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25 Dec 2006

Volume 89, Issue 26, Articles (26xxxx)

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Appl. Phys. Lett. 89, 263110 (2006); http://dx.doi.org/10.1063/1.2424541 (3 pages)

Ch. Deneke, U. Zschieschang, H. Klauk, and O. G. Schmidt
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Current-voltage behavior in hole-only single-carrier devices with self-assembling dipole molecules on indium tin oxide anodes

Chimed Ganzorig, Masaru Sakomura, Kazuyoshi Ueda, and Masamichi Fujihira

Appl. Phys. Lett. 89, 263501 (2006); http://dx.doi.org/10.1063/1.2420792 (3 pages) | Cited 13 times

Online Publication Date: 26 December 2006

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The authors report the use of chemically modified indium tin oxide (ITO) with different binding groups (–COCl and –PO2Cl2) of p-chlorobenzene derivatives forming effective monolayers to control the work function of ITO and hence to enhance the hole injection. The enhanced hole injection is studied by measuring current density–voltage (J-V) characteristics. The behavior of J-V characteristics caused by varying the ITO work function in hole-only single-carrier devices with a hole transport layer of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine is examined. Upon grafting with p-chlorophenylphosphoryl dichloride, the J-V characteristics show a space-charge-limited conduction behavior. Such modified ITO anodes lead to improvements in the device properties.
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72.20.Fr Low-field transport and mobility; piezoresistance
72.20.Ee Mobility edges; hopping transport
72.20.Ht High-field and nonlinear effects
73.30.+y Surface double layers, Schottky barriers, and work functions
85.60.Jb Light-emitting devices

Grating-bicoupled plasmon-resonant terahertz emitter fabricated with GaAs-based heterostructure material systems

Taiichi Otsuji, Yahya Moubarak Meziani, Mitsuhiro Hanabe, Takuma Ishibashi, Tomohiro Uno, and Eiichi Sano

Appl. Phys. Lett. 89, 263502 (2006); http://dx.doi.org/10.1063/1.2410228 (3 pages) | Cited 23 times

Online Publication Date: 26 December 2006

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A grating-bicoupled plasmon-resonant terahertz emitter was fabricated using InGaP/InGaAs/GaAs heterostructure material systems. The device structure is based on a high-electron mobility transistor and incorporates doubly interdigitated grating gates that periodically localize the two-dimensional (2D) plasmon in 100 nm regions with a submicron interval. Photoexcited electrons, injected to the 2D plasmon cavities, extensively promoted the plasmon instability, resulting in observation of emission of terahertz electromagnetic radiation at room temperature.
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85.30.Tv Field effect devices
84.40.-x Radiowave and microwave (including millimeter wave) technology

Temperature effect on impact ionization characteristics in metamorphic high electron mobility transistors

Po-Hsien Lai, Ssu-I Fu, Ching-Wen Hung, Yan-Ying Tsai, Tzu-Pin Chen, Chun-Wei Chen, and Wen-Chau Liu

Appl. Phys. Lett. 89, 263503 (2006); http://dx.doi.org/10.1063/1.2410233 (3 pages) | Cited 6 times

Online Publication Date: 26 December 2006

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Comprehensive studies of temperature-dependent gate-metal-related impact ionization in metamorphic high electron mobility transistors (MHEMTs) are demonstrated. It is known that, from experimental results, the electric field and temperature dependences of impact ionization mechanisms in MHEMT’s operation are dominated by the ionization threshold energy and hot electron population. The peak impact ionization-induced gate current could be substantially decreased by the presence of specific gate contact with a higher Schottky barrier height. Therefore, the suppressions of bell-shaped behavior and related impact ionization effect are observed by using the appropriate Schottky gate contacts.
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85.30.Tv Field effect devices
85.40.Ls Metallization, contacts, interconnects; device isolation

Surface potential shield with aperture structure to improve potential measurement accuracy of scanning Kelvin probe microscopy

Zhitao Yang and Michael G. Spencer

Appl. Phys. Lett. 89, 263504 (2006); http://dx.doi.org/10.1063/1.2424281 (3 pages) | Cited 3 times

Online Publication Date: 27 December 2006

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Scanning Kelvin probe microscopy is widely used to measure surface work functions and electrostatic potentials. However, its measurement accuracy suffers from a cantilever effect. The authors introduce a surface potential shield with aperture structure to eliminate cantilever effect. By varying dc biases on the shield, the strength of cantilever effect is deliberately moderated and linear regression can be used to extract the true surface potential. Experimental results show that this approach yields accurate potential measurement, especially when there is only a single potential domain within aperture. A mobile potential shield structure mounted on a micromanipulator can make this approach more versatile.
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73.30.+y Surface double layers, Schottky barriers, and work functions
68.37.-d Microscopy of surfaces, interfaces, and thin films

