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14 Feb 2013

Volume 113, Issue 6, Articles (06xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 113, 064301 (2013); http://dx.doi.org/10.1063/1.4789897 (11 pages)

Y. G. Marinov, G. B. Hadjichristov, A. G. Petrov, S. Marino, C. Versace, and N. Scaramuzza
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back to top Lasers, Optics, and Optoelectronics

Effect of growth temperature and post-growth annealing on luminescence properties of molecular beam epitaxy grown single layer Ge quantum dots

S. Das, S. Manna, R. K. Singha, R. Aluguri, and S. K. Ray

J. Appl. Phys. 113, 063101 (2013); http://dx.doi.org/10.1063/1.4790593 (8 pages)

Online Publication Date: 8 February 2013

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We report the detailed structural and luminescence properties of Si-embedded single layer Ge nanoislands grown via Stranski-Krastanov mechanism using molecular beam epitaxy. The luminescence energy peak positions correlated with structural properties have been studied as a function of the growth temperature and post-growth annealing. The photoluminescence peak for the annealed sample (500 °C grown) is blue shifted with increasing anneal temperature compared to the as grown sample due to intermixing of Si/Ge. Electroluminescence from the single layer Ge islands in the wavelength range 1.53 to 1.67 μm has been demonstrated using a metal-insulator-semiconductor structure, making it attractive for quantum dot light emitting devices.
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61.72.Cc Kinetics of defect formation and annealing
78.55.Ap Elemental semiconductors
78.66.Db Elemental semiconductors and insulators
78.67.Hc Quantum dots
81.07.Ta Quantum dots
68.55.ag Semiconductors

Intensity dependent spectral features in high harmonic generation

Khuong Ba Dinh, Peter Hannaford, and Lap Van Dao

J. Appl. Phys. 113, 063102 (2013); http://dx.doi.org/10.1063/1.4790708 (6 pages)

Online Publication Date: 8 February 2013

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We study the influence of the driving laser intensity on the spectral features of high harmonic generation in a semi-infinite gas cell. The effects of the harmonic dipole phase and the dispersion phase mismatches induced by the ionized medium during the harmonic generation process are revealed and the interplay between the macroscopic response and the single-atom response is discussed. We consider the conditions for generation of a narrow bandwidth, bright, and highly coherent high harmonic source.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.72.-g Optical sources and standards
42.60.Fc Modulation, tuning, and mode locking
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

Polarization-independent rapidly responding phase grating based on hybrid blue phase liquid crystal

Yi-Ting Lin, Hung-Chang Jau, and Tsung-Hsien Lin

J. Appl. Phys. 113, 063103 (2013); http://dx.doi.org/10.1063/1.4790375 (4 pages)

Online Publication Date: 11 February 2013

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This work demonstrates a polymer-stabilized blue phase (PSBP) liquid crystal phase grating, which is made of hybrid PSBPs with two different Kerr constants. The Kerr constant of a PSBP is related to the morphology of the polymer network which can be controlled by the phase separation temperature. Owing to the non-patterned electrode and the optical isotropy of the PSBP, the diffraction effect can be completely switched off when the voltage is absent. The diffraction intensity increases when a uniform applied electrical field induces the phase difference in the hybrid PSBP. The phase grating is completely independent of the polarization of the incident light. Furthermore, the response time to switching is in the sub-millisecond range.
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42.79.Dj Gratings
78.20.Jq Electro-optical effects
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.70.Df Liquid crystals

Rate equation analysis of injection-locked quantum cascade lasers

Cheng Wang, Frédéric Grillot, Vassilios Kovanis, and Jacky Even

J. Appl. Phys. 113, 063104 (2013); http://dx.doi.org/10.1063/1.4790883 (6 pages)

Online Publication Date: 12 February 2013

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The modulation properties of optical injection-locked quantum cascade lasers (QCLs) are investigated theoretically via a simple low dimensional rate equation model. It is found that both strong injection level and positive optical frequency detuning increase the modulation bandwidth, while a large linewidth enhancement factor (LEF) contributes to the enhancement of the peak magnitude in the intensity modulation (IM) response. As opposed to conventional injection-locked interband lasers, it is demonstrated that no dip occurs in the QCL's IM response, which is beneficial for a series of broadband microwave photonic applications. Computations also show that the value of the LEF can critically modify both the locking and stability regions on the optical frequency detuning injection level map.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Fc Modulation, tuning, and mode locking
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Effects of band non-parabolicity on cavity modes in photonic crystals

