Structural study of polar MgO(111) epitaxial thin films grown on SrTiO3(111)

We have studied the structures of polar MgO(111) thin films grown on SrTiO${}_{3}$(111) substrates to elucidate the effects of polarity in oxide thin films. MgO(111) films as well as MgO(100) films have been grown by pulsed laser deposition and have been characterized by x-ray diffraction, reflection high-energy electron diffraction, and high-resolution transmission electron microscopy. The lattice expansion up to \ensuremath{\sim}3$%$ and the enhanced structural disorder have been observed in the thin (100 nm) MgO(111) films, indicating an enhanced formation of the $F$ center. With increasing film thickness, the bulk lattice constants have been recovered and the structural disorder has been suppressed. The present findings show that MgO(111) thick films containing a finite structural disorder can be metastably realized.