Synthesis And Photoelectrical Properties Of Bismuth Titanate Ferroelectric Thin Films

As one of important functional materials, ferroelectric materials have widely used in the areasof microelectronics, photoelectronics and MEMS systems. Among them, bismuth layeredferroelectric materials, which have good physical properties, such as ferroelectric, optoelectric,dielectric and piezoelectric properties, have been focused on by many groups. Especially, moreand more work has been done on the photoelectrical properties of ferroelectric materials.As a typical lead-free and environment-friendly ferroelectric material, Bi4Ti3O12(BTO) hasattracted much attention. Previous studies was mainly focused on ferroelectric, dielectric andphotoluminescence properties of BTO, photovoltaic properties about BTO film has been seldomreported. In this manuscript, photovoltaic properties of undoped BTO film and the La doped BTO(BLTO) one were investigated. The main research contents are listed as follows:1. BTO targets were successfully sintered at about900oC by a conventional solid statereaction method and then the pure phase BTO film was deposited on FTO glass substrate by thePLD method. XRD pattern of the BTO film is almost the same with that of the BTO target.Piezoelectric force microscopy (PFM) confirmed the existence of ferroelectric properties in boththe target and the film. From the J-V curves, photoconductivity of the BTO film was observed. Itis suggested that the difference of Schottky barrier in the two interfaces derived from the differentwork functions between FTO and Pt are the major cause for the photoconductivity.2. BTO thin film was successfully grown on the FTO glass substrate by a sol-gel method. TheJ-V characteristic curve demonstrates there is a good photovoltaic effect in BTO film, and themaximum conversion efficiency of0.00026%. It is proposed that the contribution of polarizationto the photovoltaic effect is negligible, the Schottky barrier is the main driving force for theseparation of electron-hole pairs. The ON-OFF curves show the photocurrent increase with lightintensity, while the photocurrent density keeps almost constant with the extension of time, indicating that the thermal effect of light is almost negligible. Our experimental results show theBTO film has great potential application in the field of photoelectric device.3. La doped BTO thin film was grown on the FTO glass substrate by a sol-gel method. Thecrystal structure of BTO did not almost change after La substitution. Compared to BTO, theferroelectric properties of BLTO film was improved. It is observed that the remnant polarizationand coercive field of the films are gradually increased with the increase of the applied field. Fromthe J-V curves under the light illumination, it is found that the open-circuit voltage andshort-circuit current are approximately0.08V and700nA/cm2, respectively. The calculatedmaximum power efficiency is about0.00135%, which is50times higher than that of undopedBTO (0.00026%). The doping of La leads to the reduction of concentration of oxygen vacancies,improving the macroscopic ferroelectric properties of the BLTO film. Meanwhile, thedepolarization field of the film increase. It is proposed that the driving force of thephoto-generated carriers come from the cooperative action of depolarization field and interfaceSchottky barriers.