Papers by Neophytos Neophytou
Applied Physics Letters, 2016
IEEE Transactions on Nanotechnology, 2016
Journal of Computational Electronics, 2016
Journal of Applied Physics, 2015
Journal of Computational Electronics, Jan 8, 2008
The ballistic performance of graphene nanoribbon (GNR) MOSFETs with different width of armchair G... more The ballistic performance of graphene nanoribbon (GNR) MOSFETs with different width of armchair GNRs is examined using a real-space quantum simulator based on the Non-equilibrium Green's Function (NEGF) approach, self-consistently coupled to a 3D Poisson's equation for electrostatics. GNR MOSFETs show promising device performance, in terms of low subthreshold swing and small drain-induced-barrier-lowing due to their excellent electrostatics and gate control (single monolayer). However, the quantum tunneling effects play an import role in the GNR device performance degradation for wider width GNR MOSFETs due to their reduced bandgap. At 2.2 nm width, the OFF current performance is completely dominated by tunneling currents, making the OFF-state of the device difficult to control.
Nano Letters, Aug 1, 2008
The effects of the various contact types and shapes on the performance of Schottky barrier graphe... more The effects of the various contact types and shapes on the performance of Schottky barrier graphene nanoribbon field-effect-transistors (GNRFETs) have been investigated using a real-space quantum transport simulator based on the NEGF approach self-consistently coupled to a three-dimensional Poisson solver for treating the electrostatics. The device channel considered is a double gate semiconducting armchair nanoribbon. The types of contacts considered are (a) a semi-infinite normal metal, (b) a semi-infinite graphene sheet, (c) finite size rectangular shape armchair graphene contacts, (d) finite size wedge shape graphene contacts, and (e) zigzag graphene nanoribbon contacts. Among these different contact types, the semi-infinite graphene sheet contacts show the worst performance because of their very low density of states around the Dirac point resulting in low transmission possibility through the Schottky barrier, both at ON and OFF states. Although all other types of contacts can have significant enhancement in I ON to I OFF ratio, the zigzag GNR contacts show promising and size invariant performance due to the metallic properties.
ABSTRACT As devices scale towards atomistic sizes, researches in silicon electronic device techno... more ABSTRACT As devices scale towards atomistic sizes, researches in silicon electronic device technology are investigating alternative structures and materials. As predicted by the International Roadmap for Semiconductors, (ITRS), structures will evolve from planar devices into devices that include 3D features, strong channel confinement, strain engineering, and gate all around placement for better electrostatic control on the channel. Alternative channel materials such as carbon nanotubes (CNT), nanowires (NW) and III-V based channel materials are considered to be possible candidates for future device technology nodes because of their potentially superior to silicon transport properties. For nanoscale dimensions, and under the operating conditions mentioned above, both atomistic and quantum effects become important in determining the electronic structure and transport properties of the devices. Detailed modeling and simulation that capture these new physics will be essential in providing understanding and guidance to the device operation and optimization. We have used the non-equilibrium Green's function (NEGF) formalism for quantum transport simulations and real space atomistic tight-binding techniques (p z , sp3d5s*-SO) to investigate transport properties in CNT, NW and III-V HEMT field-effect transistors. Specifically, we have investigated the effect of atomistic defects such as atomic vacancies, and charged impurities in 1D CNT, and dangling bonds in NW channels. It was found that the presence of single defects, severely degrades the transport performance of 1D channels. We have further investigated the effect of physical quantization on the electronic structure of NW field-effect transistors and identified the main electronic structure factors that influence their performance. It was found that structural and quantization below 10nm can severely affect the electronic properties of NW channels by changing the effective masses and altering degeneracies through valley splitting. Different wire orientations have different transport properties. The [110] and secondly [100] oriented nanowires are found to perform better than the [111] wires in terms of ON-current capabilities for n-type wires, whereas the [111] and [110] significantly outperform the [100] wires in the case of p-type nanowires. Explanations for this behavior can be extracted from the non-parabolicity and anisotropy of the Si 3D bulk E(k) . Finally, we present an analysis of recent experimental data for III-V HEMT devices using the NEGF formalism and address several issues related to the operation of HEMT devices. Interestingly, a 60nm HEMT device can be though to first order as a ballistic channel connected to two series resistances.
