bluuebunny/crossref_metadata_2025 huggingface dataset:
Efficient modified “walk on spheres” algorithm for the linearized Poisson–Bolzmann equation
Authors: Hwang Chi-Ok , Mascagni Michael
Published: July 2002

A discrete random walk method on grids was proposed and used to solve the linearized Poisson–Boltzmann equation (LPBE) [R. Ettelaie, J. Chem. Phys. 103, 3657 (1995)]. Here, we present an efficient grid-free random walk method. Based on a modified “walk on spheres” algorithm [B. S. Elepov and G. A. Mihailov, Sov. Math. Dokl. 14, 1276 (1973)] for the LPBE, this Monte Carlo algorithm uses a survival probability distribution function for the random walker in a continuous and free diffusion region. This simulation method is illustrated by computing four analytically solvable problems. In all cases, excellent agreement is observed.
Creating process margin in laser thermal processing: Application to formation of titanium silicide
Authors: Verma G. , Talwar S. , Bravman J. C.
Published: July 2002

Recently, we have demonstrated that, due to differential thermal budget, laser silicidation is an attractive alternative for deep submicron metal-oxide field effect transistors. In laser thermal processing, any spatial beam nonuniformities or pulse to pulse energy fluctuations lead to varying Si melt depth and hence variations in the silicide depth. In this letter, we report that amorphization is a possible solution for this problem. We demonstrate that stochiometric titanium disilicide can be fabricated using laser thermal processing. We also show that the depth of the silicide can be defined by amorphization and that process margin can be created in laser thermal processing.
Fabrication of GaP/Al–oxide distributed Bragg reflectors for the visible spectrum
Authors: Pickrell G. W. , Lin H. C. , Chang K. L. , Hsieh K. C. , Cheng K. Y.
Published: July 2002

Using very-low temperature (VLT) molecular-beam epitaxy (MBE), polycrystalline GaP/Al–oxide distributed Bragg reflectors (DBRs) have been fabricated. The use of high-energy band gap materials, such as GaP, allows for applications in the visible spectrum with minimal absorption of photons in the DBR. Through the use of VLT-MBE and control of the group-V overpressure, the microstructure can be controlled, resulting in either amorphous or polycrystalline material. Due to the nature of the amorphous material, the requirement of lattice matching is relaxed with no adverse effects to the underlying single crystal material. Two DBRs were fabricated, one reflecting at a wavelength of 550 nm and the other 480 nm. Using six pairs of polycrystalline GaP/Al–oxide, a reflectivity of ∼95% was achieved indicating a high-quality DBR suitable for device use.
Effect of photoelectrochemical oxidation on properties of GaN epilayers grown by molecular beam epitaxy
Authors: Fu D. J. , Kang T. W. , Yuldashev Sh. U. , Kim N. H. , Park S. H. , Yun J. S. , Chung K. S.
Published: July 2002

GaN epilayers grown by molecular beam epitaxy were photoelectrochemically (PEC) oxidized in an aqueous KOH solution. The oxidation effect was investigated by defect-related photoconductivity and photoluminescence. The PEC treated GaN show decreased extrinsic photoresponse and concentration of deep level states in comparison with the as-grown sample. The PEC process also results in enhanced donor-bound exciton photoluminescence at 3.47 eV and restrained 3.4 eV band. No strain is detected in the PEC oxidized GaN. The 3.4 eV band is related to structural defects instead of oxygen impurities. Rather, the defects can be passivated by the PEC oxidation.
Molecular-beam epitaxy of conductive CdF2 films on Si substrates by simultaneous Cd exposure
Authors: Kobori Toshimitsu , Tsutsui Kazuo
Published: July 2002

Conductive CdF2 films were grown on CaF2/Si(111) substrates by molecular-beam epitaxy with simultaneous exposure to a pure Cd beam and a GdF3 dopant beam during CdF2 growth. CdF2 films grown at 200 °C using this method exhibited low resistivities without postannealing. CdF2 films doped with 1018–1019 cm−3 Gd at a Cd vapor pressure of 10−8–10−7 Torr exhibited low sheet resistances corresponding to a resistivity of about 0.1 Ω cm at room temperature. The temperature dependence of sheet resistance is indicative of impurity banding properties similar to those reported for bulk CdF2 crystals. This method has potential applications in the epitaxial growth of fluoride heterostructures with conductive CdF2 layers, particularly on Si substrates.
Atomic resolution structure of growth and etching patterns at the surface of microwave plasma chemical vapor deposited diamond films
Authors: Lukins P. B. , Zareie M. H. , Khachan J.
Published: July 2002

Scanning tunneling microscopy and spectroscopy of microwave chemical vapor deposited diamond films prepared using a methane/hydrogen gas mixture indicates that the predominant diamond surface structure is (001) 1×1: 2H with a wide variety of facet shapes and orientations, and that the film surface is diamond-like and semiconductive but with a surface band gap (∼1.1 eV) that is smaller than the band gap of normal bulk diamond. Significant differences are observed in the electronic properties, resolution, and contrast between newly deposited surface carbon atoms and those bound in the underlying lattice. The composition of the deposition gas mixture is important in determining both the crystal structure and the type of bond termination at the surface.
Enhanced reliability of electroluminescence from metal–oxide–silicon tunneling diodes by deuterium incorporation
Authors: Liu C. W. , Lin C.-H. , Lee M. H. , Chang S. T. , Liu Y.-H. , Chen Miin-Jang , Lin Ching-Fuh
Published: July 2002

