Pore formation in YBCO films deposited by a large-area pulsed laser deposition system

The surface morphology of YBCO films deposited by a large-area PLD system is characterized by pores of varying depths and sizes. The presence of these pores has been correlated to the yttrium-rich composition in the films. In-depth studies on the interplay of parameters including oxygen background pressure, target-substrate distance, film composition, and target stoichiometry on the porosity of the YBCO films deposited on LAO (100) substrates have been conducted. Off-stoichiometry in the composition of the films has been attributed to the scattering of the ablated species with the oxygen ambient. The relative excess of yttrium in the films causes the segregation of yttrium-rich secondary phases, one of which has been identified for the first time as barium yttrium oxide (BaY2O4). From structural characterizations, it has been found that the nucleation and enlargement of the barium yttrium oxide phase obstructs the YBCO growth and eventually causes the pores in the film to develop. Considering the deposition conditions which either promote or hinder the formation of this secondary phase during YBCO growth, a reasonably good control of pore size and density has been achieved.