bluuebunny/crossref_metadata_2025 huggingface dataset:
Induced magnetization in lithium doped YBa2Cu3Oy
Authors: Maury F. , Nicolas-Francillon M. , Nanot M. , Ollitrault-Fichet R.
Published: July 2002

The magnetization of two series of lithium doped YBCO samples, having Li molar concentrations ranging from 0 to 0.4, has been measured as a function of the magnetic field from 50 to −50 kOe at various temperatures between 5 and 30 K. The samples of the first series, annealed in oxygen, are orthorhombic and metallic. The samples of the second series, annealed in argon, are tetragonal and insulating. Above the transition temperature of the superconducting orthorhombic samples and up to 300 K, the magnetization is found to be quite similar for both series of samples, increasing with the lithium content of the sample and, in the tetragonal samples, increasing with the oxygen content of the sample. At low temperatures and for the tetragonal samples, the magnetization is found not to be proportional to the magnetic field as it should be if it were due to local moments of the order of μB. The value of 1.1μB obtained for the effective magnetic moment per Li ion from M(T) experiments carried out at H=10 kOe, which has been attributed to local moments carried by Cu ions neighboring a Li ion, may thus not have a straightforward interpretation. The shape of the M(H)curves, which exhibit a downturn at high fields, is reproduced well by the sum of a linear term and a Langevin function as if the samples displayed superparamagnetism. Some possible interpretations of this phenomenon are discussed.
Pyroelectric effect in Y–Ba–Cu–O thin films under laser illumination
Authors: Butler Donald P. , Çelik-Butler Zeynep , Adam Roman , Sobolewski Roman
Published: July 2002

The thermoelectric response of amorphous, semiconducting Y–Ba–Cu–O thin films was investigated by illumination with 150 ps optical pulses at 1064 nm and with a continuous wave at 493 nm (argon laser). The measured unbiased voltage response was consistent with pyroelectricity. At high pulse energies, the voltage response saturated due to the saturation of the spontaneous polarization. The pulsed responsivity was limited by the short pulse duration compared to the thermal time constant of the structure. The electrical time constant for the change in the spontaneous polarization was estimated to be 50 ns.
Nonlinear emission and recombination in conjugated polymer waveguides
Authors: Haugeneder A. , Neges M. , Kallinger C. , Spirkl W. , Lemmer U. , Feldmann J. , Amann M.-C. , Scherf U.
Published: July 2002

The emission dynamics in photoexcited planar conjugated polymer waveguides is investigated at high excitation densities. Using femtosecond pump/probe experiments and photoluminescence spectroscopy we investigate the interplay of nonlinear radiative and nonradiative recombination processes. Amplified spontaneous emission (ASE) leads to an ultrafast depletion of the excited state at excitation densities above 1018 cm−3 in an ladder-type poly(p-phenylene) film deposited on a glass substrate. Owing to the different waveguide structure ASE is not observed for the same polymer deposited on an indium–tin–oxide (ITO)-coated substrate. Instead, we observe nonradiative bimolecular annihilation with a coefficient γ=4.2×10−9 cm3 s−1. Our results demonstrate that even in the absence of a resonator collective stimulated emission can be much more efficient than nonradiative recombination. A mandatory prerequisite, however, is a suitable waveguide design. The use of ITO as a hole-injecting contact is problematic due to its high refractive index and its relatively high losses.
Ethyl-acetate transport in poly(ethylene terephthalate)
Authors: Ouyang Hao , Shore Shyh-Horng
Published: July 2002

The effect of annealing atmosphere on the transport of ethyl-acetate in poly(ethylene terephthalate) (PET) and related phenomena were investigated. Based on Harmon’s model for case I (Fickian), case II (swelling) and anomalous transport, the data of mass uptake were analyzed. Pure case I or case II behavior did not appear in the PET–ethyl-acetate system and is affected by the atmosphere of annealing. The variations of conformations was studied by the Fourier-transform infrared. The mass transport in PET is accompanied by a large-scale structural rearrangement, which leads to the induced crystallization of the original amorphous state. The preferred growth plane varies under the influence of different solvents. Solvent-induced crystallization was confirmed by x-ray diffraction, differential scanning calorimeter, and density measurement. The different behaviors of sorption between the solvent-treated PET crystallites and thermally treated PET can be explained in terms of the results from density measurement.
Performance predictions for a silicon velocity modulation transistor
Authors: Crow G. C. , Abram R. A.
Published: July 2002

A Monte Carlo simulation has been devised and used to model submicron Si velocity modulation transistors with the intention of designing a picosecond switch. The simulated devices have nominal top and back gate lengths of 0.1 μm, and the conduction channels have similar thickness. Mobility modulation has so far been achieved by heavily compensated doping and interface roughness at one side of the channel. The simulated devices have a high intrinsic speed; simulations performed for T=77 K suggest that current can be switched between the low and high mobility regions of the channel within 1.5 ps. However, in unstrained Si devices the main obstacle to practical device operation is the rather small current modulation factor (the ratio of the steady drain currents for the device operating in the high and low mobility regimes), which decreases towards unity with increasing drain–source bias. Such a device should work best for small electric fields along the channel (∼105 V m−1), the regime where impurity scattering has its greatest influence on the electron mobility.
Effect of laser irradiation on the luminescence of Mg and Si-doped GaN films
Authors: Zaldı́var M. Herrera , Fernández P. , Piqueras J. , Solı́s J.
Published: July 2002

