First observations of YBaCuO thin films by atomic force microscopy with conducting tips

YBaCuO thin film surfaces have been studied with an original laboratory-made attachment associated with a commercial atomic force microscope. Using a doped silicon probe coated with doped diamond, we have obtained simultaneously topographical and local contact resistance surface images within a given area of the sample. YBaCuO films on various types of substrates were observed: polycrystalline yttria-doped zirconia (PYSZ), and MgO or SrTiO3 single crystals. For YBaCuO films grown on PYSZ, the electrical image has clearly revealed the presence of electrical disconnection zones between grains, which correspond to grain boundary areas observed on the topographical image. The presence of such defects can explain the modest critical current density (Jc approximately equals 3 X 104 A/cm2 at 77 K) measured on these granular films. On the opposite, for films grown on single-crystal substrates, the electrically connected areas between grains are visible on the electrical images, that can be correlated to better electrical transport properties of the films. Moreover, island-shaped grains exhibiting terraces of one unit cell vertical height could be seen, on close inspection of films grown on PYSZ and MgO substrates. For YBaCuO elaborated on SrTiO3 substrates, the observed grain structure rather exhibited a spiral shape.