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15 Apr 2012

Volume 111, Issue 8, Articles (08xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 111, 084701 (2012); http://dx.doi.org/10.1063/1.3698319 (11 pages)

Xerxes Lopez-Yglesias, Jason M. Gamba, and Richard C. Flagan
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back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Mesoscale modeling of intergranular bubble percolation in nuclear fuels

Paul C. Millett, Michael Tonks, and S. B. Biner

J. Appl. Phys. 111, 083511 (2012); http://dx.doi.org/10.1063/1.3702872 (7 pages)

Online Publication Date: 18 April 2012

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Phase-field simulations are used to examine the variability of intergranular fission gas bubble growth and percolation on uranium dioxide grain boundaries on a mesoscopic length scale. Three key parameters are systematically varied in this study: the contact angle (or dihedral angle) defining the bubble shape, the initial bubble density on the grain boundary plane, and the ratio of the gas diffusivity on the grain boundary versus the grain interiors. The simulation results agree well with previous experimental data obtained for bubble densities and average bubble areas during coalescence events. Interestingly, the rate of percolation is found to be highly variable, with a large dependency on the contact angle and the initial bubble density and little-to-no dependency on the grain boundary gas diffusivity.
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28.41.Bm Fuel elements, preparation, reloading, and reprocessing
28.41.Te Protection systems, safety, radiation monitoring, accidents, and dismantling

The consequences of high injected carrier densities on carrier localization and efficiency droop in InGaN/GaN quantum well structures

S. Hammersley, D. Watson-Parris, P. Dawson, M. J. Godfrey, T. J. Badcock, M. J. Kappers, C. McAleese, R. A. Oliver, and C. J. Humphreys

J. Appl. Phys. 111, 083512 (2012); http://dx.doi.org/10.1063/1.3703062 (6 pages) | Cited 4 times

Online Publication Date: 18 April 2012

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There is a great deal of interest in the underlying causes of efficiency droop in InGaN/GaN quantum well light emitting diodes, with several physical mechanisms being put forward to explain the phenomenon. In this paper we report on the observation of a reduction in the localization induced S-shape temperature dependence of the peak photoluminescence energy with increasing excitation power density. This S-shape dependence is a key fingerprint of carrier localization. Over the range of excitation power density where the depth of the S shape is reduced, we also observe a reduction in the integrated photoluminescence intensity per unit excitation power, i.e., efficiency droop. Hence, the onset of efficiency droop occurs at the same carrier density as the onset of carrier delocalization. We correlate these experimental results with the predictions of a theoretical model of the effects of carrier localization due to local variations in the concentration of the randomly distributed In atoms on the optical properties of InGaN/GaN quantum wells. On the basis of this comparison of theory with experiment we attribute the reduction in the S-shape temperature dependence to the saturation of the available localized states. We propose that this saturation of the localized states is a contributory factor to efficiency droop whereby nonlocalized carriers recombine non-radiatively.
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81.05.Ea III-V semiconductors
81.07.St Quantum wells
85.60.Jb Light-emitting devices
78.67.De Quantum wells
78.55.Cr III-V semiconductors
73.63.Hs Quantum wells

Dynamic shear stress and heat transfer of solid hydrogen, deuterium, and neon

Jacob Leachman, John Pfotenhauer, and Greg Nellis

J. Appl. Phys. 111, 083513 (2012); http://dx.doi.org/10.1063/1.3703566 (8 pages)

Online Publication Date: 18 April 2012

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Solidifying and solid cryogens in steady shear are of fundamental importance to the fueling of fusion energy reactors. We present steady-state measurements of the dynamic shear stress, thermal conductivity, and heat transfer during flow of solid hydrogen, deuterium, and neon in a Couette-type viscometer cell. The measurements span a range of shear rates and temperatures from the onset of solidification to sub-cooled solid states. Normalization of the shear stress with parameters of the Lennard-Jones 6–12 potential indicates a favorable prediction of tritium properties using the quantum law of corresponding states.
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47.27.te Turbulent convective heat transfer
07.20.Mc Cryogenics; refrigerators, low-temperature detectors, and other low-temperature equipment
81.30.Fb Solidification
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves

Bandgap properties of diamond structure photonic crystals with line defects

Wei Dai, Hong Wang, and Shibin Chen

J. Appl. Phys. 111, 083514 (2012); http://dx.doi.org/10.1063/1.3703569 (5 pages) | Cited 1 time

Online Publication Date: 18 April 2012

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Three dimensional diamond structure photonic crystals with line defects fabricated by rapid prototyping and gel casting with alumina were studied. The influence of the line defect shapes, which include rectangular, ellipse, circle, and square shapes of the cross section, on the transmission properties of the electromagnetic wave was first studied. The resonant mode of the rectangular line defect is the strongest with the guiding band from 11.0 to 11.75 GHz and the normalized resonant intensity is 0.8. If the TE10 mode of the propagating electromagnetic wave is parallel to the polarization direction of the rectangular line defect, the property of the line defect will be strengthened. Two separate line defects with a point defect between them can realize the properties of a straight complete line defect through the coupling between them. The bending properties of the line defect were investigated with the bending angle from 0° to 90° at 15° interval. With the increasing of the bending angle, the guiding bandwidth was reduced slowly while the resonant intensity did not change too much.
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42.70.Qs Photonic bandgap materials
61.72.J- Point defects and defect clusters
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.25.Bs Wave propagation, transmission and absorption
81.05.ug Diamond
81.30.Fb Solidification

Tailoring electrically induced properties by stretching relaxor polymer films

V. Bobnar, X. Li, G. Casar, A. Eršte, S. Glinšek, X. Qian, and Q. M. Zhang

J. Appl. Phys. 111, 083515 (2012); http://dx.doi.org/10.1063/1.4704364 (4 pages) | Cited 1 time

Online Publication Date: 18 April 2012

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Electrically induced behavior was investigated and compared in the non-stretched and uniaxially stretched poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene), P(VDF-TrFE-CFE) terpolymer—a member of the P(VDF-TrFE)-based relaxor polymers family that exhibits fast response speeds, giant electrostriction, high electric energy density, and large electrocaloric effect. Although the temperature dependence of the low-field complex dielectric constant is almost identical in the non-stretched and stretched samples, the dc bias electric field via higher nonlinear contribution more heavily alters the dielectric response of the non-stretched terpolymer. The polarization response and, particularly, the induced electrostrictive strain are, on the other hand, much higher in the more-oriented stretched films. The changes in polar-correlation range induced by film stretching also strongly influence the directly measured electrocaloric response, which shows more pronounced temperature dependence in the stretched terpolymer. These results suggest that electrically induced properties of relaxor polymer films can be tailored by controlling the preparation conditions.
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68.55.am Polymers and organics
77.70.+a Pyroelectric and electrocaloric effects
77.22.Ch Permittivity (dielectric function)
61.41.+e Polymers, elastomers, and plastics

Mixture model for determination of shock equation of state

Jennifer L. Jordan and Melvin R. Baer

J. Appl. Phys. 111, 083516 (2012); http://dx.doi.org/10.1063/1.3702873 (10 pages)

Online Publication Date: 19 April 2012

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Simple mixture models for the prediction of shock equations of state (Hugoniot) are a necessary tool for characterization of multiple composites. A mixture model for determining the shock equation of state of composite materials is presented. The model is completely flexible allowing for multiple (>2) components. Additionally, error propagation analysis for the two component mixture model has been accomplished. The model predicts the equation of state to 5%–15% of the experimental data, which is comparable to variations realized in meso-scale modeling of similar materials.
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62.50.Ef Shock wave effects in solids and liquids
64.75.Ef Mixing
64.10.+h General theory of equations of state and phase equilibria

Key experimental information on intermediate-range atomic structures in amorphous Ge2Sb2Te5 phase change material

Shinya Hosokawa, Wolf-Christian Pilgrim, Astrid Höhle, Daniel Szubrin, Nathalie Boudet, Jean-François Bérar, and Kenji Maruyama

