Vihar P. Georgiev

Design and fabrication of memory devices based on nanoscale polyoxometalate clusters

Flash memory devices—that is, non-volatile computer storage media that can be electrically erased and reprogrammed—are vital for portable electronics, but the scaling down of metal–oxide–semiconductor (MOS) flash memory to sizes of below ten nanometres per data cell presents challenges. Molecules have been proposed to replace MOS flash memory, but they suffer from low electrical conductivity, high resistance, low device yield, and finite thermal stability, limiting their integration into current MOS technologies. Although great advances have been made in the pursuit of molecule-based flash memory, there are a number of significant barriers to the realization of devices using conventional MOS technologies. Here we show that core–shell polyoxometalate (POM) molecules can act as candidate storage nodes for MOS flash memory. Realistic, industry-standard device simulations validate our approach at the nanometre scale, where the device performance is determined mainly by the number of molecules in the storage media and not by their position. To exploit the nature of the core–shell POM clusters, we show, at both the molecular and device level, that embedding [(Se(iv)O3)2]4− as an oxidizable dopant in the cluster core allows the oxidation of the molecule to a [Se(v)2O6]2− moiety containing a {Se(v)–Se(v)} bond (where curly brackets indicate a moiety, not a molecule) and reveals a new 5+ oxidation state for selenium. This new oxidation state can be observed at the device level, resulting in a new type of memory, which we call ‘write-once-erase’. Taken together, these results show that POMs have the potential to be used as a realistic nanoscale flash memory. Also, the configuration of the doped POM core may lead to new types of electrical behaviour. This work suggests a route to the practical integration of configurable molecules in MOS technologies as the lithographic scales approach the molecular limit.

Influence of low-symmetry distortions on electron transport through metal atom chains: when is a molecular wire really "broken"?

In the field of molecular electronics, an intimate link between the delocalization of molecular orbitals and their ability to support current flow is often assumed. Delocalization, in turn, is generally regarded as being synonymous with structural symmetry, for example, in the lengths of the bonds along a molecular wire. In this work, we use density functional theory in combination with nonequilibrium Greens functions to show that precisely the opposite is true in the extended metal atom chain Cr(3)(dpa)(4)(NCS)(2) where the delocalized π framework has previously been proposed to be the dominant conduction pathway. Low-symmetry distortions of the Cr(3) core do indeed reduce the effectiveness of these π channels, but this is largely irrelevant to electron transport at low bias simply because they lie far below the Fermi level. Instead, the dominant pathway is through higher-lying orbitals of σ symmetry, which remain essentially unperturbed by even quite substantial distortions. In fact, the conductance is actually increased marginally because the σ(nb) channel is displaced upward toward the Fermi level. These calculations indicate a subtle and counterintuitive relationship between structure and function in these metal chains that has important implications for the interpretation of data emerging from scanning tunnelling and atomic force microscopy experiments.

Efficient Spin Filtering through Cobalt-Based Extended Metal Atom Chains

Density functional theory in conjunction with nonequilibrium Greens functions has been used to explore charge transport through the cobalt-based extended metal atom chain, Co(3)(dpa)(4)(NCS)(2). The isolated molecule has a doublet ground state, and the singly occupied sigma nonbonding orbital proves to be the dominant transport channel, providing spin filtering efficiencies in excess of 90%. The metal chain differs from typical organic conductors in that the pi orbitals that form the contact with the gold electrode are orthogonal to the transport channel. As a result, the rehybridization of these pi levels by the applied electric field has only a minor impact on the current, allowing spin filtering to persist even at biases in excess of 1 V.

Towards Polyoxometalate-Cluster-Based Nano-Electronics

We explore the concept that the incorporation of polyoxometalates (POMs) into complementary metal oxide semiconductor (CMOS) technologies could offer a fundamentally better way to design and engineer new types of data storage devices, due to the enhanced electronic complementarity with SiO2, high redox potentials, and multiple redox states accessible to polyoxometalate clusters. To explore this we constructed a custom-built simulation domain bridge. Connecting DFT, for the quantum mechanical modelling part, and mesoscopic device modelling, confirms the theoretical basis for the proposed advantages of POMs in non-volatile molecular memories (NVMM) or flash-RAM.

Periodic trends in electron transport through extended metal atom chains: comparison of Ru3(dpa)4(NCS)2 with its first-row analogues

Density functional theory is used to reconcile the structural, magnetic and electron transport properties of a triruthenium extended metal atom chain, Ru3(dpa)4(NCS)2. The distinct bending of the Ru–Ru–Ru core in this species is traced to strong second-order mixing between levels of σ and π symmetry that are near degenerate in the linear geometry. The dominant electron transport channel is formed by the LUMO, an orbital of π* symmetry that lies just above the Fermi level of the gold electrode. The bending has a substantial impact on electron transport in that it induces a spin crossover from a quintet to a singlet which in turn brings the LUMO much closer to the Fermi level. The presence of significant net π bonding in the metal chains also broadens the π/πnb/π* manifold, such that the channel is not strongly perturbed by the electric field, even at a bias of 1.0 V. The presence of a robust π symmetry conduction channel marks the triruthenium systems out as quite distinct from its first-row counterparts, Cr3(dpa)4(NCS)2 and Co3(dpa)4(NCS)2, where current flows primarily through the σ framework.