Pulsed laser treatments have been performed in GaN samples of both n- and p-type conductivity. The laser induced changes have been monitored by emissive mode and cathodoluminescence (CL) in a scanning electron microscope. Emissive mode observations indicate a moderate laser induced recrystallization. The luminescent emission has been characterized in both types of samples, GaN:Si and GaN:Mg. Whereas the evolution of CL in the Si doped samples could be explained by the occurrence of laser induced annealing, the luminescent behavior of the Mg doped samples upon irradiation seems to be more complex and a strong relation with the compensation or Mg activation is suggested. Several luminescence bands with maxima ranging from 3.3 to 2.7 eV and their dependence on irradiation conditions have been studied.
Tunable band offsets via control of interface atomic configuration in GaAs-on-ZnSe(001) heterovalent heterostructures
Authors: Funato Mitsuru , Aoki Satoshi , Fujita Shizuo , Fujita Shigeo
Published: July 2002

The tunability of band offsets in GaAs/ZnSe(001) heterovalent heterostructures is investigated. The surface of GaAs grown on ZnSe by metalorganic vapor phase epitaxy is atomically flat under the condition employed in this study, which is indicative of the formation of the abrupt interface. Between the growth of ZnSe and GaAs, interval is introduced for purging a Se source precursor. After the interval, the GaAs growth begins with an initial exposure of ZnSe to an As precursor. The durations of the interval and the As exposure are the parameters to control the interface chemistry. During the interval, atoms on the ZnSe surface change from Se to Zn. As the duration of the As exposure lengthens, on the other hand, either Zn or Se atoms appear alternately on the ZnSe surface, because the As precursor etches ZnSe digitally. Using these characteristics, the valence band offsets in GaAs/ZnSe(001) heterostructures are controlled between 0.6 and 1.1 eV.
Enhanced depth resolution in positron analysis of ion irradiated SiO2 films
Authors: Simpson P. J. , Spooner M. , Xia H. , Knights A. P.
Published: July 2002

We report the observation of defects following 1.7 MeV Si+ through-implantation of SiO2 films thermally grown on Si(100) substrates. Films were irradiated to fluences of 1012, 1013, and 1014 ions/cm2, and analyzed using a variable energy positron beam. Enhanced depth resolution was achieved via iterative chemical etching and measurement, for the sample irradiated to 1012 Si+/cm2, and for an unirradiated control sample. The positron annihilation “S parameter” is reduced in the implanted SiO2, due to a reduction in the formation of positronium (the atom-like positron–electron bound pair), and is increased in the Si substrate, due to the creation of vacancy-type defects. The concentration of implant-induced defects was found to be very nearly constant as a function of depth in the SiO2 film, indicating a significant contribution of both the electronic and nuclear stopping of the implanted Si+ ions to the production of positron-trapping defects. We find that the maximum overlayer thickness which allows simultaneous detection of defects in the substrate (i.e., through-oxide probing of substrate defects) is ∼600 nm.
Atomic absorption spectroscopic measurements of silicon atom concentrations in electron cyclotron resonance silicon oxide deposition plasmas
Authors: Augustyniak Edward , Chew Kok Heng , Shohet J. Leon , Woods R. Claude
Published: July 2002

The silicon atom densities in both silane/oxygen and tetraethoxysilane (TEOS)/oxygen electron cyclotron resonance (ECR) plasmas were measured as functions of microwave power, pressure, and gas flow rates. An atomic absorption spectrometer with a Si hollow-cathode lamp was constructed for these measurements. Silicon atom densities in silane/oxygen ECR discharges increase with rising plasma density, and a strong correlation was found between the Si atom gas-phase abundance and the silicon oxide film deposition rate. The measured Si concentrations [(1–7)×1010 cm−3] were high enough to account for a significant part of the film growth in the silane based chemistry. In TEOS/O2 discharges Si atom concentrations were lower by an order of magnitude, so Si is probably not a major contributor to the growth rate in that case. The internal temperature of Si atoms was found to vary from 380 to 720 K with increasing microwave power (200–650 W).
Study of sample thickness dependence in electron-beam irradiation of self-developing inorganic materials
Authors: Chen G. S. , Humphreys C. J.
Published: July 2002

In this study we apply a series of focused and overfocused electron probes with current densities ranging from 105 to 109 A m−2 to irradiate electron-gun deposited thin films of amorphous AlF3 (a-AlF3)and amorphous SiO2(a-SiO2). Statistical distributions of the time deemed necessary to produce a given amount of mass loss from the two beam-irradiated materials are measured as functions of beam current density and sample thickness. According to those results, a-AlF3 is damaged in parallel throughout the irradiated volume of the sample as indicated by no detectable thickness dependence, whereas a-SiO2 displays a distinct scaling of characteristic drilling times with thickness indicative of a combination of surface and volume mass loss processes.
Optical and electron paramagnetic resonance study of light-emitting Si+ ion implanted silicon dioxide layers
Authors: Valakh M. Ya. , Yukhimchuk V. A. , Bratus’ V. Ya. , Konchits A. A. , Hemment P. L. F. , Komoda T.
Published: July 2002

