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15 Aug 2007

Volume 102, Issue 4, Articles (04xxxx)

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Negative refractivity assisted optical power limiting

Alexander Baev, Edward P. Furlani, Marek Samoc, and Paras N. Prasad

J. Appl. Phys. 102, 043101 (2007); http://dx.doi.org/10.1063/1.2769144 (5 pages) | Cited 2 times

Online Publication Date: 16 August 2007

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We propose an approach for achieving optical power limiting (OPL) using planar multilayered media. The approach involves the use of multiple bilayer structures that consist of a negative index (NIM) layer followed by a nonlinear two-photon absorbing (TPA) layer. The NIM layers refocus light in the TPA layers to increase intensity and nonlinear absorption. We perform parametric two-dimensional finite difference time domain simulations and compute the power transmittance of the media as a function of the incident light intensity, the number of layers, the layer thickness, and the material properties. Our analysis demonstrates proof-of-concept and indicates that the TPA-based OPL can be enhanced and optimized using NIM focusing.
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42.65.-k Nonlinear optics
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
42.79.Wc Optical coatings
02.70.Bf Finite-difference methods

Multichannel filters with shape designing in two-dimensional photonic crystal slabs

Ya-Zhao Liu, Shuai Feng, Jie Tian, Cheng Ren, Haihua Tao, Zhi-Yuan Li, Bing-Ying Cheng, Dao-Zhong Zhang, and Qiang Luo

J. Appl. Phys. 102, 043102 (2007); http://dx.doi.org/10.1063/1.2769263 (6 pages) | Cited 11 times

Online Publication Date: 20 August 2007

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We investigated the influence of the irregular shape of air holes on the optical characteristics of channel-drop filters built in a two-dimensional photonic crystal slab. Three differently shaped structures are tested by experiments and simulations to demonstrate the shape-tuning capability of the optical devices. A four-channel filter based on this shape design is presented. The photonic crystal consists of a triangular array of air holes fabricated by focused ion beam lithographic technology on a silicon slab with thickness of 260 nm. The filter contains a heterostructure with different lattice constants of 420 and 430 nm. In each channel, the filtering functionality is guaranteed by the indirect resonant coupling between waveguide and cavity. The elliptical air holes that surround the cavity have different parameters, and this leads to a fine tuning of the cavity’s resonant frequencies. The performance of the multichannel filter is found to be very encouraging.
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42.79.Ci Filters, zone plates, and polarizers
42.79.Gn Optical waveguides and couplers

Analysis of laser ablation: Contribution of ionization energy to the plasma and shock wave properties

Sy-Bor Wen, Xianglei Mao, Ralph Greif, and Richard E. Russo

J. Appl. Phys. 102, 043103 (2007); http://dx.doi.org/10.1063/1.2761827 (10 pages) | Cited 3 times

Online Publication Date: 20 August 2007

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By fitting simulation results with experimentally measured trajectories of the shock wave and the vapor∕background gas contact surface, we found that inclusion of ionization energy in the analysis leads to a change in the evolution of the pressure, mass density, electron number density, and temperature of the vapor plume. The contribution of ionization energy to both the plasma and shock wave has been neglected in most studies of laser ablation. Compared to previous simulations, the densities, pressures, and temperatures are lower shortly after the laser pulse (<5 ns), but become larger (by a factor of 2) as the time after the laser pulse increases (>50 ns). The predicted laser energy conversion ratio also showed about a 20% increase (from 35% to 45%) when the ionization energy is considered. The changes in the evolution of the physical quantities result from the retention of the ionization energy in the vapor plume, which is then gradually transformed to kinetic and thermal energies. When ionization energy is included in the simulation, the vapor plume attains higher expansion speeds and temperatures for a longer time after the laser pulse. The better determination of the temperature history of the vapor plume not only improves the understanding of the expansion process of the laser induced vapor plume but also is important for chemical analysis. The accurate temperature history provides supplementary information which enhances the accuracy of chemical analysis based on spectral emission measurements (e.g., laser induced breakdown spectroscopy).
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52.20.-j Elementary processes in plasmas
52.25.Kn Thermodynamics of plasmas
52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams
52.38.Mf Laser ablation
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)

