr-ferrite sample, and a presintered sample with uniform grain size distribution and lower coercivity was gained. The classical pulverizing process is composed of coarse dry milling and fine wet milling. The results confirm that compared with wet milling, a long dry milling time promotes the development of crystal deformation and reduces the coercive force of the powders effectively; hence the viscosity is also lowered. By enhancing dry-milling time to control average particle size under 1.8 μm and fine wet milling up to 0.75 μm with optimum amount of dispersant, a well dispersed Sr-ferrite slurry is prepared. By these methods, a remarkable improvement of magnetic properties was achieved.
Void-containing nonlinear materials subject to high-rate loading
Authors: Wang Ze-Ping
Published: July 2002

A new concept of dynamic potential (macrostress potential or macrostrain-rate potential), which has two components, i.e., deformation and kinetic potentials for heterogeneous materials under intense dynamic loading, is proposed using a micromechanical approach. In particular, for voided nonlinear materials, the approximate expression of the macrostress potential is derived through an upper bound approach. Simplified physical models for ductile porous materials (aggregates of voids and ductile matrix) are employed with the matrix material taken as power-law thermal viscoplastic and incompressible. Approximate velocity fields for the matrix are adopted to derive the dynamic loading criterion and an approximate functional form of the dynamic loading criterion is developed. The inertial, rate sensitivity, and thermal effects are directly included in the expressions of the macrostress potential and the dynamic loading function. Various features of the loading function are numerically investigated in detail. Numerical analysis reveals that both the dynamic growth of voids and the loading surface have strong rate dependence and temperature dependence. Experimental data, theoretical analysis, and numerical modeling indicate that the inertial and thermal effects play a very important role in the dynamic behavior of the voided nonlinear materials.
Growth and decay of macroparticles: A feasible approach to clean vacuum arc plasmas?
Authors: Anders André
Published: July 2002

Metal and carbon plasmas produced at nonstationary cathode spots are inherently contaminated with micron-size macroparticles, and their removal is typically done by guiding the plasma through curved magnetic macroparticle filters in order to spatially separate plasma and macroparticles. In this article, a possible alternative approach is discussed in which macroparticles are destroyed rather than removed. Mass and energy transfer from the plasma to macroparticles are considered. Both cold and warm ion approximations are discussed. It was found that the desired result—complete evaporation of macroparticles before arrival at the substrate—cannot be easily achieved in any case. Ion bombardment heating is reduced by thermionic electron emission when the macroparticle approaches the temperature region of strong evaporation. The heat loss mechanisms, thermal radiation and evaporation, increase strongly with temperature. Direct heating of the macroparticle seems to be most promising. Microwave heating was ruled out because of the plasma shielding effect. The use of infrared lasers has been proposed, but this solution is likely to be uneconomical.
Stress-induced formation of high-density amorphous carbon thin films
Authors: Schwan J. , Ulrich S. , Theel T. , Roth H. , Ehrhardt H. , Becker P. , Silva S. R. P.
Published: July 2002

Amorphous carbon films with high sp3 content were deposited by magnetron sputtering and intense argon ion plating. Above a compressive stress of 13 GPa a strong increase of the density of the carbon films is observed. We explain the increase of density by a stress-induced phase transition of sp2 configured carbon to sp3 configured carbon. Preferential sputtering of the sp2 component in the carbon films plays a minor role compared to the sp2⇒sp3 transition at high compressive stress formed during the deposition process. Transmission electron microscopy shows evidence of graphitic regions in the magnetron sputtered/Ar plated amorphous carbon thin films. Differences in the microstructure of the tetrahedral amorphous carbon (ta–C) films deposited by filtered arc and mass selected ion beam; and those films deposited using magnetron sputtering combined with intense ion plating can be used to explain the different electronic and optical properties of both kinds of ta–C films.
Kinetics of fluorine atoms in high-density carbon–tetrafluoride plasmas
Authors: Sasaki K. , Kawai Y. , Suzuki C. , Kadota K.
Published: July 2002

