We study the effect of metastable states on the relaxation process (and hence information propaga... more We study the effect of metastable states on the relaxation process (and hence information propagation) in locally coupled and boundary-driven structures.
We explore the design of quantum error-correcting codes for cases where the decoherence events of... more We explore the design of quantum error-correcting codes for cases where the decoherence events of qubits are correlated. In particular, we consider the case where only spatially contiguous qubits decohere, which is analogous to the case of burst errors in classical coding theory. We present several different efficient schemes for constructing families of such codes. For example, one can find one-dimensional quantum codes of length n = 13 and 15 that correct burst errors of width b ≤ 3; as a comparison, a random-error correcting quantum code that corrects t = 3 errors must have length n ≥ 17. In general, we show that it is possible to build quantum burst-correcting codes that have near optimal dimension. For example, we show that for any constant b, there exist b-burst-correcting quantum codes with length n, and dimension k = n−log n−O(b); as a comparison, the Hamming bound for the case with t (constant) random errors yields k ≤
We explore the design of quantum error-correcting codes for cases where the decoherence events of... more We explore the design of quantum error-correcting codes for cases where the decoherence events of qubits are correlated. In particular, we consider the case where only spatially contiguous qubits decohere, which is analogous to the case of burst errors in classical coding theory. We present several different efficient schemes for constructing families of such codes. For example, one can find one-dimensional quantum codes of length n = 13 and 15 that correct burst errors of width b ≤ 3; as a comparison, a random-error correcting quantum code that corrects t = 3 errors must have length n ≥ 17. In general, we show that it is possible to build quantum burst-correcting codes that have near optimal dimension. For example, we show that for any constant b, there exist b-burst-correcting quantum codes with length n, and dimension k = n−log n−O(b); as a comparison, the Hamming bound for the case with t (constant) random errors yields k ≤
The fundamental question of how chirality affects the electronic coupling of a nanotube to metal ... more The fundamental question of how chirality affects the electronic coupling of a nanotube to metal contacts is important for the application of nanotubes as nanowires. We show that metallic-zigzag nanotubes are superior to armchair nanotubes as nanowires, by modeling the metal-nanotube interface. More specifically, we show that as a function of coupling strength, the total electron transmission of armchair nanotubes increases and tends to be pinned close to unity for a metal with Fermi wave vector close to that of gold. In contrast, the total transmission of zigzag nanotubes increases to the maximum possible value of two. The origin of these effects lies in the details of the wave function, which is explained.
Quantization in the inversion layer and phase coherent transport are anticipated to have signific... more Quantization in the inversion layer and phase coherent transport are anticipated to have significant impact on device performance in 'ballistic' nanoscale transistors. While the role of some quantum effects have been analyzed qualitatively using simple one dimensional ballistic models, two dimensional (2D) quantum mechanical simulation is important for quantitative results. In this paper, we present a framework for 2D quantum mechanical simulation of a nanotransistor / Metal Oxide Field Effect Transistor (MOSFET). This framework consists of the non equilibrium Green's function equations solved self-consistently with Poisson's equation. Solution of this set of equations is computationally intensive. An efficient algorithm to calculate the quantum mechanical 2D electron density has been developed. The method presented is comprehensive in that treatment includes the three open boundary conditions, where the narrow channel region opens into physically broad source, drain and gate regions. Results are presented for (i) drain current versus 1-1 drain and gate voltages, (ii) comparison to results from Medici, and (iii) gate tunneling current, using 2D potential profiles. Methods to reduce the gate leakage current are also discussed based on simulation results. Typeset using REVT E X 1-2 (iv) Approximate theories of quantum effects included in semi-classical MOSFET modeling tools are desirable from practical considerations because semi-classical methods are numerically less expensive, and much of the empirical and semi-classical MOSFET physics developed over the last few decades continues to hold true in many regions of a nanoscale MOS-FET. Examples of semiclassical methods that consider some quantum mechanical aspects are the density gradient, 14,15 and effective potential 16 approaches, and quantum mechanical approximations used in the Medici package. 17 Fully quantum mechanical simulations can play an important role in benchmarking such simulators. Central to quantum mechanical approaches to charge transport modeling is self-1-3 B. G r and G < : Discretized matrix equations Self-consistent solution of the Green's function and Poisson's equations requires repeated computation of the non-equilibrium charge density. This computation is often the most time consuming part in modeling the electronic properties of devices.
