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4 Jan 2010

Volume 96, Issue 1, Articles (01xxxx)

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Appl. Phys. Lett. 96, 013107 (2010); http://dx.doi.org/10.1063/1.3280900 (3 pages)

L. Fernández, M. Corso, F. Schiller, M. Ilyn, M. Holder, and J. E. Ortega
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Multiple levels in intermediate band solar cells

Antonio Luque, Pablo G. Linares, Elisa Antolín, Enrique Cánovas, Corrie D. Farmer, Colin R. Stanley, and Antonio Martí

Appl. Phys. Lett. 96, 013501 (2010); http://dx.doi.org/10.1063/1.3280387 (3 pages) | Cited 11 times

Online Publication Date: 4 January 2010

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The presence of multiple energy levels in the intermediate band solar cell (IBSC) is studied by detailed balance calculations under ideal conditions. Multiple levels are found experimentally in IBSCs made with quantum dots (QDs) which act to reduce the limiting efficiency determined from detailed balance calculations. JL-VOC measurements up to 1000 suns on IBSCs are presented together with their fitting to modified detailed balance calculations. It is found that the introduction of the QDs degrades the performance of the host cell but the sub-bandgap cell operates close to ideality.
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88.40.J- Types of solar cells
72.40.+w Photoconduction and photovoltaic effects
73.21.La Quantum dots

Room-temperature terahertz mixer based on the simultaneous electronic and optical excitations of plasma waves in a field effect transistor

H. Marinchio, L. Chusseau, J. Torres, P. Nouvel, L. Varani, G. Sabatini, C. Palermo, P. Shiktorov, E. Starikov, and V. Gružinskis

Appl. Phys. Lett. 96, 013502 (2010); http://dx.doi.org/10.1063/1.3282798 (3 pages) | Cited 4 times

Online Publication Date: 4 January 2010

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A method for the heterodyne detection of terahertz (THz) signals is proposed. A high electron mobility transistor is used as a nonlinear element, while the optical beating of two laser beams exciting plasma waves in the transistor channel plays the role of the THz local oscillator. High efficiency and room-temperature operation of such a mixer are demonstrated by numerical simulations.
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85.30.Tv Field effect devices
84.30.Ng Oscillators, pulse generators, and function generators
84.40.-x Radiowave and microwave (including millimeter wave) technology

Low-temperature electroluminescence quenching of AlGaN deep ultraviolet light-emitting diodes

J. C. Zhang, Y. Sakai, and T. Egawa

Appl. Phys. Lett. 96, 013503 (2010); http://dx.doi.org/10.1063/1.3284521 (3 pages)

Online Publication Date: 4 January 2010

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The temperature-dependent electroluminescence (EL) properties of AlGaN deep ultraviolet light-emitting diodes (LEDs) have been studied. The low-temperature EL quenching is observed in 265 nm LEDs with p-type AlGaN heterostructure, which has not previously been reported in such short wavelength devices. However, this phenomenon disappears in those with a thin i-AlN electron blocking layer (EBL). It is found that the electron overflow becomes more severe at low temperature in the LEDs without EBL, whereas it is suppressed effectively in those with AlN EBL. On the basis of a model of temperature-dependent efficiency, the EL quenching is explained by the competition of electron overflow and radiative recombination.
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85.60.Jb Light-emitting devices

Multilayer structured polymer light emitting diodes with cross-linked polymer matrices

Zhang-Lin Zhou, Xia Sheng, K. Nauka, Lihua Zhao, Gary Gibson, Sity Lam, Chung Ching Yang, James Brug, and Rich Elder

Appl. Phys. Lett. 96, 013504 (2010); http://dx.doi.org/10.1063/1.3284649 (3 pages) | Cited 6 times

Online Publication Date: 4 January 2010

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Currently, there is great interest in manufacturing multilayer polymer light emitting diode (PLED) structures via low-cost solution-based spin-casting or printing methods. The difficulty with this approach is that solvent from freshly deposited films often dissolves the underlying layers. This letter demonstrates that fully operational multilayer PLED structures can be fabricated via a solution process by embedding the hole transport material in cross-linked inert polymer matrices that protect the functional material while subsequent layers are deposited using the same solvent. The resulting devices exhibited greatly improved quantum efficiency compared with devices that did not employ cross-linked polymer matrices.
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85.60.Jb Light-emitting devices

Trapping effects in Al2O3/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistor investigated by temperature dependent conductance measurements

P. Kordoš, R. Stoklas, D. Gregušová, Š. Gaži, and J. Novák

Appl. Phys. Lett. 96, 013505 (2010); http://dx.doi.org/10.1063/1.3275754 (3 pages) | Cited 6 times

