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17 Oct 2005

Volume 87, Issue 16, Articles (16xxxx)

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Appl. Phys. Lett. 87, 161907 (2005); http://dx.doi.org/10.1063/1.2105998 (3 pages)

J. Das, K. B. Kim, F. Baier, W. Löser, and J. Eckert
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Unusual electromechanical effects in organic semiconductor Schottky contacts: Between piezoelectricity and electrostriction

Gilles Dennler, Christoph Lungenschmied, Niyazi Serdar Sariciftci, Reinhard Schwödiauer, Siegfried Bauer, and Howard Reiss

Appl. Phys. Lett. 87, 163501 (2005); http://dx.doi.org/10.1063/1.2103406 (3 pages) | Cited 18 times

Online Publication Date: 10 October 2005

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The converse electromechanical response in an organic Schottky contact follows a power law dependence S∝Vα of the mechanical strain S versus the applied voltage V, with an exponent α = 1.5, in between linear piezoelectricity and quadratic electrostriction. The experimental result is discussed within the frame of a model, where the Coulomb attraction between charged impurities present in the depletion zone of the Schottky contact and the charges accumulated in the metal at the interface with the semiconductor is considered. Electromechanical responses of such devices appear to be of fundamental interest for the investigation of nonuniform electric field distributions and for potential practical applications as electromechanical transducers.
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77.84.Jd Polymers; organic compounds
77.65.-j Piezoelectricity and electromechanical effects
73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts

Sodium hydroxide/aluminum bilayer cathode for enhancing the performance of blue polymer light-emitting diodes

Liang Ma, Zhiyuan Xie, Jun Liu, Junwei Yang, Yanxiang Cheng, Lixiang Wang, and Fosong Wang

Appl. Phys. Lett. 87, 163502 (2005); http://dx.doi.org/10.1063/1.2108116 (3 pages) | Cited 4 times

Online Publication Date: 11 October 2005

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Sodium hydroxide/aluminum (NaOH/Al) bilayer cathode has been developed for improving the performance of blue polymer light-emitting diodes (PLEDs) based on poly(9,9-dioctylfluorene). In comparison to the control device with Ca/Al as the cathode, the performance of the PLED with NaOH/Al cathode, including driving voltage, light-emitting efficiency, and color purity, is significantly improved. For achieving a luminance of 500 cd/m2, the driving voltage is reduced from 8.3 to 5.3 V and the light-emitting efficiency is increased from 0.46 to 0.67 cd/A. Moreover, the device with NaOH/Al cathode showed a pure blue emission with Commission Internationale de l’Eclairage coordinates of (0.17, 0.09) as compared to that of (0.18, 0.13) for control device operated at same condition. These improvements are attributed to the use of an efficient bilayer NaOH/Al electron injection contact that also prevents the formation of Ca-induced fluorenone occurred in PLEDs with Ca/Al cathode.
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82.45.Fk Electrodes
85.60.Jb Light-emitting devices

Improved oxide passivation of AlGaN/GaN high electron mobility transistors

B. P. Gila, M. Hlad, A. H. Onstine, R. Frazier, G. T. Thaler, A. Herrero, E. Lambers, C. R. Abernathy, S. J. Pearton, T. Anderson, S. Jang, F. Ren, N. Moser, R. C. Fitch, and M. Freund

Appl. Phys. Lett. 87, 163503 (2005); http://dx.doi.org/10.1063/1.2105987 (3 pages) | Cited 17 times

