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5 Dec 2011

Volume 99, Issue 23, Articles (23xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 99, 233701 (2011); http://dx.doi.org/10.1063/1.3651756 (3 pages)

Melis Hazar, Robert L. Steward, Jr., Chia-Jung Chang, Cynthia J. Orndoff, Yukai Zeng, Mon-Shu Ho, Philip R. LeDuc, and Chao-Min Cheng
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Droop improvement in blue InGaN/GaN multiple quantum well light-emitting diodes with indium graded last barrier

Chang Sheng Xia, Z. M. Simon Li, Wei Lu, Zhi Hua Zhang, Yang Sheng, and Li Wen Cheng

Appl. Phys. Lett. 99, 233501 (2011); http://dx.doi.org/10.1063/1.3665252 (3 pages) | Cited 6 times

Online Publication Date: 5 December 2011

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Blue InGaN/GaN multiple quantum well light-emitting diodes with the conventional and graded last barriers (GLB) are numerically investigated. When the last GaN barrier is replaced by a linearly graded InxGa1 − xN barrier with increasing indium composition in the growth direction, the forward voltage is reduced from 3.60 V to 3.25 V, and the efficiency droop is improved from 36% to 13%. Simulation results indicate that these improvements can be attributed to the formation of a deep potential well in the GLB which enhances the electron confinement and improves the hole injection efficiency.
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85.60.Jb Light-emitting devices

Effects of W diffusion barrier on inhibition of AlN formation in Ti/W/Al ohmic contacts on N-face n-GaN

Yang Hee Song, Jun Ho Son, Buem Joon Kim, Hak ki Yu, Chul Jong Yoo, and Jong-Lam Lee

Appl. Phys. Lett. 99, 233502 (2011); http://dx.doi.org/10.1063/1.3665623 (3 pages)

Online Publication Date: 5 December 2011

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We investigate the effect of W diffusion barrier in Ti/W/Al ohmic contacts formed on N-face n-GaN. The contacts exhibit contact resistivity of as low as 2.3 × 10−4 Ω cm2 and better thermal stability than Ti/Al contacts. Cross-sectional transmission electron microscopy micrographs reveal that in-diffused Al atoms on the n-GaN surface react with N atoms to form an AlN layer in Ti/Al contacts, resulting in upward band bending, and consequently, a high contact resistivity. The use of a 10-nm-thick W layer suppresses the in-diffusion of Al atoms to n-GaN, thereby preventing the formation of AlN and enhancing the thermal stability of Ti/W/Al contacts.
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73.40.Ns Metal-nonmetal contacts
68.35.Fx Diffusion; interface formation

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)-based organic, ultrathin, and transparent sound-emitting device

He Tian, Dan Xie, Yi Yang, Tian-Ling Ren, Ting-Ting Feng, Yu-Feng Wang, Chang-Jian Zhou, Ping-Gang Peng, Li-Gang Wang, and Li-Tian Liu

Appl. Phys. Lett. 99, 233503 (2011); http://dx.doi.org/10.1063/1.3666224 (3 pages) | Cited 4 times

Online Publication Date: 7 December 2011

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An organic, ultrathin, and transparent sound-emitting device was fabricated using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic) (PEDOT:PSS) films. This sound-emitting device was easily processed by spin coating. Sound emission from PEDOT:PSS was measured as a function of power, distance, and frequency. The sound frequency spectrum was found to be ultra-flat in a wide sound frequency range (10–45 kHz). Theoretical analysis showed that the working principle was the thermoacoustic effect. The PEDOT:PSS sound-emitting device has potential applications in the acoustic field. In particular, it can be integrated with a liquid crystal display to realize sound emission and image display at the same time.
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43.38.-p Transduction; acoustical devices for the generation and reproduction of sound

Demonstration of forward inter-band tunneling in GaN by polarization engineering

Sriram Krishnamoorthy, Pil Sung Park, and Siddharth Rajan

Appl. Phys. Lett. 99, 233504 (2011); http://dx.doi.org/10.1063/1.3666862 (3 pages) | Cited 3 times

Online Publication Date: 7 December 2011

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We report on the design, fabrication, and characterization of GaN interband tunnel junction showing forward tunneling characteristics. We have achieved very high forward tunneling currents (153 mA/cm2 at 10 mV, and 17.7 A/cm2 peak current) in polarization-engineered GaN/InGaN/GaN heterojunction diodes grown by plasma assisted molecular beam epitaxy. We also report the observation of repeatable negative differential resistance in interband III-Nitride tunnel junctions, with peak-valley current ratio of 4 at room temperature. The forward current density achieved in this work meets the typical current drive requirements of a multi-junction solar cell.
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81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
52.77.Dq Plasma-based ion implantation and deposition

Graphene based Schottky junction solar cells on patterned silicon-pillar-array substrate

Tingting Feng, Dan Xie, Yuxuan Lin, Yongyuan Zang, Tianling Ren, Rui Song, Haiming Zhao, He Tian, Xiao Li, Hongwei Zhu, and Litian Liu

