Inter-wire Coupling Research Articles

Innovative basis and weight functions for wire junctions in time domain moment method

A wireframe implementation of the method of moments in time domain is presented, with particular attention to wire junctions. Peculiar base and weight functions will be defined at the junctions to implicitly account for current continuity and correctly take into account inter-wire coupling. This will allow implementation of both an explicit and an implicit solution. Results obtained are validated with existing frequency domain solutions.

A coupling and crosstalk-considered timing-driven global routing algorithm for high-performance circuit design

With the exponential reduction in the scaling of feature size, inter-wire coupling capacitance becomes the dominant part of load capacitance. Two problems are introduced by couplings delay deterioration and crosstalk. This paper presents a timing-driven global routing algorithm along with consideration of coupling effects and crosstalk avoidance. Our work differs from the existing ones in that we design a global routing “framework” which performs well in terms of routablity, timing, and also facilitates the detailed routing in crosstalk avoidance. Experimental results on industrial circuits show that the algorithm leads to substantial delay reduction and effective crosstalk elimination.

Optical routing and sensing with nanowire assemblies

The manipulation of photons in structures smaller than the wavelength of light is central to the development of nanoscale integrated photonic systems for computing, communications, and sensing. We assemble small groups of freestanding, chemically synthesized nanoribbons and nanowires into model structures that illustrate how light is exchanged between subwavelength cavities made of three different semiconductors. The coupling strength of the optical linkages formed when nanowires are brought into contact depends both on their volume of interaction and angle of intersection. With simple coupling schemes, lasing nanowires can launch coherent pulses of light through ribbon waveguides that are up to a millimeter in length. Also, interwire coupling losses are low enough to allow light to propagate across several right-angle bends in a grid of crossed ribbons. The fraction of the guided wave traveling outside the wire/ribbon cavities is used to link nanowires through space and to separate colors within multiribbon networks. In addition, we find that nanoribbons function efficiently as waveguides in liquid media and provide a unique means for probing molecules in solution or in proximity to the waveguide surface. Our results lay the spadework for photonic devices based on assemblies of active and passive nanowire elements and presage the use of nanowire waveguides in microfluidics and biology.

Electron transport in crossed nano-wire junctions

Electron transport in a crossed nano-wire junction is studied using a tight-binding model. Increase in the inter-wire tunneling coupling strength suppresses the transmissions through the junction around the first subband edge. The junction's transport characteristics can be explained by the formation of bound or quasi-bound states at the junction.

Magnetism of monoatomic wires on vicinal surfaces

There are three key steps in the development of one-dimensional nanostructures: fabrication, characterization and application. In our contribution we concentrate on the second step, the characterization of the structure and magnetic behavior of monoatomic wires grown on Cu and Ag surfaces with a high density of steps from the theoretical point of view. At first, the geometry of clean Cu(11n) and Cu(10n) vicinal surfaces is optimized by means of ab initio density-functional calculations, and the formation energies of steps along the 〈110〉 and 〈100〉 directions are compared with experimental estimates. In the following, the monoatomic Fe wires are placed at various positions onto Cu substrate. The preferential positions of wires are predicted to be the inner corner sites of the steps. From the total energy differences between ferromagnetic and antiferromagnetic configurations the effective intrawire and interwire magnetic coupling constants are estimated. In all investigated cases the resulting magnetic order is ferromagnetic. Finally, the possibility of magnetism in monoatomic wires made of fifth and sixth row elements, which are nonmagnetic in a solid, is explored and discussed. The obtained results are in complete agreement with the known trends that a reduced dimensionality promotes magnetism.

Control signal layout ordering scheme minimisingcross-coupling effect in deep-submicrometre datapath design

In deep submicrometre technology, an inter-wire coupling capacitance dominates wire loading and makes interconnect delays very data-dependent. Reducing this cross-coupling effect is crucial for high-speed lower-power operation. A layout scheme is proposed for datapath control signals which reduces the switching power by 10% and wire delays by 15% for 0.25 µm microprocessor examples.

