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15 Aug 2012

Volume 112, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 112, 041101 (2012); http://dx.doi.org/10.1063/1.4740023 (20 pages)

Iain A. Anderson, Todd A. Gisby, Thomas G. McKay, Benjamin M. O’Brien, and Emilio P. Calius
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Multi-functional dielectric elastomer artificial muscles for soft and smart machines

Iain A. Anderson, Todd A. Gisby, Thomas G. McKay, Benjamin M. O’Brien, and Emilio P. Calius

J. Appl. Phys. 112, 041101 (2012); http://dx.doi.org/10.1063/1.4740023 (20 pages) | Cited 5 times

Online Publication Date: 28 August 2012

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Dielectric elastomer (DE) actuators are popularly referred to as artificial muscles because their impressive actuation strain and speed, low density, compliant nature, and silent operation capture many of the desirable physical properties of muscle. Unlike conventional robots and machines, whose mechanisms and drive systems rapidly become very complex as the number of degrees of freedom increases, groups of DE artificial muscles have the potential to generate rich motions combining many translational and rotational degrees of freedom. These artificial muscle systems can mimic the agonist-antagonist approach found in nature, so that active expansion of one artificial muscle is taken up by passive contraction in the other. They can also vary their stiffness. In addition, they have the ability to produce electricity from movement. But departing from the high stiffness paradigm of electromagnetic motors and gearboxes leads to new control challenges, and for soft machines to be truly dexterous like their biological analogues, they need precise control. Humans control their limbs using sensory feedback from strain sensitive cells embedded in muscle. In DE actuators, deformation is inextricably linked to changes in electrical parameters that include capacitance and resistance, so the state of strain can be inferred by sensing these changes, enabling the closed loop control that is critical for a soft machine. But the increased information processing required for a soft machine can impose a substantial burden on a central controller. The natural solution is to distribute control within the mechanism itself. The octopus arm is an example of a soft actuator with a virtually infinite number of degrees of freedom (DOF). The arm utilizes neural ganglia to process sensory data at the local “arm” level and perform complex tasks. Recent advances in soft electronics such as the piezoresistive dielectric elastomer switch (DES) have the potential to be fully integrated with actuators and sensors. With the DE switch, we can produce logic gates, oscillators, and a memory element, the building blocks for a soft computer, thus bringing us closer to emulating smart living structures like the octopus arm. The goal of future research is to develop fully soft machines that exploit smart actuation networks to gain capabilities formerly reserved to nature, and open new vistas in mechanical engineering.
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07.07.Tw Servo and control equipment; robots
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back to top Lasers, Optics, and Optoelectronics

Polarization of glass containing fast and slow ions

M. I. Petrov, Ya. A. Lepen’kin, and A. A. Lipovskii

J. Appl. Phys. 112, 043101 (2012); http://dx.doi.org/10.1063/1.4742975 (8 pages)

Online Publication Date: 16 August 2012

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Polarization of glasses containing two types of differing in mobility positive ions is modeled both for blocking and non-blocking anode conditions. Formation of slower ions concentration peak is demonstrated. Comparison of the model with known experimental results shows a good coincidence. The model is applicable to glass-based integrated optics.
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77.22.Ej Polarization and depolarization
42.70.Ce Glasses, quartz

Thickness dependence of temperature-induced emission mechanism in InGaN/AlGaN short-period superlattices

Young-Kyun Noh, Jeong-Han Seo, Hyo-Seok Choi, Moon-Deock Kim, and Jae-Eung Oh

J. Appl. Phys. 112, 043102 (2012); http://dx.doi.org/10.1063/1.4746744 (5 pages)

Online Publication Date: 16 August 2012

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An analysis of temperature-dependent photoluminescence (PL) spectra for a series of InGaN/AlGaN short-period superlattices (SP-SLs) with different well and barrier thickness is presented. A quantitative model, based on Gaussian-like function of localized electronic states, to fit the temperature-dependent emission peak energy gives good fits over an extended temperature range for all samples. It is found that, among all parameters in the model, the degree of broadening of the Gaussian distribution is strongly dependent of the structural parameters of SP-SLs and determines the anomalous “S-shape” behavior of the temperature-dependent emission energy. In thin well and barrier samples with higher broadening parameter, the temperature-dependence of emission energy is different from those of typical “S-shape” behavior, which is characterized by the bigger red-shift with no blue shift in the temperature range used. The depth of localization, Ea-Eo, is smaller than the corresponding activation energy obtained from the thermal quenching of the PL intensity, thus, indicating that the thermal quenching activation energy and the localization due to band-gap fluctuation most likely have different origins. We demonstrate that, in the InGaN/AlGaN SP-SLs, the interface characteristics also contributes to the temperature-induced PL emission shift as much as the compositional fluctuation does.
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78.20.N- Thermo-optic effects
68.65.Cd Superlattices
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials

Indirect optical absorption in silicon via thin-film surface plasmon

Mads L. Trolle and Thomas G. Pedersen

J. Appl. Phys. 112, 043103 (2012); http://dx.doi.org/10.1063/1.4746699 (5 pages)

Online Publication Date: 17 August 2012

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Optical excitations in semiconductors have been extensively studied in the dipole limit, neglecting the spatial variation of the exciting field. This is a very good approximation in most cases. However, when approaching the regime of nano optics and plasmonics, optical fields of extreme spatial confinement is found, e.g., surface plasmon polaritons (SPPs). Here we investigate the optical absorption of a SPP bound to an Ag thin-film embedded in Si at photon energies below the direct band gap of Si. In this regime, all absorbtion processes in Si must be assisted by a source of crystal momentum to overcome the momentum mismatch of the indirect transition. While phonon-assisted processes are the obvious example, the optical wave vector can also supply the needed momentum, provided the optical field displays sufficient spatial variation. We apply a simple model for the electric field and a band theoretical model response function for Si, including both frequency and wave vector dependence, and find that absorption in the Si medium relative to the Ag film may be increased significantly due to indirect transitions facilitated by the optical wave vector.
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78.66.Db Elemental semiconductors and insulators
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
73.20.At Surface states, band structure, electron density of states
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Film thickness and grating depth variation in organic second-order distributed feedback lasers

Víctor Navarro-Fuster, Igor Vragovic, Eva M. Calzado, Pedro G. Boj, José A. Quintana, José M. Villalvilla, Aritz Retolaza, Aritz Juarros, Deitze Otaduy, Santos Merino, and María A. Díaz-García

J. Appl. Phys. 112, 043104 (2012); http://dx.doi.org/10.1063/1.4745047 (12 pages) | Cited 3 times

Online Publication Date: 21 August 2012

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We report on the preparation and characterization, under optical pump, of second-order one-dimensional distributed feedback (DFB) lasers based on polystyrene films doped with a perylenediimide derivative, as active media. The DFB gratings were engraved on the substrates (SiO2) by thermal nanoimprint lithography, followed by reactive ion etching. Laser emission wavelength was tuned from 554 to 583 nm by changing film thickness (h) between 240 and 1200 nm. The effect on the performance (emission wavelength, threshold, slope efficiency, number of modes, and spectral shape) of varying the grating depth (d) from 30 to 240 nm, for the whole range of h values, has been investigated. Although there is extensive work in the literature aiming to tune the emission wavelength of organic DFB lasers by h variation, the effect of changing d systematically has not been previously studied. Experimental results have been interpreted by models that take into account the presence of the grating by averaging either h or the effective refractive index. Single-mode emission (λ0) was observed for h < 1000 nm, while for thicker films lasing appeared at two different wavelengths (λ0 and λ1). Models indicate that λ0 and λ1 correspond to the TE0 and TE1 waveguide modes, respectively. It was found that d plays an important role in determining the DFB thresholds and slope efficiencies for two h regimes: (i) For h < 350 nm, lowest thresholds and highest slopes efficiencies were obtained with the shallower gratings; and (ii) for h > 1000 nm, d affects significantly the losses associated with the TE1 mode, so single mode emission was achieved at λ0 or at λ1 for deep and shallow gratings, respectively. Finally, the shape of the emission spectra, both below and above threshold, has also been analyzed in order to clarify the physical mechanisms responsible for the existence of gain. Bragg dips were observed in the spectra below threshold only for devices with d/h larger than around 0.3 and their width increased with increasing d/h. In these cases, single-mode DFB emission appeared at the long-wavelength edge of the Bragg dip, indicating that index-coupling modulation contributes significantly to the gain process. On the other hand, for smaller d/h values, Bragg dips became too small to be detected, so gain coupling becomes the dominant mechanism accounting for the presence of gain.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.60.Fc Modulation, tuning, and mode locking
42.79.Dj Gratings
42.82.Cr Fabrication techniques; lithography, pattern transfer
81.65.Cf Surface cleaning, etching, patterning

Light emission lifetimes in p-type δ-doped GaAs/AlAs multiple quantum wells near the Mott transition

