bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication and characterization of erbium-doped toroidal microcavity lasers
Authors: Kalkman J. , Tchebotareva A. , Polman A. , Kippenberg T. J. , Min B. , Vahala K. J.
Published: June 2006

Erbium-doped SiO2 toroidal microcavity lasers are fabricated on a Si substrate using a combination of optical lithography, etching, Er ion implantation, and CO2 laser reflow. Erbium is either preimplanted in the SiO2 base material or postimplanted into a fully fabricated microtoroid. Three-dimensional infrared confocal photoluminescence spectroscopy imaging is used to determine the spatial distribution of optically active Er ions in the two types of microtoroids, and distinct differences are found. Microprobe Rutherford backscattering spectrometry indicates that no macroscopic Er diffusion occurs during the laser reflow for preimplanted microtoroids. From the measured Er doping profiles and calculated optical mode distributions the overlap factor between the Er distribution and mode profile is calculated: Γ=0.066 and Γ=0.02 for postimplanted and preimplanted microtoroids, respectively. Single and multimode lasing around 1.5μm is observed for both types of microtoroids, with the lowest lasing threshold (4.5μW) observed for the preimplanted microtoroids, which possess the smallest mode volume. When excited in the proper geometry, a clear mode spectrum is observed superimposed on the Er spontaneous emission spectrum. This result indicates the coupling of Er ions to cavity modes.
Photoelectron emission properties of hydrogen terminated intrinsic diamond
Authors: Takeuchi D. , Nebel C. E. , Yamasaki S.
Published: June 2006

Hydrogen terminated intrinsic diamond is characterized using photoelectron emission spectroscopy. Samples have been annealed at temperatures Ta from 200to1000°C. The electron emission characteristics can be divided into three regimes: (I) (Ta<300°C) is governed by surface band bending which allows only excitons to reach the surface with subsequent dissociation. (II) (300°C⩽Ta⩽650°C) shows increased electron emission and the surface band bending has been removed. Here, excitons and electrons contribute. (III) (Ta>650°C) additional rise of emission is detected. The spectra are dominated by photon-phonon interactions which are discussed in detail.
Spectroscopic assessment of Tm3+:GdAl3(BO3)4 crystal as a potential diode pumped laser near 1.9μm
Authors: Jia Guohua , Tu Chaoyang , Li Jianfu , Lu Xiuai , Zhu Zhaojie , You Zhenyu , Wang Yan , Wu Baichang
Published: June 2006

The spectroscopic properties of GdAl3(BO3)4:Tm3+ single crystal for the Tm3+ transition near 1.9μm are presented. The absorption cross section, the emission cross section, and the potential laser gain near 1.9μm are investigated for both σ and π polarizations. The broad absorption band at about 797nm and high value of the above cross section are promising features for obtaining a laser diode pumped Tm3+ laser. The maximum emission cross sections are 1.29×10−20 and 6.60×10−20cm2 at 1.85μm for the σ and π polarizations, respectively. The potential laser gain curves indicate that more than 200nm tunability range is expected for this material.
Phonon scattering cross section of polydispersed spherical nanoparticles
Authors: Kim Woochul , Majumdar Arun
Published: June 2006

An approximate analytical solution is proposed to estimate the phonon scattering cross section of polydispersed spherical nanoparticles. Using perturbation of the Hamiltonian due to differences in mass and bond stiffness between a host medium and a spherical nanoparticle, an analytical solution is obtained for the scattering cross section in the Rayleigh limit when the size parameter approaches zero. In the geometrical scattering limit, when the size parameter approaches infinity, the van de Hulst approximation for anomalous diffraction is used to estimate the scattering cross section as a function of acoustic impedance mismatch between the host medium and the spherical nanoparticle. Finally, these two limiting cases are bridged by a simple expression to estimate the scattering cross section for intermediate values of the size parameter. Using this, the scattering cross section for a polydispersed distribution of spherical nanoparticles was also estimated as a function of the parameters defining the statistical size distribution.
Synthesis and characterization of oriented graphitelike B–C–N hybrid
Authors: Uddin Md. Nizam , Shimoyama Iwao , Baba Yuji , Sekiguchi Tetsuhiro , Nath Krishna G. , Nagano Masamitsu
Published: June 2006

In order to clarify the structure of graphitelike B–C–N hybrid thin films, borazine (B3N3H6) ion plasma was implanted in highly oriented pyrolytic graphite at 800°C and room temperature. The films were characterized in situ by x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) using linearly polarized synchrotron radiation. The XPS study suggested that B atoms in the deposited films display in a wide variety of chemical bonds, e.g., B–C, B–N, and B–C–N. In the polarization-dependent NEXAFS spectra at the B K edge, the resonance peaks from B 1s to both π*-like and σ*-like orbitals are clearly observed, which suggest the existence of sp2 configuration around the boron atoms. On the basis of the graphitelike polarization dependencies observed in the B 1s→π* resonance peaks for B–C–N films deposited at 800°C, we conclude that the highly oriented graphitelike B–C–N hybrids are stable at low boron content.
Characterization of porous low-k films using variable angle spectroscopic ellipsometry
Authors: Othman M. T. , Lubguban J. A. , Lubguban A. A. , Gangopadhyay S. , Miller R. D. , Volksen W. , Kim H.-C.
Published: June 2006

