Nucleation processes in the growth of hcp titanium

Six distinct orientations of hcp titanium have been grown as high-quality single crystals by molecular-beam epitaxy. The substrates were freshly grown bcc metals on sapphire. We identify four different nucleation mechanisms that compete to select the eventual crystal orientation. In one novel case, the growth of tilted Ti(101{}2) is identified with nucleation at step edges rather than on terraces of the stepped Ta(211) substrate.