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15 Dec 2012

Volume 112, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 112, 121301 (2012); http://dx.doi.org/10.1063/1.4754272 (47 pages)

William T. Coffey and Yuri P. Kalmykov
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Thermal fluctuations of magnetic nanoparticles: Fifty years after Brown

William T. Coffey and Yuri P. Kalmykov

J. Appl. Phys. 112, 121301 (2012); http://dx.doi.org/10.1063/1.4754272 (47 pages) | Cited 2 times

Online Publication Date: 17 December 2012

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The reversal time, superparamagnetic relaxation time, of the magnetization of fine single domain ferromagnetic nanoparticles owing to thermal fluctuations plays a fundamental role in information storage, paleomagnetism, biotechnology, etc. Here a comprehensive tutorial-style review of the achievements of fifty years of development and generalizations of the seminal work of Brown [Phys. Rev. 130, 1677 (1963)] on thermal fluctuations of magnetic nanoparticles is presented. Analytical as well as numerical approaches to the estimation of the damping and temperature dependence of the reversal time based on Brown's Fokker-Planck equation for the evolution of the magnetic moment orientations on the surface of the unit sphere are critically discussed while the most promising directions for future research are emphasized.
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75.75.-c Magnetic properties of nanostructures
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Jk Magnetization reversal mechanisms
75.70.Rf Surface magnetism
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back to top Lasers, Optics, and Optoelectronics

Effect of low-Raman window position on correlated photon-pair generation in a chalcogenide Ge11.5As24Se64.5 nanowire

J. He, C. Xiong, A. S. Clark, M. J. Collins, X. Gai, D.-Y. Choi, S. J. Madden, B. Luther-Davies, and B. J. Eggleton

J. Appl. Phys. 112, 123101 (2012); http://dx.doi.org/10.1063/1.4769740 (5 pages)

Online Publication Date: 18 December 2012

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We investigated correlated photon-pair generation via spontaneous four-wave mixing in an integrated chalcogenide Ge11.5As24Se64.5 photonic nanowire. The coincidence to accidental ratio, a key measurement for the quality of correlated photon-pair sources, was measured to be only 0.4 when the photon pairs were generated at 1.9 THz detuning from the pump frequency due to high spontaneous Raman noise in this regime. However, the existence of a characteristic low-Raman window at around 5.1 THz in this material's Raman spectrum and dispersion engineering of the nanowire allowed us to generate photon pairs with a coincidence to accidental ratio of 4.5, more than 10 times higher than the 1.9 THz case. Through comparing the results with those achieved in chalcogenide As2S3 waveguides which also exhibit a low Raman-window but at a larger detuning of 7.4 THz, we find that the position of the characteristic low-Raman window plays an important role on reducing spontaneous Raman noise because the phonon population is higher at smaller detuning. Therefore the ultimate solution for Raman noise reduction in Ge11.5As24Se64.5 is to generate photon pairs outside the Raman gain band at more than 10 THz detuning.
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78.35.+c Brillouin and Rayleigh scattering; other light scattering
63.22.Gh Nanotubes and nanowires
78.67.Uh Nanowires

Morphological and spatial control of InP growth using closed-space sublimation

Daisuke Kiriya, Maxwell Zheng, Rehan Kapadia, Junjun Zhang, Mark Hettick, Zhibin Yu, Kuniharu Takei, Hsin-Hua Hank Wang, Peter Lobaccaro, and Ali Javey

J. Appl. Phys. 112, 123102 (2012); http://dx.doi.org/10.1063/1.4768836 (6 pages) | Cited 1 time

Online Publication Date: 18 December 2012

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Scalable growth of high quality III-V semiconductor thin films on non-epitaxial substrates is of profound interest for photovoltaic applications. Here, we demonstrate growth of indium phosphide (InP) crystals directly on metal foils using closed-space sublimation (CSS) method. CSS allows effective transfer of source material to the substrate due to a small (∼2 mm gap between source and substrate) sublimation space. The crystallization kinetics are found to be dependent on the substrate temperature and pressure of the system. Importantly, experiments revealed that both InP nanowires and polycrystalline films could be obtained by tuning the growth conditions. Furthermore, utilizing a silicon dioxide mask, selective nucleation of InP on metal substrates was obtained. Photoluminescence measurements depict the high optical quality of the CSS grown InP.
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81.16.-c Methods of micro- and nanofabrication and processing
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
68.55.ag Semiconductors
78.67.Uh Nanowires
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Negative effect of crystallization on the mechanism of laser damage in a HfO2/SiO2 multilayer

