1 V. LECA 1,2* G. VIŞĂNESCU 1 C. BACK 1 R. KLEINER 1 D. KOELLE 1 Growth mechanism, microstructure and transport properties of Sr 1-x La x CuO 2 (x=0.10-0.15) thin films 1 Physikalisches Institut – Experimentalphysik II, Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen, Germany 2 Faculty of Applied Chemistry and Materials Science, Polytechnic University of Bucharest, Str. Gheorghe Polizu 1-7, 011061 Bucharest, Romania ABSTRACT Sr 1-x La x CuO 2 (x=0.10-0.15) thin films with infinite-layer type structure were grown on BaTiO 3 buffered (001) SrTiO 3 substrates by pulsed laser deposition (PLD). The evolution of the growth front was monitored, in-situ, by high-pressure reflection high-energy electron diffraction (RHEED), while the surface morphology was analyzed by means of atomic force microscopy (AFM), ex-situ. X-ray diffraction (XRD) was used to determine the evolution of the film structure with deposition and cooling parameters, as well as to study the type and level of epitaxial strain in the Sr 1-x La x CuO 2 films. The RHEED data showed that the Sr 1-x La x CuO 2 films grow on BaTiO 3 /SrTiO 3 following a 2D or Stranski- Krastanov mechanism, depending on the La doping level. The transition point (critical thickness d c ) from layer-by-layer like (2D) to island (3D) growth depends on film stoichiometry: decreasing the La doping concentration x from 0.15 to 0.10, the critical thickness d c increases from ~45 nm to ~75 nm. In order to induce superconductivity, the Sr 1-x La x CuO 2 films were cooled down under reduction conditions. The as-deposited films showed semiconducting or metallic behavior, the resistivity decreasing with increasing La concentration. Post-deposition vacuum annealing resulted in a superconducting transition onset (but no zero resistance down to 4.2 K) only for some of the x=0.15 Sr 1-x La x CuO 2 films. PACS 74.25.Fy, 74.78.Bz, 81.15.Fg 1 Introduction Sr 1-x Ln x CuO 2 1 (Ln=La, Nd, Pr, Ga) compounds with an infinite-layer (IL) type structure in which the substitution of Ln for Sr adds electrons to the CuO 2 planes are stable when synthesized by a high- pressure high-temperature method [1-3]. They show n-type superconductivity with maximum T c of 43 K [2, 3]. Substitution of Ln for Sr can reduce the CuO 2 planes without changing the oxygen content. Addition of electrons into the CuO 2 sheets stretches the Cu-O bonds and increases the in-plane cell parameter a, while the replacement of the larger Sr 2+ * E-mail: leca@uni-tuebingen.de by a smaller La 3+ ion decreases the spacing between the adjacent CuO 2 sheets and hence the lattice parameter c. The properties of the high-T c copper oxides are dependent on the Cu-O bond length and the oxygen coordination number of the copper atoms. In the ideal IL structure there is only one Cu-O bond equal to one half of the lattice parameter a. The existence of interstitial oxygen ions in the Sr(Ln) layers, and its influence on the superconductivity in the IL structure is not fully understood, and the reports are contradictory, requiring future investigations. While there is only one oxygen position in the ideal IL structure (in the CuO 2 plane), the incorporation of excess oxygen to the interstitial sites is not restricted. The extra charge of rare-earth doping would be compensated by the oxygen ions in the Sr (Ln) layers and few or no doping electrons would be generated. A reduction atmosphere is used to expel the oxygen ions from the Sr (Ln) layers and only under such conditions superconductivity is considered to arise in the electron-doped IL compounds [1-3]. However, cooling down in low oxygen partial pressure also resulted in superconductivity, as reported for Ln=La thin films grown by PLD [4]. Thin film growth enables the preparation of these phases by considering the epitaxial effect of the substrate [4-7]. PLD is one of the techniques suitable for the preparation of tetragonal Sr 1-x Ln x CuO 2 phases [4, 7-9]. Markert et al. [10] predicted that substrates with in-plane cell parameters of ∼3.99 Å are required in order to provide sufficient extension of the Cu-O bond to promote adequate electron doping in the Sr 1-x Ln x CuO 2 compounds. BaTiO 3 satisfies the cell parameters requirements (a∼4.02 Å, bulk value) and it is chemically compatible with Sr 1-x Ln x CuO 2 . IL films on SrTiO 3 have a smaller in-plane lattice constant a than the bulk value being epitaxially strained, which is considered the main reason for the inferior superconducting properties of the films. In case of films grown on BaTiO 3 buffered SrTiO 3 the CuO 2 planes are expected to stretch and, consequently, electrons can be easily transferred to these planes. In this paper the growth mechanism and properties of Sr 1-x La x CuO 2 (x=0.10-0.15) thin films