bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-dimensional, three-velocity particle-in-cell with Monte Carlo collisions simulations of LANSCE’s H− source
Authors: Chacon-Golcher Edwin , Bowers Kevin J.
Published: March 2006

A two-dimensional, three-velocity particle-in-cell with Monte Carlo collisions (MCC) gun code is under development at LANSCE to study surface conversion H− ion source physics from a first-principles approach. The code is electromagnetostatic and is based on a sine transform/tridiagonal matrix Poisson solver implemented in axisymmetric geometry. A filament discharge is simulated by the injection of seed electrons at a biased filament surface. These electrons drive the discharge through a collection of (presently 22) reactions implemented as a MCC package. The simulation region is rectangular with internal electrodes of arbitrary shape. Given the nature of the code, no ad hoc models are necessary to model sheath physics, diffusion across magnetic-field lines, ion extraction, or other plasma phenomena.
Pre- and postextraction analyses of different charge state ion components produced in a laser ion source
Authors: Belloni F. , Doria D. , Lorusso A. , Nassisi V. , Krasa J.
Published: March 2006

Knowledge of the relative abundance and the energy distributions of various ion species produced in laser ablation plasma is essential both in the fundamental physics ground, to understand the thermalization and expansion dynamics of the plasma plume, and in the applied physics ground, to obtain an efficient and versatile ion source. We present measurements and analyses related to such aspects, both for plasma in free expansion and for the extracted ion bunch. In particular, we characterized the former by means of ion current measurements, employing appropriate Faraday cups and electrostatic spectrometers. With regard to the latter, we performed time-of-flight spectrometry and depth profiling of implanted substrates. The extracted bunch signal showed modulation on time of flight due to the presence of multiply charged ions; depth profiling by x-ray photoelectron spectroscopy also showed a maximum penetration range and a profile modulation compatible with a multienergetic beam. In these experiments we used ultraviolet excimer lasers, at irradiance values in the range of 0.1–10GW∕cm2, to generate plasma from Cu and Al targets. The ion beams were extracted at voltages of tens of kV, dc.
Production of energetic proton beams with lasers
Authors: Malka V. , Faure J. , Fritzler S. , Manclossi M. , Guemnie-Tafo A. , d’Humières E. , Lefebvre E. , Batani D.
Published: March 2006

A new method for accelerating proton beams, based on the use of intense and short laser pulses, is presented. It is shown that by focusing an ultrashort laser pulse onto a thin foil target, a proton beam with energy up to tens of MeV is produced. Due to the original properties of these beams unexplored fields in science will be discovered. In addition, this compact and low cost approach would probably be of great interest for medical applications.
Beam deflection by plasma grid filter current in the negative-ion source for JT-60U neutral beam injection system
Authors: Umeda N. , Ikeda Y. , Hanada M. , Inoue T. , Kawai M. , Kazawa M. , Komata M. , Mogaki K. , Ohga T.
Published: March 2006

In the JT-60U negative-ion-based neutral beam injection system, the effect of negative ion and electron deflection by the plasma grid (PG) magnetic filter was studied. After a long-pulse operation of up to 19s, a local melting was observed on the beamline near the ion source, facing the electron drift side of the PG magnetic field. It is confirmed that the experimental deflection of the negative-ion beam agrees well with the three-dimensional beam simulation result by taking account of the measured magnetic field. By using the code, it is found that some stripped electrons produced in the first acceleration gap pass through the down pitch of the multiple apertures in the next stage of acceleration grids, and then collide on the beamline around the melted location.
Influence of geometry of the discharge interval on distribution of ion and electron streams at surface of the Penning source cathode
Authors: Egiazaryan G. A. , Khachatrian Zh. B. , Badalyan E. S. , Ter-Gevorgyan E. I. , Hovhannisyan V. N.
Published: March 2006

In the discharge of oscillating electrons, the mechanism of the processes, which controls the distribution of the ion and electron streams over the cathode surface, is investigated experimentally. The influence of the length of the discharge interval on value and distribution of the ion and electron streams is analyzed. The distribution both of ion and electron streams at the cathode surface is determined at different conditions of the discharge. It is shown that for given values of the anode diameter da=31mm and the gas pressure P=5×10−5Torr, the intensive stream of positive ions falls entirely on the cathode central area in the whole interval of the anode length variation (ℓa=1–11cm). At the cathode, the ion current reaches the maximal value at a certain (optimal) value of the anode length that, in turn, depends on the anode voltage Ua. The intensive stream of longitudinal electrons forms in the short anodes only (ℓa=2.5–3.5cm) and depending on the choice of the discharge regime, may fall both on central and middle parts of the cathode.
Pressure optimization for H− ion production in an electron cyclotron resonance-driven and a filamented source
Authors: Svarnas P. , Breton J. , Bacal M. , Mosbach T.
Published: March 2006

The negative ion density as a function of the hydrogen pressure (1–8mTorr) in the electron cyclotron resonance-driven version of the magnetic multipole volume source “Camembert III” is measured by means of the photodetachment technique. An optimum value is observed between 4 and 5mTorr, yielding a H− ion density of about 1.5×109cm−3 in the center of the source. The electron density monotonously increases in the range ∼(0.5–2.5)×1010cm−3 and the electron temperature decreases (∼1.25–0.5eV). The optimum pressure for H− production is equally reported for a conventional filamented multipole source, in which the influence of rovibrationally excited hydrogen molecules in the electronic ground state on the formation of H− is analyzed. The physical mechanism which determines the existence of this ion density maximum is discussed.
Numerical analysis of the spatial nonuniformity in a Cs-seeded H− ion source
Authors: Takado N. , Hanatani J. , Mizuno T. , Katoh K. , Hatayama A. , Hanada M. , Seki T. , Inoue T.
Published: March 2006

