bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical conduction of carbon nanotube atomic force microscopy tips: Applications in nanofabrication
Authors: Austin Alexander J. , Nguyen Cattien V. , Ngo Quoc
Published: June 2006

This paper reports the electrical transport properties of the interface of a multiwalled carbon nanotube (MWNT) in physical end contact with a hydrogen-passivated Si surface and a Pt surface. The electrical measurement was performed in an atomic force microscope (AFM) with a MWNT attached to a scanning probe in contact mode at approximately 50% relative humidity. AFM force-distance spectroscopy was employed to set the degree of contact between the MWNT tip with the surface. The tip-substrate interface dominates the electrical measurement in this configuration, showing electrical conductivity characteristics indicative of the tip-substrate junction. MWNT tips in contact with a Pt surface exhibit a linear I-V behavior with electrical resistances in the range of 30–50kΩ, demonstrating the metallic nature of the MWNT. Results are presented for the investigation of the current-induced joule heating limitations of MWNT tips under ambient conditions. Thinning of the outer walls through a current-induced thermal oxidation process is observed at a current greater than 5μA, exhibiting a current density of greater than 106A∕cm2. For a MWNT tip in end contact with a highly p-doped silicon surface, a diode-like metal-insulator-semiconductor (MIS) junction is measured. Modeling of the MIS junction is presented and compared to the experiments.
Measurements of shock initiation in the tri-amino-tri-nitro-benzene based explosive PBX 9502: Wave forms from embedded gauges and comparison of four different material lots
Authors: Gustavsen R. L. , Sheffield S. A. , Alcon R. R.
Published: June 2006

We have completed a series of ambient temperature (23±2°C) shock initiation experiments on four lots (batches) of the insensitive high explosive PBX 9502. PBX 9502 consists by weight of 95% dry-aminated tri-amino-tri-nitro-benzene (TATB) and 5% of the plastic binder Kel-F 800, a 3∕1 copolymer of chloro-trifluoro-ethylene and vinylidene-fluoride. Two of the four lots were manufactured using the “virgin” process. Both of these lots had few fine TATB particles. One virgin lot was stored the majority of its life (>15yr) as a molding powder and pressed as a 240mm diameter by 130mm thick cylinder. The other virgin lot was stored the majority of its life as a hollow hemispherical pressing. Two lots were manufactured using the “recycle” process and had many fine TATB particles. One recycled lot was stored the majority of its life as a molding powder, while the other was stored as a pressed charge. Shock initiation experiments were performed using precisely characterized planar shocks generated by impacting an explosive sample with a projectile accelerated in a two-stage gas gun. The evolution of the shock into a detonation was measured using 10 or 11 embedded electromagnetic particle velocity gauges and three “shock tracker” gauges. Results include the following: (1) high quality particle velocity wave forms which should be useful for calibrating reactive burn models, (2) no difference in the sustained shock initiation response between lots regardless of material processing or storage history, (3) responses for all lots equivalent to those measured by Dick et al. [J. Appl. Phys. 63, 4881 (1988)], additional Hugoniot and Pop-plot data for PBX 9502, and (5) the short shock response which, when compared to the sustained shock response, shows no extension in the run distance unless the rarefaction overtakes the shock front prior to the distance it would have run towards a detonation as a sustained shock.
Grazing incidence small-angle x-ray scattering from defects induced by helium implantation in silicon
Authors: Babonneau D. , Beaufort M.-F. , Declémy A. , Barbot J.-F. , Simon J.-P.
Published: June 2006

The formation and growth of defects, including nanocavities and extended interstitial-type defects, created by helium implantation in silicon (50keV, 7.1015cm−2) in the temperature range of 100–550°C has been investigated by grazing incidence small-angle x-ray scattering. We show that quantitative information can be obtained on the size distribution, shape dispersion (i.e., anisotropy and faceting versus the size), and depth profile of the nanocavities from the near surface to deeply buried regions up to 1μm. It is thus demonstrated that low temperature implantations (⩽200°C) lead to the formation of spherical nanocavities with a heterogeneous depth distribution whereas implantations at high temperatures (⩾300°C) cause growth of the cavities, broadening of the size distribution, and size-dependent faceting leading to a size-dependent shape anisotropy. Furthermore, we show that the method allows to characterize the morphology (i.e., width and thickness) and the evolution of extended planar {113} defects created during the implantation process at high temperatures.
Phase selectivity of microwave heating evidenced by Raman spectroscopy
Authors: Vaucher Sébastien , Catala-Civera José-Manuel , Sarua Andrei , Pomeroy James , Kuball Martin
Published: June 2006

Phase-selective temperature evolution of silicon-diamond powder mixtures under 2.45GHz microwave irradiation is followed using a Raman microscope fitted onto a microwave cavity. This spectroscopic method is shown to enable the collection of thermal information for both the silicon and the diamond phase selectively. An initial difference up to 100°C is found between the thermal trajectories extracted from silicon and diamond. A thermal equilibration is observed, subsequently suggesting that heat is flowing from hot silicon towards the colder diamonds. The existence of microscopic thermal gradients reflects the specific interaction of each of the two components with the applied electromagnetic field, governed here by the free carrier concentration. In addition to phase-specific temperature measurements, monitoring of chemical changes during microwave heating is also illustrated for magnetite powder. Raman spectroscopy turns out to be a powerful contact-free, noninterfering tool to follow and analyze on-line microwave heating processing of heterogeneous solid mixtures.
Effects of hydrogen bond redistribution on photoluminescence of a-SiC:H films under thermal treatment
Authors: Vasin A. V. , Kolesnik S. P. , Konchits A. A. , Kushnirenko V. I. , Lysenko V. S. , Nazarov A. N. , Rusavsky A. V. , Ashok S.
Published: June 2006

