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14 Apr 2008

Volume 92, Issue 15, Articles (15xxxx)

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Appl. Phys. Lett. 92, 153101 (2008); http://dx.doi.org/10.1063/1.2907577 (3 pages)

H. Mino, Y. Kouno, K. Oto, K. Muro, R. Akimoto, and S. Takeyama
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Functionalization of 6H-SiC surfaces with organosilanes

S. J. Schoell, M. Hoeb, I. D. Sharp, W. Steins, M. Eickhoff, M. Stutzmann, and M. S. Brandt

Appl. Phys. Lett. 92, 153301 (2008); http://dx.doi.org/10.1063/1.2908871 (3 pages) | Cited 1 time

Online Publication Date: 14 April 2008

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We demonstrate the successful covalent functionalization of n-type 6H-SiC with organosilanes. In particular, wet-chemically processed self-assembled layers of octadecyltrimethoxysilane and amino-propyldiethoxymethylsilane were investigated. The structural and chemical properties of these layers were studied by contact angle measurements, atomic force microscopy, thermal desorption, and x-ray photoelectron spectroscopy. The organic layers are smooth and wetting angles up to 100° are observed. Desorption temperatures in the range of 830 K prove the covalent bonding of the organic molecules to the SiC surface. By utilizing self-assembled layers with functional end groups, attachment of complex molecules was demonstrated by immobilization of proteins on micropatterned organic layers.
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68.43.Vx Thermal desorption
68.37.Ps Atomic force microscopy (AFM)
79.60.Dp Adsorbed layers and thin films
87.14.E- Proteins
68.47.Fg Semiconductor surfaces
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
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Submicrometer polymer transistors fabricated by a mask-free photolithographic self-alignment process

Lukas Bürgi, Reto Pfeiffer, and Carsten Winnewisser

Appl. Phys. Lett. 92, 153302 (2008); http://dx.doi.org/10.1063/1.2904969 (3 pages)

Online Publication Date: 16 April 2008

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A simple method for patterning down to 400 nm gaps between two thin-film electrodes is presented. The edge of the first electrode defines the gap via a photolithographic step using off-normal through-substrate exposure. It is demonstrated that the gap width can be controlled simply by adjusting the exposure angle and/or photoresist thickness. Using this process, poly(3-hexylthiophene) field-effect transistors with a submicrometer channel and a large channel width to length ratio of up to 5000 have been fabricated. Since the process is mask free, contactless, highly parallel, and offers high resolution, it is potentially suited for the high-throughput, low-cost fabrication of electronic circuits.
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85.30.Tv Field effect devices
85.40.Hp Lithography, masks and pattern transfer
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Modulations of photoinduced magnetoconductance for polymer diodes

Tsung-Hsun Lee, Tzung-Fang Guo, J. C. A. Huang, and Ten-Chin Wen

Appl. Phys. Lett. 92, 153303 (2008); http://dx.doi.org/10.1063/1.2912011 (3 pages) | Cited 9 times

Online Publication Date: 16 April 2008

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This study elucidates the magnetoconductance (MC) response of polymer diodes, which can be modulated by photoillumination, varying the composition of the active layer and applying electrical bias. Under illumination, the short-circuit current of polymer diodes with active layers of regioregular poly(3-hexylthiophene) (P3HT) exhibits an MC response of approximately −4.0% in a magnetic field B = 1000 mT at room temperature. The MC response drastically reverses from negative to positive (−23.0% to 2.0%) as the electrical bias is increased from nearly below to above the open-circuit voltage. However, it is quenched (<0.1%) when the illumination is turned off or an electron acceptor material, [6,6]-phenyl C61-butyric acid methyl ester, is blended with P3HT as the active layer, suggesting that, in nature, the MC response is associated with the illumination and excitonic states. The annihilation of triplet excitons probably dominates the negative MC response.
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78.20.Ls Magneto-optical effects
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Planar-type spin valves based on low-molecular-weight organic materials with La0.67Sr0.33MnO3 electrodes

Tomonori Ikegami, Iwao Kawayama, Masayoshi Tonouchi, Satoru Nakao, Yoshiro Yamashita, and Hirokazu Tada

Appl. Phys. Lett. 92, 153304 (2008); http://dx.doi.org/10.1063/1.2905288 (3 pages) | Cited 13 times

Online Publication Date: 17 April 2008

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The spin injection and transport properties of low-molecular-weight organic semiconductors such as pentacene and bis(l,2,5-thiadiazolo)-p-quinobis(l,3-dithiole) (BTQBT) were investigated utilizing planar-type spin-valve devices with half-metallic La0.67Sr0.33MnO3 electrodes. The devices showed clear spin-valve characteristics with a magnetoresistance (MR) ratio of up to 29% at 5 K. The MR ratio was found to depend on the gap spacing of the electrodes, the applied bias voltage, temperature, and the crystallinity of the films. It was also affected by gas adsorption onto the films, indicating that the spins were scattered by carriers and/or radical ions in the films generated through charge transfer from gas molecules.
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85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.Lx Magnetic oxides
73.61.Ph Polymers; organic compounds
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
72.25.Mk Spin transport through interfaces
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Impact of semiconductor/contact metal thickness ratio on organic thin-film transistor performance

