Controlled Sign Reversal of Electroresistance in Oxide Tunnel Junctions by Electrochemical-Ferroelectric Coupling

The persistence of ferroelectricity in ultrathin layers relies critically on screening or compensation of polarization charges which otherwise destabilize the ferroelectric state. At surfaces, charged defects play a crucial role in the screening mechanism triggering novel mixed electrochemical-ferroelectric states. At interfaces, however, the coupling between ferroelectric and electrochemical states has remained unexplored. Here, we make use of the dynamic formation of the oxygen vacancy profile in the nanometer-thick barrier of a ferroelectric tunnel junction to demonstrate the interplay between electrochemical and ferroelectric degrees of freedom at an oxide interface. We fabricate ferroelectric tunnel junctions with a ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ bottom electrode and ${\mathrm{BaTiO}}_3$ ferroelectric barrier. We use poling strategies to promote the generation and transport of oxygen vacancies at the metallic top electrode. Generated oxygen vacancies control the stability of the ferroelectric polarization and modify its coercive fields. The ferroelectric polarization, in turn, controls the ionization of oxygen vacancies well above the limits of thermodynamic equilibrium, triggering the build up of a Schottky barrier at the interface which can be turned on and off with ferroelectric switching. This interplay between electronic and electrochemical degrees of freedom yields very large values of the electroresistance (more than ${10}^6\%$ at low temperatures) and enables a controlled switching between clockwise and counterclockwise switching modes in the same junction (and consequently, a change of the sign of the electroresistance). The strong coupling found between electrochemical and electronic degrees of freedom sheds light on the growing debate between resistive and ferroelectric switching in ferroelectric tunnel junctions, and moreover, can be the source of novel concepts in memory devices and neuromorphic computing.

Figure 1

Interface structure. (a) Atomic resolution HAADF image of the ${\mathrm{SrTiO}}_3(100)\mathrm{//}25\mathrm{nm}$ ${\mathrm{La}}_{0.7}{\mathrm{Sra}}_{0.3}{\mathrm{MnO}}_3(\mathrm{LSMO})/10\mathrm{nm}$ ${\text{BaTiO}}_3$ heterostructure down the [110] direction. The inset shows an EELS Ti $L_{2,3}$ (blue), La $M_{4,5}$ (red) integrated intensity map acquired from the marked area at the $\mathrm{LSMO}/\mathrm{BTO}$ interface. (b) Ti and Mn oxidation states and (c) normalized integrated intensity profiles along the STO//LSMO/BTO heterostructure. (d) AFM image showing atomically flat surfaces displaying STO surface terraces. Piezoresponse [amplitude (e) and phase (f)] hysteresis loop measured on a selected location of the sample. Amplitude (g) and phase (h) PFM images showing that stable polarization states can be written with the PFM tip using small voltages. Panels (g) and (h) show ferroelectric domains generated applying $\pm 2\mathrm{V}$ (as labeled) tip voltages.

Figure 2

Ferroelectric and oxygen vacancy resistance loops. Electroresistance loops of an $\mathrm{Ag}/\mathrm{BTO}/\mathrm{LSMO}$ bilayer with initial state written with electric field pointing down (negative voltage) (a), and with electric fields pointing up (positive voltage) (b). After applying each $V_{\mathrm{bias}}$, junction resistance is measured under $V_{\mathrm{read}}=10\mathrm{mV}$ (a) and $V_{\mathrm{read}}=0.7\mathrm{V}$ (b). The different resistance states are identified according to the orientation up ($P\uparrow$) or down ($P\downarrow$) of the ferroelectric polarization and to the location up ($\square \uparrow$) or down ($\square \downarrow$) of oxygen vacancies (i.e., oxygen vacancies pushed by electric field against the top or the bottom BTO interface). (c) I-V curves of an $\mathrm{Ag}/\mathrm{BTO}/\mathrm{LSMO}$ sample measured at 100 K. Blue (red) curves are measured after initial resistance states written in electric fields pointing down (up). Larger amplitude sweeps with negative starting voltage (cyan curve) and with positive starting voltage (light magenta curve) switch between the two families of curves (blue and red). (d) Enlarged view of the I-V curves at positive voltages to observe resistance switches.

Figure 3

Resistive and ferroelectric switching of $\mathrm{Ag}/\mathrm{BTO}/\mathrm{LSMO}$ bilayers. I-V curves of an $\mathrm{Ag}/\mathrm{BTO}/\mathrm{LSMO}$ sample measured at 100 K analyzed with the Brinkmann-Dynes-Rowell model (Ref. [41]) in the tunneling regime [panels (a)–(c)] and with a Schottky model [panel (d)]. Upper and lower sketches in panels (a) and (b) illustrate the modulation of the tunnel barrier due to the asymmetric screening at the interfaces according to the interface polarization model [9]. Sketches in panels (c) and (d) illustrate the crossover from tunnel regime (c) to Schottky regime (d). Voltage in panel (d) is swept from negative to positive.

Figure 4

Hysteretic capacitance in the Schottky regime. Low voltage (20 mV) capacitance (a) and conductance (b) loops recorded at 100 K at various frequencies: 1 kHz (green symbols), 10 kHz (red symbols), and 100 kHz (blue symbols).