bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of porosity on infrared-absorption spectra of silicon dioxide
Authors: Chou Jenq-Shiuh , Lee Si-Chen
Published: July 2002

The infrared-absorption spectra of silicon dioxide have been studied for many years and most of the peaks have been identified. During the investigation of silicon dioxide deposited by the liquid-phase-deposition technique, an interesting phenomenon was observed. It was found that the intensity ratio between the side lobe (1200 cm−1) and main peak (1090 cm−1) varies and depends on both the concentration of boric acid and the dilution ratio of the growth solution. Measurement of the refractive index shows that the material with larger absorption at 1200 cm−1 has a smaller index and thus more porous structure; therefore, the peak at 1200 cm−1 is suggested to arise from porous oxide, i.e., Si—O—Si, in a large void.
Modification of structural and opto-electronic properties of CdS thin films by Cu doping
Authors: Sebastian P. J. , Ocampo M.
Published: July 2002

The formation of p-type CdS thin films in a chemical bath doped with Cu is reported. Cu-doped films showed an amorphous nature and exhibited high electrical conductivity and quick photoresponse characteristics at high and low levels of Cu doping, respectively. Undoped CdS films were near stoichiometric, crystalline in nature, and exhibited slow photodecay. Cu doping influenced the photoresponse characteristics, structural, electrical, and optical properties of the films. The optical band gap of CdS film changed from 2.35 to 2 eV after Cu doping. These films may be used as a second layer on top of a lower band gap material in thin film solar cell structures and in wide range photo-IR detector structures.
Ferromagnetic resonance study on Fe-SiO2 granular films
Authors: Wang Wen-Nai , Jiang Zheng-Sheng , Du You-Wei
Published: July 2002

Ferromagnetic resonance (FMR) measurement was performed at the frequency of 9.8 GHz and at the room temperature on a series of Fe-SiO2 granular films fabricated by the Ar+-beam sputtering technique. Our results indicated that when the iron volume fraction (fv) of the films exceeds 0.28, the FMR spectra are superpositions of volume modes and surface modes. The analysis on the effective anisotropy field suggests that the intrinsic anisotropy (including the volume and interface one) becomes maximal when fv near the percolation threshold (fp). In particular, it was found for the films with fv=0.42 that the spin wave resonance modes satisfy the relation Hres∝n with a separation field of about 174 Gs between the successive modes. Our study reveals that Fe particles are coupled by a yet unknown interparticle interaction when fv is just below fp.
Energetics and geometry of 90° domain structures in epitaxial ferroelectric and ferroelastic films
Authors: Pertsev N. A. , Zembilgotov A. G.
Published: July 2002

Statics of 90° domain structures formed in a thin tetragonal film epitaxially grown on a cubic substrate is studied theoretically on the basis of a rigorous solution of the associated elasticity problem. Inhomogeneous internal stresses existing in the strained polydomain epitaxy are calculated by the method of fictitious dislocations distributed along domain boundaries and the film/substrate interface. The calculation properly takes into account the influence of the film free surface on the field of internal stresses. For both possible periodic laminar structures, the energies of elastic strain fields existing in the film and the substrate are evaluated. These results are used to compute the equilibrium geometric parameters of periodic domain structures that minimize the total internal energy of a polydomain epitaxy. The dependencies of the equilibrium parameters on the film thickness and the relative misfit strain between the substrate and film are described. By comparing the minimum internal energies associated with various domain configurations, a diagram of equilibrium domain patterns is developed. In the range of relative misfit strains lower than some critical value, two threshold film thicknesses exist, separating three stability regions of domain patterns which correspond to a monovariant film and two periodic domain structures. In the rest of the diagram, a pattern consisting of equivalent domains with the tetragonal axes parallel to the interface is stable, irrespective of the film thickness.
Direct calculation of two-dimensional collection probability in pn junction solar cells, and study of grain-boundary recombination in polycrystalline silicon cells
Authors: Al-Omar Abdul-Azeez S. , Ghannam Moustafa Y.
Published: July 2002

A new method for the direct calculation of the two-dimensional collection probability in pn junction solar cells is presented. Based on its reciprocity properties, the inhomogeneous continuity equation for excess carriers is transformed to a homogeneous partial differential equation (PDE) for the probability of carriers being collected in the external circuit. The new PDE is easier to solve and directly gives the short-circuit current. The method is applied to study the impact of grain-boundary recombination on the performance of polycrystalline silicon solar cells. A critical grain width of four times the carrier diffusion length in the base is found to be the limiting boundary between polycrystalline behavior and monocrystalline behavior of the cell. The sensitivity of short-circuit AM1.5 collection efficiency to the grain width Wg, the grain-boundary recombination velocity Sg, minority-carrier diffusion lengths, and surface recombination velocities, is quantified for a variety of cell types and recombination parameters. The sensitivity analysis indicates that AM1.5 collection efficiency is most sensitive to grain width in narrow grain cells, and to base diffusion length in wide grain cells.
p-type doping of CdTe with a nitrogen plasma source
Authors: Oehling S. , Lugauer H. J. , Schmitt M. , Heinke H. , Zehnder U. , Waag A. , Becker C. R. , Landwehr G.
Published: July 2002

