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Top 20 Most Read Articles
February 2010
The 20 articles with the most full-text downloads during the month, in descending order.
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Why nitrogen cannot lead to p-type conductivity in ZnO Appl. Phys. Lett. 95, 252105 (2009); http://dx.doi.org/10.1063/1.3274043 (3 pages) Online Publication Date: 22 December 2009
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Based on electronic structure and atomic size considerations, nitrogen has been regarded as the most suitable impurity for p-type doping in ZnO. However, numerous experimental efforts by many different groups have not resulted in stable and reproducible p-type material, casting doubt on the efficacy of nitrogen as a shallow acceptor. Based on advanced first-principles calculations we find that nitrogen is actually a deep acceptor, with an exceedingly high ionization energy of 1.3 eV, and hence cannot lead to hole conductivity in ZnO. In light of this result, we reexamine prior experiments on nitrogen doping of ZnO.
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Stabilization effect in ferroelectric materials during aging in ferroelectric state Appl. Phys. Lett. 87, 142903 (2005); http://dx.doi.org/10.1063/1.2084343 (3 pages) Online Publication Date: 29 September 2005
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We report that aging in ferroelectric state results in an increase of reverse transition temperature Tf in both BaTiO3 single crystal and Pb0.84La0.16Ti0.96O3 polycrystal. This indicates that the ferroelectric phase is gradually stabilized during aging below Curie temperature. The evolution of the stabilization with aging time in these two different systems obeys the same kinetic function, but with different relaxation time. This indicates that the stabilization effect of the two systems stems from a common origin. We suggest that the stabilization is due to a short-range ordering of point defects, driven by a symmetry-conforming tendency of point defects.
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Parasitic capacitance effect on programming performance of phase change random access memory devices Appl. Phys. Lett. 96, 043506 (2010); http://dx.doi.org/10.1063/1.3298368 (3 pages) Online Publication Date: 29 January 2010
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Parasitic capacitance has increasing implications on the programming performance of phase change random access memory (PCRAM) devices due to increased scaling and high frequency operation. PCRAM devices with larger parasitic capacitance were found to require higher applied voltage to amorphize due to a larger leakage current. The quenching time is also increased due to a longer voltage fall time during amorphization, resulting in a partially crystallized amorphous state. This partial amorphous state requires a lower applied voltage for crystallization, which means improved crystallization performance at the expense of amorphization. Multilevel devices could be implemented by varying the parasitic capacitance to achieve different amorphous resistance.
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Wavelength-stable cyan and green light emitting diodes on nonpolar m-plane GaN bulk substrates Appl. Phys. Lett. 96, 051101 (2010); http://dx.doi.org/10.1063/1.3299257 (3 pages) Online Publication Date: 1 February 2010
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We report the development of 480 nm cyan and 520 nm green light emitting diodes (LEDs) with a highly stable emission wavelength. The shift is less than 3 nm when the drive current density is changed from 0.1 to 38 A/cm2. LEDs have been obtained in GaInN-based homoepitaxy on nonpolar m-plane GaN bulk substrates. For increasing emission wavelength we find a large number of additional dislocations generated within the quantum wells (2×108 to ∼ 1010 cm2) and a decrease in the electroluminescence efficiency. This suggests that the strain induced generation of defects plays a significant role in the performance limitations.
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Atomically-thin crystalline films and ribbons of bismuth telluride Appl. Phys. Lett. 96, 053107 (2010); http://dx.doi.org/10.1063/1.3280078 (3 pages) Online Publication Date: 1 February 2010
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The authors report on “graphene-like” exfoliation of the large-area crystalline films and ribbons of bismuth telluride with the thicknesses of a few atoms. It is demonstrated that Bi2Te3 crystal can be mechanically separated into its building blocks—Te–Bi–Te–Bi–Te atomic fivefolds—with the thickness of ∼ 1 nm and even further—to subunits with smaller thicknesses. The atomically-thin films can be structured into suspended crystalline ribbons providing quantum confinement in two dimensions. The quasi two-dimensional crystals of bismuth telluride revealed high electrical conductivity and low thermal conductivity. The proposed atomic-layer engineering of bismuth telluride opens up a principally new route for drastic enhancement of the thermoelectric figure of merit.
