| Resistive switching random access memory (RRAM), as one of the most potential techniques for the next generation non-volatile memories, has attracted increasing attention in the last decade. Since resistive switching thin film plays an important role in the performance of RRAM, the switching properties and switching mechanisms of various thin film materials have been intensively investigated. In the present work, polycrystalline BiFeO3 (BFO) thin films were deposited onto Pt/Ti/SiO2/Si substrates by pulsed laser deposition (PLD) and Au/BFO/Pt resistive switching cells were fabricated, the influence of thin film growth conditions on the switching properties were studied, and the fundamental mechanisms behind the switching behaviours were investigated. The main results are as follows:(1) The BFO thin films deposition conditions on Pt/Ti/SiO2/Si substrates by PLD were systematically studied with emphasises on the influence of substrate temperature and oxygen partial pressure on the crystallization quality and microscopic morphology. It has been observed that the BFO thin films were well crystallized with polycrystalline structure when the substrate temperature is in the range of 550-750 ℃. The SEM images show that BFO thin films with flat surface and dense structure can be obtained at a substrate temperature of 650 ℃. While the oxygen partial pressure does not obviously influence the thin films morphology, however, Bi-rich phase was formed and the growth rate was decreased with higher oxygen partial pressure during the deposition. Moreover, it has been shown that the grain size of the BFO thin films increases by increasing the film thickness.(2)The resistive switching properties of BFO thin films were characterized, including the I-V, retention, and endurance tests. The experiment data showed that the ON/OFF ratio is strongly correlated with the substrate temperature and oxygen partial pressure. Higher ON/OFF ratio has been achieved at relatively higher substrate temperature, but the leakage current increases rapidly and leads to hard breakdown by further increase the substrate temperature. Meanwhile, the oxygen partial pressure needs careful optimization in order to obtain high ON/OFF ratio, long time retention, and good endurance. The tests indicate that the resistance states became unstable and the endurance was deteriorated with much higher oxygen partial pressure. On the other hand, the ON/OFF ratio is significantly decreased when much lower oxygen partial pressure was used during the deposition. Furthermore, larger ON/OFF ratio and better stability of resistance states can be obtained by decreasing the thin film thickness.(3)The switching mechanism of BFO thin films was investigated based on the analysis of conduction behaviour. The results indicate that the resistance of the whole Au/BFO/Pt capacitor-like cell is contributed simultaneously by the Au/BFO Schottky barrier and the bulk resistance of BFO thin film. The resistive switching is attributed to the modulation of bulk thin film resistance, which is caused by the charge trapping and activation near oxygen vacancies. Besides, a non-volatile change of the depletion thickness at the Au/BFO interface has also been observed, which is associated with the resistive switching. A mixed interface-bulk resistance mechanism involving both of thin film bulk resistance and interface barrier was proposed, which is in good agreement with the experimental data.(4)The multi-states resistive switching on BFO thin films was studied, and it indicates that the multiple resistance states were achieved by continuously tuning the resistance of BFO thin film Au/BFO depletion thickness, and each resistance state corresponds to a capacitance state. Furthermore, the CAFM measurement excludes the possibility that the multi-states switching is due to the formations of multiple local conductive filaments, and the basic unit for the resistive switching is the grain of BFO thin films, the resistively switchable grains are randomly distributed, and the distribution homogeneity of those switchable grains can be improved by decreasing the grain size. Finally, it has been shown that the ferroelectricity of BFO thin films does not dominate the resistive switching in Au/BFO/Pt cells by measuring the temperature-dependent I-V and C-V, which further supports the interface-bulk resistance mechanism based on the charge trapping and activation.
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