Variable angle spectroscopic ellipsometry (VASE™) is used as a tool to characterize properties such as optical constant, thickness, refractive index depth profile, and pore volume fraction of single and bilayer porous low-k films. The porous films were prepared using sacrificial pore generator (porogen) approach. Two sets of porous films with open- and closed-pore geometries were measured. Three models were used for data analysis: Cauchy, Bruggeman effective medium approximation (BEMA), and graded layer. Cauchy, a well-known model for transparent films, was used to obtain thickness and optical constant, whereas BEMA was utilized to calculate the pore volume fraction from the ellipsometric data. The Cauchy or BEMA models were then modified as graded layers, resulting in a better fit and a better understanding of the porous film. The depth profile of the porous film implied a more porous layer at the substrate-film interface. We found 3%–4% more porosity at the interface compared with the bulk for both films. This work shows that VASE™, a nondestructive measurement tool, can be used to characterize single- and multigraded layer porous films quickly and effectively.
High sensitivity and nonlinearity of carbon nanotube charge-based sensors
Authors: Guo Jing , Kan Edwin C. , Ganguly Udayan , Zhang Yuegang
Published: June 2006

A self-consistent atomistic simulation method is established to investigate carbon nanotubes (CNTs) as the charge sensing channels in nonvolatile memory and sensor applications. The theoretical simulation agrees with the experimental results, demonstrating that the sensor can achieve a high sensitivity down to a single electron charge at room temperature. The electrical response of the CNT charge sensor strongly depends on the position of discrete sensing charges even under low drain biases due to the near ballistic transport and quantum interference in a submicron meter CNT channel. This result differs from those for conventional complementary metal-oxide semiconductor charge-based sensors, in which the carrier transport is diffusive and the electrical response is expected to be insensitive to the position variation of the sensing charges. The high sensitivity and nonlinearity of CNT charge-based sensors are important features that need to be considered in the design of CNT memories and sensors.
Large apparent inductance in organic Schottky diodes at low frequency
Authors: Takshi Arash , Madden John D.
Published: June 2006

A large low frequency inductance is found in a Schottky diode composed of regioregular poly(3-hexylthiophene) and aluminum. This apparent inductance is evident in response to both swept frequency sinusoidal, ramp and step voltage inputs above a threshold voltage. The constant slope of the current in response to a voltage step suggests an incredibly large inductance (a few hundred megahenry) in a device that is only 2000μm3 in size. A number of potential mechanisms including chemical reactions, barrier modulation, and memory effects are evaluated in order to find a suitable explanation for the inductive behavior. Similarity in the dc characteristics of the organic Schottky diode and organic bistable devices that are being applied as memory suggests that the current leads the voltage due to increments in tunneling current that occur as charges are gradually stored in localized states.
First-principles investigation of the WC∕HfO2 interface properties
Authors: Knizhnik A. A. , Safonov A. A. , Iskandarova I. M. , Bagatur’yants A. A. , Potapkin B. V. , Fonseca L. R. C. , Stoker M. W.
Published: June 2006

The thermodynamic and electronic properties of tungsten carbide surfaces and interfaces with monoclinic hafnia (WC∕m-HfO2) are investigated through first-principles calculations. We show that oxidation of the WC surface and of the WC∕m-HfO2 interface is energetically favorable. An oxygen monolayer on the W-terminated WC(0001) surface gives rise to a larger vacuum work function than that for the C-terminated WC(0001) surface, while the opposite result is obtained for the WC(0001) effective work function on hafnia: a carbon intermediate layer results in larger work function than an oxygen intermediate layer. This result is explained by the atomic structure of the intermediate layers neighboring the interface which differ if the interface is O or C rich.
Improved calculation of the backscattering factor for quantitative analysis by Auger electron spectroscopy
Authors: Ding Z. J. , Tan W. S. , Li Y. G.
Published: June 2006

Based on a Monte Carlo simulation method, an improved calculation of the backscattering factor in quantitative analysis by Auger electron spectroscopy has been performed by integrating several aspects of recent progresses in the related fields. The calculation used a general definition of backscattering factor, more accurate ionization cross section, up-to-date Monte Carlo model of electron inelastic scattering, and a large number of electron trajectories to ensure less statistical error. The results reveal several noticeable properties of backscattering factor, i.e., its slow variation with primary energy at higher overvoltage ratios, and dependence on the geometrical configuration of a detector. However, only for large emission angles of Auger signals a considerable angular dependence of backscattering factor is found. Specifically a calculation is carried out for detection in the solid angles of a cylindrical mirror analyzer. This backscattering factor can be less than unity for very low primary energies closing to ionization energy and/or for large incident angles. The physical cause has been detailed and analyzed.
Extended Mie-Grüneisen molecular model for time dependent dielectric breakdown in silica detailing the critical roles of O–SiO3 tetragonal bonding, stretched bonds, hole capture, and hydrogen release
Authors: McPherson J. W.
Published: June 2006

An extended Mie-Grüneisen molecular model is presented, which describes a bond-breakage process for O–SiO3 tetragonal molecules in silica and the trap-generation process that occurs during time dependent dielectric breakdown (TDDB) testing. This quantitative molecular model correctly describes important physics routinely reported for silica TDDB testing: the generation of E′ centers, a breakdown strength of Ebd∼15MV∕cm, an effective dipole-moment range of peff=7–13eÅ, and a zero-field activation energy range for bond breakage of ΔHo*=1−2eV. The bond-breakage/trap-generation mechanism is shown to occur when the Si ion transitions from its primary energy minimum (with fourfold coordination) to a secondary saddle point (with threefold coordination). The molecular model also shows clearly that current induced hole capture and hydrogen release can play critically important roles in the TDDB process.
Thermal conductivity of nanoparticle suspensions
Authors: Putnam Shawn A. , Cahill David G. , Braun Paul V. , Ge Zhenbin , Shimmin Robert G.
Published: June 2006

