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2 Aug 2004

Volume 85, Issue 5, pp. 701-848

Issue Cover Spotlight Figure

Appl. Phys. Lett. 85, 807 (2004); http://dx.doi.org/10.1063/1.1777817 (3 pages)

Henry J. Liu and Kyeongjae Cho
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Low-frequency noise in AlSb∕InAs high-electron-mobility transistor structure as a function of temperature and illumination

W. Kruppa, M. J. Yang, B. R. Bennett, and J. B. Boos

Appl. Phys. Lett. 85, 774 (2004); http://dx.doi.org/10.1063/1.1777390 (3 pages) | Cited 8 times

Online Publication Date: 27 July 2004

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Measurements of the low-frequency noise in AlSb∕InAs high-electron-mobility transistor structures over the temperature range between 60 and 300 K are reported. Without illumination, the slope of the noise level with frequency was found to be close to 1∕f with a Hooge parameter, αH, of 9×10−3 at room temperature. With broad-spectrum visible-light illumination at lower temperatures, however, the noise level increases greatly and displays a strong Lorentzian component with the characteristic 1∕f2 slope above the corner frequency. The associated sheet resistance also increases greatly, consistent with previously observed negative photoconductivity in AlSb∕InAs quantum wells.
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85.30.Tv Field effect devices
73.40.Cg Contact resistance, contact potential
72.40.+w Photoconduction and photovoltaic effects
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High-temperature ferromagnetism in manganese-doped indium–tin oxide films

John Philip, Nikoleta Theodoropoulou, Geetha Berera, Jagadeesh S. Moodera, and Biswarup Satpati

Appl. Phys. Lett. 85, 777 (2004); http://dx.doi.org/10.1063/1.1773617 (3 pages) | Cited 60 times

Online Publication Date: 27 July 2004

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High-temperature ferromagnetism is demonstrated in Mn-doped indium–tin oxide (ITO) films deposited using reactive thermal evaporation. These films were grown on sapphire (0001), Si∕SiO2 as well as Si (100) substrates with the highest magnetic moment observed around 0.8 μB∕Mn in 5% Mn-doped ITO films. The electrical conduction is n type and the carrier concentration is ∼2.5×1019 cm−3 for 5% Mn doping. An anomalous Hall effect is observed in magnetotransport measurements, showing that the charge carriers are spin polarized, revealing the magnetic interaction between itinerant electrons and localized Mn spins. The carrier concentration can be varied independent of the Mn concentration in this transparent ferromagnetic semiconductor for its easy integration into magneto-optoelectronic devices.
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75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
68.55.A- Nucleation and growth
75.70.Ak Magnetic properties of monolayers and thin films
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
72.20.My Galvanomagnetic and other magnetotransport effects
75.30.Cr Saturation moments and magnetic susceptibilities
75.47.Lx Magnetic oxides
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Room-temperature magneto-optical activity of InMnAs thin films

P. T. Chiu, S. J. May, and B. W. Wessels

Appl. Phys. Lett. 85, 780 (2004); http://dx.doi.org/10.1063/1.1773618 (3 pages) | Cited 5 times

Online Publication Date: 27 July 2004

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Room-temperature magneto-optical activity of In1−xMnxAs heteroepitaxial thin films grown on InAs (001) and GaAs (001) substrates has been studied using the longitudinal magneto-optical Kerr effect. For single-phase layers grown on InAs and GaAs with x ranging from 0.03 to 0.09, square hysteresis loops that indicate ferromagnetic order were observed. The magnitude of the saturation Kerr rotation of all the InMnAs samples ranged from 0.29 to 1.1 mdeg, and scaled with the manganese concentration. The shape of the hysteresis curves for samples of InMnAs grown GaAs was invariant with respect to rotation about the sample normal, indicating in-plane magnetic isotropy. In contrast, InMnAs layers on InAs exhibited strong uniaxial in-plane anisotropy with the easy axis of magnetization along the [110] and [mathmath0] directions. The magnetic anisotropy is attributed to the effect of epitaxial strain.
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75.50.Pp Magnetic semiconductors
75.70.Ak Magnetic properties of monolayers and thin films
68.55.A- Nucleation and growth
78.20.Ls Magneto-optical effects
75.30.Gw Magnetic anisotropy
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase

