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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 1052-1062 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We describe investigations of the effects of inserting a thin, low-doped layer into the emitter of an InP/In0.53Ga0.47As heterojunction phototransistor (HPT). This high-low emitter structure has improved sensitivity and bandwidth over conventional structures at low input optical power by decreasing the bulk recombination current at the heterointerface. Experimental data show that the photocurrent gain is independent of the incident optical power at high input powers, corresponding to a heterojunction ideality factor of 1. At low input power, the gain is found to have a small power dependence, with an ideality factor of 1.25. A current gain as high as 260 is obtained at an input power of only 40 nW. These results, which are consistent with numerical simulations of the HPTs, give direct evidence that bulk recombination in the space- charge region at the emitter/base junction is the major source of recombination current for an InP/In0.53Ga0.47As HPT. A second structure is also proposed to improve the sensitivity by inserting a heavily doped layer into the base.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 342-351 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Interface charge densities and conduction-band offset energies for liquid-phase epitaxially grown n-N isotype In0.53Ga0.47As/InP heterojunctions have been measured using capacitance-voltage methods. Extremely low interface charge densities have been obtained in some samples, and they are found to be independent of both the measurement temperature and the magnitude of lattice mismatch. Our samples show a clear peak and notch in the apparent free-carrier concentration profile at temperatures as low as 83 K. This is in contrast to results reported previously where the notch, due to the carrier depletion at the heterojunction, was observed to vanish at low temperature. An electron trap has been identified in one of the samples. The trap is uniformly distributed within the bulk of the In0.53Ga0.47As layer at a density of 5×1014 cm−3. In spite of the presence of this relatively low density defect, the heterojunctions grown for this study apparently have considerably lower interface defect densities than observed by others.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 4076-4084 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Vertical stacking of organic light emitting devices (OLEDs) that emit the three primary colors is shown to be a means for achieving efficient and bright full-color displays. In Paper I, we addressed stacked OLED (SOLED) design and fabrication principles to optimize emission colors, operating voltage, and efficiency. Here, we present results on two different (metal-containing and metal-free cathode) SOLED structures that exhibit performance suitable for many full-color display applications. The operating voltages at 10 mA/cm2 (corresponding to video display brightnesses) are 6.8, 8.5, and 12.1 V for the red (R), green (G), and blue (B) elements of the metal-containing SOLED, respectively. The respective subpixel luminous efficiencies are 0.53, 1.44, and 1.52 cd/A, and the Commission Internationale de L'Éclairage (CIE) chromaticity coordinates are (0.72, 0.28), (0.42, 0.56), and (0.20, 0.22). In the high transparency metal-free SOLED, an insulating layer was inserted between the two upper subpixels to allow for independent grounding of all color emitters in the stack. At operating voltages of 12–14 V, video display brightnesses were achieved with luminous efficiencies of 0.35, 1.36, and 1.05 cd/A for the R, G, and B subpixels, respectively. The respective CIE coordinates for R, G, and B emissions are (0.72, 0.28), (0.26, 0.63), and (0.17, 0.28) in the normal viewing direction, shifting inperceptibly as the viewing angle is increased to as large as 60°. Finally, we discuss addressing schemes of SOLED displays, and compare them with other strategies for achieving full-color, OLED-based displays. © 1999 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3324-3327 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We demonstrate that ordered microlens arrays with 10 μm diam poly-dimethyl-siloxane lenses attached to glass substrates increase the light output of organic light emitting devices (OLED) by a factor of 1.5 over unlensed substrates. The lenses, which are considerably smaller than, and not aligned to the OLEDs, outcouple light that is emitted outside of the escape cone of the substrate. We show that an electrophosphorescent device based on a fac tris(2-phenylpyridine)Iridium (Ir(ppy)3) doped emitting layer has its external quantum efficiency increased from 9.5% using a flat glass substrate, to 14.5% at low current densities using a substrate with microlenses. No change in the emission spectrum is observed for different viewing angles using the lens arrays. © 2002 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 4986-4992 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We study the effects of lithium (Li) incorporation in the cathodes of organic light-emitting devices. A thermally evaporated surface layer of metallic Li is found to diffuse through, and subsequently dope, the electron transporting organic semiconducting thin films immediately below the cathode, forming an Ohmic contact. A diffusion length of ∼700 Å is inferred from analyses of the current–voltage and secondary ion mass spectrometry data. The conductivity of the Li-doped organic films is ∼3×10−5 S/cm. Photoemission spectroscopy suggests that Li lowers the barrier to injection at the organic/cathode interface, introduces gap states in the bulk of the organic semiconductor, and dopes the bulk to facilitate efficient charge transport. © 2001 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 7991-8006 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We measure the current–voltage and electroluminescence characteristics of single-heterojunction, vacuum-deposited organic light-emitting devices (OLEDs) over a wide range of materials, temperatures, and currents. We find that the current is limited by a large density of traps with an exponential energy distribution below the lowest unoccupied molecular orbital. The characteristic trap depth is 0.15 eV. Furthermore, in metal–quinolate-based devices, electroluminescence originates from recombination of Frenkel excitons, and its temperature dependence is consistent with the excitons being formed by Coulombic relaxation of the trapped electrons with holes injected from the counter electrode. By semiempirical molecular orbital modeling, we find that the trap distribution obtained from the current–voltage characteristics is consistent with a distribution in the metal–quinolate molecular conformations which result in a continuous, exponential distribution of allowed states below the lowest unoccupied molecular orbital. We discuss the implications of the intrinsic relationship between electroluminescence and current transport in OLEDs for the optimization of efficiency and operating voltage in these devices. © 1996 American Institute of Physics.
