Effects of TiO2 crystallinity and oxygen composition on forming characteristics in Pt/TiO2/Pt resistive switching cells

“Forming” is a stage in resistive switching (RS) devices that occurs before switching and represents an important physical phenomenon in the universal operating mechanism of such devices. Forming in a resistance change material appears to be a kind of dielectric breakdown. In this study, we performed time-dependent forming (TDF) characterization of Pt/TiO2/Pt resistive switching cells with TiO2 layers of different crystallinities and oxygen compositions. We prepared two samples in which the grain boundary density and the density of oxygen vacancies differ, while both the TiO2 layers exhibited the similar crystal structures. Our results reveal that the Weibull slope and variation of time to forming are determined by the deposition method of the Pt bottom electrode (BE) films. Moreover, the initial cell resistance and distribution of the TDF characteristics depend not only on the crystallinity but also on the oxygen composition of the TiO2 layers. The variation of time to forming increases as the distribution of initial resistance is reduced in Pt/NiO/Pt resistive switching cells with different NiO crystallinities. Conversely, the variation of time to forming decreases as the distribution of the initial resistance is reduced in the case of the Pt/TiO2/Pt cells. These results reflect differences in both the grain boundary density (crystallinity) and the density of oxygen vacancies (oxygen composition) of resistance change materials used in the resistive switching cells. The clear difference of crystallinities and oxygen compositions might originate from differences in the oxide deposition mode during reactive sputtering.