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22 Jan 2007

Volume 90, Issue 4, Articles (04xxxx)

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Appl. Phys. Lett. 90, 043507 (2007); http://dx.doi.org/10.1063/1.2435508 (3 pages)

D. Y. Kim and A. J. Steckl
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On the origin of the 1/f noise in shallow germanium p+-n junctions

R. M. Todi, S. Sonde, E. Simoen, C. Claeys, and K. B. Sundaram

Appl. Phys. Lett. 90, 043501 (2007); http://dx.doi.org/10.1063/1.2431759 (3 pages) | Cited 2 times

Online Publication Date: 23 January 2007

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The low-frequency noise of shallow germanium p+-n junctions is studied, for diodes with or without a nickel-germanide Ohmic contact. It is shown that the application of NiGe not only reduces the series resistance, resulting in a higher forward current, but also results in a lower 1/f noise at forward bias. From the observed geometry dependence, it is concluded that germanidation suppresses the 1/f noise generated in the series resistance, leaving surface-state-assisted generation-recombination at the junction perimeter as the dominant flicker noise source.
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85.30.Kk Junction diodes

High field-effect mobility ZnO thin-film transistors with Mg-doped Ba0.6Sr0.4TiO3 gate insulator on plastic substrates

KyongTae Kang, Mi-Hwa Lim, Ho-Gi Kim, Il-Doo Kim, and Jae-Min Hong

Appl. Phys. Lett. 90, 043502 (2007); http://dx.doi.org/10.1063/1.2434150 (3 pages) | Cited 14 times

Online Publication Date: 23 January 2007

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The authors report on the influence of Mg acceptor doping on the markedly reduced leakage current characteristics (<5×10−8 A/cm2 at 2 MV/cm) of Ba0.6Sr0.4TiO3 thin films. The suitability of room temperature deposited Mg-doped Ba0.6Sr0.4TiO3 films as gate insulators for low-voltage ZnO thin-film transistors (TFTs) (<6 V) was investigated. All room temperature processed ZnO-TFTs on plastic substrates exhibited a high field-effect mobility of 16.3 cm2/V s and a current on/off ratio of 6.4×104. The threshold voltage and subthreshold swing were 2.8 V and 400 mV/decade, respectively.
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85.30.Tv Field effect devices

Theoretical and experimental investigation of valence band offsets for direct silicon bond hybrid orientation technology

Sachin Joshi, Bhagwan Sahu, Sanjay K. Banerjee, Adrian Ciucivara, Leonard Kleinman, Rick Wise, Rinn Cleavelin, Angelo Pinto, Mike Seacrist, Mike Ries, Y.-T. Huang, Mike Ma, and C.-T. Lin

Appl. Phys. Lett. 90, 043503 (2007); http://dx.doi.org/10.1063/1.2434164 (3 pages) | Cited 5 times

Online Publication Date: 23 January 2007

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Direct silicon bonding (DSB) for hybrid orientation technology has recently generated a lot of interest due to the significant performance enhancements reported for p-channel metal oxide semiconductor devices fabricated on alternative substrate orientations. This letter reports on the experimental observation and density functional theory (DFT) based theoretical prediction of a valence band offset between the (100) and (110) silicon surfaces directly bonded to each other. This constitutes a different type of junction created by the presence of two different surface orientations in close proximity to each other and not by doping or material variations. Experimentally, this band offset was observed as an asymmetry in the forward and reverse current-voltage characteristics of a two terminal device designed to flow a current across the DSB interface. Further, the valence band offset obtained from DFT simulations was used in a conventional device simulator (TAURUS-MEDICI) to simulate the behavior of this structure. MEDICI results are in qualitative agreement with the predicted band offset of ∼ 85 meV between the (110) and (100) surfaces which result in a diodelike behavior of the current-voltage characteristics across the (110)/(100) interface. Temperature dependence of the I-V characteristics is also discussed.
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71.20.Mq Elemental semiconductors
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Solvent-vapor treatment induced performance enhancement of poly(3-hexylthiophene):methanofullerene bulk-heterojunction photovoltaic cells

