Interface-charge-coupled polarization response of Pt-BaTiO ₃-ZnO-Pt heterojunctions: A physical model approach

Heterojunctions composed of wurtzite-structure (piezoelectric) ZnO and perovskite-structure (ferroelectric) BaTiO₃ are very interesting because of the previously observed ionic lattice polarization coupling at their interfaces. We report electric Sawyer-Tower polarization hysteresis measurements and analysis of a ZnO-BaTiO₃ heterostructure with Pt front and back contacts deposited by pulsed laser deposition onto a (001) silicon substrate. The ZnO layer is n-type (N _c = 5.5 × 10¹⁶ cm ^-3), and the BaTiO₃ (BTO) layer is highly resistive. We observe a strong asymmetric ferroelectric hysteresis, which we attribute to a rectifying depletion layer formation between the ZnO and BaTiO₃ layers. The coupling between the wurtzite-structure and perovskite-structure interface polarization influences the depletion layer formation. We develop a physical model for the electric Sawyer-Tower measurements. Our model includes the effects of the depletion layer formation inside the ZnO layer, the interface charge coupling between the ZnO and BaTiO₃ layers, and the field-dependent ferroelectric polarization inside the BTO. We obtain a very good agreement between our model-generated data and our experiment. We identify voltages in forward and reverse direction at which the depletion layer opens or closes. These voltages are asymmetric, and reveal the effect of the spontaneous piezoelectric (nonswitchable) interface charge of ZnO, which we determine from our analysis here as P _sz = -4.1 μC/cm².