bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical modeling of magnetoelectric effect in a composite structure
Authors: Liu Y. X. , Wan J. G. , Liu J.-M. , Nan C. W.
Published: October 2003

The mechanical coupling effect in a magnetoelectric (ME) composite structure in which a magnetostrictive component is bonded with a piezoelectric one is simulated by numerical technique, focusing on an optimization of the magnetoelectric coupling output. The simulation starts from an experimentally developed ME composite structure and takes into account the mechanical coupling mechanism between the two components. A numerical optimization algorithm is developed, predicting a significant enhancement of the ME output by optimizing the component dimension. This algorithm can also be used for optimum design of other ME composite structures in terms of the largest ME output.
Arsenic incorporation into InGaAsP grown by low-pressure metalorganic vapor phase epitaxy using tertiarybutylarsine and tertiarybutylphosphine in N2 ambient
Authors: Huang G. S. , Tang X. H. , Zhang B. L. , Tjin Swee Chuan
Published: October 2003

In x Ga 1−x As y P 1−y epilayers have been grown by low-pressure metalorganic vapor phase epitaxy (LPMOVPE) using tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) as group V precursors and nitrogen as the carrier gas. Arsenic incorporation into InxGa1−xAsyP1−y films grown by LPMOVPE as a function of the gas phase composition ratio and V/III ratio has been systematically studied. With optimized growth conditions, the arsenic composition of the epilayers does not change linearly with the TBA source flow. It is observed that the arsenic incorporation becomes saturated when the gas phase composition TBA/(TBA+TBP) increases to 0.4. The incorporation kinetics in MOVPE growth of InxGa1−xAsyP1−y alloy has been analyzed by using an adsorption-trapping model. The As composition (y) of the InxGa1−xAsyP1−y films varies with the TBA gas phase composition ξ=TBA/(TBA+TBP) according to the expression y=2Ns*/aN0(1−e−θβξ). It is demonstrated that with the optimized growth conditions, TBA has a higher incorporation efficiency than TBP in MOVPE growth of InxGa1−xAsyP1−y films.
Model of the polarization response of single-crystal lead magnesium niobate-lead titanate for transducer applications
Authors: Piquette Jean C. , Amin Ahmed H. , McLaughlin Elizabeth A.
Published: October 2003

A theoretical model is developed and applied to experimentally measured polarization acquired from lead magnesium niobate lead titanite (PMN–PT) single crystals. The model is shown to describe well the polarization behavior with respect to changes in temperature and applied electric field at a variety of levels of applied prestress. Including the effects of prestress is important if the model is to be used in transducer design and output control. Large prestresses are applied to the active materials used in transducers in order to prevent the driver from extending. (Damage to the driver is much more likely should it go into extension during operation, and large applied prestress reduces this possibility.) Physically, the model accounts for both rotation and stretching of elementary dipoles, and realistic values for the elementary dipole moment and dipole concentration are deduced from the measurements using the model. Reasonable dipole behavior with respect to changes in temperature, stress, and electric field is also seen. The model has been shown to successfully interpolate and extrapolate measurements in temperature and electric field at various levels of applied prestress, thus allowing a significant reduction in the number of measurements that must be acquired to characterize single-crystal PMN–PT for transducer applications.
Invertibility of current density from near-field electromagnetic data
Authors: Sheltraw D. J. , Coutsias E. A.
Published: October 2003

The problem of determining a current density confined to a volume from measurements of the magnetic and electric fields it produces exterior to that volume is known to have nonunique solutions. Despite the nonuniqueness of the inversion we show that one may nevertheless uniquely determine certain moments of the vector spherical harmonic expansion of the current. It is demonstrated that the determination of these moments allows for the unique inversion of a current density confined to a spherical shell. Although unique the inversion may be ill conditioned and require a regularization of the inversion as demonstrated in an example numerical inversion.
Electric field induced piezoelectric resonance in the micrometer to millimeter waveband in a thin film SrTiO3 capacitor
Authors: Morito Kentaro , Iwazaki Yoshiki , Suzuki Toshimasa , Fujimoto Masayuki
Published: October 2003

Thin film SrTiO3 metal–insulator–metal (MIM) capacitors were fabricated in order to characterize the piezoelectric resonance in the micrometer to millimeter waveband arising from electric field induced ferroelectricity in the SrTiO3 film. The specimens showed a second-order phase transition, and the piezoelectric resonance appeared when an electric field of 250 kV/cm was applied at room temperature. Finite element method (FEM) analysis was used to interpret the piezoelectric resonance observed in the capacitors. The FEM analysis data basically agrees well with the experimental data, and the few differences between the theoretical and experimental data are interpreted mainly as artifacts caused by overlapping of resonant/antiresonant peaks during the measurement. The piezoelectric resonance of thin film MIM capacitors is strongly influenced by the longitudinal stacked structure of the MIM.
Preparation and characterization of nanostructured film of graphitized diamond crystallites for field electron emission
Authors: Chen Jian , Deng S. Z. , Chen Jun , She J. C. , Xu N. S.
Published: October 2003

