bluuebunny/crossref_metadata_2025 huggingface dataset:
The structure and properties of ion-beam-synthesized boron nitride films
Authors: Sainty W. G. , Martin P. J. , Netterfield R. P. , McKenzie D. R. , Cockayne D. J. H. , Dwarte D. M.
Published: July 2002

Films containing boron and nitrogen were prepared by electron-beam evaporation of boron and bombardment of the growing film with nitrogen ions of energy up to 1500 eV. Hard films of high transparency (extinction coefficient <0.01) were prepared with nitrogen-to-boron atomic arrival ratios greater than one. The optical constants in the visible part of the spectrum were determined as a function of B-to-N atomic ratio in the film and substrate temperature using optical photometry. Measurements of the optical constants were extended to 40 eV using a Kramers–Kronig analysis of electron-energy-loss spectra. Boron-to-nitrogen atomic arrival rates were determined and show that as ion energy and substrate temperature are increased ion flux must be increased to achieve the same stoichiometry. Film structure was imaged using high-resolution electron microscopy, and the radial distribution function (RDF) was determined. The RDF of stoichiometric films showed that high substrate temperatures increased the size of the ordered regions. The RDF and high-resolution imaging of all films were consistent with the presence of the hexagonal form of BN with substoichiometric films containing amorphous boron as the other constituent. This conclusion is consistent with the analysis derived from the sum rule obtained from the optical constants. No evidence could be found for the cubic form of BN.
Interfacial reactions on annealing molybdenum-silicon multilayers
Authors: Holloway Karen , Do Khiem Ba , Sinclair Robert
Published: July 2002

The structure and interfacial reaction in sputtered Mo-Si multilayers have been studied using cross-section transmission electron microscopy, electron diffraction, Rutherford backscattering, and low-angle x-ray diffraction. Low-temperature (T<550 °C) annealing was performed in a rapid-thermal-annealing furnace and in situ in the microscope. No solid-state amorphization was observed, in spite of the presence of amorphous alloy interfacial layers in the as-deposited structure. Instead, the amorphous interlayers crystallize, and growth of the crystalline product, hexagonal-MoSi2, proceeds. The bilayer period contracts during the reaction, as the disilicide is more dense than its constituents.
A mathematical model for a plasma-assisted downstream etching reactor
Authors: Park Sang-Kyu , Economou Demetre J.
Published: July 2002

A mathematical model was developed for a plasma-assisted downstream etching reactor of the impinging jet configuration. Finite-element methods were employed to solve for the two-dimensional fluid velocity, temperature, and active species concentration distributions. Etching of a polymer (e.g., photoresist) using pure oxygen was analyzed with emphasis on the effect of reactor design and operating conditions on etching rate and uniformity. For a given flow rate, an optimum value of pressure was identified which maximized the etching rate. The etching rate increased monotonically with power, but decreased exponentially with distance between the plasma and the wafer. Under the conditions examined, the etching rate was found to be highest at the wafer center. Local loading was observed around the periphery of the wafer at high wafer temperature. The etching uniformity was found to depend on the gas-flow distribution. A new reactor design was proposed to achieve efficient gas dissociation in the plasma, rapid transport of the dissociated gas to the etching chamber, and nearly uniform flow distribution over the wafer. The new design resulted in improvement in both etching rate and uniformity.
Analysis of multilayer thin-film structures containing magneto-optic and anisotropic media at oblique incidence using 2×2 matrices
Authors: Mansuripur M.
Published: July 2002

A complete analysis of multilayer structures containing an arbitrary number of dielectric, metal, magnetic, and birefringent/dichroic layers is presented. An algorithm, based on simple 2×2 matrices, is derived which allows reflection, transmission, absorption, magneto-optic conversion, birefringence, and dichroism of the structure to be computed on a personal computer. The incident beam is assumed to be plane monochromatic with arbitrary angle of incidence. There are no approximations involved, and the results are direct consequences of Maxwell’s equations.
Trap-induced photoconductivity in semi-insulating GaAs
Authors: Desnica U. V. , Šantić B.
Published: July 2002

Photoconductivity of semi-insulating gallium arsenide illuminated with monochromatic photons in the 0.7–1.8 eV range has been studied. It has been found that photoconductivity strongly depends on the occupancy of deep traps present in the material, so that the photoconductivity measured for the full trap regime is several orders of magnitude larger than one taken when the traps are empty. By selective emptying or filling of traps it was possible to identify the contribution of each particular trap to the photoconductivity enhancement in different temperature intervals.
A transmission electron microscopy and reflection high-energy electron diffraction study of the initial stages of the heteroepitaxial growth of InSb on GaAs (001) by molecular beam epitaxy
Authors: Zhang X. , Staton-Bevan A. E. , Pashley D. W. , Parker S. D. , Droopad R. , Williams R. L. , Newman R. C.
Published: July 2002

I n situ reflection high-energy electron diffraction and cross-sectional and plan-view transmission electron microscopy have been used to investigate the initial stages of InSb growth on GaAs(001), by molecular-beam epitaxy. Growth of the InSb commences with the formation of rectangular-based islands, having flat tops and sloping sides, with facets on certain planes of types {111} and {113}. The islands show near normal lattice spacings, with no significant straining. As deposition proceeds, islands coalesce and, after the equivalent of 40 monolayers of deposition, form a connected network. Complete coverage of the GaAs substrate is achieved after ≂300 monolayers of deposition. This places a lower limit on the thickness of InSb layers, which may be considered in the design of optoelectronic devices.
A constitutive model for the dynamic response of brittle materials
Authors: Addessio Frank L. , Johnson James N.
Published: July 2002

