Oxygen partial pressure dependences of a–c phase ratio, crystallinity, surface roughness and in-plane orientation in YBCO thin film depositions by IBS

Abstract

YBa2Cu3OX thin films were grown on MgO by ion beam sputtering at various substrate temperatures TS and oxygen partial pressures PO with a supply of either oxygen molecules or plasma (PL) to study fundamental crystal growth of perovskite oxide. Growth of the a-phase dominates at low TS while that of the c-phase dominates at high TS. This indicates that a thermal surface migration is an important mechanism for the a/c orientations. The c-phase dominates at lower PO, and it decreases while the a-phase increases with increasing PO at TS = 600 °C. This is interpreted by a particle energy assisted surface migration controlled by collision between sputtered particles and the supplied oxygen species. The two results indicate that the surface migration is a crucial factor for the a/c orientations. The a-phase growth is enhanced by PL due to two mechanisms, the collision retarded surface migration, and plasma energy assisted atomic rearrangement and surface energy reduction. Intragrain and intergrain crystallinities show a regular correlation of PO dependences at TS = 600 °C, while a PO dependence of surface roughness shows an inverse correlation with the intragrain and intergrain crystallinities. The mixed film can release a strain energy from interior grain to ambient media, then it has good crystallinity but a rough surface. In contrast, the pure single a-phase film has poor crystallinities but the smoothest surface because the strain energy is stored in the interior. In-plane orientation is improved by increasing the a-phase ratio, and in-plane crystallinity can be improved by PL.