International Journal of High Speed Electronics and Systems

Nanoelectronics (including nanomagnetics and nanophotonics) generally refers to nanometer scale devices, and to circuits and architectures which are composed of these devices. Continued scaling of the devices into the nanometer range leads to enhanced information processing systems. Generally, this scaling has arisen from three major sources, one of which is reduction of the physical gate length of individual transistors. Until recently, this has also allowed an increase in the clock speed of the chip, but power considerations have halted this to levels around 4 GHz in Si. Indeed, there are indications that scaling itself may be finished by the end of this decade. Instead, there are now pushes to seek alternative materials for nano-devices that may supplement the Si CMOS in a manner that allows both higher speeds and lower power. In this paper, we will cover some of the impending limitations, and discuss some alternative approaches that may signal continued evolution of integrated circuits beyond the end of the decade.

We consider properties of junctions for the FET geometry were molecular crystals or conducting polymers are used as semiconducting layers. In the molecular crystal Coulomb interaction of free electrons with surface polar phonons of the dielectric layer can lead to selftrapping of carriers and to the formation of a strongly coupled long-range surface polaron. The effect is further enhanced at presence of the bias electric field. The pronounced polaronic effects in conducting polymers change drastically the contact properties of these materials with respect to traditional semiconductors. Instead of the usual band banding near the contact interface, new allowed electronic bands appear inside the band gap. As a result the bias electric field and the injected charge penetrate into the polymer via creation of the soliton lattice which period changes with the distance from the contact surface. The performed studies open the possibility to describe the stationary characteristics and the hysteresis of the FET junctions and the Schottky diodes as well as to explain the photoluminescence suppression or enhancement under the bias electric field.

Here we discuss the use of the Cellular Monte Carlo (CMC) method for full band simulation of semiconductor transport and device modeling. The electronic band structure and phonon spectra are used as direct inputs to the program for both cubic, hexagonal, and strained crystal structures using both empirical and ab initio methods. As a particular example, this method is applied to study high field transport in GaN and GaN/AlGaN heterostructures, where good agreement is obtained between the simulated results, and experimental pulse I-V measurements of transport. For device simulation, the CMC algorithm is coupled to an efficient 2D/3D multi-grid Poisson solver. We discuss the application of this algorithm to several technological problems of interest, including ultra-short channel Si/Ge MOSFETs, III-V compound HEMTs, and AlGaN/GaN HEMTs.

Coherent transport in mesoscopic devices is well described by the Schrödinger equation supplemented by open boundary conditions. When electronic devices are operated at room temperature, however, a realistic transport model needs to include carrier scattering. In this work the kinetic equation for the Wigner function is employed as a model for dissipative quantum transport. Carrier scattering is treated in an approximate manner through a Boltzmann collision operator. A Monte Carlo technique for the solution of this kinetic equation has been developed, based on an interpretation of the Wigner potential operator as a generation term for numerical particles. Including a multi-valley semiconductor model and a self-consistent iteration scheme, the described Monte Carlo simulator can be used for routine device simulations. Applications to single barrier and double barrier structures are presented. The limitations of the numerical Wigner function approach are discussed.

Undesirable short-channel effects associated with the relentless downscaling of conventional CMOS devices have led to the emergence of new classes of MOSFETs. This has led to new and unprecedented challenges in computational nanoelectronics. The device sizes have already reached the level of tens of nanometers where quantum nature of charge-carriers dominates the device operation and performance. The goal of this paper is to describe an on-going initiative on nanoHUB.org to provide new models, algorithms, approaches, and a comprehensive suite of freely-available web-based simulation tools for nanoscale devices with capabilities not yet available commercially. Three software packages nanoFET, nanoMOS and QuaMC are benchmarked in the simulation of a widely-studied high-performance novel MOSFET device. The impact of quantum mechanical effects on the device properties is elucidated and key design issues are suggested.