J. Appl. Phys. 111, 083517 (2012); http://dx.doi.org/10.1063/1.3703570 (9 pages) | Cited 1 time

Online Publication Date: 19 April 2012

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Laser-induced crystalline-amorphous phase change of Ge-Sb-Te alloys is the key mechanism enabling the fast and stable writing/erasing processes in rewritable optical storage devices, such as digital versatile disk (DVD) or blu-ray disk. Although the structural information in the amorphous phase is essential for clarifying this fast process, as well as long lasting stabilities of both the phases, experimental works were mostly limited to the short-range order by x ray absorption fine structure. Here we show both the short and intermediate-range atomic structures of amorphous DVD material, Ge2Sb2Te5 (GST), investigated by a combination of anomalous x ray scattering and reverse Monte Carlo modeling. From the obtained atomic configurations of amorphous GST, we have found that the Sb atoms and half of the Ge atoms play roles in the fast phase change process of order-disorder transition, while the remaining Ge atoms act for the proper activation energy of barriers between the amorphous and crystalline phases.
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42.62.-b Laser applications
78.70.Dm X-ray absorption spectra
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
64.60.Cn Order-disorder transformations
02.50.-r Probability theory, stochastic processes, and statistics
78.70.Ck X-ray scattering

Polarization and angular effects of femtosecond laser-induced conical microstructures on Ni

Taek Yong Hwang and Chunlei Guo

J. Appl. Phys. 111, 083518 (2012); http://dx.doi.org/10.1063/1.4704394 (4 pages)

Online Publication Date: 19 April 2012

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In this paper, through femtosecond laser pulse irradiation with various polarizations and incident angles, we create arrays of nanostructure-covered conical microstructures (NC-CMs) on Ni. We show that the shape of CMs depends significantly on the polarization and incident angle of the laser beam, and find that the size of nanostructures is distributed asymmetrically on the CMs at off normal incidence. We suggest that nonuniform energy deposition owing to the polarization and the incident angle of laser beam plays an important role in the asymmetric nanostructure distribution and shape of CMs on Ni.
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78.47.J- Ultrafast spectroscopy (<1 psec)
81.65.-b Surface treatments
42.62.-b Laser applications

Thermal poling of alkaline earth boroaluminosilicate glasses with intrinsically high dielectric breakdown strength

Nicholas J. Smith, Michael T. Lanagan, and Carlo G. Pantano

J. Appl. Phys. 111, 083519 (2012); http://dx.doi.org/10.1063/1.3703679 (9 pages) | Cited 1 time

Online Publication Date: 20 April 2012

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Per the rectification model of thermal poling, it has been proposed that intrinsic breakdown strength plays a strong limiting role in the internal DC fields supported by the glass from the poling process. One might therefore hypothesize proportionately larger second-order nonlinearity (SON) in glasses with intrinsically high dielectric breakdown strength. We test these ideas by thermal poling of two different commercial alkali-free alkaline-earth boroaluminosilicate display glasses—one with barium only (AF45 from Schott), and the other with a mixture of alkaline-earth ions (OA-10 G from NEG). Not only are such compositions relevant from a commercial standpoint, they are also interesting in that they have been recently shown to exhibit remarkably high intrinsic dielectric breakdown strengths of 11–14 MV/cm. Quantitative Maker fringe and stack Maker-fringe measurements provide an accurate evaluation of the poling-induced SON susceptibilities, and indicate maximum χ(2) values of 0.44 and 0.26 pm/V in these glasses. These values are comparable to those reported for silica and other multicomponent glasses. Thus, the hypothesis that higher χ(2) would be observed in high intrinsic breakdown strength glasses was not validated. Based on our application of the rectification model, internal fields of the order 2–4 MV/cm were calculated, which are well below the measured intrinsic breakdown strengths at room temperature. The most plausible explanation for these observations is nonlinear electronic conduction effects taking place within the depletion region at the poling temperature, limiting internal fields to a fraction of the breakdown field.
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77.22.Ej Polarization and depolarization
77.22.Jp Dielectric breakdown and space-charge effects

Kinetic model for dependence of thin film stress on growth rate, temperature, and microstructure

E. Chason, J. W. Shin, S. J. Hearne, and L. B. Freund

J. Appl. Phys. 111, 083520 (2012); http://dx.doi.org/10.1063/1.4704683 (9 pages)

Online Publication Date: 20 April 2012

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During deposition, many thin films go through a range of stress states, changing from compressive to tensile and back again. In addition, the stress depends strongly on the processing and material parameters. We have developed a simple analytical model to describe the stress evolution in terms of a kinetic competition between different mechanisms of stress generation and relaxation at the triple junction where the surface and grain boundary intersect. The model describes how the steady state stress scales with the dimensionless parameter D/LR where D is the diffusivity, R is the growth rate, and L is the grain size. It also explains the transition from tensile to compressive stress as the microstructure evolves from isolated islands to a continuous film. We compare calculations from the model with measurements of the stress dependence on grain size and growth rate in the steady state regime and of the evolution of stress with thickness for different temperatures.
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68.60.Bs Mechanical and acoustical properties
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep
61.72.Mm Grain and twin boundaries
68.55.A- Nucleation and growth
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances

Identification of visible emission from ZnO quantum dots: Excitation-dependence and size-dependence

Xiaoyong Xu, Chunxiang Xu, Zengliang Shi, Chi Yang, Bin Yu, and Jingguo Hu

J. Appl. Phys. 111, 083521 (2012); http://dx.doi.org/10.1063/1.4705395 (6 pages) | Cited 1 time

Online Publication Date: 23 April 2012

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ZnO quantum dots (QDs) with uniform shape and different sizes were synthesized by a simple sol-gel method. The visible emission of the ZnO QDs displays highly both excitation-dependent and size-dependent behaviors. The results indicate that the green emission should be attributed to the transition of electrons from the conduction band to a certain deep trap related mainly to defects on the surface, while the violet emission may correspond to the transition of electrons from the shallow donor levels to the valence band. This work is favor to clarify the transition mechanism of visible emission and to extend optical and electronic applications. Particularly, the importance of combining the excitation effect with quantum size effect for investigating photoluminescence of QDs is first highlighted.
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78.67.Hc Quantum dots
81.07.Ta Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
71.20.Nr Semiconductor compounds
81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
78.55.Et II-VI semiconductors

Shock compression response of α″-Fe16N2 nanoparticles

Christopher Wehrenberg, Brian Zande, Satoru Simizu, R. T. Obermyer, S. G. Sankar, and Naresh Thadhani

J. Appl. Phys. 111, 083522 (2012); http://dx.doi.org/10.1063/1.4704368 (6 pages) | Cited 1 time

Online Publication Date: 23 April 2012

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The shock compression response of α''-Fe16N2 nanoparticles is studied in order to evaluate the thermo-mechanical stability of the α'' phase under shock loading. Shock compression experiments were performed using the parallel plate impact gas gun set up at velocities near 850 m/s. The experiments were performed at −21 °C and −126 °C in order to minimize temperature excursions and thereby determine the thermal effects on the phase stability. The α''-Fe16N2 phase is observed to partially decompose into α-Fe and γ'-Fe4N in both samples, as detected by the XRD analysis. The lower temperature produces a higher percentage of γ' phase in addition to the γ''-Fe50N50 phase. The pressure-volume-temperature response of the material is modeled using the dynamic mechanical properties of high nitrogen steel. The model is then used to evaluate the temperature excursions and their effect on the phase transitions observed.
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81.40.Gh Other heat and thermomechanical treatments
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Tt Fine-particle systems; nanocrystalline materials
75.75.-c Magnetic properties of nanostructures
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Characteristic free volume change of bulk metallic glasses

Qiang Hu, Xie-Rong Zeng, and M. W. Fu

J. Appl. Phys. 111, 083523 (2012); http://dx.doi.org/10.1063/1.4704688 (9 pages) | Cited 1 time

Online Publication Date: 23 April 2012

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The free volume change ΔVf(T) of bulk metallic glasses (BMGs) relative to a hypothesized amorphous reference state was measured using the thermal dilatation method. The characteristic free volume change, i.e., the free volume released in structural relaxation ΔVf-sr, was identified quantitatively from the ΔVf(T) curve. For a Fe-based BMG, it was found that ΔVf-sr increases with decreases in the sample diameter and heating rate. ΔVf-sr measured under the same sample diameter and heating rate conditions allowed the convenient comparison of different BMGs. The comparison revealed that the glass-forming ability (GFA) enhancement of each of two Pd-, Mg-, Cu-, Zr-, Ti-, and Fe-based BMGs can be sensitively reflected in the decrease in ΔVf-sr and the narrowing of the difference between the peak temperature of the thermal expansion coefficient and the end temperature of the glass transition process. In addition, for these twelve typical BMGs, there is a good linear relationship between ΔVf-sr and LogDc2 or LogDc, where Dc is the critical diameter. ΔVf-sr is thus sensitive to and has a close correlation with GFA. Furthermore, the ΔVf-sr measurement results are in good agreement with the free volume change measured with the specific heat capacity, room temperature density, and positron annihilation lifetime methods. In the study of the relationship between the structure and properties of BMGs, ΔVf-sr thus plays an important role given its comparability and convenience.
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61.43.Fs Glasses
64.70.pe Metallic glasses
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
78.70.Bj Positron annihilation