The H− ion production and transport processes are numerically simulated to clarify the origin of H− beam nonuniformity. The three-dimensional transport code using the Monte Carlo method has been applied to H0 atoms and H− ions in the large “JAERI 10A negative ion source” under the Cs-seeded condition, in which negative ions are dominantly produced by the surface production process. The results show that a large fraction of hydrogen atoms is produced in the region with high electron temperature. This leads to a spatial nonuniformity of H0 atom flux to the plasma grid and the resultant H− ion surface production. In addition, most surface-produced H− ions are extracted even through the high Te region without destruction. These results indicate a correlation between the production process of the H− ion and the spatial nonuniformity of the H− ion beam.
Synthesis and thermoelectric properties of Sr-filled skutterudite SryCo4Sb12
Authors: Zhao X. Y. , Shi X. , Chen L. D. , Zhang W. Q. , Zhang W. B. , Pei Y. Z.
Published: March 2006

Strontium-filled skutterudites SryCo4Sb12 have been synthesized by a melting method. The filling fraction of Sr in CoSb3 skutterudite is up to y=0.40, closely consistent with the calculated value by density functional theory methods. The lattice parameters increase linearly with the increase of Sr content, and the relative change in lattice parameters is in agreement with theoretical prediction. Hall effect measurements have been performed by Van de Pauw method at room temperature. All samples filled with Sr atom exhibit n-type conduction. The thermal and electrical transport properties have been measured in the temperature range of 300–850K. The lattice thermal conductivity of SryCo4Sb12 is significantly depressed as compared to that of the unfilled CoSb3. The dimensionless thermoelectric figure of merit, ZT, increases with increasing temperature and reaches a maximum value of 0.9 for Sr0.28Co4Sb12 at 850K.
SPOCK.CI: A multireference spin-orbit configuration interaction method for large molecules
Authors: Kleinschmidt Martin , Tatchen Jörg , Marian Christel M.
Published: March 2006

We present SPOCK.CI, a selecting direct multireference spin-orbit configuration interaction (MRSOCI) program based on configuration state functions. It constitutes an extension of the spin-free density functional theory/multireference configuration interaction (DFT/MRCI) code by Grimme and Waletzke [J. Chem. Phys. 111, 5645 (1999)] and includes spin-orbit interaction on the same footing with electron correlation. Key features of SPOCK.CI are a fast determination of coupling coefficients between configuration state functions, the use of a nonempirical effective one-electron spin-orbit atomic mean-field Hamiltonian, the application of a resolution-of-the-identity approximation to computationally expensive spin-free four-index integrals, and the use of an efficient multiroot Davidson diagonalization scheme for the complex Hamiltonian matrix. SPOCK.CI can be run either in ab initio mode or as semiempirical procedure combined with density functional theory (DFT/MRSOCI). The application of these techniques and approximations makes it possible to compute spin-dependent properties of large molecules in ground and electronically excited states efficiently and with high confidence. Second-order properties such as phosphorescence rates are known to converge very slowly when evaluated perturbationally by sum-over-state approaches. We have investigated the performance of SPOCK.CI on these properties in three case studies on 4H-pyran-4-thione, dithiosuccinimide, and free-base porphin. In particular, we have studied the dependence of the computed phosphorescence lifetimes on various technical parameters of the MRSOCI wave function such as the size of the configuration space, selection of single excitations, diagonalization thresholds, etc. The results are compared to the outcome of extensive quasidegenerate perturbation theory (QDPT) calculations as well as experiment. In all three cases, the MRSOCI approach is found to be superior to the QDPT expansion and yields results in very good agreement with experimental findings. For molecules up to the size of free-base porphin, MRSOCI calculations can easily be run on a single-processor personal computer. Total CPU times for the evaluation of the electronic excitation spectrum and the phosphorescence lifetime of this molecule are below 40 h.
Hugoniot of beta-SiAlON and high-pressure phase transitions
Authors: Sekine Toshimori , Kobayashi Takamichi , Li Xijun
Published: March 2006

Hugoniot equation-of-state data of shock compressed β-Si4Al2O2N6 have been determined to a pressure of 120 GPa. The relationship between shock velocity (Us) and particle velocity (Up) is expressed by a linear equation Us(km∕s)=5.62+0.96Up(km∕s) above a pressure of ∼40GPa beyond the Hugoniot elastic limit (15–16 GPa). A phase transition to spinel is observed at the onset pressure of 32 GPa. According to the systematics of high-pressure phase transitions of spinels that indicate a phase transition to CaTi2O4-type phases at high pressures, β-Si4Al2O2N6 may also be transformed into a CaTi2O4-type phase with further increasing pressure. The previous recovery experiments on β-SiAlON indicated the formation of spinel and amorphous phases at shock pressures of 41–63 GPa. A comparison of the Hugoniot data and recovered samples suggests that the high-pressure CaTi2O4-type phase is not quenchable and quenched as an amorphous phase although a large hysteresis is detected in the release path.
Comparison of nanostructure characteristics of ZnO grown on GaN and sapphire
Authors: Shiao Wen-Yu , Chi Chun-Yung , Chin Shu-Cheng , Huang Chi-Feng , Tang Tsung-Yi , Lu Yen-Cheng , Lin Yu-Li , Hong Lin , Jen Fang-Yi , Yang C. C. , Zhang Bao-Ping , Segawa Yusaburo
Published: March 2006

