Improvement of Conductance Modulation Linearity in a Cu²⁺-Doped KNbO₃ Memristor through the Increase of the Number of Oxygen Vacancies

Abstract

The Pt/KNbO3/TiN/Si (KN) memristor exhibits various biological synaptic properties. However, it also displays nonlinear conductance modulation with the application of identical pulses, indicating that it should be improved for neuromorphic applications. The abrupt change of the conductance originates from the inhomogeneous growth/dissolution of oxygen vacancy filaments in the KN film. The change of the filaments in a KN film is controlled by two mechanisms with different growth/dissolution rates: a redox process with a fast rate and an oxygen vacancy diffusion process with a slow rate. Therefore, the conductance modulation linearity can be improved if the growth/dissolution of the filaments is controlled by only one mechanism. When the number of oxygen vacancies in the KN film was increased through doping of Cu2+ ions, the growth/dissolution of the filaments in the Cu2+-doped KN (CKN) film was mainly influenced by the redox process of oxygen vacancies. Therefore, the CKN film exhibited improved conductance modulation linearity, confirming that the linearity of conductance modulation can be improved by increasing the number of oxygen vacancies in the memristor. This method can be applied to other memristors to improve the linearity of conductance modulation. The CKN memristor also provides excellent biological synaptic characteristics for neuromorphic computing systems.

Effect of Oxygen Vacancy on the Conduction Modulation Linearity and Classification Accuracy of Pr0.7Ca0.3MnO3 Memristor

“…Therefore, conduction modulation linearity is a highly important property that a RRAM memristor must possess for use in a neuromorphic computing system. The conduction modulation linearity of a RRAM memristor whose switching characteristics are determined by oxygen vacancy (OV) filaments is influenced by the enlargement (or disruption) of these OV filaments [ 18 , 26 , 27 , 28 , 29 , 30 ]. Changes in the size of the OV filaments in a RRAM memristor are generally explained by two reactions: a redox reaction (fast reaction), and OV diffusion (slow reaction) [ 18 , 26 , 29 , 30 ].…”

“…The conduction modulation linearity of a RRAM memristor whose switching characteristics are determined by oxygen vacancy (OV) filaments is influenced by the enlargement (or disruption) of these OV filaments [ 18 , 26 , 27 , 28 , 29 , 30 ]. Changes in the size of the OV filaments in a RRAM memristor are generally explained by two reactions: a redox reaction (fast reaction), and OV diffusion (slow reaction) [ 18 , 26 , 29 , 30 ]. In general, a RRAM memristor has nonlinear conduction modulation because the enlargement (or disruption) of an OV filament is influenced by these two processes with their different reaction rates [ 18 , 26 , 29 , 30 ].…”

“…Changes in the size of the OV filaments in a RRAM memristor are generally explained by two reactions: a redox reaction (fast reaction), and OV diffusion (slow reaction) [ 18 , 26 , 29 , 30 ]. In general, a RRAM memristor has nonlinear conduction modulation because the enlargement (or disruption) of an OV filament is influenced by these two processes with their different reaction rates [ 18 , 26 , 29 , 30 ]. A RRAM memristor, in which the enlargement of the OV filaments is determined by only one of these processes, exhibits linear conduction modulation [ 18 , 26 , 29 , 30 ].…”

“…In general, a RRAM memristor has nonlinear conduction modulation because the enlargement (or disruption) of an OV filament is influenced by these two processes with their different reaction rates [ 18 , 26 , 29 , 30 ]. A RRAM memristor, in which the enlargement of the OV filaments is determined by only one of these processes, exhibits linear conduction modulation [ 18 , 26 , 29 , 30 ]. In the case of a small number of OVs, enlargement of the OV filaments is mainly determined by the diffusion of OVs, with linear conduction modulation as the result [ 18 ].…”

“…According to a previous study, the insertion of an additional SiO 2 diffusion-limiting layer reduced the number of OVs in TaO 1−x film, resulting in linear conduction modulation in the TaO 1−x memristor [ 18 ]. In contrast, as the RRAM memristor contains numerous OVs, the redox reaction becomes a major growth mechanism of the OV filaments, leading to linear conduction modulation [ 29 , 30 ]. A CuO-added KNbO 3 memristor and a NaNbO 3 memristor annealed at reduced atmosphere showed linear conduction modulation because of the existence of a large number of OVs in the memristors [ 29 , 30 ].…”