The growth of c-axis oriented BaTiO$_{3}$ films on Si and Ge for a non-volatile field-effect transistor*

The reorientable polarization of a ferroelectric material can be utilized in a variety of applications, including the development of novel memory devices. Of particular interest is the use of a ferroelectric's spontaneous polarization to maintain ``on'' and ``off'' conductivity states in a field effect transistor. BaTiO$_{3}$ has been proposed as a Pb-free ferroelectric for such an application. Direct coupling of the ferroelectric polarization with the channel of a field effect transistor requires c-axis oriented BaTiO$_{3}$ films to be grown on Si or Ge. However, due to the small thermal expansion coefficients of Si and Ge, BaTiO$_{3}$ films tend to be a-axis oriented, having the polarization lying in the plane of the film. In order to achieve c-axis oriented BaTiO$_{3}$ films, we have developed a graded buffer layer that imparts in-plane compressive strain to overcome the incompatibility in thermal expansion. Ferroelectric, c-axis oriented, BaTiO$_{3}$ films with thicknesses exceeding 120 nm have been achieved. We will discuss the growth and characterization of these films in the development of a non-volatile, ferroelectric transistor.