Enhancement of the Curie temperature in GaMnAs∕InGaMnAs superlattices

A. Koeder, W. Limmer, S. Frank, W. Schoch, V. Avrutin, R. Sauer, A. Waag, K. Zuern, and P. Ziemann

Appl. Phys. Lett. 85, 783 (2004); http://dx.doi.org/10.1063/1.1771802 (3 pages) | Cited 3 times

Online Publication Date: 27 July 2004

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We report on an enhancement of the Curie temperature in GaMnAs∕InGaMnAs superlattices grown by low-temperature molecular beam epitaxy which is due to thin InGaMnAs or InGaAs films embedded into the GaMnAs layers. The pronounced increase of the Curie temperature is strongly correlated to the In concentration in the embedded layers. Curie temperatures up to 110 K are observed in such structures compared to 60 K in GaMnAs single layers grown under the same conditions. A further increase in TC up to 130 K can be achieved using postgrowth annealing at temperatures near the growth temperature. Pronounced thickness fringes in the high-resolution x-ray diffraction spectra indicate good crystalline quality and sharp interfaces in the structures.
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75.50.Pp Magnetic semiconductors
68.65.Cd Superlattices
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
61.72.Cc Kinetics of defect formation and annealing
81.40.Gh Other heat and thermomechanical treatments

Investigation on the magnetic and electrical properties of crystalline Mn0.05Si0.95 films

F. M. Zhang, X. C. Liu, J. Gao, X. S. Wu, Y. W. Du, H. Zhu, J. Q. Xiao, and P. Chen

Appl. Phys. Lett. 85, 786 (2004); http://dx.doi.org/10.1063/1.1775886 (3 pages) | Cited 65 times

Online Publication Date: 27 July 2004

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The magnetic and electrical properties of crystalline Mn0.05Si0.95 films prepared by post-thermal treatment of the as-deposited amorphous Si-Mn (95 at.%−5 at.%) have been investigated. Both the temperature dependence and field dependence of magnetization were measured using superconducting quantum interference devices, and it has been indicated that the film materials are ferromagnetic with Curie temperature over 400 K. X-ray diffraction analysis revealed full crystallization of the films and the incorporation of Mn into the host crystalline Si lattice. Behavior of thermally activated conduction processes of the films has been evinced by electrical property measurement for the films.
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75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
73.61.Jc Amorphous semiconductors; glasses
81.05.Gc Amorphous semiconductors
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
75.70.Ak Magnetic properties of monolayers and thin films
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
64.70.K- Solid-solid transitions
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Rapidly solidified (FePt)70P30 alloy with high coercivity

A. A. Kündig, N. Abe, M. Ohnuma, T. Ohkubo, H. Mamiya, and K. Hono

Appl. Phys. Lett. 85, 789 (2004); http://dx.doi.org/10.1063/1.1776333 (3 pages) | Cited 11 times

Online Publication Date: 27 July 2004

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The alloy Fe35Pt35P30, whose composition is close to a ternary eutectic, was rapidly solidified by melt spinning and the ribbon exhibited a high coercivity, exceeding 20 kOe after annealing. The alloy was mainly composed of L10 ordered FePt and PtP2 with an average grain size of about 50 nm. In the as-cast state, the alloy was comprised of about 20-nm-diam FePt and PtP2 grains supersaturated with P and Fe, respectively, and the coercivity was only 150 Oe. The high coercivity obtained following annealing is discussed on the basis of the microstructural observations.
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75.50.Bb Fe and its alloys
81.05.Bx Metals, semimetals, and alloys
81.30.Fb Solidification
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Vv High coercivity materials
81.40.Gh Other heat and thermomechanical treatments

Tailoring magnetic properties of core∕shell nanoparticles

Hao Zeng, Shouheng Sun, J. Li, Z. L. Wang, and J. P. Liu

Appl. Phys. Lett. 85, 792 (2004); http://dx.doi.org/10.1063/1.1776632 (3 pages) | Cited 50 times