The reliability of electroluminescence from metal–oxide–silicon (MOS) tunneling diodes was improved by the incorporation of deuterium. The deuterium was incorporated by the deuterium prebake and the postoxide deuterium annealing. At constant current stress of 100 mA, a deuterium-treated n-channel MOS tunneling light-emitting diode shows that the integrated light emission intensity increases slightly about 6% after 10 000 s operation, while the hydrogen-treated device shows a 30% decrease of the integrated light emission intensity. The hydrogen release by the electrons tunneling from the gate electrode to Si and the formation of interface defects are responsible for the degradation of light output in the hydrogen-treated samples. An annealing model is also given to explain the slight increase of light output in the deuterium-treated samples.
Oscillation of the coercive force for ultrathin Ag/Co/Cu(111) films
Authors: Tsay J. S. , Yao Y. D. , Lin J. Y. , Yang C. S.
Published: July 2002

The magnetic properties of ultrathin Ag/Co/Cu(111) films have been investigated using the surface magneto-optical Kerr effect technique. Along polar direction with a harder axis of magnetization, Ag overlayer-induced oscillation of the coercive force shows a strong in-plane anisotropy for complete Ag atomic layers. It is consistent with the enhanced longitudinal Kerr intensity. A more significant change of the magnetic properties for a thinner film was observed and this should be attributed to a larger ratio of the magnetic–nonmagnetic interface to the volume. The relatively easy axis of the magnetization is along the longitudinal direction for the Ag/Co/Cu(111) system.
Photovoltaic effects on pinch-off voltage and open-circuit voltage in high-electron-mobility-transistor and Schottky-diode configurations
Authors: Kim D. M.
Published: July 2002

Photovoltaic effects on the pinch-off voltage (VP) in the high-electron-mobility-transistor (HEMT) and the open-circuit photovoltage (Vopt,OC) in the Schottky-diode configurations are characterized as a function of the optical input (Popt). The open-circuit photovoltage (Vopt,OC) in a Schottky-diode configuration, which has only a vertical field and every photogenerated excess minority carrier contributes to the photovoltage, can be described by Vopt,OC=nVth ln(Popt/Pref). However, the photovoltage (Vopt,FET≡|VP-VPO|) in HEMT configuration, which has a lateral field as well as a vertical field to drift excess minority carriers to the source contact and results in reduced photovoltage development, can be modeled as Vopt,FET=V0(Popt/Prefo)γ.
Preisach analysis of two ferromagnetic perovskites: La0.95Mg0.05MnO3 and La0.5Sr0.5CoO3
Authors: Zhao J. H. , Song T. , Kunkel H. P. , Zhou X. Z. , Williams G. , Roshko R. M.
Published: July 2002

Measurements of the major hysteresis loop and of the temperature dependence of the field cooled and zero field cooled moment in applied fields less than the coercive field have been performed on two ferromagnetic perovskites La0.95Mg0.05MnO3 and La0.5Sr0.5CoO3. The data have been analyzed within the framework of a generalized Preisach model which includes thermal fluctuations, critical effects, and a temperature dependent distribution of free energy barriers. The analysis shows that the response functions of the manganese perovskite are dominated by the growth of the anisotropy barriers, while in the cobalt perovskite, thermal fluctuations and barrier growth play a roughly equal role.
Structured doping with light forces
Authors: Schulze Th. , Müther T. , Jürgens D. , Brezger B. , Oberthaler M. K. , Pfau T. , Mlynek J.
Published: July 2002

Light forces are a powerful tool for neutral atom manipulation and have been used previously to focus an atomic beam onto a substrate to create periodic nanostructures. We utilize the material-selective characteristic of the atom–light interaction to structure the material composition of a film during growth. A host and a dopant material are evaporated simultaneously, but only the dopant is focused by the light field. The dopant concentration varies laterally on a sub-100-nm-length scale. This technique can be extended to three-dimensional patterning and opens up ways to engineer the photonic, electronic, or magnetic features of a solid.
Cobaltous oxide infiltrated yttrium iron garnet thin films as high-coercivity media for data storage
Authors: Rastogi A. C. , Moorthy V. N. , Dhara Sandip
Published: July 2002

Significant enhancement in the saturation magnetization Ms and coercivity Hc of yttrium iron garnet (YIG) films has been obtained by a simple method of surface infiltration of CoO. Deposition of a CoO overlayer under specific conditions by a low-pressure chemical-vapor deposition process over the crystallized YIG film has been observed to show high out-of-plane Hc and Ms values of 282.7 and 85.7 kA/m, respectively. The effect of the CoO deposition rate, temperature dependence of coercivity Hc, and annealing studies have been described that indicate CoO infiltration during the growth, resulting in a Co-rich YIG layer, and generation of interfacial stress are responsible for this unusual effect. Delineation of the interface by ion-beam etching in conjunction with magnetic studies has led to the understanding of the nature of the Co-rich YIG interfacial layer. Enhanced Hc and Ms values without the usual method of Ce4+ compensation of Co-doped YIG films described in this letter are of considerable use for application in high-density magneto-optic recording media.
Investigation of quantum-confinement effect and Stokes shift in strained Ga1−xInxN/GaN double quantum wells by spectroscopic ellipsometry and photoluminescence
Authors: Lee Myoung Hee , Kim Kwang Joo , Oh Eunsoon
Published: July 2002

