Investigation On The Hysteresis Loop And Polarization Stability Of Self-polarized BiFeO₃ Thin Film

Ferroelectric thin film is promising for the applications in piezoelectric and pyroelectric microdevices. In general, the ferroelectric thin films must be polarized to obtain desirable device performance. The cost of the device can be reduced if the ferroelectric thin films are self-polarized. Recently, improving the self-polarization stability becomes a hot topic in the ferroelectric field. Traditional ferroelectric testors have some drawbacks in characterizing self-polarized thin films. The results obtained at low voltages(particularly less than coercive voltage) can not represent the real status of self-polarized thin films. The thesis is focused on the calibration of the hystersis loop, evaluation and improvement of the self-polarization stability of BiFeO3 thin film. The BiFeO3 thin film was prepared by a sol-gel method. A reasonable calibrating method for the hystersis loop measured at low voltages is presented based on the saturation degree of the hystersis loops. The shortest delay time for fully backswitching of ferroelectric domains is obtained by investigating the effect of delay time on the gap in the hystersis loops. Comparing study has been conducted in the voltage sequence applied in test of the hystersis loop. The distribution of defect complexes in the film and its effect on the self-polarization stability is deduced.The ferroelectric hystersis loops obtained after 20 minutes and 24 hours were compared with the original ones. The effect of the direction of the preset voltage on the hystersis loops was studied. According to the coercive voltage and gap of loop, the self-polarization stability was analyzed. The relation between the self-polarization stability and the direction of the preset voltage was explained based on the defect chemistry. A reasonable method for improving the stability of the self-polarization is presented based on the data analysis. In addition, a new concept based on a defect complex gradient is given for further improving the piezo- and pyroelectric properties of ferroeletric thin films.In this thesis, considering that the original status of self-polarization can be changed during the hystersis loop measurement at high voltages, the switching and backswitching behaviors of the self-polarized BiFeO3 thin film under low and high voltages are investigated. A solid base has been set up in theory and experiment for searching the methods to improve the stability of self-polarization in the future.