Yun Zhao, Zhiyuan Xie, Yao Qu, Yanhou Geng, and Lixiang Wang

Appl. Phys. Lett. 90, 043504 (2007); http://dx.doi.org/10.1063/1.2434173 (3 pages) | Cited 96 times

Online Publication Date: 23 January 2007

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The authors report enhanced poly(3-hexylthiophene) (P3HT):methanofullerene (PCBM) bulk-heterojunction photovoltaic cells via 1,2-dichlorobenzene (DCB) vapor treatment and thermal annealing. DCB vapor treatment can induce P3HT self-organizing into ordered structure leading to enhanced absorption and high hole mobility. Further annealing the device at a high temperature, PCBM molecules begin to diffuse into aggregates and together with the ordered P3HT phase form bicontinuous pathways in the entire layer for efficient charge separation and transport. Compared to the control device that is merely annealed, optical absorption, short-circuit current, and power conversion efficiency are increased for the DCB vapor-treated cell.
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84.60.Jt Photoelectric conversion
72.80.Le Polymers; organic compounds (including organic semiconductors)
72.20.Fr Low-field transport and mobility; piezoresistance
78.40.Me Organic compounds and polymers
61.72.Cc Kinetics of defect formation and annealing

Insights on the physical mechanism behind negative bias temperature instabilities

M. Houssa, V. V. Afanas’ev, A. Stesmans, M. Aoulaiche, G. Groeseneken, and M. M. Heyns

Appl. Phys. Lett. 90, 043505 (2007); http://dx.doi.org/10.1063/1.2434176 (3 pages) | Cited 6 times

Online Publication Date: 23 January 2007

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Negative bias temperature instabilities are studied on metal-oxide-semiconductor structures, with hydrogen-passivated and depassivated (H-free) (111) and (100)Si/SiO2 interfaces. By combining electrical and electron-spin resonance measurements, it is found that Pb/Pb0 centers are generated on passivated surfaces, while their density is reduced on depassivated surfaces. These results suggest the liberation of atomic hydrogen species in the gate stack followed by their transport towards the Si/SiO2 interface and reaction with the Pb and PbH centers. It is shown that these species are likely not released from the Si substrate. Alternatively, the authors propose that they may result from the cracking of hydrogen- and water-related species at or close to the metal/SiO2 interface when the device is subjected to negative bias temperature stress.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
76.30.-v Electron paramagnetic resonance and relaxation

Wideband backward coupling based on anisotropic left-handed metamaterial

Jingjing Zhang, Hongsheng Chen, Yu Luo, Yu Yuan, Linfang Shen, Lixin Ran, and Jin Au Kong

Appl. Phys. Lett. 90, 043506 (2007); http://dx.doi.org/10.1063/1.2435323 (3 pages) | Cited 2 times

Online Publication Date: 23 January 2007

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Waveguides using a slab of left-handed metamaterial (LHM) adjacent to a slab of right-handed material (RHM) have shown backward coupling properties over the frequency range in the LHM passband where both permittivity and permeability are negative. In this letter the authors further show that when both slabs are LHMs with different passbands, the resultant backward coupling frequency band is a union of both passbands. The curious fact that the stop band of a metamaterial where permittivity is negative and permeability is positive can support propagating wave is shown theoretically. Backward couplers are demonstrated experimentally to have wideband and dual-band properties by judiciously choosing LHM-LHM and LHM-RHM slabs. This proposed method offers flexibility in producing wideband/multiband coupling and can be used to design and fabricate microwave devices.
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84.40.Az Waveguides, transmission lines, striplines

Liquid-state field-effect transistors using electrowetting

D. Y. Kim and A. J. Steckl

Appl. Phys. Lett. 90, 043507 (2007); http://dx.doi.org/10.1063/1.2435508 (3 pages) | Cited 4 times

