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22 Sep 2008

Volume 93, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 93, 123101 (2008); http://dx.doi.org/10.1063/1.2981705 (3 pages)

Ulas C. Coskun, Henok Mebrahtu, Paul B. Huang, Jeremy Huang, David Sebba, Adriana Biasco, Alex Makarovski, Anne Lazarides, Thom H. LaBean, and Gleb Finkelstein
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Assembly of arbitrary and vertical optical couplers using flexible polymer micro/nanowires

Jinchun Shi, Menglin Guo, and Baojun Li

Appl. Phys. Lett. 93, 121101 (2008); http://dx.doi.org/10.1063/1.2989130 (3 pages) | Cited 3 times

Online Publication Date: 22 September 2008

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Two simple structures, a star topology network and a tree topology network, were assembled for arbitrary and vertical optical coupler applications using flexible polymer micro/nanowires, which were fabricated by a direct drawing technique from polymer solution. Experimental demonstration shows that the loss of the wires is less than 0.11 dB/mm at 650 nm wavelength. The insertion loss of the two simple structures, which were assembled by intersecting two wires, is 0.6–0.86 dB. The insertion losses of the star topology network and the tree topology network, which was assembled by knotting three wires, are 1.14 and 1.31 dB, respectively.
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42.82.Ds Interconnects, including holographic interconnects
42.82.Et Waveguides, couplers, and arrays
42.70.Jk Polymers and organics
42.82.Cr Fabrication techniques; lithography, pattern transfer

An electrodynamically confined single ZnO tetrapod laser

J. P. Mondia, R. Sharma, J. Schäfer, W. Smith, Y. P. Zhao, Z. H. Lu, and L. J. Wang

Appl. Phys. Lett. 93, 121102 (2008); http://dx.doi.org/10.1063/1.2987520 (3 pages) | Cited 7 times

Online Publication Date: 22 September 2008

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We experimentally show the viability of isolating and micropositioning ZnO-based nanostructures in an electrodynamic trap to investigate their intrinsic optical nature under atmospheric conditions. An electrospray technique is used to spray a dilute solution of ZnO tetrapods (in methanol) into an electrodynamic “endcap” trap. Subsequent tuning of trapping parameters, as the methanol evaporates, leads to the stable confinement of a single ZnO tetrapod in free space. Using this method, we trap a single ZnO tetrapod structure, manipulate its position, and study its lasing characteristics.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Electro-optical characteristics of reflective liquid crystal mode using π-cell for low power operation

Jin Seog Gwag, Jaehoon Park, You-Jin Lee, and Jae-Hoon Kim

Appl. Phys. Lett. 93, 121103 (2008); http://dx.doi.org/10.1063/1.2988266 (3 pages) | Cited 3 times

Online Publication Date: 22 September 2008

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We demonstrated a reflective type of liquid crystal (LC) display with low power consumption using 180° twist and bend states in the π-cell. The driving voltage of the proposed operational mode depends on the cell and the LC parameters, such as the pretilt angle, the sample thickness, and the ratio of the sample thickness to the helical pitch of the LC. Under optimized conditions, this mode operates with less than 1.3 V and it exhibits excellent optical characteristics at all operational wavelengths.
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42.79.Kr Display devices, liquid-crystal devices

Nanowire grid polarizer for energy efficient and wide-view liquid crystal displays

Zhibing Ge and Shin-Tson Wu

Appl. Phys. Lett. 93, 121104 (2008); http://dx.doi.org/10.1063/1.2988267 (3 pages) | Cited 10 times

Online Publication Date: 22 September 2008

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We report a liquid crystal display (LCD) using a nanowire grid polarizer (NWGP) to replace the bottom sheet linear polarizer (LP). The top LP and bottom NWGP configurations enable backlight recycling for enhancing optical efficiency while keeping a high contrast ratio and wide viewing angle. The electro-optic performance of this device configuration is studied based on the effective-medium theory and 4×4 matrix method. Results show that this configuration exhibits a 100:1 contrast ratio over 75° viewing cone in a film-compensated multidomain vertical alignment LCD and 10:1 over 65° viewing cone in a fringe-field switching LCD without any compensation film.
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42.79.Kr Display devices, liquid-crystal devices
85.60.Pg Display systems
42.79.Ci Filters, zone plates, and polarizers