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Three-dimensional photovoltaics Appl. Phys. Lett. 96, 071902 (2010); http://dx.doi.org/10.1063/1.3308490 (3 pages) Online Publication Date: 16 February 2010
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The concept of three-dimensional (3D) photovoltaics is explored computationally using a genetic algorithm to optimize the energy production in a day for arbitrarily shaped 3D solar cells confined to a given area footprint and total volume. Our simulations demonstrate that the performance of 3D photovoltaic structures scales linearly with height, leading to volumetric energy conversion, and provides power fairly evenly throughout the day. Furthermore, we show that optimal 3D structures are not simple box-like shapes, and that design attributes such as reflectivity could be optimized using three-dimensionality.
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Magnetic logic element based on an S-shaped Permalloy structure Appl. Phys. Lett. 96, 072501 (2010); http://dx.doi.org/10.1063/1.3314301 (3 pages) Online Publication Date: 16 February 2010
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Magnetic devices have shown the potential to be used not only as storage elements but also as nonvolatile and programmable logic devices. We present a magnetic logic device element—the S state element—that consists of a single magnetic layer. Its output can be controlled by orthogonal magnetic inputs. The reconfigurable logic element can be easily integrated with common magnetoresistive device concepts, such as spin valves or magnetic tunnel junctions. Using Permalloy as an example, we demonstrate the feasibility of magnetologic operation through micromagnetic simulations.
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Group velocity dispersion and self phase modulation in silicon nitride waveguides Appl. Phys. Lett. 96, 061101 (2010); http://dx.doi.org/10.1063/1.3299008 (3 pages) Online Publication Date: 8 February 2010
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The group velocity dispersion (GVD) of silicon nitride waveguides, prepared using plasma enhanced chemical vapor deposition, is studied and characterized experimentally in support of nonlinear optics applications. We show that the dispersion may be engineered by varying the geometry of the waveguide and demonstrate measured anomalous GVD values as high as −0.57 ps2/m and normal GVD values as high as 0.86 ps2/m. We also experimentally demonstrate the absence of any observed nonlinear loss at the telecommunications wavelength at peak intensities of up to 12 GW/cm2. Spectral broadening due to self phase modulation in silicon nitride waveguides with a nonlinear parameter of 1.4 W−1/m is also demonstrated.
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Mechanics of hemispherical electronics Appl. Phys. Lett. 95, 181912 (2009); http://dx.doi.org/10.1063/1.3256185 (3 pages) Online Publication Date: 5 November 2009
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A simple analytical model is established for the development of hemisphere electronics, which has many important applications in electronic-eye cameras and related curvilinear systems. The photodetector arrays, made in planar mesh layouts with conventional techniques, are deformed and transferred onto a hemisphere. The model gives accurately the positions of photodetectors on the hemisphere, and has been validated by experiments and finite element analysis. The results also indicate very small residual strains in the photodetectors. The model provides a tool to define a pattern of photodetectors in the planar, as-fabricated layout to yield any desired spatial configuration on the hemisphere.
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Single step, complementary doping of graphene Appl. Phys. Lett. 96, 063104 (2010); http://dx.doi.org/10.1063/1.3308482 (3 pages) Online Publication Date: 10 February 2010
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A single-step doping method capable of high resolution n- and p-type doping of large area graphene is presented. Thin films of hydrogen silsesquoxane on exfoliated graphene are used to demonstrate both electron and hole doping through control of the polymer cross-linking process. This dual-doping is attributed to the mismatch in bond strength of the Si–H and Si–O bonds in the film as well as out-gassing of hydrogen with increasing cross-linking. A high-resolution graphene p-n junction is demonstrated using this method.
