bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of a random array of fine silver particles from a silver film: Preparation of the frequency selective screen
Authors: Nakashima Hiroshi , Omoto Hideo , Wakabayashi Hideaki
Published: May 2004

In the investigation for preparing frequency selective screen which reflects near-infrared rays and transmits radio waves, we found a phenomenon in which a continuous silver film deposited on sheet glass changes into an array of silver particles on heating. A silver film changes first into a layer consisting of connective networks, then into worm-like particles, and finally into a random array of isolated particles. The silver particles have a hemisphere shape, and the area occupied by particles is about 1/4 of the area of the original film. The average diameter of the bottom of particles linearly increases with increasing thickness of the original silver film. The growth pattern of silver particles was described by an equation derived based on the assumption that fluctuations in the film thickness cause silver atoms to migrate to the position of a larger thickness, that is, the elevated area. Simulation of the process indicated that worm-like particles completely change to a random array of isolated particles when the ratio of the number of spaces occupied by Ag atoms to the total number of spaces on a computed cross section is decreased to about 1/4, and intervals between the position of larger thickness increase with increasing thickness of the original silver film while the above ratio is kept constant. These indications were experimentally confirmed.
Phase transformation dynamics during welding of Ti–6Al–4V
Authors: Elmer J. W. , Palmer T. A. , Babu S. S. , Zhang W. , DebRoy T.
Published: May 2004

In situ time-resolved x-ray diffraction (TRXRD) experiments were used to track the evolution of the α→β→L→β→α/α′ phase transformation sequence during gas tungsten arc welding of Ti–6Al–4V. Synchrotron radiation was employed for the in situ measurements in both the fusion zone (FZ) and the heat-affected zone (HAZ) of the weld, providing information about transformation rates under rapid heating and cooling conditions. The TRXRD data were coupled with the results of computational thermodynamic predictions of phase equilibria, and numerical modeling of the weld temperatures. The results show that significant superheat is required above the β transus temperature to complete the α→β transformation during weld heating, and that the amount of superheat decreases with distance from the center of the weld where the heating rates are lower. A Johnson–Mehl–Avrami phase transformation model yielded a set of kinetic parameters for the prediction of the α→β phase transformation during weld heating. Corresponding TRXRD measurements were made during weld cooling. In the HAZ, the β→α transformation during weld cooling was shown to initiate at the β transus temperature and terminate below the Ms temperature, resulting in a microstructure containing a substantial fraction of α′ martensite. In the FZ, the β→α transformation during weld cooling was shown to initiate below the Ms temperature, and to completely transform the microstructure to α′ martensite.
Mechanism of solid-state amorphization of Se induced by mechanical milling
Authors: Zhao Y. H. , Zhu Y. T. , Liu T.
Published: May 2004

In this work, the solid-state amorphization process of elemental trigonal Se via mechanical milling was studied. Trigonal Se has a unique crystal structure consisting of helical [–Se–]n chains with strong intrachain covalent bonds and with weak interchain van der Waals bonds. It was found that the interchain coordination distance increased while the intrachain coordination distance decreased with increasing milling time. The crystalline Se transformed to the amorphous state once the interchain coordination distance reached a critical value. The intrachain coordination distance of amorphous Se continued to decrease with milling time, suggesting that molecular chain length decreased with milling time. Combined with the x-ray absorption near-edge structure calculations, it was concluded that the mechanical milling destroyed the interchain bonds of crystalline Se, resulting in the amorphization, while the intrachain bonds were strengthened during amorphization. The present results support a previous crystallite-destabilization model for solid-state amorphization.
Electrical, structural, and chemical analysis of silicon carbide-based metal-oxide-semiconductor field-effect-transistors
Authors: Chang K.-C. , Porter L. M. , Bentley J. , Lu C.-Y. , Cooper J.
Published: May 2004

In this study we investigated the morphology and interfacial chemistry of (0001) 4H-SiC-based metal-oxide-semiconductor field-effect transistors (MOSFETs) as a function of post-oxidation annealing in nitric oxide (NO) following wet oxidation. Energy-filtered transmission electron microscopy analyses showed enhanced C/Si concentrations (up to 13%) at distinct locations along the SiO2/SiC interface in the MOSFETs that were not annealed in NO. In contrast, regions of enhanced C/Si concentration were not detected in the MOSFETs that were annealed in NO; instead, these samples showed a trace amount of interfacial N. The introduction of N may therefore be associated with a reduction of C in these samples and may contribute to the higher channel mobility (∼38 cm2/V s) in the samples annealed in NO relative to the samples that were not annealed in NO (∼9 cm2/V s). Rough SiO2/4H-SiC interfaces and nonuniform oxide thickness were observed on both the NO- and the non-NO-annealed samples. The rough interfaces shown in the transmission electron microscopy bright field images may also be an important factor limiting the channel mobility in SiC-based MOSFETs.
Discrete Fourier transform in nanostructures using scattering
Authors: Leuenberger Michael N. , Flatté Michael E. , Loss Daniel , Awschalom D. D.
Published: May 2004