A single-pixel terahertz imaging system based on compressed sensing

Wai Lam Chan, Kriti Charan, Dharmpal Takhar, Kevin F. Kelly, Richard G. Baraniuk, and Daniel M. Mittleman

Appl. Phys. Lett. 93, 121105 (2008); http://dx.doi.org/10.1063/1.2989126 (3 pages) | Cited 54 times

Online Publication Date: 23 September 2008

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We describe a terahertz imaging system that uses a single pixel detector in combination with a series of random masks to enable high-speed image acquisition. The image formation is based on the theory of compressed sensing, which permits the reconstruction of a N-by-N pixel image using much fewer than N2 measurements. This approach eliminates the need for raster scanning of the object or the terahertz beam, while maintaining the high sensitivity of a single-element detector. We demonstrate the concept using a pulsed terahertz time-domain system and show the reconstruction of both amplitude and phase-contrast images. The idea of compressed sensing is quite general and could also be implemented with a continuous-wave terahertz source.
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84.40.Ua Telecommunications: signal transmission and processing; communication satellites
07.57.Hm Infrared, submillimeter wave, microwave, and radiowave sources
02.30.Mv Approximations and expansions

GaInSb/AlInSb multi-quantum-wells for mid-infrared lasers

M. Yin, G. R. Nash, S. D. Coomber, L. Buckle, P. J. Carrington, A. Krier, A. Andreev, S. J. B. Przeslak, G. de Valicourt, S. J. Smith, M. T. Emeny, and T. Ashley

Appl. Phys. Lett. 93, 121106 (2008); http://dx.doi.org/10.1063/1.2990224 (3 pages) | Cited 3 times

Online Publication Date: 23 September 2008

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Photoluminescence (PL) from GaInSb/AlInSb type I multi-quantum-wells, grown on GaAs, has been investigated as a function of strain in the quantum wells. Luminescence, between 3 and 4 μm, was observed for all samples, with good agreement between the measured and calculated peak emission energies. Analysis of the temperature dependence of the luminescence suggests that population of excited quantum well hole subbands occurs at high temperature, leading to a reduction in the PL signal. Room temperature luminescence was obtained from a sample with ∼ 0.8% strain in the quantum wells. Preliminary results from laser diodes fabricated from companion wafers indicate lasing up to 220 K.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
73.21.Fg Quantum wells

On the efficiency droop in InGaN multiple quantum well blue light emitting diodes and its reduction with p-doped quantum well barriers

Jinqiao Xie, Xianfeng Ni, Qian Fan, Ryoko Shimada, Ümit Özgür, and Hadis Morkoç

Appl. Phys. Lett. 93, 121107 (2008); http://dx.doi.org/10.1063/1.2988324 (3 pages) | Cited 76 times

Online Publication Date: 23 September 2008

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Multiple quantum well (MQW) InGaN light emitting diodes with and without electron blocking layers, with relatively small and large barriers, with and without p-type doping in the MQW region emitting at ∼ 420 nm were used to determine the genesis of efficiency droop observed at injection levels of approximately ≥ 50 A/cm2. Pulsed electroluminescence measurements, to avoid heating effects, revealed that the efficiency peak occurs at ∼ 900 A/cm2 current density for the Mg-doped barrier, near 550 A/cm2 for the lightly doped n-GaN injection layer, meant to bring the electron injection level closer to that of holes, and below 220 A/cm2 for the undoped InGaN barrier cases. For samples with GaN barriers (larger band discontinuity) or without p-AlGaN electron blocking layers the droop occurred at much lower current densities ( ⩽ 110 A/cm2). In contrast, photoluminescence measurements revealed no efficiency droop for optical carrier generation rates corresponding to the maximum current density employed in pulsed injection measurements. All the data are consistent with heavy effective mass of holes, low hole injection efficiency (due to relatively lower p-doping) leading to severe electron leakage being responsible for efficiency droop.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Carrier dynamics in InGaAs with embedded ErAs nanoislands

