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15 Dec 2008

Volume 104, Issue 12, Articles (12xxxx)

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J. Appl. Phys. 104, 121301 (2008); http://dx.doi.org/10.1063/1.3041061 (17 pages)

Junhang Dong, Y. S. Lin, Masakoto Kanezashi, and Zhong Tang
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Modeling heat generation in a submicrometric n+−n−n+ silicon diode

Orazio Muscato and Vincenza Di Stefano

J. Appl. Phys. 104, 124501 (2008); http://dx.doi.org/10.1063/1.3041474 (10 pages) | Cited 7 times

Online Publication Date: 16 December 2008

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In this paper a hydrodynamic model for electron and phonon transport in silicon semiconductors has been formulated on the basis of the maximum entropy principle to describe off-equilibrium phenomena in submicron devices. One dimensional steady-state simulations of a n+−n−n+ silicon diode have been carried out.
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85.30.De Semiconductor-device characterization, design, and modeling
63.20.kd Phonon-electron interactions
85.30.Kk Junction diodes

Low frequency noise analysis on organic thin film transistors

Lin Ke, Surani Bin Dolmanan, Lu Shen, Chellappan Vijila, Soo Jin Chua, Rui-Qi Png, Perq-Jon Chia, Lay-Lay Chua, and Peter K.-H. Ho

J. Appl. Phys. 104, 124502 (2008); http://dx.doi.org/10.1063/1.3044440 (6 pages) | Cited 6 times

Online Publication Date: 16 December 2008

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Bottom-contact organic field-effect transistors (OFETs) based on poly(3-hexylthiophene) with different channel lengths were fabricated under different substrate pretreatment process conditions. These OFET devices were characterized using low frequency noise (LFN) spectroscopy, and the device performance parameters were correlated with the level of LFN. It is observed that the devices with higher noise levels showed poorer device properties when compared with the devices operated at same Ids of the same channel length. It is also observed that the noise level increased with the increase in channel length for devices with the same pretreatment conditions, which is due to increased trapping and detrapping in the channel material interface domain. The OFET device operating around the threshold voltage Vth will have a 1/f noise slope that is flatter, having a gradient that is smaller in magnitude. The threshold voltage of a device can thus be observed to be at the gate voltage in which 1/f noise intensity is the highest at low frequencies.
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85.30.Tv Field effect devices

Ohmic contacts to SnS films: Selection and estimation of thermal stability

M. Devika, N. Koteeswara Reddy, F. Patolsky, and K. R. Gunasekhar

J. Appl. Phys. 104, 124503 (2008); http://dx.doi.org/10.1063/1.3041622 (6 pages) | Cited 2 times

Online Publication Date: 17 December 2008

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Selection of Ohmic contacts to SnS films is one of the crucial tasks for the fabrication of efficient SnS devices. Thus, we examined different nonalloyed metallization schemes, namely, Ag, Al, In, and Sn to SnS films since their Fermi level pins exactly in between the conduction and valance bands of SnS. To explore the Ohmic behavior of M/SnS (M = Ag, Al, In, Sn) structures, the electrical properties of as-grown structures have been studied at different temperatures. From these studies it is noticed that at room temperature all M/SnS structures, except Ag/SnS, have an excellent Ohmic behavior over the voltage range from −10 to 10 V. However, Ag/SnS structures showed Ohmic trend only in the voltage range of ±6 V. The stability of the M/SnS structures was also examined by annealing them at different temperatures (300–500 °C) and the obtained peculiar results are reported.
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73.40.Ns Metal-nonmetal contacts
71.20.Nr Semiconductor compounds
85.40.Ls Metallization, contacts, interconnects; device isolation
68.55.ag Semiconductors
68.60.Dv Thermal stability; thermal effects
61.72.Cc Kinetics of defect formation and annealing

A comparative study of plasma-enhanced chemical vapor gate dielectrics for solution-processed polymer thin-film transistor circuit integration