We describe an optical beam deflection technique for measurements of the thermal diffusivity of fluid mixtures and suspensions of nanoparticles with a precision of better than 1%. Our approach is tested using the thermal conductivity of ethanol-water mixtures; in nearly pure ethanol, the increase in thermal conductivity with water concentration is a factor of 2 larger than predicted by effective medium theory. Solutions of C60–C70 fullerenes in toluene and suspensions of alkanethiolate-protected Au nanoparticles were measured to maximum volume fractions of 0.6% and 0.35vol%, respectively. We do not observe anomalous enhancements of the thermal conductivity that have been reported in previous studies of nanofluids; the largest increase in thermal conductivity we have observed is 1.3%±0.8% for 4nm diam Au particles suspended in ethanol.
Direct imaging of electron-beam interaction region
Authors: Luber D. R. , Haegel N. M.
Published: June 2006

We demonstrate a method for charge recombination imaging in the scanning electron microscope and employ this technique to directly image the two-dimensional projection of the generation volume as a function of electron energy and probe current. Using GaAs and SiC bulk samples, we observe the lateral extent of the generation volume increasing as a function of increasing e-beam accelerating voltage, in excellent agreement with previous work. This work also reveals significant variations in minority carrier distribution within the volume for low-Z compared to the high-Z material. The roles of sample geometry and carrier diffusion are discussed. The direct imaging technique used in this work is well suited for further studies of the nature of the effective interaction region for cathodoluminescence or electron-beam induced current studies in any luminescent material.
In situ simple method for measuring size and density of nanoparticles in reactive plasmas
Authors: Nunomura Shota , Kita Makoto , Koga Kazunori , Shiratani Masaharu , Watanabe Yukio
Published: June 2006

A laser-light-scattering (LLS) method for measuring the size and density of nanoparticles generated in reactive plasmas has been developed. The size and density of the nanoparticles are determined from their thermal coagulation that takes place after turning off the discharge. The measurable size and density range of the LLS method is np⪆1013(m−3∕2)×dp−5∕2L−2ng−1, where np, dp, L, and ng are the density, size, and diffusion length of the nanoparticles, and the density of a background gas, respectively. The method has been demonstrated by measurement of the size and density of nanoparticles formed by the radio-frequency discharge of dimethyldimethoxysilane Si(CH3)2(OCH3)2 diluted with Ar. Using a simple optical setup for the LLS measurement, nanoparticles are detected down to ≈1nm in size when they are generated at a density of ≈1012cm−3. The developed method is widely applicable to other systems in which thermal coagulation takes place.
Influence of thermal annealing on the bulk scattering in giant-magnetoresistance spin-valve with nano-oxide layers
Authors: Nam Chunghee , Jang Youngman , Lee Ki-Su , Shim Jungjin , Cho B. K.
Published: June 2006

Based upon a bulk scattering model, we investigated the variation of giant magnetoresistance (GMR) behavior after thermal annealing at Ta=250°C as a function of the top free layer thickness of a GMR spin valve with nano-oxide layers (NOLs). It was found that the enhancement of GMR ratio after thermal annealing is explained qualitatively in terms of the increase of active GMR region in the free layer and, simultaneously, the increase of intrinsic spin-scattering ratio. These effects are likely due to the improved specular reflection at the well-formed interface of NOL. Furthermore, we developed a modified phenomenological model for sheet conductance change (ΔG) in terms of the top free layer thickness. This modified model was found to be useful in the quantitative analysis of the variation of the active GMR region and the intrinsic spin-scattering properties. The two physical parameters were found to change consistently with the effects of thermal annealing on NOL.
Stranski-Krastanov GaN∕AlN quantum dots grown by metal organic vapor phase epitaxy
Authors: Simeonov D. , Feltin E. , Carlin J.-F. , Butté R. , Ilegems M. , Grandjean N.
Published: June 2006

We report a thorough study of the growth of self-assembled GaN∕AlN quantum dots (QDs). These QDs were grown on sapphire substrates using the Stranski-Krastanov (SK) growth mode by means of low-pressure metal organic vapor phase epitaxy. The influence of the V∕III ratio on the QD height, shape, and density is studied. Emissions above (3.8eV) and below (2.8eV) the band gap of GaN are observed for small dots (1.3nm) and large ones (4.4nm), respectively. Special emphasis was given to the SK growth mode transition and the parameters influencing the QD formation. The two-dimensional–three-dimensional transition is studied continuously by photoluminescence mapping showing its high sensitivity to the V∕III ratio.
High-field magnetization and coercivity of hard magnetic mold-cast Nd80Fe20
Authors: Kumar G. , Kerschl P. , Rößler U. K. , Nenkov K. , Müller K.-H. , Schultz L. , Eckert J.
Published: June 2006

