Full-band quantum transport in nanowire transistors

Semiconductor nanowires may be the core components of next generation processors and memories. In effect, several groups already demonstrated the feasibility of Si or Ge nanowire field-effect transistors (FETs). However, the fabrication of novel devices is always a difficult and expensive process. The recourse to technology computer aided design can facilitate the development of new structures and help reducing the inherent costs. In this article a full-band quantum transport (QT) solver dedicated to nanowire transistors is presented. The semi-empirical sp 3 d 5 s * tight-binding (TB) method is chosen as bandstructure model for its accuracy to reproduce the bulk properties, for its straight forward extension to nanostructures, and for its atomic description of the simulation domain. The calculation of multi-band open boundary conditions (OBCs) and their integration into a three-dimensional Schrödinger-Poisson or Non-equilibrium Green's Function solver are fundamental in the development of a ballistic QT simulator. Different approaches are investigated and compared in this work. They all allow transport with any channel orientation, material composition, and cross section shape. The computational burden restricts most of them to the simulation of small nanowire structures. However, some advanced numerical techniques open promising perspectives towards realistic device

Influence of cross-section geometry and wire orientation on the phonon shifts in ultra-scaled Si nanowires

Engineering of the cross-section shape and size of ultra-scaled Si nanowires (SiNWs) provides an attractive way for tuning their structural properties. The acoustic and optical phonon shifts of the free-standing circular, hexagonal, square and triangular SiNWs are calculated using a Modified Valence Force Field (MVFF) model. The acoustic phonon blue shift (acoustic hardening) and the optical phonon red shift (optical softening) show a strong dependence on the cross-section shape and size of the SiNWs. The triangular SiNWs have the least structural symmetry as revealed by the splitting of the degenerate flexural phonon modes and The show the minimum acoustic hardening and the maximum optical hardening. The acoustic hardening, in all SiNWs, is attributed to the decreasing difference in the vibrational energy distribution between the inner and the surface atoms with decreasing cross-section size. The optical softening is attributed to the reduced phonon group velocity and the localization of the vibrational energy density on the inner atoms. While the acoustic phonon shift shows a strong wire orientation dependence, the optical phonon softening is independent of wire orientation.Comment: 10 figures, 4 Tables, submitted to JAP for revie

Ab-initio quantum transport simulation of self-heating in single-layer 2-D materials

Through advanced quantum mechanical simulations combining electron and phonon transport from first-principles self-heating effects are investigated in n-type transistors with a single-layer MoS2, WS2, and black phosphorus as channel materials. The selected 2-D crystals all exhibit different phonon-limited mobility values, as well as electron and phonon properties, which has a direct influence on the increase of their lattice temperature and on the power dissipated inside their channel as a function of the applied gate voltage and electrical current magnitude. This computational study reveals (i) that self-heating plays a much more important role in 2-D materials than in Si nanowires, (ii) that it could severely limit the performance of 2-D devices at high current densities, and (iii) that black phosphorus appears less sensitive to this phenomenon than transition metal dichalcogenides

Electronic Properties of Lithiated SnO-based Anode Materials

In this paper we use an ab-initio quantum transport approach to study the electron current flowing through lithiated SnO anodes for potential applications in Li-ion batteries. By investigating a set of lithiated structures with varying lithium concentrations, it is revealed that LixSnO can be a good conductor, with values comparable to bulk $\beta$-Sn and Li. A deeper insight into the current distribution indicates that electrons preferably follow specific trajectories, which offer superior conducting properties than others. These channels have been identified and it is shown here how they can enhance or deteriorate the current flow in lithiated anode materials

Simulation of Nanowire Tunneling Transistors: From the Wentzel-Kramers-Brillouin Approximation to Full-Band Phonon-Assisted Tunneling

Nanowire band-to-band tunneling field-effect transistors TFETs are simulated using the Wentzel– Kramers–Brillouin WKB approximation and an atomistic, full-band quantum transport solver including direct and phonon-assisted tunneling PAT. It is found that the WKB approximation properly works if one single imaginary path connecting the valence band VB and the conduction band CB dominates the tunneling process as in direct band gap semiconductors. However, PAT is essential in Si and Ge nanowire TFETs where multiple, tightly-coupled, imaginary paths exist between the VB and the CB. © 2010 American Institute of Physics. doi:10.1063/1.338652