Hydrogenated amorphous silicon carbide (a-SiC:H) films have been deposited using magnetron sputtering technique. An integrated investigation of the effect of vacuum annealing temperature on photoluminescence properties and paramagnetic defects and its correlation with structural transformation of a-SiC:H has been performed. Significantly enhanced light emission efficiency after low-temperature vacuum treatment (450°C) is found due to enhanced passivation of paramagnetic defects associated with carbon-rich chemically disordered structure. Subsequent high-temperature vacuum annealing results in a decrease of luminescent intensity that is associated with an increase of carbon-related paramagnetic defect states, shown to be the primary nonradiative recombination centers.
Triple-biprism electron interferometry
Authors: Harada Ken , Matsuda Tsuyoshi , Tonomura Akira , Akashi Tetsuya , Togawa Yoshihiko
Published: June 2006

Triple-biprism electron interferometry has been developed to flexibly control all interference parameters, i.e., interference region (size and shape), fringe spacing, and fringe angle. Each filament electrode of the upper and middle electron biprisms is installed in the individual image planes of the objective and first magnifying lenses, respectively. One more filament electrode of the lower biprism is set between the crossover plane and image plane behind the second magnifying lens. The azimuth angle between the upper and middle filaments is set at 90°, and lower filament is set at about 45° for both of the above filaments. Among many possible optical setups, we chose the setup with the easiest and most flexible control of all the interference parameters through theoretical investigation and experimental confirmation. The interferometer was constructed in a 1×106V field emission electron microscope, and the overall performance was demonstrated and investigated experimentally.
Donor/acceptor type photodetectors: Role of substitution in acceptor material
Authors: Sahu Satyajit , Pal Amlan J.
Published: June 2006

We report fabrication and characterization of photodetectors based on organic semiconductors. Single and multiple heterostructures between electron donor and acceptor materials are used. Moreover, by varying the substitutes of the acceptor molecule, five different donor/acceptor combinations are presented. Photocurrent depends on the choice of the acceptor material, showing importance of energy mismatch between lowest unoccupied molecular orbital (and highest occupied molecular orbital) levels of donor and acceptor molecules. Additionally, the photocurrent also depends on the number of donor/acceptor interfaces. The results can be explained in terms of an energy band diagram of the devices. From the impedance spectroscopy, we show that the change in dielectric constant of the active material due to illumination has a direct relevance to the external current of the photodetectors.
Electrical breakdown of a bubble in a water-filled capillary
Authors: Bruggeman P. J. , Leys C. A. , Vierendeels J. A.
Published: June 2006

In this Communication, the electrical breakdown of a static bubble in a water-filled capillary generated in a dc electrical field is studied. We present experimental results which indicate that the liquid layer between capillary and bubble wall can have an important influence on the breakdown mechanism of the bubble. The breakdown electrical field (atmospheric pressure) without a liquid layer in a (vapor) bubble is 18kV∕cm. When a liquid layer is present, the electrical breakdown of an air bubble is observed at electrical fields typically two times smaller. Local plasma formation is observed in this case possibly due to bubble deformation.
Thermal transport and grain boundary conductance in ultrananocrystalline diamond thin films
Authors: Angadi Maki A. , Watanabe Taku , Bodapati Arun , Xiao Xingcheng , Auciello Orlando , Carlisle John A. , Eastman Jeffrey A. , Keblinski Pawel , Schelling Patrick K. , Phillpot Simon R.
Published: June 2006

Although diamond has the highest known room temperature thermal conductivity, k∼2200W∕mK, highly sp3 amorphous carbon films have k<15W∕mK. We carry out an integrated experimental and simulation study of thermal transport in ultrananocrystalline diamond (UNCD) films. The experiments show that UNCD films with a grain size of 3–5nm have thermal conductivities as high as k=12W∕mK at room temperature, comparable with that of the most conductive amorphous diamond films. This value corresponds to a grain boundary (Kapitza) conductance greater than 3000MW∕m2K, which is ten times larger than that previously seen in any material. Our simulations of both UNCD and individual diamond grain boundaries yield values for the grain boundary conductance consistent with the experimentally obtained value, leading us to conclude that thermal transport in UNCD is controlled by the intrinsic properties of the grain boundaries.
Twisting effects of carbon nanotube bundles subjected to axial compression and tension
Authors: Liew K. M. , Wong C. H. , Tan M. J.
Published: June 2006