In this paper, we present a full 3-D real-space quantum-transport simulator based on the Green's ... more In this paper, we present a full 3-D real-space quantum-transport simulator based on the Green's function formalism developed to study nonperturbative effects in ballistic nanotransistors. The nonequilibrium Green function (NEGF) equations in the effective mass approximation are discretized using the control-volume approach and solved self-consistently with the Poisson equation in order to obtain the electron and current densities. An efficient recursive algorithm is used in order to avoid the computation of the full Green function matrix. This algorithm, and the parallelization scheme used for the energy cycle, allow us to compute very efficiently the current-voltage characteristic without the simplifying assumptions often used in other quantum-transport simulations. We have applied our simulator to study the effect of surface roughness and stray charge on the I D -V G characteristic of a 6-nm Si-nanowire transistor. The results highlight the distinctly 3-D character of the electron transport, which cannot be accurately captured by using 1-D and 2-D NEGF simulations, or 3-D mode-space approximations.
Utilizing sp3d5s* tight-binding band structure and wave functions for electrons and holes we show... more Utilizing sp3d5s* tight-binding band structure and wave functions for electrons and holes we show that acoustic phonon limited hole mobility in [110] grown silicon nanowires (SiNWs) is greater than electron mobility. The room temperature acoustically limited hole mobility for the SiNWs considered can be as high as 2500 cm 2 /V s, which is nearly three times larger than the bulk acoustically limited silicon hole mobility.
We use a simple picture based on the π electron approximation to study the bandgap variation of c... more We use a simple picture based on the π electron approximation to study the bandgap variation of carbon nanotubes with uniaxial and torsional strain. We find (i) that the magnitude of slope of bandgap versus strain has an almost universal behaviour that depends on the chiral angle, (ii) that the sign of slope depends on the value of (n − m) mod 3 and (iii) a novel change in sign of the slope of bandgap versus uniaxial strain arising from a change in the value of the quantum number corresponding to the minimum bandgap. Four orbital calculations are also presented to show that the π orbital results are valid.
The effect of the surface roughness on the electron transport of a double gate nano MOSFETs has b... more The effect of the surface roughness on the electron transport of a double gate nano MOSFETs has been investigated. The study has been carried out using a simulator based on the twodimensional Non-Equilibrium Green's Function (NEGF) formalism coupled self-consistently with Poisson's equation. An appropriate control volume discretisation scheme for the Poisson and Green's function equations has been implemented in order to describe properly the surface roughness. The twodimensional electron and current density landscape for the device with surface roughness exhibit strong inhomogeneity as compare with the smooth interface case Devices with different randomly generated surface roughness patterns have been compared. At nanodevice scale the effects of the specific profile of the surface roughness do not self-average. The total macroscopic current pattern follows the microscopic detail of the roughness. While the related threshold voltage fluctuations are in the range of 100mV, the subthreshold slope remains quite similar between the different devices.
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 model the influence of scattering along the channel and extension regions of dual gate nanotra... more We model the influence of scattering along the channel and extension regions of dual gate nanotransistor. It is found that the reduction in drain current due to scattering in the right half of the channel is comparable to the reduction in drain current due to scattering in the left half of the channel, when the channel length is comparable to the scattering length. This is in contrast to a popular belief that scattering in the source end of a nanotransistor is significantly more detrimental to the drive current than scattering elsewhere. As the channel length becomes much larger than the scattering length, scattering in the drain-end is less detrimental to the drive current than scattering near the source-end of the channel.
