Bulk and surface structure of epitaxial YBa 2Cu 3O 7−δ thin films grown on Mg 2TiO 4 (001) substrate layers Part I: Bulk structure investigated by TEM

Abstract

Epitaxial YBa 2Cu 3O 7−δ thin films have been grown by pulsed laser deposition onto single crystal Mg 2TiO 4 (001) substrate layers. The Mg 2TiO 4 layers have been prepared by a topotaxial solid-state reaction between MgO (001) single crystal surfaces and a TiO 2 vapour. The latter was obtained by electron beam evaporation of anatase powder targets. The Mg 2TiO 4 layers have been shown to be genuinely single crystalline, well-oriented and free of defects except cationic antiphase boundaries forming a regular network. The bulk structure of the YBa 2Cu 3O 7−δ films grown under various growth conditions has been investigated by transmission electron microscopy and selected area electron diffraction. Films grown under non-optimum conditions displayed a non-homogeneous bulk structure characterized by film regions of different orientation and by a high density of droplets. Under optimum growth conditions the YBa 2Cu 3O 7−δ films are epitaxially oriented throughout, with the [001] axis perpendicular to the film plane and with good in-plane alignment. The number of droplets is low in these films, which are superconductive at T c(onset) ≈ 88.5 K. These films also showed thickness variations, which can be attributed to the spiral growth hills detected by STM and described in part II.