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Top 20 Most Read Articles
June 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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Enhanced outcoupling from organic light-emitting diodes using aperiodic dielectric mirrors Appl. Phys. Lett. 90, 241112 (2007); http://dx.doi.org/10.1063/1.2748859 (3 pages) Online Publication Date: 14 June 2007
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Aperiodic dielectric stacks between the substrate and transparent anode in organic light-emitting diodes are used to improve the optical outcoupling efficiency. The authors demonstrate that a nine-layer SiO2/SiNx aperiodic dielectric stack improves the brightness by 80% within a 60° viewing cone for a red-emitting organic light-emitting diode, while maintaining a Lambertian emission pattern. As the refractive index contrast between the two materials used in a two-component multilayer dielectric stack is increased, a brightness improvement of 170% in a 60° viewing cone is achievable while maintaining a Lambertian emission profile.
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Light-emission properties of quantum dots embedded in a photonic double-heterostructure nanocavity Appl. Phys. Lett. 90, 231101 (2007); http://dx.doi.org/10.1063/1.2746059 (3 pages) Online Publication Date: 4 June 2007
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The authors report on the investigation of the light-emission properties of quantum dots in a photonic double-heterostructure nanocavity. The emission spectrum clearly allows the identification of the cavity and waveguide band edge modes. The frequency and polarization characteristics are in good agreement with three-dimensional finite-difference time-domain calculations. Resonant waveguide band edge mode excitation is demonstrated to reduce the background intensity that does not originate from the cavity and consequently cavity mode properties become evident. Finally, the Q value is estimated and discussed.
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40% efficient metamorphic GaInP/GaInAs/Ge multijunction solar cells Appl. Phys. Lett. 90, 183516 (2007); http://dx.doi.org/10.1063/1.2734507 (3 pages) Online Publication Date: 4 May 2007
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An efficiency of 40.7% was measured and independently confirmed for a metamorphic three-junction GaInP/GaInAs/Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0 W/cm2, AM1.5D, low aerosol optical depth, 25 °C). This is the initial demonstration of a solar cell with over 40% efficiency, and is the highest solar conversion efficiency yet achieved for any type of photovoltaic device. Lattice-matched concentrator cells have now reached 40.1% efficiency. Electron-hole recombination mechanisms are analyzed in metamorphic GaxIn1−xAs and GaxIn1−xP materials, and fundamental power losses are quantified to identify paths to still higher efficiencies.
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Towards controlled bottom-up architectures in organic heterostructures Appl. Phys. Lett. 90, 243104 (2007); http://dx.doi.org/10.1063/1.2748211 (3 pages) Online Publication Date: 11 June 2007
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The authors report a study on the growth of organic heterostructures based on di-indenoperylene (DIP) (p-type conduction) deposited on top of F16CuPc (n-type conduction) by real-time x-ray scattering and atomic force microscopy. Above a threshold temperature of 90 °C, the formation of highly crystalline DIP islands takes place via Stranski-Krastanov growth. The temperature dependence of the growth behavior can be exploited to control the resulting morphologies, ranging from well ordered layered heterostructures to the self-organization of highly crystalline DIP islands with tunable size. This self-organization scenario of organic ambipolar nanostructures carries the potential for bottom-up growth of organic architectures for solar cell structures.
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Appl. Phys. Lett. 90, 231106 (2007); http://dx.doi.org/10.1063/1.2746940 (3 pages) Online Publication Date: 4 June 2007
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Large improvement in random lasing action at ultraviolet wavelength has been achieved from highly disordered ZnO films with Ag coating. The lasing threshold can be reduced by two times and slope efficiency can be increased by 5.5 times. The improvement is due to the presence of Ag coating, which enhances the surface coupling of lasing emission from the ZnO films by surface-plasmon resonance and reduces the scattering loss experienced by the random cavity modes. Furthermore, the enhancement of lasing efficiency is dependent on the Ag coating’s surface roughness, which can be controlled through the surface morphology of ZnO films.
