B-site substitution in LaTiO$_{3}$ thin films: influence on the titanium oxidation state.

LaTiO$_{3}$ (LTO) is a Mott insulator, antiferromagnetic at RT, and exhibits a Metal Insulator Transition (MIT) at the Neel temperature. Despite its complex chemistry, it is an interesting candidate to fabricate field-effect devices. A full device requires the deposition of a dielectric in contact with the LTO thin film such as HfO$_{2}$. This choice will be discussed, and we will present issues related to a possible interdiffusion. Adding Hf in the LTO layer leads to a clear change of the resistivity measured as a function of temperature, and strongly influences the MIT temperature. Starting from a semiconducting LTO, 20{\%} Hf substitution on the perovskite B-site makes the layer become metallic from RT down to 4.2K. The average valence of Ti is increasing from Ti$^{3+}$ towards Ti$^{4+}$ with the substitution of Hf, as shown by XPS. Several explanations can be proposed beyond a real incorporation of Hf into the LTO matrix. Besides a pure electronic effect, structural and catalytic effects have been then investigated in details by means of XRD, XPS and HRTEM. Multilayers as well as single-phase thin films have been fabricated to disentangle these different effects. Our results will be discussed taking also into account a possible material loss in the structure. We will in particular explore the behavior of La-deficient structure to qualitatively explain our data.