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29 Oct 2007

Volume 91, Issue 18, Articles (18xxxx)

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Appl. Phys. Lett. 91, 183501 (2007); http://dx.doi.org/10.1063/1.2801554 (3 pages)

Aurelien Du Pasquier, Daniel D. T. Mastrogiovanni, Lauren A. Klein, Tong Wang, and Eric Garfunkel
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Photoinduced charge transfer between poly(3-hexylthiophene) and germanium nanowires

Aurelien Du Pasquier, Daniel D. T. Mastrogiovanni, Lauren A. Klein, Tong Wang, and Eric Garfunkel

Appl. Phys. Lett. 91, 183501 (2007); http://dx.doi.org/10.1063/1.2801554 (3 pages) | Cited 10 times

Online Publication Date: 29 October 2007

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Germanium nanowires (GeNWs) were used to enhance the properties of organic photovoltaic devices. GeNWs were grown to a length of 1–5 μm on SiO2 by the vapor-liquid-solid method catalyzed by 20 nm Au seeds. Once grown, the GeNWs were dispersed in solution with poly(3-hexylthiophene) and spin cast into films. The photoluminescence and external quantum efficiency of the films indicated a significant increase in exciton dissociation and photocurrent generation. The results imply that the GeNWs may act as an electron acceptor for bulk heterojunction hybrid-inorganic/organic photovoltaic devices. The impacts of GeNW on device characteristics are discussed.
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73.63.Nm Quantum wires
78.67.Lt Quantum wires
73.21.Hb Quantum wires
85.60.-q Optoelectronic devices

Resistive switching effect in SrTiO3−δ/Nb-doped SrTiO3 heterojunction

M. C. Ni, S. M. Guo, H. F. Tian, Y. G. Zhao, and J. Q. Li

Appl. Phys. Lett. 91, 183502 (2007); http://dx.doi.org/10.1063/1.2803317 (3 pages) | Cited 19 times

Online Publication Date: 29 October 2007

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The authors report on the fabrication and properties of SrTiO3−δ/Nb-doped SrTiO3 heterojunctions. The current-voltage curves of these junctions show hysteresis and remarkable resistive switching behavior. Hysteresis was also observed in the capacitance-voltage curves of these junctions. Upon applying voltage pulses, the resistance of the heterojunctions can be switched between different states and the relaxation of the junction current after switching follows the Curie–Von Schweidler law. The results were discussed by considering the role of defects in the interfacial depletion region of the heterojunctions. This work indicates that heterojunctions composed of two oxides can also show the switching effect, which is useful for applications.
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73.40.-c Electronic transport in interface structures

Superconducting array for high-field magnetic resonance imaging

J. Wosik, L. Xue, L.-M. Xie, M. R. Kamel, K. Nesteruk, and James A. Bankson

Appl. Phys. Lett. 91, 183503 (2007); http://dx.doi.org/10.1063/1.2801384 (3 pages) | Cited 2 times

Online Publication Date: 30 October 2007

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We report on the design of a planar 200 MHz superconducting two-resonator array for magnetic resonance imaging (MRI) applications. The array was made out of a double-sided thin YBa2Cu3O7−x film on r-cut sapphire substrate and consists of two 24 mm diameter resonators with built-in planar capacitors for coupling to the tuning and matching electronics. Required for the performance of the MRI array, rf isolation of two resonators was accomplished by built-in planar capacitors, and the mechanism of resonator-to-resonator decoupling was analyzed. The signal-to-noise gain as a result of using high-Tc superconductor resonators/arrays was calculated and compared with experimental data.
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85.25.-j Superconducting devices
84.32.Tt Capacitors

Highly efficient white organic light-emitting diodes with single small molecular emitting material

Lei Wang, Mei-Fang Lin, Wai-Kwok Wong, Kok-Wai Cheah, Hoi-Lam Tam, Zhi-Qiang Gao, and Chin H. Chen

