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25 Sep 2006

Volume 89, Issue 13, Articles (13xxxx)

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Appl. Phys. Lett. 89, 131108 (2006); http://dx.doi.org/10.1063/1.2356892 (3 pages)

Paul E. Barclay, Kartik Srinivasan, Oskar Painter, Benjamin Lev, and Hideo Mabuchi
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Effects of metal penetration into organic semiconductors on the electrical properties of organic thin film transistors

Jeong Ho Cho, Do Hwan Kim, Yunseok Jang, Wi Hyoung Lee, Kyuwook Ihm, Jin-Hee Han, Sukmin Chung, and Kilwon Cho

Appl. Phys. Lett. 89, 132101 (2006); http://dx.doi.org/10.1063/1.2357155 (3 pages) | Cited 28 times

Online Publication Date: 25 September 2006

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To investigate the effects of metal penetration into organic semiconductors on the electrical properties of organic thin film transistors, gold was deposited onto pentacene films at various deposition rates. The sharp interface between the gold electrode and the pentacene film that results from a fast deposition rate was found to produce lower contact resistance and an increase in the field-effect mobility.
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85.30.Tv Field effect devices
73.40.Ns Metal-nonmetal contacts
73.40.Cg Contact resistance, contact potential

Examination of surface acoustic wave reflections by observing acoustoelectric current generation under pulse modulation

M. R. Astley, M. Kataoka, R. J. Schneble, C. J. B. Ford, C. H. W. Barnes, D. Anderson, G. A. C. Jones, H. E. Beere, D. A. Ritchie, and M. Pepper

Appl. Phys. Lett. 89, 132102 (2006); http://dx.doi.org/10.1063/1.2355444 (3 pages) | Cited 8 times

Online Publication Date: 25 September 2006

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The authors investigate surface acoustic wave (SAW) reflections using the frequency dependence of the acoustoelectric current through a one-dimensional channel. By pulse modulating the transducer power with a short pulse width (0.2 μs) and matching the pulse repetition period to the delay time of a specific reflection path, the authors detect individual reflected SAW wave packets. Fourier transform results show that reflections from the sample edge behind the transducer and Bragg reflections from the second transducer have the strongest effects. The reflection power coefficients for these are estimated to be ≈ 0.29 and ≈ 0.99, respectively.
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43.25.Fe
43.38.Fx
43.38.Rh
43.20.El
85.50.-n Dielectric, ferroelectric, and piezoelectric devices

Minority-carrier characteristics of InGaAs metal-oxide-semiconductor structures using atomic-layer-deposited Al2O3 gate dielectric

Y. Xuan, P. D. Ye, H. C. Lin, and G. D. Wilk

Appl. Phys. Lett. 89, 132103 (2006); http://dx.doi.org/10.1063/1.2357566 (3 pages) | Cited 8 times

Online Publication Date: 25 September 2006

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Atomic layer deposition (ALD) provides a unique opportunity to integrate high-quality gate dielectrics on III-V compound semiconductors. ALD Al2O3 is a high-quality gate dielectric on III-V compound semiconductor with low interface trap density, low gate leakage, and high thermal stability. The authors study the minority-carrier response of Al2O3/InGaAs metal-oxide-semiconductor (MOS) structures, which sheds light on the device physics for realizing high-performance inversion-type metal-oxide-semiconductor field-effect-transistor. The minority carriers in InGaAs do not respond to a small ac signal down to 100 Hz at 300 K, while they respond to up to 100 kHz at 500 K. Temperature dependent capacitance-voltage (C-V) measurement on the InGaAs MOS structure reveals the activation energy (Ea) of the minority-carrier recombination to be about 0.62 eV.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.A- Nucleation and growth

Observation of standing waves at steps on the GaN(0001) pseudo-(1×1) surface by scanning tunneling spectroscopy at room temperature

M. L. Harland and L. Li

Appl. Phys. Lett. 89, 132104 (2006); http://dx.doi.org/10.1063/1.2357570 (3 pages) | Cited 4 times

Online Publication Date: 25 September 2006

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Standing waves formed at steps of the GaN(0001) pseudo-(1×1) surface are observed with scanning tunneling spectroscopy (STS) at room temperature. The oscillatory behavior of the dI/dV calculated for free electrons scattered by atomic steps agrees well with that measured by STS suggesting that surface state of the pseudo-(1×1) structure is nearly free-electron-like thereby confirming the metallic state of the GaN(0001) pseudo-(1×1) surface.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.35.B- Structure of clean surfaces (and surface reconstruction)

