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27 Aug 2007

Volume 91, Issue 9, Articles (09xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 093110 (2007); http://dx.doi.org/10.1063/1.2775801 (3 pages)

M. Schmidbauer, Zh. M. Wang, Yu. I. Mazur, P. M. Lytvyn, G. J. Salamo, D. Grigoriev, P. Schäfer, R. Köhler, and M. Hanke
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Mechanism of surface conduction in the vicinity of Schottky gates on AlGaN/GaN heterostructures

Junji Kotani, Masafumi Tajima, Seiya Kasai, and Tamotsu Hashizume

Appl. Phys. Lett. 91, 093501 (2007); http://dx.doi.org/10.1063/1.2775834 (3 pages) | Cited 17 times

Online Publication Date: 28 August 2007

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Lateral surface leakage current (Is) on an AlGaN/GaN heterostructure was systematically investigated by using a two-parallel gate structure with a gap distance (LGG) of 200 nm–5 μm. The surface current Is systematically increased as LGG decreased. A simple resistive layer conduction that should show 1/LGG dependence failed to account for the drastic increase in Is when LGG was reduced to less than 1 μm. However, no dependence on LGG was seen in vertical current that flows in the Schottky interface. The Is showed a clear temperature dependence proportional to exp(−T −1/3), indicating two-dimensional variable-range hopping through high-density surface electronic states in AlGaN. A pronounced reduction in surface current of almost four orders of magnitude was observed in a sample with SiNx passivation.
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73.25.+i Surface conductivity and carrier phenomena
73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

High brightness stable white and yellow light-emitting diodes from ambipolar polyspirofluorenes with high charge carrier mobility

Kang-Yung Peng, Chih-Wei Huang, Ching-Yang Liu, and Show-An Chen

Appl. Phys. Lett. 91, 093502 (2007); http://dx.doi.org/10.1063/1.2776349 (3 pages) | Cited 10 times

Online Publication Date: 28 August 2007

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We have used two polyspirofluorenes, spirodiethylhexyloxylpolyfluorene (spiro-diEHPF) and triphenylamine-grafted spirodiethylhexyloxylpolyfluorene (spiro-TPA50-diEHPF), which exhibit rather high charge mobilities in the order of 10−3–10−4 cm2/V s. The light-emitting devices with rubrene-doped spiro-diEHPF show maximal luminances of 36 000 and 70 000 cd/m2 with maximal efficiencies 3.5 and 9 cd/A for the white and yellow emissions, respectively. For rubrene-doped triphenylamine-grafted polymer, spiro-TPA50-diEHPF, the maximal luminances and efficiencies are 56 000 cd/m2 and 9 cd/A for white-emitting devices and 72 000 cd/m2 and 14 cd/A for yellow emission. Furthermore, the electroluminescence profiles of the investigated devices show nearly independent of the applied voltages.
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85.60.Jb Light-emitting devices

Thermal gate SiO2 for Ge metal-oxide-semiconductor capacitors fabricated on Si substrate

Yung-Hsien Wu, Jia-Rong Wu, and Min-Lin Wu

Appl. Phys. Lett. 91, 093503 (2007); http://dx.doi.org/10.1063/1.2776352 (3 pages) | Cited 6 times

Online Publication Date: 28 August 2007

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With the Si substrate, a process to fabricate Ge metal-oxide-semiconductor (MOS) capacitors with thermally grown SiO2 as the gate dielectric has been presented. The good crystallinity of the epitaxial Ge, the thermal oxide with tiny Ge content, and the smooth interface between oxide and Ge layer demonstrate the eligibility for device operation. From the electrical characterization of the MOS capacitors, enhanced dielectric constant of the gate oxide without deteriorating the leakage current and hysteresis can be achieved by proper NH3 nitridation and subsequent N2O treatment which shows the high potential to be employed in the fabrication of high performance Ge metal-oxide-semiconductor field-effect transist.
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84.32.Tt Capacitors

Current injection and transport in polyfluorene

Chieh-Kai Yang, Chia-Ming Yang, Hua-Hsien Liao, Sheng-Fu Horng, and Hsin-Fei Meng

Appl. Phys. Lett. 91, 093504 (2007); http://dx.doi.org/10.1063/1.2759951 (3 pages) | Cited 5 times