J. Kundrotas, A. Čerškus, G. Valušis, L. H. Li, E. H. Linfield, A. Johannessen, and E. Johannessen

J. Appl. Phys. 112, 043105 (2012); http://dx.doi.org/10.1063/1.4745893 (5 pages) | Cited 1 time

Online Publication Date: 21 August 2012

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The time resolved photoluminescence of beryllium δ-doped GaAs/AlAs multiple quantum wells have been studied over a range of doping concentrations, in order to investigate possible mechanisms for the carrier radiative recombination, both above and below the Mott metal-insulator transition. It was found that at doping concentrations near the Mott transition (NBe ∼ 3 × 1012 cm−2), the radiative recombination of excitons-bound-to-acceptor impurities as well as free electrons with acceptor impurities, dominated in the Be δ-doped GaAs/AlAs MQWs (LW = 15 nm) that were used in this study. Above the Mott transition, the major contribution was from radiative recombination of free electrons with a two-dimensional hole gas. The radiative lifetime would therefore exhibit different behavior with doping. In lightly doped GaAs/AlAs MQWs, this changed from 0.3–1 ns at 3.6 K to 8 ns at 300 K, whilst in quantum wells above the Mott transition, it changed from ∼0.36 ns at 3.6 K to ∼1 ns at 300 K, and was also weakly dependent on the concentrations of acceptor doping.
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78.55.Cr III-V semiconductors
78.67.De Quantum wells
78.47.jd Time resolved luminescence
71.10.Ca Electron gas, Fermi gas
71.30.+h Metal-insulator transitions and other electronic transitions
71.35.-y Excitons and related phenomena

The influence of charged ions on the electro-optical properties of polymer-dispersed liquid crystal films prepared by ultraviolet-initiated cationic polymerization

Ping Song, Lilong Yu, Aijun Jiao, Feifei Wang, Fang Liu, Cuihong Zhang, Chaoyong Yang, Hui Cao, and Huai Yang

J. Appl. Phys. 112, 043106 (2012); http://dx.doi.org/10.1063/1.4745907 (7 pages)

Online Publication Date: 23 August 2012

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Polymer-dispersed liquid crystal (PDLC) films based on epoxy resins were prepared by UV-initiated cationic polymerization using UVI-6974 as UV-initiator. The charged ions arose from photolysis of the UV-initiator remained in the liquid crystal (LC) droplet of the polymer matrix. Optical devices based on PDLC films derive their functional properties from the electric field induced reorientation of LC droplets. These PDLC films doped with ions showed different reorientation of LC molecules and electro-optical properties than those prepared without ionic dopants. When an electric field was applied across these PDLC films, the charged ions in the LC droplet migrated along the direction of the electric field in inducement of the electric field and assembled at the LC droplet/polymer interface. According to the situation mentioned above, the model of charged double layers at the LC droplet/polymer interface was set up in our study. The behavior was attributed to the charged double layers that cancel the field strength within the “bulk” of the liquid crystal in time. By using this model, the electric field and initiator concentration dependent results and the difference of the PDLC with and without ions were investigated and explained.
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78.20.Jq Electro-optical effects
82.35.-x Polymers: properties; reactions; polymerization
61.30.Vx Polymer liquid crystals

Spontaneous emission and optical gain characteristics of blue InGaAlN/InGaN quantum well structures with reduced internal field

Seoung-Hwan Park and Doyeol Ahn

J. Appl. Phys. 112, 043107 (2012); http://dx.doi.org/10.1063/1.4747841 (5 pages)

Online Publication Date: 27 August 2012

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Spontaneous emission and optical gain characteristics of blue InGaAlN/InGaN quantum well (QW) structures with reduced internal field were investigated by using the non-Markovian model with many-body effects. The spontaneous emission coefficient of the InGaAlN/InGaN system with reduced internal field is shown to be increased by 30% compared to that of the conventional InGaN/GaN system. This is mainly due to the increase in the optical matrix element by the reduced internal field effect. If the threshold optical gain is assumed to be about 13000cm−1, the InGaAlN/InGaN QW structure is expected to have smaller threshold current density the InGaN/GaN QW structure. In the case of the InGaAlN/InGaN system, the transition wavelength is a weak function of the carrier density.
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78.67.De Quantum wells
78.66.Fd III-V semiconductors

Efficient excitation of a monopole optical transducer for near-field recording

Chubing Peng

J. Appl. Phys. 112, 043108 (2012); http://dx.doi.org/10.1063/1.4747912 (6 pages) | Cited 1 time

Online Publication Date: 28 August 2012

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An optical near-field transducer composed of a rectangular patch and a protruded peg has been numerically studied for heat-assisted magnetic recording. This transducer strongly interacts with a planar solid immersion focusing field and efficiently couples optical energy into a recording medium in a region determined by the peg cross-section. The transducer is excited through the electric field predominantly normal to its edges. The optimal size of the rectangular patch is found to be a half-wave optical antenna in height and between half-wave and full-wave in width.
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07.07.Mp Transducers
02.60.-x Numerical approximation and analysis
84.40.Ba Antennas: theory, components and accessories

Dependences of Brillouin frequency shift on strain and temperature in optical fibers doped with rare-earth ions

