bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural modifications of alumina implanted with zirconium, copper, and titanium ions
Authors: Bigarré J. , Fayeulle S. , Tréheux D. , Moncoffre N.
Published: July 2002

Microstructural modifications (amorphization, lattice deformation, phase transformations) in alumina induced by implantation of zirconium, copper, or titanium ions and by postimplantation thermal annealings were studied using grazing incidence x-ray diffraction. It was shown that the amount of lattice deformation and the type of damage resulting in the lattice depend on the ion implanted. When zirconium was implanted, the alumina lattice was highly deformed. Amorphization was observed when a high ion dose was implanted. Copper implantation led to the formation of gamma alumina. With titanium ions, very high strain was created and delta alumina was formed. After postimplantation annealings, lattices returned to their equilibrium state through crystallization of alpha alumina and precipitation of oxides of the implanted species (ZrO2, CuO and CuAl2O4, and TiO2).
Sputtered lead scandium tantalate thin films: Pb4+ in B sites in the perovskite structure
Authors: Whatmore R. W. , Huang Z. , Todd M.
Published: July 2002

Perovskite lead scandium tantalate (PST)—PbSc1/2Ta1/2O3—thin films have been prepared from lead-rich targets by rf magnetron sputtering onto a variety of substrates (e.g., MgO-on-sapphire and platinized silicon). Detailed microstructural chemical analysis of the films using transmission electron microscopy and energy dispersive x-ray spectroscopy techniques showed that they contained large amounts of excess lead, ranging from 7 to over 70 at. % and that this was located within the crystallite grains making up the films rather than solely at grain boundaries. Measurement of the lattice parameters of the films using x-ray diffraction and using the underlying materials as internal standards have shown that the lattice parameters of the films are consistently larger than those for bulk PST. Comparison of the film lattice parameters with composition and with the lattice parameter/composition trend of a large number of lead containing perovskites strongly suggests that the excess lead is present on the B sites of the structure as Pb4+. The consequences for this on the structural and electrical properties of these films and PST films deposited by other workers are discussed.
Minima of dissipated power in magnetic levitation
Authors: Clemente R. A. , Tessarotto M.
Published: July 2002

An approximate model for calculating the average lifting force and absorbed power, for a magnetically levitated metallic sphere, is presented. It is shown that by imposing suitable boundary conditions, expressed in terms of matching conditions for the magnetic field, it is possible to express the average absorbed power in terms of appropriate physical parameters. For a given conductive sphere, three cases of interest are analyzed, namely: (i) fixed frequency and resistivity; (ii) fixed external currents and resistivity; (iii) fixed external currents and frequency. The analysis displays the relevant parametric dependence and proves the existence of minima for the absorbed power in parameter space. The results are of potential interest for improving the temperature control of the metallic sample, which is of fundamental importance both in levitation melting and undercooling processes.
Influences of initial nitridation and buffer layer deposition on the morphology of a (0001) GaN layer grown on sapphire substrates
Authors: Fuke Shunro , Teshigawara Hidekazu , Kuwahara Kazuhiro , Takano Yasushi , Ito Takahiro , Yanagihara Masataka , Ohtsuka Kohji
Published: July 2002

Hexagonal GaN is grown on (0001) sapphire substrates using an atmospheric pressure organometallic vapor phase epitaxy method. We investigate the influences of the initial treatment of sapphire substrate, such as initial nitridation and low-temperature GaN buffer layer deposition, upon the surface morphology and crystallinity. The thermal stability of grown GaN layers is also investigated using a thermal etching process in a H2 atmosphere in order to obtain the information concerning the surface polarity of GaN(0001) layers. When sapphire substrates are initially nitrided, highly crystalline GaN layers with large hexagonal facets are obtained and its surface appears to be (0001)N. On the other hand, the deposition of a thicker buffer layer on the nitrided sapphire substrates improves the surface morphology, and the surface polarity of the mirror surface appears to be (0001)Ga. The initial nitridation of sapphire substrates and the GaN buffer layer deposition are considered to be important processes from viewpoints of the (0001) surface polarity.
Static space charge in evaporated ZnS:Mn alternating-current thin-film electroluminescent devices
Authors: Hitt J. C. , Keir P. D. , Wager J. F. , Sun S. S.
Published: July 2002

The operation of alternating-current thin-film electroluminescent (ACTFEL) devices may be strongly affected by the presence of dynamic or static positive space charge within the phosphor layer during device operation. Dynamic space charge is a positive charge in the phosphor layer which is periodically created and annihilated during each period of the applied voltage waveform. In contrast, static space charge is a positive space charge in the phosphor layer whose charge state does not change appreciably during steady-state operation of the ACTFEL device. The static space charge density of evaporated ZnS:Mn ACTFEL devices is estimated to be ∼7×1016 cm−3 from measured trends in the phosphor clamping field as a function of phosphor thickness. This static space charge density estimate implies a cathode clamping field of ∼ 2.2 MV/cm and a clamping interface trap depth of ∼1.5 eV. Furthermore, from transferred charge trends as a function of the phosphor thickness it is concluded that the static space charge in evaporated ZnS:Mn ACTFEL devices arises from metastable hole trapping in the phosphor.
A local selection criterion for the composition of graded thermoelectric generators
Authors: Schilz J. , Helmers L. , Müller W. E. , Niino M.
Published: July 2002

