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Journal of Applied Physics

Issue 3 • Date Aug 2008

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Displaying Results 1 - 25 of 179
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

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  • Electrical detection of biomaterials using AlGaN/GaN high electron mobility transistors

    Page(s): 031101 - 031101-11
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    Chemical sensors can be used to analyze a wide variety of environmental and biological gases and liquids and may need to be able to selectively detect a target analyte. Different methods, including gas chromatography, chemiluminescence, selected ion flow tube, and mass spectroscopy, have been used to measure biomarkers. These methods show variable results in terms of sensitivity for some applications and may not meet the requirements for a handheld biosensor. A promising sensing technology utilizes AlGaN/GaN high electron mobility transistors (HEMTs). HEMT structures have been developed for use in microwave power amplifiers due to their high two dimensional electron gas (2DEG) mobility and saturation velocity. The conducting 2DEG channel of AlGaN/GaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes. HEMT sensors can be used for detecting gases, ions, pH values, proteins, and DNA. In this paper we review recent progress on functionalizing the surface of HEMTs for specific detection of glucose, kidney marker injury molecules, prostate cancer, and other common substances of interest in the biomedical field. View full abstract»

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  • High sensitivity two-frequency paired polarized interferometer in Faraday rotation angle measurement of ambient air with single-traveling configuration

    Page(s): 033101 - 033101-4
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    High sensitivity detection on Faraday rotation and Verdet constant of ambient air under weak applied magnet field and single-pass configuration specimen is setup in which a two-frequency paired linear polarized interferometer (TPPI) coupled with balanced detector shown at shot-noise-limited detection is demonstrated. The Verdet constant of ambient air at 1.3×10-6 rad/mTm and its sensitivity at 4.3×10-8 rad/mTm were measured. Additionally, this method also is able to extend into a broad spectral range on Faraday rotation angle or Verdet constant measurement. Finally, the enhancement on detection sensitivity by integrating a Fabry–Pérot cavity into TPPI is discussed. View full abstract»

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  • Numerical and experimental investigation of wedge tip radius effect on wedge plasmons

    Page(s): 033102 - 033102-6
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    We report numerical analysis and experimental observation of strongly localized plasmons guided by triangular metal wedges and pay special attention to the effect of smooth (nonzero radius) tips. Dispersion, dissipation, and field structure of such wedge plasmons are analyzed using the compact two-dimensional finite-difference time-domain algorithm. Experimental observation is conducted by the end-fire excitation and near-field scanning optical microscope detection of the predicted plasmons on 40° silver nanowedges with the wedge tip radii of 20, 85, and 125 nm that were fabricated by the focused-ion beam method. The effect of smoothing wedge tips is shown to be similar to that of increasing wedge angle. Increasing wedge angle or wedge tip radius results in increasing propagation distance at the same time as decreasing field localization (decreasing wave number). Quantitative differences between the theoretical and experimental propagation distances are suggested to be due to a contribution of scattered bulk and surface waves near the excitation region as well as the addition of losses due to surface roughness. The theoretical and measured propagation distances are several plasmon wavelengths and are useful for a range of nano-optical applications. View full abstract»

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  • Tight-binding calculations of ZnSe/Si wurtzite superlattices: Electronic structure and optical properties

    Page(s): 033103 - 033103-9
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    Our study is devoted to the theoretical investigation of the electronic and optical properties of (ZnSe)n/(Si2)m (0001) wurtzite (WZ) superlattices (SLs) with the range n=m=1–18, giving special attention to the role of interface states at the Zn–Si and Se–Si polar interfaces. The calculations are performed by means of a semiempirical tight-binding model with an sp3s* basis. The procedure involves the construction of a tight-binding Hamiltonian model of WZ SLs from the WZ bulk in the (0001) direction with different n and m layers. For (ZnSe)16/(Si2)16 SL, we found that the energy band gap is close to 1.665 eV, with the conduction-band minimum located at the Γ point. The states at the conduction- and valence-band edges are confined two dimensionally in the Si layers. For a valence-band discontinuity ΔEv=1.09 eV given by Harrison theory, the band gap between the confined band edges states increases (2.37 eV at the Γ point for n=m=2) by decreasing the superlattice period. It is shown that the heterointerface bond relaxation strongly affects interface band in the band gap. In the (ZnSe)10/(Si2)10 SL, the relaxed Si bonds at the heterointerface induce a vacant interface band and a filled interface band in the band gap. The band structures of (ZnSe)n/(Si2)m (0001) (WZ) (SLs) with different layer thickness are used to determine the electron and hole effective masses. Furthermore, the calculated absorption spectra o- f the superlattices are found to be quite different from those of bulk ZnSe and Si but fairly close to their average. The electronic structure of the superlattice turns out to be quite sensitive to the combination of the well and barrier layer thickness. This sensitivy suggests the possibility of designing suitable band structures for device application. View full abstract»

