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10 Jul 2006

Volume 89, Issue 2, Articles (02xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 89, 023901 (2006); http://dx.doi.org/10.1063/1.2219984 (3 pages)

Nikhil Ganesh, Ian D. Block, and Brian T. Cunningham
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Spontaneous pulse generation using silicon controlled rectifier

Gamani Karunasiri

Appl. Phys. Lett. 89, 023501 (2006); http://dx.doi.org/10.1063/1.2220528 (3 pages)

Online Publication Date: 10 July 2006

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Spontaneous generation of voltage pulses was observed in a circuit consists of a silicon controlled rectifier (SCR) connected in series with a parallel RC circuit under dc bias. The pulse rate found to increase with the dc bias and saturated as pulse period reached the RC time constant. The injection of current into the SCR using a photodiode connected to the gate terminal increased the pulse rate with light intensity. This suggests that direct illumination of the reversed biased p-n junction of the SCR with light will allow the circuit to function as a pulse mode light detector.
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84.30.Ng Oscillators, pulse generators, and function generators
84.30.Jc Power electronics; power supply circuits
85.30.-z Semiconductor devices

Effect of the oxidation process on the electrical characteristics of 4H–SiC p-channel metal-oxide-semiconductor field-effect transistors

Mitsuo Okamoto, Mieko Tanaka, Tsutomu Yatsuo, and Kenji Fukuda

Appl. Phys. Lett. 89, 023502 (2006); http://dx.doi.org/10.1063/1.2221400 (3 pages) | Cited 11 times

Online Publication Date: 10 July 2006

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We fabricated inversion-type 4H–SiC p-channel metal-oxide-semiconductor field-effect transistors (MOSFETs). The electrical properties of the p-channel MOSFETs depended on the gate oxidation process. We found that wet oxidation process was effective for improving the channel mobility of the p-channel MOSFET, and obtained the field-effect channel mobility μFE of 15.6 cm2/V s by using that process. Measurements of the p-type MOS capacitors implied that the SiO2/SiC interface states around the valence band affected the channel mobility and threshold voltage of the p-channel MOSFETs. We also investigated temperature dependences.
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85.30.Tv Field effect devices
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
84.32.Tt Capacitors
81.65.Mq Oxidation

Stacked white organic light-emitting devices based on a combination of fluorescent and phosphorescent emitters

Hiroshi Kanno, Noel C. Giebink, Yiru Sun, and Stephen R. Forrest

Appl. Phys. Lett. 89, 023503 (2006); http://dx.doi.org/10.1063/1.2219725 (3 pages) | Cited 72 times

Online Publication Date: 11 July 2006

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We demonstrate a white stacked organic light-emitting device (WSOLED) employing the blue fluorescent emitter, 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl, and the green and red phosphorescent emitters, fac-tris(2-phenylpyridinato-N,C2′) iridium (III) and iridium (III) bis(2-phenyl quinolyl-N,C2′) acetylacetonate, respectively. The charge generation region consists of a Li-doped electron transport layer and a highly transparent MoOx thin film. For a two-element white SOLED (2-WSOLED), the combination of red and green phosphors with a blue fluorophore yields maximum external quantum and power efficiencies of ηext = 23%±2% at a current density of J = 1 mA/cm2 and ηp = 14±1 lm/W at J = 0.17 mA/cm2, respectively. Due to the low optical and electrical losses of the charge generation layer, the efficiencies scale approximately linearly with the number of independent emissive elements in the WSOLED. Hence, for a 3-WSOLED, the total external and power efficiencies estimated for operation of the device in a light fixture are ηext,tot = 57%±6% and ηp,tot = 22±2 lm/W, respectively, at a luminance of 1000 cd/m2, with Commission Internationale de L’Eclairage chromaticity coordinates of (x = 0.38, y = 0.44), and a color rendering index of 82. The high-efficiency, high brightness, stacked white OLED is potentially useful for solid state lighting applications.
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85.60.Jb Light-emitting devices

Thermal stability of magnetic tunneling junctions with MgO barriers for high temperature spintronics

Xiaoyong Liu, Dipanjan Mazumdar, Weifeng Shen, B. D. Schrag, and Gang Xiao

Appl. Phys. Lett. 89, 023504 (2006); http://dx.doi.org/10.1063/1.2219997 (3 pages) | Cited 11 times

Online Publication Date: 11 July 2006

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Thermal stability of MgO-based magnetic tunnel junctions has been investigated from room temperature up to 500 °C, in both the memory and sensor configurations. Junctions showed magnetoresistances of over 200% at room temperature and over 100% at 300 °C. Below 375 °C, the resistance of the parallel state remains constant, while the antiparallel state resistance linearly decreases with temperature. Above that, a rapid increase in the resistance of both states was observed, along with an irreversible loss of magnetoresistance. Junctions in the sensor configuration exhibited a constant sensitivity of 1.0%/Oe at temperatures up to 300 °C before getting degraded.
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85.75.Dd Magnetic memory using magnetic tunnel junctions
75.47.Pq Other materials
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Double-injector source of spin polarized current with controllable polarization

J. Zou, I. Sosnin, and V. T. Petrashov

Appl. Phys. Lett. 89, 023505 (2006); http://dx.doi.org/10.1063/1.2220547 (3 pages) | Cited 1 time

