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5 Dec 2005

Volume 87, Issue 23, Articles (23xxxx)

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Appl. Phys. Lett. 87, 234101 (2005); http://dx.doi.org/10.1063/1.2140082 (3 pages)

Ben McMillen, Chuck Jewart, Michael Buric, Kevin P. Chen, Yuankun Lin, and Wei Xu
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One THz harmonic oscillation of resonant tunneling diodes

N. Orihashi, S. Suzuki, and M. Asada

Appl. Phys. Lett. 87, 233501 (2005); http://dx.doi.org/10.1063/1.2139850 (3 pages) | Cited 49 times

Online Publication Date: 29 November 2005

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One THz harmonic oscillation was observed in a sub-THz oscillating GaInAs/AlAs resonant tunneling diode integrated with a slot antenna. The fundamental and third-harmonic frequencies were 342 GHz and 1.02 THz, respectively, for a 50 μm long antenna. The maximum output power of the fundamental mode was around 23 μW, and that of the third-harmonic component was 2.6% of the fundamental. Theoretical analysis with the van der Pole equation qualitatively explained the measured results.
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85.30.Kk Junction diodes
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
84.40.-x Radiowave and microwave (including millimeter wave) technology

Ambipolar organic thin-film transistors using C60/pentacene structure: Characterization of electronic structure and device property

S. J. Kang, Y. Yi, C. Y. Kim, K. Cho, J. H. Seo, M. Noh, K. Jeong, K.-H. Yoo, and C. N. Whang

Appl. Phys. Lett. 87, 233502 (2005); http://dx.doi.org/10.1063/1.2138810 (3 pages) | Cited 15 times

Online Publication Date: 29 November 2005

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We fabricated ambipolar organic thin-film transistors (OTFTs) using C60 and pentacene. The electronic structure of the interface was investigated by using ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy. The magnitude of the interface dipole and the band bendings at the interface was determined, and the complete energy level diagram for C60 on pentacene (C60/pentacene) was obtained. The lowered band offsets, due to the enhanced charge redistribution in C60/pentacene relative to pentacene on C60, are favorable for the ambipolar OTFTs. The measured field-effect mobilities were 0.017 cm2/V s and 0.007 cm2/V s for the p-channel and the n-channel operations, respectively. The threshold voltages were −2 V for the p channel and 15.6 V for the n channel, comparable to those of unipolar OTFTs using C60 or pentacene.
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85.30.Tv Field effect devices

Synthesis and ethanol sensing characteristics of single crystalline SnO2 nanorods

Y. J. Chen, X. Y. Xue, Y. G. Wang, and T. H. Wang

Appl. Phys. Lett. 87, 233503 (2005); http://dx.doi.org/10.1063/1.2140091 (3 pages) | Cited 73 times

Online Publication Date: 30 November 2005

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In a basic water-alcohol mixing solution without any other toxically organic solvents, the single crystalline SnO2 nanorods with diameters of 4–15 nm and lengths of 100–200 nm were synthesized using SnCl4 as a precursor. The sensors fabricated from the nanorods exhibited the sensitivity of 31.4 for 300 ppm of ethanol. Both the response and recovery time are short, around 1 s. Moreover, a linear dependence of the sensitivity on the ethanol concentration was observed. These behaviors were well explained by considering the high surface-to-volume ratio of the nanorods.
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81.07.Bc Nanocrystalline materials
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
81.16.-c Methods of micro- and nanofabrication and processing
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
82.80.-d Chemical analysis and related physical methods of analysis
61.46.-w Structure of nanoscale materials

Temperature-insensitive flexible polymer wavelength filter fabricated on polymer substrates

Sung-Hyun Nam, Jae-Wook Kang, and Jang-Joo Kim

Appl. Phys. Lett. 87, 233504 (2005); http://dx.doi.org/10.1063/1.2140469 (3 pages) | Cited 6 times

Online Publication Date: 30 November 2005

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Temperature-insensitive polymer wavelength filters with silicon nitride gratings were fabricated on a polymer substrate. Polymer waveguides on polymer substrates were fabricated on top of the Si wafer with a water soluble poly(vinylalcohol) (PVA) interlayer between the Si wafer and polymer waveguides. After the completion of the waveguides, the polymer waveguides are separated from the substrate by dissolving the PVA layer to form all polymer waveguides. The novel process provides a way to overcome the problem of the thermal instability of polymer substrate when the polymer waveguides are directly fabricated on polymer substrates. The polymeric wavelength filter fabricated on a polymer substrate exhibits one order of magnitude lower Bragg wavelength shift with temperature than the device on the Si-wafer in the temperature range of 25–70 °C, which is consistent with theoretical prediction.
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42.79.Gn Optical waveguides and couplers
42.70.Jk Polymers and organics
42.79.Ci Filters, zone plates, and polarizers

