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25 December 2006

Volume 89, Issue 26,  Articles (26xxxx)


Cover image from Ch. Deneke, U. Zschieschang, H. Klauk, and O. G. Schmidt, Appl. Phys. Lett. 89, 263110 (2006).

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LASERS, OPTICS, AND OPTOELECTRONICS

Impact of disorder on high quality factor III-V nitride microcavities

G. Christmann, D. Simeonov, R. Butté, E. Feltin, J.-F. Carlin, and N. Grandjean

Appl. Phys. Lett. 89, 261101 (2006) (3 pages)

Online Publication Date: 26 December 2006

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The authors report on the micron scale characterization of a monolithic GaN microcavity (MC) with lattice matched AlInN/GaN distributed Bragg reflectors by means of a microtransmission setup. This technique allows extracting very high quality factors (Q up to 2800), in accordance with theoretical predictions, contrary to what was previously reported for nitride based MCs. Furthermore, two-dimensional mappings of the MC transmission spectrum allow probing the disorder in this MC. The direct relationship between an increased disorder and a reduction in the Q factor is clearly observed.
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42.82.Gw, 85.60.-q

Nanowire metamaterials with extreme optical anisotropy

Justin Elser, Robyn Wangberg, Viktor A. Podolskiy, and Evgenii E. Narimanov

Appl. Phys. Lett. 89, 261102 (2006) (3 pages)

Online Publication Date: 26 December 2006

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The authors study perspectives of nanowire metamaterials for negative-refraction waveguides, high-performance polarizers, and polarization-sensitive biosensors. They demonstrate that the behavior of these composites is strongly influenced by the concentration, distribution, and geometry of the nanowires, derive an analytical description of electromagnetism in anisotropic nanowire-based metamaterials, and explore the limitations of their approach via three-dimensional numerical simulations. Finally, they illustrate the developed approach on the examples of nanowire-based high-energy-density waveguides and nonmagnetic negative-index imaging systems with far-field resolution of one-sixth of vacuum wavelength.
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61.46.-w, 42.70.-a, 42.79.Gn, 87.80.-y

Three-dimensional self-organization of crystalline gold nanoparticles in amorphous alumina

Chunming Jin, Honghui Zhou, Wei Wei, and Roger Narayan

Appl. Phys. Lett. 89, 261103 (2006) (3 pages)

Online Publication Date: 26 December 2006

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Multilayered heterostructures containing gold nanoparticles embedded in amorphous alumina matrices were deposited on silicon (001) substrates using pulsed laser deposition. The three-dimensional ordering of gold nanoparticles within these multilayered heterostructures was investigated using cross-sectional transmission electron microscopy and image Fourier transformation. Self-organization of gold nanoparticles along the vertical direction was observed in films grown at 20 and at 320  °C. Self-organization occurred by means of two different growth modes; both vertically correlated growth (top-on-top) and anticorrelated growth (top-on-middle) mechanisms were observed. The results of these studies suggest that the driving force for vertical ordering in this material is related to the long-range elastic interactions among the nanoparticles within the growing films.
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81.07.Bc, 81.05.Bx, 81.16.Dn, 81.16.Mk, 81.15.Fg

Femtosecond laser fabrication of apertures on two-dimensional microlens arrays

G. Langer, D. Brodoceanu, and D. Bäuerle

Appl. Phys. Lett. 89, 261104 (2006) (3 pages)

Online Publication Date: 26 December 2006

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Two-dimensional lattices of microspheres formed by self-assembly from colloidal solutions can be employed as microlens arrays. With applications in confocal microscopes, some types of parallel laser-surface processing or devices based on anomalous optical transmission, it is useful to improve the imaging properties of microspheres by generating an aperture on each of these lenses. This has been achieved by means of laser-induced forward transfer using femtosecond Ti:sapphire laser radiation. Some of the peculiarities observed in the experiments can be understood on the basis of field intensity calculations.
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42.62.-b, 42.86.+b, 42.65.Re, 42.79.Bh, 07.60.Pb

Mask-shift filtering for forming microstructures with irregular profile

Xiaochun Dong, Chunlei Du, Changtao Wang, Qiling Deng, Yudong Zhang, and Xiangang Luo

Appl. Phys. Lett. 89, 261105 (2006) (3 pages)

Online Publication Date: 26 December 2006

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A method of mask-shift filtering is presented for forming microstructures with irregular profile. When a three-dimensional (3D) microstructure is quantized to a number of microslices in one direction with equal interval, their two-dimensional average projections are calculated, respectively. A shift filtering mask is constructed by arranging the submasks converted from the corresponding subprojections. Filtering is carried out during exposure by moving the mask in the quantized direction to form a 3D microprofile on resist. The experimental results for forming irregular continuous relief profile in the larger areas are demonstrated and analyzed in this letter.
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42.86.+b, 81.16.Nd

Pulse generation and compression via ground and excited states from a grating coupled passively mode-locked quantum dot two-section diode laser

Jimyung Kim, Myoung-Taek Choi, and Peter J. Delfyett

Appl. Phys. Lett. 89, 261106 (2006) (3 pages)

Online Publication Date: 27 December 2006

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The authors generate and compress short pulses via the ground and excited state transitions from a passively mode-locked grating coupled quantum dot two-section diode laser at a repetition rate of 2.7  GHz. The selection and isolation of either the ground state or excited state transition were performed by an angle tuning of the grating. The externally compressed pulse widths are 970  fs from the ground state and 1.2  ps from the excited state transition. The sign of the chirp is up chirped for both state transitions.
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42.55.Px, 42.60.Fc, 42.60.By, 42.65.Re, 42.79.Dj

Demonstrated fossil-fuel-free energy cycle using magnesium and laser

T. Yabe, S. Uchida, K. Ikuta, K. Yoshida, C. Baasandash, M. S. Mohamed, Y. Sakurai, Y. Ogata, M. Tuji, Y. Mori, Y. Satoh, T. Ohkubo, M. Murahara, A. Ikesue, M. Nakatsuka et al.

Appl. Phys. Lett. 89, 261107 (2006) (3 pages)

Online Publication Date: 27 December 2006

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The authors propose an energy cycle based on a renewable fuel. Magnesium is chosen as an energy carrier and is combusted with water to retrieve energy using many power devices. MgO, the combustion residue, is reduced back to Mg by laser radiation generated from solar and other renewable energy sources. They have achieved an energy recovery efficiency of 42.5% for converting MgO to magnesium, using a laser. Combined with a demonstrated 38% efficiency for converting an artificial sunlight source (metal halide lamp) into laser output energy indicates that the proposed energy cycle is already in a feasible range for practical use.
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84.60.Bk, 89.30.Cc

Goos-Hänchen shift surface plasmon resonance sensor

Xiaobo Yin and Lambertus Hesselink

Appl. Phys. Lett. 89, 261108 (2006) (3 pages)

Online Publication Date: 27 December 2006

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A high resolution surface plasmon resonance sensor is proposed and demonstrated by measuring the plasmon resonance enhanced Goos-Hänchen effect at attenuated total internal reflection. The giant Goos-Hänchen shift results from the singular phase retardation at the resonance which leads to the superior sensitivity of the sensor.
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07.07.Df, 73.20.Mf, 78.68.+m

Brownian noise in radiation-pressure-driven micromechanical oscillators

Hossein Rokhsari, Mani Hossein-Zadeh, Ali Hajimiri, and Kerry Vahala

Appl. Phys. Lett. 89, 261109 (2006) (3 pages)

Online Publication Date: 27 December 2006

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The authors demonstrate Brownian-noise-limited operation of an optomechanical oscillator, wherein mechanical oscillations of a silica optical microcavity are sustained by means of radiation pressure. Using phase noise measurement above threshold, it has been shown that the short-term linewidth of mechanical oscillations is fundamentally broadened, limited by thermal equipartition of energy.
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85.85.+j, 05.40.Jc, 05.40.Ca

Electric field induced blueshift of the e1-hh1 exciton transition in a GaAs1−xNx/GaAs (x<1%) stepped quantum well

A. C. H. Lim, R. Gupta, S. K. Haywood, M. J. Steer, M. Hopkinson, and G. Hill

Appl. Phys. Lett. 89, 261110 (2006) (3 pages)

Online Publication Date: 28 December 2006

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The authors present room temperature photoluminescence and photocurrent measurements on a three layer GaAsN/GaAs/Al0.35Ga0.65As (N<=1%) stepped quantum well. A blueshift in the ground state transition energy is observed with the application of an electric field. A net blueshift of >10  meV was obtained at 40  kV/cm. These results suggest a type II band lineup in the GaAsN/GaAs heterojunction for nitrogen compositions below 1%.
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78.67.De, 78.55.Cr, 73.63.Hs, 71.35.-y, 72.40.+w, 68.65.Fg

Photorefractive effect in relaxor ferroelectric 0.62Pb(Mg1/3Nb2/3)O3–0.38PbTiO3 single crystal

Chongjun He, Zhongxiang Zhou, Dajun Liu, Xiangyong Zhao, and Haosu Luo

Appl. Phys. Lett. 89, 261111 (2006) (3 pages)

Online Publication Date: 28 December 2006

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The photorefractive properties of 0.62Pb(Mg1/3Nb2/3)O3–0.38PbTiO3 single crystals have been investigated by the two-wave mixing experiment. The maximal gain coefficient is 5.4  cm−1, the effective trap density is 1.2×1016  cm−3, and the normalized photorefractive response time under 1  W/cm2 illumination is 1.5  s at a wavelength of 632.8  nm. The dominant charge carrier was identified as holes from the direction of two-wave mixing energy transfer.
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77.84.Dy, 42.65.Hw, 78.20.-e, 72.20.Jv

Electrical properties of Ohmic contacts to ZnSe nanowires and their application to nanowire-based photodetection

J. Salfi, U. Philipose, C. F. de Sousa, S. Aouba, and H. E. Ruda

Appl. Phys. Lett. 89, 261112 (2006) (3 pages)

