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1 Jan 2007

Volume 90, Issue 1, Articles (01xxxx)

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Appl. Phys. Lett. 90, 012105 (2007); http://dx.doi.org/10.1063/1.2428402 (3 pages)

Jan Bauer, Frank Fleischer, Otwin Breitenstein, Luise Schubert, Peter Werner, Ulrich Gösele, and Margit Zacharias
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Tuning the threshold voltage of organic field-effect transistors by an electret encapsulating layer

M. Scharnberg, V. Zaporojtchenko, R. Adelung, F. Faupel, C. Pannemann, T. Diekmann, and U. Hilleringmann

Appl. Phys. Lett. 90, 013501 (2007); http://dx.doi.org/10.1063/1.2426926 (3 pages) | Cited 17 times

Online Publication Date: 2 January 2007

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Here, the authors present a technique to adjust the threshold voltage of an organic field-effect transistor (OFET) using a design similar to a dual-gate structure with an insulating Teflon-based electret layer as a second gate. The threshold voltage of a pentacene bottom gate OFET was shifted from +13.1 to −2.3 V by depositon of a 1.7 μm thick electret layer, proving the principal feasibility of this approach. This controlled tuning of the threshold voltage compensates one of the main drawbacks of organic electronics and even allows switching from a depletion to an enhancement-type transistor behavior.
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85.30.Tv Field effect devices

High-quality thin-film passivation by catalyzer-enhanced chemical vapor deposition for organic light-emitting diodes

Han-Ki Kim, Myung Soo Kim, Jae-Wook Kang, Jang-Joo Kim, and Min-Su Yi

Appl. Phys. Lett. 90, 013502 (2007); http://dx.doi.org/10.1063/1.2425021 (3 pages) | Cited 14 times

Online Publication Date: 2 January 2007

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The thin-film passivation of organic light-emitting diodes (OLEDs) by a SiNx film grown by catalyzer-enhanced chemical vapor deposition was investigated. Using a tungsten catalyzer connected in series, a high-density SiNx passivation layer was deposited on OLEDs and bare polycarbonate (PC) substrates at a substrate temperature of 50 °C. Despite the low substrate temperature, the single SiNx passivation layer, grown on the PC substrate, exhibited a low water vapor transmission rate of (2–6)×10−2 g/m2/day and a high transmittance of 87%. In addition, current-voltage-luminescence results of an OLED passivated with a 150-nm-thick SiNx film compared to nonpassivated sample were identical indicating that the performance of an OLED is not critically affected by radiation from tungsten catalyzer during the SiNx deposition. Moreover, the lifetime to half initial luminance of an OLED passivated with the single 150-nm-thick SiNx layer was 2.5 times longer than that of a nonpassivated sample.
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81.65.Rv Passivation
68.55.A- Nucleation and growth
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence

Impact of epitaxial growth at the heterointerface of a-Si:H/c-Si solar cells

Hiroyuki Fujiwara and Michio Kondo

Appl. Phys. Lett. 90, 013503 (2007); http://dx.doi.org/10.1063/1.2426900 (3 pages) | Cited 34 times

Online Publication Date: 2 January 2007

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The authors have demonstrated that interface structures of heterojunction solar cells consisting of hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) have quite large impact on the solar cell performance. In particular, unintentional epitaxial growth was found to occur during an intended a-Si:H i-layer growth on c-Si in plasma-enhanced chemical vapor deposition (PECVD). By the formation of the epitaxial layer at the interface, the solar cell efficiency decreases significantly. Their result shows that the epitaxial layer is formed rather easily in PECVD, even without the presence of H2 gas, and may have affected many previous studies on a-Si:H/c-Si solar cells seriously.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
52.77.Dq Plasma-based ion implantation and deposition
84.60.Jt Photoelectric conversion

Large enhancement of intersystem crossing in polyfluorenes by iridium-complex doping

Hua-Hsien Liao, Chia-Ming Yang, Chi-Hui Wu, Sheng-Fu Horng, Wei-Shan Lee, Hsin-Fei Meng, Jow-Tsong Shy, and Chain-Shu Hsu

Appl. Phys. Lett. 90, 013504 (2007); http://dx.doi.org/10.1063/1.2424667 (3 pages) | Cited 6 times

Online Publication Date: 3 January 2007

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Photoinduced absorption is used to study the intersystem crossing (ISC) rate of two polyfluorenes doped with iridium (III) tris[2-(4-totyl)pyridinato-N,C2]. It is found that the triplet exciton lifetime of polyfluorene is reduced by the dopants. But instead of decreasing, the population density of polyfluorene triplet exciton increases by almost one order of magnitude. The finding shows that the ISC rate can increase over 100 times due to the spin-orbital interaction with the Ir ion even though it is covered by the ligands. Specifically, the ISC lifetime changes from the intrinsic value of 62 ns to as short as 0.28 ns upon 10% doping.
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71.35.-y Excitons and related phenomena
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
61.72.up Other materials

Depolarization effects in (11math2)-oriented InGaN/GaN quantum well structures

Seoung-Hwan Park and Doyeol Ahn

Appl. Phys. Lett. 90, 013505 (2007); http://dx.doi.org/10.1063/1.2420795 (3 pages) | Cited 28 times

