Large lattice relaxation in a multielectron system - Binding energies and criteria for negative effective U.

The properties of a metastable defect observed in n-InP are reviewed. This defect is bistable and it has been established that one of its configurations has negative U character. The determination of the binding energies that define true level positions for this system is described. The kinetic criteria often used to establish negative U character for defects are shown to be inconclusive. To establish negative U character the binding energies for the levels involved should be compared. The microscopic mechanisms that may be responsible for large lattice relaxation are described in a general context.