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Journal of Applied Physics

Issue 7 • Date Apr 2011

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Displaying Results 1 - 25 of 661
  • Understanding junction breakdown in multicrystalline solar cells

    Page(s): 071101 - 071101-10
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    Extensive investigations on industrial multicrystalline silicon solar cells have shown that, for standard 1 Ω cm material, acid-etched texturization, and in absence of strong ohmic shunts, there are three different types of breakdown appearing in different reverse bias ranges. Between -4 and -9 V there is early breakdown (type 1), which is due to Al contamination of the surface. Between -9 and -13 V defect-induced breakdown (type 2) dominates, which is due to metal-containing precipitates lying within recombination-active grain boundaries. Beyond -13 V we may find in addition avalanche breakdown (type 3) at etch pits, which is characterized by a steep slope of the I-V characteristic, avalanche carrier multiplication by impact ionization, and a negative temperature coefficient of the reverse current. If instead of acid-etching alkaline-etching is used, all these breakdown classes also appear, but their onset voltage is enlarged by several volts. Also for cells made from upgraded metallurgical grade material these classes can be distinguished. However, due to the higher net doping concentration of this material, their onset voltage is considerably reduced here. View full abstract»

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  • Enhancement of airborne shock wave by laser-induced breakdown of liquid column in laser shock cleaning

    Page(s): 073101 - 073101-6
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    In laser shock cleaning (LSC), the shock wave is generated by laser-induced breakdown of the ambient gas. The shock wave intensity has thus been a factor limiting the performance of the LSC process. In this work, a novel method of amplifying a laser-induced plasma–generated shock wave by the breakdown of a liquid column is proposed and analyzed. When the laser beam is focused on a microscale liquid column, a shock wave having a significantly amplified intensity compared to that generated by air breakdown alone can be generated in air. Therefore, substantially amplified cleaning force can be obtained. The dynamics of a shock wave induced by a Q-switched Nd:YAG laser was analyzed by laser flash shadowgraphy. The peak pressure of the laser-induced shock wave was approximately two times greater than that of air breakdown at the same laser fluence. The proposed method of shock wave generation is expected to be useful in various applications of laser shock processing, including surface cleaning. View full abstract»

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  • Ultracompact surface-plasmon-polariton splitter based on modulations of quasicylindrical waves to the total field

    Page(s): 073102 - 073102-4
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    By coating a metal with a finite-thickness dielectric film, evident differences in the wave-vector magnitude between surface plasmon polaritons (SPPs) and quasicylindrical waves (quasi-CWs) emerge. This brings modulation patterns to the total field on the metal surface near the electromagnetic source. Based on such an effect, an ultracompact SPP splitter with a lateral dimension of only 800 nm is experimentally demonstrated at wavelengths of 740 nm and 832 nm in a dielectric-film-coated asymmetric single nanoslit. These results imply that the additional modulation of quasi-CWs to the total field provide new possibilities for the design of ultracompact plasmonic devices. View full abstract»

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  • Shock induced damage and damage threshold of optical K9 glass investigated by laser-driven shock wave

    Page(s): 073103 - 073103-6
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    The shock wave driven by short laser pulse is used to study the damage of brittle material K9 glass. The damage morphology of K9 glass surface indicates that the material has experienced different loading modes, respectively, at the central area and the surrounding area of the shock wave. At the central area of shock wave, the wavefront is plane and has a uniform pressure distribution, the material mainly suffers a longitudinal shock pressure; but on the edge the shock wave, the wavefront is approximately spherical, besides longitudinal pressure, transverse tensile stress will emerge inside the material. In the latter case, the damage threshold of the material is much smaller than that in the case of compressing by longitudinal pressure only. According to the relationship between damage area and shock pressure, an experimental method is proposed to measure the damage threshold of materials under shock loading. The damage threshold of K9 glass under spherical shock wave is measured to be about 1.12 GPa; and the damage threshold under plane shock wave is estimated to be between 1.82 and 1.98 GPa. They are much bigger than the damage threshold under static pressure. This method could also be used to measure the damage threshold of other materials when loaded by dynamic pressure. View full abstract»

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  • The equivalent structure and some optical properties of the periodic-defect photonic crystal

