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Journal of Applied Physics

Issue 4 • Date Aug 2003

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Displaying Results 1 - 25 of 108
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Effects of having two populations of defects growing in the cavity of a semiconductor laser

    Page(s): 2155 - 2161
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    In this study, we show the effects of the growth of two noninteracting populations of defects in the cavity of a semiconductor laser diode, induced by accelerated lifetesting. The development of one type of defect is considered to give rise to a particular failure mode or mechanism. Using a multicomponent model, we demonstrate that the evolution of the threshold current as a function of time is strongly affected by the operating temperature. We describe the progression of the degradation in terms of different thermal activation energies associated with each of the different failure modes. This observation explains why a common shape of the aging curve is seldom observed from one experiment to another and why it is difficult to have one universal threshold-aging model that everyone agrees on. In addition, the study offers insight on the estimation of the lifetime of a semiconductor laser diode. © 2003 American Institute of Physics. View full abstract»

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  • Investigation of circular Bragg reflection in an azo polymer with photoinduced chirality

    Page(s): 2162 - 2166
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    There has been recent interest in photoinduced chirality in azo polymers upon irradiation with circularly polarized light. The mechanism of ordering is thought to involve successive twisting of the achiral photoactive azo-containing side chains in response to the handedness of the pump polarization. In this study, a thin film of a liquid-crystalline azo polymer with a room-temperature nematic phase is irradiated with circularly polarized light and a circular Bragg reflection is observed in the absorption spectrum. When probed at normal incidence, the evolution of this selective reflection is monitored with increasing exposure. Then, the probing is carried out for various angles of incidence and the change in peak shape is studied. In the last experiment, the position of the Bragg reflection is monitored as the wavelength of the pump laser is varied. We propose that while the induced chirality is due to the circularly polarized light, the associated pitch may be large; the induced periodicity corresponding to the Bragg reflection may be the result of partial erasure due to a standing wave created by back reflection in thin films. © 2003 American Institute of Physics. View full abstract»

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  • Carrier dynamics in nitride-based light-emitting p-n junction diodes with two active regions emitting at different wavelengths

    Page(s): 2167 - 2172
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    The carrier transport and recombination dynamics of monolithic InGaN/GaN light-emitting p-n junction structures with two active regions are investigated. Room-temperature and low-temperature photoluminescence and room-temperature electroluminescence measurements show two emission bands originating from the two active regions. In electroluminescence, the intensity ratio of the two emission bands is independent of injection current. In contrast, the intensity ratio depends strongly on the excitation intensity in photoluminescence measurements. The dependency of the emission on excitation is discussed and attributed to carrier transport between the two active regions and to the different carrier injection dynamics in photoluminescence and electroluminescence. The luminous efficacy of a Gaussian dichromatic white-light source is calculated assuming a line broadening ranging from 2kT to 10kT. Luminous efficacies ranging from 380 to 440 lm/W are obtained for broadened dichromatic sources. © 2003 American Institute of Physics. View full abstract»

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  • Disorder effects on the imaging of a negative refractive lens made by arrays of dielectric cylinders

    Page(s): 2173 - 2176
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    By a numerical simulation based upon multiple scattering theory, we confirm that, for certain frequencies, crystal structures formed by dielectric cylinders embedded in parallel in a uniform medium can indeed act as a negative refractive lens. Some agreement with and differences from previous studies are elaborated upon. The refractive index is also estimated from ray optics approximation. The tolerance of the lens to positional disorders is then discussed. © 2003 American Institute of Physics. View full abstract»

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  • Modeling of photonic crystals using a real-valued transmission line matrix method

    Page(s): 2177 - 2182
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    In this article, we present a stable real-valued transmission line matrix (TLM) method for simulating periodic photonic structures. Unique to this method is the circuit-type implementation of the periodicity (Bloch) condition at the sidewalls of the unit cell under simulation. Unlike previously published methods for implementing periodic boundary conditions in real-valued TLM networks, our circuit representation is shown to be passive since no energy is introduced from the boundaries of the simulation domain. The enforcement of Bloch boundary conditions at the sidewalls of the simulation domain, while preserving energy, results in an unconditionally stable TLM method free of high-frequency noise. Our approach is validated by simulating a two-dimensional array of dielectric materials, a structure usually referred to as a photonic crystal. The results are compared with those obtained by the plane wave expansion method and good agreement is found. © 2003 American Institute of Physics. View full abstract»

