bluuebunny/crossref_metadata_2025 huggingface dataset:
Electromagnetic absorption in transparent conducting films
Authors: Porch Adrian , Morgan D. Vernon , Perks Richard M. , Jones Martin O. , Edwards Peter P.
Published: April 2004

In this article we calculate the skin depth of indium tin oxide (ITO) films (and related materials) as a result of free electron absorption within the visible spectrum using the simple Drude model. We also discuss the consequences of finite skin depth for the transparency of current spreading layers for light emitting diode (LED) applications. Low sheet resistances are highly desirable for these layers, but the free electron density n in ITO cannot be increased much beyond 2×1021 cm−3 without pulling the plasma frequency into the red end of the visible spectrum (thus making it highly reflective); furthermore, any increases in the film thickness cause reduced transparency due to the finite skin depth δ. However, above the plasma frequency for n≈1021 cm−3 we find that increases in the electron mobility μe cause increases in δ, since then approximately δ∝μe. Therefore, if μe in ITO can be increased above present state-of-the-art values around 50 cm2 V−1 s−1 to intrinsic limiting values around 100 cm2 V−1 s−1 by improved film processing, then substantial increases in transparency are possible whilst not sacrificing the high conductivity. The output optical power of a LED using an ITO current spreading layer with high n is also approximately proportional to μe, so mobility increases also have a direct impact on the external power efficiency of these devices.
Exciton freeze-out and thermally activated relaxation at local potential fluctuations in thick AlxGa1−xN layers
Authors: Bell A. , Srinivasan S. , Plumlee C. , Omiya H. , Ponce F. A. , Christen J. , Tanaka S. , Fujioka A. , Nakagawa Y.
Published: April 2004

Al x Ga 1−x N layers with 0.05⩽x⩽0.25 were studied using spectrally and time resolved cathodoluminescence (CL). Continuous wave spectra were taken at temperatures ranging from 5 to 300 K. The near-band-edge peak emission energy exhibits an s-shaped temperature dependence characteristic of disordered systems. This effect is quantitatively explained within a model of potential fluctuations caused by alloy disorder. An s-shape temperature dependence has been observed in other alloy systems including InGaN, however, no systematic study exists for AlGaN. In this work, the s-shape temperature dependence is systematically analyzed as a function of aluminum content and quantitatively correlated with a model of alloy disorder. The shift in the luminescence peak position with respect to the usual temperature dependence of the band gap has been quantified by −σE2/kBT, where σE is the standard deviation of the potential fluctuations. Its dependence on aluminum concentration, x, was found to systematically increase from 7 meV at x=0.05 to 21 meV at x=0.25, following the theory for alloy disorder. The recombination and relaxation kinetics investigated using time-resolved CL are fully consistent with our potential fluctuation model. At 5 K, when the excitons are strongly localized, the exciton lifetime increases monotonically with aluminum content. At elevated temperatures, when the excitons are delocalized, the decay is significantly faster and preferentially nonradiative, regardless of the aluminum content.
Relaxation of a collisionless ion matrix sheath
Authors: Israel D. , Riemann K.-U. , Tsendin L.
Published: April 2004

An analytical fluid model describing the time evolution of a collisionless ion sheath in front of a plane absorbing wall biased to pulsed high negative voltage is presented. The model utilizes the Lagrangian formulation of hydrodynamics and allows one to extend and generalize previous results for the matrix relaxation process obtained from the usual Euler formulation of hydrodynamics. Investigating the sheath evolution, the different phases of the matrix extraction and sheath expansion are characterized and treated separately. Considering the matrix extraction phase, a description is developed that, in principle, is suitable to account for arbitrary inhomogeneous initial conditions. Explicit results are given for two initial conditions of special interest. The end of the matrix extraction phase manifests itself in a distinct “kink” in the numerical solutions. By an investigation of the transition between the matrix extraction and sheath expansion phases, it is shown that this kink is due to a changing type of the ion orbits striking the wall. In order to describe the late quasistatic sheath expansion phase, the sheath boundary evolution is approximated and corresponding solutions are presented. The analytical results are compared with numerical solutions of the fluid equations as well as of a kinetic particle-in-cell/Monte-Carlo simulation and show convincing agreement.
Linewidth measurement of free-running, pulsed, distributed feedback quantum cascade lasers
Authors: Beyer Thomas , Braun Marcus , Hartwig Susanne , Lambrecht Armin
Published: April 2004

In gas spectroscopy with distributed feedback quantum cascade lasers (DFB-QCLs), linewidth is an important measurement parameter. However, in standard usage, these lasers are used in pulsed operation, so that the laser linewidth can no longer be assumed to remain constant over the length of the pulse. For the instantaneous measurement of such dynamic linewidths, a suitable measurement technique was still lacking. Frequently, pulse lengths of 5 to 10 ns are used to obtain laser linewidths that will be suitable for gas measurements. In these experiments, standard photoconductive or photovoltaic mid-infrared detectors with bandwidths in the 1 MHz range are employed, so that the linewidth is averaged over the tuning range of the QCL. The linewidth averaging could be reduced by shorter pulses, but for pulses shorter than 5 ns, the Fourier limitation increases the linewidth again. We present a technique to measure the dynamic laser linewidth continuously within a laser pulse of 30 ns to several μs. Due to the use of high-bandwidth detectors working from 2 to 12 μm, laser linewidths can be determined much more accurately than with short pulses. Instantaneous linewidths of DFB-QCLs down to 60 MHz are obtained. This shows that QCLs are well suited for gas measurement in the mid-infrared.
Excimer laser manipulation and pattering of gold nanoparticles on the SiO2/Si surface
Authors: Yang D.-Q. , Meunier M. , Sacher E.
Published: April 2004