Ryo Tateno, Hajime Okada, Tomohito Otobe, Keigo Kawase, James K. Koga, Atsushi Kosuge, Keisuke Nagashima, Akira Sugiyama, and Kunihiro Kashiwagi

J. Appl. Phys. 112, 123103 (2012); http://dx.doi.org/10.1063/1.4767231 (3 pages)

Online Publication Date: 18 December 2012

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Elucidation of the mechanisms of laser damage is indispensable in realizing high resistance mirrors for the next generation of ultra-short pulse high intensity lasers. In this study, the surface and a section of the laser-damaged area of a laser mirror were observed with a laser microscope and a transmission electron microscope (cross-sectional TEM), respectively. A grain boundary of HfO2 microcrystal was observed in the damaged area. This observation and an evaluation of the mirror's damage resistance showed that the formation of crystals in the multilayer mirror is one of the major determinants of damage resistance.
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79.20.Ds Laser-beam impact phenomena
64.70.dg Crystallization of specific substances
61.72.Mm Grain and twin boundaries
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Effect of betanin natural dye extracted from red beet root on the non linear optical properties ZnO nanoplates embedded in polymeric matrices

Aparna Thankappan, Sheenu Thomas, and V. P. N. Nampoori

J. Appl. Phys. 112, 123104 (2012); http://dx.doi.org/10.1063/1.4768930 (5 pages)

Online Publication Date: 18 December 2012

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In this article, we have investigated the effect of betanin natural dye extracted from red beetroot on nonlinear optical properties of ZnO nanoplates embedded in polymeric matrices through the Z-scan technique using an Nd: YAG laser (532 nm, 7 ns, 10 Hz). We observed reverse saturable absorption (RSA) at 532 nm for dye and ZnO nanoplates. A strong influence on RSA behavior of nanoplates-PVA matrix was observed by adding betanin natural dye. The influence of betanin on the nonlinear character of ZnO-PVA system leads to saturable absorption and again to RSA on increasing input fluence. Such a change over in the sign of the nonlinearity is due to the interplay of the exciton bleaching and optical limiting mechanisms, and probably due the presence of sucrose. Theoretical analysis has been performed using a model based on nonlinear absorption coefficient and saturation intensity. The result of present study gives an additional mechanism for the gain enhancement in dye doped ZnO matrix.
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78.66.Hf II-VI semiconductors
42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption
42.55.Rz Doped-insulator lasers and other solid state lasers
42.65.-k Nonlinear optics
71.35.-y Excitons and related phenomena
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Optical absorption in highly strained Ge/SiGe quantum wells: The role of Γ→Δ scattering

L. Lever, Z. Ikonić, A. Valavanis, R. W. Kelsall, M. Myronov, D. R. Leadley, Y. Hu, N. Owens, F. Y. Gardes, and G. T. Reed

J. Appl. Phys. 112, 123105 (2012); http://dx.doi.org/10.1063/1.4768935 (7 pages) | Cited 1 time

Online Publication Date: 18 December 2012

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We report the observation of the quantum-confined Stark effect in Ge/SiGe multiple quantum well heterostructures grown on Si0.22Ge0.78 virtual substrates. The large compressive strain in the Ge quantum well layers caused by the lattice mismatch with the virtual substrate results in a blue shift of the direct absorption edge, as well as a reduction in the Γ-valley scattering lifetime because of strain-induced splittings of the conduction band valleys. We investigate theoretically the Γ-valley carrier lifetimes by evaluating the Γ→L and Γ→Δ scattering rates in strained Ge/SiGe semiconductor heterostructures. These scattering rates are used to determine the lifetime broadening of excitonic peaks and the indirect absorption in simulated absorption spectra, which are compared with measured absorption spectra for quantum well structures with systematically varied dimensions. We find that Γ→Δ scattering is significant in compressively strained Ge quantum wells and that the Γ-valley electron lifetime is less than 50 fs in the highly strained structures reported here, where Γ→Δ scattering accounted for approximately half of the total scattering rate.
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78.20.Jq Electro-optical effects
78.67.De Quantum wells
81.07.St Quantum wells
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Mathematical estimation of melt depth in conduction mode of laser spot remelting process

