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20 Aug 2007

Volume 91, Issue 8, Articles (08xxxx)

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Appl. Phys. Lett. 91, 083504 (2007); http://dx.doi.org/10.1063/1.2772752 (3 pages)

Jeong-M. Choi, Jae Hoon Kim, and Seongil Im
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Spinodally synthesized magnetoelectric

Shenqiang Ren and Manfred Wuttig

Appl. Phys. Lett. 91, 083501 (2007); http://dx.doi.org/10.1063/1.2767174 (3 pages) | Cited 7 times

Online Publication Date: 20 August 2007

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Lead zirconium titanate/nickel ferrite (PZT/NFO) composites have been produced by crystallizing and spinodally decomposing a gel in a magnetic field below the Curie temperature of NFO. The gel had been formed by spinning a sol onto a silicon substrate. The ensuing microstructure, characterized by atomic force microscopy, magnetic force microscopy, (Lorentz) transmission electron microscopy, and scanning electron microscopy, is nanoscopically periodic and, determined by the direction of magnetic annealing field, anisotropic. The wavelength of the PZT/NFO alternation, 25 nm, agrees within a factor of 2 with the theoretically estimated value. The macroscopic ferromagnetic and magnetoelectric responses correspond qualitatively and semiquantitatively to the features of the nanostructure. The maximum of the field dependent magnetoelectric susceptibility equals 1.8 V/cm Oe.
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75.80.+q Magnetomechanical effects, magnetostriction
75.70.Ak Magnetic properties of monolayers and thin films
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Nt Magnetic annealing and temperature-hysteresis effects
75.30.Cr Saturation moments and magnetic susceptibilities
75.30.Gw Magnetic anisotropy

Pentacene-based thin-film transistors with multiwalled carbon nanotube source and drain electrodes

Chia-Hao Chang, Chao-Hsin Chien, and Jung-Yen Yang

Appl. Phys. Lett. 91, 083502 (2007); http://dx.doi.org/10.1063/1.2771532 (3 pages) | Cited 15 times

Online Publication Date: 20 August 2007

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In this letter, the authors propose a practical and reliable approach—using deposited multiwalled carbon nanotubes (MWCNTs) as source and drain electrodes—for reducing the contact resistance (Rc) in pentacene-based bottom-contact thin-film transistors. The value of Rc of the devices was closely linked to the resultant length of the deposited MWCNTs; the lowest value was 3×108 Ω μm. The largest saturation mobility was 0.14 cm2/V s; this value reached up to three times higher when the threshold voltage was determined using the maximum transconductance (Gm,max) extrapolation method, rather than the constant current method. The on/off ratio was more than 106.
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85.30.Tv Field effect devices

Optical limitations in thin-film low-band-gap polymer/fullerene bulk heterojunction devices

Nils-Krister Persson, Xiangjun Wang, and Olle Inganäs

Appl. Phys. Lett. 91, 083503 (2007); http://dx.doi.org/10.1063/1.2736280 (2 pages) | Cited 8 times

Online Publication Date: 20 August 2007

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Photovoltaic devices from the low-band-gap alternating copolymer APFO-Green1, blended with the fullerene derivative BTPF70 as electron acceptor, show a pronounced variation of the external quantum efficiency with varying thickness. Device simulation, based on ellipsometric characterization, reveals that this behavior is to be expected and valid also for most low-band-gap polymers and that it can be explained by optical interference. Requirements for materials suitable for wide spectral coverage in thin-film organic solar cells are delineated. Furthermore, the internal quantum efficiency is calculated to be ≈ 0.4.
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84.60.Jt Photoelectric conversion

High-gain pentacene-based inverter achieved through high and low energy ultraviolet treatments

Jeong-M. Choi, Jae Hoon Kim, and Seongil Im

Appl. Phys. Lett. 91, 083504 (2007); http://dx.doi.org/10.1063/1.2772752 (3 pages) | Cited 13 times

