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15 May 2006

Volume 99, Issue 10, Articles (10xxxx)

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Aluminum incorporation into AlGaN grown by low-pressure metal organic vapor phase epitaxy

G. S. Huang, H. H. Yao, T. C. Lu, H. C. Kuo, and S. C. Wang

J. Appl. Phys. 99, 104901 (2006); http://dx.doi.org/10.1063/1.2199972 (5 pages) | Cited 1 time

Online Publication Date: 24 May 2006

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Aluminum (Al) incorporation in AlxGa1−xN films grown by low-pressure metal organic vapor phase epitaxy using trimethylaluminum (TMAl) and trimethylgallium as group III precursors has been systematically studied. The solid phase Al composition of the AlxGa1−xN films varied nonlinearly with the Al gas phase composition. The incorporation kinetics of AlxGa1−xN alloy has been analyzed by using an adsorption-trapping model. Two parameters were used to characterize the properties of Al incorporation, i.e., the capture radius and the adsorption time of Al atoms. An exponential function of the Al composition of the AlxGa1−xN films versus the TMAl gas flow rate was obtained. It was demonstrated that the adsorption time of the Al atom was larger than the growth time of one atomic layer. The effects of ammonia flow rate, crystal growth rate, and growth temperature on the adsorption parameters were also discussed.
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81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.43.Mn Adsorption kinetics

Effect of solvents on the chemical and physical properties of ionic polymer-metal composites

Sia Nemat-Nasser, Shahram Zamani, and Yitzhak Tor

J. Appl. Phys. 99, 104902 (2006); http://dx.doi.org/10.1063/1.2194127 (17 pages) | Cited 24 times

Online Publication Date: 25 May 2006

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Ionic polymer-metal composites (IPMCs) consist of a perfluorinated ionomer membrane (usually Nafion® or Flemion®) plated on both faces with a noble metal such as gold or platinum and neutralized with a certain amount of counterions that balance the electrical charge of anions covalently fixed to the backbone ionomer. IPMCs are electroactive materials with potential applications as actuators and sensors. Their electrical-chemical-mechanical response is dependent on the cations used, the nature and the amount of solvent uptake, the morphology of the electrodes, the composition of the backbone ionomer, the geometry and boundary conditions of the composite element, and the magnitude and spatial and time variation of the applied potential. With water as the solvent, the applied electric potential must be limited to less than 1.3 V at room temperature, to avoid electrolysis. Moreover, water evaporation in open air presents additional problems. These and related factors limit the application of IPMCs with water as the solvent. We present the results of a series of tests on both Nafion- and Flemion-based IPMCs with ethylene glycol, glycerol, and crown ethers as solvents. IPMCs with these solvents have greater solvent uptake and can be subjected to relatively high voltages without electrolysis. They can be actuated in open air for rather long time periods, and at low temperatures. They may be good actuators when high-speed actuation is not necessary. In addition, their slow response in open air allows direct observation of the physical characteristics of the cathode and anode surfaces of a cantilever during actuations. This can provide additional clues for unraveling the underpinning micromechanisms of their actuation. Remarkably, solvents are found to have profound effects on the nature of the IPMCs’ actuation. For example, Nafion-based IPMCs in Li+ form show very small back relaxation when hydrated, but extensive back relaxation with all other solvents that we have considered. On the other hand, the same membrane in the K+ form has extensive back relaxations when solvated with water, or ethylene glycol, or glycerol, but none with 18-Crown-6.
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81.05.Qk Reinforced polymers and polymer-based composites
07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
82.45.Mp Thin layers, films, monolayers, membranes

Life cycle of a tungsten cold field emitter

K. S. Yeong and J. T. L. Thong

J. Appl. Phys. 99, 104903 (2006); http://dx.doi.org/10.1063/1.2197267 (6 pages) | Cited 6 times

Online Publication Date: 25 May 2006

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This paper studies a tungsten cold field emitter throughout its life cycle. The field emission properties and corresponding tungsten emitter evolution throughout its life cycle fall into three distinct regions which are discussed. It was found that adsorbate dynamics affect field emission current stability significantly, while adsorbate induced current instability showed different behaviors in different regions of the life cycle. It was found that emitter failure at the end of the lifetime was due to adsorbate related nanoprotrusion buildup on the emitter surface, a failure mechanism that is different from models previously reported in the literature. The buildup process of this nanoprotrusion and how it initiates arcing and destroys the tungsten tip are reported and discussed.
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79.70.+q Field emission, ionization, evaporation, and desorption

