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Applied Physics Letters

Issue 24 • Date Dec 2002

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Displaying Results 1 - 25 of 58
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Highly-bright white organic light-emitting diodes based on a single emission layer

    Page(s): 4499 - 4501
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    A very bright white organic light-emitting diode (OLED) was fabricated with a thin layer of 4-{4-[N-(1-naphthyl)-N-phenylaminophenyl]}-1,7-diphenyl-3,5-dimethyl-1,7-dihydro-dipyrazolo [3,4-b;43-e]pyridine (PAP-NPA) doped with rubrene as the source of the white emission. Thus, with a simple three-layer structure of ITO/NPB(40 nm)/PAP-NPA:0.5% rubrene(20 nm)/TPBI(40 nm)/Mg:Ag, a white light with Commission Internationale de l’Eclairage (CIE) coordinates of (0.31, 0.33) were generated. The device gave a maximum brightness of ∼42 000 cd/m2 at 14 V, and maximum luminance efficiencies of 2.92 lm/W at 6.5 V and 6.11 cd/A at 7.0 V. The CIE coordinates stayed virtually constant when the voltage increased from 8 to 12 V. Furthermore, with a two-layer structure of ITO/PAP-NPA:0.4%rubrene(40 nm)/TPBI(40 nm)/Mg:Ag, the device also reached a stable white color with maximum brightness of ∼37 000 cd/m2 and maximum luminance efficiencies of 2.51 lm/W at 6.5 V and 5.57 cd/A at 8.5 V. The stability of the white color is attributed to the confinement of charge recombination zone in a single layer. © 2002 American Institute of Physics. View full abstract»

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  • Si-based omnidirectional reflector and transmission filter optimized at a wavelength of 1.55 μm

    Page(s): 4502 - 4504
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    Omnidirectional (omni-) reflector and transmission filter optimized at a wavelength of 1.55 μm have been realized using Si/SiO2 one-dimensional photonic crystals (1D PCs). Photonic band structures in the PCs with and without a defect layer have been also analyzed. The 1D PCs fabricated have very large omnidirectional photonic band gaps (omni-PBGs), and their measured reflectance spectra are in very good agreement with the calculated results. The omni-PBG in a six-pair Si/SiO2 1D PC (no defect layer, a filling factor η=0.406) exists in a normalized frequency range of 0.223–0.297. Introducing a defect layer generates a defect state in the PBG, and it behaves like electronic impurity levels in the gap with a change in thickness of the defect layer (dD). A six-pair Si/SiO2 1D PC with the structure parameters of η=4.060, dH (Si thickness)=95.3 nm, dL (SiO2 thickness)=234.9 nm, and dD=2.336∙dH could act as a 1.55 μm-transmission filter for normal incidence light. © 2002 American Institute of Physics. View full abstract»

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  • Light-induced absorption changes in In-doped congruent LiNbO3

    Page(s): 4505 - 4507
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    We investigate visible and near-infrared light-induced absorption changes caused by intrinsic defect centers in congruently melting and reduced LiNbO3:In with a single-shot picosecond pulse at 532 nm. It is found that the dark decay dynamics of optically induced shallow centers strongly depend on In concentrations. Two types of optically induced shallow centers having dark decay time constants on the orders of milliseconds and tens of microseconds are observed in LiNbO3 doped with In below the damage-resistant threshold concentrations. Only one of the two types is observed in LiNbO3 doped with In above it. The origin of these shallow centers is also discussed. © 2002 American Institute of Physics. View full abstract»

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  • Switchable organic electroluminescence