Thermally grown SiO2 on Si substrates implanted with Si+ ions with a dose of 6×1016 cm−2 were studied by the techniques of photoluminescence, electron paramagnetic resonance (EPR), and low-frequency Raman scattering. Distinct oxygen-vacancy associated defects in SiO2 and nonbridging oxygen hole centers were identified by EPR. The luminescence intensity in the 620 nm range was found to correlate with the number of these defects. The low-frequency Raman scattering technique was used to estimate the average size of the Si nanocrystallites formed after the implantation and thermal annealing at T>1100 °C, which are responsible for the photoluminescence band with a maximum at 740 nm. The intensity of this band can be significantly enhanced by an additional treatment in a low-temperature rf plasma.
Thermal stress in a trigonal crystal rod
Authors: Greninger Charles E. , Rodriguez Sergio E.
Published: July 2002

The thermally induced stresses and strains in a freely supported long trigonal crystal rectangular rod are presented. Careful grouping of the stress, strain relationships casts the equations into the form of the usual logarithmic potential solution with new coefficients that are simple functions of the stiffness matrix elements. Then a logarithmic potential solution is used to find analytic expressions for the particular stresses. Addition of two ordinary isothermal stress fields allows the particular stresses to be corrected so that (a) the surface stresses average to zero on each lateral boundary and (b) the plane strain condition is removed and the average longitudinal stress and its moments are zero. A method to zero the residual surface stress is mentioned and numerical test cases and an example are given.
High frequency tribological investigations on quartz resonator surfaces
Authors: Laschitsch Alexander , Johannsmann Diethelm
Published: July 2002

We report on the application of quartz crystal resonators for friction measurements in the MHz regime. The dissipative processes are studied by approaching a small sphere to the quartz surface and measuring the shifts in frequency and bandwidth as a function of the sphere–sample distance. Once elastic contact is established, the frequency and the bandwidth can be well described by an increased stiffness of the quartz–sphere system. In the range of distances where contact is just being formed, we observe strongly different behavior for high-friction metal–metal interfaces and low-friction fluoropolymer interfaces. For high-friction interfaces there is an excess dissipation of energy which we attribute to frictional processes in the contact zone.
Scaling and data collapse in magnetic viscosity
Authors: Mayergoyz I. D. , Adly A. , Korman C. , Huang Mingwei , Krafft C.
Published: July 2002

This article presents experimental evidence for scaling and data collapse in magnetic viscosity. It is demonstrated that, by using appropriate scaling, the “bell-shaped” viscosity coefficient curves observed for different temperatures collapse onto one universal curve. This data collapse reveals a self-similar structure of the viscosity coefficient as a function of two variables: magnetic field and temperature.
Effect of cluster/particle deposition on atmospheric pressure chemical vapor deposition of SiO2 from four gaseous organic Si-containing precursors and ozone
Authors: Fujimoto Toshiyuki , Okuyama Kikuo , Yamada Satoshi , Adachi Motoaki
Published: July 2002

In order to analyze the particle generation and its effect on the SiO2 thin film in an atmospheric pressure chemical vapor deposition (APCVD) process using four organic silicon vapors and ozone gas, gas-phase particle generation, growth, transportation and vapor-cluster/particle codeposition processes were studied experimentally and theoretically using a flow-type vertical tube reactor. Decomposition reaction rates of four organic silicon vapors (tetraethylorthosilicate, triethoxysilane, tetramethylorthosilicate, and octamethylcyclotetrasiloxane) due to the O⋅oxidation were determined by arranging the number concentrations of the generated particles in Arrhenius plots. The obtained activation energies and frequency factors of reaction rate constants were used to simulate the vapor-cluster/particle codeposition in the CVD process. In the numerical simulation, computational fluid dynamics equations (continuity, momentum, and energy conservation equations) were solved to evaluate the gas velocity, vapor concentration, and temperature profile inside the reactor. Particle population balance equations based on discrete-sectional presentation for the particle size spectrum were solved coupling with diffusion equations of vapors and clusters/particles. This numerical simulation code could explain the vapor-cluster/particle codeposition in the thin film preparation by the APCVD process. The film growth rate and the surface morphology of the film could be reasonably explained by the deposition flux and the size of deposited clusters/particles obtained as numerical simulation results, respectively.
Annealing effects on the coercivity of SmCo5 nanoparticles
Authors: Chowdary K. M. , Giri A. K. , Pellerin K. , Majetich S. A. , Scott J. H. J.
Published: July 2002

The enhanced coercivity of ball milled nanoparticles of SmCo5 has been attributed to the reduced particle size, but the milling process introduces additional strain which can also affect the coercivity. Here the effects of size and strain are decoupled from each other using a combination of x-ray diffraction, electron microscopy, and coercivity measurements. Ball milled SmCo5 nanoparticles were annealed under different temperature, time, and atmospheric conditions to minimize the strain without chemically altering the sample, and with minimal grain growth and sintering. X-ray diffraction was used to determine if phase changes had occurred, and to monitor the average grain size and strain. From scanning electron microscopy the log-normal size distributions were found and no evidence of sintering was revealed. The sample coercivities were measured by superconducting quantum interference device following annealing.
Structural, magnetic, and Mössbauer spectral properties of Pr2Fe17Hx where x is 1, 2, and 3
Authors: Grandjean F. , Hautot Dimitri , Long Gary J. , Isnard O. , Miraglia S. , Fruchart D.
Published: July 2002

