Polarization Orientation in Lead Zirconate Titanate (001) Thin Films Driven by the Interface with the Substrate

We investigate the effect of the nature of the substrate and the bottom interface on the out-of-plane polarization orientation of ultrathin (10-nm) lead zirconate titanate (PZT) thin films of (001) orientation by photoelectron spectroscopy of samples without surface contamination. The substrate nature is varied between insulator (strontium titanate, STO) and semiconductor (Nb-doped STO, STON) and finally to a metal with a work function lower than that of PZT (strontium ruthenate, SRO). Outward polarization is obtained for PZT/STON(001) and inward polarization is obtained for PZT/STO(001) and PZT/SRO(001). Explanations are given for all these typical cases, the main elements being charge accumulation for compensation of the depolarization field, self-doping of PZT films, and the interface electric field driving the orientation of the polarization of the ferroelectric films. We find p-type self-doping is correlated with the inward polarization, and the driving field is formed between a negatively charged region with negatively ionized acceptors near the interface with the substrate and the p-type degenerate region with holes accumulated inside, toward the surface. This mechanism may be reversed under the assumption of n-type self-doping, positively ionized donors near the interface, and accumulated electrons toward the surface in the case of an interface with a substrate with a higher work function, being in line with recent data (PZT/Pt or BaTiO3/SRO).