Impact of as-synthesized and radiation-induced defects in two-dimensional vertical heterostructures

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Perini, Christopher
Vogel, Eric M.
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In recent years, transition metal dichalcogenides (TMDs) have shown promise as a next generation class of semiconducting two-dimensional materials for use in electronic devices. Due to their low dimensionality, TMDs are appealing materials for a variety of applications, including flexible electronics, digital and analogue electronics, optoelectronic uses, and sensors. The absence of out-of-plane dangling bonds in 2D materials enables the potential for arbitrary stacks of 2D layers, yielding a 2D vertical heterostructure. The stacking arrangement of 2D layers in these heterostructures can be tailored to yield a number of different device characteristics, from steep slope tunnel transistors to resonant tunnel junctions. However, the majority of studies that explore TMDs for various applications obtain films using methods that are not scalable, such as mechanical exfoliation or synthesis at high temperatures (T > 450 oC). In order for TMDs to be integrated into industrial back-end-of-line (BEOL) processes, films must be able to be synthesized using conditions that are compatible with complementary metal oxide semiconductor (CMOS) BEOL process limitations, namely low synthesis temperature. To achieve compatibility with BEOL limitations, this work demonstrates low temperature synthesis of TMDs utilizing plasma-assisted synthesis techniques. Physical characterization yields information on the stoichiometry, crystallinity, thickness, and electronic structure of the films, while electrical measurements are used to correlate the electronic transport through the films to material quality and defect structure. In particular, the temperature dependence of in and out-of-plane conductivities provide information on conduction mechanisms through the material, as well as injection at the metal/semiconductor interface. TMD films are synthesized on different substrates in order to enable direct layer-by-layer construction of heterostructures, removing the need for transfer processes that introduce contamination at the interfaces between layers. MoS2/high-k dielectric/MoS2 and MoS2/WS2 heterostructures are constructed from low temperature synthesized films, and metal/MoS2/metal heterostructures are constructed from both low and high temperature synthesized films; all of which are used to investigate tunneling and injection mechanisms, rectification, pinning effects, and switching behavior. Certain heterostructures are exposed to ionizing radiation to induce various defects into the different heterostructure layers, so that individual defect types can be correlated to changes in resulting device behavior. Hydrogen impurities and oxygen complexes at the 2D/oxide interface are found to dope and degrade device performance, and passivated oxygen vacancies in the high-k dielectric interlayer can contribute to trap-assisted tunneling across the tunnel junction. The work presented in this thesis establishes a basis for low-temperature synthesis of TMDs and demonstrates methods for how restrictions on synthesis conditions can be overcome. An understanding of how defects and material quality influence resulting device performance was performed through a combination of physical characterization and device characteristics. In addition, the interaction between the TMD and metal contacts is explored in the context of Fermi level pinning. In summary, this work demonstrates low temperature synthesis of TMDs, providing a path for 2D heterostructure implementation into BEOL processes, and explores the implications of resulting material quality and defect structure on heterostructure device performance.
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