Change of growth orientation in Pt films epitaxially grown on MgO(001) substrates by sputtering

A drastic change of the growth orientation in epitaxial Pt films grown by sputtering on MgO(001) was observed. At higher substrate temperatures, practically pure (001) epitaxial Pt films grow. On the other hand, epitaxial (111) Pt films grow at lower substrate temperatures. Interestingly, the gradual transition from (111) to (001) orientation occurs at lower temperatures when applied at a lower deposition rate. The degree of supersaturation in growth conditions is proposed as a key driver of the orientation change. When homogeneous nucleation occurs under a higher supersaturation, a large number of nuclei grow at the flat (001) terraces possessing (111) tetrahedral orientation. Under a lower supersaturation, a small number of nuclei aligned in (001) orientation to each other form dominantly at surface defects, resulting in a (001) epitaxial film with no grains. Our results suggest that one can selectively prepare either (001) or (111) epitaxial Pt films by properly adjusting substrate temperature and/or deposition rate.