The evolution of the optical absorption and emission properties of strained Ga1−xInxN/GaN double quantum wells grown on (0001)-oriented sapphire substrates with varying well width has been investigated by spectroscopic ellipsometry (SE) and photoluminescence (PL). The SE result shows that the band-gap absorption energy of the wells shifts to higher energies as the well width decreases, indicating a quantum-confinement effect. The decreasing trend agrees with the result of one-dimensional square-well potential calculations. The PL result shows a Stokes shift of the emission edge from the corresponding absorption edge, attributable to the combined effects of the strain-induced piezoelectric potential and the In-fluctuation potential in the well. The blueshift of the emission edge with increasing PL excitation density further supports the existence of a strong piezoelectric field in the well.
Study of the ground state properties of the perovskites R(Y, Pr)BaCuT(Fe, Co)O5+y within the Hubbard model
Authors: Tzavellas A. , Trohidou K. N.
Published: July 2002

The phase diagram and the ground state properties of the R(Y, Pr)BaCuT(Fe, Co)O5+y perovskites, where y is the concentration of the extra oxygen, have been studied using the Hubbard model. We consider two chains of magnetic ions and allow electron hopping between sites on different chains, modeling in this way the effect of the extra oxygen atoms in the system. By doping the system with oxygen, narrow regions of competing magnetic structures appear in the phase diagram and the magnetic ordering is rapidly destroyed with increasing oxygen concentration y. This is in agreement with the experimental results which show that magnetic ordering exists only for low extra oxygen concentration y (y<0.08). The magnetic properties of the system are only locally affected by the oxygen impurities clustering.
Surface impedance tensor in amorphous wires with helical anisotropy: Magnetic hysteresis and asymmetry
Authors: Makhnovskiy D. P. , Panina L. V. , Mapps D. J.
Published: July 2002

This article concerns the investigation of the magnetic behavior of the surface impedance tensor final_sigmâ in CoSiB amorphous wires having a residual torsion stress and a helical anisotropy. The full tensor final_sigmâ involving three different components is found by measuring the S21 parameter at a required excitation with a Hewlett-Packard network/spectrum analyzer at MHz frequencies. In general, the impedance plots versus axial magnetic field Hex exhibit a hysteresis related to that for the case of static magnetization. The diagonal components of final_sigmâ (longitudinal final_sigmazz and circular final_sigmaφφ) show a sharp peak in a narrow field interval where the domain walls form and contribute to the ac magnetization dynamics. This peak is not seen for the off-diagonal component final_sigmazφ (final_sigmaφz) since the existence of the domain structure suppresses it. Applying a dc bias current results in a gradual transition to a nonhysteretic asymmetrical behavior with an enhanced sensitivity. The portions of the experimental plots associated with the rotational dynamic process are in qualitative agreement with the theory based on a single-domain model.
Automatic parameter identification for the multilayer media Preisach model
Authors: Garg Shailendra , Della Torre Edward , Cardelli Ermanno
Published: July 2002

A proposed Preisach model for multilayer media previously presented accurately described first-order reversal curves as well as the major loop. The model used a moving Preisach model with graduated parameters for each layer in the media to account for the gradual loss of epitaxy of subsequent layers. A variable variance model was used to differentiate between major loop and reversal curve differences in curvature. The identification of parameters was complicated by their interaction, which made it difficult to fit all the data. This article suggests a technique for effectively reducing the interaction between the parameters, thereby simplifying the identification procedure.
Reflectionless tunneling in planar Nb/GaAs hybrid junctions
Authors: Giazotto Francesco , Cecchini Marco , Pingue Pasqualantonio , Beltram Fabio , Lazzarino Marco , Orani Daniela , Rubini Silvia , Franciosi Alfonso
Published: July 2002

Reflectionless tunneling was observed in Nb/GaAs superconductor/semiconductor junctions fabricated by a two-step procedure. First, periodic δ-doped layers were grown by molecular-beam epitaxy near the GaAs surface, followed by an As cap layer to protect the surface during ex situ transfer. Second, Nb was deposited by dc-magnetron sputtering onto a GaAs(001) 2×4 surface in situ after thermal desorption of the cap layer. The magnetotransport behavior of the resulting hybrid junctions was successfully analyzed within the random matrix theory of phase-coherent Andreev transport. The impact of junction morphology on reflectionless tunneling and the applicability of the fabrication technique to the realization of complex superconductor/semiconductor mesoscopic systems are discussed.
Modification of refractive index in Ag/Na ion-exchanged glasses by vacuum-ultraviolet pulse laser irradiation
Authors: Ruschin S. , Sugioka K. , Yarom G. , Akane T. , Midorikawa K.
Published: July 2002

Laser-induced refractive index modification in Ag+ ion exchanged waveguides on glass substrates was observed. Waveguiding effects were measured, and an increase in refractive index of more than 2×10−3 was deduced. Refractive index profiles show that the maximum radiation-induced difference is attained 4–5 μm below the surface. Possible mechanisms for the material modification are discussed.
Relaxation oscillation phenomena in cryogenic Si diodes
Authors: Merz Bernhard P. , Treumann Rudolf A.
Published: July 2002

We present observations of unusual oscillatory behavior of Si diodes at low temperatures. The oscillations observed are stable, highly reproducible and obey a frequency-current relation that is linear over nearly eight decades in current strength allowing for precision measurement of currents over the whole range down to picoampere currents.
Photoinduced surface deformations in ion-conducting Ag–As–S glasses. II. Anisotropic deformation produced by large light spots
Authors: Gotoh T. , Tanaka K.
Published: July 2002