Terahertz parametric gain in semiconductor superlattices in the absence of electric domains

Timo Hyart, Natalia V. Alexeeva, Ahti Leppänen, and Kirill N. Alekseev

Appl. Phys. Lett. 89, 132105 (2006); http://dx.doi.org/10.1063/1.2357579 (3 pages) | Cited 7 times

Online Publication Date: 25 September 2006

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The authors theoretically show that conditions for terahertz gain and conditions for formation of destructive electric domains in semiconductor superlattices are fairly different in the case of parametric generation and amplification. Action of an unbiased high-frequency electric field on a superlattice causes a periodic variation of energy and effective mass of miniband electrons. This parametric effect can result in a significant gain at some even harmonic of the pump frequency without the formation of electric domains and corruption from pump harmonics.
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73.63.-b Electronic transport in nanoscale materials and structures
72.30.+q High-frequency effects; plasma effects

Carrier mobility of organic thin films using lateral electrode structure with optical slits

Atsushi Kuwahara, Shigeki Naka, Hiroyuki Okada, and Hiroyoshi Onnagawa

Appl. Phys. Lett. 89, 132106 (2006); http://dx.doi.org/10.1063/1.2352731 (3 pages) | Cited 6 times

Online Publication Date: 25 September 2006

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The authors have studied the carrier mobilities of organic thin films using a time-of-flight (TOF) technique with lateral electrode structures and optical slits. For a triphenylamine derivative (TPD), which is a typical hole transport material, the obtained mobility was similar to the results of the conventional TOF measurement using a sandwiched structure with a thick film. Moreover, they have succeeded in evaluating the carrier mobility of an iridium complex [Ir(ppy)3], which is a phosphorescent material, and hole mobility on the order of 10−3 cm2/V s was obtained. This technique, which has the merits of free alignment of excitation light and no need for the preparation of a thick sample, is effective for the evaluation of carrier mobilities in thin organic materials.
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73.61.Ph Polymers; organic compounds
73.50.Dn Low-field transport and mobility; piezoresistance

Al0.15Ga0.85N/GaN high electron mobility transistor structures grown on p-type Si substrates

C.-T. Liang, Kuang Yao Chen, N. C. Chen, P. H. Chang, and Chin-An Chang

Appl. Phys. Lett. 89, 132107 (2006); http://dx.doi.org/10.1063/1.2357005 (3 pages) | Cited 3 times

Online Publication Date: 26 September 2006

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We report on experimental studies of Al0.15Ga0.85N/GaN high electron mobility transistor (HEMT) structures grown on p-type Si (111) substrates. By introducing an ultrathin SiN layer during the crystal growth, the Hall mobility of the HEMT structure can be greatly enhanced (greater than three times). This SiN treatment technique also allows the observation of Shubnikov–de Haas oscillations which is not possible in the untreated HEMT structure. Our experimental results pave the way for the integration of AlxGa1−xN/GaN HEMT structures with the mature Si technology in industry.
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85.30.Tv Field effect devices

Dark spot formation mechanism in organic light emitting diodes

Soo Young Kim, Kwang Young Kim, Yoon-Heung Tak, and Jong-Lam Lee

Appl. Phys. Lett. 89, 132108 (2006); http://dx.doi.org/10.1063/1.2357568 (3 pages) | Cited 12 times

Online Publication Date: 26 September 2006

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The authors report electroluminescence degradation in organic light emitting diode operated at a high current density of 500 mA/cm2 under nitrogen ambient. The turn-on voltage increased from 6 to 10 V, and a number of dark spots were produced when the device was operated for 420 min. Microscope image showed that dark spots are related to many protrusions and hollows formed on Al electrode. X-ray diffraction and scanning photoemission microscope spectra indicate that the Joule heat from high electric field induced the crystallization of 4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl, forming dark spots via peeling off of the Al cathode.
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85.60.Jb Light-emitting devices

Terahertz transmission characteristics of high-mobility GaAs and InAs two-dimensional-electron-gas systems

N. A. Kabir, Y. Yoon, J. R. Knab, J.-Y. Chen, A. G. Markelz, J. L. Reno, Y. Sadofyev, S. Johnson, Y.-H. Zhang, and J. P. Bird

