Process Control of Atomic Layer Deposition Molybdenum Oxide Nucleation and Sulfidation to Large-Area MoS2 Monolayers

Recent advances in the field of two-dimensional (2D) transition metal dichalcogenide (TMD) materials have indicated that atomic layer deposition (ALD) of the metal oxide and subsequent sulfidation could offer a method for the synthesis of large area two-dimensional materials such as MoS2 with excellent layer control over the entire substrate. However, growing large area oxide films by ALD with sub 1 nm nucleation coalescence remains a significant challenge, and the necessary steps are unexplored. In this work, we demonstrate the necessary process improvements required to achieve sub 1 nm nucleation control by characterization of nucleation domains formed by oxide deposition. Synthesis of the TMD MoS2 from sulfidation of oxide deposited by both thermal ALD from (tBuN)2(NMe2)2Mo and O3 and plasma enhanced ALD (PEALD) from (tBuN)2(NMe2)2Mo and remote O2 plasma was performed. Large uniform MoS2 areas were achieved by optimizing the effects of various growth process conditions and surface treatments on the ALD nucleation and growth of Mooxide and the postsulfidation of MoS2. In addition to insights into the control of the oxide deposition, film chemistry analysis during a multistep sulfidation based on less toxic sulfur as compared to H2S was performed for several temperature profiles revealing sulfur incorporation and molybdenum reduction at low temperatures but higher temperatures required for 2H crystal structure formation. The knowledge gained of the ALD, PEALD, and postsulfidation was leveraged to demonstrate tunable film thickness and centimeter-scale monolayer growth. Material quality can be studied independently of the MoS2 layer count as demonstrated by the control of the monolayer photoluminescence intensity by the temperature ramp rate during sulfidation. M and few layer transition metal dichalcogenides (TMDs) including MoS2 have attracted attention as materials for transistors, photovoltaics, sensors, and flexible systems, due to their tunable and unique electronic properties. The direct technological application of such films relies on the availability of large area, high quality materials with precise layer control. To date, a number of methods have been used to synthesize films of MoS2 and other TMDs including chemical vapor deposition (CVD). However, for applications including integrated circuits which require large area, complete and/or conformally grown films, an alternative approach under investigation for the deposition of MoS2 and other TMDs centers around the deposition of a thin film of the transition metal or its oxide and subsequent exposure to a sulfur containing vapor at high temperature. While this approach is limited by the uniformity, thickness, and continuity of the starting oxide or metal, for this very reason, it offers a number of potential advantages compared to CVD growth if that oxide can be deposited with exceptional control. For example, extremely large area, complete films, as well as conformal monolayer coatings and abrupt vertical heterostructures may be comparatively easy to achieve. While atomic layer deposition (ALD) has been recently used to deposit extremely uniform and thin oxide films to generate large area layer control for WS2, 22 MoS2, and WxMo1−xS2 alloys, given how promising this approach is, it is surprising that it has not received much greater attention. This is in part because the oxide deposition must be highly controlled. A detailed understanding of the specific deposition conditions is required to accelerate nucleation such that film coalescence occurs at a thinner film than the amount required for a monolayer of MoS2. This requirement is challenging even with the highly reactive precursors used for oxide synthesis due to the extreme thinness of monolayer MoS2. Previous work 23 with Mo(CO)6 and oxygen plasma enhanced ALD (PEALD) Received: September 16, 2016 Revised: February 3, 2017 Article