bluuebunny/crossref_metadata_2025 huggingface dataset:
Selective-area atomic layer epitaxy growth of ZnO features on soft lithography-patterned substrates
Authors: Yan M. , Koide Y. , Babcock J. R. , Markworth P. R. , Belot J. A. , Marks T. J. , Chang R. P. H.
Published: July 2002

Templated ZnO thin-film growth from the vapor phase is achieved on docosyltrichloro- silane-patterned Si substrates using atomic layer epitaxy (ALE) combined with soft lithography. Patterned hydrophobic self-assembled monolayers (SAMs) are first transferred to single-crystal Si surfaces by hot microcontact printing. Using diethylzinc and water as ALE precursors, crystalline ZnO layers are then grown selectively on the SAM-free surface regions where native hydroxy groups nucleate growth from the vapor phase. High-resolution ZnO patterns with 1.0–40 μm feature sizes are readily achieved, demonstrating that soft lithography combined with ALE is a simple and promising methodology for selective area in situ vapor phase fabrication of patterned oxide thin films.
Influence of laser intensity and pulse duration on the extreme ultraviolet yield from a water jet target laser plasma
Authors: Vogt U. , Stiel H. , Will I. , Nickles P. V. , Sandner W. , Wieland M. , Wilhein T.
Published: July 2002

Extreme ultraviolet (EUV) emission in the 11–15 nm wavelength range from a thin liquid water jet target under illumination with a high repetition rate, high average power laser (Nd-YLF) has been studied. To find the optimum conversion efficiency of laser light into EUV radiation, different laser parameters were applied. The laser intensity was varied between 1011 and 1015 W/cm2, and pulse duration in the range from 30 ps to 3 ns. A maximum conversion efficiency of 0.12% in 2.2% bandwidth and 4π steradian at 13 nm was achieved at a repetion rate of 250 kHz, and a strong dependence of the conversion efficiency on both laser intensity and pulse duration was found.
Polymer bonding process for nanolithography
Authors: Borzenko T. , Tormen M. , Schmidt G. , Molenkamp L. W. , Janssen H.
Published: July 2002

We have developed a lithography process which originates from imprint lithography and offers advantages over it. Unlike imprint lithography, not only the sample but also the mold is covered with a thermoplastic polymer. The mold and sample are brought into contact, pressed together and heated above the glass transition temperature of the thermoplast, causing the two polymer layers to become bonded (glued) together. A special treatment of the mold and sample surface causes the polymer film to stick only to the substrate after cooling. The bonding occurs at pressures and temperatures lower than those usually applied in imprint technology, and eliminates problems in conventional imprint technology that are related to lateral transport of the polymer.
Atomic force microscope chamber for in situ studies of ice
Authors: Zepeda Salvador , Yeh Yin , Orme Christine A.
Published: July 2002

To investigate the surface morphologies of biological systems in a controlled gaseous environment (e.g., the temperature, humidity and composition), most commercial atomic force microscopes require modification. We have designed a double-jacketed environmental chamber specifically for a Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) force microscope. We use cold nitrogen and thermoelectric devices to control the temperature in the chamber; the nitrogen simultaneously serves to create an inert environment. We have also designed a temperature controlled sample stage utilizing thermoelectric devices for fine temperature regulation. A variation of this sample stage allows us to image samples in fluids at cold temperatures with an O-ringless configuration. The relative humidity within the chamber is also measured with commercially available relative humidity sensors. We investigate the surface morphology of ice Ih in its pure phase and shall extend the study to ice in the presence of biological molecules, such as antifreeze proteins. We present a detailed description of our design and our first images of polycrystalline ice and single crystals of ice grown in situ from the vapor.
Full-wave verification of the fundamental properties of left-handed materials in waveguide configurations
Authors: Caloz C. , Chang C.-C. , Itoh T.
Published: July 2002

The fundamental electromagnetic properties of left-handed materials (LHMs) are reviewed and verified by finite-element method full-wave analysis using rectangular waveguide structures loaded by a LHM and adopting an effective medium approach. The negative phase velocity, positive intrinsic impedance, and modified boundary conditions at an interface with a right-handed medium are verified by loading a waveguide section with a LHM that has edges perpendicular to the waveguide axis. In addition, the negative angle of refraction is demonstrated by loading the junction of a T-junction waveguide with a LHM having one edge 45° with respect to the waveguide axis. These properties are shown by the evolution of wave fronts in the LHM and by analysis of the S-parameters of the waveguide structures.
Monte Carlo simulation of the irreversible growth of magnetic thin films
Authors: Candia Julián , Albano Ezequiel V.
Published: July 2002

The growth of magnetic films with ferromagnetic interactions between nearest-neighbor spins is studied in (d+1)-dimensional rectangular geometries for d=1,2. Magnetic films are grown irreversibly by adding spins at the boundaries of the growing interface. The orientation of the added spins depends on both the energetic interaction with already deposited spins and the temperature through a Boltzmann factor. At low temperatures thin films, of thickness L, are constituted by a sequence of well ordered magnetic domains. Spins belonging to each domain, of average length lD≫L, have mostly the same orientation, but consecutive domains have opposite magnetization. This kind of “spontaneous magnetization reversal” during the growth process has a short characteristic length lR, such that lD≫lR∼L. At higher temperatures, a transition between ordered and disordered states is also observed. The emerging behavior is compared to that of the equilibrium Ising model.
High-efficiency white organic light-emitting devices with dual doped structure
Authors: Huang Yen-Shih , Jou Jwo-Huei , Weng Wen-Kuo , Liu Jia-Ming
Published: July 2002

