The critical terrace width $\lambda$ for 2D island nucleation and growth (2DNG) on large-scale atomically flat terraces of a step-bunched Si(111)-($7\times 7$) 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 $720°\mathrm{C}$. At this temperature, the Arrhenius dependencies $\ln {\lambda }^2(1/T)$ change their slope, so that the effective 2DNG activation energy $E_{2\mathrm{D}}$ drops from 2.4 eV down to 0.5 eV at $T>720°\mathrm{C}$. We first show that the $E_{2\mathrm{D}}$ 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 $d_{-}\sim {10}^{5}a$ and the preferential step-down attachment of atoms to steps limited by an energy barrier $E_{\mathrm{ES}}^{-}\approx 0.9\mathrm{eV}$.

• Received 24 September 2012

DOI:https://doi.org/10.1103/PhysRevLett.111.036105