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 c
entre is of considerable importance since it appears to act as a compensati
ng centre which is (at least in part) responsible for the difficulties of o
btaining effective acceptor concentrations in excess of 10(18) cm(-3). Seve
ral models have been suggested, the most favoured of which is that of a sel
enium vacancy associated with a substitutional nitrogen. However, although
very recent total energy calculations support such a model, no conclusive e
vidence for this identification exists. Electrons trapped by the centre hav
e a gyromagnetic ratio of 1.38, in contrast to the value of 1.11 for electr
ons 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 Ram
an scattering experiments to determine how this g-value depends on composit
ion in the nitrogen-doped ternary alloy ZnSxSe1-x, for x in the range 0 to
0.1. Surprisingly, we find that the g-value remains constant, in contrast t
o that for the shallow donors, which increases by 0.12 over the same compos
ition range. The results present a significant challenge for theoretical ca
lculation, since any microscopic model has to be reconciled with this invar
iance.