Scanning electron microscopy investigation of laser ablated oxide targets

Abstract

The surface morphology of ZnO, CeO2 and Pb(Ti0.5Zr0.5)O3 (PZT) targets after laser ablation has been found to greatly depend on the laser wavelength. For laser photons with an energy lower than the target optical bandgap, h<Eg, a highly perturbed surface morphology which exhibits micrometre sized round-shaped cavities formed by the release of gases beneath the surface has been observed, whereas for shorter wavelengths (h>Eg) the surface was less perturbed and retained its smooth morphology. The formation of deep cavities is a general effect which has always been observed for all the materials studied in this work when h<Eg and irradiation fluences of several J cm-2, the usual values employed for pulsed laser deposition (PLD), have been used. Simultaneous to the cavity formation, the droplet density on the surface of films grown using this technique was seen to increase by at least two to three orders of magnitude. Temperature profile estimations inside the targets during the action of the laser pulse, obtained by a numerical solution of the heat diffusion equation, were also quite different for the two cases. These results show that there is a clear connection between the optical absorption coefficient of the target material, the temperature profile, the presence of the deep cavities on the target surface and a high droplet density on the surface of the grown film. Based on these results, we suggest that an explosive subsurface boiling mechanism may appear under laser irradiation conditions when h<Eg.