Gyromagnetic ratio of the compensating donor centre in nitrogen-doped ZnS_xSe_1-x

The nature of the so-called deep donor that appears at a depth of about 50 meV when ZnSe is doped with nitrogen remains a matter of controversy. The centre is of considerable importance since it appears to act as a compensating centre which is (at least in part) responsible for the difficulties of obtaining effective acceptor concentrations in excess of 10¹⁸ cm^-3. Several models have been suggested, the most favoured of which is that of a selenium vacancy associated with a substitutional nitrogen. However, although very recent total energy calculations support such a model, no conclusive evidence for this identification exists. Electrons trapped by the centre have a gyromagnetic ratio of 1.38, in contrast to the value of 1.11 for electrons bound at the normal, shallow (26 meV) donors in this material. In order to obtain a better understanding of the centre, we have used spin-flip Raman scattering experiments to determine how this g-value depends on composition in the nitrogen-doped ternary alloy ZnS_xSe_1-x, for x in the range 0 to 0.1. Surprisingly, we find that the g-value remains constant, in contrast to that for the shallow donors, which increases by 0.12 over the same composition range. The results present a significant challenge for theoretical calculation, since any microscopic model has to be reconciled with this invariance.