Au nanoparticles were deposited onto SiO2/Si by evaporation, after which x-ray photoelectron spectroscopy (XPS) indicated some interfacial interaction between the Au and the Si. The sample was then processed by KrF pulsed excimer laser radiation (248 nm) in air, at a fluence of ∼20 mJ/cm2. Following this laser irradiation, XPS indicated the loss of substrate/nanoparticle interaction, resulting in the loss of cluster adhesion to the substrate and the formation of larger, isolated, spherical Au particles through rapid cluster coalescence. UV-visible spectral measurements indicated the absence of a surface plasmon resonance peak before irradiation, due to the small nanoparticle size (<3 nm); however, such a peak appeared at ∼550 nm after irradiation, when the particle size increased to ∼5.5 nm. Using 200-mesh transmission electron microscope grids as irradiation masks, we obtained Au nanoparticles in the unpatterned areas after irradiation. Such Au nanoparticle patterning may be used in biomolecular detector-based plasmon image-type sensors.
Relationships among equivalent oxide thickness, nanochemistry, and nanostructure in atomic layer chemical-vapor-deposited Hf–O films on Si
Authors: Dey S. K. , Das A. , Tsai M. , Gu D. , Floyd M. , Carpenter R. W. , De Waard H. , Werkhoven C. , Marcus S.
Published: April 2004

The relationships among the equivalent oxide thickness (EOT), nanochemistry, and nanostructure of atomic layer chemical-vapor-deposited (ALCVD) Hf–O-based films, with oxide and nitrided oxide interlayers on Si substrates, were studied using x-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM) in annular dark-field imaging (ADF), and parallel electron energy-loss spectroscopy (PEELS), capacitance–voltage, and leakage-current–voltage measurements. The XPS (Hf 4f binding energy shift) studies indicated the formation of Hf–O–Si bonds in as-deposited amorphous films, the amount of which was influenced by the interlayer composition and annealing conditions. After post-deposition annealing in N2 and O2, the Hf–O layers were nanocrystalline. Although HRTEM images showed a structurally sharp interface between the Hf–O layer and the interlayer, angle-resolved XPS, ADF imaging, and PEELS in the STEM revealed a chemically diffused HfSiOx region in between. This interdiffusion was observed by the detection of Si (using Si L edge) and Hf (using Hf O2,3 edge) in the Hf–O layer and the interlayer. For an annealed Hf–O/interlayer stack, with an ALCVD target thickness of 4.0 nm for the Hf–O layer on 1.2 nm of nitrided chemical oxide, the experimentally measured EOT and leakage current (at −1 V) were 1.52 nm and ∼10−8 A/cm2. A three-layer (1.2 nm interlayer of nitrided chemical oxide/compositionally graded, 2 nm region of HfSiOx/2 nm HfO2 layer) capacitor model was used to determine the respective contributions to the measured EOT, and the dielectric permittivity of the interlayer was found to be 6.06. These studies clearly indicate that a total EOT of 1 nm and below is attainable in the Hf–N–O–Si/Si–N–O system.
Theory of spin injection into conjugated organic semiconductors
Authors: Ruden P. P. , Smith D. L.
Published: April 2004

We present a theoretical model to describe electrical spin injection from a ferromagnetic contact into a conjugated organic semiconductor. In thermal equilibrium the magnetic contact is spin polarized, whereas the organic semiconductor is unpolarized. The organic semiconductor must be driven far out of local thermal equilibrium by an electric current to achieve significant spin current injection. However, if the injecting contact has metallic conductivity, its electron distribution cannot be driven far from thermal equilibrium by practical current densities. Thus, quasi-equilibration between the conjugated organic semiconductor and the metallic contact must be suppressed to achieve effective spin injection. This requires a spin-dependent barrier to electrical injection that may be due either to tunneling through the depletion region of a large Schottky barrier or to tunneling through a thin, insulating, interface layer. Schottky barrier formation on conjugated organic semiconductors differs from that on inorganic semiconductors inasmuch as contacts made to organic semiconductors often follow near-ideal Schottky behavior, thus permitting the energy barrier to electrical injection to be varied over a wide range by using metals with different work functions. In addition, insulating tunnel barriers to organic semiconductors based on organic molecules can be conveniently fabricated using self-assembly techniques.
Josephson fluxon flow and phase diffusion in thin-film intrinsic Josephson junctions
Authors: Warburton P. A. , Kuzhakhmetov A. R. , Chana O. S. , Burnell G. , Blamire M. G. , Schneidewind H. , Koval Y. , Franz A. , Müller P. , Hyland D. M. C. , Dew-Hughes D. , Wu H. , Grovenor C. R. M.
Published: April 2004