We present a procedure that locally determines the composition (from a given set of available materials) of a segmented or graded thermoelectric generator leg in order to maximize its output power to a matched load. The method is based on a new interpretation of the electrical output relation, which is organized to resemble a differential output power along the device coordinate. The procedure is applied to calculate the segment length and output power of a segmented bismuth-telluride/iron-disilicide based thermoelectric generator leg.
Importance of ideal grain boundaries of high remanent composite permanent magnets
Authors: Fischer R. , Kronmüller H.
Published: July 2002

The excellent magnetic properties of nanoscaled composite magnets can only be obtained by imposing strong requirements on the grain structure. The soft magnetic phase with a maximum extension smaller than 20 nm should be embedded into a hard magnetic environment. Furthermore the grain boundaries should be as ideal as possible. These requirements for the grain structure guarantee an optimal exchange hardening of the soft magnetic phase and therefore a high remanence and a sufficiently high coercivity. In the framework of computational micromagnetism it is possible to investigate the effect of nonideal grain boundaries on the magnetization reversal process of composite magnets in detail. Nonideal grain boundaries can be simulated by intergranular layers with material parameters deviating from their bulk values. It turns out that the remanence and the coercivity decrease approximately linearly with the simultaneous reduction of the exchange and the anisotropy constants near the grain boundaries. For a reduction of these intergranular material parameters smaller than 10% of their initial values, the remanence and coercivity strongly decrease by a factor of about 2.
Three-dimensional site dependence of single-ion-induced charge collection at a p-n junction—role of funneling and diffusion processes under different ion energy
Authors: Matsukawa T. , Mori S. , Tanii T. , Arimura T. , Koh M. , Igarashi K. , Sugimoto T. , Ohdomari I.
Published: July 2002

The amount of charges induced by high energy single ion irradiation at a p-n junction diode has been measured and its dependence on the ion incident position has been evaluated by using single-ion microprobe technique. By irradiating single He ions with various incident energy (1.4–4.05 MeV), dependence of the profiles of collected charges on the ion incident energy has been investigated. Origins of the different profiles among the different incident energy are discussed in terms of different contribution of two mechanisms of charge collection, namely, field funneling and diffusion of carriers. In the case of the ion incidence within the junction area, dependence of the charge collection profile on the ion incident energy comes from different contribution of the funneling in the charge collection process, while difference in the collection efficiency of the diffused charges affects the profile in the case of the ion incidence at outside of the junction.
Response to “Comment on ‘A model of hole trapping in SiO2 films on silicon’ ” [J. Appl. Phys. 83, 5591 (1998)]
Authors: Lenahan P. M. , Conley J. F.
Published: July 2002

We disagree with the comment of R. A. B. Devine, W. L. Warren, and S. Karna [J. Appl. Phys. 83, 5591 (1998)].
Mechanically alloyed nanocomposite magnets (invited)
Authors: McCormick P. G. , Miao W. F. , Smith P. A. I. , Ding J. , Street R.
Published: July 2002

Nanocomposites, consisting of a hard magnetic rare earth–transition metal phase exchange coupled to soft magnetic α-Fe or α-(Fe,Co), exhibit enhancement of the remanent magnetization due to exchange coupling across interfaces between grains. Modeling studies have shown that crystallite sizes of less than 20 nm are generally required for significant remanence enhancement and values of remanent magnetization equal to 70%–80% of saturation magnetization have been reported in mechanically alloyed two phase mixtures of α-Fe and a hard magnetic phase, such as Nd2Fe14B. Studies of microstructural evolution during mechanical alloying have shown that as-milled structures consist of a magnetically soft two phase mixture of α-Fe and an amorphous phase. Similar microstructures are observed regardless of whether mechanical milling or mechanical alloying has been carried out. Heat treatment above a critical temperature is required to crystallize grains of the hard magnetic phase. The formation of metastable intermediate phases with interesting magnetic properties may precede formation of the equilibrium phase. It is found that the crystallization temperature is an important parameter determining the grain size of the soft magnetic phase and, hence, magnetic properties. Recent measurements of the reversible and irreversible magnetization behavior of this novel class of permanent magnet are also discussed.
Minimum temperature in electromagnetic levitation melting under terrestrial gravity
Authors: Clemente Roberto A.
Published: July 2002

Recently, the existence of minima for the dissipated power in electromagnetic levitation melting has been reported. Such results will be used in order to estimate the temperature of the surface of the sample, when the only allowed mechanism for energy transport from the levitated drop is radiation cooling. It is also possible to estimate the difference in temperature, between the bulk and the surface of the specimen, when the principal mechanism for energy transport inside the liquid metal is thermal conductivity. The results suggest that even in experiments under terrestrial gravity, it should be possible to reach large undercooling for some metals of a high melting point, high total hemispherical emissivity, relatively low resistivity and density, without any need for a cooling gas atmosphere.
Plasma assisted molecular beam epitaxy of ZnO on c -plane sapphire: Growth and characterization
Authors: Chen Yefan , Bagnall D. M. , Koh Hang-jun , Park Ki-tae , Hiraga Kenji , Zhu Ziqiang , Yao Takafumi
Published: July 2002