Abul K. Azad, Rohit P. Prasankumar, Diyar Talbayev, Antoinette J. Taylor, Richard D. Averitt, Joshua M. O. Zide, Hong Lu, Arthur C. Gossard, and John F. O’Hara

Appl. Phys. Lett. 93, 121108 (2008); http://dx.doi.org/10.1063/1.2989127 (3 pages) | Cited 7 times

Online Publication Date: 24 September 2008

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Using time-resolved optical-pump terahertz-probe spectroscopy, we study the ultrafast carrier dynamics in In0.53Ga0.47As:ErAs, a potential candidate for 1550 nm based terahertz photoconductive detectors. Material growth is performed by codepositing ErAs nanoislands with Be-compensated InGaAs on an InP:Fe substrate using molecular beam epitaxy. The material shows a rapid photoconductivity response following optical excitation. Photoexcitation with ∼ 0.5 μJ/cm2 800 nm femtosecond laser pulses yields a 3.2 ps carrier lifetime in optical-pump terahertz-probe experiments. We also measure the carrier lifetime using a 1550 nm femtosecond optical pump-probe system, and it is found to agree well with the terahertz measurements. These short lifetimes demonstrate significant potential for implementing terahertz systems using telecommunication based technologies.
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73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.61.Ey III-V semiconductors
78.70.Gq Microwave and radio-frequency interactions
73.50.Pz Photoconduction and photovoltaic effects
78.47.D- Time resolved spectroscopy (>1 psec)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Ultrafast imaging of transient electronic plasmas produced in conditions of femtosecond waveguide writing in dielectrics

W. Gawelda, D. Puerto, J. Siegel, A. Ferrer, A. Ruiz de la Cruz, H. Fernández, and J. Solis

Appl. Phys. Lett. 93, 121109 (2008); http://dx.doi.org/10.1063/1.2988275 (3 pages) | Cited 18 times

Online Publication Date: 24 September 2008

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Femtosecond laser-induced plasmas in bulk dielectrics are imaged under waveguide writing conditions, for different polarizations, pulse durations, and processing depths, and their temporal evolution is measured using ultrafast pump-probe microscopy. The irradiation beam profile is elliptically shaped yielding a disklike focal volume. We demonstrate for doped phosphate glass that increasing the pulse duration improves the spatial distribution of deposited energy by minimizing beam filamentation and prefocal depletion effects. As a consequence, energy deposition in the desired volume is greatly enhanced. Our results identify key parameters for optimizing femtosecond laser processing of dielectrics and different strategies to minimize energy loss channels.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
42.79.Gn Optical waveguides and couplers
42.70.Ce Glasses, quartz

Silicon nanowire detectors showing phototransistive gain

Arthur Zhang, Sifang You, Cesare Soci, Yisi Liu, Deli Wang, and Yu-Hwa Lo

Appl. Phys. Lett. 93, 121110 (2008); http://dx.doi.org/10.1063/1.2990639 (3 pages) | Cited 15 times

Online Publication Date: 25 September 2008

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Nanowire photodetectors are shown to function as phototransistors with high sensitivity. Due to small lateral dimensions, a nanowire detector can have low dark current while showing large phototransistive gain. Planar and vertical silicon nanowire photodetectors fabricated in a top-down approach using an etching process show a phototransistive gain above 35 000 at low light intensities. Simulations show that incident light can be waveguided into vertical nanowires resulting in up to 40 times greater external quantum efficiency above their physical fill factor. Vertical silicon nanowire phototransistors formed by etching are attractive for low light level detection and for integration with silicon electronics.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.05.Cy Elemental semiconductors
85.60.Dw Photodiodes; phototransistors; photoresistors

