It was shown that iso-electronic substitutional C atoms, which were present as an impurity in crystalline material, split the no-phonon luminescence line that was produced by the decay of excitons which were bound to P donors. The centroid of the doublet moved to lower photon energies, to a degree that was proportional to the C concentration. The splitting and shifting were attributed to a perturbation of the transition, due to strain that was produced by the C.

A.N.Safonov, G.Davies, E.C.Lightowlers: Physical Review B, 1996, 54[7], 4409-12