Online Publication Date: 28 December 2006

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Multilayer Ti/Au contacts were fabricated on individual, unintentionally doped zinc selenide nanowires with 80  nm nominal diameter. Four-terminal contact structures were used to independently measure current-voltage characteristics of contacts and nanowires. Specific contact resistivity of Ti/Au contacts is 0.024  Omega  cm2 and intrinsic resistivity of the nanowires is approximately 1  Omega  cm. The authors have also measured the spectral photocurrent responsivity of a ZnSe nanowire with 2.0  V bias across Ti/Au electrodes, which exhibits a turnon for wavelengths shorter than 470  nm and reaches 22  A/W for optical excitation at 400  nm.
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73.40.Cg, 73.40.Ns, 73.63.Rt, 85.35.Be, 85.60.Gz, 73.63.Nm, 85.30.De

Evanescent wave magnetometer

K. F. Zhao and Z. Wu

Appl. Phys. Lett. 89, 261113 (2006) (3 pages)

Online Publication Date: 28 December 2006

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The authors describe an atomic magnetometer, the evanescent wave magnetometer, which uses an evanescent wave to measure the Larmor frequency of Rb atoms near the cell surface. The submicron penetration depth of the probe beam allows the evanescent wave magnetometer to achieve a spatial resolution of several tens of microns or better, and greatly reduces the inhomogeneous broadeningdue to magnetic field inhomogeneities. Its noise density in the present experiment is ~100  pT/sqrt(Hz)for frequencies between 1 and 10  mHz, and decreases to less than 10  pT/sqrt(Hz) as the frequency increases to 25  Hz.
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07.55.Ge, 32.60.+i

Vertical AlGaN deep ultraviolet light emitting diode emitting at 322  nm fabricated by the laser lift-off technique

Koji Kawasaki, Choshiro Koike, Yoshinobu Aoyagi, and Misaichi Takeuchi

Appl. Phys. Lett. 89, 261114 (2006) (3 pages)

Online Publication Date: 28 December 2006

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A vertical AlGaN deep ultraviolet (DUV) light emitting diode (LED) emitting at 322  nm was fabfricated by the laser lift-off technique. The emission area extended to the entire electrode uniformly, and the current crowding was suppressed effectively in the devices. As a result, the differential conductance of the vertical LED was improved by a factor of 5 and the operation voltage was reduced to half, compared to that of the lateral LED. The self-heating effect was effectively suppressed even at high-current-density operation. The vertical structure in the high resistive AlGaN LED has potential application in high-power AlGaN DUV devices.
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85.60.Jb, 42.72.Bj, 42.62.-b

Phase locking of a frequency agile laser

Vincent Crozatier, Guillaume Gorju, Fabien Bretenaker, Jean-Louis Le Gouët, Ivan Lorgeré, Claude Gagnol, and Eric Ducloux

Appl. Phys. Lett. 89, 261115 (2006) (3 pages)

Online Publication Date: 28 December 2006

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The authors report on the development and phase locking of a frequency agile laser. The use of a simple unbalanced Mach-Zehnder interferometer together with a wideband phase-locked loop permits to control very fast frequency chirps (up to 3  GHz in 5  µs) with an excellent precision (frequency error less than 100  kHz). The servoloop could be applied to many tunable lasers.
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42.60.Fc, 42.55.Ah, 07.60.Ly, 42.65.Re

Phase-sensitive detection in Raman tweezers

G. Rusciano, A. C. De Luca, A. Sasso, and G. Pesce

Appl. Phys. Lett. 89, 261116 (2006) (3 pages)

Online Publication Date: 28 December 2006

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The authors discuss on a method to acquire the Raman spectrum of a single optically trapped particle. The method makes use of two laser beams: the first laser traps the particle and moves it back and forth in a plane perpendicular to the laser beam propagation; a second laser acts as Raman probe and it is fixed in space. The Raman spectrum is obtained by phase detecting the backscattered Raman photons using a lock-in amplifier. Within this approach, the background due to the scattering of the environment is completely removed. The authors apply this method to 4.25  µm diameter polystyrene beads in aqueous solution.
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07.60.-j, 42.50.Vk, 42.62.Eh

First-principles modeling of nonlinear optical properties of C3N4 polymorphs

Jianming Hu, Wendan Cheng, Shuping Huang, Dongsheng Wu, and Zhi Xie

Appl. Phys. Lett. 89, 261117 (2006) (3 pages)

Online Publication Date: 29 December 2006

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Using the first-principles method combined with the anharmonic oscillator model, the electronic, linear, and nonlinear optical properties of four C3N4 polymorphs are investigated. The calculations show that the third-order susceptibilities and nonlinear refractive indices increase with the increase of volume of C3N4 polymorphs below 1.5  eV, whereas the plasmon frequencies and relativistic nonlinear refractive indices decrease. The obtained results indicate that the spinel C3N4 has some potential applications in nonlinear optics.
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42.65.An, 42.65.Hw, 78.20.Ci, 71.45.Gm

Efficient solid-state host-guest light-emitting electrochemical cells based on cationic transition metal complexes

Hai-Ching Su, Chung-Chih Wu, Fu-Chuan Fang, and Ken-Tsung Wong

Appl. Phys. Lett. 89, 261118 (2006) (3 pages)

Online Publication Date: 29 December 2006

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The authors demonstrate highly efficient solid-state light-emitting electrochemical cells (LECs) consisting of green-emitting [Ir(dFppy)2(SB)]+(PF<sub>6</sub><sup>-</sup>) as the host and orange-emitting [Ir(ppy)2(SB)]+(PF<sub>6</sub><sup>-</sup>) as the guest [where dFppy is 2-(2,4-difluorophenyl)pyridine, SB is 4,5-diaza-9,9[prime]-spirobifluorene, and ppy is 2-phenylpyridine]. Photophysical studies show that with the optimized host-guest compositions, the emission is mainly from the guest and photoluminescence quantum yields are largely enhanced over those of pure host and guest films due to suppressed intermolecular interactions. Correspondingly, LECs based on such host-guest cationic complex systems show substantially enhanced quantum efficiencies (power efficiencies) of up to 10.4% (36.8  lm/W), representing a 1.5 times enhancement compared to those of pure host and guest devices. Such results indicate that the host-guest system is essential and useful for achieving highly efficient solid-state LECs.
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85.60.Jb, 82.47.-a

Beam splitting with a modified metallic nano-optic lens

Zhijun Sun

Appl. Phys. Lett. 89, 261119 (2006) (3 pages)

Online Publication Date: 29 December 2006

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In this letter, the authors show with finite-difference time-domain simulations the beam splitting of a metallic nano-optic lens structure modified by loading a metal block adjacent to the nanoslits. A model that considers the nearby space between the metal block and metallic lens as an equivalent surface-plasmon-waves-coupled subwavelength cavity is proposed. In the model, bound surface plasmon mode evolves into unbound free space mode with widening of the cavity, and conditional diffraction of optical wave through the lateral openings (size ~ wavelength) of the cavity leads to shaping of split beams in the intermediate zone.
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42.79.Fm, 42.79.Bh
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PLASMAS AND ELECTRICAL DISCHARGES

Scaling laws for dielectric window breakdown in vacuum and collisional regimes

Y. Y. Lau, J. P. Verboncoeur, and H. C. Kim

Appl. Phys. Lett. 89, 261501 (2006) (3 pages)

Online Publication Date: 29 December 2006

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The scaling laws for the initiation time of radio frequency (rf) window breakdown are constructed for three gases: Ar, Xe, and Ne. They apply to the vacuum, to the multipactor-triggered regime, and to the collisional rf plasma regime, and they are corroborated by computer simulations of these three gases over a wide range of pressures. This work elucidates the key factors that are needed for the prediction of rf window breakdown in complex gases, such as air, at various pressures.
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52.80.Vp, 52.80.Pi, 52.20.Fs, 52.20.Hv

Frequency coupling in dual frequency capacitively coupled radio-frequency plasmas

T. Gans, J. Schulze, D. O'Connell, U. Czarnetzki, R. Faulkner, A. R. Ellingboe, and M. M. Turner

Appl. Phys. Lett. 89, 261502 (2006) (3 pages)

Online Publication Date: 29 December 2006

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An industrial, confined, dual frequency, capacitively coupled, radio-frequency plasma etch reactor (Exelan®, Lam Research) has been modified for spatially resolved optical measurements. Space and phase resolved optical emission spectroscopy yields insight into the dynamics of the discharge. A strong coupling of the two frequencies is observed in the emission profiles. Consequently, the ionization dynamics, probed through excitation, is determined by both frequencies. The control of plasma density by the high frequency is, therefore, also influenced by the low frequency. Hence, separate control of plasma density and ion energy is rather complex.
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52.50.Dg, 52.80.Pi, 52.77.Bn, 52.70.Kz, 52.25.Os, 52.25.Jm

Xenon ion beam characterization in a helicon double layer thruster

C. Charles, R. W. Boswell, and M. A. Lieberman

Appl. Phys. Lett. 89, 261503 (2006) (3 pages)

Online Publication Date: 29 December 2006

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A current-free electric double layer is created in a helicon double layer thruster operating with xenon and compared to a recently developed theory. The Xe+ ion beam formed by acceleration through the potential drop of the double layer is characterized radially using an electrostatic ion energy analyzer. For operating conditions of 500  W rf power, 0.07  mTorr gas pressure, and a maximum magnetic field of 125  G, the measured beam velocity is about 6  km  s−1, the beam area is about 150  cm2, and the measured beam divergence is less than 6°.
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52.75.Di, 52.40.Kh, 52.40.Mj, 52.70.Nc
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STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER

Compression behavior of nanosized nickel and molybdenum

Selva Vennila. R, Shrinivas R. Kulkarni, Surendra K. Saxena, Hans-Peter Liermann, and Stanislav V. Sinogeikin

Appl. Phys. Lett. 89, 261901 (2006) (3 pages)