    Page(s): 073104 - 073104-9
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    The concept of the effective medium is used to deal with problems of defects in photonic crystals (PhCs) throughout this paper. First, scattering phenomena in PhCs with defects are investigated and can be very well replaced by those in an effective medium with effective defects in interesting frequency region. It is based on the fact that the Bloch wave in the defect-free PhC very approximates to the wave in the effective medium. Then a periodic-defect PhC created by adding periodic defects into the defect-free PhC is proposed. It can be replaced with an effective PhC by utilizing the concept of effective periodic defects embedded into an effective medium. We verify the equivalence between the periodic-defect PhC and the effective PhC by comparing their photonic band structure (PBS) and photonic band gaps (PBGs) as well as transmissions. Next, the effective PhCs instead of the periodic-defect PhCs are used to investigate the variance of the PBGs by applying different periodic defects. The substitution allows us successfully predicting the negative refraction in the periodic-defect PhC through the PBS of the effective PhC. Finally, the field distributions and Fourier coefficients of waveguide modes in the effective PhC and the periodic-defect PhC are equivalent by the supercell method confirmations. View full abstract»

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  • The effect of dielectric spacer thickness on surface plasmon enhanced solar cells for front and rear side depositions

    Page(s): 073105 - 073105-8
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    The excitation of surface plasmons on metallic nanoparticles has the potential to significantly improve the performance of solar cells, in particular thin-film structures. In this article, we investigate the effect of the dielectric spacer layer thickness on the photocurrent enhancement of 2 μm thick, thin-film poly-Si on glass solar cells, due to random arrays of self-assembled Ag nanoparticles deposited on the front or the rear of the cells. We report a strong asymmetry in the external quantum efficiency (EQE) of the cell for front and rear located particles for different spacer thicknesses, which is attributed to differences in the scattering behavior of the nanoparticles. We find that for random arrays, with spectrally broad scattering resonances, the strength of the driving field and the coupling efficiency are more important for light trapping than the resonance wavelength. For particles located on the front of the cells it is desirable to have a thin dielectric spacer layer to enhance the scattering from the Ag nanoparticles. Additionally, light trapping provided by the random sized particles on the front can overcome suppression of light transmitted in the visible wavelength regions for thin layers of Si, to result in overall EQE enhancements. However, for particles deposited on the rear it is more beneficial to have the particles as close to the Si substrate as possible to increase both the scattering and the coupling efficiency. View full abstract»

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  • Influence of indium composition in the prestrained InGaN interlayer on the strain relaxation of InGaN/GaN multiple quantum wells in laser diode structures

    Page(s): 073106 - 073106-5
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    Prestrained InGaN layers with different indium composition were grown by metalorganic chemical vapor deposition as an interlayer before the growth of InGaN/GaN multiple quantum wells (MQWs) in laser diode structures. The strain relaxation of the MQWs with different indium composition prestrained InGaN interlayer was investigated by the grazing incidence x-ray diffraction method. Comparing to the low indium composition (about 3%) case, the strain relaxation occurred in the sample with high indium composition (about 10%) prestrained interlayer. The piezoelectric field in the MQW is also proved to be much smaller after inserting a high indium composition prestrained InGaN interlayer by a measurement of photoluminescence (PL) peak shift as a function of reverse bias voltage, indicating a reduction of the quantum-confined Stark effect (QCSE). Room temperature PL and temperature-dependent PL measurements showed that the PL intensity at 300 K and the internal quantum efficiency of the MQW sample with high indium composition prestrained interlayer were increased by 123% and 177%, respectively, compared with the low indium composition case. Therefore, the prestrained InGaN interlayer containing high indium composition is beneficial to the strain relaxation in the MQW layer and the enhancement of light emission efficiency due to the reduction of QCSE. View full abstract»

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  • Influence of viscoelastic property on laser-generated surface acoustic waves in coating–substrate systems

