Crystal Structure and Orientation in Thin Films

IN crystal growth, the well-known phenomenon of pseudomorphism extends to all three dimensions. Recently we have found that thin films of aluminium on platinum, of zinc oxide on zinc and of magnesium oxide on magnesium possess abnormal crystal structures. For example, aluminium, normally of face-centred cubic structure (a = 4.05 A.), when deposited as a sufficiently thin layer on face-centred cubic platinum (a = 3.91 A.) acquired a face-centred tetragonal structure with basal axis 3.90 A. and major axis 4.02 A. Thus, in its basal dimensions the aluminium was pseudomorphic with the platinum substrate. A similar effect was observed in the case of a thin film of zinc oxide on zinc (triangular close-packed lattice, a = 2.685 ± 0.002 A.; c = 1.86). In this case, whilst the basal dimensions of the zinc oxide were similar to those of the zinc substrate, the major axis had increased from 5.178 A. (normal zinc oxide) to 6.873 ± 0.002 A. Thus, the volume of the abnormal zinc oxide cell, basally pseudomorphic with normal zinc, was nearly equal to that of the normal zinc oxide cell.