Role of TiO_2 Seed Layer Thickness on the Nanostructure Evolution and Phase Transformation Behavior of Sputtered PZT Thin Films During Post-Deposition Air-Annealing

摘要

An optimum TiO_2 seed layer is said to facilitate nucleation and subsequent growth of perovskite phase in Pb(Zr,Ti)O_3 (PZT) films during annealing. The actual causes that prevent perovskite growth, particularly in sputtered PZT films on thick TiO_2 seed layers are not yet understood clearly. Herein, based on the results of X-ray diffractometry (XRD), field-emission scanning electron microscopy (FESEM), and analytical scanning/transmission electron microscopy (S/TEM) with semi-quantitative elemental mapping using energy dispersive X-ray (EDX) spectrum imaging, the post-annealing crystallization trend of r.f. magnetron sputter-deposited amorphous PZT films has been scrutinized as a function of TiO_2 seed layer thickness. Upon annealing, PZT films on thin TiO_2 layers (～20-100 nm) partially crystallize into perovskite PZT via a transient pyro-chlore/fluorite phase. However, no perovskite phase forms on thicker (～550 nm) TiO_2 layer. Thickness of underlying TiO_2 seed layer strongly influences the crystallization, phase formation, texture, and surface morphology of the PZT films. During crystallization, Pb diffuses away from the PZT film to form solid solution with TiO_2 seed layer. Perovskite PZT phase transformation requires a minimum Pb-concentration within the transient pyrochlore/fiuorite phase. During the growth of perovskite grains, Zr gets segregated, resulting in Zr-concentration build-up within the surrounding disordered fee fluorite phase.