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16 Oct 2006

Volume 89, Issue 16, Articles (16xxxx)

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Appl. Phys. Lett. 89, 164101 (2006); http://dx.doi.org/10.1063/1.2362602 (3 pages)

Yen-Wen Lu and Chang-Jin(CJ) Kim
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On the function of a bathocuproine buffer layer in organic photovoltaic cells

M. Vogel, S. Doka, Ch. Breyer, M. Ch. Lux-Steiner, and K. Fostiropoulos

Appl. Phys. Lett. 89, 163501 (2006); http://dx.doi.org/10.1063/1.2362624 (3 pages) | Cited 53 times

Online Publication Date: 16 October 2006

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The role of bathocuproine (BCP) buffer layer inserted between active layer and Al contact in photovoltaic cells based on phthalocyanine (Pc) and C60 was investigated. Photoluminescence (PL) experiments show exciton quenching at the C60–Al interface to be strongly reduced by inserting BCP. Current-voltage characteristics of photovoltaic cells with planar geometry front electrode/Pc/C60/BCP/Al show that BCP dramatically improves electron transport out of the C60 film into the Al electrode. The tenfold increase in power conversion efficiency (η) with BCP can mostly be attributed to the latter effect. BCP does not improve η in photovoltaic cells with blend film geometry front electrode/Pc:C60/Al.
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68.55.-a Thin film structure and morphology
78.55.Kz Solid organic materials
71.35.-y Excitons and related phenomena
78.66.Tr Fullerenes and related materials
78.66.Qn Polymers; organic compounds
73.40.Ns Metal-nonmetal contacts

Production of a microbeam of slow highly charged ions with a tapered glass capillary

Tokihiro Ikeda, Yasuyuki Kanai, Takao M. Kojima, Yoshio Iwai, Tadashi Kambara, Yasunori Yamazaki, Masamitsu Hoshino, Takuya Nebiki, and Tadashi Narusawa

Appl. Phys. Lett. 89, 163502 (2006); http://dx.doi.org/10.1063/1.2362642 (3 pages) | Cited 44 times

Online Publication Date: 16 October 2006

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The authors have developed a method to produce a microbeam of slow highly charged ions based on a self-organized charge-up inside a tapered glass capillary. A transmission of 8 keV Ar8+ beam through the capillary 5 cm long with 800/24 μm inlet/outlet inner diameters was observed stably for more than 1200 s. The transmitted beam had the same size as the outlet with a beam density enhancement of approximately 10 and a divergence of ±5 mrad. The initial beam was guided through a capillary tilted by as large as ±100 mrad, and it still kept the incident charge.
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41.85.-p Beam optics
07.77.Ka Charged-particle beam sources and detectors

Separate domain formation in Ge2Sb2Te5–SiOx mixed layer

Tae-Yon Lee, Sung-Soo Yim, Dongbok Lee, Min-Hyun Lee, Dong-Ho Ahn, and Ki-Bum Kim

Appl. Phys. Lett. 89, 163503 (2006); http://dx.doi.org/10.1063/1.2362981 (3 pages) | Cited 19 times

Online Publication Date: 16 October 2006

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We report separate domain formation in cosputtered Ge2Sb2Te5–SiOx mixed layer, with SiOx amount less than 10 mol %. As-prepared Ge2Sb2Te5–SiOx layer exhibits amorphous phase with separate domains smaller than 20 nm. The separation maintains after thermal annealing, which results in crystallization into fcc phase. The crystallization activation energies of Ge2Sb2Te5–SiOx are obtained as 4.99 and 6.44 eV for mixed layers containing 5.3 and 8.4 mol % SiOx, respectively. Those are larger than 2.75 eV of pure Ge2Sb2Te5. Furthermore, the mixed layer exhibits sublimation at increased temperature. These are interpreted as formation of Ge2Sb2Te5-rich domains separated from each other by SiOx-rich domains.
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61.43.Dq Amorphous semiconductors, metals, and alloys
61.72.Cc Kinetics of defect formation and annealing
64.70.K- Solid-solid transitions
84.30.Sk Pulse and digital circuits