Influence of the annealing temperature and silicon concentration on the absorption and emission properties of Si nanocrystals

A. Podhorodecki, G. Zatryb, J. Misiewicz, J. Wojcik, and P. Mascher

J. Appl. Phys. 102, 043104 (2007); http://dx.doi.org/10.1063/1.2772501 (5 pages) | Cited 8 times

Online Publication Date: 23 August 2007

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Silicon nanocrystals embedded in a silicon-rich silicon-oxide matrix have been fabricated at different silicon contents (38%, 40%, and 49%) using plasma-enhanced chemical vapor deposition and annealing at different temperatures in the range from 900 °C to 1100 °C. Their optical properties have been investigated by photoluminescence and transmittance measurements. Strong, room-temperature emission bands at ∼ 1.6 eV have been observed for all samples, with intensities dependent on the annealing temperature and Si content of the samples. From transmittance measurements, a redshift of the absorption edge has been detected when increasing the annealing temperature or Si content.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
61.72.Cc Kinetics of defect formation and annealing
81.07.Bc Nanocrystalline materials
81.05.Cy Elemental semiconductors
81.16.-c Methods of micro- and nanofabrication and processing
78.55.Ap Elemental semiconductors

Control of polarization and mode mapping of small volume high Q micropillars

A. Daraei, D. Sanvitto, J. A. Timpson, A. M. Fox, D. M. Whittaker, M. S. Skolnick, P. S. S. Guimarães, H. Vinck, A. Tahraoui, P. W. Fry, S. L. Liew, and M. Hopkinson

J. Appl. Phys. 102, 043105 (2007); http://dx.doi.org/10.1063/1.2769803 (6 pages) | Cited 5 times

Online Publication Date: 24 August 2007

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We show that the polarization of the emission of a single quantum dot embedded within a microcavity pillar of elliptical cross section can be completely controlled and even switched between two orthogonal linear polarizations by changing the coupling of the dot emission with the polarized photonic modes. We also measure the spatial profile of the emission of a series of pillars with different ellipticities and show that the results can be well described by simple theoretical modeling of the modes of an infinite length elliptical cylinder.
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81.07.Ta Quantum dots
78.67.Hc Quantum dots
78.55.Cr III-V semiconductors
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
68.65.Hb Quantum dots (patterned in quantum wells)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Role of oxygen desorption during vacuum annealing in the improvement of electrical properties of aluminum doped zinc oxide films synthesized by sol gel method

Mingwei Zhu, Hui Huang, Jun Gong, Chao Sun, and Xin Jiang

J. Appl. Phys. 102, 043106 (2007); http://dx.doi.org/10.1063/1.2772591 (6 pages) | Cited 6 times

Online Publication Date: 24 August 2007

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Aluminum doped zinc oxide (ZAO) films were prepared by sol gel dip-coating process. Several annealing treatments were carried out to improve the conductivity of ZAO films. The results show that the annealing temperature and the cooling rate have significant effects on the resistivity of ZAO films. A minimum resistivity of 5.97×10−3 Ω cm with the transmission in visible light region above 85% can be obtained after vacuum annealing. The desorption of oxygen that is chemisorbed on the surface and at the grain boundary is thought to be responsible for the observed improvement.
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73.61.Ga II-VI semiconductors
72.80.Ey III-V and II-VI semiconductors
68.43.Mn Adsorption kinetics
81.40.Gh Other heat and thermomechanical treatments
61.72.uj III-V and II-VI semiconductors
61.72.Mm Grain and twin boundaries

