Formation and interaction of epitaxial Ge structures on Ge(001)

Abstract

The initial stages of Ge(001) homoepitaxy were studied using scanning tunneling microscopy. When 0.12 ML of Ge was deposited at 310 K, [130]- and [310]-oriented metastable clusters dominated the surface. Increasing the Ge coverage to 0.18 ML increased the density of epitaxial dimer rows. At 420 K, the metastable clusters were completely replaced by epitaxial structures, including islands that were smaller than the metastable clusters. These results were explained by two competing processes: (1) formation of metastable clusters followed by their conversion to epitaxial islands; and (2) conversion of metastable dimers to epitaxial on-top (B) and trough (D) dimers which then collide to directly nucleate stable islands. The latter dominated at elevated temperatures. At 420 K a myriad of epitaxial structures were seen including single buckled B dimers, single unbuckled dimer rows with ends terminated by either B and D dimers or only D dimers; single buckled dimer rows; pairs of buckled dimer rows with local $c(4\ifmmode\times\else\texttimes\fi{}2)$ symmetry; pairs composed of one buckled and one unbuckled dimer row; and larger epitaxial islands. The islands were elongated along the substrate $2\ifmmode\times\else\texttimes\fi{}\mathrm{direction}$ even though diffusion along the $1\ifmmode\times\else\texttimes\fi{}\mathrm{direction}$ is faster. This was attributed to an asymmetric capture probability due to the lower energy of ${S}_{A}$ steps oriented parallel to the upper terrace dimer rows versus the orthogonal ${S}_{B}$ steps. The islands also displayed step structures that do not exist on vicinal and singular Ge(001) surfaces including unbuckled ${S}_{A}$ and nonbonded ${S}_{B}$ steps. The interactions between the epitaxial islands and the substrate were also studied and it was found that these interactions led to $p(2\ifmmode\times\else\texttimes\fi{}2)$ symmetry due to dimer buckling rather than the favored $c(4\ifmmode\times\else\texttimes\fi{}2)$ symmetry.