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12 Mar 2007

Volume 90, Issue 11, Articles (11xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 90, 113108 (2007); http://dx.doi.org/10.1063/1.2714186 (3 pages)

Samuel L. Mensah, Vijaya K. Kayastha, Ilia N. Ivanov, David B. Geohegan, and Yoke Khin Yap
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Bipolar resistive switching in polycrystalline TiO2 films

K. Tsunoda, Y. Fukuzumi, J. R. Jameson, Z. Wang, P. B. Griffin, and Y. Nishi

Appl. Phys. Lett. 90, 113501 (2007); http://dx.doi.org/10.1063/1.2712777 (3 pages) | Cited 59 times

Online Publication Date: 12 March 2007

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Bipolar resistive switching was found in thin polycrystalline TiO2 films formed by the thermal oxidation of sputtered Ti films. With a Ag top electrode, TiO2 film, and Pt bottom electrode, bistable resistive switching with a low operating voltage and a good uniformity was observed repeatedly without an initial electrical “forming” process. This switching phenomenon might be described as the formation and rupture of a filamentary conductive path consisting of a chain of Ag atoms. The temperature dependence of the switching voltage is discussed in terms of interstitial ionic diffusion of Ag in the TiO2 matrix.
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84.30.Sk Pulse and digital circuits
85.40.Ry Impurity doping, diffusion and ion implantation technology
85.40.Sz Deposition technology
81.65.Mq Oxidation

Self-focused high frequency ultrasonic transducers based on ZnO piezoelectric films

Q. F. Zhou, C. Sharp, J. M. Cannata, K. K. Shung, G. H. Feng, and E. S. Kim

Appl. Phys. Lett. 90, 113502 (2007); http://dx.doi.org/10.1063/1.2712813 (3 pages) | Cited 19 times

Online Publication Date: 12 March 2007

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A micromachined self-focusing high frequency ultrasonic transducer was fabricated with a 13 μm thick ZnO film deposited on a silicon substrate by sputtering. X-ray diffraction shows that the film has a high (002) orientation. The element aperture size of the transducer was 2.5 mm, and the fundamental resonant frequency was designed to be over 200 MHz with approximately 28% bandwidth through only one matching layer. Experimental results show that this type of focused high frequency ultrasound device may have potential for cellular microstructure imaging and skin cancer detection.
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43.38.Fx Piezoelectric and ferroelectric transducers
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
87.63.D- Ultrasonography
43.80.Qf Medical diagnosis with acoustics
87.17.-d Cell processes
43.80.Vj Acoustical medical instrumentation and measurement techniques

Enhanced pyroelectric sensitivity using ferroelectric active mode detection

Joseph V. Mantese, Adolph L. Micheli, Norman W. Schubring, Michael W. Putty, Margarita P. Thompson, Scott Chang, John R. Troxell, Larry Oberdier, Jolanta Celinska, and Carlos Paz de Araujo

Appl. Phys. Lett. 90, 113503 (2007); http://dx.doi.org/10.1063/1.2713133 (3 pages) | Cited 2 times

Online Publication Date: 12 March 2007

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Active pyroelectric detection (APD), using ferroelectrics as the sensing materials, is described and compared to traditional-passive pyroelectric modes of operation. The active approach yields a number of distinct advantages over its passive counterparts, including greater effective pyroelectric coefficient and improved signal to noise ratio. Thin film test structures formed from strontium bismuth tantalate (SrBi2Ta2O9) are used to demonstrate the APD principle.
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77.55.-g Dielectric thin films
77.70.+a Pyroelectric and electrocaloric effects
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors

Thermal instability and the growth of the InGaAs/AlGaAs pseudomorphic high electron mobility transistor system

Joseph G. Pellegrino, Syed B. Qadri, Nadeemullah A. Mahadik, Mulpuri V. Rao, Wen F. Tseng, Robert Thurber, Donald Gajewski, and Jonathan Guyer

Appl. Phys. Lett. 90, 113504 (2007); http://dx.doi.org/10.1063/1.2713165 (3 pages)

