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15 Oct 2008

Volume 104, Issue 8, Articles (08xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 104, 081301 (2008); http://dx.doi.org/10.1063/1.2977587 (42 pages)

W. F. van Dorp and C. W. Hagen
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A critical literature review of focused electron beam induced deposition

W. F. van Dorp and C. W. Hagen

J. Appl. Phys. 104, 081301 (2008); http://dx.doi.org/10.1063/1.2977587 (42 pages) | Cited 81 times

Online Publication Date: 17 October 2008

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An extensive review is given of the results from literature on electron beam induced deposition. Electron beam induced deposition is a complex process, where many and often mutually dependent factors are involved. The process has been studied by many over many years in many different experimental setups, so it is not surprising that there is a great variety of experimental results. To come to a better understanding of the process, it is important to see to which extent the experimental results are consistent with each other and with the existing model. All results from literature were categorized by sorting the data according to the specific parameter that was varied (current density, acceleration voltage, scan patterns, etc.). Each of these parameters can have an effect on the final deposit properties, such as the physical dimensions, the composition, the morphology, or the conductivity. For each parameter-property combination, the available data are discussed and (as far as possible) interpreted. By combining models for electron scattering in a solid, two different growth regimes, and electron beam induced heating, the majority of the experimental results were explained qualitatively. This indicates that the physical processes are well understood, although quantitatively speaking the models can still be improved. The review makes clear that several major issues remain. One issue encountered when interpreting results from literature is the lack of data. Often, important parameters (such as the local precursor pressure) are not reported, which can complicate interpretation of the results. Another issue is the fact that the cross section for electron induced dissociation is unknown. In a number of cases, a correlation between the vertical growth rate and the secondary electron yield was found, which suggests that the secondary electrons dominate the dissociation rather than the primary electrons. Conclusive evidence for this hypothesis has not been found. Finally, there is a limited understanding of the mechanism of electron induced precursor dissociation. In many cases, the deposit composition is not directly dependent on the stoichiometric composition of the precursor and the electron induced decomposition paths can be very different from those expected from calculations or thermal decomposition. The dissociation mechanism is one of the key factors determining the purity of the deposits and a better understanding of this process will help develop electron beam induced deposition into a viable nanofabrication technique.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
41.75.Fr Electron and positron beams
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Refractive-index nonlinearities of intersubband transitions in GaN/AlN quantum-well waveguides

Yan Li, Anirban Bhattacharyya, Christos Thomidis, Yitao Liao, Theodore D. Moustakas, and Roberto Paiella

J. Appl. Phys. 104, 083101 (2008); http://dx.doi.org/10.1063/1.2996107 (6 pages) | Cited 5 times

Online Publication Date: 16 October 2008

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The refractive-index nonlinearities of intersubband transitions in GaN/AlN quantum-well waveguides are investigated. A large spectral broadening of TM-polarized near-infrared pulses is observed after propagation through these devices due to intersubband self-phase modulation. From the measured data, a nonlinear refractive index n2 of 1.8×10−12 cm2/W is estimated at the operating wavelength of 1550 nm. A detailed theoretical model of the intersubband refractive index as a function of wavelength and optical intensity is then presented. This model assumes an inhomogeneously broadened transition line and is based on experimentally determined material and device parameters. The results of this study are finally used to discuss the prospects of nitride quantum wells for all-optical switching via cross-phase modulation.
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42.79.Gn Optical waveguides and couplers
42.65.Wi Nonlinear waveguides

Multicenter structure of the optical spectra and the charge-compensation mechanisms in Nd:SrWO4 laser crystals

