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Title: Two-dimensional electronics : from material synthesis to device applications
Author: Zheng, Shan
ISNI:       0000 0004 7652 5057
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2018
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Two-dimensional (2D) materials have attracted extensive research interest in recent years. Among them, graphene and the semiconducting transition metal dichalcogenides (TMDs) are considered as promising candidates for future device applications due to their unique atomic thickness and outstanding properties. The study on graphene and TMDs has demonstrated great potential to further push the scaling of devices into the sub-10 nanometer regime and enable endless opportunities of novel device architectures for the next generation. In this thesis, crucial challenges facing 2D materials are investigated from material synthesis to electronic applications. A comprehensive review of the direct synthesis of graphene on arbitrary substrates with an emphasis on the metal-catalyst-free synthesis is given, followed by a detailed study of the contact engineering in TMDs with a focus on the strategies to lower the contact resistance. Effective approaches have been demonstrated to solve these issues. These include: (1) metal-catalyst-free synthesis of graphene on various insulating substrates; (2) Fermi level pinning observed in TMDs and integration of graphene contact to lower the contact resistance; and (3) application of metal-insulator-semiconductor (MIS) contact in TMD field-effect transistors (FETs). First, a direct low-temperature synthesis of graphene on insulators without any metal catalysts has been realized. The effects of carbon sources, NH3/H2 concentrations, and insulating substrates on the material synthesis have been systematically investigated. Graphene transistors based on the as-grown material have been fabricated to study the electronic properties, which can further confirm the nitrogen-doped graphene has been synthesized from the electrical characterizations. Then electronic devices focusing on the semiconducting TMDs has been studied. The Fermi level pinning has been observed and studied in WS2 FETs with four metal materials. A novel method of using graphene as an insertion layer between the metal and TMDs has been proven to effectively reduce the contact resistance. Owing to the benefit of tuning the graphene work function via the electric field, the contact resistance can further be reduced. Finally, the effectiveness of MIS contacts in WS2 FETs has been demonstrated. A thickness dependence research has been conducted to find the optimal thickness of the inserted insulator. Moreover, the possible physical mechanism of how this MIS contact reduces the contact resistance in 2D materials has been discussed.
Supervisor: Robertson, John Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Graphene ; TMD ; FET ; Contact resistance ; Schottky barrier height ; CVD