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15 Mar 2008

Volume 103, Issue 6, Articles (06xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 103, 061301 (2008); http://dx.doi.org/10.1063/1.2840049 (26 pages)

K. L. Klein, A. V. Melechko, T. E. McKnight, S. T. Retterer, P. D. Rack, J. D. Fowlkes, D. C. Joy, and M. L. Simpson
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Surface characterization and functionalization of carbon nanofibers

K. L. Klein, A. V. Melechko, T. E. McKnight, S. T. Retterer, P. D. Rack, J. D. Fowlkes, D. C. Joy, and M. L. Simpson

J. Appl. Phys. 103, 061301 (2008); http://dx.doi.org/10.1063/1.2840049 (26 pages) | Cited 17 times

Online Publication Date: 17 March 2008

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Carbon nanofibers are high-aspect ratio graphitic materials that have been investigated for numerous applications due to their unique physical properties such as high strength, low density, metallic conductivity, tunable morphology, chemical and environmental stabilities, as well as compatibility with organochemical modification. Surface studies are extremely important for nanomaterials because not only is the surface structurally and chemically quite different from the bulk, but its properties tend to dominate at the nanoscale due to the drastically increased surface-to-volume ratio. This review surveys recent developments in surface analysis techniques used to characterize the surface structure and chemistry of carbon nanofibers and related carbon materials. These techniques include scanning probe microscopy, infrared and electron spectroscopies, electron microscopy, ion spectrometry, temperature-programed desorption, and atom probe analysis. In addition, this article evaluates the methods used to modify the surface of carbon nanofibers in order to enhance their functionality to perform across an exceedingly diverse application space.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.43.Vx Thermal desorption
68.37.-d Microscopy of surfaces, interfaces, and thin films
01.30.Rr Surveys and tutorial papers; resource letters
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Eigenmode mediated excitation and radiation in a planar microcavity

A. A. Dukin, N. A. Feoktistov, S. A. Grudinkin, A. V. Medvedev, A. B. Pevtsov, V. G. Golubev, M. Centini, and C. Sibilia

J. Appl. Phys. 103, 063101 (2008); http://dx.doi.org/10.1063/1.2894596 (5 pages)

Online Publication Date: 18 March 2008

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We studied the effect of planar microcavity eigenmodes on the optical excitation efficiency and the luminescence of emission centers (erbium ions) introduced in the active layer of the microcavity structure. We show experimentally that, if pumping conditions are chosen in order to excite the microcavity eigenmode, the emission intensity from erbium ions is enhanced by an order of magnitude compared to the case of pumping via excitation of photonic band states. The observed photoluminescence enhancement was theoretically and quantitatively estimated by analyzing the overlap integral between the distribution profiles of the electromagnetic fields inside the active layer of the microcavity.
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78.55.-m Photoluminescence, properties and materials
61.80.-x Physical radiation effects, radiation damage

Low- and high-order nonlinear optical properties of Au, Pt, Pd, and Ru nanoparticles

R. A. Ganeev, M. Suzuki, M. Baba, M. Ichihara, and H. Kuroda

J. Appl. Phys. 103, 063102 (2008); http://dx.doi.org/10.1063/1.2887990 (7 pages) | Cited 25 times

Online Publication Date: 19 March 2008

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We investigate third- and higher-order nonlinear optical processes in Au, Pt, Pd, and Ru nanoparticles. We measured the nonlinear refractive indices and nonlinear absorption coefficients of the suspensions of these nanoparticles using the 792 nm femtosecond and picosecond pulses. The highest values of these parameters were observed in the case of Au nanoparticles (−8×10−14 m2 W−1 and 1.7×10−10 m W−1), when the 210 ps pulses were used as a probe radiation. The high-order harmonic generation was studied during the propagation of the 120 fs pulses through the plasmas produced on the surfaces containing the nanoparticles. The highest harmonic cutoff (33rd order) was observed in the case of gold nanoparticle-contained plasma. These results were compared with the harmonics generating from the plasma produced on the surfaces of the bulk materials of the same origin. A two- to sixfold enhancement of harmonic yield was observed in the case of nanoparticle-contained plumes with regard to the bulk materials.
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42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)

Optical absorption of carbon nanotube diodes: Strength of the electronic transitions and sensitivity to the electric field polarization

