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12 May 2008

Volume 92, Issue 19, Articles (19xxxx)

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Appl. Phys. Lett. 92, 193301 (2008); http://dx.doi.org/10.1063/1.2920199 (3 pages)

Wei Chen, Hong Liang Zhang, Han Huang, Lan Chen, and Andrew Thye Shen Wee
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Twisting of nanowires induced by anisotropic surface stresses

Jian-Shan Wang, Xi-Qiao Feng, Gang-Feng Wang, and Shou-Wen Yu

Appl. Phys. Lett. 92, 191901 (2008); http://dx.doi.org/10.1063/1.2928221 (3 pages) | Cited 13 times

Online Publication Date: 12 May 2008

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Many natural and synthetic quasi-one-dimensional materials are of helical or twisting shape and understanding the physical mechanisms underlying the asymmetric shape is of both theoretical and technological significances. In this letter, we pointed out that anisotropic surface stresses present as a possible reason for the formation of some micro-/nanohelices. Using Gurtin’s theory of surface elasticity, we quantitatively investigated the twisting deformation of nanowires due to anisotropic surface stresses. The present model can also elucidate the formation of some other helical materials at micro- and nanoscales, e.g., twisting lamellae in polymer spherulites, spiraled bacteria, and flagella.
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61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
68.35.Gy Mechanical properties; surface strains
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Enhanced mechanical strength and ductility of metal-repaired defective carbon nanotubes: A density functional study

G. P. Zheng and H. L. Zhuang

Appl. Phys. Lett. 92, 191902 (2008); http://dx.doi.org/10.1063/1.2924275 (3 pages)

Online Publication Date: 12 May 2008

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Metal atoms are filled into the defective sites of single-walled carbon nanotube (SWCN) containing vacancy defects, resulting in a stable repaired SWCN. The tensile deformation of the repaired SWCN is investigated by spin-polarized density functional theory. Compared to the defective SWCN, the repaired CN shows significant enhancements in mechanical strength and ductility that are close to those of pristine CN. The underlying physics of these behaviors are analyzed by the structural transformation, electronic structures, and spin and charge distributions during the tensile tests. A strong magnetomechanical coupling effect is found to be responsible for the enhanced mechanical behaviors of metal-CN hybrid structures.
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81.07.De Nanotubes
62.25.-g Mechanical properties of nanoscale systems
73.22.-f Electronic structure of nanoscale materials and related systems
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
81.40.Lm Deformation, plasticity, and creep
75.80.+q Magnetomechanical effects, magnetostriction

{110}-facets formation by hydrogen thermal etching on sidewalls of Si and strained-Si fin structures

Tsutomu Tezuka, Norio Hirashita, Yoshihiko Moriyama, Naoharu Sugiyama, Koji Usuda, Eiji Toyoda, Ken Murayama, and Shin-ichi Takagi

Appl. Phys. Lett. 92, 191903 (2008); http://dx.doi.org/10.1063/1.2924281 (3 pages) | Cited 2 times

Online Publication Date: 12 May 2008

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Si-fin structures for multigate metal-oxide-semiconductor field effect transistors (MOSFETs) with smooth and vertical sidewalls composed of {110} facets were obtained by an anisotropic gas etching in atmospheric hydrogen ambient at 925–1000 °C on strained and unstrained (001) Si-on-insulator (SOI) substrates. {110} facets emerged due to higher etching rate for higher-order crystalline plane adjacent to {110} planes. The facet formation effectively eliminated the fin-width variation originating from the lithography process. The uniaxial stress along the fins on the strained SOI substrate was found to be preserved during the process, indicating the availability of this technique for fabrication of multigate MOSFETs with strained-fin channels.
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85.30.Tv Field effect devices
81.65.Cf Surface cleaning, etching, patterning
85.40.Hp Lithography, masks and pattern transfer

One-step preparation of Ca-α-SiAlON:Eu2+ fine powder phosphors for white light-emitting diodes

Takayuki Suehiro, Naoto Hirosaki, Rong-Jun Xie, Ken Sakuma, Mamoru Mitomo, Masahiro Ibukiyama, and Suzuya Yamada

Appl. Phys. Lett. 92, 191904 (2008); http://dx.doi.org/10.1063/1.2920209 (3 pages) | Cited 8 times