Molecular dynamics simulations were performed on twisted carbon nanotube (CNT) bundles, which comprise seven (5,5) single-walled carbon nanotubes (SWCNTs) that are spaced 0.34nm apart, under axial compression and tension. A twisting load was applied to six of the (5,5) SWCNTs that surround a core SWCNT to form a twisted CNT bundle. The interaction force between the atoms was modeled using Brenner’s “second generation” potential together with the van der Waals force as characterized by the Lennard-Jones (12,6) potential. The critical buckling and failure loads and the compressive and tensile strains were obtained for both the axial compression and tension scenarios, and it was found that severely twisted CNT bundles suffered badly in both cases. During tensile loading, the intertube distance decreases to 0.2nm lower, which gave rise to intertube repulsive energy. At the same time, the attractive van der Waals force on the opposite faces of each individual SWCNT contributes to the collapse of the twisted CNT bundle.
Quantum-mechanical modeling of current-voltage characteristics of Ti-silicided Schottky diodes
Authors: Saha A. R. , Dimitriu C. B. , Horsfall A. B. , Chattopadhyay S. , Wright N. G. , O’Neill A. G. , Bose C. , Maiti C. K.
Published: June 2006

Based on the interfacial-layer and quantum-mechanical (QM) carrier transport approach, a theoretical model is proposed to predict the anomalous behavior of low-temperature current-voltage (I-V) characteristics of Ti-silicided Schottky diodes. Physical parameters such as barrier height, ideality factor, series resistance, and effective Richardson constant of silicided Schottky diodes are extracted from the forward experimental I-V characteristics. Simulations of both the forward and reverse I-V characteristics have also been performed using extracted parameters. Results are compared with the models, such as, thermionic-emission-diffusion and thermionic-emission with barrier lowering reported in the literature. It is shown that for Ti-silicided Schottky diodes, the use of QM transport model provides a better agreement with the experimental data.
Fatigue effect in ferroelectric PbZr1−xTixO3 thin films
Authors: Schorn P. J. , Bräuhaus D. , Böttger U. , Waser R. , Beitel G. , Nagel N. , Bruchhaus R.
Published: June 2006

Pb Zr 1 − x Ti x O 3 (PZT) is one preferred ferroelectric material being used in nonvolatile ferroelectric random access memory devices. The use of oxide electrodes like IrO2 or SrRuO3 (SRO) is necessary to suppress the serious loss of polarization due to bipolar voltage cycling. Although, there are a number of models under discussion, the origin of the fatigue phenomenon is still not completely understood. In this paper, the fatigue effect of ferroelectric Pb(Zr0.40,Ti0.60)O3 thin films has been studied in detail. To achieve a deeper understanding of the effect, several PZT samples with different electrode materials were investigated. After determining the dependence of the single fatigue parameters, a simulation approach was made to analyze the fatigue effect qualitatively. A sample with SRO electrodes was measured up to 1013cycles and no fatigue of the switchable polarization was observed.
Arsenic uphill diffusion during shallow junction formation
Authors: Ferri M. , Solmi S. , Parisini A. , Bersani M. , Giubertoni D. , Barozzi M.
Published: June 2006

The behavior during annealing of low-energy As-implanted Si have been investigated by comparing secondary ion mass spectrometry (SIMS) and simulated profiles. Z-contrast scanning transmission electron microscopy (STEM) imaging has also been used to determine the As local distribution in proximity of the sample surface. The implants have been performed with energies between 1 and 10keV both through a thermally grown 11nm thick oxide and without any oxide mask. SIMS and STEM profiles show, after short annealing at 800–1000°C, an As pileup in the first nanometers of the Si matrix in proximity of the SiO2∕Si interface. We demonstrate that this phenomenon can be explained with a “Fickian” standard diffusion by assuming the presence of unspecified “dopant traps” near the SiO2∕Si interface that cause a drastic reduction of the dopant able to diffuse inside the bulk. We have also verified that removing before annealing the superficial 4nm of Si does not eliminate the As pileup. Different mechanisms proposed in literature to explain the uphill diffusion are discussed. Furthermore, the availability of a suitable simulation model allows us to evaluate the dopant diffusivity during the annealing and investigate the transient enhanced diffusion (TED) phenomena.
Analytical modeling of laser pulse heating of embedded biological targets: An application to cutaneous vascular lesions
Authors: Mirkov Mirko , Sherr Evan A. , Sierra Rafael A. , Lloyd Jenifer R. , Tanghetti Emil
Published: June 2006

Detailed understanding of the thermal processes in biological targets undergoing laser irradiation continues to be a challenging problem. For example, the contemporary pulsed dye laser (PDL) delivers a complex pulse format which presents specific challenges for theoretical understanding and further development. Numerical methods allow for adequate description of the thermal processes, but are lacking for clarifying the effects of the laser parameters. The purpose of this work is to derive a simplified analytical model that can guide the development of future laser designs. A mathematical model of heating and cooling processes in tissue is developed. Exact analytical solutions of the model are found when applied to specific temporal and spatial profiles of heat sources. Solutions are reduced to simple algebraic expressions. An algorithm is presented for approximating realistic cases of laser heating of skin structures by heat sources of the type found to have exact solutions. The simple algebraic expressions are used to provide insight into realistic laser irradiation cases. The model is compared with experiments on purpura threshold radiant exposure for PDL. These include data from four independent groups over a period of 20 years. Two of the data sets are taken from previously published articles. Two more data sets were collected from two groups of patients that were treated with two PDLs (585 and 595nm) on normal buttocks skin. Laser pulse durations were varied between 0.5 and 40ms; radiant exposures were varied between 3 and 20J∕cm2. Treatment sites were evaluated 0.5, 1, and 24 hours later to determine purpuric threshold. The analytical model is in excellent agreement with a wide range of experimental data for purpura threshold radiant exposure. The data collected by independent research groups over the last 20 years with PDLs with wavelengths ranged from 577 to 595nm were described accurately by this model. The simple analytical model provides an accurate description of a wide range of experimental data. The model can be used to guide the development of future laser designs and help refine laser parameters.
Effects of postgrowth annealing treatment on the photoluminescence of zinc oxide nanorods
Authors: Wang D. , Seo H. W. , Tin C.-C. , Bozack M. J. , Williams J. R. , Park M. , Sathitsuksanoh N. , Cheng An-jen , Tzeng Y. H.
Published: June 2006