Online Publication Date: 21 August 2007

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The authors report on the fabrication of pentacene-based inverter with two p-channel thin-film transistors (TFTs) on polymer/AlOx bilayer dielectric, which has been patterned by high energy ultraviolet (UV) (254 nm) illumination. After pentacene channel growth on the dielectric, the inverter showed a high voltage gain of ∼ 10 under −6 V supply voltage (VDD) but at a transition voltage of −1 V which is too marginal to guarantee a desirable inverter operation between 0 and −6 V. When low energy UV (352 nm) was applied onto one of the two p TFTs, which plays as a load in the inverter circuit, the transition voltage shifted to an adequate value (−3 V) because the UV changes the threshold voltage of the load TFT to be lower. The UV-treated inverter demonstrated a high voltage gain of ∼ 150 under a VDD of −30 V.
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84.30.Jc Power electronics; power supply circuits
84.30.Qi Modulators and demodulators; discriminators, comparators, mixers, limiters, and compressors
85.30.Tv Field effect devices

Parylene cantilevers integrated with polycrystalline silicon piezoresistors for surface stress sensing

Rakesh Katragadda, Zhuo Wang, Waqas Khalid, Yuefa Li, and Yong Xu

Appl. Phys. Lett. 91, 083505 (2007); http://dx.doi.org/10.1063/1.2772189 (3 pages) | Cited 9 times

Online Publication Date: 22 August 2007

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Polymer based microcantilevers are known to have higher sensitivity than their silicon counterparts for surface stress sensing, owing to the lower Young’s modulus of polymers. This letter reports the development of parylene cantilevers integrated with polycrystalline silicon piezoresistors for the detection of chemical or biological molecules based on the surface stress sensing principle. The justification of using parylene is presented with theoretical calculations along with finite element simulation. The fabrication process employed for making these devices is also reported. The functionality of the sensor was preliminarily proven by the detection of octanethiol vapors. The stability of the sensor in water was demonstrated as well.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
82.80.-d Chemical analysis and related physical methods of analysis
07.10.Cm Micromechanical devices and systems
85.30.-z Semiconductor devices
84.32.Ff Conductors, resistors (including thermistors, varistors, and photoresistors)

On the origin of bistable resistive switching in metal organic charge transfer complex memory cells

T. Kever, U. Böttger, C. Schindler, and R. Waser

Appl. Phys. Lett. 91, 083506 (2007); http://dx.doi.org/10.1063/1.2772191 (3 pages) | Cited 26 times

Online Publication Date: 22 August 2007

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Electrical characteristics of Cu:tetracyanoquinodimethane (TCNQ) devices with different electrodes were studied. The comparison of impedance spectroscopic measurements on devices with Al and Pt top electrodes proved the existence of a high resistive interface layer between Cu:TCNQ and Al. An equivalent circuit was modeled and the resulting values suggest that the interface layer is composed of naturally formed aluminum oxide. Devices with deliberately formed aluminum oxide and without Cu:TCNQ were fabricated and revealed a similar behavior. The authors propose that the switching effect in Cu:TCNQ thin film devices is a Cu ion based electrochemical effect occurring in a thin aluminum oxide layer.
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84.30.Sk Pulse and digital circuits
85.40.Ls Metallization, contacts, interconnects; device isolation

Achieving ambipolar vertical organic transistors via nanoscale interface modification

Sheng-Han Li, Zheng Xu, Liping Ma, Chih-Wei Chu, and Yang Yang

Appl. Phys. Lett. 91, 083507 (2007); http://dx.doi.org/10.1063/1.2773749 (3 pages) | Cited 16 times

Online Publication Date: 22 August 2007

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Organic field-effect transistors have been the subject of much recent inquiry due to their unique properties. Here, the authors report an ambipolar vertical organic field-effect transistor, which consists of a capacitor cell vertically stacked with an organic active cell, separated by a thin source electrode. By inserting a nanoscale transition-metal-oxide layer at the source/organic interface, the authors fabricated the organic ambipolar transistors with low working voltage and high current output. The thin transition-metal oxide and partial oxidization metal grains form a unique nanostructure that balances the injection barrier height of two types of carriers at the source/organic contact.
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85.30.Tv Field effect devices
85.35.-p Nanoelectronic devices

