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Quantum simulation of electron transport in disordered two-dimensional transition metal dichalcogenides

Abstract : The discovery of graphene in 2004 has inspired a great interest in two-dimensional (2D) materials. In recent years, semiconducting 2D materials, in particular, are in the limelight for their potential use in electronics and optoelectronics. From the perspective of metal-oxide-semiconductor field-effect transistors, their atomic thickness allows an enhanced electrostatic control and their self-passivated surface reduces the potential presence of charge traps. Most importantly, the presence of a bandgap, contrary to graphene, facilitates a high on/off ratio in logic devices. Among these semiconducting materials, transition metal dichalcogenides (TMDs), with their large variety of band alignments and bandgaps, have attracted great attention for their possible use in transistors, both as monolayer materials or combined in van der Waals heterostructures. For such applications, the TMD quality is a priority, since the presence of defects might significantly affect electron transport thus leading to performance degradation.The present thesis reports on the impact of various defects, which are often observed in experimental samples, on the transport properties of TMDs. The study is based on quantum transport simulations, which combine an atomistic tight-binding description of the system and the Green’s function formalism.The first part of the thesis briefly introduces 2D materials, including their properties, synthesis, and applications. The basics of the simulation approach are also detailed. In particular, a thorough review of model Hamiltonians for TMDs, with a specific focus on tight-binding models, is presented. Moreover, the Green’s function formalism, which is the methodology adopted for the quantum transport simulations performed in the present thesis, is briefly reviewed.In the second part of the thesis, two types of typical TMD defects are simulated, and the results physically interpreted.The first study concerns edge roughness in MoS2 ribbons, which play an important role in the miniaturization of TMD-based transistors. The second study focuses on twin grain boundaries, which are often present in polycrystalline MoS2 obtained by large-scaling synthesis approaches, as chemical vapor deposition or molecular beam epitaxy. The role of spin-orbit coupling, which is significantly large in TMDs, is also taken into account. The results of these studies are quantitatively analyzed in terms of quasi-ballistic, diffusive, and localized transport regimes.The main outcome of this thesis is a better understanding and prediction of the impact of defects on the transport properties of TMDs, with possible applications in the design of performant TMD-based devices.
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Submitted on : Friday, October 23, 2020 - 11:30:42 AM
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Jejune Park. Quantum simulation of electron transport in disordered two-dimensional transition metal dichalcogenides. Micro and nanotechnologies/Microelectronics. Université Grenoble Alpes [2020-..], 2020. English. ⟨NNT : 2020GRALT008⟩. ⟨tel-02976072⟩

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