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  • Experimental study and numerical simulation of the propulsion of microbeads by femtosecond laser filament

    Page(s): 033104 - 033104-5
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    The light filament formed by intense femtosecond laser pulses in air can be used to generate the effective impulse to propel a micro glass bead. In this report, through both experimental studies and the corresponding numerical simulations that involve the dynamics of the nonlinear propagation of light and the laser ablation mechanism, we confirm that this propulsion scheme is based on the laser ablation of the target material. The fundamental characteristics of laser propulsion using a single ultrafast laser filament is also revealed. View full abstract»

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  • Experimental investigation of a metamaterial omnidirectional reflector

    Page(s): 033105 - 033105-4
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    We demonstrate experimentally the existence of an omnidirectional reflection band of a two-dimensional metamaterial structure consisting of split ring resonators and thin conducting wires. Experimental angular reflectance data agree reasonably well with simulations using model dispersion relations for the effective dielectric permittivity and magnetic permeability that were fitted to the experimentally retrieved material parameters. The tunability of the reflection bandwidth is demonstrated by changing the effective electric plasma frequency. These findings are also relevant to optical frequencies where similar metamaterial composites have been fabricated. View full abstract»

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  • Light scattering and diffuse light propagation in sintered porous silicon

    Page(s): 033106 - 033106-16
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    The scattering coefficient and the refractive index of sintered porous silicon are deduced from measurements on 1–4 μm thick freestanding films. Mie’s theory is applied to describe the light scattering by the spherical pores. Using a reduced effective refractive index for the host medium in Mie’s theory accounts for the close spacing of the pores and results in an agreement between the measured and calculated scattering coefficients. A coherent calculation for the specular nonscattered radiation is combined with a model that describes the propagation of the scattered diffuse light flux. For this diffuse model two approaches, the Kubelka Munk theory and a Lambertian model developed in this work, are compared. The combined model reproduces both, the specular as well as the diffuse component of the measured reflection and transmission. View full abstract»

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  • Tuning the wavelength of lasing emission in organic semiconducting laser by the orientation of liquid crystalline conjugated polymer

    Page(s): 033107 - 033107-6
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    We report the optical pumping of one-dimensional distributed feedback (DFB) conjugated polymer devices using a uniaxially aligned liquid crystalline polymer, poly(9,9-dioctylfluorene). We can independently select the alignment direction (via a rubbed polyimide layer) and the DFB structure (via nanoimprinting). In comparison with unaligned film, we show that lasing threshold is substantially reduced when absorption is parallel to the aligned direction (∼20.0 μJcm-2pulse-1). This is mainly due to the higher absorption coefficient estimated in the table by calculating the exciton densities at each threshold value. We also report the control of lasing wavelength through independent selection of alignment direction and DFB orientation, which is achieved through the control of the effective refractive index of waveguide (neff). View full abstract»

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  • Switching of off-axis viewing quality in twisted nematic liquid crystal display by controlling phase retardation of additional liquid crystal layers

    Page(s): 033108 - 033108-7
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    This study examined the viewing angle control of twisted nematic liquid crystal displays (TN-LCDs). Conventional TN mode has intrinsic characteristics, such as a narrow viewing angle along the vertical direction and a relatively wide viewing angle along the horizontal and diagonal directions. Our study shows that the viewing angle of the TN-LCD can be made wider and smaller than that of a normal TN cell by adding one or two homogeneously aligned liquid crystal layers between the TN cell and polarizers, and controlling their retardation with an applied voltage. View full abstract»

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  • Liquid crystal optical phase plate with a variable in-plane gradient

    Page(s): 033109 - 033109-7
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    We propose a nematic liquid crystal (LC) optical phase plate, with a large continuous in-plane gradient that is variable, and its application to a beam steering device with high efficiency. The device is a vertically aligned, continuous phase, optical phased array (V-COPA) that uses a negative dielectric anisotropy LC material. High steering efficiency of over 95% is demonstrated by modeling the LC director field and its effect on transmitted light. The period of the V-COPA grating can be varied by adjusting an applied voltage profile, which allows for continuous angular control of the diffraction angle. View full abstract»

