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Applied Physics Letters

Issue 17 • Date Oct 2002

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Displaying Results 1 - 25 of 63
  • Issue Cover

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

    Page(s): toc1
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  • Optical speedup at transparency of the gain recovery in semiconductor optical amplifiers

    Page(s): 3119 - 3121
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    Experimental demonstration of optical speedup at transparency (OSAT) has been performed on a 1 mm long semiconductor optical amplifiers (SOA). OSAT is a recently proposed scheme that decreases the recovery time of an SOA while maintaining the available gain. It is achieved by externally injecting into the SOA the beam of a separate high power laser at energies around the transparency point. Even though the experimental conditions were not optimal, a beam of 100 mW decreases the recovery time by a third when it is injected in the vicinity of the material transparency point of the device. This acceleration of the device response without detrimental reduction of the gain is found to be effective over a broad wavelength window of about 20 nm around transparency. The injection of the accelerating beam into the gain region is a less efficient solution not only because the gain is then strongly diminished but also because speeding is reduced. This originates from the reduction of the amplified spontaneous emission power in the device, which counterbalances the speeding capabilities of the external laser beam. Another advantage of the OSAT scheme is realized in relatively long SOAs, which suffer from gain overshoot under strong current injection. Simulations show that OSAT decreases the gain overshoot, which should enable us to use OSAT to further speedup the response of long SOAs. © 2002 American Institute of Physics. View full abstract»

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  • Three-dimensional photonic crystals for optical wavelengths assembled by micromanipulation

    Page(s): 3122 - 3124
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    We have established a profitable fabrication technique for three-dimensional (3D) photonic crystals for optical wavelengths. In our method, two-dimensional photonic plates, which serve as unit parts for 3D structures, are prepared by the semiconductor nanofabrication technique. Then, these plates are assembled into 3D structures by micromanipulation. Accurate lamination of the plates is assured by linking fiducial holes of neighboring plates with matching microspheres. With this technique, we have succeeded in fabricating 3D photonic crystals with one to four layers of woodpile structures. From scanning electron microscope observation of the crystals, the periodic error was determined to be within 50 nm. The optical properties of the crystals indicate existence of the photonic band gap at the expected wavelength of 3–4 μm. © 2002 American Institute of Physics. View full abstract»

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  • Red-emitting electroluminescent devices based on osmium-complexes-doped blend of poly(vinylnaphthalene) and 1,3,4-oxadiazole derivative

    Page(s): 3125 - 3127
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    Efficient red electrophosphorescence was achieved from double-layer light-emitting devices using osmium (Os)-complexes-doped blend of poly(vinylnaphthalene) (PVN) and 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PVN:PBD) as the emitting layer. Blending PVN with PBD greatly suppresses the electromer emission of PVN. The PVN:PBD blend emanates a short-wavelength electroluminescence emission peaking at around 375 nm, which overlaps well with the absorption spectra of the Os complexes and ensures very efficient energy transfer to the Os-complex dopants. The best external quantum efficiency of the double-layer devices was 2.2%, with a photometric efficiency of 1.9 cd/A. © 2002 American Institute of Physics. View full abstract»

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  • Harmonic frequency locking in a semiconductor laser with delayed negative optoelectronic feedback

    Page(s): 3128 - 3130
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    The locking states of a delayed negative optoelectronic feedback system are studied experimentally. Harmonic locking is observed in this system. Instead of locking to the frequency of the delay loop, the pulsing frequency of the system locks to a harmonic of the loop frequency. Moreover, a period-adding route of the locking states is found and locking states of Farey fractions up to order 17 are reported. The plot of the rotation number of the locking states shows a Devil’s staircase structure. © 2002 American Institute of Physics. View full abstract»

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  • Fully transparent, organic light-emitting electrochemical cells

    Page(s): 3131 - 3133
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    We report the fabrication and performance of fully transparent, organic blue light-emitting electrochemical cells (OLECs), in which both the anode and cathode are made of indium tin oxide. The active layer is a blend of polyfluorene with long and flexible alkyl side chains grafted on the 9,9 position and of a molten salt. Two identical spin-coated active layers are laminated together at high temperature to form the OLECs. The electroluminescence threshold is around 3.3 V and the light intensity exceeds 10 μW/cm2 at 5 V. © 2002 American Institute of Physics. View full abstract»

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  • Photonic pseudo-gap-based modification of photoluminescence from CdS nanocrystal satellites around polymer microspheres in a photonic crystal