Davide Mencarelli, Luca Pierantoni, and Tullio Rozzi

J. Appl. Phys. 103, 063103 (2008); http://dx.doi.org/10.1063/1.2890392 (5 pages) | Cited 4 times

Online Publication Date: 20 March 2008

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Aim of this work is to model electrostatically doped carbon nanotubes (CNT), which have recently proved to perform as ideal PN diodes, also showing photovoltaic properties. The new model is able to predict the optical absorption of semiconducting CNT as function of size and chirality. We justify theoretically, for the first time, the experimentally observed capability of CNTs to detect and select not only a well defined set of frequencies, as resulting from their discrete band structure, but also the polarization of the incident radiation. The analysis develops from an approach proposed in a recent contribution. The periodic structure of CNTs is formally modeled as a photonic crystal, that is characterized by means of numerical simulators. Longitudinal and transverse components of the electric field are shown to excite distinct interband transitions between well defined energy levels. Equivalently, for a given energy of the incident radiation, absorption may show polarization ratios strongly exceeding unity.
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85.35.Kt Nanotube devices

Field localization inside a lossy dielectric slab by means of cantor dielectric multilayers

Francesco Chiadini, Antonio Scaglione, Vincenzo Fiumara, and Ilaria Gallina

J. Appl. Phys. 103, 063104 (2008); http://dx.doi.org/10.1063/1.2894905 (6 pages) | Cited 1 time

Online Publication Date: 20 March 2008

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A Cantor multilayer is an electromagnetic band-gap structure consisting of a stack of alternating high-/low-permittivity dielectric layers whose optical lengths follow the fractal Cantor construction. The deepest stop band is bounded by two narrow transmission peaks centered at frequencies where a strong field localization occurs inside the structure. This phenomenon also occurs if a lossy dielectric slab is inserted at the midpoint of the multilayer as a defect layer and can be conveniently used to make the field intensity inside the slab significantly stronger than the intensity of the field incident on the Cantor structure.
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73.21.Ac Multilayers
77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials

X-ray luminescence of LaF3:Tb3+ and LaF3:Ce3+,Tb3+ water-soluble nanoparticles

Yuanfang Liu, Wei Chen, Shaopeng Wang, Alan G. Joly, Sarah Westcott, and Boon Kuan Woo

J. Appl. Phys. 103, 063105 (2008); http://dx.doi.org/10.1063/1.2890148 (7 pages) | Cited 14 times

Online Publication Date: 21 March 2008

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We report x-ray luminescence from LaF3:Ce3+,Tb3+ and LaF3:Tb3+ water-soluble nanoparticles. The x-ray luminescence is dominated by emission from Tb3+ ions, similar to photoluminescence spectra of the nanoparticle aqueous solutions and spectra from nanoparticle powders precipitated from the aqueous samples. Coating the nanoparticles with an insulating inorganic LaF3 or organic H2N–(CH2)10–COOH layer can enhance the x-ray luminescence from the aqueous nanoparticles. This enhancement is most likely due to the decreased energy loss due to the particle-solvent interactions.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Hx Other solid inorganic materials
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Evidences of the simultaneous presence of bow-tie and diamond scars in rare-earth doped amorphous silicon microstadium resonators

D. S. L. Figueira and N. C. Frateschi

J. Appl. Phys. 103, 063106 (2008); http://dx.doi.org/10.1063/1.2895387 (4 pages) | Cited 3 times

Online Publication Date: 21 March 2008

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Microdisks and microstadium resonators were fabricated on erbium doped amorphous hydrogenated silicon (a‐Si:H〈Er〉) layers sandwiched in air and native SiO2 on Si substrates. Annealing condition is optimized to allow large emission at 1550 nm for samples with erbium concentrations as high as 1.02×1020 atoms/cm3. Near field scanning optical microscopy shows evidence of the simultaneous presence of bow-tie and diamond scars. These modes indicate the high quality of the resonators and the potentiality for achieving amorphous silicon microcavity lasers.
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07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Demonstration of enhanced population feeding of the 1.53 μm emitting level of Er3+ in TeO2–WO3–Li2O–P2O5 glasses using upconversion luminescence spectroscopy