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Organic light-emitting diodes containing multilayers of organic single crystals Appl. Phys. Lett. 96, 053301 (2010); http://dx.doi.org/10.1063/1.3298558 (3 pages) Online Publication Date: 1 February 2010
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Double-heterostructure (DH) organic light-emitting diodes (OLEDs) with thick carrier transport layers based on organic single crystals have been fabricated. Although the total thickness of the organic layers ( ∼ 1.4 μm) is one order of magnitude greater than that of conventional thin-film OLEDs, a current density of 100 mA/cm2 was achieved at 19 V. The major advantage of the single crystal-based DH-OLED was that a very low applied electric field ( ∼ 104 V/cm) was required for the onset of carrier injection and transport, because of the high carrier mobilities of the single crystal layers.
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Tunable interfacial properties of epitaxial graphene on metal substrates Appl. Phys. Lett. 96, 053109 (2010); http://dx.doi.org/10.1063/1.3309671 (3 pages) Online Publication Date: 3 February 2010
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We report on tuning interfacial properties of epitaxially-grown graphenes with different kinds of metal substrates based on scanning tunneling microscopy experiments and density functional theory calculations. Three kinds of metal substrates, Ni(111), Pt(111), and Ru(0001), show different interactions with the epitaxially grown graphene at the interfaces. The different interfacial interaction making graphene n-type and p-type doped, leads to the polarity change of the thermoelectric property of the graphene/metal systems. These findings may give further insights to the interfacial interactions in the graphene/metal systems and promote the use of graphene-based heterostructures in devices.
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Mobility analysis of highly conducting thin films: Application to ZnO Appl. Phys. Lett. 96, 062102 (2010); http://dx.doi.org/10.1063/1.3310043 (3 pages) Online Publication Date: 8 February 2010
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Hall-effect measurements have been performed on a series of highly conductive thin films of Ga-doped ZnO grown by pulsed laser deposition and annealed in a forming-gas atmosphere (5% H2 in Ar). The mobility as a function of thickness d is analyzed by a simple formula involving only ionized-impurity and boundary scattering and having a single fitting parameter, the acceptor/donor concentration ratio K = NA/ND. For samples with d = 3–100 nm, Kavg = 0.41, giving ND = 4.7×1020 and NA = 1.9×1020 cm−3. Thicker samples require a two-layer formulation due to inhomogeneous annealing.
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Surface plasmon enhanced band edge luminescence of ZnO nanorods by capping Au nanoparticles Appl. Phys. Lett. 96, 071107 (2010); http://dx.doi.org/10.1063/1.3323091 (3 pages) Online Publication Date: 17 February 2010
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The author observe sixfold enhancement in the near band gap emission of ZnO nanorods by employing surface plasmon of Au nanoparticles, while the defect-related emission is completely suppressed. Time-resolved photoluminescence indicates that the decay process becomes much faster by Au capping. The remarkable enhancement of the ultraviolet emission intensities and transition rates is ascribed to the charge transfer and efficient coupling between ZnO nanorods and Au surface plasmons. The suppression of the green emission might be due to a combined effect of Au surface plasmon and passivation of the ZnO nanorod surface traps.
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Appl. Phys. Lett. 96, 051113 (2010); http://dx.doi.org/10.1063/1.3302458 (3 pages) Online Publication Date: 3 February 2010
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We report on Mg doping in the barrier layers of InGaN/GaN multiple quantum wells (MQWs) and its effect on the properties of light-emitting diodes (LEDs). Mg doping in the barriers of MQWs enhances photoluminescence intensity, thermal stability, and internal quantum efficiency of LEDs. The light output power of LEDs with Mg-doped MQW barriers is higher by 19% and 27% at 20 and 200 mA, respectively, than that of LEDs with undoped MQW barriers. The improvement in output power is attributed to the enhanced hole injection to well layers in MQWs with Mg-doped barriers.