Online Publication Date: 4 January 2007

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Depolarization effects on electrical and optical properties of (11math2)-oriented wurtzite (WU) InGaN/GaN quantum well (QW) were investigated using the multiband effective-mass theory. These results are compared with those of (0001)- and (10math0)-oriented WU InGaN/GaN QW structures. The internal field is shown to become zero for (11math2) crystal orientation near the crystal angle of 56°, irrespective of the In composition in the well. This is because the sum of the piezoelectric and spontaneous polarizations in the barrier is equal to that in the well. The optical gain of the (11math2)-oriented QW is significantly larger than that of the (0001)-oriented QW. This is caused mainly by the increase of the optical matrix element due to the disappearance of the internal field. Also, the (11math2)-oriented QW is found to have the optical gain comparable to that of the (10math0)-oriented QW.
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68.65.Fg Quantum wells
78.67.De Quantum wells
77.22.Ej Polarization and depolarization
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
68.55.-a Thin film structure and morphology
77.65.Ly Strain-induced piezoelectric fields
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials

High capacitance density metal-insulator-metal structure based on Al2O3–HfTiO nanolaminate stacks

V. Mikhelashvili, G. Eisenstein, and A. Lahav

Appl. Phys. Lett. 90, 013506 (2007); http://dx.doi.org/10.1063/1.2425030 (3 pages) | Cited 16 times

Online Publication Date: 4 January 2007

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The authors describe metal-insulator-metal (MIM) capacitors comprising a five layer Al2O3–HfTiO nanolaminate stack with a total thickness of ∼ 29 nm. At room temperature they demonstrate a capacitance density of ∼ 14 fF/μm2, a leakage current density of ∼ 1×10−6 A/cm2 at 1 V, a temperature capacitance coefficient of 380 ppm/°C as well as corresponding linear and quadratic voltage coefficients of 350 ppm/V and 570 ppm/V2, respectively. They compare the structure to a MIM capacitor having a uniform HfTiO insulator film. That device exhibits a larger capacitance density ( ∼ 28 fF/μm2) but its electrical properties are found to be poorer than those of the structures utilizing the nanolaminate stack.
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84.32.Tt Capacitors
85.30.Tv Field effect devices

Effect of metal electrodes on rubrene single-crystal transistors

Taishi Takenobu, Tetsuo Takahashi, Jun Takeya, and Yoshihiro Iwasa

Appl. Phys. Lett. 90, 013507 (2007); http://dx.doi.org/10.1063/1.2408642 (3 pages) | Cited 33 times

Online Publication Date: 4 January 2007

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The authors herein have investigated the effect of the metal work function on the performance of rubrene single-crystal transistors using gold and calcium metal electrodes. The current-voltage characteristic is controlled by the metal work function, which offers the possibility of controlling the Schottky barrier height by the choice of the metal. In the process of the study of metal-rubrene contacts, the authors have realized an ambipolar transistor and a Schottky diode in an identical single-crystal device with asymmetric electrodes. These data provide direct evidence of the weak Fermi level pinning and formation of depletion layer on metal-rubrene contacts.
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85.30.Pq Bipolar transistors
85.30.Kk Junction diodes
85.30.Hi Surface barrier, boundary, and point contact devices

Signal amplification in a nanomechanical Duffing resonator via stochastic resonance

Ronen Almog, Stav Zaitsev, Oleg Shtempluck, and Eyal Buks

Appl. Phys. Lett. 90, 013508 (2007); http://dx.doi.org/10.1063/1.2430689 (3 pages) | Cited 21 times

Online Publication Date: 5 January 2007

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The authors experimentally study stochastic resonance in a nonlinear bistable nanomechanical doubly clamped beam resonator, which is capacitively excited by an adjacent gate electrode. The resonator is tuned to its bistability region by an intense pump near a point of equal transition rates between its two metastable states. The pump is amplitude modulated, inducing modulation of the activation barrier between the states. When noise is added to the excitation, resonator’s displacement exhibits noise dependent amplification of the modulation signal. They measure resonator’s response in the time and frequency domains, the spectral amplification, and the statistical distribution of the jump time.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
42.65.Pc Optical bistability, multistability, and switching, including local field effects
02.50.Ey Stochastic processes

Negative differential resistance in La0.67Ca0.33MnO3−δ/Nb–SrTiO3 p-n junction

Y. S. Xiao, X. P. Zhang, and Y. G. Zhao

Appl. Phys. Lett. 90, 013509 (2007); http://dx.doi.org/10.1063/1.2430266 (3 pages) | Cited 3 times

Online Publication Date: 5 January 2007

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La0.67Ca0.33MnO3−δ/Nb–SrTiO3 p-n junction was fabricated by pulsed laser deposition. The I-V curves of this junction show rectifying behavior. Negative differential resistance (NDR) was observed at low temperatures under large bias voltages and NDR becomes more remarkable with decreasing temperature. In addition to NDR, the I-V curves also show remarkable hysteresis. The results were explained in terms of the effect of local Joule heating on the phase separation in the strained ultrathin La0.67Ca0.33MnO3−δ thin film.
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72.20.Ht High-field and nonlinear effects
73.50.Fq High-field and nonlinear effects
73.40.Ei Rectification
64.75.-g Phase equilibria
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