Eric Polizzi

A Three-Dimensional quantum simulation of Silicon nanowire transistors with the effective mass approximation

The silicon nanowire transistor (SNWT) is a promising device structure for future integrated circuits, and simulations will be important for understanding its device physics and assessing its ultimate performance limits. In this work, we present a three-dimensional (3D) quantum mechanical simulation approach to treat various SNWTs within the effective-mass approximation. We begin by assuming ballistic transport, which gives the upper performance limit of the devices. The use of a mode space approach (either coupled or uncoupled) produces high computational efficiency that makes our 3D quantum simulator practical for extensive device simulation and design. Scattering in SNWTs is then treated by a simple model that uses so-called Buttiker probes, which was previously used in metal-oxide-semiconductor field effect transistor simulations. Using this simple approach, the effects of scattering on both internal device characteristics and terminal currents can be examined, which enables our simulator to be used f...

Density-matrix-based algorithm for solving eigenvalue problems

A new numerical algorithm for solving the symmetric eigenvalue problem is presented. The technique deviates fundamentally from the traditional Krylov subspace iteration based techniques (Arnoldi and Lanczos algorithms) or other Davidson-Jacobi techniques, and takes its inspiration from the contour integration and density matrix representation in quantum mechanics. It will be shown that this new algorithm - named FEAST - exhibits high efficiency, robustness, accuracy and scalability on parallel architectures. Examples from electronic structure calculations of Carbon nanotubes (CNT) are presented, and numerical performances and capabilities are discussed.

Theoretical investigation of surface roughness scattering in silicon nanowire transistors

Using a full three-dimensional (3D), quantum transport simulator, we theoretically investigate the effects of surface roughness scattering (SRS) on the device characteristics of Si nanowire transistors (SNWTs). The microscopic structure of the Si/SiO2 interface roughness is directly treated by using a 3D finite element technique. The results show that (1) SRS reduces the electron density of states in the channel, which increases the SNWT threshold voltage, and (2) the SRS in SNWTs becomes less effective when fewer propagating modes are occupied, which implies that SRS is less important in small-diameter SNWTs with few modes conducting than in planar metal-oxide-semiconductor field-effect-transistors with many transverse modes occupied.

A self-consistent transport model for molecular conduction based on extended Hückel theory with full three-dimensional electrostatics

We present a transport model for molecular conduction involving an extended Hückel theoretical treatment of the molecular chemistry combined with a nonequilibrium Greens function treatment of quantum transport. The self-consistent potential is approximated by CNDO (complete neglect of differential overlap) method and the electrostatic effects of metallic leads (bias and image charges) are included through a three-dimensional finite element method. This allows us to capture spatial details of the electrostatic potential profile, including effects of charging, screening, and complicated electrode configurations employing only a single adjustable parameter to locate the Fermi energy. As this model is based on semiempirical methods it is computationally inexpensive and flexible compared to ab initio models, yet at the same time it is able to capture salient qualitative features as well as several relevant quantitative details of transport. We apply our model to investigate recent experimental data on alkane dithiol molecules obtained in a nanopore setup. We also present a comparison study of single molecule transistors and identify electronic properties that control their performance.

A computational study of ballistic silicon nanowire transistors

Using a rigorous 3D quantum simulator, we report a computational study of ballistic silicon nanowire transistors with arbitrary cross sections (i.e., triangular, rectangular or cylindrical). In comparison with the planar double-gate MOSFET, the silicon nanowire transistor shows promise (e.g., better electrostatic scaling for a given Si body thickness) and may provide a manufacturable opportunity to scale silicon transistors down below the scaling limit of planar MOSFETs.

Terahertz detection in single wall carbon nanotubes

It is reported that terahertz radiation from 0.69to2.54THz has been sensitively detected in a device consisting of bundles of carbon nanotubes containing single wall metallic carbon nanotubes, quasioptically coupled through a lithographically fabricated antenna, and a silicon lens. The measured data are consistent with a bolometric detection process in the metallic tubes and the devices show promise for operation well above 4.2K.

