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Top 20 Most Read Articles
December 2010
The 20 articles with the most full-text downloads during the month, in descending order.
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Pulsed laser deposition of SrTiO3/LaGaO3 and SrTiO3/LaAlO3: Plasma plume effects Appl. Phys. Lett. 97, 252105 (2010); http://dx.doi.org/10.1063/1.3529487 (3 pages) Online Publication Date: 20 December 2010
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The pulsed laser deposition of SrTiO3/LaGaO3 and SrTiO3/LaAlO3 interfaces is analyzed with a focus on the kinetic energy of the ablated species. LaGaO3 and LaAlO3 plasma plumes were studied by fast photography and space-resolved optical emission spectroscopy. Reflection high energy electron diffraction was performed proving a layer-by-layer growth up to 10−1 mbar oxygen pressure. The role of the energetic plasma plume on the two-dimensional growth and on the presence of interfacial defects at different oxygen growth pressures is discussed in connection with the conducting properties of the polar/nonpolar interfaces.
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Color display using micromechanically coupled mirrors Appl. Phys. Lett. 97, 251105 (2010); http://dx.doi.org/10.1063/1.3529468 (3 pages) Online Publication Date: 20 December 2010
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We present interferometric modulators, reproducing green, blue, and red colors using selective actuation of mechanically coupled mirror arrays having an identical air gap for simple fabrication process. The present interferometric modulators reproduce green, blue, and red colors at switching modes of (000), (010), and (101), where the spectrum peaks for the colors are measured at wavelengths of 511±5, 478±3, and 644±9 nm with maximum intensities of 77±5%, 73±2%, and 81±5%, respectively. Therefore, we experimentally demonstrated the color reproduction capability of the present interferometric modulators for display applications.
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Carrier lifetime in erbium-doped GaN waveguide emitting in 1540 nm wavelength Appl. Phys. Lett. 97, 241105 (2010); http://dx.doi.org/10.1063/1.3527089 (3 pages) Online Publication Date: 16 December 2010
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We report the characteristics of an erbium-doped GaN semiconductor waveguide amplifier grown by metal-organic chemical vapor deposition. We demonstrated that both 980 and 1480 nm optical pumping were efficient to create population inversion between the 4I13/2 and 4I15/2 energy levels. The carrier lifetime in the 4I13/2 energy band was measured to be approximately 1.5 ms in room temperature, which is slightly shorter than that in erbium-doped silica due to the interaction between the erbium ions and the semiconductor lattice structure. But it is significantly longer than the carrier lifetime in a typical semiconductor optical amplifier which is in the nanosecond regime.
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Band alignment of atomic layer deposited HfO2 on clean and N passivated germanium surfaces Appl. Phys. Lett. 97, 242108 (2010); http://dx.doi.org/10.1063/1.3524262 (3 pages) Online Publication Date: 15 December 2010
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Hard x-ray photoelectron spectroscopy has been used to study the band alignment between atomic layer deposited HfO2 on clean Ge (100) and nitrogen treated Ge (100) surfaces. The position of the valence-band maximum was determined by convolving theoretically calculated density of states from first-principles calculations and comparing with experimental valence-band data. Using Kraut’s method, the valence-band offsets were found to be 3.2±0.1 and 3.3±0.1 eV for the samples grown on clean and N passivated Ge, respectively. The oxide charge measured from capacitance-voltage measurements shows a significant increase between the two samples; however, the small change in the band offset between the two systems strongly indicates negligible contribution of the interface to the conduction/valence-band barrier and the band alignment of the heterojunctions.