A. Lupei, V. Lupei, C. Gheorghe, L. Gheorghe, and A. Achim

J. Appl. Phys. 104, 083102 (2008); http://dx.doi.org/10.1063/1.2998909 (7 pages) | Cited 2 times

Online Publication Date: 20 October 2008

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An analysis of the high resolution optical spectra (10 and 300 K) of the self-stimulated Raman scattering active Nd:SrWO4 crystal was performed. Crystals doped only with low concentrations of Nd or codoped with Na+ or Nb5+ grown in N2 or air atmosphere were investigated. The single Nd-doped crystals show five main centers, whereas codoping with Na+ in the ratio Na/Nd = 3/1 (in melt) simplifies considerably the spectra. Codoping with Nb5+ does not lead to a dominant center and introduces a new one. Growth in air revealed the role of the oxygen in the charge compensation and multicenter structure of the spectra. The spectral characteristics of the nonequivalent centers (levels, intensities, and polarization) function on concentration of Nd3+ and of codoping ions and growth conditions were analyzed. The polarization data show that only one center has ideal local symmetry S4. Based on the crystal structure, previous results, and the new spectral data, an attempt to connect various spectral nonequivalent centers to the charge compensators (Sr2+ vacancies, interstitial oxygen, Na+, and Nb5+) is made and structural models for various centers are proposed. The implications of these findings on the laser emission characteristics and on the selection of the pumping transitions are also discussed.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.60.By Design of specific laser systems
61.66.Fn Inorganic compounds
61.72.jj Interstitials
61.72.sd Impurity concentration
42.70.Hj Laser materials

Bragg scattering of x-rays in multiwalled carbon nanotubes

S. Y. Ong, P. A. Childs, D. C. Herbert, and A. G. O’Neill

J. Appl. Phys. 104, 083103 (2008); http://dx.doi.org/10.1063/1.2993754 (6 pages)

Online Publication Date: 21 October 2008

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In the past few years it has been shown theoretically that carbon nanotubes coated with various materials have the potential to act as waveguides at x-ray frequencies. At these frequencies the angle of incidence relative to the axis of the nanotube is a few milliradians, creating significant challenges for the experimental confirmation of mode formation. Recent developments in the growth of multiwalled carbon nanotubes with WS2 walls suggest that they have the potential to act as Bragg fibers at x-ray frequencies. In this paper we use the scattering matrix method to study mode formation in multiwalled carbon nanotubes coated with gold. It is found that they are capable of acting as Bragg fibers but the wall thickness and the number of bilayers must be increased in order to obtain mode confinement.
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42.81.-i Fiber optics
42.79.Dj Gratings

100 W-class solar pumped laser for sustainable magnesium-hydrogen energy cycle

T. Yabe, B. Bagheri, T. Ohkubo, S. Uchida, K. Yoshida, T. Funatsu, T. Oishi, K. Daito, M. Ishioka, N. Yasunaga, Y. Sato, C. Baasandash, Y. Okamoto, and K. Yanagitani

J. Appl. Phys. 104, 083104 (2008); http://dx.doi.org/10.1063/1.2998981 (8 pages) | Cited 4 times

Online Publication Date: 21 October 2008

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A solar pumped laser system with 7%–9% slope efficiencies has been developed. A Fresnel lens (2×2 m, f = 2000 mm) is mounted on a two-axis sun tracker platform and focuses solar radiation toward laser cavity, which embraces Cr:Nd:yttrium aluminum garnet ceramic rod. The maximum emitted laser power is 80 W corresponding to maximum total area performance of 20 W/m2 for the Fresnel lens area. This solar laser system would be used as a section of power plant in a magnesium energy cycle as a cost-efficient solar energy converter. Using direct solar radiation into laser, 4.3% net conversion efficiency has been achieved.
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42.79.Ek Solar collectors and concentrators
42.62.-b Laser applications
42.79.Bh Lenses, prisms and mirrors
42.55.Rz Doped-insulator lasers and other solid state lasers
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
88.40.-j Solar energy

Kerr and free carrier ultrafast all-optical switching of GaAs/AlAs nanostructures near the three photon edge of GaAs

Alex Hartsuiker, Philip J. Harding, Yoanna-Reine Nowicki-Bringuier, Jean-Michel Gérard, and Willem L. Vos

J. Appl. Phys. 104, 083105 (2008); http://dx.doi.org/10.1063/1.3000098 (7 pages) | Cited 7 times

Online Publication Date: 21 October 2008

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We performed nondegenerate pump-probe experiments on a GaAs/AlAs photonic cavity structure. We switched the photonic properties using the optical Kerr effect and free carriers excited by three photon absorption. The structure was probed at 1150–1640 nm, in the telecom spectral range below the stop gap. In the measurements we observe surprisingly large nondegenerate electronic Kerr coefficients over a broad wavelength range. We also extracted the three photon absorption coefficient for GaAs at three wavelengths in the near infrared. We conclude that the electronic Kerr effect is so large that the resonance of a moderate Q (Q>1000) photonic cavity could be successfully switched instantaneous.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.47.J- Ultrafast spectroscopy (<1 psec)
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.70.Qs Photonic bandgap materials
78.30.Fs III-V and II-VI semiconductors

Suppressed bimodal size distribution of InAs quantum dots grown with an As2 source using molecular beam epitaxy