Transport in double-crossed Luttinger liquids

We study transport through two Luttinger liquids (one-dimensional electrons interacting through a Coulomb repulsion in a metal) coupled together at two points. External voltage biases are incorporated through boundary conditions. We include density-density couplings as well as single-particle hops at the contacts. For weak repulsive interactions, transport through the wires remains undisturbed by the interwire couplings, which renormalize to zero. For strong repulsive interactions, the interwire couplings become strong. For symmetric barriers and no external voltage bias, a single gate voltage is sufficient to tune for resonance transmission in both wires. However, for asymmetric couplings or for finite external biases, the system is insulating.

Optical characterization of GaAs/AlGaAs quantum well wires fabricated using arsenic implantation induced intermixing

We report the fabrication of GaAs/AlGaAs quantum well wires using implantation of As at 45 keV to induce quantum well intermixing. The intermixing process was first characterized giving optimized annealing parameters of 875 °C for 30 s and an implantation dose of 1×1013 cm−2. Wire widths from 35 to 1000 nm were defined using e-beam lithography followed by lift-off. Photoluminescence spectra from the lateral wells and barriers were observed from samples with wires as narrow as 50 nm. The energies of the lateral wells were found to remain constant for wire widths between 1000 and 150 nm, and start to shift significantly towards high energy for 80 nm wires, the signal from the lateral well eventually merging with that from the lateral barrier for 35 nm wires. An intermixing radius of about 17 nm was estimated for the process. Photoreflectance measurements were also carried out on these wire samples, showing that the wires appear to have a parabolic lateral potential and clear interwire coupling was observed from samples with barriers narrower than 50 nm.

Metal-insulator transition in highly conducting oriented polymers

We suggest that highly conducting oriented polymers with a fibril structure can be modeled by a regular lattice of disordered metallic wires with a random first-neighbor interwire coupling which mimics the cross links between fibrils. We determine the position of the metal-insulator transition (MIT) as a function of interwire cross-link concentration, interwire coupling $J$, and number $M$ of polymer chains in a wire. Two different approaches are used. The first one is based on the self-consistent diagrammatic theory of Anderson localization. In the second approach, we show that the MIT can be described by a nonlinear $\ensuremath{\sigma}$ model. For $M=1$, we find that a small value of $J$ favors the metallic state while a large value of $J$ induces localization in agreement with recent numerical calculations. When $M\ensuremath{\gg}1$, an increase of $J$ always favors a delocalization of the electronic states in agreement with a previous analytical analysis.

Organized growth of lateral superlattices on vicinal surfaces: where are the limits?

Abstract We report on three issues concerning the growth of GaAs AlAs lateral superlattices on vicinal surfaces to obtain quantum wires with negligible inter-wire tunnel coupling. We show that the ledge roughness increases with increasing terrace length and decreases for slower growth rates. The tilt angle of lateral superlattices is not the relevant parameter as long as electronic properties are concerned: one has to consider the product of the coverage error by the lateral superlattice thickness. The Ga flux stability is a critical parameter to be controlled. The use of dual-filament cells improves it by a factor two compared to single-filament ones.

Spontaneous charge polarization in single-electron tunneling through coupled nanowires

Recent observations of periodic anomalies in conductance of two-dimensional arrays of densely packed metal or semiconductor nanowires give some indication on the importance of collective charge excitations in such systems. These structures can be viewed as parallel arrays of double-tunnel-junction systems with nano- wires in the middle which are electrostatically coupled to each other. To assess possible effects of the interwire coupling on the electron transport in such arrays, we investigate the electrical behavior of a simpler system of two coupled double-junction systems under the condition of Coulomb-controlled tunneling. Using Monte Carlo simulations of the electron transport through the system and the master equation analysis, we find that a system of two coupled nanowires exhibits a spontaneous polarization of charge where the accumulation of excessive electrons on one wire is accompanied by the hole accumulation on the neighboring wire. This yields considerable net charge polarization in the transverse direction, which stochastically oscillates in time but depends periodically on the applied voltage, with both the average polarization and the polarization noise decreasing with increasing temperature. The effect gives rise to a number of changes to the well-known Coulomb-Blockade features of a double-junction system, and may lead to the appearance of periodic polarization structures or polarization waves in nanowire arrays which might be detected externally.

Exciton binding and delocalization in T-shaped quantum wires

Effective mass models with screened pair interaction give reasonable energies for excitons in T-shaped wires provided the effects of complex geometry are included. A sizable shift between exciton states in wells and T-shaped wires occurs due to enhancement in binding energy, confinement in the wires, and interwire coupling.

