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1 Mar 2010

Volume 107, Issue 5, Articles (05xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 107, 051101 (2010); http://dx.doi.org/10.1063/1.3304835 (14 pages)

Brian D. Sosnowchik, Liwei Lin, and Ongi Englander
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Localized heating induced chemical vapor deposition for one-dimensional nanostructure synthesis

Brian D. Sosnowchik, Liwei Lin, and Ongi Englander

J. Appl. Phys. 107, 051101 (2010); http://dx.doi.org/10.1063/1.3304835 (14 pages) | Cited 8 times

Online Publication Date: 5 March 2010

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Localized heating has emerged as a viable technique for the site specific synthesis of one-dimensional (1D) nanostructures. By localizing the heat source, the extent of chemical vapor deposition synthesis reactions can be confined to well-defined, microscale regions. Resistive heating has been extensively used to realize highly localized regions of elevated temperature while maintaining a microelectronics-compatible thermal environment elsewhere. Other localized heating methods are being pursued as well. Overall, the approach is simple, flexible, and robust, and offers unique opportunities in 1D nanostructure synthesis, characterization, and integration. Herein, the recent progress of these techniques is reviewed and discussed.
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81.07.Bc Nanocrystalline materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
07.20.Hy Furnaces; heaters
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Extraordinary transmission in the ultraviolet range from subwavelength slits on semiconductors

M. A. Vincenti, D. de Ceglia, M. Buncick, N. Akozbek, M. J. Bloemer, and M. Scalora

J. Appl. Phys. 107, 053101 (2010); http://dx.doi.org/10.1063/1.3318460 (6 pages) | Cited 7 times

Online Publication Date: 1 March 2010

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In this paper, we describe a way to achieve the extraordinary transmission regime well into the UV range using subwavelength slits carved on semiconductor substrates. Unlike metals, the dielectric permittivity of typical semiconductors like GaAs or GaP is negative beginning in the extreme UV range (λ ≤ 270 nm) and is characterized by large absorption. We show that the metallike response of bulk semiconductors exhibits surface plasmon waves that lead to extraordinary transmission in the UV and soft x-ray ranges, in spite of the large absorption. The importance of realistic material response versus perfect conductors is also discussed. We show that the use of perfect conductor boundary conditions can lead to field amplitudes that may be underestimated by several orders of magnitudes. These findings may be important in high resolution photolithography, near-field optical devices, and ultrahigh density optical storage in wavelength ranges, where it may not be possible to utilize more traditional plasmonic materials like noble metals.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
77.22.Ch Permittivity (dielectric function)

White-emitting oxidized silicon nanocrystals: Discontinuity in spectral development with reducing size

K. Dohnalová, L. Ondič, K. Kůsová, I. Pelant, J. L. Rehspringer, and R.-R. Mafouana

J. Appl. Phys. 107, 053102 (2010); http://dx.doi.org/10.1063/1.3289719 (6 pages) | Cited 7 times

Online Publication Date: 1 March 2010

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Small oxidized silicon nanocrystals of average sizes below 3.5 nm are prepared using modified electrochemical etching of a silicon wafer. Modifications introduced in the etching procedure together with postetching treatment in H2O2 lead to a decrease in the nanocrystalline core size and also, to some extent, to changes in the surface oxide. The interplay between these two factors allows us to blueshift the photoluminescence (PL) spectrum from 680 down to 590 nm, which is accompanied by changes in PL dynamics. This continual development, however, stops at about 590 nm, below which abrupt switching to fast decaying blue emission band at about 430 nm was observed. Discontinuity of the spectral shift and possible relation between both bands are discussed.
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81.07.Bc Nanocrystalline materials
81.65.Cf Surface cleaning, etching, patterning
61.46.-w Structure of nanoscale materials
81.16.-c Methods of micro- and nanofabrication and processing
78.55.Ap Elemental semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters

Energy level decay and excited state absorption processes in dysprosium-doped fluoride glass

