Ohmic contact to monolayer semiconductors

Abstract

Scaling of silicon transistors is reaching its physical limits due to severe short channel effects, threatening to end Moore's law. Advanced beyond-silicon electronic technology requires discoveries of both new channel materials and ultralow-resistance contacts. The aim of this thesis is to develop an understanding of the science and engineering in applying synthetic monolayer semiconductors as the channel materials in field-effect transistors and in achieving excellent electrical contacts to these emerging materials. Monolayer semiconductors such as MoS₂,WS₂ and WSe₂ not only offer atomically thin thicknesses for device minimization, but also embrace desirable physical properties such as dandling-bond-free and atomically flat surface, sizable bandgap, and relatively heavier carrier effective mass to enhance transistor performance at the atomic limit.

We start by developing scalable methods for synthesizing high-quality monolayer MoS₂ that exhibits near intrinsic characteristics and moderate electron mobility at room temperature. These developments improve the fundamental transport property and reduce the unwanted impurity doping of the synthetic monolayer MoS₂ for electronic applications. Next, we further demonstrate a method to eliminate the detrimental defect states in monolayer MoS₂, which mitigates the defect-state induced Fermi-level pinning at the metal-MoS₂ interface. The monolayer MoS₂ transistors fabricated through this technique exhibit a lowered Schottky barrier and a reduced contact resistance at the metal-MoS₂ interface. This study provides an insight into the defect properties of MoS₂ that are fundamentally related to the device performance.

Finally, we provide a deeper understanding of the ohmic contact nature at metal-monolayer semiconductor interfaces and propose a new contact paradigm, gap-state saturation, to overcome the in-gap Fermi-level pinning and eventually eliminate the Schottky barrier. We experimentally apply this new ohmic contact technology to a variety of monolayer semiconductors and demonstrate high-performance monolayer semiconductor transistors. We have achieved record-low contact resistance approaching the quantum limit among all the metal-2D semiconductor interfaces. Our monolayer semiconductor transistors have also reached record-high ON-state current density among all the monolayer devices. We show that monolayer semiconductors can have a performance on par with the state-of-the-art silicon-based technology, reaching the goal of next-generation transistor technologies.