Anisotropic surface deformations appear in AgAsS2 chalcogenide films when exposed to linearly polarized 2.0 eV light spots with diameters of 5–200 μm. The anisotropic shape reflects the polarization direction. The shape also changes with the direction of light incidence. Illumination on the free surface of the films gives a craterlike deformation, and illumination through a substrate gives an anticrater deformation. In bulk samples, under any exposure conditions, an isotropic convex structure appears. The formation mechanisms of these patterns are discussed taking into account the photoinduced migration of Ag+ ions.
Phase diagram of the one-dimensional periodic Anderson model
Authors: Luo Yan , Kioussis Nicholas
Published: July 2002

We have calculated the phase diagram of the symmetric one-dimensional Anderson lattice using the local mean-field method, which can reveal the basic properties of the system throughout the entire parameter space. Near quarter and half filling the antiferromagnetic phase is the ground state. For intermediate band filling there is a continuous second-order phase transition to a ferromagnetic state. At quarter filling there is a transition from a metallic paramagnetic state to an insulating antiferromagnetic state as the on-site Coulomb interaction increases.
Field-induced transition of the Haldane antiferromagnets
Authors: Sakai Tôru
Published: July 2002

The field-induced long-range antiferromagnetic order in the quasi-one-dimensional S=1 antiferromagnet is investigated with the exact diagonalization of finite chains and the mean field approximation for the interchain interaction. In the presence of the single-ion anisotropy D, we present the phase diagrams in the plane of the magnetic field H and the temperature T for H∥D and H⊥D. It is found that the two boundary curves of the ordered phase for H∥D and H⊥D have two intersections in the case of D>0 (XY like), while no intersection in the case of D<0 (Ising like).
Layer-resolved spin polarization in Sb overlayers on NiMnSb
Authors: Skomski R. , Komesu T. , Borca C. N. , Jeong H.-K. , Dowben P. A. , Ristoiu D. , Nozières J. P.
Published: July 2002

The magnetism of antimony overlayers on a ferromagnetic substrate is investigated by spin-polarized inverse photoemission and explained in terms of a spin-dependent envelope-function approximation (SDEFA). The atomic structure of the films, which were deposited by sputtering Sb onto a NiMnSb(001) substrate, is characterized by a unique combination of three features: (i) NiMnSb is a highly spin-polarized semi-Heusler alloy predicted to be halfmetallic, (ii) antimony is a semimetal, exhibiting a band structure reminiscent of indirect-gap semiconductors, and (iii) the small lattice mismatch ensures a well-controlled interface. Combined x-ray absorption spectroscopy and spin-polarized inverse photoemission yield a layer-resolved spin polarization decaying on a length scale of the order of 1 nm. The unusual range of the spin polarization in the paramagnetic overlayer is explained by considering the alloy–antimony interface as a spin-dependent perturbation potential and taking into account the low effective masses of the Sb conduction electrons (only about 0.1 for both electrons and holes).
Lorentz microscopy study of magnetization reversal mechanism in magnetic tunnel junction elements
Authors: Shang Ping , Hogwood Andrew , Petford-Long Amanda K. , Anthony Thomas C.
Published: July 2002

The magnetization reversal mechanism of four different shaped (rectangular, elliptical, trapezoidal, and hexagonal) tunnel junction elements with area ranging from 0.04 to 12 μm2 has been investigated using Lorentz transmission electron microscopy. It has been found that the reversal field is influenced by element size and aspect ratio but is not strongly dependent upon the element shape. 360° domain walls were often observed to form in the elements and were sustained to a high field when the relative magnetization configuration of the free and pinned layers changed from parallel to antiparallel. The formation of 360° domain walls depends strongly on the shape and aspect ratio of the element, with a lower probability of formation in elements with either an aspect ratio of 1:1 or a hexagonal shape.
Berry’s phase for compact Lie groups
Authors: Strahov E.
Published: July 2002

The methods of Kähler geometry are applied to generalize the results of Berry obtained for SU(2) (namely, the existence of a geometrical part in the adiabatic phase) to any compact Lie group. We obtain explicit expressions for Berry’s geometric phases, Berry’s connections, and Berry’s curvatures in terms of parameters of the corresponding Lie algebra valued Hamiltonian. It is demonstrated that the parameter space of the Hamiltonian in the general theory is essentially a homogeneous Kähler manifold. The fundamental Kähler potentials of this manifold completely determine Berry’s phase. A general approach is exemplified by the Lie algebra Hamiltonians corresponding to SU(2) and SU(3) evolution groups.
Hot-carrier-induced oxide charge trapping and interface trap creation in metal–oxide–semiconductor devices studied by hydrogen/deuterium isotope effect
Authors: Cheng Kangguo , Lee Jinju , Hess Karl , Lyding Joseph W.
Published: July 2002

A method based on the hydrogen/deuterium isotope effect is proposed to separate and quantify the effects of interface trap creation and oxide charge trapping on hot-carrier-induced degradation in n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs). Hydrogenated and deuterated transistors were subjected to hot-carrier stress with a fixed drain voltage Vds and various gate voltages Vgs. The threshold voltage Vt and interface trap density Nit were recorded as a function of stress time. It is found that at low Vgs stress when equal numbers of interface traps are created, the shift of Vt is larger in hydrogenated transistors than in deuterated transistors. Increasing Vgs to 1/3Vds produces no noticeable difference of Vt shift in these two kinds of transistors. However, further increasing Vgs results in larger Vt shift in deuterated transistors than in hydrogenated ones. From a quantitative analysis, the contribution of oxide charge trapping to Vt shift is separated from the contribution of interface trap creation. The results suggest that interface trap creation is the dominant mechanism for hot-carrier-induced degradation in n-channel MOSFETs.
Shunt screening, size effects and I /V analysis in thin-film photovoltaics
Authors: Karpov V. G. , Rich G. , Subashiev A. V. , Dorer G.
Published: July 2002