    Page(s): 3134 - 3136
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    We report the combination of microsphere self-organization to form a photonic crystal, providing spectrally and angularly dependent electromagnetic structural resonances; with nanocrystal growth in situ on microsphere surfaces, providing optical functionalization with spectral control achieved through the quantum size effect. We demonstrate this material system using CdS nanocrystals coating the surfaces of poly(methyl methacrylate)–poly(methacrylic acid) (PMMA–PMAA) micrometer spheres. The in situ synthesis of the CdS nanocrystals on the surface of the PMMA/PMAA microspheres preserves the propensity of the hybrid microspheres to form ordered colloid arrays. Luminescence from surface states ensures that light is emitted at energies significantly below the absorption edge of the emitting species. Transmission and photoluminescence measurements reveal the interaction of the photonic stop band with photoluminescence from the nanocrystals. © 2002 American Institute of Physics. View full abstract»

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  • A possible mechanism for enhanced electrofluorescence emission through triplet–triplet annihilation in organic electroluminescent devices

    Page(s): 3137 - 3139
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    We demonstrate here that luminance increased more than linearly with an increase in current density of tris(8-hydroxyquinoline) aluminum (Alq3)-based electroluminescent (EL) devices and the EL efficiency reached ∼5 cd A-1 at 250 mA cm-2 when electron and hole injection was well balanced. The luminance–current curves were well fitted with a combination of a linear and a quadratic function of the current. The quadratic component can be attributed to additional singlet excited state (1Alq3*) formation through triplet–triplet (T–T) annihilation of triplet excited states (3Alq3*). The requirement of the well-balanced charge injection implies that the long-lived 3Alq3* was quenched efficiently by energy transfer to excess and colored Alq3-∙ anion or Alq3+∙ cation radicals in the emission zone when the charge injection was unbalanced. The short-lived 1Alq3* was not quenched appreciably. © 2002 American Institute of Physics. View full abstract»

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  • Cascaded electro-optic scanning of laser light over large angles using domain microengineered ferroelectrics

    Page(s): 3140 - 3142
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    We present a device concept based on cascaded electro-optic deflection in a domain microengineered ferroelectric chip. In our design, large deflection angles are achieved by cascading several smaller scanners in a single ferroelectric chip, such that each successive scanner stage builds upon the deflection of the previous stage. We demonstrate the basic concept using a two-stage device fabricated in a single crystal wafer of ferroelectric LiTaO3. By operating the device using a specially designed programmable multichannel driver that provides ±1.1 kV per stage, a total scan angle of 25.4° at 5 kHz was demonstrated. Even larger angles of deflection are possible with additional scanner stages. © 2002 American Institute of Physics. View full abstract»

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  • Super-resolution digital holographic imaging method

    Page(s): 3143 - 3145
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    A method to realize super-resolution digital holographic imaging is demonstrated experimentally. By placing a grating in front of the specimen, more object waves can reach the charge-coupled devices target during the recording of a digital hologram, and the image reconstructed from the digital hologram has a resolution over the diffraction limit of the ordinary holographic system. This method overcomes the crucial drawback of digital holography, and may find many applications in three-dimensional digital imaging and microscopy. © 2002 American Institute of Physics. View full abstract»

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  • Room-temperature 2.5 μm InGaAsSb/AlGaAsSb diode lasers emitting 1 W continuous waves

    Page(s): 3146 - 3148
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    We have characterized 2.5-μm-wavelength InGaAsSb/AlGaAsSb/GaSb two-quantum-well diode lasers that emit 1 W continuous waves from a 100-μm-wide aperture at a temperature of 12 °C. The threshold current density is 250 A/cm2, and the external quantum efficiency near threshold is 0.36. The wall–plug efficiency reaches a maximum of 12% at a current of 2 A. Operating in the pulsed-current mode, the devices output nearly 5 W at 20 °C. These lasers exhibit internal losses of about 4 cm-1 and differential series resistances of about 0.1 Ω. A broad-waveguide design lowers internal losses, and highly doped transition regions between the cladding layers and the GaSb reduces series resistance. © 2002 American Institute of Physics. View full abstract»

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  • Nanosecond time-resolved multiprobe imaging of laser damage in transparent solids

    Page(s): 3149 - 3151
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    We report the nanosecond time-scale and micron length-scale dynamics of elastic wave, plastic deformation, melt-front and crack propagation during laser-induced damage in the bulk of an optically transparent crystalline solid. Time-delayed probe pulses with wavelengths corresponding to RGB-color filters are used to obtain multiple images during a single damage event. This technique enables “real-time” dynamical studies of complex transient phenomena. © 2002 American Institute of Physics. View full abstract»

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  • Microstructure of GaAs/GaN interfaces produced by direct wafer fusion

