# Recent Advances in Flexible Resistive Random Access Memory
## 3 Material System of Flexible Resistive Memory
### Flexible Dielectric Materials
#### 3.1.1 Flexible Inorganic Materials
The report inferred the flexibility is attributed to the high uniformity and compactness of the SiO\({}_{\text{x}}\) layer from vacuum ultraviolet irradiation. The SiO\({}_{\text{x}}\) layer prepared by this approach not only avoids interface mismatch but also guarantees low internal stress inherently due to its mass increment process. In addition, another material that is ideal for use as flexible RRAM is graphene oxide (GO). In the latest study, high-performance flexible RRAM with GO was reported by [PERSON] [46]. This graphene together with its distinctive inter-layer oxygen diffusion path enables excellent oxygen ion/vacancy diffusion. Without an interfacial redox reaction, oxygen ions can diffuse to form conductive filaments with two inert metal electrodes by applying bias voltage. Meanwhile, the hydroxyl increases the bending modulus of GO, making it more flexible than graphene [50]. The device has shown superior performance, including a high ON/OFF ratio of \(10^{5}\), a long-term retention of \(10^{6}\) s, reproducibility over \(10^{4}\) cycles and long-term flexibility at a bending radius of 4 mm, indicating that the material has great potential for flexible RRAM. Although inorganic storage media have standard semiconductor compatibility, stable chemical properties and high ON/OFF ratios, their inherent fragility makes devices work only under relatively low bending conditions, which cannot meet the bending radius requirements of flexible memory [51, 52]. To improve the flexibility of inorganic storage media, researchers have introduced nanostructures. [PERSON] [52] used an electrochemical anodizing process to convert the metal film into metal oxide, which makes WO\({}_{\text{x}}\) resulting in nano porous (NP) structures. NP structures have the potential to be used in flexible RRAM as mobile ion or charge trap sites and vacancy-assisted switching materials. The device using this structure shows an ON/OFF ratio of \(10^{5}\) and a stable retention time over 5 \(\times\)\(10^{5}\) s, with an endurance over \(10^{3}\) cycles. More importantly, the device has a bending radius of 5.53 mm and shows robust switching behavior after \(10^{3}\) bending cycles (Figure 5). A novel HfO\({}_{\text{x}}\) medium with amorphous nanostructures was developed by [PERSON] [19]. The device can continuously erase more than \(10^{7}\) at a bend radius of 6 mm, and the randomness of resistance is less than 4.3%, but at the bending radius of 4.5 mm, the reset voltage fluctuates and the device becomes unstable. In addition, according to the report's extrapolation results, the device can stably store data at a bend radius of 6 mm for more than 10 years (about 3 \(\times\)\(10^{8}\) s). In these nanostructures, amorphous regions often appear between the nanograin boundaries, which accumulate defects such as oxygen vacancies. They help to release the strain accumulated during the bending operation and  
Figure 4: Al\({}_{2}\)O\({}_{3}\)/CdS film: (**a**) R = 5 mm; (**b**) the I-V characteristics before and after 40 or 100 bending cycles; (**c**) endurance characteristics up to 100 bending cycles [51]. induce the stable evolution of the nano conductive filaments, thus improving the bending performance of the device.