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  • Photon force analysis for a spherical particle near a substrate illuminated by a tightly focused laser beam

    Page(s): 2183 - 2187
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    We present an analysis of the radiation force acting on a spherical particle near a substrate that is illuminated by a tightly focused laser beam. The particle may be trapped at multiple positions due to multiscattering between the particle and the substrate. The particle is attracted to the substrate when the two are very close to each other, because the multiscattering produces a high-intensity region on the substrate surface. In many instances we found that a precise control of the focal position was necessary in order to scan the particle on the substrate surface. On the basis of the results of our analysis, we also compare the potentials of radiation forces on the particle with and without a substrate. © 2003 American Institute of Physics. View full abstract»

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  • Slab-thickness dependent band gap size of two-dimensional photonic crystals with triangular-arrayed dielectric or magnetic rods

    Page(s): 2188 - 2191
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    The band structures of two-dimensional photonic crystals are investigated numerically. The examined photonic crystal has a substrate-slab-cover architecture, wherein the rods in the slab are either dielectric or magnetic and are arrayed triangularly. The material used for the substrate, the cover and the surrounding medium in the slab is the same. It was found that the bandgap size of zodd modes initially becomes larger as slab thickness increases, reaching a maximum value, after which, the band gap size reduces. A similar behavior for the slab thickness-dependent band gap size was also observed for slabs containing rods with higher magnetic permeability and lower dielectric permeability, except that there was a shift toward lower values for the band gap size. A detailed discussion on the influence of the magnetic permeability of rods on the band gap size of zodd modes is also given. © 2003 American Institute of Physics. View full abstract»

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  • Investigation of carrier transport and carrier distribution in GaAs/(Al,Ga)As quantum-cascade structures

    Page(s): 2192 - 2198
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    We have investigated the current density–electric field (j-F) characteristics and photoluminescence (PL) spectra of several GaAs/(Al,Ga)As quantum-cascade structures (QCS’s) up to the threshold field strength to obtain information on the carrier distribution and field inhomogeneities. Due to the complex structure of each single period of the cascaded device, the dark j-F characteristics of undoped QCS’s exhibit distinct current maxima, resulting in regions of negative differential conductivity, while a plateaulike feature appears under illumination. For doped QCS’s, a plateau with sawtoothlike structures appears, which are well known from doped, weakly coupled superlattices under electric-field-domain formation. However, in contrast to the splitting of the PL line that is observed for doped, weakly coupled superlattices, no splitting has been observed in the doped QCS’s. The j-F characteristic of an undoped QCS with thicker barriers than the original QCS exhibits a much more pronounced current maximum in the dark. Under illumination, the j-F characteristic of this structure shows a clear plateaulike feature, which contains additional fine structure indicating the existence of electric-field inhomogeneities within each period due to the separation of electrons and holes. The PL spectra of the undoped and doped QCS’s show emission from only the widest quantum well (QW) because the holes accumulate very quickly in this QW resulting from the strong coupling within the entire QCS. However, in the PL spectra of the undoped QCS with thicker barriers, PL lines from several quantum wells can be identified. The carrier distribution process in QCS’s is also discussed as a function of the applied electric field and compared for the different structures. © 2003 American Institute of Physics. View full abstract»

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  • Characterization of BCl3/N2 plasmas

