Properties of Bi2+xSr2-xCuO6+delta thin films obtained by MBE

We report on the fabrication of epitaxial, c-axis oriented Bi(2+x)Sr(2-x)CuOb(6+delta) (2201) thin films deposited by molecular beam epitaxy on MgO and SrTiO3 substrates, using co-evaporation method, reflection high energy electron diffraction (RHEED) has been employed to monitor the surface structure of the samples during the growth. The morphology and the electronic properties of the films surface have been analyzed after the deposition by scanning tunneling microscopy (STM) at room temperature in ultra high vacuum conditions. The presence of well defined strikes in the RHEED patterns indicates a 2D nucleation growth, confirmed by STM analysis which shows very smooth surfaces on micron scales with the presence of half or one unit cell high terraces. STM spectroscopic measurements shows that the 2201 terminating layer is formed by BiO plane. The RHEED patterns show the presence of a superstructure along the b direction, due to an excess of oxygen in the BiO surface layer, whose periodicity is about twice that usually observed for BSCCO compound. The dependence of the c-axis length, obtained by X-ray diffraction patterns, has been studied as a function of the sample stoichiometry. It has been observed that a change in the Bi/Sr ratio strongly influence both the c-axis length and the electrical properties of the samples. Superconductivity has been detected in the samples with a little excess of Bi respect to Sr independently on the oxygenation conditions.