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Applied Physics Letters

Issue 18 • Date Nov 1998

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Displaying Results 1 - 25 of 54
  • 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
  • Tunable wavelength filters with Bragg gratings in polymer waveguides

    Page(s): 2543 - 2545
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    Tunable wavelength filters are demonstrated based on the thermo-optic refractive index change of the polymer waveguide with Bragg reflection grating. For the low-loss waveguide operating around 1.55 μm, fluorinated polymers are incorporated. Bragg reflection gratings are fabricated using a phase mask and a high-index polymer. The Bragg reflector exhibits a narrow bandwidth of less than 1.0 nm, a crosstalk of -20 dB, an insertion loss of 3.2 dB, and a flat-top passband. The peak wavelength of Bragg reflection is shifted over 11 nm with a slight insertion loss change. The thermo-optic tuning efficiency is 22 nm/W and the peak shift is linearly proportional to the heating power. © 1998 American Institute of Physics. View full abstract»

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  • Effect of a local electric field on photogeneration efficiency in a photorefractive polymer

    Page(s): 2546 - 2548
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    The effect of a local electric field upon photogeneration efficiency was investigated through analyses of the dependence of photocurrent, photorefractive gain, diffraction efficiency, and birefringence on the applied field of a novel photorefractive polymer containing an ionic tri(bispyridyl) ruthenium complex. It was found that since this polymer system possesses a low glass transition temperature, the dipole moments formed between the counter ions are readily aligned and generate ionic dipole field which screen photogeneration sites from the applied field. This local field lowers the photogeneration efficiency and results in the saturation of photocurrent and photorefractive gain at high applied field. This local field’s effect on an ionic polymer is further confirmed experimentally by comparing the photoconduction of a similar nonionic polymer. © 1998 American Institute of Physics. View full abstract»

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  • Beam quality enhancement for a radio-frequency excited annular CO2 laser

    Page(s): 2549 - 2551
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    We report the preliminary experimental results obtained from an annular radio-frequency excited CO2 laser with a Talbot cavity and a phase correcting mirror external to the cavity. The Talbot cavity is adopted in order to reduce the number of oscillating azimuthal modes. The use of the external profile-modulated mirror is aimed at reducing the beam phase modulation and thus increasing the amount of fundamental annular mode energy content. The beam quality results to be sensibly increased. © 1998 American Institute of Physics. View full abstract»

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  • Stability of above threshold ionization spectrum during intense-field ionization of H2+ near Rc

    Page(s): 2552 - 2554
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    With the collinear model for H2+, we have numerically investigated the photoelectron spectrum of hydrogen molecular ions exposed to an ultrashort intense laser pulse. Above-threshold ionization (ATI) peaks and their temporal evolution are carefully analyzed. We find that once the photon-energy-spaced photoelectron peaks appear, their positions will not shift during the interaction. The stability of ATI spectrum is attributed to the fact that the Stark-shifted ionization potential of H2+ exhibits a minimum plateau during the internuclear distance Rc=3.6–6 bohr in which the stretching H2+ is significantly ionized. © 1998 American Institute of Physics. View full abstract»

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  • Faster grating buildup characteristics in low silanol-containing polysilane-based photorefractive polymer

    Page(s): 2555 - 2557
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    The effect of silanol content on the grating buildup dynamics was examined by preparing two different lots of polymethylphenylsilane (PMPS). While the high silanol-containing PMPS showed very sluggish grating formation time, the lower silanol-containing PMPS exhibited very rapid grating formation in a diffraction efficiency measurement. The diffraction efficiency, on the other hand, was found to be unaffected. The results suggest that the silanol is acting as hole trap and a reduction of its content is a key to improving the response characteristics of polysilane-based photoconducting electro-optic polymer composite for photorefractive applications. © 1998 American Institute of Physics. View full abstract»

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  • High-speed maskless laser patterning of indium tin oxide thin films

    Page(s): 2558 - 2560
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    Patterning characteristics of indium tin oxide thin films using different wavelengths of a diode-pumped Q-switched Nd:YLF and a flashlamp-pumped Nd:YAG laser have been studied. While a ripplelike structure in the etched line was formed due to incomplete material removal when the first harmonic of the Nd:YLF or Nd:YAG laser was used, a residue-free line could be obtained using the fourth harmonic of the Nd:YLF laser even at higher scan speeds. The observed differences in the morphology could be attributed to different absorption characteristics at the infrared and ultraviolet wavelengths. High process speeds in excess of 1 m/s could be achieved. © 1998 American Institute of Physics. View full abstract»

