The gDFTB method applied to transport in Si nanowires and carbon nanotubes

Alessandro Pecchia, pecchia@ing.uniroma2.it, Dep. of electronics engineering,
University of Rome "Tor Vergata", Via del Politecnico 1, Rome, 00133, Italy,
Luca Latessa, Electronics Engineering, Università di Roma "Tor Vergata", Roma,
00133, Italy, Thomas Frauenheim, frauenheim@phys.upb.de, Bremen Center for
Computational Materials Science, Bremen University, Bibliothekstrasse 1, Bremen,
28359, Germany, and Aldo Di Carlo, dicarlo@ing.uniroma2.it, Department of
Electronic Engeneering, University of Rome "Tor Vergata", Rome, Italy.

The gDFTB code [1-2] is an extension to non-equilibrium Green's functions
formalism of the density-functional tight-binding (DFTB) method for quantum
transport calculations. We examine the state of the art of the development of
such simulator, including parallel distribution strategies and sparse algorithms
for calculations of the Green's functions. We then show applications to
electronic transport of carbon nanotubes and Si nanowires, promising candidates
for future nanoelectronic devices. A detailed analysis of the I-V
characteristics and subthreashold behaviour of coaxially gated field-effect
transistors is considered. Our atomistic DFT-based calculations include the
effect of reduced screening of such one-dimensional systems, including the
presence of classical and quantum capacitance. Incidentally the simulator is
able to catch a physically interesting negative quantum-capacitance regime [3]
which appears in CNT of small diameter and is due to exchange effects of the
electron-electron interactions. Similar calculations have also been applied to
gated Si nanowires of different diameters and growth directions. Here the
effects of quantum capacitance are less strong, and essentially limited only by
the density of states. Device characteristics of short wires are studied in
detail, including an analysis of quantum transport and gate control of the
device. We also show calculations including incoherent transport due to
electron-phonon scattering treated within the NEGF formalism and using realistic
phonons of the one dimensional wire.