Reaction processes of fluorine (F) atoms in high-density carbon–tetrafluoride (CF4) plasmas were investigated using vacuum ultraviolet absorption spectroscopy. A scaling law nF∝(nenCF4)0.5–0.7 was found experimentally, where nF is the F atom density and ne and nCF4 stand for the electron and parent gas (CF4) densities, respectively. The lifetime measurement in the afterglow showed that the decay curve of the F atom density was composed of two components: a rapid decay in the initial afterglow and an exponential decrease in the late afterglow. The decay time constant in the initial afterglow τ1 satisfied the scaling law τ1∝(nenCF4)−(0.3–0.4), which is a consistent relationship with the scaling law for the F atom density. The two scaling laws and the lifetimes of CFx radicals suggest that the major loss process of F atoms in the initial afterglow is the reaction with CFx radicals (probably, x=3) on the wall surface. The loss process in the late afterglow was simple diffusion to the wall surface. The surface loss probability of F atoms on the chamber wall was evaluated from the decay time constant in the late afterglow, and was on the order of 10−3.
Naphthalimide side-chain polymers for organic light-emitting diodes: Band-offset engineering and role of polymer thickness
Authors: Cacialli F. , Friend R. H. , Bouché C.-M. , Le Barny P. , Facoetti H. , Soyer F. , Robin P.
Published: July 2002

We report the fabrication of efficient green light-emitting diodes using a side-chain random polymer based on a high electron affinity (EA) naphthalimide moiety (PNI). The chromophore is attached to a polymethacrylate backbone through a spacer, and emits in the green with high efficiency (30% photoluminescence quantum yield). In single-layer light-emitting diodes (LEDs), we find that the electroluminescence quantum efficiency is not limited by Al cathodes as for poly(p-phenylene vinylene), PPV, and we attribute this to the increased EA. We also report maximum internal quantum efficiencies of about 1.7% for Ca and 0.9% for Al in double-layer devices where PPV serves as both hole injector and emitter. Compared to some oxadiazole based electron injection/transport layers, PNI gives higher efficiencies at high currents, and longer lifetimes. Tuning of emission in the red is possible by dye doping (at high concentration) the PNI and causing the emission to happen in this layer. We discuss the properties of the different device configurations from the viewpoint of the electronic structure of the materials and, in particular, the influence of the thickness of the individual layers on both quantum (photon/electron) and luminous (Lumen/W) efficiency and driving conditions. Unexpectedly, we find that not only does the dye doping of PNI redshift the emission spectrum, but also affects significantly the charge transport properties, and in particular reduces the driving field necessary for electroluminescence in both single and double-layer LEDs.
Development of Terfenol-D transducer material
Authors: Lindgren E. A. , Haroush S. , Poret J. C. , Mazzatesta A. D. , Rosen M. , Wun-Fogle M. , Restorff J. B. , Clark A. E. , Lindberg J. F.
Published: July 2002

Terfenol-D, Tb1−xDyxFe2 (x≅0.7), is successfully used as a magnetostrictive transducer material for low-frequency applications. To extend the frequency range of magnetostrictive transducers into the high kHz and MHz range, new materials processing techniques must be developed to minimize eddy current losses. The method that has demonstrated the greatest potential to date is based on rapid solidification by melt spinning to obtain thin strips of material. The objective of this program includes refining the casting procedure to yield ribbons with the [111] direction oriented along the length of the ribbon, with the (110) plane in the plane of the ribbon. The crystallographic orientation of the Terfenol-D ribbons have been controlled by varying the cooling rate of the melt-spinning process. The cooling rates were controlled by a number of parameters in the melt-spinning process and ribbons have been obtained with an enhancement of the (110) plane parallel to the surface of the ribbon. Magnetostrictive measurements under tensile stresses up to 3 MPa and magnetic fields up to 4 kOe yielded magnetostrictive displacements of 850×10−6.
Tunneling magnetoresistance and current distribution effect in spin-dependent tunnel junctions
Authors: Sun J. J. , Sousa R. C. , Galvão T. T. P. , Soares V. , Plaskett T. S. , Freitas P. P.
Published: July 2002

