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9 Jul 2007

Volume 91, Issue 2, Articles (02xxxx)

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Appl. Phys. Lett. 91, 023101 (2007); http://dx.doi.org/10.1063/1.2755879 (3 pages)

M. Fendrich and T. Kunstmann
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Polyacrylonitrile as a gate dielectric material

Hui-Lin Hsu, Wei-Chang Yang, Ya-Lien Lee, and Tri-Rung Yew

Appl. Phys. Lett. 91, 023501 (2007); http://dx.doi.org/10.1063/1.2753696 (3 pages) | Cited 3 times

Online Publication Date: 10 July 2007

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A soluble organic material, polyacrylonitrile (PAN), was investigated for its feasibility of gate dielectric applications. The solution-processed PAN was spin coated in air. Results show that leakage current density as low as 0.3 nA/cm2 for 50 nm PAN dielectrics could be achieved via process optimization. Functional transistor characteristics were achieved in air for the implementation of PAN in organic thin-film transistors using pentacene or poly(3-hexylthiophene) as a semiconductor material.
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77.55.-g Dielectric thin films
77.84.Jd Polymers; organic compounds
73.61.Ph Polymers; organic compounds
85.30.Tv Field effect devices

High-efficiency GaInP/GaAs/InGaAs triple-junction solar cells grown inverted with a metamorphic bottom junction

J. F. Geisz, Sarah Kurtz, M. W. Wanlass, J. S. Ward, A. Duda, D. J. Friedman, J. M. Olson, W. E. McMahon, T. E. Moriarty, and J. T. Kiehl

Appl. Phys. Lett. 91, 023502 (2007); http://dx.doi.org/10.1063/1.2753729 (3 pages) | Cited 30 times

Online Publication Date: 10 July 2007

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The authors demonstrate a thin, Ge-free III–V semiconductor triple-junction solar cell device structure that achieved 33.8%, 30.6%, and 38.9% efficiencies under the standard 1 sun global spectrum, space spectrum, and concentrated direct spectrum at 81 suns, respectively. The device consists of 1.8 eV Ga0.5In0.5P, 1.4 eV GaAs, and 1.0 eV In0.3Ga0.7As p-n junctions grown monolithically in an inverted configuration on GaAs substrates by organometallic vapor phase epitaxy. The lattice-mismatched In0.3Ga0.7As junction was grown last on a graded GaxIn1−xP buffer. The substrate was removed after the structure was mounted to a structural “handle.” The current-matched, series-connected junctions produced a total open-circuit voltage over 2.95 V at 1 sun.
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84.60.Jt Photoelectric conversion
81.05.Ea III-V semiconductors

High-efficiency nondoped white organic light-emitting devices

Qing-Xiao Tong, Shiu-Lun Lai, Mei-Yee Chan, Jian-Xin Tang, Hoi-Lun Kwong, Chun-Sing Lee, and Shuit-Tong Lee

Appl. Phys. Lett. 91, 023503 (2007); http://dx.doi.org/10.1063/1.2756137 (3 pages) | Cited 25 times

Online Publication Date: 10 July 2007

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High-efficiency nondoped white organic light-emitting devices (WOLEDs) were demonstrated by using both the intrinsic and exciplex emissions from a single electroluminescent material, 4,4′,4″-trispyrenylphenylamine (TPyPA). The simple device structure of indium tin oxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine/TPyPA/4,7-diphenyl-1,10-phenanthroline/LiF/Al exhibited a luminance of 10 000 cd/m2 at a low driving voltage of 4.5 V, and high current and power efficiencies of 9.4 cd/A and 9.0 lm/W, respectively. Such WOLED showed excellent color stability and purity with the Commission Internationale de L’Eclairage coordinates of (0.31, 0.35), which remained unchanged over a wide range of luminance from 100 to 20 000 cd/m2.
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85.60.Jb Light-emitting devices

