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15 Aug 2008

Volume 104, Issue 4, Articles (04xxxx)

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Effects of UV cure on glass structure and fracture properties of nanoporous carbon-doped oxide thin films

David M. Gage, Jonathan F. Stebbins, Luming Peng, Zhenjiang Cui, Amir Al-Bayati, Kenneth P. MacWilliams, Hichem M’Saad, and Reinhold H. Dauskardt

J. Appl. Phys. 104, 043513 (2008); http://dx.doi.org/10.1063/1.2968438 (8 pages) | Cited 14 times

Online Publication Date: 22 August 2008

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The effects of UV radiation curing on the glass structure and fracture properties were examined for a class of nanoporous organosilicate low dielectric constant films. A detailed characterization by nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy showed significant changes in the glass structure with increasing curing time, marked by the removal of terminal organic groups and increased network-forming bonds following the initial removal of porogen material. The higher degree of film connectivity brought about by an increased cure duration is demonstrated to significantly enhance adhesive fracture properties and to moderately improve cohesive fracture resistance. Explanations for the enhanced fracture behavior are considered in terms of the glass structure. The important role of crack path selection during adhesive and cohesive fracture processes is used to rationalize the observed behavior.
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81.05.Rm Porous materials; granular materials
77.84.Jd Polymers; organic compounds
77.55.-g Dielectric thin films
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.mm Fracture

Room temperature tunable blue-green luminescence in nanocrystalline (Pb1−xSrx)TiO3 thin film grown on yttrium-doped zirconia substrate

L. Luo, H. Z. Ren, X. G. Tang, C. R. Ding, H. Z. Wang, X. M. Chen, J. K. Jia, and Z. F. Hu

J. Appl. Phys. 104, 043514 (2008); http://dx.doi.org/10.1063/1.2969030 (5 pages) | Cited 8 times

Online Publication Date: 22 August 2008

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Room temperature tunable blue-green photoluminescence was observed in nanocrystalline (Pb1−xSrx)TiO3 thin film under UV excitation. Its emission energy increases from 2.42 (at x = 0.6) to 2.76 eV (at x = 0.4), while the band gap decreases from 3.6 to 3.3 eV. Thin films of (Pb1−xSrx)TiO3 were prepared on yttrium-doped zirconia substrate by a simple sol-gel technique with spinning-coating process. Atom force microscope micrographs and crystallographic studies revealed the polycrystalline perovskite-type structure of the thin films. The observed optical properties are attributed to distorted octahedral due to different cation substitutions. The work shows that this kind of wide band gap and low cost nanocrystalline thin films is a very promising material for flat panel display applications and integrated light emission devices.
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68.60.-p Physical properties of thin films, nonelectronic
78.55.-m Photoluminescence, properties and materials
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures

X-ray diffraction imaging of strain fields in a domain-inverted LiTaO3 crystal

Kh. Hassani, M. Sutton, M. Holt, Y. Zuo, and D. Plant

J. Appl. Phys. 104, 043515 (2008); http://dx.doi.org/10.1063/1.2968224 (5 pages)

Online Publication Date: 22 August 2008

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We investigate lattice orientation and strain fields across ferroelectric domain walls in a single crystal lithium tantalate using x-ray diffraction imaging (topography). The sample is an actual voltage-operated optical switch consisting of a series of triangular polarization-inverted domains formed in an originally poled single crystal. By applying an electric field only about 2% of the coercive field in the forward and reverse directions, we observed asymmetric lattice rotation of about 10−6 rad, and normal strain variation in the order of 10−5 with reference to the zero-field state. Our results confirm that in congruent LiTaO3 crystals there is unexpectedly large strain field expanding several micrometers across the domain walls, in contrast with the widely accepted theoretical fact that in this material the polarization reversal establishes over only a few lattice constants, resulting in small and localized lattice distortions.
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77.80.Dj Domain structure; hysteresis
77.80.Fm Switching phenomena
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.22.Ej Polarization and depolarization
61.66.Fn Inorganic compounds

High-temperature AlN interlayer for crack-free AlGaN growth on GaN

Qian Sun, Jianfeng Wang, Hui Wang, Ruiqin Jin, Desheng Jiang, Jianjun Zhu, Degang Zhao, Hui Yang, Shengqiang Zhou, Mingfang Wu, Dries Smeets, and Andre Vantomme