Yosuke Mizuno, Neisei Hayashi, and Kentaro Nakamura

J. Appl. Phys. 112, 043109 (2012); http://dx.doi.org/10.1063/1.4747926 (5 pages) | Cited 1 time

Online Publication Date: 29 August 2012

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Brillouin scattering properties in rare-earth-doped fibers, including Nd3+-doped, Tm3+-doped, Sm3+-doped, and Ho3+/Tm3+ co-doped fibers, can potentially be controlled at high speed by pumping, but there has been no report on their detailed investigations. In this study, as the first step toward this goal, the Brillouin gain spectra (BGS) in such rare-earth-doped single-mode fibers are measured, for the first time, at 1.55 μm without pumping, and the Brillouin frequency shift (BFS) and its dependences on strain and temperature are investigated. Clear BGS was observed for the Nd3+-doped and Tm3+-doped fibers, but BGS was not detected for the Sm3+-doped and Ho3+/Tm3+ co-doped fibers probably because of their extremely high propagation losses at 1.55 μm and small Brillouin gain coefficients. The BFS of the Nd3+-doped fiber was ∼10.82 GHz, and its strain and temperature coefficients were 466 MHz/% and 0.726 MHz/K, respectively. As for the Tm3+-doped fiber, the BFS was ∼10.90 GHz, and its strain and temperature coefficients were 433 MHz/% and 0.903 MHz/K, respectively. These measurement results are compared with those of silica fibers.
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42.81.Dp Propagation, scattering, and losses; solitons
42.81.Cn Fiber testing and measurement of fiber parameters

Holographic modification of TiO2 nanostructure for enhanced charge transport in dye-sensitized solar cell

Jinsoo Lee, Junghwan Yoon, Minhea Jin, and Myeongkyu Lee

J. Appl. Phys. 112, 043110 (2012); http://dx.doi.org/10.1063/1.4747939 (4 pages) | Cited 1 time

Online Publication Date: 29 August 2012

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We show that the photocurrent and energy conversion efficiency of dye-sensitized solar cells can be greatly enhanced with holographic modification to the morphology of TiO2 electrode. The nanoporous electrode coated onto conducting glass was irradiated by three interfering laser beams at 1064 nm incident from the backside of the substrate. This generated two-dimensional periodic pillars of higher density in the electrode, through which the photoexcited electrons could be extracted more effectively. The cells fabricated with modified electrodes exhibited average photocurrent and efficiency of 17.14 mA/cm2 and 9.03%, while 14.91 mA/cm2 and 7.83% were obtained from the reference cells. It was attributed to the enhanced charge transport accompanied by the reduction of internal resistance of the electrode.
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88.40.J- Types of solar cells
61.43.Gt Powders, porous materials
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
72.40.+w Photoconduction and photovoltaic effects
73.61.Ng Insulators

Tunability of terahertz random lasers with temperature based on superconducting materials

Abbas Ghasempour Ardakani, Ali Reza Bahrampour, Seyed Mohammad Mahdavi, and Mehdi Hosseini

J. Appl. Phys. 112, 043111 (2012); http://dx.doi.org/10.1063/1.4747837 (8 pages) | Cited 1 time

Online Publication Date: 31 August 2012

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We theoretically demonstrate the tunabiltiy of terahertz random lasers composed of high temperature superconductor YBCO and ruby layers as active medium. The considered system is a one-dimensional disordered medium made of ruby grain and YBCO. Finite-difference time domain method is used to calculate the emission spectrum and spatial distribution of electric field at different temperatures. Our numerical results reveal that the superconductor based random lasers exhibit large temperature tunability in the terahertz domain. The emission spectrum is significantly temperature dependent, the number of lasing modes and their intensities increase with decreasing temperature. Also, we make some discussion to explain the reason for the observed tunability and the effect of temperature variation on the spatial distribution of the electric field in the disordered active medium.
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42.55.Rz Doped-insulator lasers and other solid state lasers
84.40.-x Radiowave and microwave (including millimeter wave) technology
42.60.Fc Modulation, tuning, and mode locking
42.60.By Design of specific laser systems

Investigation of thermal properties of mid-infrared AlGaAs/GaAs quantum cascade lasers

K. Pierściński, D. Pierścińska, M. Iwińska, K. Kosiel, A. Szerling, P. Karbownik, and M. Bugajski

J. Appl. Phys. 112, 043112 (2012); http://dx.doi.org/10.1063/1.4746791 (12 pages) | Cited 1 time

Online Publication Date: 31 August 2012

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We report on detailed experimental investigation of thermal properties of AlGaAs/GaAs quantum cascade lasers (QCLs) emitting at wavelength of 9.4 μm. Different mounting options and device geometries are compared in terms of their influence on the relative increase of the active region temperature. High resolution, spatially resolved thermoreflectance is used for mapping temperature distribution over the facet of pulse operated QCLs. The devices’ thermal resistances are derived from experimental data. We also develop a numerical thermal model of QC lasers, solving heat transport equation in 2D and 3D, which includes anisotropy of thermal conductivity. By combining experimental and numerical results, an insight into thermal management in QCLs is gained. Thermal optimization of the design focuses on improving heat dissipation in the device, which is essential to increase the maximal operation temperature of the devices.
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42.55.Px Semiconductor lasers; laser diodes
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
78.20.N- Thermo-optic effects
back to top Plasmas and Electrical Discharges