Online Publication Date: 27 July 2004

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Bimagnetic FePt∕MFe2O4(M=Fe,Co) core∕shell nanoparticles are synthesized via high-temperature solution phase coating of 3.5 nm FePt core with MFe2O4 shell. The thickness of the shell is controlled from 0.5 to 3 nm. An assembly of the core∕shell nanoparticles shows a smooth magnetization transition under an external field, indicating effective exchange coupling between the FePt core and the oxide shell. The coercivity of the FePt∕Fe3O4 particles depends on the volume ratio of the hard and soft phases, consistent with previous theoretical predictions. These bimagnetic core∕shell nanoparticles represent a class of nanostructured magnetic materials with their properties tunable by varying the chemical composition and thickness of the coating materials.
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75.50.Cc Other ferromagnetic metals and alloys
81.07.Wx Nanopowders
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
61.46.-w Structure of nanoscale materials
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Quantitative analysis of nanoscale switching in SrBi2Ta2O9 thin films by piezoresponse force microscopy

Sergei V. Kalinin, Alexei Gruverman, and Dawn A. Bonnell

Appl. Phys. Lett. 85, 795 (2004); http://dx.doi.org/10.1063/1.1775881 (3 pages) | Cited 41 times

Online Publication Date: 27 July 2004

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Local switching properties in SrBi2Ta2O9 thin films have been studied by spatially resolved piezoresponse force microscopy (PFM) and spectroscopy. Variations in PFM contrast of individual grains due to their random crystallographic orientation are consistent with the grain switching behavior examined via vertical and lateral hysteresis loops. Theoretical description of vertical hysteresis loop shape obtained in the point-charge approximation is shown to be in good agreement with the experimental data. Dependence of the hysteresis loop parameters on the grain crystallographic orientation is analyzed. It has been found that grain deviation from the ideal (010) orientation when the polar axis is normal to the film plane results in the decrease of the PFM signal and increase of the coercive voltage in agreement with theoretical predictions.
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77.55.-g Dielectric thin films
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.Fm Switching phenomena
77.80.Dj Domain structure; hysteresis
68.55.-a Thin film structure and morphology
68.37.Ps Atomic force microscopy (AFM)
68.35.B- Structure of clean surfaces (and surface reconstruction)

Study of surface acoustic phonon modes in relaxor Pb(Mg1∕3Nb2∕3)O3

V. Sivasubramanian, R. Kesavamoorthy, V. S. Sastry, and V. Subramanian

Appl. Phys. Lett. 85, 798 (2004); http://dx.doi.org/10.1063/1.1776328 (3 pages) | Cited 2 times

Online Publication Date: 27 July 2004

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Surface acoustic phonon modes in pelletized polycrystalline Pb(Mg1∕3Nb2∕3)O3 are investigated in the frequency range from 5 to 40 cm−1 at various temperatures from 25 to 550 °C. These spectra support the hypothesis of the core-shell nanoclusters with 1:1 ordered, negatively charged nanocore surrounded by the charge compensating positively charged disordered nanoshell. These surface acoustic modes exist even up to 550 °C revealing the existence of the charged core-shell nanoclusters which is well beyond the dissolution temperature of the polar clusters in the material.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.-e Ferroelectricity and antiferroelectricity
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
78.30.Hv Other nonmetallic inorganics

Dielectric relaxor behavior of Cd2Nb2O7

Zhi Yu and Chen Ang

Appl. Phys. Lett. 85, 801 (2004); http://dx.doi.org/10.1063/1.1775038 (3 pages) | Cited 3 times

Online Publication Date: 27 July 2004

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The dc electric-field dependence of the dielectric properties of cadmium niobate Cd2Nb2O7 single crystals is studied, and characteristics of the dielectric behavior of Cd2Nb2O7 under dc bias are revealed. The temperature (Tm) where the dielectric relaxation mode occurs is independent of dc bias levels, while its intensity is greatly suppressed and finally eliminated with increasing dc bias from 0 to 15   kV∕cm. The observed dielectric relaxation features of Cd2Nb2O7 can be attributed to “dielectric relaxor” behavior, rather than “ferroelectric relaxor” behavior previously designated in literature. The physical nature of the dielectric relaxor behavior is briefly discussed.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.22.Gm Dielectric loss and relaxation
77.80.B- Phase transitions and Curie point

Ferroelectric and electrical behavior of (Na0.5Bi0.5)TiO3 thin films

Z. H. Zhou, J. M. Xue, W. Z. Li, J. Wang, H. Zhu, and J. M. Miao

Appl. Phys. Lett. 85, 804 (2004); http://dx.doi.org/10.1063/1.1771808 (3 pages) | Cited 35 times