Appl. Phys. Lett. 91, 183504 (2007); http://dx.doi.org/10.1063/1.2804003 (3 pages) | Cited 9 times

Online Publication Date: 30 October 2007

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We demonstrate a highly efficient white organic light emitting device with fluorescent small molecule 4,4′-bis(9-(1-naphthyl)anthracene-10-yl)biphenyl (BUBH-3). With a simple device architecture of indium tin oxide/tris 4,4′,4″-tris-N-naphthyl-N-phenylamino-triphenylamine (60 nm)/N,N′-bis-(1–naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (10 nm)/BUBH-3 (45 nm)∕Alq3 (15 nm)/LiF (1 nm)/Al (150 nm), a white light with CIEx,y color of (0.31,0.36) was generated. The device achieved one of the best single-emitting-material electroluminescence performance of white organic light-emitting devices with efficiencies of 7.0 cd/A and 3.17 lm/W at 6.9 V.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence

Optical bandwidth enhancement by operation and modulation of the first excited state of a transistor laser

H. W. Then, M. Feng, and N. Holonyak, Jr.

Appl. Phys. Lett. 91, 183505 (2007); http://dx.doi.org/10.1063/1.2805014 (3 pages) | Cited 8 times

Online Publication Date: 30 October 2007

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A model is developed to explain the large increase in the high-speed performance of the transistor laser (TL) operating on the first excited state (λ = 980 nm) compared to ground state (1000 nm). The model shows that the bandwidth of the TL (absent resonance peaks) increases as much as twofold when it shifts operation from ground to first excited state. No assumption of upper subband state filling is necessary in the calculation consistent with no “pileup” of charge in the upper subbands because of the transistor boundary condition at the electrical collector of zero charge density (collector current IC ≠ 0) resulting in a “tilted” emitter-to-collector population distribution.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Fc Modulation, tuning, and mode locking
85.30.Pq Bipolar transistors

Subthreshold field emission from thin silicon membranes

Hua Qin, Hyun-Seok Kim, Robert H. Blick, Michael S. Westphall, and Lloyd M. Smith

Appl. Phys. Lett. 91, 183506 (2007); http://dx.doi.org/10.1063/1.2805015 (3 pages) | Cited 4 times

Online Publication Date: 30 October 2007

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We report on strongly enhanced electron multiplication in thin silicon membranes. The device is configured as a transmission-type membrane for electron multiplication. A subthreshold electric field applied on the emission side of the membrane enhances the number of electrons emitted by two orders of magnitude. This enhancement stems from field emitted electrons stimulated by the incident particles, which suggests that stacks of silicon membranes can form ultrasensitive electron multipliers.
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79.70.+q Field emission, ionization, evaporation, and desorption
79.20.Hx Electron impact: secondary emission
73.30.+y Surface double layers, Schottky barriers, and work functions
81.05.Cy Elemental semiconductors
85.45.Db Field emitters and arrays, cold electron emitters
85.30.De Semiconductor-device characterization, design, and modeling

Origin of efficiency droop in GaN-based light-emitting diodes

Min-Ho Kim, Martin F. Schubert, Qi Dai, Jong Kyu Kim, E. Fred Schubert, Joachim Piprek, and Yongjo Park

Appl. Phys. Lett. 91, 183507 (2007); http://dx.doi.org/10.1063/1.2800290 (3 pages) | Cited 271 times

Online Publication Date: 30 October 2007

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The efficiency droop in GaInN/GaN multiple-quantum well (MQW) light-emitting diodes is investigated. Measurements show that the efficiency droop, occurring under high injection conditions, is unrelated to junction temperature. Furthermore, the photoluminescence output as a function of excitation power shows no droop, indicating that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MQW region. Simulations show that polarization fields in the MQW and electron blocking layer enable the escape of electrons from the MQW region and thus are the physical origin of the droop. It is shown that through the use of proper quaternary AlGaInN compositions, polarization effects are reduced, thereby minimizing droop and improving efficiency.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Stability of pentacene top gated thin film transistors