Online Publication Date: 23 January 2007

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The authors report the demonstration of transistor action in the liquid state. The control of current flow in a liquid field-effect transistor (LiquiFET) was achieved by electrowetting between competitive insulating/conducting fluids. The LiquiFET structure included dielectric/hydrophobic layers, source/drain regions, a gate electrode, and hydrophilic/hydrophobic grids to contain the liquids. For a 400 μm long channel, turn-on occurs at 2.5–3 V drain voltage. On/off current ratios >10 000:1 were measured. Linear gate voltage control over drain current was obtained with a transconductance up to 40 nS. A calculated channel mobility of ∼ 1 cm2/V s indicates that electronic charge transport dominates transistor operation.
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85.30.Tv Field effect devices
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Measurement of piezoelectric coefficient of gallium nitride using metal-insulator-semiconductor capacitors

Vinayak Tilak, Paolo Batoni, Jie Jiang, and Aaron Knobloch

Appl. Phys. Lett. 90, 043508 (2007); http://dx.doi.org/10.1063/1.2434180 (3 pages) | Cited 1 time

Online Publication Date: 24 January 2007

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Gallium nitride cantilevers with metal-insulator-semiconductor capacitors were fabricated and characterized as a function of strain to determine the effective piezoelectric constant. These cantilevers were tested at room temperature through a combination of cantilever bending experiments, strain gauge measurements, and finite element modeling. By measuring the shift in flatband voltage to applied strain the magnitude of the piezoelectric charge induced is determined. The effective piezoelectric constant e31′ was determined from these measurements to be −0.57±0.03 C/m2.
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77.65.Bn Piezoelectric and electrostrictive constants
85.30.Tv Field effect devices
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Gate capacitance-voltage characteristics of submicron-long-gate diamond field-effect transistors with hydrogen surface termination

M. Kasu, K. Ueda, Y. Yamauchi, and T. Makimoto

Appl. Phys. Lett. 90, 043509 (2007); http://dx.doi.org/10.1063/1.2436649 (3 pages) | Cited 9 times

Online Publication Date: 25 January 2007

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The radio-frequency characteristics of p-type diamond field-effect transistors with hydrogen surface termination were numerically analyzed using an equivalent-circuit model. From the gate-source capacitance (CGS)-voltage (VGS) results extracted from measured s parameters, the authors found a plateau in CGS within a certain VGS range. This means that a two-dimensional hole gas channel forms parallel to the surface and that the channel is separated by a thin energy-barrier layer with an infinite height from the gate metal. At a high negative VGS, as negative VGS is increased, CGS increases steeply. This results from holes penetrating the energy barrier.
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85.30.Tv Field effect devices

Two-dimensional analysis for emitter ledge thickness of InGaP/GaAs heterojunction bipolar transistors

Shiou-Ying Cheng, Kuei-Yi Chu, Li-Yang Chen, Lu-Ann Chen, and Chun-You Chen

Appl. Phys. Lett. 90, 043510 (2007); http://dx.doi.org/10.1063/1.2432224 (3 pages) | Cited 3 times

Online Publication Date: 25 January 2007

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In this letter, the influence of various emitter ledge thicknesses on the performances of InGaP/GaAs heterojunction bipolar transistors is investigated based on the simulation and experimental data. The undesired surface channel phenomenon at the exposed base surface between base contact and emitter ledge is comprehensively analyzed. Moreover, improper thickness of emitter ledge passivations would cause serious surface recombination at the edge of emitter ledge. Therefore, the thickness of emitter ledge is a critical and should be carefully considered. From simulated and experimental results, the optimum emitter ledge thickness of InGaP/GaAs heterojunction bipolar transistor is 100–200 Å.
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85.30.Pq Bipolar transistors
85.30.De Semiconductor-device characterization, design, and modeling

Characterization of proton irradiated copolymer thin films for microelectromechanical system applications

T. Y. Lam, S. T. Lau, C. Chao, H. L. W. Chan, C. L. Choy, W. Y. Cheung, and S. P. Wong

Appl. Phys. Lett. 90, 043511 (2007); http://dx.doi.org/10.1063/1.2435342 (3 pages) | Cited 1 time

