REACTIVE CODEPOSITION OF INSITU Y-BA-CU-O SUPERCONDUCTING FILMS USING DILUTE MIXTURES OF OZONE IN OXYGEN

This investigation reports the effect of dilute additions of ozone in molecular oxygen on the limiting reactive gas pressures required for in situ deposition of YBa2Cu3O7-x thin films. Thermodynamic analysis indicates that the high oxidizing potential of ozone assures stability at equilibrium under virtually all experimental conditions of ozone pressure and substrate temperature. Thus the generation of a minor fraction of ozone in oxygen enables in situ deposition to be carried out without further enrichment at gas pressures well below those required in pure molecular oxygen. In situ superconductor deposition at various rates using coevaporated metals revealed a lower limit at an ozone-to-copper ratio of approximately unity. Superconducting films deposited at O3/Cu flux ratios above this threshold were highly (00l) oriented with critical temperatures of 87-90 K and critical currents of 2 x 10(6) A/cm2 that exhibited only small or ambiguous trends with increasing ozone above the threshold. Films deposited at O3/Cu ratios below the threshold were visually dissimilar (transparent-brown), semiconducting, and marked by conspicuous disappearance of the characteristic x-ray spectra. This threshold is a nonthermodynamic limit below which the supply of ozone is insufficient to maintain stoichiometry during continuous deposition. Since ozone molecules donate only single oxygen atoms in reaction, the observed limiting O3/Cu flux ratio supplies no more than one-half the total oxygen required to form tetragonal YBa2Cu3O6, which is the entry level phase for in situ deposition. It follows that molecular oxygen continues to function as a principal oxidant in reactive deposition even at reduced operating pressures given the stabilizing presence of ozone at a sufficient flux to elevate copper to its higher oxidation state.