Simulation Study of the Impact of Quantum Confinement on the Electrostatically Driven Performance of n-type Nanowire Transistors

In this paper, we have studied the impact of quantum confinement on the performance of n-type silicon nanowire transistors (NWTs) for application in advanced CMOS technologies. The 3-D drift-diffusion simulations based on the density gradient approach that has been calibrated with respect to the solution of the Schrödinger equation in 2-D cross sections along the direction of the transport are presented. The simulated NWTs have cross sections and dimensional characteristics representative of the transistors expected at a 7-nm CMOS technology. Different gate lengths, cross-sectional shapes, spacer thicknesses, and doping steepness were considered. We have studied the impact of the quantum corrections on the gate capacitance, mobile charge in the channel, drain-induced barrier lowering, and subthreshold slope. The mobile charge to gate capacitance ratio, which is an indicator of the intrinsic speed of the NWTs, is also investigated. We have also estimated the optimal gate length for different NWT design conditions.

Impact of Precisely Positioned Dopants on the Performance of an Ultimate Silicon Nanowire Transistor: A Full Three-Dimensional NEGF Simulation Study

In this paper, we report the first systematic study of quantum transport simulation of the impact of precisely positioned dopants on the performance of ultimately scaled gate-all-around silicon nanowire transistors (NWTs) designed for digital circuit applications. Due to strong inhomogeneity of the selfconsistent electrostatic potential, a full 3-D real-space nonequilibrium Green function formalism is used. The simulations are carried out for an n-channel NWT with 2.2 × 2.2 nm2 cross section and 6-nm channel length, where the locations of the precisely arranged dopants in the source-drain extensions and in the channel region have been varied. The individual dopants act as localized scatters, and hence, impact of the electron transport is directly correlated to the position of the single dopants. As a result, a large variation in the ON-current and a modest variation of the subthreshold slope are observed in the ID-VG characteristics when comparing devices with microscopically different discrete dopant configurations. The variations of the current-voltage characteristics are analyzed with reference to the behavior of the transmission coefficients.

A Survey of Carbon Nanotube Interconnects for Energy Efficient Integrated Circuits

This article is a review of the state-of-art carbon nanotube interconnects for Silicon application with respect to the recent literature. Amongst all the research on carbon nanotube interconnects, those discussed here cover 1) challenges with current copper interconnects, 2) process & growth of carbon nanotube interconnects compatible with back-end-of-line integration, and 3) modeling and simulation for circuit-level benchmarking and performance prediction. The focus is on the evolution of carbon nanotube interconnects from the process, theoretical modeling, and experimental characterization to on-chip interconnect applications. We provide an overview of the current advancements on carbon nanotube interconnects and also regarding the prospects for designing energy efficient integrated circuits. Each selected category is presented in an accessible manner aiming to serve as a survey and informative cornerstone on carbon nanotube interconnects relevant to students and scientists belonging to a range of fields from physics, processing to circuit design.

Nanowire transistor solutions for 5nm and beyond

In this paper we present a comprehensive computational study of silicon nanowire transistor (SNT) and a SNM SRAM cell based on advanced design technology co-optimization (DTCO) TCAD tools. Utilizing this methodology, we provide guidelines and solutions for 5 nm and beyond in CMOS technology. At first, drift-diffusion (DD) results are fully calibrated against a Poisson-Schrodinger (PS) solution to calibrate density-gradient quantum corrections, and ensemble Monte Carlo (EMC) simulations to calibrate transport models. The calibrated DD gives us the capability to simulate statistical variability in nanowire transistors of the 5nm node and beyond accurately and efficiently. Various SNT structures are evaluated in terms of device figures of merit, and optimization of SNTs in terms of electrostatics driven performance is carried out. A variability-aware hierarchical compact model approach for SNT is adopted and used for statistical SRAM simulation near the “scaling limit”. The scaling of SNTs beyond the 5 nm is also discussed.

Correlation between gate length, geometry and electrostatic driven performance in ultra-scaled silicon nanowire transistors

In this work we have investigated the impact of quantum mechanical effects on the device performance of n-type silicon nanowire transistors (NWT) for possible future applications. For the purpose of this paper we have simulated Si NWTs with six different cross-section shapes. However for all devices the cross-sectional area is kept constant in order to provide fair comparison. Additionally we have expanded the computational experiment by including different gate length and gate materials for each of these six Si NWTs. As a result we have established a correlation between the mobile charge distribution in the channel and gate capacitance, drain induced barrier lowering (DIBL) and the sub-threshold slope (SS). The mobile charge to gate capacitance ratio, which is an indicator of the intrinsic speed of the NWTs, is also have been investigated. More importantly all calculations are based on quantum mechanical description of the mobile charge distribution in the channel. This description is based on Schrodinger equation, which is indeed mandatory for nanowires with such ultra-scale dimensions.