The bipolar resistive switching characteristics of a Pt/ZnO/Pt structure were investigated in this study. It is found that low resistance state can be only formed at a positive bias while high resistance state can be only formed at a negative bias with the bipolar switch characteristics. The conduction mechanism at different resistance states also were discussed. The resistive switch mechanism may be relate to the generation and oxidation of filaments and the switch polarity is likely resulted from the geometric effect due to the asymmetrical structure of electrodes, resulting in the flux of defects near the edge of top electrode caused by local enhancement of electric field.
The reversion of polarity within bipolar resistive switching operation occurs in Pt/HfO2/TiN and Pt/Hf/HfO2/TiN resistive random access memory devices. This reversion of voltage polarity is the result of interface generation which induces a conduction mechanism transformation from Poole-Frenkel emission to space charge limited current mechanism. To prove the reversion of polarity, this study uses curve fitting of I-V relations to verify the conduction mechanism theoretically and physical analysis to verify the oxygen ion distribution practically. The proposed Pt/Hf/HfO2/TiN devices exhibit good resistive switching characteristics, such as good uniformity, low voltage operation, robust endurance (10(3) dc sweep), and long retention (3 x 10(4) s at 85 degrees C).
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