Online Publication Date: 28 August 2007

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A comprehensive numerical model is established for the electrical processes in a sandwich organic semiconductor device with high carrier injection barrier. The charge injection at the anode interface with 0.8 eV energy barrier is dominated by the hopping among the gap states of the semiconductor caused by disorders. The Ohmic behavior at low voltage is demonstrated to be not due to the background doping but the filaments formed by conductive clusters. In bipolar devices with low work function cathode it is shown that near the anode the electron traps significantly enhance hole injection through Fowler-Nordheim tunneling, resulting in rapid increases of the hole carrier and current in comparison with the hole-only devices.
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73.61.Ph Polymers; organic compounds
85.30.De Semiconductor-device characterization, design, and modeling

Hybrid cross-linkable polymer/sol-gel waveguide modulators with 0.65 V half wave voltage at 1550 nm

Y. Enami, D. Mathine, C. T. DeRose, R. A. Norwood, J. Luo, A. K.-Y. Jen, and N. Peyghambarian

Appl. Phys. Lett. 91, 093505 (2007); http://dx.doi.org/10.1063/1.2776369 (3 pages) | Cited 40 times

Online Publication Date: 28 August 2007

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The authors report on a hybrid cross-linked electro-optic (EO) polymer/sol-gel Mach-Zehnder waveguide modulator with a half wave voltage (Vπ) of 0.65 V at 1550 nm. The low Vπ was achieved by a design that (1) combines both physical vertical tapers in the sol-gel core and photobleached index tapers in the EO polymer and (2) reduces the thickness of the device to 8 μm. These combined physical and index tapers result in improved optical mode confinement in the EO polymer with low adiabatic optical transition loss. The reduced thickness results in a larger field across the EO polymer for the same voltage, enabling a lower Vπ.
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42.79.Gn Optical waveguides and couplers
42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency

Surface plasmon polariton mediated energy transfer in organic photovoltaic devices

T. D. Heidel, J. K. Mapel, M. Singh, K. Celebi, and M. A. Baldo

Appl. Phys. Lett. 91, 093506 (2007); http://dx.doi.org/10.1063/1.2772173 (3 pages) | Cited 27 times

Online Publication Date: 28 August 2007

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The performance of a phthalocyanine-based photovoltaic is boosted in the absorption gap between the phthalocyanine Q and Soret bands. Light absorption is decoupled from exciton diffusion using a light absorbing “antenna” layer external to the conventional charge generating layers. Radiation absorbed by the antenna is transferred into the charge generating layers via surface plasmon polaritons in an interfacial thin silver contact. The peak efficiency of energy transfer is measured to be at least (51±10)%.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
78.66.Qn Polymers; organic compounds

Carbon nanotube biosensors: The critical role of the reference electrode

Ethan D. Minot, Anne M. Janssens, Iddo Heller, Hendrik A. Heering, Cees Dekker, and Serge G. Lemay

Appl. Phys. Lett. 91, 093507 (2007); http://dx.doi.org/10.1063/1.2775090 (3 pages) | Cited 37 times

Online Publication Date: 28 August 2007

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Carbon nanotube transistors show tremendous potential for electronic detection of biomolecules in solution. However, the nature and magnitude of the sensing signal upon molecular adsorption have so far remained controversial. Here, the authors show that the choice of the reference electrode is critical and resolves much of the previous controversy. The authors eliminate artifacts related to the reference electrode by using a well-defined reference electrode to accurately control the solution potential. Upon addition of bovine serum albumin proteins, the authors measure a transistor threshold shift of −15 mV which can be unambiguously attributed to the adsorption of biomolecules in the vicinity of the nanotube.
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87.80.-y Biophysical techniques (research methods)
87.85.Qr Nanotechnologies-design
87.85.Rs Nanotechnologies-applications

Thermal energy harvesting device using ferromagnetic materials

M. Ujihara, G. P. Carman, and D. G. Lee

Appl. Phys. Lett. 91, 093508 (2007); http://dx.doi.org/10.1063/1.2775096 (3 pages) | Cited 8 times

Online Publication Date: 28 August 2007

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A unique concept for harvesting electrical energy from thermal energy is presented. A thermomechanical actuator was fabricated using ferromagnetic material. The device converts thermal energy into mechanical energy, which can be converted into electrical energy using piezoelectric materials. Magnetic force and operating frequency were measured on the device. Results show that the current power density at ΔT = 50 K is between 1.85 and 3.61 mW/cm2. A thermal finite element analysis model is also presented to understand the influence of thermal interface, suggesting that increases of 18.5 mW/cm2 or higher are achievable.
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85.80.Fi Thermoelectric devices

