Observation of self poling in BaTiO3 thin films

The dependence of ferroelectric properties of sol-gel derived BaTiO3 thin ( 0.35 mu m) films on various Si substrates was investigated. It was found th at only films deposited on p(+) Si (10(20) cm(-3) B, 2% Ge) are (110) orien ted and possess a few percent of the spontaneous polarization of single-cry stal BaTiO3. The direction of polarization self restored after being revers ed by external bias or heated above the Curie temperature. It was found tha t the pyroelectric coefficient was not zero above the Curie point, which in dicated that the films are permanently subjected to an electric field, whic h originates from the contact potential between the film and the substrate. This conclusion was confirmed by surface photovoltage spectroscopy. Orient ation of the films was attributed to the presence of Ge in the substrate, w hich decreased lattice mismatch between the (100) vector of the Si substrat e and (110) vector of BaTiO3. The possibility of a paraelectric to ferroele ctric transition driven by an electric field due to the contact potential d ifference must be taken into account for dynamic random access memory appli cations. (C) 1999 American Institute of Physics. [S0021-8979(99)00708-2].