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 tunneling microscopy. The analysis of the saturation island density as a function of substrate temperature and deposition rate reveals that the smallest stable island abruptly changes from a dimer to a tetramer. From the Arrhenius plot, the migration barrier Em5~0.3560.02! eV, as well as the dimer bond energy Eb5~0.4660.19! eV, has been deduced. For low ratios between the migration constant D and flux R (D/R,10), nucleation and island growth take place not only during, but also after deposition. In this postnucleation regime, the final island density and island size distribution are no more determined by the competition between flux and monomer migration, but solely by the monomer concentration present immediately after deposition. Therefore, the island density becomes independent of substrate temperature and flux, and the scaled island size distribution closely resembles that of statistic growth ~adatom smallest stable island!. The experimental results are compared with simulations using rate equations. @S0163-1829~96!10948-6#