Observation of hexagonal nuclei in the once melt-quenched Ge2Sb2Te5 phase change contact dimensions

Min Soo Youm, Yong Tae Kim, and Man Young Sung

Appl. Phys. Lett. 91, 083508 (2007); http://dx.doi.org/10.1063/1.2773758 (3 pages) | Cited 1 time

Online Publication Date: 22 August 2007

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The microstructures of once melt-quenched Ge2Sb2Te5 (GST) phase change contact dimensions are directly investigated with high resolution transmission electron microscopy (HR-TEM) by applying reset pulse of 7–13 V. The ovonic threshold switching voltage is decreased from 4.1 to 2.8 V when the as-deposited GST cells are once melt quenched by 10 V. HR-TEM reveals that there are hexagonal nuclei in the once melt-quenched GST and the GST can be partially left in not the amorphous but the crystalline state when the molten GST is not swiftly quenched, which is an origin of the switching failure.
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81.05.Gc Amorphous semiconductors
81.10.Fq Growth from melts; zone melting and refining
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
64.70.K- Solid-solid transitions

Widely tunable parametric amplifier based on a superconducting quantum interference device array resonator

M. A. Castellanos-Beltran and K. W. Lehnert

Appl. Phys. Lett. 91, 083509 (2007); http://dx.doi.org/10.1063/1.2773988 (3 pages) | Cited 45 times

Online Publication Date: 22 August 2007

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The authors create a Josephson parametric amplifier from a transmission line resonator whose inner conductor is made from a series of superconducting quantum interference device (SQUID) array. By changing the magnetic flux through the SQUID loops, they are able to adjust the circuit’s resonance frequency and the center of the amplified band between 4 and 7.8 GHz. They observe that the amplifier has gains as large as 28 dB and infers that it adds less than twice the input vacuum noise.
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84.30.Le Amplifiers
85.25.Dq Superconducting quantum interference devices (SQUIDs)
84.40.Az Waveguides, transmission lines, striplines

Evidence for attainability of negative differential conductance in tunnel Schottky structures with two-dimensional channels

Michael N. Feiginov and Igor N. Kotel’nikov

Appl. Phys. Lett. 91, 083510 (2007); http://dx.doi.org/10.1063/1.2773935 (3 pages) | Cited 3 times

Online Publication Date: 23 August 2007

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Experimental data, which demonstrate conductance decrease in the tunnel Al/GaAs Schottky structure with delta-n-doped two-dimensional channel, are presented. The conductance decreases due to the increase in the tunnel-barrier height with bias and the corresponding drop in the barrier tunnel transparency. Theoretical calculations show that the mechanism should lead to the negative value of the differential conductance if the separation between the subbands in the two-dimensional channel is sufficiently large. On the basis of calculations, an (ErAs)Al/InAlGaAs/InAlAs/InP heterostructure is suggested, where the negative differential conductance should be reached.
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73.40.Ns Metal-nonmetal contacts
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.30.+y Surface double layers, Schottky barriers, and work functions

Relationship between host energy levels and device performances of phosphorescent organic light-emitting diodes with triplet mixed host emitting structure

Sung Hyun Kim, Jyongsik Jang, and Jun Yeob Lee

Appl. Phys. Lett. 91, 083511 (2007); http://dx.doi.org/10.1063/1.2773941 (3 pages) | Cited 39 times

Online Publication Date: 23 August 2007

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Green phosphorescent organic light-emitting diodes (PHOLEDs) with a triplet mixed host emitting layer were developed and device performances were studied by changing host materials in light-emitting layer. Power efficiency of green PHOLEDs could be improved from 12.7 to 29.1 lm/W by using triplet mixed host emitting layer. Combination of hole-transport-type host with good hole injection properties and electron-transport-type host with good electron injection properties was effective to get high efficiency in triplet mixed host devices.
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85.60.Jb Light-emitting devices

