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15 May 2007

Volume 101, Issue 10, Articles (10xxxx)

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Wave propagation characteristics of a figure-eight shaped nanoaperture

G. S. Eom, D. Yang, E. Lee, S. Park, Y. Lee, and J. W. Hahn

J. Appl. Phys. 101, 103101 (2007); http://dx.doi.org/10.1063/1.2732412 (4 pages) | Cited 5 times

Online Publication Date: 16 May 2007

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Wave propagation characteristics of apertures were analyzed to explain the light transmission of metallic nanoapertures. Based on Maxwell’s equations, the wave dispersion relations of wave propagation modes in nanoapertures were derived. The resonance frequency shift of the aperture and the variation of the spot size are explained with the dispersion relations. The relationship between near-field and far-field light transmission power throughput and spot size is also shown with the wave mode change predicted by the dispersion relations.
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78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Characterization of excitonic features in self-assembled InAs/GaAs quantum dot superlattice structures via surface photovoltage spectroscopy

C. H. Chan, C. H. Lee, Y. S. Huang, J. S. Wang, and H. H. Lin

J. Appl. Phys. 101, 103102 (2007); http://dx.doi.org/10.1063/1.2733992 (5 pages) | Cited 4 times

Online Publication Date: 18 May 2007

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This work systematically investigates the influence of InAs growth conditions and superlattice parameters on the optical properties of InAs/GaAs quantum dot (QD) superlattice structures grown by molecular beam epitaxy. Using surface photovoltage spectroscopy, one directly obtains the absorption spectra up to the highest confined QD levels at room temperature. Based on photoluminescence measurements at different excitation wavelengths, a feature below the fundamental transition is attributed to the transition from uncoupled dots in the bottom layers. The QD transition energy shift was found to be correlated with material intermixing, driven by enhanced strain strength with the increase of layer number and the decrease of spacer thickness, and the growth rate of InAs. A blueshift observed in low growth rate samples is indicative of greatly enhanced intermixing. For QD superlattices grown at a relatively high deposition rate of InAs, the material intermixing effect is gradually enhanced with increasing layer numbers, which will compensate the electronically coupled effect and will prevent further energy shift toward the lower energy side. The absorption wavelengths in 10- and 30-period InAs/GaAs QD superlattices with higher growth rates are near 1.32 μm. The results suggest that these QD superlattice structures can be used as promising active media for long-wavelength QD lasers operating at room temperature.
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81.07.Ta Quantum dots
78.67.Hc Quantum dots
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.55.Cr III-V semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)
42.55.Px Semiconductor lasers; laser diodes

Fabrication and analysis of one-dimensional defect-induced ultrawide photonic band gaps

Y. H. Lu, M. D. Huang, S. Y. Park, P. J. Kim, Y. P. Lee, J. Y. Rhee, C. K. Hwangbo, and L. Y. Chen

J. Appl. Phys. 101, 103103 (2007); http://dx.doi.org/10.1063/1.2732691 (5 pages) | Cited 1 time

Online Publication Date: 21 May 2007

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An approach named the defect-induced extension of photonic band gaps (PBGs) is proposed, in which a stratified periodic structure is combined with another stratified structure containing defect layers. For comparison, three structures composed of TiO2 and SiO2 alternating layers, perfect, five-defect and combined-photonic crystals (PCs), were fabricated by using an electron-beam evaporation system. The measured PBG width of combined-PCs extended to 370 nm at the normal incidence, nearly three times more than that of perfect ones, 130 nm, but a few ripples are observed within the PBG. In order to elucidate the origin of the ripples, the influence of the refractive-index contrast on the reflectance was analyzed theoretically. When the refractive-index contrast nH/nL ≥ 1.7, the ripples are eliminated. More importantly, an overdoubled omnidirectional PBG is realized at nH/nL = 2.56, covering the visible region completely. This approach is considered to be based on the light localization and a heterostructure resulting from introducing the impurity band and compensating for it using PCs with a strictly periodic structure, in which the thickness of every layer remains unchanged. These characteristics distinguish it from other methods proposed before, with the fabrication much easier.
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42.70.Qs Photonic bandgap materials
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.72.-y Defects and impurities in crystals; microstructure

Single-photon source characterization with twin infrared-sensitive superconducting single-photon detectors

