Investigating electron transport in chemical vapor deposition graphene nanostructures
This thesis investigates electron transport properties in chemical vapor deposition (CVD) graphene-related nanostructures. There are many potential electronic and optoelectronic applications envisioned for graphene, due to its two-dimensional character and exceptional properties. However, the lack of scalability of exfoliated graphene and the high cost of epitaxial graphene on silicon carbide remain the major obstacles for further commercialization of graphene devices. Different approaches to solve this problem have been proposed for different applications and graphene grown by CVD stands out as a useful alternative and proves to be one of the viable routes towards scalable high quality electronics. This thesis presents a study of scalable nanostructured devices based on CVD graphene, with the purpose of understanding the quantum physics of electron transport and demonstrating the potential for nano-electronic applications. First, this thesis demonstrates a scalable approach towards encapsulating and passivating high quality CVD graphene field effect transistors (FETs), and electron scattering processes are explored by studying electrical characterisation and magnetotransport phenomena in encapsulated CVD AB stack and large twist angle (30◦) bilayer graphene FETs, as well as monolayer graphene FETs for reference. The result has significant impact on the widespread implementation of graphene for its scalable device applications. Second, in order to enhance spin-orbit coupling (SOC) in graphene for spin transport study and spintronics applications, a graphene - transition metal dichalcogenide (TMD) heterostructure is investigated. Phase coherence length is reduced in the heterostructure and a special transition from weak localization (WL) to weak antilocalization (WAL) is found around a certain carrier concentration due to surface roughness induced patches. This result provides insight into fabrication and operation of scalable graphene spintronic devices. Moreover, to further elucidate single-electron behaviours as well as solve the lack of bandgap issues, graphene is studied by being patterned into various quantum dot structures, such as nanoribbon multiple quantum dots, quantum Hall antidots, and double quantum dots (DQDs). The presence of multiple quantum dots in series is exhibited in a bilayer SiC epitaxial graphene nanoribbon, due to the interplay between disorder and quantum confinement. As an alternative to etched quantum dots in graphene, antidots in the quantum Hall regime can take advantage of Landau gaps in graphene and are explored via magnetotransport measurements at millikelvin temperature. Single-electron behaviors such as Aharonov-Bohm effect and Coulomb blockade effect are observed, whereas signatures of the effective antidots proved elusive, probably due to the disorder-broadening of the Landau levels. Finally, for the purpose of fast readout of charge and spin states, radio-frequency (RF) reflectometry technique is developed in GaAs antidots and graphene double quantum dots, corresponding to capacitive and resistive couplings to the devices respectively. This attempt paves a way for characterizing the time scale of the charge transfer and spin dephasing in graphene nanodevices. All the quantum dots studies in a scalable style lay the foundation for further quantum metrology and quantum computation applications. The research in this thesis enable us to better understand the quantum physics in CVD graphene, and the fabrication and operature of CVD graphene nanostructures are highly promising for future electronics.
graphene, chemical vapor deposition, nanostructure, Hall bar, antidots, radio-frequency reflectometry, quantum Hall effect, weak localization
Networked Digital Library of Theses and Dissertations