Irradiation induced pulsations of reverse biased metal oxide/silicon structures

D. Fink, A. Kiv, D. Fuks, M. Tabacnics, M. de A. Rizutto, A. de O. D. Silva, A. Chandra, V. Golovanov, M. Ivanovskaya, and L. Khirunenko

Appl. Phys. Lett. 91, 083512 (2007); http://dx.doi.org/10.1063/1.2773950 (3 pages) | Cited 1 time

Online Publication Date: 23 August 2007

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Specific electronic features have been found in structures consisting of metal oxide layers on silicon substrates upon swift heavy ion irradiation. These features are linked to the appearance of radiation-induced negative differential resistances in the structures. In the reversed bias direction they show high frequency current pulsations at around ∼ 10 kHz frequency. Their amplitude increases with increasing applied voltage. The pulsation frequency also shows a small increase. The current amplitude depends on the ion fluence and flux.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
61.80.Jh Ion radiation effects

Depletion width measurement in an organic Schottky contact using a metal-semiconductor field-effect transistor

Arash Takshi, Alexandros Dimopoulos, and John D. Madden

Appl. Phys. Lett. 91, 083513 (2007); http://dx.doi.org/10.1063/1.2773953 (3 pages) | Cited 13 times

Online Publication Date: 23 August 2007

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Although the capacitance measurement is a common method to obtain the depletion width in a Schottky contact, the method is challenging in an organic Schottky junction since the capacitance is a combination of the capacitances associated with the trapped charges, bulk semiconductor, and the depletion region. The authors have implemented a metal-semiconductor field-effect transistor structure in order to estimate the depletion width in an organic Schottky contact. In the transistor the depletion width is calculated from the drain current at a small drain-source voltage. The result indicates a nonquadratic relation between the voltage and the depletion width.
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85.30.Tv Field effect devices

Ge nitride formation in N-doped amorphous Ge2Sb2Te5

M.-C. Jung, Y. M. Lee, H.-D. Kim, M. G. Kim, H. J. Shin, K. H. Kim, S. A. Song, H. S. Jeong, C. H. Ko, and M. Han

Appl. Phys. Lett. 91, 083514 (2007); http://dx.doi.org/10.1063/1.2773959 (3 pages) | Cited 26 times

Online Publication Date: 23 August 2007

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The chemical state of N in N-doped amorphous Ge2Sb2Te5 (a-GST) samples with 0–14.3 N at. % doping concentrations was investigated by high-resolution x-ray photoelectron spectroscopy (HRXPS) and Ge K-edge x-ray absorption spectroscopy (XAS). HRXPS showed negligible change in the Te 4d and Sb 4d core-level spectra. In the Ge 3d core-level spectra, a Ge nitride (GeNx) peak developed at the binding energy of 30.2 eV and increased in intensity as the N-doping concentration increased. Generation of GeNx was confirmed by the Ge K-edge absorption spectra. These results indicate that the N atoms bonded with the Ge atoms to form GeNx, rather than bonding with the Te or Sb atoms. It has been suggested that the formation of Ge nitride results in increased resistance and phase-change temperature.
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61.72.S- Impurities in crystals
79.60.Bm Clean metal, semiconductor, and insulator surfaces
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
78.70.Dm X-ray absorption spectra

Efficient deep blue emitters for organic electroluminescent devices

Meng-Huan Ho, Yao-Shan Wu, Shih-Wen Wen, Teng-Ming Chen, and Chin H. Chen

Appl. Phys. Lett. 91, 083515 (2007); http://dx.doi.org/10.1063/1.2773962 (3 pages) | Cited 16 times

Online Publication Date: 23 August 2007

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Highly efficient deep blue organic light emitting devices have been fabricated with a 4-(styryl)biphenyl-core based fluorescent dopant (SK-1) in the wide band gap 2-methyl-9,10-di(1-naphthyl)anthracene (α,α-MADN) host system which achieved an electroluminescence efficiency of 5.0 cd/A and an external quantum efficiency of 4.2% at 20 mA/cm2 with a saturated blue Commission Internationale de l’Eclairage coordinates of (0.15, 0.14) and a half-decay lifetime of 8000 h at an initial brightness of 100 cd/m2. The current efficiency and electroluminescent color of SK-1 doped devices have been shown to be essentially immune to drive current density.
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85.60.Jb Light-emitting devices

