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22 Nov 2004

Volume 85, Issue 21, pp. 4831-5106

Issue Cover Spotlight Figure

Appl. Phys. Lett. 85, 4845 (2004); http://dx.doi.org/10.1063/1.1823019 (3 pages)

Wounjhang Park and Jeong-Bong Lee
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Delay effect of switch-on in a supertwisted nematic cell

Fuzi Yang, Youmei Dong, L. Z. Ruan, and J. R. Sambles

Appl. Phys. Lett. 85, 5070 (2004); http://dx.doi.org/10.1063/1.1826240 (3 pages)

Online Publication Date: 23 November 2004

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By using a convergent beam system and the fully leaky guided mode technique the switch-on dynamics of an 180° supertwisted nematic have been studied. Using the Ericksen–Leslie theory and analyzing the guided mode data taken from the cell, the director structure in the cell at different times during switch-on is obtained. For three different applied voltages it is found that the switch-on time is strongly dependent on the applied field—the higher voltage corresponds to faster switching, with no evidence of backflow. A delay at the beginning of the switch-on process has been found and explored for different applied fields. This leads to a suggestion for increasing the switch-on speed of such devices by 25%.
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42.79.Kr Display devices, liquid-crystal devices

Semiconducting-polymer-based position-sensitive detectors

D. Kabra, Th. B. Singh, and K. S. Narayan

Appl. Phys. Lett. 85, 5073 (2004); http://dx.doi.org/10.1063/1.1823597 (3 pages) | Cited 21 times

Online Publication Date: 23 November 2004

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We demonstrate the utility of the organic semiconducting polymers as active media for light-sensitive position-sensitive detectors (PSDs). The characteristics of these PSDs include reasonable linearity and photoresponsivity over a large range of distance, photovoltaic mode of operation, and other advantages such as nonrigid substrates and absence of any other transporting-conducting coating layer. These devices utilize the formation of Al-polymer Schottky-type photoactive interface as a common backcontact. We demonstrate results for poly(3-hexylthiophene)-based PSDs of different interelectrode spacings where the photovoltaic signals as a function of incident-beam position are linear over the entire range.
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06.30.Bp Spatial dimensions (e.g., position, lengths, volume, angles, and displacements)
73.50.Pz Photoconduction and photovoltaic effects
85.60.Gz Photodetectors (including infrared and CCD detectors)

Valence-band tunneling induced low frequency noise in ultrathin oxide (15 math) n-type metal-oxide-semiconductor field effect transistors

J. W. Wu, J. W. You, H. C. Ma, C. C. Cheng, C. S. Chang, G. W. Huang, and T. Wang

Appl. Phys. Lett. 85, 5076 (2004); http://dx.doi.org/10.1063/1.1827930 (2 pages) | Cited 2 times

Online Publication Date: 23 November 2004

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Low frequency flicker noise in n-type metal-oxide-semiconductor field effect transistors (n-MOSFETs) with 15 math gate oxide is investigated. A noise generation mechanism resulting from valence band tunneling is proposed. In strong inversion condition, valence-band electron tunneling takes place and results in the splitting of electron and hole quasi-Fermi levels in the channel. The excess low frequency noise is attributed to electron and hole recombination at interface traps between the two quasi-Fermi levels. Random telegraph signal in a small area device is characterized to support our model.
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85.30.Tv Field effect devices
73.50.Td Noise processes and phenomena
73.40.Gk Tunneling
73.20.At Surface states, band structure, electron density of states
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Effects of polarized organosilane self-assembled monolayers on organic single-crystal field-effect transistors

J. Takeya, T. Nishikawa, T. Takenobu, S. Kobayashi, Y. Iwasa, T. Mitani, C. Goldmann, C. Krellner, and B. Batlogg

Appl. Phys. Lett. 85, 5078 (2004); http://dx.doi.org/10.1063/1.1826239 (3 pages) | Cited 96 times

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The surface conductivity is measured by a four-probe technique for pentacene and rubrene single crystals laminated on polarized and nearly unpolarized molecular monolayers with application of perpendicular electric fields. The polarization of the self-assembled monolayers (SAMs) shifts the threshold gate voltage, while maintaining a very low subthreshold swing of the single-crystal devices (0.11 V∕decade). The results, excluding influences of parasitic contacts and grain boundaries, demonstrate SAM-induced nanoscale charge injection up to ∼1012 cm−2 at the surface of the organic single crystals.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.25.+i Surface conductivity and carrier phenomena
85.30.Tv Field effect devices
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
85.65.+h Molecular electronic devices
72.25.Hg Electrical injection of spin polarized carriers
73.61.Ph Polymers; organic compounds
73.50.Dn Low-field transport and mobility; piezoresistance