We report here on the growth and the characterization of p-type CdTe grown by molecular beam epitaxy on (001) Cd0.96Zn0.04Te substrates. Nitrogen has been used as a dopant, which is activated in an electron cyclotron resonance plasma source. The carrier concentration was determined using a C/V profiler. Nitrogen has been successfully incorporated substitutionally and hole densities up to 2.6×1017 cm−3 have been achieved. In addition we present data from x-ray diffraction and photoluminescence, which demonstrate the effect of self-compensation on the nitrogen-doped CdTe layers.
Transport properties of GaAs layers grown by molecular beam epitaxy at low temperature and the effects of annealing
Authors: Luo J. K. , Thomas H. , Morgan D. V. , Westwood D.
Published: July 2002

The effects of growth temperature and subsequent annealing temperatures on the electrical properties of the low temperature (LT) grown GaAs have been investigated. It was found that the resistivity of the as-grown LT-GaAs layer increased with increasing growth temperature, but was accompanied by a reduction of breakdown voltage over the same temperature range. Thermal annealing of the samples caused the resistivity to rise exponentially with increasing annealing temperature TA, giving an activation energy of EA=2.1 eV. The transport of the LT-GaAs layers grown at Tg≤250 °C was found to be dominated by hopping conduction in the entire measurement temperature range (100–300 K), but following annealing at TA≳500 °C, the resistivity-temperature dependence gave an activation energy of ∼0.7 eV. The breakdown voltage VBD, for as-grown LT-GaAs was enhanced on lowering the measurement temperature, but conversely, decreased over the same temperature range following annealing at TA≳500 °C. The hopping conduction between arsenic defects, or arsenic clusters in annealed samples, is believed to be responsible for the observed electrical breakdown properties. Since the resistivities of the as-grown LT-GaAs layers are dependent, solely, on the excess arsenic, which in turn depends on the growth temperature, then the resistivities obtained can be used as a measure of the growth temperature.
Electron traps in Ca2+- or Sr2+-doped BaFBr:Eu2+ x-ray storage phosphors
Authors: Schweizer S. , Willems P. , Leblans P. J. R. , Struye L. , Spaeth J.-M.
Published: July 2002

The x-ray storage phosphor BaFBr:Eu2+ may be made sensitive to stimulation further into the infrared by doping with Ca2+ or Sr2+. This enables the use of light-emitting diodes instead of gas lasers for photostimulation. It is shown by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and optically detected EPR and ENDOR that upon x irradiation FA(Br−, Ca2+) centers are formed as photostimulable electron traps in Ca2+-doped BaFBr, in which one of the two nearest Ba2+ neighbors along the c axis is replaced by Ca2+. The FA(Br−, Ca2+) center has tetragonal symmetry as has the F(Br−) center. The optical absorption band of those FA(Br−, Ca2+) centers is red shifted. No FA(Br−, Ca2+) centers have been detected upon additive coloration only. The red shift of the photostimulation spectrum is caused by formation of the FA(Br−, Ca2+, or Sr2+) centers as electron traps.
Electro-refraction and electro-absorption in poled polymer Fabry–Perot étalons
Authors: O’Brien Ned F. , Dominic Vince , Caracci Stephen
Published: July 2002

We present a simple experimental procedure that uses a slowly rotating étalon to measure simultaneously the electro-refraction and electro-absorption in a poled polymer. Both effects generally contribute to the measured signal from such material systems and can be distinguished by rotating the sample and observing asymmetric peaks in the signal. The experimental results show the expected increase in both electro-refraction and electro-absorption as the probe wavelength approaches the absorption band of the chromophore. Furthermore, the dispersion of the complex electro-optic coefficient displays a periodic variation that we attribute to multiple-étalon interference. The stratified nature of the thin-film structure causes the multiple-reflection interference. This artifact will pollute most of the standard electro-optic characterization techniques for poled-polymer films.
Photostimulation redshift for nonstoichiometric Ba1−xSrxFBr:Eu2+
Authors: Dietze C. , Hangleiter Th. , Willems P. , Leblans P. J. R. , Struye L. , Spaeth J.-M.
Published: July 2002

The oxygen-free x-ray storage phosphor BaFBr, doped with Sr2+ and having Br− vacancies due to fluorine excess, is shown to have a high photostimulability shifted appreciably to lower photon energy compared to nondoped BaFBr because of the predominant generation of FA(Sr2+) centers on the Br− sublattice upon room-temperature x irradiation. The FA(Br−,Sr2+) centers are less stable than unperturbed F(Br−) centers. Therefore, the redshift of the photostimulability disappears upon thermal activation above room temperature. From the failure to observe an F center infrared emission, it is concluded that F and FA centers are spatially correlated to hole centers of yet unknown nature.
Single electron transistor using a molecularly linked gold colloidal particle chain
Authors: Sato Toshihiko , Ahmed Haroon , Brown David , Johnson Brian F. G.
Published: July 2002

