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8 Oct 2012

Volume 101, Issue 15, Articles (15xxxx)

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Appl. Phys. Lett. 101, 153501 (2012); http://dx.doi.org/10.1063/1.4756693 (3 pages)

Brandon G. Cook, William R. French, and Kálmán Varga
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Electron transport properties of carbon nanotube–graphene contacts

Brandon G. Cook, William R. French, and Kálmán Varga

Appl. Phys. Lett. 101, 153501 (2012); http://dx.doi.org/10.1063/1.4756693 (3 pages)

Online Publication Date: 8 October 2012

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The properties of carbon nanotube-graphene junctions are investigated with first-principles electronic structure and electron transport calculations. Contact properties are found to be key factors in determining the performance of nanotube based electronic devices. In a typical single-walled carbon nanotube-metal junction, there is a p-type Schottky barrier of up to ∼ 0.4 eV which depends on the nanotube diameter. Calculations of the Schottky barrier height in carbon nanotube-graphene contacts indicate that low barriers of 0.09 eV and 0.04 eV are present in nanotube-graphene contacts ((8,0) and (10,0) nanotubes, respectively). Junctions with a finite contact region are investigated with simulations of the current-voltage characteristics. The results suggest the suitability of the junctions for applications and provide insight to explain recent experimental findings.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.30.+y Surface double layers, Schottky barriers, and work functions

Capacitive micromachined ultrasonic transducer based tilt sensing

Hongbin Yu, Bin Guo, Kuruveettil Haridas, Tsu-Hui Lin, Jia Hao Cheong, Ming Lin Tsai, and Tack Boon Yee

Appl. Phys. Lett. 101, 153502 (2012); http://dx.doi.org/10.1063/1.4757998 (4 pages)

Online Publication Date: 8 October 2012

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In this paper, a tilt sensing mechanism based on the capacitive micromachined ultrasound transducers (CMUTs) is presented. By measuring the difference in the time of flight of various pulse-echo signals from different CMUT transmitting elements to one common receiving element in the oil bath, the tilt angle of the oil surface can be determined. With the proposed device, the maximum tilt angles of 20° and 28° have been measured in the clockwise and counterclockwise directions, respectively, and the difference between the measured and the theoretical values of the tilt angle was found to be within 0.05° during the whole test.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.07.Mp Transducers

A 670 GHz gyrotron with record power and efficiency

M. Yu. Glyavin, A. G. Luchinin, G. S. Nusinovich, J. Rodgers, D. G. Kashyn, C. A. Romero-Talamas, and R. Pu

Appl. Phys. Lett. 101, 153503 (2012); http://dx.doi.org/10.1063/1.4757290 (4 pages) | Cited 1 time

Online Publication Date: 9 October 2012

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A 670 GHz gyrotron with record power and efficiency has been developed in joint experiments of the Institute of Applied Physics, Russian Academy of Sciences (Nizhny Novgord, Russia), and the University of Maryland (USA) teams. The magnetic field of 27–28 T required for operation at the 670 GHz at the fundamental cyclotron resonance is produced by a pulsed solenoid. The pulse duration of the magnetic field is several milliseconds. A gyrotron is driven by a 70 kV, 15 A electron beam, so the beam power is on the order of 1 MW in 10–20 ms pulses. The ratio of the orbital to axial electron velocity components is in the range of 1.2–1.3. The gyrotron is designed to operate in the TE31,8-mode. Operation in a so high-order mode results in relatively low ohmic losses (less than 10% of the radiated power). Achieved power of the outgoing radiation (210 kW) and corresponding efficiency (about 20%) represent record numbers for high-power sources of sub-THz radiation.
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84.40.Ik Masers; gyrotrons (cyclotron-resonance masers)

Poole Frenkel current and Schottky emission in SiN gate dielectric in AlGaN/GaN metal insulator semiconductor heterostructure field effect transistors

Mina J. Hanna, Han Zhao, and Jack C. Lee

Appl. Phys. Lett. 101, 153504 (2012); http://dx.doi.org/10.1063/1.4758995 (4 pages)

