Breakdown of the vacancy model for impurity-vacancy defects in diamond

The vacancy model for impurity vacancy defects in semiconductors assumes that the ground and low energy excited states are derivable from the four sp³–hybrid orbitals on the atoms bordering the vacancy. There are many cases where this model works but we describe here a counter-example concerning the lowest excited state of the [V-N₃] defect in diamond. It is shown that a shallow electron trap, localised outside the vacancy, is involved in the first excited state and responsible for the N2 and N4 optical bands associated with this defect.