Metastable misfit-induced relaxation structures of close-packed heteroepitaxial films

Conventional and O-mediated Cu film growth on Ru(0001) is studied by means of STM and LEED at temperatures between 300 and 550 K. At temperatures above 500 K and for a Cu film thickness above three monolayers, misfit-induced strain is almost completely released via a slight rotation of the film lattice inducing a stable moiré-like reconstruction. At lower temperatures, two metastable relaxation structures are observed belonging to a non-rotated and slightly strained Cu film. In cases where the relaxation proceeds more locally, a tile-like honeycomb pattern of domains aligned along [112]-like directions is developed (unit mesh 60 Å). Large-scale relaxation gives rise to a hexagonal pattern of triangular domains (side length 25 Å) aligned along [110]-like directions (unit mesh 64 Å). In both structures energetically unfavorable on-top positions are avoided by forming domain boundaries and dislocations. The structures are elucidated by simple hard-sphere models.