In this article, we show that the discrete Fourier transform (DFT) can be performed by scattering a coherent particle or laser beam off an electrically controllable two-dimensional (2D) potential that has the shape of rings or peaks. After encoding the initial vector into the two-dimensional potential by means of electric gates, the Fourier-transformed vector can be read out by detectors surrounding the potential. The wavelength of the laser beam determines the necessary accuracy of the 2D potential, which makes our method very fault-tolerant. Since the time to perform the DFT is much smaller than the clock cycle of today’s computers, our proposed device performs DFTs at the frequency of the computer clock speed.
FePt nanoparticles formed in Al2O3 by ion beam synthesis: Annealing environment effects
Authors: White C. W. , Withrow S. P. , Williams J. M. , Budai J. D. , Meldrum A. , Sorge K. D. , Thompson J. R. , Boatner L. A.
Published: May 2004

The properties of FePt nanoparticles formed by the implantation of Fe+Pt into c-axis-oriented Al2O3 single crystals followed by thermal annealing are shown to be strongly dependent on the annealing environment. Annealing in a reducing environment (flowing Ar+4% H2, or ultrahigh vacuum) gives rise to ferromagnetic FePt nanoparticles with the L10 structure and very high magnetic coercivity (greater than 20 kOe). FePt alloy formation does not occur during annealing in an oxidizing environment. Instead, the implanted Pt precipitates out forming oriented Pt nanoparticles and the implanted Fe redistributes with ∼40% segregating to the surface where it forms epitaxial α-Fe2O3 precipitates at the surface; the remainder of the implanted Fe remains in the bulk, most likely in solid solution in the matrix. Results obtained by sequential annealing of Fe+Pt implanted samples in reducing (oxidizing) environments followed by annealing in an oxidizing (reducing) environment suggest that equilibrium, rather than kinetic, effects are responsible for the observed microstructures.
Measurements of optical losses in mid-infrared semiconductor lasers using Fabry–Pérot transmission oscillations
Authors: Revin D. G. , Wilson L. R. , Carder D. A. , Cockburn J. W. , Steer M. J. , Hopkinson M. , Airey R. , Garcia M. , Sirtori C. , Rouillard Y. , Barate D. , Vicet A.
Published: May 2004

We present a Fabry–Pérot resonator technique for room temperature optical loss measurements on mid-infrared (λ∼2–4 μm) lasers. The quality of optical waveguides for λ≈2.3 μm InGaAsSb/AlGaAsSb/GaSb interband lasers and a λ≈3.7 μm strain-compensated InGaAs/InAlAs/InP quantum cascade laser have been estimated using this method. The optical losses for these lasers lie in the range 15–25 cm−1 for interband lasers and 4–5 cm−1 (transverse electric polarization) and 21–23 cm−1 [transverse magnetic (TM) polarization] for the quantum cascade laser. The considerably higher losses for TM polarization in the case of quantum cascade laser are explained by intersubband absorption in the active layers. The method may be applied to structures with only a minimum amount of device processing, facilitating rapid progress in development of mid infrared laser designs in new materials systems.
Experimental studies on relationships between the electron work function, adhesion, and friction for 3d transition metals
Authors: Li Yanping , Li D. Y.
Published: May 2004

Experimental studies on the relationships between the electron work function (EWF), adhesion and friction for 3d transition metals were reported. EWF was measured using a scanning Kelvin probe. Adhesion and friction under light loads between a silicon nitride tip and sample surfaces were evaluated employing an atomic force microscope (AFM) that may also function as a lateral force microscope (LFM). Friction of the samples under a sharp tip at higher load levels was also measured using a microtribometer. It was demonstrated that there was strong correlation between the EWF, adhesion and friction. Higher EWF corresponded to lower adhesive force and thus to lower friction when measured under light loads using the AFM/LFM. Such a correlation stems from the fact that the measured adhesion and friction are largely attributed to the intrinsic interaction between the sample and AFM/LFM tip, which is governed by the electronic behavior reflected by the EWF. However, such a correlation became weaker when the friction was evaluated under a sharp tip at higher load levels. In this case, the friction behavior of a target metal was mainly determined by its mechanical behavior.
X-ray strain analysis at high pressure: Effect of plastic deformation in MgO
Authors: Li Li , Weidner Donald J. , Chen Jiuhua , Vaughan Michael T. , Davis Maria , Durham William B.
Published: May 2004

The factors that control the stress–strain state of a polycrystal under differential stress depend on whether or not plastic deformation has occurred in the solid. If not, then the elastic properties with the constraints of the Reuss–Voigt bounds limit this relationship. If plastic deformation becomes important then the Taylor and Sachs models are relevant. These models assume that the plastic process is enabled by dislocation flow on specific lattice planes and specific Burger’s vectors. Then, the relationship between stress and strain is controlled by the orientation of an individual grain with respect to the stress field, von Mises criterion, and the critical resolved stress on the dislocation that is necessary for flow. We use a self-consistent model to predict the flow stress during the plastic deformation of polycrystalline MgO with a slip system of {110}〈11̄0〉, {111}〈11̄0〉, and {100}〈011〉 at different critical resolved shear stress ratios for the different slip systems. The prediction of the models is correlated with the results of x-ray diffraction measurements. Uniaxial deformation experiments on polycrystalline and single-crystal MgO samples were conducted in situ using white x-ray diffraction with a multielement detector and multianvil high-pressure apparatus at a pressure up to 6 GPa and a temperature of 500 °C. A deformation DIA was used to generate pressure and control at a constant deformation rate. Elastic strains and plastic strains were monitored using x-ray diffraction spectra and x-ray imaging techniques, respectively. The correlation of the data and models suggests that the plastic models need to be used to describe the stress–strain observations with the presence of plasticity, while the Reuss and Voigt models are appropriate for the elastic region of deformation, before the onset of plastic deformation. The similarity of elastic strains among different lattice planes suggests that the {111} slip system is the most significant slip system in MgO at high pressure and high temperature.
341 nm emission from hydride vapor-phase epitaxy ultraviolet light-emitting diodes
Authors: Smith G. A. , Dang T. N. , Nelson T. R. , Brown J. L. , Tsvetkov D. , Usikov A. , Dmitriev V.
Published: May 2004

