RECENT RESEARCH PROJECTs
Quantum transport theory and simulation
Electronic transport in semiconductor nanostructures is characterized by the interplay of scattering, tunneling, and confinement. We are especially interested in time-dependent transport in nanostructures, such as the ultrashort-time dynamics that reveals the physics of contacts and various relaxation processes, intrinsic current oscillations, and interaction of highly delocalized particles with electromagntic fields. We work on adapting the non-Markovian reduced-statistical operator formalism to transport in nanostructures and on implementing the stochastic solution to the Wigner-Boltzmann transport equation for realistic structures with pronounced tunneling.
High-frequency semiclassical transport
Interaction of charge carriers with electromagnetic fields is widely used to probe microscopic phenomena. As the EM field frequency becomes comparable to the rates of carrier scattering from lattice vibrations or various defects, the common Drude model for frequency-dependent conductivity no longer holds. We have developed a multiphysics computational technnique (EMC/FDTD/MD) that couples electronic transport with full-wave electrodynamics and can accurately describe intraband transport in semiconductors in the presence of fast-varying fields and scattering.
Electronic transport in graphene and GNRs
Graphene, a two-dimensional material with a honeycomb lattice and a gapless electronic band structure, possesses superior electronic and thermal transport properties when suspended. However, once graphene is placed on a substrate, its 2D nature makes it very sensitive to the substrate properties. We explore phenomena such as screening, puddle formation, as well as ac and dc response of graphene carriers via EMC/FDTD/MD.
Thermal transport in graphene and GNRs
Graphene has 6 phonon branches, four of which can carry appreciable heat in suspended graphene. The out-of-plane (flexural) modes are strongly suppressed in supported grapehene. We investiagte the interplay between the anisotropic phonon dispersion, substrate scattering, and edge roughness scattering of phonons in graphene nanoribbons.
transport in rough semicondcuctor nanostructures
Thermal transport in nanostructures is an active area of study. Among recent experimental surprises are the thermal conductivities in ultrarough silicon nanostructures measured to be considerably below the so-called Casimir limit (the thermal conductivity limit derived under the assumption that phonon momentum is fully randomized upon scattering). We are trying to understand how realistic roughness features, accessible in experiments, affect therm al transport in nanostructures.
Thermoelectric properties of semiconductor
Thermoelectric applications, such as Peltier cooling and thermoelectric energy harvesting, require materials that are excellent electrical but poor thermal conductors. In recent years, semiconductor nanostructures have been increasingly investigated as thermoelectric building blocks, owing largely to the drastic thermal conductivity reduction. We investigate the role of confinement and roughness on both electronic and thermal transport in semiconductor nanowires (Si, GaN) and SiGe-based superlattices by solving the Boltzmann transport equations for both electrons and phonons via the stochastic Monte Carlo technique.
Quantum cascade lasers rely on quantum confinement and tunneling to achieve lasing in the infrared and terahertz parts of the electromangenic spectrum. A big problem with the so-called electrically pumped lasers, which require applied voltage and current flow to achieve lasing, is that they get excessively heated. Both electrons and phonons in QCLs are very far from equilibrium, and play off of one other in ways that are not fully understood or characterized. We are trying to understand this interplay by means of a comprehensive coupled simulation of electron and phonon transport in QCLs.
in carbon nanotube bundles
Carbon nanotubes are promising building blocks for organic photovoltaic devices. We are working to understand the generation of excitons (bound electron-hole pairs) in CNTs and the mechanisms of exciton hopping betwen CNTs of different orientation and bandgaps, in order to describe exction diffusion in disordered CNT systems.