ZnO single crystal thin films were grown on c-plane sapphire using oxygen microwave plasma assisted molecular beam epitaxy. Atomically flat oxygen-terminated substrate surfaces were obtained by pre-growth cleaning procedures involving an oxygen plasma treatment. A two dimensional nucleation during the initial growth which is followed by a morphology transition to three dimensional nucleation was observed by in situ reflection high energy electron diffraction. X-ray diffraction (XRD) and photoluminescence investigations suggest that the ZnO epilayer consists of a high quality layer on top of a transition layer containing a high density of defects in the interfacial region. A full width at half maximum (FWHM) of 0.005° is obtained for the ZnO(0002) diffraction peak in an XRD rocking curve, while a broad tail extending from the peak can also be observed. The photoluminescence spectra exhibit dominant bound exciton emission with a FWHM of 3 meV at low temperatures and free exciton emission combined with a very weak deep level emission at room temperature. Recently, these high quality ZnO epilayers have allowed the observation of optically pumped lasing at room temperatures as well as stimulated emission up to 550 K, both of which are due to an exciton related mechanism.
Effect of Co, Pt, and Au additions on the stability and epitaxy of NiSi2 films on (111)Si
Authors: Mangelinck D. , Gas P. , Gay J. M. , Pichaud B. , Thomas O.
Published: July 2002

We studied the effect of the addition of cobalt, platinum, or gold on the cell parameter of NiSi2 deposited epitaxially on (111)Si. Namely, the formation and the microstructure of NiSi2 films containing one of these elements are compared to those of the pure disilicide. The solubility of Co, Pt, and Au in NiSi2 ranges from a total substitution to nickel in the case of cobalt to a very weak quantity (less than 1%) in the case of platinum. An intermediate behavior was observed for gold which can occupy more than 10% of the metal sites. This important solubility has been confirmed by analysis of bulk Ni(Au) disilicides and is understood as a consequence of the metastability of gold silicides. Cobalt, Pt, and Au additions change the temperature of formation of NiSi2: from 800 °C for reaction with pure nickel, this temperature is lowered to 650 °C by Co and Au additions while it is increased to 950 °C for Pt. These modifications are consistent with the nucleation controlled formation of NiSi2 and the effect of added elements on both the interfacial energy and the stability of the disilicide. Measurements of the cell parameters (perpendicular and parallel) by x-ray diffraction revealed the relaxation state and the cell evolution of NiSi2 films. Platinum addition does not modify the cell in agreement with a very weak incorporation of platinum in NiSi2. The rigid interface model is well adapted to the growth of NiSi2, Ni(Au)Si2, and Ni(Co)Si2 films. According to this model and in spite of a large rhombohedral deformation, the system which better accommodates silicon is Ni(Co)Si2: in this case, there is practically no misfit between the film and the substrate. We also showed that misfit dislocations are generated principally during the NiSi2 formation and that their density evolves very little afterwards.
The effects of biaxial stress on Barkhausen noise signals when the magnetic field is noncoaxial with the stress axes
Authors: Sablik M. J. , Smith Marina Q. , Waldhart C. J. , McKee D. A. , Augustyniak B.
Published: July 2002

The Barkhausen noise amplitude was measured under conditions of biaxial stress in steel pipe for the case of a magnetic field noncoaxial with the stress axes. The stress axes for stresses σ1 and σ2 were orthogonal to each other. In particular, σ1 was the axial stress and σ2 was the hoop stress. Various angles were used for the field direction, along with various stress magnitudes, both compressive and tensile. The stress σ2 was always tensile, but σ1 was both compressive and tensile. A model for this biaxial stress situation, based on the Sablik–Jiles magnetomechanical model, was formulated. Using a model for the Barkhausen noise deriving from the Alessandro et al. model, the Barkhausen noise power maximum amplitude was computed for various field angles and stresses σ1 and σ2. The numerical results from this model calculation agreed qualitatively with many features of the experimental results. Thus, one found both numerically and experimentally that with field direction at small angles from the σ1 axis, the Barkhausen noise amplitude increased as the stress σ1 was increased from negative to positive. At large angles (generally greater than 45°), the reverse was true and the Barkhausen noise amplitude decreased as stress σ1 was increased. Also, the curves for the various angles tended to intersect when σ1 was set equal to σ2. Differences between numerical and experimental results are discussed, and suggestions are made for further improvement of the modeling.
Simple formula for the switching field of ultrathin small magnetic structures
Authors: Chui S. T. , Ryzhov V. N.
Published: July 2002

Mechanisms for the switching of the magnetization in small structures are summarized. We derive an analytic formula for the switching field in terms of the system variables and test it against results from Monte Carlo simulations.
Photoluminescence properties of MgxCd1−xSe mixed single crystals
Authors: Kim Chang-Dae , Shin Dong-Ho , Jang Hyang-Hee , Jeong Hae-Mun , Kim Duck-Tae , Yoon Chang-Sun , Park Hong-Lee , Kim Wha-Tek
Published: July 2002