Laércio Gomes, André Felipe Henriques Librantz, and Stuart D. Jackson

J. Appl. Phys. 107, 053103 (2010); http://dx.doi.org/10.1063/1.3311561 (8 pages) | Cited 1 time

Online Publication Date: 2 March 2010

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The primary excited state decay processes relating to the math13/2→math15/2 ∼ 3 μm laser transition in singly Dy3+-doped fluoride (ZBLAN) glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the math9/2, math7/2 energy levels at 1125 nm and math11/2, math9/2 energy levels at 1358 nm established that the energy levels above the math11/2 level, excluding the math9/2 level, are entirely quenched by multiphonon emission in ZBLAN glass. The math11/2 and math13/2 energy levels emit luminescence with peaks at ∼ 1700 and ∼ 2880 nm, respectively, but at low quantum (luminescence) efficiencies. The quantum efficiency of the math11/2 level and math13/2 level is ∼ 9×10−5 and ∼ 1.3×10−2, respectively, for [Dy3+] = 0.5 mol % based on calculations of the radiative lifetimes using the Judd–Ofelt theory. Excited state absorption (ESA) was detected by monitoring the rise time of the 1700 nm luminescence after tuning the probe wavelength across the spectral range from 1100 to 1400 nm. As a result of nonradiative decay of the higher excited states, ESA contributes to the heating of ∼ 3 μm fiber lasers based on Dy3+-doped fluoride glass. For [Dy3+] up to 4 mol %, we found no evidence of energy transfer processes between Dy3+ ions that influence the decay characteristics of the math11/2 and math13/2 energy levels.
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78.55.Qr Amorphous materials; glasses and other disordered solids
78.55.Hx Other solid inorganic materials
78.47.da Excited states

Analytical solution for mode-mismatched thermal lens spectroscopy with sample-fluid heat coupling

Luis C. Malacarne, Nelson G. C. Astrath, Paulo R. B. Pedreira, Renio S. Mendes, Mauro L. Baesso, Prakash R. Joshi, and Stephen E. Bialkowski

J. Appl. Phys. 107, 053104 (2010); http://dx.doi.org/10.1063/1.3309762 (7 pages) | Cited 1 time

Online Publication Date: 3 March 2010

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This paper presents an improved theoretical description of the mode-mismatched thermal lens effect using models that account for heat transport both within the sample and out to the surrounding coupling medium. Analytical and numerical finite element analysis (FEA) solutions are compared and subsequently used to model the thermal lens effect that would be observed using continuous laser excitation. FEA model results were found to be in excellent agreement with the analytical solutions. The model results show that heat transfer to the air coupling medium introduces only a minor effect when compared with the solution obtained without considering axial air-sample heat flux for practical examples. On the other hand, the thermal lens created in the air coupling fluid has a relatively more significant effect on the time-dependent photothermal lens signals.
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78.20.N- Thermo-optic effects
42.65.-k Nonlinear optics

Position-dependent coupling between a channel waveguide and a distorted microsphere resonator

Ganapathy Senthil Murugan, Yuwapat Panitchob, Elizabeth J. Tull, Philip N. Bartlett, Daniel W. Hewak, Michalis N. Zervas, and James S. Wilkinson

J. Appl. Phys. 107, 053105 (2010); http://dx.doi.org/10.1063/1.3327418 (9 pages) | Cited 5 times

Online Publication Date: 4 March 2010

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Glass microsphere resonators have the potential to add significant functionality to planar lightwave circuits when coupled to waveguides where they can provide wavelength filtering, delay and low-power switching, and laser functions. Design of such photonic circuits requires precise coupling between spheres and waveguides to allow control of Q-factor and hence of stored energy and resonator bandwidth. In this paper an erbium-doped silicate glass microsphere is coupled to an ion-exchanged glass waveguide, and excitation spectra for the sphere whispering-gallery modes are determined as a function of spatial separation. Modal assignment allows extraction of the physical parameters of the microsphere and the dependence of Q-factor with separation is compared with theory. All practical microspheres exhibit a small degree of ellipticity and the effects of this upon whispering-gallery mode excitation and wavelength splitting are explored. It has been shown that appropriate displacement and orientation of slightly deformed microspheres with respect to the waveguide can be used to control the effective Q-factor and optimize the spectral shape of the optical devices. This can result in either single high-Q peaks or substantially broadened and spectrally flattened resonances.
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84.40.Az Waveguides, transmission lines, striplines
42.79.Gn Optical waveguides and couplers
42.82.Et Waveguides, couplers, and arrays
42.55.-f Lasers
42.82.-m Integrated optics

Thermal annealing of arsenic tri-sulphide thin film and its influence on device performance

Duk-Yong Choi, Steve Madden, Douglas Bulla, Rongping Wang, Andrei Rode, and Barry Luther-Davies