Iraj Hadi

J. Appl. Phys. 112, 123106 (2012); http://dx.doi.org/10.1063/1.4769868 (10 pages)

Online Publication Date: 20 December 2012

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A one-dimensional mathematical model based on the front tracking method was developed to predict the melt depth as a function of internal and external parameters of laser spot remelting process in conduction mode. Power density, pulse duration, and thermophysical properties of material including thermal diffusivity, melting point, latent heat, and absorption coefficient have been taken into account in the model of this article. By comparing the theoretical results and experimental welding data of commercial pure nickel and titanium plates, the validity of the developed model was examined. Comparison shows a reasonably good agreement between the theory and experiment. For the sake of simplicity, a graphical technique was presented to obtain the melt depth of various materials at any arbitrary amount of power density and pulse duration. In the graphical technique, two dimensionless constants including the Stefan number (Ste) and an introduced constant named laser power factor (LPF) are used. Indeed, all of the internal and external parameters have been gathered in LPF. The effect of power density and pulse duration on the variation of melt depth for different materials such as aluminum, copper, and stainless steel were investigated. Additionally, appropriate expressions were extracted to describe the minimum power density and time to reach melting point in terms of process parameters. A simple expression is also extracted to estimate the thickness of mushy zone for alloys.
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64.70.dj Melting of specific substances
65.40.G- Other thermodynamical quantities
66.70.Df Metals, alloys, and semiconductors
66.30.Xj Thermal diffusivity
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Characteristics of built-in polarization potentials in vertically and laterally arranged InGaN/GaN quantum dots

Seoung-Hwan Park, Woo-Pyo Hong, and Jong-Jae Kim

J. Appl. Phys. 112, 123107 (2012); http://dx.doi.org/10.1063/1.4770301 (4 pages)

Online Publication Date: 20 December 2012

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Built-in polarization potentials for vertically and laterally arranged InGaN/GaN quantum dot (QD) structures are investigated using the finite-element method based on the theory of continuum elasticity. The potential for vertically arranged QD structures is shown to depend significantly on the number of arranged QD. On the other hand, in the case of laterally arranged QD structures, the potential is shown to be nearly independent of the number of arranged QD. In the case of three vertically arranged QDs, the potential in the central QD is greatly reduced. Similar result is observed in structures with more QDs than three. On the other hand, the reduction effect is not observed in the case of two QDs. The electric field in the central QD region for three vertically arranged QDs is nearly constant and is shown to be smaller compared to that in the left or right QD region. We observe that the potential in the central QD increases with increasing spacer layer thickness.
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81.05.Ea III-V semiconductors
81.07.Ta Quantum dots
73.63.Kv Quantum dots

μ-Raman spectroscopy characterization of LiNbO3 femtosecond laser written waveguides

M. R. Tejerina, D. Jaque, and G. A. Torchia

J. Appl. Phys. 112, 123108 (2012); http://dx.doi.org/10.1063/1.4769869 (8 pages)

Online Publication Date: 21 December 2012

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In this paper, we present an iterative method which merges experimental μ-Raman measurements and numerical simulations to describe femtosecond written waveguides in LiNbO3 crystals. This method is based on the deformation potential theory, and uses the finite element method to analyze elastic deformations after femtosecond laser micro-explosions in x-cut Mg:LiNbO3 crystals. The resultant strain and refractive index field after laser interaction were estimated and yielded similar values to those obtained in other works. The LiNbO3 Raman deformation potential constants were also estimated in this work.
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42.79.Gn Optical waveguides and couplers
78.35.+c Brillouin and Rayleigh scattering; other light scattering
02.60.-x Numerical approximation and analysis
02.70.Dh Finite-element and Galerkin methods
42.62.-b Laser applications

Diamond heat sinking of terahertz antennas for continuous-wave photomixing

T. Ackemann, M. Alduraibi, S. Campbell, S. Keatings, P. Luke Sam, H. Fraser, A. S. Arnold, E. Riis, and M. Missous

J. Appl. Phys. 112, 123109 (2012); http://dx.doi.org/10.1063/1.4770460 (6 pages)

Online Publication Date: 21 December 2012

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The generation of cw Terahertz radiation from photomixing in low-temperature grown GaAs is limited by the thermal load for single emitters. We propose a new heat sinking scheme based on high thermal conductivity, transparent crystalline heat spreaders as diamond in direct contact with the active zone. A first working device based on recessed electrodes is demonstrated and has a significant power dissipation capability of more than 0.75 W (average). The electrical and terahertz characteristics are analyzed.
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84.40.-x Radiowave and microwave (including millimeter wave) technology
84.40.Ba Antennas: theory, components and accessories
85.60.-q Optoelectronic devices