Postgrowth annealing was carried out to investigate the photoluminescence of zinc oxide (ZnO) nanorods synthesized using a thermal chemical vapor deposition method. The observed change in photoluminescence after the annealing processes strongly suggests that positively charged impurity ions or interstitial Zn ions are the recombination centers for green luminescence observed in the present sample. A model based on the interplay between the band bending at the surface and the migration of positively charged impurity ions or Zn ions was proposed, which satisfactorily explains the observed photoluminescence.
Electric transport properties of YBa2Cu3O7−δ thin-film bridges with laser-written channels of easy vortex motion
Authors: Jukna A. , Barboy I. , Jung G. , Abrutis A. , Li X. , Wang D. , Sobolewski Roman
Published: June 2006

Electric properties of YBa2Cu3O7−δ (YBCO) superconducting thin-film bridges with artificial 5-μm-wide channels for easy motion of Abrikosov vortices have been investigated. Vortex channels have been laser written into the bridge constrictions by means of heat-induced, partial depletion of oxygen from the laser-illuminated areas. Insertion of the channel into a bridge structure increases its resistivity in the normal state, broadens the superconducting transition, and dramatically reduces the critical current density. Experimentally observed features have been ascribed to the vortex channeling mechanism, which reduces the pinning efficiency of the extended growth defects in YBCO.
An improved correlation between impression and uniaxial creep
Authors: Hsueh Chun-Hway , Miranda Pedro , Becher Paul F.
Published: June 2006

A semiempirical correlation between impression and uniaxial creep has been established by Hyde et al. [Int. J. Mech. Sci. 35, 451 (1993)] using finite element results for materials exhibiting general power-law creep with the stress exponent n in the range 2 ⩽n⩽ 15. Here, we derive the closed-form solution for a special case of viscoelastic materials, i.e., n=1, subjected to impression creep and obtain the exact correlation between impression and uniaxial creep. This analytical solution serves as a checkpoint for the finite element results. We then perform finite element analyses for the general case to derive a semiempirical correlation, which agrees well with both analytical viscoelastic results and the existing experimental data. Our improved correlation agrees with the correlation of Hyde et al. for n⩾4, and the difference increases with decreasing n for n<4.
Optical properties and energy transfer processes in (Tm3+, Nd3+) doped tungstate fluorophosphate glass
Authors: Brandão Marconi J. S. , de Araújo Cid B. , Poirier Gael , Messaddeq Younes , Poulain M.
Published: June 2006

We investigate the linear optical properties and energy transfer processes in tungstate fluorophosphate glass doped with thulium (Tm3+) and neodymium (Nd3+) ions. The linear absorption spectra from 370to3000nm were obtained. Transitions probabilities, radiative lifetimes, and transition branching ratios were determined using the Judd-Ofelt [Phys. Rev. 127, 750 (1962); J. Chem. Phys. 37, 511 (1962)] theory. Frequency up-conversion to the blue region and fluorescence in the infrared were observed upon pulsed excitation in the range of 630–700nm. The excitation spectra of the luminescence were obtained to understand the origin of the signals. The temporal decay of the fluorescence was measured for different concentrations of the doping ions. Energy transfer rates among the Tm3+ and Nd3+ ions were also determined.
Effect of hydrostatic pressure on the current-voltage characteristics of GaN∕AlGaN∕GaN heterostructure devices
Authors: Liu Y. , Kauser M. Z. , Schroepfer D. D. , Ruden P. P. , Xie J. , Moon Y. T. , Onojima N. , Morkoç H. , Son K.-A. , Nathan M. I.
Published: June 2006

The current-voltage characteristics of n-GaN∕u-AlGaN∕n-GaN heterostructure devices are investigated for potential pressure sensor applications. Model calculations suggest that the current decreases with pressure as a result of the piezoelectric effect, and this effect becomes more significant with thicker AlGaN layers and increasing AlN composition. The change in current with pressure is shown to be highly sensitive to the change in interfacial polarization charge densities. The concept is verified by measuring the current versus voltage characteristics of an n-GaN∕u-Al0.2Ga0.8N∕n-GaN device under hydrostatic pressure over the range of 0–5kbars. The measured current is found to decrease approximately linearly with applied pressure in agreement with the model results. A gauge factor, which is defined as the relative change in current divided by the in-plane strain, approaching 500 is extracted from the data, demonstrating the considerable potential of these devices for pressure sensing applications.
A room-temperature continuous-wave operating midinfrared light emitting device
Authors: Weik Fritz , Steinmeyer Günter , Tomm Jens W. , Glatthaar Regine , Vetter Uwe , Nurnus Joachim , Lambrecht Armin
Published: June 2006