Online Publication Date: 26 December 2006

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X-ray diffraction studies of nanomolybdenum and nanonickel were conducted using a synchrotron source under nonhydrostatic compression up to pressures of 37 and 56  GPa, respectively. No phase transitions were found up to the maximum pressure studied. Bulk modulus and its pressure derivative were calculated from the pressure-volume data using the third-order Birch-Murnaghan equation of state. A decrease in compressibility in both systems with decrease in particle size has been observed. Compressibility behavior in nanometals studied is similar to the effect explained by Hall [Proc. Phys. Soc. London, Sect. B 64, 747 (1951)] and Petch [J. Iron Steel Inst., London 174, 25 (1953)].
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62.25.+g, 81.40.Lm, 81.40.Jj, 62.20.Dc, 62.20.Fe, 64.30.+t

Temperature response of 13C atoms in amorphized 6H–SiC

W. Jiang, Y. Zhang, V. Shutthanandan, S. Thevuthasan, and W. J. Weber

Appl. Phys. Lett. 89, 261902 (2006) (3 pages)

Online Publication Date: 26 December 2006

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Implantation of 13C2+ ions was employed to produce a concentration profile in 6H–SiC at 140  K. In situ study of 13C-implanted species was performed using the resonant reaction of 13C(p,gamma)14N at Ep=1.748  MeV. Significant 13C diffusion in the amorphized SiC does not occur up to 1130  K. The presence of Au implants (1.9  at.  %) does not affect the 13C behavior. High-energy H+ irradiation also does not promote the 13C diffusion. The results suggest that C atoms are readily trapped locally in the SiC structure during disordering, which is important in understanding the amorphization and recrystallization processes in SiC.
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61.72.Ww, 66.30.Jt, 81.40.Ef

Molecular beam epitaxy of phase pure cubic InN

J. Schörmann, D. J. As, K. Lischka, P. Schley, R. Goldhahn, S. F. Li, W. Löffler, M. Hetterich, and H. Kalt

Appl. Phys. Lett. 89, 261903 (2006) (3 pages)

Online Publication Date: 26 December 2006

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Cubic InN layers were grown by plasma assisted molecular beam epitaxy on 3C-SiC (001) substrates at growth temperatures from 419  to  490  °C. X-ray diffraction investigations show that the layers have zinc blende structure with only a small fraction of wurtzite phase inclusions on the (111) facets of the cubic layer. The full width at half maximum of the c-InN (002) x-ray rocking curve is less than 50  arc  min. The lattice constant is 5.01±0.01  Å. Low temperature photoluminescence measurements yield a c-InN band gap of 0.61  eV. At room temperature the band gap is about 0.56  eV and the free electron concentration is about n~1.7×1019  cm−3.
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81.15.Hi, 78.55.Cr, 68.55.-a, 71.20.Nr, 52.77.Dq

Enhanced plasticity in Mg-based bulk metallic glass composite reinforced with ductile Nb particles

D. G. Pan, H. F. Zhang, A. M. Wang, and Z. Q. Hu

Appl. Phys. Lett. 89, 261904 (2006) (3 pages)

Online Publication Date: 26 December 2006

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The authors report the synthesis of Mg-based metallic glass composite reinforced with Nb particles which are simply added during melting process. The ductile Nb particles effectively impede shear band propagation and upon yielding, deformed Nb particles distribute the load uniformly to the surrounding glassy matrix to promote the initiation and branching of abundant secondary shear bands. In contrast to the previous Mg-based metallic glass composites which fracture with very little plasticity, the composite shows great resistance to crack growth. The high strength of 900  MPa and large plasticity of 12.1±2% have made it comparable to excellent Zr- or Ti-based metallic glass composite.
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81.40.Lm, 62.20.Fe, 68.35.Gy, 81.05.Kf, 81.05.Bx

Direct observation of the epitaxial growth of molecular layers on molecular single crystals

A. Sassella, A. Borghesi, M. Campione, S. Tavazzi, C. Goletti, G. Bussetti, and P. Chiaradia

Appl. Phys. Lett. 89, 261905 (2006) (3 pages)

Online Publication Date: 26 December 2006

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In this letter the authors use the evolution of reflectance anisotropy spectra with film thickness during the growth of organic molecular films of a prototype molecular system (alpha-quarterthiophene grown onto a single crystal of the same material) to demonstrate homoepitaxy. The real time monitoring of the optical anisotropy of a thin film during deposition by organic molecular beam epitaxy is assessed as a powerful tool to achieve an effective in situ control of the growth starting from the very early deposition stages.
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68.55.Ac, 81.15.Hi, 78.66.Qn

Raman scattering and electroluminescence of ZnS:Cu,Cl phosphor powder

Yung-Tang Nien and In-Gann Chen

Appl. Phys. Lett. 89, 261906 (2006) (3 pages)

Online Publication Date: 26 December 2006

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Raman spectra of ZnS:Cu,Cl powders show a structural transition from hexagonal to cubic and a crystal growth with increasing Cu addition (>400  ppm), which is supposed to result from Cu incorporation or CuxS precipitation. The redshift of longitudinal optical mode in ZnS:Cu,Cl with increased amounts of Cu suggests that the excess Cu ions were located interstitially in the lattice and created a tensile strain. Under an electric field (3.75×106  V/m), a broad emission spectrum superposing of green, self-activated blue, and blue bands can be observed in the powders (Cu>=400  ppm), which agrees with the luminescence centers of Cu<sub>Zn</sub><sup>+</sup>, vacancies, and Cu<sub>i</sub><sup>+</sup>, respectively.
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78.30.Fs, 78.60.Fi, 64.70.Kb, 81.30.Mh, 61.72.Ji

Insight into the periodicity of Schallamach waves in soft material friction

Charles J. Rand and Alfred J. Crosby

Appl. Phys. Lett. 89, 261907 (2006) (3 pages)

Online Publication Date: 27 December 2006

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A dominant mechanism in friction of soft material interfaces is the onset and propagation of Schallamach waves. Schallamach waves are “tunnels” of air that provide relative displacement between the slider and the substrate rather than the instantaneous interfacial failure involved with stick-slip. Here, through model experiments and analysis, the authors present a fundamental relationship between the periodicity of Schallamach waves (lambda) and the ratio of interfacial adhesion ([script G]c) over complex elastic modulus (E*). This deconvolution of bulk and interfacial contributions to the friction of soft materials leads to interesting predictions that will impact material design for a wide range of applications.
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81.40.Pq, 81.40.Jj, 62.20.Qp, 62.20.Dc, 68.35.Af, 68.35.Np, 62.30.+d

Influence of stoichiometry on phase transition pressure of LiNbO3

A. Suchocki, W. Paszkowicz, A. Kaminska, A. Durygin, S. K. Saxena, L. Arizmendi, and V. Bermudez

Appl. Phys. Lett. 89, 261908 (2006) (3 pages)

Online Publication Date: 27 December 2006

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Results of Raman and x-ray diffraction studies under high pressures generated in diamond anvil cell of pure lithium niobate with different stoichiometries are reported. It is shown that the pressure at which the phase transition occurs depends on crystal stoichiometry and is strongly reduced as compared with those for LiNbO3 with congruent composition.
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62.50.+p, 64.70.Kb, 61.50.Nw, 78.30.Hv

Temperature dependence of serrated flows in compression in a bulk-metallic glass

W. H. Jiang, F. Jiang, F. X. Liu, H. Choo, P. K. Liaw, and K. Q. Qiu

Appl. Phys. Lett. 89, 261909 (2006) (3 pages)

Online Publication Date: 27 December 2006

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The authors investigated the effect of temperature on the plastic-flow and shear-banding behaviors of a Zr-based bulk-metallic glass in compression. The results indicate that at a subambient temperature (194.5  K) the plastic deformation of the bulk-metallic glass is still inhomogeneous and is characterized by the serrated flow and shear banding. But, compared to the ambient-temperature results, the subambient temperature moderates serrations in the plastic flow and leads to the formation of fewer shear bands. In view of the spatiotemporality of shear banding, the authors correlate the plastic-flow behaviors to shear-band patterns.
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81.40.Lm, 62.20.Fe

Influence of the substrate orientation on the electronic and optical properties of InAs/GaAs quantum dots

V. Mlinar and F. M. Peeters

Appl. Phys. Lett. 89, 261910 (2006) (3 pages)

Online Publication Date: 27 December 2006

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Using three-dimensional k·p calculation including strain and piezoelectricity the authors predict the variation of electronic and optical properties of InAs/GaAs quantum dots (QDs) with the substrate orientation. The QD transition energies are obtained for high index substrates, [11k] where k=1,2,3, and are compared with [001]. They find that the QD size in the growth direction determines the degree of influence of the substrate orientation: the flatter the dots, the larger the difference from the reference [001] case.
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73.21.La, 78.67.Hc, 71.15.-m, 77.65.-j

Biologically inspired polymer microfibers with spatulate tips as repeatable fibrillar adhesives

Seok Kim and Metin Sitti

Appl. Phys. Lett. 89, 261911 (2006) (3 pages)

Online Publication Date: 27 December 2006

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Being inspired by gecko foot hairs, microfibers with flat spatulate tips are proposed as repeatable adhesives. They are fabricated by molding a master template fabricated using deep reactive ion etching and the notching effect. Fabricated polyurethane fiber arrays with 4.5  µm fiber and 9  µm tip diameter demonstrated macroscale adhesion pressures up to 18  N/cm2 and overall work of adhesion up to 11  J/m2 on a 6  mm diameter glass hemisphere for a preload pressure of 12  N/cm2. These results show around four times higher adhesion and five times higher overall work of adhesion as compared to the flat polyurethane surface.
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81.05.Lg, 81.20.Hy, 81.65.Cf, 68.35.Np

Method to analyze dislocation injection from sharp features in strained silicon structures

Zhen Zhang, Juil Yoon, and Zhigang Suo

Appl. Phys. Lett. 89, 261912 (2006) (3 pages)