Online Publication Date: 12 May 2008

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A simple methodology for producing yellow-emitting Ca-α-SiAlON:Eu2+ phosphors for white light-emitting diodes has been developed, which consists of postsynthesis activation of fine α-SiAlON powders prepared by the gas-reduction-nitridation method. Phase-pure, fine Ca-α-SiAlON:Eu2+ phosphors possessing the particle size ranging ∼ 0.9–2.8 μm and high quantum efficiencies of ∼ 44%–55% under near-UV to blue light excitation are obtained as the process developed. The chromaticity coordinate of the synthesized Ca-α-SiAlON:Eu2+ can be tailored by simply controlling the activator concentration, enabling the creation of a broad range of warm-white light with the correlated color temperatures of ∼ 3500–2600 K.
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85.60.Jb Light-emitting devices

Radiative recombination dynamics in tetrapod-shaped CdTe nanocrystals: Evidence for a photoinduced screening of the internal electric field

Giovanni Morello, Davide Tarì, Luigi Carbone, Liberato Manna, Roberto Cingolani, and Milena De Giorgi

Appl. Phys. Lett. 92, 191905 (2008); http://dx.doi.org/10.1063/1.2924306 (3 pages) | Cited 1 time

Online Publication Date: 13 May 2008

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We study the radiative recombination processes in CdTe tetrapod nanocrystals at 10 K. Two intrinsic emission bands, namely the ground state (GS) and the excited state (EX), decay with three time constants, due to a power dependent Auger-like recombination process (tens of picoseconds), to the intrinsic emission of the two states (hundreds of picoseconds) and to emission from defect states (a few nanoseconds). The existence of an internal electric field originating from the e-h separation induced by the peculiar symmetry of the GS is demonstrated by a dynamical shift of the GS emission energy that is correlated to the EX population.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
71.55.Gs II-VI semiconductors
78.55.Et II-VI semiconductors
73.61.Ga II-VI semiconductors
78.66.Hf II-VI semiconductors

Intersubband transitions in ZnO multiple quantum wells

M. Belmoubarik, K. Ohtani, and H. Ohno

Appl. Phys. Lett. 92, 191906 (2008); http://dx.doi.org/10.1063/1.2926673 (3 pages) | Cited 7 times

Online Publication Date: 13 May 2008

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Intersubband transitions in ZnO/MgZnO multiple quantum wells (MQWs) are investigated by a photocurrent spectroscopy. Photocurrent peaks are observed in the energy range from 300 to 400 meV and shifted to higher energy by reducing the ZnO well thickness. Polarization-resolved photocurrent spectra show that these peaks are observed when the polarization of incident lights is TM mode, following the intersubband selection rule. Calculation indicates that the photocurrent peaks are the intersubband transition from the first to the third subband in ZnO/MgZnO MQWs.
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73.63.Hs Quantum wells
72.40.+w Photoconduction and photovoltaic effects
73.21.Fg Quantum wells

Energy bandgap bowing of InAlN alloys studied by spectroscopic ellipsometry

E. Iliopoulos, A. Adikimenakis, C. Giesen, M. Heuken, and A. Georgakilas

Appl. Phys. Lett. 92, 191907 (2008); http://dx.doi.org/10.1063/1.2921783 (3 pages) | Cited 25 times

Online Publication Date: 14 May 2008

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InxAl1−xN films were heteroepitaxially grown on AlN/Al2O3 (0001) templates by molecular beam epitaxy. The compositions studied spanned the whole ternary range. The complex dielectric function of the films was investigated by variable angle spectroscopic ellipsometry in the energy range from 0.55 to 6 eV. The energy bandgap bowing parameter was found to strongly depend on composition, monotonically increasing with decreasing InN mole fraction. This behavior is in agreement with theoretical predictions of large charge transfer contributions to bandgap bowing.
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71.20.Nr Semiconductor compounds
68.55.ag Semiconductors
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
78.66.Fd III-V semiconductors
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

First principles study of Ge/Si exchange mechanisms at the Si(001) surface

F. Zipoli, S. Cereda, M. Ceriotti, M. Bernasconi, Leo Miglio, and F. Montalenti

Appl. Phys. Lett. 92, 191908 (2008); http://dx.doi.org/10.1063/1.2926683 (3 pages) | Cited 10 times

Online Publication Date: 14 May 2008

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Based on density functional theory calculations, we show that an isolated Ge adatom on Si(001) triggers an exchange mechanism involving three atoms, which leads to the formation of a Si adatom and a mixed SiGe surface dimer. The activation energy calculated from first principles is sufficiently low (0.8 eV) to make such a process viable down to the lowest temperature (330 K) at which intermixing was reported. A second mechanism, with a higher barrier, is also proposed and shown to possibly contribute to the incorporation of Ge into deeper layers as experimentally observed at higher temperatures.
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68.35.Fx Diffusion; interface formation
68.35.Md Surface thermodynamics, surface energies

