9:00–10:00 am GCIS E223
First-Principles Simulations of Quantum Electron Transport in Two-Dimensional Semiconductor Nanodevices
Conventional first-principles density-functional theory (DFT) simulation is a robust tool for modeling structural and electronic properties of nanoscale systems. However, its heavy computational cost prevents its scaling to realistic device dimensions. In this thesis, we developed a computational pipeline for efficient and accurate atomistic full-band simulations of electron transport in nanoscale devices based on the Keldysh Non-equilibrium Green’s Function (NEGF) formalism. By transforming the delocalized plane-wave states into maximally localized Wannier functions (MLWFs) that serve as the localized basis for tight-binding parameterization, we are able to model quantum electron transport in realistic device structures at the same accuracy of a full DFT simulation but using three orders of magnitude less time. We then applied our methods as implemented in our custom-built open-source software to several realistic nanostructures of interest, including the two-dimensional (2D) metal-semiconductor contacts. The Metal-semiconductor contact is a major factor limiting the shrinking of transistor dimension to further increase device performance. We investigated two main types of contact strategies, namely top contacts and edge contacts. While 2D top contacts can achieve both low contact resistances and small volumes, they usually suffer from weak van der Waals coupling as revealed by our analysis on graphene-\ch{MoS2} top contacts. The transfer efficiency is compromised dramatically below a transfer length of tens of nm scale. On the other hand, in-plane edge contacts have the potential to achieve lower contact resistance due to stronger orbital hybridization compared to conventional top contacts. By self-consistently solving the Poisson equation for electrostatics together with transport, we are able to accurately model the graphene/\ch{MoS2} edge contact and find ohmic behavior in its I-V characteristics, which agrees with experiments. Our results also demonstrate the role played by trapped charges in the formation of a Schottky barrier, and how one can reduce the Schottky barrier height (SBH) by adjusting the relevant parameters of the edge contact system. Our framework can be extended conveniently to incorporate more general nanostructure geometries and could have broad implications in the design and fabrication of next-generation semiconductor nanodevices.
