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14 May 2007

Volume 90, Issue 20, Articles (20xxxx)

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Appl. Phys. Lett. 90, 201101 (2007); http://dx.doi.org/10.1063/1.2739308 (3 pages)

Luca Sapienza, Angela Vasanelli, Cristiano Ciuti, Christophe Manquest, Carlo Sirtori, Raffaele Colombelli, and Ulf Gennser
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Si/SiGe n-type resonant tunneling diodes fabricated using in situ hydrogen cleaning

Z. Suet, D. J. Paul, J. Zhang, and S. G. Turner

Appl. Phys. Lett. 90, 203501 (2007); http://dx.doi.org/10.1063/1.2739089 (3 pages) | Cited 5 times

Online Publication Date: 14 May 2007

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In situ hydrogen cleaning to reduce the surface segregation of n-type dopants in SiGe epitaxy has been used to fabricate Si/SiGe resonant tunneling diodes in a joint gas source chemical vapor deposition and molecular beam epitaxial system. Diodes fabricated without the in situ clean demonstrate linear current-voltage characteristics, while a 15 min hydrogen clean produces negative differential resistance with peak-to-valley current ratios up to 2.2 and peak current densities of 5.0 A/cm2 at 30 K. Analysis of the valley current and the band structure of the devices suggest methods for increasing the operating temperature of Si/SiGe resonant tunneling diodes as required for applications.
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85.30.Kk Junction diodes
81.65.Cf Surface cleaning, etching, patterning

Highly transparent microcrystalline silicon carbide grown with hot wire chemical vapor deposition as window layers in n-i-p microcrystalline silicon solar cells

Y. Huang, A. Dasgupta, A. Gordijn, F. Finger, and R. Carius

Appl. Phys. Lett. 90, 203502 (2007); http://dx.doi.org/10.1063/1.2739335 (3 pages) | Cited 15 times

Online Publication Date: 14 May 2007

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Microcrystalline silicon carbide (μc-SiC) films were prepared using hot wire chemical vapor deposition at low substrate temperature. The μc-SiC films were employed as window layers in microcrystalline silicon (μc-Si:H) n-i-p solar cells. Quantum efficiency (QE) and short circuit current density (JSC) in these n-side illuminated n-i-p cells were significantly higher than in standard p-i-n cells. A high QE current density of 26.7 mA/cm2 was achieved in an absorber layer thickness of 2 μm. The enhanced JSC was attributed to the wide band gap of the μc-SiC layer and a sufficiently high hole drift mobility in μc-Si:H absorber layer.
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84.60.Jt Photoelectric conversion
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Resonant detection of modulated terahertz radiation in micromachined high-electron-mobility transistor

V. Ryzhii, M. Ryzhii, Y. Hu, I. Hagiwara, and M. S. Shur

Appl. Phys. Lett. 90, 203503 (2007); http://dx.doi.org/10.1063/1.2734372 (3 pages) | Cited 10 times

Online Publication Date: 14 May 2007

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The authors develop a device model for a resonant detector of modulated terahertz radiation based on a micromachined high-electron-mobility transistor with the microcantilever serving as the gate. The device model accounts for mechanical motion of the microcantilever and plasma effects of the two-dimensional electron channel. It is demonstrated that at a combined resonance when the carrier terahertz frequency and the modulation frequency coincide with the plasma resonant frequency and the mechanical resonant frequency, respectively, the amplitude of the output gate and drain ac currents and, hence, the detector resonsivity exhibit sharp and high maximum.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
07.10.Cm Micromechanical devices and systems

Multipath-switching device utilizing a GaAs-based multiterminal nanowire junction with size-controlled dual Schottky wrap gates

Seiya Kasai, Tatsuya Nakamura, and Yuta Shiratori

Appl. Phys. Lett. 90, 203504 (2007); http://dx.doi.org/10.1063/1.2739085 (3 pages) | Cited 2 times

