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24 May 2010

Volume 96, Issue 21, Articles (21xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 96, 213701 (2010); http://dx.doi.org/10.1063/1.3431628 (3 pages)

Satish Rao, Saurabh Raj, Stefan Balint, Carlota Bardina Fons, Susana Campoy, Montserrat Llagostera, and Dmitri Petrov
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Conduction band discontinuity and electron confinement at the SixGe1−x/Ge interface

G. Mazzeo, E. Yablonovitch, H. W. Jiang, Y. Bai, and E. A. Fitzgerald

Appl. Phys. Lett. 96, 213501 (2010); http://dx.doi.org/10.1063/1.3432066 (3 pages) | Cited 1 time

Online Publication Date: 24 May 2010

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Germanium rich heterostructures can constitute a valid alternative to Silicon for the confinement of single electron spins. The conduction band discontinuity in SiGe/Ge heterostructures grown on pure germanium substrate is predicted to allow the confinement of electrons in the germanium, and the conduction band profile of germanium rich heterostructures allow the implementation of g-factor modulation devices not possible in Silicon. We here prove that electrons can indeed be trapped at the Si0.1Ge0.9/Ge interface and we measure the height of the energy barrier to 0.55±0.05 eV by measuring the tunneling time of electrons as a function of the electric field.
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73.20.At Surface states, band structure, electron density of states
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Carrier dynamics of terahertz emission based on strained SiGe/Si single quantum well

K. M. Hung, J.-Y. Kuo, C. C. Hong, H. H. Cheng, G. Sun, and R. A. Soref

Appl. Phys. Lett. 96, 213502 (2010); http://dx.doi.org/10.1063/1.3432075 (3 pages)

Online Publication Date: 24 May 2010

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We report analysis of the carrier distribution during terahertz emission process with carrier–phonon interaction based on p-doped strained SiGe/Si single quantum-well. The results of this analysis show that a considerable number of carriers can penetrate the phonon wall to become “hot” carriers on an approximately picosecond timescale. These hot carriers relax after the removal of the applied voltage, generating a “second” emission in the measurement. This investigation provides an understanding of the carrier dynamics of terahertz emission and has an implication for the design of semiconductor terahertz emitters.
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68.65.Fg Quantum wells
81.07.St Quantum wells
61.72.U- Doping and impurity implantation
72.20.Ht High-field and nonlinear effects

Enhanced conversion efficiencies for pillar array solar cells fabricated from crystalline silicon with short minority carrier diffusion lengths

Heayoung P. Yoon, Yu A. Yuwen, Chito E. Kendrick, Greg D. Barber, Nikolas J. Podraza, Joan M. Redwing, Thomas E. Mallouk, Christopher R. Wronski, and Theresa S. Mayer

Appl. Phys. Lett. 96, 213503 (2010); http://dx.doi.org/10.1063/1.3432449 (3 pages) | Cited 33 times

Online Publication Date: 24 May 2010

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Radial n+–p+ junction solar cells composed of densely packed pillar arrays, 25-μm-tall and 7.5 μm in diameter, fabricated from p-type silicon substrates with extremely short minority carrier diffusion lengths are investigated and compared to planar cells. To understand the two times higher AM 1.5 efficiencies of the pillar array cells, dark and light I-V characteristics as well as spectral responses are presented for the two structures. The higher pillar array cell efficiencies are due to the larger short-circuit currents from the larger photon absorption thickness and the shorter carrier collection length, with a significant additional contribution from multiple reflections in the structure.
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88.40.hj Efficiency and performance of solar cells
88.40.jj Silicon solar cells

Efficient light management scheme for thin film silicon solar cells via transparent random nanostructures fabricated by nanoimprinting

Corsin Battaglia, Karin Söderström, Jordi Escarré, Franz-Josef Haug, Didier Dominé, Peter Cuony, Mathieu Boccard, Grégory Bugnon, Céline Denizot, Matthieu Despeisse, Andrea Feltrin, and Christophe Ballif

Appl. Phys. Lett. 96, 213504 (2010); http://dx.doi.org/10.1063/1.3432739 (3 pages) | Cited 19 times

