Controlling the Polarization in Ferroelectric PZT Films via the Epitaxial Growth Conditions

Abstract

The integration of thin-film ferroelectrics with reliable properties into oxide electronics requires accomplishing deterministic polarization states. Since ferroelectricity emerges during thin-film synthesis already, it is essential to elucidate how the interplay of different growth parameters affects the polarization. Here, the polarization of fully strained Pb(Zr0.2Ti0.8)O3 (PZT) films is accessed in situ, during epitaxial growth. Surprisingly, it is found that the orientation of the out-of-plane polarization during growth may differ from the one after growth completion and it strongly depends on the substrate temperature and the oxygen partial pressure. Increasing the growth temperature and/or the oxygen partial pressure favors a uniform downward-oriented polarization, independent of the direction of polarization during growth. Specifically, for films with an emerging upward-oriented polarization, a polarization reversal and a downward-oriented polarization after cool-down is observed. The in situ measurements obtained by optical second harmonic generation (SHG) in conjunction with ex situ piezoresponse force microscopy (PFM) and X-ray diffraction (XRD) measurements point to the temperature- and pressure-dependent formation of a charged Pb defect gradient toward the film surface as the responsible mechanism for the polarization reorientation.