Nanoscale Resistive Switching in Ultrathin PbZr 0.2 Ti 0.8 O 3 –La 0.7 Sr 0.3 MnO 3 Bilayer (original) (raw)

2020, physica status solidi (b)

Electron tunneling through a ferroelectric barrier has received increasing interest in recent years because the polarization in the barrier was found to control the tunneling current. [1-11] Early work on tunneling through a ferroelectric started with theoretical considerations. [1] Multiferroic four-state tunnel junctions have been introduced by combining the ferroelectric barrier with spin-polarized tunneling in magnetic tunnel junctions. [5,7,11] Both BaTiO 3 and PbZr x Ti 1Àx O 3 (PZT, x ¼ 0-0.5) have been used as ferroelectric for the tunnel barrier. [8-11] For proper function as a tunnel barrier, ferroelectric layers of about 1-5 nm thickness need to be reversibly switched. This poses the challenge of applying very large electric fields to the barrier, because the coercive fields of ferroelectric films grow strongly with reduced thickness. [12] Often, the required electric field is of the order of 1-10 MV cm À1. An electric field of this magnitude can drive processes which are of electrochemical nature and change the chemical composition in or near the tunnel barrier. Such phenomena are part of another very active research area: resistive switching based on voltage-driven ionic movements has been vastly explored for memory applications in the last 15 years. [13] Until now, ferroelectric PbZr x Ti 1Àx O 3 (PZT, x ¼ 0-0.5) has very rarely been studied with respect to resistive switching based on ionic motions. Choi et al. measured electrical transport and switching of PZT (x ¼ 0.3) capacitors with a Pt (top) and a La 0.5 Sr 0.5 CoO 3 (bottom) electrode, varying the thickness of the PZT layer between 17 and 160 nm. [14] They found bipolar resistive switching instead of a polarization reversal for a PZT thickness of ≤34 nm. The characteristics of bipolar resistive switching can be described as follows: during cycling the voltage applied to the Pt electrode from zero to positive maximum, negative maximum, and back to zero, the resistance drops at a positive threshold voltage indicating a transition to a so-called low-resistance state (LRS). The LRS turns back to the high-resistance state (HRS) at a negative threshold voltage. The current versus voltage (I-V) loop thus roughly follows the shape of the number eight, starting with a counterclockwise loop at positive voltages. (Our measurements presented here are unipolar; nevertheless, Figure 1b can be used for an impression of how the positive half of the bipolar I-V loop would look like.) The work of Choi et al. showed that thin PZT layers display similar bipolar resistive switching such as SrTiO 3 which has often been studied and utilized in resistive switching elements. [15-17] Qin et al. studied tunnel barriers made of PZT and BaTiO 3 as well as dielectric SrTiO 3 in nominally symmetric electrode configurations with LSMO on both sides and find bipolar resistive switching without polarization reversal. [18] All three titanates show similar behavior despite their different electric nature. Notably, the application of a positive threshold voltage