Online Publication Date: 27 July 2004

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Sodium bismuth titanate (Na0.5Bi0.5)TiO3 (NBT) of perovskite structure is among the best known lead-free piezoelectric∕ferroelectric that promises a number of applications in sensors and actuators. However, NBT in thin film form has not been properly investigated, although NBT in bulk ceramic form has been widely studied. In this letter, we report the growth of polycrystalline NBT thin films by radio-frequency magnetron sputtering and their ferroelectric behavior. The NBT thin films exhibit a well-defined hysteresis loop, with a remanent polarization of 11.9 μC∕cm2 and coercive field of 37.9 kV∕cm when measured at room temperature. There is a steady decrease of dielectric constant in the range of 650–470 over the frequency range of 10–105 Hz. A change in the controlling mechanism of electrical behavior from the grain interior to the grain boundary is observed for the NBT thin film with increasing temperature. Hopping of oxygen vacancies trapped at the grain boundaries is responsible for the high dielectric loss at low frequencies and high dc conductivity observed.
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77.55.-g Dielectric thin films
77.80.Dj Domain structure; hysteresis
81.15.Cd Deposition by sputtering
61.72.Mm Grain and twin boundaries
61.72.J- Point defects and defect clusters
77.22.Ch Permittivity (dielectric function)
77.22.Gm Dielectric loss and relaxation
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A molecular dynamics study of round and flattened carbon nanotube structures

Henry J. Liu and Kyeongjae Cho

Appl. Phys. Lett. 85, 807 (2004); http://dx.doi.org/10.1063/1.1777817 (3 pages) | Cited 9 times

Online Publication Date: 27 July 2004

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Large diameter single wall carbon nanotubes exist with both round right cylinder and barbell-like flattened cross sections. Using an adaptive intermolecular reactive bond order potential, we examine the characteristics of both configurations by using molecular dynamics simulations. We then continue to examine the dynamical transition from the flattened to round state by charge injection. Predictions based on these simulations provide a basis for the design of active fluid transport devices, and nanoscale mechanical systems/motors.
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61.46.-w Structure of nanoscale materials

Field emission of carbon nanotubes grown on carbon cloth

S. H. Jo, D. Z. Wang, J. Y. Huang, W. Z. Li, K. Kempa, and Z. F. Ren

Appl. Phys. Lett. 85, 810 (2004); http://dx.doi.org/10.1063/1.1776330 (3 pages) | Cited 44 times

Online Publication Date: 27 July 2004

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Field emission from carbon nanotubes grown on carbon cloth has been studied. An extremely low electric field of less than 0.4 V∕μm is required to reach an emission current density of 1 mA∕cm2. This ultralow operating electric field of carbon nanotubes grown on carbon cloth is mainly due to a very high field enhancement factor of 1.882×104, which is the result of geometrical configuration of the carbon nanotubes and the substrate. In addition to the field enhancement, the highly disordered microstructure of carbon nanotubes grown on carbon cloth plays an important role to field emission. This unexpected result indicates that the roughness of the substrates on which carbon nanotubes grow is very important. This result also brings us significantly closer to practical applications such as highly efficient lamps, field emission displays, micro vacuum electron sources, etc.
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79.70.+q Field emission, ionization, evaporation, and desorption
61.46.-w Structure of nanoscale materials
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.07.De Nanotubes
61.43.-j Disordered solids

Chiral and quantum size effects of single-wall carbon nanotubes on field emission

Shi-Dong Liang, N. Y. Huang, S. Z. Deng, and N. S. Xu

Appl. Phys. Lett. 85, 813 (2004); http://dx.doi.org/10.1063/1.1776337 (3 pages) | Cited 21 times

Online Publication Date: 27 July 2004

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The emission current of a single-wall carbon nanotube (SWNT) in field emission is studied by the tunneling theory with the tight-binding approach. The emission current is almost independent of the chiral angle of SWNT in low fields, but increases with increase of chiral angles in very high fields. We found a room-temperature quantum size effect of SWNT on field emission. As the diameters of SWNTs increase, the current densities decrease for metallic tubes, but increase for semiconducting tubes. When the diameters of SWNTs are larger than 2 nm the current densities of metallic and semiconducting tubes are very close. These chiral and quantum size effects are originated from the energy band structure of nanotubes.
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79.70.+q Field emission, ionization, evaporation, and desorption
73.22.-f Electronic structure of nanoscale materials and related systems
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
61.46.-w Structure of nanoscale materials
73.40.Gk Tunneling
73.20.At Surface states, band structure, electron density of states

