Critical Terrace Width for Two-Dimensional Nucleation during Si Growth on Si(111)-(7 x 7) Surface

The critical terrace width lambda for 2D island nucleation and growth (2DNG) on large-scale atomically flat terraces of a step-bunched Si(111)-(7 x 7) surface has been studied by in situ ultrahigh vacuum reflection electron microscopy as a function of the substrate temperature T and Si deposition rate R. The dependence of lambda(2)(R) is characterized by a power law with scaling exponent chi = 1.36-1.46, validating an attachment limited (AL) growth kinetics up to 720 degrees C. At this temperature, the Arrhenius dependencies ln lambda(2) (1/T) change their slope, so that the effective 2DNG activation energy E-2D drops from 2.4 eV down to 0.5 eV at T > 720 degrees C. We first show that the E-2D change is caused by a transition between AL and DL (diffusion limited) growth kinetics accompanied by a step shape transformation. The AL growth mode is characterized by kinetic length d- similar to 10(a)(5) and the preferential step-down attachment of atoms to steps limited by an energy barrier E-ES(-) approximate to 0.9 eV.