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  • Dual-color polymer light-emitting pixels processed by hybrid inkjet printing

    Page(s): 2561 - 2563
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    A hybrid inkjet printing (HIJP) technology, which combines a pin-hole free polymer buffer layer and an inkjet printed polymer layer, allows the patterning of high quality polymer light-emitting devices. In this letter, we present a successful demonstration of controllable patterning of dual-color polymer light-emitting pixels using this HIJP technique. In this demonstration, the polymer buffer layer is a wide band gap, blue emitting semiconducting polymer prepared by the spin-casting technique. The inkjet printed layer is a red-orange semiconducting polymer which was printed onto the buffer layer. When a proper solvent was selected, the printed polymer diffused into the buffer layer and efficient energy transfer took place generating a red-orange photoluminescence and electroluminescence from the inkjet printed sites. Based on this principle, blue and orange-red dual-color polymer light-emitting pixels were fabricated on the same substrate. The use of this concept represents an entirely new technology for fabricating polymer multicolor displays with high-resolution, lateral patterning capability. © 1998 American Institute of Physics. View full abstract»

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  • 1.3 μm room-temperature GaAs-based quantum-dot laser

    Page(s): 2564 - 2566
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    Room-temperature lasing at the wavelength of 1.31 μm is achieved from the ground state of an InGaAs/GaAs quantum-dot ensemble. At 79 K, a very low threshold current density of 11.5 A/cm2 is obtained at a wavelength of 1.23 μm. The room-temperature lasing at 1.31 μm is obtained with a threshold current density of 270 A/cm2 using high-reflectivity facet coatings. The temperature-dependent threshold with and without high-reflectivity end mirrors is studied, and ground-state lasing is obtained up to the highest temperature investigated of 324 K. © 1998 American Institute of Physics. View full abstract»

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  • Characterization of catastrophic optical damage in Al-free InGaAs/InGaP 0.98 μm high-power lasers

    Page(s): 2567 - 2569
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    Catastrophic optical damage (COD) in Al-free InGaAs/InGaP 0.98 μm lasers has been investigated using real-time electroluminescence (EL) and transmission electron microscopy (TEM). From EL images, we observed that multiple bright spots initiated from one of the facets and then propagated to the center of the cavity during the COD process. It is clarified by the TEM analysis that the propagation of bright spots resulted in 60-nm-wide Moiré fringe along the cavity and the crystalline phase of the active area became polycrystalline. Highly nonradiative polycrystalline phase of the active area is the major cause of COD failure in the Al-free 0.98 μm lasers. © 1998 American Institute of Physics. View full abstract»

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  • Change in luminescence properties of porous Si by F2 and D2O exposure: In situ photoluminescence, Raman, and Fourier-transform infrared spectral study

    Page(s): 2570 - 2572
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    In situ photoluminescence (PL), Raman, and infrared (IR) spectra of porous Si (PS) during F2/D2O exposures were investigated. F2 exposure at 298 K resulted in a peak shift of PL band from 750 to 670 nm with an intensity reduction. IR spectra revealed that the surface hydrogenated Si of the PS was displaced by fluorinated one. By subsequent D2O exposure, the PL band further shifted to a shorter wavelength with a significant intensity increase: IR bands due to surface oxides as well as SiD and SiOD bonds were observed after the exposure. On the contrary, the average size of the Si crystallites in the PS evaluated from Raman spectra remained almost unchanged throughout the exposures. These results suggest that surface chemistry plays a crucial role in the PL of the PS. © 1998 American Institute of Physics. View full abstract»

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  • A displacement amplifier using mechanical demodulation

    Page(s): 2573 - 2575
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    Piezoelectric ceramics can provide electromechanical transduction with high stresses but low displacements. To obtain larger displacements, several mechanical amplifying structures have been used. High alternating displacements can be obtained using resonant structures. We propose a displacement amplifier based on a mechanical rectifier, able to take advantage of the high displacements of resonant structures, and capable of obtaining a continuous displacement. A prototype composed of a Langevin resonator coupled to a mass–spring–damper system, acting as the rectifier, was constructed. Interferometric measurements on the proposed mechanical demodulator were carried out, validating the principle of operation. The possible applications are in the field of fast valves and micrometric positioning. © 1998 American Institute of Physics. View full abstract»

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  • Dynamics of electron beam ablation of silicon dioxide measured by dye laser resonance absorption photography

