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10 Dec 2007

Volume 91, Issue 24, Articles (24xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 241101 (2007); http://dx.doi.org/10.1063/1.2821382 (3 pages)

S. N. Goda, S. Sensarn, M. Y. Shverdin, and G. Y. Yin
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An effective light trapping configuration for thin-film solar cells

Seung-Bum Rim, Shanbin Zhao, Shawn R. Scully, Michael D. McGehee, and Peter Peumans

Appl. Phys. Lett. 91, 243501 (2007); http://dx.doi.org/10.1063/1.2789677 (3 pages) | Cited 58 times

Online Publication Date: 10 December 2007

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Many thin-film solar cells make a compromise between achieving complete optical absorption using films that are thicker than the optical absorption length and achieving efficient conversion of the absorbed photons into photocurrent which is favored in thinner structures. We evaluate the performance of a V-shaped light trapping configuration that substantially increases the photocurrent generation efficiency for all angles of incidence and that is applicable to a broad class of low-cost thin-film solar cells. We experimentally demonstrate its effectiveness for small molecular weight and polymer organic solar cells. A 52% efficiency enhancement is obtained for a 170-nm-thick polymer cell.
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84.60.Jt Photoelectric conversion

Origin of the open circuit voltage of donor-acceptor solar cells: Do polaronic energy levels play a role?

Antonio Cravino

Appl. Phys. Lett. 91, 243502 (2007); http://dx.doi.org/10.1063/1.2817930 (3 pages) | Cited 21 times

Online Publication Date: 10 December 2007

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This letter analyzes the influence of the energy of the highest-energy occupied molecular orbital (HOMO) and the single occupied molecular orbital (SOMO) (i.e., the low-energy polaronic level) of different electron donors (D) on the open circuit voltage (VOC) value delivered by bilayer and bulk-heterojunction solar cells containing fullerenes as acceptor (A). The analysis further confirms that the VOC of these devices scales with the energy difference between the HOMO of the D and the lowest-energy unoccupied molecular orbital of the A. As observed, a better relationship is obtained if the SOMO energy of the D—extracted from photoinduced absorption spectra—is considered.
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84.60.Jt Photoelectric conversion
71.55.Ht Other nonmetals
71.38.-k Polarons and electron-phonon interactions

AlN avalanche photodetectors

R. Dahal, T. M. Al Tahtamouni, J. Y. Lin, and H. X. Jiang

Appl. Phys. Lett. 91, 243503 (2007); http://dx.doi.org/10.1063/1.2823588 (3 pages) | Cited 15 times

Online Publication Date: 10 December 2007

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Deep ultraviolet (DUV) avalanche photodetectors (APDs) based on an AlN/n-SiC Schottky diode structure have been demonstrated. The device with a mesa diameter of ∼ 100 μm exhibits a gain of 1200 at a reverse bias voltage of −250 V or a field of about 3 MV/cm. The cut-off and peak responsivity wavelengths of these APDs were 210 and 200 nm, respectively. This is the highest optical gain and shortest cut-off wavelength achieved for III-nitride based DUV APDs. It was also observed that the reverse breakdown voltage increases with decreasing device size, which suggests that the device performance is limited by the presence of dislocations. The breakdown voltage for dislocation-free AlN was deduced to be about 4.1 MV/cm. The present results further demonstrate the potential of AlN as an active DUV material for future optoelectronic device applications.
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85.60.Gz Photodetectors (including infrared and CCD detectors)

Accumulated body ultranarrow channel silicon transistor with extreme threshold voltage tunability

Ali Gokirmak and Sandip Tiwari

Appl. Phys. Lett. 91, 243504 (2007); http://dx.doi.org/10.1063/1.2818663 (3 pages) | Cited 2 times