By applying a dithiol (1,6-hexanedithiol) treatment, it was observed that a submonolayer of gold colloidal particles deposited by using an aminosilane adhesion agent [i.e., 3-(2-aminoethylamino) propyltrimethoxysilane] transform themselves into chains consisting of a few gold colloidal particles. In those chains, gold colloidal particles are believed to be linked by alkane chains derived from the dithiol molecules. The particle chain was formed on a SiO2 substrate with source, drain, and gate metal electrodes defined by electron beam lithography. It was observed that the gold particle chain bridged a gap between the source and drain forming a single electron transistor with a multi-tunnel junction in the particle chain. The electron conduction through the chain exhibited a clear Coulomb staircase and the periodic conductance oscillation as a function of gate voltage. These measurement results corresponded closely to the results of a simulation based on the orthodox theory.
Electric field induced phase transition of antiferroelectric lead lanthanum zirconate titanate stannate ceramics
Authors: Park Seung-Eek , Pan Ming-Jen , Markowski Kelley , Yoshikawa Shoko , Cross L. Eric
Published: July 2002

The electric field induced phase transition behavior of lead lanthanum zirconate titanate stannate (PLZTS) ceramics was investigated. PLZTS undergoes a tetragonal antiferroelectric (AFETet) to rhombohedral ferroelectric (FERh) phase transition with the application of an electric field. The volume increase associated with this antiferroelectric (AFE)–ferroelectric (FE) phase transition plays an important role with respect to actuator applications. This volume increase involves an increase in both transverse and longitudinal strains. The E field at which the transverse strain increases is accompanied by an abrupt jump in polarization. The longitudinal strain, however, lags behind this polarization jump exhibiting a slight decrease at the onset of phase switching. This decoupling was related to the preferentially oriented AFE domain configuration, with its tetragonal c-axis perpendicular to the applied electric field. It is suggested that phase switching involves multiple steps involving both structural transformation and domain reorientation.
Structural, electrical, and low-temperature dielectric properties of sol–gel derived SrTiO3 thin films
Authors: Thomas Reji , Dube D. C. , Kamalasanan M. N. , Chandra Subhas , Bhalla A. S.
Published: July 2002

Strontium titanate sol was prepared using strontium ethyl haxanoate and titanium isopropoxide. The sol was then spin coated on fused silica, p-type single-crystal silicon wafers (100) and stainless-steel substrates and annealed to give polycrystalline, transparent, and crack-free films. The surface morphology and structural properties of the films were studied using scanning electron microscopy and x-ray diffraction, respectively, and differential thermal analysis was used to observe structural transition. The dielectric measurements were conducted on films with metal–insulator–metal and metal–insulator–semiconductor configurations. Capacitance–voltage (C–V) measurements were carried out and the effect of the annealing temperature was studied. The dielectric constant and loss tangent at 1 MHz at room temperature were found to be 105 and 0.02, respectively, for 1.1 μm thick films. These measurements were also carried out at low temperatures down to 20 K. There are indications for a phase transition from a cubic perovskite to tetragonal perovskite structure at about 100 K where the tan δ shows some fluctuation, a characteristic of such transitions. The absence of a peak in the dielectric constant and the absence of hysteresis below the transition temperature have been explained on the basis of the low value of the tetragonal distortion (c/a=1.003) reported on bulk material.
Characterization of GaAs wire crystals grown on porous silicon by Raman scattering
Authors: da Silva S. W. , Lubyshev D. I. , Basmaji P. , Pusep Yu. A. , Pizani P. S. , Galzerani J. C. , Katiyar R. S. , Morell G.
Published: July 2002

We measured the Raman spectra of GaAs wirelike crystals grown on porous silicon (PS) using two different excitation radiations which probe the near surface and the bulk. The transverse optic and longitudinal optic vibrational bands appear redshifted and broadened when compared to bulk GaAs, and with shoulders on their low frequency sides. These features are attributed to a disorder-induced relaxation in the selection rules for Raman activity arising from point defects. In addition, the bands show a strong frequency and bandwidth dependence on excitation energy. Differences in penetration depth together with a gradient in defect density are invoked to account for the observations. This analysis yields a picture in which there is a predominantly amorphous GaAs region buried at the PS–GaAs interface followed by microcrystalline GaAs cylindrical structures that become less defective as they grow farther from the interface. The near surface tends to approach the low defect density of crystalline GaAs.
Thermodynamics of polydomain heterostructures. II. Effect of microstresses
Authors: Roytburd Alexander L.
Published: July 2002

The thermodynamic effects of microstresses on the formation of polydomain heterostructures are considered. The microstresses arise due to the periodic deviation of the actual misfit from the average misfit on the interfaces between a polydomain layer and neighboring layers. The thermodynamic analysis is restricted to heterostructures with D/h<1, where D is the period of the polydomain structure, and h is the thickness of the thinner layer of the heterostructure, either active or passive. In this case the long-range interactions between interfaces through their microstresses can be neglected. A simple and effective approximation for the microstress energy in polydomain heterostructures is proposed. Using this approximation, the effect of thickness on the polydomain structure can be included in the thermodynamic theory through the adjustment of the parameter of incompatibility between the domains. If the domain interfaces are mobile, the critical behavior of compliance and susceptibility should be observed at the critical thicknesses of the polydomain layer. These critical thicknesses correspond to loss of stability of the polydomain structure or change of polydomain symmetry.
Holographic storage media based on optically active bistable defects
Authors: Linke R. A. , Redmond I. , Thio T. , Chadi D. J.
Published: July 2002

