In this study, the resistive switching and synaptic properties of a complementary metal-oxide semiconductorcompatible Ti/a-BN/Si device are investigated for neuromorphic systems. A gradual change in resistance is observed in a positive SET operation in which Ti diffusion is involved in the conducting path. This operation is extremely suitable for synaptic devices in hardware-based neuromorphic systems. The isosurface charge density plots and experimental results confirm that boron vacancies can help generate a conducting path, whereas the conducting path generated by a Ti cation from interdiffusion forms is limited. A negative SET operation causes a considerable decrease in the formation energy of only boron vacancies, thereby increasing the conductivity in the low-resistance state, which may be related to RESET failure and poor endurance. The pulse transient characteristics, potentiation and depression characteristics, and good retention property of eight multilevel cells also indicate that the positive SET operation is more suitable for a synaptic device owing to the gradual modulation of conductance. Moreover, pattern recognition accuracy is examined by considering the conductance values of the measured data in the Ti/a-BN/Si device as the synaptic part of a neural network. The linear and symmetric synaptic weight update in a positive SET operation with an incremental voltage pulse scheme ensures higher pattern recognition accuracy.
By introducing a thin non-stoichiometric CeO2-x switching layer between the high oxygen affinity metal TaN top electrode and the TiO2 layer in a TaN/CeO2-x/TiO2/Pt bilayer (BL) device, it is possible to enhance the endurance characteristics and overcome the reliability issue. Compared with a single layer device, a BL device significantly enhances the number of direct current overswitching cycles to >1.2 × 104, non-destructive retention (>104 s), and switching uniformity. A TaON interface layer is formed which served as a reservoir of oxygen ions (O2−) in the SET-process and acts as an O2− supplier to refill the oxygen vacancies in the RESET-process and so plays a key role in the formation and rupture of conductive filaments. This study demonstrates that simply introducing a thin non-stoichiometric CeO2-x switching layer into TiO2-based devices can facilitate the enhancement of the endurance property for future nonvolatile memory applications.
In this letter, we report the coexistence of unipolar and bipolar switching in a solution-based nanocrystalline spinel ferrite ZnFe2O4 thin film prepared by the sol-gel method. It is seen that the Au/ZnFe2O4/Pt device could be activated between unipolar and bipolar switching modes just by choosing RESET-voltage polarity. Conversions between unipolar to bipolar switching modes are reversible and controllable. The results show that the SET-voltage of unipolar switching is smaller than that of bipolar switching, while memory windows (ON/OFF ratio) are identical. Furthermore, filaments are induced by the migration of oxygen vacancies (VOs), which are responsible for reducing variations in SET voltages of unipolar switching. By analyzing the current transport conduction mechanism, the electrode-limited Schottky emission mechanism is dominated in the high field region. Temperature dependence of low and high resistance states indicates that conductive filaments are composed of VOs and metallic Zn atoms, involving Joule heating and electrochemical redox reaction effects. Investigation on coexisting both unipolar and bipolar switching modes in a single Au/ZnFe2O4/Pt memory cell would open a pathway for spinel ferrite based low-cost nonvolatile memory.
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