    Page(s): 2199 - 2202
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    Optical emission spectroscopy, quadrupole mass spectrometry, and electron density measurements were used to study the effect of the percentage of N2 on the characteristics of BCl3/N2 plasmas and their resulting etch processes. The etch rate of GaAs increased from 80 Å/min in pure BCl3 to over 1000 Å/min in a 40:60 BCl3:N2 mixture (15 mTorr, 50 W, 20 sccm). The optical emission intensities of both molecular and atomic chlorine exhibited maxima near 30% N2, and an argon actinometer indicated a large increase in argon emission as a function of the increase in N2 percentage. Microwave measurements indicated that the average electron density increased only slightly with an increase in nitrogen percentage up to 60% N2. Mass spectrometric analysis of the plasmas showed that both the dissociation of BCl3 and the production of molecular chlorine were significantly enhanced by the addition of N2. These results suggest that an increase in the electron temperature as a result of electron attachment heating (and possibly energy transfer from N2 metastables) is responsible for the increased dissociation and enhanced production of etch species. © 2003 American Institute of Physics. View full abstract»

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  • Comparison of the ablation plumes arising from ArF laser ablation of graphite, silicon, copper, and aluminum in vacuum

    Page(s): 2203 - 2211
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    The ablation plumes arising after irradiation of graphite, silicon, copper and aluminum with a pulsed nanosecond ArF (λ=193 nm) laser at fluences between 2 and 20 J cm-2 in vacuum are studied and compared. The neutral and ionic components in the ablation plume have been measured via quadrupole mass spectrometry and ion probes, respectively. Additional information about the degree of ionization and the velocities of singly and multiply charged ions in the plume have been deduced via optical emission spectrometry, and the electron velocity distributions have been measured with Langmuir probes. Probing the plasma properties with this range of techniques is shown to provide a rather detailed picture of the ablation characteristics. The velocity distributions of the neutral atoms are comparatively narrow (∼1 km s-1 full width at half maximum) and peaked at a center of mass velocity of ∼3–4 km s-1. Their general form is reminiscent of those of species expanding supersonically from a pulsed nozzle. The electron and ion velocity distributions are much broader, and centered at much higher velocities (and kinetic energies). The relative ion yield, and the overall degree of ionization, both increase with increasing fluence and scale inversely with the ionization potentials of the respective target materials. Both charged components are found to be accelerating at short distances from the target. Such effects have been rationalized by assuming that incident laser radiation ionizes (by multiphoton ionization) neutral species ablated from the target surface, and that these ions and electrons then act as “seeds” for subsequent plume heating, ionization and plasma formation by inverse bremsstrahlung. This a- bsorption due to inverse bremsstrahlung ensures the incident laser intensity is highest at the outer edge of the expanding plume. The outer region thus receives preferential excitation and heating—traditionally pictured in terms of the so-called two electron temperature model. Some of the resulting “hot” electrons escape from this coronal region, leading to an overall charge imbalance within the plasma, which manifests itself as an acceleration (driven by Coulombic interactions) of the charged components within the plume. © 2003 American Institute of Physics. View full abstract»

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  • Role of the fast Ar atoms, Ar+ ions, and metastable Ar atoms in a hollow cathode glow discharge: Study by a hybrid model

    Page(s): 2212 - 2222
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    The role of the fast Ar atoms, Ar+ ions, and metastable Ar atoms in a cylindrical hollow cathode discharge (HCD) is studied based on a self-consistent model. The model comprises submodels based on the principle of Monte Carlo and fluid simulations. With Monte Carlo models the movement of the fast electrons, fast Ar atoms, and fast Ar+ ions as particles is described, while with the fluid models, the slow electrons, ions, and metastable atoms are treated as a continuum. Typical results are, among others, the fast atom, fast ion, and fast electron excitation and ionization rates, the electron, ion, and metastable atom densities and fluxes, the energy distribution function of the fast atoms, fast ions, and fast electrons, and the electric field and potential distribution. Also the relative importance of different processes determining the metastable density in an Ar HCD is analyzed, as well as the influence of the fast atoms and fast ions on the discharge properties. © 2003 American Institute of Physics. View full abstract»

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  • Harmonic content and time variation of electron energy distributions in high-plasma-density, low-pressure inductively coupled discharges

