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26 June 2006

Volume 88, Issue 26,  Articles (26xxxx)


Cover image from Stanley S. Hong, Berthold K. P. Horn, Dennis M. Freeman, and Michael S. Mermelstein, Appl. Phys. Lett. 88, 261107 (2006).

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LASERS, OPTICS, AND OPTOELECTRONICS

Terahertz quantum cascade lasers with double-resonant-phonon depopulation

Benjamin S. Williams, Sushil Kumar, Qi Qin, Qing Hu, and John L. Reno

Appl. Phys. Lett. 88, 261101 (2006) (3 pages)

Online Publication Date: 26 June 2006

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We present two different terahertz quantum cascade laser (QCL) designs based on GaAs/Al0.3Ga0.7As heterostructures that feature a depopulation mechanism of two longitudinal-optical phonon scattering events. This scheme is intended to improve high temperature operation by reducing thermal backfilling of the lower radiative state. The better of these two devices displays a threshold current density of 170  A/cm2 at 5  K and lases up to 138  K in pulsed mode and 105  K in continuous-wave mode. However, contrary to expectation, we observed no improvement in temperature performance compared to single-resonant-phonon designs, which suggests that the thermal backfilling is not yet a limiting factor for high temperature terahertz QCL operation.
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42.55.Px, 42.60.By

Light-emitting defects and epitaxy in alkali-ion-implanted alpha quartz

J. Keinonen, S. Gasiorek, P. K. Sahoo, S. Dhar, and K. P. Lieb

Appl. Phys. Lett. 88, 261102 (2006) (3 pages)

Online Publication Date: 27 June 2006

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Light-emitting centers in alkali-ion-implanted alpha quartz have been investigated with respect to the solid phase epitaxial growth of the ion irradiation induced amorphous zone. Cathodoluminescence was studied under the conditions of chemical epitaxy in annealing the samples, implanted with 2.5×1016 50  keV Na ions/cm2 or 175  keV Rb ions/cm2, in 18O2 atmosphere in the temperature range of 673–1173  K. In addition to the known intrinsic subbands at 2.40, 2.79, and 4.30  eV, which previously were associated with specific defects in the silica matrix, a strong violet band at 3.65  eV and a band at 3.25  eV have been identified. Both are intimately correlated with the presence of the implanted alkali atoms and recrystallization process. With respect to the 3.25  eV band reported in the literature, they are discussed to be correlated with the presence of nanoclusters in Si-enriched, and Ge- and Sn-implanted SiO2 structures.
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78.60.Hk, 61.80.Jh, 61.72.Ww, 81.40.Gh, 61.46.Bc

Terahertz emission profile from laser-induced air plasma

Hua Zhong, Nick Karpowicz, and X.-C. Zhang

Appl. Phys. Lett. 88, 261103 (2006) (3 pages)

Online Publication Date: 27 June 2006

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We report the characterization of the emission profile of terahertz waves generated by four-wave mixing in the presence of laser-induced plasma. Highly directional terahertz waves with a divergence angle smaller than 10° are measured in the presence of long plasmas (>10  mm). Frequency-dependent interference structures in the angular distribution of the radiation are observed under tighter focusing conditions and are explained by intense self-phase modulation of the optical pulse in the plasma. This study reveals that terahertz generation from four-wave mixing in air plasmas is a promising source for spectroscopy and imaging in the terahertz range.
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52.25.Os, 52.50.Jm, 51.70.+f, 42.65.Hw, 42.65.Jx

Enhanced second harmonic generation from nanoscale double-hole arrays in a gold film

Antoine Lesuffleur, L. Kiran Swaroop Kumar, and Reuven Gordon

Appl. Phys. Lett. 88, 261104 (2006) (3 pages)

Online Publication Date: 27 June 2006

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We present enhanced second harmonic generation (SHG) from arrays with a basis of overlapping double holes. The arrays were created by focused-ion beam milling through a gold film, and the measurements were performed in the transmission geometry. By fixing the array periodicity and varying the spacing between the holes, the SHG was enhanced by an order of magnitude for the single case where the apexes of the double-hole structure were nearly touching. Numerical calculations showed a local electric field enhancement that agrees with the SHG observations. This work shows the potential of double-hole structures for nonlinear optics at the nanoscale.
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42.65.Ky, 78.66.Bz

Spin dynamics in a diluted magnetic semiconductor quantum well studied by pump-probe absorption spectroscopy: Magnetic-field-induced suppression of electron-spin relaxation

A. Murayama, K. Seo, K. Nishibayashi, I. Souma, and Y. Oka

Appl. Phys. Lett. 88, 261105 (2006) (3 pages)

Online Publication Date: 27 June 2006

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Exciton spin dynamics is studied in a diluted magnetic semiconductor quantum well of Cd0.95Mn0.05Te by pump-probe absorption spectroscopy under magnetic fields. The time dependences of the saturated absorbance for the higher- and lower-energy spin states of heavy-hole (hh) excitons clarify the following exciton-spin relaxation process in magnetic fields: ultrafast hh-spin relaxation with the formation of dark excitons and subsequent electron-spin relaxation. The electron-spin relaxation due to the s-d exchange mechanism involving Mn spins is suppressed in a high magnetic field by field-induced pinning of the Mn spins.
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75.50.Pp, 75.40.Gb, 73.21.Fg, 71.35.-y, 75.30.Et

Reliability of strain-balanced Ga0.331In0.669As/Al0.659In0.341As/InP quantum-cascade lasers under continuous-wave room-temperature operation

A. Evans and M. Razeghi

Appl. Phys. Lett. 88, 261106 (2006) (3 pages)

Online Publication Date: 27 June 2006

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The reliability of high power strain-balanced Ga0.331In0.669As/Al0.659In0.341As/InP intersubband quantum cascade lasers (QCLs) emitting at a wavelength of 4.6  µm has been studied. Constant current aging is reported for two randomly selected high-reflectivity-coated QCLs with an output power over 100  mW. QCLs are tested under continuous-wave operation at a heat sink temperature of 298  K(25  °C) corresponding to an internal temperature of 378  K (105  °C). Over 4000  h of continuous testing is reported without any decrease in output power.
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42.55.Px, 42.60.By

Lensless focusing with subwavelength resolution by direct synthesis of the angular spectrum

Stanley S. Hong, Berthold K. P. Horn, Dennis M. Freeman, and Michael S. Mermelstein

Appl. Phys. Lett. 88, 261107 (2006) (3 pages)

Online Publication Date: 29 June 2006

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We consider the coherent superposition of unfocused wave fronts for lensless focusing of electromagnetic waves with subwavelength resolution. Near the focal point, intensity distributions generated using the approach approximate those generated using lenses. Far from the focal point, discretization of spatial frequencies results in a trade-off between the number of wave fronts and the accuracy of the approximation. We experimentally demonstrate the feasibility of the approach by generating an approximation of an azimuthally polarized Bessel beam with a focal spot diameter (full width at half maximum intensity) of 0.37lambda.
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42.60.Jf, 42.25.Kb, 42.25.Ja

Observation of optical gain in solutions of CdS quantum dots at room temperature in the blue region

Qusai Darugar, Wei Qian, and Mostafa A. El-Sayed

Appl. Phys. Lett. 88, 261108 (2006) (3 pages)

Online Publication Date: 29 June 2006

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The optical gain dynamics has been studied for two CdS quantum dot samples dispersed in toluene at room temperature. This was carried out by using femtosecond transient absorption technique with an excitation at 400  nm and gain measurement was studied at the fluorescence maxima (440 and 460  nm). The optical gain lifetime was found to be as long as 20  ps under pump fluence as low as 0.77  mJ/cm2. The low threshold is the result of long lifetime of electrons and holes and narrow emission bandwidth. These results suggest that CdS quantum dots in solution are excellent gain media for optically pumped high power blue lasers.
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78.67.Hc, 78.66.Hf, 78.55.Et, 78.67.Bf, 78.47.+p

Plasma dynamics of water breakdown at a water surface induced by femtosecond laser pulses

C. Sarpe-Tudoran, A. Assion, M. Wollenhaupt, M. Winter, and T. Baumert

Appl. Phys. Lett. 88, 261109 (2006) (3 pages)

Online Publication Date: 29 June 2006

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Femtosecond laser pulse induced ultrafast plasma dynamics studies of water breakdown in the range up to 250  ps are reported. We combine transient imaging techniques together with spectrally resolved reflection spectroscopy to monitor the early breakdown dynamics at the water surface with a laser intensity being 1.5 above threshold. We observe a 20  ps delay before the plasma expands with an initial velocity of 5900  m/s. The transient electron density after formation of the plasma is 1.2×1021/cm3. A recombination on a picosecond time scale with a rate of (1.6×10–9±0.3×10–9)  cm3/s is found.
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52.80.Wq, 52.50.Jm, 52.70.Kz, 52.30.-q, 52.25.Os

Control of subband energy levels of quantum dots using InGaAs gradient composition strain-reducing layer

T. Amano, S. Yamauchi, T. Sugaya, and K. Komori

Appl. Phys. Lett. 88, 261110 (2006) (3 pages)

Online Publication Date: 30 June 2006

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We propose a method to control the subband energy levels of quantum dots (QDs) using an InGaAs gradient composition strain-reducing layer (GC-SRL). A large band shift of 70  meV was realized using a GC-SRL at the fourth-order energy level in both the samples. In addition, the QDs with and without a GC-SRL exhibited an exponential and constant increase in the subband space, respectively. These results indicate square-well-shaped and crucible-shaped potential band structures. The GC-SRL enabled the control of not only the subband energy but also the confinement energy of these potential structures.
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73.21.La

Color-center waveguides in low-energy electron-bombarded lithium fluoride

Alessio Rocchetti, Gaetano Assanto, Rosa Maria Montereali, Enrico Nichelatti, and Fabrizia Somma