A dual doped structure was proposed for obtaining organic light-emitting devices with high-efficiency and full-spectrum white light emission. The device structure included indium tin oxide glass substrate/120 Å copper phthalocyanine hole injection layer/500 Å N, N′-bis-(1-naphthy)-N, N′diphenyl-1,1′-biphenyl-4-4′-diamine hole transport layer/110 Å 1,4-bis(2,2-diphenylvinyl) biphenyl doped with 0.025% 4-Dicyanomethylene-2-methyl-6- [2-(2,3,6,7-tetra-hydro-1H,5Hbenzo[ij]quinolizin-8-yl)vinyl]-4H-pyran+tris- (8-hydroxyquinoline) (Alq3) doped with (0.1%) coumarin 6 emitting layer/200 Å Alq3 electron transport layer/5 Å lithium fluoride (LiF)/aluminum (Al). A stable white emission [Commission Internationale de l’Eclairage 1931 chromaticity coordinate (X=0.30, Y=0.36)] was obtained for luminance ranging from 100 to 1000 cd/m2. The device turned on at 2.5 V. Its luminance was 8800 cd/m2 at 9 V, and the maximum efficiency was 5 lm/W.
Polarization reversal in congruent and stoichiometric lithium tantalate
Authors: Shur V. Ya. , Nikolaeva E. V. , Shishkin E. I. , Kozhevnikov V. L. , Chernykh A. P. , Terabe K. , Kitamura K.
Published: July 2002

Switching kinetics has been compared in congruent (CLT) and stoichiometric (SLT) lithium tantalate by simultaneous recording of instantaneous domain patterns and switching current. A mechanism of fast domain kinetics in CLT driven by domain merging was revealed. The important information about the domain kinetics has been obtained by the statistical analysis of current noise in SLT. A model of jerky domain wall motion is proposed.
High spatial resolution thermal conductivity of bulk ZnO (0001)
Authors: Florescu D. I. , Mourokh L. G. , Pollak Fred H. , Look D. C. , Cantwell G. , Li X.
Published: July 2002

We measured high spatial/depth resolution 300 K thermal conductivity κ of the Zn and O surfaces of two bulk n-type ZnO (0001) samples, grown by a vapor-phase transport method, using scanning thermal microscopy (SThM). The thermal investigation was performed in both point-by-point (∼2 μm resolution) and area-scan modes. On the first sample κ=1.16±0.08 (Zn face)/1.10±0.09 (O face) W/cm K while for the second material κ=1.02±0.07 (Zn face)/0.98±0.08 (O face) W/cm K. These are the highest κ values reported on ZnO. A correlation between SThM area-scan readings and surface topography was established by simultaneously performing atomic force microscopy scans. The influence of surface roughness on the effective thermal conductivity (i.e., heat flow) is discussed.
Characterization of the bonding strength and interface current of p-Si/n-InP wafers bonded by surface activated bonding method at room temperature
Authors: Howlader M. M. R. , Watanabe T. , Suga T.
Published: July 2002

Bonding between p-Si and n-InP was performed through the surface activated bonding method at room temperature. Tensile results show that the samples were visibly separated from the bonded interface, indicating a weak bonding strength. The cause of the weak bonding strength was intensively investigated. Consistent results between x-ray photoelectron spectroscopy and atomic force microscope investigations show that a weak phase of indium is terminated on the InP due to the depletion of phosphorus in the sputtered surface. Existence of indium layers on the debonded Si surface indicate that the samples were separated from the interface of In/InP, but not across the bonded interface of Si and indium. Typical pn junction current–voltage behavior indicates no high resistance interface layer that can withstand the flow of current through the interface. Remarkably, sputtering time as well as energy dependence on the interface current is found to be due to the accumulation of sputtering induced defects.
Controlled two-dimensional distribution of nanoparticles by spin-coating method
Authors: Hong Young-Kyu , Kim Hanchul , Lee Geunseop , Kim Wondong , Park Jong-Il , Cheon Jinwoo , Koo Ja-Yong
Published: July 2002

We demonstrate that the controlled distribution of nanoparticles can be achieved by employing the spin-coating method. The Co and Ag nanoparticles were uniformly distributed on the Si and SiO2 substrates with this method. The particle density was controllable by varying the concentration of colloids. The spatial distribution of the nanoparticles within the patterned area was also shown to be uniform with small boundary effect, which is favorable for current microelectronics technology. We propose that the spin-coating method can be utilized in developing mass production processes for future nanodevices.
Magnetic and Mössbauer studies on oxygen deficient perovskite, La0.6Sr0.4FeO3−δ
Authors: Yang J. B. , Yelon W. B. , James W. J. , Zhou X. D. , Xie Y. X. , Anderson H. U. , Chu Z.
Published: July 2002

Samples of La0.6Sr0.4Fe3−δ with varying oxygen vacancy contents were prepared by heating them in different gas flows. Magnetization measurement showed that samples with low oxygen vacancies have a magnetic ordering temperature in the range of 300–325 K while those with 9%–12% oxygen vacancies have a magnetic ordering temperature of 800 K and higher. Mössbauer spectra at 300 K exhibit paramagnetic or weak magnetic characteristics for the N2, O2, and air-quenched samples, whereas an average hyperfine field of 52 T is found for the CO/CO2 reduced samples. The heat treatment in the reducing atmosphere creates oxygen vacancies and increases unit cell volume. However, the Fe–O bond length remains nearly constant, resulting in distortion/rotation of the oxygen octahedra which increases the Fe–O–Fe bond angle as much as 12 degrees. This dramatically affects the Fe–O–Fe superexchange coupling and plays a key role in the increase of the Nèel temperatures.
A DNA-based molecular device switchable between three distinct mechanical states
Authors: Simmel Friedrich C. , Yurke Bernard
Published: July 2002

