C-AXIS EXPANSION IN RF-SPUTTERED YBA2CU3OX THIN-FILMS

The c-axis lattice parameter of YBa2Cu3Ox thin films deposited on SrTiO3(100) and MgO(100) has been carefully measured and compared with the critical temperature and normalized resistance of a number of films. The films were prepared in situ using RF magnetron off-axis sputter deposition in the high-pressure regime. The T(c) dependence on the c-axis expansion has previously been reported to be much weaker in thin films than in bulk material and considered not to be entirely due lo 0 deficiency in the chain sites. However, the change in c is less than 1% and conflicting results have previously been obtained. By comparing x-ray diffraction peak shifts relative to substrate peaks and also detailed analysis of (001) data we have obtained unambiguous evidence for the c-axis expansion. We find that the T(c) versus c-axis expansion tends to follow bulk values for films on MgO(100) but for films sputtered onto SrTiO3(100) the T(c) is fairly constant with c-axis expansion. In the latter films we also find a-axis-orientated grains, which account for higher resistance values and the shape of the R/T curves. The stability of T(c) with c-axis expansion is considered to be partly due to the presence of the a-axis-orientated grains.