Appl. Phys. Lett. 88, 261111 (2006) (3 pages)

Online Publication Date: 30 June 2006

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We employ a differential version of m-line spectroscopy through grating coupling in order to measure the refractive index of low-energy electron irradiated lithium fluoride crystal channel waveguides for broadband emission. Using photoresist films and a holographic setup for grating fabrication, we perform an accurate characterization of bidimensional structures and a direct quantitative comparison between treated and blank materials.
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42.79.Gn, 42.70.-a, 42.79.Dj, 61.72.Ji, 78.20.Ci

Frequency mixing of off-axis focused Gaussian beams: An approach to measure the phase velocity distribution

Daquan Lu, Liejia Qian, Yongzhong Li, and Dianyuan Fan

Appl. Phys. Lett. 88, 261112 (2006) (3 pages)

Online Publication Date: 30 June 2006

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In this letter we theoretically study the frequency mixing of two off-axis focused Gaussian beams. The optimal phase mismatch corresponding to maximized conversion efficiency depends on the separation of off-axis beams and can be expressed in terms of the phase velocities analytically, which provides an effective approach to measure the phase velocity distribution of a Gaussian beam.
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42.60.Jf, 42.65.Hw

Fiber-coupled single-photon detectors based on NbN superconducting nanostructures for practical quantum cryptography and photon-correlation studies

W. Slysz, M. Wegrzecki, J. Bar, P. Grabiec, M. Górska, V. Zwiller, C. Latta, P. Bohi, I. Milostnaya, O. Minaeva, A. Antipov, O. Okunev, A. Korneev, K. Smirnov, B. Voronov et al.

Appl. Phys. Lett. 88, 261113 (2006) (3 pages)

Online Publication Date: 30 June 2006

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We have fabricated and tested a two-channel single-photon detector system based on two fiber-coupled superconducting single-photon detectors (SSPDs). Our best device reached the system quantum efficiency of 0.3% in the 1540-nm telecommunication wavelength with a fiber-to-detector coupling factor of about 30%. The photoresponse consisted of 2.5-ns-wide voltage pulses with a rise time of 250  ps and timing jitter below 40  ps. The overall system response time, measured as a second-order, photon cross-correlation function, was below 400  ps. Our SSPDs operate at 4.2  K inside a liquid-helium Dewar, but their optical fiber inputs and electrical outputs are at room temperature. Our two-channel detector system should find applications in practical quantum cryptography and in antibunching-type quantum correlation measurements.
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85.25.Pb, 42.81.Pa, 03.67.Dd
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PLASMAS AND ELECTRICAL DISCHARGES

Effect of wire number on x-pinch discharges

J. S. Green, S. N. Bland, M. Collett, A. E. Dangor, K. Krushelnick, F. N. Beg, and I. Ross

Appl. Phys. Lett. 88, 261501 (2006) (3 pages)

Online Publication Date: 26 June 2006

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An investigation has been carried out into the x-ray emission and plasma dynamics of a "tabletop" x-pinch plasma generated by passing a 40  ns, 35  kA current pulse through two or four 5  µm thick tungsten wires. It was found that a four-wire x pinch consistently resulted in an order of magnitude higher energy hard x-ray yield (>3.5  keV) than a two-wire pinch. The total x-ray emission from a four-wire pinch also had a shorter pulse length such that the total x-ray power was increased by more than a factor of 2. Observations of x-pinch discharges driven by a higher energy 160  kA, 80  ns generator are also presented. In this case x pinches with two, four, and six wires were made using thicker aluminum and molybdenum wires and the number of wires is shown to play a significant role in the plasma dynamics. Increasing the number of wires enhanced the reproducibility of the discharge and the x-ray pulse.
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52.58.Lq, 52.59.Qy, 52.25.Os, 52.59.Px, 52.50.Dg, 52.70.Kz

Effect of collisions in the stratified presheath in electronegative plasmas

J. I. Fernández Palop, J. Ballesteros, R. Morales Crespo, and M. A. Hernández

Appl. Phys. Lett. 88, 261502 (2006) (3 pages)

Online Publication Date: 28 June 2006

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The effect of collisions in the formation of a stratified presheath in electronegative plasmas is analyzed. Collisions do not affect to the sheath-presheath structure in pure electropositive plasmas; however, in the case of electronegative plasmas, they can produce a large effect, changing the electric potential distribution in the presheath from monotonic to oscillatory, and viceversa. The influence of the collision rate on the plasma parameter space region in which the presheath becomes stratified is analyzed.
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52.40.Kh, 52.20.Hv, 52.20.Fs, 52.35.Fp
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STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER

Injection dependence of spontaneous radiative recombination in crystalline silicon: Experimental verification and theoretical analysis

P. P. Altermatt, F. Geelhaar, T. Trupke, X. Dai, A. Neisser, and E. Daub

Appl. Phys. Lett. 88, 261901 (2006) (3 pages)

Online Publication Date: 26 June 2006

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The radiative recombination coefficient B in crystalline bulk silicon is enhanced by the Coulomb attraction between electrons and holes. This effect is weakened at high carrier densities due to screening. We measure the resulting dependence of B on the free-carrier density (i) by reinterpreting published data and (ii) with photoluminescence and photovoltaic measurements. We calculate the Coulomb enhancement by determining the electron-hole pair correlation function at zero interparticle distance, assuming a Debye interaction potential. Both bound and scattering state contributions are fully taken into account. Due to screening, B decreases with increasing free-carrier density.
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72.20.Jv, 78.55.Ap, 72.40.+w

Evidence for adiabatic heating during fracture of W-reinforced metallic glass composites

Min Ha Lee and Daniel J. Sordelet

Appl. Phys. Lett. 88, 261902 (2006) (3 pages)

Online Publication Date: 27 June 2006

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At appropriate strain rates below their glass transition temperature, amorphous alloys deform inhomogeneously by strain localization within narrow shear planes. In the current study, we present experimental evidence of viscous flow by adiabatic heating at the fracture surface of W-reinforced Hf-based metallic glass composites under quasistatic uniaxial compressive deformation. Also, based on microstructural analysis, estimation of the temperature rise in a shear band shows that when fracture occurs, it releases a high level of strain energy, which when converted to heat, reaches temperatures sufficiently high enough to induce melting or softening of the W in the composite, producing vein patterns.
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81.40.Lm, 81.40.Gh, 81.40.Np, 62.20.Mk, 62.20.Fe, 64.70.Pf

In situ neutron diffraction measurement of transient temperature and stress fields in a thin plate

Wanchuck Woo, Zhili Feng, Xun-Li Wang, Ke An, Camden R. Hubbard, Stan A. David, and Hahn Choo

Appl. Phys. Lett. 88, 261903 (2006) (3 pages)

Online Publication Date: 27 June 2006

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Separating the elastic and thermal strains has been a long standing problem for in situ neutron diffraction measurement of transient temperature and stress fields. Using the plane stress condition, we demonstrate a method to decompose the thermal and elastic strains from the measured lattice spacing changes. The method was validated using a thin plate subjected to a local moving heat source. The methodology developed herein has practical applications for a variety of materials processing technologies such as welding, forming, and heat treatment.
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81.40.Lm, 62.20.Fe

Electronic and thermodynamic properties of beta-Ga2O3

Haiying He, Miguel A. Blanco, and Ravindra Pandey

Appl. Phys. Lett. 88, 261904 (2006) (3 pages)

Online Publication Date: 27 June 2006

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Electronic and thermodynamic properties of beta-Ga2O3 are investigated in the framework of density functional theory. The equilibrium structural parameters and Debye temperature are obtained through fitting of the energy surface to the equation of state. Analysis of the band structure shows the valence band maximum to be degenerate at Gamma and M, whereas the conduction band minimum is predicted to be at Gamma. The valence band is almost flat, indicating a rather large effective mass for holes, whereas the calculated electron effective mass comes out to be about 0.12, expressed in units of the free electron mass.
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71.20.Nr, 64.30.+t, 71.15.Mb, 63.70.+h, 71.18.+y

Role of alloy fluctuations in photoluminescence dynamics of AlGaN epilayers

E. Kuokstis, W. H. Sun, M. Shatalov, J. W. Yang, and M. Asif Khan

Appl. Phys. Lett. 88, 261905 (2006) (3 pages)

Online Publication Date: 27 June 2006

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The near-band-edge (NBE) photoluminescence (PL) of AlGaN layers with different Al content was analyzed in a wide range of excitation intensities and temperatures. The PL peculiarities indicated that tails of density of states are formed in AlGaN alloys due to the fluctuation of the alloy composition. The model involving recombination through one type of nonradiative center is proposed. The dependence of NBE PL integrated intensity on excitation power for AlGaN is weaker than that for GaN, which is attributed to carrier localization in alloys with compositional fluctuations and, thus, reduction of probability of nonradiative recombination.
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78.66.Fd, 78.55.Cr, 82.80.-d, 71.20.Nr, 72.20.Jv, 73.50.Gr

Relationship between binding site and pressure dependence for defect-hydrogen complexes in ZnO

M. G. Wardle, J. P. Goss, and P. R. Briddon

Appl. Phys. Lett. 88, 261906 (2006) (3 pages)

Online Publication Date: 27 June 2006

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The effect of hydrostatic pressure on local vibrational modes of hydrogen defects in ZnO has been studied by first-principles methods. We find that the sign and magnitude of the frequency shift rate are strongly dependent on the local environment. In the case of isolated hydrogen, the bond centered (BC) and antibonding (AB) configurations lead to positive and negative pressure shifts, in agreement with previous work. However, this result cannot be extended to defect-hydrogen complexes. In general, the sign of the shift does not indicate whether the hydrogen atom is located at a BC or AB site.
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63.20.Pw, 61.72.-y

Long dephasing time in self-assembled InAs quantum dots at over 1.3  µm wavelength