Nd 80 Fe 20 bulk specimens were prepared by copper mold casting and their structure and magnetic properties were studied. The specimens contain 73wt% elemental Nd and 27wt% hard magnetic Fe–Nd regions, which are a mixture of amorphous and nanocrystalline phases. The average composition of the Fe–Nd regions is Nd45Fe55. The presence of elemental Nd leads to multiple magnetic transitions below 30K whereas the Fe–Nd regions have a Curie temperature of 529K. Above 30K, the specimens show a single-phase hard magnetic behavior superimposed on the large susceptibility due to paramagnetic Nd and a noncollinear magnetic structure of Fe moments. At T⩽30K, two-phase demagnetization curves were observed due to the additional ferromagnetic contribution of metastable fcc Nd. The coercivity of the hard magnetic Fe–Nd regions is strongly temperature dependent, decreasing from 6.4T at 4.2K to almost zero at temperatures close to the Curie temperature. The shape of the initial magnetization curve and the remanence values of about half of the maximum magnetization (at 40T) suggest coherent rotation as the magnetization reversal mechanism for the Fe–Nd regions. The demagnetization curves can be fitted by a modified Stoner-Wohlfarth model of weakly coupled single-domain particles with sizable high-field susceptibility to account for the noncollinear magnetic order. The saturation magnetization, the anisotropy fields, and the high-field susceptibility values of the hard magnetic Fe–Nd regions are deduced from the fits in the temperature range of 75–290K.
Low temperature impact ionization in indium antimonide high performance quantum well field effect transistors
Authors: Orr J. M. S. , Buckle P. D. , Fearn M. , Giavaras G. , Wilding P. J. , Bartlett C. J. , Emeny M. T. , Buckle L. , Jefferson J. H. , Ashley T.
Published: June 2006

The observation of a kink effect in the output characteristic of an InSb∕AlInSb quantum well field effect transistor structure at low temperature (1.6K) is reported. The effect is strongly temperature dependent, and while just discernible at room temperature, it is greatly enhanced below ∼120K. At 1.6K strong hysteresis is observed in the (output) forward characteristic of the device when sweeping the drain bias up and down. Corresponding instability is also observed in the gate leakage current as a function of gate voltage. We explain the effect by comparing with Monte Carlo simulations, observing strong hole accumulation under the gate region of the device as a result of significant impact ionization in the drain region. This accumulation is enhanced compared to the more common InAs∕AlSb type II system, due to the fact that InSb∕AlInSb has a type I band alignment. This inhibits the loss of holes to the gate contact, a significant leakage mechanism in type II systems. We show that the extent of the hysteresis is a good measure of the charge accumulation under the gate region and estimate accumulation by examination of the hysteresis compared with the results of the Monte Carlo simulation.
The chemisorption of dibenzo[a,j]coronene on Si(001)-2×1
Authors: Suzuki T. , Sorescu D. C. , Jordan K. D. , Yates J. T.
Published: June 2006

Adsorption structures of the dibenzo[a,j]coronene (C32H16) molecule on the clean Si(001)-2×1 surface were investigated by scanning tunneling microscopy (STM) in conjunction with electronic structure calculations. The dibenzo[a,j]coronene molecules were found to adsorb on three different sites: one major adsorption site and two minor adsorption sites. The formation of four to eight Si–C covalent bonds is responsible for the different surface bonding structures observed. Bond strain effects due to out-of-plane bending of the molecule play a significant role in governing the surface bond energies. The geometries of the three adsorption sites were established by comparison of the experimental and simulated STM images. By applying an electrical pulse, the molecule can be made to hop from one site to another site without breaking the dibenzo[a,j]coronene molecular structure.
Fisher information of D-dimensional hydrogenic systems in position and momentum spaces
Authors: Dehesa J. S. , López-Rosa S. , Olmos B. , Yáñez R. J.
Published: June 2006

The spreading of the quantum-mechanical probability distribution density of D-dimensional hydrogenic orbitals is quantitatively determined by means of the local information-theoretic quantity of Fisher in both position and momentum spaces. The Fisher information is found in closed form in terms of the quantum numbers of the orbital.
Origin of room temperature ferromagnetism in ZnO:Cu films
Authors: Herng T. S. , Lau S. P. , Yu S. F. , Yang H. Y. , Ji X. H. , Chen J. S. , Yasui N. , Inaba H.
Published: June 2006

Copper-doped ZnO (ZnO:Cu) films were prepared on silicon substrates by filtered cathodic vacuum arc technique at room temperature using a Zn target containing 5at.% of Cu. Room temperature ferromagnetism was observed in the ZnO:Cu films with saturation magnetization of 0.037μB∕Cu atom. The origin of the ferromagnetism in ZnO:Cu was mainly due to Cu ions substituted into the ZnO lattice. X-ray diffraction, x-ray photoelectron spectroscopy, and transmission electron microscopy revealed that no ferromagnetic-related secondary phase could be detected in ZnO:Cu.
Phase stability in ceria-zirconia binary oxide nanoparticles: The effect of the Ce3+ concentration and the redox environment
Authors: Zhang Feng , Chen Chih-Hao , Raitano Joan M. , Hanson Jonathan C. , Caliebe Wolfgang A. , Khalid Syed , Chan Siu-Wai
Published: June 2006

