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27 Jun 2011

Volume 98, Issue 26, Articles (26xxxx)

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Appl. Phys. Lett. 98, 263101 (2011); http://dx.doi.org/10.1063/1.3587576 (3 pages)

Jing Ye, Yu Zhao, Libin Tang, Li-Miao Chen, C. M. Luk, S. F. Yu, S. T. Lee, and S. P. Lau
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Comparative study of n-MgZnO/p-Si ultraviolet-B photodetector performance with different device structures

Y. N. Hou, Z. X. Mei, H. L. Liang, D. Q. Ye, S. Liang, C. Z. Gu, and X. L. Du

Appl. Phys. Lett. 98, 263501 (2011); http://dx.doi.org/10.1063/1.3600789 (3 pages) | Cited 1 time

Online Publication Date: 27 June 2011

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A comparative study of n-MgZnO/p-Si UV-B photodetector performance was carried out with different device structures. The experimental results demonstrate superior photoresponse characteristics of the p-n heterojunction detector against the Schottky type metal-semiconductor-metal counterpart, including a sharper cutoff wavelength at 300 nm, a larger peak photoresponsivity of 1 A/W, and a faster response speed. The role of built-in field and low interface scattering in p-n heterojunction is explored, and the energy band diagram of n-MgZnO/p-Si is employed to interpret the efficient suppression of visible light photoresponse from Si substrate, revealing the applicability of this heterostructure in fabrication of deep ultraviolet detectors.
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85.60.Gz Photodetectors (including infrared and CCD detectors)

Hafnium dioxide as a passivating layer and diffusive barrier in ZnO/Ag Schottky junctions obtained by atomic layer deposition

Tomasz A. Krajewski, Grzegorz Luka, Sylwia Gieraltowska, Adam J. Zakrzewski, Petro S. Smertenko, Piotr Kruszewski, Lukasz Wachnicki, Bartlomiej S. Witkowski, Elzbieta Lusakowska, Rafal Jakiela, Marek Godlewski, and Elzbieta Guziewicz

Appl. Phys. Lett. 98, 263502 (2011); http://dx.doi.org/10.1063/1.3604796 (3 pages) | Cited 1 time

Online Publication Date: 27 June 2011

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This paper reports on ZnO/Ag Schottky junctions obtained by the low temperature atomic layer deposition process. Introducing the thin (from 1.25 to 7.5 nm) layer of hafnium dioxide between the ZnO layer and evaporated Ag Schottky contact improves the rectification ratio to about 105 at 2V. For the ZnO/Ag junctions without the HfO2 interlayer, the rectification ratio is only 102. We assign this effect to the passivation of ZnO surface accumulation layer that is reported for ZnO thin films.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.ag Semiconductors

Electrostatic effects on contacts to carbon nanotube transistors

Aron W. Cummings and François Léonard

Appl. Phys. Lett. 98, 263503 (2011); http://dx.doi.org/10.1063/1.3605586 (3 pages)

Online Publication Date: 27 June 2011

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We use numerical simulations to investigate the effect of electrostatics on the source and drain contacts of carbon nanotube field-effect transistors. We find that unscreened charge on the nanotube at the contact-channel interface leads to a potential barrier that can significantly hamper transport through the device. This effect is largest for intermediate gate voltages and for contacts near the ohmic-Schottky crossover, but can be mitigated with a reduction in the gate oxide thickness. These results help to elucidate the important role that contact geometry plays in the performance of carbon nanotube electronic devices.
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85.30.Tv Field effect devices
85.35.Kt Nanotube devices

Enhanced conversion efficiency of InGaN multiple quantum well solar cells grown on a patterned sapphire substrate

Ya-Ju Lee, Min-Hung Lee, Chun-Mao Cheng, and Chia-Hao Yang

Appl. Phys. Lett. 98, 263504 (2011); http://dx.doi.org/10.1063/1.3605244 (3 pages) | Cited 1 time

Online Publication Date: 28 June 2011

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This study demonstrated the enhanced conversion efficiency of an indium gallium nitride (InGaN) multiple quantum well (MQW) solar cell fabricated on a patterned sapphire substrate (PSS). Compared to conventional solar cells grown on a planar sapphire substrate, threading dislocation defects were found to be reduced from 1.28 × 109 to 3.62 × 108 cm−2, leading to an increase in short-circuit current density (JSC = 1.09 mA·cm−2) of approximately 60%. In addition, the open-circuit voltage and fill factor (VOC = 2.05 V; FF = 51%) of the solar cells grown on PSS were nearly identical to those of conventional devices. The enhanced performance is primarily due to improvements in the crystalline quality of the epitaxial layers, reducing the trapping of photogenerated electrons and holes by nonradiative recombination centers in MQW, with a corresponding increase in the transport efficiency of the carriers outside the device.
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88.40.jm Thin film III-V and II-VI based solar cells
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
81.07.St Quantum wells

Ultrasonic control of terahertz radiation via lattice anharmonicity in LiNbO3

R. H. Poolman, A. L. Ivanov, and E. A. Muljarov

Appl. Phys. Lett. 98, 263505 (2011); http://dx.doi.org/10.1063/1.3605569 (3 pages)

Online Publication Date: 29 June 2011

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We propose a tunable terahertz (THz) filter using the resonant acousto-optic (RAO) effect. We present a design based on a transverse optical (TO) phonon mediated interaction between a coherent acoustic wave and the THz field in LiNbO3. We predict a tunable range for the filter of up to 4 THz via the variation of the acoustic frequency between 0.1 and 1 GHz. The RAO effect in this case is due to cubic and quartic anharmonicities between TO phonons and the acoustic field. The effect of the interference between the anharmonicities is also discussed.
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42.79.Jq Acousto-optical devices
43.38.Zp Acoustooptic and photoacoustic transducers
43.35.Sx Acoustooptical effects, optoacoustics, acoustical visualization, acoustical microscopy, and acoustical holography
42.79.Ci Filters, zone plates, and polarizers