    Page(s): 4508 - 4510
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    The vacuum deposited amorphous organic thin film melamine cyanurate (MC) is found to possess an electrical switching property. The as-fabricated fresh thin film is at a semiconductor state and shows a rectification property in a range of several voltages. Under a higher applied voltage, the resistance of the film changes to a lower value, and shows linearity in its current–voltage relationship. After the application of a negative driving voltage for no more than 100 s, the MC film returns from the resistor state to the semiconductor state with a higher impedance. Based on this reproducible switching phenomenon, a type of voltage-controlled switchable organic electroluminescence device was fabricated by inserting the MC layer between the anode indium tin oxide (ITO) and the hole transport layer N,N-bis (3-methylphenyl)-N,N-diphenyl-benzidine (TPD). In the forward driving voltage scan (for example, increasing the driving voltage from 0 to 16 V), the impedance is higher and the current density is lower; then the scan is backward (decreasing the driving voltage from 16 to 0 V), the impedance becomes lower and the current density becomes higher. Corresponding to the two different current density states at the same driving voltage, the device luminance also reaches two different states, one is lower and another is higher. This transition from a high (ON) to low luminance (OFF) renders a prototype to realize a switchable organic electroluminescence. © 2002 American Institute of Physics. View full abstract»

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  • Two-color hologram multiplexing from the colored state in stoichiometric LiNbO3:Tb,Fe

    Page(s): 4511 - 4513
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    We have performed two-color hologram multiplexing in stoichiometric LiNbO3:Tb,Fe, which exhibits a photochromic effect. One hundred angle-multiplexed holograms of nearly equivalent strength have been recorded from the colored state by use of 850 nm recording beams and a 436 nm gating light. The stored holograms were highly resistant against erasure during continuous readout. It was estimated that 80 million readouts with a bit rate of 1 Gbit/s is possible before the diffracted signal intensity decays to half of its initial value. © 2002 American Institute of Physics. View full abstract»

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  • Dropping of electromagnetic waves through localized modes in three-dimensional photonic band gap structures

    Page(s): 4514 - 4516
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    We experimentally demonstrate trapping and dropping of photons through localized cavity modes in three-dimensional layer-by-layer photonic crystal structures. By creating acceptor- and donor-like cavities which are coupled to a highly confined waveguide (HCW), we drop selected frequencies from the waveguide mode. Tunability of the demultiplexing structures can be achieved by changing the properties of cavities and the coupling between the cavity and the HCW. © 2002 American Institute of Physics. View full abstract»

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  • Measurements of α-factor in 2–2.5 μm type-I In(Al)GaAsSb/GaSb high power diode lasers

    Page(s): 4517 - 4519
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    Spectra of the linewidth enhancement factor (α) of room-temperature-operated high-power 2–2.5 μm In(Al)GaAsSb/GaSb type-I quantum-well (QW) lasers were measured using Hakki–Paoli technique. Values of α at threshold were in the range 2.5 to 4 for all devices under study. Devices emitting 1 W cw power at λ=2.5 μm have 1.5%–1.6% compressively strained QW active region and the lowest α equal to 2.5. Measured average filament spacings in near field are in rough agreement with predictions given by experimental α-factor values. © 2002 American Institute of Physics. View full abstract»

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  • Supermode-noise suppression using a nonlinear Fabry–Pérot filter in a harmonically mode-locked fiber ring laser

    Page(s): 4520 - 4522
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    A simple efficient method for stabilizing a harmonically mode-locked fiber ring laser is proposed. In this method, a linear optical filter and a nonlinear Fabry–Pérot filter in which the refractive index is optical intensity dependent are located in the laser cavity. The linear filter is used to select a fixed lasing wavelength, and the Fabry–Pérot filter introduces a negative all-optical feedback mechanism that is able to suppress pulse-to-pulse amplitude fluctuations in the laser cavity. The scheme was experimentally demonstrated using a fiber Bragg grating as the linear filter and a laser diode biased below threshold as the nonlinear Fabry–Pérot, and stable harmonically mode-locked pulses with a supermode noise suppression ratio ≫55 dB were obtained. © 2002 American Institute of Physics. View full abstract»

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  • Transverse mode-locking in microcavity lasers

    Page(s): 4523 - 4525
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    We experimentally demonstrate mode-locking between the transverse modes of a laser. A vertical-cavity surface-emitting laser with evenly-spaced transverse modes is shown to emit a train of 2.1±0.1 ps pulses with an 11 ps repetition rate and a timing jitter of 235±30 fs. Transverse mode-locking in microcavity lasers has potential to improve the compactness, stability, integrability, repetition rate tunability, and efficiency of ultrafast optical communication sources. © 2002 American Institute of Physics. View full abstract»