    Page(s): 073107 - 073107-7
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    Taking account of the viscoelasticity of materials, the pulsed laser generation of surface acoustic waves in coating–substrate systems has been investigated quantitatively by using the finite element method. The displacement spectra of the surface acoustic waves have been calculated in frequency domain for different coating–substrate systems, in which the viscoelastic properties of the coatings and substrates are considered separately. Meanwhile, the temporal displacement waveforms have been obtained by applying inverse fast Fourier transforms. The numerical results of the normal surface displacements are presented for different configurations: a single plate, a slow coating on a fast substrate, and a fast coating on a slow substrate. The influences of the viscoelastic properties of the coating and the substrate on the attenuation of the surface acoustic waves have been studied. In addition, the influence of the coating thickness on the attenuation of the surface acoustic waves has been also investigated in detail. View full abstract»

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  • Blackbody-like emission of terahertz radiation from AlGaN/GaN heterostructure under electron heating in lateral electric field

    Page(s): 073108 - 073108-6
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    The authors report on the observation and study of terahertz radiation emission from modulation-doped AlGaN/GaN heterostructure under conditions of heating of a two-dimensional electron gas in the lateral electric field. The experimental results are compared with the theoretical model of blackbody-like emission from hot two-dimensional electrons. Complementary transport measurements and a theoretical simulation were carried out to determine the dependence of effective electron temperature on electric field. The role of nonequilibrium optical phonon accumulation is discussed. View full abstract»

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  • Femtosecond inverse Faraday effect in magnetic ionic liquid [bmim]FeCl4

    Page(s): 073109 - 073109-4
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    The nonthermally photoinduced magnetization in a magnetic ionic liquid 1-butyl-3-methylimidazolium tetrachloroferrate ([bmim]FeCl4) is investigated via the inverse Faraday effect on the subpicosecond time scale at room temperature. The pump beam induced the ellipticity changes of probe beam are revealed to arise from the contributions of both the inverse Faraday effect and the optical Kerr effect. The formation of about 1.8 THz coherent superposition between magnetic sublevels of the Fe3+-ion’s ground-state multiplet is observed with a circular pump polarization, which is assigned to the allowed impulsively stimulated Raman scattering in the magnetic ion liquid. View full abstract»

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  • Near field and cavity effects on coupling efficiency of one-dimensional metal grating for terahertz quantum well photodetectors

    Page(s): 073110 - 073110-5
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    The Modal Method is employed to simulate the coupling between the terahertz wave and one-dimensional (1D) transmissive metal grating on the top of terahertz quantum well photodetectors (THzQWPs). Electrical field patterns and behaviors of 1D grating at different frequencies and device thicknesses are systematically studied. The results show that, the coupling efficiency is not only determined by the grating parameters, multiple reflections in the device and the subsequent multiple diffractions at the grating also play an important role. Different diffracted modes interact with each other, and near field effect caused by the evanescent waves are essential in evaluating the coupling efficiency especially when the frequencies are below the cutoff of the gratings. The optimization conditions of the performance of 1D metal grating coupled THzQWPs are also discussed. View full abstract»

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  • Thermo-optic microring resonator switching elements made of dielectric-loaded plasmonic waveguides

    Page(s): 073111 - 073111-9
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    Thermo-optic switching elements made of dielectric-loaded plasmonic (DLSPP) waveguides are theoretically investigated by utilizing the three-dimensional vector finite element method. The configurations considered employ microring resonators, whose resonant frequency is varied by means of thermal tuning. First, a classic add-drop filter with parallel access waveguides is examined. Such a component features very poor drop port extinction ratio (ER). We therefore extend the analysis to add-drop filters with perpendicular access waveguides, which are found to exhibit superior drop port ERs, due to interference effects associated with the drop port transmission. In the process, the performance of a DLSPP waveguide crossing is also assessed, since it is a building block of those filters whose bus waveguides intersect. An elliptic tapering scheme is proposed for minimizing cross talk and its effect on the filter performance is explored. The dual-resonator add-drop filter with perpendicular bus waveguides and an untreated waveguide crossing of Sec. V can act as an efficient 2×2 switching element (the single-resonator variant can only act as a 1×2 switch due to structure asymmetry), possessing two equivalent input ports and featuring high ERs for both output ports over a broad wavelength range. Specifically, an extinction ratio of at least 8 dB can be attained for both output ports over a wavelength range of 3.2 nm, accommodating four 100-GHz-spaced channels. Switching times are in the order of a few microseconds, rendering the aforementioned structure capable of handling real-world routing scenarios. View full abstract»

