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Applied Physics Letters

Issue 19 • Date May 1998

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Displaying Results 1 - 25 of 47
  • Issue Cover

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

    Page(s): toc1
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  • Stable polarization self-modulation in vertical-cavity surface-emitting lasers

    Page(s): 2355 - 2357
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    The characteristics of polarization self-modulation in a vertical-cavity surface-emitting laser (VCSEL) were studied for frequencies up to 9 GHz both experimentally and theoretically. Polarization self-modulation was obtained by rotating the linearly polarized output of the VCSEL by 90° and reinjecting it into the laser. Experimentally we simultaneously recorded time traces, optical and radio-frequency spectra. We found for increasing modulation frequencies that the output characteristics changed from square-wave to sinusoidal and the VCSEL system assumed new polarization eigenstates that are different from the free-running VCSEL eigenstates. We modeled polarization self-modulation as an interband process and found a good qualitative agreement between our experimental and numerical results. © 1998 American Institute of Physics. View full abstract»

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  • A scaling law of the second-order hyperpolarizability in armchair nanotube

    Page(s): 2358 - 2360
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    The static second-order hyperpolarizabilities γ of armchair nanotubes C60+i×10 are studied. Their magnitudes obey a scaling law given by γ/γ60=(1+i/6)3.15, where γ60 is the static γ magnitude of C60. Based on this law, as an example, we predict that the static γ magnitude of armchair nanotube with a length of 40 Å generally seen in experiment is about 3.5×10-29 esu, which is an appropriate value needed in photonic devices, and implies that armchair nanotube is an ideal candidate among all third-order materials for photonic applications. © 1998 American Institute of Physics. View full abstract»

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  • Gas source molecular beam epitaxy of GaN with hydrazine on spinel substrates

    Page(s): 2361 - 2363
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    Growth of high quality wurtzite-structure GaN layers on (111) MgAl2O4 by gas source molecular beam epitaxy is described. Hydrazine was used as a source of active nitrogen. In situ reflection high energy electron diffraction was used to monitor the growth mode. Two-dimensional growth was obtained at temperatures above 750 °C on multi-step GaN buffer layers. The resulting GaN films show excellent luminescence properties. © 1998 American Institute of Physics. View full abstract»

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  • Birefringence phase-matched blue light second-harmonic generation in a KNbO3 ridge waveguide

    Page(s): 2364 - 2366
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    Ridged channel waveguides in KNbO3 were fabricated by a technique using He+ ion implantation, photolithographic masking, and subsequent Ar+ ion sputtering. A continuous-wave second-harmonic output power of 14 mW at 438 nm was obtained with an in-coupled fundamental power of 340 mW in a 0.73 cm long waveguide. Phase matching was provided by material birefringence without need of periodic poling. © 1998 American Institute of Physics. View full abstract»

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  • Raman scattering investigation of KH2PO4 subsequent to high fluence laser irradiation

    Page(s): 2367 - 2369
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    The spectral characteristics of the internal (PO4 tetrahedron) modes of fast-grown KH2PO4 crystals under sub-damage threshold, 10 ns, 355 nm laser irradiation have been investigated. Pump-and-probe Raman spectroscopy indicates transient changes of the intensity of the 915 cm-1, PO4 internal mode. This change is attributed to a transient increase of the absorption due to generation by the 355 nm pump pulse of electronic defects in the bulk of the crystal. © 1998 American Institute of Physics. View full abstract»

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  • Near-room-temperature mid-infrared interband cascade laser

    Page(s): 2370 - 2372
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    A 25-stage interband cascade laser with a W active region and a third hole quantum well for the suppression of leakage current has exhibited lasing in pulsed mode up to 286 K. A peak output power of 160 mW/facet and a slope efficiency of 197 mW/A per facet (1.1 photons per injected electron) were measured at 196 K. Above 200 K, the characteristic temperature was higher (T0=53 K) and the threshold current densities lower than for a previously reported W interband cascade laser without the third hole quantum well. © 1998 American Institute of Physics. View full abstract»

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  • Persistent high density spectral holeburning in CaS:Eu and CaS:Eu,Sm phosphors