We present an analytical model that quantitatively describes the physics behind shunting in thin film photovoltaics and predicts size-dependent effects in the I/V characteristics of solar cells. The model consists of an array of microdiodes and a shunt in parallel between the two electrodes, one of which mimics the transparent conductive oxide and has a finite resistance. We introduce the concept of the screening length L, over which the shunt affects the system electric potential. The nature of this screening is that the system generates currents in response to the point perturbation caused by the shunt. L is expressed explicitly in the terms of the system parameters. We find the spatial distribution of the electric potential in the system and its I/V characteristics. The measured I/V characteristics depend on the relationship between the cell size l and L, being markedly different for the cases of small (l≪L) and large (l≫L) cells. We introduce a new regime of the large photovoltaic cell where all the characteristics are calculated analytically. Our model is verified both numerically and experimentally: good agreement is obtained.
Observation and modeling of random telegraph signals in the gate and drain currents of tunneling metal–oxide–semiconductor field-effect transistors
Authors: Avellan Alejandro , Krautschneider Wolfgang , Schwantes Stefan
Published: July 2002

Parallel measurements of random telegraph signals (RTS) in the gate and drain currents of n-metal–oxide–semiconductor field-effect transistors with 1.3-nm-thin gate oxides are presented. RTS appear simultaneously in both currents. Contrary to what could be expected, the signals have opposite signs in the gate and drain currents. A model is proposed to explain this phenomenon by the Schottky effect. The relative amplitude of the signal fluctuation in the gate current is significantly higher than that in the drain current. Therefore, the gate current is a much more sensitive indicator for RTS than the drain current.
Casimir energy of a semi-circular infinite cylinder
Authors: Nesterenko V. V. , Lambiase G. , Scarpetta G.
Published: July 2002

The Casimir energy of a semi-circular cylindrical shell is calculated by making use of the zeta function technique. This shell is obtained by crossing an infinite circular cylindrical shell by a plane passing through the symmetry axes of the cylinder and by considering only half of this configuration. All the surfaces, including the cutting plane, are assumed to be perfectly conducting. The zeta functions for scalar massless fields obeying the Dirichlet and Neumann boundary conditions on the semi-circular cylinder are constructed exactly. The sum of these zeta functions gives the zeta function for the electromagnetic field in question. The relevant plane problem is considered also. In all the cases the final expressions for the corresponding Casimir energies contain the pole contributions which are the consequence of the edges or corners in the boundaries. This implies that further renormalization is needed in order for the finite physical values for vacuum energy to be obtained for given boundary conditions.
Unambiguous observation of subband transitions from longitudinal valley and oblique valleys in IV–VI multiple quantum wells
Authors: Wu H. Z. , Dai N. , Johnson M. B. , McCann P. J. , Shi Z. S.
Published: July 2002

PbSe/PbSrSe multiple-quantum-well (MQW) structures were grown on BaF2(111) substrates by molecular-beam epitaxy and characterized by Fourier transform infrared transmission spectroscopy. To reduce unwanted Fabry–Pérot interference fringes, the top surface of the MQW samples was coated with an anti-interference film, enabling clear observation of subband transitions without superposed interference fringes. Transition energies involving longitudinal and oblique valleys were unambiguously resolved and are in good agreement with calculations made using the envelope wave function approximation.
Effects of BaBi2Ta2O9 thin buffer layer on crystallization and electrical properties of CaBi2Ta2O9 thin films on Pt-coated silicon
Authors: Kato Kazumi , Suzuki Kazuyuki , Nishizawa Kaori , Miki Takeshi
Published: July 2002

Non-c-axis oriented CaBi2Ta2O9 (CBT) thin films have been successfully deposited via the triple alkoxide solution method on Pt-coated Si substrates by inserting BaBi2Ta2O9 (BBT) thin buffer layers. The BBT thin buffer layer, which was prepared on Pt-coated Si, was a key material for suppression of the nonpolar c-axis orientation and promoting the ferroelectric structure perpendicular to the in-plane direction of CBT thin film. The annealing temperature and thickness of the BBT thin buffer layers affected the dielectric, ferroelectric, and fatigue properties of the stacked CBT/BBT thin films. The resultant 650 °C annealed CBT/BBT(30 nm) thin film exhibited good P–E hysteresis properties and fatigue behaviors.
Magnetization measurement on the S=1 quasi-one-dimensional Heisenberg antiferromagnet Ni(C5H14N2)2N3(PF6)
Authors: Honda Z. , Katsumata K.
Published: July 2002

