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Journal of Applied Physics

Issue 11 • Date Jun 2007

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Displaying Results 1 - 25 of 188
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

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  • Cathodoluminescence spectroscopy of epitaxial-lateral-overgrown nonpolar (11-20) and semipolar (11-22) GaN in relation to microstructural characterization

    Page(s): 113101 - 113101-6
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    Low temperature spatially resolved cathodoluminescence was carried out on GaN films grown by the epitaxial-lateral-overgrowth (ELO) technique with the nonpolar (11-20) and the semipolar (11-22) orientations on R- and M-sapphires, respectively. Defect related optical transitions were identified and their localization was correlated to different regions of ELO. The sample microstructure was further investigated by plan-view and cross-section transmission electron microscopies. It is shown that the defect related emissions are mainly localized in the seed of the samples, but different defects occur as well in the wings, especially in the case of nonpolar GaN. The structural defect densities are lowest in the overgrown wings of semipolar GaN. In particular, the [0001] wing region of semipolar ELO-GaN is almost defect-free with a cathodoluminescence spectrum dominated by the GaN band-edge emission at 3.476 eV. View full abstract»

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  • Recombination dynamics in ultraviolet light-emitting diodes with Si-doped AlxGa1-xN/AlyGa1-yN multiple quantum well active regions

    Page(s): 113102 - 113102-5
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    Ultraviolet (UV) light-emitting diodes with AlxGa1-xN/AlyGa1-yN multiple quantum well active regions, doped in the barriers with different Si doping levels, show a sharp near-band edge emission line (UV luminescence). Some samples have a broad subband gap emission band centered at about 500 nm (green luminescence) in addition to the near-band edge emission. The electroluminescence intensities of the UV and green emission line are studied as a function of the injection current. For the sample grown on the AlN substrate under optimized growth conditions, the UV luminescence intensity increases linearly with the injection current, following a power law with an exponent of 1.0, while the green luminescence intensity increases sublinearly with the injection current. On the contrary, the samples grown on the sapphire substrate show a superlinear (to the power of 2.0) and linear (to the power of 1.0) dependence on the injection current for the UV and green luminescence, respectively. A theoretical model is proposed to explain the relationship between the luminescence intensities and the injection current. The results obtained from the model are in excellent agreement with the experimental results. The model provides a method to evaluate the dominant recombination process by measuring the exponent of the power-law dependence. View full abstract»

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  • Synthesis and spectroscopic properties of neodymium doped lead chloride

    Page(s): 113103 - 113103-7
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    The crystal growth and infrared spectroscopic properties of Nd doped lead chloride (Nd:PbCl2) are reported. Lead halide based materials have recently emerged as laser hosts with low maximum phonon energies. In this work, Nd:PbCl2 crystals were grown by a self-seeded Bridgman technique. Following optical pumping at 750 and 808 nm, Nd:PbCl2 exhibited several infrared (IR) emission lines between 800 and 1600 nm as well as a broad mid-IR band centered at ∼5.19 μm. It was found that for Nd3+ concentrations larger than ∼1×1019 cm-3, the mid-IR emission is predominantly due to the transition 4I11/24I9/2. From a Judd-Ofelt analysis, the radiative quantum efficiency of the 5.19 μm emission was determined to be ∼27%. The multiphonon decay rates of several closely spaced Nd3+ transitions were modeled using the well known energy-gap law and the host dependent parameters B and β were determined to be 9.5×109 s-1 and 1.26×10-2 cm, respectively. The obtained energy-gap law parameters were subsequently used to describe the temperature dependence of the 5.19 μm mid-IR emission lifetime for a ran- ge from 77 to 450 K. View full abstract»

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  • Accurate evaluation of nonlinear absorption coefficients in InAs, InSb, and HgCdTe alloys

