bluuebunny/crossref_metadata_2025 huggingface dataset:
Inverse problem of remote neutrino diagnostics of intrareactor processes
Authors: Rusov V. D. , Zelentsova T. N. , Tarasov V. A. , Litvinov D. A.
Published: July 2004

Using the well-known experimental data, the inverse problem of neutrino diagnostics of reactor core is considered. The solution of this problem makes it possible to determine distantly the current value of nuclear density of each nuclear fuel components with known accuracy and also opens up the possibility for the development of neutrino on-line technology of temporal evolution of nuclear fuel isotopic structure and reactor power.
Edge quantum yield in n-channel metal-oxide-semiconductor field-effect transistor
Authors: Kang Ting-Kuo , Su K. C. , Chang Yih J. , Chen Ming-Jer , Yeh Shih-Hsin
Published: July 2004

The quantum yield of impact ionization is performed on an n-channel metal-oxide-semiconductor field-effect transistor (n-MOSFET) by a carrier separation measurement. When the n-MOSFET is biased in accumulation, the carrier separation measurement demonstrates that the gate current mainly originates from the electrons injected into the underlying drain/source overlap regions. The measured substrate current is due to excess holes originating from the impact ionization of the injected electrons in the overlap regions. Therefore, the quantum yield in the overlap regions can be determined by the ratio of the substrate current to the gate current. It is well matched with the theoretical calculation of quantum yield.
Assignment of deep levels causing yellow luminescence in GaN
Authors: Soh C. B. , Chua S. J. , Lim H. F. , Chi D. Z. , Tripathy S. , Liu W.
Published: July 2004

The deep levels in GaN associated with yellow luminescence transitions have been investigated using photoluminescence, Hall measurements, and deep level transient spectroscopy (DLTS). Hall measurements on Si-doped GaN show the presence of donor levels at ∼18, ∼35, and ∼70 meV, which are respectively associated with the Si shallow donors, O impurities, and the nitrogen vacancies (VN). DLTS measurements, on the other hand, reveal trap levels at Ec−0.1 eV, Ec−(0.2–0.24) eV, and Ev+0.87 eV. The trap level at Ec−0.1 eV obtained from DLTS can be correlated to the 70 meV deep donor (VN) obtained from Hall measurements. The deep donor band at Ec−(0.2–0.24) eV is attributed to the ON related defect complex decorated along dislocation sites while the hole level at Ev+0.87 eV is attributed to the Ga vacancy (VGa). Thermal annealing at 750 °C in nitrogen ambient results in reduction of yellow luminescence, which could be due to decrease in the concentration of VN and ON-related defect complexes. From these observations, we propose that yellow luminescence in GaN arises from the transitions from the Ec−(0.2–0.24) eV levels to the deep level at Ev+0.87 eV.
Quantitative determination of the magnetization of proton irradiated spots in graphite with magnetic force microscopy
Authors: Han K.-H. , Esquinazi P.
Published: July 2004

Using the point probe approximation of magnetic force microscopy (MFM) and measurements of the MFM signal as a function of the tip-to-sample distance, we have determined quantitatively the magnetization of proton irradiated spots in highly oriented pyrolytic graphite. From different spots produced with ion fluences ranging from 0.05 to 75 nC/μm2 we obtained magnetization values of the order of 106 A/m. These values are in the same range of those from soft magnetic materials.
Application of the pressure wave propagation method for adhesion defects detection in very thin multilayer structures
Authors: Aı̈nouche Leı̈la , Alquié Claude
Published: July 2004

Recently it was shown that the pressure wave propagation method, usually used to study electrical behavior of dielectric materials, can be applied to the nondestructive detection and quantitative analysis of adhesion defects in bilayer structures. In this paper, we investigate the possibilities open by the same method to detect and quantify adhesion defects in trilayer structures. After a brief recall of the principle of the method and of the main results obtained for bilayer structures, we describe the trilayer structures to be tested and present preliminary measurements of the acoustical and electrical properties of the materials constituting these samples. Measurements were carried out on kapton (52 μm)–adhesive (57 μm)–kapton (130 μm) and polyetheretherketone (PEEK) (138 μm)–adhesive (52 μm)–PEEK (138 μm) transparent samples. The signals are established and analyzed, with the help of simulations, in the cases of structures perfectly bonded, totally disbonded at one interface, and partially disbonded at one or at two interfaces. The excellent agreement between the measurements carried out on perfectly bonded and totally disbonded samples and simulations assesses the correctness of the signal expression for a multilayer structure. By analyzing the signals in the time domain, in a very simple manner, it is possible to determine at which interface the disbonds occur. In the case of partially disbonded structures at one interface, the value of the percentage of totally disbonded area in the tested zone determined by measurements or simulations is very close to that deduced from a photograph of the sample. If the transit time of the pressure pulse in each medium is larger than 11 ns measurements and simulations show that, owing to this method, it is possible to detect and localize in trilayer structures submicron gaps and a percentage of totally disbonded area as low as 10% in the tested zone, which is approximately equal to 30 mm2. In the case of partially disbonded samples at the two interfaces, it is possible to make a qualitative analysis of the measurements.
Electrical properties of GaSe doped with Er
Authors: Hsu Yu-Kuei , Chang Chen-Shiung , Huang Wen-Chang
Published: July 2004