The immature developments of p-type ZnO and ZnO-related ternary or quaternary compound and the small lattice mismatch between ZnO and GaN stimulate interest in the hybrid growth of ZnO and GaN. In this research, we compared the nanostructures and optical properties of two ZnO thin-film samples grown under the same conditions but on different underlying materials (sapphire and GaN). With the high growth temperature of 450°C, both samples show domain structures. However, in contrast to the sample of ZnO on sapphire, the sample of ZnO on GaN starts with smaller domains (a few tens of nanometers in width) of almost vertical boundaries at the interface and evolves into larger domains (around 100nm in width). The 30° twist of the basal plane in ZnO with respect to its underlying sapphire does not occur in the sample of ZnO on GaN. Within a domain of ZnO on sapphire, misfit and threading dislocations can be periodically observed along the interface with the separation of about 8nm. The transition of the lattice structure across the interface between ZnO and GaN was quite smooth, indicating the high heterojunction quality in this sample. The x-ray diffraction results showed that the crystalline quality of ZnO on GaN is slightly better than that of ZnO on sapphire. However, based on the thermal quenching behavior of integrated photoluminescence intensity, we estimated that the optical quality of the two samples is about the same.
Extremely wideband signal shaping using one- and two-dimensional nonuniform nonlinear transmission lines
Authors: Afshari E. , Bhat H. S. , Hajimiri A. , Marsden J. E.
Published: March 2006

We propose a class of electrical circuits for extremely wideband (EWB) signal shaping. A one-dimensional, nonlinear, nonuniform transmission line is proposed for narrow pulse generation. A two-dimensional transmission lattice is proposed for EWB signal combining. Model equations for the circuits are derived. Theoretical and numerical solutions of the model equations are presented, showing that the circuits can be used for the desired application. The procedure by which the circuits are designed exemplifies a modern, mathematical design methodology for EWB circuits.
Magnetic force microscopy observations of Co nanoparticles grown on annealing Co10Cu90 melt spun ribbons
Authors: Vergara J. , Madurga V. , Eames P. , Dan Dahlberg E.
Published: March 2006

The growth of magnetic clusters by annealing melt spun Co10Cu90 ribbons has been investigated with magnetic force microscopy (MFM) and conventional magnetometry. No clear magnetic structures were observed by MFM in the as-quenched samples. After annealing the samples at 500°C, magnetic Co particles with a mean size of 110±30nm were observed. Further annealing to 750°C increased the Co particle size, which reaches a maximum value of 600nm, and multidomain magnetic structures were observed. The magnetization reversal of these Co nanoparticles was observed by MFM with in situ applied magnetic fields. The MFM observations of an increase in both the density and size of stable Co nanoparticles with increased annealing temperatures are in qualitative agreement with the magnetometry data which indicate a doubling of the magnetization of the Co10Cu90 ribbons after annealing at 750°C.
Electroabsorption and electrorefraction in InAs∕GaAs and InAs∕InP quantum dots
Authors: Prasanth R.
Published: March 2006

Integrated optical cross connects and add-drop multiplexers require low loss polarization independent phase shifting elements. In a composite quantum well, a 0.46mm phase shifter provides a π∕4 phase shift by combining the quantum confined Stark effect (QCSE) and carrier depletion effect. All-optical switching due to state filling effect is also experimentally investigated recently. We investigate whether the discrete energy levels and the high peak absorption in quantum dots (QDs) provide an opportunity for increasing the electrorefraction. The electrorefraction in strained InAs∕GaAs and InAs∕InP quantum dots (QDs) is explored using a numerical model based on the 4×4 Luttinger-Kohn Hamiltonian. The excitonic states are calculated by matrix diagonalization with plane-wave basis states. We observe that the QCSE sharply increases with the height of the QD and is also optimized for small radius QDs. The QCSE in pyramidal QDs is considerably larger than in squares or cylinders. We finally present large electrorefraction in cone shaped pyramidal QDs. State filling effect in QDs further resulted to an electrorefraction higher than an order of magnitude compared to that by QCSE for an acceptable waveguide absorption loss.
The origin of (001) texture evolution in FePt thin films on amorphous substrates
Authors: Kim Jae-Song , Koo Yang-Mo , Lee Byeong-Joo , Lee Seong-Rae
Published: March 2006

A theoretical study has been performed to rationalize the strong evolution of (001) texture during postannealing of deposited Fe50Pt50 thin films on amorphous substrates, by comparing calculated strain energies of several crystals with different orientations under presumed strain conditions. An atomistic calculation method based on an empirical interatomic potential (MEAM) was used to calculate strain and surface energies and atomic force microscope experiments were carried out to confirm the surface energy calculation. The (001) texture evolution could not be explained using traditional factors, the surface energy anisotropy and the in-plane strain. It was found that the strain from the L10 ordering transformation that occurs during postannealing can make the (001) crystal (crystal with [001] crystallographic orientation into the surface normal) energetically most stable among those with various orientations. It is proposed that the occurrence of anisotropic strain due to ordering transformations should be considered as a key factor that affects the texture evolution and that enhanced ordering and recrystallization kinetics is necessary to maximize the strain effect.
Local oxidation of Ga[Al]As heterostructures with modulated tip-sample voltages
Authors: Graf D. , Frommenwiler M. , Studerus P. , Ihn T. , Ensslin K. , Driscoll D. C. , Gossard A. C.
Published: March 2006