    Page(s): 2373 - 2375
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    Persistent spectral hole-burning has been reported for singly, Eu-doped, and doubly, Eu- and Sm-doped, CaS phosphors. Efficient photon gated holeburning in the 4f7 (8S7/2)-4f65d1 transition of Eu2+ is a result of photoionization of Eu2+ to Eu3+. These holes have a width of ≪5 GHz (2 K), survive thermal cycling of the phosphor up to the room temperature, 300 K, and have no detectable deterioration over more than a day of storage time at low temperature (2 K). Although self-gated holeburning is observed with the reading laser at higher powers, the photon budget for reading these holes is so small that in excess of 1000 reading cycles can be performed without destroying the optical signal. The nature of holes burned by photon-gating is found to be very different from the self-gated holes. The characteristics for the holeburning are the same in singly and doubly doped phosphors, suggesting that under the conditions of our experiments, Sm traps do not play any significant role in spectral holeburning. Possibilities of high density optical memory storage using photon-gated holeburning in this THz broad transition are discussed. © 1998 American Institute of Physics. View full abstract»

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  • Resonant cavity enhanced detectors embedded in photonic crystals

    Page(s): 2376 - 2378
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    We report a resonant cavity enhanced (RCE) detector built around a three-dimensional photonic band gap crystal. The RCE detector was built by placing a monopole antenna within the localized modes of planar and boxlike defect structures. The enhanced electric field around these defect structures were then measured by a microwave detector and a network analyzer. We measured a power enhancement factor of 3450 for planar cavity structures. A Fabry–Perot cavity model was used to understand and predict resonant cavity enhancement in this structure. The tuning bandwidth of the RCE detector extends from 10.5 to 12.8 GHz, which corresponds to the full photonic band gap by the crystal. These RCE detectors have increased sensitivity and efficiency when compared to conventional detectors, and can be used for various applications. © 1998 American Institute of Physics. View full abstract»

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  • Light-coupling masks for lensless, sub-wavelength optical lithography

    Page(s): 2379 - 2381
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    Light-coupling masks (LCMs) based on structured organic polymers that make conformal contact with a substrate can constitute an amplitude mask for light-based lithographies. The LCM is exposed through its backside, from where the light is differentially guided by the structures towards the substrate. Images of arbitrarily shaped features having dimensions much smaller than that of the vacuum wavelength of the exposing light are formed in the resist in a 1:1 correspondence to their size in light-guiding portions of the mask. LCMs allow pattern replication at high resolution and densities over large areas in photoresist without the need for elaborate projection optics. © 1998 American Institute of Physics. View full abstract»

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  • Use of ionomer as an electron injecting and hole blocking material for polymer light-emitting diode

    Page(s): 2382 - 2384
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    Light-emitting diodes of poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEHPPV) are fabricated using sodium sulfonated polystyrene (SSPS) ionomer containing 10 mol % ionic groups as an electron injecting and hole blocking material. When an electric field stronger than 2.7×107V/m is applied, SSPS in the indium–tin oxide ITO/MEH-PPV/SSPS/Al system causes a bridging effect between sodium ions and the Al cathode at the SSPS/Al interface, leading to excellent electron injection. Furthermore, the ionomer has a high band gap energy of ∼5 eV, resulting in hole blocking. The operating voltage for the ITO/MEH-PPV/SSPS/Al is reduced by ∼60% and the relative quantum efficiency is enhanced by three orders of magnitude compared with those of the corresponding single-layer MEH-PPV device. © 1998 American Institute of Physics. View full abstract»

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  • Femtosecond laser ablation of gallium arsenide investigated with time-of-flight mass spectroscopy

    Page(s): 2385 - 2387
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    We have investigated femtosecond laser-induced ablation of gallium arsenide using time-of-flight mass spectroscopy. At the ablation threshold, we estimated surface temperatures on the order of 3500 K. We observed a clear thresholdlike effect in the number of detected particles and with increasing fluence free flight desorption transforms into a collisional expansion process. Above the ablation threshold, the behavior of gallium particles can be quantitatively described through Knudsen-layer theory. © 1998 American Institute of Physics. View full abstract»

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  • High-power inter-miniband lasing in intrinsic superlattices

    Page(s): 2388 - 2390
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    We report the realization of a mid-infrared (λ≃7 μm) quantum-cascade laser, in which the emission process takes place between the two lowest minibands of an intrinsic superlattice. Contrary to previous lasers based on doped superlattices, here the dopants are located only inside suitably designed injector regions, where positive ionized donors and negative electrons are arranged to compensate the applied external field across the superlattices. This reduces impurity scattering and translates into low threshold currents (4.2 kA/cm2 at 10 K) and into room temperature operation, without compromising the large current-carrying capabilities of the minibands. Peak powers of ∼1.3 W per facet have been obtained from broad-area devices at 10 K, with still more than 1 W at 120 K and 400 mW at 200 K. Effects related to the finite size of the superlattices become visible in the spectral properties, owing to the reduced broadening, and have to be taken into account to accurately describe the laser’s behavior. © 1998 American Institute of Physics. View full abstract»

