Towards Atomic Level Simulation of Electron Devices Including the Semiconductor-Oxide Interface

We report a milestone in device modeling whereby a planar MOSFET with extremely thin silicon on insulator channel is simulated at the atomic level, including significant parts of the gate and buried oxides explicitly in the simulation domain, in ab initio fashion, i.e without material or geometrical parameters. We use the density-functional-based tight-binding formalism for constructing the device Hamiltonian, and non-equilibrium Green's functions formalism for calculating electron current. Simulations of Si/SiO2 super-cells agree very well with experimentally observed band-structure phenomena in SiO2-confined sub-6 nm thick Si films. Device simulations of ETSOI MOSFET with 3 nm channel length and sub-nm channel thickness also agree well with reported measurements of the transfer characteristics of a similar transistor.published_or_final_versio

A Combined First Principles and Kinetic Monte Carlo study of Polyoxometalate based Molecular Memory Devices

In this paper, we combine Density Functional Theory with Kinetic Monte Carlo methodology to study the fundamental transport properties of a type of polyoxometalate (POM) and its behaviour in a potential flash memory device. DFT simulations on POM molecular junctions helps us demonstrate the link between underlying electronic structure of the molecule and its transport properties. Furthermore, we show how various electrode-molecule contact configurations determine the electron transport through the POM. Also, our work reveals that the orientation of the molecule to the electrodes plays a key role in the transport properties of the junction. With Kinetic Monte Carlo we extend this investigation by simulating the retention time of a POM-based flash memory device. Our results show that a POM based flash memory could potentially show multi-bit storage and retain charge for up to 10 years

Atomic level modeling of extremely thin silicon-on-insulator MOSFETs including the silicon dioxide: Electronic structure

© 1963-2012 IEEE. Ultimate scaling of Si MOSFETs leads to extremely thin and short channels, which are justifiably modeled at the atomic level. Currently, hydrogen passivation of the channel is used in device models, as a compromise between efficiency and accuracy. This paper advances the state of the art by adopting a density-functional tight-binding Hamiltonian, permitting the inclusion of the confining amorphous oxide explicitly in the simulation domain in a way similar to ab initio approaches. Band structure of silicon-on-insulator films of different thicknesses is studied with this method, showing good agreement with the experiment and revealing large quantitative differences in comparison with simulations of H-passivated Si film.published_or_final_versio

An Approximate Framework for Quantum Transport Calculation with Model Order Reduction

A new approximate computational framework is proposed for computing the non-equilibrium charge density in the context of the non-equilibrium Green's function (NEGF) method for quantum mechanical transport problems. The framework consists of a new formulation, called the X-formulation, for single-energy density calculation based on the solution of sparse linear systems, and a projection-based nonlinear model order reduction (MOR) approach to address the large number of energy points required for large applied biases. The advantages of the new methods are confirmed by numerical experiments

Noncovalent Molecular Electronics

Molecular electronics covers several distinctly different conducting architectures, including organic semiconductors and single-molecule junctions. The noncovalent interactions, abundant in the former, are also often found in the latter, i.e., the dimer junctions. In the present work, we draw the parallel between the two types of noncovalent molecular electronics for a range of π-conjugated heteroaromatic molecules. In silico modeling allows us to distill the factors that arise from the chemical nature of their building blocks and from their mutual arrangement. We find that the same compounds are consistently the worst and the best performers in the two types of electronic assemblies, emphasizing the universal imprint of the underlying chemistry of the molecular cores on their diverse charge transport characteristics. The interplay between molecular and intermolecular factors creates a spectrum of noncovalent conductive architectures, which can be manipulated using the design strategies based upon the established relationships between chemistry and transport

Electron Transport and Ion Diffusivity through the Solid Electrolyte Interphase in Lithium Ion Batteries with Silicon Anodes

Lithium-ion batteries (LIB) are the best option among batteries for portable electronic, power tools, and electric vehicles due to their higher energy storage, higher power, and lighter weight than other battery technologies such as Ni-based or lead acid. However, Li-ion batteries still face challenges such as safety, life, performance, and cost. One way to contribute to the solutions of these challenges and, consequently, improve the performance of Li-ion cells is to develop and design more stable passivation films at the electrode-electrolyte interface. Therefore, having a better understanding of the molecular processes that lead to the nucleation, growth, structure and morphology, as well as the electron and ion transport properties of the solid electrolyte interphase (SEI) is highly important for the development of new or improved lithium-ion batteries. In this work, computational methods, which allow studying phenomena not easily observable with experimental techniques, are used to study the electron transfer characteristics and the lithium ion diffusivity of the materials found in the SEI film formed in LIB with silicon anodes. First, ab initio computational methods are used to study the electron transfer through selected finite models of SEI films formed at the anode-electrolyte interface. A combined ab initio density functional theory (DFT) and Green’s functions approach, as implemented in the Generalized Electron Nano-Interface Program (GENIP), is used to calculate the current-voltage characteristics of selected SEI configurations. The models studied consist of a LixSiy cluster, a SEI product (LiF, Li2O or Li2CO3), and an electrolyte component, ethylene carbonate (EC). Various parameters are considered in the investigation including: various lithiated states for the anode; several thicknesses and configurations for the SEI layer; and the presence of surface oxides (SiO2 and Li2Si2O5). The trend of conductance is found to be Li2O > SiO2 > LiF > Li2CO3 > Li2Si2O5, at the same applied voltage and anode configuration. Then, lithium-ion diffusion is studied in the main components of the SEI layer using classical molecular dynamics (MD) simulations in order to provide insights and to calculate the diffusion coefficients of Li-ions at temperatures in the range of 250 K to 400 K. The compounds studied are lithium fluoride (LiF), lithium oxide (Li2O) and lithium carbonate (Li2CO3). A slight increase in the diffusivity as the temperature increases is found and since diffusion is noticeable at high temperatures, Li-ion diffusion in the range of 1300 to 1800 K is also studied and the diffusion mechanisms involved in each SEI compound are analyzed. The mechanisms of Li-ion diffusion observed include vacancy assisted and knock-off diffusion in LiF, direct exchange in Li2O, and vacancy and knock-off in Li2CO3. Moreover, the effect that an applied an electric field has in the diffusion of Li-ions at room temperature is also evaluated. The long-term goal is to eventually have more control over interface parameters such as composition, structure, porosity and thickness, and thus accurately design SEI films and therefore better Li-ion batteries. This work is a step towards this ultimate goal