Hydride vapor-phase epitaxy (HVPE) was used to grow aluminum gallium nitride-based p–n diode structures on sapphire (0001) substrates. In the diode structure, an n-type AlGaN photon-emitting layer was sandwiched between n-type and p-type AlGaN cladding layers that contained higher AlN concentrations. These diode structures were processed and subsequently packaged into transistor outline cans. The light-emitting diodes (LEDs) were characterized using a state-of-the-art spectroradiometer. LED spectral emission occurred at a peak wavelength of 341 nm with a typical full width at half-maximum of approximately 14 nm. Pulsed injection currents of 110 mA resulted in greater than 2 mW of optical output power at a wavelength of 341 nm. This HVPE-grown LED’s pulsed optical power output level is comparable to reported results on similar sized metalorganic chemical vapor deposition grown ultraviolet LEDs with emission wavelengths near 340 nm.
The role of contaminants in the variation of adhesion, friction, and electrical conduction properties of carbide-coated scanning probe tips and Pt(111) in ultrahigh vacuum
Authors: Enachescu M. , Carpick R. W. , Ogletree D. F. , Salmeron M.
Published: May 2004

Scanning probe microscopy was used to investigate the tribological properties of nanoscale tips in contact with a Pt(111) single-crystal surface under ultrahigh vacuum conditions. The tips were coated with a tungsten carbide film, which contained a significant fraction of oxygen. The electrically conductive tip made it possible to alternate between contact measurements and noncontact scanning tunneling microscopy. Several types of interfaces were found depending on the chemical state of the surfaces. The first type is characterized by strong irreversible adhesion followed by material transfer between tip and sample. Low adhesion and no material transfer characterize a second type of contact, which are associated with the presence of passivating adsorbates in both (full passivation) or in one of the two contacting surfaces (half-passivation). Half-passivated contacts in which the clean side is the Pt(111) sample gave rise to periodic stick-slip friction behavior with a period equal to the atomic lattice constant of the Pt(111) surface. Local electrical conductivity measurements show a clear correlation between electronic and friction properties, with ohmic behavior on clean regions of the Pt surface and semiconductor-like behavior on areas covered with adsorbates.
Characterization of the interface region during the agglomeration of silicon nanocrystals in silicon dioxide
Authors: Pi X. D. , Coleman P. G. , Harding R. , Davies G. , Gwilliam R. M.
Published: May 2004

Si nanocrystals embedded in thermally grown SiO2 have been annealed at temperatures between 400 and 900 °C in a variety of atmospheres. Positron annihilation spectroscopy has been employed to study changes in the interface regions between nanocrystalline Si (nc-Si) and SiO2 with the support of photoluminescence measurements. We find that nitrogen and oxygen are trapped in the voids around nc-Si at low annealing temperatures. High-temperature annealing during the formation of nc-Si causes hydrogen originally residing in the SiO2/substrate region to enter the SiO2 structure. Hydrogen diffuse back to the SiO2/substrate region on annealing in vacuum at 400 °C because no other impurities block its diffusion channels. At annealing temperatures above 700 °C, both nitrogen and oxygen react with nc-Si, resulting in a volume increase. This introduces stress in the SiO2 matrix, which is relaxed by the shrinkage of its intrinsic open volume. The present data suggest that nitrogen suppresses Si diffusion in SiO2, so that the agglomeration of nc-Si is slower during annealing in nitrogen than in oxygen or vacuum.
Artificial dielectric properties of microscopic metallized rodlike bacteria cells in composites
Authors: Cai Jun , Li Yaqin , Zhang Deyuan
Published: May 2004

Bacteria cells have great potential in manufacturing complex microstructures because of their various shapes. Recently, bacteria cells have been metal plated to produce soft-core monomers by an electroless deposition technique. This article investigates the dielectric properties of CoNiP-coated rodlike bacteria cells embedded in epoxy resin. The results show that the coating was thick enough to form a continuous film, and the increase of the coating thickness played little role in the permittivity. The real part of the permittivity increased monotonically as the concentration of bacteria cells increased, whereas the imaginary part of the permittivity remained small. This indicated that all the samples had bacteria loadings below the percolation threshold concentration.
Influence of surface roughness and internal strain on defect spectrum and intensity of low-temperature photoluminescence of thin Si1−xGex layers
Authors: Ligatchev V. , Wong T. K. S. , Yoon S. F.
Published: May 2004