Online Publication Date: 16 May 2007

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A multipath-switching device using a multiterminal nanowire junction with size-controlled dual gates is proposed and demonstrated experimentally. The device switches a number of output terminals according to multiple-valued input voltages for electrons entering from a root terminal. The switching function is implemented by dual gating on multiple nanowires with different threshold voltages Vth. Systematic Vth shift is made by changing gate lengths in nanometer scale. A triple-path-switching device is fabricated using AlGaAs/GaAs etched nanowires and nanometer-scale Schottky wrap gates. Its correct operation is confirmed at room temperature. Obtained results are explained by a simple analytical model.
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85.30.-z Semiconductor devices
84.32.Dd Connectors, relays, and switches

Improved efficiency of zinc phthalocyanine/C60 based photovoltaic cells via nanoscale interface modification

Z. R. Hong, B. Maennig, R. Lessmann, M. Pfeiffer, K. Leo, and P. Simon

Appl. Phys. Lett. 90, 203505 (2007); http://dx.doi.org/10.1063/1.2739364 (3 pages) | Cited 31 times

Online Publication Date: 16 May 2007

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The photovoltaic performance of zinc phthalocyanine (ZnPc)/C60 heterojunctions is improved by alternately depositing ultrathin active layers between the two bulk layers. Using a proper multilayer structure, a 60% improvement in conversion efficiency under AM1.5 illumination is obtained, in comparison with that of flat heterojunctions. Transmission electron microscope investigations reveal an effective phase separation of ZnPc and C60 and a high degree of crystalline ordering of C60 in this system. The authors propose that an interpenetrating interface is formed, which facilitates both exciton separation and charge transport, and enhances the photocurrent from ZnPc due to an extended photoactive region.
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85.60.Bt Optoelectronic device characterization, design, and modeling

Extremely small hole capture cross sections in HfO2/HfxSiyOz/p-Si structures

M. Y. A. Yousif, M. Johansson, and O. Engström

Appl. Phys. Lett. 90, 203506 (2007); http://dx.doi.org/10.1063/1.2740188 (3 pages) | Cited 1 time

Online Publication Date: 16 May 2007

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Defects in Al/HfO2/HfxSiyOz/p-Si capacitors have been characterized using thermally stimulated current at temperatures between 30 and 300 K. The hole activation energy and capture cross section were extracted from the results. The authors observed shallow traps that move with changing the discharging voltage, giving rise to activation energies in the range 0.03–0.14 eV. Postmetallization anneal passivated these traps and a deeper trap appears with a significantly lower shift with the discharging voltage. Very small apparent capture cross sections (capture cross section times tunneling probability) have been extracted (10−26–10−18 cm2). Simulations agree very well with experimental data.
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84.32.Tt Capacitors

Highly efficient white organic light-emitting diodes using two emitting materials for three primary colors (red, green, and blue)

Ji Hoon Seo, Ji Hyun Seo, Jung Hyun Park, Young Kwan Kim, Jun Ho Kim, Gun Woo Hyung, Kum Hee Lee, and Seung Soo Yoon

Appl. Phys. Lett. 90, 203507 (2007); http://dx.doi.org/10.1063/1.2740191 (3 pages) | Cited 63 times

Online Publication Date: 16 May 2007

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The authors have demonstrated highly efficient white organic light-emitting diodes (WOLEDs) by using two emissive materials as a dopant, 1,4-bis[2-(7-N-diphenyamino-2-(9,9-diethyl-9H-fluoren-2-yl)) vinyl] benzene (DAF-ph) and iridium(III) bis(5-acetyl-2-phenylpyridinato-N,C2′) acetylacetonate ((acppy)2Ir(acac)). It was found that the OLED fabricated in this study emitted a white color consisting of three primary colors (red, green, and blue). The luminance-voltage (L-V) characteristics of the WOLEDs showed the maximum luminance of 30 500 cd/m2 at 14 V and the maximum luminous efficiency of 38.0 cd/A, respectively. The CIEx,y coordinates of the WOLED also showed (x = 0.33, y = 0.40) at 10 V.
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85.60.Jb Light-emitting devices
78.66.Qn Polymers; organic compounds

