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13 Feb 2006

Volume 88, Issue 7, Articles (07xxxx)

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Appl. Phys. Lett. 88, 072905 (2006); http://dx.doi.org/10.1063/1.2172744 (3 pages)

Y. L. Li and L. Q. Chen
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Optical upconverter with integrated heterojunction phototransistor and light-emitting diode

H. Luo, D. Ban, H. C. Liu, Z. R. Wasilewski, and M. Buchanan

Appl. Phys. Lett. 88, 073501 (2006); http://dx.doi.org/10.1063/1.2162685 (3 pages) | Cited 6 times

Online Publication Date: 13 February 2006

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We report an optical upconversion device that converts input 1.5 μm light to output 0.87 μm light with a built-in gain mechanism. The device consists of an InGaAs/InP heterojunction phototransistor (HPT) integrated with a GaAs/AlGaAs light-emitting diode (LED) by wafer fusion process. Incoming 1.5 μm optical radiation is absorbed by the HPT, generating an amplified photocurrent. The resultant photocurrent drives the LED that emits at 0.87 μm, which could be detected by a conventional silicon charge-coupled device. Upconversion is demonstrated at room temperature with a gain of 20 from the HPT and an overall external upconversion efficiency of 0.07 W/W.
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85.60.Dw Photodiodes; phototransistors; photoresistors
85.60.Jb Light-emitting devices

Silicon-on-insulator control impact-ionization-avalanche transistor

V. Dobrovolsky, S. Pavljuk, V. Rossokhaty, and S. Cristoloveanu

Appl. Phys. Lett. 88, 073502 (2006); http://dx.doi.org/10.1063/1.2177367 (3 pages) | Cited 1 time

Online Publication Date: 14 February 2006

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In thin silicon-on-insulator structures, the gate effectively controls the longitudinal component of the electric field intensity in the pn+ junction, and thus the impact avalanche ionization of carriers. The present work proposes a device based on this operation principle: the control-impact-ionization-avalanche transistor, which achieves a transconductance of 0.14 (A/V)/mm. According to the developed theoretical model and preliminary experimental data, the device can be optimized to achieve very high transconductance and frequencies such as several (A/V)/mm at frequencies of the order of 100 MHz and lower, and about 1 (A/V)/mm in 0.1–1 THz range.
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85.30.Pq Bipolar transistors

Reversible metal-semiconductor transitions for microwave switching applications

M. Dragoman, A. Cismaru, H. Hartnagel, and R. Plana

Appl. Phys. Lett. 88, 073503 (2006); http://dx.doi.org/10.1063/1.2177369 (3 pages) | Cited 15 times

Online Publication Date: 14 February 2006

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This letter presents an original approach regarding the switching at high frequencies using reversible metal-semiconductor transitions displayed by various materials termed as Mott materials. The Mott materials experience a reversible semiconductor-metal transition when an external parameter (temperature, dc bias, hydrogenation, etc.) is varied. This transition can be used to allow or to stop the propagation of high-frequency fields, when a thin film of a Mott material is integrated with a planar high-frequency waveguide. This effect could have important applications in the area of communications, computing, and sensors.
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71.30.+h Metal-insulator transitions and other electronic transitions
72.60.+g Mixed conductivity and conductivity transitions

Organic metal electrodes for controlled p- and n-type carrier injections in organic field-effect transistors

Y. Takahashi, T. Hasegawa, Y. Abe, Y. Tokura, and G. Saito

Appl. Phys. Lett. 88, 073504 (2006); http://dx.doi.org/10.1063/1.2173226 (3 pages) | Cited 51 times

Online Publication Date: 14 February 2006

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Fine control of p-, n-, and ambipolar-type field-effect transistor (FET) operations is successfully demonstrated in prototypical single-crystal organic FETs with use of chemically tunable nature of Fermi energy in tetrathiafulvalene-tetracyanoguinodimethane-based organic metal electrodes. Carrier-type preference and rectifying nature in the organic-organic contacts are revealed in terms of the FET operations as well as of the all-organic Schottky diode characteristics.
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85.30.Hi Surface barrier, boundary, and point contact devices
85.30.Tv Field effect devices
85.30.Kk Junction diodes
73.30.+y Surface double layers, Schottky barriers, and work functions

