STRAIN RELIEF VIA ISLAND RAMIFICATION IN SUBMONOLAYER HETEROEPITAXY

The mechanisms of strain relief in submonolayer heteroepitaxy of Cu/Ni (100) are studied using variable temperature scanning tunneling micros copy and high resolution low energy electron diffraction. It is demons trated that pseudomorphic copper islands, as they grow in size, underg o a spontaneous shape transition. Below a critical island size of abou t 500 atoms the islands have a compact shape, while above this size th ey become ramified. The shape transition of the coherently strained is lands, predicted theoretically by Tersoff and Tromp, is driven by the size-dependent outward relaxation of the step edge atoms due to the po sitive lattice mismatch. The ramified island shape, which reflects the energy minimum of binding and strain energy, is characterized by only one parameter: the arm width of the monolayer-high copper islands w = (22 +/- 1) atoms.