J. Appl. Phys. 104, 043516 (2008); http://dx.doi.org/10.1063/1.2968546 (4 pages) | Cited 2 times

Online Publication Date: 22 August 2008

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This paper presents a study of the transformation of high-temperature AlN (HT-AlN) interlayer (IL) and its effect on the strain relaxation of Al0.25Ga0.75N/HT-AlN/GaN. The HT-AlN IL capped with Al0.25Ga0.75N transforms into AlGaN IL in which the Al composition increases with the HT-AlN IL thickness while the total Ga content keeps nearly constant. During the HT-AlN IL growth on GaN, the tensile stress is relieved through the formation of V trenches. The filling up of the V trenches by the subsequent Al0.25Ga0.75N growth is identified as the Ga source for the IL transformation, whose effect is very different from a direct growth of HT-AlGaN IL. The a-type dislocations generated during the advancement of V trenches and their filling up propagate into the Al0.25Ga0.75N overlayer. The a-type dislocation density increases dramatically with the IL thickness, which greatly enhances the strain relaxation of Al0.25Ga0.75N.
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68.35.Ct Interface structure and roughness
64.70.kg Semiconductors
62.20.mt Cracks

First-principles study of structural and energetic properties of A2Hf2O7 (A = Dy, Ho, Er) compounds

X. T. Zu, N. Li, and F. Gao

J. Appl. Phys. 104, 043517 (2008); http://dx.doi.org/10.1063/1.2969662 (4 pages) | Cited 2 times

Online Publication Date: 25 August 2008

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The structural and energetic properties of A2Hf2O7 (A = Dy, Ho, Er) compounds have been investigated by means of ab initio total energy calculations. For Er2Hf2O7 the defect fluorite structure is predicted to be more stable than pyrochlore structure, agreeing well with previous experimental [ J. Inorg. Nucl. Chem. 31, 2367 (1969) ; J. Less Common Met. 14, 435 (1968) ] and theoretical [ J. Am. Ceram. Soc. 85, 2139 (2002) ] investigations. In contrast to Er2Hf2O7 pyrochlore, Dy2Hf2O7 and Ho2Hf2O7 prefer pyrochlore structures, which is consistent with the recent experimental observations of Dy2Hf2O7 compound [ J. Solid State Chem. 179, 1990 (2006) ].
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71.15.Nc Total energy and cohesive energy calculations
71.15.-m Methods of electronic structure calculations
61.72.jd Vacancies
71.20.Ps Other inorganic compounds

Enhanced broadband near-infrared emission from Bi-doped glasses by codoping with metal oxides

Q. Qian, Q. Y. Zhang, G. F. Yang, Z. M. Yang, and Z. H. Jiang

J. Appl. Phys. 104, 043518 (2008); http://dx.doi.org/10.1063/1.2970167 (3 pages) | Cited 6 times

Online Publication Date: 25 August 2008

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An intense broadband and flat 1.3 μm emission from Bi-doped germanate glass has been measured upon excitation with a commercial laser diode, which makes the glasses more attractive for their use in broadband optical fiber amplifiers. The 1.3 μm emission exhibits a distinctive fine luminescence shape with four bands centered at 1225, 1320, 1370, and 1390 nm, respectively. It is noted that the addition of CeO2 (2 mol %), As2O5 (2 mol %), or Y2O3 (1 mol %) into Bi-doped glasses could remarkably enhance the luminescence intensity by a factor of 4, 12, and 23, respectively. Whereas, no luminescence has been observed for the glass prepared under a controlled redox condition. The optical properties of the Bi-doped glasses and the possible mechanism involved have been explained.
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78.55.Qr Amorphous materials; glasses and other disordered solids
61.43.Fs Glasses
61.72.up Other materials

Modeling deflagration-to-detonation transition in granular explosive pentaerythritol tetranitrate