Silicon light-emitting transistor for on-chip optical interconnection

Shin-ichi Saito, Digh Hisamoto, Haruka Shimizu, Hirotaka Hamamura, Ryuta Tsuchiya, Yuichi Matsui, Toshiyuki Mine, Tadashi Arai, Nobuyuki Sugii, Kazuyoshi Torii, Shin’ichiro Kimura, and Takahiro Onai

Appl. Phys. Lett. 89, 163504 (2006); http://dx.doi.org/10.1063/1.2360783 (3 pages) | Cited 11 times

Online Publication Date: 16 October 2006

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The authors propose a light-emitting field-effect transistor with the active layer made of the ultrathin single crystal silicon with the (100) surface orientation. The ambipolar carrier injections from the highly impurity doped regions to the ultrathin silicon are achieved in complementary-metal-oxide-semiconductor compatible planar structures and the optical intensities are controlled by the gate voltage. By using the device, they have demonstrated that a simple electrical signal can be transferred by light and detected on the same silicon chip as photocurrents controlled by the gate bias.
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85.30.Tv Field effect devices
85.60.Jb Light-emitting devices
85.40.Ls Metallization, contacts, interconnects; device isolation
42.82.Ds Interconnects, including holographic interconnects

Buffer layer effect on the structural and electrical properties of rubrene-based organic thin-film transistors

J. H. Seo, D. S. Park, S. W. Cho, C. Y. Kim, W. C. Jang, C. N. Whang, K.-H. Yoo, G. S. Chang, T. Pedersen, A. Moewes, K. H. Chae, and S. J. Cho

Appl. Phys. Lett. 89, 163505 (2006); http://dx.doi.org/10.1063/1.2363940 (3 pages) | Cited 21 times

Online Publication Date: 16 October 2006

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The structural and electrical properties of organic thin-film transistors with rubrene/pentacene and pentacene/rubrene bilayered structures were investigated using x-ray diffraction, atomic force microscopy, and x-ray emission spectroscopy. High-quality rubrene thin films with orthorhombic structure were obtained in the rubrene/pentacene bilayer while the pentacene/rubrene bilayer only had an amorphous rubrene phase present. The rubrene/pentacene thin-film transistor shows more desirable current-voltage characteristics compared to the pentacene/rubrene transistor. The overall results suggest that the presence of a chemically active organic buffer layer and its associated crystal structure are crucial in enhancing the structural and electrical properties of rubrene-based transistors.
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85.30.Tv Field effect devices

Nonconductive polymer microresonators actuated by the Kelvin polarization force

Silvan Schmid, Michael Wendlandt, David Junker, and Christofer Hierold

Appl. Phys. Lett. 89, 163506 (2006); http://dx.doi.org/10.1063/1.2362590 (3 pages) | Cited 9 times

Online Publication Date: 17 October 2006

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The authors present a method, based on the Kelvin polarization force, to actuate nonconductive polymer microstructures. A proof of principle was conducted by finite element simulations. Microresonators made of SU-8 were fabricated and characterized under resonant conditions at applied ac voltage of 5 Vpp. A quality factor of Q = 87 in vacuum and a square dependence of the force on the applied voltage were obtained. The presented actuator design and fabrication do not require additional electrodes on the movable structure for actuation and thus allow for the full exploration of the exceptional variety of polymer materials for microscaled actuators and sensors.
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07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Quantitative analysis of ferroelectric domain imaging with piezoresponse force microscopy

Tobias Jungk, Ákos Hoffmann, and Elisabeth Soergel

Appl. Phys. Lett. 89, 163507 (2006); http://dx.doi.org/10.1063/1.2362984 (3 pages) | Cited 29 times

Online Publication Date: 18 October 2006

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The contrast mechanism for ferroelectric domain imaging via piezoresponse force microscopy (PFM) is investigated. A vectorial description of PFM measurements is presented which takes into account the background caused by the experimental setup. This allows a quantitative, frequency independent analysis of the domain contrast which is in good agreement with the expected values for the piezoelectric deformation of the sample and satisfies the generally required features of PFM imaging.
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77.80.Dj Domain structure; hysteresis
68.37.-d Microscopy of surfaces, interfaces, and thin films
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
77.65.-j Piezoelectricity and electromechanical effects