Elastic stability of a dual-coated optical fiber of finite length

E. Suhir

J. Appl. Phys. 102, 043107 (2007); http://dx.doi.org/10.1063/1.2764216 (9 pages) | Cited 3 times

Online Publication Date: 24 August 2007

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Dual-coated optical fibers are fabricated at elevated temperatures and operated at low temperature conditions. It is imperative that the fibers remain elastically stable, i.e., do not buckle as a result of the thermal contraction mismatch of the dissimilar materials of the fiber and the secondary coating or, in the case of an optical fiber interconnect, of the fiber and the material of an enclosure, if any. We develop a simple and easy-to-use predictive model that enables one to establish the condition of elastic stability for a dual-coated optical fiber of finite length, and/or to select the coating materials with the most favorable physical properties for such a fiber, and/or to establish the adequate thickness of the primary coating. The fiber is idealized as a free-free beam (i.e., a beam not supported at the ends), and the effect of the embedding medium is mimicked by a continuous elastic foundation. The obtained results can be applied also, with some minor modifications, for the evaluation of the elastic stability of short fibers in fiber-reinforced composites, including nanocomposites.
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42.81.Bm Fabrication, cladding, and splicing
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
62.20.D- Elasticity
62.20.F- Deformation and plasticity

Electronic structure and efficient carrier injection in low-threshold T-shaped quantum-wire lasers with parallel p- and n-doping layers

Shu-man Liu, Masahiro Yoshita, Makoto Okano, Toshiyuki Ihara, Hirotake Itoh, Hidefumi Akiyama, Loren N. Pfeiffer, Ken W. West, and Kirk W. Baldwin

J. Appl. Phys. 102, 043108 (2007); http://dx.doi.org/10.1063/1.2772544 (7 pages)

Online Publication Date: 27 August 2007

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We report on the electronic structure, efficient carrier injection, and quantitative lasing characteristics of T-shaped GaAs/AlGaAs quantum-wire-laser diodes with parallel p- and n-doping layers grown by a cleaved-edge-overgrowth method with molecular-beam epitaxy. Continuous single-mode lasing from the ground subband of the quantum wires was demonstrated between 30 and 70 K in laser diodes with high-reflectivity Au coating on both cavity facets. The lowest threshold of 0.27 mA and the highest differential quantum efficiency of 12% were achieved at 30 K. Micro-photoluminescence measurements demonstrated the high optical quality of the quantum wires with narrow linewidth of 0.9 meV and provided electronic structures of surrounding layers. Microscopic electroluminescence (EL) imaging measurements demonstrated the efficient carrier injection into the quantum wires at 30 K. These two factors, i.e., high material quality and efficient carrier injection, contribute to the low threshold current and high efficiency of the laser device. The result of EL imaging at 5 K indicates an inefficient carrier injection into the active region, which limits the operating temperature of the devices.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Photoconductivity in sprayed β-In2S3 thin films under sub-band-gap excitation of 1.96 eV

R. Jayakrishnan, Tina Sebastian, Teny Theresa John, C. Sudha Kartha, and K. P. Vijayakumar

J. Appl. Phys. 102, 043109 (2007); http://dx.doi.org/10.1063/1.2770830 (8 pages) | Cited 2 times

Online Publication Date: 27 August 2007

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β-In2S3 thin films with a band gap of ∼ 2.67 eV exhibited persistent photoconductivity when excited using photons with energy of 1.96 eV. The photoconductive response to extrinsic photoexcitation could be removed when the film stoichiometry was changed. Photoluminescence studies in the films revealed an emission of 1.826 eV, due to donor–acceptor pair (DAP) recombination, which was absent in the film not responding to extrinsic excitation. Hence, it was concluded that presence of the DAP was responsible for the extrinsic photoconductivity under the 1.96 eV excitation. This study can initiate further a methodology for tailoring the photoresponse of this semiconducting thin film by spatially controlling the film stoichiometry.
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72.40.+w Photoconduction and photovoltaic effects
71.20.Nr Semiconductor compounds
78.55.Hx Other solid inorganic materials
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
61.66.Bi Elemental solids
61.66.Dk Alloys

Two staged laser acceleration with a static magnetic field

Z. Chen, Y. K. Ho, Q. Kong, P. X. Wang, W. Wang, and J. J. Xu

J. Appl. Phys. 102, 043110 (2007); http://dx.doi.org/10.1063/1.2764241 (3 pages) | Cited 1 time

Online Publication Date: 28 August 2007

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The use of a static magnetic field with a modest intensity in the conventional vacuum laser acceleration scheme has been investigated. It has been found that the applied magnetic field can break the symmetry of the laser acceleration and deceleration phases experienced by the electrons after they leave the focal region, allowing the electrons to be accelerated in the focal region to gain more energy from the combined fields of the laser and the static magnetic field. The later process is the second stage acceleration taking place in the region outside the laser focal area. Explanations of these interaction features based on analytical calculations and simulations are presented.
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41.75.Jv Laser-driven acceleration
42.62.-b Laser applications