J. Ishi-Hayase, K. Akahane, N. Yamamoto, M. Sasaki, M. Kujiraoka, and K. Ema

Appl. Phys. Lett. 88, 261907 (2006) (3 pages)

Online Publication Date: 28 June 2006

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Excitonic dephasing is investigated in InAs self-assembled quantum dots fabricated by the strain-compensation technique. The exciton ground-state emission is centered at the wavelength of 1420  nm at 5  K. Transient four-wave mixing measurements under resonant excitation clearly demonstrate a long dephasing time of 1.09  ns at 5  K, corresponding to the homogeneous broadening of 1.2  µeV. The extrapolated zero-temperature homogeneous broadening is limited only by the population lifetime of the exciton ground state. At slightly increased temperatures, the acoustic-phonon broadening becomes dominant on dephasing.
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78.67.Hc, 73.21.La

Modeling heterostructures of nanophononic crystals by continuum model with microstructures

G. L. Huang and C. T. Sun

Appl. Phys. Lett. 88, 261908 (2006) (3 pages)

Online Publication Date: 28 June 2006

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The application of nanophononic heterogeneous materials requires efficient and accurate modeling and analysis tools. For nanostructured materials, the use of atomistic simulation may be a potential solution. However, it is well known that this approach is beset with prohibitive computing time and an astronomical amount of data generated. In this letter, a microstructure continuum theory is employed to describe wave propagation in these materials. The key step in this approach is the establishment of a relationship between the local kinematics and the global continuum variables so that the characteristics of the nanostructure are captured. Propagation of longitudinal harmonic waves in nanophononic media is considered and the resulting dispersion curves are used to evaluate the accuracy of the microstructure continuum model.
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63.22.+m, 61.46.-w, 61.72.-y

Size dependence of photoluminescence and resonant Raman scattering from ZnO quantum dots

Hsin-Ming Cheng, Kuo-Feng Lin, Hsu-Cheng Hsu, and Wen-Feng Hsieh

Appl. Phys. Lett. 88, 261909 (2006) (3 pages)

Online Publication Date: 28 June 2006

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ZnO quantum dots (QDs) of controlled sizes have been fabricated by a simple sol-gel method. The blueshift of room-temperature photoluminescence measurement from free exciton transition are observed decreasing with the QD size that is ascribed to the quantum confinement effect. From the resonant Raman scattering, the coupling strength between electron and longitudinal optical phonon, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminishes with reducing the ZnO QD diameter. The size dependence of electron-phonon coupling is principally a result of the Fröhlich interaction.
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81.07.Ta, 81.05.Dz, 78.55.Et, 78.30.Fs, 73.21.La, 78.67.Hc

Epitaxial growth of GaN on copper substrates

S. Inoue, K. Okamoto, N. Matsuki, Tae-Won Kim, and H. Fujioka

Appl. Phys. Lett. 88, 261910 (2006) (3 pages)

Online Publication Date: 28 June 2006

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We have grown GaN films on Cu(111) substrates using pulsed laser deposition. We have found that GaN(0001) grows epitaxially on Cu(111) when employing low temperature grown AlN buffer layers with an in-plane epitaxial relationship of AlN[11[overline 2]0]||Cu[1[overline 1]0] Reflection high-energy electron diffraction images have exhibited sharp streaky patterns, indicating that GaN grows with a flat surface. Electron backscattering diffraction observations have revealed that neither 30° rotational domains nor cubic phase domains exist in the GaN films. Spectroscopic ellipsometry measurements have shown that the heterointerfaces in the GaN/AlN/Cu structure are abrupt. The epitaxial growth of GaN on Cu substrates is likely to raise the power limit for future light emitting and electron devices due to the high thermal conductivity of Cu.
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81.05.Ea, 81.15.Fg, 68.55.Ac, 72.15.Eb, 79.20.Kz

Study of pore structure and stability in porous low-k interconnects using electrolyte voltammetry

D. M. Meng, N. L. Michael, Choong-Un Kim, and Young-Joon Park

Appl. Phys. Lett. 88, 261911 (2006) (3 pages)

Online Publication Date: 28 June 2006

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This letter presents a step-mode voltammetry method which uses ion diffusivity to characterize pore structure in both dense and porous low dielectric constant materials (low k) in patterned interconnect structures. Findings reveal that the intramolecular space in dense low k acts like a small physical pore network. It is determined that electrolyte ions can migrate through such space in dense low k, but with higher activation energy than in porous low k or the bulk solution, 0.31  eV vs 0.18–0.19  eV. Also, this study finds that the pores in ultralow k are not stable but can either coalesce or collapse depending on stress conditions.
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61.43.Gt, 85.40.Ls, 77.55.+f, 82.45.Rr, 66.30.Hs, 77.22.Ch

Facets evolution and surface electrical properties of nonpolar m-plane ZnO thin films

J. Zúñiga-Pérez, V. Muñoz-Sanjosé, E. Palacios-Lidón, and J. Colchero

Appl. Phys. Lett. 88, 261912 (2006) (3 pages)

Online Publication Date: 28 June 2006

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ZnO thin films have been grown along the nonpolar [10[overline 1]0] direction by metal organic vapor phase epitaxy. The ZnO (10[overline 1]0) surface develops well defined facets. The orientation of the topographic normals reveals that the inclination angle of the facets increases as thicker films are considered, attaining a maximum value of about 28.4°. This angle corresponds to {10[overline 1]1}- and {10[overline 1][overline 1]}-type facets. The origin of this faceting is discussed in terms of thermodynamic stability and kinetics arguments. The surface electrical properties of the facets have been studied by Kelvin probe microscopy, showing that the surface has different contact potential domains that alternate along the polar [0001] direction.
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81.05.Dz, 73.25.+i, 68.47.Fg, 73.61.Ga, 68.55.Ac, 68.55.Jk

Thermal stability and its prediction of bulk metallic glass systems

Mingxu Xia, Shuguang Zhang, Jianguo Li, and Chaoli Ma

Appl. Phys. Lett. 88, 261913 (2006) (3 pages)

Online Publication Date: 28 June 2006

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We report a linear relationship between the thermal stability (DeltaTx) and the amorphous formation enthalpy (DeltaHam) of typical bulk metallic glasses (BMGs). The value of the slope of DeltaTx vs DeltaHam reflects the sensitivity of thermal stability to composition and the sign of the slope implies whether the local atomic structure of amorphous phase is similar to that of primary crystalline phase. A positive slope suggests a similarity between them and vice versa. We also demonstrate that the thermal stability can be fastly assessed and predicted by DeltaHam at less cost using Ti53Cu15Ni18.5Al7M3Si3B0.5 (M=Hf,Sc,Ta,Nb) BMGs.
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61.43.Fs, 82.60.Cx

Thermal conductivity enhancement in water-in-FC72 nanoemulsion fluids

B. Yang and Z. H. Han

Appl. Phys. Lett. 88, 261914 (2006) (3 pages)

Online Publication Date: 28 June 2006

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The strategy of adding solid particles to fluids for improving thermal conductivity has been pursued for nearly a century. In this work, a concept of using liquid nanodroplets for enhancing thermal conductivity has been developed and was demonstrated in water-in-FC72 suspensions, called "nanoemulsion fluids." The thermal conductivity of FC72 is found to be increased by up to 52% for a nanoemulsion fluid containing 12  vol  % water nanodroplets of radius 9.8  nm. Such types of nanoemulsion fluids possess long-term stability and can be mass produced because of no needs for solid nanoparticles. The development of nanoemulsion fluids would open a direction for thermal fluids studies.
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66.60.+a, 82.70.Kj

Real time multicomponent echo particle image velocimetry technique for opaque flow imaging

Hairong Zheng, Lingli Liu, Logan Williams, Jean R. Hertzberg, Craig Lanning, and Robin Shandas

Appl. Phys. Lett. 88, 261915 (2006) (3 pages)

Online Publication Date: 29 June 2006

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This letter reports on a contrast-based ultrasonic particle imaging technique (echo PIV) for measuring multicomponent velocity vectors in opaque flows with excellent temporal (up to 0.5  ms) and spatial (up to 0.4  mm) resolution. Ultrasound contrast microbubbles are used as flow tracers, and digitally acquired rf data are converted into B-mode images for PIV analysis. Here, velocity fields from various flow patterns (including rotating and transient vortex flows) that are difficult to measure using other opaque flow methods such as ultrasound Doppler or magnetic resonance imaging are measured using echo PIV. This nonintrusive technique should be a promising addition to opaque flow diagnostics.
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47.80.Jk, 43.58.-e, 43.60.Lq, 47.32.Ef

Stress control structures for microelectromechanical systems using structural mechanics approach

T. Krupenkin, V. A. Lifton, A. Tran, J. Vuillemin, and D. W. Carr

Appl. Phys. Lett. 88, 261916 (2006) (3 pages)

Online Publication Date: 30 June 2006

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A method of stress control in microelectromechanical systems (MEMS) devices is presented that consists of creating counterbalancing structures to position stressed layers at the neutral plane of the device, eliminating the bending momentum acting on the device. Upon metallization, many MEMS elements such as silicon membranes show substantial bow under the stress developed as a result of the difference in the thermal expansion coefficients of a metal and silicon. The proposed membranes with the counterbalancing structures remain flat in the entire test temperature range (25–150  °C). The method gives material-independent solution to stress-induced curvature problems in a variety of ultrathin devices.
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07.10.Cm, 85.85.+j, 81.40.Lm, 62.20.Fe, 65.40.De, 85.40.Ls

Optical properties of inorganic AgSb recording thin film

Y. H. Fang, P. C. Kuo, P. W. Chen, Wei-Chih Hsu, C. Y. Chou, and T. H. Wu

Appl. Phys. Lett. 88, 261917 (2006) (3 pages)