K. Diallo, M. Erouel, J. Tardy, E. André, and J.-L. Garden

Appl. Phys. Lett. 91, 183508 (2007); http://dx.doi.org/10.1063/1.2802039 (3 pages) | Cited 11 times

Online Publication Date: 30 October 2007

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We report on the stability of top gated pentacene field effect transistors processed on Kapton™ with Parylene-C as gate dielectric. The influence of bias stress and ambient atmosphere on device characteristics were investigated. Combined influence of moisture and gate bias stress led to an increase of depletion current and subthreshold slope as well as a drift of onset voltage and threshold voltage. We show that devices stressed in the off state exhibit a high stability.
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85.30.Tv Field effect devices

Very thin photoalignment films for liquid crystalline conjugated polymers: Application to polarized light-emitting diodes

Kenji Sakamoto, Kazushi Miki, Masahiro Misaki, Koichi Sakaguchi, Masayuki Chikamatsu, and Reiko Azumi

Appl. Phys. Lett. 91, 183509 (2007); http://dx.doi.org/10.1063/1.2802572 (3 pages) | Cited 14 times

Online Publication Date: 30 October 2007

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Photoaligned polyimide films with different film thicknesses were prepared on quartz substrates, and uniaxially aligned glassy poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) layers were formed on top. The photoluminescence polarization ratio of the PFO layer rapidly increased with increasing polyimide film thickness, and beyond a thickness of 1.6 nm, it was saturated at ∼ 11. This result shows that the 1.6-nm-thick photoaligned polyimide film works as a good alignment layer for PFO. We succeeded in fabricating a polarized light-emitting diode with a polarization ratio of 29 at 459 nm and a brightness of 700 cd/m2 by using a 2.8-nm-thick polyimide photoalignment layer.
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78.66.Qn Polymers; organic compounds
78.55.-m Photoluminescence, properties and materials
61.30.Vx Polymer liquid crystals
85.60.Jb Light-emitting devices

In-plane excitation of thin silicon cantilevers using piezoelectric thin films

Glenn J. T. Leighton, Paul B. Kirby, and Colin H. J. Fox

Appl. Phys. Lett. 91, 183510 (2007); http://dx.doi.org/10.1063/1.2805070 (3 pages) | Cited 6 times

Online Publication Date: 31 October 2007

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This paper deals with the actuation of in-plane and out-of-plane motions of silicon cantilevers, using a single thin film of lead zirconate titanate with a divided electrode configuration. In-plane actuation is demonstrated practically, and excellent agreement is obtained between theoretically predicted and experimentally measured resonant amplitudes, for the fundamental out-of-plane and in-plane modes of vibration of the fabricated test cantilevers.
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77.55.-g Dielectric thin films
77.65.-j Piezoelectricity and electromechanical effects
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
85.50.-n Dielectric, ferroelectric, and piezoelectric devices

Negative transconductance in double-gate germanium-on-insulator field effect transistors

A. Zaslavsky, S. Soliveres, C. Le Royer, S. Cristoloveanu, L. Clavelier, and S. Deleonibus

Appl. Phys. Lett. 91, 183511 (2007); http://dx.doi.org/10.1063/1.2802074 (3 pages) | Cited 1 time

Online Publication Date: 31 October 2007

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Transport in double-gate (DG) transistors offers unusual properties due to the coupling between the two channels. We report on room-temperature negative transconductance in germanium-on-insulator DG transistors in the subthreshold regime. The effect is due to the coupling between conducting channels, analogous to the velocity modulation transistor (VMT). Unlike the VMT, our effect can be induced by either of the gates and arises not from a difference in the channel mobilities but from partial electric field screening at low channel densities combined with the density dependence of mobility. The negative transconductance becomes weaker as gate length LG is reduced.
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85.30.Tv Field effect devices

