TEM STUDY OF EPITAXIAL YBA2CU3O7-X THIN-FILMS DEPOSITED ON GAAS WITH MGO BUFFER LAYERS

Epitaxial, c-oriented YBa2Cu3O7-x (YBCO) thin films yielding large critical transport currents (j(c)>1.2 x 10(6) 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-degrees-C whereas the YBa2Cu3O7-x was deposited at 620-degrees-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 parallel-to < 100 > GaAs orientational relationship. the GaAs-MgO 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 MgO-YBa2Cu3O7-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.