J. Appl. Phys. 107, 053106 (2010); http://dx.doi.org/10.1063/1.3310803 (6 pages) | Cited 4 times

Online Publication Date: 4 March 2010

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Arsenic tri-sulphide (As2S3) thin film waveguides have been used successfully as nonlinear optical devices for all-optical signal processors. For such devices, low propagation loss is vital if high performance is to be obtained. In this study, thermal annealing was employed not only to stabilize the physical properties of the films, but also to reduce the sources of light attenuation in the as-deposited material. Here we investigated heat-induced changes to the microstructure and optical properties of As2S3 thin films and, based on this information, determined the best annealing conditions. The refractive index of the films rises with annealing due to thermal densification and increased heteropolar bond density. The growth of surface roughness and thermal stress in the film, however, limits the annealing temperature to ∼ 130 °C. We fabricated and analyzed waveguides produced from as-deposited and annealed films and found that the propagation loss of the guides dropped by ∼ 0.2 dB/cm as a result of appropriate annealing. Rayleigh scattering and absorption from defects associated with phase separation, homopolar bonds, voids, and dangling bonds in the as-deposited film are shown to contribute to the higher light attenuation in unannealed films.
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78.66.Jg Amorphous semiconductors; glasses
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.35.+c Brillouin and Rayleigh scattering; other light scattering
71.55.Ht Other nonmetals
61.72.Cc Kinetics of defect formation and annealing
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Laser soft marking on silicon wafer

L. E. Khoong, Y. C. Lam, H. Y. Zheng, and X. Chen

J. Appl. Phys. 107, 053107 (2010); http://dx.doi.org/10.1063/1.3319611 (7 pages)

Online Publication Date: 5 March 2010

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A laser soft marking technique is developed for laser markings on a silicon wafer. Due to negligible surface modification, the laser soft wafer markings are invisible by naked eyes under room condition and are undetectable using sophisticated instruments. However, these laser markings are found to be visible to naked eyes through a differential condensation of water droplets on the laser-marked and unmarked silicon surfaces. To understand this phenomenon, a model is established to study the condensation of water droplets on laser-marked and unmarked silicon surfaces. Experimental observations and simulation results indicate that the laser soft marking could have modified the silicon surface with a thin polycrystalline silicon layer which has a much lower conductivity than the crystalline silicon. In addition, this thin layer exhibits a thermal conductivity which is approximately two orders of magnitude lower than that of its equivalent bulk material. As a result, heat transfer on the laser-marked silicon surface is much lower than the crystalline silicon and thus makes these laser soft markings easily visible visually under condensation.
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81.05.Cy Elemental semiconductors
81.65.-b Surface treatments
66.70.Df Metals, alloys, and semiconductors

Second-harmonic generation in one-dimensional metal gratings with dual extraordinary transmissions

Ming Kang, Yongnan Li, Kai Lou, Si-Min Li, Qiang Bai, Jing Chen, and Hui-Tian Wang

J. Appl. Phys. 107, 053108 (2010); http://dx.doi.org/10.1063/1.3327215 (8 pages)

Online Publication Date: 5 March 2010

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We investigate the enhanced second-harmonic generation (SHG) in nonlinear metal gratings with simultaneously extraordinary optical transmissions (EOTs) for the fundamental and the second-harmonic wavelengths, i.e., dual EOTs. We show that the strongly temporal and spatial dispersions at Wood’s anomalies, the asymmetry in the grating structure and the intrinsic dispersion of the media are of great importance in achieving dual-EOT SHG. Metal gratings with dual EOTs are present and the maximum enhancement on SHG is around 20. Weak points of dual-EOT SHG, potential improvement and future applications are discussed.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.79.Dj Gratings

Si/SiGe quantum cascade superlattice designs for terahertz emission

G. Matmon, D. J. Paul, L. Lever, M. Califano, Z. Ikonić, R. W. Kelsall, J. Zhang, D. Chrastina, G. Isella, H. von Känel, E. Müller, and A. Neels

