Atomistic Simulation of Nanoscale Devices

Device simulation is nowa-days fully integrated into the production tool chain of transistors. The geometry of the latter can be carefully optimized, possible design pitfalls can be identified early on, and the obtained experimental data can be analyzed in detail thanks to state-of-the-art technology computer aided design tools. However, on the one hand, the dimensions of transistors are reaching the atomic scale. On the other hand, novel functionalities (e.g., light emission/detection) and materials, for example III-V semiconductors, are being added to silicon-based chips. To cope with these challenges it is crucial that device simulators go beyond classical theories, pure electronic transport, and continuum models. The inclusion of quantum mechanical phenomena, electro-thermal effects, and light-matter interactions in systems made of thousands of atoms and of various materials has become critical. In this paper, we review one approach that satisfies all these requirements, the Non-equilibrium Green's Function (NEGF) formalism, focusing on its combination with ab initio bandstructure models. The NEGF method allows to treat electrical, thermal, and optical transport at the quantum mechanical level in multi-material, multi-functional devices, without any empirical parameters. Besides advanced logic switches, it can be used to simulate e.g., photo-detectors, thermoelectric generators, or memory cells composed of almost any materials, in the ballistic limit of transport and in the presence of scattering. The key features of NEGF are summarized first, then selected applications are presented, finally challenges and opportunities are discussed.