Application of non-metal doped titania for inverted polymer solar cells

Katarzyna Siuzdak, Mamatimin Abbas, Laurence Vignau, Mélanie Devynck, Galina V. Dubacheva, and Anna Lisowska-Oleksiak

J. Appl. Phys. 112, 123110 (2012); http://dx.doi.org/10.1063/1.4770490 (5 pages)

Online Publication Date: 26 December 2012

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Inverted bulk-heterojunction polymer solar cells have been fabricated applying non-metal doped TiO2 as electron extraction buffer layers. Spin-coated films from nitrogen, sulphur, and iodine doped TiO2 nanoparticles dispersed in dimethyl sulphoxide showed comparable roughness and uniformity as those from the pure TiO2 nanoparticles. The highest power conversion efficiency (PCE) of 1.67% was obtained for N-doped TiO2, whereas in the case of pure TiO2, PCE was around 1%. The highest short circuit current density (Jsc = 10.66 mA cm−2) was achieved for I-doped TiO2. Moreover, it was observed that devices with doped TiO2 exhibit better stability under constant illumination comparing to the control devices with pure TiO2.
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88.40.H- Solar cells (photovoltaics)
88.40.J- Types of solar cells

Polarity-controlled ultraviolet/visible light ZnO nanorods/p-Si photodetector

Yong Xie, Manfred Madel, Yujie Li, Wanqi Jie, Benjamin Neuschl, Martin Feneberg, and Klaus Thonke

J. Appl. Phys. 112, 123111 (2012); http://dx.doi.org/10.1063/1.4771696 (6 pages)

Online Publication Date: 27 December 2012

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Vertically aligned ZnO nanorods of high quality were grown on p-type silicon substrate by a modified chemical vapor phase process. Low temperature photoluminescence measurements show a near band gap donor bound exciton full width at half maximum of less than 500 μeV, and only weak green defect luminescence proofing the superior optical quality of the ZnO structures. n-ZnO/p-Si photodetector devices were processed from these upright standing nanorods. The devices show bias dependent selective photon response: under forward bias, an ultraviolet to visible sensitivity ratio of three orders of magnitude was achieved, while under reverse bias ultraviolet and visible light contributed similarly to the electrical readout current. Thus, the characteristics of this detector element can be switched between “solar blind” and “broadband.”
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85.60.Gz Photodetectors (including infrared and CCD detectors)

Theoretical photo-thermo-hydrodynamic approach to the laser ablation of metals

Mihai Stafe

J. Appl. Phys. 112, 123112 (2012); http://dx.doi.org/10.1063/1.4771692 (7 pages)

Online Publication Date: 28 December 2012

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Here, we employ theory and experiments to investigate the laser ablation process of a metal (Al) using nanosecond laser-pulses at 532 nm wavelength in atmospheric air. We analyze experimentally the dependence of the ablation rate of Al on laser fluence which is varied over a wide range (from ∼4 to 500 J/cm2) by changing the laser pulses energy. The experimental data indicate that the ablation rate increases approximately linearly to ∼1.5 μm/pulse when increasing the fluence to ∼50 J/cm2, whereas further increase of the laser fluence leads to a much slower, non-linear increase of the ablation rate. By extrapolating towards zero the linear fitting curve, we find the ablation threshold fluence of Al to be ∼1.9 J/cm2. To understand and control the underlying phenomena involved in laser ablation, we propose a robust and efficient theoretical model for reliable yet fast calculation of the ablation rate of metals. We use a one-dimensional photo-thermo-hydrodynamic model that accounts for the material heating, melting, evaporation, melt ejection, and ablation plasma shielding during nanosecond laser irradiation. The model considers that the reflectivity of the target surface, the heat capacity, thermal conductivity, and the mass density of the metallic material depend on the aggregation state. The non-linear heat equation of the model is solved numerically in a multi-step iterative method. The solution of the heat equation gives the time evolution of the temperature within the target, leading further to the evaporation and melt ejection velocities, and to the ablation rate. There is a good agreement between the numerical and the experimental results on the ablation rate for a very wide fluence range, i.e., up to 150 J/cm2 when phase separation occurs in the ablating layer whose average temperature during the laser pulse reaches the Al critical temperature. The validity fluence range of the present model is approximately one order of magnitude wider than in previous theoretical works on nanosecond laser ablation of metals.
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79.20.Eb Laser ablation
02.60.-x Numerical approximation and analysis
47.55.P- Buoyancy-driven flows; convection
65.40.Ba Heat capacity
64.70.F- Liquid-vapor transitions
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
back to top Plasmas and Electrical Discharges

