Paper SS1-ThM9
In-Situ Polarization and Structure of 4 and 10 Layer Epitaxial BaTiO₃ Films

Thursday, October 18, 2007, 10:40 am, Room 608

Ultrathin films of ferroelectric materials lose their ferroelectric properties due to the depolarizing field opposing the polarization. It is important to understand the limits and conditions limiting nanoscale ferroelectrics, which are being considered for a number of sensors, memories, and transistor devices. For BaTiO₃, ferroelectricity has previously been observed experimentally down to 12 layers and predicted by first-principles calculations in 6 layer films. Film environment and interfaces play a critical role in ferroelectric properties. We have grown BaTiO3 ultra-thin films SrRuO₃/SrTiO₃ using laser-MBE in high oxygen pressures (10 mTorr). The large 2.3% lattice mismatch in this system requires careful choice of growth conditions. For ultrathin films, RHEED oscillations and patterns during growth show that layer-by-layer growth of flat, highly strained films is possible. We report in-situ, ultrahigh vacuum characterization of epitaxial films using low energy electron diffraction (LEED I-V) and scanning tunneling spectroscopy (STS). Films produce sharp (1x1) LEED patterns, indicating a well-ordered tetragonal phase structure. Comparison of observed diffraction intensities for 4 and 10 layer films at 130 and 300 K with calculated intensities reveals a vertical displacement of the central Ti, corresponding to a polarization consistent with compressive strain. Structures and polarization change dramatically after exposure to small quantities of water. STS shows discontinuous jumps at +/- 2.5 V that may indicate polarization switching. Research was sponsored by the Division of Materials Sciences and Engineering and the Center for Nanophase Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.