Measurements of the Hall effect and Er-related luminescence were made on Er-doped GaSe. Deep-level transient spectroscopy (DLTS) was also performed. Hall measurements show that hole concentrations of 0.2%, 0.5%, and 1% Er-doped GaSe samples are 1.5×1017–6×1017 cm−3 at room temperature, and that the mobility of these holes is in the range 22–34 cm2/V s. The temperature dependence of the hole concentration is explained using the two-acceptor model, in which one acceptor level is at around 65 meV above the valence band and the other one is at ∼158 meV. The DLTS measurements yield similar results. Furthermore, the shallow acceptor impurities contribute free hole carriers and act as radiative centers; the deep acceptor impurities are nonradiative centers, which are responsible for the quenching behavior of Er-related luminescence. The temperature dependence of the hole mobility can be understood as the combined scatterings of homopolar optical phonons and ionized impurities.
Study of the structural evolution in ZnO thin film by in situ synchrotron x-ray scattering
Authors: Hur Tae-Bong , Hwang Yoon-Hwae , Kim Hyung-Kook , Park Hong-Lee
Published: July 2004

The evolution of surface roughness and strain relaxation as a function of film thickness of ZnO films grown on sapphire(0001) were studied by in situ synchrotron x-ray scattering and atomic force microscopy measurements. The well-aligned two-dimensional (2D) planar layer dominated in layer-by-layer growth at the highly strained initial growth stage. As the film thickness increased, the discrete nucleations on the 2D planar layer continuously grew until the ZnO film reached the strain relaxed steady-state regime. When the 3D islands were quickly developed by the strain relaxation, the dynamic scaling exponent β was roughly 1.579. The strain relaxed steady-state regime was described as β∼0.234.
Hot-phonon-induced velocity saturation in GaN
Authors: Ridley B. K. , Schaff W. J. , Eastman L. F.
Published: July 2004

In highly polar semiconductors with electron densities typically found in heterostructure field-effect transistors (HFETs), transport cannot be described without taking hot phonons into account. Here we describe a simple analytical model applied to the case of bulk GaN, taking the nonparabolicity of the conduction band into account, and show that the production of longitudinal optical (LO) phonons reduces the mobility and causes the drift velocity to saturate at a value around 107 cm/s, depending on the density of electrons. Transfer of electrons to higher valleys is expected to be delayed to much higher fields than commonly predicted. The effect of possible hot products of the LO decay is also considered. We relate the results for bulk material to the situation in HFETs, in which real-space transfer is inhibited, by considering the effect of spatial spreading of the channel electrons.
Effects of ultrastrong gravitational field on the crystalline state of a Bi-Sb alloy
Authors: Huang Xinsheng , Mashimo Tsutomu , Ono Masao , Tomita Takeshi , Sawai Tomotsugu , Osakabe Toyotaka , Mori Nobuo
Published: July 2004

An atomic-scale graded structure has been formed previously in a Bi70Sb30 (at. %) alloy that is miscible in all proportions, by sedimentation of substitutional solute atoms under an ultrastrong gravitational field up to 1×106 G at 220–240 °C [T. Mashimo, T. Ikeda, and I. Minato, J. Appl. Phys. 90, 741 (2001)]. In this study, additional megagravity field experiments were performed on the Bi70Sb30 alloy and pure Bi at different temperatures below their melting points, to investigate the change in crystalline state under the ultrastrong gravitational field. For the Bi70Sb30 alloy ultracentrifuged at 191–205 °C, no change in composition was observed, and the grain sizes of the crystals decreased from several millimeters to tens of micrometers, while no distinct change in grain size was observed for the pure Bi ultracentrifuged under the same experimental conditions. The Bi70Sb30 alloy ultracentrifuged at 220–240 °C consisted of two regions with different morphologies–fine-grained crystals with grain sizes in the range of tens of micrometers in the low gravity region, and large crystals with grain sizes several millimeters long and hundreds of micrometers wide along the direction of gravity in the high gravity region, where sedimentation of atoms was confirmed. The large crystals with hexagonal structures were formed by preferential crystal growth roughly along the c axes, and a large strain that increased as the gravitational field increased existed inside these crystals. Formation of this anomalous crystal state might be correlated with the sedimentation of atoms.
Enhanced low field magnetoresistance of Al2O3-La0.7Sr0.3MnO3 composite thin films via a pulsed laser deposition
Authors: Yan L. , Kong L. B. , Yang T. , Goh W. C. , Tan C. Y. , Ong C. K. , Rahman Md. Anisur , Osipowicz T. , Ren M. Q.
Published: July 2004

Al 2 O 3 -doped La0.7Sr0.3MnO3(Al2O3-LSMO) thin films were deposited on Si(111) substrate via a pulsed laser deposition. The deposited Al2O3-LSMO composite thin films were characterized by x-ray diffraction, scanning electron microscopy, and electro- and magneto-transport measurements. The main phase in the Al2O3-LSMO composite films was the perovskite phase. Texturelike microstructure was observed in the Al2O3-LSMO composite films while the average grain size remained almost unchanged compared to the pure samples. The metal-insulator transition temperature decreased as a result of the addition of Al2O3 and further reduced with increasing Al2O3 content. A maximum low field mangetoresistance of ∼15% was achieved in the 2∕20Al2O3-LSMO thin films, which could be well explained in terms of the grain boundary tunneling effect. The composition of the composite thin films can be easily tuned by adjusting the target composition. This method is believed to be applicable to exploring the combinations of other manganites and insulators.
Optical spin orientation in strained superlattices
Authors: Subashiev A. V. , Gerchikov L. G. , Ipatov A. N.
Published: July 2004

