Overlapping growth windows to build complex oxide superlattices

Abstract

Perovskite oxide superlattices are of particular interest due to novel phenomena emerging at interfaces which are beyond the bulk properties of the constituent layers. However, building perovskite superlattices comprised of stoichiometric layers with sharp interfaces has proven challenging. Here, the synthesis of a series of high quality (SrTiO3)n/(CaTiO3)n superlattice structures grown on LSAT substrates is demonstrated by employing hybrid molecular beam epitaxy, where Ti was supplied using metal-organic titanium tetraisopropoxide (TTIP), and Sr and Ca were supplied using conventional effusion cells. By careful adjustment of the cation fluxes of Sr and Ca with respect to the TTIP flux, the growth windows of SrTiO3 and CaTiO3 were overlapped, allowing us to grow the individual superlattice layers with self-regulated stoichiometry. Stable and repeatable reflection high-energy electron diffraction oscillations during the entire ∼2.5 h growth period indicated good source flux stability. The structural quality of the superlattice films were determined by scanning transmission electron microscopy and synchrotron-based X-ray diffraction, revealing periodic, phase pure, homogenous superlattice structures with abrupt interfaces. Utilization of perovskite stoichiometric growth windows offers great potential for accessing and realizing interface driven phenomena in versatile perovskite superlattice materials with chemistries beyond titanates.