Takeyoshi Sugaya, Takeru Amano, and Kazuhiro Komori

J. Appl. Phys. 104, 083106 (2008); http://dx.doi.org/10.1063/1.3000456 (5 pages) | Cited 8 times

Online Publication Date: 21 October 2008

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We demonstrate the suppressed bimodal size distribution of InAs quantum dots (QDs) grown with an As2 source using molecular beam epitaxy. QDs grown with an As2 source have single mode photoluminescence (PL) spectra, whereas QDs grown with an As4 source have bimodal spectra. The PL intensities of QDs grown with an As2 source are higher than those grown with an As4 source. The density of QDs grown with an As2 source decreases at high As pressure, whereas the density of As4 grown QDs increases with As pressure. These results indicate that the surface migration of In atoms is enhanced at a higher As2 pressure, resulting in improved optical properties. The enhanced surface migration is thought to be due to the stable As–As dimer structures under an As2 source.
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68.65.Hb Quantum dots (patterned in quantum wells)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.67.Hc Quantum dots
78.55.Cr III-V semiconductors

Plasmon resonances of aluminum nanoparticles and nanorods

Y. Ekinci, H. H. Solak, and J. F. Löffler

J. Appl. Phys. 104, 083107 (2008); http://dx.doi.org/10.1063/1.2999370 (6 pages) | Cited 13 times

Online Publication Date: 23 October 2008

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We report experimental and theoretical analysis of the plasmonic resonances of Al nanoparticles and nanorods. Ordered nanoparticle arrays with well-defined shapes and narrow size distributions are fabricated on quartz substrates over large areas using extreme ultraviolet interference lithography. The structures, which have sizes down to 40 nm, exhibit strong and sharp particle plasmon resonances in the near and deep-UV ranges. A comprehensive theoretical analysis carried out using dipolar approximation and finite-difference time-domain methods shows good overall agreement with measurements while revealing the dependence of the optical response of Al structures on the fabrication conditions. The results demonstrate the suitability of using Al as a plasmonic material in the UV range and the feasibility of extending applications of plasmonics, such as surface-enhanced Raman spectroscopy, down to the deep-UV range.
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73.22.Lp Collective excitations
78.40.Kc Metals, semimetals, and alloys
78.30.Er Solid metals and alloys
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons

Terahertz transmission lines based on surface waves in plasmonic waveguides

Behnood G. Ghamsari and A. Hamed Majedi

J. Appl. Phys. 104, 083108 (2008); http://dx.doi.org/10.1063/1.3000444 (9 pages) | Cited 7 times

Online Publication Date: 23 October 2008

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This paper studies the guided-wave transmission of terahertz electromagnetic waves along the surface waveguides and parallel-plate waveguides (PPWGs). Field analysis is presented to show that low-loss and quasi-TEM propagation is only possible when the waveguides are operating in their plasmonic modes. It is shown that the observed low-loss and quasi-TEM propagation of terahertz waves in the PPWGs is due to resonant coupling of surface plasmons between the two plates. Transmission line models based on the peculiar characteristics of the surface waves associated with the plasmonic modes of the guides are developed and the distinctions with the conventional transmission line theory are highlighted.
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84.40.Az Waveguides, transmission lines, striplines

An accurate determination of the electronic transitions of InAs/InGaAs/InP quantum dots for midinfrared lasers using simultaneous complementary spectroscopic techniques

T. K. Sharma, T. J. C. Hosea, S. J. Sweeney, and X. Tang

J. Appl. Phys. 104, 083109 (2008); http://dx.doi.org/10.1063/1.3005903 (8 pages) | Cited 5 times

Online Publication Date: 31 October 2008

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InAs/InGaAs/InP quantum dots (QDs) emitting at ∼ 2 μm for midinfrared laser applications are studied using the complementary spectroscopic techniques of photoluminescence (PL), photoreflectance (PR), and surface photovoltage spectroscopy (SPS). We use a procedure that ensures that the same sample spot is studied virtually simultaneously by these three different spectroscopic techniques under almost identical conditions. We are able to measure the ground and excited states transitions of the InAs QDs without any ambiguity, thus providing a complete and clear understanding of the electronic transitions. Temperature dependent PL, SPS, and PR measurements provide a systematic thermal evolution of the ground and excited states. However, the QD transitions are not all seen together at any given temperature in either the PL or SPS measurements. By contrast, the PR technique can measure the complete set of ground and two excited state QD transitions and also the InGaAs barrier energy at all studied temperatures.
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78.67.Hc Quantum dots
78.55.Cr III-V semiconductors
73.63.Kv Quantum dots
73.21.La Quantum dots
78.20.-e Optical properties of bulk materials and thin films
72.40.+w Photoconduction and photovoltaic effects
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Theory analysis of wavelength dependence of laser-induced phase explosion of silicon

