Field-Effect Transistors and Optoelectronic Devices Based on Emerging Atomically Thin Materials

Abstract

Development of field-effect transistors and their applications is advancing at a relentless pace. Since the discovery of graphene, a single layer of carbon atoms, the ability to isolate and fabricate devices on atomically thin materials has marked a paradigm shift in the timeline of transistor technologies. In this thesis, electrical and optical properties of atomically thin structures of graphene and tungsten disulfide (WS2) are investigated. Transport in graphene side-gated transistors and contact resistance at the metal-WS2 interface are presented. Finally, the optoelectronic performance of the hybrid graphene-WS2 devices is examined. Presently, atomically thin semiconductors grown by chemical vapour deposition are of growing interest by a broad scientific community. For this work of thesis, an air stable material which requires non-toxic gases for the growth such as WS2 is selected. A considerable contact resistance at the metal/WS2 interface is found to hamper the electrical performance of WS2 transistors. The possible origin of this contact resistance is presented in this thesis. The graphene field-effect transistors with graphene side gates are fabricated by a single step of electron beam lithography and an O2 etching procedure. A comparative study of the electrical transport properties as a function of a bias applied to the side and back gate is conducted. The side gates allow for a much more efficient modulation of the charge density in the graphene channel owing to the larger maximum electric field which can experimentally be accomplished. Furthermore, the leakage between the side gate and the graphene channel is studied in a vacuum environment. It is found that the transport between graphene and the side gate is associated with Fowler-Nordheim tunnelling and Frenkel-Poole transport. More specifically, for voltages less than 60 V, the Frenkel-Poole transport dominates the transport, whereas the Fowler-Nordheim tunnelling governs the transport at higher bias. Finally, optoelectronic properties of graphene-WS2 heterostructure are explored. An ionic polymer is used as a top gate to enhance the screening of long-lived trap charges. Responsivities as large as 10^6 A/W under illumination with 600 nm wavelength of light are demonstrated at room temperature. The fall and rise time are in the order of milliseconds due to the screening of the traps by the ionic polymer. This study is the first presentation of the transition metal dichalcogenide (TMDC)-graphene hybrid heterostructure with such a high photoresponsivity and fast response times.