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29 Aug 2005

Volume 87, Issue 9, Articles (09xxxx)

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Appl. Phys. Lett. 87, 093109 (2005); http://dx.doi.org/10.1063/1.2035332 (3 pages)

J. Noborisaka, J. Motohisa, S. Hara, and T. Fukui
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Anisotropy in ordered sexithiophene thin films studied by angle-resolved photoemission using combined laser and synchrotron radiation

C. E. Heiner, J. Dreyer, I. V. Hertel, N. Koch, H.-H. Ritze, W. Widdra, and B. Winter

Appl. Phys. Lett. 87, 093501 (2005); http://dx.doi.org/10.1063/1.2034105 (3 pages) | Cited 13 times

Online Publication Date: 22 August 2005

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We present angle-resolved photoemission (PE) spectra of ordered multilayer sexithiophene (6T) films, 200 nm thick, grown on a Au(110) single crystal. However, the measurement of sharp and nonshifted PE spectral features from the low-conducting organic material is only possible if the positive surface charge, generated in the PE process, is fully compensated. We have accomplished this by simultaneous laser irradiation. On the basis of the resulting data we found that for these thick films the 6T molecules are preferentially oriented with their long axes nearly normal to the surface.
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79.60.Fr Polymers; organic compounds
61.82.Pv Polymers, organic compounds
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.80.Cb X-ray effects
73.40.-c Electronic transport in interface structures

Performance of a diamond x-ray sensor fabricated with metal-less graphitic contacts

P. J. Sellin and A. Galbiati

Appl. Phys. Lett. 87, 093502 (2005); http://dx.doi.org/10.1063/1.2035885 (3 pages) | Cited 9 times

Online Publication Date: 23 August 2005

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We report the x-ray photocurrent response of a coplanar chemical vapor deposition diamond detector fabricated using a metal-less graphitic ohmic contact. Ion implantation of 70 keV boron ions to a dose of 2×1016 cm−2 was performed through a patterned photoresist to produce a coplanar graphitic contact structure. The device photocurrent showed a fast response to pulsed x-ray irradiation, and showed no evidence of photocurrent persistence that is observed in devices fabricated using metal Schottky contacts. The graphite-contact device also showed no extrinsic photoconductivity when illuminated with white light.
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07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
85.60.Gz Photodetectors (including infrared and CCD detectors)
73.50.Pz Photoconduction and photovoltaic effects

Suns-photoluminescence: Contactless determination of current-voltage characteristics of silicon wafers

T. Trupke, R. A. Bardos, M. D. Abbott, and J. E. Cotter

Appl. Phys. Lett. 87, 093503 (2005); http://dx.doi.org/10.1063/1.2034109 (3 pages) | Cited 16 times

Online Publication Date: 23 August 2005

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In good silicon solar cells, the separation of the quasi-Fermi energies Δη in the bulk is equivalent to the cell voltage. Photoluminescence is used to measure Δη in both bifacial solar cells and partly processed solar cells. The bifacial cells are used to demonstrate that simultaneous measurement of the photoluminescence signal and of the variable incident light intensity yields pseudo current-voltage characteristics, equivalent to Suns-open circuit voltage (VOC) measurements, but in contactless mode. The applicability of this method to unfinished solar cells, without the need for a solar cell structure, is demonstrated on silicon wafers after various processing steps.
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84.60.Jt Photoelectric conversion
78.55.Ap Elemental semiconductors

Effective intermediate layers for highly efficient stacked organic light-emitting devices

J. X. Sun, X. L. Zhu, H. J. Peng, M. Wong, and H. S. Kwok

Appl. Phys. Lett. 87, 093504 (2005); http://dx.doi.org/10.1063/1.2035320 (3 pages) | Cited 34 times

Online Publication Date: 24 August 2005

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Effective intermediate electrode layers comprising of LiF(1 nm)/Ca(25 nm)/Ag(15 nm) or LiF(1 nm)/Al(3 nm)/Au(15 nm) were studied in stacked organic light-emitting devices (OLEDs). Stacked OLEDs with two identical emissive units consisting of NPB/Alq3: C545T/BCP exhibited superior luminous efficiency-current density characteristics over conventional single-unit devices. At 20 mA/cm2, the luminous efficiency of the stacked OLEDs using the intermediate layers of LiF/Ca/Ag and LiF/Al/Au were about 19.6 cd/A and 17.5 cd/A, respectively, almost doubling that of the corresponding control devices, as expected.
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85.60.Jb Light-emitting devices

Interaction of organic surfaces with active species in the high-vacuum environment

V. Podzorov, E. Menard, S. Pereversev, B. Yakshinsky, T. Madey, J. A. Rogers, and M. E. Gershenson

Appl. Phys. Lett. 87, 093505 (2005); http://dx.doi.org/10.1063/1.2035323 (3 pages) | Cited 9 times