Quanming Lu, Samuel S. Mao, Xianglei Mao, and Richard E. Russo

J. Appl. Phys. 104, 083301 (2008); http://dx.doi.org/10.1063/1.2978369 (7 pages) | Cited 2 times

Online Publication Date: 21 October 2008

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Wavelength dependence of laser ablation of silicon was investigated with nanosecond ultraviolet, visible, and infrared laser pulses in the irradiance range from 3×1010 to 1×1012 W/cm2. For 266 and 532 nm laser pulses, the depth of laser-produced crater shows a dramatic increase at a laser irradiance threshold of approximately 2×1010 and 4×1011 W/cm2 respectively, above which, large micron-sized particulates were observed to eject from the target about 300–400 ns after the laser pulse. In contrast, for 1064 nm pulse, this dramatic increase was not observed. The underlying mechanism for the observed threshold phenomenon is presented in this study, which can be attributed to the thermal diffusion and subsequent explosive boiling after the completion of the interaction between the nanosecond laser pulse and silicon. Based on our delayed phase explosive model, the ablation depths were calculated for different wavelengths and compared to experimental results. Plasma shielding during laser irradiation was included in the model, which plays a key role to the coupling of laser energy to the irradiated material.
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79.20.Ds Laser-beam impact phenomena
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Fk Semiconductors
66.70.Df Metals, alloys, and semiconductors
52.40.Hf Plasma-material interactions; boundary layer effects

Plasma etching of SiO2 using remote-type pin-to-plate dielectric barrier discharge

Jae Beom Park, Se Jin Kyung, and Geun Young Yeom

J. Appl. Phys. 104, 083302 (2008); http://dx.doi.org/10.1063/1.2999645 (5 pages) | Cited 4 times

Online Publication Date: 24 October 2008

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Atmospheric pressure plasma etching of SiO2 was examined using a modified remote-type dielectric barrier discharge (DBD), called “pin-to-plate DBD.” The effect of adding four gases CF4, C4F8, O2, and Ar to the base gas mixture containing N2 (60 slm) (slm denotes standard liters per minute)/NF3 (600 SCCM) (SCCM denotes cubic centimeter per minute at STP) on the SiO2 etch characteristics was investigated. The results showed that the SiO2 etch rate decreased continuously with increasing C4F8 (200–800 SCCM) addition, whereas the SiO2 etch rate increased with increasing CF4 (1–10 slm) addition up to 7 slm CF4. This increase in the SiO2 etch rate up to 7 slm CF4 was attributed to the effective removal of Si in SiO2 by F atoms through the removal of oxygen in SiO2 by carbon in the CFX in the plasma. However, the decrease in SiO2 etch rate with further increases in CF4 flow rate above 7 slm was attributed to the formation of a thick C–F polymer layer on the SiO2 surface. A SiO2 etch rate of approximately 243 nm/min was obtained with a gas mixture of N2 (60 slm)/NF3 (600 SCCM)/CF4 (7 slm), and an input voltage and operating frequency to the source of 10 kV and 30 kHz, respectively. The addition of 200 SCCM Ar to the above gas mixture increased the SiO2 etch rate to approximately 263 nm/min. This is possibly due to the increased ionization and dissociation of reactive species through penning ionization of Ar.
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81.65.Cf Surface cleaning, etching, patterning
81.20.-n Methods of materials synthesis and materials processing

Electrical characterization of an inductively coupled gaseous electronics conference reference cell