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  • Linear and nonlinear optical properties of gold nanoparticle-Eu oxide composite thin films

    Page(s): 033110 - 033110-5
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    In this work, nanogold particles incorporated onto europium oxide films at levels of 4% and 7% were fabricated by a vacuum evaporation technique on glass and silicon substrates held at 200 °C. Samples were investigated by x-ray diffraction and linear and nonlinear optical absorption. The linear optical absorption data were measured in the UV-visible-near infrared spectral regions and from these data the energy gap and the surface plasmon resonance were determined. The third order nonlinear optical properties of the nanogold particles incorporated onto europium oxide films were measured using the Z-scan technique. Nonlinear absorption and refraction were performed using a continuous wave laser at 633 nm. A large value of third order nonlinearities was obtained with the samples. View full abstract»

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  • Intense narrow band terahertz generation via type-II difference-frequency generation in ZnTe using chirped optical pulses

    Page(s): 033111 - 033111-4
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    We developed a tabletop source of intense, narrow band terahertz pulses via type-II difference-frequency generation in ZnTe crystal using two linearly chirped and orthogonally polarized optical pulses. The pulse energy is in the range of 1–3 nJ depending on the variable pulse duration from 1 to 5 ps. The amplitude of electric field reaches ∼10 kV/cm. The central frequency of the spectrum is continuously tunable from 0.3 to 2.5 THz with the bandwidth at 0.2–0.5 THz. View full abstract»

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  • Selection of a single femtosecond high-order harmonic using a zone plate based monochromator

    Page(s): 033112 - 033112-4
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    We report the use of zone plate optics as a monochromator for the spectral selection of a single high-order harmonic of a femtosecond laser generated in a rare gas medium. We show that this is a convenient way of monochromatizing the vacuum ultraviolet (VUV) pulses in the photon energy range from 30 up to 70 eV while keeping the pulse duration in the femtosecond range. We measure the emission spectra with a typical energy resolution of E/ΔE=60. Comparably high transmission and diffraction efficiencies allow the record of images of different harmonics using a camera within a few seconds of acquisition time. This is our first step toward a VUV microscope with a submicrometer spatial resolution and femtosecond time resolution. Possible applications of this setup are also discussed. View full abstract»

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  • Time delays and energy transport velocities in three dimensional ideal cloaking devices

    Page(s): 033113 - 033113-6
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    We obtain the energy transport velocity distribution for a three dimensional ideal cloak explicitly. Near the operation frequency, the energy transport velocity has a rather peculiar distribution. The transport velocity along the direction pointing at the center of the cloak is found to be a constant, while the transport velocity approaches zero at the inner boundary of the cloak. A ray pointing right into the center of the cloak will experience abrupt changes in velocities when it impinges on the inner surface of the cloak. This peculiar distribution causes long time delays for beams passing through the ideal cloak within a geometric optics description. View full abstract»

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  • Thermo-optical modeling of high power operation of 2 μm codoped Tm,Ho solid-state lasers

    Page(s): 033114 - 033114-9
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    The results of coupled thermo-optical modeling of Tm,Ho solid-state laser operation are reported. A rate dynamics model integrated together with a TEM00 distribution for the total number of stimulated photons is coupled with a two-dimensional time-dependent heat transfer model. The heat transfer model includes absorption, heat release, and transfer inside the crystal as well as the thermal effect of the spontaneously emitted infrared radiation. In water cooled crystal operation this radiation is shown to be absorbed within the water boundary layer, producing significant inhibition of dissipation of the heat released inside the crystal. This effect leads to crystal superheating and significant inhibition of laser energy output. Absorption loss, in particular, due to water vapor present in the cavity is found to decrease significantly laser pulse energy. Numerical results are compared to previous (Tm,Ho:yttrium lithium fluoride) and new (Tm,Ho:ceramic yttrium aluminum garnet) experimental data and discussed within the framework of the computational model and simplified analytical approximations. View full abstract»

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  • The effect of current crowding on the heat and light pattern in high-power AlGaAs light emitting diodes

    Page(s): 033115 - 033115-5
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    The results of the light and temperature micromapping in AlGaAs light emitting diodes grown by liquid phase epitaxy as double heterostructures and emitting at λ∼0.87 μm are presented. At a driving current well above the safe operating limit (≫300 mA), the nonuniform light pattern and local self-heating (with temperature gradient of about 950 °C/cm) followed by catastrophic degradation of a device were detected with the charge coupled device and infrared microscopes operating in a pulsed mode. These were shown to result from the current crowding effect in the active and contact areas of a device. Good agreement between the theory and experiment was found. View full abstract»