Yongshi Luo, Jiahua Zhang, Xia Zhang, and Xiao-jun Wang

J. Appl. Phys. 103, 063107 (2008); http://dx.doi.org/10.1063/1.2891788 (5 pages)

Online Publication Date: 25 March 2008

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P2O5 is introduced into the Er3+-doped tungsten tellurite glasses to speed up the 4I11/2‐4I13/2 relaxation. The properties of infrared to visible upconverted luminescence as a function of P2O5 contents are studied to demonstrate the enhanced feeding rate of the 1.53 μm emitting level of Er3+, which is in good agreement with that obtained using infrared emission spectra. The contributions of excited state absorption and cooperation energy transfer to the red and green upconverted luminescences are quantitatively evaluated. The Er3+-doped telluride glasses containing P2O5 could be a promising candidate as a host for broadband erbium-doped fiber amplifiers.
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78.55.Qr Amorphous materials; glasses and other disordered solids
78.40.Ha Other nonmetallic inorganics
78.30.Hv Other nonmetallic inorganics
61.43.Fs Glasses
61.72.up Other materials

Magnetometry based on nonlinear magneto-optical rotation with amplitude-modulated light

S. Pustelny, A. Wojciechowski, M. Gring, M. Kotyrba, J. Zachorowski, and W. Gawlik

J. Appl. Phys. 103, 063108 (2008); http://dx.doi.org/10.1063/1.2844494 (7 pages) | Cited 4 times

Online Publication Date: 25 March 2008

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We report on an all-optical magnetometric technique based on nonlinear magneto-optical rotation with amplitude-modulated light. The method enables sensitive magnetic field measurements in a broad dynamic range. We demonstrate the sensitivity of 4.3×10−9 G/math at 10 mG and the magnetic field tracking in a range of 40 mG. The fundamental limits of the method sensitivity and factors determining current performance of the magnetometer are discussed.
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07.55.Ge Magnetometers for magnetic field measurements
32.60.+i Zeeman and Stark effects

Dynamics of the second-order nonlinearity induced in Suprasil glass thermally poled with continuous and alternating fields

A. Kudlinski, G. Martinelli, and Y. Quiquempois

J. Appl. Phys. 103, 063109 (2008); http://dx.doi.org/10.1063/1.2901056 (6 pages) | Cited 6 times

Online Publication Date: 27 March 2008

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We report results of thermal poling experiments performed with continuous and alternating electric fields in Suprasil glass. Firstly, the second-order nonlinearity induced with a continuous voltage is fully characterized, and the nonlinear region is found to extend to almost 80 μm under the anode surface. Then, we focus our study on experiments realized with a square alternating poling field. For particular frequencies in the millihertz regime, we found a 30 times increase of the generated second harmonic signal. The nonlinear layer created under these conditions is completely characterized and a χ(2) value of 0.3 pm/V is reached for the first time in a Suprasil glass. This χ(2) coefficient is one order of magnitude greater than best results reported so far in Suprasil glasses.
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77.22.Ej Polarization and depolarization
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation

A method to retrieve optical and geometrical characteristics of three layer waveguides from m-lines measurements

T. Schneider, D. Leduc, C. Lupi, J. Cardin, H. Gundel, and C. Boisrobert

J. Appl. Phys. 103, 063110 (2008); http://dx.doi.org/10.1063/1.2885147 (7 pages) | Cited 1 time

Online Publication Date: 31 March 2008

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We consider three layer optical waveguides and present a method to measure simultaneously the refractive index and the thickness of each layer with m-lines spectroscopy. We establish the three layer waveguide modal dispersion equations and describe a numerical method to solve these equations. The accuracy of the method is evaluated by numerical simulations with noisy data and experimentally demonstrated using a PZT thin film placed between two ZnO layers.
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42.79.Gn Optical waveguides and couplers
42.70.-a Optical materials
42.79.Wc Optical coatings
42.25.Gy Edge and boundary effects; reflection and refraction
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Current constriction of high-current vacuum arc in vacuum interrupters

Lijun Wang, Shenli Jia, Ling Zhang, Dingge Yang, Zongqian Shi, Francois Gentils, and Benoît Jusselin