S. V. Singh and C. Pargmann

J. Appl. Phys. 104, 083303 (2008); http://dx.doi.org/10.1063/1.3000667 (11 pages) | Cited 6 times

Online Publication Date: 24 October 2008

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Plasma parameters and particularly the external electrical operational parameters are examined with respect to mode transition and hysteresis. The external electrical parameters such as antenna current (Ic) and antenna voltage (Vc) amplitudes are measured after the matching network and discussed for an inductively coupled argon discharge. A wide range of discharge conditions by varying applied power (up to 150 W at 13.56 MHz), gas pressure (0.7–7.5 Pa), and electrostatic coupling strength are measured for a gaseous electronics conference radio frequency reference cell. The effect on the power coupling efficiency by varying electrostatic coupling strength is studied via implementing two distinct grounded Faraday shields, in addition to the original nonshielded condition. A brief discussion on the evolution of floating potential, plasma potential, electron density, and electron energy distribution function with power and pressure is also presented mainly in context to mode transitions and hysteresis. Relatively smooth transitions in the plasma parameters and in the external electrical parameters are measured close to H to E mode transition region. Contrary to plasma parameters, however, the reverse transition from E to H mode was found to be abrupt in external parameters. The plasma parameters are measured using a commercial Langmuir probe, whereas the antenna currents and voltages are measured using a homebuilt Rogowski coil and capacitor divider, respectively.
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52.25.Fi Transport properties
52.80.Pi High-frequency and RF discharges
52.50.Dg Plasma sources
52.70.Ds Electric and magnetic measurements

A simple model of the positive ion sheath in front of a plane probe in weakly ionized electropositive plasmas

R. Morales Crespo, J. I. Fernández Palop, M. A. Hernández, M. V. Lucena-Polonio, and J. Ballesteros

J. Appl. Phys. 104, 083304 (2008); http://dx.doi.org/10.1063/1.3000669 (4 pages) | Cited 4 times

Online Publication Date: 24 October 2008

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This paper describes the positive ion sheath in front of a plane probe in weakly ionized electropositive plasmas. The results obtained are the potential profile in front of the probe and the probe positive ion current to voltage characteristic curves that have been analytically fitted as a function of the probe potential and the plasma ionization rate. Finally, the floating potential for an argon plasma has been calculated by considering a parametrization of the model obtained from the fitted characteristics.
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52.40.Kh Plasma sheaths
52.70.Ds Electric and magnetic measurements
52.25.Fi Transport properties
52.25.Jm Ionization of plasmas
52.27.Ep Electron-positron plasmas
52.27.Aj Single-component, electron-positive-ion plasmas

Ion composition produced by high power impulse magnetron sputtering discharges near the substrate

A. P. Ehiasarian, A. Vetushka, A. Hecimovic, and S. Konstantinidis

J. Appl. Phys. 104, 083305 (2008); http://dx.doi.org/10.1063/1.3000446 (8 pages) | Cited 18 times

Online Publication Date: 27 October 2008

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Plasma composition near the substrate was investigated in a high power impulse magnetron sputtering (HIPIMS) discharge using Langmuir probe analysis, mass spectroscopy, and atomic absorption spectroscopy. The HIPIMS discharge was operated in nonreactive Ar atmosphere at a pressure of 2.66 Pa and the magnetron cathode was furnished with Ti target. Plasma density, metal ion-to-neutral ratio, and gas ion-to-metal ion ratio were studied as a function of discharge current. At peak discharge current densities of ∼ 1 A cm−2, the results show that a dense plasma (ne ∼ 1018 m−3) expanded from the target toward the substrate and lasted more than 330 μs after the supplied power was turned off. The shape of the time-averaged ion energy distribution function of sputtered material exhibited a transition from Thompson to Maxwellian distribution, indicating efficient energy transfer in the discharge. The metal content in the plasma monotonically increased with discharge current and the metal ion-to-neutral ratio reached approximately 1:1 in the postdischarge plasma at peak current density of 5 A cm−2.
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52.80.-s Electric discharges
52.70.Ds Electric and magnetic measurements
52.25.-b Plasma properties

Electromagnetic field distribution calculation in solenoidal inductively coupled plasma using finite difference method

W. P. Li, Y. Liu, Q. Long, D. H. Chen, and Y. M. Chen

J. Appl. Phys. 104, 083306 (2008); http://dx.doi.org/10.1063/1.3000672 (7 pages) | Cited 2 times

Online Publication Date: 29 October 2008

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The electromagnetic field (both E and B fields) is calculated for a solenoidal inductively coupled plasma (ICP) discharge. The model is based on two-dimensional cylindrical coordinates, and the finite difference method is used for solving Maxwell equations in both the radial and axial directions. Through one-turn coil measurements, assuming that the electrical conductivity has a constant value in each cross section of the discharge tube, the calculated E and B fields rise sharply near the tube wall. The nonuniform radial distributions imply that the skin effect plays a significant role in the energy balance of the stable ICP. Damped distributions in the axial direction show that the magnetic flux gradually dissipates into the surrounding space. A finite difference calculation allows prediction of the electrical conductivity and plasma permeability, and the induction coil voltage and plasma current can be calculated, which are verified for correctness.
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52.25.Fi Transport properties
52.80.-s Electric discharges
52.65.-y Plasma simulation

