bluuebunny/crossref_metadata_2025 huggingface dataset:
Resistive switching characteristics and set-voltage dependence of low-resistance state in sputter-deposited SrZrO3:Cr memory films
Authors: Park Jae-Wan , Jung Kyooho , Yang Min Kyu , Lee Jeon-Kook , Kim Dal-Young , Park Jong-Wan
Published: August 2006

60 - nm -thick Cr-doped SrZrO3 thin films with polycrystalline structure were fabricated on SrRuO3∕SrTiO3 (100) substrates at 450°C by off-axis radio-frequency sputtering. From room temperature current-voltage measurements of Pt∕Cr-doped SrZrO3∕SrRuO3 structures, reproducible bistable resistive switching behavior was observed. The dominant conduction mechanisms of the high-resistance state were Ohmic conduction in the low-electric-field region and Frenkel-Poole emission in the high-electric-field region, while the low-resistance state fully followed Ohmic conduction. These bulk-limited conduction mechanisms imply that resistive switching phenomena may be related to the conducting path in the SrZrO3:Cr film matrix. In addition, the initial-resistance value of as-deposited SrZrO3:Cr films was much higher than that of the high-resistance state, indicating that the first soft-breakdown behavior (defined as the resistance change from the initial-resistance state of as-deposited film to the low-resistance state) is the forming process which generates the conducting path in the SrZrO3:Cr film matrix. As the set voltage (=switching voltage from the high- to low-resistance state) was increased, both the low-resistance state current and the reset voltage (=switching voltage from the low- to high-resistance state) increased due to the generation of a stronger or higher-density conducting path. We suggest that the mechanism of resistive switching is related to the formation of the conducting path in the SrZrO3:Cr matrix, and that the on-state current is determined by the set voltage which controls the generation of the conducting path.
Isentropic compression of cyclotetramethylene tetranitramine (HMX) single crystals to 50GPa
Authors: Hooks Daniel E. , Hayes D. B. , Hare D. E. , Reisman David B. , Vandersall Kevin S. , Forbes Jerry W. , Hall Clint A.
Published: August 2006

Single crystals of cyclotetramethylene tetranitramine (HMX) were isentropically compressed perpendicular to (010) and (011) faces at the Sandia Z-Machine. A 50GPa ramped magnetic pressure load of about 200ns rise time loaded four specimens of each orientation. HMX specimens were from 300–600μm thick. Velocity histories at the rear of each crystal were measured by Doppler velocimetry. Although a phase change in HMX at 27GPa has been proposed based upon isothermal data, no evidence of this change is seen in our analyses between 5 and 50GPa along the isentrope. Previous isentropic loading experiments on HMX had not shown evidence of a phase change either, but those experiments were complicated by the use of NaCl interferometer windows that have a phase change near the pressure of interest. The experiments described in this paper employed LiF interferometer windows that are known to be absent phase changes in the regime of application. Accurate determination of isentropic compressibility for HMX was not possible using data from these experiments owing to uncertainty in the very large compressibility at low pressures.
High electron mobility of indium oxide grown on yttria-stabilized zirconia
Authors: Koida T. , Kondo M.
Published: August 2006

In 2 O 3 heteroepitaxial layers of improved surface morphology and mobility were obtained by pulsed-laser-deposition method supplying proper oxygen to indium ratio on the growing surface, combined with the suppression of thermal decomposition of In2O3 layers. In situ monitoring of reflection high-energy electron diffraction patterns and ex situ monitoring of growth rate of the epilayers grown under different oxygen pressures and growth temperatures revealed that thermal decomposition occurred during the high-temperature growth and the use of significant oxygen pressure could suppress decomposition allowing for an increase in obtainable growth temperatures. As a result of the decomposition control and growth optimization, undoped In2O3 epilayers exhibited atomically flat surfaces, improved Hall mobility of 110cm2∕Vs, and carrier density of 6.6×1018cm−3. The influences of growth and postgrowth conditions on the electrical properties of epilayers were investigated based on charged and neutral impurity scattering models. The origins of the scattering centers of electrons were considered to be the same for both epilayers grown under different growth conditions (oxygen pressure and growth temperature) and annealed in reduced atmosphere after growths, and ionized impurity scatterings such as oxygen vacancy are the most probable candidates; however, postgrowth annealing process under 1atm oxygen was considered to generate another scattering center, leading to the decrease in both mobility and carrier density.
Tuning the exchange bias of soft metallic antiferromagnets by inserting nonmagnetic defects
Authors: Papusoi Cristian , Hauch Jan , Fecioru-Morariu Marian , Guntherodt Gernot
Published: August 2006

The effect of enhancing exchange bias (EB) by nonmagnetic defects in high-anisotropy insulating antiferromagnets (AFMs), such as CoO, is tested in low-anisotropy metallic AFMs, such as γ-Fe50Mn50. The temperature dependence of EB and coercivity of Ni80Fe20∕(Fe50Mn50)1−xCux ferromagnetic/antiferromagnetic (FM/AFM) bilayers, where x is the defect concentration in the AFM, is investigated. We show both by experiment and Monte Carlo simulations that the EB presents a maximum as a function of x. However, the increasing role of thermal relaxation of the AFM spin structure leads to a decrease of the FM/AFM blocking temperature with increasing x. The simulations indicate the occurrence of a fast thermal relaxation in the AFM at temperatures below the isolated spin blocking temperature. This result explains the experimental observation that with increasing x the temperature dependence of the FM/AFM coercivity changes from a monotonous decrease to a nonmonotonous dependence, exhibiting a maximum near the blocking temperature.
The effect of surface roughness on the resistivity increase in nanometric dimensions
Authors: Marom H. , Eizenberg M.
Published: August 2006

