Resistive switching of anodic TiO2-based Memristors

In recent years, memristors have attracted great attention owing to their simple fabrication process, high scalability, good compatibility with the CMOS technology, high switching speed, low power consumption and low cost for next-generation non-volatile memory technology [1]. The basic cell structure of a memristor is an insulator sandwiched between two metal electrodes. Among the materials being studied for memristors fabrication, binary metal oxides, such as TiO2, are most favourable because of their simple constituents, compatible with CMOS processes, and resistive to thermal/chemical damages. Anodizing is a an electrochemical low cost process carried out at room temperature to grow oxides on valve metals (such as Ti) that can be used in electronic devices [2]. Furthermore, anodizing is a viable tool to grow oxides whose composition, thickness and structures can be easily tuned by selecting the metallic substrate and the electrochemical conditions (i.e. formation voltage, growth bath, growth time) [3]. In this work the fabrication and characterization of microscale Ti/anodic-TiO2/Cu memristors is presented. The fabricated devices (see Fig. 1), ranging from 1 × 1 m2 to 10 × 10 m2, are forming-free and show a bipolar behaviour with a gradual switching from the high resistance state (HRS) to the low resistance state (LRS) and vice versa (see Fig. 2). Moreover, the I–V curves exhibit asymmetric rectifying characteristics with higher current at negative voltages and a rectifying ratio larger than 102 for the sample with the thickest oxide (~ 29 nm). This could be profitably used in crossbar configurations for addressing the sneak path issue. A study in terms of the anodic TiO2 preparation conditions (i.e. formation voltage, growth time) and of devices size (see Fig. 3) reveals finally that the thickest oxide film sample, exhibits the highest ROFF/RON ratio regardless of the devices size and that the largest ROFF/RON ratio (~ 80) is obtained for the 2 × 2 μm2 devices.