Enhanced transmission through quasirandom nanostructured dielectric interfaces via supercritical angle scattering

S. Riyopoulos, T. D. Moustakas, and J. S. Cabalu

J. Appl. Phys. 102, 043111 (2007); http://dx.doi.org/10.1063/1.2753578 (14 pages) | Cited 1 time

Online Publication Date: 30 August 2007

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Greatly enhanced radiation extraction has been observed through randomly textured interfaces with feature sizes comparable to the wavelength. Ray tracing fails in this regime and a different analytic treatment is presented based on supercritical scattering. A quasiperiodic interface destroys the k∥ invariance, introducing wave numbers shifted by structure period harmonics, and allows transmission for supercritical incidence. Converting the interface boundary conditions into effective current source terms yield simple integral formulas for the transmitted fraction including quasiperiodic, subwavelength surface features with random variation of the feature size and period. Supercritical transmission occurs for single monochromatic wave incidence, regardless of the angle, causing enhanced radiation extraction. Further enhancement follows from multiple bounces between opposed surfaces. The role of phase diffusion during multiple reflections is also addressed.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.35.+c Brillouin and Rayleigh scattering; other light scattering
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials

Infrared surface plasmon resonances due to Er-V semimetallic nanoparticles in III-V semiconductor matrices

M. P. Hanson, A. C. Gossard, and E. R. Brown

J. Appl. Phys. 102, 043112 (2007); http://dx.doi.org/10.1063/1.2761846 (8 pages) | Cited 5 times

Online Publication Date: 30 August 2007

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Absorption resonances attributed to surface plasmons are observed in various Er-V nanoparticle/III-V semiconductor superlattices grown by molecular beam epitaxy. ErAs particles in GaAs and GaAs0.5Sb0.5 as well as ErSb particles in GaSb and GaAs0.5Sb0.5 are investigated. Transmission measurements indicate resonant absorption peaks from 1.2 to 2.4 μm depending on growth conditions for ErAs particles in GaAs. In the case of ErSb particles in GaSb these features are observed from about 2.4 to 4.5 μm. For the ErAs or ErSb particles grown in GaAs0.5Sb0.5 the resonances were found to straddle the band gap of the semiconductor matrix around 1.5 μm. Growth conditions are used to engineer the nanoparticles’ shape and density in order to tune the wavelength and intensity of the resonance. The asymmetry of the ErSb particles, which preferentially elongate along the [0math1] direction, leads to a dependence of the resonance on polarization of the incident light. The absorption peak for light polarized parallel to the long axis of the particles is found to occur at longer wavelengths than those for that for light polarized perpendicular to the long axis of the particles.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Optical characterization of Nd3+- and Er3+-doped lead-indium-phosphate glasses

Taísa B. Brito, M. V. D. Vermelho, E. A. Gouveia, M. T. de Araujo, I. Guedes, C.-K. Loong, and L. A. Boatner

J. Appl. Phys. 102, 043113 (2007); http://dx.doi.org/10.1063/1.2756705 (7 pages) | Cited 7 times

Online Publication Date: 31 August 2007

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In this work, Judd–Ofelt analysis is applied to rare-earth-doped lead-indium-phosphate glasses (RE-PbInPO4, where RE = Er3+ and Nd3+) in order to evaluate their potential as both glass laser systems and amplifier materials. The phenomenological Judd–Ofelt parameters Ω(2), Ω(4), and Ω(6) are determined for both rare-earth ions together with their quality factors and compared with the equivalent parameters for several other host glasses. The spontaneous emission probabilities and the lifetimes of the Nd3+ 4F3/2 laser transitions are determined and analyzed as a function of the optical quality factor. For Nd3+-PbInPO4, glass fluorescence emission (890, 1058, and 1330 nm) lines are observed. Highly efficient infrared-to-visible frequency up-conversion at 530, 550, and 670 nm as well as an intense infrared fluorescence emission ( ∼ 1540 nm) is observed in Er3+-doped PbIn(PO4) glasses pumped using 800 nm radiation excitation.
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42.70.Ce Glasses, quartz
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.55.Hx Other solid inorganic materials
42.55.Rz Doped-insulator lasers and other solid state lasers
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
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Simple analytical expression for electron temperature in an alternating current plasma display discharge

