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12 Jun 2006

Volume 88, Issue 24, Articles (24xxxx)

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Appl. Phys. Lett. 88, 243101 (2006); http://dx.doi.org/10.1063/1.2211007 (3 pages)

G. H. Du, F. Xu, Z. Y. Yuan, and G. Van Tendeloo
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Light switching via thermo-optic effect of micromachined silicon prism

J. Li, A. Q. Liu, X. M. Zhang, and T. Zhong

Appl. Phys. Lett. 88, 243501 (2006); http://dx.doi.org/10.1063/1.2212278 (3 pages) | Cited 4 times

Online Publication Date: 12 June 2006

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This letter presents an optical switch using total internal reflection (TIR) via thermo-optic effect (TOE). The switching function is obtained by changing the refractive index of silicon prism via TOE to switch the light across the TIR state. The structure is fabricated by microelectromechanical systems technology. The switch measures a cross talk of >30 dB and an extinction ratio of >40 dB but exhibits high insertion losses of 31.67 and 18.95 dB in the TIR and transmission states, respectively. Compared with conventional micromechanical and light-tuned switches, this switch is unique in mechanism and free of mechanical instability.
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42.79.Ta Optical computers, logic elements, interconnects, switches; neural networks
42.79.Bh Lenses, prisms and mirrors
07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Thickness dependence of the efficiency of polymer:fullerene bulk heterojunction solar cells

M. Lenes, L. J. A. Koster, V. D. Mihailetchi, and P. W. M. Blom

Appl. Phys. Lett. 88, 243502 (2006); http://dx.doi.org/10.1063/1.2211189 (3 pages) | Cited 43 times

Online Publication Date: 12 June 2006

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We study the thickness dependence of the performance of bulk heterojunction solar cells based on poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] as electron donor and [6,6]-phenyl C61 butyric acid methyl ester as electron acceptor. Typically, these devices have an active layer thickness of 100 nm at which only 60% of the incoming light is absorbed. Increasing device thickness results in a lower overall power conversion efficiency, mainly due to a lowering of the fill factor. We demonstrate that the decrease in fill factor and hence device efficiency is due to a combination of charge recombination and space-charge effects.
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84.60.Jt Photoelectric conversion

Femtogram mass sensing platform based on lateral extensional mode piezoelectric resonator

Wei Pang, Le Yan, Hao Zhang, Hongyu Yu, Eun Sok Kim, and William C. Tang

Appl. Phys. Lett. 88, 243503 (2006); http://dx.doi.org/10.1063/1.2213975 (3 pages) | Cited 19 times

Online Publication Date: 12 June 2006

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This letter introduces a resonant mass sensor that is based on a lateral extensional mode piezoelectric resonator and has a minimum detectable mass of 10−15 g at room temperature and atmospheric pressure. The resonator with size of about 200×50×1 μm3 has a quality factor (Q) of >1400 at 60 MHz, and as small as 0.1 ppm shift of its resonant frequency can be detected. The 0.1 ppm detection capability corresponds to a mass uncertainty of only about 4.6 fg. We have experimentally demonstrated a minimum detectable frequency shift of ∼ 1.6 ppm due to absorption of isopropanol vapor (73 fg) on the sidewalls of the parylene-coated lateral extensional mode piezoelectric resonator.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
77.65.Fs Electromechanical resonance; quartz resonators
85.50.-n Dielectric, ferroelectric, and piezoelectric devices

High electrical power density from PbTe-based quantum-dot superlattice unicouple thermoelectric devices

T. C. Harman, R. E. Reeder, M. P. Walsh, B. E. LaForge, C. D. Hoyt, and G. W. Turner

Appl. Phys. Lett. 88, 243504 (2006); http://dx.doi.org/10.1063/1.2209707 (3 pages) | Cited 8 times

Online Publication Date: 13 June 2006

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We describe the first demonstration of PbTe-based, cross-plane quantum-dot superlattice (QDSL) unicouple thermoelectric generator devices fabricated from nanostructured, thick-film materials ( ∼ 100 μm). Both n- and p-type QDSL materials were investigated. With ∼ 220 K temperature difference across small thermoelements ( ∼ 95 μm length, 4 mm2 cross-sectional area), electrical power outputs up to 89 mW and power densities up to 2.2 W/cm2 have been demonstrated for both n- and p-type materials. The devices consist of a substrate-free, bulklike slab of molecular beam epitaxy grown PbSeTe/PbTe QDSL material as the n- or p-type leg and a copper wire as the other p-type leg.
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73.63.Kv Quantum dots
73.61.Le Other inorganic semiconductors
85.80.Fi Thermoelectric devices
84.60.Rb Thermoelectric, electrogasdynamic and other direct energy conversion