The stability of the cubic phase (c′) of Ce1−xZrxO2−y nanoparticles was studied by x-ray absorption near edge spectroscopy (XANES), time-resolved high temperature x-ray diffraction (XRD), and room temperature XRD. A clear relationship between the phase stability of the c′ phase and the Ce3+ concentration was found in zirconia-doped ceria, prepared in an oxidizing environment. The percentage of the 3+ oxidation state of cerium was measured from the relative Ce3+ peak intensity at the CeLIII edge in XANES. A concentration of 11% of the larger Ce3+ ions, among all cerium ions, helps to release the local stress induced by the smaller Zr4+ ions and stabilizes the c′ phase even under high zirconia concentrations of 40%–60%. A smaller particle size enhances this effect. XANES results at the ZrLIII edge supported the cubic phase stabilization. When the homogenization anneal was performed under a reducing environment instead of in air, the solubility limit of the cubic phase Ce1−xZrxO2−y was extended to above 90% zirconia, while the Ce3+ concentration increased, reaching 94% of all [Ce] in Ce0.1Zr0.9O2−y, which confirms the crucial role of Ce3+ ions in stabilizing the cubic phase as well as the importance of the gaseous environment during the homogenization anneal.
High frequency complex permeability of iron particles in a nonmagnetic matrix
Authors: Wu L. Z. , Ding J. , Jiang H. B. , Neo C. P. , Chen L. F. , Ong C. K.
Published: June 2006

The effective permeability (μeff) was measured and calculated for composites consisting of micron- or submicron-sized nanocrystalline iron particles embedded in a nonmagnetic matrix. The intrinsic permeability of iron particles was obtained from the calculation for a random spatial distribution of magnetic domains and its analytical model is derived from the Landau-Lifshitz-Gilbert equation. In the calculation, each grain is assumed to be a single magnetic domain because of its nano size. The effective permeability was calculated using three methods—Bruggeman’s effective medium theory, extended Bruggeman’s effective medium theory with the consideration of the skin effect, and a simulation method which was developed in the present work. The skin effect was considered in our simulation work. Our simulation agrees well with the experimental data. Our work has shown clearly that the magnetic domain structure with a random spatial distribution of magnetic easy axes and the skin effect need to be considered to calculate the complex permeability of polycrystalline magnetic materials.
Oxygen release and structural changes in TiO2 films during photocatalytic oxidation
Authors: Yoshida Kenta , Nanbara Takahiro , Yamasaki Jun , Tanaka Nobuo
Published: June 2006

Changes in the crystal structure and grain modifications in titanium oxide (TiO2) thin films were observed during the photocatalytic oxidation of hydrocarbons. When the hydrocarbon and collodion films were irradiated, single crystalline titanium oxide transformed into polycrystals. The titanium oxide films gradually became network aggregates. These changes were analyzed with a dedicated in situ transmission electron microscope and observed three dimensionally by electron tomography. A detailed analysis of electron energy loss spectra of the samples also revealed that the changes were associated with the loss of oxygen atoms in the TiO2 crystal lattice. Correlations between the polycrystalline grain size of TiO2 and its catalyst activity were discussed based on the measured data.
Microstructure and electrical properties of CaCu3Ti4O12 ceramics
Authors: Shao S. F. , Zhang J. L. , Zheng P. , Zhong W. L. , Wang C. L.
Published: June 2006

Ca Cu 3 Ti 4 O 12 (CCTO) ceramics are prepared by the conventional solid-state reaction method under various sintering temperatures from 1000to1120°C at an interval of 10°C. Microstructures and crystalline structures are examined by scanning electronic microscopy and x-ray diffraction, respectively. Dielectric properties and complex impedances are investigated within the frequency range of 40Hz–110MHz over the temperature region from room temperature to 350°C. It has been disclosed that the microstructures can be categorized into three different types: type A (with the small but uniform grain sizes), type B (with the bimodal distribution of grain sizes) and type C (with the large and uniform grain sizes), respectively. The largeness of low-frequency dielectric permittivity at room temperature is closely related to the microstructure. Ceramics with different types of microstructures show the diverse temperature-dependent behaviors of electrical properties. However, the existence of some common characteristics is also found among them. For all of the ceramics, a Debye-type relaxation emerges in the frequency range of 100Hz–100kHz at high measuring temperatures, which has the larger dielectric dispersion strength than the one known in the frequency range above 100kHz. Thus, the high-temperature dielectric dispersion exhibits a large low-frequency response and two Debye-type relaxations. Furthermore, all of the ceramics show three semicircles in the complex impedance plane. These semicircles are considered to represent individually different electrical mechanisms, among which the one in the low-frequency range arises most probably from the contribution of the domain boundaries, and the other two are ascribed to the contributions of the domains and the grain boundaries, respectively.
Surface reactions during etching of organic low-k films by plasmas of N2 and H2
Authors: Ishikawa Kenji , Yamaoka Yoshikazu , Nakamura Moritaka , Yamazaki Yuichi , Yamasaki Satoshi , Ishikawa Yasushi , Samukawa Seiji
Published: June 2006

Surface reactions during etching of organic low-k film by N2 and H2 plasmas were studied through observations of the surface resident species using in situ infrared spectroscopy and in vacuo electron-spin-resonance techniques. We observed surface modifications by the formation of CN and NH bonds after exposure to plasmas generated from N2 and H2. The number of carbon dangling bonds were greater in processes where H2 was present. The passivation of carbon dangling bonds leads to CH3, NH3, and CN functionalities, which are the precursors for etching products that are desorbed, which includes volatile forms such as HCN and C2N2.
Generation of ion-acoustic waves in an inductively coupled, low-pressure discharge lamp
Authors: Camparo J. C. , Klimcak C. M.
Published: June 2006