Flora M. Li, Arokia Nathan, Yiliang Wu, and Beng S. Ong

J. Appl. Phys. 104, 124504 (2008); http://dx.doi.org/10.1063/1.3029704 (12 pages) | Cited 4 times

Online Publication Date: 17 December 2008

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This paper considers plasma-enhanced chemical vapor deposited (PECVD) silicon nitride (SiNx) and silicon oxide (SiOx) as gate dielectrics for organic thin-film transistors (OTFTs), with solution-processed poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene] (PQT-12) as the active semiconductor layer. We examine transistors with SiNx films of varying composition deposited at 300 °C as well as 150 °C for plastic compatibility. The transistors show over 100% (two times) improvement in field-effect mobility as the silicon content in SiNx increases, with mobility (μFE) up to 0.14 cm2/V s and on/off current ratio (ION/IOFF) of 108. With PECVD SiOx gate dielectric, preliminary devices exhibit a μFE of 0.4 cm2/V s and ION/IOFF of 108. PQT-12 OTFTs with PECVD SiNx and SiOx gate dielectrics on flexible plastic substrates are also presented. These results demonstrate the viability of using PECVD SiNx and SiOx as gate dielectrics for OTFT circuit integration, where the low temperature and large area deposition capabilities of PECVD films are highly amenable to integration of OTFT circuits targeted for flexible and lightweight applications.
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85.30.Tv Field effect devices
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
77.55.-g Dielectric thin films

p-type doping in organic light emitting diodes based on fluorinated C60

Young-Jun Yu, Olga Solomeshch, Helena Chechik, A. A. Goryunkov, R. F. Tuktarov, Dong Hoon Choi, Jung-Il Jin, Yoav Eichen, and Nir Tessler

J. Appl. Phys. 104, 124505 (2008); http://dx.doi.org/10.1063/1.3043872 (3 pages) | Cited 6 times

Online Publication Date: 17 December 2008

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We report on hybrid organic light emitting diodes based on spin coated PVK (poly(vinylcarbazole))/poly-TPD (poly(triphenyldiamine)) formulation electron blocking and conjugated peptide emitter layers while the hole blocking, BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline), layer is vacuum sublimed. The device structure is realized through the use of fluorinated C60 as a p-dopant in a cross-linked hole transporting formulation. The lowering of the turn-on voltage is demonstrated using a conjugated peptide as the emitter layer. We suggest that fluorinated C60s could play a major role as tunable dopants in organic electronics.
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85.60.Jb Light-emitting devices
61.72.up Other materials

Modeling of electrical characteristics of midwave type II InAs/GaSb strain layer superlattice diodes

V. Gopal, E. Plis, J.-B. Rodriguez, C. E. Jones, L. Faraone, and S. Krishna

J. Appl. Phys. 104, 124506 (2008); http://dx.doi.org/10.1063/1.3042232 (6 pages) | Cited 10 times

Online Publication Date: 18 December 2008

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This paper reports the results of modeling of electrical characteristics of midinfrared type II InAs/GaSb strain layer superlattice (SLS) diode with p-on-n polarity. Bulk based model with the effective band gap of SLS material has been used in modeling of the experimental data. Temperature dependence of zero-bias resistance area product (R0A) and bias dependent dynamic resistance of the diode have been analyzed in detail to investigate dark current contributing mechanisms that are limiting the electrical performance of the diode. R0A of the diode is found to be limited by thermal diffusion currents at higher temperatures and Ohmic shunt resistance contribution limits it at low temperatures ∼ 82 K.
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85.30.Kk Junction diodes

Undulator radiation in a periodic magnetic field with a constant component

G. Dattoli, V. V. Mikhailin, and K. V. Zhukovsky

J. Appl. Phys. 104, 124507 (2008); http://dx.doi.org/10.1063/1.3039094 (8 pages) | Cited 2 times

Online Publication Date: 22 December 2008

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We present an analytical investigation of radiation emitted by an electron moving in a planar undulator with a constant magnetic field component added to the periodic one. In order to calculate the spectrum and angular distribution of radiation, we use the technique of generalized Bessel functions. In particular, this method is effectively applied for the study of the influence of the constant magnetic field on the undulator radiation emitted both along and off its axis. The analytical formulation of the effect of the weak constant field enables us to estimate its critical magnitude that may produce a significant distortion in the undulator radiation distributions.
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41.60.-m Radiation by moving charges
02.30.Gp Special functions
41.75.Fr Electron and positron beams

Comparative study of organic thin film tandem solar cells in alternative geometries

