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Applied Physics Letters

Issue 9 • Date Mar 1998

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Displaying Results 1 - 25 of 45
  • 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
  • Low-threshold InAlGaAs vertical-cavity surface-emitting laser arrays using transparent contacts

    Page(s): 1001 - 1003
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    We present top-emitting all-epitaxial planar laterally oxidized vertical-cavity surface-emitting lasers employing transparent indium–tin–oxide electrodes. The transparent contacts facilitate device fabrication and offer significantly denser device packing than similar planar laterally oxidized structures using metal contacts. The InAlGaAs-based devices operate at a wavelength of 817 nm with a minimum threshold current of 175 μA. © 1998 American Institute of Physics. View full abstract»

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  • X-ray characterization of GaN/AlGaN multiple quantum wells for ultraviolet laser diodes

    Page(s): 1004 - 1006
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    GaN/Al0.20Ga0.80N (50 Å/50 Å) multiple quantum wells (MQW) with 15 periods were grown on (0001) sapphire substrates by molecular beam epitaxy and evaluated by x-ray diffraction. To simulate an ultraviolet laser diode structure, the substrate was coated first with n-GaN as the bottom contact layer and n-Al0.25Ga0.75N as the corresponding cladding layer. The crystal structure of this system was investigated by studying the reciprocal lattice map of off-axis diffraction peaks as well as the θ–2θ pattern around the (0002) reflection. The MQW was found to be coherent and has the a-lattice parameter of the underlying Al0.25Ga0.75N. The good agreement between experimental and theoretical data in the relative intensity of up to third-order satellite peaks supports that the interfaces of the MQW are abrupt, and thus, interdiffusion of Ga and Al atoms at the growth temperature was negligible. © 1998 American Institute of Physics. View full abstract»

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  • Efficient frequency doubling of 1.5 μm femtosecond laser pulses in quasi-phase-matched optical fibers

    Page(s): 1007 - 1009
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    Second-order nonlinear gratings in optical fibers have been produced for efficient quasi-phase-matched frequency conversion around 1.5 μm. Periodic poling was achieved by defining a patterned electrode on a D-shape fiber via standard lithography and applying high voltage (4–5 kV) at elevated temperature (270–280 °C). This fabrication technique has allowed us to produce gratings uniform over 7.5 cm, as indicated by the shape and the bandwidth of the phase-matching curve. By frequency doubling ∼100 fs pulses in a grating ∼4 cm long and with a period of 57.15 μm, ∼1.05 mW average power at 768 nm has been generated, with an average conversion efficiency ∼1.2%. © 1998 American Institute of Physics. View full abstract»

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  • Heterostructure optoelectronic switch with lightcontrollable S-shaped negative differential resistance

    Page(s): 1010 - 1012
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    A heterostructure optoelectronic switch, grown by molecular beam epitaxy, has been fabricated. Owing to the carrier confinement and avalanche multiplication in the transport mechanism, S-shaped negative-differential-resistance performances are observed in the current–voltage (I–V) characteristics. The device shows a flexible optical function related to the potential barrier height and breakdown voltage controllable by incident light. The dependence of the I–V characteristics on illumination is attributed to the carrier confinement effect in the device operation. © 1998 American Institute of Physics. View full abstract»

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  • Tayloring of the optical properties by symmetry modification of substituted quaterthiophene single crystals

    Page(s): 1013 - 1014
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    We report an optical study of polymorphic single crystals of a polysubstituted α-conjugated quaterthiophene. The crystals exhibit two crystalline symmetries (monoclinic and triclinic) with different energy gaps and lifetimes of the elementary excitations. Their optical properties are correlated to the molecular structure, showing that the functionalization process determines an intrinsic tuning of the crystal properties. © 1998 American Institute of Physics. View full abstract»

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  • On the optical properties of InAs/InP systems: The role of two-dimensional structures and three-dimensional islands

    Page(s): 1015 - 1017
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    We investigate the effects of the interface morphology on the electronic properties of InAs/In systems using in-air atomic force microscopy and low temperature photoluminescence. Atomic force microscopy results show that the distribution of InAs strained film into three-dimensional islands and the two-dimensional wetting layer—typical of the Stranski–Krastanov growth mode—is strongly affected by the characteristics of the substrate and by the morphology of the InP buffer layer. The differences in the optical data are correlated to the different interface characteristics observed by atomic force microscopy. We discuss the origin of emission peaks taking into account the diffusion process of adsorbed atoms on the different types of surface. © 1998 American Institute of Physics. View full abstract»

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  • Optical second-harmonic probe for ultra-high frequency on-chip interconnects with benzocyclobutene