The 295 K unit cell parameters of Pr2Fe17Hx, where x is 0, 1, 2, and 3, have been determined by x-ray diffraction. Upon hydrogenation, the unit cell volume expands anisotropically with a 1.2% expansion of the a axis and no expansion of the c axis for x=3. The Curie temperatures are 286, 374, 440, and 484 K, for x=0, 1, 2, and 3, respectively. The Mössbauer spectra have been measured between 4.2 and 295 K, and the spectra of Pr2Fe17H and Pr2Fe17H2, obtained above 80 K, and those of Pr2Fe17H3, obtained above 120 K, have been analyzed with a seven sextet model, a model which corresponds to basal magnetic anisotropy. The Mössbauer spectra of Pr2Fe17H3 change dramatically upon cooling below 80 K, and between 4.2 and 75 K they are well fit with a four sextet model which corresponds to axial magnetization. Between 80 and 120 K, the spectra are very complex and, hence, the Mössbauer spectra reveal a spin reorientation which occurs between 80 and 120 K in Pr2Fe17H3.
A microdeposition technique for carbon nanotubes based on electron beam lithography
Authors: Ahlskog M. , Seynaeve E. , Vullers R. J. M. , Van Haesendonck C.
Published: July 2002

We describe a deposition technique for either individual or small amounts of multiwalled carbon nanotubes which is based on electron beam lithography. Carbon nanotubes are deposited from the solution phase onto a substrate through lithographically determined openings in an electron beam resist layer. The openings vary in size from a few μm to arbitrary size. It is possible to have one or a few nanotubes in the smaller openings. Using alignment markers, the technique has been applied to deposit individual nanotubes on top of gold electrodes.
Effects of growth rate of a GaN buffer layer on the properties of GaN on a sapphire substrate
Authors: Kim Ki Soo , Oh Chang Seok , Lee Kang Jea , Yang Gye Mo , Hong Chang-Hee , Lim Kee Young , Lee Hyung Jae , Yoshikawa Akihiko
Published: July 2002

We studied the effects of the growth rate of a GaN buffer layer grown on a GaN epilayer. It was found that this growth rate plays a key role in improving the quality of the GaN film on a sapphire substrate and an optimum growth rate exists that yields the best crystal quality. A GaN film grown on a buffer layer with the optimum growth rate of 18.3 nm/min has an electron Hall mobility of 539 cm2/V s and a dislocation density of approximately 2×108 cm−2. These improvements of GaN film qualities are illustrated by the promotion of the lateral growth mode.
Formation of Si islands from amorphous thin films upon thermal annealing
Authors: Wakayama Yutaka , Tagami Takashi , Tanaka Shun-ichiro
Published: July 2002

The mechanism of crystalline Si island formation from an amorphous film was discussed in relation to the free energy of the Si/SiO2 system. Agglomeration of the Si crystallite occurred forming islands, thus preventing a further increase in free energy. Crystal growth proceeded in two steps. At first, the size distribution of Si islands was unimodal but finally became bimodal during the crystallization process.
Brillouin scattering study of bulk GaN
Authors: Yamaguchi M. , Yagi T. , Sota T. , Deguchi T. , Shimada K. , Nakamura S.
Published: July 2002

High-resolution Brillouin scattering experiments have been performed for a high-quality free-standing gallium nitride (GaN) substrate. Elastic stiffness constants are reported. A comparison is made with the results of an earlier study for a GaN thin film on sapphire substrate.
Ultrashallow thermal donor formation in silicon by annealing in ambient oxygen
Authors: Åberg D. , Linnarsson M. K. , Svensson B. G. , Hallberg T. , Lindström J. L.
Published: July 2002

Czochralski-grown silicon wafers doped with phosphorus (∼1014 cm−3) have been annealed in nitrogen, wet nitrogen, oxygen, argon, and vacuum ambients at 470 °C for times up to 500 h. Sample characterization was made using predominantly electrical techniques such as admittance spectroscopy and thermally stimulated capacitance measurements but also secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy were employed. In all samples, an increasing concentration of free carrier electrons is observed with increasing annealing time, reaching a maximum of ∼1016 cm−3 at 100 h. For durations in excess of 100 h gradual decrease of the free electron concentration takes place except for the samples treated in wet nitrogen and oxygen atmospheres, which display donors stable even after 200 h. These stable centers are found to have shallower donor level positions in the energy band gap (∼25 meV below the conduction band edge Ec) than those of the centers formed in vacuum, argon, and nitrogen atmospheres (∼35 meV below Ec). The latter centers are associated with the well-established shallow thermal donors (STDs) while the origin of the former ones, which are labeled ultrashallow thermal donors (USTDs) is less known. However, on the basis of a wealth of experimental results we show that the USTDs are most likely perturbated STDs modified through interaction with fast-in diffusing oxygen species, possibly oxygen dimers. Further, comparison between the electrical data and the SIMS measurements reveals unambiguously that neither the STD nor the USTD centers involve nitrogen, in contrast to recent suggestions in the literature.
Elasticity of ultrathin copper-phthalocyanine Langmuir–Blodgett films by picosecond ultrasonics
Authors: Rouaı̈ L. , Bonello B. , Louis G. , Perrin B. , Peretti P.
Published: July 2002