S. Gowrisanker, Y. Ai, M. A. Quevedo-Lopez, H. Jia, H. N. Alshareef, E. Vogel, and B. Gnade

Appl. Phys. Lett. 92, 153305 (2008); http://dx.doi.org/10.1063/1.2904968 (3 pages) | Cited 8 times

Online Publication Date: 17 April 2008

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Pentacene-based organic thin-film transistors have been fabricated using photolithography and the bottom contact structure using parylene as the gate dielectric. Device performance was optimized by varying the thickness ratio of pentacene to Au contacts. Contact resistance dependence on the pentacene/Au thickness ratio (r = tpen/tAu) was extracted using the transfer-length method [ D. K. Schroder, Semiconductor Material and Device Characterization (Wiley, New York, 2006), pp. 184–199; Necliudov et al., Solid-State Electron. 47, 259 (2003) ]. In this paper, we show that the effect of parasitic resistance and pentacene film morphology on device performance can be decoupled. Film morphology and microstructure are reported as a function of pentacene thickness and the correlation with transistor field-effect mobility is discussed.
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85.30.Tv Field effect devices
73.40.Cg Contact resistance, contact potential
68.55.ag Semiconductors
68.55.jd Thickness
73.40.Ns Metal-nonmetal contacts
85.30.De Semiconductor-device characterization, design, and modeling
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Photoemission measurement of extremely insulating materials: Capacitive photocurrent detection in photoelectron yield spectroscopy

Yasuo Nakayama, Shinichi Machida, Daisuke Tsunami, Yasuo Kimura, Michio Niwano, Yutaka Noguchi, and Hisao Ishii

Appl. Phys. Lett. 92, 153306 (2008); http://dx.doi.org/10.1063/1.2908888 (3 pages) | Cited 2 times

Online Publication Date: 17 April 2008

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The electronic structures of various materials have been investigated using photoemission measurements. The sample charge-up problem, however, limits the application of these measurements to insulating materials. In this study, we propose a capacitive photocurrent detection method that permits photoelectron yield spectroscopy measurements of extremely insulating materials in both vacuum and ambient pressure conditions. The mechanism of detection is discussed, and the application to gold and rubrene films on mica substrates is demonstrated.
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79.60.Bm Clean metal, semiconductor, and insulator surfaces
71.20.-b Electron density of states and band structure of crystalline solids
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Bipolar space charge formation and switching effect in thin polymer films

V. R. Nikitenko, A. R. Tameev, A. V. Vannikov, A. N. Lachinov, and H. Bässler

Appl. Phys. Lett. 92, 153307 (2008); http://dx.doi.org/10.1063/1.2912374 (3 pages)

Online Publication Date: 17 April 2008

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A theoretical model of bipolar charge formation in a polymer film is proposed. High density of metastable charge pairs within a thin trapping layer results from their delayed recombination. The conditions for the highest density of charge pairs are discussed. The formation of conductive channels by charge pairs starting from the trapping layer together with the high mobility of “Coulomb holes” in the channels can switch the structure to the highly conductive state.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.61.Ph Polymers; organic compounds
73.50.Dn Low-field transport and mobility; piezoresistance
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
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Transport properties of organic field effect transistors modified by quantum dots

Masaya Nishioka, Yu Chen, and A. M. Goldman

Appl. Phys. Lett. 92, 153308 (2008); http://dx.doi.org/10.1063/1.2912345 (3 pages)

Online Publication Date: 17 April 2008

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We have investigated the transport properties of organic field effect transistors (OFETs) with quantum dot layers between the organic semiconductor and the dielectric. An insulating dot layer changes the properties of OFETs, suggesting that OFET mobility will always be reduced by additional physical disorder. However, a metal dot layer causes a much larger reduction of the mobility and an increase of the activation energy. This phenomenon may be explained by more charge being induced on metal dots causing the carriers to be more localized.
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85.30.Tv Field effect devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
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Temperature dependent Schottky barrier height and Fermi level pinning on Au/HBC/GaAs diodes

Soner Özcan, Jürgen Smoliner, Thomas Dienel, and Torsten Fritz

Appl. Phys. Lett. 92, 153309 (2008); http://dx.doi.org/10.1063/1.2912062 (3 pages) | Cited 1 time

