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Applied Physics Letters

Issue 9 • Date Aug 2010

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Displaying Results 1 - 25 of 84
  • A frequency-addressed plasmonic switch based on dual-frequency liquid crystals

    Page(s): 091101 - 091101-3
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    A frequency-addressed plasmonic switch was demonstrated by embedding a uniform gold nanodisk array into dual-frequency liquid crystals (DFLCs). The optical properties of the hybrid system were characterized by extinction spectra of localized surface plasmon resonances (LSPRs). The LSPR peak was tuned using a frequency-dependent electric field. A ∼4 nm blueshift was observed for frequencies below 15 kHz, and a 23 nm redshift was observed for frequencies above 15 kHz. The switching time for the system was ∼40 ms. This DFLC-based active plasmonic system demonstrates an excellent, reversible, frequency-dependent switching behavior and could be used in future integrated nanophotonic circuits. View full abstract»

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  • Surface plasmon enhanced responsivity in a waveguided germanium metal-semiconductor-metal photodetector

    Page(s): 091102 - 091102-3
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    The authors report on high transverse magnetic (TM)-mode responsivity in a waveguided germanium Schottky-barrier metal-semiconductor-metal photodetector on silicon-on-insulator substrate for operating wavelength at 1550 nm. The employed aluminum interdigitated electrodes act as a one-dimensional rectangular grating above the depletion layer. By means of properly designed finger dimensions, surface plasmon polariton resonances can be excited at the interface of metal and silicon interfacial layer due to grating coupling. The resulting strong field intensities reach into active region, enabling high absorption under TM injection. At a voltage of 1 V, the TM-mode photocurrent is measured over three times than that of transverse electric mode, in spite of the relatively larger TM insertion loss in the silicon waveguide. View full abstract»

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  • The effect of mode spacing on the speed of quantum-well microcavity lasers

    Page(s): 091103 - 091103-3
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    Oxide-confined quantum-well microcavity vertical-cavity surface-emitting lasers (VCSELs) of three-diameters (aperture size dA∼2, 2.5, and 3.5 μm) have been fabricated that operate as nearly single-mode lasers at ultralow thresholds ITH=0.15, 0.16, and 0.20 mA. Relative spectral intensities are measured at a set higher bias current I=0.8 mA for the three VCSEL sizes to determine the dependence on mode spacing between the fundamental and second order modes, which at increasing diameter are Δλ∼2.2, 1.6, and 1.0 nm. By studying the side-mode suppression ratio and the optical microwave frequency response of the microcavity VCSELs throughout a spread-out group of modes, we are able to resolve the dependence of signal amplitude and time response on the difference in mode spacing, Δλ, higher speed response occurring at larger Δλ. View full abstract»

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  • Light-hole and heavy-hole transitions for high-temperature long-wavelength infrared detection

    Page(s): 091104 - 091104-3
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    Hole transitions from the heavy-hole (hh) to the light-hole (lh) band contributing to the 4–10 μm response range are reported on p-GaAs/AlGaAs detectors. The detectors show a spectral response up to 16.5 μm, operating up to a temperature of 330 K where the lh-hh response is superimposed on the free-carrier response. Two characteristic peaks observed between 5–7 μm are in good agreement with corresponding energy separations of the lh and hh bands and thus originated from lh-hh transitions. Results will be useful for designing multi-spectral detection which could be realized on a single p-GaAs structure. View full abstract»

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  • Coulomb effect inhibiting spontaneous emission in charged quantum dot

    Page(s): 091105 - 091105-3
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    We investigate the emission dynamics of InAs/GaAs quantum dots (QDs) coupled to an InGaAs quantum well in a tunnel injection scheme by means of time-resolved photoluminescence. Under high-power excitation we observe a redshift in the QD emission of the order of 20 meV. The optical transition intensity shows a complex evolution, where an initial plateau phase is followed by an increase in intensity before a single-exponential decay. We attribute this behavior to the Coulomb interactions between the carriers in a charged QD and corroborate the experimental results with both a rate equation model and self-consistent eight-band kp calculations. View full abstract»

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  • Substrate-induced bianisotropy in metamaterials

    Page(s): 091106 - 091106-3
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    We demonstrate that the presence of a supporting substrate can break the symmetry of a metamaterial structure, changing the symmetry of its effective parameters, and giving rise to bianisotropy. This indicates that magnetoelectric coupling will occur in all metamaterials fabricated on a substrate, including those with symmetric designs. View full abstract»