Robert H. Hadfield, Martin J. Stevens, Richard P. Mirin, and Sae Woo Nam

J. Appl. Phys. 101, 103104 (2007); http://dx.doi.org/10.1063/1.2717582 (7 pages) | Cited 13 times

Online Publication Date: 21 May 2007

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We report on the high fidelity characterization, via spontaneous emission lifetime and g(2)(τ) measurements, of a cavity-coupled quantum dot single-photon source at 902 nm using a pair of nanowire-based superconducting single-photon detectors (SSPDs). We analyze the suitability of the twin SSPD scheme reported here for the characterization of single-photon sources at telecommunications wavelengths (1310 and 1550 nm).
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07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.25.Pb Superconducting infrared, submillimeter and millimeter wave detectors
85.60.Gz Photodetectors (including infrared and CCD detectors)

Enhancement of upconverted luminescence in Er3+:TeO2 glass on increasing the temperature and concentration of Yb3+ beyond the critical limit

Anant Kumar Singh, Garima Tripathi, S. B. Rai, and Anita Rai

J. Appl. Phys. 101, 103105 (2007); http://dx.doi.org/10.1063/1.2727451 (5 pages) | Cited 2 times

Online Publication Date: 22 May 2007

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Enhancement of upconverted luminescence in Er3+/Yb3+:TeO2 has been observed with increase in the concentration of Yb3+ on excitation with 780 nm laser radiation. It is further noted that the upconversion is increased even above the quenching limit on increasing the temperature of the glass. The incident radiation excites Yb3+ indirectly via Er3+ resulting in a strong blue emission from Yb3+ and green and red emissions from Er3+. The UV emission from Er3+ at 380 nm involves three near infrared photons when there is no Yb3+ in the glass but when Yb3+ is also present, this behaves as a two photon process. The lifetime of the 4S3/2 level of Er3+ in glasses containing only Er3+ and also those containing both Er3+ and Yb3+ has also been measured in different conditions.
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78.55.Qr Amorphous materials; glasses and other disordered solids
63.50.-x Vibrational states in disordered systems

Crystal growth and properties of LiAlO2 and nonpolar GaN on LiAlO2 substrate

Mitch M. C. Chou, D. R. Hang, H. Kalisch, R. H. Jansen, Y. Dikme, Michael Heuken, and G. P. Yablonskii

J. Appl. Phys. 101, 103106 (2007); http://dx.doi.org/10.1063/1.2713942 (5 pages) | Cited 8 times

Online Publication Date: 22 May 2007

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In this study, the growth and properties of LiAlO2 material and a nonpolar GaN-based light-emitting-diode (LED) structure on LiAlO2 have been investigated. The LiAlO2 material is grown by the Czochralski pulling technique and is used as a substrate for nonpolar nitride growth. An improved surface roughness can be obtained by a four-step polishing process. With subsequent nitridation treatment, a pure M-plane (10math0) GaN can be obtained. An electron microscope shows an abundance of cracks that are oriented parallel to the (001) and (100) planes of the LiAlO2 substrate on the rear surface of GaN. The absence of the polarization-induced electric field of a GaN-based LED structure on LiAlO2 was shown by using photoluminescence measurements. Therefore, this approach is promising to further increase the luminescence performance of GaN-based LEDs.
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81.10.Fq Growth from melts; zone melting and refining
81.65.Ps Polishing, grinding, surface finishing
81.65.Lp Surface hardening: nitridation, carburization, carbonitridation
68.47.Fg Semiconductor surfaces
68.35.Gy Mechanical properties; surface strains
78.55.Cr III-V semiconductors

Second harmonic generation in (111)-oriented InP-based quantum cascade laser

Marcella Giovannini, Mattias Beck, Nicolas Hoyler, and Jérôme Faist

J. Appl. Phys. 101, 103107 (2007); http://dx.doi.org/10.1063/1.2730570 (4 pages) | Cited 6 times

Online Publication Date: 24 May 2007

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We show the realization of quantum cascade lasers (QCL) grown on (111)-oriented InP substrates instead of the usual (100)-oriented surface. Growth along [111] orientation allows the excitation of the nonlinear susceptibility for the transverse magnetic wave produced by the QCL. Two devices were realized, at 6 and 10 μm wavelengths. They are compared with the same structures grown on the usual (100)-oriented surface. Second harmonic generation is demonstrated in one of the devices.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.65.An Optical susceptibility, hyperpolarizability
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation

Auger recombination and state filling of resonantly excited ground state in CdSe quantum dots

J. H. Kim, K. Kyhm, S. M. Kim, and Ho-Soon Yang

J. Appl. Phys. 101, 103108 (2007); http://dx.doi.org/10.1063/1.2736342 (4 pages) | Cited 5 times

Online Publication Date: 24 May 2007

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Dynamics and state filling of the resonantly excited ground state (1se‐1S3/2h) in a strongly confined CdSe quantum dot were investigated by degenerate pump-probe measurements. With increasing the electron-hole (e-h) pairs per dot the state filling and bleaching were observed with ∼ 2.7 e-h pairs. While radiative recombination was dominant with a small number of the e-h pairs (⪡1), the Auger-type recombination became significant near the bleaching number of e-h pairs ( ∼ 2.7). This result suggests that, for resonant excitation, either cold electron or hole of the ground state is scattered into the higher-energy states of a ZnS shell via Auger recombination process ( ∼ 28 ps).
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73.63.Kv Quantum dots
78.67.Hc Quantum dots
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.21.La Quantum dots

High gain oscillators: Pulse propagation and saturation

G. Dattoli, P. L. Ottaviani, and S. Pagnutti

J. Appl. Phys. 101, 103109 (2007); http://dx.doi.org/10.1063/1.2435960 (6 pages)

Online Publication Date: 25 May 2007

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Semianalytical formulas have been used to model the pulse propagation dynamics of low gain free electron laser oscillators. Most of the scaling relations, including pulse propagation effects, are limited to the low gain regime. We discuss the problem of oscillator devices operating with small signal gain coefficients larger than 1. We comment on the relevant physical aspects and show that gain and saturated power can be reproduced by a set of simple formulas, analogous to those employed for the low gain case.
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41.60.Cr Free-electron lasers

Anchoring energy and cell gap effects on liquid crystal response time

Xiangyi Nie, Ruibo Lu, Haiqing Xianyu, Thomas X. Wu, and Shin-Tson Wu

J. Appl. Phys. 101, 103110 (2007); http://dx.doi.org/10.1063/1.2734870 (5 pages) | Cited 31 times

Online Publication Date: 25 May 2007

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The anchoring energy and cell gap effects on liquid crystal response time (τ0) is analyzed theoretically and validated experimentally. Analytical expressions are derived using two different approaches: effective cell gap and surface dynamic equation methods. Consistent results are deduced from these two approaches. A simplified equation τ0 ∼ dx also fits the experimental data well, where d is the liquid crystal cell gap and x is the exponent. Under two extreme (strong and weak) anchoring limits, the exponent x approaches 2 and 1, respectively. This information is helpful for optimizing liquid crystal devices for display applications.
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61.30.Cz Molecular and microscopic models and theories of liquid crystal structure
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions
42.70.Df Liquid crystals
68.03.Cd Surface tension and related phenomena

Interdiffusion in narrow InGaAsN/GaAs quantum wells

W. Liu, D. H. Zhang, Z. M. Huang, S. Z. Wang, S. F. Yoon, W. J. Fan, C. J. Liu, and A. T. S. Wee

J. Appl. Phys. 101, 103111 (2007); http://dx.doi.org/10.1063/1.2736943 (6 pages) | Cited 1 time

Online Publication Date: 29 May 2007

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Interdiffusion in In0.32Ga0.68As0.984N0.016/GaAs multiple quantum wells with well widths of 2 and 4 nm, respectively, was investigated both experimentally and theoretically. Maximum blueshifts of 206 and 264 meV in the photoluminescence spectra were observed. Secondary ion mass spectrometry showed that both In–Ga and N–As interdiffusions played key roles for the large blueshifts. The significant In–Ga interdiffusion occurred at 650 °C while the N diffusion occurred at a temperature above 700 °C. The theoretical results are in good agreement with the experimental observations.
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66.30.Ny Chemical interdiffusion; diffusion barriers
68.35.Fx Diffusion; interface formation
78.67.De Quantum wells
78.55.Cr III-V semiconductors
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Silicon nanoparticles: Absorption, emission, and the nature of the electronic bandgap