Online Publication Date: 17 April 2008

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Au/hexa-peri-hexabenzocoronene [C42H18/(HBC)]/GaAs heterostructures were investigated by ballistic electron emission microscopy. At room temperature, the Schottky barrier height at the Au/HBC interface was measured to be 1.3 eV, while the Fermi level at the HBC-GaAs interface is pinned 1.2 eV below the GaAs conduction band. Decreasing the temperature down to T = 10 K, the Au/HBC Schottky barrier height increases to 1.55 eV and the Fermi level pinning at the HBC-GaAs interface reaches a value of 1.4 eV, close to the valence band of GaAs. These high values make HBC a promising interfacial layer in order to increase, for example, the open circuit voltage of GaAs Schottky barrier solar cells.
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85.30.Kk Junction diodes
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Microring resonator based modulator made by direct photodefinition of an electro-optic polymer

M. Balakrishnan, M. Faccini, M. B. J. Diemeer, E. J. Klein, G. Sengo, A. Driessen, W. Verboom, and D. N. Reinhoudt

Appl. Phys. Lett. 92, 153310 (2008); http://dx.doi.org/10.1063/1.2908914 (3 pages) | Cited 1 time

Online Publication Date: 18 April 2008

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A laterally coupled microring resonator was fabricated by direct photodefinition of negative photoresist SU8, containing tricyanovinylidenediphenylaminobenzene chromophore, by exploiting the low ultraviolet absorption window of this chromophore. The ring resonator was first photodefined by slight cross-linking. Thereafter, poling (to align the chromophores) and further cross-linking (to increase the glass transition temperature) were simultaneously carried out. The material showed excellent photostability and the electro-optic modulation with an r33 of 11 pm/V was demonstrated at 10 MHz.
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42.79.Hp Optical processors, correlators, and modulators
42.82.Cr Fabrication techniques; lithography, pattern transfer
42.82.Gw Other integrated-optical elements and systems
42.70.Jk Polymers and organics
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Highly efficient near-infrared hybrid organic-inorganic nanocrystal electroluminescence device

K. N. Bourdakos, D. M. N. M. Dissanayake, T. Lutz, S. R. P. Silva, and R. J. Curry

Appl. Phys. Lett. 92, 153311 (2008); http://dx.doi.org/10.1063/1.2909589 (3 pages) | Cited 1 time

Online Publication Date: 18 April 2008

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We report the use of PbS nanocrystals within a hybrid device that emits 1.2 μm electroluminescence with an external quantum efficiency of 1.15% corresponding to an internal quantum efficiency of ∼ 5%–12% thus demonstrating a viable, low-cost, highly efficient near infrared organic electroluminescent device. Direct generation of the excited state on the nanocrystal result in eliminating competing processes that have previously led to the low reported efficiencies in near-infrared light emitting devices. Furthermore, the emission wavelength can be tuned to cover a wide range of wavelengths including the 1.3–1.5 μm region without significant change of the efficiency.
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85.60.Jb Light-emitting devices
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Single zinc-doped indium oxide nanowire as driving transistor for organic light-emitting diode

Wenfeng Zhang, Jiansheng Jie, Zhubing He, Silu Tao, Xia Fan, Yechun Zhou, Guodong Yuan, Linbao Luo, Wenjun Zhang, Chun-Sing Lee, and Shuit-Tong Lee

Appl. Phys. Lett. 92, 153312 (2008); http://dx.doi.org/10.1063/1.2909716 (3 pages) | Cited 6 times

Online Publication Date: 18 April 2008

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Zn-doped In2O3 nanowires (NWs) were prepared by simple chemical vapor deposition and were systematically characterized. Field-effect transistors (FETs) constructed from the Zn-doped In2O3 nanowires exhibit excellent performance characteristics such as high mobility, “high-on-state” current of 105 A and large on/off current ratio of 107. Single-NW-FETs can successfully drive an organic light-emitting diode, revealing the application potential of Zn-doped In2O3 NW-FETs in high-performance displays.
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85.30.Tv Field effect devices
85.60.Jb Light-emitting devices
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Molecular-scale structural distortion near vacancies in pentacene

S. Seo, L. C. Grabow, M. Mavrikakis, R. J. Hamers, N. J. Thompson, and P. G. Evans

Appl. Phys. Lett. 92, 153313 (2008); http://dx.doi.org/10.1063/1.2913687 (3 pages) | Cited 1 time

Online Publication Date: 18 April 2008

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Molecular vacancies form in both of the crystallographic basis sites of thin pentacene crystals. Features in scanning tunneling microscopy images of these crystals correspond to the exposed terminal atoms of molecules. The (001) and (00math) surfaces of pentacene are distinguishable, which allows for the identification of the absolute orientation of crystals and for the unambiguous assignment of the position of molecules relative to each vacancy. For vacancies in each molecular basis site of the pentacene (001) surface, the image feature associated with one molecular nearest neighbor is displaced by significantly more than other molecules.
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61.72.jd Vacancies
68.35.bg Semiconductors
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