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25 Jun 2007

Volume 90, Issue 26, Articles (26xxxx)

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Appl. Phys. Lett. 90, 262501 (2007); http://dx.doi.org/10.1063/1.2752015 (3 pages)

Ian Appelbaum and Douwe J. Monsma
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Improved dye-sensitized solar cells with a ZnO-nanoflower photoanode

C. Y. Jiang, X. W. Sun, G. Q. Lo, D. L. Kwong, and J. X. Wang

Appl. Phys. Lett. 90, 263501 (2007); http://dx.doi.org/10.1063/1.2751588 (3 pages) | Cited 136 times

Online Publication Date: 26 June 2007

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In this letter, the authors report a dye-sensitized solar cell (DSSC) using a ZnO-nanoflower film photoanode, which was grown by a hydrothermal method at 95 °C. The dye used was cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bis-tetrabutylam-monium (N-719). At AM1.5G irradiation with 100 mW/cm2 light intensity, the DSSC based on ZnO-nanoflower film showed an energy conversion efficiency of 1.9%, which is much higher compared to that (1.0%) of the control device constructed using a photoanode of upstanding ZnO-nanorod array fabricated by hydrothermal method as well. The better performance of ZnO-nanoflower DSSC was due to a better dye loading and light harvesting of the ZnO-nanoflower film. The results demonstrate potential application of ZnO-nanoflower array for efficient dye-sensitized solar cells.
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84.60.Jt Photoelectric conversion
82.45.Fk Electrodes

Lateral variation of target poisoning during reactive magnetron sputtering

D. Güttler, R. Grötzschel, and W. Möller

Appl. Phys. Lett. 90, 263502 (2007); http://dx.doi.org/10.1063/1.2752019 (3 pages) | Cited 5 times

Online Publication Date: 26 June 2007

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The reactive gas incorporation into a Ti sputter target has been investigated using laterally resolving ion beam analysis during dc magnetron deposition of TiN in an Ar/N2 atmosphere. At sufficiently low reactive gas flow, the nitrogen incorporation exhibits a pronounced lateral variation, with a lower areal density in the target racetrack compared to the target center and edge. The findings are reproduced by model calculations. In the racetrack, the balance of reactive gas injection and sputter erosion is shifted toward erosion. The injection of nitrogen is dominated by combined molecular adsorption and recoil implantation versus direct ion implantation.
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81.15.Cd Deposition by sputtering
81.40.Pq Friction, lubrication, and wear
62.20.Qp Friction, tribology, and hardness
61.80.Jh Ion radiation effects
68.55.A- Nucleation and growth
68.43.Mn Adsorption kinetics

Nanocrystalline CdS-water-soluble conjugated-polymers: High performance photoelectrochemical cells

Wonjoo Lee, Rajaram S. Mane, Sun-Ki Min, Tae Hyun Yoon, Sung-Hwan Han, and Soo-Hyoung Lee

Appl. Phys. Lett. 90, 263503 (2007); http://dx.doi.org/10.1063/1.2752021 (3 pages) | Cited 14 times

Online Publication Date: 26 June 2007

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Enhanced photoelectrochemical cell performance of nanocrystalline CdS-water-soluble conjugated-polymer sensitizers was demonstrated. The water-soluble polymers with quaternary pyridinium salts can be easily layered after dipping nanocrystalline CdS films in the aqueous polymer solution. The 2.37% energy conversion efficiency of CdS-poly(2-ethynyl-N-carboxypropylpyridiniumbromide) (LM 3) conjugated-polymer sensitizer was significantly higher than that of the bare (0.57%) and CdS-poly(2-ethynyl-N-aminopropylpyridiniumbromide) (LM 2) conjugated-polymer sensitizer (2.15%) under AM1.5 condition (80 mW/cm2).
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84.60.Jt Photoelectric conversion
82.47.Jk Photoelectrochemical cells, photoelectrochromic and other hybrid electrochemical energy storage devices