The details are given of the preparation technique that has been developed for depositing crystalline-nanodiamond particles on large-area silicon substrates, based on an dielectrophoresis process. A patented technique was adapted to prepare the substrate surface with high density of uniformly distributed nanoprotrusions that provide local field enhancement necessary for dielectrophoresis deposition. The nanodiamond crystallites were treated at elevated temperatures to have their surfaces graphitized. High-resolution transmission microscopy and electron diffraction techniques as well as micro Raman analysis were used to study the properties of the so-prepared nanostructured film. Stable field electron emission from these films with current density of >5 mA/cm2 may be regularly obtained at fields as low as ∼3 V/μm. The current–voltage characteristics and the corresponding Fowler–Nordheim plots, emission uniformity, and current stability were studied. The findings suggest that this type of film is a potential candidate as a large-area cold cathode. A model based on emission associated nanoscale graphite network and nanotriple junction is presented.
Influence of local heating on micro-Raman spectroscopy of silicon
Authors: Kouteva-Arguirova S. , Arguirov Tz. , Wolfframm D. , Reif J.
Published: October 2003

In micro-Raman spectroscopy strong local heating by the tightly focused laser beam results in an unexpectedly small temperature shift of the Raman peak. The experimental results of shift, broadening, and asymmetry are discussed in view of restricted thermal expansion and a temperature dependent Raman cross section. The Raman signal originates mostly in the wings of the beam profile, where the temperature is substantially lower than at the center. Consequently, the Raman peak shift cannot be used to estimate the maximal surface temperature: despite local melting in the center of the probed spot, the maximal detected shift is only about 2 cm−1, 10 times less than expected from the center temperature.
Decrease of breakdown voltages for micrometer-scale gap electrodes for carbon dioxide near the critical point: Temperature and pressure dependences
Authors: Ito Tsuyohito , Fujiwara Hideyuki , Terashima Kazuo
Published: October 2003

We performed measurements of breakdown voltages as a function of environmental pressure with 1-μm-gap tungsten electrodes for high-pressure carbon dioxide up to supercritical conditions at different temperatures (305.65, 308.15, and 313.15 K). Breakdown voltage curves exhibit an inflection at around 3 MPa and a drastic decrease near the critical point. The location of the drastic decrease shifts to the high-pressure range and the sharpness and depth decrease with increased temperature. The breakdown voltage in pressure environments higher than that at the inflection point was analyzed systematically using the Townsend theory and density fluctuations. Moreover, comparison with breakdown voltage measurements by 10-μm-gap electrodes indicates that one factor inducing the inflection and the decrease might be electron attachment to existing clusters in dense carbon dioxide.
Current and spin-filtering dual diodes based on diluted magnetic semiconductor heterostructures with a nonmagnetic barrier
Authors: Zhai Feng , Guo Yong , Gu Bing-Lin
Published: October 2003

We have investigated the ballistic spin-polarized transport through a diluted magnetic semiconductor heterostructure with the inclusion of a nonmagnetic barrier. It is found that at suitable magnetic fields, the output current of the system exhibits a nearly 100% spin polarization and large values under the forward bias, while under the reverse bias it is small and shows a weak polarization. Such current and spin-filtering dual diode functions are robust under the change of the nonmagnetic barrier width.
Linewidth enhancement factor of a type-I quantum-cascade laser
Authors: Lerttamrab M. , Chuang S. L. , Gmachl C. , Sivco D. L. , Capasso F. , Cho A. Y.
Published: October 2003

Experimental results using the amplified spontaneous emission spectroscopy of a type-I quantum-cascade laser are presented. Using the Hakki–Paoli method, the optical gain spectra of the laser are extracted for the wavelength of 8.2 μm at various subthreshold current levels. The change in refractive index with increased bias current is obtained from the peak wavelength shifts of the Fabry–Pérot spectrum. A low value of −0.5 for the linewidth enhancement factor is found. A group index of around 3.47 has also been determined from Fabry–Pérot modal spacings.
InSb high-speed photodetectors grown on GaAs substrate
Authors: Kimukin Ibrahim , Biyikli Necmi , Ozbay Ekmel
Published: October 2003

We report InSb-based high-speed photodetectors grown on GaAs substrate. The p-i-n type photodetectors can operate at room temperature. Room-temperature dark current was 4 mA at 1 V reverse bias, and the differential resistance at zero bias was 65 Ω. At liquid nitrogen temperature, the dark current was 41 μA at 1 V reverse bias and the differential resistance at zero bias was 150 kΩ. Responsivity measurements were performed at 1.55 μm wavelength at room temperature. The responsivity increased with applied bias. At 0.6 V, responsivity was 1.3 A/W, where unity quantum efficiency was observed with internal gain. Time-based high-speed measurements were performed using a pulsed laser operating at 1.55 μm. The detectors showed electrical responses with 40 ps full width at half maximum, corresponding to a 3 dB bandwidth of 7.5 GHz.
Self-organized etching technique for fabricating a quasiregular array of MnAs nanoislands
Authors: Takagaki Y. , Wiebicke E. , Hesjedal T. , Kostial H. , Herrmann C. , Däweritz L. , Ploog K. H.
Published: October 2003

We present a method to fabricate arrays of MnAs islands having diameters of less than 100 nm without using nanofabrication techniques. The strain balance in the MnAs layer grown epitaxially on GaAs substrates collapses when the heterostructure is immersed in a wet-chemical etch solution. As a consequence, the MnAs layer develops a regular row of cracks and submicron-wide strips are carved from it. The strips are additionally sliced into islands during etching since two phases of MnAs, α-MnAs and β-MnAs, which alternate in a periodic way along the strips, are etched at different rates. The period of the quasiregular array of MnAs islands can be adjusted through the thickness of the MnAs layer. We also show that MnAs islands can serve as a nearly ideal etch mask to create GaAs columns by dry etching.
Metallic precipitate contribution to generation and recombination currents in p-n junction devices due to the Schottky effect
Authors: Negoita M. D. , Tan T. Y.
Published: October 2003

