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4 Oct 2004

Volume 85, Issue 14, pp. 2679-2983

Issue Cover Spotlight Figure

Appl. Phys. Lett. 85, 2860 (2004); http://dx.doi.org/10.1063/1.1799245 (3 pages)

Priya Mahadevan and Alex Zunger
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Change of the emission spectra in organic light-emitting diodes by layer thickness modification

C. H. Cheung, A. B. Djurišić, C. Y. Kwong, H. L. Tam, K. W. Cheah, Z. T. Liu, W. K. Chan, P. C. Chui, J. Chan, and A. D. Rakić

Appl. Phys. Lett. 85, 2944 (2004); http://dx.doi.org/10.1063/1.1802386 (3 pages) | Cited 4 times

Online Publication Date: 14 October 2004

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Electroluminescence and photoluminescence of organic light-emitting diodes consisting of an indium tin oxide anode, N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine as a hole transport layer, tris (8-hydroxyquinoline) aluminum as emitting layer, and an Ag cathode were measured for different layer thickness values. It was found that, for a certain range of thickness values, multiple peak emission can be achieved. In addition, the emission spectra were dependent on the viewing angle. For the optimized thickness values, normal incidence chromaticity coordinates achieved were 0.32 and 0.43. Possible explanations for observed unexpected behavior are discussed.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.66.-w Optical properties of specific thin films
78.55.Hx Other solid inorganic materials
68.55.-a Thin film structure and morphology
72.20.Fr Low-field transport and mobility; piezoresistance
73.50.Dn Low-field transport and mobility; piezoresistance

Performance enhancement of high-speed SiGe-based heterojunction phototransistor with substrate terminal

Jin-Wei Shi, Z. Pei, F. Yuan, Y.-M. Hsu, C.-W. Liu, S. C. Lu, and M.-J. Tsai

Appl. Phys. Lett. 85, 2947 (2004); http://dx.doi.org/10.1063/1.1799237 (3 pages) | Cited 1 time

Online Publication Date: 14 October 2004

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Heterojunction phototransistors (HPTs) are requested to have high electrical bandwidth (∼1 GHz) performance for their application of high-speed digital fiber communication. In this letter, a method is disclosed to enhance the speed performance of Si∕SiGe-based HPTs, which can overcome the low quantum efficiency drawback (∼0.1 A∕W) of Si-based high-speed photodetectors at the wavelength of 850 nm due to its large internal gain. By use of the substrate terminal of HPT, the speed performance can be enhanced greatly with much less reduction in optical gain as compared to the traditional technique with base-terminal-bias. Under proper optical power excitation and the common ground of substrate and emitter terminals, we can achieve 1.8 GHz fast-Fourier-transformed electrical bandwidth with 0.7 A∕W responsivity simultaneously. The demonstrated device structure can serve as a key component in the short-reach fiber communication system.
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85.60.Dw Photodiodes; phototransistors; photoresistors
85.60.Gz Photodetectors (including infrared and CCD detectors)
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Photogenerated hole carrier injection to YBa2Cu3O7−x in an oxide heterostructure

Y. Muraoka, T. Muramatsu, J. Yamaura, and Z. Hiroi

Appl. Phys. Lett. 85, 2950 (2004); http://dx.doi.org/10.1063/1.1803616 (3 pages) | Cited 22 times

Online Publication Date: 14 October 2004

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We have fabricated a YBa2Cu3O7−x∕SrTiO3:Nb heterostructure and measured the current–voltage and photovoltaic properties under ultraviolet light irradiation at room temperature. A large photovoltage of 0.8 V is observed and is positive to the film. The photovoltage appears under illumination of light with photon energy larger than 3.2 eV. These results indicate that photogenerated hole carriers in the SrTiO3:Nb substrate are injected to the film. The maximum surface hole density is attained to be 3.5×1013 cm−2 at a light power of 44 mW∕cm2. The present photocarrier injection technique could apply to many transition metal oxides to control the hole carrier density externally.
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74.72.-h Cuprate superconductors
74.78.-w Superconducting films and low-dimensional structures
72.40.+w Photoconduction and photovoltaic effects
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
74.25.Jb Electronic structure (photoemission, etc.)
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
73.25.+i Surface conductivity and carrier phenomena
73.50.Pz Photoconduction and photovoltaic effects

Effect of keto defects on the electrical properties of fluorene-based oligomers

Yong-Young Noh, Dong-Yu Kim, Yuji Yoshida, Kiyoshi Yase, Byung-Jun Jung, Eunhee Lim, Hong-Ku Shim, and Reiko Azumi

Appl. Phys. Lett. 85, 2953 (2004); http://dx.doi.org/10.1063/1.1787939 (3 pages) | Cited 6 times

Online Publication Date: 14 October 2004

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The effect of ketonic defects on electrical properties, i.e., the performance of organic field-effect transistors (OFETs) was examined in fluorene end capped fused bithiophene oligomers (BFTT). The long wavelength emission at 2.1–2.3 eV resulting from the ketonic defects was observed in photoluminescence spectra of BFTT films after UV irradiation in air. In addition, the peak corresponding to the carbonyl stretching mode of the fluorenone moiety at 1721 cm−1 was also apparent after UV irradiation for periods longer than 6 h in air. These observations confirm that ketonic defects are present in the fluorene units of BFTT after photo-oxidation. The threshold voltage (Vth), i.e., switch-on voltage, of OFETs was increased and field-effect mobility (μFET) was decreased after the formation of the ketonic defects, since these defects induce the formation of numerous trap sites in the bandgap of the semiconducting conjugated oligomer.
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85.30.Tv Field effect devices
71.20.Rv Polymers and organic compounds
73.61.Ph Polymers; organic compounds
78.66.Qn Polymers; organic compounds
78.55.Kz Solid organic materials
68.55.-a Thin film structure and morphology
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.20.Fr Low-field transport and mobility; piezoresistance
73.50.Dn Low-field transport and mobility; piezoresistance

