PROBING QUANTUM TRANSPORT IN THREE-TERMINAL NANOJUNCTIONS

Abstract

One-dimensional (1D) nanoscale systems—structures with the lateral dimensions ranging from 1 nm to 100 nm — have received significant research interest due to their unique structure-guided properties that promise functionalities far more superior than their bulk counterparts. The quantum confinement effect in 1D nanostructures provides us with a very powerful tool to tune their electrical, magnetic, optical and thermal properties and opens the gateway for their multifunctional usages in next-generation electronics. In particular, carbon nanotubes and semiconductor nanowires are found to offer tremendous opportunities to form the junction devices with controlled electronic and optoelectronic properties crucial to predictable device functions. Along with the experimental progress in synthesis and fabrication techniques leading to nano-dimensional devices with diverse applications, theoretical insights at the level of electronic structure is equally important to tune various material properties for achieving greater device performance coupled with a wider range of functionalities. This thesis provides a theoretical description of the quantum transport properties in semiconductor core-shell nanowire field effect transistors (FETs) and (8,0) single-wall carbon nanotube contacted to ferromagnetic electrodes using the first principles density functional theory (DFT) in conjunction with the coherent single-particle many-body Green’s functions approach. The first project of the thesis outlines the superior performance of a semiconductor Si-Ge core-shell nanowire quantum dot FET over its pristine Si nanowire counterpart. In this work, we have unlocked the switching mechanism responsible for the superior performance of the Si-Ge nanowire FET with the pz-orbitals in the (outer)shell-layer providing the carrier pathway in both nanowire FETs. This is followed by a work on charge transport in semiconductor Ge-Si core-shell nanowire quantum dot FETs of two different Ge-core diameters. Here, we have identified the most probable tunneling pathway of electrons in Ge-Si FETs with an orbital spatial level resolution which demonstrates the gate-bias-driven decoupling of carrier transport between the core and shell-region. Our calculations hold a qualitative agreement with the experimentally reported results. Irrespective of the Ge core diameter, we observed excellent FET characteristics within a certain threshold gate bias after which the drain current is found to drop rapidly leading to the negative differential resistance (NDR). An orbital level analysis reveals a strong coupling between the pz-orbitals of the core-Ge and the s-orbitals of the gold electrode giving rise to the peak state of NDR; no such coupling is found at the valley NDR state for which the contribution comes solely from the pz-orbitals of the shell-Si.