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Top 20 Most Read Articles
June 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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EXPERIMENTAL VERIFICATION OF CALCULATED MATRIX ELEMENTS OF THE MAGNETIC MOMENT OF RUBY Appl. Phys. Lett. 1, 1 (1962); http://dx.doi.org/10.1063/1.1777355 (2 pages) Online Publication Date: 23 December 2004
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GaN/In1−xGaxN/GaN/ZnO nanoarchitecture light emitting diode microarrays Appl. Phys. Lett. 94, 213101 (2009); http://dx.doi.org/10.1063/1.3139865 (3 pages) Online Publication Date: 26 May 2009
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We studied the fabrication and electroluminescent (EL) characteristics of GaN/In1−xGaxN/GaN/ZnO nanoarchitecture light emitting diode (LED) microarrays consisting of position-controlled GaN/ZnO coaxial nanotube heterostructures. For the fabrication of nanoarchitecture LED arrays, n-GaN, GaN/In0.24Ga0.76N multiquantum well (MQW) structures and p-GaN layers were deposited coaxially over the entire surface of position-controlled ZnO nanotube arrays grown vertically on c-plane sapphire substrates. The nanoarchitecture LEDs exhibited strong green and blue emission from the GaN/GaN/In0.24Ga0.76N MQWs at room temperature. Furthermore, the origins of dominant EL peaks are also discussed.
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Surface-emitting quantum cascade lasers with metallic photonic-crystal resonators Appl. Phys. Lett. 94, 221101 (2009); http://dx.doi.org/10.1063/1.3143652 (3 pages) Online Publication Date: 1 June 2009
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Surface emitting photonic-crystal quantum cascade lasers operating at λ ≈ 7.3 μm are demonstrated. The photonic crystal resonator is written solely on the top metallization layer. The mismatch between the modes supported by metallized and nonmetallized regions yields enough optical feedback to achieve laser action. The devices exhibit single-mode emission with a side mode suppression ratio of ≈ 20 dB, the wavelength is lithographically tunable across a range of almost 70 cm−1, and the radiation is emitted from the surface. The maximum operating temperature is 220 K. The divergence of the output beam, which is doughnut-shaped, is approximately 9°.
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Appl. Phys. Lett. 94, 223511 (2009); http://dx.doi.org/10.1063/1.3143621 (3 pages) Online Publication Date: 4 June 2009
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We have fabricated high-performance ultraviolet (UV) detectors with high-quality undoped and B-doped homoepitaxial diamond layers which were sequentially grown on a high-pressure/high-temperature-synthesized (HPHT) type-Ib (100) substrate by means of a high-power microwave-plasma chemical vapor deposition method. The detector performance measured had large quantum efficiencies due to an effective built-in current amplification function, fast temporal responses, and high UV/visible sensing ratios although the HPHT substrate used had considerable amounts of various defects inducing visible light absorptions and slow detector responses. The usefulness of the bilayer detector structure employed is discussed.
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4.3 GHz optical bandwidth light emitting transistor Appl. Phys. Lett. 94, 241101 (2009); http://dx.doi.org/10.1063/1.3153146 (3 pages) Online Publication Date: 15 June 2009
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We demonstrate a quantum-well base heterojunction bipolar light emitting transistor (HBLET) operating in the common collector configuration with a 3 dB optical response bandwidth f3 dB of 4.3 GHz. The HBLET has a current gain, β ( = |ΔIC/ΔIB|) as high as 30, and can be operated as a three-port device to provide simultaneously an optical and electrical output with gain. The f3 dB of 4.3 GHz corresponds to an effective carrier recombination lifetime of 37 ps, and shows that “fast” spontaneous recombination can be harnessed for high-speed modulation.
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Improved bulk heterojunction organic solar cells employing C70 fullerenes Appl. Phys. Lett. 94, 223307 (2009); http://dx.doi.org/10.1063/1.3148664 (3 pages) Online Publication Date: 2 June 2009
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We show that the fullerene C70 is suitable to replace fullerene C60, which is commonly used as electron transporter and acceptor in small-molecule organic solar cells. It is shown that the higher absorption of C70 leads to high external quantum efficiencies of over 50% in the spectral range of 500–700 nm. By optimizing the energy level alignment to hole transport layers, the absorption, and the ratio of C70:zinc phthalocyanine (ZnPc) in a bulk heterojunction solar cell, an efficiency of η = 2.87% is achieved. This is a substantial improvement over an identical solar cell employing C60 having η = 2.27%. The efficiency increase is due to a higher photocurrent, while fill factor and open-circuit voltage for C70 and C60-containing organic solar cells remain comparable.
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Imaging by silicon on insulator waveguides Appl. Phys. Lett. 94, 231101 (2009); http://dx.doi.org/10.1063/1.3141480 (3 pages) Online Publication Date: 8 June 2009
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We present multiphoton imaging based on semiconductor planar waveguide technology which can be used as a transmitter and receiver simultaneously. In particular, silicon on insulator waveguides with p-i-n diode structures are used to demonstrate <5 μm resolution three-photon imaging of Er3+:Y2O3 microparticles by using 1550 nm excitation. Additional theoretical study has been performed to demonstrate the proposed scheme for three-dimensional tomography of micron-sized objects, which could be realized by using multiple transmitter-detector pairs.