Cedrik Meier, Andreas Gondorf, Stephan Lüttjohann, Axel Lorke, and Hartmut Wiggers

J. Appl. Phys. 101, 103112 (2007); http://dx.doi.org/10.1063/1.2720095 (8 pages) | Cited 32 times

Online Publication Date: 30 May 2007

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Silicon nanoparticles synthesized in the gas phase are studied. From time-resolved photoluminescence measurements we determine, quantitatively, the size-dependence of the oscillator strength of the nanoparticles. We investigate experimentally the absorption and photoluminescence emission of nanoparticle ensembles with a broad size distribution. Using a model which accounts for size-effects in both oscillator strength and quantum-confinement, we are able to calculate absorption and emission spectra of ensemble samples. From these results we have determined, whether silicon nanoparticles should be regarded as indirect or direct semiconductors. Moreover, we systematically study the influence of the particle size-distribution on the optical spectra.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Ap Elemental semiconductors
78.47.-p Spectroscopy of solid state dynamics
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
71.20.Mq Elemental semiconductors
73.22.-f Electronic structure of nanoscale materials and related systems

On the role of chemical reactions in initiating ultraviolet laser ablation in poly(methyl methacrylate)

Manish Prasad, Patrick F. Conforti, and Barbara J. Garrison

J. Appl. Phys. 101, 103113 (2007); http://dx.doi.org/10.1063/1.2740340 (15 pages) | Cited 13 times

Online Publication Date: 31 May 2007

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The role of chemical reactions is investigated versus the thermal and mechanical processes occurring in a polymer substrate during irradiation by a laser pulse and subsequent ablation. Molecular dynamics simulations with an embedded Monte Carlo based reaction scheme were used to study ultraviolet ablation of poly(methyl methacrylate) at 157 nm. We discuss the onset of ablation, the mechanisms leading to ablation, and the role of stress relaxation of the polymer matrix during ablation. Laser induced heating and chemical decomposition of the polymer substrate are considered as ablation pathways. It is shown that heating the substrate can set off ablation via mechanical failure of the material only for very short laser pulses. For longer pulses, the mechanism of ejection is thermally driven limited by the critical number of bonds broken in the substrate. Alternatively, if the photon energy goes towards direct bond breaking, it initiates chemical reactions, polymer unzipping, and formation of gaseous products, leading to a nearly complete decomposition of the top layers of substrates. The ejection of small molecules has a hollowing out effect on the weakly connected substrates which can lead to lift-off of larger chunks. Excessive pressure buildup upon the creation of gaseous molecules does not lead to enhanced yield. The larger clusters are thermally ejected, and an entrainment of larger polymer fragments in gaseous molecules is not observed.
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81.65.-b Surface treatments
82.35.-x Polymers: properties; reactions; polymerization
52.38.Mf Laser ablation
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Pv Polymers, organic compounds
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Role of ambient gas and laser fluence in governing the dynamics of the plasma plumes produced by laser blow off of LiF–C thin film

R. K. Singh, Ajai Kumar, B. G. Patel, and K. P. Subramanian

J. Appl. Phys. 101, 103301 (2007); http://dx.doi.org/10.1063/1.2732446 (9 pages) | Cited 17 times

Online Publication Date: 18 May 2007

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The time- and space-resolved emission profiles of Li I and Li II emission lines from the laser-blow-off plumes of a multilayered LiF–C thin film have been studied using spectroscopic technique. The evolution features were analyzed in different ambient environments ranging from high vacuum to 3 mbars of argon pressures and at various fluences of the ablating laser. During the evolution of the plume, a transition region was found to exist between 4 and 6 mm. Here, the plume dynamics changed from free expansion to collisional regime, where the plume experienced viscous force of the medium. The enhancement observed in neutral lines, in comparison with ionic lines, is explained in terms of the yield difference in electron impact excitation and ionization processes. Substantial difference in the arrival time distribution of the plume species was observed for Li I and Li II lines at high ambient pressures. Three expansion models are invoked to explain the evolution of the plume in different ambient conditions. The laser fluence was found to control the ratio of ions and neutrals.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.38.Mf Laser ablation
52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions

Density effect on proton acceleration from carbon-containing high-density thin foils irradiated by high-intensity laser pulses