Accelerating dislocations to transonic and supersonic speeds in anisotropic metals

Helio Tsuzuki, Paulo S. Branicio, and José Pedro Rino

Appl. Phys. Lett. 92, 191909 (2008); http://dx.doi.org/10.1063/1.2921786 (3 pages) | Cited 6 times

Online Publication Date: 15 May 2008

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The dynamics of stress-accelerated dislocations in copper is investigated using molecular dynamics simulations. The structure and motion of dissociated edge dislocations are analyzed using the common neighborhood parameter and local stresses. Dislocations are accelerated by high shear stresses and reach stable velocities in the two transonic regimes. Supersonic dislocations are generated but are transient, as they require high stresses, which trigger nucleation of multiple dislocation dipoles. A velocity plateau in the first transonic regime indicates a radiation-free state in agreement with theoretical predictions.
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61.72.Lk Linear defects: dislocations, disclinations
61.72.Bb Theories and models of crystal defects

Resonant Raman scattering probe of alloying effect in ZnMgO thin films

J. F. Kong, W. Z. Shen, Y. W. Zhang, C. Yang, and X. M. Li

Appl. Phys. Lett. 92, 191910 (2008); http://dx.doi.org/10.1063/1.2930676 (3 pages) | Cited 9 times

Online Publication Date: 15 May 2008

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We have presented a detailed resonant Raman scattering investigation for the alloying effect in hexagonal Zn1−xMgxO (x ⩽ 0.323) thin films grown by pulsed laser deposition. Alloy-induced longitudinal optical (LO) phonon resonance effect has been achieved from the Raman peak shift, lineshape, and intensity through changing the Mg composition and temperature to tune the ZnMgO bandgap. By the aid of theoretical analysis combining with the extrinsic Fröhlich interaction mediated via a localized exciton, we demonstrate the pronounced outgoing resonance behavior for the LO phonons in ZnMgO, where the localized exciton due to alloy disorder dominates the resonance processes.
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78.66.Hf II-VI semiconductors
78.30.Fs III-V and II-VI semiconductors
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
71.20.Nr Semiconductor compounds

Thermal conductivity reduction in oxygen-deficient strontium titanates

Choongho Yu, Matthew L. Scullin, Mark Huijben, Ramamoorthy Ramesh, and Arun Majumdar

Appl. Phys. Lett. 92, 191911 (2008); http://dx.doi.org/10.1063/1.2930679 (3 pages) | Cited 22 times

Online Publication Date: 15 May 2008

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We report significant thermal conductivity reduction in oxygen-deficient lanthanum-doped strontium titanate (Sr1−xLaxTiO3−δ) films as compared to unreduced strontium titanates. Our experimental results suggest that the oxygen vacancies could have played an important role in the reduction. This could be due to the nature of randomly distributed and clustered vacancies, which would be very effective to scatter phonons. Our results could provide a pathway for tailoring the thermal conductivity of complex oxides, which is very beneficial to various applications including thermoelectrics.
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73.61.Ng Insulators
73.50.Lw Thermoelectric effects
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
61.72.jd Vacancies
63.20.-e Phonons in crystal lattices
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Effect of the barrier composition on the polarization fields in near UV InGaN light emitting diodes

A. Knauer, H. Wenzel, T. Kolbe, S. Einfeldt, M. Weyers, M. Kneissl, and G. Tränkle

Appl. Phys. Lett. 92, 191912 (2008); http://dx.doi.org/10.1063/1.2930686 (3 pages) | Cited 17 times

Online Publication Date: 15 May 2008

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The electroluminescence from near ultraviolet (UV) light emitting diodes containing InGaN multiple quantum wells (MQWs) with GaN, AlGaN, and InAlGaN barriers was investigated. Based on band-structure calculations the observed wavelength shift in the peak emission with increasing injection current is attributed to the screening of the polarization fields and to band gap renormalization. InGaN MQWs with almost zero net polarization have been realized. No blueshift in the emission spectra of these devices was observed over the entire current range.
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85.60.Jb Light-emitting devices

Young’s modulus of VO2 thin films as a function of temperature including insulator-to-metal transition regime

Nelson Sepúlveda, Armando Rúa, Rafmag Cabrera, and Félix Fernández

Appl. Phys. Lett. 92, 191913 (2008); http://dx.doi.org/10.1063/1.2926681 (3 pages) | Cited 10 times

