Polarity dependent predictable and random resistive switching in forming-free unipolar memristors

Polycrystalline, hexagonal HoMnO3 thin films sandwiched between an un-patterned Pt/Ti bottom electrode and a circularly patterned Al top electrode reveal forming-free, polarity dependent predictable and random, unipolar resistive switching with stable retention, and a resistance ratio between high resistance state (HRS) and low resistance state (LRS) larger than 104. An equivalent circuit model is proposed to describe the HoMnO3 memristor as a three-layered capacitor structure. Based on the energy band diagram of the HoMnO3 structure under applied negative and positive write bias, polarity dependent, predictable, and random resistive switching is explained. Furthermore, the conductance in LRS until RESET and in HRS until SET has been investigated. The conductance in LRS is large and constant until RESET bias is reached. The conductance in HRS increases from nearly zero to quantum conductance G0 (2e2/h) until SET bias is reached.