The photoluminescence spectra of MgxCd1−xSe (0.00⩽x⩽0.46) mixed single crystals grown by the chemical transport reaction technique were investigated. We observed an emission band (I2) due to excitons bound to a neutral donor, an emission band (DA) due to donor-acceptor pair recombination with LO-phonon replicas and a self-activated emission band. These emission bands were shifted to higher energy with increasing x. For the crystals with x=0.46, the donor-acceptor emission band was observed at 2.661 eV in the blue region.
Dependence of aftereffect on magnetization history
Authors: Rugkwamsook Pattana , Korman Can E. , Bertotti Giorgio , Pasquale Massimo
Published: July 2002

This paper presents the dependence of magnetic aftereffect decay rates on the past input history. Experiments performed on a metal evaporated (ME) sample have shown that the decay coefficient gradually decreases as the sample undergoes many magnetization cycles. Two promising aftereffect models, namely the Preisach model driven by stochastic inputs developed by Mayergoyz and Korman, and the thermodynamic generalization of Preisach hysteresis developed by Bertotti are employed to compare simulated results with those obtained from aftereffect measurements for different cases of magnetization history. Simulation results from both models show good agreement with experiment.
Anti-phase domains and magnetism in epitaxial magnetite layers
Authors: Hibma T. , Voogt F. C. , Niesen L. , van der Heijden P. A. A. , de Jonge W. J. M. , Donkers J. J. T. M. , van der Zaag P. J.
Published: July 2002

Recent studies show that the magnetic properties of epitaxial thin films of magnetite (Fe3O4) deviate strongly from bulk behavior: it is difficult to saturate thin films, ultrathin films may become super paramagnetic, their saturation magnetization drops to zero, and the local magnetic moments are oriented out of plane in zero field. The possible relationship between this anomalous behavior and the occurrence of anti-phase boundaries (APBs) is discussed. Transmission electron microscopy images confirm that APBs are present in our Fe3O4 films grown by molecular beam epitaxy on MgO(100). Only APBs with out-of-plane shift vectors are visible. The much higher APB density found in sputtered films suggests that preparation conditions are important. To explain the deviating saturation and the superparamagnetic behavior of thin Fe3O4 films at the same time, the magnetic coupling over the APB must be dramatically reduced due to spin disorder along the boundaries.
Fabrication and characterization of high aspect ratio perpendicular patterned information storage media in an Al2O3/GaAs substrate
Authors: Wong Joyce , Scherer Axel , Todorovic Mladen , Schultz Sheldon
Published: July 2002

In a new approach, we have fabricated 6:1 aspect ratio magnetic nanocolumns, 60–250 nm in diameter, embedded in a hard aluminum-oxide/gallium-arsenide (Al2O3/GaAs) substrate. The fabrication technique uses the highly selective etching properties of GaAs and AlAs, and highly efficient masking properties of Al2O3 to create small diameter, high aspect ratio holes. Nickel (Ni) is subsequently electroplated into the holes, followed by polishing, which creates a smooth and hard surface appropriate for future reading and writing of the columns as individual bits for high density information storage. We have used magnetic force microscopy and scanning magneto-resistance microscopy to characterize the resulting magnets. We find the columns more magnetically stable than previously achieved with magnets embedded in a SiO2 substrate. Such stability is necessary before further writing of perpendicular patterned media can be demonstrated.
Magnetic and crystallographic structure of Nd5Fe17
Authors: Yelon W. B. , Luo H. , Chen M. , Missell F. P.
Published: July 2002

Neutron diffraction has been used to refine the crystallographic and magnetic structure of Nd5Fe17. The results of Rietveld refinement yield improved precision for the Fe coordinates relative to the x-ray study. The magnetic moments are refined by assuming that all Fe atoms in comparable environments have the same moment or by grouping sites by average Fe–Fe bond length. Each model leads to five distinct moments, in which the magnitude of the moment is correlated with both the average Fe–Fe bond lengths and the near-neighbor distribution. The results of the refinement are in reasonable agreement with the analysis of the Mössbauer spectra.
Spectroscopic ellipsometry analyses of sputtered Si/SiO2 nanostructures
Authors: Charvet S. , Madelon R. , Gourbilleau F. , Rizk R.
Published: July 2002

A spectroscopic ellipsometry technique is used to attempt a quantitative analysis of thin Si/SiO2 nanocomposite films obtained by magnetron co-sputtering. The layers are first fabricated with varying values of deposition temperature and sputtered silicon area, before being annealed at different temperatures. Using an effective medium model, the ellipsometry tool allowed the estimate of the thickness and the volume fraction of silicon agglomerates or grains, in addition to the optical parameters of the layer through a tentative determination of the dielectric function of the Si nanostructures. The Si content obtained by the ellipsometry approach agrees with those previously determined by direct measurements. Besides the obvious increase of incorporated Si with the sputtered area of this material, we noticed a maximum excess of Si for a deposition temperature in the 400–500 °C range. In this respect, the peak position of the photoluminescence (PL) detected only in the annealed samples is found to be closely connected with the amount of excess Si, and in consequence, with the mean size of the thermally grown nanocrystallites. The systematic red shift of maximum PL with the rise of Si content, and implicitly with the increase of the particle size, regardless of the deposition conditions used for the fabrication of the original layer, is liable to allow conclusive argument as for the attribution of the PL in the 1.30–1.65 eV range to the quantum confinement effect. Besides the main contribution of the nanoparticle density, the quality of the Si/SiO2 interface, as examined by infrared absorption spectroscopy, appears determining for the PL efficiency, and is increasingly improved towards stoichiometric SiO2 when the annealing temperature is increased up to 1100 °C.
Large magnetostriction effect in bulk (La1−xSmx)2/3Sr1/3MnO3
Authors: Cao Q. Q. , Wu J. , Gu K. M. , Zhang S. Y. , Du Y. W.
Published: July 2002