Infrared photocurrent spectral response from plastic solar cell with low-band-gap polyfluorene and fullerene derivative

Xiangjun Wang, Erik Perzon, Juan Luis Delgado, Pilar de la Cruz, Fengling Zhang, Fernando Langa, Mats Andersson, and Olle Inganäs

Appl. Phys. Lett. 85, 5081 (2004); http://dx.doi.org/10.1063/1.1825070 (3 pages) | Cited 101 times

Online Publication Date: 23 November 2004

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Plastic solar cells were fabricated using a low-band-gap alternating copolymer of fluorene and a donor–acceptor–donor moiety (APFO-Green1), blended with [6,6]-phenyl-C61-butyric acid methylester or 3′-(3,5-Bis-trifluoromethylphenyl)-1′-(4-nitrophenyl)pyrazolino[60]fullerene as electron acceptors. The polymer shows optical absorption in two wavelength ranges from 300<λ<500 nm and 650<λ<1000 nm. Devices based on APFO-Green1 blended with the later fullerene exhibit an outstanding photovoltaic behavior at the infrared range, where the external quantum efficiency is as high as 8.4% at 840 nm and 7% at 900 nm, while the onset of photogeneration is found at 1 μm. A photocurrent density of 1.76 mA∕cm2, open-circuit voltage of 0.54 V, and power conversion efficiency of 0.3% are achieved under the illumination of AM1.5 (1000 W∕m2) from a solar simulator.
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84.60.Jt Photoelectric conversion
78.30.Na Fullerenes and related materials
72.40.+w Photoconduction and photovoltaic effects
78.30.Jw Organic compounds, polymers
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
71.55.Ht Other nonmetals

Unique architecture and concept for high-performance organic transistors

Liping Ma and Yang Yang

Appl. Phys. Lett. 85, 5084 (2004); http://dx.doi.org/10.1063/1.1821629 (3 pages) | Cited 51 times

Online Publication Date: 23 November 2004

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We report an organic transistor with a vertically stack structure, which consists of a layer-by-layer active cell (drain/organics/source) on top of a capacitor cell (source/dielectrics/gate); the middle source electrode is shared by the capacitor cell and active cell. Three unique characteristics of this transistor, (a) its very thin and rough middle source electrode; (b) its capacitor cell with high charge-storage capability, allow the active cell to be influenced when the gate is biased; and (c) the large cross-section area and small distance between the source and the drain allow current flowing between the source and drain electrodes. Devices have been fabricated by thermal evaporation with the source-drain current well modulated by the gate potential. We have achieved organic transistors with low working voltage (less than 5 V) and high current output (up to 10 mA or 4 A∕cm2) and an ON/OFF ratio of 4×106. A model is proposed for the device operation mechanism. The demonstrated device with its enhanced operating characteristics may open directions for organic transistors and their applications.
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85.30.Tv Field effect devices
84.32.Tt Capacitors
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy

Gate-alloy-related kink effect for metamorphic high-electron-mobility transistors

Y. J. Chen, W. C. Hsu, C. S. Lee, T. B. Wang, C. H. Tseng, J. C. Huang, D. H. Huang, and C. L. Wu

Appl. Phys. Lett. 85, 5087 (2004); http://dx.doi.org/10.1063/1.1823600 (3 pages) | Cited 12 times

Online Publication Date: 23 November 2004

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Gate-metal-related kink effects in InAlAs∕InGaAs∕GaAs metamorphic high-electron-mobility transistors have been investigated. Improvements on the kink effect have been observed by using the higher Schottky barrier height gate alloys, including Ti∕Au, Ni∕Au, and Pt∕Au, as compared to the use of the conventional Au gate metal. In comparison with gate alloy combinations, the devices with Ti∕Au alloy exhibit superior noise characteristics, whereas those with Ni∕Au alloy demonstrate the highest power characteristics. With the gate dimensions of 1.2×200 μm2, the device minimum noise figure, NFmin, is 1.17 dB at 2.4 GHz by using Ti∕Au and the output power is 13.14 dBm at 2.4 GHz by using Ni∕Au. Significant rf characteristics have also been improved upon that with Au gate.
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85.30.Tv Field effect devices
73.40.Ns Metal-nonmetal contacts
73.30.+y Surface double layers, Schottky barriers, and work functions
72.20.Ht High-field and nonlinear effects
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