    Page(s): 3152 - 3154
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    Results of a transmission electron microscopy (TEM) study of directly bonded GaAs/GaN structures obtained by wafer fusion are presented. A large fraction of the interface area was found to be well bonded, with the presence of a thin (1–2 nm) amorphous layer of a native oxide at the bonded interface. The dominant defects found in nonbonded areas were cavities elongated along the [110]GaAs direction. Plan-view TEM study of well-bonded regions showed the presence of two dislocation networks. The first, formed at the interface to accommodate an unintentional tilt between the fused crystals, consists of a one-dimensional array of wavy dislocations located at interface steps. The second dislocation network, formed to relieve additional strain from the thermal processing of the samples, consists of a two-dimensional, irregularly spaced grid of dislocations formed on the GaAs side that runs parallel to either the [110]GaAs or [11_0]GaAs directions. © 2002 American Institute of Physics. View full abstract»

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  • Formation of interface defects by enhanced impurity diffusion in microcrystalline silicon solar cells

    Page(s): 3155 - 3157
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    The origin of the difference in the optimum processing temperature of microcrystalline silicon (μc-Si:H) solar cells depending on the device structure has been studied. The two different type structures have been compared with each other, a superstrate-type cell, and a substrate-type cell where the order of the deposited layer is p-i-n and n-i-p, respectively on the substrate. We fabricate μc-Si:H solar cells at various deposition temperatures of the i layer under contamination-free conditions using an ultrahigh vacuum system. We found that the optimum temperature for the superstrate-type cell is lower than that for the substrate-type cell and that a significant degradation of the cell performance occurs during the deposition of the i layer at higher temperatures. We ascribed the degradation to the formation of the defect at the p/i interface due to the enhanced diffusion of the boron from the underlying layer to the intrinsic layer. We also found that this degradation and the enhanced diffusion do not take place for phosphorous or postdeposition annealing. We speculate that the enhanced boron diffusion is mediated by the SiSi bond breaking and its accompanying structural relaxation caused by atomic hydrogen. © 2002 American Institute of Physics. View full abstract»

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  • Optical properties of donor-triad cluster in GaAs and GaN

    Page(s): 3158 - 3160
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    The effect of the transition energy of three-donor clusters on far infrared absorption in n-type semiconductor materials has been investigated by a multiconfigurational self-consistent-field model calculation and applied to GaAs and GaN systems. We show that it is crucial to consider the many-particle correlation effects within three-donor clusters. With electron correlation taken into account, the present results support the interpretation of a very recent unidentified peak energy observed in absorption measurement of GaN as due to electronic transitions in these clusters. We also corroborate the suggestion that the X line in GaAs arises from such transitions. © 2002 American Institute of Physics. View full abstract»

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  • Carrier diffusion and radiative recombination in CdTe thin films

    Page(s): 3161 - 3163
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    We employed cathodoluminescence spectroscopy and imaging to investigate the carrier diffusion and radiative recombination in CdTe thin films. We observed that carriers excited by the electron beam diffuse by excitons or by free electrons via donor states at low temperatures. The distribution and concentration of these states in the CdTe films were estimated from the dependence on the excitation level of the donor-to-acceptor radiative recombination. © 2002 American Institute of Physics. View full abstract»

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  • Power dependence of defect formation in SiO2 glass by F2 laser irradiation

    Page(s): 3164 - 3166
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    The dependence of defect formation in a high-purity synthetic SiO2 glass on F2 laser power was studied. Above the threshold value of ∼10 mJ cm-2pulse-1 (∼0.5 MW cm-2), the concentration of the laser-induced E center created by the dissociation of SiOSi bond increased as a function of the F2 laser power squared. The quantum yield of the E center formed by the high-power F2 laser irradiation was ∼3 orders of magnitude larger than that formed via two-photon absorption processes of KrF or ArF laser pulses. This strongly suggests that irradiating with the high-power F2 laser creates the E center via two-step absorption processes. © 2002 American Institute of Physics. View full abstract»

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  • X-ray diffuse scattering study of the kinetics of stacking fault growth and annihilation in boron-implanted silicon

    Page(s): 3167 - 3169
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    Stacking faults in boron-implanted silicon give rise to streaks or rods of scattered x-ray intensity normal to the stacking fault plane. We have used the diffuse scattering rods to follow the growth of faults as a function of time when boron-implanted silicon is annealed in the range of 925 to 1025 °C. From the growth kinetics we obtain an activation energy for interstitial migration in silicon: EI=1.98±0.06 eV. Fault intensity and size versus time results indicate that faults do not shrink and disappear, but rather are annihilated by a dislocation reaction mechanism. © 2002 American Institute of Physics. View full abstract»

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  • Pressure behavior of Te isoelectronic centers in ZnS:Te