Basic data of ions in He-air mixtures for fluid modeling of low temperature plasma jets

M. Yousfi, A. Hennad, M. Benhenni, O. Eichwald, and N. Merbahi

J. Appl. Phys. 112, 043301 (2012); http://dx.doi.org/10.1063/1.4747936 (11 pages)

Online Publication Date: 28 August 2012

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The basic ion data such as interaction potential parameters, elastic and inelastic collision cross sections, transport coefficients (reduced mobility and diffusion coefficients) and reaction coefficients have been analysed and determined for the case of He+, N2+, and O2+ in He-dry air mixtures. The ion transport and reaction coefficients have been determined from an optimized Monte Carlo simulation using calculated elastic and experimentally fitted inelastic collision cross sections. The elastic momentum transfer cross sections have been calculated from a semi-classical JWKB (Jeffreys Wentzel Kramers Brillouin) approximation based on a (6-4) rigid core interaction potential model. The inelastic cross sections have been fitted using the measured reaction coefficients, such as, for instance, the non resonant charge transfer coefficients. The cross section sets involving elastic and inelastic processes were then validated using either the measured reduced mobility whenever available in the literature or the zero-field mobility calculated from Satoh’s relation, and potential parameters available in the literature. From the sets of elastic and inelastic collision cross sections thus obtained for the first time for He+/N2, He+/O2, N2+/He, and O2+/He systems, the ion transport and reaction coefficients were calculated in the pure gases over a wide range of the density reduced electric field E/N. Then, from the present cross section and other literature sets, the ion mobility and the longitudinal and transverse diffusion coefficients were calculated for different concentrations of air in He in the case of He+, N2+, O2+, and also O− ions.
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52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.25.Fi Transport properties
52.30.-q Plasma dynamics and flow
52.65.Pp Monte Carlo methods
back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Disorder-activated Raman spectra of cubic rocksalt-type Li(1−x)/2Ga(1−x)/2MxO (M = Mg, Zn) alloys

Li Lei, Hiroaki Ohfuji, Tetsuo Irifune, Jiaqian Qin, Xinyu Zhang, and Toru Shinmei

J. Appl. Phys. 112, 043501 (2012); http://dx.doi.org/10.1063/1.4739766 (6 pages) | Cited 1 time

Online Publication Date: 16 August 2012

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Disorder-activated phonon behaviors of the cubic rocksalt-type semiconductor alloy Li(1−x)/2Ga(1−x)/2MxO (M = Mg, Zn) prepared by high-pressure and high-temperature method has been studied by Raman scattering analysis. The LO Raman phonon in Li(1−x)/2Ga(1−x)/2MxO was found to exhibit a distinct two-mode behavior. The compositional dependence of Raman frequency, peak-width, and intensity has been discussed. A model associated with a composite mode of the Brillouin zone center and edge phonons combined with phonon dispersion curves determined by first-principle calculations were employed to explain the asymmetric broadening of the LO phonon mode. The broadening and asymmetric Raman line-shape in Li(1−x)/2Ga(1−x)/2MxO can be interpreted as a composite mode of the softening Brillouin zone center mode and the Brillouin zone edge mode.
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78.30.Hv Other nonmetallic inorganics
81.05.Hd Other semiconductors
62.50.-p High-pressure effects in solids and liquids
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
71.15.-m Methods of electronic structure calculations
71.20.Nr Semiconductor compounds

Thermal transport in graphene supported on copper

Liang Chen and Satish Kumar

J. Appl. Phys. 112, 043502 (2012); http://dx.doi.org/10.1063/1.4740071 (7 pages) | Cited 2 times

Online Publication Date: 16 August 2012

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We investigate the thermal transport in isolated single layer graphene (SLG) and SLG supported on Cu substrate using equilibrium molecular dynamics simulations and relaxation time approximation (RTA) method. We observe significant changes in the SLG dispersion curve in low frequency and low wave-vector region due to the interaction with Cu substrate. Several new phonon modes related to out-of-plane vibrations appear at the low frequency and small wave vector regions, but their contribution to graphene thermal conductivity is negligible. The thermal conductivity of graphene decreases by 44% due to the interactions with Cu substrate for high interaction strength parameter in Lennard-Jones potential formulation for graphene-Cu interaction. The phonon mode analysis through the RTA approach shows that the acoustic phonons dominate the thermal transport for both isolated and supported graphenes. The longitudinal acoustic (LA), transverse acoustic (TA), and out-of-plane acoustic (ZA) phonons contribute 654, 330, and 361 W/mK to the lattice thermal conductivity of isolated graphene, respectively. The phonon life time of ZA modes decreases by order of magnitude due to the interactions with Cu substrate and ZA mode contribution to SLG thermal conductivity decreases by 282 W/mK, while the contributions of LA and TA phonons reduce by 77.4 W/mK and 82.9 W/mK, respectively.
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66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
65.80.Ck Thermal properties of graphene
81.05.ue Graphene
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Effect of adatom surface diffusivity on microstructure and intrinsic stress evolutions during Ag film growth