A self-assembled DNA-based molecular device is described which can be switched between three distinct mechanical states. The device consists of two supramolecular “arms” connected by a hinge. The arms can be in a closed, relaxed, or stretched configuration. The operation of the device is monitored by fluorescence resonance energy transfer. Variations in the design of the device have a strong influence on the operation kinetics of the device.
Observation of novel disordered rhombohedral R2Fe17 (R=rare earth) based compounds
Authors: Yelon W. B. , James W. J. , Yang J. B. , Kamaraju K. , Cai Q. , Chu Z. , Malik S. K.
Published: July 2002

A recent study of (SmGd)Fe14Si3 showed an anomalous x-ray diffraction pattern [P. C. Ezekwenna, G. K. Marasinghe, J.-H. Nam, W. J. James, W. B. Yelon, M. Ellouze, and Ph. I’Héritier, J. Appl. Phys. 87, 6716 (2000)]. Although all observed peaks could be indexed to the rhombohedral 2:17 phase (R-3m) many lines were strongly reduced in intensity. Subsequently, the same phenomenon was seen in neutron diffraction patterns from Nd2−xDyxFe17−ySiy alloys and conventional refinement using the rhombohedral phase left significant residuals. Fourier mapping revealed additional density along the c-axis chains of Fe dumbbells and rare earth atoms. The perfect a-b-c stacking of the CaCu5 layers with regular 1/3 replacement of the rare earth atoms by Fe dumbbells appears to be broken. Detailed analysis shows that excess Fe dumbbells are incorporated into the structure and some reverse substitution is seen. In addition, the weak intensities are associated with a near equivalence of the 9d and 18h Fe sites, as in the parent CaCu5 phase. The relationship of this new structure to the other CaCu5 derived phases is described. The high Fe concentration creates the possibility of high TC and higher remanence than in the ordered 2:17 compounds.
Possible ordering of Ru and Cu in the charge-reservoir of magneto-superconductor RuSr2GdCu2O8 (Ru-1212): Magnetic, transport, and TEM microstructural studies
Authors: Awana V. P. S. , Ichihara S. , Nakamura J. , Karppinen M. , Yamauchi H. , Yang Jinbo , Yelon W. B. , James W. J. , Malik S. K.
Published: July 2002

Magnetization vs temperature behavior of RuSr2GdCu2O8−δ (Ru-1212) measured in an field of 5 Oe, shows a clear branching of zero-field-cooled (ZFC) and field-cooled (FC) curves around 140 K, a cusp at 135 K, and a diamagnetic transition around 20 K (in the ZFC branch). The cusp at 135 K is due to the antiferromagnetic ordering of the Ru moments. The magnetization-field isotherms, below 50 K, show a nonlinear contribution from a ferromagnetic component. The resistance vs temperature behavior of the compound, in applied fields of 0, 3, and 7 T, confirms that the sample is superconducting at around 20 K. The superconducting transition exhibits field broadening of a type different than that known for conventional high Tc superconductors. The magnetoresistance (MR) is negative above the Ru magnetic ordering temperature of 135 K, while below this temperature, MR displays a positive peak in low fields and becomes negative in higher fields. A maximum of 2% is observed for the negative MR value at the Ru magnetic ordering temperature. An electron diffraction pattern obtained for this Ru-1212 sample shows two types of superstructure; one with a weak spot at the center of the a–b rectangle and the other only along the b direction. It is possible that either Ru/Cu or Ru4+/Ru5+ ordering of 2b periodicity takes place along the b direction.
Annealing study of Fe2O3 nanoparticles: Magnetic size effects and phase transformations
Authors: Xu X. N. , Wolfus Y. , Shaulov A. , Yeshurun Y. , Felner I. , Nowik I. , Koltypin Yu. , Gedanken A.
Published: July 2002

Sonochemically synthesized Fe2O3 nanoparticles were annealed in air or in vacuum while their magnetization was continuously recorded. Annealing in vacuum at temperatures Ta between 240 and 450 °C produced nanophases of γ-Fe2O3 with average particle size ranging from 4 to 14 nm, depending on Ta. Phase transformation into α-Fe2O3 occurred directly by annealing in air, or via an intermediate Fe3O4 phase by annealing in vacuum at temperatures higher than 450 °C. Mapping the correlation between the magnetic properties and the annealing conditions, enables control of the annealing process to obtain nanocrystals of γ-Fe2O3, α-Fe2O3, or Fe3O4 with different particle size and magnetic properties.
Effect of Ti seed layer on the magnetization reversal process of Co/NiFe/Al-oxide/NiFe junction films
Authors: Yu Andrew C. C. , Petford-Long Amanda K. , O’Grady Kevin , Miyazaki Terunobu
Published: July 2002