Materials with nanometric dimensions exhibit higher electrical resistivity due to additional scattering centers for the conduction electrons, mainly from surfaces and grain boundaries. In this study we focus on the effect of surfaces by implementing an experimental technique in which the resistivity of thin films is measured during and after etching them inside a solution. This technique enables to analyze the contribution of surfaces to the resistivity and gives a unique insight as for the effect of surface roughness. It is shown that the scattering of electrons from annealed copper films with smooth enough surfaces is mostly specular and that the resistivity in this case is dominated by the effect of grain boundaries. However, when the roughness of the surface becomes larger than the de Broglie wavelength of the electrons, a substantial increase in resistivity occurs. This roughness-induced resistivity is analyzed and shown to be much larger in certain cases than the resistivity predicted for a flat surface, even when all electron scatterings are assumed to be completely diffused.
Assessment of material damage in a nickel-base superalloy using nonlinear Rayleigh surface waves
Authors: Herrmann Jan , Kim Jin-Yeon , Jacobs Laurence J. , Qu Jianmin , Littles Jerrol W. , Savage Michael F.
Published: August 2006

A reliable laser-based ultrasonic technique is developed to measure the second order harmonic amplitude of a Rayleigh surface wave propagating in metallic specimens. Rayleigh waves are experimentally generated with a wedge transducer and detected with a heterodyne laser interferometer. The capability of this system to measure the nonlinear contribution present in Rayleigh surface waves is demonstrated, and these results are interpreted in terms of a parameter developed for Rayleigh surface waves which corresponds to the nonlinear parameter of a longitudinal wave, β. The proposed measurement technique is used to assess damage in nickel-base high temperature alloy specimens, and the evolution of material nonlinearity under various loading conditions is quantitatively measured in terms of the increasing amplitude of the second order harmonic. These results show that there is a significant increase in the second order harmonic amplitude at monotonic tensile loads above the material’s yield stress, and that during low cycle fatigue tests, the increase in the second order harmonic amplitude is considerable, although less significant than that in the monotonic loading case. The results from this study show the major role that accumulated plasticity plays in the increase in material nonlinearity, and demonstrate the effectiveness of the proposed experimental procedure to track damage in high temperature alloys.
Factors driving c-axis orientation and disorientation of LiNbO3 thin films deposited on TiN and indium tin oxide by electron cyclotron resonance plasma sputtering
Authors: Akazawa H. , Shimada M.
Published: August 2006

We clarified critical factors affecting the crystallographic orientation of LiNbO3 (LN) thin films deposited by electron cyclotron resonance plasma sputtering on transparent conductive oxides, i.e., TiN and indium tin oxide (ITO). When LN films were crystallized during sputtering on amorphous TiN, insufficient crystallization (400°C), a roughened TiN surface by crystallization (460 and 530°C), and oxidization of TiN (600°C) interfered with the production of c-axis oriented textured film. Solid-phase crystallization of amorphous LN film on amorphous TiN through rapid thermal annealing provided a solution to disorientation factors. Because of the low crystallization temperature of ITO, both crystallization during sputtering and solid-phase crystallization by postannealing yielded polycrystalline LN for LN film on amorphous ITO, but with a substantial number of c-axis oriented domains. For both amorphous LN film on polycrystalline TiN and polycrystalline ITO, however, nucleation assisted by seeds at the interface resulted in a completely random orientation.
The resonant electromagnetic fields of an array of metallic slits acting as Fabry-Perot cavities
Authors: Hibbins Alastair P. , Lockyear Matthew J. , Sambles J. Roy
Published: August 2006

Fabry-Perot cavities are perhaps the best known of the optical transmission resonators, with cavity field enhancement accomplished by two parallel and partially reflecting planes. Recently it has been shown that arrays of narrow slits cut into a metal substrate are similarly able to exhibit resonant transmission modes. An analysis of the field solutions and transmission properties of this resonant array is compared to the well-known etalon and dielectric slab geometries, revealing a most elegant illustration of the principles of Maxwell’s electromagnetism. It is demonstrated that the matching of the propagating field to each slit-cavity mode is made possible through strong diffraction at each end. Furthermore, the interface between the slit cavities and semi-infinite space beyond acts as a high-impedance surface on resonance, reflecting the field with a positive reflection-amplitude coefficient. Metallic slit arrays have several advantages over conventional Fabry-Perot resonators with interesting application potential.
Refractive indices and band-gap properties of rocksalt MgxZn1−xO (0.68⩽x⩽1)
Authors: Schmidt-Grund Rüdiger , Carstens Anke , Rheinländer Bernd , Spemann Daniel , Hochmut Holger , Zimmermann Gregor , Lorenz Michael , Grundmann Marius , Herzinger Craig M. , Schubert Mathias
Published: August 2006