The contribution of metallic precipitates to recombination and to generation currents has been modeled for pn junction devices fabricated using Si, with the precipitate located in the space charge region of the device. The physical mechanism responsible for the electrical activity of the metallic precipitate is attributed to the Schottky junction property between the precipitate and the Si matrix materials. In steady states, it is found that the precipitate changes from a highly effective carrier recombination center to a carrier generation center when the junction bias is changed from forward to reverse biasing conditions. Based on the physical model, numerical simulation results showed that the precipitate electrical behavior resembles that of classical Shockley–Read–Hall recombination/generation centers, with a much larger activity.
Photothermal investigations of thermal and optical properties of GaAlAsSb and AlAsSb thin layers
Authors: Saadallah Faycel , Yacoubi Noureddine , Genty Frédéric , Alibert Claude
Published: October 2003

Investigation of the optical and thermal properties of GaAlAsSb/GaSb and AlAsSb/GaSb heterostructures by photothermal deflection spectroscopy (PDS) is presented. From a comparison of the amplitude and phase of the experimental PDS signal with the corresponding theoretical ones, we have determined the thermal conductivities and thickness, as well as the variations versus wavelength of the optical absorption coefficients and refractive indices of the GaAlAsSb and AlAsSb layers. The refractive indices measured in the transparency region and the small values of the thermal conductivity are in good agreement with theoretical interpolation reported in previous literature.
The electronic structure and lithiation of electrodes based on vanadium–oxide nanotubes
Authors: Augustsson A. , Schmitt T. , Duda L.-C. , Nordgren J. , Nordlinder S. , Edström K. , Gustafsson T. , Guo J.-H.
Published: October 2003

The effect of lithium insertion (lithiation) in electrodes of vanadium oxide nanotubes has been studied using resonant soft-x-ray emission spectroscopy (RSXES) and soft x-ray absorption spectroscopy (SXAS). Spectra at the V L edges were recorded ex-situ on a series of cycled electrodes that had been discharged to different potentials (3.0, 2.5, 2.0, and 1.8 V). Different chemical states upon discharge effectively correspond to different doping levels: the lower the cell potential, the higher the lithiation. The RSXES and SXAS spectra reflect the occupied and unoccupied electronic states, respectively. Spectral changes are observed as a function of the reduction process of vanadium by the electrochemical process. Lithiation induces a gradual enhancement of features attributed to occupied V 3d states in the RSXES spectra, indicating that the intercalated charges occupy bands of V 3d. Both SXAS and RSXES results suggest a partial reduction to V3+ at potentials below 2.0 V and the presence of a mixture of three different oxidation states. RSXES spectra also reveal the low energy-loss feature of 1 eV, suggesting strong correlation of the V 3d orbitals.
Leakage current and relaxation characteristics of highly (111)-oriented lead calcium titanate thin films
Authors: Tang X. G. , Wang J. , Zhang Y. W. , Chan H. L. W.
Published: October 2003

Highly (111)-oriented (Pb0.76Ca0.24)TiO3 (PCT) thin films were grown on Pt/Ti/SiO2/Si substrates by a sol–gel process. The Au/PCT/Pt metal–insulator–metal film capacitor showed well-saturated hysteresis loops at an applied field of 800 kV/cm with remanent polarization (Pr) and coercive electric field (Ec) values of 18.2 μC/cm2 and 210 kV/cm, respectively. The leakage current depended on the voltage polarity. At low electrical field and with Pt electrode biased negatively, the Pt/PCT interface exhibits a Schottky emission characteristics. The Au/PCT interface forms an ohmic contact. The conduction current when the Au electrode is biased negatively shows a space-charge-limited behavior. The dielectric relaxation current behavior of Au/PCT/Pt capacitor obeys the well-known Curie–von Schweidler law at low electric field. At higher fields, the currents have contributions to both dielectric relaxation current and leakage current.
Electron beam generation in a diode with different ferroelectric cathodes
Authors: Krasik Ya. E. , Chirko K. , Sayapin A. , Gleizer J. , Krokhmal A. , Felsteiner J.
Published: October 2003

We present experimental results of electron beam generation in a diode with cathodes made of BaTi solid solution and prepoled and unpoled PZT ferroelectrics without a screening grid in front of them. The diode operates with an accelerating pulse of ∼200 kV and ∼300 ns duration. It was found that although a surface plasma is always formed as a result of the application of a driving pulse, the beginning of the electron emission does not always coincide with the start of the accelerating pulse. Namely, it was shown that the application of the accelerating pulse at the same time with the driving pulse leads to simultaneous electron emission from the surface plasma only in the case of coincidence of the driving and accelerating electric field directions. In the opposite case, electron emission starts only at the end or at the fall of the driving pulse for the BaTi and PZT samples, respectively. Also, it was found that the electron beam current density distribution corresponds to the plasma emission spots which appear at the ferroelectric surface.
Origin of hole-like peaks in current deep level transient spectroscopy of n-channel AlGaAs/GaAs heterostructure field-effect transistors
Authors: Cavallini A. , Verzellesi G. , Basile A. F. , Canali C. , Castaldini A. , Zanoni E.
Published: October 2003