Current-voltage characteristics of p‐InGaN/n‐GaN vertical conducting diodes on n+‐SiC substrates

Atsushi Nishikawa, Kazuhide Kumakura, Tetsuya Akasaka, and Toshiki Makimoto

Appl. Phys. Lett. 87, 233505 (2005); http://dx.doi.org/10.1063/1.2140483 (3 pages) | Cited 5 times

Online Publication Date: 30 November 2005

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p‐InGaN/n‐GaN vertical conducting diodes have been grown on n+‐SiC substrates by low-pressure metalorganic vapor phase epitaxy and their current-voltage characteristics have been investigated. The typical forward voltage drop was 3.8–4.0 V at a forward current density of 100 A/cm2 with an on-state resistance of ∼ 1.3 mΩ cm2. The ideality factor of the samples was ∼ 2, meaning that the tunneling current through defects is small enough in these devices. The breakdown voltage (VB) increased with increasing n‐GaN layer thickness, while it increased with decreasing carrier concentration of the layer by substituting undoped GaN for n‐GaN. When the undoped GaN layer thickness was increased to 1800 nm, the highest breakdown voltage of 305 V was obtained with a low on-state resistance (Ron) of 1.51 mΩ cm2, leading to the figure-of-merit, (VB)2/Ron, of 62 MW/cm2.
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85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase

Direct resistance profile for an electrical pulse induced resistance change device

X. Chen, N. J. Wu, J. Strozier, and A. Ignatiev

Appl. Phys. Lett. 87, 233506 (2005); http://dx.doi.org/10.1063/1.2139843 (3 pages) | Cited 39 times

Online Publication Date: 2 December 2005

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We report the direct microscale resistance profile measurements on a symmetric thin-film electrical pulse induced resistance change (EPIR) device composed of a Pa0.7Ca0.3MnO3 (PCMO) active layer, using surface scanning Kelvin probe microscopy. The resistance switching is found to be an integration of the resistance changes from three parts of the device: the two interface regions within ∼ 1–3 μm of the electrical contacts, and the bulk PCMO material. Such a symmetric EPIR device showed a “table leg” resistance switching hysteresis loop under electric pulsing at room temperature. The symmetric EPIR device may be used as a resistive random access memory nonvolatile memory device with different operation modes by controlling electric pulse voltage.
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73.61.Ng Insulators
75.47.Gk Colossal magnetoresistance
68.37.-d Microscopy of surfaces, interfaces, and thin films
84.30.Sk Pulse and digital circuits

Magnetically steered liquid crystal-nanotube switch

Ingo Dierking and S. Eren San

Appl. Phys. Lett. 87, 233507 (2005); http://dx.doi.org/10.1063/1.2140069 (3 pages) | Cited 28 times

Online Publication Date: 2 December 2005

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A magnetically steered electric switch based on nematic liquid crystal-single-wall carbon nanotube dispersions is demonstrated. The device exploits the conductivity anisotropy of nanotubes in combination with the magnetic Freedericksz transition of a nematic liquid crystal. The performance is characterized with respect to the electric measuring field amplitude, frequency and sample cell gap. The dynamic behavior indicates a response time of approximately 4 s, and possible applications for magnetic field sensors are discussed.
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42.79.Kr Display devices, liquid-crystal devices
85.35.Kt Nanotube devices

Organic solar cells with sensitivity extending into the near infrared

Barry P. Rand, Jiangeng Xue, Fan Yang, and Stephen R. Forrest

Appl. Phys. Lett. 87, 233508 (2005); http://dx.doi.org/10.1063/1.2140075 (3 pages) | Cited 61 times

Online Publication Date: 2 December 2005

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We describe an organic photovoltaic cell based on a tin(II) phthalocyanine (SnPc)/C60 donor/acceptor heterojunction with sensitivity at wavelengths of λ>900 nm. We find that the low hole mobility in polycrystalline thin films of SnPc, μh = (2±1)×10−10 cm2/V s, prevents the use of thick layers, leading to low fill factors and therefore low-power conversion efficiencies. However, owing to its large absorption coefficient, a 50-Å-thick layer of SnPC yields solar cell external quantum efficiencies of up to 21% at λ = 750 nm. With the double heterostructure of indium-tin oxide/100 Å copper phthalocyanine/50 Å SnPc/540 Å C60/75 Å bathocuproine/Ag, we obtain a power conversion efficiency of (1.0±0.1)% under 1 sun standard AM1.5G solar illumination and efficiencies of (1.3±0.1)% under intense (10 suns) standard AM1.5G solar illumination.
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84.60.Jt Photoelectric conversion
85.60.-q Optoelectronic devices
73.61.Ph Polymers; organic compounds
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
78.66.Qn Polymers; organic compounds
73.50.Dn Low-field transport and mobility; piezoresistance
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Device structure for electronic transport through individual molecules using nanoelectrodes

