Experimental Band Structure of Pb(Zr,Ti)O3: Mechanism of Ferroelectric Stabilization

Pb(Zr,Ti)O3 (PZT) is the most common ferroelectric (FE) material widely used in solid‐state technology. Despite intense studies of PZT over decades, its intrinsic band structure, electron energy depending on 3D momentum k, is still unknown. Here, Pb(Zr0.2Ti0.8)O3 using soft‐X‐ray angle‐resolved photoelectron spectroscopy (ARPES) is explored. The enhanced photoelectron escape depth in this photon energy range allows sharp intrinsic definition of the out‐of‐plane momentum k and thereby of the full 3D band structure. Furthermore, the problem of sample charging due to the inherently insulating nature of PZT is solved by using thin‐film PZT samples, where a thickness‐induced self‐doping results in their heavy doping. For the first time, the soft‐X‐ray ARPES experiments deliver the intrinsic 3D band structure of PZT as well as the FE‐polarization dependent electrostatic potential profile across the PZT film deposited on SrTiO3 and LaxSrMn1−xO3 substrates. The negative charges near the surface, required to stabilize the FE state pointing away from the sample (P+), are identified as oxygen vacancies creating localized in‐gap states below the Fermi energy. For the opposite polarization state (P−), the positive charges near the surface are identified as cation vacancies resulting from non‐ideal stoichiometry of the PZT film as deduced from quantitative XPS measurements. The P+ ferroelectric state, pointing away from the sample requires negative charge accumulation close to the surface (a) while positive charges accumulate close to the surface of P− samples (b). For P+ samples, such off‐stoichiometry reveals in‐gap, oxygen vacancies‐related band (c), absent for P− samples (d). X‐ray photoelectron spectroscopy measurements reveal Ti/Zr vacancies at the P− surface.