Photocurrent gain mechanism in Schottky barrier photodiodes with negative average electric field

C. Rivera, J. L. Pau, and E. Muñoz

Appl. Phys. Lett. 89, 263505 (2006); http://dx.doi.org/10.1063/1.2424447 (3 pages) | Cited 5 times

Online Publication Date: 27 December 2006

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A photocurrent gain mechanism which takes advantage of the piezoelectric fields present in devices based on polar heterostructures is proposed. Piezoelectrically induced electric fields can be designed to generate a negative average electric field (NAF) region under certain bias conditions. For carrier transport limited by the barrier formed by the NAF region, photoinduced screening will result in photocurrent gain. This mechanism allows one to explain the experimental results obtained in Schottky barrier photodiodes with (In,Ga)N/GaN multiple quantum wells embedded in their active region. Responsivities higher than 1 A/W and low dark currents below 10 nA/mm2 at forward voltage were obtained.
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85.60.Dw Photodiodes; phototransistors; photoresistors

Transient thermal properties of high-power diode laser bars

Mathias Ziegler, Fritz Weik, Jens W. Tomm, Thomas Elsaesser, Włodzimierz Nakwaski, Robert P. Sarzała, Dirk Lorenzen, Jens Meusel, and Anna Kozłowska

Appl. Phys. Lett. 89, 263506 (2006); http://dx.doi.org/10.1063/1.2424644 (3 pages) | Cited 14 times

Online Publication Date: 27 December 2006

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The transient thermal properties of high-power diode laser bars with active and passive cooling are analyzed. Both thermal imaging and the analysis of the thermal wavelength tuning behavior are employed to extract the device temperature as a function of time. A steady-state thermal situation is established with rise times of about 10 and 60 ms for active and passive cooling, respectively. The latter number, however, is substantially increased by the particular properties of the external heat sink. Such results are confirmed by model calculations based on the finite element method.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Fc Modulation, tuning, and mode locking

Low hysteresis pentacene thin-film transistors using SiO2/cross-linked poly(vinyl alcohol) gate dielectric

Dong-Wook Park, Cheon An Lee, Keum-Dong Jung, Byung-Gook Park, Hyungcheol Shin, and Jong Duk Lee

Appl. Phys. Lett. 89, 263507 (2006); http://dx.doi.org/10.1063/1.2424666 (3 pages) | Cited 14 times

Online Publication Date: 27 December 2006

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Low hysteresis pentacene organic thin-film transistors (OTFTs) were fabricated using plasma-enhanced chemical vapor deposition SiO2/cross-linked poly(vinyl alcohol) gate dielectrics. The hysteresis was considerably reduced by optimizing the thickness of the SiO2 layer. The balancing of the electrons injected from the gate electrode into the gate dielectric and the trapped holes from the channel under the optimum condition is considered to be the mechanism by which the hysteresis is reduced. In addition, the optimized OTFTs with the hybrid gate dielectric show a high dielectric constant and reliable leakage characteristics due to the advantages afforded by each gate dielectric material. The device with W/L = 300/5 shows a field-effect mobility of 0.12 cm2/V s, a subthreshold slope of 0.4 V/decade, and an on/off ratio of 2.3×106. The bias stress experiments conducted for the OTFTs confirmed the effect of the injected charges and hole traps on the threshold voltage shift.
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85.30.Tv Field effect devices
85.40.Sz Deposition technology

Control of electro-osmostic flow by light

László Oroszi, András Dér, Huba Kirei, Pál Ormos, and Vilmos Rakovics

Appl. Phys. Lett. 89, 263508 (2006); http://dx.doi.org/10.1063/1.2424648 (3 pages) | Cited 5 times

Online Publication Date: 28 December 2006

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The author introduce a technique for the optical control of electro-osmosis. One wall of the microfluidic channel is formed by a photoconductive material. Illumination of the photoconductor results in a local drop of the electric field in the microchannel, reducing the electro-osmotic driving force. Fluid flow can be effectively modulated by light utilizing this effect. They verified the phenomenon in linear channels and further developed the method by building a Y junction, an optically controlled microfluidic switch. This unit is the basis of more complex flow patterns: it demonstrates the feasibility of dynamic optical control of microfluidic devices.
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82.45.-h Electrochemistry and electrophoresis
47.85.Np Fluidics
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.10.Cm Micromechanical devices and systems
47.60.-i Flow phenomena in quasi-one-dimensional systems