Online Publication Date: 27 December 2006

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A microelectronic device usually contains sharp features (e.g., edges and corners) that may intensify stresses, inject dislocations into silicon, and fail the device. The authors describe a method to analyze dislocation injection on the basis of singular stress fields near the sharp features, and apply the method to interpret available experiments of nitride pads on silicon substrates.
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61.72.Hh, 68.35.Gy

Infrared emission from Ge metal-insulator-semiconductor tunneling diodes

M. H. Liao, T.-H. Cheng, and C. W. Liu

Appl. Phys. Lett. 89, 261913 (2006) (3 pages)

Online Publication Date: 28 December 2006

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The Ge light-emitting diode with ~1.8  µm strong infrared emission is demonstrated using a metal-insulator-semiconductor tunneling structure. The intensity of a Ge device is one order of magnitude stronger than a similar Si device. At the positive gate bias, the holes in the Al gate electrode tunnel to the n-type Ge through the ultrathin oxide and recombine radiatively with electrons. An electron-hole-plasma model can be used to fit all the emission spectra from room temperature down to 65  K. From the measurement temperature range, the extracted band gap is ~40  meV lower than the reported band gap data, and the linewidth drops from 70  to  25  meV. The longitudinal acoustic phonon (~28  meV) and/or the band gap renormalization at high carrier density are proposed to be responsible for the reduction of photon energy. The band gap reduction on the mechanically strained n-type Ge and Si is also investigated experimentally and theoretically.
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85.60.Jb, 85.30.Mn

Configurations and diffusion of point defects in two-dimensional colloidal crystals

Tian Hui Zhang and Xiang Yang Liu

Appl. Phys. Lett. 89, 261914 (2006) (3 pages)

Online Publication Date: 28 December 2006

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Configurations and diffusion of crystal defects are studied in a two-dimensional colloidal model system in the presence of an attraction. Monomer vacancies are immobile and have identical symmetry with the underlying triangular lattice. Both dimer vacancies and trimer vacancies have two different configurations and the configurations with higher symmetry are more stable. Dimer vacancies in the authors' our experiments exhibit the highest diffusivity, whereas the global diffusion of vacancies of larger clusters, such as trimer vacancies, is inhibited. Compared with previous studies, it is found that defect dynamics is strongly dependent on the nature of the interaction potential.
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82.70.Dd, 66.30.Lw, 61.72.Ji

Red photoluminescence in praseodymium-doped titanate perovskite films epitaxially grown by pulsed laser deposition

Hiroshi Takashima, Kazushige Ueda, and Mitsuru Itoh

Appl. Phys. Lett. 89, 261915 (2006) (3 pages)

Online Publication Date: 28 December 2006

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Intense red photoluminescence (PL) under ultraviolet (UV) excitation was observed in epitaxially grown Pr-doped Ca0.6Sr0.4TiO3 perovskite films. The films were grown on SrTiO3 (100) substrates by pulsed laser deposition, and their epitaxial growth was confirmed by x-ray diffraction and reflected high-energy electron diffraction. The observed sharp PL peak centered at 610  nm was assigned to the transition of Pr3+ ions from the 1D2 state to the 3H4 state. The PL intensity was markedly enhanced by postannealing treatments at 1000  °C, above the film-growth temperature of 600 or 800  °C. Because the excitation and absorption spectra are similar to each other, it was suggested that the UV energy absorbed by the host lattice was transferred to the Pr ions, resulting in the red luminescence.
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78.66.-w, 78.55.Hx, 78.40.Ha

Optical measurements of the thermal properties of nanofluids

Roberto Rusconi, Erica Rodari, and Roberto Piazza

Appl. Phys. Lett. 89, 261916 (2006) (3 pages)

Online Publication Date: 28 December 2006

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The authors show that the thermal conductivity and diffusivity of colloidal particle dispersions can be rapidly obtained with high accuracy and reproducibility by exploiting a noninvasive, all-optical thermal lensing method. Applications of this technique to model suspensions of spherical monodisperse particles suggest that classical models for the effective properties of composite media hold up to rather high volume fractions, while no “anomalous” thermal conductivity effects are found.
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66.60.+a, 78.20.Nv, 82.70.Dd, 82.70.Kj

Structural short-range order of the beta-Ti phase in bulk Ti–Fe–(Sn) nanoeutectic composites

J. Das, J. Eckert, and R. Theissmann

Appl. Phys. Lett. 89, 261917 (2006) (3 pages)

Online Publication Date: 28 December 2006

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The authors report lattice distortion and “omega-like” structural short-range order (SRO) of the beta-Ti phase in a Ti–Fe–(Sn) bulk nanoeutectic composite prepared by slow cooling from the melt. The nanoeuetctic phases are chemically homogeneous, but the addition of Sn releases the local lattice strain, modifies the structural SRO, and prevents the formation of stacking faults in the body centered cubic (bcc) beta-Ti phase resulting in improved plastic deformability. The elastic properties and the structural SRO of the beta-Ti phase are proposed to be important parameters for developing advanced high strength, ductile Ti-base nanocomposite alloys.
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81.40.Jj, 81.40.Lm, 62.20.Fe, 61.66.Dk, 62.20.Dc, 68.35.Gy

Twisted phase gratings induced by photoregulated mesogenic molecules on the surface of photoreactive polymer liquid crystal film

Hiroshi Ono, Tomoyuki Sasaki, Akira Emoto, Emi Uchida, and Nobuhiro Kawatsuki

Appl. Phys. Lett. 89, 261918 (2006) (3 pages)

Online Publication Date: 29 December 2006

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Three-dimensional phase gratings with twisted structures were self-organized by photoregulated mesogenic molecules on the surface of photocrosslinkable polymer liquid crystal film. The molecules are selectively photoreacted only at the surface of the film under the polarization holographic illumination, and the resultant photocrosslinked mesogenic molecules induce the three-dimensional twisted gratings in the film during annealing process. The diffraction properties were well explained by means of Jones calculus and diffraction theory.
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42.79.Dj, 42.65.Hw, 42.70.Df, 42.70.Gi, 42.70.Nq, 61.30.Vx

Influence of cohesive energy on unit cell volume of perovskite manganites La1−xMxMnO3

G. D. Tang, D. L. Hou, Z. Z. Li, X. Zhao, W. H. Qi, S. P. Liu, and F. W. Zhao

Appl. Phys. Lett. 89, 261919 (2006) (3 pages)

Online Publication Date: 29 December 2006

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Ionic size is considered to be an important factor influencing the unit cell volume of manganites with an ABO3 structure. For La1−xSrxMnO3, however, although the average effective ion radius of all cations taken together increases with increasing x, the unit cell volume decreases for x<0.5. In the case of La1−xNaxMnO3, the unit cell volume reaches a minimum when x=0.3. Up to now, no satisfactory explanation of these phenomena has been found. In this letter, an explanation based on the ionic cohesive energy with a small additional metallic cohesive energy is described.
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61.50.Lt, 71.15.Nc, 61.50.Nw, 61.66.Fn, 82.80.-d
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ELECTRONIC TRANSPORT AND SEMICONDUCTORS

Subpicosecond time-resolved Raman studies of field-induced transient transport in an InxGa1−xAs-based p-i-n semiconductor nanostructure

K. T. Tsen, Juliann G. Kiang, D. K. Ferry, and H. Morkoç

Appl. Phys. Lett. 89, 262101 (2006) (3 pages)

Online Publication Date: 26 December 2006

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Electron transient transport in an InxGa1−xAs-based (x=0.53) p-i-n nanostructure under the application of an electric field has been studied by time-resolved Raman spectroscopy on a subpicosecond time scale and at T=300  K. The experimental results reveal the time evolution of the electron distribution function and electron drift velocity with subpicosecond time resolution. These experimental results are compared with those of both InP-based and GaAs-based p-i-n nanostructures and provide a consistent understanding and better insight of electron transient transport phenomena in semiconductors.
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73.40.Kp, 73.63.-b, 78.67.-n, 78.30.Fs, 78.47.+p

Electron transport in naphthylamine-based organic compounds

S. C. Tse, K. C. Kwok, and S. K. So

Appl. Phys. Lett. 89, 262102 (2006) (3 pages)

Online Publication Date: 26 December 2006

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Two naphthylamine-based hole transporters, namely, N,N[prime]-diphenyl-N,N[prime]-bis(1-naphthyl)(1,1[prime]-biphenyl)-4,4[prime]diamine (NPB) and 4,4[prime],4[double-prime]-tris(n-(2-naphthyl)-n-phenyl-amino)-triphenylamine (2TNATA), were found to possess electron transporting (ET) abilities. From time-of-flight measurements, values of electron mobilities for NPB and 2TNATA are (6–9)×10−4 and (1–3)×10−4  cm2/V  s, respectively, under an applied electric field range of 0.04–0.8  MV/cm at 290  K. An organic light-emitting diode that employed NPB as the ET material was demonstrated. The electron conducting mechanism of NPB and 2TNATA in relation to the Marcus theory [Rev. Mod. Phys. 65, 599 (1993)] from quantum chemistry will be discussed.
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72.20.Ee, 72.80.Le, 85.60.Jb

Selective epitaxial growth of 4H-SiC at reduced temperatures using halo-carbon precursor

Bharat Krishnan, Hrishikesh Das, Huang-De Lin, and Yaroslav Koshka

Appl. Phys. Lett. 89, 262103 (2006) (3 pages)

Online Publication Date: 26 December 2006

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Selective homoepitaxial growth of 4H-SiC polytype at temperatures down to 1300  °C was investigated using novel halo-carbon epitaxial growth technique. More than three times higher growth rate than in blanket epitaxy without morphology degradation was due to the local loading effect, which demonstrated that the growth rate of SiC homoepitaxy at low temperatures is limited by the gas-phase reaction mechanisms rather than surface mechanisms. It was established that the step-flow growth without undesirable nucleation may be maintained at growth rates in excess of 6  µm/h. Strong dependence of the growth rate on the ratio of the seed area to the masked area indicated that the growth rate is predominantly mass transport limited. Facet formation and conditions for suppressing defect generation at the mesa walls were investigated.
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68.55.Ac, 81.15.Gh, 81.15.Kk, 68.55.Ln