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Using competitive protein adsorption to measure fibrinogen in undiluted human serum Appl. Phys. Lett. 97, 253701 (2010); http://dx.doi.org/10.1063/1.3529445 (3 pages) Online Publication Date: 20 December 2010
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We report a unique sensing mechanism based on competitive protein adsorption to measure fibrinogen, a cardiovascular biomarker, in undiluted human serum. The method uses physical adsorption of proteins to a surface rather than complex and time-consuming immobilization procedures. Two fibrinogen concentrations were differentiated in spiked in human serum [3.0 mg/ml (normal concentration) versus 3.2 mg/ml (abnormal concentration with heart disease)]. Real-time surface plasmon resonance signals were monitored as fibrinogen displaced a preadsorbed protein, IgM, on a hydrophobic gold surface. The relatively strong-affinity protein, IgM, was displaced primarily by fibrinogen and much less by other proteins in human serum.
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Extraction of contact resistance in carbon nanofiber via interconnects with varying lengths Appl. Phys. Lett. 97, 253109 (2010); http://dx.doi.org/10.1063/1.3527927 (3 pages) Online Publication Date: 20 December 2010
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A method to extract the contact resistance and bulk resistivity of vertically grown carbon nanofibers (CNFs) or similar one-dimensional nanostructures is described. Using a silicon-compatible process to fabricate a terrace test structure needed for the CNF length variation, the contact resistance is extracted by measuring in situ the resistances of individual CNFs with different lengths and within a small range of diameters using a nanoprober inside a scanning electron microscope. Accurate determination of contact resistances for various combinations of catalysts and underlayer metals can lead to eventual optimization of materials’ growth and device fabrication processes for CNF via interconnects.
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Single-pixel coherent diffraction imaging Appl. Phys. Lett. 97, 241101 (2010); http://dx.doi.org/10.1063/1.3525583 (3 pages) Online Publication Date: 13 December 2010
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We demonstrate single-pixel coherent diffraction imaging, whereby broadband terahertz waveforms passed through a slanted phase retarder (SPR), diffracted from an object, were measured by a terahertz detector located in the far field. For one dimensional imaging, the fixed-location single-pixel broadband detector simultaneously measured all the spatial frequency components of the object because the frequency components of the source maintain a one-to-one correspondence with the object’s spatial frequency. For two dimensional imaging, the angular position of the SPR enabled the diffracted terahertz wave to carry an angular projection image of the object.
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Passivated TiN nanocrystals/SiN trapping layer for enhanced erasing in nonvolatile memory Appl. Phys. Lett. 97, 152112 (2010); http://dx.doi.org/10.1063/1.3501129 (3 pages) Online Publication Date: 14 October 2010
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We present chemical vapor deposition of titanium nitride nanocrystals (ncs) on silicon nitride (SiN). Ncs are passivated in situ by a silicon shell and encapsulated in SiN. High density (3×1012 cm−2), crystalline and isolated ncs are observed by transmission electron microscopy and characterized by x-ray photoelectron spectroscopy. TiN ncs/SiN are integrated as charge trapping layer in a nonvolatile memory. Devices show large memory window (10 V) and fast erasing compared to devices using pure SiN trapping layer, explained by enhanced electrical field in SiN. Acceptable reliability in terms of cycling and data retention is also demonstrated.
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Appl. Phys. Lett. 97, 252101 (2010); http://dx.doi.org/10.1063/1.3525588 (3 pages) Online Publication Date: 20 December 2010
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We report the observation of the integer quantum Hall states at Landau level fillings of ν = 2, 6, and 10 in a Hall bar device made of a single-layer epitaxial graphene film on the silicon-face of silicon-carbide prepared via argon-assisted graphitization. The two-dimensional electron gas exhibits a low-temperature (at 4 K) carrier mobility of ∼ 14 000 cm2/V s at the electron density of 6.1×1011 cm−2. Furthermore, the sheet resistance obtained from four-probe measurements across the whole area (12×6 mm2) of another specimen grown under similar condition displays roughly uniform values ( ∼ 1600 Ω/square), suggesting that the macroscopic steps and accompanying multilayer graphene domains play a minor role in the low-temperature electronic transport.
