  Ge hut clusters, which appear during heteroepitaxy of Ge on Si(001), are
prototypical examples of islands formed through strain. Experimentally,
complete facets are observed to form rapidly, though the mechanism is unknown.
We model the growth of new faces on Ge hut clusters, using linear scaling DFT.
We build realistic small huts on substrates, and show that the growth of
\{105\} facets proceeds from top-to-bottom, even for these small huts. The
growth of the facet is driven by the reconstruction on the \{105\} facet, and
nucleates at the boundaries of the facet. We thus resolve any ambiguities in
kinetic models of hut cluster growth.