    Page(s): 3170 - 3172
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    ZnS:Te epilayers with Te concentration from 0.5% to 3.1% were studied by photoluminescence under hydrostatic pressure at 15 K. Two emission bands related to the isolated Te1 and Te2 pair isoelectronic centers were observed in the samples with Te concentrations of 0.5% and 0.65%. For the samples with Te concentrations of 1.4% and 3.1%, only the Te2-related peak was observed. The pressure coefficients of all the Te1-related bands were found to be unexpectedly much larger than that of the ZnS band gap. The pressure coefficients for all the Te2-related bands are, however, rather smaller than that of ZnS band gap as usually observed. Analysis based on a Koster–Slater model indicates that an increase of the valence bandwidth with pressure is the main reason for the faster pressure shift of the Te1 centers, and the huge difference in the pressure behavior of the Te1 and Te2 centers is due mainly to the difference in the pressure-induced enhancement of the impurity potential on the Te1 and Te2 centers. © 2002 American Institute of Physics. View full abstract»

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  • Chemical structure of Al/LiF/Alq interfaces in organic light-emitting diodes

    Page(s): 3173 - 3175
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    Al/LiF cathode/organic is known to form an excellent interface for electron injection into the organic active layer, resulting in excellent performing organic light-emitting diodes (OLEDs). Here, the chemical structure of the interface between the Al/LiF bilayer cathode and tris (8-hydroxyquinoline) aluminum (Alq) of working OLED devices was investigated by using x-ray photoelectron spectroscopy (XPS). Using a in situ peel-off technique, we are able to characterize the buried interface structure without disturbing the chemical states of each element probed. The data show that there are two types of F at the interface; one is attributed to LiF and the other to F attached to the Alq. This F-doped Alq layer could induce a downshift in molecular orbital levels and thus leads to a reduced electron injection barrier. XPS depth profile results show significant O diffusion through Al layer to the interface, and the diffusion of O ends abruptly at the Al/LiF interface. © 2002 American Institute of Physics. View full abstract»

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  • Template-directed convective assembly of three-dimensional face-centered-cubic colloidal crystals

    Page(s): 3176 - 3178
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    We demonstrate that square two-dimensional grating templates can drive the growth of three-dimensional, face-centered-cubic (fcc) colloidal crystals by convective assembly. The square symmetry [i.e., (100) planes parallel to the substrate] of the underlying template was transferred to the colloidal crystal and maintained throughout its growth of ∼50 layers. We characterized crystals grown on flat and on templated substrates using electron microscopy and small-angle x-ray scattering (SAXS). SAXS measurements of the templated samples clearly show fourfold diffraction patterns that arise from fcc domains without stacking faults. © 2002 American Institute of Physics. View full abstract»

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  • Optical properties of the neodymium-containing transparent dielectrics for plasma display panel

    Page(s): 3179 - 3181
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    We have prepared transparent dielectric thick films for the plasma display panel (PDP) by adding Nd2O3 into transparent dielectric materials and investigated their optical properties. The experimental results revealed that, owing to absorptions originating in the intratransition within the 4f shell of the Nd3+ ion, the neodymium-containing transparent dielectrics act not only as dielectrics, but also as a band rejection color filter. As a result, introduction of the neodymium-containing transparent dielectrics to PDP device yields a wider color gamut and higher color purity. © 2002 American Institute of Physics. View full abstract»

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  • Atomic arrangement at the AlN/ZrB2 interface

    Page(s): 3182 - 3184
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    Low-dislocation-density GaN films (∼108cm-2) have been grown on closely lattice-matched ZrB2 substrates by metalorganic vapor phase epitaxy using low-temperature AlN as a buffer layer. High-resolution electron microscopy images of the AlN/ZrB2 interface region reveal that the AlN buffer layer does not grow directly on the ZrB2 substrate. Instead, the existence of an unintentional intermediate cubic-phase layer (approximately 2 nm thick) has been observed. Misfit dislocations are evident at both interfaces of the intermediate layer. Our analysis indicates that the intermediate layer has a lattice constant a=4.6 Å, and that it is a ternary alloy of ZrxByNz, which should result from a transformation from the hexagonal phase of ZrB2 due to interdiffusion of nitrogen and boron at the elevated temperature required for growth of GaN. This intermediate cubic-phase layer of ZrxByNz appears to have been so far unavoidable in the growth of high-quality GaN epilayers on ZrB2 substrates by our technique. © 2002 American Institute of Physics. View full abstract»

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  • Structural studies of Fe0.81Ga0.19 by reciprocal space mapping

    Page(s): 3185 - 3187
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    Reciprocal lattice mapping has been performed on Fe0.81Ga0.19 crystals by ω–ω/2θ, Ψ–Φ, and ω–Φ scans. A strong elongation of the <001>c peak was found along the <110>c direction. ω scans revealed short lateral correlation lengths ξ along <110>c and strong diffuse scattering along the <001>c. Multiple domains with monoclinic symmetry (angle ∼190°) were observed by Ψ–Φ and ω–Φ scans on the (001)c face, and were also tilted with respect to each other. The results show an average cubic structure with orthorhombic structural modulations, and two structural domain states that result in a limiting monoclinic symmetry. © 2002 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