Online Publication Date: 24 May 2010

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We propose the use of transparent replicated random nanostructures fabricated via nanoimprinting on glass as next-generation superstrates for thin film silicon solar cells. We validate our approach by demonstrating short-circuit current densities for p-i-n hydrogenated microcrystalline silicon solar cells as high as for state-of-the-art nanotextured ZnO front electrodes. Our methodology opens exciting possibilities to integrate a large variety of nanostructures into p-i-n solar cells and allows to systematically investigate the influence of interface morphology on the optical and electronic properties of the device in order to further improve device performance.
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88.40.jj Silicon solar cells
81.07.Oj Nanoelectromechanical systems (NEMS)
81.16.Nd Micro- and nanolithography
81.05.Dz II-VI semiconductors
82.45.Fk Electrodes
85.30.Kk Junction diodes

Fully lithography independent fabrication of nanogap electrodes for lateral phase-change random access memory application

Jiayong Zhang, Xiaofeng Wang, Xiaodong Wang, Huili Ma, Kaifang Cheng, Zhongchao Fan, Yan Li, An Ji, and Fuhua Yang

Appl. Phys. Lett. 96, 213505 (2010); http://dx.doi.org/10.1063/1.3431297 (3 pages) | Cited 2 times

Online Publication Date: 24 May 2010

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A nanogap electrode fabrication method was developed and nanogap electrode as small as 17 nm was achieved based on sacrificial spacer process and conventional lithography. We have transferred this method to lateral phase-change random access memory (PCRAM) device fabrication. The electrical characterizations of 4.6 μm gap width using conventional lithography and 88 nm width based on this technology are shown. It is found that the threshold voltage and the dc power consumption are remarkably decreased due to nanogap electrode process. Our method cannot only improve the fabrication efficiency of PCRAM but also be easily transferred to other nanoelectronics applications.
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85.40.-e Microelectronics: LSI, VLSI, ULSI; integrated circuit fabrication technology

Enhanced dissociation of charge-transfer states in narrow band gap polymer:fullerene solar cells processed with 1,8-octanedithiol

D. J. D. Moet, M. Lenes, M. Morana, H. Azimi, C. J. Brabec, and P. W. M. Blom

Appl. Phys. Lett. 96, 213506 (2010); http://dx.doi.org/10.1063/1.3435468 (3 pages) | Cited 11 times

Online Publication Date: 25 May 2010

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The improved photovoltaic performance of narrow band gap polymer:fullerene solar cells processed from solutions containing small amounts of 1,8-octanedithiol is analyzed by modeling of the experimental photocurrent. In contrast to devices that are spin coated from pristine chlorobenzene, these cells do not produce a recombination-limited photocurrent. Modeling of the experimental data reveals that a sixfold reduction in the decay rate of photogenerated bound electron–hole pairs can account for the marked increase in short-circuit current density and fill factor. At short-circuit conditions, the dissociation probability of bound pairs is found to increase from 48% to 70%.
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88.40.jr Organic photovoltaics
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions

Pretilt angle control and multidomain alignment of liquid crystals by using polyimide mixed with liquid crystalline prepolymer

Ki-Han Kim, Jong-In Baek, Byoung-Ho Cheong, Hwan-Young Choi, Sung Tae Shin, Jae Chang Kim, and Tae-Hoon Yoon

Appl. Phys. Lett. 96, 213507 (2010); http://dx.doi.org/10.1063/1.3435483 (3 pages) | Cited 4 times

Online Publication Date: 25 May 2010

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We propose a method to control the pretilt angle of liquid crystals by employing a mixture of the vertical alignment polyimide and the liquid crystalline prepolymer (LCP). The pretilt angle between 10° and 90° can be controlled continuously by changing the mixing ratio of the LCP. Multidomains with different pretilt angles can be formed because the pretilt angle between 50° and 90° can be obtained by varying the UV exposure time region by region under a fixed mixing ratio. We confirmed experimentally that proposed alignment layers are thermally stable.
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61.30.Vx Polymer liquid crystals
64.75.Cd Phase equilibria of fluid mixtures, including gases, hydrates, etc.

Photocurrent increase in n-i-p thin film silicon solar cells by guided mode excitation via grating coupler

Karin Söderström, Franz-Josef Haug, Jordi Escarré, Oscar Cubero, and Christophe Ballif

Appl. Phys. Lett. 96, 213508 (2010); http://dx.doi.org/10.1063/1.3435481 (3 pages) | Cited 26 times

Online Publication Date: 26 May 2010

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Angle resolved measurements of the external quantum efficiency of n-i-p single junction amorphous solar cell deposited on a grating structure show clearly defined peaks of enhanced photocurrent in the weakly absorbing region between 1.6 and 2.15 eV. We explain these absorption phenomena and their angular variation with the excitation of guided modes via grating coupling. Calculation using an equivalent flat multilayer system permits to relate the theoretical values with the experimental data.
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88.40.jj Silicon solar cells

