Electron transport in semiconducting transition metal dichalcogenide quantum devices

• Main Author: Krishnan, Radha
• Other Authors: Bent Weber
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Physics::Atomic physics::Solid state physics
• Online Access: https://hdl.handle.net/10356/162006
• Institution: Nanyang Technological University

Atomically thin semiconductors with hexagonal lattices, amongst them the transition metal dichalcogenides (TMDCs), are exciting candidates for electronic and optoelectronic applications due to their large and tuneable band gap, strong spin-orbit coupling and nonequivalent valleys. Broken inversion symmetry and time reversal symmetry further causes the spin and valley degrees of freedom to be coupled which may allow for novel avenues of control of spin-valley states towards realizing robust qubits and potentially the enhancement of coherence lifetimes. Amongst the many challenges in realizing TMDC based quantum devices, high contact resistance has hampered the investigation of their quantum properties at ultralow temperatures using transport spectroscopy techniques. In this thesis, we report a strategy to engineer tuneable low resistance Ohmic contacts to molybdenum disulphide (MoS2) down to monolayer thickness and at milli-Kelvin temperatures. We achieved this by employing separate gates for independent electrostatic doping of contact and channel regions allowing for high-quality n-type contacts to the MoS2 channel even in the low carrier density limit. As a proof of concept, we apply the contact scheme developed to a few-layer MoS2 device with an electrostatically confined split gate channel. Temperature dependent conductance measurements show that electronic states near the conduction band tail are confined to an impurity band, in which conduction proceeds via Mott variable range hopping (VRH). At low temperatures, a transition from Mott to Efros-Shklovskii (ES) hopping is observed, indicating that long-range Coulomb interaction between localized states become dominant. Sub-threshold, single-charge transitions evident from Coulomb blockade oscillations likely arise from tunneling though localized states from single defect/impurities whose excited state spectrum we resolve as a function of magnetic field. The results of our investigation may provide insights into the spin-valley states of confined charges and spin towards their application as spin or spin-valley qubits.