    Page(s): 1018 - 1020
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    We report the application of electric field-induced second-harmonic (EFISH) generation for time-resolved measurement of ultra-fast electrical pulses propagating on thin-film microstrip lines fabricated with a polymer based on benzocyclobutene as a dielectric layer on a silicon substrate. This contactless field detection enables the characterization of electric pulses on the transmission line up to 1.95 THz. In comparison to external electro-optic sampling the EFISH technique provides a better characterization tool concerning time-resolution and noninvasiveness. © 1998 American Institute of Physics. View full abstract»

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  • Large near-resonance third-order nonlinearity in an azobenzene-functionalized polymer film

    Page(s): 1021 - 1023
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    The third-order nonlinear optical response of a thin film containing the azobenzene dye Disperse Red 1 was studied using the z-scan technique with tunable picosecond pulses. A nonlinear refractive index of -5.0 cm2/GW, corresponding to a Re χ(3)=-3.0×10-15 m2/V2 (2.1×10-7 esu), has been measured at 570 nm. The observed nonlinearity is attributed to the change in refractive index induced by the trans-cis transition in the dye molecule. © 1998 American Institute of Physics. View full abstract»

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  • All-optical wavelength switching in a semiconductor laser using self-seeding and external injection-seeding

    Page(s): 1024 - 1026
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    We developed an approach for efficient optical switching by employing the mechanism of gain competition between two different wavelengths. The lasing wavelengths are obtained by self-seeding and external injection-seeding of the same gain-switched Fabry–Pérot laser diode. A direct comparison shows that the latter yields a faster switching response owing to the injection of continuous oscillations. Optical switching has been demonstrated over a 20 nm tuning range with an injection power as small as 10 μW. An operating bandwidth in the tens of MHz frequency range can be expected using the switching scheme. © 1998 American Institute of Physics. View full abstract»

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  • Optical absorption of composites of nanocrystalline silver prepared by electrodeposition

    Page(s): 1027 - 1029
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    Silver particles of average diameters in the range 10.3–25.7 nm have been grown within a gel medium by an electrodeposition technique. Detailed optical absorption characteristics in the wavelength range 250–600 nm have been investigated for nanocomposites comprising these particles dispersed in a polystyrene matrix. Absorption maximum occurs at a wavelength around 350 nm, which increases as the metal particle size is increased. Mie theory with the incorporation of a distribution of particle size gives remarkable agreement with the experimental data. The electrical conductivity as extracted from the theoretical analysis for particles with diameters ∼3 nm is found to be less than Mott’s minimum metallic conductivity. This indicates the possibility of a metal insulator transition in this system, which appears to be consistent with earlier electrical conductivity measurements. © 1998 American Institute of Physics. View full abstract»

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  • Laser generation of ultrasound using a modulated continuous wave laser diode

    Page(s): 1030 - 1032
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    A modulated continuous wave diode laser was used to generate ultrasonic Lamb waves in a thin steel plate. No surface damage was evident on the sample, a feature due to the low power density of the acoustic source in contrast to the high power pulsed laser systems usually employed for laser-ultrasound generation. The Lamb waves were monitored using a surface bonded optical fiber interferometer whose output was correlated with the pseudorandom binary sequence used to modulate the laser diode. In this fashion, it was possible to record a time domain history of the ultrasonic propagation in the sample. © 1998 American Institute of Physics. View full abstract»

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  • Brominated plastic equation of state measurements using laser driven shocks

    Page(s): 1033 - 1035
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    In order for brominated plastic (CHBr) to be used in future large lasers, such as the National Ignition Facility, capsule design, and equation of state (EOS) data are needed to address uncertainties in modeling. We have performed CHBr EOS measurements using the impedance matching technique. Laser beams spatially smoothed, and giving a spot size of 400 μm and intensities ≤5×1013W/cm2, produced high-quality shock waves allowing the simultaneous measurements of the shock velocities in two materials, one used as reference. Results are compared to other experiments and to EOS calculations. We obtained very good agreement with the theoretical curve for pressures ranging from 1 to 3 Mbar. © 1998 American Institute of Physics. View full abstract»

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  • n-type electrical conduction in transparent thin films of delafossite-type AgInO2