We describe a family of reversible holographic storage materials which exploit the bistability of the crystal defect known as the “DX” center. Crystals containing these defects have the characteristics of local photorefractive materials in that their refractive index is modified in proportion to the local optical energy absorbed. This refractive index change, which results from the release of electrons from the DX deep trap states into the conduction band, is persistent at low temperatures due to a capture barrier, Ecap, which limits reformation of the DX centers. The effect is reversed by heating above an annealing temperature, which scales with Ecap and varies with the crystal host and active dopant. A number of DX materials have now been identified with long-term persistence temperatures ranging from 50 to 180 K. In this paper, we briefly review the physics of the DX center and present theoretical estimates of several important optical properties of these materials based on a simple model. We calculate spatial resolution, maximum refractive index shift, and sensitivity, and compare our predictions with measurements on one member of the DX family, AlGaAs:Te. In a 345 μm thick sample of this material doped at 9×1017 cm−3, we find a refractive index shift, Δn, of 2×10−3 and an exposure sensitivity, S, of 0.012 cm3/J. Our expectation that the maximum refractive index change scales linearly with the doping density is consistent with our previous measurement of Δn=1.1×10−2 obtained for a sample of AlGaAs:Si doped at 4×1018 cm−3. The measured values of Δn and S, are, respectively, two and three orders of magnitude larger than corresponding values for the photorefractive material LiNbO3, and are shown here to be independent of exposing irradiance from 10−3 to 108 W/cm2. At the latter irradiance, the refractive index shift is shown to occur with a material response time shorter than our measurement limit of several picoseconds. Thus, this material exhibits high sensitivity, large refractive index change, and fast write time, all desirable properties of an optical holographic storage medium. Phase gratings written in AlGaAs:Te using low-power (mW) beams from infrared diode lasers give diffraction efficiencies from 30% to 55% for grating periods from 0.13 to 15 μm. No degradation of sensitivity is observed after large numbers of exposure–erasure cycles. Experiments with multiple-hologram exposures show that the DX materials require no exposure schedule: equal strength holograms are obtained using equal exposures. Binary data have been stored in the form of multiplexed two-dimensional arrays of pixel bits. Required material and system parameters are estimated for a 1 Tbyte holographic storage device based on angle multiplexing in a DX material.
Percolation study on electrical resistivity of SiC/Si3N4 composites with segregated distribution
Authors: Kim W. J. , Taya M. , Yamada K. , Kamiya N.
Published: July 2002

Experimental and theoretical studies are made to account for the formation of segregated networks in SiC/Si3N4 particulate composite. It was observed in the present experimental work that fine SiC particles are uniformly dispersed along boundaries of Si3N4 matrix grains. The proposed theoretical model takes into account the volume fraction of dispersed particles necessary to form a percolating network as a function of the size ratio of SiC particle to Si3N4 grain. Threshold particle loading for percolation is estimated by applying a modified two-dimensional bond percolation model and then electrical resistivity of the composite by using percolation and general effective media equations. Both predicted values of threshold particle loading and electrical resistivity were found to agree closely with the present experimental values.
Crystal ion slicing of single-crystal magnetic garnet films
Authors: Levy M. , Osgood R. M. , Kumar A. , Bakhru H.
Published: July 2002

Epitaxial liftoff has been used for achieving heterogeneous integration of many III-V and elemental semiconductor systems. However, it has been heretofore impossible to integrate devices of many other important material systems. A good example of this problem has been the integration of single-crystal transition metal oxides on semiconductor platforms, a system needed for on-chip thin film optical isolators. We report here an implementation of epitaxial liftoff in magnetic garnets. Deep ion implantation is used to create a buried sacrificial layer in single-crystal yttrium iron garnet (YIG) and bismuth-substituted YIG (Bi-YIG) epitaxial layers grown on gadolinium gallium garnet (GGG). The damage generated by the implantation induces a large etch selectivity between the sacrificial layer and the rest of the garnet. Ten-micron-thick films have been lifted off from the original GGG substrates by etching in phosphoric acid. Millimeter-size pieces of excellent quality have been transferred to the silicon and gallium arsenide substrates. Study of the magnetic domain structure in the detached epilayers by Faraday contrast shows no changes in film anisotropy. Optical insertion loss measurements are also presented.
Resonant tunneling in step-barrier structures under an applied electric field
Authors: Guo Yong , Gu Bing-Lin , Yu Jing-Zhi , Zeng Zhong , Kawazoe Yoshiyuki
Published: July 2002

