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Top 20 Most Read Articles
November 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 99, 172103 (2011); http://dx.doi.org/10.1063/1.3656703 (3 pages) Online Publication Date: 26 October 2011
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Two anomalous bias dependent resistive peaks induced by the SrTiO3 structural phase transitions at 55 and 110 K were observed in a LaAlO3/SrTiO3 and Nb:SrTiO3 rectifying junction when the LaAlO3/SrTiO3 was depleted under reverse bias. At these transition temperatures, the barrier between LaAlO3/SrTiO3 and Nb:SrTiO3 showed abrupt changes in the tunneling energy under forward bias. The peak at 110 K was an insulator-metal phase transition while the peak at 55 K was a metal-insulator one. We propose that the phase transitions of the SrTiO3 substrate influence the charge transfer to the LaAlO3/SrTiO3 layer, giving rise to these anomalous resistive peaks.
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Temperature-dependence of the internal efficiency droop in GaN-based diodes Appl. Phys. Lett. 99, 181127 (2011); http://dx.doi.org/10.1063/1.3658031 (3 pages) Online Publication Date: 4 November 2011
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The temperature dependence of the measured internal efficiencies of green and blue emitting InGaN-based diodes is analyzed. With increasing temperature, a strongly decreasing strength of the loss mechanism responsible for droop is found which is in contrast to the usually assumed behavior of Auger losses. However, the experimental observations can be well reproduced assuming density activated defect recombination with a temperature independent recombination time.
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Photo-induced spin filtering in a double quantum dot Appl. Phys. Lett. 99, 192101 (2011); http://dx.doi.org/10.1063/1.3660227 (3 pages) Online Publication Date: 7 November 2011
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We investigate the spin-dependent electron dynamics in a double quantum dot driven by sub-picosecond asymmetric electromagnetic pulses. We show analytically that applying the appropriate pulses, specified here, allows a spin separation on a femtosecond time scale in the sense that states with a desired spin projection are localized mainly on one of the dots. It is shown how to maintain in time this photo-induced spin-dependent filtering.
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Electric control of magnetization relaxation in thin film magnetic insulators Appl. Phys. Lett. 99, 162511 (2011); http://dx.doi.org/10.1063/1.3654148 (3 pages) Online Publication Date: 20 October 2011
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Control of magnetization relaxation in magnetic insulators via interfacial spin scattering is demonstrated. The experiments use nanometer-thick yttrium iron garnet (YIG)/Pt layered structures, with the Pt layer biased by an electric voltage. The bias voltage produces a spin current across the Pt thickness. As this current scatters off the YIG surface, it exerts a torque on the YIG surface spins. This torque can reduce or enhance the damping and thereby decrease or increase the ferromagnetic resonance linewidth of the YIG film, depending on the field/current configuration.
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Two-dimensional electron gas related emissions in ZnMgO/ZnO heterostructures Appl. Phys. Lett. 99, 211906 (2011); http://dx.doi.org/10.1063/1.3662964 (3 pages) Online Publication Date: 22 November 2011
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Radiative recombination of two-dimensional electron gas (2DEG), induced by polarization and validated by Hall effect measurements, is investigated in ZnMgO/ZnO heterostructures grown by metal-organic chemical vapor deposition. The Mg composition, the depth profile distribution of Mg, the residual strain in ZnMgO caplayer, and the thickness of caplayer all significantly influence the 2DEG-related transitions in ZnMgO/ZnO heterostructures. Below or above ZnO donor bound exciton, three additional broad emissions persisting up to 100 K are assigned to the spatially indirect transitions from 2DEG electrons to the photoexcited holes towards the ZnO flat-band region or remaining at the heterointerface.