Online Publication Date: 30 June 2006

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50  nm Ag1–xSbx (x=10.8–25.5) thin films were prepared by magnetron sputtering. Thermal analysis shows that the phase change occurs around 250  °C. The optical property analysis show that the as-deposited Ag80.9Sb19.1 films have high reflectivity of about 62%–73%. After heat treatment at 300  °C, the contrast of Ag80.9Sb19.1 film is 12.5%–17% for wavelengths between 400 and 800  nm. Dynamic test shows that using the Ag80.9Sb19.1 film as the memory layer of write once optical disk, a carrier-to-noise ratio of about 45  dB can be achieved at lambda=657  nm, numerical aperture of 0.65, and a linear velocity of 3.5  m/s.
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42.70.Nq, 81.40.Gh, 42.79.Vb

Characteristic fragment size distributions in dynamic fragmentation

Fenghua Zhou, Jean-François Molinari, and K. T. Ramesh

Appl. Phys. Lett. 88, 261918 (2006) (3 pages)

Online Publication Date: 30 June 2006

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The one-dimensional fragmentation of a dynamically expanding ring (Mott's problem) is studied numerically to obtain the fragment signatures under different strain rates. An empirical formula is proposed to calculate an average fragment size. Rayleigh distribution is found to describe the statistical properties of the fragment populations.
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81.40.Np, 62.20.Mk

Higher lying conduction band in GaN and AlN probed by photoluminescence spectroscopy

N. Nepal, K. B. Nam, J. Li, M. L. Nakarmi, J. Y. Lin, and H. X. Jiang

Appl. Phys. Lett. 88, 261919 (2006) (3 pages)

Online Publication Date: 30 June 2006

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Photoluminescence spectroscopy has been employed to study the band edge emissions in GaN and AlN epilayers up to 800  K. Two distinctive activation processes have been observed in both GaN and AlN. The first process occurring below Tt=325  K (Tt=500  K) for GaN (AlN) is due to the activation of free excitons to free carriers, whereas the second occurring above Tt with an activation energy of 0.29  eV (0.3  eV) for GaN (AlN) is believed to be associated with a higher lying conduction band (Gamma3) at about 0.3  eV above the conduction band minimum (Gamma1). An emission line at about 0.29  eV above the dominant transition in GaN was also observed at 700  K, corroborating the assignment of Gamma3. The values of Tt are a direct measure of the onset temperature at which free excitons dissociate into free carriers.
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71.20.Nr, 78.55.Cr, 71.35.-y

Tailored emission color synthesis using microfacet quantum wells consisting of nitride semiconductors without phosphors

M. Funato, T. Kotani, T. Kondou, Y. Kawakami, Y. Narukawa, and T. Mukai

Appl. Phys. Lett. 88, 261920 (2006) (3 pages)

Online Publication Date: 30 June 2006

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A color synthesis based on InGaN/GaN quantum wells (QWs) grown on GaN microfacets formed by regrowth on SiO2 mask stripes is demonstrated. The microfacet structure is composed of (0001), {11[overline 2]2}, and {11[overline 2]0} planes, and the InGaN well thickness and composition are spatially inhomogeneous due to the diffusion of the adatoms among the facets. These properties allow microfacet QWs, which, for example, emit yellow from the (0001) facet and blue from the {11[overline 2]2} facet, to be fabricated, of which the luminescence appears white due to the additive color mixing. Using a mask pattern that consists of regions with and without stripes, the emissions from the microfacet QWs and from planar QWs are synthesized to produce the desired apparent output colors.
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81.07.St, 81.05.Ea, 78.67.De, 78.55.Cr, 68.65.Fg, 68.43.Jk
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ELECTRONIC TRANSPORT AND SEMICONDUCTORS

Effect of point defect and Mn concentration in time-resolved differential reflection in GaMnAs

Shin Kim, Eunsoon Oh, J. U. Lee, D. S. Kim, S. Lee, and J. K. Furdyna

Appl. Phys. Lett. 88, 262101 (2006) (3 pages)

Online Publication Date: 26 June 2006

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We measured the transmission spectra and the time-resolved differential reflectivity DeltaR in Ga1–xMnxAs for x<=0.05 for several excitation wavelengths. The sign of DeltaR in Ga1–xMnxAs (x=0.015 and x=0.03) was negative for photon energy larger than band gap at room temperature. The negative component of DeltaR was explained by defect induced absorption and/or the reduction of exciton bleaching, rather than by the change in density of near band edge states associated with Mn incorporation. For Ga0.95Mn0.05As, absorption edge broadening was observed in the transmission spectra and the induced absorption effect was reduced by the screening of Mn local potential by photocarriers.
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75.50.Pp, 78.47.+p, 78.66.Fd, 71.20.Nr, 75.50.Dd, 61.72.Ji

Effect of oxygen plasma treatment on reduction of contact resistivity at pentacene/Au interface

Woong-Kwon Kim and Jong-Lam Lee

Appl. Phys. Lett. 88, 262102 (2006) (3 pages)

Online Publication Date: 27 June 2006

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We report the reduction of contact resistivity between Au and pentacene by O2 plasma treatment. Contact resistance dramatically reduced from 5.65  to  0.22  MOmega  cm by the treatment. O2 plasma treatment transformed Au to AuOx, increasing the surface energy from 45.1  to  71.5  mJ/m2. Molecular adsorption geometry of pentacene on AuOx changed from a planar structure to an upright type, improving crystallinity and molecular packing. Thus, defects and traps at the interface were reduced, decreasing the contact resistance between Au and pentacene.
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73.40.Cg, 52.77.-j, 68.35.Md

p-type behavior in nominally undoped ZnO thin films by oxygen plasma growth

Y. J. Zeng, Z. Z. Ye, W. Z. Xu, J. G. Lu, H. P. He, L. P. Zhu, B. H. Zhao, Y. Che, and S. B. Zhang

Appl. Phys. Lett. 88, 262103 (2006) (3 pages)

Online Publication Date: 28 June 2006

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We report on intrinsic p-type ZnO thin films by plasma-assisted metal-organic chemical vapor deposition. The optimal results give a resistivity of 12.7  Omega  cm, a Hall mobility of 2.6  cm2/V  s, and a hole concentration of 1.88×1017  cm–3. The oxygen concentration is increased in the intrinsic p-type ZnO, compared with the n-type layer. Two acceptor states, with the energy levels located at 160 and 270  meV above the valence band maximum, are identified by temperature-dependent photoluminescence. The origin of intrinsic p-type behavior has been ascribed to the formation of zinc vacancy and some complex acceptor center.
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81.05.Dz, 73.61.Ga, 52.77.Dq, 81.15.Gh, 81.15.Kk, 78.66.Hf

Alternative precursor metal-organic chemical vapor deposition of InGaAs/GaAs quantum dot laser diodes with ultralow threshold at 1.25  µm

A. Strittmatter, T. D. Germann, Th. Kettler, K. Posilovic, U. W. Pohl, and D. Bimberg

Appl. Phys. Lett. 88, 262104 (2006) (3 pages)

Online Publication Date: 28 June 2006

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Laser diodes based on InGaAs quantum dots (QDs) operating at 1250  nm with ultralow threshold current densities of 66  A/cm2, transparency current densities of 10  A/cm2 per quantum dot layer, and high internal quantum efficiencies of 94% have been realized using alternative precursor metal-organic chemical vapor deposition. Photoluminescence of the active QD stacks clearly indicates the requirement of varying growth parameters for subsequently deposited QD layers. The excellent performance of the QD lasers was obtained by adjusting the number of stacked QD layers to a limit given by the In content of the InGaAs strain-reducing layers grown on the QDs and individual durations of the growth interruption after deposition for each QD layer.
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42.55.Px, 42.60.By, 81.15.Gh, 78.55.Cr

Effects of As and Mn doping on microstructure and electrical conduction in ZnO films

K. Lord, T. M. Williams, D. Hunter, K. Zhang, J. Dadson, and A. K. Pradhan

Appl. Phys. Lett. 88, 262105 (2006) (3 pages)

Online Publication Date: 29 June 2006

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We report the synthesis of epitaxial As-doped ZnO and Mn-doped (ZnAs)O films by pulsed-laser deposition technique. The grain size in (ZnAs)O films decreases from 40 to less than 10  nm upon Mn doping, illustrating that Mn acts as a potential catalyst to create nanosize grains. Temperature dependent electrical resistance shows metal-insulator transition and metal-semiconductor transition (MST) at 165 and 115  K, respectively, in (ZnAs)O, although Mn doping suppresses MST completely. Both ionization efficiency on oxygen vacancies and percolation of charge carriers may be responsible for such transitions. In addition, electrical conduction in these films shows strong aging effects.
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73.61.Ga, 61.72.Vv, 68.55.Ln, 72.60.+g, 61.72.Ji, 81.40.Cd

Thermoelectric properties of doped titanium disulfides

Edward E. Abbott, Joseph W. Kolis, Nathan D. Lowhorn, William Sams, Apparao Rao, and Terry M. Tritt

Appl. Phys. Lett. 88, 262106 (2006) (3 pages)

Online Publication Date: 30 June 2006

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We report herein the doping of titanium disulfide (TiS2) with the pnictides (Pn): P, As, and Sb. The incorporation of these pnictides into titanium disulfide (TiS2–xPnx) is performed at extremely low concentrations (x~0.2%). The effects on the electronic transport of titanium disulfide by doping with arsenic is quite profound, reducing the resistivity and thermopower to 0.2  mOmega  cm and –35  µV/K at 300  K, respectively, from 1.8  mOmega  cm and –170  µV/K at 300  K for the parent compound TiS2. For a wide range of thermopower values we find that the thermopower (alpha) of these doped titanium disulfides is linearly related to the infrared reflectivity minimum and can be correlated by the experimentally determined proportionality of lambda=–0.0457alpha, where lambda is the wavelength of the minimum.
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72.20.Pa, 61.72.Ss, 78.30.Hv
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MAGNETISM AND SUPERCONDUCTIVITY

