Synthesis of two-dimensional MoS2 based on chemical vapor deposition and its electronic devices fabrication

by Wai Ching Cho

Institution: Hong Kong University of Science and Technology
Year: 2016
Keywords: Molybdenum disulfide; Chemical vapor deposition
Posted: 02/05/2017
Record ID: 2122071
Full text PDF: http://dx.doi.org/10.14711/thesis-b1584855


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…