Fabrication of Zr–N codoped p-type ZnO thin films by pulsed laser deposition

H. Kim, A. Cepler, M. S. Osofsky, R. C. Y. Auyeung, and A. Piqué

Appl. Phys. Lett. 90, 203508 (2007); http://dx.doi.org/10.1063/1.2739363 (3 pages) | Cited 15 times

Online Publication Date: 16 May 2007

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N-doped and Zr–N codoped p-type ZnO films were grown on sapphire substrates by pulsed laser deposition. The carrier type and conduction are very sensitive to N2O deposition pressure. p-type conduction is observed only for films grown at an intermediate pressure range (5×10−5–5×10−4 Torr). The Zr–N codoped ZnO films grown at 500 °C in 5×10−5 Torr of N2O show p-type conduction behavior with a low resistivity of 0.026 Ω cm, a carrier concentration of 5.5×1019 cm−3, and a mobility of 4.4 cm2 V−1 s−1. The p-type conduction behavior of Zr–N codoped ZnO films is also confirmed by the rectifying I-V characteristics of p-n heterojunctions (p-ZnO/n-Si).
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73.61.Ga II-VI semiconductors
81.15.Fg Pulsed laser ablation deposition
72.20.Fr Low-field transport and mobility; piezoresistance

Efficient electrofluorescent organic light-emitting diodes by sequential doping

Y. Divayana and X. W. Sun

Appl. Phys. Lett. 90, 203509 (2007); http://dx.doi.org/10.1063/1.2728753 (3 pages) | Cited 6 times

Online Publication Date: 17 May 2007

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The authors reported a highly efficient electrofluorescent organic light-emitting diode fabricated by sequential doping. An archetypal device utilizing 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl aminostyryl)-4H-pyran as dopant in a matrix of tris-(8-hydroxyquinoline) aluminum was investigated. The emission layer consists of a few repeating cells, similar to a multiple quantum well structure, which are made of sequentially evaporated host and dopant. An external quantum efficiency as high as 3.38% photons/electron was obtained. By avoiding coevaporation, sequential doping should render better control on device performance and day-to-day repeatability.
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85.60.Jb Light-emitting devices

Efficiency enhancement and voltage reduction in white organic light-emitting devices

S. L. Lai, M. Y. Chan, M. K. Fung, C. S. Lee, and S. T. Lee

Appl. Phys. Lett. 90, 203510 (2007); http://dx.doi.org/10.1063/1.2740482 (3 pages) | Cited 23 times

Online Publication Date: 17 May 2007

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High-efficiency and low operating voltage fluorescent white organic light-emitting devices (WOLEDs) have been realized by doping either 4,7-diphenyl-1,10-phenanthroline (BPhen) or N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) into the blue light-emissive layer. Devices doped with BPhen (or NPB) exhibited a maximum power efficiency of 8.7 lm/W (7.6 lm/W), about 74% higher than that of the reference device (5.0 lm/W). Such performance improvement is ascribed to the incorporation of a better electron-transporting layer and an improved carrier transport through the emissive layer by mixing with the higher drift mobility materials. It provides a simple and general means to improve the power efficiency of WOLED.
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85.60.Jb Light-emitting devices

Lifetime improvement of green phosphorescent organic light-emitting diodes by charge confining device structure

Sung Hyun Kim, Jyongsik Jang, and Jun Yeob Lee

Appl. Phys. Lett. 90, 203511 (2007); http://dx.doi.org/10.1063/1.2740587 (3 pages) | Cited 15 times

Online Publication Date: 17 May 2007

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Lifetime improvement of green phosphorescent organic light-emitting diodes by charge confinement inside an emitting layer was investigated. Excitons were confined within the emitting layer by using a charge confining structure with a high doping concentration at the center of the emitting layer. The lifetime of green devices could be improved by more than five times by confining the excitons at the center of the emitting layer.
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85.60.Jb Light-emitting devices

Hole mobility of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine investigated by using space-charge-limited currents

Ta-Ya Chu and Ok-Keun Song

Appl. Phys. Lett. 90, 203512 (2007); http://dx.doi.org/10.1063/1.2741055 (3 pages) | Cited 70 times

