The Study On Resistive Switching Effect And Its Control Of Oxygen Defect In BNT Films

Recently, ferroelectric thin films based resistive switching(RS) memories show a huge potential application for their simple device structure, excellent scaling capacity, high speed, high resistive ratio, and high reliability. Meanwhile, ferroelectric thin films offer a good opportunity to use in the multi-functionalities memory and in space owing to the great variability in their electrical properties and their good radiation-proof ability. However, the underlying mechanism of the RS is still unclear, which hinders the commercial application. Theoretically, the RS of ferroelectric is based on the intrinsic switching of ferroelectric domains without invoking of charged defect migration. However, experimental data need to be carefully examined as different switching mechanisms, such as the RS induced by oxygen defect, are also possible in these devices for the oxygen defect in ferroelectric oxide is difficult to avoid. Therefore, here, the RS of Bi3.15Nd0.85Ti3O12(BNT) thin films fabricated on Pt/Ti/SiO2/Si(100) substrates by chemical solution deposition technique were studied. The relationship between the RS behavior and the oxygen defect is studied systematically to powerful understand the RS mechanism. Meanwhile, the ways to enhance the RS properties fundamentally were explored in the term of controling the distribution of the oxygen defect. The main contents and results are summerized as following:(1) The density of oxygen defect regulated by the annealing temperature and atmosphere can change the RS mechanism in the BNT polycrystalline thin ?lms.First, the polycrystalline BNT thin films with a few of oxygen and large residual polarization(2Pr = 55 μC/cm2) were prepared using one-time annealing in oxygen. The BNT thin film show a RS effect induced by ferroelectric polarization switching. The ON/OFF ratio of RS is more than 3 orders during the retention capacity measurement of 2×103 s. Second, after the BNT films post-annealed in nitrogen at 700 ℃ for 2 h to increase the oxygen defect, the RS behaviors disappear. Third, the BNT films with medium oxygen defect annealed layer-by-layer in air show a RS induced by the formation/rupture of the conductive ?lament consisting of oxygen defects, where the polarization affect the distribution of the oxygen defect.(2) A ferroelectric/dielectric hetero-junction structure is designed to enhance RS mainly induced by the oxygen defects for the inserted of dielectric can restrain the ferroelectric polarization and its effect on the distribution of the oxygen defect.The Pt/BNT/HfO2/Pt devices were fabricated by inserting the HfO2 dielectric layer propared by pulsed laser deposition in the Pt/BNT/Pt capacitor. Then, the effect of thickness of HfO2 on the ferroeletric polarization of BNT was studied. When the HfO2 is 20 nm, the Pt/BNT/HfO2/Pt devices show no hysteretic polarization, which facilitates the accumulation of oxygen defect in grain boundary to form the conductive ?lament. The ON/OFF ratio of the RS in Pt/BNT/HfO2(20 nm)/Pt enlarged to 4 orders during the retention test of 104 s.(3) The stability of the RS can be improved in amorphous BNT without grain boundary and hysteretic polarization which affect the distribution of oxygen defect.The BNT thin films show amorphous structure with no ferroelectric characteristics below the annealing temperature of 600 ℃. Then, the Pt/amorphous BNT/Pt devices were fabricated based on the BNT films annealed at 400 ℃. Without the grain boundary and hysteretic polarization which affect the distribution of oxygen defect, the RS mainly induced by the formation/rupture of conductive filament for the Pt/amorphous BNT/Pt devices is more stable than in polycrystalline BNT thin films based devices during the test of repeatability and retention. The results demonstrate a possibility to fabricate the RS memories using the ferroeletric thin films by a low cost preparation annealing at low temperature.(4) The unipolar RS can been achieved in the porous BNT thin films for a space electric field concentrated effect which modulates the distribution of oxygen defect.First, the porous BNT thin films were prepared using PEG template into BNT precursor. Then, for a space electric field concentrated effect which make the distribution of oxygen defect more concentrating around the pores, the unipolar RS effect induced by the formation/rupture of conductive filament is observed in the porous BNT thin films. The RS characteristics can be adjusted by the pores and oxygen defects modulated by the annealing temperature and dwell time. The porous BNT thin films annealed at 720 ℃ for 10 min show large RS of 4 orders during retention test of 104 seconds with low operating voltage of 5 V for SET and 1.5 V for RESET. The present study should have the potential applications to develop a RS memory using the porous ferroelectric thin films.