D. Flötotto, Z. M. Wang, L. P. H. Jeurgens, E. Bischoff, and E. J. Mittemeijer

J. Appl. Phys. 112, 043503 (2012); http://dx.doi.org/10.1063/1.4746739 (9 pages) | Cited 2 times

Online Publication Date: 16 August 2012

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The effect of the adatom surface diffusivity on the evolution of the microstructure and the intrinsic stress of thin metal films was investigated for the case of growth of polycrystalline Ag films on amorphous SiO2 (a-SiO2) and amorphous Ge (a-Ge) substrates, with high and low Ag adatom surface diffusivity, respectively. The surface diffusivity of the deposited Ag adatoms on the a-Ge substrate is suppressed also after coalescence of Ag islands due to the continuous (re)segregation of Ge at the surface of the growing film as evidenced by in-situ XPS. An assessment could be made of the role of adatom surface diffusivity on the microstructural development and the intrinsic stress evolution during film growth. As demonstrated by ex-situ TEM and ex-situ XRD, the Ag films grown on the a-SiO2 and a-Ge substrates possess strikingly different microstructures in terms of grain shape, grain size, and crystallographic texture. Nevertheless, the real-time in-situ stress measurements revealed a compressive → tensile → compressive stress evolution for the developing Ag films on both types of substrates, however on different time scales and with stress-component values of largely different magnitudes. It was concluded that (i) the microstructural development of metallic thin films is predominated by the surface diffusivity of the adatoms and (ii) the intrinsic stress evolution is largely controlled by the developing microstructure and the grain-boundary diffusivity.
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68.35.Fx Diffusion; interface formation
68.55.-a Thin film structure and morphology
79.60.Bm Clean metal, semiconductor, and insulator surfaces
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
68.35.bd Metals and alloys
66.30.Fq Self-diffusion in metals, semimetals, and alloys

Local structure, paramagnetic properties, and porosity of natural coals: Spectroscopic studies

A. A. Konchits, B. D. Shanina, M. Ya. Valakh, I. B. Yanchuk, V. O. Yukhymchuk, A. D. Alexeev, T. A. Vasilenko, A. N. Molchanov, and A. K. Kirillov

J. Appl. Phys. 112, 043504 (2012); http://dx.doi.org/10.1063/1.4745015 (9 pages)

Online Publication Date: 17 August 2012

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Using methods of the scanning electron microscopy, Raman scattering of light(RS), and electron paramagnetic resonance (EPR), consistent research of the local structure and magnetic features of different types of raw coal samples from Donetsk basin is carried out. It is established that the ratio of the main peak intensities of RS spectrum D and G is inversely related to the volatile substance amount Vdaf in the coal samples. The study of the kinetic behavior of the EPR line width in hydrogen, oxygen, and methane sorption-desorption processes in each coal sample helped determine that the diffusion coefficient value for hydrogen in coal at room temperature is equal to DН = (2 ÷ 7) × 10−5 cm2/s. It is demonstrated that the oxygen diffusion occurs with time according to two different exponential laws with diffusion coefficients DO,1 = 5 × 10−6 cm2/s and DO,2 = 5.5 × 10−7 cm2/s, respectively. The smaller coefficient corresponds to the diffusion caused by the hopping process. Finally, it is established that the anthracite is a unique type of coal which does not possess the ability “to conserve” the significant EPR line width after oxygen pumping out from the samples.
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61.43.Gt Powders, porous materials
75.20.-g Diamagnetism, paramagnetism, and superparamagnetism
76.30.-v Electron paramagnetic resonance and relaxation
78.35.+c Brillouin and Rayleigh scattering; other light scattering
66.30.H- Self-diffusion and ionic conduction in nonmetals
68.43.Nr Desorption kinetics

