Surface Stress and Lattice Dynamics in Oxide Ultrathin Films

The lattice misfit between the substrate and an epitaxial film leads in general to static forces, which define the interface stress, and dynamic responses that modify the thin-film lattice dynamics. Although these are both fundamental concepts that are important for film growth and thin-film properties, they have not been investigated in a combined way so far. Therefore, herein, surface stress experiments in combination with surface phonon studies for three different, cubic oxide ultrathin film systems are reviewed. Within the class of binary oxides, NiO(001) grown on Ag(001) is chosen, which exhibits a -2.2% lattice mismatch, and BaO(001) on Pt(001), a system with a negligible lattice mismatch. For the ternary oxides, perovskite thin films of BaTiO3 grown epitaxially on Pt(001) with a lattice mismatch of -2.3% are focused upon. The surface stress experiments are conducted with an optical two-beam curvature technique under in situ growth conditions. Surface and thin-film phonons are determined by high-resolution electron energy loss spectroscopy. Surface stress and lattice dynamics are discussed in the range from the oxide monolayer to thin films of about 20 unit cell in thickness.