J. Appl. Phys. 107, 053109 (2010); http://dx.doi.org/10.1063/1.3319653 (7 pages) | Cited 1 time

Online Publication Date: 8 March 2010

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Quantum cascade lasers (QCLs) are compact sources that have demonstrated high output powers at terahertz (THz) frequencies. To date, all THz QCLs have been realized in III-V materials. Results are presented from Si1−xGex quantum cascade superlattice designs emitting at around 3 THz which have been grown in two different chemical vapor deposition systems. The key to achieving successful electroluminescence at THz frequencies in a p-type system has been to strain the light-hole states to energies well above the radiative subband states. To accurately model the emission wavelengths, a 6-band k⋅p tool which includes the effects of nonabrupt heterointerfaces has been used to predict the characteristics of the emitters. X-ray diffraction and transmission electron microscopy have been used along with Fourier transform infrared spectroscopy to fully characterize the samples. A number of methods to improve the gain from the designs are suggested.
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42.55.Px Semiconductor lasers; laser diodes
71.15.-m Methods of electronic structure calculations
78.30.-j Infrared and Raman spectra
78.60.Fi Electroluminescence
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
42.60.By Design of specific laser systems

Properties of TiO2-based transparent conducting oxide thin films on GaN(0001) surfaces

J. Kasai, T. Hitosugi, M. Moriyama, K. Goshonoo, N. L. H. Hoang, S. Nakao, N. Yamada, and T. Hasegawa

J. Appl. Phys. 107, 053110 (2010); http://dx.doi.org/10.1063/1.3326943 (4 pages) | Cited 7 times

Online Publication Date: 9 March 2010

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Anatase Nb-doped TiO2 transparent conducting oxide has been formed on GaN(0001) surfaces using a sputtering method. Amorphous films deposited at room temperature were annealed at a substrate temperature of 500 °C in vacuum to form single-phase anatase films. Films with a thickness of 170 nm exhibited a resistivity of 8×10−4 Ω cm with absorptance less than 5% at a wavelength of 460 nm. Furthermore, the refractive index of the Nb-doped TiO2 was well matched to that of GaN. These findings indicate that Nb-doped TiO2 is a promising material for use as transparent electrodes in GaN-based light emitting diodes (LEDs), particularly since reflection at the electrode/GaN boundary can be suppressed, enhancing the external quantum efficiency of blue LEDs.
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85.60.Jb Light-emitting devices
68.55.A- Nucleation and growth
78.66.Jg Amorphous semiconductors; glasses
81.15.Cd Deposition by sputtering
78.66.Fd III-V semiconductors

Europium luminescence enhancement induced by a resonant mode in a waveguide of planar metallic walls

Aldo S. Ramírez-Duverger, R. Aceves, Raúl García-Llamas, and Jorge A. Gaspar-Armenta

J. Appl. Phys. 107, 053111 (2010); http://dx.doi.org/10.1063/1.3309839 (4 pages)

Online Publication Date: 10 March 2010

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We report the experimental results of emission from europium ions inside a planar waveguide. The waveguide is constituted by a MgF2:Eu2+ film between Al metallic walls, with a film thickness appropriate to support a guided mode with wavelength inside of the excitation spectrum of the Eu ions. A guided mode is produced by incident light through a thin metallic film at an angle of incidence such that its wave vector component parallel to the waveguide couples resonantly to the mode. The luminescence of Eu2+ in the waveguide of polycrystalline MgF2 was found to have a maximum at 440 nm. This emission was ascribed to electronic transitions of Eu2+ ions dipoles on a lattice of MgF2. For an excitation light of 360 nm, the integrated emission versus angle of incidence showed a maximum for an incident light angle of 20°. This behavior is explained because of the excitation light, for those wavelength and angle of incidence, couple to a mode along the waveguide and more emitters are excited in the optical path of the guided light.
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78.55.Hx Other solid inorganic materials
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
84.40.Az Waveguides, transmission lines, striplines

Light emission polarization properties of semipolar InGaN/GaN quantum well

Hung-Hsun Huang and Yuh-Renn Wu

J. Appl. Phys. 107, 053112 (2010); http://dx.doi.org/10.1063/1.3327794 (7 pages) | Cited 5 times

Online Publication Date: 11 March 2010

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As many reports show that the InGaN quantum wells grown on semipolar substrate have better efficiency in the green spectrum, it is important to understand the light emission properties of these semipolar quantum wells. In this paper, we have studied the optical characteristics of a semipolar InGaN/GaN quantum well with different growth orientations. Also, the most common growth directions such as (10mathmath) and (11math2) planes are studied in details. The self-consistent Poisson and 6×6 k⋅p Schrödinger solver has been applied to study the band structure of the semipolar InGaN-based quantum well. We find that the light emission polarization ratio has a very interesting switching behavior under different conditions of indium compositions, quantum well widths, and injection carrier densities. Our results show that the semipolar InGaN quantum well has a potential to be a polarized light source under certain conditions.
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78.67.De Quantum wells
81.07.St Quantum wells
73.63.Hs Quantum wells
73.21.Fg Quantum wells
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Photoinitiation study of Irgacure 784 in an epoxy resin photopolymer