Online Publication Date: 10 December 2007

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A side-gated ultranarrow channel (width <10 nm) silicon field effect transistor (FET) with extreme threshold voltage (Vt) tunability is described. A narrow inversion layer is formed on the top interface controlled by the top gate. The device body and side interfaces are accumulated by up to 1019 cm−3 holes, drawn from the substrate by negatively biased side gates (Vside), increasing Vt by 3 V, suppressing peripheral leakage currents and short channel effects. Vt response to Vside follows a square root behavior, dVt/dmath = 3.73 V/math, similar to that of body doping. Maximum linear tunability (dVt/dVside) exceeds −2 V/V, average dVt/dVside is −1.67 V/V.
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85.30.Tv Field effect devices

Device physics and guiding principles for the design of double-gate tunneling field effect transistor with silicon-germanium source heterojunction

Eng-Huat Toh, Grace Huiqi Wang, Lap Chan, Ganesh Samudra, and Yee-Chia Yeo

Appl. Phys. Lett. 91, 243505 (2007); http://dx.doi.org/10.1063/1.2823606 (3 pages) | Cited 14 times

Online Publication Date: 11 December 2007

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The device physics and guiding principles for the design of the double-gate tunneling field-effect transistor with silicon-germanium (SiGe) heterojunction source are discussed. Two dimensional device simulations were employed to study the influence of the position of the SiGe/Si heterojunction on band-to-band tunneling and device performance. It is established that band-to-band tunneling occurs at a distance of ∼ 4 nm from the gate edge in the source region. In order for the narrower bandgap of SiGe to play a dominant role, the overlap between the SiGe region and the gate should be such that the whole tunneling path of the electrons is located in SiGe. To harness the maximum benefits of the high band-to-band tunneling rate in SiGe, an overlap of ∼ 2 nm between the SiGe region and the gate would be required.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
81.05.Hd Other semiconductors

Blue-emitting InGaN–GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200 A/cm2

N. F. Gardner, G. O. Müller, Y. C. Shen, G. Chen, S. Watanabe, W. Götz, and M. R. Krames

Appl. Phys. Lett. 91, 243506 (2007); http://dx.doi.org/10.1063/1.2807272 (3 pages) | Cited 138 times

Online Publication Date: 12 December 2007

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Auger recombination is determined to be the limiting factor for quantum efficiency for InGaN–GaN (0001) light-emitting diodes (LEDs) at high current density. High-power double-heterostructure (DH) LEDs are grown by metal-organic chemical vapor deposition. By increasing the active layer thickness, DH LEDs can reach a maximum in quantum efficiency at current densities above 200 A/cm2. Encapsulated thin-film flip-chip DH LEDs with peak wavelength of 432 nm have an external quantum efficiency of 40% and output power of 2.3 W at 2 A.
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85.60.Jb Light-emitting devices

Effect of mechanical strain on mobility of polycrystalline silicon thin-film transistors fabricated on stainless steel foil

Po-Chin Kuo, Abbas Jamshidi-Roudbari, and Miltiadis Hatalis

Appl. Phys. Lett. 91, 243507 (2007); http://dx.doi.org/10.1063/1.2824812 (3 pages) | Cited 5 times

Online Publication Date: 12 December 2007

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The effect of uniaxial tensile strain parallel to the channel on mobility of polycrystalline silicon thin-film transistors (TFTs) on stainless steel foil has been investigated. The electron mobility increases by 20% while the hole mobility decreases by 6% as the strain increases to 0.5%, and both followed by saturation as the strain increases further. The off current decreases for both types of TFTs under strain. All TFTs remained functional at the applied strain of 1.13%.
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85.30.Tv Field effect devices

Collector characteristics and the differential optical gain of a quantum-well transistor laser

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

Appl. Phys. Lett. 91, 243508 (2007); http://dx.doi.org/10.1063/1.2824817 (3 pages) | Cited 7 times

Online Publication Date: 12 December 2007

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The collector electrical characteristics (I-V) of a transistor laser (TL) manifest directly the transport and recombination dynamics of the vital quantum-well (QW) base region, including as described here QW recombination state changes. By employing the continuity equations and an extension of the classic charge control model, we extract from the dc I-V characteristics the differential optical gain of a single QW TL showing the QW state changes. The results agree in form with calculations employing Fermi’s golden rule and the “staircaselike” density of states of a QW.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Temperature-mediated saturation and current-induced recovery of the Vf drift in 4H-SiC p-i-n diodes

