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Applied Physics Letters

Issue 4 • Date Jul 1997

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Displaying Results 1 - 25 of 49
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Time-resolved photoluminescence studies of InGaN/GaN single-quantum-wells at room temperature

    Page(s): 425 - 427
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    We present a room-temperature study of the well-width-dependent carrier lifetimes in InGaN single-quantum wells. At room temperature, carrier recombination was found to be dominated by interface-related nonradiative processes. The dominant radiative recombination at room temperature was through band-to-band free carriers. For the sample grown at a higher growth rate, we observed a longer luminescence lifetime, which was attributed to an improved quantum well interface. © 1997 American Institute of Physics. View full abstract»

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  • Preparation and characterization of sol-gel derived Er3+: Al2O3–SiO2 planar waveguides

    Page(s): 428 - 430
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    Er3+ doped aluminosilicate thin films were prepared on silica and silica Si substrates by the sol-gel method. The Er ion concentration was varied from 0.25% to 1%. Fluorescences of Er ions in the visible and infrared regions were characterized. Green and red up-conversion fluorescences centered at 548 and 655 nm for, respectively, the (2H11/2+4S3/2)→4I15/2 and 2F9/24I15/2 were observed. A broadband peak was observed at 1531 nm corresponding to the 4I13/24I15/2 transition, with the full width at half-maximum of 47 nm comparable to those prepared by other methods. The fluorescence lifetime of the 4I13/2 excited state was found to be constant and equal to 3.5 ms until a high Er doping of 0.5 at. %.© 1997 American Institute of Physics. View full abstract»

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  • Changes in electrical conductance of hydrogenated amorphous silicon deposited on optical waveguides in glass

    Page(s): 431 - 433
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    A study of electrical conductance of hydrogenated amorphous silicon (a-Si:H) films deposited on optical waveguides in a soda-lime glass (SLG) substrate is carried out, from the viewpoint of electrical instability of a-Si:H caused by the penetration of Na ions from the glass into the film. The optical waveguides were prepared by K+Na+ or Ag+Na+ ion exchange using thermal or field-assisted methods. The effective thickness of optical waveguides was of several micrometers. The obtained results show that in the case of a-Si:H film deposited on silver waveguide there is a dependence of electrical conductance on measurement duration, if strongly weakened as compared with film deposited on the original SLG substrate. The a-Si:H films deposited on potassium waveguide and on Na-extracted surface are as stable, as the referent samples with Corning 7059 (Na-free) glass substrate. © 1997 American Institute of Physics. View full abstract»

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  • Ultrafast all-optical switch using complex refractive index changes of thin films containing photochromic dye

    Page(s): 434 - 436
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    An all-optical switch has been proposed based on photoinduced changes of an imaginary part of complex refractive index in a composite thin film made by silver and polymer thin films containing photochromic dye. Very fast switching on and switching off of a reading beam was demonstrated at the incident angle of guided wave mode using a photochromic spiropyran-doped polystyrene thin film and a ns pulsed laser as a writing beam. These responses were caused by the changes of an extinction coefficient due to photochromism. No applied power was required to hold the switched state. © 1997 American Institute of Physics. View full abstract»

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  • Use of solid electrolytic erosion for generating nano-aperture near-field collectors

    Page(s): 437 - 439
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    A new way in the fabrication of metallized nano-apertures for near-field optical microscopy is proposed. The method is based on the electrolysis between a silver metallized probe and an electrolytic glass. By dry electrolytic erosion, the metal on the tip apex is removed and lead to a very small metal free aperture. This method applied here to tapered fibers, can be extended to any other metallized tip in a rather reproducible way. © 1997 American Institute of Physics. View full abstract»

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  • Electrically pumped tunable terahertz emitter based on intersubband transition

    Page(s): 440 - 442
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    An electrically pumped three-level system was designed and fabricated using an AlGaAs/GaAs multiple quantum well structure. Under appropriate biases, the structure emits terahertz (THz) radiation, as a result of diagonal (or interwell) intersubband transition. The emission spectra were resolved using an external Fourier transform infrared spectrometer. The center frequency of the emission was voltage tunable. The emission spectra and the power-voltage relation showed clear evidence that the middle level was depopulated by fast longitudinal optical-phonon scattering, thus, a population inversion between the two upper levels is feasible. © 1997 American Institute of Physics. View full abstract»

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  • Active mode locking of a p-Ge hot hole laser

