Unexplored properties of defects with triply degenerate gap states in semiconductors: The role of lattice distortion

The existence of different charge states for point defects in semiconductors characterized by a threefold-degenerate gap state is discussed. It is shown that the importance of Jahn-Teller distortions has been previously underestimated. For instance, in symmetric situations where one can allow the distortion coordinate to change its sign, a negative-$U$ situation can occur much more easily than in cases previously analyzed. In nonsymmetric situations the balance between electron-electron repulsion and lattice distortion can be such that only one stable state exists. In this state two electrons of opposite spin are bound to the defect preventing any spectroscopic observation. This general mechanism is proposed as a possible explanation for the fact that there is no direct evidence concerning some interstitials in zinc-blende semiconductors.