Suppression of gas-phase reactions in WS2 metalorganic chemical vapor deposition

Metal-organic chemical vapor deposition of transition metal dichalcogenides is often accompanied by gas-phase reactions, which may lead to metal-based cluster formation in the gas phase and reduced deposition on the substrate surface. This study demonstrates that the cluster formation can be suppressed by keeping the residence time of metal precursor molecules in the reaction chamber significantly shorter than the typical collision time between such molecules. The rate of cluster formation was quantified by a single dimensionless parameter, the reduced residence time τr, which is the ratio of the average residence time to the intermolecular collision time. From an experimental study of WS2 metal-organic chemical vapor deposition using W(CO)6 and H2S precursors, the deposition rate is mass-transport limited at low W(CO)6 concentrations and process pressures, corresponding to τr ≪ 1. However, for τr ≥ 1, the deposition rate drops sharply, indicating cluster formation.