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 (Ferrolecetric random access memory) has been shown to have advantages over traditional DRAM (dynamic random access memory) 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 every substrate’s crystal structure and symmetry is essential in undrestanding what plane is required to grow on. The goalis finding a cubic or tetragonal unit cell with a lattice parameter having a ~5-7% mismatch. Beyond finding the appropriate plane, examining the substrate surface for growth ensued. BaTiO3 Oxide Substrate Tensile Compressive Tensile The Stranski-Krastanov growth mode forms self-assembled nanodots due to strain built up from lattice mismatch to the substrate. Barium titanate (BTO) is deposited onto an oxide substrate utilizing the molecular beam epitaxy (MBE) technique. Dots can form from both compressive (negative mismatch) and tensile (positive mismatch) strain applied from an appropriate substrate. YAlO3 (220) LaAlO3 (110) • Summary • Preliminary growths indicate BTO dot formation • Optimize substrate surfaces for BTO dot growth • Optimizie BTO dot growth conditions • 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 impurities 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)