Light-powered micromotor driven by geometry-assisted, asymmetric photon-heating and subsequent gas convection

Li-Hsin Han, Shaomin Wu, J. Christopher Condit, Nate J. Kemp, Thomas E. Milner, Marc D. Feldman, and Shaochen Chen

Appl. Phys. Lett. 96, 213509 (2010); http://dx.doi.org/10.1063/1.3431741 (3 pages) | Cited 2 times

Online Publication Date: 27 May 2010

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multimedia

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We report on the design, fabrication, and analysis of a light-driven micromotor. The micromotor was created from a nanoporous polymer with close-packed gold nanoparticles which generate heat by absorbing light. The blades of the micromotor were curved, forming convex and concave sides. Upon lateral irradiation, by geometric effect the convex side transfers more photon-generated heat to the surrounding gas molecules, causing a convective motion of gas and leading to the rotation of the micromotor. The light-driven motions of gas molecules were analyzed using molecular dynamics modeling.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
84.50.+d Electric motors

Digital circuits using self-shunted Nb/NbxSi1−x/Nb Josephson junctions

David Olaya, Paul D. Dresselhaus, Samuel P. Benz, Anna Herr, Quentin P. Herr, Alexander G. Ioannidis, Donald L. Miller, and A. W. Kleinsasser

Appl. Phys. Lett. 96, 213510 (2010); http://dx.doi.org/10.1063/1.3432065 (3 pages) | Cited 2 times

Online Publication Date: 27 May 2010

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Superconducting digital circuits based on Josephson junctions with amorphous niobium-silicon (a-NbSi) barriers have been designed, fabricated, and tested. Single-flux-quantum (SFQ) shift registers operated with ±30% bias margins, confirming junction reproducibility and uniformity. Static digital dividers operated up to 165 GHz for a single value of bias current, which was only marginally slower than circuits fabricated with externally shunted AlOx-barrier junctions having a comparable critical current density of 4.5 kA/cm2. In comparison, self-shunted a-NbSi junctions enabled a doubling in circuit density. This and their relatively thick 10 nm barriers could increase the yield of complex SFQ circuits.
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74.50.+r Tunneling phenomena; Josephson effects
74.25.Sv Critical currents
84.30.Sk Pulse and digital circuits

Improvement in the photon-induced bias stability of Al–Sn–Zn–In–O thin film transistors by adopting AlOx passivation layer

Shinhyuk Yang, Doo-Hee Cho, Min Ki Ryu, Sang-Hee Ko Park, Chi-Sun Hwang, Jin Jang, and Jae Kyeong Jeong

Appl. Phys. Lett. 96, 213511 (2010); http://dx.doi.org/10.1063/1.3432445 (3 pages) | Cited 28 times

Online Publication Date: 27 May 2010

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This study examined the impact of the passivation layer on the light-enhanced bias instability of Al–Sn–Zn–In–O (AT–ZIO) thin film transistors. The suitably passivated device exhibited only a threshold voltage (Vth) shift of 0.72 V under light-illuminated negative-thermal stress conditions, whereas the device without a passivation layer suffered from a huge negative Vth shift of >11.5 V under identical conditions. The photocreated hole trapping model could not itself explain this behavior. Instead, the light-enhanced Vth instability of the unpassivated device would result mainly from the photodesorption of adsorbed oxygen ions after exposing the AT–ZIO back-surface in an ambient atmosphere.
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85.30.Tv Field effect devices

Current-induced degradation of high performance deep ultraviolet light emitting diodes

Craig G. Moe, Meredith L. Reed, Gregory A. Garrett, Anand V. Sampath, Troy Alexander, Hongen Shen, Michael Wraback, Yuriy Bilenko, Maxim Shatalov, Jinwei Yang, Wenhong Sun, Jianyu Deng, and Remis Gaska

Appl. Phys. Lett. 96, 213512 (2010); http://dx.doi.org/10.1063/1.3435485 (3 pages) | Cited 10 times

Online Publication Date: 27 May 2010

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Lifetime measurements on single-chip, packaged 285 nm light-emitting diodes (LEDs) performed under constant current injection at 20 and 75 mA, were compared to the performance of unbiased LEDs baked at the equivalent operating junction temperatures. The thermally stressed devices showed a lesser degradation than those electrically stressed, indicating that elevated temperature alone does not cause degradation. Despite a decay to less than half of the initial power under current injection, time-resolved photoluminescence of the active region exhibits little change, while capacitance-voltage measurements imply that the reduced efficiency and power decay originate from the generation of point defects near the p-side of the p-n junction.
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85.60.Jb Light-emitting devices