Juan A. Sáenz and D. Scott Stewart

J. Appl. Phys. 104, 043519 (2008); http://dx.doi.org/10.1063/1.2970168 (14 pages) | Cited 1 time

Online Publication Date: 25 August 2008

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Based on an approach suggested by Stewart et al. [Phys. Fluids 6, 2515 (1994) ] we develop a model to simulate deflagration-to-detonation transition (DDT) in pentaerythritol tetranitrate (PETN) powders. The model uses a continuum mechanics formulation of conservation laws for a mixture of solid reactants and gas products, written in terms of mixture quantities plus two independent variables used to account for exothermic conversion of solid reactants into gas products, and compaction associated with pore collapse and grain rearrangement. We propose a simple empirical dependence of the reaction rate on the initial bed compaction that allows us to calibrate the model for a wide range of initial conditions. For the solid reactants we use a wide-ranging equation of state (EOS) developed by Davis and co-workers in a series of papers [ Proceedings of the Tenth International Symposium on Detonation, 1993, pp. 369–376; Explosive Effects and Applications (Springer, New York, 1998), Chap. 1, Combust. Flame 120, 399 (2000) ; Proceedings of the 12th International Symposium Detonation, San Diego, CA, 2002, pp. 624–631 ; . ONR 333-05-2 ; Proceedings of the Eighth Detonation Symposium, 1985, pp. 785–795 ; Proceedings of the 11th International Symposium on Detonation, 1998, pp. 303–308 ]. The EOS for powder uses the P-α model of Herrmann [ J. Appl. Phys. 40, 2490 (1969) ] and Carrol and Holt [ J. Appl. Phys. 43, 759 (1972) ]. To close the system, we suggest phenomenological closure relations, consistent with the limit of a compressible inert material and of a solid fully reactive material, such that the EOS can be posed only in terms of mixture quantities and the reaction and compaction variables. We demonstrate the model’s ability to capture DDT in PETN powders by matching transients typically observed in experiments through simulation. We show that for flows calculated using nonideal EOSs and complex reaction kinetics such as those formulated in our model, it is possible to define a separatrix, i.e., the C+ characteristic that separates the C+ characteristics that evolve into the detonation front from those that evolve away from it. We comment on the effects that the variability in the grain microstructure in PETN explosive powder beds can have on the overall mechanics of DDT and discuss possible ways to model this.
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82.33.Vx Reactions in flames, combustion, and explosions
82.20.Pm Rate constants, reaction cross sections, and activation energies
64.30.Jk Equations of state of nonmetals

In situ studies on the kinetics of formation and crystal structure of In4Sn3O12 using high-energy x-ray diffraction

G. B. González, J. S. Okasinski, T. O. Mason, T. Buslaps, and V. Honkimäki

J. Appl. Phys. 104, 043520 (2008); http://dx.doi.org/10.1063/1.2969913 (8 pages) | Cited 3 times

Online Publication Date: 25 August 2008

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High-energy x-ray diffraction was utilized to study in situ the formation temperature and crystal structure of the rhombohedral phase In4Sn3O12. The kinetics of In4Sn3O12 formation from bixbyite In2O3 and tetragonal SnO2 nanopowders were investigated during isothermal annealing treatments ranging from 1335 to 1400 °C. The transformation data exhibited two regimes, well described with a two-exponent kinetics model. The first regime followed a Johnson–Mehl–Avrami–Kolmogorov (JMAK) behavior until 75% of the In4Sn3O12 phase formed and was modeled with a modified JMAK equation. The formation of the first grains of In4Sn3O12 at 1345 °C was observed in situ using diffraction two-dimensional images. Structural results obtained from Rietveld analysis include atomic positions, phase analysis compositions of the samples, and lattice parameters during heating, cooling, and isothermal conditions. Linear and volume coefficients of thermal expansion were determined for the In4Sn3O12 phase.
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61.66.Fn Inorganic compounds
65.40.De Thermal expansion; thermomechanical effects
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization

Variable temperature photoluminescence of pulsed laser deposited ZnO thin films

J. B. Cui, Y. C. Soo, A. Thomas, H. Kandel, T. P. Chen, and C. P. Daghlian

J. Appl. Phys. 104, 043521 (2008); http://dx.doi.org/10.1063/1.2970075 (6 pages) | Cited 7 times

Online Publication Date: 25 August 2008

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Variable temperature photoluminescence of ZnO thin films deposited by a reactive laser ablation of metallic zinc was investigated. Free and bound exciton emissions are absent at cryogenic temperature, and the near band edge (NBE) emission is independent of measurement temperature for the ZnO thin film deposited at room temperature. Annealing at 700 °C results in the removal of defects, reappearance of exciton emission, and a temperature dependent NBE emission. The experimental data suggest that defects play an important role in the band edge emission in terms of both spectra shape and temperature dependence. Our observations will have an impact on device applications using ZnO, especially for optoelectronics that utilizes the exciton emission.
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78.55.Et II-VI semiconductors
78.66.Hf II-VI semiconductors
81.15.Fg Pulsed laser ablation deposition
71.35.-y Excitons and related phenomena
81.40.Gh Other heat and thermomechanical treatments
68.55.-a Thin film structure and morphology