Effects of magnetic substrates on ac losses of YBa2Cu3O7 films in perpendicular ac magnetic fields

Masaki Suenaga and Qiang Li

Appl. Phys. Lett. 88, 262501 (2006) (3 pages)

Online Publication Date: 27 June 2006

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Effects of a magnetic substrate on the ac losses of superconducting films were investigated by measuring the losses for octagonal disks of an YBa2Cu3O7 layer on a magnetic Ni–5  at.  % W substrate in perpendicular ac magnetic fields at 20  Hz and 77  K. At low fields, the losses depended on ac magnetic field amplitude B and film thickness t as [proportional]B3/t instead of [proportional]B4/t3 for a superconducting film on a nonmagnetic substrate. These results are described by considering the formation of a virtual infinite stack of superconducting films due to the magnetic mirror effect.
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74.78.Bz, 74.25.Ha, 74.72.Bk

Double-side superconducting MgB2 films for microwave and electronic applications

Li-ping Chen, Li-li Ding, Qing-rong Feng, Guang-cheng Xiong, Yun-fei Wang, Sheng Luo, Xue-qiang Zhang, and Yu-sheng He

Appl. Phys. Lett. 88, 262502 (2006) (3 pages)

Online Publication Date: 28 June 2006

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Double-side superconducting MgB2 films have been grown on double-side polished sapphire substrates by using hybrid physical-chemical vapor deposition method. The zero resistance temperatures TC(0)>37  K and the critical current densities JC(5  K,0  T)>3×106  A/cm2 have been achieved for both sides. In the first 8.73  G MgB2 microstrip resonator fabricated by a double-side film, an unloaded Q value of 3400 at 11  K was obtained corresponding to surface resistance of 1.2  mOmega for the MgB2/Al2O3 interface. By simply using a sapphire substrate with two polished holes, superconducting connection has been achieved for the double-side MgB2 film. The results show potential applications for the superconducting MgB2 films in microwave and electronic devices.
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74.78.-w, 74.25.Fy, 81.15.Gh, 74.25.Sv, 74.25.Nf

High tunnel magnetoresistance in fully epitaxial magnetic tunnel junctions with a full-Heusler alloy Co2Cr0.6Fe0.4Al thin film

Takao Marukame, Takayuki Ishikawa, Ken-Ichi Matsuda, Tetsuya Uemura, and Masafumi Yamamoto

Appl. Phys. Lett. 88, 262503 (2006) (3 pages)

Online Publication Date: 28 June 2006

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Fully epitaxial magnetic tunnel junctions (MTJs) were fabricated with a Co-based full-Heusler alloy Co2Cr0.6Fe0.4Al (CCFA) thin film, whose composition was close to the stoichiometric one, and a MgO tunnel barrier. Cross-sectional high-resolution transmission electron microscope observations indicated that all layers of the CCFA/MgO/Co50Fe50 MTJ layer structure were grown epitaxially and were single crystalline. The microfabricated CCFA/MgO/Co50Fe50 MTJs exhibited high tunnel magnetoresistance (TMR) ratios of 90% at room temperature and 240% at 4.2  K. A high tunneling spin polarization of 0.79 at 4.2  K was obtained for the epitaxial CCFA films from the TMR ratios.
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73.50.Jt, 75.47.Np, 75.47.Lx, 72.25.Mk, 68.55.Jk, 73.61.At

Detection of 0.5  THz radiation from intrinsic Bi2Sr2CaCu2O8 Josephson junctions

I. E. Batov, X. Y. Jin, S. V. Shitov, Y. Koval, P. Müller, and A. V. Ustinov

Appl. Phys. Lett. 88, 262504 (2006) (3 pages)

Online Publication Date: 28 June 2006

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We report the detection of electromagnetic radiation at about 500  GHz from current-biased intrinsic Bi2Sr2CaCu2O8 single crystal Josephson junctions. We used two silicon lenses to quasioptically couple radiation from our samples to an integrated superconducting heterodyne receiver. The estimated maximum Josephson radiation power which reached the receiver antenna was about 1  pW. We attribute the observed radiation to individual Josephson junctions of the stack and discuss a possibility of the phase locking of a larger number of junctions.
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85.25.Pb, 85.25.Cp

Low power scaling using parallel coupling for toggle magnetic random access memory

David W. Abraham and D. C. Worledge

Appl. Phys. Lett. 88, 262505 (2006) (3 pages)

Online Publication Date: 29 June 2006

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Toggle-switched magnetic random access memory devices offer wide write margins and robustness against thermally activated switching but are as yet unproven regarding the feasibility of scaling down from present sizes with acceptable write currents. We present a strategy for reducing switching fields by using parallel coupling between the two magnetic free layers, in contrast to the usual antiferromagnetic coupling previously discussed. Combined with the proper free layer magnetic material and offset fields from a carefully imbalanced pinned layer, we present experimental verification of toggling of 130  nm diameter magnetic tunnel junctions at fields of less than 50  Oe.
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85.75.Dd, 85.70.Kh, 84.30.Sk

Large room temperature magnetization in nanocrystalline zinc ferrite thin films

Murtaza Bohra, Shiva Prasad, Naresh Kumar, D. S. Misra, S. C. Sahoo, N. Venkataramani, and R. Krishnan

Appl. Phys. Lett. 88, 262506 (2006) (3 pages)

Online Publication Date: 29 June 2006

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Zinc ferrite films were deposited on fused quartz substrate at different temperatures using pulsed laser ablation (PLA) and rf sputtering. X-ray diffraction indicated that all the films were single phase ZnFe2O4 with grain growing in the range of 8–80  nm with substrate temperature. The nanocrystalline films were found to be magnetic and the spontaneous magnetization showed a strong dependence on the grain size, dropping sharply for films with larger grains. A PLA thin film deposited in vacuum at 500  °C exhibited a room temperature magnetization value of 5560  G.
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75.50.Gg, 75.50.Tt, 81.07.Bc, 81.15.Cd, 81.15.Fg, 68.55.Ac
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DIELECTRICS AND FERROELECTRICITY

Noncontact electrical metrology of Cu/low-k interconnect for semiconductor production wafers

Vladimir V. Talanov, André Scherz, and Andrew R. Schwartz

Appl. Phys. Lett. 88, 262901 (2006) (3 pages)

Online Publication Date: 26 June 2006

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We have demonstrated a technique capable of in-line measurement of dielectric constant of low-k interconnect films on patterned wafers utilizing a test key of ~50×50  µm2 in size. The test key consists of a low-k film backed by a Cu grid with >50% metal pattern density and <0.25  µm pitch, which is fully compatible with the existing dual-damascene interconnect manufacturing processes. The technique is based on a near-field scanned microwave probe and is noncontact, noninvasive, and requires no electrical contact to or grounding of the wafer under test. It yields <0.3% precision and ±2% accuracy for the film dielectric constant.
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85.40.Ls, 84.37.+q, 77.55.+f, 07.50.-e, 77.22.Ch

Ferroelectricity in the Dion-Jacobson CsBiNb2O7 from first principles

Craig J. Fennie and Karin M. Rabe

Appl. Phys. Lett. 88, 262902 (2006) (3 pages)

Online Publication Date: 26 June 2006

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We have studied ferroelectricity in Dion-Jacobson CsBiNb2O7 from first principles. Using group-theoretical analysis and first-principles density functional calculations of the total energy and phonons, we perform a systematic study of the energy surface around a paraelectric prototypic phase. Our results suggest that CsBiNb2O7 is a ferroelectric with a polarization of Ps=40  µC  cm–2. We propose further experiments to clarify this point.
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77.84.Dy, 77.80.-e, 63.20.-e, 68.35.Md, 77.22.Ej

Surface scanning probe microscopy investigation of solution deposited BiFeO3 thin films

S. Habouti, C. Solterbeck, and M. Es-Souni

Appl. Phys. Lett. 88, 262903 (2006) (3 pages)

Online Publication Date: 27 June 2006

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Thin films of the multiferroic perovskite-type oxide BiFeO3 were processed via solution deposition. The annealing schedule was chosen so as to give large and fine grained thin films. Microstructure, polarization, and dielectric properties are reported. It is shown that the large grained film is characterized by poor dielectric properties. Both microstructures show, however, poor polarization characteristics. Surface scanning potential microscopy reveals areas of high and low surface potentials in both specimens with a pronounced effect in the large grained films. It is thought that these areas are indirectly responsible for the poor polarization properties of the processed BiFeO3 films.
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77.84.Dy, 77.55.+f, 77.80.-e, 77.22.Ej, 81.40.Gh, 68.37.-d

Application of the interface capacitance model to thin-film relaxors and ferroelectrics

M. Tyunina and J. Levoska

Appl. Phys. Lett. 88, 262904 (2006) (3 pages)

Online Publication Date: 27 June 2006

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The interface capacitance model is briefly revised. It is emphasized that this is a device model related to the boundary condition for polarization and unable to explain the thickness evolution of the ferroelectric properties. The model can be applied to extract the properties of the film from those measured in the capacitor. The interface parameters are found to be temperature independent. The coupling of the film with the interface and the electrode is shown to result in the measured frequency dependent permittivity totally different from that of the film.
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77.80.-e, 77.55.+f, 77.22.Ch, 77.22.Ej

Local structure and medium-range ordering in relaxor ferroelectric Pb(Zn1/3Nb2/3)O3 studied using neutron pair distribution function analysis

I.-K. Jeong and J. K. Lee

Appl. Phys. Lett. 88, 262905 (2006) (3 pages)

Online Publication Date: 28 June 2006

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We studied an evolution of local structure and medium-range ordering in relaxor ferroelectric Pb(Zn1/3Nb2/3)O3 (PZN) from 550 to 15  K using neutron pair distribution function analysis. We show that the local structure of PZN is distorted at all temperatures studied. With decreasing temperature, a medium-range ordering of local polarizations develops with no global rhombohedral phase transition below TM. Instead, the crystal structure can be described as a mixture of polar nanoregions in a disordered lattice, similar to the case of Pb(Mg1/3Nb2/3)O3.
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77.84.Dy, 77.80.-e, 61.66.Fn, 64.70.Kb