Ti/Co/NiFe/Al-oxide/NiFe (F1) and Co/NiFe/Al-oxide/NiFe (F2) junction films were characterized using high-resolution electron microscopy (HREM), Lorentz transmission electron microscopy (LTEM), and alternating gradient force magnetometry (AGFM). HREM images showed that the Ti seed layer induced a strong 〈111〉 texture in the bottom Co/NiFe bilayer. The ferromagnet/Al-oxide interfaces in F1 showed correlated waviness, while the interface waviness in F2 appeared uncorrelated. Thus, “orange-peel” coupling effect was more significant in F1 than in F2, which was confirmed by the steep slope of the magnetization curve in the “antiparallel” magnetization configuration for F1. The LTEM in situ magnetizing experiment results and the AGFM measurement of magnetization curves showed that both junction films possessed a two-stage magnetization reversal characteristic—magnetization of the top NiFe layer reversed first followed by the reversal of the bottom Co/NiFe bilayer. LTEM observation revealed that the magnetization reversal of the top NiFe layer was via domain wall motion, while the reversal of the bottom Co/NiFe bilayers was mainly by wall motion together with a small degree of moment rotation. Domain wall mobility in the Co/NiFe bilayer of F1 was higher due to the strong crystallographic texture and large grain size appeared in the bilayer.
How does the chain extension of poly (acrylic acid) scale in aqueous solution? A combined study with light scattering and computer simulation
Authors: Reith Dirk , Müller Beate , Müller-Plathe Florian , Wiegand Simone
Published: July 2002

This work addresses the question of the scaling behavior of polyelectrolytes in solution for a realistic prototype. We show results of a combined experimental (light scattering) and theoretical (computer simulations) investigation of structural properties of the sodium salt of poly (acrylic acid) (Na-PAA). Experimentally, we determined the molecular weight (MW) and the hydrodynamic radius (RH) by static and dynamic light scattering for six different Na-PAA samples in aqueous NaCl-containing solution (0.1–1 mol/l) of polydispersity DP between 1.5 and 1.8. On the computational side, three different variants of a newly developed mesoscopic force field for Na-PAA were employed to determine RH for monodisperse systems of the same MW as in the experiments. The force field effectively incorporates atomistic information and one coarse-grained bead corresponds to one PAA monomer. We find that RH matches with the experimental data for all investigated samples. The effective scaling exponent for RH is found to be around 0.55, which is well below its asymptotic value for good solvents. Additionally, data for the radius of gyration (RG) are presented.
Simple phase bias for superconducting circuits
Authors: Majer J. B. , Butcher J. R. , Mooij J. E.
Published: July 2002

A superconducting phase-bias tool, based on a trapped fluxoid in a ring, is proposed and demonstrated. It can provide arbitrary phase values and is simple to fabricate. The phase bias has been realized in two superconducting quantum interference devices, where the critical current versus magnetic flux is shown to be shifted by π/2 and π.
Pore-size dependence of the self-diffusion of hexane in silica gels
Authors: Baumert J. , Asmussen B. , Gutt C. , Kahn R.
Published: July 2002

Quasielastic neutron scattering has been used to investigate the diffusion of hexane molecules adsorbed in mesoporous silica gels with pore diameters of 20, 40, and 60 Å, respectively. Within the temperature range 180 K⩽T⩽240 K molecular reorientations, which are best described by a tumbling rod, and translational diffusion of the molecular center of mass could be observed. In each investigated sample two different environments were identified for the adsorbed molecules: the vicinity of the pore walls and the center of the pores. The hindering potentials for both translation and rotation are found to be considerably stronger for the molecules on the pore walls. The fraction of this molecule type decreases with increasing pore size. The self-diffusion coefficients derived from our data range from Dtrans=0.6 to 2.4×10−6 cm2 s−1. The activation energies of the translational diffusion are within the range 139 meV⩽Ea⩽302 meV. They decrease with increasing pore size with a clear tendency towards the activation energy of liquid (bulk) hexane.
Gas detection with quantum cascade lasers: An adapted photoacoustic sensor based on Helmholtz resonance
Authors: Barbieri Stefano , Pellaux Jean-Paul , Studemann Eric , Rosset Daniel
Published: July 2002

A photoacoustic gas sensor exploiting a quantum cascade laser as a radiation source is demonstrated. A detection limit of ∼1 ppm with 1 ms response time is found using a Peltier-cooled Fabry–Pérot InGaAs-based quantum cascade laser emitting at 9.4 μm, and a commercial microphone as a detector. The photoacoustic cell consists of a Helmholtz resonator preceeded by a low-pass acoustic filter. This geometry is well adapted to the shape of the laser beam and allows for an effective filtering of ambient acoustical noise. The relative simplicity of the system is particularly attractive for applications where sensitivity, robustness, and ease of fabrication are all fundamental requirements.
Ultrafast x-ray pulse generation by focusing femtosecond infrared laser pulses onto aqueous solutions of alkali metal chloride
Authors: Hatanaka Koji , Miura Toshifumi , Fukumura Hiroshi
Published: July 2002

X-ray pulses were generated by focusing femtosecond infrared laser pulses (775 nm, 130 fs, 0.58 mJ/pulse) onto an alkali metal (Cs or Rb) chloride aqueous solution jet. The mechanism of x-ray pulse generation was studied by measuring x-ray emission spectra and transmitted laser pulse spectra. Addition of CsCl to distilled water was found to enhance markedly x-ray intensity and x-ray cutoff energy. The frequency-up-shift observed in transmitted laser pulses was larger in CsCl solution than in distilled water. The results suggested that a high-density plasma-like medium was produced in CsCl solution due to secondary electrons.
Single-photon detector in the microwave range
Authors: Astafiev O. , Komiyama S. , Kutsuwa T. , Antonov V. , Kawaguchi Y. , Hirakawa K.
Published: July 2002

Single-photon counting at microwave frequencies around 500 GHz is demonstrated by using a single-electron transistor (SET) formed by two capacitively coupled GaAs/AlxGa1−xAs parallel quantum dots (QDs). A point contact separating the double QDs allows the prompt escape of an excited electron from one of the QDs to another. The resulting long-lived photoinduced ionization of the QD is detected as a change in the SET current.
Experiments on arrays of globally coupled chaotic electrochemical oscillators: Synchronization and clustering
Authors: Wang Wen , Kiss István Z. , Hudson J. L.
Published: July 2002