Resonantly enhanced reflection of quadrupole polaritons in Cu2O

J. I. Jang, Y. Sun, S. Mani, and J. B. Ketterson

Appl. Phys. Lett. 93, 121111 (2008); http://dx.doi.org/10.1063/1.2990660 (3 pages) | Cited 3 times

Online Publication Date: 25 September 2008

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Using resonant two-photon excitation, we create a coherently propagating polariton wave packet and measure its reflectance (R) and transmittance (T) at the boundary opposite to the incoming surface. We find an enhanced reflection of polaritons from sample surfaces such that the ratio T/R deviates significantly from the present theory. This anomalous boundary effect most likely arises from the quadrupole excitonic component of polaritons.
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71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
71.35.-y Excitons and related phenomena

Dichromatic color tuning with InGaN-based light-emitting diodes

Natalie N. Fellows, Hitoshi Sato, You-da Lin, Roy B. Chung, Steven P. DenBaars, and Shuji Nakamura

Appl. Phys. Lett. 93, 121112 (2008); http://dx.doi.org/10.1063/1.2990761 (3 pages) | Cited 3 times

Online Publication Date: 25 September 2008

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Color tuning GaN based light-emitting diodes (LEDs) both electrically and optically was investigated. Color mixing of two LED dies, one nonpolar (λp = 467 nm) and one semipolar (λp = 574 nm), produced white light. Electrically, the correct current was supplied to each die in order to change its correlated color temperature and Commission Internationale de l’Eclairage (CIE) chromaticity coordinates from 3287 K and (0.425, 0.413) to 7242 K and (0.303, 0.315). The optical polarization anisotropy inherent in nonpolar and semipolar wurtzite GaN allowed color tuning optically with the use of a polarizer. Several regions of the CIE diagram were explored using this method and are discussed.
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85.60.Jb Light-emitting devices
78.66.Fd III-V semiconductors
42.25.Ja Polarization

High-power 1 kHz laser-plasma x-ray source for ultrafast x-ray absorption near-edge spectroscopy in the keV range

F. Dorchies, M. Harmand, D. Descamps, C. Fourment, S. Hulin, S. Petit, O. Peyrusse, and J. J. Santos

Appl. Phys. Lett. 93, 121113 (2008); http://dx.doi.org/10.1063/1.2991293 (3 pages) | Cited 7 times

Online Publication Date: 26 September 2008

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A high average power broadband x-ray source is developed in the multi-keV range, based on the thermal emission of plasmas produced with a 1 kHz fs laser focused on high Z element target. This compact ultrafast x-ray source is used to measure the x-ray absorption near-edge spectroscopy of aluminum K-edge (1.559 keV) with noise lower than 1% of the absorption edge when accumulating laser shots over a few tens of seconds. That demonstrates its suitability to study atomic and electronic structures of matter during ultrafast phase transitions among solid, liquid, or higher energy density states.
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07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.59.-f Intense particle beams and radiation sources
78.47.J- Ultrafast spectroscopy (<1 psec)
78.70.Dm X-ray absorption spectra
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)

Antireflection coatings for two-dimensional photonic crystals using a rigorous impedance definition

Felix J. Lawrence, Lindsay C. Botten, Kokou B. Dossou, and C. Martijn de Sterke

Appl. Phys. Lett. 93, 121114 (2008); http://dx.doi.org/10.1063/1.2992066 (3 pages) | Cited 14 times

Online Publication Date: 26 September 2008

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We show that no consistent scalar definition of impedance is generally possible for photonic crystals. Instead, we present a rigorous semianalytic matrix definition of impedance for square lattice photonic crystals, defined in terms of Bloch modes. We then apply our definition to design a range of multilayer photonic crystal antireflection coatings efficiently.
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42.70.Qs Photonic bandgap materials
42.79.Wc Optical coatings