Improved organic thin-film transistor performance using novel self-assembled monolayers

M. McDowell, I. G. Hill, J. E. McDermott, S. L. Bernasek, and J. Schwartz

Appl. Phys. Lett. 88, 073505 (2006); http://dx.doi.org/10.1063/1.2173711 (3 pages) | Cited 79 times

Online Publication Date: 14 February 2006

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Pentacene-based organic thin-film transistors have been fabricated using a phosphonate-linked anthracene self-assembled monolayer as a buffer between the silicon dioxide gate dielectric and the active pentacene channel region. Vast improvements in the subthreshold slope and threshold voltage are observed compared to control devices fabricated without the buffer. Both observations are consistent with a greatly reduced density of charge trapping states at the semiconductor-dielectric interface effected by introduction of the self-assembled monolayer.
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85.30.Tv Field effect devices

Electric-field-induced steering of conducting polymer dispersion in microchannels

K. S. Narayan and Manohar Rao

Appl. Phys. Lett. 88, 073506 (2006); http://dx.doi.org/10.1063/1.2171797 (3 pages) | Cited 2 times

Online Publication Date: 16 February 2006

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We demonstrate electric-field-induced spreading of drops, composed of conducting polymer blend in an aqueous dispersion, on a hydrophobic polymer surface in a confined microchannel geometry. The field-induced wetting characteristics results in a controlled formation of dried-narrow, strips of the conducting polymer. We discuss the formation of such conducting polymer films along with their electrical transport characteristics and potential applications in fabricating lithography-free structures.
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73.61.Ph Polymers; organic compounds
68.08.Bc Wetting
68.55.-a Thin film structure and morphology
47.60.-i Flow phenomena in quasi-one-dimensional systems

Biosensor based on magnetostrictive microcantilever

Suiqiong Li, Lisa Orona, Zhimin Li, and Z.-Y. Cheng

Appl. Phys. Lett. 88, 073507 (2006); http://dx.doi.org/10.1063/1.2174092 (3 pages) | Cited 18 times

Online Publication Date: 16 February 2006

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Magnetostrictive microcantilever (MSMC) as remote biosensor platform is reported. The mass sensitivity of the MSMCs is simulated and compared with the other microcantilevers. MSMCs with a thickness of 30–35 μm and different lengths and widths were fabricated from the magnetostrictive metal glass coated with a copper layer by sputtering. The resonance behavior of the MSMCs was experimentally determined. It is experimentally found that the MSMCs work well in either air or liquid. For MSMCs operated in air, a Q value of more than 500 was obtained. For MSMCs operated in water, the Q value reaches more than 30. The application of a MSMC as a biosensor platform is demonstrated by in situ detection of the yeast cells in water using the MSMC sensor.
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87.80.-y Biophysical techniques (research methods)
87.17.-d Cell processes
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
07.55.-w Magnetic instruments and components
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Transition metal oxides as the buffer layer for polymer photovoltaic cells

Vishal Shrotriya, Gang Li, Yan Yao, Chih-Wei Chu, and Yang Yang

Appl. Phys. Lett. 88, 073508 (2006); http://dx.doi.org/10.1063/1.2174093 (3 pages) | Cited 246 times

Online Publication Date: 16 February 2006

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Polymer-based photovoltaic cells have been fabricated by inserting a thin, transparent, transition metal oxide layer between the transparent anode (indium tin oxide) and the polymer layer. Two different transition metal oxides, namely vanadium oxide and molybdenum oxide, were used and the device performance was compared. The surface of the oxide films and the interface between the polymer and the oxide was studied with the help of atomic force microscopy. The effect of the thickness of the oxide layer on electrical characteristics of the device was also studied and optimized thickness was achieved to give high power conversion efficiency of 3.3% under simulated AM1.5G illumination of 100 mW/cm2.
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85.60.Bt Optoelectronic device characterization, design, and modeling
68.37.Ps Atomic force microscopy (AFM)

