**7. Interface controlled resistance (ICR)**

The redox reaction driven resistive switching is dominated by the mechanism in which the charged defects or impurities rearrange at the metal electrode and oxide interface. Such a rearrangement of mobile defects reduce the contact resistance due to large interface charge concentration which ultimately decreases the contact barrier height as well as width. This process is examined through the Mott–Schottky theory dealing with the contact formed among metal and non-metal elements. For nanoionic devices, the resistance switching mechanism based on the formation of an interfacial layer has been investigated in a wide variety of metal oxides including TiO2 [54], HfO2 [55, 56], ZrO2 [57, 58] Ta2O5 etc. It is well known that the metal oxides are more prone to oxygen ion i.e. ease of defect formation and hence, largely show nonstoichiometric nature. Therefore, the inherent nonstoichiometric nature of metal oxides produce several lattice point defects or impurities like vacancies and/or interstitials of metal and oxygen ions. Such defects present in metal oxides can be similar to acceptors or donors and are capable of modifying the electronic properties and the switching characteristics of device. Owing to sufficient concentration of charge carriers i.e. defects, such metal oxides can act equivalent to extrinsic semiconductor and thus, the classical semiconductor model governs the electronic conduction.

In order to highlight three main differences between nonstoichiometric metal oxide and classical semiconductor, researchers categorized them into a particular group known as mixed ionic–electronic conductors or chemiconductors [59, 60]. Followings are the distinctions used for classification:


Apart from the above differences, the classical semiconductor acts similar to chemiconductors possessing sufficient nonstoichiometric feature. Therefore, their electrical characteristics are described in the framework of the Mott–Schottky model producing Schottky barriers at the interface (**Table 5**).


**Table 5.** *Comparison for interface controlled resistance RS devices.*