    Page(s): 2223 - 2233
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    Plasma kinetics models which are commonly used to model rf discharges often produce electron energy distributions (EEDs) averaged over the rf cycle. In doing so, the time dynamics of the EEDs are lost. In this article we describe a technique to recover these time dynamics by using an “on-the-fly” (OTF) Monte Carlo method. The OTF method directly computes the harmonic content of the EEDs using Fourier transform techniques during advancement of the trajectories of pseudoelectrons. The OTF method was incorporated into a two-dimensional plasma equipment model to investigate the harmonic content of the EEDs and their time dependencies in inductively coupled low-pressure (≤10 mTorr) and low-frequency (≤16 MHz) plasmas sustained in Ar. The computational results demonstrated that the second harmonic dominates the time dynamics of the EEDs, and that this harmonic content mostly occurs at higher energies. The harmonic content at low energies is significant only in the electromagnetic skin layer. We also found that the time evolution of the second harmonic of the EED involves the simultaneous transit of several pulses in energy and space. These pulses are attributed to the nonlinear Lorentz force resulting from the rf magnetic field. © 2003 American Institute of Physics. View full abstract»

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  • Occupation probability for acceptor in Al-implanted p-type 4H–SiC

    Page(s): 2234 - 2241
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    Al-implanted p-type 4H–SiC layers with different implantation and annealing temperatures are formed, and the temperature dependence of the hole concentration p(T) is obtained by Hall-effect measurements. The Al acceptor level in SiC is deep (∼180 meV), and its first excited state level calculated by the hydrogenic model is still deep (∼35 meV), which is close to the acceptor level of B in Si. Therefore, in order to determine the reliable acceptor density (NA) from p(T), the Fermi–Dirac distribution function is not appropriate for Al acceptors in SiC, and a distribution function including the influence of the excited states of the Al acceptor is required. It is demonstrated that the proposed distribution function is suitable for obtaining the actual relationship between NA and p(T) in p-type 4H–SiC.© 2003 American Institute of Physics. View full abstract»

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  • Influence of the ion mean free path and the role of oxygen in nitriding processes

    Page(s): 2242 - 2247
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    In this article we report the mechanism involved in the nitriding process of stainless steel by ion implantation. The importance of the nitrogen ion mean-free path on the stainless steel nitrated layer obtained by using a broad ion source is established. The energy distribution of the nitrogen ions arriving at the substrate is basically determined by the inelastic scattering suffered by the ions on the way to the material surface, i.e., the ion mean-free-path λ. Besides this effect, the ion current density arriving at the sample surface is modified by the dispersion introduced by the collisions of the nitrogen ions with the chamber background molecules. This multiple scattering process is modeled assuming a stochastic phenomenon and its conclusions used to explain experimental results of hardness, diffusion profile, and nitrated layer thickness. A controlled oxygen-background partial pressure is also introduced and its role on the nitrated layer reported. At relatively low ion energies and oxygen partial pressures, both the diffusion zone and nitrated layer thickness are controlled by the ion current density. Indeed, they follow a linear relationship, provided that the composition of the nitrated layer does not change, i.e., the amount of incorporated nitrogen does not modify the original material crystalline phase. © 2003 American Institute of Physics. View full abstract»

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  • Spectroscopic ellipsometry study of a self-organized Ge dot layer

    Page(s): 2248 - 2253
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    We report on the determination of the dielectric function of a Ge dot layer obtained by epitaxial growth on Si(001) in the presence of Sb as surfactant and capped by Si. After growth, spectroscopic ellipsometry reveals a SiGe alloy containing Ge rich regions. After etching the Si cap, the dielectric function of the Ge rich regions exhibits critical points located at 2.92, 3.65, and 4.25 eV. It is shown that this dielectric function does not correspond either to an alloy or to bulk Ge, and is affected by confinement. © 2003 American Institute of Physics. View full abstract»

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  • Structure of GaN quantum dots grown under “modified Stranski–Krastanow” conditions on AlN