    Page(s): 1036 - 1038
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    Thin films of AgInO2 were prepared to find a transparent and n-type conducting oxide with a delafossite structure. This is a candidate material for fabricating a pn junction with the recently found p type conducting and transparent CuAlO2 delafossite. Nondoped and 5% Sn-doped thin films were deposited on a silica glass substrate by radio-frequency sputtering. The crystalline phase in the films was identified to be the delafossite structure by x-ray diffraction and chemical composition was confirmed to be an Ag/In ratio=1.00/0.97 by inductively coupled plasma emission spectroscopy. The optical band gap was estimated from absorption spectra to be ∼4.4 eV, and the thin films were transparent up to near ultraviolet region. Electrical conductivities of the nondoped and 5% Sn-doped AgInO2 films at room temperature were 1×10-5 and 6×100S cm-1, respectively. Measurements of Hall voltage and Seebeck coefficient (-50 μV K-1) suggested the conduction in the Sn-doped film to be n type. Carrier concentration and Hall mobility in the doped film were 2.7×1019cm-3 and 0.47 cm2 V-1 s-1, respectively. © 1998 American Institute of Physics. View full abstract»

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  • Formation of visible light emitting porous GaAs micropatterns

    Page(s): 1039 - 1041
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    Pore growth on n-type GaAs (100) can be initiated in 1 M HCl solution by electrochemical polarization of the material anodic to a critical potential value—the pore formation potential (PFP). At surface defects, however, the PFP is significantly lower (shifted cathodically). Focused ion beam, implantation of Si++ was used to create defined patterns in the substrate. At these implant sites, the growth of porous GaAs was selectively achieved by polarization below the overall PFP. From the porous GaAs patterns visible photoluminescence at green-yellow wavelengths can be observed. This technique, thus, allows the production of light emitting porous GaAs micropatterns of arbitrary shape by a direct writing process. © 1998 American Institute of Physics. View full abstract»

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  • Measurement of the electrostrictive coefficients of modified lead magnesium niobate using neutron powder diffraction

    Page(s): 1042 - 1044
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    Neutron powder diffraction measurements and a constitutive model were used to determine the bulk longitudinal and transverse electrostrictive coefficients (Qijkl) for polycrystalline samples of the subject material. Effective single-crystal Qijkl were calculated from the powder neutron diffraction data. The resulting values of the three independent Qijkl, as determined from the cubic symmetry of the underlying lattice, are Q3333=2.1×10-2m4/C2, Q3322=-0.87×10-2m4/C2, and Q3232=1.2×10-2m4/C2. Using these effective single-crystal values, a random ensemble average was used to predict the electromechanical performance of the polycrystalline material. Predicted values of bulk Q3333=1.86×10-2m4/C2 and Q3322=-0.78×10-2m4/C2 are 89% and 87%, respectively, of the effective single-crystal values for Q3333 and Q3322, while measured values of polycrystalline specimens are only ∼70% of the single-crystal values. © 1998 American Institute of Physics. View full abstract»

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  • Anisotropic grain noise in eddy current inspection of noncubic polycrystalline metals

    Page(s): 1045 - 1047
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    This letter discusses the role electrical anisotropy plays in the structural integrity assessment of polycrystalline titanium alloys from the standpoint of fatigue crack detection and the related issue of microstructural noise. In eddy current inspection of noncubic crystallographic classes of polycrystalline metals the electric anisotropy of individual grains produces an inherent microstructural variation or noise that is very similar to the well-known acoustic noise produced by the elastic anisotropy of both cubic and noncubic materials in ultrasonic characterization. The presented results demonstrate that although the electrical grain noise is clearly detrimental in eddy current nondestructive testing for small flaws, it can be also exploited for characterization of the microstructure in noncubic polycrystalline materials such as titanium alloys in the same way acoustic grain noise is used for ultrasonic characterization of the microstructure in different materials. © 1998 American Institute of Physics. View full abstract»

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  • Neutron diffraction study of electrostrictive coefficients of prototype cubic phase of relaxor ferroelectric PbMg1/3 Nb2/3 O3

    Page(s): 1048 - 1050
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    The electrostrictive coefficients of prototype cubic phase of PbMg1/3Nb2/3O3 are studied using a high-resolution neutron powder diffraction experiment. The key idea of the approach is to “eliminate” from the material the inhomogeneity related to the presence of the double-phase mixture “polar regions/nonpolar matrix,” and to study two structurally homogeneous states: prototype cubic at temperatures above 600 K and rhombohedral ferroelectric at temperatures below 220 K. The obtained value of volumetric electrostrictive coefficient, Qh=(8.3±1.0)×10-2 ×(m4/C2), is consistent with those of other perovskite ferroelectrics with a prototype cubic phase. © 1998 American Institute of Physics. View full abstract»

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  • X-ray photoelectron spectroscopy study of Bi4Ti3O12 ferroelectric ceramics