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  • Determination of negative ion density with optical emission spectroscopy in oxygen afterglow plasmas

    Page(s): 2391 - 2393
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    A diagnostic scheme for the measurement of the O- density in afterglow plasmas is proposed and demonstrated. The present method is based on the fact that excited oxygen atoms (O*) are produced by mutual neutralization between positive and negative ions (O++O-O*+O). The absolute value of the emission coefficient εn due to O*O+hν is determined by visible emission spectroscopy with a calibrated detection system. The O+ density nO+ is measured by a Langmuir probe combined with mass spectrometry. With the rate coefficient for the above reaction kn, the O- density can be evaluated as nO-n/(knnO+). In the present letter, the rate coefficient kn has been determined experimentally from nO- measured by probe-assisted laser photodetachment. © 1998 American Institute of Physics. View full abstract»

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  • Measurements of negative ion densities by absorption spectroscopy

    Page(s): 2394 - 2396
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    A novel diagnostic technique for the measurement of negative ions is presented. The absorption of a cw-laser beam with a wavelength close to the maximum of the photodetachment cross section is measured. The laser beam undergoes multiple reflections through the discharge volume using a multipass cell (here a Herriott arrangement). Proof-of-principle measurements of negative hydrogen ions in a hybrid volume source are presented. The measured H- density is in agreement with results of conventional, Langmuir probe-based photodetachment measurements. © 1998 American Institute of Physics. View full abstract»

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  • Hydrogen poor cationic silicon clusters in an expanding argon–hydrogen–silane plasma

    Page(s): 2397 - 2399
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    Cationic silicon clusters SinHm+ with up to ten silicon atoms have been detected mass spectrometrically in an expanding argon–hydrogen–silane plasma used for fast deposition of amorphous hydrogenated silicon. A reaction pathway is proposed in which initial silane ions are produced by dissociative charge exchange between argon and hydrogen ions emanating from the plasma source and the admixed silane followed by chain reactions of the created ions with silane. The silicon clusters are hydrogen poor, which is ascribed to the high gas temperature as the initial argon–hydrogen plasma is thermal in origin. © 1998 American Institute of Physics. View full abstract»

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  • Sound velocity of AlxGa1-xN thin films obtained by surface acoustic-wave measurements

    Page(s): 2400 - 2402
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    To determine the sound velocity in wurtzite AlxGa1-xN, we have used surface acoustic-wave (SAW) delay lines on AlxGa1-xN/c-Al2O3. AlxGa1-xN films with compositions from x=0 to x=1 were grown by plasma-induced molecular beam epitaxy. Starting from published data, we fine tuned the values of the elastic moduli used in numerical calculations such that the simulated and measured dispersion of the SAW were in good agreement. Based on these values, the surface and bulk acoustic-wave velocities of single-crystal AlxGa1-xN were determined as functions of the composition. The resulting SAW velocities ranged from 3700 to 5760 m/s for GaN and AlN, respectively. © 1998 American Institute of Physics. View full abstract»

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  • Mechanism of unidirectional liquid-crystal alignment on polyimides with linearly polarized ultraviolet light exposure

    Page(s): 2403 - 2405
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    Unidirectional liquid-crystal (LC) alignment by a linearly polarized ultraviolet light (UV) exposure was examined using two types of polyimide (PI). PI with a fluorene unit incorporated in a side chain showed LC alignment perpendicular to those of conventional PIs, i.e., perpendicular to the rubbing direction and parallel to the exposure polarization of UV. The results of the dichroic ratio measurement of LC cells, UV absorption spectra, birefringence, infrared absorption spectra, and PI conformation calculations using molecular mechanics suggest that the photodecomposition of PI by UV exposure produces anisotropic van der Waals forces, which align LC along its optical axis. © 1998 American Institute of Physics. View full abstract»

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  • Emission of silicon cluster ions by molecular ion bombardment

    Page(s): 2406 - 2408
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    Silicon cluster ions (Sin+) with n≤8 emitted from a Si(100) surface under bombardment with 4 keV SF5+, SF+, F+, and Xe+ ions have been observed. It was shown that, in all cases, the relative cluster yields (Y) roughly exhibited a power-law dependence, Y(n)∝n, where n is the number of atoms in the cluster ion. The exponents δ of the power law were in the order of δ(SF5+)≪δ(SF+)≪δ(Xe+)≪δ(F+), which was the same order as that of the size of these ions. The results indicate that the size of the bombarded ions greatly affects the cluster-ion emission. For the SF5+ bombardments, the intensity of even-n clusters (Si2+, Si4+, Si6+) was greater than that of the adjacent cluster ions with odd-n, which is in good agreement with the calculated stability of silicon cluster ions- - in the gas phase. It is concluded that the silicon cluster ions are effectively produced by the polyatomic SF5+ bombardment through the formation of high-density collision cascades at the irradiated local area. © 1998 American Institute of Physics. View full abstract»