Fabrication and properties of nanoporous GaN films

Y. D. Wang, S. J. Chua, M. S. Sander, P. Chen, S. Tripathy, and C. G. Fonstad

Appl. Phys. Lett. 85, 816 (2004); http://dx.doi.org/10.1063/1.1774273 (3 pages) | Cited 30 times

Online Publication Date: 27 July 2004

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Nanopore arrays with pore diameters of approximately 75 nm were fabricated in GaN films by inductively coupled plasma etching using anodic aluminum oxide (AAO) films as etch masks. Nanoporous AAO films were formed on the GaN surface by evaporating an Al film onto a GaN epilayer and subsequently anodizing the aluminum. To minimize plasma-induced damage, the template was exposed to CF4-based plasma conditions. Scanning electron microscopy analysis shows that the diameter and the periodicity of the nanopores in the GaN were directly transferred from the original anodic alumina template. The pore diameter in the AAO film can be easily controlled by tuning the anodization conditions. Atomic force microscopy, photoluminescence, and micro-Raman techniques were employed to assess the etched GaN nanopore surface. This cost-effective, nonlithographic method to produce nano-patterned GaN templates is expected to be useful for growth and fabrication of nitride-based nanostructures and photonic band gap materials.
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81.05.Ea III-V semiconductors
81.05.Rm Porous materials; granular materials
68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
61.43.Gt Powders, porous materials
78.55.Mb Porous materials
52.77.Bn Etching and cleaning
81.65.Cf Surface cleaning, etching, patterning
68.37.Ps Atomic force microscopy (AFM)
78.30.Fs III-V and II-VI semiconductors
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)

Characterizing quantum-dot blinking using noise power spectra

Matthew Pelton, David G. Grier, and Philippe Guyot-Sionnest

Appl. Phys. Lett. 85, 819 (2004); http://dx.doi.org/10.1063/1.1779356 (3 pages) | Cited 48 times

Online Publication Date: 27 July 2004

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Fluctuations in the fluorescence from macroscopic ensembles of colloidal semiconductor quantum dots have the spectral form of 1∕f noise. The measured power spectral density reflects the fluorescence intermittency of individual dots with power-law distributions of “on” and “off” times, and can thus serve as a simple method for characterizing such blinking behavior.
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78.67.Hc Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
82.70.Dd Colloids
78.55.Hx Other solid inorganic materials

Acoustic phonon mode splitting behavior of an asymmetric y-branch three terminal junction

Wen-Xia Li, Ke-Qiu Chen, Wenhui Duan, Jian Wu, and Bing-Lin Gu

Appl. Phys. Lett. 85, 822 (2004); http://dx.doi.org/10.1063/1.1779339 (3 pages) | Cited 30 times

Online Publication Date: 27 July 2004

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The acoustic phonon transport through an asymmetric y-branch semiconductor quantum structure is investigated by use of the scattering-matrix method. It is found that the quantum structure exhibits mode-splitting behavior, resonant transmission, and the noninteger quantized thermal conductance at very low temperatures. The thermal transport behavior sensitively depends on the geometric configuration of the structure. Our work suggests that such a y-branch structure with specific structural parameters could be used as a splitter of phonon modes.
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63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
63.20.K- Phonon interactions

A phonon depletion effect in ultrathin heterostructures with acoustically mismatched layers

Evgenii P. Pokatilov, Denis L. Nika, and Alexander A. Balandin

Appl. Phys. Lett. 85, 825 (2004); http://dx.doi.org/10.1063/1.1775033 (3 pages) | Cited 17 times

Online Publication Date: 27 July 2004

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We demonstrate theoretically that modification of the acoustic phonon spectrum in semiconductor heterostructures with large acoustic impedance mismatch between the core and cladding layers may lead to strong phonon depletion in the core layer. The latter is achieved if the heterostructure parameters are properly tuned, i.e., the structure thickness is in nanometer scale to ensure phonon quantization and the cladding layers are acoustically “softer” than the core layer. Using a numerical solution of the elasticity equation, we show that one can achieve conditions when almost all acoustic phonon modes are squeezed in the cladding layers with the exception of a small fraction of phonons with very small wave vectors (q⩽0.3 nm−1). The predicted phonon depletion effect in the core layer of the acoustically mismatched heterostructures may lead to increased carrier mobility in certain regions of the heterostructure as well as improved thermal management of heterostructure-based devices.
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63.20.D- Phonon states and bands, normal modes, and phonon dispersion
68.35.Ja Surface and interface dynamics and vibrations

