In the course of atomistic simulations of the dislocation array at the
Ge/Si(001) interface, we have generated a new closed symmetric defect
structure comprising eighteen atoms that may be found in a variety of
circumstances including dislocation intersections and grain boundarie
s. The structure maintains tetrahedral bonding with reasonable changes
in bond lengths and angles, and may have interesting electronic prope
rties. At the Ge/Si interface, these extended point defects may reach
a very high planar concentration of approximately 10(12)/cm2.