bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of green light-induced infrared absorption in KTiOPO4 and periodically poled KTiOPO4
Authors: Wang S. , Pasiskevicius V. , Laurell F.
Published: August 2004

Induced thermal-lens spectroscopy with a common-path interferometer has been used for high-sensitivity measurements of green-light induced absorption dynamics in single-domain and periodically poled KTiOPO4. Although the linear infrared absorption in both materials is the same, the saturated green-light induced absorption is consistently higher in periodically poled crystals. Initial relaxation of the induced absorption is also different in these materials where periodically poled KTP does not exhibit as fast initial relaxation characteristic as the single domain KTP. These differences in the absorption are explained by higher concentration of native defects associated with weakly bonded K+ ions, which stabilize electron and hole traps forming color centers in the material. Unannealed as-poled crystals also exhibit characteristic overshoot dynamics in the induced thermal lens signal, which signifies remnant stress in the domain walls. Investigations showed that the lattice vibration energy generated by the dissociation of induced color centers is sufficient for complete relaxation of this remaining stress. The sensitivity of the measurement technique is demonstrated by the measurement of small signal absorption in the blue spectral region arising during continuous wave quasi-phase-matched second harmonic generation in periodically poled KTP.
1.3 to 1.5 μm light emission from InGaAs/GaAs quantum wells
Authors: Wang S. M. , Zhao Q. X. , Wang X. D. , Wei Y. Q. , Sadeghi M. , Larsson A.
Published: August 2004

We propose using dipole δ-doping across highly strained InGaAs/GaAs quantum wells (QWs) to achieve light emission at wavelengths in the range of 1.3–1.55 μm. For In0.3Ga0.7As/GaAs single QWs, we demonstrate that the photoluminescence (PL) wavelength increases with the doping concentration. With a transition energy reduction as large as 370 meV, PL emission at 1.548 μm at room temperature is realized. Broad area lasers with dipole δ-doping reveal longer lasing wavelengths than those of undoped lasers, although the wavelength redshift is much less than in the PL case.
Optical and magnetic properties of Mn+-implanted neutron-transmutation-doped GaAs bulks
Authors: Kwon Y. H. , Shon Y. , Lee W. C. , Fu D. J. , Jeon H. C. , Kang T. W. , Kim T. W. , Fan X. J.
Published: August 2004

( Ga 1−x Mn x ) As thin films with both semiconducting and magnetic properties were formed by using Mn+ ion implantation and thermal annealing with a goal of producing (Ga1−xMnx)As with a high ferromagnetic transition temperature Tc. Energy dispersive x-ray fluorescence measurements showed that the Mn+-implanted neutron-transmutation-doped (NTD) GaAs samples were (Ga1−xMnx)As thin films, and photoluminescence spectra showed that the annealed (Ga1−xMnx)As thin films were p-type semiconductors. The magnetization curve as a function of the magnetic field showed that ferromagnetism existed in the (Ga1−xMnx)As thin films, and the magnetization curve as a function of the temperature showed that the Tc was ≈140 K. The present results indicate that high-Tc (Ga1−xMnx)As thin films with both semiconducting and magnetic properties can be formed from NTD-GaAs bulks by using Mn+ implantation and annealing method.
Simulations of the effects of tin composition gradients on the superconducting properties of Nb3Sn conductors
Authors: Cooley L. D. , Fischer C. M. , Lee P. J. , Larbalestier D. C.
Published: August 2004

In powder-in-tube Nb3Sn composites, the A15 phase forms between a central tin-rich core and a coaxial Nb tube, thus causing the tin content and superconducting properties to vary with radius across the A15 layer. Since this geometry is also ideal for magnetic characterization of the superconducting properties with the field parallel to the tube axis, a system of concentric shells with varying tin content was used to simulate the superconducting properties, the overall severity of the Sn composition gradient being defined by an index N. Using well-known scaling relationships and property trends developed in an earlier experimental study, the critical current density for each shell was calculated, and from this the magnetic moment of each shell was found. By summing these moments, experimentally measured properties such as pinning-force curves and Kramer plots could be simulated. We found that different tin profiles have only a minor effect on the shape of Kramer plots, but a pronounced effect on the irreversibility fields defined by the extrapolation of Kramer plots. In fact, these extrapolated values HK are very close to a weighted average of the superconducting properties across the layer for all N. The difference between HK and the upper critical field commonly seen in experiments is a direct consequence of the different ways measurements probe the simulated Sn gradients. Sn gradients were found to be significantly deleterious to the critical current density Jc, since reductions to both the elementary pinning force and the flux pinning scaling field HK compound the reduction in Jc. The simulations show that significant gains in Jc of Nb3Sn strands might be realized by circumventing strong compositional gradients of tin.
Resonant Raman scattering of a single layer of Si nanocrystals on a silicon substrate
Authors: Wellner A. , Paillard V. , Coffin H. , Cherkashin N. , Bonafos C.
Published: August 2004