The features of current deep level transient spectroscopy (I-DLTS) spectra are investigated in AlGaAs/GaAs heterostructure field-effect transistors both through experiments and two-dimensional numerical device simulations. Differently from electron traps located in the n-type semiconductor bulk, which can only be detected in emission-mode by I-DLTS, deep levels located at the ungated device surface are shown to be revealed both in emission and capture transients by I-DLTS. Experimental and simulation results indicate that this behavior originates from inherent physical properties of surface deep levels interacting with holes attracted at the ungated surface by the negative charge associated with ionized traps.
Effect of metal underlayers on low temperature silicon growth
Authors: Xu K. , Pradhan A. , Shah S. Ismat
Published: October 2003

Silicon films were deposited on bare glass, copper- and gold-coated glass substrates at 200 °C. X-ray diffraction (XRD) showed that the films deposited on substrates with gold underlayer were polycrystalline while those deposited on bare glass and copper-coated glass had no identifiable crystalline silicon XRD peak. Raman spectroscopy was used to confirm the film’s crystalline properties. The Raman spectra indicated that films deposited on gold-coated glass substrates were composed of predominantly crystalline silicon with small amounts of amorphous silicon. Atomic force microscopy (AFM) was used to study the topography and adatom diffusion on the surface. AFM micrographs showed that the polycrystalline silicon films had grain size up to 95 nm.
Allowed mesoscopic point group symmetries in domain average engineering of perovskite ferroelectric crystals
Authors: Hatch D. M. , Stokes H. T. , Cao W.
Published: October 2003

In multivariant systems, several energetically degenerate low temperature domain states can be produced at the structural phase transition. Coexistence of these domain states can produce mesoscopic structures that possess symmetries distinct from the microscopic single domain crystal symmetry. Such engineered domain structures in certain ferroelectric materials have been proven to give superior piezoelectric properties and extremely soft shear moduli. The objective of this article is to consider the variety of symmetries that can be produced through domain average engineering in proper ferroelectric systems arising from the cubic Pm3̄m symmetry perovskite structure.
Optomechanical effect in ferroelectric liquid crystal freely suspended films
Authors: Yablonskii Serguei V. , Nakano Kazuyuki , Ozaki Masanori , Kozlovsky Mikhail V. , Yoshino Katsumi
Published: October 2003

Optomechanical effect in vibrating ferroelectric liquid crystal freely suspended films have been observed. In the experiment the liquid crystal films are doped with a photochromic dye which undergoes trans-cis isomerization. Photoisomerization induces the change both in a switching potential of the host liquid crystal and the film surface tension. We observed also a nonlinear dependence of photoinduced variation of the surface tension as function of the dye concentration. Such dependence strongly supports the possibility of amplification of photochemical effect in liquid crystals. The optomechanical effect might be exploited for the remote control of the sensors using freely suspended films, and more generally for the investigation of the cooperative phenomena in liquid crystals.
Photocarrier transport in undoped microcrystalline silicon studied by the modulated photocurrent technique
Authors: Hattori K. , Musa Y. , Murakami N. , Deguchi N. , Okamoto H.
Published: October 2003

A detailed investigation of photocarrier transport and localized states in undoped hydrogenated microcrystalline silicon (μc-Si:H) has been performed using the modulated photocurrent (MPC) technique. The MPC-derived drift mobility shows a systematic variation with crystalline grain size and volume fraction, as well as a distinct anisotropy. The physical implications of these observations are discussed in accordance with models that consider connectivity of crystalline grains and geometry of individual grains. The frequency spectra of MPC mobility and their dependence on excitation intensity are also analyzed in comparison with those observed for hydrogenated amorphous silicon (a-Si:H), leading to a common property of localized states that can explain the experimental results for these materials.
Direct observation of alloy scattering of two-dimensional electrons in AlxGa1−xAs
Authors: Li Wanli , Csáthy G. A. , Tsui D. C. , Pfeiffer L. N. , West K. W.
Published: October 2003

We have measured the alloy scattering of two-dimensionally (2D) confined electrons in AlxGa1−xAs in the dilute Al concentration limit. Alloy scattering is independent of temperature below 4.2 K, and from its linear dependence on the Al concentration we obtain a scattering rate of 35 ns−1 per 1% Al impurities. The alloy scattering potential determined from our data, using the Fang–Howard approximation of the wave function for the 2D electrons, is 1.13 eV.
Frequency dependence of the dielectric properties of La-doped Pb(Zr0.35Ti0.65)O3 thin films
Authors: Zhang Z. G. , Chu D. P. , McGregor B. M. , Migliorato P. , Ohashi K. , Hasegawa K. , Shimoda T.
Published: October 2003

Frequency dependence of the dielectric properties of La-doped lead zirconate titanate thin films with Pt and Ir electrodes and two different La doping levels is studied by impedance spectrometry between 10−3 and 104 Hz. A constant phase element with an impedance of Z=A0(jω)−α is introduced into the equivalent circuit, which fits very well both the real and imaginary components of the measured impedance over the full frequency range, including the resonance, using a single α (0≪α<1). Our analysis shows that α is not related to the ferroelectric properties of the films, but it has a monotonic correlation with the real dielectric constant. The close resemblance to the well-known butterfly-shaped C–V curve shown by α with the applied dc bias (α-V) suggests that α may be affected by the density of the domain walls in the film, in addition to various dielectric relaxation processes.
Indium nitride quantum dots grown by metalorganic vapor phase epitaxy
Authors: Briot O. , Maleyre B. , Ruffenach S.
Published: October 2003