The picosecond ultrasonic technique is used to investigate the elasticity of Langmuir–Blodgett submicronic films of copper (II) octabutoxyphthalocyanine. The amplitude of the acoustic echoes is measured against the number of monolayers (1–24, i.e., from a pure bidimensional behavior to a tridimensional one). The longitudinal sound velocity and the ultrasonic attenuation for frequencies up to about 40 GHz are deduced from this behavior. For the thinnest samples, the acoustic echoes are observed with a delay whose origin is analyzed. We show that the accuracy of the method is only limited by the precise knowledge of the density of the material.
Electrical fluctuations and photoinduced current transients in CdxHg1−xTe long wavelength epilayers
Authors: Paul Nichols , Van Vliet Carolyne M. , Mergui Sylvia
Published: July 2002

Measurements are described of I-V characteristics, resistance versus T−1, noise spectra and photoresponse for frequencies of 10 Hz-1 MHz in the temperature range 300–50 K for cadmium-mercury telluride long-wavelength infrared detectors having 12μ epitaxial layers on wide band gap material. The noise consists of 1/f noise and one or two Lorentzians, attributed to shallow hole traps, probably associated with Hg vacancies. The results are discussed and analyzed after a brief resume of generation-recombination noise. The photoinduced current transient spectroscopy response curves are attributed to tunneling to interface states.
Influence of the spatial laser intensity distribution on laser nitriding of iron
Authors: Landry F. , Lieb K.-P. , Schaaf P.
Published: July 2002

Laser nitriding of iron and other metals is governed by the complicated interplay of the laser–plasma–solid interactions which lead to a superposition of several mechanisms. This work reports on the drastic influence of the spatial laser intensity distribution on the nitriding process. The effects of the lateral laser intensity on the nitrogen lateral and depth profiles, the phase formation, the surface topology, and the microhardness are revealed by resonant nuclear reaction analysis, Mössbauer spectroscopy, surface profilometry, and nanoindentation. Homogeneous laser beams lead to a strong reduction or almost the absence of the piston mechanism, thus confining the nitriding and the transport processes to the laser spot and avoiding the fallout. The details are discussed in relation to the results obtained for the raw-beam irradiations. Much higher nitrogen saturation concentrations can be achieved with a homogenized beam, but the surface hardness and the hardening depth are lower than in the case of irradiation with the raw beam. The ε nitride is only found for raw-beam irradiation.
Longitudinal piezoelectric coefficient measurement for bulk ceramics and thin films using pneumatic pressure rig
Authors: Xu F. , Chu F. , Trolier-McKinstry S.
Published: July 2002

A pneumatic pressure rig was designed to measure the effective d33 coefficient of thin film piezoelectrics by applying a known stress and monitoring the induced charge. It was found that the stress state imposed included components both perpendicular and parallel to the film plane. The later were due to friction and could largely be relieved through sliding of the O-rings to their equilibrium positions for a given pressure. The induced charge stabilized as equilibrium was reached and most of it was produced by the normal component of the stress. By minimizing the surface friction and compensating for the remnant in-plane stress, very good agreement was obtained among the d33 values measured by the Berlincourt method, double-beam interferometry and this method for a bulk lead zirconate titanate (PZT) sample. The d33 value of PZT thin films made by sol-gel processing was also measured. The as deposited films usually showed very weak piezoelectricity with d33 values ranging from 0 to 10 pC/N, indicating little pre-existing alignment of the domains. With increasing poling field, the d33 value also increased and saturated at poling fields exceeding three times the coercive field. Typically, films with thicknesses around 1 μm had d33 values of 100 pC/N. Good agreement between double-beam interferometry and this technique was also obtained for thin films. The small difference between the two measurements is attributed to the effect of mechanical boundary conditions on the effective d33 coefficient.
Electrical and dielectric properties in double doped BaTiO3 showing relaxor behavior
Authors: Mahesh Kumar M. , Suresh M. B. , Suryanarayana S. V.
Published: July 2002

BaTiO 3 , when doped with minute concentrations of Bi+3 and Fe+3, was found to exhibit relaxor behavior. The data on dielectric constant performed as a function of frequency and temperature indicate a reduction in the transition temperature with increase in frequency. A poled sample indicated a decrease in the transition temperature compared to an unpoled sample. It was found that the grain boundary capacitance were negligible and the relaxations occurred are only due to the bulk of the sample. The results have been discussed using impedance and modulus spectroscopy.
Surface nonlocality effects on an optically active waveguide
Authors: Nelson D. F.
Published: July 2002

A new wave-vector-space method capable of finding electromagnetic wave propagation in bounded nonlocal media without using boundary conditions is used to find the modes of an optically active planar dielectric waveguide. Optical activity involves first-order wave-vector dispersion (nonlocality). The method finds how the altered nonlocal interaction close to the surfaces affects the waveguide modes. It is found that first-order effects of the surface layer nonlocality enter both the dispersion relation and the field profiles. The waveguide geometry is also used as an impetus to generalize several aspects of the wave-vector-space method.
Selective adsorption of metallocenes on clean and chemically modified Si(111) surfaces
Authors: Lin J.-L. , Rauscher H. , Kirakosian A. , Himpsel F. J. , Dowben P. A.
Published: July 2002

Metallocene adsorption on clean Si(111) and CaF2/CaF1/Si(111) substrates has been investigated with scanning tunneling microscopy. The surface chemical composition is found to strongly change the adsorption site selectivity, leading to an enhanced edge selectivity on modified substrates. Templates with well-defined local chemical reactivity have been created via self-assembly. The selective adsorption of metallocenes on such tailored substrates facilitates patterning ordered arrays of magnetic nanowires and stripes on the single digit nanometer scale.
Peculiar sponge-like structure of yttrium–iron–garnet nanocrystals on quartz substrate surface
Authors: Taketomi Susamu , Yokoyama Hiroyuki , Miyajima Hideki
Published: July 2002