Excitons in complex quantum nanostructures

A theory for excitons in complex quantum nanostructures with complicated geometries and strong coupling between nanostructures is presented. Effective mass models with screened Coulomb interaction give reasonable ground-state energies for excitons in T-shaped quantum wires, coupled T-quantum wires and quantum dot quantum wells, provided the effects of complex geometry are included. A sizable shift between exciton states in quantum wells and T-shaped wires occurs due to a 40% enhancement in binding energy, confinement in the wire, and significant interwire coupling.

Nonlithographic nano-wire arrays: fabrication, physics, and device applications

A novel system of nanostructures is described consisting of nonlithographically produced arrays of nano-wires directly electrodeposited into porous anodic aluminum oxide templates. Using this method regular and uniform arrays of metal or semiconductor nano-wires or nano-dots can be created with diameters ranging from /spl sim/5 nm to several hundred nanometers and with areal pore densities in the /spl sim/10/sup 9/-10/sup 11/ cm/sup -2/ range. We report on the present state of their fabrication, properties, and prospective device applications. Results of X-ray diffraction, Raman and magnetic measurements on metal (Ni, Fe) and semiconductor (CdS, CdSe, CdS/sub x/Se/sub 1-x/, Cd/sub x/Zn/sub 1-x/S and GaAs) wires are presented. The I-V characteristics of two terminal devices made from the nano-arrays are found to exhibit room temperature periodic conductance oscillations and Coulomb-blockade like current staircases. These observations are likely associated with the ultra-small tunnel junctions that are formed naturally in the arrays. Single-electron tunneling (SET) In the presence of interwire coupling in these arrays is shown to lead to the spontaneous electrostatic polarization of the wires. Possible device applications such as magnetic memory or sensors, electroluminescent flat-panel displays, and nanoelectronic and single-electronic devices are also discussed.

Metal-insulator transition in an irregular structure of coupled metallic chains

A model of the random network of coupled metallic wires is suggested to describe transport properties of highly conducting fibril-form polymers. The suppression of closed paths in the model enables us to study exactly the transition from the dielectric to the conductive state. The critical concentration of the interwire crosslinks determining the transition depends on the localization length of a single wire and on the interwire coupling. The temperature dependence of the network conductivity near the transition is discussed.

Localization transition in a random network of metallic wires: A model for highly conducting polymers.

We consider the Anderson metal-insulator transition in a network of randomly coupled metallic wires, which is suggested to describe transport properties of highly conducting fibril-form polymers. The absence of closed paths in the model enables us to study the transition exactly. The critical concentration of the cross-links determining the transition depends on the localization length of a single wire and on the interwire coupling. Application of a magnetic field extends the area of the metallic phase.

Theory of weak localization in a lateral superlattice

A theory of weak localization for the parallel and transverse conductivity in lateral semiconductor superlattices is presented. Based on our previous studies of planar superlattices, we assume that there exists coherent transport between quantum wires. The theory explicitly accounts for the superlattice periodic structure. Small collisional broadening calculations are performed as quantum corrections to Boltzmann transport. Similar to planar superlattices, weak localization corrections to the conductivity depend on the width of the superlattice miniband. This theory allows for (i) direct determination of the effective interwire electron coupling by measurement of the parallel conductivity, and (ii) computation of the electron dephasing time (such as the electron-electron scattering time) for this quantum structure.

Low-Current, High-Speed Magnetic Memory Array Utilizing Evaporated Matrix Wiring

Planar, thin film magnetic memory arrays of 4×4 and 64×24 elements have been fabricated from evaporated matrix wiring and insulating layers. Four metallic and three insulating layers were evaporated sequentially in a vacuum chamber without breaking the vacuum. The magnetic element is an electroplated ternary alloy of nickel-iron-phosphorus 0.13 mm (0.005-in.) in diameter and a few hundred Å thick. The film has an anisotropy field of about 1 Oe and switches in 10 nsec with an applied word drive current of 35 mA. Sense and cancellation lines and dummy bit lines are used to cancel interwire coupling. Disturb tests were performed with up to 104 disturb pulses being applied along the easy axis. The outputs from films with Hc/Hk≈2 were decreased by less than 10% when disturbed in this manner. An evaporated memory array made with highly inverted films may be usable as a high-speed, low-current memory storage device.