Strong magnetic scattering from TiOx adhesion layers

A. Trionfi, S. Lee, and D. Natelson

Appl. Phys. Lett. 89, 262104 (2006) (3 pages)

Online Publication Date: 26 December 2006

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Electronic phase coherence in normal metals is incredibly sensitive to magnetic scattering. As a result, the weak localization magnetoresistance and time-dependent universal conductance fluctuations are powerful probes of magnetic impurities. The authors report measurements of these effects in Au and Ag nanowires with a 1.5  nm thick Ti adhesion layer underneath the deposited metal. The results indicate an anomalously large magnetic impurity concentration due to the Ti layer. The results also suggest that this magnetic scattering and its evolution are related to the oxidation state of the Ti.
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72.15.Gd, 75.30.Hx

Defect reduction in (11[overline 2]0) a-plane GaN by two-stage epitaxial lateral overgrowth

X. Ni, Ü. Özgür, Y. Fu, N. Biyikli, J. Xie, A. A. Baski, H. Morkoç, and Z. Liliental-Weber

Appl. Phys. Lett. 89, 262105 (2006) (3 pages)

Online Publication Date: 26 December 2006

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The authors report a two-stage epitaxial lateral overgrowth (ELO) method to get uniformly coalesced (11[overline 2]0) a-plane GaN using metal organic chemical vapor deposition by employing a relatively lower growth temperature in the first stage followed by conditions leading to enhanced lateral growth in the second. Using a two-stage ELO method the average Ga-polar to N-polar wing growth rate ratio has been reduced from 4–6 to 1.5–2, which consequently reduced the height difference between the two approaching wings at the coalescence front that resulted from the wing tilt (0.44° for Ga and 0.37° for N wings, measured by x-ray diffraction), thereby making their coalescence much easier. Transmission electron microscopy showed that the threading dislocation density in the wing areas was 1.0×108  cm−2, more than two orders of magnitude lower than that in the window areas (4.2×1010  cm−2). However, high density of basal stacking faults of 1.2×104  cm−1 was still present in the wing areas as compared to c-plane GaN where they are rarely observed away from the substrate. Atomic force microscopy and photoluminescence measurements on the coalesced ELO a-plane GaN sample also indicated improved material quality.
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68.55.Ac, 81.15.Gh, 78.66.Fd, 81.05.Ea, 78.55.Cr, 68.55.Ln

Investigation of void nucleation and propagation during electromigration of flip-chip solder joints using x-ray microscopy

S. H. Chiu and Chih Chen

Appl. Phys. Lett. 89, 262106 (2006) (3 pages)

Online Publication Date: 27 December 2006

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X-ray microscopy was employed to investigate void nucleation and propagation during electromigration in solder joints. The shape of the voids at various stages can be clearly observed. The voids became irregular when they propagated to deplete the contact opening. Growth velocity at the early stage was found to be 1.3  µm/h under 6.5×103  A/cm2 at 150  °C, and it decreased to 0.3  µm/h at later stages. Formation of intermetallic compound (IMC) and compositional changes at the interface of solder/IMC on the chip side were attributed to the retarded growth rate at later stages.
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85.40.Ls, 66.30.Qa

Carrier concentration induced band-gap shift in Al-doped Zn1−xMgxO thin films

J. G. Lu, S. Fujita, T. Kawaharamura, H. Nishinaka, Y. Kamada, and T. Ohshima

Appl. Phys. Lett. 89, 262107 (2006) (3 pages)

Online Publication Date: 27 December 2006

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Transparent conducting Al-doped Zn1−xMgxO thin films were grown on glass substrates by chemical vapor deposition. The resistivity could be lowered to 10−3  Omega  cm with optical transmittance above 85% in visible regions. The influence of carrier concentration on band-gap shift in Zn1−xMgxO alloys was systematically studied. The shift of energy gap could be fully explained by the Fermi-level band filling and band-gap renormalization effects. As the Mg content increased, the electron effective masses in Zn1−xMgxO (x=0–0.21) alloys increased from 0.30m0 to 0.49m0. The Al-doping efficiency was reduced with the increase in alloy composition.
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73.61.Le, 78.66.Li, 71.18.+y, 71.20.Ps, 61.72.Ww

Control of structure, conduction behavior, and band gap of Zn1−xMgxO films by nitrogen partial pressure ratio of sputtering gases

C. X. Cong, B. Yao, G. Z. Xing, Y. P. Xie, L. X. Guan, B. H. Li, X. H. Wang, Z. P. Wei, Z. Z. Zhang, Y. M. Lv, D. Z. Shen, and X. W. Fan

Appl. Phys. Lett. 89, 262108 (2006) (3 pages)

Online Publication Date: 27 December 2006

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Zn1−xMgxO films were grown by radio-frequency reactive magnetron sputtering using mixture of nitrogen and argon as sputtering gases. It was found that Mg concentration, structures, electrical properties, and band gaps of the films can be tuned by changing nitrogen partial pressure ratio of the sputtering gases. The Zn1−xMgxO film consists of wurtzite phase at the ratios from 0% to 50%, mixture of wurtzite and cubic phases at the ratios of 78% to 83%, and cubic phase at 100%. The Mg concentration increased linearly with increasing the ratio. The band gap increases from 3.64  eV at x=0.172  to  4.02  eV at x=0.44 for the wurtzite Zn1−xMgxO and reaches 6.30  eV for cubic Zn1−xMgxO with x=0.84. All the as-grown Zn1−xMgxO films show high resistivity at room temperature, but transform into p-type conduction after annealing at 600  °C for 30  min under 10−4  Pa, except for the film grown at the ratio of zero.
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68.55.-a, 73.61.Ga, 71.20.Nr, 81.15.Cd, 61.50.Nw, 61.72.Cc

Experimental study of the subwavelength imaging by a wire medium slab

Pavel A. Belov, Yan Zhao, Sunil Sudhakaran, Akram Alomainy, and Yang Hao

Appl. Phys. Lett. 89, 262109 (2006) (3 pages)

Online Publication Date: 27 December 2006

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An experimental investigation of subwavelength imaging by a wire medium slab is performed. A complex-shaped near field source is used in order to test imaging performance of the device. It is demonstrated that the ultimate bandwidth of operation of the constructed imaging device is 4.5% that coincides with theoretical predictions [P. A. Belov and M. G. Silveirinha, Phys. Rev. E 73, 056607 (2006)]. Within this band the wire medium slab is capable of transmitting images with lambda/15 resolution irrespective of the shape and complexity of the source. Actual bandwidth of operation for particular near-field sources can be larger than the ultimate value, but it strongly depends on the configuration of the source.
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78.20.Ci, 42.70.-a, 07.60.-j

True dipole at the indium tin oxide/organic semiconductor interface

Yow-Jon Lin, Jia-huang Hong, Yi-Chun Lien, and Bei-Yuan Liu

Appl. Phys. Lett. 89, 262110 (2006) (3 pages)

Online Publication Date: 28 December 2006

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There has been long-standing interest in the development of organic optoelectronic devices. However, the authors find that the previously reported interface-dipole calculations seem to be inaccurate, owing to a persistent neglect of the induced band bending of indium tin oxide (ITO) by coating of organic semiconductors on ITO. In this study, the correlation between the induced band bending of ITO in the presence of organic semiconductors on ITO and the dipole at the interface was examined.
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73.20.At, 73.30.+y, 79.60.Jv, 61.50.Lt

Effect of grain alignment on interface trap density of thermally oxidized aligned-crystalline silicon films

Woong Choi, Jung-Kun Lee, and Alp T. Findikoglu

Appl. Phys. Lett. 89, 262111 (2006) (3 pages)

Online Publication Date: 28 December 2006

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The authors report studies of the effect of grain alignment on interface trap density of thermally oxidized aligned-crystalline silicon (ACSi) films by means of capacitance-voltage (C-V) measurements. C-V curves were measured on metal-oxide-semiconductor (MOS) capacitors fabricated on <001>-oriented ACSi films on polycrystalline substrates. From high-frequency C-V curves, the authors calculated a decrease of interface trap density from 2×1012  to  1×1011  cm−2  eV−1 as the grain mosaic spread in ACSi films improved from 13.7° to 6.5°. These results demonstrate the effectiveness of grain alignment as a process technique to achieve significantly enhanced performance in small-grained (<=1  µm) polycrystalline Si MOS-type devices.
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73.40.Qv, 84.32.Tt

Identification of oxygen and zinc vacancy optical signals in ZnO

T. Moe Børseth, B. G. Svensson, A. Yu. Kuznetsov, P. Klason, Q. X. Zhao, and M. Willander

Appl. Phys. Lett. 89, 262112 (2006) (3 pages)

Online Publication Date: 28 December 2006

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Photoluminescence spectroscopy has been used to study single crystalline ZnO samples systematically annealed in inert, Zn-rich and O-rich atmospheres. A striking correlation is observed between the choice of annealing ambient and the position of the deep band emission (DBE) often detected in ZnO. In particular, annealing in O2 results in a DBE at 2.35±0.05  eV, whereas annealing in the presence of metallic Zn results in DBE at 2.53±0.05  eV. The authors attribute the former band to zinc vacancy (VZn) related defects and the latter to oxygen vacancy (VO) related defects. Additional confirmation for the VO and VZn peak identification comes from the observation that the effect is reversible when O- and Zn-rich annealing conditions are switched. After annealing in the presence of ZnO powder, there is no indication for the VZn- or VO-related bands, but the authors observe a low intensity yellow luminescence band peaking at 2.17  eV, probably related to Li, a common impurity in hydrothermally grown ZnO.
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78.55.Et, 61.72.Ji, 61.72.Cc

Adjustable metal-semiconductor transition of FeS thin films by thermal annealing

Ganhua Fu, Angelika Polity, Niklas Volbers, Bruno K. Meyer, Boris Mogwitz, and Jürgen Janek

Appl. Phys. Lett. 89, 262113 (2006) (3 pages)

