Fig.2 Schematic illustration of oxygen vacancy conductive filament mechanism

Fig.3 Schematic illustration of interfacial electron effect conduction mechanism

Fig.4 EPIR measurement curves （A） and the variation of EPIR value with different stimulating voltages （B） under the 4-wire mode［26］

Fig.5 （A） The depth profiles of Ce， Ti， O and N［16］； （B） The atomic ratios of O/Ce and Ti/N for CeO x /TiN stacked film［16］；（C） Schematic configuration of SDC∶SrTiO3 （STO） vertical heteroepitaxial nanocomposite（VHN） device［31］； （D） Electroforming-free R-V hysteresis loops in SDC∶STO VHN device［31］

Fig.6 （A） Retention for long-term memory stability after repeated pulsing +10 V for 10 ms［32］； （B） I-V curves as repeating voltage sweep of 0→+5→0 V five times first， and 0→-5→0 V five times second， and then finally 0→+5→0 V once at the temperature of 300 and 380 K［33］； I-V curves of （C） Pt/CeO2/Pt reference device during five repeated voltage sweeps of 0→+5→0 V and five successive sweeps of 0→-?5→0 V and （D） Pt/ITO/CeO2/Pt during ten repeated voltage sweeps of 0→+8→0 V and ten successive sweeps of 0→-4→0 V［34］

Fig.7 Bipolar switching curves of the neodymium oxide thin films for （A） 40% and （B） 60% （volume fraction） oxygen concentration parameters［18］； XPS （C） Nd3d states and （D） O1s states［38］

Fig.8 （A） The endurance of the Pt/SGO/Pt ReRAM device over 120 cycles with different ICC； （B） The retention characteristic of different resistance states for the device［40］

Fig.9 （A） The relaxation process of STP with respect to time and this is analogous to human-memory “forgetting process”， inset shows a schematic illustration of synapses［41］； （B） Normalized O1s， Y3p and Hf4d XPS spectra for the YDH thin film［42］； （C） Optical image of a fabricated flexible ITO/Y2O3/Ag RRAM device， inset shows a schematic of the fabricated devices［43］； （D） Representative I-V curve in the log scale， inset shows an I-V curve in the linear scale［43］

Fig.10 （A） I-V hysteresis loops of post-annealed devices for difference voltage and current conditions［48］； （B） I-V curves of resistive switching behavior in the Ru/RE2O3/TaN RRAM devices using Tm2O3，Yb2O3 and Lu2O3 thin films［49］