Experiments on chaotically oscillating arrays of 64 nickel electrodes in sulfuric acid were carried out. External resistors in parallel and series are added to vary the extent of global coupling among the oscillators without changing the other properties of the system. The array is heterogeneous due to small variations in the properties of the electrodes and there is also a small amount of noise. The addition of global coupling transforms a system of independent elements to a state of complete synchronization. At intermediate coupling strengths stable clusters, or condensates of elements, form. All the elements in a cluster follow the same chaotic trajectory but each cluster has its own dynamics; the system is thus temporally chaotic but spatially ordered. Many cluster configurations occur under the same conditions and transitions among them can be produced. For values of the coupling parameter on either side of the stable cluster region a non-stationary behavior occurs in which clustered and synchronized states alternately form and break up. Some statistical properties of the cluster states are determined.
Studies on the nonlinear piezoelectric response of polyvinylidene fluoride
Authors: Hahn Bernd R.
Published: July 2002

Simple mechanical models can be useful in investigating the structural dependence of the piezoelectric activity of uniaxially oriented polyvinylidene fluoride. An experimentally observed strong nonlinear relationship between the mechanical load and the transverse piezoelectric coefficient d32 is presented. This new behavior is discussed on the basis of a parallel arrangement of crystalline and amorphous regions perpendicular to the orientation axis. The coefficient d32 changes its sign also, the deformation being less than 3% in all cases. A mechanical series model is applied for the coefficient d31 whose behavior is very linear with the applied mechanical load. Furthermore, d32 shows a significant time dependence after a change in the mechanical load applied. These nonlinear effects depend upon structural features such as crystal perfection and were attributed to processes occurring in the crystalline regions and in their interfaces with the amorphous surrounding, respectively. Also, the technical importance of these effects is obvious.
Iron and the iron-boron complex in silicon
Authors: Brotherton S. D. , Bradley P. , Gill A.
Published: July 2002

Iron has been diffused into p- and n-type silicon containing various concentrations of carbon and oxygen. Apart from the established iron interstitial level and the iron-boron complex, no new centers were detected involving iron complexing with either the carbon or the oxygen. The iron-boron level was shown to dissociate by a recombination-enhanced mechanism and a deep acceptor level of this complex was detected at Ec −0.29 eV, which must be the recombination level rather than the well-established level at Ev +0.1 eV.
Electron tunneling experiments using Nb-Sn ‘‘break’’ junctions
Authors: Moreland John , Ekin J. W.
Published: July 2002

An Nb-Sn filament mounted on a flexible glass beam can be broken to form an electron tunneling junction between the fracture elements. Breaking the filament in liquid helium prevents oxidation of the freshly exposed fracture surfaces. A sharp superconducting energy gap in the I-V characteristics measured at 4 K indicates the formation of a high-quality tunneling barrier between the fracture elements. The resistance of the junction can be continuously adjusted by varying the surface bending strain of the beam. An estimated 0.1 nm change in the barrier thickness produces about an order of magnitude change in the resistance over the range from 105 to 108 Ω. The exponential character of this dependence shows that the tunnel junction is freely adjustable without intimate contact of the junction elements. ‘‘Break’’ junctions made in this way offer a new class of tunneling experiments on freshly exposed surfaces of a fractured sample without the oxide barrier previously required for junction stability. Such experiments provide a simple technique for tunneling to new materials and may eliminate complications that can be encountered during interpretation of data obtained using oxide barriers.
Explosive crystallization in silicon
Authors: Geiler H.-D. , Glaser E. , Götz G. , Wagner M.
Published: July 2002

A survey of the possible autocatalytic crystallization processes called explosive crystallization in liquid and solid states is given. The explosive liquid-phase epitaxy with laterally moving coupled interfaces of melting and crystallization in amorphous silicon layers on insulators is investigated by the use of an experimental equipment consisting of three synchronized lasers supplying the temperature pulses for ignition, spreading, and stopping of the explosive front. The velocity of this explosive crystallization front measured by use of time-resolved reflectivity of a test beam is compared with the results of model calculations. The results are in good agreement. The crystal structure was investigated by optical and transmission electron micrography and represents crystalline laminae grown preferentially in the 〈110〉 direction over a distance of about 100 μm. Areas of some millimeters in diameter can be crystallized by this method.
1.5-μm infrared excitation of visible luminescent in Y1−xErxF3 and Y1−x−yErxTmyF3 via resonant-energy transfer
Authors: van der Ziel J. P. , Van Uitert L. G. , Grodkiewicz W. H. , Mikulyak R. M.
Published: July 2002

Intense visible emission in the red, green, and blue has been observed from infrared to visible multiphoton Y1−xErxF3 upconversion phosphors pumped with 1.5-μm radiation from InGaAsP-laser sources. With 0.01<x<0.2, the dominant emission is in the green corresponding to transitions from the 4S3/2 level of Er3+. The addition of Tm3+ in the 0.001–0.02 range preferentially quenches the emission from the higher-energy Er3+ levels resulting in a red visible emission originating on the Er3+ 4F9/2 level. The phosphors are useful visible monitors of focused 1.5-μm semiconductor laser and unfocused F-center laser radiation. The unexpectedly high brightness in the visible may be attributed to thermal equilibration, where the excitation is trapped in the higher lying states after each upconversion step.
Electrical conductivity of nonideal plasmas
Authors: Zollweg R. J. , Liebermann R. W.
Published: July 2002