Ultrafast magnetization dynamics in magnetic tunneling junctions

Xiaojing Zou and Gang Xiao

Appl. Phys. Lett. 98, 263506 (2011); http://dx.doi.org/10.1063/1.3605557 (3 pages)

Online Publication Date: 30 June 2011

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To understand the dynamic magnetic properties and its relationship to ultrafast field sensing, we performed micromagnetic simulations on actual MgO-based magnetic tunneling junction (MTJ) structures. The results indicate that an MTJ with a larger aspect ratio yields a smaller response time under a sudden change of an external magnetic field. Such an effect is mainly due to the coherent rotation from the central cores inside the junction element. Damping coefficient is shown to play an important role in ultrafast sensing and the optimal value need to be a few times larger than typical values found in common ferromagnetically soft materials. Our calculations further show that the response time can be reduced by increasing the free layer thickness and/or its saturation magnetization. Finally, we have obtained the dependence of intrinsic resonance frequency on the sensor size while keeping the same aspect ratio.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.78.Cd Micromagnetic simulations
75.78.Jp Ultrafast magnetization dynamics and switching
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Self-consistent physical modeling of set/reset operations in unipolar resistive-switching memories

Marc Bocquet, Damien Deleruyelle, Christophe Muller, and Jean-Michel Portal

Appl. Phys. Lett. 98, 263507 (2011); http://dx.doi.org/10.1063/1.3605591 (3 pages) | Cited 2 times

Online Publication Date: 30 June 2011

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This Letter deals with a self-consistent physical model for set/reset operations involved in unipolar resistive switching memories integrating a transition metal oxide. In this model, set operation is described in terms of a local electrochemical reduction of the oxide leading to the formation of metallic conductive filaments. Beside, reset operation relies on the thermally assisted destruction of the formed metallic filaments by Joule heating effect. An excellent agreement is demonstrated with numerous published experimental data suggesting that this model can be confidently implemented into circuit simulators for design purpose.
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84.30.Sk Pulse and digital circuits
73.40.Rw Metal-insulator-metal structures

Polymer stabilized chiral nematic liquid crystals for fast switching and high contrast electro-optic devices

Damian J. Gardiner, Stephen M. Morris, Flynn Castles, Malik M. Qasim, Wook-Sung Kim, Su Seok Choi, Hyun-Jin Park, In-Jae Chung, and Harry J. Coles

Appl. Phys. Lett. 98, 263508 (2011); http://dx.doi.org/10.1063/1.3605597 (3 pages) | Cited 3 times

Online Publication Date: 30 June 2011

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A fast switching electro-optic device, based upon the in-plane addressing of very short pitch polymer stabilized chiral nematic liquid crystals, is presented. Polymer stabilization of the standing helical arrangement is essential to prevent the appearance of defects above the in-plane electrodes. Response times as short as 50 μs are observed at room temperature along with contrast ratios greater than 3000:1 owing to the high optical extinction at visible wavelengths in the “Off” state. The combination of these fast response times with such high contrast ratios is of great importance for next generation electro-optical elements.
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42.70.Df Liquid crystals
42.79.-e Optical elements, devices, and systems
78.40.Me Organic compounds and polymers

Enhanced infrared light harvesting of inorganic nanocrystal photovoltaic and photodetector on graphene electrode

Chih-Cheng Lin, Di-Yan Wang, Kun-Hua Tu, You-Ting Jiang, Meng-Hsiang Hsieh, Chia-Chun Chen, and Chun-Wei Chen

Appl. Phys. Lett. 98, 263509 (2011); http://dx.doi.org/10.1063/1.3605682 (3 pages) | Cited 1 time

Online Publication Date: 30 June 2011

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We demonstrate an enhancement of infrared light harvesting of inorganic PbS nanocrystal photovoltaic and photodetector devices based on the transparent graphene electrode. Due to high infrared transparency of the graphene electrode with respect to indium tin oxide (ITO), the infrared photoresponse of the graphene-based device is superior to the ITO-based counterpart, in spite of a higher sheet resistance of the graphene electrode. The outstanding infrared characteristics of the devices based on the graphene electrode make it a promising candidate for infrared optoelectronic applications such as solar cells, imaging and sensing, or optical communication.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
84.60.Jt Photoelectric conversion
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors

Superlensing effect of an anisotropic metamaterial slab with near-zero dynamic mass

Xiaoming Zhou and Gengkai Hu

Appl. Phys. Lett. 98, 263510 (2011); http://dx.doi.org/10.1063/1.3607277 (3 pages) | Cited 4 times

Online Publication Date: 1 July 2011

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A metamaterial slab of anisotropic mass with one diagonal component being infinity and the other being zero is demonstrated to behave as a superlens for acoustic imaging beyond the diffraction limit. The underlying mechanism for extraordinary transmission of evanescent waves is attributed to the zero mass effect. Microstructure design for such anisotropic lens is also presented. In contrast to the anisotropic superlens based on Fabry-Pérot resonant mechanism, the proposed lens operates without the limitation on lens thickness, thus more flexible in practical applications. Numerical modeling is performed to validate the proposed ideas.
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42.70.-a Optical materials
42.79.Bh Lenses, prisms and mirrors
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