We report Raman spectra of a single layer of silicon nanoparticles, spatially ordered in SiO2 at a tunneling distance from a silicon substrate. This is achieved by exploiting effects which enhance the nanocrystal signal, while suppressing the substrate one. The method is applied to investigate the structure of ion-implantation-produced Si nanoparticles annealed under different conditions. The results, which are in good agreement with transmission electron microscopy data, are used to explain photoluminescence measurements.
Structural and electrical characterization of dense lead zirconate titanate ceramics synthesized by the oxidant-peroxo wet-chemical route
Authors: Camargo Emerson R. , Souza Flavio L. , Leite Edson R. , Kakihana Masato
Published: August 2004

Powders wet-chemically synthesized by the “oxidant-peroxo method” with PbZr05Ti0.5O3 composition were sintered at 1000 and 1100°C for 2 and 4h, resulting in dense ceramic bodies that were structurally characterized by Raman scattering spectroscopy and by x-ray diffraction at room temperature. Their electrical properties (dielectric constant and dielectric loss) were measured in the temperature range from 25 to 450°C at different current frequencies (1, 10, and 100kHz). Microstructures were observed using a scanning electronic microscopy equipped with a x-ray energy dispersive spectrometer for chemical analysis. It was observed that the sintered bodies show tetragonal structure, and apparent densities calculated as 90%–94% of the theoretical density.
Color tuning and efficiency enhancement through interfacial energy transfer and thickness control in light-emitting diodes
Authors: Ding Liming , Karasz Frank E.
Published: August 2004

We report a possible interfacial energy transfer phenomenon in double-layer light-emitting diodes (LEDs) consisting of a poly(p-phenylene vinylene) (PPV) hole-transport layer and an emitting polymer layer containing either of two blue emitting polymers (I and II). The polymers in film form have almost the same photoluminescence (PL) spectra though they have different band gaps as determined from their absorption and PL spectra. This is due to intermolecular∕intramolecular energy transfer in polymer II films since the latter structure contains two differing oligomeric chromophores regularly interspersed by long nonconjugated blocks. In double-layer LEDs, PPV functions not only as a hole-transport layer which improves hole injection but also as a chromophore participating in the tuning of the LED emission color through interfacial energy transfer. The significant spectral overlap between the absorption spectrum of PPV and the emission spectra of I or II supports this energy transfer. The presence of PPV can modify the LED color from blue to blue-green or green and also contributes to a high external quantum efficiency. Poly(3,4-ethylenedioxythiophene)∕poly(styrene sulfonate) was also used as a hole-transport layer and its LEDs were compared to PPV-containing devices. Thicker films of the conjugated polymer chromophores produced red shifts in both PL and electroluminescence (EL) spectra which was attributed to molecular packing and self-absorption effects. At higher applied voltage, these LEDs showed blue shifts, and the emission corresponding to a higher energy transition was enhanced at the expense of emissions from the lower energy transitions. Chromophore heating is also held responsible for some of the blueshifts in EL. The thicker LEDs showed lower efficiency because of self-absorption.
Study of structure and luminescence of CdSe Nanocrystals obtained by ball milling
Authors: Urbieta A. , Fernández P. , Piqueras J.
Published: August 2004

The effect of mechanical milling on the structure and luminescent properties of CdSe powder has been investigated by means of cathodoluminescence (CL) in the scanning electron microscope, transmission electron microscopy (TEM) and x-ray diffraction. The starting powders were ball milled for times between 5 and 20h. TEM of the milled samples reveals the presence of nanocrystals with a wide range of sizes including nanocrystals of 10nm and smaller. The CL spectra of the untreated samples show the characteristic band edge emission centered at 1.8eV. In the milled samples a band centered at about 2.5eV is observed which appears to be associated to the presence of nanocrystals. The evolution of defect structure with milling time has been monitored by recording infrared cathodoluminescence spectra. A partial phase transformation from wurtzite to zinc blende structure also has been observed.
Interfacial properties of strained piezoelectric InGaAs∕GaAs quantum wells grown by metalorganic vapor phase epitaxy on (111)AGaAs
Authors: Cho Soohaeng , Sanz-Hervás A. , Kim Jongseok , Majerfeld A. , Kim B. W.
Published: August 2004

In this work we employed photoreflectance spectroscopy over the temperature range 11-300K to investigate the heterointerfaces of a strained piezoelectric InGaAs∕GaAs single quantum well structure grown on a (111)AGaAs substrate by metalorganic vapor phase epitaxy. Photoreflectance spectroscopy measurements in combination with a theoretical analysis using the quantum well structural parameters obtained by high-resolution x-ray diffractometry enabled us to evaluate separately the abruptness and roughness of the quantum well interfaces. The excellent agreement between the experimental and calculated transition energies for a quantum well structure with a well width of 41Å and 13% In demonstrates that the heterointerfaces are abrupt. From a theoretical analysis of the temperature dependence of the photoreflectance broadening parameters, based on the Bose-Einstein phonon-coupling model, we determined the longitudinal optical phonon energy and the electron-phonon coupling strength. This analysis shows an interface roughness of less than ±1 monolayer. These results in conjunction with the observation of a narrow photoluminescence linewidth of 9.1meV indicate the achievement of essentially atomically smooth interfaces in a highly strained (∼1%) piezoelectric InGaAs∕GaAs quantum well structure.
Helical multishell structures of magnesium nanowires
Authors: Hui Li , Pederiva F. , Guanghou Wang , Baolin Wang
Published: August 2004

