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12 Dec 2005

Volume 87, Issue 24, Articles (24xxxx)

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Appl. Phys. Lett. 87, 243101 (2005); http://dx.doi.org/10.1063/1.2147713 (3 pages)

Y.-S. Choi, K. Hennessy, R. Sharma, E. Haberer, Y. Gao, S. P. DenBaars, S. Nakamura, E. L. Hu, and C. Meier
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Electrorheological-fluid-based microvalves

Xize Niu, Weijia Wen, and Yi-Kuen Lee

Appl. Phys. Lett. 87, 243501 (2005); http://dx.doi.org/10.1063/1.2140070 (3 pages) | Cited 17 times

Online Publication Date: 5 December 2005

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We present the successful design and fabrication of push-and-pull microvalves that use a giant electrorheological (GER) fluid. Our multilayer microvalves, including the GER fluid control channel, the electrode, the flow channel, and the flexible membrane, are fabricated with polydimethylsioxane-based materials by soft lithography techniques. The GER effect is able to provide high-pressure changes in GER control channel so as to fully close and open an associated flow channel. The fast response time of the GER fluid and the push-and-pull valve design adopted assure fast switching time of the valve less than 10 ms and sound reliability. This GER-fluid-based microvalve has other advantages of easy fabrication and biocompatibility and is suitable for most microfluidic applications.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
83.80.Gv Electro- and magnetorheological fluids

Simulation of hybrid ZnO/AlGaN single-heterostructure light-emitting diode

Kirill A. Bulashevich, Igor Yu. Evstratov, Vladislav N. Nabokov, and Sergey Yu. Karpov

Appl. Phys. Lett. 87, 243502 (2005); http://dx.doi.org/10.1063/1.2140873 (3 pages) | Cited 4 times

Online Publication Date: 5 December 2005

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Using simulations, we have examined specific features of a hybrid n-ZnO/p-AlGaN light-emitting diode (LED) operation, originated from a type-II band alignment and a negative polarization charge at the ZnO/AlGaN interface. These factors are found to improve the carrier confinement near the interface and to affect significantly the light emission spectra and internal quantum efficiency of the LED. The theoretical predictions are compared with available observations.
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85.60.Jb Light-emitting devices
85.30.De Semiconductor-device characterization, design, and modeling

Highly polarized polymer light-emitting diodes utilizing friction-transferred poly(9,9-dioctylfluorene) thin films

Masahiro Misaki, Yasukiyo Ueda, Shuichi Nagamatsu, Masayuki Chikamatsu, Yuji Yoshida, Nobutaka Tanigaki, and Kiyoshi Yase

Appl. Phys. Lett. 87, 243503 (2005); http://dx.doi.org/10.1063/1.2142082 (3 pages) | Cited 29 times

Online Publication Date: 6 December 2005

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Polarized polymer light-emitting diodes (PLEDs) have been constructed utilizing friction-transferred poly(9,9-dioctylfluorene) (PFO) thin films. The friction transfer technique allows oriented PFO to be deposited directly onto an indium tin oxide anode without an alignment layer such as polyimide. Polarized absorption and photoluminescence spectra revealed that the polymer backbones are highly aligned in the friction direction. We fabricated PLEDs consisting of friction-transferred PFO as an emissive layer, vacuum-deposited bathocuproine as an electron transport and hole-blocking layer, and a vacuum-deposited LiF/Al cathode. Highly polarized blue emission with an integrated polarization ratio of 31 and a luminance of up to 300 cd/m2 was observed from the PLEDs.
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85.60.Jb Light-emitting devices
78.66.Qn Polymers; organic compounds
42.70.Jk Polymers and organics

All-organic flexible polymer microcavity light-emitting diodes using 3M reflective multilayer polymer mirrors

Lintao Hou, Qiong Hou, Yueqi Mo, Junbiao Peng, and Yong Cao

Appl. Phys. Lett. 87, 243504 (2005); http://dx.doi.org/10.1063/1.2119416 (3 pages) | Cited 16 times