A real-space quantum transport simulator for carbon nanoribbon (CNR) MOSFETs has been developed. ... more A real-space quantum transport simulator for carbon nanoribbon (CNR) MOSFETs has been developed. Using this simulator, the performance of carbon nanoribbon (CNR) MOSFETs is examined in the ballistic limit. The impact of quantum effects on device performance of CNR MOSFETs is also studied. We found that 2D semi-infinite graphene contacts provide metal-induced-gap-states (MIGS) in the CNR channel. These states would provide quantum tunneling in the short channel device and cause Fermi level pining. These effects cause device performance degradation both on the ON-state and the OFF-state. Pure 1D devices (infinite contacts), however, show no MIGS. Quantum tunneling effects are still playing an important role in the device characteristics. Conduction due to band-to-band tunneling is accurately captured in our simulations. It is important in these devices, and found to dominate the off-state current. Based on our simulations, both a 1.4nm wide and a 1.8nm wide CNR with channel length of ...
The European Physical Journal B, 2015
ABSTRACT As devices scale towards atomistic sizes, researches in silicon electronic device techno... more ABSTRACT As devices scale towards atomistic sizes, researches in silicon electronic device technology are investigating alternative structures and materials. As predicted by the International Roadmap for Semiconductors, (ITRS), structures will evolve from planar devices into devices that include 3D features, strong channel confinement, strain engineering, and gate all around placement for better electrostatic control on the channel. Alternative channel materials such as carbon nanotubes (CNT), nanowires (NW) and III-V based channel materials are considered to be possible candidates for future device technology nodes because of their potentially superior to silicon transport properties. For nanoscale dimensions, and under the operating conditions mentioned above, both atomistic and quantum effects become important in determining the electronic structure and transport properties of the devices. Detailed modeling and simulation that capture these new physics will be essential in providing understanding and guidance to the device operation and optimization. We have used the non-equilibrium Green's function (NEGF) formalism for quantum transport simulations and real space atomistic tight-binding techniques (p z , sp3d5s*-SO) to investigate transport properties in CNT, NW and III-V HEMT field-effect transistors. Specifically, we have investigated the effect of atomistic defects such as atomic vacancies, and charged impurities in 1D CNT, and dangling bonds in NW channels. It was found that the presence of single defects, severely degrades the transport performance of 1D channels. We have further investigated the effect of physical quantization on the electronic structure of NW field-effect transistors and identified the main electronic structure factors that influence their performance. It was found that structural and quantization below 10nm can severely affect the electronic properties of NW channels by changing the effective masses and altering degeneracies through valley splitting. Different wire orientations have different transport properties. The [110] and secondly [100] oriented nanowires are found to perform better than the [111] wires in terms of ON-current capabilities for n-type wires, whereas the [111] and [110] significantly outperform the [100] wires in the case of p-type nanowires. Explanations for this behavior can be extracted from the non-parabolicity and anisotropy of the Si 3D bulk E(k) . Finally, we present an analysis of recent experimental data for III-V HEMT devices using the NEGF formalism and address several issues related to the operation of HEMT devices. Interestingly, a 60nm HEMT device can be though to first order as a ballistic channel connected to two series resistances.
Journal of Computational Electronics, 2008
The ballistic performance of graphene nanoribbon (GNR) MOSFETs with different width of armchair G... more The ballistic performance of graphene nanoribbon (GNR) MOSFETs with different width of armchair GNRs is examined using a real-space quantum simulator based on the Non-equilibrium Green's Function (NEGF) approach, self-consistently coupled to a 3D Poisson's equation for electrostatics. GNR MOSFETs show promising device performance, in terms of low subthreshold swing and small drain-induced-barrier-lowing due to their excellent electrostatics and gate control (single monolayer). However, the quantum tunneling effects play an import role in the GNR device performance degradation for wider width GNR MOSFETs due to their reduced bandgap. At 2.2 nm width, the OFF current performance is completely dominated by tunneling currents, making the OFF-state of the device difficult to control.