The structures of free-standing magnesium nanowires are systematically studied by using genetic algorithm simulations based on molecular dynamics with a tight-binding many body potential. Several helical multishell cylindrical growth sequences are observed for magnesium nanowires. The numbers of atomic strands show the even-odd or odd-even coupling between the inner and outershell with the differences of five strands, These multishell structures are made up of coaxial tubes in the three- and four-strands helical, centered pentagonal and hexagonal, and double-chain-core parallel curved surface epitaxy. The average binding energy and coordination numbers are calculated. The angular correlation functions and vibrational properties of magnesium nanowire have also been studied.
Back-switching of ferroelectric polarization in two-component systems
Authors: Fedosov Sergei N. , Seggern Heinz von
Published: August 2004

It is shown by the example of poly(vinylidene fluoride) that back-switching of ferroelectric polarization in two-component systems during short circuiting after initial poling depends on the poling time and exhibits a maximum in samples poled for about 100μs. The switched back polarization decreases to zero for poling times smaller than 1μs and times exceeding 1s. The phenomenon is explained on the basis of a model proposed earlier for polarization buildup in two-component ferroelectrics taking into account the role of conductivity induced interfacial charge accumulation at the boundaries between ferroelectric and nonferroelectric components.
Improved spectral robustness of triple tandem solar cells by combined series/parallel interconnection
Authors: Trupke T. , Würfel P.
Published: August 2004

An improved interconnection of individual solar cells within a two-terminal triple-tandem solar cell is discussed. Limiting efficiency calculations for this modified triple-tandem solar cell, in which two series-connected bottom cells are connected in parallel to the top cell, are presented and compared to the results that are obtained for a conventional fully series-connected tandem cell. It is shown that the variation of the limiting efficiency with changing illumination conditions is smaller with the improved interconnection scheme, i.e., the latter is spectrally more robust, which is particularly important for terrestrial applications. It is also shown that, fundamentally, high efficiencies may be obtained with this scheme over a much wider range of material combinations, compared to a series-connected tandem. The limiting efficiency calulations are presented for different degrees of idealization and some technical issues involved in the realization of the suggested scheme are discussed.
Ferroelectric properties and leakage current characteristics of radio-frequency-sputtered SrBi2(V0.1Nb0.9)2O9 thin films
Authors: Ezhilvalavan S. , Samper Victor , Seng Toh Wei , Junmin Xue , Wang John
Published: August 2004

The ferroelectric properties and leakage current mechanisms of polycrystalline SrBi2(V0.1Nb0.9)2O9(SBVN) thin films, which were deposited on Pt∕SiO2∕n-Si substrate by rf-magnetron sputtering and then annealed at 700°C for 60min in air, were investigated. These SBVN films showed excellent ferroelectric properties in terms of a large remnant polarization (2Pr) of ∼25μC∕cm2(2Ec∼200kV∕cm), fatigue free characteristics up to ⩾108 switching cycles and a low current density of 10−8A∕cm2 at 100kV∕cm. X-ray diffraction and scanning electron microscope investigations indicate that the sputtered films exhibit a dense, well crystallized microstructure having random orientations and with a rather smooth surface morphology. The improved ferroelectric and leakage current characteristics obtained at the low processing temperature are attributed to the larger polarizability attained through increased rattling space in the distorted Nb(V)O6 of the perovskite block due to the partial substitution of Nb with smaller V ions. The leakage current density of the SBVN thin films was studied at higher temperatures and the data were fitted with the Schottky and Poole-Frenkel emission models.
Molecular dynamics simulations of boron diffusion in SiGe
Authors: Wang Liguo , Clancy Paulette
Published: August 2004

Transient enhanced diffusion (TED) of boron poses a limit to the formation of ultrashallow junctions. To suppress TED of boron, other species (e.g., Ge) can be introduced into the Si substrate. Experiments have shown that boron diffusivity decreased rapidly as Ge concentration increased to 40% Ge. There is no consensus on the cause for this decrease in diffusivity. Here, an ab initio derived energy database was used to fit classical potential models in order to simulate boron diffusion in SiGe on larger length and time-frames than are possible using ab initio models. In this paper, a set of Stillinger-Weber potential parameters for Ge-B and Si-Ge-B has been constructed, allowing a molecular dynamics study of boron diffusion in SiGe alloys to be carried out. Molecular dynamics simulations of boron B diffusion in Si compared to that in SiGe alloys suggest that different trapping mechanisms dominate: B in Si is trapped in substitutional positions, whereas B in SiGe alloys is trapped in interstitial positions. The number of boron interstitials increases as Ge concentration increases, reaches a maximum at 50% Ge, and then decreases as the amount of Ge increases to 100%. Concordantly, the number of Si∕Ge interstitials followed the opposite trend: Their numbers decreased, reached a minimum at 50% Ge and then increased again as the Ge concentration increased. This confirmed ab initio predictions and provided an exploration to the origin of retarded boron diffusion in SiGe alloys.
Electrical conductance of carbon nanotori in contact with single-wall carbon nanotubes
Authors: Chou Y. Y. , Guo G.-Y. , Liu Lei , Jayanthi C. S. , Wu S. Y.
Published: August 2004