Ultrafast high-field carrier transport in GaAs measured by femtosecond pump-terahertz probe spectroscopy

Yulei Shi, Qing-li Zhou, Cunlin Zhang, and Bin Jin

Appl. Phys. Lett. 93, 121115 (2008); http://dx.doi.org/10.1063/1.2992067 (3 pages) | Cited 5 times

Online Publication Date: 26 September 2008

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Femtosecond pump-terahertz probe studies of carrier dynamics in semi-insulating GaAs have been investigated in detail for various pump powers and at electric fields up to 15 kV/cm. The pump-induced attenuation of terahertz transmission reduces obviously at high field, and the carrier relaxation time is also found to be correlated with photoinjected carrier density and electric field. These effects can be fully explained in terms of the carrier intervalley scattering and the surface states filling in GaAs, which may influence the carrier recombination process. Moreover, the carrier screening effect at high pump powers is also discussed.
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78.47.J- Ultrafast spectroscopy (<1 psec)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.80.Ey III-V and II-VI semiconductors
73.20.At Surface states, band structure, electron density of states

Laser activation of dopants for nanowire devices on glass and plastic

Nipun Misra, Costas P. Grigoropoulos, David P. Stumbo, and Jeffrey N. Miller

Appl. Phys. Lett. 93, 121116 (2008); http://dx.doi.org/10.1063/1.2988826 (3 pages) | Cited 5 times

Online Publication Date: 26 September 2008

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We report postgrowth doping of silicon nanowires (SiNWs) through ion implantation and subsequent annealing with nanosecond pulsed laser light. The green laser annealing process allows for polarization selective localized heating and enables highly efficient activation of implanted boron and arsenic in the SiNWs as revealed by electrical resistivity measurements. Transistor devices fabricated by this technique show reduced parasitic series resistance and higher drive currents making the process suitable for fabrication of high-performance NW based electronics on glass and plastics.
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42.62.-b Laser applications
61.72.uf Ge and Si
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
73.63.Nm Quantum wires
85.30.Tv Field effect devices

Ultraviolet generation in periodically poled lithium tantalate waveguides

A. C. Busacca, E. D’Asaro, A. Pasquazi, S. Stivala, and G. Assanto

Appl. Phys. Lett. 93, 121117 (2008); http://dx.doi.org/10.1063/1.2992201 (3 pages) | Cited 14 times

Online Publication Date: 26 September 2008

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We demonstrate ultraviolet generation in lithium tantalate channel waveguides for frequency doubling via quasi-phase-matching. The samples, proton exchanged and nanostructured by electric-field assisted surface periodic poling with domains as deep as 40 μm, yield continuous wave light at 365.4 nm with conversion efficiencies larger than 7.5% W−1 cm−2.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.70.Mp Nonlinear optical crystals
42.65.Wi Nonlinear waveguides
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Axis-encircling electron beam generation using a smooth magnetic cusp for gyrodevices

W. He, C. G. Whyte, E. G. Rafferty, A. W. Cross, A. D. R. Phelps, K. Ronald, A. R. Young, C. W. Robertson, D. C. Speirs, and D. H. Rowlands

Appl. Phys. Lett. 93, 121501 (2008); http://dx.doi.org/10.1063/1.2988259 (3 pages) | Cited 11 times

Online Publication Date: 22 September 2008

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The generation of an annular-shaped axis-encircling electron beam using a smooth magnetic cusp was studied through numerical simulations and experiments for harmonic operation of a gyrodevice. Two magnetic coils were used to form a magnetic cusp located just downstream from the velvet cathode of an accelerator diode. An electron beam of current 34 A and voltage 130 kV with an adjustable velocity ratio α up to 1.2 was fully transported to the downstream uniform magnetic field region and used to drive a gyrotron traveling wave amplifier into saturation.
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41.75.Fr Electron and positron beams
84.40.Ik Masers; gyrotrons (cyclotron-resonance masers)
84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)
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Quantitative determination of Ge profiles across SiGe wetting layers on Si (001)