Energy fluxes in a radio-frequency magnetron discharge for the deposition of superhard cubic boron nitride coatings

S. Bornholdt, J. Ye, S. Ulrich, and H. Kersten

J. Appl. Phys. 112, 123301 (2012); http://dx.doi.org/10.1063/1.4769800 (9 pages) | Cited 1 time

Online Publication Date: 19 December 2012

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Energy flux measurements by a calorimetric probe in a rf-magnetron plasma used for the deposition of super-hard c-BN coatings are presented and discussed. Argon as working gas is used for sputtering a h-BN target. Adding a certain amount of N2 is essential for the formation of stoichiometric BN films, since a lack of nitrogen will lead to boron rich films. Subsequently, the contributions of different plasma species, surface reactions, and film growth to the resulting variation of the substrate temperature in dependence on nitrogen admixture are estimated and discussed. In addition, SRIM simulations are performed to estimate the energy influx by sputtered neutral atoms. The influence of magnetron target power and oxygen admixture (for comparison with nitrogen) to the process gas on the total energy flux is determined and discussed qualitatively, too. The results indicate that variation of the energy influx due to additional nitrogen flow, which causes a decrease of electron and ion densities, electron temperature and plasma potential, is negligible, while the admixture of oxygen leads to a drastic increase of the energy influx. The typical hysteresis effect which can be observed during magnetron sputtering in oxygen containing gas mixtures has also been confirmed in the energy influx measurements for the investigated system. However, the underlying mechanism is not understood yet, and will be addressed in further investigations.
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68.55.ag Semiconductors
61.66.Bi Elemental solids
61.66.Dk Alloys
81.05.Ea III-V semiconductors
81.15.Cd Deposition by sputtering

Analysis of cathode geometry to minimize cathode erosion in direct current microplasma jet

Federica Causa, Francesco Ghezzi, David Dellasega, Roberto Caniello, and Giovanni Grosso

J. Appl. Phys. 112, 123302 (2012); http://dx.doi.org/10.1063/1.4771876 (7 pages)

Online Publication Date: 27 December 2012

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Microplasma jets are now widely used for deposition, etching, and materials processing. The present study focuses on the investigation of the influence of cathode geometry on deposition quality, for microplasma jet deposition systems in low vacuum. The interest here is understanding the influence of hydrogen on sputtering and/or evaporation of the electrodes. Samples obtained with two cathode geometries with tapered and rectangular cross-sections have been investigated experimentally by scanning electron microscopy and energy dispersion X-ray spectroscopy. Samples obtained with a tapered-geometry cathode present heavy contamination, demonstrating cathode erosion, while samples obtained with a rectangular-cross-section cathode are free from contamination. These experimental characteristics were explained by modelling results showing a larger radial component of the electric field at the cathode inner wall of the tapered cathode. As a result, ion acceleration is larger, explaining the observed cathode erosion in this case. Results from the present investigation also show that the ratio of radial to axial field components is larger for the rectangular geometry case, thus, qualitatively explaining the presence of micro-hollow cathode discharge over a wide range of currents observed in this case. In the light of the above findings, the rectangular cathode geometry is considered to be more effective to achieve cleaner deposition.
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52.75.-d Plasma devices
52.77.Bn Etching and cleaning
52.80.Hc Glow; corona

Doppler-shifted optical absorption characterization of plume-lateral expansion in laser ablation of a cerium target