Resonant tunneling in step-barrier structures is investigated by using the transfer-matrix technique. The formulas for the transmission coefficient and the current density are derived when taking into account the coupling between components of the motion of an electron in directions parallel and perpendicular to the interfaces. By making a detailed comparison of resonant tunneling among single square-barrier structures, asymmetric double-barrier structures, and step-barrier structures, the tunneling properties in step-barrier structures are revealed. It is shown that the global behavior of step-barrier structures obtained resembles that of asymmetric double-barrier structures, and step-barrier structures are superior to both single- and double-barrier structures in many aspects. In comparison to asymmetric double-barrier structures, step-barrier structures have several features, such as a wider negative-differential resistance region, easier fabrication, high-speed response, and a relatively lower transmission coefficient and current peak-to-valley ratios. Moreover, higher resonant bias is required in order to obtain optimal transmission resonances in the step-barrier structure. The results shown in this work not only shed new light on the physics of resonant tunneling in electric-barrier structures but are also helpful in designing quantum devices based on step-barrier tunneling structures.
A surface and a gas-phase mechanism for the description of growth on the diamond(100) surface in an oxy-acetylene torch reactor
Authors: Okkerse M. , de Croon M. H. J. M. , Kleijn C. R. , van den Akker H. E. A. , Marin G. B.
Published: July 2002

A gas-phase and a surface mechanism were developed, suitable for multidimensional simulations of diamond oxy-acetylene torch reactors. The gas-phase mechanism was obtained by reducing a 48 species combustion chemistry mechanism to a 27 species mechanism with the aid of sensitivity analysis. The surface mechanism for growth on monocrystalline (100) surfaces developed, was based on literature quantum-mechanical calculations by Skokov et al. It consists of 67 elementary reaction steps and 41 species, and contains CH3 and C2H2 as gas-phase growth precursors and atomic hydrogen and oxygen to etch carbon from the surface. The gas-phase and surface chemistry models were tested in one-dimensional simulations, yielding dependencies of the growth rate on feed composition and surface temperature that are in qualitative agreement with the experiments. A more detailed study of the surface chemistry showed that, compared to CH3, acetylene contributes very little to diamond growth. Furthermore, molecular and atomic oxygen do not affect the diamond surface as much as atomic hydrogen because of their low concentrations.
Ferroelectric thin films grown on tensile substrates: Renormalization of the Curie–Weiss law and apparent absence of ferroelectricity
Authors: Pertsev N. A. , Zembilgotov A. G. , Hoffmann S. , Waser R. , Tagantsev A. K.
Published: July 2002

A thermodynamic theory is used to calculate the dielectric properties of epitaxial thin films grown on cubic substrates imposing biaxial tension on the film prototypic cubic state. For such “tensile” substrates, the theory shows that, in a conventional plate–capacitor setup, the films should display only a monotonic variation or a broad maximum in the temperature dependence of the permittivity and an apparent absence of the hysteretic polarization behavior. Changes of the Curie–Weiss temperature and constant caused by the mechanical film/substrate interaction are also predicted. Theoretical results agree with the observed properties of columnar-structured BaTiO3 and SrTiO3 thin films grown on Si.
Two-channel AlGaN/GaN heterostructure field effect transistor for high power applications
Authors: Gaska R. , Shur M. S. , Fjeldly T. A. , Bykhovski A. D.
Published: July 2002

We report on a double-channel AlGaN/GaN heterostructure field-effect transistor (HFET) for high power applications, where the bottom channel is formed by a GaN/AlGaN/GaN semiconductor–insulator–semiconductor structure. The band structure and the charge distribution are strongly influenced by the piezoelectric effect caused by the mismatch between AlGaN and GaN. This new design demonstrates that the current carrying capability of AlGaN/GaN HFETs can be enhanced using multichannel structures.
Evidence of chemical reactions in the hydroxyapatite laser ablation plume with a water atmosphere
Authors: Serra P. , Morenza J. L.
Published: July 2002

The expansion dynamics of the ablation plume generated by KrF laser irradiation of hydroxyapatite targets in a 0.1 mbar water atmosphere has been studied by fast intensified charge coupled device imaging with the aid of optical bandpass filters. The aim of the filters is to isolate the emission of a single species, which allows separate analysis of its expansion. Images obtained without a filter revealed two emissive components in the plume, which expand at different velocities for delay times of up to 1.1 μs. The dynamics of the first component is similar to that of a spherical shock wave, whereas the second component, smaller than the first, expands at constant velocity. Images obtained through a 520 nm filter show that the luminous intensity distribution and evolution of emissive atomic calcium is almost identical to those of the first component of the total emission and that there is no contribution from this species to the emission from the second component of the plume. The analysis through a 780 nm filter reveals that atomic oxygen partially diffuses into the water atmosphere and that there is a contribution from this species to the emission from the second component. The last species studied here, calcium oxide, was analyzed by means of a 600 nm filter. The images revealed an intensity pattern more complex than those from the atomic species. Calcium oxide also contributes to the emission from the second component. Finally, all the experiments were repeated in a Ne atmosphere. Comparison of the images revealed chemical reactions between the first component of the plume and the water atmosphere.
Constrained density functional theory for first principles spin dynamics
Authors: Újfalussy B. , Wang Xin-Dong , Nicholson D. M. C. , Shelton W. A. , Stocks G. M. , Wang Yang , Gyorffy B. L.
Published: July 2002