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Light emission enhancement in blue InGaAlN/InGaN quantum well structures Appl. Phys. Lett. 99, 181101 (2011); http://dx.doi.org/10.1063/1.3657141 (3 pages) Online Publication Date: 31 October 2011
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Optical properties of blue AlInGaN/InGaN quantum well (QW) structures with a quaternary AlInGaN well layer were investigated by using the non-Markovian gain model with many-body effects. The band-gap expression of the AlInGaN materials was determined through a comparison with experimental results. We found that the emission peak can be enhanced by using quaternary AlInGaN well and is sensitive on In composition in the InGaN barrier. For example, the spontaneous emission coefficient for Al0.08In0.22Ga0.67 N/InxGa1−xN QW structures shows a maximum at In composition of 0.13 in the barrier and gradually decreases with increasing In composition. This is attributed to the fact that the quasi-Fermi-level separation linearly decreases with increasing In composition in the barrier due to the decrease in the conduction and valence band offsets. The AlInGaN/InGaN system with zero internal field is found to have smaller emission peak than the AlInGaN/InGaN system with nonzero internal field due to smaller band offsets.
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Appl. Phys. Lett. 99, 201102 (2011); http://dx.doi.org/10.1063/1.3647979 (3 pages) Online Publication Date: 14 November 2011
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To realize nanophotonic devices that operate in both the infrared and visible wavelength ranges on a single wafer, we investigated the optical characteristics of silicon carbide (SiC)-based photonic crystal nanocavities. By fabricating nanocavities with lattice constants ranging from 150 to 600 nm, we experimentally demonstrated resonant wavelengths of individual cavities ranging from 550 to 1450 nm on a single SiC wafer. Furthermore, this ultra-broadband operation reveals the material dispersion of the thin SiC wafer, which is estimated as nSiC = 2.34 + 9.18 × 104/λ2, over the wide range of aforementioned wavelengths.
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Ag-nanowire films coated with ZnO nanoparticles as a transparent electrode for solar cells Appl. Phys. Lett. 99, 183307 (2011); http://dx.doi.org/10.1063/1.3656973 (3 pages) Online Publication Date: 2 November 2011
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We demonstrate that solution-processible silver-nanowire films coated with zinc-oxide-nanoparticles (ZnO-NPs) can be used as transparent electrodes in organic photovoltaic devices. The ZnO-NP coating acts as electron extraction layer and as encapsulating agent, protecting the wires from oxidation and improving their mechanical stability. Scanning photocurrent microscopy showed photocurrent generation to be more efficient at the active material surrounding the wires. Ultra-violet illumination as present in the solar spectrum was found to enhance photocurrent by improving the ZnO in-layer conductivity through the photoconductive effect. Inverted polythiophene:fullerene devices using ZnO-NP coated silver-nanowires or indium-tin-oxide as transparent electrode reached power conversion efficiencies of 2.4%.
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Plasmonic reflection grating back contacts for microcrystalline silicon solar cells Appl. Phys. Lett. 99, 181105 (2011); http://dx.doi.org/10.1063/1.3657513 (3 pages) Online Publication Date: 31 October 2011
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We report on the fabrication and optical simulation of a plasmonic light-trapping concept for microcrystalline silicon solar cells, consisting of silver nanostructures arranged in square lattice at the ZnO:Al/Ag back contact of the solar cell. Those solar cells deposited on this plasmonic reflection grating back contact showed an enhanced spectral response in the wavelengths range from 500 nm to 1000 nm, when comparing to flat solar cells. For a particular period, even an enhancement of the short circuit current density in comparison to the conventional random texture light-trapping concept is obtained. Full three-dimensional electromagnetic simulations are used to explain the working principle of the plasmonic light-trapping concept.
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Appl. Phys. Lett. 99, 173505 (2011); http://dx.doi.org/10.1063/1.3657137 (3 pages) Online Publication Date: 26 October 2011
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A high-aspect-ratio 3D multi-gate n-channel fin-shaped field effect transistor (FinFET) has been integrated with a stressor of a highly tensile nitride film as the initial inter layer dielectric capping layer upon a (110)-orientated silicon-on-insulator wafer. Drastically enhanced electrical performances, such as 190% enhancement of peak channel mobility, 91% of peak transconductance, and 34% of saturation current, etc., are achieved for an NMOS FinFET with a gate length of 90 nm. The Ioff-Ion universal curve also demonstrates an extraordinary drive current gain of 26%. Moreover, the hot carrier injection lifetime can be increased from 7.78 × 102 to 5.26 × 103 year (yr) due to the incorporation of this high-tensile contact etching stop layer and relaxation of the Si crystalline channel layer.