    Page(s): 113104 - 113104-12
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    We present a full band structure calculation of temperature- and wavelength-dependent two-photon absorption (TPA) coefficients and free carrier absorption (FCA) cross sections in InAs, InSb, and Hg1-xCdxTe alloys. The wavelength dependence of the TPA coefficients agrees well with a widely used analytical expression. However, the magnitudes of the TPA coefficients obtained here are smaller by a factor of 1.2–2.5 than the analytical values. In addition, the TPA coefficient is found to depend sensitively on the photoexcited carrier density in small gap materials. The FCA is found to arise predominantly from hole absorption. The FCA cross section is found to be independent of the carrier density, but is strongly dependent on the temperature. The calculated TPA, FCA coefficients, and lifetimes are fitted to closed-form expressions and are used in solving the rate equations to obtain the transmitted pump and probe intensities as functions of incident intensity and sample thickness. The calculated pump transmission and time-dependent probe transmission in InAs agree very well with the measured values. View full abstract»

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  • Imaging properties of a metallic photonic crystal

    Page(s): 113105 - 113105-5
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    Imaging effects in metallic photonic crystals (PCs) are examined theoretically based on the finite difference time-domain method. The analysis shows that, in metallic PC-based systems, far-field images do form at the opposite side of the PC “lens” and more importantly, follow the rule of geometric optics with respect to the changes in the source position as a direct proof of negative refraction. However, the comparison of ideal left-handed media with a metallic PC suggests that the focusing effect in the PC based system is different from that of the ideal left-handed media in many aspects, due to the inhomogeneous nature of the PC. Particularly, strong dependence on the individual geometry as well as the frequency in the PC-based system renders the effective index sensitive to the variations and potentially limits its application as a superlens. View full abstract»

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  • Thermal loading of laser induced plasma shockwaves on thin films in nanoparticle removal

    Page(s): 113106 - 113106-9
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    Damage concerns, such as substrate/film material alterations, damage, and delamination of thin films, have become a central problem in sub-100 nm particle removal applications. In the laser induced plasma (LIP) removal technique both LIP shockwave and radiation heating are potential sources of thermomechanical damage. The specific objective of current study is to conduct a computational investigation of the LIP shockwave effect on the thermoelastic response of a thin chromium (Cr) film deposited on a quartz substrate and to identify the conditions leading to the onset of plastic film deformations. The experimentally characterized shockwave pressure and temperature (approximated from gas dynamic relations) were prescribed as boundary conditions in the computational analysis. From the shockwave arrival times for different travel distances, the shockwave radius as well as the velocity were obtained as a function of the shockwave propagation time. Radial (and circumferential) stresses, caused by thermal expansion of the Cr film, were most dominant and, hence, of damage concern. It is determined that the resultant temperature rise utilizing a 1064 nm Nd:yttrium-aluminum-garnet (YAG) laser (450 mJ) due to the film-shockwave interactions was not sufficiently high to initiate film and/or substrate damage. No material alteration/damage of the Cr film is predicted due to the temperature and pressure of LIP shockwaves at the firing distance of 2 mm, with a high strain rate gain factor of two (minimum), though damage was observed experimentally for 1064 nm Nd:YAG laser at the pulse energy of 370 mJ. Reported results indicate that the leading cause of observed thin film damage during nanoparticle removal is almost certainly radiation heating from the LIP core. View full abstract»

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  • Comprehensive analysis and optimal design of top-emitting organic light-emitting devices

    Page(s): 113107 - 113107-6
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    We present an accurate analysis of light emission in top-emitting organic light-emitting devices (TOLEDs) by explicitly considering the Purcell effect. TOLEDs are optimized separately for maximum zero-degree luminance, maximum electroluminescence (EL) efficiency, and wide viewing angle with high EL efficiency. For fluorescent material with an internal quantum efficiency int0) of 0.25, the maximum zero-degree luminance and EL efficiency can be achieved by locating the emitters around the first antinode of the microcavity while for phosphorescent material with ηint0=1.0, the maximum zero-degree luminance and EL efficiency are around the second antinode. Through relaxing the efficiency by 10%–20%, the angular intensity distribution can be even better than the Lambertian distribution; meanwhile, the color shows only a small variation over an angle range of 150°. Our results, which are in good agreement with experiments, show that the Purcell effect on TOLED performances is significant and should be carefully examined in studying TOLEDs. View full abstract»

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  • Topology optimization of waveguide bends with wide, flat bandwidth in air-bridge-type photonic crystal slabs