An optimized light source for the 4–5μm wavelength region with a continuous-wave output power of 2mW at ambient temperature is introduced. The device is based on a narrow-gap lead salt semiconductor chip, which is optically pumped by a standard high-power diode laser array. Surface patterning is employed and leads to an up to sixfold increase in out-coupling efficiency compared to untreated chip surfaces. A model is presented that explains this enhancement of the light extraction. Output efficiencies of 4×10−4 are achieved.
Template induced chiral ordering in nematic liquid crystalline materials: A deuterium nuclear magnetic resonance study
Authors: Elias Anastasia L. , Brett Michael J. , Sousa Matthew E. , Woltman Scott J. , Crawford Gregory P. , Bastiaansen Cees W. M. , Broer Dirk J.
Published: June 2006

Glancing angle deposition is used to create porous liquid crystal alignment layers with high aspect-ratio, three-dimensional features. We use solid-state deuterium nuclear magnetic resonance to determine the alignment of alpha deuterated 4′-pentyl-4-cyanobiphenyl in a 3.2μm thick SiO2 film comprised of independent, helical structures. Comparing measured spectra with models, we determine that the liquid crystalline molecules exhibit chiral ordering dictated by the features of the template films. In the plane of deposition the liquid crystalline molecules tilt away from the substrate normal at an average angle of 51°, which is commensurate with the helical features of the chiral template.
Large magnetoresistance in La2∕3Ca1∕3MnO3 thin films induced by metal masked ion damage technique
Authors: Zhang M. J. , Li J. , Peng Z. H. , Zheng D. N. , Jin A. Z. , Gu C. Z.
Published: June 2006

We have developed a simple process to obtain large magnetoresistance (MR) in perovskite manganite thin films by a combination of focused ion beam milling and 120keV H2+ ion implantation. Metal slits about 70nm in width were printed by 30kV focused Ga ion beam nanolithography on a 4μm track, and the materials in these slits are then irradiated by the accelerated H2+ ions. Using this method, in a magnetic field of 5T we can get a MR>60% over a 230K temperature scope, with a maximum value of 95% at around 70K. This technique is very promising in terms of its simplicity and flexibility of fabrication and has potential for high-density integration.
Nanoporous structure of low-dielectric-constant films: A process compatibility study
Authors: Wang C. L. , Weber M. H. , Lynn K. G.
Published: June 2006

Positron annihilation lifetime spectroscopy (PALS) was used to study the nanoporous structure of spin-on and chemical vapor deposited low dielectric constant (low-k) films. Orthopositronium (o-Ps) lifetimes provided information on pore size, open and closed porosities, and depth profile of pore sizes in low-k films. In combination with three-photon (3γ) yield obtained from positron annihilation spectroscopy, the formation, diffusion, and annihilation of o-Ps are correlated with the chemical composition of matrix. The weak interaction between o-Ps and matrix with the network structure composed of Si, C, O, and H atoms allows us to extract nanoporous information from o-Ps lifetime and 3γ yield. The effects of pore generator (porogen) chemistry, thin film capping, plasma treatment, and positron radiation on nanoporous structure in several low-k films were demonstrated, which provided the information on process compatibility of low-k films. Unsolved issues related with Ps probing nano-structure of porous low-k films are summarized.
Application of phase-retrieval x-ray diffractometry to carbon doped SiGe(C)∕Si(C) superlattice structures. II. High resolution reconstruction using neural network root finder technique
Authors: Dilanian Ruben A. , Nikulin Andrei Y. , Darahanau Aliaksandr V. , Hester James , Zaumseil Peter
Published: June 2006

A neural network root finder approach for finding complex roots of high-degree complex polynomials was applied as part of the phase-retrieval x-ray diffractometry technique to reconstruct strain profiles in SiGe(C)∕Si(C) superlattice structures. The high spatial resolution, 5Å, as a result of significantly higher degree of the complex polynomial, 1400 roots, allowed us to obtain more accurate results for the strain profile distribution in SiGe(C)∕Si(C) superlattice structures. Previously, such high quality analysis was fundamentally impossible due to the limitations imposed by conventional numerical methods of finding complex polynomial roots.
Influence of carrier injection on the electromodulation response of trap-rich polymer light-emitting diodes
Authors: Brewer P. J. , deMello A. J. , deMello J. C. , Lane P. A. , Bradley D. D. C. , Fletcher R. , O’Brien J.
Published: June 2006

We investigate the influence of carrier injection on the electric field distribution in polyfluorene-based polymer light-emitting diodes containing poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS). The devices show strong charge-induced electromodulation spectra due to the accumulation of trapped electrons close to the PEDOT:PSS/polyfluorene interface. The trapped electrons cause the potential to drop preferentially at the interface, enhancing hole injection and substantially reducing the magnitude of the electric field in the bulk semiconductor. The detailed operating mechanisms of such “trap-rich” devices are poorly understood, and in this paper we perform a series of temperature-dependent current-voltage sweeps and electromodulation measurements to clarify the role of the injected charge. We find that the devices show strong field redistribution only at room temperature and that devices operating at lower temperatures (<100K) resemble trap-free light-emitting diodes with a uniform electric field that extends through the bulk. We consider also the effects of pixel aging and show that field redistribution effects are reduced after extended device operation.
Hot electron noise in n-type GaAs in crossed electric and magnetic fields
Authors: Ciccarello F. , Zarcone M.
Published: June 2006