Online Publication Date: 28 December 2006

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FeS polycrystalline thin films were prepared on float glass at 500  °C by radio-frequency reactive sputtering. The influence of vacuum annealing on the metal-semiconductor transition of FeS films was investigated. It has been found that with the increase of the annealing temperature from 360  to  600  °C, the metal-semiconductor transition temperature of FeS films first decreases and then increases, associated with first a reduction and then an enhancement of hysteresis width. The thermal stress is considered to give rise to the abnormal change of the metal-semiconductor transition of the FeS film during annealing.
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73.61.Ng, 81.15.Cd, 71.30.+h, 81.40.Gh, 68.60.Bs, 68.55.Ac

Enhanced charge mobility in a molecular hole transporter via addition of redox inactive ionic dopant: Implication to dye-sensitized solar cells

Henry J. Snaith and Michael Grätzel

Appl. Phys. Lett. 89, 262114 (2006) (3 pages)

Online Publication Date: 28 December 2006

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Upon the addition of lithium salts to the hole-transporter matrix, 2,2[prime],7,7[prime]-tetrakis(N,N-di-p-methoxypheny-amine)-9,9[prime]-spirobifluorene (spiro-MeOTAD), the authors observe a 100-fold increase in conductivity through spiro-MeOTAD within a TiO2 mesoporous network. The authors demonstrate this to be a bulk effect and not due to improved injection at the electrodes. By testing “hole-only” diodes of pure spiro-MeOTAD and those doped with lithium salts, the authors calculate that the hole mobility increases from 1.6×10−4  to  1.6×10−3  cm2/V  s. The authors discuss the possible mechanisms for this significant enhancement in charge mobility and its implication to the dye-sensitized solar cell operation.
Show PACS
73.61.Ph, 73.50.Dn, 72.20.Fr, 61.72.Ww, 85.30.Kk, 84.60.Jt

Formation mechanism of cerium oxide-doped indium oxide/Ag Ohmic contacts on p-type GaN

Dong-Seok Leem, Tae-Wook Kim, Takhee Lee, Ja-Soon Jang, Young-Woo Ok, and Tae-Yeon Seong

Appl. Phys. Lett. 89, 262115 (2006) (3 pages)

Online Publication Date: 28 December 2006

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See Also: Erratum

Show Abstract
The authors report on the formation of cerium oxide-doped indium oxide(2.5  nm)/Ag(250  nm) contacts to p-GaN. The contacts become Ohmic with a specific contact resistance of 3.42×10−4  Omega  cm2 upon annealing at 530  °C in air. X-ray photoemission spectroscopy (XPS) Ga 3d core levels obtained from the interface regions before and after annealing indicate a large band bending of p-GaN (about 1.7–1.8  eV), namely, an increase of Schottky barrier height. Based on the XPS, secondary ion mass spectroscopy, and capacitance-voltage data, possible transport mechanisms for the annealed contacts are described and discussed.
Show PACS
73.40.Ns, 73.40.Cg, 61.72.Cc, 79.60.Jv, 73.30.+y, 79.20.Rf

Quantitative observation and discrimination of AlGaN- and GaN-related deep levels in AlGaN/GaN heterostructures using capacitance deep level optical spectroscopy

A. Armstrong, A. Chakraborty, J. S. Speck, S. P. DenBaars, U. K. Mishra, and S. A. Ringel

Appl. Phys. Lett. 89, 262116 (2006) (3 pages)

Online Publication Date: 28 December 2006

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Deep levels were observed using capacitance deep level optical spectroscopy (DLOS) in an AlGaN/GaN heterostructure equivalent to that of a heterojunction field effect transistor. Band gap states were assigned to either the AlGaN or GaN regions by comparing the DLOS spectra in accumulation and pinch-off modes, where the former reflects both AlGaN- and GaN-related defects, and the latter emphasizes defects residing in the GaN. A band gap state at Ec−3.85  eV was unambiguously identified with the AlGaN region, and deep levels at Ec−2.64  eV and Ec−3.30  eV were associated with the GaN layers. Both the AlGaN and GaN layers exhibited additional deep levels with large lattice relaxation. The influence of deep levels on the two-dimensional electron gas sheet charge was estimated using a lighted capacitance-voltage method.
Show PACS
73.21.-b, 71.55.Eq, 71.20.Nr

Near-field scanning optical microscopy and time-resolved optical characterization of epitaxial lateral overgrown c-plane and a-plane GaN

Ü. Özgür, X. Ni, Y. Fu, H. Morkoç, and H. O. Everitt

Appl. Phys. Lett. 89, 262117 (2006) (3 pages)

Online Publication Date: 29 December 2006

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Epitaxial lateral overgrowth (ELO) was employed for both c-plane and a-plane GaN layers on sapphire, and a more pronounced optical improvement was observed for the a-plane GaN as evidenced by the significantly increased band edge photoluminescence (PL). Room temperature near-field scanning optical microscopy studies explicitly showed enhanced optical quality in the wing regions of the overgrown GaN due to reduced density of dislocations, and for the a-plane ELO GaN sample the wings and the windows were clearly discernible from PL mapping. Time-resolved PL measurements revealed biexponential decays with time constants that were significantly enhanced for the a-plane ELO GaN (tau1=0.08  ns, tau2=0.25  ns) when compared to the non-ELO control sample but were still much shorter than those for the c-plane ELO GaN (tau1=0.26  ns, tau2=0.90  ns).
Show PACS
78.66.Fd, 78.47.+p, 78.55.Cr, 61.72.Ff

p-type conductivity and donor-acceptor pair emission in Cd1−xFexS dilute magnetic semiconductors

X. J. Wu, D. Z. Shen, Z. Z. Zhang, J. Y. Zhang, K. W. Liu, B. H. Li, Y. M. Lu, D. X. Zhao, and B. Yao

Appl. Phys. Lett. 89, 262118 (2006) (3 pages)

Online Publication Date: 29 December 2006

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Cd1−xFexS thin films with different Fe contents were grown on c-plane sapphire by low-pressure metal organic chemical vapor deposition. The resistivity of the thin films was found to increase with the addition of more Fe contents into the Cd1−xFexS thin films by elevating the growth temperature, which was attributed to the ionization of holes from trivalent Fe ions. High density Fe doping eventually reversed the conductivity of Cd1−xFexS thin film from n to p type. With increasing Fe content, the band-to-band transition at 2.5  eV was suppressed while the emission from the donor-acceptor pairs at 2.0–2.4  eV finally dominated the photoluminescence spectra.
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81.05.Dz, 73.61.Ga, 78.66.Hf, 75.50.Pp, 75.70.Ak, 78.55.Et

Photocurrent and transmission spectroscopy of direct-gap interband transitions in Ge/SiGe quantum wells

S. Tsujino, H. Sigg, G. Mussler, D. Chrastina, and H. von Känel

Appl. Phys. Lett. 89, 262119 (2006) (3 pages)

Online Publication Date: 29 December 2006

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The authors studied the direct-gap interband transitions in strain-compensated Ge/SiGe quantum wells grown by low energy plasma enhanced chemical vapor deposition. A series of excitonic interband absorptions from the quantized hole states to the quantized electronic states at the Gamma<sub>7c</sub><sup>-</sup> edge in the Ge wells is observed up to room temperature by photocurrent and transmission spectroscopy. The results are compared with theory. At low temperature, the quantum confined Stark effect is demonstrated in a Schottky diode geometry.
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78.66.Db, 73.63.Hs, 78.67.De, 72.40.+w, 73.21.Fg

Hysteresis mechanism in pentacene thin-film transistors with poly(4-vinyl phenol) gate insulator

Cheon An Lee, Dong-Wook Park, Keum-Dong Jung, Byung-ju Kim, Yoo Chul Kim, Jong Duk Lee, and Byung-Gook Park

Appl. Phys. Lett. 89, 262120 (2006) (3 pages)

Online Publication Date: 29 December 2006

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The hysteresis mechanism is studied in pentacene organic thin-film transistors (OTFTs) with poly(4-vinyl phenol) (PVP) gate insulator by examining OTFTs with an oxide/PVP double layer gate insulator. The oxide thickness affects the direction of the hysteresis as well as its magnitude. This result can be explained on the basis of the charge injection and trapping mechanism rather than slow polarization or ion migration. The hysteresis occurs mainly due to the charges which could be injected from the gate electrode and trapped in the PVP. As the thickness of the oxide layer is increased, the gate charge injection is blocked and the effect of the charges from the channel increases.
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85.30.Tv, 85.30.De

Intrinsic crystalline-to-amorphous transition above 400  °C in plasma-deposited Si thin films

Toshihiro Kamei

Appl. Phys. Lett. 89, 262121 (2006) (3 pages)

Online Publication Date: 29 December 2006

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There has been long debate concerning why microcrystalline Si formation is disrupted at a deposition temperature above around 400  °C: is it due to the loss of surface hydrogen coverage or to the incorporation of oxygen impurities? The authors reduced the atmospheric concentration of such impurities as O, C, and N in this work by using an ultraclean plasma deposition apparatus and demonstrate that microcrystalline Si films showed improved crystallinity at a deposition temperature of 350  °C, resulting in a mobility of 5. As the deposition temperature was raised to 450  °C, however, the Raman spectrum shows an amorphous structure of the film under such clean conditions. According to secondary ion mass spectrometry, the O, C, and N concentrations in this film were as low as 1017, 1016, and 1017  cm−3, respectively, demonstrating that the crystalline-to-amorphous transition around 400  °C should be intrinsic under the present conditions and may be correlated with thermal hydrogen desorption on a film-growing surface.
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81.05.Cy, 81.05.Gc, 81.15.Jj, 52.77.Dq, 68.55.Ac, 68.55.Ln
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MAGNETISM AND SUPERCONDUCTIVITY

Magnetostriction studies in an antiferromagnetic polycrystalline Mn42Fe58 alloy

W. Y. Peng and J. H. Zhang

Appl. Phys. Lett. 89, 262501 (2006) (3 pages)