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  • Finite difference time domain analysis of the light extraction efficiency in organic light-emitting field-effect transistors

    Page(s): 033116 - 033116-5
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    The authors report on three-dimensional numerical optical simulations of the emission outcoupling efficiency in light-emitting devices with a field-effect carrier transport. The finite difference time domain method is applied to organic thin film structures on a silicon substrate with metal and metal oxide electrodes. Simulations are performed for Au, Ag, and indium tin oxide electrodes in a bottom gate, bottom contact geometry. Additional attention is paid to the dependence on electrode thickness and contact shape. We demonstrate that in unipolar driven devices with Si gate, silicon dioxide insulator, and 40 nm thick organic films, the maximum outcoupling efficiency is below 10%. This value can be increased by the implementation of a metal reflecting layer on the Si substrate. In further studies, the emission efficiency in the ambipolar regime is investigated. The result presents the dependence of light extraction on the light source-electrode distance for rectangular and wedge shaped contacts. View full abstract»

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  • Light emission and enhanced nonlinearity in nanophotonic waveguide circuits by III–V/silicon-on-insulator heterogeneous integration

    Page(s): 033117 - 033117-7
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    The heterogeneous integration of a III–V thin film on top of a silicon-on-insulator (SOI) optical waveguide circuit by means of adhesive divinylsiloxane-benzocyclobutene (DVS-BCB) die-to-wafer bonding is demonstrated, thereby achieving light emission and enhanced nonlinearity in ultracompact SOI cavities. This approach requires ultrathin DVS-BCB bonding layers to allow the highly confined optical mode to overlap with the bonded III–V film. The transfer of sub-100-nm III–V layers using a 65 nm DVS-BCB bonding layer onto SOI racetrack resonator structures is demonstrated. Spontaneous emission coupled to a SOI bus waveguide, spectrally centered around the resonator resonances, is observed by optically pumping the III–V layer. Strong carrier-induced nonlinearities are observed in the transmission characteristics of the III–V/SOI resonator structure. The all-optical control of an optical signal in these III–V/SOI resonators is demonstrated. View full abstract»

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  • Effect of annealing on the spectral and nonlinear optical characteristics of thin films of nano-ZnO

    Page(s): 033118 - 033118-9
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    The annealing effect on the spectral and nonlinear optical (NLO) characteristics of ZnO thin films deposited on quartz substrates by sol-gel process is investigated. As the annealing temperature increases from 300–1050 °C, there is a decrease in the band gap, which indicates the changes of the interface of ZnO. ZnO is reported to show two emission bands, an ultraviolet (UV) emission band and another in the green region. The intensity of the UV peak remains the same while the intensity of the visible peak increases with increase in annealing temperature. The role of oxygen in ZnO thin films during the annealing process is important to the change in optical properties. The mechanism of the luminescence suggests that UV luminescence of ZnO thin films is related to the transition from conduction band edge to valence band, and green luminescence is caused by the transition from deep donor level to valence band due to oxygen vacancies. The NLO response of these samples is studied using nanosecond laser pulses at off-resonance wavelengths. The nonlinear absorption coefficient increases from 2.9×10-6 to 1.0×10-4 m/W when the annealing temperature is increased from 300 to 1050 °C, mainly due to the enhancement of interfacial state and exciton oscillator strength. The third order optical susceptibility χ(3) increases with increase in annealing temperature (T) within the range of our investigations. In the weak confinement regime, T2.4 dependence of χ(3) is obtained for ZnO thin films. The role of annealing temperature on the optical limiting response is also studied. View full abstract»

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  • Plasma-enabled growth of ultralong straight, helical, and branched silica photonic nanowires

    Page(s): 033301 - 033301-7
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    This article reports on the low-temperature inductively coupled plasma-enabled synthesis of ultralong (up to several millimeters in length) SiO2 nanowires, which were otherwise impossible to synthesize without the presence of a plasma. Depending on the process conditions, the nanowires feature straight, helical, or branched morphologies. The nanowires are amorphous, with a near-stoichiometric elemental composition ([O]/[Si]=2.09) and are very uniform throughout their length. The role of the ionized gas environment is discussed and the growth mechanism is proposed. These nanowires are particularly promising for nanophotonic applications where long-distance and channelled light transmission and polarization control are required. View full abstract»