Theory of the dynamic response of a coplanar grid semiconductor detector

A. G. Kozorezov, J. K. Wigmore, A. Owens, and A. Peacock

Appl. Phys. Lett. 91, 023504 (2007); http://dx.doi.org/10.1063/1.2755931 (3 pages) | Cited 2 times

Online Publication Date: 11 July 2007

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The authors have developed a theoretical model for the response of a coplanar grid semiconductor detector to hard x- and γ-ray radiation. Carrier drift trajectories were obtained by solving the coupled dynamical equations for carriers driven by electrostatic fields of the coplanar grid configuration. The pulse spectra calculated by summing the individual contributions for all carriers are compared to experimental results for a large volume optimized cadmium zinc telluride coplanar grid detector and good agreement is obtained.
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29.40.Wk Solid-state detectors

White organic light-emitting devices with a bipolar transport layer between blue fluorescent and orange phosphorescent emitting layers

Ping Chen, Wenfa Xie, Jiang Li, Tao Guan, Yu Duan, Yi Zhao, Shiyong Liu, Chunsheng Ma, Liying Zhang, and Bin Li

Appl. Phys. Lett. 91, 023505 (2007); http://dx.doi.org/10.1063/1.2757096 (3 pages) | Cited 17 times

Online Publication Date: 11 July 2007

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White organic light-emitting devices based on an orange phosphorescent iridium complex, bis(2-(2-fluorphenyl)-1,3-benzothiozolato-N,C2′) iridium (acetylacetonate) [(F-BT)2Ir(acac)] and blue fluorescent 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl are reported. By introducing a bipolar transport 4,4′N,N′-dicarbazole-biphenyl layer between the fluorescent and the phosphorescent emission layers, additional light emission from (F-BT)2Ir(acac) is observed. The authors attributed it to the elimination of the Dexter energy transfer between the two emitters. Pure white emission with Commission Internationale de I’Eclairage coordinates of (0.33, 0.34) and a maximum luminance of 40960 cd/m2 were obtained. The maximum current efficiency and the color rendering index of the device are 13.4 cd/A and 71, respectively.
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85.60.Jb Light-emitting devices
85.30.De Semiconductor-device characterization, design, and modeling

Threshold voltage shifts in Si passivated (100)Ge p-channel field effect transistors: Insights from first-principles modeling

G. Pourtois, M. Houssa, B. De Jaeger, B. Kaczer, F. Leys, M. Meuris, M. Caymax, G. Groeseneken, and M. M. Heyns

Appl. Phys. Lett. 91, 023506 (2007); http://dx.doi.org/10.1063/1.2756367 (3 pages) | Cited 7 times

Online Publication Date: 11 July 2007

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An asymmetric shift of the capacitance-voltage characteristics of n-Ge/Si/SiOx/HfO2/TaN p-channel field effect transistor is reported, namely, a shift of the threshold voltage toward positive values in inversion, while the flatband voltage remains constant. First-principles calculations on silicon-passivated germanium surfaces reveal the formation of a dipole layer at the germanium/silicon interface, which leads to a decrease of the substrate work function/threshold voltage by 0.4–0.5 V. Silicon-induced surface states are also found in the germanium band gap. When the substrate Fermi level is located near these states, electrons are transferred to the silicon layer and compensate the work function shift, explaining the absence of flatband voltage shift.
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85.30.Tv Field effect devices

No bias pi cell using a dual alignment layer with an intermediate pretilt angle

Jong Bok Kim, Kyung Chan Kim, Han Jin Ahn, Byoung Har Hwang, Jong Tae Kim, Sung Jin Jo, Chang Su Kim, Hong Koo Baik, Chu Ji Choi, Min Kyoung Jo, Youn Sang Kim, Jin Seol Park, and Daeseung Kang

Appl. Phys. Lett. 91, 023507 (2007); http://dx.doi.org/10.1063/1.2757121 (3 pages) | Cited 29 times

