At low fluences, the [Sr]/[Ti] ratio of laser deposited SrTiO3 films appears to be a function of the laser fluence. The deviation from stoichiometry is remarkably constant in time. From an analysis of both the composition of the film and the irradiated target, we deduce a volume-diffusion-assisted preferential ablation process. At high fluences (above 1.3 J/cm2), stoichiometric SrTiO3 films are obtained. This is not due to a change in ablation mechanism, but follows from the fact that at 1.3 J/cm2 the calculated diffusion length of Sr within the irradiated target, becomes of the order of the ablation rate per shot.@en

We provide evidence that non-stoichiometric ablation of YBa₂Cu₃O_7-δ at low fluences is due to a phase separation of the target surface. On the other hand, in SrTiO₃ we find at low fluences evidence for preferential ablation which is assisted by volume-diffusion. As a result, the Sr/Ti ratio of the ablated films can be tuned by choosing the appropriate fluence.

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The interaction of a spatially uniform 248 nm excimer laser beam with targets of various oxides (YBa ₂ Cu ₃ O _{7 - δ} , Y ₂ BaCuO ₅ , Y ₂ O ₃ and SrTiO ₃ ) is investigated. We identify two different mechanisms for non-stoichiometric ablation. Evidence is found that in YBa ₂ Cu ₃ O _{7 - δ} and Y ₂ BaCuO ₅ the non-stoichiometric ablation at low fluences is due to a phase transformation of the target surface. This is a gradual process, only observable after prolonged irradiation of the target. For congruently melting SrTiO ₃ such a phase transformation is neither expected nor observed. We find, however, that at low fluences within 100 laser shots the target surface composition equilibrates to a stable, Ti-rich composition. In this case, the mechanism we propose is preferential ablation, which is accompanied by bulk diffusion to compensate the induced surface depletion.

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