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  • Nonlinear transport in quantum-cascade lasers: The role of electric-field domain formation for the laser characteristics

    Page(s): 073112 - 073112-6
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    We report on terahertz quantum-cascade lasers exhibiting discontinuities in the continuous-wave (cw) current-voltage and output power characteristics, which are related to the presence of electric-field domains (EFDs). The investigated lasers are based on a GaAs/Al0.25 Ga0.75As heterostructure emitting between 4.2 and 4.4 THz and operate in cw mode up to 50 K. The observed effects related to EFDs are similar to those occurring in weakly coupled superlattices and can be described by the same equations, using an effective drift velocity-field relation. A qualitative agreement between experiments and simulations has been found. View full abstract»

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  • Enhancement of (nearly) homogeneous fields in a (effective) zero-index cavity

    Page(s): 073113 - 073113-4
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    Enhanced homogeneous fields are realized in a zero-index medium embedded in Bragg reflectors. When the zero-index medium has Kerr-type nonlinearity, the threshold for optical bistability can be reduced noticeably due to the enhanced uniform fields. Based on a transmission line, we fabricate an effective zero-index medium sandwiched in electric and magnetic walls and experimentally demonstrate the enhanced nearly uniform fields. View full abstract»

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  • Direct measurement of internal quantum efficiency in light emitting diodes under electrical injection

    Page(s): 073114 - 073114-14
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    A method is presented for the direct measurement of the internal quantum efficiency in light emitting diodes (LEDs), based on the ratio of the measured external quantum efficiency and the calculated light extraction efficiency. The external quantum efficiency is measured from a single facet of the device in a simple, well-defined geometry, for which the light extraction efficiency can be calculated with good accuracy. In the proposed method, all LED facets are coated with highly absorbing material which suppresses any light that is not directly emitted into a small aperture on the top facet of the LED. We present a full wave optical model for a multilayer LED structure, from which we derive and validate an approximate model to easily calculate the extraction efficiency through the top facet of the LED. Because a current spreading electrode, often metallic, is required for uniform injection, we show that its impact on the extraction efficiency can be simply modeled through a separate transmission function calculated from the complex index of refraction of the electrode material. The various assumptions made to justify the direct emission model through a single facet (absence of photon backscatter, no photon recycling, simplified device layer model) are discussed and evaluated. The model is applied to a specific GaN LED structure. View full abstract»

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  • Angular and energy distribution of Sn ion debris ejected from a laser-produced plasma source, for laser power densities in the range suitable for extreme ultraviolet lithography

    Page(s): 073301 - 073301-8
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    In this paper, experimental results are presented for the spatial and energy distributions of charge-discriminated Sn ions ejected from laser-produced plasmas. The plasmas were formed on solid, planar Sn targets, irradiated with a Nd:YAG laser. Ions were investigated using a calibrated electrostatic sector analyzer, scanning an energy-to-charge ratio range of 0.22 to 2.2 keV/e for emission angles between 20 and 80 degrees relative to target normal. Results were obtained for three laser power densities, in the region suitable for inducing significant extreme ultraviolet emission, of the order 1.5–8.1 × 1011 W/cm2. The fully differentiated data were found to be well characterized by Gaussian fits, which allowed trends in the emission profiles to be readily quantified. Ions of set energy and charge were observed to possess a preferential angle of emission, the superposition of which yields a physical basis for the total angular emission observed previously and in this work. The experimental results obtained have been related to physical processes within the plasma that influence the energy and angle of ejection of ions from laser produced plasmas. View full abstract»

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  • Ion distribution measurements to probe target and plasma processes in electronegative magnetron discharges. I. Negative ions

    Page(s): 073302 - 073302-10
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    Mass and energy spectra of negative ions in magnetron sputtering discharges have been investigated with an energy-dispersive mass spectrometer. The dc magnetrons have been operated in the same reactive Ar/O2 atmosphere but with three different target materials: Cu, In, and W. Besides negative ions of the working gas, a variety of target metal containing negative molecular ions were found in the discharge. Their occurrence is strongly dependent on the target material. It has been correlated to the electron affinity and the bond strength of the molecules which has been calculated by density functional theory. Energy spectra of the negative ions exhibit three contributions that are clearly distinguishable. Their different origin is discussed as electron attachment in the gas phase and at the target surface, and molecule fragmentation during transport from target to substrate. The latter two contributions again significantly deviate for different target material. The high-energy part of the spectra has been analyzed with respect to the energy the particles gain upon release from the surface. It suggests that bigger molecules formed on the surface are released by ion-assisted desorption. View full abstract»