Online Publication Date: 12 July 2007

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The authors fabricated a no-bias pi cell using a dual alignment layer with an intermediate pretilt angle via a rubbing. In the dual alignment layer system, the competition between crest region favoring the vertical alignment and trough region favoring planar alignment made it possible to achieve various pretilt angles, and adjusted pretilt angle from 90° to 20° with rubbing. In addition, as the intermediate pretilt angle plays a role in eliminating the activation energy and thus allowing formation of the initial bend state in pi cell fabrication, this approach achieved a no-bias pi cell for a liquid crystal display with both low power consumption and fast response.
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42.79.Kr Display devices, liquid-crystal devices
85.60.Pg Display systems

Oxygen plasma treatment and postaging of pentacene field-effect transistors for improved mobility

Kwonwoo Shin, Sang Yoon Yang, Chanwoo Yang, Hayoung Jeon, and Chan Eon Park

Appl. Phys. Lett. 91, 023508 (2007); http://dx.doi.org/10.1063/1.2756321 (3 pages) | Cited 5 times

Online Publication Date: 13 July 2007

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The authors fabricated pentacene transistors with high mobilities by controlling the morphology of pentacene film through adjustments to the surface energy of the gate dielectrics with oxygen plasma treatment, and then by improving the interfacial properties through postaging. The increased surface energy of poly(methylmethacrylate) dielectric that results from the oxygen plasma treatment improves the interconnections between grains and enlarges the grain size. The postaging of transistors is presumed to rearrange the interface functional groups and as a result decrease the polar functionality without changing pentacene film morphology, which reduces the number of trap states and increases the mobility to 0.73 cm2/V s.
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85.30.Tv Field effect devices

Performance improvement of polymer solar cells by using a solution processible titanium chelate as cathode buffer layer

Zhan’ao Tan, Chunhe Yang, Erjun Zhou, Xiang Wang, and Yongfang Li

Appl. Phys. Lett. 91, 023509 (2007); http://dx.doi.org/10.1063/1.2757125 (3 pages) | Cited 22 times

Online Publication Date: 13 July 2007

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A solution processible titanium chelate, titanium (diisopropoxide) bis (2,4-pentanedionate) (TIPD), was used as the cathode buffer layer in the polymer solar cells (PSCs) based on the blend of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] and [6,6]-phenyl-C61-butyric acid methyl ester. Introducing TIPD buffer layer reduced the interface resistance between the active layer and Al electrode, leading to a lower device resistance. The power conversion efficiency of the PSC with TIPD buffer layer reached 2.52% under the illumination of AM1.5, 100 mW/cm2, which is increased by 51.8% in comparison with that (1.66%) of the device without TIPD buffer layer under the same experimental conditions.
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84.60.Jt Photoelectric conversion

Design of high-efficiency GaN-based light emitting diodes with vertical injection geometry

Hyunsoo Kim, Kyoung-Kook Kim, Kwang-Ki Choi, Hyungkun Kim, June-O Song, Jaehee Cho, Kwang Hyeon Baik, Cheolsoo Sone, Yongjo Park, and Tae-Yeon Seong

Appl. Phys. Lett. 91, 023510 (2007); http://dx.doi.org/10.1063/1.2756139 (3 pages) | Cited 39 times

Online Publication Date: 13 July 2007

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The authors report on the design and fabrication of high-efficiency GaN-based light emitting diodes (LEDs) with vertical-injection geometry. Based on the analyses of LED test patterns fabricated with various n-electrode dimensions, a design rule for vertical LEDs is proposed. It is found that the suppression of the vertical current under n electrodes and the efficient injection of the spreading current across the n layers are essential to fabricate high-efficiency LEDs. Introduction of the current blocking layer along with well-designed branched n electrodes results in a large enhancement of power efficiency by a factor of 1.9, compared with that of reference LEDs.
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85.60.Jb Light-emitting devices
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