Two-Dimensional Electronic Materials and Devices: Opportunities and Challenges
The unprecedented growth of the Internet of Things (IoT) and the 4th Industrial Revolution (Industry 4.0) not only demands dimensional scaling of device technologies but also new types of applications beyond today’s electronics. Two-dimensional (2D) materials, a group of layered crystals (such as graphene and MoS2) with unique properties, have emerged as promising candidates for IoT and Industry 4.0 since they can, not only extend the scaling with unprecedented performance and energy efficiency but also exhibit high potential for novel electronic devices. However, such nanomaterials suffer from significant challenges in process integration, especially in the modules that involves the formation of interfaces between 2D materials and conventional bulk materials. Thus, realizing high-performance energy-efficient 2D electronic devices has been challenging. This dissertation focuses on understanding the fundamental issues in such 2D materials (such as contacts, interfaces and doping) and in identifying applications uniquely enabled by these materials.
First, a comprehensive treatment of metal contacts to 2D semiconductors, which has been a huge hurdle for 2D electronic technologies, will be presented. As a pioneering study, new interface physics originating from the unique dimensionality and surface properties have been revealed . Solutions to minimize contact resistance are described though techniques of interface hybridization  and seamless contacts , . These techniques transform 2D semiconductors from solely scientifically-interesting materials into high-performance field-effect transistor (FET) technologies, such as MoS2 FETs with record-low contact resistances ,  and WSe2 FETs with record-high drive current and mobility .
Beyond metal interfaces, dielectric interface is crucial for preserving the carrier mobility in 2D channels, for which a solution enabled by buffer layers has been proposed . On the other hand, the vertical van der Waals interfaces between 2D and 3D semiconductors, which retain the advantages of pristine ultra-thin 2D films as well as maximized tunneling area/field, have been studied and exploited into a novel beyond-silicon transistor technology – the first 2D channel tunnel FET (TFET) , which beat the fundamental limitation in the switching behavior of transistors. Recent results from the engineering of such 2D-3D semiconductor interfaces by surface reduction/passivation are described, showing a significant boost of drive current.
While conventional diffusion/ion implantation methods are infeasible for 2D materials, two efficient doping techniques that are specific for 2D materials – surface doping ,  and intercalation doping  are presented. The theoretical study of surface doping using ab-initio methods helped develop a novel doping scheme that uniquely exploits the Lewis-base like pedigree of 2D semiconductors without disturbing the structural integrity of the 2D atomic layer configuration , as well as a novel electrocatalyst based on MoS2 that achieved record high hydrogen evolution reaction (HER) performance . On the other hand, intercalation doping has been employed to demonstrate graphene based transparent electrodes with the best combination of transmittance and sheet resistance , and also the first graphene interconnects with excellent performance, reliability and energy-efficiency , .
Moreover, by uniquely exploiting the high kinetic inductance and conductivity of intercalation doped graphene, a fundamentally different on-chip inductor has been demonstrated , , with both small form-factors and high inductance values, that were once thought unachievable in tandem. This 2D technique provides an attractive solution to the longstanding scaling problem of analog/radio-frequency electronics and opens up an unconventional pathway for the development of future ultra-compact wireless communication systems.
Finally, a novel dissipative quantum transport methodology based on Büttiker probes with band-to-band tunneling capability is developed for 2D FETs . Subsequently, gate-induced-drain-leakage (GIDL), one of the main leakage mechanisms in FETs especially access transistors, is evaluated for the first time for 2D FETs. The results establish the advantages of certain 2D semiconductors in greatly reducing GIDL and thereby support use of such materials in future memory technologies.
The dissertation concludes with a vision for how a smart life can be realized in the future by harnessing the capabilities of various 2D technologies in the era of IoT and Industry 4.0.
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 J. Kang et al., under review.
 J. Kang et al., under review.