Temperature dependence of semiconducting BiFeO₃ thin films deposited by pulsed laser deposition for resistive memory application

Full text: Conducting ferroelectric materials have exhibited emergent electronic properties, including significant electroresistance effects in switchable ferroelectric diodes and multiferroic/ferroelectric tunnel junctions and intriguing charge conduction in ferroelectric domain walls, which have attracted considerable attention in high-speed switching and high-density memories as well as in ultrafast electro-optical devices. Ferroelectric-resistive memories based on ferroelectric diodes have demonstrated the ability of high resistive ON/OFF ratios, high speeds and low write powers with the high reproducibility by controlling polarization orientation. Here we fabricated semiconducting (00𝑙) BiFeO₃ (BFO) thin films with the thicknesses of 30-40 nm on SrRuO₃/SrTiO₃ substrates using a pulsed laser deposition technique. The growth mode of BFO thin film can been controlled by the underlay SrRuO₃ bottom electrodes. It was found that the SrRuO₃ thin film was 3D island formation at laser energy density of 2.5 J/cm^-2 but a 2D layer-by-layer growth mode when the energy density was reduced down to 1.5 J/cm^-2. Above-layer growth modes of BFO thin films match with SrRuO₃ exactly, though the growing conditions are the same. The 2D thin film at 300 K shows switchable ON/OFF diode currents upon polarization flipping near a negative coercive voltage, which is nevertheless absent from the 3D thin film. From a positive-up-negative-down pulse characterization technique, we measured domain switching current transients in both semiconducting thin films after deduction of time-dependent leakage currents, where doman switching time can be adjusted in the range of ns-ms through the resistance adjustment of the circuit RC time constant. These current transients can be further transformed into standard polarization-voltage (𝑃-𝑉) hysteresis loops. The 𝑃-𝑉 hysteresis loops after 1 𝜇s-retention time show the preferred domain orientation pointing to bottom electrodes in a 3D thin film. This poor polarization retention can be improved considerably in a 2D thin film. From these P-V hysteresis loops, we extracted domain switching time dependence of coercive voltage at temperatures of 78 - 300 K. It is found that the coercive voltages in semiconducting ferroelectric thin films are much higher than those in insulators. Finally, an equivalent resistance model in description of free-carrier compensation of the domain boundary charge during domain growth is developed to interpret this difference. Thes semiconducting thin films can be further integrated into the elements of ferroelectric resistive memories with the sizes of a few tens of nanometers with readout currents on the order of nA. (author)