We have measured the transport properties of intrinsic Josephson junction arrays as a function of both temperature and magnetic field, the field being applied perpendicular to the transport current. We show that they depend upon whether the junction width exceeds or is less than the Josephson penetration depth. For widths greater than the Josephson penetration depth, the transport properties are dominated by Josephson vortex flow. For widths less than the Josephson penetration depth, the zero-field dissipation is dominated by Josephson phase diffusion. For non-zero applied fields, there is a good fit to the classical “Fraunhofer” dependence of the critical current upon field in addition to a competition between Josephson phase diffusion and Josephson vortex flow.
The response of hydroxy-terminated polybutadiene to one-dimensional shock loading
Authors: Millett J. C. F. , Bourne N. K. , Akhavan J.
Published: April 2004

Hydroxy-terminated polybutadiene (HTPB) is commonly used as a binder system in rocket propellants and plastic bonded explosives (PBXs). As such, the investigation of this material under high-strain-rate and shock-loading conditions is of importance if the response of propellants and PBXs is to be understood. Therefore, the Hugoniot of two different HTPB compositions has been investigated, using manganin stress gauges. Both materials have a linear Us–up relationship, with the material having a higher plasticizer content having a lower value of c0. It has been suggested that additions of plasticizer increase the compliance of the material. In addition, in one composition, shock recovery experiments have been performed. Results indicate that no changes in glass transition temperature, decomposition temperature, or molecular weight occur in the range of shock stresses investigated.
Blueshift of near band edge emission in Mg doped ZnO thin films and aging
Authors: Shan F. K. , Kim B. I. , Liu G. X. , Liu Z. F. , Sohn J. Y. , Lee W. J. , Shin B. C. , Yu Y. S.
Published: April 2004

Pure and Mg doped ZnO thin films were deposited at 400 °C on glass substrates by pulsed laser deposition. An x-ray diffractometer (XRD) was used to investigate the structural properties of the thin films. It is found that all the thin films have a preferred (002) orientation. The peak position of (002) orientation is found to shift from 34.39° to 34.55°. The lattice constants of ZnO thin films were also obtained from XRD data. It is found that, with the increase of the dopant concentration, the lattice constant a decreases from 3.25 to 3.23 Å, and c decreases from 5.20 to 5.16 Å. From the spectrophotometer transmittance data, the band gap energies of the thin films were calculated by a linear fitting process. The band gap energy of Mg doped ZnO thin film increases with increasing dopant concentration. In photoluminescence (PL) spectra, two PL emission peaks are found in pure ZnO thin films, one is the near band edge (NBE) emission at 3.28 eV, and the other is green-yellow-red emission at around 2.4 eV. However, with the increase of the dopants, no green-yellow-red emissions are found in PL of Mg doped ZnO thin films. The NBE emission has a blueshift compared with that of pure ZnO thin film (as much as 0.12 eV). As time goes on, NBE emission in pure ZnO thin film is enhanced, and the green-yellow-red emissions disappear.
Beam study of the Si and SiO2 etching processes by energetic fluorocarbon ions
Authors: Toyoda H. , Morishima H. , Fukute R. , Hori Y. , Murakami I. , Sugai H.
Published: April 2004

Energy-controlled and mass-selected beam of fluorocarbon ion (CF3+,CF2+) at energies from 50 to 400 eV are directed at Si and SiO2 surfaces, in order to elucidate etch mechanisms and SiO2/Si etch selectivity. During the ion beam injection, the time evolution of neutral radicals desorbed from the etching surface is measured by appearance mass spectrometry. At the same time, in situ surface analysis is also carried out by x-ray photoelectron spectroscopy. The etching rate of a clean Si surface by CF3+ injection is initially high but decreases with time due to the formation of a thin surface layer containing F and C atoms. Finally, a few-nanometers-thick stationary SixCyFz overlayer is formed on the Si surface for ion fluence >3×1016 cm−2, along with saturation of CF2 and SiF2 radical desorption from the surface. Comparison is made with molecular dynamics simulations of CF3+ interactions with Si surfaces. CF2+ injection on the Si surface yields rapid formation of the SixCyFz layer and lower desorption of the CF2 radical compared with the CF3+ injection case. On the other hand, SiO2 etching by CF3+ takes place at an energy above 50 eV. During steady-state etching, a SixOyFz overlayer is formed with C atoms hardly observed. The measured desorption rates of etch products (CO and SiF2) show the same energy dependence of SiO2 etching yield per ion. CF2 desorption during the SiO2 etching is very low compared with Si etching.
Spatial distribution of cavities in silicon formed by ion implantation generated excess vacancies
Authors: Peeva A. , Kögler R. , Skorupa W. , Christensen J. S. , Kuznetsov A. Yu.
Published: April 2004

The spatial distribution of nanosized cavities in silicon formed by high energy Ge ion implantation and annealing is determined. The cavities are directly observed by transmission electron microscopy without any metal decoration. They are shown to be the agglomerates of implantation-induced excess vacancies. The concentration depth profile of the vacancies bound in the cavities agrees well with the excess vacancy profile calculated for the implantation process. Almost all the generated excess vacancies agglomerate in cavities after annealing at 900 °C for 30 s. The vacancy profile shape coincides with the depth profile of Cu that was intentionally introduced in the cavity region. The perfect match of vacancy and Cu distribution indicates the cavities are the determining gettering centres for Cu atoms in ion implanted Si.
Modified Deal Grove model for the thermal oxidation of silicon carbide
Authors: Song Y. , Dhar S. , Feldman L. C. , Chung G. , Williams J. R.
Published: April 2004