We have performed magnetization (M) measurements on the S=1 quasi-one-dimensional Heisenberg antiferromagnet, Ni(C5H14N2)2N3(PF6) (NDMAP) with a vibrating sample magnetometer in a static field (H). In the temperature (T) dependence of the susceptibility (M/H), we observed that a transition from the gapped phase to a gapless one occurs at a finite H. On increasing H further, we observed a bending in the M/H–T curve and the M/H takes almost constant value below the bending temperature (TB). Since TB corresponds to the critical temperature determined from heat capacity measurements, we identify the T independent susceptibility to the perpendicular susceptibility of an anisotropic antiferromagnet. We also observed a change in the M–H curves of NDMAP at finite H. The value of H where M changes corresponds to the phase boundary between the field induced long-range ordered and paramagnetic phases determined from the heat capacity measurement.
Transmission electron microscopy observations of ferroelectric domains in the tetragonal Pb(Ni1/3Nb2/3)O3–PbZrO3–PbTiO3 ceramics modified by bismuth and zinc
Authors: Zhu Xinhua , Zhu Jianming , Zhou Shunhua , Li Qi , Liu Zhiguo , Ming Naiben
Published: July 2002

Ferroelectric domains of the tetragonal (Pb0.985Bi0.01)(Ni1/4Zn1/12Nb2/3)0.2(ZrσTi1−σ)0.8O3 (σ=0.30 and 0.40) ceramics were investigated by transmission electron microscopy and electron diffraction. It was observed that the 90° macrodomains displayed the shape of simple lamella twinning, and that the δ-fringe contrast was frequently observed within these configurations of 90° domains. The 180° domain walls with zigzag structure between 90° domain walls were also observed. The formation sequences of the 90° and 180° domain structures were discussed in terms of energy minimization during the phase transition from cubic to tetragonal ferroelectric phase on cooling.
Influence of surface acoustic waves on lateral forces in scanning force microscopies
Authors: Behme G. , Hesjedal T.
Published: July 2002

We present a detailed study of the influence of ultrasonic surface acoustic waves (SAWs) on point-contact friction. Lateral force microscopy (LFM) and multimode scanning acoustic force microscopy (SAFM) were used to measure and to distinguish between the influence of in-plane and vertical surface oscillation components on the cantilever’s torsion and bending. The experiments show that friction can locally be suppressed by Rayleigh-type SAWs. Through the mapping of crossed standing wave fields, the wave amplitude dependence of the friction is visualized within microscopic areas without changing other experimental conditions. Above a certain wave amplitude threshold, friction vanishes completely. We found that the friction reduction effect is caused by the vertical oscillation components of the SAW. Purely in-plane polarized Love waves do not give rise to a significant friction reduction effect. Thus, we conclude that the mechanical diode effect, i.e., the effective shift of the cantilever off of the oscillating surface, is responsible for the SAW-induced lubrication. This explanation is supported by vertical and lateral SAFM measurements: in areas with completely vanishing friction, low frequency vertical cantilever oscillations are still observable, whereas lateral (torsional) cantilever oscillations are no longer excited. Additionally, at very high Rayleigh wave amplitudes an effect of lateral force rectification was observed. It results in a scan direction-independent appearance of the LFM traces.
Surface structure of niobium-dioxide overlayer on niobium(100) identified by scanning tunneling microscopy
Authors: Li Y. , An B. , Xu X. , Fukuyama S. , Yokogawa K. , Yoshimura M.
Published: July 2002

The surface structures of an oxide overlayer on Nb(100) single crystal formed by numerous cycles of Ar-ion sputtering and flash annealing at 1973 K in an ultra-high-vacuum chamber have been investigated by scanning tunneling microscopy. Flat terraces with a monatomic step height of Nb(100) are observed; where two orthogonal domains are alternately evident. A ladder-like (3×10) structure of Nb(100) with no long-range order is identified at atomic resolution in the domain on the terrace. The ladder-like (3×10) structure is interpreted to be a modulated NbO2(010) overlayer on the Nb(100) surface and an atomic structural model of the ladder-like (3×10) structure is proposed.
Strain relaxation of GaN nucleation layers during rapid thermal annealing
Authors: Yi M. S. , Noh D. Y.
Published: July 2002

The strain relaxation of GaN nucleation layers grown on sapphire (0001) during rapid thermal annealing was studied in a synchrotron x-ray scattering experiment. The as-grown GaN nucleation layer is compressively strained. Upon annealing to 750 °C, the lattice strain first changes to tensile. This tensile strain is released progressively as the annealing temperature increases. The nucleation layer sublimates significantly at 1050 °C where it becomes mostly strain-free hexagonal GaN.
Ballistic photoconductivity in nanostructures
Authors: Ruda H. , Shik A.
Published: July 2002

A model for photoconductivity of nanostructures, in which carriers move collisionlessly between electrodes or regions of intensive recombination, is presented. Detailed analysis was performed for two particular cases: a thin film between two bulk contacts and low-temperature grown GaAs containing As clusters. The photocurrent dependence on the applied voltage V was found to be essentially nonlinear for small V and tend to saturation for large V. The spectrum of photoconductivity differs drastically from that of optical absorption and is characterized by a maximum near the absorption edge, with peak position depending on V.
Investigation on morphological stability of ultrathin Pb films
Authors: Zhang L. H. , Sui M. L. , Zhang L. , Hu K. Y. , Li D. X.
Published: July 2002

Ultrathin Pb films supported by Al film were made by using cold-rolling and ion-beam thinning techniques. The morphological instability of the Pb film under electron-beam irradiation was investigated by means of in situ transmission electron microscopy observations. It has been found that under electron-beam irradiation, Pb films with an incoherent Pb/Al interface spheroidized into Pb particles, but those with a semicoherent Pb/Al interface were stable in morphology. The morphological stability of thin films depends on the microstructure and the thermodynamic property of the interphase boundary. A critical interfacial energy for the spheroidization of thin films was determined based on a thermodynamics analysis.
Spatial control of second-order optical susceptibility induced in thermally poled glasses
Authors: Guillet de Chatellus H. , Montant S. , Freysz E.
Published: July 2002

