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Applied Physics Letters

Issue 25 • Date Dec 2001

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Displaying Results 1 - 25 of 63
  • 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
  • GaInSb/InAs/AlGaAsSb “W” quantum-well light-emitting diodes

    Page(s): 4073 - 4075
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    We report on the characterization of molecular-beam epitaxy-grown Ga0.75In0.25Sb/InAs/Al0.15Ga0.85As0.05Sb0.95 “W” quantum-well light-emitting diodes (LED). The devices demonstrated room-temperature emission with a peak wavelength of 3.3 μm. A pulsed current output power of 55 μW with a slope efficiency of 98 μW/A was measured and the 300 K internal efficiency was estimated to be 2.5%. Comparative power analysis suggests that LED and laser diode efficiencies are limited by the same thermally activated nonradiative mechanism, probably Auger recombination. © 2001 American Institute of Physics. View full abstract»

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  • Design and performance analysis of deep-etch air/nitride distributed Bragg reflector gratings for AlInGaN laser diodes

    Page(s): 4076 - 4078
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    The key parameters in the fabrication of deep-etch high-order λ/4 Bragg gratings for short-wavelength nitride-based lasers are investigated. Calculations indicate that, for an air-gap thickness of 1.73 μm and single-spot Gaussian beam profile, the reduction in grating reflectivity due to light diffraction in the air gaps is only 17% with respect to a first-order structure with 0.1 μm air gaps. Scanning electron microscopy and microphotoluminescence characterizations confirm the validity of the numerical predictions and show that the 28%–38% reflectivity obtained from prototype focused-ion-beam-etched air/nitride gratings is mainly limited by imperfections and material disorder due to etching. Improving the etching technique would, therefore, allow standard lithographic fabrication of reduced-threshold GaN lasers. © 2001 American Institute of Physics. View full abstract»

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  • High single-mode power observed from a coupled-resonator vertical-cavity laser diode

    Page(s): 4079 - 4081
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    We report a monolithic coupled-resonator vertical-cavity laser with an ion-implanted top cavity and a selectively oxidized bottom cavity which exhibits single fundamental-mode operation. The output powers are as high as 6.1 mW with side mode suppression ratios greater than 30 dB. The sizes of the implant and oxide current apertures are shown to be important for demonstrating the required selectivity for the fundamental lasing mode. With a fixed bias current on the implant cavity and increasing oxide cavity current, mode switching from single-mode operation to multimode operation and back to single-mode operation was observed. The intensities of the fundamental and first transverse modes were calculated by solving a set of multimode rate equations. The calculation indicates that the observed mode switching can be identified with changes in the optical length of the oxide cavity with increasing pump current. The observed mode dynamics are unique to coupled-resonator vertical-cavity lasers. © 2001 American Institute of Physics. View full abstract»

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  • Single-mode tunable organic laser based on an electroluminescent oligothiophene

    Page(s): 4082 - 4084
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    Single-mode tunable laser emission is obtained from a stable cavity based on a prototype compound of a class of functionalized electroluminescent oligothiophenes. Laser emission is demonstrated in the red spectral region with a tunability wavelength range of 30 nm, centered at 607 nm. The laser exhibits a well-defined pump threshold and good emission characteristics. The results suggest the use of the family of functionalized oligothiophenes as optical amplifiers covering the whole visible spectral region. © 2001 American Institute of Physics. View full abstract»

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  • Lithium metal for x-ray refractive optics

    Page(s): 4085 - 4087
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    Lithium metal is the best material for refractive lenses that must focus x-rays with energies below 15 keV, but to date no lens from Li has been reported. This letter demonstrates focusing of 10 keV x-rays with a one-dimensional sawtooth lens made from Li. The lens’ theoretical gain is 4.5, with manufacturing imperfections likely responsible for the threefold gain that is observed. Despite the Li reactivity the lens is stable over months of operation if kept under vacuum. © 2001 American Institute of Physics. View full abstract»

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  • Continuous wave waveguide laser at room temperature in Nd3+-doped Zn:LiNbO3

    Page(s): 4088 - 4090
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    This work reports continuous laser action at room temperature in LiNbO3:Nd3+ channel waveguides, fabricated by Zn diffusion. The absorbed pump power at the threshold was 1.25 mW and a slope efficiency of 20% was obtained. With our available pump power the laser could emit up to 0.14 mW without exhibiting any photorefractive damage. © 2001 American Institute of Physics. View full abstract»