Nano Letters, 2008
The effects of the various contact types and shapes on the performance of Schottky barrier graphe... more The effects of the various contact types and shapes on the performance of Schottky barrier graphene nanoribbon field-effect-transistors (GNRFETs) have been investigated using a real-space quantum transport simulator based on the NEGF approach self-consistently coupled to a three-dimensional Poisson solver for treating the electrostatics. The device channel considered is a double gate semiconducting armchair nanoribbon. The types of contacts considered are (a) a semi-infinite normal metal, (b) a semi-infinite graphene sheet, (c) finite size rectangular shape armchair graphene contacts, (d) finite size wedge shape graphene contacts, and (e) zigzag graphene nanoribbon contacts. Among these different contact types, the semi-infinite graphene sheet contacts show the worst performance because of their very low density of states around the Dirac point resulting in low transmission possibility through the Schottky barrier, both at ON and OFF states. Although all other types of contacts can have significant enhancement in I ON to I OFF ratio, the zigzag GNR contacts show promising and size invariant performance due to the metallic properties.
Journal of Applied Physics, 2007
... D.Nikonov, and M.Lundstrom, Theoretical Study of Graphene Nanoribbon Field-Effect Transistors... more ... D.Nikonov, and M.Lundstrom, Theoretical Study of Graphene Nanoribbon Field-Effect Transistors, Proceeding ... assisted band-to-band tunneling in carbon nanotube field-effect transistors, Appl ... S.Datta, Quantum Transport: Atom to Transistor, 2nd ed. (Cambridge University Press ...
IEEE Transactions On Nanotechnology, 2000
We explore the three-dimensional (3-D) electrostatics of planar-gate carbon nanotube field-effect... more We explore the three-dimensional (3-D) electrostatics of planar-gate carbon nanotube field-effect transistors (CNTFETs) using a self-consistent solution to the Poisson equation with equilibrium carrier statistics. We examine the effects of the gate insulator thickness and dielectric constant and the source/drain contact geometry on the electrostatics of bottom-gated (BG) and top-gated (TG) devices. We find that the electrostatic scaling length is mostly determined by the gate oxide thickness, not by the oxide dielectric constant. We also find that a high-k gate insulator does not necessarily improve short-channel immunity because it increases the coupling of both the gate and the source/drain contact to the channel. It also increases the parasitic coupling of the source/drain to the gate. Although both the width and the height of the source and drain contacts are important, we find that for the BG device, reducing the width of the 3-D contacts is more effective for improving short channel immunity than reducing the height. The TG device, however, is sensitive to both the width and height of the contact. We find that one-dimensional source and drain contacts promise the best short channel immunity. We also show that an optimized TG device with a thin gate oxide can provide near ideal subthreshold behavior. The results of this paper should provide useful guidance for designing high-performance CNTFETs.
A real-space quantum transport simulator for carbon nanoribbon (CNR) MOSFETs has been developed. ... more A real-space quantum transport simulator for carbon nanoribbon (CNR) MOSFETs has been developed. Using this simulator, the performance of carbon nanoribbon (CNR) MOSFETs is examined in the ballistic limit. The impact of quantum effects on device performance of CNR MOSFETs is also studied. We found that 2D semi-infinite graphene contacts provide metal-induced-gap-states (MIGS) in the CNR channel. These states would provide quantum tunneling in the short channel device and cause Fermi level pining. These effects cause device performance degradation both on the ON-state and the OFF-state. Pure 1D devices (infinite contacts), however, show no MIGS. Quantum tunneling effects are still playing an important role in the device characteristics. Conduction due to band-to-band tunneling is accurately captured in our simulations. It is important in these devices, and found to dominate the off-state current. Based on our simulations, both a 1.4nm wide and a 1.8nm wide CNR with channel length of 12.5nm can outperform ultra scaled Si devices in terms of drive current capabilities and electrostatic control. Although subthreshold slopes in the forward-bias conduction are better than in Si transistors, tunneling currents are important and prevent the achievement of the theoretical limit of 60mV/dec.