Liquid transport based on electrostatic deformation of fluid interfaces

Januk Aggarwal, Andrzej Kotlicki, Michele Mossman, and Lorne Whitehead

J. Appl. Phys. 99, 104904 (2006); http://dx.doi.org/10.1063/1.2193650 (6 pages) | Cited 1 time

Online Publication Date: 25 May 2006

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We have developed a method for moving liquid along a surface using purely electrostatic effects, without the need for mechanically moving parts. In this approach, liquid drops are confined to specific regions of the substrate by a printed pattern that has the appropriate hydrophilic and hydrophobic wetting characteristics. Using a variation of well-known electrowetting techniques, the shape of the droplet can be changed by applying an electric field that changes the surface energy relationship. Specifically, a bead of oil confined to a hydrophobic region of the surface can be pinched into drops using localized electrostatic fields, and by changing the applied field pattern, these oil drops can be moved to cause net liquid flow. This approach may have useful applications in heat transfer. We have demonstrated that with drops traveling at approximately 15 cm/s, it is possible to transport heat more effectively than by using an equivalent volume of solid copper.
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68.08.Bc Wetting
47.55.D- Drops and bubbles
68.03.Cd Surface tension and related phenomena

Magnetic force microscopy sensors using iron-filled carbon nanotubes

Andreas Winkler, Thomas Mühl, Siegfried Menzel, Radinka Kozhuharova-Koseva, Silke Hampel, Albrecht Leonhardt, and Bernd Büchner

J. Appl. Phys. 99, 104905 (2006); http://dx.doi.org/10.1063/1.2195879 (5 pages) | Cited 50 times

Online Publication Date: 25 May 2006

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Probes for magnetic force microscopy (MFM) were prepared by pinning iron-filled multiwall carbon nanotubes to conventional scanning force microscopy probes. These nanotube MFM probes reveal a great potential for high spatial resolution of both topography and magnetic stray field. The ends of the high aspect ratio iron nanowires within the nanotubes can be considered as stationary effective magnetic monopole moments which opens the possibility of quantitative stray field measurements in a straightforward manner. The carbon shells around the iron nanowires provide wear resistance and oxidation protection.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.55.-w Magnetic instruments and components
07.79.Pk Magnetic force microscopes
07.79.Lh Atomic force microscopes
68.37.Ps Atomic force microscopy (AFM)
06.30.Ka Basic electromagnetic quantities

An x-ray topographic study of diamond anvils: Correlation between defects and helium diffusion

Agnès Dewaele, Paul Loubeyre, Ramesh André, and Jürgen Härtwig

J. Appl. Phys. 99, 104906 (2006); http://dx.doi.org/10.1063/1.2197265 (5 pages) | Cited 3 times

Online Publication Date: 26 May 2006

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X-ray topographic images of several dozens of diamonds have been recorded before and after their use as anvils in a diamond anvil high pressure cell. The aim of this study is to better understand and prevent the breakage of diamond anvils when they are used in contact with a helium sample. Indeed, helium is a good hydrostatic medium for very high pressure studies but many anvils are subjected to helium embrittlement. X-ray topographic observations suggest that helium atoms enter the diamond anvil by a diffusion process enhanced by subsurface strains and/or extended preexisting defects and nonhydrostatic stress. Then these atoms open cracks during nonhydrostatic load of the diamond anvil. The diamond anvils can be classified into three types depending on the nature of the defects. An associated criterion for the selection of stones resistant to helium is proposed.
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81.05.U- Carbon/carbon-based materials
66.30.Lw Diffusion of other defects
68.35.Fx Diffusion; interface formation
68.35.Gy Mechanical properties; surface strains
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials

Reevaluation of the mechanism for ultrananocrystalline diamond deposition from Ar/CH4/H2 gas mixtures