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  • Ion distribution measurements to probe target and plasma processes in electronegative magnetron discharges. II. Positive ions

    Page(s): 073303 - 073303-10
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    Spectra of the ion mass and energy distributions of positive ions in reactive (Ar/O2) and nonreactive (Ar) dc magnetron sputtering discharges have been investigated by energy-resolved mass spectrometry. The results of three sputter target materials, i.e., Cu, In, and W are compared to each other. Besides the main gas constituents, mass spectra reveal a variety of molecular ions which are dependent on the target material. In reactive mode, ArO+ is always observed in Ar/O2 but molecules containing Ar and the metal were exclusively found for the Cu target. The occurrence of the different ions is explained in the context of their bond strengths obtained from density functional theory calculations. The energy spectra generally contain the known low-energy peak corresponding to the plasma potential. Differently extended high-energy tails due to sputtered material were observed for the different targets. Besides these, high-energetic ions were detected with up to several 100 eV. Their energies are significantly different for Ar+ and O+ with Ar+ strongly depending on the target material. The spectra are discussed together with results from transport of ions in matter (TRIM) calculation to elucidate the origin of these energetic ions. View full abstract»

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  • Electron density dependence of intensity ratio for FeXXII extreme ultraviolet emission lines arising from different ground levels in electron beam ion trap and large helical device

    Page(s): 073304 - 073304-9
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    Extreme ultraviolet (EUV) spectra of highly charged iron ions in a wavelength range of 100–300 Å have been observed from two different plasma sources of the Tokyo Electron Beam Ion Trap (Tokyo-EBIT) with a monoenergetic electron beam and a Large Helical Device (LHD) with Maxwellian electron energy. The excitation process of the spectral lines is compared between the two plasmas, and it is found that the excitation process for Fe XIX - Fe XXII ions is clearly different. Namely, the EUV emission lines from the EBIT plasma are only dominated by electron impact excitation connected to the ground state, but the excitation mechanism is not so simple in the LHD plasma. The difference in the excitation process is studied by measuring the intensity ratio of EUV emission lines (114.412 Å [1s22s2p2 2P3/2 → 1s22s22p 2P3/2]/117.144 Å [1s22s2p2 2P1/2 → 1s22s22p 2P1/2]) arising from different ground levels in the Fe XXII ions. The line intensity ratio has an extremely small value of 0.2 in the EBIT plasma with a low beam current of 30 mA and a beam energy of 2 keV, while the ratio varies with the electron density ne in the LHD plasmas, i.e., 0.35 for ne = 1 × 1013 cm-3 and 0.65 for ne = 4 × 1013 cm-3. Here, the electron density of the EBIT plasma is estimated to be smaller than 1012 cm-3 and the electron temperature of the LHD plasmas is 2 keV. The dependence of the line intensity ratio on the observed electron density is analyzed for both the EBIT and the LHD plasmas using several collisional-radiative (CR) models. The present experimental data can easily be reproduced by the analysis when the thermal pr- ton impact excitation is taken into account. The importance of the proton impact excitation is also experimentally verified by injecting an iron pellet into the LHD plasmas and changing the ratio of the proton density to the electron density. View full abstract»

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  • Determination of sputtered atom densities and velocities via simulation

    Page(s): 073305 - 073305-7
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    Transport of sputtered atoms in a plasma sputtering unit is simulated using two known elastic collision models. The methods of studying the collision of two atoms are isotropic random scattering angle in the center of mass frame and the so-called center to center collision model in which the impact parameter is ignored and the two particles collide such as two material points. The obtained results for the density and velocity of sputtered atoms in both models are compared with some experimental reports. This investigation shows that the copper and argon atoms present pointlike behavior in the collision process. View full abstract»

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  • Field dependence on the molecular ionization potential and excitation energies compared to conductivity models for insulation materials at high electric fields