With respect to growing indium nitride quantum dots with very low surface densities for quantum cryptography applications, we have studied the metalorganic vapor phase epitaxy of InN onto GaN buffer layers. From lattice mismatch results the formation of self-assembled dots. The effects of the growth temperature, V/III molar ratio, and deposition time are studied, and we demonstrate that quantum-sized dots of InN can be grown with a material crystalline quality similar to the quality of the GaN buffer layer, in densities of 107 to 108 cm−2. Such low densities of dots allow for the realization of experiments or devices in which a single dot is isolated, and may be used in the near future to produce single-photon sources.
Spin-dependent conductance minima in magnetic tunnel junctions
Authors: Xiang X. H. , Zhu T. , Landry G. , Du J. , Zhao Yuwen , Xiao John Q.
Published: October 2003

We present a study on the positions of conductance minima for the parallel and antiparallel magnetization configurations in magnetic tunnel junctions, both experimentally and theoretically. It is found that the conductance minima can have as much as a 100-mV shift from zero bias, and the shifts are spin dependent. These behaviors have not been observed in tunnel junctions based on nonmagnetic electrodes. By considering the voltage dependent density of states of ferromagnetic electrodes in the Brinkman model, the difference can be well explained. The results demonstrate that the density of states of the ferromagnetic electrodes play an important role in defining the bias dependence behaviors in magnetic tunnel junctions.
Improved ferroelectric and pyroelectric properties in Mn-doped lead zirconate titanate thin films
Authors: Zhang Q. , Whatmore R. W.
Published: October 2003

We have investigated the effects of Mn doping on the ferroelectric and pyroelectric properties of Pb(Zr0.3Ti0.7)O3 (PZT) thin films on substrates Pt/Ti/SiO2/Si. The Mn-doped (1 mol %) PZT (PMZT) showed almost no hysteretic fatigue up to 1010 switching bipolar pulse cycles, coupled with excellent retention properties. We present evidence that while a low permittivity interfacial layer forms between the Pt electrode and PZT films, this does not occur in PMZT. We propose that Mn dopants are able to reduce oxygen vacancy mobility in PZT films and Mn2+ ions consume the oxygen vacancies generated during repeated switching, forming Mn4+ ions. These mechanisms are probably responsible for their low observed fatigue characteristics. Mn doping brings additional benefits to the electrical properties of PZT films. The relevant pyroelectric coefficients (p) of a 700 nm thick film are 3.52×10−4 C m−2 K−1 and detectivity figures of merit FD=3.85×10−5 Pa−0.5 at 33 Hz for Mn-doped PZT, compared with p=2.11×10−4 C m−2 K−1 and FD=1.07×10−5 Pa−0.5 for the undoped PZT films. This means that the Mn-doped PZT thin films are excellent candidates as device materials for both memory and pyroelectric applications.
Nanofluids containing multiwalled carbon nanotubes and their enhanced thermal conductivities
Authors: Xie Huaqing , Lee Hohyun , Youn Wonjin , Choi Mansoo
Published: October 2003

Multiwalled carbon nanotubes (CNTs) as produced are usually entangled and not ready to be dispersed into fluids. We treated CNTs by using a concentrated nitric acid to disentangle CNT aggregates for producing CNT nanofluids. Oxygen-containing functional groups have been introduced on the CNT surfaces and more hydrophilic surfaces have been formed during this treatment, which enabled to make stable and homogeneous CNT nanofluids. Treated CNTs were successfully dispersed into polar liquids like distilled water, ethylene glycol without the need of surfactant and into nonpolar fluid like decene with oleylamine as surfactant. We measured the thermal conductivities of these nanotube suspensions using a transient hot wire apparatus. Nanotube suspensions, containing a small amount of CNTs, have substantially higher thermal conductivities than the base fluids, with the enhancement increasing with the volume fraction of CNTs. For the suspensions with the same loading, the enhanced thermal conductivity ratios are reduced with the increasing thermal conductivity of the base fluid. Comparison between the experimental data and the theoretical model indicates that the thermal conductivities of nanotube suspensions seem to be very dependent on the interfacial layer that exists between the nanotube and the liquid.
Transition metal ion implantation into AlGaN
Authors: Frazier R. M. , Thaler G. T. , Abernathy C. R. , Pearton S. J. , Nakarmi M. L. , Nam K. B. , Lin J. Y. , Jiang H. X. , Kelly J. , Rairigh R. , Hebard A. F. , Zavada J. M. , Wilson R. G.
Published: October 2003

n- and p-type AlxGa1−xN (x=0.38 for n-type, x=0.13 for p-type) layers grown on Al2O3 substrates were ion implanted with the transition metals Mn, Cr, and Co at high concentrations (peak doping levels ∼3 at. %). After implantation and annealing at 1000 °C, only impurity transitions at ∼2.9 and 3.9 eV and no band-edge photoluminescence could be observed in all the samples. X-ray diffraction did not detect any peaks associated with second phase formation. Room-temperature hysteresis loops were obtained for Co-implanted n-type AlGaN, while there was no convincing evidence for ferromagnetism in the Mn- or Cr-implanted n-AlGaN. By sharp contrast, Mn implantation in p-AlGaN did produce ferromagnetic behavior and 300 K hysteresis. Both carrier type and crystalline quality can influence the resulting magnetic properties.
Secondary electron emission and self-consistent charge transport and storage in bulk insulators: Application to alumina
Authors: Meyza X. , Goeuriot D. , Guerret-Piécourt C. , Tréheux D. , Fitting H.-J.
Published: October 2003