Negative index material composed of electric and magnetic resonators

Ruopeng Liu, Aloyse Degiron, Jack J. Mock, and David R. Smith

Appl. Phys. Lett. 90, 263504 (2007); http://dx.doi.org/10.1063/1.2752120 (3 pages) | Cited 24 times

Online Publication Date: 26 June 2007

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Negative index-of-refraction materials (NIMs) require both the electric permittivity and the magnetic permeability of the material to be negative. Most reported NIMs have combined conducting split ring resonators (SRRs) to realize the magnetic response and nonresonant wires to realize the electric response. Here, however, the authors demonstrate an impedance-matched, relatively broadband NIM which makes use of SRRs and electric resonators, tuned such that they resonate at the same frequency and with nearly the same frequency dispersion. The NIM properties are confirmed by direct measurements of the phase variation and field distributions, obtained by field mapping.
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42.70.-a Optical materials
42.79.-e Optical elements, devices, and systems
77.22.Ch Permittivity (dielectric function)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Hybrid AlN–SiC deep ultraviolet Schottky barrier photodetectors

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

Appl. Phys. Lett. 90, 263505 (2007); http://dx.doi.org/10.1063/1.2752126 (3 pages) | Cited 10 times

Online Publication Date: 26 June 2007

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Deep ultraviolet (DUV) Schottky barrier photodetectors have been demonstrated by exploiting the epitaxial growth of high quality AlN epilayer on n-type SiC substrate. The fabricated AlN/n-SiC hybrid Schottky barrier detectors exhibited a peak responsivity at 200 nm with very sharp cutoff wavelength at 210 nm, very high reverse breakdown voltages (>200 V), very low dark currents (about 10 fA at a reverse bias of 50 V), and high responsivity and DUV to UV/visible rejection ratio. These outstanding features are direct attributes of the fundamental material properties and high quality of AlN epilayers. The fabricated photodetectors also have a thermal energy limited detectivity at zero bias of about 1.0×1015 cm Hz1/2 W−1. These results demonstrated that AlN epilayers are an excellent candidate as an active material for DUV optoelectronic device applications.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
73.30.+y Surface double layers, Schottky barriers, and work functions
72.40.+w Photoconduction and photovoltaic effects

Ultrathin organic transistors for chemical sensing

Richard D. Yang, T. Gredig, Corneliu N. Colesniuc, Jeongwon Park, Ivan K. Schuller, William C. Trogler, and Andrew C. Kummel

Appl. Phys. Lett. 90, 263506 (2007); http://dx.doi.org/10.1063/1.2749092 (3 pages) | Cited 26 times

Online Publication Date: 26 June 2007

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Ultrathin cobalt phthalocyanine transistors of 4 ML have been fabricated for chemical sensing. Compared to 50 ML devices, the ultrathin transistors show faster response times, higher base line stabilities, and sensitivity enhancements of 1.5–20 for the five analytes tested. The enhanced response for the ultrathin transistors provides insight into the device physics. The absorption of analytes changes the surface doping level and trap energies. The changes in surface trap energies perturb the charge transport properties of the ultrathin devices, thereby, making these devices more sensitive.
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85.30.Tv Field effect devices
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
82.80.-d Chemical analysis and related physical methods of analysis

Device physics and design of double-gate tunneling field-effect transistor by silicon film thickness optimization

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

Appl. Phys. Lett. 90, 263507 (2007); http://dx.doi.org/10.1063/1.2748366 (3 pages) | Cited 12 times

Online Publication Date: 27 June 2007

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The device physics of the double-gate tunneling field-effect transistor (DG TFET) is explored through two dimensional device simulations. The on-state drain current Ion of the DG TFET, which is based on band-to-band tunneling, has a strong dependence on the silicon film thickness TSi and the physics governing it is detailed. It is established that band-to-band tunneling at the surface is very strong and accounts for a large part of the total drain current. However, a substantial part of the total drain current Ids is contributed by a subsurface portion of the silicon film. Detailed potential distributions show that the coupling of two gate electrodes in the DG TFET could effectively reduce the tunneling width ωT at the center of the silicon film up to an optimum TSi where maximum drain current is obtained.
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85.30.Tv Field effect devices