Application of a self-breakdown hydrogen spark gap switch on high power pulse modulator

Jin-liang Liu, Yi Yin, Tian-wen Zhan, Jia-huai Feng, Hui-huang Zhong, and Xin-xin Wang

J. Appl. Phys. 104, 083307 (2008); http://dx.doi.org/10.1063/1.3005902 (5 pages)

Online Publication Date: 30 October 2008

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Hydrogen has the best recovery property compared to other gases such as N2 and SF6. Therefore, it has a potential application on the multipulse or high repetitive pulse modulator. This paper introduced a high pressure hydrogen gas switch with two cylindrical structures. The stress of the switch under high pressure was analyzed theoretically, and the hydrogen gas switch was hydrostatically tested with pressure up to 30 atm. Such a switch was employed on a high power pulse modulator using water as the dielectric of pulse forming line. At the switch breakdown voltage of 520 kV and the pressure of hydrogen 12 atm, 230 kV, 31 kA, and 60 ns pulse width electron beams were obtained at the field emission diode. Furthermore, when the switch was filled with different gases such as H2, N2, and SF6, the rise times of diode voltage of modulator had been compared at the gas breakdown voltage 400 kV. The results showed that the rise times of diode voltage were reduced obviously when hydrogen was used as the dielectric of spark gap switch.
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52.80.Mg Arcs; sparks; lightning; atmospheric electricity
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Metal-organic vapor phase epitaxy of crystallographically oriented MnP magnetic nanoclusters embedded in GaP(001)

S. Lambert-Milot, C. Lacroix, D. Ménard, R. A. Masut, P. Desjardins, M. Garcia-Hernandez, and A. de Andres

J. Appl. Phys. 104, 083501 (2008); http://dx.doi.org/10.1063/1.2992558 (7 pages) | Cited 6 times

Online Publication Date: 17 October 2008

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Hybrid ferromagnetic-semiconductor GaP:MnP thin films were grown at 650 °C by metal-organic vapor phase epitaxy on GaP(001) using trimethylgallium, tertiarybutylphosphine, and methyl cyclopentadienyl manganese tricarbonyl (MCTMn). Overall Mn concentrations in the hybrid films, determined by Rutherford backscattering spectrometry, were found to be nearly proportional to the MCTMn precursor gas flow rate and ranged from 2 to 3.5 at. %. Cross-sectional transmission electron microscopy (TEM) analyses revealed the presence of a homogeneous distribution of 15–30 nm wide nanoclusters in a dislocation-free GaP matrix that is fully coherent with the substrate. The nanocluster facets are predominantly aligned along the (220) planes of the GaP matrix and selected-area electron diffraction patterns in TEM indicate that the nanoclusters are coherent (or semicoherent) with the single-crystal GaP matrix. The Mn:P composition ratio in the nanoclusters was determined to be 1.00±0.05 from parallel electron energy loss spectroscopy analyses. Increasing the MCTMn flow rate during film growth resulted in an increased concentration of MnP nanoclusters in the epilayer while their dimensions remained virtually unchanged. Magnetometric characterization indicates a ferromagnetic order, with a Curie temperature of about 294 K, originating from the MnP clusters.
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81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.05.Ea III-V semiconductors
68.55.ag Semiconductors
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
75.70.Ak Magnetic properties of monolayers and thin films
75.50.Tt Fine-particle systems; nanocrystalline materials

Two dimensional mesoscale simulations of projectile instability during penetration in dry sand

