Uniform-height island growth of Pb onSi(111)−Pb(3×3)at low temperatures

Abstract

We have studied the growth of Pb on $\mathrm{Si}(111)\ensuremath{-}\mathrm{P}\mathrm{b}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ at low temperatures, $T<250\mathrm{K},$ and for coverages up to $\ensuremath{\theta}<20\mathrm{ML}.$ The study is carried out with the complementary techniques of high-resolution low-energy electron diffraction and variable-temperature scanning tunneling microscopy. Uniform-height Pb islands, of flat tops and steep edges, are observed on this phase [similar to the uniform island growth observed on the $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)].$ The origin of this unusual growth is quantum size effects (i.e., the dependence of the electron energy on the island thickness because of the quantization of the electron energy levels in the island). The preferred island height is five steps on the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ and seven steps on the $(7\ifmmode\times\else\texttimes\fi{}7)$ (for the same growth conditions T, \ensuremath{\theta}) because of the different electronic structure at the two interfaces. Since different types of phases [i.e., $(1\ifmmode\times\else\texttimes\fi{}1),\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3},$ which differ in the Pb stoichiometry] can form from the initial $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase, with increasing coverage, the growth on these phases can be also used to study island-height selection. Laterally larger uniform-height islands are observed on the $(1\ifmmode\times\else\texttimes\fi{}1)$ at lower temperatures, $T<170\mathrm{K};$ growth on the $\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ at higher temperatures, $T>195\mathrm{K},$ shows an open film with preference of the islands to grow to larger heights with uncovered regions of the $\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase between the islands, even after deposition of 20 ML of Pb.