In this paper, spin-dependent tunnel junctions fabricated by shadow mask (junction area 0.25 mm2) and by lithography (junction area down to 9×2 μm2) were studied. The junctions have NiFe and CoFe electrodes and the insulating barrier Al2O3 is formed by depositing a 10–30 Å thick Al layer, followed by a 1–3 min plasma oxidation in an O2 atmosphere. The mm2-size junctions show tunneling magnetoresistances (TMR) of 10%–13.5% at room temperature (RT), with 50% decrease in TMR for a bias voltage of 220 mV. The junction resistances range from hundreds of Ω to tens of kΩ. The analysis of current distribution indicates that no geometrically enhanced magnetoresistance occurs in the cross-shaped mm2-size junctions when the measured junction resistance is five times larger than the electrode resistance over junction area. The μm2-size junctions show TMR of 17%–24% at RT, independent of the junction area, and have a resistance between 90 kΩ and 1 MΩ for the 9×2 μm2 size (resistance-area products of ∼3 MΩ×μm2). The μm2-size junctions show 50% decrease in TMR for a bias voltage of 430 mV, and high sensitivity (>20%/Oe).
Photoluminescence and photoluminescence excitation study of CuGaTe2
Authors: Krustok J. , Collan H. , Hjelt K. , Yakushev M. , Hill A. E. , Tomlinson R. D. , Mändar H. , Neumann H.
Published: July 2002

Photoluminescence (PL) and photoluminescence excitation measurements over a temperature range 12 K<T<300 K on high quality CuGaTe2 crystals grown by the vertical Bridgman method were completed. The whole PL spectrum consists of two regions. The first region includes PL bands E1 at 1.431 eV, E2 at 1.426 eV, and E3 at 1.417 eV, together with their phonon replicas while in the second region we have a PL band D0 at 1.338 eV with its well-resolved LO-phonon replicas (ℏωLO=26.5 meV). All these PL bands appear to be at higher energy than the lowest (fundamental) band gap energy EgA. The possible origin of observed PL bands is discussed.
H
2
S
destruction in 50 Hz and 25 kHz gliding arc reactors
Authors: Dalaine V. , Cormier J. M. , Pellerin S. , Lefaucheux P.
Published: July 2002

The destruction of foul smelling gases is an important subject involving a number of industrial applications. Increased interest is being devoted to technologies based on the properties of cold plasmas, which are the subject of a large body of research. Among all possible techniques, the sliding discharge is one of the simplest to utilize. We present a study of the destruction of H2S diluted in air by this type of discharge at two frequencies; 50 Hz and 25 kHz. We first describe the electric power supplies, their characteristics, and the design of the reactor. The properties of the discharge and its changes are then described at the above two frequencies. We performed measurements of voltage, current, velocities, and length of the discharge. After describing the instrumentation used for chemical diagnoses, the main experimental results are presented. They show the changes in the conversion rate as a function of the flow rate, of the gas to treat and of the frequency. It was thus possible to determine how destruction efficiency is affected by voltage, flow rates, and frequency of the power supply. The experimental results obtained show that the efficiency is greater at 50 Hz. This effect is not produced by the dependence of chemical mechanisms with the frequency. At low frequency it is possible to obtain greater lengths of the discharge than those obtained with high frequencies, which is why the destruction is better. At the same time, energy cost was compared for both frequencies. In favorable cases, we observed that about 500 eV per molecule permit a conversion rate of 75% at 50 Hz and only 55% at 25 kHz. This cost, which seems very high compared to other techniques, is due to the important dilution of the gliding discharge device, but the order of magnitude of the power consumption is 1 W/ℓ of gas treated.
Thermal conductivity of uranium dioxide up to 2900 K from simultaneous measurement of the heat capacity and thermal diffusivity
Authors: Ronchi C. , Sheindlin M. , Musella M. , Hyland G. J.
Published: July 2002