B. Viktor Andersson, Nils-Krister Persson, and Olle Inganäs

J. Appl. Phys. 104, 124508 (2008); http://dx.doi.org/10.1063/1.3050346 (6 pages) | Cited 7 times

Online Publication Date: 22 December 2008

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Optical modeling of one folded tandem solar cell and four types of stacked tandem solar cells has been performed using the finite element method and the transfer matrix method for the folded cell and the stacked cells, respectively. The results are analyzed by comparing upper limits for short circuit currents and power conversion efficiencies. In the case of serial connected tandems all of the five cell types may be compared, and we find that the folded cells are comparable to stacked tandem cells in terms of currents and power conversion efficiencies.
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84.60.Jt Photoelectric conversion

Thermally activated field assisted carrier generation and transport in N,N′-di-[(1-naphthalenyl)-N,N′-diphenyl]-(1,1′ biphenyl)-4,4′-diamine doped with 2,3,5,6-tetrafluoro-7,7′,8,8′-tetracyanoquinodimethane

Gayatri Chauhan, Ritu Srivastava, Virendra Kumar Rai, Arunandan Kumar, S. S. Bawa, P. C. Srivastava, and M. N. Kamalasanan

J. Appl. Phys. 104, 124509 (2008); http://dx.doi.org/10.1063/1.3050305 (5 pages) | Cited 1 time

Online Publication Date: 23 December 2008

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Current density-voltage (J-V) characteristics of N,N′-di-[(1-naphthalenyl)-N,N′-diphenyl]-(1.1′ biphenyl)-4,4′-diamine (α-NPD) doped with 2,3,5,6-tetrafluoro-7,7′,8,8′-tetracyanoquinodimethane have been studied as a function of doping concentration (0–0.8 wt %) and temperature (105–300 K). The current density was found to increase with increase in doping concentration. In the doped samples as field increases above 3.3×104 V/cm the current abruptly starts increasing at a higher rate, which is ascribed as due to increased free charge carrier generation in the bulk. The enhanced free charge carrier generation is due to field assisted thermal dissociation of donor-acceptor pairs (Poole–Frenkel process) as well as charge injection at the interface. The released carriers increase the charge carrier density which brings the Fermi level near the highest occupied molecular orbital level of the α-NPD and reduces the space charge region near the interface favoring the tunneling of charge carrier across the interface, which is enough to support Ohmic conduction. The carrier generation has been found to be a thermally activated process. At higher fields (i.e., above 1.52×105 V/cm) the nonlinear J-V characteristics have been explained as due to field dependent mobility of holes.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.80.Le Polymers; organic compounds (including organic semiconductors)
61.72.U- Doping and impurity implantation
73.40.Gk Tunneling
71.20.Nr Semiconductor compounds
72.20.Ht High-field and nonlinear effects

Schrödinger solitons in left-handed SiO2–Ag–SiO2 and Ag–SiO2–Ag plasmonic waveguides calculated with a nonlinear transmission line approach

Attieh Shahvarpour, Shulabh Gupta, and Christophe Caloz

J. Appl. Phys. 104, 124510 (2008); http://dx.doi.org/10.1063/1.3048548 (5 pages) | Cited 1 time

Online Publication Date: 23 December 2008

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The existence of solitons in SiO2–Ag–SiO2 and Ag–SiO2–Ag plasmonic waveguides (PWGs) is investigated using a nonlinear transmission line approach. Both the SiO2–Ag–SiO2 and the Ag–SiO2–Ag, the former in its TM even mode and the latter in its odd mode, are shown to support bright solitons as solutions to the nonlinear Schrödinger equation. The SiO2–Ag–SiO2 even mode has low left-handed (LH) loss but is undesirably mixed with a right-handed mode, while the Ag–SiO2–Ag odd mode exhibits low-loss pure LH characteristics and is therefore the most suited mode for soliton propagation. Soliton PWGs may find applications in compact switching, pulse shaping, and pulse compressing devices.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
42.65.Tg Optical solitons; nonlinear guided waves
84.40.Az Waveguides, transmission lines, striplines