A Monte Carlo analysis of hot electron noise properties of bulk n-type GaAs in crossed electric and magnetic fields is presented. Effects due to the nonparabolicity of the conduction band are properly taken into account by means of a local parabolic approximation. Stochastic properties of electron transport are investigated by computing the velocity autocorrelation function as well as the spectral density of its fluctuations. It is found that these functions are significantly affected by the presence of the magnetic field and exhibit signatures of nonparabolicity and of occurrence of intervalley scattering between nonequivalent valleys. At suitable electric field strengths noise is lowered in a wide range of frequencies with a simultaneous decreasing of its total power.
High spatial resolution mapping of partially strain-compensated SiGe:C films in the presence of postannealed defects
Authors: Darahanau A. V. , Benci A. , Nikulin A. Y. , Etheridge J. , Hester J. , Zaumseil P.
Published: June 2006

An experimental-analytical technique for the model-independent nondestructive characterization of single-crystal alloys is applied to partially strain-compensated SiGe:C∕Si single layer structures with high concentrations of Ge. The studies were performed on pre- and postannealed SiGe:C∕Si samples. X-ray Bragg diffraction profiles were collected at a synchrotron radiation source near the absorption edge of Ge. The studies have allowed the reconstruction of the complex crystal structure factor as a function of crystal depth, permitting direct observation of the effect of the thermal annealing on lattice strain and structural composition in the SiGe:C layer. The technique was shown to be applicable to the analysis of both perfect crystals and crystal structures containing a low defect concentration.
Electron beam charging of insulators: A self-consistent flight-drift model
Authors: Touzin M. , Goeuriot D. , Guerret-Piécourt C. , Juvé D. , Tréheux D. , Fitting H.-J.
Published: June 2006

Electron beam irradiation and the self-consistent charge transport in bulk insulating samples are described by means of a new flight-drift model and an iterative computer simulation. Ballistic secondary electron and hole transport is followed by electron and hole drifts, their possible recombination and/or trapping in shallow and deep traps. The trap capture cross sections are the Poole-Frenkel-type temperature and field dependent. As a main result the spatial distributions of currents j(x,t), charges ρ(x,t), the field F(x,t), and the potential slope V(x,t) are obtained in a self-consistent procedure as well as the time-dependent secondary electron emission rate σ(t) and the surface potential V0(t). For bulk insulating samples the time-dependent distributions approach the final stationary state with j(x,t)=const=0 and σ=1. Especially for low electron beam energies E0<4keV the incorporation of mainly positive charges can be controlled by the potential VG of a vacuum grid in front of the target surface. For high beam energies E0=10, 20, and 30keV high negative surface potentials V0=−4, −14, and −24kV are obtained, respectively. Besides open nonconductive samples also positive ion-covered samples and targets with a conducting and grounded layer (metal or carbon) on the surface have been considered as used in environmental scanning electron microscopy and common SEM in order to prevent charging. Indeed, the potential distributions V(x) are considerably small in magnitude and do not affect the incident electron beam neither by retarding field effects in front of the surface nor within the bulk insulating sample. Thus the spatial scattering and excitation distributions are almost not affected.
Brownian motion field dependent mobility theory of hopping transport process
Authors: Ke Lin , Chua Soo Jin , Han Ronald Cai Cheng , Ting Lin Ting , Vijila Chellapan
Published: June 2006

A Brownian motion theory of hopping mobility has been formulated based on the one-dimensional hopping conduction model between localized states. The probability of hopping in the direction of the applied electric field and the duration of the hop between the localized states are assumed to be field dependent and thermally activated. The general form of the Brownian motion mobility model fitted well with the time of flight results measured in the low field regime and for most part of the mobility data extracted from the space charge limited conduction applied to tris-(8-oxyquinolato) aluminum (Alq3) in higher field regime. The Brownian motion model can be modified in order to account for the dependence of charge mobility in the higher electric field regime and at higher temperatures. The variation of charge mobility with applied electric field was fitted using the Brownian motion theory. The hopping time and the hopping distance were extracted from the fit and found to be about 3ps and 0.9nm, respectively for Alq3 at room temperature.
Structural investigations of Pt∕TiOx electrode stacks for ferroelectric thin film devices
Authors: Cao Jiang-Li , Solbach Axel , Klemradt Uwe , Weirich Thomas , Mayer Joachim , Horn-Solle Herbert , Böttger Ulrich , Schorn Peter J. , Schneller T. , Waser Rainer
Published: June 2006

