Thin Films of Layered-MoS2: ALD of Molybdenum and Sulfurization a Novel Approach

Abstract

Thin layers of two dimensional (2D) materials mainly transition metal dichalcogenides (TMDs) and more specifically ultra-thin layered-MoS2 semiconductor possess exceptional properties such as electrical, optical, magnetic, mechanical and chemical properties. This allows the exploration of internal quantum degrees of freedom of electrons and their potential for use in semiconductor microelectronics, optoelectronic, energy, and sensor and detector applications. These exciting results are being achieved mostly by using exfoliation of flecks from bulk MoS2 crystal. However, the biggest challenge in realizing TMDs full potential has been the lack of scalable material synthesis methods for such films with high uniformity, conformality and interfacing with other materials such as oxides, metals and its process compatibility. Among the various thin film deposition methods, atomic layer deposition (ALD) offers the best combination of precisely controlled layer-by-layer thin film growth at low temperature with very high conformality on complex substrates. Here we will present the growth of layered-MoS2 thin films. To grow high quality layered–MoS2 thin films, we have developed an ALD-based two step processing approach: firstly the growth of well controlled ultra-thin layer of Mo metal using ALD followed by the sulfurization of the ALD Mo layer at various temperatures. This two-steps processing results in high quality layered-MoS2 thin films on large substrates. For the Mo ALD process we used molybdenum hexafluoride (MoF6) and Si2H6 precursors. We used in-situ QCM measurements to study interfacial and nucleation effects in the formation of continuous ultra-thin metal layer of Mo. The composition of both the ALD Mo and the layered MoS2 layers was determined by X-ray photoelectron spectroscopy (XPS). Further, cross-sectional transmission electron microscopy (TEM) was performed to confirm the formation of layered MoS2 on high aspect ratio trenches and Raman analysis to verify the signature of E12g blue shift and A1g red shift in the MoS2 structure. Here, we will discuss the details of the two-step thin film growth process for creating layered MoS2 layers via ALD Mo and subsequent sulfurization as well as the properties of the MoS2 films.