Method for determination of recombination activity of cylindric conduction channels for back-contacted solar cells

Nicola Mingirulli, Daniel Biro, Ralf Preu, Stefan W. Glunz, and Stephan Riepe

Appl. Phys. Lett. 91, 183512 (2007); http://dx.doi.org/10.1063/1.2805215 (3 pages) | Cited 3 times

Online Publication Date: 1 November 2007

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A method for the determination of the surface recombination velocity of cylindric vias for the application in back-contacted silicon solar cells is proposed. By applying a theoretical framework for the lifetime dependence on dislocation density, the measured minority carrier lifetime as a function of via density can be described accurately; thus the value for the surface recombination velocity at the cylinder barrel Svia is extracted. Accordingly, Svia after SiNx deposition from the front and the rear of the wafers systematically decreases if material surrounding the laser drilled via is removed prior to passivation by etching from 106 to 100 cm/s.
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61.72.Lk Linear defects: dislocations, disclinations
68.47.Fg Semiconductor surfaces
81.65.Cf Surface cleaning, etching, patterning
81.65.Rv Passivation
84.60.Jt Photoelectric conversion
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Theoretical dynamic response of electrostatic parallel plate

Ki Bang Lee

Appl. Phys. Lett. 91, 183513 (2007); http://dx.doi.org/10.1063/1.2803323 (3 pages)

Online Publication Date: 1 November 2007

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Theoretical dynamic response of an electrostatic parallel plate to a time varying voltage has been obtained as a function of the applied voltage. The nonlinear equation of motion is theoretically solved for the dynamic response when ac drive voltage with dc bias voltage actuates the movable plate of the parallel plate with small amplitude. The dynamic response is expressed in a closed form to perform research on a variety of microactuators and sensors employing parallel plates.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.10.Cm Micromechanical devices and systems
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

High-performance fullerene C60 thin-film transistors operating at low voltages

Masatoshi Kitamura, Yasutaka Kuzumoto, Masakazu Kamura, Shigeru Aomori, and Yasuhiko Arakawa

Appl. Phys. Lett. 91, 183514 (2007); http://dx.doi.org/10.1063/1.2804004 (3 pages) | Cited 12 times

Online Publication Date: 2 November 2007

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Low-voltage operation of fullerene C60 thin-film transistors (TFTs) has been realized using zirconium-silicon oxide (ZSO) as a gate insulator. The gate insulator consisted of triple layers of SiO2/ZSO/SiO2 deposited by rf sputtering. The C60 TFTs with the insulators operated at a low voltage of 5 V. The surface of the insulator was treated with hexamethyldisilazane (HMDS) or octadecyltrimethoxysilane (ODS). The C60 TFT with the ODS-treated insulator exhibited higher performance than that with the HMDS-treated insulator, and it had a field-effect mobility of 1.46 cm2/V s, threshold voltage of 1.9 V, and a current on/off ratio of 2×106.
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85.65.+h Molecular electronic devices
85.30.Tv Field effect devices

X-ray diamond detectors with energy resolution

G. Conte, M. Girolami, S. Salvatori, and V. Ralchenko

Appl. Phys. Lett. 91, 183515 (2007); http://dx.doi.org/10.1063/1.2805221 (3 pages) | Cited 3 times

Online Publication Date: 2 November 2007

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Polycrystalline diamond detectors with energy resolving capability of the impinging beam were realized and tested by using a miniature pyroelectric x-ray pulse generator. Microstrip structures were defined by photolithography aimed to reduce parasitic capacitances and to perform characterization measurements in a sandwich configuration. Leakage currents as low as 20 pA at 500 V were measured on a 270 μm thick device. Pulse height distributions were carried out around Ta Lα (8.14 keV) and Cu Kα (8.05 keV) characteristic lines of the source. Energy resolution at 200 V was found equal to 9% with an increase to 11% at 500 V. When the bias was increased to the maximum voltage the sample shows an Ohmic behavior.
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07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
41.50.+h X-ray beams and x-ray optics

