ISLAND SHAPE TRANSITION IN HETEROEPITAXIAL METAL GROWTH ON SQUARE LATTICES

A novel mechanism for ramified island growth in the initial stages of metal heteroepitaxy is reported. Scanning tunneling microscopy measure ments reveal that copper islands on Ni(100), as they grow in size, und ergo a shape transition. Below a critical size of approximate to 480 a toms, compact islands form, while above this size they develop a ramif ied shape. This effect is not of kinetic origin and has been observed in an extended range of growth temperature (250-370 K) and deposition Aux (10(-5)-10(-2) monolayer/s). The shape transition is ascribed to t he island size dependent strain relaxation.