Ian Campbell1,Douglas Heine1,Ageeth Bol1
University of Michigan–Ann Arbor1
Ian Campbell1,Douglas Heine1,Ageeth Bol1
University of Michigan–Ann Arbor1
Sulfur-containing transition metal dichalcogenides (TMDs), like molybdenum sulfide, have proven to be promising materials for electrocatalysis, energy storage, and microelectronics applications that are demanding increasingly thinner films and smaller particles over large areas. Useful films of TMDs can be prepared via the process of exfoliating layers within TMDs, which suffers from poor scalability, or through approaches like chemical vapor deposition and atomic layer deposition (ALD). ALD is capable of depositing atomically thin layers of material by using self-limiting, surface-saturating reactions that make it uniquely capable of synthesizing subnanometer-scale layers and particles over large areas with variable topography. Currently, the dominant method for preparing sulfide TMDs via ALD is to alternately expose a substrate to a metalorganic precursor and a sulfur source, which is often hydrogen sulfide (H<sub>2</sub>S) or a plasma containing H<sub>2</sub>S. H<sub>2</sub>S is a corrosive, toxic, and flammable gas that is heavier than air, which makes it hazardous and expensive to store, install, and transport. Additionally, the odor of H<sub>2</sub>S is imperceptible at life-threatening concentrations, making it even more dangerous. Alternative sulfur precursors in the solid or liquid phase would be beneficial in terms of cost and safety and would require the installation of no additional hardware for most ALD reactors. In this contribution, the widely researched ALD process using bis(tert-butylimido)bis(dimethylamino)molybdenum(IV) ((<sup>t</sup>BuN)<sub>2</sub>(NMe<sub>2</sub>)<sub>2</sub>Mo) and H<sub>2</sub>S plasma is compared to a novel ALD process using (<sup>t</sup>BuN)<sub>2</sub>(NMe<sub>2</sub>)<sub>2</sub>Mo, hydrogen plasma, and di-tert-butyl disulfide (TBDS), which is an inexpensive, liquid-phase sulfur precursor. The growth behavior of both ALD processes is characterized via continuous in-situ spectroscopic ellipsometry, which reveals saturating reactions for H<sub>2</sub>S-plasma and TBDS exposures. The composition and stoichiometry of the deposited films are determined using X-ray photoelectron spectroscopy, Rutherford backscattering, and elastic recoil detection. The crystallographic structures of the films are assessed using Raman spectroscopy and selected-area electron diffraction in conjunction with transmission electron microscopy. Molybdenum sulfide films prepared using both sulfur precursors are compared for their abilities to catalyze the hydrogen evolution reaction and for their electronic resistivities and charge carrier mobilities as determined by van der Pauw measurements. Finally, it is demonstrated that the TBDS-based ALD process is transferrable to the synthesis of other TMDs like niobium sulfide.