Appl. Phys. Lett. 89, 132109 (2006); http://dx.doi.org/10.1063/1.2357605 (3 pages) | Cited 6 times

Online Publication Date: 26 September 2006

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Frequency-dependent complex conductivity of high-mobility GaAs and InAs two-dimensional-electron-gas (2DEG) systems is studied by terahertz time domain spectroscopy. Determining the momentum relaxation time from a Drude model, the authors find a lower value than that from dc measurements, particularly at high frequencies/low temperatures. These deviations are consistent with the ratio τt/τq, where τq is the full scattering time. This suggests that small-angle scattering leads to weaker heating of 2DEGs at low temperatures than expected from dc mobility.
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73.61.Ey III-V semiconductors
78.70.Gq Microwave and radio-frequency interactions
73.50.Dn Low-field transport and mobility; piezoresistance

Schottky barrier height of boride-based rectifying contacts to p-GaN

L. Stafford, L. F. Voss, S. J. Pearton, J.-J. Chen, and F. Ren

Appl. Phys. Lett. 89, 132110 (2006); http://dx.doi.org/10.1063/1.2357855 (3 pages) | Cited 4 times

Online Publication Date: 26 September 2006

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Schottky contact formation on p-GaN using a W2B-based metallization scheme was investigated using x-ray photoelectron spectroscopy (XPS), current-voltage (I-V), and capacitance-voltage (C-V) measurements. The Schottky barrier height (SBH) determined from XPS is 2.7 eV, whereas fitting of the I-V’s gives 1.2 and 3.8 eV depending on the assumed mechanism of forward current flow. While the C-V’s and the measurement temperature dependence of the I-V’s support tunneling as being the dominant transport mechanism, this latter approach overestimates the true SBH of W2B/p-GaN contacts due to the presence of an interfacial layer acting as an additional barrier to carrier transport.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ei Rectification
79.60.Jv Interfaces; heterostructures; nanostructures

Anisotropic resistivity of (100)-oriented mesoporous silicon

S. Borini, L. Boarino, and G. Amato

Appl. Phys. Lett. 89, 132111 (2006); http://dx.doi.org/10.1063/1.2357882 (3 pages) | Cited 4 times

Online Publication Date: 26 September 2006

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The resistivity of (100)-oriented mesoporous silicon has been studied using two different electrode configurations. The authors observed that the electronic transport along the longitudinal direction (parallel to the sample surface) is strongly inhibited at room temperature but not along the perpendicular direction. They show that such electrical anisotropy can be removed by heating the material, reporting an increase of six orders of magnitude of the longitudinal conductivity when the temperature rises from 20 to 100 °C. These experimental findings are interpreted on the basis of the material morphology and nanostructuration, which determine the availability of percolative pathways for free charge carriers.
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73.63.-b Electronic transport in nanoscale materials and structures
61.43.Gt Powders, porous materials

Interplay between s-d exchange interaction and Rashba effect: Spin-polarized transport

W. Yang, Kai Chang, X. G. Wu, H. Z. Zheng, and F. M. Peeters

Appl. Phys. Lett. 89, 132112 (2006); http://dx.doi.org/10.1063/1.2357888 (3 pages) | Cited 3 times

Online Publication Date: 26 September 2006

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The authors investigate the spin-polarized transport properties of a two-dimensional electron gas in a n-type diluted magnetic narrow gap semiconductor quantum well subjected to perpendicular magnetic and electric fields. Interesting beating patterns in the magnetoresistance are found which can be tuned significantly by varying the electric field. A resonant enhancement of spin-polarized current is found which is induced by the competition between the s-d exchange interaction and the Rashba effect [ Y. A. Bychkov and E. I. Rashba, J. Phys. C 17, 6039 (1984) ].
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75.50.Pp Magnetic semiconductors
73.63.Hs Quantum wells
75.30.Et Exchange and superexchange interactions
72.25.Dc Spin polarized transport in semiconductors
72.20.My Galvanomagnetic and other magnetotransport effects

Two-dimensional electron gas in Zn polar ZnMgO/ZnO heterostructures grown by radical source molecular beam epitaxy