The thermal diffusivity and heat capacity of uranium dioxide have been measured from 500 to 2900 K with an advanced laser-flash technique. These two quantities were determined simultaneously by means of an accurate numerical fitting of the experimental thermograms. At high temperatures the precision of the method used is much better than that associated with conventional laser-flash measurements. It was found that the heat capacity continues to increase even at temperatures above the expected lambda transition (2670 K). The inverse of the thermal diffusivity increases linearly with temperature up to 2600 K, whilst at higher temperatures the slope markedly decreases. A new expression for the thermal conductivity as a function of temperature is proposed, which is corroborated by some theoretical considerations on the underlying heat transport mechanisms.
Optical properties of graphite
Authors: Djurišić Aleksandra B. , Li E. Herbert
Published: July 2002

Optical constants of graphite for ordinary and extraordinary waves are modeled with a modified Lorentz–Drude model with frequency-dependent damping. The model enables the shape of the spectral line to vary over a range of broadening functions with similar kernels and different wings, the broadening type being its adjustable parameter. The model parameters are determined by the acceptance-probability-controlled simulated annealing algorithm. Good agreement with the experimental data is obtained in the entire investigated spectral range (0.12–40 eV for ordinary wave and 2.1–40 eV for extraordinary wave). The significant discrepancies between the experimental data obtained by the reflectance measurements and the electron-energy-loss spectroscopy data are analyzed in details. Inconsistency in terms of unsatisfied Kramers–Kronig relations is discovered in the index of refraction data derived from reflectance measurements, and a method for correcting the data is proposed.
Strong enhancement of optical absorbance from ionic self-assembled multilayer thin films of nanocluster Pt and polymer dye
Authors: Liu Yanjing , Claus Richard O.
Published: July 2002

The observation of unusually enhanced optical absorbance spectra of ionic self assembled multilayer (ISAM) thin films composed of alternating layers of Pt nanoclusters (<1 nm dimension) and polymer dye is reported. The first bilayer absorbance is found to be considerably larger than that of several succeeding bilayers even though there is no difference in composition for each bilayer. A layer-by-layer-dependent redshift in maximum wavelength position due to the strong coupling of metals and polymer molecules is observed. The saturation absorption is obtained for the first time when the required thickness of the ISAM film is deposited. The large and unusual enhancement effects are attributed to both the charge-transfer mechanism and very large local fields and collective phenomena near the surface of the small metal clusters/electrolytes and at the interfaces between the cluster/polymer multilayers.
Reduction of the energy gap pressure coefficient of GaN due to the constraining presence of the sapphire substrate
Authors: Perlin Piotr , Mattos Laila , Shapiro Noad A. , Kruger Joachim , Wong William S. , Sands Tim , Cheung Nathan W. , Weber Eicke R.
Published: July 2002

We have performed a detailed investigation of the photoluminescence pressure dependence of heteroepitaxial GaN thin films on sapphire substrates. A comparison between as grown GaN on sapphire and free-standing GaN membranes, created using a laser assisted substrate liftoff process, revealed that the presence of the sapphire substrate leads to an energy gap pressure coefficient reduction of approximately 5%. This result agrees with the numerical simulations presented in this article. We established that the linear pressure coefficient of free-standing GaN is 41.4±0.2 meV/GPa, and that the deformation potential of the energy gap is −9.36±0.04 eV. Our results also suggest a new, lower value of the pressure derivative for the bulk modulus of GaN (B′=3.5).
One-dimensional magnetophotonic crystals
Authors: Inoue Mitsuteru , Arai Ken'ichi , Fujii Toshitaka , Abe Masanori
Published: July 2002