The realization of the potential of carbon nanotori as elements of nanoscale devices based on their recently predicted unusual properties requires a thorough understanding of contacting these nanostructures. We carried out a series of calculations of the electric conductance of carbon nanotori contacted by single-wall carbon nanotubes to shed light on the effects of the geometry as well as the chemistry of the contacts. The relaxed structures of the contacted nanotori were determined by an order-N nonorthogonal tight-binding molecular dynamics scheme. The conductance was calculated using the Landauer-Büttiker formula based on a π-orbital Hamiltonian. We found that the conductance of the contacted carbon nanotorus is very sensitive to the transparency (chemistry) of the contacts. We also found that the equivalence (chemistry as well as geometry) of the contacts plays an important role in the transport properties. For example, a difference in the right contact and left contact will diminish the constructive quantum interference of the transmission as compared to the situation when the two contacts are equivalent. This conclusion is general and is expected to be applicable for any metallic contacts to the contacted carbon nanotori.
Nondestructive determination of layers producing Franz-Keldysh oscillations appearing in photoreflectance spectra of heterojunction bipolar transistor structures based on their line-shape analysis
Authors: Takeuchi Hideo , Yamamoto Yoshitsugu , Nakayama Masaaki
Published: August 2004

We report on the demonstration that layers producing Franz-Keldysh oscillations (FKOs) in photoreflectance (PR) spectra of multiple-layer structures can be nondestructively determined through the line-shape analysis. At first, we discuss PR spectra of two kinds of AlGaAs∕GaAs heterojunction bipolar transistor (HBT) structures with a difference in the base-layer thickness. We have found that the phase of FKOs is sensitive to the total layer thickness over the interface associated with the FKOs. In order to analyze the FKO phase, we have derived a calculation model for the line shape of the FKOs, taking account of the probe-light interference effect. The phase of the calculated FKO profile agrees with that of the measured spectra, which supports the appropriateness of the present model. The calculation model has been also applied to the analysis of FKOs from an InGaP∕GaAs HBT structure. The calculation model well reproduces the phase of the FKOs from the base-emitter region as well as that of the FKOs from the base-collector region. In addition, we have demonstrated that the band-gap energy of the InGaP emitter layer can be precisely estimated from the FKOs with the use of the phase factor taking account of the interference effect.
Optimizing the nematic liquid crystal relaxation speed by magnetic field
Authors: Wang Bin , Wang Xinghua , Bos Philip J.
Published: August 2004

Nematic liquid crystalline materials have been widely used in electro-optical control of visible light. However, when IR light is considered, the thickness of the liquid crystal layer must be increased which causes the relaxation time of the material to be slower than required for many applications. In this paper we use a magnetic field to increase the speed of thick nematic devices. We show that above a particular magnetic field strength, thicker cells relax more quickly than thinner ones. Also, we find that there exists an optimal voltage range for devices of a particular thickness and with a particular applied magnetic field. Devices that allow a half-wave modulation of 1.55μm light in less than 5ms are shown to be possible with the use of a 2T magnetic field.
Electromagnetic resonance in deformed split ring resonators of left-handed meta-materials
Authors: Hsu Yi-Jang , Huang Yen-Chun , Lih Jiann-Shing , Chern Jyh-Long
Published: August 2004

Recently, structures called deformed split ring resonators have been proposed to achieve the smallest volume of a unit cell, while retaining negative permeability. With a combination of wire structures, the characteristics of a negative index of refraction, or left-handed meta-materials, can be seen from the transmission spectrum. This electromagnetic resonance, including its orientation tolerance and angular dependence on the incident beam, is explored in detail in the microwave range.
Effect of induced anisotropy on magnetoimpedance characteristics in NiFe∕Au∕NiFe sandwich thin films
Authors: Makhnovskiy D. P. , Fry N. , Panina L. V. , Mapps D. J.
Published: August 2004

The high frequency magnetoimpedance (MI) has been measured in NiFe∕Au∕NiFe narrow sandwich thin films with transverse, longitudinal, and crossed magnetic anisotropies over a frequency range of 50–500MHz. The MI elements have an opened structure with lengths of 2 or 5 mm, widths of 50–200μm and a total film thickness of 1.5μm. The magnetoimpedance characteristics with different field curve shapes, including asymmetrical, have been realized for certain magnetic anisotropy using a dc bias current. Initially the different MI field characteristics have been demonstrated in ferromagnetic microwires with circumferential, helical, and longitudinal anisotropies. Here we have reintroduced these field characteristics for the MI sandwich thin film elements. In an attempt to induce a particular anisotropy in the films the magnetic layers were rf sputtered in the presence of a strong magnetic field, with a further thermal restress treatment of the final MI elements. The use of thin film technology for MI is preferable in many applications because of its compatibility with integrated circuit technology which enables miniaturization, avoiding alignment issues, and wire soldering.
Oxygen impurity doping into ultrapure hydrogenated microcrystalline Si films
Authors: Kamei Toshihiro , Wada Takehito
Published: August 2004