A microphysically based material model for the dynamic inelastic response of a brittle material is developed. The progressive loss of strength as well as the post-failure response of a granular material with friction are included. Crack instability conditions (an inelastic surface in stress space) and inelastic strains are obtained by considering the response of individual microcracks to an applied stress field. The assumptions of material isotropy and an exponential distribution for the crack radius are invoked to provide a tractable formulation. The constitutive model requires a minimal number of physical parameters, is compatible with a previously developed ductile fracture model [J. Appl. Phys. 64, 6699 (1988)] that utilizes inelastic surfaces, and can be formulated as an efficient, robust numerical algorithm for use in three-dimensional computer codes. The material model is implemented into a Lagrangian computer formulation for the demonstration of material response to dynamic loading conditions. Comparisons with one-dimensional, uniaxial impact experiments are provided.
Hyperbolic heat conduction due to axisymmetric continuous or pulsed surface heat sources
Authors: Kim W. S. , Hector L. G. , Özisik M. N.
Published: July 2002

Irradiation of metals with laser pulses in the nanosecond to picosecond range may give rise to temperature fields which are significantly different from those predicted by the classical Fourier heat conduction model. In this work we examine the very short-time temperature response of a semi-infinite region to an axisymmetric laser surface source, which is either continuous or activated for a period Δt and has a spatial profile which is Gaussian, doughnut, or a combination of the Gaussian and doughnut modes. The material properties are assumed constant, and radiation and convection from the irradiated surface are neglected. The non-Fourier temperature responses are compared with their diffusive counterparts and shown to give rise to steep local temperature concentrations.
Ion implantation effects in polycrystalline WO3 thin films
Authors: Tate T. J. , Garcia-Parajo M. , Green Mino
Published: July 2002

Ion implantation is a possible technique to introduce electrochromically active dopants into fine-grained WO3 thin films. However, to be of use, we must be able to distinguish, and erase, effects due to collisional damage. Argon implants were performed in order to investigate damage effects. Pure damage was found to cause oxygen deficiency, leading to a broadband grey coloration. The formation of hydrogen bronzes (due to water vapor absorption), and of grain-grown material, are reported. Beam heating effects were found to be important due to the fine-grained nature of the material, and need to be suppressed in order for ion implantation to be a useful technique in this area.
Internal reflection effects on transient photothermal reflectance
Authors: Chen G. , Tien C. L.
Published: July 2002

Transient photothermal reflectance may provide valuable mechanistic and diagnostic information during laser-material interaction. Fast laser heating, however, often generates a large temperature gradient and stratifies the heated region through the temperature-dependent optical properties. An electromagnetic wave propagating through such a stratified medium experiences internal reflection so that the reflectance from a surface depends on the profile of the optical properties beneath it. The internal reflection process also redistributes the energy absorption and thus the temperature field. To evaluate the effects of internal reflection on the reflectance and temperature distribution, the one-dimensional energy equation is solved numerically, coupled with the matrix method in optical multilayer theory, for nickel (Ni), silicon (Si), and germanium (Ge). The computational results agree well with the picosecond photothermal reflectance experimental results on Ni. For Si, internal reflection does not have appreciable effects on the reflectance up to the melting point, while for Ge it significantly changes the signal and the temperature distribution. Criteria are established to determine when internal reflection should be considered.
Techniques to minimize DX center deleterious effects in III-V device performance
Authors: Muñoz E. , Calleja E. , Izpura I. , García F. , Romero A. L. , Sánchez-Rojas J. L. , Powell A. L. , Castagné J.
Published: July 2002

The properties of deep donor states (DX centers) in III-V alloys are discussed in relation to their influence on device characteristics and performance. The techniques to avoid or minimize such deleterious effects in AlGaAs-based devices are discussed, along with their physical basis, and some guidelines for improved III-V device design are established. New results about the benefits of proper donor selection, the role of In alloying, the advantage of δ doping in layers and in modulation-doped devices, and the use of AlInAs and InGaP as alternative wide band-gap III-V alloys are presented.
Forced translational vibrations of 90° domain walls and the dielectric dispersion in ferroelectric ceramics
Authors: Pertsev N. A. , Arlt G.
Published: July 2002

