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Top 20 Most Read Articles
January 2013
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 102, 011102 (2013); http://dx.doi.org/10.1063/1.4755756 (4 pages) Online Publication Date: 2 January 2013
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Graphene recently has been demonstrated to support surface-enhanced Raman scattering. Here, we show that the enhancement of the Raman signal of methylene blue on graphene can be tuned by using either the electric field effect or chemical doping. Both doping experiments show that hole-doped graphene yields a larger enhancement than one which is electron-doped; however, chemical doping leads to a significantly larger modulation of the enhancements. The observed enhancement correlates with the changes in the Fermi level of graphene, indicating that the enhancement is chemical in nature, as electromagnetic enhancement is ruled out by hybrid electrodynamical and quantum mechanical simulations.
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Ultra-thin perfect absorber employing a tunable phase change material Appl. Phys. Lett. 101, 221101 (2012); http://dx.doi.org/10.1063/1.4767646 (5 pages) Online Publication Date: 26 November 2012
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We show that perfect absorption can be achieved in a system comprising a single lossy dielectric layer of thickness much smaller than the incident wavelength on an opaque substrate by utilizing the nontrivial phase shifts at interfaces between lossy media. This design is implemented with an ultra-thin (∼λ/65) vanadium dioxide (VO2) layer on sapphire, temperature tuned in the vicinity of the VO2 insulator-to-metal phase transition, leading to 99.75% absorption at λ = 11.6 μm. The structural simplicity and large tuning range (from ∼80% to 0.25% in reflectivity) are promising for thermal emitters, modulators, and bolometers.
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Three-dimensional micro electromechanical system piezoelectric ultrasound transducer Appl. Phys. Lett. 101, 253101 (2012); http://dx.doi.org/10.1063/1.4772469 (5 pages) Online Publication Date: 17 December 2012
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Here we present the design and experimental acoustic test data for an ultrasound transducer technology based on a combination of micromachined dome-shaped piezoelectric resonators arranged in a flexible architecture. Our high performance niobium-doped lead zirconate titanate film is implemented in three-dimensional dome-shaped structures, which form the basic resonating cells. Adjustable frequency response is realized by mixing these basic cells and modifying their dimensions by lithography. Improved characteristics such as high sensitivity, adjustable wide-bandwidth frequency response, low transmit voltage compatible with ordinary integrated circuitry, low electrical impedance well matched to coaxial cabling, and intrinsic acoustic impedance match to water are demonstrated.
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Continuous-wave terahertz system with a 60 dB dynamic range Appl. Phys. Lett. 86, 204104 (2005); http://dx.doi.org/10.1063/1.1935032 (3 pages) Online Publication Date: 13 May 2005
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We have developed a high-performance continuous-wave terahertz imaging system based on photomixing. The emitter and detector are driven by compact, unstabilized, single-mode diode lasers. The all-optoelectronic, homodyne detection scheme yields both amplitude and phase information, and with careful optimization and matching of both emitter and receiver, a 60 dB dynamic range, at 0.53 THz, can be routinely achieved. This replicates the performance of established pulsed THz imagers at this frequency.
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Novel millimeter‐wave near‐field resistivity microscope Appl. Phys. Lett. 68, 1579 (1996); http://dx.doi.org/10.1063/1.116685 (3 pages)
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We demonstrate a technique for contactless mapping of resistivity or dielectric constant of surfaces and films with a spatial resolution better than 100 μm. This technique may be used for the nondestructive testing of semiconducting wafers, conducting polymers, oxide superconductors, and printed circuits. The principle of operation consists of the scanning of a tiny millimeter‐wave antenna at a very small height above an inhomogeneous conducting surface and measuring the intensity and phase of the reflected (transmitted) wave. We use a specially designed resonant slit antenna and achieve subwavelength spatial resolution of λ/50. © 1996 American Institute of Physics. |
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Quantitative self-calibrating lock-in carrierographic lifetime imaging of silicon wafers Appl. Phys. Lett. 101, 242107 (2012); http://dx.doi.org/10.1063/1.4772207 (4 pages) Online Publication Date: 12 December 2012
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Quantitative self-calibrating lock-in carrierography (LIC) imaging of crystalline silicon wafers is introduced using an InGaAs camera and a spread super-bandgap illumination laser beam. Images at several modulation frequencies and a simplified model based on photocarrier radiometric theory are used to construct the effective carrier lifetime image from the phase-frequency dependence. The phase image data at several frequencies and at selected locations on a wafer were compared to frequency scans obtained with a single-element InGaAs detector, and good agreement was found. The quantitative LIC lifetime imaging capability demonstrated in this work is self-calibrating and eliminates the requirement for calibration in conventional photoluminescence imaging.
