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15 May 2007

Volume 101, Issue 10, Articles (10xxxx)

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Droplet dynamics in a microchannel subjected to electrocapillary actuation

Suman Chakraborty and Rajat Mittal

J. Appl. Phys. 101, 104901 (2007); http://dx.doi.org/10.1063/1.2732431 (8 pages) | Cited 7 times

Online Publication Date: 16 May 2007

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In this paper, a theoretical approach is devised to analyze the droplet dynamics in a microchannel subjected to electrocapillary actuation mechanisms. Specific attention is devoted to estimate the influences of the applied potentials and the hydrophobic insulation thicknesses, so as to obtain suitable interelectrode spacings, for a given droplet size. It is revealed that there exists a threshold limit on the value of the interelectrode pitch, beyond which the overall microfluidic transport rates are substantially lowered. On the other hand, with interelectrode spacings below the threshold limit, unnecessarily large number of electrodes might be necessary to achieve a desired droplet transport. Further, the optimal interelectrode spacings are found to be strong functions of the applied potentials and the dielectric layer thicknesses, based on which suitable design guidelines can be proposed.
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47.55.dm Thermocapillary effects
47.55.nb Capillary and thermocapillary flows
47.61.Fg Flows in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS)
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.85.ld Boundary layer control
47.85.Np Fluidics

Characterization of multiscale surface evolution of polycrystalline copper thin films

J. J. Yang and K. W. Xu

J. Appl. Phys. 101, 104902 (2007); http://dx.doi.org/10.1063/1.2732440 (7 pages) | Cited 3 times

Online Publication Date: 16 May 2007

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Surface morphologies of Cu films with different thicknesses varying from 110 to 660 nm deposited on Ta-covered Si(111) substrates at 700 K by magnetron sputtering were investigated by atomic force microscopy (AFM). The global surface fluctuation image of the film was obtained directly from AFM measurement with larger scan size. Multiresolution wavelet transform approach incorporating power spectrum density analysis was proposed to extract the local surface fluctuation image. From these images, the dynamical exponents of both global and local surface fluctuations were calculated in terms of dynamic scaling theory. The multiscale surface evolution of Cu film could be characterized by a set of local exponent values αl ≈ 0.87 and βl ≈ 0.22, and global exponent values αg ≈ 0.83 and βg ≈ 0.85. The dynamic evolution of local surface fluctuations is consistent well with that predicted by linear surface diffusion-dominated growth equation, while the dynamic evolution of global surface fluctuations exhibits anomalous scaling behavior due to the presence of nonlocal bulk diffusion.
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68.55.-a Thin film structure and morphology
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.35.Ja Surface and interface dynamics and vibrations
68.35.Fx Diffusion; interface formation
68.47.De Metallic surfaces

Friction as the fundamental factor controlling the pretilt angle of homeotropic liquid crystal cells: A microrubbing investigation

Michinori Honma and Toshiaki Nose

J. Appl. Phys. 101, 104903 (2007); http://dx.doi.org/10.1063/1.2732436 (7 pages) | Cited 2 times

Online Publication Date: 17 May 2007

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The variation of a pretilt angle on polyimide films, which are rubbed using a tiny metal ball (1–8 mm diameter), is evaluated. During the rubbing process, a vertical load and a frictional force are simultaneously measured, and the relationship between the pretilt angle and frictional work is investigated under the various rubbing conditions such as the vertical load, the number of rubbings, the moving speed, and the ball diameter. Particularly, the dominant factor affecting the variation of the pretilt angle is explored. We propose that the averaged frictional work can be expressed as the sum of contributions of elastic and plastic deformations. It is clarified that the frictional work of plastic deformation is the most fundamental and most important factor for controlling the pretilt angle under the various rubbing conditions.
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61.30.-v Liquid crystals
68.15.+e Liquid thin films
62.10.+s Mechanical properties of liquids
68.60.Bs Mechanical and acoustical properties