N.-Y. Lue, Y.-S. Chen, H.-S. Wei, and G. Y. Wu

J. Appl. Phys. 113, 063105 (2013); http://dx.doi.org/10.1063/1.4789945 (6 pages)

Online Publication Date: 13 February 2013

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We include the effect of band non-parabolicity on photonic defect states within the Wannier theory, which improves the quadratic approximation adopted by Painter et al. [Phys. Rev. B 68, 035214 (2003)] for large-size defects, as well as extends the theory to a wider range of defect size. A 2D hexagonal photonic crystal is considered, and analyzed for the origin and degree of non-parabolicity in the 1st TE band around J-point, and for the effect of non-parabolicity on acceptor type cavity modes. The non-parabolicity is shown to derive primarily from the inherent anisotropy of band dispersion around the point. Overall, with the inclusion of non-parabolicity, (i) mode degeneracy is lowered, (ii) the “binding energy” of a cavity mode is increased, and (iii) the cut-off of defect size for a given cavity mode is reduced, by as much as 35% in certain cases, in comparison with that calculated without the non-parabolicity. A simple “overall effective mass” picture is provided for the understanding of non-parabolicity effects.
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42.70.Qs Photonic bandgap materials

Numerical simulation of stimulated emission and lasing in dye doped cholesteric liquid crystal films

L. Penninck, J. Beeckman, P. De Visschere, and K. Neyts

J. Appl. Phys. 113, 063106 (2013); http://dx.doi.org/10.1063/1.4790873 (8 pages)

Online Publication Date: 13 February 2013

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Dye-doped chiral-nematic liquid crystal lasers have great potential as small size, low-cost, widely tunable lasers. We present a numerical model for stimulated emission and lasing in liquid crystal films based on thin film optics. The gain threshold is modelled and the results are confirmed experimentally. The effect of the orientation of dye molecules and the matching of the photonic bandgap to the dye spectrum on the threshold for lasing is discussed.
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42.55.-f Lasers
42.70.Df Liquid crystals
42.70.Hj Laser materials
02.60.-x Numerical approximation and analysis

Enhancement of momentum coupling coefficient by cavity with toroidal bubble for underwater laser propulsion

Jun Chen, Bei-Bei Li, Hong-Chao Zhang, Hao Qiang, Zhong-Hua Shen, and Xiao-Wu Ni

J. Appl. Phys. 113, 063107 (2013); http://dx.doi.org/10.1063/1.4792041 (5 pages)

Online Publication Date: 14 February 2013

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High-speed photography method is employed to study underwater laser propulsion using targets with and without cavity as well as the effect of the cavity depth. The shapes and motions of bubbles generated from the target tail by Nd: YAG laser are recorded by high-speed camera. Then, the influence of different bubble shapes on the laser propulsion is analyzed. Besides, the velocity and momentum coupling coefficient of the targets are investigated. The results show that the bubble is hemispherical in the case of target without cavity, while it is toroidal in the case of target with cavity. Experiments verify that compared with hemispherical bubble, the toroidal bubble is more conducive to laser propulsion in water, which means the target with cavity obtains more momentum than the target without cavity. In addition, the momentum coupling coefficient increases with laser energy first, and then it levels out and decreases a bit for the target with cavity. There is little effect of the cavity depth on propulsion.
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47.55.dd Bubble dynamics
47.80.Jk Flow visualization and imaging
47.60.-i Flow phenomena in quasi-one-dimensional systems
42.62.Eh Metrological applications; optical frequency synthesizers for precision spectroscopy

Study on phosphor sedimentation effect in white light-emitting diode packages by modeling multi-layer phosphors with the modified Kubelka-Munk theory