Self-assembled three-dimensional conducting network of single-wall carbon nanotubes

Graciela B. Blanchet, Shekhar Subramoney, R. K. Bailey, G. D. Jaycox, and C. Nuckolls

Appl. Phys. Lett. 85, 828 (2004); http://dx.doi.org/10.1063/1.1776619 (3 pages) | Cited 19 times

Online Publication Date: 27 July 2004

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We described here the self-assembling of a three-dimensional array of single-wall nanotubes (SWNTs). The distinctive choice of materials allowed for the self-assembly of SWNTs with low resistance conducting polymer links into a conducting network that when embedded into an insulating host shows no disruption of the conduction path. The ability to control network formation independently of the electrical properties of the host drastically changes the design of these conducting organic networks. Thus, enabling the tailoring of their electrical properties while addressing issues of film processability relevant for their application as printable conductors in organic electronic applications. These networks provide opportunities for applications in micro- and nanoelectronics.
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61.46.-w Structure of nanoscale materials
73.61.Wp Fullerenes and related materials
68.37.Lp Transmission electron microscopy (TEM)
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Micropatterning of metal films coated on polymer surfaces with epoxy mold and its application to organic field effect transistor fabrication

Zhe Wang, Rubo Xing, Jian Zhang, Jianfeng Yuan, Xinhong Yu, and Yanchun Han

Appl. Phys. Lett. 85, 831 (2004); http://dx.doi.org/10.1063/1.1776325 (3 pages) | Cited 11 times

Online Publication Date: 27 July 2004

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In this letter, a simple and versatile approach to micropatterning a metal film, which is evaporated on a Si substrate coated with polymer, is demonstrated by the use of a prepatterned epoxy mold. The polymer interlayer between the metal and the Si substrate is found important for the high quality pattern. When the metal–polymer–Si sandwich structure is heated with the temperature below Tm but above Tg of the polymer, the plastic deformation of the polymer film occurs under sufficiently high pressure applied. It causes the metal to crack locally or weaken along the pattern edges. Further heating while applying a lower pressure results in the formation of an intimate junction between the epoxy stamp and the metal film. Under these conditions the epoxy cures further, ensuring adhesion between the stamp and the film. The lift-off process works because the adhesion between the epoxy and the metal film is stronger than that between the metal film and the polymer. A polymer field effect transistor is fabricated in order to demonstrate potential applications of this micropatterning approach.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
82.35.Gh Polymers on surfaces; adhesion
68.55.A- Nucleation and growth
85.30.Tv Field effect devices
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep
62.20.M- Structural failure of materials
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
68.35.Np Adhesion
81.40.Gh Other heat and thermomechanical treatments

Reduction of the number of electrons emitted backwards in back-gated devices for field emission: A theoretical study

V. P. Mammana and L. R. C. Fonseca

Appl. Phys. Lett. 85, 834 (2004); http://dx.doi.org/10.1063/1.1776612 (3 pages) | Cited 3 times

Online Publication Date: 27 July 2004

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The use of back-gated geometry (BGG) for field emission devices promises improved robustness, lower power consumption, and simpler manufacturing process. The BGG is a configuration in which the cathode is positioned between anode and gate, different from conventional approaches. Using a cylinder∕plane model to represent rows of cathode lines and the back-gate it is demonstrated that this geometry combined with an appropriate effective work-function modulation along the cathode surface can reduce the amount of electrons emitted backwards.
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79.20.Kz Other electron-impact emission phenomena
68.49.Jk Electron scattering from surfaces
79.70.+q Field emission, ionization, evaporation, and desorption
73.30.+y Surface double layers, Schottky barriers, and work functions

Efficient bottom cathodes for organic light-emitting devices

Jie Liu, Anil R. Duggal, Joseph J. Shiang, and Christian M. Heller

Appl. Phys. Lett. 85, 837 (2004); http://dx.doi.org/10.1063/1.1776620 (3 pages) | Cited 12 times