Nanoimprinting-lithography-induced self-aligned liquid crystals for novel multifunctional optical films

Chih-Ho Chiu, Hui-Lung Kuo, Pin-Cheng Chen, Chun-Hsiang Wen, Yi-Chun Liu, and Huang-Ming Philip Chen

Appl. Phys. Lett. 88, 073509 (2006); http://dx.doi.org/10.1063/1.2173222 (3 pages) | Cited 13 times

Online Publication Date: 16 February 2006

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Polymerizable nematic liquid crystal (LC) was adopted as a resist for our newly developed nanoimprinting lithography process. Liquid crystal molecules were self-aligned during imprinting process without precoated alignment layer. The large patterned area was able to achieve 4×4 cm2 at very low impriting pressure, 1.5 bar. The optical anisotropic was verified through polarized optical microscopy and the retardation was measured by polarized ultraviolet/visible spectrometer. The micrograted LC films prepared by this novel method offer the potential applications for a LC alignment layer within LC cells, as well as preparation of optical anisotropic films.
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61.30.Vx Polymer liquid crystals
81.16.Nd Micro- and nanolithography
85.40.Hp Lithography, masks and pattern transfer
42.70.Df Liquid crystals
78.40.Pg Disordered solids
61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures

GaN light-emitting diodes with Archimedean lattice photonic crystals

Aurélien David, Tetsuo Fujii, Elison Matioli, Rajat Sharma, Shuji Nakamura, Steven P. DenBaars, Claude Weisbuch, and Henri Benisty

Appl. Phys. Lett. 88, 073510 (2006); http://dx.doi.org/10.1063/1.2168673 (3 pages) | Cited 22 times

Online Publication Date: 16 February 2006

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We study GaN-based light emitting diodes incorporating an omnidirectional photonic crystal with Archimedean lattice. Photonic bands are observed over several Brillouin zones, revealing reciprocal space symmetries and evidencing the omnidirectionality of the photonic crystal. Intensities of the diffracted bands are found to agree with the Fourier transform of the crystal lattice, and confirm its Archimedean nature.
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42.70.Qs Photonic bandgap materials
85.60.Jb Light-emitting devices

Field emission characteristics of a graphite nanoneedle cathode and its application to scanning electron microscopy

Yoichiro Neo, Hidenori Mimura, and Takahiro Matsumoto

Appl. Phys. Lett. 88, 073511 (2006); http://dx.doi.org/10.1063/1.2173712 (3 pages) | Cited 5 times

Online Publication Date: 16 February 2006

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A high-brightness electron beam of more than 1011 A sr−1 m−2 was achieved from a graphite nanoneedle cathode, which was fabricated by simple hydrogen plasma etching of a graphite rod. A field emission was obtained at a high residual pressure of 10−6 Torr. The performance of this cold cathode was demonstrated by the fabrication of a scanning electron microscope, which was operated at a high residual pressure of 10−5–10−6 Torr. The brightness of this cathode offers a convenient field electron emission source that does not require a massive ultrahigh vacuum system.
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79.70.+q Field emission, ionization, evaporation, and desorption
81.07.Bc Nanocrystalline materials
52.77.Bn Etching and cleaning
81.16.-c Methods of micro- and nanofabrication and processing

Efficient hole injection in organic light-emitting diodes using C60 as a buffer layer for Al reflective anodes

Jun Yeob Lee

Appl. Phys. Lett. 88, 073512 (2006); http://dx.doi.org/10.1063/1.2174838 (3 pages) | Cited 30 times

Online Publication Date: 16 February 2006

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The hole injection of the organic light-emitting diodes with Al as a reflective anode for top-emitting devices was improved by using C60 as a thin buffer layer between Al and a hole transport layer. The driving voltage of the devices with C60 buffer layer was 5.5 V compared with 11 V for the devices without C60 buffer layer. The decrease of interfacial energy barrier by interface dipole formation between Al and C60 contributed to the low driving voltage of the devices.
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85.60.Jb Light-emitting devices