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  • Red, green, and blue upconversion luminescence of trivalent-rare-earth ion-doped Y2O3 nanocrystals

    Page(s): 4526 - 4528
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    Trivalent-rare-earth ion-doped Y2O3 nanocrystals have been synthesized, and their photoluminescence properties have been studied under 980 nm laser diode excitation. The crystallite size estimated by x-ray diffractometry and scanning electron microscopy was about 30–40 nm. In Yb3+ and Tm3+ codoped Y2O3 nanocrystals, the bright blue emissions near 450 and 480 nm have been noticeable due to the 1D23F4 and 1G43H6 transitions of Tm3+, respectively. The bright green emissions of Er3+ doped Y2O3 nanocrystals appeared near 530 and 550 nm were assigned to the 2H11/24I15/2 and 4S3/24I15/2 transitions of Er3+, respectively. The ratio of the intensity of green luminescence to that of red luminescence has decreased with an increase of concentration of Yb3+ in Er3+ doped Y2O3 nanocrystals. In sufficient quantities of Yb3+ to Er3+, the bright red emission near 660 nm has been predominant due to the 4F9/24I15/2 transition of Er<- - /roman>3+. The primary color components are in these red, green, and blue emissions, from which a wide spectrum of colors, including white, would be produced by appropriate mixing them. © 2002 American Institute of Physics. View full abstract»

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  • Photodetection in the visible, ultraviolet, and near-infrared with silicon microdischarge devices

    Page(s): 4529 - 4531
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    The photosensitivity of 50 and 100 μm square microdischarge devices having an inverted pyramidal Si cathode has been demonstrated and characterized in the ultraviolet, visible, and near-infrared (350–1200 nm). When operating with 500 Torr of Ne, (100 μm)2 microdischarge devices exhibit responsivities of 700±150 and 570±120 A W-1 at 850 and 660 nm, respectively, or more than an order of magnitude larger than peak values typical of commercially available Si avalanche photodiodes. Maximum sensitivity occurs in the 800–900 nm region for the larger (100 μm square) devices but is blueshifted to 600–650 nm for (50 μm)2 detectors. The peak response for the 50 μm square device is 950±250 A W-1 (at ∼625 nm), an increase of ∼35% over that for the (100 μm)2 detector. The experimental results indicate that the semiconductor photocathode determines the spectral response of the device whereas the plasma serves as an electron multiplier. The sensitivity and dynamic range of this new hybrid semiconductor/plasma photodetector suggest that arrays of microdischarge photodetectors are potentially valuable for spectroscopic and biochemical applications. © 2002 American Institute of Physics. View full abstract»

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  • Fabrication technique for filling-factor tunable titanium dioxide colloidal crystal replicas

    Page(s): 4532 - 4534
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    Filling-factor tunable titanium dioxide replicas of colloidal crystals were fabricated by using a liquid phase deposition (LPD) technique. Ammonium hexafluorotitanate [(NH4)2TiF6] was used as a precursor, which was converted slowly to titanium dioxide by hydrolysis with boric acid. After the oxide was formed in voids of latex colloidal crystals, the latex was removed by calcination. By applying the LPD process to the replica again, the filling factor of the replica could be increased in a controllable fashion. With 203 nm template spheres, the peak shift in the ∼490 nm stop band depended linearly on the deposition time for the first 20 min, at a rate of 4.4 nm/min. Thus the position of the Bragg reflection peak could be adjusted precisely by controlling the filling factor of the replica structure. © 2002 American Institute of Physics. View full abstract»

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  • Images of dopant profiles in low-energy scanning transmission electron microscopy

