Switching between electronic and ionic-driven synaptic functionality in non-volatile memristors

The observed multilevel resistive switching in non-volatile $\text{T}\text{i}\text{O}_{2}/\text{A}1_{2}\text{O}_{3}$ memristors strongly correlates with the structure and properties of the functional metal-oxide layers that form this memristive composition. The structure of the 30 nm-thick TiO2 layer drives the physical mechanism underlying the non-volatile resistive switching, which can be changed from electronic to ionic, enabling the synaptic behavior emulation. When the resistive switching mechanism is induced by electronic processes, the resistance state of $\text{T}\text{i}\text{O}_{2}/\text{A}1_{2}\text{O}_{3}$ structures can be electrically tuned over seven orders of magnitude. In this case, the range of non-volatile resistance tuning is mainly determined by properties of 5 nm-thick $\text{A}1_{2}\text{O}_{3}$ layer, specifically by electronic transport mechanism associated with hopping via trap states in the band gap. In this paper, based on the results of local electrical property investigation of $\text{T}\text{i}\text{O}_{2}$ layer of bilayer structures carried out using conductive atomic force microscopy and combined with I–V curve measurements, we experimentally prove that the necessary condition for the implementation of the resistive switching mechanism driven by electronic processes is a formation of p-n junction between n-type $\text{A}1_{2}\text{O}_{3}$ layer and p-type $\text{T}\text{i}\text{O}_{2}$ layer.