ON THE ORIGIN OF SPATIAL NONUNIFORMITIES IN THE COMPOSITION OF PULSED-LASER-DEPOSITED FILMS

A model is proposed for the origin of spatial nonuniformities in the composition of pulsed laser-deposited films derived from multicomponent targets. It is based on the idea that the forward peaking of each species in the plume depends on the species mass via the ratio of two mass-dependent velocities: the flow velocity, which characterizes the directed forward motion, and the mean random thermal velocity, which characterizes lateral motion. The on-axis enrichment of light-mass species observed in films deposited at moderate laser fluences (e.g., Cu in depositions of YBa2Cu3O7) is attributed to the dual effects of a mass-dependent collision rate and collision effectiveness. In weak expansions, these effects leave the lower mass species with a lower temperature, a higher flow velocity, and a relatively more forward-peaked distribution than their high-mass counterparts. The improved compositional uniformity observed for depositions at higher laser fluences is attributed to an incomplete transition to a stronger expansion regime in which all the species in the plume have the same flow velocity. In this regime, the high-mass species are relatively more forward peaked due to their lower mean thermal velocity which scales as (mass) -1/2.