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Title: Lateral and vertical nanoelectronic devices using chemical vapour deposition grown 2D materials
Author: Zhou, Yingqiu
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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In this thesis, field effect transistors (FETs), tunnel barrier transistors (TBTs), and photodetector based on monolayer and bilayer molybdenum disulphide (MoS2) and tungsten disulphide (WS2) 2D crystals, all synthesized using the bottom-up and scalable approach of chemical vapour deposition method (CVD), are studied. The devices are investigated in both lateral devices and vertical device configurations. The first chapter of results probes the changes in FET performance at the grain boundaries (GBs) of WS2 crystals, which are intrinsically formed during material growth and correlates it to the photoluminescence spectroscopy (PL) and Kevin Probe Microscopy (KPM) results. Nanoelectronic FETs are fabricated in pristine area, parallel to GBs, as well as perpendicular to GBs in order to understand the impact of GBs on the charge transport characteristics. The threshold voltage, which is a key FET variable is observed to shift towards the negative direction when a GB is parallel to the conductive channel compared with that of devices based on pristine WS2, demonstrating that along the GB, the electronic properties are distinct and this causes modulation in the intrinsic FET behaviour. The second chapter of results explores the use of graphene electrodes with a unique vertical stacked cross bar geometry. WS2 is employed as the semiconducting photoactive layer sandwiched between two graphene electrodes to form vertical TBTs that also acts as photodetectors. The photo-generated charge carriers in WS2 tunnel to top and bottom graphene electrodes with different probabilities, resulting in a net photocurrent. Increasing the thickness of WS2 from monolayer to bilayer in the graphene/WS2/graphene vertical devices leads to strong photovoltaic behaviour, which is observed due to the long lifetime of the interlayer exciton formed in photo-active bilayer as opposed to monolayer WS2. The third chapter of results demonstrates mixed heterobilayer (MoS2:WS2) stacks for vertical TBTs. In order to enhance the photovoltaic effect observed in the previous chapter in vertical devices, the bilayer photo-active WS2 layer is substituted by MoS2/WS2. An atomic sharp type-II band alignment formed between the MoS2/WS2 interface is found to facilitate the formation of interlayer excitons with long lifetime through charge separation, which is understood to enhance strong photovoltaic effect. By reversing the MoS2:WS2 stack direction the direction of the photocurrent is observed to be dictated by the type-II interface rather than the differences in the magnitude of the tunnel barriers at the graphene interfaces.
Supervisor: Warner, Jamie H. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available