Nanolithography based on local oxidation with a scanning force microscope has been performed on an undoped GaAs wafer and a Ga[Al]As heterostructure with an undoped GaAs cap layer and a shallow two-dimensional electron gas. The oxide growth and the resulting electronic properties of the patterned structures are compared for the constant and modulated voltages applied to the conductive tip of the scanning force microscope. All the lithography has been performed in noncontact mode. Modulating the applied voltage enhances the aspect ratio of the oxide lines, which significantly strengthens the insulating properties of the lines on GaAs. In addition, the oxidation process is found to be more reliable and reproducible. Using this technique, a quantum point contact and a quantum wire have been defined and the electronic stability, the confinement potential and the electrical tunability are demonstrated to be similar to the oxidation with constant voltage.
Theoretical study of ion desorption from poly-(methyl methacrylate) and poly-(isopropenyl acetate) thin films through core excitation
Authors: Takahashi Osamu , Tabayashi Kiyohiko , Wada Shin-ichi , Sumii Ryouhei , Tanaka Kenichiro , Odelius Michael , Pettersson Lars G. M.
Published: March 2006

Site-specific chemical reactions following core excitation of poly-(methyl methacrylate) (PMMA) and poly-(isopropenyl acetate) (PiPAc) thin films were investigated. New x-ray absorption spectra of PMMA and PiPAc at the C and O K edges and theoretical spectra within the framework of density functional theory using model molecules were reported, and some new peak assignments were proposed for these spectra. Core-hole excited state molecular dynamics simulations were performed to discuss dissociation dynamics for the target systems, and some specific reaction mechanisms were discussed and explained theoretically; for example, the amount of CH3 ion fragments for PMMA was enhanced at the C and O K edges through the existence of the repulsive σ*(O–CH3) excited state.
Influence of single and double deposition temperatures on the interface quality of atomic layer deposited Al2O3 dielectric thin films on silicon
Authors: Dueñas S. , Castán H. , García H. , de Castro A. , Bailón L. , Kukli K. , Aidla A. , Aarik J. , Mändar H. , Uustare T. , Lu J. , Hårsta A.
Published: March 2006

An electrical characterization of Al2O3 based metal-insulator-semiconductor structures has been carried out by using capacitance-voltage, deep level transient spectroscopy, and conductance-transient (G-t) techniques. Dielectric films were atomic layer deposited (ALD) at temperatures ranging from 300 to 800 °C directly on silicon substrates and on an Al2O3 buffer layer that was grown in the same process by using 15 ALD cycles at 300 °C. As for single growth temperatures, 300 °C leads to the lowest density of states distributed away from the interface to the insulator [disorder-induced gap states (DIGS)], but to the highest interfacial state density (Dit). However, by using 300∕500°C double growth temperatures it is possible to maintain low DIGS values and to improve the interface quality in terms of Dit. The very first ALD cycles define the dielectric properties very near to the dielectric-semiconductor interface, and growing an upper layer at higher ALD temperature produces some annealing of interfacial states, thus improving the interface quality. Also, samples in which the only layer or the upper one was grown at the highest temperature (800 °C) show the poorest results both in terms of Dit and DIGS, so using very high temperatures yield defective dielectric films.
Leakage current of multiferroic (Bi0.6Tb0.3La0.1)FeO3 thin films grown at various oxygen pressures by pulsed laser deposition and annealing effect
Authors: Wang Can , Takahashi Mitsue , Fujino Hidetoshi , Zhao Xia , Kume Eiji , Horiuchi Takeshi , Sakai Shigeki
Published: March 2006

Thin films of multiferroic (Bi0.6Tb0.3La0.1)FeO3 were grown on Pt∕Ti∕SiO2∕Si substrate under various oxygen pressures by pulsed laser deposition technique. X-ray diffraction patterns show that the crystallinity of the thin film is improved with decreasing depositing oxygen pressure and the thin film grown at a lower oxygen pressure of 0.01torr exhibits a single perovskite phase with preferred (001) orientation. Leakage current of the as-deposited thin films decreases with decreasing grown oxygen pressure. Significantly, the leakage current of the thin films can be reduced largely by an annealing process at 800°C with flowing oxygen. The annealed thin films show a relatively high resistivity and stable polarization loops with double remnant polarization of about 3μC∕cm2. The conduction properties of the thin films have been well analyzed, and it is indicated that the dominant conduction mechanism is the space-charge-limited conduction for the thin film grown at higher oxygen pressure and the Poole-Frenkel conduction for the thin films grown at lower oxygen pressure.
Near infrared light emission quenching in organolanthanide complexes
Authors: Quochi F. , Orrù R. , Cordella F. , Mura A. , Bongiovanni G. , Artizzu F. , Deplano P. , Mercuri M. L. , Pilia L. , Serpe A.
Published: March 2006

We investigate the quenching of the near infrared light emission in Er3+ complexes induced by the resonant dipolar interaction between the rare-earth ion and high frequency vibrations of the organic ligand. The nonradiative decay rate of the lanthanide ion is discussed in terms of a continuous medium approximation, which depends only on a few, easily accessible spectroscopic and structural data. The model accounts well for the available experimental results in Er3+ complexes, and predicts an ∼100% light emission quantum yield in fully halogenated systems.
Dependence of the high-field grain-boundary magnetoresistance of ferromagnetic manganites on Curie temperature
Authors: Gangineni R. B. , Dörr K. , Kozlova N. , Nenkov K. , Müller K.-H. , Schultz L. , Seetha Lakshmi L.
Published: March 2006