M. Brehm, M. Grydlik, H. Lichtenberger, T. Fromherz, N. Hrauda, W. Jantsch, F. Schäffler, and G. Bauer

Appl. Phys. Lett. 93, 121901 (2008); http://dx.doi.org/10.1063/1.2988261 (3 pages) | Cited 13 times

Online Publication Date: 22 September 2008

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The peak positions in photoluminescence spectra of Ge wetting layers (WL) deposited at 700 °C were measured versus the Ge coverage with an extremely high relative resolution of 0.025 monolayers. A nearly linear redshift of the peaks with increasing Ge coverage is observed. We derived quantitative WL composition profiles by fitting this shift, and its dependence on the deposition temperature of the capping layer (Tc), to results of band structure calculations. Despite the high growth temperature, the Ge content in the WL exceeds 80%. It is shown that the composition profile is dominated by surface segregation of Ge on Si.
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78.55.Ap Elemental semiconductors
71.20.Mq Elemental semiconductors
68.35.Dv Composition, segregation; defects and impurities

Phase stability and elastic modulus of Ti alloys containing Nb, Zr, and/or Sn from first-principles calculations

Qing-Miao Hu, Shu-Jun Li, Yu-Lin Hao, Rui Yang, Börje Johansson, and Levente Vitos

Appl. Phys. Lett. 93, 121902 (2008); http://dx.doi.org/10.1063/1.2988270 (3 pages) | Cited 4 times

Online Publication Date: 22 September 2008

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The alloying effects of Nb, Zr, and/or Sn on the phase stability and elastic properties of Ti are investigated by using a first-principles method. Our calculation results indicate that a carefully designed Ti–Nb–Zr–Sn system can be a good candidate for low modulus biomedical materials. We find that the well-known correlation between the e/a ratio and both elastic and phase stabilities for Ti alloyed with transition metal elements breaks down for the Ti–Sn alloy.
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81.05.Bx Metals, semimetals, and alloys
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli

Molecular beam epitaxial growth of InAsN:Sb for midinfrared Optoelectronics

Q. Zhuang, A. Godenir, A. Krier, G. Tsai, and H. H. Lin

Appl. Phys. Lett. 93, 121903 (2008); http://dx.doi.org/10.1063/1.2988281 (3 pages) | Cited 10 times

Online Publication Date: 22 September 2008

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We report molecular beam epitaxial growth and characterization of dilute nitride InAsN:Sb. X-ray diffraction, energy dispersive x-ray spectrometry, and electron probe microanalysis revealed that nitrogen incorporation is significantly enhanced by introduction of Sb flux during growth, together with a dramatic improvement of the photoluminescence. These observations were attributed to the surfactant effect of Sb which suppresses the surface diffusion length of nitrogen and improves the homogeneity of the alloy. Sb incorporation is enhanced with the presence of nitrogen which was associated with the surface kinetic of growth. InAsN:Sb/InAs p-i-n light emitting diodes operating near 4.0 μm were also realized.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.55.Cr III-V semiconductors
68.55.A- Nucleation and growth
68.35.Fx Diffusion; interface formation
85.60.Jb Light-emitting devices

Plasmonic nanoparticle enhanced light absorption in GaAs solar cells

Keisuke Nakayama, Katsuaki Tanabe, and Harry A. Atwater

Appl. Phys. Lett. 93, 121904 (2008); http://dx.doi.org/10.1063/1.2988288 (3 pages) | Cited 154 times

