Phase-Engineered Field-Effect Transistors Based on Two-Dimensional Transition Metal Dichalcogenides

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are layered materials in which each unit consists of a transition metal (Mo, and W) layer sandwiched between two chalcogen (S, Se, and Te) atomic layers. They can exist in either a semiconducting or metallic phase. Semiconducting TMDC based field-effect transistors (FETs) are considered promising candidates for post-Si electronics owing to their ultra-thin body enabling ultimate scalability. However, the large contact resistance at the metal/TMDC interface limits their drive current for high-performance applications. The large contact resistance is due to an enlarged schottky barrier height resulting from strong fermi-level pinning and the existence of van der Waal gaps at the metal/TMDC interface and in-between TMDC layers. The ultimate solution to the contact issue is to utilize lateral metallic-semiconducting TMDC junctions as 2D edge contacts. Thanks to the small free energy difference between its semiconducting 2H and metallic 1T’ phases, lateral 2H/1T’ MoTe2 homojunctions can be synthesized in situ by flux-controlled phase engineering. In this dissertation, a comprehensive study combining detailed structural and electrical properties of in-situ-grown lateral MoTe2 homojunctions formed via the flux-controlled technique is presented. MoTe2 p-MOSFETs with phase-engineered 1T’ contacts which showed significantly improved performance over devices with metal/2H contacts are demonstrated. In order to employ this flux-controlled technique for large-scale CMOS fabrication, a two-step lithographic synthesis approach for creating various lateral TMDC junctions is developed. Lateral 2H/1T′ MoTe2 homojunction p-MOSFETs and 2H-MoS2/1T′-MoTe2 heterojunction n-MOSFETs are fabricated using the two-step approach. Besides, a reversible phase transition between 2H and 1T’ MoS2 by gate-controlled Li+ intercalation through a solid PEO:LiClO4 electrolyte is demonstrated, which could allow the use of lateral metallic-semiconducting TMDC junctions for phase-change memory applications.