Microstructure, surface roughness, morphology, defect spectrum, and low-temperature photoluminescence of thin (10–125 nm) strained Si1−xGex layers (0.1⩽x⩽0.3), deposited by chemical vapor deposition (CVD) at 650 °C on silicon wafers have been studied. Nominally undoped layers with crystalline orientations of 〈100〉 and 〈111〉 have been investigated. Local strain within the layers was estimated from x-ray diffraction data. It decreases with the layer thickness in the 〈100〉-oriented samples, but rises in the 〈111〉-oriented ones. Nanoscale (∼10–30 nm) and microscale (∼0.2–1 μm) morphologies have been found on the surface of the Si1−xGex layers by atomic-force microscopy. The lateral sizes of the morphologies and surface roughness depend on the thickness, germanium concentration x, and crystalline orientation of the layers. The spectrum of defect states N(E) in the band gap of these samples has been experimentally studied by the deep-level-transient-spectroscopy (DLTS) technique. The standard D1(P1), D2, P3, and P4 defect peaks were observed. The N(E) spectrum is strongly influenced by germanium concentration, crystalline orientation, and surface roughness of the films (especially at Ec−E<0.4 eV). Photoluminescence (PL) was excited with argon ion (Ar+) laser at a sample temperature of about 5 K. Both “no-phonon” and phonon-assisted PL peaks around 1.1 eV, as well as a strong peak at 0.80 eV were observed. These peaks originated, respectively, from the no-phonon line from the Si substrate, transverse optical/acoustical phonon replica and dislocation-related Si1−xGex band, D1. Intensities of these PL peaks are influenced by the layer thickness, internal strain, surface roughness, and germanium concentration x. Possible mechanisms of relationship between the local strain, film roughness, the defect spectrum N(E), and the D1 line strength are discussed.
Optical dispersion of ternary II–VI semiconductor alloys
Authors: Liu Xinyu , Furdyna J. K.
Published: May 2004

The wavelength dependence of the refractive indices n of a series of II–VI ternary alloys—ZnCdSe, ZnBeSe, ZnMgSe, ZnMnSe, ZnCdTe, ZnMnTe, ZnMgSe, and ZnSeTe—were measured at frequencies below their respective energy gaps using the combined techniques of optical reflectivity and the prism coupler method. To facilitate the analysis of the results—including those obtained in the wavelength region near the fundamental energy gap—we have modified the semiempirical single-effective-oscillator (SEO) model of n by introducing an additional term that explicitly accounts for interband transitions at the fundamental gap. Using the SEO model modified in this manner to fit the wavelength dependence of n, a set of semiempirical parameters was established for the above ternary II–VI-based alloys, where the fitting parameters for each alloy family are themselves expressed as functions of the alloy composition. The availability of these parameters makes it possible to calculate the index of refraction of any given II–VI ternary alloy for any composition and at any wavelength. Furthermore, these parameters provide valuable physical insights, such as the relationship between the covalency (or ionicity) of the material and its refractive index. In addition to its fundamental usefulness, this approach can—by appropriate extrapolation—also be used for obtaining the dispersion properties for “hypothetical” zinc blende compounds that do not form under equilibrium crystal growth conditions (such as MnTe, MnSe, or BeSe).
Nondestructive spectroscopic method to detect MnAs metallic nanocrystals in annealed GaAs:Mn
Authors: Seo S. S. A. , Noh T. W. , Kim Y.-W. , Lim J. D. , Park Y. D. , Kim Y. S. , Khim Z. G. , Jeon H. C. , Kang T. W. , Pearton S. J.
Published: May 2004

We report an optical spectroscopic method to monitor NiAs-type MnAs (α-MnAs) nanocrystals in (Ga,Mn)As diluted magnetic semiconductors. We utilize Mn ion implantation of low temperature (LT) GaAs epitaxial thin films followed by rapid thermal annealing (RTA) to yield embedded ferromagnetic α-MnAs nanoclusters in a GaAs:Mn matrix. As-implanted samples are paramagnetic and become ferromagnetic with Curie temperature of ∼320 K after RTA at 750 °C. No peaks of potential secondary phases could be observed in x-ray diffraction measurements. However, in optical spectra, the annealed samples show resonant absorption at 0.9 eV photon energy, due to resonant surface plasma oscillation of spherical metallic phases embedded in LT GaAs. Since the absorption peak position in the photon energy has a direct relation to the value of the plasma frequency of metallic inclusions, the metallic clusters in LT GaAs are identified as α-MnAs nanocrystals by comparing them with simulations based on Maxwell–Garnett theory. We suggest that this optical method can be applied to various granular systems and diluted magnetic semiconductors as a nondestructive way to detect and quantify metallic nanoclusters.
Ferromagnetic resonance study of diluted Fe nanogranular films
Authors: Tomita Satoshi , Hagiwara Masayuki , Kashiwagi Takanari , Tsuruta Chusei , Matsui Yoshio , Fujii Minoru , Hayashi Shinji
Published: May 2004

Ferromagnetic resonance (FMR) in diluted Fe nanogranular films with low volume fractions of Fe (v) has been studied. Films prepared by a co-sputtering technique are composed of amorphous Fe nanoparticles embedded in SiO2 glass matrices. The resonance of the FMR uniform mode is found to be strongly affected by the v, the temperature, and the angle between the film plane and the applied magnetic field. The present study suggests that the values of these parameters should be carefully selected in order to realize left-handed materials using diluted ferromagnetic-metal nanogranular films.
Spalling of Cu3Sn intermetallics in high-lead 95Pb5Sn solder bumps on Cu under bump metallization during solid-state annealing
Authors: Jang Jin-Wook , Ramanathan Lakshmi N. , Lin Jong-Kai , Frear Darrel R.
Published: May 2004

