Multiscale Approaches for Nano-Electonic Devices: From Density Functional Tight Binding to Continuous Models
University of Roma, IT
Keywords: Density Functional, Tight Binding
Abstract:Technological advances in fabrication, characterization and control at the nanoscale level have enabled the manufacturing of a variety of new organic and inorganic nanostructures with a good degree of reproducibility. In many cases the active devices involve few thousands of atoms, requiring the application of atomistic simulations. In recent years we have seen the first striking successes of this technology and the demonstration of its potentialities. Such a new class of devices requires new simulation approaches, since the inherent quantum-mechanical physics involved must be treated properly. The debate over the exact nature of the transport mechanisms in many of such systems still remains open, such as the influence of correlation and the thermal dissipation. Transport calculations can be obtained via self-consistent density-functional method coupled with non-equilibrium Greenís function approaches [1-2]. In particular, density functional tight-binding techniques  are very promising due to their intrinsic efficiency. This scheme allows treatment of systems comprising a large number of atoms. We will give a description of the DFTB + NEGF methodology, the present status of the research in this field and show applications to molecular systems , carbon nanotubes and inorganic nanowires. Energy dissipation via phonon scattering will also be investigated by a proper definition of self-energy terms[6-8]. This method allows to study explicitly the influence of each individual vibrational mode and allows for a detailed analysis of the power dissipated in the molecular wire. As a final discussion, we will present a new approach of mixing atomistic and effective medium calculations within the multiscale-multiphysics description of modern nanostructured devices which has been recently developed in the TiberCAD simulator .