Magnetic field argon ion filtering for pulsed magnetron sputtering growth of two-dimensional MoS2

Abstract

Plasma assisted physical vapor deposition processes can provide alternative and scalable approaches for synthesis of two-dimensional (2D) materials. While plasma species with high kinetic energies and chemical activities are beneficial for reduced temperature growth of crystalline 2D layers, precise control of these characteristics for reduced defect generation under ion bombardment is one of the main challenges for plasma assisted growth processes. In this study a variable intensity magnetic field was used to control chemistry, energy, and spatial density distribution of plasma produced by pulsed magnetron sputtering of MoS2 in 2.0Pa argon. A magnetic coil was used to deflect plasma flux and control electron and ion densities at the substrate surface. Plasma emission and mass-spectrometry studies showed the abundance of excited neutrals and positive ions of Ar, Mo and S with no evidence for MoS radicals and negative S ions. Ar1+ abundance was several orders of magnitude above that of Mo1+ and S1+ and argon ions were therefore the primary source of the substrate bombardment. Wavelength specific plasma imaging and mass/energy analysis demonstrated that the tunable magnetic field can effectively steer ionized Ar species away from the substrate with about a factor of three reduction of their abundance at the condensation surface. Spatial distributions of sputtered Mo and S species were also influenced but to a different degree, which was dependent on their ionization state. Tunable magnetic filtering helped to reduce unnecessary Ar bombardment by maintaining incident ion energies <8eV to minimize point defect generation in hexagonal 2D MoS2 films. The argon ion filtering approach was verified by producing three monolayer thick polycrystalline 2D MoS2 films over wafer-scale areas allowing for a scalable direct synthesis needed for device manufacturing.