# Recent Advances in Flexible Resistive Random Access Memory
## 3 Material System of Flexible Resistive Memory
### Flexible Dielectric Materials
#### 3.1.1 Flexible Inorganic Materials
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Dielectric** & & & & & & \\
**Materials** & **Polarity** & **ON/OFF Ratio** & **Durability** & **Retention** & **Number of** & **Bending** \\  & & & & & & \\  & & & & & & \\ \hline TiO\({}_{2}\)[21] & bipolar & \(10^{3}\) & \(10^{2}\) & \(2\times 10^{3}\) s & 40 & 9 mm \\ NiO\({}_{\text{x}}\)[22; 23] & bipolar & \(10^{6}\) & \(10^{2}\) & \(10^{4}\) s & 120 & 10 mm \\ ZnO\({}_{\text{x}}\)[24] & bipolar & \(10^{6}\) & \(10^{2}\) & \(10^{4}\) s & 98 & 20 mm \\ AlO\({}_{\text{x}}\)[25; 26] & bipolar & \(10^{6}\) & \(30\) & \(3\times 10^{4}\) s & 100 & 10 mm \\ WO\({}_{\text{x}}\)[28; 29] & bipolar & \(10^{5}\) & \(2\times 10^{3}\) & \(10^{5}\) s & 100 & 15 mm \\ HfO\({}_{\text{x}}\)[30; 31] & bipolar & \(1.2\times 10^{3}\) & \(5\times 10^{8}\) & \(10^{4}\) s & \(10^{3}\) & 5 mm \\ MoO\({}_{\text{x}}\)[32; 33] & unipolar & \(2.38\times 10^{5}\) & \(50\) & \(2\times 10^{5}\) s & 100 & 10 mm \\ \hline \hline \end{tabular}  
* Durability is measured by the number of switching cycles the device undergoes under DC conditions; N/R: not reported.  
\end{table} 
Table 1: Performance comparison of several representative binary metal oxide dielectric materials. requirements for its flexibility are low, relatively. However, the composition content of RRAM based on multicomponent oxides is difficult to control in the fabrication process, the fabrication process is complex and most of them are incompatible with the CMOS process, so multicomponent metal oxides are rarely used as dielectric materials. Solid electrolytes are usually ionic conductors with high electron mobility. The resistance transformation mechanism of the material is based on the formation and disconnection of conductive metal filaments caused by the redox reaction of cations in electrolytes. Its outstanding characteristic is high flexibility, and it is also a kind of flexible RRAM with great potential. The chalcogenides most commonly used as solid electrolytes, including Ag\({}_{2}\)Se [36], Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\)[37], WSe\({}_{2}\)[38], etc., are all storage media with excellent performance. Their ON/OFF ratio is generally more than \(10^{4}\) and their durability is generally more than \(10^{6}\). In order to further improve the mechanical properties of flexible RRAM devices, a kind of inorganic sulfide compound and its composite materials with inorganic oxides have been developed. Using Al\({}_{2}\)O\({}_{3}\) and CdS as the storage medium by Ju [49], the medium can withstand 100 times of repeated bending, and the minimum bending radius can be reduced to 5 mm (Figure 4). The Al\({}_{2}\)O\({}_{3}\) layer between the Ag top electrode and the CdS layer act as the barrier that prevents Ag from significantly migrating into the CdS layer. The Al\({}_{2}\)O\({}_{3}\) layer with appropriate thickness prevents the unnecessary migration of a large amount of Ag into the CdS layer to control the thickness of filaments, which prevents delamination and reduces thermal effects. Although solid electrolytes have excellent performance, chalcogenides are not easy to manufacture. They are complex and costly, and their performance is similar to that of binary metal oxides. Therefore, solid electrolytes are not widely used relatively. Figure 3: \(W/TiO_{2}/HfO_{2}/TaN\) device: (**a**) schematic illustration displaying the cross-array structure; (**b**) DC endurance characteristics for \(10^{3}\) cycles; (**c**) excellent pulse endurance for over \(10^{7}\) switching cycles attained with 10 μs programming pulses; (**d**) retention tests made at LRS and HRS show no evident change for over \(10^{4}\) s at \(125\)\({}^{\circ}\)C [34]. As for other flexible dielectric materials, SiO\({}_{\text{x}}\)-based flexible RRAM was reported by [PERSON] [42], with the ON/OFF ratio of \(10^{5}\) after 2000 bending cycles at a 2.5 mm bending radius. The device uses vacuum ultraviolet irradiation to prepare SiO\({}_{\text{x}}\) films from perhydropolysilazane.