It was shown, using heavily N-doped material, how iso-electronic doping resulted in an impurity band and how this was reflected by a large band-gap reduction and enhanced band-edge absorption. Heavily-doped GaP, or GaPN, exhibited properties which were characteristic of both direct and indirect gap semiconductors. Exciton bound states which were associated with perturbed N pair centres and larger GaN clusters were observed. It was concluded that, in order to describe correctly the properties of an impurity band, a hierarchy of impurity complexes had to be considered. The data also suggested that the excitonic effect played a role in impurity-band formation and band-gap reduction.

Optical Transitions in the Isoelectronically Doped Semiconductor GaP:N: an Evolution from Isolated Centres, Pairs and Clusters to an Impurity Band. Y.Zhang, B.Fluegel, A.Mascarenhas, H.P.Xin, C.W.Tu: Physical Review B, 2000, 62[7], 4493-500