Online Publication Date: 27 July 2004

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Bilayers of aluminum and an alkali fluoride are well-known top cathode contacts for organic light-emitting devices but have never been successfully applied as bottom contacts. We describe a bilayer bottom cathode contact for organic electronic devices based on reversing the well-known top cathode structure such that the aluminum, rather than the alkali fluoride, contacts the organic material. Electron-only devices were fabricated showing enhanced electron injection from this bottom contact. Kelvin probe, x-ray photoelectron spectroscopy experiments, and thermodynamic calculations suggest that the enhancement results from n doping of the organic material by dissociated alkali metals.
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85.60.Jb Light-emitting devices
65.40.G- Other thermodynamical quantities
79.60.-i Photoemission and photoelectron spectra
73.50.Dn Low-field transport and mobility; piezoresistance
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)

Improved characteristics of organic light-emitting devices by surface modification of nickel-doped indium tin oxide anode

Ching-Ming Hsu and Wen-Tuan Wu

Appl. Phys. Lett. 85, 840 (2004); http://dx.doi.org/10.1063/1.1777416 (3 pages) | Cited 18 times

Online Publication Date: 27 July 2004

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This letter presents the optoelectrical performance of an organic light-emitting diode (OLED) through the elevation of indium tin oxide (ITO) anode work function by Ni co-sputter surface doping and additional O2 plasma treatment. The turn-on voltage of OLED devices can be reduced by 2.3 V for Ni atomic concentration greater than 1.8% and by 2.7 V for the additional O2 plasma treatment. Devices with Ni(2.6%)-doped and O2 plasma treated ITO anodes perform the highest luminance efficiency (0.91 lm∕W), three times larger than undoped ITO (0.31 lm∕W) at 250 cd∕m2.
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85.60.Jb Light-emitting devices
81.15.Cd Deposition by sputtering
52.77.Bn Etching and cleaning
73.30.+y Surface double layers, Schottky barriers, and work functions

35 GHz mode-locking of 1.3 μm quantum dot lasers

M. Kuntz, G. Fiol, M. Lämmlin, D. Bimberg, M. G. Thompson, K. T. Tan, C. Marinelli, R. V. Penty, I. H. White, V. M. Ustinov, A. E. Zhukov, Yu. M. Shernyakov, and A. R. Kovsh

Appl. Phys. Lett. 85, 843 (2004); http://dx.doi.org/10.1063/1.1776340 (3 pages) | Cited 47 times

Online Publication Date: 27 July 2004

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35 GHz passive mode-locking of 1.3 μm (InGa)As∕GaAs quantum dot lasers is reported. Hybrid mode-locking was achieved at frequencies up to 20 GHz. The minimum pulse width of the Fourier-limited pulses was 7 ps with a peak power of 6 mW. Low uncorrelated timing jitter below 1 ps was found in cross correlation experiments. High-frequency operation of the lasers was eased by a ridge waveguide design that includes etching through the active layer.
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42.60.Fc Modulation, tuning, and mode locking
42.60.By Design of specific laser systems
42.55.Px Semiconductor lasers; laser diodes
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Real-time three-dimensional optoacoustic imaging using an acoustic lens system

J. J. Niederhauser, M. Jaeger, and M. Frenz

Appl. Phys. Lett. 85, 846 (2004); http://dx.doi.org/10.1063/1.1777820 (3 pages) | Cited 25 times

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In medical optoacoustics (photoacoustics), absorbing structures, such as blood vessels, hidden inside scattering media are illuminated with short laser pulses resulting in the generation of thermoelastic pressure transients. This initial three-dimensional (3D) acoustic pressure distribution, which exactly resembles the absorption distribution, was imaged into a water container with a 4f acoustic lens system. An optical dark-field stereo imaging system using a 30 ns flash illumination light was used to capture a snapshot of the pressure-induced refraction index changes in the water container at a predetermined time after the original laser pulse. The imaging system works at 20 Hz frame rate and was designed toward a theoretical resolution of 50 μm. The proposed method directly provides 3D images of absorbing structures without the need of computational reconstruction algorithms.
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87.63.Hg Thermography
43.80.Qf Medical diagnosis with acoustics
43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect
43.58.-e Acoustical measurements and instrumentation
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