Examination Committee

Prof Ning WANG, PHYS/HKUST (Chairperson)
Prof Kei May LAU, ECE/HKUST (Thesis Supervisor)
Prof Qingbin ZHENG, MAE/HKUST
 

Abstract

Since the disclosure of the excellent carrier mobility in ultrathin monolayer graphene in 2004, an astounding amount of attention in the area of ultrathin two-dimensional (2D) semiconductor materials has been aroused. Molybdenum disulfide (MoS2), as a typical member of 2D transition-metal dichalcogenides (TMDs), has attracted significant research interest due to it prevailing over graphene in terms of acquiring a semiconducting band gap with layer-number-dependent properties. Most of the findings on MoS2-based devices were usually firstly demonstrated by exfoliated MoS2 because this is a fast and easy method of investigating the properties of MoS2 prototypes. Nevertheless, devices fabricated using micro-mechanical exfoliation or layer transfer are not preferred in manufacturing as the processes involved are complicated and result in an uncertain size, layer number and location of transferred MoS2. Chemical vapor deposition (CVD) has been indicated as a reliable and promising synthesis method of MoS2 with a comparable electrical performance to that of its natural crystals. This method can be used for deposited monolayers, discrete triangular islands and also continuous films from thin layers to thick layers.
 
In this thesis, MoS2 isolated triangular islands on SiO2/Si substrate and large area continuous thin film on SiO2/p-GaN/Si substrate were grown independently using selectable CVD growth of MoS2 in a home-built furnace set-up. Back-gated FETs as well as top-gated FETs based on as-grown MoS2 triangular islands were fabricated and investigated on SiO2/Si substrate, while top-gated FETs and semiconductor/insulator/semiconductor (SIS) heterojunction diodes constructed from as-grown MoS2 few-layer film were also prepared and studied on SiO2/p-GaN/Si substrate. The FET devices on both substrates show a clear current saturation characteristic, which is crucial for its utilization in most circuit applications. For the SIS heterojunction diodes on SiO2/p-GaN/Si substrate, clear current-rectifying characteristics were shown, which are barely seen for devices based on as-grown MoS2 without layer transfer. This sheds light on future applications for large scale 2D heterostructure devices.