We analyze the high-field magnetoresistance of polycrystalline ferromagnetic manganites of varied composition in magnetic fields up to μ0H=47T. Small to medium deviations from a linear field dependence of the conductance {as discovered for a La0.7Sr0.3MnO3 film recently, [N. Kozlova et al., J. Magn. Magn. Mater. 261, 48 (2003)]} are typical. The conductance is always well described by a quadratic polynomial G(H)=a+bH+cH2 with temperature-dependent coefficients a, b, and c. Both b and c increase with decreasing Curie temperature (TC) of the samples. The conductance slope 1∕G0 dG∕dH is related to an average susceptibility of the magnetically disturbed layer at the grain boundaries. The nonsaturated magnetic order of grain-boundary (GB) spins at 5K and 47T means the presence of strong antiferromagnetic interactions. The observation of a systematic correlation between GB magnetoresistance and bulk TC indicates that magnetic order at GBs is governed by parameters such as doping and average ionic size (electronic band width). Furthermore, the temperature dependence of linear and quadratic magnetoconductance contributions is investigated; here, the weak temperature dependence for compounds with high TC is notable.
Polarized superconductors in nanostructures
Authors: Bergmann Gerd , Lu Jia G. , Mueller Robert
Published: March 2006

Single electron transistors with two ferromagnetic electrodes in the antiferromagnetic alignment and a superconducting island can induce spin polarization in the superconductor. The effect of spin polarization on the superconducting properties is studied theoretically. Spin injection is an interesting alternative to high magnetic fields to induce large spin polarization in a superconductor. Since this method is not restricted to extremely thin superconductors, it provides a mean to verify exotic states in s superconductors, such as the Fulde-Ferrel-Larkin-Ovchinnikov state. The combination of spin injection and nanosized islands promises an approach to investigate superconductors with high spin polarization.
Reliability screening of diode lasers by multispectral infrared imaging
Authors: Kozlowska Anna , Wawrzyniak Piotr , Malag Andrzej , Teodorczyk Marian , Tomm Jens W. , Weik Fritz
Published: March 2006

Multispectral infrared (IR) imaging is used for reliability screening of diode lasers. The signal detected by the camera in the near IR region is found to be mainly affected by midgap deep level luminescence, whereas the thermal radiation is the main signal contribution in the mid IR. The information from both spectral channels is used for preselection of potentially long-lived devices. The devices with enhanced luminescence from deep levels and elevated temperature of the active region are the candidates for faster degradation. Defects such as hot spots can be discarded extremely quickly by using thermographic inspection. The thermographic results are complemented with spectral Fourier transform photocurrent measurements and low-current I-V characterization. The results of aging experiments of diode lasers are presented. A correlation between measurements of unaged devices and lifetest results is found and discussed in detail. The proposed preselection methodology is an alternative to conventional burn-in tests for high-power diode lasers.
Short-ranged structural rearrangement and enhancement of mechanical properties of organosilicate glasses induced by ultraviolet radiation
Authors: Iacopi F. , Travaly Y. , Eyckens B. , Waldfried C. , Abell T. , Guyer E. P. , Gage D. M. , Dauskardt R. H. , Sajavaara T. , Houthoofd K. , Grobet P. , Jacobs P. , Maex K.
Published: March 2006

The short-ranged bonding structure of organosilicate glasses can vary to a great extent and is directly linked to the mechanical properties of the thin film material. The combined action of ultraviolet (UV) radiation and thermal activation is shown to generate a pronounced rearrangement in the bonding structure of thin organosilicate glass films involving no significant compositional change or film densification. Nuclear magnetic resonance spectroscopy indicates loss of –OH groups and an increase of the degree of cross-linking of the organosilicate matrix for UV-treated films. Fourier transform infrared spectroscopy shows a pronounced enhancement of the Si–O–Si network bond structure, indicating the formation of more energetically stable silica bonds. Investigation with x-ray reflectivity and ellipsometric porosimetry indicated only minor film densification. As a consequence, the mechanical properties of microporous organosilicate dielectric films are substantially enhanced while preserving the organosilicate nature and pristine porosity of the films. UV-treated films show an increase in elastic modulus and hardness of more than 40%, and a similar improvement in fracture energy compared to untreated films. A minor increase in material dielectric constant from 3.0 to 3.15 was observed after UV treatment. This mechanism is of high relevance for the application of organosilicate glasses as dielectric materials for microelectronics interconnects, for which a high mechanical stability and a low dielectric constant are both essential film requirements.
Raman investigation of strain in Si∕SiGe heterostructures: Precise determination of the strain-shift coefficient of Si bands
Authors: Nakashima S. , Mitani T. , Ninomiya M. , Matsumoto K.
Published: March 2006

Raman scattering experiments were carried out on Si∕SiGe heterostructures. The strain in both the top Si layer, and the Si1−xGex buffer layers with various Ge compositions was evaluated using several excitation sources, together with x-ray diffraction and secondary ion mass spectrometry. The strain-shift coefficient, which is a necessary quantity to evaluate the strain by Raman spectroscopy, was precisely determined. The dependence of the Si–Si band frequency on the Ge composition in the SiGe alloy was also examined. We found that the strained top-Si layers with a thickness below 25nm experience coherent growth on Si1−xGex buffer layers with composition x<0.35.
Structure, phase transformation, and magnetic properties of SmCo7−xCrx magnets
Authors: Yao Q. , Liu W. , Zhao X. G. , Li D. , Zhang Z. D.
Published: March 2006

