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2 Oct 2006

Volume 89, Issue 14, Articles (14xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 89, 142101 (2006); http://dx.doi.org/10.1063/1.2358202 (3 pages)

H. C. Lin, P. D. Ye, Y. Xuan, G. Lu, A. Facchetti, and T. J. Marks
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Performance of carbon nanotube-dispersed thin-film transistors

S. Kumar, G. B. Blanchet, M. S. Hybertsen, J. Y. Murthy, and M. A. Alam

Appl. Phys. Lett. 89, 143501 (2006); http://dx.doi.org/10.1063/1.2357852 (3 pages) | Cited 10 times

Online Publication Date: 2 October 2006

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A numerical technique that relies on modifying the organic semiconducting host with metallic carbon nanotubes (CNTs) to increase the transconductance or, equivalently, reduce effective channel length (Leff) has recently been proposed. The authors use an extensive set of experimental data to analyze the performance of these transistors using the theory of heterogeneous two-dimensional percolating networks of metal-semiconducting CNTs embedded in the organic host. Their analysis (i) reproduces experimental characteristics, (ii) shows that Leff scales as a power law of CNT-doping density (ρ), (iii) illustrates the importance of an active subpercolating network of semiconducting CNTs in an organic host, and (iv) establishes the upper limit of transistor count for an integrated circuit based on this technology as a function of ρ, on current (Ion), and circuit-failure probability (F).
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
85.35.Kt Nanotube devices

Pressure sensing by flexible, organic, field effect transistors

I. Manunza, A. Sulis, and A. Bonfiglio

Appl. Phys. Lett. 89, 143502 (2006); http://dx.doi.org/10.1063/1.2357924 (3 pages) | Cited 26 times

Online Publication Date: 2 October 2006

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A mechanical sensor based on a pentacene field effect transistor has been fabricated. The pressure dependence of the output current has been investigated by applying a mechanical stimulus by means of a pressurized air flow. Experimental results show a reversible current dependence on pressure. Data analysis suggests that variations of threshold voltage, mobility and contact resistance are responsible for current variations. Thanks to the flexibility of the substrate and the low cost of the technology, this device opens the way for flexible mechanical sensors that can be used in a variety of innovative applications such as e-textiles and robotic interfaces.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
85.30.Tv Field effect devices

Dual-color ultraviolet metal-semiconductor-metal AlGaN photodetectors

Mutlu Gökkavas, Serkan Butun, HongBo Yu, Turgut Tut, Bayram Butun, and Ekmel Ozbay

Appl. Phys. Lett. 89, 143503 (2006); http://dx.doi.org/10.1063/1.2358206 (3 pages) | Cited 1 time

Online Publication Date: 2 October 2006

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Backilluminated ultraviolet metal-semiconductor-metal photodetectors with different spectral responsivity bands were demonstrated on a single AlxGa1−xN heterostructure. This was accomplished by the incorporation of an epitaxial filter layer and the recess etching of the surface. The 11 nm full width at half maximum (FWHM) responsivity peak of the detector that was fabricated on the as-grown surface was 0.12 A/W at 310 nm with 10 V bias, whereas the 22 nm FWHM responsivity peak of the detector fabricated on the recess-etched surface was 0.1 A/W at 254 nm with 25 V bias. Both detectors exhibited excellent dark current characteristics with less than 10 fA leakage current.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
81.65.Cf Surface cleaning, etching, patterning
72.40.+w Photoconduction and photovoltaic effects

Simple and controlled single electron transistor based on doping modulation in silicon nanowires

M. Hofheinz, X. Jehl, M. Sanquer, G. Molas, M. Vinet, and S. Deleonibus

Appl. Phys. Lett. 89, 143504 (2006); http://dx.doi.org/10.1063/1.2358812 (3 pages) | Cited 29 times