Appl. Phys. Lett. 99, 233505 (2011); http://dx.doi.org/10.1063/1.3665404 (3 pages) | Cited 8 times

Online Publication Date: 7 December 2011

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Graphene-on-silicon Schottky junction solar cells were prepared with pillar-array-patterned silicon substrate. Such patterned substrate showed an anti-reflective characteristic and led to an absorption enhancement of the solar cell, which showed enhanced performance with short-circuit current density, open-circuit voltage, fill factor, and energy conversion efficiency of 464.86 mV, 14.58 mA/cm2, 0.29, and 1.96%, respectively. Nitric acid was used to dope graphene film and the cell performance showed a great improvement with efficiency increasing to 3.55%. This is due to the p-type chemical doping effect of HNO3 which increases the work function and the carrier density of graphene.
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88.40.H- Solar cells (photovoltaics)
88.40.J- Types of solar cells

Resonant-tunnelling-diode oscillators operating at frequencies above 1.1 THz

Michael Feiginov, Cezary Sydlo, Oleg Cojocari, and Peter Meissner

Appl. Phys. Lett. 99, 233506 (2011); http://dx.doi.org/10.1063/1.3667191 (3 pages) | Cited 9 times

Online Publication Date: 8 December 2011

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We present resonant-tunnelling-diode (RTD) oscillators operating at the fundamental frequency of 1111 GHz. We show that our RTDs and RTD oscillators have much room for further improvement of their parameters and for further increase of their operating frequencies. The operating frequencies of several THz should be achievable with RTD oscillators. Our study also shows that operation of RTDs beyond the relaxation-time limit at THz frequencies should be possible. RTD oscillators under study are extremely compact (less than a square millimeter) room-temperature sources of coherent cw THz radiation. Such sources should enable plenty of real-world THz applications.
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85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)

Spin injection from two-dimensional electron and hole gases in resonant tunneling diodes

Y. Galvão Gobato, H. V. A. Galeti, L. F. dos Santos, V. López-Richard, D. F. Cesar, G. E. Marques, M. J. S. P. Brasil, M. Orlita, J. Kunc, D. K. Maude, M. Henini, and R. J. Airey

Appl. Phys. Lett. 99, 233507 (2011); http://dx.doi.org/10.1063/1.3668087 (4 pages)

Online Publication Date: 8 December 2011

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We have investigated the polarized-resolved photoluminescence from the contact layers and the quantum-well in an n-type GaAs/GaAlAs resonant tunneling diode for magnetic fields up to 19 T. The optical emission from the GaAs contact layers comprises the recombination from highly spin-polarized two-dimensional electron and hole gases with free tunneling carriers. Both the energy position and intensity of this indirect recombination are voltage-dependent and show remarkably abrupt variations near scattering-assisted tunneling resonances. Our results show that these two dimensional gases act as spin-polarized sources for carriers tunneling through the well in resonant tunneling diodes.
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72.25.-b Spin polarized transport
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
78.55.Cr III-V semiconductors
85.30.Kk Junction diodes
68.65.Fg Quantum wells

Probing the electrostatics of self-assembled monolayers by means of beveled metal-oxide-semiconductor structures

Lior Kornblum, Yair Paska, Jonathan A. Rothschild, Hossam Haick, and Moshe Eizenberg

Appl. Phys. Lett. 99, 233508 (2011); http://dx.doi.org/10.1063/1.3665630 (3 pages) | Cited 3 times

Online Publication Date: 9 December 2011

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A method is proposed for characterization of the electrostatics of self-assembled monolayers (SAMs). The method is based on the extraction of the metal’s effective work function in metal-oxide-semiconductor capacitors, where the SAM is positioned at the metal-oxide interface. Hexyltrichlorosilane molecules assembled on SiO2 are used as a model system for this method. A band offset of 0.5 ± 0.15 eV is observed in the SAM sample when compared to a reference with no molecules. Spectroscopy is employed to confirm the presence of silane anchoring groups after metal deposition.
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68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
73.40.Cg Contact resistance, contact potential
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)

Low-field mobility in ultrathin silicon nanowire junctionless transistors

Bart Sorée, Wim Magnus, and William Vandenberghe

Appl. Phys. Lett. 99, 233509 (2011); http://dx.doi.org/10.1063/1.3669509 (3 pages)

Online Publication Date: 9 December 2011

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We theoretically investigate the phonon, surface roughness and ionized impurity limited low-field mobility of ultrathin silicon n-type nanowire junctionless transistors in the long channel approximation with wire radii ranging from 2 to 5 nm, as function of gate voltage. We show that surface roughness scattering is negligible as long as the wire radius is not too small and ionized impurity scattering is the dominant scattering mechanism. We also show that there exists an optimal radius where the ionized impurity limited mobility exhibits a maximum.
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85.30.-z Semiconductor devices
63.22.Gh Nanotubes and nanowires
68.35.bg Semiconductors
72.20.Fr Low-field transport and mobility; piezoresistance
73.63.Nm Quantum wires
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