    Page(s): 113108 - 113108-6
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    We employed the topology optimization (TO) method to improve the transmission bandwidth of waveguide bends in air-bridge-type, two-dimensional photonic crystal slabs. We experimentally confirmed that bend loss at longer wavelengths in the vicinity of the band edge was suppressed by using TO. The optimized bends showed good performance, comparable to that of straight waveguides. View full abstract»

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  • Characterization of mixed Nd:LuxGd1-xVO4 laser crystals

    Page(s): 113109 - 113109-7
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    A series of laser crystals Nd:LuxGd1-xVO4 (x=0.14,0.32,0.50,0.61,0.70,0.80) was grown by the Czochralski method. The thermal properties, including the average linear thermal expansion coefficients, thermal diffusion coefficients, specific heats, and thermal conductivities, of the mixed crystals were obtained. The material constants Ms for the thermal stress resistance figure were calculated and showed that the thermal fracture limits of the mixed crystals should be comparable with that of Nd:YVO4. The polarization absorption spectra from 240 to 1000 nm were measured at room temperature and the absorption cross sections at 809 nm were calculated. Using the Judd-Ofelt theory, the theoretical radiative lifetimes were calculated and compared with the experimental results. Continuous wave laser performances were achieved with the mixed crystals at the wavelength of 1.06 μm when they were pumped by a laser diode. Thermal, optical, and laser properties have shown variation as a function of x and proved that the mixed crystals are good laser materials. View full abstract»

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  • Bichromatic laser emission from dipyrromethene dyes incorporated into solid polymeric media

    Page(s): 113110 - 113110-9
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    Bichromatic laser emission from dipyrromethene-based solid-state dye lasers is reported. The dependence of this dual emission on different factors and its origin and causes are discussed in the light of different models proposed in the literature. Our experimental results indicate that the long-wavelength emission can be explained in terms of reabsorption/reemission effects and inhomogeneous broadening of the S0-S1 transition. The short-wavelength emission corresponds to the usual S0-S1 transition and dominates at low dye concentration. View full abstract»

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  • Composition dependent ultraviolet photoresponse in MgxZn1-xO thin films

    Page(s): 113111 - 113111-6
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    MgxZn1-xO (0.0≤x≤0.12) thin films have been deposited on glass substrates by sol-gel process. The hexagonal wurtzite structure of ZnO is retained in the Mg-substituted films. From the photoresponse measurements, it is observed that both ZnO and substituted films are UV sensitive and with increasing Mg content, the sensitivity shifts toward the shorter wavelength side with a gradual decrease in magnitude. The UV-to-visible current ratio remains almost constant up to x=0.08 and thereafter decreases. The decay time for 90% photocurrent gradually decreases to less than 4 s for x=0.05 followed by an increase for higher values of x. The decay time is faster than the growth time for all the substituted films. The change in the photoconductivity with Mg content (x) is correlated to the microstructural change. A trap level, found to be involved in the photoconductivity, is located at ∼0.8 eV below the conduction band. The photo-to-dark current ratio (Iph/Id) gradually decreases from three orders of magnitude to one order with the increase in the Mg content. View full abstract»

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  • Multiple-channel mid-infrared optical parametric oscillator in periodically poled MgO:LiNbO3

    Page(s): 113112 - 113112-5
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    We report a compact all-solid-state mid-infrared optical parametric oscillator with wide tunability using periodically poled 5 mol % MgO-doped LiNbO3 with a multiple-channel structure design and pumped by a compact diode-pump solid-state laser operating at 1.064 μm. Wide tunability from 1.44 to 1.58 μm at the signal beam wavelength and from 3.28 to 4.11 μm at the idler beam wavelength was achieved by both varying the temperature and translating the crystal through the resonator and the pump beam with no realignment required. The tuning performance and its related factors were analyzed, and at the same time the output performance as well as the effect of mid-infrared absorption of idler beam was investigated. View full abstract»

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  • Modeling the sequential electric field assisted diffusion into glass of two ion species

    Page(s): 113113 - 113113-6
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    The sequential electric field assisted diffusion of two ion species into glass is modeled in this work. Using standard assumptions, two coupled nonlinear drift-diffusion equations are derived for the ion concentration distributions. These equations are solved numerically for the case where the two ion species each have a lower mobility than the indigenous sodium ions in the glass. It is shown that stationary state distributions form if the final species injected into the glass is the less mobile of the two. Analytical expressions are derived for the stationary state distributions. A possible application in integrated optics involving Ag+ and Cu+ ion diffusion is considered. View full abstract»