The room-temperature optical pseudo-dielectric-functions of single-phase, single-crystalline rocksalt-structure MgxZn1−xO with Mg-content x between 0.68 and 1 were determined in the photon energy range from 0.75to9.10eV using spectroscopic ellipsometry. The refractive index determined in the spectral region below the fundamental absorption edge decreases with increasing Mg content. The pseudo-dielectric-functions reveal structures caused in critical points due to electronic band-to-band transitions and free exciton formation at the fundamental band-gap transition. Standard model dielectric function approaches were applied for line shape analysis. Upon increase of the bond ionicity with increasing Mg content the energies of the band-to-band transitions as well as the fundamental-band-gap exciton binding energy parameters increase, while the Γ-point spin-orbit-splitting energy parameter decreases. We compare our results with the band-gap properties of wurtzite-structure MgxZn1−xO with Mg-content x between 0 and 0.5 obtained previously, and we discuss discontinuities across the phase transition. We provide estimates for the band gap bowing parameter of rocksalt-structure MgxZn1−xO and the band-gap value of rocksalt-structure ZnO.
Effects of growth conditions on InAs quantum dot formation by metal-organic chemical vapor deposition using tertiarybutylarsine in pure N2 ambient
Authors: Yin Zongyou , Tang Xiaohong , Zhao Jinghua , Deny Sentosa
Published: August 2006

Self-assembled InAs quantum dots (QDs) on GaAs grown by metal-organic chemical vapor deposition using tertiarybutylarsine as arsenic source in pure nitrogen ambient have been investigated in detail. Effects of the growth conditions, e.g., substrate temperature and inlet precursor flux, on the QD formations have been studied. A temperature window for growing high density (1.3×1010cm−2) of QDs with the trimethylindium (TMIn) flow flux of 30SCCM (SCCM denotes cubic centimeter per minute at STP) is observed from 440to460°C. High density, ⩾1.3×1010cm−2, of the QDs have been grown at higher temperature, 500°C, with the higher TMIn flux of 75SCCM, but the formed QDs are not very uniform. To improve the QD uniformity and density further, a two-step growth method has been proposed and investigated. Using the two-step growth method, the dot density achieves 30% higher than that of the QDs grown by using the normal Stranski-Krastanow growth method. The size of the QDs formed by the two-step growth is larger and more uniform.
Understanding the role of annealing temperature and ion energy in the growth of Au clusters
Authors: Reinke P. , Howe J. , Eswaramoorthy S. , Thune E.
Published: August 2006

Many of the established processes in the fabrication of thin films use plasma or ion beam assisted techniques to control film composition and structure. To improve the understanding of the role of low energy ions we investigate the influence of temperature (273–873K) on the growth of Au clusters, which are deposited on amorphous carbon substrates. The use of a mass selected ion beam facility allowed to control the ion energies and afforded a narrow energy distribution. The Au+ energy was adjusted between 320 and 20eV, a substantial structural modification of the carbon layer surface due to irradiation or annealing can be excluded. The films were characterized with transmission electron microscopy (TEM), and photoelectron spectroscopy (PES) and subjected to equivalent annealing cycles in situ. In the core level PES a dynamic final state effect which leads to a cluster size dependent peak shift was exploited to determine cluster sizes in the subnanometer regime. TEM and PES are established successfully as complementary methods and allow to access the complete size range and implantation depth of the Au ions. Annealing leads to a substantial increase in the number of small clusters, a cluster ripening is only observed towards the end of the annealing cycle. At the highest ion energies of 200 and 320eV, the initial distribution contains only very small clusters (a few atoms) and the growth proceeds faster for 200eV. A qualitative model, which considers the particle fluxes within the system, is used to analyze these results and confirms the critical role of the subsurface Au reservoir. The initial ion energy determines the Au implantation depth and controls the Au concentration in the reservoir. The Au concentration then determines the diffusive flux of atoms to the surface and thus drives the nucleation of new, small clusters and controls the growth rate. Only the presence of such a reservoir allows the nucleation of new Au clusters as the annealing temperature is increased. These results indicate pathways to the control of cluster size and impact on the interpretation and control of thin film deposition.
Field emission from reconstructed heavily phosphorus-doped homoepitaxial diamond (111)
Authors: Yamada Takatoshi , Okano Ken , Yamaguchi Hisato , Kato Hiromitsu , Shikata Shin-ichi , Nebel Christoph E.
Published: August 2006

We report about field emission from reconstructed phosphorus-doped diamond surfaces. In order to reconstruct the surface, annealing at 950°C for 60min in a high vacuum system has been applied. Field emission shows the lowest threshold field for the reconstructed surface of 16V∕μm, while the threshold fields for oxidized and hydrogen-terminated surface are 28 and 44V∕μm, respectively. A model is introduced to discuss these results, which takes into account effective electron affinities and tunneling of electrons from conduction band and donor levels.
Random laser action in ZnO doped polymer
Authors: Vutha Amar C. , Tiwari S. K. , Thareja R. K.
Published: August 2006

We report on random lasing action in zinc oxide microparticles dispersed in organic disordered hybrid polymer. Lasing action from zinc-oxide–polymer hybrid is reported at temperature ranging from 6to250K. Coherent backscattering measurements reveal that the transport of light in the sample is diffusive and feedback for lasing is incoherent. The zinc oxide particles provide both the gain and the scattering whereas polymer matrix offers ease of material fabrication.
Effects of intertube coupling and tube chirality on thermal transport of carbon nanotubes
Authors: Yan X. H. , Xiao Y. , Li Z. M.
Published: August 2006

