Atomic force microscopy study of the surface micro structure of YBa2Cu3O7-δ thin films on MgO substrates

The surface microstructure of c-axis oriented epitaxial YBa2Cu3O7-δ-thin films (Tc=88 K, ΔTC = 0.2–1 K, Jc ≥ 2 × 106 A cm−2 at 77 K) deposited on MgO substrates was studied by atomic force microscopy (AFM) operating in air at room temperature. Polished MgO substrates were annealed at 1200°C for 2 h in flowing oxygen and the induced modification on the surface microstructure was detected by AFM. Steps and screw dislocation loops, observed for the substrates annealed at 1200°C, were found to act as preferential nucleation sites for YBa2Cu3O7-δ thin film growth and lead to a controlled growth with a surface roughness lower than 10 Å. We propose a film growth model to describe the controlled epitaxial film growth on the substrates with steps on the surface. Within this model, thin films grown on substrates with surface-steps will have a smoother surface when compared with the films deposited on substrates without steps. This is in agreement with our experimental observations.