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Metal-rich Au-silicide nanoparticles for use in nanotechnology Appl. Phys. Lett. 94, 233101 (2009); http://dx.doi.org/10.1063/1.3148782 (3 pages) Online Publication Date: 8 June 2009
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We present a route to functionalize chemically and magnetically silicon surfaces by a local passivation, taking advantage of Stranski–Krastanov growth mode of the Au–Si(111) system. Metal-rich Au-silicide nanoparticles, supported on a Si-rich two-dimensional Au-silicide layer, are obtained. Subsequently deposited Co is used to form magnetic nanostructures. The two Au silicides display a different chemical reactivity with Co enabling the fabrication of localized magnetic Co nanodots. These magnetic nanostructures can be aligned along step bunches of a vicinal Si(111) surface. By varying the growth parameters, the particle density can be tuned from 109 to the low 1012 dots/in.2.
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Appl. Phys. Lett. 94, 211101 (2009); http://dx.doi.org/10.1063/1.3142421 (3 pages) Online Publication Date: 26 May 2009
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The impact of dielectric intermediate reflectors on the light absorption in the top cell of an a-Si:H/μc-Si:H tandem solar cell comprising randomly textured surfaces was investigated by rigorous diffraction theory. Despite the strong light scattering, we found Fabry–Pérot oscillations for the absorption with a decreasing modulation for an increasing thickness of the intermediate layer, a larger oscillation period when compared to thin films and a homogenization of the absorption profile. Optimized intermediate reflectors generate an absorption enhancement in the a-Si:H film, which varies between a factor of 2 and more than 3 for wavelengths of strong and weak absorption, respectively.
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A highly efficient two level diamond based single photon source Appl. Phys. Lett. 94, 203107 (2009); http://dx.doi.org/10.1063/1.3141450 (3 pages) Online Publication Date: 20 May 2009
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An unexplored diamond defect center that is found to emit stable single photons at a measured rate of 1.6 MHz at room temperature is reported. The center, identified in chemical vapor deposition grown diamond crystals, exhibits a sharp zero phonon line at 734 nm with a full width at half maximum of ∼ 4 nm. The photon statistics confirm that the center is a single emitter and provides direct evidence of a true two level single quantum system in diamond.
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Appl. Phys. Lett. 94, 223504 (2009); http://dx.doi.org/10.1063/1.3148341 (3 pages) Online Publication Date: 1 June 2009
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A metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge triple-junction solar cell is shown to provide current-matching of all three subcells and thus composes a device structure with virtually ideal band gap combination. We demonstrate that the key for the realization of this device is the improvement of material quality of the lattice-mismatched layers as well as the development of a highly relaxed Ga1−yInyAs buffer structure between the Ge substrate and the middle cell. This allows the metamorphic growth with low dislocation densities below 106 cm−2. The performance of the approach has been demonstrated by a conversion efficiency of 41.1% at 454 suns (454 kW/m2, AM1.5d ASTM G173–03).
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Appl. Phys. Lett. 94, 251101 (2009); http://dx.doi.org/10.1063/1.3153276 (3 pages) Online Publication Date: 22 June 2009
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We report experimental demonstration of directional light emission from limaçon-shaped microcavity semiconductor lasers. Quantum cascade lasers (QCLs) emitting at λ ≈ 10 μm are used as a model system. Both ray optics and wave simulations show that for deformations in the range 0.37<ε<0.43, these microcavities support high quality-factor whispering gallerylike modes while having a directional far-field profile with a beam divergence θ∥ ≈ 30° in the plane of the cavity. The measured far-field profiles are in good agreement with simulations. While the measured spectra show a transition from whispering gallerylike modes to a more complex mode structure at higher pumping currents, the far field is insensitive to the pumping current demonstrating the predicted “universal far-field behavior” of this class of chaotic resonators. Due to their relatively high quality factor, our microcavity lasers display reduced threshold current densities compared to conventional ridge lasers with millimeter-long cavities. The performance of the limaçon-shaped QCLs is robust with respect to variations of the deformation near its optimum value of ε = 0.40.
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Efficient semitransparent inverted organic solar cells with indium tin oxide top electrode Appl. Phys. Lett. 94, 243302 (2009); http://dx.doi.org/10.1063/1.3154556 (3 pages) Online Publication Date: 15 June 2009
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We reported on highly efficient semitransparent polymer solar cells comprising a transparent sputtered indium tin oxide (ITO) top electrode. We used an inverted cell structure with titanium dioxide prepared by atomic layer deposition as electron selective layer and molybdenum oxide (MoO3) as hole extraction layer. Moreover, the MoO3 layer prevents damage to the organic active materials due to the ITO sputtering process. For the semitransparent device, power conversion efficiencies of 1.9% were achieved with a high transmittance of 80% in the red region of the visible spectrum.
