**5. Ferroelectric resistive switching**

In the late year *1970*, the first experimental work by Esaki reported the resistive switching behavior utilizing reversal of polarization in ferroelectric material [40]. This report followed many other ferroelectric based resistive switching materials such as the perovskitetitanates including PbTiO3 [41]**,** BaTiO3 [42], and multiferroic system e.g. BiFeO3 [43]. With consistent investigations, researchers have explained the resistive switching in ferroelectrics [40–43]. Usually, there exists two different types of ferroelectric based resistive switching memory. Considering the conduction mechanism, first is the ferroelectric tunneling junction and other is known as ferroelectric diode type memory. An ultrathin tunneling ferroelectric barrier is the primary component of the ferroelectric tunneling junctions. For such junctions, one can observe significant modification in tunnel oxide barrier height because of polarization reversal occurring in the ferroelectric materials. The ferroelectric tunnel barrier exhibits potential distribution of asymmetric type which evokes change in the height of barrier when reversal of polarization takes place. For memory cells, when both the metal electrodes are different that impose distinct screening lengths, the asymmetric potential distribution is achieved [44, 45]. The asymmetric behavior of potential has also been examined after placing a non-ferroelectric ultrathin layer onto the ferroelectric oxide layer [46]. The presence of dielectric ultrathin layer acts as a separation wall which essentially divides polarization charge in ferroelectric oxide layer and screening charge in metal electrode. Therefore, the non-ferroelectric i.e. dielectric layer possesses a certain distribution of the potential. In case of ferroelectric diode, the interface between metal electrode and ferroelectric layer observes the formation of Schottky type barrier. Upon the reversal of polarization in ferroelectric layer, the height of Schottky type barrier correspondingly changes (**Table 3**) [41].


## **Table 3.**

*Comparison of switching voltages for ferroelectric type RS devices.*
