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Applied Physics Letters

Issue 23 • Date Jun 2003

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Displaying Results 1 - 25 of 65
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • AlGaInP/mirror/Si light-emitting diodes with vertical electrodes by wafer bonding

    Page(s): 4011 - 4013
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    In a previous study, we reported a highly efficient AlGaInP light-emitting diode (LED) with a Au/AuBe/SiO2/Si mirror substrate (MS) fabricated by wafer bonding, where a planar electrode structure is used. In view of the more efficient epilayer area utilized, AlGaInP/mirror/barrier/Si LEDs with vertical electrodes are proposed in this work. A variety of barrier layers (Pt/Ti, TaN/Ta, and TiN/Ti) have been incorporated into the mirror structure. The mirror quality after bonding is a confirmed key issue in obtaining vertical MS–LEDs with high brightness. It is found that AuBe thickness has a large effect on the final MS–LED performance due to the difference in the interdiffusion of Be atoms in each mirror structure. The diffusion of excess Be atoms diffusing to the mirror side results in a rougher surface and inferior reflectivity. The luminance intensity of an AlGaInP LED chip (626 nm) with an optimum AuBe thickness can reach a maximum of ∼165 mcd at 20 mA with a forward voltage of 2.1 V. After encapsulation into lamps, the peak power efficiency can reach 21.7%, which corresponds to a 9 mW output at 20 mA. Therefore, the MS structure can be extended to fabricate high-brightness AlGaInP LEDs on Si with conventional vertical electrodes. © 2003 American Institute of Physics. View full abstract»

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  • Ultrafast sampling measurements using the photomodulated kinetic inductance in high-Tc superconductors

    Page(s): 4014 - 4016
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    We provide a demonstration of ultrafast sampling measurements using the photokinetic effect in high-Tc superconductors to observe fast electrical signals in and rf resonator structure. We observe direct mixing of an applied optical sampling pulse with an ultrafast electrical signal capacitatively coupled to the sampling microbridge. The structures used here are 10 μm wide bridges lithographically defined in thin Tl2Ba2CaCu2O10 films. The time resolution and sensitivity of the technique are evaluated and the effects of bias currents on signal strengths are explored. © 2003 American Institute of Physics. View full abstract»

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  • Room-temperature low-voltage electroluminescence in amorphous carbon nitride thin films

    Page(s): 4017 - 4019
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    White-blue electroluminescent emission with a voltage bias less than 10 V was achieved in rf sputter-deposited amorphous carbon nitride (a-CN) and amorphous silicon carbon nitride (a-SiCN) thin-film-based devices. The heterojunction structures of these devices consist of: Indium tin oxide (ITO), used as a transparent anode; amorphous carbon film as an emission layer, and aluminum as a cathode. The thickness of the carbon films was about 250 Å. In all of the produced diodes, a stable visible emission peaked around 475 nm is observed at room temperature and the emission intensity increases with the current density. For an applied voltage of 14 V, the luminance was about 3 mCd/m2. The electroluminescent properties of the two devices are discussed and compared. © 2003 American Institute of Physics. View full abstract»

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  • Temperature transients and thermal properties of GaAs/AlGaAs quantum-cascade lasers

    Page(s): 4020 - 4022
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    The use of a gated interband photoluminescence technique has allowed the study of the temperature transients in the active volume of a quantum-cascade laser during pulsed operation. The active region was found to rise by ∼40 K during a 150 ns current pulse, returning to the heat-sink temperature in ∼500 ns. The measured temperature transients were fitted, using a one-dimensional heat diffusion model, giving a value of kAR=1.36±0.2 W cm-1K-1 for the active region thermal conductivity, at 30 K. This is approximately ten times lower than literature values for bulk AlGaAs alloys of equivalent composition and has significant implications on the prospects for high-power continuous-wave device operation. © 2003 American Institute of Physics. View full abstract»

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  • Polymer laser fabricated by a simple micromolding process

    Page(s): 4023 - 4025
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    We report polymer distributed feedback lasers fabricated using solvent-assisted microcontact molding. The poly[2-methoxy-5-(3,7-dimethyloctyloxy) paraphenylenevinylene] film is patterned by placing it in conformal contact with an elastomeric mould inked with a suitable solvent. When the resulting microstructured film is pumped with the 532 nm pulsed output of a microchip laser, we observe lasing above a threshold pump energy of 225 nJ. Above threshold the emission narrows to a linewidth of less than 0.6 nm at a wavelength of 638 nm. This micromolding technique may find application to a wide range of wavelength-scale microstructured organic photonic devices. © 2003 American Institute of Physics. View full abstract»