Online Publication Date: 26 December 2006

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Magnetostriction “jump” effect often occurs in ferromagnetic materials. However, in this letter, the jump effect is observed in antiferromagnetic face-centered-cubic austenitic polycrystalline Mn42Fe58 alloy at room temperature. The magnetostriction under an applied field of 1.0  T is 169  ppm parallel to the field. The jump magnetostriction of −581  ppm, which is antiparallel to the field, is obtained when the sample is subjected to the compressive stress of 1.52  MPa. The relation between the magnetostriction and the field strength is discussed.
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75.80.+q, 75.50.Ee, 62.50.+p, 81.40.Vw

Enhancement of exchange coupling between GaMnAs and IrMn with self-organized Mn(Ga)As at the interface

H. T. Lin, Y. F. Chen, P. W. Huang, S. H. Wang, J. H. Huang, C. H. Lai, W. N. Lee, and T. S. Chin

Appl. Phys. Lett. 89, 262502 (2006) (3 pages)

Online Publication Date: 26 December 2006

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An atomically flat and uniform reaction layer of Mn(Ga)As was found to self-organize at the (Ga,Mn)As/IrMn interface by postannealing. The Mn(Ga)As layer exhibits strong ferromagnetic characteristics up to the measured 300  K. In particular, the manifested horizontal shift of field-cooled hysteresis loops shows a clear signature of exchange bias attributable to the exchange coupling between IrMn and Mn(Ga)As. Implication from composition analyses, exchange-bias effect, and thickness dependence of the Mn(Ga)As layer versus annealing conditions is also discussed.
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75.50.Pp, 75.50.Dd, 75.70.Cn, 75.60.Ej, 68.35.Fx, 61.72.Cc

Structural and magnetic properties of epitaxial thin films of the ordered double perovskite La2CoMnO6

H. Z. Guo, A. Gupta, T. G. Calvarese, and M. A. Subramanian

Appl. Phys. Lett. 89, 262503 (2006) (3 pages)

Online Publication Date: 26 December 2006

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Epitaxial thin films of the ordered ferromagnetic double perovskite La2CoMnO6 have been grown on (001)-oriented SrTiO3 substrates by pulsed laser deposition. X-ray photoemission spectroscopy analysis suggests that the Mn and Co ions are primarily in the 4+ and 2+ valence states. Magnetic measurements indicate the presence of two phases: a majority and a minority ferromagnetic phase with transition temperatures of 230 and 80  K, respectively. A spin-glass-type magnetic behavior is also indicated for the films based on time-dependent measurements. The spin-glass behavior likely arises from a competition between ferromagnetic and antiferromagnetic interactions because of local disorder, even in films grown under optimal conditions.
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75.70.Ak, 68.55.Jk, 75.50.Dd, 75.50.Lk, 75.50.Ee, 81.15.Fg

Vacancy induced structural and magnetic transition in MnCo1−xGe

Jian-Tao Wang, Ding-Sheng Wang, Changfeng Chen, O. Nashima, T. Kanomata, H. Mizuseki, and Y. Kawazoe

Appl. Phys. Lett. 89, 262504 (2006) (3 pages)

Online Publication Date: 26 December 2006

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The authors report ab initio total energy calculations on the first-order structural transition of the ferromagnetic MnCo1−xGe(0.00<=x<=0.25) intermetallic compound. They show that increasing Co vacancies induce a transition from an orthorhombic structure at 0<=x<=0.08 to a hexagonal structure at x>0.08. A concomitant high-to-low moment magnetic transition and a large magnetovolume effect occur due to the change of the symmetry and the resulting coupling distance between the magnetic atoms. These results provide an excellent account for the experimental results and reveal the crucial role of the Co vacancies in determining the relative structural stability and the magnetic properties of MnCo1−xGe.
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64.70.Kb, 75.30.Kz, 61.72.Ji, 75.40.Cx, 75.50.Cc

Prospects for detection of spin accumulation using submicron planar Andreev array spectroscopy

F. Magnus, K. A. Yates, B. Morris, Y. Miyoshi, Y. Bugoslavsky, L. F. Cohen, G. Burnell, M. G. Blamire, and P. W. Josephs-Franks

Appl. Phys. Lett. 89, 262505 (2006) (3 pages)

Online Publication Date: 27 December 2006

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Andreev spectroscopy can be employed to measure transport spin polarization. Planar Andreev devices have an advantage over point-contact Andreev spectroscopy as they offer greater control over interface quality and the possibility of spatially resolved information about spin polarization using submicron Andreev junction arrays. The authors compare the performance of Pb point contacts onto Cu and Co with that of large area and submicron planar junctions and singularly connected nanoarrays. Planar structures compare favorably to point contacts although the fabrication method influences extracted parameters. The authors find that submicron planar junction reproducibility does not adversely affect the prospects for developing arrays for detection of spin accumulation.
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74.50.+r, 74.70.Ad, 74.25.Ha, 74.25.Fy

Strain induced magnetic anisotropy in highly epitaxial CoFe2O4 thin films

W. Huang, J. Zhu, H. Z. Zeng, X. H. Wei, Y. Zhang, and Y. R. Li

Appl. Phys. Lett. 89, 262506 (2006) (3 pages)

Online Publication Date: 27 December 2006

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Cobalt ferrite (CoFe2O4) thin films were epitaxially grown on (001) SrTiO3 and (001) MgO by laser molecular beam epitaxy. Microstructural studies indicate that the CoFe2O4 grown on (001) SrTiO3 with compressive strain are c-oriented island growth mode with rough surface morphology, whereas the films on (001) MgO with tensile strain become c oriented with layer-by-layer mode. Magnetic property studies reveal that the compressive strained CoFe2O4 films on (001) SrTiO3 can significantly enhance out-of-plane magnetization (190  emu/cm3) with a large coercivity (3.8  kOe). In contrast, the tensile strained CoFe2O4 films on (001) MgO exhibit weak magnetic anisotropy. These results suggest that strong magnetic anisotropy is highly dependent on the lattice mismatch induced strain.
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75.50.Gg, 75.70.Ak, 75.30.Gw, 75.60.Ej, 68.60.Bs, 68.35.Bs

Dynamics of vortex core switching in ferromagnetic nanodisks

Q. F. Xiao, J. Rudge, B. C. Choi, Y. K. Hong, and G. Donohoe

Appl. Phys. Lett. 89, 262507 (2006) (3 pages)

Online Publication Date: 28 December 2006

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Dynamics of magnetic vortex core switching in nanometer-scale Permalloy disk, having a single vortex ground state, was investigated by micromagnetic modeling. When an in-plane magnetic field pulse with an appropriate strength and duration is applied to the vortex structure, additional two vortices, i.e., a circular and an antivortex, are created near the original vortex core. Sequentially, the vortex-antivortex pair annihilates. A spin wave is created at the annihilation point and propagated through the entire element; the relaxed state for the system is the single vortex state with a switched vortex core.
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75.40.Gb, 75.30.Ds, 75.25.+z, 75.50.Bb, 75.50.Tt

Improvement of magnetic properties of granular perpendicular recording media by using a fcc nonmagnetic intermediate layer with stacking faults

Atsushi Hashimoto, Shin Saito, Norikazu Itagaki, and Migaku Takahashi

Appl. Phys. Lett. 89, 262508 (2006) (3 pages)

Online Publication Date: 28 December 2006

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Various binary systems are investigated as face-centered cubic (fcc) bases for the nonmagnetic intermediate layer (NMIL) in granular perpendicular recording media. Loss of stacking order on the (111) plane in fcc NMIL is found to enhance the perpendicular magnetic anisotropy energy (K<sub>u</sub><sup>RL</sup>) of the overlying medium. One persuasive reason for this enhancement is the preferred epitaxial growth of magnetic grains on only the (111) atomic terrace of the fcc NMIL due to collapse of the sixfold symmetry of ([overline 1]11), (1[overline 1]1), and (11[overline 1]) atomic terraces of the fcc NMIL.
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75.50.Ss, 75.70.Ak, 75.30.Gw, 61.72.Nn

Trends in spin-transfer-driven magnetization dynamics of CoFe/AlO/Py and CoFe/MgO/Py magnetic tunnel junctions

G. Finocchio, G. Consolo, M. Carpentieri, A. Romeo, B. Azzerboni, L. Torres, and L. Lopez-Diaz

Appl. Phys. Lett. 89, 262509 (2006) (3 pages)

Online Publication Date: 28 December 2006

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A spin-polarized current is able to excite magnetization dynamics in nanomagnets. A detailed theoretical study of dynamics in low and high field regimes in AlO and MgO magnetic tunnel junctions (MTJs) is presented, considering the maximum value of the applied current which comes from the breakdown voltage of the tunnel barrier. In low field regime, dynamics with a well-defined peak in frequency is observed. In high field regime, AlO MTJ presents the same behavior, while the magnetization in the MgO MTJ shows chaotic motion with a noisy spectrum. Lastly, an effect of the presence of a pinhole in the tunnel barrier is discussed.
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75.70.Cn, 72.25.Mk, 75.50.Tt, 75.60.Ej

Interfacial and quantum well effects on ac magnetotransport of La0.7Sr0.3MnO3/La1.4Sr1.6Mn2O7 composites

C. Y. Hsu, Hsiung Chou, B. Y. Liao, and J. C. A. Huang

Appl. Phys. Lett. 89, 262510 (2006) (3 pages)

Online Publication Date: 29 December 2006

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The field-dependent complex impedance (CI) spectroscopy is employed to clarify the origins of ac magnetoimpedance (MI) effects at room temperature near the conduction threshold of [La0.67Sr0.33MnO3(113)]1−x/[La1.67Sr1.33Mn2O7(327)]x composites. The field-dependent CI spectra in 113-327 granular composites are further transformed to field-dependent magnetoconductance (MC) spectra. The frequency-dependent high-field and low-field MC spectra can be well interpreted by including the trap-state contributions in 113-327 interfaces and 327 grain boundaries, together with the leakage-region-induced spin-dependent quantum well state effect. The principle of this work could be applied to study the magnetoresistance and MI effects of magnetic granular composites.
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75.47.Lx, 73.63.Hs, 73.43.Qt, 75.60.Ej, 61.72.Mm