Online Publication Date: 2 October 2006

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A simple and highly reproducible single electron transistor (SET) has been fabricated using gated silicon nanowires. The structure is a metal-oxide-semiconductor field-effect transistor made on silicon-on-insulator thin films. The channel of the transistor is the Coulomb island at low temperature. Two silicon nitride spacers deposited on each side of the gate create a modulation of doping along the nanowire that creates tunnel barriers. Such barriers are fixed and controlled, like in metallic SETs. The period of the Coulomb oscillations is set by the gate capacitance of the transistor and therefore controlled by lithography. The source and drain capacitances have also been characterized. This design could be used to build more complex SET devices.
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85.35.Gv Single electron devices
85.30.Tv Field effect devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
81.16.Nd Micro- and nanolithography
85.40.Ry Impurity doping, diffusion and ion implantation technology
85.40.Hp Lithography, masks and pattern transfer

Probing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of Au/GaN Schottky contacts

R. X. Wang, S. J. Xu, S. L. Shi, C. D. Beling, S. Fung, D. G. Zhao, H. Yang, and X. M. Tao

Appl. Phys. Lett. 89, 143505 (2006); http://dx.doi.org/10.1063/1.2358207 (3 pages) | Cited 7 times

Online Publication Date: 3 October 2006

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Under identical preparation conditions, Au/GaN Schottky contacts were prepared on two kinds of GaN epilayers with significantly different background electron concentrations and mobility as well as yellow emission intensities. Current-voltage (I-V) and variable-frequency capacitance-voltage (C-V) characteristics show that the Schottky contacts on the GaN epilayer with a higher background carrier concentration and strong yellow emission exhibit anomalous reverse-bias I-V and C-V characteristics. This is attributed to the presence of deep level centers. Theoretical simulation of the low-frequency C-V curves leads to a determination of the density and energy level position of the deep centers.
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81.05.Ea III-V semiconductors
73.61.Ey III-V semiconductors
71.55.Eq III-V semiconductors
73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts
78.66.Fd III-V semiconductors

Low half-wave voltage and high electro-optic effect in hybrid polymer/sol-gel waveguide modulators

Y. Enami, C. T. DeRose, C. Loychik, D. Mathine, R. A. Norwood, J. Luo, A. K.-Y. Jen, and N. Peyghambarian

Appl. Phys. Lett. 89, 143506 (2006); http://dx.doi.org/10.1063/1.2354440 (3 pages) | Cited 8 times

Online Publication Date: 3 October 2006

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The authors report on hybrid electro-optic (EO) polymer–sol-gel modulators with low half-wave voltage (Vπ) and low insertion loss. Larger EO coefficient r33 results from the high poling field achieved when EO polymer is sandwiched between sol-gel cladding layers. The reduced interelectrode distance (d) resulting from the elimination of the sol-gel core layer in the active region further reduces Vπ. Straight channel phase modulators operate with Vπ = 4.2 V at 1550 nm using a reduced d of 11.5 μm, which corresponds to an r33 of 78 pm/V, among the highest r33 reported. The authors also examine a Mach-Zehnder modulator with Vπ = 3.9 V using a conventional d of 15 μm.
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42.79.Hp Optical processors, correlators, and modulators
42.82.Et Waveguides, couplers, and arrays
42.79.Gn Optical waveguides and couplers

Polymer field-effect transistor gated via a poly(styrenesulfonic acid) thin film

Elias Said, Xavier Crispin, Lars Herlogsson, Sami Elhag, Nathaniel D. Robinson, and Magnus Berggren

Appl. Phys. Lett. 89, 143507 (2006); http://dx.doi.org/10.1063/1.2358315 (3 pages) | Cited 24 times

Online Publication Date: 3 October 2006

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A polyanionic proton conductor, named poly(styrenesulfonic acid) (PSSH), is used to gate an organic field-effect transistor (OFET) based on poly(3-hexylthiophene) (P3HT). Upon applying a gate bias, large electric double layer capacitors (EDLCs) are formed quickly at the gate-PSSH and P3HT-PSSH interfaces due to proton migration in the polyelectrolyte. This type of robust transistor, called an EDLC-OFET, displays fast response (<1 ms) and operates at low voltages (<1 V). The results presented are relevant for low-cost printed polymer electronics.
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85.30.Tv Field effect devices
84.32.Tt Capacitors

Fast ac electro-osmotic micropumps with nonplanar electrodes

John Paul Urbanski, Todd Thorsen, Jeremy A. Levitan, and Martin Z. Bazant

Appl. Phys. Lett. 89, 143508 (2006); http://dx.doi.org/10.1063/1.2358823 (3 pages) | Cited 60 times