Sm Co 7 − x Cr x  (x=0–0.6) magnets have been synthesized by mechanical alloying and subsequent annealing. A small amount of Cr favors the formation of Th2Ni17-type 2:17 phase and increasing the Cr content can extend the temperature range in which the hexagonal 2:17 phase remains stable. The coercivities of the SmCo7−xCrx magnets are significantly enhanced by increasing the Cr content. The highest coercivity of 15.8kOe is obtained for the SmCo6.4Cr0.6 alloy annealed at 700°C for 30min. The transformation of Th2Ni17-type structure to Th2Zn17-type structure is observed when increasing the annealing temperature. The magnetization reversal process of the alloys has been discussed, according to the measurement of initial magnetization curves and minor hysteresis loops.
Influence of GaNAs strain-compensation layers on the optical properties of GaIn(N)As∕GaAs quantum wells upon annealing
Authors: Liu H. F. , Xiang N.
Published: March 2006

Ga In ( N ) As ∕ Ga As and GaIn(N)As∕GaNAs∕GaAs quantum well (QW) samples, with and without GaNAs strain-compensating layers (SCLs), were grown on GaAs (001) substrates by molecular beam epitaxy. Photoluminescence (PL) was used to study the effects of the GaNAs SCL on the properties of the Ga(In)NAs QWs upon annealing. We observed that the insertion of GaNAs SCL produced a distinct increase in the PL blueshift as a function of annealing time. X-ray diffraction from the strain-compensated GaIn(N)As QWs before and after annealing showed no N atom diffusion, but exhibited Ga–In atom interdiffusion across the QW interfaces. We compared the effects of the GaNAs SCL on the PL blueshift with those of the SiO2 encapsulant upon annealing. The increased PL blueshift caused by the GaNAs SCL for tann⩽40s is attributed to the further GamIn4−m–N (0⩽m⩽4) changes due to greater local strain caused by GaNAs (SCL) quantum barriers as compared with GaAs barriers. For tann>40s, the nonsaturable blueshift caused by GaNAs SCL is attributed to defect-assisted (especially, Ga vacancies) Ga∕In interdiffusion, since the density of Ga vacancy defects in the GaNAs SCLs is quite high.
Characterization of HfSiON gate dielectrics using monoenergetic positron beams
Authors: Uedono A. , Ikeuchi K. , Otsuka T. , Shiraishi K. , Yamabe K. , Miyazaki S. , Umezawa N. , Hamid A. , Chikyow T. , Muramatsu T. Ohdaira M. , Suzuki R. , Inumiya S. , Kamiyama S. , Akasaka Y. , Nara Y. , Yamada K.
Published: March 2006

The impact of nitridation on open volumes in thin HfSiOx films fabricated on Si substrates by atomic layer deposition was studied using monoenergetic positron beams. For HfSiOx, positrons were found to annihilate from the trapped state due to open volumes which exist intrinsically in an amorphous structure. After plasma nitridation, the size of open volumes decreased at a nitrogen concentration of about 20at.%. An expansion of open volumes, however, was observed after postnitridation annealing (PNA) (1050°C, 5s). We found that the size of open volumes increased with increasing nitrogen concentration in HfSiOx. The change in the size of open volumes was attributed to the trapping of nitrogen by open volumes, and an incorporation of nitrogen into the amorphous matrix of HfSiOx during PNA. We also examined the role of nitrogen in HfSiOx using x-ray photoelectron spectroscopy and first principles calculations.
Fourier-transform infrared and photoluminescence spectroscopies of self-assembled monolayers of long-chain thiols on (001) GaAs
Authors: Ding Ximing , Moumanis Khalid , Dubowski Jan J. , Tay Lilin , Rowell Nelson L.
Published: March 2006

Self-assembled monolayers (SAMs) of various thiols have shown the potential to protect freshly fabricated or chemically cleaned GaAs surfaces from oxidization, adsorption of foreign atoms, and∕or surface defect formation. We have employed an attenuated total reflection Fourier-transform infrared spectroscopic technique to investigate the process of formation of long-chain thiols, comprising ten or more methylene chains, on the surface of (001) GaAs. A strong infrared (IR) signal was measured for all the investigated GaAs-thiol interfaces. Varying the type of terminal groups, from hydrophilic to hydrophobic, significantly changes the IR intensity of the methylene stretching vibration, indicating different methylene chain orientation in SAMs. Consequently, these SAMs exhibited different passivation performance to the (001) GaAs surface as judged by the intensity of the GaAs-related photoluminescence signal.
Growth of Ge quantum dots on Si(100)-(2×1) by pulsed laser deposition
Authors: Hegazy M. S. , Elsayed-Ali H. E.
Published: March 2006