M. Miyabe, M. Oba, H. Iimura, K. Akaoka, Y. Maruyama, H. Ohba, M. Tampo, and I. Wakaida

J. Appl. Phys. 112, 123303 (2012); http://dx.doi.org/10.1063/1.4771879 (10 pages) | Cited 1 time

Online Publication Date: 27 December 2012

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The temporal evolution of the ablation plume of cerium was investigated by absorption spectroscopy. Cerium oxide pellets were ablated in a helium atmosphere by second-harmonic radiation (532 nm) from a Nd:YAG laser at a fluence of 0.5 J/cm2. The lateral velocity (expansion velocity horizontal to the sample surface) of the plume was determined from the magnitude of the Doppler splitting of the absorption spectra measured close to the sample surface. The lateral velocities of neutral and singly ionized atoms were systematically investigated by varying several parameters, such as ambient gas pressure, ablation laser fluence, observation timing, and observation height. In addition, temporal profiles of the absorption signal were measured by detuning the probe laser frequency from the atomic resonant frequency in order to obtain the temporal variation of the velocity. On the basis of the drag force model, the slowing coefficients for atomic and ionic species in a helium atmosphere were evaluated along with lateral velocity in a vacuum. This study may help in understanding the plume dynamics effect on deposited film properties as well as optimizing experimental conditions for ablation-based spectroscopic analysis.
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79.20.Eb Laser ablation
68.55.-a Thin film structure and morphology
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.30.Er Solid metals and alloys
78.40.Kc Metals, semimetals, and alloys
78.66.Bz Metals and metallic alloys
back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Intermetallic compound formation at Cu-Al wire bond interface

In-Tae Bae, Dae Young Jung, William T. Chen, and Yong Du

J. Appl. Phys. 112, 123501 (2012); http://dx.doi.org/10.1063/1.4768835 (8 pages)

Online Publication Date: 17 December 2012

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Intermetallic compound (IMC) formation and evolution at Cu-Al wire bond interface were studied using focused ion beam /scanning electron microscopy, transmission electron microscopy (TEM)/energy dispersive x-ray spectroscopy (EDS), nano beam electron diffraction (NBED) and structure factor (SF) calculation. It was found that discrete IMC patches were formed at the Cu/Al interface in as-packaged state and they grew toward Al pad after high temperature storage (HTS) environment at 150 °C. TEM/EDS and NBED results combined with SF calculation revealed the evidence of metastable θ′-CuAl2 IMC phase (tetragonal, space group: Imathm2, a = 0.404 nm, c = 0.580 nm) formed at Cu/Al interfaces in both of the as-packaged and the post-HTS samples. Two feasible mechanisms for the formation of the metastable θ′-CuAl2 phase are discussed based on (1) non-equilibrium cooling of wire bond that is attributed to highly short bonding process time and (2) the epitaxial relationships between Cu and θ′-CuAl2, which can minimize lattice mismatch for θ′-CuAl2 to grow on Cu.
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81.05.Bx Metals, semimetals, and alloys
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
61.66.Dk Alloys

Influence of noble gas ion polishing species on extreme ultraviolet mirrors

A. J. R. van den Boogaard, E. Zoethout, I. A. Makhotkin, E. Louis, and F. Bijkerk

J. Appl. Phys. 112, 123502 (2012); http://dx.doi.org/10.1063/1.4768915 (4 pages)

Online Publication Date: 17 December 2012

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Low energy ion polishing is attractive in thin films because of the small interaction zone with the treated material. In this context, various noble gases (Ne, Ar, Kr, and Xe) have been applied for low energy ion polishing of interfaces in nanoscale optical Mo/Si multilayers in order to mitigate the evolving roughness during the deposition process. The interface morphology has been studied by grazing incidence small angle x-ray scattering, the multilayer composition by x-ray photoelectron spectroscopy, and the general performance by extreme ultraviolet (EUV) reflectometry. Both the average roughness level and the vertical correlation length of the roughness can be reduced significantly by increasing the atomic mass of the ion species applied for polishing. Maximum EUV reflectance is observed for Kr+-polishing, while Xe+-polishing shows a superior structure. This apparent contradiction is explained by taking into account the optical absorption from noble gas residuals in the amorphous silicon layers.
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42.79.Bh Lenses, prisms and mirrors
61.05.cf X-ray scattering (including small-angle scattering)
42.70.-a Optical materials
42.79.-e Optical elements, devices, and systems

The influence of material properties on the elastic band structures of one-dimensional functionally graded phononic crystals

Xing-liang Su, Yuan-wen Gao, and You-he Zhou

J. Appl. Phys. 112, 123503 (2012); http://dx.doi.org/10.1063/1.4768934 (8 pages)

Online Publication Date: 17 December 2012

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We study the influence of material parameters on elastic band gaps of one-dimensional functionally graded phononic crystals (FGPCs). By using plane-wave expansion, we calculate the first four band structures of FGPCs consisting of functionally graded materials (FGMs). These structures vary exponentially. We systematically study the influence of material parameters for four different FGPC models. Compared with traditional phononic crystals (PCs), the FGPC band gaps are clearly changed by FGMs. We also consider the influence of material composition, material properties and geometrical parameters on band gaps. Results show that different FGM properties can change the band structures remarkably. Our work can facilitate the design of vibration filters and noise insulators and provide more design freedom in engineering.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
42.70.Qs Photonic bandgap materials
71.20.-b Electron density of states and band structure of crystalline solids
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Selective nucleation induced by defect nanostructures: A way to control cobalt disilicide precipitation during ion implantation

