Epitaxy of ultrathin films of YBa 2Cu 3O 7−δ on SrTiO 3(001) investigated with X-ray standing waves

Abstract

Ultrathin films (40 Å and 80 Å) of YBa 2Cu 3O 7−δ were grown on SrTiO 3(001) by molecular beam epitaxy and characterized using the X-ray standing wave technique. The results show that the epitaxy of the superconductor starts on the TiO 2 terminated surface with a perovskite-like BaTi(Cu)O 3 unit cell, i.e. the Ba binds to the TiO 2 surface layer. The next constituent of the thin film is either a BaCuO or YCuO perovskite-like unit cell. This alternative permits smooth overgrowth of SrTiO 3(001) step edges. Strong perpendicular strain ( ε ⊥ = −2.0% ± 0.1%) in the 40 Å films proofs almost pseudomorphic growth whereas the 80 Å film ( ε ⊥ = −0.8% ± 0.1%) has significantly relaxed towards the bulk c-axis length.