Online Publication Date: 25 January 2007

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The electrostrictive response of proton irradiated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] thin films, deposited on silicon (Si) substrates, has been characterized. By applying an ac field to the copolymer films, the induced surface displacement of the film along the thickness direction was measured using a laser interferometer. After the proton irradiation, the electric field induced strain response of the copolymer film is enhanced. Since the copolymer films are laterally clamped by the rigid substrate, the films cannot vibrate freely. At a proton dose of 50 Mrad, the effective electrostrictive coefficient Meff,33 of P(VDF-TrFE) thin film on Si is 0.07×10−18 V2/m2 which is ∼ 25 times smaller than that of bulk sample under the same irradiation dose (M33 = 1.76×10−18 V2/m2). Using bulk micromachining to etch away most of the Si substrate, an actuator in the form of a suspended membrane was fabricated. The effective electrostrictive coefficient at the center of the membrane Meff,33′ increased to 0.67×10−18 V2/m2 due to the weakening of the substrate clamping effect. The resonance characteristics of the actuators based on these irradiated copolymer films were studied.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.10.Cm Micromechanical devices and systems

Influence of water on the work function of conducting poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)

N. Koch, A. Vollmer, and A. Elschner

Appl. Phys. Lett. 90, 043512 (2007); http://dx.doi.org/10.1063/1.2435350 (3 pages) | Cited 32 times

Online Publication Date: 25 January 2007

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The influence of water exposure on the work function (ϕ) and surface composition of conducting poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDT:PSS) was investigated with ultraviolet and x-ray photoelectron spectroscopies. It was found that annealing PEDT:PSS in vacuum to 220 °C yields a high ϕ of 5.65 eV. Subsequent exposure to water vapor or air reduces ϕ to ∼ 5.15 eV, and the film surface becomes enriched with PEDT. These observations were fully reversible for repeated annealing–water exposure cycles. The reduction in ϕ is attributed to (i) the inclusion of water leading to a larger dielectric constant and (ii) polymer swelling-induced rearrangements of surface dipoles.
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73.61.Ph Polymers; organic compounds
73.30.+y Surface double layers, Schottky barriers, and work functions
79.60.Bm Clean metal, semiconductor, and insulator surfaces
81.40.Gh Other heat and thermomechanical treatments
77.22.Ch Permittivity (dielectric function)

Impact of interfacial roughness on tunneling conductance and extracted barrier parameters

Casey W. Miller, Zhi-Pan Li, Johan Åkerman, and Ivan K. Schuller

Appl. Phys. Lett. 90, 043513 (2007); http://dx.doi.org/10.1063/1.2431443 (3 pages) | Cited 13 times

Online Publication Date: 26 January 2007

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The net tunneling conductance of metal-insulator-metal tunnel junctions is studied using a distribution of barrier thicknesses consistent with interfacial roughness typical of state-of-the-art tunnel junctions. Moderate amounts of roughness cause the conductance to resemble that of much thinner and taller barriers. Fitting numerically generated conductance data that include roughness with models that assume a single-thickness barrier leads to erroneous results for both the barrier height and width. Rules of thumb are given that connect the roughness to the real space mean thickness and the thickness inferred from fitting the net conductance with traditional tunneling models.
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73.40.Gk Tunneling
68.35.Ct Interface structure and roughness
73.40.Rw Metal-insulator-metal structures

Electrophosphorescent devices from a poly(9-vinylcarbazole)/tris(2-phenylpyridine)iridium(III) bilayer with a concentration gradient

Jong Hyeok Park, Dong Hee Yu, O. Ok Park, and Jai Kyeong Kim

Appl. Phys. Lett. 90, 043514 (2007); http://dx.doi.org/10.1063/1.2432271 (3 pages) | Cited 4 times

