Why are elemental dopants for Silicon on Germanium usually chosen from group XIII on group XV ?

Explanation

While preparing N or P type semiconductor from pure semiconductor, size of impurity atoms should be nearly equal to the size of Si or Ge atoms (so that symmentry of crystalline structure, would be maintained before and after the doping.) Also, the type of impurity atoms should be such that after its bonding through covalent bonds with the host crystal, we should get required free charge carriers.
Above conditions are fulfilled by elements in `13^(th)` and `15^(th)` group (XIII and XVth group) and so they are selected for preparing N and P type semiconductors.