The Spitzer formula for the electrical conductivity of fully ionized gases is modified to permit its application in the nonideal plasma region. A rigorous method of calculating electrical conductivities in partially ionized gases in both ideal and nonideal plasmas is also described. These models are based on the classical Chapman–Enskog approximation with the superposition of binary collisions without the assumption of additional electron scattering mechanisms, such as scattering by plasma oscillations. Good agreement with experiment is obtained in the nonideal region largely through a more adequate treatment of electron screening of the ionic Coulomb potential. The large electron scattering cross section for attractive ion potentials found by Mason et al. [E. A. Mason, R. J. Munn, and F. J. Smith, Phys. Fluids 10, 1827 (1967)] is found to decrease the conductivity in the nonideal region compared to the cross sections of Liboff.
The effect of InP substrate misorientation on GaInAs-AlInAs interface and alloy quality
Authors: Brown A. S. , Mishra U. K. , Henige J. A. , Delaney M. J.
Published: July 2002

The quality of GaInAs-AlInAs epitaxial layers is found to be critically dependent on the degree of (100)-InP substrate misorientation. The alloy quality of both materials is improved when the substrate is misoriented 4° off the (100). The heterojunction interface quality as determined by the full width at half-maximum of quantum-well photoluminescence is also improved when a substrate misoriented by 4° is used. A degradation of both alloy and interface quality as compared to material on (100) InP is observed when the misorientation is 2°. These effects are also observed for strained quantum-well structures.
The characteristics of amorphous silicon carbide hydrogen alloy
Authors: Tsai Hsiung-Kuang , Lin Wei-Liang , Sah Wen Jyh , Lee Si-Chen
Published: July 2002

Various properties, including the chemical composition, optical gap, infrared and photoluminescence spectra, dark and photoconductivity, and activation energy of the intrinsic and doped amorphous silicon carbide hydrogen alloy prepared by radio-frequency glow-discharge decomposition of a silane-methane mixture have been studied in detail. The gas-phase composition of CH4, i.e., Xg=CH4/(CH4+SiH4), was varied from 0 to 0.8 and the B2H6 and PH3 mole fraction in the gas phase was varied from 0.09% to 2%. It is found that the carbon mole fraction in the film is smaller than 0.12 for Xg≤0.8 and the incorporation of carbon for Xg≤0.4 makes the film inhomogeneous, i.e., forming Si- and SiC-rich regions. The observed variation of optical and electrical properties as a function of doping and carbon mole fraction are explained in a consistent way.
Raman study of crystalline structure and resonant behavior in (AlxGa1−x)0.5In0.5P quaternary alloys
Authors: Kubo Minoru , Mannoh Masaya , Narusawa Tadashi
Published: July 2002

Raman study of the crystalline structure and resonant scattering behavior in (AlxGa1−x)0.5In0.5P alloys is presented. The first-order longitudinal-optical phonon line shape shows asymmetric broadening which is explained by the spatial correlation model. The correlation length in this model is related to the size of domains due to the generation of the ordered structure. In resonant Raman scattering in (AlxGa1−x)0.5In0.5P alloys, we observe drastic enhancement of the Raman intensities. It is called scattered photon resonance, and the peaks are around the energy of interband transition related to the E0/E0+Δ0 at the Γ point.
Monte Carlo study of critical relaxation in the 3D Heisenberg model
Authors: Peczak P. , Landau D. P.
Published: July 2002

Employing Monte Carlo simulations, the equilibrium dynamic critical behavior or L×L×L simple cubic classical Heisenberg models with periodic boundary conditions are determined. From the long-time exponential decay of the time-displaced correlation function of the magnetization the linear (equilibrium) relaxation time τ is also determined. It is found that at the critical point Tc, τ grows like τ≂Lz with z=1.94±0.06. In the paramagnetic region a power-law divergence τ≂(T/Tc−1)−Δ was found with Δ=zν=1.38±0.04, which implies a dynamical critical exponent z=1.96±0.07. Both values are consistent with theoretical predictions and results of experimental studies concerning the dynamical critical behavior of Heisenberg magnets perturbed by order-parameter nonconserving forces.
Depth uniformity of electrical properties and doping limitation in neutron-transmutation-doped semi-insulating GaAs
Authors: Satoh M. , Kuriyama K. , Kawakubo T.
Published: July 2002

Depth uniformity of electrical properties has been evaluated for neutron-transmutation-doped (NTD), semi-insulating GaAs irradiated with thermal neutrons of 1.5×1018 cm−2 by the van der Pauw method combined with iterative etching of the surface. In NTD-GaAs wafers (thickness ∼410 μm) annealed for 30 min at 700 °C, the depth profiles of the resistivity, the carrier concentration, and the Hall mobility show constant values of 1×10−2 Ω cm, 2.0×1017 cm−3, and 3100 cm2/V s, respectively, within an experimental error of 5%. In an annealing process, the redistribution and/or the segregation of NTD impurities is not observed. We also discuss the limitations of low-level NTD in semi-insulating GaAs. It is suggested that the activation of the NTD-impurities below ∼1×1016 cm−3 is mainly restricted by the presence of the midgap electron trap (EL2).
Experimental and theoretical analysis of transient grating generation and detection of acoustic waveguide modes in ultrathin solids
Authors: Meth Jeffrey S. , Marshall C. D. , Fayer M. D.
Published: July 2002

