Area of work

My primary interest is in applying quantum mechanical and chemical methods within the field of electron transport calculations in mesoscopic systems. My work aims at understanding the electronic structure of nano materials and establishing a link between structures and properties. With this in mind, I employ tools and methods of theoretical and computational chemistry and physics, such as Density Functional Theory (DFT), Non-Equlibrium Green's Function (NEGF) formalism and numerous ab initio techniques. I am also interested in performing calculations to explore possible new materials for solar cells, batteries and non-CMOS devices. I have a broad interest in drug design and modeling, transition-metal and organometallic chemistry, molecule sensors and electronics. A guiding principle in my work is that theory and calculations should be used in synergy with experiments, addressing topical problems and providing insight that informs experimental practice.

  • Transport in mesoscopic systems
  • Quantum Chemistry 
  • Molecular electronics and sensors
  • Transition-metal and organometallic chemistry
  • Material science 

'Excellent computer simulations are done for a purpose. The most valid purposes are to explore uncharted territory, to resolve a well-posed scientific or technical question, or to make a good design choice.' 

L.P. Kadanoff 

 

 

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Recent grants worked on

2011 -

NEGF simulator development

The project is based on research in NEGF quantum transport simulation technology and its incorporation in efficient 3D simulation tools. The work includes improvement of efficiency of 3D NEGF simulation using couple mod simulation techniques and incorporation of the quantum transport simulation module in the 3D Glasgow 'atomistic' simulator. The emphasis is on the improved speed and convergence of trapped charges and interface roughness. Nanowire and FinFET transistors are considered. 

 

 

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2011 -

 

Molecular-Metal-Oxide-nanoelectronicS (M-MOS): Achieving the Molecular Limit

The main aim of the project is to demonstrate functional circuits using molecular metal-oxides (MMOs), connecting self-assembled MMOs into top-down, lithographically defined CMOS architectures with the ultimate aim of achieving the molecular limit in data storage and processing: i.e., realising inorganic, single molecule transistors. This project is highly creative and adventurous, proposing that inorganic molecules could be reliably used in the fabrication of nano-electronic devices that take advantage of the intrinsic electronic properties of molecules as switchable molecular semiconductors.

 

 

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Link to journal article at nature.com

 

2007 - 2011

Electronic structure/function relationships inmetal nanowires: components for molecularelectronics

The project revolves around understanding the link between electronic structure and electron transport properties of extended metal atom chain (EMAC). These complexes are built from  helical array of oligo-α-pyridyl ligands which is used to support a frame of metal centres. These structures have been the subject of a protracted debate in the inorganic chemistry community due to their polymorphism – they exist is symmetric and unsymmetic forms. My current objective is to relate the fundamental electronic structure of these EMAC complexes to their conductance as measured, for example, by STM. Ultimately, an understanding of these phenomena will be essential to the development of new computer architectures based on molecular-scale components.

   

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Publications

2015

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

Y. Wang, T. Al-Ameri, X. Wang, V. P. Georgiev et. al.,  IEEE Transaction on Electron Devices, 2015, 62 (10), 3229 - 3236. 

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.

Comparison of Si <100> and <110> crystal orientation nanowire transistor reliability using Poisson–Schrödinger and classical simulations

L. Gerrer, V. P. Georgiev, S. M. Amoroso, E. Towie, A. Asenov,  Microelectronics Reliability, 2015, 55, 1307-1322. 

In this paper we perform trap sensitivity simulation analysis of square nanowire transistors (NWT), comparing Poisson–Schrödinger (PS) and classical solutions. Both approaches result in a very different electrostatic behaviour due to strong quantum confinement effects in ultra-scaled NWTs such as the Si NWTs presented in this work. Statistical distributions of traps are investigated, modelling the steady state impact of Random Telegraph Noise and Bias Temperature Instabilities for two crystal orientations. Statistical simulations are performed to evaluate the reliability impact on threshold voltage and ON current, emphasising the importance of both confinement and trap distribution details for the proper assessment of reliability in nanowire transistors.

Multi-scale Computational Framework for Evaluating of the Performance of Molecular Based Flash Cells

V. P. Georgiev and A. Asenov,  Numerical Methods and Aplications Lecgture Notes in Computer Science2015, 8962, 196 - 203

Interplay between quantum mechanical effects and a discrete trap position in ultra-scaled FinFETs

V. P Georgiev, S. M Amoroso, L. Gerrer, F. Adamu-Lema, A. Asenov,2015 International Conference on Simulation of Semiconductor Processes and Devices (SISPAD), 2015, 246-249.