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.
We study the effect of metastable states on the relaxation process (and hence information propaga... more We study the effect of metastable states on the relaxation process (and hence information propagation) in locally coupled and boundary-driven structures.
We explore the design of quantum error-correcting codes for cases where the decoherence events of... more We explore the design of quantum error-correcting codes for cases where the decoherence events of qubits are correlated. In particular, we consider the case where only spatially contiguous qubits decohere, which is analogous to the case of burst errors in classical coding theory. We present several different efficient schemes for constructing families of such codes. For example, one can find one-dimensional quantum codes of length n = 13 and 15 that correct burst errors of width b ≤ 3; as a comparison, a random-error correcting quantum code that corrects t = 3 errors must have length n ≥ 17. In general, we show that it is possible to build quantum burst-correcting codes that have near optimal dimension. For example, we show that for any constant b, there exist b-burst-correcting quantum codes with length n, and dimension k = n−log n−O(b); as a comparison, the Hamming bound for the case with t (constant) random errors yields k ≤
We explore the design of quantum error-correcting codes for cases where the decoherence events of... more We explore the design of quantum error-correcting codes for cases where the decoherence events of qubits are correlated. In particular, we consider the case where only spatially contiguous qubits decohere, which is analogous to the case of burst errors in classical coding theory. We present several different efficient schemes for constructing families of such codes. For example, one can find one-dimensional quantum codes of length n = 13 and 15 that correct burst errors of width b ≤ 3; as a comparison, a random-error correcting quantum code that corrects t = 3 errors must have length n ≥ 17. In general, we show that it is possible to build quantum burst-correcting codes that have near optimal dimension. For example, we show that for any constant b, there exist b-burst-correcting quantum codes with length n, and dimension k = n−log n−O(b); as a comparison, the Hamming bound for the case with t (constant) random errors yields k ≤
The fundamental question of how chirality affects the electronic coupling of a nanotube to metal ... more The fundamental question of how chirality affects the electronic coupling of a nanotube to metal contacts is important for the application of nanotubes as nanowires. We show that metallic-zigzag nanotubes are superior to armchair nanotubes as nanowires, by modeling the metal-nanotube interface. More specifically, we show that as a function of coupling strength, the total electron transmission of armchair nanotubes increases and tends to be pinned close to unity for a metal with Fermi wave vector close to that of gold. In contrast, the total transmission of zigzag nanotubes increases to the maximum possible value of two. The origin of these effects lies in the details of the wave function, which is explained.
Quantization in the inversion layer and phase coherent transport are anticipated to have signific... more Quantization in the inversion layer and phase coherent transport are anticipated to have significant impact on device performance in 'ballistic' nanoscale transistors. While the role of some quantum effects have been analyzed qualitatively using simple one dimensional ballistic models, two dimensional (2D) quantum mechanical simulation is important for quantitative results. In this paper, we present a framework for 2D quantum mechanical simulation of a nanotransistor / Metal Oxide Field Effect Transistor (MOSFET). This framework consists of the non equilibrium Green's function equations solved self-consistently with Poisson's equation. Solution of this set of equations is computationally intensive. An efficient algorithm to calculate the quantum mechanical 2D electron density has been developed. The method presented is comprehensive in that treatment includes the three open boundary conditions, where the narrow channel region opens into physically broad source, drain and gate regions. Results are presented for (i) drain current versus 1-1 drain and gate voltages, (ii) comparison to results from Medici, and (iii) gate tunneling current, using 2D potential profiles. Methods to reduce the gate leakage current are also discussed based on simulation results. Typeset using REVT E X 1-2 (iv) Approximate theories of quantum effects included in semi-classical MOSFET modeling tools are desirable from practical considerations because semi-classical methods are numerically less expensive, and much of the empirical and semi-classical MOSFET physics developed over the last few decades continues to hold true in many regions of a nanoscale MOS-FET. Examples of semiclassical methods that consider some quantum mechanical aspects are the density gradient, 14,15 and effective potential 16 approaches, and quantum mechanical approximations used in the Medici package. 17 Fully quantum mechanical simulations can play an important role in benchmarking such simulators. Central to quantum mechanical approaches to charge transport modeling is self-1-3 B. G r and G < : Discretized matrix equations Self-consistent solution of the Green's function and Poisson's equations requires repeated computation of the non-equilibrium charge density. This computation is often the most time consuming part in modeling the electronic properties of devices.