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Polarization and plasmon effects in nanowire arrays Appl. Phys. Lett. 90, 223106 (2007); http://dx.doi.org/10.1063/1.2744481 (3 pages) Online Publication Date: 30 May 2007
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The charge distribution and dipole moment in a system of interacting metallic nanowires in external electric field were found and used to calculate the plasmon energies governing optical properties of these nanostructures. The in-line alignment of nanowires caused a redshift of the plasmon peak while their parallel arrangement caused a blueshift. The dependence of these shifts on the separation between nanowires was calculated and compared with the recent experimental data on linear chains of Au nanorods.
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Optofluidic ring resonator based dye laser Appl. Phys. Lett. 90, 221101 (2007); http://dx.doi.org/10.1063/1.2743884 (3 pages) Online Publication Date: 29 May 2007
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The authors demonstrate a microfluidic dye laser using a liquid core optical ring resonator (LCORR). The LCORR is made of a fused silica capillary with a wall thickness of a few microns. The circular cross section of the capillary forms a ring resonator that supports whispering gallery modes (WGMs) and provides an optical feedback for lasers. Due to the high Q factor of the WGM (107), a low lasing threshold is achieved (1 μJ/mm2). In addition, they show that the laser can be coupled out via a tapered fiber in touch with the LCORR, thus providing a mechanism for easy laser delivery.
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Correlation between optoelectronic and structural properties and epilayer thickness of AlN Appl. Phys. Lett. 90, 241101 (2007); http://dx.doi.org/10.1063/1.2747662 (3 pages) Online Publication Date: 11 June 2007
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AlN epilayers were grown by metal organic chemical vapor deposition on sapphire substrates. X-ray diffraction measurements revealed that the threading dislocation (TD) density, in particular, the edge TD density, decreases considerably with increasing the epilayer thickness. Photoluminescence results showed that the intensity ratio of the band edge emission to the defect related emission increases linearly with increasing the epilayer thickness. Moreover, the dark current of the fabricated AlN metal-semiconductor-metal deep ultraviolet (DUV) photodetectors decreases drastically with the AlN epilayer thickness. The results suggested that one effective way for attaining DUV optoelectronic devices with improved performance is to increase the thickness of the AlN epilayer template, which results in the reduction of the TD density.
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Appl. Phys. Lett. 90, 233110 (2007); http://dx.doi.org/10.1063/1.2746946 (3 pages) Online Publication Date: 6 June 2007
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The authors report the fabrication of a p-field effect transistor (FET) and an n-FET with a silicon nanowire channel and doped silicon source and drain regions. The silicon nanowires were synthesized by the vapor-liquid-solid method. For p-FETs the source and drain regions were formed by adding boron doped silicon to the unintentionally doped nanowire body at predefined locations using in situ doped silicon epitaxy. For n-FETs the epitaxial source and drain regions were grown undoped and were later implanted with P and As. The measured Id-Vg characteristics of the devices exhibited unipolar transport, while reference FETs made with nanowires from the same batch but with Schottky (metal) contacts exhibited ambipolar characteristics.
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Fabrication and characterization of an electrically variable, nanospring based interferometer Appl. Phys. Lett. 90, 253101 (2007); http://dx.doi.org/10.1063/1.2749418 (3 pages) Online Publication Date: 18 June 2007
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The authors report on an electrically variable Fabry-Pérot interferometer constructed from the organic material tris(8-hydroxyquinoline) aluminum (Alq3). A two-turn helical Alq3 film with a 300 nm pitch is deposited via glancing angle deposition. Electrostatic compression of the nanosprings between parallel, partially reflective mirrors controls the peak transmission wavelength. Contact mode atomic force microscope measurements using a conductive tip indicate a spring compression of 1.2 nm when a voltage of 6 V is applied across the structure. Spectroscopic measurements show a 1.6 nm shift in the peak transmission wavelength. These results demonstrate precise control of the transmission wavelength of an electrically variable nanometer scaled interferometer.
