Simulation Models for Silicon Nanowire Field-effect Transistors

Silicon nanowires offer a promising and reliable way to build transistors with improved device performances. Using suitable contact materials, the fabrication of reconfigurable transistors becomes feasible. Such transistors can be reprogrammed to support either electron or hole currents, allowing more flexible circuit designs. The atomic structure of the contact interface and of the nanowire itself is becoming increasingly important to enable highly optimized devices.

To understand the relation between the atomic interface structure and the resulting transistor characteristics, a first-principles model based on density functional theory was developed. It was shown how different orientations and types of the NiSi2-silicon interface result in a different symmetry between the on-currents in the electron or hole program. In case of {111} interfaces, for example, a rotation of the silicon part by 180° can change the on-currents by about one order of magnitude. The transport was related to fundamental material properties, for which one example is the Schottky barrier height at the contacts.

To engineer the properties of silicon nanowires and their contacts, the lattice can be mechanically strained by oxidizing the nanowire. Using molecular dynamics, we studied the resulting strain profiles as well as the reduction of the nanowire diameter. Quantum and surface effects become increasingly important for smaller diameters. These effects were studied using density functional theory. It was shown how the electronic transport properties differ between nanowire center and surface. Additionally, the increase of the band gap due to the decreasing diameter was calculated.