Contactless technique to quantify the edge-junction recombination in solar cells

Florence W. Chen and Jeffrey E. Cotter

Appl. Phys. Lett. 89, 263509 (2006); http://dx.doi.org/10.1063/1.2424676 (3 pages) | Cited 3 times

Online Publication Date: 28 December 2006

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This letter presents a method to determine the recombination properties of exposed edges of p-n-junction silicon devices, particularly, solar cells. The method is based on the extended analysis of injection level dependent carrier lifetime data of samples with various degrees of intentionally created edges. Its application to laser-cut edges is shown here. Values of per-length space-charge-region recombination current densities of approximately 2×10−8 and 2×10−9 A/cm have been determined for laser-cut- and laser-cut-and-etched edges. The technique is beneficial for both understanding edge recombination in solar cells and developing improved edge passivation techniques.
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84.60.Jt Photoelectric conversion
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
81.65.Rv Passivation

Optical evidence of Stark effect in single-walled carbon nanotube transistors

Taishi Takenobu, Yuji Murayama, and Yoshihiro Iwasa

Appl. Phys. Lett. 89, 263510 (2006); http://dx.doi.org/10.1063/1.2425009 (3 pages) | Cited 14 times

Online Publication Date: 29 December 2006

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The effect of an externally applied electric field in single-walled carbon nanotubes was studied using a thin-film transistor configuration. Under the electric field, the optical spectra displayed redshifts and broadening. These phenomena present evidence of the Stark effect in single-walled carbon nanotubes. The finding of the Stark effect suggests the potential use of carbon nanotubes in electro-optic devices for optical communication.
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85.30.Tv Field effect devices
85.35.Kt Nanotube devices

Permittivity enhancement of aluminum oxide thin films with the addition of silver nanoparticles

R. Ravindran, K. Gangopadhyay, S. Gangopadhyay, N. Mehta, and N. Biswas

Appl. Phys. Lett. 89, 263511 (2006); http://dx.doi.org/10.1063/1.2425010 (3 pages) | Cited 17 times

Online Publication Date: 29 December 2006

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Multilayer reactive electron-beam evaporation of thin aluminum oxide layers with embedded silver nanoparticles (Ag-nps) has been used to create a dielectric thin film with an enhanced permittivity. The results show a frequency dependent increase of the dielectric constant κ. Overall stack κ of the control sample was found to be 7.7–7.4 in the 1 kHz–1 MHz range. This is in comparison with κ = 16.7–13.0 over the same frequency range in the sample with Ag-nps. Capacitance-voltage and conductance-voltage measurements indicate the presence of charge capture resulting from the Ag-nps. The authors attribute this dielectric constant enhancement to dipole and space charge polarization mechanisms.
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77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)
77.22.Jp Dielectric breakdown and space-charge effects
77.22.Ej Polarization and depolarization
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy

Quantum efficiency enhancement in top-emitting organic light-emitting diodes as a result of enhanced intrinsic quantum yield

Qiang Huang, Sebastian Reineke, Karsten Walzer, Martin Pfeiffer, and Karl Leo

Appl. Phys. Lett. 89, 263512 (2006); http://dx.doi.org/10.1063/1.2425014 (3 pages) | Cited 22 times

Online Publication Date: 29 December 2006

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The authors compare red, green, and blue top- and conventional bottom-emitting organic light-emitting diodes (OLEDs) based on iridium complexes. Surprisingly, red top-emitting OLEDs are much more efficient than their bottom-emitting counterpart, while for green and blue devices, top and bottom emitters are comparable. The authors explain these results by an improved internal quantum efficiency only in the red top-emitting OLED: Due to strong microcavity effect, spontaneous emission of emitters in all top-emitting devices is enhanced. However, only the internal quantum yield of the red emitters is then improved due to its comparable radiative and nonradiative decay rates. They also show that the efficiency roll-off at higher current densities is reduced in top-emitting OLEDs.
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85.60.Jb Light-emitting devices

Investigating the stability of zinc oxide thin film transistors

R. B. M. Cross and M. M. De Souza

Appl. Phys. Lett. 89, 263513 (2006); http://dx.doi.org/10.1063/1.2425020 (3 pages) | Cited 93 times

Online Publication Date: 29 December 2006

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The stability of thin film transistors incorporating sputtered ZnO as the channel layer is investigated under gate bias stress. Positive stress results in a positive shift of the transfer characteristics, while negative stress results in a negative shift. Low bias stress has no effect on the subthreshold characteristics. This instability is believed to be a consequence of charge trapping at/near the channel/insulator interface. Higher biases and longer stress times cause degradation of the subthreshold slope, which is thought to arise as a consequence of defect state creation within the ZnO channel material. After all stress measurements, the devices recover their original characteristics at room temperature without any annealing.
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85.30.Tv Field effect devices
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