Online Publication Date: 12 March 2007

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The effects of temperature overshoot during molecular beam epitaxy growth on the transport properties of conventionally and delta-doped pseudomorphic high electron mobility transistor (pHEMT) structures have been examined. A diffuse reflectance spectroscopy (DRS)-controlled versus a thermocouple (TC)-controlled, growth scheme is compared. Several advantages of the DRS-grown pHEMTs over the TC-controlled version were observed. Modest improvements in mobility, on the order of 2%–3%, were observed in addition to a 20% reduction in carrier freeze-out for the DRS-grown pHEMTs at 77 K.
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85.30.Tv Field effect devices
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.05.Ea III-V semiconductors

Operation of a reversed pentacene-fullerene discrete heterojunction photovoltaic device

D. M. Nanditha, M. Dissanayake, Ross A. Hatton, Richard J. Curry, and S. R. P. Silva

Appl. Phys. Lett. 90, 113505 (2007); http://dx.doi.org/10.1063/1.2713345 (3 pages) | Cited 24 times

Online Publication Date: 12 March 2007

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The photoresponse of reversed bilayer organic photovoltaic device based on pentacene and C60 is examined, and the mechanism of photocurrent generation is shown to be different to that in conventional heterojunction devices, with free charge carriers generated at the electrode-organic interfaces rather than the organic heterojunction. This hypothesis is tested with silver nanoclusters incorporated at the organic heterojunction to quench excitons and facilitate recombination of free charge carriers, which shows a predicted increase in Jsc. The large Voc in this reversed cell structure is also rationalized in the context of the model proposed.
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85.60.-q Optoelectronic devices
85.65.+h Molecular electronic devices
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Energy harvesting with piezoelectric drum transducer

Sheng Wang, Kwok Ho Lam, Cheng Liang Sun, Kin Wing Kwok, Helen Lai Wa Chan, Ming Sen Guo, and Xing-Zhong Zhao

Appl. Phys. Lett. 90, 113506 (2007); http://dx.doi.org/10.1063/1.2713357 (3 pages) | Cited 15 times

Online Publication Date: 12 March 2007

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Piezoelectric materials can convert ambient vibrations into electrical energy. In this letter, the capability of harvesting the electrical energy from mechanical vibrations in a dynamic environment through a piezoelectric drum transducer has been investigated. Under a prestress of 0.15 N and a cyclic stress of 0.7 N, a power of 11 mW was generated at the resonance frequency of the transducer (590 Hz) across an 18 kΩ resistor. It is found that the energy from the transducer increases while the resonance frequency of the transducer decreases when the prestress increases. The results demonstrate the potential of the drum transducer in energy harvesting.
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85.50.-n Dielectric, ferroelectric, and piezoelectric devices
46.40.-f Vibrations and mechanical waves

In situ electrical conductivity measurement of high-pressure molten (Mg0.875,Fe0.125)2SiO4

Ming Li, Chunxiao Gao, Yanzhang Ma, Duojun Wang, Yanchun Li, and Jing Liu

Appl. Phys. Lett. 90, 113507 (2007); http://dx.doi.org/10.1063/1.2712811 (3 pages) | Cited 7 times

Online Publication Date: 13 March 2007

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In situ resistance measurement of mantle mineral under high temperature and pressure has been considered an important method for studying the electrical properties and thermal states of Earth’s interior. Here the authors report the results of the electrical conductivity of molten olivine [(Mg0.875,Fe0.125)2SiO4] on diamond anvil cell with pressure at 13.2 GPa and temperature at 3720 K. The results indicate that the activation enthalpy of molten olivine is much less than that of solid, and its conductivity is relatively insensitive to temperature. Moreover, at the given temperature range the conductivity of molten olivine exhibits Arrhenius behavior perfectly. Compared to the results of Hawaiian tholeiite provided by Tyburczy and Waff [J. Geophys. Res. 88, 1413 (1983)] at lower pressure and temperature, the pressure effect on molten olivine conductivity is slightly weaker. This method for electrical conductivity measurement on laser-heated diamond anvil cell allows the environment simulation study of unresearched regions of Earth, no longer restricted to large cavity press.
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65.40.G- Other thermodynamical quantities
72.80.Sk Insulators

Influence of interface layer composition on the electrical properties of epitaxial Gd2O3 thin films for high-K application

Apurba Laha, H. J. Osten, and A. Fissel

Appl. Phys. Lett. 90, 113508 (2007); http://dx.doi.org/10.1063/1.2713142 (3 pages) | Cited 27 times