A modified Deal Grove model for the oxidation of 4H-SiC is presented, which includes the removal of the carbon species. The model is applied to data on the oxidation rates for the (0001) Si, (0001̄) C, and (112̄0) a faces, which are performed in 1 atm dry oxygen and in the temperature range 950–1150 °C. Analysis within the model provides a physical explanation for the large crystal-face dependent oxidation rates observed.
Effects of ferroelectric switching on the piezoelectric small-signal response (d33) and electrostriction (M33) of lead zirconate titanate thin films
Authors: Gerber P. , Kügeler C. , Böttger U. , Waser R.
Published: April 2004

The effects of an increasing small signal amplitude on the piezoelectric small-signal response and electrostriction of tetragonal Pb(Zrx, Ti1−x)O3 thin films are investigated. The piezoelectric small-signal coefficient d33, piezoelectric large signal-strain S, and electrostriction coefficient M33 are measured using a double-beam laser interferometer. A continuously increasing influence of the small signal amplitude is found starting at very low values. In particular, the impact on the measured coercive field Ec is found to be stronger than the impact of the cycling frequency of the applied bias field. Also, unexpected electrostrictive behavior is investigated and explained by the influence of ferroelectric switching on the intrinsic piezoelectric lattice strain. Furthermore, the influence of an applied small-signal on the piezoelectric large-signal response is investigated.
Terahertz radiation from InAs with various surface orientations under magnetic field irradiated with femtosecond optical pulses at different wavelengths
Authors: Takahashi Hiroshi , Sakai Masahiro , Quema Alex , Ono Shingo , Sarukura Nobuhiko , Nishijima Gen , Watanabe Kazuo
Published: April 2004

We present the magnetic-field dependence of terahertz (THz)-radiation power from femtosecond-laser-irradiated InAs with various surface orientations. Under 800 nm optical excitation, the magnetic field that provides the maximum THz-radiation power is found to be affected by the surface orientation, and InAs (111) exhibits it at lower magnetic fields than that of the other surfaces. In contrast, under 1560 nm excitation, the dependence on the surface orientation almost disappeared, and saturation is observed at a much smaller magnetic field than that in the 800 nm excitation case. Additionally, from the results of magnetic-field dependence up to 14 T, the shift of the peak in the THz-radiation spectrum toward lower frequency is confirmed, depending on the magnetic field applied, which is possibly induced by the emergence of a magnetoplasma effect.
Electroreflectance studies of Stark shifts and polarization-induced electric fields in InGaN/GaN single quantum wells
Authors: Kaplar R. J. , Kurtz S. R. , Koleske D. D. , Fischer A. J.
Published: April 2004

To observe the effects of polarization fields and screening, we have performed contacted electroreflectance (CER) measurements on In0.07Ga0.93N/GaN single quantum well light emitting diodes for different reverse bias voltages. Room-temperature CER spectra exhibited three features which are at lower energy than the GaN band gap and are associated with the quantum well. The position of the lowest-energy experimental peak, attributed to the ground-state quantum well transition, exhibited a limited Stark shift except at large reverse bias when a redshift in the peak energy was observed. Realistic band models of the quantum well samples were constructed using self-consistent Schrödinger–Poisson solutions, taking polarization and screening effects in the quantum well fully into account. The model predicts an initial blueshift in transition energy as reverse bias voltage is increased, due to the cancellation of the polarization electric field by the depletion region field and the associated shift due to the quantum-confined Stark effect. A redshift is predicted to occur as the applied field is further increased past the flatband voltage. While the data and the model are in reasonable agreement for voltages past the flatband voltage, they disagree for smaller values of reverse bias, when charge is stored in the quantum well, and no blueshift is observed experimentally. To eliminate the blueshift and screen the electric field, we speculate that electrons in the quantum well are trapped in localized states.
Direct determination of volume changes in ion-beam-irradiated SiC
Authors: Jiang W. , Wang C. M. , Weber W. J. , Engelhard M. H. , Saraf L. V.
Published: April 2004

A single crystal 6H-SiC wafer was sequentially implanted in two areas at 873 and 295 K using 2.0 MeV Au2+ ions under off-axis conditions. Identical Au depth profiles, as a function of atomic areal density, were obtained for both irradiation temperatures. X-ray photoelectron spectroscopy (XPS) and analytical electron microscopy were used to determine the one-dimensional expansion in the amorphous state produced at 295 K relative to that in the slightly damaged state produced at 873 K, based on the Au reference markers. In addition, the redshift of the plasmon-loss peak in the electron energy loss spectroscopy (EELS) was used to measure the local density changes. Comparison of the three methods indicates that the XPS and EELS methods were the most reliable, yielding a volume expansion of (11.5±2)% for the amorphous state in 6H-SiC at 295 K. The volume expansion in the slightly damaged state at 873 K was determined to be 0.9% by EELS.
Realistic evaluation of power losses in solar cells by using thermographic methods
Authors: Isenberg Joerg , Warta Wilhelm
Published: April 2004