Run Hu, Yiman Wang, Yong Zou, Xing Chen, Sheng Liu, and Xiaobing Luo

J. Appl. Phys. 113, 063108 (2013); http://dx.doi.org/10.1063/1.4792051 (6 pages)

Online Publication Date: 14 February 2013

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In this study, we studied the phosphor sedimentation effect in white phosphor-converted light-emitting diode packages by modeling the multi-layer phosphors with gradient concentrations. The essence of phosphor sedimentation can attribute to the variation of phosphor concentrations. By modifying the Kubelka-Munk theory, we built a multi-layer phosphor model with considering the light scattering, light absorption, and light conversion process simultaneously. With a brief review of Kubelka-Munk theory, multi-layer phosphors were modeled on the basis of single-layer phosphor model. The phosphor sedimentation effect was characterized by modeling multi-layer phosphors with gradient concentrations, whereas keeping the total amount of phosphors at the same level. It is found from the five calculation cases that phosphor sedimentation will cause the drop of light extraction efficiency (LEE) by 13.04%. Furthermore, the phosphor layer with inverse-gradient concentrations will enhance the LEE 16.56%. To figure out the reasons, the light losses were calculated, and it is proved that the light loss is enhanced when phosphor sedimentation happens.
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85.60.Jb Light-emitting devices

Polymeric photovoltaics with various metallic plasmonic nanostructures

Beibei Zeng, Qiaoqiang Gan, Zakya H. Kafafi, and Filbert J. Bartoli

J. Appl. Phys. 113, 063109 (2013); http://dx.doi.org/10.1063/1.4790504 (10 pages)

Online Publication Date: 14 February 2013

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Broadband light absorption enhancement is numerically investigated for the active light harvesting layer of an organic photovoltaic (OPV), which consists of a blend of poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Periodic plasmonic nanostructures placed above and below the active layer incorporate Ag, Al, Au, or a combination of two different metals. Three dimensional (3D) full-field electromagnetic simulations are applied to determine the effect of varying the metal employed in the plasmonic nanostructures on the absorption enhancement of the OPV. In addition, the geometric parameters (e.g., film thickness, period, and diameter) of the symmetrically distributed top and bottom metal (Ag, Al, or Au) nanostructures were varied to optimize the device structure and delineate the mechanism(s) leading to the absorption enhancement. A spectrally broadband, polarization-insensitive, and wide-angle absorption enhancement is obtained using a double plasmonic nanostructure and is attributed to the combined excitation of localized and single-interface surface plasmon polariton modes. The total photon absorption of the OPV with the optimized double plasmonic Ag nanostructures was found to be enhanced by as much as 82.8% and 80.4% under normal (0°) and 60° light incidence, respectively.
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88.40.jr Organic photovoltaics
85.65.+h Molecular electronic devices
88.40.hj Efficiency and performance of solar cells
back to top Plasmas and Electrical Discharges

Joule heat generation in thermionic cathodes of high-pressure arc discharges

M. S. Benilov and M. D. Cunha

J. Appl. Phys. 113, 063301 (2013); http://dx.doi.org/10.1063/1.4790709 (11 pages)

Online Publication Date: 8 February 2013

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The nonlinear surface heating model of plasma-cathode interaction in high-pressure arcs is extended to take into account the Joule effect inside the cathode body. Calculation results are given for different modes of current transfer to tungsten cathodes of different configurations in argon plasmas of atmospheric or higher pressures. Special attention is paid to analysis of energy balances of the cathode and the near-cathode plasma layer. In all the cases, the variation of potential inside the cathode is much smaller than the near-cathode voltage drop. However, this variation can be comparable to the volt equivalent of the energy flux from the plasma to the cathode and then the Joule effect is essential. Such is the case of the diffuse and mixed modes on rod cathodes at high currents, where the Joule heating causes a dramatic change of thermal and electrical regimes of the cathode. The Joule heating has virtually no effect over characteristics of spots on rod and infinite planar cathodes.
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52.80.Mg Arcs; sparks; lightning; atmospheric electricity
84.47.+w Vacuum tubes
52.25.Mq Dielectric properties
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.40.Hf Plasma-material interactions; boundary layer effects
52.50.Lp Plasma production and heating by shock waves and compression

Deposition and tuning of nanostructured hydrocarbon deposits: From superhydrophobic to superhydrophilic and back