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  • Phenomenological theory of bistability in polariton diodes

    Page(s): 091107 - 091107-3
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    Polariton diodes have been recently shown to exhibit pronounced bistability induced by the control of the light-matter coupling via an external bias and optical pumping [D. Bajoni, E. Semenova, A. Lemaître, S. Bouchoule, E. Wertz, P. Senellart, S. Barbay, R. Kuszelewicz, and J. Bloch, Phys. Rev. Lett. 101, 266402 (2008)]. In the present paper, we detail the phenomenological theory developed to describe the observed hysteresis cycles and present additional experimental data confirming the validity of the present model. View full abstract»

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  • Transition in electron transport in a cylindrical Hall thruster

    Page(s): 091501 - 091501-3
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    Through the use of high-speed camera and Langmuir probe measurements in a cylindrical Hall thruster, we report the discovery of a rotating spoke of increased plasma density and light emission which correlates with increased electron transport across the magnetic field. As cathode electron emission is increased, a sharp transition occurs where the spoke disappears and electron transport decreases. This suggests that a significant fraction of the electron current might be directed through the spoke. View full abstract»

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  • Electron-beam generation in a wide-aperture open gas discharge: A comparative study for different inert gases

    Page(s): 091502 - 091502-3
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    In the present study, electron-beam generation by open discharges was examined. The study was performed at gas pressures up to 20 Torr, and covered all inert gases. At voltages up to 8 kV, electron-beam currents up to 1600 A with current density ∼130 A/cm2 and a beam generation efficiency in excess of 93% were obtained. The production of electrons from cold cathode was concluded to be of photoemissive nature, enabling the production of high-intensity electron beams in any noble gas or in a mixture of a noble gas with molecular gases irrespective of cathode material. View full abstract»

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  • Structural origin of the high glass-forming ability in Gd doped bulk metallic glasses

    Page(s): 091901 - 091901-3
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    The structural origin of the high glass-forming ability (GFA) in Gd-doped CuZrAl bulk metallic glasses was investigated via synchrotron radiation techniques combined with simulations. It is found that 2 at. % Gd addition increases and stabilizes the solute-centered clusters, which leads to the relatively high atomic- and cluster-level packing efficiency and GFA in Cu46Zr45Al7Gd2 alloy. However, the presence of Al–Gd solute-solute bonding in Cu46Zr42Al7Gd5 decreases the packing efficiency, deteriorating the GFA of this alloy. These findings may extend our understanding on the sensitive dependence of GFA on the concentration of doping atoms in a number of multicomponent bulk metallic glasses. View full abstract»

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  • Accurate ab initio predictions of III–V direct-indirect band gap crossovers

    Page(s): 091902 - 091902-3
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    We report the compositional dependence of the electronic band structure for a range of III–V alloys. Standard density functional theory is insufficient to mimic the electronic gap energies at different symmetry points of the Brillouin zone. The Heyd–Scuseria–Ernzerhof hybrid functional with screened exchange accurately reproduces the experimental band gaps and, more importantly, the alloy concentration of the direct-indirect gap crossovers for the III–V alloys studied here: AlGaAs, InAlAs, AlInP, InGaP, and GaAsP. View full abstract»

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  • Enhancement of optical quality in metamorphic quantum wells using dilute nitride buffers

    Page(s): 091903 - 091903-3
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    Strong enhancement of optical quality in quantum wells by incorporating nitrogen in metamorphic InGaAs buffers grown on GaAs substrates is demonstrated. This has resulted in 3.7 or 5.4 times enhancement of photoluminescence intensity from the metamorphic quantum wells when using dilute nitride superlattice alone or adding nitrogen in a strain compensated GaAs/In0.3Al0.7As superlattice, respectively. This study shows great potentials by incorporating N in metamorphic buffers to further improve the quality of metamorphic optoelectronic devices. View full abstract»

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  • Optical constants of graphene measured by spectroscopic ellipsometry

    Page(s): 091904 - 091904-3
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    A mechanically exfoliated graphene flake (∼150×380 μm2) on a silicon wafer with 98 nm silicon dioxide on top was scanned with a spectroscopic ellipsometer with a focused spot (∼100×55 μm2) at an angle of 55°. The spectroscopic ellipsometric data were analyzed with an optical model in which the optical constants were parameterized by B-splines. This parameterization is the key for the simultaneous accurate determination of the optical constants in the wavelength range 210–1000 nm and the thickness of graphene, which was found to be 3.4 Å. View full abstract»