Effective suppression of Fermi level pinning in polycrystalline-silicon/high-k gate stack by using polycrystalline-silicon-germanium gate electrode

Xiongfei Yu, Chunxiang Zhu, and Mingbin Yu

Appl. Phys. Lett. 89, 163508 (2006); http://dx.doi.org/10.1063/1.2363144 (3 pages) | Cited 3 times

Online Publication Date: 18 October 2006

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In this study, the crucial issue of unacceptably high threshold voltage (Vth) induced by Fermi level pinning in poly-Si/high-k complementary metal-oxide-semiconductor field-effect transistor (MOSFET) was effectively suppressed by inserting a poly-SiGe gate electrode. The Vth of −1.02 V in poly-Si/HfO2 p-channel MOSFET was tuned to −0.81 V in poly-Si/Al2O3/HfO2 and further reduced to −0.49 V in poly-Si/poly-SiGe/Al2O3/HfO2 gate stack. Meanwhile, the Vth of 0.3 V was achieved in the n-channel MOSFET with the poly-SiGe gate. Moreover, transconductance and Vth stability in the MOSFETs with poly-SiGe gate were remarkably improved compared to poly-Si/HfO2 and poly-Si/Al2O3/HfO2 devices. The low Vth and good Vth stability observed in the devices with poly-SiGe gate may be mainly attributed to the suppressed formation of oxygen vacancies in high-k gate dielectric, which is commonly believed to cause the Fermi level pinning effect in poly-Si/high-k device.
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73.20.At Surface states, band structure, electron density of states
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
77.55.-g Dielectric thin films
85.30.Tv Field effect devices

Superconducting memory based on ferromagnetism

R. Held, J. Xu, A. Schmehl, C. W. Schneider, J. Mannhart, and M. R. Beasley

Appl. Phys. Lett. 89, 163509 (2006); http://dx.doi.org/10.1063/1.2362870 (3 pages) | Cited 9 times

Online Publication Date: 19 October 2006

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Answering to the need for dense superconducting memories, the authors propose a memory concept that combines ferromagnetic dots for the storage of the data and Josephson junctions for their readout. Good scalability is expected for large scale integration. Exploratory memory cells have been implemented using 3 μm Nb technology and Permalloy dots. Nonvolatile data storage at 300 K was demonstrated.
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84.30.Sk Pulse and digital circuits
85.25.Hv Superconducting logic elements and memory devices; microelectronic circuits
85.40.-e Microelectronics: LSI, VLSI, ULSI; integrated circuit fabrication technology

Formation and characterization of silicon/carbon nanotube/silicon heterojunctions by local synthesis and assembly

Takeshi Kawano, Dane Christensen, Supin Chen, Chung Yeung Cho, and Liwei Lin

Appl. Phys. Lett. 89, 163510 (2006); http://dx.doi.org/10.1063/1.2364151 (3 pages) | Cited 15 times

Online Publication Date: 19 October 2006

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This work investigates the formation of silicon/multiwalled carbon nanotube/silicon heterojunctions by in situ synthesizing carbon nanotubes between two heavily doped, suspended silicon microstructures that are separated 5–10 μm apart using the techniques of localized heating and electric-field-assisted self-assembly. The local electric field has the strength of 0.2–1 V/μm. Tip- and root-grown carbon nanotubes are observed to form two different heterojunction morphologies at the tips as the former stop to grow and the latter continue to grow as the growth tips of carbon nanotubes reach the cold silicon. Experimental measurements of the silicon/carbon nanotube/silicon system show linear current-voltage characteristics indicating Ohmic contact behavior.
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81.16.Dn Self-assembly
73.40.Ns Metal-nonmetal contacts
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Recombination zone in mixed-host organic light-emitting devices

Chih-Hung Hsiao, Yan-Hau Chen, Tien-Chun Lin, Chia-Chiang Hsiao, and Jiun-Haw Lee

Appl. Phys. Lett. 89, 163511 (2006); http://dx.doi.org/10.1063/1.2361266 (3 pages) | Cited 22 times