Three-dimensional theory of the Cerenkov free-electron laser

H. L. Andrews and C. A. Brau

J. Appl. Phys. 101, 104904 (2007); http://dx.doi.org/10.1063/1.2734540 (6 pages) | Cited 4 times

Online Publication Date: 21 May 2007

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We describe an analytical theory for the gain of a Cerenkov free-electron laser including diffraction of the optical beam in the direction transverse to the electron beam, parallel to the surface of the dielectric. Since the width of the optical beam depends on the gain, the usual cubic dispersion relation of two-dimensional slow-wave structures is replaced by a 5/2-power dispersion relation, but three of the five roots are extraneous. The results show that for a narrow electron beam, the optical beam is much wider than the electron beam. This reduces the gain by an order of magnitude. Moreover, in the three-dimensional theory the allowed roots of the dispersion relation have positive real parts, so they correspond to slow waves; when transverse diffraction is included, fast waves are forbidden.
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41.60.Cr Free-electron lasers
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
41.75.Fr Electron and positron beams

Mechanism of solid phase crystallization of prepatterned nanoscale α-Si pillars

Hyun-Jin Cho, Brian J. Greene, Judy L. Hoyt, and James D. Plummer

J. Appl. Phys. 101, 104905 (2007); http://dx.doi.org/10.1063/1.2734531 (5 pages) | Cited 1 time

Online Publication Date: 22 May 2007

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We have investigated the mechanism of crystallite nucleation and growth in prepatterned nanoscale α-Si pillars using transmission electron microscopy. The number of α-Si pillars that crystallize during annealing depends upon the pillar diameter and the density of nucleation sites at the α-Si/oxide interface, in the as-deposited film. These nucleation sites are presumed to be clusters of atoms exhibiting short-range order that are formed during the initial deposition of α-Si. Their density depends upon the specific deposition conditions. The density of nucleation sites is extracted from the measured pillar crystallization statistics using a Poisson distribution model. It is also observed that the orientation dependence of the crystal growth rate enhances the formation of a single grain inside each pillar. Significant reduction of defect density is achieved with high temperature annealing of sub-100-nm pillars where the surface to bulk ratio is high.
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81.05.Gc Amorphous semiconductors
81.05.Cy Elemental semiconductors
64.70.K- Solid-solid transitions
61.72.Cc Kinetics of defect formation and annealing
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.37.Lp Transmission electron microscopy (TEM)

Transition from tungsten erosion to carbon layer deposition with simultaneous bombardment of tungsten by helium and carbon

Ivan Bizyukov and Karl Krieger

J. Appl. Phys. 101, 104906 (2007); http://dx.doi.org/10.1063/1.2727381 (6 pages) | Cited 6 times

Online Publication Date: 23 May 2007

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Simultaneous bombardment of tungsten layers with helium and carbon ions leads to both erosion of tungsten and implantation of carbon. The underlying processes have been investigated numerically and experimentally as a function of the carbon fraction in the incident ion flux for ion energies in the kilo electron volt range. Tungsten layers were deposited on polished single-crystal silicon substrate by magnetron sputter deposition to eliminate the influence of surface roughness on the experimental results. The fluence-dependent dynamics of the surface composition was measured in situ by ion beam analysis. The given projectile-target system is subject only to kinematic processes, and is therefore particularly suitable for benchmarking of purely kinematic simulations based on the binary-collision approximation, as implemented in the TRIDYN code. TRIDYN calculations match the experimental results very well, which demonstrates the validity of the kinematic description. In particular, the simulations allow us to predict the change of surface composition by bombardment with gaseous and nonvolatile ions, as well as the transition point from W erosion to C deposition as function of the carbon fraction in the incident ion flux.
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61.80.Jh Ion radiation effects
61.82.Bg Metals and alloys
61.72.up Other materials
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
68.55.Nq Composition and phase identification
68.35.Dv Composition, segregation; defects and impurities

The effect of size on the quantitative estimation of defect depth in steel structures using lock-in thermography