Constrained density functional theory is used to formulate a theory of general noncollinear spin systems which makes it possible to implement first principles spin dynamics in a manner that is firmly grounded in density functional theory. At each time step, local constraining fields are calculated from a self-consistent algorithm. In addition to discussing the conceptual basis of the resulting constrained local moment model we illustrate the theory by explicit calculations for the relative rotation of the corner and body center moments of bcc iron.
Generating random media from limited microstructural information via stochastic optimization
Authors: Cule D. , Torquato S.
Published: July 2002

Random media abound in nature and in manmade situations. Examples include porous media, biological materials, and composite materials. A stochastic optimization technique that we have recently developed to reconstruct realizations of random media (given limited microstructural information in the form of correlation functions) is investigated further, critically assessed, and refined. The reconstruction method is based on the minimization of the sum of squared differences between the calculated and reference correlation functions. We examine several examples, including one that has appreciable short-range order, and focus more closely on the kinetics of the evolution process. The method is generally successful in reconstructing or constructing random media with target correlation functions, but one must be careful in implementing an earlier proposed time-saving step when treating random media possessing significant short-range order. The issue of the uniqueness of the obtained solutions is also discussed.
Temperature quenching of photoluminescence intensities in undoped and doped GaN
Authors: Leroux M. , Grandjean N. , Beaumont B. , Nataf G. , Semond F. , Massies J. , Gibart P.
Published: July 2002

This work discusses the temperature behavior of the various photoluminescence (PL) transitions observed in undoped, n- and p-doped GaN in the 9–300 K range. Samples grown using different techniques have been assessed. When possible, simple rate equations are used to describe the quenching of the transitions observed, in order to get a better insight on the mechanism involved. In undoped GaN, the temperature dependence of band edge excitonic lines is well described by assuming that the A exciton population is the leading term in the 50–300 K range. The activation energy for free exciton luminescence quenching is of the order of the A rydberg, suggesting that free hole release leads to nonradiative recombination. In slightly p-doped samples, the luminescence is dominated by acceptor related transitions, whose intensity is shown to be governed by free hole release. For high Mg doping, the luminescence at room temperature is dominated by blue PL in the 2.8–2.9 eV range, whose quenching activation energy is in the 60–80 meV range. We also discuss the temperature dependence of PL transitions near 3.4 eV, related to extended structural defects.
Transparent stacked organic light emitting devices. I. Design principles and transparent compound electrodes
Authors: Gu G. , Parthasarathy G. , Burrows P. E. , Tian P. , Hill I. G. , Kahn A. , Forrest S. R.
Published: July 2002

Vertical stacking of organic light emitting devices (OLEDs) that emit the three primary colors is a means for achieving full-color flat panel displays. The physics, performance, and applications of stacked OLEDs (SOLEDs) are discussed in this and the following paper (Papers I and II, respectively). In Paper I, we analyze optical microcavity effects that can distort the emission colors of SOLEDs if not properly controlled, and describe design principles to minimize these parasitic effects. We also describe the fabrication and operating characteristics of transparent contacts that are an integral part of SOLEDs. We demonstrate that both metal-containing and metal-free transparent electrodes can serve as efficient electron and hole injectors into the stacked organic semiconductor layers. Two different transparent SOLED structures (metal-containing and metal-free) that exhibit sufficient performance for many full-color display applications will be discussed in Paper II.
Maximum entropy principle for nonparabolic hydrodynamic transport in semiconductor devices
Authors: Trovato M. , Falsaperla P. , Reggiani L.
Published: July 2002

A closed hydrodynamic approach for a full nonparabolic band model is developed from the maximum entropy principle. Generalized kinetic fields are introduced within a total average-energy scheme. Numerical calculations for bulk and submicron Si structures are found to compare well with those obtained by ensemble Monte Carlo simulators thus validating the proposed approach.
e
31
piezoelectric constant measurement of lead zirconate titanate thin films
Authors: Cattan E. , Haccart T. , Rémiens D.
Published: July 2002

The piezoelectric constant e31 measurement procedure is presented for sputtered lead zirconate titanate (PZT) thin films deposited on silicon substrates. With a simple cantilever used in the direct piezoelectric effect, it is possible to measure piezoelectric coefficients of unpoled and poled films. We discuss the poling treatment effects; applied electric field amplitude and duration on the piezoelectric response. We have registered a quasistatic piezoelectric hysteresis loop, and will show that the poling procedure can modify the symmetry of the hysteresis loop. The thin films were piezoelectric in a virgin state, which causes the offset of the piezoelectric hysteresis loops. The maximum remanent piezoelectric constant e31 estimated for sputtered PZT thin film is −4 C/m2.
Influence of environmental conditions on shear–force distance control in near-field optical microscopy
Authors: Brunner R. , Marti O. , Hollricher O.
Published: July 2002

In our experiments we show, that a contaminating water film is very important for the shear–force distance control in near-field optical microscopy. This is demonstrated at the transition between a hydrophilic glass surface and a hydrophobic Langmuir–Blodgett film of arachidic acid at different relative humidities. This contaminating water film is one, if not the important reason for the damping of an oscillating fiber during surface approach. It is further shown, that the bulk viscosity of water alone cannot be responsible for the observed damping effect. A thickness dependent viscosity of this water film is proposed. These observations can also explain, why the shear–force distance control works on nearly all surfaces at ambient conditions, but fails to work at very low temperatures.
Laser induced removal of spherical particles from silicon wafers
Authors: Lu Y. F. , Zheng Y. W. , Song W. D.
Published: July 2002