A theory of collective translational vibrations of 90° domain walls (DWs) in ferroelectric ceramics is presented. Vibrational motions of DWs forming a regular domain structure of a representative grain are assumed to be completely correlated but independent of DW oscillations in other grains. A dynamic mechanical stress field appearing in a ceramic because of DW vibrations is calculated. In contrast to former studies, this calculation takes into account effects due to the lagging of sound waves emitted by oscillating DWs and gives a general expression for the dynamic mechanical restoring force acting on DWs. From this expression we derive the equation of sustained forced DW vibrations in an oscillating external electric field that is valid for a wide frequency range including microwave frequencies. A general solution of this equation is found, which enables us to compute numerically the dependencies of amplitude and phase of DW vibrations on the frequency ω of the applied electric field. It is shown that in the low-frequency range ω<ω*=ct/g (ct=velocity of transverse sound wave, g=grain size) the general equation of DW vibrations can be reduced to a simplified equation that includes the static restoring force, the inertial reaction, and the radiation reaction self-force of the DWs emitting sound waves. Analytic expressions are derived for the DW effective mass and for the factors characterizing the static restoring force and the radiation reaction. The contribution of DW vibrations to the complex dielectric constants of ferroelectric ceramics is calculated. It is predicted that at very high frequencies ω≫ω* the DW contribution to the real part of permittivity strongly decreases due to clamping of DWs. In this frequency range a peak of dielectric losses should also arise being caused by the emission of sound waves from oscillating DWs. It is emphasized that the above effects can be correctly described on the base of the general equation of DW vibrations only.
The microwave absorbing and resonance phenomena of Y-type hexagonal ferrite microwave absorbers
Authors: Kwon H. J. , Shin J. Y. , Oh J. H.
Published: July 2002

The microwave absorbing characteristics and resonance of Y-type hexagonal ferrite–rubber composites were investigated. The complex permeability and permittivity of Ni2−xZnxY ferrite bodies were measured using a network analyzer in the frequency range of 200 MHz–14 GHz. Two types of resonance, the domain wall and the spin rotational resonance, were observed. With a ferrite particle with a diameter of about 1 μm, only spin rotational resonance was observed. The first matching frequency, found in the ferrite–rubber composites, which was higher than that of spin rotational resonance, increased with spin rotational resonance frequency. It was also found that domain wall resonance had no effects on the microwave absorbing characteristics. Based on these findings, it could be concluded that the microwave absorbing characteristics were caused by only one type of resonance, the spin rotational resonance.
Film stress of sputtered W/C multilayers and strain relaxation upon annealing
Authors: Geisz J. F. , Kuech T. F. , Lagally M. G. , Cardone F. , Potemski R. M.
Published: July 2002

Sensitive curvature measurements were performed on 40 Å period W/C multilayer structures on Si substrates using a two beam laser reflection technique. A compressive stress of approximately 1530 MPa was measured in these sputtered multilayer films. Thermal annealing to 500 °C resulted in very little strain relaxation in the multilayers but x-ray diffraction data show a slight increase of the multilayer period. Significant strain relaxation, though, was observed when a 400 Å W buffer layer was included. Thermal annealing of these samples to 400–500 °C resulted in large strain relaxation due to the growth of α-W crystals in the buffer layer. Large oxide formation on air annealed samples did not significantly change the strain state of the films.
Conversion of chemically deposited photosensitive CdS thin films to n-type by air annealing and ion exchange reaction
Authors: Nair M. T. S. , Nair P. K. , Zingaro R. A. , Meyers E. A.
Published: July 2002

A method is presented for the deposition of CdS thin films of 0.05–0.7 μm thickness from solutions at 50–70 °C containing citratocadmium(II) complex ions and thiourea. The films show an optical band gap Eg≳2.6 eV. Optical transmittance is about 80% for photon energy <Eg. The dark conductivity of the films is of the order of 10−8 Ω−1 cm−1. The photosensitivity of these films is high, 106–107 under illumination with tungsten halogen light of 1 kW m−2. Annealing in air at 400–500 °C for 1 h converts the films to n type. It is possible to obtain sheet resistances of about 150 Ω for a 0.2 μm film (i.e., conductivity of 300 Ω−1 cm−1) by this process. Conversion of the films to n type is possible also by immersing the film in a 0.01 M HgCl2 solution for 15 min followed by air annealing for 1 h at 200 °C. The films show n-type dark conductivity of ≂0.05 Ω−1 cm−1 and photoconductivity of ≂1 Ω−1 cm−1. X-ray diffraction and x-ray photoelectron spectroscopic depth profile studies on the films show that the modification of the electrical characteristics is brought about through changes in composition of the surface layers in the films.
Theoretical and experimental study of low-temperature, capacitively coupled, radio-frequency helium plasmas
Authors: Riley M. E. , Greenberg K. E. , Hebner G. A. , Drallos P.
Published: July 2002

A comparison is made of one-dimensional theoretical simulations with measured densities of electrons and He metastables in a Gaseous Electronics Conference Reference Cell. The theory attempts to be as ab initio and complete as necessary for describing a He plasma. The basic theoretical description incudes the time-dependent, electric-field-consistent Boltzmann equation for the electrons, inertial fluid equations for the heavy particles, and a multilevel model of the He excitations. The novel numerical algorithms are rapidly convergent. The experimental and theoretical absolute number densities are generally within a factor of 2 for both the electron plasma density and the metastable densities. In addition the spatial variation of the metastable populations (singlet and triplet) between the electrodes is well reproduced.
A neutron diffraction and Mössbauer spectral study of the structure and magnetic properties of the Y2Fe14−xSixB solid solutions
Authors: Marasinghe G. K. , Pringle O. A. , Long Gary J. , Yelon W. B. , Grandjean F.
Published: July 2002

