Thickness-dependent structural phase transition of strained SrRuO3ultrathin films: The role of octahedral tilt

Abstract

We grew epitaxial SrRuO${}_{3}$ (SRO) films on SrTiO${}_{3}$ (STO) (001) substrates with SRO layer thicknesses ($t$) between 10 and 200 pseudocubic unit cells (uc). Using the square net of the cubic STO surface, we were able to epi-stabilize the tetragonal SRO phase at room temperature for ultrathin films with $t$ \ensuremath{\leqslant} 17 uc. On the other hand thicker films with $t$ \ensuremath{\geqslant} 19 uc have an orthorhombic crystal structure similar to that of bulk SRO at room temperature. With increasing temperature, the orthorhombic films undergo a structural transition to the tetragonal phase at ${T}_{\mathrm{OT}}$. The value of ${T}_{\mathrm{OT}}$ and the orthorhombicity factor at room temperature are reduced with decreasing film thickness. We also observed half-order Bragg reflections, indicating that the tetragonal structure arises from the suppression of the tilt angle of RuO${}_{6}$ octahedra. The observed critical thickness around ${t}_{c}$ \ensuremath{\sim} 18 uc is much larger than the recent theoretical prediction (i.e., less than 2 uc) [J. He, A. Borisevich, S. V. Kalinin, S. J. Pennycook, and S. T. Pantelides, Phys. Rev. Lett. 105, 227203 (2010)]. This work thus demonstrates that the lattice symmetry mismatch at the interface plays an important role in determining the structural properties of perovskite films.