Heteroepitaxial growth is a process of profound fundamental importance as well as
an avenue to realize nanostructures such as Ge/Si quantum dots (QDs), with appeal-
ing properties for applications in opto- and nanoelectronics. However, controlling
the Ge/Si QD size, shape, and composition remains a major obstacle to their practi-
cal implementation. Here, Ge nanostructures on Si(111) were investigated in situ and
in real-time by low energy electron microscopy (LEEM), enabling the observation
of the transition from wetting layer formation to 3D island growth and decay. The
island size, shape, and distribution depend strongly on the growth temperature. As
the deposition temperature increases, the islands become larger and sparser, consis-
tent with Brownian nucleation and capture dynamics. At 550◦C, two distinct Ge/Si
nanostructures are formed with bright and dark appearances that correspond to flat,
atoll-like and tall, faceted islands, respectively. During annealing, the faceted islands
increase in size at the expense of the flat ones, indicating that the faceted islands are
thermodynamically more stable. In contrast, triangular islands with uniform mor-
phology are obtained from deposition at 600◦C, suggesting that the growth more
closely follows the ideal shape. During annealing, the islands formed at 600◦C ini-
tially show no change in morphology and size and then rupture simultaneously, sig-
naling a homogeneous chemical potential of the islands. These observations reveal
the role of dynamics and energetics in the evolution of Ge/Si QDs, which can serve
as a step towards the precise control over the Ge nanostructure size, shape, compo-
sition, and distribution on Si(111).