The spatially resolved evaluation of power losses in solar cells is a key issue in identifying technological and material quality problems and realistically judging their influence on solar cell performance. Up to now a spatially resolved image of leakage currents could only be obtained by lock-in thermography measuring the distribution of the dark leakage current in solar cells. However, it is well known from simulations that the dark and illuminated current paths in solar cells may differ considerably. In this article spatially resolved images of the power losses in solar cells measured under realistic operation conditions, that is at the maximum power point and under illumination, are presented. This improvement was made possible by the development of a measurement technique called illuminated lock-in thermography. The underlying physics of this technique are discussed and it is shown that this method gives an image, which is directly proportional to the local power losses in the solar cell. Different loss mechanisms as, e.g., losses in the emitter sheet resistance and due to recombination at grain boundaries could be visualized in two-dimensional images. A comparison and quantification of different loss mechanisms under operation conditions was obtained. It is found that, e.g., the losses due to recombination in the “striation rings” of Czochralski-silicon are clearly underrated as compared to losses via point-like “shunts” if a measurement of the dark characteristics of the cell is used. The comparison of dark and illuminated leakage current images gives laterally resolved experimental evidence for the differences in dark and illuminated current paths in solar cells predicted by device simulations.
On the crystallization of thin films composed of Sb3.6Te with Ge for rewritable data storage
Authors: Kooi B. J. , De Hosson J. Th. M.
Published: April 2004

This article addresses the crystallization of amorphous Sb3.6Te films (40 nm thick) and 5 at. % Ge containing Sb3.6Te films (10, 20, and 40 nm thick) as studied with transmission electron microcopy using in situ annealing. These materials exhibit growth-dominated crystallization, in contrast to the usual Ge2Sb2Te5 that shows nucleation-dominated crystallization. Particularly the crystal-growth velocity in these systems has been measured as a function of temperature from which the activation energy for growth can be derived. The strong effect of the 5 at. % Ge addition on the total crystallization behavior is revealed by the following four phenomena: Ge increases the crystallization temperature (from 95 to 150 °C), increases the activation energy for growth (from 1.58 to 2.37 eV), increases the nucleation rate and decreases the growth anisotropy. The crystallites have a special transrotational structure and a mechanism responsible for the development of this special structure is delineated.
Asymmetrically kinked hysteresis loops in exchange biased NiFe/IrMn rings
Authors: Guo Z. B. , Zheng Y. K. , Li K. B. , Liu Z. Y. , Luo P. , Wu Y. H.
Published: April 2004

Magnetic properties of a series of NiFe/IrMn rings with inner diameters from 0.35 to 1.50 μm, while keeping the outer diameter of 2.0 μm, have been studied by magnetic force microscopy and hysteresis loop measurements at various temperatures. The enhancements in exchange bias and coercivity have been observed in the patterned ring structures. Asymmetrical, kinked hysteresis loops are present in the NiFe/IrMn rings with inner diameters less than 1.50 μm. The kinks in hysteresis loops in the increasing field branch correspond to the presence of a flux closure state. The presence of the flux closure state has been attributed to the formation of antiferromagnetic domain walls in the magnetization reversal which acts as an energy barrier and traps the flux closure state.
Thermally induced optical effects in optical fibers by inverse methodology
Authors: Yang Yu-Ching , Chu Shao-Shu , Chang Win-Jin
Published: April 2004

A conjugate gradient method based on an inverse algorithm is applied in this study to estimate the unknown time-dependent convection heat transfer coefficient and microbending loss in carbon-coated optical fibers by reading the transient temperature measurement data at the fiber surface, when the optical fibers are subjected to transient thermal loading. No prior information is available on the functional form of the unknown convection heat transfer coefficient in the present study; thus, it is classified as the function estimation in inverse calculation. The accuracy of the inverse analysis is examined by using the simulated exact and inexact temperature measurements. Results show that an excellent estimation on time-dependent convection heat transfer coefficient, temperature distributions, and thermally induced microbending loss can be obtained for all the test cases considered in this study. The individual effect of the thickness, Young’s modulus, Poisson’s ratio, and thermal expansion coefficient of the carbon coating on the transient microbending loss of the optical fiber is also investigated. According to the numerical results, there exists an optimum value for the thickness of the carbon coating to reduce the transient microbending loss and to sustain the mechanical force at the same time.
Effect of internal strain on the propagation of failure in shock loaded soda-lime glass
Authors: Millett J. C. F. , Bourne N. K.
Published: April 2004

Failure wave propagation in shocked soda-lime glass has been investigated as a function of internal strain. This has been varied by either examining the material in the as-received or a heat-treated state. Results show that while material strength does not seem to be significantly effected, failure wave velocity is reduced by an annealing treatment. Annealing has been shown to reduce the amount of internal strain, and thus the amount of available energy for crack propagation is likewise reduced.
Electrically active defects induced by sputtering deposition on silicon: The role of hydrogen
Authors: Volpi F. , Peaker A. R. , Berbezier I. , Ronda A.
Published: April 2004