Subhasis Ghosh, Henny Halimun, Ajit Kumar Mahapatro, Jaewon Choi, Saurabh Lodha, and David Janes

Appl. Phys. Lett. 87, 233509 (2005); http://dx.doi.org/10.1063/1.2140470 (3 pages) | Cited 27 times

Online Publication Date: 2 December 2005

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We present a simple and reliable method for making electrical contacts to small organic molecules with thiol endgroups. Nanometer-scale gaps between metallic electrodes have been fabricated by passing a large current through a lithographically-patterned Au-line with appropriate thickness. Under appropriate conditions, the passage of current breaks the Au-line, creating two opposite facing electrodes separated by a gap comparable to the length of small organic molecules. Current-voltage characteristics have been measured both before and after deposition of short organic molecules. The resistance of single 1,4-benzenedithiol and 1,4-bezenedimethanedithiol molecules were found to be 9 MΩ and 26 MΩ, respectively. The experimental results indicate strong electronic coupling to the contacts and are discussed using a relatively simple model of mesoscopic transport. The use of electrodes formed on an insulating surface by lithography and electromigration provides a stable structure suitable for integrated circuit applications.
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85.65.+h Molecular electronic devices

Quantum efficiencies of PtSi/p-Si composites in the 3–5 μm wavelength range

Clayton W. Bates and Alphonso Hendricks

Appl. Phys. Lett. 87, 233510 (2005); http://dx.doi.org/10.1063/1.2140881 (3 pages) | Cited 2 times

Online Publication Date: 2 December 2005

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A formula for the quantum efficiency of Schottky barrier metal-semiconductor composite photon detectors is derived. It includes the two factors that completely describe the quantum efficiency of a planar Schottky barrier detector. It has in addition a third factor, that can be larger or smaller than one depending on the ratio of the mean-free path of the photoexcited carriers to the dispersed metal particle sizes. Using this formula, quantum efficiencies, due to photoexcited holes in experimentally prepared PtSi/p-Si composite films, were calculated in the 3–5 μm micron wavelength range using measured absorption and particle size data. The results provide evidence for the contribution of particle size effects to the efficiencies in PtSi/p-Si composite detectors and are consistent with previous particle size effects in other metal-semiconductor composite materials.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
73.30.+y Surface double layers, Schottky barriers, and work functions
73.61.-r Electrical properties of specific thin films
78.66.Sq Composite materials
78.30.-j Infrared and Raman spectra
73.40.Ns Metal-nonmetal contacts
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Exciton quenching in poly(phenylene vinylene) polymer light-emitting diodes

D. E. Markov and P. W. M. Blom

Appl. Phys. Lett. 87, 233511 (2005); http://dx.doi.org/10.1063/1.2139622 (3 pages) | Cited 21 times

Online Publication Date: 2 December 2005

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The quenching of excitons at the metallic cathode of a polymer light-emitting diode (PLED) has been investigated by time-resolved photoluminescence. The decay of the luminescence is analyzed including both nonradiative energy transfer to the metallic cathode and exciton diffusion. Incorporation of the resulting exciton density profiles into a PLED device model consistently describes the reduction of the quantum efficiency at low bias voltage.
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85.60.Jb Light-emitting devices

Copper(I) complex employed in organic light-emitting electrochemical cells: Device and spectra shift

Yuan-Min Wang, Feng Teng, Yan-Bing Hou, Zheng Xu, Yong-Sheng Wang, and Wen-Fu Fu

Appl. Phys. Lett. 87, 233512 (2005); http://dx.doi.org/10.1063/1.2139987 (3 pages) | Cited 12 times

Online Publication Date: 2 December 2005

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Copper(I) complex employed in organic light-emitting electrochemical cells (OLECs) are reported. During the study we found that the electroluminescence (EL) spectra under forward voltage redshifted compared with the photoluminescence spectra of the film. Moreover, the EL spectra under reverse voltage also redshifted compared with the forward EL spectra. Based on the electric-field theory and the device mechanism of OLECs, we suggest that the spectra shift is ascribed to the polarization effect of molecular orbitals under high electric field in the device. The polarization and radiation models under electric field have been proposed.
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85.60.Jb Light-emitting devices
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