C. T. Zhou, M. Y. Yu, and X. T. He

J. Appl. Phys. 101, 103302 (2007); http://dx.doi.org/10.1063/1.2730565 (6 pages) | Cited 4 times

Online Publication Date: 22 May 2007

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The acceleration of protons in dense plastic foils irradiated by ultrahigh intensity laser pulses is simulated using a two-dimensional hybrid particle-in-cell scheme. For the chosen parameters of the overdense foils of densities ρ = 0.2, 1, and 3 g/cm3 and of an ultrahigh intensity (2×1020 W/cm2) laser pulse, our simulations illustrate that a high-density target is favorable to high collimation of the target-normal-sheath acceleration protons but less energy for a short acceleration time (<100 fs). In particular, the difference of strong local heating of the carbon ion for different plasma densities is clearly observed at both the front and rear surfaces of thin solid targets, suggesting that the effect of the density and composition of the targets are also important for correctly simulating energetic ion generation in ultraintense laser-solid interactions.
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52.38.Kd Laser-plasma acceleration of electrons and ions
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.65.Rr Particle-in-cell method
52.40.Kh Plasma sheaths
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.38.Ph X-ray, γ-ray, and particle generation

Two-dimensional electromagnetic model of a microwave plasma reactor operated by an axial injection torch

R. Álvarez and L. L. Alves

J. Appl. Phys. 101, 103303 (2007); http://dx.doi.org/10.1063/1.2732508 (6 pages) | Cited 10 times

Online Publication Date: 23 May 2007

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This paper presents a two-dimensional electromagnetic model for a microwave (2.45 GHz) plasma reactor operated by an axial injection torch. The model solves Maxwell’s equations, adopting a harmonic time description and considering the collision dispersion features of the plasma. Perfect-conductor boundary conditions are satisfied at the reactor walls, and absorbing boundary conditions are used at the open end of the coaxial waveguide powering the system. Simulations yield the distribution of the electromagnetic fields and the average power absorbed by the system for a given spatial profile of the plasma density (tailored from previous experimental measurements), with maximum values in the range 1014−1015 cm−3. Model results reveal that the system exhibits features similar to those of an air-filled, one-end-shorted circular metal waveguide, supporting evanescent or oscillatory solutions for radial dimensions below or above a critical radius, respectively. Results also show that the fractional average power absorbed by the plasma is strongly influenced by the system dimensions, which play a major role in defining the geometry pattern of the electromagnetic field distribution. Simulations are used to provide general guidelines for device optimization.
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52.75.Hn Plasma torches
52.50.Dg Plasma sources
52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.25.Fi Transport properties
52.40.Fd Plasma interactions with antennas; plasma-filled waveguides

Effect of water on sulfur dioxide (SO2) and nitrogen oxides (NOx) removal from flue gas in a direct current corona discharge reactor

Jiaxiang Yang, Xiaochun Chi, and Limin Dong

J. Appl. Phys. 101, 103304 (2007); http://dx.doi.org/10.1063/1.2733762 (5 pages) | Cited 1 time

Online Publication Date: 23 May 2007

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A direct current (dc) corona discharge reactor composed of needle-plate electrodes in a glass container filled with flue gas was designed. To clarify the influence of water on discharge characteristics, water was introduced in the plasma reactor as electrode where plate electrode is immersed, under the application of dc voltage. Experiment results show that (1) corona wind forming between high-voltage needle electrode and water by corona discharge enhances the cleaning efficiency of flue gas due to the existence of water and the cleaning efficiency will increase with the increase of applied dc voltage within definite range and (2) both removal efficiencies of NOx and SO2 increased in the presence of water, which reach up to 98% for SO2, and about 85% for NOx under suitable conditions. These results play an important role in flue gas cleanup research.
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52.80.Hc Glow; corona
52.50.Dg Plasma sources
52.77.Bn Etching and cleaning
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)

Optical emission spectroscopy characterization of oxygen plasma during degradation of Escherichia coli