Dielectric and optical properties of epitaxial rare-earth scandate films and their crystallization behavior

H. M. Christen, G. E. Jellison, Jr., I. Ohkubo, S. Huang, M. E. Reeves, E. Cicerrella, J. L. Freeouf, Y. Jia, and D. G. Schlom

Appl. Phys. Lett. 88, 262906 (2006) (3 pages)

Online Publication Date: 29 June 2006

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Rare-earth scandates (ReScO3, with Re=Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, i.e., the entire series for which the individual oxides are chemically stable in contact with Si) were deposited in a temperature-gradient pulsed laser deposition system onto LaAlO3 substrates. The crystallization temperature depends monotonically on the Re atomic number and the Goldschmidt tolerance factor, with crystallization temperatures as low as 650  °C for LaScO3 and PrScO3. The dielectric constants of the crystalline films K[approximate]30 (determined by microwave microscopy) are significantly larger than those of their amorphous counterparts. In combination with the large observed band gaps (Eg>5.5  eV, determined by ellipsometry), these results indicate the potential of these materials as high-K dielectrics for field-effect transistor applications.
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77.84.Dy, 77.55.+f, 77.22.Ch, 81.15.Fg, 78.66.Nk

Ferroelectric behavior in nominally relaxor lead lanthanum zirconate titanate thin films prepared by chemical solution deposition on copper foil

Taeyun Kim, Jacqueline N. Hanson, Alexei Gruverman, Angus I. Kingon, and S. K. Streiffer

Appl. Phys. Lett. 88, 262907 (2006) (3 pages)

Online Publication Date: 29 June 2006

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We demonstrate that (Pb0.9La0.1)(Zr0.65Ti0.35)0.975O3 (PLZT) (10/65/35) thin films that have a nominally relaxor composition and that are deposited by chemical solution deposition onto copper foil show polarization hysteresis. Ferroelectric domain switching and a shift in Curie temperature are also observed. This is in contrast to the non-hysteretic behavior of films with identical composition prepared on Pt/SiO2/Si substrates. This suggests that the mismatch in coefficient of thermal expansion between PLZT and copper induces a compressive strain in the PLZT during cooling after high temperature crystallization under low pO2, and causes an out-of-plane polarization.
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77.84.Dy, 77.55.+f, 77.80.Bh, 77.80.Dj, 77.80.Fm, 77.22.Ej

Room temperature ferroelectric properties of Mn-substituted BiFeO3 thin films deposited on Pt electrodes using chemical solution deposition

S. K. Singh, H. Ishiwara, and K. Maruyama

Appl. Phys. Lett. 88, 262908 (2006) (3 pages)

Online Publication Date: 30 June 2006

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Mn-substituted BiFeO3 (BFO) thin films were fabricated by chemical solution deposition on Pt/Ti/SiO2/Si(100) structures. X-ray diffraction analysis revealed that the BFO lattice was somewhat distorted when 5% of Fe atoms were substituted with Mn atoms, but no secondary phase appeared by Mn substitution up to 20%. The leakage current density at higher electric field than 0.6  MV/cm decreased by Mn substitution of 3%–5%, compared with a pure BFO film. Because of the low leakage current density in the high electric field region, well saturated polarization hysteresis loops with remanent polarization of 100  µC/cm2 were observed in the 5% Mn-substituted BFO films at a measurement frequency of 1  kHz.
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77.84.Bw, 77.55.+f, 81.05.-t, 77.80.-e, 81.15.Lm, 68.55.Ac

Study on the interface thermal stability of metal-oxide-semiconductor structures by inelastic electron tunneling spectroscopy

Chih-Feng Huang, Bing-Yue Tsui, Pei-Jer Tzeng, Lurng-Shehng Lee, and Ming-Jinn Tsai

Appl. Phys. Lett. 88, 262909 (2006) (3 pages)

Online Publication Date: 30 June 2006

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The bias polarity-dependent inelastic electron tunneling spectroscopy is employed to detect the thermal stability of the Ta–Pt/SiO2/Si and Ta–Pt/HfO2/Si structures. This work provides a direct evidence that the Fermi-level pinning of metal gates is counted for the generation of extrinsic states due to interface interaction. A Ta2O5 layer forms at the Ta–Pt/SiO2 interface during thermal annealing whereas only an intermittent Ta–O bond is observed at the Ta–Pt/HfO2 interface. Although the heat of formation of HfO2 is lower than that of SiO2, Si presumably diffuses into HfO2 layer and replaces Hf atoms during the high-temperature annealing.
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68.60.Dv, 73.20.At, 61.72.Cc, 68.35.Fx, 66.30.Ny
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NANOSCALE SCIENCE AND DESIGN

High-temperature ferromagnetism in Mn-doped ZnO nanowires

U. Philipose, Selvakumar V. Nair, Simon Trudel, C. F. de Souza, S. Aouba, Ross H. Hill, and Harry E. Ruda

Appl. Phys. Lett. 88, 263101 (2006) (3 pages)

Online Publication Date: 26 June 2006

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We have observed ferromagnetism in dilute (~1–4  at.  %) Mn-doped crystalline ZnO nanowires at temperatures up to 400  K. Arrays of freestanding single crystal ZnO:Mn nanowires were fabricated by Au-catalyzed vapor-liquid-solid growth. Structure and compositional analyses revealed that Mn was incorporated into the ZnO lattice. From the observed saturation magnetization, the magnetic moment per Mn atom is estimated to be between 0.3µB and 1.2µB. Photoluminescence measurements show a strong suppression of defect related midgap emission, indicative of an interplay between Mn doping and native point defects.
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75.50.Dd, 75.50.Pp, 75.60.Ej, 75.30.Cr, 75.50.Tt, 78.67.Lt

Room-temperature low-power hydrogen sensor based on a single tin dioxide nanobelt

L. L. Fields, J. P. Zheng, Y. Cheng, and P. Xiong

Appl. Phys. Lett. 88, 263102 (2006) (3 pages)

Online Publication Date: 26 June 2006

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Hydrogen sensors have been fabricated from single SnO2 nanobelts synthesized via catalyst-free thermal evaporation. Ohmic contacts were defined on individual SnO2 nanobelts by photolithography and pulsed laser deposition of RuO2. The sensitivity and response time of the sensors, without any catalyst on the surface, to 2% hydrogen at temperatures between 25 and 80  °C were measured. Sensitivity greater than 50%, response time <220  s, and power consumption <10  nW at room temperature were demonstrated.
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07.07.Df, 82.80.-d

Analysis of the three-dimensional ordering of epitaxial Ge quantum dots using focused ion beam tomography

Alan J. Kubis, Thomas E. Vandervelde, John C. Bean, Derren N. Dunn, and Robert Hull

Appl. Phys. Lett. 88, 263103 (2006) (3 pages)

Online Publication Date: 26 June 2006

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Buried layers in quantum dot (QD) superlattices influence the position of QDs in the subsequently grown layers through strain field interactions. Since the strain interactions are complex, a three-dimensional reconstruction of the superlattice can enhance the fundamental understanding of self-organization mechanisms. We have studied the three-dimensional relationship of QDs using focused ion beam tomography. Analysis of the reconstruction is consistent with earlier models for self-organization. QDs on successive layers form above buried QDs. In certain cases, successive QDs in a column decrease in size, resulting in the elimination of the column while QDs in other columns grow in size.
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68.65.Hb, 68.65.Cd, 81.15.Hi

Guided three-dimensional molecular self-assembly on silicon substrates

Chia-Ching Chang, Kien Wen Sun, Lou-Sing Kan, and Chieh-Hsiung Kuan

Appl. Phys. Lett. 88, 263104 (2006) (3 pages)

Online Publication Date: 27 June 2006

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We demonstrate three-dimensional (3D) self-assemble growth of the metallothionein (Mn,Cd-MT-2) molecules on patterned semiconductor substrates. The MT molecules deposited on the patterned substrates were found to grow into 3D rod or ring-type nanostructures, depending on the shape of patterned nanostructures on the substrates. Dense arrays of 3D molecular nanorods or rings with an area density close to 1010  cm–2 were demonstrated with a pore size of 20  nm and a pitch size of 100  nm. Those engineered molecular nanostructures provide an excellent opportunity for biological applications, sensing sources of nanodevices, biochemical reactions on surfaces, and even single molecule studies.
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81.16.Dn, 81.07.-b, 61.46.-w, 68.65.-k, 61.43.Gt, 87.15.-v

Diffusion mediated growth of (111) oriented silver nanoparticles in polyvinyl alcohol film under 6  MeV electron irradiation

K. A. Bogle, S. D. Dhole, and V. N. Bhoraskar

Appl. Phys. Lett. 88, 263105 (2006) (3 pages)

Online Publication Date: 27 June 2006

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Silver (111) nanoparticles were synthesized by diffusing silver from a solution into polyvinyl alcohol (PVA) films under 6  MeV electron irradiation at room temperature (~25  °C). The diffusion of silver in the PVA was confirmed by the Rutherford backscattering spectroscopy and scanning electron microscopy techniques. The plasmon absorption peak at ~426  nm was an evidence for the initiation of the diffusion mediated growth of silver nanoparticles. The x-ray diffraction results and the blueshift in the plasmon absorption peak reveal that the size of silver nanoparticles could be tailored in the range from 35  to  15  nm by varying the electron fluence over the range of 1014–1015  e/cm2.
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81.07.Bc, 61.46.Df, 78.67.Bf, 66.30.Jt, 61.82.Pv, 82.80.Yc

Water electrolysis activated by Ru nanorod array electrodes

Seongyul Kim, Nikhil Koratkar, Tansel Karabacak, and Toh-Ming Lu

Appl. Phys. Lett. 88, 263106 (2006) (3 pages)