We present a study of the electrically active defects produced in p-type silicon crystals underneath a titanium layer deposited by sputtering to form a Schottky contact. The defects were investigated by Deep Level Transient Spectroscopy and free carrier profiles were determined by Capacitance–Voltage measurements. The sputtering deposition experiments were performed on silicon crystals grown either by Molecular Beam Epitaxy (MBE) or by the Czochralski technique (Cz). All the layers contained hydrogen as a result of normal cleaning and etching procedures. In MBE-grown silicon, four defects were observed [referred to as H(0.61), H(0.52), H(0.33), and H(0.23)]. H(0.23) has been identified as the donor level of the divacancy, while it has been shown that the three other defects were boron-related. In the Cz-grown silicon crystals only H(0.52) has been observed. This defect is hydrogen-related in addition to its correlation with boron. In both MBE and Cz grown crystals, the H(0.52) defect displayed a bell-shape distribution while the three other defects follow profiles decaying exponentially from the surface. We propose that H(0.52) is Bi–Bs–H formed by the capture of Bi at a hydrogen passivated Bs site.
A comparative model of two mechanisms from which a magnetic field arises in the heart
Authors: Murdick R. A. , Roth B. J.
Published: April 2004

There are two different mechanisms from which a magnetic field can arise in the heart. The first mechanism has the source of the magnetic field as a current dipole oriented perpendicular to the action potential wave front with the “return” currents flowing in an adjacent volume conductor. The second mechanism is one with no adjacent volume conductor and has a magnetic field that arises due to the anisotropy of the tissue. We see that the two mechanisms produce magnetic fields of similar magnitudes and similar falloff rates as distance from the tissue increases, indicating that both mechanisms are important.
A comparison of volume compressions of silver and gold up to 150 GPa
Authors: Akahama Yuichi , Kawamura Haruki , Singh Anil K.
Published: April 2004

Samples of silver and gold have been compressed together in a diamond anvil cell up to ∼150 GPa and the x-ray diffraction patterns recorded using a synchrotron radiation source. The parameters αt for silver and gold, where α is a factor that determines the relative weights of iso-stress and iso-strain conditions and t is the difference between the axial and radial stress components, are estimated from the analysis of the diffraction data. The αt-values are small and increase with pressure reaching 2 GPa and 0.5 GPa at 150 GPa for silver and gold, respectively. The effects of varying α between 1 and 0.5 on the hydrostatic volume compression derived from the measured data and the pressures computed therefrom are discussed. The compression data for silver and gold, corrected for nonhydrostatic compression effects assuming iso-stress condition (α=1), fit the relation (V/V0)Ag=1.5946(V/V0)Au3−3.0024(V/V0)Au2+2.8285(V/V0)Au−0.4207. The pressures computed from (V/V0)Ag and (V/V0)Au using equations of state from different sources have been compared. A fit of the Vinet equation to (V/V0)Ag data and pressures derived from the (V/V0)Au data using a recently proposed equation of state gives 5.47(2) for the pressure derivative of bulk modulus if the bulk modulus is constrained at 98.0 GPa, a value obtained from the ultrasonic measurements.
Disorder effects on infrared reflection spectra of InN films
Authors: Thakur J. S. , Auner G. W. , Haddad D. B. , Naik R. , Naik V. M.
Published: April 2004

The effect of plasmon and LO-phonon damping on the optical measurements of InN films is discussed. Phonon and plasmon damping dramatically modifies the spectral features of the optical spectra and destabilizes the coupled modes of the system. Phonon damping affects the optical properties in a qualitatively different way from plasmon damping. Increased phonon damping leads to a merging of the coupled modes for a certain range of carrier density. Plasmon damping broadens the spectral linewidths of both of the coupled modes when plasmon energies are of the order of LO-phonon energies. However, when plasmon energies are larger than LO-phonon energies, increasing plasmon damping makes the higher-energy mode completely degenerate with plasmon, and may even have lower energies than plasmon. In weakly damped situations, we also discuss the low-energy optical transmission region that forms in between the coupled modes. Finally, the effect of plasmon and LO-phonon coupling on the analyses of the experimental infrared reflection data of InN films (for different electron densities) is discussed.
Misfit strain relaxation by dislocations in SrRuO3/SrTiO3 (001) heteroepitaxy
Authors: Oh Sang Ho , Park Chan Gyung
Published: April 2004

Misfit relaxation by dislocations in perovskite SrRuO3/SrTiO3 (001) heterostructure with low lattice mismatch (f=0.64%) was studied. Pure edge misfit dislocations (MDs) with a Burgers vector of the a〈011〉 type were found to be the major interfacial defects responsible for the misfit relief. They were introduced by half-loops expansion from the film surface as well as by extension of pre-existing dislocations in the substrate. These 45°-MDs formed along the 〈100〉 directions in a rectangular grid pattern, preferentially at surface steps of the TiO2-terminated SrTiO3 (STO) substrate. At film thicknesses much higher than the critical film thickness (hc), however, the MD spacing was not reduced but saturated in a nearly constant value far above the equilibrium prediction (∼61.4 nm). The saturated spacing of MDs corresponds to roughly the ledge distances on the STO surface (120–150 nm). It is suggested that difficulties in the multiplication of MDs by the already-formed ones lead to the abnormally high residual strain. Further accumulation of residual strain results in microstructural modifications such as surface undulations and the two-layered structure along the growth direction.
Coercivity and exchange bias near the compensation temperature for inhomogeneous Fe/Gd ferrimagnets
Authors: Demirtas S. , Koymen A. R.
Published: April 2004