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  • Single-mode operation of coupled-cavity lasers based on two-dimensional photonic crystals

    Page(s): 4091 - 4093
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    We report the fabrication of short-cavity lasers with highly reflective two-dimensional photonic crystal mirrors on an InGaAsP/InP laser structure emitting at 1.57 μm. An intracavity photonic crystal mirror creates two coupled cavities, which provide additional longitudinal mode selection for stable single-mode operation with side-mode suppression ratios exceeding 35 dB. The shortest lasers with l=100 μm overall length have a threshold current of 13 mA and provide more than 4 mW power under continuous wave operation. Longer devices with l=200 μm deliver up to 9 mW. A maximum modulation bandwidth of 7.9 GHz was determined by relative intensity noise measurements. © 2001 American Institute of Physics. View full abstract»

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  • Quantum effects on bremsstrahlung spectrum from electron–positron plasmas

    Page(s): 4094 - 4096
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    Quantum effects on the nonrelativistic electron–positron bremsstrahlung process in electron–positron plasmas are investigated using the corrected Kelbg potential taking into account the classical effect as well as the quantum-mechanical effect. The Born approximation is assumed for the initial and final states of the collision system. The results show that the electron–positron bremsstrahlung radiation cross section decreases with increasing thermal de Broglie wavelength (λ), i.e., decreasing plasma temperature (T). It is also found that the quantum effects on the bremsstrahlung radiation cross section increase with increasing radiation photon energy. © 2001 American Institute of Physics. View full abstract»

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  • High-repetition-rate ferroelectric-cathode gyrotron

    Page(s): 4097 - 4099
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    The intensive research on ferroelectric electron-emission mechanisms in the last decade has resulted in a wide understanding of the physics and characteristics of this plasma-assisted electron source. Nevertheless, practical devices employing this cathode were hardly introduced. In this experimental study, a high-repetition-rate microwave oscillator based on a ferroelectric electron gun has been developed. The device operates as a cyclotron-resonance maser in the gyrotron mode. Microwave pulses exceeding 1.5 kW at ∼7 GHz are measured in repetition rates above 3 MHz and duty cycles of ∼50%. These experimental results encourage the implementation of ferroelectric cathodes in practical high-power microwave tubes. © 2001 American Institute of Physics. View full abstract»

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  • A pump–probe investigation of laser-droplet plasma dynamics

    Page(s): 4100 - 4102
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    The temporal character of the plasma formed by the irradiation of micron-sized argon and krypton droplets with intense 100-ps laser pulses is investigated using a pump–probe scheme. The evolution of the droplet plasma is assessed by monitoring delay-dependent x-ray and extreme ultraviolet (EUV) emission, and by imaging frequency-doubled probe light scattered from the interaction region. Depending on the spectral region of interest, the type of emission, and the droplet characteristics, the effective plasma lifetime can extend from a few hundred picoseconds to as long as several nanoseconds. Knowledge of the droplet plasma lifetime is important in optimizing the efficiency of this laser plasma as an EUV or x-ray source. © 2001 American Institute of Physics. View full abstract»

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  • Systematic measurement of AlxGa1-xN refractive indices

    Page(s): 4103 - 4105
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    Dispersion of the ordinary and extraordinary indices of refraction have been measured systematically for wurtzitic AlxGa1-xN epitaxial layers with 0.0≤x≤1.0 throughout the visible wavelength region. The dispersion, measured by a prism coupling waveguide technique, is found to be well described by a Sellmeier relation. Discrepancies among previous measurements of refractive index dispersion, as a consequence of different growth conditions and corresponding band gap bowing parameter, are reconciled when the Sellmeier relation is parameterized not by x but by band gap energy. © 2001 American Institute of Physics. View full abstract»

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  • Evaluation of the precipitate contribution to the infrared absorption in interstitial oxygen measurements in silicon

    Page(s): 4106 - 4108
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    The spurious contribution of oxide precipitates to the infrared absorption of interstitial oxygen in silicon is determined from spectra collected at 7 K for samples with different initial interstitial oxygen Oi concentration subjected to a three-step thermal treatment. These data can be used to correct the residual Oi values determined at room temperature following the standard procedure from the intensity of the 1107 cm-1 absorption band. The error in residual Oi is found to reach values on the order of 2×1017 atoms/cm3 for samples with initial Oi content higher than 6.5×1017 atoms/cm3. © 2001 American Institute of Physics. View full abstract»

