Structural transformations in the Stranski-Krastanov growth of Mg on Mo(001)

The epitaxy of Mg, a hexagonal-closed-packed (hcp) metal in the bulk, onto a room-temperature body-centered-cubic Mo(001) substrate is investigated. A Stranski-Krastanov, layer by layer followed by islands, growth mode is observed including the initial stabilization of a metastable face-centered-cubic (fcc) phase of Mg. Layer by layer growth persists for four layers. During growth of the first and second layer the highly corrugated surface potential and lattice mismatch both act to limit island size, and growth proceeds via nucleation and growth of small two-dimensional islands. Third- and fourth-layer growth proceeds by step flow. The increase in the diffusion constant necessary for step flow may be aided by the presence of a stress relieved c(2x2) surface reconstruction in this regime. The reconstruction may result from a buckling of the Mg lattice to conform to the smaller Mo(001) spacing. Measurement of atomic step heights confirms the growth of fcc Mg(001). Beyond four layers a transition to three-dimensional (3D) island growth occurs. Associated with this transition is a change in crystal structure. The 3D islands exhibit hexagonal symmetry with two domains rotated by 30 degree with respect to one another. Measurement of emergent screw dislocations at the surface of the islands confirms the growth of hcp Mg(0001). Evidence of edge dislocations aligned with steps in the underlying Mo substrate are also observed. copyright 1999 The American Physical Society