We investigate the coercivity and exchange bias effect in Fe/Gd multilayers. Antiferromagnetically coupled (Fe 4 nm/Gd 4 nm)4 multilayer has a compensation temperature (Tcomp) of approximately 90 K. We observe that the coercivity of the inhomogeneous Fe/Gd ferrimagnet makes a local minimum near the Tcomp. When a 16-nm-thick Gd layer is deposited over a (Fe 4 nm/Gd 4 nm)4 multilayer, exchange bias emerges due to the interfacial interaction of the thick Gd overlayer and the Fe surface of the multilayer. Negative and positive exchange bias were observed below and above the Tcomp, respectively. In addition, there is a continuous transition from negative exchange bias to positive exchange bias across the Tcomp where coercivity makes the local minimum.
Submicron ripple formation on glass surface upon laser-nanosphere interaction
Authors: Theppakuttai Senthil , Chen Shaochen
Published: April 2004

Submicron ripples have been observed on a borosilicate glass surface when irradiated by a nanosecond Nd: yttritium–aluminum–garnet laser (10 ns, 1064 nm), using silica nanospheres. The ripples thus obtained do not satisfy Rayleigh’s diffraction condition in that (a) the ripple spacing is different from the value predicted by the classical model, (b) the spacing is independent of the incident angle, and (c) the orientation is not always perpendicular to the laser polarization. Also, the ripple characteristics are not dependent on the diameter of the spheres used and the ripples have almost the same periodicity irrespective of the experimental parameters. Photoionization followed by a self-organization process due to nonlinear absorption of the enhanced optical field between the spheres and glass sample is believed to be the primary reason for the creation of ripples on the glass substrate.
Measuring the plasma density of a ferroelectric plasma source in an expanding plasma
Authors: Dunaevsky A. , Fisch N. J.
Published: April 2004

The initial density and electron temperature at the surface of a ferroelectric plasma source were deduced from floating probe measurements in an expanding plasma. The method exploits negative charging of the floating probe capacitance by fast flows before the expanding plasma reaches the probe. The temporal profiles of the plasma density can be obtained from the voltage traces of the discharge of the charged probe capacitance by the ion current from the expanding plasma. The temporal profiles of the plasma density, at two different distances from the surface of the ferroelectric plasma source, could be further fitted by using the density profiles for the expanding plasma. This gives the initial values of the plasma density and electron temperature at the surface. The method could be useful for any pulsed discharge, which is accompanied by considerable electromagnetic noise, if the initial plasma parameters might be deduced from measurements in expanding plasma.
A phase-field approach to the simulation of the excimer laser annealing process in Si
Authors: La Magna Antonino , Alippi Paola , Privitera Vittorio , Fortunato Guglielmo , Camalleri Marco , Svensson Bengt
Published: April 2004

We present a phase-field methodology applied to the simulation of dopant redistribution in Si during an excimer laser annealing process. The kinetic model derived in the framework of the Ginsburg–Landau thermodynamic formalism is made up of three coupled equations that rule the concurrent evolution of the thermal, phase, and impurity fields. The model was solved numerically by considering, as the initial conditions, the generic material modification due to an ion implant process, i.e., the implanted impurity profile in a SiO2/a–Si/c–Si stack. The model is parametrized for the cases of As and B doping, considering the thermal properties of the materials in the stack and the impurity-dependent diffusivity in the solid, liquid, and interfacial regions (the latter is characterized by a finite dimension). Simulated profiles are compared with the experimental results that have been obtained by secondary ion mass spectrometry and spreading resistance profiling. These comparisons demonstrate the reliability of the theoretical methodology. The model features are discussed in detail, especially with a view to the extension of the method to other impurity atoms and to the two-dimensional case.
Formation of ultrahigh density Ge nanodots on oxidized Ge/Si(111) surfaces
Authors: Nakamura Yoshiaki , Nagadomi Yasushi , Sugie Kaoru , Miyata Noriyuki , Ichikawa Masakazu
Published: April 2004

Ge nanodots with a typical size of ∼4 nm and ultrahigh density (>1012 cm−2) were grown on ultrathin SixGe1−x oxide films made by oxidizing Ge wetting layers grown on Si (111)–(7×7) surfaces. The density of the nanodots was independent of the Ge deposition rate, indicating that the chemical reaction between Ge atoms and the oxide films determined the nucleation of Ge nanodots. The size and density of Ge nanodots grown on the ultrathin SixGe1−x oxide films depended on the growth temperature and deposition amount. These dependences indicate that we can form the Ge nanodots with ultrahigh density controllably by using ultrathin SixGe1−x oxide films.
Hole mobility in zincblende c–GaN
Authors: Rodrigues C. G. , Fernandez J. R. L. , Leite J. R. , Chitta V. A. , Freire V. N. , Vasconcellos A. R. , Luzzi R.
Published: April 2004

We consider the nonequilibrium thermodynamic state of carriers in III-nitrides, and calculate the mobility of holes in cubic GaN layers under electric fields of low intensity. The contribution of different scattering mechanisms to the mobility is analyzed, and the relevance of each one is characterized. Satisfactory agreement with recently published experimental data is obtained.
Quantum mechanical modeling of gate capacitance and gate current in tunnel dielectric stack structures for nonvolatile memory application
Authors: Koh B. H. , Chim W. K. , Ng T. H. , Zheng J. X. , Choi W. K.
Published: April 2004