A series of polycrystalline bulk (La1−xSmx)2/3Sr1/3MnO3 samples over a wide composition range (x=0,0.1,0.33,…,1) was prepared by solid-phase reaction sintering technique. The x-ray diffraction pattern indicates that all the samples have an orthorhombically distorted perovskite structure. A large linear magnetostriction with a value up to −180×10−6 was observed at the temperature of 77 K under the maximum magnetic field of 1.5 T. The linear magnetostriction and magnetoresistance effect of our samples has similar dependence on the Sm content x. At room temperature, the sample with a Curie temperature of 320 K, and a Sm content of x=0.33, shows both maximum linear magnetostriction value of −40×10−6 and a maximum magnetoresistance value of −12%.
Layered manganites: Magnetic structure at extreme doping levels
Authors: Mitchell J. F. , Millburn J. E. , Medarde M. , Argyriou Dimitri N. , Jorgensen J. D.
Published: July 2002

We report powder neutron diffraction results on the crystal and magnetic structures of the bilayer Ruddlesden–Popper phase Sr3Mn2O7−δ (δ=0.0, 0.45) and correlate these structures with their magnetic and transport properties. The δ=0.45 compound contains a large number of oxygen vacancies that are disordered in the MnO2 planes. As a result of this disordered vacancy structure, Sr3Mn2O6.55 is a nonmagnetic insulator. Sr3Mn2O7.0 (δ=0) is an antiferromagnetic insulator whose magnetic structure is related to that of the SrMnO3 perovskite. Comparison of this end-member compound to its doped congeners in the La2−2xSr1+2xMn2O7 series highlights the extreme sensitivity of magnetic structure to dopant concentration in these layered materials.
Probing thermal conductivity variations in excimer laser irradiated polyimide foils
Authors: Walther H. G. , Kitzing T. , Bozoki Z. , Liakhou G. L. , Paoloni S.
Published: July 2002

Polyimide polymers can be converted from an insulator to a conductor by suitably pulsed excimer laser irradiation. The ultraviolet light is strongly absorbed by the polymer and pyrolyzes the material into carbon clusters. The formation of those clusters is accompanied by an increase of both electric and thermal conductivity. We observed photothermally the steep rise of thermal diffusivity when the density of the electrically conducting clusters exceeded the percolation threshold. This finding illustrates the charge carriers contribution to the kinetic energy transport in disordered materials.
One- and two-dimensional simulations of imploding metal shells
Authors: Keinigs R. K. , Atchison W. L. , Faehl R. J. , Thomas V. A. , Mclenithan K. D. , Trainor R. J.
Published: July 2002

We report results of one- and two-dimensional (2D) magnetohydrodynamic simulations of imploding, cylindrical metal shells. One-dimensional simulations are used to calculate implosion velocities of heavy liners driven by 30 MA currents. Accelerated by the j×B force, 45 g aluminum/tungsten composite liners achieve velocities on the order of 13 km/s. Used to impact a tungsten target, the liner produces shock pressures of approximately 14 Mbar. The first 2D simulations of these liners are also described. These simulations have focused on two problems: (1) the interaction of the liner with the electrically conducting glide planes, and (2) the effect of realistic surface perturbations on the dynamics of the implosion. The former interaction is confined primarily to the region of the contact point between the liner and glide plane, and does not seriously affect the inner liner surface. However a 0.2 μm surface perturbation has a significant effect on the implosion dynamics.
Theoretical and experimental study of charge carrier kinetics in crystalline silicon
Authors: Hahneiser Olaf , Kunst Marinus
Published: July 2002

A model simulating excess charge carrier kinetics in Si wafers was developed taking into account space charge fields and surface recombination. This model was applied to experimental data obtained by contactless transient and frequency resolved photoconductivity measurements of silicon wafers with accumulation or depletion layers at the surface. It is shown that a surface accumulation layer has only a minor influence and surface recombination can be analyzed with a surface recombination velocity weakly depending on the excess carrier concentration. At low injection level a surface depletion layer leads to a strongly nonlinear behavior of the photoconductivity due to excess charge carriers stored in the space charge region. The presence of these charge carriers is also revealed by the tail of the transient photoconductivity signal characterized by a decay time longer than the volume lifetime. At higher injection level the contribution of these charge carriers to the photoconductivity can be neglected and the data can be analyzed with the (differential) surface recombination velocity.
Unambiguous distinction between diffusion length and surface recombination velocity of solar cells at different excitation levels
Authors: Wawer P. , Rochel M. , Wagemann H.-G.
Published: July 2002