Effects of the thermal treatment and the fabrication process of Pb(Zr0.3Ti0.7)O3 (PZT) thin films using chemical solution deposition on Pt∕TiOx electrode stacks were investigated using complementary analytical techniques including atomic force microscopy (AFM), x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and grazing incidence x-ray reflectivity of synchrotron radiation. The surface and interface structures of the Pt∕TiOx electrode stacks with different thermal treatments, and the PZT∕Pt∕TiOx sample were examined. The propagation of Pt hillocks on the bare Pt∕TiOx electrode stacks upon the annealing was observed. AFM observations also revealed that the upper surface of the Pt bottom electrode under PZT thin film became rougher than that of the bare Pt electrode with the same thermal history. Global structural information including the density, surface or interface root-mean-square roughness, and thickness of each constituent layer in the samples were determined using x-ray reflectivity. A density decrease of the Pt layer upon the annealing or during the fabrication of PZT thin films was found from fitting the specular reflectivity, and further confirmed by the negative shift of the Yoneda peak of Pt in the diffuse reflectivity. The formation of Pt hillocks on the bare Pt electrodes was attributed to the compressive stress during the high-temperature annealing caused by the limited incorporation of Ti and O into the Pt layer. Roughening of the PZT/Pt interface was ascribed to the interaction between the compressive stress in Pt and the indentation by the PZT crystallization and grain growth during the annealing.
Preparation of silicon carbide nanotubes by hydrothermal method
Authors: Pei L. Z. , Tang Y. H. , Chen Y. W. , Guo C. , Li X. X. , Yuan Y. , Zhang Y.
Published: June 2006

One-dimensional SiC nanotubes with small diameter without metallic catalysts were prepared under supercritically hydrothermal conditions. The SiC nanotubes were characterized by x-ray diffraction, energy dispersive x-ray spectrum, selected area electron diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and Raman and infrared spectra. The results show that the samples are SiC nanotubes composed with crystalline β-SiC structure and some residual carbon nanoscale grains. The SiC nanotubes with small inner pores have an outer diameter of less than 10nm and length of micrometers. The structures of SiC nanotubes are hollow inner pores, multiwall crystalline SiC layers, and thin amorphous silicon oxide outer layers. The substitution reaction model is proposed to explain the formation of SiC nanotubes. C atoms diffuse into Si nanotubes and Si atoms in Si nanotubes are partially substituted for C atoms by substitution chemical reaction, resulting in the formation of SiC nanotubes.
Electrical properties of (Ba,Sr)TiO3 thin films revisited: The case of chemical vapor deposited films on Pt electrodes
Authors: Ehrhart Peter , Thomas Reji
Published: June 2006

Due to the dependence on both bulk and interface properties neither the effective dielectric constant ε nor the leakage current J can be scaled in a straightforward manner with film thickness for high-ε thin film capacitors. Based on detailed investigations of different thickness series of (Ba,Sr)TiO3 films on platinized substrates the bulk and interfacial properties are separated. An approach to estimate the apparent interfacial layer thickness is discussed. The behavior of the leakage current is divided in two regions: for low voltages, ⩽1V, the currents are very low, ⩽10−10A∕cm2, and dominated by the relaxation currents (Curie–von Schweidler behavior). At higher voltages the change to a very strong power law dependence is observed, J∼E16. The thickness dependence is removed by scaling with the internal field or dielectric displacement of the film, D=ε0εE. Hence, a direct connection between the increase in ε and the increase in leakage with film thickness is revealed. This behavior is accompanied by a larger scatter of the data and seems to be controlled by a more inhomogeneous or local conductivity. Influences of the measuring temperature and of stoichiometry and interfacial properties are discussed.
Influence of perovskite termination on oxide heteroepitaxy
Authors: Schmidt D. A. , Ohta Taisuke , Yu Q. , Olmstead Marjorie A.
Published: June 2006

We report a combined high-temperature scanning tunneling microscopy, ion scattering spectroscopy, and photoelectron spectroscopy study of bare lanthanum aluminate [LaAlO3(LAO)] and of the initial stages of anatase TiO2 growth on LAO(001). LAO(001) exhibits mixed La–O and Al–O2 surface terminations at 400°C. Heteroepitaxial TiO2, grown by evaporating Ti metal in O2, nucleates near step edges, growing out to cover both upper and lower terraces uniformly, regardless of termination, indicating that the substrate cations and perovskite surface polarity play little direct role in controlling the morphology of this single-metal oxide heteroepitaxy. TiO2 films of 1.5nm in thickness exhibit a surface reconstruction similar to the bulk anatase (1×4).
Praseodymium silicate films on Si(100) for gate dielectric applications: Physical and electrical characterization
Authors: Lupina G. , Schroeder T. , Dabrowski J. , Wenger Ch. , Mane A. U. , Müssig H.-J. , Hoffmann P. , Schmeisser D.
Published: June 2006

Praseodymium (Pr) silicate dielectric layers were prepared by oxidation and subsequent N2 annealing of thin Pr metal layers on SiO2∕Si(100) substrates. Transmission electron microscopy studies reveal that the resulting dielectric has a bilayer structure. Nondestructive depth profiling by using synchrotron radiation x-ray photoelectron spectroscopy shows that, starting from the substrate, the dielectric stack is composed of a SiO2-rich and a SiO2-poor Pr silicate phase. Valence and conduction band offsets of about 2.9 and 1.6eV, respectively, between the dielectric and the Si(100) substrate bands were deduced. Pr silicate films with an equivalent oxide thickness of 1.8nm show approximately three orders of magnitude lower leakage currents than silicon oxynitride references. Capacitance versus voltage measurements of the Pr silicate/Si(100) system report a flat band voltage shift of 0.22V, an effective dielectric constant of about 11 and a reasonably good interface quality with an interface state density on the order of 1011cm−2. Experimental results are supplemented by ab initio considerations which review the most probable mechanisms of fixed charge formation in the Pr silicate layers.
Analytical and numerical calculations of the magnetic force microscopy response: A comparison
Authors: Engel-Herbert R. , Schaadt D. M. , Hesjedal T.
Published: June 2006

