If useful electronic devices are to be fabricated from thin films of the high critical temperature oxide superconductors, such as yttrium barium copper oxide (YBCO), then films must be formed on substrates with dielectric properties which permit device operation at high frequencies, such as sapphire. It is, however, difficult to form high quality YBCO on sapphire because of reaction with the substrate and because epitaxy is not readily achieved.

We report the deposition of epitaxial thin films of magnesium oxide, by UHV reactive d.c. magnetron sputtering, on variously orientated substrates of sapphire, and the characterisation of the MgO/sapphire system by x-ray and TEM techniques.

Onto such films of MgO on sapphire we have deposited films of YBCO which were strongly textured with the c-axis perpendicular to the plane of the substrate and which were superconducting without the need for a high temperature post deposition anneal.