We study the thermal conductivity of single-walled carbon nanotube bundles and multi-walled carbon nanotubes. It is shown that, for an individual single-walled carbon nanotube, its thermal conductivity is both diameter and chirality dependent. In defect-free bundles and multi-walled carbon nanotubes, the phonon essentially does not transcend the bonds between the constituent walls due to the weak intertube interaction. If, however, the intertube coupling is strong, a substantial reduction in the thermal conductivity maybe implied. Such a low thermal conductivity can be found in several thermal transport experiments of carbon nanotube mats.
Elucidating the mechanism of transient loss of phosphorus due to interface segregation
Authors: Chang R. D. , Tsai J. R. , Ho L. W.
Published: August 2006

The mechanism of the transient dose loss of phosphorus from silicon was experimentally investigated to understand mechanisms of dopant segregation at a SiO2∕Si interface. Phosphorus is lost and then partially recovered during annealing after implantation. The rate of dose recovery depends on the concentration of phosphorus at the silicon surface. Dopant clustering at the interface is proposed to explain the fact that the surface concentration remains nearly constant during dose recovery. When the dose recovery rate decreases with the surface concentration declines, the trapping of dopants by individual interface defects is believed to be the major mechanism of segregation.
History effects on the viscous motion of acoustically forced particles
Authors: Allen J. S. , Kobayashi M. H. , Coimbra C. F. M.
Published: August 2006

We examine memory viscous effects on the motion of a particle in the vicinity of nodal points. The Basset history force has a fractional derivative dependence of order 3∕2 on the particle displacement, and its contribution to the dynamics of acoustically forced particles is demonstrated for typical conditions occurring in acoustic separation devices. The inclusion of the Basset history force in the formulation has the net effect of changing the eigenfunctions of the problem from exponential to Mittag-Leffler functions, thus affecting the overall balance of forces considered.
A comparative study of the electrical properties of heavily Al implanted, single crystalline and nanocrystalline SiC
Authors: Heera V. , Madhusoodanan K. N. , Skorupa W. , Dubois C. , Romanus H.
Published: August 2006

The electrical properties of heavily Al doped single and nanocrystalline 4H–SiC layers on semi-insulating 4H–SiC substrate, prepared by multienergy, high-fluence Al implantation and subsequent furnace annealing, are investigated by sheet resistance and Hall effect measurements. Ion beam induced crystallization is used to prepare the nanocrystalline SiC layers. The doping levels are chosen around the solid solubility limit of 2×1020cm−3 in the range from 5×1019to1.5×1021cm−3. The comparison of the results shows that heavily Al doped single crystalline SiC layers have superior conduction properties. The lowest resistivities measured at room temperature are 0.08 and 0.8Ωcm for the single crystalline and nanocrystalline samples, respectively. Recent results on enhanced Al acceptor activation in nanocrystalline SiC cannot be confirmed. There is an upper limit for the hole concentration in the nanocrystalline samples independent of the Al supersaturation level in the as-implanted state due to outdiffusion of Al in excess to the solid solubility limit during annealing. In contrast to the nanocrystalline SiC layers the as-implanted Al profile in single crystalline material remains stable after annealing even for concentrations above the solid solubility limit. Therefore, in single crystalline material efficient impurity band conduction due to strongly interacting acceptors can be achieved in the range of supersaturation. For lower doping levels impurity band conduction is more effective in nanocrystalline SiC.
Effects of nitrogen doping and illumination on lattice constants and conductivity behavior of zinc oxide grown by magnetron sputtering
Authors: Yao B. , Shen D. Z. , Zhang Z. Z. , Wang X. H. , Wei Z. P. , Li B. H. , Lv Y. M. , Fan X. W. , Guan L. X. , Xing G. Z. , Cong C. X. , Xie Y. P.
Published: August 2006

A yellow-orange nitrogen-doped zinc oxide (ZnO:N) film was deposited on a quartz glass substrate at 510K by reactive radio-frequency magnetron sputtering of a ZnO target with sputtering gas of nitrogen. The lattice constants of the as-grown ZnO:N are much larger than those of undoped ZnO, and decrease with increasing annealing temperature due to escape of the nitrogen from the ZnO:N and decrease of tensile stress, accompanied with color change from yellow-orange to pale yellow. The nitrogen occupies two chemical environments in the ZnO:N based on x-ray photoelectron spectroscopy measurement. One is NO acceptor formed by substitution of N atom for O sublattice, and another is (N2)O double donors produced by substitution of N molecular for O site, which make the lattice constants expanded. The as-grown ZnO:N film shows insulating, but behaves p-type conduction in the dark after annealed at 863K for 1h under 10−3Pa. Unfortunately, the p-type conduction is not stable and reverts to n type soon. However, after illuminated by sunlight for several minutes, the n-type ZnO:N transforms into p type again. The mechanism of the transformation of the conductivity behavior is discussed in the present work.
Identification of sulfur double donors in 4H-, 6H-, and 3C-silicon carbide
Authors: Reshanov S. A. , Pensl G. , Nagasawa H. , Schöner A.
Published: August 2006