Bipolar Monte Carlo simulation of hot carriers in III-N LEDs

P. Kivisaari, T. Sadi, J. Li, V. P. Georgiev, J. Oksanen, P. Rinke, J. Tulkki, 2015 International Conference on Simulation of Semiconductor Processes and Devices (SISPAD), 2015, 393-396.

Multi-Subband Ensemble Monte Carlo simulation of Si nanowire MOSFETs

L. Donetti, C. Sampedro, F. Gamiz, A. Godoy, F. J Garcia-Ruiz, E. Towiez, V. P Georgiev, S. M. Amoroso, C. Riddet, A. Asenov 2015 International Conference on Simulation of Semiconductor Processes and Devices (SISPAD), 2015, 353-356.

2014

Design and fabrication of memory devices based on nanoscale polyoxometalate clusters

C. Busche, L. Vila-Nadal, J. Yan, H. N. Miras, D.-L. Long, V. P. Georgiev, A. Asenov, R. H. Pedersen, N. Gadegaard, M. Mirza, D. J. Paul, J. M. Poblet and L. Cronin, Nature, Nov. 2014, 515 (7528), 545 - 549

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.

Comparison Between Bulk and FDSOI POM Flash Cell: A Multiscale Simulation Study

V. P. Georgiev, S. M. Amoroso, T. Ali, L. Vila-Nadal, C. Busche, L Cronin, A. Asenov, IEEE Transaction on Electron Devices, 2014, 62 (2), 680 - 684 

In this brief, we present a multiscale simulation study of a fully depleted silicon-on-insulator (FDSOI) nonvolatile memory cell based on polyoxometalates (POMs) inorganic molecular clusters used as a storage media embedded in the gate dielectric of flash cells. In particular, we focus our discussion on the threshold voltage variability introduced by random discrete dopants (random dopant fluctuation) and by fluctuations in the distribution of the POM molecules in the storage media (POM fluctuation). To highlight the advantages of the FDSOI POM flash cell, we provide a comparison with an equivalent cell based on conventional (BULK) transistors. The presented simulation framework and methodology is transferrable to flash cells based on alternative molecules used as a storage media.

FDSOI molecular flash cell with reduced variability for low power flash applications

V. P. Georgiev, S. M. Amoroso, T. Ali, L. Vila-Nadal, C. Busche, L Cronin, A. Asenov, 2013 Proceedings of the European Solid-State Device Research Conference (ESSDERC), 2014, 978-1-4799-4378-4

Inverse Scaling Trends for Charge-Trapping-Induced Degradation of FinFETs Performance

S. M. Amoroso, V. P. Georgiev, L. Gerrer, E. Towie, X. Wang, C. Riddet, A. R. Brown and A. Asenov, IEEE Transaction on Electron Devices, 2014, 61 (12), 353 - 356 

In this brief, we investigate the impact of a singlediscrete charge trapped at the top oxide interface on theperformance of scaled nMOS FinFET transistors. The chargetrapping-inducedgate voltage shift is simulated as a functionof the device scaling and for several regimes of conduction-fromsubthreshold to ON-state. Contrary to what is expected for planarMOSFETs, we show that the trap impact decreases with scalingdown the FinFET size and the applied gate voltage. By comparingdrift-diffusion with nonequilibrium Green functions simulations,we show that quantum effects in the charge distribution andtransport can reduce or amplify the impact of discrete traps insimulation of reliability resilience of scaled FinFETs.

3D Multi-Subband Ensemble Monte Carlo Simulator of FinFETs and Nanowire Transistors

C. Sampedro, L. Donetti, F. Gámiz, A. Godoy, F.J. Garcia-Ruiz, V.P. Georgiev, S. M. Amoroso, C. Riddet, E.A. Towie, A Asenov2014 International Conference on Simulation of Semiconductor Processes and Devices (SISPAD), 2014, 978-1-4799-5288-5/14

Metamorphosis of a nano wire: A 3-D coupled mode space NEGF study

Salvatore Maria Amoroso, Vihar P Georgiev, Ewan Towie, Craig Riddet, Asen Asenov, 2014 International Workshop on Computational Electronics (IWCE), 2014, 978-1-4799-5433-9/14. 

Optimization and Evaluation of Variability in the Programming Window of a Flash Cell With Molecular Metal-Oxide Storage

Vihar P. Georgiev, Stanislav Markov, Laia Vilà-Nadal,Christoph Busche, Leroy Cronin, Asen Asenov IEEE Transaction on Electron Devices, 2014, 6 (61), 2019-2026.