K. Muraoka, K. Suzuki, M. Azumi, Y. Yamagata, and M. Yagi

J. Appl. Phys. 102, 043301 (2007); http://dx.doi.org/10.1063/1.2769268 (4 pages)

Online Publication Date: 16 August 2007

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A simple analytical expression was derived for electron temperature Te in discharge conditions typical for an alternating current plasma display panel discharge, in which the least possible number of parameters was introduced so as to provide good physical insight into the mechanisms of determining Te under various discharge conditions. From this derivation, an explicit dependence of Te on discharge parameters (gas composition and pressure, and the strength of the electric field E) has been derived. It was tested for a discharge condition studied in our previous article on the discharge-radiation dynamics, yielding reasonable values for a cathode fall and a positive column. Also, the values of Te obtained experimentally using laser Thomson scattering in a positive column are well predicted from this analysis.
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52.75.-d Plasma devices
85.60.Pg Display systems

Time-resolved triple probe investigations of a pulsed magnetron discharge

P. M. Bryant, S. A. Voronin, J. W. Bradley, and A. Vetushka

J. Appl. Phys. 102, 043302 (2007); http://dx.doi.org/10.1063/1.2769805 (10 pages) | Cited 6 times

Online Publication Date: 17 August 2007

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Time-resolved measurements of the electron temperature Te and density ne at the centerline of a bipolar pulsed dc magnetron argon discharge were obtained using a triple probe. Two electron temperature spikes at the pulse transients were observed and are interpreted as being due to the presence of energetic electrons generated during these periods. During the off time the observed rapid decay of Te and gradual decay of ne are shown to be a consequence of enhanced plasma retention due to the magnetized electrons. The rapid rise in ne during the on time was observed to reach a maximum, coinciding with a minimum in Te (with Te decaying rapidly), probably due to enhanced ionization by the energetic electrons. Throughout the rest of the pulse period Te increased slightly whereas ne decreased due to global collisional heating of electrons with an additional energetic electron group formed during the on time. The results also show that the electron temperature and plasma density increase with decreasing duty cycle. The plasma density increased linearly with the total energy input per pulse E and increases with pressure. The electron temperature decreases towards the higher pressures and was found to be approximately independent of E. The calculated ion power flux density to a floating substrate (averaged over one pulse cycle and being proportional to the ion-to-atom arrival ratio) was found to be higher by a factor between 2 and 4 than during dc at the same discharge conditions. The power flux was also found to increase linearly with time-averaged power with the steepest rise at the lowest duty cycle. Decreasing the duty cycle and increasing the time-averaged power will lead to the rise in the ion-to-atom arrival ratio and generally improve the quality of the deposited thin films. Finally, these results show the triple probe to be a reliable and efficient method to measure the temporal evolution of the plasma parameters in the pulsed magnetron.
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52.25.-b Plasma properties
52.70.Ds Electric and magnetic measurements

Production of neutrals and their effects on the ion charge states in cathodic vacuum arc plasmas