The self-consistent charge transport in bulk alumina samples during electron beam irradiation is described by means of an iterative computer simulation. Ballistic electron and hole transport as well as their recombination and trapping are included. As a main result the time-dependent secondary electron emission rate σ(t) and the spatial distributions of currents j(x,t), charges ρ(x,t), the field F(x,t), and the potential slope V(x,t) are obtained. For bulk insulating samples, the time-dependent distributions approach the final stationary state with j(x,t)=const=0 and σ=1. Especially for low electron beam energies E0=1 keV, the incorporation of charges can be controlled by the potential VG of a vacuum electrode in front of the target surface. Finally, for high electron beam energies, the real negative surface potential V0<0 is measured by x-ray bremsstrahlung spectra and the shift of the short wavelength edge. For the initial beam energy E0=30 keV, the experimental value V0=−16 kV is still in good agreement with our simulations.
Effect of magnetic bias field on magnetoelectric coupling in magnetoelectric composites
Authors: Liu Y. X. , Wan J. G. , Liu J.-M. , Nan C. W.
Published: October 2003

The effect of dc magnetic bias field on the magnetoelectric coupling of a two-component magnetoelectric composite structure is investigated numerically using the finite-element method, in which the nonlinear magnetostress coupling for the magnetostrictive component is taken into account. It is shown that the magnetostress coupling coefficient increases first and then falls down with increasing of the bias field, and this behavior is argued to be responsible for the dependence of magnetoelectric yield on the bias field. The numerical modeling using the ANSYS5.5 finite element algorithm for the Tb0.3Dy0.7Fe1.9-epoxy/Pb(Zr0.52Ti0.48)O3-epoxy composite structure gives fairly consistent results with the experiments.
Anomalous temperature dependence of optical emission in visible-light-emitting amorphous silicon quantum dots
Authors: Kwack Ho-Sang , Sun Yuanping , Cho Yong-Hoon , Park Nae-Man , Park Seong-Ju
Published: October 2003

We have investigated optical properties of amorphous silicon quantum dots (a-Si QDs) embedded in a silicon nitride grown on (100) silicon substrates by plasma-enhanced chemical vapor deposition. The photoluminescence (PL) emission colors of red, orange, and blue were shown for the a-Si QDs with average dot sizes of 5.8, 2.4, and 1.3 nm, respectively. The increase in the PL peak energy with decreasing QD size was attributed to the quantum confinement effect in a-Si QDs. An anomalous temperature dependence of the PL intensity and PL peak energy was observed for a-Si QDs. The visible-light PL emission from a-Si QDs is most efficient at near room temperature, and the PL spectral shape and peak energy are almost independent of temperature, which are most favorable for light-emitting device applications. From time-resolved PL experiments, an enhancement of phonon-assisted indirect radiative recombination was proposed to explain the anomalous temperature dependence.
Avalanche ballistic electron emission microscopy with single hot-electron sensitivity
Authors: Heller E. R. , Tivarus C. , Pelz J. P.
Published: October 2003

We discuss an implementation of ballistic electron emission microscopy (BEEM), in which the metallic or metal–insulator “stack” of interest is formed directly over an avalanche p–n diode. This allows nanometer-resolution studies of hot-electron transport through technologically important device stacks with up to single electron sensitivity and >10 kHz measurement bandwidth when the avalanche diode is cooled to <200 K.
Measurement of gigawatt radiation pulses from a vacuum and extreme ultraviolet free-electron laser
Authors: Richter M. , Gottwald A. , Kroth U. , Sorokin A. A. , Bobashev S. V. , Shmaenok L. A. , Feldhaus J. , Gerth Ch. , Steeg B. , Tiedtke K. , Treusch R.
Published: October 2003

In order to measure the photon flux of highly intense and extremely pulsed vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) radiation in absolute terms, we have developed a gas-monitor detector which is based on the atomic photoionization of a rare gas at low particle density. The device is indestructible and almost transparent. By first pulse-resolved measurements of VUV free-electron laser radiation at the TESLA test facility in Hamburg, a peak power of more than 100 MW was detected. Moreover, the extended dynamic range of the detector allowed its accurate calibration using spectrally dispersed synchrotron radiation at much lower photon intensities.
Magnetic properties and glass formability of Fe61Co10Zr5W4B20 bulk metallic glassy alloy
Authors: Pawlik P. , Davies H. A. , Gibbs M. R. J.
Published: October 2003

Amorphous melt spun ribbon samples of Fe61Co10Zr5W4B20 alloy of various thicknesses up to ∼350 μm and suction cast 1 mm diameter rod and 2, 3, and 4 mm outside diameter tube samples of the same alloy were produced. The experimental results for the rod samples indicate a relatively high saturation polarization μ0Ms=0.8 T and a Curie temperature TC=615 K, as well as a very low coercive field Hc=1.4±0.5 A/m with high anisotropy that lowers the permeability of the samples. Thermal analysis by differential scanning calorimetry carried out on ribbon and rod samples revealed a large supercooled liquid region ΔTx of ∼60 K for this alloy.
Alternated high- and low-pressure nitriding of austenitic stainless steel: Mechanisms and results
Authors: Gomes G. F. , Ueda M. , Reuther H.
Published: October 2003