Online Publication Date: 3 October 2006

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This letter demonstrates dramatic improvements in flow rate and frequency range over conventional planar ac electro-osmotic (ACEO) pumps by exploiting three-dimensional (3D) stepped electrodes. A 3D ACEO pump was fabricated by electroplating steps on a symmetric electrode array and tested against a state-of-the-art asymmetric planar ACEO pump in a microfluidic loop. For all frequencies (0.1–100 kHz), the 3D pump had a faster flow rate, in some cases by an order of magnitude. Their experimental results suggest that, after some optimization, mm/s velocities will be attainable with alternating battery voltages, which presents an exciting opportunity for microfluidics.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

High performance mathF-diamond thermal neutron detectors

Marco Marinelli, E. Milani, G. Prestopino, M. Scoccia, A. Tucciarone, G. Verona-Rinati, M. Angelone, M. Pillon, and D. Lattanzi

Appl. Phys. Lett. 89, 143509 (2006); http://dx.doi.org/10.1063/1.2356993 (3 pages) | Cited 19 times

Online Publication Date: 4 October 2006

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Fabrication reproducibility and high performance reliability were obtained in fissile-material-free thermal neutron detectors based on chemical vapor deposited diamond in a multilayered p-type/intrinsic/metal design. Under α particle irradiation, all the detectors (more than ten) have shown 100% charge collection efficiency and approximately 1.5% energy resolution. A mathF layer was deposited on the detector surface as converting material for thermal neutrons through the math(n,α)T nuclear reaction. Both the 2.73 MeV tritium and the 2.06 MeV α peaks are detected and clearly resolved. Stable performance and excellent linear behavior of the count rate versus the incident neutron flux were observed.
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29.40.Wk Solid-state detectors

Rectifying and magnetotransport properties of the heterojunction of Co-doped and undoped TiO2−δ with La0.69Ca0.31MnO3 single crystal

C. M. Xiong, Y. G. Zhao, Z. H. Zhao, Z. Q. Kou, Z. H. Cheng, H. F. Tian, H. X. Yang, and J. Q. Li

Appl. Phys. Lett. 89, 143510 (2006); http://dx.doi.org/10.1063/1.2357036 (3 pages) | Cited 6 times

Online Publication Date: 4 October 2006

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Electron-doped wide-band-gap dilute magnetic semiconductor Ti0.93Co0.07O2−δ and TiO2−δ were grown on a hole-doped La0.69Ca0.31MnO3 single crystal to form heterojunctions. These junctions exhibit good rectifying properties and magnetoresistance effect over a relatively wide temperature range. The results for TiO2−δ were similar to that of Ti0.93Co0.07O2−δ in all respects. A schematic band structure of the junction was proposed to account for the results. This work indicates that manganite single crystals can be used as substrates for integration with other materials, which may open an alternative avenue for the exploitation of the manganite-based devices.
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75.50.Pp Magnetic semiconductors
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.40.Ei Rectification
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
73.20.At Surface states, band structure, electron density of states

Improving the performance of hot-electron bolometers and solid state coolers with disordered alloys

L. J. Taskinen and I. J. Maasilta

Appl. Phys. Lett. 89, 143511 (2006); http://dx.doi.org/10.1063/1.2357555 (3 pages) | Cited 2 times

Online Publication Date: 4 October 2006

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Normal metal-insulator-superconductor tunnel junctions were used as thermometers at sub-Kelvin temperatures to study the electron-phonon (e-p) interaction in thin aluminum films doped with manganese, as a function of manganese concentration. The temperature dependence of the e-p interaction is consistent with an existing theory for disordered metals. The strength of the interaction decreases with increasing manganese concentration, providing a means to improve sensitivity of detectors and the base temperature of solid state coolers.
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07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
07.20.Dt Thermometers
63.50.-x Vibrational states in disordered systems
71.38.-k Polarons and electron-phonon interactions
72.10.Di Scattering by phonons, magnons, and other nonlocalized excitations
61.72.up Other materials

Plasmon enhanced electron drag and terahertz photoconductance in a grating-gated field-effect transistor with two-dimensional electron channel

