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Applied Physics Letters

Issue 17 • Date Oct 2001

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Displaying Results 1 - 25 of 61
  • 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
  • Near-field microscopy and spectroscopy of third-harmonic generation and two-photon excitation in nonlinear organic crystals

    Page(s): 2681 - 2683
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    Near-field microscopy and spectroscopy of third-harmonic generation and two-photon excitation are demonstrated on a subdiffraction-limited scale. Near-field measurements of both the intensity dependence and the spectral dependence are performed on nonlinear organic nanocrystals of 4-(N, N-diethylamino)-β-nitrostyrene. The in-plane local anisotropies of third-order susceptibilities are observed, which is revealed to be associated to two-photon allowed π–electron transition. The local spectra exhibit the crystal symmetry on a nanometric scale. © 2001 American Institute of Physics. View full abstract»

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  • Wide-range-tunable laterally coupled distributed feedback lasers based on InGaAsP–InP

    Page(s): 2684 - 2686
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    We have investigated tunable distributed feedback (DFB) lasers based on InGaAsP quantum wells grown by molecular-beam epitaxy. Two-section tunable DFB lasers were fabricated by patterning laterally gain coupling binary superimposed gratings perpendicular to the ridge waveguide. Side mode suppression ratios of up to 42 dB have been achieved. The tuning range covers 25 nm. © 2001 American Institute of Physics. View full abstract»

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  • Dynamics of the second-order nonlinearity in thermally poled silica glass

    Page(s): 2687 - 2689
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    We study the temporal evolution of both the second-order nonlinear coefficient and of the nonlinear thickness in thermally poled silica-glass slides by using a high-resolution all-optical technique. A time delay in the nonlinearity formation is observed, followed by an increase to a maximum, and a final decrease. The thickness is shown to increase at a rate that differs significantly from that reported for the corresponding ionic charge fronts. Our measurements also show strong dependencies on sample thickness and these can be attributed to different electric fields in the depletion region. © 2001 American Institute of Physics. View full abstract»

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  • Surface-emitting channel drop filters using single defects in two-dimensional photonic crystal slabs

    Page(s): 2690 - 2692
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    We report a theoretical analysis of trapping and emission of photons by a single defect in a two-dimensional photonic crystal slab. We show that the tuning of emission wavelength is possible by selecting appropriate geometry of structure while keeping the emission efficiency maximal. The results suggest the possibility of applying it to ultra-small channel add/drop device in wavelength division multiplexed optical communications. © 2001 American Institute of Physics. View full abstract»

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  • Strain-induced channel waveguiding in bulk sapphire substrates

    Page(s): 2693 - 2695
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    We report on the two-dimensional confinement of light in the channel region of a bulk sapphire substrate, which was formed by utilizing the photoelastic effect in sapphire induced by a sputter-deposited SiO2 film. The experimental result, combined with the simulation results, shows that the sputter-deposited SiO2 films are compressively stressed up to 10 GPa level and that a 1.0-μm-thick film, for example, induces an index change of 5×10-3 in sapphire in the vertical direction. This amount of index change is found sufficient to support a fundamental mode at wavelengths of 1.54 μm or below. © 2001 American Institute of Physics. View full abstract»

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  • 206 W continuous-wave TEM00 mode 1064 nm beam generation by a laser-diode-pumped Nd:YAG rod laser amplifier

    Page(s): 2696 - 2697
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    We demonstrate a 206 W 1064 nm beam generation at a beam quality of M2=1.34 with a Nd:YAG laser amplifier system. We also report on an efficient 150 W TEM00 mode diode-pumped Nd:YAG rod laser. © 2001 American Institute of Physics. View full abstract»

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  • Reduction of microtrenching and island formation in oxide plasma etching by employing electron beam charge neutralization

    Page(s): 2698 - 2700
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    During plasma etching of oxide thin-film patterns, nonuniform charge buildup within etching features results in formation of microtrenches. Near the etch endpoint, the underlying film layer adjacent to the feature edges is exposed first, leaving an oxide island in the feature center and potentially causing underlayer damage before the endpoint is reached. Herein, a directional electron flux is added to the plasma ion flux incident on the etching substrate with the goal of minimizing microtrenching and oxide island formation. Scanning electron microscopic images of patterns etched with added electron irradiation show a reduction in microtrenching and oxide island formation as compared to patterns etched under identical conditions without electron irradiation. A computer simulation shows that the added electron irradiation reduces microtrenching by allowing more uniform ion flux to reach the feature bottom. © 2001 American Institute of Physics. View full abstract»

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  • Inhomogeneous broadening in quantum dots with ternary aluminum alloys

    Page(s): 2701 - 2703
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    We study how the optical properties of InAs self-assembled quantum dots (QDs) grown on GaAs substrate are affected when using AlGaAs barriers to increase the carrier confinement. The inhomogeneous broadening of the QD ensemble is found to increase when ternary aluminum alloys are used next to or within the QDs. By growing thin GaAs spacers to separate the QDs from the AlGaAs barriers, we obtain QD ensembles which exhibit little photoluminescence quenching and well-defined excited states up to room temperature. Postgrowth rapid thermal annealing is then used to intermix these InAs/GaAs/AlGaAs QDs and diffuse the Al towards the QDs. In contrast with QDs having thick binary GaAs barriers, the inhomogeneous broadening of QDs with nearby AlGaAs barriers is not decreased with intermixing, leading to unresolved excited state peaks when the interdiffusion length becomes comparable to the GaAs spacer thickness. © 2001 American Institute of Physics. View full abstract»

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  • Distinguishing surface and bulk contributions to third-harmonic generation in silicon

    Page(s): 2704 - 2706
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    We report measurements of third-harmonic generation from ultrathin crystalline silicon layers of gradually varying thickness. Both the angular and thickness dependence of the third-harmonic light generated in transmission at normal incidence are consistent with negligible surface contribution to third-harmonic generation in silicon, even under tight focusing. This work illustrates a method for distinguishing surface and bulk contributions to harmonic generation. © 2001 American Institute of Physics. View full abstract»

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  • Millisecond dynamics of thermal expansion of mechanically controllable break junction electrodes studied in the tunneling regime

    Page(s): 2707 - 2709
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    The thermal expansion dynamics of W, Pt–Ir, and Au mechanically controllable break junction electrodes was studied in the millisecond range. By measuring a transient tunnel current as a function of time, we found that, at low temperatures, the electrode elongation Δs∼t1/2 due to the large values of thermal diffusivity of metals. The magnitude of Δs varies in direct proportion to the power P dissipated in the electrodes. © 2001 American Institute of Physics. View full abstract»

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  • Periodic composition modulations in InGaN epitaxial layers

    Page(s): 2710 - 2712
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    InGaN epitaxial layers grown by metalorganic chemical vapor deposition were investigated in order to understand the occurrence of composition modulations in the GaN–InN system. The In contents of the samples were determined to be x=0.21 and 0.31. Transmission electron microscopy was performed on [0001], [101¯0], and [112¯0] zone-axis specimens. Plan-view images display a domain structure, representing regions in which the directions of the modulations differ. Intersections between domains occur in <101¯0>, and <112¯0>, and other directions. Satellite spots appear in selected-area diffraction patterns. These observations can be explained by diffraction effects resulting from periodic composition modulations. An equation was derived relating the spacing between the satellites and the reflections to the wavelength of the modulations in the wurtzite structure. The sample with x=0.21 had a wavelength of λ=3.1±1.3 nm and the one with x=0.31 had λ=3.2±1.3 nm. Since Young’s modulus is isotropic in the (0001) plane, no particular direction is favored for the modulations based on strain energy considerations. This result is consistent with the observation of the variously oriented domains and satellites. © 2001 American Institute of Physics. View full abstract»

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  • Effects of step-graded AlxGa1-xN interlayer on properties of GaN grown on Si(111) using ultrahigh vacuum chemical vapor deposition

    Page(s): 2713 - 2715
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    We report the growth of high-quality GaN on a Si(111) substrate using a five step-graded AlxGa1-xN (x=0.87–0.07) interlayer between GaN epilayer and AlN buffer layer by ultrahigh vacuum chemical vapor deposition. The crack density and the surface roughness of the GaN layer grown on the graded AlxGa1-xN interlayer were substantially reduced, compared to those of GaN grown on an AlN buffer layer. Significant improvement in the structural and optical properties of the GaN layer was also achieved by the use of a graded interlayer. These results are attributed to the decrease of the lattice mismatch between GaN and AlN layer, and the reduction of the thermal stress by the graded interlayer. © 2001 American Institute of Physics. View full abstract»

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  • Surface hydroxyl formation on vacuum-annealed TiO2(110)

    Page(s): 2716 - 2718
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    The change in surface composition and structure of a rutile TiO2(110) surface during thermal annealing in an ultrahigh vacuum was studied by coaxial impact–collision ion scattering spectroscopy and time-of-flight elastic recoil detection analysis. When the clean TiO2(110) surface with a 1×1 bridging-oxygen-rows structure was obtained by annealing at 730 °C, about one monolayer of hydrogen atoms still resided on the surface. These hydrogen atoms were assigned to surface hydroxyls as an ingredient of the TiO2(110)1×1 structure, which was formed in the self-restoration process of surface oxygen vacancy defects by dissociative adsorption of water molecules during thermal annealing. © 2001 American Institute of Physics. View full abstract»

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  • High-temperature metastability of cubic spinel Si3N4

    Page(s): 2719 - 2721
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    The metastability of cubic spinel Si3N4 has been investigated at high temperature, up to about 1800 K, and in Ar atmosphere by quench experiments and differential thermal analyses coupled with thermogravimetry. The results indicate that the enthalpy change from spinel-type Si3N4 to β-Si3N4 is -29.2±3.5 kJ/mol and that the transformation starts at 1670 K. The high-temperature metastability of spinel Si3N4 may provide various directions for industrial applications. © 2001 American Institute of Physics. View full abstract»

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  • Comparison of light- and heavy-ion-irradiated quantum-wells for use as ultrafast saturable absorbers

    Page(s): 2722 - 2724
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    We have compared light- and heavy-ion irradiation of InGaAs/InAlAs multiple-quantum wells for ultrafast saturable absorption applications. Under heavy-ion impacts, defect clusters were produced, as observed via transmission electronic microscopy. By contrast, in proton-irradiated samples, only point defects were formed. Nonlinear absorption measurements were performed with excitonic resonance pumping. The relaxation time of absorption saturation (minimum value 2 ps) did not depend on the irradiating ion, and was practically independent of the pulse repetition rate (up to 10 GHz) and optical excitation fluence (0.1 mJ/cm2). We conclude that irradiating multiple-quantum wells with light ions is as effective as using heavy ions, when fabricating ultrafast saturable absorber devices operating at high bit rate and near bandedge wavelength. © 2001 American Institute of Physics. View full abstract»

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  • Hillock formation on copper at room temperature by cleaning in ammonia vapor

    Page(s): 2725 - 2727
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    Rapidly solidified copper particles are formed by electron-beam decomposition of copper hydride. When exposed to aqueous ammonia vapor at room temperature, the surface of the particles is cleaned and etched, and it develops hillocks and incipient whiskers. Damage of this kind is associated with compressive stress in integrated-circuit metallization. The development of such damage without any elevated temperature appears facilitated by the surface cleaning, and may have implications for device processing. Some links with surface diffusivity and its proposed role in device reliability are also explored. © 2001 American Institute of Physics. View full abstract»

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  • Tellurium antisites in CdZnTe

    Page(s): 2728 - 2730
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    The electrical properties of CdTe and Cd1-xZnxTe crystals grown under excess tellurium by a modified Bridgman technique are critically dependent on the zinc content. Below an x value of 0.07, the as-grown CdZnTe crystals are n type while, above this value, CdZnTe crystals are p type. The origin of the shallow donor level at 0.01 eV below the conduction band is most likely singly ionized Te antisites (Te at Cd sites). The origin of the deep donor level at 0.75 eV below the conduction band is therefore doubly ionized tellurium antisites. Based on this model, the conduction type of CdZnTe crystals is determined by the results of compensation between the shallow donors of Te antisites and the shallow acceptors of Cd vacancies. High resistivity Cd0.9Zn0.1Te crystals are produced by compensating the p-type crystals with indium impurity at a low doping level of 1–5×1015cm-3. At room temperature, CdZnTe radiation detectors can resolve the six low energy peaks in the 241Am spectrum, a performance comparable to that of the best CdZnTe detectors reported. © 2001 American Institute of Physics. View full abstract»

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  • Hydrostatic-pressure dependence of the photoconductivity of single-crystal pentacene and tetracene

    Page(s): 2731 - 2733
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    Pentacene and tetracene show readily observable photoconductivity when illuminated with light in the blue part of the visible spectrum. We measured the change of photoconductivity with hydrostatic pressure in single-crystal samples of both materials. Possible mechanisms for the observed increase in photoconductivity with pressure are discussed. We conclude that a carrier-mobility increase under pressure is most likely to cause the increase in photoconductivity in the case of pentacene. For tetracene, changes in the absorption spectrum in the range of the excitation wavelengths may also be significant. We also observe a phase transition near 0.3 GPa in tetracene, in agreement with previous results. © 2001 American Institute of Physics. View full abstract»

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  • Influence of microstructure on the carrier concentration of Mg-doped GaN films

    Page(s): 2734 - 2736
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    Room-temperature Hall effect measurements of (0001) Mg-doped GaN films grown on sapphire substrates by metalorganic chemical vapor deposition show a reduction in hole concentration for Mg concentrations greater than 1020cm-3. A combination of secondary ion mass spectrometry and transmission electron microscopy indicates a steadily increasing Mg incorporation during growth and the formation of inversion domains at these high concentrations. We discuss mechanisms that could give rise to a reduction of the hole concentration at high Mg doping levels. © 2001 American Institute of Physics. View full abstract»

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  • Improved mobilities and resistivities in modulation-doped p-type AlGaN/GaN superlattices

    Page(s): 2737 - 2739
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    The transport properties of modulation, shifted modulation, and uniformly doped Al0.20Ga0.80N/GaN superlattices are presented. The modulation-doped sample is doped only in the AlGaN barriers. The shifted-modulation-doped sample has its dopants shifted by one-quarter period. Measurements reveal a strong improvement in mobility and resistivity for the modulation-doped and shifted-modulation-doped structures versus the uniformly doped structure. The modulation-doped sample has a mobility of 9.2 and 36 cm2/V s at 300 and 90 K respectively and a very low resistivity of 0.20 and 0.068 Ω cm at 300 and 90 K, respectively. Capacitance–voltage profiling shows multiple two-dimensional hole gases. The results are consistent with a reduction of neutral impurity scattering for modulation-doped structures as compared to uniformly doped structures. © 2001 American Institute of Physics. View full abstract»

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  • Controlled oxygen doping of GaN using plasma assisted molecular-beam epitaxy

    Page(s): 2740 - 2742
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    High-quality (0001) and (0001¯)-GaN films were grown by plasma-assisted molecular-beam epitaxy to study the dependence of oxygen incorporation on polarity and oxygen partial pressure. Oxygen incorporates at a rate ten times faster on nitrogen-polar GaN than on the Ga polarity. Oxygen doping is controllable, reproducible, and produces low compensation material up to concentrations of at least 1018cm-3 with higher levels showing significant compensation. Layers containing oxygen at levels above 1022cm-3 exhibit severe cracking while oxygen concentrations less than 1021cm-3 do not introduce significant strain. The oxygen incorporation rate has a weak dependence on Ga overpressure during Ga-stable growth but dramatically increases for conditions approaching N-stable growth. © 2001 American Institute of Physics. View full abstract»

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  • Self-assembled heterojunction between electrodeposited PbS nanoparticles and indium tin oxide substrate

    Page(s): 2743 - 2745
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    Self-assembled heterojunction was fabricated by means of an electrochemical deposition of PbS nanoparticles on indium tin oxide substrate. The current–voltage and capacitance–voltage studies confirmed the formation of a heterojunction. A large current and large capacitance were observed in the case of a device from particle of smaller size which is believed to be due to the large surface area contact. The rectifying behavior of the heterojunction was found to be weak as compared to the usual p–n junction. © 2001 American Institute of Physics. View full abstract»

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  • Impurity-controlled dopant activation: Hydrogen-determined site selection of boron in silicon carbide

    Page(s): 2746 - 2748
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    The geometry and formation energy of substitutional B and Al dopants as well as their complexes with hydrogen have been calculated in 4H–SiC using first-principles methods. Our results show that boron selecting the silicon site and, therefore, getting activated as a shallow acceptor depends on the presence of hydrogen which is promoted into the crystal by boron itself. Without hydrogen, boron would mostly be incorporated at the carbon site. Aluminum does not show this behavior: it always selects the silicon site and is incorporated independently of hydrogen. © 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