“Hump” characteristics and edge effects in polysilicon thin film transistors

A. Valletta, P. Gaucci, L. Mariucci, G. Fortunato, and F. Templier

J. Appl. Phys. 104, 124511 (2008); http://dx.doi.org/10.1063/1.3050323 (6 pages) | Cited 2 times

Online Publication Date: 23 December 2008

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Transfer characteristics of polycrystalline silicon (polysilicon) thin film transistors (TFTs) often show a “hump” in subthreshold regime. This effect, also observed in silicon-on-insulator (SOI) transistors, can be attributed to the presence of an enhanced electrical field at edges of the channel, which is related to the specific shape of the edge and its surrounding oxide. In this paper we attempt an analysis of the hump effect in polysilicon TFTs combining electrical measurements and two dimensional numerical simulations, and considering several geometries of the channel edge. The transfer characteristics showing the hump effect are analyzed in terms of a parallel of the main (“bulk”) transistor with two parasitic transistors located at the channel edges. The main and parasitic transistors have different threshold voltages and subthreshold swings and the equivalent parallel circuit reproduces very well the experimental transfer characteristics. The effect on the hump of interface states and oxide fixed charge, localized at the edge regions, is also analyzed and it is found that a degradation of the edge interfaces can easily lead to a hump reduction, thus explaining the large variability in this effect observed for different devices and different processes.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling

Photonic crystal superlattice employed as left-handed planoconcave lens

Y. Saado, Y. Neve-Oz, M. Golosovsky, D. Davidov, and A. Frenkel

J. Appl. Phys. 104, 124512 (2008); http://dx.doi.org/10.1063/1.3041630 (5 pages) | Cited 1 time

Online Publication Date: 24 December 2008

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We demonstrate focusing of the microwave radiation by a planoconcave lens based on photonic crystal superlattice. The superlattice consists of dielectric rods of two different diameters. The structural modulation results in a narrow transmission subband inside the band gap. The subband is characterized by a negative refractive index and high transmission efficiency. This negative refractive index arises from the Brillouin zone folding in the lattice with the composite unit cell. The full width at half maximum of the focal spot of our lens is 0.36λ×0.42λ, which is close to the Abbe limit, and the power enhancement is 2.2.
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42.25.Bs Wave propagation, transmission and absorption
63.20.-e Phonons in crystal lattices
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.05.Zx New materials: theory, design, and fabrication

Effect of transverse mode structure on the far field pattern of metal-metal terahertz quantum cascade lasers

P. Gellie, W. Maineult, A. Andronico, G. Leo, C. Sirtori, S. Barbieri, Y. Chassagneux, J. R. Coudevylle, R. Colombelli, S. P. Khanna, E. H. Linfield, and A. G. Davies

J. Appl. Phys. 104, 124513 (2008); http://dx.doi.org/10.1063/1.3043795 (4 pages) | Cited 5 times

Online Publication Date: 24 December 2008

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We elucidate the effects of the lateral mode structure on the far field pattern of metal-metal ridge-waveguide terahertz quantum cascade lasers. By introducing a 6-μm-wide metal gap on the top metal contact, we suppress odd-parity lateral modes and drastically modify the far field pattern. Measurements are in good qualitative agreement with full three-dimensional finite-difference-time-domain modeling. Experimental evidence of nonuniform current pumping on the intensity distribution of the guided mode (and hence the far field pattern) is also presented and explained in terms of gain guiding.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
84.40.Az Waveguides, transmission lines, striplines

Analytical ballistic theory of carbon nanotube transistors: Experimental validation, device physics, parameter extraction, and performance projection

Deji Akinwande, Jiale Liang, Soogine Chong, Yoshio Nishi, and H.-S. Philip Wong

J. Appl. Phys. 104, 124514 (2008); http://dx.doi.org/10.1063/1.3050345 (7 pages) | Cited 9 times

Online Publication Date: 29 December 2008

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We developed a fully analytical ballistic theory of carbon nanotube field effect transistors enabled by the development of an analytical surface potential capturing the temperature dependence and gate and quantum capacitance electrostatics. The analytical ballistic theory is compared to the experimental results of a ballistic transistor with good agreement. The validated analytical theory enables intuitive circuit design, provides techniques for parameter extraction of the bandgap and surface potential, and elucidates on the device physics of drain optical phonon scattering and its role in reducing the linear conductance and intrinsic gain of the transistor. Furthermore, a threshold voltage definition is proposed reflecting the bandgap-diameter dependence. Projections for key analog and digital performances are discussed.
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85.35.Kt Nanotube devices
85.30.Tv Field effect devices
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