Modulation spectroscopy studies of defects

We examine some relationships between defect characteristics and the modulated optical properties of solids. In particular, the effects of an electric field ℰ on the N absorption in GaP are presented. The crystallographic-orientation dependence is used to determine symmetry properties of the impurity electron states and their interaction with the host band structure. These data also yield an impurity concentration which is in agreement (to within a factor of 2) with results obtained using other techniques. The Franz-Keldysh mechanism responsible for the electro-absorption suggests that in indirect gap semiconductors, an electric field should double the luminescence. This enhancement will be greatest for ℰ along the orbital axis of the luminescent center. Generalizations of this work to other chemical and structural defects are discussed.