Optical orientation in the strained semiconductor superlattices (SL) is investigated theoretically. The dependence of the features in spin-polarization spectra on the structure parameters is clarified. The value of polarization in the first polarization maximum in the SL structures is shown to grow with the splitting between the hh and lh states of the valence band, the joint strain and confinement effects on the hh1-lh1 splitting being strongly influenced by the tunneling in the barriers. In strained structures with high barriers for the holes initial polarization can exceed 95%. Calculated polarization spectra are close to the experimental spectra of polarized electron emission.
Suppression and promotion of convection in water by use of radial components of the magnetization force
Authors: Maki Syou , Ataka Mitsuo
Published: July 2004

Three-dimensional numerical computations of convection in water were carried out in the presence of the radial components of the magnetization force in a horizontal bore of a superconducting magnet. It is shown that the convection suppression is possible, when an upward radial component is used in combination with a residual axial component. Convection promotion is also possible when a laterally eccentric position is used.
Statistical geometry of random fibrous networks, revisited: Waviness, dimensionality, and percolation
Authors: Yi Y. B. , Berhan L. , Sastry A. M.
Published: July 2004

Waviness alters both geometric and mechanical properties of stochastic fibrous networks and significantly affects overall mechanical response, but few results are available in the literature on the subject. In this work, we explore the importance of the dimension of constituent fibers (1D vs 2D) in determination of percolation thresholds, and other fundamental statistical properties of fibers having geometric waviness, in adaptation of classical theories on random lattices to practical applications, including analysis of nanotube ropes and collagen bundles. Although the so-called “curl ratio” clearly affects the statistical properties, as evaluated by Kallmes and Corte a few decades ago, we have found some results in this classic work to be inaccurate for systems containing fibers of moderate waviness. Our main findings include the independence of the mean number of crossings with regard to waviness, as well as the nonlinear dependence of probability of intersection on waviness. Our investigation of percolation in wavy fiber networks reveals that the percolation threshold is significantly increased, with increasing curl ratio. In addition, several nontrivial results related to network properties of infinite straight lines are also described, some of which are believed to have wide applications in practice.
Damage accumulation in Si during high-dose self-ion implantation
Authors: Zhong Y. , Bailat C. , Averback R. S. , Ghose S. K. , Robinson I. K.
Published: July 2004

Accumulation and annealing of damage in Si implanted with self-ions to high doses were investigated using a combination of grazing incidence diffuse x-ray scattering, high-resolution x-ray diffraction scans, and transmission electron microscopy. During implantation at 100°C, small vacancy and interstitial clusters formed at low doses, but their concentrations saturated after a dose of ≈3×1014cm−2. The concentration of Frenkel defects at this stage of the implantation was ≈1×10−3. At doses above 1×1015cm−2, the concentration of implanted interstitial atoms began to exceed the Frenkel pair concentration, causing the interstitial clusters to grow, and by ≈3×1015cm−2, these clusters formed dislocation loops. Kinematical analysis of the rocking curves illustrated that at doses above 1×1015cm−2 the “plus one” model was well obeyed, with one interstitial atom being added to the dislocation loops for every implanted Si atom. Measurements of Huang scattering during isochronal annealing showed that annealing was substantial below 700°C for the specimens irradiated to lower doses, but that little annealing occurred in the other samples owing to the large imbalance between interstitial and vacancy defects. Between 700 and 900°C a large increase in the size of the interstitial clusters was observed, particularly in the low-dose samples. Above 900°C, the interstitial clusters annealed.
Electronic structure and magnetic properties of MnFe3N
Authors: Paduani C.
Published: July 2004

The electronic structure and magnetic properties of MnFe3N have been studied using the full potential–linearized augmented plane-wave method and the generalized gradient corrections. The stability of the magnetic ordering is investigated as a function of the unit cell volume. This compound has been reported as ferrimagnetic in the literature. However, the present calculations indicate that a ferromagnetic ordering is the stable phase at low temperatures. With lattice contraction a low-spin phase is identified, where the Fe moment is decreased at face-centered sites. The Fe and Mn moments are strongly correlated in this nitride. A linear relation between the magnetization and the magnetic hyperfine field (Fermi contact term) with respect to changes in the unit cell volume is observed.
Control of growth mode in SrTiO3 homoepitaxy under 500°C
Authors: Li Yan Rong , Li Jin Long , Zhang Ying , Wei Xian Hua , Deng Xin Wu , Liu Xing Zhao
Published: July 2004

Homoepitaxial SrTiO3 thin films were grown by laser molecular beam epitaxy. The growth mode was determined by in-situ reflective high energy electron diffraction, and the surface of the films was studied by ex-situ atomic force microscopy. At the deposition rate of 0.16Å∕sec and the laser energy density of 6J∕cm2, layer-by-layer growth was observed above 460°C substrate temperature, while the Stranski-Krastanov growth mode, that is layer-by-layer growth plus island growth mode, prevailed between 460°C and 410°C. On further decreasing the substrate temperature, the island growth was determined under 410°C. With the optimization of deposition process in terms of laser energy density and deposition rate, the lowest crystallization temperatures of SrTiO3 films grown in layer-by-layer growth mode were obtained as low as 280°C. The effects of laser energy density on growth temperature were studied.
X-ray-absorption fine-structure study of ZnSexTe1−x alloys
Authors: Pellicer-Porres J. , Polian A. , Segura A. , Muñoz-Sanjosé V. , Di Cicco A. , Traverse A.
Published: July 2004

