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Applied Physics Letters

Issue 16 • Date Apr 2001

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Displaying Results 1 - 25 of 49
  • 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
  • Quantitative thermal imaging by synchronous thermoreflectance with optimized illumination wavelengths

    Page(s): 2267 - 2269
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    Using thermoreflectance microscopy with a camera, we have designed a system which delivers submicronic images of the alternative temperature variations in integrated circuits working in a modulated regime. A careful choice of the illumination wavelength permits us to highlight the heating in chosen parts of the sample and to optimize the thermoreflectance signal. We measure and explain the modifications of the photothermal response which are induced by the presence of a passivation layer. A calibration conducted on various materials with a thermocouple gives access to the absolute alternative temperature variations in integrated circuits working at frequencies between 0.1 Hz (quasipermanent regime) and 5 MHz. © 2001 American Institute of Physics. View full abstract»

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  • Monolithic-integrated two-wavelength laser diodes for digital-versatile-disk/compact-disk playback

    Page(s): 2270 - 2272
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    We have developed a monolithic two-wavelength laser diode, which emits 650 and 780 nm wavelengths. This device, which has a separated-confinement-heterostructure multi-quantum-well active region and a gain-guiding tapered-stripe structure, is fabricated using only two steps of metal organic chemical vapor deposition. The operating currents at 5 mW are 57.0 and 61.5 mA for the 650 and 780 nm elements, respectively. The relative intensity noise of the 650 and 780 nm elements was below -130 dB/Hz up to 70 °C without high-frequency superposition circuits. © 2001 American Institute of Physics. View full abstract»

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  • Fabrication of arrays of two-dimensional micropatterns using microspheres as lenses for projection photolithography

    Page(s): 2273 - 2275
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    This letter demonstrates the use of an array of transparent microspheres in forming repetitive, micrometer-scale patterns in photoresist, starting from masks with centimeter-scale patterns. A transparent microsphere with diameter d≫1.5 μm acts as a lens and reduces centimeter-scale images into micrometer-scale images on its image plane. A planar array of microspheres projects the image of an illuminated mask onto a corresponding array of micropatterns on their common image plane. We have prepared arrays of polystyrene microspheres (d=1.5–10 μm) embedded in a transparent membrane to generate repetitive patterns in photoresist, and have transferred the resulting patterns into metal films by liftoff. The optical system of this technique is related to that used in conventional projection photolithography, but differs in that the lens that accomplishes size reduction is positioned within 10 μm of the photoresist. The microspheres generate uniform patterns over an area of ∼2 cm2, using a mask with area ∼25×25 cm2 illuminated with a white light source. This method can generate submicron features either within a micropattern or between neighboring patterns. © 2001 American Institute of Physics. View full abstract»

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  • Enhanced peak power and short pulse operation of planar waveguide CO2 lasers

    Page(s): 2276 - 2278
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    We report a large peak power enhancement and reduction in pulse width for planar waveguide carbon dioxide lasers. Gain modulation through rf discharge power switching produces trains of laser pulses with peak power levels at up to 38 times the cw power level, with a pulse duration as low as 10 μs. Operation at repetition rates in the kHz region preserves the average power (100 W) of the normal cw/long pulse mode of operation. The laser is shown to operate close to the predicted boundaries dictated by thermal loading of the discharge. © 2001 American Institute of Physics. View full abstract»

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  • Photonic crystal microcavities with self-assembled InAs quantum dots as active emitters

    Page(s): 2279 - 2281
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    We report the use of self-assembled InAs quantum dots as active emitters in a photonic crystal microcavity. We have fabricated defect microcavities by removing 37 and 61 air holes from a triangular lattice in a photonic crystal membrane, and obtained quality factors in excess of 1000. © 2001 American Institute of Physics. View full abstract»

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  • Nondestructive readout of photochromic optical memory using photocurrent detection

    Page(s): 2282 - 2284
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    We proposed and demonstrated a nondestructive readout method using photocurrent detection for photon-mode photochromic memory. The principle of this readout method, which utilized the ionization potential change according to photoisomerzation reaction, was confirmed by using a medium with a photochromic diarylethene layer and phthalocyanine photoabsorbing layer, and by using a near-infrared readout light. We demonstrated perfect nondestructive readout operations over 106 times. © 2001 American Institute of Physics. View full abstract»

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  • Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films