A neutron diffraction and Mössbauer spectral study of Y2Fe14−xSixB shows that silicon preferentially occupies the 4c, and to a lesser extent, the 8j1 sites in Y2Fe14−xSixB. The trend in the site occupancy is the same as in Nd2Fe14−xSixB. The Curie temperature of Y2Fe14−xSixB increases with increasing silicon content. Neutron diffraction data show that the increase in Curie temperature is accompanied by a contraction of the unit cell. Wigner–Seitz cell calculations, using the Y2Fe14−xSixB lattice and positional parameters obtained by neutron diffraction, show that the silicon site occupancy is correlated with the rare earth contact area of that site. The Mössbauer spectra of Y2Fe14−xSixB have been fit with a model which takes into account the distribution of near-neighbor environments of an iron atom due to the presence of silicon. The weighted average of Hmax, the average hyperfine field of the iron sites with no silicon near neighbors, decreases by 42 and 45 kOe per silicon substitution per formula unit at 85 and 295 K, respectively. The weighted average of ΔH, the average reduction in the hyperfine field caused by the addition of one silicon near neighbor to the near neighbor environment, is about 13.5 and 18 kOe at 85 and 295 K, respectively, and does not show an appreciable dependence on the silicon content. The isomer shifts obtained from these fits suggest an increase in covalency of bonding on silicon substitution, an increase which is consistent with the preference of silicon to bond with yttrium.
Comparison of the dynamic magnetomechanical properties of Tb0.27Dy0.73Fe2 and Tb0.30Dy0.70Fe2
Authors: Kendall D. , Piercy A. R.
Published: July 2002

Comparison is made between the magnetomechanical properties of Tb0.27Dy0.73Fe2 (27 Tb) and Tb0.30Dy0.70Fe2 (30 Tb), two commercially available compositions of the giant magnetostrictive alloy Terfenol-D. The quasi-static magnetostriction (as a function of bias field) and the dynamic strain coefficient (as a function of bias field and frequency) are shown for the two compositions at room temperature, using stress bias values of 0 and 9 MPa. The energy loss per cycle as a function of frequency is also given. The data for the static magnetostriction versus bias field shows 30 Tb to exhibit a significantly greater increase in initial slope and saturation strain when stress bias is applied compared to that seen for 27 Tb, which possesses superior magnetostriction at zero stress bias. A similar trend is observed in the data for the dynamic strain coefficient with 30 Tb again performing better under stress bias. The widening of the static strain hysteresis loop observed for stressed 30 Tb is seen to manifest itself in the low frequency energy loss per cycle; however 27 Tb is observed to possess the higher eddy current loss at 1 kHz. Data for the ratio d/χ, a quantity which is dependent only on magnetization processes and material constants, are shown which suggests that the application of a stress bias affects the magnetization processes, but differently for the two compositions.
Magnetic and structural instabilities of ultrathin Fe(100) wedges (invited)
Authors: Bader S. D. , Li Dongqi , Qiu Z. Q.
Published: July 2002

An overview is provided of recent efforts to explore magnetic and related structural issues for ultrathin Fe films grown epitaxially as wedge structures onto Ag(100) and Cu(100). Experiments were carried out utilizing the surface magneto-optic Kerr effect. Ordinary bcc Fe is lattice matched to the primitive unit cell of the Ag(100) surface. Fe wedges on Ag(100) can be fabricated whose thick end has in-plane magnetic easy axes due to the shape anisotropy, and whose thin end has perpendicular easy axes due to the surface magnetic anisotropy. A spin-reorientation transition can thus be studied in the center of the wedge where the competing anisotropies cancel. The goal is to test the Mermin–Wagner theorem which states that long-range order is lost at finite temperatures in an isotropic two-dimensional Heisenberg system. Fe wedges on Cu(100) can be studied in like manner, but the lattice matching permits fcc and tetragonally distorted fcc phases to provide structural complexity in addition to the interplay of competing magnetic anisotropies. The results of these studies are new phase identifications that help both to put previous work into perspective and to define issues to pursue in the future.
CdTe nanostructures prepared by thermal annealing
Authors: Ramírez-Bon R. , Espinoza-Beltrán F. J. , Arizpe-Chávez H. , Zelaya-Angel O. , Sánchez-Sinencio F.
Published: July 2002

Homogeneous thin films of a-CdTe:O were thermal annealed in an Ar flux. The as-grown samples have an amorphous structure. The annealed films crystallize to a mixture of both CdTe and CdTeO3. For low annealing temperatures (less than 100 °C) it is possible to get CdTe crystallites with sizes smaller than 14 nm. In this work we report measurements of x-ray diffraction and optical absorption spectra of these films. The x-ray diffraction patterns of the films were used to study the CdTe crystallization process as a function of the annealing temperature. The CdTe crystallite size of the films was determined from the diffraction patterns using the Debye–Scherrer formula. It was observed that the CdTe crystallite size increases with annealing temperature. The optical absorption spectra of the films with the smallest CdTe crystallite size show how the absorption band edge of CdTe is shifted by the effects of the crystallite size.
Effect of the surface roughness on the spectral distribution of photoemission current at the silver/solution contact
Authors: Kostecki R. , Augustynski J.
Published: July 2002