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Chemical coating of large-area Au nanoparticle two-dimensional arrays as plasmon-resonant optics Appl. Phys. Lett. 97, 221101 (2010); http://dx.doi.org/10.1063/1.3518469 (3 pages) Online Publication Date: 29 November 2010
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Innovative nanophotonic applications require a technique for generating not a nanometer-scale point but a large-area (mm2−m2) near-field light source. We succeeded in developing a large-area near-field light source that is densely constructed of uniform-size gold nanoparticles (AuNPs) two-dimensionally arrayed with regular interparticle gaps, which has tunable localized surface plasmon resonance bands (600–1100 nm). The near-field excitation properties based on the optical tunability of the AuNP two-dimensional arrays demonstrate that our chemical coating of large-area near-field light sources is widely applicable such as for high-sensitivity optical sensors and high-efficiency solar cells.
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Carbon impurities and the yellow luminescence in GaN Appl. Phys. Lett. 97, 152108 (2010); http://dx.doi.org/10.1063/1.3492841 (3 pages) Online Publication Date: 13 October 2010
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Using hybrid functional calculations we investigate the effects of carbon on the electrical and optical properties of GaN. In contrast to the currently accepted view that C substituting for N (CN) is a shallow acceptor, we find that CN has an ionization energy of 0.90 eV. Our calculated absorption and emission lines also indicate that CN is a likely source for the yellow luminescence that is frequently observed in GaN, solving the longstanding puzzle of the nature of the C-related defect involved in yellow emission. Our results suggest that previous experimental data, analyzed under the assumption that CN acts as a shallow acceptor, should be re-examined.
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Loss mechanisms in hydrazine-processed Cu2ZnSn(Se,S)4 solar cells Appl. Phys. Lett. 97, 233506 (2010); http://dx.doi.org/10.1063/1.3522884 (3 pages) Online Publication Date: 8 December 2010
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We present a device characterization study for hydrazine-processed kesterite Cu2ZnSn(Se,S)4 (CZTSSe) solar cells with a focus on pinpointing the main loss mechanisms limiting device efficiency. Temperature-dependent study and time-resolved photoluminescence spectroscopy on these cells, in comparison to analogous studies on a reference Cu(In,Ga)(Se,S)2 (CIGS) cell, reveal strong recombination loss at the CZTSSe/CdS interface, very low minority-carrier lifetimes, and high series resistance that diverges at low temperature. These findings help identify the key areas for improvement of these CZTSSe cells in the quest for a high-performance indium- and tellurium-free solar cell.
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Catastrophic optical damage at front and rear facets of diode lasers Appl. Phys. Lett. 97, 231101 (2010); http://dx.doi.org/10.1063/1.3524235 (3 pages) Online Publication Date: 6 December 2010
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Single-pulse tests of the catastrophic optical damage (COD) are performed for three batches of diode lasers with different gain-regions. The tests involve in situ inspection of front, rear, and side of the devices by a thermocamera. Devices with an Al-containing gain-region show COD at the front facet, as expected for strong facet heating via surface recombination and reabsorption of laser light. In contrast, Al-free devices with low surface recombination rates tend to fail at the rear facet, pointing to a different heating scenario. The high carrier density at the rear facet favors heating and COD via Auger recombination processes.
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Appl. Phys. Lett. 97, 233101 (2010); http://dx.doi.org/10.1063/1.3523252 (3 pages) Online Publication Date: 6 December 2010
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Using the ab initio pseudopotential density functional method, we investigate the functionalization of halogen molecules into graphene-based nanostructures with zigzag and armchair edges. We find that halogen molecules adsorb through chemisorption on the zigzag edge carbon atoms with a binding energy of ∼ 1–5 eV, in sharp contrast to physisorption on the armchair edge and elsewhere where they adsorb with a binding energy of ∼ 0.07 eV. Our findings would be utilized for an approach to the separation of zigzag graphene nanoribbons with regular edges with the change of the solubility of the functionalized nanoribbons.