Online Publication Date: 6 December 2005

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We have developed an electrically-driven flexible polymer microcavity with green- or red-emitting polymer sandwiched between the 3M multilayer flexible polymer distributed Bragg reflector and the aluminum mirror. The nearly all-polymeric flexible planar polymer microcavity of a Fabry-Perot resonator is fabricated by employing transparent conducting polyaniline as an anode and a thin barium layer followed by a thick aluminum layer as a cathode. The all-flexible polymer microcavity is characterized by external quantum efficiency, spectral narrowing, and angle dependence of resonance peaks. Reduced angular dependence of the emission in microcavity with 3M polymer reflector is observed and the reasons for such a reduction are discussed.
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85.60.Jb Light-emitting devices
42.79.Bh Lenses, prisms and mirrors
42.82.Gw Other integrated-optical elements and systems
42.79.Wc Optical coatings

Highly stable organic light-emitting devices with a uniformly mixed hole transport layer

Chih-Hung Tsai, Chi-Hung Liao, Meng-Ting Lee, and Chin H. Chen

Appl. Phys. Lett. 87, 243505 (2005); http://dx.doi.org/10.1063/1.2137453 (3 pages) | Cited 6 times

Online Publication Date: 7 December 2005

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Highly stable organic light-emitting devices were made by using a uniformly mixed hole transport layer (UM-HTL) composed of a mixture of 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) and N,N′-bis(1-naphthyl)-N,N′-diphenyl,1,1′-biphenyl-4,4′-diamine (NPB) in a 3:7 (MADN:NPB) ratio. The lifetime of 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]-pyrano[6,7,8-ij]quinolizin-11-one doped green device with UM-HTL can be greatly improved to 2.7 times longer than that of the conventional device (NPB based HTL) without impacting on its driving voltage and emissive color significantly. This improvement in stability can be attributed to the fact that the unstable [Alq3+] species formed by electro-oxidation have been effectively suppressed.
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85.60.Jb Light-emitting devices

Electrical characterization of tunnel insulator in metal/insulator tunnel transistors fabricated by atomic force microscope

Fu-Chien Chiu, Shih-Kai Fan, Kwang-Cheng Tai, Joseph Ya-min Lee, and Ya-Chang Chou

Appl. Phys. Lett. 87, 243506 (2005); http://dx.doi.org/10.1063/1.2143127 (3 pages) | Cited 5 times

Online Publication Date: 7 December 2005

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Metal-insulator-tunnel-transistors (MITTs) that operate by varying the gate voltage to control the current flow through a tunnel insulator were fabricated. In this work, the local Ti/TiOx/Ti tunnel junctions were formed by tip-induced anodic oxidation using conducting-tip atomic force microscope. Experimental results show that the dominant conduction mechanism of the Ti/TiOx/Ti structure at 300 K is Schottky emission at low electric field and Fowler–Nordheim tunneling at high electric field, respectively. The Ti/TiOx barrier height and the electron effective mass in TiOx are evaluated using both the intercept of Schottky plot and the slope of Fowler–Nordheim plot. The electron effective mass in TiOx and the extracted Ti/TiOx barrier height were determined to be 0.48 m0 and 95 meV, respectively. The Ids-Vds characteristics show that the MITTs with 58.5 nm channel length can operate with a current on/off ratio of about 107.
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85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
73.30.+y Surface double layers, Schottky barriers, and work functions
81.65.Mq Oxidation

Saturated deep blue organic electrophosphorescence using a fluorine-free emitter

R. J. Holmes, S. R. Forrest, T. Sajoto, A. Tamayo, P. I. Djurovich, M. E. Thompson, J. Brooks, Y.-J. Tung, B. W. D’Andrade, M. S. Weaver, R. C. Kwong, and J. J. Brown

Appl. Phys. Lett. 87, 243507 (2005); http://dx.doi.org/10.1063/1.2143128 (3 pages) | Cited 76 times