    Page(s): 2576 - 2578
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    The dynamics of electron beam ablation plumes have been characterized through the application of dye laser resonance absorption photography. The ablation of fused silica by a channelspark electron beam was studied by probing the near-ground state, 3p21D-4s 1P0 neutral Si transition at 288.158 nm. Necessary background gases (Ar or N2) were tested at pressures of 15 or 30 mTorr. A two-lobed, Si atom plume shape was discovered that is hydrodynamically more complex than laser ablation plumes. These plumes merge into a single-lobed plume at about 400 ns after the e-beam current pulse rise. Plume front expansion velocities of Si atoms were measured at nearly 1 cm/μs, and are comparable to the expansion of laser ablated metal atom plumes with laser fluences of a few J/cm2. © 1998 American Institute of Physics. View full abstract»

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  • Stress and relief of misfit strain of Ge/Si(111)

    Page(s): 2579 - 2581
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    The intrinsic stress and morphology of the Stranski–Krastanow system Ge/Si(111) have been investigated at deposition temperatures of 700–950 K. In a broad range of intermediate temperatures, only one distinct decline of stress is observed at the onset of three-dimensional islanding. Supported by a recent transmission electron microscopy study, the results demonstrate that the strain of Ge/Si(111), where the substrate surface in contrast to Ge/Si(001) is the glide plane for dislocations, is relieved by incorporation and continuous rearrangement of dislocations during the island stage. © 1998 American Institute of Physics. View full abstract»

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  • Microstructure of Ti/Al ohmic contacts for n-AlGaN

    Page(s): 2582 - 2584
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    Transmission electron microscopy was employed to evaluate the microstructure of Al/Ti ohmic contacts to AlGaN/GaN heterostructure field-effect transistor structures. Contact resistance was found to depend on the structure and composition of the metal and AlGaN layers, and on atomic structure of the interface. A 15–25-nm-thick interfacial AlTi2N layer was observed at the contact-AlGaN interface. Formation of such nitrogen-containing layers appears to be essential for ohmic behavior on n-type III-nitride materials suggesting a tunneling contact mechanism. Contact resistivity was found to increase with Al fraction in the AlGaN layer. © 1998 American Institute of Physics. View full abstract»

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  • Polytypoid structures in annealed In2O3–ZnO films

    Page(s): 2585 - 2587
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    Atomic-resolution Z-contrast images demonstrate unambiguously that the annealed, metalorganic chemical vapor deposition derived transparent In2O3–ZnO films have a polytypoid microstructure, consisting of ZnO slabs of variable width separated by single In–O octahedral layers. These In–O layers induce a polarity inversion in the two adjacent ZnO layers, which is reversed again by a mirror domain boundary inside each ZnO slab. © 1998 American Institute of Physics. View full abstract»

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  • Observation of CuPt-A type atomic ordering in AlxIn1-xAs alloys

    Page(s): 2588 - 2590
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    CuPt-A type atomic ordering, which has been reported for only Al0.5In0.5P and Ga0.5In0.5P, was observed in Al-rich AlxIn1-xAs grown on (001) InP substrates by solid-source molecular-beam epitaxy; only the triple-period-A type ordering in the [111]A directions has been previously reported for this alloy system grown under a similar growth condition. While the triple-period-A type ordering was observed in Al0.5In0.5As grown with a (2×3) surface reconstruction, the CuPt-A type ordering was observed in Al-rich AlxIn1-xAs when the surface showed a (1×2) surface reconstruction during growth. This observation provides strong support for the previous inference that the (1×2) surface reconstruction gives rise to CuPt-A type ordering. © 1998 American Institute of Physics. View full abstract»

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  • Structure and optical properties of amorphous diamond films prepared by ArF laser ablation as a function of carbon ion kinetic energy

    Page(s): 2591 - 2593
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    Amorphous carbon films with variable sp3 content were produced by ArF (193 nm) pulsed laser deposition. Electron energy loss spectroscopy and spectroscopic ellipsometry were employed to systematically study changes in the bonding and optical properties of the carbon films as a function of the kinetic energy of ablated C ions, which was measured using an ion probe. The measurements reveal that the films with the most diamond-like properties are obtained at the C ion kinetic energy of ∼90 eV. In contrast to measurements made as a function of laser fluence, ion probe measurements of kinetic energy are a convenient as well as more accurate and fundamental method for monitoring deposition conditions, with the advantage of being readily transferable for interlaboratory comparisons. © 1998 American Institute of Physics. View full abstract»