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  • Dielectric constant model for environmental effects on the exciton energies of single wall carbon nanotubes

    Page(s): 091905 - 091905-3
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    The excitonic optical transition energies Eii of single wall carbon nanotubes, that are modified by surrounding materials around the tubes (known as the environmental effect), can be reproduced by defining a dielectric constant κ which depends on the subband index, nanotube diameter, and exciton size. The environmental effects on excitons can be recognized on a plot of the functional form of κ simply by the different linear slopes obtained for different samples. This treatment should be very useful for calculating Eii for any type of nanotube environment, hence providing an accurate assignment of many nanotube (n,m) chiralities. View full abstract»

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  • Mid- and far-infrared intersubband absorption in quantum dash nanostructures

    Page(s): 091906 - 091906-3
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    The authors calculate and analyze intersubband absorption in InAs/GaAs quantum dash ensemble. The absorption spectrum dominantly occupies the mid- and extends to the far-infrared region. As far as the wetting layer is taken into account, the electron concentration significantly affects the absorption and the spectrum can be tailored by adjusting spacer layers. It is shown that size fluctuation of dashes causes spectrum broadening and its asymmetry, while the increase in the wetting layer thickness leads to a redshift in the spectrum and decrease in the absorption. View full abstract»

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  • Hydrogen related defect complexes in ZnO nanoparticles

    Page(s): 091907 - 091907-3
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    Hydrogen related local vibrational modes (LVMs) of ZnO nanoparticles have been studied using Fourier transform infrared spectroscopy and Raman spectroscopy in as prepared and high temperature annealed samples. The obtained experimental results confirm the presence of cationic vacancies (VZn) in addition to unintentional hydrogen doping and their complex defects such as VZnHi and VZnHO. After high temperature annealing, hydrogen related LVMs and multiphonon modes disappear. The presence of these complex defects determines the nonradiative and multiphonon recombination processes in the band gap of ZnO due to carrier trapping at deep levels. View full abstract»

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  • Micropump based on liquid marbles

    Page(s): 091908 - 091908-2
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    A micropump based on a pair of liquid marbles coated with various powders and connected with a capillary tube is presented. The idea of the micropump is based on the difference of the Laplace pressures in the marbles. The initial stream was supported by the pressure instability developed under water overflow. The reported experiments validate the concept of the effective surface tension of liquid marbles. The micropump could be used for precise delivery of small quantities of liquids, the design of microreactors and microfluidics applications. View full abstract»

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  • Raman scattering determination of the energy difference between Γ and L conduction band minima in Ga1-xInxAsySb1-y

    Page(s): 091909 - 091909-3
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    We report a Raman scattering determination of the energy difference EΓ-L between the Γ conduction-band minimum and the L valley minima in n-type Ga1-xInxAsySb1-y lattice matched to GaSb (x=0.15, y=0.13). A frequency downshift in the L+ phonon–plasmon coupled mode is observed between 80 K and room temperature that is attributed to electron transfer from the Γ to the L valleys. We use the L+ frequency shift to evaluate EΓ-L by performing Lindhard–Mermin L+ line-shape fits for different EΓ-L values. The EΓ-L value increases with electron concentration due to band gap renormalization. A value EΓ-L=154 meV is derived for intrinsic material. View full abstract»

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  • Low temperature deformation of the R-phase in a NiTiFe shape memory alloy

    Page(s): 091910 - 091910-3
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    Deformation in the P3 phase (R-phase) of NiTiFe was investigated by in situ neutron diffraction during compressive loading at cryogenic temperatures. At 216 K, upon loading the R-phase detwinned and subsequently underwent a reversible stress-induced transformation to the B19 phase (martensite). At 92 K on the other hand, detwinning was suppressed and the stress-induced martensite formed did not transform back upon unloading. The experiments also directly observed a hitherto theoretically predicted B33 phase. Rietveld refinement of the neutron diffraction spectra were used to determine lattice parameters of the B33 and R-phases. Plane-specific elastic moduli were also determined for the R-phase. View full abstract»

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  • Annealing induced anomalous electrical transport behavior in SnO2 thin films prepared by pulsed laser deposition

