In-situ Surface X-ray Diffraction Study of Ruddlesden-Popper Series Thin Film Growth

The layered Ruddlesden-Popper phases of A$_{n+1}$B$_{n}$O$_{3n+1}$, such as Sr$_{2}$TiO$_{4}$ and La$_{2}$NiO$_{4}$, have attracted much attention as potential materials for solid-oxide fuel cell cathodes and thermoelectrics. To understand the fundamentals of this class of layered oxide thin films, we studied the growth of (001)-oriented Sr$_{2}$TiO$_{4}$ and La$_{2}$NiO$_{4}$ on SrTiO$_{3}$ substrates by using oxide molecular beam epitaxy with in-situ surface x-ray diffraction. For Sr$_{2}$TiO$_{4}$, the synthesis of the double SrO layer followed by TiO$_{2}$ dynamically reconstructs back into the SrTiO$_{3}$ phase, which demonstrates that during thin film deposition other pathways under growth conditions can give rise to new structural arrangements. In contrast with Sr$_{2}$TiO$_{4}$, the growth of La$_{2}$NiO$_{4}$ involves the stacking of polar LaO$^{+}$ and NiO$_{2}^{-}$ layers. This raises the question of how polarity mismatch at the interface with the SrTiO$_{3}$ substrate will influence the growth process. A detail comparison of these two cases will be discussed. Work at the Advanced Photon Source, Argonne is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.