Joshua D. Caldwell, Orest J. Glembocki, Robert E. Stahlbush, and Karl D. Hobart

Appl. Phys. Lett. 91, 243509 (2007); http://dx.doi.org/10.1063/1.2824391 (3 pages) | Cited 5 times

Online Publication Date: 13 December 2007

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4H-SiC is desirable for high power and temperature electronic applications. However, for bipolar devices, device operation induces Shockley stacking fault (SSF) expansion and a drift in the forward voltage drop (Vf). Here, we present results showing that the electrically induced SSF expansion and Vf drift saturate. The saturation voltage is strongly temperature dependent and decreases tenfold at 247 °C from 30 °C. In heavily faulted diodes, the injection of relatively high currents (<35A/cm2) at elevated temperatures ( ∼ 245 °C) actually induced a partial recovery of the drift. A possible driving force mechanism for SSF expansion based on these results is then presented.
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85.30.Kk Junction diodes

External electroluminescence measurements of InGaAs/InAlAs avalanche photodiodes

Hod Finkelstein, Sanja Zlatanovic, Yu-Hwa Lo, Sadik C. Esener, and Kai Zhao

Appl. Phys. Lett. 91, 243510 (2007); http://dx.doi.org/10.1063/1.2824463 (2 pages)

Online Publication Date: 14 December 2007

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The external efficiency of electroluminescence resulting from hot-carrier recombination has been studied in an InGaAs/InAlAs avalanche photodiode. An analytical model that quantifies this emission is presented. Experimental data suggest that the emission originates from an intrinsic layer above the multiplication region. This electroluminescence mechanism offers a novel way for frequency upconversion, where the upconverted frequency can be controlled with proper choice of device layers. Lastly, we report for the first time the optical absorption properties of In0.52Al0.48As.
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85.60.Dw Photodiodes; phototransistors; photoresistors
78.60.Fi Electroluminescence

Nanoscale field effect transistor for biomolecular signal amplification

Yu Chen, Xihua Wang, Mi K. Hong, Shyamsunder Erramilli, Pritiraj Mohanty, and Carol Rosenberg

Appl. Phys. Lett. 91, 243511 (2007); http://dx.doi.org/10.1063/1.2822445 (3 pages) | Cited 9 times

Online Publication Date: 14 December 2007

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We report amplification of biomolecular recognition signal in lithographically defined silicon nanochannel devices. The devices are configured as field effect transistors (FET) in the reversed source-drain bias region. The measurement of the differential conductance of the nanowire channels in the FET allows sensitive detection of changes in the surface potential due to biomolecular binding. Narrower silicon channels demonstrate higher sensitivity to binding due to increased surface-to-volume ratio. The operation of the device in the negative source-drain region demonstrates signal amplification. The equivalence between protein binding and change in the surface potential is described.
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85.30.Tv Field effect devices

Undoped polythiophene field-effect transistors with mobility of 1 cm2 V−1 s−1

B. H. Hamadani, D. J. Gundlach, I. McCulloch, and M. Heeney

Appl. Phys. Lett. 91, 243512 (2007); http://dx.doi.org/10.1063/1.2824845 (3 pages) | Cited 63 times

Online Publication Date: 14 December 2007

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We report on charge transport in organic field-effect transistors based on poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) as the active polymer layer with saturation field-effect mobilities as large as 1 cm2 V−1 s−1. This is achieved by employing Pt instead of the commonly used Au as the contacting electrode and allows for a significant reduction in the metal/polymer contact resistance. The mobility increases as a function of decreasing channel length, consistent with a Poole-Frenkel model of charge transport, and reaches record mobilities of 1 cm2 V−1 s−1 or more at channel lengths on the order of few microns in an undoped solution-processed polymer cast on an oxide gate dielectric.
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85.30.Tv Field effect devices