Online Publication Date: 12 October 2005

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MgO has proven effective in the past as a surface passivation layer to minimize current collapse in AlGaN/GaN high electron mobility transistors (HEMTs). However, MgO is not environmentally stable and more stable oxides need to be developed. MgCaO can be produced that is lattice matched to the GaN. Three samples were grown with 0%, 50% and 75% of Ca, which had respective lattice mismatches of −6.5% for MgO, −1% for Mg0.50Ca0.50O and +4% for Mg0.25Ca0.75O. Drain saturation current in HENTs had increases of 4.5% and 1%, respectively, for Mg0.5Ca0.5O and Mg0.25Ca0.75O passivated devices. However, there was a 10% decrease for the device passivated with pure MgO. This was due to strain applied on the nitride HEMT by the oxide, which is consistent with the piezoelectric effect in the nitride HEMT by the oxide, which is consistent with the piezoelectric effect in the nitride form the lattice mismatch between AlGaN and GaN. From pulsed measurements, HEMTs passivated with Mg0.5Ca0.5O and Mg0.25Ca0.75O showed higher passivation effectiveness (90% of dc current) then the MgO passivated HEMTs (83% dc current). This is due to the closer lattice matching of these calcium containing oxides and the reduction in interface traps associated with lattice mismatch.
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85.30.Tv Field effect devices
81.65.Rv Passivation
77.65.-j Piezoelectricity and electromechanical effects

Laser-ablation-assisted microparticle acceleration for drug delivery

V. Menezes, K. Takayama, T. Ohki, and J. Gopalan

Appl. Phys. Lett. 87, 163504 (2005); http://dx.doi.org/10.1063/1.2093930 (3 pages) | Cited 15 times

Online Publication Date: 12 October 2005

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Localized drug delivery with minimal tissue damage is desired in some of the clinical procedures such as gene therapy, treatment of cancer cells, treatment of thrombosis, etc. We present an effective method for delivering drug-coated microparticles using laser ablation on a thin metal foil containing particles. A thin metal foil, with a deposition of a layer of microparticles is subjected to laser ablation on its backface such that a shock wave propagates through the foil. Due to shock wave loading, the surface of the foil containing microparticles is accelerated to very high speeds, ejecting the deposited particles at hypersonic speeds. The ejected particles have sufficient momentum to penetrate soft body tissues, and the penetration depth observed is sufficient for most of the pharmacological treatments. We have tried delivering 1 μm tungsten particles into gelatin models that represent soft tissues, and liver tissues of an experimental rat. Sufficient penetration depths have been observed in these experiments with minimum target damage.
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87.80.-y Biophysical techniques (research methods)
42.62.Be Biological and medical applications
82.70.Gg Gels and sols

Numerical simulations of contact resistance in organic thin-film transistors

I. G. Hill

Appl. Phys. Lett. 87, 163505 (2005); http://dx.doi.org/10.1063/1.2112189 (3 pages) | Cited 34 times

Online Publication Date: 12 October 2005

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The origin of the source/drain contact resistance reported in studies of pentacene-based organic thin-film transistors (OTFTs) has been investigated using numerical device simulations. Quantitative agreement with published contact resistance values is obtained, using reasonable values for the physical parameters describing both the semiconductor material and the metal/organic interfaces. In particular, the difference in contact resistance measured in top and bottom contact OTFTs has been reproduced.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
73.40.Cg Contact resistance, contact potential

Semimetal-semiconductor rectifiers for sensitive room-temperature microwave detectors

A. C. Young, J. D. Zimmerman, E. R. Brown, and A. C. Gossard

Appl. Phys. Lett. 87, 163506 (2005); http://dx.doi.org/10.1063/1.2112201 (3 pages) | Cited 9 times

Online Publication Date: 14 October 2005

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In contrast to traditional metal-semiconductor (i.e., Schottky) junctions, ErAs:InAlGaAs heterojunctions have recently been shown to provide highly “engineerable” electrical rectification characteristics through the tuning of the Schottky barrier height and differential resistance, while maintaining the very low specific capacitance for which Schottky diodes are famous. We demonstrate that these new rectifiers can operate effectively at zero bias and can be extremely sensitive at microwave frequencies. We report a tangential sensitivity of −63 dBm at 3.1 GHz in an 8.0 KHz bandwidth, corresponding to a noise-equivalent power (NEP) of 8.9×10−13 W/Hz1/2. We attribute this noise performance to the high rf-to-dc current responsivity ( ∼ 8 A/W), zero bias operation, and noise source reduction. The new diodes are found to be ∼ 19 dB more sensitive than the best available Hewlett–Packard zero-bias diodes at comparable frequencies.
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85.30.Kk Junction diodes
85.30.Hi Surface barrier, boundary, and point contact devices
73.40.Ns Metal-nonmetal contacts
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