Atomic control of layer-by-layer epitaxial growth onSrTiO3(001):Molecular-dynamics simulations

Molecular-dynamics simulations were performed to clarify the structures of SrO and BaO layers on a ${\mathrm{SrTiO}}_3\mathrm{}\left(001\right)\mathrm{}$ substrate at the atomic level, and to predict an appropriate buffer layer for ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}/{\mathrm{SrTiO}}_3$ heterojunction. The atomic structure of these layers grown on a ${\mathrm{SrTiO}}_3\mathrm{}\left(001\right)\mathrm{}$ substrate terminated on the ${\mathrm{TiO}}_2$ atomic plane was investigated. From the analysis of the angle distribution of Sr-O-Sr and the radial distribution between Sr and O, the first single SrO layer on the ${\mathrm{SrTiO}}_3\mathrm{}\left(001\right)\mathrm{}$ substrate was found to keep a perfect NaCl-type structure. However, the structure of the second SrO layer deviated from a NaCl-type structure. This result suggests that only a single SrO layer is able to grow epitaxially and uniformly on the ${\mathrm{SrTiO}}_3\mathrm{}\left(001\right)\mathrm{}$ substrate terminated on the ${\mathrm{TiO}}_2$ atomic plane. Since a BaO layer is one component of the ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ layered structure, a detailed understanding of the ${\mathrm{B}\mathrm{a}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_3\mathrm{}\left(001\right)\mathrm{}$ heterojunction has been desired. Here, the stress induced by the lattice mismatch of the ${\mathrm{B}\mathrm{a}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_3\mathrm{}\left(001\right)\mathrm{}$ and ${\mathrm{B}\mathrm{a}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_3\mathrm{}\left(001\right)\mathrm{}$ heterojunction was evaluated. The ${\mathrm{B}\mathrm{a}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_3\mathrm{}\left(001\right)\mathrm{}$ gained 1.2 GPa stress, while surprisingly the ${\mathrm{B}\mathrm{a}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_3\mathrm{}\left(001\right)\mathrm{}$ did not have any stress. Moreover, the BaO layer was found to grow epitaxially and uniformly on the ${\mathrm{S}\mathrm{r}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_3\mathrm{}\left(001\right)\mathrm{}.$ Note that ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ is expected to grow epitaxially on a BaO layer since the BaO layer is a part of the ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ layered structure. Hence, we suggest that BaO/SrO is a suitable buffer layer for the ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}/{\mathrm{SrTiO}}_3$ heterojunction.