F. Fortuna, M.-A. Nguyen, M.-O. Ruault, M. A. Kirk, V. A. Borodin, and M. G. Ganchenkova

J. Appl. Phys. 112, 123504 (2012); http://dx.doi.org/10.1063/1.4769213 (15 pages)

Online Publication Date: 17 December 2012

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In this paper, we show a way to control cobalt disilicide precipitation during Co ion implantation at high temperatures (650 °C) by affecting radiation defects involved in precipitate nucleation and growth. We demonstrate that the relative shares of different precipitate types nucleated by implantation are strongly affected by defect microstructures deliberately created in investigated samples prior to cobalt implantation. Especially interesting is the effect of a dense ensemble of extremely small (1-3 nm) cavities, which promotes the formation of a relatively uniform layer of coherent cobalt disilicide precipitates with a narrow size distribution. In order to better understand the mechanism of the microstructural influence on the precipitate nucleation modes during Co implantation, we investigate the disilicide precipitation using different implantation setups and compare the results with those for cavity-free Si specimens implanted in similar conditions.
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64.60.qj Studies of nucleation in specific substances
61.46.-w Structure of nanoscale materials
61.72.Qq Microscopic defects (voids, inclusions, etc.)
61.80.Jh Ion radiation effects
81.30.Mh Solid-phase precipitation

Ultrafast transient spectroscopy of nano-domains of polymer/fullerene blend for organic photovoltaic applications

Sanjeev Singh, Bill Pandit, Golda Hukic-Markosian, Tek P. Basel, Z. Valy Vardeny, Sergey Li, and Darin Laird

J. Appl. Phys. 112, 123505 (2012); http://dx.doi.org/10.1063/1.4769211 (6 pages)

Online Publication Date: 18 December 2012

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We measured the picoseconds (ps) transient photomodulation (PM) dynamics of photoexcitations in blends of regio-regular poly(3-hexyl-thiophene) [RR-P3HT] (donors-D) and indene-C60 bisadduct (fullerene derivative) [ICBA] (acceptor-A) that phase-separate into D- and A-nano-domains, in a broad spectral range from 0.25 to 2.5 eV; in comparison with steady state PM spectra. We correlate our measurements with organic photovoltaic solar cell performance made from the same D and A materials. In D-A blends of RR-P3HT/ICBA with (1.2:1) weight ratio having solar cell power conversion efficiency of ∼5.1%, we found that although the intrachain excitons in the polymer nano-domains decay within ∼10 ps, no charge polarons are generated on their expense up to ∼1 ns. Instead, there is a built-up of charge-transfer (CT) excitons at the D-A domain interfaces that occurs with the same kinetics as the exciton decay. The CT excitons dissociate into separate polarons in the D- and A-nano-domains at a much later time (≫1 ns). This “two-step” charge photogeneration process is typical in organic bulk heterojunction cells. Our results emphasize the important role of the CT state in generating free charge polarons in organic solar cells.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells

Predictive calculation of the lattice thermal conductivity with temperature-dependent vibrational parameters

Z. Alameh and M. Kazan

J. Appl. Phys. 112, 123506 (2012); http://dx.doi.org/10.1063/1.4769429 (9 pages)

Online Publication Date: 18 December 2012

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This paper presents a predictive model for the lattice thermal conductivity. The model is based on Callaway's solution to the Boltzmann equation for phonons which discriminates between the natures of the resistive and nonresistive phonon processes. However, the present model uses temperature-dependent lattice vibrational parameters and sound group velocities calculated on the basis of a dynamical matrix. No adjustment to thermal conductivity measurements is required. The model requires only the material mechanical properties as inputs to yield the material thermal conductivity as a function of temperature. A precise transmission probability function is introduced in the model in order to widen its application for the cases where interfaces are present. The importance of all the features of the developed model is demonstrated clearly with reference to reported data regarding the effects of surface orientation and isotope composition in single crystals, the effect of alloy composition in alloys, and the effect of grains boundaries in polycrystals. Namely, the developed model accounts for (i) the effects of surface orientation and isotope composition on the thermal conductivity of silicon and germanium single crystals, (ii) the effect of alloy composition on the thermal conductivity of silicon-germanium alloys, and (iii) the effect of phonon scattering at grains boundaries on the thermal conductivity of polycrystalline silicon.
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66.70.Df Metals, alloys, and semiconductors
61.72.Mm Grain and twin boundaries
62.65.+k Acoustical properties of solids
81.05.Cy Elemental semiconductors
82.20.Tr Kinetic isotope effects including muonium
63.20.K- Phonon interactions

