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Theoretical Researches on Electronic Transport in Nanoscale Devices

Author: LanJin
Tutor: SuiWenQuan; HanYan
School: Zhejiang University
Course: Integrated Circuit Design
Keywords: Nanoscale electronic devices current imaging graphene ac transport spin-dependentac transport spin accumulation
CLC: TN101
Type: PhD thesis
Year: 2013
Downloads: 15
Quote: 0
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Abstract

As the feature size of recent electronic devices decreases continuingly into the nanoscale, the quantum effects inside these devices comes out to play an important role in these devices. As a result of the introduction of these quantum effects, the electron transport in the nano-devices behaves differently with these classical devices, and thus can not be described by the classical transport models, but must be described by the quantum transport theory.This work mainly studies some properties of the electron transport in nano-devices, through the dc and ac transport theory. First, basic dc and ac transport theory is introduced, these transport theories are usually built upon various theoretical tools. In this work, we are focused on the the introduction of the scattering matrix and Green’s function method, and connections with these two methods are discussed. The connection enable us to compare these two transport theory and pro-vide us a deeper understanding about their application ranges, limitations and the approximations.Based on these basic transport theories, we employed the current imaging technique to study the effects of the third probe in a three-terminal nano-device on transport properties. Demonstrating devices’internal current images under different configuration, we discussed the influences of the relative angle and coupling strength between the third probe and the original two-terminal device on the transport processes. The impressive current profile also shows to be a powerful complementary to the analysis of the external conductances.We also discussed the transport properties of the graphene nano-ribbon based T-stub stru-cutures. As is known, in graphene nanoribbons, there are two different boundary configurations with distinguishing properties, which are armchair edge type and the zigzag edge type. Through the analysis on the ac conductances, we find that the ac behaviours shows very strong bound-ary dependencies, which agrees well with the corresponding dc transport properties. Due to the intrinsically included contact and scattering information, though the distribution of local partial density of states, we could perform more detailed analysis on the device’s transport properties. We discussed the real space distributions on the local partial density of states at some character- istic energy points, and further demonstrated some typical properties in armchair and zigzag type graphene nano-ribbons, such as the fast-oscillating transverse modes in armchair type and the edge state in zigzag type graphene nano-ribbon.Based the charge transport theory, we introduced the spin index into the ac transport theory. Spin accumulation is included in by considering the exchange interaction effects. The exchange interaction makes the internal potential in the device split into a spin-related parameter, this fea-ture corresponds well with the new introduced spin degree freedom. We demonstrated the effects of exchange interaction on ac spin interaction by two models, a two-terminal model and a three-terminal model with gate. In two-terminal model, we demonstrated the basic spin-dependent ac transport with exchange interaction included, and the properties of the spin-related internal poten-tial are discussed. And in the gated three-terminal model, we introduced a similar spin-capacitive effects imitating the charge capacitive effects of the gate. Comparing these two capacitive effect-s, the role of exchange interaction on spin-dependent ac transport are further demonstrated. The cross-influences of exchange interaction and Coulomb interaction on both charge and spin ac con-ductances show that, charge and spin both as the intrinsic property of the electron, are inseparable in transport processes.

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