A full theoretical analysis of and experimental evidence for the optical generation and detection of acoustic waveguide modes (Lamb waves) in ultrathin solids using the transient grating (TG) technique is presented. The driving force due to the TG excitation is derived for a free, isotropic plate. In contrast to a bulk isotropic material in which a single wave is excited, the TG excites a number of modes with a variety of frequencies but with the same tangential component of the wavevector. The frequencies beat, resulting in a complex time-dependent signal. Experimental results are presented for anthracene sublimation flakes. In addition to discussing the general features of Lamb wave generation, we also discuss the effects of mounting the crystal on a substrate, of varying the fringe spacing, of resonant probing, and of polarized detection. The nature of Lamb waves in anisotropic materials is illustrated, and the extent to which the isotropic theory can be applied to anisotropic systems is discussed.
Monte Carlo simulation for the ion implantation of silicide heterostructures
Authors: Chang Chin-Liang , Yang Jeng-Rern , Lue Juh Tzeng
Published: July 2002

A Monte Carlo simulation is used to solve the ion-implantation process with three-dimensional boundaries. The projected range, standard deviation, skewness, kurtosis, and damage distributions after ion implanation for TiSi2 heterostructures and the feasibility of the silicide doping process are simulated in this work.
Sb adsorption on Si 〈111〉 analyzed by ellipsometry and reflection high-energy electron diffraction: Consequences for Sb doping in Si molecular-beam epitaxy
Authors: Andrieu Stéphane
Published: July 2002

In this paper, two distinct studies are presented: Sb adsorption on Si 〈111〉 analyzed by both ellipsometry and electron diffraction in real time, and Sb doping in Si molecular-beam epitaxy (MBE). In the adsorption study, saturation is shown to occur at one monolayer of equivalent silicon. Superstructures do not appear at the same coverages and temperatures during adsorption and desorption. It is also confirmed that the anomalies we have previously observed in the desorption kinetics are due to superstructures. A detailed study of the sticking coefficient explains both the differences in adsorption and desorption phase diagrams and the complex evolution of Sb doping in Si MBE. Variations of the doping level with the growth rate and incident dopant flux support this result.
Uniform deposition of diamond films using a flat flame stabilized in the stagnation-point flow
Authors: Murayama Motohide , Kojima Shinji , Uchida Kiyoshi
Published: July 2002

For uniform deposition of diamond films by acetylene/oxygen combustion flames, the flat flame stabilized in the stagnation-point flow in front of the substrate has been realized by adding hydrogen to reduce the burning velocity. The flat flame stability was examined for the hydrogen/acetylene/oxygen ratio and the flow velocity of the reactant gas. In the flat flame, fields of temperature and concentrations of radical species which take part in diamond deposition, are homogeneous on the surface of the substrate. It has been confirmed by scanning electron microscopy and Raman spectroscopy that diamond film deposited by the round flat flame with a diameter of about 7 mm, are uniform over area with nearly the same diameter of the flame.
Stress and plastic flow in silicon during amorphization by ion bombardment
Authors: Volkert C. A.
Published: July 2002

In situ wafer curvature measurements were performed during amorphization of silicon by MeV ion implantation. These measurements provide information about density changes and plastic phenomena in the implanted region. Experiments were performed for a variety of ions, a range of fluxes, and for temperatures between −175 and 200 °C. In all cases, the implanted region expanded due to the creation of damaged crystal, creating compressive stress in the implanted region on the order of 108 N/m2. Once heavily damaged or amorphous regions were formed, radiation-enhanced plastic flow of material out of the plane of the wafer occurred in order to relieve the stress created by the expansion. The value of the shear viscosity responsible for this phenomena could be measured by comparing samples with the same history but different stresses. For 2-MeV Xe implantation at room temperature and 1011 ions/cm2 s, the radiation-enhanced shear viscosity is ∼ 1013 Ns/m2, which is at least four orders of magnitude smaller than the thermally activated shear viscosity. Possible contributions to flow from a homogeneous distribution of broken bonds and from fluid-like collision cascade regions are discussed.
Mössbauer effect and magnetization studies of α-FeMn alloys
Authors: Paduani C. , Galvão da Silva E. , Perez-Alcazar G. A. , McElfresh M.
Published: July 2002

The bcc phase of Fe-Mn alloys was studied by means of the Mössbauer effect, x-ray diffraction, thermogravimetry analysis, and magnetization measurements for Mn concent- rations up to 17.5 at.%. These alloys are disordered over this composition range and both the average hyperfine field and critical temperature (Tc) decrease with increasing Mn concentration. Above 5 at.% Mn about 3 at.% Ti was added to each sample in order to stabilize and retain the α-phase at room temperature. The HF distribution shows peaks spaced 24 kOe apart corresponding to the substitution of each Fe nearest neighbor by a Mn atom. From the magnetization measurements we have found that the mean magnetic moment decreases at a slow rate for Mn contents up to 11 at.% followed by a fast decrease for larger Mn concentration. The usual interpretation of a constant magnetic moment for Fe and a small one for Mn cannot be used to interpret the present results.
The use of liquid jets to simulate angled drop impact
Authors: Hand R. J. , Field J. E. , Townsend D.
Published: July 2002

The liquid jet impact technique has been used extensively for the quantitative study of rain impact. Most of this work has been for normal impact. Although some angled impact studies have been pursued using jets, these have been purely of a qualitative nature. This paper develops the theory of angled drop impact. This allows the establishment of a method of correlating between angled drop and jet impact and obtaining ‘‘equivalent drop’’ curves for jet impact. The correlation is also applied to normal impact. For normal impact it is shown that the previously published equivalent drop curves need to be modified especially at low velocities. The results for both normal and angled impact are discussed with reference to the details of the jet profile, and it is shown that angled jets may be considered as having an average equivalent drop size. The theory of angled impact also explains the shape of damage marks produced by impact and why the amount of damage decreases so rapidly with increasing angle.
The effect of field-dependent heat capacity on the characteristics of the ferromagnetic Ericsson refrigeration cycle
Authors: Yan Zijun , Chen Jincan
Published: July 2002

