TEM study of epitaxial YBa₂Cu₃O_7-x thin films deposited on GaAs with MgO buffer layers

Epitaxial, c-oriented YBa₂Cu₃O_7-x (YBCO) thin films yielding large critical transport currents (j_c > 1.2×10⁶ A/cm² at 77 K) were deposited on GaAs using MgO buffer layers. The MgO was deposited by electron gun evaporation at a substrate temperature of 460°C whereas the YBa₂Cu₃O_7-x was deposited at 620°C by reactive thermal coevaporation. The low deposition temperatures which can be applied for the thermal co-evaporation deposition to obtain high-quality YBCO thin films, are particularly important for the deposition on GaAs substrates. High-resolution transmission electron microscopy was used to image the various interfaces and to investigate the quality of the deposited films. In spite of a significant mismatch between MgO and GaAs (a_MgO=0.4213 nm and a_GaAs=0.565 nm) the MgO film grows in single orientation with a 〉100〈_MgO∥〉100〈_GaAs orientational relationship. the GaAsMgO interface was very smooth and had a roughness of 1-2 nm. The MgO buffer layer was significantly strained and contained planar defects parallel to the (100) plane. The MgOYBa₂Cu₃O_7-x interface was surprisingly smooth and planar. In the c-oriented film, stacking faults, dislocations and a-oriented areas a few nm in size were found.