Online Publication Date: 10 October 2012

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We analyze the anomalous I-V behavior in SiN prepared by plasma enhanced chemical vapor deposition for use as a gate insulator in AlGaN/GaN metal insulator semiconductor heterostructure filed effect transistors (HFETs). We observe leakage current across the dielectric with opposite polarity with respect to the applied electric field once the voltage sweep reaches a level below a determined threshold. This is observed as the absolute minimum of the leakage current does not occur at minimum voltage level (0 V) but occurs earlier in the sweep interval. Curve-fitting analysis suggests that the charge-transport mechanism in this region is Poole-Frenkel current, followed by Schottky emission due to band bending. Despite the current anomaly, the sample devices have shown a notable reduction of leakage current of over 2 to 6 order of magnitudes compared to the standard Schottky HFET. We show that higher pressures and higher silane concentrations produce better films manifesting less trapping. This conforms to our results that we reported in earlier publications. We found that higher chamber pressure achieves higher sheet carrier concentration that was found to be strongly dependent on the trapped space charge at the SiN/GaN interface. This would suggest that a lower chamber pressure induces more trap states into the SiN/GaN interface.
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85.30.Tv Field effect devices
73.50.Fq High-field and nonlinear effects
73.30.+y Surface double layers, Schottky barriers, and work functions
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Origin of kink effect in AlGaN/GaN high electron mobility transistors: Yellow luminescence and Fe doping

N. Killat, M. J. Uren, D. J. Wallis, T. Martin, and M. Kuball

Appl. Phys. Lett. 101, 153505 (2012); http://dx.doi.org/10.1063/1.4757993 (3 pages)

Online Publication Date: 10 October 2012

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AlGaN/GaN high electron mobility transistors with different Fe-doping density were studied using electrical and optical analysis to gain insight into the nature of traps responsible for the kink effect in electrical characteristics. Kink effect has been previously suggested to result from direct trapping of carriers in defects related to yellow luminescence (YL) centers. However, the results demonstrate that YL is suppressed by Fe doping, whereas the kink effect is not affected to the same extent. YL related defect states are therefore not exclusively responsible for the kink effect, suggesting a more complex trapping mechanism to affect device output characteristics.
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85.30.Tv Field effect devices
85.40.Ry Impurity doping, diffusion and ion implantation technology

Laser-induced quantum pumping in graphene

Pablo San-Jose, Elsa Prada, Henning Schomerus, and Sigmund Kohler

Appl. Phys. Lett. 101, 153506 (2012); http://dx.doi.org/10.1063/1.4758695 (3 pages) | Cited 3 times

Online Publication Date: 10 October 2012

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We investigate non-adiabatic electron pumping in graphene generated by laser irradiation with linear polarization parallel or perpendicular to the transport direction. Transport is dominated by the spatially asymmetric excitation of electrons from evanescent into propagating modes. For a laser with parallel polarization, the pumping response exhibits a subharmonic resonant enhancement which directly probes the Fermi energy; no such enhancement occurs for perpendicular polarization. The resonance mechanism relies on the chirality of charge carriers in graphene.
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71.20.Tx Fullerenes and related materials; intercalation compounds
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Hybrid pentacene/a-silicon solar cells utilizing multiple carrier generation via singlet exciton fission

Bruno Ehrler, Kevin P. Musselman, Marcus L. Böhm, Richard H. Friend, and Neil C. Greenham

Appl. Phys. Lett. 101, 153507 (2012); http://dx.doi.org/10.1063/1.4757612 (3 pages)

Online Publication Date: 10 October 2012

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Silicon dominates the solar cell market because of its abundance, mature production processes, and high efficiencies, with the best solar cells approaching the Shockley-Queisser limit. Multiple exciton photogeneration provides a route to solar cells that surpass the Shockley-Queisser limit, and we report the use of pentacene, for which photogenerated singlet excitons rapidly convert into two lower-energy spin-triplet excitons. We report solar cells that couple amorphous silicon to pentacene. We show that a thin layer of nanocrystals between silicon and pentacene allows simultaneously harnessing low-energy photons absorbed in silicon and high-energy photons absorbed in pentacene, generating two excitons via singlet fission.
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88.40.jj Silicon solar cells
88.40.hj Efficiency and performance of solar cells

Electrical and microstructural analyses of 200 MeV Ag14+ ion irradiated Ni/GaN Schottky barrier diode

Ashish Kumar, A. Hähnel, D. Kanjilal, and R. Singh

Appl. Phys. Lett. 101, 153508 (2012); http://dx.doi.org/10.1063/1.4758929 (4 pages)

