INITIAL-STAGES OF CU EPITAXY ON NI(100) - POSTNUCLEATION AND A WELL-DEFINED TRANSITION IN CRITICAL ISLAND SIZE

We present a comprehensive study of the nucleation kinetic of Cu on Ni (100) using variable-temperature scanning tunnelling microscopy. The a nalysis of the saturation island density as a function of substrate te mperature and deposition rate reveals that the smallest stable island abruptly changes from a dimer to a tetramer. From the Arrhenius plot, the migration barrier E(m)=(0.35+/-0.02) eV, as well as the dimer bond energy E(b)=(0.46+/-0.19) eV, has been deduced. For low ratios betwee n the migration constant D and flux R (D/R<10(4)), nucleation and isla nd growth take place not only during, but also after deposition. In th is postnucleation regime, the final island density and island size dis tribution are no more determined by the competition between flux and m onomer migration, but solely by the monomer concentration present imme diately after deposition. Therefore, the island density becomes indepe ndent of substrate temperature and Aux, and the scaled island size dis tribution closely resembles that of statistic growth (adatom smallest stable island). The experimental results are compared with simulations using rate equations.