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6 Dec 2004

Volume 85, Issue 23, pp. 5499-5791

Issue Cover Spotlight Figure

Appl. Phys. Lett. 85, 5694 (2004); http://dx.doi.org/10.1063/1.1828575 (3 pages)

M. Y. Shen, C. H. Crouch, J. E. Carey, and E. Mazur
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Current collapse-free i-GaN∕AlGaN∕GaN high-electron-mobility transistors with and without surface passivation

S. Arulkumaran, T. Hibino, T. Egawa, and H. Ishikawa

Appl. Phys. Lett. 85, 5745 (2004); http://dx.doi.org/10.1063/1.1830677 (3 pages) | Cited 4 times

Online Publication Date: 8 December 2004

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Drain current (ID) collapse-free i-GaN∕AlGaN∕GaN high-electron-mobility transistors (HEMTs) with and without surface passivation (electron-beam evaporated SiO2) were demonstrated using dc and pulsed (120 Hz) IDS–VDS characteristics up to the drain supply voltage of 40 V. The observation of small ID transients and negligibly small hysteresis widths with small white light illumination effects on both passivated and unpassivated i-GaN∕AlGaN∕GaN HEMTs confirms the suppression of collapse related traps. Three and two thermally activated trap levels were observed in passivated (+0.395, −0.079, and −0.949 eV) and unpassivated (−0.066 and −0.368 eV)AlGaN∕GaN HEMTs, respectively. However, i-GaN∕AlGaN∕GaN HEMTs with and without surface passivation exhibited only one trap level at −0.161 eV. These results show that the addition of thin cap layer i-GaN screens the collapse-related surface states/traps from channel.
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85.30.Tv Field effect devices
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
81.65.Rv Passivation

Normally closed microgrippers using a highly stressed diamond-like carbon and Ni bimorph structure

J. K. Luo, A. J. Flewitt, S. M. Spearing, N. A. Fleck, and W. I. Milne

Appl. Phys. Lett. 85, 5748 (2004); http://dx.doi.org/10.1063/1.1833555 (3 pages) | Cited 11 times

Online Publication Date: 8 December 2004

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A normally closed microgripper with a radius of curvature of 18–50 μm using a diamond-like carbon (DLC) and stress free electroplated Ni bimorph structure has been demonstrated. The large curvature in the fingers of the microgrippers is due to the high compressive stress of the DLC layer. The radius of curvature of the figures can be adjusted by the thickness ratio, and the closure of the devices can also be adjusted by varying the finger length. This device works much more efficiently than other bimorph structures due to the large difference in thermal expansion coefficients between the DLC and the Ni layers. Preliminary electrical tests have shown these microgrippers can be opened by 60°–90° at an applied power of <20 mW.
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07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
68.60.Bs Mechanical and acoustical properties
65.40.De Thermal expansion; thermomechanical effects

Organic vertical-channel transistors structured using excimer laser

R. Parashkov, E. Becker, G. Ginev, T. Riedl, M. Brandes, H.-H. Johannes, and W. Kowalsky

Appl. Phys. Lett. 85, 5751 (2004); http://dx.doi.org/10.1063/1.1833551 (3 pages) | Cited 8 times

Online Publication Date: 8 December 2004

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Low-cost, large-area patterning of organic field-effect transistors with high-resolution is a subject of ongoing investigations. Here, we present a concept of patterning vertical-channels organic transistors using excimer laser. The channel length is controlled by the thickness of the dielectric polymer layer between the drain and source electrodes. We demonstrate that, by using this method, patterning of transistors with either metal or polymer contacts with resolutions as high as 2 μm is possible. Experimental data of vertical-channel pentacene transistors with either gold or poly (3,4-ethylenedioxythiophene) as drain-source contacts are reported. Field effect mobilities of 1×10−3 and 3×10−4 cm2∕V s, respectively, have been measured in these devices.
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85.30.Tv Field effect devices
72.20.Fr Low-field transport and mobility; piezoresistance
73.40.Ns Metal-nonmetal contacts
42.62.-b Laser applications

Dye-sensitized near-infrared room-temperature photovoltaic photon detectors

P. V. V. Jayaweera, A. G. U. Perera, M. K. I. Senevirathna, P. K. D. D. P. Pitigala, and K. Tennakone

Appl. Phys. Lett. 85, 5754 (2004); http://dx.doi.org/10.1063/1.1831562 (3 pages) | Cited 5 times

Online Publication Date: 8 December 2004

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Dye molecules bonded to a semiconductor surface could inject carriers to a band on photoexcitation. This process known as dye-sensitization is used for extending the sensitivity of silver halide emulsions. More recently, dye-sensitization has been adopted to devise solar cells. A near-infrared (NIR) sensitive heterojunction n‐TiO2∕D∕p‐CuSCN (where D denotes a NIR absorbing dye) is developed to examine the possibility of using dye-sensitization for IR detection. Although the responsivity is lower and response slow compared to silicon detectors, dye-sensitized detectors would be cost effective, especially for large area devices. They are operable at room temperature and have the advantage of insensitivity to noise induced by band-gap excitations (providing high specific detectivity of ∼1011). Furthermore, the spectral response can be adjusted by choosing the appropriate dye.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.30.Tv Field effect devices

Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions

Jiangeng Xue, Soichi Uchida, Barry P. Rand, and Stephen R. Forrest

Appl. Phys. Lett. 85, 5757 (2004); http://dx.doi.org/10.1063/1.1829776 (3 pages) | Cited 256 times

Online Publication Date: 8 December 2004

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We demonstrate high-efficiency organic photovoltaic cells by stacking two hybrid planar-mixed molecular heterojunction cells in series. Absorption of incident light is maximized by locating the subcell tuned to absorb long-wavelength light nearest to the transparent anode, and tuning the second subcell closest to the reflecting metal cathode to preferentially absorb short-wavelength solar energy. Using the donor, copper phthalocyanine, and the acceptor, C60, we achieve a maximum power conversion efficiency of ηP=(5.7±0.3)% under 1 sun simulated AM1.5G solar illumination. An open-circuit voltage of VOC⩽1.2 V is obtained, doubling that of a single cell. Analytical models suggest that power conversion efficiencies exceeding 6.5% can be obtained by this architecture.
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84.60.Jt Photoelectric conversion
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Hydrostatic-pressure dependence of organic thin-film transistor current versus voltage characteristics

Zhenlin Rang, Marshall I. Nathan, P. Paul Ruden, Reid Chesterfield, and C. Daniel Frisbie

Appl. Phys. Lett. 85, 5760 (2004); http://dx.doi.org/10.1063/1.1829388 (3 pages) | Cited 12 times

Online Publication Date: 8 December 2004

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We report results of electrical output and transfer characteristics for two top-contact pentacene thin-film transistors under hydrostatic pressure at room temperature. Strong reversible increases of the drain current and the field-effect hole mobility with increasing pressure were observed, in particular for a device with relatively low current at atmospheric pressure.
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85.30.Tv Field effect devices
73.50.Dn Low-field transport and mobility; piezoresistance
62.50.-p High-pressure effects in solids and liquids
73.61.Ph Polymers; organic compounds

Metal∕organic∕metal bistable memory devices

D. Tondelier, K. Lmimouni, D. Vuillaume, C. Fery, and G. Haas

Appl. Phys. Lett. 85, 5763 (2004); http://dx.doi.org/10.1063/1.1829166 (3 pages) | Cited 85 times

Online Publication Date: 8 December 2004

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We report a bistable organic memory made of a single organic layer embedded between two electrodes, and compare to the organic∕metal nanoparticle∕organic tri-layers device [Ma, Liu, and Yang, Appl. Phys. Lett. 80, 2997 (2002)]. We demonstrate that the two devices exhibit similar temperature-dependent behaviors, a thermally activated behavior in their low conductive state (off-state) and a slightly “metallic” behavior in their high conductive state (on-state). This feature emphasizes a similar origin for the memory effect. Owing to their similar behavior, the one layer memory is advantageous in terms of fabrication cost and simplicity.
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73.40.Rw Metal-insulator-metal structures

Tuning the oxide/organic interface: Benzene on SnO2(101)

Matthias Batzill, Khabibulakh Katsiev, and Ulrike Diebold

Appl. Phys. Lett. 85, 5766 (2004); http://dx.doi.org/10.1063/1.1831565 (3 pages) | Cited 13 times

Online Publication Date: 8 December 2004

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Two different SnO2(101) bulk terminations have been prepared in order to demonstrate the impact of the oxide surface composition on the interface properties between SnO2(101) and an organic film. The change in work function causes a rigid shift of the molecular orbitals of the condensed organic film by 1 eV with respect to the valence band of SnO2. This change in the band alignment between an organic film and an oxide electrode material allows tuning of the barriers for charge transfer across this interface in molecular electronics applications.
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73.20.At Surface states, band structure, electron density of states
68.35.Dv Composition, segregation; defects and impurities
73.30.+y Surface double layers, Schottky barriers, and work functions
79.60.Jv Interfaces; heterostructures; nanostructures
79.60.Fr Polymers; organic compounds
79.60.Dp Adsorbed layers and thin films

Reduction of operating voltage in organic light-emitting diode by corrugated photonic crystal structure

Masayuki Fujita, Tetsuya Ueno, Kuniaki Ishihara, Takashi Asano, Susumu Noda, Hiroshi Ohata, Taishi Tsuji, Hitoshi Nakada, and Noriyuki Shimoji

Appl. Phys. Lett. 85, 5769 (2004); http://dx.doi.org/10.1063/1.1836867 (3 pages) | Cited 18 times

Online Publication Date: 8 December 2004

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A reduction of the operating voltage is achieved for an organic light-emitting diode containing a corrugated photonic crystal structure fabricated by the etching of an indium-tin-oxide anode layer. This is due to a partial reduction in the thickness of the organic layer. The light extraction efficiency can be also improved due to the diffraction of confined light by the photonic crystal effect. The voltage reduction is demonstrated in combination with an improvement in the luminance efficiency at constant current for the fabricated device.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
81.65.Cf Surface cleaning, etching, patterning
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