Second-order susceptibility thermally induced in poled glass samples was erased in doped borophosphate, lead silica, and fused silica glasses using an optical method. A simple law describes the temporal evolution and the spatial profile of the erased nonlinear region versus the intensity and profile of the erasure laser beam. A comparison is made between one and two photon absorption induced spatial profiles.
Electrode sheath voltage in pulsed high-pressure mercury arcs
Authors: Kloss A. , Schneidenbach H. , Hess H.
Published: July 2002

The electric field strength and the electrode sheath voltage in high-pressure mercury arcs were experimentally determined for operation in a current-pulse mode. The current was sinusoidally alternating with a basic frequency of 50 Hz. Each fifth half cycle was pulsed with different ratios of the maximum current in the pulse to those in the other four half cycles. The experimental results quite well agree with model calculations considering radiation transport. The most impressive feature of the electrode sheath voltage is a very pronounced minimum immediately after the reignition peak which can reach high negative values.
Strong influence of supercooled liquid on crystallization of the Al85Ni5Y4Nd4Co2 metallic glass
Authors: Louzguine Dmitri V. , Inoue Akihisa
Published: July 2002

The present letter aims to report the effect of supercooled liquid region on crystallization behavior of an Al85Ni5Y4Nd4Co2 metallic glass produced by rapid solidification of the melt. It is found that crystallization behaviors of this alloy above and below the glass-transition temperature are completely different. Formation of the primary nanoscale α-Al particles observed during continuous heating or after annealing above glass-transition temperature does not occur during isothermal annealing below glass-transition temperature when an unidentified intermetallic compound, and α-Al phases are formed simultaneously.
Negative thermal expansion and magnetic properties of Y2Al3Fe14−xMnx compounds
Authors: Hao Yanming , Gao Yan , Wang Bowen , Qu Jingping , Li Yangxian , Hu Jifan , Deng Jiachun
Published: July 2002

The structure and magnetic properties of Y2Al3Fe14−xMnx (x=0–12) compounds have been investigated by means of x-ray diffraction and magnetization measurements. The Y2Al3Fe14−xMnx compounds have a hexagonal Th2Ni17-type structure. Their unit-cell volumes increase slowly with increasing x first, then increases rapidly with the further increase of x. This implies that there exists a positive spontaneous volume magnetostriction in the magnetic state of Y2Al3Fe14−xMnx compounds. X-ray diffraction of the Y2Al3Fe11Mn3 compound from 150 to 300 K shows that there appears a negative coefficient of thermal expansion, ᾱ≈−7.5×10−5/K, from 185 to 200 K. The Curie temperature and the saturation magnetization of these compounds show a rapid drop with increasing x.
Submicron three-dimensional infrared GaAs/AlxOy-based photonic crystal using single-step epitaxial growth
Authors: Sabarinathan Jayshri , Bhattacharya Pallab , Zhu Donghai , Kochman Boaz , Zhou Weidong , Yu Pei-Chen
Published: July 2002

A relatively simple technique is demonstrated to fabricate three-dimensional face-centered-cubic infrared photonic crystals with submicron feature sizes using GaAs-based technology, single-step epitaxial growth, and lateral wet oxidation. The photonic crystals were fabricated with feature sizes (a) of 1.5 and 0.5 μm. Transmission measurements reveal a stopband centered at 1.0 μm with a maximum attenuation of 10 dB for the submicron (a=0.5 μm) photonic crystal. This technique is scalable to small photonic crystal periodicity and hence to shorter wavelengths.
Atomic equilibrium concentrations in (InGa)As quantum dots
Authors: Galluppi M. , Frova A. , Capizzi M. , Boscherini F. , Frigeri P. , Franchi S. , Passaseo A.
Published: July 2002

In InxGa1−xAs quantum dots (QDs), the plot of peak emission energies versus the total amount of indium exhibits a well defined pattern, which is independent of the indium concentration. Moreover, photoluminescence spectra of InAs QDs grown by atomic layer molecular beam epitaxy (ALMBE) roughly coincide with those of In0.5Ga0.5As QDs grown by metalorganic vapor phase deposition. We suggest that the total amount of In rather than the nominal In concentration determines the emission energy of these two sets of QDs, and that In interdiffusion is rather strong in ALMBE growth.
Index and density changes induced by proton radiation in lanthanum crown glass
Authors: Gusarov A. , Doyle D. , Hermanne A. , Berghmans F.
Published: July 2002

We studied the refractive index and density changes in cerium-doped lanthanum crown glass irradiated in air with 38.5 MeV protons. At a low proton fluence, the index change is linear with dose, and reaches 2.4×10−5 at 2.7×1012 p/cm2 (0.65 Mrad dose). The rate of index change decreases with the proton fluence, but at high total dose (1014 p/cm2) radiation-induced surface dilatation and birefringence start to appear. Our results show that the present concept of radiation-hard optical glass should be revised and that a high dose of proton radiation can produce index changes sufficient for the fabrication of integrated optical devices.
Absolute and convective nature of the modulational and Raman instabilities in the relativistic regime
Authors: Couairon A. , Mora P.
Published: July 2002