    Page(s): 443 - 445
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    The generation of 200 picosecond pulses of far-infrared radiation from a p-Ge hot hole laser (50–140 cm-1) was achieved due to active mode locking by electrical intracavity modulation of the gain. © 1997 American Institute of Physics. View full abstract»

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  • An optically addressed modulator based on low-temperature-grown multiple quantum well GaAlAs

    Page(s): 446 - 448
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    We report an optically addressed multiple quantum well spatial light modulator with large modulation gain which can be used in applications such as amplified incoherent-to-coherent conversion. The novel device design utilizes separate detector and modulator regions to obtain large gain. The design also incorporates low-temperature-grown quantum wells in the modulator region to reduce the effects of high-intensity readout. Our preliminary data show that modulation gain of 45 and modulation depth of 40% can be obtained by simply applying a dc bias. However, our data also show that the modulation gain decreases at higher writing frequencies, indicating that refreshing the device is required after each write-read operation. © 1997 American Institute of Physics. View full abstract»

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  • THz optical beat frequency generation from a single mode locked semiconductor laser

    Page(s): 449 - 451
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    We demonstrate ultrahigh repetition rate beat signal generation by filtering two modes from the optical spectrum of a 110 GHz pulse train produced by a 1.56 μm wavelength, subharmonic hybrid mode locked monolithic semiconductor distributed Bragg reflector laser. The beat signal repetition rate can be tuned to any higher harmonic of the 110 GHz frequency up to 1.34 THz. Furthermore, the filtering technique facilitates low phase noise and synchronization with these signals. © 1997 American Institute of Physics. View full abstract»

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  • Demonstration of a micro far-infrared Smith–Purcell emitter

    Page(s): 452 - 454
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    A diffraction grating mounted in the focal region of a modified scanning electron microscope has been used to produce far-infrared Smith–Purcell radiation. The electron beam radius, voltage, and current ranges were 3–27 μm, 27–40 kV, and 10–140 μA, respectively. The micron scale of the electron beam was well matched to the evanescent surface waves of the grating. The measured wavelength range extended from 288 to 803 μm, and both the wavelengths and the intensities observed were in accord with expectations based on theory. The details of the experiment and a summary of the results will be discussed. © 1997 American Institute of Physics. View full abstract»

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  • Optical metastability in bulk GaN single crystals

    Page(s): 455 - 457
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    Bulk GaN single crystals were grown from cold pressed GaN powder by sublimation in flowing ammonia. Optical transmission measurements indicated that the absorption coefficient for the transparent samples is 50 cm-1 in the wavelength region from 650 to 400 nm. Optical metastability in bulk GaN crystals was studied through time dependent photoluminescence both at room and liquid–nitrogen temperatures. The observation included decreasing output intensity of the ultraviolet emission attributed to the band edge and increasing output intensity of a new emission band centered at 378 nm at room temperature. At liquid–nitrogen temperature, the photoinduced emission band consisted of at least one LO-phonon replica of the zero-phonon line centered at 378 nm. The ratio of output intensities of the photoinduced band to the band edge increased by a factor of 10 during 27 min of exposure time. The photoinduced effect is attributed to the metastable nature of traps in bulk GaN. © 1997 American Institute of Physics. View full abstract»

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  • Aspect ratio independent etching of dielectrics

    Page(s): 458 - 460
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    Monte Carlo simulations of pattern-dependent charging during oxide etching predict that the etch rate scaling with aspect ratio breaks down when surface discharge currents are significant. Under conditions of ion-limited etching and no inhibitor deposition, the etch depth depends on the maximum incident ion energy, reaction threshold, and surface discharge threshold, and is the same irrespective of the trench width (0.5 μm). © 1997 American Institute of Physics. View full abstract»

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  • Switching behavior of surface stabilized ferroelectric liquid crystals induced by pulse voltages

    Page(s): 461 - 463
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    The switching behavior of surface stabilized ferroelectric liquid crystals has been studied using a simple uniform switching model in an applied electric field. We measured the response time as a function of pulse voltage for a ferroelectric liquid crystal material with a negative dielectric anisotropy, which shows a minimum in the response time-voltage curve, and found good agreement between the theory and the experimental results. Accordingly, the parameters appearing in the switching model, the initial azimuthal angle, rotational viscosity, and surface anchoring energy can be determined. © 1997 American Institute of Physics. View full abstract»

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  • Bulk amorphous alloys with high mechanical strength and good soft magnetic properties in Fe–TM–B (TM=IV–VIII group transition metal) system