On the scaling of steady structured waves in heterogeneous materials

T. J. Vogler, J. P. Borg, and D. E. Grady

J. Appl. Phys. 112, 123507 (2012); http://dx.doi.org/10.1063/1.4768705 (18 pages)

Online Publication Date: 18 December 2012

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Large amplitude steady waves in materials have been observed to display certain scaling relationships between the strain rate and the stress amplitude. In many homogeneous materials, strain rate scales with stress to the fourth power. However, scaling of strain rate with stress to the first, second, and fourth power has been found for different classes of heterogeneous materials. We examine wave structures for three classes of heterogeneous materials through mesoscale simulations that resolve the scale of heterogeneity explicitly. We utilize these simulations to gain insight into the scaling phenomena observed and to identify the critical non-dimensional parameters for the phenomena. These parameters are then applied to the available experimental data for the three classes. The same set of non-dimensional groups is found to be appropriate for layered and particulate composite materials, while somewhat different groups are found for granular materials. Two different types of simulations lead to different conclusions on the need for the inclusion of strength in the non-dimensionalization for granular materials. The groups formed are found to collapse the experimental data quite well when the strength parameter is not included. Finally, a simple model for granular materials demonstrates that the crucial aspect of their behavior that controls the scaling of waves is the need for mass transfer to close voids in the material.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.F- Deformation and plasticity
61.72.Qq Microscopic defects (voids, inclusions, etc.)
81.05.Rm Porous materials; granular materials

Thermal conductivity of sawtooth-like graphene nanoribbons: A molecular dynamics study

Hui-Sheng Zhang, Zhi-Xin Guo, Xin-Gao Gong, and Jue-Xian Cao

J. Appl. Phys. 112, 123508 (2012); http://dx.doi.org/10.1063/1.4768445 (4 pages)

Online Publication Date: 18 December 2012

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In this work, we have employed the classical non-equilibrium molecular dynamics method to investigate the thermal conductivity of sawtooth-like graphene nanoribbons (SGNRs). It is found that the thermal conductivity of SGNRs is much smaller than that of straight armchair-GNRs. When the length of SGNRs is fixed, the thermal conductivity of SGNRs is very sensitive to n (length of the segmented graphene nanoribbons (GNRs)). Our results indicate that the thermal conductivity of SGNRs reduces remarkably at first and then increases with n increasing. The length and width dependence of thermal conductivity for SGNRs is further explored. Interestingly, we find the thermal conductivity of SGNRs is nearly independent on the length, while it is more sensitive to the width. Our results provide a better understanding for GNR and GNR based nanostructures in thermal management.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
61.48.Gh Structure of graphene
61.46.-w Structure of nanoscale materials

Fabrication of large-grained thin polycrystalline silicon films on foreign substrates by titanium-assisted metal-induced layer exchange

T. Antesberger, T. A. Wassner, M. Kashani, M. Scholz, R. Lechner, S. Matich, and M. Stutzmann

J. Appl. Phys. 112, 123509 (2012); http://dx.doi.org/10.1063/1.4768542 (8 pages)

Online Publication Date: 18 December 2012

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Metal-induced layer exchange (MILE) is a well-known method to grow large-grained high quality polycrystalline silicon on foreign substrates. We have modified the commonly used layer stack by an additional titanium interfacial layer (substrate/metal/titanium/oxide/amorphous silicon). The resulting layer exchange process is called titanium-assisted metal-induced layer exchange (Ti.MILE). For the investigated metals, Al (Ti.ALILE) and Ag (Ti.AgILE), the additional Ti layer does not affect the overall layer exchange process but results in a strong enlargement of the grains in the resulting polycrystalline silicon layer up to 250 μm. We have investigated the influence of the titanium interfacial layer on the process dynamics and grain growth. Furthermore, the structural and optical properties of the resulting polycrystalline silicon layer are investigated by means of different analysis methods.
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81.05.Cy Elemental semiconductors
81.15.Np Solid phase epitaxy; growth from solid phases
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