J. Berndt, H. Acid, E. Kovacevic, C. Cachoncinlle, Th. Strunskus, and L. Boufendi

J. Appl. Phys. 113, 063302 (2013); http://dx.doi.org/10.1063/1.4789949 (7 pages)

Online Publication Date: 11 February 2013

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Carbonaceous fluorine free nanoparticles synthesized in a low temperature acetylene discharge are used in a first step for the production of (super)hydrophobic coatings. In a second step, the influence of different plasma and UV induced functionalizations on the wetting characteristics of these materials is investigated. The experiments show that the superhydrophobic surfaces can be turned continuously and reversibly into hydrophilic (superhydrophilic) surfaces by means of the different treatment methods. The reversibility of these processes is studied in a third step. It is shown that the changes of the surface which are induced by the plasma treatment can be undone by means of EUV irradiation. The switchability of the surface due to external stimuli can be easily used for the controlled production of patterned surfaces. This is demonstrated by means of one simple example.
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81.65.-b Surface treatments
68.55.am Polymers and organics
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Rx Nanocrystalline materials
68.08.Bc Wetting

High-temperature inert gas plasma magnetohydrodynamic energy conversion by using linear-shaped Faraday-type channel

Tomoyuki Murakami, Yunqin Zhuang, and Yoshihiro Okuno

J. Appl. Phys. 113, 063303 (2013); http://dx.doi.org/10.1063/1.4792056 (6 pages)

Online Publication Date: 14 February 2013

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We describe high-density magnetohydrodynamic (MHD) energy conversion in a high-temperature seed-free argon plasma, for which a compact linear-shaped Faraday-type MHD electrical power generator is used. Short-time-duration single-pulse shock-tunnel-based experiments demonstrate the MHD energy conversion with varying total inflow temperature up to 9000 K and applied magnetic-flux density up to 4.0 T. The high-temperature plasma is transformed from the thermal-equilibrium state at the entrance to the weak-nonequilibrium state in the supersonic MHD channel. The discharge structure is reasonably homogeneous without suffering from serious streamer development. The power generation performance is monotonically improved by increasing total inflow temperature and strength of magnetic field. The enthalpy extraction efficiency of 13.1% and overall power density of 0.16 GW/m3 are attained. The local power density at the middle of the channel reaches 0.24 GW/m3.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Tc Shock waves and discontinuities
52.80.-s Electric discharges
52.25.Fi Transport properties
52.25.Kn Thermodynamics of plasmas

Linear stability of electron flow produced by field emission

A. Rokhlenko and J. L. Lebowitz

J. Appl. Phys. 113, 063304 (2013); http://dx.doi.org/10.1063/1.4792059 (8 pages)

Online Publication Date: 14 February 2013

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A linear stability analysis of the planar one dimensional space charge limited flow is performed when the current is determined by a current-field relation, e.g., the Fowler-Nordheim or any other emission model. The initial velocity is assumed the same for all emitted electrons. The flow is shown to be stable with decaying oscillations depending on the nature of the emission law, including in some situations non-oscillating slowly decaying modes. When the emission variations are due only to changes of the initial flow velocity, the time of decay can be much longer than the electron transit time for a given flow setup.
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79.70.+q Field emission, ionization, evaporation, and desorption
77.22.Jp Dielectric breakdown and space-charge effects
back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Effects of parameter variations on negative effective constitutive parameters of non-metallic metamaterials

Yang Li and Nicola Bowler

J. Appl. Phys. 113, 063501 (2013); http://dx.doi.org/10.1063/1.4790714 (12 pages)

Online Publication Date: 8 February 2013

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Analytical expressions describing the variability of effective constitutive parameters of non-metallic metamaterials, as a function of the constituent geometric and material parameters and their variations, have been developed from the total differential of Clausius-Mossotti expressions (using Mie dipole polarizabilities) for the effective (bulk) constitutive parameters of the metamaterial. In practice, these expressions are important for estimating the performance of a metamaterial with particular variations in the parameters of its constituents that arise during the fabrication process, and can be used to guard against extinction of desired double negative (DNG) behavior. With the derived expressions, the effects of parameter variations on effective constitutive parameters of non-metallic metamaterials have been analyzed for three types of metamaterials: (i) cubic arrays of identical magnetodielectric spheres; (ii) cubic arrays of dielectric spheres with equal radius but two different permittivities; and (iii) cubic arrays of dielectric spheres with equal permittivity but two different radii. These effects are evaluated in terms of the calculated variations in values of the effective constitutive parameters of the metamaterial in the vicinity of the DNG or single negative (SNG) band for particular geometric and material parameters and their variations. Results show that variation in the following parameters impacts DNG bandwidth. Listed in order from greatest to least influence: (i) sphere radius; (ii) sphere permittivity and permeability; (iii) lattice constant of the array; and (iv) the constitutive parameters of the array medium, all impact the width of the achievable DNG band. For particular cases studied here, results also show that the DNG behavior may be extinguished if there are 0.78%, 0.016%, and 0.016% variations in all parameters of metamaterial types (i), (ii), and (iii), respectively, as defined above. For the design of non-metallic metamaterials with inclusions, having arbitrary material parameters, in either periodic or random arrangement, the presented results can give a qualitative guide on the level of fabrication tolerances that should be achieved in order to observe the predicted SNG or DNG behavior experimentally.
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81.05.Xj Metamaterials for chiral, bianisotropic and other complex media
61.72.Qq Microscopic defects (voids, inclusions, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
77.22.Ch Permittivity (dielectric function)
75.85.+t Magnetoelectric effects, multiferroics