C. Wallbrink, S. A. Wade, and R. Jones

J. Appl. Phys. 101, 104907 (2007); http://dx.doi.org/10.1063/1.2732443 (8 pages) | Cited 9 times

Online Publication Date: 23 May 2007

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An investigation into the effect of size on the quantitative estimation of defect depth in a steel specimen has been undertaken using lock-in thermography. Phase contrast measurements over circular defects of varying diameter and depth are presented for a range of excitation frequencies. It was found that the diameter of a defect had an appreciable effect on the observed phase angle which consequently has significant implications with regard to estimating defect depth. Phase contrast measurements for a range of defects in a 10 mm steel specimen indicate that an excitation frequency of 0.02 Hz is the optimal frequency for defect detection. Results obtained with an excitation frequency of 0.02 Hz are used to discuss the limitations of determining the size and depth of defects. A finite element analysis was found to have good correlation with experimental data and thus demonstrates potential in providing improved estimates of defect depth.
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81.70.Fy Nondestructive testing: optical methods

An examination of facet formation during solid phase epitaxy of line-shaped amorphized regions in (001) and (011) Si

K. L. Saenger, K. E. Fogel, J. A. Ott, D. K. Sadana, and H. Yin

J. Appl. Phys. 101, 104908 (2007); http://dx.doi.org/10.1063/1.2736343 (4 pages) | Cited 9 times

Online Publication Date: 23 May 2007

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The shapes of the recrystallization fronts observed during solid phase epitaxy (SPE) in line-shaped amorphized Si (a-Si) regions in single-crystal (001) and (011) Si are shown to exhibit a complex time evolution inconsistent with simple models in which regrowth behavior is determined exclusively by the competition between fixed-growth-rate lateral and vertical SPE. We find that the main features of our data for ⟨110⟩-aligned a-Si lines may be explained by the propensity of the lateral and vertical growth fronts to form stable a-Si/{111} interfaces at their edges. These same a-Si/{111} interfaces have been previously implicated as the root cause of the trench-edge defects produced during the recrystallization of patterned a-Si regions bounded laterally by oxide-filled trenches. An extension of a nanofacet model recently developed to explain the trench-edge defects is shown to explain many of the observed recrystallization behaviors.
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68.55.-a Thin film structure and morphology
68.55.A- Nucleation and growth
81.15.Np Solid phase epitaxy; growth from solid phases
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization

Fabrication of porous alumina on quartz crystal microbalances

R. J. Lazarowich, P. Taborek, B.-Y. Yoo, and N. V. Myung

J. Appl. Phys. 101, 104909 (2007); http://dx.doi.org/10.1063/1.2730563 (7 pages) | Cited 2 times

Online Publication Date: 25 May 2007

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Nanoporous alumina structures were fabricated on quartz crystal microbalances (QCMs) by electrochemically anodizing aluminum electrodes which had a typical thickness of 13 μm. By varying the temperature and voltage of anodization, the diameter of the cylindrical pores could be controlled in the range of 12–40 nm. Properties of the porous films were determined from scanning electron microscopy images and by analyzing isotherms of nitrogen, propane, and water. The isotherms showed signatures of capillary condensation and hysteresis. The mass sensitivity of the QCMs with porous alumina electrodes was increased by a factor of up to 120 compared to conventional QCMs with flat electrodes. Measurements at atmospheric pressure and room temperature in which the relative humidity was varied showed that QCMs with porous alumina electrodes are very sensitive to the partial pressure of water, and can be used as humidity sensors.
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81.05.Rm Porous materials; granular materials
81.07.-b Nanoscale materials and structures: fabrication and characterization
81.16.-c Methods of micro- and nanofabrication and processing
82.45.Yz Nanostructured materials in electrochemistry
61.43.Gt Powders, porous materials
82.45.Fk Electrodes

Measurement of high piezoelectric response of strontium-doped lead zirconate titanate thin films using a nanoindenter