For a number of years it has been known that the alkali rf-discharge lamps used in atomic clocks can exhibit large amplitude intensity oscillations. These oscillations arise from ion-acoustic plasma waves and have typically been associated with erratic clock behavior. Though large amplitude ion-acoustic plasma waves are clearly deleterious for atomic clock operation, it does not follow that small amplitude oscillations have no utility. Here, we demonstrate two easily implemented methods for generating small amplitude ion-acoustic plasma waves in alkali rf-discharge lamps. Furthermore, we demonstrate that the frequency of these waves is proportional to the square root of the rf power driving the lamp and therefore that their examination can provide an easily accessible parameter for monitoring and controlling the lamp’s plasma conditions. This has important consequences for precise timekeeping, since the atomic ground-state hyperfine transition, which is the heart of the atomic clock signal, can be significantly perturbed by changes in the lamp’s output via the ac-Stark shift.
Laser-induced forward transfer of liquids: Study of the droplet ejection process
Authors: Colina M. , Duocastella M. , Fernández-Pradas J. M. , Serra P. , Morenza J. L.
Published: June 2006

Laser-induced forward transfer (LIFT) is a laser direct-write technique that offers the possibility of printing patterns with a high spatial resolution from a wide range of materials in a solid or liquid state, such as conductors, dielectrics, and biomolecules in solution. This versatility has made LIFT a very promising alternative to lithography-based processes for the rapid prototyping of biomolecule microarrays. Here, we study the transfer process through the LIFT of droplets of a solution suitable for microarray preparation. The laser pulse energy and beam size were systematically varied, and the effect on the transferred droplets was evaluated. Controlled transfers in which the deposited droplets displayed optimal features could be obtained by varying these parameters. In addition, the transferred droplet volume displayed a linear dependence on the laser pulse energy. This dependence allowed determining a threshold energy density value, independent of the laser focusing conditions, which acted as necessary conditions for the transfer to occur. The corresponding sufficient condition was given by a different total energy threshold for each laser beam dimension. The threshold energy density was found to be the dimensional parameter that determined the amount of the transferred liquid per laser pulse, and there was no substantial loss of material due to liquid vaporization during the transfer.
Plasma oscillations in high-electron-mobility transistors with recessed gate
Authors: Ryzhii V. , Satou A. , Knap W. , Shur M. S.
Published: June 2006

We calculate the plasma oscillation spectrum in high-electron-mobility transistors (HEMTs) with recessed gate having the highly doped caps adjacent to the source and drain contacts and the windows between the caps and the gate. The resonant plasma frequencies are found as functions of the lengths of the gate, cap, and window regions, the electron concentration in the transistor channel, and the gate voltage. We demonstrate that the effect of cap region can result in a significant reduction of the resonant frequencies in comparison with those calculated for simplified HEMT model. This can provide a plausible explanation of the data obtained in recent experimental studies of the detection of terahertz radiation in and its emission from HEMTs.
Electrical characteristics of Si nanocrystal distributed in a narrow layer in the gate oxide near the gate synthesized with very-low-energy ion beams
Authors: Ng C. Y. , Chen T. P. , Zhao P. , Ding L. , Liu Y. , Tseng Ampere A. , Fung S.
Published: June 2006

A nanocrystal Si (nc-Si) distributed in a narrow layer in the gate oxide close to the gate is synthesized with Si ion implantation at 2keV, and the electrical characteristics of the nc-Si structure are investigated. The onset voltage of the Fowler-Nordheim tunneling of the structure is lower than that of pure SiO2 structure, and it decreases with the nc-Si concentration. The phenomenon is attributed to the reduction of the effective thickness of the tunneling oxide. The application of a positive or negative voltage causes electron or hole trapping in the nc-Si, leading to a positive or negative flatband voltage shift, respectively. A steplike flatband voltage shift as a function of charging voltage is observed, suggesting single electron or hole trapping in the nc-Si at room temperature. On the other hand, the nc-Si structure shows good charge-retention characteristics also.
Microwave absorption of nanoscale CoNi powders
Authors: Kurlyandskaya G. V. , Bhagat S. M. , Luna C. , Vazquez M.
Published: June 2006

Ferromagnetic resonance (FMR) and microwave magnetoabsorption can be expected to be powerful tools for characterization of magnetic nanoparticles. In the present work, we have studied the FMR and magnetic field dependence of nonresonant microwave losses in CoNi nanoparticles of about 45, 100, and 200nm caliper sizes prepared by the polyol technique. Contrary to our expectation, FMR lines, although wide, show that the particles are not truly spherical. Subsequent scanning and transmission electron microscopy investigations have confirmed this finding. Further, as before, all the powders are found to have large zero field absorption which reduces on application of a direct current (dc) field parallel to the microwave magnetic field but is relatively insensitive to a dc field if the powder is located in the microwave electric field, hence defining a magnetoimpedance for the system.
Electronic transport of n-type CdSe quantum dot films: Effect of film treatment
Authors: Yu Dong , Wehrenberg Brian L. , Jha Praket , Ma Jiasen , Guyot-Sionnest Philippe
Published: June 2006