Sulfur ions are implanted into 6H-, 4H-, and 3C-SiC. Admittance and double correlated deep level transient spectroscopy investigations reveal that S atoms form double donors in SiC, which reside on lattice sites. The number of double donors observed corresponds to the number of inequivalent lattice sites of the particular SiC polytype. The temperature dependence of the electron capture cross section is determined by the filling pulse method. It is shown that the capture of electrons into S donors is dominated by the cascade mechanism.
Metal/molecule/p-type GaAs heterostructure devices
Authors: Lodha Saurabh , Janes David B.
Published: August 2006

Molecular devices in a metal/molecule/p-type GaAs configuration were fabricated, electrically characterized, and analyzed using an electrostatic model. Various alkane and aromaticthiols were self-assembled on GaAs substrates and the top metal contact was formed by a low energy, indirect path technique. Spectroscopic and surface characterization results indicate the formation of a nonpenetrative, robust metal contact on smooth, uniform, and crystalline monolayers. Molecular devices with a Au top contact exhibit increased conductivity and less rectification than the control Schottky devices. The observed temperature and voltage dependence of the current is consistent with direct tunneling transport in the case of all the molecular samples. An electrostatic model which considers the dielectric constant and dipole charge of the molecular layer has been developed to estimate the GaAs depletion barrier at the molecule-GaAs interface. A semiquantitative model has been developed for device conductivity, which assumes that the conductivity is proportional to the product of the molecular density of states and the strength of molecular coupling to the GaAs contact. It uses an analytical thermionic-field emission model for the semiconductor depletion barrier to estimate the strength of molecular coupling to the GaAs contact. The relative conductances of the molecules, after accounting for the effects of the semiconductor portion of the barrier, are in line with reported values for the densities of states of the molecules and calculated conductances for corresponding metal-molecule-metal devices.
Free-carrier absorption and active layer heating in large optical cavity high-power diode lasers
Authors: Ryvkin B. S. , Avrutin E. A.
Published: August 2006

An analytical theory is developed for the thermal properties of a large optical cavity semiconductor laser high above threshold. The performance of symmetric and asymmetric waveguide structures is analyzed and compared. Narrow asymmetric waveguides are predicted to be advantageous from the thermal point of view.
Interaction between long-crested random waves and a submerged horizontal cylinder
Authors: Arena Felice
Published: August 2006

This paper is concerned with the interaction between long-crested random waves and a submerged horizontal cylinder in deep water. The analytical linear predictions are obtained by extending the classical linear solution for the diffraction of monochromatic waves by a horizontal submerged cylinder, to the random waves by applying the theory of wind-generated waves. It is obtained that the wave pressure, which represents a random Gaussian process, has an amplitude that increases on the upper half-cylinder and decreases on the lower half-cylinder, with respect to the wave pressure amplitude at the same depth in an undisturbed wave field. The random wave force is derived by integration of the wave pressure on the cylinder. Both the horizontal and the vertical force components have equal standard deviation and very narrow spectra. By applying the quasideterminism theory, is then obtained that both the wave pressure and the wave force on the cylinder, when a very high wave occurs, are quasi-impulsive in the time domain. Finally, the analytical predictions are compared with the data of a small-scale field experiment. It is shown that the linear theory well predicts the drop of the propagation speed of the wave pressure on the cylinder. Some discrepancies have been noted for the standard deviation of the random wave force, which is overestimated by analytical predictions.
Scanning transmission electron microscopy investigations of interfacial layers in HfO2 gate stacks
Authors: Agustin Melody P. , Bersuker Gennadi , Foran Brendan , Boatner Lynn A. , Stemmer Susanne
Published: August 2006

Electron energy-loss spectroscopy combined with high-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy was used to investigate the chemistry of interfacial layers in HfO2 gate stacks capped with polycrystalline Si gate electrodes. To interpret the energy-loss near-edge fine structure (ELNES) obtained from the interfacial layers, reference spectra were obtained from single crystal hafnium silicate (HfSiO4), monoclinic HfO2 powder, and amorphous SiO2. No bulk-like silicate bonding could be detected in the ELNES of Si L2,3 and O K edges recorded from layers at the Si substrate interface. Compared to bulk SiO2, the interfacial ELNES showed additional features that were caused by overlap of signals from Si, HfO2, and SiO2, despite a relatively small electron probe size of ∼3Å. HAADF showed that interfacial roughness caused the projected thickness of nominally pure SiO2 (within the detection limit of the method) to be as small as ∼5Å in many locations.
Propagation dependent exciton-spin dephasing and relaxation due to exchange interaction
Authors: Rahimpour Soleimani H. , Ostatnicky T. , Cronenberger S. , Gallart M. , Gilliot P. , Hönerlage B.
Published: August 2006