FEAST As A Subspace Iteration Eigensolver Accelerated By Approximate Spectral Projection

The calculation of a segment of eigenvalues and their corresponding eigenvectors of a Hermitian matrix or matrix pencil has many applications. A new density-matrix-based algorithm has been proposed recently and a software package FEAST has been developed. The density-matrix approach allows FEASTs implementation to exploit a key strength of modern computer architectures, namely, multiple levels of parallelism. Consequently, the software package has been well received, especially in the electronic structure community. Nevertheless, theoretical analysis of FEAST has lagged. For instance, the FEAST algorithm has not been proven to converge. This paper offers a detailed numerical analysis of FEAST. In particular, we show that the FEAST algorithm can be understood as an accelerated subspace iteration algorithm in conjunction with the Rayleigh-Ritz procedure. The novelty of FEAST lies in its accelerator which is a rational matrix function that approximates the spectral projector onto the eigenspace in question. Analysis of the numerical nature of this approximate spectral projector and the resulting subspaces generated in the FEAST algorithm establishes the algorithms convergence. This paper shows that FEAST is resilient against rounding errors and establishes properties that can be leveraged to enhance the algorithms robustness. Finally, we propose an extension of FEAST to handle non-Hermitian problems and suggest some future research directions.

Efficient estimation of eigenvalue counts in an interval

Summary Estimating the number of eigenvalues located in a given interval of a large sparse Hermitian matrix is an important problem in certain applications, and it is a prerequisite of eigensolvers based on a divide-and-conquer paradigm. Often, an exact count is not necessary, and methods based on stochastic estimates can be utilized to yield rough approximations. This paper examines a number of techniques tailored to this specific task. It reviews standard approaches and explores new ones based on polynomial and rational approximation filtering combined with a stochastic procedure. We also discuss how the latter method is particularly well-suited for the FEAST eigensolver. Copyright

Electrostatics of nanowire transistors

The charge transfer doping in metal wire-carbon nanotube junctions has been demonstrated to be important before. In this paper, we investigate the effect of contact geometry, metal work function and insulator dielectric constant on the charge transfer doping into a nanowire channel. We then explore the effect of charge transfer doping on the operation of nanowire transistors. We show that the nanowire transistors with large gate underlap can still deliver an appreciable amount of on-current, which provides a possible explanation for a recent experiment by Javey et al. At the same time, charge transfer doping also imposes constraints on the transistor design.

Influence of defects on nanotube transistor performance

Carbon nanotube field effect transistors CNTFETs have excellent device characteristics and are candidates for future digital switches and rf transistors. 1–5 Simple circuits based on CNTFETs have already been demonstrated. 6 An important consideration in the design and reliability of circuits is the role of defects, impurities, and parameter fluctuations in affecting the device characteristics. In this letter, we investigate how vacancies and charged impurities affect the device characteristics of CNTFETs. Vacancies arise in graphite at low concentrations during growth and are part of the thermal equilibrium concentration. 7,8 They are believed to be the predominant defects on irradiated graphite surfaces and CNTs Refs. 9–12 and stable on long time scale. Charged impurities usually consist of ions, molecules, alkali metals, or dopants that exchange charge with the CNT or electrostatically interact with the nanotube. 13–16 The model device considered consists of a 13,0 zigzag CNT, with 1 nm diameter and 0.8 eV band gap Fig. 1. The length of the undoped channel and source/drain extension regions are 25 and 22.5 nm, respectively. The source/drain regions are doped uniformly with ND =1 0 9 dopants/ m. The surrounding gate oxide is a 4 nm thick HfO2 high dielectric material =1 6, whereas the interior of the CNT is vacuum =1 . We chose a gate work function that produces flatband conditions at VG =� Eg / 4, where Eg is the band gap