Two types of one-dimensional photonic crystals composed of magnetic and dielectric materials (magnetophotonic crystals) driven, respectively, by Kerr (reflection) and Faraday (transmission) modes were constructed. Their optical and magneto-optical (MO) properties were studied in detail to confirm our theoretical results showing the large Kerr and Faraday effects of the media originating in the localization of light. For the Kerr-mode operation, films with (SiO2/SiN)×k/Co/(SiN/SiO2)×k (k: number of layers) structures were fabricated, while for the Faraday-mode operation, films with (SiO2/Ta2O5)×k/Bi:DyIG/(Ta2O5/SiO2)×k structures were formed. Excellent agreement between the theoretical and experimental results was obtained, where large enhancement in both Kerr and Faraday rotations appeared originating in the localization of light in the vicinity of the magnetic layers. Since the localized state of light can be controlled artificially, the one-dimensional magnetophotonic crystals will impact for various MO applications.
Near-field optical microscopy of thin photonic crystal films
Authors: Phillips P. L. , Knight J. C. , Mangan B. J. , Russell P. St. J. , Charlton M. D. B. , Parker G. J.
Published: July 2002

Near-field optical microscopy is used to image the light propagating in a Si3N4-on-silica film with a strip of deeply etched submicron holes in the center. Fringe patterns created by interference between incident and diffracted guided modes are observed in the smooth waveguide surrounding the large air-hole photonic film. Observation of the fields in the photonic crystal regions indicates that out-of-plane scattering at the smooth/periodic transitions and leakage caused by grating coupling play major roles. The effects of different air-hole sizes and lattice pitches are also explored.
Shallow impurities in semiconductor superlattices: A fractional-dimensional space approach
Authors: Reyes-Gómez E. , Oliveira L. E. , de Dios-Leyva M.
Published: July 2002

A thorough detailed study of donor and acceptor properties in doped GaAs–(Ga,Al)As semiconductor superlattices is performed within the fractional-dimensional approach, in which the real anisotropic “impurity+semiconductor superlattice” system is modeled through an effective isotropic environment with a fractional dimension. In this scheme, the fractional-dimensional parameter is chosen via an analytical procedure and involves no ansatz, and no fittings either with experiment or with previous variational calculations. The present fractional-dimensional calculated results for the donor and acceptor energies in GaAs–(Ga,Al)As semiconductor superlattices are found in quite good agreement with previous variational calculations and available experimental measurements.
Paramagnetism, superparamagnetism, and spin-glass behavior in bulk amorphous Pd–Ni–Fe–P alloys
Authors: Shen T. D. , Schwarz R. B. , Thompson J. D.
Published: July 2002

We have investigated the magnetic properties of bulk amorphous Pd40Ni40−xFexP20 (x=0–17.5) alloys. For Pd40Ni40P20 (x=0), the magnetic susceptibility consists of temperature-independent and Curie–Weiss-type terms. Alloys with x⩾5 are paramagnetic at high temperatures. With decreasing temperature, the amorphous alloys become superparamagnetic. At even lower temperatures, and under a weak applied magnetic field, these alloys are spin glasses, as evidenced by static and dynamic magnetic measurements. The spin-freezing temperature increases with increasing iron content and this is attributed to the role of the Ruderman–Kittel–Kasuya–Yosida interaction in creating the spin-glass state. The occurrence of a reentrant spin-glass behavior on cooling (superparamagnetism-to-ferromagnetism-to-spin-glass transition) is also observed for x=17.5 at a field ⩾50 Oe. An unexpected result is that the ferromagnetic state in the present bulk metallic glasses is field induced. Evidence for the field-induced ferromagnetic-like order is obtained from (a) straight regions in the susceptibility versus temperature curves measured at various fields, (b) an Arrott plot, and (c) time-independent magnetization. With increasing applied field, the spin-freezing temperature decreases and the Curie temperature increases, broadening the temperature range of the field-induced ferromagnetic-like state. The temporal decay of the thermoremanent magnetization in the amorphous alloy with x=17.5 is slower than that in typical crystalline spin glasses. The spin-freezing temperature of the amorphous alloy with x=17.5 decreases approximately logarithmically with applied field, which differs from the prediction of Néel’s model for spin glasses.
Transient electron transport in wurtzite GaN, InN, and AlN
Authors: Foutz Brian E. , O’Leary Stephen K. , Shur Michael S. , Eastman Lester F.
Published: July 2002