Online Publication Date: 19 October 2006

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In this letter, the authors had quantitatively investigated the recombination zone in the mixed-host (MH) emitting layer (EML) of an organic light-emitting device with different mixed ratios experimentally and theoretically. The MH-EML consisted of a hole-transport layer (HTL) and an electron-transport layer fabricated by coevaporation. When the mixed ratio of the HTL in the EML increases, the driving voltage increases then decreases; this can be well demonstrated by an electrical model with different carrier mobilities. A blueshift was also observed due to the solid state solvation effect combined with the exciton shift from the anode to the cathode side.
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85.60.Jb Light-emitting devices

High-mobility transistors based on nanoassembled carbon nanotube semiconducting layer and SiO2 nanoparticle dielectric layer

Wei Xue, Yi Liu, and Tianhong Cui

Appl. Phys. Lett. 89, 163512 (2006); http://dx.doi.org/10.1063/1.2361278 (3 pages) | Cited 16 times

Online Publication Date: 19 October 2006

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The authors report the fabrication and characterization of high-mobility thin-film transistors (TFTs) using layer-by-layer (LBL) nano self-assembled single-walled carbon nanotubes (SWCNTs) as the semiconducting material and SiO2 nanoparticles as the gate dielectric material. The channel length and the effective thickness of the SWCNT semiconductor layer are 50 μm and 38 nm, respectively. The effective thickness of the SiO2 dielectric layer is 180 nm. The SWCNT TFT exhibits p-type semiconductor characteristics and operates in the accumulation mode, with a hole mobility (μp) of 168.5 cm2/V s, a normalized transconductance (gm/W) of 0.5 S/m, a threshold voltage (Vth) of −3 V, and an on/off current ratio (Ion/off) of 4.2. The combination technique with LBL nano self-assembly and microlithography provides a simple, low-temperature, and highly efficient approach to fabricate inexpensive TFT devices.
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85.30.Tv Field effect devices
85.35.Kt Nanotube devices
81.16.Dn Self-assembly
85.40.Hp Lithography, masks and pattern transfer

Lateral reduction of random percolative networks formed by nanocrystals: Possibilities for a new concept electronic device?

Giovanni Pennelli

Appl. Phys. Lett. 89, 163513 (2006); http://dx.doi.org/10.1063/1.2361282 (3 pages) | Cited 4 times

Online Publication Date: 19 October 2006

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Percolative threshold dependence on the reduction of lateral dimension in random site networks is investigated by Monte Carlo simulations. Conductance of a large amount of random grain (nanocrystal) sets has been calculated as a function of the transverse dimension. Effect of a transverse applied field, acting on the grain charging energies, is evaluated and compared with the lateral reduction. Active devices (transistors) based on this concept could be fabricated and employed for multilevel device integration.
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72.10.-d Theory of electronic transport; scattering mechanisms
61.46.Hk Nanocrystals
73.63.Bd Nanocrystalline materials
85.35.-p Nanoelectronic devices
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Extraction of nitride trap density from stress induced leakage current in silicon-oxide-nitride-oxide-silicon flash memory

Shaw-Hung Gu, Tahui Wang, Wen-Pin Lu, Yen-Hui Joseph Ku, and Chih-Yuan Lu

Appl. Phys. Lett. 89, 163514 (2006); http://dx.doi.org/10.1063/1.2360180 (3 pages) | Cited 11 times

Online Publication Date: 20 October 2006

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The authors propose a technique to extract a silicon nitride trap density from stress induced leakage current in a polycrystalline silicon-oxide-nitride-oxide-silicon flash memory cell. An analytical model based on the Frenkel-Poole emission is developed to correlate a nitride trap density with stress induced leakage current. The extracted nitride trap density is 7.0×1012 cm−2 eV−1. They find that nitride trapped charges have a rather uniform distribution in an energy range of measurement ( ∼ 0.2 eV).
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84.32.Tt Capacitors
84.30.Sk Pulse and digital circuits
85.30.Tv Field effect devices

Application of metal-doped organic layer both as exciton blocker and optical spacer for organic photovoltaic devices

M. Y. Chan, S. L. Lai, K. M. Lau, C. S. Lee, and S. T. Lee

Appl. Phys. Lett. 89, 163515 (2006); http://dx.doi.org/10.1063/1.2362974 (3 pages) | Cited 31 times

