|Ph.D Thesis||Department of Materials Science and Engineering|
|Supervisor:||Assoc. Prof. Berger Shlomo|
This work shows, for the first time, ferroelectric behavior in thin films having a thickness below the known critical size.
Microstructure studies of ultra-thin BaTiO3 films (2-10nm thick) show nano ferroelectric domains having a width as small as one unit cell. HRTEM investigation revealed only 180o nano-domains formed in multi-domain structures. The domains are mostly oriented in parallel to the film plane, but few out of plane orientations were also observed. The ferroelectric behavior was characterized as a function of thickness in vertical and parallel to the films plane; extreme polarization anisotropy between the parallel and vertical directions was observed.
The dielectric breakdown voltage of the 2-4 nm thick BTO film under dc electric fields at room temperature was found to be of about 60V (an electric field of 150-300 MV/cm), higher by two orders of magnitude than that reported for BaTiO3 bulk materials.
The values of the leakage current density, fatigue edge and switching time of the nano-domains are comparable to those reported for bulk-size BaTiO3 domains. Two temperature-dependent peaks of dielectric constant were observed; these peaks are attributed to two transition temperatures from the paraelectric to the ferroelectric phase, which depend on the orientation of the domains relative to the film plane and internal stress direction.
Piezoelectric response was detected in ultra-thin BaTiO3 films, having a thickness of 2-4 nm. The piezoelectric coefficient d31 was found to be smaller by one order of magnitude than that reported for the bulk.
A correlation between microstructure, film thickness, film stress and ferroelectric properties is demonstrated and discussed. Thermodynamic criterions for the formation of nano-domains having a one unit cell size are given.