The use of an aqueous electrolyte containing dissolved CO2 enabled revealing of an unusually intense photoemission from a bulk silver electrode. The observed cathodic photocurrent exhibits a sharp peak at ∼360 nm corresponding to the energy of surface plasmons in silver. The amount and the spectral distribution of the photocurrent are strongly affected by the extent of roughness of the silver surface. In particular, an electrochemical roughening procedure, analogous to that leading to the surface enhancement of Raman scattering (SERS) at silver, causes appearance of a long tail on the low-energy side of the photoemission spectra. In consequence, significant photocurrents, corresponding to a quantum yield of ∼7×10−3, are observed for the deliberately roughened silver electrodes illuminated with the wavelength of 514 nm, most frequently used in SERS experiments. It is postulated that this intense photoemission, discovered in the presence of CO2 in the solution, equally takes place in the absence of scavengers. However, in the latter case no significant photocurrents can be detected as most of the photoemitted electrons (solvated or not) return back to the electrode.
Near-perfect quantum wells on vicinal surfaces
Authors: Wood Colin E. C. , Srinivas Vivek , Midadye Eyel V. , Empson Kevin L.
Published: July 2002

Molecular-beam-epitaxial growth conditions for GaAs/AlGaAs quantum wells with near-perfect interfaces on (100) GaAs substrates misoriented 4° to the (111)A, and essentially free from defects and impurities, are determined from kinetic and thermodynamic arguments. 4 K luminescence spectra of nominally 200 Å wells grown under such conditions are dominated by free-exciton recombinations with ∼120 μeV half-peak width.
Criteria for stability in bistable electrical devices with S- or Z-shaped current voltage characteristic
Authors: Wacker A. , Schöll E.
Published: July 2002

Electronic devices exhibiting bistability in the current-voltage characteristics are considered from a unified viewpoint. We obtain simple relations for the stability of the different branches in the current-voltage characteristics. Criteria for oscillatory instabilities are discussed, and special conclusions for elements with S- or Z-shaped characteristics are drawn. The stabilization of the middle branch of the double-barrier resonant-tunneling diode in a circuit with effectively negative capacitance and negative resistance is derived in a simple way.
Thermal stability of nanostructured Sm2Fe17Cx compounds prepared by ball milling
Authors: Mao O. , Altounian Z. , Yang J. , Ström-Olsen J. O.
Published: July 2002

The thermal stability of ball-milled nanocrystalline Sm2Fe17Cx interstitial compounds has been investigated. Sm2Fe17Cx decomposes into a mixture of phases upon annealing above a certain decomposition temperature, Td1, which depends on the carbon concentration in the compound. Annealing above a certain temperature Td2(≳Td1), the Sm2Fe14C phase starts to form. The amount of the Sm2Fe14C phase increases with increasing annealing temperature at the expense of the Sm2Fe17 carbide until Tr1, at which temperature Sm2Fe14C starts to recombine with other phases to form the Sm2Fe17 carbide. For the starting composition with x<1.5, the Sm2Fe17Cx single phase can be recovered above the recovery temperature Tr2(≳Tr1).
Design of ion energy distributions by a broad beam ion source
Authors: Zeuner Michael , Meichsner Jürgen , Neumann Horst , Scholze Frank , Bigl Frieder
Published: July 2002

We characterize the performance of a built-in hot filament broad beam ion source by mass spectrometry, energy analysis, and beam profile measurements. In the ion energy distribution we detect various peak structures which can be explained by the potential across the ion source and different charge transfer processes. Depending on the typical cross sections for these processes, differences between the energy distributions of the ion species are observed. The total ion current obtained with the source is determined by the ionization rate in the discharge and the current share directed toward the extraction grid system. The performance of the source is strongly dependent on the process gas used. We observe much broader energy distributions in oxygen and nitrogen than in argon. This broadening is explained by spatial inhomogeneities in the discharge region and can be reduced by a suitable setting of the source parameters. The main contribution to the ion flux is caused by species generated directly from the process gas, with important impurities from source materials appearing only with additional chemical reactions in oxygen.
Metal surface nitriding by laser induced plasma
Authors: Thomann A. L. , Boulmer-Leborgne C. , Andreazza-Vignolle C. , Andreazza P. , Hermann J. , Blondiaux G.
Published: July 2002

We study a nitriding technique of metals by means of laser induced plasma. The synthesized layers are composed of a nitrogen concentration gradient over several μm depth, and are expected to be useful for tribological applications with no adhesion problem. The nitriding method is tested on the synthesis of titanium nitride which is a well-known compound, obtained at present by many deposition and diffusion techniques. In the method of interest, a laser beam is focused on a titanium target in a nitrogen atmosphere, leading to the creation of a plasma over the metal surface. In order to understand the layer formation, it is necessary to characterize the plasma as well as the surface that it has been in contact with. Progressive nitrogen incorporation in the titanium lattice and TiN synthesis are studied by characterizing samples prepared with increasing laser shot number (100–4000). The role of the laser wavelength is also inspected by comparing layers obtained with two kinds of pulsed lasers: a transversal-excited-atmospheric-pressure–CO2 laser (λ=10.6 μm) and a XeCl excimer laser (λ=308 nm). Simulations of the target temperature rise under laser irradiation are performed, which evidence differences in the initial laser/material interaction (material heated thickness, heating time duration, etc.) depending on the laser features (wavelength and pulse time duration). Results from plasma characterization also point out that the plasma composition and propagation mode depend on the laser wavelength. Correlation of these results with those obtained from layer analyses shows at first the important role played by the plasma in the nitrogen incorporation. Its presence is necessary and allows N2 dissociation and a better energy coupling with the target. Second, it appears that the nitrogen diffusion governs the nitriding process. The study of the metal nitriding efficiency, depending on the laser used, allows us to explain the differences observed in the layer features as purity, thickness, and surface morphology.
Effect of strain relaxation on forward bias dark currents in GaAs/InGaAs multiquantum well p–i–n diodes
Authors: Griffin P. R. , Barnes J. , Barnham K. W. J. , Haarpaintner G. , Mazzer M. , Zanotti-Fregonara C. , Grünbaum E. , Olson C. , Rohr C. , David J. P. R. , Roberts J. S. , Grey R. , Pate M. A.
Published: July 2002

