Structure of III-Sb(001) Growth Surfaces: The Role of Heterodimers

We have determined the structure of AlSb and GaSb (001) surfaces prepared by molecular beam epitaxy under typical Sb-rich device growth conditions. Within the range of flux and temperature where the diffraction pattern is nominally $(1\ifmmode\times\else\texttimes\fi{}3)$, we find that there are actually three distinct, stable $(4\ifmmode\times\else\texttimes\fi{}3)$ surface reconstructions. The three structures differ from any previously proposed for these growth conditions, with two of the reconstructions incorporating mixed III-V dimers within the Sb surface layer. These heterodimers appear to play an important role in island nucleation and growth.