J. Appl. Phys. 103, 063301 (2008); http://dx.doi.org/10.1063/1.2875813 (8 pages) | Cited 1 time

Online Publication Date: 17 March 2008

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Compared with previous paper [ L. Wang et al., J. Appl. Phys. 100, 113304 (2006) ], higher-current vacuum arc is simulated and analyzed based on magnetohydrodynamics model, and current constriction phenomenon in arc column is mainly paid attention to and analyzed in this paper. According to simulation results, it can be found that significant current constriction only appears near anode regions for lower-current vacuum arc. However, with the increase of arc current, current constriction also appears near the cathode side, and with the further increase of arc current, current constriction near the cathode side can become more significant than that near the anode side. The current constriction near the cathode side can be mainly caused by very high current level. The increase of axial magnetic field (AMF) strength will inhibit current constriction in the whole arc column. For influence of AMF distribution, saddle-shaped distributed AMF can more efficiently inhibit current constriction of arc column than bell-shaped AMF. The phenomenon of current constriction near the cathode side has also been found by many experiments, which also can verify the correctness of simulation results.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.80.Mg Arcs; sparks; lightning; atmospheric electricity

Experimental study of hydrogen plasma reforming by intermittent spark discharges

R. Barni, A. Quintini, M. Piselli, and C. Riccardi

J. Appl. Phys. 103, 063302 (2008); http://dx.doi.org/10.1063/1.2838177 (9 pages) | Cited 1 time

Online Publication Date: 17 March 2008

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We present an experimental analysis of an air/methane high-voltage discharge at atmospheric pressure which operates in an intermittent spark regime. The aim is to study the feasibility of hydrogen reforming through plasma processing. The structure of the spark and the electrical characteristics of the discharge were studied as a function of the air/methane mixture composition, the gas flow rate, the average current, and the external circuitry resistance. Discharge spectra in the visible and near-ultraviolet range show the production of several radicals and excited species which provide a clue to the relevant chemical kinetics in the plasma state. Detailed mass analysis of the composition of the exhaust gas flow shows a significant production of hydrogen and high conversion efficiency of methane. The dependence on the discharge operating parameters was investigated in order to clarify the optimal setup design for an efficient plasma reformer.
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82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
52.50.Dg Plasma sources
52.80.Mg Arcs; sparks; lightning; atmospheric electricity

Seed electron production from O− ions under high-power microwave excitation

G. F. Edmiston, A. A. Neuber, H. G. Krompholz, and J. T. Krile

J. Appl. Phys. 103, 063303 (2008); http://dx.doi.org/10.1063/1.2841504 (6 pages) | Cited 4 times

Online Publication Date: 18 March 2008

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Surface and volume breakdown formation during pulsed high-power microwave (HPM) excitation can severely limit the power densities which can be transmitted into an atmospheric medium. Recent studies in this area have focused on developing models which accurately predict flashover formation at either dielectric/air interfaces or in the gas volume directly adjacent to these interfaces. These models are typically validated through comparison with experimentally gathered data. With respect to HPM surface flashover, experiments in the S-band at 5 MW power levels have reported on the contributing factors to flashover development including the effects of gas type, pressure, and relative humidity. A Monte Carlo-type electron motion simulation code, MC, has been developed to calculate the increasing electron density during flashover formation in this case. Results from the MC code have exhibited a quantitative agreement with experimental data over a wide range of atmospheric conditions. A critical parameter to flashover development is the stochastic process involving the appearance of initiatory or “seed” electrons, as seen by the reduction in flashover delay time by approximately 10−20% in the presence of external ultraviolet illumination. While the current version of the MC code seeds the flashover location with electron densities on the order of background ion densities produced by cosmic radiation, it fails to incorporate the field-assisted collisional detachment processes which are often assumed to be the primary origin of these electrons on the time scales of interest. Investigation of these processes and development of more accurate seeding in the MC code is a key step toward predicting HPM flashover over a wide range of parameters, particularly in the presence of highly electronegative gases such as SF6 or O2, in which there is an absence of free electrons with zero applied field.
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52.80.Pi High-frequency and RF discharges
52.65.Pp Monte Carlo methods
52.25.Fi Transport properties
52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions

Electrohydrodynamic interaction induced by a dielectric barrier discharge

C. A. Borghi, M. R. Carraro, A. Cristofolini, and G. Neretti

J. Appl. Phys. 103, 063304 (2008); http://dx.doi.org/10.1063/1.2888354 (11 pages) | Cited 4 times

Online Publication Date: 21 March 2008

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In the present work an experimental investigation on the electrohydrodynamic (EHD) interaction effect, induced by means of a dielectric barrier discharge on a 1 atm subsonic air flow, is described. The air flow is obtained in an open circuit blowing wind tunnel at different air speeds. A plane plasma panel equipped with a dielectric discharge barrier system is immersed in the flow. A single phase sinusoidal power supply system, a superposition of a single phase sinusoidal system with a dc voltage supply, and a three phase symmetrical power supply configuration are considered. Electric, fluid dynamic, and spectroscopic diagnostic techniques are utilized. Pitot probe measurements are performed in the boundary layer on the plasma panel surface. Schlieren imaging is done to visualize the plasma sheath. Vibrational and rotational temperatures are evaluated by means of spectroscopic techniques. The force induced on the flow by the dielectric barrier discharge acts on the ions of the non-neutral sheath ahead of the plasma filament when travelling on the panel surface only. This causes a reduction of the time interval during which the force is active. The EHD effect and the momentum transfer to the gas flow in the boundary layer are reduced when the flow speed grows. An increase of the supply frequency and of the supply voltage causes an increase of the momentum transfer. In these experiments no significant increase of the efficiency of EHD effect is observed when utilizing the superposition of a dc voltage to a single phase power supply or a three phase power supply.
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47.65.-d Magnetohydrodynamics and electrohydrodynamics
52.40.Hf Plasma-material interactions; boundary layer effects
52.70.Ds Electric and magnetic measurements
52.40.Kh Plasma sheaths
47.40.Dc General subsonic flows
52.30.-q Plasma dynamics and flow

Calculation of the gas temperature in a throughflow atmospheric pressure dielectric barrier discharge torch by spectral line shape analysis

A. Ionascut-Nedelcescu, C. Carlone, U. Kogelschatz, D. V. Gravelle, and M. I Boulos

J. Appl. Phys. 103, 063305 (2008); http://dx.doi.org/10.1063/1.2891419 (9 pages) | Cited 7 times

Online Publication Date: 31 March 2008

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An analysis of spectral line profiles is used to calculate the gas temperature and to estimate the upper limit of the electron density in an atmospheric pressure dielectric barrier discharge torch. Two transitions are studied, that of helium (He) at 587.5 nm and that of hydrogen (Hβ) at 486.1 nm, both observed in the spectra of the light emitted from the gap-space region. Relevant broadening mechanisms including the Doppler and Stark effects, as well as the collision processes between an emitter and a neutral particle, are reviewed. It is deduced that the main contribution to the broadened profiles is due to collisions. Through knowledge of the van der Waals interaction potential, a general expression for determining the gas temperature is derived and applied to each transition. The results obtained from both lines are in agreement; i.e., the gas temperature is found to be 460±60 K at the highest voltage applied. This value is consistent with the experimental observation that at these conditions the afterglow plasma cannot ignite paper, whose ignition temperature is 507 K. Since no signature of the Stark effect can be detected either in He or Hβ transition, the upper limit of the electron density, estimated from the uncertainty on the Hβ linewidth, is 4×1012 cm−3. The generality of the method allows one to determine the temperature as a function of other parameters, such as voltage and flow rate. Concerning the applied voltage, the gas temperature increases linearly from 315±30 to 460±60 K, as derived from both lines. Over the same voltage range, a similar behavior is found for the rotational temperature, as deduced from the first negative B(2Σu+,v = 0)→X(2Σg+,v = 0) transition of the molecular nitrogen ion. However, the temperature varies between 325±30 and 533±15 K, indicating an overestimation of the gas temperature. On the other hand, the gas temperature derived from each of the lines does not show a significant variation with the He flow rate in the range of 5–40 l/min.
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52.80.-s Electric discharges
52.75.Hn Plasma torches
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.30.-q Plasma dynamics and flow
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
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The mechanism of nickel sulfide induced rapid crystallization of highly textured tungsten disulfide (WS2) thin films: An in situ real-time diffraction study

S. Brunken, R. Mientus, S. Seeger, and K. Ellmer

J. Appl. Phys. 103, 063501 (2008); http://dx.doi.org/10.1063/1.2875679 (6 pages) | Cited 4 times

Online Publication Date: 17 March 2008

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Highly (001)-textured, photoactive tungsten disulfide (WS2) thin films have been grown by nickel sulfide induced rapid crystallization of amorphous sulfur rich tungsten sulfide (WS3+x) films. This paper focuses on the mechanism of the crystal growth. The rapid crystallization was monitored by real-time in situ energy dispersive x-ray diffraction. Provided that a thin nickel film is deposited prior to the deposition of WS3+x, the films crystallize rapidly (>20 nm/s). The crystallization takes place at a temperature of about 650 °C, i.e., slightly above the Ni–S eutectic temperature of 637 °C. After crystallization, isolated hexagonal NiSx crystallites are located on the surface of the WS2 layer, which was observed by scanning and transmission electron microscopy. These results lead to the model that the rapid crystallization occurs by liquid-phase crystal growth from NiSx droplets floating on top of the crystallized WS2 films. Nickel sulfide induced crystallized WS2 layers exhibit a pronounced (001) orientation with crystallite sizes up to 3 μm diameter.
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68.55.ag Semiconductors
64.70.kg Semiconductors