The characteristics of a magnetic Ericsson refrigeration cycle are investigated on the basis of the thermodynamic properties of the ferromagnetic material. The effect of field-dependent heat capacity on regeneration is discussed. The coefficients of performance of the Ericsson magnetic refrigeration cycle are derived. Finally it is pointed out that, according to theoretical analysis, the Ericsson magnetic refrigeration cycle can be expected to reach or approach perfect regeneration by using a mixture of several magnetic materials as the working substance.
Structural imperfections in silica glasses with an optical absorption peak at 3.8 eV
Authors: Awazu Koichi , Harada Koushi , Kawazoe Hiroshi , Muta Ken-ichi
Published: July 2002

Structural imperfections in silica glasses with an optical absorption peak at 3.8 eV were studied. The 3.8-eV peak was assigned to Cl2 molecules trapped in the glasses. The peak absorption was increased with increasing partial pressure of chlorine for sintering. The peak and its full width at half maxima were the same as those of Cl2 molecules in the gas phase. ≡SiOH was generated in the glasses by annealing in H2 ambient. After irradiation with a KrF excimer laser (5 eV), an absorption peak at 4.8 eV and red emission were observed. The properties were the same as the photochemical reaction of O2 molecules in the gas phase. We proposed that O2 molecules were trapped in the glass network when silica glasses were sintered under Cl2 ambient. The reaction is expressed as ≡SiOH+HOSi≡+Cl2→≡SiCl+ClSi≡+H2O↑+1/2O2 (trapped).
An annealing study of nickel point defects in high-pressure synthetic diamond
Authors: Lawson Simon C. , Kanda Hisao
Published: July 2002

Results of an annealing study, carried out in the temperature range 1500–1900 °C, of nickel-related optical centers in high-pressure synthetic diamond are presented. It is established that the well-known 1.883 and 2.51 eV systems anneal out during the temperature regime of nitrogen aggregation and the concurrent growth of an array of structure, which extends throughout the visible region of the absorption spectrum, and gives the previously bright-yellow-colored diamonds a rich golden-yellow color, is observed. By carrying out the annealing sequence on diamonds grown using various solvent catalysts a correlation is found between preanneal 1.883 eV absorption and maximized absorption of some of the annealed structure. From the results it is proposed that centers associated with nickel and nitrogen are produced and their possible natures are speculated on. It is found that the defect-induced one-phonon spectra of the diamonds examined may be satisfactorily decomposed into three components. To account for certain changes in the infrared spectra during the annealing sequences, and using previously reported results, it is proposed that one of these components may result from nitrogen in a positive charge state.
Optically induced gratings in Fe- and Mn-doped Bi4Ge3O12 single crystals
Authors: Zaldo C. , Moya E. , Magaña L. F. , Kovács L. , Polgár K.
Published: July 2002

Optically induced gratings have been created in Bi4Ge3O12 single crystals doped with Fe and Mn impurities. The writing kinetics of the holograms depends on the previous illumination with ultraviolet (UV) light, λ=310 nm. It has been concluded that there is a photorefractive contribution due to Fe impurities ([Fe]=2.7×1017 atoms/cm3) which can be observed when the holograms are written with 457.9 nm light. This contribution is essentially independent of the previous UV illumination, the steady state diffraction efficiency η of a 633 nm He-Ne laser beam is ≊2×10−4, and the corresponding modulation of the refractive index Δn at λ=633 nm is ≊4×10−6. The UV illumination induces a transient behavior of the diffraction efficiency with ηmax≊8×10−4, this regime is dominated by the photorefractive effect. After this maximum, η decreases to ≤0.3×10−4 and a quasi steady state is reached; in this steady state regime the contribution of a photochromic grating dominates the value of η. Mn-doped samples ([Mn]=4.3×1017 atoms/cm3) also show the transient behavior and the photochromic contribution.
The initial magnetization curve after alternating current demagnetization
Authors: Seeck St. , Lambeck M.
Published: July 2002

Investigating the initial curve of various ferromagnetic materials a novel effect was found: when a specimen is demagnetized by an ac field with decreasing amplitude using a small number of cycles, the initial magnetization curve exhibits a distinct peaky structure. This structure can be explained by the Preisach model [see Mayergoyz, Mathematical Models of Hysteresis (Springer, New York, 1991), e.g., p. 1] thus providing new means of testing and applying this model using a noncyclic measurement.
Relation between the Ω6 intensity parameter of Er3+ ions and the 151Eu isomer shift in oxide glasses
Authors: Tanabe S. , Ohyagi T. , Todoroki S. , Hanada T. , Soga N.
Published: July 2002

The Ωt intensity parameters (t=2,4,6) of Er3+ ions in several oxide glasses and the isomer shift (IS) of 151Eu Mössbauer spectra in glasses of the same composition were determined. Among these Ωt’s, the Ω6 parameter was found to have a good relation with IS; Ω6 decreases with an increase of IS which reflects the 6s electron density of rare-earth ions. From the theoretical expression of Ωt, Ω6 is considered to be more affected by the overlap integrals of the 4f and 5d orbitals than Ω2 and Ω4, and to increase with an increase of these overlap integrals. These overlap integrals are supposed to decrease when the 6s electron density is larger, since the 6s electron density shields the 5d electron orbital.