The nature (convective or absolute) of the modulational and Raman instabilities is investigated in the relativistic regime, for a large amplitude electromagnetic wave propagating in a plasma of arbitrary density. The present paper extends previous results by Guérin et al. [Phys. Plasmas 2, 2807 (1995)]. Stability diagrams are obtained and compared to the stability properties found by a classical temporal stability analysis. It is shown that the modulational and the (forward and backward) Raman instabilities are individually convective. However, the Raman instability is absolute for large density and high intensity when the forward and backward Raman branches have merged into a hybrid Raman branch. Near quarter critical density, this hybrid branch is destabilized and is absolute from the threshold. The plasma equilibrium is convectively unstable between quarter critical and critical density at low intensity.
Linear and nonlinear electrostatic modes in a nonuniform magnetized electron plasma
Authors: Vranješ J. , Shukla P. K. , Kono M. , Poedts S.
Published: July 2002

Linear and nonlinear low-frequency modes in a magnetized electron plasma are studied, taking into account a proper description of the equilibrium plasma state that is inhomogeneous. Assuming a homogeneous magnetic field and sheared plasma flows, flute-like perturbations are studied in the presence of density and potential gradients. Linear analysis reveals the presence of a streaming instability and depicts conditions for global linear spiral mode. In the nonlinear domain, a tripolar vortex, which is driven and carried by the flow, is found. Also investigated are the consequences of a magnetic shear as well as nonuniformities along the magnetic field lines, which are shown to be responsible for the possible annulment of the magnetic shear effects. Streaming along the lines of the sheared magnetic field is also studied. A variety of nonlinear structures (viz. global multipolar vortices, local vortex chains, and tripolar vortices) is shown to be the consequence of the simultaneous action of the parallel and perpendicular flows.
Band alignment in organic devices: Photoemission studies of model oligomers on In2O3
Authors: Blyth R. I. R. , Duschek R. , Koller G. , Netzer F. P. , Ramsey M. G.
Published: July 2002

The interfaces of In2O3, a model for indium–tin–oxide (ITO), with benzene, thiophene, and benzaldehyde, models for technologically important organic molecules, are studied using angle resolved ultraviolet photoemission and work function measurements. Band alignment diagrams for hypothetical Al/organic/ITO devices have been drawn, using values determined from this work and previously published studies of these molecules on Al(111). The similarity between the bonding of benzene and thiophene on Al(111) and In2O3, i.e., largely electrostatic, leads to near identical alignment at both metal and oxide interfaces. This indicates that clean Al and ITO will make a very poor electron/hole injecting pair. We suggest that the apparent efficiency of Al as an electron injecting contact in real devices is due to the presence of oxygen at the Al/organic interface. For benzaldehyde the interaction with In2O3 is largely electrostatic, in contrast to the covalent bonds formed on Al(111). This leads to very different alignment at the Al and oxide interfaces, showing the importance of the particular organic–inorganic interaction in determining band alignment.
Effect of annealing treatments on positive temperature coefficient of resistance properties of barium titanate ceramics and a new model for the positive temperature coefficient of resistance effect
Authors: Gallego Marina Martinez , West Anthony R.
Published: July 2002

Alternating current impedance has shown the presence of three electrical components in a semiconducting barium titanate ceramic that has been processed to give a positive temperature coefficient of resistance (PTCR) effect. Two of the component resistances, R1 and R2, each present a PTCR effect and, from the temperature dependence of their capacitances, they are attributed to different regions, nonferroelectric and ferroelectric, respectively, at the surfaces of individual grains or in the region of inter-grain contacts. The influence of isothermal annealing in air at 1000, 1100, and 1200 °C on these PTCR effects was investigated. At the resistance maxima, Rmax and Tmax, two types of dependence were found for both R1 and R2. At short times, Rmax decreased and Tmax shifted to higher temperatures; at long times, the reverse occurred and Rmax increased while Tmax shifted to lower temperature. Capacitance values for both regions were constant at short times but decreased at long times, indicating a thickening of the regions responsible for R1 and R2 at long times. The room temperature resistance R25 was constant at short times but increased dramatically at long times, leading to a decrease in the magnitude of the PTCR effect. A model to account for the observed changes is proposed. Freshly fired ceramics are oxygen deficient and semiconducting; grain surfaces are Ti rich. Oxidative annealing creates an oxidized surface layer and an electron depletion layer just inside the grain surfaces. Counter diffusion through these layers of Ba2+ and O2− ions then occurs so as to partially restore electroneutrality; consequently, the concentration of surface acceptor states, Ns, decreases initially. On prolonged annealing, further oxidation occurs: oxide ions diffuse into the grain cores, with counter diffusion of electrons to the grain surfaces where they are trapped by additional absorbed oxygens leading to an increase in Ns. An explanation is given as to how both ferroelectric and nonferroelectric regions of the ceramics exhibit a PTCR effect. The key parameter that controls the activation barrier to conduction appears to be the polarization of the lattice associated with dipolar fluctuations and ferroelectric domains rather than with the polarizability of the different regions.
Melting of indium at high pressure determined by monochromatic x-ray diffraction in an externally-heated diamond anvil cell
Authors: Shen Guoyin , Sata Nagayoshi , Rivers Mark L. , Sutton Stephen R.
Published: July 2002

The melting behavior of indium at high pressure has been studied in an externally heated diamond anvil cell (DAC) using x-ray diffraction measurements. Melting at high pressure was identified by the appearance of diffuse scattering from the melt with the simultaneous disappearance of crystalline diffraction signals. The observed melting curve is in good agreement with previous determinations based on resistivity measurements in a piston cylinder apparatus. These results demonstrate the successful melting experiments in a DAC using the x-ray diffuse scattering as the melting criterion.