Solid films of CdSe colloidal quantum dots are conductive when reduced by electrochemistry. The dark conductivity and the photoconductivity of these n-type CdSe nanocrystalline solids have been investigated as a function of film treatment. When the films are treated with NaOH, a carrier mobility of ∼10−2cm2∕Vs and a charging/discharging time of ∼10ms are obtained. Under an excitation intensity of ∼0.75mW∕cm2, a significant photoconductivity of ∼10−3Scm−1 has been observed. The high mobility allowed demonstrating the operation of a field effect transistor.
The determination of all syzygies for the dependent polynomial invariants of the Riemann tensor. II. Mixed invariants of even degree in the Ricci spinor
Authors: Carminati J. , Lim A. E. K.
Published: June 2006

We continue our analysis of the polynomial invariants of the Riemann tensor in a four-dimensional Lorentzian space. We concentrate on the mixed invariants of even degree in the Ricci spinor ΦABȦḂ and show how, using constructive graph-theoretic methods, arbitrary scalar contractions between copies of the Weyl spinor ΨABCD, its conjugate Ψ¯ȦḂĊḊ and an even number of Ricci spinors can be expressed in terms of paired contractions between these spinors. This leads to an algorithm for the explicit expression of dependent invariants as polynomials of members of the complete set. Finally, we rigorously prove that the complete set as given by Sneddon [J. Math. Phys. 39, 1659–1679 (1998)] for this case is both complete and minimal.
Surface deformation behavior of beta solution treated and overaged Ti–6Al–4V during laser shock processing
Authors: El-Dasher Bassem S. , Zaleski Tania M. , Gray Jeremy J. , Rybak Sergiusz J. , Chen Hao-Lin
Published: June 2006

The surface of a beta solution treated and overaged Ti–6Al–4V alloy specimen deformed by laser shock processing was studied using electron backscatter diffraction, scanning electron microscopy, and atomic force microscopy. Slip steps were observed within grains oriented with their c axis nearly parallel to the specimen surface normal. Based on the slip step traces and orientation information, the slip planes were determined to be {112¯2} for grains with their c axis within 15° of the specimen surface normal and {112¯1} for grains with their c axis between 15° and 40° away from the specimen surface normal. Although both these planes are known to belong to twinning systems, {112¯2}⟨112¯3¯⟩ and {112¯1}⟨112¯6¯⟩, respectively, the latter has not been observed to operate as a slip system. Examination of the Taylor factors associated with these slip systems shows that the grains with slip steps have the lowest Taylor factors. Determination of localized lattice rotations showed a unique behavior in grains with slip steps, such that all the lattice rotations were concentrated about the steps, with almost no orientation variations in between slip steps. This distribution indicates that stress concentrations exist at the slip steps, which could potentially affect the performance of the material.
Room temperature air oxidation of nanostructured Si thin films with varying porosities as studied by x-ray photoelectron spectroscopy
Authors: Yang D.-Q , Meunier M. , Sacher E.
Published: June 2006

The room temperature air oxidation of nanostructured Si thin films, with varying porosities, has been followed by x-ray photoelectron spectroscopy (XPS), using films deposited by KrF excimer (248nm) laser ablation in He gas ambients. The overall oxidation, determined from the Si2p XPS spectrum, was shown to be amenable to time-porosity superposition, with the extracted shift factors indicating that oxidation is controlled by the permeability of air in the pores. A model has been developed to describe the room temperature air oxidation process as a function of time and porosity, in accord with the experimental findings. Efforts to determine O:Si atomic ratios from O1s:Si2p spectral ratios have revealed the effect of porosity on both the photoelectron attenuation lengths and the size-dependent photoemission intensities of the nanoparticles that compose the samples.
Liquid transport based on electrostatic deformation of fluid interfaces
Authors: Aggarwal Januk , Kotlicki Andrzej , Mossman Michele , Whitehead Lorne
Published: June 2006

We have developed a method for moving liquid along a surface using purely electrostatic effects, without the need for mechanically moving parts. In this approach, liquid drops are confined to specific regions of the substrate by a printed pattern that has the appropriate hydrophilic and hydrophobic wetting characteristics. Using a variation of well-known electrowetting techniques, the shape of the droplet can be changed by applying an electric field that changes the surface energy relationship. Specifically, a bead of oil confined to a hydrophobic region of the surface can be pinched into drops using localized electrostatic fields, and by changing the applied field pattern, these oil drops can be moved to cause net liquid flow. This approach may have useful applications in heat transfer. We have demonstrated that with drops traveling at approximately 15cm∕s, it is possible to transport heat more effectively than by using an equivalent volume of solid copper.
Thermal motion of carbon clusters and production of carbon nanotubes by gravity-free arc discharge
Authors: Mieno T. , Takeguchi M.
Published: June 2006

Thermal and diffusion properties of hot gas around a dc arc discharge under a gravity-free condition are investigated using a jet plane in order to improve the arc production of carbon clusters. Spherically symmetric temperature distribution of He gas around the arc plasma and monotonic slow expansion of the high-temperature region are observed. By means of the passive-type Mie scattering method, random slow diffusion of carbon clusters around the arc plasma is clearly observed under the gravity-free condition. This indicates that carbon clusters including single-walled carbon nanotubes are synthesized around the arc plasma where the He temperature is higher than 1000K. It is confirmed that large bundles of fatter single-walled carbon nanotubes are produced under the gravity-free condition.
Nonperturbative dynamic photon absorption of quantum wells
Authors: Fu Y. , Lu W. , Jiang J. , Wang M. K. , Yang X. P. , Wu G. , Fan Y. H. , Li Y. G.
Published: June 2006

