Thickness-dependent phase boundary in Sm-doped BiFeO3 piezoelectric thin films on Pt/Ti/SiO2/Si substrates.

Sm-doped BiFeO3 thin films were fabricated on platinized silicon substrates via a sol-gel method. Sm contents and thicknesses were varied in a wide range to investigate their effects on the phase structure and piezoelectricity. X-ray diffraction and Raman spectroscopy experiments revealed a rhombohedral to orthorhombic phase transition and the co-existence of both phases in a certain compositional vicinity. It is found that the proportion of a rhombohedral phase increased with film thickness at the compositions corresponding to the phase transition boundary, indicating the influence of the film thickness on the phase structure. The phase transition phenomenon and film thickness effect on the boundary were also studied by piezoresponse force microscopy. Based on the structure analysis and piezoelectric characterization results, a phase diagram of thickness versus composition was proposed, in which the morphotropic phase boundary was located at 9% to 11% in thinner Sm-doped films and shifted towards the Sm-rich side with increasing thickness.