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High quality factor single-crystal diamond mechanical resonators Appl. Phys. Lett. 101, 163505 (2012); http://dx.doi.org/10.1063/1.4760274 (4 pages) Online Publication Date: 18 October 2012
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Single-crystal diamond is a promising material for microelectromechanical systems (MEMs) because of its low mechanical loss, compatibility with extreme environments, and built-in interface to high-quality spin centers. But its use has been limited by challenges in processing and growth. We demonstrate a wafer bonding-based technique to form diamond on insulator, from which we make single-crystal diamond micromechanical resonators with mechanical quality factors as high as 338 000 at room temperature. Variable temperature measurements down to 10 K reveal a nonmonotonic dependence of quality factor on temperature. These resonators enable integration of single-crystal diamond into MEMs technology for classical and quantum applications.
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Optical conductivity of highly mismatched GaP alloys Appl. Phys. Lett. 102, 023901 (2013); http://dx.doi.org/10.1063/1.4773526 (4 pages) Online Publication Date: 14 January 2013
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Highly mismatched alloys are promising for applications to intermediate-band (IB) solar cells. Here, we report first-principles prediction of intermediate bands in GaP on the basis of hybrid-density-functional theory, which enables to handle large supercells including defects with much better accuracy than semilocal functionals. Calculated optical conductivity reveals that the intermediate states due to co-doped Mg and O have sufficiently high optical transition probability. The multiple gaps are robust against thermalization. Intermediate-band states become more delocalized by hybridization with phosphorus-vacancy states, increasing the optical transition probability.
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Continuous diffraction patterns from circular arrays of carbon nanotubes Appl. Phys. Lett. 101, 251102 (2012); http://dx.doi.org/10.1063/1.4770503 (4 pages) Online Publication Date: 17 December 2012
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We report the remarkable diffraction effects produced from circular patterned arrays of multiwalled carbon nanotubes (MWCNTs). Highly ordered circular arrays of multiwalled carbon nanotubes (with inter-nanotube spacings of 633 nm) display optical dispersion effects similar to compact discs. These arrays display remarkable diffraction patterns in the far field which are spatially continuous. High quality diffraction patterns were obtained experimentally which are in excellent agreement with the theoretical calculations. The achieved continuous diffraction patterns pave the way towards the utilization of engineered carbon nanotube arrays in applications like three dimensional holograms.
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Increasing recoverable energy storage in electroceramic capacitors using “dead-layer” engineering Appl. Phys. Lett. 101, 242909 (2012); http://dx.doi.org/10.1063/1.4772016 (4 pages) Online Publication Date: 12 December 2012
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The manner in which ultrathin films of alumina, deposited at the dielectric-electrode interface, affect the recoverable energy density associated with (BiFeO3)0.6–(SrTiO3)0.4 (BFST) thin film capacitors has been characterised. Approximately 6 nm of alumina on 400 nm of BFST increases the maximum recoverable energy of the system by around 30% from ∼13 Jcc−1 to ∼17 Jcc−1. Essentially, the alumina acts in the same way as a naturally present parasitic “dead-layer,” distorting the polarisation-field response such that the ultimate polarisation associated with the BFST is pushed to higher values of electric field. The work acts as a proof-of-principle to illustrate how the design of artificial interfacial dielectric “dead-layers” can increase energy densities in simple dielectric capacitors, allowing them to compete more generally with other energy storage technologies.