Self-assembled germanium quantum dots (QDs) were grown on Si(100)-(2×1) by pulsed laser deposition. In situ reflection-high energy electron diffraction (RHEED) and postdeposition atomic force microscopy are used to study the growth of the QDs. Several films of different thicknesses were grown at a substrate temperature of 400°C using a Q-switched Nd:yttrium aluminum garnet laser (λ=1064nm, 40ns pulse width, 23J∕cm2 fluence, and 10Hz repetition rate). At low film thicknesses, hut clusters that are faceted by different planes, depending on their height, are observed after the completion of the wetting layer. With increasing film thickness, the size of the clusters grows and they gradually lose their facetation and become more rounded. With further thickness increase, the shape of these clusters becomes domelike with some pyramids observed among the majority of domes. The effect of the laser fluence on the morphology of the grown clusters was studied. The cluster density was found to increase dramatically while the average cluster size decreased with the increase in the laser fluence. For a laser fluence of 70J∕cm2, dome- shaped clusters that are smaller than the large huts formed at 23J∕cm2 were observed. At a substrate temperature of 150°C, misoriented three-dimensional (3D) clusters are formed producing only a RHEED background. At 400 and 500°C, huts and a lower density of domes are formed, respectively. Above 600°C, 3D clusters are formed on top of a discontinuous textured layer.
A theoretical insight into the photophysics of psoralen
Authors: Serrano-Pérez Juan José , Serrano-Andrés Luis , Merchán Manuela
Published: March 2006

Psoralen photophysics has been studied on quantum chemistry grounds using the multiconfigurational second-order perturbation method CASPT2. Absorption and emission spectra of the system have been rationalized by computing the energies and properties of the low-lying singlet and triplet excited states. The S1 ππ* state has been determined to be responsible of the lowest absorption and fluorescence bands and to initially carry the population in the photophysical processes related to the phototherapeutic properties of psoralen derivatives. The low-lying T1 ππ* state is, on the other hand, protagonist of the phosphorescence, and its prevalent role in the reactivity of psoralen is suggested to be related to the elongation of the pyrone ring C3–C4 bond, where the spin density is distributed on both carbon atoms. Analysis of energy gaps and spin-orbit coupling elements indicates that the efficient photophysical process leading to the population of the lowest triplet state does not take place at the Franck-Condon region but along the S1 relaxation path.
Analytic model of electron pulse propagation in ultrafast electron diffraction experiments
Authors: Michalik A. M. , Sipe J. E.
Published: March 2006

We present a mean-field analytic model to study the propagation of electron pulses used in ultrafast electron diffraction experiments (UED). We assume a Gaussian form to characterize the electron pulse, and derive a system of ordinary differential equations that are solved quickly and easily to give the pulse dynamics. We compare our model to an N-body numerical simulation and are able to show excellent agreement between the two result sets. This model is a convenient alternative to time consuming and computationally intense N-body simulations in exploring the dynamics of UED electron pulses, and as a tool for refining UED experimental designs.
Nonlinear characteristics of pseudo-Y-junction single-walled carbon nanotubes
Authors: Kim Do-Hyun , Huang Jun , Rao Bangalore K. , Choi Wonbong
Published: March 2006

We fabricated nanoscale field-effect transistors based on Y-junction single-walled carbon nanotubes (SWCNTs) and characterized their performance. The Y-junction transistor was modulated by a metallic branch of the Y-SWCNTs and exhibited the characteristics of an ambipolar field-effect transistor (FET) at room temperature. We obtained a subthreshold swing of 700mV∕decade and an Ion∕off ratio of 105 with a low off-state leakage current of ∼10−13A. These results are of fundamental interest in nano-FET as well as CNT-mat study and may suggest reliable applications in nanoelectronics and sensors.
Indium arsenide quantum wire trigate metal oxide semiconductor field effect transistor
Authors: Gilbert M. J. , Ferry D. K.
Published: March 2006

We present the results of a three-dimensional, self-consistent ballistic quantum mechanical simulation of an indium arsenide (InAs) quantum wire metal oxide semiconductor field effect transistor with channel lengths of 30 and 10nm. We find that both devices exhibit exceptional Ion∕Ioff ratio, reasonable subthreshold swing, and reduced threshold voltage variation. Furthermore, we find that the current in the 30nm case is reduced at the high voltage end of the gate sweep due in part to interference due to lateral states set up in the channel of the device, but in the shorter channel case we do not find this effect for the voltages swept. This effect can be easily seen in the electron density as the perturbations in the density. We also find these states present in the drain voltage sweeps as well. These states present a possible problem for use in complementary metal oxide semiconductor architectures. Finally, we compare the performance of the 10nm InAs trigate device to a similar silicon device. We find that, when a suitable gate material and doping density is chosen, the InAs devices perform comparably to silicon devices in the ballistic limit.
Floating potential and sheath thickness for cylindrical and spherical probes in electronegative plasmas
Authors: Morales Crespo R. , Fernández Palop J. I. , Hernández M. A. , Borrego del Pino S. , Díaz-Cabrera J. M. , Ballesteros J.
Published: March 2006

In this paper, the floating potential, for cylindrical and spherical Langmuir probes immersed into an electronegative plasma, is determined by using a radial model described in a previous paper. This floating potential is determined for several probe radius values and ranks of plasma electronegativity, from almost electropositive plasmas to high electronegative plasmas. The thickness of the positive ion sheath is also determined for this kind of probes in electronegative plasmas, as well as the analytical expressions fitting this thickness, showing its dependence on the probe radius and electric potential.
Effect of interfacial stresses on the elastic behavior of nanocomposite materials
Authors: Yang Fuqian
Published: March 2006

