The current carrying capacity of ballistic electrons in carbon nanotubes that are coupled to idea... more The current carrying capacity of ballistic electrons in carbon nanotubes that are coupled to ideal contacts is analyzed. At small applied voltages, where electrons are injected only into crossing subbands, the differential conductance is $4e^2/h$. At applied voltages larger than $\Delta E_{NC}/2e$ ($\Delta E_{NC}$ is the energy level spacing of first non crossing subbands), electrons are injected into non crossing subbands. The contribution of these electrons to current is determined by the competing processes of Bragg reflection and Zener type inter subband tunneling. In small diameter nanotubes, Bragg reflection dominates, and the maximum differential conductance is comparable to $4e^2/h$. Inter subband Zener tunneling can be non negligible as the nanotube diameter increases because $\Delta E_{NC}$ is inversely proportional to the diameter. As a result, with increasing nanotube diameter, the differential conductance becomes larger than $4e^2/h$, though not comparable to the large number of subbands into which electrons are injected from the contacts. These results may be relevant to recent experiments in large diameter multi-wall nanotubes that observed conductances larger than $4e^2/h$.
Carbon nanotubes (CNTs) are amongst the most explored one-dimensional nanostructures and have att... more Carbon nanotubes (CNTs) are amongst the most explored one-dimensional nanostructures and have attracted tremendous interest from fundamental science and technological perspectives. Albeit topologically simple, they exhibit a rich variety of intriguing electronic properties, such as metallic and semiconducting behaviour. Furthermore, these structures are atomically precise, meaning that each carbon atom is still three-fold coordinated without any dangling bonds. CNTs have been used in many laboratories to build prototype nanodevices. These devices include metallic wires, field-effect transistors, electromechanical sensors and displays. They potentially form the basis of future all-carbon electronics. This review deals with the building blocks of understanding the device physics of CNT-based nanodevices. There are many features that make CNTs different from traditional materials, including chirality-dependent electronic properties, the one-dimensional nature of electrostatic screening and the presence of several direct bandgaps. Understanding these novel properties and their impact on devices is crucial in the development and evolution of CNT applications.
We have developed physical approximations and computer code capable of realistically simulating 2... more We have developed physical approximations and computer code capable of realistically simulating 2-D nanoscale transistors, using the non-equilibrium Green's function (NEGF) method. This is the most accurate full quantum model yet applied to 2-D device simulation. Open boundary conditions and oxide tunneling are treated on an equal footing. Acoustic phonon scattering is included, causing transport to deviate from ballistic in a realistic manner. Electrons in the ellipsoids of the conduction band are treated within the anisotropic effective mass approximation. Self consistent solution of Poisson-NEGF equations is numerically intensive because of the number of spatial and energy coordinates involved. This makes the use of parallel/distributed computing imperative.
We study the conductance of carbon nanotube wires in the presence of disorder, in the limit of ph... more We study the conductance of carbon nanotube wires in the presence of disorder, in the limit of phase coherent transport. For this purpose, we have developed a simple numerical procedure to compute transmission through carbon nanotubes and related structures. Two models of disorder are considered, weak uniform disorder and isolated strong scatterers. In the case of weak uniform disorder, our simulations show that the conductance is not significantly affected by disorder when the Fermi energy is close to the band center. Further, the transmission around the band center depends on the diameter of these zero bandgap wires. We also find that the calculated small bias conductance as a function of the Fermi energy exhibits a dip when the Fermi energy is close to the second subband minima. In the presence of strong isolated disorder, our calculations show a transmission gap at the band center, and the corresponding conductance is very small.
We computationally study the electrostatic potential profile and current carrying capacity of car... more We computationally study the electrostatic potential profile and current carrying capacity of carbon nanotubes as a function of length and diameter. Our study is based on solving the non equilibrium Green's function and Poisson equations self-consistently, including the effect of electron-phonon scattering. A transition from ballistic to diffusive regime of electron transport with increase of applied bias is manifested by qualitative changes in potential profiles, differential conductance and electric field in a nanotube. In the low bias ballistic limit, most of the applied voltage drop occurs near the contacts. In addition, the electric field at the tube center increases proportionally with diameter. In contrast, at high biases, most of the applied voltage drops across the nanotube, and the electric field at the tube center decreases with increase in diameter. We find that the differential conductance can increase or decrease with bias as a result of an interplay of nanotube length, diameter and a quality factor of the contacts. From an application view point, we find that the current carrying capacity of nanotubes increases with increase in diameter. Finally, we investigate the role of inner tubes in affecting the current carried by the outermost tube of a multiwalled nanotube.