High temperature power performance of AlGaN/GaN high-electron-mobility transistors on high-resistivity silicon

S. Arulkumaran, G. I. Ng, Z. H. Liu, and C. H. Lee

Appl. Phys. Lett. 91, 083516 (2007); http://dx.doi.org/10.1063/1.2773987 (3 pages) | Cited 7 times

Online Publication Date: 23 August 2007

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High temperature power performance of AlGaN/GaN high-electron-mobility transistor (HEMT) on high-resistivity-(HR) Si for different microwave frequencies of 3, 6, and 8 GHz was studied. The device output power density (Pout) reduction with temperature for HR-Si-based AlGaN/GaN HEMTs is almost equivalent to the Pout reduction with temperature for semi-insulating–SiC substrate based AlGaN/GaN HEMTs. After high temperature stress, very small degradation was observed in the power performance at 3 GHz. Moreover, the rate of Pout decrease with the temperature is not much affected by measurement frequencies. The conduction mechanism of trap assisted impact ionization and temperature assisted tunneling were identified on Si3N4 passivated AlGaN/GaN HEMTs. The AlGaN/GaN HEMTs on HR-Si can also be used for high-power and high-frequency applications at elevated temperatures.
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85.30.Tv Field effect devices

Scattering of surface acoustic waves by a phononic crystal revealed by heterodyne interferometry

Kimmo Kokkonen, Matti Kaivola, Sarah Benchabane, Abdelkrim Khelif, and Vincent Laude

Appl. Phys. Lett. 91, 083517 (2007); http://dx.doi.org/10.1063/1.2768910 (3 pages) | Cited 15 times

Online Publication Date: 24 August 2007

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Surface acoustic wave propagation within a two-dimensional phononic band gap structure has been studied using a heterodyne laser interferometer. Acoustic waves are launched by interdigital transducers towards a square lattice of holes etched in a piezoelectric medium. Interferometer measurements performed at frequencies lying below, within, and above the expected band gap frequency range provide direct information of the wave interaction with the phononic crystal, revealing anisotropic scattering into higher diffraction orders depending on the apparent grating pitch at the boundary between the phononic crystal and free surface. Furthermore, the measurements also confirm the existence of an elastic band gap, in accordance with previous electrical measurements and theoretical predictions.
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63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
46.40.Cd Mechanical wave propagation (including diffraction, scattering, and dispersion)
77.65.Dq Acoustoelectric effects and surface acoustic waves (SAW) in piezoelectrics
62.65.+k Acoustical properties of solids
68.35.Iv Acoustical properties
78.20.hb Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects

Control of open-circuit voltage in organic photovoltaic cells by inserting an ultrathin metal-phthalocyanine layer

Yoshiki Kinoshita, Taku Hasobe, and Hideyuki Murata

Appl. Phys. Lett. 91, 083518 (2007); http://dx.doi.org/10.1063/1.2775085 (3 pages) | Cited 36 times

Online Publication Date: 24 August 2007

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The authors develop organic photovoltaic cells with multicharge separation (MCS) interfaces by inserting a very thin layer of metal phthalocyanine. The devices with MCS interface allow one to control short-circuit current density (Jsc) and open-circuit voltage (Voc). The power conversion efficiency (ηp) of the device with MCS interface (Cu-phthalocyanine/C60 and pentacene/C60) is enhanced compared with that of the device with single charge separation interface (pentacene/C60). The enhancement of ηp is attributable to the increase in Voc with maintaining the Jsc. By using Zn-phthalocyanine, which possesses longer excited lifetime compared with Cu-phthalocyanine, both Jsc and Voc have been improved simultaneously and the ηp reaches 2.04%.
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84.60.Jt Photoelectric conversion
73.40.Ns Metal-nonmetal contacts
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