Dušan Sabol, Michael R. Gleeson, Shui Liu, and John T. Sheridan

J. Appl. Phys. 107, 053113 (2010); http://dx.doi.org/10.1063/1.3276173 (8 pages) | Cited 4 times

Online Publication Date: 12 March 2010

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A deeper understanding of the processes, which occur during free radical photopolymerization, is necessary in order to develop a fully comprehensive model, which represents their behavior during exposure. One of these processes is photoinitiation, whereby a photon is absorbed by a photosensitizer producing free radicals, which can initiate polymerization. These free radicals can also participate in polymer chain termination (primary termination), and it is therefore necessary to understand their generation in order to predict the temporally varying kinetic effects present during holographic grating formation. In this paper, a study of the photoinitiation mechanisms of Irgacure 784 photosensitizer, in an epoxy resin matrix, is presented. We report our experimental results and present a theoretical model to predict the physically observed behavior.
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42.70.Jk Polymers and organics
82.35.-x Polymers: properties; reactions; polymerization
82.50.-m Photochemistry
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions

Measurement of extraction and absorption parameters in GaN-based photonic-crystal light-emitting diodes

Elison Matioli, Blaise Fleury, Elizabeth Rangel, Evelyn Hu, James Speck, and Claude Weisbuch

J. Appl. Phys. 107, 053114 (2010); http://dx.doi.org/10.1063/1.3309837 (6 pages) | Cited 3 times

Online Publication Date: 12 March 2010

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The light extraction efficiency of photonic-crystal (PhC) light-emitting diodes (LEDs) relies on the competition between the PhC extraction and dissipation mechanisms of the guided light within the LED. This work presents the experimental determination of the PhC extraction length of each guided mode and the absorption coefficient of the active region (AR) and quantum wells (QWs) from the observation of the LED far-field emission using a high-resolution angle-spectrum-resolved measurement. The angular and spectral linewidths of the extracted guided modes reveal, depending on the spectral range, the modal extraction length of the PhCs, the AR absorption length, or a combination of both. Modes with a high confinement with the QWs presented a shorter absorption length compared with their extraction length by a shallow surface PhC (95-nm-deep), meaning that the AR absorption was a more efficient mechanism than the PhC extraction. The measured modal extraction length of the shallow surface PhC varied in the range of 55–120 μm, which determines the minimum dimensions of the device and the maximum acceptable dissipation length for an efficient extraction of the guided light by the PhCs. This paper presents also a discussion on the PhC designs that yield PhC extraction lengths shorter than other dissipation lengths, a fundamental requirement for high-efficiency PhC LEDs. The same technique was also applied to estimate the absorption coefficient of the InGaN-based QWs, and can be extended to experimentally determine losses by metallic layers from electrical contacts or other dissipation mechanisms, which are parameters of interest to a broader class of optoelectronic devices, not only PhC LEDs.
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85.60.Jb Light-emitting devices
42.70.Qs Photonic bandgap materials

All-optical switching with bacteriorhodopsin protein coated microcavities and its application to low power computing circuits

Sukhdev Roy, Mohit Prasad, Juraj Topolancik, and Frank Vollmer

J. Appl. Phys. 107, 053115 (2010); http://dx.doi.org/10.1063/1.3310385 (9 pages) | Cited 5 times

Online Publication Date: 12 March 2010

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We show all-optical switching of an input infrared laser beam at 1310 nm by controlling the photoinduced retinal isomerization to tune the resonances in a silica microsphere coated with three bacteriorhodopsin (BR) protein monolayers. The all-optical tunable resonant coupler re-routes the infrared beam between two tapered fibers in 50 μs using a low power (<200 μW) green (532 nm) and blue (405 nm) pump beams. The basic switching configuration has been used to design all-optical computing circuits, namely, half and full adder/subtractor, de-multiplexer, multiplexer, and an arithmetic unit. The design requires 2n−1 switches to realize n bit computation. The designs combine the exceptional sensitivities of BR and high-Q microcavities and the versatile tree architecture for realizing low power circuits and networks (approximately mW power budget). The combined advantages of high Q-factor, tunability, compactness, and low power control signals, with the flexibility of cascading switches to form circuits, and reversibility and reconfigurability to realize arithmetic and logic functions, makes the designs promising for practical applications. The designs are general and can be implemented (i) in both fiber-optic and integrated optic formats, (ii) with any other coated photosensitive material, or (iii) any externally controlled microresonator switch.
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42.79.Ta Optical computers, logic elements, interconnects, switches; neural networks
42.79.Gn Optical waveguides and couplers
42.65.Pc Optical bistability, multistability, and switching, including local field effects

