MOLECULAR-BEAM EPITAXY OF BI2SR2CUOX AND BI2SR2CA0.85SR0.15CU2OX ULTRA THIN-FILMS AT 300-DEGREES-C

Molecular beam epitaxy of ultra thin films of Bi2Sr2CuO6 (2201 phase) and Bi2Sr2(Ca.85Sr0.15)Cu2O8 (2212 phase) is realized on SrTiO3(100) at a substrate temperature of 300-degrees-C, using 10(-5) Pa of NO2 as an oxidant. The structure of the ultra thin film formed is confirmed by X-ray diffraction. An in-situ RHEED (reflection high energy electron diffraction) study reveals that the film formed has the same in-plane lattice constant as that of the SrTiO3(100) substrate, namely the a and b axes are 0.39 nm. Because the a and b axes are lengthened approximately 3%, the c axis of the ultra thin 2201 film was 2.41 nm, shorter than the value for the bulk material. The crystallinity of the film strongly depended on the sequence of the oxidation process, revealing that the elementary unit of the epitaxial growth is the sub-unit of the perovskite structure. In the Bi cuprate series, Sr-Cu-Sr is the elementary unit. As a result, the sequential supply of Bi, Sr, Cu and the oxidant NO2 is necessary to realize low temperature epitaxy.