Online Publication Date: 11 July 2006

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We present low-temperature measurements of Co/Al spin valves with a double-injector source of spin polarized current. Using an in-plane magnetic field, the injector electrodes were magnetized in opposite directions. The spin polarization of the injected current was controlled by changing the ratio of currents through the two electrodes and was continuously varied from zero up to the maximum spin polarization of Co. This result was verified by measuring the spin valve signal, using the detector electrode magnetized to align with one of the injectors. This source can be used for spintronic applications as well as in research on hybrid ferromagnet/superconductor structures.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Cc Other ferromagnetic metals and alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)

Self-exciting, self-sensing PbZr0.53Ti0.47O3/SiO2 piezoelectric microcantilevers with femtogram/Hertz sensitivity

Zuyan Shen, Wan Y. Shih, and Wei-Heng Shih

Appl. Phys. Lett. 89, 023506 (2006); http://dx.doi.org/10.1063/1.2219994 (3 pages) | Cited 25 times

Online Publication Date: 12 July 2006

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Piezoelectric microcantilever sensors (PEMSs) consisting of a piezoelectric layer bonded to a nonpiezoelectric layer offer the advantages of electrical self-actuation and self-detection. Here we report PEMSs 60–300 μm in length fabricated from 1.5-μm-thick sol-gel PbZr0.53Ti0.47O3 (PZT) films with a 2 μm grain size, a dielectric constant of 1600, and a saturation polarization of 55±5 μC/cm2. The PEMSs exhibited up to four resonance peaks with quality factors Q ranging from 120 to 320. In humidity sensing tests, a PEMS with a 60×25 μm PZT/SiO2 section and a 24×20 μm SiO2 extension exhibited 1×10−15 g/Hz mass sensitivity, two orders of magnitude better than the sensitivity of the current PZT PEMS.
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85.50.-n Dielectric, ferroelectric, and piezoelectric devices
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.10.Cm Micromechanical devices and systems

Impact of gate materials on positive charge formation in HfO2/SiO2 stacks

C. Z. Zhao, J. F. Zhang, M. B. Zahid, G. Groeseneken, R. Degraeve, and S. De Gendt

Appl. Phys. Lett. 89, 023507 (2006); http://dx.doi.org/10.1063/1.2220484 (3 pages) | Cited 3 times

Online Publication Date: 12 July 2006

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The use of metal gates removes gate depletion and allows aggressive downscaling of equivalent oxide thickness. The impact of changing gate from poly-Si to metal on device reliability has not been fully addressed. This work focuses on how TaN gate affects the positive charge formation in HfO2/SiO2 stacks. Under positive gate bias stresses, the results show that positive charge formation in metal-gated samples is significantly higher than in poly-Si gated samples. However, positive charge formation is similar in these two types of samples, when stressed under negative gate bias. The results are explained by assuming that there are more hydrogenous species at the metal/dielectric interface and the hydrogen release from the anode dominates the positive charge formation. The behavior of positive charges in the stack is also compared with that in a single SiO2 layer.
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77.55.-g Dielectric thin films

Titania-nanotube-array-based photovoltaic cells

H. Wang, C. T. Yip, K. Y. Cheung, A. B. Djurišić, M. H. Xie, Y. H. Leung, and W. K. Chan

Appl. Phys. Lett. 89, 023508 (2006); http://dx.doi.org/10.1063/1.2221502 (3 pages) | Cited 32 times

Online Publication Date: 12 July 2006

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Dye-sensitized solar cells based on perpendicular titania nanotube arrays were fabricated. Titania nanotube arrays were prepared by anodization of Ti foil. The cell performance as a function of nanotube length and anodization method was investigated. Short circuit current density and cell efficiency increased with the nanotube length. Device performance was also affected by anodization method, spacer layer thickness, and annealing conditions. With optimized device structure and titania annealing procedure, short circuit current density of 3.28 mA/cm2 could be achieved under AM 1.5 simulated solar irradiation.
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84.60.Jt Photoelectric conversion
85.35.Kt Nanotube devices

Interface charge compensation in InP based heterojunction bipolar transistors with implanted subcollectors

Navin Parthasarathy, Christoph Kadow, Zach Griffith, Mark J. W. Rodwell, Miguel Urteaga, Keisuke Shinohara, Richard Pierson, and Bobby Brar

Appl. Phys. Lett. 89, 023509 (2006); http://dx.doi.org/10.1063/1.2221512 (3 pages)

Online Publication Date: 13 July 2006

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We report InP/In0.53Ga0.47As/InP double heterojunction bipolar transistors (DHBTs) with implanted subcollectors. We demonstrate the compensation of charge at the regrowth interface by the use of a blanket Fe implant. An isolated N++ subcollector is then formed by a patterned Si implant. With the compensation of the interface charge, this patterned subcollector eliminates the extrinsic base-collector capacitance Ccb associated with the base interconnect pad over the entire range of bias voltages. These implanted subcollector DHBTs with the shallow Fe implant have 363 GHz fτ and 410 GHz fmax. The dc current gain β ∼ 40, BVceo = 4.9 V, BVcbo = 5.4 V, and Icbo<70 pA at Vcb = 0.3 V.
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85.30.Pq Bipolar transistors
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