Laser-induced removal of spherical silica particles from silicon wafers was investigated. The cleaning efficiency and laser cleaning thresholds for particles with diameters of 0.5, 1.0, 2.5, and 5.0 μm were carefully measured. It is found that the cleaning efficiency is more sensitive to laser fluence than laser pulse number and repetition frequency. The particle ejecting energies were found to increase with laser fluence. The threshold laser fluences for removing particles with sizes of 0.5 and 1.0 μm are 225 and 100 mJ/cm2, respectively, when KrF excimer laser is used. The threshold laser fluences are only a value below 5.0 mJ/cm2 for particles with a size of 2.5 and 5 μm. A model including both linear expansion and elastic deformation model was proposed to explain the experimental results. With this model, the particle movement and deformation in laser cleaning process were calculated. The expressions for threshold laser fluences were derived. The theoretical predictions are found to be greater than the experimental results. The difference can be explained by the enhancement of light intensity near the contacting area, due to the focusing and scattering by spherical particles. This model is useful to the study of laser cleaning as well as particle adhesion and deformation on solid surfaces.
Effect of arsenic precipitates on Fermi level in GaAs grown by molecular-beam epitaxy at low temperature
Authors: Chen Y. H. , Wang Z. G. , Yang Z.
Published: July 2002

A simple model is presented to discuss the effect of As precipitates on the Fermi level in GaAs grown by molecular-beam epitaxy at low temperature (LT-GaAs). This model implements the compensation between point defects and the depletion of arsenic precipitates. The condition that the Fermi level is pinned by As precipitates is attained. The shifts of the Fermi level in LT-GaAs with annealing temperature are explained by our model. Additionally, the role of As precipitates in conventional semi-insulating GaAs is discussed.
Magnetic structure of LaFe10.8Al2.2 and LaFe10.8Al2.2N3 cluster compounds
Authors: Moze O. , Kockelmann W. , Liu J. P. , de Boer F. R. , Buschow K. H. J.
Published: July 2002

The magnetic structure at 15 K of LaFe10.8Al2.2 and its ternary nitride LaFe10.8Al2.2N3 have been determined by high resolution time-of-flight neutron powder diffraction. Magnetism in these materials is determined by clusters formed by a central Fe atom surrounded by Fe atoms with icosahedral symmetry. This icosahedral symmetry is not affected by N uptake since both compounds are confirmed to crystallize in the cubic NaZn13-type structure. The Al atoms are statistically distributed over the 96i cluster site only, while N atoms are found to fully occupy the highly symmetrical 24d interstitial hole site. Charging with N gives rise to an 8% increase in the unit cell volume of the parent compound. The magnetic structures are overall ferromagnetic for both compounds, with each of the two Fe sublattices aligned parallel to each other. The influence of N uptake on the magnitudes of the cluster Fe moments will be discussed.
Influence of plasma parameters on the growth and properties of magnetron sputtered CNx thin films
Authors: Hellgren Niklas , Macák Karol , Broitman Esteban , Johansson Mats P. , Hultman Lars , Sundgren Jan-Eric
Published: July 2002

Carbon nitride CNx thin films were grown by unbalanced dc magnetron sputtering from a graphite target in a pure N2 discharge, and with the substrate temperature Ts kept between 100 and 550 °C. A solenoid coil positioned in the vicinity of the substrate was used to support the magnetic field of the magnetron, so that the plasma could be increased near the substrate. By varying the coil current and gas pressure, the energy distribution and fluxes of N2+ ions and C neutrals could be varied independently of each other over a wide range. An array of Langmuir probes in the substrate position was used to monitor the radial ion flux distribution over the 75-mm-diam substrate, while the flux and energy distribution of neutrals was estimated through Monte Carlo simulations. The structure, surface roughness, and mechanical response of the films are found to be strongly dependent on the substrate temperature, and the fluxes and energies of the deposited particles. By controlling the process parameters, the film structure can thus be selected to be amorphous, graphite-like or fullerene-like. When depositing at 3 mTorr N2 pressure, with Ts>200 °C, a transition from a disordered graphite-like to a hard and elastic fullerene-like structure occurred when the ion flux was increased above ∼0.5–1.0 mA/cm2. The nitrogen-to-carbon concentration ratio in the films ranged from ∼0.1 to 0.65, depending on substrate temperature and gas pressure. The nitrogen film concentration did, however, not change when varying the nitrogen ion-to-carbon atom flux ratios from ∼1 to 20.
Influence of interface and Al structure on layer exchange during aluminum-induced crystallization of amorphous silicon
Authors: Nast Oliver , Hartmann Andreas J.
Published: July 2002

