Molecular Beam Epitaxy and Microstructural Study of La(2-x)Sr(1+x)Cu(2)O(6+y) Thin Films

Thin films of La(2)SrCu(2)O(6+y) with varying Sr/La ratios have been grown by MBE on (001) SrTiO(3); their structural as well as their transport properties are reported. Under appropriate conditions, thin films with a metal-like resistivity down to 8 K are obtained, i.e. these films are closer to the onset of superconductivity than previous results indicate for bulk samples. This observation might be explained by a thin-film-specific alteration in site occupation of the La and Sr atoms in the lattice. In addition, the defect structures of a typical sample have been studied by transmission electron microscopy and the thin-film/substrate interface structure is determined. Out-of-phase boundaries with displacement vectors R = [00 1/5] occur throughout the entire film thickness. Such interfaces, which possibly originate at substrate surface steps, disconnect the CuO(2) layers and negatively influence eventual superconductivity. The films contain copper oxide outgrowths and nano-sized a-axis grains at the surface. Locally a 2(2**1/2)a(p) x 2(2**1/2)a(p) modulation is observed; we have determined its unit cell and compared it with other so-called 2(2**1/2)a(p) x 2(2**1/2)a(p) structures occurring in other superconducting compounds such as YBa(2)Cu(3)O(7-d) and Bi(1.8)Pb(0.4)Sr(2)Ca(2)Cu(3)O(10+x).