Online Publication Date: 20 October 2006

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An effective optical spacer based on doping of ytterbium (Yb) metal into bathophenanthroline (BPhen) has been developed for applications in organic photovoltaic (OPV) devices. Utilizing Yb:BPhen as an optical spacer in standard copper phthalocyanine/C60 photovoltaic devices, power efficiency can be increased by four times to 3.42%. Ultraviolet photoemission spectroscopy measurements reveal that the good electron transport between C60 and Yb:BPhen is mainly related to the suitable energy level alignment at the interface. Combining with its high optical transparency and electrical conductivity, the Yb:BPhen film provides a useful means for maximizing the power conversion efficiency of OPV devices.
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78.66.Qn Polymers; organic compounds
61.72.up Other materials
73.61.Ph Polymers; organic compounds
79.60.Dp Adsorbed layers and thin films
79.60.Fr Polymers; organic compounds
71.35.-y Excitons and related phenomena

Air-stable n-channel copper hexachlorophthalocyanine for field-effect transistors

Mang-Mang Ling, Zhenan Bao, and Peter Erk

Appl. Phys. Lett. 89, 163516 (2006); http://dx.doi.org/10.1063/1.2362976 (3 pages) | Cited 17 times

Online Publication Date: 20 October 2006

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The authors report air-stable n-channel transistor performance of copper hexachlorophthalocyanine (Cl16CuPc) organic semiconductor. The charge carrier mobility is about 0.01 cm2/V s for thin films deposited at an elevated substrate temperature. Effects of substrate temperature, substrate surface chemical treatment, thin film morphology, and crystallinity on device performance are investigated. No significant decrease in mobility and on/off ratio was observed for devices stored in ambient environment.
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73.61.Ph Polymers; organic compounds
85.30.Tv Field effect devices
73.50.Dn Low-field transport and mobility; piezoresistance
72.20.Fr Low-field transport and mobility; piezoresistance
81.65.-b Surface treatments

InGaAs metal-oxide-semiconductor capacitors with HfO2 gate dielectric grown by atomic-layer deposition

N. Goel, P. Majhi, C. O. Chui, W. Tsai, D. Choi, and J. S. Harris

Appl. Phys. Lett. 89, 163517 (2006); http://dx.doi.org/10.1063/1.2363959 (3 pages) | Cited 37 times

Online Publication Date: 20 October 2006

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The influence of various process conditions on the structural integrity and electrical properties of Al/HfO2/p-In0.13Ga0.87As metal-oxide-semiconductor capacitors was investigated. Room temperature capacitance voltage measurements revealed postdielectric deposition anneal reduced hysteresis by more than 0.5 V and sulfur passivation of InGaAs improved the capacitance frequency dispersion properties as well as reduced interface trap density. At V = VFB−1 V, the leakage current densities ∼ 1.3×10−7, 0.4×10−6, and 1.3×10−6 A/cm2 were measured in devices with annealed HfO2 (110 and 32 Å) and sulfur-passivated InGaAs (110 Å unannealed HfO2), respectively. Transmission electron microscopy revealed sharp epitaxial InGaAs/crystalline HfO2 and GaAs/InGaAs interfaces.
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84.32.Tt Capacitors
85.30.Tv Field effect devices

Physics of ultrathin photovoltaics

V. G. Karpov, M. L. C. Cooray, and Diana Shvydka

Appl. Phys. Lett. 89, 163518 (2006); http://dx.doi.org/10.1063/1.2364136 (3 pages) | Cited 5 times

Online Publication Date: 20 October 2006

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The authors consider physical properties of ultrathin photovoltaics with thickness (≲1 μm) smaller than both the depletion width and diffusion length, applicable to the cases of amorphous, polycrystalline, and nanostructured devices. Three phenomena underlie the unique physics of such systems: (1) lateral screening by conducting electrodes, (2) leakiness due to defect assisted tunneling, and (3) gigantic capacitive energy conducive to shunting breakdown. The authors give numerical estimates and discuss practical implications of these phenomena.
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85.60.-q Optoelectronic devices
85.30.-z Semiconductor devices
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