Since 2004, the discovery of graphene with its one-atom thickness and outstanding carrier mobility, has initiated tremendous amount of research works in two-dimensional (2D) materials. However, the absence of an intrinsic band gap limits graphene in its use for logic electronics. Molybdenum disulfide (MoS₂), a member of 2D transition metal dichalcogenides (TMDCs), is regarded as one of the alternative candidates beyond graphene. With its layer dependent properties and existence of a semiconducting band gap, 2D MoS₂ shows the potential to be the building blocks for future electronics. In 2011, exfoliated monolayer MoS₂ field effect transistors (FETs) with dielectric engineering were first demonstrated with an enhanced mobilities of 217 cm²/V.s and good immunity to the short chann...[ Read more ]

Since 2004, the discovery of graphene with its one-atom thickness and outstanding carrier mobility, has initiated tremendous amount of research works in two-dimensional (2D) materials. However, the absence of an intrinsic band gap limits graphene in its use for logic electronics. Molybdenum disulfide (MoS₂), a member of 2D transition metal dichalcogenides (TMDCs), is regarded as one of the alternative candidates beyond graphene. With its layer dependent properties and existence of a semiconducting band gap, 2D MoS₂ shows the potential to be the building blocks for future electronics. In 2011, exfoliated monolayer MoS₂ field effect transistors (FETs) with dielectric engineering were first demonstrated with an enhanced mobilities of 217 cm²/V.s and good immunity to the short channel effect. Afterwards, a numbers of prototypes were also demonstrated for nano-electronics such as integrated circuits (ICs) and logical operators based on exfoliated 2D MoS₂. However, in manufacturing, micromechanical exfoliation is not a reliable production method of MoS₂ as the size and thickness of the products are uncontrollable. Recently, chemical vapor deposition (CVD) has been demonstrated as the most promising synthesis method, achieving a reasonable electrical performance compared to that of natural MoS₂ crystals. However, it still remains challenging to produce high quality and continuous MoS₂ film up to wafer scale, meaning that the development of MoS₂ based electronics is still in its early stage.

In this thesis, selectable CVD growth of MoS₂ isolated triangular islands and large area continuous thin film on SiO₂/Si substrates were performed independently in a home-built furnace system using MoO₃ and sulfur as precursors, by controlling the timing of sulfur vaporization and the substrates positions. Hydrofluoric acid (HF) substrate pre-treatment was found to improve the adhesion and modulate the size of the MoS₂ triangular islands. MoS₂ triangular islands are monolayers with a grain size of up to ~70 μm and with outstanding PL signals, whereas the MoS₂ continuous thin film was composed of small grain size MoS₂ nanosheets, with smooth surface morphology and large covered area over centimeters. Natural MoS₂ samples prepared by micromechanical exfoliation were also characterized and fabricated as field effect transistors (FETs) as the references. Back gated FETs based on as-grown MoS₂ triangular islands demonstrated comparably higher current densities and estimated mobilities of up to ~ 16 cm²/V.s due to their single crystal nature; while MoS₂ continuous thin film FETs showed lower mobilities of up to ~ 9 cm²/V.s. Both the CVD MoS₂ devices showed a remarkable current on/off ratio of up to more than 10⁸ and clear current saturation. Thanks to the high-k environment, passivation with a 50 nm Al₂O₃ over-layer deposited by atomic layer deposition (ALD) of MoS₂ triangular islands back-gated FETs experiences less hysteresis and a significant increase in current densities of up to few times. Also, the corresponding top gated device with the Al₂O₃ as the gate dielectric showed an improved subthreshold slope, compared to those of back gated devices and enhanced current densities. Finally, simple MoS₂/p-GaN heterostructures were prepared by depositing natural MoS₂ by micromechanical exfoliation and CVD-growing of MoS₂ continuous thin film on p-GaN/Si substrates correspondingly. A clear current rectifying effect with a rectification ratio of up to ~40 was shown in the fabricated natural MoS₂/p-GaN heterostructure devices, while the heterostructure devices based on CVD-grown MoS₂ continuous thin film on p-GaN experienced abnormal reverse rectification, which may be due to the changes of their intrinsic properties during the CVD growth and transport direction issue of MoS₂.