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Unique prospects for graphene-based terahertz modulators Appl. Phys. Lett. 99, 113104 (2011); http://dx.doi.org/10.1063/1.3636435 (3 pages) Online Publication Date: 12 September 2011
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The modulation depth of two-dimensional electron-gas (2DEG) based terahertz (THz) modulators using AlGaAs/GaAs hetero-structures with metal gates is inherently limited to <30%. The metal gate not only attenuates the THz signal but also severely degrades modulation depth. Metal losses can be significantly reduced employing an alternative material with tunable conductivity. Graphene presents a unique solution to this problem due to its symmetric band structure and extraordinarily high hole mobility. In this work, we show that it is possible to achieve a modulation depth of >90% while simultaneously minimizing signal attenuation to <5% by tuning the Fermi level at its Dirac point.
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Near-infrared absorbing semitransparent organic solar cells Appl. Phys. Lett. 99, 193307 (2011); http://dx.doi.org/10.1063/1.3660708 (3 pages) Online Publication Date: 11 November 2011
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We present efficient, semitransparent small molecule organic solar cells. The devices employ an indium tin oxide-free top contact, consisting of thin metal films and an additional organic capping layer for enhanced light in/outcoupling. The solar cell encorporates a bulk heterojunction with the donor material Ph2-benz-bodipy, an infrared absorber. Combination of Ph2-benz-bodipy with C60 as acceptor leads to devices with high open circuit voltages of up to 0.81 V and short circuit current densities of 5-6 mA/cm2, resulting in efficiences of 2.2%-2.5%. At the same time, the devices are highly transparent, with an average transmittance in the visible range (400-750 nm) of up to 47.9%, with peaks at 538 nm of up to 64.2% and an average transmittance in the yellow-green range of up to 61.8%.
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Appl. Phys. Lett. 99, 181116 (2011); http://dx.doi.org/10.1063/1.3658392 (3 pages) Online Publication Date: 3 November 2011
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We demonstrate the surface plasmon (SP) enhanced n-ZnO/AlN/p-GaN light-emitting diodes (LEDs) by inserting the Ag nanoparticles (NPs) between the ZnO and AlN layers. The ultraviolet/violet near band edge emission of the device is significantly enhanced while the green defect-related emission is modestly suppressed compared to the LEDs without Ag NPs. The red-shift of electroluminescence (EL) peak and the reduced photoluminescence decay lifetime of ZnO suggest that the improved EL performance of the device with Ag NPs is attributed to the resonant coupling between excitons in ZnO and localized SPs in Ag NPs.
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Appl. Phys. Lett. 99, 193301 (2011); http://dx.doi.org/10.1063/1.3658875 (3 pages) Online Publication Date: 7 November 2011
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About 67% increase in power conversion efficiency (PCE) of copper phthalocyanine/C60 based organic solar cells was demonstrated by doping 4 wt % iridium(III)bis(3-(2-benzothiazolyl)-7-(diethylamino)-2H-1-benzopyran-2-onato-N’,C4)(acetyl acetonate) (IrC6) into C60 acceptor layer. The raised PCE was proved to result from the efficient photo absorption of IrC6 followed by the energy and electron transfer from IrC6 to C60 due to the matched energy level alignment between these two species. Besides, IrC6 could also increase the exciton dissociation efficiency at the active interface of the solar cells. The more detail improvement mechanisms were also discussed.
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Optical fiber tips functionalized with semiconductor photonic crystal cavities Appl. Phys. Lett. 99, 191102 (2011); http://dx.doi.org/10.1063/1.3660278 (3 pages) Online Publication Date: 7 November 2011
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We demonstrate a simple and rapid epoxy-based method for transferring photonic crystal (PC) cavities to the facets of optical fibers. Passive Si cavities were measured via fiber taper coupling as well as direct transmission from the fiber facet. Active quantum dot containing GaAs cavities showed photoluminescence that was collected both in free space and back through the original fiber. Cavities maintain a high quality factor (2000-4000) in both material systems. This design architecture provides a practical mechanically stable platform for the integration of photonic crystal cavities with macroscale optics and opens the door for innovative research on fiber-coupled cavity devices.