We have systematically performed oxygen impurity doping into ultrapure hydrogenated microcrystalline Si (μc-Si:H) films grown at ∼200°C where the lowest spin densities are obtained. Two threshold oxygen concentrations [O] were observed: no crystallinity deterioration below [O]≈1×1020cm−3 and rapid carrier density increase above [O]≈1×1018cm−3. In these cases, electron carrier density roughly obeys a 1.4th power law with respect to [O] in the range between these two thresholds, while crystallinity remains almost unchanged. An implication of these results is discussed that includes plausible microscopic structures of oxygen donors and their locations.
The role of nitrogen-nitrogen pairs in the deviation of the GaAsN lattice parameter from Vegard’s law
Authors: Wang S. Z. , Yoon S. F. , Fan W. J. , Loke W. K. , Ng T. K. , Wang S. Z.
Published: August 2004

We propose several physical mechanisms that may account for the difference between the nitrogen composition in GaAsN materials measured by secondary ion mass spectroscopy and x-ray diffraction. A simplified model proposed here proves that Vegard’s law remains valid as long as all nitrogen atoms in the GaAsN alloys are located substitutionally at the arsenic sites. The theoretical results based on N-N pair defects are in good agreement with the experimental data, suggesting that the N-N pairs are the predominant nitrogen-related defects that cause deviation from the GaAsN lattice constant predicted by Vegard’s law.
Threshold voltage change due to organic-inorganic interface in pentacene thin-film transistors
Authors: Lee Jiyoul , Kim J. H. , Im Seongil , Jung Duk-Young
Published: August 2004

We have constructed pentacene-based organic thin-film transistors (OTFTs) using 270nm thick Al2O3+x gate dielectric deposited on indium tin oxide glass. Two different deposition techniques have been employed for the solid pentacene channel of our OTFTs: traditional thermal evaporation (TE) and energetic cluster evaporation (ECE). The TE-deposited pentacene channel appeared superior to the ECE-deposited pentacene in regard to crystallinity and hole mobility. However, the ECE-prepared OTFT showed an earlier turn on with smaller threshold voltage than the TE-prepared OTFT. This threshold voltage shift appeared more prominent with proper chemical treatment on the surface of our Al2O3+x gate oxide.
Quantum transport in a nanosize double-gate metal-oxide-semiconductor field-effect transistor
Authors: Croitoru M. D. , Gladilin V. N. , Fomin V. M. , Devreese J. T. , Magnus W. , Schoenmaker W. , Sorée B.
Published: August 2004

A model for the nanosize double-gate silicon-on-insulator metal-oxide-semiconductor field-effect transistors is developed, which enables both physical accuracy and flexibility. The quantum Liouville equation in the Wigner function representation has been used to deal with the quantum transport problem. Accounting for electron scattering due to ionized impurities, acoustic phonons, and surface roughness at the Si∕SiO2 interface, device characteristics are obtained as a function of structure parameters. From the analysis of the Wigner function, the coexistence of diffusive and ballistic transport naturally emerges. The obtained I-V characteristics show that the double-gate device is a potential structure for ultimate complementary metal-oxide-semiconductor scaling.
Modal foundations of close-packed optical arrays with particular application to infrared and millimeter-wave astronomical interferometry
Authors: Withington Stafford , Hobson Michael P. , Campbell Edward S.
Published: August 2004

A scheme is presented for modeling the behavior of multimode, multielement, close-packed optical systems. The scheme is of particular importance when describing the operation of astronomical interferometers. The basic idea is to find, through singular value decomposition, the fields on the sky and at the detectors, to which the individual telescopes of an array can couple. In the paper, we call these fields eigenfields. Eigenfields are more fundamental than eigenmodes, which are the field distributions that can pass through an optical system unchanged in form, and provide mutually orthogonal basis sets for propagating the second-order statistical properties of a source through the individual telescopes, through the beam combiners, and onto the detectors. Using our scheme it is possible to model many different kinds of interferometer, to include the noise generated by the optical components themselves and to calculate the power coupled into coherent, partially coherent, and incoherent detectors.
High oxygen-pressure annealing effects on the ferroelectric and structural properties of PbZr0.3Ti0.7O3 thin films
Authors: Escote M. T. , Pontes F. M. , Leite E. R. , Longo E. , Jardim R. F. , Pizani P. S.
Published: August 2004

We report on the synthesis of polycrystalline thin films of PbZr0.3Ti0.7O3 (PZT) by the so-called chemical solution deposition technique. The thin films were deposited on Pt∕Ti∕SiO2∕Si substrates by the spin-coating method, and were heat treateded at 700°C in air and under several oxygen pressures (10<PO2<60bars). The structural, morphological, and ferroelectric properties were characterized by x-ray diffraction (XRD), infrared and Raman spectroscopy, atomic force microscopy (AFM), and polarization-electric-field hysteresis loop measurements. The XRD and the Raman spectroscopy results revealed that the films heat treated in air and at low oxygen pressures (PO2<40bars) are single phase. However, analysis of the data indicated a clear decreasing of the crystallization degree of the films with increasing oxygen pressure. AFM results showed the PZT films display a rosette structure embedded in a matrix, which comprises grains with an average grain size ranging from 60 to 120nm. Ferroelectric hysteresis loops’ measurements performed on these PZT films exhibited a clear decrease of the remnant polarization with increasing oxygen-pressure PO2. This study indicated some important effects of the high oxygen-pressure annealing on the physical properties of PZT thin films.
Structural analysis of pure and LiCF3SO3-doped amorphous WO3 electrochromic films and discussion on coloration kinetics
Authors: Bueno P. R. , Pontes F. M. , Leite E. R. , Bulhões L. O. S. , Pizani P. S. , Lisboa-Filho P. N. , Schreiner W. H.
Published: August 2004