André Anders, Efim M. Oks, and Georgy Yu. Yushkov

J. Appl. Phys. 102, 043303 (2007); http://dx.doi.org/10.1063/1.2769789 (7 pages) | Cited 13 times

Online Publication Date: 20 August 2007

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Cathodic arc plasmas are considered fully ionized and they contain multiply charged ions, yet gaseous and metal neutrals can be present. It is shown that they can cause a significant reduction of the ion charge states as measured far from the cathode spots. Several cathode materials were used to study the evolution of the mean ion charge state as a function of time after arc ignition. The type of cathode material, arc current amplitude, intentionally increased background gas, additional surfaces placed near the plasma flow, and other factors influence the degree of charge state reduction because all of these factors influence the density of neutrals. In all cases, it was found that the mean ion charge state follows an exponential decay of first order, math(t) = A exp(−t/τ)+mathss, where A is a parameter describing the importance of the decay, τ is the characteristic decay time, and mathss is a steady-state value approached for continuous arc operation. The extrapolated values math(t→0) indicate surprisingly high mean charge states as produced at cathode spots and not “skewed” by charge exchange collisions with neutrals.
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52.80.Mg Arcs; sparks; lightning; atmospheric electricity
52.80.Vp Discharge in vacuum
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.25.Ya Neutrals in plasmas

Determining parameter sets for low-frequency-oscillation-free operation of Hall thruster

Taichiro Tamida, Takafumi Nakagawa, Ikuro Suga, Hiroyuki Osuga, Toshiyuki Ozaki, and Katsuaki Matsui

J. Appl. Phys. 102, 043304 (2007); http://dx.doi.org/10.1063/1.2771039 (6 pages) | Cited 7 times

Online Publication Date: 22 August 2007

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We performed a theoretical and experimental study of low-frequency oscillation in a Hall thruster. The oscillation phenomenon depends on various external control parameters. However, we found that such dependence can be observed very clearly using certain expressions, which are combinations of external control parameters. Using these expressions, one can observe the oscillation strength and oscillation mode for a given set of external control parameters. We considered that low-frequency oscillation can be essentially suppressed by a method of determining parameter sets. The method of determining the parameter sets is very important for Hall thruster design and control; in particular, it makes it easy to manage electromagnetic compatibility with the other equipments in a satellite, using a power supply control.
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97.60.-s Late stages of stellar evolution (including black holes)
89.40.Dd Air transporation
07.50.Hp Electrical noise and shielding equipment

Deposition of nanocrystalline silicon films at room temperature

Radhika C. Mani, Ioana Pavel, and Eray S. Aydil

J. Appl. Phys. 102, 043305 (2007); http://dx.doi.org/10.1063/1.2770824 (7 pages) | Cited 8 times

Online Publication Date: 24 August 2007

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Bond rearrangements, facilitated by H insertion into strained Si–Si bonds have been shown to result in H-induced crystallization of amorphous silicon films. Whether such H-induced rearrangements can lead to synthesis of nanocrystalline films at room temperature has remained an open question. In this article, the authors demonstrate the deposition of thin films containing nanocrystals of silicon using inductively coupled SiH4/H2 plasma at room temperature. Real time in situ spectroscopic ellipsometry and ex situ transmission electron microscopy revealed that the silicon nanocrystals nucleate below the surface, and grow beneath an amorphous silicon crust. This observation validates the hydrogen-induced crystallization model. Analysis of the crystal size distribution shows that the distribution depends on the growth duration rather than the substrate temperature. Observation of crystals as large as 100–150 nm at room temperature indicates that silicon nanocrystals not only nucleate but also grow substantially in the bulk even at room temperature.
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81.05.Cy Elemental semiconductors
81.07.Bc Nanocrystalline materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.A- Nucleation and growth
64.70.K- Solid-solid transitions
61.46.Hk Nanocrystals
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Using atomistic computer simulations to analyze x-ray diffraction data from metallic glasses

M. I. Mendelev, D. J. Sordelet, and M. J. Kramer

J. Appl. Phys. 102, 043501 (2007); http://dx.doi.org/10.1063/1.2769157 (7 pages) | Cited 28 times

Online Publication Date: 16 August 2007

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We propose a method of using atomistic computer simulations to obtain partial pair correlation functions from wide angle diffraction experiments with metallic liquids and their glasses. In this method, a model is first created using a semiempirical interatomic potential and then an additional atomic force is added to improve the agreement with experimental diffraction data. To illustrate this approach, the structure of an amorphous Cu64.5Zr35.5 alloy is highlighted, where we present the results for the semiempirical many-body potential and fitting to x-ray diffraction data. While only x-ray diffraction data were used in the present work, the method can be easily adapted to the case when there are also data from neutron diffraction or even in combination. Moreover, this method can be employed in the case of multicomponent systems when the data of several diffraction experiments can be combined.
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61.43.Fs Glasses

