# Multilevel Bipolar Electroforming-Free Resistive Switching Memory Based on Silicon Oxynitride
## 3 Results and Discussion
### Resistive Switching Mechanism
The defects, which are a combination of silicon dangling bonds, are considered as traps in the bulk of the initial SiO\({}_{\mathrm{x}}\)N\({}_{\mathrm{y}}\) layer [39]. The combination of oxygen and hydrogen with silicon dangling bonds results in a reduction of dangling bonds and traps [34; 36]. Silicon dangling bonds and oxygen vacancies appear to play major roles for electroforming-free resistive switching. A gas mixture of N\({}_{\mathrm{2}}\)O:SiH\({}_{\mathrm{4}}\):NH\({}_{\mathrm{3}}\):N\({}_{\mathrm{2}}\) at a ratio of 3:20:1:30 and a nitrous oxide (N\({}_{\mathrm{2}}\)O) flow rate of 60 SCCM were applied during the PECVD deposition process, which resulted in an oxygen and nitrogen defect-rich SiO\({}_{\mathrm{x}}\)N\({}_{\mathrm{y}}\) layer with a low amount of hydrogen. This improved the electrical performance of the SiO\({}_{\mathrm{x}}\)N\({}_{\mathrm{y}}\) layer to the degree that it did not need any additional treatment to achieve electroforming-free resistive switching behavior in the device [34; 36]. The switching mechanism is inherently related to the oxygen and nitrogen content in the film. When a positive voltage is increased beyond the V\({}_{\mathrm{SET1}}\) value and reaches the V\({}_{\mathrm{SET2}}\), resistive switching from IRS to LRS takes place, and conductive filaments of oxygen vacancies (V\({}_{\mathrm{0}}\)\({}^{+2}\)) are formed in the interfacial region between the Ni and SiO\({}_{\mathrm{x}}\)N\({}_{\mathrm{y}}\) layers. The Ni layer acts as a charge reservoir, because of its two stable (Ni\({}^{2+}\)/Ni\({}^{4+}\)) oxidation states [40; 41]. Oxygen ions travel from the bulk SiO\({}_{\mathrm{x}}\)N\({}_{\mathrm{y}}\) layer to the Ni layer under the influence of the applied positive voltage to form nonstoichiometric NiO\({}_{\mathrm{x}}\). Depending on their availability, oxygen ions can bond with Ni to form NiO, a Ni(O\({}_{\mathrm{2}}\)) complex, and/or nickel dioxide (ONiO) [42]. During the formation of NiO and NiO\({}_{\mathrm{2}}\) the changes in the standard molar Gibbs free energy are \(-211.7\) and \(-199.0\) kJ mol\({}^{-1}\), respectively, at 289 K and 1 atmospheric pressure [43]. In electrochemical voltage units, the equivalents of these formation energies are 2.19 and 2.06 V, respectively [44]. The negative Gibbs free energies indicate that the formation of NiO\({}_{\mathrm{x}}\) is spontaneous. The high oxygen vacancy density in SiO\({}_{\mathrm{x}}\)N\({}_{\mathrm{y}}\) can be attributed to Ni oxidation, which accumulates oxygen. Oxygen migration to the Ni layer causes a redox reaction within the conducting filaments, which stimulates the formation of more conductive oxygen vacancy filaments. It is possible that the formation of these conducting filaments occurs within or near the interface layer [45; 46]. In any case,  
Figure 5: Schematics of the proposed switching mechanism of an Au/Ni/SiO\({}_{\mathrm{x}}\)N\({}_{\mathrm{y}}\)/p\({}^{+}\)-Si memory device. the resistance of the device switches from the IRS to LRS when conducting filaments form in the oxygen-terminated interfacial region (Figure 5c). To break the NiO\({}_{\text{x}}\) bonds, the same amount of formation energy must be applied in the negative bias direction. The dissociation electrochemical voltages for NiO and NiO\({}_{2}\) are \(-2.19\) and \(-2.06\) V, respectively [43]. When a negative bias voltage is applied and approaches those values, the oxygen ions travel back from the Ni layer to the interface and recombine with the oxygen vacancies. This capture process ruptures the conducting oxygen vacancy filaments [47; 48; 49], leading to resistive switching from the LRS to IRS (Figure 5d). Therefore, no additional electroforming process is required to break the ultrathin NiO\({}_{\text{x}}\) layer which is formed in the interfacial region between the Ni and SiO\({}_{\text{x}}\)N\({}_{\text{y}}\) layers. Regular bias voltage is enough to form and break the conduction filament through the ultrathin NiO\({}_{\text{x}}\) layer.