Transient electron transport and velocity overshoot in wurtzite GaN, InN, and AlN are examined and compared with that which occurs in GaAs. For all materials, we find that electron velocity overshoot only occurs when the electric field is increased to a value above a certain critical field, unique to each material. This critical field is strongly dependent on the material, about 4 kV/cm for the case of GaAs but much higher for the III–nitride semiconductors: 140 kV/cm for GaN, 65 kV/cm for InN, and 450 kV/cm for AlN. We find that InN exhibits the highest peak overshoot velocity and that this velocity overshoot lasts over the longest distances when compared with GaN and AlN. Finally, using a one-dimensional energy–momentum balance approach, a simple model is used to estimate the cutoff frequency performance of nitride based heterojunction field effect transistors (HFETs) and a comparison is made to recently fabricated AlGaN/GaN HFETs.
X-ray induced luminescence of high-purity, amorphous silicon dioxide
Authors: Miller A. J. , Leisure R. G. , Austin Wm. R.
Published: July 2002

A comprehensive study of x-ray stimulated luminescence has been carried out on four types of high-purity, amorphous silica (a-SiO2). Both high OH and low OH as well as oxygen-excess and oxygen-deficient materials were studied. The room-temperature, visible x-radio luminescence (XRL) was measured continuously as a function of x-ray dose from zero to 400 Mrad volume average dose. In addition to the XRL measurements, electron paramagnetic resonance (EPR) was used to determine the concentrations of the two key radiation-induced defects, the E′ center and the nonbridging oxygen hole center (NBOHC). The XRL spectra were deconvolved into four Gaussian components with centers at 1.9, 2.2, 2.6, and 2.75 eV. The same centers and widths could be used to describe the spectra in all four types of a-SiO2, only the intensities varied. The 2.6 and 2.75 eV lines are strongly dose dependent, rising from near zero intensity at zero dose in all four materials. These two lines are strongly correlated with each other; they have essentially the same dependence on dose and sample type. This correlation suggests that these two lines are due to the same radiation-induced defect, or to closely related defects. The dose dependence and sample-to-sample variation of these two lines bear some similarities to the E′ concentrations. In contrast to the 2.6 and 2.75 eV lines, the 1.9 eV line has a high intensity at the lowest doses measurable. A simple phenomenological model is proposed to describe the 1.9 eV XRL line. This model involves two populations of defects; one population is present at zero dose and is assumed to be dose independent, while the second population is dose dependent. Evidence is presented that the dose-dependent defect is the NBOHC. The XRL due to the dose-independent population may be associated with a transient response to the x rays, or to a metastable defect; this population may not be observable in post-irradiation experiments such as EPR and conventional photoluminescence. Similar to the 1.9 eV line, the 2.2 eV line also has relatively high intensity at the lowest measurable x-ray dose. The behavior of this line is in general agreement with the self-trapped exciton model.
Structure and magnetization of arrays of electrodeposited Co wires in anodic alumina
Authors: Strijkers G. J. , Dalderop J. H. J. , Broeksteeg M. A. A. , Swagten H. J. M. , de Jonge W. J. M.
Published: July 2002

We have produced arrays of Co nanowires in anodic porous alumina filters by means of electrodeposition. The structure and magnetization behavior of the wires was investigated with nuclear magnetic resonance (NMR) and magnetization measurements. NMR shows that the wires consist of a mixture of fcc and hcp texture with the (0001) texture of the hcp fraction oriented preferentially perpendicular to the wires. The magnetization direction is determined by a competition of demagnetizing fields and dipole–dipole fields and can be tuned parallel or perpendicular to the wires by changing the length of the wires.
Wide angle distributed Bragg reflectors for 590 nm amber AlGaInP light-emitting diodes
Authors: Chiou S. W. , Lee C. P. , Huang C. K. , Chen C. W.
Published: July 2002