Microstructural evolution and atomic transport by thermomigration in eutectic tin-lead flip chip solder joints

Dan Yang, B. Y. Wu, Y. C. Chan, and K. N. Tu

J. Appl. Phys. 102, 043502 (2007); http://dx.doi.org/10.1063/1.2769270 (6 pages) | Cited 9 times

Online Publication Date: 16 August 2007

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The thermomigration behavior of eutectic tin-lead flip chip solder joints at an ambient temperature of 150 °C was investigated in terms of microstructural evolution, atomic transport, and numerical simulation. Pb accumulation and phase separation were observed in solder joints near a melting temperature after 50 h, which was supported by energy dispersive x-ray and element mapping analysis. It is believed that Pb atoms migrated from the chip side (the hot side) to the substrate side (the cold side) under a temperature gradient. Thermal electrical finite element simulation for the real flip chip test structure showed the existence of a temperature difference between the substrate side and the chip side. In addition, a temperature gradient above 1000 °C/cm across the adjacent unpowered solder joints was predicted. This was also verified by temperature measurements with thermocouples. The atomic flux of Pb due to thermomigration was calculated here, which was agreeable with the values originally reported. Also, the driving force of thermomigration was estimated to be 10−17 N, even approaching the same order with that of electromigration under a current density of 104 A/cm2.
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72.15.Eb Electrical and thermal conduction in crystalline metals and alloys
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
61.72.-y Defects and impurities in crystals; microstructure
64.75.-g Phase equilibria
66.30.Qa Electromigration
85.40.Ls Metallization, contacts, interconnects; device isolation

Dielectrophoresis force driven dynamics of carbon nanotubes in liquid crystal medium

Anoop Kumar Srivastava, Seok Jin Jeong, Myong-Hoon Lee, Seung Hee Lee, Seok Ho Jeong, and Young Hee Lee

J. Appl. Phys. 102, 043503 (2007); http://dx.doi.org/10.1063/1.2769341 (5 pages) | Cited 12 times

Online Publication Date: 17 August 2007

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This work reports the translation motion of carbon nanotubes (CNTs) dispersed in nematic liquid crystal (NLC) under an ac electric field. This effect was studied for homogeneously and vertically aligned NLC cells driven by in-plane field and vertical electric field, respectively. Long axis of the CNTs is aligned along the liquid crystal director, and above the critical field, the director of the NLC is distorted due to translation motion of CNTs in NLC. The amplitude of this translation motion decreases with increasing frequency, whereas the critical field increases with increasing frequency. We present the mechanism of translation motion of CNTs and model this observed phenomenon based on dielectrophoretic force.
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82.45.-h Electrochemistry and electrophoresis
61.30.-v Liquid crystals

The C1s core line in irradiated graphite

Giorgio Speranza, Luca Minati, and Mariano Anderle

J. Appl. Phys. 102, 043504 (2007); http://dx.doi.org/10.1063/1.2769332 (7 pages) | Cited 6 times

Online Publication Date: 17 August 2007

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Recently, plasma deposited amorphous carbon films have been the subject of extensive experimental and theoretical investigations aimed at correlating their electronic, structural, and mechanical properties to growth parameters. To investigate these properties, different spectral parameters reflecting the electronic structure of carbon-based materials are proposed in literature. The effects of various electronic configurations on the carbon photoelectron spectra are analyzed here with particular attention to C1s core line with the aim to better interpret its structure. The latter is commonly fitted under the assumption that it can be described by using just two spectral components related to sp2 and sp3 hybrids. Their relative intensities are then used to estimate the sp2 and sp3 phases. We show that, in the presence of an amorphous network, the C1s line shape is the result of a more complex mixture of electronic states. Ar+ irradiated graphite and successive oxidation was used to identify spectral features to better describe the C1s line shape.
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71.20.Tx Fullerenes and related materials; intercalation compounds
79.60.Bm Clean metal, semiconductor, and insulator surfaces
79.60.Ht Disordered structures
61.80.Jh Ion radiation effects
61.82.Ms Insulators
61.43.Er Other amorphous solids