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  • 7
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ultraviolet (UV) optical pumping followed by fluorescence wavelength downconversion of thin film organic light emitting materials deposited directly on the surface of Si p-n junction diodes is found to be an accurate and rapid means to determine the film internal fluorescence efficiency. By measuring the photoresponse of the Si detectors in the UV, we find that the organic light emitting films of aluminum tris-(8-hydroxyquinoline) (Alq3), N,N′-diphenyl-N,N′-bis-(3-methylphenyl)- 1,1′-biphenyl-4,4′-diamine, and bis-(8-hydroxyquinaldine)-chlorogallium (Gaq2′Cl), have internal fluorescence efficiencies of 0.30±0.05, 0.35±0.03, and 0.36±0.03, respectively. It has also been found that the organic films can be grown to a thickness which optimizes UV light conversion and can, at the same time, serve as antireflection coatings in the visible spectral region, thereby resulting in enhanced Si photodiode sensitivity extending from the UV to the infrared. © 1996 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 58 (1991), S. 562-564 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Optical waveguiding in a crystalline organic semiconductor, namely, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA), has been demonstrated in slab and rib waveguides using both end-fire and grating coupling at λ 1.064 μm. The effective indices of the transverse electric (TE) modes measured by means of grating coupling are in good agreement with theoretical prediction. A very low propagation loss of 2.5 dB/cm in a PTCDA rib waveguide has been determined from measurements of the scattered light intensity from the top surface of the guide.
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 57 (1990), S. 469-471 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The conduction-band discontinuities and interface charge densities of several n-N isotype InxGa1−xAs/InP (x(approximately-equal-to)0.53) heterojunctions with lattice mismatches (Δa/a) ranging from +0.26 to −0.24% were measured using capacitance-voltage techniques. To facilitate these measurements, organic-on-inorganic contact barrier diodes were used. Extremely low interface charge densities (〈1×1010 cm−2) are obtained for all the samples, which are approximately one order of magnitude lower than previously reported values for these heterojunctions. We find that the interface charge density is independent of the magnitude of lattice mismatch and temperature. All the samples show a clear peak-and-notch in their apparent free-carrier concentration profiles at temperatures as low as 83 K. This is in contrast to results reported previously where the notch is observed to disappear at low temperature. The measured heterojunction conduction-band discontinuity is also found to be temperature independent, with a value of 0.22±0.02 eV.
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 74 (1999), S. 305-307 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report the demonstration of a transparent, completely metal-free, full-color stacked organic light-emitting device (SOLED). The SOLED emits light from both top and bottom (substrate) surfaces with total external quantum efficiencies of 0.65%, 1.3%, and 2.2% for the green, blue, and red stacked subpixels, respectively. The respective top emission quantum efficiencies for the three subpixels are 0.23%, 0.63%, and 1.6%. The angular dependence of emission colors due to microcavity effects is weak when viewed from the top device surface. This metal-free SOLED is from 21% to 50% transparent over the entire visible spectral range. Capability for top emission makes this device suitable for integration with electronic components in active matrix display backplanes. © 1999 American Institute of Physics.
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