Online Publication Date: 22 September 2008

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We demonstrate an improvement in efficiency of optically thin GaAs solar cells decorated with size-controlled Ag nanoparticles fabricated by masked deposition through anodic aluminum oxide templates. The strong scattering by the interacting surface plasmons in densely formed high aspect-ratio nanoparticles effectively increases the optical path of the incident light in the absorber layers resulting in an 8% increase in the short circuit current density of the cell. The nanoparticle array sheet conductivity also reduces the cell surface sheet resistance evidenced by an improved fill factor. This dual function of plasmonic nanoparticles has potential to enable thinner photovoltaic layers in solar cells.
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84.60.Jt Photoelectric conversion
81.16.-c Methods of micro- and nanofabrication and processing
73.25.+i Surface conductivity and carrier phenomena
73.22.Lp Collective excitations

Liquid Al80Cu20: Atomic diffusion and viscosity

J. Brillo, S. M. Chathoth, M. M. Koza, and A. Meyer

Appl. Phys. Lett. 93, 121905 (2008); http://dx.doi.org/10.1063/1.2977863 (3 pages) | Cited 6 times

Online Publication Date: 22 September 2008

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Properties of mass transport in liquid Al80Cu20 were measured over a broad temperature range of more than 500 K by means of oscillating cup viscometry and quasielastic neutron scattering. The shear viscosity and the coefficient of the Cu self-diffusion exhibit an Arrhenius-type temperature dependence. The activation energy for the viscous flow is 2.4 times smaller than that of the Cu self-diffusion. Below 1400 K, the Cu self-diffusion becomes increasingly smaller than expected from the viscosity data rescaled via the Stokes–Einstein relation.
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61.25.Mv Liquid metals and alloys
66.10.cg Mass diffusion, including self-diffusion, mutual diffusion, tracer diffusion, etc.
66.20.Ej Studies of viscosity and rheological properties of specific liquids

Photoluminescence enhancement in thin films of PbSe nanocrystals

C. G. Christova, J. W. Stouwdam, T. J. Eijkemans, A. Yu. Silov, R. W. van der Heijden, M. Kemerink, R. A. J. Janssen, and H. W. M. Salemink

Appl. Phys. Lett. 93, 121906 (2008); http://dx.doi.org/10.1063/1.2989131 (3 pages) | Cited 2 times

Online Publication Date: 23 September 2008

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Remarkable photoluminescence enhancement (PLE) in submonolayer films of PbSe nanocrystals (NCs) upon continuous illumination was observed. The intensity increase from films on InP substrates was highest in vacuum, while for films on Si/SiO2 substrates the PLE was stronger in air. The magnitude of the PLE was found to depend on the excitation intensity, being higher for a weaker irradiation power. The possible mechanisms behind the phenomenon of the PLE are discussed and it is suggested to originate mainly from charge trapping outside the NCs core.
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78.55.Hx Other solid inorganic materials
78.66.Li Other semiconductors
81.16.-c Methods of micro- and nanofabrication and processing
61.46.Hk Nanocrystals
68.55.-a Thin film structure and morphology

Photoexcitation screening of the built-in electric field in ZnO single quantum wells

T. Makino, Y. Segawa, A. Tsukazaki, A. Ohtomo, and M. Kawasaki

Appl. Phys. Lett. 93, 121907 (2008); http://dx.doi.org/10.1063/1.2981523 (3 pages) | Cited 12 times

Online Publication Date: 23 September 2008

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ZnO/Mg0.22Zn0.78O quantum wells were studied by excitation-intensity-dependent luminescence at 10 K. The samples were grown by laser molecular-beam epitaxy on ScAlMgO4 substrates to evaluate the well width dependence (1 to 10 nm) of exciton transition energies. Under weak excitation, the photoluminescence shows a quantum-confined Stark effect for the wide wells. The well width dependences of the experimental transition energies are compared with previously reported calculations to evaluate the electric field due to spontaneous and piezoelectric polarizations. The internal electric field is comparable with 650 kV/cm. With an increase in excitation intensity, blueshift of the luminescence was observed, suggesting photoexcitation screening of electric fields.
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78.67.De Quantum wells
71.35.-y Excitons and related phenomena
78.55.Et II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.65.Fg Quantum wells
81.07.St Quantum wells
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