H. Tampo, H. Shibata, K. Matsubara, A. Yamada, P. Fons, S. Niki, M. Yamagata, and H. Kanie

Appl. Phys. Lett. 89, 132113 (2006); http://dx.doi.org/10.1063/1.2357588 (3 pages) | Cited 29 times

Online Publication Date: 27 September 2006

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A two-dimensional electron gas was observed in Zn polar ZnMgO/ZnO (ZnMgO on ZnO) heterostructures grown by radical source molecular beam epitaxy. The electron mobility of the ZnMgO/ZnO heterostructures dramatically increased with increasing Mg composition and the electron mobility (μ ∼ 250 cm2/V s) at RT reached a value more than twice that of an undoped ZnO layer (μ ∼ 100 cm2/V s). The carrier concentration in turn reached values as high as ∼ 1×1013 cm−2 and remained nearly constant regardless of Mg composition. Strong confinement of electrons at the ZnMgO/ZnO interface was confirmed by C-V measurements with a concentration of over 4×1019 cm−3. Temperature-dependent Hall measurements of ZnMgO/ZnO heterostructures also exhibited properties associated with well defined heterostructures. The Hall mobility increased monotonically with decreasing temperature, reaching a value of 2750 cm2/V s at 4 K. Zn polar “ZnMgO on ZnO” structures are easy to adapt to a top-gate device. These results open new possibilities for high electron mobility transistors based upon ZnO-based materials.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
72.20.Ee Mobility edges; hopping transport
72.20.Fr Low-field transport and mobility; piezoresistance
72.20.My Galvanomagnetic and other magnetotransport effects

Ohmic contacts to plasma etched n-Al0.58Ga0.42N

M. A. Miller, S. E. Mohney, A. Nikiforov, G. S. Cargill, and K. H. A. Bogart

Appl. Phys. Lett. 89, 132114 (2006); http://dx.doi.org/10.1063/1.2357867 (3 pages) | Cited 8 times

Online Publication Date: 28 September 2006

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Plasma etching is required to expose n-AlxGa1−xN layers for bottom-emitting ultraviolet light emitting diodes grown on sapphire. However, etching can increase the difficulty of forming Ohmic contacts. X-ray photoelectron spectroscopy and cathodoluminescence reveal how the semiconductor changes with etching and help explain why it becomes more difficult to form an Ohmic contact. A V/Al/V/Au metallization has been investigated for Ohmic contacts to n-Al0.58Ga0.42N etched with a BCl3/Cl2/Ar chemistry. Increased V thickness and higher annealing temperatures were required to obtain a specific contact resistance of 4.7×10−4 Ω cm2 for etched n-Al0.58Ga0.42N compared to optimized contacts on unetched films.
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73.40.Ns Metal-nonmetal contacts
85.60.Jb Light-emitting devices
52.77.Bn Etching and cleaning
81.65.Cf Surface cleaning, etching, patterning
73.40.Cg Contact resistance, contact potential
85.40.Ls Metallization, contacts, interconnects; device isolation

Low hysteretic behavior of Al/AlOx/Al Josephson junctions

S. V. Lotkhov, E. M. Tolkacheva, D. V. Balashov, M. I. Khabipov, F.-I. Buchholz, and A. B. Zorin

Appl. Phys. Lett. 89, 132115 (2006); http://dx.doi.org/10.1063/1.2357915 (3 pages) | Cited 6 times

Online Publication Date: 28 September 2006

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All-aluminum Josephson junctions with high-transparency barriers were fabricated using the shadow-evaporation technique and measured at low temperatures, T ≈ 25 mK. Due to the high junction transparency, the IV characteristics showed only small hysteresis with a retrapping-to-switching current ratio of up to 80%. The observed critical currents were as large as 80%-100% of the Ambegaokar-Baratoff values. High barrier quality was confirmed by the low subgap leakage currents in the quasiparticle branches, which makes the low hysteretic Al junctions promising for application in integrated rapid single-flux quantum - qubit circuitry.
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74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
74.25.F- Transport properties
74.50.+r Tunneling phenomena; Josephson effects
74.25.Sv Critical currents

Are electrical switching and rectification inherent properties of carbon nanotube Y junctions?