Local stress determination in chromia-former thanks to micro-Raman spectroscopy: A way to investigate spontaneous delamination processes

M. Guerain, P. Goudeau, B. Panicaud, and J. L. Grosseau-Poussard

J. Appl. Phys. 113, 063502 (2013); http://dx.doi.org/10.1063/1.4790308 (7 pages)

Online Publication Date: 11 February 2013

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Spontaneous delamination process for α-Cr2O3 thermal oxide films growing on NiCr-30 alloys is studied thanks to micro Raman spectroscopy. In particular, stress maps are performed through and around buckles developed on chromia films. Depending on the cooling rate at the end of the oxidation process, different buckle types appear which are investigated. Associated residual stress distribution clearly evidences the stress release field. In addition, geometrical features are determined for the different buckle types, and from comparison with modelling describing buckle formation and propagation, it is possible to get the interface toughness distribution.
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68.60.Bs Mechanical and acoustical properties
78.30.Hv Other nonmetallic inorganics
78.66.Nk Insulators
81.40.Lm Deformation, plasticity, and creep
62.20.mq Buckling

The impact of substrate miscut on the morphology of InGaN epitaxial layers subjected to a growth interruption

Nicolas Jouvet, Menno J. Kappers, Colin J. Humphreys, and Rachel A. Oliver

J. Appl. Phys. 113, 063503 (2013); http://dx.doi.org/10.1063/1.4790311 (5 pages)

Online Publication Date: 11 February 2013

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To investigate the formation of gross well width fluctuations in InGaN quantum wells subjected to a growth interruption or temperature ramp, the impact of substrate miscut on the morphology of thin InGaN epitaxial layers grown on GaN pseudo-substrates has been studied. Following a growth interruption, the InGaN layer morphology consists of interlinking strips of InGaN separated by elongated troughs. The troughs tend to align along the same direction as the step edges in the underlying GaN pseudo-substrate and the spacing of the troughs varies in direct proportion to the step edge spacing. However, the troughs are on average more widely spaced than the terrace edges, and analysis of the distribution of trough spacings and step edge spacings suggests that it may be double-monolayer steps which most influence the trough formation. A mechanism for trough formation is proposed in which indium is preferentially incorporated at double-monolayer step edges and that the resulting indium-rich regions decompose during the growth interruption, forming a trough.
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68.55.ag Semiconductors
68.60.-p Physical properties of thin films, nonelectronic
68.65.Fg Quantum wells
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Morphological evolution of InGaN/GaN light-emitting diodes grown on free-standing m-plane GaN substrates

R. M. Farrell, D. A. Haeger, K. Fujito, S. P. DenBaars, S. Nakamura, and J. S. Speck

J. Appl. Phys. 113, 063504 (2013); http://dx.doi.org/10.1063/1.4790636 (7 pages)

Online Publication Date: 11 February 2013

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We report on the morphological evolution of InGaN/GaN light-emitting diodes (LEDs) grown on nominally on-axis and intentionally misoriented free-standing m-plane GaN substrates. Large variations in p-n junction depth (±175nm) were observed for LEDs grown on nominally on-axis substrates, while negligible variations in junction depth (±20 nm) were observed for LEDs grown on intentionally misoriented substrates. The surfaces of LEDs grown on the nominally on-axis substrates were characterized by the presence of a high density of pyramidal hillocks [Hirai et al., Appl. Phys. Lett. 91, 191906 (2007)], while the surfaces of the LEDs grown on the intentionally misoriented substrates were relatively smooth and free of pyramidal hillocks. Detailed measurements indicated that the variations in junction depth observed for LEDs grown on nominally on-axis substrates were related to an evolution in the shape of individual pyramidal hillocks during the growth of the LEDs. These results indicate that growing LEDs on intentionally misoriented substrates is an effective way to eliminate the pyramidal hillocks and variations in junction depth associated with growth on nominally on-axis substrates.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes

Dielectric function of Cu(In, Ga)Se2-based polycrystalline materials

Shota Minoura, Keita Kodera, Takuji Maekawa, Kenichi Miyazaki, Shigeru Niki, and Hiroyuki Fujiwara

J. Appl. Phys. 113, 063505 (2013); http://dx.doi.org/10.1063/1.4790174 (14 pages)

Online Publication Date: 11 February 2013

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The dielectric functions of Cu(In, Ga)Se2(CIGS)-based polycrystalline layers with different Ga and Cu compositions have been determined by applying spectroscopic ellipsometry (SE) in a wide energy range of 0.7–6.5 eV. To suppress SE analysis errors induced by rough surface and compositional fluctuation, quite thin CIGS layers (<60 nm) with high uniformity toward the growth direction have been characterized using a self-consistent SE analysis method. We find that the optical model used in many previous studies is oversimplified particularly for the roughness/overlayer contribution, and all the artifacts arising from the simplified analysis have been removed almost completely in our approach. The CIGS dielectric functions with the variation of the Ga composition [x = Ga/(In + Ga)] revealed that (i) the whole CIGS dielectric function shifts toward higher energies with x, (ii) the band gap increases linearly with x without the band-gap bowing effect, and (iii) the overall absorption coefficients are significantly smaller than those reported earlier. Furthermore, the reduction of the Cu composition [y = Cu/(In + Ga)] leads to (i) the linear increase in the band-edge transition energy and (ii) the decrease in the absorption coefficient, due to the smaller interaction of the Cu 3d orbitals near the valence band maximum in the Cu-deficient layers. When y > 1, on the other hand, the free-carrier absorption increases drastically due to the formation of a semi-metallic CuxSe phase with a constant band gap in the CIGS component. In this study, by using a standard critical-point line-shape analysis, the critical point energies of the CIGS-based layers with different Ga and Cu compositions have been determined. Based on these results, we will discuss the optical transitions in CIGS-based polycrystalline materials.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.05.Hd Other semiconductors
68.35.bg Semiconductors
71.20.Nr Semiconductor compounds

Intrinsic degradation mechanism of nearly lattice-matched InAlN layers grown on GaN substrates

Guillaume Perillat-Merceroz, Gatien Cosendey, Jean-François Carlin, Raphaël Butté, and Nicolas Grandjean

J. Appl. Phys. 113, 063506 (2013); http://dx.doi.org/10.1063/1.4790424 (10 pages)

Online Publication Date: 11 February 2013

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Thanks to its high refractive index contrast, band gap, and polarization mismatch compared to GaN, In0.17Al0.83N layers lattice-matched to GaN are an attractive solution for applications such as distributed Bragg reflectors, ultraviolet light-emitting diodes, or high electron mobility transistors. In order to study the structural degradation mechanism of InAlN layers with increasing thickness, we performed metalorganic vapor phase epitaxy of InAlN layers of thicknesses ranging from 2 to 500 nm, on free-standing (0001) GaN substrates with a low density of threading dislocations, for In compositions of 13.5% (layers under tensile strain), and 19.7% (layers under compressive strain). In both cases, a surface morphology with hillocks is initially observed, followed by the appearance of V-defects. We propose that those hillocks arise due to kinetic roughening, and that V-defects subsequently appear beyond a critical hillock size. It is seen that the critical thickness for the appearance of V-defects increases together with the surface diffusion length either by increasing the temperature or the In flux because of a surfactant effect. In thick InAlN layers, a better (worse) In incorporation occurring on the concave (convex) shape surfaces of the V-defects is observed leading to a top phase-separated InAlN layer lying on the initial homogeneous InAlN layer after V-defects coalescence. It is suggested that similar mechanisms could be responsible for the degradation of thick InGaN layers.
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81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.35.bg Semiconductors
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)

