Multilevel polarization switching in ferroelectric thin films

Abstract

Ferroic order is characterized by hystereses with two remanent states and therefore inherently binary. The increasing interest in materials showing non-discrete responses, however, calls for a paradigm shift towards continuously tunable remanent ferroic states. Device integration for oxide nanoelectronics furthermore requires this tunability at the nanoscale. Here we demonstrate that we can arbitrarily set the remanent ferroelectric polarization at nanometric dimensions. We accomplish this in ultrathin epitaxial PbZr_0.52Ti_0.48O₃ films featuring a dense pattern of decoupled nanometric 180° domains with a broad coercive-field distribution. This multilevel switching is achieved by driving the system towards the instability at the morphotropic phase boundary. The phase competition near this boundary in combination with epitaxial strain increases the responsiveness to external stimuli and unlocks new degrees of freedom to nano-control the polarization. We highlight the technological benefits of non-binary switching by demonstrating a quasi-continuous tunability of the non-linear optical response and of tunnel electroresistance.

Fig. 1. Formation of nanoscale 180° domains in strained PZT_MPB thin films.

a ISHG signal evolution during the ongoing growth of PZT_MPB on SRO-buffered NSO (red symbols) and at halted growth (black symbols). The insets illustrate the prevailing domain configurations during and after growth. b Reciprocal space map (out-of plane Q_⊥ vs. in-plane Q_||) around NSO 420 and PZT_MPB 103. The PZT_MPB film is fully strained with an extracted tetragonality c/a of 1.04. The dashed vertical lines indicate the main peak and satellite peak positions. c Cross-section at fixed Q_⊥ across the intensity distribution around the PZT_MPB 103 reflection. d HAADF-STEM image with overlaid ferroelectric dipole map viewed along the [010] zone axis. The yellow arrows reveal the presence of oppositely polarized 180° domains delimited by the dashed white lines. The white arrows represent the net polarization of each nanodomain. Scale bar, 4 nm.

Fig. 2. Local switching characteristics of the PZT_MPB thin films.

a The vPFM image after locally applying +4 V (outer square) and −2 V (inner square) between the scanning tip and the SRO bottom electrode. b Averaged vPFM signal across the dashed outline in (a). c The vPFM image after applying the incremental reverse poling scheme detailed in (d). d Poling voltage as function of the sample location for the two-step poling scheme applied on the image area in (c). e Averaged horizontal vPFM cross-section over the entire image in (c).

Fig. 3. Continuous tunability of the SHG emission via DC voltage application.

a Schematic SHG imaging setup in 45° reflection. b SHG image after applying −3.0/+1.5 V to a box-in-box region. The inner box shows the reversibly depolarized SHG-inactive state. c Cross-section of the SHG signal across the dashed outline in (b). d SHG image containing four neighboring 15 × 15 μm² square areas, where the poling voltage is gradually increasing from left to right. Scale bars in b, d 10 μm. e Spatially averaged SHG signal for each poled area and the pristine background in d as a function of the applied voltage with indicated standard error.

Fig. 4. Variable electroresistance in ultrathin PZT_MPB films.

a The vPFM image on a 4 nm PZT_MPB film displays a gradual polarization switching. b Conductive atomic force microscopy map on the same area as in a with a 100 mV DC reading voltage. c Averaged vPFM signal (black) and resistance change (blue) cross-sections across the poled stripes in a, b, respectively. The blue line shows the three-point moving average of the blue data points. The gray line serves as a guide to the eye to show the PFM signal and resistance of the as-grown state.