S. K. Dwivedi, R. D. Teeter, C. W. Felice, and Y. M. Gupta

J. Appl. Phys. 104, 083502 (2008); http://dx.doi.org/10.1063/1.2999391 (10 pages) | Cited 1 time

Online Publication Date: 20 October 2008

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To gain insight into the instability and trajectory change in projectiles penetrating dry sand at high velocities, two dimensional plane strain mesoscale simulations were carried out using representative models of a particulate system and of a small projectile. A program, ISP-SAND, was developed and used to generate the representative particulate system with mean grain sizes of 60 and 120 μm as well as ±30% uniform size distribution from the mean. Target porosities ranged from 30% to 40%. The penetration of ogive nose steel projectiles with caliber radius head of 3.5 and length-to-diameter (l/d) ratio of 3.85 was simulated using the updated Lagrangian explicit parallel finite element code ISP-TROTP. Deformation of the projectile and individual sand grains was analyzed using a nonlinear elastic-inelastic model for these materials. Grain-grain and grain-projectile interactions were analyzed using a contact algorithm with and without friction. Projectile instability was quantified and compared using the lateral displacement of the center of mass, lateral force acting on the projectile, and its rotational momentum about the center of mass. The main source of projectile instability and the ensuing trajectory change in the penetration simulations was found to be the inhomogeneous loading of the projectile due to the heterogeneities and randomness inherent in a particulate media like sand. The granularity of the media has not been considered explicitly in previous work. Projectile instability increased with impact velocity, as expected. However, it also increased for the case of elastic impactor that preserved the nose shape, with an increase in grain size, and for uniform grain sizes. Moreover, friction, inherently present in geologic materials, was found to be a major contributor to instability. Conclusions derived from one projectile depth simulations were confirmed by two deeper penetration simulations considering up to three full lengths of penetration (requiring a larger sand target). The deep penetration simulation predicted considerable instability with a trajectory change of approximately 45° when friction was considered in the dry sand medium. An overall conclusion of this work is that projectile penetration studies in geologic materials need to explicitly consider the heterogeneous or particulate nature of these materials.
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62.50.Ef Shock wave effects in solids and liquids
62.20.F- Deformation and plasticity
46.15.-x Computational methods in continuum mechanics
46.40.Cd Mechanical wave propagation (including diffraction, scattering, and dispersion)

Interfacial thermal conductance between silicon and a vertical carbon nanotube

Ming Hu, Pawel Keblinski, Jian-Sheng Wang, and Nachiket Raravikar

J. Appl. Phys. 104, 083503 (2008); http://dx.doi.org/10.1063/1.3000441 (4 pages) | Cited 13 times

Online Publication Date: 20 October 2008

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Molecular simulations are used to evaluate thermal resistance between crystalline silicon and a vertically oriented carbon nanotube (CNT). Without chemical bonds between CNT and Si the thermal resistance is high and its values are consistent with that measured in experiment on vertical CNT arrays. With chemical bonds the thermal resistance is reduced by two orders of magnitude demonstrating significant potential of CNT arrays for thermal management applications. The underlying mechanism for the very large effect of chemical bonding is revealed by simulations of individual phonon scattering across the interface and understood within an analytical solution of a simple spring-mass chain model.
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73.40.-c Electronic transport in interface structures
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials

Light emission properties and mechanism of low-temperature prepared amorphous SiNX films. I. Room-temperature band tail states photoluminescence

M. Wang, M. Xie, L. Ferraioli, Z. Yuan, D. Li, D. Yang, and L. Pavesi

J. Appl. Phys. 104, 083504 (2008); http://dx.doi.org/10.1063/1.2996292 (4 pages) | Cited 3 times

Online Publication Date: 21 October 2008

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A room-temperature photoluminescence (PL) study of amorphous nonstoichiometric silicon nitride (SiNX) films prepared under low temperature is reported. PL peak position can be tuned from 1.90 to 2.90 eV by adjusting the film composition. The luminescence lifetime is within the nanosecond range. The dependence of the PL lifetime on the emission energy suggests that band tail states are involved in the thermalization and recombination of photon-generated carriers. This is further supported by the correlation between the optical band gap, the PL peak energy, and the width of the PL spectrum. We propose that optical transitions among band tail states are the main light emission mechanisms.
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78.66.-w Optical properties of specific thin films
78.55.Hx Other solid inorganic materials
71.23.-k Electronic structure of disordered solids
61.43.Er Other amorphous solids
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Light emission properties and mechanism of low-temperature prepared amorphous SiNX films. II. Defect states electroluminescence

M. Wang, J. Huang, Z. Yuan, A. Anopchenko, D. Li, D. Yang, and L. Pavesi

J. Appl. Phys. 104, 083505 (2008); http://dx.doi.org/10.1063/1.2996299 (4 pages) | Cited 6 times

Online Publication Date: 21 October 2008

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In this paper, we present a room-temperature electroluminescence (EL) study of amorphous nonstoichiometric silicon nitride (SiNX) films. The light-emitting device is formed by an ITO/SiNX/p-type silicon structure. EL shows a yellowish broad emission spectrum with a power efficiency of 10−6. The EL peak energy depends on the bias voltage rather than on the silicon content in SiNX. By fitting the current-voltage characteristic with existing models, we found that under high voltages the Poole–Frenkel hole conduction is the main carrier transport mechanism in these devices. Injected electrons are captured by silicon dangling bonds (K center) and recombine with holes, which are localized in valence band tail states. Unbalanced hole and electron injection and nonradiative recombination are the main constraints on the EL efficiency of SiNX.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
71.23.-k Electronic structure of disordered solids
78.66.Nk Insulators
73.61.Ng Insulators