    Page(s): 464 - 466
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    New bulk amorphous alloys exhibiting a wide supercooled liquid region before crystallization were found in Fe–(Co,Ni)–(Zr,Nb,Ta)–(Mo,W)–B systems. The Tg is as high as about 870 K and the supercooled liquid region reaches 88 K. The high thermal stability of the supercooled liquid enabled the production of bulk amorphous alloys with diameters up to 6 mm. These bulk amorphous alloys exhibit a high compressive strength of 3800 MPa, high Vickers hardness of 1360, and high corrosion resistance. Besides, the amorphous alloys exhibit a high magnetic-flux density of 0.74–0.96 T, low coercivity of 1.1–3.2 A/m, high permeability exceeding 1.2×104 at 1 kHz, and low magnetostriction of about 12×10-6. © 1997 American Institute of Physics. View full abstract»

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  • Thermal stability of Ir/polycrystalline-Si structure for bottom electrode of integrated ferroelectric capacitors

    Page(s): 467 - 469
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    Ir films deposited on polycrystalline silicon (poly-Si), with and without barrier layer, were annealed and the thermal stability was investigated to check the feasibility of the structure for bottom electrode of integrated ferroelectric capacitors. Ir did not form silicide up to 700 °C and did not get oxidized up to 550 °C. It was found that Ir prevented diffusion of oxygen through it when annealed at 700 °C. Ir/poly-Si is believed to be most promising for bottom electrode structure from the results. © 1997 American Institute of Physics. View full abstract»

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  • Powder diffraction from compressed molecular hydrogen in a diamond-anvil cell

    Page(s): 470 - 472
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    We report the first observation of x-ray powder-diffraction from molecular hydrogen in a diamond-anvil cell obtained at the European Synchrotron Radiation Facility using image-plate recording techniques. The sample was prepared by compression of hydrogen together with nickel powder in the high-pressure chamber. This result suggests that structural phase transitions in solid hydrogen could be studied directly with powder x-ray techniques. © 1997 American Institute of Physics. View full abstract»

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  • Optimized growth conditions for the epitaxial nucleation of β-GaN on GaAs(001) by molecular beam epitaxy

    Page(s): 473 - 475
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    Ultrathin (5–7 monolayers) nucleation layers of GaN are deposited on (2×4)-GaAs(001) by plasma-assisted molecular beam epitaxy. Reflection high-energy electron diffraction applied in situ reveals these layers to be epitaxial β-GaN. Transmission electron microscopy confirms this result and reveals in addition that the layers are highly connected and have an atomically abrupt interface to the GaAs substrate. The rms roughness of these layers, as measured by atomic force microscopy, is as low as 1.4 Å. © 1997 American Institute of Physics. View full abstract»

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  • Fracture toughness and fatigue-crack propagation in a Zr–Ti–Ni–Cu–Be bulk metallic glass

    Page(s): 476 - 478
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    The recent development of metallic alloy systems which can be processed with an amorphous structure over large dimensions, specifically to form metallic glasses at low cooling rates (∼10 K/s), has permitted novel measurements of important mechanical properties. These include, for example, fatigue-crack growth and fracture toughness behavior, representing the conditions governing the subcritical and critical propagation of cracks in these structures. In the present study, bulk plates of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy, machined into 7 mm wide, 38 mm thick compact-tension specimens and fatigue precracked following standard procedures, revealed fracture toughnesses in the fully amorphous structure of KIc55 MPam, i.e., comparable with that of a high-strength steel or aluminum alloy. However, partial and full crystallization, e.g., following thermal exposure at 633 K or more, was found to result in a drastic reduction in fracture toughness to ∼1 MPa√m, i.e., comparable with silica glass. The fully amorphous alloy was also found to be susceptible to fatigue-crack growth under cyclic loading, with growth-rate properties comparable to that of ductile crystalline metallic alloys, such as high-strength steels or aluminum alloys; no such fatigue was seen in the partially or fully crystallized alloys which behaved like very brittle ceramics. Possible micromechanical mechanisms for such behavior are discussed. © 1997 American Institute of Physics. View full abstract»

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  • Effects of rapid thermal annealing on GaInP/AlGaInP lasers grown by all-solid-source molecular beam epitaxy