Structural recovery of ion implanted ZnO nanowires

G. Perillat-Merceroz, F. Donatini, R. Thierry, P.-H. Jouneau, P. Ferret, and G. Feuillet

J. Appl. Phys. 111, 083524 (2012); http://dx.doi.org/10.1063/1.4704697 (5 pages)

Online Publication Date: 23 April 2012

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Ion implantation is an interesting method to dope semiconducting materials such as zinc oxide provided that the implantation-induced defects can be subsequently removed. Nitrogen implantation followed by anneals under O2 were carried out on zinc oxide nanowires in the same conditions as in a previous study on bulk ZnO [Perillat-Merceroz et al., J. Appl. Phys. 109, 023513 (2011)], allowing a direct comparison of the defect recovery mechanisms. Transmission electron microscopy and cathodoluminescence were carried out to assess the effects of nitrogen implantation and of subsequent anneals on the structural and optical properties of ZnO nanowires. Defect recovery is shown to be more effective in nanowires compared with bulk material due to the proximity of free surfaces. Nevertheless, the optical emission of implanted and annealed nanowires deteriorated compared to as-grown nanowires, as also observed for unimplanted and annealed nanowires. This is tentatively attributed to the dissociation of excitons in the space charge region induced by O2 adsorption on the nanowire surface.
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81.16.-c Methods of micro- and nanofabrication and processing
61.72.U- Doping and impurity implantation
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)
78.60.Hk Cathodoluminescence, ionoluminescence
71.35.-y Excitons and related phenomena
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Temperature dependent elastic constants for crystals with arbitrary symmetry: Combined first principles and continuum elasticity theory

Tianjiao Shao, Bin Wen, Roderick Melnik, Shan Yao, Yoshiyuki Kawazoe, and Yongjun Tian

J. Appl. Phys. 111, 083525 (2012); http://dx.doi.org/10.1063/1.4704698 (7 pages) | Cited 2 times

Online Publication Date: 23 April 2012

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To study temperature dependent elastic constants, a new computational method is proposed by combining continuum elasticity theory and first principles calculations. A Gibbs free energy function with one variable with respect to strain at given temperature and pressure was derived; hence, the minimization of the Gibbs free energy with respect to temperature and lattice parameters can be put into effective operation by using first principles. Therefore, with this new theory, anisotropic thermal expansion and temperature dependent elastic constants can be obtained for crystals with arbitrary symmetry. In addition, we apply our method to hexagonal beryllium, hexagonal diamond, and cubic diamond to illustrate its general applicability.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.dq Other elastic constants
65.40.De Thermal expansion; thermomechanical effects
61.66.Bi Elemental solids

Multiple excitation process in deep-ultraviolet emission from AlGdN thin films pumped by an electron beam

Shinya Iwahashi, Naohiro Kishi, Shinya Kitayama, Takashi Kita, Yoshitaka Chigi, Tetsuro Nishimoto, Hiroyuki Tanaka, Mikihiro Kobayashi, Tsuguo Ishihara, and Hirokazu Izumi

J. Appl. Phys. 111, 083526 (2012); http://dx.doi.org/10.1063/1.4705416 (4 pages)

Online Publication Date: 25 April 2012

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We studied the deep-ultraviolet emission properties of Al0.999Gd0.001 N thin films pumped by an electron beam. The Al0.999Gd0.001 N thin films were grown on fused silica substrates using an ultra-pure reactive sputtering technique. The intra-orbital electron transition of the Gd3+ ions in Al0.999Gd0.001 N showed an extremely narrow luminescence line at 318 nm. We fabricated field-emission devices using an Al0.999Gd0.001 N phosphor thin film and analyzed the dependence of the device characteristics on the injected current and acceleration voltage. The maximum output power was 1.0 mW/cm2. The excitation cross section was of the order of 10−13 cm2 and was found to depend on the acceleration voltage. These results indicate that injected high-energy electrons multiply excite Gd3+ ion.
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81.15.Cd Deposition by sputtering
78.66.Fd III-V semiconductors
81.05.Ea III-V semiconductors
68.55.ag Semiconductors
78.55.Cr III-V semiconductors

