We have undertaken a new set of experiments to investigate the behavior of adsorbed‐impurity induced reconstructions at growth interfaces. We have observed a striking difference in the stability of the B(3)1/2×(3)1/2and Ga(3)1/2×(3)1/2two‐dimensional structures at the interface between Si(111) anda‐Si, and in their segregation behavior during molecular beam epitaxy crystal growth. This leads to a new model of dopant behavior in silicon molecular beam epitaxy.