Shape & Structure

1. Growth of Self-Assembled BaTiO3Nanodots using Tensile Strain Timothy A. Morgan, Zhaoquan Zeng, Robert J. Sleezer, Gregory J. Salamo Department of Physics, University of Arkansas, Fayetteville, Arkansas Institute for Nanoscale Materials and Engineering, University of Arkansas, Fayetteville, Arkansas Motivation • Shape & Structure Ferroelectric materials has found application in non-volatile especially FeRAM. Theoretical investigation (Bellaiche, Nature, 2004) of Ferroelectric nanostructures has found the possibility of exploiting polarization vortex states to increase the density of FeRAM to 60 Tbit/in2. The pursuit of creating material to exploit this phenomenon is the goal of this work. • Atomic force microscopy (AFM) revealed dots • Avg. height 1.3 nm, Avg. radius 18 nm, Avg. Volume 3,1457 nm3, Density 5 x 1011 cm-2 • The volume of the dots only accounts for ~1 monolayer deposited • Cross-sectional transmission electron microscopy (XTEM) confirms a layer ~14 monolayers (5.6 nm) thick exists beneath the dots Growth Mechanism • Tensile Strain (+5.5% Mismatch) • BTO stretches laterally • Strain Builds Up • BTO relaxes into islands Growth Approach Molecular Beam Epitax y & Shuttered RHEED • Riber MBE32 • Addon Oxygen Plasma • DCA Titanium Cell • RHEED • kSa BandiT Tempearture Monitoring • Composition • X-ray photoelectron spectroscopy (XPS) revealed BTO stoiciometry to be 1:1:3 for Ba:Ti:O Only Ba & O Cells Open Only Ti & O Cells Open b a BaO layer starts TiO2 layer starts Ti 2p1/2 2p3/2 MgO [100] MgO [110] c d • RHEED: The Transition to Nanodots • Reflection High Energy Electron Diffraction (RHEED) monitoring during layer by layer growth • Kikuchi lines on MgO indicate clean, flat surface (Figures a, b) • Spotty RHEED pattern at 15 monolayers of BTO growth indicates nanodot formation (Figures c,d) BTO [110] BTO [100] Ba 3d5/2 O 1s Summary • Molecul;ar beam epitaxy has been used to grow self-assembled barium titanate (BTO) nanodots on MgO (001). This is shown by: • RHEED confirms the tensile strain allows the BaTiO3 to transition from a two dimensional (2D) film to three dimensional (3D) islands • AFM confirms BTO nanodots and reveals an average height and diameter of 1.3 nm and 36 nm, respectively • XPS confirmed the stoichiometry of the grown material as 1:1:3 for Ba:Ti:O • XTEM revealed a layer between 4-6 nm thick lies beneath the dots • ACKNOWLEDGEMENTS: Department of the Army, Army Research Lab, Contract No: W911NF-08-2-0006