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  • Design of inexpensive diffraction limited focal plane arrays for millimeter wavelength and terahertz radiation using glow discharge detector pixels

    Page(s): 033302 - 033302-4
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    Development of focal plane arrays (FPAs) for millimeter wavelength and terahertz radiation is presented in this paper. FPA is based on an inexpensive glow discharge detector (GDD) that serves as a pixel in the FPA. It was shown in previous investigations [A. Abramovich etal, Appl. Opt. 46, 7207 (2007)] that those inexpensive neon indicator lamp GDDs are quite sensitive to millimeter wavelength and terahertz radiation. The diameter of the GDD lamp is 6 mm and thus the FPA can be diffraction limited. Development of a FPA using such devices as detectors is advantageous since the cost of such lamps is around $0.2–0.5 per lamp, and it also serves as a room temperature detector. Experimental results at 100 GHz show that the responsivity of the terahertz FPA 4×4 GDD pixel is three times better than in previous measurements of A. Abramovich etal [Appl. Opt. 46, 7207 (2007)].The addition of a parabolic reflector improves the accuracy of the noise equivalent power measurement which was found to be 6×10-9 W/√Hz for a 1 kHz modulation. However, it is expected to be considerably less at higher modulation frequencies because of much reduced noise. View full abstract»

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  • Xe-excitation efficiency and plasma saturation in plasma display microdischarges

    Page(s): 033303 - 033303-7
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    Plasma display panel (PDP) efficiency data are correlated with panel emission measurements. A large visible/infrared (vis/IR) ratio of the phosphor emission in the visible to the Xe emission in the infrared indicates a high Xe-excitation efficiency. Monitoring the changes in the vis/IR ratio allows a decomposition of the discharge efficiency into Xe-excitation efficiency and electron heating efficiency contributions. For several different PDP efficiency dependencies on sustain voltage and frequency, consistent trends in Xe-excitation efficiency and electron heating efficiency are found. In addition, in order to follow the discharge development, the time dependence and the spatial distribution of the Xe emission are monitored. The combined results show that plasma saturation is significant to low Xe-content panels in default operation conditions and that plasma saturation decreases with the high voltage high frequency operation of high Xe-content panels. These driving conditions, which are especially suited for high Xe-content panels, govern a fast and spatially distributed discharge development with a lower effective electron temperature, increased Xe-excitation efficiency, and decreased plasma saturation. View full abstract»

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  • Optical emission study of a doped diamond deposition process by plasma enhanced chemical vapor deposition

    Page(s): 033304 - 033304-10
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    Standard H2/CH4/B2H6 plasmas (99% of H2 and 1% of CH4, with 0–100 ppm of B2H6 added) used for doped diamond film growth are studied by optical emission spectroscopy in order to gain a better understanding of the influence of boron species on the gas phase chemistry. Only two boron species are detected under our experimental conditions (9/15/23 W cm-3 average microwave power density values), and the emission spectra used for studies reported here are B(2S1/2-2P1/2,3/20) and BH[A 1Π-X 1Σ+(0,0)]. Variations of their respective emission intensities as a function of the ratio B/C, the boron to carbon ratio in the gas mixture, are reported. We confirmed that the plasma parameters (Tg, Te, and ne) are not affected by the introduction of diborane, and the number densities of B atoms and BH radical species were estimated from experimental measurements. The results are compared to those obtained from a zero-dimensional chemical kinetic model where two groups of reactions are considered: (1) BHx+HBHx-1+H2 (x=1–3) by analogy with - the well-known equilibrium CHx+H set of reactions, which occurs, in particular, in diamond deposition reactors; and (2) from conventional organic chemistry, the set of reactions involving boron species: BHx+C2H2 (x=0–1). The results clearly show that the model based on hydrogen and boron hydrides reactions alone is not consistent with the experimental results, while it is so when taking into account both sets of reactions. Once an upper limit for the boron species number densities has been estimated, axial profiles are calculated on the basis of the plasma model results obtained previously in Laboratoire d’Ingénierie des Matériaux et des Hautes Pressions, and significant differences in trends for different boron species are found. At the plasma-to-substrate boundary, [BH] and [B] drop off in contrast to [BH2], which shows little decrease, and [BH3], which shows little increase, in this region. 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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Editor
P. James Viccaro
Argonne National Laboratory