Plasma nitriding process by direct current glow discharge at low temperature increasing the thermal diffusivity of AISI 304 stainless steel

L. V. Prandel, A. Somer, A. Assmann, F. Camelotti, G. Costa, C. Bonardi, A. R. Jurelo, J. B. Rodrigues, and G. K. Cruz

J. Appl. Phys. 113, 063507 (2013); http://dx.doi.org/10.1063/1.4790631 (5 pages)

Online Publication Date: 12 February 2013

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This work reports for the first time on the use of the open photoacoustic cell technique operating at very low frequencies and at room temperature to experimentally determine the thermal diffusivity parameter of commercial AISI304 stainless steel and AISI304 stainless steel nitrided samples. Complementary measurements of X-ray diffraction and scanning electron microscopy were also performed. The results show that in standard AISI 304 stainless steel samples the thermal diffusivity is (4.0 ± 0.3) × 10−6 m2/s. After the nitriding process, the thermal diffusivity increases to the value (7.1 ± 0.5) × 10−6 m2/s. The results are being associated to the diffusion process of nitrogen into the surface of the sample. Carrying out subsequent thermal treatment at 500 °C, the thermal diffusivity increases up to (12.0 ± 2) × 10−6 m2/s. Now the observed growing in the thermal diffusivity must be related to the change in the phases contained in the nitrided layer.
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81.65.Lp Surface hardening: nitridation, carburization, carbonitridation
52.77.-j Plasma applications
66.30.Xj Thermal diffusivity
68.35.Fx Diffusion; interface formation
81.40.Gh Other heat and thermomechanical treatments

Correlation between structural parameters and the magnetocaloric effect in epitaxial La0.8Ca0.2MnO3/LaAlO3 thin film

J. C. Debnath, J. H. Kim, Y. Heo, A. M. Strydom, and S. X. Dou

J. Appl. Phys. 113, 063508 (2013); http://dx.doi.org/10.1063/1.4790876 (6 pages)

Online Publication Date: 12 February 2013

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An epitaxial La0.8Ca0.2MnO3/LaAlO3 (LCMO/LAO) thin film was fabricated using the pulsed laser deposition technique to evaluate the correlation between the crystal structure and the magnetocaloric effect. In our study, the LCMO film was 200 nm in thickness and appeared to have a strong out-of plane texture. We found that each column in the LCMO thin film layer is a domain which has a different ordering direction. These microscopic feature results in anisotropic properties of magnetization, entropy, and relative cooling power. The film exhibited a paramagnetic-to-ferromagnetic second order phase transition at 249 K. The lack of any hysteresis loss also confirmed that the material is intrinsically reversible. In addition, the large magnetization of the thin film results in an entropy change larger than those of all other perovskite type materials. Consequently, the relative cooling power is significantly enhanced.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Ak Magnetic properties of monolayers and thin films
81.15.Fg Pulsed laser ablation deposition
68.55.aj Insulators
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

A comparative study of two molecular mechanics models based on harmonic potentials

Junhua Zhao, Lifeng Wang, Jin-Wu Jiang, Zhengzhong Wang, Wanlin Guo, and Timon Rabczuk

J. Appl. Phys. 113, 063509 (2013); http://dx.doi.org/10.1063/1.4791579 (12 pages) | Cited 1 time

Online Publication Date: 12 February 2013

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We show that the two molecular mechanics models, the stick-spiral and the beam models, predict considerably different mechanical properties of materials based on energy equivalence. The difference between the two models is independent of the materials since all parameters of the beam model are obtained from the harmonic potentials. We demonstrate this difference for finite width graphene nanoribbons and a single polyethylene chain comparing results of the molecular dynamics (MD) simulations with harmonic potentials and the finite element method with the beam model. We also find that the difference strongly depends on the loading modes, chirality and width of the graphene nanoribbons, and it increases with decreasing width of the nanoribbons under pure bending condition. The maximum difference of the predicted mechanical properties using the two models can exceed 300% in different loading modes. Comparing the two models with the MD results of AIREBO potential, we find that the stick-spiral model overestimates and the beam model underestimates the mechanical properties in narrow armchair graphene nanoribbons under pure bending condition.
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81.40.Lm Deformation, plasticity, and creep
62.20.de Elastic moduli
62.20.F- Deformation and plasticity
62.25.-g Mechanical properties of nanoscale systems
61.48.Gh Structure of graphene
81.40.Jj Elasticity and anelasticity, stress-strain relations