Structural differences between pure and LiCF3SO3-doped amorphous WO3 thin films were studied by Raman spectroscopy and X-ray photoelectron spectroscopy analysis. Based on our interpretation of reports in the literature on Raman spectra of WO3 thin films, we found that the LiCF3SO3-doped amorphous WO3 film presented greater amounts of oxygen vacancies, W+4 valence state and W+6=O surface vibration modes. These structural differences served as the basis to make inferences about the dissimilarities also found in the kinetic coloration observed during the electroinsertion reaction, focusing particularly on the dynamics of Li+ insertion into the two different structures.
Optical modulation in photonic band gap structures by surface acoustic waves
Authors: Krishnamurthy Srinivasan , Santos Paulo V.
Published: August 2004

We investigate theoretically the modulation of light beams in photonic band gap (PBG) structures by surface acoustic waves (SAWs). In these structures, the propagation of light beams can be actively controlled through an external acoustic stimulus. We have extended the mathematically rigorous transfer matrix method to calculate the light flow in PBG structures subjected to the spatial- and time-dependent dielectric function modulation induced by the SAW. The calculational procedure, which applies for SAWs with a frequency much smaller than that of the light, is employed to determine the transmission spectra of a one-dimensional Bragg stack with a cavity and that of a two-dimensional GaAs PBG structure with periodic air holes. We demonstrate that these two structures can be configured as an on∕off optical switch with very high contrast ratios and, in the two-dimensional case as an efficient wavelength-tunable optical filter.
Energy and dose characteristics of ion bombardment during pulsed laser deposition of thin films under pulsed electric field
Authors: Fominski V. Yu. , Nevolin V. N. , Smurov I.
Published: August 2004

Experiments on pulsed laser deposition of Fe films on Si substrates were performed with the aim to analyze the role of factors determining the formation of an energy spectrum and a dose of ions bombarding the film in strong pulsed electric fields. The amplitude of the high-voltage pulse (−40kV) applied to the substrate and the laser fluence at the Fe target were fixed during the deposition. Owing to the high laser fluence (8J∕cm2) at a relatively low power (20mJ), the ionization of the laser plume was high, but the Fe vapor pressure near the substrate was low enough to avoid arcing. Electric signals from a target exposed to laser radiation were measured under different conditions (at different delay times) of application of electric pulses. The Si(100) substrates were analyzed using Rutherford ion backscattering∕channeling spectrometry. The ion implantation dose occurred to be the highest if the high-voltage pulse was applied at a moment of time when the ion component of the plume approached the substrate. In this case, the implanted ions had the highest energy determined by the amplitude of the electric pulse. An advance or delay in applying a high-voltage pulse caused the ion dose and energy to decrease. A physical model incorporating three possible modes of ion implantation was proposed for the interpretation of the experimental results. If a laser plume was formed in the external field, ions were accelerated from the front of the dense plasma, and the ion current depended on the gas-dynamic expansion of the plume. The application of a high-voltage pulse, at the instant when the front approached the substrate, maintained the mode that was characteristic of the traditional plasma immersion ion implantation, and the ion current was governed by the dynamics of the plasma sheath in the substrate-to-target gap. In the case of an extremely late application of a high-voltage pulse, ions retained in the entire volume of the experimental chamber (as a result of the laser-plume expansion) are involved in the implantation process. Therefore, the spread in implanted-ion energies depends on the configuration of the electric field inside the chamber.
Mechanisms of vapor permeation through multilayer barrier films: Lag time versus equilibrium permeation
Authors: Graff G. L. , Williford R. E. , Burrows P. E.
Published: August 2004

Multilayer, thin-film organic/inorganic composite barrier layers have recently been reported to achieve water vapor permeation rates of <10−5g∕m2∕day at 25°C∕40%RH on polyethylene terephthalate substrates. Using both transient and steady-state vapor permeation measurements combined with classical Fickian diffusion models, we determine the mechanism of vapor permeation through such barrier structures and show that results obtained to date are limited not by equilibrium diffusion but by lag-time effects caused by the extremely long effective path length for the diffusing gas. The implications for further improvement of flexible thin-film vapor barriers are discussed.
Photoelectrochemical etching of n-type 4H silicon carbide
Authors: Shishkin Y. , Choyke W. J. , Devaty R. P.
Published: August 2004

Photoelectrochemical etching of highly doped n-type 4H SiC in dilute hydrofluoric acid along different crystallographic orientations under low voltage and/or low current conditions is studied. Scanning electron microscope images show that anodization of the hexagonal polytype 4H SiC with subsequent pore formation proceeds anisotropically. It is proposed that under uv illumination the crystallographic planes terminated with silicon atoms are more resistant to electrolytic attack than the planes terminated with carbon and mixed silicon-carbon atoms. This model is used to explain the observed triangular-channel pore morphologies. A clear indication was found that the resultant pore structure does not depend on the direction of the external electric field applied to the sample. Electrical parameters recorded as part of the photoelectrochemical etching process are described and interpreted.
Magnetocrystalline anisotropy of Fe3Ge single crystal: Effect of pressure and Mn substitution for Fe
Authors: Albertini F. , Negri D. , Pareti L. , Watts E. B. , Arnold Z. , Kamarad J. , Calestani G. , Deriu A. , Besseghini S.
Published: August 2004