S. Sriram, M. Bhaskaran, A. S. Holland, K. T. Short, and B. A. Latella

J. Appl. Phys. 101, 104910 (2007); http://dx.doi.org/10.1063/1.2735407 (5 pages) | Cited 11 times

Online Publication Date: 29 May 2007

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Strontium-doped lead zirconate titanate (PSZT) is reported to have a high piezoelectric coefficient (d33) in the range of 200−600 pm/V, when in the form of ceramic disks or pellets. This article reports piezoelectric response results for PSZT thin films deposited by rf magnetron sputtering on gold-coated silicon substrates. The compositions of the deposited thin films have been found to be uniform with depth, using secondary ion mass spectroscopy. The surfaces of the deposited thin films have been studied using an atomic force microscope and observed to be regular and nanostructured in nature. The piezoelectric response of the thin films, using the inverse piezoelectric effect, has been measured using a nanoindenter. Values of thin film d33 up to 608 pm/V were obtained, which is much higher than previously reported values of d33 for any thin film. The high values can be attributed to optimized deposition conditions and the low stress measured for the thin film arrangement on the substrate. The technique has been verified by obtaining a null response for silicon dioxide and by measuring d33 values of similar magnitude for PSZT thin films using an atomic force microscope in the same testing arrangement. The piezoelectric response has been mapped to study variations across the thin film and with distance from the top electrode.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.65.-j Piezoelectricity and electromechanical effects
77.55.-g Dielectric thin films
68.60.Bs Mechanical and acoustical properties
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Fracture strength in randomly perforated plates

Y. Yanay, A. Goldsmith, M. Siman, R. Englman, and Z. Jaeger

J. Appl. Phys. 101, 104911 (2007); http://dx.doi.org/10.1063/1.2733623 (8 pages) | Cited 3 times

Online Publication Date: 29 May 2007

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We have measured the forces that need to be applied quasistatically to randomly perforated rectangular aluminum plates to tear them apart. Concomitantly, for identical geometrical structures, we have carried out numerical simulations of the tearing process. The two results agree quite reasonably, and especially so for perforated area densities (P) that are rather less than the critical percolation densities. A given value of P can be achieved through various different realizations (patterns of perforations), each with a somewhat different fracture (or failure) strength F. Our results predict empirically an “expected F” [math(P)] and an expected standard deviation [root variance, S(P)] as functions of P. This enables the risk analysis of failures in plates under tensile stress, which is needed in, e.g., the estimation of the flying safety of a spacecraft subject to multiple perforations.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials
46.50.+a Fracture mechanics, fatigue and cracks

Direct correlation between ferrite microstructure and electrical resistivity

R. J. Cernik, R. Freer, C. Leach, C. Mongkolkachit, P. Barnes, S. Jacques, K. Pile, and A. Wander

J. Appl. Phys. 101, 104912 (2007); http://dx.doi.org/10.1063/1.2735400 (6 pages) | Cited 4 times

Online Publication Date: 30 May 2007

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Variations in the composition and microstructure of Mn-Zn soft ferrites have been directly correlated with spatial variations in electrical resistivity, which were both observed to occur on a length scale of approximately 500 μm. Tomographic energy dispersive diffraction imaging (TEDDI) was used to determine the nonsystematic change in the lattice parameter across the sample volume (8.48±0.05 Å) at a spatial resolution of 50 μm. We have used a microprobe contact technique to measure the local electrical resistivity ( ∼ 35 Ω cm) and density functional theory to model the band structure. The band structure calculations directly utilized the experimentally measured lattice parameters from the TEDDI measurements and were in good agreement with the measured resistivity. The mean band gap shrinkage was found to be 0.02 eV. This value for Eg was found to account well for the observed 10−20 Ω cm resistivity variations.
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71.20.Ps Other inorganic compounds
61.66.Fn Inorganic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
72.80.-r Conductivity of specific materials

Study of electro-osmotic flows in microchannels packed with variable porosity media via lattice Boltzmann method

Zhenhua Chai, Zhaoli Guo, and Baochang Shi

J. Appl. Phys. 101, 104913 (2007); http://dx.doi.org/10.1063/1.2735403 (8 pages) | Cited 10 times

Online Publication Date: 30 May 2007

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In this article, electro-osmotic flow (EOF) in microchannels packed with a variable porosity medium is studied using the lattice Boltzmann method (LBM). The present lattice Boltzmann model is constructed based on the generalized porous medium equation for EOF and validated by comparing the numerical solution with the approximate analytical solution. A detailed parametric study has been presented for EOF in microchannels filled with a variable porosity medium. It is found that the variations of porosity, particle size, ζ potential, applied electric field strength, and tortuosity significantly affect the flow pattern. Numerical results also indicate that the variation of the porosity near the wall has an important influence on the velocity profile, and should not be neglected in practice.
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47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.61.Fg Flows in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS)
47.56.+r Flows through porous media
47.11.Qr Lattice gas
47.85.Np Fluidics