Organic light-emitting devices with a mixture emitting layer of tris-(8-hydroxyquinoline) aluminum and 4,4′-bis(carbazol-9-yl)-biphenyl

B. J. Chen, X. W. Sun, and K. R. Sarma

Appl. Phys. Lett. 88, 243505 (2006); http://dx.doi.org/10.1063/1.2209708 (3 pages) | Cited 9 times

Online Publication Date: 13 June 2006

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Organic light-emitting devices with a mixture of tris-(8-hydroxyqunoline) aluminum (Alq3) and 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) as the emitting layer have been fabricated. The devices were fabricated in the same run with a standard device without CBP for comparison, with an identical structure of indium tin oxide (ITO)/m-MTDATA (80 nm)/NPB (20 nm)/CBP:Alq3 (40 nm)/BCP (10 nm)/Alq3 (60 nm)/Mg:Ag (200 nm), where m-MTDATA is 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine, which is used to improve hole injection; NPB is N,N′-di(naphth-2-yl)-N,N′-diphenyl-benzidine; and BCP is 2,9-Dimethyl-4,7-diphenyl-1,110-phenanthorline. The ratio of CBP to Alq3 in mixture was varied from 0 to 2. For device with a ratio of 0.5, the current efficiency and power efficiency were significantly improved by 35% and 32%, respectively, compared to the standard device with Alq3 only as emitting layer. By increasing the ratio to 2, the current efficiency and power efficiency were dropped by 20% and 11%, respectively. The electroluminescence spectra showed a slight blueshift with the increase of CBP to Alq3 ratio.
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78.66.Qn Polymers; organic compounds
85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence

Energy-dispersive coherent scatter computed tomography

J. Delfs and J.-P. Schlomka

Appl. Phys. Lett. 88, 243506 (2006); http://dx.doi.org/10.1063/1.2209710 (3 pages) | Cited 2 times

Online Publication Date: 13 June 2006

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We present a reconstructive x-ray imaging technique that yields the spatially resolved coherent scatter form factor of macroscopic objects. High spectral resolution is obtained by the energy and angle analyses of coherently scattered photons. In comparison with transmission x-ray computed tomography structural information about the materials under investigation is obtained. The combination of the attenuation contrast and the scatter contrast obtained with this technique can be applied, e.g., in medical imaging or baggage inspection.
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81.70.Tx Computed tomography
87.59.bd Computed radiography
07.85.-m X- and γ-ray instruments

Control of single-wall-nanotube field-effect transistors via indirect long-range optically induced processes

K. S. Narayan, Manohar Rao, R. Zhang, and P. Maniar

Appl. Phys. Lett. 88, 243507 (2006); http://dx.doi.org/10.1063/1.2209712 (3 pages) | Cited 5 times

Online Publication Date: 13 June 2006

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We observe significant changes in the response of single-wall-carbon-nanotube-based field-effect transistors upon photoexcitation in the presence of optically active conjugated polymer network. The primary features observed are in the form of an increase in the current in the depletion mode upon photoexcitation. Pulsed measurements indicate that the transistor enters the depleted state prior to the rise in current brought about by the transfer of the photogenerated carriers from the semiconducting polymer to the nanotube under depletion bias.
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85.35.Kt Nanotube devices
85.30.Tv Field effect devices
73.63.Fg Nanotubes
81.07.De Nanotubes

Substrate patterning for liquid crystal alignment by optical interference

Xuemin Lu, Fuk Kay Lee, Ping Sheng, H. S. Kwok, V. Chigrinov, and Ophelia K. C. Tsui

Appl. Phys. Lett. 88, 243508 (2006); http://dx.doi.org/10.1063/1.2209713 (3 pages) | Cited 8 times

Online Publication Date: 13 June 2006

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Inhomogeneous liquid crystal (LC) alignment surfaces comprising a succession of microdomains favoring different LC alignment directions have been demonstrated for a number of optoelectronic applications. However, the prevalent method used to fabricate these surfaces is time consuming and produce functional areas that are too small for practical use. Here, we demonstrate a simple method based on photopatterning of an azodye layer with an interference pattern produced by intercepting two coherent UV beams. This method can produce alignment patterns within seconds with a practical size of ∼ (0.5 cm)2.
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42.70.Df Liquid crystals
61.30.-v Liquid crystals
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
81.65.Cf Surface cleaning, etching, patterning
42.25.Hz Interference