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Appl. Phys. Lett. 97, 221102 (2010); http://dx.doi.org/10.1063/1.3521464 (3 pages) Online Publication Date: 29 November 2010
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Three-dimensional direct laser writing is commonly associated with tightly focused femtosecond laser pulses. Although few reports have used continuous-wave lasers instead, it is unclear whether state-of-the-art three-dimensional submicron structures for photonics can be fabricated along these lines. Here, we systematically investigate the underlying mechanisms using a 532 nm continuous-wave laser operating at power levels of only some 10 mW and three different commercially available photoresists. Body-centered cubic woodpile photonic crystals composed of 24 layers with rod spacings as small as 450 nm serve as a demanding benchmark example for illustrating “state-of-the-art.”
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Surface plasmon enhanced UV emission in AlGaN/GaN quantum well Appl. Phys. Lett. 97, 221104 (2010); http://dx.doi.org/10.1063/1.3515419 (3 pages) Online Publication Date: 1 December 2010
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The surface plasmon (SP) energy for resonant enhancement of light has shown to be modified by the epitaxial substrate and the overlying metal thin film. The modification of SP energy in AlGaN/GaN epitaxial layers is studied using spectroscopic ellipsometry for enhanced UV-light emission. Silver induced SP can be extended to the UV wavelength range by increasing the aluminum concentration in AlxGa1−xN epilayer. A threefold increase in the UV-light emission is observed from AlGaN/GaN quantum well due to silver induced SP. Photoluminescence lifetime measurements confirm the resonant plasmon induced increase in Purcell factor as observed from the PL intensity measurements.
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High switching endurance in TaOx memristive devices Appl. Phys. Lett. 97, 232102 (2010); http://dx.doi.org/10.1063/1.3524521 (3 pages) Online Publication Date: 6 December 2010
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We demonstrate over 1×1010 open-loop switching cycles from a simple memristive device stack of Pt/TaOx/Ta. We compare this system to a similar device stack based on titanium oxides to obtain insight into the solid-state thermodynamic and kinetic factors that influence endurance in metal-oxide memristors.
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Nongeminate carrier recombination rates in organic solar cells Appl. Phys. Lett. 97, 233302 (2010); http://dx.doi.org/10.1063/1.3524025 (3 pages) Online Publication Date: 7 December 2010
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Ambipolar organic thin-film transistors and lateral resistor structures have been used to study the transport of charge carriers in bulk heterojunction (BHJ) organic photovoltaic devices. Active layers of a phase-separated blend of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester were chosen due to their wide use in BHJ solar cell devices. A new method for determining nongeminate recombination rates is reported. Field dependent measurements of the recombination rate show that recombination decreases with increasing electric field.
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Light out-coupling enhancement of organic light-emitting devices with microlens array Appl. Phys. Lett. 97, 223303 (2010); http://dx.doi.org/10.1063/1.3521413 (3 pages) Online Publication Date: 1 December 2010
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Light out-coupling efficiency of organic light-emitting devices from high-index glass substrate into air is enhanced by attaching ordered microlens arrays, which are fabricated by a roll-to-roll mold transfer process. The dependence of microlens geometries on light extraction is analyzed experimentally and theoretically. An increase of 60% in the light out-coupling with an optimized elliptical microlens array is achieved over a conventional device without affecting the electroluminescent spectrum.
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Highly conducting zinc oxide thin films achieved without postgrowth annealing Appl. Phys. Lett. 97, 241903 (2010); http://dx.doi.org/10.1063/1.3525575 (3 pages) Online Publication Date: 13 December 2010
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Heavily doped zinc oxide thin films were grown by radio frequency magnetron sputtering. High level of Al doping has been achieved at high growth temperatures (TG), thereby avoiding the need for postgrowth annealing. It is further shown that due to ionized impurity scattering, mobility decreases with increasing TG. Optical transmission spectra show that the bandgap varies from 3.22 to 3.68 eV with increasing Al content due to the combined effect of Burstien–Moss effect and bandgap renormalization.
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