Helium irradiation effects in polycrystalline Si, silica, and single crystal Si

K. J. Abrams, J. A. Hinks, C. J. Pawley, G. Greaves, J. A. van den Berg, D. Eyidi, M. B. Ward, and S. E. Donnelly

J. Appl. Phys. 111, 083527 (2012); http://dx.doi.org/10.1063/1.4705450 (6 pages)

Online Publication Date: 26 April 2012

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Transmission electron microscopy (TEM) has been used to investigate the effects of room temperature 6 keV helium ion irradiation of a thin (≈55 nm thick) tri-layer consisting of polycrystalline Si, silica, and single-crystal Si. The ion irradiation was carried out in situ within the TEM under conditions where approximately 24% of the incident ions came to rest in the specimen. This paper reports on the comparative development of irradiation-induced defects (primarily helium bubbles) in the polycrystalline Si and single-crystal Si under ion irradiation and provides direct measurement of a radiation-induced increase in the width of the polycrystalline layer and shrinkage of the silica layer. Analysis using TEM and electron energy-loss spectroscopy has led to the hypothesis that these result from helium-bubble-induced swelling of the silicon and radiation-induced viscoelastic flow processes in the silica under the influence of stresses applied by the swollen Si layers. The silicon and silica layers are sputtered as a result of the helium ion irradiation; however, this is estimated to be a relatively minor effect with swelling and stress-related viscoelastic flow being the dominant mechanisms of dimensional change.
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61.80.Jh Ion radiation effects
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
71.55.Cn Elemental semiconductors

Molecular dynamics simulation study on heat transport in monolayer graphene sheet with various geometries

Ajing Cao

J. Appl. Phys. 111, 083528 (2012); http://dx.doi.org/10.1063/1.4705510 (9 pages) | Cited 2 times

Online Publication Date: 26 April 2012

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Using non-equilibrium molecular dynamics (MD) simulations, we study heat transport in monolayer graphene sheet. We show that the thermal transport in monolayer graphene sheet exhibits a strong length dependence on thermal conductivity, reaching 2360 W/mK at 2.8 μm. By modeling a two-dimensional heat spread type of heat conduction mimicking the experimental probing using the excitation laser light focused on a graphene, the isotropic nature of heat flow in graphene is revealed, which is in support of recent experimental probing. The T−1 dependence of thermal conductivity is observed at temperatures above room temperature. A peak value at 300 K is observed with further decreasing T, in good agreement with that of carbon nanotubes reported experimentally. Thermal conductivity of graphene nanoribbons (GNRs) strongly depends on the ribbon width, which is attributed to arise from the surface phonon scattering. Furthermore, the nonlinear temperature profile is revealed for asymmetric GNRs. A fitting approach for the MD obtained temperature profile based upon the analytic solution is proposed to obtain the thermal conductivity of GNRs of asymmetric geometry. These findings shed light on tuning thermal properties of GNRs with geometry optimizations.
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65.80.Ck Thermal properties of graphene
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
61.48.Gh Structure of graphene
63.22.Rc Phonons in graphene

Optimizing the crystal environment through extended x-ray absorption fine structure to increase the luminescent lifetimes of Er3+ doped Y2O3 nanoparticles

James A. Dorman, Ju H. Choi, Gregory Kuzmanich, John R. Bargar, and Jane P. Chang

