  Calculations of the potential energy surface for tracer Ga and In adatoms
above three GaAs (111)A surface reconstructions are presented in order to
understand the growth conditions required to form axial heterostructures in
GaAs/InGaAs nano-pillars. In all calculations the Ga adatom has a stronger bond
energy to the surface than the In adatom. The diffusion barriers for Ga adatoms
are 140meV larger than for In adatoms on the Ga vacancy surface, but they are
comparable on the As trimer surface. Also the binding energy for an In adatom
is closer to that of a Ga adatom on the As trimer surface. We conclude that the
As trimer surface is preferable for adsorption of In and thus for selective
formation of hetero-interfaces on (111) facets. This work helps explain the
recent successful formation of axial GaAs/InGaAs hetero-interfaces in catalyst
free nano-pillars.
