All-Epitaxial Ferroelectric Tunnel Junctions with Ultrathin BaTiO3

Ferroelectric tunnel junctions (FTJs) are a promising route toward the development of high density, non volatile memories with non-destructive readout [1]. The principle of operation is polarization-dependent tunneling electroresistance (TER). The direction of polarization in the ferroelectric layer defines high and low resistance states. So far, the most impressive results regarding TER ON/OFF ratios have been either without a top electrode, or using a top electrode of a non-oxide metal. However, defects in the ferroelectric [2] or interfacial layer can reduce performance. To overcome these limitations, we have fabricated fully-strained epitaxial FTJs using perovskite oxides for all layers: La$_{\mathrm{0.67}}$Sr$_{\mathrm{0.33}}$MnO$_{\mathrm{3}}$/BaTiO$_{\mathrm{3}}$/SrRuO$_{\mathrm{3}}$//SrTiO$_{\mathrm{3}}$. The heterostructures are grown by pulsed laser deposition and show high structural quality, sharp interfaces, and very smooth surfaces. Top electrodes are patterned using e-beam lithography. Piezoresponse force microscopy shows that ferroelectricity is maintained for a barrier thickness as low as 3 unit cells. We present our results on TER performance and the dependence of switching properties on the BaTiO$_{\mathrm{3}}$ thickness. [1] V. Garcia, et al. Nature 460, 81-84 (2009). [2] M. Dawber, K. M. Rabe, J.F. Scott, Rev. Mod. Phys. 77, 1083 (2005).