Atomistic modeling and quantum transport simulation of nano-scaled devices based on the density functional theory Hamiltonian

Abstract

In this thesis, the methodology is developed to eliminate the uncertainties in the density functional theory Hamiltonian-based non-equilibrium Green's function method and is applied to the simulation of atomistic-level quantum mechanical transport simulation. In the existing methodology, device characteristics are calculated by dividing the effects of electrons and holes using arbitrary reference energy. This approach works efficiently and accurately for simple structures such as n-type field-effect transistors (FETs) or p-type FETs, where the reference energy can be easily established. However, in the case of modern devices, naturally occurring states such as metal-induced gap states in metal-semiconductor junctions or gap states by single-atom defects can be present. The presence of such states can result in significant differences in the transport properties depending on the selection of the division line between electrons and holes. To address this issue, we employed a method of setting the core charge value based on the electronic distribution of the device under undoped and unbiased conditions. Combining the device's equilibrium charge with the suggested core charge, the mode space method, crucial for simulating realistic-sized devices, can be applied. To validate the proposed algorithm, we compared it with the existing methodology in a Si nanowire structure. We found no significant difference between the proposed algorithm and the existing methodology for simple structure, as expected. Additionally, through calculations on small-sized devices, we successfully confirmed the applicability of the mode space transformation method to the proposed algorithm. In the case of tunneling FET structures with atomic-level defects, the present approach shows significant variations depending on the division line between electrons and holes. Using the developed methodology, we successfully extracted the Schottky barrier height of the metal-semiconductor junction and confirmed that the transport characteristics of the metal/semiconductor junction exhibit rectifying behavior. Using the proposed algorithm in this paper, we can efficiently address various problems in modern nanoscale devices without ambiguity.