Aps Meeting Abstracts, Mar 1, 2006
The Network for Computational Nanotechnology (NCN) is a multi-university, NSF-funded initiative w... more The Network for Computational Nanotechnology (NCN) is a multi-university, NSF-funded initiative with a mission to lead in research, education, and outreach deploying a unique web-based infrastructure (http://nanoHUB.org) to serve the nation's National Nanotechnology Initiative. Around 30 research codes/community tools are available and all the NCN services are free of charge. One such community tool is the CNTFET simulator based on NEGF techniques and the Finite-Element-Method (FEM) to treat three-dimensional (3D) electrostatics. We are able to simulate electronic transport in experimentally demonstrated 3D CNT devices with atomistic potential and charge resolution. Currently, we are investigating the effects of atomistic defects in the CNT devices such as vacancies and charged impurities.
The progress in nanomaterials' synthesis allows the realization of thermoelectric devices based o... more The progress in nanomaterials' synthesis allows the realization of thermoelectric devices based on low dimensional nanostructures. In these confined systems the electrical and thermal conductivity, and the Seebeck coefficient can be designed to some degree independently, providing enhanced ZT values compared to their bulk material's value. We calculate the electrical conductivity, the Seebeck coefficient, and the electronic thermal conductivity of scaled silicon and germanium nanowires using an atomistic sp3d5s*-spin-orbit-coupled tight-binding model. This atomistic model accurately captures the electronic structure of the nanowires while being computationally affordable. We examine n-type and p-type nanowires of diameters/thicknesses from D=3nm to 12nm with various aspect ratios, in [100], [110] and [111] transport orientations, at different doping levels and temperatures. Using experimentally measured values for the lattice thermal conductivity, the expected ZT values of the nanowires are estimated.
Bulletin of the American Physical Society, Mar 5, 2015
Optimization of thermoelectric properties in cross-plane superlattices -A 1D NEGF Study MISCHA TH... more Optimization of thermoelectric properties in cross-plane superlattices -A 1D NEGF Study MISCHA THESBERG, MAHDI POURFATH, Vienna Univ of Technology, NEOPHYTOS NEOPHYTOU, University of Warwick, HANS KOSINA, Vienna Univ of Technology -Thermoelectric materials utilize carrier energy filtering through potential barriers to achieve improvements in the Seebeck coefficient. Barriers, however, tend to drastically reduce the electrical conductivity, and power factor improvements are difficult to be realized. In this work we present a fully quantum mechanical simulation study of thermoelectric transport in the presence of barriers for energy filtering. For this, we use the non-equilibrium Green's function (NEGF) method, including both acoustic and optical phonons. We show that power factor improvements can be achieved by properly adjusting a series of interrelated parameters: i) the position of the Fermi level, ii) the width, size and shape of the barriers as well as the separation between them, iii) the optical phonon energies. Our results provide insight on how to optimize superlattices and nanocomposite materials for enhanced thermoelectric properties.
Proceedings of the 9th Joint Eurographics Ieee Vgtc Conference on Visualization, May 23, 2007
Volumetric Subdivision (VS) is a powerful paradigm that enables volumetric sculpting and realisti... more Volumetric Subdivision (VS) is a powerful paradigm that enables volumetric sculpting and realistic volume deformations that give rise to the concept of "virtual clay". In VS, volumes are commonly represented as a space-filling set of deformed polyhedra, which can be further decomposed into a mesh of tetrahedra for rendering. Images can then be generated via tetrahedral projection or raycasting. A current shortcoming in VS-based operations is the need for a very high level of subdivision to represent fine detail in the mesh and to obtain a high-fidelity visualization. However, we have discovered that the subdivision process itself can be closely simulated with radial basis functions (RBFs), making it possible to replace the finer subdivision levels by a coarser aggregation of RBF kernels. This reduction to a simplified assembly of RBFs subsequently enables interactive rendering of volumetric subdivision shapes within a GPU-based volume splatting framework.
Uploads
Papers by Neophytos Neophytou