Critical layer thickness in Stranski-Krastanow growth of Ge on Si(001)

The physical factors that control the critical wetting layer thickness in Stranski-Krastanow growth have remained unresolved. Here, we report results of density-functional calculations that demonstrate a key role played by the surface reconstruction of the initial layers. We show that, in the prototype Ge/Si(001) system, the assumption that the (2 x 1) reconstruction of the Si(001) substrate is preserved during the growth leads to an underestimation of h_c. Proper inclusion of (2 x N) reconstruction and buckling as stress-relieving mechanisms, however, leads to delayed islanding with h_c equal to the experimental value of three monolayers