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Photon management by metallic nanodiscs in thin film solar cells Appl. Phys. Lett. 94, 213102 (2009); http://dx.doi.org/10.1063/1.3141402 (3 pages) Online Publication Date: 26 May 2009
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We investigate the absorption enhancement by metallic nanodiscs in thin-film amorphous silicon solar cells. The effect is quantitatively evaluated by rigorously solving Maxwell’s equations. We show that 50% more photons can be absorbed using geometries accessible for current nanofabrication technologies. Moreover, the thinner the solar cell, the larger the absorption enhancement. Detailed investigations prove that the enhancement can be related to the excitation of localized plasmon polaritons. The simultaneous enhancement in both the near-field amplitude and the scattering cross section at resonance as the leading physical mechanism is discussed in detail.
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Aluminum doped zinc oxide for organic photovoltaics Appl. Phys. Lett. 94, 213301 (2009); http://dx.doi.org/10.1063/1.3142423 (3 pages) Online Publication Date: 29 May 2009
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Aluminum doped zinc oxide (AZO) was grown via magnetron sputtering as a low-cost alternative to indium tin oxide (ITO) for organic photovoltaics (OPVs). Postdeposition ozone treatment resulted in devices with lower series resistance, increased open-circuit voltage, and power conversion efficiency double that of devices fabricated on untreated AZO. Furthermore, cells fabricated using ozone treated AZO and standard ITO displayed comparable performance.
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Appl. Phys. Lett. 94, 231123 (2009); http://dx.doi.org/10.1063/1.3153508 (3 pages) Online Publication Date: 12 June 2009
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The effect of an electron blocking layer (EBL) on the efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes (LEDs) is investigated. At low current density, the LEDs with a p-AlGaN EBL show a higher external quantum efficiency (EQE) than LEDs without an EBL. However, the EQE of LEDs without an EBL is higher than LEDs with an EBL as injection current density is increased. The improved EQE of LEDs without an EBL at high current density is attributed to the increased hole injection efficiency.
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Mapping cavity modes of ZnO nanobelts Appl. Phys. Lett. 94, 231103 (2009); http://dx.doi.org/10.1063/1.3151866 (3 pages) Online Publication Date: 8 June 2009
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ZnO nanostructures attract current interest because they have the potential to implement cavity quantum electrodynamics at room temperature. We report a photoluminescence mapping of ZnO nanobelts both at room temperature and 4.2 K. The multicavity modes were observed all over the belt surface, which were induced by Fabry–Pérot interference. The emission from the belt surface is enhanced at both the ends and the sides of the belt, and is highly linearly polarized in the direction perpendicular to the long axis of the belt. The results are explained using finite-difference time-domain simulations.
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Metal-oxide interfaces at the nanoscale Appl. Phys. Lett. 94, 233115 (2009); http://dx.doi.org/10.1063/1.3154546 (3 pages) Online Publication Date: 10 June 2009
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In contrast to the 6×7 spacing registry that yields a minimum coincidence misfit, we find that the nanoscale Cu2O–Cu interface formed during initial oxidation of Cu(111) surfaces adopts a 5×6 coincidence site lattice that is accommodated by an increased lattice misfit strain. A simple analysis on the equilibrium elastic strain in epitaxial oxide nanoislands reveals a previously unnoticed correlation between the interface structure and surface stresses at the nanoscale.
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High efficiency organic photovoltaic cells based on a vapor deposited squaraine donor Appl. Phys. Lett. 94, 233304 (2009); http://dx.doi.org/10.1063/1.3152011 (3 pages) Online Publication Date: 9 June 2009
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2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) is used as a donor material in vapor deposited organic heterojunction photovoltaic cells. Devices with the structure indium tin oxide/SQ (x)/C60 (400 Å)/bathocuproine (100 Å)/Al (1000 Å), where x = 65, 110, 150, and 200 Å were compared. Devices with x = 65 Å exhibited a power conversion efficiency of 3.1% under 1 sun, AM1.5G simulated solar irradiation, giving an open circuit voltage of 0.76±0.01 V, a short circuit current of 7.01±0.05 mA/cm2, and a fill factor of 0.56±0.05. Thicker SQ films lead to lower short circuit currents and fill factors, giving conversion efficiencies in the range of 2.6% to 3.2%. The demonstration of sublimable SQ as a donor material opens up a family of compounds for use in small molecule based heterojunction photovoltaics.
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Organic/inorganic hybrid solar cells with vertically oriented ZnO nanowires Appl. Phys. Lett. 94, 173107 (2009); http://dx.doi.org/10.1063/1.3126955 (3 pages) Online Publication Date: 28 April 2009
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Hybrid photovoltaic (PV) devices based on copper-phthalocyanine and fullerene combined with ZnO nanowire array as direct charge transport paths were fabricated. The optimized device with ZnO nanowire has a more than fourfold increase in PV performance than the device without nanowire. The former has an open circuit voltage of 0.46 V, a short circuit current of 3.86 mA/cm2, a fill factor of 0.30, and a power conversion efficiency of 0.53%. The hybrid device based on ZnO nanowires with improved PV performance suggests a way for fabrication of PV device with more exciton dissociation interface area and continuous carrier transport paths.
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