X-ray-absorption fine-structure experiments at different temperatures in ZnSexTe1−x (x=0, 0.1, 0.2, 0.55, 0.81, 0.93, 0.99, and 1.0) have been performed in order to obtain information about the structural relaxation and disorder effects occurring in the alloys. First and second neighbor distance distributions have been characterized at the Se and Zn K edges, using multiple-edge and multiple-scattering data analysis. The first neighbor distance distribution was found to be bimodal. The static disorder associated with the Zn–Te distance variance did not depend appreciably on composition. On the other hand, the static disorder associated with the Zn–Se distance increased as the Se content diminished. Using the bonding angle information provided by our experiments the point of view of the anion has been related to that of the cation. The resulting structural model indicates that Zn tetrahedra surrounding the anions remain essentially undistorted, but forced to tilt from their ideal zincblende orientation to accommodate the minority element. The main origin of structural disorder is suggested.
Hydrogen emission by Nd-YAG laser-induced shock wave plasma and its application to the quantitative analysis of zircalloy
Authors: Kurniawan Koo Hendrik , Lie Tjung Jie , Idris Nasrullah , Kobayashi Takao , Maruyama Tadashi , Suyanto Hery , Kagawa Kiichiro , OnTjia May
Published: July 2004

An experiment was carried out to demonstrate the detection of a hydrogen emission line, HI656.2nm (Hα), in a plasma induced by a Q-switched Nd-YAG (YAG, yttrium aluminium garnet) laser in a low pressure gas on various types of samples, such as zinc, a glass slide, and a zircalloy tube. Contribution by surface water could be suppressed by a laser cleaning treatment and the resulting calibration curve obtained for zircalloy tube samples doped with various concentrations of hydrogen (0, 200, 540, and 960) suggest potential applications to the quantitative analysis of hydrogen. A study of the dynamic process represented by the time profiles of the hydrogen emission, in comparison with those for zinc atomic emission, revealed a specific feature that is related to the small mass of hydrogen. This specific feature can be explained by the shock wave excitation mechanism in terms of new hypothetical process, namely, a mismatch between the movement of ablated hydrogen atoms and the formation of the shock wave.
Aspect ratio of liquid phase epitaxial SiGe∕Si(001) islands as probed by high resolution x-ray diffraction
Authors: Hanke M. , Schmidbauer M. , Grigoriev D. , Köhler R.
Published: July 2004

X-ray diffuse scattering is used to probe size, shape, and strain distribution of self-organized SiGe∕Si(001) islands, which were grown by liquid phase epitaxy. The SiGe islands show a truncated pyramidal shape with {111} side facets and a (001) top facet and they are highly uniform in size. With an averaged island base width of 130nm and a corresponding height of 65nm all the islands have a characteristic geometrical base-to-height aspect ratio of about 2. X-ray diffuse scattering is used to locally probe the elastically relaxed regions inside the island apex and the strongly strained regions near the substrate-island interface. It is found that the geometrical aspect ratio has a large impact on the x-ray diffuse intensity pattern in reciprocal space. By performing corresponding kinematical x-ray simulations this fact can be utilized to determine the aspect ratio with high sensitivity.
Inhomogeneous strain state in a rectangular InGaAs quantum wire/GaAs barrier specimen prepared for cross sectional high-resolution transmission electron microscopy
Authors: Yoo Yo-Han , Lee Woong , Shin Hyunho
Published: July 2004

Inhomogeneous strain state in a rectangular InGaAs quantum wire (QWR) embedded in GaAs matrix has been comparatively studied, via finite element analysis, for the cases of specimen prepared for high-resolution transmission electron microscopy (HRTEM) and the bulk specimen. The quantum wire is extruded from the surface of the HRTEM specimen due to the inherent lattice-mismatch strain, which yields an inhomogeneous strain state in the HRTEM specimen. Some strain components are significantly relaxed in the HRTEM specimen, while other one is comparable to the strain state in bulk counterpart, depending on strain-monitoring location. Thus, careful selection of the strain component and measuring location is necessary in order to get meaningful bulk strain information from the HRTEM specimen, which is informative for the analysis and design of the QWR nanostructures.
High power ferromagnetic resonance and spin wave instability processes in Permalloy thin films
Authors: An Sung Yong , Krivosik Pavol , Kraemer Michael A. , Olson Heidi M. , Nazarov Alexey V. , Patton Carl E.
Published: July 2004

The high power ferromagnetic resonance (FMR) response, as well as butterfly curves of the spin wave instability threshold microwave field amplitude hcrit versus in-plane static field H profiles, have been measured for Permalloy films with thicknesses of 104, 128, and 270nm at a nominal pumping frequency of 9.37GHz. The hcrit values range from about 1 to 7Oe. Both the resonance saturation response at the FMR field and the subsidiary absorption (SA) response for static fields below the FMR field are similar in appearance to those for bulk ferrites. Butterfly curves over the SA response region, while similar to those for ferrites, exhibit a film thickness dependent band edge cutoff effect not found in bulk ferrites. The SA butterfly curve data were analyzed on the basis of a spin wave instability theory adapted to thin films. The observed shift in the SA band edge cutoff with thickness agrees with calculations based on the thin film dispersion response and the assumption of first order instability processes with critical modes at one half the pumping frequency. The fitted SA spin wave linewidths give values consistent with metallic relaxation processes, but indicate critical modes with wave vectors that always make relatively small 0°–20° angles with the static field, very different from the critical modes for bulk ferrites. Three key conclusions from this work are (1) the nonlinear microwave FMR response in Permalloy films is a threshold effect related to well established spin wave instability processes, (2) the details of the SA response are controlled largely by the thin film spin wave dispersion, and (3) these nonlinear processes occur for very small precession angles.
Strain relaxation of pseudomorphic Si1−xGex∕Si(100) heterostructures after Si+ ion implantation
Authors: Holländer B. , Buca D. , Mörschbächer M. , Lenk St. , Mantl S. , Herzog H.-J. , Hackbarth Th. , Loo R. , Caymax M. , Fichtner P. F. P.
Published: July 2004