A combined surface modification treatment consisting of ion nitriding at high pressure and high temperature, followed by a cycle at low pressure, both cycles using a gas mixture of (N2/H2):(50/50) in pressure, was applied to stainless-steel AISI 304. In the first cycle, in a glow discharge at 4×10−1 mbar and temperatures of 400–450 °C, high-pressure nitriding was applied to the samples. In the second cycle, in a glow discharge at 8×10−4 mbar, low-pressure nitriding was applied to the samples. Applying this sequential hybrid treatment alternately, good nitriding was obtained. X-ray diffraction (XRD) measurements showed the effects of this hybrid ion nitriding in the AISI 304 surface, indicating thick nitrided layers, confirmed by Auger electron spectroscopy (AES). Conversion electron Mössbauer spectroscopy, combined with AES and XRD, showed phases and compounds induced by such treatments. Vickers hardness measurements showed great enhancement in the surface hardness. Applying other combinations of gas mixtures and cycles, produced diverse results in the surface, like the induction of α- and ε-phases.
Spatial diffusion of excitons in n-type modulation-doped (Cd,Mn)Te/(Cd,Mg)Te single quantum wells under magnetic fields
Authors: Takano Fumiyoshi , Akinaga Hiro , Tokizaki Takashi , Kuroda Shinji , Takita Kôki
Published: October 2003

Time-resolved and spatially resolved photoluminescence (PL) measurements were performed on the (Cd,Mn)Te-based two-dimensional (2D) electron gas system at 4.2 K under magnetic fields up to 0.5 T. The detailed characteristics of the PL spatial extent under these magnetic fields were strongly dependent upon the intrinsic 2D carrier density. We found that, in the lower carrier density sample, the PL spatial extent increased with an increase in the magnetic fields. This is contrary to the result for the nonmagnetic system. These phenomena are qualitatively interpreted by considering the competition of the mobile neutral exciton (X), localized negatively-charged exciton (X−) and, furthermore, the suppression of a free-exciton-magnetic-polaron formation, which is one of the characteristic properties of diluted magnetic semiconductors such as (Cd,Mn)Te.
Analysis of the radio-frequency single-electron transistor with large quality factor
Authors: Turin Valentin O. , Korotkov Alexander N.
Published: October 2003

We have analyzed the response and noise-limited sensitivity of the radio-frequency single-electron transistor (rf-SET), extending the previously developed theory to the case of arbitrary large quality factor Q of the rf-SET tank circuit. It is shown that while the rf-SET response reaches the maximum at Q roughly corresponding to the impedance matching condition, the rf-SET sensitivity worsens monotonically with the increase of Q. In addition, we propose an operation mode in which an overtone of the incident rf wave is in resonance with the tank circuit.
Inhomogeneous evolution of a glass surface via free, rapid expansion
Authors: Sharma Anju , Jain Himanshu , Carnali Joseph O. , Jain Isha H.
Published: October 2003

We have proposed a mechanism of inhomogeneous evolution of a viscous fluid at elevated temperature from rapid, free expansion/stretching, which gives rise to regular structural and/or chemical defects on the surface. Support for this phenomenon is demonstrated by a simple experiment consisting of rapidly blowing a film-coated glass tube and studying its microstructure in detail. The surface of the blown glass exhibits a regular pattern of “surface cracking,” indicating nonuniform expansion of the molten glass. This cracking is attributed to the strong dependence of viscosity on temperature, and is likely to occur in a variety of viscoelastic-forming operations.
Formation of porous silicon through the nanosized pores of an anodized alumina template
Authors: Das Biswajit , McGinnis S. P.
Published: October 2003

This letter demonstrates the ability to fabricate porous silicon through the nanoscale pores of an anodized alumina template formed on a silicon substrate. The alumina template acts as a protective coating on the porous silicon surface increasing its mechanical integrity, thus eliminating the need for delicate handling that is usually required for this material system. The optical properties of the porous silicon formed through the template show characteristics similar to that of normally formed porous silicon. In addition, the porous silicon optical properties show very little degradation with time; the alumina coating is believed to contribute to this improved stability. The results are significant and can increase the likelihood of commercial applications of this material.
Pressure-induced re-entrant electronic and magnetic state in Pr0.7Ca0.3MnO3 manganite
Authors: Cui Congwu , Tyson Trevor A.
Published: October 2003

In Pr0.7Ca0.3MnO3, pressure induces re-entrant magnetic and electronic state changes in the range from 1 atm to ∼6 GPa. The metal–insulator and magnetic transition temperatures coincide from ∼1 to 5 GPa, decouple outside of this range, and do not change monotonically with pressure. The effects may be explained by pressure-tuned competition between double exchange and super exchange. The insulating state induced by pressure above ∼5 GPa is possibly ferromagnetic, different from the ferromagnetic and antiferromagnetic phase-separated insulating state below ∼0.8 GPa.
Functional field transformation with irregular waveguide structures
Authors: Yang Ming-Chuan , Li Jia-Han , Webb Kevin J.
Published: October 2003

We demonstrate functional field transformation in irregular waveguide structures having strong scatter from a stepped conducting wall geometry. The resulting degrees of freedom and significant evanescent field content make it possible to accomplish virtually perfect transformation in a distance of several wavelengths. A microwave experiment verifies frequency-dependent transformation, suggesting that wavelength division multiplexing based on this principle is possible. A mode-selective transformation, which exhibits disparate reflectivities for different modes at the same frequency, may find application in multimode laser cavities.
Observation of 430 nm electroluminescence from ZnO/GaN heterojunction light-emitting diodes
Authors: Alivov Ya. I. , Van Nostrand J. E. , Look D. C. , Chukichev M. V. , Ataev B. M.
Published: October 2003