We report the spalling of Cu3Sn intermetallics in high-lead 95Pb5Sn solder bumps on Cu under bump metallization (UBM) during solid state annealing. Upon reflow, the Cu3Sn intermetallics formed on Cu UBM. However, after solid state annealing at 170 °C, the Cu3Sn intermetallics spalled off from Cu UBM and the Pb phase filled the gap between the Cu3Sn intermetallics and Cu UBM. This is primarily explained by the loss of chemical adhesion between the Cu3Sn intermetallics and Cu UBM due to no additional chemical reaction. Thermodynamic principles are used to interpret the spalling phenomenon and the analysis showed that the interfacial free energy without spalling is greater than that with spalling after solid-state annealing. Spalling of the Cu3Sn intermetallics initiated at an open interface such as the edge of Cu UBM and finally extended to the flat interface at a slower rate.
Resonance magnetoelectric effect in bulk composites of lead zirconate titanate and nickel ferrite
Authors: Zeng M. , Wan J. G. , Wang Y. , Yu H. , Liu J.-M. , Jiang X. P. , Nan C. W.
Published: May 2004

Magnetoelectric (ME) Pb(Zr0.52Ti0.48)O3 (PZT)–NiFe2O4 bulk composites with various PZT volume fractions were prepared. The ME coupling coefficient, impedance, and flux density versus the frequency were measured. The magnetic bias and PZT volume fraction in the composites are investigated to optimize the ME output. It is revealed that ME resonances are caused by electromechanical resonance in the piezoelectric phase and magnetomechanical resonance in the magnetostrictive phase. Maximum magnetoelectric voltage coefficient resonance values of 6.7 V/cm Oe at 132.6 kHz and 6.12 V/cm Oe at 427.2 kHz for the composite with optimized PZT volume fraction of 0.55 were obtained at optimized magnetic bias of 0.6 kOe.
Critical thickness of equilibrium epitaxial thin films using finite element method
Authors: Subramaniam Anandh
Published: May 2004

On growth beyond critical thickness, interfacial misfit dislocations nucleate, and this partially relaxes the strain due to lattice mismatch in epitaxially grown thin films. It has been observed that, in metallic thin films, global minimization of energy can explain the experimentally determined critical thickness values and these are referred to as equilibrium thin films. In this work, using a combination of finite element simulation of a coherently strained growing thin film and simple edge dislocation, the critical thickness for dislocation nucleation is determined. The edge dislocation and the growing thin film are modeled by feeding in the appropriate stress free Eshelby misfit strain in suitable regions of the model. The critical thickness of various metallic thin films is calculated and compared with standard theory and with available experimental results.
Structural disorder produced in KH2PO4 by light-ion bombardment
Authors: Kucheyev S. O. , Felter T. E.
Published: May 2004

We study structural disorder produced in tetragonal KH2PO4 (KDP) single crystals at room temperature by irradiation with MeV light ions. Results show that electronic energy loss plays a major role in the production of lattice defects in KDP. The effective diameters of ion tracks depend superlinearly on the electronic stopping power of energetic light ions. Structural lattice disorder is also accompanied by the formation of a network of cracks and blisters on the sample surface. Such irradiation-induced cracking and blistering typically evolve over extended periods of time (e.g., days) after bombardment and are strongly affected by ion irradiation and sample storage conditions.
Kink-effect related noise in short-channel InAlAs/InGaAs high electron mobility transistors
Authors: Vasallo B. G. , Mateos J. , Pardo D. , González T.
Published: May 2004

We perform a microscopic analysis of the noise associated with the kink effect in short-channel InAlAs/InGaAs lattice-matched high electron mobility transistors (HEMTs) by using a semiclassical two-dimensional (2-D) ensemble Monte Carlo simulator. The kink effect in HEMTs has its origin in the pile up of holes (generated by impact ionization) taking place under the source side of the gate, that leads to a reduction of the gate-induced channel depletion and results in a drain current enhancement. Our results indicate that the generation of holes by impact ionization and their further recombination lead to fluctuations in the charge of the hole pile up which provoke an important increase of the drain-current noise, even when the kink effect is hardly perceptible in the output characteristics. In addition, shot noise related to the hole gate leakage current is found at the gate terminal, which can further degrade the global noise performance of HEMTs.
Effect of Ga+ ion irradiation on the structural and magnetic properties of CoFe/IrMn exchange biased bilayers
Authors: McGrouther D. , Chapman J. N. , Vanhelmont F. W. M.
Published: May 2004

The effects of focused beam Ga+ ion irradiation on the physical microstructure and magnetic properties of CoFe/IrMn exchange biased bilayers have been investigated by transmission electron microscopy. Only for Ga+ ion doses >1014 ions cm−2 was the microstructure detectably altered with increases in both the bilayer mean grain size and texture being observed. At larger doses (>1015 ions cm−2) radical alterations to the grain morphology and polycrystalline nature of the film were observed including formation of a remarkable needle-like phase at 3×1015 ions cm−2. The magnetic properties and magnetization reversal behavior of the bilayer were studied using the Fresnel mode of Lorentz microscopy. Ga+ ion doses >1013 ions cm−2 were found to progressively reduce the bias field strength and coercivity with the former reaching half original strength at 2×1014 ions cm−2. Higher doses were found to alter the reversal mechanism accompanied by progressive degradation of the magnetic properties. It is likely that the observed magnetic property changes are caused by intermixing of atoms between the various layers.
Role of growth temperature and the presence of dopants in layer-by-layer plasma deposition of thin microcrystalline silicon (μc-Si:H) doped layers
Authors: Gordijn A. , Rath J. K. , Schropp R. E. I.
Published: May 2004