Nanoscale amorphous yttrium–iron–garnet (YIG) particles were prepared by the alkoxide method. They were dispersed in a kerosene solvent, coated on a quartz plate substrate, and calcined at a temperature of 1273 K for 2 h. Surface morphology and cross-sectional microstructure of the thin coated films were examined by atomic force microscopy and transmission electron microscopy, respectively. During the calcination, amorphous YIG particles were transformed to YIG nanocrystals of ≈25 nm in mean diameter, and no extended grain growth or fusion of the multigrains was observed. Each particle was individually crystallized, but interconnected to each other, forming a sponge-like structure of 600 nm in thickness. Electron diffraction and energy dispersion x-ray analysis verified that the sponge-like layer consisted of YIG nanocrystalline particles. A rather dense intermediate layer of ≈100 nm in thickness was formed as a result of interfacial reactions between YIG and SiO2 decomposing to α-Fe2O3 and Y2Si2O7. The change in the Si concentration across the interlayer depth was modeled by thermal diffusion. This peculiar sponge-like structure of YIG nanoparticles supports our previous interpretation of the shift of the light absorption spectral peak, i.e., due to this peculiar structure, electrons are localized in each YIG particle, which act as a quantum dot, attributing to the quantum size effect observed in the spectral shift.
Equilibrium island ridge arrays in strained solid films
Authors: Rudin C. D. , Spencer B. J.
Published: July 2002

The equilibrium shape and chemical potential of an array of island ridges in an epitaxially strained solid film are determined from a continuum model describing the Stranski–Krastonow morphology of a thin layer on a substrate. This model incorporates the conditions for mechanical and chemical equilibrium, isotropic surface energy, and misfit strain. Solutions corresponding to periodic arrays of island ridges separated by wetting layers are found in the limit of island height much less than the island width. Our results describe how the island width, shape, and chemical potential vary as the distance between the ridges varies. The scaled island shape is shown to remain relatively constant as the distance between the islands increases.
Measurement of ion energy distributions in the bias enhanced nucleation of chemical vapor deposited diamond
Authors: Kátai Sz. , Tass Z. , Hárs Gy. , Deák P.
Published: July 2002

In situ, real-time, mass selective energy analysis of the incoming ions has been carried out during bias enhanced nucleation for diamond chemical vapor deposition. The dependence of ion energies and fluxes as a function of the bias voltage and CH4 concentration was measured. The main ionic species that strike the substrate surface during bias enhanced nucleation have energies between 50 and 80 eV and are characterized by a low H content. When the bias is lowered to the value at which nucleation enhancement becomes negligible (100 V in our system), ions have an energy of about 30–40 eV, which is the critical value for penetrating below the surface of a hydrogenated amorphous carbon layer. These findings strongly support the subplantation model of bias enhanced nucleation.
An improved model for ultraviolet- and x-ray-induced electron emission from CsI
Authors: Boutboul T. , Akkerman A. , Gibrekhterman A. , Breskin A. , Chechik R.
Published: July 2002

A microscopic theoretical model is proposed for calculating the characteristics of ultraviolet photoemission and x-ray secondary electron emission induced from CsI photoconverters. This approach is based on a realistic picture of the basic interactions of photons and induced electrons within the material. Both differential and integral emission characteristics, such as energy spectra and quantum efficiencies, are estimated according to the model and are found to agree, in general, with experimental data. The model-calculated photoemission enhancement under high external electric fields is also considered and is fairly compatible with measured values. The applicability of the model in the field of radiation detectors incorporating solid photoconverters is discussed.
Comparison of the adhesion of diamond coatings using indentation tests and micro-Raman spectroscopy
Authors: Fan Qi Hua , Grácio J. , Pereira E.
Published: July 2002

Adhesion of diamond films deposited on Ti interlayer coated copper substrates is compared according to acoustic emission during indentation loading and according to Raman peak shift. The acoustic emission from samples with and without annealing process shows different critical loads, which cause the film delamination and the delamination propagation. A higher annealing temperature and/or a longer annealing time lead to lower critical load or worse adhesion, that is proposed as a result of structure change at the diamond film/substrate interface during annealing. The films exhibiting lower critical load show lower diamond Raman peak shift, which is proportional to residual biaxial stresses in the films. The decrease in the residual stresses is attributed to weakening in the coating adhesion. Therefore, the coating adhesion can be also compared using Raman spectroscopy with the advantage of being nondestructive.
Measurement of intermolecular electron current in supramolecular assemblies created by the Langmuir–Blodgett technique
Authors: Donovan K. J. , Elliott J. E. , Scott K. , Wilson E. G.
Published: July 2002

Intermolecular photoelectron transfer in a vector ordered molecular assembly, produced as a Langmuir–Blodgett multilayer, causes currents and voltage changes at electrodes. A comprehensive account is given of the experimental determination of the voltage changes, which can be done, in principal, with a time resolution of 25 ps. The present experimental limitations to this time resolution are described. The experiments allow determination of the time evolution of the charge-separation product. The means of determining the intermolecular transfer time of the carriers in such multilayers is described.
Temperature dependence of the fundamental absorption edge in Cu2GeSe3
Authors: Marcano G. , Nieves L.
Published: July 2002