AlGaN/GaN/AlN quantum-well field-effect transistors with highly resistive AlN epilayers

Z. Y. Fan, J. Li, M. L. Nakarmi, J. Y. Lin, and H. X. Jiang

Appl. Phys. Lett. 88, 073513 (2006); http://dx.doi.org/10.1063/1.2174847 (3 pages) | Cited 9 times

Online Publication Date: 16 February 2006

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AlGaN/GaN/AlN quantum-well field-effect transistors have been demonstrated. By replacing a semi-insulating GaN epilayer with a highly resistive AlN epilayer in the device structure, parasitic conduction in the GaN epilayer, leakage current through the GaN epilayer, and the channel electrons spillover into the GaN epilayer have been completely eliminated and the drain current collapse has been reduced. The fabricated devices on sapphire substrate with 1 μm gate length show a high saturation current (>1 A/mm), and excellent gate control capability with a pinch-off voltage of −6 V. Even without passivation, the devices exhibit small drain current collapse (<10%) under 1 μs pulse gate driving.
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85.30.Tv Field effect devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Open circuit voltage of stacked bulk heterojunction organic solar cells

Kenji Kawano, Norihiro Ito, Taisuke Nishimori, and Jun Sakai

Appl. Phys. Lett. 88, 073514 (2006); http://dx.doi.org/10.1063/1.2177633 (3 pages) | Cited 45 times

Online Publication Date: 17 February 2006

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We investigate the open circuit voltage (VOC) of stacked bulk heterojunction (BHJ) organic solar cells based on conjugated polymer and fullerene derivative by spin-coating method. Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) blended with [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) is used as a photoactive layer. The structure is ITO(lower BHJ cell)PEDOT:PSS/MDMO-PPV:PCBM/ITO/(upper BHJ cell)PEDOT:PSS/MDMO-PPV:PCBM/Al. VOC of the stacked cell is 1.34 V and is about 1.6 times as large as that of single BHJ organic solar cell. VOC of the stacked cell is not proportional to the number of photoactive layers, but is the sum of the VOC values of the upper and the lower BHJ cells.
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84.60.Jt Photoelectric conversion
85.65.+h Molecular electronic devices

Photoconductivity and highly selective ultraviolet sensing features of amorphous silicon carbon nitride thin films

Chun-Wei Chen, Cheng-Chia Huang, Yun-Yue Lin, Wei-Fang Su, Li-Chyong Chen, and Kuei-Hsien Chen

Appl. Phys. Lett. 88, 073515 (2006); http://dx.doi.org/10.1063/1.2178406 (3 pages) | Cited 3 times

Online Publication Date: 17 February 2006

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Photoconductivity of amorphous silicon carbon nitride (a-SiCN) as a function of incident photon energies has been studied. A metal-semiconductor-metal photodetector device based on the a-SiCN thin film demonstrates excellent selective ultraviolet sensing features. A large photo-to-dark current ratio about 5000 and a relative quantum efficiency about ∼ 105 under illumination of the 250 nm light source and a bias voltage of 5 V were observed. A model based on the heterogeneous structure in the a-SiCN thin film which consists of π−π* bands and σ−σ* bands was introduced to account for the observed photoconductive transport properties.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
72.40.+w Photoconduction and photovoltaic effects

Plasma-free hydrogenation of ultralow-temperature polycrystalline silicon thin-film transistors with SiNx:H as interlayer dielectric

Choong-Heui Chung, Yong-Hae Kim, Jaehyun Moon, Myung-Hee Lee, Jung Wook Lim, Sun Jin Yun, Dong-Jin Park, Dae-Won Kim, and Jin Ho Lee

Appl. Phys. Lett. 88, 073516 (2006); http://dx.doi.org/10.1063/1.2178407 (3 pages) | Cited 3 times