Short-wavelength infrared second harmonic generation in quantum cascade lasers

Yong-Hee Cho (조용희) and Alexey Belyanin

J. Appl. Phys. 107, 053116 (2010); http://dx.doi.org/10.1063/1.3309870 (7 pages)

Online Publication Date: 15 March 2010

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We propose an electrically pumped intersubband laser capable of operating at short infrared wavelengths ∼ 1.5–2.5 μm. Short-wavelength operation is achieved via resonant intracavity second harmonic generation in quantum cascade (QC) lasers based on high band offset heterostructures. This approach overcomes the fundamental problem of intervalley scattering that prohibits direct lasing in QC lasers in this spectral range. The proposed devices can be modulated at a rate exceeding 100 GHz, which may be interesting for a variety of applications. We present detailed calculations of bandstructure design, phase-matched waveguide, and current-output power dependence for the devices based on GaInAs/AlAsSb/InP heterostructures. Accurate position of highly excited subbands is determined by calculating the conduction band (CB) dispersion with the energy-dependent 14-band effective mass, in which matrix elements are adjusted to fit the CB structure obtained with a 30-band k⋅p method. We also discuss the controversial location of lateral valleys, which imposes the limitation on the fundamental laser transition energy.
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42.55.Px Semiconductor lasers; laser diodes
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.25.Fx Diffraction and scattering

Selective absorption enhancement in organic solar cells using light incoupling layers

Jan Meiss, Mauro Furno, Steffen Pfuetzner, Karl Leo, and Moritz Riede

J. Appl. Phys. 107, 053117 (2010); http://dx.doi.org/10.1063/1.3311559 (7 pages) | Cited 7 times

Online Publication Date: 15 March 2010

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We show that capping layers of tris-(8-hydroxy-quinolinato)-aluminum Alq3 enable increased absorption and photocurrent in organic solar cells (OSCs) when using transparent metal films as top electrodes. Furthermore, by varying the capping layer thickness, the optical field in the OSC is tuned for selective wavelengths, opening a possibility of influencing the external quantum efficiency for specific absorber materials. It is described how a second maximum of the optical field intensity can be utilized, which is a concept significant for tandem solar cells. Indium tin oxide (ITO)-free OSCs are presented which show the influence of capping layer on efficiency, saturation, fill factor, and open-circuit voltage, with numerical calculations supporting the experimental evidence of layer-selective enhancement.
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88.40.hj Efficiency and performance of solar cells
back to top Plasmas and Electrical Discharges

Plasma enhanced chemical vapor deposition of wear resistant gradual a-Si1−x:Cx:H coatings on nickel-titanium for biomedical applications

Benedikt Niermann, Marc Böke, Janine-Christina Schauer, and Jörg Winter

J. Appl. Phys. 107, 053301 (2010); http://dx.doi.org/10.1063/1.3310641 (6 pages)

Online Publication Date: 1 March 2010

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Plasma enhanced chemical vapor deposition has been used to deposit thin films with gradual transitions from silicon to carbon on Cu, Ni, stainless steel, and NiTi. Thus show low stress, elasticity, and wear resistance with excellent adhesion on all metals under investigation. Already at low Si concentrations of 10 at. % the intrinsic stress is considerably reduced compared to pure diamondlike carbon (DLC) films. The deposition process is controlled by optical emission spectroscopy. This technique has been applied to monitor the growth precursors and to correlate them with the film composition. The compositions of the films were determined by Rutherford backscattering spectroscopy and XPS measurements. Due to the elastic properties of the gradual transition and the excellent biocompatibility of DLC, the described film systems present a useful coating for biomedical applications.
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68.55.ag Semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.40.Pq Friction, lubrication, and wear
68.60.Bs Mechanical and acoustical properties
87.85.jj Biocompatibility
52.77.Dq Plasma-based ion implantation and deposition