    Page(s): 092101 - 092101-3
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    SnO2 thin films were deposited on quartz substrates by pulsed laser deposition and postannealed at different temperatures in oxygen ambience. X-ray diffraction, Hall measurement, and x-ray photoelectron spectroscopy were employed to investigate the properties of the annealed SnO2 thin films. An anomalous electrical transport behavior as a function of the annealing temperature was observed. Both the growth of the crystal grain and oxygen vacancy density variation in the annealing process have been identified to be responsible for the transition of electrical transport properties. View full abstract»

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  • Electron spin resonance of Zn1-xMgxO thin films grown by plasma-assisted molecular beam epitaxy

    Page(s): 092102 - 092102-3
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    Zn1-xMgxO thin films with a Mg content x between 0 and 0.42 grown by plasma-assisted molecular beam epitaxy on c-plane sapphire substrates were investigated by electron spin resonance at 5 K. Above band gap illumination induces a persistent resonance signal, which is attributed to free conduction band electrons. The g-factors of the Zn1-xMgxO epitaxial layers and their anisotropy were determined experimentally and an increase from g||=1.957 for x=0 to g||=1.970 for x=0.42 was found, accompanied by a decrease in anisotropy. A comparison with g-factors of the AlxGa1-xN system is also given. View full abstract»

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  • Formation of low-resistance Ohmic contacts to N-face n-GaN for high-power GaN-based vertical light-emitting diodes

    Page(s): 092103 - 092103-3
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    We report on the formation of low-resistance Ohmic contacts to N-face n-GaN for high-power vertical light-emitting diodes using an Al–Ga solid solution (50 nm)/Ti(30 nm)/Al(200 nm) scheme and compare them with Ti(30 nm)/Al(200 nm) contacts. The Al–Ga solid solution layer is introduced to minimize the formation of Ga vacancies near the N-face n-GaN surface. It is shown that, unlike the Ti/Al contacts, the Al–Ga solid solution/Ti/Al contacts exhibit Ohmic behavior with a resistivity of 4.1×10-4 Ω cm2, even after annealing at 250 °C. X-ray photoemission spectroscopy and secondary ion mass spectrometry examinations are performed to understand the temperature dependence of the electrical properties. View full abstract»

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  • Room temperature negative differential resistance characteristics of polar III-nitride resonant tunneling diodes

    Page(s): 092104 - 092104-3
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    III-nitride resonant tunneling diodes (RTDs), consisting Al0.2Ga0.8N/GaN double-barrier (DB) active layers, were grown on c-plane lateral epitaxial overgrowth (LEO) GaN/sapphire and c-plane freestanding (FS) GaN. RTDs on both templates, fabricated into mesa diameters ranging from 5 to 35 μm, showed negative differential resistance (NDR) at room temperature. NDR characteristics (voltage and current density at NDR onset and current-peak-to-valley ratio) were analyzed and reported as a function of device size and substrate choice. Our results show that LEO RTDs perform as well as FS ones and DB active layer design and quality have been the bottlenecks in III-nitride RTDs. View full abstract»

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  • Solvent-mediated threshold voltage shift in solution-processed transparent oxide thin-film transistors

    Page(s): 092105 - 092105-3
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    We investigated solvent-mediated threshold voltage (VTH) shift in solution-processed zinc–tin oxide (ZTO) thin film transistors (TFTs). The ZTO TFTs showed negative VTH shift when exposed to various organic solvents such as hexane, isopropanol, and chlorobenzene. Additionally the magnitude of the shift showed a close relationship with the dielectric constant or electronegativity of the solvent molecules. From the experiments, one of the origins of the VTH shift in the transparent oxide TFTs appears to be closely correlated with the dipole interaction of the solvent molecules and ZTO back channel surface. View full abstract»

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  • Abrupt resistivity decrease and other unexpected phenomena in a doped amorphous ternary metal oxide

    Page(s): 092106 - 092106-3
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    The phenomenon of insulator-metal transition introduced in amorphous binary gallium oxide as discussed by Nagarajan etal [Nature Mater. 7, 391 (2008)] is demonstrated to be also achievable in an amorphous ternary metal oxide system. The annealing method used in amorphous binary gallium oxide is not applicable to the ternary system. A different approach, aluminum incorporation in amorphous indium zinc oxide (IZO), is adopted in achieving insulator-metal transition or sharp resistance decrease in the amorphous ternary oxide. In addition to sharp resistance decrease, some other unexpected phenomena, such as different partial crystallization, are also reported. View full abstract»

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Applied Physics Letters, published by the American Institute of Physics, features concise, up-to-date reports on significant new findings in applied physics.

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Nghi Q. Lam
Argonne National Laboratory