Online Publication Date: 27 June 2006

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Efficient hydrogen production is critical to fuel cell operation. One of the most convenient methods to produce hydrogen is via water electrolysis. However, overpotential losses at the cell electrodes results in poor efficiency. In this study we carried out water electrolysis experiments with ruthenium (Ru) nanorod arrays as the cathode. We show up to 25% reduction in overpotential and 20% reduction in energy consumption by use of the Ru nanorod cathode compared to the planar Ru cathode. We attribute the improvement to the increased active area of the nanostructured electrode which reduces the operating current density of the electrolyzer.
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82.45.Hk, 82.45.Yz, 82.45.Fk, 82.47.-a, 84.60.-h

Porous InP array-directed assembly of InAs nanostructure

Xiao-Ling Che, Lu Li, Feng-Qi Liu, Xiu-Qi Huang, and Zhan-Guo Wang

Appl. Phys. Lett. 88, 263107 (2006) (3 pages)

Online Publication Date: 27 June 2006

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Fascinating features of porous InP array-directed assembly of InAs nanostructures are presented. Strained InAs nanostructures are grown by molecular-beam epitaxy on electrochemical etched porous InP substrate. Identical porous substrate with different pore depths defines different growth modes. Shallow pores direct the formation of closely spaced InAs dots at the bottom. Deep pores lead to progressive covering of the internal surface of pores by epitaxial material followed by pore mouth shrinking. For any depth an obvious dot depletion feature occurs on top of the pore framework. This growth method presages a pathway to engineer quantum-dot molecules and other nanoelements for fancy physical phenomena.
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81.05.Ea, 61.46.-w, 81.15.Hi, 81.07.Ta

Single-electron tunneling force spectroscopy of an individual electronic state in a nonconducting surface

E. Bussmann and C. C. Williams

Appl. Phys. Lett. 88, 263108 (2006) (3 pages)

Online Publication Date: 28 June 2006

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A tunneling spectroscopy technique to measure the energy level of an electronic state in a completely nonconducting surface is demonstrated. Spectroscopy is performed by electrostatic force detection of single-electron tunneling between a scanning probe and the state as a function of an applied voltage. An electronic state near the surface of a SiO2 film is found 5.5±0.2  eV below the conduction band edge. A random telegraph signal, caused by sporadic back-and-forth single-electron tunneling, is observed as the probe Fermi level passes through the state energy.
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07.79.Cz, 68.37.Ef, 73.40.Gk, 73.20.At, 71.20.Ps

Modified atomic force microscope for high-rate dynamic force spectroscopy

A. Ptak, M. Kappl, and H.-J. Butt

Appl. Phys. Lett. 88, 263109 (2006) (3 pages)

Online Publication Date: 28 June 2006

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Usually adhesion forces between interfaces increase with the rate of separating the interfaces. One of the most popular instruments for such rate-dependent adhesion experiments (also called dynamic force spectroscopy) is the atomic force microscope (AFM). A limitation of this as well as that of other experimental techniques is the maximum loading rate that can be applied. We extended the range of loading rates of a commercial AFM by up to three orders of magnitude by integration of a small additional piezoactuator with high resonance frequency. Performance of the instrument is demonstrated for a model system of self-assembled monolayers of thiols.
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07.79.Lh, 68.37.Ps, 85.50.-n, 68.47.Pe, 68.35.Np

Ge-dot/Si multilayered structures through Ni-induced lateral crystallization

Bo Yan, Yi Shi, Lin Pu, Jianmin Zhu, Kuangji Zhang, Guobin Ma, Ping Han, Rong Zhang, and Youdou Zheng

Appl. Phys. Lett. 88, 263110 (2006) (3 pages)

Online Publication Date: 28 June 2006

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We demonstrate a method for fabricating high-quality Ge-dot/Si multilayered structures. High-density self-assembled Ge dots are grown on amorphous Si layer periodically by low-pressure chemical vapor deposition, and then the amorphous Si are crystallized through Ni-based metal-induced lateral crystallization. Optical microscopy, micro-Raman spectroscopy, and electron microscopy observations reveal that the crystallized Si film has large leaflike grains elongated along the lateral crystallization direction, which shows (110) preference. Furthermore, this preference is found to deliver to different Si layers. The strain shift of Ge dots deduced from Raman spectroscopy reveals a formation of a high-quality interface between the crystallized Si and Ge dot.
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81.15.Gh, 68.65.Ac, 78.67.Pt, 78.30.Am, 64.70.Kb

Precise localization and correlation of single nanoparticle optical responses and morphology

Rongchao Jin, Justin E. Jureller, and Norbert F. Scherer

Appl. Phys. Lett. 88, 263111 (2006) (3 pages)

Online Publication Date: 28 June 2006

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We demonstrate nanometer scale localization of the nonlinear optical response of single nanoparticles and aggregates and correlate this with their morphology. The essence of our approach is to create position markers on an optical and electron-transparent substrate (Si3N4 thin film) that allows optical measurements and transmission electron microscopy (TEM) imaging of the identical nanoparticles or aggregates. The second harmonic activity optical image of individual Ag nanostructures is registered with the TEM image. Centroid localization of the optical signals allows correlation with better than 25  nm precision. This is sufficient to determine the origin of optical "hot spots" within multiparticle aggregates.
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78.67.Bf, 61.46.Df, 42.65.-k, 42.70.Nq, 68.37.Lp

Directional random lasing in dye-TiO2 doped polymer nanowire array embedded in porous alumina membrane

Hee-Won Shin, Seung Yeon Cho, Kyong-Hoon Choi, Seung-Lim Oh, and Yong-Rok Kim

Appl. Phys. Lett. 88, 263112 (2006) (3 pages)

Online Publication Date: 29 June 2006

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We have demonstrated the lasing in the porous alumina membrane filled with hybrid polymer nanowires which consisted of poly(N-vinylcarvazole), rhodamine 6G, and TiO2 nanoparticles. The angle-resolved photoluminescence measurement suggested that lasing had a strong directionality along the hybrid polymer nanowires which were embedded within the nanochannels of the membrane. Although wavelengths of the lasing peaks were not affected by excitation and detection angles, lasing behavior strongly depended on the pore diameters of the membranes utilized. It is suggested that the closed loops for lasing are formed via multiple scattering induced by TiO2 nanoparticles embedded in the hybrid polymer nanowires.
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42.55.Rz, 42.55.Mv, 42.60.By

Nanoresolution profiling of metal-metal interfaces from x-ray Fraunhofer diffraction data

A. V. Darahanau, A. Y. Nikulin, R. A. Dilanian, B. C. Muddle, A. Souvorov, Y. Nishino, and T. Ishikawa

Appl. Phys. Lett. 88, 263113 (2006) (3 pages)

Online Publication Date: 29 June 2006

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High-angular-resolution Fraunhofer diffraction data were collected from several samples with interfaces between dissimilar metals using synchrotron x-radiation. The refractive index profile in the vicinity of the interface of each sample was reconstructed with spatial resolution of about 80  nm by the phase retrieval x-ray diffractometry technique, using only limited a priori knowledge of the sample. These studies have demonstrated the viability of the technique as a nondestructive method of characterization of internal interfaces within multiphase materials.
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68.35.Ct, 81.70.Ex, 78.20.Ci

Coercivity enhancement in the SrRuO3/SrMnO3 superlattices

P. Padhan and W. Prellier

Appl. Phys. Lett. 88, 263114 (2006) (3 pages)

Online Publication Date: 29 June 2006

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Magnetic superlattices consisting of ferromagnetic SrRuO3 and antiferromagnetic SrMnO3 have been grown on (001)-oriented SrTiO3 substrates using the pulsed laser deposition technique. Zero-field-cooled and field-cooled out-of-plane hysteresis loops with different range of magnetic fields have been measured for the series of superlattices. The coercive field of the ferromagnetic layer in SrRuO3–SrMnO3 superlattices is 12 times larger than that of the thin film of SrRuO3. This enhancement in the coercive field can be explained using the concept of the interplay of the pinning energy and the microscopic biased field.
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75.70.Cn, 75.60.Ej, 75.50.Dd, 75.50.Ee

Selecting the growth sites of carbon nanotubes on silicon substrates by ion implantation

Yang Yue, Zhengjun Zhang, and Chao Liu

Appl. Phys. Lett. 88, 263115 (2006) (3 pages)

Online Publication Date: 29 June 2006

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Surface modification by xenon ion implantation could significantly influence the growth of carbon nanotubes on silicon substrates. By chemical vapor deposition using a mixture of ferrocene and xylene as the precursor, carbon nanotubes preferably grew on silicon substrates at areas implanted by xenon ions, leaving the unimplanted areas blank. This is due to the nanometer-scale (4–5  nm) surface roughness induced by ion implantation and oxidation of the roughened surface during implantation and carbon nanotube growth. This study provides an alternative idea to predetermine the growth sites of nanotubes in fabricating arrays of aligned nanotubes on silicon substrates.
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81.07.De, 81.05.Uw, 81.16.-c, 81.15.Gh, 61.72.Ww, 81.65.Mq

Ordering Fe nanowire on stepped Cu (111) surface

X. Tan, G. Ouyang, and G. W. Yang

Appl. Phys. Lett. 88, 263116 (2006) (3 pages)

Online Publication Date: 29 June 2006

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We describe formation of Fe nanowires with varying widths at the upper step edges on a vicinal Cu (111) surface via kinetic Monte Carlo simulations on the basis of step decoration effects. The existence of an optimal temperature and deposition flux for the formation of the well-ordered Fe nanowires on stepped Cu (111) surface is found. The physical origin of the formation of Fe nanowires on stepped Cu (111) surfaces is pursued.
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68.65.La, 61.46.-w

Band gap engineering and spatial confinement of optical phonon in ZnO quantum dots