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  • Long-range surface plasmon-polariton waveguides and devices in lithium niobate

    Page(s): 113114 - 113114-12
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    Long-range surface plasmon-polariton waveguides and devices based on a thin narrow Au stripe buried in z-cut LiNbO3 claddings and designed for operation at free-space optical wavelengths near 1550 nm are described and discussed. Parallel or antiparallel crystal orientations for the claddings lead to phase modulation or intensity modulation via mode cutoff, respectively. Theoretical results are given for straight waveguides, curved waveguides, and Bragg gratings in symmetric claddings, and for straight waveguides in electro-optically induced asymmetric claddings. The main theoretical findings are that 1 dB/mm of attenuation (or less) is achievable using Au stripes of reasonable dimensions (0.5–1 μm wide, 20–40 nm thick), that low overlap losses (≪1 dB) to large and small modes are achievable for the same stripe thickness, that radii of curvature in the range of 10–30 mm are required for stripes having a moderate (10-3) confinement, that first order gratings having a reflectance of 0.9 and a bandwidth of 0.75 nm are achievable and that they are electro-optically tunable over a 2.9 nm range, and that an electro-optically induced index asymmetry of about 4×10-4 is sufficient to cut off weakly (10-4) confined modes. Structures were fabricated by direct wafer bonding and thinning to form the lithium niobate claddings. The measured optical insertion loss of 2 mm - long waveguides varied from 10 to 17 dB, which are somewhat higher than theoretical expectations. Low frequency electro-optic mode cutoff measurements produced extinction ratios near 12 dB and a linear transfer characteristic, thus demonstrating intensity modulation via this mechanism. The results suggest that bulk values for the optical and electro-optic properties of the LiNbO3 claddings have been retained. View full abstract»

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  • Absorption intensities and emission cross section of intermanifold transition of Er3+ in Er3+:Y2O3 nanocrystals

    Page(s): 113115 - 113115-5
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    The room temperature absorption intensities of Er3+(4f11) transitions in synthesized Er3+:Y2O3 nanocrystals have been analyzed using the Judd-Ofelt (J-O) model in order to obtain the phenomenological intensity parameters. The J-O intensity parameters are subsequently used to determine the radiative decay rates, radiative lifetimes, and branching ratios of the Er3+ transitions from the upper multiplet manifolds to the corresponding lower-lying multiplet manifolds 2S+1LJ of Er3+(4f11). The emission cross section of the important intermanifold Er3+ 4I13/24I15/2 (1.5 μm) transition has been determined. The room temperature fluorescence lifetime of this transition in Er3+:Y2O3 nanocrystals was measured. From the radiative lifetime determined from the J-O model and measured fluorescence lifetime, the quantum efficiency of this material was determined. The comparative study of Er3+(4f11) ions suggests that synthesized Er3+:Y2O3 nanocrystals could be an excellent alternative to single crystal Er3+:Y2O3 for certain applications especially in the near infrared region. View full abstract»

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  • Spectral analysis of synthesized nanocrystalline aggregates of Er3+:Y2O3

    Page(s): 113116 - 113116-6
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    The absorption and fluorescence spectra of nanocrystalline aggregates of Y2O3 doped with Er3+ are reported between 8 K and room temperature. The nanocrystalline particles were synthesized from a homogenous solution of the metal ions and urea at elevated temperatures in order to control the precipitation of the mixed hydroxides by a slow uniform reaction throughout the solution. The morphology of the calcinated materials revealed uniformly spherical aggregates 200 nm or less depending on the ratio of the metal ions in the initial solution. Spectra obtained from these particles were analyzed in detail for the crystal-field splitting of the 2S+1LJ multiplet manifolds of Er3+(4f11) including the ground-state manifold 4I15/2, and excited manifolds 4I9/2, 4F9/2, 4S3/2, 2H11/2, 4F7/2, 4F5/2, and 4F3/2. Fluorescence lifetimes and results from an analysis of the intensities of manifold-to-manifold transitions are also reported. The sharp-line absorption and emission spectra are comparable to spectra reported earlier for Er3+:Y2O3 grown as large single crystals by a flame fusion method. The results described in the present study suggest that the simple, inexpensive method of preparation that is reported will lead to further investigation of these nanocrystals for thei- r optical properties, especially in the near infrared region of the spectrum. View full abstract»

