arXiv subjects

Supriyo Datta

Publications and source records attributed to Supriyo Datta.

88 records · Page 5Linked to original sources

arXiv2004

A Quantum Mechanical Approach for the Simulation of Si/SiO2 Interface Roughness Scattering in Silicon Nanowire Transistors

In this work, we present a quantum mechanical approach for the simulation of Si/SiO2 interface roughness scattering in silicon nanowire transistors (SNWTs). The simulation domain is discretized with a three-dimensional (3D) finite element mesh, and the microscopic structure of the Si/SiO2 interface roughness is directly implemented. The 3D Schrodinger equation with open boundary conditions is solved by the non-equilibrium Green's function method together with the coupled mode space approach. The 3D electrostatics in the device is rigorously treated by solving a 3D Poisson equation with the finite element method. Although we mainly focus on computational techniques in this paper, the physics of SRS in SNWTs and its impact on the device characteristics are also briefly discussed.

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arXiv2004

Electrical resistance: an atomistic view

This tutorial article presents a "bottom-up" view of electrical resistance starting from something really small, like a molecule, and then discussing the issues that arises as we move to bigger conductors. Remarkably, no serious quantum mechanics is needed to understand electrical conduction through something really small, except for unusual things like the Kondo effect that are seen only for a special range of parameters. This article starts with energy level diagrams (section 2), shows that the broadening that accompanies coupling limits the conductance to a maximum of q^2/h per level (sections 3, 4), describes how a change in the shape of the self-consistent potential profile can turn a symmetric current-voltage characteristic into a rectifying one (sections 5, 6), shows that many interesting effects in molecular electronics can be understood in terms of a simple model (section 7), introduces the non-equilibrium Green function (NEGF) formalism as a sophisticated version of this simple model with ordinary numbers replaced by appropriate matrices (section 8) and ends with a personal view of unsolved problems in the field of nanoscale electron transport (section 9). Appendix A discusses the Coulomb blockade regime of transport, while appendix B presents a formal derivation of the NEGF equations. MATLAB codes for numerical examples are listed in the appendix C.

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arXiv2004

Gating of a molecular transistor: Electrostatic and Conformational

We derive a general result that can be used to evaluate and compare the transconductance of different field-effect mechanisms in molecular transistors, both electrostatic and conformational. The electrostatic component leads to the well-known thermal limit in the absence of tunneling. We show that in a standard three-terminal geometry and in the absence of strong electron-phonon coupling, the conformational component can lead to significant advantages only if the molecular dipole moment μis comparable to et_ox, t_ox being the thickness of the oxide. Surprisingly this conclusion is independent of the ``softness'' of the conformational modes involved, or other geometrical factors. Detailed numerical results for specific examples are presented in support of the analytical results.

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arXiv2004

Electrostatic potential profiles of molecular conductors

The electrostatic potential across a short ballistic molecular conductor depends sensitively on the geometry of its environment, and can affect its conduction significantly by influencing its energy levels and wave functions. We illustrate some of the issues involved by evaluating the potential profiles for a conducting gold wire and an aromatic phenyl dithiol molecule in various geometries. The potential profile is obtained by solving Poisson's equation with boundary conditions set by the contact electrochemical potentials and coupling the result self-consistently with a nonequilibrium Green's function (NEGF) formulation of transport. The overall shape of the potential profile (ramp vs. flat) depends on the feasibility of transverse screening of electric fields. Accordingly, the screening is better for a thick wire, a multiwalled nanotube or a close-packed self-assembled monolayer (SAM), in comparison to a thin wire, a single-walled nanotube or an isolated molecular conductor. The electrostatic potential further governs the alignment or misalignment of intramolecular levels, which can strongly influence the molecular I-V characteristic. An external gate voltage can modify the overall potential profile, changing the current-voltage (I-V) characteristic from a resonant conducting to a saturating one. The degree of saturation and gate modulation depends on the metal-induced-gap states (MIGS) and on the electrostatic gate control parameter set by the ratio of the gate oxide thickness to the channel length.

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arXiv2003

Towards Multi-Scale Modeling of Carbon Nanotube Transistors

Multiscale simulation approaches are needed in order to address scientific and technological questions in the rapidly developing field of carbon nanotube electronics. In this paper, we describe an effort underway to develop a comprehensive capability for multiscale simulation of carbon nanotube electronics. We focus in this paper on one element of that hierarchy, the simulation of ballistic CNTFETs by self-consistently solving the Poisson and Schrodinger equations using the non-equilibrium Greens function (NEGF) formalism. The NEGF transport equation is solved at two levels: i) a semi-empirical atomistic level using the pz orbitals of carbon atoms as the basis, and ii) an atomistic mode space approach, which only treats a few subbands in the tube-circumferential direction while retaining an atomistic grid along the carrier transport direction. Simulation examples show that these approaches describe quantum transport effects in nanotube transistors. The paper concludes with a brief discussion of how these semi-empirical device level simulations can be connected to ab initio, continuum, and circuit level simulations in the multi-scale hierarchy.