Microcrystalline silicon (μc-Si:H) p- and n-type layers have been developed by Layer-by-Layer (LbL) deposition at high temperatures. The LbL deposition consists of alternating boron or phosphorus doped amorphous silicon depositions and hydrogen plasma treatments by Very High Frequency Chemical Vapor Deposition (VHF PECVD). The layers are developed to be resistant to the temperature and hydrogen flux of a micro- of polycrystalline intrinsic layer grown at a high deposition rate in a p-i-n or an n-i-p solar cell device. It is concluded that the LbL method is suitable to produce device quality μc-Si:H p- and n-type doped layers in a temperature range from 250 to 400 °C. This is not possible with standard continuous PECVD employing high hydrogen dilution of silane, where the addition of dopants reduces the crystallinity. An optimum effective thickness per deposition cycle (total thickness divided by the number of cycles) of 1.5 nm/cycle is needed for the crystallization. This optimal effective sub layer thickness is independent of dopants and deposition temperature. However, a minimum thickness of the first layer is needed for a sustaining growth in the LbL process. The doped layers grown by LbL are smoother than reference samples grown by continuous wave (cw). The doping efficiencies in our LbL deposited layers are structurally higher than those in cw deposition (for p layers a doping efficiency of 39% in case of LbL, compared to 1% for cw). The properties of the best high-temperature doped layers are as follows: for LbL p-type μc-Si:H (Ts=350 °C, 29 nm), activation energy=0.11 eV and dark conductivity=0.1 Ω−1 cm−1; for LbL n-type μc-Si:H (Ts=400 °C, 31 nm), activation energy=0.056 eV and dark conductivity=2.7 Ω−1 cm−1. Test solar cells have been deposited using Hot-Wire CVD (HWCVD) and VHF PECVD deposited μc-Si:H i-layers on top of the high-temperature LbL μc-Si:H n-type doped layer in an n-i-p configuration on a stainless steel substrate without a back reflector. A high open circuit voltage of 0.56 V and a fill factor of 0.7 show the high doping efficiency and crystallinity of the n-type doped layer and the resistance to the impinging atomic hydrogen during the HWCVD deposition. The mechanism behind the LbL μc-Si:H growth phenomenon is a controversial subject. We studied the LbL growth and nucleation mechanism as well as the incorporation of dopant atoms in the μc-Si:H layers. Etching, abstraction, and hydrogen diffusion are analyzed and it is concluded that our observations support the nucleation model that is based on hydrogen diffusion, while chemical transport and epitaxial growth are excluded to be the mechanism behind the crystallization.
Interplay between GaN polarity and surface reactivity towards atomic hydrogen
Authors: Losurdo Maria , Giangregorio Maria M. , Capezzuto Pio , Bruno Giovanni , Namkoong Gon , Doolittle W. Alan , Brown April S.
Published: May 2004

GaN epitaxial layers have been grown by molecular beam epitaxy (MBE). Low (200 °C) and high (700 °C) temperature nitrided sapphire substrates, and GaN and AlN buffer layers have been used as the platforms for investigating polarity selection processes of GaN grown by RF-MBE, aimed at presenting a comprehensive understanding of the issues of GaN polarity and growth conditions. The results show that Ga-polar GaN results from the use of AlN buffer layers, and when the substrate/bulk interface has a Ga- or Al-rich interface. In contrast, a large density of N-polar domains is found when GaN epitaxial films are grown on GaN buffer layers on sapphire when nitrided at 200 °C. The impact of the polarity of GaN on the interaction of GaN surfaces with atomic hydrogen is also studied. A different reaction rate and reaction extent is found for N- and Ga-polar GaN with atomic hydrogen, with N-polar GaN exhibiting greater reactivity. It is also demonstrated that the reactivity of GaN with atomic hydrogen coupled with surface potential measurements exploiting Kelvin probe microscopy is an alternative approach to determining the polarity of GaN films, and identifying the presence of inversion domains. Finally, the impact of GaN polarity on electrical properties of AlGaN/GaN heterojunction structures is also shown.
Probing highly deep levels in solid C70 with isothermal capacitance transience-time spectroscopy
Authors: Ran G. Z. , Chen K. M. , Zhang X. L. , Liu H. F.
Published: May 2004

A technique, referred to as the isothermal capacitance transience-time spectroscopy, has been developed to probe the deep levels at insulator/semiconductor interfaces or in semiconductor materials with fairly wide band gaps held at moderately elevated temperatures. The highly deep levels in the solid C70/p-GaAs heterojunction were studied with this technique. Two discrete hole traps, H1 and H2, were found to exist in the solid C70 layer at the positions of Ev+0.856 eV and Ev+1.037 eV, respectively.
Dynamics of the pump-probe reflectivity spectra in GaAs and GaN
Authors: Prabhu S. S. , Vengurlekar A. S.
Published: May 2004