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Room temperature deposited indium zinc oxide thin film transistors Appl. Phys. Lett. 90, 232103 (2007); http://dx.doi.org/10.1063/1.2746084 (3 pages) Online Publication Date: 4 June 2007
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Depletion-mode indium zinc oxide (IZO) channel thin film transistors were fabricated on glass substrates from layers deposited at room temperature using rf magnetron sputtering. The threshold voltage was in the range from −5.5 to −6.5 V depending on gate dielectric (SiO2) thickness and the drain current on-to-off ratio was ∼ 105. The maximum field effect mobility in the channel was ∼ 4.5 cm2 V−1 s−1, lower than the Hall mobility of ∼ 17 cm2 V−1 s−1 in the same layers, suggesting a strong influence of scattering due to trapped charges at the SiO2-IZO interface. The low deposition and processing temperatures make these devices suitable for applications requiring flexible substrates.
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Room-temperature ferromagnetic and ferroelectric behavior in polycrystalline ZnO-based thin films Appl. Phys. Lett. 90, 222110 (2007); http://dx.doi.org/10.1063/1.2745247 (3 pages) Online Publication Date: 31 May 2007
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Polycrystalline ZnO-based thin films with Li and/or Co doping have been prepared by a sol-gel spin-coating method on silicon substrates. Magnetization measurements reveal that Li-doped ZnO film shows paramagnetic behavior. However, the Co-doped ZnO thin films show obvious room-temperature ferromagnetic properties, and ferromagnetic properties can be enhanced by the Li codoping, which may be ascribed to indirect exchange via Li-related defects. All ZnO-based films exhibit ferroelectric behavior, and ferroelectric properties can be tuned by the dopants.
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Contrast in transmission spectroscopy of a single quantum dot Appl. Phys. Lett. 90, 221106 (2007); http://dx.doi.org/10.1063/1.2743750 (3 pages) Online Publication Date: 30 May 2007
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The authors perform transmission spectroscopy on single quantum dots and examine the effects of a resident carrier’s spin, the incident laser spot size, polarization, and power on the experimental contrast. They demonstrate a factor of 5 improvement in the maximum contrast by using a solid immersion lens to decrease the spot area. This increase yields a maximum signal to noise ratio of ∼ 2000 Hz−1/2, which will allow for megahertz detection frequencies. The authors anticipate that this improvement will allow further investigation of spectral fluctuation and open up the feasibility for an all-optical readout of an electron spin in a quantum dot.
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Spin transport through multilayer graphene Appl. Phys. Lett. 90, 252505 (2007); http://dx.doi.org/10.1063/1.2750397 (3 pages) Online Publication Date: 19 June 2007
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The magnetoresistance (MR) of Co/multilayer graphene/Co spin valve structures has been studied. A positive MR of up to 0.39% at 2 K with switching at magnetic fields that are consistent with the magnetization orientations inferred from anisotropic MR data obtained for the Co contacts has been observed. The positive MR decreases with increasing excitation current and eventually changes sign. The positive MR decreases with increasing temperature, but the negative MR has no temperature dependence.
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Metal-like conductivity in transparent Al:ZnO films Appl. Phys. Lett. 90, 252108 (2007); http://dx.doi.org/10.1063/1.2749836 (3 pages) Online Publication Date: 20 June 2007
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This letter reports on the metal-like conductivity down to 125 K in highly crystalline transparent ( ≥ 85% in the visible region) Al:ZnO films grown on sapphire and glass substrates by pulsed-laser deposition technique. Significantly different surface morphologies were found on both types of films. Temperature dependent resistivity measurements of the films grown at 450 °C on sapphire and glass show metal-like conductivity with electrical resistivity, ∼ 1.77×10−4 and ∼ 3.92×10−4 Ω cm, respectively, at room temperature followed by either a residual conductivity or a metal-semiconductor transition at low temperature due to the localization effect caused by the defects.