    Page(s): 2285 - 2287
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    Photoluminescence and cathodoluminescence (CL) spectra of stoichiometric and oxygen-deficient ZnO films grown on sapphire were examined. It was found that the intensities of the green and yellow emissions depend on the width of the free-carrier depletion region at the particle surface; the thinner the width, the larger the intensity. Experimental results and spectral analyses suggest that the mechanism responsible for the green (yellow) emission is the recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the single ionized oxygen vacancy Vo+ (the single negatively charged interstitial oxygen ion Oi-) center in the particle. © 2001 American Institute of Physics. View full abstract»

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  • Screw dislocations in GaN: The Ga-filled core model

    Page(s): 2288 - 2290
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    First-principles total energy calculations performed for [0001] screw dislocations in GaN with |b|=c indicate that a model with a helical Ga-filled core is more stable than the hollow core model in Ga-rich conditions. This model gives rise to electronic states dispersed throughout the band gap. Such a dislocation is therefore expected to be a very strong center for nonradiative recombination and a pathway for current leakage. © 2001 American Institute of Physics. View full abstract»

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  • Growth of aligned carbon nanotubes by biasing during growth

    Page(s): 2291 - 2293
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    Well aligned multiwalled carbon nanotube (CNT) growth was achieved by positively biasing the substrate during growth. Growth was performed in a flowing mixture of 7% CH4 in Ar onto Co covered Si held at 800 °C with and without the presence of an electric field. High resolution scanning electron microscopy shows that the tube alignment occurs only when a positive bias is applied to the substrate whereas no aligned growth occurs under a negative bias and no tube growth is observed, under the presently applied conditions, with no field. This finding may open up the possibility of realizing cold electron emitting devices based on CNTs with a large electric field enhancement. In particular, it may be possible to utilize the same gate which is needed to turn the device on also to obtain field assisted aligned carbon nanotube growth into the desired regions. Alternatively, due to the fact that no CNTs grow under the conditions of this experiment without bias, selected area biasing may permit selected area growth of vertically aligned carbon nanotubes, a process that may find many applications. © 2001 American Institute of Physics. View full abstract»

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  • Roughness evolution in polyimide films during plasma etching

    Page(s): 2294 - 2296
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    We report an experimental study on the evolution of etch front roughness in fluorinated polyimide films in oxygen based plasmas. For standard low-pressure (40 mT) etching conditions, the root-mean-square roughness, w, of the polymer surface increases with the amount of material etched, d, as w=0.0265(d-116)β with β=1, independent of etch rate, rf power, and gas composition. The etched surfaces can be described by the statistics of self-affine surfaces with scaling exponent, α=0.6±0.1 and lateral correlation length, ξ, of ∼0.3 μm. A dramatic reduction in roughness is observed under higher pressure etching conditions of 1000–2000 mT. © 2001 American Institute of Physics. View full abstract»

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  • Characterization of free-standing hydride vapor phase epitaxy GaN

    Page(s): 2297 - 2299
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    A free-standing GaN template grown by hydride vapor phase epitaxy has been characterized by transmission electron microscopy (TEM). The TEM investigation was augmented by x-ray diffraction, defect delineation etching process followed by imaging with atomic force microscopy and variable temperature photoluminescence. The density of dislocations near the N face was determined to be, in order, 3±1×107, 4±1×107, and about 1×107 cm-2 by cross-sectional TEM, plan-view TEM, and a defect revealing etch, respectively. The same methods on the Ga face revealed the defect concentration to be, in order, less than 1×107 cm-2 by plan-view TEM, less than 5×106 cm-2 by cross-sectional TEM, and 5×105 cm-2 by defect revealing hot H3PO4 acid, respectively. The full width at half maximum of the symmetric (0002) x-ray diffraction peak was 69 and 160 arc sec for the Ga and N faces, respectively. That for the asymmetric (101_4) peak was 103 and 140 arc sec for Ga and N faces, respectively. The donor bound exciton linewidth was about 1 meV each at 10 K, and a green band centered at about 2.44 eV was observed. © 2001 American Institute of Physics. View full abstract»

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  • High-performance blue electroluminescent devices based on hydroxyphenyl-pyridine beryllium complex

    Page(s): 2300 - 2302
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    Blue light organic electroluminescent (EL) devices with high-performance were fabricated by using a highly fluorescent material, bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp2). The double layer devices with a structure of [indium tin oxide/N, N-di(α-naphthyl)-N-N-diphenyl(1, 1-biphenyl)-4, 4-diamine (600 Å)/Bepp2 (500 Å)/LiF (10 Å)/Al (2000 Å)] exhibited a maximum luminance of 15 000 cd/m2 and a maximum electroluminescent efficiency of 3.43 lm/w (3.8 cd/A). © 2001 American Institute of Physics. View full abstract»