Online Publication Date: 5 January 2010

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Frequency dependent conductance measurements at varied temperature between 25 and 260 °C were performed to analyze trapping effects in the Al2O3/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors. The trap states with a time constant τT,f ≅ (0.1–1) μs (fast) and τT,s = 10 ms (slow) were identified. The conductance measurements at increased temperatures made it possible to evaluate the fast trap states in about a four times broader energy range than that from room temperature measurement. The density of the fast traps decreased from 1.4×1012 cm−2 eV−1 at an energy of 0.27 eV to about 3×1011 cm−2 eV−1 at ET = 0.6 eV. The density of the slow traps was significantly higher than that of the fast traps, and it increased with increased temperature from about 3×1012 cm−2 eV−1 at 25–35 °C to 8×1013 cm−2 eV−1 at 260 °C.
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85.30.Tv Field effect devices

Piezoelectric accelerometers for ultrahigh temperature application

Shujun Zhang, Xiaoning Jiang, Michael Lapsley, Paul Moses, and Thomas R. Shrout

Appl. Phys. Lett. 96, 013506 (2010); http://dx.doi.org/10.1063/1.3290251 (3 pages) | Cited 7 times

Online Publication Date: 6 January 2010

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High temperature sensors are of major importance to aerospace and energy related industries. In this letter, a high temperature monolithic compression-mode piezoelectric accelerometer was fabricated using YCa4O(BO3)3 (YCOB) single crystals. The performance of the sensor was tested as function of temperature up to 1000 °C and over a frequency range of 100–600 Hz. The accelerometer prototype was found to possess sensitivity of 2.4±0.4 pC/g, across the measured temperature and frequency range, indicating a low temperature coefficient. Furthermore, the sensor exhibited good stability over an extended dwell time at 900 °C, demonstrating that YCOB piezoelectric accelerometers are promising candidates for high temperature sensing applications.
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06.30.Gv Velocity, acceleration, and rotation
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
07.20.Ka High-temperature instrumentation; pyrometers

Efficient heterojunction solar cells on p-type crystal silicon wafers

Qi Wang, M. R. Page, E. Iwaniczko, Yueqin Xu, L. Roybal, R. Bauer, B. To, H.-C. Yuan, A. Duda, F. Hasoon, Y. F. Yan, D. Levi, D. Meier, Howard M. Branz, and T. H. Wang

Appl. Phys. Lett. 96, 013507 (2010); http://dx.doi.org/10.1063/1.3284650 (3 pages) | Cited 22 times

Online Publication Date: 6 January 2010

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Efficient crystalline silicon heterojunction solar cells are fabricated on p-type wafers using amorphous silicon emitter and back contact layers. The independently confirmed AM1.5 conversion efficiencies are 19.3% on a float-zone wafer and 18.8% on a Czochralski wafer; conversion efficiencies show no significant light-induced degradation. The best open-circuit voltage is above 700 mV. Surface cleaning and passivation play important roles in heterojunction solar cell performance.
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88.40.H- Solar cells (photovoltaics)
81.65.Rv Passivation
81.65.Cf Surface cleaning, etching, patterning
81.10.Fq Growth from melts; zone melting and refining

Large enhancement in photon detection sensitivity via Schottky-gated CdS nanowire nanosensors

Te-Yu Wei, Chi-Te Huang, Benjamin J. Hansen, Yi-Feng Lin, Lih-Juann Chen, Shih-Yuan Lu, and Zhong Lin Wang

Appl. Phys. Lett. 96, 013508 (2010); http://dx.doi.org/10.1063/1.3285178 (3 pages) | Cited 22 times

Online Publication Date: 6 January 2010

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The Schottky contact based photon detection was demonstrated using CdS (visible light responsive), silicon (indirect n-type oxygen-non-adsorbing), and CuO (indirect p-type oxygen-adsorbing) nanowire nanosensors. With changing one of the two nanowire-electrode contacts from ohmic to Schottky, detection sensitivities as high as 105% were achieved by the CdS nanowire nanosensor operated at the reverse bias mode of −8 V, which was 58 times higher than that of the corresponding ohmic contact device. The reset time was also significantly reduced. In addition, originally light nonresponsive silicon and CuO nanowires became light responsive when fabricated as a Schottky contact device. These improvements in photon detection can be attributed to the Schottky gating effect realized in the present nanosensor system by introducing a Schottky contact.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
68.43.Mn Adsorption kinetics
85.60.Gz Photodetectors (including infrared and CCD detectors)
73.30.+y Surface double layers, Schottky barriers, and work functions
07.10.Cm Micromechanical devices and systems
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Flexible high-frequency microwave inductors and capacitors integrated on a polyethylene terephthalate substrate

Lei Sun, Guoxuan Qin, Hai Huang, Han Zhou, Nader Behdad, Weidong Zhou, and Zhenqiang Ma

Appl. Phys. Lett. 96, 013509 (2010); http://dx.doi.org/10.1063/1.3280040 (3 pages) | Cited 11 times