We investigate the domain structure of submicrometer sized ferromagnetic stripes exhibiting in-plane and out-of-plane magnetized areas with magnetic force microscopy (MFM). Two simulation approaches are used to calculate the observed MFM response. The first relies on an analytical solution for the stray field of a bar magnet and the subsequent modeling of the sample as an arrangement of bar magnets. The MFM response is calculated for a realistic tip shape incorporating a distribution of magnetic dipoles. The second, numerical approach is based on a discretization scheme, breaking the tip-sample problem up into cells and then calculating the energy of the magnetic tip-sample interaction. The MFM responses obtained for the ferromagnetic stripe structure are compared. A discussion of the advantages and limitations of the two methods is given in terms of precision, computing time, and flexibility. The numerical method offers shorter computing times and greater flexibility, opening the door for realistic three-dimensional MFM response simulations. The advantage of the analytical method is the investigation of small structures, as its precision is higher for the comparable computational effort.
Synthesis and characterization of polyureasilazane derived SiCN ceramics
Authors: Andronenko Sergey I. , Stiharu Ion , Misra Sushil K.
Published: June 2006

Samples of SiCN ceramics were synthesized by thermal treatment of commercially available CERASET™ polyureasilazane, used as liquid-polymer precursor, at pyrolysis temperatures of 1000, 1050, 1100, 1150, and 1200°C. Electron paramagnetic resonance (EPR) signals due to sp2-hybridized carbon-related dangling bonds were recorded over the 4–300K temperature range at X band (9.6GHz), and the spectra showed the presence of an intense EPR line with g=2.0027 at room temperature for all samples; at liquid helium temperature an additional line was seen present as a shoulder to main line. These two signals are due to carbon-related dangling bonds present as (i) defects on the free-carbon phase and (ii) within the bulk of SiCN ceramic network. The value of the antiferromagnetic exchange constant between dangling bonds in the various samples, not hitherto available in the literature, was estimated from the temperature variation of the EPR linewidth to be anywhere from J=−12 to J=−15K in the samples synthesized at 1000, 1100, and 1150°C. The EPR linewidth of the samples decreased with increasing pyrolysis temperature of a sample, being 0.07mT at X band at room temperature for the samples synthesized at 1150 and 1200°C, which can be used potentially as stable g markers, due to their very narrow linewidth. The W-band (95GHz) room-temperature spectrum of the sample pyrolyzed at 1100°C exhibited only one line consistent with the X-band spectrum. At higher frequency of G band (170GHz) the EPR spectra are better resolved, clearly showing the presence of a strong (g=2.0027) and a weak (g=2.0032) EPR line at room temperature, and the latter corresponds to the line present at X band as shoulder at liquid-helium temperature. In addition, nuclear magnetic resonance measurements were carried out to confirm the existence of free-carbon phase and oxygen atoms in the samples.
Three-dimensional analytical modeling of nanocrystal memory electrostatics
Authors: Ganguly Udayan , Narayanan Venkat , Lee Chungho , Hou Tuo-Hung , Kan Edwin C.
Published: June 2006

The integration of a two-dimensional distribution of discrete nanoscale floating gates in the nonvolatile memory gate stack produces significant three-dimensional (3D) electrostatic effects in contrast to the conventional flash memory modeling where a one-dimensional (1D) treatment is often sufficient. We have developed an analytical model for 3D electrostatics, which can not only enhance design intuition for device optimization but also provide convenient integration with a Schrödinger solver for self-consistent transport calculations since it is independent of discretization requirements. The model is validated by comparing with a finite-element Maxwell equation solver. The 3D analytical model has a much lower root-mean-square error than the 1D formulation for electrostatic potentials and fields in the tunneling path.
Effects of rapid thermal annealing on device characteristics of InGaAs∕GaAs quantum dot infrared photodetectors
Authors: Fu L. , Tan H. H. , McKerracher I. , Wong-Leung J. , Jagadish C. , Vukmirović N. , Harrison P.
Published: June 2006

In this work, rapid thermal annealing was performed on InGaAs∕GaAs quantum dot infrared photodetectors (QDIPs) at different temperatures. The photoluminescence showed a blueshifted spectrum in comparison with the as-grown sample when the annealing temperature was higher than 700°C, as a result of thermal interdiffusion of the quantum dots (QDs). Correspondingly, the spectral response from the annealed QDIP exhibited a redshift. At the higher annealing temperature of 800°C, in addition to the largely redshifted photoresponse peak of 7.4μm (compared with the 6.1μm of the as-grown QDIP), a high energy peak at 5.6μm (220meV) was also observed, leading to a broad spectrum linewidth of 40%. This is due to the large interdiffusion effect which could greatly vary the composition of the QDs and thus increase the relative optical absorption intensity at higher energy. The other important detector characteristics such as dark current, peak responsivity, and detectivity were also measured. It was found that the overall device performance was not affected by low annealing temperature, however, for high annealing temperature, some degradation in device detectivity (but not responsivity) was observed. This is a consequence of increased dark current due to defect formation and increased ground state energy.