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  • High figure-of-merit porous Pb1-xCaxTiO3 thin films for pyroelectric applications

    Page(s): 2409 - 2411
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    The pyroelectric properties of Pb1-xCaxTiO3 (x=0–0.3) thin films have been greatly improved by including porosity in the microstructure. Control of nucleation and growth by variation of the heating rate was used to process porous, low dielectric constant films on platinized silicon wafers. Electrical characterization of poled films showed pyroelectric coefficients of up to ∼200 μC/m2 K and permitivities as low as Єr≈55. The resulting high figure-of-merit p/Єr in combination with small dielectric losses of 0.5%–2.0% at 1 kHz make the films good candidates for pyroelectric thin-film devices. Relationships between microstructure, composition, and properties were established. © 1998 American Institute of Physics. View full abstract»

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  • Channeled ion beam synthesis of heteroepitaxial Nd0.32Y0.68Si1.7 layers

    Page(s): 2412 - 2414
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    Heteroepitaxial Nd0.32Y0.68Si1.7 layers with good crystalline quality (χmin of Nd and Y is 3.5% and 4.3%, respectively) have been prepared by high dose 65 keV Y and 80 keV Nd implantation into a Si(111) substrate using channeled ion beam synthesis. Although the disilicide of Nd only exists in a tetragonal or an orthorhombic phase which cannot be grown epitaxially on a Si(111) substrate, our results show that the addition of Y to the Nd–Si system forces the latter into a hexagonal structure. Rutherford backscattering/channeling and x-ray diffraction studies reveal that the lattice parameters of the Nd0.32Y0.68Si1.7 epilayer are aepi=0.3915 nm and cepi=0.4152 nm and that the epilayer is stable up to 950 °C. Annealing at 1000 °C results in partial transformation into other unidentified phases. © 1998 American Institute of Physics. View full abstract»

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  • Recurrent local resistance breakdown of epitaxial BaTiO3 heterostructures

    Page(s): 2415 - 2417
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    Leakage current through epitaxial BaTiO3 films was investigated to clarify the difference between the characteristics of nanometer and millimeter-size metal contacts. SrTiO3:Nb bottom electrode revealed genuine properties of a single metal/BaTiO3 contact and demonstrated that breakdown voltage and leakage current density at both nanometer and millimeter-size contacts were controlled by the Schottky barrier. However, in marked contrast with millimeter-size contacts, nanometer-size contacts conducted little current below breakdown voltage and repeatedly exhibited abrupt breakdowns having a giant current density ≫10 A mm-2. The breakdown field was as high as 0.45 MV cm-1 at the forward bias, while no breakdown occurred up to 0.5 MV cm-1 at the reverse bias. © 1998 American Institute of Physics. View full abstract»

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  • The influence of cavities and point defects on boron diffusion in silicon

    Page(s): 2418 - 2420
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    Cavities, formed in Si by hydrogen implantation and subsequent annealing, can provide strong sinks for metal interstitials and are ideal gettering sites for metal impurities. This letter reports the effect of cavities on the transient enhanced diffusion (TED) of boron. Boron implantation was carried out into wafers containing pre-formed cavities and TED of boron was suppressed during subsequent annealing. In some cases, the boron was introduced into an amorphous layer and the presence of cavities was also observed to reduce the amount of the transient enhanced diffusion occurring. © 1998 American Institute of Physics. View full abstract»

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  • Crystal orientation dependence of piezoelectric properties of lead zirconate titanate near the morphotropic phase boundary

    Page(s): 2421 - 2423
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    The piezoelectric and dielectric constants in different crystal orientations of the lead zirconate titanate (PZT) have been phenomenologically calculated for the compositions near the morphotropic phase boundary at room temperature. For a tetragonal PZT, the effective piezoelectric constant d33 monotonously decreases as the crystal cutting angle from the spontaneous polarization direction [001] increases. However, for a rhombohedral PZT, the effective piezoelectric constant d33[001]// along the perovskite [001] direction was found to be much larger than those along the spontaneous polarization direction [111]. This crystal orientation-related enhancement is emphasized as the composition approaches the morphotropic phase boundary. This suggests that by adopting the perovskite [001] orientation with a rhombohedral composition near the morphotropic phase boundary, the piezoelectric constant d33 for PZT can be greatly enhanced. © 1998 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