| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Wednesday Sessions |
| Session TF-WeP |
| Session: | Thin Films Posters Session II |
| Presenter: | John Conley, Jr., Oregon State University, USA |
| Authors: | A. Valdivia, Oregon State University, USA J.F. Conley, Jr., Oregon State University, USA |
| Correspondent: | Click to Email |
The synthesis of high quality single layer 2D MoS2 on large area substrates remains challenging. Mechanical exfoliation is capable of producing high quality material, but it is limited to small areas, requires transfer to the device substrate, is not scalable, and is impractical for manufacturing. Chemical vapor deposition (CVD) has been shown to yield MoS2 on a variety of substrates, but is limited by non-uniform electrical properties, poor process stability, and high deposition temperature (>600°C). A natural method for the synthesis of 2D materials is atomic layer deposition (ALD) in which alternating, purge separated, self-limiting surface reactions allow for precise thickness control, high conformality, and scalability to large surface areas. Although reports of ALD MoS2 are beginning to emerge, ALD of single layer MoS2 has typically required specialty sapphire substrates and high temperature (800°C) post deposition anneals and/or has been limited to small diameter wafers.
In this work, we demonstrate low temperature ALD of single to few monolayer MoS2 uniformly across 150 mm diameter patterned SiO2/Si and quartz substrates.1 Purge separated cycles of MoCl5 and H2S precursors were used at ALD reactor temperatures ranging from 375°C to 475°C. Raman scattering measurements show clearly the in-plane (E12g) and out-of-plane (A1g) modes for ALD films deposited at 375°C or 475°C, indicating the presence of mono- to few layer MoS2. The separation of the E12g and A1g peaks is a function of the number of ALD cycles, shifting closer together with fewer layers. Films deposited at 475°C are of much higher quality than films deposited at 375°C. While the E12g-A1g peak separation of the 375 °C film corresponds to bulk-like MoS2, the smaller peak separation of the 475°C film suggests a thickness of approximately two monolayers. Raman polarization tests confirm the MoS2 crystals have the desired orientation parallel to the surface. High temperature H2S and sulfur annealed films produce a sharpening of the E12g and A1g peaks as well as the appearance of the band edge PL and spin orbit splitting peaks, a further indication of the presence of monolayer MoS2. High resolution transmission electron microscopy images confirms the presence of monolayer to bilayer MoS2 films. X-ray photoelectron spectroscopy indicates that a sub-stoichiometric sulfur ratio in the as-deposited films is increased to the stoichiometric S/Mo ratio after annealing in H2S at 600°C and above. Results suggest that ALD may be a promising method for production of 2D MoS2.
Support provided by Sharp Labs of America.