Stretchable and Highly Bendable Thin Film Transistors Based on Atomically Thin Chemical Vapor Deposited Molybdenum Disulfide

Two-dimensional (2D) materials have received tremendous research attention recently, as they possess peculiar physical properties in their monolayer and few-layer forms, which further lead to novel applications. A wide range of 2D materials covers insulators, metals, superconductors and semiconductors with various bandgap, giving a fresh platform to reinvent conventional electronic and optoelectronic devices with new advantages. Molybdenum disulfide (MoS2), belonging to the group of transition metal dichalcogenides (TMDCs), has been widely studied for its particular physical properties and potential applications as a leading example among 2D semiconductors. In this thesis, we focus on the applications ofMoS2 in microelectronic and micro-electromechanical devices. The MoS2 we studied is grown specifically by chemical vapor deposition, which offers the highest potential in large area growth. Utilizing the flexible nature such as high in-plane fracture strain and low bending stiffness of atomically thinMoS2, we developed stretchable thin film transistors (TFTs) on polydimethylsiloxane (PDMS) and highly bendable thin film transistors on polyimide. The fabrication process is ameliorated and designed to provide good chemical and temperature compatibility to the polymer substrates. Electrical performance of the flexible devices under different degrees of bending and stretching has been studied. The stretchableMoS2 TFTs could sustain up to 4% of uniaxial tensile strain maintaining device function, and the failure occurs at 5% of strain due to the fracture in gate dielectric layer. The highly bendableMoS2 TFTs with parylene-C gate dielectric layer exhibit tolerance to the smallest bending radius of 0.625 mm. Different encapsulation layers such as HfO2, Cytop and SU8 have been implemented on the bendable devices to improve the performance represented by the increase in field effect mobility, reduction in hysteresis and better endurance to mechanical deformation. The resulting flexibleMoS2 TFTs show field effect mobility of 21.4 cm^2V^-1s^-1 and a large on-off ratio of 106 in air. The SU8 encapsulation layer is further demonstrated as controllable n-type doping strategy with low temperature process, which is ideal for flexible MoS2 devices as it additionally exhibits good stability in air and water.