D. Vujošević, M. Mozetič, U. Cvelbar, N. Krstulović, and S. Milošević

J. Appl. Phys. 101, 103305 (2007); http://dx.doi.org/10.1063/1.2732693 (7 pages) | Cited 20 times

Online Publication Date: 31 May 2007

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Optical emission spectroscopy was applied for plasma characterization during sterilization of substrates contaminated with bacteria. The amount of 1010/ml cells of Escherichia coli was carefully applied to glass substrates and exposed to oxygen plasma glow discharge at different pressures between 30 and 200 Pa. Plasma was created in a glass discharge tube by an inductively coupled rf generator at the frequency of 27.12 MHz and output power of about 250 W. The electron temperature and plasma density were estimated with a double Langmuir probe. They were between 3 and 5 eV and 2 and 35×1015 m−3. Density of neutral oxygen atoms was measured with a catalytic probe, and was between 2 and 6×1021 m−3. Optical emission spectroscopy was performed with a low resolution spectrometer. The emission from carbon monoxide and nitrogen molecules was used to monitor the evolution of bacteria degradation. Both signals expressed a well defined maximum corresponding to peak erosion of bacteria by plasma radicals. As the sterilization was accomplished, both CO and N2 lines fell below the detection limit of the spectrometer. The bacteria degradation was also monitored by scanning electron microscope (SEM) and culturing. The SEM images corresponded well with the evolution of CO and N2 lines so the optical emission spectroscopy found a reliable tool for monitoring the sterilization process.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
51.70.+f Optical and dielectric properties
52.77.-j Plasma applications
52.25.-b Plasma properties
52.70.Ds Electric and magnetic measurements

Electrowetting on plasma-deposited fluorocarbon hydrophobic films for biofluid transport in microfluidics

P. Bayiati, A. Tserepi, P. S. Petrou, S. E. Kakabakos, K. Misiakos, and E. Gogolides

J. Appl. Phys. 101, 103306 (2007); http://dx.doi.org/10.1063/1.2735682 (9 pages) | Cited 8 times

Online Publication Date: 31 May 2007

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The present work focuses on the plasma deposition of fluorocarbon (FC) films on surfaces and the electrostatic control of their wettability (electrowetting). Such films can be employed for actuation of fluid transport in microfluidic devices, when deposited over patterned electrodes. Here, the deposition was performed using C4F8 and the plasma parameters that permit the creation of films with optimized properties desirable for electrowetting were established. The wettability of the plasma-deposited surfaces was characterized by means of contact angle measurements (in the static and dynamic mode). The thickness of the deposited films was probed in situ by means of spectroscopic ellipsometry, while the surface roughness was provided by atomic force microscopy. These plasma-deposited FC films in combination with silicon nitride, a material of high dielectric constant, were used to create a dielectric structure that requires reduced voltages for successful electrowetting. Electrowetting experiments using protein solutions were conducted on such optimized dielectric structures and were compared with similar structures bearing commercial spin-coated Teflon® amorphous fluoropolymer (AF) film as the hydrophobic top layer. Our results show that plasma-deposited FC films have desirable electrowetting behavior and minimal protein adsorption, a requirement for successful transport of biological solutions in “digital” microfluidics.
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52.77.Dq Plasma-based ion implantation and deposition
68.08.Bc Wetting
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
68.55.-a Thin film structure and morphology
68.03.Cd Surface tension and related phenomena
68.35.B- Structure of clean surfaces (and surface reconstruction)

Optical and electrical characterization of an atmospheric pressure microplasma jet for Ar/CH4 and Ar/C2H2 mixtures