High-pressure x-ray diffraction and Raman spectra study of indium oxide

D. Liu, W. W. Lei, B. Zou, S. D. Yu, J. Hao, K. Wang, B. B. Liu, Q. L. Cui, and G. T. Zou

J. Appl. Phys. 104, 083506 (2008); http://dx.doi.org/10.1063/1.2999369 (5 pages) | Cited 10 times

Online Publication Date: 21 October 2008

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High-pressure synchrotron radiation x-ray diffraction and Raman spectroscopy measurements of indium oxide (In2O3) were carried out at room temperature up to 27.8 and 26.2 GPa, respectively. A pressure-induced phase transition from cubic-phase (Iamath) was observed at a pressure above 12.8–15.3 GPa, which disagrees with earlier theoretical prediction (3.8 GPa). According to the x-ray diffraction experimental data, the high-pressure phase is isostructural with hexagonal corundum-type structure (Rmathc symmetry). However, broad peaks observed in Raman spectra suggest that the high-pressure structure is disordered. The volume change from cubic phase to corundum phase is about 4% and the axial ratio c/a in the corundum phase decreases with increasing pressure.
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78.30.Hv Other nonmetallic inorganics
62.50.-p High-pressure effects in solids and liquids
64.70.kg Semiconductors

High angle annular dark field and electron energy loss spectroscopy study on the substitution and distribution of cobalt in ZnO by multilayer growth

Shu-Fang Chen, Chuan-Pu Liu, H. S. Hsu, and J. C. A. Huang

J. Appl. Phys. 104, 083507 (2008); http://dx.doi.org/10.1063/1.3000450 (4 pages) | Cited 1 time

Online Publication Date: 21 October 2008

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Co-doped ZnO films were synthesized by ion beam sputtering using multilayer (ZnO/Co) growth. Both the distribution and the chemical states of Co in ZnO can be well controlled by varying the ratio of the nominal layer thickness of ZnO to Co. Transmission electron microscopy indicated that all of the as-deposited Zn1−x(Co)xO films were polycrystalline with a (0002) preferred orientation. The local microstructures and chemical states were identified by Z-contrast imaging and electron energy loss spectroscopy. In ZnO (1.5 nm)/Co (0.1 nm), homogeneous Co-doped ZnO was observed to have been formed through interdiffusion. However, decreasing or increasing the thickness of ZnO leads to the formation of Co clusters in the ZnO matrix or Zn1−x(Co)xO multilayers, respectively. For ZnO thickness≧1.5 nm, Co is substituted for Zn, and its valence state is 2+. All Co-doped ZnO films show room-temperature ferromagnetic behavior, which appears to depend strongly on the Co distribution.
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81.15.Cd Deposition by sputtering
68.37.Og High-resolution transmission electron microscopy (HRTEM)
66.30.Ny Chemical interdiffusion; diffusion barriers
68.55.ag Semiconductors
79.20.Uv Electron energy loss spectroscopy
75.50.Pp Magnetic semiconductors

Growth-interruption-induced low-density InAs quantum dots on GaAs

L. H. Li, N. Chauvin, G. Patriarche, B. Alloing, and A. Fiore

J. Appl. Phys. 104, 083508 (2008); http://dx.doi.org/10.1063/1.3000483 (4 pages) | Cited 3 times

Online Publication Date: 21 October 2008

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We investigate the use of growth interruption to obtain low-density InAs quantum dots (QDs) on GaAs. The process was realized by Ostwald-type ripening of a thin InAs layer. It was found that the optical properties of the QDs as a function of growth interruption strongly depend on InAs growth rate. By using this approach, a low density of QDs (4 dots/μm2) with uniform size distribution was achieved. As compared to QDs grown without growth interruption, a larger energy separation between the QD confined levels was observed, suggesting a situation closer to the ideal zero-dimensional system. Combining with an InGaAs capping layer such as In-rich QDs enable 1.3 μm emission at 4 K.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)
78.67.Hc Quantum dots
68.37.Ps Atomic force microscopy (AFM)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
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