Online Publication Date: 11 October 2012

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Ni/GaN Schottky barrier diodes were irradiated with 200 MeV Ag ions up to fluence of 1 × 1011 ions/cm2. The current-voltage measurements showed that the ideality factor, n, increased and the reverse leakage current, IR, decreased with increase in fluence. But, Schottky barrier height increased only marginally with increase in fluence. Cross-sectional transmission electron microscopy images revealed the presence of defect clusters in bulk GaN after irradiation. However, the Ni/GaN interface did not show any intermixing or degradation after the irradiation.
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85.30.Hi Surface barrier, boundary, and point contact devices
85.30.Kk Junction diodes

Trap-assisted tunneling resistance switching effect in CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3 heterostructure

X. G. Chen, J. B. Fu, S. Q. Liu, Y. B. Yang, C. S. Wang, H. L. Du, G. C. Xiong, G. J. Lian, and J. B. Yang

Appl. Phys. Lett. 101, 153509 (2012); http://dx.doi.org/10.1063/1.4760221 (4 pages) | Cited 1 time

Online Publication Date: 11 October 2012

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We reported the resistance switching (RS) behavior in the epitaxially grown CeO2/ La0.7(Sr0.1Ca0.9)0.3MnO3 (CeO2/LSCMO) heterojunctions on SrTiO3 substrate. The CeO2/LSCMO device displayed improved switching characteristics as compared to that of metal/manganite device. The switching threshold voltage showed a strong dependence on the thickness of the CeO2 layer, where a minimum/maximum thickness was required for the appearance of the resistance switching. Both set and reset threshold voltages increase with the increase of the CeO2 layer thickness due to the trap-assisted electron tunneling effect. In the meantime, the defects or vacancies in the CeO2 films, in particular, the concentration of the defects or vacancies in the interface between CeO2 and LSCMO, have a significant impact on the switching effect. These results suggest that the electron tunneling accompanied by a trapping/detrapping process at the interface is likely responsible for the RS effect in the insulator/manganite system.
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84.30.Sk Pulse and digital circuits
73.40.Gk Tunneling
72.60.+g Mixed conductivity and conductivity transitions
61.72.jd Vacancies

Nonadiabatic fiber taper-based Mach-Zehnder interferometer for refractive index sensing

Linlin Xu, Ying Li, and Baojun Li

Appl. Phys. Lett. 101, 153510 (2012); http://dx.doi.org/10.1063/1.4759041 (3 pages)

Online Publication Date: 12 October 2012

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A Mach-Zehnder interferometer based on a nonadiabatic fiber taper is proposed for refractive index (RI) sensing. Only the first order cladding guided mode is excited in the down-taper. The radius of the fiber core in the interference region is too thin (<1.64 μm) to confine the fundamental mode in the fiber core, the fundamental core guided mode is converted to the fundamental cladding guided mode and interference occurs between it and the first order cladding guided mode. The sensitivity of the sensor is 980 nm per refractive index unit for the surrounding refractive index ranging from 1.332 to 1.392.
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42.81.Pa Sensors, gyros
07.60.Ly Interferometers
42.81.Bm Fabrication, cladding, and splicing

The origin of broad distribution of breakdown times in polycrystalline thin film dielectrics

Muhammad Masuduzzaman, Sujing Xie, Jayhoon Chung, Dhanoop Varghese, John Rodriguez, Srikanth Krishnan, and Muhammad Ashraful Alam

Appl. Phys. Lett. 101, 153511 (2012); http://dx.doi.org/10.1063/1.4758684 (5 pages) | Cited 1 time

Online Publication Date: 12 October 2012

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The distribution of breakdown times of thin film dielectrics, stressed in a constant voltage mode, is generally interpreted in terms of percolation theory of dielectric breakdown. The percolation model suggests that relative distribution of failure times (normalized to the mean) should narrow down considerably for thicker dielectrics. Explicitly contradicting this prediction, we find a larger distribution of failure times even for relatively thick polycrystalline oxides. We use atomic force microscopy and conductive AFM measurements to confirm that breakdown in these films are primarily localized in the grain boundaries, decorated with large number of pre-existing defects. The classical percolation model—adapted to this specific situation of spatially localized trap generation—offers an intuitive explanation of the breadth of the failure time distribution in thick polycrystalline dielectric. The theory offers an opportunity to optimize the intrinsic trade-off between variability and reliability in polycrystalline films.
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77.55.-g Dielectric thin films
77.22.Jp Dielectric breakdown and space-charge effects
61.72.Mm Grain and twin boundaries
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
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