    Page(s): 479 - 481
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    We have examined the influence of rapid thermal annealing on the performance characteristics of GaInP/AlGaInP quantum well lasers which were grown by all-solid-source molecular beam epitaxy. It was found that when the laser structures were annealed the threshold current densities of the lasers decreased significantly. This improvement in lasing performance could be associated with the possibility that annealing removed nonradiative recombination centers from the quantum wells. The emission wavelength, differential quantum efficiency, and characteristic temperature were not affected to any remarkable extent, indicating that the interdiffusion of group-III elements did not damage the structures. © 1997 American Institute of Physics. View full abstract»

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  • Photoluminescence of CuInS2 thin films and solar cells modified by postdeposition treatments

    Page(s): 482 - 484
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    The photoluminescence of CuInS2 thin films and solar cells is investigated as a function of postdeposition treatments for different temperatures and excitation intensities. Annealing in hydrogen atmosphere causes an increase of PL intensity at 1.445 eV by more than a factor of 100, while subsequent annealing in oxygen or air ambient passivates this transition, which is ascribed to a donor-acceptor pair recombination between a sulphur vacancy and a copper vacancy. A defect mechanism is suggested that assumes the passivation of sulphur vacancies by oxygen in grain surfaces which can be activated by hydrogen annealing. © 1997 American Institute of Physics. View full abstract»

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  • The effect of nitrogen ions emitted from a plasma source on molecular beam epitaxial growth of p-ZnSe:N

    Page(s): 485 - 487
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    Excited neutral nitrogen species emitted from a rf plasma source were characterized by the laser-induced fluorescence (LIF) spectroscopy, while nitrogen ions were detected by the ion counting method. The LIF intensity for nitrogen molecules increases monotonously up to the rf power of 100 W and saturates over 100 W. On the contrary, ion count of nitrogen ions shows a gradual increase up to 100 W, then rapidly increases above 100 W. The correlation between the number of excited nitrogen species and the net acceptor concentration (NA-ND) of nitrogen doped ZnSe epitaxial layers for various rf powers has been studied. We confirm that the excited neutral nitrogen molecules are effective for acceptor doping, while nitrogen ions enhance carrier compensation presumably due to degradation of crystal quality. We show that the activation ratio {(NA-ND)/[N]} of p-ZnSe:N is greatly improved by removing ions from the nitrogen plasma. © 1997 American Institute of Physics. View full abstract»

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  • Laser emission from photonic dots

    Page(s): 488 - 490
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    Laser emission was observed in photonic semiconductor dots with a discretized optical mode spectrum. The photonic dots with lateral sizes between 1 and 5 μm provide a three-dimensional optical confinement by using in the vertical direction AlAs/GaAs Bragg mirrors and in the lateral directions the refractive index discontinuity at the etched surfaces. In the optically pumped structures, the laser emission takes place on the fundamental mode of the microcavities. External threshold excitation densities of 200 W/cm2, which correspond to a very low internal optical excitation power of 0.15 μW per microcavity post, were measured for microcavity structures with a lateral size of 2.7 μm. © 1997 American Institute of Physics. View full abstract»

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  • Auger recombination dynamics of Hg0.795Cd0.205Te in the high excitation regime

    Page(s): 491 - 493
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    A direct measurement of carrier recombination, far from equilibrium, in Hg0.795Cd0.205Te (Nd-Na=3.3×1014cm-3) has been made on a picosecond time scale with a pump–probe technique using a free-electron laser. Over the range of carrier densities (5×1016–3×1017cm-3) and at the temperatures (50–300 K) studied experimentally, contributions to the recombination from Auger, Shockley–Read–Hall, and radiative mechanisms were calculated using an analytic approximation, with carrier degeneracy included, Auger-1 (CCCH) recombination rates were calculated, which also gave the Auger-7 (CHHL) rates via a simple relationship. Excellent agreement was obtained, with Auger-1 dominant at all temperatures and, significantly, for T≫225 K when the sample is intrinsic, the Auger-7 contribution was found to be important. © 1997 American Institute of Physics. View full abstract»

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  • AlGaAs/GaAs high electron mobility transistor with a low-temperature grown GaAs ion damage blocking layer

    Page(s): 494 - 496
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    We have successfully used a thin layer (∼200 Å) of annealed low-temperature GaAs (LT-GaAs) to reduce ion damage that would occur during the formation of a dry-etch gate recess of a high electron mobility transistor. Compared to structures without an ion damage blocking layer, the devices with a thin layer of LT-GaAs are more robust against ion damage. This is important for the application of ion-assisted processing to the fabrication of electronic devices, such as dry etching used to achieve gate recessing. © 1997 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