Observation of self poling in BaTiO₃ thin films

The dependence of ferroelectric properties of sol-gel derived BaTiO₃ thin (0.35 μm) films on various Si substrates was investigated. It was found that only films deposited on p⁺ Si (10²⁰cm^-3 B, 2% Ge) are (110) oriented and possess a few percent of the spontaneous polarization of single-crystal BaTiO₃. The direction of polarization self restored after being reversed by external bias or heated above the Curie temperature. It was found that the pyroelectric coefficient was not zero above the Curie point, which indicated that the films are permanently subjected to an electric field, which originates from the contact potential between the film and the substrate. This conclusion was confirmed by surface photovoltage spectroscopy. Orientation of the films was attributed to the presence of Ge in the substrate, which decreased lattice mismatch between the (100) vector of the Si substrate and (110) vector of BaTiO₃. The possibility of a paraelectric to ferroelectric transition driven by an electric field due to the contact potential difference must be taken into account for dynamic random access memory applications.