We present results and describe progress we have made in the development of our fully quantum mec... more We present results and describe progress we have made in the development of our fully quantum mechanical two dimensional device simulator. The simulator is based on the non equilibrium Greens function method (NEGF), which in the absence of many body effects (electron-phonon and electron-electron interactions) is equivalent to Schrodinger's equation with open boundaries. We discuss issues faced with regards to open boundary conditions, computational resource requirements, and their relationship to self consistent solution of the Poisson-NEGF equations.
Atomistic simulations using a combination of classical forcefield and Density-Functional-Theory (... more Atomistic simulations using a combination of classical forcefield and Density-Functional-Theory (DFT) show that carbon atoms remain essentially sp2 coordinated in either bent tubes or tubes pushed by an atomically sharp AFM tip. Subsequent Green's-function-based transport calculations reveal that for armchair tubes there is no significant drop in conductance, while for zigzag tubes the conductance can drop by several orders of magnitude in AFM-pushed tubes. The effect can be attributed to simple stretching of the tube under tip deformation, which opens up an energy gap at the Fermi surface.
Quantum mechanical confinement effects, gate, hand-to-hand and source-to-drain tunnelling will dr... more Quantum mechanical confinement effects, gate, hand-to-hand and source-to-drain tunnelling will dramatically affect the characteristics of future generation nanometre scaled devices. It has been demonstrated already that first-order quantum corrections, which satisfactorily describe quantum confinement effects, can be introduced into efficient TCAD orientated drift-diffusion simulators using the density gradient approach. In this paper we refer to Non-Equilibrium Green's Function simulations in order to calibrate the density gradient formalism in respect of both confinement and source-to-drain tunnelling using different effective masses in directions normal and parallel to the conducting channel. We demonstrate that the density gradient formalism can describe accurately the current characteristics in sub 20 nm double gate MOSFETs.
The current carrying capacity of ballistic electrons in carbon nanotubes that are coupled to idea... more The current carrying capacity of ballistic electrons in carbon nanotubes that are coupled to ideal contacts is analyzed. At small applied voltages, where electrons are injected only into crossing subbands, the differential conductance is $4e^2/h$. At applied voltages larger than $\Delta E_{NC}/2e$ ($\Delta E_{NC}$ is the energy level spacing of first non crossing subbands), electrons are injected into non crossing subbands. The contribution of these electrons to current is determined by the competing processes of Bragg reflection and Zener type inter subband tunneling. In small diameter nanotubes, Bragg reflection dominates, and the maximum differential conductance is comparable to $4e^2/h$. Inter subband Zener tunneling can be non negligible as the nanotube diameter increases because $\Delta E_{NC}$ is inversely proportional to the diameter. As a result, with increasing nanotube diameter, the differential conductance becomes larger than $4e^2/h$, though not comparable to the large number of subbands into which electrons are injected from the contacts. These results may be relevant to recent experiments in large diameter multi-wall nanotubes that observed conductances larger than $4e^2/h$.
Carbon nanotubes (CNTs) are amongst the most explored one-dimensional nanostructures and have att... more Carbon nanotubes (CNTs) are amongst the most explored one-dimensional nanostructures and have attracted tremendous interest from fundamental science and technological perspectives. Albeit topologically simple, they exhibit a rich variety of intriguing electronic properties, such as metallic and semiconducting behaviour. Furthermore, these structures are atomically precise, meaning that each carbon atom is still three-fold coordinated without any dangling bonds. CNTs have been used in many laboratories to build prototype nanodevices. These devices include metallic wires, field-effect transistors, electromechanical sensors and displays. They potentially form the basis of future all-carbon electronics. This review deals with the building blocks of understanding the device physics of CNT-based nanodevices. There are many features that make CNTs different from traditional materials, including chirality-dependent electronic properties, the one-dimensional nature of electrostatic screening and the presence of several direct bandgaps. Understanding these novel properties and their impact on devices is crucial in the development and evolution of CNT applications.