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arXiv2003

Predicted Performance Advantages of Carbon Nanotube Transistors with Doped Nanotubes as Source/Drain

Most carbon nanotube field-effect transistors (CNTFETs) directly attach metal source/drain contacts to an intrinsic nanotube channel. When the gate oxide thickness is reduced, such transistors display strong ambipolar conduction, even when the Schottky barrier for electrons (or for holes) is zero. The resulting leakage current, which increases exponentially with the drain voltage, constrains the potential applications of such devices. In this paper, we use numerical simulations to show that if CNT based metal-oxide-semiconductor (MOS) FETs can be achieved by using heavily doped CNT sections as source and drain, ambipolar conduction will be suppressed, leakage current will be reduced, and the scaling limit imposed by source-drain tunneling will be extended. By eliminating the Schottky barrier between the source and channel, the transistor will be capable of delivering more on-current. The leakage current of such devices will be controlled by the full bandgap of CNTs (instead of half of the bandgap for SB CNTFETs) and band-to-band tunneling. These factors will depend on the diameter of nanotubes and the power supply voltage.

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arXiv2003

A Numerical Study of Scaling Issues for Schottky Barrier Carbon Nanotube Transistors

We performed a comprehensive scaling study of Schottky barrier carbon nanotube transistors using self-consistent, atomistic scale simulations. We restrict our attention to Schottky barrier carbon nanotube FETs whose metal source/drain is attached to an intrinsic carbon nanotube channel. Ambipolar conduction is found to be an important factor that must be carefully considered in device design, especially when the gate oxide is thin. The channel length scaling limit imposed by source-drain tunneling is found to be between 5nm and 10nm, depending on the off-current specification. Using a large diameter tube increases the on-current, but it also increases the leakage current. Our study of gate dielectric scaling shows that the charge on the nanotube can play an important role above threshold.

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arXiv2002

Current-voltage Characteristics of Molecular Conductors: Two versus Three Terminal

This paper addresses the question of whether a ``rigid molecule'' (one which does not deform in an external field) used as the conducting channel in a standard three-terminal MOSFET configuration can offer any performance advantage relative to a standard silicon MOSFET. A self-consistent solution of coupled quantum transport and Poisson's equations shows that even for extremely small channel lengths (about 1 nm), a ``well-tempered'' molecular FET demands much the same electrostatic considerations as a ``well-tempered'' conventional MOSFET. In other words, we show that just as in a conventional MOSFET, the gate oxide thickness needs to be much smaller than the channel length (length of the molecule) for the gate control to be effective. Furthermore, we show that a rigid molecule with metallic source and drain contacts has a temperature independent subthreshold slope much larger than 60 mV/decade, because the metal-induced gap states in the channel prevent it from turning off abruptly. However, this disadvantage can be overcome by using semiconductor contacts because of their band-limited nature.

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arXiv2002

Resistance of a Molecule

In recent years, several experimental groups have reported measurements of the current-voltage (I-V) characteristics of individual or small numbers of molecules. Our purpose in this chapter is to provide an intuitive explanation for the observed I-V characteristics using simple models to illustrate the basic physics. In contrast to the MOSFET, whose I-V is largely dominated by classical electrostatics, the I-V characteristics of molecules is determined by a more interesting interplay between nineteenth century physics (electrostatics) and twentieth century physics (quantum transport) and it is important to do justice to both aspects. We start with a qualitative discussion of the main factors affecting the I-V characteristics of molecular conductors, using a simple toy model to illustrate their role. Including the effects of: (1) Shift in the energy level due to charging effects and (2) broadening of the energy levels due to the coupling to the two contacts we obtain realistic I-V plots, even though the toy model assumes that conduction takes place independently through individual molecular levels. In general, however, the full non-equilibrium Green's function (NEGF) formalism is needed. Here, we describe the NEGF formalism as a generalized version of the one-level model. This formalism provides a convenient framework for describing quantum transport and can be used in conjunction with ab initio or semi-empirical Hamiltonians. A simple semi-empirical model for a gold wire serves to illustrate the full NEGF formalism. This example is particularly instructive because it shows the lowest possible "Resistance of a 'Molecule'" per channel.

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arXiv2002

Charge transfer in molecular conductors -- oxidation or reduction?

We discuss the nature of charge transfer in molecular conductors upon connecting to two metallic contacts and imposing a voltage bias across them. The sign of the charge transfer (oxidation vs. reduction) depends on the position of the metal Fermi energy with respect to the molecular levels. In addition, the charge transfer depends on the strength of the coupling (chemisorption vs. physisorption) with the contacts. A convenient way to establish the nature and onset of the charge transfer and the corresponding features in the I-V is to draw an energy level diagram for each spin species. Starting from such a level diagram, we argue that transport in the Tour-Reed switching molecules, which consist of a central phenyl ring with a nitroamine redox center, involves the oxidation of a highest occupied molecular orbital (HOMO)-based level.