Viscoplastic analysis of cyclic indentation behavior of thin metallic films

C. B. Blada and Y.-L. Shen

J. Appl. Phys. 113, 063510 (2013); http://dx.doi.org/10.1063/1.4792034 (5 pages)

Online Publication Date: 12 February 2013

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The indentation behavior of metallic thin film is studied numerically, with attention devoted to cyclic response under a fixed load amplitude. The finite element model consists of an aluminum (Al) film above a silicon (Si) substrate, subject to indentation by a diamond indenter. The rate-dependent viscoplastic model of Al is employed. It is shown that, toward the end of the reloading phase of each cycle, plastic deformation continues to occur and the indenter moves deeper into the material. The advancing rate decreases with time, leading to a stabilized response after about ten cycles, in agreement with experimental measurement. Tracking of local deformation history is carried out, to corroborate with the macroscopic indentation response obtained from modeling and experiment.
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81.40.Lm Deformation, plasticity, and creep
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.fq Plasticity and superplasticity
62.20.Qp Friction, tribology, and hardness
68.60.-p Physical properties of thin films, nonelectronic

X-ray photoelectron diffraction study of dopant effects in La0.7X0.3MnO3 (X = La, Sr, Ca, Ce) thin films

C. Raisch, C. Langheinrich, R. Werner, R. Kleiner, D. Koelle, M. Glaser, T. Chassé, and A. Chassé

J. Appl. Phys. 113, 063511 (2013); http://dx.doi.org/10.1063/1.4789988 (7 pages)

Online Publication Date: 13 February 2013

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We present and discuss element-specific x-ray photoelectron diffraction (XPD) patterns of La, Mn, O and the dopant ions Ca, Sr and Ce of various La0.7X0.3MnO (LXMO) films grown epitaxially on SrTiO3(001) substrates. The recorded XPD polar scans are explained in the framework of multiple-scattering cluster calculations, where in general a very good agreement between experimental and theoretical data has been found. Results for all thin films are compatible with a tetragonally distorted cubic perovskite structure with similar MnO6 network. Strong evidence for Mn-site termination was found in all thin films. Dopant locations on A-type sites were clearly confirmed for Sr in LSMO and Ca in LCMO films by means of XPD polar scans. The absence of surface-sensitive Ce3d diffraction features for Ce in LCeMO points to non-equivalent Ce sites and related near-surface disorder.
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79.60.-i Photoemission and photoelectron spectra
81.15.Fg Pulsed laser ablation deposition
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
61.72.sh Impurity distribution
68.55.aj Insulators

Viscoelastic characterization of low-velocity impact of a solid ball on an agar gel

K. Ara and H. Katsuragi

J. Appl. Phys. 113, 063512 (2013); http://dx.doi.org/10.1063/1.4790872 (10 pages)

Online Publication Date: 13 February 2013

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A viscoelastic characterization method using low-velocity impact is experimentally studied. A steel ball is dropped from a certain height and impacts on an agar gel target with 1–4 m/s velocity. The motion of the impactor ball is captured by a high-speed camera. Instantaneous penetration depth, velocity, and acceleration of the impactor are computed from the high-speed video data. The obtained kinematic data are analyzed in terms of the equation of motion of the impactor. Specifically, we compute the impact viscosity and impact elasticity, assuming a simple impact drag force model. The impact drag force model consists of a linear viscous term, a linear elastic term, and a constant term. From the estimated impact viscosity, we confirm that the Reynolds number is relatively low (less than 10). This low Reynolds number is consistent with the simple linear viscous assumption. From the estimated impact elasticity, we can calculate the speed of sound and the strength of target agar gel. In order to examine the velocity dependence of the elasticity, we also perform very slow (less than 0.1 mm/s) penetration tests using the same agar gel samples. The comparison between impact elasticity and slow penetration elasticity reveals the weak velocity strengthening of agar gel.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
81.70.Bt Mechanical testing, impact tests, static and dynamic loads
62.20.D- Elasticity
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
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