We use the invariant expansion of an effective Hamiltonian to investigate exciton-spin dephasing and relaxation in a simple two-band bulk semiconductor. When compared to the situation at the Γ point, the point group symmetry of a crystal can be broken at finite wave vectors. As a consequence, depending on the propagation direction, the exchange interaction between electrons and holes in conjunction with the center of mass motion of the excitons can mix different states. This leads to spin beating of excitons. Here we consider exchange terms which are linear, quadratic, and cubic in the wave vector Q and their effect for different directions of propagation. Since dipole active excitons have either transverse or longitudinal character, their total angular momentum (pseudospin) has to be defined with respect to the direction of propagation of the quasiparticles. Therefore, scattering processes, in which the direction of Q is changed, result in a variation of the quantization direction and lead to spin relaxation. In low symmetry directions, both processes contribute to the time evolution of the spin state.
Negative differential conductance and bistability in undoped GaAs∕(Al,Ga)As quantum-cascade structures
Authors: Lu S. L. , Schrottke L. , Teitsworth S. W. , Hey R. , Grahn H. T.
Published: August 2006

We discuss the mechanisms for negative differential conductance (NDC) and bistable behavior observed in the current-density–electric-field (j-F) characteristics of undoped GaAs∕Al0.45Ga0.55As quantum-cascade structures (QCSs). While the j-F characteristic of a QCS with a single period exhibits NDC without bistability, NDC and a bistable behavior are observed for a QCS with 20 periods of the same design. Calculations using a scattering-rate model neglecting any field inhomogeneities show that the interplay of resonant tunneling between the states in the QCS with resonant scattering of longitudinal optical phonons can lead to NDC in both structures. However, the bistable behavior in the QCS with 20 periods can only be explained if an inhomogeneous field distribution due to charge accumulation is taken into account in addition to the NDC. The abrupt decrease of the current density at the field strength of the bistability is attributed to a rapid decrease of the accumulated charge, which is confirmed by electric-field-dependent photoluminescence spectroscopy.
Spintronic investigation of the phase separated manganite (La,Ca)MnO3
Authors: Hueso L. E. , Granja L. , Levy P. , Mathur N. D.
Published: August 2006

We report magnetic and electrical transport studies of an epitaxially grown trilayer thin film structure comprising La0.59Ca0.41MnO3 sandwiched between La0.7Ca0.3MnO3 electrodes. Since La0.59Ca0.41MnO3 lies at the edge of the thin film ferromagnetic metallic phase field, phase separation effects are expected. These effects can explain the observed magnetic isotropy of the middle layer. By contrast, the electrode material is magnetically uniaxial. Easy axis magnetic field sweeps of the trilayer produce two sharp magnetic transitions, but only one sharp transition in current-in-plane resistance measurements.
Simulation of a direct current microplasma discharge in helium at atmospheric pressure
Authors: Wang Qiang , Economou Demetre J. , Donnelly Vincent M.
Published: August 2006

A numerical simulation of a dc microplasma discharge in helium at atmospheric pressure was performed based on a one-dimensional fluid model. The microdischarge was found to resemble a macroscopic low pressure dc glow discharge in many respects. The simulation predicted the existence of electric field reversals in the negative glow under operating conditions that favor a high electron diffusion flux emanating from the cathode sheath. The electric field adjusts to satisfy continuity of the total current. Also, the electric field in the anode layer is self adjusted to be positive or negative to satisfy the “global” particle balance in the plasma. Gas heating was found to play an important role in shaping the electric field profiles both in the negative glow and the anode layer. Basic plasma properties such as electron temperature, electron density, gas temperature, and electric field were studied. Simulation results were in good agreement with experimental observations.
Emission enhancement from metallodielectric-capped ZnO films
Authors: Ni W. H. , An J. , Lai C. W. , Ong H. C. , Xu J. B.
Published: August 2006

Emission enhancement from ZnO thin films coated with Al∕AlOx has been studied by photoluminescence spectroscopy. While Al exhibits a moderate enhancement on ZnO, the introduction of an ultrathin AlOx spacer can increase the luminescence significantly. By examining the dependence of light emission of Al∕AlOx∕ZnO on AlOx thickness, we found, other than the surface plasmon mediation, a short-ranged nonradiative channel that plays a crucial role in determining that the overall emission enhancement is also present. Based on a simple analytical model, it is found that the nonradiative process is strongly dependent on (AlOxthickness)−6 and therefore its origin is suggested to be of Förster type. Finally, an AlOx spacer with thickness of 5nm is found to be adequate to eliminate the unwanted quenching effect.
Improved stability of nanocrystalline porous silicon after coating with a polymer
Authors: Mandal N. P. , Sharma Ashutosh , Agarwal S. C.
Published: August 2006

The effect of light soaking (LS) on nanocrystalline porous silicon (nc-Si) is found to depend on the ambient conditions. Light soaking in air decreases the photoluminescence (PL) intensity and increases the number of dangling bond density measured by electron spin resonance (ESR) monotonically as the exposure time is increased. In vacuum, however, short light exposures increase the PL intensity and decrease the ESR signal, but longer exposures have the opposite effect. Thus PL intensity has a maximum as a function of exposure time. The temperature dependence of LS in vacuum is measured at 300, 325, and 350K. We found that the maximum PL intensity occurs at smaller exposure times as the temperature is increased. These results are explained in terms of two kinds of photostructural changes having opposite LS effects on the PL. Coating nc-Si with a thin layer of polystyrene stabilizes PL against light soaking as well as water vapor, without affecting the PL intensity significantly. Modified bonding configurations at the nc-Si/polystyrene interface, as observed by Fourier transform infrared, seem responsible for the improved stability.
Electrodeless measurement of charge carrier mobility in pentacene by microwave and optical spectroscopy techniques
Authors: Saeki Akinori , Seki Shu , Tagawa Seiichi
Published: August 2006