Oxygen transport and reaction mechanisms in rhenium gate contacts on hafnium oxide films on Si

R. P. Pezzi, M. Copel, M. Gordon, E. Cartier, and I. J. R. Baumvol

Appl. Phys. Lett. 88, 243509 (2006); http://dx.doi.org/10.1063/1.2209720 (3 pages) | Cited 8 times

Online Publication Date: 13 June 2006

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Oxygen transport and incorporation were investigated following postdeposition annealing of metal-oxide-semiconductor structures having ultrathin rhenium films as metal electrode and HfO2 films as dielectric on Si(001). Isotopic tracing, nuclear reaction analysis, narrow resonant nuclear reaction profiling, and x-ray photoelectron spectroscopy were used to pursue this investigation. For annealing temperatures below 400 °C, oxygen from the gas phase incorporates mainly in near-surface regions of the overlying Re cap. Significant oxygen incorporation into the HfO2 films is observed only after annealing at 500 °C. The present results are discussed considering that supplying oxygen to the metal/dielectric interface can cause device threshold voltage shifts.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
77.55.-g Dielectric thin films
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
61.72.Cc Kinetics of defect formation and annealing
82.80.-d Chemical analysis and related physical methods of analysis
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
79.60.Jv Interfaces; heterostructures; nanostructures

Modeling of annealing kinetics for hydrogenated-amorphous-silicon-based solar cells using two-component metastable defects

Seung Yeop Myong and Koeng Su Lim

Appl. Phys. Lett. 88, 243510 (2006); http://dx.doi.org/10.1063/1.2213507 (3 pages) | Cited 6 times

Online Publication Date: 14 June 2006

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We propose a kinetic model for the annealing in p-i-n-type hydrogenated-amorphous-silicon (a-Si:H)-based solar cells using a combination of the fill factor and “fast” and “slow” metastable defect states in their absorbers. Reported annealing data are simulated on p-i-n-type a-Si:H-based solar cells using the proposed model in order to confirm its validity. The recovery kinetic dependence on the thermal annealing temperature, biased voltage, and phase of the absorber controlled by the hydrogen dilution ratio during deposition are reviewed. Furthermore, we suggest a recovery mechanism for the solar cells based on long range hydrogen diffusion.
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84.60.Jt Photoelectric conversion
61.72.Cc Kinetics of defect formation and annealing
71.55.Cn Elemental semiconductors

Efficient organic light-emitting devices using an iridium complex as a phosphorescent host and a platinum complex as a red phosphorescent guest

Toshimitsu Tsuzuki, Yuji Nakayama, Junji Nakamura, Takeshi Iwata, and Shizuo Tokito

Appl. Phys. Lett. 88, 243511 (2006); http://dx.doi.org/10.1063/1.2213017 (3 pages) | Cited 13 times

Online Publication Date: 15 June 2006

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We demonstrated efficient organic light-emitting devices (OLEDs) using a phosphorescent host/guest system consisting of bis(2-phenylpyridinato-N,C2′)iridium(acetylacetonate) [(ppy)2Ir(acac)] as a host and a platinum complex (Pt-SA-1) as a guest. The OLED using (ppy)2Ir(acac) film doped with Pt-SA-1 (1 wt %) showed an ideal red emission via efficient energy transfer from the host to the guest. The external quantum efficiency of the device was as high as 8.3%. The driving voltage was significantly reduced compared with a device using a conventional host of 4,4′-di(carbazole-9-yl)biphenyl, which resulted from the enhancement of the hole injection from the hole-transport layer to the host.
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85.60.Jb Light-emitting devices

Highly efficient solution processed blue organic electrophosphorescence with 14 lm/W luminous efficacy

Mathew K. Mathai, Vi-En Choong, Stelios A. Choulis, Benjamin Krummacher, and Franky So

Appl. Phys. Lett. 88, 243512 (2006); http://dx.doi.org/10.1063/1.2212060 (3 pages) | Cited 62 times