The strain relaxation of pseudomorphic Si1−xGex layers (x=0.21,…,0.33) was investigated after low-dose Si+ ion implantation and annealing. The layers were grown by molecular-beam epitaxy or chemical vapor deposition on Si(100) or silicon-on-insulator. Strain relaxation of up to 75% of the initial strain was observed at temperatures as low as 850°C after implantation of Si ions with doses below 2×1014cm−2. We suggest that the Si implantation generates primarily dislocation loops in the SiGe layer and in the underlying Si which convert to strain relaxing misfit segments. The obtained results are comparable to strain relaxation achieved after He+ implantation with doses of 1–2×1016cm−2.
Anomalous switching in submicrometer magnetic tunnel junction arrays arising from magnetic vortex and domain wall pinning
Authors: Park Wanjun , Hwang I. J. , Kim Taewan , Lee K. J. , Kim Young Keun
Published: July 2004

We present switching characteristics of patterned submicrometer magnetic tunnel junction arrays containing NiFe and CoFe free layers. The resemblance of magnetization and magnetoresistance (MR) curves was studied by micromagnetic calculations and experimental measurements. Upon analyzing the MR transfer curves, the magnetic vortex and domain wall pinning effects on anomalous switching of each magnetic tunnel junction can be distinguished by remanent states. Data indicates that the low saturation magnetization of the free layer increases domain wall pinning and decreases trapped magnetization vortices.
Surface and interface chemical composition of thin epitaxial SrTiO3 and BaTiO3 films: Photoemission investigation
Authors: Amy F. , Wan A. , Kahn A. , Walker F. J. , McKee R. A.
Published: July 2004

In this work, we use SrTiO3 and BaTiO3 films (thickness <10nm) epitaxially grown on Si(100) substrates, and x-ray and ultraviolet (UV) photoemission spectroscopy to investigate the effect of surface preparation on chemical and electronic film and interface properties. Depending on the surface treatment, e.g., ex situ UV generated ozone or annealing in oxygen or vacuum, the valence band maximum position shifts by more than 2eV, whereas the oxide core levels shift by less than 0.6eV. These findings indicate that extremely careful cleaning procedures must be applied, as surface composition and film morphology are of paramount importance in the determination of the electronic structure of the crystalline oxides. In addition, the interfaces between SrTiO3 and BaTiO3 and Si are shown to be unstable at annealing temperatures required to thoroughly clean and order the surface.
All-optical light modulation in pharaonis phoborhodopsin and its application to parallel logic gates
Authors: Sharma Parag , Roy Sukhdev
Published: July 2004

All-optical light modulation in pharaonis phoborhodopsin (ppR) protein has been analyzed considering its ppRO state dynamics based on nonlinear intensity-induced excited-state absorption. Amplitude modulation of a cw probe laser beam transmission at 560nm corresponding to the peak absorption of ppRO intermediate state through ppR, by a modulating cw pump laser beam at 498nm corresponding to the peak absorption of initial ppR state has been analyzed considering all six intermediate states in its photocylce using the rate equation approach. The transmission characteristics have been shown to exhibit a dip at relatively lower pump intensity values compared to bacteriorhodopsin, which is sensitive to normalized small-signal absorption coefficient (β), rate constants of ppRM and ppRO states, and absorption of the ppRO state at 498nm. There is an optimum value of β for a given pump intensity range for which maximum modulation can be achieved. It is shown that 100% modulation can be achieved if the initial state of ppR does not absorb the probe beam. The results have been used to design low power all optical parallel NOT, AND, OR, XNOR, and the universal NAND and NOR logic gates for two cases: (i) only changing the output threshold and (ii) considering a common threshold with different β values. At typical parameters, wild-type (WT) ppR based logic gates can be realized at considerably lower pump powers than WT-bR.
Enhanced coupling of light from organic light emitting diodes using nanoporous films
Authors: Peng H. J. , Ho Y. L. , Yu X. J. , Kwok H. S.
Published: July 2004

We have demonstrated an approach to enhance the light extraction efficiency for organic light emitting diode (OLED). Nanoporous alumina film was used to modify the optical wave propagation and coupling of light from the OLED. Experimental results showed an increase of over 50% in the coupling efficiency of the nanoporous device, without affecting the electrical properties of the OLED. Effective medium theory can be used to model the optical properties of the nanoporous media and simulated the optical characteristics of the OLED successfully.
Wavelength control of 1.3–1.6 μm light emission from the quantum dots self-formed in GaAs∕InAs short-period superlattices grown on InP (411)A substrates
Authors: Mori J. , Nakano T. , Shimada T. , Hasegawa S. , Asahi H.
Published: July 2004