Numerical analysis of acoustic wave propagation in layered carbon nanofiber reinforced polymer composites

Li Sun, Yong Yu, Gangbing Song, and Jihua Gou

J. Appl. Phys. 104, 043522 (2008); http://dx.doi.org/10.1063/1.2973039 (5 pages) | Cited 1 time

Online Publication Date: 27 August 2008

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Polymer composites reinforced by carbon nanofibers (CNFs) in the form of paper sheet show significant vibration and acoustic damping improvement when compared to pure matrix materials. Without looking into the microscopic energy dissipation mechanisms, this paper analyzes the wave propagation in the composites from a macroscopic point of view. The CNF nanocomposites in this study were treated as stacking of alternating layers of pure polymer and CNF reinforced polymer. Analyses of acoustic wave propagation focused on revealing the effects of acoustic impedance discontinuity at the interfaces of the layered structure. Plane wave transmission coefficient has been calculated as a function of the number of the layer repeats and thickness at different wave frequencies. Oscillations in the transmission coefficient have been observed when the acoustic wavelength is on the same order of the bilayer thickness, indicating the possibility of designing the nanocomposite structure to optimize noise reduction characteristics. The numerical analysis converges with effective media theory when acoustic wavelength is much larger than the layer thickness.
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62.65.+k Acoustical properties of solids
63.22.Np Layered systems
62.30.+d Mechanical and elastic waves; vibrations

Optical properties and phase change transition in Ge2Sb2Te5 flash evaporated thin films studied by temperature dependent spectroscopic ellipsometry

J. Orava, T. Wágner, J. Šik, J. Přikryl, M. Frumar, and L. Beneš

J. Appl. Phys. 104, 043523 (2008); http://dx.doi.org/10.1063/1.2970069 (10 pages) | Cited 9 times

Online Publication Date: 27 August 2008

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We studied the optical properties of as-prepared (amorphous) and thermally crystallized (fcc) flash evaporated Ge2Sb2Te5 thin films using variable angle spectroscopic ellipsometry in the photon energy range 0.54–4.13 eV. We employed Tauc–Lorentz (TL) model and Cody–Lorentz (CL) model for amorphous phase and TL model with one additional Gaussian oscillator for fcc phase data analysis. The amorphous phase has optical bandgap energy Egopt = 0.65 eV (TL) or 0.63 eV (CL) slightly dependent on used model. The Urbach edge of amorphous thin film was found to be ∼ 70 meV. Both models behave very similarly and accurately fit to the experimental data at energies above 1 eV. The CL model is more accurate in describing dielectric function in the absorption onset region. The thickness decreases ∼ 7% toward fcc phase. The bandgap energy of fcc phase is significantly lower than amorphous phase, Egopt = 0.53 eV. The temperature dependent ellipsometry revealed crystallization in the range 130–150 °C. The bandgap energy of amorphous phase possesses temperature redshift −0.57 meV/K (30–110 °C). The crystalline phase has more complex bandgap energy shift, first +0.62 meV/K (150–180 °C) followed by −0.29 meV/K (190–220 °C). The optical properties (refractive index, extinction coefficient, and optical bandgap energy) of as-prepared and fcc flash evaporated Ge2Sb2Te5 thin films are very similar to those values previously reported for sputtered thin films.
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78.66.Jg Amorphous semiconductors; glasses
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.ag Semiconductors

Anisotropic photonic structures induced by three-dimensional vector holography in dye-doped liquid crystals

Tomoyuki Sasaki, Hiroshi Ono, and Nobuhiro Kawatsuki

J. Appl. Phys. 104, 043524 (2008); http://dx.doi.org/10.1063/1.2970172 (6 pages) | Cited 3 times

Online Publication Date: 27 August 2008

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Periodic anisotropic structures were induced by means of a three-dimensional vector holographic technique in an azo-dye-doped liquid crystal composite with uniaxial alignment. The three-dimensional vector hologram was fabricated by both the polarization interference and the polarization propagation in the anisotropic recording medium. In order to obtain clear insight into the optical properties of three-dimensional vector holograms, various types of structures were induced by changing the polarization states and incident angles of the recording beams. The diffraction properties of various types of three-dimensional vector holograms were calculated by the finite-difference time-domain method, and the theoretical explanations were in good agreement with the experimental results.
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42.40.Eq Holographic optical elements; holographic gratings
42.70.Qs Photonic bandgap materials
42.25.Ja Polarization
61.30.-v Liquid crystals