Online Publication Date: 13 March 2007

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The authors report on the impact of interface layer composition on electrical properties of epitaxial Gd2O3 thin films on Si(001) substrates. The electrical properties of epitaxial Gd2O3 thin films were improved significantly by controlled modification of interface layer composition. The minimum capacitance equivalent thickness estimated for Pt/Gd2O3/Si metal oxide semiconductor structures was as low as 0.76 nm with leakage current density of 15 mA/cm2 at (Vg−VFB) = 1 V. The corresponding density of interface states was found to be 2.3×1012 cm−2 eV−1. The authors also find that a change in the interface layer composition significantly alters band alignment of Gd2O3 layer with respect to Si substrates.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.20.At Surface states, band structure, electron density of states
73.61.Cw Elemental semiconductors
77.55.-g Dielectric thin films

Origin of the drain current bistability in polymer ferroelectric field-effect transistors

R. C. G. Naber, J. Massolt, M. Spijkman, K. Asadi, P. W. M. Blom, and D. M. de Leeuw

Appl. Phys. Lett. 90, 113509 (2007); http://dx.doi.org/10.1063/1.2713856 (3 pages) | Cited 30 times

Online Publication Date: 13 March 2007

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The authors present measurements that elucidate the mechanism behind the observed drain current bistability in ferroelectric field-effect transistors based on the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) as the gate dielectric. Capacitance-voltage measurements on metal-insulator-semiconductor diodes demonstrate that the bistability originates from switching between two states in which the ferroelectric gate dielectric is either polarized or depolarized. Pulsed charge displacement measurements on these diodes enable a direct measurement of the accumulated charge in the polarized state of 40±3 mC/m2.
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85.50.-n Dielectric, ferroelectric, and piezoelectric devices
85.30.Tv Field effect devices
77.80.Fm Switching phenomena
77.22.Ej Polarization and depolarization

Red and near infrared polarized light emissions from polyfluorene copolymer based light emitting diodes

Abay Gadisa, Erik Perzon, Mats R. Andersson, and Olle Inganäs

Appl. Phys. Lett. 90, 113510 (2007); http://dx.doi.org/10.1063/1.2713139 (3 pages) | Cited 18 times

Online Publication Date: 15 March 2007

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The authors report polarized red, electroluminescence peak at 705 nm and near infrared, electroluminescence peak at 950 nm, light emission from light emitting diodes based on two polyfluorene copolymers. The copolymers are synthesized from a fluorene monomer combined with donor-acceptor-donor comonomers and designed to have a low band gap and form birefringent liquid crystalline phases. Emission occurs from aligned thin films of polymer layers. The emissive layers are aligned by spin coating on a layer of rubbed conducting polymer poly(3,4-ethylene dioxythiophene)-poly(styrene sulphonate) and thermally converted into glassy nematic liquid crystalline state.
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85.60.Jb Light-emitting devices
42.79.Kr Display devices, liquid-crystal devices

Polarization transfer from photon to electron spin in g factor engineered quantum wells

H. Kosaka, Y. Mitsumori, Y. Rikitake, and H. Imamura

Appl. Phys. Lett. 90, 113511 (2007); http://dx.doi.org/10.1063/1.2709966 (3 pages) | Cited 8 times

Online Publication Date: 15 March 2007

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The authors demonstrate polarization transfer from a photon to an electron spin intermediated by a light-hole exciton in a GaAs/AlGaAs quantum well, which has an engineered electron g factor of less than 0.01 for an in-plane magnetic field. Negative spin polarization was clearly observed at the selective excitation of the light-hole exciton from two-color time-resolved Kerr rotation. This demonstration is a necessary step towards demonstrating coherence transfer from a photon to an electron spin, which is necessary for building a quantum repeater used for long distance quantum communications.
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73.63.Hs Quantum wells
72.25.Dc Spin polarized transport in semiconductors
78.67.De Quantum wells
71.35.-y Excitons and related phenomena
73.21.Fg Quantum wells
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation

Analytical description of the motion of an acoustic-driven atomic force microscope cantilever in liquid

Cédric Jai, Touria Cohen-Bouhacina, and Abdelhamid Maali

Appl. Phys. Lett. 90, 113512 (2007); http://dx.doi.org/10.1063/1.2713860 (3 pages) | Cited 15 times