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  • Electrically tunable waveguide laser based on ferroelectric liquid crystal

    Page(s): 4026 - 4028
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    Laser action in a waveguide configuration has been demonstrated in a planar alignment cell of dye-doped chiral smectic liquid-crystal mixtures with a short pitch helical structure. In this configuration, doped dye can effectively be excited by a pump beam illuminating perpendicularly the helical axis and the laser light emitted along the helical axis propagates in the waveguide. Lasing wavelength can be tuned by adjusting the periodicity of the helical pitch upon applying the electric field. © 2003 American Institute of Physics. View full abstract»

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  • Monte Carlo simulation of tunable mid-infrared emission from coupled Wannier–Stark ladders in semiconductor superlattices

    Page(s): 4029 - 4031
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    We present a theoretical and experimental study on the mid-infrared electroluminescence associated with transitions between electric-field-induced conduction states, forming the Wannier–Stark ladder, in strongly coupled GaAs/AlAs superlattices. The interwell and intrawell radiative transitions in the whole range of electric fields from the moderate localization to the resonance-induced delocalization regimes have been experimentally investigated. Monte Carlo simulations show a very good agreement between experimental and theoretical electroluminescence spectra. Results show that the application of an electric field in the range from 100 to 250 kV/cm shifts the emission peak, related with interwell diagonal transitions between Stark-localized ground states of two adjacent wells, up to the limit corresponding to the merging of this electroluminescence peak with the intersubband emission between excited and ground state of the same well. The theoretical investigation indicates that interwell scattering via LO phonons is responsible for the population of the excited state of the ladder. © 2003 American Institute of Physics. View full abstract»

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  • Photoluminescence enhancement of colloidal quantum dots embedded in a monolithic microcavity

    Page(s): 4032 - 4034
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    We demonstrate an enhancement of the spontaneous emission from colloidal CdSe quantum dots embedded in a half-wavelength one-dimensional cavity. When embedded in the cavity, the emission of the quantum dots is enhanced by a factor of 2.7. We also show a strong amplification by one order of magnitude in the absorption of the CdSe quantum dots due to the cavity effect. © 2003 American Institute of Physics. View full abstract»

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  • Enhanced room-temperature emission in Cr4+ ions containing alumino-silicate glasses

    Page(s): 4035 - 4037
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    The Bi2O3 modified alumino-silicate glasses, when excited at 800 nm exhibited a broadband emission of Cr4+ in the range of 1.2 to ∼1.6 μm at room temperature. The mean lifetime of (1E-3A2)→3T1 transition in Cr4+-doped glass at RT was as long as ∼426 μs, enabling the realization of both tunable lasers and broadband fiber amplifiers. The Cr4+ emission at RT also showed the dependency on the excitation wavelength and site distribution within the chosen glass host. A photoinduced charge transfer process from the Cr3+ ion into Cr4+ ion appears to be responsible for the excitation of Cr4+. © 2003 American Institute of Physics. View full abstract»

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  • Extremely low threshold-current-density InGaAs quantum-well lasers with emission wavelength of 1215–1233 nm

    Page(s): 4038 - 4040
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    Extremely low threshold-current-density In0.4Ga0.6As quantum-well (QW) lasers have been realized in the 1215–1233 nm wavelength regime. The measured room-temperature threshold current density of the InGaAs QW lasers with a cavity length of 1000 μm is only 90 A/cm2 at an emission wavelength of 1233 nm. © 2003 American Institute of Physics. View full abstract»

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  • Is Cu60Ti10Zr30 a bulk glass-forming alloy?

    Page(s): 4041 - 4043
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    The microstructures of Cu60Ti10Zr30 alloys fabricated by using two different methods, (rods of 2.5 mm in diameter prepared by a copper-mold casting method, and ribbons of about 0.03 mm in thickness prepared by the melt-spinning method), have been investigated by transmission electron microscopy and high-resolution transmission electron microscopy. Surprisingly, we found that the alloy in both geometries contains cubic nanometer-sized crystals of about 5–7 nm in diameter with a lattice parameter of 0.45 nm for ribbons and 7–15 nm in diameter with a lattice parameter of 0.42 nm for rods. Nanocrystals with a significant volume fraction are randomly distributed in the amorphous matrix. The copper element is enriched in nanocrystals while a slightly high zirconium content is found in the matrix. We classify that the Cu60Ti10Zr30 alloy prepared by both of the aforementioned methods is a nanocomposite: Nanocrystals embedded in an amorphous matrix. © 2003 American Institute of Physics. View full abstract»