Resistive switching devices based on nanocrystalline solid electrolyte (AgI)0.5(AgPO3)0.5

H. X. Guo, B. Yang, L. Chen, Y. D. Xia, K. B. Yin, Z. G. Liu, and J. Yin

Appl. Phys. Lett. 91, 243513 (2007); http://dx.doi.org/10.1063/1.2825273 (3 pages) | Cited 12 times

Online Publication Date: 14 December 2007

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Resistive switching devices with a sandwich structure Ag/(AgI)0.5(AgPO3)0.5/Pt were fabricated on silicon(001) wafer by using the pulsed laser deposition method and the focused ion beam nanofabrication technique. (AgI)0.5(AgPO3)0.5 films deposited at room temperature show a nanocrystal structure and the composition of the films was identified by using x-ray photoelectron spectroscopy. The current-voltage characteristics of the Ag/(AgI)0.5(AgPO3)0.5/Pt memory units show good switching behaviors. The ratio of the conductance between the “ON” state (high conductance) and the “off” state (low conductance) reaches 1×106. The ON and OFF states can be effectively written, read, and erased up to 4×105 scanning cycles by using a set of voltage pulses with an amplitude less than 3 V. It also could be observed that the time for the writing and erasing operations could be less than 150 ns. The switching mechanism of the Ag/(AgI)0.5(AgPO3)0.5/Pt memory devices was ascribed to the possible formation and dissolution of Ag filaments in (AgI)0.5(AgPO3)0.5 films induced by the applied electrical pulses with different polarities.
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66.30.Pa Diffusion in nanoscale solids
84.30.Sk Pulse and digital circuits
85.35.-p Nanoelectronic devices

Electric-field-driven dielectric breakdown of metal-insulator-metal hafnium silicate

Byoung Hun Lee, Chang Yong Kang, Paul Kirsch, Dawei Heh, Chadwin D. Young, Hongbae Park, Jiwoon Yang, Gennadi Bersuker, Siddarth Krishnan, Rino Choi, and Hi-Deok Lee

Appl. Phys. Lett. 91, 243514 (2007); http://dx.doi.org/10.1063/1.2825288 (3 pages) | Cited 9 times

Online Publication Date: 14 December 2007

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The breakdown characteristics of the crystalline and amorphous hafnium (Hf) silicates have been studied using metal-insulator-metal capacitors. It is found that, although the distribution of time-dependent dielectric breakdown and charge to breakdown (QBD) are affected by crystallization, the breakdown is primarily controlled by the electric field rather than charge fluence.
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73.40.Rw Metal-insulator-metal structures
77.22.Jp Dielectric breakdown and space-charge effects
84.32.Tt Capacitors

Transport properties of field-effect transistor with Langmuir-Blodgett films of C60 dendrimer and estimation of impurity levels

Naoko Kawasaki, Takayuki Nagano, Yoshihiro Kubozono, Yuuki Sako, Yu Morimoto, Yutaka Takaguchi, Akihiko Fujiwara, Chih-Chien Chu, and Toyoko Imae

Appl. Phys. Lett. 91, 243515 (2007); http://dx.doi.org/10.1063/1.2824818 (3 pages) | Cited 12 times

Online Publication Date: 14 December 2007

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Field-effect transistor (FET) device has been fabricated with Langmuir-Blodgett films of C60 dendrimer. The device showed n-channel normally off characteristics with the field-effect mobility of 2.7×10−3 cm2 V−1 s−1 at 300 K, whose value is twice as high as that (1.4×10−3 cm2 V−1 s−1) for the FET with spin-coated films of C60 dendrimer. This originates from the formation of ordered π-conduction network of C60 moieties. From the temperature dependence of field-effect mobility, a structural phase transition has been observed at around 300 K. Furthermore, the density of states for impurity levels was estimated in the Langmuir-Blodgett films.
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85.75.Hh Spin polarized field effect transistors
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
81.05.ub Fullerenes and related materials
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
85.65.+h Molecular electronic devices
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