We report a modeling study of a conceptual nonvolatile memory cell based on inorganic molecular metal–oxide clusters as a storage media embedded in the gate dielectric of a MOSFET. For the purpose of this paper, we developed a multiscale simulation framework that enables the evaluation of variability in the programming window of a flash cell with sub-20-nm gate length. Furthermore, we studied the threshold voltage variability due to random dopant fluctuations and fluctuations in the distribution of the molecular clusters in the cell. The simulation framework and the general conclusions of our work are transferrable to flash cells based on alternative molecules used for a storage media.

2013

Towards Polyoxometalate-Cluster-Based Nano-Electronics

Laia Vilà-Nadal, Scott G. Mitchell, Stanislav Markov, Christoph Busche, Vihar P. Georgiev, Asen Asenov, Leroy Cronin, Chem. Eur. J, 2013, 19 (49), 6502.

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.

Multi-scale computational framework for the evaluation of variability in the programing window of a flash cell with molecular storage

Vihar P. Georgiev, Stanislav Markov, Laia Vilà-Nadal, Asen Asenov, Leroy Cronin, 2013 Proceedings of the European Solid-State Device Research Conference (ESSDERC), 2013, 978-1-4799-0649-9/13. 

Molecular-Metal-Oxide-nanoelectronicS (M-MOS): Achieving the Molecular Limit

Vihar P. Georgiev, Stanislav Markov, Laia Vilà-Nadal, Asen Asenov, Leroy Cronin, 2013 International Workshop on Computational Electronics (IWCE)2013, 978-3-901578-26-7. 

Interactions Between Precisely Placed Dopants and Interface Roughness in Silicon Nanowire Transistors: Full 3-D NEGF Simulation Study

Vihar P. Georgiev, Ewan A. Towie, Asen Asenov, 2014 International Conference on Simulation of Semiconductor Processes and Devices (SISPAD), 2013, 978-1-4673-5736-4, 416.

Simulation of a single dopant nanowire transistor

Asen Asenov and Vihar P. Georgiev, MRS Online Proceedings Library2013, 1564, mrss13-1564-gg03-01

Nowadays the silicon technology is capable of delivering sub-10 nm devices  where 'every atom counts'. Manipulation of atoms with high precision on such a scale, in principle, can lead to technological innovations, such as transistors with extremely short gate length, quantum computing components and optoelectronic devices. One possible strategy to create this next generation of devices is to precisely place individual discrete dopants (such as phosphorous atoms) in a nanoscale transistor.

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

Vihar P. Georgiev, Ewan A. Towie, Asen Asenov, IEEE Transaction on Electron Devices2013, 60 (3), 965-971

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 self-consistent 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 x 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.

Low-symmetry distortions in Extended Metal Atom Chains (EMACS): origins and consequences for electron transport

Vihar P. Georgiev, P. J. Mohan, Daniel DeBrincat, John E. McGrady, Coordination Chemistry Reviews2013, 1 (257), 290-298 

The extended metal atoms chains (EMACs) consitute a diverse group of molecules differing in both length and identity of the constituent metal ions. They therefore present important challenges to our understanding of metal-metal bonding and its relationship to structural, magnetic and electron transport properties. In this review we summarise our current understanding of the origins of low-symmetry distortions in these chains and their impact on current flow through the molecule. We conclude that structure, magnetochemistry and resistance are all intimately related through a shared dependence on the energies and orientations of the frontier orbitals.

2012

Attenuation of Conductance in Extended Metal Atom Chains

Vihar P. Georgiev, W. M. C. Sameera, John E. McGrady, Phys. Chem. C 2012, 116 (38), pp 20163–20172

Density functional theory, in conjunction with non-equilibrium Green’s functions, is used to explore the attenuation of the resistance of Cox wires along the series Co3(dpa)4(NCS)2, Co5(tpda)4(NCS)2, Co7(teptra)4(NCS)2. At very low bias (0 < V < 25 mV) the conductance, G, decreases in the order G(Co3) > G(Co5) > G(Co7), consistent with experiment and with an anticipated inverse relationship between conductance and chain length. At higher voltages, however, the current-voltage responses of all three are striking non-linear, and above 50 mV G(Co5) > G(Co3) > G(Co7). The very different behavior of the members of this homologous series can be traced to the different symmetries and multiplicities of their respective ground states, which in turn control the properties of the dominant transport channels.

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

P. J. Mohan, Vihar P. Georgiev, John E. McGrady, Chem. Sci.20123, 1319-1329.

Density functional theory, in conjunction with non-equilibrium Green’s functions, 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.

Electron transport through molecular wires based on a face-shared bioctahedral motif

Vitesh Mistry, Vihar P. Georgiev, John E. McGrady, Comptes Rendus Chimie201215(2-3), 176 - 183.