G. R. Aizin, V. V. Popov, and O. V. Polischuk

Appl. Phys. Lett. 89, 143512 (2006); http://dx.doi.org/10.1063/1.2358836 (3 pages) | Cited 8 times

Online Publication Date: 4 October 2006

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The authors present a theory of dc photoresponse in two-dimensional (2D) electron channel in the grating-gated field-effect transistor irradiated by an electromagnetic wave of terahertz frequency. The authors determine photoinduced dc correction to the source-drain voltage and demonstrate that it has resonant peaks when the frequency of an external radiation coincides with 2D plasmon frequencies. The photoresponse is shown to depend on the asymmetric electron drag in the 2D channel with constant bias current. The amplitude of the resonant peaks has nonmonotonic temperature dependence with a maximum at elevated temperatures. The results explain qualitatively some important features of the photoresponse observed in recent experiments.
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85.30.Tv Field effect devices
85.60.-q Optoelectronic devices

Demonstration of microfiber knot laser

Xiaoshun Jiang, Qing Yang, Guillaume Vienne, Yuhang Li, Limin Tong, Junjie Zhang, and Lili Hu

Appl. Phys. Lett. 89, 143513 (2006); http://dx.doi.org/10.1063/1.2359439 (3 pages) | Cited 34 times

Online Publication Date: 4 October 2006

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The authors demonstrate a 1.5 μm wavelength microfiber laser formed by tightening a doped microfiber into a knot in air. The 2-mm-diameter knot, assembled using a 3.8-μm-diameter microfiber that is directly drawn from Er:Yb-doped phosphate glass, serves as both active medium and resonating cavity for lasing. Single-longitudinal-mode laser with threshold of about 5 mW and output power higher than 8 μW is obtained. Their initial results suggest a simple approach to highly compact lasers based on doped microscale optical fibers.
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42.55.Wd Fiber lasers
42.60.By Design of specific laser systems
42.55.Sa Microcavity and microdisk lasers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Impact of Si substrate orientations on electrical properties of crystalline Gd2O3 thin films for high-K application

Apurba Laha, H. J. Osten, and A. Fissel

Appl. Phys. Lett. 89, 143514 (2006); http://dx.doi.org/10.1063/1.2360209 (3 pages) | Cited 22 times

Online Publication Date: 5 October 2006

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The authors compare the properties of epitaxial Gd2O3 thin films grown on silicon substrates with three different orientations for high-K application. Pt/Gd2O3/Si(111) and Pt/Gd2O3/Si(110) metal oxide semiconductor heterostructures show promising electrical properties and hence, could be considered for future generation of complementary metal oxide semiconductor devices. Capacitance equivalent oxide thicknesses estimated from capacitance versus voltage characteristics are 0.97, 1.12, and 0.93 nm for the films grown on Si(001), Si(111), and Si(110) substrates, respectively. The films exhibit good insulating property with leakage current densities of 0.4, 0.5, and 4.5 mA/cm2, respectively, at (Vg−VFBV) = −1 V.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
68.55.-a Thin film structure and morphology
77.55.-g Dielectric thin films
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
73.61.Ng Insulators
73.61.Cw Elemental semiconductors

Towards the textile transistor: Assembly and characterization of an organic field effect transistor with a cylindrical geometry

Maurizio Maccioni, Emanuele Orgiu, Piero Cosseddu, Simone Locci, and Annalisa Bonfiglio

Appl. Phys. Lett. 89, 143515 (2006); http://dx.doi.org/10.1063/1.2357030 (3 pages) | Cited 26 times

Online Publication Date: 5 October 2006

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Cylindrical organic field effect transistors have been obtained starting from a metallic fiber used in textile processes. The metal core of the yarn, covered with a thin polyimide layer, is the gate of the structure. A top-contact device can be obtained by depositing a layer of organic semiconductor followed by the deposition of source and drain top contacts, made by metals or conductive polymers, deposited by evaporation or soft lithography. Thanks to the flexibility of the structure and the low cost of technologies, this device is a meaningful step towards innovative applications of textile electronics.
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85.30.Tv Field effect devices

White organic light-emitting device based on a compound fluorescent-phosphor-sensitized-fluorescent emission layer