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Electrical excitation and detection of magnetic dynamics with impedance matching Appl. Phys. Lett. 101, 182402 (2012); http://dx.doi.org/10.1063/1.4764519 (4 pages) Online Publication Date: 2 November 2012
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Motivated by the prospects of increased measurement bandwidth, improved signal to noise ratio, and access to the full complex magnetic susceptibility we develop a technique to extract microwave voltages from our high resistance ( ∼ 10kΩ) (Ga,Mn)As microbars. We drive magnetization precession with microwave frequency current, using a mechanism that relies on the spin orbit interaction. A capacitively coupled λ/2 microstrip resonator is employed as an impedance matching network, enabling us to measure the microwave voltage generated during magnetisation precession.
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Enhanced external quantum efficiency in rectangular single nanowire solar cells Appl. Phys. Lett. 102, 021101 (2013); http://dx.doi.org/10.1063/1.4775578 (4 pages) Online Publication Date: 14 January 2013
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Single-nanowire solar cells (SNSCs) in lying configuration can have external quantum efficiency (EQE) over 100% but always in narrowbands with EQE peaks slightly exceeding unit. We presented a rectangular gallium arsenide (GaAs) SNSC, which provides light absorption efficiency (Qabs) and EQE far beyond 100% for both transverse electric and magnetic illuminations, by optimally engineering the nanowires and introducing an advanced nanoshell design. Electromagnetic and carrier transport calculations show that Qabs and EQE peaks of the designed SNSCs can both be over 200% with averaged EQE ∼ 150% in most of the active spectral band of GaAs.
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Appl. Phys. Lett. 102, 011105 (2013); http://dx.doi.org/10.1063/1.4773558 (4 pages) Online Publication Date: 4 January 2013
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A hole injection layer (HIL) is designed in GaN-based light emitting diodes (LEDs) between multiple quantum wells and p-AlGaN electron blocking layer (EBL). Based on numerical simulation by apsys, the band diagram is adjusted by HIL, leading to the improved hole-injection efficiency. The designed HIL is a p-GaN buffer layer grown at low temperature (LT_pGaN) on last quantum barrier before p-AlGaN EBL. The output power of the fabricated GaN-based LED device with LT_pGaN HIL is enhanced by 128% at 100 A/cm2, while the efficiency droop is reduced by 33% compared to the conventional LED.
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Appl. Phys. Lett. 102, 033901 (2013); http://dx.doi.org/10.1063/1.4758300 (5 pages) Online Publication Date: 22 January 2013
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An approach for an all lattice-matched multijunction solar cell optimized design is presented with 5.807 Å lattice constant, together with a detailed analysis of its performance by means of full device modeling. The simulations show that a (1.93 eV)In0.37Al0.63As/(1.39 eV)In0.38Ga0.62As0.57P0.43/(0.94 eV)In0.38Ga0.62As 3-junction solar cell can achieve efficiencies >51% under 100-suns illumination (with Voc = 3.34 V). As a key proof of concept, an equivalent 3-junction solar cell lattice-matched to InP was fabricated and tested. The independently connected single junction solar cells were also tested in a spectrum splitting configuration, showing similar performance to a monolithic tandem device, with Voc = 1.8 V.
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A laser-driven nanosecond proton source for radiobiological studies Appl. Phys. Lett. 101, 243701 (2012); http://dx.doi.org/10.1063/1.4769372 (4 pages) Online Publication Date: 10 December 2012
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Ion beams are relevant for radiobiological studies and for tumor therapy. In contrast to conventional accelerators, laser-driven ion acceleration offers a potentially more compact and cost-effective means of delivering ions for radiotherapy. Here, we show that by combining advanced acceleration using nanometer thin targets and beam transport, truly nanosecond quasi-monoenergetic proton bunches can be generated with a table-top laser system, delivering single shot doses up to 7 Gy to living cells. Although in their infancy, laser-ion accelerators allow studying fast radiobiological processes as demonstrated here by measurements of the relative biological effectiveness of nanosecond proton bunches in human tumor cells.