    Page(s): 073306 - 073306-14
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    The aim is to improve the understanding of high-field phenomena (such as preinception currents/conduction, streamer initiation and propagation) in insulating materials in terms of the molecular properties of the substances involved. In high electric fields, ionization is a likely process, and in all such processes, the ionization potential is an important parameter. A fundamental question is how these processes depend on the electric field, and therefore, based on the interaction between a negative point charge and a molecular cation as modeled by density functional theory, a field-dependent model for the ionization potential is developed. In addition, the first excitation energies as a function of the electric field are calculated using time-dependent density functional theory. It is demonstrated that empirical high-field conduction models for cyclohexane and n-tridecane can be explained in terms of the difference between the ionization potential and the first excitation energy. It is also suggested that the reduction of the ionization potential with electric fields, can help explain how fast-mode streamers propagate. View full abstract»

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  • Numerical investigation of the ion temperature effects on magnetized DC plasma sheath

    Page(s): 073307 - 073307-8
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    The properties of a magnetized plasma sheath with finite ion temperature is considered. The effects of the external magnetic field and the ion temperature on the sheath parameters are examined. It is found that by increasing the ion temperature and the magnetic field strength there is an increase in the positive space charge and the ion energy and a decrease in the sheath thickness. Furthermore, the ion temperature has a direct effect on the ion flux toward the wall. View full abstract»

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  • Production of atmospheric pressure diffuse nanosecond pulsed dielectric barrier discharge using the array needles-plate electrode in air

    Page(s): 073308 - 073308-5
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    In this paper, a bidirectional high pulse voltage with 20 ns rising time is employed to generate an atmospheric pressure diffuse dielectric barrier discharge using the array needles-plate electrode configuration. Both double needle and multiple needle electrode configurations nanosecond pulsed dielectric barrier discharges are investigated. It is found that a diffuse discharge plasma with low gas temperature can be obtained, and the plasma volume increases with the increase of the pulse peak voltage, but remains almost constant with the increase of the pulse repetition rate. In addition to showing the potential application on a topographically nonuniform surface treatment of the discharge, the multiple needle-plate electrode configuration with different needle-plate electrode gaps are also employed to generate diffuse discharge plasma. View full abstract»

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  • Energetic deposition of carbon in a cathodic vacuum arc with a biased mesh

    Page(s): 073309 - 073309-6
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    Carbon films were deposited in a filtered cathodic vacuum arc with a bias potential applied to a conducting mesh mounted in the plasma stream between the source and the substrate. We determined the stress and microstructural properties of the resulting carbon films and compared the results with those obtained using direct substrate bias with no mesh. Since the relationship between deposition energy and the stress, sp2 fraction and density of carbon are well known, measuring these film properties enabled us to investigate the effect of the mesh on the energy and composition of the depositing flux. When a mesh was used, the film stress showed a monotonic decrease for negative mesh bias voltages greater than 400V, even though the floating potential of the substrate did not vary. We explain this result by the neutralization of some ions when they are near to or passing through the negatively biased mesh. The microstructure of the films showed a change from amorphous to glassy carbonlike with increasing bias. Potential applications for this method include the deposition of carbon films with controlled stress on low conductivity substrates to form rectifying or ohmic contacts. View full abstract»

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  • Physics and modeling of an end-Hall (gridless) ion source

    Page(s): 073310 - 073310-11
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    In an end-Hall source, an ion beam is extracted from a magnetized plasma and accelerated by the plasma electric field without grids. The principle of end-Hall sources is similar to that of Hall effect thrusters (or closed-drift thrusters), but their design is optimized for processing applications (ion beam assisted deposition or substrate cleaning) rather than propulsion. The beam divergence is larger in end-Hall ion sources, and these sources can operate at low ion energies. Although end-Hall sources are commonly used in the surface processing industry, no detailed modeling of these sources is available, and their operation is quite empirical. In this paper, a self-consistent, two-dimensional, quasineutral model of an end-Hall ion source is developed and used in order to improve the understanding of the basic physics of these plasma sources and to quantify the parameters controlling the properties of the extracted ion beam. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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P. James Viccaro
Argonne National Laboratory