The pump-probe reflectivity (PPR) technique is a quick way to characterize the short carrier lifetime in materials which may be potentially good terahertz (THz) emitters or detectors. Here, we study the PPR signal in semiconductors theoretically in the frequency domain (at various energies above and below the band gap) as a function of pump-probe delay. We consider two conditions of carrier relaxation. In one, the carriers are assumed to form a hot, thermalized energy distribution during excitation itself and then to cool via phonon emission, as is expected in the case of high density excitation in GaAs. In the other case, the carriers essentially remain in a nonequilibrium, nonthermal state even as they relax. This can happen when the carrier-longitudinal optical phonon interaction is stronger than carrier–carrier scattering, as is likely in GaN even at moderately high densities. In addition, effects of carrier trapping and recombination determining the carrier lifetime are included. The calculation takes into account the effect of Sommerfeld factor and pump induced modulation of the probe reflectivity due to band filling (BF), band gap renormalization (BGR), and free carrier absorption. Signatures of carrier cooling and decay can be identified from the delay dependence of the PPR signal at high enough carrier densities (⩾1×1018 cm−3) when the carrier cooling rate is comparable with the decay rate. In that case, carrier cooling shows up in the reflectivity signal as a rise in the time evolution whereas the signal decay is mainly related to carrier decay, albeit in a nonexponential way. However, at lower densities, the signal evolution with delay is rather complex. There, it is not possible to identify the signature of carrier cooling and the decay of the signal is not governed simply by the carrier decay rate. We point out that in general, the magnitude and signature of the PPR signal at different delays are governed by an interplay between the BGR and BF effects. The delay dependence of the signal is a very sensitive function of the form of BGR used to describe its density dependence at low densities. We find that the delay and frequency dependence of the PPR signal is different for a thermalized, cooling distribution from that for the relaxing, nonthermalized distribution. Thus, PPR experiments may be able to distinguish rapid carrier relaxation via a cascade emission of longitudinal optical phonons due to stronger Fröhlich coupling in GaN from cooling of hot, thermalized carriers in GaAs.
Microstructure of precipitated Au nanoclusters in TiO2
Authors: Wang C. M. , Zhang Y. , Shutthanandan V. , Thevuthasan S. , Duscher G.
Published: May 2004

Gold nanoclusters dispersed in TiO2(110) single crystal have been formed by 2 MeV Au2+ implantation to an ion fluence of 6.0×1016 cm−2 at 300 and 975 K followed by annealing at 1275 K for 10 h. The morphological features, size, crystallographic orientation of the Au nanoclusters with respect to the TiO2 matrix, and the interface structure between the Au nanoclusters and TiO2 have been investigated using conventional transmission electron microscopy (TEM), high-resolution TEM (HRTEM), electron diffractions, and high angle annular dark-field (HAADF) imaging in an aberration corrected scanning TEM (STEM). STEM-HAADF image directly reveals that Au atoms are in the substitutional Ti atomic columns in the TiO2 lattice prior to nucleation of Au cluster. An Atomic structural model of the interface between Au and TiO2 was established based on HRTEM and image simulations. The precipitated Au clusters show typical (111) twins. Au clusters are faceted along Au{112}, Au{111}, and Au{220} planes. Two types of orientation relationship can be identified, Au〈110〉//TiO2[001] and Au{111}//TiO2(200), and Au〈110〉//TiO2[001] and Au{111}//TiO2(110). These orientation relationships as well as the {111} twining feature in Au clusters are similarly observed for Au clusters grown on stoichiometric TiO2(110) free surface, indicating that the presently established orientation corresponds to the lowest interfacial energy for Au contacted with TiO2. This is essential for understanding the catalytic properties of Au supported on TiO2.
Control of Coulomb blockade in a mesoscopic Josephson junction using single electron tunneling
Authors: Hassel J. , Seppä H. , Delahaye J. , Hakonen P.
Published: May 2004

We study a circuit where a mesoscopic Josephson junction (JJ) is embedded in an environment consisting of a large bias resistor and a normal-insulator-superconductor (NIS) junction. The effective Coulomb blockade of the JJ can be controlled by the tunneling current through the NIS junction leading to transistor-like characteristics. We show using phase correlation theory and numerical simulations that substantial current gain with low current noise (in≲1 fA/Hz) and noise temperature (≲0.1 K) can be achieved. Good agreement between our numerical simulations and experimental results is obtained.
Triplet exciton confinement and unconfinement by adjacent hole-transport layers
Authors: Goushi Kenichi , Kwong Raymond , Brown Julie J. , Sasabe Hiroyuki , Adachi Chihaya
Published: May 2004

To understand confinement of the triplet exciton of Ir(ppy)3 by hole-transport layers, we compared energy-dissipative processes of the triplet exciton of Ir(ppy)3 which is doped into 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD), 4,4′-bis [N-(p-tolyl)-N- phenyl-amino]biphenyl (TPD), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), and 4,4′-N,N′-dicarbazole-biphenyl hosts. Significant energy transfer from Ir(ppy)3 into the triplet levels of α-NPD was observed. In the case of the TPD host, however, partial confinement of the Ir(ppy)3 triplet exciton was observed. This result suggests both forward and backward energy transfer from Ir(ppy)3 to the TPD triplet levels. Furthermore, employing TAPC as a hole-transport layer achieved strong confinement of the Ir(ppy)3 triplet exciton. One conclusion from these results is that electrophosphorescence efficiency is well correlated with the triplet energy level of the hole-transport layer host materials.
Influence of defect states on the secondary electron emission yield γ from MgO surface
Authors: Motoyama Yasushi , Hirano Yoshikuni , Ishii Keiji , Murakami Yukio , Sato Fumio
Published: May 2004