Equation of state for diamond in wide ranges of pressure and temperature

John S. Tse and Wilfried B. Holzapfel

J. Appl. Phys. 104, 043525 (2008); http://dx.doi.org/10.1063/1.2969909 (10 pages) | Cited 2 times

Online Publication Date: 28 August 2008

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Theoretical total energy and phonon calculations for diamond under strong compressions up to 3 TPa are analyzed with different analytical forms for the representation of p-V isotherms. A simple model with only two characteristic (volume dependent) phonon parameters—an effect acoustic Debye temperature and an additional Einstein temperature of the optical phonons—provides a complete modeling of all the thermophysical properties of diamond in this extended range of pressure up to the melting curve. The comparison with thermophysical data at ambient pressure provides additional information on weak intrinsic anharmonic contributions and gives the frame for the complete modeling up to 3 TPa in pressure and temperatures up to the melting curve.
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64.30.Jk Equations of state of nonmetals
62.50.-p High-pressure effects in solids and liquids
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
63.20.Ry Anharmonic lattice modes
71.15.Nc Total energy and cohesive energy calculations
64.70.dj Melting of specific substances

Helium implanted gallium nitride evidence of gas-filled rod-shaped cavity formation along the c-axis

Jean-François Barbot, Frédéric Pailloux, Marie-Laure David, Laurent Pizzagalli, Erwan Oliviero, and Guillaume Lucas

J. Appl. Phys. 104, 043526 (2008); http://dx.doi.org/10.1063/1.2970062 (7 pages) | Cited 1 time

Online Publication Date: 28 August 2008

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The structural defects induced by He implantation in GaN epilayer at high fluence (1×1017He/cm2) and elevated temperature (750 °C) have been studied by conventional and high resolution transmission electron microscopy. In addition to the planar interstitial-type defects lying in the basal plane usually observed after high fluence implantation into GaN, a continuous layer of bubbles arranged in rows parallel to the implanted surface is observed in the region of maximum He concentration. This arrangement of bubbles is ascribed to interactions with dislocations. Beyond, one dimensional rod-shaped defects appear perpendicular to the implanted surface. Contrast analysis of high resolution images and atomistic simulations gives converging results in the determination of the nature and structure of these defects, i.e., gas-filled rod-shaped cavities in an overpressurized state.
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61.72.Qq Microscopic defects (voids, inclusions, etc.)
68.37.Lp Transmission electron microscopy (TEM)
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations

Vacancy-mediated intermixing in InAs/InP(001) quantum dots subjected to ion implantation

C. Dion, P. Desjardins, F. Schiettekatte, M. Chicoine, M. D. Robertson, N. Shtinkov, P. J. Poole, X. Wu, and S. Raymond

J. Appl. Phys. 104, 043527 (2008); http://dx.doi.org/10.1063/1.2970093 (10 pages)

Online Publication Date: 28 August 2008

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We have investigated the influence of defects emanating from phosphorus implantation damage in the InP capping layer on postgrowth thermally induced intermixing in self-assembled InAs/InP(001) quantum dots (QDs). Photoluminescence (PL) spectra from as-grown samples could be described as the superposition of separate PL peaks where each peak corresponded to emission from an ensemble of QDs with a particular height ranging from 4 to 13 ML. Blueshift of up to 270 meV and significant bandwidth broadening were observed in the PL spectra after ion implantation with a fluence of 5×1011−1014 cm−2 and subsequent annealing at temperatures ranging from 450 to 600 °C. From the analysis of the evolution of the QD peaks upon intermixing, which revealed the coexistence of intact QD PL and a broad PL feature related to heavily intermixed QDs, it was suggested that the bandwidth broadening resulted from spatial inhomogeneity in the compositional intermixing. In order to better understand the mechanism responsible for the ion-implantation-induced intermixing, samples capped with a stack of compressively strained In0.75Ga0.25As/InP quantum wells (QWs) were prepared to trap vacancies released by the implantation damage while not inhibiting the effect of the interstitials. Both blueshift and bandwidth broadening were suppressed in samples containing the strained InGaAs QWs, whereas the evolution of the PL spectra from the QDs behaves as expected for interstitial-mediated intermixing. The vacancies were thus believed to be trapped in the QWs and indicated that intermixing in ion-implanted InP capped samples is mediated by vacancies. The shape of the QDs changed from a truncated pyramid in the as-grown state to a double convex lens structure after intermixing as confirmed by cross-sectional scanning transmission electron microscopy imaging. Furthermore, the change in shape and compositional intermixing of the QDs were attributed to vacancy trapping in the vicinity of the QDs as based on atomistic strain calculations.
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61.72.uj III-V and II-VI semiconductors
61.72.jd Vacancies
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
61.72.jj Interstitials
61.80.Jh Ion radiation effects
78.55.Cr III-V semiconductors
78.67.Hc Quantum dots