Channel stress measurements of 45 nm node transistors with embedded silicon-germanium source and drain using ultraviolet Raman spectroscopy

C. P. Wong, J. Kasim, J. P. Liu, A. See, and Z. X. Shen

Appl. Phys. Lett. 96, 213513 (2010); http://dx.doi.org/10.1063/1.3431295 (3 pages) | Cited 1 time

Online Publication Date: 27 May 2010

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We report the use of ultraviolet Raman spectroscopy to measure the average channel stress for 45 nm node transistors with embedded silicon-germanium (SiGe) source and drain. Direct probing of the channel for stress measurements was made possible by removing the polycrystalline silicon gate using a simple and cost-free approach. We demonstrate the feasibility of this method for measuring channel stress of dense transistor structures with varying pitch lengths, with ∼ 80 nm SiGe selectively grown in the source and drain regions.
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78.30.-j Infrared and Raman spectra
85.30.Tv Field effect devices
78.30.Er Solid metals and alloys
78.40.Kc Metals, semimetals, and alloys
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Modeling the capacitance-voltage response of In0.53Ga0.47As metal-oxide-semiconductor structures: Charge quantization and nonparabolic corrections

T. P. O’Regan, P. K. Hurley, B. Sorée, and M. V. Fischetti

Appl. Phys. Lett. 96, 213514 (2010); http://dx.doi.org/10.1063/1.3436645 (3 pages) | Cited 10 times

Online Publication Date: 27 May 2010

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The capacitance-voltage (C-V) characteristic is calculated for p-type In0.53Ga0.47As metal-oxide-semiconductor (MOS) structures based on a self-consistent Poisson–Schrödinger solution. For strong inversion, charge quantization leads to occupation of the satellite valleys which appears as a sharp increase in the capacitance toward the oxide capacitance. The results indicate that the charge quantization, even in the absence of interface defects (Dit), is a contributing factor to the experimental observation of an almost symmetric C-V response for In0.53Ga0.47As MOS structures. In addition, nonparabolic corrections are shown to enhance the depopulation of the Γ valley, shifting the capacitance increase to lower inversion charge densities.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
81.05.Ea III-V semiconductors

Influence of uniaxial mechanical stress on the high frequency performance of metal-oxide-semiconductor field effect transistors on (100) Si wafer

Younggun Han, Masaaki Koganemaru, Toru Ikeda, Noriyuki Miyazaki, Woon Choi, and Hajime Tomokage

Appl. Phys. Lett. 96, 213515 (2010); http://dx.doi.org/10.1063/1.3428793 (3 pages) | Cited 1 time

Online Publication Date: 27 May 2010

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The effects of uniaxial mechanical stress on the radio frequency performance of n- and p-metal-oxide-semiconductor field effect transistors (MOSFETs) are investigated up to 10 GHz. Under tensile stress, the gate transconductance (gm) increases in the n-MOSFETs while it decreases in the p-MOSFETs, whereas the results were vice versa for compressive stress. The total gate capacitance (CG) extracted from scattering parameters increases (decreases) under tensile (compressive) stress for both n- and p-MOSFETs, which is explained by the variation in the effective mass perpendicular to the Si/SiO2 interface. The cut-off frequency (fT) varies in inverse proportion to the CG variation.
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85.30.Tv Field effect devices

Improvement of infrared detection using Ge quantum dots multilayer structure

M. Kolahdouz, A. Afshar Farniya, L. Di Benedetto, and H. H. Radamson

Appl. Phys. Lett. 96, 213516 (2010); http://dx.doi.org/10.1063/1.3441120 (3 pages) | Cited 7 times

Online Publication Date: 28 May 2010

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Monocrystalline SiGe/Si multiquantum dot and well structures have been manufactured/compared as thermistor materials for infrared detection. The performance of the devices (both the thermal and electrical) has been very sensitive to the quality of the epitaxial layers which is evaluated by the interfacial roughness and strain amount. This study demonstrates that the devices containing quantum dots have higher thermal coefficient resistance 3.4%/K with a noise constant (K1/f) value of 2×10−9.
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07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
68.55.ag Semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)
68.65.Fg Quantum wells
68.35.Ct Interface structure and roughness
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