Solid solutions based on the perovskite ferroelectrics Bi₀.₅Na₀.₅TiO₃ (BNT) and Bi₀.₅K₀.₅TiO₃ (BKT) might someday replace current Pb-based ferroelectric and piezoelectric devices. This is one goal of the Restrictions on Hazardous Substances (RoHS) guidelines seeking to limit Pb in consumer devices. Although the Bi-based ferroelectrics are well suited to the task for their overall high Curie temperature and good piezoelectric properties, there are still questions about reliability. The primary goal of this research was to advance the understanding of long term reliability in polycrystalline BNT-BKT thin films fabricated by chemical solution deposition (CSD). The constituent cations are highly volatile at the crystallization temperatures, and oxygen vacancies are common to all oxide perovskites. The resulting defects are associated with higher leakage currents, which can reduce long term stability by increasing the frequency of early failures due to localized breakdown events. Research focused on several topics related to electronic and ionic conduction in BNT-BKT thin films.
Mn-doping is a well known technique utilized to decrease electronic current in many perovskite ferroelectrics. A study of the steady state leakage current in Mn-doped 80BNT-20BKT films was performed, including 0 up to 2 mol% Mn. Space charge-limited conduction was found in all films, although the onset of strong injection increased with dopant concentration. The 2 mol% Mn films showed only Ohmic conduction beyond 180 °C and 400 kV/cm. Additionally, ionic conduction processes also play a role
in fatigue and resistance degradation. High temperature transient currents are believed to be directly related to ionic migration. Analysis of these peaks reveal activation energies and mobilities consistent with migration of oxygen vacancies in the 80BNT-20BKT films. However, the transients for Mn-doped films displayed some unusual characteristic of very short, temperature insensitive transient times. This may indicate that a different mechanism is operating in those films.
The binary system (100–x)BNT-xBKT for x=10, 20, 30, and 40 was studied for the steady state leakage current and ionic transport properties. Leakage current decreased dramatically on moving from the rhombohedral x=10~20 films to the tetragonal x=30~40 films. Other correlations included some evidence of incipient texturing of the tetragonal films, as well as decreasing roughness with larger x. The ionic transport properties were again measured in these films. They appear to show a decrease in the mobility of oxygen vacancies with increasing x. This may have implications for improving reliability in the future.
The effects of post-annealing at different oxygen partial pressures (pO₂) was attempted in order to determine the majority electronic carrier type. All films tested, which included 80BNT-20BKT, 95(80BNT-20BKT)-5Bi(Ni₀.₅Ti₀.₅)O₃, and 95(80BNT-20BKT)-5BaTiO₃, produced results consistent with n-type conduction. Also, while the current is generally space charge-limited, there are several key features of the current density-electric field characteristics which were affected by the post-anneal pO₂. Information on the trap density and distribution and the carrier density can be gleaned from analyzing these curves.