Luminescence evolution of ZnO single crystal under low-energy electron beam irradiation

B. Dierre, X. L. Yuan, and T. Sekiguchi

J. Appl. Phys. 104, 043528 (2008); http://dx.doi.org/10.1063/1.2973190 (4 pages) | Cited 7 times

Online Publication Date: 29 August 2008

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The effects of electron beam irradiation on the luminescence of ZnO single crystals were investigated by cathodoluminescence. We have found that the evolution of the intensity during the e-beam irradiation depends on the surface polarity. For O-face, the ultraviolet (UV) emission decreases exponentially and approaches an asymptotic value. For Zn-face, it first increases and then decreases. The decrease components are similar in both faces. If we halt the e-beam irradiation, the UV intensity recovers partially. These results suggest that the decrease in the UV evolution is related to metastable bulk defect reactions at the subsurface region while the increase is related to surface reaction such as electron-stimulated desorption.
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78.60.Hk Cathodoluminescence, ionoluminescence
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
61.80.Fe Electron and positron radiation effects
78.55.Et II-VI semiconductors

Influence of ZnO on the crystallization kinetics and properties of diopside-Ca-Tschermak based glasses and glass-ceramics

Ashutosh Goel, Dilshat U. Tulyaganov, Essam R. Shaaban, Rajendra N. Basu, and José M. F. Ferreira

J. Appl. Phys. 104, 043529 (2008); http://dx.doi.org/10.1063/1.2970170 (11 pages) | Cited 3 times

Online Publication Date: 29 August 2008

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We report on the influence of ZnO on the structural, thermal, and crystallization behavior of the diopside-Ca-Tschermak based glasses, and on the processing, microstructure, and the properties of the sintered glass ceramics. Four glasses with nominal compositions of CaMg0.8Al0.4Si1.8O6, CaMg0.75Zn0.05Al0.4Si1.8O6, CaMg0.70Zn0.10Al0.4Si1.8O6, and CaMg0.60Zn0.20Al0.4Si1.8O6 were obtained by melting at 1580 °C for 1 h. Structural and thermal behavior of the glasses was investigated by Fourier-transform infrared spectroscopy, density measurements, dilatometry, and differential thermal analysis. Nonisothermal crystallization kinetics has been employed to study the crystallization mechanism in the glasses. Sintering, crystallization, microstructure, and properties of the glass ceramics were investigated under nonisothermal heating conditions in the temperature range of 850–1000 °C.
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61.43.Fs Glasses
78.35.+c Brillouin and Rayleigh scattering; other light scattering
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
81.05.Pj Glass-based composites, vitroceramics
64.70.dj Melting of specific substances
64.70.kj Glasses
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Electron microscopy analysis of crystalline silicon islands formed on screen-printed aluminum-doped p-type silicon surfaces

Robert Bock, Jan Schmidt, Rolf Brendel, Henning Schuhmann, and Michael Seibt

J. Appl. Phys. 104, 043701 (2008); http://dx.doi.org/10.1063/1.2963192 (5 pages) | Cited 8 times

Online Publication Date: 19 August 2008

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The origin of a not yet understood concentration peak, which is generally measured at the surface of aluminum-doped p+ regions produced in a conventional screen-printing process is investigated. Our findings provide clear experimental evidence that the concentration peak is due to the microscopic structures formed at the silicon surface during the firing process. To characterize the microscopic nature of the islands (lateral dimensions of 1–3 μm) and line networks of self-assembled nanostructures (lateral dimension of ≤ 50 nm), transmission electron microscopy, scanning electron microscopy, scanning transmission electron microscopy, and energy dispersive x-ray analysis are combined. Aluminum inclusions are detected 50 nm below the surface of the islands and crystalline aluminum precipitates of ≤ 7 nm in diameter are found within the bulk of the islands. In addition, aluminum inclusions (lateral dimension of ∼ 30 nm) are found within the bulk of the self-assembled line networks.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
61.66.Bi Elemental solids
81.16.Dn Self-assembly
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Mechanisms of unexpected reduction in hole concentration in Al-doped 4H-SiC by 200 keV electron irradiation