High lateral density quantum dot (QD) structures are self-formed by growing GaAs∕InAs short-period superlattices (SLs) on InP (411)A substrates by gas source molecular beam epitaxy. Multilayer QD structures sandwiched with InP barrier layers showed a strong photoluminescence emission. The 1.3–1.6μm wavelength emission was easily obtained and precisely controlled by regulating the SL period as well as the number of InAs monolayers (layer thickness). This wavelength control can be understood by considering the carrier confinement along the growth direction as well as the effective well depth change.
Impurity band characteristics near the band edge of Al-doped ZnO
Authors: Hur Tae-Bong , Hwang Yoon-Hwae , Kim Hyung-Kook
Published: July 2004

The characteristics of impurity bands near the band edge of Al-doped and undoped ZnO ceramics were investigated by photoluminescence, photoluminescence excitation, and x-ray diffraction. We found that Al(0.6%)-doped ZnO had two impurity bands whose binding energies were roughly 13 and 99meV below the effective band edge. Also, we found that Al(1.1%)-doped ZnO had an impurity band of ∼80meV binding energy below the effective band edge. As the doping concentration of ZnO increases, Al-impurity bands degenerated from two localized levels to the single localized level. The green band emission of Al(0.6%)-doped ZnO is stronger than those of the pure and Al(1.1%)-doped ZnO because of the high charge transfer rate to the effective band.
Photoresponsive character of double-doped La2∕3(Ca1∕3Sr2∕3)1∕3MnO3 film
Authors: Jin K. X. , Chen C. L. , Wang S. L. , Wang Y. L. , Wang Y. C. , Song Z. M. , Ren R.
Published: July 2004

The transient resistance change induced by modulated laser pulse has been observed in the double-doped perovskite La2∕3(Ca1∕3Sr2∕3)1∕3MnO3 thin film prepared by rf magnetron sputtering method. The time response character of photoinduced voltage signal is independent of the bias current, but its photoinduced relative change in resistance is nonlinearly dependent on the bias current. All the phenomena in the perovskite film is explained by the double exchange model and the small polaron theory. This is very important in the practical application of a colossal magnetoresistance photoelectric switch.
Zinc oxide as an ozone sensor
Authors: Martins R. , Fortunato E. , Nunes P. , Ferreira I. , Marques A. , Bender M. , Katsarakis N. , Cimalla V. , Kiriakidis G.
Published: July 2004

This work presents a study of intrinsic zinc oxide thin film as ozone sensor based on the ultraviolet (UV) photoreduction and subsequent ozone re oxidation of zinc oxide as a fully reversible process performed at room temperature. The films analyzed were produced by spray pyrolysis, dc and rf magnetron sputtering. The dc resistivity of the films produced by rf magnetron sputtering and constituted by nanocrystallites changes more than eight orders of magnitude when exposed to an UV dose of 4mW∕cm2. On the other hand, porous and textured zinc oxide films produced by spray pyrolysis at low substrate temperature exhibit an excellent ac impedance response where the reactance changes by more than seven orders of magnitude when exposed to the same UV dose, with a response frequency above 15kHz, thus showing improved ozone ac sensing discrimination.
Near-field enhancement and imaging in double planar polariton-resonant structures
Authors: Maslovski Stanislav , Tretyakov Sergei , Alitalo Pekka
Published: July 2004

It is shown that a system of two coupled planar material sheets possessing surface mode (polariton) resonances can be used for the purpose of evanescent field restoration and, thus, for sub-wavelength near-field imaging. The sheets are placed in free space so that they are parallel and separated by a certain distance. Due to interaction of the resonating surface modes (polaritons) of the sheets an exponential growth in the amplitude of an evanescent plane wave in the system can be achieved. This effect was predicted earlier for backward-wave (double-negative or Veselago) slab lenses. The alternative system considered here is proved to be realizable at microwaves by grids or arrays of resonant particles. The necessary electromagnetic properties of the resonating grids and the particles are investigated and established. Theoretical results are supported by microwave experiments that demonstrate amplification of evanescent modes.
Photothermal depth profilometry of heat-treated hardened 0.15%–0.2%C, 0.6%–0.9%Mn Steels
Authors: Liu Yue , Baddour Natalie , Mandelis Andreas , Beingessner Clare
Published: July 2004

The thermal-diffusivity depth-profilometric properties of hardened AISI1018 steel samples (0.15%–0.2%C and 0.6%–0.9%Mn) are investigated from experimental data and compared to their microhardness depth profiles. To fully understand the effect of the individual steps of the heat-treating process on the thermal-diffusivity of the steels, the thermal-diffusivity depth profiles are reconstructed using laser infrared photothermal radiometry. The inverted depth profiles are compared to the results of microhardness testing after each step of carbonitriding and quenching. The comparison shows that there is a good to excellent anticorrelation between hardness and thermal-diffusivity profiles for both carbonitrided and quenched samples with 0.02in. case depth and gradually worsening anticorrelation trends for 0.04 and 0.06in. case depths. It is concluded that for this particular steel, both carbon diffusion and the complicated carbonitrided microstructure affect the absolute values of the thermal-diffusivity profiles as well as their depth distribution.
Microstructural studies of (PbLa)(ZrTi)O3 ceramics using complex impedance spectroscopy
Authors: Dutta Soma , Choudhary R. N. P. , Sinha P. K. , Thakur Awalendra K.
Published: July 2004