Upconversion injection in rubrene/perylene-diimide-heterostructure electroluminescent diodes

Ajay K. Pandey and Jean-Michel Nunzi

Appl. Phys. Lett. 90, 263508 (2007); http://dx.doi.org/10.1063/1.2752540 (3 pages) | Cited 14 times

Online Publication Date: 27 June 2007

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The authors implement and demonstrate in this letter a scheme that permits to drive electroluminescence with an extremely low turn-on voltage. The device behaves like compound semiconductors, in which the electroluminescence turn-on voltage is about the same as the open circuit voltage for the photovoltaic effect. However, the electroluminescence turn-on voltage is about half of the band gap of the emitting material that cannot be explained using current models of charge injection in organic semiconductors. The authors hereby propose explanation through an Auger-type two-step injection mechanism.
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85.30.Kk Junction diodes
85.60.Jb Light-emitting devices

High efficiency protocrystalline silicon/microcrystalline silicon tandem cell with zinc oxide intermediate layer

Seung Yeop Myong, Kobsak Sriprapha, Shinsuke Miyajima, Makoto Konagai, and Akira Yamada

Appl. Phys. Lett. 90, 263509 (2007); http://dx.doi.org/10.1063/1.2752736 (3 pages) | Cited 16 times

Online Publication Date: 27 June 2007

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The authors develop a hydrogenated protocrystalline silicon (pc-Si:H)/hydrogenated microcrystalline silicon (μc-Si:H) double-junction solar cell structure employing a boron-doped zinc oxide (ZnO:B) intermediate layer. Highly stable intrinsic pc-Si:H and μc-Si:H absorbers are prepared by a 60 MHz very-high-frequency plasma-enhanced chemical vapor deposition technique. Degenerate ZnO:B intermediate and back reflectors are deposited via a metal organic chemical vapor deposition technique. Because the ZnO:B intermediate layer reduces the potential thickness for the pc-Si:H absorber in the top cell, this double-juncion structure is a promising candidate to fabricate highly stable Si-based thin-film solar cells. Consequently, the high conversion efficiency of 12.0% is achieved.
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84.60.Jt Photoelectric conversion

Undoped homojunction chalcogen thin-film transistors on glass

Ki-Bong Song, Sang-Su Lee, Kyung-Am Kim, Jeong-Dae Suh, Jun-Ho Kim, Taek-Sung Lee, Byung-ki Cheong, and Won-Mock Kim

Appl. Phys. Lett. 90, 263510 (2007); http://dx.doi.org/10.1063/1.2753102 (3 pages) | Cited 3 times

Online Publication Date: 28 June 2007

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A technique to develop an undoped homojunction chalcogen thin-film transistor (UHJ-c-TFT) on glass is described. The UHJ-c-TFT is based on ternary chalcogenide alloy amorphous (α) Ge2Sb2Te5 (GST) and the positively cooled crystalline (χ) GST. The α-GST and the positively cooled χ-GST are used as a channel layer and ohmic contact layer, respectively. In the UHJ-c-TFT, the authors realize electric rectification by the energy difference from the Fermi level of the α-GST and the Fermi level of the positively cooled χ-GST. The UHJ-c-TFT shows the clear gate characteristics of a typical p-channel enhancement mode.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling

Enhanced efficiency of GaN-based light-emitting diodes with periodic textured Ga-doped ZnO transparent contact layer

Jinn-Kong Sheu, Y. S. Lu, Min-Lum Lee, W. C. Lai, C. H. Kuo, and Chun-Ju Tun

Appl. Phys. Lett. 90, 263511 (2007); http://dx.doi.org/10.1063/1.2753110 (3 pages) | Cited 28 times

