Substrate stoichiometry changes during pulsed laser deposition: a case study on SrTiO 3

The influence of UV illumination by the plasma plume on the substrate stoichiometry during pulsed laser deposition (PLD) was examined on the model perovskite S r T i O 3 (STO) by the means of in situ impedance spectroscopy during pulsed laser deposition (IPLD). In this manner, the evolution of the STO bulk conductivity was tracked at 300 ∘ C during STO thin film deposition and during deposition on a quartz cover to isolate illumination effects from deposition effects. These measurements revealed an increasing bulk conductivity during covered measurements. Impedance spectroscopy under applied bias voltages indicates that these changes are not caused by photovoltages but by an enhanced oxygen incorporation compensated by electron hole generation under UV illumination. This enhanced conductivity persists after illumination. The combination of across-plane and in-plane measurements further indicates the formation of a layered system, allowing to estimate conductivity and thickness of the newly formed oxygen vacancy deficient layer. This results in a ∼ 30 times more conductive top layer with a thickness of ∼ 40 μ m . The driving force induced by the UV illumination which is responsible for this stoichiometry change corresponds to a p(O 2 ) difference of around six orders of magnitude. Results of in situ measurements show that the real deposition of a thin film leads to a more complex layered system where the growing film interacts with the illuminated top layer of the substrate and possibly introduces additional oxygen vacancies. These changes have a strong influence on any grown thin film either by oxidation/reduction potential or lattice parameter changes.