Direct correlation between the internal quantum efficiency and photoluminescence lifetime in undoped ZnO epilayers grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy

D. Takamizu, Y. Nishimoto, S. Akasaka, H. Yuji, K. Tamura, K. Nakahara, T. Onuma, T. Tanabe, H. Takasu, M. Kawasaki, and S. F. Chichibu

J. Appl. Phys. 103, 063502 (2008); http://dx.doi.org/10.1063/1.2841199 (4 pages) | Cited 18 times

Online Publication Date: 17 March 2008

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The equivalent internal quantum efficiency (ηinteq) at 300 K of the near-band-edge excitonic photoluminescence (PL) peak in ZnO epilayers grown by plasma-assisted molecular beam epitaxy on Zn-polar ZnO substrates was directly correlated with the PL lifetime (τPL) for the first time. This relation seems to be universal for O-polar ZnO films grown by other methods. Present homoepitaxial ZnO epilayers grown above 800 °C exhibited atomically flat surfaces, and the best full-width-at-half-maximum value of (0002) ZnO x-ray diffraction ω-rocking curves was 17.6 arcsec. The high-temperature growth also led to a long τPL of 1.2 ns at 300 K. As a result, a record high ηinteq value (9.6%) was eventually obtained under an excitation density of 5 W/cm2 (He–Cd, 325.0 nm). The homoepitaxial Zn-polar ZnO films grown by molecular beam epitaxy are coming to be used for p-n junction devices.
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78.66.Hf II-VI semiconductors
78.55.Et II-VI semiconductors
68.55.ag Semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.15.Jj Ion and electron beam-assisted deposition; ion plating
52.77.Dq Plasma-based ion implantation and deposition

First-order perturbation solutions of faceted nanostructures in an electric field

Cheng-hsin Chiu and Zhijun Huang

J. Appl. Phys. 103, 063503 (2008); http://dx.doi.org/10.1063/1.2841696 (8 pages) | Cited 2 times

Online Publication Date: 17 March 2008

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This paper presents the first-order perturbation solution to the two-dimensional electrostatics problems of faceted wires on a film-substrate system under the influence of a patterned electric plate. The solutions are accurate to the first order of the characteristic slopes of the wire and the pattern; the expressions of the solutions are concise, and the solutions are valid for single and multiple wires and patterns. The perturbation solutions provide an efficient scheme for studying the effects of a patterned electric plate on the formation of wires on the film-substrate systems.
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41.20.Cv Electrostatics; Poisson and Laplace equations, boundary-value problems
81.07.-b Nanoscale materials and structures: fabrication and characterization

Investigation of the occupation behavior for oxygen atoms in AlN films using Raman spectroscopy

E. F. McCullen, J. S. Thakur, Y. V. Danylyuk, G. W. Auner, and L. W. Rosenberger

J. Appl. Phys. 103, 063504 (2008); http://dx.doi.org/10.1063/1.2894588 (4 pages) | Cited 3 times

Online Publication Date: 17 March 2008

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We investigated the behavior of Raman modes for AlN thin films fabricated with plasma source molecular beam epitaxy method having high levels of oxygen contamination. Oxygen atoms occupy different lattice sites depending on their at. % value and, thus, strongly influence spectral features of certain Raman modes. We studied the variations in the width of nonpolar E2low and E2high modes which represent mainly the vibrations of Al sublattice and N atoms, respectively, in the AlN lattice. When oxygen occupies a N site, it affects the width of the E2high mode, and at the same time, the charge neutrality constraint creates an Al vacancy and, thus, simultaneously affects the width of the E2low mode. We found that for our films whose oxygen concentration vary from 1 to 10 at. %, the width of both the E2high and E2low modes varies linearly with the oxygen contamination levels suggesting that even at such high levels of oxygen contamination, oxygen atoms still prefer to occupy the N site. This is contrary to previous studies of single crystal AlN. The oxygen contamination in our films was determined by x-ray photoemission spectroscopy measurements.
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63.22.Dc Free films
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.jd Vacancies
78.30.Hv Other nonmetallic inorganics
79.60.-i Photoemission and photoelectron spectra
33.60.+q Photoelectron spectra