High-indium-content InGaAs metal-oxide-semiconductor capacitor with amorphous LaAlO3 gate dielectric

N. Goel, P. Majhi, W. Tsai, M. Warusawithana, D. G. Schlom, M. B. Santos, J. S. Harris, and Y. Nishi

Appl. Phys. Lett. 91, 093509 (2007); http://dx.doi.org/10.1063/1.2776846 (3 pages) | Cited 23 times

Online Publication Date: 29 August 2007

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The structure and electrical properties of LaAlO3/n-In0.53Ga0.47As metal-oxide-semiconductor capacitors deposited by molecular-beam epitaxy were investigated. Transmission electron microscopy revealed a sharp interface between the dielectric and InGaAs. Postdeposition annealing at 440–500 °C significantly reduced the capacitive equivalent thickness and frequency dispersion. A hysteresis of 15 mV–0.1 V, a dielectric permittivity of 17±1, and a dielectric strength of ∼ 4.3 MV/cm were measured. Additionally, a high loss in the parallel conductance and gate-bias independence in the inversion region was observed, implying the fast generation rate of minority carriers in In0.53Ga0.47As.
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84.32.Tt Capacitors
85.30.Tv Field effect devices

Defects around self-organized InAs quantum dots measured by slow positron beam

X. Q. Meng, Z. Q. Chen, P. Jin, Z. G. Wang, and Long Wei

Appl. Phys. Lett. 91, 093510 (2007); http://dx.doi.org/10.1063/1.2776861 (3 pages) | Cited 3 times

Online Publication Date: 29 August 2007

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Self-organized InAs quantum dots (QDs) have been fabricated by molecular beam epitaxy. The authors try to use a slow positron beam to detect defects in and around self-organized QDs, and point defects are observed in GaAs cap layer above QDs. For the self-organized InAs QDs without strain-reducing layer, it is free of defects. However, by introducing a strain-reducing layer, the density of point defects around larger sized InAs QDs increased. The above results suggest that low energy positron beam measurements may be a good approach to detect depth profiles of defects in QD materials.
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71.55.Eq III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

On-chip matching networks for radio-frequency single-electron transistors

W. W. Xue, B. Davis, Feng Pan, J. Stettenheim, T. J. Gilheart, A. J. Rimberg, and Z. Ji

Appl. Phys. Lett. 91, 093511 (2007); http://dx.doi.org/10.1063/1.2776888 (3 pages) | Cited 4 times

Online Publication Date: 29 August 2007

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In this letter, the authors describe operation of a radio-frequency superconducting single electron transistor (rf-SSET) with an on-chip superconducting LC matching network consisting of a spiral inductor L and its capacitance to ground Cp. The superconducting network has a lower Cp and gives a better matching for the rf-SSET than does a commercial chip inductor. Moreover, the superconducting network has negligibly low dissipation, leading to sensitive response to changes in the rf-SSET impedance. The charge sensitivity δq = 2.4×10−6 e/math in the subgap region and energy sensitivity δε = 1.9ℏ indicate that the rf-SSET is operating in the vicinity of the shot noise limit.
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85.25.Cp Josephson devices
84.32.Hh Inductors and coils; wiring
85.25.Hv Superconducting logic elements and memory devices; microelectronic circuits
84.40.Lj Microwave integrated electronics

Small molecule based and solution processed highly efficient red electrophosphorescent organic light emitting devices

Heekyung Kim, Younghun Byun, Rupasree Ragini Das, Byoung-Ki Choi, and Pil-Soo Ahn

Appl. Phys. Lett. 91, 093512 (2007); http://dx.doi.org/10.1063/1.2776016 (3 pages) | Cited 27 times

Online Publication Date: 29 August 2007

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The authors report the very high luminous efficiency in solution processed red electrophosphorescent organic light emitting devices using small molecular host and guest materials. The maximum luminous efficiency reached 12.7 cd/A, corresponding to external quantum efficiency of 15.1%, with its emission peak wavelength of 620 nm and the Commission Internationale de l’Eclairage coordinates of (0.65, 0.33). Along with these excellent performances of the solution processed device, which were comparable to those of the vacuum deposited counterpart device with similar structure and materials, the comparative study on both devices suggests the merits of the solution process adopting robust small molecular materials only.
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85.60.Jb Light-emitting devices