Kuo-Feng Lin, Hsin-Ming Cheng, Hsu-Cheng Hsu, and Wen-Feng Hsieh

Appl. Phys. Lett. 88, 263117 (2006) (3 pages)

Online Publication Date: 30 June 2006

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Both band gap engineering and spatial confinement of optical phonon were observed depending upon the size of ZnO quantum dots at room temperature. Size-dependent blueshifts of photoluminescence and absorption spectra reveal the quantum confinement effect. The measured Raman spectral shift and asymmetry for the E2(high) mode caused by localization of optical phonons agree well with that calculated by using the modified spatial correlation model.
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73.21.La, 78.55.Et, 63.22.+m, 78.67.Hc, 78.30.Fs, 63.20.Pw

Hydrogen plasma dry etching method for field emission application

T. C. Cheng, J. Shieh, W. J. Huang, M. C. Yang, M. H. Cheng, H. M. Lin, and M. N. Chang

Appl. Phys. Lett. 88, 263118 (2006) (3 pages)

Online Publication Date: 30 June 2006

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The ability to fabricate large-area, uniform emitters is an important factor in many vacuum microelectronics applications, especially for field emission displays. In this letter, we measured the field emission properties of uniform silicon nanowire emitters prepared by hydrogen plasma etching using in situ high-resolution scanning electron microscopy and a tungsten anode of 1  µm diameter. Our results indicate that the field emission properties are improved upon increasing the etching time; this process sharpens the nanowires' geometry and lowers their work function. These highly uniform (with respect to length, diameter, and distribution) nanowires display great potential for application within many field emission nanoelectronics devices.
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81.05.Cy, 81.07.Bc, 81.65.Cf, 52.77.Bn, 79.70.+q, 73.63.Bd

Optical luminescence from alkyl-passivated Si nanocrystals under vacuum ultraviolet excitation: Origin and temperature dependence of the blue and orange emissions

Y. Chao, A. Houlton, B. R. Horrocks, M. R. C. Hunt, N. R. J. Poolton, J. Yang, and L. Siller

Appl. Phys. Lett. 88, 263119 (2006) (3 pages)

Online Publication Date: 30 June 2006

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The origin and stability of luminescence are critical issues for Si nanocrystals which are intended for use as biological probes. The optical luminescence of alkyl-monolayer-passivated silicon nanocrystals was studied under excitation with vacuum ultraviolet photons (5.1–23  eV). Blue and orange emission bands were observed simultaneously, but the blue band only appeared at low temperatures (<175  K) and with high excitation energies (>8.7  eV). At 8  K, the peak wavelengths of the emission bands were 430±2  nm (blue) and 600±2  nm (orange). The orange and blue emissions originate from unoxidized and oxidized Si atoms, respectively.
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78.67.Bf, 78.55.Ap, 81.65.Rv, 61.46.-w

Compound nanostructures formed by metal nanoparticles dispersed on nanodendrites grown on insulator substrates

Guoqiang Xie, Minghui Song, Kazuo Furuya, Dmitri V. Louzguine, and Akihisa Inoue

Appl. Phys. Lett. 88, 263120 (2006) (3 pages)

Online Publication Date: 30 June 2006

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A composite nanostructure consisting of Pt nanoparticle/W nanodendrite was fabricated on an insulator Al2O3 substrate using an electron-beam-induced deposition process combined with an ion sputtering method. W nanodendrites with the tips of 3  nm were grown self-standing at the edge of the Al2O3 substrate. The observed morphology is attributed to a growth mechanism involving an electrical charge-up on the substrate surface, movement of charges, and their accumulation on the convex surface of the substrate as well as at the tips of the deposits. Pt nanoparticles with an equilibrium face-centered-cubic structure were uniformly distributed on the W nanodendrites. These composite nanostructures have potential for application in the fields of catalysis and nanodevices.
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81.16.-c, 68.70.+w, 61.46.Df, 81.15.Ef, 81.15.Cd
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DEVICE PHYSICS

Pixel-encapsulated flexible displays with a multifunctional elastomer substrate for self-aligning liquid crystals

Yeun-Tae Kim, Jong-Ho Hong, Tae-Young Yoon, and Sin-Doo Lee

Appl. Phys. Lett. 88, 263501 (2006) (3 pages)

Online Publication Date: 26 June 2006

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We report on a pixel-encapsulated flexible liquid crystal display (LCD) based on an elastomer substrate of self-aligning LC molecules. The elastomer substrate, fabricated by a replica molding technique, has pixel-encapsulating walls that serve as spacers and allow for mechanical stability and reproducibility against bending deformations. Our pixel-encapsulated LCD provides great flexibility, durability, and excellent electro-optic performances in a highly bent environment.
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42.79.Kr, 42.70.Df, 85.60.Pg, 61.30.Vx

Temperature-dependent dark current measurements in GaAsN heterojunction diodes

J. M. Luther, S. W. Johnston, S. R. Kurtz, R. K. Ahrenkiel, and R. T. Collins

Appl. Phys. Lett. 88, 263502 (2006) (3 pages)

Online Publication Date: 26 June 2006

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Temperature- and bias-dependent current measurements were performed on n+-GaAs/p-GaAs1–xNx heterojunction diodes. The samples studied are in the dilute regime and contain less than 1.7% nitrogen with respect to arsenic. Current-voltage, thermally stimulated current, and current transient (after voltage change) measurements provide unique insight into the defect participation in carrier transport within the depletion region. We will present the data obtained from these measurements, which show an activation energy of 0.21  eV and may be related to a key defect measured by capacitance transients in the literature.
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85.30.Kk

High-performance blue electroluminescence devices based on distyrylbenzene derivatives

Yu Duan, Yi Zhao, Ping Chen, Jiang Li, Shiyong Liu, Feng He, and Yuguang Ma

Appl. Phys. Lett. 88, 263503 (2006) (3 pages)

Online Publication Date: 26 June 2006

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Amorphous molecular, distyrylbenzene derivative 2,5,2[prime],5[prime],2[double-prime],5[double-prime]-hexastyryl-[1,1[prime];4[prime],1[double-prime]] terphenyl (HSTP) is sandwiched between NPB and Alq3 as blue light-emitting material in typical multilayer organic light-emitting devices, where NPB and Alq3 are 1,4-bis(1-naphylphenylamino)biphenyl and tris(8-hydroxyquinoline)aluminum, respectively. Formation of exciplex at the interface of NPB and HSTP layer is verified by study on photoluminescence and electroluminescence (EL) spectra. The performance of EL can be greatly improved by optimizations of devices; a pure blue device with Internationale de l'Eclairage coordination (0.16, 0.13), maximum brightness of 15  830  cd/m2, and current efficiency of 4.88  cd/A is obtained.
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85.60.Jb

Double-gated field emitter array with carbon nanotubes grown by chemical vapor deposition

Young Chul Choi, Kwang Seok Jeong, In Taek Han, Ha Jin Kim, Yong Wan Jin, Jong Min Kim, Byong Gon Lee, Jong Hwan Park, and Deok Hyoen Choe

Appl. Phys. Lett. 88, 263504 (2006) (3 pages)

Online Publication Date: 26 June 2006

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We fabricated 4.75  in. diagonal field emitter arrays with a double-gated structure in which the carbon nanotubes were synthesized by chemical vapor deposition using CO and H2 as feed gases. The nanotubes grown directly inside gate holes were used as an emitter. The diameter of the gate hole opening was as small as 4  µm, accompanied with a large number of gate holes in a pixel. The electron beam spreading was minimized by employing a focus electrode. It was found that neither anode voltage nor focus electrode voltage had a strong influence on the anode current. The pixel-to-pixel uniformity of the fabricated structure was measured to be about 91%. Considering the limitation of the structure that has only a vertical resistive layer, it can be said that the fabricated field emitter shows quite a good uniformity.
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85.45.Db, 81.15.Gh, 85.35.Kt

Highly directional acoustic wave radiation based on asymmetrical two-dimensional phononic crystal resonant cavity

Manzhu Ke, Zhengyou Liu, Pei Pang, Wengang Wang, Zhigang Cheng, Jing Shi, Xingzhong Zhao, and Weijia Wen

Appl. Phys. Lett. 88, 263505 (2006) (3 pages)

Online Publication Date: 26 June 2006

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The radiation properties of an asymmetrical two-dimensional phononic crystal resonant cavity with a point source inside are investigated experimentally. The resonant cavity is formed by two separated phononic crystals of different thickness, both of which consist of the same square array of steel rods in water. We observe highly directional acoustic wave radiation when a point acoustic source is put inside the cavity. The radiation field has a half-power beam width less than 6°. This design may serve as a highly directional acoustic source in applications.
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62.65.+k, 63.20.-e

Experimental investigation of transport properties in chalcogenide materials through 1/f noise measurements

P. Fantini, A. Pirovano, D. Ventrice, and A. Redaelli

Appl. Phys. Lett. 88, 263506 (2006) (3 pages)

Online Publication Date: 26 June 2006

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Low-frequency noise spectral density in chalcogenide-based phase-change memory cells has been measured, discussing the role of trapping centers and static disorder as responsible for a noise level in the vitreous insulating state two orders of magnitude higher than in the ordered conducting polycrystalline one. The magnitude of 1/f noise has been also studied as a function of the applied voltage and exploited to experimentally investigate the transport mechanisms in chalcogenide alloys, showing that the exponential increase of noise spectral density with voltage can be quantitatively explained by considering an avalanchelike multiplication phenomenon.
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73.61.Jc, 72.70.+m, 84.30.Sk

Material structure and metastability of hydrogenated nanocrystalline silicon solar cells

Guozhen Yue, Baojie Yan, Gautam Ganguly, Jeffrey Yang, Subhendu Guha, and Charles W. Teplin

Appl. Phys. Lett. 88, 263507 (2006) (3 pages)

Online Publication Date: 26 June 2006

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