In this work we present a conclusive separation of bulk and surface recombination properties of solar cells. For this purpose, bifacial silicon solar cells were fabricated. The backside differential spectral response of the cells has been measured in the presence of bias light, both with and without backside passivation by means of corona charging on top of a thermal oxide. Employing the common one-dimensional Shockley model, the measurement curves have been simulated. This enables the base diffusion length to be distinguished from the backside surface recombination velocity. As such, their values have been determined individually. Repeating this procedure for different intensities of bias light has yielded the nonlinear behavior of the recombination mechanisms. By applying the Schockley–Read–Hall recombination theory, it was deduced that Fe interstitials presumably are the predominant bulk recombination centers.
Ti–O coordination at a Pb(Zr,Ti)O3/Pt interface annealed in a hydrogen-containing ambient analyzed using spatially resolved electron energy-loss spectroscopy
Authors: Ikarashi Nobuyuki , Hosoi Nobuki
Published: July 2002

We have investigated how annealing in a hydrogen-containing ambient changes the Ti–O coordination structure in a Pb(Zr,Ti)O3 (PZT) film fabricated on a Pt electrode by using spatially resolved electron energy-loss spectroscopy (EELS). Experimental results show that the Ti–O coordination structure was changed by the annealing at the PZT/Pt interface. However, there were no clear changes in the coordination number of O atoms to a Ti atom at the interface. Our EELS spectrum analysis using DV-Xα molecular orbital calculations indicates that the orbital overlaps between the 3d orbitals of a Ti atom and the 2p orbitals of the neighboring O atoms differ at the interface from those in the bulk PZT crystal. Therefore, our results indicate that the H2 annealing deformed the PZT crystal structure at the PZT/Pt interface, changing the Ti–O orbital overlaps. This structural change in the PZT film should be attributed to the marked decrease in Pb concentration at the H2-annealed PZT/Pt interface [N. Ikarashi, Appl. Phys. Lett. 73, 1955 (1998)].
Intermixing-induced resonance shift in GaAs/AlxOy distributed Bragg resonators
Authors: Cohen M. I. , Tan H. H. , Jagadish C.
Published: July 2002

The effect of ion implantation-induced interdiffusion on the resonant wavelength of GaAs/AlxOy distributed Bragg reflectors is investigated. As interdiffusion becomes stronger, the resonant wavelength is seen to redshift. Shifts of more than 60 nm could be achieved for center wavelengths around 800 nm. A model is proposed to explain this behavior. This model agrees well with previous lateral oxidation studies.
Atomic structural study of a Br-chemisorbed Si(111)-7×7 surface by using scanning tunneling microscopy
Authors: Mochiji Kozo , Ichikawa Masakazu
Published: July 2002

The atomic structure of a Br-chemisorbed Si(111)-7×7 surface is investigated by scanning tunneling microscopy. At low coverage, Br-reacted adatoms are distinguishable from unreacted adatoms in topographic images. The bias-dependent imaging suggests that the lowest tail of the antibonding state of the Br–Si bond is located about 1.5 eV above the Fermi level. At saturation coverage, a 7×7 structure is perfectly retained at room temperature and most adatoms remain after annealing at 400–630 °C. These results indicate that additional Br atoms cannot insert themselves into the Si–Si backbond between a Br-bonded adatom and a rest atom; therefore, SiBr is the only surface species present even at saturation coverage, which is not the case for F- or Cl-saturated Si(111) surfaces.
Role of Ar2+ and Ar2+ ions in a direct current argon glow discharge: A numerical description
Authors: Bogaerts Annemie , Gijbels Renaat
Published: July 2002

A two-dimensional model has been developed for calculating the behavior of Ar2+ and Ar2+ ions in a direct current argon glow discharge, by the use of balance equations describing the various production and loss processes for these species, as well as their transport by diffusion and migration. These balance equations are coupled to the equations for the Ar+ ions and electrons and solved simultaneously with Poisson’s equation, to obtain a self-consistent description of the charged particles behavior and the electrical characteristics in the glow discharge. Moreover, this model is combined with the other models that we have developed previously for the Ar atoms in various excited levels and the Cu atomic and ionic species, to obtain an overall description of the direct current argon glow discharge. The model is applied to typical conditions used for glow discharge mass spectrometry (pressure of 50–100 Pa, voltage of 600–1400 V, and current of 0.4–15 mA). Typical calculation results include the densities and fluxes of these ionic species, as well as the relative contributions of their production and loss processes. The Ar2+ ions are almost exclusively formed by two-electron ionization from Ar0 atoms, and they become primarily lost by diffusion and subsequent recombination at the cell walls. The Ar2+ ions are mainly created by Hornbeck–Molnar and metastable-metastable associative ionization, whereas atom to molecule conversion seems to play only a minor role at the discharge conditions under study. Loss of these Ar2+ ions is caused primarily by diffusion and recombination at the cell walls, but dissociative recombination in the plasma plays also a significant role. We found that the ratios of Ar2+/Ar+ and Ar2+/Ar+ ion densities and fluxes were in the order of 1%–10%, which is in good agreement with experimental observations.
Comparison of the magnetic and transport properties of Ga-, Ni-, and Fe-doped perovskite manganese oxides
Authors: Chen Xin , Wang Zhihong , Cai Jianwang , Shen Baogen , Zhan Wenshan , Chen Jinsong
Published: July 2002