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Efficiency droop in 245–247 nm AlGaN light-emitting diodes with continuous wave 2 mW output power Appl. Phys. Lett. 96, 061102 (2010); http://dx.doi.org/10.1063/1.3302466 (3 pages) Online Publication Date: 9 February 2010
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We report on 245–247 nm AlGaN-based deep ultraviolet (DUV) light-emitting diodes with continuous wave output power up to 2 mW. DUV diodes with peak emission wavelength of 245 and 247 nm exhibit turn-on voltage less than 10 V. At room temperature and cw operation the maximum external quantum efficiency was close to 0.18%, which is the highest value published to date for devices with peak emission wavelength shorter than 250 nm. A large external efficiency droop observed at current densities above 100 A/cm2 is attributed to self-heating, carrier spillover from the QWs into the barrier layers or the p-type cladding layer, and/or Auger recombination. A semiempirical equation was proposed to describe the efficiency droop in DUV diodes at a high current injection.
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Appl. Phys. Lett. 96, 051109 (2010); http://dx.doi.org/10.1063/1.3304004 (3 pages) Online Publication Date: 2 February 2010
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Periodic triangle pyramidal array patterned sapphire substrates (PSSs) with various slanted angles were fabricated by wet etching. It was found beside normal wurtzite GaN, zinc blende GaN was found on the sidewall surfaces of PSS. The crystal quality and performance of PSS-LEDs improved with decrease in slanted angle from 57.4° to 31.6°. This is because most of the growth of GaN was initiated from c-planes. As the growth time increased, GaN epilayers on the bottom c-plane covered these pyramids by lateral growth causing the threading dislocation to bend toward the pyramids.
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Appl. Phys. Lett. 96, 053102 (2010); http://dx.doi.org/10.1063/1.3299269 (3 pages) Online Publication Date: 1 February 2010
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We demonstrated a simple and low-cost fabrication of ZnO p-n homojunction. The junction consists of n-type ZnO nanowires array by a hydrothermal method covered with p-type Al, N co-doped ZnO film by a sol-gel method. The junction exhibits good rectification characteristics, with reverse leakage current and rectification ratio of ∼ 5 μA and ∼ 150 at bias of 3 V, respectively. The junction is operated as a photodetector when light radiation is shined on the glass-side of the device. The photodetector shows a peak responsivity at 384 nm with UV-visible responsivity ratio (R384 nm/R550 nm) of ∼ 70 at an operating bias of −3 V.
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Self-assembled templating for the growth of molecular nanodots Appl. Phys. Lett. 96, 063101 (2010); http://dx.doi.org/10.1063/1.3308512 (3 pages) Online Publication Date: 8 February 2010
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A checkerboard pattern of α-sexithiophene (α-6T) with an approximate lattice size of 2.6×3.3 nm2 has been achieved on a Ag(110) surface at monolayer coverage, which is formed through a lateral ordering of two oriented molecules and vacancy defects. We find that this self-assembled pattern provides a template structure for the formation of molecular nanodots and nanocolumns. Second and third-layer islands of α-6T are formed within the checkerboard lattice, leading to a uniform molecular nanodot array, and a nanosized columnar structure is expected if growth is continued into the multilayer regime.
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How many surface plasmons are locally excited on the ridges of metallic lamellar gratings? Appl. Phys. Lett. 96, 051115 (2010); http://dx.doi.org/10.1063/1.3304021 (3 pages) Online Publication Date: 3 February 2010
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In contrast to earlier classical studies that analyze the surface Bloch modes supported by metallic gratings composed of slits as collective surface-plasmon-polaritons (SPPs) resonances (or poles) of the entire periodic problem, we study the normalized rate of SPPs that are locally launched on every individual ridge of metallic lamellar gratings. With this “microscopic” description at the unit-cell level, we further explain how these individual SPPs constructively interfere to build up the classical collective resonances. The approach, which combines analytical treatments and fully-vectorial computations, shines new light on an important and classical phenomenon of grating diffraction.
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