Optical photoresponse of quantum wells in the nonperturbative regime has been studied by high-power and ultrafast-oscillating free-electron laser (FEL) experiments, which revealed a profound deviation of the nonlinear power from conventional perturbative multiple photon absorption processes. By combining the experimental and theoretical works we have shown that the nonperturbative solution of the time-dependent Schrödinger equation is essential to understand the experimental observations. Optical transitions of electrons are dynamic. One photon is absorbed or emitted when an electron transits from one electron state to the other. The rates of absorption and emission are proportional to the time interval in the femtosecond time scale. In the picosecond time scale, multiphoton processes emerge. The strong and fast-oscillating FEL source intensifies the dynamic photon absorption and emission processes in the quantum wells, resulting in a much enhanced nonlinearity in the photoresponse spectrum.
Hydrogen induced roughening and smoothing in surface morphology during synchrotron-radiation-excited GeH4-source homoepitaxy on Ge(001)
Authors: Akazawa Housei
Published: June 2006

The kinetics of roughening and smoothing in Ge(001) surface morphology during synchrotron-radiation-excited GeH4-source homoepitaxy has been investigated. At high GeH4 pressures and low substrate temperatures, two-dimensionally flat surface morphology is maintained while passivating the epitaxially growing surface with a Ge hydride layer. After exceeding the critical temperature (330°C), the delivery of GeHx species to the partially dangling bond terminated surface and the accelerated migration of Ge atoms result in roughening from the initial stage of growth, the onset of which can be detected in terms of a small loop in real-time ψ-Δ trajectories of ellipsometric monitoring. The roughening tendency is enhanced with increasing synchroton radiation beam intensity and substrate temperature and with decreasing GeH4 pressure. Submicrometer tall islands consisting of a (001) top surface surrounded by (113) sidewalls are produced. Roughening is terminated when all dangling bonds are tied up with H atoms, i.e., when the surface free energy is minimized. Then, preferential embedding into valleys between islands yields sinusoidal-like ripple morphology, which is eventually converted into a flat Ge epilayer with no inner defects.
Influence of doping density on electron dynamics in GaAs∕AlGaAs quantum cascade lasers
Authors: Jovanović V. D. , Höfling S. , Indjin D. , Vukmirović N. , Ikonić Z. , Harrison P. , Reithmaier J. P. , Forchel A.
Published: June 2006

A detailed theoretical and experimental study of the influence of injector doping on the output characteristics and electron heating in midinfrared GaAs∕AlGaAs quantum cascade lasers is presented. The employed theoretical model of electron transport was based on a fully nonequilibrium self-consistent Schrödinger-Poisson analysis of the scattering rate and energy balance equations. Three different devices with injector sheet doping densities in the range of (4–6.5)×1011cm–2 have been grown and experimentally characterized. Optimized arsenic fluxes were used for the growth, resulting in high-quality layers with smooth surfaces and low defect densities. A quasilinear increase of the threshold current with sheet injector doping has been observed both theoretically and experimentally. The experimental and calculated current-voltage characteristics are in a very good agreement. A decrease of the calculated coupling constant of average electron temperature versus the pumping current with doping level was found.
Optical properties of amorphous GeTe, Sb2Te3, and Ge2Sb2Te5: The role of oxygen
Authors: Olson J. K. , Li Heng , Ju T. , Viner J. M. , Taylor P. C.
Published: June 2006

Amorphous films of GeTe, Sb2Te3, and Ge2Sb2Te5 were grown to thicknesses of 0.3–3μm using rf sputtering. The optical properties of these films are influenced by the presence of oxygen impurities. The absorption edge in these glasses is sometimes broader than in “standard” chalcogenide glasses, such as GeSe2 or As2Se3. This result implies either that the valance band consists of highly strained bonds or that large densities of defects exist. In some samples, there exists an electron paramagnetic resonance signal in the absence of any optical excitation, which implies that a large defect density (∼1019cm−3) exists within the energy gap. Below the optical gap the refractive index for Ge2Sb2Te5 is approximately 3.5. Electron spin resonance signals associated with the glassy SiO2 interface with the chalcogenide films are also observed.
Effect of growth temperature on closely lattice-matched GaAsSbN intrinsic layer for GaAs-based 1.3μm p-i-n photodetector
Authors: Wicaksono S. , Yoon S. F. , Loke W. K. , Tan K. H. , Ng B. K.
Published: June 2006

GaAsSbN layers closely lattice-matched to GaAs were studied for application as the intrinsic layer in GaAs-based 1.3μm p-i-n photodetector. The GaAsSbN was grown as the intrinsic layer for the GaAs∕GaAsSbN∕GaAs photodetector structure using solid-source molecular beam epitaxy in conjunction with a radio frequency plasma-assisted nitrogen source and valved antimony cracker source. The lattice mismatch of the GaAsSbN layer to GaAs was kept below 4000ppm, which is sufficient to maintain coherent growth of ∼0.45μm thick GaAsSbN on the GaAs substrate. The growth temperature of the GaAsSbN layer was varied from 420–480°C. All samples exhibit room temperature photocurrent response in the 1.3μm wavelength region, with dark current density of ∼0.3–0.5mA∕cm2 and responsivity of up to 33mA∕W at 2V reverse bias. Reciprocal space maps reveal traces of point defects and segregation (clustering) of N and Sb, which may have a detrimental effect on the photocurrent responsivity.