Online Publication Date: 7 December 2005

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We demonstrate saturated, deep blue organic electrophosphorescence using the facial- and meridianal- isomers of the fluorine-free emitter tris(phenyl-methyl-benzimidazolyl)iridium(III)(f-Ir(pmb)3 and m-Ir(pmb)3, respectively) doped into the wide energy gap host, p-bis(triphenylsilyly)benzene (UGH2). The highest energy electrophosphorescent transition occurs at a wavelength of λ = 389 nm for the fac- isomer and λ = 395 nm for the mer- isomer. The emission chromaticity is characterized by Commission Internationale de l’Eclairage coordinates of (x = 0.17,y = 0.06) for both isomers. Peak quantum and power efficiencies of (2.6±0.3)% and (0.5±0.1)lm/W and (5.8±0.6)% and (1.7±0.2)lm/W are obtained using f-Ir(pmb)3 andm-Ir(pmb)3 respectively. This work represents a departure from previously explored, fluorinated blue phosphors, and demonstrates an efficient deep blue/near ultraviolet electrophosphorescent device.
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78.60.Fi Electroluminescence

Roll-type thermoelectric devices with InN thin films

Ryohei Izaki, Nakaba Kaiwa, Masayuki Hoshino, Tadashi Yaginuma, Shigeo Yamaguchi, and Atsushi Yamamoto

Appl. Phys. Lett. 87, 243508 (2005); http://dx.doi.org/10.1063/1.2143110 (3 pages) | Cited 11 times

Online Publication Date: 7 December 2005

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In terms of the approach to the integration of thermoelectric devices, we proposed a roll-type thermoelectric device. We studied the load characteristics of the thermoelectric microdevices using InN thin films prepared by reactive radio-frequency sputtering. It consisted 60-pair or 120-pair InN-chromel films, which were deposited on polyimide film. For the 120-pair device, the maximum open output voltage and the maximum output power were 210 mV and 65 nW, respectively, at temperature difference of 168 K.
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85.80.Fi Thermoelectric devices
73.50.Lw Thermoelectric effects
72.20.Pa Thermoelectric and thermomagnetic effects
81.15.Cd Deposition by sputtering

Low-voltage polymer thin-film transistors with a self-assembled monolayer as the gate dielectric

Yeong Don Park, Do Hwan Kim, Yunseok Jang, Minkyu Hwang, Jung Ah Lim, and Kilwon Cho

Appl. Phys. Lett. 87, 243509 (2005); http://dx.doi.org/10.1063/1.2143113 (3 pages) | Cited 38 times

Online Publication Date: 7 December 2005

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By a simple process, we manufactured polymer thin-film transistors (PTFTs) using a 2.6 nm thick self-assembled monolayer (SAM) of alkyl chains as the gate dielectric to reduce the operating voltage of the device. These manufactured PTFTs operate with supply voltages of less than 2 V. A densely packed SAM of docosyltrichlorosilanes (DCTS) was a very efficient insulating barrier due to the very limited penetration of polymer transistor molecules into the SAM insulator. The present results show that a DCTS monolayer is suitable for use as a gate dielectric. These results enhance the prospects of using polymer TFTs with a SAM gate dielectric in low-power applications such as identification tags.
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85.30.Tv Field effect devices
73.61.Ph Polymers; organic compounds

Vanadium-doped indium tin oxide as hole-injection layer in organic light-emitting devices

T.-H. Chen, Y. Liou, T. J. Wu, and J. Y. Chen

Appl. Phys. Lett. 87, 243510 (2005); http://dx.doi.org/10.1063/1.2137892 (3 pages) | Cited 4 times

Online Publication Date: 8 December 2005

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Organic light-emitting devices were fabricated by using vanadium-doped indium tin oxide (ITO) as the hole-injection layers between the hole transport layer, N,N′-dia(1-napthl)-N,N′-diphenyl benzidine and the ITO anode. The vanadium-doped ITO layer was 15-nm thick with three different vanadium concentrations (6, 10.5, and 12.5 mol %). Three different resistivities (10, 500, and 10 000 Ω cm) and work functions (5, 5.2, and 5.4 eV) were obtained. The device with 6 mol % V-doped ITO layer possessing the least resistivity (10 Ω cm) and work function (5 eV) has the lowest turn-on voltage (below 3 V), the lowest operating voltage (below 7 V), the highest luminance (1000 cd/m2 below 7 V), and the highest power efficiency (>5 lm/W at 10 mA/cm2) among all. Such performance was attributed to the balance between the carrier concentration and the energy barrier for the hole injection.
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85.60.Jb Light-emitting devices