Photoinduced transient conductivity of pentacene thin films prepared by thermal vapor deposition is investigated using time-resolved microwave conductivity (TRMC) measurements, giving intrinsic charge carrier mobility in highly ordered structures without any electrode attached. The real and imaginary conductivity values are analyzed and compared with those predicted by molecular orbital calculations. The effects of substrate surface treatment by hexamethyldisilazane are discussed on the basis of kinetic traces of conductivity and morphology. A mobility of >0.7cm2∕Vs was obtained from the TRMC measurements and analysis of transient absorption spectra. The measurement of field-effect-transistor mobility in single-crystal domains requires not only complicated fabrication techniques but also many assumptions on the carrier channels, density, injection, etc. The combination of TRMC and transient optical spectroscopy provides an approach for addressing this issue. The present technique is applicable to a wide variety of organic semiconducting materials. Moreover, it is the only technique revealing the intrinsic potentials of mobility in materials that is supported by complete experimental and quantitative procedures not based on any assumptions.
Stokes flow through a tube with bumpy wall
Authors: Wang C. Y.
Published: August 2006

The Stokes flow through a tube with a bumpy wall is solved through a perturbation in the small amplitude of the three-dimensional bumps. The solution can be expressed in terms of modified Bessel functions. For given area or volume of a bump, there exists an optimum circumferential wave number and an optimum aspect ratio for which the flow rate is maximized.
Second axial Fe3+ center in stoichiometric lithium tantalate
Authors: Malovichko G. , Petersen R. , Bäuman Ch. , Grachev V.
Published: August 2006

The axial Fe3+ center Fe1 with the crystal field parameter b20≈3130×10−4cm−1 is well studied in congruent lithium tantalate crystals. The second axial Fe3+ center Fe2 was discovered and investigated by the electron paramagnetic resonance in stoichiometric samples prepared by vapor transport equilibrium treatment. The crystal field parameter of the Fe2 center (b20≈2050×10−4cm−1) is significantly smaller than for Fe1. The electron nuclear double resonance measurements have shown that hyperfine interactions of the Fe3+ electrons with the surrounding Li nuclei for Fe2 are stronger than for Fe1. Therefore, the conclusion was made that in the case of Fe2 center the iron ion substitutes for Ta and has Li nuclei in the nearest neighborhood, whereas in the case of Fe1 center it substitutes for Li, has Ta nuclei as nearest neighbors and Li nuclei in the second shell only.
Computational modeling of process induced damage during plasma clean
Authors: Rauf S. , Haggag A. , Moosa M. , Ventzek P. L. G.
Published: August 2006

When partially completed circuits come in contact with plasmas during integrated circuit fabrication, current from the plasma can potentially damage active devices on the wafer. A suite of computational models is used in this article to investigate damage to ultrathin (1.0–5.5nm) transistor gate dielectric (SiO2) during Ar∕O2 based plasma cleaning in a capacitively coupled plasma reactor. This modeling infrastructure includes a two-dimensional plasma equipment model for relating process control parameters to ion and electron currents, a three-dimensional model for flux density calculation within a circular via, an electrostatic model for computing potential across the gate dielectric, and a percolation model to investigate dielectric damage characteristics. Computational results show that when the plasma current comes in contact with the gate dielectric, the gate dielectric rapidly charges up and the potential difference across the dielectric saturates at the level necessary to support the plasma induced current. The steady-state voltage across the dielectric determines the propensity of irreversible damage that can occur under this electrical stress. Gate dielectric damage was found to be most sensitively linked to dielectric thickness. As thin dielectrics (<2.0nm) are leaky, direct tunneling current flow ensures that the potential drop across the gate dielectric remains small. As a consequence, the dielectric is able to withstand the plasma current and the probability of damage is small. However, for thicker dielectrics where Fowler-Nordheim tunneling is dominant, a large voltage builds up across the gate dielectric due to the plasma induced current. The probability of thicker dielectrics getting damaged during the plasma process is therefore high. For given plasma conditions and gate dielectric thickness, current collection area (i.e., antenna size) determines the voltage buildup across the gate dielectric. Damage probability increases with the size of the antenna connected to the transistor gate electrode. Via aspect ratio and plasma process condition variations (around a given set of process conditions) have a relatively smaller effect on plasma induced device damage.
Gallium nitride nanowire nonvolatile memory device
Authors: Cha Ho-Young , Wu Huaqiang , Chae Soodoo , Spencer Michael G.
Published: August 2006

We report the fabrication and characterization of gallium nitride (GaN) nanowire nonvolatile memory field-effect transistors (FETs). The memory device was implemented using a top-gate GaN nanowire FET with an oxide layer as a storage node. A 40nm thick silicon dioxide layer was embedded between the top metal gate and the nanowires, which was deposited using plasma enhanced chemical vapor deposition. Charges were stored in and released from the oxide layer by applying negative and positive gate biases, respectively. It is suggested that charge transport at the gate edge is responsible for the write and erase mechanisms. The locally enhanced electric field at the gate edge induces Fowler-Nordheim tunneling from the metal gate, while the much lower field near the interface between the oxide and the nanowire channel suppresses charge transport.
Microstructure dependent physical properties of evaporated tin sulfide films
Authors: Devika M. , Ramakrishna Reddy K. T. , Koteeswara Reddy N. , Ramesh K. , Ganesan R. , Gopal E. S. R. , Gunasekhar K. R.
Published: August 2006