Antonis N. Andriotis and Madhu Menon

Appl. Phys. Lett. 89, 132116 (2006); http://dx.doi.org/10.1063/1.2354014 (3 pages)

Online Publication Date: 28 September 2006

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Recent experimental results on carbon nanotube Y junctions have indicated ballistic rectification and switching, making them promising candidates for molecular device applications. The question still persists, however, whether this switching is the result of inherent nanotube properties or caused by the remnants of catalyst particles in the junction region of the nanotubes. In the present work, through a detailed theoretical calculation of quantum conductance of Y junctions with and without the presence of catalyst particles blocking the junction region the authors show that rectification and switching properties can be attributable entirely to the inherent nanotube properties and not to the catalysts.
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73.40.Ei Rectification
73.63.Fg Nanotubes
73.23.Ad Ballistic transport

Origin of forward leakage current in GaN-based light-emitting devices

S. W. Lee, D. C. Oh, H. Goto, J. S. Ha, H. J. Lee, T. Hanada, M. W. Cho, T. Yao, S. K. Hong, H. Y. Lee, S. R. Cho, J. W. Choi, J. H. Choi, J. H. Jang, J. E. Shin, et al.

Appl. Phys. Lett. 89, 132117 (2006); http://dx.doi.org/10.1063/1.2357930 (3 pages) | Cited 32 times

Online Publication Date: 28 September 2006

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The authors fabricated GaN-based light-emitting diodes (LEDs) on two different GaN templates with the same LED structure. One on thin GaN template ( ∼ 2 μm) with high dislocation density [low (109 cm−2)] grown by metal-organic vapor-phase epitaxy (sample A) and the other on thick GaN template ( ∼ 20 μm) with comparatively low dislocation density [high (108 cm−2)] by hydride vapor-phase epitaxy (sample B). In order to understand the mechanism of leakage current in LEDs, the correlation between current-voltage characteristics and etch pit density of LEDs was studied.
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85.60.Jb Light-emitting devices

Quantitative analysis of hysteresis in carbon nanotube field-effect devices

Swastik Kar, Aravind Vijayaraghavan, Caterina Soldano, Saikat Talapatra, Robert Vajtai, Omkaram Nalamasu, and Pulickel M. Ajayan

Appl. Phys. Lett. 89, 132118 (2006); http://dx.doi.org/10.1063/1.2358290 (3 pages) | Cited 28 times

Online Publication Date: 29 September 2006

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The authors present a model to analyze hysteresis in transfer characteristics (TCs) of single-wall carbon nanotube field-effect transistors, based on capacitive charging of the surrounding dielectric by charges injected out of the nanotube. The model identifies the extent and time scale of the hysteresis and correctly describes the dependence of the magnitude of hysteresis on common experimental parameters. The authors propose and experimentally establish a “time-decay” experiment for obtaining accurate device properties in hysteresis-affected devices without actually measuring TCs. The authors further show that values obtained by this method can be used to precisely predict TCs for the same device under different experimental parameters.
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85.35.Kt Nanotube devices
85.30.Tv Field effect devices

Optical and magnetic properties of Eu-doped GaN

J. Hite, G. T. Thaler, R. Khanna, C. R. Abernathy, S. J. Pearton, J. H. Park, A. J. Steckl, and J. M. Zavada

Appl. Phys. Lett. 89, 132119 (2006); http://dx.doi.org/10.1063/1.2358293 (3 pages) | Cited 32 times

Online Publication Date: 29 September 2006

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GaN films were doped with Eu to a concentration of ∼ 0.12 at. % during growth at 800 °C by molecular beam epitaxy, with the Eu cell temperature held constant at 470 °C. All samples were postannealed at 675 °C. The films exhibited strong photoluminescence (PL) in the red (622 nm) whose absolute intensity was a function of the Ga flux during growth, which ranged from 3.0×10−7 to 5.4×10−7 Torr. The maximum PL intensity was obtained at a Ga flux of 3.6×10−7 Torr. The samples showed room temperature ferromagnetism with saturation magnetization of ∼ 0.1–0.45 emu/cm3, consistent with past reports where the Eu was found to be predominantly occupying substitutional Ga sites. There was an inverse correlation between the PL intensity and the saturation magnetization in the films. X-ray diffraction showed the presence of EuGa phases under all the growth conditions but these cannot account for the observed magnetic properties.
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78.66.Fd III-V semiconductors
75.70.Ak Magnetic properties of monolayers and thin films
75.50.Dd Nonmetallic ferromagnetic materials
78.55.Cr III-V semiconductors
68.55.-a Thin film structure and morphology
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
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