Online Publication Date: 28 June 2007

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GaN-based light-emitting diodes (LEDs) with indium tin oxide (ITO)/Ga-doped ZnO (GZO) composite oxide films serving as a transparent contact layer (TCL) were demonstrated. In this study, the wall-plug efficiency of LEDs (LED-III) with textured ITO/GZO composite TCL can be markedly improved by 200% and 45% of magnitude as compared to conventional LEDs with Ni/Au TCL(LED-II) and planar ITO/GZO TCL(LED-I), respectively. Compared to LED-II, this enhancement is due to the enhanced light extraction efficiency of ITO/GZO composite TCL with high transparency. Compared to LED-I, ZnO-based TCL with a higher refractive index (n ∼ 2.0) allows further enhancement of light extraction through the creation of a textured structure on transparent conductive oxide TCL deposited on the top surface of LEDs. In addition, the ITO/GZO composite TCL with a thickness of 550 nm is far larger than that of Ni/Au TCL with a thickness of approximately 15 nm. Therefore, in addition to the effect of high transparency, the thicker ITO/GZO TCL with low lateral resistance would also act as a current-spreading layer leading to an enhancement of light extraction.
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85.60.Jb Light-emitting devices

Vibration energy harvesting device based on air-spaced piezoelectric cantilevers

Zhuo Wang and Yong Xu

Appl. Phys. Lett. 90, 263512 (2007); http://dx.doi.org/10.1063/1.2752726 (3 pages) | Cited 11 times

Online Publication Date: 28 June 2007

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This letter reports a piezoelectric device based on air-spaced composite cantilevers for vibration energy harvesting. With air-spaced piezoelectric cantilevers, the voltage generated due to vibrations is larger since the distance between the piezoelectric layer and the neutral plane is increased. In addition, the energy conversion efficiency is also increased. A simplified analytical model was developed to explain the advantages and facilitate the selection of design parameters. Finite element simulation was also carried out to verify the analytical results. A prototype was successfully assembled and tested.
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85.50.-n Dielectric, ferroelectric, and piezoelectric devices
02.70.Dh Finite-element and Galerkin methods

Wet oxidation of nitride layer implanted with low-energy Si ions for improved oxide-nitride-oxide memory stacks

V. Ioannou-Sougleridis, P. Dimitrakis, V. Em. Vamvakas, P. Normand, C. Bonafos, S. Schamm, N. Cherkashin, G. Ben Assayag, M. Perego, and M. Fanciulli

Appl. Phys. Lett. 90, 263513 (2007); http://dx.doi.org/10.1063/1.2752769 (3 pages) | Cited 2 times

Online Publication Date: 29 June 2007

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An alternative method for the formation of the top oxide in oxide-nitride-oxide dielectric stacks is proposed. This method combines low-energy (1 keV) silicon ion implantation into a thin nitride-oxide stack and subsequent low-temperature wet oxidation (850 °C for 15 min). Transmission electron microscopy shows that for an implanted dose of 1.5×1016 Si cm−2, an 8-nm-thick silicon oxide layer develops on the surface of the nitride-oxide stack. Time of flight secondary ion mass spectrometry reveals: (1) transformation of the implanted silicon nitride to an oxygen-rich-silicon nitride layer and (2) pilling up of nitrogen atoms at the bottom silicon/oxide-substrate interface. The resulting oxide-nitride-oxide stack exhibits strong charge storage effects and excellent charge retention properties leading to a 1.5 V, 10 yr extrapolated memory window at 125 °C. These results suggest that the proposed fabrication route may lead to gate dielectric stacks of substantial potential impact for mainstream nitride-based memory devices.
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81.65.Mq Oxidation
61.72.up Other materials
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
64.70.K- Solid-solid transitions
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
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