A comparative study on magnetic and transport properties has been performed for B-site substituted perovskites La0.67Sr0.33Mn0.90B0.10O3 (B=Ga, Ni, and Fe). The doped samples show a notable decrease of the Curie Temperature, from 365 K of the undoped sample to 290 K (Ga), 292 K (Ni), and 265 K (Fe). Furthermore, the metal–semiconductor transition peaks in the doped samples shift to lower temperature, from above 300 K (undoped) to 212 K (Ga), 237 K (Ni), and 195 K (Fe). This considerable differences in magnetic and transport properties between doped and undoped samples and the diversity among the doped samples can be explained by the destruction of the partial double-exchange interactions and the exchange couplings between Mn and doped ions.
Modeling of the field reversal in a capacitively coupled high frequency sheath
Authors: Valentini H.-B.
Published: July 2002

Dealing with the collisionless plane sheath of a capacitively coupled high frequency (hf) discharge a two-point boundary value problem of the standard form is elaborated for a set of ordinary differential equations. The numerical solution yields that the direction of the electric field is reversed in a short interval of the period if the hf amplitude exceeds a threshold. The sheath model hitherto often used is extended to include the effect of the field reversal in a relatively simple way. At the electrode various boundary conditions are taken into account. It is shown that the different screening of the time-averaged component and of the high frequent components of the electric field near the electrode and in the interior of the sheath is the physical reason for the occurrence of the field reversal. In front of the electrode, the field reversal decreases the electron density and the electron current and causes an additional step in the displacement current. Apparently, recent experiments known from the references confirm the theoretical results.
Electrical characterization of electron cyclotron resonance deposited silicon nitride dual layer for enhanced Al/SiNx:H/InP metal–insulator–semiconductor structures fabrication
Authors: Peláez R. , Castán E. , Dueñas S. , Barbolla J. , Redondo E. , Mártil I. , González-Dı́az G.
Published: July 2002

We report a study of metal–insulator–semiconductor (MIS) structures on InP. The interfacial state density and deep levels existing in MIS structures were measured by deep level transient spectroscopy (DLTS) technique. The electrical insulator properties were measured by current–voltage techniques. MIS structures were fabricated on InP substrates by direct deposition of silicon nitride (SiNx:H) thin films by electron cyclotron resonance chemical vapor deposition. In this work, we show that interfacial state density can be diminished, without degrading electrical insulator properties, by fabricating MIS structures based on a dual layer insulator with different compositions and with different thickness. The effect of rapid thermal annealing treatment has been analyzed in detail in these samples. Interface state densities as low as 3×1011 cm−2 eV−1 were measured by DLTS in some structures. Conductance transients caused by disorder-induced gap states have been observed and analyzed providing some information about interface width. Finally, deep levels induced in the substrate have been investigated. Three deep levels at energies of 0.19, 0.24, and 0.45 eV measured from the conduction band have been found, and their dependence on the rapid thermal annealing process has been analyzed.
The influence of image forces on the extraction of physical parameters in Schottky barrier diodes
Authors: Mikhelashvili V. , Eisenstein G.
Published: July 2002

We present a method for extraction of the Schottky barrier diode parameters from current–voltage (I–V) characteristics in the presence of barrier height changes with voltage induced by image forces. The techniques, which allow for the inclusion of a series resistance Rs yield the built-in voltage Vbi, the saturation current IS (zero bias barrier height ΦB) the image force potential ΔΦB, the donor concentration ND and the voltage dependence of the ideality factor β. The method separates the effects on the I–V characteristic of an image force from the effect of current flow mechanisms other than thermionic emission. The proposed procedure determines a general voltage dependence of the effective ideality factor β(V) which is not limited to Schottky diodes but is also applicable to other diode types based on PN junction.
Electron beam ablation of materials
Authors: Kovaleski S. D. , Gilgenbach R. M. , Ang L. K. , Lau Y. Y.
Published: July 2002

The channelspark, a low accelerating voltage, high current electron beam accelerator, has been used for ablation of materials applied to thin film deposition. The channelspark operates at accelerating voltages of 10 to 20 kV with ∼1500 A beam currents. The electron beam ionizes a low-pressure gas fill (10–20 mTorr Ar or N2) to compensate its own space charge, allowing ion focused transport. Ablation of TiN, Si, and fused silica has been studied through several plasma diagnostics. In addition, thin films of SiO2 have been deposited and analyzed. Strong optical emission from ionized species, persisting for several microseconds, was observed in the electron beam ablated plumes. Free electron temperatures were inferred from relative emission intensities to be between 1.1 and 1.2 eV. Dye-laser-resonance-absorption photography showed Si atom plume expansion velocities from 0.38 to 1.4 cm/μs for several pressures of Ar or N2 background gas. A complex, multilobed plume structure was also observed, yielding strong indications that an electron beam instability is occurring, which is dependent upon the conductivity of the target. Nonresonant interferometry yielded line-averaged electron densities from 1.6 to 3.7×1023 m−3 near the target surface. Resonant UV interferometry performed on Si neutral atoms generated in the ablation plumes of fused silica targets measured line integrated densities of up to 1.6×1016 cm−2, with the total number of ablated silicon neutrals calculated to be in the range 2.0×1015 to 5.0×1013. Electron beam deposited films of fused silica were microscopically rough, with a thickness variation of 7%. The average SiO2 deposition rate was found to be about 0.66 nm/shot. The electron beam-deposited fused silica films had accurately maintained stoichiometry. Ablated particulate had an average diameter near 60 nm, with a most probable diameter between 40 and 60 nm. For SiO2 targets, the mass of material ablated in the form of particulate made up only a few percent of the deposited film mass, the remainder being composed of atomized and ionized material.
Laser ablation dynamics of YBa2Cu3O7−x in an oxygen jet
Authors: Park Seung Min , Moon Ji Youn
Published: July 2002