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  • Influence of the dipole interaction energy on clustering in InxGa1-xN alloys

    Page(s): 2303 - 2305
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    The interactions of the polarization-induced dipole moments at each unit cell in InxGa1-xN alloys have been studied using analytical electrostatic dipole energy calculations to determine their potential influence on local material composition. Clustering of In atoms along the [0001] direction and anticlustering of In in the (0001) plane are shown to be energetically favorable. Comparisons of the dipole interaction energy to the strain energy and the energy associated with the decreased entropy of the ordered configurations show that the dipole interaction energy could have a substantial influence on local composition in InxGa1-xN alloys. © 2001 American Institute of Physics. View full abstract»

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  • Near-field photoreflectance spectroscopy of quantum well structures

    Page(s): 2306 - 2308
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    We present near-field photoreflectance (NPR) spectroscopic studies of GaAs/Al0.3Ga0.7As quantum well structures using a near-field scanning optical microscope with either an uncoated or a metal-coated tapered optical fiber probe. The NPR method provides advantages over conventional optical microscopic ones: (i) higher signal-to-noise ratio, (ii) lower temperature-sensitivity of the signal, and (iii) more information about higher electronic energy states. We also discuss the feasibility of a imaging with high resolution and contrast by using the NPR method. © 2001 American Institute of Physics. View full abstract»

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  • Atomically resolved structure of InAs quantum dots

    Page(s): 2309 - 2311
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    InAs was grown by molecular-beam epitaxy onto GaAs(001) until quantum dots (QDs) formed. At this point, the growth was interrupted and the uncovered QDs were investigated in situ by scanning tunneling microscopy (STM). Atomically resolved STM images of the QDs revealed that four dominating bounding facets occur, whose Miller indices were identified to be {137}. The assignment of the facet orientation was based on experiments on planar high Miller index GaAs surfaces. In addition, the latter experiments indicated that {137} facets are thermodynamically stable only up to a certain size. This conclusion is assumed to explain the sharp size distribution of InAs QDs. © 2001 American Institute of Physics. View full abstract»

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  • First principles study of the initial stages of SiC growth on Si(001)

    Page(s): 2312 - 2314
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    The initial stages of SiC growth on Si(001) are studied via ab initio molecular dynamics simulations at finite temperature. Several C coverages are considered, at various adsorption sites. At low T, C is adsorbed at the surface, with Si–C bond lengths close to that of bulk SiC. When increasing temperature, C adatoms are incorporated in the substrate subsurface layers, giving rise to the carbonization process. On the contrary, C dimers do not penetrate the substrate and remain stable even at the highest temperatures considered: our results point at radicals with single C atoms as efficient precursors for SiC growth. © 2001 American Institute of Physics. View full abstract»

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  • Hexagonal growth spirals on GaN grown by molecular-beam epitaxy: Kinetics versus thermodynamics

    Page(s): 2315 - 2317
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    GaN grown by molecular-beam epitaxy on Ga-polar GaN templates prepared by metal organic chemical vapor deposition shows a variety of morphologies that depend on defects and growth conditions. We measured the mean terrace widths of hexagonal growth spirals or hillocks versus ammonia and Ga fluxes and substrate temperature. The measurements were compared to a near equilibrium model of the growth. The results indicate that under excess Ga growth conditions, Ga-polar GaN(0001) has a mean step-edge energy of 0.27 eV/Å. © 2001 American Institute of Physics. View full abstract»

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  • Natural oxides on air-exposed and chemically treated InGaP surfaces grown by metalorganic vapor phase epitaxy

    Page(s): 2318 - 2320
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    Chemical properties of natural oxides on air-exposed and chemically treated In0.49Ga0.51P surfaces grown by metalorganic vapor phase epitaxy were systematically investigated by x-ray photoelectron spectroscopy. An air-exposed sample exhibited a highly In-rich surface which included a large amount of natural oxides. From the valence-band spectra and energy separations between core levels, it was found that the InPO4-like chemical phase was dominant in natural oxides of air-exposed InGaP surfaces. Chemical surface treatments in HCl and HF solutions were effective in reducing natural oxide and in recovering the surface stoichiometry. © 2001 American Institute of Physics. View full abstract»

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  • Retardation of boron diffusion in silicon by defect engineering