Online Publication Date: 17 February 2006

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Plasma-free defect passivation is achieved on polycrystalline silicon thin-film transistors fabricated below 150 °C by annealing and extracting H from SiNx:H interlayer dielectric. By annealing at 250 °C without a plasma application, VT and μFE were improved from 11.5 V to 3.5 V and from 86 cm2/V s to 212 cm2/V s, respectively. Improvement in performance is attributed to defect passivation by H diffusing out from SiNx:H. Dangling bonds and strained bonds can be acceptably passivated around 170 °C, and 205 °C, respectively. The activation energy for the diffusion of H into polycrystalline silicon was estimated to be 0.87 eV.
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85.30.Tv Field effect devices
81.65.Rv Passivation
61.72.Cc Kinetics of defect formation and annealing

High-efficiency microcavity top-emitting organic light-emitting diodes using silver anode

Huajun Peng, Jiaxin Sun, Xiuling Zhu, Xiaoming Yu, Man Wong, and Hoi-Sing Kwok

Appl. Phys. Lett. 88, 073517 (2006); http://dx.doi.org/10.1063/1.2172734 (3 pages) | Cited 44 times

Online Publication Date: 17 February 2006

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Top-emitting organic light-emitting diodes (TOLEDs) employing highly reflective Ag as anode and semitransparent LiF/Al/Ag as cathode were fabricated. The hole injection efficiency of Ag anode can be significantly improved with surface modification using a CF4 plasma. With C545T-doped Alq3 emitter, the top-emitting device shows a low turn-on voltage of 2.65 V. The optimized microcavity TOLED shows a current efficiency enhancement of 65% and a total outcoupling efficiency enhancement of 35%, compared with a conventional OLED. No color variation was observed in the forward 140° forward viewing cone. Strong dependence of efficiency on Ag cathode thickness was observed, in good agreement with numerical simulations.
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85.60.Jb Light-emitting devices

1/f noise in all-epitaxial metal-semiconductor diodes

A. C. Young, J. D. Zimmerman, E. R. Brown, and A. C. Gossard

Appl. Phys. Lett. 88, 073518 (2006); http://dx.doi.org/10.1063/1.2174837 (3 pages) | Cited 8 times

Online Publication Date: 17 February 2006

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In contrast to traditional metal-semiconductor (i.e., Schottky) junctions, molecular beam epitaxy (MBE)-grown ErAs:InAlGaAs heterojunctions have recently been shown to provide highly “engineerable” electrical rectification characteristics through the tuning of the Schottky barrier height, while maintaining the very low specific capacitance. This letter reports an approximate 10× improvement in the low-frequency noise performance by using MBE-grown ErAs as the Schottky contact instead of evaporated aluminum. The low-frequency noise power spectrum of ErAs devices has been observed to have a 1/f1.0 frequency dependence. Constant-current bias-dependent measurements have shown a 1.8 power law dependence of the noise spectral density on dc current.
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85.30.Kk Junction diodes
85.30.De Semiconductor-device characterization, design, and modeling

Lifetime of organic thin-film transistors with organic passivation layers

Seung Hoon Han, Jun Hee Kim, Jin Jang, Sang Mi Cho, Myung Hwan Oh, Sun Hee Lee, and Dong Joon Choo

Appl. Phys. Lett. 88, 073519 (2006); http://dx.doi.org/10.1063/1.2174876 (3 pages) | Cited 36 times

Online Publication Date: 17 February 2006

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We fabricated a high-performance pentacene organic thin-film transistor (OTFT) on plastic and passivated it with polyvinylalcohol and photosensitive acryl layers. The pentacene was grown on the thin-film transistor area using a self-organized process. The OTFT after the passivation exhibited the field-effect mobility of 0.80 cm2/V s, threshold voltage of −9.2 V, and on/off current ratio of 108. The field-effect mobility and threshold voltage change exponentially with time in air. The lifetime, defined as the time required to decrease the on current by one-half, was found to be ∼ 11 000 h in ambient air. The absolute value of threshold voltage of the unpassivated OTFT increases with time in air, but it decreases with time in the passivated OTFT. This indicates that the origins for the degradation in the performances of the two OTFTs are different. H2O in unpassivated and O2 in passivated OTFT appear to be the major origin for the degradation.
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85.30.Tv Field effect devices
81.65.Rv Passivation
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