Lanthanum modified lead zirconate titanate (PLZT:8∕60∕40) ceramics prepared by a sol-gel route showed a well-defined microstructure comprising of grains separated by boundaries. Complex impedance spectroscopy has provided a convincing evidence for the existence of both grain (bulk) and grain-boundary effects that were separated in the frequency domain in impedance spectrum. The impedance analysis further provided the value of relaxation frequency, which was a characteristic intrinsic property of the material and was independent of sample geometrical factors. Relaxation frequency calculated at different temperatures was used to evaluate bulk dielectric constant (εb), which was compared with the real part of the dielectric constant (ε′). The temperature variation of the bulk electrical conductivity (σdc) indicated an evidence of Arrhenius-type thermally activated process showing a linear variation up to a temperature of 713°K and was predominantly governed by grain boundary conduction showing a plateau region beyond 713°K.
Electrical and optical properties of CdxZn1−xTe single crystals for applications as terahertz electro-optic sensors
Authors: Kang H. S. , Lee H. I. , Kim T. W.
Published: July 2004

Cd x Zn 1 − x Te single crystals were grown by using the Bridgman method. Hall-effect measurements showed that the carrier type and carrier concentration of unintentionally CdxZn1−xTe crystals were p type and between approximately 1014 and 1015, respectively. Phototransmission spectra showed that the position of the band edge emission shifted to higher wavelength with increasing Cd mole fraction, and photoluminescence spectra showed that the peak corresponding to the excitons bound to neutral acceptors shifted to lower energy with increasing Cd mole fraction. The electro-optic sensors fabricated utilizing the CdxZn1−xTe single crystals were operated in the terahertz spectrum range. These results can help improve the understanding of CdxZn1−xTe single crystals for applications in terahertz electro-optic sensors.
Influence of process parameters on the morphology of Au∕SiO2 nanocomposites synthesized by radio-frequency sputtering
Authors: Barreca Davide , Gasparotto Alberto , Tondello Eugenio , Bruno Giovanni , Losurdo Maria
Published: July 2004

Metal nanoparticles on oxide matrices have gained a markedly increasing consideration with regard to both scientific and applicative purposes, thanks to the possibility of tailoring the system characteristics by a proper choice of the preparation route and the processing conditions. In the present work, Au∕SiO2 nanocomposites were prepared by radio-frequency (rf) sputtering of gold from Ar plasmas on amorphous silica substrates. Particular attention was devoted to the influence of the synthesis parameters on the chemicophysical properties of the final nanosystems. To this regard, both in situ and ex situ characterization techniques were adopted. In particular, laser reflection interferometry was employed for an in situ monitoring of growth processes, while ex situ analyses were specifically dedicated to the investigation of Au∕SiO2 nanostructure, chemical composition, optical properties, and surface morphology (glancing-incidence x-ray diffraction, transmission electron microscopy, spectroscopic ellipsometry, x-ray photoelectron spectroscopy, ultraviolet-visible absorption, and atomic force microscopy). The obtained results showed the possibility of tailoring the Au∕SiO2 morphology from clusterlike to islandlike systems or continuous films, with consequent modification of the optical properties, by a proper choice of total pressure, rf power, deposition time, and growth temperature.
Charge trapping in metal-ferroelectric-insulator-semiconductor structure with SrBi2Ta2O9∕Al2O3∕SiO2 stack
Authors: Xu Zhen , Kaczer Ben , Johnson Jo , Wouters Dirk , Groeseneken Guido
Published: July 2004

The charge trapping is studied in metal-ferroelectric-insulator-semiconductor (MFIS) capacitors with SrBi2Ta2O9(SBT)∕Al2O3∕SiO2 gate stack by high-frequency and pulsed capacitance-voltage (CV) measurements. The ferroelectric polarization is observed by high-frequency CV. Under fast gate voltage sweep in pulsed CV, the delay of electron trapping detrapping in the buffer layer induces an opposite CV hysteresis direction than that of the ferroelectric polarization. For memory programming, the hole trapping in the gate stack limits the electric field in SBT. Furthermore, the electron trapping during stress induces serious threshold voltage instability as well as erratic memory read out. All these charge trapping problems are important for the practical application and reliability of the memory with MFIS structure.
Distinguishing the effect of surface passivation from the effect of size on the photonic and electronic behavior of porous silicon
Authors: Pan L. K. , Sun Chang Q. , Yu G. Q. , Zhang Q. Y. , Fu Y. Q. , Tay B. K.
Published: July 2004

CF 4 plasma-passivation enhanced size dependence of the blueshift in photoemission and photoabsorption, E2p-level shift, and band-gap expansion of porous silicon has been measured and analyzed numerically based on the recent “bond order-length-strength” correlation [C. Q. Sun, Phys. Rev. B 69, 045105 (2004)]. Matching predictions to the measurements conducted before and after fluorination reveals that fluorination further enhances both the crystal binding intensity that determines the band gap and core level shift and the electron-phonon coupling that contributes to the energies of photoemission and photoabsorption. This approach enables us to discriminate the effect of surface-bond contraction from the effect of surface-bond nature alteration on the unusual behavior of photons, phonons, and electrons in nanosolid Si.
X-ray and neutron diffraction investigations of the structural phase transformation sequence under electric field in 0.7Pb(Mg1∕3Nb2∕3)-0.3PbTiO3 crystal
Authors: Bai Feiming , Wang Naigang , Li Jiefang , Viehland D. , Gehring P. M. , Xu Guangyong , Shirane G.
Published: July 2004