In the field of photovoltaics, semiconductors of the III-V group such as GaAs and InP have been considered as the most efficient absorber materials due to their direct energy band gap and high mobility. In these compounds, arsenic and phosphorus are highly toxic and expensive. In this work we present systematic preparation of low cost SnS thin films and characterize these films to test their suitability for photovoltaic applications. We have observed that the films (with thickness ≅0.5μm) grown at the substrate temperature of 275°C exhibit a low resistive single SnS phase and have a direct optical band gap of 1.35eV with an absorption coefficient of ∼105cm−1. SnS films could be alternative semiconductor materials as absorbers for the fabrication of photovoltaic devices.
Ultraviolet pulse laser induced modifications of native silicon/silica interfaces analyzed by optical second harmonic generation
Authors: Scheidt T. , Rohwer E. G. , von Bergmann H. M. , Stafast H.
Published: August 2006

Native silicon/silica (Si∕SiO2) interfaces are investigated by electric field induced second harmonic (EFISH) generation employing near infrared femtosecond laser pulses (782.8nm, 80fs, 10nJ, 80MHz repetition rate). Here, the temporal EFISH evolution induced by the femtosecond laser irradiation is recorded with a time resolution of 0.2s. Comparative EFISH studies are performed with Si∕SiO2 interfaces, preexposed to ultraviolet (UV) laser pulses (308nm, 16ns, 0.45–2.8J∕cm2) as well as virgin Si∕SiO2. After UV irradiation the femtosecond laser induced electron injection and trapping in the ultrathin oxide is found to be drastically accelerated in contrast to the essentially unaffected interfacial hole dynamics. This result is explained by an enhanced interfacial electron trap density caused in most cases by the UV laser induced melting and recrystallization of the near-interface silicon. Furthermore, three-dimensional second harmonic imaging reveals a saturation effect of the UV induced sample modification due to the melting of the near-interface silicon, for which a threshold fluence of 0.47J∕cm2 is extracted for xenon chloride (XeCl) laser irradiation. Below this threshold an accumulative behavior of the UV induced modification is observed for repeated exposure to several UV laser pulses. This is pointing to a dose dependent modification process presumably due to thermally driven interface chemistry and/or microscopic structural and electronic changes of the Si∕SiO2 interface.
Effective thermal conductivity in nanofluids of nonspherical particles with interfacial thermal resistance: Differential effective medium theory
Authors: Zhou Xiao Feng , Gao Lei
Published: August 2006

By taking into account the interfacial thermal resistance across the solid particles and the host liquids, we present differential effective medium theory to estimate the effective thermal conductivity in nanofluids of nonspherical solid particles. It is found that high enhancement of effective thermal conductivity can be achieved when the nanoparticles’ shape is deviated much from the spherical one. On the other hand, increasing the interfacial thermal resistance results in an appreciable degradation in the thermal conductivity enhancement. To one’s interest, our theoretical results are in good agreement with recent experimental data on nanofluids. In particular, our theoretical predictions successfully show the nonlinear dependence of effective thermal conductivity on the volume fractions of nanotubes.
Nanopore sculpting with noble gas ions
Authors: Cai Qun , Ledden Brad , Krueger Eric , Golovchenko Jene A. , Li Jiali
Published: August 2006

We demonstrate that 3keV ion beams, formed from the common noble gases, He, Ne, Ar, Kr, and Xe, can controllably “sculpt” nanometer scale pores in silicon nitride films. Single nanometer control of structural dimensions in nanopores can be achieved with all ion species despite a very wide range of sputtering yields and surface energy depositions. Heavy ions shrink pores more efficiently and make thinner pores than lighter ions. The dynamics of nanopore closing is reported for each ion species and the results are fitted to an adatom diffusion model with excellent success. We also present an experimental method for profiling the thickness of the local membrane around the nanopore based on low temperature sputtering and data is presented that provides quantitative measurements of the thickness and its dependence on ion beam species.
Effects of large density variation on strongly heated internal air flows
Authors: Bae Joong Hun , Yoo Jung Yul , Choi Haecheon , McEligot Donald M.
Published: August 2006

Direct numerical simulation (DNS) is employed to examine the effects of large density variation of strongly heated air flowing in a vertical pipe on turbulent heat and momentum transfer. The predictions of the heat transfer and skin friction coefficients as well as the mean velocity and temperature profiles are in excellent agreement with the existing experimental data. Like some previous studies on heated air flows, the present study shows that the flow is laminarized with heat flux because a large reduction occurs in turbulence intensity. Unlike the velocity fluctuations, however, the thermal turbulence intensities such as the normalized density and temperature fluctuations remain as relatively insensitive to the heating conditions. Moreover, it is observed that the mean velocity and temperature profiles become dissimilar to each other during the process of laminarization with heating. In the present study we elucidate these anomalous behaviors of the strongly heated air flows with a notion of similarity breakdown between the mean velocity and temperature profiles, which occurs due to large density variations, and by examining the production rates of the turbulence kinetic energy and thermal turbulence fluctuation variance.