YBa 2 Cu 3 O X thin films were grown on MgO by ion beam sputtering at various substrate temperatures T S and oxygen partial pressures P O 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 T S while that of the c-phase dominates at high T S . This indicates that a thermal surface migration is an important mechanism for the a/c orientations. The c-phase dominates at lower P O , and it decreases while the a-phase increases with increasing P O at T S = 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 P O dependences at T S = 600 • C, while a P O 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.

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