Growth of graphene structure on 6H-SiC(0001): Molecular dynamics simulation

Chao Tang, Lijun Meng, Huaping Xiao, and Jianxin Zhong

J. Appl. Phys. 103, 063505 (2008); http://dx.doi.org/10.1063/1.2894728 (5 pages) | Cited 6 times

Online Publication Date: 17 March 2008

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The growth process of graphene structure on 6H-SiC(0001) surface has been studied using the classical molecular dynamics simulation and the simulated annealing technique. Effects of annealing temperature and coverage of carbon atoms on the formation of graphene have been investigated. We found that two layers of carbon atoms of the 6H-SiC(0001) subsurface after sublimation of Si atoms undergo a transformation from a diamondlike phase to a graphenelike structure at annealing temperature above 1500 K. This transformation temperature is in good agreement with experimental observations. We also found that the formation of graphene structure strongly depends on the number of carbon layers. Two layers of carbon atoms result in large graphene clusters and four layers of carbon lead to the formation of graphene bilayer sheets. However, a single layer of carbon only forms chainlike and ringlike clusters without the hexagonal ordering. Our results provide atomic-level information about the graphitization of the 6H-SiC(0001) surface, which is useful in controlling the growth of graphene sheets.
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68.35.bg Semiconductors
68.35.Rh Phase transitions and critical phenomena

The role of lattice mismatch and kinetics in texture development: Co1−xNixSi2 thin films on Si(100)

D. Smeets, A. Vantomme, K. De Keyser, C. Detavernier, and C. Lavoie

J. Appl. Phys. 103, 063506 (2008); http://dx.doi.org/10.1063/1.2888554 (11 pages) | Cited 9 times

Online Publication Date: 17 March 2008

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Mixed Co1−xNixSi2 films (0 ≤ x ≤ 1) were grown by solid phase reaction of homogeneous Co1−xNix metal films, codeposited on Si(100). The texture of these films was contemplated using complementary experimental techniques: Rutherford backscattering and channeling spectrometry, x-ray pole figure measurements, and orientation imaging with electron backscattering diffraction. Based on the increasing Co1−xNixSi2 lattice parameter with increasing Ni concentration, a gradual, continuous improvement of the epitaxial quality of the film would be expected. The observed trend is significantly different. The epitaxial quality of the disilicide film indeed improves with increasing Ni concentration, but only up to 15% Ni. Moreover, the increasing epitaxial quality is due to a large volume fraction of (110)-oriented grains, instead of the anticipated (100) orientation. The most abundant texture component is not necessarily the one with the best in-plane match with the substrate, i.e., epitaxy, nor the one which assures the continuity of crystallographic planes across the plane of the interface, i.e., axiotaxy. Clearly, geometrical arguments alone cannot account for the observed large size and high volume fraction of (110)-oriented grains. On the other hand, we demonstrate that growth kinetics plays an important role in texture development and epitaxial growth during the solid phase reaction. Above 15% Ni, the epitaxial quality rapidly decreases and a polycrystalline film is formed for 40% Ni. This decrease is explained by a gradual shift of the disilicide nucleation site from the interface with the substrate to the surface of the thin film. For high Ni concentrations, i.e., ≥ 50% Ni, the (100) orientation dominates the thin-film texture, due to the growth of a NiSi2-rich film at the substrate interface. The changing nucleation site, due to this phase separation, and the differing growth kinetics can significantly alter the texture of ternary films. These two factors should be taken into consideration when implementing ternary alloys in devices, since their physical properties, stability, roughness, resistance, etc., depend critically on the texture of the films.
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82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis
68.55.jm Texture
68.55.A- Nucleation and growth
64.75.St Phase separation and segregation in thin films
68.60.-p Physical properties of thin films, nonelectronic
61.85.+p Channeling phenomena (blocking, energy loss, etc.)

