Ion-assisted deposition of MoSx films from laser-generated plume under pulsed electric field

Abstract

The thickness profiles and compositional distributions of MoSx films deposited from a plume generated by pulsed laser irradiation of the MoS2 target were investigated at a varying fluence and constant laser pulse energy. It was shown that films with stoichiometric composition were formed at sufficiently low fluence (near the ionization threshold), and increasing fluence caused intricate nonmonotonic variations of the compositional distribution. A substantial deviation of the film composition from stoichiometric and a significant radial gradient of the sulfur concentration over the substrate surface (1<x<3, where x is the ratio of concentrations of S and Mo atoms, x=S/Mo) were found. These phenomena were caused by: (1) the incongruent target evaporation; (2) the mass dependence of the angular distribution of the ablated particles; and (3) the selective sulfur sputtering and desorption induced by energetic particles (ions, excited atoms) of the laser-generated plume. When the laser fluence was low, films of stoichiometric composition were bombarded by energetic particles with a low intensity, and the number of displacements was inadequate for formation of the basal-oriented structure (≪1 displacement per atom). Raising the laser fluence, it is possible to increase the bombardment dose; however, the effect of the factors (1)–(3) has turned the selection of optimal conditions for stoichiometric film deposition into a rather complicated problem. A new technique is proposed for pulsed laser deposition of high-purity MoSx films with varying sulfur concentration and the required ion-irradiation dose. The technique consists in accelerating the ion component of the plume by a pulsed electric field up to an energy of a few tens of kilo-electron-volts, thus making it possible to significantly increase the number of displaced atoms in the MoSx film without noticeable change in its composition and to induce ion mixing at the MoSx-substrate interface. The positive voltage pulse of up to 100 kV amplitude was applied to the target during the plume expansion from the target towards the substrate which was grounded. The modification of the MoSx films by accelerated ions resulted in noticeable improvement of their lubrication performance.