Mass spectroscopic studies on a reactive laser ablation of YBa2Cu3O7−x at 266 and 1064 nm in a pulsed oxygen jet have been performed by direct sampling of ions escaping from a laser-induced plasma (plume). Energetics of the ions is studied by using a time-of-flight (TOF) quadrupole mass spectroscopy. TOF spectra of metal ions consist of a fast and a slow component, but TOF spectra of metal oxide ions show single broad distributions. Significant changes in the relative enrichment of metal oxide and oxygen ions and their TOF distributions are observed by adding oxygen molecules to the plume. Formation mechanisms of each ion are investigated by analyzing the TOF spectra with O2 pulse on and off. The dependences of the TOF spectra on the laser wavelengths and their fluences are also examined to elucidate the acceleration mechanisms of ions as well as photochemical phenomena in the plume.
Frequency response of microwave dielectric Bi2(Zn1/3Nb2/3)2O7 thin films laser deposited on indium–tin oxide coated glass
Authors: Cheng Hsiu-Fung , Chen Yi-Chun , Lin I.-Nan
Published: July 2002

Bi 2 (Zn 1/3 Nb 2/3 ) 2 O 7 (BZN) thin films were prepared by using a pulsed laser deposition technique. For films in situ deposited on indium–tin oxide (ITO) coated glass substrates, the crystalline phase can be obtained by growing at a substrate temperature (Ts) higher than 475 °C. Too low a substrate temperature (Ts<400 °C) results in the amorphous phase, whereas too high a temperature (Ts>600 °C) leads to substantial interaction between the BZN film and the ITO layer. For the films deposited at a 500 °C substrate temperature, the texture characteristics change with their thickness. The films are (222) preferentially oriented when they are thin, and (400) preferentially oriented when they are thick. The optical properties, measured using optical spectroscopy, reveal that the index of refraction (n) and absorption coefficient (κ) vary between n=2.08−2.51 and κ=1.22×10−5−1.88×10−4 nm−1, respectively. These optical parameters do not change significantly with the preferred orientation and thickness of the films. However, the low frequency dielectric properties are closely correlated with the material’s characteristics. The crystalline BZN films have a markedly larger dielectric constant than the amorphous films.
Effect of growth interruption time and growth temperature on the natural formation of InGaAs/AlGaAs quantum disk structures on GaAs (311)B substrates
Authors: Ogawa Tomoya , Akabori Masashi , Motohisa Junichi , Fukui Takashi
Published: July 2002

We studied the effect of growth interruption time and growth temperature on the natural formation of InGaAs/AlGaAs quantum disk structures grown on high-index GaAs (311)B substrates by metalorganic vapor phase epitaxy at about 800 °C. The InGaAs/AlGaAs quantum disks were uniformly grown only with growth interruption time of 0 s. With increasing growth interruption time, the self-organized growth mode disappeared with the growth interruption time of 20 s. During growth interruption, strain for the natural formation of quantum disks was released by intermixing between In and Al. With decreasing growth temperature, the quantum disk did not show a hexagonal shape with clear facets. The quantum disks were formed at temperature range between 700 and 830 °C. The self-organization appeared through strain-driven mass transport. The strain-driven mass transport seems to have two characteristic lengths such as the amplitude and the periodicity determined by In composition and the InGaAs layer thickness from electron-beam induced mass transport phenomena.
Nitrogen and oxygen transport and reactions during plasma nitridation of zirconium thin films
Authors: Pichon L. , Straboni A. , Girardeau T. , Drouet M. , Widmayer P.
Published: July 2002

Zirconium nitride (ZrN) is a refractory material with good mechanical and thermal properties. It is therefore a good candidate for hard surface treatment at high temperature. In this work, we report the growth and characterization of ZrN by plasma assisted thermal nitridation of zirconium films in a NH3 atmosphere. The process was monitored by in situ monochromatic ellipsometry and the nitrides grown were profiled and analyzed by Auger electron spectroscopy. By using temperatures in the 700–800 °C range, the material obtained is quite close to ZrN, but, depending on experimental conditions, residual oxygen (impurities) can be easily incorporated by reaction with zirconium. The analysis of the ellipsometric data has shown that the nitridation did not occur by simple growth of nitride on zirconium. Auger profiles confirmed the presence of an oxidized zirconium layer localized between the nitrided surface and the remaining metal. This oxidation was observed to occur preferentially during temperature ramping, that is, in the low temperature regime. At high temperature, nitridation is dominant and the incorporated oxygen is exchanged with nitrogen. Oxygen is then partly rejected by diffusion out of the film through the ZrN surface layer and partly by diffusion in the deep zirconium sublayer. By using these observations, a new model of growth with a layered ZrN/ZrOx/Zr film was used to describe in situ ellipsometric data. By comparing the pure thermal and the plasma treatments, the advantages of the plasma assisted treatment become clearly: complete nitridation of the zirconium layer was achieved and the oxygen amounts in the film were substantially reduced.