Density functional theory in conjunction with non-equilibrium Green's functions is used to explore the electron transport properties of a series of molecules based on the face-shared bioctahedral (M2Cl9) motif. The metal-metal bond orders in the chosen molecules, [Rh2Cl9]3–, [Ru2Cl9]3– and [Mo2Cl9]3– vary from 0 (Rh) to 1 (Ru) and 3 (Mo), and the calculations indicate that there is a direct correlation between conductance and bond order. The [Mo2Cl9]3– case is particularly interesting as it is well known from crystallographic studies to be very flexible, the Mo–Mo bond length varying over a range of ∼0.35 Å depending on cation. The upper limit of this range marks the point where homolytic cleavage of the δπ components of the triple bond is complete, and this has a marked impact on electron transport. The localization of the metal-based orbitals means that those on the left (source) and right (drain) sides respond very differently to applied bias, giving rise to resonance effects at particular bias voltages, and hence to negative differential resistance effects.

2011

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

Vihar P. Georgiev, John E. McGrady, J. Am. Chem. Soc.2011133(32), 12590–12599.

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 Green’s functions to show that precisely the opposite is true in the extended metal atom chain Cr3(dpa)4(NCS)2 where the delocalized π framework has previously been proposed to be the dominant conduction pathway. Low-symmetry distortions of the Cr3 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 σnb 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.

2010

Efficient Spin Filtering through Cobalt-Based Extended Metal Atom Chains

Vihar P. Georgiev and John. E. McGrady, Inorg. Chem., 201049(12), 5591- 5597.

Density functional theory in conjunction with nonequilibrium Green’s functions has been used to explore charge transport through the cobalt-based extended metal atom chain, Co3(dpa)4(NCS)2. The isolated molecule has a doublet ground state, and the singly occupied σ 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 π orbitals that form the contact with the gold electrode are orthogonal to the transport channel. As a result, the rehybridization of these π 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.

Boron-nitrogen- and boron-substituted anthracenes and -phenanthrenes as models for doped carbon-based materials

M. Velinova, V. Georgiev, T. Todorova, G. Madjarova, A. Ivanova, A. Tadjer, Journal of Molecular Structure: THEOCHEM2010955, 97–108.

The aim of this theoretical study is the investigation of hydrocarbon stability as a function of BN- and B-substitution degree. The targets are anthracene and phenanthrene because they resemble the zig-zag and armchair peripheries of graphene sheets. The computations are made at the RHF and MP2 level with various basis sets. The conclusions drawn are based on the results from calculations for more than 600 different structures created in a systematic way where pairs of carbon atoms are replaced by the respective number of B and N, or solely B-atoms. A common feature of all stable substituted molecules is the tendency for least fragmentation of the carbon skeleton, particularly, for preservation of maximum number of intact carbon aromatic cycles. The major dissimilarity between the BN- and B-substituted molecules are the preferred doping patterns and positions: the favorable structures feature alternation of B and N in the BN-derivatives and disjoint borons separated by C2 fragments in the B-ones; the nitrogen atoms substitute predominantly secondary carbons, while borons prefer tertiary sites in the BN-substituted hydrocarbons and secondary positions in the B-modified structures. Overall, the anthracene isomers are more stable. The energy and AO contributions to the LUMOs of the studied species are rationalized in terms of electron-accepting properties of the latter, estimating their potential performance as oxidants. Energy depression of LUMOs is achieved mainly in B-substituted hydrocarbons due to large coefficients on the borons causing pronounced delocalization of the orbital. The findings can be used for directed synthesis of novel modified carbon-based materials for practical purposes.

2004

Structure and properties of non-classical polymers. XIV. Heteronuclear 1-D polymers with 2-azaphenalenyl radicals 

F. Dietz, K. Mullen, M. Baumgarten, V. Georgiev, N. Tyutyulkov, Chem.Phys.Letters, 2004389, 135-139.

The magnetic properties of a class of 1-D organic polymers with stable 2-azaphenalenyl radicals within the elementary units have been investigated theoretically. The nature and magnitude of the magnetic coupling between the electrons within the half-filled band (HFB) of the polymers were determined using the band theory. In all cases the π-electrons within the HFB are ferromagnetically coupled with the main contribution of the Coulomb- and indirect exchange to the effective Heisenberg integral.

Personal Details

University Web-page

Lecturer

E-mail: Vihar.Georgiev@glasgow.ac.uk

Ph.D. in Computational Inorganic Chemistry,  University of Oxford, UK

M.Sc. in Computational Chemistry, Sofia University, Bulgaria

B.Sc.  in Chemistry, Sofia University, Bulgaria

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