Abstract
The absorption spectra of ZnSe:Co and ZnS:Co contain sharp weak lines in the vicinity of 2.5 eV (20000 cm-1) in addition to the quartet-quartet bands of Co d7. Photocapacitance measurements on ZnSe:Co reveal a deep level in the lower half of the gap approximately 2.2 eV (17700 cm-1) below the conduction band edge. When the level is filled, cobalt is in the neutral d7 configuration. Both the electron and the hole photoionisation cross section spectra have been measured for this level. On the basis of the results, a model for the new absorption lines is proposed in which some quartet-doublet transitions gain intensity from the degeneracy of the final states with the conduction band while a phonon barrier decreases the auto-ionisation probability.