The effect of the dislocation line density produced by the relaxation of strain in GaAs/InxGa1−xAs multiquantum wells where x=0.155–0.23 has been studied. There is a strong correlation between the dark line density, observed by cathodoluminescence, before processing of the wafers into photodiode devices, and the subsequent low forward bias (<1.5 V) dark current densities of the devices. A comparison is made of the correlation between the reverse bias current density and dark line density and it is found that, in this range of strain, the forward bias current density varies more. Two growth methods, molecular beam epitaxy and metal organic vapor phase epitaxy, have been used to produce the wafers and no difference between the growth methods has been found in dark line or current density variations with strain.
The mechanism of abnormal grain growth in Sr0.6Ba0.4Nb2O6 ceramics
Authors: Lee Han-Young , Freer R.
Published: July 2002

Strontium barium niobate, Sr0.6Ba0.4Nb2O6 (SBN40) ceramics were prepared by the mixed oxide route; specimens sintered at 1300 °C–1450 °C for 2–4 h had densities of ∼95% theoretical. The use of “conventional’’ conditions (1300 °C; ∼200 °C below the melting temperature) gave rise to a duplex structure comprising a mixture of small grains and abnormally large grains (in excess of 100 μm). Transmission electron microscopy studies revealed a Nb-rich, Ba-poor phase at the grain boundaries; the low melting temperature of this phase caused localized liquid phase sintering, resulting in abnormal grain growth. Dual-stage sintering (sintering at 1250 °C for 4 h, followed by sintering at a temperature in the range 1350 °C–1450 °C) inhibited abnormal grain growth. The use of a fast heating rate (600 °C/h) and dual-stage sintering (1250 °C and 1450 °C) yielded samples with good transparency and preferential orientation along the c-direction of the tetragonal unit cell. The dielectric properties depended sensitively on microstructure. Specimens with the highest densities had the highest relative permittivities (2400–2750); specimens with more uniform distribution of grain sizes had sharper dielectric maxima. A mechanism for abnormal grain growth is proposed.
Luminescence from plasma deposited silicon films
Authors: Edelberg Erik , Bergh Sam , Naone Ryan , Hall Michael , Aydil Eray S.
Published: July 2002

We report the observation of room-temperature and low-temperature visible photoluminescence from nanocrystalline silicon (nc-Si) thin films produced by plasma-enhanced chemical vapor deposition (PECVD) through a gas discharge containing SiH4 diluted in Ar and H2. The nanocrystalline silicon films were characterized using transmission electron microscopy, spectroscopic ellipsometry, infrared and Raman spectroscopy, and were examined for photoluminescence. Luminescent films consisted of dense silicon nanocrystals that grew in a columnar structure with approximately 20%–30% void space dispersed inside the film. Aside from having small crystalline silicon regions, the structure of the nc-Si films is different than that of porous Si, another luminescent Si material generally produced by electrochemical anodization. Yet, the photoluminescence spectra of the thin nc-Si films were found to be similar to those observed from porous silicon. This similarity suggests that the same mechanism responsible for light emission from porous silicon may also be responsible for emission from nc-Si. The photoluminescence spectra are analyzed in terms of a simple quantum confinement model. Although the mechanism of visible luminescence from porous Si is still a point of controversy, our results support the hypothesis that some of the luminescence from porous silicon and nc-Si films is due to quantum confinement of electrons and holes in crystals with dimensions 2–15 nm.
Mechanics of coherent and dislocated island morphologies in strained epitaxial material systems
Authors: Johnson H. T. , Freund L. B.
Published: July 2002

A combined analytical and computational model is developed to study the mechanics of strained epitaxial island growth in typical semiconductor systems. Under certain growth conditions in systems with a film/substrate lattice mismatch, deposited material is known to aggregate into islandlike shapes with geometries having arc shaped cross-sections. A two-dimensional model assuming linear elastic behavior is used to analyze an isolated arc shaped island with elastic properties similar to those of the substrate. The substrate is assumed to be much larger than the island. Finite element analysis shows that in order to minimize the total energy, which consists of strain energy, surface energy, and film/substrate interface energy, a coherent island will adopt a particular height-to-width aspect ratio that is a function of only the island volume. It is then shown that for an island with volume greater than a certain critical size, the inclusion of a mismatch strain relieving edge dislocation is favorable. The criterion for the critical size is based on a comparison of the configurational forces acting on the edge of the island in the presence of an edge dislocation. Finally, a finite element calculation combined with an analytical treatment of the singular dislocation fields is used to determine the minimum energy island aspect ratio for the dislocated island/substrate system. The combination of the minimum energy morphology studies for the coherent and dislocated systems with the dislocation nucleation criterion gives a complete model for strained epitaxial island growth which can serve as a basis for interpretation of experiments.
Carrier transport mechanism of Ohmic contact to p-type diamond
Authors: Yokoba M. , Koide Yasuo , Otsuki A. , Ako F. , Oku T. , Murakami Masanori
Published: July 2002