    Page(s): 2321 - 2323
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    By judiciously placing vacancy and interstitial defects at different depths, we are able to enhance or retard boron diffusion. This opens up a new approach for the formation of shallow P+n junction in silicon. After preimplantation with 50 or 500 keV Si+ ions to produce a surface vacancy-rich region, we studied the diffusion of deposited B on predamaged samples with annealing between 900 and 1010 °C. Boron diffusion retardation was observed in both implantation conditions after low temperature annealing with enhancement occurring in a 50 keV implanted sample at high temperature. Choosing high energy implantation to separate vacancies and interstitials can reduce the boron diffusion significantly. Such suppression became more obvious with higher implant doses. A junction less than 10 nm deep (at 1×1017cm-3 according to carrier concentration profiles) can be formed. © 2001 American Institute of Physics. View full abstract»

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  • Phase field microelasticity theory and modeling of multiple dislocation dynamics

    Page(s): 2324 - 2326
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    The phase field theory and model of a multidislocation dynamics in an elastically anisotropic crystal under applied stress is developed. The proposed three-dimensional (3D) model is a particular case of our phase field microelasticity model of the stress-induced martensitic transformations. A spontaneous self-organization of dislocations in the evolving ensemble, which involves multiplication/annihilation and movement of dislocations controlled by their elastic interaction, is described by the Ginzburg–Landau kinetic equation. Examples of 3D computer simulation of dislocation dynamics are considered. © 2001 American Institute of Physics. View full abstract»

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  • Midinfrared absorption and photocurrent spectroscopy of InAs/GaAs self-assembled quantum dots

    Page(s): 2327 - 2329
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    We report on a comparison between the midinfrared absorption and the photocurrent response of n-doped InAs/GaAs self-assembled quantum dots. The absorption, resonant at 160 meV, is polarized along the z growth axis of the dots. The photocurrent is dominated by a z-polarized resonance around 220 meV (5.6 μm wavelength). A weaker component of the photocurrent is observed for an in-plane polarized excitation. The photoresponse can be measured for a 0 V applied bias. The photoresponsivity is investigated as a function of the applied bias. The responsivity and the dark current exhibit an asymmetric profile versus the external bias. This asymmetry is correlated to the structural asymmetry of the quantum dot layers. © 2001 American Institute of Physics. View full abstract»

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  • Passivation of oxygen-related donors in microcrystalline silicon by low temperature deposition

    Page(s): 2330 - 2332
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    Low-temperature processing for high-performance solar cells based on hydrogenated microcrystalline silicon (μc-Si:H) has been developed using a conventional rf plasma-enhanced chemical vapor deposition (PECVD) technique at an excitation frequency of 13.56 MHz under a high deposition pressure condition. Among pin type solar cells, it is found that deposition temperature of i-layer at 140 °C is effective particularly for improving open circuit voltage (Voc), surprisingly without deteriorating short circuit current or fill factor. Carrier density of undoped μc-Si abruptly decreases for deposition temperatures lower than 180 °C, and the improvement of Voc is ascribed to a decrease of shunt leakage current arising from the oxygen-related donors. This implies that oxygen-related donors can be passivated at low deposition temperatures and that hydrogen plays an important role for the passivation. We propose a simple model for the hydrogen passivation of oxygen related donors. We apply this passivation technique to solar cells, and consequently a conversion efficiency of 8.9% (Voc=0.51 V, Jsc=25 mA/cm-1, FF=0.70) has been obtained in spite of an oxygen concentration of 2×1019 cm-3 in combination with device optimization such as a p-layer. Effect of deposition temperature of i-layer upon other solar cell parameter, short circuit current, and fill factor is also discussed. © 2001 American Institute of Physics. View full abstract»

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  • Room-temperature excitons in wide-gap layered-oxysulfide semiconductor: LaCuOS

    Page(s): 2333 - 2335
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    Sharp optical absorption and emission peaks near the band gap (Eg≈3.1 eV) were observed in LaCuOS polycrystalline thin films at room temperature. The absorption peak was able to be detected at temperatures as high as 490 K, and its intensity remarkably increased with decreasing temperature. The spectral position of the absorption peak and its temperature dependence almost agreed with those of the emission peak. It was concluded that the sharp absorption and emission peaks originate from excitons. On the basis of semiquantitative consideration about the excitons, it is suggested that the electronic-structure characteristic of the layered-crystal structure of LaCuOS is responsible for the stability of the excitons. The observation of the exciton absorption and emission at room temperature revealed that LaCuOS is a promising material for optoelectronic applications such as light-emitting devices. © 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