J. Appl. Phys. 111, 083529 (2012); http://dx.doi.org/10.1063/1.3702789 (8 pages)

Online Publication Date: 26 April 2012

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To predict and optimize luminescence efficiency of rare-earth ion doped (RE) nanophosphors, a relationship between the RE-concentration and the luminescent parameters is often obtained by Judd-Ofelt analysis, where the quality factor (χ = Ω4/Ω6) depends on the Er interactions with other RE elements in the second nearest neighboring shell. In this work, a detailed analysis of the local bonding environment by extended x-ray absorption fine structure (EXAFS) analyses is shown as effective as the Judd-Ofelt analysis to quantify the Er↔RE interaction in the second nearest neighboring shell (ρN = IREr↔RE2/IREr↔RE1). As the physical basis of ρN is consistent to that of χ, the EXAFS analysis becomes a viable alternative to replace Judd-Ofelt analysis to predict the optimum dopant concentration. This approach was corroborated based on analysis of Er3+:Y2O3 and core-shell Er3+:Y2O3|Y2O3 (5 nm shell) nanoparticles (NPs), with Er3+ concentrations up to 20 mol %. The ρN ratio from EXAFS analysis was shown to strongly correlate to the lifetimes extracted from the Judd-Ofelt analysis, both predicting the optimal dopant concentrations to be at 5 mol % and 2 mol % for the Er3+:Y2O3 and core-shell NPs, respectively. This confirms that EXAFS analysis can be used as a more time efficient method to achieve the same outcome typically obtained by Judd-Ofelt analysis, enabling the optimization of the luminescent lifetimes of RE doped nano-phosphors.
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78.55.Hx Other solid inorganic materials
78.70.Dm X-ray absorption spectra
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
61.72.up Other materials

Impact of the Ga/In ratio on the N incorporation into (In,Ga)(As,N) quantum dots

R. Gargallo-Caballero, A. Guzmán, J. M. Ulloa, A. Hierro, M. Hopkinson, E. Luna, and A. Trampert

J. Appl. Phys. 111, 083530 (2012); http://dx.doi.org/10.1063/1.4706559 (7 pages)

Online Publication Date: 26 April 2012

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In this work, we demonstrate the dependence of the nitrogen incorporation on the Ga/In content into (In,Ga)(As,N) quantum dots (QDs) grown on GaAs (100) by radio-frequency plasma assisted molecular beam epitaxy (MBE). Morphological analysis by atomic force microscopy and cross-sectional transmission electron microscopy, together with an estimation of the transition thickness, monitored in situ during the growth, predict a maximum in the N incorporation for 30% Ga content. This result is confirmed by photoluminescence measurements of the as-grown and post-growth annealed samples. We attribute this behavior to a trade off between two mechanisms depending on the Ga/In content: one related to the stability of the Ga-N bond, and the other related to the surface strain and/or In segregation.
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81.07.Ta Quantum dots
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.05.Ea III-V semiconductors

Deflection of a cantilever rectangular plate induced by surface stress with applications to surface stress measurement

Xianwei Zeng, Jiaquan Deng, and Xi Luo

J. Appl. Phys. 111, 083531 (2012); http://dx.doi.org/10.1063/1.4706562 (8 pages)

Online Publication Date: 26 April 2012

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Surface stress plays important roles in the fabrication and applications of thin-film substrate systems. Bending test of cantilever microbeams has been commonly applied to characterize the surface stress. Stoney’s equation, ideally valid for completely unconstrained plates, is typically used to convert the measured deflection to a surface stress. To assess the validity of Stoney’s equation for the more complicated case of a plate with a clamped end, an analytical solution has been obtained in this study for the deflection of a cantilever rectangular plate due to surface stresses at its upper and lower surfaces. The analytical solution is given by the summation of single Fourier cosine series in the length and the width directions of the plate and a lower order polynomial. Numerical results for the deflection, slope, and curvature for the midpoint of the free end are presented for cantilever plates with aspect ratios ranging from 0.1 to 10 and for different Poisson’s ratios. In most practical measurements of surface stress, the aspect ratio is greater than one and the maximum percentage errors of Stoney’s equation for the deflection, slope, and curvature for the midpoint of the free end are 16%, 16%, and 10%, respectively. The present analytical solution based on Fourier cosine series with the first two leading terms can provide a significant improvement over Stoney’s equation. The maximum percentage errors for the deflection, slope, and curvature for the midpoint of the free end are reduced to 3%, 2%, and 3%, respectively.
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46.70.De Beams, plates, and shells
46.80.+j Measurement methods and techniques in continuum mechanics of solids
02.10.De Algebraic structures and number theory
46.25.Cc Theoretical studies

Characterization and luminescence properties of AlON:Eu2+ phosphor for white-emitting-diode illumination