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  • Structures of nitrided Si(001) surfaces: First-principles theoretical study

    Page(s): 4044 - 4046
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    First-principles theoretical calculations for nitrided Si(001) surfaces are presented. We examined the structure and stability of ordered nitrided surfaces with various coverages of N and H atoms. For the 2×2 ordered reconstruction observed in very recent experiments, we propose a structural model, where N atoms are adsorbed into Si dimer centers and also into back bonds, and those in the dimer centers are dimerized. We also discuss the nitridation process. © 2003 American Institute of Physics. View full abstract»

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  • Parameters of wires during electric explosion

    Page(s): 4047 - 4049
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    We use the nucleation theory of electric explosion of wire to estimate wire parameters at the explosion for a wide range of experimental conditions. We analyze a number of the major parameters including the limit value of energy deposited during the initial resistive phase of the explosion and discuss the correspondence of these results to previously published experimental data. © 2003 American Institute of Physics. View full abstract»

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  • Orientation control of poly(vinylidenefluoride-trifluoroethylene) crystals and molecules using atomic force microscopy

    Page(s): 4050 - 4052
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    We have developed an aligning technique for polymer crystals and molecular chains utilizing contact-mode atomic force microscopy (AFM). We have aligned lamellar crystals and molecular chains of poly(vinylidenefluoride-trifluoroethylene) thin films. By scanning the film surface using an AFM cantilever tip at the temperature range of 70–100 °C, we aligned the crystals to the scan direction. Moreover, we successfully aligned the molecular chains to the scan direction by scanning at a higher temperature (135 °C). The aligned chains subsequently formed large lamellar crystals, which were still ferroelectric. © 2003 American Institute of Physics. View full abstract»

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  • Oxygen vacancy ordering in epitaxial layers of yttrium oxide on Si (001)

    Page(s): 4053 - 4055
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    The origin of the superstructure observed in epitaxial yttrium oxide (Y2O3) layers on Si (001) is determined by electron energy loss spectroscopy (EELS). The oxygen K edge is measured both in the superstructure and a defect-free region of the Y2O3 layers and they are compared to EELS spectra obtained from bulk stoichiometric and reduced Y2O3. It is shown that as a result of the epitaxial growth, oxygen vacancies order into a superstructure creating nonstoichiometric regions in an otherwise stoichiometric Y2O3 layer. Furthermore, it is shown that oxygen deficiency introduces a change of the density of states of the lower conduction band of Y2O3. © 2003 American Institute of Physics. View full abstract»

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  • Photoluminescence of a superficial Si nanolayer and an example of its use

    Page(s): 4056 - 4058
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    A characteristic photoluminescence of a superficial Si nanolayer realized by ion implantation has been observed. This effect, being totally independent of those shown recently for a nanoscale Si-layered system, is similar to that produced by Si nanocrystals (Si nc). To visualize the nature and give evidence of this effect, we fabricated samples in two different ways: (i) by incorporation of Si nc into thin SiO2 films deposited on Si wafer by the spin-on-glass method and (ii) by a nanoscale superficial crystalline-Si modification using medium-energy ion implantation and thermal treatment. In both cases the UV-to-red light conversion has been observed to be independent of wafer post-implantation damage. To show the UV-to-red conversion contribution, we use the ion modified superficial Si layer with its well-defined potential barrier, the so-called carrier collection limit. Such a modified Si structure gives us a method of deconvoluting several optoelectronic features observed experimentally on modified Si. The practical realization is compatible with well-established Si technology. © 2003 American Institute of Physics. View full abstract»

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  • Optical phonon spectra of GaP nanoparticles prepared by nanochemistry

    Page(s): 4059 - 4061
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    Gallium phosphide (GaP) nanoparticles have been synthesized by colloidal nanochemistry with two different surfactants: trioctylphosphine oxide and dodecylamine. Transverse optical (bulk) and surface optical phonons associated with the GaP nanoparticles were observed and studied experimentally by infrared transmission spectroscopy of a solid dispersion of these nanoparticles in cesium iodide pellets. These vibrational properties of the nanoparticles were used to obtain information about the crystallinity and surface interactions. The crystallinity and the stoichiometry of the samples were also examined and characterized by transmission electron microscopy, electron diffraction, and energy dispersive x-ray spectroscopy. © 2003 American Institute of Physics. View full abstract»

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  • Periodical poling characteristics of congruent MgO:LiNbO3 crystals at elevated temperature

    Page(s): 4062 - 4064
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    A structure for quasi-phase matching of large cross section using 5 mol % MgO-doped LiNbO3 (MgO:LN) crystal for high-power wavelength conversion was realized. We have found that the coercive field to invert polarization of the crystal reduces drastically with elevated temperature. The coercive field reduces to 1.2 kV/mm at 250 °C, which is about 1/4 compared with that for MgO:LN at room temperature (RT) and about 1/17 of that for LN at RT. 3 mm thick periodically poled MgO:LN has been fabricated with 30 μm period. © 2003 American Institute of Physics. View full abstract»

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  • Midinfrared continuous-wave photoluminescence of lead–salt structures up to temperatures of 190 °C

    Page(s): 4065 - 4067
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    Continuous-wave photoluminescence in the midinfrared for PbSe/PbEuSe and PbTe/PbEuTe multiquantum well structures as well as for PbSe and PbTe bulklike structures, excited by a semiconductor laser diode, is investigated. All samples are grown by molecular-beam epitaxy on BaF2(111) substrates under the same growth conditions. Both for the Te-based systems and for the Se-based systems, it turns out that bulklike structures show photoluminescence up to higher temperatures than multiquantum well structures. In particular, emission spectra from PbTe/PbEuTe multiquantum wells are obtained up to temperatures of 200 K and from PbSe/PbEuSe multiquantum well structures up to 60 °C whereas for bulklike PbSe photoluminescence at temperatures as high as 190 °C is demonstrated. © 2003 American Institute of Physics. View full abstract»

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  • Experimental evidence of polarization dependence in the optical response of opal-based photonic crystals

    Page(s): 4068 - 4070
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    We report angle resolved reflectivity measurements showing the polarization dependence of the pseudogap in artificial opals. This kind of photonic crystals consist of silica spheres ordered in a face-centered-cubic lattice. The analyzed gap originates from the (111) family of planes. It is shown that the width of the Bragg peak observed in the reflectance spectra follows the bands determining the pseudogap, which are selectively excited according to the polarization (s or p) of the light impinging on the opal. Moreover, it is found that the coupling of light with the photonic bands occurs according to their predicted symmetry, which was assigned by arguments based on group theory. © 2003 American Institute of Physics. View full abstract»

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  • High-resolution near-field spectroscopy investigation of GaN laterally overgrown structures on SiC

    Page(s): 4071 - 4073
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    We investigated epitaxial lateral overgrown GaN structures with different wing tilt using a spectroscopic scanning near-field optical microscope (spectroscopic SNOM), which takes a complete optical spectrum at each point of a sample surface. From these measurements, we obtain information about strain at different points of the surface, and comparing emission intensity between regions of lateral growth and vertical growth, we directly see the efficiency of defect density reduction. For the high wing-tilt sample, an increased defect density at the window–wing interface could be identified. © 2003 American Institute of Physics. View full abstract»

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  • Si dangling-bond-type defects at the interface of (100)Si with ultrathin HfO2

    Page(s): 4074 - 4076
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    Electron spin resonance analysis of (100)Si/HfO2 interfaces prepared by chemical vapor deposition of the oxide using three chemically different precursors reveals that the trivalent Si defects common for Si/SiO2 interfaces—Pb0 and Pb1 centers—are universally observed upon hydrogen detachment. The density of the Pb0 is higher than in the (100)Si/SiO2 structures and is sensitive to the deposition process. However, the density can be significantly reduced by annealing of the Si/HfO2 structures in O-containing ambient, likely through re-establishing the Si/SiO2 interface. Also, the Pb-type centers can be entirely passivated by hydrogen already at 400 °C. The density of fast interface states closely follows the variations in the Pb0 center density, suggesting it as the dominant contribution to the fast interface states. © 2003 American Institute of Physics. View full abstract»

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  • Temperature dependence of carrier mobility in Si wafers measured by infrared photocarrier radiometry

    Page(s): 4077 - 4079
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    A recently introduced infrared photocarrier radiometry technique has been used to determine the temperature dependence of carrier mobility in Si wafers. In addition, its potential to determine simultaneously the carrier lifetime, diffusion coefficient, and surface recombination velocity is reported. This noncontact, nonintrusive, and all-optical technique relies on the detection of infrared radiation from harmonically excited free carriers (pure electronic diffusion-wave detection). Using a multiparameter fitting to a complete theory, the results showed that the lifetime increases with temperature, the diffusion coefficient decreases [D(T)∼T-1.5], and the temperature dependence of carrier mobility is μ(T)=(1.06±0.07)×109×T-2.49±0.01 cm2/V s. © 2003 American Institute of Physics. 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