In this paper, we present a full 3-D real-space quantum-transport simulator based on the Green's ... more In this paper, we present a full 3-D real-space quantum-transport simulator based on the Green's function formalism developed to study nonperturbative effects in ballistic nanotransistors. The nonequilibrium Green function (NEGF) equations in the effective mass approximation are discretized using the control-volume approach and solved self-consistently with the Poisson equation in order to obtain the electron and current densities. An efficient recursive algorithm is used in order to avoid the computation of the full Green function matrix. This algorithm, and the parallelization scheme used for the energy cycle, allow us to compute very efficiently the current-voltage characteristic without the simplifying assumptions often used in other quantum-transport simulations. We have applied our simulator to study the effect of surface roughness and stray charge on the I D -V G characteristic of a 6-nm Si-nanowire transistor. The results highlight the distinctly 3-D character of the electron transport, which cannot be accurately captured by using 1-D and 2-D NEGF simulations, or 3-D mode-space approximations.
Utilizing sp3d5s* tight-binding band structure and wave functions for electrons and holes we show... more Utilizing sp3d5s* tight-binding band structure and wave functions for electrons and holes we show that acoustic phonon limited hole mobility in [110] grown silicon nanowires (SiNWs) is greater than electron mobility. The room temperature acoustically limited hole mobility for the SiNWs considered can be as high as 2500 cm 2 /V s, which is nearly three times larger than the bulk acoustically limited silicon hole mobility.
We use a simple picture based on the π electron approximation to study the bandgap variation of c... more We use a simple picture based on the π electron approximation to study the bandgap variation of carbon nanotubes with uniaxial and torsional strain. We find (i) that the magnitude of slope of bandgap versus strain has an almost universal behaviour that depends on the chiral angle, (ii) that the sign of slope depends on the value of (n − m) mod 3 and (iii) a novel change in sign of the slope of bandgap versus uniaxial strain arising from a change in the value of the quantum number corresponding to the minimum bandgap. Four orbital calculations are also presented to show that the π orbital results are valid.
The effect of the surface roughness on the electron transport of a double gate nano MOSFETs has b... more The effect of the surface roughness on the electron transport of a double gate nano MOSFETs has been investigated. The study has been carried out using a simulator based on the twodimensional Non-Equilibrium Green's Function (NEGF) formalism coupled self-consistently with Poisson's equation. An appropriate control volume discretisation scheme for the Poisson and Green's function equations has been implemented in order to describe properly the surface roughness. The twodimensional electron and current density landscape for the device with surface roughness exhibit strong inhomogeneity as compare with the smooth interface case Devices with different randomly generated surface roughness patterns have been compared. At nanodevice scale the effects of the specific profile of the surface roughness do not self-average. The total macroscopic current pattern follows the microscopic detail of the roughness. While the related threshold voltage fluctuations are in the range of 100mV, the subthreshold slope remains quite similar between the different devices.
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 model the influence of scattering along the channel and extension regions of dual gate nanotra... more We model the influence of scattering along the channel and extension regions of dual gate nanotransistor. It is found that the reduction in drain current due to scattering in the right half of the channel is comparable to the reduction in drain current due to scattering in the left half of the channel, when the channel length is comparable to the scattering length. This is in contrast to a popular belief that scattering in the source end of a nanotransistor is significantly more detrimental to the drive current than scattering elsewhere. As the channel length becomes much larger than the scattering length, scattering in the drain-end is less detrimental to the drive current than scattering near the source-end of the channel.
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