Enhancement of double-layer capacitance behavior and its electrical conductivity in layered poly (3, 4-ethylenedioxythiophene)-based nanocomposites

A. Vadivel Murugan, Annamraju Kasi Viswanath, Guy Campet, Chinnakonda S. Gopinath, and K. Vijayamohanan

Appl. Phys. Lett. 87, 243511 (2005); http://dx.doi.org/10.1063/1.2140468 (3 pages) | Cited 20 times

Online Publication Date: 8 December 2005

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In this letter, we report on the enhanced double-layer capacitance of a layered poly (3, 4-ethylene dioxythiophene) PEDOT-MoO3 nanocomposite, which has been synthesized by a novel microwave irradiation method. The x-ray photoelectron spectroscopy analysis shows the changes in electron density and the shift in binding energy suggesting charge transfer from sulfur atoms upon PEDOT intercalation between MoO3 layers. The room-temperature conductivity for the PEDOT-MoO3 composite is found to be 1.82×10−1 S cm−1, which is four orders of magnitude higher than that of the pristine oxide (3.78×10−5 S cm−1). The enhanced double-layer capacitance of the PEDOT-MoO3 nanocomposite ( ∼ 300 F g−1) compared to that ( ∼ 40 mF g−1) of pristine MoO3 is attributed to higher electronic conductivity, enhanced bidimensionality, and increase in surface area of the nanocomposite.
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73.63.Bd Nanocrystalline materials
79.60.Fr Polymers; organic compounds
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions

Electron traps and hysteresis in pentacene-based organic thin-film transistors

Gong Gu, Michael G. Kane, James E. Doty, and Arthur H. Firester

Appl. Phys. Lett. 87, 243512 (2005); http://dx.doi.org/10.1063/1.2146059 (3 pages) | Cited 115 times

Online Publication Date: 8 December 2005

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In the absence of charge storage or slow polarization in the gate dielectric, the hysteresis in the current-voltage (I−V) characteristics of pentacene-based organic thin-film transistors (OTFTs) is dominated by trapped electrons in the semiconductor. The immobile previously stored negative charge requires extra holes to balance it, resulting in the early establishment of the channel and extra drain current. Inferred from I−V characteristics, this simple electrostatic model qualitatively explains memory effects in pentacene-based OTFTs, and was verified by a time domain measurement.
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85.30.Tv Field effect devices

Enhanced higher-harmonic imaging in tapping-mode atomic force microscopy

M. Balantekin and A. Atalar

Appl. Phys. Lett. 87, 243513 (2005); http://dx.doi.org/10.1063/1.2147708 (3 pages) | Cited 11 times

Online Publication Date: 8 December 2005

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Higher-harmonics generation in a tapping-mode atomic force microscope is a consequence of the nonlinear tip-sample interaction force. The higher harmonics contain important information about the materials’ nanomechanical properties. These harmonics can be significantly enhanced by driving the cantilever close to a submultiple of its resonant frequency. We present the results of enhanced higher-harmonic imaging experiments on several samples. The results indicate that enhanced higher harmonics can be utilized effectively for both material characterization and surface roughness analysis with a high signal-to-noise ratio.
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07.79.Lh Atomic force microscopes
43.60.-c

Plasma oscillations of two-dimensional electron stripe

Alexander Dmitriev and Michael S. Shur

Appl. Phys. Lett. 87, 243514 (2005); http://dx.doi.org/10.1063/1.2138677 (3 pages) | Cited 3 times

Online Publication Date: 9 December 2005

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We show that the gated two-dimensional (2D) electron gas stripe supports a high frequency plasma mode corresponding to its displacement as a whole with respect to the gate (in addition to conventional plasma waves). Similar modes should exist in multiple (parallel) 2D electron layer stripes.
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72.30.+q High-frequency effects; plasma effects
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
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