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M.; Hofmann, Tino; Brandt, M.; Lorenz, M.; Grundmann, M.; Ashkenov, N.; Schmidt, H.; Ianno, Natale J.; and Schubert, Mathias, "Interface polarization coupling in piezoelectric-semiconductor ferroelectric heterostructures" (2010 We present a dielectric continuum model approach for studying the electrical polarization properties of interface polarization coupled BaTiO 3 , BaTiO 3 -ZnO, and ZnO-BaTiO 3 -ZnO thin-film structures consisting of several hundred nanometer thick layers. Our model augments the effects of electric field driven switchable polarization and depletion layer formation with spontaneous interface polarization coupling. Wurtzite-structure ͑piezoelectric͒ n-type ZnO and perovskite-structure ͑ferroelectric͒ highly insulating BaTiO 3 layers were prepared and investigated. The coupling between the nonswitchable spontaneous polarization of ZnO and the electrically switchable spontaneous polarization of BaTiO 3 causes strong asymmetric polarization hysteresis behavior. The n-type ZnO reveals hysteresis-dependent capacitance variations upon formation of depletion layers at the ZnO/BTO interfaces. We obtain a very good agreement between our model generated data and our experiment. Our model approach allows for derivation of the amount and orientation of the spontaneous polarization of the piezoelectric constituents and can be generalized toward multiple-layer piezoelectricsemiconductor ferroelectric heterostructures. We identify interface polarization coupled triple-layer ZnO-BTOZnO heterostructures as two-terminal unipolar ferroelectric Bi-junction transistor for use in memory storage.

Heterojunctions composed of wurtzite-structure (piezoelectric) ZnO and perovskite-structure (ferroelectric) BaTiO 3 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-BaTiO 3 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 · 10 16 cm -3 ), and the BaTiO 3 (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 BaTiO 3 layers. The coupling between the wurtzite-structure and perovskitestructure 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 BaTiO 3 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 lC/cm 2 .

We report on capacitance-voltage, current-voltage, Sawyer-Tower, and transient current switching measurements for a ZnO-BaTiO 3-ZnO heterostructure deposited on ͑001͒ silicon by using pulsed laser deposition. The triple-layer structure reveals asymmetric capacitance-and current-voltage hysteresis and cycling-voltage dependent Sawyer-Tower polarization drift. We explain our findings by coupling of the ferroelectric ͑BaTiO 3 ͒ and piezoelectric ͑ZnO͒ interface charges and parallel polarization orientation of the ZnO layers causing asymmetric space charge region formation under positive and negative bias. The transient current characteristics suggest use of this structure as nonvolatile memory device.

We report on electrooptic reflection‐type spectroscopic ellipsometry investigations of a wurtzite structure (ZnO) and perovskite structure (BaTiO3) thin film heterostructure between Pt contacts. The ZnO spontaneous polarization biases the ferroelectric polarization domain switching in BaTiO3, and causes a strong asymmetric electrooptic hysteresis behavior, reflected by an overall change of sample effective thickness and effective index of refraction. This hysteresis behavior is concordant with our previously observed electric hysteresis behavior of the same sample structure.We explain the hysteresis behavior by piezoelectric strain within the ferroelectric layer, by interface lattice charge coupling between the BaTiO3 and ZnO layer, and the previously identified depletion layer formation within the n ‐type doped ZnO layer. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)