Aluminum-induced crystallization of amorphous silicon (a-Si) is studied using various microscopy techniques and x-ray photoelectron spectroscopy. During the isothermal annealing of subsequently deposited aluminum and a-Si films on glass, a layer exchange process is induced, while a continuous polycrystalline silicon film (poly-Si) on glass is formed within the initial metal layer and therefore displaces it. This crystallization process is conducted at temperatures ranging from 350 °C to 500 °C, significantly below the eutectic temperature of the Si–Al binary system of 577 °C. The results presented focus on the influences of the polycrystalline structure of the evaporated Al, the Si–Al layer sequence, and the interface layer between the Al and Si films on the overall crystallization process. They reveal that the larger the Al grain size of the initial polycrystalline Al layer, the larger the grain size of the final poly-Si film and the slower the entire layer exchange process. It is further shown that the layer sequence, although influencing the speed of the poly-Si formation, has little impact on the overall layer exchange process. Additionally, evidence is given that an Al oxide interface layer separates the continuous poly-Si layer from the Al, independent of the original layer sequence. The analyzed oxide interface layer remains at its position throughout the entire Al and Si layer exchange process. An existing phenomenological model of the diffusion-controlled crystallization during the layer exchange is extended to embrace the role of the parameters discussed in this paper.
Study of block copolymer interface by pulsed NMR
Authors: Tanaka Hajime , Nishi Toshio
Published: July 2002

The polymer–polymer interface, i.e., the interface of the styrene-diene block copolymer, is detected by use of pulsed NMR, probably for the first time. The thickness of the interface estimated from NMR is about 20 Å in all the observed block copolymers, which is consistent with the results of the small angle x-ray scattering. The interface is detected by the NMR measurement through the two effects: (1) intramolecular effect and (2) intermolecular effect. The intramolecular effect is related to the restriction of the degree of molecular motion of the part of the chains bound to the phase with low mobility. This effect is characteristic in polymers because polymers have very large molecular weights and nuclei in polymers are correlated with each other on account of its connectivity. On the other hand, the intermolecular effect is directly related to the state of mixing of the two different polymers. The intramolecular effect seems to be mainly active for reducing the mobility of the part of the molecules in the interface in block copolymers.
Molecular conformational equilibria in liquids
Authors: Zichi Dominic A. , Rossky Peter J.
Published: July 2002

The extended RISM integral equation theory is employed to study molecular conformational equilibria in liquids. Solvent induced torsional free energy surfaces are computed for the solutes n-butane and 1, 2-dichloroethane at infinite dilution in apolar and polar model liquids, CCl4 and water. In addition, the intramolecular distribution of conformers for the neat molecular liquids n-butane and 1, 2-dichloroethane are determined. The qualitative features of these free energies agree well with available simulation results. Quantitatively, the absolute solvent effects are consistently overestimated; however, the differences between polar and apolar systems are accurately predicted. The thermodynamic components of the solvent induced free energies for the dilute solutions are analyzed. For the systems examined here, the internal energy dominates the torsional free energies. In particular, for n-butane in water, the increase in gauche conformers is in accord with the notion of a hydrophobic interaction. However, the thermodynamic behavior found here opposes the usual belief that such association is an entropy driven process.
Magnetic coupling and dynamics in solid α and β-O2. I. An a
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o theoretical approach
Authors: Jansen A. P. J. , van der Avoird A.
Published: July 2002

This paper describes a new approach to the dynamic and magnetic properties of solid α and β oxygen which is based on two theoretical developments. First, we have constructed the lattice and spin Hamiltonian for solid O2 by including explicitly the interactions between the triplet ground state O2 molecules as obtained mainly from recent ab initio calculations. The spin coupling parameters in this Hamiltonian, especially the Heisenberg exchange parameter J, are strongly dependent on the positions and orientations of the molecules. Secondly, we have developed an integrated scheme for lattice dynamics and spin wave calculations which uses this Hamiltonian. The actual mixing between the lattice modes, phonons and librons, and the magnons appears to be small; their interaction can be largely taken into account by renormalization of the coupling terms. In the lattice dynamics part of the calculation it is essential to include the Heisenberg term, since it is the extremely strong anisotropy of the coupling parameter J that explains the anomalously large libron splitting in α-O2. The spin-wave calculation with the Hamiltonian averaged over the lattice vibrations yields reasonable values for the magnon frequencies with no empirical fit parameters.
Isotope effects in the fragmentation of water: The photodissociation of HOD in the first absorption band
Authors: Engel V. , Schinke R.
Published: July 2002

We investigate the photofragmentation of HOD in the first absorption band. Full three-dimensional quantum mechanical calculations on an ab initio excited state potential are presented. They are based on the IOS approximation for the rotational (bending) degree of freedom. The remaining two-dimensional problem is treated exactly using hyperspherical (polar) coordinates. HOD absorption spectra are compared with recently published results for H2O and D2O. Dissociation of the vibrational ground state favors the production of OD fragments, in agreement with physical intuition. The final vibrational distributions for OH and OD are relatively broad indicating strong final state interaction. They are broader for OD but inverted for OH products. Cross sections for the dissociation of vibrationally excited stretching states are also compared with those for H2O and a strong isotope effect is found. The branching ratio depends significantly on the initial vibrational state of the parent molecule. The quantum mechanical results are compared with simple classical calculations and generally very good agreement is found. Within the classical limit all data are readily understandable.