Low efficiency roll off at high current densities in Ir-complex based electrophosphorescence diode with exciton diffusing and fluorescence compensating layers

Dongyu Zhang, Wenlian Li, Bei Chu, Jianzhuo Zhu, Tianle Li, Lianglaing Han, Defeng Bi, Xiao Li, Dongfang Yang, Fei Yan, Huihui Liu, Dan Wang, and Taiju Tsuboi

Appl. Phys. Lett. 91, 183516 (2007); http://dx.doi.org/10.1063/1.2805740 (3 pages) | Cited 11 times

Online Publication Date: 2 November 2007

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We demonstrate a fac-tris(2-phenylpyridine) iridium-based electrophosphorescent organic green-light emitting diode with a considerably reduced current-efficiency roll off at high current density. Such a low roll off of efficiency was achieved by inserting nondoped 4,4′-N,N′-dicarbazole-biphenyl (CBP) layer and a tris-(8-hydroxy-quinoline) aluminum (Alq3) layer between the emitting and electron-transporting layers to diffuse excitons from the emitting layer. The Alq3 layer is found to contribute as a complementary green fluorescent emitter at high current density. Thus, only a small decrease from 20.7 to 16.7 cd/A was detected in current efficiency when the current density increases from 3.9 to 100 mA/cm2. A high current efficiency of 12.6 cd/A was achieved even at 350 mA/cm2, indicating that the efficiency roll off was drastically reduced when compared with the device without CBP and Alq3 layers.
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85.60.Jb Light-emitting devices

High performance thin film transistor with low temperature atomic layer deposition nitrogen-doped ZnO

S. J. Lim, Soon-ju Kwon, Hyungjun Kim, and Jin-Seong Park

Appl. Phys. Lett. 91, 183517 (2007); http://dx.doi.org/10.1063/1.2803219 (3 pages) | Cited 51 times

Online Publication Date: 2 November 2007

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High performance thin film transistor (TFT) with atomic layer deposition (ALD) nitrogen doped ZnO (ZnO:N) as an active layer is demonstrated. The electrical properties of ZnO thin films were effectively controlled by in situ nitrogen doping using NH4OH as a source for reactants. Especially, the electron concentration in ZnO was lowered to below 1015 cm−3. Good device characteristics were obtained from the inverted staggered type TFTs with ZnO:N channel and ALD Al2O3 gate insulator; μsat = 6.7 cm2/V s, Ioff = 2.03×10−12 A, Ion/off = 9.46×107, and subthreshold swing = 0.67 V/decade. The entire TFT fabrication processes were carried out at below 150 °C, which is a favorable process for plastic based flexible display.
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85.30.Tv Field effect devices
81.05.Dz II-VI semiconductors
68.55.-a Thin film structure and morphology
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Acoustic cloaking in three dimensions using acoustic metamaterials

Huanyang Chen and C. T. Chan

Appl. Phys. Lett. 91, 183518 (2007); http://dx.doi.org/10.1063/1.2803315 (3 pages) | Cited 130 times

Online Publication Date: 2 November 2007

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A scheme to achieve two dimensional (2D) acoustic cloaking is proposed by Cummer and Schurig [New J. Phys. 9, 45 (2007)] by mapping the acoustic equations to Maxwell’s equations of one polarization in the 2D geometry. We find that the acoustic equation can be mapped to the direct current conductivity equation in three dimensions, which then allows the design of three-dimensional acoustic cloaking using the coordinate transformation scheme. The perfect cloaking effect is confirmed by solving for the scattering problem using the spherical-Bessel function series expansion method.
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43.20.Fn Scattering of acoustic waves
03.50.De Classical electromagnetism, Maxwell equations
42.70.-a Optical materials
02.30.Gp Special functions
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