Hiroshi Kanno, Yiru Sun, and Stephen R. Forrest

Appl. Phys. Lett. 89, 143516 (2006); http://dx.doi.org/10.1063/1.2357038 (3 pages) | Cited 32 times

Online Publication Date: 5 October 2006

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The authors demonstrate a combination fluorescent and phosphor-sensitized-fluorescent white organic light-emitting device (WOLED), employing the conductive host material, 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl, doped with the phosphorescent green, and the fluorescent red and blue emitters, fac-tris(2-phenylpyridinato-N,C2′) iridium (III), 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran, and 4,4′-bis (9-ethy-3-carbazolvinylene)-1-1′-biphenyl, respectively. Although two fluorescent dopants are employed along with only a single phosphor, this simple structure can, in principle, achieve 100% internal quantum efficiency. In the prototype, the phosphor-sensitized WOLED exhibits total external quantum and power efficiencies of ηext,tot = 13.1±0.5% and ηp,tot = 20.2±0.7 lm/W, respectively, at a luminance of 800 cd/m2 with Commission Internationale de L’Eclairage chromaticity coordinates of (x = 0.38, y = 0.42) and a color rendering index of 79.
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85.60.Jb Light-emitting devices

Inverted bulk-heterojunction organic photovoltaic device using a solution-derived ZnO underlayer

M. S. White, D. C. Olson, S. E. Shaheen, N. Kopidakis, and D. S. Ginley

Appl. Phys. Lett. 89, 143517 (2006); http://dx.doi.org/10.1063/1.2359579 (3 pages) | Cited 63 times

Online Publication Date: 5 October 2006

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Inverted organic photovoltaic devices based on a blend of poly(3-hexylthiophene) and a fullerene have been developed by inserting a solution-processed ZnO interlayer between the indium tin oxide (ITO) electrode and the active layer using Ag as a hole-collecting back contact. Efficient electron extraction through the ZnO and hole extraction through the Ag, with minimal loss in open-circuit potential, is observed with a certified power conversion efficiency of 2.58%. The inverted architecture removes the need for the use of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) as an ITO modifier and for the use of a low-work-function metal as the back contact in the device.
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85.60.-q Optoelectronic devices

Photon crystal waveguide-based surface plasmon resonance biosensor

Maksim Skorobogatiy and Andrei V. Kabashin

Appl. Phys. Lett. 89, 143518 (2006); http://dx.doi.org/10.1063/1.2360186 (3 pages) | Cited 32 times

Online Publication Date: 6 October 2006

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The concept of a photonic crystal waveguide-based surface plasmon resonance sensor is proposed, in which plasmons on a surface of a thin metal film are excited by a Gaussian-like leaky mode of an effectively single mode photonic crystal waveguide. The authors demonstrate that effective refractive index of a waveguide core mode can be designed to be considerably smaller than that of a core material, enabling efficient phase matching with plasmons at any wavelength of choice, while retaining highly sensitive response to changes in the refractive index of an analyte layer. This is attractive for the development of portable surface plasmon resonance biochemical sensors.
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42.79.Gn Optical waveguides and couplers
42.70.Qs Photonic bandgap materials
87.80.-y Biophysical techniques (research methods)
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.68.+m Optical properties of surfaces

Use of cross-linkable polyfluorene in the fabrication of multilayer polyfluorene-based light-emitting diodes with improved efficiency

A. Charas, H. Alves, L. Alcácer, and J. Morgado

Appl. Phys. Lett. 89, 143519 (2006); http://dx.doi.org/10.1063/1.2360243 (3 pages) | Cited 12 times

Online Publication Date: 6 October 2006

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The authors report the use of a cross-linkable polyfluorene to fabricate multilayer light-emitting diodes (LEDs), thereby avoiding the restriction to combine polymeric solutions in different solvents. In particular, we find that for LEDs fabricated with a hole-injection layer of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT), with magnesium cathodes and with poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), as emissive layer, its electroluminescence efficiency increases from 2 to 5.5 cd/A upon insertion of the cross-linked polyfluorene between PEDOT and F8BT. This efficiency increase is attributed to an improvement of charge carrier balance within the F8BT emissive layer and a reduction of exciton quenching at PEDOT interface.
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85.60.Jb Light-emitting devices
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