Observation of magnetism in the nanoscale amorphous ruthenium clusters prepared by ion beam mixing

W. C. Wang, Y. Kong, X. He, and B. X. Liu

Appl. Phys. Lett. 89, 262511 (2006) (3 pages)

Online Publication Date: 29 December 2006

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A unique composite of nanoscale amorphous Ru clusters embedded in the amorphous Ru/Ta matrix was observed in the Ru/Ta multilayered films upon ion irradiation. It was found that the magnetic moment of the Ru nanoclusters varied with cluster size, i.e., when the diameter of the clusters changed from 1  to  5  nm, the magnetic moment decreased from 0.24µB (parallel) and 0.27µB (vertical) to 0µB and 0µB, respectively, according well with the prediction from ab initio calculations.
Show PACS
75.75.+a, 75.50.Tt, 75.50.Kj, 75.70.Cn, 75.30.Cr, 61.46.Bc, 82.80.Ej
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DIELECTRICS AND FERROELECTRICITY

Mechanism of leakage current reduction of tantalum oxide capacitors by postmetallization annealing

W. S. Lau, K. K. Khaw, Taejoon Han, and Nathan P. Sandler

Appl. Phys. Lett. 89, 262901 (2006) (3 pages)

Online Publication Date: 26 December 2006

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In this letter, the authors will point out that defect states related to impurities or structural defects in tantalum oxide capacitors can be passivated by hydrogen during postmetallization anneal (PMA) while oxygen vacancies are enhanced by PMA such that some will observe a decrease while other may observe an increase in the leakage current after PMA. The PMA process can be tuned such that the hydrogen passivation of defect states dominates over the enhancement of oxygen vacancies, resulting in significant leakage current reduction.
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84.32.Tt, 81.65.Rv, 85.40.Ls

Strain distribution in epitaxial SrTiO3 thin films

Z. Y. Zhai, X. S. Wu, Z. S. Jiang, J. H. Hao, J. Gao, Y. F. Cai, and Y. G. Pan

Appl. Phys. Lett. 89, 262902 (2006) (3 pages)

Online Publication Date: 27 December 2006

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The lattice strain distributions of epitaxial SrTiO3 films deposited on LaAlO3 were investigated by in situ x-ray diffraction at the temperature range of 25–300  K, grazing incident x-ray diffraction, and high resolution x-ray diffraction. The nearly linear temperature dependence of the out-of-plane lattice constant of SrTiO3 was observed in the measured temperature range. The depth distribution of the lattice strain at room temperature for SrTiO3 films includes the surface layer, strained layer, and interface layer.
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68.60.Bs, 61.66.Fn, 68.55.Jk

Structural properties of SrO thin films grown by molecular beam epitaxy on LaAlO3 substrates

O. Maksimov, V. D. Heydemann, P. Fisher, M. Skowronski, and P. A. Salvador

Appl. Phys. Lett. 89, 262903 (2006) (3 pages)

Online Publication Date: 27 December 2006

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SrO films were grown on LaAlO3 substrates by molecular beam epitaxy and characterized using reflection high-energy electron diffraction (RHEED) and x-ray diffraction (XRD). The evolution of the RHEED pattern is discussed as a function of film thickness. 500  Å thick SrO films were relaxed and exhibited RHEED patterns indicative of an atomically smooth surface having uniform terrace heights. Films had the epitaxial relationship (001)SrO||(001)LaAlO3; [010]SrO||[110]LaAlO3. This 45° in-plane rotation minimizes mismatch and leads to films of high crystalline quality, as verified by Kikuchi lines in the RHEED patterns and narrow rocking curves of the (002) XRD peak.
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81.05.-t, 68.55.Jk, 81.15.Hi, 68.55.Ac, 68.35.Bs

Oxygen vacancy in monoclinic HfO2: A consistent interpretation of trap assisted conduction, direct electron injection, and optical absorption experiments

Peter Broqvist and Alfredo Pasquarello

Appl. Phys. Lett. 89, 262904 (2006) (3 pages)

Online Publication Date: 27 December 2006

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The authors calculate energy levels associated with the oxygen vacancy in monoclinic HfO2 using a hybrid density functional which accurately reproduces the experimental band gap. The most stable charge states are obtained for varying Fermi level in the HfO2 band gap. To compare with measured defect levels, they determine total energy differences specific to the considered experiment. Their results show that the oxygen vacancy can consistently account for the defect levels observed in (Poole-Frenkel-type) trap assisted conduction, direct electron injection, and optical absorption experiments.
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71.55.Ht, 72.20.Ht, 72.25.Hg, 61.72.Ji

Low symmetry phase in Pb(Zr0.52Ti0.48)O3 epitaxial thin films with enhanced ferroelectric properties

Li Yan, Jiefang Li, Hu Cao, and D. Viehland

Appl. Phys. Lett. 89, 262905 (2006) (3 pages)

Online Publication Date: 28 December 2006

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The authors report the structural and ferroelectric properties of Pb(Zr0.52Ti0.48)O3 (PZT) epitaxial thin films grown on (001), (110), and (111) SrRuO3/SrTiO3 substrates by pulsed laser deposition. A monoclinic C (Mc) phase has been found for (101) films, whereas (001) and (111) ones were tetragonal (T) and rhombohedral (R), respectively. The authors find that the ferroelectric polarization of the Mc phase is higher than that in either the T or R ones. These results are consistent with predictions (i) of epitaxial phase diagrams and (ii) that the enhanced ferroelectric properties of morphotropic phase boundary PZT are related to a low symmetry monoclinic phase.
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77.55.+f, 77.84.Dy, 68.55.Jk, 77.22.Ej, 81.30.Dz

Controlling interfacial reactions between HfO2 and Si using ultrathin Al2O3 diffusion barrier layer

Rajesh Katamreddy, Ronald Inman, Gregory Jursich, Axel Soulet, and Christos Takoudis

Appl. Phys. Lett. 89, 262906 (2006) (3 pages)

Online Publication Date: 29 December 2006

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The authors investigated the effectiveness of atomic layer deposited (ALD) aluminum oxide barrier layer in controlling the interfacial reaction between ALD HfO2 film and Si substrate. The HfO2 was observed to form silicate and silicide at its interface with Si during 5  min postdeposition annealing in Ar at 800 and 1000  °C. A 0.5-nm-thick Al2O3 barrier layer was found to control interfacial reactions between HfO2 and Si during annealing at 800  °C, but not at 1000  °C, whereas a 1.5-nm-thick barrier of Al2O3 was needed to prevent interfacial reaction up to an annealing temperature of 1000  °C.
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82.65.+r, 68.35.Fx, 68.55.Ac, 81.15.Ef, 61.72.Cc

Domain reversal and relaxation in LiNbO3 single crystals studied by piezoresponse force microscope

Yi Kan, Xiaomei Lu, Xiumei Wu, and Jinsong Zhu

Appl. Phys. Lett. 89, 262907 (2006) (3 pages)

Online Publication Date: 29 December 2006

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The LiNbO3 crystal was polarized and characterized by the piezoresponse force mode of scanning probe microscope. By using the Kolmogorov-Avrami-Ishibashi [Izv. Akad. Nauk, USSR: Ser. Math. 3, 355 (1937); J. Chem. Phys. 8, 212 (1940); J. Phys. Soc. Jpn. 63, 1031 (1994); 63, 1601 (1994)] theory to analyze the relaxation process after domain switching, it was found that (1) the percent of final switched domains after poling increased with the enhancing poling voltage, (2) the nucleation time was a constant to a certain sample, (3) the dimension of domain growth decreased with the enhancing poling voltage, and (4) the relaxation time had a maximum at a medial voltage. The corresponding mechanism for domain switching was discussed, which is hopefully useful for domain engineering.
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77.80.Dj, 77.80.Fm, 77.22.Gm

Deaging of heat-treated iron-doped lead zirconate titanate ceramics

T. Granzow, E. Suvaci, H. Kungl, and M. J. Hoffmann

Appl. Phys. Lett. 89, 262908 (2006) (3 pages)

Online Publication Date: 29 December 2006

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Doping strongly influences the properties of ferroelectric perovskite materials. One striking difference between donor- and acceptor-doped materials is the transition from “soft” to “hard” ferroelectric behavior. The physical reasons for this phenomenon are still unclear. The authors present measurements of the ferroelectric hysteresis and deaging behavior of iron-doped lead zirconate titanate after adjusting the defect structure by heat treatment in an oxygen-depleted atmosphere. Contrary to expectations, the introduction of extra oxygen vacancies makes the material “softer.” This effect, which is discussed based on a model of defect dipoles, sheds new light on the unsolved problem of hardening and aging.
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77.84.Dy, 77.80.Dj, 81.05.Je, 81.40.Cd, 81.40.Gh, 61.72.Ji
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NANOSCALE SCIENCE AND DESIGN

Doping of germanium nanowires grown in presence of PH3

E. Tutuc, J. O. Chu, J. A. Ott, and S. Guha

Appl. Phys. Lett. 89, 263101 (2006) (3 pages)

Online Publication Date: 26 December 2006

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The authors study the Au-catalyzed chemical vapor growth of germanium (Ge) nanowires in the presence of phosphine (PH3), used as a dopant precursor. The device characteristics of the ensuing nanowire field effect transistors (FETs) indicate n-type, highly doped nanowires. Using a combination of different nanowire growth sequences and their FET characteristics, the authors determine that phosphorus incorporates predominately via the conformal growth, which accompanies the acicular, nanowire growth. As such, the Ge nanowires grown in the presence of PH3 contain a phosphorus doped shell and an undoped core. The authors determine the doping level in the shell to be ~=(1−4)×1019  cm−3.
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61.72.Tt, 61.72.Ss, 68.65.La, 81.15.Gh, 85.30.Tv