High-desity nonvolatile ferroelectric resisitive memory

Binary data in 1T1C or 2T2C non-volatile ferroelectric random access memories, which are represented by the polarization charges of switching and non-switching events, are retrieved when a reading current pulse exceeds the sense amplifier threshold for discrimination of nonswitching pulses, and a reset pulse is required after the destructive readout. Downscaling the dimensions of such memory cells reduces the measurable polarization charge proportional to the capacitor area, and data are no longer detected reliably in high-density memories. The nondestructive readout of the binary information is possible from the high- and low-conductance bipolar switching of a ferroelectric diode under two opposite poling fields, where the direction of the diode effect switches with polarization reversal. Our study of BiFeO3 thin-film capacitors shows measured polarization-modulated diode currents for encoding information within high-density memories. We used two in-plane nano-electrodes fabricated on the film surface in gaps of a few tens of nanometers. As an in-plane field is applied antiparallel to the domain orientation above a coercive field, the partial domains near the film surface layer are switched firstly to form conductive charged walls with respect to the underlying unswitched bulk matrix, so that we can observed an “on” current from current-voltage loops. As the switching field is high enough, the underlying domains throughout the whole film thickness are switched. In this case, there was no charged domain wall, and thus, we observed an “off” current. Both “on” and “off” currents with the ratio of 100 are stable over laboratory time with write cycles larger than 2×105 and write time as short as 10 ns, which provide the proof of ferro-resistive switching and independent adjustments of ferroelectricity and conductivity in a single-phase ferroelectric in promotion of the integration density of the memory with high performance.