Online Publication Date: 16 June 2006

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We report highly efficient solution processed blue electrophosphorescent organic light emitting diodes (PHOLEDs) utilizing a phosphorescent dye and a nonconjugated polymer host, molecularly doped with electron transporting molecules. Based on a bilayer device architecture blue PHOLEDs with luminous efficacy of 14 lm/W at luminous efficiency reaching 22 cd/A are demonstrated. Analysis of device performance indicates that this high efficiency is achieved by a combination of improved charge balance and light outcoupling efficiency. Our results demonstrate that simple solution processed devices can have efficiencies similar to those published to date for small molecule multilayer PHOLEDs based on the same emitter.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.55.Kz Solid organic materials
61.72.up Other materials

Low-voltage high-mobility pentacene thin-film transistors with polymer/high-k oxide double gate dielectrics

D. K. Hwang, Kimoon Lee, Jae Hoon Kim, Seongil Im, Chang Su Kim, Hong Koo Baik, Ji Hoon Park, and Eugene Kim

Appl. Phys. Lett. 88, 243513 (2006); http://dx.doi.org/10.1063/1.2206555 (3 pages) | Cited 32 times

Online Publication Date: 16 June 2006

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We report on the fabrication of pentacene-based thin-film transistors (TFTs) with poly-4-vinylphenol (PVP)/yttrium oxide (YOx) double gate insulator films. The minimum PVP and YOx layer thicknesses were chosen to be 45 and 50 nm, respectively. The PVP and YOx double dielectric layers with the minimum thicknesses exhibited a high dielectric capacitance of 70.8 nF/cm2 and quite a good dielectric strength of ∼ 2 MV/cm at a leakage current level of ∼ 10−6 A/cm2 while the leakage current from either PVP or YOx alone was too high. Our pentacene TFTs with the 45 nm thin PVP/50 nm thin YOx films operated at −5 V showing a high field effect mobility of 1.74 cm2/V s and a decent on/off current ratio of 104. Our work demonstrates that the PVP/YOx double layer is a promising gate dielectric to realize low-voltage high-mobility organic TFTs.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling

Ruthenium gate electrodes on SiO2 and HfO2: Sensitivity to hydrogen and oxygen ambients

L. Pantisano, T. Schram, Z. Li, J. G. Lisoni, G. Pourtois, S. De Gendt, D. P. Brunco, A. Akheyar, V. V. Afanas’ev, S. Shamuilia, and A. Stesmans

Appl. Phys. Lett. 88, 243514 (2006); http://dx.doi.org/10.1063/1.2212288 (3 pages) | Cited 18 times

Online Publication Date: 16 June 2006

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The incorporation of chemical species during annealing at the interface between the Ru gate electrode and the dielectric perturbs the electrostatic potential, thus affecting the effective work function of the metal. For both SiO2 and HfO2 gate dielectrics, O2 anneal at 550 °C leads to Ru oxidation (i.e., to RuOx formation) at the Ru/dielectric interface, resulting in a higher effective work function than that of metallic Ru. In turn, when RuOx is exposed to hydrogen, a work function reduction occurs, caused by either a reduction of RuOx into Ru or hydroxyl incorporation at the interface.
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73.40.Ns Metal-nonmetal contacts
81.40.Gh Other heat and thermomechanical treatments
77.55.-g Dielectric thin films
73.30.+y Surface double layers, Schottky barriers, and work functions
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
81.65.Mq Oxidation

High-k and low-k nanocomposite gate dielectrics for low voltage organic thin film transistors

Chang Su Kim, Sung Jin Jo, Sung Won Lee, Woo Jin Kim, Hong Koo Baik, Se Jong Lee, D. K. Hwang, and Seongil Im

Appl. Phys. Lett. 88, 243515 (2006); http://dx.doi.org/10.1063/1.2213196 (3 pages) | Cited 20 times

Online Publication Date: 16 June 2006

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CeO2–SiO2 nanocomposite films were used as the gate dielectrics in organic thin film transistors (OTFTs) with pentacene active semiconductor. CeO2–SiO2 composite films exhibited a high dielectric capacitance of 57 nF/cm2 with exceptionally low leakage current. Good device characteristics were obtained with saturation at low operating voltages ( ∼ 2 V) and with a field effect mobility of 0.84 cm2 V−1 s−1, a threshold voltage of ∼ 0.25 V, an on/off current ratio of 103, and a subthreshold slope of 0.3 V/decade, whereas the gate leakage current density is considerably lowered. These results should therefore increase the prospects of using OTFTs in low power applications such as portable devices.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.55.-g Dielectric thin films
85.30.Tv Field effect devices
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