P. W. May, J. N. Harvey, J. A. Smith, and Yu. A. Mankelevich

J. Appl. Phys. 99, 104907 (2006); http://dx.doi.org/10.1063/1.2195347 (11 pages) | Cited 44 times

Online Publication Date: 30 May 2006

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Various mechanisms for the growth and renucleation of ultrananocrystalline diamond (UNCD) films are discussed and evaluated in the light of experimental and theoretical evidences in recent publications. We propose that the most likely model for UNCD growth is that where most of the diamond is formed via a similar mechanism to that of microcrystalline diamond films, i.e., gas phase H atoms abstracting surface hydrogens, followed by a CHx, x = 0–3, addition. Calculations of the gas composition close to the substrate surface in the microwave plasma reactor for both the microcrystalline diamond and the UNCD growth, at substrate temperatures of 1073 and 673 K, suggest that CH3 and C atoms are the most likely precursors for the growth of UNCD. However, the deposition is interrupted by an event which prevents the smooth growth of a continuous layer, and instead creates a surface defect which changes the growth direction and acts as a renucleation site. The possible nature of this event is discussed in detail. Using estimates for reaction rates of various species (including H atoms, Ar* metastables, Ar+ and ArH+ ions) on the diamond surface, a number of mechanisms are discussed and discounted. We propose that the most likely causes for the renucleation required for the UNCD growth are (i) the attachment of C1 species (especially C atoms) followed by local surface restructuring, (ii) the reduction of the efficiency of the β-scission reaction resulting in an increase in the number of long-chained hydrocarbons on the surface, or (iii) a combination of these two processes.
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68.55.A- Nucleation and growth
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.47.Fg Semiconductor surfaces
81.05.Cy Elemental semiconductors
68.35.B- Structure of clean surfaces (and surface reconstruction)

Interpretation of an anomalous peak in low-temperature photoluminescence measurements of bulk GaAs1−xNx on GaAs

W. K. Cheah, W. J. Fan, S. F. Yoon, W. K. Loke, R. Liu, and A. T. S. Wee

J. Appl. Phys. 99, 104908 (2006); http://dx.doi.org/10.1063/1.2199976 (5 pages) | Cited 2 times

Online Publication Date: 30 May 2006

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Low-temperature (10 K) photoluminescence measurements of GaAs1−xNx epitaxial layers grown on GaAs reveal an anomalous second peak in solid-source molecular beam epitaxy. Rapid thermal annealing (RTA) of a specific GaAsN sample reveals a lower energy peak (γ) which redshifts and a higher energy peak (α) which blueshifts under increasing annealing temperature. The band-anticrossing model is used to identify the origins of the two peaks and we propose a model to explain the RTA observations by the concept of increased confinement in areas of higher N concentrations by trapped N localized states. The γ peak is due to the accumulation of N content near the GaAs/GaAsN interface. Hence, this abnormal annealing behavior occurs in layers with nonuniform N concentration at the GaAsN/GaAs interface.
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81.05.Ea III-V semiconductors
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.A- Nucleation and growth
82.80.-d Chemical analysis and related physical methods of analysis

1 Å-resolution chemical imaging by phase contrast technique

C. Iwamoto, S. Statonaka, T. Yamamoto, Y. Ikuhara, and H. Matsuhata

J. Appl. Phys. 99, 104909 (2006); http://dx.doi.org/10.1063/1.2199979 (6 pages)

Online Publication Date: 31 May 2006

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Atomic-resolution high-voltage electron microscopy (ARHVEM) was applied to map chemical compositions of GaN/AlN/AlGaN layers. Image simulation showed that image variation of the GaN was faster than that of the AlN with an increase in the sample thickness. However, at less than approximately 3 nm thickness, images of the GaN and AlN closely resembled their atomic structures simultaneously at an adequate defocus condition. Under this experimental condition, the GaN and AlN layers, both having the same wurtzite structure, were discriminated by contrasts of the cations. Contrasts of the N enabled us to determine thickness variation between the GaN and AlN. ARHVEM observation showed that, although the morphology of the interface between the GaN and AlN seems to be rough, the interface is locally sharp on an atomic scale.
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68.65.Ac Multilayers
68.35.Ct Interface structure and roughness
68.37.-d Microscopy of surfaces, interfaces, and thin films
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