Ultrasensitive radio-frequency pseudomorphic high-electron-mobility-transistor readout for quantum devices

Nikolai Oukhanski and Eckhardt Hoenig

Appl. Phys. Lett. 85, 2956 (2004); http://dx.doi.org/10.1063/1.1790598 (3 pages) | Cited 5 times

Online Publication Date: 14 October 2004

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Two versions of a cryogenic multistage pseudomorphic high-electron-mobility field-effect transistor amplifier (based on the AlGaAs/InGaAs/GaAs heterostructure) have been designed for quantum device readout and tested at an ambient temperature ∼380 mK. The minimum noise temperature of the first amplifier version is below 110±25 mK(∼80±20 hf∕kB) at 28.6 MHz, estimated from the noise of input 10 kΩ resistance and coupled input tank circuit with an active resistance at the resonant frequency RS(f0)≈17.9 kΩ. Its minimum voltage spectral noise density, with respect to the input, is about 200 pV∕(Hz)1∕2 and the corner frequency of the 1∕f noise is close to 300 kHz. For the amplifier with the lowest designed back action, the minimum noise temperature below 130±30 mK(∼100±25 hf∕kB) at 26.8 MHz was estimated when coupled to an input tank circuit with RS(f0)≈61.8 kΩ. The power consumption of the amplifiers is in the range of 100–600 μW.
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85.30.Tv Field effect devices
84.30.Le Amplifiers
72.70.+m Noise processes and phenomena
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Magnetocurrent in a bipolar spin transistor at room temperature

Y. W. Huang, C. K. Lo, Y. D. Yao, L. C. Hsieh, J. J. Ju, D. R. Huang, and J. H. Huang

Appl. Phys. Lett. 85, 2959 (2004); http://dx.doi.org/10.1063/1.1796522 (3 pages) | Cited 6 times

Online Publication Date: 14 October 2004

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A spin transistor which consists of a metallic giant magnetoresistance emitter, a copper base, and a p-n junction was prepared on a Si(100) wafer. The emitter current changes from 1 mA at a magnetically parallel state to 0.968 mA at a magnetically antiparallel state. At the same states the base currents were 29.3 μA and 333 nA, respectively, which gave a magnetocurrent ratio of ∼8600% and a transfer ratio of 3×10−2 at room temperature for a common collector configuration. The sensitivity of this spin device is higher than 4000%∕Oe. The memory effect and the high performance make it possible for practical usage. The working principle of this kind of three-terminal spin device can be simply described by circuit theory.
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85.30.Pq Bipolar transistors
75.47.De Giant magnetoresistance
72.25.Mk Spin transport through interfaces

Pressure-induced changes in the conductivity of AlGaN∕GaN high-electron mobility-transistor membranes

B. S. Kang, S. Kim, F. Ren, J. W. Johnson, R. J. Therrien, P. Rajagopal, J. C. Roberts, E. L. Piner, K. J. Linthicum, S. N.G. Chu, K. Baik, B. P. Gila, C. R. Abernathy, and S. J. Pearton

Appl. Phys. Lett. 85, 2962 (2004); http://dx.doi.org/10.1063/1.1800282 (3 pages) | Cited 39 times

Online Publication Date: 14 October 2004

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AlGaN∕GaN high-electron-mobility transistors (HEMTs) show a strong dependence of source∕drain current on the piezoelectric-polarization-induced two-dimensional electron gas. The spontaneous and piezoelectric-polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of pressure changes. The changes in the conductance of the channel of a AlGaN∕GaN high electron mobility transistor (HEMT) membrane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in the ambient pressure. The conductivity of the channel shows a linear change of −(+)6.4×10−2 mS∕bar for application of compressive (tensile) strain. The AlGaN∕GaN HEMT membrane-based sensors appear to be promising for pressure sensing applications.
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85.30.Tv Field effect devices
72.80.Ey III-V and II-VI semiconductors
77.22.Ej Polarization and depolarization

Highly-efficient broadband waveguide outcoupling in light-emitting diodes with self-organized polymer blends

N. Corcoran, P. K. H. Ho, A. C. Arias, J. D. Mackenzie, R. H. Friend, G. Fichet, and W. T. S. Huck

Appl. Phys. Lett. 85, 2965 (2004); http://dx.doi.org/10.1063/1.1801680 (3 pages) | Cited 7 times

Online Publication Date: 14 October 2004

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Self-organized, two-dimensional micron-scale photonic structures have been fabricated within the emissive layer of polymer blend light-emitting diodes (LEDs). The relief-and-phase grating is achieved by phase separation of two semiconducting polymers directed by a surface chemical pattern. Short-wavelength oscillations are found pinned (and are thus phase-locked) to the domain boundaries. These high-frequency harmonics mimic short-period gratings in providing efficient waveguide outcoupling but without spectral dispersion. This provides a general way to harness the waveguide modes trapped in polymer LEDs, doubling their external quantum and power efficiencies while maintaining spectral integrity with viewing angle.
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85.60.Jb Light-emitting devices
42.79.Gn Optical waveguides and couplers
64.75.-g Phase equilibria
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