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  • Carbon loss induced by plasma beam irradiation in porous silica films

    Page(s): 113301 - 113301-6
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    Plasma-induced damages of porous silica films during plasma processes were investigated by using a plasma beam irradiation apparatus. We used the porous silica films incorporated with methyl groups to achieve high hydrophobicity. The carbon (methyl group) reductions in the film as an index of the level of damages induced by Ar, He, O2, H2, and N2 plasma irradiations were examined by x-ray photoelectron spectroscopy and secondary ion mass spectroscopy. The damage due to Ar and He plasma bombardment increased with an increase in the ion dosage, although it was not strongly affected by the ion energy in the range higher than 130 eV. Furthermore, it was found that the damage near the film surface was influenced more by metastable He atoms than by metastable Ar atoms. Both O ions and O atoms caused severe damage. N atoms did not affect the decrease of carbon content but reacted with carbon to form CN bonds. H atoms decreased carbon content slightly, but the amount of decrease was saturated by the further irradiation of H atoms. View full abstract»

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  • Experimental and theoretical investigation of the effects of sample size on copper plasma immersion ion implantation into polyethylene

    Page(s): 113302 - 113302-5
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    Polymers are frequently surface modified to achieve special surface characteristics such as antibacterial properties, wear resistance, antioxidation, and good appearance. The application of metal plasma immersion ion implantation (PIII) to polymers is of practical interest as PIII offers advantages such as low costs, small instrument footprint, large area, and conformal processing capability. However, the insulating nature of most polymers usually leads to nonuniform plasma implantation and the surface properties can be adversely impacted. Copper is an antibacterial element and our previous experiments have shown that proper introduction of Cu by plasma implantation can significantly enhance the long-term antibacterial properties of polymers. However, lateral variations in the implant fluence and implantation depth across the insulating substrate can lead to inconsistent and irreproducible antibacterial effects. In this work, the influence of the sample size on the chemical and physical properties of copper plasma-implanted polyethylene is studied experimentally and theoretically using Poisson’s equation and plasma sheath theory. Our results indicate that the sample size affects the implant depth profiles. For a large sample, more deposition occurs in the center region, whereas the implantation to deposition ratio shows less variation across the smaller sample. However, the Cu elemental chemical state is not affected by this variation. Our theoretical study discloses that nonuniform metal implantation mainly results from the laterally different surface potential on the insulating materials due to surface charge buildup and more effective charge transfer near the edge of the sample. View full abstract»

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  • Spectroscopic study of carbonaceous dust particles grown in benzene plasma

    Page(s): 113303 - 113303-8
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    Carbonaceous dust particles have been synthesized from benzene using an rf glow discharge. Scanning electron microscope inspection revealed that the plasma-synthesized dust particles can be classified into two types. Shell-structured dust particles showed a wide size distribution from 3 to40 μm. The other type, with different degrees of aggregation, appeared to be dense and spherical with a very distinctive yellow color and size distribution from 100 nm to 2 μm. Analyses using micro-Raman and Fourier transform infrared microscopy indicated that the main components of the dust particles are polyphenyls and hydrogenated amorphous carbon (HAC). The luminescence background in Raman spectra and the infrared C–H stretching vibrational features observed around 3.4 μm for the dust particles are attributed to HAC. The formation mechanisms and spectroscopic characterization of carbonaceous dust particles are discussed. View full abstract»

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  • Near infrared long lasting emission of Yb3+ and its influence on the optical storage ability of Mn2+-activated zinc borosilicate glasses