Micro thermoindicators and optical-electronic temperature control for microfluidic applications

Liyu Liu, Suili Peng, Weijia Wen, and Ping Sheng

Appl. Phys. Lett. 91, 093513 (2007); http://dx.doi.org/10.1063/1.2776848 (3 pages) | Cited 5 times

Online Publication Date: 29 August 2007

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The authors report the design and implementation of sensing and control of local temperature in polydimethylsiloxane microfluidic reaction chip, based on the fabrication of a microtemperature sensor with thermochromic color bars and the associated optical and electronic feedback controls. The thermochromic color bar demonstrates easy and accurate local temperature monitoring. In combination with a microheater, this contactless microchip temperature control approach may have wide application potentials in microchemical and microbiological analyses.
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07.20.Dt Thermometers
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Organic light-emitting diodes with structured cathode

Deang Liu, Michael Fina, Zhaoyang Chen, Xiaobo Chen, Gao Liu, Steven Johnson, and Samuel S. Mao

Appl. Phys. Lett. 91, 093514 (2007); http://dx.doi.org/10.1063/1.2776018 (3 pages) | Cited 11 times

Online Publication Date: 29 August 2007

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Organic light-emitting diode (OLED) devices were fabricated with a structured polymer-cathode interface. The devices have a layered structure indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/poly(2-methoxy-5-[(2′-ethylhexyl)oxy]-p-phenylenevinylene)/Al. The light-emitting polymer layers were patterned via a stamp imprinting process, followed by vapor deposition of the cathode. Devices employing the structured cathode showed improved electron injection and increased brightness when compared to conventional flat cathode devices. Electrostatic modeling of the structured cathode geometry revealed localized increase in electric field leading to enhanced electron tunneling through the polymer-cathode interface. Thus, improved OLED performance was demonstrated from cathode microstructuring.
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85.60.Jb Light-emitting devices

Transparent organic light-emitting diodes using resonant tunneling double barrier structures

Seung Yoon Ryu, Sung Jin Jo, Chang Su Kim, Sang Hun Choi, Joo Hyon Noh, Hong Koo Baik, Hee Seong Jeong, Dong Won Han, Seung Yong Song, and Kyu Sung Lee

Appl. Phys. Lett. 91, 093515 (2007); http://dx.doi.org/10.1063/1.2776347 (3 pages) | Cited 12 times

Online Publication Date: 29 August 2007

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A semitransparent cathode of indium tin oxide (ITO)/Ag/ITO was developed as a resonant tunneling double barrier structure for transparent organic light-emitting diodes. A weak negative differential resistance was observed in devices using a 100 nm thick ITO/Ag/ITO layer as a cathode in combination with a thin LiF/Al layer. The current injection of devices was dominated by resonant tunneling, which induced no luminance at low voltage. This was achieved by employing an e-beam evaporated ITO/Ag/ITO cathode due to the double quantum barriers of ITO and the quantum well of Ag. The authors also applied the multilayer cathode to small molecule devices, which showed the same resonant tunneling currents.
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73.40.Gk Tunneling
85.60.Jb Light-emitting devices

High response organic visible-blind ultraviolet detector

Debdutta Ray and K. L. Narasimhan

Appl. Phys. Lett. 91, 093516 (2007); http://dx.doi.org/10.1063/1.2778545 (3 pages) | Cited 22 times

Online Publication Date: 30 August 2007

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The authors demonstrate a high efficiency visible-blind ultraviolet organic photodetector with a response of 30 mA/W. The active layer is a blend of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) and tris(8-hydroxyquinoline) aluminum (Alq3). The authors show that the spontaneous as well as the electric field induced carrier generation efficiencies in the blend are enhanced over its constituents. The spontaneous carrier generation efficiency measured from total carrier collection measurements is 30% in the blend. The photoluminescence of the blend shows an efficient energy transfer from the TPD to Alq3 molecule. The mobility-lifetime (μτ) product in the blend is 2.2×10−12 cm2/V. The μτ product is weakly dependent on temperature.
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85.60.Gz Photodetectors (including infrared and CCD detectors)

