**1. Introduction**

Resistive random access memory (RRAM) is a kind of memory in which, according to the different voltage applied to the metal oxide, the resistance of the material changes correspondingly between the high resistance state (HRS) and the low resistance state (LRS), so as to open or block the current flow channel and use this property to store various information [1]. RRAM can significantly increase durability and data transmission speed compared with flash memory devices. The main factor affecting the performance of RRAM is the RS layer, and the performance of different RS layers varies greatly. A variety of materials can be applied as the resistive switching layers of RRAM, such as HfO2, SnWO4, ZrO2, and CuO [2–6], among which binary metal oxides like HfO2 are widely regarded as the most promising resistive switching layer [1,7]. The conduction mechanisms of RRAM have been studied in depth, among which Ohmic conduction, Schottky emission, space-charge-limited conduction (SCLC), and trap-assisted tunneling are the most popular [1,8–11]. The conductive filament (CF) model has also been one of the most recognized models [8]. With the development of science and technology, flexible memory has also been extensively studied in the past decade [12,13]. Due to the advantages of their being inexpensive and lightweight, flexible memristors are more widely used than non-flexible devices such as disposable sensors [14] or indenofluorene-based monomers [15].

Although flexible electronic devices have promising applications in wearable devices, few papers have reported on the RS characteristics of flexible films deposited on mica substrates [16–18]. Mica substrates are cheap, easy to prepare, and satisfy the demands of industrial production, which makes them an excellent candidate for preparing flexible RRAM substrates. In this paper, HfO2 thin films were grown on flexible mica substrates by the sol-gel method. For comparison of different substrates, HfO2 films were also deposited on Pt/Ti/SiO2/Si (100) substrates. As a kind of ordinary semiconductor compound, HfO2 film has a high dielectric constant and desirable light transmittance with a simple preparation [19]. Due to its thermal stability and excellent retention performance [2,20–22], HfO2 has been widely studied in the field of RRAM in recent years [23], and is one of the most promising candidates for the resistive switching layer. The results show that the ratio of HRS to LRS exceeded 100 in the HfO2-based-non-flexible structure, with excellent stability. In contrast to non-flexible resistive switching, the HfO2-based flexible structure demonstrated a pretty good resistive switching characteristic, but its endurance was inferior to non-flexible resistive switching. This HfO2-based flexible device has a simple preparation method (sol-gel), inexpensive cost, and excellent flexibility not existing in an HfO2-based-non-flexible structure, which conforms to the developing requirements of our time for flexible RRAM.