Hideharu Matsuura, Nobumasa Minohara, and Takeshi Ohshima

J. Appl. Phys. 104, 043702 (2008); http://dx.doi.org/10.1063/1.2969788 (6 pages) | Cited 2 times

Online Publication Date: 25 August 2008

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The hole concentration in Al-doped p-type 4H-SiC was found to be significantly reduced by electron irradiation when compared to the hole concentration in Al-doped p-type Si; this is an unexpected result. The temperature dependence of the hole concentration p(T) in Al-doped 4H-SiC irradiated with 200 keV electrons at various fluences was measured. Only substitutional C atoms in SiC can be displaced by irradiation with 200 keV electrons. The reduction in p(T) by the electron irradiation was found to be mainly due to a decrease in Al acceptors and not due to an increase in defects (e.g., C vacancies) located around the middle of the bandgap in SiC. Based on the analysis of p(T), two types of acceptor species were detected and the density and energy level of each acceptor species were determined. An equation describing the fluence dependence of each acceptor density is proposed. The results suggest that for the 200 keV electron irradiation, a substitutional C atom bonded to an Al acceptor was more easily displaced than a substitutional C atom bonded to four Si atoms.
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71.55.Ht Other nonmetals
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

The electronic and transport properties of a molecular junction studied by an integrated piecewise thermal equilibrium approach

M.-H. Tsai, T.-H. Lu, and Y.-H. Tang

J. Appl. Phys. 104, 043703 (2008); http://dx.doi.org/10.1063/1.2970164 (8 pages) | Cited 2 times

Online Publication Date: 25 August 2008

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An integrated piecewise thermal equilibrium approach based on the first-principles calculation method has been developed to calculate bias dependent electronic structures and current- and differential conductance-voltage characteristics of the gold-benzene-1,4-dithiol-gold molecular junction. The calculated currents and differential conductance have the same order of magnitude as experimental ones. An electron transfer was found between the two electrodes when a bias is applied, which renders the two electrodes to have different local electronic structures. It was also found that when Au 5d electrons were treated as core electrons the calculated currents were overestimated, which can be understood as an underestimate of the Au–S covalent bonding and consequently the contact potential barrier and the replacement of delocalized Au 5d carriers by more itinerant delocalized Au 6sp carriers in the electrodes.
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85.65.+h Molecular electronic devices
71.15.-m Methods of electronic structure calculations

Soft x-ray induced Ag diffusion in amorphous pulse laser deposited As50Se50 thin films: An x-ray photoelectron and secondary ion mass spectroscopy study

M. Kalyva, A. Siokou, S. N. Yannopoulos, T. Wagner, Krbal, J. Orava, and M. Frumar

J. Appl. Phys. 104, 043704 (2008); http://dx.doi.org/10.1063/1.2970104 (7 pages) | Cited 2 times

Online Publication Date: 25 August 2008

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In the present paper, x-ray photoelectron spectroscopy (XPS) is used to induce and study the Ag diffusion and dissolution in pulsed laser deposited As50Se50 amorphous chalcogenide films. Dynamic secondary ion mass spectroscopy (SIMS) is also employed to investigate the Ag atomic concentration in depth. Dynamic SIMS measurements reveal that even before x-ray irradiation a considerable percentage of the total silver amount diffuses into the matrix forming an ∼ 70 nm mixed Ag–Se–As layer. XPS analysis shows that x-ray irradiation induces further diffusion of silver into the chalcogenide matrix. At the end of the procedure silver is found to be homogeneously dissolved into the matrix leaving only a 5–7 nm thick surface layer with excess silver concentration. In this surface layer stable Ag2Se clusters existing probably in quasicrystalline form prohibit further diffusion. The origin of the mechanism of the x-ray induced Ag diffusion and dissolution in amorphous chalcogenides is discussed in light of the present results.
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66.30.hh Glasses
79.60.Ht Disordered structures
61.80.Cb X-ray effects
81.15.Fg Pulsed laser ablation deposition
68.55.ag Semiconductors
64.75.Bc Solubility