We report a non-monotonic and strongly temperature dependent magneto-resistance observed in clean quantum point contacts. At the same time the conductance of the point contact varies linearly with temperature. This unexpected behavior may be related to electron-electron interactions.

Device parameter fluctuations, which arise from both the stochastic nature of the manufacturing process and more fundamentally from the intrinsic discreteness of charge and matter, are a dominant source of device mismatch in nano-CMOS devices, and a bottleneck to the future yield and performance of circuits and systems. The impact of such parameter fluctuations is investigated for circuits — with a specific exemplar of 6-T SRAM — whose devices scale from 35 nm gate length. We posit a change in design approach to include the use of statistical compact models as a starting point for the development of cell libraries containing fluctuation information necessary for design under the constraints of parameter fluctuations, and novel Technology Aided System Design tools.

The terahertz region is one of the unexplored bands. This paper first reviews the present status of conventional high-speed devices, especially InP-based high electron mobility transistors (HEMTs), and addresses the technological problems facing the goal of terahertz operation. As an alternative approach to solve these problems, we developed a plasmon-resonant photomixer for realizing a coherent terahertz continuous-wave source. Preliminary results on electromagnetic response to impulsive photoexcitation at room temperature are reported briefly.

We overview the plasma properties of a two-dimensional electron gas in semiconductor heterostructures as well as new concepts and proposals of novel terahertz devices (detectors, sources utilizing plasma instabilities, and photomixers) based on these heterostructures.

We numerically investigated the possibility of terahertz polarization controller based on electronic dispersion control of two dimensional (2D) plasmon gratings in semiconductor heterostructure material systems. Taking account of the Mikhailov's dispersive plasmonic conductivity model, the electromagnetic field emission properties of the gated 2D plasmon gratings were numerically analyzed with respect to the density (n) of electrons by using in-house Maxwell's FDTD (finite difference time domain method) simulator. When n is low under a constant drift-velocity condition, the fundamental plasmon mode is excited, being coupled with the radiative zeroth mode of transverse electric (TE) waves. When n exceeds a threshold level, the second harmonic mode of plasmon is predominantly excited, being coupled with the non-radiative first mode of TE waves. We numerically demonstrated that if a grating mesh of 2D plasmons is formed where two independent 2D plasmon gratings are combined orthogonally, the structure can act as a polarization controller by electronically controlling the two axial plasmonic dispersions.

Terahertz (THz) response spectra of GaN-based field-effect transistor (FET) arrays are calculated in a self-consistent electromagmetic approach. Two types of FET arrays are considered: (i) FET array with a common channel and a large-area grating gate, and (ii) array of FET units with separate channels and combined intrinsic source and drain contacts. It is shown that the coupling between plasmons and THz radiation in the FET array can be strongly enhanced as compared to a single-unit FET. The computer simulations show that the higher-order plasmon modes can be excited much more effectively in the array of FET units with separate channels and combined source and drain contacts then in FET array with a common channel and a large-area grating gate.

We have observed that Terahertz (THz) irradiation to a carbon nanotube quantum dot (CNT-QD) leads to the generation of an excess current in the Coulomb blockade regime. It was found that this THz detected signal survives even when the incident THz wave is extremely weak (~1 fW). This means that the CNT-QD could works as a highly sensitive THz detector.

We present numerical investigation of anomalous internal photoelectric effect which is realized in thin film (< 100 nm) structures by surface plasmon (SP) excitation and its interaction with primary laser radiation. SP electric field gain and electron temperature in the SP field have been calculated.

Widely-tunable voltage-controlled oscillator using resonant tunneling diodes in a distributed manner is discussed. The circuit configuration and the principle of operation of the VCO are described together with several results of numerical calculations, which include necessary condition of permanent oscillation and how much the oscillation frequency is tuned by the voltage.