Coarsening kinetics of topologically highly correlated grain boundary networks

Ming Tang, Bryan W. Reed, and Mukul Kumar

J. Appl. Phys. 112, 043505 (2012); http://dx.doi.org/10.1063/1.4740070 (14 pages) | Cited 1 time

Online Publication Date: 21 August 2012

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We apply phase-field simulations in two dimensions to study the thermal coarsening of grain boundary (GB) networks with high fractions of twin and twin-variant boundaries, which for example are seen in grain-boundary-engineered FCC materials. Two types of grain boundary networks with similar starting special boundary fractions but different topological features were considered as initial conditions for the grain growth simulations. A lattice Monte Carlo method creates polycrystalline microstructures (Reed and Kumar (RK)), which exhibit hierarchical organization of random and special coincidence site lattice boundaries. The other type of microstructures (randomly distributed (RD)) contains random distributions of special boundaries subject only to crystallographic constraints. Under the assumption that random boundaries have larger energy and much higher mobility than special boundaries, simulations show that increasing the initial special boundary fraction in both microstructures slows down grain growth. However, the two starting microstructures exhibit very different behavior in the evolution of GB character and triple junction (TJ) distributions. The RD networks coarsened more slowly than the RK networks with comparable initial fractions of special boundaries. The observed trend in the evolution of the RK microstructures is explained by an extended von Neumann-Mullins analysis. This study demonstrates that the special boundary fraction is not a sufficient indicator of the coarsening behavior of twinned GB networks; the network topology must also be considered to correctly predict the grain growth kinetics.
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61.72.Mm Grain and twin boundaries

Compliant Si nanostructures on SOI for Ge nanoheteroepitaxy—A case study for lattice mismatched semiconductor integration on Si(001)

P. Zaumseil, G. Kozlowski, Y. Yamamoto, J. Bauer, M. A. Schubert, T. U. Schülli, B. Tillack, and T. Schroeder

J. Appl. Phys. 112, 043506 (2012); http://dx.doi.org/10.1063/1.4748341 (5 pages)

Online Publication Date: 23 August 2012

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The integration of lattice mismatched semiconductors on Si(001) is of fundamental importance to further increase the performance and/or functionality of today’s Si integrated circuits. The theory of compliant substrate effects offers the vision to integrate defect-free alternative semiconductor structures on Si. This concept is based on balancing the mismatch strain between the overgrowing epitaxial semiconductor and the Si substrate by a strain partitioning phenomenon. Using the Ge/Si heterosystem as a case study, we report by advanced 3rd generation synchrotron as well as laboratory techniques for materials characterization on the nanoscale clear experimental evidence for the compliance of Si nanoislands on SOI for selective Ge nanoheteroepitaxy. This integration concept is not limited to Ge but extendable to semiconductors like III-V and II-VI materials.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
61.46.-w Structure of nanoscale materials
68.55.-a Thin film structure and morphology

Red-IR stimulated luminescence in K-feldspar: Single or multiple trap origin?

Martin Thalbitzer Andersen, Mayank Jain, and Peter Tidemand-Lichtenberg

J. Appl. Phys. 112, 043507 (2012); http://dx.doi.org/10.1063/1.4745018 (11 pages) | Cited 1 time

Online Publication Date: 24 August 2012

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We investigate on the origins of the infra-red stimulated luminescence (IRSL) signals in 3 potassium feldspars based on IR-red spectroscopy (∼700–1050 nm) using a fiber-coupled tunable Ti:Sapphire laser, in combination with different thermal and optical (pre)treatments of the samples. We also measure dose-response curves with different wavelengths and at different stimulation temperatures so as to be able to distinguish between traps based on their electron trapping cross-sections. Our data suggest that the dosimetric signals, IRSL, and the post IR-IRSL in K-feldspars arise from a single electron trapping centre.
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78.55.Hx Other solid inorganic materials
81.40.Gh Other heat and thermomechanical treatments
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.80.Sk Insulators
78.30.Hv Other nonmetallic inorganics
78.45.+h Stimulated emission

Impact fragmentation of aluminum reactive materials

Joseph P. Hooper

J. Appl. Phys. 112, 043508 (2012); http://dx.doi.org/10.1063/1.4746788 (7 pages)

Online Publication Date: 24 August 2012

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We report the fragmentation of brittle, granular aluminum spheres following high velocity impact (0.5-2.0 km/s) on thin steel plates. These spheres, machined from isostatically pressed aluminum powder, represent a prototypical metallic reactive material. The fragments generated by the impacts are collected in a soft-catch apparatus and analyzed down to a length scale of 44μm. With increasing velocity, there is a transition from an exponential Poisson-process fragment distribution with a characteristic length scale to a power-law behavior indicative of scale-invariance. A normalized power-law distribution with a finite size cutoff is introduced and used to analyze the number and mass distributions of the recovered fragments. At high impact velocities, the power-law behavior dominates the distribution and the power-law exponent is identical to the universal value for brittle fragmentation discussed in recent works. The length scale at which the power-law behavior decays is consistent with the idea that the length of side microbranches or damage zones from primary cracks is governing this cutoff. The transition in fragment distribution at high strain-rates also implies a significant increase in small fragments that can rapidly combust in an ambient atmosphere.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.mj Brittleness
62.20.mt Cracks
61.43.Gt Powders, porous materials