The carrier transport mechanism through the p-diamond/metal interface was studied by measuring specific contact resistances (ρc) using a transmission line method for Ti, Mo, and Cr (carbide forming metals) and Pd and Co (carbon soluble metals) metals contacting to the boron-doped polycrystalline diamond films. Schottky barrier heights (φB) of around 0.5 eV were measured for the annealed contacts. The present result indicates that formation of thermally stable graphite layers at the diamond/metal interfaces upon annealing would pin the Fermi level of the p-diamond. This model led to the preparation of in situ Ohmic contacts by depositing a thin diamondlike carbon on the p-diamond surface prior to metal deposition, and also to excellent Schottky contacts with breakdown voltages higher than 900 V. The present experiment concluded that the existence of a graphite layer at the diamond/metal interface controlled the electrical properties through the p-diamond/metal interface.
Salicidation process using NiSi and its device application
Authors: Deng F. , Johnson R. A. , Asbeck P. M. , Lau S. S. , Dubbelday W. B. , Hsiao T. , Woo J.
Published: July 2002

Self-aligned silicidation is a well-known process to reduce source, drain, and gate resistances of submicron metal-oxide-semiconductor devices. This process is particularly useful for devices built on very thin Si layers (∼1000 Å or less) on insulators because of the large source and drain resistances associated with the thin Si layer. NiSi is a good candidate for salicidation process due to its low resistivity, low formation temperature, little silicon consumption, and large stable processing temperature window. In this article, the formation of nickel mono-silicide (NiSi) using rapid thermal annealing, the thermal stability of NiSi on n+ poly-Si and the contact resistance of NiSi on n+ Si layers in a SIMOX structure were investigated. NiSi salicidation process was, then, incorporated into a NMOS/SIMOX device fabrication for partial and full consumption of the Si in the source and drain regions during the salicidation process. The effects of void formation and silicide encroachment on the device performance were also studied.
Shock-induced chemical reactions in titanium–silicon powder mixtures of different morphologies: Time-resolved pressure measurements and materials analysis
Authors: Thadhani N. N. , Graham R. A. , Royal T. , Dunbar E. , Anderson M. U. , Holman G. T.
Published: July 2002

The response of porous titanium (Ti) and silicon (Si) powder mixtures with small, medium, and coarse particle morphologies is studied under high-pressure shock loading, employing postshock materials analysis as well as nanosecond, time-resolved pressure measurements. The objective of the work was to provide an experimental basis for development of models describing shock-induced solid-state chemistry. The time-resolved measurements of stress pulses obtained with piezoelectric polymer (poly-vinyl-di-flouride) pressure gauges provided extraordinary sensitivity for determination of rate-dependent shock processes. Both techniques showed clear evidence for shock-induced chemical reactions in medium-morphology powders, while fine and coarse powders showed no evidence for reaction. It was observed that the medium-morphology mixtures experience simultaneous plastic deformation of both Ti and Si particles. Fine morphology powders show particle agglomeration, while coarse Si powders undergo extensive fracture and entrapment within the plastically deformed Ti; such processes decrease the propensity for initiation of shock-induced reactions. The change of deformation mode between fracture and plastic deformation in Si powders of different morphologies is a particularly critical observation. Such a behavior reveals the overriding influence of the shock-induced, viscoplastic deformation and fracture response, which controls the mechanochemical nature of shock-induced solid-state chemistry. The present work in conjunction with our prior studies, demonstrates that the initiation of chemical reactions in shock compression of powders is controlled by solid-state mechanochemical processes, and cannot be qualitatively or quantitatively described by thermochemical models.
Magneto-optical enhancement in sputtered epitaxial films of praseodymium-substituted yttrium iron garnet
Authors: Gomi M. , Toyoshima H.
Published: July 2002

Faraday rotation and ellipticity were measured at room temperature in the photon energy range of 1.5–4.2 eV on Y3−xPrxFe5O12 (x=0–2.0) epitaxial films grown by rf sputtering. The Pr3+ substitution induced negative rotation and ellipticity which satisfy a paramagnetic dispersion relation caused by an electronic transition centered at 3.0 eV. The maximum value of the Pr3+ contribution was −2.5×104 deg/cm in rotation and −4.1×104 deg/cm in ellipticity for x=2.0. Optical absorption measurements showed that this is a new transition induced by the coexistence of Pr3+ and Fe3+, and it is neither 4f–5d intra-atomic transition nor 4f–4f transition originating from Pr ions as previously supposed.
Intrinsic dead layer effect and the performance of ferroelectric thin film capacitors
Authors: Zhou C. , Newns D. M.
Published: July 2002

