Preparation and transport properties of Ba1-xSrxCuO2+δ infinite layer films grown by molecular-beam epitaxy

Thin films of Ba1-xSrxCuO2+δ in the infinite layer structure were prepared by molecular beam epitaxy on SrTiO3 substrates. Excellent in-plane order during growth was shown by RHEED. The lattice constant in c-direction was determined by x-ray diffraction. It changed from 0.404 nm to 0.345 nm when x increased from 0 to 1. The film surfaces were smooth with some outgrowths as revealed by atomic force microscopy. Excellent crystal structure and epitaxy of the films was demonstrated by high resolution transmission electron microscopy. The room temperature dc resistivities of the films varied from 10-3 Ωcm to 102 Ωcm, depending on x and on the oxidation conditions during growth. The resistivities of most films showed negative temperature coefficients and obeyed the conduction model of variable range hopping at low temperatures. In the composition range x = 0.5-0.8, however, an anomalous resistance dependence on temperature was observed in many samples. The resistivities started to deviate from the monotonic behaviour just below 200 K and in some cases dropped remarkably at temperatures below 140 K. (orig.)