Online Publication Date: 6 January 2010

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This letter reports the realization of bendable inductors and capacitors integrated on a polyethylene terephthalate substrate that can operate at high microwave frequencies. A low-temperature fabrication process compatible with flexible thin-film transistors (TFTs) was developed. By employing bendable dielectric materials, spiral inductors and metal-insulator-metal capacitors with high quality factors and high resonance frequencies were demonstrated. The effects of mechanical bending on the performance of inductors and capacitors were also measured and analyzed. These demonstrations combined with previously demonstrated microwave TFTs will lead to flexible radio-frequency and microwave systems in the future.
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84.40.Lj Microwave integrated electronics
84.32.Hh Inductors and coils; wiring
84.32.Tt Capacitors

Inverse temperature dependence of subthreshold slope in graphene nanoribbon tunneling transistors

Youngki Yoon and Sayeef Salahuddin

Appl. Phys. Lett. 96, 013510 (2010); http://dx.doi.org/10.1063/1.3280379 (3 pages) | Cited 3 times

Online Publication Date: 7 January 2010

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By performing an atomistic transport calculation, we examine the temperature dependence of the tunneling current in a graphene nanoribbon band-to-band tunneling transistor. The subthreshold swing is shown to be a nonlinear function of temperature, and in stark contrast to a conventional FET, the swing versus temperature shows a negative slope below a certain drain current. The nonlinear threshold voltage shift with temperature is also examined. A method to capture the distinguishing nonlinearity in the voltage shift with temperature and drain current is proposed.
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85.30.Tv Field effect devices
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
81.05.ue Graphene

Carbon nanotube film piezoresistors embedded in polymer membranes

Kangwon Lee, Seung S. Lee, Jung A Lee, Kwang-Cheol Lee, and Seungmuk Ji

Appl. Phys. Lett. 96, 013511 (2010); http://dx.doi.org/10.1063/1.3272686 (3 pages) | Cited 4 times

Online Publication Date: 7 January 2010

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We present carbon nanotube film (CNF) piezoresistors embedded in polymer membranes. CNFs by vacuum filtration are patterned on an Au-deposited Si-wafer and transferred onto the poly-dimethylsiloxane (PDMS) using the weak adhesion property between Au-layer and Si-wafer. Transmittance and I-V characteristic are measured to confirm transferred CNFs as transparent electrodes. The pressure sensor consists of CNF piezoresistors embedded in 130 μm thick circular PDMS membranes. The gauge factor of CNFs at different thickness is obtained around 10–20 when the resistance increases from 2.7 to 5.6 kΩ with applied pressure, which shows that CNFs can be used as transparent piezoresistors in polymer-based microelectromechanical systems.
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85.35.Kt Nanotube devices
07.10.Cm Micromechanical devices and systems
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
84.32.Ff Conductors, resistors (including thermistors, varistors, and photoresistors)

Contact resistance in few and multilayer graphene devices

A. Venugopal, L. Colombo, and E. M. Vogel

Appl. Phys. Lett. 96, 013512 (2010); http://dx.doi.org/10.1063/1.3290248 (3 pages) | Cited 45 times

Online Publication Date: 8 January 2010

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The contact resistance of metals on backgated graphene field-effect transistors is studied. The residual resistance obtained at high backgate voltage is found to be in excellent agreement with the extracted values of contact resistance from transfer length measurements on graphene flakes. The contact resistance is found to be a significant contributor to the total resistance of graphene-based devices. The specific contact resistance is shown to be independent of the applied backgate voltage and the number of graphene layers.
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85.30.Tv Field effect devices
81.05.ue Graphene
73.40.Cg Contact resistance, contact potential

Investigation of terbium scandate as an alternative gate dielectric in fully depleted transistors

M. Roeckerath, J. M. J. Lopes, E. Durğun Özben, C. Urban, J. Schubert, S. Mantl, Y. Jia, and D. G. Schlom

Appl. Phys. Lett. 96, 013513 (2010); http://dx.doi.org/10.1063/1.3275731 (3 pages) | Cited 4 times

Online Publication Date: 8 January 2010

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Terbium scandate thin films were deposited by e-gun evaporation on (100) silicon substrates. Rutherford backscattering spectrometry and x-ray diffraction studies revealed homogeneous chemical compositions of the films. A dielectric constant of 26 and CV-curves with small hystereses were measured as well as low leakage current densities of <1 nA/cm2. Fully depleted n-type field-effect transistors on thin silicon-on-insulator substrates with terbium scandate gate dielectrics were fabricated with a gate-last process. The devices show inverse subthreshold slopes of 80 mV/dec and a carrier mobility for electrons of 225 cm2/V∙s was extracted.
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68.55.aj Insulators
68.49.Sf Ion scattering from surfaces (charge transfer, sputtering, SIMS)
77.55.-g Dielectric thin films
77.80.Dj Domain structure; hysteresis
85.30.Tv Field effect devices
73.61.Ng Insulators
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