Online Publication Date: 26 January 2007

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Electrophosphorescence and the dynamics of charge carrier recombination in poly(9-vinylcarbazole) (PVK)/tris(2-phenylpyridine)iridium [Ir-(ppy)3] bilayer electroluminescence devices were investigated. Because the second layer [Ir-(ppy)3] is spin cast from a solvent that slightly swells the PVK layer, the two layers are partially intermixed at the bilayer interface. Indium tin oxide (ITO)/3,4-polyethylenedioxythiophene-polystyrenesulfonate (PEDOT)/[PVK/Ir(ppy)3] bilayer/2,9-dimethyl-4,7-diphenyl-1, 10-phenanthroline (BCP)/Ca/Al ultilayer devices showed similar emission intensity and color quality to those of the PVK:Ir(ppy)3 blended device. However, turn-on and driving voltage were decreased dramatically compared with those of the ITO/PEDOT/PVK:Ir(ppy)3 blend/BCP/Ca/Al.
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85.60.Jb Light-emitting devices

Variable liquid crystal pretilt angles on various compositions of alignment layers

Jong Bok Kim, Kyung Chan Kim, Han Jin Ahn, Byoung Har Hwang, Dong Choon Hyun, and Hong Koo Baik

Appl. Phys. Lett. 90, 043515 (2007); http://dx.doi.org/10.1063/1.2432272 (3 pages) | Cited 28 times

Online Publication Date: 26 January 2007

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The authors introduce variable liquid crystal (LC) pretilt angles via ion beam (IB) irradiation of silicon carbide (SiC) layers of various compositions. To control the composition of the SiC layer, the authors altered the rf power ratio between the graphite target and silicon target. The pretilt angle of the silicon-rich SiC layer was constant regardless of IB irradiation angle; however, the carbon-rich SiC layer showed variable pretilt angles, depending on IB irradiation angle. The authors attribute variable pretilt angle to the competition between van der Waals interactions, favoring the vertical alignment, and pi-pi interactions, favoring the LC alignment parallel to IB direction.
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61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures
61.80.Jh Ion radiation effects

Enhanced NO2 selectivity of hybrid poly(3-hexylthiophene): ZnO-nanowire thin films

Vibha Saxena, D. K. Aswal, Manmeet Kaur, S. P. Koiry, S. K. Gupta, J. V. Yakhmi, R. J. Kshirsagar, and S. K. Deshpande

Appl. Phys. Lett. 90, 043516 (2007); http://dx.doi.org/10.1063/1.2432279 (3 pages) | Cited 16 times

Online Publication Date: 26 January 2007

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Sensing characteristics of the drop-cast thin films of poly(3-hexylthiophene) (P3HT), ZnO nanowires, and P3HT:ZnO-nanowire hybrids with different compositions have been studied for various gases, namely, NO2, H2S, NH3, CH4, and CO. At room temperature, pure P3HT and ZnO-nanowire films were highly sensitive to NO2 and H2S, moderately sensitive to CO, and nearly insensitive to NH3 and CH4. P3HT:ZnO-nanowire hybrid films, with increasing ZnO-nanowire content, exhibited gradual enhancement in NO2 sensitivity, while reducing sensitivity for H2S. P3HT:ZnO-nanowire (1:1 ratio by weight) films were found to be highly selective for NO2. Plausible mechanisms explaining the enhanced NO2 selectivity by hybrid films are discussed.
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82.80.-d Chemical analysis and related physical methods of analysis
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Influence of the electron beam on electromigration measurements within a scanning electron microscope

B. Stahlmecke and G. Dumpich

Appl. Phys. Lett. 90, 043517 (2007); http://dx.doi.org/10.1063/1.2432304 (3 pages) | Cited 4 times

Online Publication Date: 26 January 2007

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In situ electromigration studies on polycrystalline gold nanowires are performed using a high resolution scanning electron microscope. Simultaneously, the resistance of the gold nanowires is recorded during current stressing. The nanowires are prepared by electron beam lithography (EBL) and subsequent thermal evaporation of gold onto the resist mask. In a further EBL-process contact leads are attached to the gold nanowires to determine resistance changes of better than 5×10−4. The authors observe small resistance oscillations which occur during the scanning process. They show that these oscillations are due to the scanning process of the electron beam rather than founded on periodic changes of the void morphology.
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66.30.Qa Electromigration
61.80.Fe Electron and positron radiation effects
73.63.-b Electronic transport in nanoscale materials and structures
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