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Appl. Phys. Lett. 90, 233504 (2007); http://dx.doi.org/10.1063/1.2746418 (3 pages) Online Publication Date: 5 June 2007
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Blue InGaN/GaN multiple-quantum-well light emitting diodes with a peak emission wavelength of 444 nm were grown on low extended defect density semipolar (10
 ) bulk GaN substrates by conventional metal-organic chemical vapor deposition. The calculated external quantum efficiency and output power at a drive current of 20 mA under pulsed operations (10% duty cycle) were 29% and 16.21 mW, respectively. The device exhibited virtually no peak electroluminescence wavelength shift with increasing drive currents, indicating a significant reduction of polarization-related internal electric fields. |
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Appl. Phys. Lett. 90, 243103 (2007); http://dx.doi.org/10.1063/1.2748096 (3 pages) Online Publication Date: 11 June 2007
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The authors demonstrate the highly reproducible fabrication of n-channel depletion-mode (D-mode) and enhancement-mode (E-mode) field effect transistors (FETs) created from ZnO nanowires (NWs). ZnO NWs were grown by the vapor transport method on two different types of substrates. It was determined that the FETs created from ZnO NWs grown on an Au-coated sapphire substrate exhibited an n-channel D mode, whereas the FETs of ZnO NWs grown on an Au-catalyst-free ZnO film exhibited an n-channel E mode. This controlled fabrication of the two operation modes of ZnO NW-FETs is important for the wide application of NW-FETs in logic circuits.
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Appl. Phys. Lett. 90, 243106 (2007); http://dx.doi.org/10.1063/1.2748333 (3 pages) Online Publication Date: 12 June 2007
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The authors report on the electrical and ultraviolet (UV) photoresponse properties of quasialigned ZnO nanowires (NWs)/p-Si heterojunction grown by a low-temperature solvothermal technique. The current-voltage characteristic of a single ZnO NW/p-Si junction measured by a scanning tunneling microscope shows a rectifying behavior with a rectification ratio IF/IR of 33 at 5 V. The current transport is dominated by the recombination-tunneling mechanism for 0.4 V<V<1.5 V while by the space-charge-limited current conduction beyond 1.5 V. The heterojunction is sensitive to the UV light with the faster rise and decay time constants of 360 and 280 ms, respectively.
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Surface plasmon polariton waveguiding: From multimode stripe to a slot geometry Appl. Phys. Lett. 90, 251104 (2007); http://dx.doi.org/10.1063/1.2749864 (3 pages) Online Publication Date: 19 June 2007
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Surface plasmon polariton (SPP) photonic waveguiding along laterally wide and 20 nm thick gold stripes, at λ = 1.55 μm, exhibited multimodal behavior with some localizations of the field near the stripe’s edges. This was leveraged in a configuration comprised of two adjacent edges, creating slot waveguide geometry. The measured slot mode was predominantly a TM mode—which corresponds to the coupled fields of the two edges as was also confirmed by numerical simulation.
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In situ study of the growth of ZnO nanosheets using environmental scanning electron microscope Appl. Phys. Lett. 90, 233104 (2007); http://dx.doi.org/10.1063/1.2746947 (3 pages) Online Publication Date: 4 June 2007
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The authors demonstrated an investigation of the real-time growth of ZnO nanosheets via in situ observation in an environmental scanning electron microscope. It was found that ZnO nanosheets were grown in a two-stage process: [2
 0]-oriented nanowires were grown first, and then ZnO nanosheets grew perpendicularly on the parent nanowires along the [0001] axis. The authors’ results revealed that the change of the ZnO growth direction from [20] (a axis) to [0001] (c axis) can be achieved by control of the growth temperature. |
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