Graduate Thesis Or Dissertation

 

Atomic Layer Deposition of Two Dimensional MoS2 on 150 mm Substrates Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/sb397c47d

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  • Two-dimensional transition metal dichalcogenides (TMDs) have recently come under intense investigation as building blocks for van der Waals heterostructure electronics. One of the most promising TMDs is MoS₂, which transitions from an indirect bandgap (1.3 eV) in its bulk state to a direct band gap (1.8 eV) in its single layer state making it suitable for optoelectronic and transistor applications. The synthesis of high quality single layer MoS₂ on large substrates, however, remains a challenge. Although, mechanical exfoliation is capable of producing the highest quality material, it is limited by small surface areas and is not scalable. Chemical vapor deposition (CVD) is also widely utilized but exhibits a lack of thickness control, poor process stability, and requires high deposition temperatures (typically above 650 °C). Atomic layer deposition (ALD) is natural technique for the synthesis of 2D materials. ALD is a CVD technique in which reactants are introduced to the chamber sequentially rather than simultaneously. Sequential self-limiting surface reactions allow for precise thickness control, high conformality, and scalability to large surface areas. The objective of this work is to demonstrate low temperature atomic layer deposition (ALD) of monolayer to few layer MoS₂ uniformly across 150 mm diameter SiO₂/Si and quartz substrates. Purge separated cycles of MoCl₅ and H₂S precursors were used at reactor temperatures of up to 475 °C. Raman scattering studies show clearly the in-plane (E¹₂g) and out-of-plane (A₁g) modes of MoS₂. The separation of the E¹₂g and A₁g peaks is shown to be a function of the number of ALD cycles, shifting closer together with fewer layers. X-ray photoelectron spectroscopy (XPS) indicates that stoichiometry is improved by post deposition annealing in a sulfur ambient. High resolution transmission microscopy (TEM) confirmed the atomic spacing of monolayer MoS₂ thin films signaling successful deposition of monolayer to few layer MoS₂ thin films.
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