A. Yanguas-Gil, K. Focke, J. Benedikt, and A. von Keudell

J. Appl. Phys. 101, 103307 (2007); http://dx.doi.org/10.1063/1.2714646 (8 pages) | Cited 31 times

Online Publication Date: 31 May 2007

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A rf microplasma jet working at atmospheric pressure has been characterized for Ar, He, and Ar/CH4 and Ar/C2H2 mixtures. The microdischarge has a coaxial configuration, with a gap between the inner and outer electrodes of 250 μm. The main flow runs through the gap of the coaxial structure, while the reactive gases are inserted through a capillary as inner electrode. The discharge is excited using a rf of 13.56 MHz, and rms voltages around 200–250 V and rms currents of 0.4–0.6 A are obtained. Electron densities around 8×1020 m−3 and gas temperatures lower than 400 K have been measured using optical emission spectroscopy for main flows of 3 slm and inner capillary flows of 160 SCCM. By adjusting the flows, the flow pattern prevents the mixing of the reactive species with the ambient air in the discharge region, so that no traces of air are found even when the microplasma is operated in an open atmosphere. This is shown in Ar/CH4 and Ar/C2H2 plasmas, where no CO and CN species are present and the optical emission spectroscopy spectra are mainly dominated by CH and C2 bands. The ratio of these two species follows different trends with the amount of precursor for Ar/CH4 and Ar/C2H2 mixtures, showing the presence of distinct chemistries in each of them. In Ar/C2H2 plasmas, CHx species are produced mainly by electron impact dissociation of C2H2 molecules, and the CHx/C2Hx ratio is independent of the precursor amount. In Ar/CH4 mixtures, C2Hx species are formed mainly by recombination of CHx species through three-body reactions, so that the CHx/C2Hx ratio depends on the amount of CH4 present in the mixture. All these properties make our microplasma design of great interest for applications such as thin film growth or surface treatment.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.70.Ds Electric and magnetic measurements
52.50.Dg Plasma sources
52.80.Pi High-frequency and RF discharges
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
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Isotope effects between hydrogen and deuterium microwave plasmas on chemical vapor deposition homoepitaxial diamond growth

N. Mizuochi, J. Isoya, J. Niitsuma, T. Sekiguchi, H. Watanabe, H. Kato, T. Makino, H. Okushi, and S. Yamasaki

J. Appl. Phys. 101, 103501 (2007); http://dx.doi.org/10.1063/1.2727380 (5 pages) | Cited 1 time

Online Publication Date: 16 May 2007

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This article shows that replacing hydrogen with deuterium improves the quality of microwave plasma-assisted chemical vapor deposition homoepitaxial diamond. Suppression of point defects in the bulk and of nonepitaxial crystallites and increasing of free-exciton emission intensity were revealed by electron paramagnetic resonance, optical microscopy, and cathodoluminescence, respectively. The isotope effects on the etching rate of diamond by deuterium are also revealed. The isotope effects are discussed from the viewpoint of etching effects.
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52.77.Dq Plasma-based ion implantation and deposition
52.77.Bn Etching and cleaning
81.05.U- Carbon/carbon-based materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.A- Nucleation and growth
81.65.Cf Surface cleaning, etching, patterning

Reactive magnetron sputtering of tungsten disulfide (WS2−x) films: Influence of deposition parameters on texture, microstructure, and stoichiometry

V. Weiß, S. Seeger, K. Ellmer, and R. Mientus

J. Appl. Phys. 101, 103502 (2007); http://dx.doi.org/10.1063/1.2716395 (9 pages) | Cited 4 times

Online Publication Date: 16 May 2007

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Tungsten disulfide (WS2−x) films (0.07 ⩽ x ⩽ 0.7) were prepared by reactive magnetron sputtering from a tungsten target in rare gas/H2S atmospheres and at substrate temperatures up to 620 °C. The nucleation and growth of the films were investigated by in situ energy dispersive x-ray diffraction (EDXRD) and by ex situ techniques such as electron microscopy, elastic recoil detection analysis, and x-ray reflectivity. From the EDXRD analysis it was found that the films always nucleate with the (001) planes, i.e., the van der Waals planes, parallel to the substrate surface. For high deposition rates and/or low substrate temperatures a texture crossover from the (001) to the (100) crystallite orientation occurs during the growth. High deposition rates, low substrate temperatures, or low sputtering pressures lead to a significant lattice expansion of the crystallites in the c direction (up to 3%). This is most probably caused by a disturbed or turbostratic film growth induced by the energetic bombardment during film deposition. Reflected and neutralized energetic ions (Ar0,S0) from the tungsten target and negative ions (S−) accelerated in the cathode dark space constitute the main sources of the energetic bombardment leading to crystallographic defects. The energy of these particles can be tailored by (i) thermalization between target and substrate in the sputtering gas or (ii) by a reduction of the discharge or target voltage, respectively, by high frequency excitation of the plasma. Films deposited under favorable conditions with respect to low particle energies and at substrate temperatures higher than 200 °C exhibit a significant sulfur deficiency of up to about 5 at. % compared to the stoichiometric composition of WS2. This is ascribed to an energetic particle bombardment-induced sulfur desorption from the growing films.
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81.15.Cd Deposition by sputtering
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.82.-d Radiation effects on specific materials
61.80.Jh Ion radiation effects

Excitation-wavelength-dependent photoluminescence evolution of CdSe/ZnS nanoparticles

