Interface-Charge-Coupled Polarization Response of Pt-BaTiO3-ZnO-Pt Heterojunctions: A Physical Model Approach

Abstract

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 .

Resistive hysteresis and interface charge coupling in BaTiO3-ZnO heterostructures

“…We developed a dielectric continuum model that describes the asymmetric polarization hysteresis behavior and thereby confirmed the influence of the spontaneous polarization of ZnO on the BTO switchable ferroelectric spontaneous polarization. 13 Our model revealed the existence of a barrier between the ZnO and BTO interface, the formation of a depletion layer within the ZnO layer at the interface, and predicted as a fact that interface charge coupling between the ZnO and BTO layers offset ͑bias͒ the switching points ͑voltages͒ of the ferroelectric BTO polarization. The model predicted further a diodelike rectifying behavior, where in "forward" direction electrons can freely move within the proposed n-type ZnO layer and in "reverse" direction electrons are drained from the ZnO layer leaving behind a space charge region of width w at the BTO-ZnO interface.…”

“…The BTO and ZnO layers were highly textured polycrystalline ͓BTO with ͑111͒ and ZnO with ͑0001͒ texture͔. Electrical Hall-effect, infrared ellipsometry, 8 and our dielectric continuum model analysis 13 obtained that the ZnO layer is n-type conductive, with free electron concentration N c Ϸ 5.5ϫ 10 16 cm −3 , while the BTO layer is highly resistive. The current-voltage ͑I-V͒ measurements were recorded at different sweeping ͑maximum͒ voltages ͑V max ͒.…”

“…We identified voltages in forward and reverse direction at which the depletion layer occurs or disappears. 13 These voltages reveal the formation of the ferroelectric spontaneous polarization biased by the ZnO spontaneous polarization. From this model we calculated the amount of the spontaneous polarization of ZnO as P sz = −4.1 C / cm 2 , which showed to be in good agreement with previous theoretical predictions.…”

“…This value matches with the previously observed voltage shift in electric polarization loops obtained by S-T polarization measurements of our sample. 13 For small voltage loops ͑V max =1 V͒ the log͉I͉-V resembles the varying resistance due to slight variation in the ZnO depletion layer at the BTO-ZnO interface. Due to the ZnO spontaneous interface charge, this depletion layer exists already at zero negative bias, and does not disappear for small negative bias.…”

Interface polarization coupling in piezoelectric-semiconductor ferroelectric heterostructures

“…Our model approach was briefly mentioned in two previous proceedings publications. 57,60 In this work, we present our model approach in detail and give a quantitative comparison with experimental data. Section II details the model approach.…”