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Culturing photosynthetic bacteria through surface plasmon resonance Appl. Phys. Lett. 101, 253701 (2012); http://dx.doi.org/10.1063/1.4771990 (4 pages) Online Publication Date: 17 December 2012
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In this work, cultivation of photosynthetic microbes in surface plasmon enhanced evanescent fields is demonstrated. Proliferation of Synechococcus elongatus was obtained on gold surfaces excited with surface plasmons. Excitation over three days resulted in 10 μm thick biofilms with maximum cell volume density of 20% vol/vol (2% more total accumulation than control experiments with direct light). Collectively, these results indicate the ability to (1) excite surface-bound cells using plasmonic light fields, and (2) subsequently grow thick biofilms by coupling light from the surface. Plasmonic light delivery presents opportunities for high-density optofluidic photobioreactors for microalgal analysis and solar fuel production.
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Appl. Phys. Lett. 102, 023112 (2013); http://dx.doi.org/10.1063/1.4776707 (4 pages) Online Publication Date: 17 January 2013
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A high-quality graphene transparent conductive film was fabricated by roll-to-roll chemical vapor deposition (CVD) synthesis on a suspended copper foil and subsequent transfer. While the high temperature required for the CVD synthesis of high-quality graphene has prevented efficient roll-to-roll production thus far, we used selective Joule heating of the copper foil to achieve this. Low pressure thermal CVD synthesis and a direct roll-to-roll transfer process using photocurable epoxy resin allowed us to fabricate a 100-m-long graphene transparent conductive film with a sheet resistance as low as 150 Ω/sq, which is comparable to that of state-of-the-art CVD-grown graphene films.
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Organic electroluminescent diodes Appl. Phys. Lett. 51, 913 (1987); http://dx.doi.org/10.1063/1.98799 (3 pages)
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A novel electroluminescent device is constructed using organic materials as the emitting elements. The diode has a double‐layer structure of organic thin films, prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium‐tin‐oxide anode and an alloyed Mg:Ag cathode. Electron‐hole recombination and green electroluminescent emission are confined near the organic interface region. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W), and brightness (>1000 cd/m2) are achievable at a driving voltage below 10 V. |
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Appl. Phys. Lett. 102, 012113 (2013); http://dx.doi.org/10.1063/1.4774400 (4 pages) Online Publication Date: 10 January 2013
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ZnO films deposited by plasma-enhanced atomic layer deposition (PEALD) have been used to investigate resistive memory behavior. The bipolar resistance switching properties were observed in the Al/PEALD-ZnO/Pt devices. The resistance ratio for the high and low resistance states (HRS/LRS) is more than 103, better than ZnO devices deposited by other methods. The dominant conduction mechanisms of HRS and LRS are trap-controlled space charge limited current and Ohmic behavior, respectively. The resistive switching behavior is induced upon the formation/disruption of conducting filaments. This study demonstrated that the PEALD-ZnO films have better properties for the application in 3D resistance random access memory.
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Appl. Phys. Lett. 102, 022908 (2013); http://dx.doi.org/10.1063/1.4776735 (4 pages) Online Publication Date: 16 January 2013
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Ultra-low-k time-dependent dielectric breakdown (TDDB) is one of the most important reliability issues in Cu/low-k technology development due to its weaker intrinsic breakdown strength compared to SiO2 dielectrics. With continuous technology scaling, this problem is further exacerbated for Cu/ultra-low-k interconnects. In this letter, the TDDB degradation behavior of ultra-low-k dielectric in Cu/ultra-low-k interconnects will be investigated by a method consisting of a combination of Raman with Fourier transform infrared vibrational microscopes. In TDDB tests on Cu/low-k interconnect, it was found that intrinsic degradation of the ultra-low-k dielectric would first occur under electrical field stress. Upon further electrical field stress, the ultra-low-k dielectric degradation would be accelerated due to Ta ions migration from the Ta/TaN barrier bi-layer into the ultra-low-k dielectrics. In addition, no out-diffusion of Cu ions was observed in our investigation on Cu/Ta/TaN/SiCOH structures.
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