High-concentration doping effects of aliovalent Al and Ga on ferroelectric properties of BaTiO3 Films

Abstract

BaTiO3 is a promising ferroelectric material with controllable properties through elemental doping. Doping aliovalent elements such as Al3+ and Ga3+ into the Ti4+ sites of BaTiO3 has been also studied, demonstrating the potential enhancement of its ferroelectric properties. However, the doping concentration has been limited to small values. In this study, we significantly expand the doping levels of aliovalent Al3+ and Ga3+ ions on Ti4+ sites in BaTiO3 films up to 20 % using substrate-induced strain effect, and investigate their effects on the structural, ferroelectric, and dielectric properties. Ga 20 %-doped films exhibit room-temperature ferroelectricity with a spontaneous polarization of 20 μC/cm2, closely resembling BaTiO3 (24 μC/cm2). In contrast, ferroelectricity disappears in Al 20 %-doped BaTiO3 films even at 10 K, indicating the importance of size of dopants for maintaining the ferroelectricity. Regarding the structure, Al and Ga doping promote the formation of a locked phase; the observed critical thickness, 20–100 nm, is much thicker than that of non-doped BaTiO3 film, 10 nm. The c/a – 1 ratio in the locked phase reaches 7 %. Because of the coexistence of the relaxed and locked phases, the doped films exhibit flexoelectricity, generating a spontaneous electric field along the out-of-plane direction.