We have developed physical approximations and computer code capable of realistically simulating 2... more We have developed physical approximations and computer code capable of realistically simulating 2-D nanoscale transistors, using the non-equilibrium Green's function (NEGF) method. This is the most accurate full quantum model yet applied to 2-D device simulation. Open boundary conditions and oxide tunneling are treated on an equal footing. Acoustic phonon scattering is included, causing transport to deviate from ballistic in a realistic manner. Electrons in the ellipsoids of the conduction band are treated within the anisotropic effective mass approximation. Self consistent solution of Poisson-NEGF equations is numerically intensive because of the number of spatial and energy coordinates involved. This makes the use of parallel/distributed computing imperative.
We study the conductance of carbon nanotube wires in the presence of disorder, in the limit of ph... more We study the conductance of carbon nanotube wires in the presence of disorder, in the limit of phase coherent transport. For this purpose, we have developed a simple numerical procedure to compute transmission through carbon nanotubes and related structures. Two models of disorder are considered, weak uniform disorder and isolated strong scatterers. In the case of weak uniform disorder, our simulations show that the conductance is not significantly affected by disorder when the Fermi energy is close to the band center. Further, the transmission around the band center depends on the diameter of these zero bandgap wires. We also find that the calculated small bias conductance as a function of the Fermi energy exhibits a dip when the Fermi energy is close to the second subband minima. In the presence of strong isolated disorder, our calculations show a transmission gap at the band center, and the corresponding conductance is very small.
We computationally study the electrostatic potential profile and current carrying capacity of car... more We computationally study the electrostatic potential profile and current carrying capacity of carbon nanotubes as a function of length and diameter. Our study is based on solving the non equilibrium Green's function and Poisson equations self-consistently, including the effect of electron-phonon scattering. A transition from ballistic to diffusive regime of electron transport with increase of applied bias is manifested by qualitative changes in potential profiles, differential conductance and electric field in a nanotube. In the low bias ballistic limit, most of the applied voltage drop occurs near the contacts. In addition, the electric field at the tube center increases proportionally with diameter. In contrast, at high biases, most of the applied voltage drops across the nanotube, and the electric field at the tube center decreases with increase in diameter. We find that the differential conductance can increase or decrease with bias as a result of an interplay of nanotube length, diameter and a quality factor of the contacts. From an application view point, we find that the current carrying capacity of nanotubes increases with increase in diameter. Finally, we investigate the role of inner tubes in affecting the current carried by the outermost tube of a multiwalled nanotube.
We present results and describe progress we have made in the development of our fully quantum mec... more We present results and describe progress we have made in the development of our fully quantum mechanical two dimensional device simulator. The simulator is based on the non equilibrium Greens function method (NEGF), which in the absence of many body effects (electron-phonon and electron-electron interactions) is equivalent to Schrodinger's equation with open boundaries. We discuss issues faced with regards to open boundary conditions, computational resource requirements, and their relationship to self consistent solution of the Poisson-NEGF equations.
Atomistic simulations using a combination of classical forcefield and Density-Functional-Theory (... more Atomistic simulations using a combination of classical forcefield and Density-Functional-Theory (DFT) show that carbon atoms remain essentially sp2 coordinated in either bent tubes or tubes pushed by an atomically sharp AFM tip. Subsequent Green's-function-based transport calculations reveal that for armchair tubes there is no significant drop in conductance, while for zigzag tubes the conductance can drop by several orders of magnitude in AFM-pushed tubes. The effect can be attributed to simple stretching of the tube under tip deformation, which opens up an energy gap at the Fermi surface.
Quantum mechanical confinement effects, gate, hand-to-hand and source-to-drain tunnelling will dr... more Quantum mechanical confinement effects, gate, hand-to-hand and source-to-drain tunnelling will dramatically affect the characteristics of future generation nanometre scaled devices. It has been demonstrated already that first-order quantum corrections, which satisfactorily describe quantum confinement effects, can be introduced into efficient TCAD orientated drift-diffusion simulators using the density gradient approach. In this paper we refer to Non-Equilibrium Green's Function simulations in order to calibrate the density gradient formalism in respect of both confinement and source-to-drain tunnelling using different effective masses in directions normal and parallel to the conducting channel. We demonstrate that the density gradient formalism can describe accurately the current characteristics in sub 20 nm double gate MOSFETs.
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Papers by M. P. Anantram