This work analyzes the effective bulk modulus of a composite material consisting of spherical inclusions at dilute concentrations. By introducing the theory of surface elasticity and accounting for the contribution of interfacial stresses, a closed-form expression for the effective bulk modulus is derived. The analysis shows that the dependence of the elastic response on the size of the embedded inclusions in the composite material is different from the classic results obtained in the theory of linear elasticity. This is because of shrinkage of the inclusions caused by the interfacial stresses. The interfacial stresses can either enhance or reduce the effective bulk modulus depending on the bulk modulus ratio of matrix to inclusion.
Visible and infrared emissions from c-axis oriented AlN:Er films grown by magnetron sputtering
Authors: Liu F. S. , Liu Q. L. , Liang J. K. , Luo J. , Zhang H. R. , Zhang Y. , Sun B. J. , Rao G. H.
Published: March 2006

Visible and infrared photoluminescence was observed from AlN:Er films grown by reactive radio-frequency magnetron sputtering under different powers. X-ray diffraction indicates that the films have a c-axis oriented hexagonal wurtzite structure with an average crystallite size of about 59–83nm and a strain of about (1.12–1.69)×10−3. Green emissions (∼540 and 558nm) from the AlN:Er films are due to the intra-4fn transition of Er3+ from H11∕22 and S3∕24 to I15∕24, and infrared (∼1535nm) emissions correspond to I13∕24 to I15∕24 transitions. Increasing the sputtering power results in larger grain size, lower strain, and enhancement of the luminescence intensity.
Theoretical investigation of near gap electronic states of Si∕SiGe multiple quantum wells on (001)-Si or SiGe substrates
Authors: Virgilio Michele , Farchioni Riccardo , Grosso Giuseppe
Published: March 2006

We present a theoretical study of the near gap electronic states of Si∕Ge based multiple quantum well systems composed of Si and Si1−xGex alloys coherently grown on (001)-Si or SiGe substrates. We interpret the experimental photoluminescence spectra recently reported [S. R. Sheng et al., Appl. Phys. Lett. 83, 857 (2003); 83, 2790 (2003)] in terms of direct or indirect k-space transitions. The effect of the spatial localization of the valence and conduction states is analyzed. We investigate the structures in the experiments within the tight binding renormalization method. Strain conditions, spin orbit effects, and quantum confinement are fully considered. Our calculations give an accurate description of the near gap experimental photoluminescence peaks.
Broadband multilayer polarizers for the extreme ultraviolet
Authors: Wang Zhanshan , Wang Hongchang , Zhu Jingtao , Wang Fengli , Gu Zhongxiang , Chen Lingyan , Michette Alan G. , Powell A. Keith , Pfauntsch Sławka J. , Schäfers Franz
Published: March 2006

Nonperiodic molybdenum/silicon broadband multilayer polarizers have been designed using numerical optimization algorithm and fabricated using direct current magnetron sputtering. Their performances have been characterized using the high precision eight-axis soft x-ray polarimeter at the BESSY facility. Different multilayers have measured s-polarized reflectivities of 27% at 13.1nm and higher than 15% over the wavelength range of 13–19nm. Nearly constant s reflectivity, up to 37%, is observed over the 15–17nm wavelength range, where the degree of polarization is more than 98%. Furthermore, these multilayer polarizers also show high s reflectivity and polarization over a broad angular range at fixed wavelength.
Three-dimensional integration of metal-oxide-semiconductor transistor with subterranean photonics in silicon
Authors: Indukuri Tejaswi , Koonath Prakash , Jalali Bahram
Published: March 2006

Monolithic integration of photonics and electronics has been achieved in silicon by three-dimensionally integrating metal-oxide-semiconductor field-effect transistors and waveguide-coupled microdisk resonators. Implantation of oxygen ions into a silicon-on-insulator substrate with a patterned thermal oxide mask followed by a high temperature anneal was utilized to realize the buried photonic structures. This results in the formation of vertically stacked silicon layers separated from each other by an intervening oxide layer. Transistors are fabricated on the surface silicon by conventional processing techniques. Optical and electronic functionalities are thus separated into two different layers of silicon, paving the way toward dense three-dimensional optoelectronic integration.
Control of magnetic-field effect on electroluminescence in Alq3-based organic light emitting diodes
Authors: Odaka Hidefumi , Okimoto Yoichi , Yamada Toshikazu , Okamoto Hiroshi , Kawasaki Masashi , Tokura Yoshinori
Published: March 2006

The magnetic-field effect on electroluminescence (EL) has been investigated for the tris-(8-hydroxyquinolino) aluminum (Alq3)-based organic light emitting diode. The EL intensity sharply increases up to 8% with increasing magnetic field to 500Oe at room temperature. The magnetic field effect on EL depends on the interface structure between a hole transporting and a light emitting layers, indicating the importance of the spin-state dynamics of the electron-hole pairs at the interface.
Multifrequency electron paramagnetic resonance analysis of polycrystalline gadolinium-doped PbTiO3—Charge compensation and site of incorporation
Authors: Eichel Rüdiger-A. , Meštrić Hrvoje , Kungl Hans , Hoffmann Michael J.
Published: March 2006

Gadolinium(III)-modified polycrystalline lead titanate (PbTiO3) of 1.0mol% dopant level was investigated by means of multifrequency electron paramagnetic resonance spectroscopy in order to determine sign and size of the local-environment sensitive finestructure parameter B20. The Gd3+ ions were assigned to substitute for Pb2+ at the A site of the perovskite ABO3 lattice, providing donor centers. The obtained value amounts to B20=0.776(2)GHz at ambient temperature and 0.860(2)GHz at 5K, thus showing a considerable temperature dependence. A charge-compensation mechanism is proposed that is based solely on the creation of lead and oxygen vacancies.