The temperature dependence of the magnetocrystalline anisotropy and the effects of pressure on the intrinsic magnetic properties of the hexagonal DO19 phase of Fe3Ge single crystals have been studied. The easy-magnetization direction is in the basal plane at low temperatures and a spin reorientation transition from plane to axis occurs at 377K, with increasing temperature. The thermal dependence of the magnetic anisotropy of Fe3Ge has been determined and described in the frame of the Carr model for localized moments. The partial substitution of Fe by Mn, besides reducing both the saturation magnetization and Curie temperature, extends the axial region widely. Pressure has little influence on the magnetic moment but it markedly affects the crystalline anisotropy. The planar character of the anisotropy is indeed reduced in both pure and substituted compounds. The application of high pressure, the partial substitution of Fe by Mn and the increase of temperature have similar effects on the magnetocrystalline anisotropy of Fe3Ge—all induce a reduction of its planar character, favoring a plane to axis spin reorientation transition.
Fabrication of nickel oxide nanoparticles in SiO2 by metal-ion implantation combined with thermal oxidation
Authors: Amekura H. , Umeda N. , Takeda Y. , Lu J. , Kishimoto N.
Published: August 2004

A method is proposed to synthesize oxide nanoparticles in insulators, using metal-ion implantation and following thermal oxidation, which introduces less damage compared to the sequential implantation of metal ions and oxygen ions. Ni-oxide nanoparticles are formed in O2 gas flow at ∼800°C for 1h, through thermal oxidation of Ni metal nanoparticles, which were introduced in SiO2 by charging-free negative ion implantation of 60keV. After the oxidation, optical absorption in the visible region, which is due to Ni metal nanoparticles in the specimen, disappears, and a steep absorption edge of insulator NiO appears around ∼4eV. Simultaneously, the large magnetization of Ni metal nanoparticles changes to a weak magnetization of antiferromagnetic NiO nanoparticles. The nanoparticle formation is confirmed by transmission electron microscopy observation.
Atomic-layer-deposited Al2O3 thin films with thin SiO2 layers grown by in situ O3 oxidation
Authors: Kim Seong Keun , Hwang Cheol Seong
Published: August 2004

The growth, thermal annealing behaviors, and electrical properties of Al2O3 thin films grown by atomic layer deposition (ALD) on bare (100)Si and various oxidized Si wafers, by in situ O3 oxidation at 400°C and ex situ rapid thermal annealing (RTA) under O2 atmosphere at 900°C, were investigated. The ALD process was performed using Al(CH3)3 and high concentration of O3(400gm3). The high oxidation potential of O3 oxidized the Si surface at a very early stage of film growth and eliminated the incubation period even on a bare Si surface. The as-grown Al2O3 films had excess oxygen in the films, which diffused to the film Si interface and increased the interfacial layer by oxidizing the Si substrates during postannealing. The Al2O3 films grown on a bare Si substrate had the highest concentration of excess oxygen which resulted in the largest increase in the interfacial layer thickness during postannealing. As a result, the initial oxidation of the Si wafer did not significantly decrease the capacitance density compared to the films grown on a nonoxidized Si wafer at the as-deposited and postannealed states. Therefore, the Al2O3 layers grown using a high concentration of O3 oxidant on the in situ O3 oxidized Si wafers showed real high-k gate dielectric performance although the dielectric constants of the Al2O3 films were rather small (∼9) compared to other high-k gate dielectric films.
The open circuit voltage in amorphous silicon p-i-n solar cells and its relationship to material, device and dark diode parameters
Authors: Dutta U. , Chatterjee P.
Published: August 2004

We review the sensitivity of the open-circuit voltage (Voc) to material, device and dark diode parameters, and try to assess the possibilities of improving this quantity in hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells, having a wide band-gap emitter layer. We find that parameters that affect Voc can broadly be classified into two categories: those that alter the built-in potential (Vbi) appreciably, and those that produce small or no change in Vbi, but can still alter Voc by changing the dark recombination current in the intrinsic absorber or the interfaces. The study is carried out using an electrical-optical model based on the solution of the Poisson’s and the continuity equations. In agreement with existing work, we find that Voc is very sensitive to all parameters that appreciably alter Vbi, viz., the transparent conducting oxide∕P contact barrier height, the P layer thickness and its activation energy, although Voc is found to be more sensitive to these parameters than Vbi itself. However, we cannot correlate such changes in Voc to the dark diode parameters. On the other hand, parameters that have little effect on Vbi, but alter the dark recombination current, viz., the density of states, capture cross-sections and the Urbach edge, have only a minor influence on Voc, although these changes can qualitatively be linked to changes in the dark current. Also we find that the common belief that Voc decreases as the reverse bias generation current increases is not always true. Boron profiling of the P∕I interface and its effect on Voc has also been studied. All in all our study indicates that in a-Si:H p-i-n cells under AM1.5 light, there is little scope of further improving Voc, while retaining the cell efficiency.
Polarizability of conducting sphere-doublets using series of images
Authors: Wallén Henrik , Sihvola Ari
Published: August 2004