    Page(s): 4535 - 4537
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    A scanning electron microscope is used in transmission mode. The image is formed with secondary electrons, collected by the standard detector, resulting from the conversion of transmitted electrons on a circular disk, covered with MgO smoke, located below the thinned specimen, and centered on the optical axis. Operating in this mode, bright-field images of As dopant profiles in Si, having a peak concentrations of 5 and 2.5 at. % and a spatial extension of about 40 nm, have been observed in cross sectioned specimens. The description of the dopant profiles has a resolution of 6 nm as defined by the spot size of the microscope, equipped with a LaB6 tip, and operating at 30 keV. © 2002 American Institute of Physics. View full abstract»

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  • Atomic structure of steps and defects on the clean diamond (100)-2×1 surface studied using ultrahigh vacuum scanning tunneling microscopy

    Page(s): 4538 - 4540
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    We report ultrahigh vacuum scanning tunneling microscopy studies of the clean nonhydrogen-terminated diamond (100)-2×1 surface showing single- and double-layer steps that are rebonded. The main defects observed are single, multiple, and row dimer vacancies, and antiphase boundaries. Buckling of dimers is not observed, consistent with symmetric dimers. © 2002 American Institute of Physics. View full abstract»

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  • Observation of coreless edge and mixed dislocations in Mg-doped Al0.03Ga0.97N

    Page(s): 4541 - 4543
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    Transmission electron microscopy has been used to examine the core structure of threading dislocations in heavily Mg-doped Al0.03Ga0.97N thin films. It is shown that edge and mixed, but not screw, dislocations have hollow cores with diameters typically 1–5 nm. A model is presented where hollow core formation is initiated by the segregation and subsequent precipitation of Mg at these dislocation cores. The implications for understanding the formation of hollow core dislocations in other GaN films are discussed. © 2002 American Institute of Physics. View full abstract»

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  • Extraction of height–height correlation function of random surfaces from the average intensity of image speckles

    Page(s): 4544 - 4546
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    We present a method for the extraction of the height–height correlation function of random surfaces from the average intensity of image speckles. The setup of a Fourier transforming and imaging system with a variable aperture is used for both the theoretical analysis and experimental performances. Based on the analytical expression of the image intensity, an algorithm is developed to formulate numerically the intensity data versus the aperture radius into the pair of Bessel–Fourier transform and the inversion, from which the height–height correlation function is reconstructed. Three samples of Gaussian correlation are used for the experimental demonstration. The extracted height–height correlation function and the random surface parameters obtained thereby conform with those obtained by atomic force microscopy. © 2002 American Institute of Physics. View full abstract»

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  • Highly crystalline 5H-polytype of sp3-bonded boron nitride prepared by plasma-packets-assisted pulsed-laser deposition: An ultraviolet light emitter at 225 nm

    Page(s): 4547 - 4549
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    Highly crystalline 5H-polytypic form of sp3-bonded boron nitride (BN) was grown by pulsed-laser-vaporization of BN, where synchronous reactive-plasma packets assisted the crystal growth in the vapor phase. The structure of the product crystallites (∼5 μm) was confirmed by using transmission electron diffraction and electron energy loss spectroscopy. This material proved to have a sharp and dominant band at 225 nm by cathodoluminescence at room temperatures and corresponding monochromatic images revealed that they uniformly emitted the ultraviolet light. Considering that cubic BN has already been doped as p- and n-type semiconductors, this material may be applied to the light-emitting devices working at almost the deepest limit of the UV region that is functional without vacuum. © 2002 American Institute of Physics. View full abstract»

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  • Damp-heat induced sulfate formation in Cu(In,Ga)(S,Se)2-based thin film solar cells

    Page(s): 4550 - 4552
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    To investigate the impact of damp heat treatments on the electronic and chemical structure of Cu(In,Ga)(S,Se)2-based thin film solar cells, we have performed a detailed soft x-ray emission study of the ZnO/CuIn(S,Se)2 and ZnO/CdS/CuIn(S,Se)2 interfaces. By comparing the sulfur L2,3 emission spectra of pristine and damp-heat treated samples, we find a sulfate formation at the ZnO/CuIn(S,Se)2 and the ZnO/CdS interface. The intensity behavior as a function of ZnO film thickness further reveals a diffusion of sulfur atoms into the ZnO film, leading to the formation of zinc sulfate in the ZnO window layer of damp-heat-treated Cu(In,Ga)(S,Se)2-based solar cells. © 2002 American Institute of Physics. View full abstract»