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arXiv2002

First-Principles Analysis of Molecular Conduction Using Quantum Chemistry Software

We present a rigorous and computationally efficient method to do a parameter-free analysis of molecular wires connected to contacts. The self-consistent field approach is coupled with Non-equilibrium Green's Function (NEGF) formalism to describe electronic transport under an applied bias. Standard quantum chemistry software is used to calculate the self-consistent field using density functional theory (DFT). Such close coupling to standard quantum chemistry software not only makes the procedure simple to implement but also makes the relation between the I-V characteristics and the chemistry of the molecule more obvious. We use our method to interpolate between two extreme examples of transport through a molecular wire connected to gold (111) contacts: band conduction in a metallic (gold) nanowire, and resonant conduction through broadened, quasidiscrete levels of a phenyl dithiol molecule. We obtain several quantities of interest like I-V characteristic, electron density and voltage drop along the molecule.

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arXiv2002

Insights into molecular conduction from I-V asymmetry

We investigate the origin of asymmetry in the measured current-voltage (I-V) characteristics of molecules with no inherent spatial asymmetry. We establish that such molecules can exhibit asymmetric I-V characteristics due to unequal coupling with the contacts. In contrast with spatially asymmetric molecules, conduction takes place through essentially the same level in both bias directions. The asymmetry arises from a subtle difference in the charging effects, which can only be captured in a self-consistent model for molecular conduction. For HOMO-based conduction, the current is smaller for positive voltage on the stronger contact, while for LUMO conduction, the sense of asymmetry is switched.

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arXiv2001

First-Principles Based Matrix-Green's Function Approach to Molecular Electronic Devices: General Formalism

Transport in molecular electronic devices is different from that in semiconductor mesoscopic devices in two important aspects: (1) the effect of the electronic structure and (2) the effect of the interface to the external contact. A rigorous treatment of molecular electronic devices will require the inclusion of these effects in the context of an open system exchanging particle and energy with the external environment. This calls for combining the theory of quantum transport with the theory of electronic structure starting from the first-principles. We present a rigorous yet tractable matrix Green's function approach for studying transport in molecular electronic devices, based on the Non-Equilibrium Green's Function Formalism of quantum transport and the density-functional theory of electronic structure using local orbital basis sets. By separating the device rigorously into the molecular region and the contact region, we can take full advantage of the natural spatial locality associated with the metallic screening in the electrodes and focus on the physical processes in the finite molecular region. This not only opens up the possibility of using the existing well-established technique of molecular electronic structure theory in transport calculations with little change, but also allows us to use the language of qualitative molecular orbital theory to interpret and rationalize the results of the computation. For the device at equilibrium, our method provides an alternative approach for solving the molecular chemisorption problem. For the device out of equilibrium, we show that the calculation of elastic current transport through molecules, both conceptually and computationally, is no more difficult than solving the chemisorption problem.

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arXiv1999

Fermi-level alignment at metal-carbon nanotube interfaces: application to scanning tunneling spectroscopy

At any metal-carbon nanotube interface there is charge transfer and the induced interfacial field determines the position of the carbon nanotube band structure relative to the metal Fermi-level. In the case of a single-wall carbon nanotube (SWNT) supported on a gold substrate, we show that the charge transfers induce a local electrostatic potential perturbation which gives rise to the observed Fermi-level shift in scanning tunneling spectroscopy (STS) measurements. We also discuss the relevance of this study to recent experiments on carbon nanotube transistors and argue that the Fermi-level alignment will be different for carbon nanotube transistors with low resistance and high resistance contacts.

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arXiv1998

Negative Differential Resistance in the Scanning Tunneling Spectroscopy of Organic molecules

The conductance-voltage spectrum of molecular nanostructures measured by scanning tunneling spectroscopy (STS) is generally assumed to reflect the local density of states of the molecule. This excludes the possibility of observing negative differential resistance (NDR). We report here the observation of NDR in the scanning tunneling microscope (STM) current-voltage (I-V) characteristics of self-assembled monolayer (SAM) of 4-p-Terphenylthiol molecules on gold substrate measured using a platinum probe. We argue that the NDR arises from narrow structures in the local density of states at the tip apex atom and show that depending on the electrostatic potential profile across the system, NDR could be observed in one or both bias directions.

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arXiv1996

Josephson Effect in Unconventional Superconductors

In this letter we present a simple relation, that allows one to predict the Josephson current between two unconventional superconductors with arbitrary band-structure and pairing symmetry. We illustrate this relation with examples of s-wave and d-wave junctions and show a detailed numerical comparison with the phenomenological Sigrist and Rice relation commonly used to interpret tunneling experiments between d-wave superconductors. Our relation automatically accounts for `mid-gap states' that occur in d-wave superconductors and clearly shows how these states lead to a temperature-sensitive contribution to the Josephson current for certain orientations of the junction. This relation should be useful in exploring the effects of disordered junctions, multiple atomic orbitals and complicated order parameters.

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