A structure for high brightness light-emitting diodes (LEDs) is demonstrated. A distributed Bragg reflector (DBR) is used to enhance the quantum efficiency of the LEDs. This unique DBR uses a composite structure that consists of two DBRs to provide both high reflectivity and wide angle reflection. For 590 nm (amber range) AlGaInP LEDs, the quantum efficiency is increased to 5.05% by using this composite DBR structure. This result is much better than those obtained from conventional DBRs, and is comparable to that of wafer bonded AlGaInP LEDs.
Neutron diffraction and Mössbauer study of the magnetic structure of YFe6Sn6
Authors: Cadogan J. M. , Suharyana  , Ryan D. H. , Moze O. , Kockelmann W.
Published: July 2002

We have used time-of-flight (TOF) neutron powder diffraction, and both Fe57 and Sn119 Mössbauer spectroscopy over the temperature range 2–600 K to determine the magnetic ordering mode of the Fe sublattice in YFe6Sn6. The crystal structure is orthorhombic (space group Immm). The Fe sublattice orders antiferromagnetically with a Néel temperature of 558(5) K. The TOF neutron diffraction patterns obtained at 4 and 293 K show that the antiferromagnetic ordering of the Fe sublattice is along [100] with a propagation vector q=[010]. The magnetic space group is IPm′m′m′. This magnetic structure is confirmed by our Sn119 Mössbauer spectra.
Effect of interface roughness on the exchange bias for NiFe/FeMn
Authors: Liu Congxiao , Yu Chengtao , Jiang Huaming , Shen Liyong , Alexander C. , Mankey G. J.
Published: July 2002

The effect of interface roughness on exchange bias for NiFe/FeMn bilayers is investigated for polycrystalline films and epitaxial films. Three different systems were investigated: polycrystalline Ta (10 nm)/Ni80Fe20 (10nm)/Fe50Mn50 (20 nm) films on oxygen plasma-etched Si(100) or Cu/H–Si(100) and epitaxial Ni80Fe20 (10nm)/Fe60Mn40 (20 nm) films on Cu/H–Si(110). For films grown on plasma-etched substrates, as the etching time is increased, film roughness increases up to 12 nm. For the polycrystalline films grown on ultrathin Cu underlayers, x-ray diffraction shows the fcc (111) texture is greatly reduced as the thickness is increased. The epitaxial Cu/Si(110) buffer layer induces fcc (111) epitaxial growth and modifies the interface morphology. The dependence of exchange bias on roughness for each set of samples is explained in terms of a competition between the interfacial exchange coupling and the af uniaxial anisotropy.
Luminescence properties of ZnS phosphor nanocrystals prepared by the laser-induced gas-evaporation method
Authors: Tanaka Masanori , Sawai Shinya , Sengoku Masaya , Kato Manabu , Masumoto Yasuaki
Published: July 2002

Nanocrystalline particles of ZnS:(Ag, Al) semiconductor phosphor, whose sizes are mostly 3–5 nm in diameter, are prepared by the gas-evaporation method with cw CO2 laser heating. The Raman scattering spectrum as well as the transmission electron microscope observation demonstrates that the crystallization of the nanoparticles was caused successfully through the gas-phase condensation. Under irradiation of ultraviolet light, the nanoparticles exhibit blue luminescence, as in the case of the starting material of ZnS:(Ag, Al) bulk powder. The peak of the luminescence spectrum of the nanoparticles shifts to lower energy with increasing delay time and also with decreasing excitation intensity, showing that the luminescence originates from the donor–acceptor pair recombination. However, it is concluded that the luminescence of the nanoparticles is not ascribed to the blue Ag luminescence mechanism responsible for the luminescence of the bulk powder, by taking into account the spatial confinement of an electron trapped at the donor and a hole at the acceptor. It is argued that the luminescence mechanism of the nanoparticles is the so called self-activated luminescence, which involves zinc vacancies.
Fixed point theory of iterative excitation schemes in NMR
Authors: Tycko R. , Pines A. , Guckenheimer J.
Published: July 2002