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  • Optical investigation of shallow acceptor states in GaN grown by hydride vapor-phase epitaxy

    Page(s): 4109 - 4111
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    The evolution of low-temperature photoluminescence (PL) spectra with the thickness of the layer (3–400 μm) is investigated on high-quality GaN grown by hydride vapor-phase epitaxy. With increasing layer thickness, three acceptor bound exciton peaks are found to reduce in intensity, although the impurity concentrations, measured by secondary ion mass spectrometry, do not depend on the sample thickness. The observed acceptor transitions are attributed to intrinsic defects, originating from the substrate/layer interface and decreasing in density with the thickness of the layer. The optical properties, studied by reflectance, temperature and excitation power dependent PL, are compared to those of homoepitaxial GaN films grown by metalorganic chemical vapor deposition. © 2001 American Institute of Physics. View full abstract»

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  • Determination of residual stresses in Pb(Zr0.53Ti0.47)O3 thin films with Raman spectroscopy

    Page(s): 4112 - 4114
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    The present work uses Raman spectra to measure residual stresses in Pb(Zr0.53Ti0.47)O3 thin films. Based on thermodynamic analysis, a linear relationship is found between the stress and the square of the Raman frequency in the A1 [transverse optical3 (TO3)] and E [longitudinal optical3 (LO3)] modes. We calibrate the linear relationship by measuring the Raman spectra of stressed bulk Pb(Zr0.53Ti0.47)O3 samples. Then, we assess residual stresses in the lead zirconate titanate thin films at different thicknesses and different annealing temperatures. The residual stresses extracted from the A1(TO3) mode are consistent with those from the E(LO3) mode, which are more or less the same as those measured by the x-ray diffraction sin2 ψ method. © 2001 American Institute of Physics. View full abstract»

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  • Ductility of ultrafine grained copper

    Page(s): 4115 - 4117
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    In this letter, we address the effect of grain refinement on room-temperature ductility of copper. Recent experimental results have shown that this material, as well as a number of other single-phase metals that are ductile when coarse grained, lose their ductility with decreasing grain size in the submicrometer range. A recently developed model in which such materials are considered as effectively two-phase ones (with the grain boundaries treated as a linearly viscous second phase) was applied to analyze the stability of Cu against ductile necking. As a basis, Hart’s stability analysis that accounts for strain-rate-sensitivity effects was used [E. W. Hart, Acta Metall. 15, 351 (1967)]. The results confirm the observed trend for reduction of room-temperature ductility with decreasing grain size. The model can be applied to predicting the grain-size dependence of ductility of other metallic materials as well. © 2001 American Institute of Physics. View full abstract»

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  • Lateral composition modulation in short period superlattices: The role of growth mode

    Page(s): 4118 - 4120
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    The role of the growth mode on lateral composition modulation is studied in short period superlattices of AlAs/InAs and GaAs/InAs. Reflection high energy electron diffraction and scanning tunneling microscopy are used to monitor the growth mode and the quality of the interfaces. Cross-sectional transmission electron microscopy indicates that samples that grow via the layer-by-layer growth mode do not exhibit lateral composition modulation and the superlattice structure is well defined. Lateral composition modulation forms when roughening occurs during growth. However, too much roughening, i.e., three-dimensional island nucleation destroys the regularity of the composition modulation in both the lateral and vertical directions. These results are in general agreement with theoretical predictions. © 2001 American Institute of Physics. View full abstract»

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  • Thermal conductivity and interfacial thermal resistance of polymeric low k films

    Page(s): 4121 - 4123
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    The effective thermal conductivity of four polymeric thin films with distinct molecular morphologies has been measured as a function of film thickness down to 70 Å using a 3ω technique. Comparing to SiO2, the intrinsic thermal conductivity of polymers is about 5–10 times smaller while the interfacial thermal resistance is about 2–10 times larger. The interfacial thermal resistances are explained in terms of the mismatches of acoustic and mechanical properties between polymers and crystalline materials. Both elastic and inelastic scatterings at the interface are examined. © 2001 American Institute of Physics. View full abstract»

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  • Energy level alignment driven by electron affinity difference at 3,4,9,10-perylenetetracarboxylic dianhydride/n-GaAs(100) interfaces

    Page(s): 4124 - 4126
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    Ultraviolet photoemission spectroscopy (UPS) was employed to investigate the electronic structure upon deposition of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on differently treated n-GaAs(100) surfaces. Interface dipoles are found to form according to the electron affinities (EA) of the substrates and PTCDA films at the interfaces and, consequently, the vacuum level alignment rule does not hold. The results demonstrate that the energy offset between the conduction band minimum of n-doped inorganic semiconductors and the lowest unoccupied molecular orbital of organic molecular films at the interfaces can be obtained using UPS by systematically varying the EA of substrates with a known band gap. © 2001 American Institute of Physics. View full abstract»

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  • Finite element analysis of epitaxial lateral overgrown GaN: Voids at the coalescence boundary

    Page(s): 4127 - 4129
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    We report on the finite element analysis of stress distribution at the coalescence boundary in epitaxial lateral overgrown (ELO) GaN related to voids. Different void geometries were considered in our model to investigate the influence of their size/shape on the stress distribution. Large compressive stress is localized in the vicinity of the voids, also an increased tensile stress is present at the corners of the SiN mask. Confocal micro-Raman mapping experiments confirm the presence of increased stress at the coalescence boundary of ELO GaN. © 2001 American Institute of Physics. View full abstract»

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  • Oxidation kinetics of tunnel barrier and its effect on exchange bias of proximity interface

    Page(s): 4130 - 4132
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    Atomic migration in metallic multilayers of magnetic tunnel junction (MTJ), and consequent compositional modulation at the interface during plasma oxidation of an Al tunnel barrier is reported. Surprising effects of such modulation in our specific MTJ appear as the systematic increase in exchange bias of the NiFe buffer layer beneath an FeMn antiferromagnet. Cation-current-limited oxidation of the Al metal layer drives atomic migration in the underlying multilayers to compensate for the cation vacancies created in the metal side of the metal/oxide interface. The condition of oxide growth which manifests different migration behavior is also discussed. © 2001 American Institute of Physics. View full abstract»

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  • Hexagonal GaN grown on GaAs{11n} substrates by metalorganic vapor-phase epitaxy using AlAs intermediate layers

    Page(s): 4133 - 4135
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    Hexagonal GaN (h-GaN) layers are grown by metalorganic vapor-phase epitaxy on GaAs{11n}A and B (n=8, 4, 3, 2, 1) substrates using AlAs intermediate layers. The best quality of h-GaN is obtained on (11¯4)B, where the crystallographic relationship is found from a pole figure to be h-GaN{0001}∥GaAs(33¯5)B and h-GaN{101¯2}∥GaAs(001). We propose a simple model that explains why such a crystallographic relationship is easily realized on {114} resulting in the superior structural and optical properties. Furthermore, from a comparison between the growth on the A and B substrates, it is pointed out that the polarity is a key factor in determining the crystallographic properties. © 2001 American Institute of Physics. View full abstract»

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  • Diagnostics of surface layer disordering using optical third harmonic generation of a circular polarized light

    Page(s): 4136 - 4138
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    A nonlinear optical technique based on a third-harmonic generation of a circular polarized light is proposed for the diagnostics of order–disorder transformations in semiconductors. Applications of this technique to study both noncentrosymmetric (GaAs) and centrosymmetric (Si) materials are demonstrated. © 2001 American Institute of Physics. View full abstract»

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  • Interactions between gallium and nitrogen dopants in ZnO films grown by radical-source molecular-beam epitaxy

    Page(s): 4139 - 4141
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    It has been recently predicted that the co-doping of an acceptor (nitrogen) and a donor (aluminum, gallium, indium) in a 2:1 ratio will dope ZnO p-type due to a reduction in the Madelung energy making the nitrogen acceptor energy level more shallow. We have been growing gallium and nitrogen co-doped ZnO films by radical-source molecular-beam epitaxy by use of oxygen and nitrogen radicals supplied via rf radical source cells. Diode-like current–voltage characteristics and donor acceptor pair-like photoluminescence emission were observed for a Ga and N doped ZnO film grown on an undoped ZnO buffer layer. However, Hall measurements revealed that the conductivity was n-type. Formation of a non-ZnO phase in the sample was confirmed by secondary ion mass spectroscopy and x-ray diffraction measurements. Zn and Zn+O secondary ion intensities fell sharply by two orders of magnitude in going from the undoped ZnO layer to the highly co-doped ZnO. X-ray diffraction measurements indicated the formation of ZnGa2O4. © 2001 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