We apply the Thomas theory of ferroelectricity to bulk and thin film perovskite ferroelectrics in the paraelectric regime above the transition temperature. From available data on bulk SrTiO3 we are able to fully determine the parameters in the Thomas theory for this material, with overall reasonable results, supporting its validity. In a new application of the Thomas theory to the surface of a thin ferroelectric film in the linear response regime, it is found that there is anticipated to be an intrinsic “dead layer effect” on the surface of a dielectric film which significantly reduces the effective dielectric constant observed in capacitor applications. Two predictions of the theory are verified from recent experimental data. An experiment is suggested to distinguish between linear and nonlinear surface effects.
Thermal expansion of vitreous silica: Correspondence between dilatation curve and phase transitions in crystalline silica
Authors: Kikuchi Yoshikazu , Sudo Hajime , Kuzuu Nobu
Published: July 2002

The linear thermal expansion coefficients of fused quartz containing 3 ppm of OH produced by electrical melting of natural quartz powder were measured using an interferometric dilatometer in the temperature range 300–970 K. The data obtained show several maxima and minima, some of which correspond to the phase transitions in crystalline silica. A sharp maximum is observed at 500 K, which corresponds to the α to β transition temperature of cristobalite. Minima at 850 and 580 K correspond to low- to high-temperature phase transitions of crystalline quartz and tridymite, respectively. This correspondence was not observed in synthetic fused silica containing about 800 ppm of OH.
Photo- and electroreflectance of cubic boron phosphide
Authors: Schroten E. , Goossens A. , Schoonman J.
Published: July 2002

In order to reveal the band structure of cubic boron phosphide a photo- and electroreflectance study has been undertaken. Towards this end, epitaxial thin films of this III–V semiconductor are grown on silicon (100) by chemical vapor deposition. Both modulated reflectance techniques reveal a critical point at 4.25 eV. The value of this first direct transition is more than 1 eV lower than previous reports, which are based on transmission spectra.
Dispersion of the nonlinear absorption of copper phthalocyanine in a silica xerogel matrix through the visible spectrum
Authors: Blanco E. , Narayana Rao D. , Aranda F. J. , Rao D. V. G. L. N. , Tripathy S. , Akkara J. A. , Litrán R. , Ramírez-del-Solar M.
Published: July 2002

Recently, macrocyclic organic dyes have attracted a lot of attention as possible candidates for applications in optical power limiters and pulse shaping devices. We present results for the effective excited state absorption cross section of copper phthalocyanine tetrasulfonate (CuPcTs) in a CuPcTs/Silica xerogel composite. The sample was prepared by the sol-gel method using ultrasound energy to promote the hydrolysis of tetramethoxysilane which was the silica precursor. The study has been performed in the visible region of the spectrum from 450 to 730 nm. This study represents a complete description of the dispersion of the nonlinear absorption for the composite. Measurements were also obtained for a solution of the CuPc in sulfuric acid for comparison purposes. We observed reverse saturable absorption in the range of 450–540 nm wavelength and saturable absorption in the range 570–730 nm wavelength for the composite.
Seeded epitaxy of Co90Fe10/Cu multilayers on MgO(001): Influence of Fe seed layer thickness
Authors: Kuch W. , Marley A. C. , Parkin S. S. P.
Published: July 2002

Epitaxial (001) Co90Fe10/Cu multilayers were grown by sputter deposition on MgO(001) substrates using bilayer seed layers containing thin Fe and Pt layers. The influence of the thickness of the Fe layer on the structure of the Co90Fe10/Cu multilayers was studied by x-ray diffraction, and compared to its effect on the giant magnetoresistance measured in these films. The structural quality, especially of the Co90Fe10/Cu layers closest to the seed bilayer, and the change in magnetoresistance depend sensitively on the Fe seed layer thickness. The largest giant magnetoresistance effect is found for Fe thicknesses around 12 Å. The influence of the Fe layer thickness is discussed in terms of lattice matching between the substrate, Fe, Pt, and Co90Fe10/Cu layers, and alloying of Fe and Pt in the seed layer.
Optical properties of silicon nanoclusters fabricated by ion implantation
Authors: Shimizu-Iwayama Tsutomu , Kurumado Norihiro , Hole David E. , Townsend Peter D.
Published: July 2002

A method for the fabrication of luminescent Si nanoclusters in an amorphous SiO2 matrix by ion implantation is reported. We have measured the dose (concentration of excess Si atoms) and annealing time dependence of the photoluminescence of Si nanoclusters in SiO2 layers at room temperature. The samples were fabricated by ion implantation and subsequent annealing. After annealing, a photoluminescence band below 1.7 eV has been observed. The peak energy of the photoluminescence is found to be almost independent of annealing time, while the intensity of the luminescence increases as the annealing time increases. Moreover, we found that the peak energy of the luminescence is strongly affected by the dose of implanted Si ions, especially in the high-dose range. We also show direct evidence of widening of the band-gap energy of Si particles of a few nanometers in size by employing photoacoustic spectroscopy. These results indicate that the photons are absorbed by Si nanoclusters, for which the band-gap energy is modified by the quantum confinement effects, and the emission is not simply due to direct electron–hole recombination inside Si nanoclusters, but is related to defects probably at the interface between the Si nanoclusters and SiO2, for which the energy state is affected by cluster–cluster interactions.