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  • Fe/ZnSe(001) Schottky-barrier height evaluated by photoemission

    Page(s): 4553 - 4555
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    We present the Schottky-barrier height determination for the Fe/ZnSe(001) system performed by core and valence level photoelectron emission spectroscopy. Above the thickness of 2 ML, the Fe–Fermi level position is stabilized at 1.6 eV above the valence-band maximum of the n-type undoped ZnSe. This corresponds to a Schottky-barrier height value of 1.1 eV. A bulk-like d-band electronic structure could be observed for thickness as thin as 2 ML of Fe. © 2002 American Institute of Physics. View full abstract»

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  • I-Line lithography of poly-(3,4-ethylenedioxythiophene) electrodes and application in all-polymer integrated circuits

    Page(s): 4556 - 4558
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    Industrialization of polymer electronics requires the use of safe solvents. To that end an I-line lithography process for conducting thin poly(3,4-ethylenedioxythiophene) films has been developed. The fully waterborne process is based on photocrosslinking using bisazide- and polyazide-type photoinitiators. The minimum feature size realized comprises 2.5 μm wide lines separated by 1 μm spacings. The sheet resistance is 1 kΩ/square. The process has been applied to fabricate all-polymer integrated circuits. The technology is demonstrated with functional 15-bit code generators. © 2002 American Institute of Physics. View full abstract»

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  • Tunable ferromagnetic resonance peak in tunneling magnetoresistive sensor structures

    Page(s): 4559 - 4561
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    Noise properties of submicron scale tunneling magnetoresistive (TMR) sensors were investigated at frequencies up to 3 GHz. Noise spectral density was measured as a function of frequency, applied field, and bias current. Noise spectral density versus frequency dependence exhibits a pronounced peak, tunable over a wide frequency range. This peak appears to originate from current-driven precession of magnetization. The peak center frequency can be as low as 200 MHz and has a strong dependence on applied field and bias current. The damping constant α of the main precession mode in the TMR sensor free layer was found to be in the range of 0.05–0.18. It is shown that the magnetic state of a magnetoresistive sensor depends on the bias current and may be characterized by noise properties. The magnetoresistive element can operate as a source of high-frequency radiation with 1 nW emitting power from a 0.1 μm2 junction and signal to noise ratio of 10 dB. © 2002 American Institute of Physics. View full abstract»

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  • Crossover of magnetotransport process toward spin-polarized tunneling in manganite thin films

    Page(s): 4562 - 4564
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    We report on the magnetoresistive features of well-characterized polycrystalline manganite thin films as a function of film thickness. A crossover of the magnetization process from domain wall displacement to magnetic rotation of each grain is observed around a thickness of 25 nm, accompanied with the maximum of the coercivity. The thickness dependence of the electrical resistivity clearly distinguishes the transport mechanism at 25 nm, below which the magnetoresistance can be well scaled by ∼(M/MS)2, using the magnetization M and the saturation magnetization MS. Spin-polarized tunneling transport between adjacent grains provides a comprehensive picture for the magnetotransport data. © 2002 American Institute of Physics. View full abstract»

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  • Thickness dependence of exchange bias and structure in MnPt and MnNi spin valves

    Page(s): 4565 - 4567
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    We describe x-ray diffraction and magnetic measurements of annealed MnPt and NiMn spin valves as a function of MnPt (NiMn) film thickness. Thin films (>~5–10 nm) are not chemically ordered and hence not antiferromagnetic, which explains the lack of exchange coupling in these films. With increasing thickness, the L10 chemical order develops (through coexistence between fcc and L10 phases) and there is a strong correlation between chemical order and exchange coupling. The lack of chemical order in the thin films is likely due to a lack of nucleation sites for the L10 phase. © 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