In order to study the dependence of the secondary electron emission yield γ on the defect states of the MgO surface in plasma display panels, cathodoluminescence (CL) spectra of MgO films, which were deposited at oxygen partial pressures of 0, 4.0×10−4, and 1.2×10−5 Torr, were measured. The CL intensities from the F (oxygen ion vacancy+one electron) and F+ (oxygen ion vacancy+one electron) centers of the MgO film that was deposited at high oxygen partial pressure (1.2×10−4 Torr), significantly increased with aging during discharge. Assuming that the probabilities of transitions are proportional to the measured CL intensities from the F and F+ bands of the MgO films, the γi values of MgO films for Ne and Xe ions, which include the F and F+ bands, were calculated. The tendencies of the breakdown voltages calculated using these γi values were consistent with those of the measured voltages of the MgO films. These calculated results suggested that the influence of the F and F+ bands on the γi values for Xe ions is large compared with that for Ne ions, and that the γi value of the MgO film for Xe ions increases with increasing numerical densities of the F and F+ centers, especially for F+ centers.
Concentration effects on lubrication rheology for polymer solution in molecularly thin film using molecular dynamics
Authors: Jeng Yeau-Ren , Chen Chia-Chen , Shyu Shiuh-Hwa
Published: May 2004

The current study utilized a molecular dynamics (MD) method to investigate the concentration effects of the mixture with polymer and monatomic liquid on the lubrication rheological properties. Fluids considered were confined in a molecularly thin film in which the number of molecular layers and film thickness are varied. In the MD simulation, polymer molecules were modeled as bead-spring chains using the finitely extendable nonlinear elastic potential. Both the Poiseuille flow and Couette flow were analyzed and the results were presented for slip ratio, viscosity, and the relation between shear stress and shear rate. It was found that the properties of polymer solution in molecularly thin film are different from the bulk properties and that the polymer concentration will affect the rheological behaviors for molecularly thin film.
Structural characteristics of ferroelectric phase transformations in single-domain epitaxial films
Authors: Alpay S. P. , Misirlioglu I. B. , Sharma A. , Ban Z.-G.
Published: May 2004

Structural characteristics of phase transformations in epitaxial ferroelectric films are analyzed via a Landau–Devonshire thermodynamic formalism. It is shown that the phase transformation temperature, the lattice parameters, and the order of the phase transformation are a strong function of the misfit strain and are considerably different compared to unconstrained, unstressed single crystals of the same composition. Depending on the internal stress state, it is possible that the structural aspects of the paraelectric–ferroelectric phase transformation may be completely obscured in the presence of epitaxial strains. The thickness dependence of epitaxial stresses due to relaxation by misfit dislocations during film deposition is incorporated into the model using an “effective” substrate lattice parameter. There is a good quantitative agreement between the theoretical analysis and experimental observations reported in the literature on the variations in the lattice parameters and the phase transformation temperature with film thickness in epitaxial BaTiO3 films.
Growth and decay dynamics of a stable microbubble produced at the end of a near-field scanning optical microscopy fiber probe
Authors: Taylor R. S. , Hnatovsky C.
Published: May 2004

Low power cw laser radiation coupled into a near-field scanning optical microscopy fiber probe has been used to generate a stable microbubble in water. A probe tip which was selectively chemically etched and metallized served as a microheater for the generation of the stable bubble. Bubble diameters in the range of 40–400 μm and lifetimes of over an hour have been obtained. The microbubble exhibited a linear growth phase over a period of a few seconds before reaching a maximum diameter which depended on the laser power. When the laser beam was blocked the microbubble decayed with a rate which was inversely proportional to the bubble diameter. The bubble lifetime depended on the square of the initial bubble diameter. Instabilities which transform a large stable bubble into a microjet stream of micron sized bubbles as the laser power was increased is also described.
Hydrogen exchange kinetics in NaAlH4 catalyzed in different decomposition states
Authors: Kircher Oliver , Fichtner Maximilian
Published: May 2004

Hydrogen absorption and desorption kinetics of NaAlH4 catalyzed with small Ti-clusters were studied. Three different kinds of samples were investigated based on undecomposed NaAlH4, Na3AlH6, and NaH+Al, the latter two both produced by decomposition of pure NaAlH4 and mechanical alloying. The samples, catalytically doped in the described decomposition state, showed different absorption kinetics and storage capacities. Whereas the same kinetic behavior was observed for the first decomposition step, the second step exhibited strong differences. The absorption behavior of the samples showing the fastest kinetics and highest storage capacities was further investigated by absorbing hydrogen in two separate steps. The first decomposition/absorption step can be well interpreted with a nucleation and growth model.
Pulsed laser deposition of marine origin material: Preparation and characterization of CaCO3 particles and CaO nanocrystals
Authors: Nave Doron , Rosenwaks Salman , Vago Razi , Bar Ilana
Published: May 2004

An attempt at using laser ablation of biofabricated coralline skeletal material Porites lutea for particle deposition is described. The deposition was performed in an evacuated chamber by pulses of the second harmonic (532 nm) of a Nd:YAG laser. The morphology and the local structure of the deposited films were characterized by scanning electron microscopy, X-ray diffraction, confocal Raman microscopy, and transmission electron microscopy. The as-deposited coatings partially retained the inorganic constituent of the CaCO3 target and mostly transformed to CaO. The crystalline phase was changed from solely aragonite in the target to a mixture of aragonite and calcite microparticles and CaO nanofeatures.