NUCLEATION AND GROWTH MECHANISMS OF A,B-AXIS-ORIENTED YBA2CU3O7-DELTA FILMS ON LAALO3

The initial stages of deposition of a,b-axis-oriented YBa2Cu3O7-delta onto LaAlO3 substrates have been examined using transmission-electron microscopy. These films have their c axis parallel to the plane of the substrate throughout their thickness, including at the film-substrate interface: no initial layer with the c axis perpendicular to the plane of the substrate is observed. Nucleation of the c-parallel material is extremely dense, such that the triple perovskite layering along the c axis is sometimes disrupted, leading to structures containing two or four perovskite blocks instead of the usual three. By a film thickness of 8 nm the proper YBa2Cu3O7-delta structure has stabilized, although the domain size is still very small, on the order of four unit cells in the c direction, or 4.7 nm. Two ledge structures are observed in projection on the film surface: (100)/(001) faces and (110)/(013) faces.