The structural phase transformations of 0.7Pb(Mg1∕3Nb2∕3)O3-0.3PbTiO3 (PMN-30%PT) have been studied using x-ray diffraction (XRD) and neutron scattering as a function of temperature and electric field. We observe the phase transformational sequence (i) cubic (C)→ tetragonal (T)→ rhombohedral (R) in the zero-field-cooled (ZFC) condition; (ii) C→T→ monoclinic (MC)→ monoclinic (MA) in the field-cooled (FC) condition; and (iii) R→MA→MC→T with increasing field at fixed temperature beginning from the ZFC condition. Upon removal of the field, the MA phase is stable at room temperature in the FC condition, and also in the ZFC condition with increasing field. Several subtleties of our findings are discussed based on results from thermal expansion and dielectric measurements, including (i) the stability of the MA phase, (ii) a difference in lattice parameters between inside bulk and outside layer regions, and (iii) a difference in the phase transition temperature between XRD and dielectric data.
Constructal multi-scale tree-shaped heat exchangers
Authors: da Silva Alexandre K. , Lorente Sylvie , Bejan Adrian
Published: July 2004

This paper describes the conceptual design and performance of balanced two-stream counterflow heat exchangers, in which each stream flows as a tree network through its allotted space. The two trees in counterflow are like two palms pressed against each other. The paper develops the relationships between effectiveness and number of heat transfer units for several tree-counterflow configurations: (i) constructal dichotomous trees covering uniformly a rectangular area, (ii) trees on a disk-shaped area, and (iii) trees on a square-shaped area. In configurations (ii) and (iii) each stream flows between the center and the periphery of the area. Configurations (i) and (ii) are trees with minimal resistance to fluid flow. Configuration (iii) is designed by minimizing the length of each duct in the network. The paper reports the formula for the number of heat transfer units in each configuration. Unlike in counterflows formed by two parallel streams, in which the longitudinal temperature gradient is constant, in the counterflow formed by two trees the longitudinal temperature gradient is steeper as one approaches the periphery of the tree canopy. The application of dendritic heat exchangers to devices with maximal transport density is discussed, e.g., electronics cooling, fuel cell architectures, etc.
Diffusion of boron in germanium at 800–900°C
Authors: Uppal Suresh , Willoughby Arthur F. W. , Bonar Janet M. , Cowern Nick E. B. , Grasby Tim , Morris Richard J. H. , Dowsett Mark G.
Published: July 2004

Diffusion of B in Ge is studied in the temperature range 800–900°C using implantation doping and B doped epitaxial Ge layers. Concentration profiles before and after furnace annealing were obtained using high resolution secondary ion mass spectroscopy (SIMS). Diffusion coefficients were calculated by fitting the annealed profiles using TSUPREM. We obtained diffusivity values which are at least two orders of magnitude lower than the lowest values previously reported in the literature. Using our values an activation energy of 4.65(±0.3)eV is calculated. Present experimental results suggest that interstitial mediated mechanism should be considered for B diffusion in Ge in accordance with recent theoretical calculations. Annealed SIMS profiles also suggest that B solid solubility in Ge is ∼2×1018cm−3 at 875°C which agrees with literature values.
Solid phase epitaxy of amorphous silicon carbide: Ion fluence dependence
Authors: Bae In-Tae , Ishimaru Manabu , Hirotsu Yoshihiko , Sickafus Kurt E.
Published: July 2004

We have investigated the effect of radiation damage and impurity concentration on solid phase epitaxial growth of amorphous silicon carbide (SiC) as well as microstructures of recrystallized layer using transmission electron microscopy. Single crystals of 6H-SiC with (0001) orientation were irradiated with 150keV Xe ions to fluences of 1015 and 1016∕cm2, followed by annealing at 890°C. Full epitaxial recrystallization took place in a specimen implanted with 1015 Xe ions, while retardation of recrystallization was observed in a specimen implanted with 1016∕cm2 Xe ions. Atomic pair-distribution function analyses and energy dispersive x-ray spectroscopy results suggested that the retardation of recrystallization of the 1016Xe∕cm2 implanted sample is attributed to the difference in amorphous structures between the 1015 and 1016Xe∕cm2 implanted samples, i.e., more chemically disordered atomistic structure and higher Xe impurity concentration in the 1016Xe∕cm2 implanted sample.
Effects of gate material on Fowler-Nordheim stress induced thin silicon dioxide degradation under negative gate bias
Authors: Samanta Piyas , Chan Mansun
Published: July 2004

An exhaustive theoretical investigation on the role of gate material as well as commonly used metal deposition processes [viz., electron beam (e-beam) evaporation and thermal evaporation] on high-field stress-induced dielectric breakdown and∕or degradation of identically thick (8–10nm) thermally grown silicon dioxide (SiO2) films used in memory devices has been reported. Gate materials studied here are n+-polycrystalline silicon (polySi) and aluminum (Al) with n-channel metal-oxide-semiconductor field effect transistor structures. Results will be shown here during constant current and constant field Fowler-Nordheim (FN) tunnel injection from the gate into SiO2. Our theoretical results establish that Al-gated structures exhibit poorer dielectric integrity compared to polySi-gated structures under both types of FN stressing technique. Furthermore, compared to thermally deposited Al-gated samples, e-beam evaporated Al-gated samples show slightly higher gate oxide deterioration in either mode of FN stressing studied here.