Online Publication Date: 15 March 2007

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In this letter the authors present an analytical description that enables determining the motion of an acoustic-driven atomic force microscope cantilever in liquid. The authors show that for low quality factors the effective driving force that acts on the deflection motion depends on the damping. The authors derived equations that accurately give the amplitude and phase of the cantilever deflection and the authors also give the expressions of the damping and stiffness of the interaction.
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07.79.Lh Atomic force microscopes

Low-threshold-switch phototransistor based on Kirk effect and float-zone silicon

D. B. Zhou, D. J. Han, C. M. Sun, R. Yang, and K. Liang

Appl. Phys. Lett. 90, 113513 (2007); http://dx.doi.org/10.1063/1.2713129 (3 pages)

Online Publication Date: 16 March 2007

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Based on the Kirk effect, a n+pn−n+ phototransistor with low optical threshold switching on high purity float-zone silicon was demonstrated. Under a collector-emitter voltage range from 3 to 15 V, the phototransistor can turn on from a lower current state (range from 0.29 to 0.65 mA) to a higher current state (range from 2.47 to 13.20 mA) when it was triggered by illumination with a small critical optical power (range from 2.0 to 3.6 μW). The theoretical calculations on critical collector current density were compared with the experimental results. Negative differential resistance and latch characteristics were observed and discussed.
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85.60.Dw Photodiodes; phototransistors; photoresistors

Ferroelectric all-polymer hollow Bragg fibers for terahertz guidance

Maksim Skorobogatiy and Alexandre Dupuis

Appl. Phys. Lett. 90, 113514 (2007); http://dx.doi.org/10.1063/1.2713137 (3 pages) | Cited 25 times

Online Publication Date: 16 March 2007

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Design of hollow all-polymer Bragg fibers using periodic multilayers of ferroelectric polyvinylidene fluoride (PVDF) polymer and a low loss polycarbonate (PC) polymer is demonstrated. Efficient band gap guiding is predicted near the transverse optical frequency of a PVDF material in the terahertz regime. Optimal reflector designs are investigated in the whole terahertz region. Depending on frequency, the lowest loss hollow Bragg fiber can be one of the following: a photonic crystal fiber guiding in the band gap regime, a metamaterial fiber with a subwavelength reflector period, a single PC, or a PVDF tube.
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42.81.Wg Other fiber-optical devices
42.70.Jk Polymers and organics
42.15.Eq Optical system design
42.70.Qs Photonic bandgap materials
77.80.-e Ferroelectricity and antiferroelectricity
77.84.Jd Polymers; organic compounds

Ultraviolet-enhanced device properties in pentacene-based thin-film transistors

Jeong-M. Choi, D. K. Hwang, Jung Min Hwang, Jae Hoon Kim, and Seongil Im

Appl. Phys. Lett. 90, 113515 (2007); http://dx.doi.org/10.1063/1.2715033 (3 pages) | Cited 8 times

Online Publication Date: 16 March 2007

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The authors report on the ultraviolet (UV)-enhanced device properties in pentacene-based thin-film transistors (TFTs). Pentacene TFTs showed a degraded mobility and lowered saturation current after illumination by a high energy UV with 254 nm wavelength. However, under 364 nm UV these devices surprisingly displayed enhanced saturation current and also showed threshold voltage shift toward lower values, maintaining their mobilities. The saturation current increase and threshold voltage shift were further related to the negative fixed charges excessively formed at the pentacene/dielectric interface by the low energy UV. The authors thus conclude that a low energy UV could rather enhance the pentacene TFT performances and also control the threshold voltage of the device.
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85.30.Tv Field effect devices
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Evidence of enhanced phonon-energy coupling in SiO2/Si

Pangleen Ong and Zhi Chen

Appl. Phys. Lett. 90, 113516 (2007); http://dx.doi.org/10.1063/1.2714197 (3 pages) | Cited 3 times

Online Publication Date: 16 March 2007

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The authors designed special pn junctions to examine the recently discovered phonon-energy-coupling enhancement effect. They found that the breakdown voltage of the silicon substrate is increased by 0.3 V after rapid thermal process (RTP), whereas it remains the same for diodes annealed in furnace with the same parameters as those in RTP. The increase in breakdown voltage of silicon is caused neither by dopant redistribution nor increased contact resistance after RTP but due to its intrinsic properties, i.e., stronger Si–Si bonds. The strengthening of Si–Si bonds is caused by coupling of the phonon energy from silicon to thin oxide.
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85.30.Kk Junction diodes
61.72.Cc Kinetics of defect formation and annealing
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