In this work, we report on the growth, fabrication, and device characterization of wide-band-gap heterojunction light-emitting diodes based on the n-ZnO/p-GaN material system. The layer structure is achieved by first growing a Mg-doped GaN film of thickness 1 μm on Al2O3(0001) by molecular-beam epitaxy, then by growing Ga-doped ZnO film of thickness 1 μm by chemical vapor deposition on the p-GaN layer. Room-temperature electroluminescence in the blue-violet region with peak wavelength 430 nm is observed from this structure under forward bias. Light–current characteristics of these light-emitting diodes are reported, and a superlinear behavior in the low current range with a slope 1.9 and a sublinear behavior with a slope 0.85 in the high current range are observed.
Strong near-infrared photoluminescence and absorption from Si/Si1−xGex type-II multiple quantum wells on bulk crystal SiGe substrates
Authors: Sheng S. R. , Rowell N. L. , McAlister S. P.
Published: October 2003

We report a study of photoluminescence (PL) and optical absorption in high-quality tensile-strained Si type-II multiple quantum wells (QWs) grown on bulk crystal SiGe substrates by using low-temperature ultrahigh-vacuum chemical vapor deposition. Detailed PL experiments, as a function of excitation density together with applied uniaxial stress, were performed to study the band alignment and to help elucidate the origin of the observed PL peaks. Spatially direct and indirect transitions of the Si QWs and an intense broad subgap PL were observed. With increasing excitation power, the PL lines of Si QWs shift to lower energies, whereas the broad PL shifts to higher energy. The applied [110] compressive stress results in a redshift of the substrate PL lines, as expected, but a blueshift of both the Si QW PL and the broad PL lines. The optical absorption data also show strong absorption in the near-infrared region where the broad PL was observed. Near the absorption edge (Eg), the energy dependence obeys a square law rather than a square root law, suggesting that Eg is associated with a quasi-direct transition. The Eg value deduced according to the square law agrees well with the broad PL peak energy provided that temperature-dependent Eg, excitonic binding energy, and exciton localization energy, as well as quantum confinement are considered.
Effect of grain structure on the onset of diffusion-controlled stress relaxation in Pt thin films
Authors: Hyun S. , Vinci R. P. , Fahey K. P. , Clemens B. M.
Published: October 2003

The thermomechanical behavior of Pt thin films was studied using the substrate curvature technique. Films of identical thickness but different grain size were prepared by sputtering directly onto a thermally grown SiO2 layer on a (100) Si wafer. Predictions from a thin-film version of the deformation mechanism map model and the diffusional wedge are compared to the experimental results. The diffusional wedge model most accurately predicts the onset of diffusion-controlled stress relaxation in columnar Pt films. The use of an effective film thickness equal to the grain size improves the match to noncolumnar microstructures.
Monolithic integration of spectrally selective uncooled lead selenide detectors for low cost applications
Authors: Diezhandino J. , Vergara G. , Pérez G. , Génova I. , Rodrigo M. T. , Sánchez F. J. , Torquemada M. C. , Villamayor V. , Plaza J. , Catalán I. , Almazán R. , Verdú M. , Rodrı́guez P. , Gómez L. J. , Montojo M. T.
Published: October 2003

We report the manufacture of an uncooled infrared detector with its natural spectral response modified as design. Thanks to a technology to process uncooled polycrystalline PbSe sensors, we have integrated a filter and a PbSe detector monolithically. Our processing is based on thermal deposition of a thin PbSe layer followed by a specific thermal treatment in an iodine-rich atmosphere that turns PbSe into an infrared sensitive material. Using this technique, we are able to process uncooled medium-wave infrared detectors directly on the last layer of an interference filter. After this achievement, polycrystalline PbSe detectors will be smarter, more reliable, and exhibit better characteristics than their direct competitors (near room-temperature cadmium mercury telluride, InAsSb, etc.). Standard photolithography, interference filter deposition, and dry etching techniques are fully compatible with this polycrystalline PbSe processing. These are essential facts in order to obtain reduced filter and detector sizes (multicolor arrays). The technology described in this work is useful in the field of low cost infrared detection.
Tunable two-photon pumped lasing using a holographic polymer-dispersed liquid-crystal grating as a distributed feedback element
Authors: He Guang S. , Lin Tzu-Chau , Hsiao Vincent K. S. , Cartwright Alexander N. , Prasad Paras N. , Natarajan L. V. , Tondiglia V. P. , Jakubiak R. , Vaia R. A. , Bunning T. J.
Published: October 2003

A holographic polymer-dispersed liquid-crystal (H-PDLC) grating film was employed as an angle-dependent and narrow spectral-band feedback control element for two-photon pumped lasing in a dye solution, 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4′-(6-hydroxyhexyl sulfonyl) stilbene (APSS) in dimethyl sulphoxide. The grating film contained about 80 layers of liquid-crystal domains periodically dispersed in an ∼15 μm thick polymer film, featuring a maximum reflectance of 75% at 561 nm position with an ∼9 nm spectral bandwidth. The output lasing wavelength could be tuned from 561.5 to 548.5 nm and the lasing bandwidth changed from 5 to 3 nm when the incidence angle on the grating film varied from 0° to 22°. The overall lasing efficiency was measured to be 10%.