Online Publication Date: 15 May 2008

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Young’s modulus of VO2 thin films has been measured for the first time through the material’s insulator-to-metal transition. The resonant frequency of silicon VO2 coated cantilevers was measured in the temperature range 30–90 °C. It has been found that during the semiconductor to metallic transition of VO2 thin films, which occurs at a temperature of 68 °C, Young’s modulus changes most dramatically with temperature, abruptly reversing its declining trend with increasing temperature. The film is stiffened through the transition and, as the temperature is further raised, the declining trend is reasserted at a similar rate.
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68.60.Bs Mechanical and acoustical properties
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
71.30.+h Metal-insulator transitions and other electronic transitions

Cutoff effect of light transmission through structured metal films

Shi-qiang Liu, Qian-jin Wang, Xiao-gang Yin, Cheng-ping Huang, and Yong-yuan Zhu

Appl. Phys. Lett. 92, 191914 (2008); http://dx.doi.org/10.1063/1.2927375 (3 pages) | Cited 1 time

Online Publication Date: 15 May 2008

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The optical transmission spectra of structured metal films, which were fabricated by depositing a thin metal layer on the perforated metal surface, have been experimentally studied in this paper. They exhibit an extraordinary cutoff effect at certain wavelength, beyond which no efficient light transmission can be achieved. A systematic study reveals that the cutoff wavelength is dependent on the lattice period but not on the hole shape or size, making a difference to the cutoff of waveguide. To understand the effect, a qualitative explanation has been proposed.
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78.66.Bz Metals and metallic alloys

In situ transmission electron microscopy study on the crystallization of GeTe binary alloy

Eun Tae Kim, Jeong Yong Lee, and Yong Tae Kim

Appl. Phys. Lett. 92, 191915 (2008); http://dx.doi.org/10.1063/1.2919048 (3 pages)

Online Publication Date: 16 May 2008

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Microstructural properties of GeTe thin films were investigated by an in situ heating method within a high voltage electron microscope (HVEM). The results confirm that the transformation from an amorphous state to a fcc crystalline state yields a GeTe binary alloy with a ring-shaped amorphous structure. The fcc structured GeTe transforms into a GeTe orthorhombic structure during the experiment. The crystallization behavior of the GeTe orthorhombic structure in the HVEM is quite different from thermal crystallization. Our observation of real-time structural change confirms that the relaxed amorphous structure participates in the crystallization process in the electron beam irradiation condition.
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68.55.-a Thin film structure and morphology
64.70.kd Metals and alloys
61.80.Fe Electron and positron radiation effects

Characterization of granular compaction by nonlinear acoustic resonance method

C. Inserra, V. Tournat, and V. Gusev

Appl. Phys. Lett. 92, 191916 (2008); http://dx.doi.org/10.1063/1.2931088 (3 pages) | Cited 6 times

Online Publication Date: 16 May 2008

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A method of nonlinear acoustic resonance is applied to monitor the evolution in the linear and nonlinear elastic and dissipative mechanical properties of a granular packing in the process of its compaction by consecutive mechanical taps. It is demonstrated that the variations in the nonlinear mechanical properties are more pronounced than those in the linear ones, indicating the advantage of nonlinear acoustic methods for the compaction control. The experimental observations revealed for the first time the modification in the statistical distribution of the grain-grain contact interaction forces by the compaction.
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81.05.Rm Porous materials; granular materials
43.25.Gf Standing waves; resonance
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
62.65.+k Acoustical properties of solids

Influence of pulse width on electroluminescence and junction temperature of AlInGaN deep ultraviolet light-emitting diodes

J. C. Zhang, Y. H. Zhu, T. Egawa, S. Sumiya, M. Miyoshi, and M. Tanaka

Appl. Phys. Lett. 92, 191917 (2008); http://dx.doi.org/10.1063/1.2931698 (3 pages) | Cited 10 times

Online Publication Date: 16 May 2008

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The behavior of electroluminescence (EL) and junction temperature of AlInGaN deep ultraviolet light-emitting diodes under pulse-width modulation is investigated. The redshift of both emissions from quantum-well (P1) and localized (P2) states in the EL spectra and the increase of intensity ratio of P1 to P2 are observed with the increase of duty cycle. The photoluminescence of p-GaN contact layer is adopted to measure the junction temperature, which shows a linear relation with the duty cycle. Meanwhile, the duty cycle coefficient of junction temperature increases with the increase of injection current. The EL behaviors are explained by the thermal effect induced by pulse current at high duty cycle.
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85.60.Jb Light-emitting devices
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