Online Publication Date: 17 May 2007

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The hole mobility of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) at various thicknesses (50–1000 nm) has been estimated by using space-charge-limited current measurements. A thin layer of buckminsterfullerene has been used for a quasi-Ohmic contact between NPB and indium tin oxide. The mobility at bulk property dominant thickness is in excellent agreement with the results from time-of-flight method. For the typical thickness of organic light-emitting devices, the hole mobility of NPB, 1.63×10−5 cm2/V s, at 50 nm is smaller than the value 7.64×10−4 cm2/V s at 1000 nm (electric field at 0.1 MV/cm). The authors suggest that the lower mobility is caused by the interfacial trap states.
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72.20.Fr Low-field transport and mobility; piezoresistance
72.20.Ht High-field and nonlinear effects

Efficient multilayer white polymer light-emitting diodes with aluminum cathodes

Xiaodi Niu, Chuanjiang Qin, Baohua Zhang, Junwei Yang, Zhiyuan Xie, Yanxiang Cheng, and Lixiang Wang

Appl. Phys. Lett. 90, 203513 (2007); http://dx.doi.org/10.1063/1.2741125 (3 pages) | Cited 27 times

Online Publication Date: 17 May 2007

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Efficient multilayer white polymer light-emitting diodes (WPLEDs) with aluminum cathodes are fabricated. The multilayer structure is composed of a water soluble hole-injection layer, a toluene-soluble emissive layer, and an alcohol-soluble emissive layer. The polarity difference of the solvents used for spin coating these polymers allows for realization of the multilayer polymer structure. The recombination zone confined at the interface of the two emissive polymers avoids exciton quenching by electrodes, and white emission is realized by harvesting photons emitted from the two emissive polymers. A maximum luminous efficiency of 16.9 cd/A and a power efficiency of 11.1 lm/W are achieved for this WPLED.
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85.60.Jb Light-emitting devices

Efficient laser textured nanocrystalline silicon-polymer bilayer solar cells

A. A. D. T. Adikaari, D. M. N. M. Dissanayake, R. A. Hatton, and S. R. P. Silva

Appl. Phys. Lett. 90, 203514 (2007); http://dx.doi.org/10.1063/1.2739365 (3 pages) | Cited 8 times

Online Publication Date: 18 May 2007

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Excimer laser textured thin film silicon and poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene bilayer solar cells are fabricated and characterized with air mass 1.5 simulated solar irradiation. The polymer layer increases the light harvesting capability of the cell and increases the shunt resistance while increasing open circuit voltage. The highest efficiency of 0.87% for the thin film silicon/polymer bilayer device is observed due to enhanced charge collection resulting from the inclusion of the polymer layer. Generation of charge carriers is proposed to be occurring mainly in the silicon layer and charge transport is explained using an energy band diagram.
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84.60.Jt Photoelectric conversion
42.62.-b Laser applications
96.60.Ub Solar irradiance
73.20.At Surface states, band structure, electron density of states

Integrated ZnO nanotips on GaN light emitting diodes for enhanced emission efficiency

J. Zhong, H. Chen, G. Saraf, Y. Lu, C. K. Choi, J. J. Song, D. M. Mackie, and H. Shen

Appl. Phys. Lett. 90, 203515 (2007); http://dx.doi.org/10.1063/1.2741052 (3 pages) | Cited 54 times

Online Publication Date: 18 May 2007

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Enhancement of light extraction from an integrated ZnO nanotips/GaN light emitting diode (LED) is demonstrated. The device is composed of a GaN LED with a Ga-doped ZnO (GZO) transparent conductive layer and ZnO nanotips grown on GZO for light extraction. The light output power of a ZnO nanotips/GZO/GaN LED exhibits 1.7 times enhancement, in comparison with a conventional Ni/Au p-metal LED. The higher emission efficiency is attributed to the enhanced light transmission and scattering in the ZnO/GaN multilayer.
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85.60.Jb Light-emitting devices
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