Access Type

Open Access Dissertation

Date of Award

January 2020

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physics and Astronomy

First Advisor

Zhixian . Zhou

Abstract

The rapid growth of modern electronics industry over the past half-century has been sustained by the continued miniaturization of silicon-based electronics. However, as fundamental limits approach, there is a need to search for viable alternative materials for next-generation electronics in the post-silicon era. Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs) have attracted much attention due to their atomic thickness, absence of dangling bonds and moderately high carrier mobility. However, achieving low-resistance contacts has been major impediment in developing high-performance field-effect transistors (FETs) based on 2D semiconductors. A substantial Schottky barrier (SB) is often present at the metal/2D-semicondcutor interface, largely due to the Fermi level pinning effect. To date, various strategies employed to reduce or eliminate the SB and ultimately reduce the contact resistance, such as phase engineering and chemical doping are still deficient. Here, we present a simple, yet effective method to significantly reduce the SB height (SBH) in TMD-based FETs by inserting ultrathin 2D semiconductors as an interlayer at the semiconductor-metal interface. Specifically, we have observed a drastic reduction in the SBH from ~ 100 meV to ~ 25 meV by inserting an ultrathin MoSe2 between the MoS2 channel and the contact metal. This improvement can be attributed to the coupling of Fermi level pinning close to the conduction band edge of the interlayer and the slightly smaller electron affinity of the MoSe2 interlayer compared to that of the MoS2 channel. Consequently, this reduction in the SBH results in over an order of magnitude decrease in the contact resistivity from ~ 6 x 10-5 Ω cm2 to 1 x 10-6 Ω cm2 and current transfer length from ~ 425 nm to ~ 60 nm. The improvements in the contact properties yield greater device performance, enhancing the two-terminal mobility from ~30 cm2V-1s-1 to ~ 60 cm2V-1s-1 at room temperature and from ~70 cm2V-1s-1 to ~ 160 cm2V-1s-1 at 160 K. This new contact engineering method presents an important advantage over previous works that utilize insulating interlayers because this method uses advantageous band alignments to reduce the SBH while minimizing the tunneling barrier at the contact interface.

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