A systematic approach to reduce macroscopic defects in c-axis oriented YBCO films

Abstract

c-axis oriented YBCO films with T c=90 K and low macroscopic defect density were grown reproducibly on STO, LAO and YSZ/Al 2O 3 with a rf-sputtering system using a p ressure controlled s elf- t emplate (PST) process under optimized conditions. We show how the hole formation in YBCO films is prevented and the target lifetime enhanced with a proper adjustment of the rf-power and the deliberate adding of H 2O molecules to the sputtering gas. Variation of the oxygen pressure demonstrates that at low pressure YBCO films with T c=85 K and a smoother surface are grown, while at high pressure films with T c=90 K and a rough surface due to CuO particles are obtained. The benefits of both pressure regimes are merged in the PST process where the growth starts at low oxygen pressure. After the growth of a few unit cells the pressure is increased and stabilized until the end of growth resulting in YBCO films with T c=90 K and a significant smoother surface. We conclude that nucleation sites of CuO are located only on the substrate surface and that it is a necessity to grow YBCO in the stability region of CuO to obtain films with T c=90 K.