    Page(s): 2254 - 2261
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    We propose a procedure to grow GaN quantum dots (QDs) on AlN by using the Ga surfactant effect in plasma-assisted molecular beam epitaxy. Self-formed GaN islands were spontaneously generated under vacuum, after evaporation of the Ga bilayer stabilizing the two-dimensional GaN layer grown under Ga-rich conditions. Island characteristics (size and density) are studied as a function of the nominal amount of GaN deposited. We demonstrate that the QD density can be controlled in the 3×1010cm-2–2×1011cm-2 range. It is shown that beyond a given amount of GaN nominally deposited, there is a coexistence between elastic and plastic relaxation, with GaN islands being formed on a partially relaxed two-dimensional GaN layer thicker than two monolayers. © 2003 American Institute of Physics. View full abstract»

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  • Nucleation and propagation of plastic collapse bands in aluminum honeycomb

    Page(s): 2262 - 2270
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    Uniaxial compression experiments on aluminum honeycomb are performed to investigate localization of deformation in cellular materials. Physical experiments, combined with numerical simulation of the honeycomb microstructure, feature the indepth analysis of the nucleation and propagation of plastic collapse bands. The onset of inelasticity is determined by the von Kármán collapse load for the honeycomb microstructure. The plastic collapse mechanism yields localization of deformation in the form of collapse bands. At the same time, microstructural imperfections are generated in the vicinity of those bands. As a result, three microstructural configurations characterize the honeycomb specimen: Uncrushed material with deformation-induced imperfections, crushed material containing folded cell walls, and an active interface between the uncrushed and crushed regions. Globally, the active interface emerges as a flat propagating crushing front that travels down the specimen. It appears that the behavior of the interface governs the macroscopic response of the honeycomb. Consequently, a one-dimensional interface-based constitutive law is developed along with an additional field equation that monitors the imperfection intensity. The results provide a basis for the mechanical modeling of materials that evolve statistically inhomogeneous microstructures during deformation. © 2003 American Institute of Physics. View full abstract»

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  • Effect of biasing field on the soft mode in the vicinity of the SmC*-SmA phase transition of a ferroelectric liquid crystal “ZLI4851.”

    Page(s): 2271 - 2276
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    Frequency (10 Hz–1 MHz) and temperature (22–80 °C) dependent dielectric constant (both real and imaginary) of a ferroelectric liquid crystal ZLI-4851 film (21 μm thickness) showing a second order SmC*-SmA phase transition around 63.2 °C have been reported. The variation of dielectric constant as a function of temperature follows the generalized Landau theoretical model. Thermal variation of the complex dielectric constant has also been measured with different bias field strengths (0–20 V) for both the SmC* and the SmA phases. Interestingly, soft mode is observed in the SmC* phase only by applying a bias field perpendicular to the helix axis. Due to the application of the bias field, the SmC* phase loses stability of the helical structure, which is considered to be responsible for the appearance of soft mode in the present ferroelectric liquid crystal system. © 2003 American Institute of Physics. View full abstract»

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  • Formation of highly uniform silicon nanoparticles in high density silane plasmas

    Page(s): 2277 - 2283
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    We have investigated silicon particles formed in an inductively coupled plasma (ICP) system and have measured their structural properties by electron microscopy. ICP silane plasmas are shown to be an interesting and versatile source of nanoparticles. Three regimes are mapped out: a regime of no observable particle growth at the lowest pressures, a regime of polydisperse and agglomerated particles at the highest pressures, and a regime yielding highly monodisperse particles at intermediate pressures. Our results suggest that an ICP allows this unique production of monodisperse nanoparticles at high concentrations. The particle size can be controlled from 15 to 200 nm and is determined by the plasma on time. For short plasma times, particle growth appears to be due primarily to coagulation, while in the second phase particle growth is attributed primarily to surface deposition. For H2 dilutions less than 92%, the growth rate is almost independent of H2 partial pressure. Transmission electron micrographs, however, indicate that the addition of hydrogen decreases the particle density. At higher dilution ratios, polycrystalline particles are obtained. Under all other conditions the particles are amorphous. © 2003 American Institute of Physics. View full abstract»

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  • Donor–acceptor pair photoluminescence spectra analysis in CdTe:Ag