Effects of dimensional nanoscaling on the optical and electrical properties of crystalline Si thin films

A. K. Sharma, P. C. Logofãtu, C. S. Mayberry, S. R. J. Brueck, and N. E. Islam

J. Appl. Phys. 101, 104914 (2007); http://dx.doi.org/10.1063/1.2737983 (9 pages) | Cited 5 times

Online Publication Date: 31 May 2007

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Thin film Si structures between 10 and 200 nm in thickness and configured into two terminal metal-semiconductor-metal structures have been characterized for optical and electrical properties. Dark currents, spectral response, dc quantum efficiency, and ultrafast time response up to 400 nm femtosecond laser illuminations at low fields have been studied. Dark currents and dc photocurrent measurements showed an increase in the film conductivity between 75 and 35 nm, suggesting an increase in the carrier effective velocities due to confinement. An increase in the carrier effective velocity below 75 nm was also confirmed through the transient response analysis. The measured spectral responses are in good agreement with Fresnel’s theoretical model for thin film coupling. The electron-limited transient signal has a full width at half maximum (FWHM) approximately 40 ps for the 10 nm Si film as compared to 490 ps for a 200 nm structure. For a hole-limited transit time signal the FWHM was about 82 ps for the 10 nm film as compared to 2.5 ns for the 200 nm film reduction in FWHM for both electrons and holes for the 10 nm film, signifying that carriers travel much faster as the film thickness is reduced.
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78.47.-p Spectroscopy of solid state dynamics
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
73.61.Cw Elemental semiconductors
68.55.-a Thin film structure and morphology
73.40.Sx Metal-semiconductor-metal structures
73.63.Bd Nanocrystalline materials

Nonlinearity and dynamic phase transition of charge-density-wave lattice

Chao-hung Du, Chung-Yu Lo, Hsiu-Hau Lin, and Shih-Lin Chang

J. Appl. Phys. 101, 104915 (2007); http://dx.doi.org/10.1063/1.2738408 (5 pages)

Online Publication Date: 31 May 2007

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We report the investigation of the dynamic behavior of charge-density waves (CDWs) in a quasi-one-dimensional material K0.3MoO3 using x-ray scattering and multiple x-ray diffraction. Under the application of voltages, we demonstrate that the occurrence of nonlinear conductivity caused by CDW is through the internal deformation of the CDW lattice, i.e., a phase jump of 2π, as the applied voltage exceeds the threshold. By measuring the evolution of peak width of satellite reflections as a function of the field strength, we also report that the CDW lattice can be driven to move and undergo a dynamic phase transition, i.e., from the disordered pinning state to ordered moving solid state, and finally, to disordered moving liquid.
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71.45.Lr Charge-density-wave systems
72.15.Nj Collective modes (e.g., in one-dimensional conductors)
78.70.Ck X-ray scattering
64.70.K- Solid-solid transitions

Ultraviolet laser-induced submicron spatially resolved superhydrophilicity on single crystal lithium niobate surfaces

A. C. Muir, S. Mailis, and R. W. Eason

J. Appl. Phys. 101, 104916 (2007); http://dx.doi.org/10.1063/1.2734539 (5 pages) | Cited 7 times

Online Publication Date: 31 May 2007

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Lithium niobate crystal surfaces become superhydrophilic after ultraviolet laser irradiation. The crystal surface hydrophilicity, which was assessed by the contact angle of a sessile drop of de-ionized water, was found to undergo a transition from mildly hydrophobic (contact angle θE ≈ 50°) to a superhydrophilic state (θE<5°). Patterning of the hydrophilicity at the micron and submicron ranges has been achieved by spatially modulating the illuminating laser beam.
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68.08.Bc Wetting
68.03.Cd Surface tension and related phenomena
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
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