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  • Influence of the water layer on the shear force damping in near-field microscopy

    Page(s): 2594 - 2596
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    The influence of the water layer on the shear force damping is investigated in the case of a perfectly flat mica surface. In ambient conditions it is shown that the damping curve exhibits three particular regimes depending on the tip-sample distance. Moreover, the damping varies significantly over the first hour, pointing out the complexity of the distance control by shear force detection. © 1998 American Institute of Physics. View full abstract»

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  • Contrast reversal in scanning capacitance microscopy imaging

    Page(s): 2597 - 2599
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    We have investigated the quantification properties of scanning capacitance microscopy (SCM) by using two dedicated test structures and highlight the response of SCM to changes in dopant density. Our results indicate that contrast reversal occurs and that the SCM output is not always a monotonically increasing signal with decreasing dopant density. Two epitaxially grown staircase structures covering the doping ranges 1014–1020cm-3 p type and 5×1014–5×1019cm-3 n type were produced for this study as the turning point in the response function typically occurs at a doping level of around 1017cm-3. Through the use of a simple simulation model we see that contrast reversal is expected due to a relative shift between the dC/dV curves for different doping levels. The onset of contrast reversal can be adjusted by changing the dc sample bias leading to a shift in the operating position of the SCM, and the significance of this point will be discussed here. © 1998 American Institute of Physics. View full abstract»

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  • Direct micropatterning of Si and GaAs using electrochemical development of focused ion beam implants

    Page(s): 2600 - 2602
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    Focused ion beam implantation of Si++ was used to write defined surface damage/implant patterns into n-type GaAs (100) and Si (100) substrates. These implant sites represent initiation sites for dissolution processes when electrochemically polarized in HCl or HF electrolytes, respectively. Selective dissolution within the patterns is achieved if anodic polarization of the n-type material is carried out in the dark at potentials below (cathodic to) the onset of dissolution potential of the unimplanted surface. Uniform etching within the implanted region takes place, when local electropolishing conditions are established. Thus, highly defined etch patterns, e.g., lines, gratings, or pits, can be produced in the submicron range. The depth of the etched patterns corresponds to the implant/damage profile created in the implantation process and etch stop occurs at less reactive crystal planes. © 1998 American Institute of Physics. View full abstract»

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  • Initial carrier relaxation dynamics in ion-implanted Si nanocrystals: Femtosecond transient absorption study

    Page(s): 2603 - 2605
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    Transient absorption spectra of ion-implanted Si nanocrystals (NCs) exhibit two picosecond photoinduced absorption features, attributed to carriers in NC quantized states (high-energy band) and Si/SiO2 interface states (low-energy band). Fast relaxation of the high-energy band indicates that populations of quantized states are short lived and decay on the sub-10-ps time scale due to efficient surface trapping. This shows that the red emission in our samples is not due to carriers in quantized states but rather is a result of deactivation of surface traps. © 1998 American Institute of Physics. View full abstract»

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  • Raman and x-ray studies of InN films grown by metalorganic vapor phase epitaxy

    Page(s): 2606 - 2608
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    Thin InN films were deposited on the (0001) sapphire substrate at various temperatures from 325 to 600 °C by metalorganic vapor phase epitaxy. We used Raman scattering and x-ray diffraction to investigate the film properties and crystalline structures. Significant line broadening, softening and intensity evolution were observed at the growth temperatures between 375 and 450 °C. This can be attributed to the formation of the mixed hexagonal and cubic structures and the related dislocation defects. As the growth temperature is further increased, the hexagonal phase is found to be dominant in the deposited InN film. © 1998 American Institute of Physics. View full abstract»

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  • Significance of tunneling in p+ amorphous silicon carbide n crystalline silicon heterojunction solar cells

    Page(s): 2609 - 2611
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    We used the internal photoemission (IPE) technique to accurately determine the valence and conduction band offsets at the a-SiC:H/c-Si interface and investigated with numerical simulations their effects on the photocarrier collection in p+ a-SiC:H/n c-Si heterojunction solar cells. The valence and conduction band discontinuities were found to be 0.60 and 0.55 eV, respectively. However, despite the large barrier at the valence band edge, 30 nm p+ a-SiC:H/n c-Si heterojunction solar cells show no collection problems due to blocking of holes (FF=0.73). Combined IPE measurements and simulation results indicate that tunneling of holes through this barrier at the valence band edge can explain the unhindered collection. © 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