    Page(s): 1051 - 1053
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    Ferroelectric Bi4Ti3O12 ceramics have been prepared by the method of reactive sintering. The ceramics exhibit good ferroelectric properties with a remanent polarization Pr=5.5×10-2 C/m2 and a coercive field Ec=3MV/m. High resolution x-ray photoelectron spectra of the atomic constituents of Bi4Ti3O12 were recorded. The obtained results indicate that oxygen vacancies are preferentially sited in the vicinity of Bi ions, in the Bi2O2 layers. © 1998 American Institute of Physics. View full abstract»

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  • Accurate extraction of the diffusion current in silicon p-n junction diodes

    Page(s): 1054 - 1056
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    An accurate method for the extraction of the reverse diffusion current component in a silicon p-n junction diode is proposed. It combines capacitance–voltage and current–voltage measurements on an array of diodes with different geometry in order to separate the peripheral and the volume leakage current components. The corrected volume capacitance is then used to calculate the depletion width as a function of the reverse bias. Extrapolation of the reverse current to zero depletion width results in the diffusion current part, both for the volume and for the peripheral component. From the temperature dependence, a thermal activation energy of 1.12 eV is obtained. The volume diffusion current density of the p-type Czochralski wafers studied, shows a pronounced substrate dependence, while the peripheral diffusion current density is constant. Finally, the implications for the extraction of the effective bulk recombination lifetime are discussed. © 1998 American Institute of Physics. View full abstract»

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  • An improved analysis for band edge optical absorption spectra in hydrogenated amorphous silicon from optical and photoconductivity measurements

    Page(s): 1057 - 1059
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    The uncertainties inherent in the normalization of subgap photoconductivity spectra to the optical absorption spectra α(hv) in a-Si:H based films have been addressed. An analysis is presented which is based on optical transitions of constant dipole matrix element between parabolic distributions of extended states and exponential distributions of localized tail states. This analysis has been used to normalize the two sets of results accurately, as verified by photothermal deflection spectroscopy measurements, and is shown to be useful in the commonly encountered cases, in which the two spectra do not overlap over an extended region. Improved quantitative fits of α(hv), for photon energy from ∼1.5 to 2.4 eV, obtained on different a-Si:H based films indicate that the localized exponential band tail regions extend ∼60–70 meV above the optical gap. © 1998 American Institute of Physics. View full abstract»

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  • Polarized electroabsorption spectra and light soaking of solar cells based on hydrogenated amorphous silicon

    Page(s): 1060 - 1062
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    We present grazing-incidence measurements of polarized electroabsorption spectra in p–i–n solar cells based on hydrogenated amorphous silicon (a-Si:H). We find a significantly stronger polarization dependence in the present measurements compared with earlier work based on electroabsorption detected using coplanar electrodes on a-Si:H thin films. We do not find any significant dependence of the polarized electroabsorption upon light soaking, although this effect was found in previous work with coplanar electrodes. © 1998 American Institute of Physics. View full abstract»

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  • Junction devices based on sulfonated polyaniline

    Page(s): 1063 - 1065
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    Schottky diodes were fabricated using aluminum/neutralized-sulfonated-polyaniline (SPAN) junctions. I–V and C–V measurements were made, and the barrier height B) and the background concentration (NB) were determined to be 0.8 V and 4×1017/cm3, respectively. Using these diodes as gate control, depletion-mode thin-film transistors were fabricated with a source and drain made of gold Ohmic contacts. The transistors were characterized by I–V measurements, and the carrier mobility determined from devices operating in the |VG|≫|VDS| “linear” regime was about 0.01 cm2/V s. This high value of mobility could be attributed to the spherulitic (partially ordered) structures observed in the SPAN thin films. Field-effect transistors were also fabricated on SPAN films deposited on an n-doped silicon substrate acting as the gate electrode with a thermally grown oxide layer. A reasonably high on/off ratio (∼4×103) was measured in these devices. © 1998 American Institute of Physics. View full abstract»

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  • Characterization of high-quality InGaN/GaN multiquantum wells with time-resolved photoluminescence

    Page(s): 1066 - 1068
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    Recombination in single quantum well and multiquantum well InGaN/GaN structures is studied using time-resolved photoluminescence and pulsed photoluminescence measurements. Room-temperature measurements show a rapid lifetime (0.06 ns) for a single quantum well structure, while an increasingly long decay lifetime is measured for multiquantum wells as more quantum wells are incorporated into the structure. Temperature-dependent lifetime measurements show that a nonradiative recombination mechanism activates above 45 K in the single quantum well but is less important in the multiquantum wells. © 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