Multilayered dielectric stack structures, with a layered or crested potential profile, have been proposed for use as the tunnel dielectric of nonvolatile memories for fast low-voltage programming and longer charge retention. In this work, self-consistent quantum mechanical (QM) numerical calculations, using an in-house developed charge quantization simulation program, were conducted to analyze the gate tunneling current and capacitance of metal–insulator–semiconductor (MIS) devices with tunnel dielectric stack structures. The self-consistent QM simulator takes into account polysilicon depletion, quantization effects on the carrier density, and wave penetration effects. The gate current density–gate voltage (Jg–Vg) simulation uses a recursive method for calculating the transmission probability through the dielectric stack structure. The physical model was used to fit with capacitance–voltage and Jg–Vg measurements on MIS devices with different single-layer dielectric and multilayered dielectric stack structures. The simulation of the Jg–Vg characteristics of a layered-barrier structure of HfO2/Al2O3/HfO2, which can be potentially applied as the tunnel dielectric of nonvolatile memory devices, is also presented and compared with results from metal–oxide–semiconductor devices with a single layer of SiO2 or HfO2 as gate dielectric. It was found that the layered-barrier structure has the steepest Jg–Vg characteristics of the three structures with identical equivalent-oxide thickness. This results in a small ratio of program voltage to retention voltage for the layered-barrier structure, which makes it attractive for nonvolatile memory application.
Photoluminescence from Si nanocrystals induced by high-temperature implantation in SiO2
Authors: Sias U. S. , Moreira E. C. , Ribeiro E. , Boudinov H. , Amaral L. , Behar M.
Published: April 2004

A systematic study of photoluminescence (PL) behavior of Si nanocrystals in SiO2 obtained by ion implantation in a large range of temperatures (−200 up to 800 °C), and subsequent furnace annealing in N2 ambient was performed. A PL signal in the wavelength range 650–1000 nm was observed. The PL peak wavelength and intensity are dependent on the fluence, implantation and annealing temperatures. It was found that after annealing at 1100 °C, both implantations of 1.5×1017 Si/cm2 at room temperature or 0.5×1017 Si/cm2 at 400 °C result in the same PL peak intensity. By varying the implantation temperature we can achieve the same PL efficiency with lower fluences showing that hot implantations play an important role for initial formation of the nanocrystals. The PL intensity evolution as a function of the annealing time was also studied. As the implantation temperature was increased, larger mean size Si nanocrystals were observed by means of dark-field transmission electron microscopy analysis.
Compositional dependence of the Raman frequencies and line shapes of ZnS1−xTex alloys
Authors: Yu Young-Moon , Kim Dae-Jung , Choi Yong Dae , Lee Ki-Seon , O Byungsung
Published: April 2004

The Raman spectra for ZnS1−xTex alloys were investigated for a wide range of Te composition x. The lattice vibration showed two-mode behavior. The asymmetric broadening of the line shape was found to be dependent on the Te composition x and could be explained by the “Spatial Correlation” model.
Reflection polarized light microscopy and its application to pyrolytic carbon deposits
Authors: Bortchagovsky E. G.
Published: April 2004

The methods for the measurement of extinction angles by reflection polarized light microscopy were analyzed with respect to the investigation of pyrolytic carbon deposits. It is demonstrated that measurements of the extinction angle for deposits with circular morphology (e.g., a pyrolytic carbon layer on a fiber) must be strictly distinguished from the standard measurements of optically uniaxial materials with spatially uniform optical properties, such as a single crystal (e.g., graphite or pyrolytic carbon on a planar substrate). On the basis of the mathematical description of the method, an expression of the extinction angle for materials with a circular morphology is derived. This expression differs from the equation describing measurements of the extinction angle for optically uniaxial materials with spatially uniform properties erroneously applied for circular morphology. It is demonstrated on the basis of the developed formalism that no discrepancy exists between measured extinction angles for circular pyrolytic carbon deposits and single crystalline graphite, which was discussed earlier due to the misinterpretation of data measured by two different methods. The physical meaning of extinction angles is discussed and approaches for the measurement of two meaningful ellipsometric parameters instead of one extinction angle are proposed.
High-quality, melt-grown ZnO single crystals
Authors: Reynolds D. C. , Litton C. W. , Look D. C. , Hoelscher J. E. , Claflin B. , Collins T. C. , Nause J. , Nemeth B.
Published: April 2004

High-quality, melt-grown ZnO crystals are reported. The reflection and emission spectra of the melt-grown samples are compared with the same spectra from high-quality, vapor-grown ZnO crystals. We isolate the reflection and emission spectra predominantly related to the intrinsic properties associated with the wurtzite structure of the crystals. The quality of the crystals is reflected in the spectral reproduction of the intrinsic properties of the crystals. Both the ground state and the n=2 state of the free excitons associated with the A, B, and C valence bands of the crystals are spectrally observed in reflection. Assuming a hydrogenic character for the free excitons, the binding energy of these excitons associated with all three valence bands was determined. For the intrinsic emission spectra, attention was focused on the A-band free excitons and related optical parameters. Both the reflection and emission spectra for the melt-grown material compared very closely with the same spectra observed from high-quality vapor-grown ZnO samples. The details of both the reflection and emission spectra verify the high-quality of the melt-grown material.