Online Publication Date: 24 August 2005

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Using single-crystal organic field-effect transistors with the conduction channel exposed to environmental agents, we have observed generation of electronic defects at the organic surface in the high-vacuum environment. Rapid decrease of the source–drain current of an operating device is observed upon exposure of the channel to the species generated by high-vacuum gauges. We attribute this effect to interaction of the organic surface with electrically neutral free radicals produced in the process of hydrocarbon cracking on hot filaments with a relatively low activation energy Ea ∼ 2.5 eV (240 kJ/mol). The reported results might be important for optimizing the high-vacuum processes of fabrication and characterization of a wide range of organic and molecular electronic devices.
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85.30.Tv Field effect devices
85.65.+h Molecular electronic devices
68.47.Fg Semiconductor surfaces
73.20.At Surface states, band structure, electron density of states
73.20.Hb Impurity and defect levels; energy states of adsorbed species

Measurement of junction temperature in GaN-based laser diodes using voltage-temperature characteristics

Han-Youl Ryu, Kyoung-Ho Ha, Jung-Hye Chae, Ok-Hyun Nam, and Yong-Jo Park

Appl. Phys. Lett. 87, 093506 (2005); http://dx.doi.org/10.1063/1.2037201 (3 pages) | Cited 20 times

Online Publication Date: 24 August 2005

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We present a method to determine junction temperature in GaN-based laser diodes (LDs) for simple, fast, and reliable characterization of thermal properties. The large change of forward operation voltage with temperature in GaN laser diodes is advantageously used to measure junction temperature. Using this method, we compare junction temperature of LD structures with different substrates and chip mounting methods. It is found that the junction temperature can be reduced considerably by employing GaN substrates or epi-down bonding. For epi-down bonded LDs, as much as two-fold reduction in junction temperature is achieved compared to epi-up bonded ones and junction temperature rise in this case is only about 13 degrees for more than 100 mW-output power.
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42.55.Px Semiconductor lasers; laser diodes

Organic heterojunction and its application for double channel field-effect transistors

Jun Wang, Haibo Wang, Xuanjun Yan, Haichao Huang, and Donghang Yan

Appl. Phys. Lett. 87, 093507 (2005); http://dx.doi.org/10.1063/1.2037204 (3 pages) | Cited 84 times

Online Publication Date: 25 August 2005

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Heterojunction organic field-effect transistor (OFET) based on p-type copper phthalocyanine (CuPc) and n-type hexadecafluorophthalocyaninatocopper (F16CuPc) was demonstrated. The heterojunction OFETs can be operated in normally-on (depletion-accumulation) mode, which attributes to the existence of a new conductive channel at the interface of heterojunction. The new channel is originated from accumulation of electrons and holes induced by the interface dipole. Compared with the device with CuPc single layer, the double channel transistor displays improved field-effect mobility from 0.017 to 0.042 cm2/V s, and threshold voltage shifts from −17 to +19 V. In addition, ambipolar electric characteristics have been observed from the heterojunction OFETs.
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85.30.Tv Field effect devices
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Incorporation of electroluminescence and electrochemiluminescence in one organic light-emitting device

Changgua Zhen, Yutao Chuai, Chunfeng Lao, Lan Huang, Dechun Zou, Do Nam Lee, and Byeong Hyo Kim

Appl. Phys. Lett. 87, 093508 (2005); http://dx.doi.org/10.1063/1.2012522 (3 pages) | Cited 6 times

Online Publication Date: 26 August 2005

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The incorporation of electroluminescence and electrochemiluminescence in one light-emitting device with the configuration of indium tin oxide (ITO)/[Ru(bpy)2(dimbpy)](PF6)2/Alq3(30 nm)/NPB(45 nm)/Ag(60 nm) is investigated. When the ITO and Ag electrodes are negatively and positively biased respectively, the color of the emission changes from green to light yellow-white within 100 s. So there is only one recombination zone in the bulk of the Alq3 at the initial stage of the operation, followed by the formation of another recombination zone in the bulk of the [Ru(bpy)2(dimbpy)](PF6)2. This directly indicates the existence of unipolar injection (electron injection) process in the ruthenium complex layer. The external quantum efficiency of the device is 1.4% compared with 0.45% of the ITO/[Ru(bpy)2(dimbpy)](PF6)2/Ag device, improved by twofold. Furthermore, this hybrid device provides a feasible way to control the emission of a wide spectrum of colors including red, green, yellow, and white.
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85.60.Jb Light-emitting devices
78.60.Ps Chemiluminescence
78.60.Fi Electroluminescence
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