Spectroscopy, energy transfer, and frequency upconversion in Tm3+-doped TeO2-PbO glass

Vineet Kumar Rai, Leonardo de S. Menezes, and Cid B. de Araújo

J. Appl. Phys. 102, 043505 (2007); http://dx.doi.org/10.1063/1.2769788 (4 pages) | Cited 6 times

Online Publication Date: 17 August 2007

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Absorption and luminescence experiments were performed in thulium (Tm3+)-doped TeO2-PbO glass samples. Spectroscopic quantities such as transition probabilities, branching ratios, and radiative lifetimes related to the Tm3+ ions were determined using the Judd-Ofelt theory. Luminescence properties were studied by exciting the samples using a pulsed laser operating at 590 nm. Stokes luminescence centered at 792 nm and frequency upconversion (UC) to the blue ( ≈ 457 nm) were observed. The intensity and the time evolution of both emissions were studied for different concentrations of Tm3+ ions. The results indicate that the dipole-dipole interaction provides the most important contribution for energy transfer among the Tm3+ ions. The origin of the UC process is attributed to energy transfer among Tm3+ ions in pairs.
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78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
78.55.Qr Amorphous materials; glasses and other disordered solids
78.30.Hv Other nonmetallic inorganics

Thermodynamics of flame impingement heat transfer

S. K. Som, G. K. Agrawal, and Suman Chakraborty

J. Appl. Phys. 102, 043506 (2007); http://dx.doi.org/10.1063/1.2769784 (9 pages)

Online Publication Date: 20 August 2007

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A theoretical model for entropy generation and utilization of work potential (exergy) in flame impingement (both premixed and diffusion) heat transfer has been developed in this article, to offer physical insights on the optimal operational regimes, depicting high values of the surface heat flux with minimal exergy destruction, within the practical constraints. The irreversibility components due to different physical processes have been evaluated from a general entropy transport equation. The velocity, temperature, and species concentration fields required for the solution of entropy transport equation have been determined from the numerical computation of flow-field in the flame. Global two-step chemical kinetics has been considered with methane (CH4) and air as fuel and oxidizer, respectively. The results have been predicted in terms of average nondimensional heat flux, expressed as Nusselt number at the target plate, the irreversibility components, and second law efficiency, as functions of the pertinent input parameters such as the jet Reynolds number and the ratio of plate separation distance to nozzle diameter (H/d). The average Nusselt number has been found to increase with an increase in jet Reynolds number and a decrease in H/d ratio, up to a value of 8. The dominant source of thermodynamic irreversibility in a premixed flame has been attributed to the thermal energy exchange whereas, in a diffusion flame, the same has been attributed to an uncontrolled exchange of electrons accompanying the reactive kinetics. The second law efficiency has been found to increase with an increase in jet Reynolds number and an increase in the H/d ratio, up to a value of 20. Values of the jet Reynolds number greater than 10 000 and H/d ratio in the tune of 15 have been observed to pertain to the regime of optimum flame impingement heat transfer, consistent with the energy and exergy balance constraints.
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47.70.Pq Flames; combustion
44.40.+a Thermal radiation

Large deformation analysis of gellan gels

Shinnosuke Kawai, Yoko Nitta, and Katsuyoshi Nishinari

J. Appl. Phys. 102, 043507 (2007); http://dx.doi.org/10.1063/1.2769143 (9 pages) | Cited 4 times

Online Publication Date: 20 August 2007

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Gellan gel, a typical polysaccharide gel, is ruptured with different deformation behaviors from gelatin gel or rubber. It exhibits both strain hardening and softening; hardening is observed for moderate strain and softening occurs for larger strain. From the analyses of stress–strain curves of gellan gels, we propose forms of strain energy function. The fit with the proposed equation was excellent, while the existing models fail because they consider only one of hardening or softening effect. Furthermore, these equations are shown to be capable of extracting the hardening and softening effects separately from the observed stress–strain curves. By using these fitting equations, the concentration dependences of hardening and softening are investigated. It is shown that the degrees of hardening and softening both increase with increasing gellan concentration.
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62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep
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