Growth and evolution of microstructure of epitaxial YBa 2Cu 3O 7− x ultrathin and thin films on MgO

Abstract

The nucleation and early stages of growth, and the evolution of surface microstructure of epitaxial c-axis YBa 2Cu 3O 7− x thin films are studied as a function of film thickness and growth temperature. The films were grown in situ on polished MgO (100) substrates by unbalanced DC magnetron sputtering at a growth rate of 3 nm/min. We use a combination of high-resolution scanning electron microscopy (SEM), scanning tunneling microscopy (STM), atomic force microscopy (AFM), X-ray diffraction, and transport-property measurements. The results from ultrathin films, 3 to 50 c⊥ unit cells (1 c⊥ unit cell equals 1.17 nm) indicate that epitaxial growth proceeds by the classical screw dislocation mediated three-dimensional island growth mode, i.e. each island grows both vertically and laterally by the incorporation of adatoms at a spirally expanding step emanating from one or more screw dislocations. The initial adatom clustering leads to a high density of nuclei (≈2×10 11 cm −2). The density of screw dislocations, ϱ screw, determined from STM images of the film surface, is found to depend on both the thickness of films and the growth temperature. At moderate growth temperatures (720–760°C) the early stage of coalescence of nuclei and island growth is characterised by a high density of screw dislocations, (3–9)×10 9 cm −2, decreasing to (2–3)×10 9 cm −2 for 200 c⊥ unit-cells films. Higher growth temperatures result in smooth films with a low density of screw dislocations, e.g.≤0.3×10 9 cm −2 at 850°C for 200 c⊥ unit-cells films. A correlation is found between high values of ϱ screw and high values of the critical current density. The ultrathin films exhibit suppression of superconductivity and broadening of the resistive transition with decreasing film thickness, so that T c0=55 K for a 3 c⊥ unit-cells film.