Oscillation of surface in-plane lattice spacing during epitaxial growth of BaTiO3 and SrTiO3 on SrTiO3(1 0 0)

In-plane lattice spacing oscillation was observed as functions of surface coverage and growing film thickness by reflection high-energy electron diffraction during the growth of strained BaTiO3 and unstrained SrTiO3 epitaxial layers on SrTiO3(1 0 0). For BaTiO3/SrTiO3, the oscillation continued until the critical thickness is reached, whereas for SrTiO3/SrTiO3, oscillation was observed only at the initial few monolayer growth. The origin of oscillations is discussed in relation to the non-tetragonal elastic distortion occurring at the free edges of 2D single monolayer islands. The amplitude of the oscillation strongly depended on the island size and density.