F. Zhang, S. Chen, J. F. Chen, H. L. Zhang, J. Li, X. J. Liu, and S. W. Wang

J. Appl. Phys. 111, 083532 (2012); http://dx.doi.org/10.1063/1.4705404 (4 pages)

Online Publication Date: 27 April 2012

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AlON phosphors with different Eu2+ doping concentrations were synthesized by solid-state reaction method. Two Eu2+ doping sites were confirmed by temperature-dependent emission intensity, band shift and biexponential decay studies. High internal quantum efficiency and tunable chromaticity coordinates make AlON:Eu2+ phosphor a suitable candidate for white light-emitting diodes application.
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85.60.Jb Light-emitting devices

Effect of gold composition on the orientations of oxide nuclei during the early stage oxidation of Cu-Au alloys

Langli Luo, Yihong Kang, Judith C. Yang, and Guangwen Zhou

J. Appl. Phys. 111, 083533 (2012); http://dx.doi.org/10.1063/1.4707929 (9 pages)

Online Publication Date: 27 April 2012

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In situ environmental transmission electron microscopy is employed to study the effect of Au composition in Cu-Au alloys on the orientations of oxide islands during the initial-stage oxidation of Cu-Au(100) alloys. An orientation transition from nucleating epitaxial Cu2O islands to randomly oriented oxide islands is observed upon increasing the oxygen gas pressure. By increasing the Au composition in the Cu-Au alloys, both the oxide nucleation time and saturation density of oxide islands increase, but the critical oxygen pressure leading to nucleating randomly oriented Cu2O islands decreases. It is shown by a kinetic model that such a dependence of the critical oxygen pressure on the alloy composition is related to its effect on two competing processes, the oxide-alloy structure match and the effective collision of oxygen atoms, in determining the overall nucleation rate of oxide islands during the oxidation.
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81.05.Bx Metals, semimetals, and alloys
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
81.65.Mq Oxidation
64.60.qj Studies of nucleation in specific substances
64.70.kd Metals and alloys

GaP heteroepitaxy on Si(001): Correlation of Si-surface structure, GaP growth conditions, and Si-III/V interface structure

A. Beyer, J. Ohlmann, S. Liebich, H. Heim, G. Witte, W. Stolz, and K. Volz

J. Appl. Phys. 111, 083534 (2012); http://dx.doi.org/10.1063/1.4706573 (6 pages) | Cited 1 time

Online Publication Date: 30 April 2012

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GaP-layers on Si(001) can serve as pseudo-substrates for a variety of novel optoelectronic devices. The quality of the GaP nucleation layer is a crucial parameter for the performance of such devices. Especially, anti-phase domains (APDs) evolving at mono-atomic steps on the Si-surface can affect the quality of a layer adversely. The size, shape, and possible charge of the APDs and their boundaries depend on the polarity of the surrounding crystal. The observed polarity of the GaP is caused by the A-type double step configuration of the Si-surface reconstruction prior to GaP growth and the prevalent binding of Ga to Si under optimized growth conditions. The polarity of the GaP-layer and hence the atomic configuration at the Si-III/V interface can be changed by altering the growth conditions. With this knowledge, defect-free GaP/Si(001) templates for III/V device integration on Si-substrates can be grown.
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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
68.55.ag Semiconductors
68.35.Ct Interface structure and roughness

Structural characterization of Ti-15Mo alloy used as biomaterial by Rietveld method

José Roberto Severino Martins, Jr. and Carlos Roberto Grandini

J. Appl. Phys. 111, 083535 (2012); http://dx.doi.org/10.1063/1.4707920 (8 pages)

Online Publication Date: 30 April 2012

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The biochemical and mechanical behavior of titanium alloys has been studied extensively for a variety of applications in the aerospace and biomedical fields. In the literature, there are studies that relate the microstructure and the phases of the material with its properties; however, there is little information that quantifies each phase and relates this to its properties. In addition, little has been done to analyze the effects of oxygen and heat treatment on the alloy’s structure. In this paper, the effect of doping with oxygen and the effect of heat treatments on structural properties of Ti-15Mo alloy used as biomaterials is examined using scanning electron microscopy, x-ray diffraction, and diffractogram analysis using the Rietveld method.
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87.85.J- Biomaterials
81.40.Gh Other heat and thermomechanical treatments
81.40.Lm Deformation, plasticity, and creep
61.72.up Other materials
62.20.F- Deformation and plasticity
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