The temperature dependence of the energy gap EG in Cu2GeSe3 has been obtained from optical absorption measurements in the temperature range from 10 to 300 K. X-ray powder diffraction analysis indicates that the compound crystallizes in an orthorhombic structure with the unit cell parameters a=11.878(8), b=3.941(3), and c=5.485(3) Å. The data were analyzed by considering several models reported in the literature for the variation of EG vs T. The best fit was obtained by using an expression proposed by Pässler [Phys. Status Solidi B 200, 155 (1997)]. From the analysis, the Debye temperature was estimated to be around 170 K. Also obtained was the fact that the effective energy of phonons that participate in the shift of the band gap with temperature is about 15 meV and corresponds to the acoustic phonon branch.
Lattice relaxation and metastability of the EL2 defect in semi-insulating GaAs and low temperature GaAs
Authors: Kowalski G. , Collins S. P. , Moore M.
Published: July 2002

Using the two-dimensional reciprocal space mapping, we have clearly established the relative positions of the arsenic atom (EL2 defect) corresponding to its stable and metastable configurations. It is shown that lowering only the temperature of the sample, before transition, induces transformation of the lattice. Characteristic temperature (120–140 K) below which such transformation takes place is the same as for thermal recovery of the defect. It is suggested that this temperature related lattice relaxation might promote a further transition to the metastable state. Full reciprocal space maps of the x-ray diffracted intensity are shown for the whole substrate-layer system.
Defects in neutron transmutation doped silicon studied by positron annihilation lifetime measurements
Authors: Coeck M. , Balcaen N. , Van Hoecke T. , Van Waeyenberge B. , Segers D. , Dauwe C. , Laermans C.
Published: July 2002

Neutron transmutation doping of n-type silicon was obtained by irradiating the samples with thermal neutrons in order to create P atoms. Positron lifetime measurements were carried out and the evolution of the induced defects was studied by thermal annealing of the samples. The annealing out of the divacancy was seen between 200 and 350 °C. Around 700 °C large vacancy clusters (containing about eight vacancies) are formed. All the defects are annealed above 800 °C.
Low-temperature formation of Si(001) 2×1 surfaces from wet chemical cleaning in NH4F solution
Authors: Thanh Vinh Le , Bouchier D. , Hincelin G.
Published: July 2002

In situ reflection high-energy electron diffraction and thermal desorption spectroscopy were used to investigate the surface structure and chemistry of Si(001) surfaces after wet chemical etching in dilute HF and ammonium fluoride (NH4F) solutions. The HF-etched surfaces were found to be rough and terminated by mono-, di-, and tri-hydride species, in good agreement with previous vibrational spectroscopy measurements. The surface roughness observed after HF treatment appears to arise from surface defects such as adstructures and kinks left on the surface after the removal of the oxide layers. The Si(001) etching processes in NH4F solution were found to be highly anisotropic, and the etching time in the solution was found to be the key parameter which determined the chemistry and the morphology of the surface. We show that for a limited etching time (∼1 min), a flat and bulk-like dihydride-terminated Si(001) surface could be formed and this surface was found to transform, upon annealing in ultrahigh vacuum, into a well-developed 2×1 reconstruction at a temperature as low as ∼400 °C. Our results also suggested that (111) microfacets were formed on Si(001) surfaces only after a prolonged etching in NH4F solution and they remained stable up to an annealing temperature of ∼650 °C.
Dependence of crystallinity on oxygen pressure and growth mode of La0.3Sr1.7AlTaO6 thin films on different substrates
Authors: Li Yijie , Wu X. J. , Wen J. G. , Takahashi Y. , Tanabe K.
Published: July 2002

The influence of oxygen pressure on epitaxial quality of La0.3Sr1.7AlTaO6 (LSAT) films deposited by laser ablation on SrTiO3(100), LaAlO3(100), and MgO(100) substrates has been investigated. X-ray diffraction shows only (00l) peaks in the oxygen pressure range from 10−3 to 500 mTorr. However, the crystallinity of LSAT films is found to be sensitive to oxygen pressure and dependent on substrates. LSAT films on SrTiO3 have the narrowest full width at half maximum (FWHM) of 0.05° at 300 mTorr. The films on LaAlO3 exhibit a FWHM of 0.3° at 50–100 mTorr. On MgO substrate, significant improvement of crystallinity is achieved at 200–300 mTorr, but rocking curves give an FWHM of 0.70°–1.00° due to the large lattice mismatch between LSAT and MgO. Atomic force microscopy analyses demonstrate smooth LSAT surfaces without identified grains. Reflection high-energy electron diffraction studies suggest a two-dimensional growth mode for LSAT films on SrTiO3 and LaAlO3 and a three-dimensional growth mode for LSAT films on MgO. Cross-sectional high-resolution electron micrographs indicate that LSAT films on SrTiO3 and LaAlO3 have defect-free epitaxial structure and atomically flat interfaces. An 80 Å thick intermediate layer with partially disordering is observed between LSAT films and MgO substrates.
Carrier transport in organic alloy light emitting diodes
Authors: Shen Jun , Yang Jie
Published: July 2002

We present theoretical models and numerical results on the carrier transport mechanisms in single-layer organic alloy light emitting diodes. A typical organic alloy consists of a mixture of electron and hole transporting materials and is modeled by assigning individual conduction channels to each type of material. The sensitivity of the diode efficiency on several key parameters (alloy composition, energy level difference, contact barrier height, and traps, etc.) is studied. The results can be used to design structures to achieve balanced carrier injection and transport by properly adjusting these parameters.
The model of solid phase crystallization of amorphous silicon under elastic stress
Authors: Kimura Yasuo , Kishi Masato , Katoda Takashi
Published: July 2002