Kazunari Ozasa, Shigeyuki Nemoto, Mizuo Maeda, and Masahiko Hara

J. Appl. Phys. 101, 103503 (2007); http://dx.doi.org/10.1063/1.2732543 (6 pages) | Cited 8 times

Online Publication Date: 16 May 2007

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The wavelength-dependent photoluminescence (PL) evolution of CdSe/ZnS (core/shell) nanoparticles (NPs) was investigated from the viewpoint of excitation wavelength dependence. In addition, the mechanisms involved in the photoinduced changes of the NP PL spectrum are discussed. The NPs of CdSe/ZnS placed on a thin film of an insulator (GaAsOx/GaAs or SiO2/Si) exhibited PL decay and blueshifts on a time scale of minutes depending on the excitation wavelength, i.e., higher photon energy induces a faster PL decay with a larger and faster blueshift. The PL decay with 435 nm excitation was a single exponential in temporal evolution, while the delayed acceleration of decay was observed with 365 nm excitation. The excitation-wavelength dependence of the blueshift was much larger than that of the PL decay, indicating that they originate from independent photoinduced reactions. This was confirmed by an ultrahigh vacuum measurement where only PL decay was observed without any blueshift. This result clearly reveals that the blueshift is caused by photoinduced oxidation with atmospheric oxygen. Similar temporal PL evolutions were observed in single-NP measurements as gradual changes in colors. The most probable mechanism for PL decay is the photoinduced extraction of surface-protecting molecules from the NPs, which induces nonradiative recombinations of photoexcited electron-hole pairs.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Et II-VI semiconductors

Correlation between luminescence and structural evolution of Si-rich silicon oxide film annealed at different temperatures

Minghua Wang, Deren Yang, Dongsheng Li, Zhizhong Yuan, and Duanlin Que

J. Appl. Phys. 101, 103504 (2007); http://dx.doi.org/10.1063/1.2732544 (4 pages) | Cited 16 times

Online Publication Date: 16 May 2007

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The visible red band light emission of the Si-rich silicon oxide films prepared by electron-beam evaporation with postannealing was investigated. The films annealed at temperatures lower than 600 °C were composed of Si nanoclusters with high fraction of boundary atoms, featuring photoluminescence (PL) band centered at 700–710 nm. By contrast, the PL bands of the films annealed at temperatures higher than 600 °C redshifted with the expansion of the Si nanoclusters. The most intense light emission was observed in the 600 °C annealed sample. The conversion of the luminescence mechanism from surface states controlled to quantum confinement controlled was proposed on the basis of the evolution of the film structure.
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78.66.Nk Insulators
78.55.Hx Other solid inorganic materials
81.05.-t Specific materials: fabrication, treatment, testing, and analysis
81.40.Gh Other heat and thermomechanical treatments
73.20.At Surface states, band structure, electron density of states
68.55.-a Thin film structure and morphology

Observation of phonon modes in epitaxial PbTe films grown by molecular beam epitaxy

Huizhen Wu, Chunfang Cao, Jianxiao Si, Tianning Xu, Hanjie Zhang, Haifei Wu, Jing Chen, Wenzhong Shen, and Ning Dai

J. Appl. Phys. 101, 103505 (2007); http://dx.doi.org/10.1063/1.2714682 (5 pages) | Cited 5 times

Online Publication Date: 17 May 2007

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Phonon modes of PbTe films grown by molecular beam epitaxy have been studied by micro-Raman scattering. On the as-grown PbTe surface, strong TeO2 phonon vibrational modes were detected, which obscured the observation of the longitudinal optical (LO) phonons of PbTe in early conventional Raman scattering experiments. Existence of a TeO2 layer on the PbTe surface is confirmed by observation with x-ray photoemission spectroscopy. After removal of TeO2 by chemical etching, the LO phonons for PbTe films were unambiguously observed. Misfit strain accommodated in the epitaxial films makes the lattice distorted from cubic structure, which lowers the crystal symmetry and leads to observation of what would normally be Raman inactive LO phonon modes for PbTe.
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63.20.D- Phonon states and bands, normal modes, and phonon dispersion
78.30.Hv Other nonmetallic inorganics
68.55.-a Thin film structure and morphology
79.60.Bm Clean metal, semiconductor, and insulator surfaces
81.65.Cf Surface cleaning, etching, patterning
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
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