Diffusive transport in graphene: The role of interband correlation

S. Y. Liu, X. L. Lei, and Norman J. M. Horing

J. Appl. Phys. 104, 043705 (2008); http://dx.doi.org/10.1063/1.2969787 (6 pages) | Cited 5 times

Online Publication Date: 26 August 2008

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We present a kinetic equation approach to investigate dc transport properties of graphene in the diffusive regime considering long-range electron-impurity scattering. In our study, the effects of interband correlation (or polarization) on conductivity are taken into account. We find that the conductivity contains not only the usual term inversely proportional to impurity density Ni but also an anomalous term that is linear in Ni. This leads to a minimum in the density dependence of conductivity when the electron density Ne is equal to a finite critical value Nc. The effects of various scattering potentials on the conductivity minimum are also analyzed. Using typical experimental parameters, we find that for random-phase-approximation–screened electron-impurity scattering, the minimum conductivity is about 4.42e2/h when Ne ≈ 0.11Ni, and the conductivity varies almost linearly with the electron density for Ne>Ni.
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73.63.Bd Nanocrystalline materials
72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect)

Solute mass diffusion coefficient: Comparison of microgravity experiments with molecular dynamic simulation and Enskog hard sphere corrected estimates

Paul J. Scott and R. W. Smith

J. Appl. Phys. 104, 043706 (2008); http://dx.doi.org/10.1063/1.2969663 (2 pages) | Cited 2 times

Online Publication Date: 27 August 2008

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Mass diffusion coefficients and their variation with temperature are necessary for the modeling of many materials processes. The experimental measurement of the diffusion coefficient is difficult because of gravity-driven convective flow. A long series of experiments has been carried out in the reduced gravity of earth orbiting space station laboratories in which the steady gravitational field is reduced to approximately 5×10−5g, where g is the gravitational acceleration at the earth’s surface. This effectively eliminates most buoyancy convection, but the results obtained may be contaminated by transient g-perturbations, called “g-jitter,” resulting from vibration of the space platform. Here we compare the results of liquid diffusion microgravity experiments using a lead 1 wt % gold alloy to estimates derived from molecular dynamics simulations and several Enskog hard sphere corrected expressions for the diffusion coefficient.
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66.10.cg Mass diffusion, including self-diffusion, mutual diffusion, tracer diffusion, etc.
61.20.Ja Computer simulation of liquid structure
47.55.P- Buoyancy-driven flows; convection

Spin accumulation and pure spin current in a three-terminal quantum dot ring with Rashba spin-orbit effect

Feng Chi, Jun Zheng, and Lian-Liang Sun

J. Appl. Phys. 104, 043707 (2008); http://dx.doi.org/10.1063/1.2973339 (4 pages) | Cited 7 times

Online Publication Date: 29 August 2008

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Based on the Keldysh nonequilibrium Green’s function technique, we study the spin accumulation and the pure spin current in a three-terminal quantum ring device, in which a quantum dot (QD) is inserted in one arm of the ring and the Rashba spin-orbit interaction (RSOI) exists in the other. We find that under certain circumstances, the pure spin current in the middle lead and the spin accumulation in the QD emerge simultaneously. The sign and the magnitude of them can be modulated by adjusting the bias voltages, the RSOI-induced phase factor and the magnetic flux penetrating the ring. The origin of the spin accumulation and the pure spin current is interpreted in terms of the spin-dependent total effective coupling strengths between the leads and the QD.
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72.25.Mk Spin transport through interfaces
73.63.Kv Quantum dots
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
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Magnetic anisotropy and spin disorder in textured MnBi crystals synthesized by a field-inducing approach at a high temperature

Yongsheng Liu, Jincang Zhang, Shixun Cao, and Zhongming Ren

J. Appl. Phys. 104, 043901 (2008); http://dx.doi.org/10.1063/1.2966456 (5 pages) | Cited 5 times

Online Publication Date: 18 August 2008

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Textured MnBi crystals in a Bi matrix are fabricated by quenching at a high temperature ( ∼ 653 K) in a magnetic field of 10 T. Microstructure observations reveal that MnBi grains are aligned along their c-axis. Magnetization measurements show a pronounced magnetic anisotropy in directions normal and parallel to the fabrication field resulting from the alignment. MnBi crystals display spin-disorder behaviors in ac magnetization, which may emerge due to the quenching processing.
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75.30.Gw Magnetic anisotropy
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
81.40.Gh Other heat and thermomechanical treatments
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