Room temperature visible photoluminescence of silicon nanocrystallites embedded in amorphous silicon carbide matrix

U. Coscia, G. Ambrosone, and D. K. Basa

J. Appl. Phys. 103, 063507 (2008); http://dx.doi.org/10.1063/1.2844477 (6 pages) | Cited 9 times

Online Publication Date: 18 March 2008

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The nanocrystalline silicon embedded in amorphous silicon carbide matrix was prepared by varying rf power in high vacuum plasma enhanced chemical vapor deposition system using silane methane gas mixture highly diluted in hydrogen. In this paper, we have studied the evolution of the structural, optical, and electrical properties of this material as a function of rf power. We have observed visible photoluminescence at room temperature and also have discussed the role played by the Si nanocrystallites and the amorphous silicon carbide matrix. The decrease of the nanocrystalline size, responsible for quantum confinement effect, facilitated by the amorphous silicon carbide matrix, is shown to be the primary cause for the increase in the PL intensity, blueshift of the PL peak position, decrease of the PL width (full width at half maximum) as well as the increase of the optical band gap and the decrease of the dark conductivity.
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78.55.Hx Other solid inorganic materials
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.66.Db Elemental semiconductors and insulators
78.66.Jg Amorphous semiconductors; glasses
78.55.Ap Elemental semiconductors

Optical properties of tungsten oxide thin films with protons intercalated during sputtering

Y. Yamada, K. Tajima, S. Bao, M. Okada, K. Yoshimura, and A. Roos

J. Appl. Phys. 103, 063508 (2008); http://dx.doi.org/10.1063/1.2887931 (4 pages) | Cited 1 time

Online Publication Date: 19 March 2008

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Tungsten oxide thin films with protons intercalated during deposition (HxWO3) were prepared using reactive direct-current-magnetron sputtering in a gas mixture of argon, oxygen, and hydrogen. The as-deposited films fabricated under suitable conditions were colored due to the formation of tungsten bronze. The concentration of intercalated protons, given by the x values in HxWO3, was evaluated by ejecting protons electrochemically from the films. The x value of the films prepared at a constant working pressure was found to be proportional to the hydrogen flow ratio during deposition. On the other hand, the x value of the films prepared at a constant hydrogen flow ratio decreased sharply with increasing working pressure during deposition. The dispersion of the extinction coefficient (κ) of the films was estimated by analyzing the experimental spectra of Ψ and Δ measured with spectroscopic ellipsometry using the model composed of a homogeneous tungsten bronze layer with an additional surface roughness layer. As a result of this analysis, the κ value was found to increase sharply with the number of intercalated protons. There was a linear dependence between the κ value and the x value for x<0.2, while for x>0.3, the absorption saturated. This indicates that it is possible to evaluate the x value of HxWO3 films using spectroscopic ellipsometry.
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81.15.Cd Deposition by sputtering
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
82.45.Gj Electrolytes
78.66.Nk Insulators

Optical and microstructural studies of atomically flat ultrathin In-rich InGaN/GaN multiple quantum wells

Soon-Yong Kwon, Hee Jin Kim, Euijoon Yoon, Yudong Jang, Ki-Ju Yee, Donghan Lee, Seoung-Hwan Park, Do-Young Park, Hyeonsik Cheong, Fabian Rol, and Le Si Dang

J. Appl. Phys. 103, 063509 (2008); http://dx.doi.org/10.1063/1.2874494 (4 pages) | Cited 10 times

Online Publication Date: 20 March 2008

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Optical and microstructural properties of atomically flat ultrathin In-rich (UTIR) InGaN/GaN multiple quantum well were investigated by means of photoluminescence (PL), time-resolved PL (TRPL), and cathodoluminescence (CL) experiments. The sample exhibits efficient trapping of the photoexcited carriers into quantum wells (QWs) and the effect of internal electric field in the QWs was found negligible by excitation power-dependent PL and TRPL. These phenomena were attributed to the nature of UTIR InGaN QWs, indicating the potential of this system for application in optoelectronic devices. Variation of TRPL lifetime across the PL band and spatially resolved monochromatic CL mapping images strongly suggest that there is micrometer-scale inhomogeneity in effective band gap in UTIR InGaN/GaN QWs, which is originated from two types of localized areas.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
78.60.Hk Cathodoluminescence, ionoluminescence
68.65.Fg Quantum wells
78.47.jd Time resolved luminescence
71.20.Nr Semiconductor compounds
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