Enhancement of thermoelectric properties of CoSb3-based skutterudites by double filling of Tl and In

Adul Harnwunggmoung, Ken Kurosaki, Atsuko Kosuga, Manabu Ishimaru, Theerayuth Plirdpring, Rattikorn Yimnirun, Jaru Jutimoosik, Saroj Rujirawat, Yuji Ohishi, Hiroaki Muta, and Shinsuke Yamanaka

J. Appl. Phys. 112, 043509 (2012); http://dx.doi.org/10.1063/1.4748340 (6 pages)

Online Publication Date: 27 August 2012

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Thermoelectric (TE) generators can directly generate electrical power from waste heat, and thus could be an important part of the solution to future power supply and sustainable energy management. The main obstacle to the widespread use of TE materials in diverse industries, e.g., for exhaust heat recovery in automobiles, is their low efficiency in converting heat to electricity. The conversion efficiency of TE materials is quantified by the dimensionless figure of merit, ZT, and the way to enhance ZT is to decrease the lattice thermal conductivity (κlat) of the material, while maintaining a high electrical conductivity, i.e., to create a situation in which phonons are scattered but electrons are unaffected. Here, we report skutterudites filled by Tl and In, Tl0.1InxCo4Sb12, which allow a dramatic reduction of κlat, yielding a ZT of 1.2 at 700 K. We demonstrate that the reduction of κlat is due to the effective phonon scattering induced both by the rattling of Tl and In and by the naturally formed In2O3 nanoparticles (<50 nm). The combined approach of double filling and self-formed nanostructures might be applicable to various clathrate compounds. Thus, our results point to a new strategy in the improvement of bulk TE materials.
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72.20.Pa Thermoelectric and thermomagnetic effects
84.60.Rb Thermoelectric, electrogasdynamic and other direct energy conversion
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
63.22.Kn Clusters and nanocrystals

Raman study of the Verwey transition in magnetite at high-pressure and low-temperature: Effect of Al doping

L. Gasparov, Z. Shirshikova, T. M. Pekarek, J. Blackburn, V. Struzhkin, A. Gavriliuk, R. Rueckamp, and H. Berger

J. Appl. Phys. 112, 043510 (2012); http://dx.doi.org/10.1063/1.4747834 (5 pages)

Online Publication Date: 28 August 2012

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We employed Raman spectroscopy to investigate how the hydrostatic pressure affects the temperature of the Verwey transition in pure (Fe3O4) and Al-doped (Fe2.8Al0.2O4) magnetite. In both samples, pressure suppresses the transition. The Al-doped sample displays no transition above 8 GPa. We do not observe such discontinuity in a pure magnetite, which suggests that the discontinuity is doping driven. Our Clausius-Clapeyron formula based analysis of the pressure–transition temperature dependence is in excellent agreement with our data in pure magnetite. The Al doping leads to a smaller entropy change and larger volume expansion consistent with partial charge ordering at the transition.
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78.30.Hv Other nonmetallic inorganics
62.50.-p High-pressure effects in solids and liquids
61.72.up Other materials
75.50.Gg Ferrimagnetics
71.30.+h Metal-insulator transitions and other electronic transitions
65.40.gd Entropy

Hydrogen diffusion in lead zirconate titanate and barium titanate

K. J. Alvine, M. Vijayakumar, M. E. Bowden, A. L. Schemer-Kohrn, and S. G. Pitman

J. Appl. Phys. 112, 043511 (2012); http://dx.doi.org/10.1063/1.4748283 (6 pages)

Online Publication Date: 28 August 2012

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Hydrogen is a potential clean-burning, next-generation fuel for vehicle and stationary power. Unfortunately, hydrogen is also well known to have serious materials compatibility issues in metals, polymers, and ceramics. Piezoelectric actuator materials proposed for low-cost, high efficiency high-pressure hydrogen internal combustion engines (HICE) are known to degrade rapidly in hydrogen. This limits their potential use and poses challenges for HICE. Hydrogen-induced degradation of piezoelectrics is also an issue for low-pressure hydrogen passivation in ferroelectric random access memory. Currently, there is a lack of data in the literature on hydrogen species diffusion in piezoelectrics in the temperature range appropriate for the HICE as charged via a gaseous route. We present 1HNMR quantification of the local hydrogen species diffusion within lead zirconate titanate and barium titanate on samples charged by exposure to high-pressure gaseous hydrogen ∼32 MPa. Results are discussed in context of theoretically predicted interstitial hydrogen lattice sites and aqueous charging experiments from existing literature.
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66.30.-h Diffusion in solids
77.65.-j Piezoelectricity and electromechanical effects
77.84.Cg PZT ceramics and other titanates
76.60.-k Nuclear magnetic resonance and relaxation
62.50.-p High-pressure effects in solids and liquids
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