Atomic force microscopy observation of Bi₂Sr₂CaCu₂O_8+δ thin films prepared by molecular beam epitaxy

Abstract

We observed the surface structures of 1200 Å-thick c-axis oriented Bi₂Sr₂CaCu₂O_8+δ thin films, which we intend to use for the base electrode of a planar type Josephson junction. The films were prepared by molecular beam epitaxy on the Nd:YAlO₃(001)substrates at a growth rate of 20 Å/min.Atomic force microscopy observation revealed that the surface of the films prepared at a substrate temperature of 780 °Cand an ozone pressure of 2.3 × 10⁻³ Pahas island-like structures. Their shape is like a stack of cylindrical plates and the diameter of the island is typically 250 nm.Most of the steps have a height of 15 ∼ 16 Å,which corresponds to the primitive-cell thickness of Bi₂Sr₂CaCu₂O8+δ.We discuss the result in comparison with that observed in Bi₂Sr₂Ca₂Cu₃O_10+δ films grown by metal organic chemical vapor deposition and YBa₂Cu₃O_7−δ films by sputtering. The occurrence of the structure with several steps is attributed to the insufficient supply of elements. The roughness less than 100 Åis favorable for the fabrication of junctions with thin barriers.