    Page(s): 2284 - 2288
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    Photoluminescence (PL) spectra of CdTe single crystals and films, undoped and Ag-doped, were studied in the range of the photon energy (hν): 1.5–1.6 eV, at 10 K. Films were doped during the growth process. Thin layers of Ag were vacuum evaporated onto the surface of undoped CdTe crystals, then were submitted to thermal annealing in a N2+2% H2 ambient at 400 °C for 3 h. PL spectra of crystals exhibit a donor–acceptor pair (DAP) emission at hν=1.491 eV (named here DAPAg2) with the highest intensity with respect to other signals. The PL of films is characterized by a peak at 1.5 eV followed by several phonon replicas. In both cases, the DAP emissions are due to the Ag-impurity presence and are accompanied by phonon replicas. Unannealed intrinsic crystals exhibit a unique broad DAPAg1 peak at hν=1.501 eV, and one month aged at room temperature Ag-doped crystals only display a wide DAPAg2 peak at hν=1.491 eV with 65% of its initial intensity, that reflects a reduced density of Ag diffused-doping levels. The origin of DAP lines in a PL spectra of CdTe:Ag has not been completely well defined until now, and we have introduced some explanatory conclusions about and a simple model for electronic transitions. This issue is important because Ag is an everpresent residual impurity in CdTe. © 2003 American Institute of Physics.   View full abstract»

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  • Hydrosilylation of crystalline silicon (111) and hydrogenated amorphous silicon surfaces: A comparative x-ray photoelectron spectroscopy study

    Page(s): 2289 - 2294
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    Alkene molecules were covalently bonded to hydrogen-terminated crystalline silicon (111) and hydrogenated amorphous silicon (a-Si:H) surfaces by thermally induced hydrosilylation. The resulting chemical surface structure was analyzed by x-ray photoelectron spectroscopy and compared to that of the corresponding silicon surfaces covered by a native oxide and terminated with hydrogen. Our results demonstrate successful hydrosilylation on both substrate materials. However, the presence of oxygen on the surface turns out to hinder the hydrosilylation reaction, as shown by the reduced concentration of hydrocarbons on the surface after prolonged exposure of the Si substrates prior to hydrosilylation. By monitoring both the O 1s and the Si 2p peaks, the oxidation kinetics of a-Si:H was found to be diffusion limited. Since stable hydrogen termination as a prerequisite of hydrosilylation can be achieved on a-Si:H surfaces with much less technological effort than on crystalline silicon surfaces, a-Si:H is a promising substrate for biofunctionalization procedures requiring less stringent process conditions. © 2003 American Institute of Physics. View full abstract»

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  • Site selective study of Eu3+-doped transparent oxyfluoride glass ceramics

    Page(s): 2295 - 2301
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    Optical properties of Eu3+ ions in oxyfluoride glasses and glass ceramics doped with two different concentrations, 0.1 and 1 mol %, have been analyzed and compared with previous results for higher concentrated samples, 2.5 mol %. The Eu3+ ions in the 0.1 mol % doped glass ceramics are diluted into like crystalline environments with higher symmetry and lower coupled phonon energy than in the precursor glasses; meanwhile in the 1 mol % doped glass ceramics the presence of EuF3 clusters has been observed in addition to diluted ions. Fluorescence line narrowing measurements indicate the presence of two main fluoride site distributions for the diluted Eu3+ ions in both glass ceramics. © 2003 American Institute of Physics. View full abstract»

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  • Photoinduced inclination of polyimide molecules containing azobenzene in the backbone structure

    Page(s): 2302 - 2307
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    We have investigated the inclined alignment of polyimide molecules (containing azobenzene in the backbone structure) induced by oblique angle irradiation of ultraviolet (UV) light. The UV irradiation was performed for a polyamic acid film, which then was thermally converted into a polyimide film. The orientation of the polyimide backbone structure was determined by measuring the polarized infrared absorption spectra as a function of the angle of incidence. We confirmed that inclined alignment of the polyimide backbone structures is obtained by oblique angle UV irradiation, and that this also induces tilted homogeneous liquid crystal (LC) alignment. For the UV irradiation conditions used in the present study, the average inclination angle of the polyimide backbone structure was about 4° from the surface plane. The pretilt angle of the LC molecules was 0.9°. © 2003 American Institute of Physics. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory