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Applied Physics Letters

Issue 5 • Date Feb 1999

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Displaying Results 1 - 25 of 50
  • Issue Cover

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

    Page(s): toc1
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  • Single-pass thin-film electro-optic modulator based on an organic molecular salt

    Page(s): 635 - 637
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    Single crystal films of 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) with excellent optical quality have been prepared by modified shear method. Electro-optic modulation for light propagating perpendicular to the film surface was measured at a wavelength (720 nm) where absorption is small. Very large electro-optic modulation (20%) was demonstrated in a 3 μm thick film at a low ac field (1 V/μm, at 4 kHz). Electro-optic modulation in such thin films is suitable for a wide range of applications in free-space interconnects and various modulator structures with light propagating perpendicular to the film. The magnitude of the electro-optic coefficient at 720 nm, r11∼530 pm/V, is significantly larger than that of bulk crystals. © 1999 American Institute of Physics. View full abstract»

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  • Long wavelength superlattice quantum cascade lasers at λ17 μm

    Page(s): 638 - 640
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    We report the realization of a semiconductor injection laser based on intraband transitions with emission wavelengths extending beyond the atmospheric windows. The structure uses the quantum cascade scheme with “chirped” superlattices as active material. Laser action in pulsed operation is achieved at λ≃17 μm up to 150 K, with peak output powers of ∼12 mW at cryogenic temperatures. © 1999 American Institute of Physics. View full abstract»

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  • White light emission induced by confinement in organic multiheterostructures

    Page(s): 641 - 643
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    A technique for inducing white-light emission from organic multiheterostructures is proposed. The configuration of organic multiheterostructure white-light emitting diodes is ITO/TPD(50 nm)/BePP2(5 nm)/TPD(4 nm)/BePP2:rubrene(5 nm)/TPD(4 nm)/Alq3(10 nm)/Al. Triphenyldiamine derivative (TPD) is used as a hole-transporting layer and the potential barrier layers. Blue fluorescent phenylpyridine beryllium (BePP2), orange fluorescent rubrene, and green fluorescent aluminum complex (Alq3) are used as three primary colors. BePP2 and BePP2 doped with rubrene act as the potential wells sandwiched between TPD barrier layers, in which excitons are confined. Alq3 is used as an electron-transporting green-color emitter. The white-light emission spectrum covers a wide range of the visible region, and the Commission Internationale de l’Eclairage coordinates of the emitted light are (0.32, 0.38) at 9 V. The maximum brightness and luminous efficiency of this device are 4000 cd/m2 (at 17 V) and 0.4 lm/W, respectively. © 1999 American Institute of Physics. View full abstract»

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  • Growth of straight nanotubes with a cobalt–nickel catalyst by chemical vapor deposition

    Page(s): 644 - 646
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    In this letter, we report the catalytic synthesis of a large amount of straight carbon nanotubes using a transition-metal cobalt–nickel/zeolite catalyst. High-resolution transmission electron microscopy images show that they are well graphitized. Raman spectrum shows its peak at 1349 cm-1 (D band) is much weaker than that at 1582 cm-1 (G band). We believe that straight carbon nanotubes contain much less defects than curved nanotubes and might have potential applications in the future. © 1999 American Institute of Physics. View full abstract»

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  • Determination of elastic constants in thin films using hydrogen loading

    Page(s): 647 - 649
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    By measuring stress and strain that build up in thin films during hydrogen absorption, the elastic constants of the films can be determined, if a one-dimensional elastic behavior occurs only. This will be demonstrated for hydrogen absorption in Nb films. The in-plane stress is determined from the substrate curvature that is measured by using a two-beam laser setup. The out-of-plane strain is measured via x-ray diffraction. Furthermore, this method allows us to distinguish whether the film is plastically or elastically deformed by checking the reversibility of the stress–strain curve. In the case of a 250-nm-thick Nb film, the elastic constants obtained are similar to that of bulk Nb. © 1999 American Institute of Physics. View full abstract»

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  • Diamond/Ir/SrTiO3: A material combination for improved heteroepitaxial diamond films

    Page(s): 650 - 652
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    Heteroepitaxial diamond films with highly improved alignment have been realized by using the layer sequence diamond/Ir/SrTiO3(001). In a first step, epitaxial iridium films with a misorientation ≪0.2° have been deposited on polished SrTiO3(001) surfaces by electron-beam evaporation. Using the bias-enhanced nucleation procedure in microwave plasma chemical vapor deposition, epitaxial diamond grains with a density of 109cm-2 could be nucleated on these substrates. The orientation relationship for this layer system is diamond(001)[100]||Ir(001)[100]||SrTiO3(001)[100]. The polar and azimuthal spread for the crystal orientation of a 600 nm thick diamond film is about in each case. For an 8 μm thick diamond film a significantly improved alignment of 0.34° (polar) and 0.65° (azimuthal) has been measured. The latter values, which to the best of our knowledge are superior to those of all former reports about epitaxial diamond films on alternative substrates, indicate the high potential of the substrate Ir/SrTiO3 for the realization of large-area single-crystalline diamond films. © 1999 American Institute of Physics. View full abstract»

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  • Selective-area formation of Si microstructures using ultrathin SiO2 mask layers

    Page(s): 653 - 655
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    We have developed a technique to form Si microstructures at preassigned positions on Si substrates. The key element of this technique is resistless patterning of ultrathin SiO2 mask layers by direct electron-beam exposure. Selective-area growth of Si was accomplished by two different chemistries: flow-modulated plasma-enhanced chemical vapor deposition (CVD) at 473 K or ultra-high-vacuum CVD at 853 K. Epitaxial deposition was achieved by the latter growth method when a mask layer with minimum thickness for deposition selectivity (approximately 0.2 nm) was employed. © 1999 American Institute of Physics. View full abstract»

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  • Finite-element modeling of diamond deformation at multimegabar pressures

    Page(s): 656 - 658
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    Finite-element modeling calculations reveal the origin of the remarkably large elastic strains in diamond observed in recent experiments at multimegabar pressures. This approach provides a means to determine the pressure dependence of the yield strength of strong materials used in the gasket, and allows us to examine quantities that are not accessible experimentally such as the stress and strain relations in diamond. Stress tensor elements are obtained near the tip where large modifications in the optical properties of diamond have been observed. © 1999 American Institute of Physics. View full abstract»

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  • Low-pressure transformation of graphite to diamond under irradiation

    Page(s): 659 - 660
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    It is demonstrated experimentally that an irradiation-induced transformation of planar as well as curved graphite to diamond can be carried out without applying pressure. A specimen containing both graphite and diamond is irradiated at high temperature with an electron beam in an electron microscope. It is observed that the diamond crystals grow on the expense of graphite, although graphite is, at the low pressure prevailing here, the stable phase of carbon. © 1999 American Institute of Physics. View full abstract»

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  • Occurrence of cubic GaN and strain relaxation in GaN buffer layers grown by low-pressure metalorganic vapor phase epitaxy on (0001) sapphire substrates

    Page(s): 661 - 663
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    Investigations on GaN buffer layers grown by low-pressure metalorganic vapor phase epitaxy on (0001) sapphire substrates indicated that the mechanisms by way of which GaN buffer layers relax stresses introduced by the lattice mismatch and thermal expansion coefficient difference between GaN epilayer and sapphire substrate are related to both the crystallographic structure of GaN and thickness of the buffer layers. Beside forming misfit dislocations, mismatch-induced stresses can also be relaxed by forming stacking faults and microtwin boundaries parallel to (11-1) of GaN near the interface between GaN and sapphire substrate in cubic GaN buffer layers. It was found that, in cubic GaN buffer layers, there exists a critical thickness within which the stacking faults and/or microtwin boundaries parallel to (11-1) of GaN can be formed. This critical value is determined to be 50 nm. © 1999 American Institute of Physics. View full abstract»

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  • High-strength materials produced by precipitation of icosahedral quasicrystals in bulk Zr–Ti–Cu–Ni–Al amorphous alloys

    Page(s): 664 - 666
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    Zr62-xTixCu20Ni8Al10 (3≤x≤5) amorphous alloys crystallize via precipitation of icosahedral quasicrystals in the primary crystallization step, leading to nano-sized quasicrystals embedded in an amorphous matrix. Ti is the decisive component favoring the precipitation of quasicrystals. The mechanical properties of the crystallized alloys with different amounts of quasicrystalline phase were measured by compression and bending tests. If the volume fraction of quasicrystalline precipitates is below about 50%, the strength increases with an increasing amount of quasicrystalline precipitates, but the ductility does not decrease significantly in comparison with the amorphous counterpart. The fracture stress reaches 1835 MPa for 50 vol % of quasicrystals. Quasicrystalline precipitates of more than 60 vol % lead to reduction of ductility and strength. This shows a way of producing bulk quasicrystalline materials of high strength by crystallization of bulk amorphous alloys. © 1999 American Institute of Physics. View full abstract»

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  • Tunneling spectroscopy and spectroscopic imaging of granular metallicity of polyaniline

    Page(s): 667 - 669
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    The band-gap electronic structures of two forms of the conducting polymer polyaniline, emeraldine base (EB) and emeraldine salt (ES), have been studied. Tunneling spectroscopy measurements show that ES is metallic in nature with a finite electron density of state at the Fermi energy (EF), while EB behaves as an insulator with a gap at EF in agreement with theoretical calculations. These results allowed us to obtain direct evidence of the granular metallicity of ES by spectroscopic imaging. The spectroscopic images show different spatial distributions of nanometer-sized metallic particles in ES and EB samples, providing important information needed in studying electrical conductivities of conducting polymers. © 1999 American Institute of Physics. View full abstract»

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  • Interfacial electronic structures in an organic light-emitting diode

    Page(s): 670 - 672
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    Direct measurements of electronic properties have been made for the metal/organic and organic/organic interfaces in a multilayer organic light-emitting diode (LED) using ultraviolet photoemission spectroscopy. The device configuration considered is indium–tin oxide (ITO)/copper phthalocyanine (CuPc)/N,N-bis-(1-naphyl)-N,N-diphenyl-1,1-biphenyl-4,4-diamne (NPB)/8-hydroxyquinoline aluminum (Alq)/Mg. For the material interfaces considered here, our result indicates that the traditional concept of vacuum-level alignment, though not valid for metal/organic interfaces, still holds at organic/organic interfaces. This implies that little charge transfer occurs at the interfaces due to the small interaction between organic molecules. The largest band offsets are observed between the lowest unoccupied state levels of the organic molecules. Based on the directly measured energy-level alignments, a model is proposed to explain the improved efficiency of multilayer organic LEDs, as compared to those with a single organic layer structure. © 1999 American Institute of Physics. View full abstract»

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  • Depth profile and lattice location analysis of Sb atoms in Si/Sb(δ-doped)/Si(001) structures using medium-energy ion scattering spectroscopy

    Page(s): 673 - 675
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    Medium-energy coaxial impact-collision ion scattering spectroscopy has been used to study the depth profile and lattice location of Sb atoms in Si/Sb(δ-doped)/Si(001) structures prepared by solid phase epitaxy. The Sb atoms are observed to diffuse into the Si capping layer at concentrations much higher than the solubility limit in a Si crystal. In addition, the concentration of diffused Sb atoms does not show a monotonic decrease with increasing distance from the δ-layer plane. The lattice locations of the diffused Sb atoms are found to be strongly dependent on the distance from the location of the original Sb δ layer. © 1999 American Institute of Physics. View full abstract»

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  • Time resolved magneto-optical spectroscopy on InGaAs nanostructures grown on (311)A and (100)-oriented substrates

    Page(s): 676 - 678
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    We present a time-resolved magneto-photoluminescence study of In0.5Ga0.5As self-organized nanostructures grown on (100) and (311)A-oriented substrates by molecular beam epitaxy. The (311)A-oriented samples have a corrugated surface realizing a sort of quantum wire array, whereas the (100) samples exhibit Stranski–Krastanow islands. The different morphology of the nanostructures is reflected in the different electron/hole wave-function confinement along the three directions (perpendicular and parallel to the growth direction). We discuss the effects of the magnetic field (up to 8 T) on the recombination mechanism in these InGaAs nanostructures and on the transient dynamics of photoluminescence. We observe a clear decrease of the photoluminescence decay time with magnetic field flux indicating the exciton nature of the radiative low-temperature recombination processes. © 1999 American Institute of Physics. View full abstract»

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  • Control of diffusion of implanted boron in preamorphized Si: Elimination of interstitial defects at the amorphous-crystal interface

    Page(s): 679 - 681
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    Transient-enhanced diffusion (TED) during thermal annealing of ion-implanted B in Si is well established and attributed to the ion-induced, excess interstitials. On the other hand, the mechanism to account for TED of B in preamorphized (PA) Si remains unclear. Enhanced diffusion of the B persists in regrown layers even though the ion-induced interstitial defects responsible for TED in B+-only implanted Si are eliminated following regrowth. To test the hypothesis that TED in PA Si results from the “excess” interstitial-type defects below the amorphous-crystalline (a-c) interface, a buried PA layer has been recrystallized from the surface inward to the SiO2 interface of silicon-on-insulator material to eliminate all possible sources of excess interstitials. The effect on B diffusion and the role of the residual interstitial-type defects will be discussed. © 1999 American Institute of Physics. View full abstract»

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  • X-ray diffraction pole figure evidence for (111) sidewall texture of electroplated Cu in submicron damascene trenches

    Page(s): 682 - 684
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    The crystallographic texture of electroplated Cu in damascene trenches has been examined by x-ray diffraction pole figure analysis. The influence of two post-plating treatments on the resulting orientation of (111) planes of the Cu inside the trenches are compared. When the as-deposited small-grained Cu is allowed to recrystallize at room temperature before chemical mechanical polishing of the overlying Cu, we observe only a (111) fiber texture of the Cu inside the trenches. In contrast, when the overlying material is polished away before recrystallization of the small-grained Cu, pole figures show evidence of sidewall texture of the (111) planes in addition to the (111) fiber texture in the as-deposited as well as the annealed state. The presence or absence of a sidewall texture component in the pole figures offers insight into the evolution of the microstructure of damascene Cu. © 1999 American Institute of Physics. View full abstract»

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  • Amphoteric behavior of arsenic in HgCdTe

    Page(s): 685 - 687
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    The properties of arsenic in HgCdTe are predicted using ab initio calculations and a statistical theory. Predictions on the amphoteric nature of arsenic are in good agreement with experimental results on material growth both by liquid phase epitaxy and molecular beam epitaxy (MBE). The experimentally observed dependence of the arsenic diffusion on mercury partial pressure is also explained by our results. A microscopic model for activating the arsenic as an acceptor is suggested, and requirements of post-MBE-growth activation anneals are identified. © 1999 American Institute of Physics. View full abstract»

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  • Study of Mg diffusion during metalorganic chemical vapor deposition of GaN and AlGaN

    Page(s): 688 - 690
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    The diffusion behaviors of Mg in GaN and AlGaN layers are investigated using otherwise undoped GaN test structures containing three Mg-dopant spikes. These simplified structures enable accurate dopant profiling by avoiding the formation of nonplanar V-shaped defects, as confirmed by atomic force microscopy measurements. We also study the Mg distribution in GaN:Mg/AlGaN heterostructures. In all cases, no significant diffusion/segregation behaviors were observed for Mg in nitride material systems at 1060 °C over a 1.25 h time. © 1999 American Institute of Physics. View full abstract»

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  • Substitutional Ge in 3C–SiC

    Page(s): 691 - 693
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    The incorporation of substitutional Ge into 3C–SiC alloys is studied theoretically with an anharmonic Keating model specifically adapted to the computation of the structural properties and the lattice dynamics of Si1-x-yGexCy alloys. Basic energy calculations show that the substitution of Si by Ge is more probable than the substitution of C by Ge in the zinc-blende silicon carbide crystal. If Ge replaces only Si, then the lattice parameter equals (0.43593±0.00002)+(0.000337±0.000002)y, where y stands for the Ge content. Hence, Vegard’s law is not applicable. The alloy is characterized by a distinct phonon spectrum whose maximum peak position in cm-1 is best described by the exponential decay (243±1)+(27±2)exp[-y/(7.5±1.2)] up to the zinc-blende GeC compound. © 1999 American Institute of Physics.   View full abstract»

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  • Observation of two atomic configurations for the {12¯10} stacking fault in wurtzite (Ga, Al) nitrides

    Page(s): 694 - 696
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    High resolution electron microscopy has been used to determine the atomic structure of the {12¯10} stacking fault which easily folds from prismatic to basal plane in wurtzite (Al, Ga)N epitaxial layers. This letter reports experimental evidence for a simultaneous occurrence of two atomic configurations for the {12¯10} stacking fault. In more than 90% of the cases, it takes the atomic structure of the 1/2<101¯1>{12¯10} stacking fault; and in very few nanometric areas, it exhibits a 1/6<202¯3> stacking fault atomic configuration. © 1999 American Institute of Physics.   View full abstract»

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  • Formation of oriented particles in an amorphous host: ZnS nanocrystals in silicon

    Page(s): 697 - 699
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    Processes for incorporating randomly oriented crystalline precipitates in an amorphous host can be traced back to the 17th century when Cassius produced “gold ruby” glass. In this glass, octahedral colloidal precipitates of gold scatter light by the Mie process to produce a deep red color. In contrast to gold ruby glass, we describe a type of material in which the crystalline precipitates are crystallographically aligned in a coherent manner—even though they are dispersed in an amorphous matrix. Ion implantation and thermal processing are first used to form zinc sulfide nanocrystals that are coherently oriented with respect to a crystalline Si host. The Si is then amorphized by ion irradiation leaving the highly radiation-resistant ZnS precipitates in an aligned crystalline state. The process is anticipated to find applications in the creation of surfaces with unique optoelectronic properties. © 1999 American Institute of Physics. View full abstract»

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  • Effect of the end-of-range loop layer depth on the evolution of {311} defects

    Page(s): 700 - 702
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    The interactions between end-of-range dislocation loops and {311} defects as a function of their proximity were studied. The dislocation loops were introduced at 2600 Å by a dual 1×1015cm-2, 30 keV and a 1×1015cm-2, 120 keV Si+ implantation into silicon followed by an anneal at 850 °C for 30 min. The depth of the loop layer from the surface was varied from 2600 to 1800 Å and 1000 Å by polishing off the Si surface using a chemical–mechanical polishing (CMP) technique. A post-CMP 1×1014cm-2, 40 keV Si+ implantation was used to create point defects at the projected range of 580 Å. The wafers were annealed at 700, 800, and 900 °C, and plan-view transmission electron microscopy study was performed. It was found that the number of interstitials in {311} defects decreased as the projected range damage was brought closer to the loop layer, while the number of rectangular elongated defects (REDs) increased. Experimental investigation showed that REDs are formed at the end of range. It is concluded that the interstitials introduced at the projected range are trapped at the end of range. The REDs are formed due to the interactions between the interstitials and the pre-existing dislocation loops. © 1999 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