research

Modern day electronic devices are reaching nanometer dimensions where atomistic effects are dominant. Present day chips have half a billion transistors, while scanning tips allow researchers to `see' and manipulate atoms and molecules to build interesting materials and devices. Fundamentally new principles are needed to understand how current flows at these nanometer length scales (1 nm ~ 10 atomic lengths), and traditional macroscopic concepts like mobility and diffusion coefficient need to be replaced by more basic concepts that require an understanding of resistance, `friction', and electron transport at their most fundamental level. Regardless of the specific form future electronic devices adopt, it is clear that we need to develop ways to describe and model the electronic properties of device structures engineered on an atomic scale. This is what our research is all about.

Specifically, my research focuses on three aspects of nanoelectronic modeling and simulation:

Fundamental physics of current flow through nanosystems:

Traditional CAD tools for electronic conduction are based on macroscopic concepts such as mobility and diffusion and continuum approaches such as effective mass theory, that do not apply at these length scales. We are exploring the novel physics arising from quantum interference, inelastic scattering, friction and heating due to vibrations and spins, strong non-equilibrium many-body effects, atomistic effects, nanoscale thermal and spin transport, hybrid electron dynamics at the nano-micro interface, as well as time-dependent effects due to hysteretic switching, memory and noise. Our aim is to understand these diverse physical phenomena, establish their formal evolution equations and predict/explain their experimental implications.

Computational modeling:

Next we translate the formal evolution equations into quantitative simulation tools, using a combination of computational materials science and quantum chemistry. This includes semi-empirical as well as first principles methods for capturing bulk and surface chemistry, interfacial bandstructure and transport, describing the nano-channels and contact surfaces atomistically. Special attention is aimed at multiscaling and embedding techniques to describe hetero-interfaces and surface states, as in hybrid molecule-silicon devices.

Device engineering:

Finally, we combine the formal equations with numerical simulation tools to identify performance advantages and limitations of nanoscale devices, such as resonant tunneling diodes, switches, conductors, interconnects, transistors and electronic sensors made out of various materials such as silicon or SiGe, molecules, nanotubes, nanowires, spintronic or magnetic elements and silicon quantum dots. Part of our current interests involve exploring hybrid devices operating on novel principles, such as gate-tunable scattering centers for characterization and detection, conformationally gated molecules for nano-relays, molecular redox centers and motors integrated on a silicon CMOS platform for memory and heat sinking.

Microscopic theory of noise for molecular characterization and passivation

As electronic devices shrink, transport becomes dominated by impurities at surfaces and interfaces, which create localized trap levels inside the energy band gap of the channel in commercial field-effect transistors (FETs). At resonance driven by a gate, the trap levels are stochastically filled and emptied by electrons, blocking and unblocking the conduction channel through long ranged electrostatic repulsion (Coulomb scattering) and short ranged quantum interference (Fano scattering). The resulting two-state random telegraph signals, also known in literature as RTS (Random Telegraph SIgnals) noise, can be used to locate the trap position both spectrally as well as spatially, providing full characterization of molecular targets attached to the transistor's channel.

Electronically addressed Rabi oscillations for optoelectronic information processing

There is a lot of interest in quantum computing, as defined on qubits and making use of quantum superposition and entanglement of states, and optoelectronics, where information in circuits is transferred by electrons as well as fast-moving photons. Thus we need a theory that combines together quantum optics with quantum transport. On the other hand, to address a few important questions associated with time-dependent phenomena (ac response and transients, single-electron pumps and turnstiles, random telegraph signals, Rabi flopping, switching rates, interconnect delays, etc.), we need time-domain formulation of the transport theory that incorporates the memory effects, taking into consideration the non-Markovian character of the conduction process. The aim of this project is to analyze optically created signal on the singly occupied two-level system, i.e. Rabi oscillations related to populations of particular energy levels due to near-resonant interaction with monochromatic light, its transfer to the channel and electronic detection in the standard field-effect transistor (FET) configuration via current-time characteristics for fixed voltages. Schematic representation of the system under investigation as well as examplary results are shown in Figue 1. Our computational scheme is based on the fully time-dependent non-equilibrium Green's functions (TD-NEGF) formalism, while numerical results are obtained within the time-domain decomposition (TDD) technique. In particular, we recognized two different and coexisting mechanims for signal transfer between two subsystems: short-range quantum interference due to state blocking (dominant for stronger coupling case), and long-range Coulomb scattering due to charge redistribution (dominant for weak coupling case).

Threshold voltage control of backgated MOSFETs by supramolecular

monolayers

We study how organic molecules can control the electronic and transport properties of an underlying semiconducting transistor to whose surface they are chemically bonded. The threshold voltage of the backgated metal-oxide-semiconductor field effect transistor (MOSFET) shifts due to a combination of the long-ranged dipolar electrostatics of the molecular head-groups, as well as shorter-ranged charge transfer and interfacial dipole driven by band-alignment between the molecular backbone and the reconstructed semiconductor surface atoms.