Iterative schemes for NMR have been developed by several groups. A theoretical framework based on mathematical dynamics is described for such iterative schemes in nonlinear NMR excitation. This is applicable to any system subjected to coherent radiation or other experimentally controllable external forces. The effect of the excitation, usually a pulse sequence, can be summarized by a propagator or superpropagator (U). The iterative scheme (F) is regarded as a map of propagator space into itself, Un+1=FUn. One designs maps for which a particular propagator Ū or set of propagators {Ū} is a fixed point or invariant set. The stability of the fixed points along various directions is characterized by linearizing F around the fixed point, in analogy to the evaluation of an average Hamiltonian. Stable directions of fixed points typically give rise to broadband behavior (in parameters such as frequency, rf amplitude, or coupling constants) and unstable directions to narrowband behavior. The dynamics of the maps are illustrated by ‘‘basin images’’ which depict the convergence of points in propagator space to the stable fixed points. The basin images facilitate the optimal selection of initial pulse sequences to ensure convergence to a desired excitation. Extensions to iterative schemes with several fixed points are discussed. Maps are shown for the propagator space SO(3) appropriate to iterative schemes for isolated spins or two-level systems. Some maps exhibit smooth, continuous dynamics whereas others have basin images with complex and fractal structures. The theory is applied to iterative schemes for broadband and narrowband π (population inversion) and π/2 rotations, MLEV and Waugh spin decoupling sequences, selective n-quantum pumping, and bistable excitation.
Monte Carlo simulation of model amphiphile-oil–water systems
Authors: Larson R. G. , Scriven L. E. , Davis H. T.
Published: July 2002

A square-lattice model of amphiphile-oil–water systems is developed in which oil and water molecules occupy single sites and amphiphiles occupy chains of sites. Energies and free energies estimated by Monte Carlo sampling of configuration space show that when the head, or water-loving portion, of the amphiphile has no tendency to hydrate or surround itself with water, as opposed to surrounding itself with other heads, the capability of even long amphiphiles to solubilize repellant oil and water into a single phase is weak. Although the Monte Carlo free energies deviate markedly from those given by quasichemical theory, the deviation of the phase behavior is modest. Computer drawings of typical equilibrium configurations show highly irregular interfaces, apparently caused by capillary waves which are pronounced in two dimensions.
State-selected reaction and relaxation of NH2[X 2B1(0,ν2,0)] radicals and NO2
Authors: Xiang Tian-Xiang , Torres Lourdes M. , Guillory William A.
Published: July 2002

The reaction of ground and vibrationally excited NH2 (ν″2 =0,1) with NO2 has been studied using the sensitive laser-induced fluorescence (LIF) technique. The state-selected NH2 radical was generated via infrared multiple-photon dissociation (IRMPD) of N2H4, NH3, and CH3NH2. The rate constants obtained at room temperature are kNO2(ν″2 =0)=(2.26±0.08)×10−11 cm3 molecule−1 s−1 and kNO2(ν″2 =1)=(1.73±0.06)×10−11 cm3 molecule−1 s−1. The most prominent feature of the results is the decrease of the reaction rate with vibrational bending mode excitation. A possible reaction mechanism is proposed to account for this observation as well as for previously reported literature values. The vibrational relaxation of NH2(ν″2=1) with CH3NH2 and NH3 have also been studied. The room temperature rate constants are kCH3NH2=(7.4±0.5)×10−11 cm3 molecule−1 s−1 and kNH3=(4.7±0.6)×10−11 cm3 molecule s−1, respectively. A near-resonant vibrational relaxation model is proposed to fit the experimental results.