Growth of apatite films by laser ablation: Reduction of the droplet areal density

The pulsed laser deposition of calcium phosphate (apatite) thin films using targets with different optical absorptivities has been studied. It was found that the surface morphology of the grown films greatly depends on the target optical properties, the higher the optical absorption coefficient a, the lower the droplet density on the surface of the deposited films. The temperature profiles inside the different targets during the action of a laser pulse were obtained by numerically solving the heat diffusion equation. These simulations indicated for the low a value targets a large volume heating with the formation of a thick layer of melted material, and the occurrence of a sub-surface superheating effect once the evaporation began. For higher a value targets, the temperature profiles corresponded to the surface heating case, where the evaporation process began at earlier times and was restricted to the outermost surface region. Time- and space-resolved optical emission spectroscopy investigations showed that there was a significant increase of electron density and temperature for the plasmas formed from targets having higher a values, in agreement with the temperature profile estimations. (C) 1996 American Institute of Physics.