Investigation of Ferroelectric Nanodots for Memory Applications

1. Investigation of Ferroelectric Nanodots for Memory Applications Timothy A. Morgan, ZhaoquanZeng, Greg Salamo Motivation Ferroelectric nanodots are an exciting material to investigate for memory applications. FeRAM has been shown to have advantages over traditional DRAM through data retention, power reduction and quicker access times. Improving properties such as fatigue and density are goals for FeRAM to improve. Theoretical work by Bellaiche et al have shown the possibility of increasing FeRAM density to 60 Tb/in2 through a vortex phase in Pb(Zr,Ti)O3nanodots that have bistable states switchable through static inhomogeneous electric fields or time varying magnetic fields1,2. Experimentally, investigating ferroelectric nanodots to realize the vortex phase and determining the switching characteristics is the goal of this research. BTO Dot Formation Technique Substrate Selection Examining commercially available substrates for appropriate mismatch is necessary. Understanding the crystal structure and its symmetry of every substrate was essential in undrestanding what plane was required to grow on. The goal was finding a cubic or tetragonal unit cell with a lattice parameter having a ~5-7% mismatch. Preparing the substrate for growing BTO is essential. BaTiO3 Oxide Substrate Tensile Compressive Tensile The Stranski-Krastanov growth mode is used to form self-assembled ferroelectric nanodots. The key is choosing appropriate substrates so that strain will build up due to the lattice mismatch and form 3D islands (nanodots). The MBE technique is utilized to control the deposition of BTO on different oxide substrates. YAlO3 (220) LaAlO3 (110) • Summary • Preliminary growths indicate BTO dot formation is possible • Optimize substrate surfaces for BTO dot growth • Optimizie growth conditions for BTO dots • Examine ferroelectric properties Substrate Preparation Obtainining a smooth surface on our substrate is our goal. We have worked on preparing MgO through annealing with oxygen flow. Surface analysis consisted of examining the roughness and composition. MBE Growth of Ferroelectric Nanodots Riber 32 MBE System Shuttered RHEED Technique MgO @ 700° C 12 hrs Growing a monolayer at a time is important for perovskite structures (materials with an ABO3 chemical formula and O in a FCC) just like superlattices in III-V semiconductors. We grow alternating layers of BaO and TiO2 for high quality material. MgO @ 850° C 12 hrs • MBE Capabilities • High purity materials • Monolayer control of deposited material • Substrate temperature variation • High crystal quality • RHEED monitoring • Acknowledgements • Rob Sleezer • David Monk • Morgan Ware • XPS Spectra • Mg & O peaks • Ca defects MgO @ 1100° C 12 hrs 1. Naumov, I. I., Bellaiche, L., and Fu, H., Nature432 (7018), 737 (2004) 2. Naumov, I., Bellaiche, L., Prosandeev, S., Ponomareva, I., Kornev, I., United States of America Patent No. US 2008/0130346 A1 (2008). STO (100) substrate BaO layer on STO (100) TiO2 layer on STO (100)