Oxygen as a dual function regulator in MoS2 CVD synthesis: enhancing precursor evaporation while modulating reaction kinetics

TL;DR

This study uncovers how oxygen plays a dual role in MoS2 chemical vapor deposition, enhancing precursor evaporation and modulating reaction kinetics, leading to improved synthesis control.

Contribution

It combines experiments and simulations to elucidate oxygen's mechanistic roles, enabling better control of MoS2 growth via optimized oxygen dosing strategies.

Findings

Oxygen increases MoO3 sublimation and Mo3O9 supply.

Sulphur oxides limit reactive MoS6 formation, affecting growth.

Optimal S:O2 ratios improve monolayer MoS2 quality.

Abstract

Molybdenum disulfide (MoS2) is a promising 2D transition metal dichalcogenide (TMD) for optoelectronics and quantum technology applications, but faces challenges in scalable synthesis and defect engineering. Oxygen-assisted chemical vapor deposition (O-CVD), which introduces in-situ oxygen during growth, shows excellent potential in resolving both issues at once. Although co-flowing oxygen shows improvement in growth, the underlying mechanistic role of oxygen remains unclear. In this work, a combination of oxygen dosing experiments, density functional theory (DFT) calculations, computational fluid dynamics (CFD) simulations, and ab initio molecular dynamics (AIMD) simulations, uncover the dual role of oxygen in O-CVD. Firstly, AIMD reveals that oxygen increases MoO3 sublimation and enhances Mo3O9 supply. Concomitantly, DFT reveals that sulphur oxides, due to their bulkier nature than pure S2, limit the formation of reactive MoS6 intermediates. Subsequently, by experimentally varying the oxygen flow-interval, flow-rate, and flow-time, and correlating them with CFD simulations, we decouple oxygen's roles in source-poisoning prevention (i.e. MoO3 evaporation) and growth regulation. We find that maintaining a low sulphur-to-oxygen (S:O2) ratio at the MoO3 boat and substrate during nucleation, and a high S:O2 ratio at the substrate during growth is the key to obtaining large-area high-quality monolayer MoS2, confirmed by our optical measurements. Based on our understanding, we present a kinetic phase diagram for MoS2 synthesis, which will enable controlled oxygen dosing as a tuning parameter for scalable, defect-controlled monolayer MoS2 synthesis.