Droplet formation during extended time pulsed laser deposition of La0.5Sr0.5CoO3 thin layers

The appearance of a strong splashing effect during extended time pulsed laser deposition of La0.5Sr0.5CoO3 (LSCO) films has been investigated. For this aim, films were grown in a base vacuum (4 x 10(-7) Torr) from targets ablated for prolonged time duration or under an oxidizing atmosphere of 10(-2) Torr of N2O from stoichiometric, as-prepared, fresh targets. The films grown under vacuum were found by Rutherford backscattering spectrometry to be oxygen deficient and transparent, while those grown under the oxidizing atmosphere were stoichiometric and opaque. The measured optical absorption coefficient (alpha) at the laser wavelength used for the ablation process (KrF, lambda = 248 nm) for these two series of films was found to be around 1 - 2 x 10(4) cm(-1) and 2 - 3 x 10(5) cm(-1), respectively. It is suggested here that, because of preferential oxygen evaporation from the surface layer of the LSCO target during prolonged laser ablation time, the value of alpha decreased, inducing a volume absorption of the laser radiation. This favored the formation on the target surface of a thick and hot layer of liquid which lasted a longer time than that formed on stoichiometric targets. Under these conditions, the occurrence of a subsurface, explosive volume boiling mechanism, which is the actual cause of the observed splashing effect, was very likely. Numerical estimations of the temperature depth profiles inside the irradiated targets, based on the one-dimensional heat diffusion equation, supported the appearance of an explosive boiling mechanism for the low alpha value targets. (C) 1999 American Institute of Physics. [S0021-8979(99)07205-9].