The classical electrostatic problem of two nonintersecting conducting spheres in a uniform incident electric field is considered. Starting from the basic Kelvin’s image principle, the two spheres are replaced with equivalent series of image sources, from which the polarizability is calculated. Explicit expressions for the axial and transversal components of the polarizability dyadic are found by solving the recurrence equations. Efficient numerical evaluation of the different series is also discussed.
Electrical activation in silicon-on-insulator after low energy boron implantation
Authors: Saavedra Antonio F. , Jones Kevin S. , Law Mark E. , Chan Kevin K. , Jones Erin C.
Published: August 2004

We have investigated the electrical activation of implanted boron in silicon-on-insulator (SOI) material using Hall effect, four-point probe, and secondary ion mass spectrometry. Boron was implanted at energies ranging from 1keVto6.5keV with a dose of 3×1014cm−2 into bonded SOI wafers with surface silicon thickness ranging from 300Åto1600Å. In one sample set, furnace anneals at 750°C were performed in a nitrogen ambient for times ranging from 5minto48h. A second sample consisted of isochronal furnace anneals performed from 450°Cto1050°C for 30min. Significantly less activation of boron is observed in SOI at temperatures below 750°C, regardless of the implant energy and surface silicon thickness. Between 750°C and 900°C, the active dose of boron in SOI is similar to that of bulk Si. As the implant energy increases, the fractional activation in thin SOI increases, due to a reduction in boron interstitial clusters (BIC) in the surface Si layer. It is concluded that an increase in the BIC population is the likely source of the low activation observed in SOI. This may be due to an increase in the interstitial supersaturation within the surface Si layer, due to the interface acting as a reflective boundary for interstitials.
Reactive nanostructured foil used as a heat source for joining titanium
Authors: Duckham A. , Spey S. J. , Wang J. , Reiss M. E. , Weihs T. P. , Besnoin E. , Knio O. M.
Published: August 2004

We have joined titanium alloy (Ti-6Al-4V) specimens at room temperature and in air by using free-standing nanostructured Al∕Ni multilayer foils to melt a silver-based braze. The foils are capable of undergoing self-sustaining exothermic reactions and thus act as controllable local heat sources. By systematically controlling the properties of the foils and by numerically modeling the reactive joining process, we are able to conclude that the temperatures reached by the foils during reaction are critical in determining the success of joining when using higher melting temperature braze layers.
Phase retrieval with two-beam off-axis x-ray holography
Authors: Kohmura Yoshiki , Sakurai Tatsuyuki , Ishikawa Tetsuya , Suzuki Yoshio
Published: August 2004

A wave front dividing x-ray interferometer with a prism was applied to two-beam off-axis holography. Complex amplitudes at the detector plane, which were derived from the hologram using the fringe scanning method, were inversely transformed to those at the exit surface of the specimen. The x-ray phase shift due to the amorphous carbon particles was quantitatively determined with the background of the phase image being as small as approximately λ∕100 at 12.4keV.
Thick M-type barium hexaferrite films grown on garnet substrates
Authors: Yoon S. D. , Vittoria C.
Published: August 2004

The deposition and characterization of thick M-type barium hexaferrite (BaFe12O19) films produced by a modified liquid phase epitaxy deposition technique are reported. The films are deposited on (111) oriented single crystal garnet (Gd3Ga5O12) substrates. The thickness of the films ranged between 45 and 80μm with growth rates of up to 40μm∕h. This growth rate is about five times greater than films grown on substrates of (111) magnesium oxide. Although the films are relatively thick, the ferrimagnetic resonance linewidth is remarkably narrow (∼50Oe) for external fields applied along the c axis.
Spectrometry of 0.46 and 13.56MHz Ar∕SF6 inductive plasma discharges
Authors: Tuszewski M. , Scarborough W. K. , White R. R.
Published: August 2004

The neutral and positive ion species of two inductively coupled plasma (ICP) discharges, operated with low-pressure argon and sulfur hexafluoride (Ar∕SF6) gas mixtures, are studied with optical emission and mass spectrometry. Similar discharges sustained in the two ICPs show significantly different species. The spectra of the 0.46MHz hemispherical ICP suggest nearly pure Ar∕SF6 discharges. The spectra of the 13.56MHz planar ICP reveal many species containing silicon and oxygen, from etching of the quartz dielectric. Etch rate measurements support these observations.
Interface structures of AlN∕MgB2 thin films sputtered on sapphire c- and r-plane
Authors: Abe H. , Naito M. , Moon W.-J. , Kaneko K. , Saito A. , Wang Z.
Published: August 2004

Structures of interfaces between AlN and MgB2 thin films were investigated by cross-sectional transmission electron microscopy. These layers were deposited on sapphire substrate with different orientations by a conventional method. A thick amorphous layer was present between films in the case of r-plane substrate, whereas many AlN crystals were grown epitaxially in the case of c-plane substrate. Then NbN∕AlN∕MgB2 trilayers were intentionally fabricated on sapphire c-plane and showed Josephson and quasiparticle tunneling property with small subgap leakage current.