Influence of inhomogeneous magnetic field on the characteristics of very high frequency capacitively coupled plasmas

Kallol Bera, Shahid Rauf, Jason Kenney, Leonid Dorf, and Ken Collins

J. Appl. Phys. 107, 053302 (2010); http://dx.doi.org/10.1063/1.3296349 (7 pages) | Cited 2 times

Online Publication Date: 2 March 2010

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The effect of inhomogeneous magnetic field on the spatial structure of very high frequency (VHF) plasmas is investigated for different coil configurations, gas pressures, high frequency bias powers, and degrees of electronegativity. The simulation results show that the electron density peaks in the center of the chamber for VHF plasmas due to the standing electromagnetic wave effect. On application of a magnetic field, the density increases near the wafer edge and decreases at the chamber center. The radial magnetic field component is found to limit electron loss to the electrodes and locally enhance the electron density. The axial magnetic field component limits plasma diffusion in the radial direction helping preserve the effect of improved electron confinement by the radial magnetic field. The peak electron density decreases with increasing magnetic field as the plasma moves toward the electrode edge occupying a larger volume. The effect of magnetic field becomes weaker at higher pressure due to the increased electron-neutral collisions which reduce the effectiveness of electron confinement around the magnetic field lines. The impact of magnetic field on plasma profile is somewhat weaker in an electronegative Ar/CF4 plasma because of the presence of less mobile and unmagnetized negative ions.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.-y Plasma simulation
52.20.Fs Electron collisions
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.55.Dy General theory and basic studies of plasma lifetime, particle and heat loss, energy balance, field structure, etc.

A reconfigurable plasma antenna

Rajneesh Kumar and Dhiraj Bora

J. Appl. Phys. 107, 053303 (2010); http://dx.doi.org/10.1063/1.3318495 (9 pages) | Cited 2 times

Online Publication Date: 3 March 2010

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An experiment aimed at investigating the antenna properties of different plasma structures of a plasma column as a reconfigurable plasma antenna, is reported. A 30 cm long plasma column is excited by surface wave, which acts as a plasma antenna. By changing the operating parameters, e.g., working pressure, drive frequency, input power, radius of glass tube, length of plasma column, and argon gas, single plasma antenna (plasma column) can be transformed to multiple small antenna elements (plasma blobs). It is also reported that number, length, and separation between two antenna elements can be controlled by operating parameters. Moreover, experiments are also carried out to study current profile, potential profile, conductivity profile, phase relations, radiation power patterns, etc. of the antenna elements. The effect on directivity with the number of antenna elements is also studied. Findings of the study indicate that entire structure of antenna elements can be treated as a phased array broadside vertical plasma antenna, which produces more directive radiation pattern than the single plasma antenna as well as physical properties and directivity of such antenna can be controlled by operating parameters. The study reveals the advantages of a plasma antenna over the conventional antenna in the sense that different antennas can be formed by tuning the operating parameters.
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52.40.Fd Plasma interactions with antennas; plasma-filled waveguides
84.40.Ba Antennas: theory, components and accessories

Characteristics of a magnetohydrodynamic electrical power generator using convexly divergent channel

Tomoyuki Murakami and Yoshihiro Okuno

J. Appl. Phys. 107, 053304 (2010); http://dx.doi.org/10.1063/1.3311567 (6 pages) | Cited 2 times

Online Publication Date: 4 March 2010

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We describe a magnetohydrodynamic (MHD) electrical power generator equipped with a convexly divergent channel, as determined through shock-tunnel-based experiments. The slight enhancement in a MHD channel divergence upstream provides boundary layer relief in a MHD flow decelerated by a retarding Lorentz force. Despite the present approach being simple and requiring a relatively minor modification of the MHD channel profile, the quality of MHD power-generating plasma and the energy conversion efficiency are improved compared to those from a previous linearly divergent channel; an excessive increase in static pressure is suppressed and a Hall field is enhanced, whereby notably high isentropic efficiency is achieved.
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84.60.Lw Magnetohydrodynamic conversion
84.70.+p High-current and high-voltage technology: power systems; power transmission lines and cables

Gas temperature and electron density profiles in an argon dc microdischarge measured by optical emission spectroscopy

Sergey G. Belostotskiy, Tola Ouk, Vincent M. Donnelly, Demetre J. Economou, and Nader Sadeghi