    Page(s): 113304 - 113304-4
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    The near infrared long lasting phosphorescence of Yb3+ is observed in Yb3+ and Mn2+ codoped zinc borosilicate glasses. Compared with the glasses solely activated by Mn2+, when the Yb3+ ion is codoped, the red long lasting phosphorescence of the samples is largely improved in both brightness and persistent time but the photostimulated long lasting phosphorescence is greatly depressed. It is considered that the appearance of the phosphorescence of Yb3+ is due to the alteration of the energy transfer channel; additionally, Yb3+ also changes the trap depth of the glasses with the shallower trap predominating therefrom the red long lasting phosphorescence is improved and the photostimulated long lasting phosphorescence is degraded. View full abstract»

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  • Robustness of a tailored hole target in laser-produced collimated proton beam generation

    Page(s): 113305 - 113305-7
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    A robustness of a thin-foil tailored hole target is demonstrated by particle simulations in laser-produced proton generation. The hole target has a hole at the target rear surface. When an intense short pulse laser illuminates the thin-foil target with the hole, transverse edge fields of an accelerated electron cloud and an ion cloud are shielded by a protuberant part of the hole so that the proton beam divergence is suppressed [Sonobe etal, Phys. Plasmas 12, 073104 (2005)]. This paper presents the robustness of the hole target against laser parameter changes in a laser spot size and in a laser pulse length against a contaminated proton source layer and against a laser alignment error. The 2.5-dimensional particle-in-cell simulations also show that a multiple-hole target is robust against a laser alignment error and a target positioning error. The multihole target may serve as a robust target for practical uses to produce a collimated proton beam. View full abstract»

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  • Effect of surface reactions of low-energy carbon ions on the secondary electron emission of TiN:O thin films

    Page(s): 113306 - 113306-6
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    Low-energy secondary electron emission coatings are required for antimultipactor applications in several important technologies and their study and development is also a matter of scientific interest. For this purpose, titanium nitride was deposited on Si(100) substrates by reactive sputtering and the influence of low-energy carbon ion bombardment on the secondary electron emission yield of TiN:O coatings was studied. The composition and chemical bonds formed in the films after carbon ion implantation were analyzed by x-ray photoelectron spectroscopy (XPS). XPS revealed the formation of both carbidic and graphitic bonds. N was displaced while Ti-C bonds were formed up to a limiting value of 0.103 for the carbidic carbon atomic fraction, beyond which a graphitic surface layer was deposited. The secondary electron emission yields for TiN:O were measured before and after low-energy CHn+ ion bombardment and air exposure, and after heating. Initially, the carbon ion implantation reduced the secondary electron emission yield. Then an increase in secondary electron emission was obtained when the excess graphitic carbon was deposited on the sample. On the other hand, subsequent thermal treatment at 700 °C of the carburized samples produced a further reduction of the secondary electron emission yield. The maximum yields are about 53% lower for thermal annealed films than for similarly treated previously measured as-deposited layers. A narrowing of XPS peak line shapes is observed as a consequence of the annealing away of structural and chemical defects in the near-surface region. In addition, secondary electron emission (SEE) yield curves were used in a simulation of multipactor discharge. Both experimental multipactor threshold tests and computer multipactor simulations indicate that SEE yield values for low primary-electron energies are the most influencing parameter- s on multipactor threshold. View full abstract»

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  • Penetration of plasma into the wafer-focus ring gap in capacitively coupled plasmas

    Page(s): 113307 - 113307-11
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    In plasma etching equipment for microelectronics fabrication, there is an engineered gap between the edge of the wafer and wafer terminating structures, such as focus rings. The intended purpose of these structures is to make the reactant fluxes uniform to the edge of the wafer and so prevent a larger than desired edge exclusion where useful products cannot be obtained. The wafer-focus ring gap (typically≪1 mm) is a mechanical requirement to allow for the motion of the wafer onto and off of the substrate. Plasma generated species can penetrate into this gap and under the beveled edge of the wafer, depositing films and possibly creating particles which produce defects. In this paper, we report on a computational investigation of capacitively coupled plasma reactors with a wafer-focus ring gap. The penetration of plasma generated species (i.e., ions and radicals) into the wafer-focus ring gap is discussed. We found that the penetration of plasma into the gap and under the wafer bevel increases as the size of the gap approaches and exceeds the Debye length in the vicinity of the gap. Deposition of, for example, polymer by neutral species inside the gap and under the wafer is less sensitive to the size of the gap due the inability of ions, which might otherwise sputter the film, to penetrate into the gap. 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