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Optical detection of deep electron traps in poly(p-phenylene vinylene) light-emitting diodes Appl. Phys. Lett. 99, 183305 (2011); http://dx.doi.org/10.1063/1.3656713 (3 pages) Online Publication Date: 1 November 2011
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The trap-limited electron currents in poly(p-phenylene vinylene) (PPV) derivatives can be modeled using a Gaussian trap distribution that is positioned approximately 0.75 eV below the lowest unoccupied molecular orbital (LUMO) of PPV. Photothermal deflection spectroscopy measurements and internal photo-emission spectroscopy measurements confirm the claim of a Gaussian shaped trap distribution centered at 0.75 eV below the LUMO of PPV. Additionally, two PPV derivatives that differ in the number of conformational defects incorporated during synthesis exhibit identical electron trapping behavior, showing that the traps do not originate from extrinsic impurities of the synthesis or defects in the polymer chains.
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Graphene magnet realized by hydrogenated graphene nanopore arrays Appl. Phys. Lett. 99, 183111 (2011); http://dx.doi.org/10.1063/1.3653286 (3 pages) Online Publication Date: 3 November 2011
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The so-called zigzag edge of graphenes theoretically has localized electrons due to the presence of flat energy bands near the Fermi level. The localized electron spins are strongly polarized, resulting in ferromagnetism. We fabricate graphenes with honeycomb-like arrays of hydrogen-terminated and low-defect hexagonal nanopores by a nonlithographic method using nanoporous alumina templates. We report large-magnitude room-temperature ferromagnetism caused by electron spins localizing at the zigzag nanopore edges. This promises to be a realization of rare-element free, controllable, transparent, flexible, and mono-atomic layer magnets and novel spintronic devices.
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Surface effect on resistive switching behaviors of ZnO Appl. Phys. Lett. 99, 192106 (2011); http://dx.doi.org/10.1063/1.3659296 (3 pages) Online Publication Date: 9 November 2011
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The interaction between chemisorbed oxygen adatoms (O2(ad)−) and oxygen vacancies associated with the formation/rupture of conductive filaments dominates the switching yield of ZnO, which is also confirmed by the fact that the reduction of SET/RESET voltage with the temperature. The pronounced surface effect-induced conductivity lowering due to O2(ad)− chemisorption leads to increased resistance of high resistance state (HRS). The current decay of the HRS with increased temperatures/times is owing to the severe O2(ad)− chemisorption as Joule heating is continuously applied. The statistical analysis for over 400 cells provides essential evidence for evaluating the surface effect on resistive switching.
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Appl. Phys. Lett. 99, 181115 (2011); http://dx.doi.org/10.1063/1.3658388 (3 pages) Online Publication Date: 2 November 2011
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GaInN/GaN light-emitting triodes having two anodes for promoting the injection of holes into the active region were fabricated and characterized. It was found that the anode-to-anode bias modulates not only the hole-injection efficiency but also the effective light-emitting area and hence the current density through the active region. As the anode-to-anode bias increases, the efficiency at the same current density increases, whereas the efficiency droop decreases substantially, indicating that the limited hole-injection efficiency is one of the dominant mechanisms responsible for the efficiency droop in GaN-based light-emitting diodes.
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Temperature-dependence of ink transport during thermal dip-pen nanolithography Appl. Phys. Lett. 99, 193101 (2011); http://dx.doi.org/10.1063/1.3657777 (3 pages) Online Publication Date: 7 November 2011
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We investigate the control of tip temperature on feature size during dip-pen nanolithography (DPN) of mercaptohexadecanoic acid (MHA) on Au. Heated atomic force microscopy (AFM) probes operated between 25 °C and 50 °C wrote nanostructures of MHA for various dwell times and tip speeds. The feature size exhibited an exponential dependence on tip temperature with an apparent activation barrier of 165 kJ/mol. Analysis of the ink transfer process shows that, while ∼1/3 of the barrier is from ink dissolution into the meniscus, the rest reflects the barrier to adsorption onto the growing feature, a process that has been ignored in previous DPN models.
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