Solid phase crystallization of an amorphous silicon (a-Si) film stressed by a Si3N4 cap was studied by laser Raman spectroscopy. The a-Si films were deposited on Si3N4 (50 nm)/Si(100) substrate by rf sputtering. The stress in an a-Si film was controlled by thickness of a Si3N4 cap layer. The Si3N4 films were also deposited by rf sputtering. It was observed that the crystallization was affected by the stress in a-Si films introduced by the Si3N4 cap layer. The study suggests that the elastic stress increases with crystallization due to the smaller elastic modulus of a-Si with respect to crystalline silicon (c-Si). It is most reasonable to think that the elastic stress does not relax and that the elastic energy increased with crystallization because the elastic modulus of a-Si is smaller than that of c-Si. The experimental data was fitted by this model and the difference of the enthalpy ΔHac between a-Si and c-Si which is the latent heat of crystallization obtained by the fitting showed good coincidence with the previously reported value.
On the prediction of electromigration voiding using stress-based modeling
Authors: Duan Q. F. , Shen Y.-L.
Published: July 2002

In modeling electromigration failure, it is common to employ the concept of a critical stress at which interconnect failure occurs. In this report, we illustrate that the atomic flux divergence, obtained directly from the one-dimensional stress-based modeling, is more appropriate in characterizing the formation of void in electromigration. A numerical analysis was carried out, to model the evolution of stress, atomic flux, and flux divergence in an aluminum line containing a fast-diffusion segment. The maximum flux divergence, not the maximum tensile stress, predicts the voiding location which is consistent with microscopic observations in the experiments of Joo et al. [Acta. Mater. 46, 1969 (1998); J. Appl. Phys. 85, 2108 (1999)] utilizing nanoindented single-crystal aluminum lines. This is because the flux divergence directly reflects the extent of matter depletion, and thus the propensity of voiding.
X-ray investigations of the molecular mobility within polymer surface gratings
Authors: Geue Th. , Schultz M. , Grenzer J. , Pietsch U. , Natansohn A. , Rochon P.
Published: July 2002

The physical origin of surface relief patterning on amorphous polymer films containing azobenzene-side chains induced by holographic exposure with visible light of about 450 nm is not yet fully understood. To understand the nature of the induced material transport is of special interest to describe the dynamic processes occurring in thin films below the glass transition temperature TG. Thus, we investigated films made from the polar (poly {4′-[2-(methacryloxy) ethyl]-ethyl}amino-4-nitroazobenzene, TG=129 °C) and less polar {poly[4-(2-methacryloxy)-ethyl] azobenzene, TG=80 °C} azobenzene side-chain homopolymers and performed temperature-resolved coherent x-ray and visible (VIS) light scattering measurements of the thermally induced erasure of the surface gratings. The simultaneous use of x-ray synchrotron light (λ=0.14 nm) and VIS laser light (λ=633 nm) allows the detection of the material flow on different lengths scales. We did not find remarkable differences in the thermal behavior of polar and nonpolar materials. Depending on the time used for inscribing the gratings the VIS signal starts vanishing at a critical temperature TK below the glass temperature TG. Up to TG the x-ray grating peak intensities increase to a maximum even if the VIS signal is almost zero. Probing the grating in a different depth below the surface, the first and second order x-ray Fourier components reach their intensity maxima at different temperatures and rise up in intensity with time constants characterized by an Arrhenius-like activation energy of about 2.6 eV. At T>TG the grating peak intensities go to zero. Our measurements can be interpreted by a model of anisotropic viscosity. At T<TG the erasing of the surface grating takes place by a material flow perpendicular to the initial surface. This is accompanied by the formation of an intrinsic density grating within the film against the shear tension of the polymer. At T>TG the created lateral density modulation becomes equalized by a lateral material flow quantified by a diffusion coefficient of about D=3×10−13 cm2 s−1.
Ion-beam modification of Co/Ag multilayers II: Variation of structural and magnetic properties with Co layer thickness
Authors: Veres T. , Cai M. , Germain S. , Rouabhi M. , Schiettekatte F. , Roorda S. , Cochrane R. W.
Published: July 2002

The structural, magnetic and transport properties of rf sputtered Co/Ag multilayers with Co-layer thicknesses ranging from 1 to 14 Å have been studied by a combination of x-ray diffraction, magnetic and transport measurements. The magnetoresistance at room temperature has a maximum value of more than 12% for a Co-layer thickness around 5 Å. Magnetic measurements demonstrate that samples near this Co-layer thickness are in the transition region from superparamagnetic to ferromagnetic behavior. X-ray analysis indicates that, during deposition, a significant quantity of Co is dispersed throughout a highly textured Ag matrix. Upon irradiation with 1 MeV Si+ ions up to a dose of 5×1016 Si+/cm2, an initial demixing of the Co is followed by segregation into grains with the same texture as the Ag. The resulting changes in the magnetization and magnetoresistance are characterized on the basis of a log-normal distribution of the volume of the magnetic particles. As the particle sizes increase, a systematic evolution towards ferromagnetic behavior for films initially in the superparamagnetic and transition regions results.