Employing Monte-Carlo simulations we investigate parameters and optimize geometry of IR quantum-dot detectors with diffusion-limited capture into the dots surrounded by potential barriers. Our results show that structures with modulation doping of interdot matrix provide an effective separation of the localized and conducting electron states. In these structures, the capture time is mainly determined by the quantum dot concentration and the height of potential barriers around dots. The capture is not sensitive to the dot positions. It also weakly depends on the electric field up to the characteristic value, at which significant electron heating allows hot electrons to overcome the barriers. Optimizing the carrier capture and transit times, we show that quantum-dot structures have a lot of potentials for increasing the photoconductive gain and for the development of IR room-temperature detectors.

The magnetoresistance of Fe/MgO/Fe magnetic tunnel junctions (MTJs) was studied taking into consideration image forces. For MTJs with an MgO insulator, explanations are given of the giant tunneling magnetoresistance (TMR) effect and the effect of the increasing TMR with an increase in MgO insulator thickness. It is demonstrated that the electron current density through MTJs can be high enough to switch the magnetization of a ferromagnetic electrode.

Quantum computation is a revolutionary new paradigm that has experienced tremendous growth since the mid 1990s and has inspired a number of ingenious schemes whose long-term goal is to realize a large-scale, fast, parallel, and easily fabricated quantum computer (QC). Silicon-based solid-state proposals using spins of dopants, such as phosphorus, as qubits are attractive because of the long spin relaxation times and the potential for scaling the device to a large number of qubits and integrating the QC with existing silicon technology. We propose a modified Kane¹ architecture in which we address the difficult fabrication and addressing problems by simplifying the device structure and utilizing an existing, reliable optical detection method. The device consists of linear arrays of P atoms, which will be entangled through their exchange interactions. Scanning tunneling microscopy is used to fabricate the device and an external magnetic field and a large field gradient enable individual spin addressability. The lone P electron spin is used to control the nuclear spin qubit orientation. Excitons, generated in the substrate via a laser, are used to probe the spin states. A He-3 cryostat-based spin manipulation and readout system has been developed but many challenges exist in making a nano-scale quantum device in a real materials system.

We review the results of our current research on quantum engineering which include the theory, modeling and simulations of quantum devices for potential applications to threat reduction and homeland security. In particular, we discuss: (i) scalable solid-state quantum computation with qubits based on (a) nuclear spins of impurity atoms in solids, (b) superconducting junctions, and (c) unpaired electron spins of spin radicals in self-assembled organic materials; (ii) quantum neural devices; (iii) quantum annealing; (iv) novel magnetic memory devices based on magnetic tunneling junctions with large tunneling magnetoresistance; (v) terahertz detectors based on microcantilever as a light pressure sensor; (vi) BEC based interferometers; (vii) quantum microscopes with a single-spin resolution based on (a) a magnetic resonant force microscopy and (b) an optically detected magnetic resonance; and (viii) novel approach for suppression of fluctuations in free space high-speed optical communication. Finally, we describe the similarities between the behavior of cross sections in reactions with heavy nuclei in the regions of strongly overlapped resonances and electron conductivity in semiconductor heterostructures.

A methodology and Monte Carlo simulation flow with the integrated LSI Logic package, Molotof, was applied to the 65 nm poly line sensitivity analysis. Strong phase shift mask (sPSM) manufacturing process specification was optimized to obtain image critical dimensions (CDs) and image placement errors (IPEs) complying with technology process tolerances. Reticle manufacturing statistical variarions of phase depth, phase width, and phase intensity imbalance were used to generate a virtual sPSM for imaging poly lines on a wafer. A criterion for qualifying reticle specification is to obtain all latent image CDs and IPEs within a photo process module allowed range for a given mask specification. The approach allows for computing reticle and litho budgets into CD imaging performance. Sensitivity to a single parameter variation and full statistical analysis of the 65 nm poly line imaging performance affected by reticle manufacturing errors is presented. The optimum reticle specification, yielded 100% of critical dimensions and image placement errors, was found in simulation and confirmed by empirical data.