Bipolar resistive switching in a single layer memory device based on a conjugated copolymer

Sang-Hyun Hong, Ohyun Kim, Seungchel Choi, and Moonhor Ree

Appl. Phys. Lett. 91, 093517 (2007); http://dx.doi.org/10.1063/1.2778549 (3 pages) | Cited 11 times

Online Publication Date: 30 August 2007

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The feasibility of employing polymer memory devices as future data-storage units was investigated by using thin films of a π-conjugated copolymer, namely, poly(o-anthranilic acid-co-o-nitroaniline), which can be obtained by low-cost solution processing. Investigations on the conjugated-copolymer film showed that the hysteresis in the current-voltage characteristics can be used to perform “write-read-erase-read” memory functions. In addition, the ratio of the on/off current depended on the switching-on compliance current and the maximum voltage applied. This study suggests that the resistive switching of the copolymer device can be attributed to trap charging and discharging, which induce localized filament formation and disappearance.
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84.30.Sk Pulse and digital circuits
85.30.-z Semiconductor devices

Unipolar accumulation-type transistor configuration implemented using Si nanowires

Yinghui Shan, S. Ashok, and Stephen J. Fonash

Appl. Phys. Lett. 91, 093518 (2007); http://dx.doi.org/10.1063/1.2778752 (3 pages) | Cited 4 times

Online Publication Date: 30 August 2007

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A unipolar accumulation-type field effect transistor (FET) is proposed, fabricated, and characterized. The device, which uses a single doping type and Ohmic contacts, relies on the nanoscale to force the only possible source-drain path to pass through an accumulated (on state) or depleted (off state) region. The transistor is demonstrated using silicon nanowires (SiNWs) grown on glass with our nanochannel-template-guided “grow-in-place” approach. The resulting SiNW FETs exhibit an on-off ratio of 106 and a subthreshold slope of 130 mV/decade. These transistors can allow unique design flexibility; for example, a NAND gate can be achieved in a uniformly doped nanowire with four contacts.
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85.30.Tv Field effect devices
85.35.-p Nanoelectronic devices

Direct water photoelectrolysis with patterned n-GaN

Ichitaro Waki, Daniel Cohen, Rakesh Lal, Umesh Mishra, Steven P. DenBaars, and Shuji Nakamura

Appl. Phys. Lett. 91, 093519 (2007); http://dx.doi.org/10.1063/1.2769393 (3 pages) | Cited 18 times

Online Publication Date: 30 August 2007

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Direct photoelectrolysis of water was observed using a structured n-GaN photoanode, aqueous NaOH solution, and platinum cathode. The structured n-GaN was formed by selective area regrowth using metal stripes defined by photolithography. Gas evolution from both the n-GaN anode and the platinum cathode was observed under UV irradiation without external bias. The metal stripes eliminated current crowding and the n-GaN regrowth increased specific surface area. As a result, distinct improvements in photocurrent were achieved. The origin of the photocurrent and the conversion efficiency are discussed based on water-splitting reactions and etching reactions.
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81.05.Ea III-V semiconductors
82.45.Vp Semiconductor materials in electrochemistry
82.45.Hk Electrolysis
81.16.Nd Micro- and nanolithography
72.40.+w Photoconduction and photovoltaic effects
81.65.Cf Surface cleaning, etching, patterning

Electrical switching and in situ Raman scattering studies on the set-reset processes in Ge–Te–Si glass

M. Anbarasu, S. Asokan, Sudakshina Prusty, and A. K. Sood

Appl. Phys. Lett. 91, 093520 (2007); http://dx.doi.org/10.1063/1.2770770 (3 pages) | Cited 8 times

Online Publication Date: 31 August 2007

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Bulk Ge15Te83Si2 glass has been found to exhibit memory-type switching for 1 mA current with a threshold electric field of 7.3 kV/cm. The electrical set and reset processes have been achieved with triangular and rectangular pulses, respectively, of 1 mA amplitude. In situ Raman scattering studies indicate that the degree of disorder in Ge15Te83Si2 glass is reduced from off to set state. The local structure of the sample under reset condition is similar to that in the off state. The Raman results are consistent with the switching results which indicate that the Ge15Te83Si2 glass can be set and reset easily.
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72.80.Ng Disordered solids
78.35.+c Brillouin and Rayleigh scattering; other light scattering
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