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Synthesis and Defect Science and Engineering of Two-Dimensional Materials


Rapid scaling of silicon-based transistors has enabled remarkable performance enhancements in integrated electronics. However, silicon-based transistors are quickly reaching theoretical scaling limits as a result of short channel effects dictated by silicon’s intrinsic material properties. As such, there is a need for alternative materials that can transcend or better address these limitations. Scaling theory suggests that atomically thin, pristine semiconductors can enable more aggressive shrinking of the gate length. Given these requirements, two-dimensional materials (2DMs) have emerged as promising alternatives because they can be scaled down to single or few atomic layers, have no dangling bonds, and exhibit unique electrical properties.

Despite the burgeoning trove of proof-of-concept demonstrations of 2DM-based devices, large-scale fabrication and commercialization remain elusive. At this time, two of the most pressing shortcomings include (1) the transfer step requirement, wherein the 2DM must be physically transferred from a growth substrate to the target substrate, and (2) the high contact resistance between the 2DM and metal contacts. These device requirements create significant bottlenecks and challenges that limit the current feasibility of 2DM implementation in everyday devices. This work focuses on two particular classes of two-dimensional materials—graphene and transition metal dichalcogenides—and highlights some of the progress made towards addressing both of these device-related issues.

To start, this work details the development of improved transfer-free graphene synthesis by chemical vapor deposition (CVD) directly on SiO2. We outline an expansion of the parameter space that optimizes process conditions, using nickel and copper as metal catalysts and gaseous methane as the carbon precursor. We introduce a mechanism based on carbon permeability that provides deeper insight into the growth process. Low-energy electron microscopy (LEEM) is used to showcase some of the intrinsic differences between nickel and copper that lead to contrasting results. In the end, we demonstrate reproducible, monolayer graphene with low defect density using nickel as a catalyst, and reproducible, 2-3 layer graphene with uniform coverage using copper as a catalyst.

To address the high metal–2DM contact resistance, mild hydrogen plasma treatment is applied to WSe2. X-ray photoemission spectroscopy (XPS) indicates that H2 plasma treatment selectively induces selenium vacancies in the WSe2 lattice, resulting in controllable

n-doping with increasing plasma treatment times. WSe2 n-FETs fabricated with H2 treatment on the contact regions demonstrate two orders of magnitude decrease in contact resistance.

By addressing some of the challenges related to graphene transfer and TMDC contact resistance, these studies help establish a foundation towards scalable integration of 2DMs in beyond-silicon electronics.

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