J. Appl. Phys. 107, 053305 (2010); http://dx.doi.org/10.1063/1.3318498 (7 pages) | Cited 7 times

Online Publication Date: 5 March 2010

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Optical emisssion spectroscopy was employed to study a high pressure (100 s of Torr), slot-type (600 μm interelectrode gap), argon dc microdischarge, with added traces of nitrogen. Spatially resolved gas temperature profiles were obtained by analyzing rovibrational bands of the N2 first positive system. The gas temperature peaked near the cathode and increased with current. The contribution of Stark broadening to the hydrogen Hβ emission lineshape was used to extract the electron density. The axial distribution of electron density as well as visual observation revealed that the microdischarge positive column was highly constricted. The electron density near the sheath edge increased with both pressure and current.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.40.Hf Plasma-material interactions; boundary layer effects
52.40.Kh Plasma sheaths
52.80.-s Electric discharges

Zeff profile diagnostics using visible bremsstrahlung continuum for nonaxisymmetric plasmas with finite β in large helical device

H. Y. Zhou, S. Morita, M. Goto, and C. F. Dong

J. Appl. Phys. 107, 053306 (2010); http://dx.doi.org/10.1063/1.3326970 (11 pages) | Cited 5 times

Online Publication Date: 9 March 2010

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An astigmatism-corrected Czerny–Turner-type visible spectrometer coupled with a charge-coupled device has been installed in large helical device (LHD) to measure visible bremsstrahlung continuum. A full vertical profile has been observed from the elliptical plasmas at horizontally elongated plasma cross section through a 44 fiber parallel array with vertical observation length of ∼ 1 m. Line emissions can be easily eliminated by use of the visible spectrometer instead of an interference filter. A nonmonotonic bremsstrahlung emission profile, which is originated in the thick ergodic layer surrounding the main plasma, has been observed for normal discharges in all the magnetic configurations of LHD. After analyzing the detailed structure, the lower half of the vertical bremsstrahlung emission profile is found to be free of the strong edge bremsstrahlung emission in inwardly shifted magnetic configurations (Rax ≤ 3.60 m). The nonmonotonic bremsstrahlung emission disappeared in extremely high-density discharges (ne ≥ 1014 cm−3) because of the reduction of the plasma outside boundary. When the local emissivity is calculated using Abel inversion, the solution is very sensitive to the distortion of the magnetic surface structure. The influence of the magnetic surface distortion based on the finite β effect is examined as error estimation in addition to unclear edge plasma boundary due to the presence of the ergodic layer. The result indicates that the determination of the normalized minor radius for each observation chord gives a larger influence on the Abel inversion rather than the determination of the chord length. When the observed chord-integrated bremsstrahlung intensity profile is flat, the resultant uncertainty seen in the bremsstrahlung emissivity profile becomes large, in particular, at the plasma center. The Zeff profile calculated with consideration of density and temperature profiles is verified in neutral-beam-heated discharges with H2 and C pellet injections, where the Zeff values should be close to 1 and 6, respectively. Analysis on the flat and hollow electron density profiles results in a fairly flat Zeff profile.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.55.Jd Magnetic mirrors, gas dynamic traps
52.40.Hf Plasma-material interactions; boundary layer effects
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Entropy change linked to the magnetic field induced martensitic transformation in a Ni–Mn–In–Co shape memory alloy

V. Recarte, J. I. Pérez-Landazábal, S. Kustov, and E. Cesari

J. Appl. Phys. 107, 053501 (2010); http://dx.doi.org/10.1063/1.3318491 (4 pages) | Cited 7 times

Online Publication Date: 2 March 2010

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Experimental results on the temperature dependence of the entropy change induced by magnetic field in a Ni–Mn–In–Co ferromagnetic shape memory alloy have been analyzed. Different behaviors of the entropy change ΔS versus temperature have been observed, depending on the value of polarizing magnetic field. In addition, the magnetocaloric effect shows over certain temperature range, a limit value corresponding to the transformation entropy ΔStr. To explain the experimental results, a model, which takes into account the value of the martensitic transformation temperature shift and the transformation temperature range, has been proposed. The model allows to estimate the entropy change as a function of temperature and applied magnetic field from a few experimental data and therefore a first estimation of the refrigerant capacity of the system can be done.
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81.30.Kf Martensitic transformations
64.70.kd Metals and alloys
75.30.Sg Magnetocaloric effect, magnetic cooling
65.40.gd Entropy
75.50.Cc Other ferromagnetic metals and alloys
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